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Report RSE 18002345

Aging tests of lithium-ion cells with different operating conditions

The document reports the results of post-mortem analyses performed on three NMC lithium-ion cells subjected to aging tests with different operating conditions, in order to verify the presence of morphological changes on the cell materials.

The analysis of degradation mechanisms, with the aim to develop aging models for the estimation of theuseful life of energy storage systems, is one of the main research topics in this sector. To understandhow the expected life of a battery changes with the working conditions, it is possible to subject similarcells to different aging cycles, monitoring the performance over time by measuring the values of someperformance parameters through specific tests.

In order to complete the results of the aging tests, thecells can be submitted to post-mortem analysis, in order to verify the state of the electrode materials andto highlight if some elements have shown a greater degradation than others. These analyses are alsoimportant to highlight degradation effects on materials that do not necessarily manifest themselvesthrough an evident performance decrease but can lead to a sudden failure of the cell, which can cause aninternal short circuit, a swelling of the cell and, in the worst cases, thermal runaway.

The document reports the results of post-mortem analyses performed on three 3.7 V - 40 Ah NMClithium-ion cells, previously subjected to aging tests with different operating conditions for 4990 cyclesand characterized by a different state of health, manifested with a different value of the maximumcapacity and internal resistance. The aim of the performed post-mortem analyses is to verify thepresence of morphological changes that may be the cause of the different state of health and explain thevariation of the performance parameters obtained in the various parametric check-ups carried out duringthe previous aging tests. The analysis of the different materials was conducted through scanning electronmicroscopy and microanalysis with energy dispersion detector. The observations of the samplesextracted from the cells have highlighted in particular a correlation between the increase of the internalresistance of the cells and the deformation of the copper foil on which the graphite of the anode isdeposited.

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