Evolution of “adsorption–insertion” K+ storage behaviors in flower-like carbons with tunable heteroatom doping and graphitic structures

Abstract

Expanding the graphitic interlayer spacing and introducing heteroatoms into the carbon structure are the two key strategies for boosting the potassium storage performance. To fundamentally clarify the relation of graphitic structures and heteroatom-doping in carbon with the potassium storage behavior, carbons with diverse heteroatom-doping levels and microstructures (N,S-CNSs) are produced. It has been recognized that the potassium storage mechanism in N,S-CNSs can be described as an “adsorption–insertion” mechanism: the highly disordered/pseudo-graphitic carbon structure with rich defective sites contributing to a high-potential sloping region by “adsorption” and the graphitic carbon structure contributing to a low-potential plateau region by “insertion”. By tuning the “adsorption” and “insertion” mechanisms, N,S-CNS-1200 with a highly developed graphitic structure can provide a satisfactory reversible specific capacity of ∼300 mA h g⁻¹ at 0.05 A g⁻¹ with a high initial coulombic efficiency of 44.9%, a high “insertion” capacity contribution of 57.9%, and a high capacity retention ratio of 81.9% after 1000 cycles. Moreover, kinetic analysis and ex situ XRD studies show that the kinetic diffusion of K⁺ in N,S-CNS-1200 is more advantageous. This work clearly clarifies the pivotal role of tuning the heteroatom doping and graphitic structure of carbons for realizing highly accessible graphitic structures for advanced potassium ion batteries.