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David Snydacker
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"The cathode shows an initial discharge capacity of 1272 mAh/g (based on the incorporated sulfur content) or 599 mAh/g (based on the compound ; Figure 3 a)." In the Li-S research world, it's common practice to normalize cathode capacity according to the active cathode mass and ignore the inactive mass. However, to calculate total cell energy density, one must consider the entire electrode mass, including both the active and inactive mass. Perhaps the inactive mass in Li-S cathodes can be reduced over time, but today, Li-S cathodes have a much higher inactive mass (~50%) than Li-ion cathodes (~15%).
First cycle coulombic efficiency is often more important than anode capacity. By boosting anode capacity, they probably reduce the first cycle coulombic efficiency. CPreme is the gold standard with 95% first cycle coulombic efficiency.
If they could deposit the platinum on carbon, instead of gold, they would have something.
Advanced battery chemistries, including next-gen Li-ion, Li-S, Li-O2, and Mg-ion, hold disruptive potential in the auto industry. If one or more of these approaches scales successfully, BEVs will be transformed and sales will surge well beyond EIA's statistical extrapolations.
The authors claim 130 mAh/g is "large capacity", but I disagree with this claim. NCA at 190 mAh/g is a large capacity material. Argonne National Lab's layered LMO at 250 mAh/g is a large capacity material. Energy = capacity * voltage. Increasing capacity is good, so long as there is not mechanical degradation. Increasing voltage is good, so long as there is not electrochemical degradation. It seems they have electrochemical degradation leading to only 80% capacity retention after 100 cycles. So this material is not commercially viable, but as you write this is a prototype. I think it would be most interesting to test this new electrolyte molecule TTFEP with truly high-capacity layered LMO materials which operate below 4.5 V.
Silicon anodes are probably the future. But as I said before, they need to offer stable capacity, not just high capacity. Check out this picture of the Solid-Electrolyte Interphase (SEI), these are lithium-rich phases that form in the anode: http://www.cse.anl.gov/cees/index.html Electrolyte additives can help form a stable SEI, but much of that lithium comes from the cathode and corresponds to reduced capacity for the full cell. I'd bet that in five years, we'll have Li-Si/Li-S or Li-Si/Li-Mn-M-O batteries so that a Tesla Model S with the same size battery will go 450 miles. Nissan Leaf will go 150 miles and charge in 15 minutes.
Mining metals is fundamentally unlike mining fuels. It concentrates the resources, it doesn't destroy them. Who would ever throw out a battery that contains $3,000 worth of nickel? Batteries, rare earths, steels, etc, can all be recycled with low environmental impact, if done right. The argument about charging EVs with coal electricity is tired. Computers were a niche market before the internet. EVs were a niche market before wind and solar power. We need to take a forward-looking systems-level perspective on sustainability.
Dave D, Cell energy density (not anode energy density) is the ultimate measure of performance. Cell energy density is calculated by adding up the size of the anode, the size of the cathode, and the size of all other inactive components in the cell. The cathode is the largest part of the cell, and the anode is relatively small. Even if you reduce the size of the anode by 75%, you've only reduced the size of your cell by 15%. Ok. Now here's the really bad news. In a working cell, lithium shuttles back-and-forth between the anode and the cathode. When the anode loses capacity, lithium dies and cannot shuttle back to the cathode. The whole cell loses capacity. So if your anode capacity fades by half, the whole cell capacity fades by half. Essentially, your anode dies and takes your cathode down with it. That extra 15% cell capacity from a smaller anode isn't worth a thing if your anode is going to eat up all the lithium from the cathode. Hope that makes things more clear. Best, Dave S
I don't see how an anode material that destroys 50% of the lithium in the cell during the first 100 cycles could ever be commercially viable. The anode is the smallest part of the cell, so it's very important to differentiate between anode capacity and the corresponding cell capacity as it fades over time. Commercial graphite anodes retain 94% of their capacity after 100 cycles. So cell capacity might fade from 100 units to 94 units. For this new silicon/silicide anode, capacity might start high at 115 units, but it would fade quickly to 63 units. 94 versus 63 - that's the relevant comparison. 1,000 mAh/g versus 350 mAh/g is not a fair comparison. Unless this anode material can be tailored so it's lithium loss (capacity fade) is minimized.
Remarkably high and stable capacity. But what is the C-rate? Capacities are reported in mAh/g but rates are in mA/cm^2. No g/cm^2 reported for the conversion?
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Apr 2, 2012