Torayev Amangeldi, Engelke Simon, Su Zeliang, Marbella Lauren E, De Andrade Vincent, Demortière Arnaud, Magusin Pieter C M M, Merlet Céline, Franco Alejandro A, Grey Clare P
Laboratoire de Réactivité et Chimie des Solides (LRCS), UMR CNRS 7314, Université de Picardie Jules Verne, Hub de l'Energie, 15 Rue Baudelocque, Amiens 80039, France.
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
J Phys Chem C Nanomater Interfaces. 2021 Mar 11;125(9):4955-4967. doi: 10.1021/acs.jpcc.0c10417. Epub 2021 Feb 25.
Li-O batteries offer a high theoretical discharge capacity due to the formation of light discharged species such as LiO, which fill the porous positive electrode. However, in practice, it is challenging to reach the theoretical capacity and completely utilize the full electrode pore volume during discharge. With the formation of discharge products, the porous medium evolves, and the porosity and tortuosity factor of the positive electrode are altered through shrinkage and clogging of pores. A pore shrinks as solid discharge products accumulate, the pore clogging when it is filled (or when access is blocked). In this study, we investigate the structural evolution of the positive electrode through a combination of experimental and computational techniques. Pulsed field gradient nuclear magnetic resonance results show that the electrode tortuosity factor changes much faster than suggested by the Bruggeman relation (an equation that empirically links the tortuosity factor to the porosity) and that the electrolyte solvent affects the tortuosity factor evolution. The latter is ascribed to the different abilities of solvents to dissolve reaction intermediates, which leads to different discharge product particle sizes: on discharging using 0.5 M LiTFSI in dimethoxyethane, the tortuosity factor increases much faster than for discharging in 0.5 M LiTFSI in tetraglyme. The correlation between a discharge product size and tortuosity factor is studied using a pore network model, which shows that larger discharge products generate more pore clogging. The Knudsen diffusion effect, where collisions of diffusing molecules with pore walls reduce the effective diffusion coefficients, is investigated using a kinetic Monte Carlo model and is found to have an insignificant impact on the effective diffusion coefficient for molecules in pores with diameters above 5 nm, , most of the pores present in the materials investigated here. As a consequence, pore clogging is thought to be the main origin of tortuosity factor evolution.
锂-氧电池由于形成了诸如LiO等轻质放电产物而具有较高的理论放电容量，这些产物填充了多孔正极。然而，在实际中，要达到理论容量并在放电过程中完全利用整个电极孔体积具有挑战性。随着放电产物的形成，多孔介质会发生演变，并且正极的孔隙率和曲折因子会通过孔隙的收缩和堵塞而改变。当固体放电产物积累时孔隙会收缩，当孔隙被填满（或通道被堵塞）时就会发生孔隙堵塞。在本研究中，我们通过实验和计算技术相结合的方法研究了正极的结构演变。脉冲场梯度核磁共振结果表明，电极曲折因子的变化比布鲁格曼关系式（一个将曲折因子与孔隙率经验性联系起来的方程）所表明的要快得多，并且电解质溶剂会影响曲折因子的演变。后者归因于溶剂溶解反应中间体的能力不同，这导致了不同的放电产物粒径：在使用0.5 M LiTFSI的二甲基乙二醚溶液放电时，曲折因子的增加比在0.5 M LiTFSI的四甘醇二甲醚溶液中放电要快得多。使用孔隙网络模型研究了放电产物尺寸与曲折因子之间的相关性，结果表明较大的放电产物会导致更多的孔隙堵塞。利用动力学蒙特卡洛模型研究了努森扩散效应，即扩散分子与孔壁的碰撞会降低有效扩散系数，结果发现对于直径大于5 nm的孔（这里研究的材料中存在的大多数孔）中的分子，该效应对有效扩散系数的影响微不足道。因此，孔隙堵塞被认为是曲折因子演变的主要原因。
