Concentration Gradient Driving Rapid Potassium Ion Diffusion in Graphite.

Great effort has been devoted to modifying the carbon structure at the molecular level to enhance the intrinsic diffusion coefficient of potassium ion to improve the power density of potassium-ion batteries (PIBs), but its energy density is traded off. Here, a concentration-gradient-driving ion diffusion strategy is proposed to overcome such a trade-off. To build up a high concentration gradient, N-doped carbon (NC) is coated on exfoliated graphite (EG), whereby a seven times enhancement in the potassium ion concentration on the EG surface is achieved before the initiation of intercalation. Resultantly, the apparent diffusion coefficient of potassium ion in the optimized sample (EG@NC-200) is increased 1000 times at the bottleneck stage of potassium ion diffusion in EG, and the transition of graphite intercalation compounds from stage 3 to 2 is also accelerated. As a result, even at 1.6 A g-1, EG@NC-200 still provides a discharge specific capacity of 134 mAh g-1 below 0.4 V, far exceeding the 8 mAh g-1 of EG. More importantly, the discharge midpoint voltage and voltage hysteresis of EG@NC-200 at 1.6 A g-1 are 0.02 and 1.72 V lower than EG, respectively. The assembled full-cell possesses an energy density of 705 Wh kg1- based on the mass of EG@NC-200.