Electrolyte strategies to minimize surface reactivity for improved reversibility of the H2–H3 phase transition†

High-nickel layered oxide cathodes are promising candidates for application in next-generation lithium-ion batteries. However, they are plagued by high surface reactivity with electrolytes and poor reversibility of the high voltage H2–H3 phase transition. While electrolytes generally impact cathode surface reactivity, herein we demonstrate that the use of advanced electrolytes can greatly improve the reversibility of the bulk H2–H3 phase transition due to a reduction in surface reactivity and resultant surface reconstruction. We compare the ability of several common electrolyte enhancement strategies to improve the reversibility of the H2–H3 phase transition with a LiNiO₂ cathode. We find that while all strategies tested in this study improve the reversibility of the phase transition, a combination of fluorinated solvents and an LiPO₂F₂ additive yields the best results in galvanostatic cycling. We quantitatively measure the capacity loss in the H2–H3 phase transition region with second derivative analysis and show that the degree of capacity fade is different in different phase transition regions. With galvanostatic intermittent titration technique and galvanostatic electrochemical impedance spectroscopy, we find that advanced electrolytes can reduce the resistance growth with cycling when passing through the H2–H3 phase transition. With cyclic step chronoamperometry, we examine the evolution of the high-rate performance of the phase transition in each electrolyte and find that a combination of surface stabilization and conductivity are needed to optimize high-rate performance.