Improvement of the Cathode Electrolyte Interphase on P2-Na_2/3Ni_1/3Mn_2/3O₂ by Atomic Layer Deposition

Atomic layer deposition (ALD) is a commonly used coating technique for lithium ion battery electrodes. Recently, it has been applied to sodium ion battery anode materials. ALD is known to improve the cycling performance, Coulombic efficiency of batteries, and maintain electrode integrity. Here, the electrochemical performance of uncoated P2-Na_2/3Ni_1/3Mn_2/3O₂ electrodes is compared to that of ALD-coated Al₂O₃ P2-Na_2/3Ni_1/3Mn_2/3O₂ electrodes. Given that ALD coatings are in the early stage of development for NIB cathode materials, little is known about how ALD coatings, in particular aluminum oxide (Al₂O₃), affect the electrode–electrolyte interface. Therefore, full characterizations of its effects are presented in this work. For the first time, X-ray photoelectron spectroscopy (XPS) is used to elucidate the cathode electrolyte interphase (CEI) on ALD-coated electrodes. It contains less carbonate species and more inorganic species, which allows for fast Na kinetics, resulting in significant increase in Coulombic efficiency and decrease in cathode impedance. The effectiveness of Al₂O₃ ALD coating is also surprisingly reflected in the enhanced mechanical stability of the particle which prevents particle exfoliation

Investigating the Energy Storage Mechanism of SnS₂‑rGO Composite Anode for Advanced Na-Ion Batteries

Tin sulfide–reduced graphene oxide (SnS₂-rGO) composite material is investigated as an advanced anode material for Na-ion batteries. It can deliver a reversible capacity of 630 mAh g^–1 with negligible capacity loss and exhibits superb rate performance. Here, the energy storage mechanism of this SnS₂-rGO anode and the critical mechanistic role of rGO will be revealed in detail. A synergistic mechanism involving conversion and alloying reactions is proposed based on our synchrotron X-ray diffraction (SXRD) and <i>in situ</i> X-ray absorption spectroscopy (XAS) results. Contrary to what has been proposed in the literature, we determined that Na₂S₂ forms instead of Na₂S at the fully discharge state. The as-formed Na₂S₂ works as a matrix to relieve the strain from the huge volume expansion of the Na–Sn alloy reaction, shown in the high resolution transmission electron microscope (HRTEM). In addition, the Raman spectra results suggest that the rGO not only assists the material to have better electrochemical performance by preventing particle agglomeration of the active material but also coordinates with Na-ions through electrostatic interaction during the first cycle. The unique reaction mechanism in SnS₂-rGO offers a well-balanced approach for sodium storage to deliver high capacity, long-cycle life, and superior rate capability

Exploring Oxygen Activity in the High Energy P2-Type Na_0.78Ni_0.23Mn_0.69O₂ Cathode Material for Na-Ion Batteries

Large-scale electric energy storage is fundamental to the use of renewable energy. Recently, research and development efforts on room-temperature sodium-ion batteries (NIBs) have been revitalized, as NIBs are considered promising, low-cost alternatives to the current Li-ion battery technology for large-scale applications. Herein, we introduce a novel layered oxide cathode material, Na_0.78Ni_0.23Mn_0.69O₂. This new compound provides a high reversible capacity of 138 mAh g^–1 and an average potential of 3.25 V vs Na⁺/Na with a single smooth voltage profile. Its remarkable rate and cycling performances are attributed to the elimination of the P2–O2 phase transition upon cycling to 4.5 V. The first charge process yields an abnormally excess capacity, which has yet to be observed in other P2 layered oxides. Metal K-edge XANES results show that the major charge compensation at the metal site during Na-ion deintercalation is achieved via the oxidation of nickel (Ni²⁺) ions, whereas, to a large extent, manganese (Mn) ions remain in their Mn⁴⁺ state. Interestingly, electron energy loss spectroscopy (EELS) and soft X-ray absorption spectroscopy (sXAS) results reveal differences in electronic structures in the bulk and at the surface of electrochemically cycled particles. At the surface, transition metal ions (TM ions) are in a lower valence state than in the bulk, and the O K-edge prepeak disappears. On the basis of previous reports on related Li-excess LIB cathodes, it is proposed that part of the charge compensation mechanism during the first cycle takes place at the lattice oxygen site, resulting in a surface to bulk transition metal gradient. We believe that by optimizing and controlling oxygen activity, Na layered oxide materials with higher capacities can be designed

Understanding the Electrochemical Mechanisms Induced by Gradient Mg²⁺ Distribution of Na-Rich Na_3+<i>x</i>V_2–<i>x</i>Mg_<i>x</i>(PO₄)₃/C for Sodium Ion Batteries

Metal-ion doping can improve the electrochemical performance of Na₃V₂(PO₄)₃. However, the reason for the enhanced electrochemical performance and the effects of cation doping on the structure of Na₃V₂(PO₄)₃ have yet been probed. Herein, Mg²⁺ is doped into Na₃V₂(PO₄)₃/C according to the first-principles calculation. The results indicate that Mg²⁺ prefers to reside in the V site and an extra electrochemical active Na⁺ is introduced to the Na₃V₂(PO₄)₃/C crystal to maintain the charge balance. The distribution of Mg²⁺ in the particle of Na₃V₂(PO₄)₃/C is further studied by electrochemical impedance spectroscopy. We find that the highest distribution of Mg²⁺ on the surface of the particles leads to facile surface electrochemical reactions for Mg²⁺-doped samples, especially at high rates