Interface Structure in Solid-State Lithium Batteries

Abstract

Although solid-state batteries had been suffering from low rate capability due to low ionic conductivities of solid electrolytes, conductivities comparable to non-aqueous liquid electrolytes used in current lithium-ion batteries have been achieved in some recently-developed solid electrolytes. Especially among sulfide systems, the highest conductivity has reached 10−2 S cm−1 [1], which is even higher that of liquid electrolytes when taking the transport number of unity into account. Consequently, ion transport in solid electrolytes can cease from rate-determining; however, it has been replaced by that across interfaces. Ionic conductors often exhibit anomalous ionic conduction at their surface or interface, which is categorized as “nanoionics” taking place in space-charge layers formed with thickness around 10 nm at the interface or surface. It appears in solid-state batteries with sulfide electrolytes as high interfacial resistances to high voltage cathodes, which result in the unimproved power densities of the solid-state batteries in spite of the high ionic conductivities of the employed solid electrolytes. Classical Nernst equation infers that the high potential of cathode makes the space-charge layers lithium-depleted and thus highly-resistive due to the depletion of charge carriers. The high interfacial resistance has been successfully reduced by an interfacial design based on the space-charge model, where an ion-conducting and electron-insulating oxide is interposed at the interface in order to shield the sulfide electrolyte from the high cathode potential and thus act as a buffer against the lithium depletion [2]. The changes in the electrode properties upon the interposition are well explainable by the space-charge layer model [3]. However, because the space-charge layer that lies between solids with only several nanometers in thickness is difficult to access, it has not revealed its presence even before advanced analyses; whereas it has emerged in computations. Electronic structure of LiFePO4 and Li3PS4, which are cathode and solid electrolyte materials, respectively, obtained by the first principles calculation suggests preferential formation of lithium vacancies on the electrolyte side of the LiFePO4/Li3PS4 interface. In addition, when FePO4/Li3PS4 interface is relaxed in order to simulate the interface structure during the battery operation, lithium ions move from the electrolyte to electrode side to bring about lithium depletion on the electrolyte side.