On Nonlinear Modelling of Structural Battery Composites

Licentiate thesis, 2024

Structural battery composites are multifunctional materials with the ability
to simultaneously store electrochemical energy and carry mechanical load. A
conventional lithium-ion battery comprises a positive electrode, a separator,
and a negative electrode. The constituents are soaked in a liquid electrolyte,
allowing lithium-ion exchange between the electrodes while electrons travel
through an external circuit via current collectors. In structural batteries,
the negative electrode is replaced by multifunctional carbon fibres, enabling
the additional function as mechanical reinforcement. The liquid electrolyte
is replaced by a porous two-phase matrix material. The solid polymer phase
transfers mechanical loads between the carbon fibres, and the lithium ions
travel in the liquid phase, occupying the pores in the polymer matrix. Redox
reactions occur on the electrode/electrolyte interfaces. For instance, reduction
of lithium ions forms neutral lithium that diffuses inside the carbon fibres
in a charge process of a structural battery full cell. Lithium insertion is accompanied
by extensive carbon fibre expansion, which causes internal stresses
and affects the elastic moduli of the carbon fibres. In contrast, applying a
mechanical load to the lithiated carbon induces a response in the electric
potential.
In the first paper, we study the effect of the carbon fibre expansion on the
surrounding matrix material in a structural negative electrode. In particular,
we investigate the significance of adopting non-linear kinematics. In this way,
features such as the internal stress state, homogenized tangent stiffness, and
expansion of the homogenized negative electrode can be predicted.
The second paper concerns multiphysics modelling of a structural battery
full cell, that is, a LiFePO4 (lithium-iron-phosphate) based positive electrode
against a carbon fibre negative electrode. Specifically, we introduce non-linear
reaction kinetics to describe the redox reactions occurring on the electrode /
electrolyte interfaces. Moreover, we discuss coupled electro-chemo-mechanical
modelling of the positive electrode and the pertinent calibration towards experimental
findings.