Grading Games in the Secondary Cell Market
Secondary cell sorting games artificially elevate capacity ratings via thermal and discharge rate manipulation, requiring strict incoming testing to avoid severe pack failure.
Parasitic Side Reactions
Meaning
This term designates the unwanted chemical and electrochemical reactions that occur inside a battery cell alongside the primary charge-transfer reactions. In lithium-ion batteries, parasitic side reactions involve the decomposition of the liquid electrolyte, the dissolution of transition metals from the cathode and the continuous growth of the solid electrolyte interphase. These secondary reactions consume active lithium ions and liquid electrolyte, leading to capacity fade, increased internal resistance and gas generation.
The parameter applies to all active electrochemical cells and is highly dependent on temperature and operating voltage. It does not apply to dry, un-electrolyte-filled cells.
Chemical Degradation
The most common of these secondary reactions is the continuous reduction of the carbonate-based electrolyte solvent at the anode surface during charging. This reaction occurs because the operating potential of the graphite anode is below the electrochemical stability window of the liquid electrolyte. This decomposition forms the solid electrolyte interphase, which is a protective layer that limits further electrolyte reaction but consumes active lithium in the process.
At the same time, high operating voltages and temperatures can trigger the oxidation of the electrolyte at the cathode surface. These parasitic side reactions are driven by thermodynamic instability and proceed continuously, even when the battery is sitting idle, resulting in self-discharge.
Performance Impact
The accumulation of these unwanted reaction products on the electrode surfaces has a severe impact on the cell’s long-term performance and safety. As these resistive films grow thicker, they increase the cell’s internal impedance, which reduces its power capability and generates more heat during operation. Additionally, the consumption of active lithium ions directly leads to capacity fade, while the loss of liquid electrolyte can result in dry-out of the separator, further increasing resistance.
Some of these reactions also generate volatile gases such as carbon dioxide and carbon monoxide, which can cause swelling and mechanical deformation in pouch cells.
Additive Design
To minimize these unwanted reactions, battery manufacturers incorporate specialized chemical additives into the electrolyte formulation. These additives are designed to decompose preferentially during the cell’s first charging cycle, forming a highly stable, uniform and thin protective layer on the electrode surfaces. This optimized layer effectively blocks the transport of electrolyte molecules to the active material while allowing the rapid passage of lithium ions.
Developing and testing these advanced additive packages is a primary focus of battery R&D, as they are necessary for enabling high-voltage, long-life cell formulations.