Structural and Electrochemical Investigation of Bisulfate and Hydroxysulfate based Polyanionic Cathodes
The discovery of LiFePO4 cathode for Li-ion battery ushered intensive study on polyanionic high-voltage battery insertion materials. Polyanionic materials offer rich crystal chemistry, robust framework, voltage tunability, and high redox potential based on the inductive effect due to the polyanionic unit [(XO4)mn-, X = S, P, Si, W, Mo, etc.] . Among them, SO4-based polyanionic systems have the advantage of higher redox potential and ease/versatility of low temperature synthesis. In this spirit, I have investigated bisulfate [A2-xM(SO4)2: A= Li, Na, K; x= 0,1] and hydroxysulfate [AMSO4OH: A = Li] type sulfate-based polyanionic frameworks. Few salient features of my thesis work are: (i) Spray drying route was used to discover a metastable monoclinic polymorph of Li2NiII(SO4)2 (s.g. P21/c). As per first principle calculations, it can work as a 5.5 V (vs. Li+/Li) cathode for Li-ion battery coupling both cationic (Ni2+/Ni3+) and anionic (O-) redox activity. The crystal chemistry, phase stability landscape and the ground state magnetic structure (A-type Antiferromagnetic spin ordering) of this novel compound have been examined [2,3]. (ii) Mineralogical exploration and synthetic preparation of naturally found minerals are strategically used to unveil battery electrode materials. Following, saranchinaite Na2Cu(SO4)2 and its hydrated derivative kröhnkite Na2Cu(SO4)2.2H2O bisulfate minerals have been prepared using the facile spray drying synthesis route. The thermodynamic phase stability landscape has been explored along with the structural effects on the Na+ ion mobility. While the presence of Cu makes them unsuitable for insertion ion chemistry, they have been reported as potential conversion type battery electrodes . (iii) The eldfellite NaVIII(SO4)2 (s.g. C2/m) is demonstrated as a versatile novel cathode material for both Li-ion (2.57 V, 80 mAh/g) and Na-ion (2.28 V, 70 mAh/g) battery at current rate of C/20 and based on solid solution reaction mechanism. (iv) Hydrothermally prepared orthorhombic polymorph of FeIIISO4OH (s.g. Pnma) has been examined as a 3.2 V (110 mAh/g, C/20) Li-ion battery cathode. I have further demonstrated the first reversible Na-ion (de)insertion in monoclinic FeIIISO4OH at ~2.9 V based on solid solution reaction mechanism with a discharge capacity of 85 mAh/g (C/100) [5,6]. Overall, this work can be suitably placed in the materials science tetrahedron encompassed by “structure-property-processing-performance”. I will elaborate the above-mentioned sulfate based polyanionic battery insertion materials.