Preparation And Property Research Of Pvdf-hfp-based Polymer Electrolyte

As critical materials used in polymer lithium-ion battery, modified microporous poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) electrolytes were prepared through blending, semi-interpenetrating networks (semi-IPN), graft copolymerization, respectively. The research was focused on their physical and electrochemical properties and their battery performance. As a result, this work is important to instruct the future development of microporous polymer electrolytes. Microporous polymer electrolyte was prepared through blending of PVDF-HFP and fully cyanoethylated cellulose derivative (DH-4-CN) by the extraction method. DH-4-CN with a high dielectric constant, can promote the adsorption of electrolyte and lithium salt dissociation. This polymer electrolyte has a wide electrochemical stability window (>4.8 V) and higher stable body resistance and interface resistance. The blend polymer electrolyte with PVDF-HFP/DH-4-CN=14:1 (w/w) has the conductivity of 4.36×10-3 S·cm-1 at 20℃. The button cell with this polymer electrolyte shows good cycle and rate performance.Poly(ethylene glycol) (PEG) with two epoxy ends was cross-linked using poly(ethyleneimine) (PEI) as a crosslinking agent. The polymer electrolyte was prepared with the semi-interpenetrating networks of PVDF-HFP and crosslinked PEG which avoided to introduce the impurities into system. The properties of the semi-IPN electrolytes were studied through X-ray diffraction, BET, scanning electron microscopy, electrochemical linear scanning, impedance, cell cycle and rate performance testing. The results showed that the semi-IPN electrolyte has the advantages of PVDF-HFP and PEG. The semi-IPN polymer electrolyte has a better stability of bulk resistance and interface resistance than PVDF-HFP. The polymer electrolyte with appropriate cross-linked PEG has a better porosity and higher conductivity as a result of good dispersion of the deionized water in PEG. The semi-IPN electrolyte with PVDF-HFP/DIEPEG+PEI=60:40 (w/w) has the conductivity of 2.30×10-3 S·cm-1 at 20℃, break strength of 8.9 MPa, elongation of 46.3%. The coin cell with this electrolyte showed the initial capacity of 120.4 mAh·g-1, and the discharge capacity fell down to 119.1 mAh·g-1 after 50 cycles. In contrast to the discharge capacity with 0.2 C, the discharge capacity with 0.5 C, 1 C, 2 C were 97.4%, 94.9%, 81.4%, respectively.The microporous electrolyte of PVDF-HFP grafted with PEG (PVDF-HFP-g- PEG) was prepared because the blend and semi-IPN polymer electrolytes appeared micro-phase separation under long-term storage or use. PVDF-HFP-g-PEG has a higher electrolyte absorption and conductivity than PVDF-HFP with a conductivity of 3.28×10-3 S·cm-1 at 20℃. The electrochemical window of this electrolyte reached up to 4.8 V. The coin cell with PVDF-HFP-g-PEG showed the initial capacity of 120.7 mAh·g-1, and the discharge capacity fell down to 115.9 mAh·g-1 after 50 cycles. In contrast to the discharge capacity with 0.2 C rate, the discharge capacity with 0.5 C, 1 C, 2 C were 96.2%, 94.5%, 81.3%, respectively.The sandwich-type polymer electrolyte has the low compatibility between the layers, which may lead to the coating layer sloughing under abusive conditions. PVDF-HFP-g-PEG as the buffer was introduced to between the inner PVDF-HFP and outer polyethlene glycol (PEO) which acted as an adhesive. EDS analysis showed that the buffer layer existed the molecular self-assembly phenomenon which is more beneficial to adhesion. Compared with the three-layer polymer electrolyte without buffer, five-layer polymer electrolyte had a more stable interfacial structure. After 500 charge-discharge cycles at 1 C rate, the three-layer polymer electrolyte showed the partial sloughing and five-layer polymer electrolyte maintained the structural stability. Five-layer electrolyte had a small change of bulk resistance and interface resistance, as well as better discharge performance. The coin cell with five-layer electrolyte showed the relatively low charge-discharge. Improvement the conductivity of five-layer electrolyte is the direction of future work.