Lithium-ion batteries experience complex degradation mechanisms when exposed to elevated temperatures, driven by chemical and physical processes that affect each cell component. The degradation pathways vary depending on the specific chemistry, but common trends emerge across most commercial systems. Understanding these mechanisms is critical for improving battery longevity and performance in high-temperature applications.

Electrolyte decomposition is one of the primary degradation pathways at elevated temperatures. Organic carbonate-based electrolytes, such as mixtures of ethylene carbonate (EC) and dimethyl carbonate (DMC), undergo thermal decomposition through several routes. At temperatures above 70°C, EC can decompose via ring-opening reactions, forming CO2 and oligomeric species. Linear carbonates like DMC decompose through transesterification, producing alcohols and esters. Lithium hexafluorophosphate (LiPF6) salt, commonly used in electrolytes, thermally decomposes into LiF and PF5, with the latter further reacting with trace water to form HF. This acidic environment accelerates the breakdown of other cell components. Additives like vinylene carbonate (VC) or fluoroethylene carbonate (FEC), while stabilizing the solid-electrolyte interphase (SEI) at room temperature, may decompose at elevated temperatures, losing their protective effects.

The solid-electrolyte interphase (SEI) layer on the anode undergoes significant changes at high temperatures. The SEI, primarily composed of lithium ethylene dicarbonate (LEDC), lithium alkyl carbonates, and LiF, begins to dissolve or decompose above 60°C. LEDC decomposes into Li2CO3 and organic compounds, reducing its passivation ability. As the SEI breaks down, fresh anode surfaces become exposed to the electrolyte, leading to further reduction reactions and SEI reformation. This continuous breakdown and repair consume active lithium, increasing impedance and reducing capacity. Graphite anodes experience additional degradation through exfoliation at high temperatures, particularly when the SEI is compromised. Silicon-containing anodes face accelerated volume expansion-related degradation, as higher temperatures reduce the mechanical integrity of the electrode structure.

Cathode materials exhibit distinct thermal degradation behaviors depending on their chemistry. Layered oxides like LiNi_xMn_yCo_zO2 (NMC) undergo several degradation processes. At elevated temperatures, the delithiated cathode becomes increasingly reactive, leading to oxygen release from the lattice structure. This oxygen loss is accompanied by phase transitions from layered to spinel or rock-salt structures, reducing lithium intercalation capacity. Transition metal dissolution, particularly manganese and nickel, accelerates with temperature, with dissolved ions migrating to the anode and catalyzing further SEI growth. Lithium iron phosphate (LFP) cathodes show better thermal stability due to strong P-O bonds, but still experience some capacity fade through iron dissolution at very high temperatures. Lithium cobalt oxide (LCO) cathodes undergo similar structural changes to NMC but at lower temperatures due to cobalt's catalytic activity.

Separator integrity is another critical factor in high-temperature degradation. Polyolefin separators, such as polyethylene (PE) and polypropylene (PP), begin to melt and shrink near their melting points (around 135°C for PE and 165°C for PP). Even below these melting temperatures, the separators undergo morphological changes that reduce mechanical strength and increase pore size, potentially leading to internal short circuits. Ceramic-coated separators show improved thermal stability but may still experience binder degradation at high temperatures.

Differential scanning calorimetry (DSC) provides quantitative data on these degradation processes by measuring heat flow during controlled temperature increases. Typical DSC curves for lithium-ion batteries show multiple exothermic peaks corresponding to different decomposition events. The first minor peak around 100-120°C often represents SEI decomposition, while larger peaks at higher temperatures correspond to electrolyte decomposition and cathode reactions. The onset temperature and peak intensity vary significantly with state of charge, as delithiated cathodes are more reactive. Fully charged NMC cathodes may show exothermic activity starting as low as 150°C, while LFP cathodes remain stable to much higher temperatures.

Accelerated rate calorimetry (ARC) data reveals the self-heating behavior of battery materials under adiabatic conditions. ARC measurements typically show multiple temperature ramps corresponding to different degradation mechanisms. The initial self-heating rate is often low, representing SEI decomposition, followed by more rapid heating as electrolyte decomposition begins. The temperature coefficient (Q10) for these reactions typically falls between 2 and 3, meaning the reaction rate doubles or triples for every 10°C increase in temperature. This Arrhenius-type behavior explains why elevated temperatures dramatically accelerate degradation.

Several side reactions contribute to the overall degradation process. Electrolyte oxidation at the cathode surface increases with temperature, producing CO2 and other gaseous products. These reactions are particularly severe at high states of charge where the cathode potential is highest. Reduction of electrolyte at the anode also increases, forming thicker and more resistive SEI layers. Transition metal dissolution follows an exponential relationship with temperature, with some studies showing dissolution rates increasing by an order of magnitude between 25°C and 60°C. The dissolved metals then migrate to the anode, where they catalyze further electrolyte reduction.

The cumulative effect of these degradation mechanisms is seen in capacity fade and impedance growth. Capacity loss occurs through multiple pathways: active lithium consumption in SEI repair, loss of active material through structural changes in electrodes, and electrical isolation of particles due to binder degradation. Impedance increases through several mechanisms: thickening of surface layers on both electrodes, increased charge transfer resistance due to degraded interfaces, and reduced ionic conductivity of the electrolyte as decomposition products accumulate.

Quantitative studies have established temperature-dependent degradation rates for various chemistries. NMC/graphite cells may lose 3-5% capacity per month when stored at 60°C, compared to less than 1% per month at 25°C. LFP cells show better stability, with capacity losses of 1-2% per month at 60°C. Cycling at elevated temperatures accelerates degradation further, with some chemistries showing capacity fade rates up to 0.5% per cycle at 45°C.

Material modifications can mitigate some thermal degradation effects. Cathode surface coatings, such as aluminum oxide or lithium phosphate, reduce transition metal dissolution and oxygen loss. Electrolyte additives that form more stable SEI layers, such as lithium difluorophosphate (LiDFP), can improve high-temperature performance. Ceramic-filled separators maintain mechanical integrity at higher temperatures. However, these modifications often involve tradeoffs with other performance parameters like low-temperature performance or power capability.

Understanding these thermal degradation mechanisms informs battery management strategies. Temperature monitoring and control systems can significantly extend battery life by preventing operation in conditions that accelerate degradation. Advanced battery management systems may incorporate models of temperature-dependent degradation to optimize charging protocols and usage patterns. Material selection for specific applications must consider the expected temperature environment and the associated degradation pathways for each chemistry.