Preparation And Performance Of Polymer Electrolyte For Li-ion Batteries

The liquid electrolytes used in high performance rechargeable lithium-ion batteries (LIBs) may still impact the safety and cycling stability because of electrolytes leakage. Polymer electrolytes are expected to be the alternative electrolytes to satisfy the needs for power storage device because of several important factors, such as high ionic conductivities, reliable safety, easy fabrication, appropriate mechanical properties along with important technological issues related to electrochemical stability. The solid polymer electrolytes prepared by dissolving the salt into the polymer framework are still found to present low ionic conductivities at ambient temperature and dependence of temperature. The ionic conductivities can be enhanced by dissolving polymer matrices and lithium salts in organic solvents. However, liquid electrolytes will deteriorate mechanical property. In this paper, we have prepared a series of polymer electrolytes with excellent thermostability, well mechanical property, wide electrochemical window and high ionic conductivity, including trimethylolpropane trimethylacrylate (TMPTMA) based GPE, triethylene glycol diacetate-2-propenoic acid butyl ester (TEGDA-BA) based GPE, polyimide (PI) based GPE, polyacrylonitrile/alumina (PAN/Al2O3) based GPE, blending TEGDA-BA/PAN based GPE.The novle plasticiser used in TMPTMA based GPE have been fabricated to solve the safety problems of liquid electrolytes. The non-ionic plastic crystal electrolytes were prepared by dissolving lithium bis-trifluoromethanesulfonimide (LiTFSI) into succinonitrile (SN). In the electrolytes, SN can cause fast mobility of LiTFSI due to its high plasticity and diffusivity. The ionic liquid electrolytes were obtained by using tributylmethylammonium bis(trifluoromethanesulfonyl) imide (N1,4,4,4TFSI), LiTFSI and dimethylcarbonate (DMC). Here, the addition of DMC helps to low the viscosity of the ionic liquid and promote the diffusion of the lithium salt. The soft matter solid electrolytes were prepared by polymerizing monomer trimethylolpropane trimethylacrylate (TMPTMA) using in situ thermal polymerization into above novle plasticiser. Addition of polymer into plastic crystal electrolytes could improve their mechanical strength. The TMPTMA based GPE could keep ingredient totally solid and exhibit high ionic conductivities and wide electrochemical window.To improve the poor thermostability and weak mechanical strength of the 1EGDA-BA based GPE, composite gel electrolytes were fabricated by compositing it with other materials, such as PI, nano Al2O3. PAN/Al2O3and PAN. The main results are as follows:(1) TEGDA-BA/PI GPE:Robust electrospun PI membranes exhibit excellent mechanical property and three-dimensional network based on uniform polymeric interconnected structures. With the presence of TEGDA-BA, the resulting TEGDA-BA/PI GPE shows high ionic conductivity up to2.0×l0-3S cm-1at25℃and good interfacial compatibility with LiFePO4and LiFePO4electrodes.(2) TEGDA-BA/Al2O3GPE:The composite polymer electrolytes (CPE) are fabricated by incorporating Al2O3nanoparticles as inorganic filler via in situ polymerization. Due to the uniformly dispersed Al2O3nanoparticles, significant improvements in the mechanical flexibility and bendability are presented in the resulting polymer electrolytes. The flexibility of CPE film is improved in terms of taking advantages of both stiffness of the nanosized fillers and flexibility of the polymer chains. The CPE with5wt%Al2O3nanoparticles exhibits the highest ionic conductivity up to6.02×10-3S cm-1at25℃and the highest Li+transference number (0.675). With the presence of Al2O3, the growth of interface resistance is retarded, which increases the interface stability. The Li|CPE|Li4Ti5O12and Li|CPE|LiFePO4cells demonstrate remarkably stable charge/discharge performance and excellent capacity retention during cycling test.(3) TEGDA-BA/PAN-Al2O3GPE:Mechanically robust PAN-A12O3composite membranes were fabricated by electrospinning mixed solution of PAN and Al2O3nanoparticles. The as-prepared composite membranes of a three-dimensional network based on uniform polymeric interconnected structures exhibit excellent mechanical properties. The introduced Al2O3nanoparticles significantly improved the electrolyte compatibility, thermal properties and wettability of membranes. The fabricated TEGDA-BA/PAN-Al2O3GPE shows high ionic conductivity up to2.35×lO-3S cm-1at25℃. The half cells based on Li[Li1/6Ni1/4Mn7/12]O7/4F1/4electrodes demonstrate remarkably stable charge/discharge performance and excellent capacity retention of～240.4mAh g-1after50cycles.(4) Blending TEGDA-BA/PAN GPE:Flexible GPE with interpenetrating cross-linked network is fabricated by blending long-chain PAN polymer matrix and short-chain TEGDA-BA framework, considering of the synergistic effect of the linear polymer and crosslinked monomers. Due to the good restriction of liquid electrolytes in interpenetrating cross-linked network, significant improvements in the mechanical bendability and interface stability are presented in the free-standing GPE film. The GPE with5wt%PAN exhibits high ionic conductivity up to5.9×10-3S cm-1at25℃and stable electrochemical window (>5.0V vs. Li/Li+). The LiFePO4|GPE|Li4Ti5O12full cells exhibit discharge capacity of125.2mAh g-1and more than98%columbic efficiency after100cycles.