Strategy to Increase Energy Density and Improve Stability of LiMPO4 (M=Co, Ni) Cathodes for All-Solid-State Li-Ion Batteries

正极材料对锂离子电池(Lib)的能量密度和寿命周期起着关键作用。它的热力学稳定性取决于本征电压极限，而本征电压极限是由相关材料中阴离子的固有氧化极限定义的。整个电池单元的外部稳定性还受到电解液的氧化还原电位和阴极/电解液界面的化学兼容性的影响。功率能力取决于Li+和/或电子通过块状阴极材料和跨越相界面的阴极/电解液的转移速率。因此，为了从根本上提高能量密度和功率密度，同时保持电池的高稳定性，必须解决以下关键问题：a)在高压充电状态下运行的电池的化学兼容性和热力学稳定性；b)Li+离子在相间和相界的高迁移率；c)电极中的高电子传导性。本项目的目标是新型5V固态电池的制备和研究。我们重点研究了LiMPO4(M=Co，Ni)橄榄石基材料，其中通过设计另一种具有更高氧化电位的聚磷酸盐相来实现在完全脱硫态下的高结构稳定性。应解决下列科学问题：a)所设计的阴极材料的异常高的电子传导性的来源是什么？B)通过聚磷酸盐相参与氧化还原反应，电池的能量密度能从根本上增加吗？C)橄榄石基材料与高锂离子导电性固体电解质在化学上是否兼容？D)电池充放电时，正极/电解液界面是否稳定，锂离子是否导通？薄膜正极材料及其界面将在超高压条件下制备，并将主要通过原位和操纵面电子能谱、HRTEM/STEM和X射线衍射法结合电子和结构性质的模拟来研究其电子和结构性质。通过这种新的方法，我们将通过监测电池运行过程中物理化学性质的变化来研究离子导体及其界面的本征和外在稳定范围。

The cathode material plays a key role in the energy density and life cycle of Lithium Ion Batteries (LIB). Its thermodynamic stability depends on the intrinsic voltage limit which is defined by the inherent oxidation limit of the anions within the relevant material. The extrinsic stability of the whole battery cell is additionally influenced by the redox potential of the electrolyte and by the chemical compatibility of the cathode/electrolyte interface. Power capability depends on the Li+ and/or electron transfer rates through the bulk cathode material and across the phase boundary cathode/electrolyte. Thus, to increase essentially the energy density and the power density, whilst maintaining a high battery stability, the following key issues have to be solved: a) chemical compatibility and thermodynamic stability of the battery cell operated at a high voltage charged state; b) high migration rate of Li+ ions through the phases and across the phase boundaries; c) high electronic conductivity in the electrode. The aim of our project is the preparation and investigation of novel 5V solid-state batteries. We focus on the LiMPO4 (M=Co, Ni) olivine-based material in which a high structural stability in the fully delithiated state is achieved via the material design with another polyphosphate phase of higher oxidation potential. The following scientific problems shall be addressed: a) what is the origin of unusually high electronic conductivity revealed for the designed cathode material? b) can the energy density of the battery be essentially increased via the involvement of the polyphosphate phase into the redox reaction? c) are the olivine-based materials chemically compatible with highly Li+ ion conductive solid electrolyte? d) is the cathode/electrolyte interface stable and Li+ ion conducting upon the charging/discharging of the battery cell? The film cathode materials and their interfaces will be prepared under UHV condition and their electronic and structural properties will be studied mostly by in situ and operando electron spectroscopy, HRTEM/STEM and XRD combined with the modelling of electronic and structural properties. With such a novel approach, we will address the intrinsic and extrinsic stability range of ion conductors and their interfaces by monitoring the evolution of the physicochemical properties during operation of the battery cell.