Defect engineering on BiFeO3 through Na and V codoping for aqueous Na-ion capacitors

Sodium with low cost and high abundance is considered as a substitute element of lithium for batteries and supercapacitors, which need the appropriate host materials to accommodate the relatively large Na+ ions. Compared to Li+ storage, Na+ storage makes higher demands on the structural optimization of perovskite bismuth ferrite (BiFeO3). We propose a novel strategy of defect engineering on BiFeO3 through Na and V codoping for high-efficiency Na+ storage, to reveal the roles of oxygen vacancies and V ions played in the enhanced electrochemical energy storage performances of Na-ion capacitors. The formation of the oxygen vacancies in the Na and V codoped BiFeO3 (denoted as NV-BFO), is promoted by Na doping and suppressed by V doping, which can be demonstrated by XPS and EPR spectra. By the first-principles calculations, the oxygen vacancies and V ions in NV-BFO are confirmed to substantially lower the Na+ migration energy barriers through the space and electric field effects, to effectively promote the Na+ transport in the crystals. Electrochemical kinetic analysis of the NV-BFO//NV-BFO capacitors indicates the dominant capacitive-controlled capacity, which depends on fast Na+ deintercalation-intercalation process in the NV-BFO electrode. The NV-BFO//NV-BFO capacitors open up a new avenue for developing high-performance Na-ion capacitors.