Transport Limitations in Binary Electrolytes and Their Influence on Lithium Plating

Abstract

While nowadays lithium ion batteries are used in a wide range of consumer electronic devices and full electric cars, they still suffer from a limited energy density, required for larger driving ranges or long-lasting mobile phone batteries. One way to improve the energy density is to increase the electrode loading, i.e., the thickness of anode and cathode coating layers. By increasing the layer thickness, the ion transport ways are prolonged and rate limitations can occur if the drawn or applied current is too large, depending on the morphology of the coating and the ion transport properties in the liquid electrolyte (i.e., conductivity, diffusion coefficient, transference number, and thermodynamic factor).. Insufficient ion transport rates can lead to the buildup of concentration gradients within the liquid electrolyte phase of the cell and thus may cause large overpotentials, which in turn can limit the charge that can be extracted/inserted before the cut-off potentials are reached.1 In the case of the charging of graphite anodes, the buildup of concentration gradients across the electrode during lithium intercalation leads to an unequal distribution of overpotentials, which ultimately can result in lithium plating a high C-rates, particularly for thick electrodes and/or at low temperatures. The occurrence of lithium plating will decrease the coulombic efficiency and thus describes an important aging mechanism for long term cycling life. Our work focuses on the understanding of ionic transport properties of commonly used lithium ion battery electrolytes2 as well as on the quantification of geometric parameters3 of porous coatings. We will analyze the influence of the effective ionic transport on the rate performance of the cell as well as the onset of lithium plating in the anode depending on the cycling C-Rate. Figure 1 exemplarily shows the anode potential (vs. Li/Li+) of a graphite electrode in a simulated NMC/graphite full cell using the transport parameters determined for an electrolyte of LiPF6 in EC:EMC (3:7, w:w) at two different temperatures at the end of a 1C charge (cell potential of 4.3 V). It can be seen, that the critical point for lithium plating is reached at different fractions of the cell’s state of charge (SOC), which is a result of the different ionic transport properties at -10°C and 50°C. Our work will help to better understand the charging rate limitations of graphite anodes as a function of operating conditions, electrolyte transport properties, and electrode morphology. This can be used to improve electrode design and to optimize charging schemes, with the aim to prevent lithium plating and to decrease cell aging. Figure 1: Simulated graphite half-cell potentials at end of 1C charge of a 3 mAh/cm² NMC/graphite full cell (at 4.3 V) using temperature dependent transport parameters to illustrate the impact of ionic transport properties on the cell performance.