Research On The Application Of Poly(Ionic Liquid)in Lithium Metal Batteries

Secondary lithium metal batteries are currently a research hotspot due to their low self-discharge rate,long cycling life,and no memory effect.However,the use of traditional flammable and explosive organic electrolytes and the uncontrollable growth of lithium dendrites greatly inhibit their commercialization process.Therefore,the development of high safety and high energy density lithium metal batteries has become a trend,such as building solid-state lithium metal batteries and high energy density lithium-sulfur batteries.However,the resulting problems include low room temperature ionic conductivity of solid electrolytes,poor electrode/electrolyte interface compatibility,and early short-circuiting of the battery caused by the growth of lithium dendrites at the electrolyte interface.In addition,the battery performance degradation caused by polysulfide shuttle in lithium-sulfur batteries is another major issue.The analysis found that there are similar solutions to the problems in different systems,such as introducing ionic liquids into polymer frameworks to obtain polymeric ionic liquid with high ionic conductivity and"designability"of ionic liquids and mechanical properties of polymers,which can be used as polymer electrolytes and binders.Based on"designability,"groups that are easy to form hydrogen bonds can be introduced into the polymer electrolyte to impart self-healing ability,thereby spontaneously eliminating cracks caused by dendrite formation and ensuring battery safety.When used as a binder for lithium-sulfur batteries,groups that can absorb polysulfides are introduced to suppress the"shuttle effect"of polysulfides and improve the performance of lithium-sulfur batteries.Based on the above problems and analysis,this paper developed a polymeric ionic liquid with both self-healing ability and polysulfide adsorption,which are used as solid-state electrolytes for lithium batteries and binders for lithium-sulfur batteries.The specific research work is as follows:（1）In response to the issues of high crystallinity and low room temperature ionic conductivity of traditional polymer electrolytes,as well as the problem of lithium dendrite growth induced by electrode/electrolyte interface defects,this paper grafted ionic liquid chain units（EMIM⁺）onto the polymer（PMMA）backbone,and prepared a new type of poly（ionic liquid）electrolyte with high ionic conductivity(1.76×10^-4 S cm^-1),wide electrochemical window（5.2 V）,and self-healing ability.Through theoretical calculations,it has been found that due to the interaction of hydrogen bonds between molecules,the electrolyte can spontaneously repair defects at the interface between the electrode and electrolyte,thereby inducing the uniform deposition of Li⁺.The Li|LiFePO₄ battery has a first discharge specific capacity of 139.7 mAh g^-1 at room temperature and 0.5 C current density,and after 200 cycles,the discharge specific capacity can still be maintained at 124.9 mAh g^-1,with a capacity retention rate of 89.4%.（2）In response to the issues of low ionic conductivity,poor mechanical properties,and almost no inhibition effect on the polysulfide shuttle of traditional PVDF binders,this project introduces the aforementioned prepared poly（ionic liquid）（PIL）as a binder.The PIL binder contains imidazolium cations,which can form"weak acid-weak base"pairs with polysulfide anions,effectively adsorbing polysulfides and improving the cycling stability of lithium-sulfur batteries.In addition,compared with PVDF binders,PIL binders have higher viscosity and can overcome the structural damage of the cathode caused by volume changes during the cycling process,maintaining the integrity of the cathode and achieving an improvement in sulfur utilization efficiency.The lithium-sulfur battery prepared using PIL binder has an initial discharge specific capacity of 1544.8 mAh g^-1 at room temperature and 0.5 C current density,and the discharge specific capacity can still reach 879.6 mAh g^-1 after 100 cycles,with a capacity retention rate of 56.9%.