锂离子电池是当今社会使用最为广泛的电化学储能器件，但是其能量密度依然无法满足我们日益增长的需求。提升锂离子电池的能量密度主要从两个角度入手：一是提升电极材料的比容量，二是拓宽电池的电压窗口。本文分别从这两个方面出发，研究了具有高比容量的LiNi0.8Co0.1Mn0.1O2（NCM811）正极材料在高压下表面正极电解液界面（CEI）膜的变化，并首次提出NCM811正极与CEI膜之间的界面过渡区域，为人工CEI膜的研究提供新的思路，以及探索能够同时对锂金属负极和高电压正极（5 V级）稳定的新型高浓度双盐锂离子电池电解液，以提升锂离子电池的能量密度。本文探索了高比容量正极材料NCM811在不同充放电区间下循环后电化学交流阻抗谱（EIS）的变化，发现在4.9 V的充电截止电压下电荷转移阻抗（Rct）出现了明显的减小，并将其与CEI膜的结构与组成相对应，提出了新形成的过渡区域打破了正极与CEI膜之间的边界，大大促进了锂离子传输与扩散。未来的人工CEI膜的设计上可以考虑打破CEI膜与正极之间的界面以减小阻抗。论文还提出了一种能够同时对锂金属负极和高电压正极（5 V级）稳定的新型高浓度双盐锂离子电池电解液。高浓度双盐电解液形成的稳定SEI膜可以有效保护高催化活性的正极，如LiNi0.5Mn1.5O4（LNMO）与LiNi0.7Co0.15Mn0.15O2（NCM71515）等。因此，Li||LNMO和Li||NCM71515电池分别实现了500圈88.5%和200圈86.2%的容量保持率，平均库仑效率分别达到了99.6%和99.5%。沉积三倍过量的金属锂作为负极的LNMO（负载量1.8 mAh/cm2）全电池成功实现了超过100圈的稳定循环，平均库仑效率为99.3%。本文中的研究工作为未来新一代能够同时适应锂金属负极和表面高催化活性的高压正极的电解液的研发提供了一个新思路。本论文的工作内容为高能量密度的锂离子电池提供了新的思路与方法。

Lithium-ion battery is the most widely used electrochemical energy storage device in today's society, but its energy density still cannot meet our growing needs. There are two ways to improve the energy density of lithium-ion batteries: one is to improve the specific capacity of the electrode material and the other is to widen the voltage window of the battery. In this paper, the changes of cathode electrolyte interphase (CEI) film on the surface of LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode with high specific capacity under high voltage were studied from these two aspects respectively, and the interface transition region between NCM811 cathode and CEI film was proposed for the first time, providing new ideas for the study of artificial CEI film, and to explore new high-concentration double-salt electrolytes that can achieve stability to both lithium metal anode and high-voltage cathode (5 V class) to improve the energy density of lithium ion batteries.This paper explores the change of the Electrochemical Impedance Spectroscopy (EIS) of NCM811 with high specific capacity under different charge and discharge interval, and there is an obvious decrease of the charge transfer impedance Rct under the charge cut-off voltage of 4.9 V, corresponding to the structure and composition of the CEI. This paper puts forward hypothesis that the new formation of the transition area breaks the boundary between cathode and CEI film, and greatly promotes the lithium ion transport and diffusion. In the future design of artificial CEI film, the interface between CEI film and cathode can be broken to reduce the impedance.A novel high-concentration double-salt lithium-ion battery electrolyte which is stable to both lithium metal anode and high voltage cathode (5 V class) is presented. The stable SEI formed by high-concentration double-salt electrolyte can effectively protect the cathode with high catalytic activity, such as LiNi0.5Mn1.5O4 (LNMO) and LiNi0.7Co0.15Mn0.15O2 (NCM71515). Therefore, LNMO||Li and NCM71515||Li batteries achieved capacity retention rates of 88.5% and 86.2% for 500 and 200 cycles respectively, and the average Coulombic efficiency reaches 99.6% and 99.5%, respectively. The full battery with LNMO (1.8 mAh/cm2) as cathode and 3x excess lithium as anode successfully achieves stable cycle of more than 100 cycles with an average Coulombic efficiency of 99.3%. The research work in this paper provides a new idea for the research and development of a new generation of electrolyte which can adapt to both lithium metal anode and high catalytic activity cathode. The work of this thesis provides new ideas and methods for high energy density lithium-ion battery.