Cr-doped Na3V2(PO4)3@C enables high-capacity with V2+/V5+ reaction and stable sodium storage

Due to its abundant sodium content and low cost, sodium-ion battery (SIB) has become an effective substitute and supplement for lithium-ion batteries, which has a broad development prospect in large-scale energy storage systems. Na-super-ionic conductor (NASICON) structural materials have stable 3D skeleton structures and open Na+ transport channels, which is a very promising SIB cathode material. But in the typical NASICON material Na3V2(PO4)3 (NVP), the number of electrons involved in NVP per formula unit is less than 2 at the stable voltage window, which limits the further improvement of battery performance. In this work, we report another NASICON structured Na3V4/3Cr2/3(PO4)3@C (NVCP@C), which is obtained by Cr-doped NVP through spray drying. By taking full advantage of the voltage platforms of V5+/4+, V4+/3+, and V3+/2+ in the window of 1.5–4.4 V, NVCP@C delivered a high discharge capacity (175 mAh g–1) and durable cyclability (86% capacity retention for 2000 cycles). In-situ X-ray diffraction results demonstrate that the reversible structural evolution accompanies by solid-solution reaction and two-phase reaction mechanisms co-exist during charge/discharge processes. When coupled with Na+ pre-embedded hard carbon (HC), the assembled NVCP@C//HC full cell delivers a high capacity (105 mAh g–1) and long cycling performance (70% after 1000 cycles). This Cr-doped NVP method offers new insights into the design of high-energy NASICON-structured cathode materials.