Meaning
Mechanical failure occurs along the boundaries of individual crystalline grains within polycrystalline active material particles during electrode operation. This process of inter-granular fracture isolates regions of the active material from both electronic and ionic conduction networks. It contributes to the rapid capacity fade observed in high-energy density insertion cathodes.
The degradation is accelerated by high state-of-charge limits and rapid charging.
Stress Origin
Anisotropic lattice expansion and contraction generate severe localized shear stresses at the interfaces between differently oriented grains. During electrochemical cycling, inter-granular fracture is initiated when the cumulative stress exceeds the mechanical cohesion of the grain boundaries. This stress is particularly high in materials that undergo abrupt volume changes during phase transitions.
As the grain boundaries split open, they create new pathways that allow liquid electrolyte to penetrate deep into the particle core.
Performance Loss
The detachment of crystalline grains prevents them from contributing to the reversible capacity of the battery cell. Over time, inter-granular fracture results in a steady increase in the cell’s internal resistance as electrical contact is lost. This loss of active material reduces the maximum power output and increases heat generation during operation.
Mitigation Strategy
Battery material developers design single-crystal active materials to eliminate grain boundaries and prevent this mechanical failure mode. Sourcing single-crystal alternatives reduces the occurrence of inter-granular fracture. This adjustment extends the cycle life of the pack.
