Mechanistic Elucidation of Conducting Binder for a Safer Potassium-Ion Battery Anode

The need to efficiently store intermittent renewable energy from nature has motivated extensive research in the field of rechargeable batteries, particularly lithium-ion batteries (LIBs) for their long cycle life, high energy density, and high efficiency. However, lithium resources are scarce and unevenly distributed globally, raising concerns over whether future supply will meet rapidly increasing demand. Sustainable potassium-ion batteries (KIBs) use earth abundant resources and operate with a similar working principle as LIBs, offering a promising complementary field of research. However, the shorter cycle life has precluded commercial success. Capacity fading is exacerbated due to significant volumetric expansion and subsequent degradation of the electrode resulting from insertion of a much larger cation. This factor has critical implications for annualized costs. Additionally, the more reductive K metal raises concerns over safety. While the literature has largely attempted to address these issues with unscalable novel carbon architectures and costly concentrated or ionic liquid electrolytes, few studies have addressed the binder, despite being a critical component of the electrode composite. To our knowledge, no studies currently exist on conductive polymer binders for KIB anodes.

We therefore substitute the standard insulating polymer binder poly(vinylidene fluoride) (PVDF) for an electronically conductive polymer mixture, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) due to its high conductivity, low cost, and large body of existing research. Thermal safety is also critical if large quantities of batteries are kept in proximity. We therefore comprehensively seek to explore the electrochemical, mechanical, and thermal safety performance of PEDOT:PSS as a binder to identify issues which can be addressed.

The continuously conductive matrix of PEDOT:PSS and carbon black (CB) enhances electrical contact, reducing the charge transfer impedance (R_CT) by 50% as measured with electrochemical impedance spectroscopy (EIS). Cycling studies also show increased initial capacity by 30% to 311 mAh g^-1, and improved 100 cycle capacity retention from 63 to 72%. The standard PVDF and conductive carbon black (CB) composite binder (Figure 1a) provides inconsistent electrical contact due to aggregation of the CB compared to the PEDOT:PSS and CB binder (Figure 1b). Irreversible capacity loss due to solid electrolyte interface (SEI) formation in the initial discharge is also reduced from 34 to 28%. Differential scanning calorimetry measures heat generation of the electrode materials and shows reduced heat generation from the SEI as well as from reaction of intercalated K with the electrode materials and electrolyte. These factors are attributed to enhanced stability of the conducting binder as well as weaker interactions with the electrolyte. Removing the CB from PEDOT:PSS binder however results in rapid capacity fading and increased R_CT, despite enhanced electrical conductivity of the PEDOT:PSS binder in ex-situ 2-probe resistance studies. We further examine the mechanical properties of PEDOT:PSS as a binder using a novel experimental method involving optical sensors and a windowed cell to observe expansion and contraction of the electrode material, as well as electrochemical properties with EIS and cyclic voltammetry (CV). Irreversible capacity loss correlates strongly with irreversible expansion for the graphite electrodes with rigid PEDOT:PSS binder. Also, we find the polymer to participate in redox reactions which decrease the conductivity at low voltages.

These results showcase a promising avenue towards improving KIBs independent of the electrode active material and identify issues towards improving lifetime. There is great potential to further enhance the electrical properties of this system with optimized doping to increase conductivity and to enhance mechanical properties with polymer composites that enable a more flexible continuously conducting matrix.