Pearl-Structure-Enhanced NASICON Cathode toward Ultrastable Sodium-Ion Batteries

High-entropy doping in NASICON cathodes: Activating the V4+/V5+ redox couple and inducing a reversible single solid-solution phase reaction for advanced sodium ion batteries

Na3V2(PO4)3 (NVP) with typical NASICON structure is highly regarded as one of the most appealing cathodes for sodium-ion batteries (SIBs) for their excellent structural stability; however, the poor electronic conductivity and irreversible phase transition at high voltage (∼4 V) for the material result in poor rate capabilities and significant capacity degradation during electrochemical reactions. Herein, the high-entropy doping strategy of introducing six types of metal ions into the V-sites in Na3V2(PO4)3 is used to design a high-performance cathode of Na3V1.73(Mn, Ca, Mg, Al, Zr)0.05Nb0.02(PO4)3 (HE-NVP) for SIBs. After the high-entropy doping, a reversible V4+/V5+ redox pair is activated at high voltage of ∼4 V, which significantly contributes to the enhancement of energy density and operating voltage. Moreover, after high entropy doping, the single solid solution phase transition in high voltage range is induced, effectively improving the material's structural stability and exhibiting very small volume change of 1.1 % during the Na+ extraction/insertion. Based on the unique advantages mentioned above, the high-entropy HE-NVP exhibits a superior discharge capacity of 122.1 mAh g-1 at a current rate of 0.5C and maintains a remarkable capacity retention of 90.1 % after 500 cycles at a rate of 5C. This work provides a useful reference for designing advanced cathodes for SIBs by regulating high entropy doping.

Superior Sodium Metal Anodes Enabled by 3D Hierarchical Metallic Scaffolds with Enhanced Sodiophilicity

Sodium-metal batteries (SMBs) are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, Na-metal systems remain challenging by critical barriers, including severe Na dendrites growth and infinite huge volume change, which limit the feasibility of SMBs. Here, this work develops a 3D conductive scaffold consisting of vertical crystalline TiO2 nanotube arrays embedded with ultrafine silver nanoparticles (denoted as Ag@TiO2 NTAs) with ultrasonication-assisted in situ deposition method for high-performance SMBs. Significantly, the hierarchical hollow nanotubes with large surface area can reduce the current density to promote compact electrodeposition and guide the parallel growth of Na. Meanwhile, the sodiophilic Ag nanocrystals with strong interactions with Na+ enable a marked reduction of the nucleation barriers. As a result, the Na metal anode with the Ag@TiO2 NTAs host delivers remarkable electrochemical properties including ultralow voltage hysteresis and prolonged cycling stability over 3600 h. By pairing with a Na3V2(PO4)3 cathode, the SMBs achieve 87% capacity retention after 2000 cycles at 8 C, suggesting its potential application for highly stable Na anodes.

Accelerating Electrochemical Responses of Na4VMn(PO4)3 via Bulk-Defects and Architecture Engineering for High-Performance Sodium-Ion Batteries

Manganese-based NASICON-type Na4VMn(PO4)3 (NVMP) has captured widespread attention in sodium-ion batteries (SIBs) due to its abundant reserves and high operating voltages. However, the low intrinsic conductivity and detrimental Jahn-teller (J-T) effect impedes its electron and ion transfer, leading to rapid structural degradation and capacity decay. Herein, a facile multiscale coupling strategy is proposed to synthesize the nanosheet-stacked rods (NVMP-NSRs) with rational defects for improving intrinsic conductivity and structural stability, thus accelerating electrochemical responses. Localized unsaturated coordination states around vanadium atoms in NVMP-NSRs are also regulated, further facilitating rapid Na+ diffusion with relieved volume expansion due to the unique architecture design. Density functional theory (DFT) calculations reveal highly rearranged interfacial charges, yielding benefits for reducing the energy barriers of Na+ migration. The innovative NVMP-NSRs with appropriate bulk defects exhibit considerable discharge capacity (120.1 mAh g-1 at 0.5C), high-rate performance (70.9 mAh g-1 at 30C), and negligible capacity decay (3000 cycles at 20C). Moreover, the assembled NVMP-NSRs//hard carbon full cells demonstrate a high energy density of 391.1 Wh kg-1 with excellent cyclic stability (91.2% after 100 cycles at 1C). The multiscale coupling strategy in this work offers new avenues to design high-performance electrode materials toward fast electrochemical responses and robust structural stability.

Bilayer Mn-based Prussian blue cathode with high redox activity for boosting stable cycling in aqueous sodium-ion half/full batteries

The Mn-based Prussian blue analogs (PBAs) have garnered significant attention due to their high specific capacity, stemming from the unique multi-electron reactions with Na+. However, the structural instability caused by multi-ion insertion impacts the cycle life, thus limiting their further application in aqueous sodium-ion batteries (ASIBs). To address this issue, this work employed an in situ epitaxial solvent deposition method to homogeneously grow Ni hexacyanoferrate (NiHCF) on the surface of MnPBA, which can effectively overcome the de-intercalation instability. The resulting heterostructured MnPBA@NiHCF integrates the multiple redox-active centers of MnPBA with the confinement ability of the outer NiHCF layer, thereby maintaining overall structural stability. As a cathode material for ASIBs, MnPBA@NiHCF achieves a reversible specific capacity of 66.2 mAh/g after 200 cycles at 1 A/g, significantly outperforming the single-component MnPBA and NiHCF, respectively. Moreover, it demonstrates ultralong cycling stability with only 0.0002 % capacity fade per cycle over 20,000 cycles at 10 A/g. Extensive kinetic analyses further confirm its superior Na+ diffusion behaviors with disclosed redox mechanism through the comprehensive in situ Raman and ex situ analysis. A full cell built with a polyimide (PI) anode achieved an energy density of up to 59.9 Wh kg-1, displaying a power output of 1200.4 W kg-1 and exceptional cycle stability. This work provides innovative insights for developing stable PBA cathodes for ASIB applications.

Architecture Engineering for Thick Electrodes in High-Energy Batteries: Challenges and Strategies

With the burgeoning demand for smart portable electronic devices and high-performance electric vehicles, there is tremendous urgency to further dramatically improve the energy density of rechargeable batteries. Although utilizing thick electrodes to improve energy density is a straightforward and productive approach, the slow reaction kinetics and inadequate mechanical strength caused by the thickness increase have hampered their development. Therefore, to break through the bottleneck of thick electrodes, we comprehensively summarize the recent progress of thick electrode architecture engineering in the field of rechargeable batteries. Considering the relationship between electrode structure and electrochemical performance, we focus on the four crucial challenges (high tortuosity, slow electron and ion transport, improper porosity, and visible cracking) and corresponding solutions (constructing vertically aligned hierarchical channels, introducing multidimensional conductive materials, regulating the degree of calendering, and so on) in constructing thick electrodes. Finally, the construction strategy of thick electrodes and the inextricable relationship of these crucial factors are summarized, and an outlook on the development and research directions toward thick electrodes is discussed, providing valuable reference for designing high-performance thick electrodes.

Optimization Strategies of Na3V2(PO4)3 Cathode Materials for Sodium-Ion Batteries

Na3V2(PO4)3 (NVP) has garnered great attentions as a prospective cathode material for sodium-ion batteries (SIBs) by virtue of its decent theoretical capacity, superior ion conductivity and high structural stability. However, the inherently poor electronic conductivity and sluggish sodium-ion diffusion kinetics of NVP material give rise to inferior rate performance and unsatisfactory energy density, which strictly confine its further application in SIBs. Thus, it is of significance to boost the sodium storage performance of NVP cathode material. Up to now, many methods have been developed to optimize the electrochemical performance of NVP cathode material. In this review, the latest advances in optimization strategies for improving the electrochemical performance of NVP cathode material are well summarized and discussed, including carbon coating or modification, foreign-ion doping or substitution and nanostructure and morphology design. The foreign-ion doping or substitution is highlighted, involving Na, V, and PO43- sites, which include single-site doping, multiple-site doping, single-ion doping, multiple-ion doping and so on. Furthermore, the challenges and prospects of high-performance NVP cathode material are also put forward. It is believed that this review can provide a useful reference for designing and developing high-performance NVP cathode material toward the large-scale application in SIBs.

From Lab to Application: Challenges and Opportunities in Achieving Fast Charging with Polyanionic Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs), recognized for balanced energy density and cost-effectiveness, are positioned as a promising complement to lithium-ion batteries (LIBs) and a substitute for lead-acid batteries, particularly in low-speed electric vehicles and large-scale energy storage. Despite their extensive potential, concerns about range anxiety due to lower energy density underscore the importance of fast-charging technologies, which drives the exploration of high-rate electrode materials. Polyanionic cathode materials are emerging as promising candidates in this regard. However, their intrinsic limitation in electronic conductivity poses challenges for synchronized electron and ion transport, hindering their suitability for fast-charging applications. This review provides a comprehensive analysis of sodium ion migration during charging/discharging, highlighting it as a critical rate-limiting step for fast charging. By delving into intrinsic dynamics, key factors that constrain fast-charging characteristics are identified and summarized. Innovative modification routes are then introduced, with a focus on shortening migration paths and increasing diffusion coefficients, providing detailed insights into feasible strategies. Moreover, the discussion extends beyond half cells to full cells, addressing challenges and opportunities in transitioning polyanionic materials from the laboratory to practical applications. This review aims to offer valuable insights into the development of high-rate polyanionic cathodes, acknowledging their pivotal role in advancing fast-charging SIBs.

Enhancing Rapid Li+/Na+ Storage Performance via Interface Engineering of Reduced Graphene Oxide-Wrapped Bimetallic Sulfide Nanocages

Transition-metal sulfide is considered to be an admirable transformational electrode material due to low cost, large specific capacity, and good reversibility in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). Herein, the reduced graphene oxide-wrapped open bimetallic sulfide (NiS2-Co3S4@rGO) nanocage, derived from nickel-cobalt Prussian blue, was obtained by two-step calcination. There are luxuriant pore structures in the nanocage composite with a specific surface area of 85.28 m2 g-1, which provides plentiful paths for rapid transmission of Li+/Na+ and alleviates the volume stress caused by insertion and extraction of alkali metal ions. The excellent interface combination of bimetallic sulfide wrapped in reduced graphene oxide improves the conductivity and overall performance of the battery. Thanks to the special interface engineering, the open NiS2-Co3S4@rGO nanocage composite displays rapid lithium storage properties with an average diffusion coefficient of 8.5 × 10-13 cm2 s-1. Moreover, after 300 cycles, the reversible capacity of the composite is 1113.2 mAh g-1 at 1 A g-1. In SIBs, the capacity of the open NiS2-Co3S4@rGO composite is 487.9 mAh g-1 when the current density is 5 A g-1. These preeminent performances demonstrate the enormous development prospects of bimetallic sulfide nanocage as anode material in LIBs and SIBs.

Fine Structure and Electrochemical Performance Investigations of Spherical LiMn0.6Fe0.4PO4/C Cathode Material Synthesized via a Spray-Drying Route at Various Calcination Temperatures

LiMnxFe1-xPO4/C is characterized by excellent multiplicative performance and high operating voltage, and as a type of cathode material, it has a high electronic conductivity and thus has received much attention. In this paper, carbon-coated LiMn0.6Fe0.4PO4/C was synthesized using glucose + PEG2000 as the carbon source by wet sanding and spray-drying. The experimental results show that the use of sanding and the spray-drying method can make the particle size distribution of LiMn0.6Fe0.4PO4/C powder more uniform. The initial discharge specific capacity of the LiMn0.6Fe0.4PO4/C battery was 144.3 mA h g-1, and after 100 cycles at 1 C current, the discharge specific capacity of the battery remained at 128.2 mA h g-1 with a cycling efficiency of 94.3%. At the same time, the oxidation states and coordination environments of the elements Fe and Mn were elucidated by X-ray absorption fine structure spectroscopy. And the ex-Fe-MS was tested under different charging and discharging conditions. The sample optimized by the orthogonal test has good cycle stability and multiplication performance.

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.

Stable Structure and Fast Ion Diffusion: A Flexible MoO2@Carbon Hollow Nanofiber Film as a Binder-Free Anode for Sodium-Ion Batteries with Superior Kinetics and Excellent Rate Capability

Designing innovative anode materials that exhibit excellent ion diffusion kinetics, enhanced structural stability, and superior electrical conductivity is imperative for advancing the rapid charge-discharge performance and widespread application of sodium-ion batteries. Hollow-structured materials have received significant attention in electrode design due to their rapid ion diffusion kinetics. Building upon this, we present a high-performance, free-standing MoO2@hollow carbon nanofiber (MoO2@HCNF) electrode, fabricated through facile coaxial electrospinning and subsequent heat treatment. In comparison to MoO2@carbon nanofibers (MoO2@CNFs), the MoO2@HCNF electrode demonstrates superior rate capability, attributed to its larger specific surface area, its higher pseudocapacitance contribution, and the enhanced diffusion kinetics of sodium ions. The discharge capacities of the MoO2@HCNF (MoO2@CNF) electrode at current densities of 0.1, 0.2, 0.5, 1.0, 2.0 and 5.0 A g-1 are 195.55 (155.49), 180.98 (135.20), 163.81 (109.71), 144.05 (90.46), 121.16 (71.21) and 88.90 (44.68) mAh g-1, respectively. Additionally, the diffusion coefficients of sodium ions in the MoO2@HCNFs are 8.74 × 10-12 to 1.37 × 10-12 cm2 s-1, which surpass those of the MoO2@CNFs (6.49 × 10-12 to 9.30 × 10-13 cm2 s-1) during the discharging process. In addition, these prepared electrode materials exhibit outstanding flexibility, which is crucial to the power storage industry and smart wearable devices.

Facilitating Rapid Na+ Storage through MoWSe/C Heterostructure Construction and Synergistic Electrolyte Matching Strategy

The realization of sodium-ion devices with high-power density and long-cycle capability is challenging due to the difficulties of carrier diffusion and electrode fragmentation in transition metal selenide anodes. Herein, a Mo/W-based metal-organic framework is constructed by a one-step method through rational selection, after which MoWSe/C heterostructures with large angles are synthesized by a facile selenization/carbonization strategy. Through physical characterization and theoretical calculations, the synthesized MoWSe/C electrode delivers obvious structural advantages and excellent electrochemical performance in an ethylene glycol dimethyl ether electrolyte. Furthermore, the electrochemical vehicle mechanism of ions in the electrolyte is systematically revealed through comparative analyses. Resultantly, ether-based electrolytes advantageously construct stable solid electrolyte interfaces and avoid electrolyte decomposition. Based on the above benefits, the Na half-cell assembled with MoWSe/C electrodes demonstrated excellent rate capability and a high specific capacity of 347.3 mA h g-1 even after cycling 2000 cycles at 10 A g-1. Meanwhile, the constructed sodium-ion capacitor maintains ∼80% capacity retention after 11,000 ultralong cycles at a high-power density of 3800 W kg-1. The findings can broaden the mechanistic understanding of conversion anodes in different electrolytes and provide a reference for the structural design of anodes with high capacity, fast kinetics, and long-cycle stability.

A high-energy-density NASICON-type Na3V1.25Ga0.75(PO4)3 cathode with reversible V4+/V5+ redox couple for sodium ion batteries

The stable three-dimensional framework and high operating voltage of sodium superionic conductor (NASICON)-type Na3V2(PO4)3 has the potential to work with long cycle life and high-rate performance; however, it suffers from the poor intrinsic electronic conductivity and low energy density. Herein, Ga3+ is introduced into Na3V2(PO4)3 to activate the V4+/V5+ redox couple at a high potential of 4.0 V for enhancing energy density of the materials (Na3V2-xGax(PO4)3). After the partial substitution of Ga3+ for V3+, three redox couples (V2+/V3+, V3+/V4+ and V4+/V5+) of V are reversibly converted in the voltage range of 1.4-4.2 V, suggesting multi-electrons (>2e-) involved in the reversible reaction, and simultaneously the electronic conductivity of the materials is effectively enhanced. As a result, the cathode with x = 0.75 exhibits excellent electrochemical properties: in the voltage range of 2.2-4.2 V, delivering an initial capacity of 105 mAh/g at 1C with a capacity retention rate of 92.3% after 400 cycles, and providing a stable reversible capacity of 88.3 mAh/g at 40C; and in the voltage range of 1.4-4.2 V, presenting the reversible capacity 152.3 mAh/g at 1C (497.6 Wh kg-1), and cycling stably for 1000 cycles at 20C with a capacity decay of 0.02375% per cycle. It is found that the Na3V2-xGax(PO4)3 cathodes possess the sodium storage mechanism of single-phase and bi-phase reactions. This investigation presents a useful strategy to enhance the energy density and cycling life of NASICON-structured polyanionic phosphates by activating high-potential V4+/V5+ redox couple.

Outstanding long cycle stability provide by bismuth doped Na3V2(PO4)3 enwrapped with carbon nanotubes cathode for sodium-ion batteries

Na3V2(PO4)3 (NVP), possessing good ionic conduction properties and high voltage plateau, has been deemed as the most prospective material for sodium ion batteries. However, the weak intrinsic electronic conductivity has hindered its further commercialization. Herein, an ingenious strategy of Bi3+ substitution at V3+ site in NVP system is proposed. The ionic radius of Bi3+ is slightly larger than that of V3+, which can further expand the crystal structure inside the NVP, thus accelerating the migration of Na+. Meanwhile, the appropriate amount of carbon coating and carbon nanotubes (CNTs) enwrapping construct an effective three-dimensional network, which provides a conductive framework for electronic transfer. Furthermore, the introduction of CNTs also inhibit the agglomeration of active grains during the sintering process, reducing the particle size and shortening the diffusion path of Na+. Comprehensively, the conductivity, ionic diffusion ability and structural stability of the modified Na3V2-xBix(PO4)3/C@CNTs (0 ≤ x ≤ 0.05) sample are significantly improved. The Na3V1.97Bi0.03(PO4)3/C@CNTs sample obtains a reversible capacity of 97.8 mAh g-1 at 12C and maintains a value of 80.6 mAh g-1 after 9000 ultra-long cycles. As for the super high rate at 80C, it exhibits a high capacity of 84.34 mAh g-1 and retains a capacity of 73.34 mAh g-1 after 6000 cycles. The superior electrochemical performance is derived from the enhancement of the crystal structure by Bi3+ doping and the highly conductive network consisting of carbon coating layers and enwrapped CNTs.

Boosted Redox Kinetics Enabling Na3V2(PO4)3 with Excellent Performance at Low Temperature through Cation Substitution and Multiwalled Carbon Nanotube Cross-Linking

The open NASICON framework and high reversible capacity enable Na3V2(PO4)3 (NVP) to be a highly promising cathode candidate for sodium-ion batteries (SIBs). Nevertheless, the unsatisfied cyclic stability and degraded rate capability at low temperatures due to sluggish ionic migration and poor conductivity become the main challenges. Herein, excellent sodium storage performance for the NVP cathode can be received by partial potassium (K) substitution and multiwalled carbon nanotube (MWCNT) cross-linking to modify the ionic diffusion and electronic conductivity. Consequently, the as-fabricated Na3-xKxV2(PO4)3@C/MWCNT can maintain a capacity retention of 79.4% after 2000 cycles at 20 C. Moreover, the electrochemical tests at -20 °C manifest that the designed electrode can deliver 89.7, 73.5, and 64.8% charge of states, respectively, at 1, 2, and 3 C, accompanied with a capacity retention of 84.3% after 500 cycles at 20 C. Generally, the improved electronic conductivity and modified ionic diffusion kinetics resulting from K doping and MWCNT interconnecting endows the resultant Na3-xKxV2(PO4)3@C/MWCNT with modified electrochemical polarization and improved redox reversibility, contributing to superior performance at low temperatures. Generally, this study highlights the potential of alien substitution and carbon hybridization to improve the NASICON-type cathodes toward high-performance SIBs, especially at low temperatures.