Jointly develop wide voltage window battery -Lithium - Ion Battery Equipment

Jointly develop wide voltage window battery electrolyte -Lithium - Ion Battery Equipment

Since the 1990s, the good oxidation resistance (~4.3V) of EC based electrolyte system and its good matching performance with graphite anode have laid the foundation for the large-scale application of commercial lithium ion batteries. However, with people's further desire for electric vehicle mileage in recent years, this system (graphite negative electrode - EC based electrolyte - NMC/LCO/LFP positive electrode) has gradually failed to meet people's expectations.(Lithium - Ion Battery Equipment)

Therefore, developing new high voltage battery systems or further improving the charging cut-off potential on the basis of the current system to improve the energy density is becoming the research focus of high energy density lithium batteries. Whether developing a new battery system with higher voltage system or building a battery with higher voltage and higher energy density based on the current positive and negative electrode materials, the narrow voltage window of electrolyte has become the key to limit the further development of the next generation of high energy density lithium ion batteries. Based on this, researcher Fan Xiulin from Zhejiang University and professor Wang Chunsheng from the University of Maryland in the United States combed the new electrolyte systems with wide voltage windows in recent years, and looked forward to them.

The review article was published on the international journal Chemical Society Review under the title of "High voltage liquid delivery roles for Libraries: progress and perspective".

First of all, it needs to be pointed out that the breakthrough of organic liquid electrolyte has widened the voltage window of secondary batteries to more than 3.0V for the first time, which has freed them from the 1.23V thermodynamic window of water batteries. This is also the basis for lithium ion batteries to stand out from many secondary batteries (as shown in Figure 1a). According to the formula 1/Q=1/Qa+1/Qc, under the existing NMC positive electrode system, when the capacity of the negative electrode exceeds 1000mAh/g, further increasing the capacity of the negative electrode has little effect on the increase of the energy density of the whole system (as shown in Fig. 1b). In order to further improve the energy density of the battery, improve the working cut-off voltage of existing cathode materials, or develop new cathode materials with higher voltage and capacity, it will be a more effective method (as shown in Figure 1b). However, the improvement of cut-off voltage is more and more restricted by the oxidation resistance of traditional commercial EC ester based electrolyte.

Recent studies have found that EC solvent is a double-edged sword in commercial electrolyte systems. Although it can form an excellent SEI film on the negative side of graphite, it is also the most oxidation resistant electrolyte matrix component in commercial electrolyte. Therefore, the high content of EC in commercial electrolyte greatly limits the application of electrolyte in the next generation of high voltage and high energy density battery systems (as shown in Figure 1c). Fortunately, we can form excellent SEI/CEI interfaces by regulating the solvated structure of electrolyte or film forming additives, which can inhibit side reactions and broaden the operating voltage window of the battery dynamically

Based on the above analysis, the author summarized several electrolyte systems that have been studied recently and can effectively improve the working window of electrolyte, including EC free electrolyte system (including EC free ester based electrolyte system, fluorinated electrolyte, sulfone electrolyte, nitrile electrolyte and other high voltage electrolytes) High concentration electrolyte and pseudo high concentration electrolyte system (including ester based high concentration, ether based high concentration, other organic high concentration, water in salt and other electrolytes with high concentration in water system), ionic liquids, electrolyte additives (unsaturated bonds, fluoride, phosphides, nitriles, organic borides, organic silicon, organic sulfides, etc.).

Several types of high voltage electrolyte systems involve a wide range of specific components and types, but the essential mechanism or design criteria are similar: inhibit the oxidation or reduction of solvent molecules, and introduce components with better film-forming properties. However, the specific positive and negative electrode materials are often different. For example, for graphite cathode, the volume expansion rate is only about 13%. The SEI generated by traditional EC decomposition and tightly bonded with graphite can withstand such a large volume expansion, so commercial lithium ion batteries can effectively cycle for thousands of cycles. For electrode materials such as Si (volume expansion>300%) and Li (volume expansion is infinite) with large volume expansion.



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