摘要
电池内部不可控的枝晶生长问题严重地影响着电池的循环性能和安全性能,这对于锂金属电池的实际应用是一个严峻的挑战.尽管已有较多的实验和理论研究工作聚焦于电极间锂离子各向异性输运特性对枝晶形貌的影响,但仍有一些开放性的问题有待进一步研究,例如,如何将枝晶生长的动态演变与电解液性质、电势分布或隔膜多孔结构诱导的锂离子各向异性输运关联起来.我们通过将锂离子在电解液中的扩散系数(DL)表示为二阶张量的形式并进行相场模拟,发现Dyy:Dxx比值的增加,以及电势诱导的电极/电解液界面锂离子快速扩散层均可以降低界面处锂离子浓度梯度和电势梯度,从而减弱枝晶生长的驱动力.我们还发现隔膜基体与y方向之间夹角的增大也会显著促进电解质中的锂离子各向异性输运特性,以利于抑制枝晶生长.籍此本文提出设计Dyy:Dxx = 10:1的电解液和基体倾斜角为arctan(0.5)的隔膜用于锂金属电池.该相场研究有望为设计具有抑制枝晶能力的电解质或隔膜提供指导.
Abstract
Lithium metal is a promising anode candidate for high-energy-density secondary batteries due to its high theoretical capacity and low electrochemical potential,while the uncontrolled dendrite growth causing poor cycling performance and safety concerns poses serious challenges for the practical application of lithium metal batteries.During the electrodeposition process,the lithium-ion(Li+)diffusion process is directly related to the electrode/electrolyte interfacial Li+ concentration gradient as well as the dendritic morphology.Regulating the anisotropic Li+ diffusion property is a convenient way to reshape its transfer behavior without introducing any external fields(e.g.,temperature field,magnetic field,acoustic field,etc.)or increasing the weight of batteries.Despite the large amount of experimental and theoretical work on the effect of the anisotropic Li+ diffusion behavior on the dendritic morphology,some open questions remain to be deliberated,e.g.,correlating the dynamic evolution of dendrite growth with the anisotropic Li+ diffusion induced by the electrolyte property,electric potential,and separator structure.In this paper,an electrochemical phase-field model is applied to explore the influences of electrolyte inherent anisotropic Li+ diffusion,electric potential-induced anisotropic Li+ diffusion,and separator-structure-induced anisotropic Li+ migration on dendrite growth via a homemade MATLAB code.Instead of a fixed numerical value,the modified Li+ diffusivity in the electrolyte(DL)is expressed as a second-order tensor by decomposing into two components along the x(Dxx)and y(Dyy)directions,which is not only able to explore the electrolyte inherent anisotropic Li+ diffusion but also easy to describe the electric potential-induced fluctuations of DL and the corresponding Li+ concentration distribution.Predicted results indicate that with the increase of Dyy:Dxx,the interfacial Li+ concentration gradient is alleviated due to the accelerated longitudinal Li+ replenishment and decelerated transversal"entrainment"phenomenon,thus decreasing the driving force of dendrite growth.Besides,the electric potential-induced interfacial Li+ fast diffusion layer can also reduce the electric potential gradients surrounding the dendrite tips and then uniform the dendrite morphologies.Surprisingly,separators with higher matrix tilt angles are demonstrated to achieve effective anisotropic Li+ diffusion in electrolyte,which can not only reduce the dendrite-growth velocity,but also extend the dendrite-growth pathway and prolong the battery short circuit time.Following this,electrolyte with the Dyy:Dxx = 10:1 and separator with the matrix tilt angle of arctan(0.5)are evaluated as promising materials for lithium metal batteries.This study provides a rational guidance for designing electrolytes or separators with dendrite-inhibiting capability.
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