I'd be inclined to skip this sort of thing on handheld-scale batteries.

It only makes sense when a large battery serves as a power store for something like a vehicle, where a fire extinguisher stands between a cabin full of concussions and a cabin full of charred corpses.

Cell phones are sold to us as disposable trash, that corporations and governments use to eavesdrop on our lives. We're trained to sneer at a cell phone more than a year old.

From Wikipedia:

  However, an increasing number of studies 
  have linked exposure to TPP with reproductive 
  and developmental toxicity, neurotoxicity, 
  metabolic disruption, endocrine effects, and 
  genotoxicity. TPP has also been found to 
  induce significant estrogenic activity

https://en.wikipedia.org/wiki/Triphenyl_phosphate

There aren't a lot of teeth to regulations for responsible battery disposal. Even in countries with healthy recycling programs. If I threw all my old cell phone batteries into landfill today, I could safely gamble with any punishment risked.

Now add unspent fire suppressant to the list of things that go in the garbage with last year's cell phone?