Studies On Synthesis And Electrochemical Properties Of Composite Polymer Electrolyte With Ionic Liquids

The rechargeable lithium polymer battery is a new type of high-energy chemical power source.It has got much attention due to its advantages such as good safety, high energy density and flexible preparing.Now the studies on polymer electrolytes are focused on two aspects:firstly,the conductivity of the polymer electrolyte at room temperature should be increased without decreasing its mechanical strength;secondly, the properties of the Li/polymer electrolyte interface must be improved.Recently, large efforts have been devoted to the investigation of ionic liquids as green chemistry materials.Incorporating ionic liquids to the polymer electrolyte to get the composite polymer electrolyte with ionic liquids is a promising method to prepare polymer electrolyte from the applying point of view.Two types of new composite polymer electrolytes containing imidazolium-based and piperidinium-based ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide(EMI-TFSI) and N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide(PP13-TFSI) are prepared by solution casting method in this paper.These two kinds of new composite polymer electrolytes with ionic liquids have been studied by different electrochemical methods and spectral methods.Furthermore,the properties of the interface of lithium and polymer electrolyte have also been studied by in-situ micro-FTIR technique.The main results are summarized as follows:1.FTIR characterization indicates that incorporating both EMI-TFSI and PP13-TFSI can change the crystalline phase structure of PEO and get more amorphous phase;can weaken the association between Li⁺ and TFSI^- and the interaction between Li⁺ and oxygen in the PEO,as a result,more free Li⁺ can be obtained.TFSI^- anion from these two kinds of ionic liquids plays a main plasticizing role in the P(EO)₂₀LiTFSI systems.The glass transition temperatures(T_g) and the crystallinity of the polymer electrolyte decrease with ionic liquids(both EMI-TFSI and PP13-TFSI) addition from the DSC results.The good reversibility of the Li deposition and dissolution,as well as a stable interracial passive film have been observed by incorporating PP13-TFSI and a relative small quantity of EMI-TFSI from CV result.Incorporating EMI-TFSI and PP13-TFSI will increase the conductivity of the polymer electrolyte obviously,and the systems with EMI-TFSI will have even better conductivity than the PP13-TFSI system.When the amount of ionic liquid in the P(EO)₂₀LiTFSI polymer electrolyte is relatively large(x=1.0),the conductivity will get more than 10^-4S/cm at 40℃,and it is two orders of magnitude bigger than the polymer electrolyte without ionic liquids.The addition of ionic liquid will lead to the decrease of transfer number slowly,but the lithium ion conductivity still increases. The addition of EMI-TFSI and PP13-TFSI to P(EO)₂₀LiTFSI polymer electrolyte result in a decrease of interracial resistance,the better interfacial stability and a wider electrochemical window.At the high ionic liquids concentrations(x=1.0),the electrochemical window reaches 5.2V,and it suggests that this composite polymer electrolytes can be applied safely in 5V lithium secondary batteries.The tests performed on Li metal/LiFePO₄ batteries containing these two types of polymer electrolytes have shown good capacity performance at moderate temperature.At least, we can get capacity of 120mAhg^-1 at 0.2C rate for both P(EO)₂₀LiTFSI+ 1.0EMI-TFSI and P(EO)₂₀LiTFSI+1.0PP13-TFSI electrolytes at 50℃and the result is satisfactory for the real application.We can consider composite polymer electrolyte with ionic liquids as "water in sponge" model.PEO is the hard sponge base,and there are Li⁺,TFSI^-,ion pairs,triplet ions,EMI and PP13 cations in the PEO.Ionic liquid as the plasticizer,due to its high dielectric constant,promotes ion pairs to dissociation and more free Li⁺ can be obtained.TFSI^- anion from ionic liquid plays main plasticizing role.It can change the crystalline phase structure of PEO,accelerate the segmental motion of the PEO-based polymer electrolyte,as a result,get more amorphous phase.The cations from ionic liquid(EMI and PP13) can interact with the oxygen in PEO,weaken the association between Li⁺ and TFSI^- and the complexation between Li⁺ and oxygen in the PEO,as a result,it facilitates the transport of lithium ion.2.A spectroelectrochemical cell was optimized,and the solid-solid interface between lithium electrode and polymer electrolytes was explored by using in situ micro-FTIR spectroscopy.Collecting reflective FTIR spectrum before and after the several CV experiments at the high temperature(50℃) with in-situ electrochemical cell is an effective method.In-situ FTIR results obtained from the polymer electrolyte containing PC and EC systems after some lithium deposition-dissolution processes, show that the surface chemistry of Li is dominated,as expected,by PC and EC reduction to ROCO₂Li and Li₂CO₃ species.