Hollandite-Type VO1.75(OH)0.5: Effective Sodium Storage for High-Performance Sodium Ion Batteries
Authors: Jo Jae Hyeon , Kim Jongsoon , Myung Seung-Taek 
Published: February 2020

Lithium-ion batteries (LIBs) have attracted significant attention as high-energy-density rechargeable power sources for electric vehicles and large-scale energy storage applications. Although LIBs have been successfully commercialized in the world battery market, the expected shortage of lithium resources resulting from the ever-increasing demand for LIBs needs to be addressed. This issue has accelerated the search for alternative power sources such as sodium-ion batteries (SIBs), which are emerging as promising candidates to replace LIBs. Sodium is one of the most plentiful resources on earth; thus, SIBs have been extensively studied because of their low cost as well as their competitiveness in terms of energy density versus LIBs. However, the ionic size of Na+ (1.02 Å) is larger than that of Li+ (0.76 Å), which affects the phase stability, transport properties, and interphase formation. The low standard redox potential of sodium (−2.71 V vs. standard hydrogen electrode (SHE)) compared with that of lithium (−3.04 V vs.
SHE) is another demerit of SIBs; hence, a high capacity is necessary to compensate for the low operation voltage to approximate the energy density levels of LIBs. Thus far, various types of electrode materials have been suggested as cathode materials for SIBs, including Na2/3MnO2, Na2/3[MxMn1
−x]O2 (M: transition metals), NaCrO2, α-MnO2, and Na3V2PO3. Vanadium oxides (VxOy) is of interest because of the wide range of oxidation states available from +2 (as in VO) to +5 (as in V2O5), which affects the structural, electrochemical, and magnetic properties. This property has led to the application of vanadium oxides in catalysts, chemical sensors, and energy storage devices such as rechargeable lithium batteries. Herein, we introduce new nanosized hollandite-type VO1.75(OH)0.5 as a novel cathode material for Na-ion batteries. Structural investigation based on X-ray diffraction (XRD), neutron diffraction (ND), and Rietveld refinement suggests the presence of numerous vacant sites for Na+ intercalation and structural information on hydrogen in the VO1.75(OH)0.5. All of the possible Na+ sites and tunnel-type Na+ diffusion pathways along the c-axis are confirmed by bond-valence-sum analyses. The nanosized hollandite-type VO1.75(OH)0.5 delivers an unexpectedly high specific capacity of approximately 351 mAh g−1 at 15.5 mA g-1 in the voltage range of 1.0–3.7 V (vs. Na+/Na), which agrees well with the results predicted by first-principles calculations. In addition, combined studies using first-principles calculations and several experimental techniques including in-situ operando X-ray diffraction and ex situ X-ray absorption spectroscopy confirm that the nanosized hollandite-type VO1.75(OH)0.5 undergoes a single-phase reaction with a capacity retention of 71% over 200 cycles. Furthermore, the open structure and nanosized particles of hollandite-type VO1.75(OH)0.5 contribute to its excellent power capability with 56% of the capacity measured at 0.05C being delivered at 7C. These results indicated the outstanding cyclability of Na
x
VO1.75(OH)0.5 as a promising cathode material for SIBs. We believe that our novel approach will provide insight for the discovery and understanding of novel electrode materials for not only SIBs but also other rechargeable batteries.

Figure 1