P2S5 Glass Research Articles

Formation Processes of a Solid Electrolyte Interphase at a Silicon/Sulfide Electrolyte Interface in a Model All-Solid-State Li-Ion Battery.

Understanding the electrochemical reactions at the interface between a Si anode and a solid sulfide electrolyte is essential in improving the cycle stabilities of Si anodes in all-solid-state batteries (ASSBs). Highly dense Si films with very low roughnesses of <1 nm were fabricated at room temperature via cathodic arc plasma deposition, which led to the formation of a Si/sulfide electrolyte model interface. Li (de)alloying through the model interface hardly occurred during the first cycle, whereas it proceeded stably in subsequent cycles. Hard X-ray photoelectron spectroscopy and neutron reflectometry directly revealed that the reduction or oxidation of the interfacial component or Li3PS4 electrolyte occurred during the first cycle. Consequently, an interfacial layer with a thickness of 13 nm and primarily composed of Li2S, SiS2, and P2S5 glasses was formed during the first cycle. The interfacial layer acted as a Li-conductive, electron-insulating solid electrolyte interphase (SEI) that provided reversible (de)lithiation. Our model interface directly demonstrates the electrochemical reaction processes at the Si/Li3PS4 interface and provides insights into the structures and electrochemical properties of SEIs to activate the (de)lithiation of Si anodes using a sulfide electrolyte.

Mitigating Chemo-Mechanical Degradation in Sulfide-Based All-Solid-State Batteries Comprising Sulfur Composite Cathode and Li–Si Alloy Anode

In this study, all-solid-state batteries consisting of an Li2S5–P2S5 (LPS) glass electrolyte, a sulfur/LPS glass/carbon composite cathode, and an Li–Si anode are prepared and tested under high cut-off voltage conditions (0.5–3.7 V) to elucidate the effect of LPS redox activity on the chemo-mechanical stability of LPS-based cells. The cell cycled in the voltage range 0.5–3.7 V exhibits unstable charging behaviors and voltage noise, with substantial LPS oxidative decomposition. X-ray photoelectron spectroscopy and impedance spectra analyses of a three-electrode system reveal that the cathode resistance is remarkably increased by the oxidative decomposition of LPS when the cell is charged to 3.7 V, which induces dendritic lithium growth at the anode side, resulting in a micro-short circuit through the electrolyte. Composite anodes of Li3PS4 glass+Li–Si alloy (Type 1) and Li3N+LiF+Li–Si alloy (Type 2) are also prepared for all-solid-state batteries (ASSBs) with LPS glass electrolyte and sulfur/LPS glass/carbon composite cathode. Using a three-electrode system, the anode and cathode potentials are separated, and their polarization resistances are individually traced. Even under high-cutoff-voltage conditions (3.7 V), Type 1 and 2 cells are stably cycled without voltage noise for >200 cycles. Although cathode polarization resistance drastically increases after 3.7 V charge owing to LPS oxidation, LPS redox behavior is fairly reversible upon discharge–charge unlike the non-composite alloy anode cell. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis reveals that the enhanced cyclability is attributed to uniform Li–Si alloying throughout the composite anode, providing more pathways for lithium ions even when these ions are over-supplied via LPS oxidation. These results imply that LPS-based cells can be reversibly cycled with LPS redox even under high-cutoff voltages, as long as non-uniform alloying (lithium dendrite growth) is prevented.

Effect of heat treatment time on the transport properties of nano-crystallized Li2O–Na2O–V2S5–P2S5 glass–ceramics

Effect of heat treatment time on the transport properties of nano-crystallized Li2O–Na2O–V2S5–P2S5 glass–ceramics

Redox activity of Li2S–P2S5 electrolyte inducing chemo-mechanical failure in all-solid-state batteries comprising sulfur composite cathode and Li–Si alloy anode

In this study, all-solid-state batteries consisting of an Li2S5–P2S5 (LPS) glass electrolyte, a sulfur/LPS glass/carbon composite cathode, and an Li–Si anode are prepared and tested under two different cut-off voltage conditions (0.5–2.7 V and 0.5–3.7 V) to elucidate the effect of LPS redox activity on the chemo-mechanical stability of LPS-based cells. In the voltage range 0.5–2.7 V, which is outside the theoretical stability window for LPS, the cell is stably cycled, indicating that there exists a practical stability window that fulfills each cell condition. On the other hand, the cell cycled in the voltage range 0.5–3.7 V exhibits unstable charging behaviors and voltage noise, with substantial LPS oxidative decomposition. X-ray photoelectron spectroscopy and impedance spectra analyses of a three-electrode system reveal that the cathode resistance is remarkably increased by the oxidative decomposition of LPS when the cell is charged to 3.7 V, which induces dendritic lithium growth at the anode side, resulting in a micro-short circuit through the electrolyte. When the cycles are repeated with voltage noise, the capacity loss becomes significant, which is attributed to cathode degradation (irreversible LPS decomposition) and cracks through the cell, as promoted by the micro-short circuit. This study shows that LPS redox activity outside the practical stability window can lead not only to oxidative degradation of the cathode itself, but also to chemo-mechanical failure in LPS-based cells (especially with high-capacity anode materials such as lithium alloys).

Importance of Li-Metal/Sulfide Electrolyte Interphase Ionic Conductivity in Suppressing Short-Circuiting of All-Solid-State Li-Metal Batteries

The electronic and ionic conductivities of the interphase that forms between Li metal and solid electrolytes (SEs) are key parameters in determining battery cell performance. In this study, we evaluated the effect of the interphase on Li dissolution/deposition behaviors. The reduction of Li2S–P2S5 glasses to Li2S and Li3P by Li metal occurred at the Li/SE interface. The Li dissolution/deposition performance at 100 °C was improved by increasing the Li3P content in the interphase, and the cell with a Li4P2S6 glass electrolyte functioned without short-circuiting at a current density of 1.3 mA cm−2. The ionic conductivity of the Li/SE interphase was evaluated by preparing Li–SE compounds using mechanochemical processing. The milled sample prepared from Li metal and Li4P2S6 glass showed a one order of magnitude higher conductivity of 10−4 S cm−1 at 100 °C than that of the Li–Li3PS4 milled sample, indicating that the ionic conductivity of the interphase formed at the Li/SE interface is an important factor for improving the short-circuiting tolerance of all-solid-state Li-metal batteries.

Influence of Crystallinity of Lithium Thiophosphate Solid Electrolytes on the Performance of Solid‐State Batteries

AbstractSolid electrolytes (SEs) largely define the properties of all‐solid‐state batteries (ASSBs) and are expected to improve their safety, stability, and performance. Their ionic conductivity has much improved in recent years, enabling higher power and energy density. However, more subtle parameters, such as crystallinity, may also influence the electrochemical performance of cells. In this work, the correlation between the performance of ASSBs and thiophosphate SEs having the same stoichiometry, but different crystallinity is investigated. In In/InLi | SE | LiCoO2@ LiNb0.5Ta0.5O3 model cells, better cycling and rate performance is achieved when using glass/glass‐ceramic SEs (e.g., 75Li2S·25P2S5 glass, 70Li2S·30P2S5 glass, and Li6PS5Cl glass‐ceramic). This can be mostly attributed to the mitigation of contact loss by the glass/glass‐ceramic SEs compared to their crystalline SE counterparts. Furthermore, the SE decomposition at typical cathode potentials is investigated by using SE and carbon composites as cathodes. Larger volume changes and more severe decomposition are observed with crystalline SEs in the SE/carbon composite cathode after cycling. The crystalline SEs show higher electronic partial conductivity which results in more degradation in the composite cathode. This work sheds light on optimized composite cathode design for ASSB by carefully choosing solid electrolytes with appropriate mechanical and (electro‐)chemical properties.

Optimization of lithium ion conductivity of Li2S-P2S5 glass ceramics by microstructural control of crystallization kinetics

To improve the Li ion conductivity in a glass-ceramic solid electrolyte, the microstructural control for Li2S-P2S5 glass ceramics was investigated based on crystallization kinetics. 75Li2S∙25P2S5 (mol.%) glass was synthesized by a ball-milling method, and glass-ceramics were prepared by crystallization with heat treatment at various temperatures with appropriate holding times. Crystallization kinetics—nucleation rate, I, and crystal growth rate, U, of 75Li2S∙25P2S5 glass—were determined by differential thermal analysis and in situ transmission electron microscopy (TEM). The obtained parameters were referred to optimize the heat treatment conditions. Through the optimization, Li ion conductivity was improved by 80% in comparison with the glass electrolyte, in which a conductivity of 1.33 × 10−3 S cm−1 was achieved at room temperature by controlling the microstructure and the formation of a network structure in the glass matrix. This approach brings out the best performance of 75Li2S∙25P2S5 glass-ceramics. The effective medium theory was adopted to discuss the formation of a fast ion-conducting network for the improvement of Li ion conductivity. The present method can provide an effective preparation route of a glass-ceramic solid electrolyte for all-solid Li-ion batteries.

Preparation and characterization of composite quasi-solid electrolytes composed of 75Li2S·25P2S5 glass and phosphate esters

Mechanical properties and ionic conductivities of solid electrolytes are important factors influencing the performance of all-solid-state batteries. This study investigates composite quasi-solid electrolytes composed of Li2S–P2S5 glass and a phosphate ester, prepared via planetary ball milling, and the relative densities of the powder-compressed pellets, formed using the composite quasi-solid electrolytes. Dense pellets of the Li2S–P2S5 glass electrolyte are obtained via addition of phosphate esters; the relative densities increase from 90.6% to 93.1%, without the ionic conductivities decreasing, upon addition of a small quantity of tris (2, 2, 2-trifluoroethyl) phosphate to the Li3PS4 glass. The addition of small quantities of phosphate esters to sulfide solid electrolytes is effective in improving the formability of these solid electrolytes.

Lithium conduction and the role of alkaline earth cations in Li2S–P2S5–MS (M = Ca, Sr, Ba) glasses

There has been considerable interest in lithium-sulfide glasses with high lithium ion conductivities because of their potential applications in solid-state electrolytes. Here, new sulfide glasses containing alkaline earth cations, Li2S–P2S5–MS (M = Ca, Sr, Ba), were synthesized by melt quenching. Furthermore, the glass-forming regions in these glasses were identified.Three samples containing equimolar alkaline earth cations (60Li2S–30P2S5–10MS) were selected and their Li+ conductivities were measured. Lower Li+ conductivities were observed in 60Li2S–30P2S5–10MS than in Li2S–P2S5. The local structures of P and Li were characterized by 31P and 6Li solid-state magic angle spinning (MAS) NMR to determine the cause for the observed decrease in the Li+ conductivities and understand the role of alkaline earth cations in the Li2S–P2S5 glasses. A higher-field shift of the PS43− was observed in the 31P MAS NMR spectra of the glasses containing alkaline earth cations, which was attributed to the formation of bridging structures between PS43− units by alkaline earth cations. Moreover, more than half of the Li had low mobility, which is consistent with the conductivity. These results highlight the presence of an effective conduction path around the PS43− in contrast to P2S74− and P2S64−.

Amorphous versus Crystalline Li3PS4: Local Structural Changes during Synthesis and Li Ion Mobility

Glass–ceramic solid electrolytes have been reported to exhibit high ionic conductivities. Their synthesis can be performed by crystallization of mechanically milled Li2S–P2S5 glasses. Herein, the a...

Operando Optical Microscopy for Graphite Negative Electrode Layers in All-Solid-State Lithium Batteries

Recently, all-solid-state lithium batteries have attracted global attentions as next-generation batteries because of their higher safety due to the use of nonflammable inorganic solid electrolytes instead of flammable organic liquid electrolytes. Bulk-type all-solid-state batteries employ composite electrodes consisting of electrode active materials and solid electrolytes. There are a lot of solid-solid interfaces in composite electrode layers resulting in inhomogeneous reaction distributions. In order to achieve higher battery performance, it is important to investigate electrochemical reaction mechanisms at the solid-solid interfaces and evaluate reaction distributions in the electrode layers. A graphite negative electrode is used in commercial lithium-ion batteries. However, there are few papers regarding all-solid-state batteries using graphite composite electrodes.[1] It is worth studying reaction mechanisms of graphite composite electrodes in all-solid-state lithium batteries for practical use in the near future. Colors of graphite particles change from black via dark blue and red to gold during a lithiation process.[2] Observation of color changes enables us to evaluate reaction distributions in a graphite electrode layer. In our previous paper, we compared reaction distributions of composite negative electrodes consisted of graphite and sulfide solid electrolytes with weight ratios of x : 100-x (x = 50, 60 and 70) by ex-situ optical microscopy.[3] The cell using the x = 50 electrode showed the highest reversible capacity of more than 250 mAh g-1 and homogeneous reaction distributions. In this study, to monitor forming of reaction distributions during charge-discharge cycles, operando optical microscopy was conducted for a graphite electrode layer in an all-solid-state cell. Composite electrodes were prepared by mixing graphite and 75Li2S·25P2S5 (mol%) glass electrolyte particles with weight ratios of 50 : 50. 75Li2S·25P2S5 glass and lithium-indium alloy were used as a solid electrolyte separator and a counter electrode, respectively. The cell (Li-In/75Li2S·25P2S5 glass/Graphite) was cut to obtain flat cross-sectional observation areas. Operando optical microscopic observation was conducted for the cross-section of the graphite electrode layer at room temperature under a current density of 0.068 mA cm-2. The cell was mounted under a low confined pressure of ca. 70 kPa in an Ar-filled vessel during optical microscopy. Optical micrographs for the graphite electrode layer showed that lithiation and delithiation proceeded preferentially for the graphite particles near the solid electrolyte layer. The color changes in the graphite particles were quantitatively evaluated to compare SOC values. During the initial lithiation process, almost all the graphite particles in the electrode layer changed their colors from black to gold. However, after the 3rd lithiation process, only the graphite particles near the electrolyte separator layer showed color changes. The SOC value for the graphite near the separator layer side was more than twice as high as that near the current collector side. This suggested that inhomogeneous reaction distributions were formed in the graphite electrode layer, which resulted in degradation of cycle performances. In addition, the thickness changes of the graphite electrode layer during charge-discharge tests were also examined. Acknowledgement: Optical microscopic observation was supported by Lasertec Corp.. Reference s: [1] K. Takada, T. Inada, A. Kajiyama, H. Sasaki, S. Kondo, M. Watanabe, and R. Kanno, Solid State Ionics, 158 (2003) 269–274. [2] S. J. Harris, A. Timmons, D. R. Baker, and C. Monroe, Chem. Phys. Lett., 485 (2010) 265–274. [3] M. Otoyama, A. Sakuda, A. Hayashi, and M. Tatsumisago, Solid State Ionics, 323 (2018) 123-129.

Na+ Ion Transport in x Na2s – (1-x) P2S5 thiophosphate Glasses: An Ab Initio Molecular Dynamics Study

Sodium is an earth abundant element, which makes sodium ion batteries a promising technology for grid-scale energy storage applications. Due to safety concerns with liquid electrolytes, extensive research has been carried out over the past decade to develop new solid-state electrolytes. However, developing solid-state electrolytes with relatively high ionic conductivity at room temperatures is a major challenge. Sulfide glasses have been reported to possess excellent ionic conductivity making them potential electrolytes for high performance sodium ion batteries. With an aim to develop novel electrolyte for high performance sodium ion batteries, our present work focuses on analyzing the local structure of sulfide glasses and its subsequent impact on transport properties through atomistic simulations, in conjunction with experimental characterization. As model systems, we studied x Na2S – (1-x) P2S5 glasses. The polarization and resonance effect of the P=S bonds in the glass complicates the parametrization of interatomic potentials necessary for classical MD simulations. Therefore, we employed ab initio MD simulations that perform quantum calculations on the fly while solving equations of motions at each MD step to accurately capture the complex chemistry. As model systems for our calculations, we considered a range of electrolytes with composition x Na2S – (1-x) P2S5 (x = 0, 0.33, 0.5, 0.6, 0.67, 0.75). The glasses were formed using melt-quench technique and the density of the glasses matched well (within 5%) with the experimentally measured value. We analyzed the local structure of these glasses by calculating the radial pair distribution functions (RDF’s) from ab initio MD and compared it to those obtained through X-ray scattering experiments to validate the model. The FTIR and NMR characterization of these glasses show presence of different structural units that effectively control the ionic conductivity of these glasses. We calculated the fractions of each of these functional units present in the glasses from ab initio MD. At 75% Na2S, all the sulfur present in the structure is non-bridging due to presence of purely [PS4]3- groups in the glass. At concentration lower than 75% Na2S, other structural units containing bridging sulfur such as [P2S7]4- dominate the local structure. On the other hand, at concentrations above 75% Na2S, additional structural units corresponding to [PS3]3- and polysulfides are observed along with [PS4]3- due to excess Na2S content. Increase in concentration of polysulfides leads to lower ionic conductivity. As a result, optimal ionic conductivity of these glasses is observed at 75 Na2S – 25 P2S5 composition. The calculated ionic conductivity of these glasses at room temperature was ~ 10-6 S/cm, which matches well in terms of order of magnitude with experimentally measured values. A relatively high ionic conductivity (~ 10-5 S/cm) can be achieved at slightly elevated temperatures of around 60 °C, making these sodium thiophosphate glasses promising electrolytes for solid-state sodium ion batteries.

Mechanical Properties of Li2S–P2S5 Glasses with Lithium Halides and Application in All-Solid-State Batteries

Mechanical properties of solid electrolytes are important as well as ionic conductivity to achieve all-solid-state batteries with large capacities and long cycle life. In this study, it is shown that Li2S–P2S5 glasses with lithium halides, especially LiI, exhibit both high ionic conductivity and favorable mechanical properties. Mechanical properties, such as Young’s modulus and formability at powder compaction, are evaluated for the solid electrolytes. The addition of lithium halides decreases the Young’s modulus of the electrolytes. In addition, fewer pores and grain boundaries were observed in the powder-pressed pellets of Li2S–P2S5 glasses with lithium halides. All-solid-state batteries using Si electrodes and glass electrolytes with lithium halides exhibited a larger capacity of 20 cycles compared to those without lithium halides. These results provide guidelines for the construction of all-solid-state batteries from the viewpoint of the mechanical properties of solid electrolytes.

Ionic conductivity and crystallization process in the Li2S–P2S5 glass electrolyte

The ionic conductivity of the Li2S–P2S5 glass electrolyte systems strongly depends on the type of precipitated crystalline phases and heat-treatment conditions. To clarify the correlation between ionic conductivity and crystallization behavior, a specific observation technique to visualize the precipitated nanocrystallites via transmission electron microscopy (TEM), and a quantitative method for evaluating the crystallization degree are developed. The 75Li2S·25P2S5 glass exhibits a high conductivity of approximately 7.5 × 10−4 S/cm at a sintering temperature between 120 °C and 180 °C. Above 180 °C, however, conductivity sharply decreases to 1.8 × 10−4 S/cm. In situ TEM observation reveals that this is attributed to the crystallization of the glasses. The crystallization process is mainly characterized by the formation of β-Li3PS4 nanocrystallites precipitated above 180 °C whose size remains intact during further heating until 250 °C. As a result, to achieve high ionic conductivity in the 75Li2S·25P2S5 glass, it is important to realize an amorphous state with the presence of nanocrystallites that would yield a crystallinity degree lower than 5%.

Direct observation of a non-crystalline state of Li2S\u2013P2S5 solid electrolytes

There are two types of solid electrolytes which has been recently expected to be applied to all-solid-state batteries. One is the glasses characterized by an amorphous state. The other is the glass ceramics containing crystalline in an amorphous matrix. However, the non-crystalline state of glasses and glass ceramics is still an open question. It has been anticipated that sea-island and core-shell structures including crystalline nanoparticles have been proposed as candidate models for glass ceramics. Nevertheless, no direct observation has been conducted so far. Here we report the non-crystalline state of Li2S–P2S5 glasses and glass ceramics, and the crystallization behavior of the glasses during heating via direct transmission electron microscopy (TEM) observation. High resolution TEM images clearly revealed the presence of crystalline nanoparticles in an amorphous region. Eventually we suggest that the precipitation and connection of crystalline nanoparticles in an amorphous matrix are key to achieving high ionic conductivity.

Synergistic effect of processing and composition x on conductivity of xLi2S-(100 − x)P2S5 electrolytes

Highly Li-ion conductive Li2S-P2S5 glass-ceramic electrolytes were prepared by controlling the compositions and heat treatment process of the glass. In the systems xLi2S·(100−x)P2S5 (65≤x≤75,mol%) glass phase substrates were prepared by mechanical ball milling. After melting, rapidly quenching and tempering, glass-ceramic electrolytes were prepared. Among them, the 70Li2S·30P2S5 sample showed a highest conductivity of 1.58×10−3Scm−1 and an activation energy of 39kJmol−1. The outstanding property was attributed to both the precipitation of the high conductivity thio-LISICON III phase and the heat treatment, which reduces the grain boundary and improves the grain morphology. Moreover, ball milling method was applied to prepare the composite cathode. Significant high capacity all-solid-state cell with commercial Li2S as active material and 70Li2S·30P2S5 glass ceramic as electrolyte was prepared. First discharge capacity of 1055mAhg−1, high capacity retention of 57.2% after ten cycle and excellent coulombic efficiency were achieved.

Raman Imaging for LiCoO2 Composite Positive Electrodes in All-Solid-State Lithium Batteries to Investigate State-of-Charge Distributions

All-solid-state batteries show higher safety with low risk of leakage and explosion because of nonflammable inorganic solid electrolytes, which are alternative to conventional organic liquid electrolytes. Bulk-type all-solid-state batteries are constructed by pressing positive and negative electrode layers and a solid electrolyte layer at room temperature. The batteries are capable of having high energy density by adding large amounts of active materials into composite electrodes, which are composed of solid electrolyte particles and active material particles. We have investigated electrochemical performances of bulk-type all-solid-state cells using a LiCoO2 composite positive electrode and a Li2S-P2S5 solid electrolyte.1 Composite positive electrodes have many solid-solid interfaces, and electrochemical reactions at the interfaces have not been studied well. Inhomogeneous reaction may occur in the composite electrodes when solid-solid contact areas are insufficient. To improve cell performance, investigation of state-of-charge (SOC) distributions in the electrodes and fabrication of electrodes showing uniform reaction distributions are important. Raman spectroscopy is a suitable technique for investigating SOC of active materials because Raman spectra are sensitive to the structural changes of active materials during charge-discharge tests. Moreover, Raman mapping technique enables us to obtain SOC distribution maps of composite electrodes. In this study, Raman mapping was carried out for charged and discharged LiCoO2 composite positive electrodes in all-solid-state cells to investigate SOC distributions. A 75Li2S·25P2S5 (mol%) glass and indium foil were used as a solid electrolyte and a negative electrode, respectively. A composite positive electrode was prepared by mixing LiCoO2 particles and 75Li2S·25P2S5 glass particles with 80:20 in weight ratio. All-solid-state cells were charged and discharged with a cut-off voltage of 2.6-4.2 V (vs. Li+/Li) at 25oC under a current density of 0.064 mA cm-2. Raman mapping was conducted for surface parts of composite positive electrodes prepared by an Ar ion-milling technique. There are two strong Raman bands at 486 and 596 cm-1, originating from the E g and A 1g modes corresponding to O-Co-O bending and Co-O stretching, respectively.2 Those peaks of E g and A 1g modes shifted to 470 cm-1 and 582 cm-1 respectively, when the cell was charged to 4.2 V (vs. Li+/Li) and returned to original peak positions after the discharging process. To evaluate SOC of the composite positive electrode, A 1g peak positions were investigated in detail. The mapping image showed that charge-discharge reactions did not proceed uniformly, and inhomogeneous reaction distributions existed at the areas of insufficient contacts between LiCoO2 particles and solid electrolyte particles.3 To achieve uniform electrochemical reactions, a composite positive electrode having sufficient interfaces between LiCoO2 and solid electrolyte particles should be prepared. To increase contact points between them, a composite positive electrode with solid electrolytes with a smaller particle size was fabricated. Raman mapping images of the charged and discharged electrodes suggest that uniform electrochemical reactions are achieved. It is noteworthy that the use of smaller solid electrolyte particles is effective way to obtain uniform reaction distributions for composite positive electrodes in all-solid-state lithium batteries. Acknowledgement This research was financially supported by JST, ALCA-SPRING.

In Situ Monitoring of Fast Li-Ion Conductor Li7P3S11 Crystallization Inside a Hot-Press Setup

Rechargeable solid-state lithium ion batteries (SSLB) require fast ion conducting solid electrolytes (SEs) to enable high charge and discharge rates. Li7P3S11 is a particularly promising lithium solid electrolyte, exhibiting very high room temperature conductivities of up to 17 mS·cm–1 and high ductility, allowing fast ion transport through the bulk and intimate contact to high surface electrodes. Here we present a novel hot-press setup that facilitates the synthesis of solid electrolytes by combining in situ electrochemical impedance spectroscopy (EIS) with simultaneous temperature- and pressure-monitoring. While a high room temperature conductivity in the order of 10 mS·cm–1 is readily achieved for phase pure Li7P3S11 with this design, it further enables monitoring of the different steps of crystallization from an amorphous Li2S–P2S5 glass to triclinic Li7P3S11. Nucleation, crystallization and—at temperatures exceeding 280 °C—decomposition of the material are analyzed in real time, enabling process opti...

Raman Mapping for LiCoO2 Composite Positive Electrodes in All-Solid-State Lithium Batteries Using Li2S-P2S5 Solid Electrolytes

All-solid-state batteries with nonflammable inorganic solid electrolytes, an alternative to conventional inflammable organic liquid electrolytes, are widely studied as next generation batteries with low risk of leakage and explosion. Bulk-type batteries use composite electrodes of active materials and solid electrolytes. Solid electrolytes play a role of delivering Li ions to active materials. Bulk-type batteries are capable of having high energy density by adding large amounts of active materials into composite electrodes. We have investigated the electrochemical performance of bulk-type all-solid-state cells using a LiCoO2 composite positive electrode and a Li2S-P2S5 solid electrolyte [1]. Composite electrodes can be fabricated by mixing LiCoO2 particles and solid electrolyte particles. There are many solid-solid interfaces in composite electrodes. It is important that electrochemical reactions at LiCoO2-electrolyte solid-solid interfaces are clarified to improve the cell performance. Raman microscopy is one of the useful methods for investigating the reactions because of its feature of high spatial resolution and surface sensitivity. Raman spectral changes of composite electrodes are closely related to the structural changes of active material during charge-discharge cycling. In this study, Raman spectroscopy was carried out for LiCoO2 composite positive electrodes in all-solid-state cells before and after the charge-discharge process. Moreover, Raman mapping was conducted to evaluate state-of-charge (SOC) distributions of each active material in the electrodes. The 75Li2S·25P2S5 (mol%) glass and indium foil were used as solid electrolyte and a negative electrode, respectively. A composite positive electrode was prepared by mixing LiCoO2 particles and 75Li2S·25P2S5 glass particles (80:20 wt.%). All-solid-state cells were charged and discharged with a cut-off voltage of 2.6-4.2 V (vs. Li+/Li) at 25 oC under a current density of 0.064 mA cm-2. Raman spectra were obtained for the surface part of composite positive electrodes prepared by an Ar ion-milling technique. There are two strong Raman peaks at 486 and 596 cm-1, originating from the E g and A 1g modes of oxygen vibration in LiCoO2, respectively. Those peaks shifted to the lower wavenumber side and their intensity decreased after the initial charging process. After the discharging process, those peaks returned to the original positions. Mapping images showed charge-discharge reactions did not proceed uniformly at the areas of insufficient contacts between LiCoO2 particles and solid electrolyte particles [2]. Our approaches to fabricate composite positive electrodes having uniform charge-discharge reactions will be demonstrated. Acknowledgement This research was financially supported by JST, ALCA-SPRING.

Structure and Ionic Conductivity of Li2S–P2S5 Glass Electrolytes Simulated with First-Principles Molecular Dynamics

Lithium thiophosphate-based materials are attractive as solid electrolytes in all-solid-state lithium batteries because glass or glass-ceramic structures of these materials are associated with very high conductivity. In this work, we modeled lithium thiophosphates with amorphous structures and investigated Li+ mobilities by using molecular dynamics calculations based on density functional theory (DFT-MD). The structures of xLi2S-(100 - x)P2S5 (x = 67, 70, 75, and 80) were created by randomly identifying appropriate compositions of Li+, PS43-, P2S74-, and S2- and then annealing them with DFT-MD calculations. Calculated relative stabilities of the amorphous structures with x = 67, 70, and 75 relative to crystals with the same compositions were 0.04, 0.12, and 0.16 kJ/g, respectively. The implication is that these amorphous structures are metastable. There was good agreement between calculated and experimental structure factors determined from X-ray scattering. The differences between the structure factors of amorphous structures were small, except for the first sharp diffraction peak, which was affected by the environment between Li and S atoms. Li+ diffusion coefficients obtained from DFT-MD calculations at various temperatures for picosecond simulation times were on the order of 10-3 - 10-5 Angstrom2/ps. Ionic conductivities evaluated by the Nernst-Einstein relationship at 298.15 K were on the order of 10-5 S/cm. The ionic conductivity of the amorphous structure with x = 75 was the highest among the amorphous structures because there was a balance between the number density and diffusibility of Li+. The simulations also suggested that isolated S atoms suppress Li+ migration.