Safety is very crucial for lithium-ion batteries (LIBs) in the application of electric vehicles (EVs), hybrid EVs (HEVs), and plug-in HEVs (PHEVs). Li₄Ti₅O₁₂ (LTO) is non-flammable, and has a safe working potential of approximately 1.55 V (vs Li⁺/Li), which prevents the formation of the solid electrolyte interphase (SEI) and suppresses lithium dendrite deposition on the surface of the anode.(1, 2) In addition, LTO possesses almost zero volume change upon lithiation and delithiation, which could lead to excellent cyclic performance.(3) However, it is still a challenge to achieve good battery performance at high charge/discharge rates using LTO as the anode material because of the low electrical conductivity (<10^-13 S cm^-1) of LTO.(4) Thus, there have been many efforts to improve the rate capability and the cycling performance of LTO anodes for LIBs at high rates.

Because of the excellent electrical conductivity of graphene, LTO/graphene composites have attracted many attention recently. Here we report on the one-step continuous preparation of spherical LTO/graphene composites through aerosolization of a graphene oxide (GO) suspension mixed with titanium(IV) bis(ammonium lactato) dihydroxide and LiOH precursors. The GO sheet is crumpled into a spherical shape when the GO-containing aerosol droplets pass through a high-temperature tube furnace; meanwhile, the Li and Ti precursors react to form LTO nanocrystals.(5)

The LTO nanocrystals are locally crystallized and grown in situ on the surface of crumpled graphene (CG) spheres during the solvent evaporation and GO crumpling process, leading to a highly stable hierarchical structure with LTO nanocrystals well distributed in the CG sphere. This type of architecture greatly improves the electrical conductivity of the LTO and facilitates the electron/charge transfer between the anode materials and the current collector. More importantly, this method is a facile one-step process with no need for subsequent treatment of the product LTO/CG. Therefore, the method is suitable for continuous production and thus promising for large-scale manufacturing. In addition, the spherical morphology of the LTO/CG favors a higher tap density for practical LIB applications.

The electrochemical performance of the LTO/CG composite was tested using a 2032-type coin cell. When the voltage range was limited between 1.0 and 2.0 V, graphene was inactive and served only as a conducting component. The LTO/CG was activated for two cycles at a current density of 12.5 mA g^-1, showing a capacity of ca. 140 mAh g^-1 and then cycled at 125 mA g^-1, delivering a capacity of ca. 120 mAh g^-1 over 500 cycles. The LTO/CG exhibited excellent rate capability, at a very high current density of 5,000 mA g^-1; the LTO/CG composite delivered a capacity of ca. 83 mAh g^-1, corresponding with 60% of the capacity obtained at 12.5 mA g^-1. The LTO/CG was cycled at 1,250 mA g^-1 to evaluate its cycle performance, showing a capacity of ca. 100 mAh g^-1 and retaining 88 mAh g^-1 after 5,000 cycles.

Considering graphene as an active anode material, the batteries were also tested with an extended lower voltage cutoff at 0.01 V. The LTO/CG composite delivered a capacity of ca. 270 mAh g^-1 at 25 mA g^-1 for the two initial cycles, and maintained 210 mAh g^-1 at 250 mA g^-1 without any decay for 500 cycles. The results indicate that the LTO/CG composite has excellent cyclic performance, even when using graphene as an active component. When operating between the cutoffs of 0.01-2.0 V, the capacities are 270, 227, 199, 187, 179, 169, 152, and 117 mAh g^-1 for the current densities of 25, 50, 125, 250, 500, 1250, 2500, 5000 mA g^-1, showing excellent rate capability.

In summary, we have synthesized the LTO/CG composite through a one-step preparation method with no need for subsequent treatment of the product. The method is capable of continuous production, thus offering significant potential for large-scale manufacturing. The LTO nanocrystals are well distributed in the CG, which significantly improves the electrical conductivity of the LTO. The batteries using the LTO/CG anode show excellent rate capability and outstanding cycling performance.

 References

1.    S. Ganapathy and M. Wagemaker, ACS Nano, 6, 8702 (2012).

2.    X. Lu, L. Zhao, X. He, R. Xiao, L. Gu, Y.-S. Hu, H. Li, Z. Wang, X. Duan, L. Chen, J. Maier and Y. Ikuhara, Adv. Mater., 24, 3233 (2012).

3.    J. Haetge, P. Hartmann, K. Brezesinski, J. Janek and T. Brezesinski, Chem. Mat., 23, 4384 (2011).

4.    L. Shen, H. Li, E. Uchaker, X. Zhang and G. Cao, Nano Lett., 12, 5673 (2012).

5.    S. Mao, X. Huang, J. Chang, S. Cui, G. Zhou and J. Chen, NPG Asia Mater, 7, e224 (2015).