Flexible–Stretchable Micro Lithium Ion Batteries for Implantable, Wearable and Embedded Electronics

Future generation electronics which will be in the form of implantable, wearable and/or embedded format will require reliable power supply. Remote wireless charging or grid power supplied battery or energy harvesting – in any form of power supply, energy storage or battery will play critical role. In that regard, coin cell batteries and/or super capacitors are not suitable due to their bulky profile and toxic materials involved in their manufacturing. From that perspective, rechargeable micro lithium ion batteries (mLIB) are promising option for such energy storage. They offer high operating voltage, long life and high energy capacity. However, one major challenge is to achieve flexible and stretchable battery which is critical physical form for implantable, wearable and/or embedded electronics. In recent years, two routes have been pursued for such flexible mLIB fabrication: (1) exploration of new types of inherently compliant materials for battery electrodes such as carbon nanotube, graphene, carbon grease and slurry mixes of nanomaterials and (2) thinned inorganic thin film based battery. Although nano-scale thin films based mLIBs are reliable from their manufacturing and performance perspective, they are fabricated on rigid substrates. One option is to use exfoliation of a mica substrate to release battery stack – however, mica substrates are unconventional and physical delamination of such substrate is a low throughput/low yield process. In this work we present a soft etch back based flexible mLIB formation process. First we fabricate the battery on bulk mono-crystalline silicon (100) using lithium based thin films including cobalt, phosphorous alloy and then we spin a soft material on top of it. Next we flip the whole system and etch back the silicon using reactive ion etching in sequential manner. Next we spin another soft material on the flipped “chip”. Finally, we remove the first soft material selectively over the thin films present and the second soft material. In this way, we obtain 40 mm thick silicon based mLIB with an enhanced normalized capacity of 146 mAh/cm² even after 120 cycles of continued operation. Comparison with a traditional rigid battery (fabricated in batch with the flexible one) shows comparable to superior performance.