Preparation Of LiP（C₂O₄）₂F₂ Based On Lithium Salt In Recycled Electrolyte And Study On The Electrochemical Properties

China has developed into the world largest producer and seller of new energy vehicle with the rapid development of the new energy vehicle industry.With the lithium-ion batteries（LIBs）commonly used in new energy vehicles are being retired on a large scale,it becomes increasingly important to recycle and reuse them.Domestic and foreign recycling technologies nowadays,whether pyrolysis or wet process,only focus on the recovery of valuable metals,ignoring the recovery of electrolytes and the high-value utilization of lithium salts in electrolyte.In particular,the fluoride and organic phosphates generated from the decomposition of LiPF₆ in the electrolyte cause significant environmental pollution and serious equipment corrosion.Based on these issues,leaching and impurity removal pretreatment of the electrolyte in retired LIBs was firstly performed in this dissertation,and then it was used as a raw material to prepare lithium difluorodioxalate phosphate（LiP（C₂O₄）₂F₂,LiDFBOP）.Additionally,the prepared LiDFBOP was applied as a lithium salt additive to S i@Graphite@C‖Li and lithium nickel-cobalt aluminate（LNCA）‖Li half cells to verify the good compatibility between LiDFBOP and the electrodes with excellent performance.Finally,LiDFBOP was applied to Si@Graphite@C‖LNCA full cells to study its electrochemical performance in full cells.Details are as follows:（1）In order to increase the content of LiPF₆ in the recycled electrolyte and meet the requirement of high-value utilization in follow-up,the optimal leaching solvents and impurity removal pretreatment of the recycled electrolyte were carried out.The leaching solvents were optimized based on the mass of LiPF₆ obtained from the electrolyte leached with every ten 18650 cylindrical batteries.The results of nuclear magnetic resonance fluorine spectroscopy(¹⁹F NMR)and inductively coupled plasma optical emission spectrometer（ICP-OES）showed that the masses of LiPF₆ leached by methyl formate（MF）,ethyl acetate（EA）,methyl acetate（MA）,and dimethyl carbonate（DMC）were 0.38,0.22,0.42,and 0.33 g,respectively,so MA has the best leaching effect。Then MA was used in the full component recycling process of retired LIBs.By controlling the temperature and pressure,the concentration of LiPF₆ in the recycled electrolyte reached 0.29 mol L^-1.Finally,the recycled electrolyte showed a reduced content of water form 80.90 mg kg^-1 to 4.56 mg kg^-1,and a reduced HF content from 1312.9051 mg kg^-1 to 291.5213 mg kg^-1 by pretreating with silane pretreatment agent,meeting the requirement of high-value utilization in follow-up.（2）In order to realize the high-value utilization of the recovered electrolyte,the synthesis temperature,reaction time and reactant molar ratio of LiDFBOP prepared from LiPF₆ were optimized.The sample purity was obtained by the peak area integral of ¹⁹F NMR,which was used as a measure to optimize the reaction conditions.The analysis results showed that the optimal synthesis temperature of LiDFBOP was 75℃,the optimal reaction time was 27 h,and the optimal reactant molar ratio[LiPF₆:bis（trimethylsilicon）oxalate（DTMSO）]was 1:3.Subsequently,the pretreated recovered electrolyte was used as a raw material,the reactant DTMSO was introduced and the above-mentioned preferred reaction conditions were applied to the preparation process.Finally,the LiDFBOP sample was successfully prepared with a purity of 98.6%.（3）The LiDFBOP lithium salt prepared above was used as an electrolyte additive,and the electrochemical performance and mechanism of action were studied by assembling Si@Graphite@C‖Li and LNCA‖Li half-cells.The results showed that the capacity retention of the Si@Graphite@C‖Li half-cell increased from 49.3%to 62.7%after 100 cycles by adding 2 wt.%LiDFBOP.After adding 2 wt.%LiDFBOP to the LNCA‖Li half-cell,the half-cell showed an increased capacity retention after 100 cycles from 69.4%to 93.6%,an increased average discharge specific capacity at 5 C rate from 95 mAh g^-1 to 144 mAh g^-1,and an increased capacity recovery rate increased from 84.3%to 94.4%when the rate was reduced to C/5.In order to further investigate the effect of this additive on the electrode/electrolyte interface,the decomposition of LiDFBOP and composition of solid electrolyte interface（SEI and CEI）were analyzed by density functional theory（DFT）calculation,atomic force microscopy,potential-re solved in-situ electrochemical impedance spectroscopy and other characterizations.The results showed that the SEI film formed in the STD+2 wt.%LiDFBOP electrolyte had a higher Young’s modulus（10.3 GPa）than the one formed in the STD electrolyte,and improvement of the cycle performance was also due to the addition of LiDFBOP.The Li（EC）₂（PF₆）₁（DFBOP）₁ solvation structure was formed in the electrolyte,which participated in and affected the formation of SEI film as well as increased the inorganic LiF component in SEI film.It also decomposed and produced organic ingredients rich in P and F atoms,which improved the flexibility of SEI film,thereby inhibiting the volume expansion of the Si@Graphite@C anode.In the LNCA‖Li half-cell,two oxalate chelating ligands of LiDFBOP undergo bidirectional cross-linking reaction,resulting in the preferential oxidative decomposition.Thus a dense and elastic CEI film can be formed in this process,which can continuously repair itself in the later stage of cycle.This property is beneficial to improve the electrochemical performance and rate performance of the cell in later stage of the cycle.（4）Based on the above three parts,it is found that LiDFBOP prepared from recovered electrolyte can not only inhibit the volume expansion of Si@Graphite@C anode as an electrolyte additive,but also has the effect of stabilizing the CEI film at LNCA/electrolyte interface.In order to make the experimental conclusions closer to the application practice,the electrochemical performance of LiDBFOP prepared from the recovered electrolyte in Si@Graphite@C‖LNCA full pouch cells were studied,and its mechanism and application prospects were verified.In this section,the electrochemical method using small rate charging and self-discharge was introduced to strength the decomposition of LiDBFOP during formation process.The integrity of this improved interface after LiDFBOP decomposition was demonstrated by scanning electron microscopy and energy dispersive X-ray spectroscopy.The results showed that the full cell in the STD+LiDFBOP electrolyte system with the help of enhanced film formation method had a higher specific capacity(156.76 Ah g^-1)compared with that of the cells with STD(132.55 Ah g^-1).After 150 cycles,the cell with STD+LiDFBOP electrolyte still showed stable cycle performance with a capacity retention rate of 81.74%.The results confirmed that the LiDFBOP prepared from recovered electrolyte in retired LIBs can effectively improve the electrochemical performance of Si@Graphite@C anode and LNCA cathode materials.Hence,this preparation method of LiDFBOP is a scalable fabrication process for electrolyte recovery from retired LIB s.