Garnet-Based Solid\nLi-Metal Batteries Operable under\nHigh External Pressure with HCOOH-Induced Electron-Blocking and Lithiophilic\nInterlayer

Despite good compatibility with Li metal, garnet solid\nelectrolytes\nsuffer from severe electron-attack-induced Li-metal penetration and\nlarge interfacial resistance. Here, a formic acid (HCOOH)-induced\nelectron-blocking and lithiophilic interlayer is created via a spontaneous\nreaction with surface Li<sub>2</sub>CO<sub>3</sub> contamination on\nthe garnet electrolyte (LLZTO) pellet. Unlike previous methods that\ninvolved immersing LLZTO in acidic solutions, this study employs a\nvolatile small-molecule organic acid that is easily removable, condensed,\nand recyclable, thus circumventing the environmental drawbacks associated\nwith acid waste. The Li symmetric cell assembled with HCOOH-treated\nLLZTO exhibits a low interfacial impedance (3 Ω cm<sup>2</sup>) and a high critical current density (1.7 mA cm<sup>–2</sup>) at room temperature, enabling the cell to cycle continuously for\nover 1000 h at 0.2 mA cm<sup>–2</sup>. Furthermore, under a\nstacking pressure of 2 MPa, stable lithium plating/stripping was achieved\nat a current density of 0.3 mA cm<sup>–2</sup> with the assistance\nof HCOOH treatment. Additionally, the battery paired with a LiFePO<sub>4</sub> cathode delivers a high capacity of 151.7 mAh g<sup>–1</sup> at 1 C and maintains 88.5% of the initial capacity after 500 cycles,\nsuggesting the feasibility of this interfacial engineering strategy\nfor garnet-based solid Li-metal batteries.