Grading and Quality Control of Zinc-Ion Battery Cells

This paper presents the analysis and results developed during a master thesis internship with the Swedish company Enerpoly. The objective of the thesis was to assess and establish a grading and quality control system for the zinc-ion battery cells manufactured by the company. For this purpose, several pouch cells were assembled and submitted to formation cycles, self-discharge, and cycle life tests. While from an initial study, analysing and relating the formation cycle parameters, it was concluded that 2 formation cycles are sufficient for a stable operation of the cell, no reliable outcomes were reached from the self-discharge tests due to a problem in the cycler. Lastly, concerning the cycle life tests, even though no strong correlations were encountered between the formation cycle parameters and the good or bad performances of the cells, significant conclusions were drawn from the data analysis. On one hand, some of the cells assembled presented very stable behaviours throughout the cycling protocol, only fading capacity as a result of external factors, as electrolyte drying or coating delamination. On the other hand, malfunctioning cells, discarded from the analysis, were always identified by either a fast rise of their internal resistance or a low initial gravimetric capacity. For these reasons, it was concluded that the grading and quality control system suggested to be implemented in the manufacturing process should consist of 2 independent stages: quality control checks on the dry cell – such as checking case sealing, absence of short circuits, and the coating quality – would ensure that no external factors influence the cell’s performance; and quality control checks on the wet cell – including internal resistance and capacity measurements before and after formation – guarantee that malfunctioning cells can be discarded and recycled before integrating the battery pack.