PEDOT:PSS Conducting Polymer As Binder and Conductive Additive for Silicon Nanoparticle-Based Lithium-Ion Battery Anodes

Abstract

Developing high performance Li-ion batteries (LiBs) is of great interest to meet a demand for primary power sources that can operate portable electronic devices, hybrid or full-electric vehicles, and renewable energy storage grids. Among the various electrode materials available for LiBs, silicon has been recognised as the most promising anode material due to its extremely high lithiation capacity (theoretically, 4200 mAh/g for Li4.4Si alloy phase), which is over 10 times higher than that of graphite (372 mAh/g), the traditional anode material in LiBs. Nevertheless, the practical application of Si in the commercialized LiBs is still challenging due to its massive volume variation (approximately 300%) that occurs during electrochemical lithiation/delithiation. The volume variation can result in the fracture and pulverization of Si particles, which eventually causes a loss of an electrical contact between the electrode materials, then finally leading rapid capacity fading during further cycling. In order to address this issue, tremendous attempts have been devoted mostly through structural modification of Si materials, including synthesis of nano-structured Si (nanowires, nanotubes, yolk-shell, and nanoporous Si) and Si-based composite materials using nano-carbon matrix (CNTs, CNFs, and graphene). Owing to the efficient accommodation of the volume variation of Si, which result from the well-designed nanostructure or the buffering effect of the nano-carbon matrix, these Si-based electrodes exhibit the significant improvements in the cycling stability. However these strategies generally involve complicated synthetic steps or require additional materials, thereby their scalability and compatibility remains a challenge under the cost-related practical point of view. Furthermore, the structure-modified Si, electrode active material, cannot stand alone to construct electrode for conventional battery cells. To support the sufficient electrical conductivity and structural robustness into the battery electrodes, a large amount, typically over 30%, of inactive components including conductive agent and polymeric binder is required. However this portion of inactive components is relatively higher than that used for the practical electrode formulations (less than total 20%) and it may result in the decrease in the energy density of LiBs. Given these limitations, increasing attention has been focused on the advancement of the inactive components of battery electrodes. Traditionally, these inactive components possess their own independent functionality. The polymeric binder covers the active materials and conductive agent to mechanically combine together in the entire electrode. And the conductive agent creates the conducting bridges between active materials for their electrochemical utilization. Such a classic electrode system that consists of tri-components (active material, conductive agent, and polymeric binder) is very typical in the conventional LiBs, but it does not suitably work for the Si-based electrodes which have huge volume variation. This is attributed to the break of connections between the non-adhesive conductive agents and the active materials by continuous volume variation. To the end, bi-component electrode system based on dual-functional conducting polymer were further developed to enhance the electrochemical performance of Si-based electrodes. Here, the conducting polymer plays two roles simultaneously not only as a conductive agent but also as a binder, and it can replace the traditional two inactive components by its dual-functionality. This nature of conducting polymer allow to more effectively maintain the continuous electrical network even during huge volume variation of active Si. Additionally, the bi-component electrode that only consists of active Si material and conducting polymer can maximize the energy density of the LiB cells by minimizing portion of total inactive components, which might be great merit of this bi-component system. Based on the above concepts, herein we propose the advanced bi-component Si electrode by employing poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), one of the most common and the successful conducting polymer. PEDOT:PSS is a commercially acquirable polyelectrolyte with excellent physical properties including high electrical conductivity, chemical stability, and mechanically robust. Owing to these great advantages, there has been several effort directed toward developing battery electrodes by exploiting the intrinsic properties of PEDOT:PSS. Here, we report the Si/PEDOT:PSS bi-components electrode via a very simple strategy; the high solubility in water caused by PSS polyanion chains enable the PEDOT:PSS solution to be directly adopted into simple slurry casting technique, the most efficient way that was used in the practical battery electrode manufacturing process. This simple synthesis strategy should allow for the development of Si-electrode, and should also serve as a guide for engineers and scientists engaged in the rational design of electrode system for LIBs.