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 (D_L) is expressed as a second-order tensor by decomposing into two components along the x (D_xx) and y (D_yy) 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 D_L and the corresponding Li⁺ concentration distribution. Predicted results indicate that with the increase of D_yy : D_xx, 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 D_yy : D_xx = 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.

Key words: Lithium dendrite, Phase-field simulation, Electrolyte, Separator, Lithium-ion diffusivity