High temperature stable lithium-ion polymer battery
Introduction
Lithium-ion batteries are presently one of the major rechargeable power sources for portable electronic devices because of their high output voltage and high energy density, based on weight and volume. The first lithium-ion cells were based on liquid electrolyte technologies and these still make up the majority of the cells currently in use. However, in recent years, there has been considerable interest in polymer lithium-ion batteries because of their enhanced design flexibility and potential safety advantages [1], [2], [3]. Among the different polymer technologies being evaluated, Telcordia’s PLION technology is one of the most promising for commercialization.
An important requirement for all rechargeable batteries for use in portable electronic applications is that they be able to cycle at elevated temperatures (at least up to 60°C). However, Telcordia-type batteries, typically do not cycle well under these conditions. One possible cause of the poor high temperature performance is the hexafluoro propylene (HFP) unit substituted poly(vinylidene fluoride-co-hexafluoro propylene) (PVDF–HFP) copolymer used in the separator membranes and as the electrode binder and which typically employs 12% HFP. The HFP makes the polymer more soluble in solvents such as acetone, thus improving its processability and increases the uptake of electrolyte solution in the cell, thus enhancing ionic conductivity. However, the relatively high level of HFP (12%) could also increase swelling and the solubility of the polymer in the electrolyte solution at elevated temperatures, thereby contributing to the observed poor cycle life performance. The purpose of the present investigation was to explore the use of PVDF–HFP polymers with lower HFP contents as a means of improving the high temperature performance of lithium-ion polymer cells.
Section snippets
Experimental
Three different PVDF–HFP compositions (Solvay) were selected for evaluation as electrode binders and separator membranes. The electrode binder compositions employed 0, 5, and 12% HFP while the separator membranes used 5% (separator C), 12% (separator B), and a 1:1 blend of 5 and 12% HFP (separator A). Electrodes were prepared by first dry mixing the active material (Seimi LiCoO2 for the cathode and Osaka MCMB for the anode) and conductive carbon [4] (Chevron Acetylene Black), followed by the
Results and discussion
Table 1 shows the measured electrical conductivities of cathode coatings on PET using the three different binder compositions. The conductivity of the coating with PVDF homopolymer was significantly lower than that of the others. We attribute this to the lower solubility of the PVDF homopolymer in NMP with the plasticizer, resulting in incomplete mixing and a nonhomogeneous distribution of the binder in the electrode coating.
The separator membrane with a 1:1 blend of 5 and 12% HFP had a melting
Conclusions
We have found the HFP content in the PVDF–HFP copolymer used as an electrode binder in polymer lithium-ion batteries to have a significant impact on high temperature performance. The 12% HFP binder typically used in Telcordia’s PLION technology provides good rate capabilities and low temperature performance, but extremely poor cycle life performance at 60°C. By reducing the HFP content to 5%, we have demonstrated significantly improved cycle life performance at 60°C with no degradation in rate
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