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In this work a new expansion measurement methodology for the detection of ageing mechanisms in lithium ion cells is described and evaluated. For this purpose, a polarization-optical homodyne Michelson interferometer with a resolution of approx. 10 nm was set up to allow measurements of different cell types, cell sizes, and cell chemistry within a Memmert IP55 climate chamber. The used interferometric measurement setup is ten times more accurate than measurement systems commonly used for battery measurements in the literature. The better spatial resolution of an interferometer setup enables the recording of the expansion response to short current pulses with low transferred charge. This offers the possibility to detect ageing mechanisms such as lithium plating within short measuring times. With this measurement setup, a lithium iron phosphate pouch cell from the company A123, with the model identification AMP20M1HD-A, is measured. First, an expansion measurement over a full charge and discharge cycle with a constant load of $\pm \frac {C}{20} (\widehat {=} {\mathrm {1\,\, \text {A}}})$ is performed. Here, the cell undergoes an expansion of about $\mathrm {42 ~\mu \text { m} }$ . Subsequently, the in this work proposed measurement methodology is presented to show the influence of effects such as the formation of overshooting graphite stages or lithium plating on cell expansion. For this purpose, at a residual capacitance of 50%, the cell is pulsed with current of different intensity $I \in \{\pm {\mathrm {20 \text {A}}}, \pm {\mathrm {80 \text {A}}}\}$ for $t = \{ {\mathrm {240\,\, \text {s}}}, {\mathrm {60\,\, \text {s}}}\}$ , at $T = \{ { {10\,\, ^{\circ}\text{C}}}, { {20\,\, ^{\circ}\text{C}}}, { {30\,\, ^{\circ}\text{C}}}\}$ , respectively, and the mechanical response is recorded using the high accuracy interferometer setup. A mechanical relaxation of the cell after the pulses with a time constant in the range between 10 s and 20 s can be observed. It is shown that this relaxation behavior can be associated with effects of the anode known from the literature, such as the formation of overshooting graphite stages or lithium plating. These effects can be recorded by the very high-resolution measurement already with little transferred charge and within a short measurement time. |
