The chemical reactions and phase transitions at high voltages are generally considered to determine the electrochemical properties of high-voltage layered cathodes such as Ni-rich rhombohedral oxides. Even if significantly higher SOCs (states-of-charge) are utilized above the capability of transition metal redox (primarily Ni and Co), the effect of oxygen redox on Ni-rich rhombohedral oxides still looks mysterious thereby necessitating research that can clarify the relationship between redox reactions and phase transitions. Here, we performed a comprehensive and comparative study of the cationic and anionic redox reactions, as well as the structural evolution of a series of commercial Ni-rich layered oxides with and without Al doping. We combined the results from X-ray spectroscopy, operando electrochemical mass spectrometry, and neutron diffraction with electrochemical properties and thereby revealed the different oxygen redox activities associated with structural and electrochemical degradations. We reveal that Al doping suppresses the irreversible oxygen release but enhances the lattice oxygen oxidization. With this modulated oxygen redox activity, the Ni-rich layered oxides’ notorious H2–H3 structural phase transition becomes highly reversible. Our findings disentangle the different oxygen redox activities during high-voltage cycling and clarify the role of dopants in the Ni-rich layered oxides in terms of structural and electrochemical stability, shedding light on the future direction of optimizing layered cathode materials for safer high energy-density secondary batteries.

中文翻译：

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控制富镍层状阴极高压充电可逆性的氧氧化还原活性


通常认为高压下的化学反应和相变决定了高压层状阴极（如富镍菱形氧化物）的电化学性质。即使使用明显更高的 SOC（电荷状态）高于过渡金属氧化还原（主要是 Ni 和 Co）的能力，氧氧化还原对富镍菱形氧化物的影响仍然看起来很神秘，因此需要研究来阐明氧化还原反应和相变之间的关系。在这里，我们对阳离子和阴离子氧化还原反应进行了全面的比较研究，以及一系列有和没有 Al 掺杂的商业富镍层状氧化物的结构演变。我们将 X 射线光谱、原位电化学质谱和中子衍射的结果与电化学性质相结合，从而揭示了与结构和电化学降解相关的不同氧氧化还原活性。我们揭示了 Al 掺杂抑制了不可逆的氧释放，但增强了晶格氧的氧化。凭借这种调制氧氧化还原活性，富镍层状氧化物臭名昭著的 H2-H3 结构相变变得高度可逆。我们的研究结果解开了高压循环过程中不同的氧氧化还原活性，并阐明了掺杂剂在富镍层状氧化物中的结构和电化学稳定性方面的作用，为优化层状正极材料以获得更安全的高能量密度二次电池的未来方向指明了方向。