Structural Lithium-ion battery: Multiphysics modeling of mechanical and electrochemical phenomena

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 on the lithium-ion battery technology. In the micro-battery, which is one of several design solutions, the carbon fiber is employed as a negative electrode of the battery and also as a composite reinforcement material. It is coated with a solid polymer electrolyte working as an ion conductor and separator whilst transferring mechanical loads. The coated fiber is surrounded by additional matrix material acting as cathode and transferring loads to the fibers, composed of conductive additives, active electrode material and electrolyte. This assembly of materials allows for the necessary 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 modeling of the effects of volume changes on internal mechanical stress state in the structural battery, potentially causing micro-damage formation in the material, which degrade both electrical and mechanical performance of the structural battery composite. In this work, 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 Li concentration distributions were used in combination with linear elastic stress 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.