Spatially-resolved degradation in high-voltage single-crystal cathodes and its mitigation via surface structural pillar and kinetic promoter

The advancement of lithium-ion batteries toward higher energy densities requires the expansion of operating voltage windows, yet this introduces complex degradation pathways. Here, we report on a newly elucidated degradation mechanism in high-voltage single-crystal cathodes: phase transition asynchrony. In situ pouch-cell diffraction and post-mortem analyses revealed that high-voltage cycling induces a profound phase transition asynchrony, which segregates the particle into a Li-poor/O1-rich shell and a Li-rich/O3-rich core. This asynchronous phase transition induces localized lattice strain, leading to particle degradation, premature capacity fade, and increased polarization. We further demonstrate a targeted mitigation method by atomic layer deposition (ALD) surface doping with aluminum ions, can mitigate lattice contraction on the cathode surface layer through a structural-pillar effect that mechanically reinforces defect sites, thereby suppressing lattice distortion, while simultaneously acting as a positive charge center that promotes and homogenizes surface Li⁺ diffusion kinetics. This dual action directly counteracts the root cause of the asynchrony. As a result, the modified cathode exhibits exceptional cycling stability, retaining 93.69 % capacity after 100 cycles at 4.55 V. This work provides a fundamental understanding of a critical high-voltage degradation mechanism and presents a precision-engineering solution with broad applicability for next-generation energy storage systems.