Electrolyte decomposition in lithium-ion batteries is a complex electrochemical process that generates gaseous byproducts, affecting cell performance and longevity. The organic carbonate-based electrolytes, typically consisting of solvents like ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), along with lithium salts such as LiPF₆, undergo reduction at the anode and oxidation at the cathode, leading to gas evolution. These reactions are influenced by operational conditions, including voltage, temperature, and the presence of impurities.

At the anode, reduction reactions occur when the electrolyte components interact with the negatively charged electrode surface, typically graphite or silicon. The solid electrolyte interphase (SEI) forms as a protective layer during initial cycles, but continued reduction leads to further decomposition. Ethylene carbonate, a common solvent, undergoes reductive decomposition, producing lithium ethylene dicarbonate (LEDC) as a primary SEI component. However, side reactions generate gases such as ethylene (C₂H₄) and hydrogen (H₂). The reduction pathway involves electron transfer to EC, breaking the carbonate ring and forming C₂H₄ and CO₃²⁻, which further reacts to produce CO₂.

Propylene carbonate (PC), if present, decomposes similarly but tends to co-intercalate into graphite, causing exfoliation and gas generation, primarily propylene (C₃H₆). Linear carbonates like DMC and EMC decompose into methane (CH₄), ethane (C₂H₆), and CO through radical pathways. The presence of lithium salts accelerates these reactions; LiPF₆ hydrolyzes to form HF, which corrodes the SEI and promotes further gas evolution.

At the cathode, oxidation reactions dominate, particularly at high voltages (>4.3 V vs. Li/Li⁺). Solvent molecules lose electrons, leading to the formation of radical cations that decompose into CO₂, CO, and other volatile organic compounds. Ethylene carbonate oxidizes to form CO₂ and oligomeric species, while linear carbonates produce aldehydes and esters. Lithium salts also participate; LiPF₆ decomposition releases PF₅, which reacts with solvents to form phosphorylated compounds and additional CO₂.

Temperature plays a critical role in accelerating these reactions. Elevated temperatures (>60°C) increase the kinetics of both reduction and oxidation processes. The Arrhenius relationship governs the temperature dependence, with activation energies for gas generation typically ranging between 50-100 kJ/mol. High temperatures also destabilize LiPF₆, increasing HF formation and subsequent SEI degradation.

Impurities such as water, even at trace levels (<50 ppm), exacerbate gas generation. Water reacts with LiPF₆ to form HF and POF₃, which further decompose solvents. Additionally, transition metal ions (Ni, Mn, Co) dissolved from the cathode catalyze electrolyte oxidation, increasing CO₂ and CO production.

Analytical techniques are essential for identifying and quantifying gaseous decomposition products. Gas chromatography-mass spectrometry (GC-MS) provides precise speciation of evolved gases, distinguishing between CO, CO₂, CH₄, and hydrocarbons. Fourier-transform infrared spectroscopy (FTIR) is used for real-time monitoring of gas evolution during cycling, detecting functional groups associated with CO₂ (2300-2400 cm⁻¹) and hydrocarbons (2800-3000 cm⁻¹). Differential electrochemical mass spectrometry (DEMS) combines electrochemical cycling with mass spectrometry, enabling correlation between charge-discharge processes and gas generation rates.

Quantitative studies reveal that CO₂ is the dominant gas at the cathode, constituting up to 70% of total gas evolution at high voltages. At the anode, C₂H₄ and H₂ are primary products, with their ratios dependent on solvent composition. For example, cells with EC-based electrolytes produce more C₂H₄, while those with linear carbonates generate higher CH₄ concentrations.

Mitigation strategies focus on electrolyte formulation improvements. Additives like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) stabilize the SEI, reducing gas generation at the anode. For cathode protection, oxidation-resistant solvents such as sulfones or ionic liquids are explored, though tradeoffs in ionic conductivity remain. Lithium salt alternatives, including LiFSI and LiTFSI, exhibit better thermal stability but may introduce other decomposition pathways.

Understanding these chemical pathways is crucial for developing next-generation electrolytes that minimize gas evolution while maintaining electrochemical performance. Advances in analytical techniques continue to provide deeper insights into decomposition mechanisms, guiding the design of safer and more durable lithium-ion batteries.