It almost certainly won't explode or catch on fire, but it might start to swell up and it might not last as long as it used to. It might also start behaving strangely, for example jumping several % instead of one at a time when discharging. If I were you I'd order a new one and just replace it. I recently replaced the battery in my OnePlus One and it cost me about $9. I would use the phone as it is while waiting for a replacement. As long as it hasn't been very bent (>30 degrees) or punctured it shouldn't pose a risk.