Performance Improvement of All-solid-state Lithium Batteries by Structural Modification of Li2S-P2S5 based Solid Electrolyte and Interfacial Control of Composite Cathode

Abstract

Demands on power sources with high energy density and high safety performance are ever increasing and it strongly motivates researchers to investigate and develop the all-solid-sate lithium ion batteries (ASS-LIBs) based on the inorganic solid electrolytes. The ASS-LIBs, in which the currently used liquid electrolytes are substituted for solid electrolyte materials, could provide a solution for safety issues stemming from the hazardous nature of conventional lithium ion batteries using highly volatile and flammable organic liquid electrolytes. However, great challenges remain for realizing the high performance ASS-LIBs using inorganic solid electrolytes. One of them is how to further increase high ionic conductivity of solid electrolytes at room temperature. Another one is how to create favorable solid–solid interface between active material and solid electrolyte in composite cathodes. To solve these issues, it is firstly essential to select a promising solid electrolyte system for all-solid-state lithium batteries. Hence, the focus of this project has been on Li2S-P2S5 system, since it is easily possible to prepare the Li2S-P2S5 glass electrolytes with high lithium content by simple mechanical milling due to good glass forming ability and electrochemical stability against electrochemical reduction. In addition, the Li2S-P2S5 glass-ceramic electrolytes prepared by crystallization of the obtained glass electrolyte exhibit an excellent mechanical properties due to low Young’s moduli as well as an electrochemical properties due to precipitation of highly lithium ion conducting crystalline phases such as thio-LISICON II or III analogues, which enables an intimate contact to be formed at interface between the active material and solid electrolyte in composite cathodes. Furthermore, the P2S5 is commercially produced as industrial material for agricultural chemicals, and then the cost of the P2S5-based solid electrolyte should be lower than that of the SiS2-based and GeS2-based solid electrolytes. Therefore, on the basis of these favorable properties, the first objective of the present project was directed towards investigating the ion conduction in the Li2S-P2S5 glass-ceramics prepared by high-energy mechanical milling and post heat-treatment, and understanding the relationships between composition-structure-property, in order to further improve the ion conductivity and structural stability. More specifically, in Chapter 3, extra interstitial lithium ions were introduced into the xLi2S∙(100-x)P2S5 (75≤x≤80) glass-ceramics with the high Li+ ion conducting crystalline phase of thio-LISICON analogues by aliovalent substitution, P5+↔5Li+, and its conductivities and basic electro- chemical properties were identified. In the binary Li2S-P2S5 system, 78.3Li2S∙21.7P2S5 glass-ceramics prepared by mechanical milling and subsequent heat-treatment at 260 oC for 3h showed the highest conductivity of 6.3×10-4 S·cm-1 at room temperature and the lowest activation energy for conduction of 30.5 kJ·mol-1. The enhancement of conductivity with increasing x up to 78.3 is probably caused by the introduction of interstitial lithium ions at the partially occupied Li sites which affects the Li ion distribution. The prepared solid electrolyte exhibited the lithium ion transport number of almost unity and voltage stability of 5 V vs. Li at room temperature. In Chapter 4, selenium with higher polarizability and larger ionic size was incorporated into the 75Li2S·25P2S5 glass and glass-ceramics by the addition of P2Se5 with the purpose of lowering the electrostatic binding energy. The structural analysis of the 75Li2S·(25-x)P2S5·xP2Se5 (0≤x≤3) glass and glass-ceramics showed that selenium is effectively incorporated into the amorphous matrix and crystal lattice of thio-LISICON III analog phase by mechanical milling and crystallization, which results in the formation of the Se incorporated network units. For both glass and glass-ceramics solid electrolytes, the ionic conductivities were enhanced with an increase of Se contents, and the glass-ceramic electrolyte with 2 mol% of P2Se5 showed the highest conductivity of 6×10-4 S cm-1 at room temperature. In Chapter 5, low thermal stability of the metastable thio-LISICON II analog phase of the 78Li2S·22P2S5 glass-ceramics was identified by gradual transformation to a thio-LISICON III analog phase over a narrow temperature range of heat treatment. However, the addition of 5 mol% Li2SO4 into the 78Li2S·22P2S5 glass-ceramics suppressed the formation of the thio-LISICON III analog phase and enhanced the structural stability of the thio-LISICON II analog phase at higher heat treatment temperatures. From X-ray diffraction (XRD) and differential thermal analysis (DTA) results, the enhanced thermal stability of the thio-LISICON II analog phase in the Li2SO4 added 78Li2S·22P2S5 glass-ceramics is closely related to the lithium content of the glass matrix and the incorporated tetrahedral SO42- anions in the crystal lattice of the thio-LISICON II analog phase. However, although the solid electrolytes with high ionic conductivity have been developed, the rate determining step is determined mainly at the interface between the cathode active material and solid electrolyte in the composite cathodes rather than Li+ ion migration in solid electrolytes itself. Therefore, in order to improve the interfacial charge-transfer reaction, an electrochemically favorable electrode (solid)-electrolyte (solid) interface in all-solid-state composite cathodes has to be created. There are two major causes, which detrimentally affect the electrochemical reaction at the electrode-solid electrolyte interface in ASS-LIBs. One is the inhomogeneous ion and electron conducting path in the all-solid-state composite cathodes. In the case of using the solid electrolytes, it is difficult to form uniform and continuous conducting network because they are not wettable and infiltrative like liquids. The other is the degradation of interfacial structure induced by formation of space-charge layer and chemical element diffusion. Therefore, the second objective of the present project was to form an efficient conducting network in the composite cathode and suppress the unfavorable side reactions at the interface based as well as understand reaction mechanism taking place within the composite cathode and at electrode/solid electrolyte interfaces. More specifically, in Chapter 6, composite cathodes comprising the solid powders of LiCoO2, electrolyte and Super P carbon are prepared using three different mixing methods to achieve uniform distribution of the constituent solid particles, and the effect of this mixing method on the microstructure and properties of the resulting composite cathode was also discussed. By applying the wet-mixing method, composite cathodes with greater homogeneous distribution of solid particles were obtained when compared to those prepared by dry-mixing. As a result of this favorable feature, a higher discharge capacity of 84 mAh·g-1 and a capacity retention of 73% were maintained at the 50th cycle in the ASS cell using the composite cathode prepared by wet-mixing with a ball mixer, which resulted from the smaller interfacial resistance for the intercalation /deintercalation of the Li+ ions in this composite cathode. This is determined to be due to the homogeneous distribution of the constituent particles leading to significantly reduced electrode polarization. In Chapter 7 and 8, in order to identify the effects of interfacial structure of cathode materials on the electrochemical performance of ASS-LIBs using Li2S-P2S5 glass-ceramic solid electrolytes, 1) over-stoichiometric Li1+xCoO2 (x = 0.1, 0.2, and 0.3) cathode materials were synthesized from an aqueous solution of lithium nitrate (LiNO3) as an excess lithium precursor, and 2) Li2CO3-coated LiCoO2 powders are prepared from a lithium hydroxide (LiOH) solution via low-temperature heat treatment, respectively. In both case, while regarded as an impurity phase in lithium battery systems using liquid electrolytes due to its detrimental effects on electrochemical performance, all the Li2CO3 formed on the surface of the over-stoichiometric Li1+xCoO2 particles and the stoichiometric LiCoO2 were identified to act as an effective coating material to suppress the interfacial side reactions without a significant decrease in interfacial kinetics for ASS-LIBs systems using the sulfide solid electrolytes.