Aging-induced impedance rise in batteries represents a critical failure mechanism that progressively degrades performance and can lead to catastrophic failure. The phenomenon manifests through several interconnected processes, including solid electrolyte interphase (SEI) growth, active material contact loss, and electrolyte depletion. These mechanisms collectively increase internal resistance, alter thermal behavior, and accelerate performance collapse. Understanding these processes requires analysis of their electrochemical signatures, particularly through differential voltage and current methods that reveal state-of-health degradation.

SEI growth constitutes a primary contributor to impedance rise, particularly in lithium-ion batteries. The SEI layer forms during initial cycles as electrolyte reduction products deposit on anode surfaces, primarily graphite or silicon. While the SEI passivates the electrode and prevents further electrolyte decomposition, it continues growing slowly throughout the battery's life. This growth consumes lithium ions and organic electrolytes, gradually increasing charge transfer resistance. SEI thickening shows distinct electrochemical signatures in differential voltage analysis, where the peaks corresponding to anode staging transitions broaden and diminish in amplitude. The process follows parabolic kinetics, with growth rate dependent on temperature and cycling conditions. Elevated temperatures above 45°C dramatically accelerate SEI growth through increased solubility of SEI components and enhanced reaction kinetics.

Contact loss between active materials and current collectors represents another impedance-increasing mechanism. Repeated volume changes during cycling, particularly in high-capacity anodes like silicon or nickel-rich cathodes, cause particle cracking and delamination. This mechanical degradation creates disconnected material islands that no longer participate in electrochemical reactions, effectively reducing active electrode area. Contact loss manifests in impedance spectroscopy as an increase in the medium-frequency semicircle associated with charge transfer resistance. Differential current analysis reveals this through decreased capacity utilization at fixed voltage windows, particularly during high-rate discharges where ohmic losses dominate. The phenomenon becomes severe after thousands of cycles or under high mechanical stress conditions, with some nickel-manganese-cobalt (NMC) cathodes showing 20-30% active material isolation after 2000 deep discharge cycles.

Electrolyte depletion compounds impedance rise through multiple pathways. Lithium inventory loss occurs through SEI growth and lithium plating, while solvent molecules decompose through oxidation at high-voltage cathodes. The resulting depletion increases electrolyte viscosity and reduces ionic conductivity, particularly at low temperatures where ion mobility already suffers. Fluoride-based salts like LiPF6 further degrade into hydrofluoric acid, which corrodes electrode materials and generates resistive byproducts. Electrolyte depletion shows in differential voltage curves as capacity shifts between electrodes and increased polarization at all state-of-charge levels. Advanced electrolytes with additives like vinylene carbonate or lithium difluorophosphate can mitigate but not eliminate these effects.

Impedance rise directly impacts heat generation through Joule heating effects. The relationship follows the fundamental I²R law, where internal resistance increases cause disproportionate heat generation during high-current operations. A battery with doubled impedance will generate four times the heat at the same current, creating positive feedback loops. Localized heating accelerates SEI growth and electrolyte decomposition, further increasing impedance. This thermal-electrochemical coupling explains the sudden performance collapses observed in aged batteries subjected to high loads. Thermal modeling shows that impedance increases above 150% of initial values frequently lead to thermal runaway conditions during fast charging or high-power discharges.

Differential voltage analysis provides critical insights into aging mechanisms by examining the incremental voltage change per unit capacity (dV/dQ). Healthy batteries show sharp peaks corresponding to phase transitions in electrode materials, while aged cells exhibit flattened and shifted features. SEI growth primarily affects the anode peaks, causing broadening and reduced amplitude. Contact loss manifests as decreased peak area proportional to active material loss. Electrolyte depletion shifts the entire curve due to increased polarization. Combining differential voltage with coulombic efficiency measurements allows precise attribution of capacity fade to specific degradation modes.

Differential current analysis complements this approach by examining current response to small voltage perturbations. The technique reveals kinetic limitations through changes in the current-voltage relationship. Aged batteries show decreased current response across all voltages, with particularly severe reductions at the extremes of the state-of-charge range where impedance effects magnify. The slope of the differential current plot directly correlates with internal resistance, providing a real-time assessment of impedance rise without requiring full electrochemical impedance spectroscopy.

Performance collapse occurs when impedance reaches critical thresholds that prevent normal operation. Electric vehicle batteries typically reach end-of-life at 20-30% capacity fade, but impedance-driven failures often occur earlier in high-power applications. The collapse follows a nonlinear trajectory where small impedance increases initially cause minor performance degradation before reaching a tipping point. This explains why calendar-aged batteries with seemingly acceptable capacity can fail catastrophically when subjected to high loads—their impedance has crossed critical thresholds while capacity appears only moderately affected.

Mitigation strategies focus on interrupting the feedback loops between impedance rise and secondary degradation mechanisms. Anode coatings can stabilize SEI layers, while elastic binders maintain particle contact during cycling. Electrolyte formulations with high oxidative stability and lithium reservoir additives address depletion issues. Battery management systems must incorporate impedance-based health indicators alongside capacity measurements to predict performance collapse accurately. Advanced thermal management becomes increasingly critical as batteries age to counteract rising heat generation.

The progression from initial impedance rise to final failure follows predictable electrochemical pathways that modern analysis methods can detect early. By quantifying SEI growth, contact loss, and electrolyte depletion through differential analysis techniques, engineers can develop targeted interventions to extend battery life and prevent catastrophic failures. These approaches move beyond simple cycle life testing to address the fundamental electrochemistry driving aging-induced impedance increases.