The importance of passive materials in thick Li-ion battery electrodes

Kurzfassung

Due to their outstanding energy and power density, Li-ion batteries are widely used in portable electronic devices and electric vehicles. The porous composite of a Li-ion battery electrode generally consists of active material (particle diameter ~ 10-20 μm), conductive carbon (particle diameter ~ 100 nm) and polymeric binder. The so-called CB domain, a microporous phase formed from carbon black and the binder, is distributed in the macro-pores of the active material network and provides better mechanical stability and electronic contact. However, at the same time the CBD increases the tortuosity of Li-ion transport pathways in the electrolyte. At high current densities, this increases mass transport limitations and reduces the performance of the battery cell. It has already been shown that the production process has a significant effect on the morphology and distribution of the CBD. For instance, harsh drying conditions can cause binder migration to the electrode surface, which amplifies transport limitations and performance losses1. In our contribution we present results of pore scale simulations of Li-ion batteries2, which explicitly consider the morphology and distribution of the CB domain. In these simulations we study 3D realizations of NMC cathodes created by a 3D stochastic microstructure generator3 with varying density and thickness. In a first step we determine effective conductivities of the virtual samples with varying CB distribution and volume fraction. This data provides insight on limiting processes during operation of the battery cell and is important input to volume-averaged models of the Newman-type. In a second step, we simulate the electrochemical performance of the virtual electrodes with an extended version of the “Battery and Electrochemistry Simulation Tool” (BEST) for microstructureresolved simulations of Li-ion batteries which was developed in our group. In order to accurately capture the influence of the CB domain on battery performance we extend our framework taking into account both the transport of electrons in the conductive carbon network as well as Li-ion transport in the available pore space. This approach allows us to evaluate the effect of inhomogeneous CB distribution arising from different conditions in the production process and gives directions for both electrode and process optimization.