Considerations in Electrode Prepartion of Si-Based Slurries for Comercial Li-Ion Batteries

Abstract

The demand for high energy density rechargeable Li-ion batteries calls for a move beyond traditionally graphite anodes. In the near-term, Si-graphite composite anodes utilizing small amounts of the alloying metal have been marked as a candidate.(1, 2) By moving to this configuration, graphite buffers the expansion of Si and also acts as a conductive network.(3) Controlling the Si to 15% by mass of the electrode allows for anodes with a theoretical capacity of 917 mAh/g active material, roughly 2.5 times greater than graphite on its own. A key to achieving this capacity comes by utilizing water based binders such as lithium substituted polyacrylic acid (LiPAA).(4) Binders, like LiPAA, interact with the hydroxyl groups on the surface Si allowing for better distribution throughout the electrode.(5) Although these binders show improvements in electrochemical performance, water-based slurries bring about a unique set of processing challenges not found with its n-methyl-2-pyrrolidone (NMP) counterpart. A commonly overlooked issue is continued oxidation of Si upon exposure to water.(6) Not only does this affect the electrochemical performance, but prolonged exposure can lead to excessive hazardous H­2gas production. In this study, we look at the reaction of Si and with water, monitoring gas production in a unique pressure vessel mimicking industrial mixing techniques used in large scale slurry production. Si is examined for changes using Si NMR, powder XRD, TEM, FTIR, and XPS. In addition to the chemical characterization of Si, industrial relevant large format (5.5cmX 8.6cm) pouch cells are assembled utilizing a LiNi0.5Mn0.3Co0.2­O2cathode against 15% Si-graphite composite anodes. Both Water and NMP based anodes are compared at several secondary drying temperatures. Uniform drying of these anodes adds an additionally layer of difficulty as water is easily bound to the surface of Si.(7) Despite these complications, Si-graphite full cells prepared with water based slurries still outperform full cells with anodes prepared using NMP based slurries, with full cell capacities ~1.5 times better after 100 cycles.