三角双锥结构NaxTiO2的第一性原理研究：钠离子电池负极应用
First-Principles Study on NaxTiO2 with Trigonal Bipyramid Structures: An Insight into Sodium-Ion Battery Anode Application
摘要：
开发具有低电极电压、高比容量和优异倍率性能的高效负极材料是商业上可行的钠离子电池（SIB）的迫切需要。为了寻找一种新型的SIB负极材料，我们采用第一性原理计算研究了具有五氧配位三角双锥（TB）结构的NaxTiO 2化合物的电化学性质.确定了四种不同的TB相，我们进行了它们的晶体结构的优化，并计算了它们的能量学，如钠结合能，形成能，电极电位和Na离子迁移的活化能。计算表明，TB-I相可以是SIB阳极材料的四个TB相中的最佳选择，这是由于在Na插入时相对低的体积变化低于4%，在1.0V以下的低电极电压，在x = 1时完全钠化有可能实现约335 mAh/g的最高比容量，以及在Na含量从x = 0.125到x = 0.5时低于0.35eV的合理低的激活势垒。通过对Na化过程中电子态密度和电荷密度差的分析，发现TB相中的NaxTiO 2化合物由于Na原子向Ti离子的电子转移，从电子绝缘材料转变为电子导电材料，有序化了Ti4 +/Ti 3+氧化还原对，有利于SIB操作。
英文摘要：
Developing efficient anode materials with low electrode voltage, high specific capacity and superior rate capability is urgently required on the road to commercially viable sodium-ion batteries (SIBs). Aiming at finding a new SIB anode material, we investigate the electrochemical properties of NaxTiO2 compounds with unprecedented penta-oxygen-coordinated trigonal bipyramid (TB) structures by using the first-principles calculations. Identifying the four different TB phases, we perform the optimization of their crystal structures and calculate their energetics such as sodium binding energy, formation energy, electrode potential and activation energy for Na ion migration. The computations reveal that TB-I phase can be the best choice among the four TB phases for the SIB anode material due to relatively low volume change under 4% upon Na insertion, low electrode voltage under 1.0 V with a possibility of realizing the highest specific capacity of ~335 mAh/g from fully sodiation at x = 1, and reasonably low activation barriers under 0.35 eV at the Na content from x = 0.125 to x = 0.5. Through the analysis of electronic density of states and charge density difference upon sodiation, we find that the NaxTiO2 compounds in TB phases change from electron insulating to electron conducting material due to the electron transfer from Na atom to Ti ion, ordering the Ti 4+/Ti 3+ redox couple for SIB operation.
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