Synthesis and Stoichiometry of Different Layered Sodium Cobalt Oxides
Synthesis and Stoichiometry of Different Layered Sodium Cobalt Oxides
复制标题
DOI：
10.1021/cm5021788
复制
发表时间：
2014-09-23
影响因子：
8.6
通讯作者：
Ceder, Gerbrand
中科院分区：
文献类型：
--
作者：
Lei, Yuechuan;Li, Xin;Ceder, Gerbrand
层状钠金属氧化物可以形成不同的晶体结构，每种晶体结构具有不同的电化学行为。作为更好地理解如何形成每个相的原型系统，我们提出了在固态合成中稳定 NaxCoO2 不同层状相的条件。通过在合成样品上使用非原位 XRD 的新颖组合，并使用原位 XRD 来监测 Na 含量和晶格参数之间的关系，我们能够在空气中 450 至 750 摄氏度之间以及 Na:Co 样品比率范围为 0.60 至 1.05 的情况下构建 NaxCoO2 的相图。根据 XRD 分析，揭示了 O3、O'3、P'3 和 P2 四个单相域。与之前的报道相反，我们发现纯O3、O'3和P'3相只能在固定化学计量x分别为x = 1.00、0.83和0.67时形成，而P2相的形成范围稍大，为0.68至0.76。 O3-Na1.00CoO2 的恒电流充电在电压分布上显示出几个平坦和倾斜的区域，其遵循 O3-O'3-P'3-P3-P'3 的顺序，并且随着板间距离的增加。我们的结果表明，电化学上重要的 P2 结构可能是通过熵来稳定的。
Layered Na-metal oxides can form in different crystal structures, each with different electrochemical behavior. As a prototype system to better understand how each phase can be formed, we present the conditions under which different layered phases of NaxCoO2 can be stabilized in solid-state synthesis. Using a novel combination of ex situ XRD on as-synthesized samples, with in situ XRD to monitor the relation between Na content and lattice parameters, we are able to construct a phase diagram of NaxCoO2 between 450 to 750 degrees C in air and for Na:Co sample ratios ranging from 0.60 to 1.05. Four single phase domains of O3, O'3, P'3, and P2 are revealed based on the XRD analysis. In contrast to previous reports it is found that pure O3, O'3 and P'3 phase can only form at a fixed stoichiometry of x = 1.00, 0.83, and 0.67, respectively, while the P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging of O3-Na1.00CoO2 shows several flat and sloping regions on the voltage profile, which follows the sequence of O3-O'3-P'3-P3-P'3, with increasing interslab distances. Our results indicate that the electrochemically important P2 structure is likely stabilized by entropy.