Research Of Bacterial Cellulose Gel Polymer Electrolyte For Lithium Battery

Because of its excellent properties, such as high power density, high output voltage, long cycle life and low self discharge properties, lithium battery has been the key storage powers for portable electronic devices, computer and satellite. Recently, gel polymer electrolyte(GPE) has attracted much attention due to its battery safety and designability. GPE is one kind of multiphase materials that electrolyte is bound in gel networks. GPE, with its higher conductivity (more than 10-3S/cm), has almost met the requirement of commercialization. The typical host polymers for GPE include PEO, PAN, PVDF and PMMA. These traditional GPE membranes prepared by solution casting have many disadvantages for lithium battery application, such as much difficulty in making films and poor mechanical property caused by adding too much plasticizer. Thus, preparation of GPE membranes with high conductivity and high mechanical properties has been the key issue for GPE lithium battery.Bacterial Cellulose (BC) membrane presents a hydrogel state, composed of more than 95% water bound in the three-dimensional networks by molecular interaction between H2O and cellulose. In this work, the GPE membranes were prepared by vacuum-impregnation method using BC hydrogel as the raw materials, and the lithium salts were incorporated simultaneously. The effects of type, adding quantity and manner of lithium salts on the conductivity and mechanical properties of GPE membranes were investigated systematically. Meanwhile, a new gel composite comprised of SiO2 nanoparticles and BC was prepared by in situ synthesis method. Then the composite gel polymer electrolyte (CGPE) was obtained by incorporating lithium salts and organic solvent into the gel composite mentioned above. Finally, lithium batteries were assembled by using the BC-GPE membrane as prepared, followed by measuring the charge-discharge cycling performance. The main conclusions of our work are summarized as follow:1. The BC organic gel membranes were successfully prepared by means of vacuum-impregnation process, which replaced the water with organic solvent. The properties were characterized using different techniques such as thermo gravimetric analysis and mechanical testing. The results showed that GPE membrane had a better thermal stability and mechanical properties when polyethylene glycol dimethyl ether (NHD) and LiClO4 were used as organic solvent and lithium salt, respectively.2. The content of residual moisture and LiClO4 in the gel were the main factors to affect the conductivity of BC-GPE. Under the same conditions, the conductivity increased with the impregnating temperature, which benefit the water removal from hydrogel, and LiClO4 content. When the impregnating temperature was 80℃and the content of LiClO4 was 23wt%, the conductivity was up to 1.6×10-2S/cm.3. With the help of the measurements of FTIR, SEM and XRD, it can be concluded that:①There was no chemical bonds between NHD, LiClO4 and BC.②The solvent and electrolyte filled into the three-dimensional network of BC.③The BC crystalline structure was not changed, but the crystallinity of BC little bit decreased. The relationship between temperature and conductivity of BC-GPE was analysised by Arrhenius equation. The fitting results showed that the hopping mechanism dominated the Li+transfer process in GPE membranes.4. The BC/SiO2 composite gel polymers with different SiO2 content were prepared by in situ synthesis method. Then the CGPE was obtained by incorporating lithium salts and organic solvent into the composite gel mentioned above. When the content of SiO2 was 12wt%, the impregnating temperature was at 80℃and the content of LiClO4 was 23wt%, the conductivity was 7.14×10-3S/cm. Under the same condition, BC/SiO2 CGPE had a wider electrochemical stability window than BC-GPE.5. A lithium battery was assembled using BC/NHD/17wt% LiC104 GPE as the electrolyte, LiCoO2 as the cathode and lithium as the anode. The average discharge voltage of the battery was about 3.9V. The rate discharge performance was satisfactory. When the discharge rate was 0.1C, the cycle performance was acceptable. At 30 cycle, the discharge capacity was 86.2mAh/g, and about 82% of the initial discharge capacity was remained.