Electrochemically activated MnO as a cathode material for sodium-ion batteries
Graphical abstract
Introduction
Sodium-ion batteries (SIBs) are regaining great attention nowadays because of the abundance of Na in the Earth's crust [1]. Numerous Na-based compounds synthesized in recent years demonstrated promising results as cathode materials for SIBs [2], [3]. However, the material design progress still falls behind the ever-increasing market demand, which calls for new materials with improved energy density. In the course of this expedition, a few groups beside us showed feasibility to trigger the electrochemical activity of a composite electrode consisting of metal oxide (MO) and lithium fluoride (LiF) in Li-ion batteries (LIBs) [4], [5], [6], [7]. More specifically, within this new finely divided composite powders prepared by mechanical ball-milling, the transition metal serves as electron reservoir and LiF provides Li+ ion to balance the charge. Such a combination of light constituents enables exceedingly high theoretical capacity over 250 mAh g− 1 based on one electron transfer of common 3d-metals (Mn, Fe, etc.).
Although not fully understood, the proposed reaction scheme can be generalized to various transition metal oxides via eco-synthesis approaches, providing new insights into material design perspective. In the present work, we adopt this concept to SIBs by switching the fluorine source to NaF and using MnO as the redox component. We show that a MnO-0.1NaF composite prepared by high-energy ball-milling for 30 min delivers 157 mAh g− 1 on the first discharge, and quickly stabilizes to 122 mAh g− 1 on succeeding cycles in NaPF6-based electrolyte. We further reveal that upon charging, the NaPF6 partially decomposes above 4.2 V releasing NaF, which can act as an internal source of F−, hence leading to in situ prepared electrodes with reversible capacity of 145 mAh g− 1.
Section snippets
Experimental
Commercial MnO (Sigma Aldrich, 99%) and NaF (Alfa Aesar, 99%) were used to prepare cathode composites. MnO-xNaF composites with various molar ratios (0, 0.1, 0.5, 1, and 1.5) were mixed in a stainless-steel vial of 10 mL with ball-to-powder ratio of 15. The vial was sealed in Ar and mounted in a SPEX high-energy ball mill adopting 875 cycles/min 3D movement for 15 min. Then, 20 wt% of conductive Csp was added and the composites were milled for another 15 min.
The structure of as-prepared composites
Results and discussion
Fig. 1a demonstrates a typical XRD pattern of the MnO-xNaF composites prepared by ball-milling for 30 min. The pattern shows broadening of the Bragg peaks pertaining to MnO, indicating reduced particle size and increased lattice strain. Peaks corresponding to NaF, added in small amounts to the composite, are also visible in the profile. This indicates that the ball milling does not trigger chemical reaction between components, which is also confirmed by the SEM (Fig. 1b) and STEM-EDX (Fig. 1c)
Conclusions
In conclusion, we have successfully generalized the redox composite model derived from lithium-ion to sodium-ion batteries. High-energy ball-milling as short as 30 min is sufficient to prepare reproducible and easily scalable MnO-xNaF composites. A reversible capacity of 122 mAh g− 1 is achieved by cycling MnO-0.1NaF in half cells. It can be further improved to 145 mAh g− 1 using NaPF6 as the sole F-source at the expense of a lengthy activation process and partial consumption of the electrolyte salt.
Acknowledgements
This work was partially supported by the Hong Kong Research Grants Council under the General Research Fund Project #611213. L.Z. thanks the HKUST for his Postgraduate Studentship.
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2019, Energy Storage MaterialsCitation Excerpt :They found that preferred orientations are existing with the detected intermediate phase by tracing the full lithiation procedure. Very recently, Tarascon's group found that the MnO-0.1NaF composite can be electrochemical activated as a potential positive material for SIB using NaPF6 as the sole F-source [62]. This electrochemically-driven synthesis would open a new a door for these 3d transition metal-based oxide materials.
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