Separation of Lithium Inventory Loss and Active Material Loss in Stationary Storage
Low-rate differential voltage analysis decouples lithium inventory depletion from active material loss, identifying capacity knee risks before failure occurs.
Biphasic Phase Coexistence
Meaning
Thermodynamic state where two distinct crystal structures of an electrode material occupy the same particle during ion insertion or extraction. Observation of biphasic phase coexistence is common in lithium iron phosphate cells during the transition between the lithiated and delithiated states. The voltage remains nearly constant as the interface between the two phases moves through the material.
Kinetic Behavior
Transport of ions across the boundary between the competing phases dictates the charging rate of the cell. In the presence of biphasic phase coexistence, the internal resistance stays relatively low until the majority of the material has transformed. This characteristic allows for the flat discharge curves that define high stability chemistries.
Material Stress
Mechanical strain develops at the junction where the lattice parameters of the two phases differ. Over time, the repeated movement of this interface can lead to microcracking and loss of electrical contact within the electrode. Managing this effect involves reducing particle sizes to minimize the distance ions must travel during the phase change.
The resulting structural integrity determines the end of life for the cell.
Monitoring Method
X-ray diffraction and operando spectroscopy reveal the shifting peaks associated with each crystal structure. These measurements help optimize the composition of the cathode to improve cycle life.