CoSe@N-Doped Carbon Nanotubes as a Potassium-Ion Battery Anode with High Initial Coulombic Efficiency and Superior Capacity Retention.

Potassium-ion batteries (KIBs) have gained significant interest in recent years from the battery research community because potassium is an earth-abundant and redox-active metal, thus having the potential to replace lithium-ion batteries for sustainable energy storage. However, the current development of KIBs is critically challenged by the lack of competitive electrode materials that can reversibly store large amounts of K+ and electrolyte systems that are compatible with the electrode materials. Here, we report that cobalt monochalcogenide (CoSe) nanoparticles confined in N-doped carbon nanotubes (CoSe@NCNTs) can be used as a K+-storing electrode. The CoSe@NCNT composite exhibits a high initial Columbic efficiency (95%), decent capacity (435 mAh g-1 at 0.1 A g-1), and stability (282 mAh g-1 2.0 A g-1 after 500 cycles) in a 1 M KPF6-DME electrolyte with K as the anode over the voltage range from 0.01 to 3.0 V. A full KIB cell consisting of this anode and a Prussian blue cathode also shows excellent electrochemical performance (228 mAh g-1 at 0.5 A g-1 after 200 cycles). We show that the NCNT shell is effective not only in providing high electronic conductivity for fast charge transfer but also in accommodating the volume changes during cycling. We also provide experimental and theoretical evidence that KPF6 in the electrolyte plays a catalytic role in promoting the formation of a polymer-like film on the CoSe surface during the initial activation process, and this amorphous film is of critical importance in preventing the dissolution of polyselenide intermediates into the electrolyte, stabilizing the Co0/K2Se interface, and realizing the reversibility of Co0/K2Se conversion.