High-performance Sodium-ion Batteries Research Articles

Integrating Multiple Redox-Active Units into Conductive Covalent Organic Frameworks for High-Performance Sodium-Ion Batteries.

The rational design of porous covalent organic frameworks (COFs) with high conductivity and reversible redox activity is the key to improving their performance in sodium-ion batteries (SIBs). Herein, we report a series of COFs (FPDC-TPA-COF, FPDC-TPB-COF, and FPDC-TPT-COF) based on an organosulfur linker, (trioxocyclohexane-triylidene)tris(dithiole-diylylidene))hexabenzaldehyde (FPDC). These COFs feature two-dimensional crystalline structures, high porosity, good conductivity, and densely packed redox-active sites, making them suitable for energy storage devices. Among them, FPDC-TPT-COF demonstrates a remarkably high specific capacity of 420 mAh g-1 (0.2 A g-1), excellent cycling stability (~87% capacity retention after 3000 cycles, 1.0 A g-1) and high rate performance (339 mAh g-1 at 2.0 A g-1) as an anode for SIBs, surpassing most reported COF-based electrodes. The superior performance is attributed to the dithiole moieties enhancing the conductivity and the presence of redox-active carbonyl, imine, and triazine sites facilitating Na storage. Furthermore, the sodiation mechanism was elucidated through in-situ experiments and density functional theory (DFT) calculations. This work highlights the advantages of integrating multiple functional groups into redox-active COFs for the rational design of efficient and stable SIBs.

Revealing the regulation mechanism of Na3V2(PO4)2O2F crystal growth in sodium alginate solution for high-performance sodium ion batteries

Na3V2(PO4)2O2F (NVPFO) attracts tremendous attention as a cathode material for sodium-ion batteries (SIBs) because of its higher capacity and structural stability. However, its inherent poor conductivity affects rate performance and long cycle performance. Herein, a special lamellar NVPFO are successfully synthesized by sodium alginate (SA) assisted hydrothermal method. SA dissolves in water to form hydrogel, due to the specific cross-linking of SA and V4+, which restricts the crystal growth of nanoparticles. Meanwhile, a conductive carbon layer for sodium ion transport channels is constructed after carbonization. Based on multidimensional experiments and density functional theory (DFT) calculations, a mechanism for formation of special lamellar NVPFO in SA solution is proposed for the first time. The results show that special lamellar morphology shortens Na+ migration path and conductive coating facilitates electronic transmission. The optimized NVPFO@C-2 has better electrochemical performance with outstanding high rate capacity (106.8 mAh g–1 at 15 C) and long-term cyclability (99.8 mAh g−1 capacity retention of 87.1 % even after 1000 cycles at 5 C).

Fluorinated ionic liquid mediated nanostructure with enhanced conductivity and fluorine retention in Na3V2(PO4)2F3 cathode toward high-performance sodium-ion batteries

Na3V2(PO4)2F3 (NVPF) is considering as a promising cathode material for sodium-ion batteries (SIBs), due to its framework robustness and high working voltage. For NVPF cathodes, the main obstacle of poor conductivity could be well-addressed by carbon hybridization or nano-crystallization strategies. However, carbon compositing process would bring up fluorine-loss issue and the designation of tailor-made nanocomposite remains challenging. Here, we realize the optimal regulation on crystal growth by virtue of a fluorinated ionic liquid usage, which could compensate the fluorine-loss during high-temperature annealing process. As a structural-directing agent, ionic liquid with low surface tension also helps to reduce surficial Gibbs free energy of NVPF crystal, facilitating to reduce the particle size. Meanwhile, the π-cation interaction between reduced graphene oxide (rGO) and cations of ionic liquid is beneficial to fabricate ultrathin rGO-wrapped NVPF nanoplates (I-NVPF@rGO). The tailored I-NVPF@rGO exhibits enhanced conductivity and optimized crystal planes, resulting in excellent rate capability and improved energy density. When coupling with MoS2/SnS/rGO anode, the MoS2/SnS/rGO||I-NVPF@rGO full battery delivers a capacity of 116 mA h g−1 at 0.2 C. This strategy offers valuable insights into the development of sodium-ion batteries.

Chloride-Ion-Enriched Solid Electrolyte Interphase with Rapid Na+ Migration toward High-Performance Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have emerged as potential alternatives to lithium-ion batteries (LIBs), particularly for large-scale applications. Alloy-type anode materials for sodium-ion batteries are esteemed as prospective candidate materials for sodium-ion anodes, owing to their elevated theoretical capacity, heightened utilization efficiency, and minimal production of insulating byproducts. However, the severe volume changes and sluggish ion diffusion kinetics can lead to irreversible particle fragmentation and reaggregation phenomena, ultimately resulting in electrode degradation. Additionally, repetitive volume changes can cause an unstable solid electrolyte interphase (SEI). This study presents the synthesis of chloride-ion-modulated bimetallic SnSb/C nanoparticle anode materials, highlighting the following advantages: (i) Designing a bimetallic SnSb alloy structure serves to buffer the structural stresses generated during sodium insertion/extraction processes, effectively mitigating particle fracture phenomena induced by electrode material expansion/contraction. (ii) Nanostructuring both alloy materials enables the full utilization of active materials and shortens diffusion pathways, thereby significantly enhancing the diffusion rate of sodium ions. (iii) Introducing a carbonaceous matrix serves to alleviate self-agglomeration phenomena of the material during charge/discharge cycles, enhancing the material's conductivity and structural stability. (iv) Utilizing chloride-ion interface modification to achieve a chloride-rich solid-electrolyte interphase (SEI) enhances battery performance.

Nanodiamond-Assisted High-Performance Sodium-Ion Batteries with Weakly Solvated Ether Electrolyte at -40°C.

Realizing high performances of sodium-ion batteries (SIBs) working at low temperatures is a pressing need for the commercial applications of SIBs. In this work, nanodiamonds (NDs) are introduced in diglyme electrolytes (ND-Diglyme) to significantly improve the low-temperature performances of SIBs. The corresponding SIB achieves an initial reversible specific capacity of 324mA h g-1 at -40°C (slightly decreased from 357mA h g-1 at 25°C) and shows a capacity retention ratio of ≈82% after 100 cycles at 0.1 A g-1. Moreover, it shows a capacity as high as 40mA h g-1 at 1 A g-1, nearly five times the date of the pure Diglyme electrolyte. Experimentally reveals that introducing NDs is helpful in inhibiting dendrite growth and improving the cyclic stability of anode at LT, because the ND with strong adsorption to sodium ions can not only assist in forming an effective solid electrolyte interface rich with NaF and Na2CO3 but also effectively reduce the activation energy (decreased from 426.68 to 370.51 meV) during the charge transfer processes. Hence, the proposed ND-assisted weakly ether electrolyte in this study presents a viable electrolyte additive solution to fulfill the rising low-temperature demands of SIBs.

N, S, Se-Codoped dual carbon encapsulation and Se substitution in pyrite-type FeS2 for high-rate and long-life sodium-ion batteries

The use of metal sulfides as anodes in sodium-ion batteries (SIBs) faces significant challenges due to slow reaction kinetics and extensive shuttling of sodium polysulfides (NaPSs). In response, a selenium-substituted iron disulfide encapsulated in a nitrogen-sulfur-selenium-codoped dual carbon framework (Se-FeS2@NSSC) has been engineered for high-performance SIBs. Selenium substitution within the FeS2 lattice improves electrical conductivity, facilitates favorable redox kinetics that reducing the possibility of NaPSs generation. Moreover, the dual NSSC encapsulation promotes Na+/electron transport and efficiently manages volume expansion during electrochemical processes. Strong interfacial interaction (Fe-S-C bonding) between Se-FeS2 and NSSC further suppress NaPSs shuttling, significantly alleviating the cell failure and thus prolonging its lifespan. Consequently, Se-FeS2@NSSC demonstrates a top-notch performance, achieving a notable capacity of 725 mAh g-1 at 0.5 A g-1, unparalleled rate capability (355.1 mAh g-1 at 30 A g-1), and sustained cyclic stability (89.6 % capacity is retained after 1250 cycles). Theoretical analyses underscore the critical role of selenium in diminishing bulk stress–strain energies, lowering energy barriers for Na+ diffusion, and decreasing Gibbs free energy for conversion reactions, thereby markedly bolstering electrochemical kinetics and overall performance of Se-FeS2@NSSC. The insights gleaned from this research foster advancements in developing next-generation anodes with high capacity and durability, representing a promising response to the existing hurdles in SIBs technology.

Synthesis of a Sodium-Ion Cathode Material Na3Fe2(SO4)3F with the Help of Fluorine Element.

The development of cathode materials has always been one of the most crucial areas of research in the field of sodium-ion batteries. Sulfate-based polyanionic materials, known for their high working voltage characteristics, have received widespread attention. In this work, a fluoro-sulfate sodium-ion battery cathode material, Na3Fe2(SO4)3F modified with carbon nanotubes, was developed using a low-temperature solid-state annealing method. This Na3Fe2(SO4)3F cathode exhibits an exceptionally high voltage of 3.77 V, excellent discharge capacity (102 mAh/g at 0.1C), and good rate capability. This material broadens the research directions for cathode materials and holds promise as a foundation for the further development of high-performance sodium-ion batteries.

Large scale atomistic and quantum mechanical study of Na+ ion transport in liquid electrolytes for batteries

Lithium–ion batteries (LIBs) have long dominated energy storage markets due to their high energy density and reliability. However, concerns over lithium scarcity and geographic distribution necessitate alternatives. Sodium-ion batteries (SIBs) offer a promising solution due to sodium's abundance, cost-effectiveness, and favorable electrochemical properties. This study investigates Na+ ion transport, aggregation, and performance in various organic electrolytes using molecular dynamics (MD) simulations and density functional theory (DFT) calculations. The electrolytes studied include ethylene carbonate (EC), propylene carbonate (PC), dimethoxyethane (DME), and dimethyl carbonate (DMC) at 320 K. We focused on conductivity, diffusivity, and survival probability of Na+ ions at different salt concentrations. Furthermore, the solvation structure and the binding energy of ions in the electrolytes are thoroughly analyzed. Key findings reveal that at 2 M salt concentration, Na+ ion diffusivity and ionic conductivity follow the order EC>PC>DME>DMC. Increasing salt concentration decreases self–diffusion coefficients of Na+ and PF6− ions across all electrolytes, affecting conductivity. EC shows the highest ionic conductivity at both 1 M and 2 M salt concentrations. These insights suggest that a 2 M NaPF6 concentration in EC optimizes ionic conductivity, making it ideal for high–performance SIBs. This study provides crucial understanding for optimizing electrolytes in SIBs, advancing their development for scalable energy storage solutions.

CoO nanoparticles anchored on porous pinecone-derived activated carbon as anodes for high-performance lithium and sodium-ion batteries

In this work, a new nanocomposite system based on CoO nanoparticles (NPs) anchored on pinecone-derived activated carbon (PAC), denoted as CoO(x)@PAC, is synthesized by facile ball-milling and sonication processes followed by annealing. H3PO4 is employed as an activation agent to activate the porous carbon derived from pinecone. The loading amount of CoO is varied to obtain the optimum electrochemical performances. Among the nanocomposites, the CoO(20)@PAC delivers a discharge capacity of 886 mA h g−1 after 100 cycles at a current density of 100 mA g−1 when employed as an anode in lithium-ion batteries (LIBs). At higher current rate of 1000 mA g−1, the anode provides an enhanced discharge capacity of 456.6 mA h g−1 after 200 cycles with 99 % capacity retention compared to the 4th cycle capacity. In sodium-ion batteries (SIBs), the CoO(20)@PAC anode delivers a discharge capacity of 290 mA h g−1 at 50 mA g−1. The same nanocomposite anode provides a discharge capacity of 120 mA h g−1 after 900 cycles at a current density of 500 mA g−1. The CoO(20)@PAC nanocomposite shows a great potential as an anode for both LIBs and SIBs, due to its higher capacity, exceptional rate capability, longer cycle life, and a Coulombic efficiency exceeding 99 %.

Synergistic effect of in-situ carbon-coated mixed phase iron oxides and 3d electrode architectures as anodes for high-performance sodium-ion batteries

Due to the high theoretical capacity, iron-oxide-based Fe2O3 (1007 mAh g−1) and Fe3O4 (926 mAh g−1) materials can be a promising conversion-type anode material for Sodium-ion batteries (SIBs). However, the low electronic conductivity (10−14 S cm−1) and 200% volume expansion of iron oxide lead to poor cycle life and capacity retention, limiting its commercial application. In this work, carbon-coated mixed-phase iron oxide C-Fe2O3-Fe3O4 (C-IO) was synthesized by pyrolysis using ferrocene precursor. Subsequently, a 3D carbon fiber (CF) network was introduced to improve the electrochemical performance of the material by resolving the volume expansion and pulverization issues. Both the CF network and Fe3O4 provide an electron transport pathway. The conventional Cu-C-IO electrodes deliver a 2nd cycle capacity of 335 mAh g−1 at 250 mA g−1 and exhibit 61.5% capacity retention after 500 cycles. In contrast, the 3D-CF-based IO electrodes show 2nd cycle capacity of 607 mAh g−1 at 50 mA g−1 with 96.4% capacity retention after 35 cycles and 420 mAh g−1 with ∼85.4% capacity retention after 500 cycles at 250 mA g−1 (w.r.t. CF and C-IO active mass, CF contribute ∼ 27% and C-IO ∼ 73% capacity), making it a potential anode for SIBs.

3D MoS2/graphene oxide integrated composite as anode for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are emerging as a promising alternative to conventional lithium-ion technology, due to the abundance of sodium resources. The major drawbacks for the commercial application of SIBs lie in the slow kinetic processes and poor energy density of the devices. Molybdenum sulfide (MoS2), a graphene-like material, is becoming a promising anode material for SIBs, because of its high theoretical capacity (670 mAh g–1) and layered structure that suitable for Na+ intercalation/extraction. However, the intrinsic properties of MoS2, such as low conductivity, slow Na+ diffusion kinetics and large volume change during charging/discharging, restrict its rate capability and cycle stability. Here, molybdenum disulfide and graphene oxide (3D MoS2/GO) with excellent conductivity were fabricated through layer-by-layer method using amino-functionalized SiO2 nanospheres as templates. The 3D MoS2/GO composite demonstrates excellent cycling stability and capacity of 525 mA h g–1 at 500 mA g–1 after 100 cycles, which mainly due to the integrated MoS2/GO components and unique 3D macroporous structure, facilitating the material conductivity and Na+ diffusion rate, while tolerating the volume expansion of MoS2 during the charge/discharge processes.

Preparation and application of high-performance sodium-ion batteries cathode material Na3.6Fe2.6(PO4)1.6P2O7 with optimized sodium-iron ratio

Na4Fe3(PO4)2P2O7 (NFPP) has been developed as the most promising polyanion cathode for sodium-ion batteries due to its excellent electrochemical performance, high safety properties, and environmental friendliness. However, the electrochemically inactive impurity (m-NaFePO4) will inevitably generate during synthesis process, which decreases the practical capacity of NFPP significantly. In this paper, a novel Na3.6Fe2.6(PO4)1.6P2O7 cathode material with excellent performance is successfully exploited by optimizing sodium-iron ratio to reduce the formation of impurity phases. In a series of as-prepared materials with sodium-iron ratio between 1.33–1.50, Na3.6Fe2.6(PO4)1.6P2O7 exhibits the highest discharge specific capacity (110.3 mAh g−1 at 0.1C), best rate capability (84.4 mAh g−1 at 30 C) and cycle performance (78.2 % capacity retention after 3000 cycles at 20 C). Furthermore, 20 Kg of the Na3.6Fe2.6(PO4)1.6P2O7 cathode material is prepared by spray drying method, which has been successfully applied in sodium-ion pouch cells (12 Ah) with a commercial hard carbon as the anode materials. The assembled pouch cells display excellent cycle performance (82.4 % capacity retention after 2000 cycles at 1 C), rate capability (88 % capacity retention at 30 C) and low-temperature performance (74 % capacity retention at −40 °C).

Triphasic heterostructured Zn–Sn–S hollow nanoboxes encapsulated by N, S-codoped carbon as anodes for high-performance sodium-ion batteries

Metal sulfides have been expected huge practical potential for sodium-ion batteries (SIBs), which are mainly owing to their admirable merits of natural abundance, low price, and high theoretical capacity. However, the inferior electrical conductivity and enormous volume variation originating from sodiation/desodiaziton reactions usually result in unsatisfied rate and cycling properties. In this work, a coprecipitation with a following sulfurization method has been rationally designed to prepare the triphasic heterostructured hollow Zn–Sn–S nanoboxes encapsulated by N, S-codoped carbon (ZSS@NCS) as anodes for SIBs. The coexistence of triphasic ZSS heterostructures that consist of ZnS, SnS2, and Sn2S3 effectively facilitates the fast Na + diffusion. The N, S-codoped carbon (NSC) derived from the polydopamine is coated outside of the ZSS heterostructures, which provides infinite affinity between ZSS and NSC that efficiently accelerates the electron transport and maintains the structural stability via the confinement effect. The synergistic effects of the heterostructured ZSS and NSC endow the ZSS@NSC anode with favorable sodium storage properties including decent discharge capacity (683.8 mAh g−1 at 0.1 A g−1), satisfying rate (232.6 mAh g−1 at 10.0 A g−1) and cycling properties (402.2 mAh g−1 over 150 cycles).

Stabilizing Interlayer Repulsion in Layered Sodium-Ion Oxide Cathodes via Hierarchical Layer Modification.

Layered sodium-ion oxides hold considerable promise in achieving high-performance sodium-ion batteries. However, the notorious phase transformation during charging, attributed to increased O2-─O2- repulsion, results in substantial performance decay. Here, a hierarchical layer modification strategy is proposed to stabilize interlayer repulsion. During desodiation, migrated Li+ from the transition metal layer and anchored Ca2+ in sodium sites maintain the cationic content within the sodium layer. Meanwhile, partial oxygen substitution by fluorine and the involvement of oxygen in redox reactions increase the average valence of the oxygen layer. This sustained cation presence and elevated anion valence collectively mitigate increasing O2-─O2- repulsion during sodium extraction, enabling the Na0.61Ca0.05[Li0.1Ni0.23Mn0.67]O1.95F0.05 (NCLNMOF) cathode to retain a pure P2-type structure across a wide voltage range. Unexpected insights reveal the interplay between different doping elements: the robust Li─F bonds and Ca2+ steric effects suppressing Li+ loss. The NCLNMOF electrode exhibits 82.5% capacity retention after 1000 cycles and a high-rate capability of 94 mAh g-1 at 1600mA g-1, demonstrating the efficacy of hierarchical layer modification for high-performance layered oxide cathodes.

Rational combination of Inverted-Pyramid structured Nickel-based Prussian blue with porous 3D carbon as high-performance sodium-ion batteries cathode

Metal based Prussian blue composite materials with heterogeneous structures possess substantial potential for enhancing both ionic and charge transfer processes, ultimately expediting electrochemical reaction kinetics for various battery devices. However, the limited bonding between carbon materials and Prussian blue analogues (PBAs) and the uncontrolled nucleation rate of metal component resulted in limited specific capacity and cyclic stability. Herein, we introduced a novel approach for the in-situ synthesis of Ni-PBA with an inverted-pyramid structure on a three-dimensional ultra-thin carbon frames (3DUC) substrate via a hydrothermal method. The inverted pyramid structure fits tightly with the 3DUC during nucleation to form a one-piece stable structure. This integration effectively curbs aggregation and hasty nucleation tendencies of NiPBA. Furthermore, the abundant voids and interconnected networks within the 3DUC structure significantly reduce ion diffusion path lengths, thereby lowering the Na diffusion barrier and enhancing the material’s capacitance contribution rate. Consequently, this cathode material exhibits commendable initial capacity (125.2 mAh/g at 50 mA g−1) and exceptional long-term cycling stability (with a capacity retention of 89.06 % after 900 cycles at 50 mA g−1). These findings hold significant promise for advancing the commercial viability of metal-PBA based electrodes by rational heterogeneous structure design.

Ribbon-Ordered Superlattice Enables Reversible Anion Redox and Stable High-Voltage Na-Ion Battery Cathodes.

High-voltage layered oxide cathodes attract great attention for sodium-ion batteries (SIBs) due to the potential high energy density, but high voltage usually leads to rapid capacity decay. Herein, a stable high-voltage NaLi0.1Ni0.35Mn0.3Ti0.25O2 cathode with a ribbon-ordered superlattice is reported, and the intrinsic coupling mechanism between structure evolution and the anion redox reaction (ARR) is revealed. Li introduction constructs a special Li-O-Na configuration activating reversible nonbonded O 2p (|O2p)-type ARR and regulates the structure evolution way, enabling the reversible Li ions out-of-layer migration instead of the irreversible transition metal ions out-of-layer migration. The reversible structure evolution enhances the reversibility of the bonded O 2p (O2p)-type ARR and inhibits the generation of oxygen dimers, thus suppressing the irreversible molecular oxygen (O2)-type ARR. After the structure regulation, the structure evolution becomes reversible, |O2p-type ARR is activated, O2p-type ARR becomes stable, and O2-type ARR is inhibited, which largely suppresses the capacity degradation and voltage decay. The discharge capacity is increased from 154 to 168 mA h g-1, the capacity retention after 200 cycles significantly increases from 35 to 84%, and the voltage retention increases from 78 to 93%. This study presents some guidance for the design of high-voltage, O3-type oxide cathodes for high-performance SIBs.

Suppressed O3–P3 phase transition of Cu/Fe/Mn-based layered oxides as cathode materials for high performance sodium-ion batteries

O3-type Cu/Fe/Mn-based layered oxides are regarded as promising cathode materials for sodium-ion batteries due to the abundance of elements in the Earth's crust and the imperative for large-scale energy storage. However, these materials still encounter some challenges, including low capacity and insufficient rate performance. In this study, Al-doped, cobalt-free, and nickel-free Na0.9Cu0.18Fe0.3Mn0.52-xAlxO2 (x = 0, 0.03, 0.05, 0.08) are successfully synthesized by using the solid-state method to introduce non-electrochemically active elements to stabilize the crystal structure and enhance the electrochemical performance. The impact of Al-doping is comprehensively analyzed by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS). Al-doping effectively enhances the diffusion coefficient of Na+, reduces the Jahn-Teller effect caused by Mn3+ and mitigates the transition from O3 to P3 phase. Finally, it stops the partial phase transitions from O3 to P3, which can be found in pristine oxide without Al-doping during charge-discharge cycling. Notably, the Na0.9Cu0.18Fe0.3Mn0.47Al0.05O2 electrode exhibits a reversible capacity of 123.89 mAh g−1 at 0.1 C, a capacity retention rate of 91.1 % after 200 cycles at 1 C, and a discharge capacity of 66.36 mAh g−1 at a high rate of 5 C.

Engineering interleaved large and small Na4MnV(PO4)3@NC microspheres cathode towards high performance sodium ion batteries

The booming consumption of energy storage devices spontaneously accelerates the urgent development of robust cathode materials for sodium-ion batteries (SIBs). In view of this, the sodium superionic conductor Na4MnV(PO4)3 (NMVP), with considerable energy density of 425 Wh kg−1, has become the focus in SIB research. Despite this, the performance of NMVP is substantially compromised by Jahn-Teller distortion of Mn3+ ions in conjunction with inherently limited electronic conductivity. In order to address this limitation, this study introduces a series of N-doped carbon (NC) coated NMVP materials with interleaved large and small microsphere morphology. Of particular note, the optimized NMVP-NC2 shows the most excellent performance, maintaining a capacity of 65.7 mAh g−1 at 5C over 500 cycles. A full cell configuration employing an NMVP-NC2 cathode and a commercial hard carbon (HC) anode manifests commendable circular stability with a capacity of 80.7 mAh g−1 underneath 100 cycles at 1C, with an impressive 95.3 % capacity preservation. This distinctive architecture imparts the materials with a high tap density and a relatively low surface area, which is beneficial to the industrial employment. In addition, density functional theory (DFT) calculations indicate that the NC regulation significantly narrows the NMVP's bandgap and facilitates electron transfer, thereby accelerating electron/ion transport kinetics. The NC coating approach combined with large-scale spray drying tactic in this work offers a clear strategy to industrialize NMVP cathode towards high-performance SIBs.

A Unique Wide-Spacing Fence-Type Superstructure for Robust High-Voltage O3-Type Sodium Layered Cathode.

Enhancing the energy density of layered oxide cathode materials is of great significance for realizing high-performance sodium-ion batteries and promoting their commercial application. Lattice oxygen redox at high voltage usually enables a high capacity and energy density. But the structural degradation, severe voltage decay, and the resultant poor cycling performance caused by irreversible oxygen release seriously restrict the practical application. Herein we introduce a novel fence-type superstructure (2a × 3a type supercell) into O3-type layered cathode material Na0.9Li0.1Ni0.3Mn0.3Ti0.3O2 and achieve a stable cycling performance at a high voltage of 4.4 V. The fence-type superstructure effectively inhibits the formation of the vacancy clusters resulting from out-of-plane Li migration and in-plane transition metal migration at high voltage due to the wide d-spacing, thereby significantly reducing the irreversible release of lattice oxygen and greatly stabilizing the crystal structure. The cathode exhibits a high energy density of 545 Wh kg-1, a high rate capability (112.8 mAh g-1 at 5C) and a high cycling stability (85.8%@200 cycles with a high initial capacity of 148.6 mAh g-1 at 1C) accompanied by negligible voltage attenuation (98.5%@200 cycles). This strategy provides a distinct spacing effect of superstructure to design stable high-voltage layered cathode materials for Na-ion batteries.

Cu-substituted Na0.75Ni0.17Cu0.08Mn0.75O2 cathode with suppressing P2-O2 phase transition and air-stable for high-performance sodium-ion batteries

P2-Na0.75Ni0.25Mn0.75O2 cathode material with high specific capacity and high operating voltage is favored by researchers. However, the complex phase transition (P2-O2) at high voltage and the rapid capacity decay caused by Na+/vacancy ordering seriously restrict its application. In this study, a series of Cu-substituted P2-type Na0.75Ni0.25-xCuxMn0.75O2 (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) cathodes are synthesized. The Na0.75Ni0.17Cu0.08Mn0.75O2 cathode achieves a high initial discharge capacity of 133.6 mAh g−1 and remains 80.5 % of this capacity after 150 cycles at 0.1C, outperforming the performance of other compositions. Investigations into the superior electrochemical performance of Na0.75Ni0.17Cu0.08Mn0.75O2 through a multi-technique approach, including in-situ X-ray diffraction (XRD), ex-situ X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT), elucidate the underlying mechanisms. The results show that the introduction of Cu2+ in Na0.75Ni0.25Mn0.75O2 can successfully regulate the ratio of Naf/Nae, with enhanced cell performance when Na+ occupies more Nae sites compared to Naf sites. In-situ XRD confirms that the Cu substitution for Ni stabilizes the P2 structure during the charge–discharge process and inhibits the unfavorable P2-O2 phase transition at high voltage. In addition, Cu-substituted cathode materials exhibit a good effect on improving air stability, attributed to the higher Cu2+/Cu3+ redox potential. This unique substitution mechanism offers a novel perspective for understanding the structure-performance relationship of P2-type cathode materials and provides important support for the design of air-stable, high-performance cathode materials.