Application Research Of Nitrile Compounds In Lithium Ion Battery Electrolyte

Lithium-ion battery electrolyte is composed of basic lithium salt,organic solvent,and functional additives.Lithium salt is easily decomposed when the electrolyte is working at high temperature,and the commercial carbonate solvent is also easily decomposed under high voltage,which will seriously reduce the overall performance of the battery.This dissertation takes 1-（2-cyanoethyl）pyrrole（CP）,1,3,6-hexanetrinitrile（HTN）and butyronitrile（BN）as three nitriles as the research objects,and used them as additives and the main solvent was used in the electrolyte.The research result show that nitrile compounds have a significant improvement effect on the thermal stability and high voltage performance of the electrolyte.The main research conclusions are as follows:1.We proposed a new electrolyte additive CP to improve the high temperature performance of LiFePO₄ batteries.The battery with 1 wt.%CP additive added to the basic electrolyte,the battery capacity retention rate increased from 38.1%to 76.7%after 200 cycles at a high temperature of 60 ℃.Theoretical calculation result show that the binding energy of CP and PF₅/HF,a byproduct of decomposition in the electrolyte,was higher than that of the solvent in the electrolyte.Electrode surface analysis show that CP can reduce the decomposition of the electrolyte,inhibit the dissolution of transition metals in the electrolyte at high temperatures,and at the same time promote the formation of a more regular and thinner CEI film on the surface of the cathode material.CP additives can effectively improve the high-temperature performance of batteries and are expected to be applied on a large scale.2.The electrolyte solvent is decomposed under high voltage,and the silicon negative electrode will expand in volume during the cycle,which will cause the performance of the battery to decrease.For the first time,we used a combination of lithium difluorophosphate（LiDFP）and HTN as a mixed functional additive to improve the performance of Si-C/LiNi_0.5Mn_1.5O₄ full battery.After 150 cycles,the capacity retention rate of the battery containing 1 wt.%LiDFP and 1 wt.%HTN increased from 34.58%to 91.57%.LiDFP can be preferentially reduced on the surface of the negative electrode to form a dense and stable film,thereby effectively improving the interface performance of the silicon negative electrode.The C≡N of HTN can remove H₂O and HF in the electrolyte,and can form a strong coordination with the metal ions of the positive electrode material to reduce side reactions on the electrode surface.Therefore,the combined use of LiDFP and HTN as electrolyte additives is expected to be widely used in high energy density batteries.3.Under low temperature conditions,the electrolyte conductivity of lithium-ion batteries decreases,and the lack of time for lithium ions to shuttle easily leads to the formation of lithium dendrites.Compared with traditional carbonate solvents,ether solvents have better reduction stability to lithium metal,but ether solvents are unstable under high voltage（>4V）.Butyronitrile has high conductivity and oxidation resistance,which can effectively compensate for the defects of ether solvents.We have designed a new electrolyte formula:1.5M LiTFSI+0.2M LiODFB+5 wt.%FEC-DME:BN（3:1/V）,which is applied to Li/LiNi_0.5Mn_0.3Co_0.2O₂batteries.The battery capacity retention rate was 88.84%after 150 cycles at-10 ℃.This formula has great significance for assisting the application of ether solvents under high voltage.