Kinetic analysis of silicon-lithium alloying reaction in silicon single crystal using soft X-ray absorption spectroscopy.

Silicon has been considered to be one of the most promising anode active materials for next-generation lithium-ion batteries due to its large theoretical capacity (4200 mA h g-1, Li22Si5). However, silicon anodes suffer from degradation due to large volume expansion and contraction. To control the ideal particle morphology, an experimental method is required to analyze anisotropic diffusion and surface reaction phenomena. This study investigates the anisotropy of the silicon-lithium alloying reaction using electrochemical measurements and Si K-edge X-ray absorption spectroscopy on silicon single crystals. During the electrochemical reduction process in lithium-ion battery systems, the continuous formation of solid electrolyte interphase (SEI) films prevents the achievement of steady-state conditions. Instead, the physical contact between silicon single crystals and lithium metals can prevent the effect of SEI formation. The apparent diffusion coefficient and the surface reaction coefficient are determined from the progress of the alloying reaction analyzed by X-ray absorption spectroscopy. While the apparent diffusion coefficients show no clear anisotropy, the apparent surface reaction coefficient of Si (100) is more significant than that of Si (111). This finding indicates that the surface reaction of silicon governs the anisotropy of practical lithium alloying reaction for silicon anodes.

Adaptive Cation Pillar Effects Achieving High Capacity in Li-Rich Layered Oxide, Li2 MnO3 -LiMeO2 (Me = Ni, Co, Mn).

Intensive research is underway to further enhance the performance of lithium-ion batteries (LIBs). To increase the capacity of positive electrode materials, Li-rich layered oxides (LLO) are attracting attention but have not yet been put to practical use. The structural mechanisms through which LLO materials exhibit higher capacity than conventional materials remain unclear because their disordered phases make it difficult to obtain structural information by conventional analysis. The X-ray total scattering analysis reveals a disordered structure consisting of metal ions in octahedral and tetrahedral sites of Li layers as a result of cation mixing after the extraction of Li ions. Metal ions in octahedral sites act as rigid pillars. The metal ions move to the tetrahedral site of the Li layer, which functions as a Li-layer pillar during Li extraction, and returns to the metal site during Li insertion, facilitating Li diffusion as an adaptive pillar. Adaptive pillars are the specific structural features that differ from those of the conventional layered materials, and their effects are responsible for the high capacity of LLO materials. An essential understanding of the pillar effects will contribute to design guidelines for intercalation-type positive electrodes for next-generation LIBs.

Anion Redox in an Amorphous Titanium Polysulfide.

Amorphous transition-metal polysulfides are promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of their high theoretical capacities. In this study, sulfur anion redox during lithiation of amorphous TiS4 (a-TiS4) was investigated by using experimental and theoretical methods. It was found that a-TiS4 has a variety of sulfur valence states such as S2-, S-, and Sδ-. The S2- species became the main component in the Li4TiS4 composition, indicating that sulfur is a redox-active element up to this composition. The simulated a-TiS4 structure changed gradually by lithium accommodation to form a-Li4TiS4: S-S bonds in the disulfide units and polysulfide chains were broken. Bader charge analysis suggested that the average S valency decreased drastically. Moreover, deep lithiation of a-TiS4 provided a conversion reaction to metallic Ti and Li2S, with a high practical capacity of ∼1000 mAh g-1 when a lower cutoff voltage was applied.

Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li1.2Ni0.2Mn0.6O2 during charge-discharge cycles.

Lithium- and manganese-rich layered oxides (LMRs) are promising positive electrode materials for next-generation rechargeable lithium-ion batteries. Herein, the structural evolution of Li1.2Ni0.2Mn0.6O2 during the initial charge-discharge cycle was examined using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy to elucidate the unique delithiation behavior. The pristine material contained a composite layered structure composed of Ni-free and Ni-doped Li2MnO3 and LiMO2 (M = Ni, Mn) nanoscale domains, and Li ions were sequentially and inhomogeneously extracted from the composite structure. Delithiation from the LiMO2 domain was observed in the potential slope region associated with the Ni2+/Ni4+ redox couple. Li ions were then extracted from the Li2MnO3 domain during the potential plateau and remained mostly in the Ni-doped Li2MnO3 domain at 4.8 V. In addition, structural transformation into a spinel-like phase was partly observed, which is associated with oxygen loss and cation migration within the Li2MnO3 domain. During Li intercalation, cation remigration and mixing resulted in a domainless layered structure with a chemical composition similar to that of LiNi0.25Mn0.75O2. After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.

Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance.

Conversion-type iron trifluoride (FeF3) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF3 operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF3 electrode material. Bulk analyses of FeF3 upon cycling reveal incomplete oxidation to Fe3+ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF3 reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF3 shows better performance in an electrolyte comprising LiBF4 solute in cyclic carbonate solvents than in chain carbonate-containing LiPF6 as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF3 electrode.

Structural characterization of an amorphous VS4 and its lithiation/delithiation behavior studied by solid-state NMR spectroscopy.

Vanadium sulfide (VS4) is one of the promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of its high theoretical capacity (1196 mA h g-1). Crystalline VS4 has a unique structure, in which the Peierls-distorted one-dimensional chains of V-V bonds along the c axis are loosely connected to each other through van der Waals interactions. In this study, an amorphous VS4 is prepared by mechanical milling of the crystalline material, and its lithiation/delithiation behavior is investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy. The amorphous VS4 shows a chain structure similar to that of crystalline VS4. The amorphous host structure is found to change drastically during the lithiation process to form Li3VS4: the V ions become tetrahedrally coordinated by S ions, in which the valence states of V and S ions simultaneously change from V4+ to V5+ and S- to S2-, respectively. When the Li insertion proceeds further, the valence state of V ions is reduced. After the 1st cycle, the amorphous VS4 recovers to the chain-like structure although it is highly disordered. No conversion to elemental V is observed, and a high capacity of 700 mA h g-1 is reversibly delivered between 1.5 and 2.6 V.

Structural Understanding of Superior Battery Properties of Partially Ni-Doped Li2MnO3 as Cathode Material.

We examined the crystal structures of Li2(NixMn1-x)O3(-δ) (x = 0, 1/10, 1/6, and 1/4) to elucidate the relationship between the structure and electrochemical performance of the compounds using neutron and synchrotron X-ray powder diffraction analyses in combination. Our examination revealed that these crystals contain a large number of stacking faults and exhibit significant cation mixing in the transition-metal layers; the cation mixing becomes significant with an increase in the Ni concentration. Charge-discharge measurements showed that the replacement of Mn with Ni lowers the potential of the charge plateau and leads to higher charge-discharge capacities. From a topological point of view with regard to the atomic arrangement in the crystals, it is concluded that substituting Mn in Li2MnO3 with Ni promotes the formation of smooth Li percolation paths, thus increasing the number of active Li ions and improving the charge-discharge capacity.

Structural and electronic features of binary Li2S-P2S5 glasses.

The atomic and electronic structures of binary Li2S-P2S5 glasses used as solid electrolytes are modeled by a combination of density functional theory (DFT) and reverse Monte Carlo (RMC) simulation using synchrotron X-ray diffraction, neutron diffraction, and Raman spectroscopy data. The ratio of PSx polyhedral anions based on the Raman spectroscopic results is reflected in the glassy structures of the 67Li2S-33P2S5, 70Li2S-30P2S5, and 75Li2S-25P2S5 glasses, and the plausible structures represent the lithium ion distributions around them. It is found that the edge sharing between PSx and LiSy polyhedra increases at a high Li2S content, and the free volume around PSx polyhedra decreases. It is conjectured that Li(+) ions around the face of PSx polyhedra are clearly affected by the polarization of anions. The electronic structure of the DFT/RMC model suggests that the electron transfer between the P ion and the bridging sulfur (BS) ion weakens the positive charge of the P ion in the P2S7 anions. The P2S7 anions of the weak electrostatic repulsion would causes it to more strongly attract Li(+) ions than the PS4 and P2S6 anions, and suppress the lithium ionic conduction. Thus, the control of the edge sharing between PSx and LiSy polyhedra without the electron transfer between the P ion and the BS ion is expected to facilitate lithium ionic conduction in the above solid electrolytes.

Local structure of magnesium in silicate glasses: a 25Mg 3QMAS NMR study.

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

Multiple Ca(2+) environments in silicate glasses by high-resolution (43)Ca MQMAS NMR technique at high and ultra-high (21.8 T) magnetic fields.

We here show the (43)Ca 5QMAS NMR spectra at high field (16.4 T) and the first 7QMAS spectrum at ultra-high field (21.8 T) for geologically important Ca-containing glasses. The high-resolution MQMAS spectra present a clear evidence of multiple Ca sites in the amorphous structures that have never been identified by other analytical methods. The present study suggests that the Ca(2+) ions are mainly in 7- and 8-fold coordination sites. This will offer valuable insights for dynamic properties of magmatic liquids. The MQMAS NMR technique at high magnetic field is a unique tool to understand the detailed structural information on a specific element in solids including organic and inorganic compounds.

Molecular dynamics study of CaSiO(3)-MgSiO(3) glasses under high pressure.

The pressure-induced structural evolutions of CaSiO(3)-MgSiO(3) glasses have been examined by means of molecular dynamics simulation. Our calculations revealed that Si coordination remained unchanged up to 15 GPa, while modifier cations caused significant changes in the short-range order structure. In the present study, we conclude that the main compression mechanisms for CaSiO(3)-MgSiO(3) glasses are: (1) the Si-O-Si angle reduction, (2) the coordination increase of Ca and Mg cations, and (3) the compaction in the medium-range scale. Furthermore, small changes in the Q(n) distribution suggest pressure-induced disproportionation reactions. Similar pressure responses between CaSiO(3)-MgSiO(3) glasses may imply that the structural changes of SiO(4) framework units are more significant than those of interstitial cations, Ca and Mg.