Debasmita Dwibedi

Na2.44Mn1.79(SO4)3: a new member of the alluaudite family of insertion compounds for sodium ion batteries

Sodium-ion batteries have been extensively pursued as economic alternatives to lithium-ion batteries. Investigating the polyanion chemistry, alluaudite structured Na2Fe2II(SO4)(3) has been recently discovered as a 3.8 V positive electrode material (Barpanda et al., Nature Commun., 5: 4358, 2014). Registering the highest ever Fe-III/Fe-II redox potential (vs. Na/Na+) and formidable energy density, it has opened up a new polyanion family for sodium batteries. Exploring the alluaudite family, here we report isotypical Na2+2xMn2-xII(SO4)(3) (x = 0.22) as a novel high-voltage cathode material for the first time. Following low-temperature (ca. 350 degrees C) solid-state synthesis, the structure of this new alluaudite compound has been solved adopting a monoclinic framework (s.g. C2/c) showing antiferromagnetic ordering at 3.4 K. Synergising experimental and ab initio DFT investigation, Na2+2xMn2-xII(SO4)(3) has been found to be a potential high-voltage (ca. 4.4 V) cathode material for sodium batteries.

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 Na2Fe(II)2(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+2xFe(II)2-x(SO4)3. It marks the first demonstration of solvothermal synthesis of alluaudite Na2+2xM(II)2-x(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 °C, the lowest temperature value until date. The current work reports the synthetic aspects of pristine and modified ionothermal synthesis of Na2+2xFe(II)2-x(SO4)3 having tunable size (300 nm ∼5 μm) 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.

Designing Novel Sulphate-based Ceramic Materials as Insertion Host Compounds for Secondary Batteries

Rechargeable batteries have propelled the wireless revolution and automobiles market over the past 25 years. Developing better batteries with improved energy density demands unveiling of new cathode ceramic materials with suitable diffusion channels and open framework structure. In this pursuit of achieving higher energy density, one approach is to realize enhanced redox voltage of insertion of ceramic compounds. This can be accomplished by incorporating highly electronegative anions in the cathode ceramics. Building on this idea, recently various sulphate- based compounds have been reported as high voltage cathode materials. The current article highlights the use of sulphate (SO4) based cathodes to realize the highest ever Fe3+/Fe2+ redox potentials in Li-ion batteries (LiFeSO4F fluorosulphate: 3.9 V vs Li/Li+) and Na-ion batteries (Na2Fe2(SO4)3 polysulphate: 3.8 V vs Na/Na+). These sulphate-based cathode ceramic compounds pave way for newer avenues to design better batteries for future applications.

From cation flexibility to multifaceted industrial adoptability: a voyage to the resourceful spinel*

ABSTRACT Signature of cation flexibility associated with the order–disorder structure of three spinel families, namely aluminates (MgAl2O4/ZnAl2O4), ferrites (NiFe2O4/ZnxNi1− xFe2O4), and titanate (Li4Ti5O12) was probed using the X-ray diffraction and Raman spectroscopy. Cation flexibility (also known as antisite defects or (dis)order structural transitions) is a unique feature of many oxide AB2O4 spinel systems in which the cations exchange their sites under external influence. This feature generates functionality in many different ways. In some cases, such as ferrites, the flexibility of zinc to occupy tetrahedral sites enhances the magnetic properties, whereas the flexibility of lithium in lithium titanate brings exotic electrochemical feature with zero strain. Similarly, cation flexibility of magnesium and aluminium makes the structure tolerant towards nuclear irradiation. Some such salient features of spinel systems are described highlighting the underlying crystal structures. Enriched with enormous chemical versatility, order–disorder structure, vacancy, interstitials and mixed-cation occupancy, ‘Spinel is a world in itself’. This is why, exploring its huge accomplishments never gets enough. Ever wonder, where from this versatility stems in! This review endeavours to seek the answer by combining the flexible order–disorder structure, supporting diagnosis tools and tuneable properties of this versatile class: The Spinels. GRAPHICAL ABSTRACT

Alluaudite class of high voltage sodium insertion materials: An interplay of polymorphism and magnetism

The research and development with sodium ion batteries has geared up manifold in last one decade, owing to their abundance, non-toxicity, uniform geographical distribution and electrochemical performance complimentary to lithium counterpart. This research often leads to various novel material discoveries such as Na2Fe2(SO4)3 sodium insertion material, which has recently registered the highest-ever Fe3+/Fe2+ redox potential (3.8 V vs. Na) having excellent cyclability and rate kinetics. This basically belongs to a family of materials-Alluaudites Na2M2(SO4)3 (M: Fe, Mn, Co, Ni). Such cathode insertion compounds are basically functional materials, involving redox active 3d transition metals that are often magnetic in nature. We have investigated the magnetic structure and properties of - Alluaudites Na2M2(SO4)3. These alluaudite shows wide structural diversity and polymorphism. Employing various experimental methods involving diffraction, magnetic susceptibility, Mossbauer spectroscopy and low temperature neu...

Alluaudite polyanionic frameworks for rechargeable sodium-ion batteries

To satiate the global energy demand, efficient energy generation and storage is crucial. Here, electrochemical energy storage in general and rechargeable batteries in particular is the most viable devices for energy storage and (mobile) delivery. While lithium-ion batteries has ushered the wireless revolution over the last three decades, ever growing concern over natural lithium reserves has stimulated major interest in sodium-ion batteries as economic alternatives, particularly for large-scale storage applications. But, be it Li-ion or Na-ion technologies, both need to be improved in order to match the energy density, operational safety and sustainability as required by myriads of energy hungry applications. Addressing such challenges “Energy Storage (R)Evolution” vastly relies on innovation of new cathode materials along with the optimization of existing ones. In this prospect, inorganic materials with three dimensional polyanionic framework are viable battery materials, due to their ability to store and transport alkali ions. In addition, the presence of channels in these polyanionic materials provides means for the insertion of alkali ions while maintaining the structural stability. Chemists have garnered great success in discovery of suites of polyanionic framework insertion materials such as borates, phosphates, fluorophosphates, pyrophosphates, silicates, sulfates, flurosulphate etc. Among them, sulfate compounds deliver the highest operation potentials due to the presence of stronger electron withdrawing sulphate groups. One of the early sulfate materials to be studied for sodium insertion capabilities was NASICON class of Fe2(SO4)3, which can intake multiple Na+ ions to form Na2Fe2(SO4)3. In a recent breakthrough, a completely different polymorph of Na2Fe2(SO4)3 was discovered by Barpanda et al. Having an alluaudite mineral structure, it has benchmarked the highest ever Fe3+/Fe2+ redox potential (ca. 3.8 V vs. Na+/Na0) with excellent cyclability and rate kinetics. It has led to unprecedented focus on ‘alluaudite’ framework insertion materials. The origin of such high redox voltage is tantalizing, which can be correlated to its crystal structure. Na2Fe2(SO4)3 stabilizes into a monoclinic framework (s.g. C2/c), which is built from unique edge sharing of transition metal octahedra (TMO6) that are in turn interconnected by SO4 tetrahedral units. This renders large open channels along b axis for sodium (de)insertion. Inspired by this beautifully channelized crystal structure of Na2Fe2(SO4)3, in the present study, we have extended the alluaudite family by discovering isostructural Na2Mn2(SO4)3 and Na2Co2(SO4)3 analogues. Synergizing X-ray diffraction, Rietveld refinement, bond valence sum analysis and electrochemical measurements, we have identified the Na-ion migration pathways that enable these alluaudite materials to act as superior cathodes for next generation rechargeable sodium batteries. We will describe the synthesis and structural aspects of various SO4− based alluaudites and compare them with PO4− based and MoO4− based alluaudite cathodes.