Research on Composite Quasi-Solid Polymer Electrolytes

Lithium-ion batteries are dominated the fields of power sources for future electrochemical device attribute to their high energy density, high working voltage, high efficiency, long life and memory-effect-free [1]. Solid-state polymer have been employed to develop rechargeable lithium-ion batteries which have little security problems caused by liquid electrolyte leakage. In recent years, Soft matter ionic conductors such as gel polymer electrolytes (GPEs) have been emerging as a suitable class of materials due to their ability to deliver substantially higher Li⁺ transport number [2]. In general, conventional GPEs are prepared using a predesigned frame via solution casting of liquid state mixtures (i.e., lithium salt and polymers dissolved in organic solvents), followed by solvent evaporation for solidification. However, the cumbersome preparation processes, and low lithium transport numbers are usually inadequate for practical application. To relieve the preparation problem, in situ synthesis process has been increasingly attracted extensive attention in the preparation of GPEs. The as prepared GPEs are also known as composite polymer electrolytes, because they combine the cohesive features typical of solid systems with liquid-like transport properties. The composite quasi-solid polymer electrolytes were prepared by in situ thermal polymerization using trimethylolpropane trimethylacrylate (TMPTMA) [3, 4] or triethylene glycol diacetate-2-propenoic acid butyl ester copolymer (TEGDA-BA) [5, 6] as monomer. The preparation processes, electrochemical performance and interfacial properties of polymer electrolytes were studied.

TMPTMA-based gel polymer electrolytes were prepared by using TMPTMA as a monomer, lauroyl peroxide (LPO) as a thermal initiator, 1M LiFP₆ / (EC:DMC:EMC =1:1:1, V/V/V) liquid electrolyte solvent  and plastic crystal electrolyte (containing 4 mol% LiTFSI in the succinonitrile) as plasticizer, respectively.

The influencing factors of the electrochemical performance, such as polymerization temperature, liquid electrolyte content and initiator content, were systematically discussed and the process parameters were optimized. The high ionic conduction (>10^-3 S cm^-1) of gel polymer electrolytes was mainly due to the liquid electrolyte trapped in the pores highly dispersed in the polymer framework.

The homogeneous and stable liquid state electrolyte was obtained by magnetic stirring the precursors. After the gel reaction, the precursor liquid electrolyte became a viscous gel and showed little flow even tipping the vial at a certain angle.

The polymer electrolytes has a wider electrochemical voltage window was stable approximately up to 5 V versus Li/Li⁺ without an irreversible oxidation.

The discharge capacity vs. the cycle number had been studied employing LiCoO₂, Li[Li_1/6Ni_1/4Mn_7/12]O₂, LiFePO₄ as the active cathode material and lithium metal as the anode. The Li/GPEs/ LiCoO₂ display a high initial discharge capacity of 134.1 mAh g^-1, as well as an excellent rate capability (83% remaining capacity after 100 cycles).

TMPTMA-based and TEGDA-BA-based composite polymer electrolytes have been successfully prepared in situ thermal polymerization. The polymer electrolytes have higher ionic conductivities, electrochemical stability and excellent charge/discharge galvanostatic cycling performance at room-temperature. The excellent properties of polymer electrolytes make it significant potential material for next generation lithium ion batteries.

Acknowledgements    

This work was supported by the 973 project (2013CB934001), NSF of China (51172024, 51372022)

References

[1] L.Z. Fan, Y.S. Hu, J. Maier. Adv. Funct. Mater., 2007, 17, 2800.

[2] E.H. Kil , K.H. Choi. K.Y.  Cho, S.Y. Lee.  Adv. Mater., 2013, 25, 1395.

[3] D. Zhou, L.Z. Fan, H.H. Fan, Q. Shi. Electrochim. Acta, 2013, 89, 334.

[4] Q.J. Wang, H.H. Fan, L.Z. Fan, Q. Shi.  Electrochim. Acta, 2013, In press.

[5] H.H. Fan, H.X. Li, L.Z. Fan. J. Power Sources, 2013, Accepted.

[6] Q.J. Wang, H.F. Ni, W.L. Song, L.Z. Fan. J. Power Sources, 2013,in press.