Sonochemical Synthesis of Na0.7MnO2.05 and Na0.44MnO2 Insertion Materials for Na-Ion Batteries and Na-Ion Capacitors

Abstract

Energy storage reign supreme in driving the modern lifestyle. Among various energy storage technologies, Lithium-ion batteries (LIBs) and Lithium-ion capacitors (LICs) have attracted much attention due to their high energy and power density respectively. The constraint in the lithium resource impels us to replace Li with other abundant elements. Sodium seems to be the most attractive element to replace lithium with comparable standard electrode potential (-2.71 V vs SHE) and practically unlimited resources making this technology cost effective. Cathode plays an important role in storage devices (batteries and supercapacitors). Various transition metal oxides and polyanionic compounds can be employed as efficient cathode materials, but oxides are favorable owing to their high energy density and ease of synthesis. Current work explores two manganese-based oxides as cathodes for Na-ion batteries (NIBs) and Na-ion Capacitors (NICs): (i) Na0.7MnO2.05 and (ii) Na0.44MnO2. Depending on the stoichiometry, sodium manganese oxides mainly have two kinds of structures: (i) layered structure (e.g. Na0.7MnO2.05) or (ii) tunnel structure (e.g. Na0.44MnO2), both having large number of vacancies accommodating constituent Na-ions. Na0.44MnO2 is particularly attractive as cathode in NIB because of its unique 3D crystal structure, which greatly facilitates Na+ mobility whereas Na0.7MnO2.05 is good candidate for cathode in NICs owing to its sloppy discharge profile. Here, ultrasonic sonochemical synthesis has been employed to obtain the target compounds, which restricts the final high-temperature annealing duration within 1-2 h. Herein, we will present (i) single crystalline Na0.7MnO2.05 nanoplates with preferential growth in (100) direction facilitating efficient Na+ (de)insertion leading to capacity over 140 mAh/g (average voltage = 2.4 V) and (ii) orthorhombic Na0.44MnO2 having a reversible capacity of 110 mAh/g (average voltage = 2.3 V). The salient features of sonochemical synthesis, structure, morphology and final electrochemical performance of NaxMnO2 oxides (both as NIB and NIC) will be presented combining experimental work as well as first-principle calculations.