LiFePO4-Coated LiNi0.5Co0.2Mn0.3O2 Cathode Materials with Improved High Voltage Electrochemical Performance and Enhanced Safety for Lithium Ion Pouch Cells

Abstract

The applicable nickel-rich cathode materials (LiNi0.5Co0.2Mn0.3O2, NCM) under high cutoff voltage for high energy density lithium ion batteries shows a broad prospect, but creates enormous challenges for safety. In this work, the LiFePO4-coated LiNi0.5Co0.2Mn0.3O2 (NCM-LFP) composites were prepared on a large scale by the high-speed dispersion and mechanical fusion method, attempting to improve the electrochemical performance and safety of pouch full cells worked under a high cutoff voltage of 4.5 V. Performances have been greatly enhanced, especially for the heat generation during the charge process: the temperature rise of a 5Ah NCM-LFP/MCMB (mesocarbon microbeads) pouch cell was 18.7°C, much lower than that of a NCM/MCMB pouch cell (42.7°C). Also, no thermal runaway was observed of NCM-LFP/MCMB pouch cells at 100% state of charge (SOC) during the extrusion and metallic nail penetration tests. The results of ex-situ impedance measurement, high magnification transmission electron microscope (HR-TEM) and in-situ X-Ray Diffraction (XRD) during heating revealed that the enhanced high voltage safety was contributed to the suppressed structure evolution from layer to spinel to rock salt, as a result of a dense and uniform LFP protective layer on NCM.