UCLA

The proliferation of wearable devices has been significantly hindered by limitations of reliable flexible power solutions. To address this challenge, this dissertation introduces the concept of a flexible Li-ion battery, featuring a partitioned cathode and anode electrode array coated on flexible composite current collectors, which is referred to as the “battlet”. Ionic liquid Lithium bis(fluorosulfonyl)imide (LiFSI) with 1-Butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI) is used as liquid electrolytes to address the flammability concern. The mechanism of mechanical failure during bending, the fabrication of partitioned electrodes, a comparison of the ionic liquid and organic electrolyte, and electrochemical performance is discussed in this work. Results show that this approach can reduce the crack propagation of the electrodes under mechanical distortion, thus improving electrochemical stability. The full cell battery achieves a 0.7 mAh/cm² capacity density and can withstand around 1000 bending cycles at a 5 mm bending radius.Additionally, using a flexible Fan-Out Wafer-Level Packaging (FOWLP) platform, FlexTrateTM, a flexible wireless charger is designed using resonant magnetic coupling. The wireless charger can withstand up to 5 mm bending radius and can deliver a constant 3.3 V output voltage and 3.9 mW peak power. The flexible wireless charger, along with the flexible battlet battery, serves as a fully wireless power solution for wearables that we call FlexPower. To integrate the FlexPower with flexible electronics, this dissertation also discusses a 2D and a 3D flexible integration approach based on FlexTrateTM. As a demonstration of the 2D approach, the FlexPower is integrated with a UV microLED display array consisting of 33 microLEDs. The power consumption of the LED array is 3 mW, and the flexible battery, with a capacity of 4.6 mWh, can power the microLEDs for more than 1.5 hours. For the 3D integration, the dissertation explores the development of flexible interconnects on the front and back sides of FlexTrateTM. A through-glass via die is used to facilitate interconnection between both sides. A detailed experimental study of the SF6/O2 plasma PDMS dry-etch method for backside contact opening, interconnect performance, and reliability is also addressed. This work represents, to the best of our knowledge, the first demonstration of a flexible battery integrated with a flexible wireless charger powering flexible µLED arrays for wearable applications.