Polyoxovanadate ionic crystals with open tunnels stabilized by macrocations for lithium-ion storage

Polyoxometalates (POMs) with multiple redox active sites have been reported as charge sponge for lithium-ion batteries (LIBs). Herein, we for the first time introduce a polyoxovanadate (POV) ionic crystals with macrocations, [Ni(Phen)₃][ClV₁₄O₃₄]Cl (NiV₁₄, Phen = 1,10-phenanthroline), as an anode material for LIBs. The existence of macrocation [Ni(Phen)₃]²⁺ stabilizes the open tunnels inside NiV₁₄. The NiV₁₄ electrode exhibits superior rate capabilities (1083 mAh·g⁻¹ at 100 mA·g⁻¹ and 384 mAh·g⁻¹ at 2000 mA·g⁻¹) due to the rapid capacitive dominated contribution and high Li⁺ ions diffusion coefficients (3.3 × 10⁻¹² cm⁻²·s⁻¹), and it delivers a remarkable cycling stability with a Coulombic efficiency of 99.7% after 1000 cycles at 2000 mA·g⁻¹. Such performance can be attributed to the stable structure of NiV₁₄ and the highly reversible valence changes of vanadium during the charge/discharge processes, which are revealed by a combination of in situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) measurements. This work not only demonstrates that NiV₁₄ with open tunnels stabilized by macrocation is a promising anode material for high performance LIBs, but also provides important references for the rational design of POMs electrode materials in advanced energy storage systems.