We recently compared our prototype cell for a wearable device with a comparable commercial Li-ion cell by deliberately creating a precarious scenario. We overcharged a conventional 130-mAh Li-ion cell and our 100-mAh silicon Li-ion cell to 250 percent of capacity and simultaneously punctured the package of each (through the standard nail-penetration test). The conventional Li-ion cell burst into flames, but our silicon Li-ion cell did not.
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. We compared a prototype with a conventional Li-ion battery having the same form factor, one designed to fit in a smart watch (that battery was 18 by 27 by 4 mm). Our internal tests showed our battery to have much higher capacity and a corresponding increase in energy density—695 Wh/L as opposed to about 460 for the conventional cell.
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.
Consumers yearn for better and more powerful batteries for their mobile devices, as survey after survey attests. Most demanding of all are the wearable devices and microsensors that are being created for the Internet of Things. Such IoT devices have even less room in them for batteries than do tablets and smartphones.
This wouldn’t be the first time that photolithography and wafer production have suddenly revamped whole industries. It happened first when computers began to use integrated circuits. These fabrication techniques were also applied to lighting, which moved from fluorescent tubes to light-emitting diodes and to video displays, which went from cathode ray tubes to liquid crystal displays.
We believe that the approach we’re pioneering will bring about a similar transformation in the market for lithium-ion batteries. The change will first appear in wearables, next in IoT and phones, and ultimately in electric vehicles and grid storage, as volumes scale up and manufacturing costs come down, as it already has in the solar industry.
With safer, thinner, and higher-energy batteries, designers will have more flexibility to create breakthrough products. Expect mobile devices to get smaller, to last longer between charges, and to continue to deliver amazing new capabilities to enhance our lives.
or whatever you'd like, just please think…