Structure Optimization And Sodium Storage Mechanism Of Manganese-Based Bimetal NASICON Cathode Materials

To overcome the low specific capacity and poor conductivity of the manganese-based bimetal NASICON nanomaterials,the structures of these materials were adjusted and optimized by in-situ carbon coating,dual carbon decoration,and three-dimensional conductive network design.Their structural characteristics can be analyzed through advanced morphology characterization technologies such as SEM,TEM.Meanwhile,the in-situ XRD can further monitor the structure evolutions of electrode materials during the charge/discharge processes,and reveal the energy storage mechanism of manganese-based bimetal NASICON cathode materials for sodium-ion batteries(SIBs).These works can not only provide theoretical guidance for improving the energy density and rate/cycling performance of SIB cathode materials,but also put forward new ideas for optimizing the electrochemical performances of sodium-ion full cells.Based on the above researches,significant achievements are briefly summarized as follows:(1)Na₃Mn Ti(PO₄)₃/C(NMTP/C)hollow microspheres with an open and stable NASICON framework were synthesized by a spray-drying-assisted process with post-annealing.After systematically comparing the structure design and micro-morphology,the three-electron redox reaction of NMTP/C-650 as cathode material for SIBs was investigated for the first time.Furthermore,the effect of sintering temperature on the electrochemical activity and cycling performance of the materials was also explored.The prepared NMTP/C demonstrated fully reversible three-electron redox reactions at 2.1,3.5,and 4.0 V(vs.Na⁺/Na),corresponding to the Ti^3+/4,Mn^2+/3+,and Mn^3+/4+redox couples,respectively.These reversible three-electron redox reactions endowed the NMTP/C a high specific capacity of 160m Ah g^-1 at 0.2 C(1 C=176 m Ah g^-1),and the highly stable NASICON framework ensured the superior cycling life of NMTP/C(?92%capacity retention after 500cycles at 2 C).In addition,the discharge capacity of NMTP/C could be recovered to92.3%when the C-rate returned back to 0.2 C after the high rate test.The in-situ XRD test indicated that both of solid-solution and two-phase reactions involved in the sodium storage process of NMTP/C.During the first charge process,2 Na⁺de-intercalates from NMTP.Upon subsequent discharge/charge,3 Na⁺reversibly intercalate into and de-intercalate from the NMTP framework.(2)Based on the above research on NMTP/C,it could be observed that low capacity/rate performance and unsatisfied cyclability limited its practical application.Therefore,we further proposed a dual carbon decoration strategy to tackle the above-mentioned issues and designed the semi-graphitic carbon and reduced graphene oxide co-functionalized NMTP(NMTP/C@r GO).The structure,morphology,and composition of the NMTP/C@r GO were systematically characterized.We also investigated the effects of dual carbon decoration and calcination temperature on sodium storage performance at high voltage(Mn^2+/4+).The NMTP/C@r GO demonstrated better sodium storage performance than the NMTP/C.NMTP/C@r GO delivers a specific capacity of?114 m Ah g^-1,reaching a high energy density of?410 Wh kg^-1 with an average discharge potential of?3.6 V at0.2 C(1 C=117 m Ah g^-1).At a relatively high rate of 1 C,the NMTP/C@r GO delivered a capacity of 98 m Ah g^-1 and capacity retention of 76.6%after 800 cycles,further testifying its excellent cyclability.The NMTP nanoparticles were stably wrapped up by semi-graphitic carbon and reduced graphene oxide,which leaded to the improved electrical conductivity,higher tap density,and effective bicontinuous conduction of electron and ion.The valence state conversion of manganese ion and the structural transformation of NMTP/C@r GO were determined by ex-situ XPS and in-situ XRD,respectively.These results demonstrated that both solid-solution and two-phase reactions were involved in the electrochemical reaction.Besides,the NMTP/C@r GO//SC full cell delivers an initial discharge capacity of 97 m Ah g^-1 at0.2 C and 73%capacity retention after 100 cycles,demonstrating excellent electrochemical performances.It was anticipated that the NMTP/C@r GO possessed great commercial potential in SIBs,and could be expected to provide new thoughts for the scalable development and application of SIBs with high energy density,excellent rate capability and long cycling scalability.(3)A combination of electrospinning and stepwise calcination technologies was used to successfully prepare a flexible self-supporting Na₄Mn V(PO₄)₃/C(FF-NMVP/C)cathode material.The structure,morphology,and element composition of the FF-NMVP/C were systematically studied.The ex-situ EIS was first used to investigate the effects of V and Mn on the ion diffusion rate during redox reactions.The FF-NMVP/C delivered a capacity of 89.5 m Ah g^-1 and 91.2%capacity retention after 150 cycles at 1 C(1 C=111 m Ah g^-1).At 5 C,the FF-NMVP/C cathode exhibited a stable Coulombic efficiency of 100%after 800 cycles,and the capacity retention of 81%(62.8 m Ah g^-1)could be achieved based on the second cycle.By ex-situ XPS and ex-situ EIS tests,the structure evolution,AC impedance and ion transportation of FF-NMVP/C under different charge/discharge states were systematically analyzed.Furthermore,a self-supporting full cell was assembled by using FF-NMVP/C as cathode and the prepared flexible self-supporting antimony-carbon(FF-Sb/C)as anode.The full cell could deliver a specific discharge capacity of 85.9 m Ah g^-1 after 100 cycles at 1 C with a capacity retention of 87.5%,and average discharge capacities of 99.8,100.2,97.8 m Ah g^-1 at 0.5,1,2 C,respectively.When the rate returned to 0.2 C,the average discharge capacity also got back to 100.8 m Ah g^-1.In this work,the carbon coating and continuous 3D conductive network for FF-NMVP/C were successfully constructed.The significantly increased electron/ion transfer rate endowed FF-NMVP/C good rate performance,stable cyclability and promising commercial prospects.