Designing advanced electrode materials for rechargeable magnesium & sodium batteries and spectroscopic investigation

Abstract

From portable electronics to grid storage, batteries act as important media to store electrical energy. Lithium-ion batteries (LIBs) are taking a dominant role in battery markets. The supply risks associated with Li resources could stimulate the development of practical post- LIBs, especially magnesium (Mg) /sodium (Na)-based batteries. Mg and Na batteries with the advantages of natural abundance and low cost of precursor materials are promising alternatives to LIBs. Rechargeable Mg batteries offer many advantages over the LIBs. The theoretical volumetric capacity of Mg metal anode (3833 mAh cm-3) is approximately two times higher than that of the Li metal (2061 mAh cm-3). Pure Mg-metal anode is dendritefree with high safety. In addition, Na-ion batteries are one of the fast-growing batteries following LIBs. Recent prototypes of Na-based batteries are competitive with some LIBs and are already being introduced into commercial products. Therefore, the cost-effective post- LIBs (Mg and Na batteries) will be promising and competitive in the future battery market. To broaden the future application of batteries, we further develop electrode materials with good ultra-low temperature tolerance. The design of organic/inorganic electrode materials and an in-depth understanding of the reaction mechanism via spectroscopies are the main objectives of this thesis. Firstly, we demonstrated a new Mg storage mechanism for Mg/Bi batteries. A latent MgBi intermediate phase is firstly captured via operando synchrotron spectroscopy and ab initio methods. We synthesized the mesoporous bismuth nanosheets (p-Bi NS) as a freestanding alternative anode to pure Mg metal. Results demonstrate that p-Bi NS outperforms reported bismuth-based materials in Mg batteries. These findings will advance the mechanistic understanding and material design principles for Mg batteries. Secondly, we in-situ constructed a nanostructured Bi anode from bismuth selenide. Through the combination of operando synchrotron X-ray diffraction, ex-situ synchrotron Xray absorption spectroscopy, and comprehensive electrochemical tests, it is demonstrated that the nanosize of the in-situ formed Bi crystals contributes to the fast Mg2+ diffusion kinetics and highly efficient Mg-Bi alloying/de-alloying. The resultant Bi anodes exhibited superior long-term cycling stability with over 600 cycles. We conclude that our findings offer a practical approach to guide the materials design of the alloy anode for highly stable rechargeable Mg batteries. Thirdly, we reported layer-structured metallic vanadium diselenide (1T-VSe2) as a cathode material for low-temperature Mg2+/Li+ hybrid batteries. We demonstrated the high electronic conductivity and fast ion diffusion kinetics for 1T-VSe2, and a highly safe 1TVSe2/ Mg battery for operation under -40 oC. The Jahn-Teller effect in compressed configuration is initiated in 1T-VSe2 with the change of electronic state on electrochemical intercalation of alkali metal ions. Using combined experimental results and theoretical studies including operando synchrotron X-ray diffraction, ex-situ X-ray absorption spectroscopy, and density functional theory (DFT) computation, we confirm that the Jahn- Teller effect contributes significantly to the fast overall kinetics and structural stability of the electrode. Lastly, we reported an organic electrode, nanosized disodium rhodizonate hybridizing with graphene oxide (nDSR/GO) as the highly efficient electrode for sodium-ion batteries with an excellent ultra-low temperature performance, in which the pseudocapacitive electrochemical behavior of the organic electrode has been fully utilized. By modulating the solution-mediated kinetics of composite electrodes, the Na-nDSR/GO battery exhibits a high capacity of 119 mAh g-1 at -50 oC. The Prussian blue analogue (PBA)-nDSR/GO full cell demonstrates an ultra-long lifespan over 2500 cycles with a capacity of 99 mAh g-1 at a high current density of 300 mA g-1. The designed Na-ion batteries perform one of the best performances under an ultra-low temperature. Therefore, this battery is promising in practical use in extreme cold conditions.