Investigation of internal gas pressure and internal temperature of cylindrical Li-ion cells to study thermodynamical and mechanical properties of hard case battery cells

This study is the first to measure the relationship between gas pressure, internal gas temperature and state of charge of cylindrical Li-ion battery cells. For this purpose, a new, simple and affordable setup for in-situ investigation of internal gas pressure and internal temperature of commercial cylindrical Li-ion cells is designed and tested. Setups to investigate the internal gas pressure have so far only been performed on pouch and prismatic cells or require complex and expensive measurement equipment. In this work, the mechanical effects of an increasing gas pressure in cylindrical Li-ion cells are addressed for the first time. It is shown that the measurement of internal gas pressure and internal temperature over multiple charging and discharging cycles is possible. Measurements at different current rates show the dependency of the gas pressure to the total amount of charge transferred and the temperature of the cell. The pressure, which depends on the degree of lithiation of the electrodes, relaxes after the Li-ion cells have been exposed to a high current. Additionally, phases of the phase transition are visible via differential capacity analysis. The differential pressure analysis shows an equivalent trend to the differential capacity. Thus demonstrates the predominant phase transformations, namely the restructuring of lithiated graphite, the lithiation of silicon, and the phase transformation of nickel–cobalt–manganese (NCM) oxide. These effects contribute significantly to the pressure change, as they cause electrode expansion, which results in a reduction of the gas volume of the Li-ion cell, leading to an increase in pressure.