Wang Renhai, Wu Shunqing, Zhang Feng, Zhao Xin, Lin Zijing, Wang Cai-Zhuang, Ho Kai-Ming
Department of Physics, University of Science and Technology of China, Hefei 230026, China.
Department of Physics, OSED, Key Laboratory of Low Dimensional Condensed Matter Physics (Department of Education of Fujian Province), Jiujiang Research Institute, Xiamen University, Xiamen 361005, China.
Phys Chem Chem Phys. 2020 Jul 1;22(25):13975-13980. doi: 10.1039/d0cp01056a.
Due to the high cost and insufficient resources of lithium, alternative sodium-ion batteries have been widely investigated for large-scale applications. NaFePO4 has the highest theoretical capacity of 154 mA h g-1 among the iron-based phosphates, which makes it an attractive cathode material for Na-ion batteries. Experimentally, LiFePO4 has been highly successful as a cathode material in Li-ion batteries because its olivine crystal structure provides a stable framework during battery cycling. In NaFePO4, maricite replaces olivine as the most stable phase. However, the maricite phase is experimentally found to be electrochemically inactive under normal battery operating voltages (0-4.5 V). We found that partial substitutions of Na with Li stabilize the olivine structure and may be a way to improve the performance of NaFePO4 cathodes. Using the previously developed structural LiFePO4 database, we examined the low-energy crystal structures in the system when we replace Li with Na. The known maricite and olivine NaFePO4 phases are reconfirmed and an unreported phase with energy between them is identified by our calculations. Besides, the Li-doped olivine type compound LixNa1-xFePO4 with mixed alkali ions retains better energetic stability compared with the other two types of structures of the same composition, as long as the proportion of Li exceeds 0.25. The thermodynamic stability of o-type LixNa1-xFePO4 can be further improved at finite temperatures. The primary limitation of the calculations is that we mainly focus on the zero-temperature condition; however, the relative stability of the structures may vary depending on the ambient temperature.
由于锂的成本高昂且资源不足，人们对用于大规模应用的替代钠离子电池进行了广泛研究。在铁基磷酸盐中，NaFePO4的理论容量最高，为154 mA h g-1，这使其成为钠离子电池有吸引力的正极材料。在实验中，LiFePO4作为锂离子电池的正极材料非常成功，因为其橄榄石晶体结构在电池循环过程中提供了稳定的框架。在NaFePO4中，镁铁矿取代橄榄石成为最稳定的相。然而，实验发现镁铁矿相在正常电池工作电压（0 - 4.5 V）下是电化学惰性的。我们发现用锂部分替代钠可稳定橄榄石结构，这可能是提高NaFePO4正极性能的一种方法。利用先前开发的结构LiFePO4数据库，我们研究了用钠替代锂时该体系中的低能量晶体结构。我们的计算重新确认了已知的镁铁矿和橄榄石NaFePO4相，并确定了它们之间能量的一个未报道的相。此外，只要锂的比例超过0.25，具有混合碱离子的锂掺杂橄榄石型化合物LixNa1-xFePO4与相同组成的其他两种结构类型相比，保留了更好的能量稳定性。在有限温度下，o型LixNa1-xFePO4的热力学稳定性可进一步提高。计算的主要局限性在于我们主要关注零温度条件；然而，结构的相对稳定性可能会因环境温度而异。
