| With the continuous expansion of 3C batteries,power batteries and energy storage grids,the demand for lithium-ion batteries has soared,making it difficult to meet the growing demand.Sodium and lithium belong to the first main group and have many similar physicochemical properties.The electrode potential is close,and the reserves are abundant and the price is low.Therefore,sodium ion batteries are one of the most likely substitutes for lithium-ion batteries.Among them,Na2MnPO4F cathode material with high working potential and high theoretical capacity has become a research hotspot,but the lower electron and ionic conductivity affect its electrochemical performance.Therefore,in this study,Na2MnPO4F/C materials with conductive carbon coating,good morphology and small particle size were prepared by spray drying and electrospinning.In addition,Na3V2(PO4)2F3 of NASICON structure was used as the second phase to composite to improve its electrochemical performanceThe hollow spherical structure of Na2MnPO4F/C was prepared by spray drying method.Compared with dense or irregular particles,the hollow sphere can increase the reaction area between the electrolyte and the electrode.The micro-sized hollow spherical shell is composed of many contact nano-sized Na2MnPO4F primary particles;in addition,each nano-sized Na2MnPO4F is surrounded by uniform carbon to form a good conductive network,which effectively improves the electrochemical properties of the material.The electrochemical results show that the sample with carbon-coated hollow spheres exhibits better performance.The sample prepared at 650℃ has a first discharge specific capacity of 102.4 mAh·g-1 at 0.05 C;after 30 cycles,the capacity retention rate is 72.4%;in addition,the charge transfer resistance(Rct)is lower than other samples,and the exchange current density(i0)is higher;moreover,the obvious charge and discharge platform(-3.6 V)is observed at room temperature.The Na2MnPO4F/C nanofiber material was prepared by electrospinning.Conductive carbon is used as a fiber matrix with a diameter of about 100 nm,and Na2MnPO4F particles with a particle size of 10～30 nm are embedded therein to form a three-dimensional hybrid conductive network structure,giving full play to the dynamic advantages of nanomaterials.It was found that only after calcination at 250 ℃ for 2 h in air and calcined not higher than 650 ℃ in argon atmosphere,the samples can maintain a good nanofiber structure.Analysis of the TG and FI-IR test data,as a raw material PVP of the nanofiber matrix needs to undergo a pre-oxidation process,a cyclo-configuration reaction occurs,in order to form a stable structure with high temperature resistance.Electrochemical tests show that only samples with good nanofiber structure can exhibit better performance.The initial discharge specific capacity(122.4 mAh·g-1)at 0.05 C is very close to the theoretical value(124.7 mAh·g-1);-3.6 V showed an obvious charge and discharge platform;the discharge specific capacity retention rate after cycling 50 times at 0.05 C was 66.2%.In addition,cyclic voltammetry tests show that the diffusion coefficient of Na ions in the electrode material is on the order of 10-13～10-14,which is about three orders of magnitude higher than reported in the literature.In addition to the modification of the Na2MnPO4F material itself,the second phase of Na3V2(PO4)2F3 introduced into the NASCION structure by sol-gel method can generate lattice defects and provide more sodium ion transport active sites,thereby improving electronic and ionic conductivity of Na2MnPO4F.The effects of Mn-doped Na3V2(PO4)2F3 and different compounding ratios on the materials were investigated in detail.The data show that Mn doping and composite Na3V2(PO4)2F3 can reduce particle size,improve morphology and improve electrochemical performance.Among them,Na3(V1-2y/3Mny)2(PO4)2F3 with a doping amount of y=0.05 and Na2MnPO4F·Na3V2(PO4)2F3/C with a compound ratio of 1:1 have the smallest pore size,the largest number of pores,and the pore walls most thin,the particles are also finer.When the doping amount is y=0.05,the specific discharge capacities at 0.1 C and 0.2 C rate are 116.7 and 113.9 mAh·g-1,respectively,and there is a more obvious charge and discharge platform than other samples.At 1:1,the first discharge specific capacity at 0.05 C rate is 126.6 mAh·g-1,which is close to the theoretical specific capacity of 126.7 mAh·g-1.The first discharge specific capacity at 1 C rate also reaches 76.9 mAh·g-1.And the sample rose from 0.05 C to 0.2 C,the specific discharge capacity was only attenuated by 5%,and the capacity retention rate at each magnification was about 98%,showing good rate performance and cycle performance.The AC impedance test shows that the exchange current density i0 increases gradually after doping with Mn;as the Na3V2(PO4)2F3 composite ratio increases,the charge transfer resistance Rct of the material decreases gradually. |
PDF Full Text Request