The Electro-Chemo-Mechanical Coupling in Lithium Alloy Electrodes and Its Origins

Abstract

A method to identify and separate the influence of changes in the surface stress from the bulk stress in a model lithium-ion battery electrode during electrochemical cycling was developed. The strategy for this separation is based on the different influence of surface and bulk stresses on the coupling between electrode potential and mechanical strain as measured by dynamic electro-chemo-mechanical analysis and the coupling between the transferred electric charge and the elastic strain as determined by wide angle X-ray scattering. Using both methods, it was possible to uncover the behavior of an apparent surface stress evoked by the bulk stress due to grain boundary alloying of lithium in a gold film. Additionally, the analysis allowed for a determination of a range in surface stress due to underpotential deposition of one monolayer of lithium as the interval between −3.1 to Lithium-ion batteries are widely used, though the amount of electric charge they can store is still far away from their theoretical capacity.1,2 In order to increase the capacity of lithium-ion battery anodes, the commonly used graphite needs to be replaced. A very promising alternative are materials which can form alloys with lithium. 3,4 There are several possible candidates. 5 Among the most important ones are silicon, germanium, tin, aluminum, and antimony. The theoretical volumetric lithium storage capacity of these materials is several times higher than that of the currently used anodes. In case of silicon, more than ten times that of graphite relative to the weight of the host material. This storage capability of the afore mentioned materials is based on their ability to store more than one, in case of silicon, germanium, and tin up to 4.4 lithium atoms per host atom. The obvious drawback of these materials is the huge volume change during lithium loading which can easily exceed 100%.If the electrode material is constrained (e.g. by the current collector) or the lithium is inhomogeneously distributed in the host material, the change in volume is accompanied by stresses.6 Possible consequences are the evolution of cracks in the material which can lead to pulverization of the electrode material and consequently loss of capacity. 6 A promising possibility to prevent pulverization is to reduce the structure size of the electrode material and to introduce high porosity. 7,8 High porosity reduces concentration gradients by reducing the diffusion lengths and it weakens the constraints by offering free space in the vicinity of the electrode. However, this strategy also increases the surface to volume ratio, and therefore drastically increases the impact of the surface stress, which can have an important impact on the stability of the electrode material.9 Additionally, the stress interacts with the chemical potential via the strain energy. 10 Therefore, the equilibrium lithium concentration is a function of the stress state which, in general, is inhomogeneous within the electrode material. Recent simulations performed by Gau and Zhou 11 indicat...

Dimensional Changes in Automotive Pouch Li-Ion Cells: A Combined Thermo-Mechanical/Electrochemical Study

“…In particular, the characterization of the mechanical and thermal stability in operando conditions is a significant subject, and some interesting papers address this problem: capacity loss is related to possible electrode side reactions inducing mechanical degradation of the solid-electrolyte-interface, 10 direct mechanical abuse of lithium batteries via multi-parameter controlled pinch test, 11,12 mechanical compression indentation by using hemispherical and conical punches, 13 induced thermal abuse, 14 changes in battery dimension durung charge/discharge, 15 non-uniform material distribution in the electrode particles inducing mechanical stress (through localized volume changes). 16,17 Moreover, lithium-ion batteries were supposed to be practically free of any memory-effect. i.e.…”