Rohit Satish

High-performance hybrid electrochemical capacitor with binder-free Nb2O5@graphene

Hybrid electrochemical capacitors (HECs) are capable of storing more energy than supercapacitors while providing more power compared to lithium-ion batteries (LIBs). The development of Li-intercalating materials is critical to organic electrolyte based HECs, which generally give larger potential output than aqueous electrolyte based HECs. This article reports on a simple binder-free Nb2O5@graphene composite that exhibited excellent HEC performance as compared with other Li intercalating electrode materials. The composite exhibited enhanced cyclability with a capacity retention of 91.2% compared to 74.4% of the pure Nb2O5 half-cell when tested at a rate of 2000 mA g−1 (10 C). The composite displayed a lower polarization effect when cycled at increasing scan rates (1–10 mV s−1). The enhanced rate capability could be ascribed to the use of a highly conductive graphene support. As a result, the HEC composed of the Nb2O5@graphene composite and activated carbon (AC) delivered a maximum energy and power density of 29 W h kg−1 and 2.9 kW kg−1. The performance is better than most reported HECs with other Li-intercalating electrode materials.

Exploring the influence of iron substitution in lithium rich layered oxides Li2Ru1−xFexO3: triggering the anionic redox reaction

Lithium rich layered materials are an interesting class of materials which exploit both anionic and cationic redox reactions to store energy upwards of 250 mA h g−1. This paper aims to understand the nature of the redox reactions taking place in these compounds. Li2RuO3 was used as the base compound, which is then compared with compounds generated by partially substituting Ru with Ti and Fe respectively. Electrochemical tests indicate that Fe substitution in the sample leads to an improvement in capacity, cycle life and reduction of potential decay. To elucidate the reason for this improvement in operando diffraction experiments were carried out, highlighting the formation of a secondary de-lithiated phase. The distortion of the pristine structure eventually induces frontier orbital reorganization leading to the oxygen redox reaction resulting in extra capacity. Local changes at Fe and Ru ions are recorded using in operando X-ray absorption spectroscopy (XAS). It was noted that while Ru undergoes a reversible redox reaction, Fe undergoes a significant irreversible change in its coordination environment during cycling. The changes in the coordination environment of oxygen and formation of O2n− type species were probed in situ using soft X-rays.