Enabling the Production of Homogeneous High-Load Positive Electrodes by Tailoring the Electrode Formulation – a Conductive Additive and Solvent Approach

Abstract

The automotive industry experiences a substantial change due to electrification of major parts of the transportation system by using lithium ion batteries (LIBs). To facilitate this transition, low cost and high energy density LIBs produced under the most sustainable conditions possible are required. These objectives are amongst others strongly related with the positive electrode. Thus, in order to achieve enhanced energy densities on cell level, the application of high-load positive electrodes for various cell systems ranging from lithium ion to lithium metal batteries is inevitable. However, to ensure high lithium ion mobility within thick composite electrodes and to obtain maximized capacity utilization, it is crucial to tailor the electrode microstructure. With regard to the production, detrimental effects like binder-migration can occur upon drying of thick electrodes inducing inhomogeneous distribution of binder and conductive additive within the electrode coating. Beyond that, the effect of lateral coating shrinkage resulting in electrode cracking during drying plays an increasingly significant role with increasing electrode thickness.The application of high solid contents (SC) during electrode paste processing can widely suppress the effects of binder migration and crack formation.[1,2] However, elevated SCs result in increasing paste viscosities leading to practical limitations for homogeneous electrode coating. By using nano-scale and micro-scale spherical, linear and three-dimensional conductive additives like carbon microfibers (CMF) or conductive graphite (CG) in addition to carbon black (CB), the adjustment of an appropriate paste viscosity can be facilitated. The addition of more carbonaceous additives acting as conductive additive as well as processing additive resolves the rheological requirements for an electrode paste with 80% SC and significantly influences the pore structure of the electrode. Thus, tailoring active and inactive components is crucial to enable processing of high SC electrode pastes with an appropriate viscosity in conjunction with the production of thick electrodes exhibiting an optimized pore structure benefiting the electrochemical performance. The additional introduction of carbon nanotubes (CNTs) leads to the formation of segregated networks providing more stability within the electrode and a favorable electrode microstructure. Moreover, the use of CNTs benefits the electrochemical performance by immensely boosting electronic conductivity resulting in higher rate capability and increased capacity retention. The various electrode formulations containing up to three different conductive additives were compared to the benchmark formulation without further additives in terms of electronic conductivity, adhesion, pore structure and electrochemical performance. Different electrode formulations were investigated and compared regarding the composite electrode adhesion strength, electronic conductivity, microstructure and electrochemical performance over 400 cycles in a coin cell setup with a graphitic negative electrode. However, the optimized formulation containing CNTs enables the production of thick positive electrodes exhibiting significantly higher areal capacities up to 8 mAh cm-2 with superior electrochemical properties and higher content of active material in the formulation resulting in higher energy densities on cell level. In a next step, the state-of-art processing solvent N-Methyl-2-pyrrolidon (NMP) was targeted with the goal of replacing NMP with a non-toxic solvent. The influence of a co-solvent on the electrode paste viscosity was investigated to further increase the SC, lower production costs and enable improved environmental benignity.A comprehensive study on tailoring the rheological, structural and electrochemical properties by processing additives is presented. The increase of the SC to 80% is a first step towards the reduction of the ecologic and economic footprint for LIB production while simultaneously enabling electrodes with high areal capacities exhibiting increased rate capability and capacity retention enabled by the addition of CNTs.[1] J. Seeba, S. Reuber, C. Heubner, A. Müller-Köhn, M. Wolter, A. Michaelis, Chemical Engineering Journal Volume 402, 2020, 125551. [2] L. Ibing, T. Gallasch, P. Schneider, P. Niehoff, A. Hintennach, M. Winter, F. M. Schappacher, Journal of Power Sources 423, 2019, 183–191.