The emergence of roll-up smartphones, flexible smart watches and portable medical de-vices leads to an urgent demand for flexible energy storage devices. Rechargeable Li ion bat-teries has been the top leader in the battery industry for almost 30 years due to its high cyclic stability, high gravimetric power density and energy density, allowing compact integration to flexible devices. However, its power density is inadequate for very large current applications such as start-up of electric vehicles. Another common power source is supercapacitors, which has an advantage of longer cycle life, high safety and much higher power density than batteries, but it suffers from low energy density. Both battery and supercapacitor cannot be singly used for applications that require both high power an...[ Read more ]

The emergence of roll-up smartphones, flexible smart watches and portable medical de-vices leads to an urgent demand for flexible energy storage devices. Rechargeable Li ion bat-teries has been the top leader in the battery industry for almost 30 years due to its high cyclic stability, high gravimetric power density and energy density, allowing compact integration to flexible devices. However, its power density is inadequate for very large current applications such as start-up of electric vehicles. Another common power source is supercapacitors, which has an advantage of longer cycle life, high safety and much higher power density than batteries, but it suffers from low energy density. Both battery and supercapacitor cannot be singly used for applications that require both high power and high energy, such as regenerative braking system in trains and electric vehicles. In order to satisfy both demands, lithium ion capacitors (LIC) are fabricated by utilizing battery materials on one electrode and capacitive materials on the other, achieving supercapacitor-type power density and battery-type energy density.

Another limiting factor for achieving flexibility is on the use of organic liquid electrolyte in electrochemical devices. Organic electrolyte is notorious of its high flammability and low flash point, contributing to strict packaging requirements, bulky devices, non-flexibility and limited geometry. Solid polymer electrolyte (SPE) is one alternative to satisfy the flexibility requirements, but its low room-temperature ionic conductivity and poor interfacial contact with electrodes limits its applications for powering up the wearable electronic devices.

To overcome the above bottlenecks, this work first develops an ionic liquid solid polymer electrolyte (IL-SPE) based LIC. The poly(vinylidene fluoride-co-hexafluoropropylene) methylpyrrolidinium bis-trifluoromethanesulfonimide (Pyr₁₄TFSI) IL-SPE is developed by solution casting method. Then, the IL-SPE is sandwiched between the Li₄Ti₅O₁₂/activated carbon anode and LiFePO₄/activated carbon cathode. This novel design of LIC is characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD). 20 mAh/g capacity can still be obtained at a charge/discharge rate of 2.0 A/g, good rate capability and 82.0% capacity retention with high Coulombic efficiency (>99%) after 1200 cycles have been achieved.

This work further applies the IL-SPE based LIC into a pouch cell with Miura-ori pattern to achieve stretching capability, even all the materials itself are not stretchable. The electro-chemical performances of the as-prepared pouch cell, after folding and after 500 times bending are compared using GCD test. To improve the interfacial contact, IL-based electrolyte interlayer is added at the interfaces, and the result is compared against the one without it. Interestingly, the interlayer enables higher specific capacity of the as-prepared pouch cell, but results in much lower capacity retention after folding and bending 500 times, possibly due to better electrode wetting, but more unwanted side reactions within the system. On the contrary, ~80% of initial capacity can be retained after 500 times bending, and overall structural stability can be achieved from the one without the interlayer. This shows promising applications of origami patterns in flexible and stretchable energy storage devices.