Petronela Gotcu-Freis

Quantitative experiments of lithiation/delithiation rates were considered for a better understanding of electrochemical intercalation/deintercalation processes in laser structured thick film cathodes. Besides galvanostatic cycling for evaluation of specific discharge capacities, a suitable quantitative approach for determining the rate of Li-ion insertion in the active material and the rate of Li-ion transport in the electrolyte is expressed by chemical diffusion coefficient values. For this purpose, the galvanostatic intermittent titration technique has been involved. It could be shown that laser structured electrodes provide an enhanced chemical diffusion coefficient and an improved capacity retention at high charging and discharging rates.

An interlaboratory study was performed to determine the heat capacity of an active material for lithium-ion batteries with layered structure and nominal composition LiNi_1/3 · Mn_1/3Co_1/3O₂ (NMC111). The commercial sample, which was characterized using powder X-ray diffraction and inductively coupled plasma–optical emission spectroscopy, is single phase (α-NaFeO₂ crystal structure) with a composition of Li_1.02Ni_0.32Mn_0.31Co_0.30O₂. Heat capacity measurements of the homogeneous sample were performed at five laboratories using different operators, methods, devices, temperature ranges, gas atmospheres and crucible materials. The experimental procedures from each laboratory are presented and the results of the individual laboratories are analyzed. Based on a comprehensive evaluation of the data from each laboratory, the heat capacity of the NMC111 sample from 315 K to 1 020 K is obtained with an expanded reproducibility uncertainty of less than 1.22 %.

Thermal behaviour and thermophysical properties of two typical cathodes for lithium-ion batteries were studied in dependence of temperature. The cathode materials are composite thick films containing a mixture of 90 wt% LiMeO₂ active material (with Me = Co or Me = Ni + Mn + Co, respectively) and additives (binder and carbon black), deposited on aluminium current collector foils. The thermal conductivity of each cathode type and their corresponding composite layers were determined up to 573 K from the measured thermal diffusivity, the specific heat capacity and the estimated density based on metallographic methods and structural investigations. In addition, the impact of lithiation degree x in Li_xMeO₂ on the transport properties of cathode samples was also investigated. The quantitative determination and the homogeneity of Li content on the surface and within the bulk of the samples were validated by laser induced breakdown spectrometry. The results presented here explain at cell component level, i.e. cathode material, the thermal runaway behaviour of lithium-ion batteries in a combined approach of application oriented and fundamental research. Therefore, these data are significant for improving the simulation studies of their thermal management, in which the bulk properties are assumed, as a common approach, temperature and lithiation degree independent.

The first experimental thermophysical properties of two cathodes containing LiCoO₂and LiMn₂O₄blended active materials combine application oriented and fundamental research.

Due to a growing demand of cost-efficient lithium-ion batteries with an increased energy and power density as well as an increased life-time, the focus is set on intercalation cathode materials like LiFePO₄. It has a high practical capacity, is environmentally friendly and has low material costs. However, its low electrical conductivity and low ionic diffusivity are major drawbacks for its use in electrochemical storage devices or electric vehicles. By adding conductive agents, the electrical conductivity can be enhanced. By increasing the surface of the cathode material which is in direct contact with the liquid electrolyte the lithium-ion diffusion kinetics can be improved. A new approach to increase the surface of the active material without changing the active particle packing density or the weight proportion of carbon black is the laser-assisted generation of 3D surface structures in electrode materials. In this work, ultrafast laser radiation was used to create a defined surface structure in LiFePO₄ electrodes. It was shown that by using ultrashort laser pulses instead of nanosecond laser pulses, the ablation efficiency could be significantly increased. Furthermore, melting and debris formation were reduced. To investigate the diffusion kinetics, electrochemical methods such as cyclic voltammetry and galvanostatic intermittent titration technique were applied. It could be shown that due to a laser generated 3D structure, the lithium-ion diffusion kinetic, the capacity retention and cell life-time can be significantly improved.