Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide

Toward Titanium Carbide Batteries Many batteries and capacitors make use of lithium intercalation as a means of storing and transporting charge. Lithium is commonly used because it offers the best energy density, but also because there are difficulties in storing larger cations without disrupting the crystal structure of the host. Lukatskaya et al. (p. 1500) developed a series of MX compounds, where M represents a transition metal and X is carbon or nitrogen.The compound Ti3C2 forms a two dimensional layered structure, which is capable of accommodating a wide range of cations, including multivalent ones, either spontaneously or electrochemically The layered material Ti3C2 can intercalate much larger cations than Li+, allowing for energy storage applications. The intercalation of ions into layered compounds has long been exploited in energy storage devices such as batteries and electrochemical capacitors. However, few host materials are known for ions much larger than lithium. We demonstrate the spontaneous intercalation of cations from aqueous salt solutions between two-dimensional (2D) Ti3C2 MXene layers. MXenes combine 2D conductive carbide layers with a hydrophilic, primarily hydroxyl-terminated surface. A variety of cations, including Na+, K+, NH4+, Mg2+, and Al3+, can also be intercalated electrochemically, offering capacitance in excess of 300 farads per cubic centimeter (much higher than that of porous carbons). This study provides a basis for exploring a large family of 2D carbides and carbonitrides in electrochemical energy storage applications using single- and multivalent ions.