Research On The Mechanism And Enhancement Technology Of Electrode/Electrolyte Interphase Film Formation Process

The properties of the solid electrolyte interface（SEI）film on the surface of graphite electrodes determine lithium-ion battery performance.Currently,introducing lithium salt-based additives into the electrolyte is one of the effective methods to improve the performance of SEI films.However,due to the sensitivity of SEI films to air,as well as the lack of effective characterization methods to in-situ analyze their formation process and mechanism,there is still no unified theory to explain the mechanism of the additive in the formation process and key influencing factors.This thesis developed a potential resolved in-situ electrochemical impedance spectroscopy（PRIs-EIS）testing technique to achieve non-destructive in-situ characterization of interface properties during electrochemical process.And on this basis,the reduction mechanism of lithium salt additives,such as lithium bis（oxalato）borate（Li BOB）,lithium difluoro（oxalato）borate（Li ODFB）and lithium bis（oxalato）fluorophosphate（Li DFBOP）,in lithium Li PF₆ based electrolytes was analyzed,elucidating the structure-performance relationship of these additives in the SEI film formation process.Accordingly,corresponding film-formation-enhancing strategies were developed to improve the film-forming efficiency of lithium salts,addressing problems such as low decomposition strength,incomplete decomposition,suppression of effective SEI film components formation,and poor interfacial stability during the film formation process for additives like Li PF₆,Li ODFB,and Li DFBOP.The main research content and conclusions are as follows:1.A PRIs-EIS technique was developed,and the physical model of the traditional equivalent circuit was refined.The evolution of the properties of SEI films formed on graphite electrode surface were explored during the formation process,which demonstrated that the time constant in the high-frequency region was related to the charge transfer process and the diffusion of lithium ions in the inorganic components of SEI films,while the time constant in the medium-frequency region was related to the diffusion of lithium ions in the organic components of SEI films.Furthermore,it was also revealed that the organic components in the SEI film formed by Li PF₆-based electrolyte undergo oxidative decomposition during the charging process,which even occurs after a long cycling.Finally,it was proposed that the low decomposition strength of Li PF₆ was the main reason for its poor film-forming ability.2.The structure-performance relationship of three lithium salt additives,Li BOB,Li ODFB and Li DFBOP,in the reduction process of Li PF₆-based electrolytes was studied,clarifying that the main reason for the poor film-forming performance of Li BOB was due to the loose and porous structure of its solid-state decomposition product,which cannot effectively inhibit its own decomposition.Li ODFB failed to form a complete film due to the accumulation of its soluble decomposition products on the electrode surface.In addition,the reduction products of Li DFBOP contained a higher proportion of rigid inorganic components,resulting in a harder and more brittle SEI film,which may be fractured under large external stresses.3.To address the the low decomposition strength of Li PF₆,a technique was developed to strengthen the lithium salt decomposition process through the enhancement of the interfacial electric field.Firstly,based on electrochemical tests and molecular dynamics simulations,it was proposed that the smaller the molecular volume of the lithium salt anion,the stronger the double layer electric field strength at the interface between the graphite electrode and the electrolyte.Based on this conclusion,Cl^-was introduced into the electrolyte as a small molecule additive to enhance the interfacial double layer electric field strength.It effectively promoted the decomposition of Li PF₆ and produced more phosphate,improving the stability of the SEI film.As a result,the capacity retention of the graphite half-cell increased from83.4%to 92.3%after 200 cycles at 0.5 C.4.To address the problem of self-inhibitory film formation caused by the soluble decomposition products of Li ODFB,a scheme was proposed to enhance the decomposition of Li ODFB and thus improve the efficiency of SEI film formation by strengthening the diffusion of soluble products.The results showed that the soluble products of Li ODFB would accumulate on the electrode surface,obstructing the self-decomposition of the lithium salt.Meanwhile,these products could also catalyze the decomposition of ethylene carbonate solvent to enhance the film-forming ability of the electrolyte.Based on this conclusion,ultrasonic treatment was applied to the electrochemical system during the Li ODFB decomposition process.It promotes the diffusion of soluble lithium salt products from the electrode surface to the electrolyte,reducing the inhibition of the products on the self-decomposition process of the lithium salt and solvent,and enhancing their decomposition strength.The results showed that the phenomenon of capacity fade during long-term cycling of the graphite half-cell was significantly suppressed with this strengthening scheme,and the capacity retention rate remained above 80%after 500 cycles.5.To leverage the advantages of the oxidation and reduction products of Li DFBOP,a gomphosis type SEI film with both rigidity and flexibility was constructed to enhance the stability of the SEI film.The oxidative and reductive products of Li DFBOP were prepared separately on a highly oriented pyrolytic graphite electrode.It was demonstrated that the Li F-rich inorganic compounds in the reduction products exhibited high rigidity and brittleness,whereas the organic and phosphate-rich oxidation products exhibited high viscoelasticity and ion conductivity.Accordingly,a SEI film,which constructed by interweaving rigid reduction products with flexible oxidation products,was designed and built.This film had both high mechanical strengths to inhibit the damage of lithium dendrites and high flexibility to avoid cracking of the SEI film under external stress,thereby improving the interfacial stability.The battery cycling and rate performance were significantly improved,and the capacity retention rate was increased from 50.2%without enhancement to 96.7%after 200 cycles at 2 C.This study comprehensively elucidated the mechanism and influencing factors of lithium salt in the film formation process at the electrode/electrolyte interface,and developed methods to enhance film formation.The results provide new ideas for the design of high-performance interface films and can further guide the design of new additive molecules,the optimization of interface properties,and the targeted enhancement of battery performance.