Understandingthe Impact of Composition on the H2–H3Phase Transition in High-Nickel Cathodes

High-nickel layered oxides are becoming the cathode of choice in electric vehicles. However, their cycle life is inhibited by the deleterious H2–H3 phase transition at voltages >4.1 V vs. Li/Li⁺. Unfortunately, avoiding this region significantly limits accessible capacity, so understanding this phase transition is essential to maximize single cycle energy. Herein, we examine the performance of the H2–H3 phase transition with 3 high-nickel cathode compositions. We find that all of the cathodes complete the phase transition at voltages >4.4 V, and that the endpoints of the phase transition cannot be determined based on the voltage plateau alone. We observe that the degree to which the material undergoes the phase transition is inversely proportional to the reversibility of the phase transition. Furthermore, we find that when charged entirely through the H2–H3 region, the cyclability of the phase transition appears to be similar in each material. With galvanostatic intermittent titration technique and second derivative analysis, we find that a significant amount of the capacity fade in the H2–H3 region can be attributed to worsening kinetics, with minimal active material loss. With ex situ X-ray diffraction, we find that increased interlayer spacing correlates to improved capacity retention in the high-voltage region. However, the low-voltage regions can suffer from worsening kinetics if the cutoff voltage is high, thus negating much of the benefit of high-voltage cycling with lower-nickel cathodes.