Unveiling the sodium adsorption behavior of controlled NaTi₂(PO₄)₃ on Ti₃C₂T_x MXene for balanced salt adsorption capacity and cycling stability

Capacitive deionization (CDI) is considered a promising desalination technology with advantages of low cost, high-energy efficiency, and easy regeneration. However, the insufficient ion capacity of conventional electrode materials is limiting the large-scale application of CDI. Herein, a new design of NaTi₂(PO₄)₃ (NTP) on Ti₃C₂T_x MXene (NTP@MXene) using an in-situ growth method assisted by polyvinylpyrrolidone as a structural stabilizer is proposed to tune the reaction kinetics and structural stability. The in-situ grown nanocubes-on-MXene structure and Ti-O bonding rivets between NTP and MXene greatly improve the structural stability and CDI performance. Electrochemical tests and kinetic analysis reveal the effect on reaction kinetics by tuning particle size and composition ratio: the increased NTP ratio enhances the reaction kinetics yet decreases the cycling stability. With an optimal ratio of 50% NTP to MXene, the NTP@MXene electrode demonstrates an outstanding salt removal capacity of 171.43 mg g⁻¹, a fast ion adsorption rate of 4.37 mg g⁻¹ min⁻¹, and excellent cycling performance (85% capacity retention after 100 cycles). This work provides insight into the properties-performance correlation of CDI electrodes to balance the Na⁺ adsorption kinetics and stability for practical CDI applications.