Converting the CHF₃ Greenhouse Gas into Nanometer-Thick LiF Coating for High-Voltage Cathode Li-ion Batteries Materials

Solving the surface (electro-)chemical instability and the fading behavior of high voltage cathode materials cycled above 4.3 V vs Li⁺/Li remains a major challenge for the next generation of high energy density Li-ion batteries. Here, we present a facile, environmentally friendly, cost effective and scalable method to address this problem by uniformly fluorinating the surface of cathode materials with a mild fluorinating agent (CHF₃) using a gas flow-type reactor. CHF₃, well known as a potent greenhouse gas, is successfully transformed into a stable ~2 nm LiF homogenous layer by converting the adventitious Li₂CO₃ layer covering the surface of the vast majority of layered-oxide cathode materials. The fluorination mechanism and the interface stability of the LiF coating layer is systematically studied on LiNi_0.8Co_0.15Al_0.05O₂ using synchrotron surface spectroscopy techniques, operando XRD and TEM. In addition, we demonstrate improved electrochemical cycling performance of the LiF coated LiNi_0.8Co_0.15Al_0.05O₂ when cycled up to 4.5 V where the impedance and overpotential decrease by 30 % and 100 mV respectively after 100 cycles, with a capacity retention better than 94 % and improved rate performance at high current density. Furthermore, the universality of the fluorination approach is validated further on Ni-rich LiNi_0.85Co_0.1Mn_0.05O₂ cathode material cycled up to 4.3 and 4.8 V vs Li⁺/Li.