Origins of irreversible capacity loss in hard carbon negative electrodes for potassium-ion batteries

Hard carbon (HC) is considered as a negative electrode material for potassium-ion batteries, but it suffers from significant irreversible capacity loss at the first discharge cycle. Here, we investigated the possible reasons of this capacity loss with a combination of in situ AFM and various ex situ TEM techniques (high resolution TEM and high angle annular dark field scanning TEM imaging, and STEM-EELS and STEM-EDX spectroscopic mapping) targeting the electrode/electrolyte interphase formation process in the carbonate-based electrolyte with and without vinylene carbonate (VC) as an additive. The studied HC consists of curved graphitic layers arranged into short packets and round cages, the latter acting as traps for K⁺ ions causing low Coulombic efficiency between cycling. Our comparative study of solid electrolyte interphase (SEI) formation in the carbonate-based electrolyte with and without the VC additive revealed that in the pristine electrolyte, the SEI consists mostly of inorganic components, whereas adding VC introduces a polymeric organic component to the SEI, increasing its elasticity and stability against fracturing upon HC expansion/contraction during electrochemical cycling. Additionally, significant K⁺ loss occurs due to Na⁺ for K⁺ exchange in Na-carboxymethyl cellulose used as a binder. These findings reflect the cumulative impact of the internal HC structure, SEI properties, and binder nature into the electrochemical functional properties of the HC-based anodes for K-ion batteries.