In recent years, continuous progress in electronic devices, especially in wearable devices, has attracted surging attention from the consumer market. Therefore, flexible energy storage was developed to fulfill the needs of new flexible devices with ultra-lightweight and small volume. The very recent products and concepts such as touch screens, roll-up displays, wearable sensors, and even implantable medical devices have shown great potential in flexible applications because of their extreme convenience. However, the development of corresponding power sources largely lags behind these emerging technologies of flexible devices. Lithium-ion batteries (LIBs), owing to high energy density and high operating voltage, have been serving as an ideal power source for flexible devices. Nevertheless, direct implementation of commercial LIBs leads to irreversible deformation of structural integrity, short-circuiting or even severe explosion hazard. Such dilemma originates from the poor flexibility of electrode and electrolyte. For electrode side, current electrode sheets used in LIBs are manufactured by holding active material particles and conductive agents by a small weight fraction of polymeric binders. Such fragile electrode structure could easily lose electrical contact under physical deformation, leading to disintegrated electrode sheets, drastic degradations of electrochemical performance, and even safety issue due to internal short-circuiting. For electrolyte side, LIBs employ nonaqueous liquid electrolyte with high ionic conductivity and excellent electrode wettability. However, the drawbacks of such electrolyte system are also evident: poor ion selectivity, flammability, and leakage issue while being deformed render unsuitability of liquid electrolyte for flexible device application.
To fabricate flexible LIBs, the current state-of-the-art research employs two design strategies involving electrode structure. One popular strategy is constructing scaffolding structure using carbonaceous materials to function as supportive matrix for active materials. Given carbon nanotubes (CNTs) as an example, the CNTs possess remarkable electrical conductivity and mechanical strength (elastic modulus: 1 TPa, tensile strength: 100 GPa), which contribute to conductive and flexible electrodes as the high-aspect ratio of CNTs can serve as threading materials. Another strategy is rational architecture design of active materials that are conventionally particulate. For example, vanadium pentoxide nanowires can be readily fabricated into free-standing and binder-free electrode membrane. Nevertheless, the most of strategies above still fall short of practicality due to reduced portion of active materials and consequently compromised energy density.
In comparison with the mobile liquid electrolyte, the emerging solid-state electrolytes could largely solve circumventing issues of ion selectivity, flammability and leakage. As one prevailing category, solid polymer electrolytes comprising polymers and lithium salts feature decent manufacturing flexibility. Meanwhile, their poor ionic conductivity (10−8 ~ 10−5S cm−1) could be ameliorated by gel polymer electrolytes with organic solvents (plasticizers) and/or inorganic solid fillers (e.g., SiO2). Nevertheless, the non-conductive fillers block ion-transport pathways while allow partial electrical conduction, limiting the interfacial engineering and compatibility with electrodes.
In this dissertation, we tackle the aforementioned critical issues of flexible batteries in two aspects. Firstly, we design and synthesize flexible electrode from prospective of material and architecture. A novel cathode constructed by entangling networks of V2O5, CNTs and polytetrafluoroethylene (PTFE) is design and fabricated. Notably, the resulting flexible battery simultaneously achieves excellent mechanical strength (800 MPa young’s module), superior cycle durability (86% retention after 1000 times bending) and intriguing capacity (300 mAh g-1 at 0.25C).
Furthermore, a Zr-based metal-organic framework (MOF) possessing open-metal sites (OMSs) was used as the microporous filler to facilitate cation (Li+) conduction in GPL. Compared with the state-of-the-art research, our work significantly enhanced tLi+ of GLP from 0.39 up to 0.66 while maintained 1.5 mS cm−1 ionic conductivity. Notably, a reduced thermal activation energy (from 113 to 76 meV) was observed, suggesting diffusion energy barriers was eased by selective promotion of Li+ conduction.
To conclude, flexible Li-ion batterie system research is still at early developing stage. Above work provides rational design and improvement of the current FLIBs system in rather facile and cost-effective way. The methodology we proposed are hoped to bring further innovation toward FLIBs field and be extended to numerous applications in the future.