Electrolyte decomposition is a critical failure mode in batteries that leads to irreversible capacity loss, safety hazards, and eventual cell failure. The electrolyte, composed of solvents, lithium salts, and additives, undergoes chemical and electrochemical degradation during operation, producing gaseous byproducts and solid deposits that compromise performance. Unlike general gas generation analysis, this discussion focuses on the failure consequences arising from these decomposition pathways, including pressure buildup, cell swelling, and venting mechanisms.
The primary decomposition mechanisms involve solvent oxidation at the cathode and reduction at the anode. At high voltages, carbonate solvents such as ethylene carbonate (EC) and dimethyl carbonate (DMC) oxidize, forming CO2, CO, and other organic compounds. This process accelerates with increasing voltage and temperature, particularly in high-energy-density cells operating above 4.3 V. On the anode side, solvent reduction occurs due to the reactive nature of the lithium surface or solid-electrolyte interphase (SEI). Reduction products include lithium alkyl carbonates, Li2CO3, and gaseous species like H2 and CH4. These reactions are exacerbated by high charging rates, low temperatures, or defects in the SEI layer.
Lithium salt degradation further contributes to failure. LiPF6, the most widely used salt, hydrolyzes in the presence of trace moisture, producing HF and PF5. HF attacks electrode materials and current collectors, while PF5 catalyzes further solvent decomposition. Alternative salts like LiFSI and LiTFSI exhibit better stability but may introduce other challenges, such as aluminum current collector corrosion at high potentials. The choice of salt significantly influences the decomposition pathway and the nature of byproducts.
Gas generation from electrolyte decomposition leads to internal pressure buildup, a direct cause of cell swelling. CO2 and H2 are the most common gases, with their generation rates dependent on cycling conditions and electrolyte composition. For instance, linear carbonates like DMC produce more CO2 than cyclic carbonates like EC. Pressure accumulation strains the cell casing, leading to mechanical deformation, delamination of electrode layers, and loss of electrical contact. In extreme cases, the pressure exceeds the venting threshold, triggering safety mechanisms that release gases but also permanently disable the cell.
Venting mechanisms are designed as a last-resort safety feature to prevent catastrophic failure. Cylindrical cells typically incorporate a scored vent cap that ruptures at a predetermined pressure, while pouch cells rely on sealed seams that split under stress. However, venting introduces new risks: the sudden release of flammable gases can ignite if exposed to sparks or high temperatures. Moreover, venting does not address the root cause of decomposition, meaning the cell remains compromised even after pressure relief.
Electrolyte formulation plays a decisive role in mitigating decomposition-related failures. Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) stabilize the SEI, reducing solvent reduction and H2 evolution. High-concentration electrolytes (HCE) and localized HCE designs limit free solvent molecules, thereby suppressing oxidation. Sulfolane-based electrolytes exhibit superior anodic stability but face challenges with low-temperature performance. Each formulation involves trade-offs between stability, conductivity, and cost.
The consequences of electrolyte decomposition extend beyond gas generation. Solid byproducts like Li2CO3 and lithium alkyl carbonates increase interfacial resistance, impairing charge transfer and reducing power capability. Metallic lithium plating, often a result of uneven SEI growth, creates hotspots that accelerate localized decomposition. These processes create positive feedback loops: increased resistance leads to higher overpotentials, which in turn drive further decomposition.
Temperature is a critical accelerant of decomposition pathways. Elevated temperatures lower the activation energy for both solvent oxidation and reduction, while low temperatures promote lithium plating and SEI fracture. Thermal gradients within large-format cells exacerbate inhomogeneous decomposition, leading to uneven aging and premature failure. Effective thermal management systems are essential to suppress these effects, particularly in high-performance applications like electric vehicles.
Failure analysis techniques for decomposition-related issues include post-mortem gas chromatography to identify gaseous products, mass spectrometry for quantifying volatile species, and X-ray photoelectron spectroscopy (XPS) to characterize solid decomposition layers. Electrochemical impedance spectroscopy (EIS) tracks the growth of resistive layers, while pressure sensors can monitor real-time gas evolution in experimental setups.
Mitigation strategies must address both chemical and mechanical aspects of decomposition-induced failure. Chemically stable electrolytes, robust SEI-forming additives, and moisture control during manufacturing reduce decomposition rates. Mechanically, reinforced casings, pressure-tolerant separators, and redundant venting systems contain the effects of gas generation. Advanced battery management systems (BMS) can detect early signs of abnormal gas production through impedance monitoring or pressure sensing, enabling preventive measures.
The interplay between decomposition mechanisms and cell design underscores the need for holistic approaches to battery development. Electrolyte stability must be evaluated in conjunction with electrode materials, operating conditions, and safety systems. As batteries push toward higher energy densities and faster charging, decomposition-related failure modes will remain a central challenge requiring continuous innovation in materials science and engineering.