High-nickel layered oxide cathodes for high-energy-density lithium-ion batteries
The thriving energy-storage market has been motivating enormous efforts to advance the state-of-art lithium-ion batteries. The development of cathode materials, in particular, holds the key to realizing the high-energy-density and low-cost promise. Among the insertion-reaction cathodes currently in play, the layered oxides, especially the LiNiO₂-based high-Ni type, are being intensively pursued as one of the most promising candidates. However, the high-Ni layered oxides inherently encounter a trade-off between capacity and stability - the higher the capacity contributed by the higher Ni content, the worse the electrochemical cyclability. This dissertation focuses on improving the stability of high-Ni layered oxide cathodes through multiple effective approaches. First, a practical doping method is presented by incorporating a small dose of Al into the layered structure, which significantly improves the electrochemical performance of the cathode. It reveals that Al-incorporation greatly enhances the stability of cathode-electrolyte-interphase (CEI) due to the modified cathode electronic structure. Furthermore, in-situ X-ray diffraction provides an operando evidence for the reduced lattice distortions during cycling with Al-incorporation. Second, lithium bis(oxalate) is employed as an effective electrolyte additive to improve the electrode-electrolyte-interphase stability. The well-tuned electrode-electrolyte interphase is featured with excellent robustness against electrochemical abuse. Moreover, the correlation between cathode-surface chemistry and anode-electrolyte interphase is revealed by studying the interphases at atomic level. Third, by constructing a dual-functional binder framework with a conductive polymer polyaniline, the high-Ni layered oxide cathodes exhibit significantly improved cyclability. This new binder framework not only promotes the rate performance even at low temperatures, but also effectively scavenges the acidic species in the electrolyte through a protonation process. Hence the cathode-surface reactivity is greatly suppressed and the rock-salt phase propagation into the bulk structure is considerably alleviated. Finally, in comparing with the state-of-art cathode (LiNi [subscript 0.8] Co [subscript 0.1] Mn [subscript 0.1] O₂), the interphasial and structural evolution processes of high-Ni layered oxides (LiNi [subscript 0.94] Co [subscript 0.06] O₂) are systematically investigated over the course of their service life (1,500 cycles). By applying advanced analytical techniques (e.g., Li-isotope labeling and region-of-interest method), the dynamic chemical evolution on the cathode surface is revealed with spatial resolution, and the correlation between lattice distortion and cathode-surface reactivity is established for the first time.