Li-KCl熔融界面上离子扩散的分子动力学研究

Investigation of ionic diffusion on molten Li-KCl interface by molecular dynamics method

采用分子动力学(MD)方法研究了熔融Li(电极)-KCl(电解质)界面上离子的扩散行为。熔融界面上离子的扩散动力学通过离子质心的均方位移(MSD)和速度自相关函数(VACF)进行研究,扩散系数由MSD(t)函数线性区间的斜率和VACF(t)函数积分得到。模拟结果表明,在熔融的Li-KCl界面上,Li+离子在浓度梯度的驱动下穿过界面发生定向迁移,导致双电层的形成和外电路上电流的输出。Li+离子的扩散系数比K+和Cl-离子的大7～8倍,说明在界面上Li+离子是主要的载荷子,热电池的电荷传输机制主要与Li+离子的扩散运动有关。由Nernst-Einstein公式对电导率进行估算,由扩散到KCl层中的Li+离子产生的电导率约为0.4 S.cm-2,对应的电流密度估算值为3.27×105A.cm-2。

Molecular dynamics（MD） simulations are used to study the ionic diffusion behavior on the molten Li（electrode）-KCI（ e- lectrolyte） interface. The dynamics of the ionic diffusion on the molten interface is investigated by the mean-square displacement （ MSD ） and the velocity autocorrelation function（VACF） for the center of mass of the ion. The diffusion coefficient is estimated from the linear slope of MSD（t）and from the integral of the VACF（t）functions. The simulation results indicate that Li＋ ions take place directional transport through the molten Li-KC1 interface driven by the concentration gradient,resulting in the formation of electrical double layer and the output of current in external circuit. The diffusion coefficient of the Li＋ions is 7 - 8 times larger than those of K＋ and Cl- ions. This means that the Li＋ ions are the main charge carriers on the molten electrode Li-electrelyte KCl interface, and the mechanism of the charge transport in the thermal battery is mainly related to the diffusion behavior of the Li＋ ions. The electrical condt, ctivity is calculated from the Nernst-Einstein formula. The electrical conductivity resulted from the Li＋ ions diffusing into the KCI layer is calculated to be approximately 0. 4 S· cm-2, and the estimated value of the corresponding current density is 3.27×10^5A.cm-2,