Unraveling the mechanism of potassium metal capacitor for highly efficient charge storage

Although hybrid metal ion capacitors(MICs) are highly desired to achieve both high power density of supercapacitors and high energy density of rechargeable batteries, the mismatch problem of electrochemical kinetics of negative and positive electrodes in MICs hampers the realization of this goal. Here, a new hybrid capacitor concept-potassium metal capacitor(PMC) is proposed for the first time, where potassium metal and commercial activated carbon(AC) without any modification are applied as negative and positive electrodes, respectively, and the electrolyte is the same as that of non-aqueous potassium ion batteries. The simplest PMC prototype exhibits a good combination of high energy density(184.9 Wh kg^(-1)) and power density(12.4 kW kg^(-1)), which benefits from the synergistic effect of potassium metal and AC electrode. The former experiences fast potassium plating/striping during charging and discharging, and the later possesses complex multiple charge behaviors driven by low potential of potassium metal. Specifically, below open-circuit voltage, transportation of solvated cations in AC pores plays an important role;beyond this voltage, synergy actions of cations and anions, including adsorption/desorption of solvated cations and anions, and ions exchange between them, dominate the capacitance contribution. This work enriches the types of MICs, and deepens the understanding of the energy storage mechanism of non-aqueous hybrid metal capacitors.

Bifunctional fluoropyridinium-based cationic electrolyte additive for dendrite-free Li metal anode

Although lithium metal has become a promising anode material for high-energy batteries owing to its high specific capacity and the lowest reduction potential,the continuous side reactions with electrolyte as well as the safety problem caused by Li dendrite growth restrict Li anode’s practical application.Herein,we demonstrate that N-fluoropyridinium(ArF^(+))bis(trifluoromethane)sulfonimide(TFSI-)as an electrolyte additive can protect the lithium metal by both solid electrolyte interphase(SEI)protection and electrostatic repulsion mechanisms.The ArF+cations not only participate in forming F,Ncontaining SEI protective layer on Li surface,but also act as a cationic repellent during Li deposition to inhibit Li dendrite growth.As a result,the cycle performance of Li symmetric cells and Li||LiFePO_(4)full cells were significantly improved by using ArFTFSIadded electrolyte.This study provides an electrolyte additive strategy for Li anode realizing SEI protection and electrostatic repulsion simultaneously.