The present work investigates the synthesis and characteristics of a Na super-ionic conductor -type Sc3+- and Yb3+-doped Na3Zr2Si2PO12 solid electrolyte for application in solid-state Na-ion batteries. A significant improvement of Na-ion conductivity in Na3Zr2Si2PO12 has been achieved through crystal engineering and microstructure refinement. The presence of the monoclinic-ZrO2 impurity phase adversely affecting the Na-ion conductivity is eliminated using the cubic-ZrO2 precursor at the place of monoclinic-ZrO2 in the conventional solid-state reaction method. Utilizing cubic-ZrO2 also refined the microstructure with thin and microcrack-free grain boundaries. A replacement of 16.5 at. % of Zr4+ by Sc3+ in Na3Zr2Si2PO12 enhances the room-temperature total ionic conductivity from 0.61 to 0.96 mS·cm–1. Replacing 11.11 at. % of Sc3+ by Yb3+ further improves the room-temperature ionic conductivity to 1.62 mS·cm–1, which is >2.5 times higher than that of bare Na3Zr2Si2PO12. The strategic approach used to raise the ionic conductivity in the current work can be applied to other materials, paving way toward realizing high-performance solid-electrolytes for viable and economic Na-ion batteries. A room-temperature conductivity of 1.51 mS·cm–1 for Sc3+/Yb3+-doped Na3Zr2Si2PO12 measured employing Na metal as electrodes confirms Na-ion conduction. Furthermore, a very low current density (∼10–7 A/cm2) in the cyclic-voltammetry profile of the Na|solid-electrolyte|Na cell demonstrates the suitability of Sc3+/Yb3+-doped Na3Zr2Si2PO12 as a solid-electrolyte for Na-ion batteries. A detailed analysis of these materials has been performed, and the possible reasons for the conductivity enhancement are discussed.

High-performance cathode materials are highly desirable for the realization of commercial Na-ion batteries. Sodium super ion conductor (NASICON)-type Na 3 V 2 (PO 4 ) 3 is one of the most promising cathode materials. In the present study, an improvement in the kinetics of the redox reactions and consequently, the electrochemical performance of Na 3 V 2 (PO 4 ) 3 cathode have been demonstrated by the partial substitution of Si for P. Nano-sized powders of Na 3+x V 2 (PO 4 ) 3−x (SiO 4 ) x (x = 0, 0.1, 0.2 and 0.4) are synthesized using the citric acid-assisted sol-gel route. A thin carbon layer of ∼8−16 nm thickness is formed on the calcined particles of the active material. The vanadium present in Na 3 V 2 (PO 4 ) 3 is shown to retain the trivalent state after the Si substitution. Coin cells are fabricated using an organic electrolyte, carbon nanotubes decorated Na 3+x V 2 (PO 4 ) 3−x (SiO 4 ) x as a cathode, and Na as an anode. Cyclic voltammetry profiles acquired on the half-cells confirmed (1) improved reversibility of the redox reactions, (2) lower polarization losses, and (3) superior sodium-ion diffusivity in the Si-substituted compounds. As a result, cells with these electrodes exhibit an enhanced electrochemical performance with a high specific capacity, excellent rate capability, and stable cycling performance. The electroactive material with Na 3.1 V 2 (PO 4 ) 2.9 (SiO 4 ) 0.1 composition exhibits ∼21% higher specific capacity than that with Na 3 V 2 (PO 4 ) 3 at 5C charging/ discharging rates, furnishing a Coulombic efficiency of >99%. The expansion of the crystal volume and the presence of excess Na-ion for ionic conduction are primarily responsible for the observed superior performance. The exceptional capacity of 85 mA h/g at 1C charging/discharging rates and superior cycling life achieved in Na 3.1 V 2 (PO 4 ) 2.9 (SiO 4 ) 0.1 /carbon nanotubes show that the strategy of expanding the crystal lattice through appropriate doping is an effective way to develop Na 3 V 2 (PO 4 ) 3 cathode for Na-ion batteries. KEYWORDS: sodium-ion battery, cathodes, NASICON, doping, Na 3 V 2 (PO 4 ) 3

In this study, we discuss the effect of different milling parameters, such as the type and concentration of surfactants, milling energy, and milling time on the structural, morphological and magnetic properties of hard magnetic SmCo5 nanoflakes prepared by surfactant assisted high energy ball milling. Two kinds of surfactants, polyvinylpyrolidone (PVP) with ethanol and oleic acid (OA) with n-heptane, were used as milling media. Increase in surfactants concentration and decrease in milling energy result in the decrease of degree of amorphization and reduction in grain size with milling time. Milling at 200 rpm results in more homogeneous and thicker flakes with fewer fractions of nanoparticles as compared to milling at 800 rpm. Increase in surfactants concentration results in the increase of the aspect ratio of flakes. Due to better capping ability of OA, the degree of flaking is higher when milling in OA than that in case of PVP. A maximum coercivity of 2.3 T was obtained after milling for 1.0 and 2.0 h for 10 and 50 wt. % of OA, respectively, at 800 rpm. A maximum (BH)max of 23.8 MGOe (188.9 kJ m−3) and degree of texture of 93% were obtained for 10 wt. % OA after 10 h of milling at 200 rpm. The pronounced anisotropy and high coercivity of the nanoflakes should prove advantageous for the preparation of textured exchange spring magnets. I. INTRODUCTIO

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