All-Solid State Na-Ion Batteries Na2-2dFe2-D(SO4)3|Na3-XMxP1-XS4 (M=Ge4+,Ti4+, Sn4+) (x=0,0.1) | Na2Ti3O7
Authors: Prasada Rao Rayavarapu , Chen Haomin , Wong Lee Loong , Adams Stefan 
Published: February 2020

Sodium ion batteries attracted large research community because of earth-abundance and low cost of sodium. Though significant advances have been made in the development of Na-ion cathodes and anodes, a major impediment to the commercial viability of rechargeable Na-ion batteries is the lack of an effective electrolyte. Current Na-ion electrolytes are mostly based on the same fundamental chemistry as Li-ion electrolytes – a mixture of cyclic and linear organic carbonates with a Na-salt. Such organic solvent-based liquid electrolytes are flammable and have limited electrochemical windows. A promising alternative architecture is all-solid-state Na-ion rechargeable batteries utilizing non-flammable ceramic Na superionic conductor electrolytes. Na solid electrolytes such as -alumina and NAtrium Superionic CONductors (NASICON) are well-studied, but they typically exhibit reasonable ionic conductivities only at higher temperatures. Here we report the synthesis, characterization of materials that from our parallel computational studies appeared to be promising components of all-solid state rechargeable Na-ion batteries with high rate capabilities.

Recently it is reported that the cubic phase of Na3PS4 (c-Na3PS4, space group: I4-3m) is a promising solid electrolyte with a room temperature Na+ ion conductivity of 0.2mS/cm. In order to increase the ionic conductivity further we prepared Na3-xMxP1-xS4 (M=Ge4+,Ti4+, Sn4+) (x=0,0.1) using mechanical milling followed by annealing at 250°C. Maximum room temperature ionic conductivity of 0.25mS/cm was observed for Sn doped NaP3S4. In situ high temperature XRD showed that all the compounds are stable up to 400°C.

When it comes to identifying cathode materials for such a NIB , a considerable fraction of research activity still focuses materials analogous to those that work well in Li-ion batteries containing costly transition metals such as Co, which appears problematic for various reasons: differences in reaction mechanisms of the Na analogues to the Li transition metal oxides, different structural requirements for Na+ mobility, the fact that a cost advantage of Na over Li will only translate into low cost batteries. Hence, research on transition metal electrode materials for Na-ion batteries will have to concentrate on abundant transition metals (essentially Fe, Ti, Mn, and possibly Cr, V). On the other hand, O3- NaFeO2, and P2-type Nax[Fe0.5Mn0.5]O2 utilizing the Fe4+/Fe3+ redox couple are known to suffer from limited reversible capacity and low operating potential. Here we report the preparation of Na2+2dFe2-d(SO4)3 by mechanical milling of Na2SO4 and FeSO4 followed by annealing at 400ºC in an argon environment. The Na2-2dFe2-d(SO4)3 formed an alluaudite-type structure in space group C2/c with lattice constants a=12.654(1)Å, b=12.761(9)Å, c=6.503(5)Å, β=115.52(1)º close those previously reported for d =0.12.

For anode transition metal oxides the number of low potential lithium insertion compounds is rather limited owing to the competition of insertion vs conversion reactions, the former being only favored for early 3d metal (Ti, V) oxides. Along that line, NaxVO2 was recently found to reversibly react with Na at ca. 1.5 V but sensibly lower operation voltages would be needed for enhanced energy density. Titanium-based systems are considered as best alternatives. Hence, Na2Ti3O7 was prepared from anatase TiO2 and anhydrous Na2CO3 mixtures with 10% excess of the latter with respect to stoichiometric amounts. These mixtures were milled and treated at 800ºC for 20 h with intermediate regrinding. X-ray diffraction pattern indicated the formation of pure crystalline phase as shown in the figure with space group P121/m.

Based on the synthesized and characterized materials we assembled all-solid state sodium-ion batteries combining the alluaudite-type cathode with Na3PS4 as the solid electrolyte and Na2Ti3O7 as the anode. Favourable rate performance of the all solid state sodium-ion battery was demonstrated at room temperature.

Figure 1