Meaning
Time-resolved derivative voltage analysis tracked during open-circuit rest periods captures the redistribution and equilibration of lithium ions within porous electrode structures. The differential capacity relaxation method evaluates potential shifts after current interruption to detect phase transitions, internal concentration gradients and parasitic reactions without destroying the test cell. This non-destructive diagnostic ceases to provide distinct phase information when cell temperature fluctuates during the relaxation window or when background self-discharge currents exceed the electrochemical relaxation rate.
Equilibration Mechanism
Current interruption halts active electrochemical flux, leaving non-uniform lithium concentration profiles across active particles and through the electrode thickness. Solid-state diffusion drives lithium ions from oversaturated particle surfaces toward depleted particle cores, while inter-particle ionic currents equalize potentials across different electrode depths. The rate of differential capacity change over rest time quantifies solid-state diffusion coefficients and identifies slow phase equilibration processes in blended cathode materials.
Plating Detection
Stripping of reversibly plated metallic lithium back into graphite during rest generates an open-circuit voltage plateau that manifests as a distinct peak in differential relaxation curves. The area under the derivative relaxation peak correlates quantitatively with the mass of reversible metallic lithium on the anode surface. Automated diagnostic routines execute this test following fast-charge cycles to verify that lithium intercalation completes without persistent metallic accumulation.
Supply Qualification
Tier-one automotive battery procurement protocols specify relaxation-based screening at cell end-of-line testing to filter out units with internal micro-shorts or abnormal phase distributions. Measuring voltage derivatives during rest periods enables pack assemblers to group cells with matched kinetic properties, reducing balancing overhead in completed modules. Suppliers unable to pass relaxation criteria face lot rejections due to the high risk of rapid capacity degradation in field deployments.
