Investigating the Electrical and Electrochemical Performance of a Conducting Binder for a Fast-Charging Potassium-Ion Battery Anode

Abstract

Energy from intermittent sources such as wind and solar requires large-scale storage to match production with consumption. Rechargeable batteries, and in particular lithium-ion batteries (LIBs), have been a strong contender for their long cycle life, high energy density, and high efficiency. However, the scarcity and uneven distribution of lithium deposits has brought forth concerns whether future supply will meet rapidly increasing demand. Sustainable potassium-ion batteries (KIBs) use earth-abundant resources and operate on a similar principle as LIBs, providing a promising complementary field of research. However, the shorter cycle life and limited rate capabilities have precluded commercial success. A larger cation impedes insertion into layered graphite, reducing kinetic capabilities. Capacity fading is also exacerbated due to significant volumetric expansion (60%). These factors have critical implications for quickly storing negatively priced surplus electricity as well as annualized costs.We therefore aim to improve the charging speed and mitigate the capacity fading mechanism through enhanced electronic properties. While the literature has largely attempted to address these issues with unscalable nanostructured carbons and expensive salt-concentrated or ionic liquid electrolytes, few studies have addressed the binder, despite being a critical component of the composite electrode. To our knowledge, no studies currently exist on conductive polymer binders for KIB anodes. To this end we substitute the standard insulating polymer binder poly(vinylidene fluoride) (PVDF) for an electronically conductive polymer mixture, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS).Substitution of PVDF for PEDOT:PSS in graphite electrodes with conductive carbon black (CB) additive shows significantly improved performance in half-cell cycling studies, with the 100 cycle capacity retention increasing from 63 to 72%. The higher initial capacity (240 vs 311 mAh g-1) at C/10 also indicates enhanced electrical contact and reduction of electronically isolated material. This capacity even exceeds the theoretical capacity of KC8 (279 mAh g-1) due to the additional capacitive charge storage. Further, through electrochemical impedance spectroscopy (EIS) we show a reduced charge transfer impedance (RCT) from 2310 to 1020 Ω, which enables faster kinetics. At a rate of C/3, the capacities are 160 and 40 mAh g-1, respectively. Removal of the carbon black (CB) for graphite with PEDOT:PSS binder however results in a high RCT, poor rate capabilities, and shortened cycle lifetime despite ex-situ results showing comparable or better electrical conductivity of the PEDOT:PSS to a PVDF/CB composite. This discrepancy is a subject of interest, as removing CB has the potential to increase the fraction of active material. In efforts to identify the underlying cause, we find poor electrolyte uptake of the PEDOT:PSS, and through EIS and half-cell cycling we show that electrical and electrolyte contact are competing factors for this system. Additionally, through cyclic voltammetry (CV) we identify reversible redox activity at 2.5 V vs. K/K+ of the PEDOT:PSS during the reductive scan as K+ coordinates with the negatively charged PSS, diminishing the concentration of charge carrying holes on the PEDOT polymer. EIS highlights a significant reversible change in electrochemical behavior when crossing this voltage threshold, supporting this claim. These factors likely contribute to the increased charge-transfer resistance. Results overall, however, showcase a promising avenue towards improving KIBs independent of the electrode active material, and identify issues to address in further improving lifetime and charging rates using this material. Figure 1