2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
3, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
4, University of Tennessee, Knoxville, Knoxville, Tennessee, United States
Increasing the energy density of the lithium-ion battery (LIB) is necessary to meet the demands for their expanding applications from portable electronics to large-scale emerging applications, such as renewable energy storage grids and electric vehicles that require acceptable driving distance upon a single charging. Among many candidates that can increase the energy density of anode, silicon (Si) is extremely compelling due to its high theoretical capacity (3579 mAh/g for Si compared to 372 mAh/g for commercial graphite anode), low operating potential, non-toxicity and worldwide abundance. However, the high specific capacity of the silicon-based electrode is typically observed only at the initial cycles, and cannot meet the long cycle life required for typical electric vehicle application. Utilization of polymeric material to hold active materials intact is a conventional approach, and the polymer binder plays even more significant role in the cell performance of the silicon-based electrodes because of their enormous volume changes during electrochemical cycling. In this presentation, two novel polymer binders for Si anode will be discussed. The first work investigated the architecture effect of synthetic polymers on the polymer binder performance for the high-mass loading silicon(15wt%)/graphite(73wt%) composite electrode (active materials > 2.5 mg/cm2). With the same chemical composition and functional-group ratio, the graft block copolymer reveals improved cycling performance in both capacity retention (495mAh/g vs 356 mAh/g at 100th cycle) and coulombic efficiency (90.3% vs 88.1% at 1st cycle) than the physical mixing of glycol chitosan (GC) and lithium polyacrylate (LiPAA). Galvanostatic results also demonstrate the significant impacts of different architecture parameters of graft copolymers, including grafting density and side chain length, on their ultimate binder performance. By simply changing the side chain length of GC-g-LiPAA, the retaining de-lithiation capacity after 100 cycles varies from 347 mAh/g to 495 mAh/g. The second approach further tailored the functionality and architecture of the polymer binder. By incorporating catechol groups and balancing subsequent crosslinking of the chitosans, the polymer binder possesses both interaction with silicon materials and mechanical robustness to withstand the volume change during charging/discharging process. The degree of functionality and cross-link was systematically studied and optimized. The obtained polymer binders enable the Si-based anode to maintain the high long-term cycling stability, i.e., the retaining de-lithiation capacity after 100 cycles is 2269 mAh/g (90% capacity retaining). This presentation will discuss the design of polymer binders for Si-anode and the effect of various parameters including compositions, interactions, and mechanical properties.