Crystal and electronic structure changes during the charge-discharge process of Na 4 Co 3 (PO 4 ) 2 P 2 O 7
Crystal and electronic structure changes during the charge-discharge process of Na 4 Co 3 (PO 4 ) 2 P 2 O 7
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DOI：
10.1016/j.jpowsour.2016.07.006
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发表时间：
2016-09
影响因子：
9.2
通讯作者：
H. Moriwake;A. Kuwabara;C. Fisher;M. Nose;H. Nakayama;Shinji Nakanishi;H. Iba;Y. Ikuhara
中科院分区：
文献类型：
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作者：
H. Moriwake;A. Kuwabara;C. Fisher;M. Nose;H. Nakayama;Shinji Nakanishi;H. Iba;Y. Ikuhara
钠离子电池为解决锂资源有限的问题提供了一个潜在的解决方案，而新开发的正极材料Na4Co3(PO4) 2p2o7与其他钠离子电池材料相比，由于其高倍率、高容量和高电压而备受关注。然而，其电子结构和充放电行为的细节仍然不确定。本文采用密度泛函理论的GGA +非形式化方法，详细计算了Na4Co3(PO4) 2p2o7的脱沉淀行为。假设一个逐步脱氢的过程，我们发现Na的去除到NaCo3(PO4) 2p2o7伴随着Co2+氧化成Co3+。进一步去除Na得到Co3(PO4) 2p2o7需要氧化p2o7多面体中的氧离子，而不是将Co3+氧化为Co4+。这样形成的孔被认为是强自困的，使它们在室温下不移动。同时，在这最后的解盐步骤中，观察到一个大的体积收缩，收缩了Na迁移通道。这两个因素可以解释从Na4Co3(PO4)2P2O7中去除所有Na在实验中遇到的困难。
Sodium-ion batteries offer a potential solution to the problem of limited lithium resources, and the newly developed positive electrode material Na4Co3(PO4)2P2O7is attracting significant attention due to its high rate, high capacity, and high voltage compared to other sodium-ion battery materials. However, details of its electronic structure and its charge/discharge behavior are still uncertain. Here we report detailed first-principles calculations of the desodiation behavior of Na4Co3(PO4)2P2O7using the GGA +Uformalism of density functional theory. Assuming a stepwise desodiation process, removal of Na down to NaCo3(PO4)2P2O7is found to be accompanied by oxidation of Co2+to Co3+. Further removal of Na to give Co3(PO4)2P2O7requires oxidation of oxygen 2porbitals in the P2O7polyhedra instead of Co3+being oxidized to Co4+. The holes thus formed are expected to be strongly self-trapped, rendering them immobile at room temperature. At the same time, a large volume shrinkage is observed during this last desodiation step, constricting the Na migration channels. These two factors may explain the difficulty encountered experimentally in removing all Na from Na4Co3(PO4)2P2O7.