Meaning
This chemical degradation process involves the physical or electrochemical disruption of the protective passivation layer formed on the anode surface of a lithium ion cell. Typically caused by high temperatures, mechanical stresses, or overcharging, solid electrolyte interphase breakdown leads to the exposure of the active anode material to the organic electrolyte. It results in further reaction between the lithium and the solvent, consuming active lithium and generating gaseous byproducts.
Sourcing teams monitor the stability of this passivation layer to select cells and materials that offer high durability and long cycle life. If this layer undergoes breakdown, it accelerates cell self discharge, increases internal resistance, and causes severe capacity fade.
Chemical Mechanism
The breakdown occurs when the delicate polymer organic structure of the passivation layer is fractured by the volume changes of the anode during lithium insertion and extraction. It can also be triggered by high temperatures, which cause the chemical compounds in the layer to decompose and dissolve back into the electrolyte. Once the protective layer is damaged, the electrolyte reacts with the exposed lithium carbon compound to form a new interphase layer.
This reconstruction consumes more active lithium ions and solvent, which reduces the total energy capacity of the cell. This process also generates gases, such as ethylene and carbon dioxide, which can cause the pouch or can to swell.
Electrochemical Consequences
The ongoing cycle of breakdown and regeneration is one of the primary causes of long term battery degradation. It leads to a steady increase in the internal resistance of the cell, which reduces the power output and increases heat generation during operation. Sourcing managers evaluate the electrochemical impedance of the cells over time to monitor the health and stability of the interphase layer.
Cells that can maintain a stable interphase are preferred for high cycle and high temperature applications. This assessment helps ensure that the chosen cells will meet the performance requirements of the customer.
Additive Sourcing
Procurement agreements often specify the chemical additives used in the electrolyte to enhance the stability of the interphase layer. These additives, such as vinylene carbonate, are chosen for their ability to form a more durable and flexible layer that can withstand mechanical stresses. Sourcing teams audit the materials of the suppliers to ensure that the proper chemical formulations are used during manufacturing.
This focus on material chemistry and stability helps protect the buyer from receiving cells that will suffer from premature degradation and failure.
