Elucidating the role of cathode identity: Voltage-dependent reversibility of anode-free batteries

The cathode material in a lithium (Li) battery determines the system cost, energy density, and thermal stability. In anode-free batteries, the cathode also serves as the source of Li for electrodeposition, thus impacting the reversibility of plating and stripping. Here, we show that the reason LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathodes deliver lower Coulombic efficiencies than LiFePO₄ (LFP) is the formation of tortuous Li deposits, acidic species in the electrolyte, and accumulation of “dead” Li⁰. Batteries containing an LFP cathode generate dense Li deposits that can be reversibly stripped, but Li is lost to the solid electrolyte interphase (SEI) and corrosion according to operando ⁷Li NMR, which seemingly “revives” dead Li⁰. X-ray photoelectron spectroscopy (XPS) and in situ ¹⁹F/¹H NMR indicate that these differences arise because upper cutoff voltage alters electrolyte decomposition, where low-voltage LFP cells prevent anodic decomposition, ultimately mitigating the formation of protic species that proliferate upon charging NMC811.