5-16. Understanding the effects of charge transfer on the lithium ion diffusion in solid-state electrolyte materials

Hong Zhu

UM-SJTU Joint Institute, Shanghai Jiao Tong University

Abstract: Replacing the currently employed flammable liquid electrolytes in lithium ion batteries (LIBs) with the solid-state electrolyte (SSE) materials and collocating with the Li metal anodes to construct the all-solid-state lithium ion batteries (ASSLIBs) not only could solve the battery safety issues, but also remarkably enhance the energy density of battery systems. The SSE materials shall achieve Li ion fast conduction with a high Li ionic conductivity at room temperature, good chemical/electrochemical stability, great mechanical properties, and compatibility with electrodes. The existing design principles for Li superionic conductors include BCC anion framework, low Li phonon density of states, and highly distorted Li-anion polyhedron. In addition, smaller anion charge has been regarded to be beneficial to the migration of lithium ions. However, our previous studies on the chalcopyrite-structured LiMS₂ (M are transition metals, from Ti to Ni) and non-spinel Li₃MI₆ (M=Sc, Y and La) compounds demonstrate that the migration barrier of Li ions could be only reduced through a comprehensive control of the anion charge and Li ion's coordination number. For a tetrahedral Li ion migration to adjacent tetrahedral site through an octahedral site, the smaller the negative anion charge is, the lower the lithium ion migration barrier will be. While for an octahedral Li ion migration to adjacent octahedral site through a tetrahedral site, the larger negative anion charge is, the lower the lithium ion migration barrier will be. Based on this design principle and effectively blocking the conducting channels, a new sulfide-based superionic conductor, a kesterite-structured Li₂CuPS₄ (LCPS) material, was proposed to be thermodynamically, dynamically and electrochemically stable, exhibiting a much higher ionic conductivity of 84.9 mS cm⁻¹ at 300 K than the state-of-the-art Li₁₀GeP₂S₁₂ material.Non-spinel Li₃LaI₆ has been predicted to have a high room temperature ionic conductivities of 1.23 mS/cm, with a higher thermal and electrochemical stability than sulfide electrolytes. Adjusting the non-lithium elements within the same crystal structure framework to obtain the desired electronegativity difference between the anion element and non-lithium cation element and hence the anion charge could be applied to the SSE material’s design.

电荷转移对固态电解质锂离子传输的影响

朱虹

上海交通大学密西根学院

摘要：将商业化锂离子电池中的液态电解质替换为固态电解质，并搭配锂金属负极组成全固态锂离子电池系统，有望从根本上解决锂离子电池系统的安全性问题并大幅提高能量密度。锂离子固态电解质材料需具备可与液态电解质比拟的室温电导率、良好的化学、电化学稳定性以及机械性能，拥有与电极材料优良的接触性和兼容性。目前报道的锂离子快离子导体设计原则主要包括BCC阴离子骨架、低锂声子态密度中心、高畸变锂多面体等。除此之外，晶体中较小的阴离子电荷也往往被认为有利于锂离子的快速传输，即负一价阴离子比负二价阴离子更有利于阳离子扩散。然而，基于黄铜矿结构硫化物和非尖晶石结构卤化物的第一性原理计算，我们发现只有综合调控阴离子电荷与锂离子配位数才能够有效降低锂离子迁移能垒。例如，对于相邻的两个锂八面体之间的锂离子扩散（需经过一个四面体过渡态），阴离子电荷越大，锂离子扩散能垒反而越低；而对于相邻的两个锂四面体之间的锂离子扩散（需经过一个八面体过渡态），阴离子电荷越小，对应的锂离子扩散能垒则越低。基于这一设计准则并选用恰当的阳离子设计隔绝电子导电通道，我们获得了四元Li₂CuPS₄固态电解质，Li₂CuPS₄材料不但具有高热力学稳定性、较好的电化学稳定性，其理论室温离子电导率更是高于Li₁₀Ge₂PS₁₂并达到84.9 mS/cm；而新发现的非尖晶石卤化物Li₃LaI₆除了具有较好的室温离子电导率1.23 mS/cm，其热力学稳定性和电化学稳定性更是远优于硫化物电解质。

同时，我们从电极/固态电解质界面电荷转移这一全新角度研究界面离子传输阻力，通过理论模拟LiCoO₂正极/L₃PS₄电解质界面以及LiCrS₂正极/Li₃PS₄电解质界面的电荷转移和锂离子迁移行为，成功阐明了氧化物正极/硫化物电解质具有高界面阻抗的原因。

因此，有效调控电解质内部及其与电极界面处的电荷转移这一新思考，有望为更好地探究和提高全固态锂离子电池的传输性能提供良好的基础。

关键词：第一性原理计算；锂离子电池；固态电解质；快离子导体；离子扩散