capacity is utilized, capacity decay after cycling becomes more pronounced. With increasing Ni content, the two-phase character from LiNiO 2 emerges at the end of charge: one is the H3 phase containing almost no Li, and the other is the H2 phase containing 25% Li (Li 0.25 NiO 2 ). Abrupt changes, such as Li layer collapse and surface reactivity increase, occur during the H2 to H3 phase transition. Cathode capacity decay primarily occurs in the Li-depleted state; lowering the upper voltage cutoff to avoid the formation of the H3 phase has been shown to greatly improve capacity retention. It has been reported that surface oxygen evolution is a major contributor to the capacity decay, particularly in single crystals where cracking is largely suppressed and losing contact with the current collector is avoided. After oxygen loss, dangling transition metals diffuse back into the Li layer, forming rock-salt-like or spinel-like surface phases. These surface phases are generally believed to increase the charge/discharge resistance. However, the quantitative relationship between surface phase structure and capacity fade remains unclear. A thorough understanding of this relationship is crucial for maximizing the cycle life and energy density of high-Ni oxides. The insights will also help rationalize surface protection strategies. Previous DFT and TEM studies have shown that the surface densified phase with open Li diffusion channels could be the β phase of composition Ni 5 O 8 , or the γ phase of composition Ni 3 O 4 . Regarding the electrochemical performance after surface densification, Xu et al. observed the formation of a fatigue phase in LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) based on lattice parameter changes, where 25% Li is trapped at the end of charge. The trapping of Li in the fatigue phase has been attributed to the lattice mismatch between the surface phase and the H3 phase. The surface phase's layer spacing does not collapse due to the high concentration of Ni in the Li layer. It was argued that the surface phase pins the Li layer spacing, thereby preventing the delithiation reaction that would otherwise trigger Li layer collapse. However, the H2 and H3 phases can coexist within a single particle despite the significant difference in layer spacing. Since H3 can grow from H2, and H2 is compatible with the surface phase, the reason why H3 cannot coexist with the surface phase becomes less clear. Ikeda et al. found that capacity decay in LiNiO 2 only occurs above 4.1 V and attributed it to the movement of Ni to tetrahedral sites. Bautista Quisbert et al. later reported a gradual shrinkage of the 4.2 V plateau in LiNiO 2 . Their GITT measurements indicated that the charge overpotential increases dramatically above 4.2 V as capacity fades, while the discharge overpotential at the same Li content remains low. This charge-discharge asymmetry suggests that kinetics plays an important role. Pei et al. recently demonstrated that Al doping in NMC811 helps mitigate the fatigue behavior. While surface densification was alleviated following Al doping, the root cause was attributed to the lattice strain. In this paper, an alternative explanation for the capacity decay is provided based on . The surface phase thicknesses are identical, each consisting of three atomic layers (∼1 nm). Both particles are charged and discharged under constant-current constant-voltage (CCCV) conditions with an upper voltage cutoff of 4.4 V, alongside a pristine particle without any surface phase for reference. All simulations are conducted at 400 K unless otherwise specified. Note that the oxygen lattice is fixed in these simulations, and O1-type stacking fault formation is not accounted for. Also, the current setup aims to provide an upper bound for Li transport behavior (fastest possible) dictated by the densified phases in the sub-surface region, without incorporating explicit surface energetics.