Study On The Effect Of Trace Water For Electrolyte And Battery Performance

At present,lithium ion batteries?LIBs?are widely used in the fields of electronic communication and transportation as energy storage devices.The requirements for energy density,power density,safety,and cycle life of LIBs are becoming more and more demanding.Impurity in the electrolyte is one of the most important reasons for the capacity fading in lithium-ion battery.Trace water,as one of the most common impurity,will lead to the hydrolysis of the lithium salt,and the further increase of battery impedance,the decrease of potential plateau,the damage of the solid electrolyte interphase film and the aggravation of safety hazard.This is also an indicator that must be strictly controlled in the production process.Therefore,the current research focuses on the failure mechanism about the cell performance caused by trace water.In this thesis,the damage mechanism of trace water on lithium salt and electrolyte was systematically studied.The main research contents are as follows:Firstly,lithium bis?oxalate?borate?LiBOB?is taken as research object to investigate the changes of physicochemical properties when stored in a humid environment.Results show that high relative humidity can accelerate the increase in weight,though a same constant weight will be achieved in the end.That is,LiBOB stored in different humid environment has similar absorbance.The increase in weight is caused by comprehensive reactions including the occurring of hydrolytic reaction,the formation of adsorbed water and crystal water.Both of the hydrolytic decomposition and the formation of adsorbed water continually occur during the whole air-exposure process.And in the later stage,stable monohydrate LiBOB·H₂O compounds are formed firstly and turned into dihydrate LiBOB·2H₂O compounds sequentially.Quantum chemical calculation results show that hydrolytic decomposition is spontaneous and irreversible at room temperature,the hydrolysis products still exists even dried at 130°C,namely,the reparability will be limited due to the formation of harmful hydrolysis products LiB?C₂O₄??OH?₂ and LiB?C₂O₄?OH,though both of the adsorbed water and the crystal water can be removed thoroughly.Moreover,we believe that the existence of hydrolysis products will lead to the sharp increase of impedance of the solid electrolyte interphase film,which will give rise to the deterioration of electrochemical performances sequentially.Secondly,the effects of different amount of trace water in LiBOB-ethylene carbonate?EC?/diethyl carbonate electrolyte?DEC?on the performance of LIBs have been systematically studied.Results show that the change H₂O content in the LiBOB-EC/DEC has different influence on the electrochemical performance of LIBs.When the H₂O content in the LiBOB-EC/DEC electrolyte is low,the original solvation structure is destroysed due to the hydrogen bond between H₂O and EC,and forms new solvates Li?EC?₃?H₂O?and Li?EC?₂?H₂O?₂.The H₂O produced by the desolvation of the new solvate,which reacts with the highly active ROCO₂Li to form an excessively thick SEI film rich in Li₂CO₃ and ROLi,hence the migration of Li⁺ions is hampered.When the content of water impurities in LiBOB-EC/DEC electrolyte is high,the hydrolysis products derived from the reaction of BOB^-and H₂O will lead to the sharp increase in resistance of the solid electrolyte interphase film,which will lead to the deterioration of electrochemical performances.However,the destructive effect of the hydrolysis reaction on the battery is stronger than that of the Li⁺solvated structure change.These results show that the storage conditions of raw materials and environmental moisture must be strictly controlled to ensure the electrochemical performance of LIBs in the production process of lithium salt and electrolyte.