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Based in Fremont, California, Amprius Technologies is positioning itself as a leader in lithium-ion battery technology and claims to produce the highest energy density cells in the world, based on its 100% silicon nanowire anode platform.
The company’s website states that they are developing cells approaching a 450 watt-hour per kilogram energy density over hundreds of cycles. This is one of the breakthroughs needed to enable eVTOL and other electric aircraft to have practical use range and endurance.
On Oct. 25, the company announced a three-year agreement with BAE Systems that will see Amprius initially work with BAE Systems’ Air business to deliver its lightweight high-energy batteries specifically developed for electrically powered flight applications.
“Amprius’ innovative silicon nanowire anode battery cells have the potential to deliver exceptional performance, and we’re keen to work with [the Amprius team] to explore opportunities for the electric products that we’re developing for military applications,” said Dave Holmes, director of advanced projects, technology and manufacturing at BAE Systems’ Air.
The Airbus Zephyr, a solar-powered stratospheric unmanned aerial vehicle (UAV) was partly powered by a 24 kg (52.9 lb) Amprius battery pack. Last August, the UAV set several aviation records, including the longest continuous flight for an uncrewed aircraft — at 64 days, 18 hours and 26 minutes. The aircraft was conducting tests in coordination with the Army Futures Command and was fitted with payloads to aid research in the Army’s high altitude surveillance requirements.
The battery developer became a publicly-traded company on Sept. 15 when Amprius completed its business combination with Kensington Capital Acquisition Corp., a special purpose acquisition company.
Why silicon anodes?
During a 2020 Stanford Energy StorageX Symposium, Dr. Kang Sun, CEO of Amprius, explained that the main advantage of using silicon anodes is that silicon has about 10 times the energy density of graphite-based anodes. However, its major drawback is that it can swell up to about 300% during cycling, which will damage the battery after just a few cycles. The technology started being investigated in the 1990s in Japan, without much success.
About 14 years ago, Stanford University professor Yi Cui proposed the silicon nanotube concept, which Sun still believes is the best available structure for silicon anodes to mitigate the expansion problem.
Amprius embraced this solution. It is achieved by having a conductive filament grown from the substrate, which does not require binders but is akin to being welded to the collector plate. It is progressively coated with low density, porous amorphous silicon and finished with a thin layer of high-density amorphous silicon.
When seen under high magnification, the resulting structure looks like a double-sided pile carpet with the individual nanowires being about 30 microns long. Sun stated that the structure is strong and damage resistant. The resulting empty spaces in the structure are exploited to allow for the silicon’s expansion.
Since the silicon nanowire structure is very different from the silicon particle structure, there is less interference and thus, Amprius claims it can reach the near theoretical capacity limit for this material.
Another advantage is that silicon anodes can be much thinner than their graphite counterparts. This means that with a silicon anode, lithium-ions have a shorter distance to travel. This, coupled with the nanowire structure, allows for the fast charge rates — another important advantage of this technology.
On the discharge side of the cycle, a graphite anode would also have to be designed to lower loading specification. The higher loading specification possible with silicon anodes results in lower manufacturing costs since the material requirements can be decreased due to less surface area required for the same power. Graphite also suffers from a slower intercalation rate due its planar structure — the reversible process of inclusion or insertion of ions within a layered structure.
If one tries to charge a cell faster than the graphite’s natural intercalation rate, the lithium-ions start piling on the surface of the electrode and start depositing, eventually forming dendrites that can grow up to a point of shorting the cell. This is exacerbated by the voltage of graphite being near the plating potential for lithium. Amprius’ silicon nanowire design is much less exposed to this effect.
One of the drawbacks of silicon anodes is the first cycle irreversible lithium loss where up to 40% of the lithium-ions remain fixed on the anode in the first charge/discharge cycle. This has an adverse impact on the cell’s capacity. The effect can be reduced by 90 to 100%, by depositing lithium directly into the anode material prior to production — a process called prelithiation.
Cycle and calendar life are the other challenges that silicon anode must take on. The 10-year life target will probably not be limiting for eVTOL applications, as the number of cycles should result in battery replacement intervals well before that threshold is reached.
The charge and discharge cycles that eVTOL aircraft will need for network optimization will require both fast charging and intermittent high power draw during take-off and landing — the latter of which will also occur at a low state of charge. These are challenging use profiles for any battery technology and chemistry.
Extreme fast charge demonstration
During an event held on Oct. 26, Amprius demonstrated an extreme fast charge of one of its cells. The target was to reach 80% charge in less than 10 minutes. The test was carried out using a 100% silicon nanowire cell with a 370 Wh/kg capacity at cell level. It is rated for greater than 100 cycles to 80% of initial capacity at 100% depth of discharge. The cell has a 2.8-amp hour capacity, weighs 28 grams, and can accept a charging current of 28 A (10 C) and 120 W charging power at peak. The cell was placed in a chamber at 22 C. The peak charge rate was 10 C.
During the test, Dr. Constantin Ionel Stefan, chief technology officer at Amprius, stated that compared to graphite anode batteries, eVTOL aircraft silicon nanowire batteries can expect up to double the flight time and faster turnarounds. At the five-minute mark, the cell had reached over 73% charge and had transitioned to a constant voltage mode of 4.35 V at around two minutes, 15 seconds. The maximum cell temperature shown in the test was 57.3 C (135.1 F), but it was not stated if this was the peak temperature. Stefan said the cell did not have active temperature control for this test, and if that were the case, it could be capable of even faster charging. The 80% charge target was reached at six minutes, three seconds.
Stefan said that the cell, as shown in the test at 370 Wh/kg, is on the lower spectrum of its energy density range but offers higher power. The 400 Wh/kg model would also be suitable for eVTOL applications, and both are designed with 100% silicon anode. The advantage is the ability to fast charge, as there is much less risk of lithium plating. The trade-off for fast charging is that the thinner anode has less energy capacity, but this is balanced by silicon’s inherent high-energy capacity.