High-voltage oxide cathodes for high-energy-density lithium-ion batteries

Abstract

The worldwide electrification of the automobile industry has been strongly pushing the advancement of lithium-ion batteries (LIBs) with high energy density and long service life. Since the cathode is currently the limiting electrode for energy density, safety, and cost of commercial LIBs, extensive efforts have been devoted into investigating next-generation high-performance cathode materials with high capacity and operating voltage. Among the pool of cathodes, high-nickel layered oxide cathodes, LiNiₓM₁₋ₓO₂ (M = Co, Mn, Al, etc.; x > 0.7), are regarded as one of the most promising candidates. However, the practical viability of high-Ni cathodes is compromised by their air instability, fast structural and interfacial deteriorations during operation, poor thermal stability, and high cost. On the other hand, another promising cathode, high-voltage spinel LiNi₀.₅Mn₁.₅O₄, exhibits better thermal and structural stabilities, but suffers from rapid performance degradations due to its high operating voltage of > 4.7 V vs. Li⁺/Li. This dissertation focuses on stabilizing the operation of high-Ni and high-voltage spinel cathodes with diverse modification strategies and advancing the understanding of the degradation mechanisms of cells with high-voltage cathodes assisted by state-of-the-art characterizations. First, the function of atomic scale zinc-doping in a high-Ni cathode LiNi₀.₉₄Co₀.₀₄Zn₀.₀₂O₁.₉₉ is investigated. The incorporation of Zn greatly mitigates the average voltage and capacity fade by ameliorating the anisotropic lattice distortion, enhancing the structural integrity, and reducing cathode-electrolyte side reactions. Moreover, Zn-doping is proved beneficial to improve the thermal stability. Second, a cobalt- and manganese-free LiNi₀.₉₃Al₀.₀₅Ti₀.₀₁Mg₀.₀₁O₂ cathode is rationally designed, synthesized, and comprehensively investigated. Collectively, the use of Al, Ti, and Mg in the cathode enables a stable operation of practical full cells over 800 cycles by alleviating electrolyte decomposition reactions, transition-metal crossover, and active lithium loss. Third, single-element doped cathodes, viz., LiNi₀.₉₅Co₀.₀₅O₂, LiNi₀.₉₅Mn₀.₀₅O₂, and LiNi₀.₉₅Al₀.₀₅O₂, along with undoped LiNiO₂, are compared through a control of cutoff energy density to elucidate the role of dopants in high-Ni cathodes. Via a group of advanced analytical techniques, it is unveiled that one critical role of dopant is regulating the state-of-charge and the occurrence of H2–H3 phase transition of high-Ni cathodes, which essentially dictates the cycle stability. Finally, electrochemical modifications on the graphite anode and high-voltage spinel cathode are performed and characterized. The results suggest that the graphite anode interphase degradations caused by acidic and transition-metal crossover species generated from the cathode predominately contribute to the cell performance deterioration. Based on in-depth analyses, pathways towards long-life high-voltage full cells are pictured.