NOVELTY - A carbon dioxide composite getter system (20) has a carbon dioxide-permeable envelope (14), defining an internal volume V, that contains powders of a first active material (12, 12', 12'') and a second active material (11, 11', 11''), whose cumulative volume is Vm, where the first active material is a hydroxide chosen from lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and strontium hydroxide and the second active material is an oxide chosen from lithium oxide, calcium oxide, magnesium oxide and/or strontium oxide.    USE - Carbon dioxide composite getter system for use in sealed system i.e. energy storage device such as a rechargeable lithium-ion battery or a supercapacitor (all claimed).    ADVANTAGE - The system has improved carbon dioxide sorption characteristics, better capacity, and is easy to manufacture.    DETAILED DESCRIPTION - A carbon dioxide composite getter system has a carbon dioxide-permeable envelope, defining an internal volume V, that contains powders of a first active material and a second active material whose cumulative volume is Vm, where the first active material is a hydroxide chosen from lithium hydroxide, sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide and strontium hydroxide and the second active material is an oxide chosen from lithium oxide, calcium oxide, magnesium oxide and/or strontium oxide, the carbon dioxide-permeable envelope has a carbon dioxide permeation coefficient of 50 cm3asteriskmm/(m2asteriskdayasteriskatm) and the ratio of the water vapor transmission rate(WVTR) at relative humidity(RH) of 90% and 38 degrees C to the WVTR at RH of 85% and 23 degrees C is less than 10, preferably less than 5. An INDEPENDENT CLAIM is also included for a sealed system.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of a carbon dioxide composite getter system.    Second active material (11, 11', 11'')    First active material (12, 12', 12'')    Carbon dioxide-permeable envelope (14)    Carbon dioxide composite getter system (20)