No abstract

A physical-chemical model is suggested, which is able to describe the enhanced discharge rate capability of lithium-ion cells by using laser-structured graphite anodes. Recently published test data of coin cells comprising unstructured and structured graphite anodes with LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathodes is used for the presented purpose of modeling, simulation and validation. To minimize computational demand, a homogenized three-dimensional model of a representative hole structure is developed, accounting for charge and mass transport throughout the cell layers and one-dimensional diffusion within radial-symmetric particles. First, a standard pseudo-two-dimensional model is calibrated against rate capability test data of coin cells with unstructured anodes. The calibrated parameter set is transferred to the three-dimensional model in order to simulate the transient voltage response and the discharged capacity depending on the applied Crate. The simulation data shows excellent agreement with experimental data for both cell types. Three stages of rate capability enhancement are identified showing an improved relative capacity retention of 11−24% at 3C. Experimental and simulation data reveal a restricted Crate window, which can be positively affected by the structuring process, whereas both shape and pattern of the structuring process can be further optimized with the model.

In the production process chain of lithium-ion battery cells, the filling, consisting of dosing and wetting steps, of the cell and its components with electrolyte liquid is eminent for the final product quality and costs. To reduce the unnecessary wetting duration between filling and formation, and thereby the production costs, a measurement method for the wetting progress is necessary. In this paper, electrochemical impedance spectroscopy (EIS) as a well-established technique is used for the first time to quantify the wetting degree of batteries during cell production. The experimental data of the EIS acquired during the dosing and subsequent wetting process is correlated to images recorded by in situ neutron radiography. Results show that the impedance of the battery cells strongly depends on the wetting degree of the cell assembly and can thus be used to determine the fully wetted state enabling faster processing.

Within this paper we report on a lithium-ion battery with laser-structured graphite anodes, alleviating current drawbacks of lithiumion batteries such as the reduced discharge capacity at high Crates and the onset of lithium-plating during fast charging. These issues are intensified at low temperatures, as reaction and diffusion kinetics decelerate, which is why a focus of the presented work lies on low temperature performance. Electrochemical impedance spectroscopy was used to show a reduction in the impedances of cells with laser-structured anodes in comparison to their conventional counterparts. The discharge capacity retention at high Crates was enhanced by up to 27% compared to conventional cells, proving potential for high power applications. For the cells with laserstructured anodes, the onset of lithium-plating at 0°C was observed at higher charging Crates by analyzing the voltage relaxation after charging. At −15°C, a smaller amount of plated lithium was detected, even though lithium-plating could not be entirely avoided. Laser structuring also enabled shorter charging times, as the upper cutoff voltage was reached at a higher SOC. The results point out that laser structuring of the anode improves the fast charging capability of lithium-ion cells, especially under demanding operating conditions.

Lithium-ion batteries are widely used as energy storage devices due to their high energy density and versatile applicability. Key components of lithium-ion batteries are electrically isolated electrodes and a liquid electrolyte solution which enables ion transport between the electrodes. Laser structuring of electrodes is a promising approach to enhance the high-current capability of lithium-ion batteries by reducing cell internal resistances, as a larger contact area of the active material with the electrolyte solution is created. In the work described here, lithium-ion battery anodes were structured by locally ablating small fractions of the coating using femtosecond laser pulses with infrared wavelengths. A study on ablation characteristics depending on different process parameters such as laser fluence and repetition rate was performed. Special focus was on the ablation efficiency, enabling an optimized process design. The influence of the electrode composition was taken into account by studying the ablation behavior at a varying binder content. Evenly distributed micro holes were chosen in order to keep active material removal at a minimum. To evaluate the effect of structured graphite anodes on the electrochemical properties of lithium-ion batteries, test cells were manufactured and galvanostatically cycled at different current rates. Results show improvements in high-current performance which is expressed by an increased discharge capacity yield.