Influence of Synthesis Parameters on the Short-Range Structure and Electrochemical Performances of Li2MnO2F.

In the last years, disordered rocksalt structure (DRS) materials were proposed as a positive electrode for lithium-ion batteries. In particular, the fluorinated DRS materials were proposed to be more stable upon cycling than pure oxide counterparts. These materials are mainly obtained by mechanosynthesis in order to incorporate a significant number of F ions and maintain a disordered structure. Since the local structural arrangement is crucial for battery application, we aim to monitor its evolution upon the synthesis of Li2MnO2F from two sets of precursors: Mn2O3, Li2O, and LiF or LiMnO2 and LiF. The synthesis progress was thus followed, by 7Li and 19F MAS NMR coupled to XRD to probe the structure at different scales. This allowed us to identify an optimal milling time to reach the final compounds. We show that they exhibit similar morphology (by SEM), medium- and short-range orders (by XRD, 7Li and 19F NMR, EXAFS), and average Mn oxidation degree (by XANES). The electrochemical performances of the two compounds are almost similar, with high specific capacities of 319 mAh·g-1 ("from LiMnO2") and 304 mAh·g-1 ("from Mn2O3") for the first charge to 4.8 V vs Li+/Li, proving their interest as post-NMC candidates as positive electrode materials.

P2-Na(x)Mn(1/2)Fe(1/2)O2 phase used as positive electrode in Na batteries: structural changes induced by the electrochemical (de)intercalation process.

The electrochemical properties of the P2-type NaxMn1/2Fe1/2O2 (x = 0.62) phase used as a positive electrode in Na batteries were tested in various voltage ranges at C/20. We show that, even if the highest capacity is obtained for the first cycles between 1.5 and 4.3 V, the best capacity after 50 cycles is obtained while cycling between 1.5 and 4.0 V (120 mAh g(-1)). The structural changes occurring in the material during the (de)intercalation were studied by operando in situ X-ray powder diffraction (XRPD) and ex situ synchrotron XRPD. We show that a phase with an orthorhombic P'2-type structure is formed for x ≈ 1, due to the cooperative Jahn-Teller effect of the Mn(3+) ions. P2 structure type stacking is observed for 0.35 < x < 0.82, while above 4.0 V, a new phase appears. A full indexation of the XRPD pattern of this latter phase was not possible because of the broadening of the diffraction peaks. However, a much shorter interslab distance was found that may imply a gliding of the MO2 slab occurring at high voltage. Raman spectroscopy was used as a local probe and showed that in this new phase the MO2 layers are maintained, but the phase exhibits a strong degree of disorder.

Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7.

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O22- dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O-• was identified in Na2-xMn3O7. Computations reveal that O-• is thermodynamically favorable over the peroxide-like O22- dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

Coulombic self-ordering upon charging a large-capacity layered cathode material for rechargeable batteries.

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as cation migration, layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Here we demonstrate a self-repairing phenomenon of stacking faults upon desodiation from an oxygen-redox layered oxide Na2RuO3, realizing much better reversibility of the electrode reaction. The phase transformations upon charging A2MO3 (A: alkali metal) can be dominated by three-dimensional Coulombic attractive interactions driven by the existence of ordered alkali-metal vacancies, leading to counterintuitive self-repairing of stacking faults and progressive ordering upon charging. The cooperatively ordered vacancy in lithium-/sodium-rich layered transition-metal oxides is shown to play an essential role, not only in generating the electro-active nonbonding 2p orbital of neighbouring oxygen but also in stabilizing the phase transformation for highly reversible oxygen-redox reactions.

Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode.

Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na2MO3 (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na2MO3 are poorly established. Here using two polymorphs of Na2RuO3, we demonstrate the critical role of honeycomb-type cation ordering in Na2MO3. Ordered Na2RuO3 with honeycomb-ordered [Na(1/3)Ru(2/3)]O2 slabs delivers a capacity of 180 mAh g(-1) (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g(-1) (1.0-electron reaction). We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2MO3 cathodes.