An Investigation On Molybdate Polyanion Cathode Materials

Nowadays,lithium-ion batteries are the ideal choice for new energy vehicles,energy storage systems and portable digital products.However,the shortage of lithium resources leads to the restrictions on the application of lithium-ion batteries.Therefore,sodium ion batteries,which are similar with lithium ion batteries,have begun large-scale research because of their abundant sodium resources.Polyanionic electrode materials have a lot of advantages,such as the high structural stability and rich diversity,high working potential,high safety,etc.,so it is suitable for sodium ion batteries as cathode materials.The little studied cathode material of Polyanion molybdate provides a new research direction for polyanion materials.This paper optimizes the preparation of material and modifies the material.Meanwhile,this paper exploring the hygroscopicity of materials.Investigating the optimal preparation time of Na_2.67Mn_1.67（MoO₄）₃ and the appropriate preparation method.Firstly,Na_2.67Mn_1.67（MoO₄）₃ was successfully prepared by solid-phase method and the preparation time was optimized.The results of XRD and SEM show that the material was calcined for 0.5 h and the material was prepared with a minimum particle size.The first discharge specific capacity reached 53.41 mAh g^-1,which is the highest among all materials.The capacity retention rate is 45.61%after 20 cycles at 0.1 C.which indicates that calcination for 0.5 h is the optimal preparation time.The XRD and SEM results of solid phase method and sol-gel method show that the material obtained by the sol-gel method has higher crystallinity and smaller particle size than those of the solid phase method.The first discharge specific capacity of Na_2.67Mn_1.67（MoO₄）₃ obtained by the sol-gel method reached 64.12 mAh g^-1 and the capacity retention rate was 55.72%after 20 cycles at 0.1 C,which was 20.05%higher than that of the solid phase method,indicating that the sol-gel method is more suitable for the preparation of Na_2.67Mn_1.67（MoO₄）₃.The carbon coating of the Na_2.67Mn_1.67（MoO₄）₃ was carried out at 400°C with sucrose as carbon source in the atmosphere of Ar.And then we study the influence on the crystal structure and electrochemical properties.Both XRD and SEM results show that the Na_2.67Mn_1.67（MoO₄）₃/C composite was successfully prepared and no impurities were formed.The composite with a carbon content of 5%has a first discharge specific capacity of 84.11mAh g^-1,which is the highest in all composites.After 20 cycles,the capacity retention rate（63.35%）is the highest,which is 31.12%higher than that of uncoated materials,indicating the coated carbon has a significant improvement in the electrochemical properties of the material.We use the GITT method to measure the diffusion coefficient of sodium ions in the composites.The diffusion coefficient is in the range of from 1.62×10^-1515 to 2.30×10^-12 cm² s^-1.This paper exploring the hygroscopicity of Na_2.67Mn_1.67（MoO₄）₃.First,the XRD analysis shows that the material exposed to air for 10 d and 15 d may react with water and produce a certain amount of impurities.The impurities may be Na₂MoO₄?2H₂O.SEM image shows that the surface of the material became rough.Meanwhile,the partially enlarged view of the SEM show that the surface of the material was significantly cracked and detached,indicating that the reaction with water caused the deterioration of crystal structural.Electrochemical data analysis found that with the increase of the time exposed to air,the first discharge specific capacity of the material gradually decreased,indicating that the electrochemical properties of the material exposed to air deteriorated.The discharge specific capacities of the 1st,5th,10th,and 20th cycle of materials exposed to air for 15 d were 41.20,42.76,33.51,and 16.74 mAh g^-1,respectively,and the capacity retention rates of the 5th,10th,and 20th were 103.77,81.33and 40.63%,respectively.During the cycle,the discharge specific capacities of the material increases firstly and then decreases.The reason why the discharge specific capacity increases may be due to that the reaction with water results in a decrease in the electrochemical activity of the material,and the material gradually activates with the cycle progresses.The rapid decay of capacity may be due to that the reaction with water and the Jahn-Teller effect together leading to the deterioration of the crystal structure,hindering the diffusion of sodium ions in the material.