Learning from biology: biomimetic carbon cells promote high-power potassium ion batteries

Abstract

Potassium ion batteries (PIBs) are gaining traction as an alternative scalable energy storage system due to a high abundance of potassium resources (1.5 wt% in the Earth’s crust), fast ion transport kinetics of K+ in electrolyte and low standard reduction potential of potassium (−2.93 V vs. standard electrode potential (E0)) [1–4]. The rapidly growing field of this rising technology has forced us to search for advanced electrode materials with superior electrochemical performance and scalable production methods. Building on the historical achievements of lithium and sodium ion batteries, inexpensive carbons are one of the most promising anode materials due to their good electrical conductivity, benign tailorable properties, eco-friendliness and high stability in electrolytes [5–7]. Among all the carbon-based materials, carbon nanotubes (CNTs), graphite, graphene, amorphous carbon and their derived 3D structures have all been considered as potential candidates that could be applied in awide range of electrochemical fields [8]. Althoughcurrent studies havemade a few breakthroughs to prolong the cycling stability of carbon-based electrodes, the capability for fast charging/discharging still needs to be improved for practical applications. More importantly, the use of carbon-based anodes in full cells is essential to help evaluate the feasibility for this un-matured technology. Inspired by biological cells demonstrating natural selection over billions of years, it has been found that metal ions selectively absorb and accumulate within the cells of halophytic plants, which then can be converted into graded 3D carbon and metal oxide nanocomposites [9]. Based on this interesting finding, Lu and his co-workers have prepared biomimetic carbon cells (BCCs) akin to the biological cells with several ion transportation channels preserved [10].The synthesis of BCCs is schematically shown in Fig. 1a. It is noted that the addition of metal Co catalysts into C3N4 intermediate prepared by heating melamine precursor could promote the growth of carbon nanotubes inside the BCC. Also, the amorphous carbon that grewon the nanotubes andgraphene could act as the shell to protect the entire BCC. Specifically, Fig. 1b exhibits some internal spaces inside the biological cell with surfaces composed of bilayer lipidmembranes.The interior of a BCC consists of open spaces that could allow ions to transfer quickly inside the composite, thus benefiting the rate capability. In addition, the internal space of the BCC could also accommodate volume variations upon cycling, which can maintain the structural integrity of the carbon material. Figure 1c shows the morphology of the BCCs, which exhibit a characteristic ellipsoidal shape similar to that of biological cells. Interestingly, the graphitic structure inside the BCC could still be maintained after cycling for 1000 cycles, as evidenced by the lattice fringe images shown in Fig. 1d. In terms of electrochemical performance, half-cell tests suggested that the BCC anode could operate over 15 months at a