Garnet-Based Solid Li-Metal Batteries Operable under High External Pressure with HCOOH-Induced Electron-Blocking and Lithiophilic Interlayer

Despite good compatibility with Li metal, garnet solid electrolytes suffer from severe electron-attack-induced Li-metal penetration and large interfacial resistance. Here, a formic acid (HCOOH)-induced electron-blocking and lithiophilic interlayer is created via a spontaneous reaction with surface Li₂CO₃ contamination on the garnet electrolyte (LLZTO) pellet. Unlike previous methods that involved immersing LLZTO in acidic solutions, this study employs a volatile small-molecule organic acid that is easily removable, condensed, and recyclable, thus circumventing the environmental drawbacks associated with acid waste. The Li symmetric cell assembled with HCOOH-treated LLZTO exhibits a low interfacial impedance (3 Ω cm²) and a high critical current density (1.7 mA cm^-2) at room temperature, enabling the cell to cycle continuously for over 1000 h at 0.2 mA cm^-2. Furthermore, under a stacking pressure of 2 MPa, stable lithium plating/stripping was achieved at a current density of 0.3 mA cm^-2 with the assistance of HCOOH treatment. Additionally, the battery paired with a LiFePO₄ cathode delivers a high capacity of 151.7 mAh g^-1 at 1 C and maintains 88.5% of the initial capacity after 500 cycles, suggesting the feasibility of this interfacial engineering strategy for garnet-based solid Li-metal batteries.