(Invited) Charge Transport Limitations to the Power Performance of LiNi 0.5 Mn 0.3 Co 0.2 O 2 Composite Electrodes
Résumé
Automotive applications of Lithium ion batteries demand two key characteristics: mileage (energy) and torque (power). Energy demand can be simple achieved by increasing electrode mass loadings and/or density. However, power suffers due to the increased electrode thickness and/or density increases the path length for ions transport, which is the key limiting factor for proper functioning at high current densities [1]. This study deals with how the different electrode parameters affect electrochemical performance especially at high current densities.
The interplay between electrode microstructure, electrochemical performance and lithium salt diffusion must be further studied to improve battery engineering. Herein, the electrochemical performance of NMC532-based positive electrodes for EV application were analyzed in relationship with their microstructure and their transport properties.
The NMC532 electrodes were designed with varying degrees of loading, mass percentages of electrode components and calendaring to evaluate electrode microstructure effects against electrode performance.
The electrode microstructures were acquired numerically in 3D at different scales by X-ray tomography and FIB-SEM tomography, which allowed their fine analysis through the quantification of several parameters such as the intra- and interconnectivity between the various phases, and the porosity distribution in size and tortuosity. The electrodes electronic transport properties were measured by using broadband dielectric spectroscopy from 40 Hz to 10 GHz [2]. The electrode ionic conductivities were evaluated by performing numerical simulations using the Fast Fourier Transform (FFT) method. Such simulations are directly computed on the grid represented by the voxels in the 3D numerized FIB-SEM volumes [3].
The electrochemical performance were measured in half-cells with LP30 electrolyte at various temperatures between 0 and 40°C. The results were analyzed with the penetration depth model from Gallagher et.al. [4]. This one proposes to relate the capacity at high C-rates (typically C or higher) with the electrode microstructure parameters, the electrolyte transport properties and the applied current density.
The decrease in specific capacity observed with the increase in current density as well as in electrode thickness, the decrease of its porosity and the temperature decrease is in agreement with the existing literature. Our results show more particularly the significant influence of the NMC532 conductivity and its modulation with the lithiation state. Moreover, we have been able to find a good agreement between the experimental results and the predictions of the penetration depth model. But, it is necessary to take into account precisely the different electrode porosities (open or closed, micrometric or nanometric) which participate with variable efficiencies in ion transport. We found finally that increased CB-PVdF content increases tortuosity due to any existing interactions between the electrolyte species and these additives, in agreement with our Pulsed Field Gradient Spin - Echo NMR (PFG-SE NMR) recent study [5].
Hence, this work provides a better understanding of how these composite electrodes perform depending on their microstructure and the active material intrinsic conductivity.