Reactive Oligomer Coating Cathode Active Material for Improved Lithium-Ion Battery Performance and Safety

Abstract

Lithium-ion battery (LIB) releases significant heat when the lithium metal oxide cathode is subjected to abnormal heating originating from irreversible chemical reactions of the electrode with electrolyte (often accompanied by gaseous combustion), thereby leading to thermal runaway and ultimately disastrous explosion [1-3]. Thermal runaway of LIBs can be triggered by several factors such as over-charge, over-temperature, mechanical damage, etc. In an attempt to resolve the safety issue of LIBs, several protection techniques have been employed externally or internally. The external protection mechanisms include the current interruptive devices, positive temperature coefficient devices, current limiting fuses and diodes (blocking/bypass). By contrast, the internal protection mechanisms focus on the individual components such as electrodes, separators and electrolytes with an aim to make the battery system intrinsically safer against hazard [4-6].Recently, in order to overcome the disastrous thermal runaway problem of LIBs, a self-terminated oligomer was prepared by the polymerization of N,N'-bismaleimide-4,4'-diphenylmethane (BMI) with barbituric acid (BTA). This unique oligomer was then coated on the pellet surface of cathode active materials to greatly reduce the risk of thermal runaway [7, 8]. However, the discharge capacity of LIBs with such oligomer being coated on the cathode active materials was lower than that of LIBs without the oligomer inside [7]. Thus, how to balance between the reduction in the risk of thermal runaway and the maintenance of the satisfactory electrochemical performance of LIBs becomes a crucial issue.In this study, we prepared novel reactive oligomer from polymerization of BMI with nucleophile (e.g., cyanuric acid (CA)) was coated on the pellet surface of cathode active materials (LiNi0.6Co0.2Mn0.2O2) for improved performance properties and high safety of LIB. The LiNi0.6Co0.2Mn0.2O2 pellets modified by 0.5-1 wt% BMI/CA oligomer exhibited much higher discharge capacity as compared to the native LiNi0.6Co0.2Mn0.2O2 at 25 and 55 oC. The solid electrolyte interface (SEI) formed on the cathode active material pellets after 100 charge/discharge cycles was characterized by X-ray photoelectron spectroscopy and scanning electron microscopy. A small amount of BMI/CA oligomer (as the cathode additive) greatly reduced undesirable side reactions between the electrode and electrolyte, thereby leading to relatively stable LIBs performance. Finally, the thermal stability of delithiated coin-cell LIBs were investigated at 150 oC. The thermal stability of the coin-cell LIB associated with 1.0 wt% BMI/CA coated LiNi0.6Co0.2Mn0.2O2 cathode material is better than that of the coin-cell LIB with pristine LiNi0.6Co0.2Mn0.2O2 cathode materials.