Interplay of Pressure and Electrolyte on the (Ir)Reversible Expansion of State-of-the-Art and Next-Generation Battery Materials using Operando Dilatometry

In this work, the correlation between mechanical pressure on the cell and volume expansion is studied for state-of-the-art (SoA) and next generation materials for lithium-ion batteries. The effect of volume changes, primarily of the anode, on the electrochemical processes occurring within the cell is investigated using a custom-built operando dilatometry test setup. With this test setup it is possible to distinguish between anode and cathode potentials by using a lithium reference electrode.

Initial investigations of different materials show that SoA materials such as graphite, lithium titanate, lithium nickel manganese cobalt oxide or lithium iron phosphate exhibit significantly lower expansion than next generation materials such as silicon or lithium metal. Lithium metal, in particular, exhibits high volumetric changes of about 7.9 µm per mAh cm^-2
during plating at high current rates and low applied external pressures. This expansion cannot be explained by the theoretical expansion of the metal during plating alone. Further increases in (irreversible) thickness are associated with SEI and the deposition of less dense, i.e., dendritic or mossy, lithium. The volumetric change during lithium stripping is only 5.4 µm per mAh cm^-2, resulting in an irreversible expansion of ~5 µm during one cycle.

By applying high pressure, the irreversible expansion can be suppressed to a large extent, especially at high current rates. The choice of electrolyte also affects the irreversible dilation of the lithium metal and therefore the cycling performance. The use of a carbonate-based electrolyte will increase irreversible dilation and decrease cell performance compared to an ether-based electrolyte.