Impact of Milling Conditions on Hard and Soft Carbon Properties and Performance in Potassium-Ion Batteries

Hard carbon (HC) and soft carbon (SC) are very attractive electrode materials for potassium-ion batteries (KIBs). However, specific attention must be devoted to electrode preparation to preserve their properties. This work reports the effects of the milling conditions (ball milling and hand grinding) used for HC and SC (or mixtures) on their properties and electrochemical performance. The properties were strongly affected by the use of ball milling instead of hand grinding. Ball milling gave a homogeneous morphology with small particles but a more disorganized structure containing inorganic impurities. A higher specific surface area, more oxygen-containing groups, and a greater active surface area were also generated. In addition, the more disorganized materials with a higher oxygen content (i.e., hard carbons) were the most influenced by ball milling. These modified properties had a negative impact on the performance. In contrast, hand grinding of the materials provided the best initial Coulombic efficiency (ICE) and greater capacity retention during cycling, i.e., 79% (HC) and 91% (SC) after 50 cycles. Post-mortem XPS studies revealed the nature of the solid electrolyte interphase (SEI) and the possible reasons for the low ICE. The SC was tested in a full cell vs C-coated KVPO4F0.5O0.5 and delivered promising performance at high C-rates.