It is a challenge to prepare organic electrodes for sodium-ion batteries with long cycle life and high capacity. The highly reactive radical intermediates generated during the sodiation/desodiation process could be a critical issue because of undesired side reactions. Here we present durable electrodes with a stabilized α-C radical intermediate. Through the resonance effect as well as steric effects, the excessive reactivity of the unpaired electron is successfully suppressed, thus developing an electrode with stable cycling for over 2,000 cycles with 96.8% capacity retention. In addition, the α-radical demonstrates reversible transformation between three states: C=C; α-C·radical; and α-C− anion. Such transformation provides additional Na+storage equal to more than 0.83 Na+ insertion per α-C radical for the electrodes. The strategy of intermediate radical stabilization could be enlightening in the design of organic electrodes with enhanced cycling life and energy storage capability.
Samples of electrochemically active materials were mixed with active material, super P and polyvinylidene fluoride in a 60:30:10 weight ratio. After ball-mixing with N-methyl-2-pyrrolidone (NMP) for 4 h, the mixture was coated uniformly using a doctor-blade on the copper foil with roughly 0.7 mg cm−2 mass loading. The electrodes were dried at 110 °C under vacuum for overnight. The electrochemical performance was evaluated using 2016 coin cells with a sodium metal anode and NaClO4 (1 M) in EC/DMC (1:1 w/w) as the electrolyte. The samples for ex situ experiment such as FT-IR, X-ray absorption near-edge structure (XNEFS), XPS, solid state NMR, TEM and EDS were prepared by replacing acetylene black with equal mass Nano-copper. The CV measurements were carried out using CHI660 electrochemical workstation. Electrochemical impedance spectroscopy was recorded on Solartron 1470E, the amplitude of the sine perturbation signal was 5 mV and the frequency was scanned from the highest (10 kHz) to the lowest (5 mHz). Galvanostatic charge discharge cycles were tested by Neware battery tester at different current densities at room temperature.
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