电解质在电池中起到离子导通的作用，是电池导电网络的重要组成部分。醚基电解质与锂金属的反应性相对较低，因而被广泛地应用于锂金属电池。其低粘度和高离子传导性有利于锂离子的快速传导和快速的界面电荷转移，而醚类溶剂的低凝固点使得电池在零度以下具有优异的性能。更重要的是，醚基电解质与锂金属负极表现出高兼容性，可以抑制充电时锂的树枝状沉积。然而，使用高度易燃的醚类溶剂时存在安全隐患：醚的沸点较低，这对高比能量电池的使用带来了安全风险。此外，醚基电解质的氧化稳定性较弱，导致在高电压下（> 4 V vs Li/Li+）正极表面的溶剂分解无法控制，大大恶化了醚基溶液的高压金属锂电池的循环性能。本文工作在实现良好的抗氧化性与对正负极成膜性能的同时，提高醚基电解质的安全性。首先，本工作合成了一种新的丁烯氧基环三磷腈（BCPN）单体，将其加入醚类电解液，加入氟化助溶剂并凝胶化处理。在协同作用下，有效地提升醚基电解质溶液在高能金属锂基电池中的应用。通过精心设计的Li+溶剂化壳与BCPN衍生的保护性界面层在，实现了高水平的正极兼容性。通过这种电解质工程策略，Li||LiNi0.8Co0.1Mn0.1O2电池可以达到高容量保持率，卓越的低温性能，高压下良好的循环性。进一步，本研究开发的含氟助溶剂和防火聚合物基体的协同作用的不可燃电解质（NGPE）可以形成稳定的电极|电解质界面层，极大地保护了滥用条件下（包括高压力，低温，过充等）电池的稳定性，并表征了电池的热失控性能。该电解质安全策略降低火灾和电解液泄漏的安全风险，并保证了滥用条件下的稳定供电。本工作的电解质设计理念为实现极端工作条件下的耐用锂金属电池提供了一条有希望的道路。最后，基于对醚类的卤素取代，本工作提高了醚类电解质的本征安全性。常规醚类采用氟取代提高其抗氧化性以及不可燃性，相同结构的氯取代具有更低的断键能，在受热时释放Cl·自由基中和燃烧所需的H·自由基等，因此具有更高的安全性。同时在负极表面形成富LiCl-LiF的SEI，提高了其离子电导率，有助于电池稳定循环。
Ether-based electrolytes are widely used in lithium metal batteries due to their relatively low reactivity with lithium metal. The low viscosity and high ionic conductivity facilitate fast conduction of lithium ions and rapid interfacial charge exchange, while the low freezing point of ether-based solvents allows for excellent battery performance below zero degrees. More importantly, ether-based electrolytes exhibit high compatibility with lithium metal anodes and can inhibit dendrite growth (lithium deposition) during charging. However, there are safety hazards in the use of highly flammable ether solvents: the low boiling points of ether pose safety risks including fire, explosion and liquid leakage for the use of high specific energy cells. Besides, the insufficient oxidation stability of ether–based electrolytes leads to uncontrollable solvent decomposition on the cathode surface under high voltage (> 4 V vs. Li/Li+), greatly deteriorating the cyclability of high–voltage Li metal–based batteries when containing ethereal solutions. In this paper, two approaches are used to improve the safety and oxidation resistance of ether-based electrolytes while achieving good film-forming properties for positive and negative electrodes, respectively.Firstly, a new butenoxycyclotriphosphazene (BCPN) monomer was synthesized in this work. The addition of the fluorinated co–solvent not only reduces the flammability of electrolyte solutions, but also tailors the Li+ solvation sheath to improve the oxidation stability. The BCPN–based polymer matrix thoroughly eliminates the safety risks of fire and electrolyte solution leakage. It is noted that being different from traditional soluble flame retardants, this non–flammable polymer matrix is immobile and electrochemically inert without any adverse effects on the Li anode surface. In a synergistic manner, the ether-based electrolyte solution is effectively enhanced for the application in high-energy lithium metal-based batteries. A high level of anode compatibility is achieved through a carefully designed Li+ solventized shell with a protective BCPN-derived interfacial layer. With this electrolyte engineering strategy, Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) cells can achieve high capacity retention, and good cycling stability at high pressure.Further, the non-flammable electrolyte (NGPE) developed in this study with the synergistic effect of fluorine co-solvent and fire-resistant polymer matrix can form a stable electrode|electrolyte interface layer, which greatly protects the stability of the battery under abusive conditions (including high pressure, low temperature, overcharge, etc.) and characterizes the thermal runaway performance of the battery using accelerated adiabatic calorimetry (ARC), etc. This electrolyte safety strategy reduces the safety risk of fire and electrolyte leakage and ensures stable power supply under abuse conditions. The electrolyte design concept of this work provides a promising path to achieve durable lithium metal batteries under extreme operating conditions.Finally, the present work improves the intrinsic safety of ether electrolytes based on the halogen substitution of ethers. Conventional ethers use fluorine substitution to improve their oxidation resistance as well as non-flammability, and the same structure of chlorine substitution has a lower bond breaking energy and releases Cl-radicals to neutralize H-radicals required for combustion, etc. when subjected to heat, thus providing non-flammability. Meanwhile, the formation of LiCl-LiF-rich SEI on the negative electrode surface improves higher ionic conductivity and contributes to stable battery cycling.
