Lithium ion is not one thing, one type of battery, but describes a very wide range of batteries. If you are willing to pay more and use more advanced materials for the difference parts of the battery, you can make it way secure.

We don't do that yet for cars as for the amount of batteries you need, it would make the car to expensive. However for medical devices, such batteries are already in us and with all the research into batteries today, this will improve even more.

I'm more concerned about my head being drilled open, metal put into my head and having a BLE connection. The battery is the least of my worries.