Liox Power, Inc., Pasadena, California 91106, United States.
J Phys Chem A. 2011 Nov 10;115(44):12399-409. doi: 10.1021/jp2073914. Epub 2011 Oct 18.
There is increasing evidence that cyclic and linear carbonates, commonly used solvents in Li ion battery electrolytes, are unstable in the presence of superoxide and thus are not suitable for use in rechargeable Li-air batteries employing aprotic electrolytes. A detailed understanding of related decomposition mechanisms provides an important basis for the selection and design of stable electrolyte materials. In this article, we use density functional theory calculations with a Poisson-Boltzmann continuum solvent model to investigate the reactivity of several classes of aprotic solvents in nucleophilic substitution reactions with superoxide. We find that nucleophilic attack by O(2)(•-) at the O-alkyl carbon is a common mechanism of decomposition of organic carbonates, sulfonates, aliphatic carboxylic esters, lactones, phosphinates, phosphonates, phosphates, and sulfones. In contrast, nucleophilic reactions of O(2)(•-) with phenol esters of carboxylic acids and O-alkyl fluorinated aliphatic lactones proceed via attack at the carbonyl carbon. Chemical functionalities stable against nucleophilic substitution by superoxide include N-alkyl substituted amides, lactams, nitriles, and ethers. The results establish that solvent reactivity is strongly related to the basicity of the organic anion displaced in the reaction with superoxide. Theoretical calculations are complemented by cyclic voltammetry to study the electrochemical reversibility of the O(2)/O(2)(•-) couple containing tetrabutylammonium salt and GCMS measurements to monitor solvent stability in the presence of KO(2)(•) and a Li salt. These experimental methods provide efficient means for qualitatively screening solvent stability in Li-air batteries. A clear correlation between the computational and experimental results is established. The combination of theoretical and experimental techniques provides a powerful means for identifying and designing stable solvents for rechargeable Li-air batteries.
越来越多的证据表明，环状和线性碳酸酯是锂离子电池电解液中常用的溶剂，在超氧阴离子存在下不稳定，因此不适合用于使用非质子电解液的可充电锂空气电池。对相关分解机制的深入了解为选择和设计稳定的电解质材料提供了重要依据。在本文中，我们使用含泊松-玻尔兹曼连续溶剂模型的密度泛函理论计算来研究几种非质子溶剂在与超氧阴离子的亲核取代反应中的反应性。我们发现，O(2)(•-)对 O-烷基碳的亲核攻击是有机碳酸酯、磺酸盐、脂肪族羧酸酯、内酯、膦酸酯、膦酸盐、磷酸盐和砜类分解的常见机制。相比之下，O(2)(•-)与羧酸酚酯和 O-烷基氟化脂肪族内酯的亲核反应通过羰基碳的攻击进行。对超氧阴离子亲核取代稳定的化学官能团包括 N-烷基取代酰胺、内酰胺、腈和醚。结果表明，溶剂反应性与在与超氧阴离子的反应中取代的有机阴离子的碱性密切相关。理论计算通过循环伏安法得到补充，以研究含有四丁基铵盐的 O(2)/O(2)(•-)偶的电化学可逆性，并通过 GCMS 测量监测 KO(2)(•)和 Li 盐存在下溶剂的稳定性。这些实验方法为在锂空气电池中定性筛选溶剂稳定性提供了有效的手段。建立了计算和实验结果之间的明确相关性。理论和实验技术的结合为识别和设计可充电锂空气电池的稳定溶剂提供了有力手段。
