基于高氟化复合固态电解质的高电压三元固态锂电池的研究

Conventional solid-state electrolytes have been limited to matching with high-voltage ternary materials to obtain higher energy density solid-state lithium batteries due to their intrinsic low oxidation decomposition potential and poor interfacial stability with high-voltage positive electrode. However, the design principles of high-voltage solid-state electrolyte and interfacial compatibility of high voltage cathode with solid-state electrolyte are still ambiguous thus far. In this project, a series of highly fluorinated composite solid electrolytes with high voltage resistance are designed and synthesized by introducing strong electron-withdrawing groups, and the stability of interface between the ternary cathode and solid-state electrolyte is improved by in-situ electrochemically forming fluorination interfacial (CEI) films. The synchrotron radiation X-ray photoelectron spectroscopy and X-ray near-edge absorption spectroscopy with chemical and spatial resolution will be initially employed to study the compositions of CEI films between highly fluorinated composite solid-state electrolyte and cathode interface, and to reveal the effects of the obtained CEI films towards the crystal structure and valence states of elements on the surface of ternary cathode materials. Furthermore, the formation mechanism of fluorinated CEI films at high voltage ternary cathode surface and its functionalization for protecting cathode materials will be clarified. The research in this project can not only pave a new way for building a new high-voltage solid-state electrolyte system and improving the interfacial stability of cathode, but also establish the theoretical foundation for promoting the development and application of high-voltage solid-state lithium batteries.

传统固态电解质自身氧化分解电压低、与正极界面稳定性差等问题一直限制其匹配高电压三元材料以获得更高能量密度的固态锂电池。然而目前对于耐高压固态电解质的设计原则及固态电解质与高电压正极界面兼容性等问题的认知尚未清楚。本项目拟引入含氟的强吸电子基团设计合成系列耐高压的高氟化复合固态电解质，并利用电化学原位形成的氟化界面（CEI）膜改善高电压三元正极与固态电解质的界面稳定性。率先运用具有化学和空间分辨能力的同步辐射X-射线光电子能谱和X-射线近边吸收光谱技术探究不同氟化程度的复合固态电解质与正极界面间CEI膜组分，揭示该CEI膜对三元正极材料表面晶体结构和元素价态的影响，进而阐明氟化CEI膜的形成机理及对正极材料的保护机制。该项目的实施不仅为构建耐高压固态电解质新体系及改善电极与电解质界面问题提供新思路，而且为推进高电压固态锂电池的研发和应用奠定新的理论基础。

采用不燃、不漏液、易封装的固态电解质代替传统的有机液态电解质有望从根本上解决传统液态锂离子电池的安全问题。然而要实现固态电解质的实际应用仍存在诸多挑战，如界面稳定性差、界面电阻高等，严重限制了其在高比能量密度固态锂电池中的性能发挥。本项目主要围绕高氟化固态电解质的设计及其界面问题，主要开展以下研究内容：.（1）通过液态流延法制备了硫化物基高氟复合电解质聚（偏氟乙烯-六氟丙烯）(PVDF-HFP)/Li10GeP2S12。低分子量的全氟聚醚由于其C–F键具有很强的电负性，作为Li10GeP2S12在注膜液中的稳定分散助剂。此外，高分子量全氟聚醚作为负极界面稳定剂，通过原位形成富含 LiF的固体电解质界层面，显著改善了Li10GeP2S12与Li的界面兼容性。该电解质表现出高室温离子电导率 (0.18 mS cm-1)，锂离子转移电子数 (0.68)，同时还具有良好的机械强度和不易燃性。组装的全固态LiFePO4ǁLi电池表现出优异的循环性能和倍率性能。.（2）采用PVDF-HFP粘接高含量的纳米级Li6.4La3Zr1.4Ta0.6O12 (LLZTO) 构建超薄复合固体电解质(CSE)。在CSE与高电压三元正极界面添加高氟含硼液体电解质(B, F-LE)，原位形成低电阻、高稳定的SLEI层。B, F-LE与CSE具有良好的化学相容性，能快速均匀地输送Li+。二者结合形成的高氟化固液混合电解质对高电压正极表现出优异的化学及电化学稳定性。最终，应用在LiNi0.6Co0.2Mn0.2O2‖Li固态电池表现出杰出的倍率性能和高电压循环稳定性。.（3）设计了一种在金属锂表面原位形成的聚合物-无机物梯度变化的SEI（PIG-SEI）膜，该人工SEI膜由富含PEGDA-co-VC的聚合物外层和富含LiF，Li3N和Cu的无机纳米颗粒内层组成。由PIG-SEI保护的金属锂负极不仅可以改善PIG-SEI膜与金属锂之间的界面接触以及SEI层的锂离子电导率，还可以提高界面柔韧性和机械强度。实验结果表明，在极少量碳酸盐电解质条件下，Li||LiFePO4和Li||LiNi0.8Co0.1Mn0.1O2电池在0.5C下循环500次仍保持122mAh g−1和123mAh g−1高的可逆比容量。