Research progress of tunnel-type sodium manganese oxide cathodes for SIBs

Understanding the Synergistic Effect of Mg/Co Ions Doping on the Layer-Tunnel Hybrid Structure of Na0.44MnO2 Cathode Materials

Sodium manganese oxide Na0.44MnO2 has become a promising cathode material for sodium ion batteries due to its stable tunnel structure, but its low Na+ content and Mn3+ induced Jahn Teller (JT) distortion pose challenges to its practical application. This study constructs a tunnel-layered hybrid material that combines the advantage of two structures through co-doping with Mg and Co ions. The Mg/Co co-substitution effect is crucial in regulating the hybrid crystal structure, as it compresses the TM-O layer (transition metal oxide plates) and expands the Na layer spacing, enhancing Na+ diffusion kinetics. This modification also mitigates lattice distortion from the JT effect, improving structural stability. Electrochemical studies reveal that the Na0.44Mn0.96Mg0.02Co0.02O2 cathode in sodium-ion batteries demonstrates a higher initial specific capacity (135.8 mAh g-1, 0.5C) and improved cycle stability. Density functional theory (DFT) calculations further confirm that Mg and Co ions reduce the band gap, enhance electronic conductivity, and improve rate performance. Additionally, the increased O 2p state density near the Fermi level favors oxygen redox reactions. This research provides a new method for the design of energy storage materials. Besides, the investigation into the mechanism of Mg and Co dual substitution offers a promising strategy for optimizing sodium-ion batteries.

Mn-based tunnel-structured Na0.44MnO2 cathode materials for high-performance sodium-ion batteries: electrochemical mechanism, synthesis and modifications

Sodium-ion batteries (SIBs) have emerged as promising and mature alternatives to lithium-ion batteries (LIBs) in the post-LIB era, necessitating the development of cost-effective and high-performance cathode materials. The unique crystal texture of Mn-based tunnel-structured cathode materials offers outstanding cycling stability, rate capability and air stability, making them a highly attractive option for sodium-ion storage applications. This comprehensive review summarizes recent advancements in the understanding of sodium-ion storage mechanism, synthesis techniques, and modification strategies for Mn-based tunnel-structured cathode materials, thereby significantly contributing to the advancement of high-performance cathodes for SIBs.

Reviving Sodium Tunnel Oxide Cathodes Based on Structural Modulation and Sodium Compensation Strategy Toward Practical Sodium-Ion Cylindrical Battery

As a typical tunnel oxide, Na0.44MnO2 features excellent electrochemical performance and outstanding structural stability, making it a promising cathode for sodium-ion batteries (SIBs). However, it suffers from undesirable challenges such as surface residual alkali, multiple voltage plateaus, and low initial charge specific capacity. Herein, an internal and external synergistic modulation strategy is adopted by replacing part of the Mn with Ti to optimize the bulk phase and construct a Ti-containing epitaxial stabilization layer, resulting in reduced surface residual alkali, excellent Na+ transport kinetics and improved water/air stability. Specifically, the Na0.44Mn0.85Ti0.15O2 using water-soluble carboxymethyl cellulose as a binder can realize a capacity retention rate of 94.30% after 1,000 cycles at 2C, and excellent stability is further verified in kilogram large-up applications. In addition, taking advantage of the rich Na content in Prussian blue analog (PBA), PBA-Na0.44Mn1-xTixO2 composites are designed to compensate for the insufficient Na in the tunnel oxide and are matched with hard carbon to achieve the preparation of coin full cell and 18650 cylindrical battery with satisfactory electrochemical performance. This work enables the application of tunnel oxides cathode for SIBs in 18650 cylindrical batteries for the first time and promotes the commercialization of SIBs.

Dual Activation for Tuning N, S Co-Doping in Porous Carbon Sheets Toward Superior Sodium Ion Storage

Porous carbon has been widely focused to solve the problems of low coulombic efficiency (ICE) and low multiplication capacity of Sodium-ion batteries (SIBs) anodes. The superior energy storage properties of two-dimensional(2D) carbon nanosheets can be realized by modulating the structure, but be limited by the carbon sources, making it challenging to obtain 2D structures with large surface area. In this work, a new method for forming carbon materials with high N/S doping content based on combustion activation using the dual activation effect of K2SO4/KNO3 is proposed. The synthesized carbon material as an anode for SIBs has a high reversible capacity of 344.44 mAh g-1 at 0.05 A g-1. Even at the current density of 5 Ag-1, the capacity remained at 143.08 mAh g-1. And the ICE of sodium-ion in ether electrolytes is ≈2.5 times higher than that in ester electrolytes. The sodium storage mechanism of ether/ester-based electrolytes is further explored through ex-situ characterizations. The disparity in electrochemical performance can be ascribed to the discrepancy in kinetics, wherein ether-based electrolytes exhibit a higher rate of Na+ storage and shedding compared to ester-based electrolytes. This work suggests an effective way to develop doubly doped carbon anode materials for SIBs.

Fe doping mechanism of Na0.44MnO2 tunnel phase cathode electrode in sodium-ion batteries

Due to its stability and low cost, the tunnel-style sodium-manganese oxide (Na0.44MnO2) material is deemed a popular cathode choice for sodium-ion rechargeable batteries. However, the Jahn-Teller effect caused by Mn3+ in the material results in poor capacity and cycling stability. The purpose of this experimental study is to partially replace Mn3+ with Fe3+, in order to reduce the Jahn-Teller effect of the material during charging and discharging process. The results of Raman spectroscopy and X-ray photoelectron spectroscopy confirmed that the content of Mn3+ decreased after Fe3+ doping. Electrochemical studies show that the Na0.44Mn0.994Fe0.006O2 cathode has better rate performance (exhibits a reversible capacity of 87.9 mAh/g at 2 C) and cycle stability in sodium-ion batteries. The diffusion coefficient of sodium ions increases by Fe3+ doping. The excellent rate performance and capacity improvement are verified by density functional theory (DFT) calculation. After doping, the band gap decreases significantly, and the results show that the state density of O 2p increases near the Fermi level, which promotes the oxidation-reduction of oxygen. This work provides a straightforward approach to enhance the performance of Na0.44MnO2 nanorods, and this performance improvement has guiding significance for the design of other materials in the energy storage domain.

Recent Advances in Sodium-Ion Batteries: Cathode Materials

Emerging energy storage systems have received significant attention along with the development of renewable energy, thereby creating a green energy platform for humans. Lithium-ion batteries (LIBs) are commonly used, such as in smartphones, tablets, earphones, and electric vehicles. However, lithium has certain limitations including safety, cost-effectiveness, and environmental issues. Sodium is believed to be an ideal replacement for lithium owing to its infinite abundance, safety, low cost, environmental friendliness, and energy storage behavior similar to that of lithium. Inhered in the achievement in the development of LIBs, sodium-ion batteries (SIBs) have rapidly evolved to be commercialized. Among the cathode, anode, and electrolyte, the cathode remains a significant challenge for achieving a stable, high-rate, and high-capacity device. In this review, recent advances in the development and optimization of cathode materials, including inorganic, organometallic, and organic materials, are discussed for SIBs. In addition, the challenges and strategies for enhancing the stability and performance of SIBs are highlighted.

Multifunctional Na2TiO3 Coating-Enabled High-Voltage and Capacitive-like Sodium-Ion Storage of Na0.44MnO2

Sodium-ion batteries, as an attractive option for large-scale energy storage, still face the problems of low energy density and unsatisfactory rate performance. Among various cathodes, the tunnel-type Na0.44MnO2 with large S-shaped Na+ transport tunnels is one of the promising cathode materials for fast and robust sodium-ion storage, yet suffering from Mn dissolution and structural collapse. Herein, a Na-rich layered oxide Na2TiO3 is first constructed as a multifunctional coating layer on the surface of the Na0.44MnO2 nanorod. Na2TiO3 not only acts as an Na+ reservoir, but also serves as a protective layer to prevent Na0.44MnO2 from electrolyte etching. Besides, the derived Ti-doped Na0.44MnO2 transition layer supplies additional Na+ diffusion pathways along the radial direction of the nanorod with a short migration distance. The optimized 3 wt % Na2TiO3-coated Na0.44MnO2 exhibits enhanced an initial capacity of 127 mAh g-1 at 2-4.5 V. In addition, it shows an ultra-high capacitive-like capacity ratio of 96.7%, hence delivering an excellent rate performance of 80.2 mAh g-1 at 20C. Long-term cycling tests indicate splendid stability against high voltage, achieving 97.7% capacity retention at 20C after 900 cycles. This work provides an effective strategy to improve the rate performance and high-voltage stability of Na0.44MnO2 for high energy and power density batteries.

Vanadium fluorophosphates: advanced cathode materials for next-generation secondary batteries

Next-generation secondary batteries including sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) are considered the most promising candidates for application to large-scale energy storage systems due to their abundant, evenly distributed and cost-effective sodium/potassium raw materials. The electrochemical performance of SIBs (PIBs) significantly depends on the inherent characteristics of the cathode material. Among the wide variety of cathode materials, sodium/potassium vanadium fluorophosphate (denoted as MVPF, M = Na and K) composites are widely investigated due to their fast ion transportation and robust structure. However, their poor electron conductivity leads to low specific capacity and poor rate capacity, limiting the further application of MVPF cathodes in large-scale energy storage. Accordingly, several modification strategies have been proposed to improve the performance of MVPF such as conductive coating, morphological regulation, and heteroatomic doping, which boost the electronic conductivity of these cathodes and enhance Na (K) ion transportation. Furthermore, the development and application of MVPF cathodes in SIBs at low temperatures are also outlined. Finally, we present a brief summary of the remaining challenges and corresponding strategies for the future development of MVPF cathodes.

Interfacial Engineering of Na3V2(PO4)2O2F Cathode for Low-Temperature (-40 °C) Sodium-Ion Batteries

Na3V2(PO4)2O2F (NVPOF) is considered a promising cathode material for sodium-ion batteries (SIBs) on account of its attractive electrochemical properties such as high theoretical capacity, stable structure, and high working platform. Nevertheless, the inevitable interface problems like sluggish interfacial electrochemical reaction kinetics and poor interfacial ion storage capacity seriously hinder its application. Construction of chemical bonding is a highly effective way to solve interface problems. Herein, NVPOF with interfacial V-F-C bonding (CB-NVPOF) is developed. The CB-NVPOF cathode exhibits high rate capability (65 mA h g-1 at 40C) and long-term cycling stability (a capacity retention of 77% after 2000 cycles at 20C). Furthermore, it shows impressive electrochemical performance at temperatures as low as -40 °C, delivering a capacity of 56 mA h g-1 at 10C and a capacity retention of ∼80% after 500 cycles at 2C. The interfacial V-F-C bond engineering significantly advances the electronic conductivity, Na+ diffusion, as well as interface compatibility at -40 °C. This study provides a novel idea for improving the electrochemical performance of NVPOF-based cathodes for SIBs aiming for low-temperature applications.