High-performance flexible energy storage and harvesting system for wearable electronics

Abstract

This paper reports on the design and operation of a flexible power source integrating a lithium ion battery and amorphous silicon solar module, optimized to supply power to a wearable health monitoring device. The battery consists of printed anode and cathode layers based on graphite and lithium cobalt oxide, respectively, on thin flexible current collectors. It displays energy density of 6.98 mWh/cm(2) and demonstrates capacity retention of 90% at 3C discharge rate and ~99% under 100 charge/discharge cycles and 600 cycles of mechanical flexing. A solar module with appropriate voltage and dimensions is used to charge the battery under both full sun and indoor illumination conditions, and the addition of the solar module is shown to extend the battery lifetime between charging cycles while powering a load. Furthermore, we show that by selecting the appropriate load duty cycle, the average load current can be matched to the solar module current and the battery can be maintained at a constant state of charge. Finally, the battery is used to power a pulse oximeter, demonstrating its effectiveness as a power source for wearable medical devices.

Figure 1

(a) Illustration of activity-tracking wristband concept containing flexible battery, PV energy harvesting module, and pulse oximeter components. (b) Diagram and (c) photograph of a flexible energy harvesting and storage system comprising PV module, battery, and surface-mount Schottky diode, showing the components and attachment points. The diode is included to prevent discharge of the battery into the PV module in low-light conditions. (d–g) Photographs of the device being flexed in the hand (d) and on various flexible and curved surfaces: jacket sleeve (e), bag (f), and travel mug (g).

Figure 2

(a) Cross-sectional schematic of the lithium ion battery. (b) Optical image of graphite electrode on nickel foil and LCO electrode on stainless steel, flat (top) and flexed over a pen with diameter of 10 mm (bottom). Cross-sectional SEM micrographs of LCO (c), and graphite (d) electrodes, respectively. Topographical SEM micrographs of LCO (e), and graphite (f) electrodes, respectively.

Figure 3

(a) 1st, 10th and 100th charge-discharge curves of the battery cycled between 4.2 and 3.0 V at C/5 rate. (b) Charge-discharge capacity (mAh) and coulombic efficiency of the battery cycled for 100 cycles at C/5 rate. (c) Discharges curves of the battery discharged at C/10, C/5, C/2, 1C, 2C, 3C and 5C rate. (d) Discharge capacity (mAh) and capacity retention of the battery discharged at C/10, C/5, C/2, 1C, 2C, 3C and 5C rate.

Figure 4

(a) Charge-discharge capacity of the battery electrochemically cycled at C/5 rate under flat state and after flexing 200 times to 3, 2 and 1 inch bending radius. (b) Charge-discharge curves of the battery under flat state and after bending for 600 cycles. (c) ElS curves of the battery under flat state and after bending for 600 times.

Figure 5. Electrical characteristics of flexible PV module charging the flexible battery.

(a) Current-voltage characteristics of the PV module and (b) battery voltage over time as it is charged by the PV module, under different illumination conditions. (c) Battery voltage profiles over 10 charge/discharge cycles. Charging is performed using the PV module under 100 mW/cm² irradiance and discharging is performed at a constant current of 20 mA. (d) Time for the battery to charge to 4.2 V and discharge to 3.6 V over the same 10 cycles as in (c).

Figure 6. Behavior of PV module, battery and load connected together.

(a) Circuit schematic of PV module, battery and load, indicating currents flowing in each component. (b) Discharge curves of the battery with two PV module irradiance conditions. The load alternates between 20 mA for 30 seconds and 1 mA for 90 seconds. (c) Closeup of the battery voltage and load current waveforms shown in (b) over a few load cycles. (d) Battery voltage and load current waveforms with the load alternating between 20 mA for 30 seconds and 1 mA for 220 seconds. The dotted black lines are a guide to the eye indicating that the battery voltage at the end of each cycle does not change.

Figure 7

(a) Diagram of the system for obtaining photoplethysmogram (PPG) signals from pulse oximeter. (b) Charge-discharge characteristic of the battery, highlighting three voltages (*a, *b, *c) at which the battery was used to power the pulse oximeter. (c) PPG signals obtained at the three battery voltages.