Lithium Iron Phosphate Cell Open Circuit Voltage Equilibrium Mapping
Lithium iron phosphate open circuit voltage equilibrium maps two-phase hysteresis using multi-hour relaxation testing for state of charge estimation
dV/dQ Peaks
Meaning
Electrochemical diagnostic indicators obtained through the differentiation of cell voltage with respect to discharged capacity provide a means to track degradation modes in lithium-ion batteries. Analysis of these features, commonly called dv/dq peaks, allows engineers to identify phase transitions in the electrode materials without destroying the cell. Individual peaks correspond to thermodynamic phase equilibria where the voltage remains relatively constant during lithium insertion or extraction.
Physical Origin
Thermodynamic phase co-existence regions produce voltage plateaus during slow charging or discharging. Differentiating the voltage profile with respect to capacity transforms these plateaus into distinct dv/dq peaks. The spacing between these peaks corresponds to the capacity of the active material participating in that specific phase regime.
Degradation Tracking
Shifted positions or reduced heights in the voltage derivative signals during cycling reveal the loss of active lithium or the degradation of active material. When active material degrades, the corresponding dv/dq peaks contract or move closer together in terms of capacity. This signal change helps researchers separate loss of active material from loss of lithium inventory.
Because each peak corresponds to a specific insertion state, tracking their evolution under different temperatures allows developers to isolate degradation mechanisms without dismantling the pack.
Measurement Requirement
High-precision coulometry is required to gather voltage data with low noise during slow galvanostatic cycles. Because noise in the voltage measurement is amplified during differentiation, digital smoothing algorithms are applied to the raw voltage curves before identifying the dv/dq peaks. Testing at low current densities ensures that kinetic polarization does not obscure the thermodynamic peaks.