- Sandwich one of these between the plastic covers and battery. No idea how effective it'll be with cooling.
- Make a foldable mylar heatshield you can deploy under the car when it's parked over hot asphalt like at my work. Like how the James Webb Space Telescope has layers of insulation to reflect the heat away.
Regarding insulation: I don't think this will work. The issue with the summer heat is the thermal mass. Even if you have a thermos-quality insulator, the summer heat average is still far too hot. For example, my LeafSpy reading shows me 38-Celcius battery temperature during the 'cooler' night (outside temp 27-Celcius).
Regarding the active cooling: Now we're talking!! The only problem is how to install that easily...
The easiest answer is to always park your car in the air-conditioned car park (or buy a Tesla )
The heat in the battery is generated by current flow through the battery when charging and discharging (driving). Insulating the battery would make things worse, not better, as the heat in the battery has to be discharged to the environment. The trouble with hot whether is that battery cooling is less effective, just as ICE cars have a tendency to boil because the radiator cooling is less effective. When driving, airflow under the car assists with cooling. I don't know whether anyone has tried using a fan to direct under the car for cooling whilst charging.
I use a variable current EVSE (6, 10 or 15 amperes) and charge at the lowest rate that will charge the car in time for the next journey, and then only charge to 80 or 90% unless the ultimate range is actually needed, as the final charging period generates the most heat. (Also the battery ages faster when fully charged or fully discharged. It is best to avoid having the battery fully charged or fully discharged for any unnecessary length of time.) Since implementing about 8,000 kilometres ago this strategy appears to have significantly slowed the rate at which the SOH (capacity) of the battery is declining according to LeafSpy.