Examining the effects of nitrogen-doped carbon coating on zinc vanadate nanoflowers towards high performance lithium anode

Coating transition metal oxides with nitrogen-doped carbon is an efficient way to enhance lithium-ion battery performance by improving the conductivity and stability of the electrodes. So far, little attention has been paid to how the calcination process affects bimetallic oxides, such as zinc vanadate, with regards to the oxidation state of the metal, the zinc/vanadium ratio and the specific surface area. In this work, we report nitrogen-doped carbon coated zinc vanadate nanoflowers (particle size: 10 nm; coating layer thickness: 20 nm) with a high specific surface area (115 m² g⁻¹) through a facile method. High-angle annular dark-field scanning transmission electron microscopy, X-ray diffraction and electron paramagnetic resonance spectroscopy reveal that V⁵⁺ from the precipitated Zn₃(OH)₂(V₂O₇)(H₂O)₂ is largely converted to V³⁺ in ZnV₂O₄. A vanadium loss of about 9% during calcination lead to increased Zn/V ratio and formation of ZnO. When applied as anode in a lithium-ion battery, the as-prepared ZnV₂O₄/ZnO@N doped C exhibits a considerable reversible specific capacity of 620 mAh g⁻¹ at a current density of 0.1 A g⁻¹ after 50 cycles, very close to the theoretical capacity (651 mAh g⁻¹) and considerably higher than the non-coated counterpart (306 mAh g⁻¹). The material is stable during extended cycling (200 cycles at 0.5 A g⁻¹). In-depth electrochemical analysis including three-electrode system testing shows that the carbon shell is crucial in maintaining the structure stability and enhancing the capacity of the active material.