Emerging Prototype Sodium-Ion Full Cells with Nanostructured Electrode Materials

Yolk-Shell Construction of Na3V2(PO4)2F3 with Copper Substitution Microsphere as High-Rate and Long-Cycling Cathode Materials for Sodium-Ion Batteries

Na3V2(PO4)2F3 (NVPF) is emerging as a promising cathode material for high-voltage sodium-ion batteries. Whereas, the inferior intrinsic electrical conductivity leading to poor rate performance and cycling stability. To address this issue, a strategy of synthesizing unique yolk-shell structured NVPF with copper substitution via spray drying method is proposed. Besides, the synergistic modulation of both crystalline structure and interfacial properties results in significantly enhanced intrinsic and interfacial conductivity of NVPF. The optimized yolk-shell structured cathode materials can possess a high capacity of 117.4 mAh g-1 at 0.1 C, and remains a high-capacity retention of 91.3% after 5000 cycles. A detailed investigation of kinetic properties combined with in situ XRD technology and DFT calculations, has been implemented, particularly with regard to electron conduction and sodium ion diffusion. Consequently, the yolk-shell structured composition of Na3V1.94Cu0.06(PO4)2F3 with nitrogen-modified carbon coating layer shows the lowest polarization potential because of the effectively enhanced electronic conductivity and Na+ diffusion process in the bulk phase. The robust electrochemical performance suggests that developing the unique yolk-shell structure with the collaboration of interface and bulk crystal properties is a favorable strategy to design cathode material with a high performance for sodium-ion batteries.

Simultaneous modification of dual-substitution with CeO2 coating boosting high performance sodium ion batteries

Na3V2(PO4)3 (NVP) is highly valued based on the stable construction among the polyanionic compounds. Nevertheless, the drawback of low intrinsic conductivity has been impeded its further application. In this paper, the internal channels of the crystal structure are extended by the introduction of larger radius Ce3+, which increases the transport rate of Na+. The introduction of Mo6+ replacing the V site leads to a beneficial n-type doping effect and facilitates the transportation of electrons. Besides, CeO2 cladding is introduced to further enhance the electronic conductivity of NVP system. Initially, CeO2 serves as an n-type semiconductor and functions as a conductive additive to significantly enhance the electronic conductivity of the electrode, thereby improving the electrochemical characteristics. Moreover, CeO2 functions as an oxygen buffer, aiding in the maintenance of active metal dispersion during operation and enabling efficient electron transfer between CeO2 and [VO6] octahedra in NVP, thus fostering outstanding electrical connectivity between the oxides. CeO2 cladding can be effectively integrated with the carbon layer to stabilize the NVP system. Comprehensively, the modified Na3V1.79Ce0.07Mo0.07(PO4)3/C@8wt.%CeO2 (CeMo0.07@8wt.%CeO2) composite exhibits excellent rate and cycling properties. It delivers a capacity of 113.4 mAh/g at 1C with a capacity retention rate of 80.3 % after 150 cycles. Even at 10C and 40C, it also submits high capacities of 84.7 mAh/g and 76 mAh/g, respectively. Furthermore, the CHC//CeMo0.07@8wt.%CeO2 asymmetric full cell possesses excellent sodium storage property, indicating its prospective application potentials.

Synergistic Manipulation of Na+ Flux and Surface-Preferred Effect Enabling High-Areal-Capacity and Dendrite-Free Sodium Metal Battery

The propensity of sodium anode to form uniform electrodeposit is bound up with the nature of electrode surface and regulation of Na-ion flux, as well as distribution of electronic field, which is quite crucial for high-areal-capacity sodium metal batteries (SMBs). Herein, a novel metallic sodium/sodium-tin alloy foil anode (Na/NaSn) with 3D interpenetrated network and porous structure is prepared through facile alloy reaction. The strong sodiophilic properties of sodium-tin alloy can lower the nucleation energy, resulting in smaller depositing potential and strong adsorption of Na+ , while synergistic effect of porous skeleton and additional potential difference (≈0.1 V) between Na and Na-Sn alloy (Na15 Sn4 ) can alleviate volume expansion, redistribute the Na-ion flux and regulate electronic field, which favors and improves homogeneous Na deposition. The as-fabricated Na/NaSn electrode can endow excellent plating/stripping reversibility at high areal capacity (over 1600 h for 4 mAh cm-2 at 1 mA cm-2 and 2 mAh cm-2 at 2 mA cm-2 ), fast electrochemical kinetics (500 h under 4 mAh cm-2 at 4 mA cm-2 ) and superior rate performances. A novel strategy in the design of high-performance Na anodes for large-scale energy storage is provided.

Co-Salen Complex-Derived CoP Nanoparticles Confined in N-Doped Carbon Microspheres for Stable Sodium Storage

The poor rate and cycle performance rooting from the inferior electrical conductivity and large volume change are bottlenecks for further application of the potential anode material in sodium-ion batteries. To address this problem, homogeneous CoP nanoparticles enwrapped in the N-doped carbon (CoP/NC) microspheres are synthesized by the simultaneous carbonization and phosphorization of Co-salen complex microspheres for the first time. The N-doped carbon enhances its conductivity and diminishes the volume stress, and the dispersed CoP nanoparticles in carbon provide more reaction sites, resulting in a superior sodium storage performance. CoP/NC microspheres exhibit the capacity of 373 mA h g^-1 at 0.1 A g^-1 after 100 cycles. Even at 2 A g^-1 for 2000 cycles, the capacity of 195 mA h g^-1 is also achieved. This work provides an excellent reference for the design and synthesis of sulfide, selenide, and other transition-metal composites. It is also beneficial to expand the application of salen complexes in the design and synthesis of catalysts and energy storage materials.

Highly Crystallized Prussian Blue with Enhanced Kinetics for Highly Efficient Sodium Storage

Prussian blue analogs (PBAs) featuring large interstitial voids and rigid structures are broadly recognized as promising cathode materials for sodium-ion batteries. Nevertheless, the conventionally prepared PBAs inevitably suffer from inferior crystallinity and lattice defects, leading to low specific capacity, poor rate capability, and unsatisfied long-term stability. As the Na⁺ migration within PBAs is directly dependent on the periodic lattice arrangement, it is of essential significance to improve the crystallinity of PBAs and hence ensure long-range lattice periodicity. Herein, a chemical inhibition strategy is developed to prepare a highly crystallized Prussian blue (Na₂Fe₄[Fe(CN)₆]₃), which displays an outstanding rate performance (78 mAh g^-1 at 100 C) and long life-span properties (62% capacity retention after 2000 cycles) in sodium storage. Experimental results and kinetic analyses demonstrate the efficient electron transfer and smooth ion diffusion within the bulk phase of highly crystallized Prussian blue. Moreover, in situ X-ray diffraction and in situ Raman spectroscopy results demonstrate the robust crystalline framework and reversible phase transformation between cubic and rhombohedral within the charge-discharge process. This research provides an innovative way to optimize PBAs for advanced rechargeable batteries from the perspective of crystallinity.

Potassium mediated Co-Fe-based Prussian blue analogue architectures for aqueous potassium-ion storage

Prussian blue analogs (PBAs) with unique structure show great potential for aqueous potassium-ion batteries (AKIBs). Herein, K2Co[Fe(CN)6] and KNi[Co(CN)6] architectures are developed as the cathode candidates for AKIBs. Moreover, the reaction kinetics detection and DFT calculations are employed to analyse the battery performances.

Sodium citrate as a self-sacrificial sodium compensation additive for sodium-ion batteries

Commercial sodium citrate is proposed as the self-sacrificial cathode additive for the first time to offset the initial sodium loss. The optimum additive can obviously increase the energy density of the as-constructed hard carbon//Na3V2(PO4)2F3/rGO full-cell by 28.9% without sacrificing its other electrochemical properties, showing promising application prospects in sodium ion batteries.

High power Na3V2(PO4)3 symmetric full cell for sodium-ion batteries

Sodium-ion batteries (SIBs) are a viable substitute for lithium-ion batteries due to the low cost and wide availability of sodium. However, practical applications require the development of fast charging sodium-ion-based full-cells with high power densities. Na₃V₂(PO₄)₃ (NVP) is a bipolar material with excellent characteristics as both a cathode and an anode material in SIBs. Designing symmetric cells with NVP results in a single voltage plateau with significant specific capacity which is ideal for a full cell. Here we demonstrate for the first time a tremendous improvement in the performance of NVP symmetric full cells by introducing an ether-based electrolyte which favors fast reaction kinetics. In a symmetric full cell configuration, 75.5% of the initial capacity was retained even after 4000 cycles at 2 A g^-1, revealing ultra-long cyclability. Excellent rate performances were obtained at current densities as high as 1000C, based on the cathode mass, revealing ultrafast Na⁺ transfer. The power density obtained for this NVP symmetric cell (48 250 W kg^-1) is the best among those of all the sodium-ion-based full cells reported to date.

Methanol-derived high-performance Na3V2(PO4)3/C: from kilogram-scale synthesis to pouch cell safety detection

Na₃V₂(PO₄)₃ (NVP) is regarded as a potential cathode material that can be applied in sodium ion batteries (SIBs) owing to its NASICON structure. However, most of the reported works have focused on the synthesis of materials and the improvement of their electrochemical properties, with little research on the design and safety of pouch cells. Herein, we implemented a cost-saving route to realize the industrial-scale synthesis of NVP cathode materials. The obtained NVP samples possess an impressive Na-ion storage capability with high reversible capacity (116.3 mA h g^-1 at 0.2 C), superior power capability (97.9 mA h g^-1 at 30 C), and long lifespan (71.6% capacity retention after 2500 cycles at 20 C). It was remarkable that industrial-scale NVP/hard carbon (HC) sodium-ion pouch cells could be designed with an 823 mA h discharge capacity at a current of 200 mA (about 0.25 C), and which possess a long life and high rate performance (1000 cycles with a little decay at a current of 4000 mA). Besides, the pouch cells also exhibit excellent thermal stability when demonstrated for application in unmanned aerial vehicles (UAVs), and puncturing experiment results can further prove the excellent safety performance of NVP-hard carbon pouch cells.

Nickel Hollow Spheres Concatenated by Nitrogen-Doped Carbon Fibers for Enhancing Electrochemical Kinetics of Sodium-Sulfur Batteries

The high energy density of room temperature (RT) sodium-sulfur batteries (Na-S) usually rely on the efficient conversion of polysulfide to sodium sulfide during discharging and sulfur recovery during charging, which is the rate-determining step in the electrochemical reaction process of Na-S batteries. In this work, a 3D network (Ni-NCFs) host composed by nitrogen-doped carbon fibers (NCFs) and Ni hollow spheres is synthesized by electrospinning. In this novel design, each Ni hollow unit not only can buffer the volume fluctuation of S during cycling, but also can improve the conductivity of the cathode along the carbon fibers. Meanwhile, the result reveals that a small amount of Ni is polarized during the sulfur-loading process forming a polar Ni-S bond. Furthermore, combining with the nitrogen-doped carbon fibers, the Ni-NCFs composite can effectively adsorb soluble polysulfide intermediate, which further facilitates the catalysis of the Ni unit for the redox of sodium polysulfide. In addition, the in situ Raman is employed to supervise the variation of polysulfide during the charging and discharging process. As expected, the freestanding S@Ni-NCFs cathode exhibits outstanding rate capability and excellent cycle performance.

Hierarchical hollow microspheres Na3V2(PO4)2F3C@rGO as high-performance cathode materials for sodium ion batteries

Nanosheet-assembled Na₃V₂(PO₄)₂F₃ microspheres were synthesized by a one-step polyol-assisted hydrothermal method and combined with graphene as the cathode material for SIBs.

Synthesis of micro/nanoscaled metal–organic frameworks and their direct electrochemical applications

Developing strategies to control the morphology and size of MOFs is important for their applications in batteries, supercapacitors and electrocatalysis. This review focuses on the design and fabrication of MOFs at the micro/nanoscale.

Nonaqueous Sodium-Ion Full Cells: Status, Strategies, and Prospects

With ever-increasing efforts focused on basic research of sodium-ion batteries (SIBs) and growing energy demand, sodium-ion full cells (SIFCs), as unique bridging technology between sodium-ion half-cells (SIHCs) and commercial batteries, have attracted more and more interest and attention. To promote the development of SIFCs in a better way, it is essential to gain a systematic and profound insight into their key issues and research status. This Review mainly focuses on the interface issues, major challenges, and recent progresses in SIFCs based on diversified electrolytes (i.e., nonaqueous liquid electrolytes, quasi-solid-state electrolytes, and all-solid-state electrolytes) and summarizes the modification strategies to improve their electrochemical performance, including interface modification, cathode/anode matching, capacity ratio, electrolyte optimization, and sodium compensation. Outlooks and perspectives on the future research directions to build better SIFCs are also provided.

Recent Progress in Rechargeable Sodium-Ion Batteries: toward High-Power Applications

The increasing demands for renewable energy to substitute traditional fossil fuels and related large-scale energy storage systems (EES) drive developments in battery technology and applications today. The lithium-ion battery (LIB), the trendsetter of rechargeable batteries, has dominated the market for portable electronics and electric vehicles and is seeking a participant opportunity in the grid-scale battery market. However, there has been a growing concern regarding the cost and resource availability of lithium. The sodium-ion battery (SIB) is regarded as an ideal battery choice for grid-scale EES owing to its similar electrochemistry to the LIB and the crust abundance of Na resources. Because of the participation in frequency regulation, high pulse-power capability is essential for the implanted SIBs in EES. Herein, a comprehensive overview of the recent advances in the exploration of high-power cathode and anode materials for SIB is presented, and deep understanding of the inherent host structure, sodium storage mechanism, Na⁺ diffusion kinetics, together with promising strategies to promote the rate performance is provided. This work may shed light on the classification and screening of alternative high rate electrode materials and provide guidance for the design and application of high power SIBs in the future.

Recent Progress of Layered Transition Metal Oxide Cathodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are attracting increasing attention and considered to be a low-cost complement or an alternative to lithium-ion batteries (LIBs), especially for large-scale energy storage. Their application, however, is limited because of the lack of suitable host materials to reversibly intercalate Na⁺ ions. Layered transition metal oxides (Na_x MO₂ , M = Fe, Mn, Ni, Co, Cr, Ti, V, and their combinations) appear to be promising cathode candidates for SIBs due to their simple structure, ease of synthesis, high operating potential, and feasibility for commercial production. In the present work, the structural evolution, electrochemical performance, and recent progress of Na_x MO₂ as cathode materials for SIBs are reviewed and summarized. Moreover, the existing drawbacks are discussed and several strategies are proposed to help alleviate these issues. In addition, the exploration of full cells based on Na_x MO₂ cathodes and future perspectives are discussed to provide guidance for the future commercialization of such systems.

Air-Stable and High-Voltage Layered P3-Type Cathode for Sodium-Ion Full Battery

The development of highly efficient and stable cathodes for sodium-ion batteries (SIBs) is strategically critical to achieving large-scale electrical energy storage. Creating air-stable and high-voltage layered cathodes for sodium-ion full batteries still remains a challenge. Herein, we describe a rational design and preparation of a stable P3-Na_2/3Ni_1/4Mg_1/12Mn_2/3O₂ cathode. The cathode displays a satisfactory working voltage of 3.6 V and excellent cyclic stability over 100 cycles at a 1 C rate without obvious capacity fading. The results of ex situ X-ray diffraction (XRD) demonstrate that the P3-type structure is well retained even when charged to 4.4 V. Furthermore, the structural characterization by XRD Rietveld refinement, scanning electron microscopy, and electrochemical testing certifies that the cathode maintains its structure commendably even when soaked in water for 12 h. In particular, the P3- Na_2/3Ni_1/4Mg_1/12Mn_2/3O₂∥hard carbon full battery exhibits a desired competitively high voltage of 3.45 V and an attractive energy density of up to 412.2 W h kg^-1 based on the cathode. The comprehensive results achieved by the specially designed strategy provide guidance toward the exploration of stable cathodes in the application of SIBs as modern energy-storage devices.

Effects of Particle Size on Mg2+ Ion Intercalation into λ-MnO2 Cathode Materials

An emergent theme in mono- and multivalent ion batteries is to utilize nanoparticles (NPs) as electrode materials based on the phenomenological observations that their short ion diffusion length and large electrode-electrolyte interface can lead to improved ion insertion kinetics compared to their bulk counterparts. However, the understanding of how the NP size fundamentally relates to their electrochemical behaviors (e.g., charge storage mechanism, phase transition associated with ion insertion) is still primitive. Here, we employ spinel λ-MnO₂ particles as a model cathode material, which have effective Mg²⁺ ion intercalation but with their size effect poorly understood to investigate their operating mechanism via a suite of electrochemical and structural characterizations. We prepare two differently sized samples, the small nanoscopic λ-MnO₂ particles (81 ± 25 nm) and big micron-sized ones (814 ± 207 nm) via postsynthesis size-selection. Analysis of the charge storage mechanisms shows that the stored charge from Mg²⁺ ion intercalation dominates in both systems and is ∼10 times higher in small particles than that in the big ones. From both X-ray diffraction and atomic-resolution scanning transmission electron microscopy imaging, we reveal a fundamental difference in phase transition of the differently sized particles during Mg²⁺ ion intercalation: the small NPs undergo a solid-solution-like phase transition which minimizes lattice mismatch and energy penalty for accommodating new phases, whereas the big particles follow conventional multiphase transformation. We show that this pathway difference is related to the improved electrochemical performance (e.g., rate capability, cycling performance) of small particles over the big ones which provides important insights in encoding within the particle dimension, that is, the single-phase transition pathway in high-performance electrode materials for multivalent ion batteries.

Ultralong Layered NaCrO2 Nanowires: A Competitive Wide-Temperature-Operating Cathode for Extraordinary High-Rate Sodium-Ion Batteries

The development of high-rate cathodes particularly with remarkable wide-temperature-tolerance sodium-storage capability plays a significant role in commercial applications of sodium-ion batteries (SIBs). Herein, we devise a scaled-up electrospinning avenue to fabricate nanocrystal-constructed ultralong layered NaCrO₂ nanowires (NWs) toward SIBs as a wide-temperature-operating cathode. The resultant one-dimensional (1D) NaCrO₂ nanoarchitecture is endowed with orientated and shortened electronic/ionic transport and remarkable structural tolerance to stress change over sodiation-desodiation processes. Benefiting from these structural superiorities, the NaCrO₂ NWs are featured with prominent Na⁺-storage behaviors in the wide-operating-temperature range from -15 to 55 °C. Promisingly, the NaCrO₂ NWs exhibit extraordinary high-rate capacities of ∼108.8 and ∼87.2 mAh g^-1 at 10 and 50 C rates at 25 °C, and even 94.6 (55 °C) and ∼60.1 (-15 °C) mAh g^-1 at 10 C, along with outstanding cyclic stabilities with capacity retentions of ∼80.6% (-15 °C), 88.4% (25 °C), and ∼86.9% (55 °C). The overall performance of our NaCrO₂ is superior to other reported NaCrO₂-based cathodes, even with conductive nanocarbon coating. Encouragingly, a competitive energy density of ∼161 Wh kg^-1 can be obtained by the NaCrO₂ NWs-based full cell. Therefore, our NaCrO₂ NWs can be highly anticipated as advanced cathode for commercial wide-temperature-tolerance SIBs.

A Multi-Ion Strategy towards Rechargeable Sodium-Ion Full Batteries with High Working Voltage and Rate Capability

Sodium-ion batteries (SIBs) are a promising alternative for the large-scale energy storage owing to the natural abundance of sodium. However, the practical application of SIBs is still hindered by the low working voltage, poor rate performance, and insufficient cycling stability. A sodium-ion based full battery using a multi-ion design is now presented. The optimized full batteries delivered a high working voltage of about 4.0 V, which is the best result of reported sodium-ion full batteries. Moreover, this multi-ion battery exhibited good rate performance up to 30 C and a high capacity retention of 95 % over 500 cycles at 5 C. Although the electrochemical performance of this multi-ion battery may be further enhanced via optimizing electrolyte and electrode materials for example, the results presented clearly indicate the feasibility of this multi-ion strategy to improve the electrochemical performance of SIBs for possible energy storage applications.

Conductivity and Pseudocapacitance Optimization of Bimetallic Antimony-Indium Sulfide Anodes for Sodium-Ion Batteries with Favorable Kinetics

Metal sulfides show promise for use in alkali-ion batteries because of their high theoretical capacities. However, their poor cycling stability and rate performance hinder their further development. To avoid these issues, In2S3 into Sb2S3 is introduced to improve its electrochemical properties by optimizing its crystal structure and sodium storage mechanism. A heterostructure composed of In2S3 and Sb2S3 shows a unique morphology of formicary microspheres, which provide abundant channels for fast transfer of sodium ions, large surface area for a high pseudocapacitance effect, and enough voids to relieve volume expansion. A sodium-ion battery containing the bimetallic sulfide anode exhibits a high reversible capacity of 400 mA h g-1 and long cycle life of about 1000 cycles. Similarly, a high capacity of ≈610 mA h g-1 is achieved for a lithium-ion battery containing the anode. During sodiation/desodiation, the synergistic effect of In2S3 and Sb2S3 enhances electronic conductivity and supports the host structure, preventing collapse. The cycling performance and rate performance of the In2S3-Sb2S3 anode are further improved by wrapping the electrode with carbon nanotubes. Even at a high current density of 3.2 A g-1, this carbon composite structure still shows a capacity of about 355 mA h g-1.

Sodium Ion Capacitor Using Pseudocapacitive Layered Ferric Vanadate Nanosheets Cathode

Sodium ion capacitors (SICs) are designed to deliver both high energy and power densities at low cost. Electric double-layer capacitive cathodes are typically used in these devices, but they lead to very limited capacity. Herein, we apply a pseudocapacitive layered ferric vanadate (Fe-V-O) as cathode to construct non-aqueous SICs with both high energy and power densities. The Fe-V-O nanosheets cathode displays remarkable rate capability and cycling stability. The pseudocapacitive sodium storage mechanism of Fe-V-O, with over 83% of total capacity from capacitive contribution, is confirmed by kinetics analysis and ex situ characterizations. The capacitive-adsorption mechanism of hard carbon (HC) anode is demonstrated, and it delivers excellent rate capability. Based on as-synthesized materials, the assembled HC//Fe-V-O SIC delivers a maximum energy density of 194 Wh kg-1 and power density of 3,942 W kg-1. Our work highlights the advantages of pseudocapacitive cathodes for achieving both high energy and power densities in sodium storage devices.

Nonhierarchical Heterostructured Fe2 O3 /Mn2 O3 Porous Hollow Spheres for Enhanced Lithium Storage

High capacity transition-metal oxides play significant roles as battery anodes benefiting from their tunable redox chemistry, low cost, and environmental friendliness. However, the application of these conversion-type electrodes is hampered by inherent large volume variation and poor kinetics. Here, a binary metal oxide prototype, denoted as nonhierarchical heterostructured Fe₂ O₃ /Mn₂ O₃ porous hollow spheres, is proposed through a one-pot self-assembly method. Beyond conventional heteromaterial, Fe₂ O₃ /Mn₂ O₃ based on the interface of (104)_Fe2O3 and (222)_Mn2O3 exhibits the nonhierarchical configuration, where nanosized building blocks are integrated into microsized spheres, leading to the enhanced structural stability and boosted reaction kinetics. With this design, the Fe₂ O₃ /Mn₂ O₃ anode shows a high reversible capacity of 1075 mA h g^-1 at 0.5 A g^-1 , an outstanding rate capability of 638 mA h g^-1 at 8 A g^-1 , and an excellent cyclability with a capacity retention of 89.3% after 600 cycles.

The re-emergence of sodium ion batteries: testing, processing, and manufacturability

With the re-emergence of sodium ion batteries (NIBs), we discuss the reasons for the recent interests in this technology and discuss the synergies between lithium ion battery (LIB) and NIB technologies and the potential for NIB as a "drop-in" technology for LIB manufacturing. The electrochemical testing of sodium materials in sodium metal anode arrangements is reviewed. The performance, stability, and polarization of the sodium in these test cells lead to alternative testing in three-electrode and alternative anode cell configurations. NIB manufacturability is also discussed, together with the impact that the material stability has upon the electrodes and coating. Finally, full-cell NIB technologies are reviewed, and literature proof-of-concept cells give an idea of some of the key differences in the testing protocols of these batteries. For more commercially relevant formats, safety, passive voltage control through cell balancing and cell formation aspects are discussed.

The State and Challenges of Anode Materials Based on Conversion Reactions for Sodium Storage

Sodium-ion batteries (SIBs) have huge potential for applications in large-scale energy storage systems due to their low cost and abundant sources. It is essential to develop new electrode materials for SIBs with high performance in terms of energy density, cycle life, and cost. Metal binary compounds that operate through conversion reactions hold promise as advanced anode materials for sodium storage. This Review highlights the storage mechanisms and advantages of conversion-type anode materials and summarizes their recent development. Although conversion-type anode materials have high theoretical capacities and abundant varieties, they suffer from multiple challenging obstacles to realize commercial applications, such as low reversible capacity, large voltage hysteresis, low initial coulombic efficiency, large volume changes, and low cycling stability. These key challenges are analyzed in this Review, together with emerging strategies to overcome them, including nanostructure and surface engineering, electrolyte optimization, and battery configuration designs. This Review provides pertinent insights into the prospects and challenges for conversion-type anode materials, and will inspire their further study.

Flexible Micro-Supercapacitors Based on Naturally Derived Juglone

Recently, great efforts have been devoted to designing and fabricating flexible, lightweight, wearable, and miniaturized supercapacitors. At the same time, the exploration of green, renewable, and biocompatible energy-storage materials has been attracting intensive attention. By taking fabrication and configuration design into consideration, the naturally derivable juglone molecule was exploited as an active charge-storage material, and integrated into flexible and micro-supercapacitor devices. The polypyrrole/juglone-composite-based supercapacitors exhibit significant energy-storage capabilities with high specific capacitance and long cyclability, which are comparable to that of conventional electrode materials. This study presents a new way for developing flexible, lightweight, portable, and/or wearable electronic devices with biocompatible and environmentally friendly attributes.

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.

Hard Carbons for Sodium-Ion Battery Anodes: Synthetic Strategies, Material Properties, and Storage Mechanisms

Sodium-ion batteries are attracting much interest due to their potential as viable future alternatives for lithium-ion batteries, in view of the much higher earth abundance of sodium over that of lithium. Although both battery systems have basically similar chemistries, the key celebrated negative electrode in lithium battery, namely, graphite, is unavailable for the sodium-ion battery due to the larger size of the sodium ion. This need is satisfied by "hard carbon", which can internalize the larger sodium ion and has desirable electrochemical properties. Unlike graphite, with its specific layered structure, however, hard carbon occurs in diverse microstructural states. Herein, the relationships between precursor choices, synthetic protocols, microstructural states, and performance features of hard carbon forms in the context of sodium-ion battery applications are elucidated. Derived from the pertinent literature employing classical and modern structural characterization techniques, various issues related to microstructure, morphology, defects, and heteroatom doping are discussed. Finally, an outlook is presented to suggest emerging research directions.

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.