Lithium-ion batteries degrade over time through complex electrochemical mechanisms that reduce capacity and increase impedance. Understanding these processes is critical for improving battery design, extending lifespan, and enabling accurate predictive modeling. The primary degradation pathways include solid-electrolyte interphase (SEI) growth, lithium plating, cathode dissolution, and electrolyte decomposition. Each mechanism involves distinct chemical and physical processes that interact with cell components and operating conditions.

The solid-electrolyte interphase forms on the anode surface during initial cycles as electrolyte reduction products accumulate. This layer is essential for passivation but continues growing slowly throughout the battery’s life. SEI growth consumes active lithium ions, directly reducing capacity. The process is driven by electron leakage through the SEI, which facilitates further electrolyte reduction. Organic components like lithium ethylene dicarbonate dominate early SEI formation, while inorganic species such as LiF become more prevalent over time. Elevated temperatures accelerate SEI growth due to increased reaction kinetics, while high states of charge exacerbate electron leakage. SEI thickening also raises interfacial resistance, contributing to impedance growth. Models often approximate SEI growth using parabolic kinetics, assuming diffusion-limited transport of solvent molecules. However, this simplification neglects SEI heterogeneity and cracking, which can lead to localized rejuvenation and nonlinear degradation.

Lithium plating occurs when lithium ions are reduced to metallic lithium on the anode surface instead of intercalating into graphite. This side reaction is favored at high charging rates, low temperatures, or when anode potential drops below 0 V versus Li/Li+. Plated lithium may react irreversibly with the electrolyte, forming dead lithium that cannot participate in cycling. It may also grow dendritically, posing safety risks. Plating is particularly problematic in fast-charging applications or energy-dense cells with thin electrodes. Models often use Butler-Volmer kinetics coupled with anode potential estimation to predict plating onset. Experimental validation relies on voltage relaxation profiles or post-mortem analysis, though in-situ detection remains challenging. The interplay between plating and SEI growth complicates degradation, as plated lithium can catalyze further electrolyte breakdown.

Cathode degradation involves multiple mechanisms depending on material chemistry. Layered oxides like NMC suffer from transition metal dissolution, where ions like manganese migrate into the electrolyte and deposit on the anode. This process destabilizes the cathode structure and poisons the anode SEI. Dissolution is aggravated by high voltages, elevated temperatures, and acidic electrolyte impurities. Spinel and phosphate cathodes exhibit different degradation modes, such as phase transitions or oxygen loss. Cathode degradation often follows reaction-driven kinetics, with models incorporating surface reactions and bulk diffusion limitations. Impedance rise at the cathode stems from particle cracking, contact loss, or surface films like lithium carbonate. Experimental techniques like XRD and XPS help quantify structural changes, while impedance spectroscopy tracks interfacial evolution.

Electrolyte decomposition occurs at both electrodes but is more pronounced at high-voltage cathodes. Oxidation of carbonate solvents generates gaseous products like CO2 and solid deposits such as polycarbonates. Additives like vinylene carbonate mitigate decomposition but may deplete over time. Electrolyte degradation depletes conductive species, increasing ionic resistance. Models often treat decomposition as a side reaction competing with intercalation, with rates tied to electrode potential and temperature. The loss of electrolyte volume also affects wetting and transport properties, indirectly accelerating other degradation modes.

Mathematical modeling of these mechanisms requires coupling electrochemical principles with material-specific degradation kinetics. Common approaches include pseudo-two-dimensional models extended with aging equations or empirical correlations based on stress factors like cycle count and depth of discharge. Continuum models solve mass and charge conservation equations with additional source terms for side reactions. These models assume homogeneous conditions within electrodes, neglecting local variations in porosity or current density. More advanced frameworks incorporate particle-scale heterogeneities or mesoscale phenomena like crack propagation. All models face trade-offs between computational cost and predictive accuracy, especially for long-term aging across diverse operating profiles.

Experimental validation combines electrochemical testing with advanced characterization. Cycling tests under controlled conditions isolate specific degradation modes, while post-mortem analysis reveals morphological and compositional changes. For example, capacity fade curves can be deconvoluted into contributions from lithium inventory loss and active material degradation. Differential voltage analysis quantifies lithium plating by identifying characteristic voltage plateaus. Impedance spectroscopy tracks the evolution of individual cell resistances over time. These measurements inform model parameterization and reveal mechanisms not captured by simulations.

The implications for lifespan prediction are significant. Accurate models must account for coupled degradation pathways and their dependence on operational history. For instance, SEI growth increases impedance, which in turn promotes lithium plating during fast charging. Such interactions lead to nonlinear aging trajectories that simple cycle-counting methods cannot capture. Physics-based models enable extrapolation beyond tested conditions, supporting the development of more durable batteries and optimized usage protocols. However, uncertainties in material properties and environmental factors limit prediction accuracy, especially for multi-year deployments.

Degradation mechanisms also influence battery management strategies. Awareness of lithium plating risks can guide charging algorithm design, while SEI stability informs temperature management requirements. Cathode-specific degradation modes may dictate voltage window restrictions. Understanding these relationships allows for adaptive control systems that balance performance and longevity.

In summary, electrochemical degradation in lithium-ion batteries arises from interrelated chemical and physical processes at electrode-electrolyte interfaces. SEI growth, lithium plating, cathode dissolution, and electrolyte decomposition collectively drive capacity fade and impedance rise. Mathematical models capture these mechanisms with varying degrees of fidelity, supported by experimental characterization techniques. Predictive capabilities continue improving as models incorporate more mechanistic details and validation datasets expand. This knowledge underpins efforts to enhance battery durability and reliability across diverse applications.