Hard carbon composites with metal oxides and metal nitrides for sodium-ion batteries

Abstract

In recent years, sodium-ion batteries (SIBs) have attracted much attention as an alternative to lithium-ion batteries. Hard carbon (HC) is a well-studied anode material for SIBs. In this thesis, the temperature dependence of the HC sodium insertion/extraction behaviour was investigated. HC composites with metal oxides and metal nitrides were synthesised and their electrochemical performance in SIBs evaluated.
HC temperature dependence was tested in sodium half-cells with a common NaClO4-based electrolyte at temperatures from 10 to 80 °C. Capacity after 20 cycles at 100 mA g−1 current varied from 90 mA h g−1 at 10 °C to 270 mA h g−1 at 60 °C. Increased temperature significantly improves the HC rate capability, with 120 mA h g−1 capacity found at 60 °C with 500 mA g−1 current. Stability was high at moderate temperature with 220 mA h g−1 capacity remaining after 200 cycles at 40 °C with a current of 100 mA g−1.
Iron oxide was combined with HC to make composites by two routes, a sol-gel and a hydrothermal process. Both routes coated iron oxide evenly on the HC surfaces and improved HC capacity with a low loading of iron oxide. The 2.5% Fe2O3-HC prepared by the sol-gel method showed 271 mA h g-1 reversible capacity at 50 mA g-1 after 20 cycles, while the unprocessed HC capacity is 222 mA h g-1. The 5% Fe2O3-HC synthesised by the hydrothermal method increased capacity by 38 mA h g-1 compared to unprocessed HC. The additional capacities were contributed by Fe2O3, which exhibited high specific capacity, 2182 and 1034 mA h g-1 for sol-gel and hydrothermal method, respectively. Iron nitride was obtained from ammonolysis of Fe2O3-HC and displayed 231 mA h g-1 reversible capacity, which has 64 mA h g-1 additional capacity compared to HC treated with the same process. All the composites showed a low operating potential below 0.1 V vs. Na/Na+. The ex-situ XRD suggested good reversibility of Fe2O3-HC and Fe2N-HC during reduction/oxidation process.
Germanium nitride, niobium oxynitride and vanadium nitride were combined with HC by a sol- gel process and evaluated in SIBs. 5 % Ge3N4-HC exhibited 185 mA h g-1 reversible capacity at 100 mA g-1, better than the analogous HC processed in the same way. 5% Nb(ON)0.425-HC increased 22 mA h g-1 capacity at 100 mA g-1 compared with the HC treated with the same synthesis process. Ge3N4 and Nb(ON)0.425 were firstly evaluated in SIBs and exhibited a high specific capacity of 557 and 857 mA h g-1. 5% VN-HC showed a 44 mA h g-1 additional capacity at 50 mA g-1 compared to the HC treated with same procedure, with 1453 mA h g-1 specific capacity attributed to VN, which is the highest specific capacity of VN reported to date. All the metal nitrides with HC composites displayed a low operating potential, suggesting suitability as a negative electrode material for SIBs.

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Identifiers

ORCID for Bowen Liu: orcid.org/0000-0003-1328-7082

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

ORCID for Richard Wills: orcid.org/0000-0002-4805-7589

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