[论文解读] Lithium Metal Penetration Induced by Electrodeposition through Solid Electrolytes: Example in Single-Crystal Li6La3ZrTaO12 Garnet

Tushar Swamy, Richard Park+7 more|arXiv (Cornell University)|Jan 1, 2018

Advancements in Battery Materials参考文献 1被引用 20

一句话总结

本研究在高电流密度（5–10 mA/cm²）下探究锂金属穿透单晶Li6La3ZrTaO12（LLZTO）石榴石固态电解质的机制，发现裂纹起始主要发生在集流体边缘，这是由于电场增强所致，而非较大预存的表面缺陷。有限元建模显示，在电极不连续处10 μm范围内电场增强达5倍，表明电场集中是电化学机械失效的主要驱动力，其影响超过较大缺陷的影响，对所有固态电池设计构成关键挑战。

ABSTRACT

Solid electrolytes are considered a potentially enabling component in rechargeable batteries that use lithium metal as the negative electrode, and thereby can safely access higher energy density than available with today's lithium ion batteries. To do so, the solid electrolyte must be able to suppress morphological instabilities that lead to poor coulombic efficiency and, in the worst case, internal short circuits. In this work, lithium electrodeposition experiments were performed using single-crystal Li6La3ZrTaO12 garnet as solid electrolyte layers to investigate the factors that determine whether lithium penetration occurs through brittle inorganic solid electrolytes. In these single crystals, grain boundaries are excluded as possible paths for lithium metal propagation. However, Vickers microindentation was used to introduce sharp surface flaws of known size. Using operando optical microscopy, it was found that lithium metal penetration sometimes initiates at these controlled surface defects, and when multiple indents of varying size were present, propagates preferentially from the largest defect. However, a second class of flaws was found to be equally or more important. At the perimeter of surface current collectors, an enhanced electrodeposition current density causes lithium metal filled cracks to initiate and grow to penetration, even when the large Vickers defects are in close proximity. Modeling the electric field concentration for the experimental configurations, it was shown that a factor of 5 enhancement in field can readily occur within 10 micrometers of current collector discontinuities, which we interpret as the origin of electrochemomechanical stresses leading to failure. Such field amplification may determine the sites where supercritical surface defects dominate lithium metal propagation during electrodeposition, overriding the presence of larger defects elsewhere.

研究动机与目标

• 理解脆性固态电解质（如单晶LLZTO石榴石）中锂金属电镀的失效机制。
• 识别在高电流密度下锂金属填充裂纹传播的主导起始位置。
• 研究表面缺陷与几何电场增强在固态电解质中裂纹起始的控制作用。
• 评估电极边缘电场集中对触发电化学机械失效的作用。
• 通过模拟电场放大效应，为全固态电池提供设计指导。

提出的方法

• 在5–10 mA/cm²电流密度下对单晶LLZTO石榴石进行恒电流电镀，模拟1C–2C充电速率。
• 采用维氏压痕在表面制造已知尺寸的可控缺陷，以隔离缺陷驱动的裂纹起始。
• 通过原位光学显微镜实时追踪电镀过程中的锂镀层及裂纹扩展。
• 通过离位显微镜及AFM/SEM表征分析失效后的微观结构与裂纹形貌。
• 利用COMSOL Multiphysics进行有限元建模，模拟电场分布，重点关注集流体边缘的电场增强。
• 通过改变电极尺寸比和电解质厚度与宽度比，评估电场增强因子。

实验结果

研究问题

• RQ1 在高电流密度电镀条件下，单晶LLZTO石榴石中锂金属穿透的起始位置由什么决定？
• RQ2 预存表面缺陷（如维氏压痕）与集流体边缘的几何电场增强相比，其影响如何？
• RQ3 电极边缘的电场放大在多大程度上驱动固态电解质中的裂纹起始与扩展？
• RQ4 电场集中效应是否足以覆盖较大人为引入的表面缺陷的影响，从而主导失效位置？
• RQ5 电极几何形状与长宽比如何影响全固态电池中锂穿透的风险？

主要发现

• 即使压痕位于边缘10 μm以内，锂金属穿透仍主要发生在金集流体边缘，而非较大预存的维氏压痕处。
• 计算表明，在集流体不连续处10 μm范围内电场增强达5倍，解释了局部裂纹起始的原因。
• 裂纹扩展稳定且为亚表面扩展，与超临界表面缺陷的应力驱动生长一致。
• 当边缘电场增强存在时，即使存在大尺寸表面缺陷，其对裂纹起始的主导作用也被覆盖，表明电场集中是主要失效触发因素。
• 有限元建模证实，降低厚度与宽度比并增大正极相对于负极的尺寸，可减少电场放大，从而降低失效风险。
• 在10 mA/cm²电流密度下，锂穿透2 mm厚LLZTO的时间不足1分钟，表明在实际充电条件下存在快速短路风险。