It's not true. According to Technical Service Manager John Hadley at Rayovac in Madison, Wisconsin, unless you're living in a jungle environment with extreme heat and humidity, keeping your batteries in the freezer may do more harm than good.
Batteries convert chemical energy into electrical energy.
Inside a battery, a chemical reaction takes place that generates electrons. Every battery has a positive and negative end, which is why, if you put a battery in your flashlight upside down, it won't work. The electrons generated by the chemical reaction build up on the negative end and flow into the flashlight, lighting it up. Batteries that are kept in the freezer should be warmed to room temperature before use, which is time-consuming and inconvenient. And even if the outside of the battery doesn't feel cold, Hadley says, the chemicals inside may not yet be warmed up, and the battery won't operate at full power.