Manganese-Rich NASICON Sodium-Ion Batteries For High-Energy Na-Ion Batteries

In recent years,sodium-ion batteries are attracting increasing attention once again from researchers.Nevertheless,the serious structural damage of electrode materials during the Na⁺extraction/insertion process is caused by the intrinsic large radius and mass of Na⁺,which extremely limits the choice of the high-performance electrode.Na₄MnV（PO₄）₃（NMVP）cathode is considered to be one of the most promising cathode materials for large-scale application in sodium-ion batteries,due to the huge abundance,low cost,relatively high voltage platform and specific capacity from Mn.However,it still delivers a limited electrochemical performance due to the low conductivity and Jahn-Teller effect.To overcome such issues,the high-performance NMVP materials were adjusted and optimized through synthesis process optimization,in-situ doping modification,hierarchical structure construction and interfacial coating modification.Meanwhile,the effect of modification for cathode materials on the sodium storage mechanism was deeply studied,which can provide a strategy to further optimize the electrochemical performance of materials and realize the practical application of modified cathode materials in large-scale sodium storage systems.The specific studies are as follows:（1）The low-cost,low-toxicity NASICON-type three-dimensional porous spherical NMVP/C-SD（Na₄MnV（PO₄）₃/C-SD）cathode materials were synthesized by a simple spray drying-assisted technique and evaluated as cathode materials for NIBs.Compared with commercial nickel substrate oxide,manganese-based Prussian white and sodium vanadium phosphate cathode materials,the NMVP/C-SD materials manifest the good potential for practical applications.Experimental results affirmed that the porous spherical morphological structure could lead to higher electrical conductivity,smaller electrochemical polarization,and stable structure compared with the irregular composites prepared by the conventional sol-gel method,resulting in superior electrochemical performance.The cycling performance of NMVP/C-SD samples far exceeded that of NMVP/C composites synthesized by the sol-gel method（72.6%v.s.31.0%capacity retention after 2000 cycles at 5 C,respectively）.The higher sodium storage performance of NMVP/C-SD broadens the development of the synthesis process of manganese-based bimetallic NASICON structured phosphate cathode materials and provides new insights to promote the application of NIBs.（2）To address the problem of electrochemical structure changes caused by Mn²⁺/Mn³⁺redox reaction,based on the above research,we have proposed the Cedoping Na_3.9Mn_0.9Ce_0.1V（PO₄）₃@CeO₂/C materials（NMCVP@CeO₂/C,Cedoping content was 0.1 at%）.Benefiting from the homogeneous distribution of Ce,the crystal structure is regulated,the conductivity is significantly improved,and the electrochemical reaction kinetics is also promoted.The mechanism of Cedoping on the structural properties and ionic/electronic conductivity of NMVP materials was predicted by DFT calculation.The shortened bond length of Mn-O in the NMCVP structure after doping corresponds to a more stable structure and a reduced Mn³⁺Jane-Teller effect,which also inhibits the solubilization of transition metals.The NMCVP@CeO₂/C cathode half-cell exhibited an exceptional initial reversible specific capacity of 121.2 m Ah g^-1 at0.2 C,and a better capacity of 50.3 m Ah g^-1 at 50 C,while the undoped NMVP/C cathode provides only 33.9 m Ah g^-1 at 50 C.Besides,the assembled NMCVP@CeO₂/C//CHC full-cell delivered an energy density of 237.9 Wh kg^-1（based on the overall mass of the electrodes）and a high-capacity retention rate of 78.9%at 2 C for 400 cycles.Meanwhile,the reversible reaction of NMCVP@CeO₂/C cathode during the charging/discharging process was confirmed by GITT and ex-situ XRD measurements.（3）To improve the conductivity and realize the electrons/ions dual continuous conduction of materials,a scalable anti-solvent method with DMF was used to induce the directional growth of Na₄MnV（PO₄）₃ from random bulk morphology to uniform multi-level nanosheet self-assembly structure（NMVP-D）,which effectively shortens the diffusion path of Na⁺and improves the kinetics of Na⁺diffusion.Based on the above,a uniform conductive network coating layer is formed by polyaniline（PANI）during the high-temperature sintering process to synthesize a nanosheet assembled hierarchical petal-like NMVP microspheres（NMVP-D@cPANI）composites,in which ultra-small nanosheets were embedded in N-doped carbon conductive network.The NMVP-D@cPANI half-cell offers a high reversible capacity of 102.5 m Ah g^-1at 0.2 C and a superior rate capacity of 61.7 m Ah g^-1 at 50 C.Moreover,its long-term cycle measurement showed the capacity retention rate was 87.1%after 2000 cycles at 5 C and 85.5%after 4000 cycles at 10 C.In contrast,the NMVP-B cathode possessed negligible capacities of 2.5 and 1.5 m Ah g^-1,at 40and 50 C,respectively,and exhibited a low-capacity retention ratio of 70.4%at5 C for 2000 cycles.The Na⁺storage mechanism was studied by kinetic analysis（pseudocapacitance calculation and GITT test）and ex-situ XRD,indicating that its superior electrochemical performance was attributed to the fast ionic/electron diffusion pathway and the ultra-small nanosheet multi-level structure with good interface compatibility.NMVP-D@cPANI//CHC full cells exhibit a high operating voltage of 3.4 V and a high energy density of 351.9Wh kg^-1（based on the cathode active material）at a power density of 84.5 W kg^-1,which further demonstrates the great potential of the material for practical applications.（4）Na₄MnV（PO₄）₃/C composites with high soften point coal pitch as the carbon source were prepared.Based on the above,a combination of simple wet process and heat treatment was used to prepare hybrid NaPO₃-Al（PO₃）₃ loading NMVP/C@Al（PO₃）₃ materials,which enhanced surface/interface properties and stability of electrode materials.The optimal 2-NMVP/C@Al（PO₃）₃ cathode exhibited retention values of 88.5%（5 C）and 89.7%（10 C）over 3000 and4000 cycles,respectively,which are much superior to those of unmodified NMVP/C cathode（45.3%at 5 C and 44.7%at 10 C）.It is found that the main reason for the improvement of cycle stability is that the mixed loading layer composed of thermally stable Al（PO₃）₃ and high ionic conductivity NaPO₃ can promote the structural stability of the electrode material and the stability of the electrode/electrolyte interface,which is confirmed by in/ex-situ X-ray diffractometry.In addition,the high ionic conductivity of NaPO₃ is conducive to improving the conductivity of cathode materials.