Nitrogen- and TiN-modified Li ₄Ti ₅O ₁₂: One-step synthesis and electrochemical performance optimization

It is believed that a TiN coating can increase the electrical conductivity, and consequently the performance, of an electrode. In this work, a simple one-step synthesis of nitrogen- and TiN-modified Li ₄Ti ₅O ₁₂, i.e. solid-state reaction of Li ₂CO ₃ and TiO ₂ anatase in an ammonia-containing atmosphere, is introduced. The reducing ammonia atmosphere could cause the partial reduction of Ti ⁴⁺ to Ti ³⁺ and the doping of nitrogen into the Li ₄Ti ₅O ₁₂ lattice, in addition to the formation of the TiN phase. By controlling the ammonia concentration of the atmosphere and using a slight Ti excess in the reactants, Li ₄Ti ₅O ₁₂, nitrogen-doped Li ₄Ti ₅O ₁₂, or TiN-coated nitrogen-doped Li ₄Ti ₅O ₁₂ were obtained. Both the electrical conductivity and the TiN thickness were closely related to the ammonia concentration in the atmosphere. Synthesis under reducing atmosphere also resulted in powders with a different plate shape particulate morphology from that synthesized in air, and such plate-shape powders had an ultrahigh tap density of ∼1.9 g cm ^-3. Interestingly, the formation of TiN was not beneficial for capacity improvement due to its insulation towards lithium ions, unlike the nitrogen doping. The sample prepared under 3% NH ₃-N ₂, which was free of TiN coating, showed the best electrode performance with a capacity of 103 mA h g ^-1 even at 20 C with only 3% capacity decay after cycling 100 times.