Role of Ligand-to-Metal Charge Transfer in O3-Type NaFeO₂–NaNiO₂ Solid Solution for Enhanced Electrochemical Properties

Na-ion batteries have been the subjects of intensive studies for grid-scale energy storage recently. O3-type NaFeO₂ is a promising candidate for the Na-ion cathode materials, though the irreversibility during Na-ion extraction/insertion seriously hinders its practical application. The present work demonstrates that partial replacement of Fe in O3-NaFeO₂ with Ni leads to the significant improvement of the electrochemical properties. The ⁵⁷Fe Mössbauer and X-ray absorption spectra show that O3-type NaFeO₂–NaNiO₂ solid solution forms hybridized frontier orbital of a Fe–O–Ni bond via ligand-to-metal charge transfer, which plays a dominant role in the charge–discharge process. The resulting O3-NaFe_0.3Ni_0.7O₂ delivers an initial discharge capacity of 135 mA·h·g^–1, most of which is in a high-voltage region of 2.5–3.8 V, with a high initial Coulombic efficiency of 93%, and shows enhanced cycle stability

Na₂FeP₂O₇: A Safe Cathode for Rechargeable Sodium-ion Batteries

Vying for newer sodium-ion chemistry for rechargeable batteries, Na₂FeP₂O₇ pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na₂FeP₂O₇ as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP₂O₇. High-temperature analyses of the desodiated state NaFeP₂O₇ show an irreversible phase transition from triclinic (<i>P</i>1̅) to the ground state monoclinic (<i>P</i>2₁/<i>c</i>) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P₂O₇)^4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na₂FeP₂O₇ as a safe cathode candidate for large-scale economic sodium-ion battery applications

Na₂FeP₂O₇: A Safe Cathode for Rechargeable Sodium-ion Batteries

Vying for newer sodium-ion chemistry for rechargeable batteries, Na₂FeP₂O₇ pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na₂FeP₂O₇ as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP₂O₇. High-temperature analyses of the desodiated state NaFeP₂O₇ show an irreversible phase transition from triclinic (<i>P</i>1̅) to the ground state monoclinic (<i>P</i>2₁/<i>c</i>) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P₂O₇)^4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na₂FeP₂O₇ as a safe cathode candidate for large-scale economic sodium-ion battery applications

Na₂FeP₂O₇: A Safe Cathode for Rechargeable Sodium-ion Batteries

Vying for newer sodium-ion chemistry for rechargeable batteries, Na₂FeP₂O₇ pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na₂FeP₂O₇ as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP₂O₇. High-temperature analyses of the desodiated state NaFeP₂O₇ show an irreversible phase transition from triclinic (<i>P</i>1̅) to the ground state monoclinic (<i>P</i>2₁/<i>c</i>) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P₂O₇)^4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na₂FeP₂O₇ as a safe cathode candidate for large-scale economic sodium-ion battery applications