Degradation of a Li_1.5Al_0.5Ge_1.5(PO₄)₃-Based Solid-State Li-Metal Battery: Corrosion of Li_1.5Al_0.5Ge_1.5(PO₄)₃against the Li-Metal Anode

Solid-state Li-metal batteries were widely studied to reach an energy density of 500 mAh kg-1 before 2030. However, the interfacial parasitic reaction between the Li1.5Al0.5Ge1.5(PO4)3 (LAGP) solid-state electrolyte and Li metal generates a mixed conducting interphase (MCI), which grows continuously and leads to the fast degradation of the battery. In previous work, the role of electron transport in the corrosion of LAGP is highlighted. Herein, it has been found that Li-ion transport also plays an important role in the corrosion of LAGP. In the degradation of LAGP, the Li-ion injection through Li1 sites on the (012) plane leads to the fast corrosion of the plane, as detected by grazing incidence X-ray diffraction. The extra Li ion brings electrons to occupy the nearby Ge4+. Simultaneously, the additional interstitial Li ion distorts the local structure and breaks the PO4 tetrahedron. As a result, the corner-shared GeO6 octahedron and PO4 tetrahedron are destructed. The decomposition of LAGP generates a Li-rich MCI, which shows increased electronic conductivity compared with pristine LAGP. The high chemical potential of the Li atom at the MCI results in the continuous corrosion of LAGP. Furthermore, it has been found that ambipolar diffusion at the interface plays an important role in the growth of MCI. The MCI grows faster when ions and electrons are diffused in the same direction motivated by the chemical potential differences of the Li atom. If the cell is cycled at a small current of 0.05 mA cm-2 to separate the diffusion of electrons and ions, the MCI grows at a slower rate. Therefore, the corrosion of LAGP can be ascribed to the chemical diffusion of the Li atom. The ion and electron transport play equally important roles in the electrochemical corrosion of LAGP.