Effects of lithium bis(oxalato)borate-derived surface coating layers on the performances of high-Ni cathodes for all-solid-state batteries

Instability of the cathode/solid electrolyte interface is a critical issue limiting the commercialization of all-solid-state batteries (ASSBs). Specifically, lithium thiophosphates (sulfide electrolytes), promising solid electrolytes, are highly reactive with oxide cathodes, leading to undesirable side reactions and electrolyte decomposition during cycling. Surface coating of oxide cathodes is an efficient approach to suppress undesirable interfacial reactions. Several ternary compounds, such as LiNbO3, LiTaO3, and Li2ZrO3, with suitable ionic conductivities have been applied as coating materials due to their appropriate stabilities against sulfide electrolytes. However, the fabrication of a coating layer containing ternary oxides requires high-temperature (over 600–800 °C) heat treatment for the homogeneous reaction of the constituent ions, which can deteriorate the performances of high-Ni cathodes. Accordingly, herein, a coating layer was synthesized using lithium bis(oxalato)borate (LiBOB) via low-temperature (below 400 °C) heat treatment. LiBOB formed thin and homogeneous coating layers on the surfaces of cathodes. LiBOB-coated cathode heat-treated at 350 °C presented higher discharge capacity, better rate capability, and lower impedance value as compared to those of the pristine cathode. Considering X-ray photoelectron spectra and energy-dispersive X-ray spectroscopy analyses of the composite cathodes, the LiBOB coating layer formed at 350 °C significantly reduced electrolyte decomposition and side reactions at the cathode/electrolyte interface, which enhanced the electrochemical performances of the LiBOB-coated cathodes. This result confirms that the LiBOB-based coating is an effective approach to improve the stability of the cathode/sulfide e Basically lectrolyte interface. Our simple and unique approach based on additives, such as LiBOB, has the potential to trigger new research on the fabrication of surface coatings at low temperatures (below 400 °C) for the cathodes of ASSBs.