Operando X-ray Studies of Ni-Containing Heteropolyvanadate Electrode for High-Energy Lithium-Ion Storage Applications.

Ni-containing heteropolyvanadate, Na 6 [NiV 14 O 40 ], was synthesized for the first time to be applied in high-energy lithium storage applications as a negative electrode material. Na 6 [NiV 14 O 40 ] can be prepared via a facile solution process that is suitable for low-cost mass production. The as-prepared electrode provided a high capacity of approximately 700 mAh g -1 without degradation for 400 cycles, indicating excellent cycling stability. The mechanism of charge storage was investigated using operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), transition X-ray microscopy (TXM), and density functional theory (DFT) calculations. The results showed that V 5+ was reduced to V 2+ during lithiation, indicating that Na 6 [NiV 14 O 40 ] is an insertion-type material. In addition, Na 6 [NiV 14 O 40 ] maintained its amorphous structure with negligible volume expansion/contraction during cycling. Employed as the negative electrode in a lithium-ion battery (LIB), the Na 6 [NiV 14 O 40 ]//LiFePO 4 full cell had a high energy density of 300 W h kg -1 . When applied in a lithium-ion capacitor, the Na 6 [NiV 14 O 40 ]//expanded mesocarbon microbead full cell displayed energy densities of 218.5 and 47.9 W h kg -1 at power densities of 175.7 and 7774.2 W kg -1 , respectively. These findings reveal that the negative electrode material Na 6 [NiV 14 O 40 ] is a promising candidate for Li-ion storage applications.