Impact of External Pressure on the Performance and Expansion of State-of-the-Art and Next-Generation Battery Materials Using Operando Dilatometry

Abstract

The demand for higher energy densities in lithium-ion batteries leads to the use of next-generation anode materials, e.g. lithium metal or silicon anodes, in order to achieve those energy targets. When comparing with today’s state-of-the-art (SoA) electrode materials, one downside of the increased energy density of the next-generation materials is the drastically increased volume changes during formation and cycling. These next-generation materials exhibit volume changes theoretically up to ~300% for silicon, for example, and ~4.9 µm per mAh cm-2 for lithium metal [1]. The volumetric changes can favor aging mechanisms and increase the electrochemical/mechanical interactions inside the cells.In this work, the interactions between (in)homogeneous mechanical pressure on the cell and volume expansion are studied for SoA materials. In addition, the volumetric changes of next-generation materials are examined and compared with SoA materials. The influence of the volume changes, primarily of the anode, on the electrochemical processes occurring in the cell is investigated using a dilation sensitive test set-up [2] based on the system reported by Sauerteig et al. [3], but with the addition of a reference electrode which allows the anode and cathode potentials to be distinguished.During initial investigations of graphite vs. NMC 111 [2], an irreversible thickness change of 8% occurs in the first cycle (see Figure 1 (a)) due to formation of the solid electrolyte interphase (SEI), particle rearrangement, delamination and graphene layer spacing in the graphite anode. In the following cycles, the irreversible thickness change decreases and a reversible thickness change between 8% - 10% is observed. Inhomogeneous pressure distribution as well as high applied pressures accelerate aging of the electrodes significantly. During CCCV charging, the anode potential drops below 0 V, causing lithium plating at an earlier state of charge for the inhomogeneous pressure distribution compared to the homogeneous pressure distribution due to localized exchange currents.In contrary to the results of SoA-material, the cycling of lithium metal reveals high volumetric changes during stripping and plating of around 6 µm per mAh cm-2 (see Figure 1 (b)) [4]. The high expansion can be explained on the one hand by the reversible dilation of the lithium metal ions and on the other hand by irreversible dilation due to the formation of the SEI and ‘dead lithium’. By applying high pressure, the irreversible dilation can be suppressed especially at high current rates. Furthermore, the choice of the electrolyte has an impact on the irreversible dilation of the lithium metal and hence the performance during cycling. With the use of a carbonate based electrolyte the irreversible dilation is enhanced and the cell performance is decreased compared to an ether based electrolyte (see Figure 1 (b)).