Poly(ionic liquid) Based Anodic Binder with Enhanced Cyclability for Lithium-Ion Batteries

The commercialization of the lithium-ion batteries (LiBs) by Sony in early 90s has led to revolution in technology in past two decades. Due to its higher gravimetric and volumetric energy, lower self-discharge, absence of memory effect and flexibility in the design has made this class of batteries very popular. In order to achieve improved efficiency and safety, a plethora of electrode materials and electrolytes are designed. In addition to these materials, binders play a very crucial role in the performance of the LiBs. Binders are polymers that maintain the integrity of electrodes by binding the electrode material together and the electrode laminate to the current collector. They assist in Li-ion mobility and solid electrolyte interface formation. Poly(vinylidene fluoride) commonly known as PVDF is the commercial choice of binder due to its cost in addition to its thermal, chemical and electrochemical stabilities. However, a major drawback of using PVDF is its dissolution into electrolyte leading to the loss of electrode integrity and capacity fade on long cycling. Also, due to the fluorinated framework, fluorinated salts are formed on electrode surface and poses an environmental threat.1 In order to mitigate this setback, a variety of both natural and synthetic polymers are currently being explored as binders for LiBs.2 Poly(ionic liquid)s (PILs) are emerging class of material with polymeric backbone and an ionic liquid unit attached to it. The structural diversity is vast due to the availability of innumerous ionic liquids and polymeric backbones. By varying the structure, tuning of the properties can be achieved. The current work focusses on developing a multifunctional binder which in addition to binding the electrode material, can assist the formation of a better electrode-electrolyte interface. An allylimidazolium tagged polystyrene derivative (PVBCAImTFSI) was prepared by quaternization reaction of the allylimidazole with poly(vinylbenzyl chloride). The obtained polymer was characterized with FT-IR spectroscopy and energy-dispersive X-ray spectroscopy (EDX). Structural optimization and electronic properties were calculated using Gaussian 09. Electrochemical characterization was carried out using 2025-coin cells with graphite anodes fabricated with PIL and PVDF binders. Electrochemical Impedance spectroscopy (EIS) and Dynamic Electrochemical Impedance Spectroscopy (DEIS) were carried out to understand the nature of the electrode-electrolyte interface. Cyclic voltammetry was carried out to understand various redox processes in the potential range of 10 – 2100 mV vs Li/Li+ at a scan rate of 0.1 mV/s. Linear-Sweep Voltammetry was carried out to evaluate the electrochemical stability of the binder in the potential window of 0 – 7000 mV vs Li/Li+. Figure shows the CV measurements which indicated a decreased irreversible electrolyte reduction (shown in the circle) for the half-cells with PVBCAImTFSI binder as compared to that of the PVDF binder. Also, the intercalation-deintercalation overpotential was reduced by about 90 mV in the former case. To further understand the diffusion of the Li+ ion in the active material, CV based scan rate studies were carried out. The diffusion coefficient calculated for the PVBCAImTFSI based electrodes was found to be 40% higher than that of the PVDF based electrodes. These indicated formation of a thinner and more robust SEI. The impedance due to the SEI, RSEI values were estimated by fitting the DEIS data to the equivalent electric circuit models (EECMs). These values for PVBCAImTFSI were found to be lesser than that of PVDF at all potentials indicating formation of better SEI in the former case. Due to the high wettability of PVBCAImTFSI and better SEI characteristics the present system led to an enhanced charge-discharge performance in terms of cyclability as well as stability. A discharge capacity of 210 mAh/g was obtained at 1C rate for PVBCAImTFSI based half-cells and about 95% capacity retention was observed after 500 cycles. Whereas, PVDF based half-cells exhibited 140 mAh/g of discharge capacity and 40% capacity retention. Even the rate studies suggest that PVBCAImTFSI based electrodes to have higher stability. In conclusion, an allylimidazolium based multifunctional poly(ionic liquid) was synthesized and used as an anodic binder for graphite electrodes. The anodic half-cells with PVBCAImTFSI displayed about 210 mAh/g of discharge capacity and a 95% retention after 500 cycles at 1C rate. Its enhanced charge-discharge performance can be attributed to its improved interaction with the electrolyte and a thinner SEI formed. The results obtained in this work indicates that PVBCAImTFSI can potentially replace PVDF as a binder in LiBs. (1) Patnaik, S. G.; Vedarajan, R.; Matsumi, N. J. Mater. Chem. A, 5 , 17909 (2017). (2) Chen, H.; Ling, M.; Hencz, L.; Ling, H. Y.; Li, G.; Lin, Z.; Liu, G.; Zhang, S. Chem. Rev., 118 (18), 8936 (2018). Figure 1