Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V. , Helmholtzstraße 20 , D-01069 Dresden , Germany.
Experimental Physics VI, Center for Electronic Correlations and Magnetism , University of Augsburg , D-86159 Augsburg , Germany.
ACS Appl Mater Interfaces. 2018 Oct 24;10(42):36108-36119. doi: 10.1021/acsami.8b11609. Epub 2018 Oct 10.
Two new structural forms of Na CoTiO, the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the Na CoTiO cathode), we observed a structural phase transformation of O3-NaCoTiO into P3-Na CoTiO, whereas no changes other than conventional unit cell volume shrinkage were detected for P3-NaCoTiO. During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na CoTiO was impeded for both materials and occurs well below 1 V versus Na/Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co → HS-Co) and by a significant unit cell volume expansion at the P3 → O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na/Na during discharging allows to complete the P3 → O3 transformation and increases the specific discharging capacity to 200 mA h g. Moreover, a quasi-symmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g.
两种新型的 NaCoTiO 结构形式，层状 O3- 和 P3-形式，被合成并进行了全面的表征。这两种材料都表现出作为钠离子电池电极的电化学活性。在电池充电过程中（NaCoTiO 阴极脱钠），我们观察到 O3-NaCoTiO 向 P3-NaCoTiO 的结构相变，而对于 P3-NaCoTiO，除了常规的单元体积收缩外，没有检测到其他变化。在 Na 插入过程中（电池放电），两种材料的 P3 型向 O3-NaCoTiO 的再转化受到阻碍，仅在 1 V 以下相对于 Na/Na 进行。再转化受到 Co 的电荷和自旋转移（LS-Co→HS-Co）以及 P3→O3 转变时单元体积显著膨胀的阻碍，这从磁化、晶体学和光谱研究中得到了揭示。由于这种转变的动力学取决于许多参数，如时间、温度和颗粒大小，因此会产生较大的电池过电势。在放电过程中，将截止电压扩展到 0.2 V 相对于 Na/Na，可以完成 P3→O3 转变，并将比放电容量提高到 200 mA h g。此外，基于 O3-和 P3-形式的准对称全电池被设计出来，消除了与钠阳极相关的安全问题，并提供了 130 mA h g 的放电容量。
