Bian Based Copolymer As Binder Material for Graphite Anode in Lithium Ion Batteries

Abstract

Lithium-ion batteries (LIBs) employing graphite carbon anodes are widely used in consumer electronics and hence over the past few years, intense research has been pursued to develop different components of LIBs. However relatively less focus have been dedicated towards development of efficient binder materials. Over the years, poly(vinylidene) fluoride (PVDF) has been the primary choice as binder in electrodes of LIBs, owing to its good electrochemical, chemical and thermal stability, acceptable adhesion to the electrode materials and current collector, and ability to absorb electrolytes. However the prime disadvantage of PVDF is that it fails to maintain a conducting linkage between the active material and the conductive additive, owing to its inherent non-conducting nature. This particular drawback is further magnified upon continuous Li insertion/de-insertion, ultimately leading to increased cell resistance[1]. The coating of conjugated polymers helps mitigating the low intrinsic electronic conductivity, builds up effective paths for electronic transport and Li ion diffusion [2][3]. Recently Liu et.al reported a conjugated polymeric binder material for silicon based anodes in LIBs. However, no such strategized effort has been put for performance enhancement of graphite anodes. Hence in this work, we put effort to develop a new class of polymeric binder material to enhance the performance of graphite anodes, addressing it from two perspectives. i. To use the polymeric binder as a part of the active material. ii. To obtain improved conductivity by exploiting the presence of inherent conjugation and redox active groups in the polymeric binder material. In this regard, compounds of the family bis(aryl)acenaphthenequinonediimine (Ar-BIAN), which have long been employed as ligands for transition metals, were considered. The rich redox chemistry of Ar-BIAN based compounds have been evaluated. Ar-BIAN based compounds are characterised by high chemical stability and have wide scope for functionalisation owing to the availability of suitable precursors. But no reports exist on application of Ar-BIAN based polymers, drawing upon their redox chemistry in energy storage devices. In an effort to capitalise on these unexplored but potential properties of these group of polymers, we utilise them as polymeric binder materials for LIBs both independently and in combination with PVDF to see their feasibility as part of active material in graphite anodes. Ar-BIAN-Dioctylfluorene (BIAN-Flu) copolymers were synthesised via Sonogashira coupling and characterised (IR, NMR, UV, GPC, MS, CV). A series of graphite/BIAN-Flu polymer composites were prepared and electrode fabrication was achieved by doctor blading the composite on copper foil. Charge discharge (C/D) behaviour of the prepared polymer as a binder as well as an active material was evaluated in an anodic half-cell at 0.05C with 0.1M LiTFSI/EC:DEC as electrolyte. The electrode reaction kinetics was also evaluated by dynamic electrochemical impedance spectroscopy (DEIS) confirming efficient Li intercalation and de-intercalation. Studies were performed with different weight percent of the polymeric additive to optimise the amount of the polymeric additive to be taken in electrode fabrication. Long term cycling studies were also undertaken with 0.025C charging and 0.05C discharging to evaluate the effect of cycling. The C/D studies indicate that the polymeric active material significantly enhanced the capacity of the anodic half cells compared to only PVDF as the binder. This can be attributed to the inherent redox activity in the polymeric active material which also contains binding sites for Li ion. The reversible capacity increased from 180 mAhg-1 (in case of only PVDF as binder) to 225 mAhg-1 (in case of the polymeric active material). Additionally, the high columbic efficiency of 99% demonstrates the supporting role of the polymeric additive material in maintaining the reversibility of the C/D process. The polymeric material thus serves two important roles, i. Enhances the capacity because of the ability to reversibly bind to Li ion. ii. Enhances the overall C/D process of the half-cell because of its inherent redox activity, reflected in relatively low over-potential in the C/D profiles and reduced cell impedance as exhibited in the DEIS studies.