[Half Decaf]

Hmm, I would use something sticky on one side and with some substance to it. Thickness depends on how cramped the gap is between battery and cable.

A felt pad of moderate thickness might work, these pads are used to line some drawers - you get that at Home Depot.

To fit a bigger gap, for a bit more thickness, you can look to the pads that are used to apply to corns, bunions, etc. These stick really good and have saved my arse in the past for sundry home fixit applications. You get these pads at the drugstore.

I wouldn't worry about a fire hazard with your phone for this application - what you need to worry about is conductance, and these materials are not very conductive.