Flexible energy storage devices based on nanocomposite paper

Abstract

There is strong recent interest in ultrathin, flexible, safe energy storage devices to meet the various design and power needs of modern gadgets. To build such fully flexible and robust electrochemical devices, multiple components with specific electrochemical and interfacial properties need to be integrated into single units. Here we show that these basic components, the electrode, separator, and electrolyte, can all be integrated into single contiguous nanocomposite units that can serve as building blocks for a variety of thin mechanically flexible energy storage devices. Nanoporous cellulose paper embedded with aligned carbon nanotube electrode and electrolyte constitutes the basic unit. The units are used to build various flexible supercapacitor, battery, hybrid, and dual-storage battery-in-supercapacitor devices. The thin freestanding nanocomposite paper devices offer complete mechanical flexibility during operation. The supercapacitors operate with electrolytes including aqueous solvents, room temperature ionic liquids, and bioelectrolytes and over record temperature ranges. These easy-to-assemble integrated nanocomposite energy-storage systems could provide unprecedented design ingenuity for a variety of devices operating over a wide range of temperature and environmental conditions.

Fig. 1.

Fabrication of the nanocomposite paper units for supercapacitor and battery. (a) Schematic of the supercapacitor and battery assembled by using nanocomposite film units. The nanocomposite unit comprises RTIL ([bmIm][Cl]) and MWNT embedded inside cellulose paper. A thin extra layer of cellulose covers the top of the MWNT array. Ti/Au thin film deposited on the exposed MWNT acts as a current collector. In the battery, a thin Li electrode film is added onto the nanocomposite. (b) Photographs of the nanocomposite units demonstrating mechanical flexibility. Flat sheet (top), partially rolled (middle), and completely rolled up inside a capillary (bottom) are shown. (c) Cross-sectional SEM image of the nanocomposite paper showing MWNT protruding from the cellulose–RTIL thin films. (Scale bar, 2 μm.) The schematic displays the partial exposure of MWNT.

Fig. 2.

Electrochemical double-layer capacitance measurements of nanocomposite paper supercapacitor. (a) Cyclic voltammograms (CV) and (b) charge–discharge behavior of the nanocomposite supercapacitor devices with KOH and RTIL electrolytes. The CV measurements are carried out at a scan rate of 50 mV/s at room temperature. The near-rectangular shape of the CV curves indicates good capacitive characteristics for the device. Charge–discharge behavior was measured at a constant current of 1 mA. (c) Power density of the supercapacitor device (using the RTIL electrolyte) as a function of operating temperature. The supercapacitor operates over a record range of temperatures (195–450 K). (d) CV of the supercapacitor by using bioelectrolytes [body fluids (blood, sweat)] measured at a scan rate of 50 mV/s at room temperature.

Fig. 3.

Electrochemical measurements of nanocomposite paper battery. (a) First charge–discharge curves of the nanocomposite thin-film battery cycled between 3.6 and 0.1 V at a constant current of 10 mA/g. (b) Charge capacity vs. number of cycles of the nanocomposite thin-film battery. (c) The flexible nanocomposite film battery used to glow a red light-emitting diode (LED). The flexible battery consists of an individual nanocomposite thin film with the Li thin film present on one side as one of the electrodes. The LED glows even when the battery device is rolled up, and the demonstration could be repeated over several tens of cycles at an initial operating voltage of 2.1 V.

Fig. 4.

Supercapacitor-battery hybrid energy devices based on nanocomposite units. (a) Schematic of a four-terminal hybrid-energy device showing the arrangement of supercapacitor and battery in parallel configuration. (b) The discharge curve of battery and supercapacitor is plotted as a function of time. The discharge of battery charges the supercapacitor, and subsequently the supercapacitor is discharged. (c) Schematic of a three-terminal hybrid energy device that can act as both supercapacitor and battery. The three terminals are defined, and the battery and supercapacitor segments of the device are shown. (d) The discharge behavior of the battery and subsequent discharge of supercapacitor are shown. The battery is discharged with terminals 1 and 2 shorted. This simultaneously charges the supercapacitor following the double-layer formation at the electrode interface. Subsequently, the supercapacitor is discharged across terminals 1 and 3. An additional separator (glass fibers) is normally added along with the excess cellulose spacer to improve behavior.