Lithium Titanate Confined in Carbon Nanopores for Asymmetric Supercapacitors

As one of the superior anode materials for high power Li-ion batteries and asymmetric supercapacitors, spinel Li4Ti5O12 (LTO), has attracted significant attention in recent years owing to its unique characteristics.[1] Li4Ti5O12 exhibits a flat lithiation potential plateau at the voltage of ~1.55 V vs. Li/Li+, which largely prevents both excessive solid electrolyte interphase (SEI) formation and lithium dendrite growth. In addition, LTO is known for its “near-zero” volume change during repeatable lithiation and delithiation, which contributes to its excellent cyclic stability. One major drawback of LTO to be qualified for high-rate performance is its poor electrical conductivity. One approach to improving electrical conductivity is doping of metal or nonmetal ions in Li, Ti or O sites, though the cyclic stability may be impaired and the resulting rate performance may still be insufficient for practical applications. Another drawback is that the Li-ion transport in LTO is slow compared with that of supercapacitor electrode materials.[2] Therefore, recent endeavors have been devoted to the design of LTO nanostructures and those with porous LTO morphologies, which demonstrated improved kinetics.[3] The synthesis of LTO nanoparticles has been extensively studied, however, it is still rather difficult to prepare uniform Li4Ti5O12 nanoparticles of small dimensions (e.g., < 5-10 nm)  due to the high annealing temperature required during synthesis (700-800 oC), at which LTO crystals aggregate and over-grow due to Ostwald ripening. In this research, we report on a novel facile strategy for the low-cost synthesis of uniform Li4Ti5O12- activated carbon nanocomposites (LTO-AC), where crystalline sub – 4 nm LTO nanoparticles are uniformly distributed in the nanopores of the carbon matrix.[4] In contrast to the nanosized particles of various shapes and sizes, which are difficult (and expensive) to handle and utilize in electrodes, micro-scale LTO-AC has the potential to serve as drop-in replacement for AC in supercapacitor electrode production. AC not only serves as spatial confinement to control the growth of LTO nanocrystals, but also as a conductive material to compensate the poor electrical conductivity of LTO. As a result, Li4Ti5O12 nanoparticles in AC pores demonstrated remarkable performance characteristics, showing more than 100 mA h g-1 at the ultra-high rate of 350C (1C= 175 mA g-1), where charge or discharge takes place in ~6 s. When compared with pure AC, the LTO-AC nanocomposites showed up to 12 times higher gravimetric capacity and 12 times higher volumetric capacity. [1] K. Zaghib, A. Mauger, H. Groult, J. Goodenough, C. Julien, Materials 2013, 6, 1028. [2] N. S. Choi, Z. H. Chen, S. A. Freunberger, X. L. Ji, Y. K. Sun, K. Amine, G. Yushin, L. F. Nazar, J. Cho, P. G. Bruce, Angewandte Chemie-International Edition 2012, 51, 9994. [3] Y. Ren, A. R. Armstrong, F. Jiao, P. G. Bruce, Journal of the American Chemical Society 2010, 132, 996. [4] E. Zhao, C. Qin, H.-R. Jung, G. Berdichevsky, A. Nese, S. Marder, G. Yushin, ACS Nano 2016. Figure 1