recent progress in solid electrolytes for energy storage

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Progress rePort

Recent Progress in Solid Electrolytes for Energy Storage Devices

Tingting Ye, Luhe Li, and Ye Zhang*

With the rapid advances in safe, flexible, and even stretchable electronic products, it is important to develop matching energy storage devices to more effectively power them. However, the use of conventional liquid electrolytes produces volatilization and leakage that are dangerous and requires strict packaging layers that are typically rigid. To this end, solid electrolytes that can overcome these problems have attracted increasing attention in recent decades. In this review article, three main types of solid electrolytes (i.e., inor-ganic, polymer, and composite electrolytes) are first described and compared in terms of their structures and properties. The advantages of solid electro-lytes to make safe, flexible, stretchable, wearable, and self-healing energy storage devices, including supercapacitors and batteries, are then discussed. The remaining challenges and possible directions are finally summarized to highlight future development in this field.

DOI: 10.1002/adfm.202000077

T. Ye, L. Li, Prof. Y. ZhangCollege of Engineering and Applied SciencesNanjing UniversityNanjing 210023, ChinaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202000077.

elasticity modulus of polymer solid elec-trolytes endow the energy storage devices with high flexibility and stretchability.[14,15] Finally, owing to the complex interactions among polymer chains, the resulting energy storage devices show more func-tions such as self-healable with the use of solid electrolytes.[16,17]

A series of energy storage devices based on solid electrolytes have been thus exten-sively explored in the past decades, and they are expected to greatly facilitate our life in the near future. For instance, light-weight, safe, and flexible energy storage devices can be directly worn on the body, and biocompatible energy storage devices can be implanted into tissues or organs to continuous provide energy for medical

devices in vivo.[10,12,18,19] It is thus necessary to carefully review the development of solid electrolytes for energy storage devices, particularly, the key advances in recent years, to show the main research directions for the next step. Here the categories, structures, properties, and ion transport mechanisms of solid electrolytes are firstly introduced. The fabrications and perfor-mances of resulting safe, flexible, stretchable, wearable, and self-healing energy storage devices including supercapacitors, metal batteries, and metal–air batteries are highlighted. Finally, the challenges and prospects of energy storage devices based on solid electrolytes are summarized for practical applications, the final goal of the concentration in this direction.

2. Solid Electrolytes

2.1. Inorganic Solid Electrolytes

Inorganic solid electrolytes have relatively high ionic conduc-tivity (>10−4 S cm−1), high thermal stability, high mechanical strength, and nonflammability.[12,14,20] Herein, for the conveni-ence of discussion, we divide inorganic solid electrolytes into three main types, including oxide, sulfide, and others (Table 1).

2.1.1. Oxide

Oxide is a typical inorganic solid electrolyte simultaneously with high chemical/electrochemical stability, mechanical strength and oxidation voltage.[14] Perovskite inorganic solid electrolytes with crystal structure of ABO3 (A = Ca, Sr, La; B = Al, Ti) can improve ionic conductivity by substituting different ions at A

1. Introduction

In recent years, the next-generation electronic devices, such as wearable bioelectronic devices,[1,2] electronic fabrics,[3,4] and implantable medical devices[5,6] are rapidly developed. Con-ceivably, these devices would be directly worn on the skin or implanted into the tissue and work stably under complex defor-mations. These wearable and implantable devices strongly demand indispensable energy storage systems that can be safe, soft, and multifunctional.[7,8] However, the toxic and flam-mable organic liquid electrolytes for commercial energy storage devices cause significant safety hazard.[9,10] In addition, it is dif-ficult to achieve the required flexibility and stretchability due to the strict encapsulation to prevent leakage of organic liquid electrolytes.[11]

Solid electrolytes with inorganic ceramics or polymers as matrices offer a better choice for energy storage devices due to their unique advantages. First, the self-supporting solid elec-trolytes simplify the packaging process and technical require-ments.[12] Second, inorganic ceramics with high mechanical strength can make solid electrolytes effectively inhibit the growth of dendrites, thereby preventing short circuits and reducing safety risks.[13] Third, the unique flexural rigidity and

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and B sites. In general, applying a large rare-earth metal ion at A site can promote the transport of ions, since the ions in the perovskite migrate along the adjacent A site vacancies and pass through a bottleneck comprising four adjacent oxygen ions.[13] The Li3xLa2/3−xTiO3 obtained by occupying the A site with Li or La has a high ionic conductivity of 10−3 S cm−1 at 25 °C.[21–23] However, the structural and chemical instabilities of Ti4+ to lithium metal make it inadaptable for lithium-ion batteries.[21]

AM2(PO4)3 (A = Li, Na; M = Ti, Ge, Zr) is the original for-mula of NASICON-type inorganic solid electrolytes where the ions transport in 3D M2(PO4)3− skeleton composed of MO6 octahedron and PO4 tetrahedron.[24] LiTi2(PO4)3 offers the most suitable lattice size for Li+ conduction but the high porosity is not conducive to ion conduction.[25,26] Partially substituting Ti4+ by Al3+, Li1+xAlxTi2−x(PO4)3 (LATP) exhibits high ionic con-ductivity of 10−3 S cm−1 due to the increased density of Li+ in skeleton and interstitial migration with lower activation ener-gies.[27–29] In addition, substituting Ti4+ with Ge4+ can produce Ti-free Li1+xAlyGe2−y(PO4)3 (LAGP) to increase the stability for lithium metal.[30]

Li5La3M2O12 (M = Nb, Ta, Zr) is a typical formula of garnet-type inorganic solid electrolytes. The Li-rich cubic Li7La3Zr2O12 (LLZO) possesses relatively high ionic conductivity of 2.44 × 10−4 S cm−1, high chemical stability to lithium metal, and wide electrochemical window.[31] However, tetragonal LLZO exhibits poor ionic conductivity due to the fully ordered arrangement of Li+.[32] Substitution is an effective strategy to stabilize the highly conductive cubic phase by creating Li vacan-cies, e.g., substituting Li sites by Al3+, Ga3+, or replacing Zr with Nb5+, Sb5+.[33–36] The Li6.55+yGa0.15La3Zr2−yScyO12 prepared by dually replacing LLZO displays high ionic conductivity of 1.8 × 10−3 S cm−1.[37] The Ga3+ inserted at Li site can stabilize the cubic crystal structure while the partially filled Sc3+ at Zr site can increase the number of charge carriers. And the pres-ence of Sc increases the disorder of the Li network, leading to an increase in local mobility of the partial Li population (Figure 1a).

2.1.2. Sulfide

Sulfide inorganic solid electrolytes possess higher ionic con-ductivities than oxide inorganic solid electrolytes because of the isovalent substitution of oxygen to larger and higher polarizability sulfur.[38] The ionic conductivity of amorphous Li2S–P2S5 varies from the preparation process and the ratio of Li2S to P2S5. The amorphous 75 Li2S–25 P2S5 (mol %) fab-ricated by mechanical milling possesses an ionic conductivity of 2.0 × 10−4 S cm−1,[39] while the glass–ceramic 70 Li2S–30 P2S5 (mol %) obtained by melt quenching and heating exhibited an extremely high ionic conductivity of 1.7 × 10−2 S cm−1.[40] Gener-ally, the amorphous Li2S–P2S5 that contains large amounts of Li2S is expected to exhibit high ionic conductivity. While the glass–ceramic Li2S–P2S5 exhibits superior ionic conductivity due to the heat treatment which eliminates the grain bounda-ries and produces Li7P3S11 crystals with high ionic conductivity.

The thio-LiSICON with a LixM1−yM′yS4 (M = Si or Ge; M′ = P, Al, Zn, Ga, or Sb) formula shows ionic conductivities ranged from 10−7 to 10−3 S cm−1.[41,42] A new type, Li10GeP2S12 (LGPS),

has an ionic conductivity of 1.2 × 10−2 S cm−1, which is

comparable to liquid electrolytes.[43] The ion migrate in LGPS through a 1D pathway which is composed of LiS6 octahedra and (Ge0.5P0.5)S4 tetrahedra (Figure 1b). Substituting Ge4+ with Si4+ or Sn4+ can solve the limitation of the low abundance and high cost of Ge. In addition, Li9.54Si1.74P1.44S11.7Cl0.3 possesses the highest conductivity of 2.5 × 10−2 S cm−1 among reported Li+ conductors due to the 3D conduction mode based on the 1D pathway of LGPS.[44]

2.1.3. Others

Composite hydrides are regarded as reliable inorganic solid electrolytes due to their low grain-boundary resistance, high stability to lithium metal, high mechanical strength and high flexibility.[14] LiNH2, Li3AlH6, and Li2NH generally exhibit poor ionic conductivities, while LiBH4 shows high ionic conductivity of 10−3 S cm−1 due to its hexagonal structure.[45] Therefore, it is desirable to synthesize stable hexagonal LiBH4 to improve the ionic conductivity through structural tuning, e.g., second-phase incorporation, aliovalent ion doping, interface engineering, etc., Na2B10H10 even exhibits superior conductivity of around 10−2 S cm−1.[46] This can be attributed to the ion conduction pathway in a spacious, vacant-rich, and interstitial network, which is provided by a large, polyhedral, orientationally mobile, B10H10

2− sublattice (Figure 1c).Halide inorganic solid electrolytes also simultaneously

demon strate high stability to lithium metal, mechanical strength, and flexibility.[14] In addition, the large radius of the monovalent halogen anion and the weak interaction with Li+ provide halide inorganic solid electrolytes high ionic conduc-tivities. For instance, Li3YBr6 possesses both high ionic con-ductivity of 1.7 × 10−3 S cm−1 and chemical/electrochemical stability due to the stable YBr6

3− octahedrons and the conduc-tion pathway incorporated with Y3+.[47] The Li+ conduction path-ways in Li3YBr6 are connected via tetrahedral interstitial sites in three directions (Figure 1d), and the tetrahedral interstitial sites adjacent to Y3+ are blocked by repulsive coulombic interactions between Y3+ and Li+.

Single-ion inorganic solid electrolytes mean that only cations are transported in electrolytes while the anions are absent or

Ye Zhang is an associate professor at the College of Engineering and Applied Sciences at Nanjing University. She obtained her Ph.D. in Macromolecular Chemistry and Physics from Fudan University and then joined Harvard Medical School as a postdoctoral research fellow. Her research focuses on the development

of flexible energy and electronic devices and their biotech-nological applications.




immobilized in the matrix to increase the ionic conductivity. Covalent organic frameworks (COFs), metal organic frameworks (MOFs) and organic microporous polymer networks (MPNs) are suitable to prepare single-ion solid electrolytes due to their crys-talline and porous network.[48–53] For example, it is promising to introduce imidazolate to the backbone of the COF to form new ionic COF with an anion alignment center.[48] The ionic COF exhibits high Li+ conductivity of 7.2 × 10−3 S cm−1 because of the loose interaction between imidazolate anion and Li+ (Figure 1e). In addition, introducing tetracoordinate borate,[54] tetra- and hexacoordinate phosphate,[55] and hexacoordinate silicon[56] ionic functionalities to form negatively charged structural units is promising strategy to synthesize single-ion solid electrolytes.

To summarize, the performances of inorganic solid electro-lytes are affected by many factors such as crystal structure, Li+ coordination number, anion valence, ionic radius, and struc-tural transformation. It is important to carefully balance the above factors to design and optimize inorganic solid electro-lytes aiming at high ionic conductivities. There are also some disadvantages of inorganic solid electrolytes, which should be considered for practical applications. For one instance, the sulfide inorganic solid electrolytes are sensitive to moisture and oxygen, and the hydride and halide inorganic solid electro-lytes are sensitive to moisture, so the resulting energy storage devices need to be carefully packaged. For another instance, the poor flexibility of perovskite inorganic solid electrolytes may


Table 1. Summary of solid electrolyte materials.

Solid electrolytes Ionic conductivity [S cm−1]

Advantages Disadvantages

Inorganic solid electrolytes

Perovskite[21–23] 10−4–10−3 High ionic conductivity High stability to airHigh mechanical strength

High thermal stability

Unstable to LiPoor flexibility

High production cost

NASICON[25–30] 10−4–10−3 High stability to airHigh mechanical strength

Wide electrochemical windowHigh thermal stability

Unstable to LiHigh production cost

Garnet[31–37] 10−4–10−3 High stability to LiHigh mechanical strength

Wide electrochemical windowHigh thermal stability

High production cost

AmorphousLi2S-P2S5

[39–40]10−4–10−2 High ionic conductivity Low grain-boundary

resistanceStable to Li

High flexibilityHigh thermal stability

Sensitive to moisture and O2

Thio-LISICON (LGPS)[41–44]

10−2 Extremely high conductivityHigh thermal stability

Sensitive to moisture and O2

Hydride[45–46] 10−7–10−2 Low grain-boundary resistanceStable to Li

High mechanical strengthHigh flexibility

Sensitive to moisturePoor compatibility with cathode materials

Halide[47] 10−8–10−3 Stable to LiHigh mechanical strength

High flexibilityHigh thermal stability

Sensitive to moistureLow oxidation voltage

Single-ion[48–53] 10−5–10−3 High ion migration number Poor electrochemical stability

Polymer solid electrelytes

Polymer[58,72–80] 10−8–10−4 High flexibilityClose contact with electrodes

MultifunctionsChemical stability

Low ionic conductivityRelatively poor mechanical strength

Gel polymer[57,81–85] 10−4–10−3 High ionic conductivityHigh flexibility

Close contact with electrodesMultifunctions

Chemical stability

Poor mechanical strengthPoor thermal stability

Composite solid electrolytes[87–98]

10−5–10−3 High ionic conductivityHigh flexibility

High mechanical strengthHigh thermal stability

Close contact with electrodes

High interfacial resistance between electrolyte layersAggregation of inorganic particles



lead to poor electrode/electrolyte interfaces, and the instability of inorganic solid electrolytes to lithium metal, such as perov-skite (LLTO), NASICON (LATP, LAGP), and sulfide (crystalline Li2S-P2S5, LGPS),[12] also result in such poor interfaces. More efforts should be made to enhance them by further optimizing moieties and structures of the inorganic solid electrolytes.

2.2. Polymer Solid Electrolytes

Compared with inorganic solid electrolytes, polymer solid elec-trolytes display promising advantages such as more simple man-ufacturing process, higher flexibility, higher chemical stability, and satisfactory compatibility with electrodes. Polymer solid electrolytes are typically prepared by uniformly dissolving metal salts in polymer matrices (Figure 2a).[57] The mostly explored polymers include poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVDF), poly(vinylidene fluoride-hexafluoropro-pylene) (PVDF-HFP), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), and poly(vinyl alcohol) (PVA).[58–62]

The formation of close electrode/electrolyte interface is the key to reduce charge transfer resistance at the interface and fur-ther ensure high stability of the interface after a high number of cycling. To date, many polymer solid electrolytes that exhibit close interfaces with electrodes, e.g., those based on PEO, PVDF, PMMA, and PVDF-HFP, have been explored for both high cycling performance and capacity retention.[63–67] Based on the high flexibility, these polymer solid electrolytes closely contact with electrodes even when they have different surface micro-structures and morphologies, and they can also adapt to the volume change of the electrodes during cycling (Figure 2b). The energy-dispersive X-ray spectra of the electrode surface, which contacts liquid and poly(ether-acrylate)-based polymer solid electrolyte, shows no significant difference after charging/dis-charging for one cycle. This may indicate that polymer solid elec-trolytes are comparable to liquid electrolytes from a viewpoint of interface with electrode, but it requires sufficient charging/dis-charging cycles to further verify the above conclusion.[68]

The close contact between electrodes and polymer solid elec-trolytes can be further verified by the cyclic voltammogram of


Figure 1. Inorganic solid electrolytes. a) The unit cell of Li54Ga1La24Zr15Sc1O96. Reproduced with permission.[37] Copyright 2017, American Chem-ical Society. b) Crystal structure of Li10GeP2S12 (the framework structure and Li+ that participate in ionic conduction) and framework structure of Li10GeP2S12. 1D chains are formed by LiS6 octahedra and (Ge0:5P0:5)S4 tetrahedra, and these chains are connected by a common corner with PS4 tetrahedra. Reproduced with permission.[43] Copyright 2011, Springer Nature. c) Related geometries of the B10H10

2− anions, with green, brown, and white spheres denoting boron, carbon, and hydrogen atoms, respectively. Reproduced with permission.[46] Copyright 2016, Wiley-VCH. d) The crystal structures of Li3YBr6. The yellow surface corresponds to the ionic conduction path, and the regions enclosed with red surfaces correspond to the stable Li+ positions. Reproduced with permission.[47] Copyright 2018, Wiley-VCH. e) The structures of imidazolate-ionic COF. Reproduced with permission.[48] Copyright 2019, American Chemical Society.



the resulting batteries. Polymer solid electrolyte exhibits an approximate overpotential compared to liquid electrolyte. The close interface between poly(ether-acrylate)-based polymer solid electrolyte and electrode is attributed to the flexible PEO which softens the rigid poly(ether-acrylate) network to ameliorate the contact between the electrode and electrolyte and reduces the interfacial resistance. Besides the close contact, polymer solid electrolytes are stable to electrodes. Polycarbonate-based solid polymer electrolytes, e.g., poly(propylene carbonate),[69] poly(ethylene carbonate)[70] and poly(vinylene carbonate),[71] typically exhibit high electrochemically stability window due to the high dielectric constant of cyclic carbonates. For example, poly(vinylene carbonate)-based polymer solid electrolyte with electrochemically stability window of 4.5 V versus Li/Li+ can fabricate lithium battery with high voltage LiCoO2 (4.3 V).[71] The capacity of LiCoO2||PVCA-SPE||Li battery retains by 84.2% after 150 cycles indicates the good electrode/electrolyte inter-face stability during the long-term cycles. In addition, there is no sharp voltage fluctuation and short circuit of Li deposi-tion/striping cycling after 600 h of polarization, indicating high compatibility between PVCA and lithium anode.

Compared with inorganic solid electrolytes, although polymer solid electrolytes possess stable interfaces with elec-trodes, they usually display relatively low ionic conductivities at room temperature (10−8 to 10−6 S cm−1) because of the dif-ferent ion transport mechanisms in polymer solid electrolytes, i.e., segmental motion of the coordination site on polymer backbone and hopping among ionic coordination sites by struc-ture rearrangement.[72] Therefore, reducing glass-transition

temperature (Tg) of polymer or introducing side chain units on polymer backbone with promoted segmental motion can significantly increase ionic conductivity. For example, poly(ethylene glycol) methyl ether methacrylate (PEGM) copo-lymerized with lithium 1-[3-(methacryloyloxy)-propylsulfonyl]-1-(trifluoromethylsulfonyl)imide (LiMTFSI) (Figure 2c) can reduce the Tg of poly(LiMTFSI) to below room temperature, and increase the ionic conductivity from 1.1 × 10−12 S cm−1 to 2.3 × 10−6 S cm−1 at 25 °C.[73] Generally, the side chain units need to possess low steric hindrance, few polar groups, and weak interchain interactions.

Another factor that influences the ionic conductivity relates to the movement of salt anions, which compete with Li+ at the transport in the electrolyte. Fixing the salt anions to polymer backbones can significantly increase the transference number of Li+.[74] Notably, the salt anions should possess weak interac-tions with Li+ to guarantee the high dissociation level deter-mined by the negative charge distribution of the salt anions. Compared with RCO2

− and RSO3−, SO2N(−)SO2CF3 and

SO2N(−)SO(NSO2CF3)CF3 exhibit improved nega-tive charge distribution, and the polymer solid electrolytes obtained by grafting the lithium salts containing these anions on PEO possess high ionic conductivities of 1.3 × 10−5 and 2.4 × 10−5 S cm−1 at 60 °C, respectively.[58,75] The salt anions can be directly used as side chains to bond on polymer chain to build low Tg single-ion polymer solid electrolytes with high ionic conductivity, but the steric hindrance and polar groups of the anions need to be considered. In addition, introducing anion acceptors on polymer matrices which immobilize anions


Figure 2. Polymer solid electrolytes. a) Schematic diagram of polymer solid electrolytes. b) Schematic of physical contact of polymer solid electrolyte and inorganic solid electrolyte with electrode material. Reproduced with permission.[101] Copyright 2018, American Chemical Society. c) The molecular formula of the poly(PEGM)-b-poly(LiMTFSI) copolymer. Reproduced with permission.[73] Copyright 2016, American Chemical Society. d) Illustration of the interaction between thioamide groups in PDTOA and anions. Reproduced with permission.[76] Copyright 2015, American Chemical Society. e) Illus-tration of the PEO-PS copolymers blended with LiTFSI (triblock copolymer, brush polymer, and hyperstar polymer, from left to right). Reproduced with permission.[80] Copyright 2019, American Chemical Society. f) Schematic diagram of gel polymer electrolytes.



on polymer can also promote the transport of Li+. For example, the thioamide groups in poly(dithiooxamide) (PDTOA) can bond anions of the lithium salt by hydrogen bonding, thus reducing the rate of anion diffusion. The ionic conductivity of PEO-b-PDTOA block copolymer reaches 2 × 10−4 S cm−1 at 45 °C (Figure 2d).[76]

Obviously, high mechanical strength is a key for polymer solid electrolyte to withstand external pressure and prevent pen-etration by dendrites. Generally, we can reduce the activity of main chains in polymers to increase their mechanical strength by varying the moieties (e.g., the use of rigid chains that contain aromatic or heterocyclic rings), moderately crosslinking polymer chains and promote polymer crystallization by improving the regularity of polymer chains.[77] However, reducing the mobility of the polymer chain also leads to a decrease in ionic conduc-tivity. Therefore, it is important to carefully make a trade-off during the design of polymer solid electrolytes.

A large number of modifications of polymer solid electro-lytes to balance the ionic conductivity and mechanical strength have been made typically on the basis of PEO that simultane-ously shows high structure stability, chemical stability, and ionic conductivity at an amorphous state. A block polymer of poly(styrene-block-ethylene oxide-block-styrene) (PS-b-PEO-b-PS) can balance the mechanical hardness and ionic conductivity of the resulting solid electrolyte.[78] Crystallization can improve the mechanical strength but is not favorable for ionic conductivity. In this system, since the hard PS block can provide mechanical strength for polymer solid electrolyte while the amorphous PEO is responsible for conducting Li+, the PS–PEO–PS can mini-mize crystallinity to optimize ionic conductivity. To optimize the block copolymers, a series of PS-b-PEO copolymers have been explored, e.g., PS with either linear or comb PEO. The linear PS–PEO–PS block polymer exhibits the best effect on balancing the ionic conductivity and mechanical hardness of these solid electrolytes. When the molecular weight of PEO is 35 kg mol−1 and the weight percentage is 78, PS–PEO–PS shows superior ionic conductivity of 2.55 × 10−4 S cm−1 at 60 °C.[79] A hyper-star polymer with hyperbranched PEO as the core ensures little crystallinity and linear PS arm physically crosslinking to pro-vide mechanical strength (Figure 2e). The ionic conductivity and storage modulus of this hyperstar polymer solid electro-lyte are 9.5 × 10−5 S cm−1 and 0.5 MPa at 60 °C, respectively.[80] These explorations provide clues to improve ionic conductivities and mechanical strengths of polymer solid electrolytes by using other polymers, or constructing single-ion polymer electrolytes based on tough-hard block copolymers. However, the increase in ionic conductivity of polymer solid electrolytes is limited.

Gel polymer electrolytes consist of polymer solid electrolytes and plasticizers (Figure 2f) exhibit high ionic conductivity from 10−4 to 10−3 S cm−1 at room temperature.[57] The gel polymer elec-trolytes also possess better electrode wettability than polymer solid electrolytes but poor mechanical property and low thermal stability. Different from polymer solid electrolytes, the trans-port of ions in the gel polymer electrolytes usually occurs in the liquid phase, while the polymer backbone only serves to provide mechanical strength and maintain the shape of gel polymer electrolytes.[19] Therefore, the polymer backbone should pos-sess properties of good film-forming ability, high film strength, and nondecomposition in semiliquid state. Organic solvents

and ionic liquids are mostly studied for plasticizers. Organic solvents, such as carbonates (propylene carbonate (PC), eth-ylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC)), can accelerate ion pair dissociation of salts and promote transport of ions.[81–83] However, the high con-tent of organic solvents reduces mechanical properties of gel polymer electrolytes, and the flammability of organic solvents results in safety problem. In contrast, ionic liquids (IL) (e.g., fluorine-containing anions and cations of imidazole and pyri-dine) are widely explored for low vapor pressure, remarkable electrochemical, high thermal stability, and nonflammability. However, there is a compatibility problem between ILs and poly mers, which may lower ionic conductivity.[84]

Gel polymer electrolytes using water as solvent are com-monly used in aqueous energy storage devices. Obviously, these electrolytes, e.g., PVA/H3PO4, PVA/KOH, and PVA/LiCl, are incompatible with lithium metal but extensively used for super-capacitors.[85] Although such electrolytes are simple to prepare, they demonstrate poor water retention and low mechanical property. But, hydrogels with 3D network structures can absorb large amounts of moisture into the network structure and exhibit improved mechanical property due to physical and chemical crosslinking of polymers.[15] In hydrogels, physical crosslinking typically involves charge interaction, hydrogen bond, and hydrophobic interaction, while chemical crosslinking is primarily caused by chemical reaction between groups or crosslinkers. Crosslinking is considered as an important strategy for modified hydrogels. Generally, polymer materials used to prepare the hydrogels are rich in hydrophilic groups, e.g., OH, COOH, CONH2, CONH, and SO3H, which produce weak physical crosslinking in polymers by hydrogen bonds. The rich hydrophilic groups are the key to increase the water content and water retention capacity of hydrogels. These abundant functional groups also provide the possibility for polymer modifications such as the formation of graft, copol-ymer, and interpenetrated network. The hydrogels with strong chemical crosslinking by adding crosslinkers is more effec-tive in the improvement of mechanical properties. However, if the degree of crosslinking is too high, the hydrogel becomes relatively rigid and brittle. Fortunately, the design of double network structure can simultaneously achieve the toughness and softness of the hydrogels.[86] The promising advantages of hydrogels lie in the thermoresponsiveness, stretchability, and self-healability, which will be discussed in the next chapter.

Although ionic conductivities of polymer solid electrolytes have been much enhanced by optimizing moieties and struc-tures of polymers, they are still not comparable to those of inorganic solid electrolytes. The use of gel polymers can greatly increase the ionic conductivity of the resulting polymer solid electrolytes, the mechanical strengths of gel polymer electro-lytes are poor. It is necessary to develop solid electrolytes for close and stable contacts onto electrodes with both high ionic conductivity and mechanical strength.

2.3. Composite Solid Electrolytes

The composite solid electrolytes that combine the advantages of inorganic and polymer solid electrolytes are expected not




only simultaneously display high ionic conductivity, mechanical strength, thermal stability, and nonflammability of the inor-ganic solid electrolyte, but also show high flexibility, high sta-bility, and close interface with electrode of the polymer solid electrolyte.

Composite solid electrolytes can be roughly divided into two main categories, i.e., 1) polymer and inorganic solid electrolytes being stacked into layered composite electrolytes; 2) polymer solid electrolytes incorporated with inorganic fillers. For layered composite electrolytes, the composition of the layered structure can be replaced according to the properties of the electrode materials and batteries. For example, adding a flexible polymer solid electrolyte layer between rigid oxide inorganic solid elec-trolyte and the electrode can improve the interface contact, while high electrochemical oxidation voltage, and mechanical strength of the oxide inorganic solid electrolyte can make up for the low strength and oxidation voltage of the polymer solid electrolyte. A typical sandwiched structure of polymer/inor-ganic/polymer composite solid electrolytes in the first category is demonstrated in Figure 3a, and it shows two advantages for energy storage devices. On the one hand, the inorganic layer reduces the double-layer electric field at the lithium/polymer interface by hindering the transport of salt anions, increasing the coulombic efficiency. On the other hand, in addition to preventing direct contact between the inorganic layer and the

lithium metal, the polymer layer can effectively inhibit dendrite nucleation and better wet the lithium metal for better interface. For example, a crosslinked poly(ethylene glycol) methyl ether acrylate (CPMEA)/Li1.3Al0.3Ti1.7(PO4)3 (LATP)/CPMEA sand-wiched composite solid electrolyte for lithium battery exhibits high coulombic efficiency of 99.8–100% with high electro-chemical stability.[87] The Li||LiFePO4 battery with CPMEA/LATP/CPMEA shows a long cycling stability of 640 cycles, while the battery with CPMEA electrolyte shows a rapid fading after 200 cycles, which proves the better electrochemical stability of CPMEA/LATP/CPMEA. This conclusion can be further demon-strated by the impedance changing during the long cycling. The battery with CPMEA exhibits an obvious impedance increase after 325 cycles in contrast to the battery with CPMEA/LATP/CPMEA which shows much lower increase even after 640 cycles. The long cycling stability of CPMEA/LATP/CPMEA also demonstrates that the composite solid electrolyte can effec-tively restrain the dendrite, since the CPMEA layer inhibits the formation of dendrite and LATP layer inhibits the growth of dendrite.

Although the above structure design combines the advan-tages of polymer and inorganic solid electrolytes, the interfa-cial resistance between two neighboring electrolyte layers may cause an increase in resistance of the composite solid electro-lytes. To this end, inorganic fillers are directly incorporated


Figure 3. Composite solid electrolytes. a) Illustration of polymer/inorganic/polymer sandwiched structure. Reproduced with permission.[87] Copyright 2016, American Chemical Society. b) Schematic of interaction mechanisms between PEO chain and SiO2 (the strongly interacting PEO–SiO2 composite with almost amorphous PEO). Two possible interaction mechanisms are shown (i.e., chemical bonding of the ends of PEO chains with hydroxyl groups on SiO2 surfaces and mechanical wrapping of PEO chains). Reproduced with permission.[91] Copyright 2017, American Chemical Society. c) Li+ transport pathways within the composite solid electrolyte. d) Li+ conduction pathways in composite polymer electrolytes with nanoparticles, random nanowires and aligned nano-wires. Compared with isolated nanoparticles, nanowires could supply a more continuous, fast conduction pathway for Li+. Reproduced with permission.[95] Copyright 2017, Springer Nature. e) Schematic representation of possible conduction mechanism in composite electrolytes with agglomerated nanoparticles and 3D continuous framework. Reproduced with permission.[98] Copyright 2018, Wiley-VCH.



into polymer solid electrolytes to prepare composite solid electrolytes. Inorganic fillers are classified as negative ones that do not participate in ionic transport and positive ones that partially involve in ionic conduction. Dispersing negative inorganic fillers such as SiO2, TiO2, Al2O3, ZrO2, and LiAlO2 nanoparticles in polymer solid electrolytes can improve ionic conductivities by 1–2 order, from 10−7 to 10−5 S cm−1.[88–90] The negative inorganic fillers that perform as crosslinking centers in polymer solid electrolytes can hinder the rearrangement of segments and reduce the crystallinity of the polymer. Mean-while, there are Lewis acid–base interactions between the functional groups of negative inorganic fillers and anions in the electrolytes, which enhance salt dissociation and further promote transport of cations. The conventional method about directly adding inorganic particles to polymer solid electrolytes has a problem of aggregation, and the effect for inhibiting crystallization of polymers is not satisfactory.[90] A uniform dispersion of ultrafine SiO2 in PEO can be achieved by in situ hydrolysis of tetraethyl orthosilicate, and it increases effective surface area for Lewis acid–base interaction.[91] In this com-posite solid electrolyte, the hydroxyl groups on the surface of SiO2 are chemically bonded to those at the ends of PEO chains, and the PEO chains can be mechanically wrapped and par-tially embedded in SiO2 spheres (Figure 3b). These two strong interactions effectively reduce the crystallization behavior of the polymer. As a result, the ionic conductivity of PEO/SiO2/LiClO4 reaches 4.4 × 10−5 S cm−1 at 30 °C and 1.2 × 10−3 S cm−1 at 60 °C. Moreover, the PEO/SiO2/LiClO4 obtained by in situ hydrolysis exhibits a high electrochemical stability window of 5.5 V versus Li/Li+, while the PEO/SiO2/LiClO4 composite solid electrolyte obtained by ex situ hydrolysis shows only 4.7 V. The high electrochemical stability window of in situ PEO/SiO2/LiClO4 is attributed to the strong adsorption of anion which inhibits the decomposition of anode at high potential, while the anodic decomposition of anion may be the main reason of electrochemical instability at high potential.[92]

Negative inorganic fillers can improve ionic conductivity by reducing the crystallinity of polymers and improve the electro-chemical and thermal stability, but they cannot directly partici-pate in ion conduction. In contrast, positive inorganic fillers not only inherit the advantages of negative inorganic fillers but also can participate in ion conduction. Positive inorganic fillers (LLTO, LLZTO, LATP, LAGP, etc.) can be directly added to the polymer solid electrolytes to improve ionic conductivity, mechanical strength, and thermal and electrochemical stability. For example, the ionic conductivity of PEO/Li6.4La3Zr1.4Ta0.6O12 (LLZTO)/LiTFSI composite solid electrolyte increases with the increase of LLZTO content, and reaches the highest ionic con-ductivity of about 1.17 × 10−4 S cm−1 at 30 °C when the content of LLZTO is 20%. In addition, the rigid LLZTO and PEO form a mechanically robust framework to resist lithium dendrites, and the composite solid electrolyte exhibits high flexibility when the LLZTO content is less than 50%. The electrochemical stability window of PEO/LLZTO/LiTFSI has been improved to 5.0 V versus Li/Li+.[93] The ion transport in composite solid electrolyte prefers along the inorganic phase and polymer–inorganic inter-face (Figure 3c). Therefore, the nanoscale inorganic fillers with large specific surface areas can enhance the ionic conductivity drastically.[94]

Based on this ion transport mechanism, constructing contin-uous pathways and large interfacial areas is more advantageous for ion transport than isolated nanoparticles where Li+ has to cross a lot of particle–particle junctions (Figure 3d). A com-posite solid electrolyte formed by dispersing 1D ceramic nano-wire filler of Li0.33La0.557TiO3 (LLTO) in PAN/LiClO4 exhibits an enhanced ionic conductivity of 2.4 × 10−4 S cm−1 at 25 °C, which is attributed to the prolonged ion pathway at the polymer–filler interface compared with the nanoparticle (Figure 3d).[94] In comparison to the randomly dispersed ion conductive nano-wires, the nanowires aligned along the same direction can fur-ther improve the ion conductivity due to the lack of crossing junctions in nanowire surfaces (Figure 3d).[95] On the basis of 0D nanoparticles and 1D nanowires, a few-layered vermicu-lite sheet PEO/LiTFSI composite exhibits ionic conductivity of 2.9 × 10−5 S cm−1, high thermal stability and high mechanical modulus. The electrochemical stability window can be stable up to 5.35 V, and the resulting battery can cyclize for two months without polarization.[96] For the same reason, the vertically aligned vermiculite sheet/PEO composite exhibits higher ionic conductivity than randomly aligned ones.[97] In order to achieve the utilization of high concentrations of inorganic fillers, a 3D nanostructured hydrogel Li0.35La0.55TiO3(LLTO) framework is constructed to composite with PEO (Figure 3e). The 3D nanostructured LLTO framework provides an interconnected 3D continuous phases that promotes the ionic conductivity to almost 10−4 S cm−1.[98] Therefore, for positive inorganic fillers, building continuous ion transmission network at nanoscale and avoiding agglomeration of fillers are the main directions for designing composite polymer composite electrolytes, while for negative inorganic fillers, the key is to build strong physics/chemical interaction between inorganic particles and polymer chains to suppress crystallinity of polymer and achieve mono-dispersity of the inorganic particles.

3. Solid Electrolytes for Advanced Energy Storage DevicesAs previously discussed, for the application in energy storage devices, solid electrolytes show promising advantages in prop-erties compared with liquid electrolytes. Five main properties of safety, flexibility, stretchability, wearability, and self-healability that have been mostly explored in the past decades are carefully summarized below.

3.1. Safety

Safety is a key for the practical application of energy storage devices. The safety issue, taking lithium-ion batteries based on organic solvents as example, is generally stemmed from the overheating caused by overcharging, short circuit, or exposing to high temperature. Along with the accumulation of heat, on the one hand, the solid electrolyte interphase (SEI) may be decomposed, causing the reaction between lithium metal and unstable organic electrolyte, which results in the release of flammable gases; on the other hand, the separators decom-pose at high temperature, further increasing the safety hazard




of energy storage devices. In addition, the flammability of the organic electrolytes makes energy storage devices extremely flammable or even explosive in a high-temperature environ-ment.[99] The high mechanical properties of solid electrolytes can resist external pressure and dendrite puncture, reducing the chance of short circuit in the energy storage devices. In addition, both high thermal stability and flame retardancy enable energy storage devices to withstand high temperature. The use of thermoresponsive polymer solid electrolyte further allows the energy storage device to work smartly, i.e., auto-matically shutting down themselves above critical temperature, which also guarantees their safety.[100]

In order to solve the safety problem, it is necessary to avoid overheating of energy storage devices during the operation. The internal short circuit is the primary reason for overheating. There are two major factors to produce short circuits inside: 1) external factors such as pressure or piercing by external forces; 2) internal factors mainly caused by the formation of lithium dendrites. For external factors, the self-supporting and shape stability of the solid electrolytes ensure that the energy storage devices retain their original structures after being pierced or cut. The pouch type battery with solid-state electrolyte remains stable without burning or exploding during nail penetration

test, and it further maintains original voltage without short circuit. In contrast, the battery with liquid electrolyte pro-duces a flame and undergoes intense expansion (Figure 4a).[71] The solid-state lithium battery with a cellulose-supported poly (propylene carbonate) polymer solid electrolyte can effectively power the LED after cutting off a portion.[69]

For internal factors, lithium dendrites are caused by uneven local current distributions due to nonuniform of the SEI. Good adhesion/wetting of the flexible polymer solid electro-lyte to the lithium metal surface can result in a more uniform lithium ion flux at the interface, which can inhibit the forma-tion of dendrites.[87] In addition, the high mechanical strength of inorganic solid electrolytes can inhibit the growth of den-drites after their formations. Therefore, an ultrathin polymer-modified rigid ceramic solid electrolyte directly contacting with the lithium anode exhibits a flat stripping/plating voltage pro-file and high dendrite-free cycling (over 3200 h), which signifi-cantly improves the safety of energy storage device.[101] In order to explain the capability of solid electrolytes to inhibit lithium dendrites, the comparing experiments on lithium deposition between traditional separator (Celgard 2500) and ion conduc-tive composite membranes which consists of a polymer and a rigid ceramic have been made. The optical image (Figure 4b)


Figure 4. Safety of energy storage devices. a) Impact of external factors on safety. The lift photograph indicates the pouch-type cells with solid electrolyte and liquid electrolyte after nail penetration test. Reproduced with permission.[71] Copyright 2017, Wiley-VCH. The right photograph indicates solid-state, soft-package lithium batteries to power a red LED after being cut. Reproduced with permission.[69] Copyright 2015, Wiley-VCH. b) Optical images of den-drite growth in symmetric Li–Li cell with Gelgard (left) and composite membrane (right). Reproduced with permission.[102] Copyright 2015, Wiley-VCH. c) Thermogravimetric analysis of the PVDF-HFP and LLZO composite electrolyte and the pure PVDF-HFP electrolyte. Reproduced with permission.[103] Copyright 2018, Elsevier. d) Photographs of flame test on a PEO/LiTFSI film and a PI film. Reproduced with permission.[106] Copyright 2019, Springer Nature. e) Schematic diagram of the working mechanism of the thermoresponsive polymer solid electrolyte.



shows that the battery with Celgard 2500 appears with signifi-cant dendrites in one minute, and the dendrites grow rapidly, puncturing the membrane within 6 min to disable the battery. In contrast, there are no obvious dendrites in the battery with composite membranes after 40 min, indicating high capability to inhibit lithium dendrites from growing due to its superior mechanical properties.[102] Therefore, the solid electrolyte is a favorable solution for short circuit caused overheating.

With the accumulation of heat in the energy storage devices, the instability of liquid electrolytes results in safety hazard for energy storage devices. Inorganic solid electrolytes (oxide, sulfide, halide) are often used in combination with polymer solid electrolytes to improve the high temperature resistance of energy storage devices due to their high thermal stability. For example, the pure PVDF-HFP electrolyte completely decom-poses at 500 °C, while the composite electrolyte consists of Li7La3Zr2O12 (LLZO) particles and PVDF-HFP polymer matrix only decomposes by about 50 wt% before 500 °C and remains stable until 800 °C. (Figure 4c).[103] Besides, the stability of elec-trolytes at high temperatures can fundamentally reduce the safety issues of energy storage devices.

Although the flammability of the electrolytes can be reduced by adding flame retardants, there is a trade-off between the flammability of electrolytes and the capacity of energy storage devices.[104,105] Therefore, self-flame retardant electrolytes are developed using self-flame retardant materials, such as poly-mers containing nitrogen, fluorine, phosphorus, and sulfur elements and inorganic ceramics. A lithium-ion battery with polymer–polymer solid electrolyte which consists of a nonflam-mable polyimide film host and a flammable PEO/LiTFSI filler exhibits remarkable safety performances (Figure 4d).[106] The composite solid electrolyte consists of 3D Li6.4La3Zr2Al0.2O12 (LLZO) network and PEO can retain the structure of the 3D LLZO network during the combustion process, in spite of the disappearance of polymer component.[107] A bipolar lithium-ion battery does not show explosion or structural disruption even after exposure to a flame, and it can still power an LED lamp, which attributed to the presence of the nonflammable sebaconitrile-based gel composite electrolyte and electrode binder.[108] Furthermore, the use of solid electrolytes can avoid the problem that lithium metal reacts with organic electrolyte to form combustible gases, which is one of the main factors for combustion.

Composite solid electrolytes are effective choices to reduce the safety hazards of energy storage devices, since most inor-ganic solid electrolytes exhibit good mechanical strength, high thermal stability and nonflammability, while the flexible polymer solid electrolytes can inhibit the formation of lithium dendrites. However, the external high temperature environment still affects the safety of devices. A thermoresponsive polymer solid electrolyte can respond to temperature change in real time to reduce the risk of energy storage devices at high temperature. The thermoresponsiveness of electrolytes is attributed to the reversible phase changes or sol–gel transitions of polymers.[100] Ions in thermoresponsive polymer solid electrolyte can migrate freely between the cathode and anode at normal working tem-perature, but when the temperature exceeds the critical tem-perature, the change of polymer molecular conformations will inhibit the ionic conduction (Figure 4e). For example, a

thermoresponsive polymer solid electrolyte obtained by copo-lymerizing poly(1,3-dioxolane) (PDOL) and poly-(lithium allyl-sulfide) (PLAS) provides an intelligent and active response.[109] The ionic conductivity of thermoresponsive polymer solid elec-trolyte and the capacity of lithium-metal battery exhibit signifi-cant drops at 70 °C, thus preventing the device from exploding at high temperature. As a demonstration, a small electric fan powered by battery spontaneously stops working at 70 °C. Con-sidering that the intelligent temperature response of energy storage devices is mainly based on polymer solid electrolytes, the further introduction of inorganic solid electrolytes seems to be an effective strategy to develop thermoresponsive polymer solid electrolytes with good mechanical strength, high thermal stability and nonflammability simultaneously.

3.2. Flexibility

The rapid development of flexible electronics such as wear-able devices, roll-up displays, and bendable mobile phones, has spurred the research on flexible energy storage devices. Flexible energy storage devices with planar sandwich structures require all components to be flexible, and the electrodes and electro-lytes need to have a good adhesion at the bent state. Polymer solid electrolytes not only solve the safety problem that caused by leakage of the liquid electrolytes, but also realize the defor-mations, such as bending, folding, and even twisting, of the energy storage device due to their high flexibility.

Flexible supercapacitors have received attentions for their high-power density, high rate of charge–discharge and long cycling lifetime. Gel polymer electrolytes with matrices of PMMA, PVDF, PAN, and PVA possess remarkable migration efficiency of ions and high flexibility, so they have been widely used to produce flexible supercapacitors. Furthermore, the gel polymer electrolytes have good wettability and adhesion to elec-trodes. Enormous researches for flexible PVA-based gel electro-lytes have been reported due to their simple preparations and high chemical stability. A redox-active gel polymer electrolyte obtained by adding 1-anthraquinone sulfonic acid sodium with abundant hydrogen bond acceptors to PVA–H2SO4 system can be twisted into a knot, indicating its both high toughness and strength.[110] As shown in Figure 5a, the supercapacitor made from the PVA-H2SO4-1-anthraquinone sulfonic acid sodium (PHA) gel electrolyte can be easily bended and folded. The flexible supercapacitor with PHA shows energy density of 30.5 Wh kg−1 with power density of 350 W kg−1, while the energy density of the supercapacitor with PVA-H2SO4 gel elec-trolyte is only 10 Wh kg−1. The cyclic voltammetry (CV) curves of the flexible supercapacitor at flat and folded states well overlap, and the specific capacitances at different current densities at a folded state are almost the same as the flat state, indicating the bending deformation has slight influence on electrochemical performance of supercapacitor. A PVA-based organohydrogel electrolyte obtained by incorporating H2O/ethylene glycol (EG) binary mixed solvent into PVA network exhibits high flexibility even at −40 °C.[111] The EG can efficiently reduce the freezing point and form abundant hydrogen bonds with PVA chains. The capacitance was retained by 90% after 1000 charge/dis-charge cycles under bending by 180° (Figure 5b). Besides, the




CV and galvanostatic charge–discharge (GCD) curves show nearly the same shapes at different bending angles, indicating high stability of the organohydrogel electrolyte.

Although PVA is a potential polymer matrix to prepare flex-ible gel electrolytes, the poor water retention property still needs to be improved. Polyzwitterions gel polymer electrolyte not only possesses robust water retention ability due to the strong elec-trostatic interactions between the charged groups and water molecules, but also exhibits a considerable flexibility.[112] More-over, the charged and polar groups in polyzwitterions result in a strengthened adhesion between the gel electrolyte and the electrode. The supercapacitor fabricated with polyzwitterions gel polymer electrolyte exhibits a slight increase capacity of 103% after 10 000 charge–discharge cycles. The CV curves of flexible supercapacitor display no significant differences before and after bending, indicating high flexibility and stability of poly zwitterions gel electrolyte. However, the above gel polymer electrolytes are corrosive or toxic. Accordingly, biocompatible and biodegradable materials, such as alginate, agar, and cellu-lose, are recently used to prepare flexible gel polymer electro-lytes, and flexible supercapacitors have been also obtained with similar high electrochemical performances.[113–115]

Compared with supercapacitors, flexible batteries usually demonstrate higher energy density and more stable oper-ating voltages. To assemble flexible batteries, polymer solid

electrolytes are the first choice due to their inherent flexibility. The low ionic conductivity of polymer solid electrolytes can be improved by incorporating inorganic solid electrolytes. For example, a 3D garnet-polymer composite electrolyte obtained by filling PEO/LiTFSI into 3D garnet-type Li6.4La3Zr2Al0.2O12 (LLZO) nanofiber networks possesses an ionic conductivity of 2.5 × 10−4 S cm−1.[107] The composite gel electrolyte, including tetraethylene glycol dimethylether, ethoxylated trimethylolpro-pane triacrylate, and Al2O3 nanoparticles, can maintain the structural integrity after single- and z-folding deformations.[116] In addition, the Li–S battery with this composite gel electrolyte exhibits high cycling performance (discharge capacity retains 680 mAh g−1 after 200 cycles) and no appreciable change in dis-charge capacity during the folding test. However, the high con-tents of inorganic solid electrolytes lead to an increase in rigidity of the electrolyte. To obtain high ionic conductivity and flex-ibility simultaneously, PEO/LiTFSI is filled into a robust poly-imide (PI) host with vertically aligned nanochannels to form an all-polymer solid electrolyte.[106] The aligned channels in PI can increase the ionic conductivity to 2.3 × 10−4 S cm−1, while the high modulus of 850 MPa can inhibit the dendrite growth. The PI/PEO/LiTFSI polymer solid electrolyte exhibits high flexibility which can regain its original shape after folding and twisting, and the button battery with unfolded PI/PEO/LiTFSI polymer solid electrolyte can work normally (Figure 5c). The


Figure 5. Flexibility of energy storage devices. a) Schematic diagram of fabricated supercapacitor with PHA gel film and its flexible behavior. Reproduced with permission.[110] Copyright 2016, Royal Society of Chemistry. b) Photograph of the PVA organohydrogel electrolyte under twisting at −40 °C and the cycling stability of the supercapacitor with PVA organohydrogel electrolyte under a 180° bending angle. Reproduced with permission.[111] Copyright 2018, Wiley-VCH. c) Photographs of PI/PEO/LiTFSI electrolyte under folding, twisting and unfolding. Reproduced with permission.[106] Copyright 2019, Springer Nature. d) Schematic illustration of the flexible zinc-ion battery. e) Capacity retention of the flexible zinc-ion battery under different bending cycles. d,e) Reproduced with permission.[117] Copyright 2018, Royal Society of Chemistry.



energy density (246 Wh kg−1) of solid state battery with PI/PEO/LiTFSI electrolyte is comparable to those with liquid electrolyte. The LiFePO4||PI/PEO/LiTFSI||Li battery exhibits stable cycling performances (>200 cycles) at C/2, while the LiFePO4||PEO/LiTFSI||Li cell decays sharply within 100 cycles. As expected, the all-solid-state pouch battery with PI/PEO/LiTFSI polymer solid electrolyte can power the LED under folding and twisting.

Gel polymer electrolytes can further improve the ionic con-ductivity, and the good wettability and adhesion to electrodes can prevent separation of the electrode from the electrolyte during deformation. However, the semiliquid gel electrolyte is too soft to resist the piercing of dendrites. A gel polymer elec-trolyte matrix with high flexibility and high strength is obtained by filling a graft copolymer of polyacrylamide (PAM) and gelatin into an electrospun polyacrylonitrile (PAN) fiber membrane.[117] The polymer grafting can significantly enhance the mechanical strength and ionic conductivity of the gel electrolyte, and the PAN fiber membrane can further increase its strength. The ionic conductivity and strength of the obtained PAM/gelatin/PAN gel electrolyte are 1.76 × 10−2 S cm−1 and 7.76 MPa, respec-tively. As shown in Figure 5d, a solid-state zinc-ion battery is assembled from flexible electrodes and PAM/gelatin/PAN gel electrolyte with a sandwiched structure in an open-air environ-ment. The zinc-ion battery possesses high capacity retention of 97% after 1000 cycles. In addition, the flexible zinc-ion battery with an ultralow thickness of 110 mm can be bended repeat-edly. After bending for 100 cycles, the flexible zinc-ion battery shows no obvious capacity loss, and the capacity can retain by 93.1% after bending for 800 cycles (Figure 5e). By introducing more hydrogen bonds, we may obtain gel polymer electrolytes

with high toughness to make flexible energy storage devices. However, these gel electrolytes can only satisfy deformations such as bending and twisting. When the energy storage devices are stretched by external forces, the gel electrolytes may crack or even break. Therefore, it is essential to further improve the mechanical properties of the gel polymer electrolytes to with-stand severer deformations.

3.3. Stretchability

In addition to flexibility, energy storage devices also need to be elastic to accommodate stretching deformations. Thus, the rigid components in the devices were replaced with stretchable materials or the devices were designed into stretchable forms. The gel polymer electrolytes with high stretchability are widely investigated for assembling elastic energy storage devices.

Due to relatively easier fabrication and simpler structure, stretchable supercapacitors have been developed rapidly com-pared with the other kinds of energy storage devices. PVA-based gel electrolytes that commonly used in supercapacitors are usu-ally prepared by blending polymers, plasticizers, and electro-lyte salts. However, the resulting gel polymer electrolytes show poor stretchability due to the weak interactions among mole-cular chains. The elongation of the PVA–ionic liquid–Li2SO4 gel polymer electrolyte can be merely up to 160%.[118] Surpris-ingly, a chemically crosslinked PVA–H2SO4 hydrogel exhibits high stretchability by 300%.[119] This can be attributed to the 3D crosslinking network structure in the PVA–H2SO4 hydrogel (Figure 6a) that creates strong interaction among the polymer


Figure 6. Stretchability of energy storage devices. a) Schematic of PVA–H2SO4 blended solution and chemical hydrogel. The blue lines represent PVA polymer chains and the red squares represent the crosslink points. Reproduced with permission.[119] Copyright 2015, Wiley-VCH. b) Recovery perfor-mance of Agar/HPAAm hydrogel under different stretching conditions. c) Schematic and photographs of supercapacitor with Agar/HPAAm hydrogel at the states of stretched and recovered. d) CV curves for Agar/HPAAm hydrogel-based supercapacitor at different strains. b–d) Reproduced with permis-sion.[122] Copyright 2019, Wiley-VCH. e) Photographs of the stretchability test of PAM-WiS hydrogel electrolyte. Reproduced with permission.[123] Copy-right 2019, Wiley-VCH. f) Schematic illustration of the stretchable lithium-ion battery. g) Stretchable lithium-ion battery lighting up an LED before and after stretching by 400%. h) Dependence of the output energy on stretching cycles. f–h) Reproduced with permission.[125] Copyright 2015, Wiley-VCH.



chains, thereby increasing the stretchability of the hydrogel. Based on this theory, building the 3D network with physical and chemical crosslinking simultaneously in hydrogels may further improve the stretchability. For example, a hydrogel polymer electrolyte with 3D network structure is formed by crosslinking poly(acrylic acid-co-acrylamide) (P(AA-co-AAm)) with divalent cobalt ions (Co2+).[120] During stretching, the weak reversible hydrogen bonds break to release energy while the strong metal-coordination between Co2+ and carboxylic ions guarantee the integrity of hydrogel polymer electrolyte. The P(AA-co-AAm)/CoCl2 hydrogel polymer electrolyte exhibits a maximal elonga-tion of 1280%.

Besides the metal-coordination, rigid crosslinkers such as vinyl hybrid silica nanoparticles (VSNPs) also can form chem-ical crosslinking among polymer chains by covalently grafting them. The VSNP–PAMgel electrolyte can be stretched to 1500% without breaking.[121] The VSNPs in hydrogel can not only serve as the crosslinking points for PAM to homogenize the PAM network, but also as stress buffers to dissipate energy, thus strengthening the gel electrolyte at large strains. An intrinsi-cally 1000% ultrastretchable supercapacitor is obtained by put-ting the polypyrrole/carbon nanotubes (CNT) electrode onto the prestretched VSNP–PAM gel polymer electrolyte, and then releasing naturally to form wavy structure. The capacitance of the stretchable supercapacitor achieves an enhancement factor of 2.6 with an increase of strain, which is attributed to the increased contact areas between electrodes and the electrolyte during stretching.

Building double crosslinked networks represents another effective method to improve stretchability. For example, a hydrogel polymer electrolyte with agar and hydrophobically associated polyacrylamide (HPAAm) double network can be stretched as large as 3400%.[122] Besides the superior stretch-ability, the agar/HPAAm gel electrolyte is highly elastic and reversibly stretchable. As shown in Figure 6b, the agar/HPAAm gel electrolyte shows only slight residual deformations after 30 stretching cycles at 100% and 300% elongation. Even when the elongation increases to 500%, the residual deformation of the gel electrolyte is only 5.4%. This is because the double net-works will rupture and dissipate energy during the stretching process, resulting in high strength and toughness of the agar/HPAAm hydrogel. On the other hand, the strong hydrophobic interactions in the HPAAm network not only dissipate energy and withstand stress, but also rebuild the entire hydrogel net-work in a short period of time, greatly improving mechanical properties, especially elasticity.

The stretchable supercapacitor is thus assembled from the soft polypyrrole film and agar/HPAAm gel polymer electrolyte. The supercapacitor can be stretched to 200% without dam-ages and easily restored to its original state (Figure 6c). The CV curves of stretchable supercapacitor at different strains exhibit similar quasi-rectangular shapes (Figure 6d), indicating the stable electrochemical performance during stretching. The capacitances of supercapacitor exhibit no differences between pristine and 100%-stretched states during 1000 cycles, demon-strating high stretchability and elasticity. According to the above discussion, hydrogels containing strong covalent crosslinks and weak intermolecular interactions have better stretchability. Besides, the rapid reconstitution of network structure after

damaging can achieve reversible stretchability of the hydrogel polymer electrolyte.

The low decomposition voltage of water limits the use of highly stretchable hydrogel polymer electrolytes in stretchable lithium-ion batteries, and thus there are relatively few reports of stretchable electrolytes for lithium-ion batteries compared to supercapacitors. The hydrogel electrolyte based on PAM uses a strategy named “water-in-salt” (WiS) to expand the working window between 1.95 and 4.93 V.[123] The PAM-WiS hydrogel can be stretched to at least 300% (Figure 6e), and the lithium-ion battery assembled with PAM-WiS hydrogel electrolyte can be stretched by up to 100%. However, the battery capacity lost 35% after 50 cycles at 50% strain, and the energy den-sity dropped from 35 to 17 Wh kg−1. As the most commonly used polymer electrolytes in solid-state lithium-ion batteries, PEO-based polymer electrolytes have been studied with cer-tain stretchability. Moreover, during the stretching process, the ionic conductivity of the electrolyte shows a steady increase due to the rearrangement of the polymer chains.[124] A gum-like stretchable lithium-ion battery assembled from a PEO-based polymer electrolyte can be stretched to 400%.[125] As shown in Figure 6f, the stretchable lithium-ion battery is fabricated by sandwiching the PEO polymer electrolyte between two arch-structured electrodes. The resulting lithium-ion battery can retain 87% capacity after 100 charge/discharge cycles. In addi-tion, even the lithium-ion battery is stretched to five times its original length, the brightness of the illuminated LED does not decrease (Figure 6g). The output energy can still be maintained at 97% after the battery has been stretched for 200 times at a strain of 400% (Figure 6h).

Since the small-molecule plasticizers such as organic sol-vents and ionic liquids detangles the polymer chains, which is disadvantageous to the mechanical strength of the gel polymer electrolyte, a polymer matrix with a high degree of crosslinking is constructed in the gel electrolyte. The crosslinked poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-co-HFP)]/ionic liquid (IL) gel polymer electrolyte can be stretched by up to 400% with a tensile stress as high as 10.6 MPa, while the P(VDF-co-HFP)/IL without crosslinking breaks at 35% with a tensile stress of 0.4 MPa.[126] And the strain of the crosslinked P(VDF-co-HFP)/IL gel polymer electrolyte can be fully recov-ered at a strain of 100%, indicating high elasticity of crosslinked P(VDF-co-HFP)/IL gel polymer electrolyte. Ureidopyrimi-dinone (UPy) units are very advantageous for building net-work structures due to their quadruple hydrogen bonds.[127] A polymer electrolyte containing UPy-based monomer (UPyMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) exhibits a strain of more than 2000%. The abundant intermo-lecular hydrogen bonds in polymer electrolytes can dissipate the mechanical energy to obtain a superior stretchability. There-fore, polymer solid electrolytes for lithium-ion batteries with high stretchability can be obtained on the basis of chemical or multiple physical cross-linking of the polymer matrices. How-ever, the resilience and lifetime of stretchable energy storage devices are important indicators for practical applications. Since the residual strain is the main reason for the reduction of resilience and lifetime of the stretchable energy storage devices, it is necessary to create rapidly reversible interactions in gel polymer electrolytes to weaken residual strains.




3.4. Wearability

The conventional planar architecture of flexible and stretchable energy storage devices may limit their applications in many emerging fields, e.g., wearable electronic devices. Compared with the planar structure, the 1D shape allows energy storage devices to be deformable in all dimensions. In addition, 1D fiber-shaped energy storage devices can be woven into textiles to better fit the human body, which makes it more attractive for wearable electronics. The use of gel polymer electrolytes in fiber-shaped energy storage devices avoids the complicated injection and packaging processes required for liquid electro-lytes, making the devices more compact. Besides, the gel elec-trolytes coated on fiber electrodes can prevent short circuits of devices during bending, twisting, and stretching.

Fiber-shaped supercapacitors and lithium-ion batteries are typically assembled with a twisting structure which refers to wind two fiber electrodes together at a certain twisting angle (Figure 7a).[128] The fiber electrodes need to be precoated a gel electrolyte layer to prevent short circuits. In order to simplify the assembly process and achieve mass production, a fiber-shaped supercapacitor is assembled through a continuous one-step process.[129] As shown in Figure 7b, the spinnable CNTs is immersed into the active material suspension to form hybrid fiber electrodes, and then drawn into a PVA/H3PO4 gel elec-trolyte. Finally, the two fiber electrodes are twisted into a fiber-shaped supercapacitor by rotating motor. The CNT/reduced graphene oxide (RGO) fiber-shaped supercapacitor shows high capacity retention after 10 000 cycles, and higher energy density and power density (volumetric energy density of 2.4 mWh cm−3 and volumetric power density of 0.016 W cm−3 at the current density of 31 mA cm−3) are produced in comparison to other fiber-shaped supercapacitors based on carbon composite fibers. The GCD curves of fiber-shaped supercapacitor exhibit little difference before and after bending to 180° for 400 and 1000 cycles, and the capacity can be maintained by more than 95% after bending for 1000 cycles.

The stretchable fiber-shaped supercapacitor can be also obtained by overtwisting the fiber electrodes into a spring-like structure. The fiber-shaped supercapacitor that contains two parallelly placed spring-like CNT electrodes precoated with PVA/H3PO4 can be bent in any direction without degradation in performance.[130] The specific capacitance can be maintained to be above 90% and 94% after being stretched to 100% and subjected to 300 stretching cycles, respectively. Introducing active nanoparticles such as Li4Ti5O12 and LiMn2O4 to the above CNT fiber electrodes produces stretchable fiber-shaped lithium-ion battery.[130] After coating PEO-succinonitrile/LiTFSI gel electrolyte, the obtained fiber-shaped lithium-ion battery can exhibit 92.1% capacity retention after 100 charge/discharge cycles, and possess an energy density of 27 Wh kg−1. In addi-tion, the capacity of the fiber-shaped battery is maintained by 85% after stretching at 100%, and the change of capacity is no more than 1% after 300 stretching cycles at the strain of 50%. These fiber-shaped energy storage devices with twisted struc-ture enable rapid, continuous, and large-scale production due to its simple operation. However, under severe deformation, the two electrodes of the twisted structure may be misaligned, and the low effective contact areas between two electrodes affect

the energy density and power density of the fiber-shaped energy storage devices.

Another structure of the fiber-shaped energy storage device is the coaxial structure comprising a fiber electrode core and an electrode sheath separated by a gel polymer electrolyte layer (Figure 7c).[128] Due to the good contact between the electrode and electrolyte, this structure exhibits high stability under deformation and large effective contact areas. Coaxial struc-ture is suitable for the assembly of fiber-shaped metal–air batteries, since the gel polymer electrolyte can protect metal anodes from corrosion by water, nitrogen, oxygen, moisture, and carbon dioxide in the air. Gel polymer electrolyte layer which tightly bonds to the lithium wire can be formed by in situ polymerization of the precursor solution on the lithium wire. For example, the precursor solution including lithium triflate (LiTF), tetraethylene glycol dimethyl ether (TEGDME), PVDF-HFP, N-methyl-2-pyrrolidinone (NMP), 2-hydroxy-2-methyl-1-phenyl-1-propanone (HMPP), and trimethylolpropane ethoxylate triacrylate (TMPET) is first coated on the lithium wire to be photopolymerized under UV irradiation to form a solidified electrolyte layer, and then the aligned CNT sheets and the perforated heat shrinkable tubes are wrapped outside to produce a fiber-shaped lithium–air battery (Figure 7d).[131] The fiber-shaped lithium–air battery exhibits a specific capacity of 12 470 mAh g−1 at current density of 1400 mA g−1 and can work for 100 cycles in air without decay. The obtained fiber-shaped lithium–air battery can be bended to various degrees without damaging and can be operated normally during the bending and releasing process. The ultraflexible fiber-shaped silicon-oxide battery is assembled by replacing the lithium wire with a more flexible lithiated silicon/CNT hybrid fiber.[132] The energy density (512 Wh kg−1) of the silicon-oxide battery is 2–10 times higher than those of the other flexible batteries. In addition, the fiber-shaped silicon-oxide battery can be bended, tied, twisted and looped (Figure 7e) without obvious changes of charge and discharge behaviors. Moreover, the fiber-shaped silicon-oxide battery displays no obvious voltage losing after 20 000 bending cycles.

Besides the high flexibility and stretchability, fiber-shaped energy storage devices can be woven into soft tex-tiles (Figure 7f–g).[129,132–134] The tight contact between the gel polymer electrolyte and the electrode allows the fiber-shaped energy storage devices to withstand various deformations without short circuits in the resulting textile during usage. Therefore, as for fiber-shaped energy storage devices, the gel polymer electrolyte layer should be formed quickly with a uni-form thickness and good contact with the electrode to prevent separation of the electrolyte from the electrode during deforma-tion due to local aggregation of the electrolyte. This can be the additional criterion for selecting gel polymer electrolytes for fiber-shaped energy storage devices, except the general require-ments for ionic conductivity and strength.

3.5. Self-Healability

Energy storage devices inevitably suffer from local stresses under various deformations. After a long period of accumula-tion, these devices may be damaged, which deteriorates the




lifespan and reliability of the devices. Therefore, energy storage devices should have the ability to self-heal the cracked inter-faces. The self-healing mechanism involves the addition of microencapsulation, molecular interdiffusion and presence of reversible chemical bonds.[16] Gel polymer electrolytes exhibit good self-healability due to the high mobility of molecular chains and the abundant reversible interactions among mole-cular chains.

In general, the self-healability of gel polymer electrolytes relies on the reversible covalent bonds and noncovalent interac-tions.[17] For instance, it may be realized through a Diels-Alder

reaction in the case of chemical bonds. As shown in Figure 8a, poly(furfuryl methacrylate-co-methyl methacrylate) (P(FMA-co-MMA)) and poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-co-HFP)) double network gel polymer electrolyte shows a satisfactory healing effect by thermally reversible Diels–Alder reaction.[135] Under heating, the broken bonds between furan and multimaleimide crosslinker reconnect to construct the chemically crosslinked network to heal the cracked interface (Figure 8b). Furthermore, this double network gel polymer elec-trolyte possesses a superior mechanical strength (failure tensile stress of 660 kPa with 268% strain; failure compressive stress


Figure 7. Wearability of energy storage devices. a) Schematic illustration of twisted structure. Reproduced with permission.[128] Copyright 2018, Royal Society of Chemistry. b) Schematic illustration of the continuous fabrication process of fiber-shaped supercapacitors. Reproduced with permission.[129] Copyright 2015, Wiley-VCH. c) Schematic illustration of coaxial structure. Reproduced with permission.[128] Copyright 2018, Royal Society of Chem-istry. d) Schematic illustration of the fabrication of the fiber-shaped lithium–air battery. Reproduced with permission.[131] Copyright 2016, Wiley-VCH. e) Photographs of the fiber-shaped battery under various deformations. Reproduced with permission.[132] Copyright 2017, Wiley-VCH. f) fiber-shaped supercapacitors being woven into flexible textiles. Reproduced with permission.[129] Copyright 2015, Wiley-VCH. g) The progress of fiber-shaped batteries being woven into a textile and the textile before and after twisting. Reproduced with permission.[132] Copyright 2017, Wiley-VCH.


2000077 (16 of 20) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2020, 2000077

of 17 MPa with 85% strain) and considerable ionic conductivity (3.3 × 10−3 S cm−1). However, the self-healability based on cova-lent bonds is referred to be nonspontaneous since external stimuli, such as heating, pH change, lighting and catalyst, are needed to initiate the healing process.

Compared with reversible covalent bonds, the self-healing based on reversible noncovalent interactions such as hydrogen bonding, metal bonding and electrostatic crosslinking are usu-ally spontaneous.[127,136,137] Among these, the self-healability of most gel polymer electrolytes is caused by hydrogen bonding.

Figure 8. Self-healability of energy storage devices. a) Schematic illustrations of the composition of the P(FMA-co-MMA)/P(VDF-co-HFP) double net-work structure and its self-healing process. b) Three fan-shaped P(FMA-co-MMA)/P(VDF-co-HFP) gel polymer electrolytes, which are dyed into three colors. They fuse together into a cylindrical shape at 100 °C for 10 min, without cracks in the healed cylindrical sample. a,b) Reproduced with permis-sion.[135] Copyright 2018, American Chemical Society. c) Schematic illustration of the synthesis and self-healing mechanism of the brush-like UPyMA-PEGMA copolymer, and optical images of the self-healing process under ambient conditions. d) Cycling performance of the LFP||Li cell at 0.1C with UPyMA-PEGMA gel polymer electrolyte (top) and healed UPyMA-PEGMA gel polymer electrolyte (bottom). The inset images show the as-fabricated and healed sample. c,d) Reproduced with permission.[127] Copyright 2018, Royal Society of Chemistry. e) Schematic diagram of self-healability PANa-Fe3+ hydrogel. f) Photographs of the self-healing aqueous battery based on PANa-Fe3+ hydrogel powering a clock before cutting (left), after cutting (middle), and after self-healing (right). e,f) Reproduced with permission.[137] Copyright 2018, Wiley-VCH.



For example, a gel polymer electrolyte with UPy-containing monomer (2-(3-(6-methyl-4-oxo-1, 4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate) (UPyMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) can heal the breakage without any stimuli within 2 h due to the quadrupole hydrogen bonds (Figure 8c).[127] The electrolyte cuts in two can self-heal completely and exhibits an elongation of 300% without any crack in the original fracture. The ionic conductivity of healed gel polymer electrolyte lost only 2% compared to the pris-tine electrolyte. And the lithium-ion battery with healed gel polymer electrolyte shows a close discharge capacity and cycle performance (initial capacity of 155 mAh g−1 and 142 mAh g−1 after 80 cycles) compared to the pristine gel electrolyte (ini-tial capacity of 157 mAh g−1 and 143 mAh g−1 after 100 cycles) (Figure 8d).

Metal bonding can construct a reversibly crosslinked network with high stability. Fe3+ as a crosslinker can form ionic bond in and between the sodium polyacrylate (PANa) to reattach the damaged surface (Figure 8e).[137] The self-healing NiCo||Zn bat-tery with this gel polymer electrolyte exhibits a high discharge capacity of 250 mAh g−1, which is much higher than those of previous solid-state Zn batteries. Moreover, the NiCo||Zn bat-tery can heal spontaneously and power a clock after healing (Figure 8f). The healing efficiency of the battery reaches 87% after 4 breaking/healing cycles.

In short, spontaneous self-healability is more suitable for practical applications, and the mechanism of spontaneous self-healability is the reorganization of the broken interactions. Therefore, the rapid movement of the molecular chain is con-ducive to accelerate the rate of self-healability. The self-healing gel polymer electrolytes greatly improve the performance and the lifetime of the energy storage devices. This feature will broaden the application of energy storage devices, especially in electronic devices that are susceptible to damages in many fields such as wearable facilities and field sports.

4. Summary and Outlook

As a promising choice for the bioelectronics devices, solid electrolytes are being developed at an unprecedented speed (Figure 9). These energy storage devices can realize the expected safety, flexibility, stretchability, wearability, and self-healability to effectively power a series of electronic devices in various fields ranged from wearable facilities to smart textiles and to artificial intelligence. One of the main development directions of solid electrolytes in the future is to make the appli-cation of solid electrolytes reach the level of liquid electrolytes, which needs to address the low ionic conductivity of solid elec-trolytes at room temperature and the insufficient interface com-patibility. The ability of ions to migrate in the electrolyte, and electrode/electrolyte interface, and the contact resistance at the electrode/electrolyte interface will affect the charge–discharge rate performance of the energy storage device. Therefore, measures need to be taken to improve the ionic conductivity of solid electrolytes, such as element replacement and isovalent element doping of oxide solid electrolytes, blending, copoly-merization, or crosslinking of polymer solid electrolytes. It is also critical to develop more general fabrication methods, i.e.,

covering stable solid electrolyte layer and introducing viscous polymer matrix and electrolyte additives, to enhance the inter-faces between solid electrolytes and electrodes. In addition to these basic characteristics, the flexibility, stretchability, and self-healability of solid electrolytes should be further improved to make them more suitable for practical applications. For example, introducing polymer chains with high mobility and constructing network structures with high kinetic assembly constants to ensure the deformation/recovery and fracture/healing efficiency of polymer electrolyte.

For device assembly, based on the self-supporting and unique formability of solid electrolytes, energy storage devices should adopt novel assembly methods to simplify the production pro-cess and achieve functionalization of devices. For example, assembling energy storage devices by in situ polymerization not only eliminates the extra electrolyte preparation step, but also forms close contact interfaces between electrodes and elec-trolytes. The close contact interfaces can be also obtained by directly depositing electrode materials on the self-supporting solid electrolyte membrane to assemble an all-in-one energy storage device, and this method can also solve the problems of delamination, displacement, and strain-accommodating engi-neering in traditional energy storage devices. Besides, a con-tinuous assembly process from electrode preparation to energy storage device fabrication should be developed to achieve rapid, large-scale, and low-cost device assembly. For example, using 3D printing technology to directly print the electrolyte on the electrodes to quickly assemble the all-in-one energy storage device can not only reduce the interface resistance, but also realize the pattern design of the energy storage devices. It should be noted that the printable electrolytes need to have an appropriate viscosity to be continuously extruded from the nozzle and stacked layer by layer, as well as to avoid structural shrinkage during solidification.

In order to meet the demands of energy storage devices for more special functionality, it is urgent to develop other intelli-gent solid electrolytes. For example, the solid electrolytes which

Figure 9. The outline and future development direction of solid electrolytes.



can apply the huge temperature differences are promising to operate in special environment, i.e., the moon and Mars. Solid electrolytes can also be given stress response characteristics. The electrolyte changes from flexible to rigid when stimulated by a specific external force, which can be applied to clothing to protect human. Besides, shape memory solid electrolytes can memorize and recover permanent shape in response to external stimuli, e.g., heating and pressure, which can avoid structural and functional fatigue caused by long-term localized stress in the energy storage device.

In summary, the unique advantages of solid electrolytes pro-vide a lot of opportunities to solve a spectrum of problems in fields mostly related to flexible electronics, microelectronics, and bioelectronics. The use of solid electrolytes represents a promising direction in energy materials and devices.

Conflict of InterestThe authors declare no conflict of interest.

Keywordsbatteries, battery safety, flexible devices, solid electrolytes, supercapacitors

Received: January 4, 2020Revised: March 2, 2020

Published online:

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