I'm currently developing a solar powered data logger that sits outside in direct sunlight for a good chunk of its life. Preliminary testing has shown that the inside of the enclosure can get up to 70°C on a hot day in the Australian Sun.
My first prototype iteration used a single 5000mAh 3.7v prismatic lithium polymer cell. This met the power requirements perfectly but on moderately hot days the 50°C over-temperature cutoff would kick in and the cell would stop charging from the solar panel and the board would fail to stay on when removed from the sun. I also have concerns about the lifespan of these cells under heat-stress and using potentially flammable li-poly cells in hot conditions.
The device requires 2-3000mAh of capacity with a board Vcc of 3v, the average current draw is <100mA. I'd prefer not to vent the enclosure as the device is used in pretty dusty conditions and I'd like to keep it as a sealed unit. By better insulating the enclosure, I may be able to bring the inside down to around 60°C if that makes things easier. The battery needs to be less than 12mm thick.
For my next iteration I'm considering different chemistries to address this issue. So far the only cells rated for such temperatures I've found are the SAFT high-temperature lithium ion cells (http://www.saftbatteries.com/force_download/VL32600_125.pdf) that are rated up to 125°C, but they don't come as a flat cell and they're not readily available in small quantities.
I've also read mixed literature about what temperatures LiFePO4 cells can be used at.
What options have those of you who've needed batteries in high ambient temperatures found?
If anyone could point me in the direction of some other chemistries or specific cells that might meet my requirements I would be very appreciative.