Spatial Thermal Conductivity Characterization in Prismatic Formats
Measuring anisotropic conductivity tensors under mechanical stack compression prevents thermal modeling errors in large prismatic lithium battery packs.
Solid Electrolyte Interphase Degradation
Meaning
Physical and chemical breakdown of the protective passivation layer on the anode surface leads to increased internal resistance and the loss of active lithium or sodium inventory. This solid electrolyte interphase degradation occurs due to the mechanical stress of electrode expansion or the chemical instability of the interface. It governs the capacity fade and the aging rate of the electrochemical cell over its lifetime.
The boundary of this term includes the dissolution of the layer and the subsequent consumption of more electrolyte.
Aging Mechanism
Mechanical cracking of the interphase layer during the intercalation process exposes fresh active material to the electrolyte. This solid electrolyte interphase degradation triggers the continuous formation of new passivation products which consumes the charge carriers. Over time, the layer becomes thicker and more resistive to ion transport.
High temperatures accelerate the chemical decomposition of the organic and inorganic components within the layer. The loss of active material and electrolyte results in a gradual decline in the energy storage capacity. Side reactions also produce gaseous byproducts that can cause the cell to swell.
Capacity Loss
Irreversible consumption of ions during the repair of the interface is the primary cause of battery performance decline. This solid electrolyte interphase degradation is particularly severe in high capacity anodes that undergo significant volume changes. The resulting increase in impedance reduces the power delivery and the efficiency of the battery.
State of health estimation algorithms monitor the increase in internal resistance to track the progress of the degradation. Maintaining a stable interface is necessary for achieving thousands of charge and discharge cycles. The loss of capacity is often used to define the end of life for the battery pack.
Stability Enhancement
Electrolyte additives and surface coatings are used to create a more durable and flexible passivation layer. This solid electrolyte interphase degradation can be mitigated by choosing solvents that form a stable and ionically conductive interface. Fluoroethylene carbonate is a common additive used to improve the toughness of the interphase in sodium ion systems.
Proper formation protocols during the first few cycles are essential for creating a uniform and sturdy layer. Atomic layer deposition can also be used to apply a protective film that prevents direct contact between the electrode and the electrolyte. Long term research focuses on understanding the molecular structure of the interphase to design better materials.
The final stability of the battery is a direct result of the integrity of this protective layer.
Three Electrode Anode Potential Tracking for Low Temperature Battery Management System Design
Direct anode potential tracking prevents sub-zero lithium plating by dynamically modulating charge current to hold graphite surface potential above zero volts vs Li/Li+.