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

High-Energy Earth-Abundant Cathodes with Enhanced Cationic/Anionic Redox for Sustainable and Long-Lasting Na-Ion Batteries

Layered iron/manganese-based oxides are a class of promising cathode materials for sustainable batteries due to their high energy densities and earth abundance. However, the stabilization of cationic and anionic redox reactions in these cathodes during cycling at high voltage remain elusive. Here, an electrochemically/thermally stable P2-Na0.67Fe0.3Mn0.5Mg0.1Ti0.1O2 cathode material with zero critical elements is designed for sodium-ion batteries (NIBs) to realize a highly reversible capacity of ≈210 mAh g-1 at 20 mA g-1 and good cycling stability with a capacity retention of 74% after 300 cycles at 200 mA g-1, even when operated with a high charge cut-off voltage of 4.5 V versus sodium metal. Combining a suite of cutting-edge characterizations and computational modeling, it is shown that Mg/Ti co-doping leads to stabilized surface/bulk structure at high voltage and high temperature, and more importantly, enhances cationic/anionic redox reaction reversibility over extended cycles with the suppression of other undesired oxygen activities. This work fundamentally deepens the failure mechanism of Fe/Mn-based layered cathodes and highlights the importance of dopant engineering to achieve high-energy and earth-abundant cathode material for sustainable and long-lasting NIBs.

In Situ Construction of Inorganics-Rich Cathode-Electrolyte Interface toward Long-Life Prussian White Cathode

Prussian white (PW) is one of the most promising candidates as a cathode for sodium-ion batteries (SIBs) because of its high theoretical capacity, excellent rate performance, and low production cost. However, PW materials suffer severe capacity decay during long-term cycling. In this work, a robust cathode electrolyte interface (CEI) is designed on the PW cathode by employing cresyl diphenyl phosphate (CDP) and adiponitrile (ADN) as electrolyte additives. CDP and ADN possess higher highest occupied molecular orbital energy levels (HOMO) than other solvents, leading to the preferential decomposition of CDP and ADN to construct an inorganics-rich CEI layer in situ on the PW cathode. Benefiting from this CEI layer, the degradation of PW is effectively inhibited during the long cycling. The Na||PW cell achieves an excellent cycling performance with a capacity retention of 85.62% after 1400 cycles. This work presented here provides a feasible strategy for improving the cycling performance of PW by electrolyte modification.

Hot-Pressing Enhances Mechanical Strength of PEO Solid Polymer Electrolyte for All-Solid-State Sodium Metal Batteries

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are widely utilized in all-solid-state sodium metal batteries (ASSSMBs) due to their excellent flexibility and safety. However, poor ionic conductivity and mechanical strength limit its development. In this work, an emerging solvent-free hot-pressing method is used to prepare mechanically robust PEO-based SPE, while sodium superionic conductors Na3 Zr2 Si2 PO12 (NZSP) and NaClO4 are introduced to improve ionic conductivity. The as-prepared electrolyte exhibits a high ionic conductivity of 4.42 × 10-4 S cm-1 and a suitable electrochemical stability window (4.5 V vs Na/Na+ ). Furthermore, the SPE enables intimate contact with the electrode. The Na||Na3 V2 (PO4 )3 @C ASSSMB delivers a high-capacity retention of 97.1% after 100 cycles at 0.5 C and 60 °C, and exhibits excellent Coulombic efficiency (CE) (close to 100%). The ASSSMB with the 20 µm thick electrolyte also demonstrates excellent cyclic stability. This study provides a promising strategy for designing stable polymer-ceramic composite electrolyte membranes through hot-pressing to realize high-energy-density sodium metal batteries.

High-Efficiency Separator Capacity-Compensation Strategy Applied to Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are expected to replace partial reliance on lithium-ion batteries (LIBs) in the field of large-scale energy storage as well as low-speed electric vehicles due to the abundance, wide distribution, and easy availability of sodium metal. Unfortunately, a certain amount of sodium ions are irreversibly trapped in the solid electrolyte interface (SEI) layer during the initial charging process, causing the initial capacity loss (ICL) of the SIBs. A separator capacity-compensation strategy is proposed, where the capacity compensator on the separator oxidizes below the high cut-off voltage of the cathode to provide additional sodium ions. This strategy shows attractive advantages, including adaptability to current production processes, no impairment of cell long-cycle life, controlled pre-sodiation degree, and strategy universality. The separator capacity-compensation strategy is applied in the NaNi1/3 Fe1/3 Mn1/3 O2 (NMFO)||HC full cell and achieve a compensated capacity ratio of 18.2%. In the Na3 V2 (PO4 )3 (NVP)||HC full cell, the initial reversible specific capacity is increased from 61.0 mAh g-1 to 83.1 mAh g-1 . The separator capacity-compensation strategy is proven to be universal and provides a new perspective to enhance the energy density of SIBs.

Research progress of organic liquid electrolyte for sodium ion battery

Electrochemical energy storage technology has attracted widespread attention due to its low cost and high energy efficiency in recent years. Among the electrochemical energy storage technologies, sodium ion batteries have been widely focused due to the advantages of abundant sodium resources, low price and similar properties to lithium. In the basic structure of sodium ion battery, the electrolyte determines the electrochemical window and electrochemical performance of the battery, controls the properties of the electrode/electrolyte interface, and affects the safety of sodium ion batteries. Organic liquid electrolytes are widely used because of their low viscosity, high dielectric constant, and compatibility with common cathodes and anodes. However, there are problems such as low oxidation potential, high flammability and safety hazards. Therefore, the development of novel, low-cost, high-performance organic liquid electrolytes is essential for the commercial application of sodium ion batteries. In this paper, the basic requirements and main classifications of organic liquid electrolytes for sodium ion batteries have been introduced. The current research status of organic liquid electrolytes for sodium ion batteries has been highlighted, including compatibility with various types of electrodes and electrochemical properties such as multiplicative performance and cycling performance of electrode materials in electrolytes. The composition, formation mechanism and regulation strategies of interfacial films have been explained. Finally, the development trends of sodium ion battery electrolytes in terms of compatibility with materials, safety and stable interfacial film formation are pointed out in the future.

Injecting Excess Na into a P2-Type Layered Oxide Cathode to Achieve Presodiation in a Na-Ion Full Cell

The initial Na loss limits the theoretical specific capacity of cathodes in Na-ion full cell applications, especially for Na-deficient P2-type cathodes. In this study, we propose a presodiation strategy for cathodes to compensate for the initial Na loss in Na-ion full cells, resulting in a higher specific capacity and a higher energy density. By employing an electrochemical presodiation approach, we inject 0.32 excess active Na into P2-type Na_0.67Li_0.1Fe_0.37Mn_0.53O₂ (NLFMO), aiming to compensate for the initial Na loss in hard carbon (HC) and the inherent Na deficiency of NLFMO. The structure of the NLFMO cathode converts from P2 to P'2 upon active Na injection, without affecting subsequent cycles. As a result, the HC||NLFMO_preNa full cell exhibits a specific capacity of 125 mAh/g, surpassing the value of 61 mAh/g of the HC||NLFMO full cell without presodiation due to the injected active Na. Moreover, the presodiation effect can be achieved through other engineering approaches (e.g., Na-metal contact), suggesting the scalability of this methodology.

Sustainable Enhanced Sodium-Ion Storage at Subzero Temperature with LiF Integration

Though layered sodium oxide materials are identified as promising cathodes in sodium-ion batteries, biphasic P3/O3 depicts improved electrochemical performance and structural stability. Herein, a coexistent P3/O3 biphasic cathode material was synthesized with "LiF" integration, verified with X-ray diffraction and Rietveld refinement analysis. Furthermore, the presence of Li and F was deduced by inductively coupled plasma-optical emission spectrometry (ICP-OES) and energy dispersive X-ray spectroscopy (EDS). The biphasic P3/O3 cathode displayed an excellent capacity retention of 85% after 100 cycles (0.2C/30 mA g^-1) at room temperature and 94% at -20 °C after 100 cycles (0.1C/15 mA g^-1) with superior rate capability as compared to the pristine cathode. Furthermore, a full cell comprising a hard carbon anode and a biphasic cathode with 1 M NaPF₆ electrolyte displayed excellent cyclic stabilities at a wider temperature range of -20 to 50 °C (with the energy density of 151.48 Wh kg^-1) due to the enhanced structural stability, alleviated Jahn-Teller distortions, and rapid Na⁺ kinetics facilitating Na⁺ motion at various temperatures in sodium-ion batteries. The detailed post-characterization studies revealed that the incorporation of LiF accounts for facile Na⁺ kinetics, boosting the overall Na storage.

Zinc Single-Atom-Regulated Hard Carbons for High-Rate and Low-Temperature Sodium-Ion Batteries

Hard carbons, as one of the most commercializable anode materials for sodium-ion batteries (SIBs), have to deal with the trade-off between the rate capability and specific capacity or initial Columbic efficiency (ICE), and the fast performance decline at low temperature (LT) remains poorly understood. Here, a comprehensive regulation on the interfacial/bulk electrochemistry of hard carbons through atomic Zn doping is reported, which demonstrates a record-high reversible capacity (546 mAh g^-1 ), decent ICE (84%), remarkable rate capability (140 mAh g^-1 @ 50 A g^-1 ), and excellent LT capacity (443 mAh g^-1 @ -40 °C), outperforming the state-of-the-art literature. This work reveals that the Zn doping can generally induce a local electric field to enable fast bulk Na⁺ transportation, and meanwhile catalyze the decomposition of NaPF₆ to form a robust inorganic-rich solid-electrolyte interphase, which elaborates the underlying origin of the boosted electrochemical performance. Importantly, distinguished from room temperature, the intrinsic Na⁺ migration/desolvation ability of the electrolyte is disclosed to be the crucial rate-determining factors for the SIB performance at LT. This work provides a fundamental understanding on the charge-storage kinetics at varied temperatures.

Native lattice strain induced structural earthquake in sodium layered oxide cathodes

High-voltage operation is essential for the energy and power densities of battery cathode materials, but its stabilization remains a universal challenge. To date, the degradation origin has been mostly attributed to cycling-initiated structural deformation while the effect of native crystallographic defects induced during the sophisticated synthesis process has been significantly overlooked. Here, using in situ synchrotron X-ray probes and advanced transmission electron microscopy to probe the solid-state synthesis and charge/discharge process of sodium layered oxide cathodes, we reveal that quenching-induced native lattice strain plays an overwhelming role in the catastrophic capacity degradation of sodium layered cathodes, which runs counter to conventional perception—phase transition and cathode interfacial reactions. We observe that the spontaneous relaxation of native lattice strain is responsible for the structural earthquake (e.g., dislocation, stacking faults and fragmentation) of sodium layered cathodes during cycling, which is unexpectedly not regulated by the voltage window but is strongly coupled with charge/discharge temperature and rate. Our findings resolve the controversial understanding on the degradation origin of cathode materials and highlight the importance of eliminating intrinsic crystallographic defects to guarantee superior cycling stability at high voltages.

Native crystallographic defects are often introduced during synthesis of battery materials, but has been overlooked. Here, using in situ synchrotron X-ray probes and electron microscopy, the authors have revealed their adverse effect during battery operation.

Electronic Effect and Regiochemistry of Substitution in Pre-sodiation Chemistry

The low oxidation potential of a pre-sodiation cathode additive intrinsically prevents decomposition of the electrolyte. Although the introduction of electron-donating substitution reduces the oxidation potential, the additional molecular weight restricts the output capacity. Herein, as theroretically predicted, the electrochemical oxidation potential of sodium carboxylate is manipulated by the electronic effect and regiochemistry of the functionality, in which the stronger electron-donating substituent, p-π conjugation, and optimized regiochemistry can dramatically lead to the lower potential originated from the elevation of the highest occupied molecular orbital level. Thus, benefiting from the para-NH₂ unit accompanied by a conjugated aromatic architecture, molecularly engineered sodium para-aminobenzoate (PABZ-Na) presents a reduced oxidation plateau of 3.45 V. Triggered by the positive compensation merit, sodium-based electrochemical storage systems manifest excellent electrochemical performances. This breakthrough sheds light into the correlation between the electronic effect of the functional group and the oxidation potential of the organic additive, affording in-depth insights into the fundamental guidance of pre-sodiation chemistry.

Computational Auxiliary for the Progress of Sodium-Ion Solid-State Electrolytes

All-solid-state sodium batteries (ASSBs) have attracted ever-increasing attention due to their enhanced safety, high energy density, and the abundance of raw materials. One of the remaining key issues for the practical ASSB is the lack of good superionic and electrochemical stable solid-state electrolytes (SEs). Design and manufacturing specific functional materials used as high-performance SEs require an in-depth understanding of the transport mechanisms and electrochemical properties of fast sodium-ion conductors on an atomic level. On account of the continuous progress and development of computing and programming techniques, the advanced computational tools provide a powerful and convenient approach to exploit particular functional materials to achieve that aim. Herein, this review primarily focuses on the advanced computational methods and ion migration mechanisms of SEs. Second, we overview the recent progress on state-of-the-art solid sodium-ion conductors, including Na-β-alumina, sulfide-type, NASICON-type, and antiperovskite-type sodium-ion SEs. Finally, we outline the current challenges and future opportunities. Particularly, this review highlights the contributions of the computational studies and their complementarity with experiments in accelerating the study progress of high-performance sodium-ion SEs for ASSBs.

Molecular grafting towards high-fraction active nanodots implanted in N-doped carbon for sodium dual-ion batteries

Sodium-based dual-ion batteries (Na-DIBs) show a promising potential for large-scale energy storage applications due to the merits of environmental friendliness and low cost. However, Na-DIBs are generally subject to poor rate capability and cycling stability for the lack of suitable anodes to accommodate large Na+ ions. Herein, we propose a molecular grafting strategy to in situ synthesize tin pyrophosphate nanodots implanted in N-doped carbon matrix (SnP2O7@N-C), which exhibits a high fraction of active SnP2O7 up to 95.6 wt% and a low content of N-doped carbon (4.4 wt%) as the conductive framework. As a result, this anode delivers a high specific capacity ∼400 mAh g-1 at 0.1 A g-1, excellent rate capability up to 5.0 A g-1 and excellent cycling stability with a capacity retention of 92% after 1200 cycles under a current density of 1.5 A g-1. Further, pairing this anode with an environmentally friendly KS6 graphite cathode yields a SnP2O7@N-C||KS6 Na-DIB, exhibiting an excellent rate capability up to 30 C, good fast-charge/slow-discharge performance and long-term cycling life with a capacity retention of ∼96% after 1000 cycles at 20 C. This study provides a feasible strategy to develop high-performance anodes with high-fraction active materials for Na-based energy storage applications.

Effects of Comonomers on the Performance of Stable Phosphonate-Based Gel Terpolymer Electrolytes for Sodium-Ion Batteries with Ultralong Cycling Stability

Gel polymer electrolyte (GPE) is one of the most promising alternatives to solve the bottlenecks of nonaqueous liquid electrolytes such as decomposition, safety hazards, and growth of dendrites. In this work, three novel methyl phosphonate-based crosslinking gel terpolymer electrolytes with different comonomers are designed and prepared by in situ radical polymerization. The gel polymer electrolytes have excellent thermal stability, wide electrochemical windows (≥4.9 V), and high ionic conductivities (±3 mS cm^-1), and may be used as less-flammable electrolytes for sodium-ion batteries. ³¹P NMR spectra, Arrhenius plot, and density functional theory (DFT) calculations confirm that multifunctional phosphonate structural units promote the dissociation of NaClO₄ and help to transport the sodium ions freely in the polymer framework. X-ray photoelectron spectroscopy (XPS) results show that the gel polymer electrolytes have the capability of inhibiting liquid electrolyte decomposition and the formation of the stable solid electrolyte interphase (SEI) film. The Na₃V₂(PO₄)₃/GPE/Na cells exhibit better ultralong cycling stability and enhanced temperature performance than those of liquid cells. Strikingly, GPE1 has the best comprehensive electrochemical performance, especially the rate performance and long-term cycling stability with a capacity retention ratio of 82.6% after 3500 cycles, which indicates that different comonomers have obvious effects on the performance. Therefore, the full cell of SnS₂/GPE1/Na₃V₂(PO₄)₃ is evaluated and delivers good cycling stability of 500 cycles, holding a great prospect for the design and production of phosphorus-containing electrolytes for safer sodium-ion batteries.

Carbon-Decorated Na3V2(PO4)3 as Ultralong Lifespan Cathodes for High-Energy-Density Symmetric Sodium-Ion Batteries

In this work, several carbon-decorated Na₃V₂(PO₄)₃ materials (NVP@C-750/800/850) are successfully fabricated using a sol-gel approach and subsequent heat treatment. When NVP@C-800 is used as a cathode, it shows an ultralong cycle life (2000 cycles) at a high rate of 10C, which is superior to the other two electrodes and those of reported NVP@C cathodes in the literature. The excellent results of NVP@C-800 are attributed to its nanostructure and the well-defined conductive carbon layer. The symmetric sodium (Na)-ion battery (SIB) with NVP@C-800 as both a cathode and an anode shows a high capacity at 40 mA g^-1 with a voltage plateau of about 1.79 V and energy density of 113 W h kg^-1, revealing that NVP@C is of great application prospect.

Fluorine Dissolution-Induced Capacity Degradation for Fluorophosphate-Based Cathode Materials

Na₃V₂(PO₄)₂F₃ has been considered as a promising cathode material for sodium-ion batteries due to its high operating voltage and structural stability. However, the issues about poor cycling performance and lack of understanding for the capacity degradation mechanism are the major hurdle for practical application. Herein, we meticulously analyzed the evolution of the morphology, crystal structure, and bonding states of the cathode material during the cycling process. We observed that capacity degradation is closely related to the shedding of the active material from the collector caused by HF corrosion. Meanwhile, HF is produced through F anion dissolution from Na₃V₂(PO₄)₂F₃ induced by trace H₂O during the cycling process. The F^- dissolution-induced degradation mechanism based on fluorine-containing cathode materials is proposed for the first time, providing a new insight for the understanding, modification, and performance improvement for fluorophosphate-based cathode materials.

Gelation of organic liquid electrolyte to achieve superior sodium-ion full-cells

The irreversible consumption of active sodium in sodium-ion full-cells (SIFCs) becomes particularly serious due to the existence of unavoidable interface or side reaction, which has become the key to restrict the development of high-performance sodium-ion batteries (SIBs). Interface design and electrolyte optimization have been proved to be effective strategies to improve or solve this problem. In this work, on the basis of traditional organic liquid electrolytes, a novel gel polymer electrolyte with high ionic conductivity (1.13 × 10^-3 S cm^-1) and wide electrochemical stability window (~4.7 V) was designed and synthesized using bacterial cellulose film as precursor. Compared with the liquid electrolyte, the obtained electrolyte can endow better sodium storage performance in both half- and full-cells. When coupled with sodium hexacyanoferrate cathode and hard carbon anode, a capacity of 94.2 mA h g^-1 can be obtained with a capacity retention of 75% after 100 cycles at a current density of 100 mA g^-1, while those of with conventional liquid electrolyte can deliver a capacity of 99.0 mA h g^-1 but only accompany 58% capacity retention under the same conditions. Significantly, when the current density increases to 800 mA g^-1, their capacity difference reaches 23.4 mA h g^-1.

Self-Template Synthesis of NaCrO2 Submicrospheres for Stable Sodium Storage

Sodium-ion batteries (SIBs) are the appropriate alternatives to lithium-ion batteries (LIBs) for the large-scale energy storage applications because of the abundant resources and wide distribution of sodium on earth. O3-NaCrO₂ is a promising cathode material for SIBs due to its stable structure and low-cost raw materials. In this paper, we design and synthesize a powder consisting of submicrometer-sized O3-NaCrO₂ spheres (s-NaCrO₂) self-assembled with nanoflakes, which exhibits faster ion migration ability and strong structure robustness. The galvanostatic intermittent titration technique test reveals the higher apparent Na⁺ diffusion coefficient of s-NaCrO₂ when compared with a normal NaCrO₂ powder with an irregular particle morphology_. The s-NaCrO₂ shows impressive electrochemical properties with a capacity of 90 mAh g^-1 at 50 C. In addition, outstanding cycling stability is shown when tested at 20 C, where a capacity of 90 mAh g^-1 is maintained with a retention of 87% after 1500 cycles. Also, s-NaCrO₂ is advantageous at high (50 °C) and low (-10 °C) temperatures. The full cells assembled employing Sb/C as the anode exhibit good rate capability with 85 mAh g^-1 obtained at 50 C.

Regulate Phosphorus Configuration in High P-Doped Hard Carbon as a Superanode for Sodium Storage

Heteroatom-doped hard carbon is a popular method to optimize the electrochemical performance of anode electrodes for sodium-ion batteries. Herein, phosphorus-doped hollow carbon nanorods (P-HCNs) are obtained by a one-step synthesis with a high phosphorus content of 7.5 atom %. By controlling the P configuration, the P-HCNs03 exhibits reversible capacity as high as 260 mA h g^-1 at the current density of 1.0 A g^-1 after 500 cycles with an initial Coulombic efficiency (ICE) of 73%. When the amount of phosphorus in the as-prepared materials is changed, the different structures of the P-doped carbon lattices are analyzed by X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. Based on the first-principles calculation, although the P-O bond has the most configurations, the excellent reversible capacity of the electrode is attributed to the strong Na-absorption ability of P═O and P-C bonds. The sodium-based dual-ion batteries (NDIBs) assembled with P-HCNs03 as an anode and expanded graphite as a cathode (P-HCNs03//EG) exhibited a high energy density of 138 W h kg^-1 at a power density of 159 W kg^-1. The results provide an important angle to optimize the performance of hard carbons with other functionalized heteroatoms.

Sponge-like NaFe2PO4(SO4)2@rGO as a high-performance cathode material for sodium-ion batteries

Sponge-like NaFe₂PO₄(SO₄)₂@reduced graphene oxide composite is prepared as cathode material for sodium-ion batteries. The corresponding full cells matched with hard carbon anode exhibit favorable rate and cyclic performance.

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.

Superior Na-Storage Properties of Nickel-Substituted Na2FeSiO4@C Microspheres Encapsulated with the In Situ-Synthesized Alveolation-like Carbon Matrix

The poor electronic conductivity of Na₂FeSiO₄ always limits its electrochemical reactivities and no effective solution has been found to date. Herein, the novel Ni-substituted Na₂Fe_1-xNi_xSiO₄@C nanospheres (50-100 nm) encapsulated with a 3D hierarchical porous skeleton (named as alveolation-like configuration) constructed using in situ carbon are first synthesized via a facile sol-gel method, and the effects of Ni substitution combined with the design of a unique carbon network on Na-storage properties are assessed systematically, focusing on alleviating the inherent defects of the Na₂FeSiO₄ cathode material. A series of characterization technologies such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and so forth, coupled with the electrochemical measurements and first-principles calculations, are used to explore the structure, morphology and electrochemical behaviors of the as-prepared materials. The results show that the synergism between Ni substitution and the special alveolation-like configuration enables fast Na ions mobility (from 10^-14 to 10^-12 cm² s^-1), reduces band gap energy (from 2.82 to 1.79 eV) and lowers Na-ion diffusion barriers, finally reciprocating the vigorous electrochemical kinetics of the electrode. Especially, the elaborately designed material-Na₂Fe_0.97Ni_0.03SiO₄@C-displays superior Na-storage properties of around 197.51 mA h g^-1 (corresponding to 1.43 Na⁺ intercalation) at 0.1 C within 1.5-4.5 V along with desirable capacity retention (84.44% after 100 cycles), and the rate capability is also markedly enhanced (a capacity of 133.62 mA h g^-1 at 2 C). Such the effective methodology employed in this work opens a potential pathway to synthesize the silicate cathode material with excellent electrochemical properties.

High-Efficiency Cathode Sodium Compensation for Sodium-Ion Batteries

Sodium-ion batteries have gained much attention for their potential application in large-scale stationary energy storage due to the low cost and abundant sodium sources in the earth. However, the electrochemical performance of sodium-ion full cells (SIFCs) suffers severely from the irreversible consumption of sodium ions of cathode during the solid electrolyte interphase (SEI) formation of hard carbon anode. Here, a high-efficiency cathode sodiation compensation reagent, sodium oxalate (Na₂ C₂ O₄ ), which possesses both a high theoretical capacity of 400 mA h g^-1 and a capacity utilization as high as 99%, is proposed. The implementation of Na₂ C₂ O₄ as sacrificial sodium species is successfully realized by decreasing its oxidation potential from 4.41 to 3.97 V through tuning conductive additives with different physicochemical features, and the corresponding mechanism of oxidation potential manipulation is analyzed. Electrochemical results show that in the full cell based on a hard carbon anode and a P2-Na_2/3 Ni_1/3 Mn_1/3 Ti_1/3 O₂ cathode with Na₂ C₂ O₄ as a sodium reservoir to compensate for sodium loss during SEI formation, the capacity retention is increased from 63% to 85% after 200 cycles and the energy density is improved from 129.2 to 172.6 W h kg^-1 . This work can provide a new avenue for accelerating the development of SIFCs.

Structural and Thermodynamic Understandings in Mn-Based Sodium Layered Oxides during Anionic Redox

A breakthrough utilizing an anionic redox reaction (O2-/On-) for charge compensation has led to the development of high-energy cathode materials in sodium-ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2-type Mn-based compound exhibits a distinct two-phase behavior preserving a high-potential anionic redox (≈4.2 V vs Na+/Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p-electron and the reversible unmixing of Na-rich and Na-poor phases are confirmed in detail. In light of the combined study, a critical role of the anion-redox-induced two-phase reaction in the positive-negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high-voltage feature in Mn-based layered cathode materials that are charge-compensated by an anionic redox reaction.

Symmetric Sodium-Ion Battery Based on Dual-Electron Reactions of NASICON-Structured Na3MnTi(PO4)3 Material

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na₃MnTi(PO₄)₃ material. The Na₃MnTi(PO₄)₃ electrode can deliver a stable capacity of up to 160 mAh g^-1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti^3+/4+, Mn^2+/3+, and Mn^3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na₃MnTi(PO₄)₃ as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g^-1 at 0.1 C and an energy density of 52 Wh kg^-1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.

High-performance sodium-ion batteries with a hard carbon anode: transition from the half-cell to full-cell perspective

Hard carbon is an appealing anode material for sodium-ion batteries (SIBs) due to renewable resources, low cost and high specific capacity. Practical full cells based on hard carbon with high energy density and long cyclability are expected to possess application interest for grid-scale energy storage. In this review, following this archetypal use scenario of SIBs, we aim at providing a quantitative full-cell metric for evaluating newly designed anodes or cathodes. Some significant problems in conventional half-cell and full-cell tests, including unfaithful prediction of capacity loss by coulombic efficiency in the full-cell and under-estimated capacity of hard carbon in the half-cell test, are discussed to better assess the actual capacity and cyclability of the hard carbon anode in sodium-matched full cells. Finally, we review rational design of hard carbon itself and the selection of electrolytes from such a full-cell perspective.

Presodiation Strategies and Their Effect on Electrode-Electrolyte Interphases for High-Performance Electrodes for Sodium-Ion Batteries

Most active materials for sodium-ion batteries suffer from the problem of low-energy efficiency in the first cycle because of the loss of active sodium ions consumed for the formation of a solid electrolyte interface. To make up for the lost sodium ion, presodiation treatments have been applied, which are effective ways to mitigate the low initial efficiency. Here, we developed a direct-contact method to achieve the presodiation for cathode and anode electrodes and demonstrated the enhanced Coulombic efficiency of the first cycle with improved cyclability and reversible capacity. Moreover, we proved the formation of a thick passivation layer at the cathode-electrolyte interface during the presodiation process; this contributes to the improved cycle stability by preventing the dissolution of the active material and its deposition on the anode surface. The direct-contact method is a simple and cost-effective way to complete presodiation, and this simple process will be widely applicable for practical battery manufacturing.

A Calcium Organic Salt/rGO composite with Low Solubility and High Conductivity as a Sustainable Anode for Sodium-Ion Batteries

Organic electrodes hold great promise for sustainable electrodes in sodium-ion batteries (SIBs) owing to their easy availability from biomass. However, traditional organic electrodes suffer from two inherent problems, high solubility in organic electrolytes and low electronic conductivity. Here, a calcium organic salt, Cabpdc (bpdc=4,4'-biphenyldicarboxylate) was designed and formed into a composite with reduced graphene oxide (rGO) to improve these two problems by a "two-in-one" approach. As expected, the Cabpdc/rGO composite displayed competitive cycle and rate performances as an anode for SIBs. Additionally, all-organic sodium-ion full cells were successfully fabricated combining this anode with a commercial organic cathode, promising applications for sustainable SIBs.