Fundamental Research On Polymer Aluminum Ion Secondary Batteries

In recent years,a majority of studies of aluminum ion secondary batteries have focused on positive electrode materials,while the liquid electrolyte system,as the "blood" of batteries,also has constraints that need to be solved urgently,such as strong corrosiveness,gas generation,sensitivity to water,etc.Furthermore,the decomposition potential of common AlCl3-based liquid electrolyte is relatively low,which limits the upper cut-off voltage when the positive electrode had high oxidation potential.Such critical issues would substantially undermine the safety and stability of aluminum ion batteries,so it is urgent to develop novel electrolyte systems.Polymer electrolyte,with the advantages of high safety factor,no leakage problem and convenient for practical application,has become the new trend of chemical power electrolyte system.However,few research of polymer aluminum ion secondary batteries has been reported.In order to design the polymer electrolyte for aluminum ion batteries,the selection of polymer matrice has been explored preferentially,accompanied by optimization from the plasticizer perspective to improve ionic conductivity and broaden the electrochemical window.The polymer quasi-solid-state aluminum ion secondary batteries have been constructed and the main work is summarized as follows.(1)The suitable polymer frameworks for aluminum secondary battery electrolyte have been investigated.The interaction between four conventional polymers(PEO,PAN,PMMA and PVDF)and chloroaluminate was studied.With electron-giving polar groups,these polymers appear to have limited opportunities to coordinate with the active AlCl4-and Al2Cl7-anions,but would react with AlCl3 which served as Lewis acid and lead to the decomposition of the electrochemically active species(AlxCly-).On the other hand,the reaction between the polymers and AlCl3 also failed to provide any AlxCly-.Polyacrylamide(PAM),a polymer with amide groups on the chain segment,have proved to show little impact on electrochemical activity of chloroaluminate.Since the monomer acrylamide(AM)could be complexed with AICl3 to provide chloroaluminate anions directly,PAM-based solid-state electrolyte could be obtained by simply initiating polymerization.Raman results confirmed the existence of AlCl4-and Al2Cl7-anions,and the ionic conductivity was measured as～10-7 S cm-1 at room temperature.(2)The gel-polymer electrolyte,with AlCl3/[EMIm]Cl ionic liquid as plasticizer,has been successfully prepared by in situ polymerization.Based on the novel gel electrolyte,the quasi-solid-state Al-C secondary batteries were constructed with flake graphite as the positive electrode material.Results demonstrated that the electrochemical window could reach 2.50 V and the ionic conductivity at room temperature was 5.77 ×10-3 S cm-1.The quasi-solid-state batteries showed excellent charge-discharge performance,with the specific discharge capacities of 123 mA h g-1 at the 1st cycle and of 110 mA h g-1 at the 100th cycle;The energy storage and release in the quasi-solid-state Al-C batteries were also based on intercalation/deintercalation of chloroaluminate anions into/out of the graphite layers during charge/discharge process.Meanwhile,such quasi-solid-state cells were able to offer relatively high energy storage performance below ice point and possessed ability to accommodate the strain upon mechanical bending to maintain stability.And the quasi-solid-state batteries also could achieve great safety under mechanical cutting and unsealed states.Moreover,the gas production in the quasi-solid-state batteries has been massively suppressed,compared to the cases in the liquid-state systems.(3)Construction of a long-life quasi-solid-state batteries aluminum ion battery based on the gel electrolytes with wide potential window.By employing AlCl3/Et3NHCl ionic liquid as plasticizer,the potential window could be achieved in the range of 0-2.96 V and the ionic conductivity at room temperature was measured as 4.52 × 10-3 S cm-1.The characterization results of the quasi-solid-state Al-C batteries highlighted the advantages of increasing upper cut-off voltages in high rate charging/discharging processes.The battery exhibited an ultra-stable cycling behavior around 800 cycles and delivered-90 mA h g-1 capacities.And the structural evolution of the positive electrode products showed that more chloroaluminate anions were intercalated into graphite layers at high charging potential.Meanwhile,the gel electrolyte established robust electrode-electrolyte interfaces,and presented high stability in mechanical bending.High electrochemical activity of the gel still existed after being exposed to air for 5 min.The comparison of gas production between the two ET-type systems declared that the gel electrolyte could partly inhibit the side reactions during charging and discharging process,effectively avoiding leaking issues.