Ti-Doped Tunnel-Type Na₄Mn₉O₁₈ Nanoparticles as Novel Anode Materials for High-Performance Supercapacitors

Nanomaterials with tunnel structures are extremely attractive to be used for electrode materials in electrochemical energy storage devices. Tunnel-structured Ti-doped Na₄Mn₉O₁₈ nanoparticles (TNMO-NPs) were synthesized by a facile and high-production method of the solid-state reaction with a high-energy ball-milling process. As electrode materials in the supercapacitor cell, the as-synthesized TNMO-NPs exhibit a high specific capacity of 284.93 mA h g^-1 (0.57 mA h cm^-2/1025.75 F g^-1). A superior rate capability with a decay of 36% is achieved by increasing the scan rates from 2 to 25 mV s^-1. To further explore the storage mechanism of Ti-doped Na₄Mn₉O₁₈ materials, density functional theory (DFT) calculations were used to calculate the activation energy for the ion immigration in the electrode, and the results show that the minimum ion diffusion barrier energy is 0.272 eV, indicating that the sodium ions could insert into the system easily. Through the scan-rate-dependent cyclic voltammetry analysis, the capacity value indicates a mixed charge storage of capacitive behavior and Na⁺ intercalation progress. A maximum energy density of 77.81 W h kg^-1 at a power density of 125 W kg^-1 is achieved, and a high energy density of 54.79 W h kg^-1 is maintained even at an ultrahigh power density of 3750 W kg^-1. The TNMO-NP supercapacitors show excellent flexibility at various bent (0-180°) states. The capacitive performance of the TNMO-NPs makes them promising cathode materials for flexible supercapacitors with high specific capacities and high energy densities.