Boosting the reactivity of Ni2+/Ni3+ redox couple via fluorine doping of high performance Na0.6Mn0.95Ni0.05O2-xFx cathode
Introduction
Sodium-ion batteries (SIBs) have attracted much attention for large-scale energy storage because of its low cost and similar properties with Li [[1], [2], [3], [4]]. P2-type layered oxides (NaxMO2, M = one or several transition metal(s)) are considered as attractive candidates cathode material for rechargeable SIBs due to the high capacity, relatively stable structure and easy scale-up preparation [[5], [6], [7], [8]]. And the simultaneous presence of Mn and Ni in this family (Na-Mn-Ni-O system) are of great interest because of their large reversible capacity and high working potential [[9], [10], [11]], among which active nickel element has a high redox potential of Ni2+/Ni3+ (3.6 V vs. Na/Na+). It is indeed true that some promising results, in terms of superior Na-ion conductivity, excellent cycling stability and high specific capacities have been achieved [12,13]. However, it still deserves great effort to fully explore the electrochemical performance potential (e.g. working potential, reversible capacity, and cycling stability.) for practical application by improving the redox activity of nickel.
To suppress the capacity fading of P2-type Na−Ni−Mn−O system, cationic doping has been widely pursued. Substitution of manganese with Mg2+, Al3+, Fe3+, Zn2+, Co3+, Cu2+, and Ti4+ have been extensively investigated [[14], [15], [16], [17], [18], [19], [20]]. And the cationic doping could stabilize the structure by suppressing phase transition. Wang et al. [21] reported that Cu-substituted Na0.67Ni0.3-xCuxMn0.7O2 material can effectively improve the structural stability by suppressing the irreversible P2–O2 phase transition and improving Na+/vacancy ordering transition. Although much progress has been made, the cationic doping mainly aims to stabilize structure and suppress phase transition [22]. And the activation of Ni2+ is still rarely noted and deserves more effort. Typically, the Ni-Mn-based oxides are composed of repeating sheets of TMO6 layers with Na ions being sandwiched in between the oxide layers [23,24]. Anion is an important part of the crystal structure. Fluorine ion, located at the adjacent site to oxygen in the element period table and contained only one negative charge, is doped in the oxygen site. The accompanied oxygen-vacancy, structure modulation and robust NiF bond may promote the Ni2+ activation and enhance the respective electrochemical performance.
Here, a series of Na0.6Mn0.95Ni0.05O2-xFx (x = 0, 0.02, 0.05, 0.08) compounds were prepared by co-precipitation route and a subsequent solid state reaction. The effects and mechanism of F− doping on the structure and electrochemical performance have been comprehensively studied. The charge/discharge profiles indicate that F-doping could realize sufficient utilization of Ni2+/Ni3+ redox couple, thereby increasing the specific capacity compared with the pristine. F−doping could improve the rate performance by increasing the Na+ diffusion coefficient is also revealed. Meanwhile, owing to the robust NaF bond, it could prevent the collapse of P2 phase structure caused by excess deintercalation of Na+ and improve its cycle stability. To our best knowledge, it is the first report on the anion doping effect to boost the redox activity of transition metal ions. We believe that the current study and scientific findings will encourage more researchers to focus on boosting the redox activity of transition metal ion more than just the synergistic action between transition metal ions during the development of high-performance SIBs cathodes.
Section snippets
Materials preparation
The precursors with a Mn:Ni molar ratio of 95:5 were synthesized by a simple co-precipitation method. Stoichiometric amount of C4H6MnO4·4H2O (KESHI, 99%) and C4H6O4Ni·4H2O (KESHI, 99%) were dissolved in distilled water. An aqueous solution containing the excess of 2 times amount of oxalic acid was added into the previous acetate solution. The co-precipitation solution was continuously stirred at 40 °C for 3 h, before evaporation at 90 °C. The resultant powder was dried in oven at 110 °C for
Results and discussion
ICP-AES and ion chromatography (IC) measurement were tested to confirm the composition of the as-prepared samples. The obtained results (Table S1) are in accordance with designed values. And the XRD patterns of Na0.6Mn0.95Ni0.05O2-xFx (x = 0.00, 0.02, 0.05, 0.08) (Fig. 1a) show strong diffraction peaks assigned to P2-type layered structure with a space group P63/mmc (JCPDS#:00-027-0751). Compared with the un-doped sample, the (002) peak gradually shifted to a lower angle, which suggest enlarge
Conclusions
In this study, F-doped P2-type Na0.6Mn0.95Ni0.05O2-xFx samples (x = 0, 0.02, 0.05, 0.08) have been comprehensively investigated. Owing to structural parameter modulation and robust NiF bond, F-doping could realize sufficient utilization of Ni2+/Ni3+ redox couple and result in higher reversible capacity. GITT and EIS results reveal that doping with fluorine could increase the Na+ diffusion coefficient. MNF5 with appropriate F content showed the best electrochemical performance, including high
Acknowledge
This work was supported by the National Natural Science Foundation of China (No. 21506133) and the Youth Foundation of Sichuan University (No. 2017SCU04a08), the National Key Research and Development of China (grant no. 2016YFD0200404), research Foundation for the Postdoctoral Program of Sichuan University (No. 2017SCU12018, 2018SCU12045).
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2023, Journal of Colloid and Interface ScienceCitation Excerpt :The oxidation peak emerges at about 0.9 V at positive scans, while the corresponding reduction peak appears at about 0.7 V. This pair of redox peaks refers to the conversion of Ni2+/Ni3+ [8,39], and the higher current density of a-NiOx/CDs demonstrates more electrons involved in the electrochemical reaction. In the electrolyte with NaBH4 and p-NP (Fig. 5d), new oxidation peaks are observed at about 0 V and −0.4 V, and they could be related to the reduction of p-NP.