Na-ion batteries are promising devices for smart grids and electric vehicles due to cost effectiveness arising from the overall abundance of sodium (Na) and its even geographical distribution. Among other factors, the energy density of Na-ion batteries is limited by the positive electrode chemistry. NaSICON-based positive electrode materials are known for their wide range of electrochemical potentials,[1],[2],[3] high ionic conductivity, and most importantly their structural and thermal stabilities. Using first- principles calculations, we chart the chemical space of 3d transition metal-based NaSICON phosphates of formula NaxMM’(PO4)3 (with M and M’= Ti, V, Cr, Mn, Fe, Co and Ni), to analyze their thermodynamic stabilities and the intercalation voltages for Na+ ions. Specifically, we computed the Na insertion voltages and related properties of 28 distinct NaSICON compositions. We investigated the thermodynamic stability of Na-intercalation in previously unreported NaxMn2(PO4)3 and NaxVCo(PO4)3. The calculated quaternary phase diagrams of the Na-P-O-Co and Na-P-O-Ni chemical systems explain the origin of the suspected instability of Ni and Co-based NaSICON compositions. From our analysis, we are also able to rationalize anomalies in previously reported experimental data in this diverse and important chemical space.

中文翻译：

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钠离子电池用NaSiCON电极材料的化学图

由于钠（Na）的总体丰度及其均匀的地理位置所带来的成本效益，Na离子电池是用于智能电网和电动汽车的有前途的设备。除其他因素外，Na离子电池的能量密度受正极化学物质的限制。基于NaSICON的正电极材料因其广泛的电化学势，[1]，[2]，[3]高离子电导率，以及最重要的是其结构和热稳定性而闻名。使用第一原理计算，我们绘制了分子式为NaxMM'（PO4）3（M和M'= Ti，V，Cr，Mn，Fe，Co和Ni）的3d过渡金属基NaSICON磷酸盐的化学空间，分析它们的热力学稳定性和Na +离子的嵌入电压。特别，我们计算了28种不同的NaSICON成分的Na插入电压和相关特性。我们调查了以前未报道的NaxMn2（PO4）3和NaxVCo（PO4）3中Na嵌入的热力学稳定性。Na-PO-Co和Na-PO-Ni化学体系的四元相图计算结果说明了Ni和Co基NaSICON组合物可能存在的不稳定性。从我们的分析中，我们还能够合理化先前报告的实验数据在这种多样且重要的化学空间中的异常。Na-PO-Co和Na-PO-Ni化学体系的四元相图计算结果说明了Ni和Co基NaSICON组合物可能存在的不稳定性。从我们的分析中，我们还能够合理化先前报告的实验数据在这种多样且重要的化学空间中的异常。Na-PO-Co和Na-PO-Ni化学体系的四元相图计算结果说明了Ni和Co基NaSICON组合物可能存在的不稳定性。从我们的分析中，我们还能够合理化先前报告的实验数据在这种多样且重要的化学空间中的异常。