Converting the CHF 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 (CHF) using a gas flow-type reactor. CHF, well known as a potent greenhouse gas, is successfully transformed into a stable ~2 nm LiF homogenous layer by converting the adventitious LiCO 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 LiNiCoAlO using synchrotron surface spectroscopy techniques, operando XRD and TEM. In addition, we demonstrate improved electrochemical cycling performance of the LiF coated LiNiCoAlO 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 LiNiCoMnO cathode material cycled up to 4.3 and 4.8 V vs Li/Li.