Compression of Si Alloy Electrodes to Increase Li-Ion Battery Energy Density

Abstract

Introduction Si is attractive for use in high energy density anode materials due to its high volumetric capacity of 2194 Ah/L (corresponding to Li15Si4) [1]. The volume expansion of Si can be diluted to improve cycle life by the use of active/inactive composite electrode materials [2, 3]. However, electrode processing conditions also plays an important role in performance improvement. For commercial energy cells, the electrode stack should have high energy density. Therefore, electrode compression or calendering is widely practiced in industry to increase energy density [4]. Si-based materials are typically hard and brittle and cannot be readily calendered. Therefore such coatings can have high porosities, which can result in low energy density.Here, we present a facile and viable method for the compression of Si alloy electrodes while maintaining their high volumetric capacity, low volume expansion and good cycling performance. Experimental Electrode slurries were made by mixing specific ratios of 3M L-20772 Si alloy [5] and LiPAA (Polyacrylic acid) solution in distilled water with/without the addition of SFG6L graphite (28% by weight). The slurries were coated on Cu foil using a 0.004 inch gap coating bar and dried at 120oC in air for 1 h. The electrode foils were then passed through a calender for calendering to ~20% prorosity. The electrode pore volume was the difference of the total coating volume minus the solids volume. Electrodes were assembled into 2325 size coin-type cells using 1M LiPF6 dissolved in EC:DEC:FEC (3:6:1 vol%) solution. Two Celgard separators and a lithium foil counter/reference electrode were used. Electrode thicknesses were measured to within ± 1μm with a Mitutoyo 293-340 precision micrometer. The morphology of electrodes was studied using the Phenom G2 pro desktop SEM. Results The porosity of an uncalendered Si alloy / LiPAA 91/9 w/w electrode is about 56%. When it is fully lithiated coin cell is disassembled, it was found the entire coating had expanded by 96% and the porosity was calculated to be 57%. Therefore the alloy does not expand into the available porosity in the coating. Instead, as shown in Figure 1, as the alloy expands by 96%, the pores expand by the same amount. The large volume fraction porosity in such electrodes makes their volumetric capacity relatively low (624 Ah/L). To increase the volumetric capacity, electrodes with various formulations were calendered to ~20% porosity.Figure 2 shows the cycling performance of some of these electrodes. Clearly, calendering has a detrimental effect on the cycling performance of Si alloy electrodes when graphite is not present. By adding graphite in the electrode, the cycling excellent cycling can result. Moreover the volumetric coating capacity is increased to 957 Ah/L while overall volume expansion is reduced to only 64%.In this manner high volumetric capacity, low volume expansion alloy electrodes with excellent cycling characteristics can be obtained. Mechanisms of volume expansion in alloy coatings and methods of improving volumetric capacity and lowering volume expansion will be discussed.