An enclosure for an energy storage device is presently disclosed. The enclosure includes a cell housing having a base portion and at least one side portion seamlessly extending from the base portion to define a volume and having a peripheral edge defining an aperture distal from the base portion through which an electrochemical cell may be disposed within the volume, and a cover securable to the peripheral edge of the housing, where the housing and cover are configured to house at least one electrochemical cell at an operating temperature greater than about 100 degrees Celsius. The enclosure may also include an environmental housing configured to nestingly receive the cell housing, and an insulating element disposed between the environmental housing and the cell housing. Also disclosed is a method of packaging the energy storage device utilizing the enclosure.