Electrolyte Design Strategies for Fast-Charging Lithium-Ion Batteries
In recent years,transportation powered mainly by oil has brought negative issues such as environmental pollution and fos-sil energy depletion,and electric vehicles powered by lithium-ion batteries have come into focus in order to realize the electrified trans-portation.Today,great strides have been achieved for electric vehicle in terms of range,safety and cost.However,electric vehicles are still criticized by consumers,because the charging time is much longer than the refueling time for internal combustion engine vehi-cles.In order to increase their market share,electric vehicles need to be charged to 80％state of charge within 5～10 min,correspond-ing to a charging rate of 5C or more,which is known as the extreme fast charging(XFC)technology.It is necessary to ensure that lithi-um ions are quickly removed from the cathode and embedded into the anode without lithium deposition during fast charging,and kinet-ics of lithium insertion into anode materials is considered as the bottleneck.The problems lie ingraphite,which is the most widely used anode materials because of its low cost and high capacity.Because the lithium insertion potential of graphite(0.1 V(vs.Li/Li+))is very close to the working potential of the lithium metal(0 V(vs.Li/Li+)).When charging at high rate or low temperature,the potential of the anode tends to polarize below 0 V,leading to lithium metal plating on the graphite surface rather than insertion.Li plating would result in active lithium loss and internal micro-short circuit of the battery.The underlying mechanism limiting fast charging can be sum-marized in two processes:mass transport,related to the transport ofLi+in the electrolyte and electrodes;and charge transfer,involv-ing in the solventization/desolvation of Li+and the diffusion of lithium ions in the solid electrolyte interface.As the pathway for ion transport between cathode and anode,electrolyte has a pivotal impact on the fast charging performance of lithium-ion batteries,and optimizing electrolyte is one of the most important methods to achieve fast charging for high energy density lithium-ion batteries.This paper reviewed the recent progress in the research of novelty electrolytes for fast charging,from the perspectives of promoting the rapid migration of lithium ions in electrolytes,reducing the energy barrier of lithium ion desolvation and designing advanced solid electrolyte interfaces.A commonly used method to improve the fast charging performance of lithium-ion batteries was to accelerate the migration rate of Li+in the electrolyte,which was mainly related to the ionic conductivity and Li+migration number of the electrolyte.The solvent largely determined the electrolyte ionic conductivity,and the use of a suitable co-solvent could significantly improve the electrolyte ion-ic conductivity.In addition,it was also important to improve the Li+migration number at a giving conductivity.Currently,one of the best ways to improve the Li+migration number was to develop new lithium salts.Secondly,before the solventized Li+was inserted into graphite,the solventized sheath must be stripped,and this step consumed a lot of energy.Reducing the energy barrier to desolvation by adjusting the solventized structure of Li+in the electrolyte was also an effective strategy to improve the charging rate.The methods mainly included high concentration,local high concentration and weakly solventized electrolytes.These well designed electrolytes could form special solventized structures,which helped to accelerate the Li+desolvation.Moreover,the solventized sheath with anions could also derive the formation of solid electrolyte inter phase(SEI)with abundant inorganic that reduced the interface impedance and facilitates fast kinetics.Finally,it was also possible to design advanced SEI in situ by adding suitable additives to the conventional electrolyte to accelerate the kinetics at the interface.This review came to a conclusion with a summary and outlook on electrolytes for fast charging.Based on commercial electrolytes,it was common and simple to use co-solvents or new lithium salts to improve ionic con-ductivity and Li+migration number of electrolytes,or use SEI formation additives to facilitate fast interfacial kinetics.Adjusting the sol-ventized structure of Li+to achieve fast desolvation was also an effective strategy.Although the fast charging performance of high con-centration electrolytes was good,their development was restricted by cost issues and safety problems.The local high concentration elec-trolytes solved the above problems to a certain extent,while more suitable diluents were still need to be explored.Weakly solventized electrolytes significantly improved the rate performance of lithium-ion batteries,but the research on fast charging of this electrolyte was lacking.In all,the development of electrolytes could lead to some major breakthroughs in fast charging for lithium-ion batteries.
fast chargingelectrolytegraphitelithium-ion batteries
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