Meaning
Internal mechanical stress within a battery electrode material causes the physical fracture of individual active particles during charge and discharge cycles. This particle cracking disrupts the continuous electrical contact between the particle and the conductive matrix of the electrode. The phenomenon forces a loss of electrochemically active surface area while also exposing fresh crystal surfaces to the electrolyte.
Constant exposure of these new surfaces accelerates the formation of an unstable solid electrolyte interphase layer. This cycle of fracture and parasitic reaction reduces the reversible capacity of the cell over its operational life.
Structural Impact
Mechanical expansion and contraction during lithium ion insertion lead to anisotropic strain within the polycrystalline lattice structure. Particle cracking follows as these internal stresses exceed the fracture toughness of the material grains. The resulting separation of grains prevents the efficient migration of charge carriers throughout the entire electrode volume.
Isolated particles become electrically inactive and contribute nothing to the measured storage capacity of the device. High discharge rates aggravate this internal degradation by increasing the magnitude of the concentration gradients within the individual particles.
Failure Mechanism
Chemical degradation at the particle boundary weakens the grain boundaries until the material succumbs to the pressure of lithiation. The process begins with microcracks forming at the interface where primary particles meet inside the larger secondary agglomerate. Electrolyte penetration into these fissures triggers secondary reactions that increase the impedance of the electrode interface.
Internal resistance rises as the structural integrity of the composite material vanishes under the strain of continuous cycling.
Performance Consequence
Permanent capacity fade happens when the total number of fractured particles reaches a threshold that limits the ion transport path. These broken connections create high resistance zones that decrease the power delivery capabilities of the battery. Thermal stability drops as the high surface area of the fragmented particles facilitates exothermic reactions within the electrolyte.
Every fracture represents a point of no return for the electrochemical potential of the cell.
