P2-Na2/3Mn(1-x)Znx(Cux)O2钠离子电池正极材料性能的研究

正极材料是制约电池能量密度的关键。基于过渡金属离子与晶格氧双重氧化还原反应的层状过渡金属氧化物具有高的比容量，是下一代高比能量钠离子电池的首选正极材料。然而，阴离子氧化还原反应的过程往往伴随着晶格结构的剧烈变化以及晶格氧的损失，导致正极材料极大的电压滞后和容量衰减。近年来的研究表明，在 P2 结构层状氧化物的电化学反应中，材料不但可能同时具有阳离子和阴离子的氧化还原活性，而且在循环过程中还能保持较高的结构稳定性。然而，关于该类型材料的研究还局限于碱金属或碱土金属掺杂的化合物中。在这种特殊的结构中，进一步拓宽该类型材料的研究范围、深入研究阴离子氧化反应的结构基础、理解阴离子氧化反应过程中稳定晶格氧的有效机制，对探索高容量电池电极材料有重要意义。因此，本文通过对 P2-Na 2/3 Mn （ 1-x ） M x O 2（M=Zn、Cu）系列材料的研究，探索其中过渡金属和氧的相互作用，并进一步加深对阴离子氧化反应机理以及晶格氧稳定机制的理解。本文研究了不同含量 Zn 掺杂对 P2-Na 2/3 MnO 2 材料结构和电化学性能的影响。结果表明，Zn 的掺入并未影响材料的长程有序结构，所有材料均属于P63/mmc 空间群。但 Zn 的掺入对材料的电化学性能有较大影响，对 P2-Na 2/3 Mn 0.72 Zn 0.28 O 2 进行深入研究，该材料具有 157 mAh/g 的高可逆比容量，其绝大部分容量由位于 4.1 V 附近的长平台、基于 O 2- /(O 2 ) n- 氧化还原所获得的容量提供。由 Zn-O 键之间弱相互作用所形成的非键态 O-2p 轨道和由缺钠结构所产生的氧氧键的缩短，一起激发了阴离子的氧化还原反应。且由于在整个充放电过程中，O 至少可以和三个在过渡金属层中的阳离子相连，使晶格氧稳定性提高。但材料发生了 P2-O2 的可逆相变，该相变不利于材料的循环稳定性。本文进一步研究了 Cu 掺杂 P2-Na 2/3 Mn 0.72 Cu 0.28 O 2 材料的结构与电化学性能，该体系中 Cu-O 键之间的相互作用要强于 Zn-O 键。该材料同属于 P63/mmc空间群，基于过渡金属离子和阴离子氧的共同氧化还原反应，该化合物具有 104 mAh/g 的可逆比容量。理论计算结果表明由于该体系中弱的 Cu 3+ -O 共价键，使得氧非键态的存在成为可能，从而激发了阴离子氧化还原反应。该材料无相变发生的特性和稳定的氧堆积顺序提高了充放电过程中的循环稳定性并减小了电压迟滞。综上所述，Zn 和 Cu 的掺杂有利于激发材料中的阴离子氧化还原反应，进一步提高材料的能量密度，而且该掺杂还在提高材料晶体结构和晶格氧的稳定性方面具有重要作用。这些发现为设计高能量密度以及高稳定性的正极材料提供了有利的参考。

Cathode materials are crucial for developing sodium-ion batteries with high energy density. Layered transition metal oxides show high specific capacity owning to the redox of transition metal and oxygen, emerging as a new path to optimize the electrochemical performance of cathodes. However, activating the anionic redox reaction of oxygen in transition metal oxides generally leads to oxygen loss and structural instability upon cycling, resulting in large voltage hysteresis and capacity fading. Recently, P2-layered oxides have been reported to have cationic and anionic redox activities and showed a high structural stability during cycles. However, the studies about P2-layered oxides with cationic and anionic redox activities are limited in compounds doped by alkaline/alkaline earth metals. It is very important to further expand the playgrounds of materials with anionic redox in this special structure and explore the effective mechanism to stabilize the lattice oxygen. Herein, we pay attention to the anionic redox reaction in a system with interaction between oxygen and transition metals, and explore the lattice oxygen stabilizaition mechanism in P2- Na 2/3 Mn (1-x) Zn(Cu) x O 2 .We focus on the structural and electrochemical performance of P2-Na 2/3 Mn (1- x) Zn x O 2 . Our results indicate that the doping of Zn has a limited influence of crystal structure and all compounds crystalize in P63/mmc space group. However, charge and discharge profiles show a huge variation with the amount of Zn elements. Na 2/3 Mn 0.72 Zn 0.28 O 2 is used as a prototype to investigate the charge compensation and structural evolution mechanism during cycles. The compound delivers a total capacity of 157 mAh/g, stemming from the redox of O 2- /(O 2 ) n- and Mn 3+ /Mn 4+ . The O-2p nonbonding states originating from the weak Zn-O interaction and the shortened O- O bonds induced by the absence of Na atoms in the alkali layers catalyze the anionic redox reaction. The sample undergoes a reversible phase transition from P2 to O2 during charge and discharge progress, which has a negative impact on its cycling stability. We investigate the structural and electrochemical performance of Na 2/3 Mn 0.72 Cu 0.28 O 2 . The compound crystalizes in P63/mmc space group and has provided a highly reversible capacity of 104 mAh/g with a smooth voltage profile originating from both cationic and anionic redox reaction. Density functional theory calculations show that the nonbonding states of O-2p states along the Cu-O bonds promote the oxygen redox activity. The non-phase transition properties and stable oxygen stacking sequence in the electrochemical cycles are beneficial to small voltage hysteresis and cycling performance. All in all, the effects of the doped Zn and Cu elements in electrochemical performance are studied. Zn and Cu ions make contributions to improve the energy density by activating anionic redox reaction in materials. These findings may provide new aspects in the anionic redox process and offer a new strategy to design cathodes with high energy density and structural stability.