In a molecular dynamics computational study by Yang and Garde, a negative charge density of −n e is uniformly distributed to all atoms of a CNT 0.67 nm in diameter and 1.1 nm in length (comprising 100 carbon atoms), so that each atom carries a charge of −n/100 e. When the surface charge is −5 e the NT interior becomes favorable to all three investigated cations, namely potassium, cesium, and sodium. In contrast, when the surface charge is reduced to −2 e no cations enter the tube. At a charge of −3 e, sodium ions take 25 times longer to enter the NT than potassium and cesium ions. Studies such as these may aid better design of high-energy storage devices.