Preparation And Properties Of Ionic Liquid-based Polymer Electrolytes

Organic electrolytes used in traditional lithium-ion batteries have characteristics of highly volatile and flammable,which would cause leakage,fire for even explosion when the related Li-ion batteris kept under high temperature,overcharge,short circuit,or damage.In addition,lithium dendrites formed spontaneously on the Li anode surface during Li⁺delposition process would pierce the separator,resulting in short circuit and potential safety hazard.The solid electrolyte has advantages of non-leakage,wide electrochemical window,and high thermal stability.Replacing organic electrolyte with solid electrolyte would fundamentally solve the safety problems,and mechanically inhibit the growth of lithium dendrite.Compared with inorganic solid electrolyte,polymer electrolyte has better flexibility and could tolerate electrode deformation during charge/discharge process.However,its low ionic conductivity impedes its further application at room temperature.To solve the above problems,ionic liquid with high ionic conductivity and low vapor pressure was introduced in the preparation of a polymer electrolyte membrane,which provides a good idea for realizing all-solid-state lithium battery at room temperature.Our work is list as follows:(1)We have adjusted the cation and anion of ionic liquid,and study their effects on the plateau and cycle performance of Li Fe PO₄/Li batteries.The experimental results showed that the battery using EMIMTFSI electrolyte exhibts the best electrochemical performance,which can steadily operate for 300 cycles stably,with the discharge capacity maintained above 100.8 m Ah g^-1.After100 cycles,the charging and discharging pleateau were 3.67 V and 3.15 V,respectively,displaying the minium polarization.The gel polymer electrolyte membrane was obtained by mixing EMIMTFSI with PVDF through the solution-casting solution.The mass ratio of ionic liquid/PVDF and the concentration of lithium salt were adjusted to optimize the preparation process of the electrolyte membrane.The results show that the Li Fe PO₄/Li battery exhibits the best electrochemical performance with the mass ratio of EMIMTFSI/PVDF and the concentration of Li TFSI keeping at 1:0.5 and 1 mmol g^-1,respectively.The obtained battery can still deliver a discharge specific capacity of 98 m Ah g^-1 after100 cycles at 0.5 C,with the coulomb efficiency over 99%.(2)In the gel electrolyte membrane prepared by the solution-casting method,ionic liquids and polymer are mainly mixed physically.The anions and cations from ionic liquid are"free",which could move without restaint and advese to Li+transport.On this basis,ionic liquid was grafted onto polymer matrix by chemical bonds,so that the ions were not affected by the potential gradient and the transport rate of lithium ions was improved.At the same time,a uniformly distributed space charge layer was formed to guide the uniform deposition/dissolution of lithium ions during charge/discharge process.EMIMTFSI was grafted onto PMMA branched chain,followed by dispersing in NMP to form homogeneous solution.An integrated electrode would be farbicated by casting the obtained soltion on the Li anode.The discharge specific capacity of the assembled Li Fe PO₄/Li battery could reach up to 107.2 m Ah g^-1 after 120 cycles at 25?and 0.5 C.No lithium dendrites were observed after cycling.