High Throughput Alumina Coatings on Graphite Anode Powders in a Continuous ALD Reactor

Atomic layer deposition (ALD) has been demonstrated by researchers in both academia and industry onto cathode and anode powders as an effective technology for improving the power and energy density, and safety of lithium-ion batteries. These improvements are critical for achieving the targeted increases in battery performance for a wide range of industries. For it to be fully adopted into lithium-ion battery products, the ALD process on electrode powders needs to be scaled to match the material capacity requirements of the industry. This means that processes need to be developed that are capable of coated hundreds to thousands of metric tons a year of electrode powders. Forge Nano (FN) has previously developed large scale rotary and fluidized bed reactors for coating powders at the grams to hundreds of kilograms scale, but a continuous process that can be directly incorporated into a production line is very desirable. To that end, FN has designed and built an atmospheric pressure continuous spatial ALD system capable of coating powders with thin alumina films, and used it to demonstrate the coating of graphite anode powder. The reactor (Figure 1) is based on a vibrating conveyor, a standard powder handling and processing operation. The powder travels down a vibrating tray with a porous stainless-steel deck. Process gas flows perpendicularly through the porous deck and the powder bed. As the powder flows down the tray, it traverses zones where the various ALD precursor exposure and purge steps occur. As built, the reactor can deposit 4 ALD cycles in a single pass. Multiple passes allow for thicker films to be deposited, but in a commercial system the reactor would be built to length for the appropriate number of cycles. FN coated two grades of commercial synthetic graphite anode powder, with throughputs ranging from 20 – 60 kg/h. Samples were analyzed using ICP to determine the amount of aluminum (as alumina) deposited by the process. Samples taken periodically during system startup showed that steady state flow and deposition were achieved after about 10 minutes of operation. The configuration of the ALD zones was varied to demonstrate that the purge length of the reactor was sufficient to prevent the deposition of excess alumina by CVD. The flow rate of the trimethyl aluminum and water precursors were independently varied to demonstrate that the deposition was self-limiting. Multiple passes of the substrates through the reactor showed that the process deposited a consistent 25 or 50 ppm of alumina (depending on substrate) per ALD cycle. These growths per cycle match the deposition seen in small scale fluidized bed and rotary reactors. Further scaling of the process to hundreds of kg/h is possible through simple increases in reactor dimensions. Coated materials were tested in coin cells as anode powders for lithium-ion batteries. First cycle coulombic efficiency (FCE), reversible capacity, and cycle life performance for the continuously produced graphite powders showed significant improvement over the uncoated materials, and were comparable to those for graphites prepared via more traditional ALD processing in fluidized bed reactors under vacuum conditions. Figure 1