Size-dependent electrochemical properties of vertically aligned MXene electrodes for fast Li-ion storage

MXenes are attractive electrode materials for batteries and pseudocapacitors because they have higher electronic conductivity than carbons or oxides and can exhibit surface-confined redox reactions. However, restacking of 2D nanosheets may limit the ionic transport in MXene electrodes and lead to sluggish charge/discharge. To understand and control the 2D mass transport properties of Ti₃C₂T_x MXene, we fabricated electrodes with different diffusion lengths and nanosheet alignments (vertical and horizontal). The electrodes were made by combining electrophoretic deposition and freeze-drying of delaminated size-regulated MXenes (95 to ∼600 nm lateral size). Their electrochemical lithiation/de-lithiation behavior was studied in a non-aqueous electrolyte. Vertically-aligned Ti₃C₂T_x MXene film with a thickness of 300 μm provided faster Li⁺ storage than horizontally-stacked MXene film with a thickness of 10 μm. Electrodes made of small-size flakes exhibited higher discharge capacity and capacity retention at high rates due to a shorter Li⁺ diffusion path. The vertically aligned MXene electrodes show high electrochemical performance and fast charge/discharge.