Activation of Sodium Storage Sites in Prussian Blue Analogues via Surface Etching

Sodium-ion battery technologies are known to suffer from kinetic problems associated with the solid-state diffusion of Na⁺ in intercalation electrodes, which results in suppressed specific capacity and degraded rate performance. Here, a controllable selective etching approach is developed for the synthesis of Prussian blue analogue (PBA) with enhanced sodium storage activity. On the basis of time-dependent experiments, a defect-induced morphological evolution mechanism from nanocube to nanoflower structure is proposed. Through in situ X-ray diffraction measurement and computational analysis, this unique structure is revealed to provide higher Na⁺ diffusion dynamics and negligible volume change during the sodiation/desodiation processes. As a sodium ion battery cathode, the PBA exhibits a discharge capacity of 90 mA h g^–1, which is in good agreement with the complete low spin Fe^LS(C) redox reaction. It also demonstrates an outstanding rate capability of 71.0 mA h g^–1 at 44.4 C, as well as an unprecedented cycling reversibility over 5000 times

In Operando Probing of Sodium-Incorporation in NASICON Nanomaterial: Asymmetric Reaction and Electrochemical Phase Diagram

NASICON-type materials are one of the most promising cathodes for sodium-ion batteries (SIBs) due to their stable structure and the three-dimensional framework for the migration of Na⁺. During the usage of SIBs, they should hold the ability to endure sudden changes in temperature and current density, which have a profound impact on battery life. However, little research focused on the reaction mechanism under the above situations. Here, the phase transformation processes of NASICON-type material, Na₃V₂(PO₄)₃, are investigated by applying high-resolution in situ X-ray diffraction and Raman coupled with electrochemical tests under different temperatures (273 and 293 K) and scan rates (0.5, 2, and 5 mV s^–1). The results demonstrate that the phase evolution process is one-phase solid solution during the desodiation process rather than the traditionally two-phase reaction at a high scan rate or low temperature. An electrochemical phase diagram is also drawn based on thein situ results, which can be used to explain the asymmetric result. This work can help with understanding the phase evolution process of NASICON-type cathodes, as well as guiding the application of SIBs in various working conditions

Acetylene Black Induced Heterogeneous Growth of Macroporous CoV₂O₆ Nanosheet for High-Rate Pseudocapacitive Lithium-Ion Battery Anode

Metal vanadates suffer from fast capacity fading in lithium-ion batteries especially at a high rate. Pseudocapacitance, which is associated with surface or near-surface redox reactions, can provide fast charge/discharge capacity free from diffusion-controlled intercalation processes and is able to address the above issue. In this work, we report the synthesis of macroporous CoV₂O₆ nanosheets through a facile one-pot method via acetylene black induced heterogeneous growth. When applied as lithium-ion battery anode, the macroporous CoV₂O₆ nanosheets show typical features of pseudocapacitive behavior: (1) currents that are mostly linearly dependent on sweep rate and (2) redox peaks whose potentials do not shift significantly with sweep rate. The macroporous CoV₂O₆ nanosheets display a high reversible capacity of 702 mAh g^–1 at 200 mA g^–1, excellent cyclability with a capacity retention of 89% (against the second cycle) after 500 cycles at 500 mA g^–1, and high rate capability of 453 mAh g^–1 at 5000 mA g^–1. We believe that the introduction of pseudocapacitive properties in lithium battery is a promising direction for developing electrode materials with high-rate capability

3.0 V High Energy Density Symmetric Sodium-Ion Battery: Na₄V₂(PO₄)₃∥Na₃V₂(PO₄)₃

Symmetric sodium-ion batteries (SIBs) are considered as promising candidates for large-scale energy storage owing to the simplified manufacture and wide abundance of sodium resources. However, most symmetric SIBs suffer from suppressed energy density. Here, a superior congeneric Na₄V₂(PO₄)₃ anode is synthesized via electrochemical preintercalation, and a high energy density symmetric SIB (Na₃V₂(PO₄)₃ as a cathode and Na₄V₂(PO₄)₃ as an anode) based on the deepened redox couple of V⁴⁺/V²⁺ is built for the first time. When measured in half cell, both electrodes show stabilized electrochemical performance (over 3000 cycles). The symmetric SIBs exhibit an output voltage of 3.0 V and a cell-level energy density of 138 W h kg^–1. Furthermore, the sodium storage mechanism under the expanded measurement range of 0.01–3.9 V is disclosed through an in situ X-ray diffraction technique