Enhancement of ionic diffusion and electrochemical performance by laser structuring of electrodes for advanced lithium-ion battery

Abstract

The thickness and porosity of electrode are critical parameters governing the performance of lithium-ion batteries. An increase of active material in the electrode by either increasing thickness or increasing density through strong calendaring results in a higher energy density, which however causes a simultaneous drop of power density due to poor ionic diffusion from the surface to deeper regions within the electrode. A thick and dense electrode can also lead to the capacity loss and decrease of productivity by poor wettability. In other words, there exists a trade-off relationship between the energy and power densities for increasing electrode thickness or decreasing electrode porosity, which makes the optimization of electrode thickness and porosity be a difficult problem. In this dissertation, laser processing of lithium-ion battery electrodes (NCM, LCO, graphite, and silicon) as a method to improve the electrode performance is investigated for advanced lithium-ion battery. The study consists of two parts: (1) laser structuring of electrode using an ultra-short pulsed laser and (2) selective rapid surface treatment using a high-energy nanosecond pulsed laser. First, uniform grooves were formed by femtosecond laser structuring with little thermal and chemical side effects on NCM, LCO, and graphite electrodes which were much thicker (100~700 μm) and denser (porosity 26~50 %) than conventional electrodes. The grooves contributed to enlarging ionic diffusion pathways, shortening diffusion path length from the electrode surface to the current collector, and reducing tortuosity thus enhancing ionic conduction within the electrode. Additionally, electronic conduction was improved due to local carbonization around the laser-irradiated regions. These improvements in laser-structured electrode resulted in the simultaneous enhancement of energy and power densities despite the significantly increased thickness and density of electrodes as compared with the conventional electrodes. A possibility of faster charge than conventional electrodes was also confirmed. The volumetric and gravimetric energy densities can also be improved by applying laser structuring to thick electrodes because a higher loading of active materials becomes possible while reducing inactive materials such as repeatedly stacked separator and current collector. It is shown that the performances of laser-structured electrodes depend on the depth and pitch of grooves and guidelines for laser structuring considering the operational conditions of lithium-ion battery are suggested. In silicon based electrode, laser treatment of dense surface layer produced during manufacturing processes using high energy pulsed laser is investigated. The removal of dense surface layer by laser treatment contributed to fast wetting with electrolyte, improvement of ionic conduction, and release of stress by volume change during cycles, thus enhancing cycle life and rate capability. Since an unfocused laser beam with a large spot diameter was used for the surface treatment, this process can be done with a high throughput. The developed laser surface treatment method is considered to be a practical approach for industry where a stringent control of manufacturing processes to prevent the formation of dense surface region is difficult. The research results in this dissertation demonstrate that laser processing is a viable approach to surpass the limitations in electrode structure of conventional lithium-ion battery, and the details on application methods of laser processing for next generation batteries.