Interplay between Elastic and Electrochemical Properties during Active Material Transitions and Aging of a Lithium‐Ion Battery

The interplay between the internal mechanical properties and external mechanical conditions of a battery cell, e. g., Young's modulus and thickness change, has a crucial impact on the cell performance and lifetime, and thus, needs to be fully understood. In this work, 12 Ah lithium-ion battery pouch cells were studied during cycling and aging by non-invasive operando ultrasonic and dilation measurements. The effective Young's modulus increases and the thickness varies the most within a single cycle during the graphite transition from stage 1L to 4, at the beginning of the 2 to 1 stage transition and at the phase transition of the nickel-rich NCM from H2 to H3. After 1000 cycles of aging, the overall effective Young's modulus of the lithium-ion battery decreases by ∼11 %-12 % and the cell thickness increases irreversibly by ∼3 %-4 %, which is mostly related to a thicker and possibly softer, more porous solid electrolyte interphase layer.