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As a promising lightweight multifunctional material, carbon fiber structural battery composites have great potentials to enable longer service life and operating distance for the rapidly increasing mobile electric technologies. While simultaneously carrying mechanical loads and storing electrical energy, the developed multifunctional composites can achieve “massless” energy storage and further extend to sensing and energy harvesting for self-powered structural health monitoring. However, it is still very challenging to predict the state-of-health of structural battery composites due to a lack of understanding of underlying coupled mechanical-electrochemical phenomena during operation. In this study, we first use a novel 3D printing method to fabricate and tailor microstructure of the multifunctional carbon fiber composites. With an optimal electrode layer thickness of 0.4 mm, the stable specific capacity at 1C reaches over 80% of the theoretical capacity of the electrode active materials (lithium iron phosphate) with an average energy density of 152 Wh/kg observed. The corresponding flexural modulus and flexural strength are 8.7 GPa and 69.6 MPa, respectively. The state-of-health of 3D printed structural battery composites under electrochemical cycling and external mechanical loadings are also investigated. The mechanical performance is not affected by the electrochemical charge-discharge processes. The structural battery composites under three-point bending testing show good capacity retention with rapid degradation of electrochemical performance observed near fracture point. The findings from this study not only provide insights for monitoring the state-of-health of structural battery but also show mechanical-electrochemical coupling as a potential way of self-powered structural health monitoring through the 3D printed multifunctional composites. 

Abstract Pellet-based extrusion deposition of carbon fiber-reinforced composites at high material deposition rates has recently gained much attention due to its applications in large-scale additive manufacturing. The mechanical and physical properties of large-volume components largely depend on their reinforcing fiber length. However, very few studies have been done thus far to have a direct comparison of additively fabricated composites reinforced with different carbon fiber lengths. In this study, a new additive manufacturing (AM) approach to fabricate long fiber-reinforced polymer (LFRP) was first proposed. A pellet-based extrusion deposition method was implemented, which directly used thermoplastic pellets and continuous fiber tows as feedstock materials. Discontinuous long carbon fibers, with an average fiber length of 20.1 mm, were successfully incorporated into printed LFRP samples. The printed LFRP samples were compared with short fiber-reinforced polymer (SFRP) and continuous fiber-reinforced polymer (CFRP) counterparts through mechanical tests and microstructural analyses. The carbon fiber dispersion, distribution of carbon fiber length and orientation, and fiber wetting were studied. As expected, a steady increase in flexural strength was observed with increasing fiber length. The carbon fibers were highly oriented along the printing direction. A more uniformly distributed discontinuous fiber reinforcement was found within printed SFRP and LFRP samples. Due to decreased fiber impregnation time and lowered impregnation rate, the printed CFRP samples showed a lower degree of impregnation and worse fiber wetting conditions. The feasibility of the proposed AM methods was further demonstrated by fabricating large-volume components with complex geometries.