Pseudocapacitive Negative Electrodes Based on Mxene Ti2CT x for High-Power Li-Ion Batteries

Abstract

Development of rapid charging systems in Li-ion batteries is one of the urgent tasks for the widespread use of environmental-friendly electric vehicles towards a sustainable and green society. However, in spite of numerous efforts devoted to realizing the high-power systems, their power density is not satisfactory, partly due to the poor rate capability of the negative electrodes. A novel family of layered compounds, MXene (M n +1X n T x : M = Ti, V, Nb, etc.; X = C, N; n = 1-3; T = surface termination groups) has attracted increasing attention as negative electrode material with high-rate capability for Li-ion batteries.[1] MXene is the derivative phase of ternary layered compounds, MAX phase (M n AX n : A = Al, Si, S, etc.), first synthesized with concentrated hydrofluoric acid (HF) solution to extract selectively the A element.[2] Due to the tunable compositions and structures in MXene, there are huge choices for the development of the high-power electrode materials. Gogotsi and colleagues have recently discovered that lithium fluoride (LiF) and hydrochloric acid (HCl) solution also selectively extracts the A element from MAX phase, and Ti3C2T x synthesized with the LiF and HCl solution exhibits higher performance as pseudocapactior in aqueous systems than HF-synthesized Ti3C2T x .[3]Therefore, the synthetic method for MXene is a key process to determine its electrochemical property. In this work, we have successfully synthesized Ti2CT x with the LiF and HCl solution. The charge-discharge measurement reveals that the Ti2CT x delivers large reversible capability of 260 mAh/g in 1M LiPF6/EC-DMC (1:1, v:v) electrolyte, which is larger than that of Ti2CT x prepared with concentrated HF solution (180 mAh/g).[4] Reaction mechanism was investigated with combined approach using XANES, XRD, and 7Li-MAS-NMR. We construct the prototype full cell with the Ti2CT x and LiNi1/3Co1/3Mn1/3O2 negative/positive electrodes, and demonstrate that the Ti2CT x is the promising electrode material for the high-power Li-ion batteries. [1] M. R. Lukatskaya, et al. and Y. Gogotsi, Science 341, 1502 (2013). [2] M. Naguib, et al. and M. W. Barsoum, Adv. Mater. 23, 4248 (2011). [3] M. Ghidiu, et al. and Y. Gogotsi, Nature 516, 78 (2014). [4] J. Come, et al. and P. Simon, J. Electrochem. Soc. 159, A1368 (2012). Figure 1. Charge-discharge profile of Ti2CT x in 1M LiPF6/EC-DMC (1:1, v:v) electrolyte and comparison in the rate capability synthesized with different solutions. Figure 1