A SAFER, MORE ENERGY-DENSE LI-ION BATTERY
Part Four – Photolithography and Wafer Production
This is the fourth (and final) in a series of posts from the IEEE Spectrum article, “How to Build a Safer, More Energy-Dense Lithium-ion Battery,” authored by Ashok Lahiri, Nirav Shah, and Cam Dales of Enovix. The article describes how we use photolithography and wafer processing techniques to fabricate our 3D Silicon™ Lithium-ion battery.
To fabricate the Enovix battery, we begin with a wafer of silicon that’s 1 millimeter thick. This doesn’t have to be the chip-grade stuff—it can be the same low-cost material that is used to produce solar cells. To the wafer we apply a photolithographic mask and etch the required pattern with typical silicon etchants borrowed from the solar industry. Because the pattern can vary in shape—square, rectangular, round, oval, hexagonal—as well as in length and width, we have the ability to form a wide variety of cell designs. The silicon that’s left behind where the mask was placed forms the anodes and “backbones” of the interlaced cell structure.
Next, we selectively deposit a thin coat of metal film onto the anodes and backbones to form current collectors and then deposit a ceramic separator around the collector on the anodes. Because the anodes and backbones are not electrically connected on the wafer, we can selectively electroplate different coatings on each. To create the cathodes, we inject a conventional cathode slurry, filling the remaining voids in the wafer. Then a laser slices off individual 1-mm thick die from the wafer, with the lateral dimensions of the die approximating the dimensions of the final battery. Positive and negative tabs are then attached to each die, which are baked to remove moisture, and stacked to form the desired height of the battery. The tabs are all connected to form a single positive and negative tab for the cell, and the resulting stacked cell is then pouched or inserted into a metal can, which is filled with electrolyte, sealed and tested.
Our architecture, silicon wafer photolithography, and etching process are comparable to what is used in three-dimensional MEMS. Hence we dubbed our device the 3D Silicon Lithium-ion battery.
Much of this manufacturing technology comes, of course, from the solar cell business. The progress in that field—fueled by immense R&D investment worldwide—at once explains the low cost of our manufacturing approach and the likelihood that it will continue to improve in efficiency and scale.