Faraday Materials Laboratory, Materials Research Centre , Indian Institute of Science , C. V. Raman Avenue , Bangalore 560012 , India.
Department of Materials Science and Engineering , National University of Singapore , 9 Engineering Drive 1 , Singapore 117546 , Singapore.
Inorg Chem. 2018 Jun 4;57(11):6324-6332. doi: 10.1021/acs.inorgchem.8b00291. Epub 2018 May 14.
Cubic-framework sodium cobalt-based metaphosphate NaCo(PO) was recently demonstrated to be an attractive Na cationic conductor. It can be potentially used in the next-generation rechargeable Na ion batteries. The crystal structure and electrical conductivity were studied and found to have a three-dimensional framework with interconnecting tunnels for Na migration ( J. Solid State Electrochem. , 2016 , 20 , 1241 ). This inspired us to study the electrochemical (de)intercalation behavior of Na in the NaCo(PO) assuming a cubic Pa3̅ framework. Herein, synergizing experimental and computational tools, we present the first report on the electrochemical activity and Na diffusion pathway analysis of cubic NaCo(PO) prepared via conventional solid-state route. The electrochemical analyses reveal NaCo(PO) to be an active sodium insertion material with well-defined reversible Co/Co redox activity centered at 3.3 V (vs Na/Na). Involving a solid-solution redox mechanism, close to 0.7 Na per formula unit can be reversibly extracted. This experimental finding is augmented with bond valence site energy (BVSE) modeling to clarify Na migration in cubic NaCo(PO). BVSE analyses suggest feasible Na migration with moderate energy barrier of 0.68 eV. Cubic NaCo(PO) forms a 3.3 V sodium insertion material.
最近的研究表明，立方骨架钴基焦磷酸盐 NaCo(PO) 是一种很有前途的钠离子导体，它有望应用于下一代可充电钠离子电池。我们对其晶体结构和电导率进行了研究，发现其具有三维骨架，其中存在相互连接的钠离子迁移隧道（J. Solid State Electrochem., 2016, 20, 1241）。这启发我们研究了 Na 在 NaCo(PO) 中的电化学（脱）嵌入行为，假设其具有立方 Pa3̅ 骨架。在此，我们综合运用实验和计算工具，首次报道了通过传统固相法制备的立方 NaCo(PO) 的电化学活性和 Na 扩散途径分析。电化学分析表明，NaCo(PO) 是一种具有活性的钠离子嵌入材料，其 Co/Co 氧化还原活性在 3.3 V（相对于 Na/Na）处具有明确的可逆性。涉及固溶体氧化还原机制，约 0.7 个 Na+/单位分子可以可逆地提取。这一实验结果通过键价态能（BVSE）模拟得到了进一步证实，该模拟澄清了立方 NaCo(PO) 中的 Na 迁移。BVSE 分析表明，Na 迁移具有可行的迁移通道，其能量势垒适中，为 0.68 eV。立方 NaCo(PO) 形成了一种 3.3 V 的钠离子嵌入材料。
