Pursuit of Sustainable Iron-Based Sodium Battery Cathodes: Two Case Studies

Rechargeable batteries have been the torchbearer electrochemical energy storage devices empowering small-scale electronic gadgets to large-scale grid storage. Complementing the lithium-ion technology, sodium-ion batteries have emerged as viable economic alternatives in applications unrestricted by volume/weight. What is the best performance limit for new-age Na-ion batteries? This mission has unravelled suites of oxides and polyanionic positive insertion (cathode) compounds in the quest to realize high energy density. Economically and ecologically, iron-based cathodes are ideal for mass-scale dissemination of sodium batteries. This Perspective captures the progress of Fe-containing earth-abundant sodium battery cathodes with two best examples: (i) an oxide system delivering the highest capacity (similar to 200 mA h/g) and (ii) a polyanionic system showing the highest redox potential (3.8 V). Both develop very high energy density with commercial promise for large-scale applications. Here, the structural and electrochemical properties of these two cathodes are compared and contrasted to describe two alternate strategies to achieve the same goal, i.e., improved energy density in Fe-based sodium battery cathodes.

Ionothermal Synthesis of High-Voltage Alluaudite Na2+2xFe2-x(SO4)(3) Sodium Insertion Compound: Structural, Electronic, and Magnetic Insights

Exploring future cathode materials for sodium-ion batteries, alluaudite class of Na2Fe2II(SO4)(3) has been recently unveiled as a 3.8 V positive insertion candidate (Barpanda et al. Nat. Commun. 2014, 5, 4358). It forms an Fe-based polyanionic compound delivering the highest Fe-redox potential along with excellent rate kinetics and reversibility. However, like all known SO4-based insertion materials, its synthesis is cumbersome that warrants careful processing avoiding any aqueous exposure. Here, an alternate low temperature ionothermal synthesis has been described to produce the alluaudite Na2+2xFe2-xII(SO4)(3). It marks the first demonstration of solvothermal synthesis of alluaudite Na2+2xM2-xII(SO4)(3) (M = 3d metals) family of cathodes. Unlike classical solid-state route, this solvothermal route favors sustainable synthesis of homogeneous nanostructured alluaudite products at only 300 degrees C, the lowest temperature value until date. The current work reports the synthetic aspects of pristine and modified ionothermal synthesis of Na2+2xFe2-xII(SO4)(3) having tunable size (300 nm similar to 5 mu m) and morphology. It shows antiferromagnetic ordering below 12 K. A reversible capacity in excess of 80 mAh/g was obtained with good rate kinetics and cycling stability over 50 cycles. Using a synergistic approach combining experimental and ab initio DFT analysis, the structural, magnetic, electronic, and electrochemical properties and the structural limitation to extract full capacity have been described.

Modulation of Buckling Dynamics in Nanoparticle Laden Droplets Using External Heating

Dynamics of contact free (levitated) drying of nanofluid droplets is ubiquitous in many application domains ranging from spray drying to pharmaceutics. Controlling the final morphology (macro to micro scales) of the dried out sample poses some serious challenges. Evaporation of solvent and agglomeration of particles leads to porous shell formation in acoustically levitated nanosilica droplets. The capillary pressure due to evaporation across the menisci at the nanoscale pores causes buckling of the shell which leads to ring and bowl shaped final structures. Acoustics plays a crucial role in flattening of droplets which is a prerequisite for initiation of buckling in the shell: Introduction of mixed nanocolloids (sodium dodecyl sulfate + nanosilica) reduces evaporation rate, disrupts formation of porous shell, and enhances mechanical strength of the shell, all of which restricts the process of buckling. Although buckling is completely arrested in such surfactant added droplets, controlled external heating using laser enhances evaporation through the pores in the shell due to thermally induced structural changes and rearrangement of SDS aggregates which reinitializes buckling in such droplets, Furthermore, inclusion of anilinium hydrochloride into the nanoparticle laden droplets produces ions which adsorb and modify the morphology of sodium dodecyl sulfate crystals and reinitializes buckling in the shell (irrespective of external heating conditions). The kinetics of buckling is determined by the combined effect of morphology of the colloidal particles, particle/aggregate diffusion rate within the droplet, and the rate of evaporation of water. The buckling dynamics leads to cavity formation which grows subsequently to yield final structures with drastically different morphological features. The cavity growth is controlled by evaporation through the nanoscate pores and exhibits a universal trend irrespective of heating rate and nanoparticle type.

Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni): towards high-voltage sodium battery applications

Sodium-ion-based batteries have evolved as excellent alternatives to their lithium-ion-based counterparts due to the abundance, uniform geographical distribution and low price of Na resources. In the pursuit of sodium chemistry, recently the alluaudite framework Na2M2(SO4)(3) has been unveiled as a high-voltage sodium insertion system. In this context, the framework of density functional theory has been applied to systematically investigate the crystal structure evolution, density of states and charge transfer with sodium ions insertion, and the corresponding average redox potential, for Na2M2(SO4)(3) (M = Fe, Mn, Co and Ni). It is shown that full removal of sodium atoms from the Fe-based device is not a favorable process due to the 8% volume shrinkage. The imaginary frequencies obtained in the phonon dispersion also reflect this instability and the possible phase transition. This high volume change has not been observed in the cases of the Co- and Ni-based compounds. This is because the redox reaction assumes a different mechanism for each of the compounds investigated. For the polyanion with Fe, the removal of sodium ions induces a charge reorganization at the Fe centers. For the Mn case, the redox process induces a charge reorganization of the Mn centers with a small participation of the oxygen atoms. The Co and Ni compounds present a distinct trend with the redox reaction occurring with a strong participation of the oxygen sublattice, resulting in a very small volume change upon desodiation. Moreover, the average deintercalation potential for each of the compounds has been computed. The implications of our findings have been discussed both from the scientific perspective and in terms of technological aspects.