高濃度電解質的傳輸性質對於鋰枝晶生長之影響

Abstract

一直以來，鋰金屬電池被認為是高能量密度之可充電電池的候選之一。然而，負極/電解質界面的不穩定反應導致鋰金屬電池在充電過程中無法避免形成鋰枝晶(dendrites)。近來，高濃度電解質（highly concentrated electrolytes, HCEs）因其在沉積過程中能減輕負極表面鋰離子消耗進而抑制鋰枝晶生長而引起了人們的關注。然而因其傳輸性質與濃度高度相依，對枝晶形成的影響仍在研究中。

本研究使用穩態濃差極化(steady-state concentration polarization)的數學模型以及一系列相場(phase-field)模擬之動態分析，研究了HCEs中濃度相依的傳輸性質對鋰枝晶形成的影響。經由穩態分析給出了HCEs在長期沉積過程中電解液濃度的分佈，並透過在模型中引入參數β以量化濃度對擴散係數的相依性，我們解得考慮濃度相依性擴散係數的濃度分佈和極限電流密度(limiting current density)的解析解，並進一步指出極限電流密度和Sand的解析解的偏差是β和初始濃度C0的函數。此外，數值模擬表明，無因次參數βC0較大時在電極附近的濃度下降有初始和緩、隨後急劇下降的趨勢，最終在較短的時間內逼近最終值。最後，以相場模擬不均勻的負極表面上的沉積趨勢，證實本研究所推導的極限電流密度是不均勻沉積的關鍵指標。我們相信這項研究為理解HCEs的行為提供了新想法，並且為未來相關的鋰枝晶形成的研究提供一些靈感。

Lithium metal batteries (LMBs) have long been regarded as one of the promising candidates for high-energy-density rechargeable batteries. However, the unstable reaction at the anode/electrolyte interface results in Li-dendrite formation in LMBs during the charging process. Recently, highly concentrated electrolytes (HCEs) have garnered attention in LMBs due to their ability to mitigate lithium-ion depletion at the anode surface during the deposition process and thus inhibit dendrite formation. However, the transport properties of HCEs are highly concentration-dependent. The exact influence on dendrite formation is still under investigation.

This study investigates the influence of the concentration-dependent transport properties in HCEs on the formation of Li-dendrite using a mathematical model of steady-state concentration polarization along with a series of phase-field simulations for dynamic analysis. Steady-state analysis reveals the variations in the electrolyte profile during the long-term deposition. Via the mathematical model with the introduction of a parameter β to quantify the dependence of concentration on diffusivity, an analytical solution for the concentration profile and limiting current density considering the concentration-dependent diffusivity is obtained. We further indicate that the deviation of limiting current density and Sand's solution is a function of β and C0. Moreover, numerical simulations reveal that the concentration near the electrode for higher dimensionless βC0 exhibits a gentler initial decrease followed by a steeper drop, ultimately reaching the final value in a shorter time. Finally, the phase-field simulation of the deposition trend on the non-homogeneous anode geometry confirms that the limiting current density derived in this study is a key indicator of non-uniform deposition. We believe that this study presents a methodology for comprehending the electrolyte behavior of HCEs and may offer inspiration for future investigations in Li-dendrite formation.