Thermally-induced degradation in battery systems is a critical factor affecting performance, safety, and longevity. Elevated temperatures accelerate multiple aging mechanisms across battery components, including electrodes, electrolytes, and separators. Understanding these processes is essential for designing effective thermal management systems and optimizing operating conditions.

Electrode degradation at high temperatures primarily involves solid electrolyte interphase (SEI) growth and active material particle cracking. The SEI layer forms on anode surfaces during initial cycles and ideally stabilizes, but elevated temperatures accelerate its continued growth. This consumes lithium inventory, increases impedance, and reduces capacity. For graphite anodes, SEI growth follows an Arrhenius relationship, with activation energies typically ranging between 50-70 kJ/mol. Lithium-ion diffusion through the SEI becomes faster at higher temperatures, exacerbating decomposition reactions. In cathodes, such as NMC or LFP, thermal stress induces microcracks due to repeated lattice expansion and contraction during cycling. These cracks expose fresh surfaces to electrolyte reactions, further accelerating degradation.

Particle cracking is particularly severe in high-nickel cathodes (e.g., NMC811), where anisotropic strain leads to grain boundary separation. At temperatures above 60°C, transition metal dissolution increases, migrating to the anode and catalyzing further SEI growth. Silicon anodes suffer from accelerated volume expansion-induced fracture at elevated temperatures, leading to rapid capacity fade.

Electrolyte decomposition is another major thermal degradation pathway. Organic carbonate-based electrolytes undergo thermal breakdown, producing gaseous byproducts such as CO₂, CH₄, and C₂H₄. At temperatures exceeding 70°C, LiPF₆ salt decomposes into PF₅, which reacts with solvents to form HF and other acidic species. These products corrode electrode materials and degrade cell performance. The onset of significant electrolyte decomposition occurs around 80-90°C for conventional LiPF₆-based systems, though advanced additives can raise this threshold.

Gas generation not only increases internal pressure but also depletes the electrolyte, leading to dry-out and increased impedance. In extreme cases, gas accumulation can cause cell swelling or venting. The rate of gas evolution follows exponential temperature dependence, with activation energies typically in the range of 80-100 kJ/mol for common electrolyte formulations.

Separators face thermal degradation through shrinkage and pore closure. Polyolefin separators (PE/PP) begin softening around 90-120°C, leading to loss of mechanical integrity. At higher temperatures, melting causes pore collapse, increasing the risk of internal short circuits. Ceramic-coated separators improve thermal stability but may still suffer from binder degradation. Separator shrinkage above 150°C can trigger catastrophic failure by enabling electrode contact.

Accelerated aging tests are used to study these mechanisms under controlled conditions. Common protocols involve storing or cycling cells at elevated temperatures (e.g., 45°C, 60°C) while monitoring capacity fade and impedance rise. The Arrhenius equation is applied to extrapolate aging rates to lower temperatures:

k = A * exp(-Ea/RT)

where k is the degradation rate, A is the pre-exponential factor, Ea is activation energy, R is the gas constant, and T is temperature in Kelvin. By measuring degradation at multiple temperatures, the activation energy for a specific failure mode can be determined. For example, SEI growth typically exhibits Ea values of 50-70 kJ/mol, while electrolyte decomposition shows higher values (80-100 kJ/mol).

Thermal modeling combines these kinetic parameters with heat generation profiles to predict lifetime under real-world conditions. Multiscale simulations account for localized heating effects, such as hot spots near current collectors or uneven cooling. Abuse conditions, including overcharge or external heating, are modeled to evaluate safety margins.

The implications for thermal management system design are significant. Effective systems must maintain cells within an optimal temperature window, typically 15-35°C for lithium-ion batteries. Active cooling (liquid, air, or phase-change materials) is necessary for high-power applications, while passive methods (thermal pads, heat sinks) may suffice for low-rate scenarios. Temperature uniformity is critical—gradients exceeding 5°C across a pack can induce uneven aging.

Operating temperature windows are determined by balancing performance and degradation. Low temperatures reduce reaction rates but increase lithium plating risk during charging. High temperatures improve kinetics but accelerate degradation. The tradeoff is quantified using Arrhenius-based lifetime projections. For example, operating at 40°C instead of 25°C may double the degradation rate, cutting cycle life by half.

Advanced thermal management strategies include predictive control algorithms that adjust cooling based on real-time degradation estimates. Materials innovations, such as thermally stable electrolytes or self-healing electrodes, can extend the allowable temperature range. However, all designs must account for the fundamental thermal degradation mechanisms to ensure safe and durable operation.

In summary, thermally-induced degradation in batteries is a complex interplay of chemical, mechanical, and electrochemical processes. Electrode aging, electrolyte decomposition, and separator breakdown each follow temperature-dependent kinetics that can be modeled using accelerated testing and Arrhenius relationships. These insights directly inform thermal management system design, ensuring optimal performance and longevity across diverse applications.