| Polymeric lithium-ion batteries(PLIBs) are constructed with three main components such as anode, cathode and polymer electrolyte. Among them, polymer electrolyte has two major functions: one is responsible for transferring Li+, and another is to separate the anode from the cathode with battery. In comparison with the liquid electrolyte, polymer electrolyte can trap the organic carbonate-based electrolyte in polymer matrix. As a result, the assembled PLIBs are more safety than the liquid lithium-ion batteries due to little volatlity of the entrapped electrolyte. Gel polymer electrolute(GPE) with bicontinuous-phase structure possesses not only high ionic conductivity but also high mechanical strength. It has been developed into a focus upon study of polymer electrolyte nowadays.It is very important to select suitable polymer matrix to prepare the polymer electrolyte with good electrochemical performances. Normally, the polymers that have low degree of crystallinity, high dielectric constant, high thermal stability and strong mechanical strength are often used as the polymer matrix of polymer electrolyte. Owing to high dielectric constant, low degree of crystallinity, good electrochemical/thermal stability and hydrophobicity, poly(vinylidenefluoride-cohexafluoropropylene)(P(VDF-HFP)) was chosen as the polymer matrix for polymer electrolyte. In this article, it was blended with polyvinyl pyrrolidone(PVP) to prepare a kind of blended gel polymer electrolyte(BGPE) by a solution casting process through three approaches such as direct blending, in-situ polymerization induced phase separation, and in-situ hydrolysis/polymerization induced phase separation. In the BGPE, the P(VDF-HFP) phase acts as mechanical supporting backbone while the PVP phase takes charge of ionic conduction. The study contents mainly include:1. Blended polymer membranes(BPMs) were prepared by a solution casting process, in which P(VDF-HFP) was directly blended with PVP with different mass ratios. The BPMs were soaked with the electrolyte Li TTSI/EMIm TFSI to from physically blended gel polymer electrolytes(PBGPEs). SEM images showed that the number of pores within the BPM varied little with the increasing amount of the PVP component. TG curves revealed that the prepared BPMs had high decomposition temperatures, indicating a good thermal stability. When the mass ratio of PVP to P(VDF-HFP) was 0.2, the obtained BPM had the maximum number of pores accompanying with 158% of electrolyte uptake and 32% of electrolyte leakage. The resultant PBGPE-2 had an ionic conductivity up to 1.112×10-3 S cm-1 at 75 ℃, which closes to an order of magnitude of the liquid electrolyte. Its onset potential added up to 5.5 V(vs. Li+/Li) in anodic direction, promising a pratical application in lithium-ion batteries. The assembled Li/Li Fe PO4 cells could deliver the initial capacities of 151.2, 147.0, 142.5 and 132.9 m Ah g-1 at the rates of 0.1 C, 0.2 C, 0.3 C and 0.5 C, respectively. After 50 cycles, the corresponding capacity retentions were 94.8%, 93.9%, 89.8% and 81.9%. The results indicated that the PBGPE had a moderate rate capability and cycling performance.2. P(VDF-HFP)/PVP blended membranes were prepared by in-situ polymerization induced phase separation method using a solution of P(VDF-HFP), which was added various amount of N-vinyl pyrrolidone(NVP) monomers. The obtained polymer membranes were immersed in the electrolyte to from chemically blended gel polymer electrolyte(CBGPE). SEM images displayed that more spheric particles with decreased particle size appeared in the polymer matrix, and the pores’ number increased with the increasing amount of NVP monomers. FTIR spectra revealed that NVP monomers had been polymerized successfully. The results of TG analysis suggested that the prepared polymer membranes with high decomposition temperatures had a good thermal stability. DSC measurement results indicated that the degree of crystallinity of polymer membranes decreased with the increasing amount of NVP monomers. When the mass ratio of P(VDF-HFP) to NVP was 2:1, the polymer membranes had a 179% of electrolyte uptake and 29% of electrolyte leakage, the resultant CBGPE-2 had an ionic conductivity of 1.277×10-3 S cm-1 at 75 ℃ and its onset potential up to 5.3 V(vs. Li+/Li) in the anodic direction. The assembled Li/Li Fe PO4 cells could deliver the initial capacities of 156.6、153.8、141.8 and 135.8 m Ah g-1 at the rates of 0.1 C, 0.2 C, 0.3 C and 0.5 C, respectively. After 50 cycles, the corresponding capacity retentions were 94.3%, 95.0%, 95.6% and 90.4%. The results suggested that the CBGPE prepared by in-situ polymerization induced phase separation exhibitted better rate capability and more stable cyclability than those of the PBGPE.3. Nanocomposite P(VDF-HFP)/PVP blended membranes were prepared by in-situ hydrolysis/polymerization induced phase separation method using a solution(the mass ratio of P(VDF-HFP) to NVP was 2:1), to which various amounts of TEOS precursor was added. The obtained polymer membranes were immersed in the electrolyte to form nanocomposite blended gel polymer electrolyte(NCBGPE). SEM images showed that the nanoparticles decreased in particle size while the pores’ number increased with the increasing amount of Si O2 at firsy and then decreased. However, there also existed many micropoes within the polymer membranes. FTIR spectra revealed that the TEOS precursor had been hydrolized to Si O2 nanoparticles, and NVP monomers had been polymerized successfully. The results of TG analysis suggested that the as-prepared polymer membranes had a good thermal stability whose decomposition temperature varied little with the increasing amount of Si O2. DSC measurement results indicated that the degree of crystallinity of polymer membranes increased with the increasing amount of Si O2 at first and then decreased. When the amount of Si O2 was 8%, the polymer membranes had a lowest degree of crystallinity, 218% of electrolyte uptake and 17% of electrolyte leakage. The resultant NCBGPE-2 had an ionic conductivity of 1.031×10-3 and 1.458×10-3 S cm-1 at 25 and 75 ℃, respectively. Its onset potential added up to 5.6 V(vs. Li+/Li) in the anodic direction. The assembled Li/Li Fe PO4 cells could deliver the initial capacities of 166.5, 160.4, 155.4, 145.1 and 137.2 m Ah g-1 at the rates of 0.1 C, 0.2 C, 0.3 C, 0.5 C and 1 C, respectively. After 50 cycles, the corresponding capacity retentions were 95.3%, 98.9%, 91.3%, 98.9% and 98.8%. The results suggested that the NCBGPE prepared by in-situ hydrolysis/polymerization induced phase separation exhibitted the best rate capability and the most stable cyclability among all kinds of the GPEs. |
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