As a result of their high energy density lithium-ion batteries have been widely applied in the fields of portable electronic, implantable organs, power tools, and more recently electric vehicles. Typically carbon is widely used as a conductive additive in electrode fabrication due to the low conductivity of the active materials, which has a negative effect on the volumetric and gravimetric energy density. As a result, exploration of new materials with high conductivity as electrodes for Li-ion batteries is highly desirable.

Titanium-based materials as lithium-ion battery negative electrode have been widely studied. The zero strain insertion leads to excellent reversibility (>10,000 times) and capacity retention rate; ^[1] the combination of high lithium mobility results in high rate ability of the batteries; ^[2] higher working potential (>1.2 V vs Li⁺/Li) brings better safety with no lithium metal deposition during discharging compared to the carbon anodes. ^[3] But the relatively low electric conductivity (e.g. Li₄Ti₅O₁₂ is only 10^-9 S·cm^-1 at 20 ℃) limits their ability and low temperature applications.

LiTi₂O₄ adopting the ramsdellite structure was firstly fabricated by D.C. Johnston. ^[4] This structure consists of distorted TiO₆ octahedra which connect together by sharing edges resulting in double linked columns. These columns form an open framework structure in which Li⁺ occupies 50% of the tetrahedral channel sites. Hence the remaining tetrahedral sites are available for Li⁺ insertion delivering a theoretical specific capacity of 161 mAh g^-1 for complete reduction to Ti³⁺ and all tetrahedral sites occupied and presumably lithium could be extracted. For titanium ramsdellite series capabilities of 180 mAh·g^-1 (theoretical 298 mAh·g^-1) for Li₂Ti₃O₇, 113 mAh·g^-1 for LiTi₂O₄ and 320 mAh·g^-1 (theoretical 335 mAh·g^-1) for TiO₂ under 0.25 mA·cm^-2 has been obtained by Gover. ^[5]

Titanium oxycarbide, TiO_xC_1-x, with a rocksalt structure formed by TiC and TiO, with O and C atoms sharing the anion sites, has a high metallic conductivity. ^[6] Thus partial replacement of O by C in transition metal oxides opens up a range of new compositions that might be expected to yield important functional properties.

In this work a novel carbon doping strategy was used to successfully fabricate LiTi₂O_4-xC_x with the Ramsdellite structure. Initially TiO_xC_1-x and Li₂Ti₃O₇ were made as raw materials by solid reaction ans then the LiTi₂O_4-xC_x was fabricated in Ar under specific temperature. After that its structure was characterised and electrochemical property was tested. Based on refiment of X-ray diffraction patterns, increased levels of carbon substitution led to an increase in a and b parameters with a contraction in c. The incorporation of carbon was further confirmed by mass spectroscopy/thermal analysis. The performance as a negative electrode material for lithium-ion batteries was enhanced by carbon substitution with increased rate capability and improved cycle ability. Especially for LiTi₂O_3.925C_0.075 capacities of 151 mAh·g^-1 and 67 mAh·g^-1 could be obtained under discharging current densities of 100 mA·g^-1 and 2000 mA·g^-1 and capacity decreased by 5.57% after 100 cycles. This results from increased conductivity and reduced cell polarization.

Keywords: titanium carbide, lithium ion battery, carbon doping

Reference

[1] T. Ohzuku, A. Ueda, N. Yamamoto, Journal of the Electrochemical Society, 142, 1431(1995).

[2] K. Ariyoshi, R. Yamato, T. Ohzuku, Electrochimica acta, 51, 1125(2005).

[3] D. Peramunage, K. Abraham, Journal of the Electrochemical Society, 145, 2615(1998).

[4] D. Johnston, Journal of Low Temperature Physics, 25, 145(1976).

[5] R. K. Gover, J. R. Tolchard, H. Tukamoto, T. Murai, J. T. Irvine, Journal of the Electrochemical Society, 146, 4348(1999).

[6] D. N. Miller, A. K. Azad, H. Delpouve, L. Quazuguel, J. Zhou, A. Sinha, P. Wormald, J. T. Irvine, Journal of Materials Chemistry A, 4, 5730(2016).