MnO₂Nanosheets on a Carbon Nanofiber Freestanding Film by Electrospinning and in Situ Spraying for Lithium and Sodium Storage

Freestanding electrodes are critical for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) with high energy density and long cycling stability. We fabricated freestanding MnO2 nanosheet-coated carbon nanofibers by synchronous in situ spraying of MnO2 nanosheets onto electrospinning polymer nanofibers, which might be a high-voltage electric field-induced assembly process. The designed film is used as anode material for LIBs and SIBs. It demonstrates good cycling stability without capacity loss after long cycling for LIBs, delivers a capacity of 256.4 mAh g-1 at 1.00 A g-1, and has an extremely stable cycle life for over 200 cycles at 0.05 A g-1. For SIBs, it delivers a capacity of 135.0 mAh g-1 at 1.00 A g-1 and has a stable cycle life for over 500 cycles at 0.50 A g-1. The capacitive contribution ratio increases from 44.4 to 76.2% (0.2-1.0 mV s-1), and the capacitive process plays a major role in the total capacity. The enhancement of the electrochemical performance is attributed to the good conductivity of the carbon nanofiber network and the one-dimensional (1D)/two-dimensional (2D) composite structure of the electrode, in which the high performance of carbon nanofibers and MnO2 nanosheets is fully exploited. Importantly, the combination of in situ spraying and electrospinning can be extended to two or more materials with different polarities, ζ-potential values, or solubilities, broadening the applications of composites.