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Ionic-liquid electrolytes can enhance battery performance and safety but are expensive. To reduce the use of ionic-liquid electrolytes, we investigated the charge/discharge properties of Si-based electrodes in an organic-liquid electrolyte, where the electrode surface was pre-coated with a film derived from an ionic-liquid electrolyte. No improvement in the electrode performance was observed compared to that of a nonmodified Si electrode. Once the modified film was broken down, a stable surface film could not be reformed in the organic-liquid electrolyte.

We investigated the impact of electrolyte difference on lithiation and delithiation properties of a Li1.00Si electrode to improve the Coulombic efficiency (CE) of Si-based electrodes. The results of X-ray diffraction, Raman spectroscopy, and soft X-ray emission spectroscopy demonstrated that a portion of the Li in Li1.00Si desorbed by simply immersing the electrode in an ionic-liquid electrolyte, that is, the phase transition of Li1.00Si to Si occurred. In contrast, this phenomenon was not confirmed in an organic-liquid electrolyte. Instead, the desorbed Li was consumed for the formation of a surface film; thus, the Li in Li1.00Si did not elute into the electrolyte. The addition of vinylene carbonate (VC) to the ionic-liquid electrolyte suppressed the phase transition of Li1.00Si to Si. Although the Li1.00Si electrode showed a low initial CE and poor cycling performance in a VC-free electrolyte, the electrode exhibited a high CE and a remarkable cycle life in the VC-added electrolyte. It was considered that no desorption of the mechanically added Li in Li1.00Si contributed to the superior cycle life; thus, the characteristic ductility, malleability, and high electrical conductivity of lithium silicide should improve the electrochemical performance.

Silicides are attractive novel active materials for use in the negative-electrodes of next-generation lithium-ion batteries that use certain ionic-liquid electrolytes; however, the reaction mechanism of the above combination is yet to be clarified. Possible reactions at the silicide electrode are as follows: deposition and dissolution of Li metal on the electrode, lithiation and delithiation of Si, which would result from the phase separation of the silicide, and alloying and dealloying of the silicide with Li. Herein, we examined these possibilities using various analysis methods. The results revealed that the lithiation and delithiation of silicide occurred.

We have previously disclosed that the ionic-liquid electrolyte sodium bis(fluorosulfonyl)amide (NaFSA)/1-methyl-1-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) can significantly improve the cycling stability of Sn4P3 negative electrodes for Na-ion batteries (NIBs). However, the strong electrostatic interaction between Na+ and FSA− in the electrolyte leads to high viscosity and low conductivity. In this study, we have tried to improve the conductivity of the electrolyte and enhance the rate capability of the Sn4P3 electrode by introducing an ether group in the side-chain of the ionic liquid cation to reduce said electrostatic interaction. Ether-substituted ionic liquid 1-methoxymethyl-1-methylpyrrolidinium (PyMOM)-FSA showed higher conductivity than Py13-FSA and the Sn4P3 electrode exhibited a higher rate capability. The differential capacity vs. potential plots suggest that the reaction between Na+ and Sn or P is promoted in the ether-substituted ionic liquid electrolyte. These results demonstrate that introduction of an ether moiety is an effective approach to improve the rate capability of the Sn4P3 electrode in NIBs.

Photoelectrochemical charge–discharge properties of TiO2/MnO2 composite electrodes were investigated in Na2SO4 aqueous solutions by using the photosynthesis-related electrolyte additives of nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP). The photovoltage significantly increased from 230 to 700 mV by adding only NADPH because it efficiently consumes photoexcited holes on TiO2 to enhance electron accumulation. By adding both ATP and NADPH, the photovoltage was further enhanced to 1400 mV. The drastic synergy effect successfully boosted the photoelectrochemical Na+ storage because the enhanced photovoltage of TiO2 could increase the Na+ adsorption amount on MnO2, demonstrating a new photoelectrochemical energy-storage system inspired by the photosynthesis.

We investigated the applicability of an ionic liquid electrolyte to a phosphorus‐doped Si (P‐doped Si) electrode to improve the performance and safety of the lithium‐ion battery. The electrode exhibited excellent cycling performance with a discharge capacity of 1000 mA h g-1 over 1400 cycles in the ionic liquid electrolyte, whereas the capacity decayed at the 170th cycle in the organic electrolyte. The lithiation/delithiation reaction of P‐doped Si occurred a localized region in the organic electrolyte, which generated a high stress and large strain. The strain accumulated under repeated charge‐discharge cycling, leading to severe electrode disintegration. In contrast, the reaction of P‐doped Si proceeded uniformly in the ionic liquid electrolyte, which suppressed the electrode disintegration. The P‐doped Si electrode also showed good rate performance in the ionic liquid electrolyte; a discharge capacity of 1000 mA h g-1 was retained at 10 C.

1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide (EMI-TFSA) is one of the promising ionic liquids as electrolyte solvent to enhance the electrochemical performance of Si electrode for Li-ion batteries (LIBs) because of its low viscosity and high conductivity. However, it has low stability against reduction and its reductive decomposition product inhibits Li+ insertion to electrodes, leading to poor cycling stability. To exert a potential of EMI-TFSA, we employed vinylene carbonate (VC) as film-forming additive. Si electrode exhibited very high cycling stability and rate capability in 20 vol.% VC-added EMI-TFSA-based electrolyte. In addition, by replacing TFSA anion with bis(fluorosulfonyl)amide (FSA) for Li salt and ionic liquid solvent, an excellent cycling performance and outstanding rate capability was achieved. VC cannot only fabricate a good surface film but also lower the interaction between Li+ and FSA-, providing smooth desolvation of FSA- to obtain better high-rate performance. Non-flammability of the VC-added electrolytes was confirmed by fire resistance test in closed-system: no ignition was observed even at 300°C. Consequently, we found that mixture electrolyte consisted of EMI-based ionic liquid and VC, especially 1 M LiFSA/EMI-FSA with 20 vol.% VC, is a prospective candidate for simultaneously enhancing the electrochemical performance of Si electrode as well as safety of LIBs.

To achieve electrode performance with both high capacity and long cycle life, we investigated the effect of the anion structure in an ionic liquid electrolyte on the electrochemical performance of an annealed Ni-P/(etched Si) negative electrode for Li-ion batteries. The electrode maintained a discharge capacity of 1890 mA h g-1 after 250 cycles in bis(fluorosulfonyl)amide-based ionic liquid electrolyte, which was approximately three times higher than that in bis(trifluoromethanesulfonyl)amide-based electrolyte.

We investigated the lithiation and delithiation properties of pure binary silicide electrodes in an ionic liquid electrolyte as novel anodes for lithium‐ion batteries. Some electrodes maintain a high reversible capacity in the electrolyte, whereas they show a poor cycling performance in an organic electrolyte. The superior performance results from the high affinity for the transition metal that composes the silicide with Li. Based on reaction behavior analysis, the crystal structure of silicide is maintained during the cycling, and phase separation does not occur. The ionic liquid electrolyte suppresses the formation of cracks and exfoliation of the silicide layer from a substrate. In addition, a surface film formed on the silicide electrode through the reductive decomposition of the electrolyte has different components than that on a Si electrode, even in the same ionic liquid electrolyte. Soft X‐ray emission spectroscopy demonstrates that the pure silicide itself reacts with Li. The obtained results will provide significant insights into novel alloy‐based anode materials for lithium‐ion batteries.

Charge-discharge performances of Sn4P3 anodes for Na‐ion battery were evaluated in an ionic liquid electrolyte using N‐methyl‐N‐propylpyrrolidinium bis(fluorosulfonyl)amide at intermediate temperatures of 60 and 90 oC. At these temperatures, the anode showed extra capacities based on the full sodiation of Sn in a potential range below 0.2 V vs. Na+/Na because its slow kinetics was improved by elevating operation temperature. Under the current density of 0.1 A g-1 (0.08 C), the Sn4P3 anode at 60 oC exhibited a large capacity of 750 mA h g-1 at the 120th cycle and high Coulombic efficiencies above 99% after the 5th cycle. On the other hand, the efficiency degraded at 90 oC by the electrolyte decomposition. At 60 oC, the anode attained an excellent rate performance with capacity of 250 mA h g-1 even at 3 A g-1 (2.65 C). These results demonstrated the promising operation at intermediate temperature at around 60 oC for Sn4P3 anode in ionic liquid electrolyte.

We prepared composite electrodes by using rutile TiO 2 particles and γ-MnO 2 particles, and evaluated their photoelectrochemical capacitor properties based on Na + adsorption by light irradiation in aqueous electrolytes. By employing different synthesis method for TiO 2 particles, we synthesized TiO 2 particles with various particle sizes and crystallite sizes. An electrode of sol–gel-synthesized TiO 2 showed higher photovoltages compared with an electrode of commercial TiO 2 . This probably originates from a larger contact area between electrode surface and electrolyte because of its smaller particle size than commercial TiO 2 's size. A further enhancement in photovoltage was attained for an electrode of a hydrothermally-synthesized TiO 2 with good crystallinity. We consider that electron−hole recombination was suppressed because hydrothermal TiO 2 has a lower density of lattice defect trapping the photoexcited carriers. As photoelectrochemical capacitor, a composite electrode consisting of hydrothermal TiO 2 and MnO 2 exhibited a 2.4 times larger discharge capacity compared with that of commercial TiO 2 and MnO 2 . This result is attributed to an increased amount of Na + adsorption induced by the enhanced photovoltage of TiO 2 .

Concentration changes of electrolyte salt in practical lithium ion batteries occur due to various factors during operation, and the changes causes serious degradation of battery performance. It is important to identify elementary factors and how each of the factors induces the concentration changes in batteries by using a simplified system. The concentration of ions in the electrolyte solution between a highly oriented pyrolytic graphite (HOPG) electrode and a lithium foil electrode in a model battery was studied during charge/discharge cycles by in situ microprobe Raman spectroscopy. The concentration of ions decreased during de-intercalation of Li+ from HOPG. The decreased concentration recovered during subsequent rest time with a time constant of several hours. The results of impedance spectroscopy showed that the cycle dependence of the resistance of the surface film on the graphite was similar to that of the time constant. On the other hand, the cycle dependence of the capacitance at the graphite surface and the cycle dependence of the charge transfer resistance at the graphite surface were similar to that of the degree of concentration change. The results suggest that the resistance of the surface film can be evaluated from the rate of the recovery.

Excellent cycling performance of an electrode composed of silicon alone was achieved in a bis(fluorosulfonyl)amide (FSA)‐based electrolyte, with a high discharge capacity of 950 mA h g−1 observed even at the 500th cycle. To elucidate the reaction behavior of the Si electrode in an FSA‐based ionic liquid electrolyte, we investigated the change in the cross‐sectional morphology of the Si‐active material layer, the distribution of Li in the layer, and the crystallinity of Si on the electrode surface. By cross‐sectional scanning electron microscopy, we confirmed that the electrode thickness increased with the cycle number. The increase in thickness was less noticeable in the FSA‐based electrolyte than in an organic electrolyte. An elemental analysis of the electrode material revealed that a film derived from the electrolyte was formed not only on the surface but also inside of the electrode. Soft X‐ray emission spectroscopy demonstrated that the distribution of Li in the FSA‐based electrolyte was more uniform for the cross‐section of the cycled electrode compared to that in an organic electrolyte. The results of Raman spectroscopy indicated that domains of amorphous Si were homogeneously distributed on the electrode surface in the FSA‐based electrolyte. The uniform distribution of the lithiation−delithiation reaction should help to suppress disintegration of the active material layer.

Lithium ion batteries have attracted much attention due to their high power density. The change in concentration of salt in the electrolyte solution and local shortage of electrolyte solution in batteries cause serious degradation of battery performance. In this work, in situ Raman spectroscopy of the electrolyte solution at different positions in a laminate lithium ion battery (a typical practical battery) was simultaneously conducted by using ultrafine multi-probes. Eight probes were aligned in deep narrow spaces between two electrodes at intervals of about 2.5 mm in a plane parallel to the surfaces of electrodes. The concentration changed differently at the positions during charging and discharging. In addition, local dry up and local refilling of the electrolyte solution were observed. These phenomena were sometimes observed at the same time at the positions of two adjacent probes, indicating that the phenomena occurred in a millimeter scale. The method used in this study is useful for in situ analysis of the electrolyte solution in deep narrow spaces in other electrochemical devices under conditions close to those in practical devices.

We evaluated the charge–discharge performance of a Sn4P3 negative electrode in an ionic liquid electrolyte comprised of N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) and NaFSA. We also conducted cyclic voltammetry and transmission electron microscopy for the Sn4P3 electrode to reveal the reaction mechanism. It was suggested that Na15Sn4 and Na3P are formed via phase separation in the first sodiation and that elemental Sn and elemental P formed by following a desodiation reaction with Na ions in the subsequent cycles. The Sn4P3 electrode exhibited a high Coulombic efficiency of 99.1% at the fourth cycle and an excellent cycling performance with a high reversible capacity of 750 mA h g–1 even at the 200th cycle. We demonstrated that there are two important factors to improve the performance: (i) higher volume fraction of Sn than P and (ii) uniform dispersion of Sn nanoparticles in a P matrix. The ionic liquid electrolyte showed good applicability to the Sn4P3 negative electrode due to i...

The concentration of ions in the electrolyte solution in lithium ion batteries changes during operation, reflecting the resistance to ion migration and the positions of diffusion barriers. The change causes various negative effects on the performance of batteries. Thus, it is important to elucidate how the concentration changes during operation. In this work, the concentration change of ions in the electrolyte solution in deep narrow spaces in a realistic battery was studied by in situ ultrafine microprobe Raman spectroscopy. Graphite composite and LiCoO 2 composite electrodes, which are the most commonly used electrodes in practical batteries, were placed facing each other and their distance was set to 80 μm, which is close to the distance between electrodes in practical batteries. After repeated charge/discharge cycles, the concentration of ions increased and decreased greatly during charging and discharging, respectively. The maximum concentration was more than three-times higher than the minimum concentration. The rate of changes in concentration increased almost linearly with increase in current density. The results have important implications about concentration changes of ions occurring in practical batteries.

Changes in the concentration of ions in the electrolyte solution between electrodes in a lithium ion battery were studied by in situ multi-microprobe Raman spectroscopy. The distance between the two electrodes was set to 190 μm. Six separator films, each with a thickness of 25 μm, were inserted between the two electrodes, and probes were inserted between the separator films at the anode side, the middle position and the cathode side. After repeated charge/discharge cycles, the concentration of ions increased and decreased during charging and discharging, respectively. Such concentration changes first started to occur at the anode side and then occurred at the middle position and finally at the cathode side. The results suggest complexity of changes in the concentration of ions in separator films with micropores in practical batteries. Keywords: Lithium ion battery, In situ analysis, Raman spectroscopy, Electrolyte, Separator, Solid electrolyte interphase

Lithium-ion batteries have attracted considerable attention due to their high power density. The change in concentration of salt in the electrolyte solution in lithium-ion batteries during operation causes serious degradation of battery performance. Herein, a new method of in situ Raman spectroscopy with ultrafine multifiber probes was developed to simultaneously study the concentrations of ions at several different positions in the electrolyte solution in deep narrow spaces between the electrodes in batteries. The total amount of ions in the electrolyte solution clearly changed during operation due to the low permeability of the solid-electrolyte interphase (SEI) at the anode for Li+ permeation. The permeability, which is a key factor to achieve high battery performance, was improved (enhanced) by adding film-forming additives to the electrolyte solution to modify the properties of the SEI. The results provide important information for understanding and predicting phenomena occurring in a battery and for designing a superior battery. The present method is useful for analysis in deep narrow spaces in other electrochemical devices, such as capacitors.

We investigated the influence of the anions in ionic liquid electrolytes on the electrochemical performance of a silicon (Si) negative electrode for a lithium-ion battery. While the electrode exhibited poor cycle stability in tetrafluoroborate-based and propylene carbonate-based electrolytes, better cycle performance was achieved in bis(fluorosulfonyl)amide (FSA – )- and bis(trifluoromethanesulfonyl)amide (TFSA – )-based electrolytes, in which the discharge capacity of a Si electrode was more than 1000 mA h g −1 at the 100th cycle. It is considered that a surface film derived from FSA – - and TFSA – -based electrolytes effectively suppressed continuous decomposition of the electrolyte. In a capacity limitation test, a discharge capacity of 1000 mA h g −1 was maintained even after about the 1600th cycle in the FSA – -based electrolyte, which corresponds to a cycle life almost twice as long as that in TFSA – -based electrolyte. This result should be explained by the high structural stability of FSA – -derived surface film. In addition, better rate capability with a discharge capacity of 700 mA h g −1 was obtained at a high current rate of 6 C (21 A g −1 ) in FSA – -based electrolyte, which was 7-fold higher than that in TFSA – -based electrolyte. These results clarified that FSA – -based ionic liquid electrolyte is the most promising candidate for Si-based negative electrodes.

The anode properties of Nb-doped rutile TiO 2 electrodes were investigated in an ionic liquid electrolyte comprised of N -methyl- N -propylpyrrolidinium cation and bis(fluorosulfonyl)amide anion for use in a safe Na-ion battery. Although the electrolyte's conductivity was lower than that of a conventional organic electrolyte at 30 °C, it showed high conductivity comparable to that of the organic electrolyte at 60 °C. The Nb-doped TiO 2 electrode showed excellent cyclability in the ionic liquid electrolyte at 60 °C: a high capacity retention of 97% was observed even at the 350th cycle, which is comparable to value in the organic electrolyte (91%). In a non-flammability test in a closed system, no ignition was observed with the ionic liquid electrolyte even at 300 °C. These results indicate that combination of a Nb-doped TiO 2 anode and ionic liquid electrolyte gives not only an excellent cyclability but also high safety for a Na-ion battery operating at a temperature below the sodium's melting point of 98 °C.

Developing lithium-ion batteries (LIBs) with a high energy density, long cycle, and high safety is essential for establishing a sustainable society because LIBs are used as electric vehicle batteries and stationary batteries for utilizing renewables. Silicon (Si) has great potential as an anode active material for next-generation LIBs due to its high theoretical capacity (3580 mA h g- 1 for Li15Si4). However, Si electrodes show poor cycle stability, which is mainly caused by a large volume change in Si during the lithiation (charge) and delithiation (discharge) reactions. Additionally, Si has other disadvantages, such as high electrical resistivity and a low Li+ diffusion coefficient. We have previously investigated the lithiation and delithiation properties of pure binary silicide (M ySiz, M : transition metal) electrodes. Some electrodes maintained a high reversible capacity in an ionic-liquid electrolyte, whereas they showed a poor cycling performance in a conventional organic-liquid electrolyte. In contrast, the effect of difference in crystal phase of silicides composed of the same elements on the lithiation and delithiation properties has not yet been clarified. Herein, we synthesized various pure nickel silicide (Ni-Si) powders. and investigated their lithiation and delithiation properties in an ionic-liquid electrolyte. Ni-Si powders (NiSi2, NiSi, Ni2Si, and Ni3Si) were synthesized by a mechanical alloying (MA) method. The obtained powders were characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD patterns were identified compared with patterns in the Inorganic Crystal Structure Database (ICSD). Each silicide electrode was prepared by a gas deposition (GD) method, which does not require a binder or conductive agent. We assembled 2032-type coin cells, which consisted of the silicide electrode as the working electrode, Li metal foil as the counter electrode and a glass fiber filter as the separator. 1 mol dm- 3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)amide (Py13-FSA) was used as an ionic-liquid electrolyte. A galvanostatic charge-discharge test was carried out in the potential range between 0.005 and 2.000 V vs. Li+/Li at 303 K. The current density was set at 50 mA g- 1. To discuss the charge density of each element in Li x Ni y Si z , a first-principle calculation based on density functional theory (DFT) was performed using the projector augmented wave (PAW) method as implemented in the plane wave code of the Vienna Ab initio Simulation Package (VASP). A generalized gradient approximation (GGA) was used as the term exchange correlation with a kinetic cutoff of 350 eV. Brillouin zone sampling was performed with an 8×8×8 k point mesh within a Gamma point centered mesh scheme. We synthesized various Ni-Si powders by a MA method. Comparing the XRD pattern of the prepared samples and the corresponding ICSD pattern, all peaks were assigned to pure Ni-Si phase. Additionally, there were no peaks arising from the raw materials (Ni and Si). In Raman spectra, peaks assigned to crystalline Si (c-Si) and amorphous Si (a-Si) appear at approximately 520 and 490 cm- 1, respectively. Each silicide also gives no Raman peaks of c-Si and/or a-Si which indicates that neither c-Si nor a-Si is included in the synthesized powders. Therefore, the MA treatment successfully produced a pure Ni-Si phase. Fig.1 shows the dependence of the gravimetric discharge capacity of various nickel silicide electrodes on the cycle number in 1 M LiFSA/Py13-FSA.The NiSi2 electrode exhibited the highest initial capacity of approximately 800 mA h g- 1, and the Ni3Si electrode showed the second highest capacity. The results demonstrated that there was no correlation between initial capacity and silicon content in Ni-Si. To reveal the difference in the initial capacity, we determined the charge density of each element in Li x Ni y Si z based on computational chemistry. Li and Si have positive charges in all silicides, whereas Ni has a negative charge in Li x Ni y Si z . These results indicate that Li has a high affinity with Ni in Li x Ni y Si z . In addition, it is suggested that the higher the affinity of Ni with Li is, the higher the initial capacity is. Furthermore, we investigated the distance between Li and close atoms (Ni or Si) in Li x Ni y Si z . The initial capacity increased with an increase in the distance. Therefore, both the affinity of Ni with Li and the distance between Li and neighboring atom should influence on the initial capacity. Figure 1

The degradation mechanism of a graphite negative-electrode in LiPF6-based electrolyte solution was investigated using the basal plane of highly oriented pyrolytic graphite (HOPG) as a model electrode. Changes in the surface morphology were observed by in situ atomic force microscopy. In the initial cathodic scan, a number of pits appeared at around 1.75 V vs. Li(+)/Li, and fine particles formed on the terrace of the HOPG basal plane at about 1.5 V vs. Li(+)/Li. The fine particles were characterized by spectroscopic analysis, such as X-ray photoelectron spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy. We added one of the components to LiClO4-based electrolyte solution, and successfully reproduced the formation of pits and fine particles on the basal plane of HOPG. Based on these results, the formation mechanisms of pits and fine particle layers were proposed.

A GexSi1−x alloy electrode is useful for addressing the shortcomings of a Si negative electrode for lithium-ion batteries. To further improve the electrochemical performance of a GexSi1−x negative electrode, a film-forming additive and the formation of a composite with LaSi2 were applied. A Ge0.1Si0.9 electrode exhibited better cyclability in the additive-containing electrolyte with a discharge capacity of 1240 mA h g−1 at the 400th cycle. In addition, a Ge0.1Si0.9/LaSi2 composite electrode showed better cycle performance than a Ge0.1Si0.9 electrode.

Article, Journal of The Electrochemical Society. 167(7): 174101 (2019)

Elemental Silicon (Si) has a high theoretical capacity of 3580 mA h g-1 and has attracted attention as an active material of negative electrode for high energy density lithium-ion batteries. However, it shows poor cycle stability, which is mainly due to a massive volume change in Si during lithiation and delithiation. Here, we report that combination an ionic liquid electrolyte with a charge capacity limit of 1000 mA h g-1 significantly suppresses Si volume expansion, improving the cycle life. On the other hand, the Si layer expands largely in an organic electrolyte even with the charge capacity limit and even in an ionic-liquid electrolyte without the limit. We demonstrated that the homogeneously distributed Si lithiation−delithiation, phase-transition control from the Si to Li-rich Li-Si alloy phases, and formation of a surface film with structural and/or mechanical stability contribute to suppressing Si volume expansion.1 A Si working electrode was fabricated by gas-deposition method, which is without any binder and conductive agent. A 2032-type coin cell was constructed, comprising a Si electrode, a glass fiber filter as the separator, and a Li metal sheet as the counter electrode. An ionic liquid electrolyte used was 1 mol dm-3 (M) lithium bis(fluorosulfonyl)amide (LiFSA) dissolved in N-methyl-N-propylpyrrodinum bis(fluorosulfonyl)amide (Py13-FSA). For comparison, 1 M lithium bis(trifluoromethanesulfonyl)amide (LiFSA) dissolved in propylene carbonate (PC) was employed as a conventional organic electrolyte. Galvanostatic charge-discharge testing was performed with a charge capacity limit of 1000 mA h g-1 unless stated otherwise. The current density was set at 0.36 A g-1 (0.1 C) during the first cycle and 1.44 A g-1 (0.4 C) during subsequent cycles. An electrode cross section was observed by field emission scanning electron microscopy (FE-SEM). The cross section was fabricated using a cross-section polisher or focused ion beam. The electrode was not exposed to the atmosphere until it was introduced into the chamber of the FE-SEM using a transfer vessel. A reversible capacity of the Si electrode decayed at around the100th cycle in the organic electrolyte (1 M LiTFSA/PC). Conversely, in the ionic-liquid electrolyte (1 M LiFSA/Py13-FSA), the Si electrode exhibited a better cycle life with a reversible capacity of 1000 mA h g-1 after the 600th cycle. To determine the difference between the cycle lives of Si electrode in organic and ionic-liquid electrolytes, we investigated the thickness of the lithiated Si active material layers by FE-SEM. The thickness of the nonlithiated Si layer before charge-discharge testing was 1.6 ± 0.3 mm.2 In the organic electrolyte, the Si layer expnads with the cycle number, developing several cracks and becoming porous after the 50th and 100th cycles. The thickness reached ca. 25 mm after the 100th cycle when the capacity faded. The amorphous Li1.0Si (a-Li1.0Si) phase may mainly form with a charge capacity limit of 1000 mA h g-1 because it has a theoretical capacity of 950 mA h g-1. The expansion rate of the Si layer was about 1460%, whereas the calculated rate of increase in thickness from Si to a-Li1.0Si phases is 17%. In the ionic-liquid electrolyte, the Si electrode retained a thickness of 2.7 mm over 300 cycles, with an expansion rate of ca. 69%. Unexpectedly, the Si layer did not expand significantly after repeated cycling. There are no reports on the significant suppression of Si volume expansion after such long-term cycling. However, the expansion rate of 69% is still above the calculated rate of 17% for the a-Li1.0Si phase. After the 600th cycle and just before capacity fading, the layer thickness increased to 13.3 mm with some void and crack formation inside the layer. We discuss differences in the performances of the Si-alone electrode in the organic and ionic-liquid electrolytes based on the Li storage distribution, phase transition, and surface film formation. Reference s : [1] Y. Domi, H. Usui, K. Yamaguchi, S. Yodoya, H. Sakaguchi, ACS Appl. Mater. Interfaces,.2019, in press. [2] K. Yamaguchi, Y. Domi, H. Usui, H. Sakaguchi, ChemElectroChem, 2017, 4, 3257-3263.

Structural changes in the surface of graphite composite electrodes in the 1st charge/discharge cycle were studied by ex situ and in situ Raman spectroscopy. The influence of the salt in the electrolyte solution and the volume of this solution injected into electrochemical cells on the structural degradation of the graphite surface was investigated. The surface crystallinity of graphite was degraded in LiPF6-based electrolyte solution with only a cycle of intercalation/de-intercalation reactions of Li+, compared to that in a LiClO4-based electrolyte. In situ Raman measurements revealed that structural disordering of the graphite surface should occur in an early stage of the initial intercalation reaction of Li+ into graphite.

Structural changes in the surface of edge plane highly oriented pyrolytic graphite (HOPG) electrodes were studied in ethylene carbonate (EC)-based electrolytes by in situ Raman spectroscopy. The Raman spectra revealed that the surface crystallinity of graphite was significantly lowered by the initial intercalation and de-intercalation reactions of Li+. This structural degradation resulted in a sluggish stage transition of Li-GIC in the vicinity of the edge plane in the subsequent potential cycle. On the other hand, when the film-forming additive vinylene carbonate was used in the EC-based electrolyte solution, the crystallinity of the edge plane HOPG was maintained even after potential cycling. In addition, the phase transition of Li-GIC during the 2nd potential cycle proceeded in the same manner as in the initial cycle. Based on the present results, we discuss the suppressive role of film-forming additives on the degradation of the surface structure as it relates to the intercalation mechanism of Li+.

Changes in the surface morphology of the edge planes of graphite during a potential sweep were studied using highly oriented pyrolytic graphite (HOPG) in an ethylene carbonate (EC) + diethyl carbonate (DEC)-based electrolyte solution by in situ atomic force microscopy (AFM). The effects of the microscopic structures of graphite, i.e., edge and basal planes, on surface film formation are discussed. The formation of fine particles and precipitates was observed depending on the electrode potential between 1.0 and 0 V. These were considered to be remnants of blisters that could be observed at the basal plane and decomposition products of the electrolyte solution. The surface films were 56 and 66 nm thick after the first and second cycles, respectively. The precipitate layer formed on the edge plane was thinner than that observed on the basal plane after the second cycle. These results enabled us to elucidate the difference in the formation of surface films on the edge and basal planes of HOPG.

The concentration change of ions in the electrolyte solution in deep narrow spaces between electrodes in batteries was studied by in situ multi-probe Raman spectroscopy. When two separator films were placed at the anode and cathode sides, the concentration change became greater, suggesting that the resistance for ion migration at the anode side increased more than that at the cathode side. Thus, there seems to be a concerted effect of the surface film at the anode [solid electrolyte interphase (SEI)] and the adjacent separator film to form an effective diffusion barrier for Li+.