z Energy Technology & Environmental Science

Lithium Ion Conduction in Diblock Polymer Electrolyte withTethered AnionYubin He, Nian Liu,* and Paul A. Kohl*[a]

Solid polymer electrolyte (SPE) may provide a path for nextgeneration high-performance, hazard-free lithium ion batteries.However, the low lithium ion transference number (tLi+) andelectrolyte conductivity limit device performance. In this study,a diblock copolymer with a polycarbonate first block and poly(sulfonated styrene) second block has been synthesized and

characterized for conductivity and shows improved tLi+. Thelow dissociation of lithium salts from the tethered sulfonateanion was overcome by incorporating plasticizers. The diblockSPE material had room temperature conductivity of up to1.95 ⋅10� 6 S/cm, and lithium transference number of up to 0.83with the tethered sulfonate anion.

1. Introduction

Conventional lithium ion batteries (LIB) have intrinsic safetyconcerns due to the use of a flammable organic liquidelectrolyte. Solid state batteries with solid polymer electrolyte(SPE) can potentially improve safety, increase the operatingtemperature, mitigate voltage droop problems (i. e. lowering ofthe voltage loss due to concentration polarization) and lowerthe manufacturing cost.[1] Polymer-based solid electrolytes aremore suitable than inorganic solid electrolytes for roll-to-rollmanufacturing because the SPE can be deposited in-situ onone of the electrodes and the polymer can mechanicallydeform to match the dimensional changes of the electrodesduring charge and discharge cycles.[2]

Steady progress has been made in SPEs to achieveconductivity >0.1 mS/cm, through optimization of the polymerarchitecture to reduce the crystallinity,[3] introduction of organicor inorganic plasticizers to enhance the polymer segmentalmotion,[4] or use of a polymer-in-salt strategy to decouple theionic mobility from segmental polymer motion.[5] Althoughthere have been improvements in overall ion conduction, onlythe lithium ion conductivity is of value in a lithium ion batterybecause the anion is not electroactive. The fundamentaldefinition of Li+ transference number (tLi+) is the ratio oflithium ion conduction to the total ion conduction in theabsence of a concentration gradient. A low transferencenumber results in concentration polarization within the batterywhich decreases the cell voltage due to Nernstian losses andlimits the maximum current density of the cell. Previouslyreported SPEs are often prepared by blending a polymer withone or more lithium salts. Polar groups within the polymer,

such as oxygen dipoles serve as a bulky solvation shell forlithium ions.[6] The counter anions are negatively charged andhave a large ionic radius making them weakly bound to thepolymer matrix. The sluggish polymer chain motion as well asstrong interaction between Li+ and the polymer matrix resultsin a low value for tLi+ (e.g., ∼0.2) for conventional polyethyleneoxide (PEO) based SPE.[7] Anion transport and build-up at theelectrode interface can lead to undesired side reactions withthe electrode in addition to concentration polarization.[8]

The lithium ion transference number can be improved byweakening the interaction between the lithium ion andpolymer backbone. For example, the PEO based electrolyte hasstrong coordination with lithium ions resulting in low tLi+.[7] Onthe other hand, a polycarbonate matrix has a much lowerinteraction strength with Li+ resulting in tLi+>0.5.[9] Similarly,decoupling the Li+ transport from polymer segmental motionvia the polymer-in-salt strategy can also leads to a tLi+ ∼0.7.[5b]

Another strategy to improve the tLi+ is immobilizing the anionby tethering it to the polymer chain. Attaching the anion to thepolymer backbone through a non-covalent bond,[10] or addingan anion-binding macrocycle host can lead to tLi+>0.6.[11]

Although these strategies can improve the tLi+, unity trans-ference numbers are difficult to achieve.

High tLi+ can be achieved by completely immobilizing theanion, such as by covalently tethering the anion to the polymerbackbone,[12] but this strategy usually leads to low ionicconductivity, on the order of 10� 7 to 10� 5 S/cm, due to lowdegree of anion-cation dissociation.[8] PEO block copolymerswith PEO being the ion solvating block and polyanions as thesecond block were developed in order to simultaneouslyachieve good conductivity and high lithium ion transferencenumber.[13] However, crystallization of PEO chains remains afundamental problem. PEO oxidation at high potential (i. e. 3.8to 4 V vs Li/Li+) limits the use of high voltage cathodematerials.[7] Low mechanical strength, especially, above thepolymer melting point (e.g., 60 °C), can lead to lithium dendriteformation and battery short circuit. Polycarbonates (PC) havebeen investigated as a promising alternative[2b,14] to PEO

[a] Dr. Y. He, Prof. N. Liu, Prof. P. A. KohlSchool of Chemical and Biomolecular EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332, USAE-mail: nian.liu@chbe.gatech.edu

kohl@gatech.edu

Supporting information for this article is available on the WWW underhttps://doi.org/10.1002/slct.202004595

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because they have extended electrochemical stability (>4.2 V)[15] and decreased polymer crystallinity.[16]

In this study, a diblock polyelectrolyte with pendantpolycarbonate as the first block and poly(sulfonated styrene) asthe second block was synthesized (PC-b-PSSLi). Free standingfilms with an optimized block ratio and plasticizer were madeby solvent casting from solution. It was found that the filmswithout plasticizer had negligible lithium ion conductivity dueto poor ion solvation. Higher conductivity, 1.95 ⋅10� 6 S/cm, wasachieved with pentanedinitrile plasticizer. The lithium iontransference number was measured with PEO and pentanedini-trile (PDN) plasticizer and found to be up to 0.83.

2. Result and discussion

Synthesis of Block Copolymer: As shown in Figure 1, the diblockcopolymer PC-b-PSSNa was synthesized via sequential Raft

polymerization. A pendant type polycarbonate was used as theion solvating first block. Polystyrene with a tethered sulfonateanion was used as the tethered anion second block. After Raftpolymerization, Na+ counter ions in the second block wereexchanged for Li+ via ion exchange in a solution with excessLiClO4.

The reaction yield, chemical structure and molecular weightof PC-b-PSSNa was characterized by 1HNMR. Figure 2 showsthe NMR spectrum after polymerization of DB-PC. Signals 1 to 3were assigned to the double bond of the unreacted monomer,signals 4 to 6 were assigned to the protons on the carbonatering. The reaction yield was calculated from the ratio of theintegrated areas for signals 1 and 4. The integrated area ofsignal 1 was close to zero, showing that the polymerizationyield was >99%. Signal 9 was assigned to residual chaintransfer agent at the end of polymer chains. The molecularweight of the first block (PC) was calculated from the ratio ofthe integrated areas for signals 4 and 9 (i. e. MWPC =172×(A1)/(A9/3)=10.2 kg/mol).

In the second polymerization step, the PC block (above)was used as a macro chain transfer agent. Sodium p-styrenesulfonate (SS) was used as the reactant for adding thesecond block. Figure 3 shows the 1HNMR spectrum of thereaction product after introducing the second block. Signals 10and 11 were assigned to the protons on the phenyl ring ofsecond block. Signals 4, 5 and 6 were assigned to protons onthe carbonate ring. The molecular weight of the second block(PSS) was calculated based on the ratio of the integrated areasof signals 4 and 10 (i. e. MWPSS = (A10/2)× (206/172)× (10.2 kg/mol/A4) =4.2 kg/mol). The total molecular weight of the PC-b-PSSNa was 14.4 kg/mol. Quantitative NMR was used todetermine the molecular weight rather than gel permeationchromatography because the polymers were not soluble inTHF.

The PC-b-PSSNa block copolymer was soaked in a saturatedsolution of LiClO4 to exchange the Na+ counter ion for Li+.Afterwards, the polymer product was purified by five cycles

Figure 1. Reaction scheme to synthesize PC-b-PSSLi diblock copolymer.

Figure 2. 1HNMR spectrum of the reaction product after Raft polymerizationof DB-PC monomer. Figure 3. 1HNMR spectrum of PC-b-PSSNa.

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consisting of dissolution in DMF followed by precipitation inacetone to completely remove residual LiClO4. As shown inFigure 4, the PC-b-PSSLi showed a high intensity 7Li signal,showing that Li+ was the counter ion. 23Na NMR spectrum wasused to confirm that virtually no sodium ions remained(Figure S4).

SPE Fabrication and Conductivity: The SPE was preparedwith PC-b-PSSLi as the polymer matrix, and dinitrile com-pounds as the plasticizer to enhance the segmental motion ofthe polymer chains[17] and provide extra ion solvatingmoieties.[18] The effect of plasticizer on the membrane formingability was firstly investigated. SN has a melting point above60 °C. The SPE membrane containing SN plasticizer appearsturbid after cooled to room temperature (Figure S2). PDN wasalso used investigated as a plasticizer and has a melting point<0 °C. Free standing membrane were obtained by solutioncasting on PTFE or SS substrates (Figure S3). Due to its goodcompatibility in the membrane, PDN plasticizer was used in thefollowing experiments, unless otherwise noted. The molar massand block ratio of PC-b-PSSLi also affects the quality of themembrane formed. A higher PSSLi mole fraction led to a morerigid and fragile membrane. In this study, the mass ratiobetween the first block and second block was set at 2.4 : 1 (i. e.,the molecular weight of PC block was 10.2 kg/mol and themolecular weight of PSSLi block was 4.2 kg/mol).

The conductivity of PC-b-PSSLi was measured as a functionof PDN plasticizer concentration using electrochemical impe-dance spectroscopy (EIS), as shown in Table 1. The ionexchange capacity (IEC, mmol/g) reflects the molar content oflithium ions per gram of polymer electrolyte. The pristine PC-b-PSSLi had an IEC of 1.41 mmol/g, however, the conductivitywas low (Figure S5) due to low degree of lithium iondissociation from the tethered sulfonate anion.[8] With highPDN concentration, the IEC is reduced due to the greater massfracture of non-ionic components in the membrane (Figure 5and Table 1), yet the conductivity is increased because theplasticizer helped in ion dissociation. The ratio of conductivityto IEC (Conductivity/IEC) reflects the conductivity of per moleof ion conducting group. The higher Conductivity/IEC value athigh plasticizer concentration shows that the presence of alithium salt is necessary but not sufficient for achieving highlithium ion conductivity. Lithium salt dissociation is a criticalfactor in achieving high conductivity. An optimized plasticizercontent of 67 wt% was found to have a conductivity of1.94 ⋅10� 6 S/cm.

The ion conductivity was also measured by the currentinterrupt method.[19] As shown in Figure 6, a small interruptcurrent (i), 10 uA, was applied to a SS/SPE/SS cell for 0.1 s (SSrefers to stainless steel). The cell voltage dropped instanta-neously by the amount of the series ohmic potential drop. Theohmic resistance of the SPE (R) was calculated from the ratiobetween voltage drop (~V) and interrupted current, Equation 1.The conductivity was approximately the same as from the EISmethod (3.7 ⋅10� 7 S/cm vs 2.4 ⋅10� 7 S/cm).

Electrochemical Stability and Transference Number: Theelectrochemical stability of as-prepared SPE was measuredusing cyclic voltammetry (CV), Figure 7. The cell configurationwas Li/UVPC/SS, where lithium metal was the reference-counterelectrode and stainless steel was the working electrode. Atpositive potentials, the as-prepared SPE showed a stableelectrochemical window up to 4.1 V above which the polycar-bonate can be oxidized.[2b] At low potentials, the anodic current

Figure 4. 7Li NMR of PC-b-PSSLi after ion exchange from Na+ to Li+.

Table 1. Composition, ion exchange capacity and room temperatureconductivity of PC-b-PSSLi with different content of PDN plasticizer.

PC-b-PSSLicontent (wt%)

PDNcontent (wt%)

IEC(mmol/g)

Conductivity(S/cm)

Conductivity/IEC

100 0 1.41 Notmeasurable

NA

67 33 0.94 Notmeasurable

NA

50 50 0.70 1.47 ⋅10� 7 2.10 ⋅10� 7

40 60 0.56 2.40 ⋅10� 7 4.28 ⋅10� 7

33 67 0.47 1.95 ⋅10� 6 4.15 ⋅10� 6Figure 5. EIS plot of PC-b-PSSLi SPE with 50 wt%, 60 wt% and 67 wt% PDNplasticizer. The cell configuration was SS/SPE/SS. SS refers to stainless steel.

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peak between � 0.5 V and 0 V was due to lithium metaloxidation. It is noted that the total charge due to lithiumoxidation was less than the charge due to lithium platting at<0 V. This result indicates that side reactions are occurringbetween the lithium metal and the SPE.

The Li+ transference number was measured by thepotentiostatic method, as described above. There are severalfactors that can affect the transference number including LiClO4

impurities after ion exchange due to the presence of mobileanions (e.g., ClO4

� ), and the presence of sodium ions which donot participate in the electrochemical reaction. In addition, astable SEI between the lithium metal electrode and SPE iscrucial for measuring the transference number because reac-tion of the plasticizer with the electrodeposited lithium metalwould accumulate decomposition products at the interface

and impede the ion transport. The transference number of PC-b-PSSLi was first measured with PDN as plasticizer (Figure 8).The lithium ion transference number was significantly higherthan observed with liquid electrolytes (e.g., 0.2) and a value of0.5 was observed. This value is lower than expected for atethered anion (c.a., 0.8 to 1) due to the side reaction of PDNplasticizer on lithium metal.[18] The PDN plasticizer electrolytedoes not include solid electrolyte interphase (SEI) formingcompounds. Reaction of PDN with lithium which changes theion content with the SEI and ohmic resistance of the electro-lyte. These factors lead to a decrease in current with time, Fig.8, and an erroneously low value for lithium transferencenumber because mobile anions including CN� werecreated.[18,20]

To more accurately measure the transference number, PEGplasticizer was used. The lithium transference number (tLi+) wasfound to be 0.83 with PEO plasticizer, as shown in Figure 9.Further improvement in the transference number (i. e., closer tounity) may be possible with a lower molecular weight PEG or

Figure 6. Current interrupt method to measure the conductivity: voltage-time profile of PC-b-PSSLi SPE with 60 wt% PDN plasticizer. The cellconfiguration was SS/SPE/SS.

Figure 7. CV profile of PC-b-PSSLi SPE at 22 °C. The scan rate was 1 mV/s. Theeffective area of SPE was 1.13 cm2.

Figure 8. Li+ transference number of PC-b-PSSLi with PDN plasticizer at50 °C. The effective area of SPE was 1.13 cm2.

Figure 9. Li+ transference number of PC-b-PSSLi using PEG250 as theplasticizer. The effective area of SPE was 1.13 cm2 at 50 °C.

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stabilization of lithium metal SEI. An improved SEI, such as bythe introduction of propylene carbonate oligomers[21] or otherSEI-forming organic and inorganic additives (e.g., fluoroethy-lene carbonate, LiNO3 and LiBOB[22]) would stabilize theelectrode-electrolyte interface and series resistance. Neverthe-less, a higher lithium ion transference number value wasmeasured with both plasticizers showing that the tetheredanion approach holds promise for mitigating voltage droopproblems.

3. Summary

In this study, a PC-b-PSSLi diblock polyelectrolyte was synthe-sized via sequential Raft polymerization followed by ionexchange from Na+ to Li+. A pendant type polycarbonate wasused as the first block, and sulfonated polystyrene was used asthe second block. Because the anions are tethered, a high Li+

transference number, 0.83, was observed. It was found thatPDN plasticizer promotes the dissociation of the lithium-sulfonate ion pair. A high conductivity of 1.95 ⋅10� 6 S/cm wasachieved.

Supporting Information

Details of the experimental procedures and characterizationdata are provided in supporting information (Figure S1 toFigure S5).

Acknowledgements

The authors gratefully acknowledge funding from Kolon Indus-tries.

Conflict of Interest

The authors declare no conflict of interest.

Keywords: block copolymers · polyanions · solid polymerelectrolytes · transference number · polycarbonate

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Submitted: December 7, 2020Accepted: January 15, 2021

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