When a lithium ion battery is charged, a current is driven into the cell an d the state of the battery can be tracked by measuring the integral of the current over time.
When the battery then is cooled, the terminal voltage is reduced, but also, the available current is reduced over time. This link shows a typical dis charge curve for multiple temperatures.
The variation in voltage can explain a reduced capacity with temperature. But I don't get the reduced mAh at colder temperatures. A battery is a che mical process that releases an electron with each ion that reaches the anod e. Charge and current should be conserved other than internal leakage whic h is typically very small in lithium ion batteries.
So how can the mAh capacity of a cell be reduced by the cold?