Structural composite batteries (SCB) are an emerging class of energy storage systems that simultaneously store electrical energy and bear mechanical loads. These systems are multifunctional composites in which lithium-ion battery chemistry is integrated into a structural composite framework. A lithium iron phosphate (LFP)-based composite slurry, consisting of LFP active material, carbon black (CB), and a polyvinylidene fluoride (PVDF) binder, is slurry-cast onto carbon fibers (CF) to form a structurally integrated cathode. Energy storage in SCB occurs through redox reactions at the electrode-electrolyte interfaces. Efficient charge transfer is therefore strongly governed by the integrity and quality of these interfaces. During electrochemical operation, interfacial resistance tends to increase progressively, while externally applied mechanical loads introduce additional stresses at the interfaces, potentially leading to interfacial damage. Such degradation affects both electrochemical performance and mechanical load-bearing capability. So, maintaining interfacial stability is critical for ensuring the long-term multifunctional performance of structural composite batteries. In this work, we develop a coupled electro-chemo-mechanical finite element framework to investigate interfacial degradation in a laminated structural battery composite employing a slurry-based LFP-coated CF cathode. The model builds upon a half-cell electro-chemo-mechanical formulation and is extended to a representative volume element (RVE) comprising the LFP slurry-based CF cathode and a structural battery electrolyte (SBE). The governing equations of mechanical equilibrium, lithium-ion transport, and charge conservation are fully coupled and implemented within COMSOL Multiphysics. This framework provides new insights into the interaction between electrochemical cycling and interfacial mechanical degradation in multifunctional structural batteries. The results provide the importance of maintaining interfacial integrity for efficient multifunctional performance. The proposed approach provides a flexible and robust method for incorporating interfacial degradation phenomena in structural battery modeling.