PVDF-HFP

The preparation and properties of a novel, commercially viable Li-ion battery based on a gel electrolyte has recently been disclosed by Bellcore (USA) [124]. The technology has, to date, been licensed to six companies and full commercial production is imminent. The polymer membrane is a copolymer based on PVdF copolymerized with hexafluoropropylene (HFP). HFP helps to decrease the crystallinity of the PVdF component, enhancing its ability to absorb liquid. Optimizing the liquid absorption ability, mechanical strength, and processability requires optimized amorphous/crystalline-phase distribution. The PVdF-HFP membrane can absorb plasticizer up to 200 percent of its original volume, especially when a pore former (fumed silica) is added. The liquid electrolyte is typically a solution of LiPF6 in 2 1 ethylene carbonate dimethyl car-... 

Gozdz et al. (of Bellcore) [25] recognized that poly (vinylidene difluoride) hexafluoropropylene (PVDF HFP) copolymers could form gels with organic solvents and developed an entire battery based on this concept. Typically, the gel separator is 50 pm thick and comprises 60wt. % polymer. In the Bellcore process the separator is laminated to the electrodes under pressure at elevated temperature. The use of the PVDF HFP gelling agent increases the resistivity of the electrolyte by about five times which limits the rate capability of such batteries. 

Doyle et al. [40] used a mathematical model to examine the effect of separator thickness for the PVDF.HFP gel electrolyte system and found that decreasing separator thickness below 52 pm caused only a minor decrease in ohmic drop across the cell. The voltage drops in the electrodes were much more significant. They state that their model predictions were confirmed experimentally. 

Tadic et al. studied the polymer poly-vynilidene fluoride/hexa-fluoropropylene ( PVdF/HFP ) containing lithium salt solution in Ethylene carbonate/diethylene carbonate ( EC/DEC )- In order to understand better the effect of anion size in the electrolyte, two Li salts were compared, namely LiN(CF3S02)2 (termed Liimide by the authors) and LiN(C2F5S02)2 (termed Libeti ). 

According to Tarascon and co-workers, the swelling of PVdF—HFP by liquid electrolytes was never complete due to the semicrystalline nature of the copolymer, which tends to microphase-separate after the activation by electrolyte. On the other hand, it is those crystalline domains in the gelled PVdF—HFP that provide mechanical integrity for the resultant GPE. Thus, a dual phase structure was proposed for the Bellcore GPE by some authors, wherein the amorphous domain swollen by a liquid electrolyte serves as the ion conduction phase, while tiny crystallites act as dimensional stabilizer. 

Apparently, the formation of the microporous structure within the PVdF—HFP copolymer was of critical importance to the success of Bellcore technology, and the ion conductivity was proportional to the uptake of the liquid electrolyte. To achieve the desired porosity of PVdF film, Bellcore researchers prepared the initial polymer blend of PVdF with a plasticizer dibutylphthalate (DBP), which was then extracted by low boiling solvents after film formation. Thus, a pore-memory would be left by the voids that were previously occupied by DBP. However, due to the incomplete dissolution of such high-melting DBP during the extraction process, the pore-memory could never be restored at 100% efficiency. Beside the total volume of pores thus created by the plasticizer. 

Figure 79. SEM of methanol-extracted separators prepared in the weight ratio of plasticizer/PVdF-HFP/Si02 of 5 3 2. The extracted plasticizer in parts a and c is DBP, and that in part b is an oligomer with molecular weight 450. Note that the scale in parts a and b is 5.0 jum and that in part c is 100 nm. (Reproduced with permission from ref 561 (Figure 2). Copyright 2000 The Electrochemical Society.)...

Improvements based on Bellcore technology were reported recently by Wunder and co-workers. Using PEG oligomers instead of DBP, they obtained the PVdF—HFP microporous films with the pore size increased from nanoscale to microscale, as shown in... 

One particular version of the lithium-ion gel polymer cells, also known as plastic lithium-ion cell (PLION). was developed by Bellcore (now Telcordia Technologies).In this case. Gozdz et al. developed a microporous plasticized PVdF—HFP based polymer electrolyte which served both as separator and electrolyte. In PLION cells, the anode and cathode are laminated onto either side of the gellable membrane. Good adhesion between the electrodes and the membranes is possible because all three sheets contain significant amounts of a PVdF copolymer that can be melted and bonded during the lamination step. 

The PVdF—HFP separators used in PLION cells were around 3 mil thick, and had poor mechanical properties. It has been reported that the major source of rate limitation in PLION cells was the separator thickness. The rate capability of these cells can be significantly improved by decreasing the separator thickness to that typically used in liquid electrolyte system. Moreover, in the absence of shutdown function. the separator does not contribute to cell safety in any way. Park et al. reported that the HFP content in separators did not have any significant impact on cell performance. The Bellcore process has proven to be an elegant laboratory process but is difficult to implement in large-scale production. 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

The evidence that the 1 couple can diffuse freely in the liquid domains entrapped by the three-dimensional network of the gelators has also been found in the case of a PVDF-HFP gel via steady-state voltammetry at ultramicroelectrodes. Quite surprisingly the voltammogramms of the liquid and of the gel are almost perfectly superimposable (Fig. 17.14) and the diffusion coefficient of the redox ions could be calculated to be 3.6 x 10 cm2/s and 4.49 x 10-6 cm2/s for I- and I3, respectively, using Equation 17.15,... 

Figure 17.14 Steady-state voltammetry of a liquid and polymer (PVDF-HFP) gel electrolyte at a Pt ultramicroelectrode. Scan speed lOmV/s. Reprinted by permission from Mac Millan Publishers Ltd Nature Materials, 2003, 2, 402.

It was possible to improve the interfacial properties of Li metal anodes in liquid electrolyte solutions using additives that modify the Li-surface chemistry, such as C02 [23-27] and HF [28,29], Using PEO-based gel electrolyte systems effectively suppressed dendritic deposition of lithium [30], In Section C we report on a very good charge-discharge performance of lithium metal anodes in PVdF-HFP gel electrolyte systems. Furthermore, addition of C02 to the PVdF-HFP gel electrolyte system considerably improves the charge/discharge characteristics [31]. 

As shown in Figure 7a, the lithium anode tested in the EC/PC-based liquid solution shows relatively high efficiency only at the first cycle. In subsequent cycles, the cycling efficiency drops to around 75%. The Li cycling efficiencies measured in EC/PC are similar to those measured in the PEO gel. It is obvious that lithium anodes in the PVdF-HFP gel electrolyte system show higher cycling efficiency. 

Figure 8 Micrographs of lithium metal anode at the first and fifth cycle in (a) PEO gel without C02, (b) PVdF-HFP gel without C02, and (c) PVdF-HFP gel with C02.

The gel polymer electrolytes studied were composed of PVdF or poly(vi-nylidene fluoride-hexafluoropropylene) (PVdF-HFP) as base materials with the addition of TEABF4/EC + PC as the plasticizer. 

In addition, combinations of activated carbon (AC) powder with PVdF-HFP gel electrolyte as a binder (30 wt.% PVdF-HFP) were prepared and investigated as electrode material for EDLC. When activated carbon, whose specific surface area was 2500 m2 g 1 was used, a capacitance of 123 F g 1 could be obtained. Such a capacity was higher than that of EDLC with liquid organic electrolyte solutions [73],... 

Many approaches have been developed for the production of ionic liquid-polymer composite membranes. For example, Doyle et al. [165] prepared RTILs/PFSA composite membranes by swelling the Nafion with ionic liquids. When 1-butyl, 3-methyl imidazolium trifluoromethane sulfonate was used as the ionic liquid, the ionic conductivity ofthe composite membrane exceeded 0.1 S cm at 180 °C. A comparison between the ionic liquid-swollen membrane and the liquid itself indicated substantial proton mobility in these composites. Fuller et al. [166] prepared ionic liquid-polymer gel electrolytes by blending hydrophilic RTILs into a poly(vinylidene fiuoridej-hexafluoropropylene copolymer [PVdF(HFP)] matrix. The gel electrolytes prepared with an ionic liquid PVdF(HFP) mass ratio of 2 1 exhibited ionic conductivities >10 Scm at room temperature, and >10 Scm at 100 °C. When Noda and Watanabe [167] investigated the in situ polymerization of vinyl monomers in the RTILs, they produced suitable vinyl monomers that provided transparent, mechanically strong and highly conductive polymer electrolyte films. As an example, a 2-hydroxyethyl methacrylate network polymer in which BPBF4 was dissolved exhibited an ionic conductivity of 10 S cm at 30 °C. 

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