By Kyle Proffitt
April 22, 2025 | Jeff Dahn, Professor at Dalhousie University, gave his annual plenary address at the International Battery Seminar last month. Dahn spoke about silicon-carbon composite anodes, comparing chemical and mechanical synthesis methods and praising carbon nanotubes for turning garbage to gold.
“Silicon-carbon (Si-C) materials are becoming popular,” Dahn says, because pure silicon expands so much. Mixing the silicon with carbon can mitigate this swelling while gaining an energy boost. As a result, “Silicon-carbon composites with about 50 weight percent silicon are available from many vendors—over 70 in China,” he said. Si-C is primarily prepared through a chemical reaction using vapor deposition; silane gas is pumped across hard carbon in a furnace. The result is particles about 10 microns in size. It can also be prepared by mechanical milling, generating particles of approximately 5-6 microns.
“We’ve looked at quite a number of chemical silicon-carbons and quite a number of mechanical silicon-carbons,” Dahn began. However, much of his work could not be disclosed; he showed only prototypical results from the two methods.
For chemical Si-C, transmission electron microscopy and X-ray scattering results show the silicon uniformly distributed in roughly 1-nanometer clusters within nanopores of the hard carbon. Chemical Si-C showed discharge capacity of about 1800 mAh/g. The mechanical Si-C Dahn showed had a lower capacity of about 1200 mAh/g, but he said it’s possible to prepare mechanical Si-C with the higher capacity.
He discussed the importance of binders when it comes to silicon anodes, highlighting the importance of maintaining an electrically conductive network. “We’ve learned… that single-walled carbon nanotubes (SWCNTs) are like magic in electrodes where particles show huge volume change; they turn garbage into gold.” He showed results from a 2024 article where the addition of just 0.5% SWCNTs enabled stable cycling with silicon monoxide (SiO) anode material, using a “simple binder”, whereas in the absence of the nanotubes, these cells dramatically lost capacity. “The same is true for chemical SiC; simple binders work when you put in SWCNTs,” Dahn said. He showed that contrary to prevailing assumptions with silicon anodes, no fluoroethylene carbonate additive is necessary with chemical Si-C, at least as long as SWCNTs are included.
In addition to the “breathing” that occurs during cycling for volume-change materials, Dahn indicated that irreversible volume changes are another major problem, where lithium inventory is lost to side reactions, thickening the SEI. His group prepared an apparatus to measure the associated pressure created from cycling and compared the irreversible pressure changes in batteries using anodes with 20% micronized silicon, chemical Si-C, or mechanical Si-C.
Remarkably, only the chemical Si-C anode avoided irreversible swelling, which corresponded to stable cycling. SEM images of these particles showed no cracking or pulverization. The cells still undergo a large, reversible volume expansion during cycling. In a mini-summary, Dahn said that cells with 20% Si-C could reach 1000+ cycles at C/3 or C/5 rates.
Focusing next on the reversible volume changes, Dahn turned to a topic discussed last year—electrolyte movement causing salt depletion at the battery ends. For high volume change materials like Si-C and SiO, higher charge rates lead to rapid capacity depletion; however, the capacity is regained after a rest, which makes sense because the rest gives the electrolyte time to re-equilibrate. Using impedance spectroscopy, his group tracked the timescale of this relaxation event in 18650 cells prepared with SiO anode material and discovered that it occurred over eleven days. “The DC resistance of the cell takes many, many days to relax back, and things are much worse in longer cells, because the time for the electrolyte salt imbalance to relax scales as the length of the cell squared,” he said. “Long cells are not a good thing if you’re going to be incorporating a large volume change material into the negative electrode.” He hinted that great care might be put into cell design to avoid such electrolyte imbalance.
Si-C Excitement
Summarizing, Dahn said that “chemical silicon carbon materials are really exciting” for their lack of irreversible swelling, their functionality without fancy electrolytes, avoidance of cracking or pulverization, and good cycle life. Furthermore, he said that chemical Si-C should be manufacturable at a price of $25/kg. Natural graphite comes in at $5/kg, but once you consider their energy densities—1800 Ah/kg vs. 360 Ah/kg—the price is exactly the same, at 1.4 cents/Ah.
As one drawback, Dahn said that chemical Si-C will never compete with graphite for extremely long cycle life if significant cycling depth is used; he pointed to his NMC532/graphite cells that have cycled 26,000 times over 7+ years (10.4 million km), evidence of his continuing quest for ultra-long cycling.
