A Comprehensive Review on Strategies for Enhancing the Performance of Polyanionic-Based Sodium-Ion Battery Cathodes

The significant consumption of fossil fuels and the increasing pollution have spurred the development of energy-storage devices like batteries. Due to their high cost and limited resources, widely used lithium-ion batteries have become unsuitable for large-scale energy production. Sodium is considered to be one of the most promising substitutes for lithium due to its wide availability and similar physiochemical properties. Designing a suitable cathode material for sodium-ion batteries is essential, as the overall electrochemical performance and the cost of battery depend on the cathode material. Among different types of cathode materials, polyanionic material has emerged as a great option due to its higher redox potential, stable crystal structure, and open three-dimensional framework. However, the poor electronic and ionic conductivity limits their applicability. This review briefly discusses the strategies to deal with the challenges of transition-metal oxides and Prussian blue analogue, recent developments in polyanionic compounds, and strategies to improve electrochemical performance of polyanionic material by nanostructuring, surface coating, morphology control, and heteroatom doping, which is expected to accelerate the future design of sodium-ion battery cathodes.

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

Unprecedented Cyclability and Moisture Durability of NaCrO2 Sodium-Ion Battery Cathode via Simultaneous Al Doping and Cr2O3 Coating

Although there are many cathode candidates for sodium-ion batteries (NIBs), NaCrO₂ remains one of the most attractive materials due to its reasonable level of capacity, nearly flat reversible voltages, and high thermal stability. However, the cyclic stability of NaCrO₂ needs to be further improved in order to compete with other state-of-the-art NIB cathodes. In this study, we show that Cr₂O₃-coated and Al-doped NaCrO₂, which is synthesized through a simple one-pot synthesis, can achieve unprecedented cyclic stability. We confirm the preferential formation of a Cr₂O₃ shell and a Na(Cr_1-2xAl_2x)O₂ core, rather than xAl₂O₃/NaCrO₂ or Na_1/1+2x(Cr_1/1+2xAl_2x/1+2x)O₂, through spectroscopic and microscopic methods. The core/shell compounds exhibit superior electrochemical properties compared to either Cr₂O₃-coated NaCrO₂ without Al dopants or Al-doped NaCrO₂ without shells because of their synergistic contributions. As a result, Na(Cr_0.98Al_0.02)O₂ with a thin Cr₂O₃ layer (5 nm) shows no capacity fading during 1000 charge/discharge cycles while maintaining the rate capability of pristine NaCrO₂. In addition, the compound is inert against humid air and water. We also discuss the reasons for the excellent performance of Cr₂O₃-coated Na(Cr_1-2xAl_2x)O₂.

Polyanion-type cathode materials for sodium-ion batteries

This review summarizes the recent progress and remaining challenges of polyanion-type cathodes, providing guidelines towards high-performance cathodes for sodium ion batteries.

Triclinic Off-Stoichiometric Na3.12Mn2.44(P2O7)2/C Cathode Materials for High-Energy/Power Sodium-Ion Batteries

The application of sodium-ion batteries (SIBs) requires a suitable cathode material with low cost, nontoxic, high safety, and high energy density, which is still a big challenge; thus, a basic research on exploring new types of materials is imperative. In this work, a manganic pyrophosphate and carbon compound Na_3.12Mn_2.44(P₂O₇)₂/C has been synthesized through a feasible sol-gel method. Rietveld refinement reveals that Na_3.12Mn_2.44(P₂O₇)₂ adopts a triclinic structure ( P1̅ space group), which possesses spacious ion diffusion channels for facile sodium migration. The off-stoichiometric phase is able to offer more reversible Na⁺, delivering an enhanced reversible capacity of 114 mA h g^-1 at 0.1 C, and because of the strong "inductive effect" that (P₂O₇)^4- groups imposing on the Mn³⁺/Mn²⁺ redox couple, Na_3.12Mn_2.44(P₂O₇)₂/C presents high platforms above 3.6 V, contributing a remarkable energy density of 376 W h kg^-1, which is among the highest Fe-/Mn-based polyanion-type cathode materials. Furthermore, the off-stoichiometric compound also presents satisfactory rate capability and long-cycle stability, with a capacity retention of 75% after 500 cycles at 5 C. Ex situ X-ray diffraction demonstrates a single-phase reaction mechanism, and the density functional theory calculations display two one-dimensional sodium migration paths with low energy barriers in Na_3.12Mn_2.44(P₂O₇)₂, which is vital for the facile sodium storage. We believe that this compound will be a competitive cathode material for large-scale SIBs.

Design of fast ion conducting cathode materials for grid-scale sodium-ion batteries

The obvious cost advantage as well as attractive electrochemical properties, including excellent cycling stability and the potential of high rate performance, make sodium-ion batteries prime candidates in the race to technically and commercially enable large-scale electrochemical energy storage. In this work, we apply our bond valence site energy modelling method to further the understanding of rate capabilities of a wide range of potential insertion-type sodium-ion battery cathode materials. We demonstrate how a stretched exponential function permits us to systematically quantify the rate performance, which in turn reveals guidelines for the design of novel sodium-ion battery chemistries suitable for high power, grid-scale applications. Starting from a diffusion relaxation model, we establish a semi-quantitative prediction of the rate-performance of half-cells from the structure of the cathode material that factors in dimensionality of Na⁺ ion migration pathways, the height of the migration barriers and the crystallite size of the active material. With the help of selected examples, we also illustrate the respective roles of unoccupied low energy sites within the pathway and temperature towards the overall rate capability of insertion-type cathode materials.

Phosphate Framework Electrode Materials for Sodium Ion Batteries

Sodium ion batteries (SIBs) have been considered as a promising alternative for the next generation of electric storage systems due to their similar electrochemistry to Li-ion batteries and the low cost of sodium resources. Exploring appropriate electrode materials with decent electrochemical performance is the key issue for development of sodium ion batteries. Due to the high structural stability, facile reaction mechanism and rich structural diversity, phosphate framework materials have attracted increasing attention as promising electrode materials for sodium ion batteries. Herein, we review the latest advances and progresses in the exploration of phosphate framework materials especially related to single-phosphates, pyrophosphates and mixed-phosphates. We provide the detailed and comprehensive understanding of structure-composition-performance relationship of materials and try to show the advantages and disadvantages of the materials for use in SIBs. In addition, some new perspectives about phosphate framework materials for SIBs are also discussed. Phosphate framework materials will be a competitive and attractive choice for use as electrodes in the next-generation of energy storage devices.

Polyanion-Type Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries, representative members of the post-lithium-battery club, are very attractive and promising for large-scale energy storage applications. The increasing technological improvements in sodium-ion batteries (Na-ion batteries) are being driven by the demand for Na-based electrode materials that are resource-abundant, cost-effective, and long lasting. Polyanion-type compounds are among the most promising electrode materials for Na-ion batteries due to their stability, safety, and suitable operating voltages. The most representative polyanion-type electrode materials are Na3V2(PO4)3 and NaTi2(PO4)3 for Na-based cathode and anode materials, respectively. Both show superior electrochemical properties and attractive prospects in terms of their development and application in Na-ion batteries. Carbonophosphate Na3MnCO3PO4 and amorphous FePO4 have also recently emerged and are contributing to further developing the research scope of polyanion-type Na-ion batteries. However, the typical low conductivity and relatively low capacity performance of such materials still restrict their development. This paper presents a brief review of the research progress of polyanion-type electrode materials for Na-ion batteries, summarizing recent accomplishments, highlighting emerging strategies, and discussing the remaining challenges of such systems.