High-density cathode structure of independently acting Prussian-blue-analog nanoparticles: a high-power Zn–Na-ion battery discharging ∼200 mA cm−2 at 1000C†
Sustainable societies demand ultrahigh-power batteries to discharge explosive electricity. Herein, we use current densities (mA cm−2) to measure high power when comparing different laboratory-scale loading amounts (several mg cm−2) of redox-active materials. Current densities are improved by increasing discharge currents (A g−1) in conjunction with loading amounts. However, maintaining the theoretical capacities of cathodes at ultrahigh rates of ≥100C remains challenging. Independently acting nanoparticles (NPs) are most effective in boosting the C-rate capabilities of cathodes by improving sluggish kinetics via shortened ion-diffusion length. Therefore, we fabricate aggregation-free cathodes through the one-step filtration of independently water-dispersed surface-modified NPs of Prussian-blue analogs (metal hexacyanoferrates (MHCFs), M = Zn, Cu, and Mn) along with single-walled carbon nanotubes. ZnHCF NPs are densely stacked between 1.0 and 1.3 g cm−3 while maintaining nanometer-scale spaces around each NP. These spaces are filled with electrolyte solutions, enabling the NPs to act independently. In Zn–Na-ion batteries, an unprecedented charge/discharge rate mismatch between cathodes and Zn-foil anodes is investigated by changing the loading amounts of NPs. The low-loading amounts of ≤0.50 mg cm−2 achieve synchronized charge/discharge profiles ranging from 100 (1.64) to 1000C (operating voltage, 1.53 V) with retained discharge capacities of ≥97%. Furthermore, we demonstrate the ability to conduct at least 150 000 charge/discharge cycles at 400C. At a high loading amount of 3.0 mg cm−2, the capacity charged to the theoretical value at 300C is almost fully discharged at 1000C/66 A g−1 with high-power outputs of 198 mA cm−2/246 mW cm−2. In Ragone plots, Zn–Na/K-ion batteries using MHCFs exhibit ultrahigh-power densities of 104–105 W kg−1 with energy densities of 90–200 W h kg−1. The unique cathode structure thereby shows a promising avenue for overcoming the tradeoff between densifying NPs and increasing current densities for future ultrahigh-power batteries.