Structural Composite Lithium-Ion Battery : Effect of Intercalation induced volumetric changes on micro-damage

Sammanfattning: The quest for lighter materials and structures to reduce climate impacts in the automotive industry has paved the way for multifunctional solutions. Mass saving on a system level can be achieved by materials or structures having more than one primary function, thus reducing the number of components used. Structural batteries are composite materials that simultaneously carry mechanical loads while delivering electrical energy. While carbon fiber is a commonly used reinforcing material in high-performance composite materials, it also possesses excellent lithium intercalation properties.Therefore,it is possible to use carbon fiber to develop structural batteries based onlithium-ion battery technology.Among several proposed solutions,the micro-battery employs the carbon fiber asa negative electrodeof the battery and also as a composite reinforcement material. The fiber is coated with a solid polymer electrolyte which works as an ion conductor and separator whilst transferring mechanical loads. The coated fiber is surrounded by additional matrix material acting asapositive electrode, composed of conductive additives, active electrode material and electrolyte. This assembly of materials allows thenecessary electrochemical processes to occur simultaneously, including electrochemical reactions at the surface of the active electrode material, mass transport within active electrode material by diffusion, mass transport in electrolyte by diffusion and migration, and electronic conduction.During electrochemical cycling the electrodes undergo volume changes as a result of lithium transport. The work in this thesis addresses the effects of volume changes on internal mechanical stress state in the structural battery. A physics-based mathematical model employing a number of coupled nonlinear differential equations has been set-up and solved numerically to investigate performance in the structural battery material. The resulting transient lithium ionconcentration distributions were used in combination with linear elastic analysis in order to assess the mechanical stresses in the fiber, coating and matrix caused by non-uniform swelling and shrinking of the micro-battery. Stress analysis shows that high hoop stress in the matrix during charging may initiate radial matrix cracks at the coating/matrix interface. Linear elastic fracture mechanics hasbeen used to analyze radial matrix crack propagation and debonding at coating/matrix interface in both unidirectional (UD) and cross-ply laminate, under electrochemical load only and combined electrochemical and thermomechanical load. Results show that for cross-ply structural battery composite the sequence of macro-scale crack forming events differs from a conventional cross-plied composite, as well as froma UD composite battery laminate. The most likely course of failure events in a cross-ply laminate were found to be: 1) radial matrix crack initiation and unstable growth; 2)matrix/coatingdebond when the matrix crack hasacertain length.