Doping Modification Study Of Li₄Ti₅O₁₂ As An Anode Material For Lithium Ion Batteries

Lithium-ion batteries（LIBs） with high energy density and long cycle life have been widely used in energy storage devices for electrical vehicles. Electric vehicles are mushrooming in 2014, and energetically promoted by Chinese policy and market requirement, the market space of power Li-ion cell in the future is huge. Anode materials are one of the four key materials for lithium-ion batteries, however, graphite, the most commonly commercial anode material, has not yet able to meet the gradually increasing requirements of powering electric vehicles. Enhancement in safety and improvement in performance are the main properties that have to be implemented. Spinel lithium titanate(Li₄Ti₅O₁₂) is one of promising anodes to substitute graphite in the future. On one hand, it possesses a relatively high potential plateau（around 1.55 V versus Li+/Li） to ensure safety of the battery by avoiding Li-metal deposition and the formation of solid electrolyte interface（SEI） layer on electrode surface. On the other hand, Li₄Ti₅O₁₂ is a zero-strain insertion material, providing excellent lithium-ion mobility and could avoid structural damage caused by charging/discharging process, showing excellent resistance to over-charge and over-discharge. Nevertheless, the commercial application of Li₄Ti₅O₁₂ in lithium-ion batteries is hindered by its kinetic problems of poor electronic conductivity（merely 10-13 S cm-1）. Thus, Li₄Ti₅O₁₂ was chosen as research subject to improve the electronic conductivity,this paper synthesized a series doped modified Li₄Ti₅O₁₂ anodes via solid-state method, the research contents are as follows:（1） Tatalum-doped Li₄Ti₅O₁₂ samples were synthesized by one step solid-state reactions. The as-prepared Li₄Ti_4.995Ta_0.005O₁₂ delivered 95.1 mAhg^-1 at 10 C and 132.2 mAhg^-1 at 5C after 100 cycles. The excellent electrochemical performance could be attributed to the improved electronic conductivity and lithium ions diffusivity by the enlarged lattice parameter and introduction of mixed valence of Ti4+/Ti³⁺ by Ta doping.（2） Nitrogen-modified Li₄Ti₅O₁₂ was synthesized by thermal decomposition of Li₄Ti₅O₁₂ and solid melamine. As a result, the TiN coated Li₄Ti₅O₁₂ exhibited higher rate capability and better cycling stability than the bare Li₄Ti₅O₁₂, the main reasons were the improved electronic and ion conductivities provided by the thin TiN surface coating layer（¹.24 nm）. The LTON12 electrode delivered a capacity of 124.2 m Ahg^-1 after 500 cycles at 5C, and exhibited a capacity of 74.3 mAhg^-1 at even 100 C with a fixed discharge rate of 1C.（3） Micron-sized Li₄Ti₅O₁₂ with both surface modification（TiN） and inner Ta⁵⁺ dopant manipulation had been synthesized via a combination of solid state reaction and surface thermal nitridation. The formation of thin amorphous TiN coating layer on Li₄Ti₅O₁₂ surface enhanced surface electronic conductivity and electrical contact between particles, while Ta⁵⁺ bulk doping in the lattice improved the intrinsic ionic conductivity and electronic conductivity inside particles. The nitridation Ta-doped LTOTa N showed a capacity of 132.7 mAhg^-1 at 10 C with a high capacity retention of 93.9% after 500 cycles. Even at 20 C the discharge capacity remains 89.5 m Ahg^-1.