Precise Al2O3 Coating on LiNi0.5Co0.2Mn0.3O2 by Atomic Layer Deposition Restrains the Shuttle Effect of Transition Metals in Li-Ion Capacitors

Lithium ion capacitors (LICs), with their high power density and acceptable energy density have become a popular energy storage device on the market. Among various electrode materials of lithium ion batteries (LIBs), LiNi0.5Co0.2Mn0.3O2 (NCM), which has both high capacity and good cycle performance, has been widely used to improve the energy density of power devices. Unfortunately, the short cycle life and low power density of NCM have become new challenges for LICs. This work deeply analyzes the capacity failure mechanism of NCM in LICs during cycling, and finds that in the cycling process, the transition metal (TM) in NCM will dissolve and produce a shuttle effect, which eventually accumulates on the anode surface. Comfortingly, the precise Al2O3 coating on the NCM surface obtained by atomic layer deposition (ALD) successfully restrains the dissolution of TM in NCM during cycling, thus preventing the shuttle effect of TM. An additional benefit of the Al2O3 coating is to reduce internal stress changes of NCM secondary particles during long-time charging and discharging, thereby ensuring a close contact inside NCM particles after multiple cycles. As a result, the LICs assembled by the Al2O3 coated NCM composite cathode exhibit an excellent cycling capability and rate performance. This work paves a way for improving the electrochemical performance of LICs with the NCM cathode, and provides a reference for the further research on the performance attenuation mechanism of other composite electrodes in LICs.