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_nT_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_nAX_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 LiNi_1/3Co_1/3Mn_1/3O₂ negative/positive electrodes, and demonstrate that the Ti₂CT_xis 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.