*Article* **High-Precision Monitoring of Volume Change of Commercial Lithium-Ion Batteries by Using Strain Gauges**

**Lisa K. Willenberg 1,2, Philipp Dechent 1,2, Georg Fuchs 1,2, Dirk Uwe Sauer 1,2,3,\* and Egbert Figgemeier 2,4,\***


Received: 17 December 2019; Accepted: 9 January 2020; Published: 11 January 2020

**Abstract:** This paper proposes a testing method that allows the monitoring of the development of volume expansion of lithium-ion batteries. The overall goal is to demonstrate the impact of the volume expansion on battery ageing. The following findings are achieved: First, the characteristic curve shape of the diameter change depended on the state-of-charge and the load direction of the battery. The characteristic curve shape consisted of three areas. Second, the characteristic curve shape of the diameter change changed over ageing. Whereas the state-of-charge dependent geometric alterations were of a reversible nature. An irreversible effect over the lifetime of the cell was observed. Third, an s-shaped course of the diameter change indicated two different ageing effects that led to the diameter change variation. Both reversible and irreversible expansion increased with ageing. Fourth, a direct correlation between the diameter change and the capacity loss of this particular lithium-ion battery was observed. Fifth, computer tomography (CT) measurements showed deformation of the jelly roll and post-mortem analysis showed the formation of a covering layer and the increase in the thickness of the anode. Sixth, reproducibility and temperature stability of the strain gauges were shown. Overall, this paper provides the basis for a stable and reproducible method for volume expansion analysis applied and established by the investigation of a state-of-the-art lithium-ion battery cell. This enables the study of volume expansion and its impact on capacity and cell death.

**Keywords:** lithium-ion batteries; ageing; mechanical fatigue; volume change; measurement technique; silicon anode
