In lithium-ion batteries, gas generation during operation remains a critical challenge affecting performance and safety. Among the various mechanisms, the oxidation of conductive additives like carbon black and carbon nanotubes at high voltages contributes significantly to carbon dioxide evolution. This process occurs when the operating potential exceeds the electrochemical stability window of these carbon-based materials, particularly under conditions where localized high potentials develop during charging.
Carbon materials oxidize through electrochemical reactions with the electrolyte, producing CO₂ as a primary gaseous product. The reaction follows pathways dependent on the surface chemistry of the carbon, the electrolyte composition, and the operating voltage. Carbon black, consisting of amorphous carbon with high surface area, exhibits greater susceptibility to oxidation compared to graphitic materials due to its higher density of edge sites and defects. Carbon nanotubes, while more resistant because of their ordered structure, still undergo oxidation at potentials above 4.5 V vs. Li/Li⁺ in conventional organic electrolytes. The oxidation process involves the formation of surface oxides, which decompose into CO₂, further accelerating degradation.
Potential-pH diagrams provide a thermodynamic framework to predict carbon oxidation behavior. In non-aqueous systems, the stability of carbon correlates with the electrolyte's electrochemical window. For instance, in carbonate-based electrolytes, carbon oxidation becomes significant above 4.7 V, where the decomposition of solvents and conductive additives occurs. The presence of trace water or acidic impurities lowers the onset potential due to catalytic effects. The following table summarizes key oxidation potentials for carbon materials in common electrolytes:
Material | Onset Potential (V vs. Li/Li⁺) | Primary Gas Product
Carbon Black | 4.5 - 4.7 | CO₂
Carbon Nanotubes | 4.7 - 5.0 | CO₂
Surface treatments and additives mitigate carbon oxidation by passivating reactive sites or altering interfacial chemistry. Phosphorus-containing compounds, for example, form protective layers that reduce direct contact between carbon and aggressive electrolyte species. Similarly, coatings of metal oxides like Al₂O₃ or ZrO₂ applied via atomic layer deposition enhance oxidation resistance by acting as physical barriers. Another approach involves doping carbon materials with heteroatoms such as nitrogen or boron, which modify electronic properties and raise the oxidation threshold.
Electrolyte additives also play a crucial role. Compounds like vinylene carbonate or lithium difluorophosphate stabilize the solid-electrolyte interphase (SEI), preventing electrolyte decomposition at high voltages. Fluorinated solvents extend the oxidative stability limit, reducing parasitic reactions with conductive additives. However, additive effectiveness depends on their concentration and compatibility with other cell components.
Gas generation from carbon oxidation impacts cell performance through increased internal pressure and impedance rise. CO₂ accumulation may lead to swelling or venting in sealed cells, while the loss of conductive additives reduces electrode conductivity, accelerating capacity fade. Advanced characterization techniques like differential electrochemical mass spectrometry (DEMS) quantify gas evolution rates, linking them to specific voltage ranges and degradation modes.
Operational factors such as temperature and charge rate influence oxidation kinetics. Elevated temperatures accelerate reaction rates, while high charging currents promote localized overpotentials that exacerbate carbon degradation. Battery management systems must therefore incorporate voltage limits and temperature controls to minimize these effects.
Research continues to explore alternative conductive materials with higher oxidation resistance, such as graphene derivatives or conductive polymers, though cost and processing challenges remain. Meanwhile, optimizing existing carbon additives through surface engineering and electrolyte formulation offers a practical route to suppress gas generation in high-voltage applications.
Understanding carbon oxidation mechanisms enables better design strategies for next-generation batteries, where higher energy densities push materials closer to their stability limits. By addressing this issue, improvements in cycle life, safety, and performance can be achieved without compromising the conductive network essential for electrode functionality. Future developments will likely focus on in-situ characterization techniques and multi-functional additives that simultaneously enhance conductivity and oxidation resistance.