Low ambient temperatures (Working and Storage): Question for Simon Hackett, page-6

The battery will be very resistant to freezing. The large volume of water will store a considerable amount of thermal energy so you would have to sustain a temperature below the electrolyte's freezing point for quite some time. Also, if the battery is operating it generates heat so the ambient temperature would have to be well below the electrolyte's freezing point to overcome the heat generated.

In situations where freezing is a risk due to sustained low temperatures and low battery activity, the battery can have a heater fitted that is powered from the battery itself. There's a cost in terms of energy drain of course, so there would have to be sufficient charging input on an ongoing basis to replenish the battery.

If it were to freeze, then yes I expect that the damage would be extreme. Typically when water solutions freeze the dissolved ions separate from the water as the crystals form. If the thing froze solid, chances are it would be a write-off.