Discovery of abnormal lithium-storage sites in molybdenum dioxide electrodes

Abstract

Developing electrode materials with high-energy densities is important for the development of lithium-ion batteries. Here, we demonstrate a mesoporous molybdenum dioxide material with abnormal lithium-storage sites, which exhibits a discharge capacity of 1,814 mAh g(-1) for the first cycle, more than twice its theoretical value, and maintains its initial capacity after 50 cycles. Contrary to previous reports, we find that a mechanism for the high and reversible lithium-storage capacity of the mesoporous molybdenum dioxide electrode is not based on a conversion reaction. Insight into the electrochemical results, obtained by in situ X-ray absorption, scanning transmission electron microscopy analysis combined with electron energy loss spectroscopy and computational modelling indicates that the nanoscale pore engineering of this transition metal oxide enables an unexpected electrochemical mass storage reaction mechanism, and may provide a strategy for the design of cation storage materials for battery systems.

Figure 1. Ordered mesoporous MoO₂ and Li-storage performance.

(a,b) SEM, (c,e) STEM, (d) TEM and (f) HRTEM images of ordered mesoporous MoO₂ materials. Electrochemical performance of ordered mesoporous MoO₂ (S_BET=115 m² g⁻¹) and bulk MoO₂ (Aldrich, S_BET=0.23 m² g⁻¹): (g) voltage profiles and (h) cycle performances at current rate of 0.1 C in 1.3 M LiPF₆ (ethylene carbonate/diethyl carbonate (EC/DEC)=3/7, by volume ratio). The value (837.93 mAh g⁻¹) represents the theoretical capacity for MoO₂, based on conversion reaction (4 mol of Li per 1 mol of MoO₂). Relationship of the Li-storage capacities of the MoO₂ electrodes with respective to their surface areas: (i) the first and (j) the tenth lithiation/delithiation. The galvanostatic lithiation/delithiation tests were replicated for three times. Error bars (i,j) represent the standard deviation of the mean about a series of the measured values. Straight line is a linear fit. Scale bars= 100 nm (a,b), 10 nm (c,d), 5 nm (e) and 1.5 nm (f).

Figure 2. Structure evolution of Li storage in ordered mesoporous MoO₂.

(a) Ex situ XRD patterns of ordered mesoporous MoO₂ during the first cycle as a function of depth of discharge and charge. (b) HRTEM images of mesoporous MoO₂ before lithiation, after full lithiation and after full delithiation. Scale bar= 1.5 nm. (c) In situ XANES patterns and (d) in situ EXAFS patterns of ordered mesoporous MoO₂ electrode for the first lithiation.

Figure 3. DFT calculations and STEM-EELS spectra.

(a) Snapshots of the ordered mesoporous MoO₂ electrode with the increase of Li inserted, calculated by DFT, and (b) top-view HRTEM image taken from a fully lithiated ordered mesoporous MoO₂, demonstrating the formation of three different phases: crystalline phase (D_CP), amorphous phase (D_AP) and SEI (D_SEI). Scale bar= 5 nm. (c) STEM-EELS spectra taken from areas D_CP, D_AP and D_SEI of the fully lithiated ordered mesoporous MoO₂. Dotted lines represent the peak position of Mo N edge (blue) and the onset of Li K-edge (black).

Figure 4. Pore structure of mesoporous MoO₂ by in operando SAXS and ex situ TEM.

(a) Color-coded 3D contour and projection map showing SAXS data collected from ordered mesoporous MoO₂ during in operando experiment. (b) The changes in lattice parameter and resolved peak relative intensity calculated from the (110) reflection with the corresponding dQ/dV plot. Representative TEM images and framework thickness of ordered mesoporous MoO₂ electrodes: (c) pristine, (d) lithiated and (e) delithiated. Scale bars= 20 nm.

Figure 5. Schematic diagram of the reaction pathways and mesoscale morphology.

Schematic diagram of the reaction pathways and the resulting products of ordered mesoporous MoO₂ with respect to the amount of Li ion inserted.