Correlating Homogeneity and Microstructure of in Situ plated Li in “Anode-Free” Solid-State Batteries

Abstract

Solid-state batteries utilizing Li metal anodes have the potential to offer increased safety and higher energy density compared to state-of-the-art Li-ion batteries. To achieve these energy density gains, the Li metal anode must be thin (<25 µm). Manufacturing of thin Li metal anodes is complicated by the reactivity of Li metal, which forms a thin passivation layer even in controlled environments. The “anode-free” cell manufacturing approach provides an alternative to these challenges. These cells are manufactured with a bare anode current collector (CC) and the Li metal anode is plated in situ at the CC/solid-state electrolyte interface during the first charging step using the Li stored in the cathode active material. While manufacturing cells with in situ formed Li anodes may offer benefits, removing all excess Li in the cell also poses many challenges. The Coulombic efficiency of these cells must be >99.99% to retain 90% capacity over 1,000 cycles [1]. Therefore, when the Li anode is formed in situ during the first charge, the temperature, pressure, and current density must be carefully tuned to optimize the reversibility of the plated Li. To achieve high reversibility of Li metal anodes, the plated Li should be dense, conformal to the current collector and solid-state electrolyte, homogeneous in thickness, and ideally have a small grain size. These objectives can be achieved by tuning the plating parameters during in situ Li deposition, including pressure, temperature, and current density. It has been previously shown that applying 5 MPa of pressure could improve the homogeneity of in situ formed Li anodes in solid-state cells [2].However, applying pressures >1 MPa is undesirable for practical applications [3]. Therefore, it is necessary to alter the in situ plated Li using other parameters, including the temperature and current density used during plating. Here, we show the effect of plating current density and temperature on the homogeneity and microstructure of in situ formed Li anodes in solid-state cells. It is demonstrated that optimized plating current protocols can improve the homogeneity of plated Li without the need for elevated temperatures. Finally, the relationship between the homogeneity of the in situ plated Li and the reversibility upon stripping is studied using unidirectional stripping experiments. These results illustrate the importance of carefully controlling the parameters for Li plating to enable high Coulombic efficiency in situ formed Li anodes in solid-state cells.