Li Ke, Subasinghege Don Visal, Gupta Chris S, David Rolf, Kumar Revati
Department of Chemistry, Louisiana State University, 232 Choppin Hall, Baton Rouge, Louisiana 70803, USA.
J Chem Phys. 2021 May 14;154(18):184505. doi: 10.1063/5.0046073.
Sodium-based rechargeable battery technologies are being pursued as an alternative to lithium, in part due to the relative abundance of sodium compared to lithium. Despite their low dielectric constant, glyme-based electrolytes are particularly attractive for these sodium-based batteries due to their ability to chelate with the sodium ion and their high electrochemical stability. While the glyme chain length is a parameter that can be tuned to modify solvation properties, charge transport behavior, reactivity, and ultimately battery performance, anion identity provides another tunable variable. Trifluoromethanesulfonate (triflate/OTf) and bis(trifluoromethane)sulfonamide (TFSI) are chemically similar anions, which are often used in battery electrolytes for lithium-based batteries. In this paper, molecular simulations are used to examine the differences in ion association and charge transport between sodium salts of these two anions at different salt concentrations in glymes with the increasing chain length. The use of the modified force field developed for NaOTf in glymes for the NaTFSI electrolytes was validated by comparing the TFSI-sodium ion radial distribution functions to the results from ab initio molecular dynamics simulations on 1.5 M NaTFSI in diglyme. While the ion association behavior as a function of salt concentration showed similar trends for both NaOTf and NaTFSI in tetraglyme and triglyme electrolytes, the dominant solvation structures for the two sets of electrolytes are distinctly different in the monoglyme and diglyme cases. The conductivity is impacted by both the ion association behavior in these electrolytes and the non-vehicular or hopping transport of the anions in these systems.
钠基可充电电池技术正被视作锂基电池的替代品加以研究，部分原因在于钠相较于锂具有相对丰富的储量。尽管基于甘醇二甲醚的电解质介电常数较低，但因其能够与钠离子螯合且具有较高的电化学稳定性，故而对这些钠基电池极具吸引力。虽然甘醇二甲醚链长是一个可调节的参数，用于改变溶剂化性质、电荷传输行为、反应活性并最终影响电池性能，但阴离子特性提供了另一个可调节变量。三氟甲磺酸根（三氟甲磺酸盐/OTf）和双（三氟甲烷）磺酰胺（TFSI）是化学性质相似的阴离子，常用于锂基电池的电解质中。本文利用分子模拟研究了这两种阴离子的钠盐在不同盐浓度下，于链长不断增加的甘醇二甲醚中离子缔合和电荷传输的差异。通过将TFSI-钠离子径向分布函数与在二甘醇二甲醚中1.5 M NaTFSI的从头算分子动力学模拟结果进行比较，验证了在甘醇二甲醚中为NaOTf开发的修正力场对NaTFSI电解质的适用性。虽然在四甘醇二甲醚和三甘醇二甲醚电解质中，NaOTf和NaTFSI的离子缔合行为随盐浓度变化呈现相似趋势，但在单甘醇二甲醚和二甘醇二甲醚情况下，这两组电解质的主要溶剂化结构明显不同。这些电解质中的离子缔合行为以及这些体系中阴离子的非载流子或跳跃传输都会影响电导率。
