A968 Journal of The Electrochemical Society, 164 (6) A968-A972 (2017)0013-4651/2017/164(6)/A968/5/$37.00 © The Electrochemical Society

Ex Situ Investigation of Anisotropic Interconnection inSilicon-Titanium-Nickel Alloy Anode MaterialJong-Soo Cho,a Pankaj Kumar Alaboina,a Chan-Soon Kang,b Seul-Cham Kim,bSeoung-Bum Son,c Soonsung Suh,d Jaehyuk Kim,d Seunguk Kwon,d Se-Hee Lee,e,∗Kyu-Hwan Oh,b and Sung-Jin Choa,z

aJoint School of Nanoscience & Nanoengineering, North Carolina A&T State University, Greensboro, North Carolina27401, USAbSchool of Material Science & Engineering, Seoul National University, Seoul 151-742, South KoreacNational Renewable Energy Laboratory, Golden, Colorado 80401, USAdSamsung SDI, 428- Gongse-dong, Yongin-si, Gyunggi-do 446-577, South KoreaeDepartment of Mechanical Engineering, University of Colorado at Boulder, Colorado 80309, USA

Herein we investigate the nanostructural evolution of Silicon-Titanium-Nickel (Si-Ti-Ni) ternary alloy material synthesized bymelt spinning process for advanced lithium-ion battery anode. The synthesized material was found to have nano-Silicon particlesdispersed in the Ti4Ni4Si7 (STN) alloy buffering matrix and was characterized by X-ray diffraction (XRD), High resolution-transmission electron microscope (HR-TEM), Scanning transmission electron microscopes - energy dispersive X-ray spectrometer(STEM-EDS), and electrochemical performance test. The role of STN matrix is to accommodate the volume expansion stresses ofthe dispersed Si nanoparticles. However, an interesting behavior was observed during cycling. The Si nanoparticles were observedto form interconnection channels growing through the weak STN matrix cracks and evolving to a network isolating the STN matrixinto small puddles. This unique nanostructural evolution of Si particles and isolation of the STN matrix failing to offer significantbuffering effect to the grown Si network eventually accelerates more volume expansions during cycling due to less mechanicalconfinement and leads to performance degradation and poor cycle stability.© 2017 The Electrochemical Society. [DOI: 10.1149/2.0221706jes] All rights reserved.

Manuscript submitted November 30, 2016; revised manuscript received February 21, 2017. Published March 10, 2017.

Li-ion batteries (LIBs) offer the highest energy density and poweras compared to any other battery technologies commercially availabletoday. Even though graphite is a conventional and widely used anodematerial for LIBs, industries are trying to develop new anode materialswith higher capacities. Silicon (hereafter denoted as Si) is one of themost promising anode materials for next generation Li-ion batteriesto replace graphite because of its natural abundance, non-toxicity, andthe highest specific capacity among the known materials to date. How-ever, the main challenge for the implementation of Si anode is theirlarge volume changes, which might be up to 300% during lithiuminsertion and de-insertion, which often causes pulverization of theactive Si particles and poor cycle stability.1–4 Extensive research hasbeen done focusing on accommodating Si volume changes issue and tobring them closer to commercial applications.5–10 Some of the popularapproaches over the recent years included the use of Si as nanowirestructures,11–13 nanoparticles,14,15 porous structures,12,16,17 and com-posites with carbon or composites with some flexible matrix.18–25 Theuse of nanoscale particles scales down the volume expansions and ac-commodates cracking, and are also well known to reduce the lithiumdiffusion lengths improving the rate performance.13,15,26 Si dispersedin the flexible superelastic matrix would provide the buffering ef-fect and absorb the massive volume expansions avoiding cracking orpulverization.18–25

As one of the solutions for the huge volume expansion issue thatleads to poor cycling performance, nanoscale Si crystalline particlesin a Ti4Ni4Si7 (STN) buffering matrix alloy was used in this work.The nickel (Ni)-titanium (Ti) alloys have attracted interest due to theirsuperelastic properties, high flexibility, and high recovery deforma-tion ability,27–29 making them the suitable choice to accommodateSi volume expansions and absorb generated stresses. Melt spinningtechnique that rapidly solidifies molten metal solutions to alloys wasemployed to produce Si nanoparticles dispersed in STN matrix. Theidea of using nanoscale Si particles is to withstand and accommodatethe volume stresses and thereby to mitigate particles cracking duringthe cycling process. The buffering matrix, Ti4Ni4Si7 (Si-Ti-Ni sys-tem, hereafter denote as STN) embedding the Si nanoparticles waschosen to fulfill the accommodation of the Si volume changes. Toour surprise, the interconnection of the embedded Si nanoparticles

∗Electrochemical Society Member.zE-mail: scho1@ncat.edu

forming channels in the matrix was observed during cycling. Hereinwe investigate the nanostructural evolution of Si in STN alloy anodeand report that formation of Si channels (or network) in the STNmatrix are triggered during lithiation and delithiation processes. Ini-tially, the nanoscale Si particles were unconnected and dispersed inthe matrix, but after cycling, the particles were found to be inter-connected via channels forming a Si network. Various microscopicinvestigations have been approached to elucidate the nanostructuralevolution and phase transition in the previous studies.21–25,30 Wanget al. studied Si/Carbon composite electrode and showed that Si lay-ers bond strongly to the carbon nanofiber (CNF) substrate and thestructure remain intact during early stages of cyclic charge/dischargeand starts deteriorating with progressive cycling.31 Liu et al. did a sim-ilar study, which shows reversible nanopore formation during cyclingprocess in Germanium nanowires.32 To the best of our knowledge, ithas never been reported in the prior works of Si matrix compositesabout the evolution of Si interconnected network. Herein we report thenanostructural evolution of Si network in STN buffering matrix alloyanode and propose a mechanism for Si interconnection network pro-gression leading to isolation of STN matrix; resulting in performancedegradation and poor cycling stability.

Experimental

The nanoscale Si embedded Si-Ti-Ni (STN) ternary alloy matrixwas synthesized by using melt spinner from MK Electron Inc., Korea.Fig. 1a shows the melt spinning schematic which rapidly solidifiesmolten metal alloy on a rapidly spinning copper roller to producemelt spun ribbons. The chemical composition of the molten STN al-loy used was 68% of Si, 16% of Ti and 16% of Ni (atomic %), whichwas quenched on the copper roller to produce the melt spun ribbons.The composition was selected based on Si content of 68 atomic %to target high capacity, high initial efficiency, and to ensure a stablephase in the STN alloy.25 Fig. 1b shows the ribbons obtained whichhad an average width size of less than 1 mm and thickness of 10–15μm. The ribbons were then pulverized to STN powder (5 μm averageparticle size) by ball milling. The melt spinning process started withSTN metal alloy solution, but the as-prepared powder obtained afterpulverization was found to have nanoscale Si crystalline particles dis-persed in the STN alloy matrix which could be attributed to the rapidquenching process. The as-prepared material was characterized using

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Journal of The Electrochemical Society, 164 (6) A968-A972 (2017) A969

Figure 1. (a) Simplified schematic and photo image of the melt spinner; (b) Photo image of melt spun Si-Ti-Ni alloy ribbon; (c) X-ray diffraction (XRD) patternof ball milled melt spun Si-Ti-Ni alloy powder that matched with Si phase (JCPDS, 27–1402) and Ti4Ni4Si7 phase (JCPDS, 80–1015); and (d) TEM image ofSi-Ti-Ni alloy showing Si nanoparticles dispersed in Ti4Ni4Si7 (STN) matrix.

X-ray diffraction (XRD), High Resolution- Transmission electron mi-croscope (HR-TEM), Scanning transmission electron microscopes -energy dispersive X-ray spectrometer (STEM-EDS) to study the phaseand structure. For the electrochemical performance investigations, theactive material slurry for anode electrode was prepared with 88 wt% ofsynthesized STN powder, 4 wt% of Ketjen Black as conductive carbonand 8 wt% of polyamide-imide (PAI) as the binder. Coin cells 2032were assembled, and 1.5M LiPF6 with ethylene carbonate/diethyl car-bonate/fluoroethylene carbonate, EC/DEC/FEC (5:70:25) was used asthe electrolyte.

Constant current-constant voltage (CC/CV) discharge-charge testswere conducted to characterize the electrochemical properties of theSTN alloy. Cells were cycled at a 0.1 C for the first cycle, 0.2 C for thesecond, and 1 C for all subsequent cycles (1 C rate corresponds to 0.88mA cm−2). The potential window for electrochemical measurementswas 0.01–1.5 V vs. Li/Li+ (all the voltages below are vs. Li/Li+).To investigate the nanomechanical evolution behavior in STN foursamples were prepared. The first sample was the fresh STN electrodesample before cycling. The remaining three samples were obtainedby tearing down the coin cells after first cycle lithiation at 0.1 C,third cycle lithiation at 1 C, and 50th cycle delithiation at 1 C. Thenanostructures of the STN samples evolved at each condition wereobserved by TEM (HR-TEM, JEOL 3000F), which also had an air-lock system to prevent the samples from being oxidized during theirtransfer from Focused Ion Beam (FIB) to TEM.

Results and Discussion

Nanostructure evolution in STN anode material duringcycling.—XRD patterns of ball-milled melt spun STN powder isshown in Fig. 1c and was found to match with Si phase (JCPDS,27–1402) and STN (Ti4Ni4Si7) phase (JCPDS, 80–1015). Using Ri-etveld analysis from the XRD patterns, the weight fraction of Si inmelt spun STN alloy was calculated to be around 46 wt%, and theremaining 54 wt% was Ti4Ni4Si7 matrix phase.21 TEM image of theas-prepared STN powder is shown in Fig. 1d. It was found to have Si

nanoparticles dispersed in the STN matrix which can be attributed tothe rapid quenching process as mentioned in the Experimental section.

STN matrix can be considered as inactive to Li alloying. The STNmatrix elements are in the stable alloy state and do not further partic-ipate to alloy with Li to contribute to capacity. From a previous workof K.J. Lee,23 the capacity of STN matrix was reported as less than50 mAh g−1 over just 5 cycles. This amount of capacity is extremelylow compared to the huge theoretical capacity of Si (3579 mAh g−1)and can be ignored with much focus giving to its primary role to serveas a buffering matrix. Based on the STN alloy composition of 46 wt%Si phase from the XRD patterns and the designed electrode composi-tion with 88 wt% STN alloy active material, the expected theoreticalcapacity was around 1448 mAh g−1. In the electrochemical perfor-mance test, during the first cycle i.e. formation at 0.1 C rate (0.088 mAcm−2) the material showed a charge and discharge capacity of 1252.8mAh g−1 and 1098.8 mAh g−1, respectively, with an efficiency of87.7%, shown in Fig. 2a. It can be observed that the experimentalcapacity is lower than the expected theoretical capacity which can beattributed to the mechanical confinement of free volume expansion ofactive Si by the buffering STN matrix limiting the extent of Li inser-tion affecting capacity while compared with unconfined Si.21 Afterthe first formation cycle which is essential for the formation of a sta-ble solid electrolyte interphase necessary for Li transportation into Siactive material., the cell was cycled at 0.2 C (0.176 mA cm−2) rate forthe second cycle and at 1 C rate (0.88 mA cm−2) for all the remainingcycles. The cycling results are shown in Fig. 2b. The reversible ca-pacity achieved at 0.2 C rate in the second cycle was 1050.5 mAh g−1

and the efficiency was improved to 98.4% after the formation cycle.The cycling results in Fig. 2b shows that the efficiency was improvedclose to 100%, but the capacity was gradually fading for all the sub-sequent cycles with a capacity fading of around 13.6% over 50 cyclesat 1 C rate. The reason for this fading and failure can be attributed tothe loss of ability of the STN matrix to accommodate the Si volumeexpansion stresses as the cell is cycled. Fig. 3a shows the close lookof the melt spun STN ribbon obtained right after the melt spinningprocess. The ribbon was observed to have two different morphologies

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A970 Journal of The Electrochemical Society, 164 (6) A968-A972 (2017)

Figure 2. (a) First cycle voltage profile of STN (Si-Ti-Ni) alloy in the potentialwindow 0.01–1.5 V at 0.1 C rate (0.088 mA cm−2); and (b) Specific capacityand Columbic Efficiency of STN alloy anode versus cycle numbers.

Figure 3. (a) Closer look of the Si-Ti-Ni (STN) ribbon cross-section obtainedafter melt spinning; (b) TEM image of STN cooled in contact with the Copperroll; and (c) TEM image of the free cooling side of the STN ribbon.

Figure 4. High Resolution-TEM images of Si-Ti-Ni (STN) anode showingthe growth of Si interconnections with increasing lithiation or delithiationcycling.

separated by a dashed line in Fig. 3a. The two morphology sectionswere both found to have Si nanoparticles dispersed in STN matrix,but they differed in shape and size of the embedded Si nanoparticles.Very fine and round shaped Si grains were observed (bright spots inFig. 3b) in the area which was in contact with the Copper roller andexperienced rapid solidification by internal water cooling of the roller.The diameter of embedded Si nanoparticles was about 20–60 nm. Incontrast, coarse Si grains in Fig. 3c with long and wire-like shapewere seen dispersed in the STN near to the free cooling area, whichhad a relatively slow cooling rate.

Fig. 4a1 and Fig. 4a2 present the TEM images of STN anode after1st Li insertion. Compared to the individually dispersed Si nanoparti-cles in STN matrix as seen in Fig. 3a and Fig. 3b, Si interconnectionchannels (or networks) were found to take off between Si particlesduring cycling. These channels were seen in the STN anode initially.Furthermore, it was observed that during long-term cycling, more andmore Si channels were formed spreading the network as can be seenin TEM images after 3rd Li insertion, Fig. 4b1 and Fig. 4b2, andafter 50th cycles, Fig. 4c1 and Fig. 4c2. Si channels were incremen-tally grown, and almost all Si grains were linked each other throughSi nanochannels over 50 cycles. This evolution of the nanoscale Siparticles to an extensive interconnection network eventually leads tothe isolation of the STN matrix draining its ability to mechanicallyconfine the nanocrystalline Si particles to accommodate and absorbSi volume stresses. This loss of the buffering ability of the STN ma-trix allows the Si to recall its huge volume expansion issue whichstarts to account for the capacity fading observed in Fig. 2b during

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Journal of The Electrochemical Society, 164 (6) A968-A972 (2017) A971

Figure 5. Scanning transmission electron microscopes - energy dispersive X-ray spectrometer (STEM-EDS) line scanning analysis on the interconnection channels(after 3rd lithiated cycle).

cycling. Initially, we expect to see the structural behavior differentlyboth on a fine grain (Fig. 3b) and coarse grain (Fig. 3a). However, theHigh-resolution TEM images in Fig. 4 shows that the degree of inter-connection and behavior of both the fine and coarse grain Si particleswere almost similar.

Fig. 5 present the STEM-EDS analysis results of the Si channelsformed during cycling. The contrast of this image shown in Fig. 5a isswitched due to STEM image mode, compared to the TEM images.EDS line scan was performed across the Si channels. It was found thatthe Si content was almost constant, but the intensities of Ti and Niwere clearly decreased in the middle of the line-scan area indicatingthe channels were mainly composed of Si.

Mechanism for nanostructure evolution.—The mechanism of theevolution from nanoscale Si particles to a large interconnection net-work during cycling is shown as a schematic in Fig. 6. The anodematerial is shown as two spherical Si nanoparticles dispersed or em-bedded in the inactive STN matrix before cycling. During cycling, thefree volume expansion of embedded nano-Si particles occurs corre-sponding to Li+ insertion. As a result, tensile stresses are producedby the Si particles, generating strains in the matrix. The STN matrixabsorbs and accommodates most of the volume stresses. When the ex-panded Si particles are confined to the inactive matrix, they are underthe condition of volumetric confinement in an isotropic elastic matrixresulting in compressive stress and limited lithiation.21,23,30 However,any weak matrix locations in the vicinity of the Si particles which failto sustain the stresses start to crack and provide a way for the Si expan-sion or channel like growth during lithiation. The internally appliedcompressive stress on the Si particles by self-expansion during Li in-sertion drives the Si atoms to move through the tensile stress appliedarea between the Si particles to release the stress, and this generatescracks. As a result, Si channels are formed through the crack orientedanisotropic expansions. As the number of cycles is increased, the Sichannels grow further more forming interconnections between theSi particles, and eventually building a big interconnected Si network

Figure 6. Schematic showing the mechanism for Silicon nanostructure net-work evolution in the Si-Ti-Ni (STN) matrix.

similar to electrochemical sintering33 and isolating the STN matrixinto small puddles. The Si network evolution and isolation of the STNmatrix can be seen in the TEM images of the Fig. 6. Now, the isolationof the STN matrix fails to offer significant buffering effect to the Sinetwork which reflects in the performance degradation as seen earlierin the poor cycling in Fig. 2a.

Conclusions

Silicon-Titanium-Nickel ternary alloy (Si-Ti-Ni or STN) anodematerial was synthesized by melt spinning method. The synthesizedmaterial was found to have nano-Si particles dispersed in the Ti4Ni4Si7

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A972 Journal of The Electrochemical Society, 164 (6) A968-A972 (2017)

(STN) buffering matrix with the objective to accommodate Si volumeexpansion issue during cycling. In the investigations, the Si particlesvolume expansions were initially found to be satisfied by the sur-rounding STN matrix. However, as the cycles were increased, theSi particles were observed to form interconnection channels growingthrough the weak STN matrix cracks generated during high tensilevolume expansion stresses. The Si particles interconnections eventu-ally evolve into a significant Si network isolating the STN matrix.This unique behavior and isolation of the STN matrix fails to offersignificant buffering effect to the Si network during volume expan-sions, due to less mechanical confinement and leads to performancedegradation and poor cycle stability.

Acknowledgment

This work was supported by the Fundamental R&D Program forTechnology for World Premier Materials funded by the Ministry ofKnowledge Economy, South Korea (10037919).

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