Sodium layered transition metal oxides have been considered as promising cathode materials for sodium ion batteries due to their large capacity and high operating voltage. However, mechanism investigations of chemical evolution and capacity failure at high voltage are inadequate. As a representative cathode, Na2/3Ni1/3Mn2/3O2, the capacity contribution at a 4.2 V plateau has long been assigned to the redox of the Ni3+/Ni4+ couple, while at the same time it suffers large irreversible capacity loss during the initial discharging process. In this work, we prove that the capacity at the 4.2 V plateau is contributed to the irreversible O2-/O2 n-/O2 evolution based on in situ differential electrochemical mass spectrometry and density functional theory calculation results. Besides, a phenomenon of oxygen release and subsequent surface lattice densification is observed, which is responsible for the large irreversible capacity loss during the initial cycle. Furthermore, the oxygen release is successfully suppressed by Fe substitution due to the formation of a unique Fe-(O-O) species, which effectively stabilizes the reversibility of the O2-/O2 n- redox at high operating voltage. Our findings provide a new understanding of the chemical evolution in layered transition metal oxides at high operating voltage. Increasing the covalency of the TM-O bond has been proven to be effective in suppressing the oxygen release and hence improving the electrochemical performance.

中文翻译：

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回顾Na2 / 3Ni1 / 3Mn2 / 3O2阴极：氧还原化学和氧释放抑制。

钠层状过渡金属氧化物由于其大容量和高工作电压而被认为是钠离子电池的有前途的阴极材料。但是，在高压下化学放出和容量破坏的机理研究不足。作为代表性的阴极Na2 / 3Ni1 / 3Mn2 / 3O2，长期以来在4.2 V平台上的容量贡献已分配给Ni3 + / Ni4 +对的氧化还原，而与此同时，它在初始放电期间遭受了不可逆的大容量损失处理。在这项工作中，我们基于原位差分电化学质谱法和密度泛函理论计算结果证明，在4.2 V平台上的容量有助于不可逆O2- / O2 n- / O2的演化。除了，观察到氧气释放现象和随后的表面晶格致密化现象，这是在初始循环期间大量不可逆容量损失的原因。此外，由于形成独特的Fe-（OO）物种，Fe取代成功地抑制了氧的释放，这有效地稳定了O2- / O2 n-氧化还原在高工作电压下的可逆性。我们的发现为在高工作电压下层状过渡金属氧化物的化学演化提供了新的认识。已证明增加TM-O键的共价键可有效抑制氧释放并因此改善电化学性能。由于形成了独特的Fe-（OO）物种，Fe取代成功地抑制了氧的释放，这有效地稳定了高工作电压下O2- / O2 n-氧化还原的可逆性。我们的发现为在高工作电压下层状过渡金属氧化物的化学演化提供了新的认识。已证明增加TM-O键的共价键可有效抑制氧释放并因此改善电化学性能。由于形成了独特的Fe-（OO）物种，Fe取代成功地抑制了氧的释放，这有效地稳定了高工作电压下O2- / O2 n-氧化还原的可逆性。我们的发现为在高工作电压下层状过渡金属氧化物的化学演化提供了新的认识。已证明增加TM-O键的共价键可有效抑制氧释放并因此改善电化学性能。