Lithium Extraction Mechanism in Li-Rich Li2MnO3 Involving Oxygen Hole Formation and Dimerization
Lithium Extraction Mechanism in Li-Rich Li2MnO3 Involving Oxygen Hole Formation and Dimerization
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DOI：
10.1021/acs.chemmater.6b02870
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发表时间：
2016-09-27
影响因子：
8.6
通讯作者：
Islam, M. Saiful
中科院分区：
文献类型：
--
作者：
Chen, Hungru;Islam, M. Saiful
具有层状结构的富锂氧化物电极吸引了相当大的兴趣，因为它们可以为锂离子电池提供高能量密度。然而，关于它们的氧化还原化学存在重大争议。很明显，从富锂Li 2 MnO 3中提取锂的机制尚未完全理解，特别是与观察到的O-2析出和结构转变有关。在这里，锂锰氧化物的脱锂和动力学过程进行了研究，采用高层次的混合泛函从头计算模拟技术，因为它们准确地再现氧空穴态的电子结构。我们发现，锂提取是通过氧化物阴离子的氧化进行电荷补偿，因此整体脱锂反应涉及晶格氧损失。氧（O-）上的局部空穴作为第一步形成，但不稳定，导致氧二聚（O-O类似于1.3埃），并最终形成分子O-2。氧二聚促进Mn迁移到空出的锂层中的八面体位点上。结果表明，可逆的氧氧化还原没有重大的结构变化是唯一可能的，如果本地化的氧空穴稳定和氧二聚抑制。这种理解对于未来优化用于高能量密度电池的新型富锂阴极材料是重要的。
Lithium-rich oxide electrodes with layered structures have attracted considerable interest because they can deliver high energy densities for lithium-ion batteries. However, there is significant debate regarding their redox chemistry. It is apparent that the mechanism of lithium extraction from lithium-rich Li2MnO3 is not fully understood, especially in relation to the observed O-2 evolution and structural transformation. Here, delithiation and kinetic processes in Li2MnO3 are investigated using ab initio simulation techniques employing high level hybrid functionals as they reproduce accurately the electronic structure of oxygen hole states. We show that Li extraction is charge-compensated by oxidation of the oxide anion, so that the overall delithiation reaction involves lattice oxygen loss. Localized holes on oxygen (O-) are formed as the first step but are not stable leading to oxygen dimerization (with O-O similar to 1.3 angstrom) and eventually to the formation of molecular O-2. Oxygen dimerization facilitates Mn migration onto octahedral sites in the vacated lithium layers. The results suggest that reversible oxygen redox without major structural changes is only possible if the localized oxygen holes are stabilized and oxygen dimerization suppressed. Such an understanding is important for the future optimization of new lithium-rich cathode materials for high energy density batteries.