Meaning
Preferential orientation of grains within a polycrystalline material influences the directional mechanical properties and the conductivity of current collectors in high power cells. Microstructural texture arises from the manufacturing history of the metal, such as the rolling direction of a sheet or the cooling rate of a cast part. This non random distribution of crystal lattices means that the material will respond differently to stress depending on the direction of the load.
In battery production, the texture of copper and aluminum foils affects how they stretch and tear during the high speed coating process. The influence of this property is reduced as the grain size becomes smaller or as the material is annealed to a more random state.
Orientation Mechanism
Deformation during cold rolling forces the individual crystals to rotate and align their slip planes with the direction of the metal flow. Microstructural texture is characterized by the specific crystallographic planes that face the surface of the sheet. For aluminum current collectors, a strong cube texture is often preferred because it provides the best balance of strength and conductivity.
This alignment is managed by controlling the reduction ratio and the temperature of the rolling mills.
Property Anisotropy
Variation in the elastic modulus and the yield strength occurs as a result of the aligned crystal structure. Microstructural texture can lead to earing during the deep drawing of cell cans, where the top edge of the cup becomes wavy rather than flat. This anisotropy must be accounted for in the forming simulations to ensure the final parts meet the required tolerances.
If the texture is too strong, the foil may be prone to splitting along the rolling direction during battery assembly.
Analysis Technique
Electron backscatter diffraction and x ray diffraction are the primary tools used to quantify the degree of orientation in a sample. These methods allow researchers to visualize the microstructural texture as a pole figure or an orientation distribution function. Data from these scans helps optimize the heat treatment cycles to produce the desired grain alignment.
If the texture is not monitored, the performance of the current collectors may vary between batches, leading to inconsistencies in cell capacity. The final texture determines the fatigue life of the metal when it is subjected to the repeated expansion and contraction of the battery during use.
