Avoiding electrochemical indentations: a CNT-cocooned LiCoO<sub>2</sub> electrode with ultra-stable high-voltage cycling

Charging LiCoO2 (LCO) to above 4.5 V induces crystal cracking and seriously deteriorates the battery cycle life. Decreasing the range of the LCO misfit strain during deep de-lithiation is useful for preventing cracks, but this is not always achievable. Here, we demonstrate that the limited electrochemical contact area between electronically conductive carbon and the LCO crystal causes “electrochemical indentations” (ECIs) during charging and discharging. Particularly in fast charging, the high local ΔcLi gradient in LCO would cause a local volume of the surficial lattice to shrink while the rest of the crystal is still under stretching, and hence, drive the ECI to cause cracking. Increasing the electrochemical contact area would reduce the ECI and cracking risk. Therefore, we developed a free-standing CNT-LCO electrode in which all of the LCO particles were intimately wrapped with a dense CNT cocoon to establish a larger true electrical contact area. The simulations demonstrated that the radial ΔcLi and ECI decreased significantly in the cocooned LCO particles. The cocooned LCO electrode maintained good morphology and retained 94% of its energy density after 400 cycles when charged to 4.55 V. By removing the need for a current collector and binder, the volumetric energy density of the CNT-LCO cathode reached 3200 Wh L−1 (electrode).