Electrochemical mechanism of high Na-content P2-type layered oxides for sodium-ion batteries
Electrochemical mechanism of high Na-content P2-type layered oxides for sodium-ion batteries
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DOI：
10.1007/s12598-020-01403-7
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发表时间：
2020-04
期刊：
影响因子：
8.8
通讯作者：
Ying Yang;Weifeng Wei
中科院分区：
文献类型：
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作者：
Ying Yang;Weifeng Wei
P2型阴极的容量和结构问题的发展迄今为止集中在离子掺杂/取代策略上。在最近发表在《美国化学学会杂志》上的一份报告中，Hu及其同事证明了高Na含量的P2型层状氧化物具有更高的容量和更高的结构稳定性。P2型层状氧化物的分子式NaxTMO 2（TM：过渡金属），使快速钠？迁移过程中，由于直接钠？TMO 2板之间的传输路径，并提供实现高循环/速率能力的机会，作为钠离子电池（SIB）的最有潜力的电极，已经获得了很多关注[1]。然而，TM氧化还原电对的高活性电压导致在4.0V以下的低容量，并且在充电/放电期间的不利相变（P2到O2或OP 4/“Z”相）导致循环不稳定性，这已经成为其实际应用的主要障碍[2，3]。许多努力致力于提高P2型材料的电化学性能。化学掺杂Mg 2？，钛4？，铜2？李？据报道，减轻但不能完全抑制结构不稳定性[4-6]。此外，TM 3？基于P2型氧化物已被证明有效地增加充电容量，但通常遭受结构转变和空气敏感性[7]。开发高Na含量的P2型材料也被认为是从根本上解决上述问题的有希望的策略。P2主体中的高Na含量确保可以减轻TMO 2板之间的静电排斥，以避免从P型到O型堆叠的结构转变[8]。此外，预期高Na含量材料在较低截止电压下显示出较高的Na储存容量，这归因于较低的平均氧化态和TM的升高的3d eg* 水平[8]。在最近发表在《美国化学学会杂志》上的一篇报告中，中国科学院大学的胡和同事报告说，利用Li？替代[8]。X射线衍射（XRD）和高分辨透射电子显微镜（HRTEM）分析表明，所制备的材料具有典型的P2型层状结构，空间群为P63/mmc。当在2.0-4.0 V电压范围内测试时，这种高Na含量的P2型阴极提供比来自原始Na 2/3 Ni 1/3 Mn 2/3 O2的 * 82 mAh 4g-1（图1 a）更高的 * 102.5 mAh 4g-1（图1 B）的可逆容量。有趣的是，这种材料所表现出的较高的容量源于较少量的Ni 2？(16与Na_（2/3）Ni_（1/3）Mn_（2/3）O_2材料（1/3 mol）相比，Na_（2/3）Ni_（1/3）Mn_（2/3）O_2材料的电子氧化速率随Na含量的增加而增加，表明Na含量越高，Ni_（2/3）Ni_（1/3）Mn_（2/3）O_2材料的电子氧化速率越快。Ni3？氧化还原电对当截止电压增加到4.6 V时，在该材料中获得 * 150 mAh 4g-1的充电容量（图1c），揭示了几乎 * 0.58 mol Na？已经被驱逐了通常，高截止电压促进更多的Na？但会导致结构降解和电解质分解[9]。然而，在材料中没有观察到显著的容量衰减。作者进行了第一性原理密度泛函理论（DFT）计算，以获得更多的见解不同的截止电压下的氧化还原活性。这是从Mn离子的磁化强度计算表明，Mn 4？在电化学过程中是不活跃的
The development on capacity and structure issues of P2-type cathode has so far focused on ion-doping/substitution strategy. In a recent report published in Journal of the American Chemical Society, Hu and colleagues demonstrated that high Na-content P2-type layered oxides exhibit higher capacities as well as great structural stability. P2-type layered oxides with the formula of NaxTMO2 (TM: transition metal), which enable the fast Na? migration process due to the direct Na? transport pathways between the TMO2 slabs and provide the opportunity to achieve high cycle/rate capabilities, have been gaining much attention as the most potential electrodes for sodiumion batteries (SIBs)[1]. However, the high active voltage of TM redox couples leads to low capacity under 4.0 V, and the unfavorable phase transitions (P2 to O2 or OP4/‘Z’phases) during charge/discharge contribute to cycling instability, which have become major obstacles to their practical applications [2, 3]. Numerous efforts have been devoted to enhancing the electrochemical properties of P2-type materials. Chemical doping with Mg2?, Ti4?, Cu2? and Li? were reported to alleviate but not completely suppress the structural instability [4–6]. In addition, the TM3?-based P2-type oxides have been proved effective to increase charge capacity, but often suffer from structural transitions and air sensitivity [7]. Developing high Na-content P2-type materials is also regarded as a promising strategy to address above issues fundamentally. High Na-content in P2 host ensures that the electrostatic repulsions between the TMO2 slabs can be alleviated to avoid the structural transition from the P-toO-type stackings [8]. Furthermore, high Na-content materials are expected to show higher Na storage capacity under lower cutoff voltage, which is ascribed to the lower average oxidation state and the raised 3d eg* level of TMs [8]. In a recent report published in Journal of the American Chemical Society, Hu and colleagues at the University of Chinese Academy of Sciences reported that a high-Na P2-type oxide with a chemical composition of Na45/54Li4/54 Ni16/54Mn34/54O2 was successfully synthesized by Li? substitution [8]. X-ray diffraction (XRD) pattern and highresolution transmission electron microscopy (HRTEM) image display that the as-prepared material is isostructural with typical P2-type layered structure with P63/mmc space group. When tested in 2.0–4.0 V voltage range, this high Na-content P2-type cathode delivers higher reversible capacity of* 102.5 mAh4g-1(Fig. 1 b) than* 82 mAh4g-1 from pristine Na2/3Ni1/3Mn2/3O2 (Fig. 1 a). Interestingly, the higher capacity exhibited by this material originates from smaller amount of Ni2?(16/54 mol) in contrast to Na2/3Ni1/3Mn2/3O2 material (1/3 mol), demonstrating that higher Na-content can promote the electron oxidation of Ni2?/Ni3? redox couple. When the cutoff voltage was increased to 4.6 V, a charge capacity of* 150 mAh4g-1 is obtained (Fig. 1 c) in this material, revealing that almost* 0.58 mol Na? has been deintercalated. Usually, a high cutoff voltage promotes more Na? to be extracted, but results in structural degradation and electrolyte decomposition [9]. However, there is no significant capacity decay observed in the material. The authors conducted the first-principles density functional theory (DFT) calculations to obtain more insights into the redox activity under different cutoff voltages. It is indicated from the calculated magnetization of Mn ions that Mn4? is inactive during the electrochemical process