Carbon Fiber-Reinforced Redox-Active Polymer Composite as a Multifunctional Cathode for Structural Batteries

Electrifying transportation is essential for moving towards a more sustainable future. Currently, lithium-ion batteries (LIBs) are widely adopted for the electrification of transportation, such as electric vehicles (EVs) and electric aircraft, due to their high energy density, cycle stability, and low self-discharge rate compared to other energy storages. However, the gravimetric energy density of conventional LIB packs is still lower than the energy efficiency of internal combustion engines, inevitably increasing the total weight of these vehicles and thereby limiting their driving ranges with the need for frequent recharges.<br/> To address this issue, the concept of structural batteries (SBs) has recently garnered considerable attention for their ability to achieve both mass and volume savings at the system level. SBs are multifunctional components that can simultaneously store electrical energy and carry mechanical load. By integrating an energy storage function into a structural composite, SBs can substantially reduce both the mass and volume of the system.<br/> Typically, SBs can be classified into two categories depending on their integration strategies: coupled and decoupled types. The decoupled types correspond to simple combinations of load-bearing structures with energy storage components, where conventional batteries are embedded between structural face sheets. Therefore, the efficiency of mass and volume savings achieved by the decoupled SBs is inconsiderable. In contrast, in the coupled SBs, each material component is designed to simultaneously perform mechanical and energy storage functions. Although the coupled strategy is more ideal for achieving both mass and volume savings, realizing the coupled SBs, which exhibit electrochemical and mechanical performance comparable to commercial LIB packs and structural composites, respectively, is still very challenging.<br/> To prepare coupled SBs, typically, conventional electrode slurries containing transition metal oxide, carbon conducting additives, and PVDF binder, are deposited on the carbon fiber (CF) current collectors, inspired by carbon fiber-reinforced polymer (CFRP) composites. However, such conventional electrode materials lack the ability to bind the CFs, leading to easy delamination even under weak mechanical stresses.<br/> Herein, we present a novel concept of electrode design for coupled SBs: a carbon fiber-reinforced redox-active polymer (CFRRAP), consisting of CFs bound by a polymer matrix exhibiting redox activity. To demonstrate the potential of this concept, we prepared a series of CFRRAP composite cathodes comprised of T300 CFs and a polyimide (PI) resin with different ratios of Super-P carbon additives via a hand lay-up process followed by hot compression molding. These composite cathodes can carry high mechanical load, which is attributed to the thermoplastic PI resin tightly binding the CFs as a matrix, comparable to conventional CFRP-based structural composites. Simultaneously, the carbonyl groups in the PI matrix can store electrical energy via reversible redox reactions accompanying the insertion/de-insertion of Li-ions. We explored the structure-property relationships between the electrochemical/mechanical properties and Super-P content in the composite cathodes, revealing a trade-off between the electrochemical performance and the electrochemical properties of the composites. At an optimized content, the PI-based CFRRAP cathode exhibits an elastic modulus of 18 GPa and a tensile strength of 213 MPa while delivering a specific capacity of 141 mAh/g with an average discharge voltage of 2.2 V. It is noteworthy that its mechanical and electrochemical properties outperform most coupled/decoupled structural cathodes reported so far. We investigated their charge/discharge mechanism and mechanical failure modes through various analysis methods. It should also be noted that the composite electrodes demonstrate flame retardation properties.