The AA lithium ion batteries getting tossed into the trash and setting garbage trucks on fire are problematic enough. The energy capacity of a cell phone is quite a bit more and correspondingly more spectacular in the combustion.
From iFixit:
https://www.ifixit.com/News/69041/how-batteries-can-catch-fi...
https://www.ifixit.com/News/34034/lithium-ion-batteries-are-...
> USA Today reports that 65 percent of fires at waste facilities in California were started by lithium-ion batteries. In a 2018 survey of 21 waste facilities across California, 86 percent reported a fire at their facility in the last two years, according to the California Products Stewardship Council (CPSC). Of those fires, 56 percent were attributed to batteries, with the remainder attributed to “traditional hazards of combustibles.” In other words, batteries are causing more fires than the oils, fuels, and other hazardous materials of waste management—combined.
> And that’s only the fires actually reported.