Nitrogen- and TiN-modified Li4Ti5O12: one-step synthesis and electrochemical performance optimization

Abstract

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 Li₄Ti₅O₁₂, i.e. solid-state reaction of Li₂CO₃ and TiO₂ anatase in an ammonia-containing atmosphere, is introduced. The reducing ammonia atmosphere could cause the partial reduction of Ti⁴⁺ to Ti³⁺ and the doping of nitrogen into the Li₄Ti₅O₁₂ 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, Li₄Ti₅O₁₂, nitrogen-doped Li₄Ti₅O₁₂, or TiN-coated nitrogen-doped Li₄Ti₅O₁₂ 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% NH₃–N₂, 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.