Laser Battery with Outstanding Liquid Electrolyte Wetting and Performance

Abstract

The performance of lithium-ion batteries (LIB) can be negatively affected by poor liquid electrolyte wetting of electrode and separator materials causing insufficient cell reliability, reduced safety, and poor electrochemical performance at high charging and discharging rates (C-rate). These drawbacks become even more substantial for large electrode areas and with an increase in electrode thickness. Within the electrolyte filling process, homogeneous wetting of electrode and separator layers is strongly requested to guarantee maximal capacity and optimal performance under challenging operation. To accelerate and homogenize the electrolyte wetting of electrode materials, cost-efficient laser-structuring processes have been recently developed. It could be shown that micro-capillary structures can be easily formed into electrodes resulting in improved electrolyte wetting (Figure 1). In order to test the improvement in wetting of laser-structured electrodes with liquid electrolyte, lithium nickel manganese cobalt oxide (NMC) cathodes were tape-casted, laser-structured, and incorporated into pouch cells vs. graphite anodes with a cell size of 5 x 5 cm2. The assembled cells were electrochemically cycled immediately after the process steps electrolyte filling, vacuuming, and final sealing to investigate the microstructures ability for electrolyte transport in the electrode. By carrying out C-rate dependent cycling it could be shown that pouch cells incorporating laser-structured electrodes exhibited about 80 % of initial discharge capacity at 2C/2C charging/discharging rate while capacity retention of about 50 % was examined for cells with unstructured electrodes. Furthermore, the long-term cycling stability was investigated for pouch cells with unstructured and laser-structured standard electrodes at 1C/1C charging/discharging rate. Also, cells with structured electrodes exhibited > 80 % of initial discharge capacity for cycle numbers > 1000 while cells with unstructured electrodes failed for cycle numbers < 200. This improvement in cycling stability was attributed to drastically accelerated electrolyte wetting in the electrodes as well as to improved interfacial reactions kinetics provided by laser-generated electrode surface structures. In order to investigate the impact of change in surface topography onto the cycling behavior, Electrochemical Impedance Spectroscopy (EIS) was carried out. EIS spectra were analyzed within C-rate dependent and long-term cycling. In addition to electrode wetting, many efforts have been undertaken to also enhance the wettability of separators by applying ceramic coatings to polymeric base materials. To further accelerate the electrolyte wetting speed, we investigated a rather new technological approach for ultrafast femtosecond laser patterning of surface-coated separators. Micro-channel structures could be formed into the thin film coating of the separators resulting in improved wetting properties. In frame of post-mortem studies, Laser-Induced Breakdown Spectroscopy (LIBS) of electrode materials is performed in order to compare “Laser-Batteries” and standard lithium-ion cells after same cycling age. LIBS is a new approach for a rapid post-mortem analysis and it enables a fast chemical mapping of electrode materials to qualitatively investigate the evaluation of degradation mechanisms in LIB´s. Finally, these laser-based processes for electrodes and separators are under intense investigation for process up-scale from laboratory-scale to industrial roll-to-roll processes with respect to processing speed, electrode area, process compatibility, and processing costs. The outstanding advantages of high-capacity “Laser-Batteries” with laser-structured electrodes and separators have to be transferred to the industrial process. Advantages which result from superwicking of laser-modified battery materials are significantly reduced cell storage time-spans, enhanced cycling life-time, and improved high C-rate behavior due to enhanced interfacial reaction kinetics. Figure 1