Exploring a sodium-enriched cathode (i.e. Na4V2(PO4)3, which differs from its traditional stoichiometric counterpart Na3V2(PO4)3 that can provide extra endogenous sodium reserves to mitigate the irreversible capacity loss of the anode material (i.e. hard carbon), is an intriguing presodiation method for the development of high energy sodium-ion batteries. To meet this challenge, herein, we first propose a redox-potential-matched chemical sodiation approach, utilizing phenazine-sodium (PNZ-Na) as the optimal reagent to sodiate the Na3V2(PO4)3 precursor into Na-enriched Na4V2(PO4)3. The spontaneous sodiation reaction enables a fast reduction of one-half V ions from V3+ to V2+, followed by the insertion of one Na+ ion into the NASICON framework, which only takes 90 s to obtain the phase-pure Na4V2(PO4)3 product. When paired with a hard carbon anode, the resulting Na4VP‖HC full cell exhibits a high energy density of 251 W h kg−1, which is 58% higher than that of 159 W h kg−1 for the Na3VP‖HC control cell. Our chemical sodiation methodology provides an innovative approach for designing sodium-rich cathode materials and could serve as an impetus to the development of advanced sodium-ion batteries.
RuO2, the benchmark catalyst for the oxygen evolution reaction (OER), has traditionally been considered Pauli paramagnetic; however, recent findings have demonstrated its antiferromagnetic (AFM) properties, hinting at the opportunity to enhance RuO2’s OER performance by manipulating its magnetic traits. In this study, we successfully induced weak ferromagnetism in commercial RuO2, transitioning it from an AFM state using an electrochemical sodiation method. This process resulted in high activity, achieving an overpotential of 145 mV to reach 10 mA cm–2 and extending the service hours by more than 13 times compared to pristine RuO2 in 0.5 M H2SO4. A combination of experimental and theoretical analyses indicated that the sodiation triggers significant surface compressive stress, leading to lattice distortion and disruption of the pristine structural symmetry of RuO2. Consequently, orbital degeneration ensues, prompting individual spin-up d electrons to jump to the high-spin state. This mechanism drives the conversion from AFM to weak FM behavior for RuO2, ultimately yielding exceptional catalytic activity and stability.
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