Meaning
This chemical reaction occurs when different organic carbonate solvents in the battery electrolyte exchange their alkyl groups in the presence of catalyst species. During exposure to elevated temperatures or active electrode surfaces, linear carbonates react to form new, unsymmetrical solvent molecules. Transesterification alters the original composition of the electrolyte, which can change its ionic conductivity, viscosity and thermal stability.
The reaction is a significant aging mechanism in lithium-ion cells that utilize mixtures of linear and cyclic carbonate solvents. It is limited to the liquid phase of the electrolyte and occurs more rapidly in the presence of basic lithium alkoxide species.
Reaction Process
The process begins when trace amounts of lithium alkoxides are generated through the reduction of carbonate solvents at the anode. These alkoxides act as highly active catalysts that promote the exchange of alkyl groups between different carbonate molecules. For example, dimethyl carbonate and diethyl carbonate can react to form ethyl methyl carbonate in a reversible chemical equilibrium.
This exchange continues until the system reaches a thermodynamic equilibrium that depends on the temperature and starting concentrations. The resulting change in solvent composition can increase the viscosity of the electrolyte, which slows down lithium ion transport.
Commercial Valuation
Sourcing departments specify the purity and stability of the electrolyte to prevent this reaction from occurring in purchased cells. The chemical stability of the solvent mixture directly affects the long-term cycle life and high-temperature storage performance of the batteries. Suppliers must provide cells with stable electrolyte formulations that use specific additives to suppress the formation of alkoxide catalysts.
Sourcing contracts often include requirements for gas chromatography testing of the electrolyte from aged cells to monitor chemical composition. This testing ensures that the battery chemistry remains stable and does not degrade prematurely under hot storage conditions.
Chemical Limits
The rate of this reaction is highly dependent on the temperature and the presence of basic impurities within the cell. Below thirty degrees Celsius, the reaction is extremely slow and has a negligible effect on the electrochemical performance of the cell. The process does not occur in electrolytes that use a single solvent species, as there are no different alkyl groups to exchange.
It cannot be easily reversed, but its progress is limited by the thermodynamic equilibrium of the specific solvent mixture. These boundary conditions make the choice of starting solvent composition and purity critical for preventing electrolyte degradation.
