Silicon has the potential to help lithium-ion batteries meet 350 Wh/kg on the cell level and 235 Wh/kg on the pack level

One thing noted in Tesla’s Model S update announcement back in July is the introduction of silicon in the anode of the cell. This change enables the battery pack to deliver 90 kWh of energy on a single charge, a roughly 6% boost from the previous 85 kWh system. Although Elon Musk was trying to keep it low by describing the move as “a baby step in the direction of using silicon”, the news had stirred up some excitement in the battery community. It had been long overdue to see silicon anode’s debut in an electric car (EV) battery.

Silicon has the potential to help lithium-ion batteries meet 350 Wh/kg on the cell level and 235 Wh/kg on the pack level (United States Advanced Battery Consortium USABC CY 2020 goals for EV batteries). The state-of-the-art graphite does not have this potential; the highest energy density reported with graphite anode was less than 250 Wh/kg to our knowledge (243 Wh/kg for Panasonic 3400mAh NCR18650B cell). Also, Argonne National Lab reported by simulation that the energy density of packs would not be able to exceed 200 Wh/kg if only graphite was used. This promise of silicon comes from the fact that 1g of silicon can store 3578 mAh of charges while graphite can only do 373 mAh/g. The energy of a cell or a pack is simply the product of the voltage and the amount of charges. It should be mentioned that this 10-fold difference in specific capacity cannot be translated into 10-fold difference in specific energy mainly due to the weight from other components in a cell.

The interest in silicon anode started in the early 80s and the nanopowder approach that prevails now was first explored by Hong Li et al in 1999. However, even up to now, silicon shows impractical short cycle life in the pilot production scale, stemming from the 280% volume change between the charged and the discharge states as well as the physical properties of silicon and its lithium alloys. Panasonic was reported in 2009 to have been developing silicon-based 4000mAh 18650 cells for fiscal 2012 volume production, but they are not yet available to our knowledge.

Currently, 5-10% of silicon can be blended with graphite as the anode material and the cell can exhibit comparable cycle life as graphite does. This probably is what “partially use silicon” meant in Elon’s comment.

 

The Model S had been using Panasonic’s 3100mAh NCR18650A before the update announcement. The cell is based on NCA cathode/synthetic graphite anode couple (NCA refers lithium nickel cobalt aluminum oxide, LiNi0.8Co0.15Al0.05O2). It should be acknowledged that Tesla represents the technical approach on using 18650 cells for EV applications.

The 3400mAh NCR18650B from 2012 is with the same electrode chemistries. It has been seen since then that 3 newer Panasonic 18650 cells might have silicon (silicon oxide actually) mixed in the anode – 3400mAh NCR18650BF, 3500mAh NCR18650GA and 3600mAh NCR18650G.

NCR18650BF is 2g lighter than NCR18650B with the same rated capacity and nominal voltage, according to the tentative specs available online. The energy density is 248 Wh/kg (a slight 2% increase based on NCR18650B). NCR18650GA’s (used also to build batteries for UAV) energy density was estimated at 255 Wh/kg (a 5% increase based on NCR18650B), although the cell is a bit heavy at 48.0g. NCR18650G has high capacity and there is some improvement on weight and dimensions from NCR18650GA, so the estimated energy density was 269 Wh/kg (a 11% increase based on NCR18650B). emvalley.com/news

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