Effect of composite electrode thickness on the electrochemical performances of all-solid-state li-ion batteries

Abstract

Several ceramic half-cells with differing electrode composite thicknesses but identical formulations were assembled using the spark plasma sintering (SPS) technique, in order to conduct comparable investigations of their kinetic limitations. The SPS technique was used to assemble the composite electrode and the electrolyte together within a few minutes. NASICON-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) ceramic was used as solid electrolyte, as it offers high ionic conductivity (3 × 10−4 S.cm−1 at 25 °C) with a Li+ transport number of 1. LiFePO4 active material was used as a model material; it offers an average flat potential of 3.45 V vs Li+/Li and a reasonably high theoretical capacity of 170 mAh.g−1. Surface capacity values (from 0.8 to 3.5 mAh.cm−2), which are proportional to electrode thickness, remained quite close to the initial values after more than 20 cycles, even for a 325 μm thick electrode (3.5 mAh.cm−2). The overpotential in the flat plateau region was proportional to the current density used, which means that it was dependent only on the cell’s ohmic drop. Performances were not limited by the ion transport into the solid electrolyte and composite electrode volume - as in classical Li-ion batteries - since the transport number of LAGP is one. Therefore, very thick electrode-enabling batteries with high-surface capacity can be considered.