Preparation Of The Cathode Material Sodium Vanadium(Fluoride) Phosphate And Study Of Its Working Mechanism For Hybrid-ion Batteries

Abstract:According to the "hybrid ion battery" concept proposed by predecessors, in this paper, we have studied the preparation of the cathode material sodium vanadium phosphate (fluoride) and its working mechanism for hybrid ion batteries.Fistly, on the basis of the traditional two-step high-temperature solid-state method, we develop a simpler and more efficient new process, mechanical activation assisted one-step solid-state carbothermal reduction (one-step method), to synthesis Na3V2(PO4)2F3material. The material prepared by one-step method exhibits better rate capability and cycle stability. At0.1C rate, the initial discharge specific capacity is112.7mAh·g-1with two voltage plateaus near4.2V and3.7V. When the discharge increased to10C, the initial specific capacity remains87.7mAh·g-1. In addition, the material also exhibit excellent cycle stability. Furtherly, the effect of F content in the material on its electrochemical performance is investigated. With the reduction of the F content (corresponding to [PO43-] increasing), the4.2V voltage plateau is gradually shortened, and the3.7V one is gradually extended. Then, the new Na3V2(PO4)3phase is obtained with single plateau and good electrochemical performance.Secondly, single factor controlled experiment is applied to determine the optimum conditions for synthesis of Na3V2(PO4)3material:glucose addition level of80g/mol Na3V2(PO4)3, mechanical activation time of6h, calcinations temperature of700℃, roasting time of6h.Finally, the working mechanism of the Na3V2(PO4)3material in the hybrid ion system is studied by the electrolyte immersion test and whole cell test, combined with the analysis of crystal structure and electrochemical charancteristics of the material. The Li+ions in the electrolyte will begain to exchange the Na+ions in the material when the two phases contact. Over the initial charging, the deintercalation of the remaining active Na+ions and exchanged Li+ions in the material co-occurs. Over the subsequent cycles, the ion conductors in the system are Na+/Li+hybrid ions. As the cycle progresses, finally, Li+ions lead the ion migration, and the Na+ions in the electrolyte just balance the charge in the system. In the whole cell, Na3V2(PO4)3material can also work stably, which proved that sodium-based materials can work not depending on a lot of active Li+ions provided by anode.