Journal of The Electrochemical Society, 161 (12) A1851-A1859 (2014) A18510013-4651/2014/161(12)/A1851/9/$31.00 © The Electrochemical Society

Interfacial Chemistry Control for Performance Enhancementof Micron Tin-Nickel/Graphite Battery AnodeSukhyun Hong,a Myeong-Ho Choo,a Yo Han Kwon,b Je Young Kim,band Seung-Wan Songa,c,∗,z

aDepartment of Energy Science and Technology, Chungnam National University, Daejeon 305-764, South KoreabBattery R & D, LG Chem, Ltd., Daejeon 305-380, South KoreacDepartment of Fine Chemical Engineering and Applied Chemistry, Chungnam National University,Daejeon 305-764, South Korea

Designing and controlling the anode–electrolyte interfacial chemistry of a micron Sn-Ni/graphite composite battery anode led tothe formation of a stable solid electrolyte interphase (SEI) layer. We utilized fluoroethylene carbonate (FEC)-based electrolyte thatis more interfacially compatible than an EC-based electrolyte, trimethyl phosphite electrolyte additive that reduces the attack ofLiPF6-derived acidic species in the electrolyte, and the addition of a low fraction of SnF2 to anode for capturing the F anions of HFpresent in the electrolyte. Mechanistic surface chemistry studies using ATR FTIR and X-ray photoelectron spectroscopy revealedthat the SnF2 transforms to SnF4 by capturing F anions, while FEC and phosphite provide a surface protective and robust SEI. Theinterfacially controlled composite anode with a tuned content of graphite exhibits good cycling stability (90% retention at the 50th

cycle) with high discharge capacity of ∼800 mAhg−1 of tin, in contrast to a rapid capacity fade in the conventional electrolyte.© 2014 The Electrochemical Society. [DOI: 10.1149/2.0661412jes] All rights reserved.

Manuscript submitted May 27, 2014; revised manuscript received July 10, 2014. Published August 29, 2014. This was Paper 912presented at the San Francisco, California, Meeting of the Society, October 27–November 1, 2013.

Tin (Sn)-based anode materials have been an attractive alterna-tive to currently commercialized graphite for high-energy Li-ionbatteries because of their large specific gravimetric capacity of Sn(∼992 mAhg−1 vs. 372 mAhg−1 for graphite) and volumetric capac-ity (2023 mAhcm−3 vs. 818 mAhcm−3),1 appropriate operating volt-age above lithium for a battery safety and high electronic conductiv-ity of metallic Sn.2–4 Typical failure modes of Sn-based electrodesare electrochemical and mechanical disintegration of particles due tolarge volume change (∼300%) upon lithiation of Sn, and interfacialinstability in the conventional electrolyte, resulting in a poor cyclingperformance and solid electrolyte interphase (SEI) stability.

Numerous efforts have been made to address these issues, with thefocus on improvement of cycling performance, such as nanosize andnanostructure of Sn,2,5,6 the use of an inactive matrix2,7,8 and func-tional binder,9 electrolyte formulation for SEI stabilization,10 and pre-lithiation of Sn for a reduced initial irreversibility.11,12 Despite notableperformance improvement with the approaches for nano-materials,there has been a trade-off with complex and expensive synthetic pro-cesses, and an uneasiness in handling the nanomaterials in the largescale. From a practical view point, micro-sized materials preparedby a simple process such as ball-milling are favored for large-scaleproduction, cost, safety, and volumetric energy density. Micron mate-rials, however, are more likely to undergo disintegration upon volumechange during cycling and more rapid performance fade,13 due to lim-ited reaction kinetics of ion/electron diffusion than nano-materials.

Control of Sn-electrolyte interfacial chemistry and SEI sta-bility is a promising approach for achieving the improved cy-cling performance.14 The Sn is known to react with LiPF6-derivedstrong acids that originate from these reactions (LiPF6 ←→ PF5

+ LiF, PF5 + H2O → 2HF + PF3O),15,16 and be deactivated.14,17,18

Surface degradation can occur by the dissolution of surface oxideby HF. Although inhibition or lowering of the acid attack is the keyparameter for performance enhancement, to date, the attainment of astable cycling performance of micron Sn-based anode through inter-facial chemistry control has not been reported.

We report here on controlling the interfacial chemistry (SEI sta-bility) of the micron tin-nickel/graphite (Sn-Ni/graphite) compositeanode prepared by ball-milling and its effect on electrochemical cy-cling performance. The strategic use of more interfacially compatiblesolvent of FEC than EC of conventional electrolyte, combined withtrimethyl phosphite (P(OCH3)3, TMP) as a Lewis base electrolyte

∗Electrochemical Society Active Member.zE-mail: swsong@cnu.ac.kr

additive that can lower the attack of acidic species, significantly re-duces the interfacial resistances and enhances cycling performance.Furthermore, by applying a small fraction (1.6 wt%) of SnF2 to theanode active material for inhibiting HF-attack, we demonstrate thatthe composite anode exhibits a remarkable performance enhancement,delivering ∼800 mAhg−1 of tin and 90% capacity retention at the 50th

cycle.

Experimental

Material preparation and characterization.— Commercial tin (Sn,∼10 μm, 99%, Aldrich), nickel (Ni, < 150 μm, 99.99%, Aldrich), tindifluoride (SnF2, 99%, Aldrich), and graphite (<20 μm, Aldrich)powders were used as received. First, Ni and graphite were each ball-milled with zirconia balls (5 mm diam) in a stainless-steel containerfor 30 min at room temperature at the oscillation frequency of 50 Hz,using a vibrational ball mill (Mini-mill Pulverisette 23, Fritsch), toreduce their particle size similar to that of Sn. Then, Sn and Ni withoutand with SnF2 powders were mixed at a ratio of 79 : 21 wt% and77.88 : 20.5 : 1.62 wt%, respectively, at a mortar for 30 min in theAr-filled glove box. Then the mixed powder and graphite powderwere combined at varied ratios from 80:20 to 50:50 wt% and ball-milled with zirconia balls for 3 h, producing SnF2-free and -addedSn-Ni/graphite composite active materials.

Phase and material composition of as-prepared composite powderswere identified with powder X-ray diffraction (XRD) analysis. XRDmeasurement was conducted in the 2θ range from 10◦ to 70◦ at thescan rate of 2◦/min with 0.02◦ step using powder X-ray diffractometer(Rigaku D/MAX-2200) with Ni-filtered Cu Kα radiation at 40 kV and40 mA.

Electrochemical characterization.— The electrochemical charge-discharge cycling performance of SnF2-free and SnF2-added Sn-Ni/graphite anodes was evaluated with 2016-type coin lithium half-cells at room temperature. The electrode slurry, which was composedof 85 wt% active material, 10 wt% poly acrylic acid binder (PAA, MW450,000, Aldrich), and 5 wt% carbon black (Super P) in N-methyl-2-pyrolidinone (NMP) solvent, was coated on copper foil. The ratio ofthe active material to NMP was 1:3. The coated slurry was dried ina vacuum oven at 110◦C for 12 h. The 2016 coin half-cell consistedof SnF2-free or -added Sn-Ni/graphite electrode as a working elec-trode, lithium metal foil as a counter electrode, 90 μL of non-aqueouscarbonate-based liquid electrolyte, and separator (Celgard C210). Twotypes of liquid electrolytes were used; ethylene carbonate (EC)-basedone, 1.0 M LiPF6/EC:EMC (3:7 volume ratio, Panax E-Tec), and

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fluoroethylene carbonate (FEC)-based one, 1.0 M LiPF6/FEC:DEC(1:1 volume ratio, Panax E-Tec), without and with 3 wt% trimethylphosphite (P(OCH3)3, TMP, Aldrich) as an electrolyte additive, re-spectively. The cells were assembled in the Ar-filled glove box withoxygen and water less than 1 ppm and tested in the voltage range of0.05–1.5 V at a constant current of 55 mAg−1 (corresponding to 0.1C)-constant voltage mode using a multichannel cycler (WBCS3000, Won-A Tech). The integrated capacities of composite anodes were normal-ized to the weight of Sn by using the capacity of 160 mAhg−1 forgraphite only electrode, which was measured at 55 mAg−1 (∼0.35Cfor graphite) separately under identical condition. Graphite-free SnF2-Ni electrode, which consists of 83 wt% SnF2 and 17 wt% Ni, was alsoprepared and tested as a reference for comparison. AC impedancemeasurement was conducted at ambient temperature after the 1st and50th cycle, using an impedance analyzer (VSP SP-150, Bio-Logic) inthe frequency range of 630 kHz to 100 mHz with an amplitude of 10mV, after the cells were fully discharged to 1.5 V at the given cyclenumber followed by letting them at open circuit voltage (OCV) for∼10 h to reach equilibrium. No voltage change was observed afterreaching the equilibrium. Impedance spectra were fitted using a modelelectric circuit till the goodness of fit to be less than 1%.

For the investigation of the SEI formation and composition, andparticle morphology change, the cycled electrodes were disassembledfrom the coin cells and rinsed with dimethyl carbonate (DMC, PanaxE-Tec) for 60 s to remove the residual electrolyte, followed by dryingat room temperature in the Ar-filled glove box. Changes with cyclingin the particle morphology of the electrodes were monitored with fieldemission scanning electron microscopy (FE SEM, JSM-7000F, JEOL)at 10 kV.

Surface characterization.— Surface characterization of pristineand cycled electrodes was performed using ex situ attenuated to-tal reflectance (ATR) infrared spectrometer (FTIR, Nicolet 6700)

equipped with a mercury−cadmium−telluride (MCT) detector. Thecycled electrodes were directly mounted on the tightly closed ATRunit in the Ar-filled glove box to avoid air atmospheric contamina-tion during transportation from glove box to dry N2-purged samplecompartment of the IR instrument as well as during IR measurement.The spectra were collected with 1024 scans and spectral resolution of4 cm−1. For comparison, we also prepared a control sample of stainlesssteel disk with a smooth surface immersed in 1 M LiPF6/FEC:DECfor 24 h without applying electrochemistry. The stainless steel diskwas separated from the electrolyte and dried at room temperature inthe glove box, then subjected to the IR measurement with varying thedrying time from 1 to 24 h, from which we obtained the drying-timedependent IR spectra of the electrolyte residue of LiPF6:FEC. It wasthen washed with DMC for 60 s.

Surface composition analysis for the electrodes before and aftercycling was conducted employing X-ray photoelectron spectroscopy(XPS, MultiLab 2000, Thermo) with Al Kα X-ray source at 15 kV.The cycled electrodes were transferred from glove box to the XPSchamber using a portable vacuum sealed-carrier container withoutexposure to air. High-resolution spectra were obtained at a power of150 W under a base pressure of 5 × 10−10 mbar. The spot size wasabout 500 μm and the pass energy was 30 eV. The binding energywas calibrated based on the C 1s level at 284.7 eV. The curve-fittingon the F 1s peak was conducted with Gaussian (80%) and Lorentzian(20%) functions and full-width-half-maximum (FWHM) of 1.85 eVafter having Shirley-type background correction, using XPSPEAK 4.1program.

Results and Discussion

Composition and structural characterization of Sn-Ni/graphite ac-tive material.— A powder XRD pattern (Fig. 1a) of ball-milled SnF2-added Sn-Ni/graphite composite (SnF2-added) active material with

Figure 1. Powder XRD pattern (a) and SEM elemental map-ping (b) of F atom for SnF2-added Sn-Ni/graphite (80/20 wt%)composite active material, and XPS spectral comparison of (c)F 1s for SnF2-added composite electrode after surface-etching(Ar-sputtering) and (d) Sn 3d5/2 for (i) bare and (ii) SnF2-addedcomposite electrodes without surface-etching.

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Journal of The Electrochemical Society, 161 (12) A1851-A1859 (2014) A1853

Figure 2. Charge-discharge voltage profiles of Sn-Ni/graphite(80/20 wt%) electrodes in the electrolyte (EL) of (a) 1MLiPF6/EC:EMC, and (b) 1M LiPF6/FEC:DEC, (c) with 3 wt%TMP additive and (d) SnF2-added electrode with TMP at 0.1Cand their dQ/dV plots (a′) – (d′).

20 wt% graphite shows the reflections from each phase of crystallineβ-Sn (JCPDS 65-0296), Ni (JCPDS 65-2865) and graphite (JCPDS65-6212) but neither intermetallics (e.g., NixSny) nor SnO2 of surfaceimpurity. The reflections from SnF2 phase are invisible due to its lowconcentration (1.62 wt%). The three components must be physicallymixed. Bare (SnF2-free) active material exhibits the same pattern (notshown).

The presence of added SnF2 was first monitored by elemental (F)mapping as shown in Fig. 1b. The surface state of the pristine SnF2-added composite electrode was investigated by XPS. The presenceof SnF2 was confirmed in the F 1s XPS spectrum (Fig. 1c) onlyafter surface-etching with Ar-sputtering (Fig. 1c), down to a depth of2 nm from the top-surface (referred to the standard material of SiO2).Without surface-etching, we were not able to see the F atom, as thetop surface of the electrode might be covered by other materials. Abroad peak ranged from 684 to 686 eV, attributed to O-Sn-F bond andSnF2,19 is indicative of the chemical nature of F atom. In the spectra ofSn 3d5/2 (Fig. 1d (i)-(ii)), the top surface of both bare and SnF2-addedelectrodes without surface-etching were covered by mainly surfaceoxide (SnO2)20 by an affinity of Sn to oxygen in the air.

Electrochemical performance of Sn-Ni/graphite compositeelectrode.— Cycling ability.— Figure 2 shows charge and dischargevoltage profiles of bare (SnF2-free) and SnF2-added Sn-Ni/graphite

(80/20 wt%) composite electrodes between 0.05 and 1.5 V at a rateof 0.1C in the EC- and FEC-based electrolytes, whose micron activematerials were prepared by a simple physical mixing with ball-milling(Fig. 1). The integrated capacities were normalized to the weight ofSn. In the initial charge curves, the plateaus at 0.67, 0.48, 0.4 V, andbelow 0.4 V, corresponding to the formation of Li2Sn5, LiSn, Li5Sn2,Li7Sn5, and Li22Sn5 phases,1 respectively, which are represented ascathodic peaks in dQ/dV plots (Fig. 2(a′)-2(d′)), are clearly observed.The contribution of graphite to the capacity below 0.2 V is relativelysmall due to its low content.

Changing the electrolyte from the EC-based (Fig. 2a) to the FEC-based (Figs. 2b–2d), and using TMP additive (Figs. 2c-2d), and addinga low fraction (1.6 wt%) of SnF2 (Fig. 2d) to the active material led toa noticeable increase in the first discharge capacity from 499 (Fig. 2a)to 785 mAhg−1 (Fig. 2d), initial coulombic efficiency from 66 to 73%,and capacity retention (Fig. 3) from 19 to 74% at the 50th cycle, withwell-maintained structural resolution of prominent peaks by lithiationand delithiation in the dQ/dV plots (Fig. 2(d′)). Also rate-capability(Fig. 4) up to 2C (1.1 Ag−1) is achieved. It is evident that both Snand graphite are more interfacially compatible with FEC than EC.Effectiveness of FEC to the SEI formation at graphite anode is wellknown,21–23 whereas its effect on the electrochemical performanceof Sn is unknown. The Sn appears to behave in a fashion similarto silicon, referred to that FEC as an additive and solvent provides

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0

200

400

600

800

1000

0 10 20 30 40 50

Cycle number

Dis

char

geca

pac

ity

(mA

hg-1

)

Sn-Ni only,EC-based EL

EC-based EL

FEC-based EL

FEC-based EL+TMP

FEC-based EL+TMP,SnF2

Figure 3. Cycling performance in different elec-trolytes at 0.1C, including a Sn-Ni only electrode.

an enhanced SEI stability and performance of silicon anode.24–27 Asfound in our earlier study, TMP as a Lewis base successfully functionsas a capturing agent of LiPF6-derived strong Lewis acids (PF5, PF3O),contributing to performance improvement.14 The role of added SnF2

is discussed below.

Changes in particle morphology.— The variation in interfacialchemistry strongly affects the particle morphology with cycling.While cycling in the EC-based electrolyte (Fig. 5b) results in a particleaggregation compared to pristine (Fig. 5a), a particle cracking occursin the FEC-based electrolytes (Fig. 5c-5d), producing secondary par-ticles consisting of spherical primary nano-particles, with retainedparticle connectivity. When adding the SnF2 (Fig. 5e-5(e′)), veryporous spherical secondary particles consisting of well-connectednano-particles form. This porous and nanoscale feature might beadvantageous for accommodating the volume change, and sustain-ing the particle connectivity and facile electrolyte wetting (interfacialcontact).

Interfacial resistance.— The effect of interfacial chemistry on in-terfacial resistance was investigated using AC impedance spectro-scopic analysis. Nyquist plots (Figs. 6a–6d) consist of two semicir-cles in the high to middle frequency region, which corresponds tothe interfacial resistances of surface film (Rf) formation and charge

0

0.5

1

1.5

2

0 200 400 600 800 1000 1200

Capacity (mAhg-1)

Vol

tage

(V) 0.1 C2 C

1 0.5 0.2

Figure 4. Rate capability of SnF2-added Sn-Ni/graphite electrode in 1MLiPF6/FEC:DEC with TMP.

transfer (Rct), respectively, and inclined straight line for Li+-ion dif-fusion process within the bulk.22,28 Their Bode plots are displayedin Fig. 6(a′)-6(d′). The obtained spectra were fitted using the equiv-alent circuit (Fig. 6a) consisting of three CPE; CPE1 is related todiffusion capacitance attributed to the Li+-ion diffusion in the SEIfilm, CPE2 to charge transfer resistance and the electric double-layer

1 µm

a b

1 µm

1 µm

1 µm

1 µm

500 nm

c d

e'e

Figure 5. SEM images of particle morphology changes from (a) Sn-Ni/graphite composite pristine electrode to the cycled electrodes in (b) 1MLiPF6/EC:EMC, and (c) 1M LiPF6/FEC:DEC and (d) with TMP additive, and(e-e′) SnF2-added electrode with TMP.

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Journal of The Electrochemical Society, 161 (12) A1851-A1859 (2014) A1855

Figure 6. Comparison Nyquist plots of bare Sn-Ni/graphite electrodes in (a)1M LiPF6/EC:EMC, and (b) 1M LiPF6/FEC:DEC, (c) with TMP and (d) SnF2-added electrode with TMP, after the 1st and 50th cycles, and their Bode plots(a′)-(d′).

capacitance of electrode-electrolyte interface, and CPE3 to the semi-infinite diffusion impedance. The fitting results (dots) match well withthe measured data (solid lines). The resultant changes in the Rf andRct with cycling are summarized and compared in Figs. 7a-7b, re-spectively. In the EC-based electrolyte, the Rf (Fig. 7a) continues toincrease with cycling, indicating that the EC does not provide a surfaceprotective SEI in the initial cycle as found before14 and electrolyte de-composition continues. However, the tendency is the opposite in theFEC-based electrolytes, probably due to the formation of a surfaceprotective SEI during initial cycling and inhibition of further elec-trolyte decomposition in the next cycles. When using TMP combinedwith SnF2-addition, overall Rf is significantly reduced. This confirmsthat the stable SEI layer produced in the initial cycle plays a surfaceprotecting role. On the other hand, while the initial Rct (Fig. 7b) issignificant in the EC-based electrolyte because of formation of numer-ous resistive surface species (e.g., LiF) as observed in the EC-basedelectrolyte29 and/or unstable SEI that degrades by the volume changeof electrode,30 the Rct at the 50th cycle in the FEC-based electrolytesremains unchanged. Upon SnF2-addition, the Rct at the first cycle isrelatively larger but becomes similar to those of bare electrodes at the50th cycle. In the view of interfacial resistance changes, the capac-ity fade (Fig. 2a and 3) of bare electrode in EC-based electrolyte iscorrelated to the continuous formation of a plenty of resistive surfacespecies with cycling and the SEI instability. This in turn demon-

Figure 7. Changes in the interfacial resistance of (a) Rf and (b) Rct, obtainedby fitting the spectra with the electric circuit in Fig. 6a, of bare Sn-Ni/graphiteelectrodes in 1M LiPF6/EC:EMC, and in 1M LiPF6/FEC:DEC without andwith TMP, SnF2-added electrode in FEC-based electrolyte with TMP, after the1st and 50th cycles.

strates that performance enhancement in the FEC-based electrolytewith TMP combined with SnF2-addition is due to the effective sur-face passivation of the electrode during initial cycling and robustnessof the SEI.

Investigation of the SEI composition and formation.— To under-stand what interfacial reaction mechanism leads to the performanceenhancement, surface chemistry studies were conducted using attenu-ated total reflection (ATR) FTIR and XPS. Figure 8 shows IR spectrafor the cycled electrodes, comparing to those of pristine electrodeand FEC-based electrolyte residue. Pristine (Fig. 8a) and all cycled(Fig. 8a (i)-(iv)) electrodes show the peaks at 2965 – 2854 cm−1 and1457 – 1407 cm−1 due to methyl and methylene groups of alkyl(CH3CH2-) functionality from PAA binder and/or newly producedorganic compounds. For bare electrode cycled in the EC-based elec-trolyte (Fig. 8a (i)), strong absorbance peaks at 1500−1400 and 876cm−1 are attributed to υ(CO3)asym, υ(CO3)sym, and δ(CO3) of lithiumcarbonate (Li2CO3), as found as a major SEI component.14,18,31,32 Alsoobserved are alkyl carbonate salt ROCO2Mn+ (M=Li/Sn)14,17,31,32 andorganic phosphorous fluorides OPF3−y(OR)y.17,18,31,32

All of the electrodes (Fig. 8a (ii)-(iv)) cycled in the FEC-basedelectrolyte exhibit two couples of the peaks at 1836-1807 cm−1 and1751–1728 cm−1 in the C=O region. The same spectral feature to theelectrolyte residue of LiPF6:FEC solvate (Fig. 8a) dictates that theyoriginate from electrolyte residue. To identify the surface species de-rived from electrochemically induced changes to an electrode in theFEC-based electrolyte, it is important to identify the spectral featuresassociated with residual electrolyte. We examined the residual elec-trolyte on an stainless steel disk after it was immersed in the electrolyteof 1M LiPF6/FEC:DEC. The IR spectra before washing with DMC,but with varying the drying time at room temperature, are shown inFig. 9a–9d. The spectral features of DEC were not discernable, asit was evaporated by drying. The relative intensity of two strong ab-sorbance peaks at 1836 and 1807 cm−1 is unique to the C=O group ofFEC, similar to the LiPF6:EC solvate. The relatively strong peak near840 cm−1 is attributable to the υ(P-F) from solvated LiPF6, as assignedas the case of EC.33,34 Of particular interest is the gradual growth ofa couple of peaks at 1751 and 1728 cm−1 with drying time, at the ex-

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Figure 8. (a) FTIR, and XPS (b) Sn 3d5/2 and (c) F 1s spectral comparisonof pristine Sn-Ni/graphite electrode and electrolyte residue of LiPF6:FEC, andbare Sn-Ni/graphite electrodes cycled in (i) 1M LiPF6/EC:EMC, and (ii) 1MLiPF6/FEC:DEC, (iii) with TMP and (iv) SnF2-added electrode with TMP.

pense of the intensity of the peaks at 1836 and 1807 cm−1. The grownpeaks are positioned at the C=O spectral region of ester group.32

This implies that the strength of interaction for Li+ · · · O=C ofFEC weakens with drying, as the type of Li+-solvation changes fromC=O of carbonate -CO3- to ester -CO2-. Because of the presence ofelectron-withdrawing group F on one side of the five-membered ringof FEC, the C=O in the other side may be preferred for Li+-solvation.In previous reports on silicon-based anode cycled with FEC additive,those peaks in the C=O region were interpreted differently as origi-nated from the SEI species of polycarbonate that was a decompositionproduct of vinylene carbonate-like intermediate produced after theloss of F from FEC.25,35 Nonetheless, all of the peaks from the elec-trolyte residue on a stainless steel disk disappear after washing withDMC (Fig. 9e). However, at our cycled electrodes (Fig. 8a (ii)-(iv)),those peaks of LiPF6:FEC residue still remain even after washing withDMC. The residue might be trapped between particles or adsorbed atthe particle surface. Remember that porous secondary particles con-sisting of well-connected porous primary nano-particles form in theFEC-based electrolyte (Fig. 5c-5(e′)), contrary to micro-particles ag-gregation in the EC-based one (Fig. 5b). Such a unique morphologicalfeature might induce the electrolyte residue to be trapped/absorbed onthe nano-particles. In addition, the presence of electron-withdrawingF atom in FEC may lead to its surface adsorption to electron-rich Sn-Ni/graphite anode particles. A facile contact between active materialand electrolyte can be advantageous for performance enhancement.

Figure 9. FTIR spectra of the electrolyte residue on stainless steel disk im-mersed in 1M LiPF6/FEC:DEC depending on drying time at room temperature:(a)-(d) before washing and (e) after washing with DMC.

Excluding the peaks of electrolyte residue, the spectra from theFEC-based electrolyte (Fig. 8a (ii)-(iv)) are similar to that from EC-based one (Fig. 8a (i)), except the absence of alkyl carbonate saltROCO2Mn+. Instead, a strong absorbance peak at 1589 cm−1, at-tributed to νsym(C=O) of carboxylate salt RCO2Mn+,18,32 is observedwith a peak near 1150 cm−1 by C-F functionality32 by FEC decom-position. The data indicate that FEC plays a role in providing a facileinterfacial contact, and producing relatively reduced (lower oxygencontent) type of organic compound and C-F containing species.

Surface chemical state of the Sn and F atoms of cycled electrodeswas investigated by performing the XPS analysis. In the Sn 3d5/2 spec-tra (Fig. 8b (i)-(ii)) of the bare electrodes without TMP, no peak isobserved. This indicates a full surface coverage by newly producedsurface species. The observation of LixSn (near 484 eV)36 with TMP(Fig. 8b (iii)-(iv)) is associated with relatively thinner SEI. The SnF2-addition results in the appearance of tiny peaks of SnF2 and SnF4 at486.2 and 487.5 eV,37,38 respectively. In the F 1s spectra, all cycledelectrodes show multiple peaks composed of PFO-containing com-pounds probably organic phosphorus fluorides and/or LiPxFyOz typespecies at 687.1 eV,38,39 SnF4 at 685.2 eV and LiF/SnF2 at 684.4 eV.Correlating to previous findings,14 curve-fitting results (Table I) re-veal that just LiF and PFO exist at the bare electrode in the EC-basedelectrolyte (Fig. 8c (i)). The use of FEC (Fig. 8c (ii)-(iv)) and TMP(Fig. 8c (iii)-(iv)) leads to a notable decrease in the amount of P-F-Ocompounds, confirming the role of TMP in capturing the acidic species(PF5, PF3O), whereas an increase in the amount of SnF2 indicates theFEC as a F-source. By SnF2-addition (Fig. 8c (iv)), SnF4 appearsat the expense of SnF2. The presence of SnF2 and SnF4 that haveelectrophilic Sn should make the electrode resistive to PF-containingLewis acids that are electrophilic as well. No discernable difference

Table I. XPS curve-fitting results of the F 1s peak for the surfaceF atom of Sn-Ni/graphite composite electrodes cycled in differentelectrolyte composition or with added-SnF2.

Electrolyte composition/SnF2-addition

SnF2/LiF(%)

SnF4(%)

P-F-O(%)

1M LiPF6/EC:EMC 79 (LiF only) 0 211M LiPF6/FEC:DEC 85 0 151M LiPF6/FEC:DEC

+ 3 wt% TMP86 0 14

1M LiPF6/FEC:DEC+ 3 wt% TMP, SnF2-added

74 14 12

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Figure 10. (a) Voltage profiles of a SnF2-Ni electrode in 1M LiPF6/FEC:DECwith 3 wt% TMP and (b) FTIR spectral comparison of (i) pristine SnF2-Ni electrode and (ii) the electrode after 50 cycles and (iii) SnF2-added Sn-Ni/graphite composite electrode after 50 cycles in 1M LiPF6/FEC:DEC with3 wt% TMP.

was there in the spectra of C, O, Li, and P atoms (not shown) despitebeing cycled in different electrolyte, consistent with IR data. We be-lieve that a significant performance enhancement and a decrease ininterfacial resistance by the use of FEC along with TMP and added-

SnF2 are ascribed to the effective surface passivation with a robust butrelatively thinner SEI especially composed of fluorinated tin and C-Fcontaining species.

Investigation of the role of SnF2.— To understand the role ofadded-SnF2 in enhancing the cycling performance, surface analysiswas carried out on SnF2-Ni only electrodes (Fig. 10a). Cycled SnF2-Ni electrode (Fig. 10b (ii)) shows exactly the same SEI components tothose of SnF2-added composite electrode (Fig. 10b (iii)), compared topristine (Fig. 10a (i)), indicating that formation of organic compoundsis governed by the interfacial reaction between Sn and FEC.

The F 1s XPS spectrum (Fig. 11a (iii)) for the electrode immersedin the FEC-based electrolyte with TMP (i.e. chemical reaction) revealsthe clear feature of SnF4, compared to pristine (Fig. 11a (i)-(ii)), re-gardless of surface-etching. This confirms that the following chemicalreaction occurs; SnF2 + 2HF → SnF4 + 2H+. The positively charged(electrophilic) Sn of SnF2 can attract and capture the nucleophilicF anions from HF in the electrolyte, forming SnF4, as illustrated inFig. 11b, then, HF is removed. Electrochemical cycling (Fig. 11a (iv))intensifies the formation of SnF4 and LiF/SnF2, because electrochem-ical reduction of LiPF6 can produce more amount of F anions.16

As suggested in the previous report,40 the following conversionreaction occurs on SnF2 during lithiation, producing LiF

SnF2+2Li+ + 2e →← Sn + 2LiF [1]

The SnF2 undergoes a large initial irreversible capacity loss, asrevealed in Fig. 10a. Nevertheless, as the reverse reaction of Eq. 1occurs, the SnF2 also exists at the electrode surface, which is confirmedwith the F 1s spectrum (Fig. 11a (iv)). Although the SnF4 could reactwith lithium

SnF4 + 2Li →← SnF2 + 2LiF [2]

in terms of formation enthalpy the SnF4 phase (-1019 kJ/mol) ismore stable than SnF2 (−508 kJ/mol).41 The SnF2 is composed ofSn4F8 tetramers, in which each Sn is surrounded by a highly distortedoctahedron with the Sn-F bond length ranged from 205 to 329 pm,whereas the Sn in SnF4 has a relatively regular octahedron with muchshorter bond length at 188–202 pm.42 Thus, once the SnF4 forms,it remains. This clarifies the role of SnF2 in capturing the F anionsforming a stable SnF4.

Control of cycling stability with graphite content.— Cycling sta-bility is further improved by tuning the content of graphite. Increasingthe graphite content from 20 to 50 wt% (Fig. 12) leads to a dramatic

Figure 11. (a) F 1s XPS spectral comparison of (i) pristine SnF2-Ni electrode and (ii) after surface-etching, and (iii) after immersing (chemical reaction) and (iv)50 cycles in 1M LiPF6/FEC:DEC with 3 wt% TMP. (b) A schematic illustration for the role of SnF2 as an F anion-capturing agent, forming SnF4.

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A1858 Journal of The Electrochemical Society, 161 (12) A1851-A1859 (2014)

0

200

400

600

800

1000

0 10 20 30 40 50

Cycle number

Dis

cha

rge

cap

aci

ty(m

Ah

g-1

) Theoretical capacity of Sn

Sn-Ni/graphite= 80/20 wt%

50/50 wt%

Figure 12. Cycling stability of SnF2-added Sn-Ni/graphite composite electrodes depending onthe content of graphite (20 and 50 wt%) in 1MLiPF6/FEC:DEC with 3 wt% TMP.

increase of capacity retention from 74 to 90% at the 50th cycle, deliv-ering capacities 832−745 mAhg−1. To date, this is the best capacityretention achieved for micron Sn-based anode. Improved cycling per-formance is ascribed to the synergistic effect of better accommodationof volume change by more amount of graphite,43 in addition to con-trolled interfacial chemistry.

Conclusions

We have reported the controlled electrode–electrolyte interfacialchemistry of micron Sn-Ni/graphite composite by reducing the attackof acidic compounds from electrolyte, while permitting a facile activematerial-electrolyte interfacial contact and formation of a robust SEI,and lowering the interfacial resistances. The synergistic effects of theuse of FEC as an alternative solvent to EC and TMP as an additive ofacid-capturing Lewis base, and the addition of SnF2 to the electrodeas an F anion-capturing agent contribute to the composite electrodeto outperform the SnF2-free electrode in the conventional electrolytethat exhibits a rapid performance fade. Moreover, tuning the contentof graphite enables a promising cycling performance. Our designand control of the interfacial chemistry and a basic understandingof interfacial reaction mechanism are believed to provide a usefulplatform for the performance control of various new battery electrodesthat suffer from interfacial instability.

Acknowledgments

This work was supported by the Ministry of Science, ICT &Future Planning (2013K000214) and by the Ministry of Education(2012026203) of Korea. Authors Sukhyun Hong and Myeong-HoChoo contributed equally to this work.

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