Abstract—

This work assesses the applicability of the EMF method for determination of the values of thermodynamic functions for the Li–S electrochemical system in lithium–sulfur cells with various states of charge. It is shown that the EMF method can be used for determination of the values of thermodynamic functions for the Li–S electrochemical systems in lithium–sulfur cells with various states of discharge only in the first charge–discharge cycle. The EMF method is inapplicable in the following charge–discharge cycles owing to disturbance of equilibrium in the electrochemical system due to the direct chemical interaction between sulfur and high-order polysulfides of lithium (Li₂S_n, n > 4) and the metallic lithium electrode. The thermodynamic functions of the Li–S system with different sulfur reduction degree are in the following ranges: ΔG = –480…–410 kJ/mol; ΔH = –490…–420 kJ/mol; ΔS = –120…–20 J/(mol K) at the temperature of 303 K. Quantum–chemical calculations of the values of thermodynamic functions are carried out for electrochemical reduction of sulfur and lithium polysulfides. The calculated values of thermodynamic functions agree reasonably with the measured values. The thermodynamic efficiency of energy conversion is estimated for discharge of lithium–sulfur cells at 30°C; it is 93–98%.

中文翻译：

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EMF法确定Li–S电化学系统热力学函数的可能性

摘要—

这项工作评估了EMF方法在确定具有不同电荷状态的锂硫电池中Li-S电化学系统的热力学函数值时的适用性。结果表明，仅在第一个充放电循环中，EMF方法可用于确定具有各种放电状态的锂硫电池中Li-S电化学系统的热力学函数值。由于硫与锂的高阶多硫化物之间的直接化学相互作用，由于电化学系统中的平衡干扰，EMF方法不适用于以下充放电循环（Li ₂ S _n，n> 4）和金属锂电极。具有不同硫还原度的Li–S系统的热力学函数在以下范围内：ΔG = –480…–410 kJ / mol；Δ ħ = -490 ... -420千焦/摩尔; Δ小号= -120 ... -20焦耳/（摩尔K）在热力学函数的值的303 K.量子化学计算的温度是为电化学还原硫和多硫化锂的情况下进行。热力学函数的计算值与测量值合理地一致。估计在30°C时锂硫电池放电的能量转换的热力学效率；是93–98％。