锂离子电池正极材料Na3V2（PO4）2F3的原位XRD及固体核磁共振研究

采用溶胶凝胶法制备Na3V2（PO4）2F3/C复合材料,该材料具有优异的电化学循环性能和倍率性能.利用电化学原位同步辐射X射线衍射（XRD）及魔角旋转固体核磁共振（MASSS-NMR）技术研究了Na3V2（PO4）2F3材料充放电过程中结构变化过程及Li/Na嵌入-脱出反应.研究结果表明,Na3V2（PO4）2F3的电极反应按嵌入-脱出反应机理进行,充放电过程中材料具有优异的结构稳定性.我们还发现Na3V2（PO4）2F3与电解液接触后与电解液中的Li＋发生部分交换反应形成LixNa3-xV2（PO4）2F3.在首次充电时,Li＋和结构中Na1位置的Na＋共同从晶格中脱出;而首次放电过程中,Na＋和Li＋共同嵌入到晶格中;充放电过程中发生的是Li＋和Na＋的共嵌入-脱出反应.

In situ XRD and solid state NMR characterization of Na3V2（PO4）2F3 as cathode material for lithium-ion batteries

In this work,Na3V2（PO4）2F3/C composite fluorophosphate cathode material is synthesized by a sol-gel process.The as prepared material shows excellent electrochemical performance.A capacity of 125 mAh/g of the material is obtained at the current density of 150 mA/g.When cycling at the current density of 3000 mA/g,it still delivers a capacity of 110 mAh/g（88% of the initial capacity at 150 mA/g）.The charging and discharging mechanism of this material is investigated by synchrotron-based in situ XRD and ex situ MAS SS-NMR.In situ XRD results indicate a single phase electrochemical delithiation/lithiation process and the material shows excellent structure stability during charge/discharge process.The ex situ MAS SS-NMR results show that Li-Na exchange happens when Na3V2（PO4）2F3 contacts with electrolyte,and the charge/discharge mechanism is：Li＋ and Na＋ in the Na1 site are deintercalated from the structure in the first charge process;Li＋ and Na＋ are co-intercalated in the structure in the following discharge process.