Inorganic-Enriched Solid Electrolyte Interphases: A Key to Enhance Sodium-Ion Battery Cycle Stability?

The characteristics of solid electrolyte interphase (SEI) at both the cathode and anode interfaces are crucial for the performance of sodium-ion batteries (SIBs). The research demonstrates the merits of a balanced organic component, specifically the organic sodium alkyl sulfonate (ROSO2Na) featured in this work, in conjunction with the inorganic sodium fluoride (NaF), to enhance the interfacial stability. Using a customized electrolyte, it has optimized the interphase, curbing excess NaF production, and created a thin and uniform NaF/ROSO2Na-rich SEI layer. It offers exceptional protection against interface deterioration, transition metal dissolution, and concurrently ensures a consistent reduction in interfacial impedance. This creative approach results in a substantial improvement in the performance of both the Na0.9Ni0.4Fe0.2Mn0.4O2 cathode and the hard carbon anode. The cathode demonstrates remarkable average Coulombic efficiency exceeding 99.9% and a capacity retention of 81% after 500 cycles. Furthermore, the Ah-level pouch cell has shown outstanding performance with an 87% capacity retention after 400 cycles. Moving beyond the prevailing focus on inorganic-rich SEI, these results highlight the effectiveness of the customized organic-inorganic hybrid SEI formulation in improving SIB technology, offering an adaptable solution that ensures superior interfacial stability.

Dual-Driven Ion/Electron Migration and Sodium Storage by In Situ Introduction of Copper Ions and a Carbon-Conductive Framework in a Tin-Based Sulfide Anode.

Tin sulfide (SnS) has emerged as a promising anode material for sodium ion batteries (SIBs) due to its high theoretical capacity and large interlayer spacing. However, several challenges, such as severe insufficient electrochemical reactivity, rapid capacity degradation, and poor rate performance, still hinder its application in SIBs. In this study, in situ introduction of copper ions and a carbon conductive framework to form SnS nanocrystals embedded in a Cu2SnS3 lamellar structure heterojunction composite (SnS/Cu2SnS3/RGO) with graphene as the supporting material is proposed to achieve dual-driven sodium ion/electron migration during the continuous electrochemical process. The designed structure facilitates the preferential electrochemical reduction of copper ions into copper nanocrystals during the discharge process and functions as a catalytically active center to promote multivalence tin sodiation reaction. Furthermore, during the charging process, the presence of copper nanocrystals also facilitates efficient desodiation of NaxSn and further activates to form higher valence state sulfides. As a result, the SnS/Cu2SnS3/RGO composite demonstrates high cycling stability with a high reversible capacity of 395 mAh g-1 at 5A g-1 after 500 cycles with a capacity retention of 85.6%. In addition, the assembled Na3V2(PO4)3∥SnS/Cu2SnS3/RGO sodium ion full cell achieves 93.7% capacity retention after 80 cycles at 0.5 A g-1.

Na3Zr2Si2PO12-polymer composite electrolyte for solid state sodium batteries.

The integration of the flexibility of organic polymer electrolyte and high ionic conductivity of the ceramic electrolyte is attempted in search of efficient and safer battery. Composite solid polymer electrolyte (CSPE) provides high ionic conductivity with a sustainable thin film of electrolyte. The CSPE is synthesized by the solution cast technique using Na3Zr2Si2PO12 (NZSP) as ceramic and poly(vinylidene fluoride-hexafluoropropylene) with Salt-Ionic liquid as polymer electrolyte. X-ray diffraction (XRD) of CSPE includes amorphous nature due to the polymer part as well as crystalline peaks of ceramic NZSP, simultaneously. The prepared CSPE sample shows homogeneous and interconnected surface morphology is observed by Scanning electron microscopy (SEM) image. Thermogravimetric analysis (TGA) shows electrolyte is thermally stable up to 200 ℃ and differential scanning calorimetry (DSC) reveals decrease in degree of crystallinity due to NZSP addition in the CSPE. By complex impedance spectroscopy (CIS), room temperature ionic conductivity of the prepared CSPE is found ~ 1.03 mS/cm. The dielectric behaviour of the prepared electrolyte is also studied to investigate the ion dynamics within the sample. The cationic transference number is 0.53 and the electrochemical stability window (ESW) of the CSPE is 4.9 V which is suitable for sodium solid-state batteries applications.

Synthesis of Iron-Based Prussian Blue Analogues with Ultralong Cycle Performance in a Novel T-Shaped Collision Microreactor.

Iron based Prussian blue analogues (Fe-PBA) hold significant promise cathode materials for sodium ion batteries due to their low cost and desirable electrochemical properties. However, their practical application is often hindered by the presence of vacancy defects and issues of oxidation that arise during the material preparation process. To overcome these challenges, this study introduces an innovative T-shaped collision microreactor aimed at promoting a uniform concentration field, narrowing the gap between material mixing rate and reaction rate, and providing an oxygen-free environment for the synthesis of Fe-PBA. Employing this microreactor, Fe-PBAs were synthesized with a meticulous approach that led to a material possessing a well-defined morphology and a stoichiometry of Na1.54Fe[Fe(CN)6]0.93. The material exhibited a notable reduction in vacancy defects and minimized oxidation levels. Upon electrochemical evaluation, the Fe-PBA crafted within the microreactor demonstrated an impressive specific capacity of 94.1 mAh/g at a current density of 0.5 C and showcased a long-term cyclability with 72.6% capacity retention over 2000 cycles. Comparative analysis revealed that the structural and electrochemical properties of Fe-PBA prepared in the microreactor outperformed those of samples prepared using traditional reactors. This work provides a new approach for the continuous and stable fabrication of high-performance battery cathode materials.

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 324 mA h g-1 at -40 °C (slightly decreased from 357 mA 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 40 mA 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.

Multifunctional Porous Organic Polymer Anode with Desired Redox Potential and Capacity for High-Performance Aqueous Sodium-Ion and Ammonium-Ion Batteries.

Aqueous sodium-ion batteries (ASIBs) and aqueous ammonium-ion batteries (AAIBs) attract great attention due to their low cost, safety, and environmental friendliness, but the lack of suitable electrodes with competitive capacity and redox potential limits their practical applications. Herein, we report a porous organic polymer (POP) with multiple redox processes as anodes for ASIBs and AAIBs. This POP displays desired redox potential and shows high reversible capacity of more than 200 mAh g-1 in both ASIBs and AAIBs. Full cells configured by this POP anode also display excellent performance (about 80% and 60% capacity retention over 1000 cycles in ASIBs and AAIBs). In addition, the intercalation chemistry of inorganic NH4+ with this POP is investigated, illustrating that the pyrazine sites in this POP are the redox centers to reversibly combine with NH4+. This work provides a promising and alternative anode for ASIBs and AAIBs and paves a new way for the development of novel organic materials for other aqueous battery systems.

Deconstruction Engineering of Lignocellulose Toward High-Plateau-Capacity Hard Carbon Anodes for Sodium-Ion Batteries.

Biomass-derived hard carbon is a promising anode material for commercial sodium-ion batteries due to its low cost, high capacity, and stable cycling performance. However, the intrinsic tight lignocellulosic structure in biomass hinders the formation of sufficient closed pores, limiting the specific capacity of obtained hard carbons. In this contribution, a mild, industrially mature pretreatment method is utilized to selectively regulate biomass components. The hard carbon with a rich closed pore structure is prepared by optimizing the appropriate ratio of biomass composition. Optimized etching conditions enhanced the closed pore volume of hard carbon from 0.15 to 0.26 cm3 g-1. Consequently, the engineered hard carbon exhibited excellent electrochemical performance, including a high reversible capacity of 346 mAh g-1 with a high plateau capacity of 254 mAh g⁻¹ at 50 mA g⁻¹, robust rate capability, and cycling stability. The optimized hard carbon shows an 88 mAh g⁻¹ increase in plateau capacity compared to hard carbon from directly carbonizing bamboo fibers. This mature approach provides an easy-to-operate industrial pathway for designing high-capacity biomass-based hard carbons for sodium-ion batteries.

Enhancing the Cycling and Rate Performance of NaNi1/3Fe1/3Mn1/3O2 Cathodes by La/Al Codoping.

O3-type layered oxides hold significant promise as the material for cathodes in sodium-ion batteries for their favorable electrochemical properties, while irreversible structural degradation and harmful phase transitions during cyclic operation limit the practical application of these materials. In this work, we proposed a La3+/Al3+ codoping strategy in O3-Na(Ni1/3Mn1/3Fe1/3)O2 cathode materials and found that batteries with the Na (Ni1/3Mn1/3Fe1/3)0.998La0.001Al0.001O2 (NFM-La/Al) cathodes exhibited not only promoted capacity from 135.80 to 170.42 mAh g-1 at 0.2 C but also significantly enhanced cycling stability, with a 10% improvement in capacity retention compared with NFM cathodes after 300 cycles. Particularly, their rate performance was significantly improved as well. XRD and XPS tests indicated that La could expand the c-axis of NFM due to its larger ionic radius and thus significantly increased Na+ ion diffusion efficiency, and in addition, Al doping could effectively increase the content of Ni2+ and Mn4+ and thus greatly alleviated the negative Jahn-Teller effect caused by Mn3+. Moreover, consistent with XRD analyses, DFT calculations further substantiated the effectiveness of the La/Al codoping strategy by demonstrating the detailed atom substitution mechanism in the NFM crystal lattice. The boosted structure stability and Na+ diffusion kinetics may enhance the potential for practical applications of O3-type oxide cathodes.

Simultaneous Tailoring of Chemical Composition and Morphology Configuration in Metal Hexacyanoferrate for Ultrafast and Durable Sodium-ion Storage.

Metal hexacyanoferrates (MHCFs) with adjustable composition and open framework structures have been considered as intriguing cathode materials for sodium-ion batteries (SIBs). Exploiting MHCFs with ultrafast and durable sodium storage capability as well as comparable capacity is always a goal that many investigators pursue, but remains challenging. Herein, simultaneous tailoring of chemical composition and morphology configuration is carried out to design a hollow monoclinic high-entropy MHCF (HMHE-HCF) assembled by nanocubes for the first time to realize the objective. The "cocktail effect" of high-entropy construction, rich sodium content of monoclinic phase, and unique hollow structure endow HMHE-HCF cathode with fast reaction kinetics and energetically stable performance during continuous charging/discharging processes. As a result, the HMHE-HCF cathode demonstrates superior rate performance up to an ultra-high rate of 100 C (71.1% retention to 0.1 C), and remarkable cycling stability with a capacity retention of 77.8% over 25,000 cycles at 100 C, outperforming most reported sodium-ion cathodes. Further, the HMHE-HCF//hard carbon full-cell delivers capacities of 99.0 and 82.3 mAh g-1 at 0.1 C and 10 C, respectively, and retains 98.1% of the initial capacity after 1,600 cycles at 5C, demonstrating its potential application for sodium-ion storage.

Electron-Spin Regulation Driving Heterointerface Electron Distribution and Phase Transition toward Ultrafast and Durable Sodium Storage.

Phase engineering is an effective strategy for modulating the electronic structure and electron transfer mobility of cobalt selenide (CoSe2) with remarkable sodium storage. Nevertheless, it remains challenging to improve fast-charging and cycling performance. Herein, a heterointerface coupling induces phase transformation from cubic CoSe2 to orthorhombic CoSe2 accompanied by the formation of MoSe2 to construct a CoSe2/MoSe2 heterostructure decorated with N-doped carbon layer on a 3D graphene foam (CoSe2/MoSe2@NC/GF). The incorporated Mo cations in the bridged o-CoSe2/MoSe2 not only act an electron donor to regulate charge-spin configurations with more active electronic states but also trigger the upshift of d/p band centers and a decreased ∆d-p band center gap, which greatly enhances ion adsorption capability and lowers the ion diffusion barrier. As expected, the CoSe2/MoSe2@NC/GF anode demonstrates a high-rate capability of 447 mAh g-1 at 2 A g-1 and an excellent cyclability of 298 mAh g-1 at 1 A g-1 over 1000 cycles. The work deepens the understanding of the elaborate construction of heterostructured electrodes for high-performance SIBs.

Tailoring Pseudo-Graphitic Domain by Molybdenum Modification to Boost Sodium Storage Capacity and Durability for Hard Carbon.

Hard carbon (HC) stands out as the most prospective anode for sodium-ion batteries (SIBs) with significant potential for commercial applications. However, some long-standing and intractable obstacles, like low first coulombic efficiency (ICE), poor rate capability, storage capacity, and cycling stability, have severely hindered the conversion process from laboratory to commercialization. The above-mentioned issues are closely related to Na+ transfer kinetics, surface chemistry, and internal pseudo-graphitic carbon content. Herein, constructing molybdenum-modified hard carbon solid spheres (Mo2C/HC-5.0), both the ion transfer kinetics, surface chemistry, and internal pseudo-graphitic carbon content are comprehensively improved. Specifically, Mo2C/HC-5.0 with higher pseudo-graphitic carbon content provides a large number of active sites and a more stable layer structure, resulting in improved sodium storage capacity, rate performance, and cycling stability. Moreover, the lower defect density and specific surface area of Mo2C/HC-5.0 further enhance ICE and sodium storage capacity. Consequently, the Mo2C/HC-5.0 anode achieves a high capacity of 410.7 mA h g-1 and an ICE of 83.9% at 50 mA g-1. Furthermore, the material exhibits exceptional rate capability and cycling stability, maintaining a capacity of 202.8 mA h g-1 at 2 A g-1 and 214.9 mA h g-1 after 800 cycles at 1 A g-1.

Regulating the Pore Structure of Biomass-Derived Hard Carbon for an Advanced Sodium-Ion Battery.

Biomass-derived hard carbon materials are attractive for sodium-ion batteries due to their abundance, sustainability, and cost-effectiveness. However, their widespread use is hindered by their limited specific capacity. Herein, a type of bamboo-derived hard carbon with adjustable pore structures is developed by employing a ball milling technique to modify the carbon chain length in the precursor. It is observed that the length of the carbon chain in the precursor can effectively control the rearrangement behavior of the carbon layers during the high-temperature carbonization process, resulting in diverse pore structures ranging from closed pores to open pores, which significantly impact the electrochemical properties. The optimized hard carbon with abundant closed pores exhibits a high specific capacity of 356 mAh g-1 at 20 mA g-1, surpassing that of bare hard carbon (243 mAh g-1) and hard carbon with abundant open pores (129 mAh g-1 at 20 mA g-1). However, the kinetic analysis reveals that hard carbon with open pores shows better sodium-ion diffusion kinetics, indicating that a balance between the closed and open pores should be considered. This research offers valuable insights into pore design and presents a promising approach for enhancing the performance of hard carbon anode materials derived from biomass precursors.

Uncovering the Nonequilibrium Evolution Mechanism between Na2+2 δFe2- δ(SO4)3 Cathode and Impurities in the Na2SO4-FeSO4·7H2O Binary System for High-Voltage Sodium-Ion Batteries.

Sodium-ion batteries are increasingly recognized as ideal for large-scale energy storage applications. Alluaudite Na2+2 δFe2- δ(SO4)3 has become one of the focused cathode materials in this field. However, previous studies employing aqueous-solution synthesis often overlooked the formation mechanism of the impurity phase. In this study, the nonequilibrium evolution mechanism between Na2+2 δFe2- δ(SO4)3 and impurities by adjusting ratios of the Na2SO4/FeSO4·7H2O in the binary system is investigated. Then an optimal ratio of 0.765 with reduced impurity content is confirmed. Compared to the poor electrochemical performance of the Na2.6Fe1.7(SO4)3 (0.765) cathode, the optimized Na2.6Fe1.7(SO4)3@CNTs (0.765@CNTs) cathode, with improved electronic and ionic conductivity, demonstrates an impressive discharge specific capacity of 93.8 mAh g-1 at 0.1 C and a high-rate capacity of 67.84 mAh g-1 at 20 C, maintaining capacity retention of 71.1% after 3000 cycles at 10 C. The Na2.6Fe1.7(SO4)3@CNTs//HC full cell reaches an unprecedented working potential of 3.71 V at 0.1 C, and a remarkable mass-energy density exceeding 320 Wh kg-1. This work not only provides comprehensive guidance for synthesizing high-voltage Na2+2 δFe2- δ(SO4)3 cathode materials with controllable impurity content but also lays the groundwork of sodium-ion batteries for large-scale energy storage applications.

Template-Assisted Epitaxial Growth of Ordered SnO2 Nanorods Arrays with Different Hollow Structures for High-Performance Sodium Storage.

Anode materials for sodium ion batteries (SIBs) are confronted with severe volume expansion and poor electrical conductivity. Construction of assembled structures featuring hollow interior and carbon material modification is considered as an efficient strategy to address the issues. Herein, a novel template-assisted epitaxial growth method, ingeniously exploiting lattice matching nature, is developed to fabricate hollow ordered architectures assembled by SnO2 nanorods. SnO2 nanorods growing along [100] direction can achieve lattice-matched epitaxial growth on (110) plane of α-Fe2O3. Driven by the lattice matching, different α-Fe2O3 templates possessing different crystal plane orientations enable distinct assembly modes of SnO2, and four kinds of hollow ordered SnO2@C nanorods arrays (HONAs) with different morphologies including disc, hexahedron, dodecahedron and tetrakaidecahedron (denoted as Di-, He-, Do-, and Te-SnO2@C) are achieved. Benefiting from the synergy of hollow structure, carbon coating and ordered assembly structure, good structural integrity and stability and enhanced electrical conductivity are realized, resulting in impressive sodium storage performances when utilized as SIB anodes. Specifically, Te-SnO2@C HONAs exhibit excellent rate capability (385.6 mAh·g-1 at 2.0 A·g-1) and remarkable cycling stability (355.4 mAh·g-1 after 2000 cycles at 1.0 A·g-1). This work provides a promising route for constructing advanced SIB anode materials through epitaxial growth for rational structural design.

Dehydration Achieving the Iron Spin State Regulation of Prussian Blue for Boosted Sodium-Ion Storage Performance.

Prussian blue analogs (PBAs) show promise as cathodes for sodium-ion batteries due to their notable cycle stability, cost-effectiveness, and eco-friendly nature, yet the presence of interstitial water limits the specific capacity and obstructs Na+ mobility within the material. Although considerable experimental efforts are focused on dehydrating water for capacity enhancement, there is still a deficiency of a comprehensive understanding of the low capacity of low-spin Fe resulting from interstitial water, which holds significance in Na+ storage. This study introduces a novel gas-assisted heat treatment method to efficiently remove interstitial water from Fe-based PBA (NaFeHCF) electrodes and combines experiments and theoretical calculations to reveal the iron spin state regulation that is related to the capacity enhancement mechanism. This dehydration strategy significantly enhances battery capacity, especially the portion at higher voltages (3.4-4.0 V). The increase in capacity is attributed to the following factors: an enhanced proportion of Fe2+, reduced water content which facilitates faster charge transfer, and the activation of low spin Fe2+. The optimized NaFeHCF demonstrated impressive half-cell performance of retaining 87.3% capacity after 2000 cycles at a 5 C rate and achieving 100 mAh g-1 capacity over 200 cycles when being paired with hard carbon, exhibiting its practical potential.

Full Exploitation of Charge Compensation of O3-type Cathode Toward High Energy Sodium-Ion Batteries by High Entropy Strategy.

O3-type cathodes with sufficient Na content are considered as promising candidates for sodium-ion batteries (SIBs). However, these cathodes suffer from insufficient utilization of the active elements, restraining the delivered capacity. In this work, a high entropy strategy is applied to a typical O3 cathode NaLi0.1Ni0.35Mn0.55O2 (NLNM), forming a high entropy oxide NaLi0.1Ni0.15Cu0.1Mg0.1Ti0.2Mn0.35O2 (Na-HE). Results show that the active elements are fully exploited in Na-HE, with a two-electron reaction by Ni2+/4+ (further extended to Cu redox and even oxygen redox), vastly different from a one-electron reaction of Ni2+/3+ in NLNM. The full utilization of the active elements dramatically improves the output capacity of the cathode (122.6 mAh g-1 of Na-HE versus 81 mAh g-1 of NLNM). Moreover, the detrimental phase transition is well suppressed in Na-HE. The cathode exhibits high capacity retention of 88.7% after 100 cycles at 130 mA g-1, compared to only 36.4% for NLNM. These findings provide new insight for the design of new cathode materials for SIBs with high energy density and robust stability.

Abnormal Gas Generation during First Discharge Process of Sodium Ion Battery.

In recent years, sodium-ion batteries (SIBs) have attracted a lot of attention and are considered an ideal alternative to lithium-ion batteries (LIBs). The hard carbon (HC) anode in SIBs presents a unique challenge for studying the formation process of the solid electrolyte interphase (SEI) during initial cycling, owing to its distinctive porous structure. This study employs a combination of ultrasonic scanning techniques and differential electrochemical mass spectrometry to conduct an in-depth analysis of the two-dimensional distribution and composition of gases during the formation process. The findings reveal distinct gas evolution behaviors in SIBs compared to LIBs during formation. Notably, significant gas evolution is observed during the discharge phase of the formation cycle in SIBs, with higher discharge rates leading to increased gas evolution rates. This phenomenon is likely attributed to the adsorption of CO2 gas by the abundant pores in HC, followed by desorption during discharge. Furthermore, the study demonstrates that the addition of 5A molecular sieves, which competitively adsorb gases, effectively reduces gas adsorption on the anode during formation, thereby significantly enhancing battery performance. This research elucidates the gas adsorption and desorption behavior at the battery interface, providing new insights into the SEI formation process in SIBs.

High-Energy Sodium Ion Batteries Enabled by Switching Sodiophobic Graphite into Sodiophilic and High-Capacity Anodes.

Owing to the crustal abundance of sodium element, sodium ion batteries (SIBs) are considered a promising complementary to lithium-ion battery for stationary energy storage applications. The cointercalation chemistry enables the use of cost-effective graphite as anodes, whereas the low capacity (<130 mAh g-1) and high redox potential (>0.6 V vs. Na/Na+) of graphite significantly limit the energy density of SIBs. Herein, we induce the high-capacity Na metal into sodiophilic ternary graphite intercalation compounds (t-GICs) via co-intercalation and deposition reactions, thereby achieving Na/t-GIC anodes with high capacities and low working voltage (0.18 V). The new anodes exhibit high coulombic efficiencies of above 99.7 % over 550 cycles and a high-rate capacity of 588.4 mAh g-1 at 6 C (10 min per charge). When it is paired with Na3V2(PO4)2F3 (NVPF) cathodes, the SIBs demonstrate a high energy density of 259 Wh kg-1both electrodes surpassing that of commercial LiFePO4//graphite batteries. The outstanding anode performance is attributed to the tailored sodiophilicity of graphite through manipulating the ether solvents and the in-situ generated space among t-GIC flakes to stably accommodate Na metal. Our findings for stable Na plating/striping on sodiophilic graphite materials provide an effective approach for developing advanced SIBs.

Leveraging Entropy and Crystal Structure Engineering in Prussian Blue Analogue Cathodes for Advancing Sodium-Ion Batteries.

The synergistic engineering of chemical complexity and crystal structures has been applied to Prussian blue analogue (PBA) cathodes in this work. More precisely, the high-entropy concept has been successfully introduced into two structure types of identical composition, namely, cubic and monoclinic. Through the utilization of a variety of complementary characterization techniques, a comprehensive investigation into the electrochemical behavior of the cubic and monoclinic PBAs has been conducted, providing nuanced insights. The implementation of the high-entropy concept exhibits crucial selectivity toward the intrinsic crystal structure. Specifically, while the overall cycling stability of both cathode systems is significantly improved, the synergistic interplay of crystal structure engineering and entropy proves particularly significant. After optimization, the cubic PBA demonstrates structural advantages, showcasing good reversibility, minimal capacity loss, high thermal stability, and unparalleled endurance even under harsh conditions (high specific current and temperature).

Polyaniline-Coated Na3V2(PO4)2F3 Cathode Enables Fast Sodium Ion Diffusion and Structural Stability in Rechargeable Batteries.

Na3V2(PO4)2F3 (NVPF), a typical sodium superionic conductor (NASICON) type structure, has attracted much interest as a potential positive electrode in sodium-ion battery. However, the inherently poor electronic conductivity of phosphates compromises the electrochemical properties of this material. Here, we develop a general strategy to improve the electrochemical performance by preparing a new composite material "polyaniline (PANI)@NVPF" using a Pickering emulsion method. The X-ray diffraction and Raman results indicated a successful PANI coating without affecting the NASICON-type structure of NVPF, and they enhanced the interfacial bonding between the two components. Also, thermogravimetric analysis and scanning electron microscopy analyses revealed that the PANI content influenced the thermal stability and morphology of the nanocomposites. As a result, the sodium test cells exhibited multielectron reactions and a better rate performance for PANI@NVPF nanocomposites as compared to NVPF. Specifically, 2%PANI@NVPF maintained 70% of its initial capacity at 5C. Ex-situ electron paramagnetic resonance revealed the existence of mixed valence states of vanadium (V4+/V3+) in both discharge and charge processes. Consequently, the successful PANI coating into the sodium superionic conductor framework improved the sodium diffusion channels with a measurable increase of diffusion coefficients with cycling (ca. 3.25 × 10-11 cm2 s-1). Therefore, PANI@NVPF nanocomposites are promising cathode candidates for high-rate sodium-ion battery applications.