Comparison of phase composition, morphology and electrochemical property for Li3−xNaxV2(PO4)3 (x=0.5, 1.5 and 2.0) as lithium storage cathode materials

Three Li 3-x Na x V 2 (PO 4 ) 3 (x=0.5, 1.5 and 2.0) samples are synthesized by a traditional solid-state reaction method in this work. Their phase composition, surface morphology and electrochemical property are described and compared by using various physical/chemical methods. Phase analysis results reveal that Li 2.5 Na 0.5 V 2 (PO 4 ) 3 consists of monoclinic Li 3 V 2 (PO 4 ) 3 , rhombohedral Li 3 V 2 (PO 4 ) 3 and rhombohedral Na 3 V 2 (PO 4 ) 3. While, both Li 1.5 Na 1.5 V 2 (PO 4 ) 3 and LiNa 2 V 2 (PO 4 ) 3 are the two-phase mixture consisted of rhombohedral Li 3 V 2 (PO 4 ) 3 and rhombohedral Na 3 V 2 (PO 4 ) 3 . Electrochemical testing results reveal that LiNa 2 V 2 (PO 4 ) 3 and Li 1.5 Na 1.5 V 2 (PO 4 ) 3 can deliver the initial discharge capacities of 101.4   mAh   g −1 and 108.6   mAh   g −1 with a long potential plateau at 3.69   V, respectively. In contrast, Li 2.5 Na 0.5 V 2 (PO 4 ) 3 presents an initial discharge capacity of 111.7   mAh   g −1 with four potential plateaus. High lithium ion diffusion coefficient in Li 2.5 Na 0.5 V 2 (PO 4 ) 3 indicates that the existence of monoclinic Li 3 V 2 (PO 4 ) 3 phase can improve the ionic conductivity and then be responsible for good electrochemical performance. Besides, in-situ X-ray diffraction observation of LiNa 2 V 2 (PO 4 ) 3 demonstrates that the phase transition is not fully reversible but quasi-reversible during the lithiation-delithiation process. The partial irreversibility of structural evolution for LiNa 2 V 2 (PO 4 ) 3 induces the capacity loss upon repeated cycles.