ABSTRACT Among available cathode materials of sodium ion batteries (SIBs), the sodium layered transition metal oxides (Na x TMO 2 ) stand out owing to their high specific capacity and suitable working voltage. However, they are commonly plagued by lattice collapse, irreversible phase transition, and poor air stability, which severely limit their cycle durability and rate capability. To address the persistent challenges of Na x TMO 2 , structural regulation strategies based on the pillar and pinning effects have emerged as effective approaches. The ions serving as pillars in the alkali metal layer can expand the interslab spacing and strengthen interlayer interactions, thus constructing efficient Na + migration pathways and establishing a robust lattice framework. Similarly, the pinning ions in the TM and/or alkali metal layers acting as nails help to stabilize phase configuration, especially in deeply sodiated and desodiated states. Both pillar and pinning effects are extensively employed to optimize the structural stability, phase evolution behavior, and electrochemical properties of Na x TMO 2 . This review systematically summarizes recent advances in Na x TMO 2 regulated by pillar and pinning effects, primarily focusing on their construction approaches and underlying enhancement mechanisms. Finally, we outline the ongoing challenges and future research directions for Na x TMO 2 modified by the pillar or pinning effects.

Sodium-ion batteries (SIBs) are promising next-generation batteries as a sustainable alternative to lithium-ion systems, yet an understanding of the solid electrolyte interphase (SEI) is far from sufficient. Here, we develop a probing approach using redox mediator molecules to characterize subnanometric SEI pores, revealing that Na+ transport occurs through diffusion channels. By electrochemical analysis, differential electrochemical mass spectrometry, and theoretical calculations, the influences of solvent salts on SEI architecture have been studied. These findings offer fundamental knowledge beyond classical SEI models and provide both a powerful characterization tool and principles for electrolyte choice for SIBs.

Limited cycle life remains a major obstacle to the practical application of high-capacity alloy negative electrodes in rechargeable batteries aimed at boosting energy density. The key challenges lie in the inherent uncontrollable volume changes and unstable electrode-electrolyte interphases. Here, we demonstrate a long-life self-construal tin (Sn) negative electrode for sodium (Na)-ion batteries enabled by in situ-formed embedded C-N anchors that integrate mechanical and chemical restrictions. This effect reshapes the alloying reactions with pronounced phase transformation hysteresis and triggers an electrochemically driven self-reconstructed structure for alloying reactions, therefore resolving the uncontrollable volume expansion and associated detrimental effects. C-N anchors also promote the formation of unique viscoelastic electrode-electrolyte interphases that comfortably accommodate large volume fluctuations for thousands of cycles. The designed Sn-based negative electrode exhibits long cycle life over 7000 cycles with a low-capacity decay rate of ~0.0036% at 2 C. Stable cycling of the Sn-based negative electrode is further confirmed in a prototype Na-ion pouch cell. This work offers an efficient design of employing the inherent volume expansion of alloy to electrochemically induce a self-constructed structure that comfortably accommodates volume changes, thereby ensuring long cycle life.

Sodium ion batteries are attracting extensive interest due to their low cost and abundant sodium resources. However, sodium ion batteries still suffer fromsevere performance degradation at low temperatures due to the conflict between ion desolvation and diffusion. Herein, we design a co-intercalation ether electrolyte to achieve solvent co-intercalation in thehard carbon negative electrode, thereby bypassing the slow desolvation process while ensuring rapid ion diffusion in electrolyte and hard carbon. The optimized solvation structure also promotes the formation of a thin, inorganic-rich solid electrolyte interface, facilitating interfacial ion transport. As a result, the co-intercalation electrolyte enables hard carbon to deliver good low-temperature performance, with an initial Coulombic efficiency of 80.5% at -50°C (20 mA g-1) and a capacity retention of 93% after 200 cycles (100 mA g-1). Moreover, an Ah-level full cell retains cell stack-level (excluding packaging) specific energy of 163 Wh kg-1 at 25 °C and 107 Wh kg-1 at -50°C (100 mA g-1), demonstrating the practical feasibility of this strategy for wide-temperature sodium ion batteries. This work has the potential to overcome the long-standing trade-off between low-temperature ion desolvation and diffusion, offering an approach for electrolyte design toward wide-temperature sodium ion batteries.

Sodium-ion batteries (SIBs) are promising for large-scale energy storage, owing to their resource abundance and low cost. However, long-term stability is constrained by complex interfacial interactions and microstructural degradation. This study investigates the mechanistic coupling between anode composition, electrolyte chemistry, and solid electrolyte interphase (SEI) evolution in full-cell SIBs employing sodium vanadium phosphate (NVP) cathodes. Pure tin (Sn), hard carbon (HC), and Sn-HC composite anodes were systematically evaluated with carbonate ester- and ether-based electrolytes. Microscopic and spectroscopic analyses reveal that Sn-rich electrodes undergo significant pulverization and unstable SEI formation, whereas HC maintains structural integrity and forms kinetically stable SEI. On the other hand, the Sn-HC composite mitigates Sn's mechanical degradation while enhancing capacity retention. Electrochemical analysis highlights the critical role of electrolyte choice in modulating redox reversibility and interfacial integrity. Accelerating rate calorimetry (ARC) links interphase behavior to distinct thermal decomposition pathways and self-heating rate. These findings provide mechanistic insights into the electro-chemo-mechanical degradation processes dictating the long-term stability and thermal safety of SIBs.

Na4Fe3(PO4)2P2O7 (NFPP) is regarded as a prospective cathode for sodium-ion batteries (SIBs) because of its high structural stability and cost-effectiveness. However, its practical application is hindered by intrinsically low electronic conductivity. Herein, an unconventional electron transfer mechanism from Ni2+ to Fe3+ ions is unveiled in Ni-doped Na4.3Fe3(PO4)2P2O7 (NFPP-Ni) cathode, which facilitates electronic coupling within the Fe-O-Ni coordination unit and thereby effectively boosts electron transport. Moreover, the redox kinetics and reversibility of NFPP materials are predominantly governed by the degree of Fe-O covalency. The intermediate eg occupancy of Fe2+, modulated by the presence of Ni2+, optimizes the overlap between Fe d and O p orbitals. The adjustment of Ni dopant strikes a balance between accelerating Na+ diffusion kinetics and mitigating lattice strain during cycling. As a result, the NFPP-Ni electrode displays impressive rate capacity (121.0 mAh g-1 at 0.1C / 80.9 mAh g-1 at 10C) and stable cyclability (89.1% capacity retention after 1000 cycles). More importantly, the relationship between Fe eg orbital occupancy and Fe-O covalency in NFPP as modulated by various transition metal cations (Ni2+, Mn2+, Zn2+, Co2+ and Cu2+) with different electron configurations are systematically elucidated, thereby providing insights for the commercial development of sodium-ion batteries (SIBs). Tuning the eg orbital occupancy of Fe in Na4.3Fe3(PO4)2P2O7 cathode can effectively optimize the spatial overlap between Fe d and O p orbitals with excellent rate capability for sodium-ion batteries. The eg could be a significant descriptor for Fe-O covalency that describes a volcano curve as a function of eg.

Sulfide-based solid electrolytes are promising for all-solid-state sodium batteries due to their high ionic conductivity and facile processability, but their practical use is limited by moisture sensitivity and poor interfacial stability. To address these issues, Na3−xPS4−xMx (M = F, Cl, Br, I) electrolytes were first synthesized as a preliminary study to evaluate the effect of halogen doping. Chlorine was identified as the most effective dopant and was therefore selected for a systematic investigation of doping concentration. Na3−xPS4−xClx (x = 0.1–0.3) electrolytes were prepared by solid-state sintering, and the optimum composition was determined to be Na2.85PS3.85Cl0.15, which achieved a high ionic conductivity of 5.5 × 10−4 S·cm−1 with a reduced activation energy of 33.3 kJ·mol−1. When employed in TiS2|Na2.85PS3.85Cl0.15|Na3Sn full cells, the optimized electrolyte enabled high initial capacity, excellent rate capability, and stable long-term cycling. These results highlight the effectiveness of Cl doping concentration control in enhancing both the intrinsic properties of Na3PS4-based electrolytes and the overall electrochemical performance of all-solid-state sodium batteries.

Growing concerns over lithium cost and supply limitations have led to increasing interest in sodium-ion batteries (SIBs). However, hard carbon (HC) anodes suffer from low initial Coulombic efficiency due to irreversible sodium loss during the formation of the solid electrolyte interphase and ion trapping, which reduces the useable capacity in full-cell systems. Various sacrificial sodium sources have been investigated, but many generate gas, react with moisture, or degrade the cathode when they are mixed directly with it. In this study, we present a presodiation strategy based on a MnO@NaF composite (MNC) coated onto the cathode-facing side of the separator (MNCS). They are inexpensive, stable in air, and compatible with standard electrode fabrication processes. The MNC releases additional sodium through NaF decomposition catalyzed by MnO with negligible gaseous byproducts. By placing the MNC on the separator rather than on the cathode, the design avoids unwanted reactions while improving sodium availability and ion transport. When applied to a full cell with an O3-type Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2 cathode and HC anode, the MNCS increased the initial discharge capacity to 169.5 mAh g-1 and maintained 69.5% of its capacity after 200 cycles. These results demonstrate the effectiveness of this approach in improving the available energy density and long-term stability in SIBs.

Natrium SuperIonic CONductor (NASICON)-Nb2(PO4)3 is regarded as a potential anode for sodium-ion batteries due to its higher storage capacity (∼150 mAh g- 1) stemming from two-electron Nb5+/Nb4+/Nb3+ redox. However, the reversibility of Nb5+/Nb4+/Nb3+ redox is limited by its structural degradation. Herein, we unveil highly reversible Nb5+/Nb4+/Nb3+ redox reactions in bulk NASICON-NaNbAl(PO4)3 (NaNbAl) anode. The introduction of Na-ions via Al3+ substitution stabilizes the NASICON framework and activates distinct Nb redox couples with Na (de)intercalation, as demonstrated via our density functional theory-based calculations. This bulk anode delivers capacities up to 90 mAh g- 1 at 5C with 86% retention after 1000 cycles. More significantly, we capture rapid sodium ion (de)intercalation in NaNbAl at high current rates using operando synchrotron X-ray diffraction which is in agreement with the calculated low migration barriers associated with Na motion within the structure. A full Na-ion cell (Na4V2(PO4)3||NaNbAl) achieves an energy density of 201 Wh kg- 1 (based on cathode mass) and retains 84% capacity over 200 cycles at 1C. This study opens new avenues for realizing reversible Nb5+/Nb4+/Nb3+ redox in bulk NASICON anodes with suitable chemical substitutions.

The unavailability of high-performance cathodes hinders large-scale adoption of sodium-ion batteries (NIBs). In this work, we report for the first time a Leidenfrost-assisted synthesis as a low-cost and scalable approach for designing In3+-doped mixed phosphate ([PO4]3--[P2O7]4-) cathodes. The strategic substitution of In3+ at the Fe site induces lattice expansion, thereby facilitating enhanced Na+ diffusion and improved electrochemical performance. The optimized cathode composition, Na4Fe2.97In0.03(PO4)2P2O7 (NFIPP03), exhibits exceptional electrochemical performance, with a specific capacity of 129.3 mAhg-1 at 0.05 C, corresponding to an energy density of 359Whkg-1, and a stable cycling performance more than 10000 cycles at 20 C. Temperature-dependent magnetic susceptibility (M-T) and electron paramagnetic resonance (EPR) measurements reveal an enhanced spin state in NFIPP03 compared to the pristine sample, as well as improved electrical conductivity. Ex situ XRD and XPS analyses confirm excellent structural and chemical stability during (de)sodiation. Furthermore, density functional theory (DFT) calculations indicate significantly widened Na+ migration pathways, reduced activation energy barrier, and an attenuated bandgap in NFIPP03 which corroborates our experimental observations. Our findings highlight the synthesis for developing cost-effective, high-performance iron-based mixed-phosphate cathodes, advancing the sustainability and scalability of NIB technology.

Abstract Morphology control is a key design parameter for sodium-ion layered oxide cathodes, yet achieving uniform precursors with balanced stoichiometry is more challenging than in lithium-ion systems due to the broader range of transition-metal chemistries involved. These complexities highlight the need for systematic comparisons of coprecipitation routes tailored to sodium-ion compositions. Here, we examine how ammonia- and citrate-based coprecipitation methods shape the morphology and composition of equimolar Ni–Fe–Mn hydroxide and oxyhydroxide precursors. We investigate how pH, ligand concentration, and temperature jointly influence precipitation onset, particle shape, and metal incorporation. In the ammonia system, precipitation proceeds readily between pH 10.5–11.5, with pH ≈ 11.0 yielding the most uniform morphology and target Ni:Fe:Mn ≈ 1:1:1 stoichiometry. Higher ammonia levels improve morphology but above ~1.0 M begin to delay Ni incorporation and introduce phase separation. In contrast, the citrate system shows delayed precipitation (threshold pH ≈ 11.8) but forms dense granular microspheres with narrow size distributions across 0.1–0.6 M citrate, with Fe enrichment emerging at higher ligand concentrations. X-ray diffraction reveals β-Ni(OH)2-type hydroxides for ammonia-derived precursors and δ-FeOOH-type oxyhydroxides for citrate-derived ones. Together, these results provide practical guidance for tailoring precursor morphology and composition in Fe- and Mn-rich sodium-ion cathode materials.

Layered sodium transition metal oxides (NaxTMO2) are one of the promising cathodes for sodium ion batteries due to the high energy density and advantages of large-scale manufacturing. Recently, P2-type oxides cathodes, as an important family of NaxTMO2, has been paid much attention because the oxygen redox chemistry can further boost the energy density of the cathodes through the special configurations of "Na-O-Li" or "Na-O-Mg". However, these materials suffer the phase transition due to highly depleting Na+ in the structure, when the oxygen redox chemistry is triggered. Past of the decade has witnessed the effective strategies to cope with the phase transition issues through elements doping, but the universal rule behind the elements doping strategies is still a mystery, which is essential to provide guidance to design the advanced cathodes. In this perspective, we present a discussion on phase transitions during charge/discharge process in P2-type oxides, drawing from the research within our group and recent publications. Phase transition process in P2-type cathodes is analyzed through the lens of structural geometry evolution toward intrinsic physical/chemical properties. Governing principles and regulation mechanisms of phase transition are elucidated as well, which can give contributions to the rational design of high-performance P2-type oxide cathodes.

All-solid-state sodium batteries (ASSSBs) stand out as a transformative energy storage technology, combining sodium's natural abundance with enhanced safety and competitive energy density. Solid electrolytes are pivotal to this innovation, with halide electrolytes emerging as prominent candidates due to their unique strengths-superior deformability for intimate electrode contact, strong cathode compatibility, and promising Na-ion conductivity. Despite recent progress, significant challenges persist in scalable synthesis, performance optimization, and mechanistic understanding of ion transport and interfacial interactions. This review comprehensively covers sodium-based halide electrolytes, including their structural chemistry, ion transport, synthesis, modification, electrochemical stability, interfacial behavior, and computational insights. We further integrate a systematic framework to elucidate intricate synthesis-structure-property relationships, enabling a holistic understanding for rational material design. Crucially, this work distinguishes itself by distilling concrete design principles for Na-halide conductors, providing quantitative insights into humidity stability, and establishing in-depth correlations between interphases/degradation modes and full-cell metrics. Moreover, a practical assessment of key performance metrics (energy density, power density, cycle life) and design guidance is presented. Finally, we pinpoint critical barriers (moisture sensitivity, anode incompatibility, and conductivity limitations) and outline a roadmap emphasizing compositional design, interface engineering, manufacturing scalability, machine learning, operando characterization, and standardized metrics to accelerate commercialization.

Cobalt-based bimetallic Prussian blue analogues modified with Selenization and carbon coating as high-performance anodes for sodium-ion batteries.

Regulating local defect energy states of O3 type layered cathode materials for better sodium ion batteries.

Mitigating Prussian blue cathode degradation in sodium-ion batteries through synergistic NaTi₂(PO₄)₃ anode and gel electrolyte design

Nano Cu introduced carbon composite derived from waste toilet paper as a sustainable material for high- performance zinc hybrid supercapacitors and sodium-ion batteries

Electronic modulation with high-valence metal doping towards high-rate Na4Fe3(PO4)2P2O7 cathode in sodium-ion batteries.

Oxygen redox pathway reconstruction via ZrO polarized layer and transition metal vacancies: a dynamic confinement strategy for high-performance sodium-ion battery.

Entropy-driven disordered surface formation with durable anti-water stability for ultra-stable layered oxide cathodes in sodium-ion batteries