Preparation of Functional Gel Polymer Electrolyte of Poly(vinylidene fluoride)-HFP/gelatin

Poly(vinylidene fluoride) (PVDF) is produced by the polymerization reaction of vinylidene fluoride monomer. This material is a member of the fluoropolymer family and has excellent physical and mechanical properties and chemical stability. The strong electronegativity of fluorine atoms in its molecular structure and the high stability of C-F bonds give PVDF pipes many excellent properties. First of all, PVDF pipes have excellent chemical resistance and can resist the erosion of most acids, alkalis, salt solutions and various organic solvents. Secondly, it has excellent heat resistance and low-temperature toughness, has a wide temperature range for long-term use, and can maintain good mechanical properties in an environment from -40°C to 150°C. In addition, PVDF pipes also exhibit the advantages of low friction coefficient, non-stickiness, high dielectric constant and dielectric strength.

A gel polymer electrolyte was prepared using electrospinning and soaking methods. This gel electrolyte utilizes the electrospinning network of PVDF-HFP and its own absorption of liquid electrolyte, making this electrolyte more conducive to ion transmission and gel formation. At the same time, GN with sol-gel characteristics improves the mechanical properties of HFP-GN GPE.

The interaction between polar groups and Li⁺ in GN was verified through XPS testing. Compared with the initial LiPF₆, in HFP-GN that absorbed LiPF₆, the peak of Li1s moved to a lower binding energy range. This shows that the electrons of the electron-donating polar group are transferred from N or O to Li⁺, increasing the electron cloud density around Li⁺. In addition, after absorbing LiPF₆, the N1s and O1s of HFP-GN are obviously shifted to higher binding energy positions than that of pure HFP-GN. This may be due to the reduced electron cloud density around the N and O atoms of the polar groups. In addition, DFT calculations verified the role of GN on the HFP-GN three-dimensional network in building Li⁺ penetration channels, indicating that there is a strong interaction between GN and Li⁺. Therefore, the introduction of GN weakens the Li-PF6 interaction and increases the dissociation degree of LiPF₆.

The ion transport capabilities of Celgard, PVDF-HFP and HFP-GN GPEs are the basis for battery electrochemical performance. At 25 ℃, the ionic conductivities of Celgard, PVDF-HFP GPE and HFP-GN GPE are 0.83×10^-3 S/cm, 1×10^-3 S/cm, and 1.27×10^-3 S/cm respectively. HFP-GN GPE has high ionic conductivity and can better meet the requirements of practical applications. The activation energy of HFP-GN GPE is 0.074 eV, which is slightly lower than Celgard and PVDF-HFP GPE. It shows that the transport of lithium ions in HFP-GN GPE requires little energy and potential barrier. In addition, the wide electrochemical stability window of HFP-GN GPE indicates that HFP-GN GPE has good electrochemical stability and oxidation resistance. More importantly, the t+ of HFP-GN GPE is 0.54, which is higher than Celgard which has no effective lithium ion transport channel. This result is due to the three-dimensional porous network structure of HFP-GN GPE and the coordination between polar groups and Li⁺.

Finally, the researchers systematically evaluated the application capabilities of HFP-GN GPE in lithium batteries. The experiment used high-voltage LiCoO₂ as the positive electrode to assemble the battery. The results show that through assembly and testing of LiCoO₂/Li batteries, LiCoO₂/HFP-GN GPE/Li always maintains excellent rate performance and cycle stability. After 400 cycles at 2 C, the capacity retention rate of the LiCoO₂/HFP-GN GPE/Li battery is as high as 74%.