Meaning
Interactions occur between the mechanical stress state and the chemical diffusion of mobile ions within battery electrodes. The phenomenon of stress coupling describes how localized mechanical forces alter the chemical potential and transport rate of lithium or sodium ions. It dictates the ionic concentration distribution in high-capacity anodes that undergo large volume changes during cycling.
The coupling applies to all solid-state and liquid-electrolyte cells.
Physical Mechanism
Mechanical tension reduces the energy barrier for ion insertion, whereas compression increases it and drives ions away. Through stress coupling, the gradient of hydrostatic stress acts as an additional driving force for diffusion, causing ions to accumulate in tension zones. This stress-induced diffusion can either stabilize or destabilize the electrode structure depending on the particle geometry.
The interaction is modeled using coupled electrochemical-mechanical equations.
Degradation Consequence
The presence of highly localized stresses accelerates the formation of microcracks and the detachment of active material particles. In stress coupling, these mechanical effects can lead to uneven current distributions and localized overcharging. This localized stress promotes the growth of lithium dendrites and reduces the safe operating envelope of the cell.
The resulting damage reduces the capacity of the cell.
Sourcing Evaluation
Cell designers utilize mechanical and electrochemical simulation tools to optimize electrode and pack geometries to minimize stress gradients. Sourcing materials that minimize stress coupling effects improves the long-term reliability of the cell. This choice minimizes the risk of early pack failure.
