This article covers various factors in battery life, including how temps affect storage degradation.
https://batteryuniversity.com/article/bu-808-how-to-prolong-lithium-based-batteries
One thing to note is that both temps AND charge level play a role in the calculations. So if you have no choice but to store the phone in the car, try to do so at a low charge level. Not too low, though, as extreme low charge levels also cause degradation. I would aim for 30-35% storage in heat.
I would also place it low and in the shade, such as under a seat, to keep it in a relatively cooler part of the car with open airflow. Maybe even invest in rain blocking visors so you can keep windows cracked to let some of the hottest air out without risking rain getting in. An exterior car sun shade could be a great option to prevent heat from getting in in the first place (interior shades don't help much as the heat has already penetrated into the interior). If you are contemplating window tinting, look at ceramic, which also blocks a lot of UV rays. I haven't tried it, but I hear a lot of people talking about how much cooler it made their vehicles in the summer.
May be a long shot, but talk to your company and see if they can set up some sort of lock box/storage area for personal electronics? It's not a situation I've had to deal with, but phones are so ubiquitous that I think most places have some sort of protective storage just for this and theft reasons.