Electromechanical Behavior of Large-Format Prismatic Lithium-Ion Batteries

Stress and strain evolution, both throughout the manufacturing process and in response to electrical loads during cell operation, may be important to the state-of-health of prismatic lithium-ion batteries for electric vehicle applications. At present the impacts of mechanical phenomena on battery performance are not very well understood. To avoid capacity fade incurred by operation near very low and very high charge states, many automotive batteries are designed to have a great deal of excess capacity, and to operate in a limited range of charge states.⁽¹⁾Understanding relationships between mechanical effects at various length scales (electrode, cell, and pack) may inform the design of control schemes that will allow more utilization of battery materials.

At every stage of automotive battery-pack assembly, the respective components are subjected to large stresses.⁽²⁾ Intercalation-deintercalation stresses that evolve during electrical cycling can cause volume expansion and fracture in the active particles, leading to degradation processes in the electrodes that ultimately reduce cycle life of the battery. Failure mechanisms such as cracking and delamination of electrodes can make new surface area for solid-electrolyte interphase (SEI) formation.⁽³⁾ Particle cracking can also reduce electrical contact with binders and conductivity-providing additives, isolating useful regions of active material and reducing apparent capacity over time.⁽⁴⁾ Additional consequences of mechanical cycling include particle rearrangement and reduced pore volume, which can both decrease Li-ion mobility through porous electrode materials.⁽⁵⁾

Mechanical effects can also be beneficial for battery health. During electrode fabrication, it is known that certain amounts of compression can lead to better-performing materials.⁽²⁾ At the cell level, a constant compressive force is placed on the battery by fitting it inside of a rigid casing, which improves contact within individual layers of the jellyroll. On the pack level, compressing stacked battery cells within a module can help to reduce problems arising from vibration or other mechanical effects.

Figure 1 shows a zero-displacement experiment, where the evolution of equilibrium stress within a constrained 3-cell battery pack is measured against SOC at a variety of ambient temperatures. In conjunction with zero-force (free-swelling) experiments, these data can be used to determine an effective bulk modulus of the battery cell, whose evolution over time may help to elucidate state-of-health. The data shown demonstrate quantitatively that thermal expansion and contraction contribute as much to mechanical forces within a battery pack as intercalation/deintercalation stresses, if not more. This effect is known but not well understood.

We also will present dilatometry measurements on the electrode level. With these data and cell-level measurements in combination, it will be possible to elucidate the transfer functions that relate material properties within the battery to deformations of its exterior.

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