The Performance of Polymer Binders in Lithium Ion Batteries

High-energy-density lithium-ion batteries (LIBs) with a long cycle life are in tremendous demand for use in portable electronics and electric vehicles [1]. Unfortunately, the practical application of high capacity anode materials in LIBs is still quite challenging. Large variation in volume occurs during charge–discharge cycling, which causes fracturing and pulverization of the anode materials and breaks the electrical contacts between active materials and conductive additives, resulting in a rapid capacity fading and a short cycle life [2]. One of the main approaches is using binders to stabilize the structure of the electrode [3]. The traditional PVDF binder, which interacts with electrode materials via weak Van der Waals forces and consequently lacks the necessary capabilities (e.g., the suppression of significant volume variations, the interface maintenance etc.), could not fulfill the high demands of batteries with high energy density. Besides, extensive usage of the PVDF binder in the lithium ion battery is costineffective and may raise environmental concerns as its handling often needs the assistance of organic solvents [4]. A traditional binder system is dual-component based, essentially with two components for two different functionalities. Polymer binders, such as polyvinylidene fluoride (PVDF), mechanically hold the active materials and additives together. Electronically conductive additives, such as acetylene black (AB), are necessary to ensure electrical conductivity of the entire electrode. In a porous composite electrode, the nonconductive polymer binder combines with AB conductive additives to maintain the electronic connection. In addition to the mechanical adhesion and electronic connection, the polymer covers the active material surfaces, so the polymer should swell in electrolyte to provide enough ionic conductivity. Although such classic dual-component binder design is popular in the current Liion batteries, it does not work well for the high-capacity electrodes with large volume change. Mechanically, highcapacity electrode materials tend to generate more than an order of magnitude higher stress in the electrode than those of graphite during lithiation. The stress disrupts the mechanical integrity, leading to electrode fracture and delamination. More seriously, the electronic integrity of electrodes relies on the connections between the nonadhesive conductive additives and active materials. Even with extensive amounts of conductive additive, this connection will break after extended cycles of large volume change. The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems [5]. In this study the goal is to show that electrochemical performance of the MOF anode with CMC binder and polyaniline binder will significantly improve compared to that of a MOF anode with a PVDF binder.