Search

The composite structure materials in sodium-ion batteries (SIBs) have received increasing attentions due to the synergistic effect. P2/P3 composite cathode with the advantages of high reversible capacity and superior reaction kinetics was regarded as one of the promising cathodes for SIBs. Crystal phase component ratio and particle morphology of hybrid structures are closely related with the electrochemical performance, especially the energy density. Herein, P2/P3 hybrid structure materials Na0.6Mn1-xNixO2 were synthesized by co-precipitation method. Furthermore, the component ratio and particle size are tuned and realized via simple Ni2+ content optimization. The targeted sample Na0.6Mn0.75Ni0.25O2 shows high tap density over 1.2 g cm−3 and excellent electrical properties with an initial capacity of 101.36 mA h g−1 at 0.2 C, corresponding to a high volumetric energy density of 512 Wh L−1 based on the cathode active material. Moreover, the long-term cycling capacity retention can reach 68 % at 1 C after 500 cycles. The present study develops a promising cathode of SIBs that maybe applied in low-speed electric vehicles. And the simultaneous optimization design represents a potential route for the regulation of composite structures to obtain high performance SIBs.

The application of Na3·32Fe2·34(P2O7)2 sodium cathodes with abundant resources and robust structure is severely hindered by the poor electronic conductivity. A multiple-dimensional carbon matrix could provide a satisfactory electronic conductivity as well as facilitate structural stability via wrapping and bridging of the active material. With an analysis of the raw materials and their respective reactions, Na3·32Fe2·34(P2O7)2@C (NFP-Cs) composites with a hybrid carbon matrix have been synthesized. The reactions in the system result in versatile N-doped carbon structures, including thin carbon layers, carbon rods, carbon nanosheets and carbon scaffolds. This integrated carbon architecture wraps and bridges the Na3·32Fe2·34(P2O7)2 particles, not only promoting electronic conductivity, but also serving as a buffer to minimize volume changes and stabilize the structural integrity of the composite. Furthermore, the nitrogen doping could generate more defects and active sites, which provide improved conductivity and better redox activity. Finally, the synthesized composite material has demonstrated impressive rate capabilities (100 mAh g−1 at 0.1 C and 66 mAh g−1 at 20 C), and outstanding cycling stability (90.5% at 1 C after 1000 cycles, 89.9% at 5 C after 2000 cycles). This study takes the utmost advantage of the synthesis, which is also easily scaled-up for mass production.

Though P2-type Na-Mn-Ni-O cathodes with low cost and environmental benign have been extensively investigated, it is still challenging to fulfill practical application with limited cycle stability and low operating potential. However, the research of the reactive Ni 2+ /Ni 3+ redox pair that enable both high working potential and high reversible capacity was almost ignored. And traditional cationic doping mainly focusses on crystal modulation and structure stabilization. In this study, an anion doping strategy (F-doping) was carried out to further boost the activity of Ni 2+ . The multi-function including structural parameter modulation and robust Ni-F bond significantly promote the activation of Ni 2+ /Ni 3+ and advance the respective working potential. Related reaction kinetics and structural evolution were also investigated to elucidate the F-doping mechanism. The Na 0.6 Mn 0.95 Ni 0.05 O 1.95 F 0.05 with appropriate F content could deliver a high capacity of 80.76 mA h g −1 at 2C with 75.0% retained after 960 cycles. The excellent rate capability and full cell performance also demonstrate the high potential. This study may provide a new sight for the development of P2-type layered Na-Mn-Ni-O cathode, which could be extended to other oxides cathode.

The peak-loading shift function of sodium-ion batteries in large-grid energy store station poses a giant challenge on the account of poor rate performance of cathodes. NASICON type Na 3 V 2 (PO 4 ) 3 with a stable three-dimensional framework and fast ion diffusion channels has been regarded as one of the potential candidates and extensively studied. Nevertheless, a multilevel integrated tactic to boost the performance of Na 3 V 2 (PO 4 ) 3 in terms of crystal structure modulation, coated carbon graphitization regulation, and particle morphology design is rarely reported and deserves much attention. In this study, organic ferric was used to prepare Fe-doped Na 3 V 2 (PO 4 ) 3 @C cathode on the account of low cost, environmental friendliness, and catalytic function of Fe on carbon graphitization. The density functional theory calculation depicts that the most stable site for Fe atom is the V site and moderate replacement of Fe at V position would reduce the band gap energy from 2.19 by 0.43 eV and improve the electron transfer, which is crucial for the intrinsic poor conductivity of Na 3 V 2 (PO 4 ) 3 . The experimental results show that Fe element can be introduced into the bulk structure successfully, modulating relevant structural parameters. In addition, the coated carbon layer graphitization degree is also regulated due to the catalysis function of Fe. And, the decomposition of organic ferric would infuse the formation of porous structure, which can promote electrolyte permeation and shorten the electron/ion diffusion. Finally, the optimized Na 3 V 1.85 Fe 0.15 (PO 4 ) 3 @C could possess a high capacity of 103.69 mA h g -1 and retain 91.45% after 1200 cycles at 1.0C as well as 94.45 mA h g -1 at 20C. In addition, the excellent performance is comprehensively elucidated via ex situ X-ray diffraction and pseudocapacitance characterization. The multifunction contribution of Fe-doping may provide new clue for designing porous electrode materials and a new sight into F

The development of prospective cathode candidates is one of the key issues for the practical application of sodium ion batteries. Although alluaudite Na2-2xFe2-x(SO4)3 (NFS) cathode combines natural abundance, environmental benign and higher theoretical energy density than conventional polyanionic cathodes, the present available performance is unsatisfying because of limited electronic conductivity. The traditional in situ carbon coating via wet chemistry and thermolysis is infeasible due to the moisture sensitivity and poor thermal stability. In this study, the above issues are simultaneously tackled via a simple but effective method assisted by graphene addition with the merits of high conductivity, hydrophobicity, and stretchable nature. Since the added graphene could affect the growth of NFS particle, the nanoparticle could be wrapped well and form NFS@graphene composite, which could keep from moisture attack. Moreover, the functional groups of the graphene precursor could boost the crystallization. The NFS@graphene could deliver an initial reversible capacity of -106 mAh g-1 at 25 °C, and a capacity retention of >98% at 0 °C is retained over 700 cycles. The inspiring results would cast insight on the graphene-assisted synthesis and were meaningful for scaling up the Na2-2xFe2-x(SO4)3 cathodes.

A novel strategy for confined synthesis of graphene wrapped nano-sized LiMn0.5Fe0.5PO4 hybrid composite has been developed, including co-precipitation and solvothermal reactions. The LiMn0.5Fe0.5PO4 nanoparticles with a constrained diameter of 20 nm are homogeneously wrapped by a continuous interconnected graphene sheets. The mechanism and composite structure evolution during the process are carefully investigated and discussed. With the shortened Li+ diffusion paths and enhanced electron conductivity, the hybrid composite shows high discharge capacity and superior rate performance with the discharge capacities of 166 mA h g−1 at 0.1 C and 90 mA h g−1 at 20 C. Excellent cycle stability is also demonstrated with only about 7.8% capacity decay after 500 cycles at 1 C.

A series of xLi1.5Ni0.25Mn0.75O2.5·(1 − x)Li0.5Ni0.25Mn0.75O2 (0 ≤ x ≤ 1) cathode materials have been synthesized. These compounds exhibit dramatic differences in structure, morphology and charge/discharge characteristics. As the x increases, the morphology shows an amazing trend: starting with an octahedral shape (x = 0), transforming to an octahedral/plate shape (0.1 ≤ x ≤ 0.9) in which both the spinel phase and the layered phase can be indexed in the XRD patterns, and ending up with a plate shape (x = 1.0). The particular layered-spinel composites xLi1.5Ni0.25Mn0.75O2.5·(1 − x)Li0.5Ni0.25Mn0.75O2 (0.1 ≤ x ≤ 0.9) exhibit better cycling stability than that of pristine spinel Li0.5Ni0.25Mn0.75O2 (x = 0) and layered Li1.5Ni0.25Mn0.75O2.5 (x = 1.0) materials. This improved cycling performance of these layered-spinel composites can be ascribed to the heterogeneous intergrowth of some layered phases and spinel phases in the parent structure as detected by TEM. Among these materials, Li0.5Ni0.25Mn0.75O2 and Li1.5Ni0.25Mn0.75O2.5 barely deliver the specific capacities of 90 mA h g−1 and 117 mA h g−1 at 5 C and show the capacity retentions of about 83% and 86% at 0.2 C after 50 cycles, respectively, while the layered-spinel 0.8Li1.5Ni0.25Mn0.75O2.5·0.2Li0.5Ni0.25Mn0.75O2 cathode shows the best rate capability of 162 mA h g−1 at 5 C and the best cycling stability of 98% after 50 cycles at 0.2 C.

A series of nanocrystalline lithium-rich cathode materials Li1.5Mn0.75Ni0.25O2.5 have been prepared by a novel synthetic process, which combines the co-precipitation method and a modified molten salt method. By using a moderate excess of 0.5LiNO3-0.5LiOH eutectic salts as molten media and reactants, the usage of deionized water or alcohol in the subsequent wash process is successfully reduced, compared with the traditional molten salt method. The materials with different excess Li salt content, Li/M (M = Ni + Mn) = 1.55, 1.65, 1.75, 1.85, 1.95, 2.05, molar ratio, show distinct differences in their structure and charge-discharge characteristics. The structural characterization demonstrates that the sample with a ratio of Li/M = 1.85 has a more well-defined alpha-NaFeO2 structure and a more enlarged Li layer spacing. It also exhibits the best comprehensive electrochemical behavior with the highest coulombic efficiency, the best rate capability and optimal cycling stability. More specifically, it delivers a dramatically improved initial coulombic efficiency of 87.86%, and a discharge capacity of 129 mA h g-1 even at an ultra-high current density of 2000 mA g-1 (10C). Meanwhile a superior cycling stability is also observed with a high discharge capacity of 251 mA h g-1 and a retention of 98% at 0.2C after 50 cycles. Our results reveal that this method is facile and feasible to synthesize a high rate and high capacity lithium-rich material.

LiMn0.5Fe0.5PO4 (LMFP) materials are synthesized by the hydrothermal approach in an organic-free and surfactant-free aqueous solution. The phase and morphological evolution of the material intermediates at different reaction temperatures and times are characterized by XRD, SEM and TEM, respectively. The results show that during temperature increase, the solubility product (Ksp) of the precursors (Li3PO4, Fe3(PO4)2 and (Mn,Fe)3(PO4)2) is the decisive parameter for the precipitation processes. Once the temperature locates at the range of 100-110 °C, the unstable precursors dissolve quickly and then LMFP nuclei are formed, followed by a dissolution-reprecipitation process. As the reaction progresses, the primary particles self-aggregate to form rod or plate particles to reduce the overall surface energy through oriented attachment (OA) and the Ostwald ripening (OR) mechanism. Moreover, the resultant concentration of the precursor significantly affects the crystal size of LMFP by altering the supersaturation degree of solution at the nucleation stage. The carbon coated LMFP nanostructure synthesized at 0.6 mol L-1 (resultant concentration of PO43-) delivers discharge capacities of 155, 100 and 81 mA h g-1 at 0.1, 5 and 20 C rate, respectively. The understanding of nanostructural evolution and its influence on the electrochemical performance will pave a way for a high-performance LMFP cathode.