Sammendrag
Na-ion batteries (NIBs) are currently an immature technology but have gained interest in research in the last years. A big field within NIBs is on the anode since there is none good enough yet. In this work, we have looked at BiFeO3 as an anode material for NIBs. Bi-based anodes have been investigated in the group before this work and have shown promising results. The choice of BiFeO3 was based on the interesting mechanism that occurs during charging and discharging when used as an anode. The mechanism is a conversion-alloying reaction and combines conversion and alloying reactions when BiFeO3 is sodiated. We have investigated this mechanism in detail with combined X-ray diffraction (XRD) and galvanostatic cycling (GS) with operando-XRD in the RECX-lab at the Department of Chemistry. BiFeO3 was synthesised by ceramic solid-state synthesis and sol-gel synthesis as part of the work. It has previously been synthesised with these syntheses but has proven to be difficult to be phase pure. The product of the syntheses has been characterised with XRD and refined with the Rietveld method. The ceramic solid-state synthesis gave a low yield of BiFeO3 and several impurities, but the sol-gel synthesis gave a high yield of BiFeO3 and few impurities. Various treatments of BiFeO3 were investigated, including ball milling, leaching and doping, to provide increased performance as a battery material. These tests showed that ball milling of the material gave the best results, but it was still not possible to find an optimisation that avoided a large drop in capacity during the first 30 cycles. To characterise BiFeO3 as an anode material, we test it in half-cells against Na metal. These half-cells were electrochemically analysed by GS and cyclic voltammetry (SV). We also tested different mixtures of electrode composition and electrolytes to optimise battery properties. These showed that NaPF6 was the best salt and propylene carbonate (PC) + 5 wt% fluoroethylene carbonate (FEC) gave the most stable battery cells. The data from the electrochemical characterisation and operando-XRD were used together to interpret the reaction mechanisms of BiFeO3. We saw from these that there were similarities in the alloying reaction between Na and Bi with other Bi-based anodes, with NaBi and Na3Bi as different stages in the reaction. However, there were no indications of the phase(s) of Fe after the reaction.
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