Lithium-ion capacitors (LICs) could combine the virtues of high power capability of conventional supercapacitors and high energy density of lithium-ion batteries. However, the lack of high-performance electrode materials and the kinetic imbalance between the positive and negative electrodes are the major challenge. In this study, Fe 3 O 4 nanoparticles encapsulated in nitrogen-rich carbon (Fe 3 O 4 @NC) were prepared through a self-assembly of the colloidal FeOOH with polyaniline (PANI) followed by pyrolysis. Due to the well-designed nanostructure, conductive nitrogen-rich carbon shells, abundant micropores and high specific surface area, Fe 3 O 4 @NC-700 delivers a high capacity, high rate capability and long cycling stability. Kinetic analyses of the redox reactions reveal the pseudocapacitive mechanism and the feasibility as negative material in LIC devices. A novel LIC was constructed with Fe 3 O 4 @NC-700 as the negative electrode and expanded graphene (EGN) as the positive electrode. The wellmatched two electrodes effectively alleviate the kinetic imbalance between the positive and negative electrodes. As a result, Fe 3 O 4 @NC-700//EGN LIC exhibits a wide operating voltage window, and thus achieves an ultrahigh energy density of 137.5 W h kg −1. These results provide fundamental insights into the design of pseudocapacitive electrode and show future research directions towards the next generation energy storage devices.
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