Sodium-Ion Intercalation Mechanism in MXene Nanosheets

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state ²³Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti₃C₂T_x in a nonaqueous Na⁺ electrolyte exhibits reversible Na⁺ intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti₃C₂T_x undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na⁺ is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na⁺ and the swelling effect of penetrated solvent molecules between the Ti₃C₂T_x sheets. Since Na⁺ intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti₃C₂T_x shows good capacity retention over 100 cycles as well as excellent rate capability.