Abstract Hard carbon represents a highly promising option for anodes in sodium-ion batteries, yet its application is constrained by insufficient plateau capacity, unsatisfactory initial Coulombic efficiency, and the still-debated sodium storage mechanism. This paper details the pioneering use of biphenyl phenolic novolac resin as a precursor for hard carbon anodes. The hard carbon derived from pyrolysis undergoes systematic structural changes, including reduced defect and oxygen contents, decreased specific surface area, enhanced local ordering, tuned interlayer spacing, and enlarged nanopore size, by controlling the pyrolysis temperature. These structural changes collectively promote both the plateau capacity and the overall reversible capacity. The optimized sample pyrolyzed at 1600 °C delivers a high reversible capacity of 387.7 mAh g–1 at 20 mA g–1, together with an initial Coulombic efficiency of 84.6%, and maintains 76% capacity retention after 2000 cycles at 500 mA g–1. Combining electrochemical kinetics analysis with in situ Raman spectroscopy, we identify an adsorption-intercalation-filling sodium storage mechanism, in which Na+ adsorption on surface defects contributes mainly to the sloping capacity, whereas Na+ intercalation into turbostratic carbon layers and subsequent filling of nanopores dominate the low-voltage plateau capacity. In addition to presenting a resin precursor for high-performance hard carbon anodes, this work provides mechanistic insights into the regulation of microstructure for plateau-dominated sodium storage.

A nano-sized spherical MnFePO 4 ·H 2 O precursor is synthesized in aqueous phase via scalable reverse-titration. Integrated with sand milling–spray drying, the LiMn 1− x Fe x PO 4 /C cathode delivers 120.5 mAh g −1 at 5C and 95.15% retention after 450 cycles.

To develop high-safety and high-performance olivine LiMn1-xFexPO4/C cathodes for lithium-ion batteries, a scalable synthetic route is established by employing KMnO4 as the oxidant to prepare the MnFePO4·H2O precursor through a reverse-titration co-precipitation strategy. Nanosized spherical MnFePO4·H2O with a uniform elemental distribution and consistent valence states is successfully obtained, effectively suppressing Mn3+ disproportionation in an aqueous co-precipitation system. During lithiation, Li+ inserts into the initially vacant M1 sites, accompanied by the reduction of Mn3+/Fe3+ to Mn2+/Fe2+, thereby converting trivalent MnFePO4·H2O into olivine LiMn1-xFexPO4. The sand-milling spray-drying (SMSD) process further enables the formation of quasi-spherical secondary particles with a narrow particle-size distribution and a continuous highly graphitized carbon coating layer, in clear contrast to the irregular agglomerates and nonuniform carbon coatings generated by conventional ball-milling/stirring-drying treatment. As a result, the L-SMSD sample delivers a high discharge specific capacity of 153.5 mAh g-1 at 0.1C, an excellent rate capacity of 120.5 mAh g-1 at 5C, and outstanding cycling stability with a capacity retention of 95.15% after 450 cycles at 1C. Kinetic analysis further reveals that L-SMSD exhibits reduced polarization, enhanced Li+ diffusivity, and improved interfacial reaction kinetics. This work provides a practical route for the industrial-scale production of LiMn1-xFexPO4/C cathodes and is of great significance for the development of next-generation lithium-ion batteries.

Hard carbon is considered one of the most promising anode materials for sodium-ion batteries. Biomass-derived carbon precursors are especially appealing owing to their renewability, natural abundance, and low cost. Herein, low-ash walnut shell is employed as a precursor to prepare hard carbon, and a melamine-assisted nitrogen-doping strategy is developed to tailor its microstructure and sodium-storage behavior. The resulting nitrogen-doped hard carbon exhibits a robust carbon framework with enlarged interlayer spacing. Moreover, pyridinic N and pyrrolic N are the predominant nitrogen configurations, contributing 52.8

Nano-structured LiMn 0.6 Fe 0.4− x Mg x PO 4 /C microspheres are prepared by using NH 4 Mn 0.6 Fe 0.4− x Mg x PO 4 ·H 2 O obtained via co-precipitation as precursor for the first time. Mg 2+ doping improves the performance, kinetics and structural stability of the cathode.

Anthracite-based carbon materials are widely regarded as practical anode materials for sodium-ion batteries due to their abundant resources, cost-effectiveness, high carbon yield, and excellent chemical stability. However, their widespread applications are hindered by issues such as poor cycling performance, limited storage capacity, and low coulombic efficiency. Herein, we propose an innovative “construction-release-repair” design strategy that leverages the synergistic effect of B/N co-doping to address these challenges. By constructing B–N bonds, releasing unstable heteroatoms, and repairing defect sites on carbon layers, this strategy significantly enhances the microstructure and surface engineering of anthracite-based hard carbon. The optimized hard carbon anode delivers high specific capacity of 344.2 mAh g−1 at 0.03 ​A ​g−1 with an initial coulombic efficiency of 85.3 ​%, and excellent cycle retention of 88.2 ​% after 500 cycles at 0.2 ​A ​g−1. In-situ Raman spectroscopy and density functional theory calculations reveal that the B/N synergistic effect enhances the sodium-ion adsorption, reduces the diffusion barrier, and enhances reaction activity, facilitating more efficient intercalation and nanopore filling in the low-voltage plateau region. This work offers valuable insights into microstructure regulation and the design of high-performance anode for sodium-ion materials.

Fluorochemicals are a rapidly expanding class of materials used in a variety of fields including pharmaceuticals, metallurgy, agrochemicals, refrigerants, and in particular, alkali metal ion batteries. However, achieving one-step synthesis of pure fluorophosphate compounds in a well-controlled manner remains a formidable challenge due to the volatilization of fluorine during the heat treatment process. One feasible method is to cleave the C-F bond in polytetrafluoroethylene (PTFE) during synthesis to create a fluorine-rich atmosphere and strongly reducing environment. However, the inert nature of the C-F bond in PTFE presents a significant obstacle, as it is the strongest single bond in organic compounds. To address this predicament, we propose a fluorine-compensating strategy that involves cleavage of the C-F bonds by nucleophilic SN2-type reactions of Brønsted base (ammonia) enabling fluorine compensation. The decomposed products (NH2· and C·) also result in the formation of micropores (via NH3 escape) and in-situ carbon coating (via C· polymerization). The resultant cathode delivers a superior potassium storage capability including high rate performance and capacity retention. This contribution not only overcomes the obstacles associated with the inert C-F bond in fluororesin, but also represents a significant step forward in the development of fluorine-containing compounds.

LiVPO4F is an attractive cathode material due to its high operating voltage, high energy density, and stable structure, but its application is still hindered by low electrical conductivity and complicated synthesis processes. Here, the microsphere LiVPO4F@C cathode material with a high discharge plateau and stable structures is synthesized by a simple hydrothermal reaction. The formation mechanisms of the microsphere LiVPO4F are clarified. Surfactant hexadecyl trimethylammonium bromide is self-assembled into a spherical micelle matrix during the hydrothermal reaction and formed the microsphere LiVPO4F@C particles through multilayer adsorption. Meanwhile, the effects of pH value and the content of LiF on the morphology, structure, and electrochemical performance of the LiVPO4F@C cathode material are investigated. When the value of pH is close to neutral, the fluorine can be effectively deposited to form spherical LiVPO4F. With the addition of LiF at a ratio of 1.4, the prepared LiVPO4F@C exhibits better crystallinity and uniform spherical morphology, with excellent rate capability and cycling stability and a capacity retention of 93.60% at 5C. This work provides a simple and effective preparation solution for exploring a high-performance LiVPO4F@C cathode material for lithium-ion batteries.

The practical applications of LiMn0.8Fe0.2PO4 have been greatly limited due to its low electrical conductivity and inferior electrochemical reaction kinetics. Herein, a Ni-doped strategy has been employed to optimize the morphology of LiMn0.8Fe0.2PO4 nanoparticles with preferred growth orientation, leads to improved electrical/ ionic conductivity and electrode kinetic. Compared with pristine sample (Ni-D0), the electrical conductivity, ion diffusion coefficient and exchange current density of 5 mol% Ni-doped sample (Ni-D5) are obviously increased to 9.57 x 10-2 S cm-1, 3.301 x 10-11 cm2 s- 1, and 0.339 mA cm-2, respectively. The first principle calculations also confirmed the enhanced electrical conductivity of LiMn0.8Fe0.15Ni0.05PO4@C. Meanwhile, Ni-D5 exhibits good rate capability and cycling stability. Furthermore, the operando XRD analysis demonstrated its highly structural stability and reversible properties. Therefore, the Ni-doped strategy is considered a promising and effective modification approach to enhance the Li-storage capability of LiMn0.8Fe0.2PO4 cathode for high performance rechargeable second batteries.

Carbon material has emerged as a highly promising anode for sodium-ion batteries (SIBs) due to its abundance of resources, cost-effectiveness, and high carbon yield. This work elaborately designs the precursor structure for the self-assembly of melamine-cyanuric acid on the anthracite surface through hydrogen bonding, and successfully constructs N-doped carbon with tailored microstructure and expanded interlayer spacing. Serving as anode for SIBs, the optimized sample delivers high specific capacity (371.3 mAh g-1 at 0.05 A g-1), superior rate capability (295.8 mAh g-1 at 10.0 A g-1), and excellent ultra-long cycling performance (the retention of 91.5% after 3000 cycles at 0.5 A g-1). The systematic investigations reveal the enhancement of sodium-ion storage in the low-voltage plateau region involving the interlayer intercalation coupled with nanopores filling. It is discovered that the microporous structure formed by the appropriate graphite sheet angle influences the migration and storage of sodium ions. Density functional theory calculations indicate that the adsorption capacity for sodium ion is enhanced and the migration energy barrier perpendicular to the graphite layer is reduced at the appropriate angle of 8 degrees. This study provides novel insights into the sodium-ion storage mechanism, offering guidance for the better design of anthracite-based carbon anode with superior performance. N-doped carbon with tailored microstructure and expanded interlayer spacing is constructed by the structural engineering design for the self-assembly of MCA molecules on the anthracite surface through hydrogen bonding, delivering a 3D conductive network and flexible framework. DFT calculations demonstrate that the microporous structure formed by the appropriate angle of 8 degrees enhances the migration and storage of sodium ions. image

The critical role of carbene-related species (:CH 2 ) in catalyzing C–F bond breakage in C n F 2 n was elucidated, which provided an effective fluorine-compensating strategy for synthesizing phase-pure fluorophosphates.

Anthracite-based carbon is considered one of the most promising anodes for sodium-ion batteries (SIBs) due to its abundant natural resource and low cost. However, their performance is very poor. In this article, the microstructure of anthracite is hybridized by phenolic epoxy resin. The cross-linking reaction between anthracite and the resin generates hybrid carbon. The structure of anthracite transforms from graphite-like phase to pseudographite phase, accompanied by the increase of interlayer spacing. Due to the formation of oxygen-containing functional groups, abundant defects are generated on the surface and inside. Phenolic epoxy resin generates a microhard carbon structure around anthracite, in which curved and intertwined disordered structure are formed. This can effectively reduce the degree of graphitization of anthracite, promote rearrangement, and inhibit the regular accumulation of carbon layers. The performance of the anthracite anode with 5% resin pyrolyzed at 1200 degrees C is improved obviously, delivering capacity of 357.7 mAh g(-1) at a current density of 50 mA g(-1) and excellent capacity retention of 89% after 500 cycles at 500 mA g(-1). According to the in situ Raman analysis, it can be seen that Na+ is mainly adsorbed on the defect sites and open pore gaps on the material surface and then inserted in the pseudographitic region. This research proposes an effective strategy to enhance the performance of natural coal carbon material, providing huge opportunity for the commercialization of low-cost anthracite anodes for SIBs.

The graphene sheets (GNs) and toluene-soluble component of pitch (TS) are used as the mixed carbon source in the synthesis of LiMn0.8Fe0.2PO4@C@GNs through different carbon-coating methods. The results show that the ball–milling (BM) method is more favorable for the uniform dispersion of GNs and reduction of agglomeration compared to mechanical agitation (MA). The bridging effect of GNs and the pyrolytic carbon of TS with a high graphitization degree constructs an intact carbon skeleton on the surface of LiMn0.8Fe0.2PO4 crystal, thus greatly improving the structural stability and electronic conductivity. Sample BM–1.5 containing 1.5 wt. LiMn0.8Fe0.2PO4@C@GNs prepared by ball-milling with the toluene-soluble component of pitch and graphene sheet as the mixed carbon source has excellent electrochemical performance.

Prussian blue analogues (PBAs) are considered as superior cathode materials for potassium-ion batteries (PIBs) because of their three-dimensional open framework structure, high stability, and low cost. However, the intrinsic lattice defects and low potassium content typically results in poor rate and cycling performance, thus limited their practical applications. In this work, high-quality K1.64FeFe(CN)6 (PW-HQ) material with less crystalline water (6.21%) and high potassium content (1.64 mol-1) was successfully synthesized by a novel coprecipitation method with potassium citrate (K-CA) and potassium chloride (KCl) addition. Specifically, the electrode delivers a reversible capacity of 113.1 mA h g-1 at the current rate of 50 mA g-1 with-100% coulombic efficiency. Besides, the electrode retained 90% reversible capac-ity at 500 mA g-1 current density after 1000 cycles, indicating only 0.01% capacity decay per cycle. Moreover, we have revealed that the introduction of K-CA controlled the chelating rate of Fe(II) and the addition of KCl increased the K+ content, hence improving the capacity and stability of the as -prepared electrodes. Structural evolution and potassium storage mechanism were further investigated by detailed ex-situ X-ray diffraction and in-situ Raman measurements, which demonstrated reversible potassiation/depotassiation behavior and negligible volume change during the electrochemical process. In general, this work provides an efficient strategy to eliminate water contents in Prussian blue cathode and improve its electrochemical performance, which plays a key role in promoting the industrialization of potassium ion batteries. (c) 2023 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by ELSEVIER B.V. and Science Press. All rights reserved.

Hard Carbon have become the most promising anode candidates for sodium-ion batteries, but the poor rate performance and cycle life remain key issues. In this work, N-doped hard carbon with abundant defects and expanded interlayer spacing is constructed by using carboxymethyl cellulose sodium as precursor with the assistance of graphitic carbon nitride. The formation of N-doped nanosheet structure is realized by the CN• or CC• radicals generated through the conversion of nitrile intermediates in the pyrolysis process. This greatly enhances the rate capability (192.8 mAh g-1 at 5.0 A g-1 ) and ultra-long cycle stability (233.3 mAh g-1 after 2000 cycles at 0.5 A g-1 ). In situ Raman spectroscopy, ex situ X-ray diffraction and X-ray photoelectron spectroscopy analysis in combination with comprehensive electrochemical characterizations, reveal that the interlayer insertion coordinated quasi-metallic sodium storage in the low potential plateau region and adsorption storage in the high potential sloping region. The first-principles density functional theory calculations further demonstrate strong coordination effect on nitrogen defect sites to capture sodium, especially with pyrrolic N, uncovering the formation mechanism of quasi-metallic bond in the sodium storage. This work provides new insights into the sodium storage mechanism of high-performance carbonaceous materials, and offers new opportunities for better design of hard carbon anode.

Potassium vanadium fluorophosphate (KVPO4F) is regarded as a promising cathode candidate for potassium-ion batteries due to its high working voltage and satisfactory theoretical capacity. However, the usage of electrochemically inactive binders and redundant current collectors typically results in inferior electrochemical performance and low energy density, thus implying the important role of rational electrode structure design. Herein, we have reported a scalable and cost-effective synthesis of a cellulose-derived KVPO4F self-supporting electrode, which features a special surface hydroxyl chemistry, three-dimensional porous and conductive framework, as well as super flexible and stable architecture. The cellulose not only serves as a flexible substrate, a pore-forming agent, and a versatile binder for KVPO4F/conductive carbon but also enhances the K-ion migration ability. Benefiting from the special hydroxyl chemistry-induced storage mechanism and electrode structural stability, the flexible freestanding KVPO4F cathode exhibits high-rate performance (53.0% capacity retention with current densities increased 50-fold, from 0.2 C to 10 C) and impressive cycling stability (capacity retention up to 74.9% can be achieved over 1,000 cycles at a rate of 5 C). Such electrode design and surface engineering strategies, along with a deeper understanding of potassium storage mechanisms, provide invaluable guidance for better electrode design to boost the performance of potassium-ion energy storage systems.

Na3V2(PO4)(2)F-3, with robust 3D structural framework and high operating potential, is regarded as a promising cathode candidate for sodium-ion batteries. However, the low electrical conductivity caused by poly-anionic polyhedron leads to slow kinetics and poor rate performances. In this work, Cr3+ doped Na3V2(PO4)(2)F-3/C is prepared by using chromium as homovalent dopant ion. The synergistic effect of Cr3+ doping and carbon coating promotes the rapid transportation of electrons in Na3V2(PO4)(2)F-3/C particles. The Rietveld refinements show that Cr3+ doping can adjust the crystal structure and accelerate the diffusion kinetics of Na+. The Cr3+ doping im-proves the electrical conductivity of Na3V2-xCrx(PO4)(2)F-3/C. In-situ electrochemical impedance spectroscopy measurements show that Cr3+ doping effectively reduces the charge transfer impedance and plays an important role in improving the charge transfer kinetics. Consequently, the optimized Na3V1.98Cr0.02(PO4)(2)F-3/C exhibits excellent rate capability of 91.2 mAh center dot g(-1) at 30 C and long cyclic stability, in which the capacity decay per cycle is 0.0029% over 1000 cycles at 10 C. The results provide a facile strategy and beneficial way to design advanced Na3V2(PO4)(2)F-3 cathode toward sodium-ion batteries.

A highly stable intermediate hydrated MnHPO 4 is used to synthesize a well-crystallized LiMn 0.8 Fe 0.2 PO 4 @C cathode, which exhibits a high electrical conductivity of 6.823 × 10 −2 S cm −1 and excellent cycling stability with a capacity retention of 98.62%.

Although hard carbon has been considered as one of the most promising anodes for sodium-ion batteries, however, the poor performance and ambiguous sodium storage mechanism limit the further development. Herein, a novel epoxy phenol novolac resin is used as precursor for the first time to construct excellent electrochemical performance anode by pyrolyze it at different temperature. The results demonstrate that the plateau region in discharge curve of hard carbon is ascribed to Na insertion into the long-range ordered carbon structure. Improving ordering degree with suitable interlayer distance can both enhance the plateau capacity and total capacity. With the increasing of pyrolysis temperature, the graphitization degree of hard carbon is gradually enhanced. The cured epoxy phenol novolac resin pyrolyzed at 1800 degrees C delivers the reversible capacity of 480.3 mAh g- 1 at 50 mA g-1, high initial coulombic efficiency of 84.6%, and capacity retention of 92% after 1000 cycles at 500 mA g-1. The "adsorption-intercalation mechanism" for Na storage in EPNHC-1800 is verified by insitu Raman spectroscopy analysis. We believe that the sodium storage mechanism of hard carbon is "adsorptionintercalation-filling", namely, the adsorption of Na-ions on the surface of carbon layers providing the slope capacity, the intercalation of Na-ions between carbon layers and filling in the nanopores providing the plateau capacity. This study offers a novel precursor to synthesize low cost and high-performance hard carbon anode material for sodium-ion batteries, delivering great opportunity for the commercialization of anode.