CO2 extinguishers are class B. They also don't work on anything other than a small battery fire because li-ions are generating their own fuel, heat, and sparks (basically almost the entire fire triangle) so they reignite easily as soon as the CO2 dissipates. If its a closed space you have a chance with CO2, if its not a closed space its probably going to reignite.So, what kind of fire extinguisher should you use in this scenario? Lithium-ion batteries are considered a Class B fire, so a standard ABC or BC dry chemical fire extinguisher should be used. Class B is the classification given to flammable liquids. Lithium-ion batteries contain liquid electrolytes that provide a conductive pathway, so the batteries receive a B fire classification.
Throwing it overboard, or dowsing with water to remove the heat is going to be more effective. Even though water is not typically used on class B fires.