Meaning
Gas release occurs when oxygen atoms leave the crystal lattice of a cathode material at high potentials. This surface oxygen evolution triggers chemical instability and contributes to the degradation of the electrolyte. High nickel cathodes are particularly prone to this effect because the metal-oxygen bonds become unstable as lithium is removed.
Controlling this release is a primary safety requirement for high energy density battery systems.
Thermal Runaway
Exothermic reactions between the released oxygen and the flammable electrolyte lead to rapid heating. When surface oxygen evolution occurs inside a sealed cell, the internal pressure rises and may cause venting. This process is a precursor to the uncontrolled combustion of the battery pack.
Lattice Vacancy
Vacant sites left behind by the departing oxygen cause the crystal structure to collapse. Following surface oxygen evolution, the material often transforms into a more stable but less active phase like rock salt. This structural change permanently reduces the amount of lithium the cathode can hold.
Safety Threshold
Operational limits are set to keep the cell voltage below the point where gas release begins. Monitoring surface oxygen evolution during laboratory testing helps determine the safe window for different electrode compositions. Additives that strengthen the oxygen bond help extend these limits and improve battery safety.
