Electromechanical Failure of NASICON-Type Solid-State Electrolyte-Based All-Solid-State Li-Ion Batteries

Although inorganic all-solid-state Li-ion batteries (ASSLiBs) using stable oxide solid electrolytes are considered to be promising candidates for future energy storage devices, their relatively high cell impedance due to the restricted contact area and interfacial stability results in unsatisfactory electrochemical performance and fast capacity fading. The mechanism limiting performance and cycle life in such ASSLiBs still lack study and hence understanding. To overcome this bottleneck, we prepared a bulk ASSLiB, where a MnO2–CNT nanocomposite is used as a high-voltage anode to prevent reduction of the electrolyte by a lithium anode, high-voltage LiNi0.5Mn1.5O4 is used as a cathode, and ambient-air stable Li1.5Al0.5Ge1.5(PO4)3 (LAGP) is chosen as a solid-state electrolyte (SSE). This ASSLiB shows a maximum discharge capacity of 82 mAh g–1 at 0.15C and 23.8 °C. The electrochemical impedance study of the cell reveals a decrease in impedance after solid interface layer formation in cycle 1 followed by an increase after electrode lithiation/delithiation. Electrochemical evaluation and first-principles calculations were used to explore the decomposition of the LAGP after charge/discharge cycles. Decomposition of LAGP with the assistance of Li ions and free electrons from voids and grain boundaries leads mainly to the formation of Li4P2O7, Li3PO4, Ge5P6O25, AlPO4, and GeO. Finite element method simulations reveal that the volume expansion due to the formation of the decomposition products Li4P2O7, Li3PO4, and AlPO4 results in a maximum internal stress of 2.5–125 GPa for various Li excess ratios ranging from 0 to 6. This by far exceeds the failure stress of LAGP and results in crack formation and growth in the SSE on multiple cycling.