Effect of 3-dimensional electrode structure on electrochemical performance of graphite anodes for lithium-ion batteries

Abstract

To fully utilize renewable resources, lithium-ion batteries (LIBs) with high energy and power densities have received great attention. With the increase of their use in industries such as electric vehicles (EVs), the demand for high performance LIBs is increasing steadily. LIBs are comprised of anodes, cathodes, electrolytes, and separators. Among them, since anode/cathode materials directly affect the cell performance such as specific capacities (mAh g-1) and energy densities, numerous studies have been conducted to search for alternative electrode materials. For example, Si anode material has been considered the most promising post-anode material due to its high theoretical capacity of 3,579 mAh g-1. However, it is still used in a limited composition for commercial graphite (Gr) anodes due to its severe degradation of Si particles during cycling. Considering the configuration of current anodes, a straightforward approach to enhance the cell performance is the modification of electrode morphology to reduce internal resistance values such as electrical resistance (Re) and electrolyte resistance in electrode pores (Rion). Since the mass transport rate is limited during fast charging and in thick electrodes, a study on the electrode architectures that are favorable to supply lithium ions is highly promising. In this study, the effects of 3-dimensional (3d) electrode architectures on the physical and electrochemical properties of Gr anodes are investigated. Since commercial electrodes consist of densely packed electrode particles, they have high Rion values that decrease the capacity utilization at high current rates or/and in thick electrodes. Two types of structured Gr anodes are fabricated to improve the cell performance of fast-charging Gr anodes and thick Gr anodes. For fast-charging Gr anodes, spherical polymer particles that are decomposed above 450 °C in an argon atmosphere to form lots of pores in Gr electrodes are employed as a pore-forming agent. They can increase electrode porosity even after being pressed to ensure physical contact between conductive carbons and electrode particles. The pore-structured Gr anodes provide new electrolyte impregnation pathways and reduce ionic transport length, preventing the depletion of lithium ions even at high current rates. As a result, the typical Gr delivers 92 mAh g-1 after 100 cycles at a current rate of 2C, whereas pore-structured Gr shows 233 mAh g-1. Such improved electrochemical properties are ascribed to the decrease in ionic transport length. To increase the capacity utilization of thick Gr anodes, the binder carbonization and laser-structuring are employed to reduce electrode tortuosity and increase electrode porosity. Since the conductive carbons are densely positioned among electrode particles, they disturb electrolyte penetration and ionic migration during charging/discharging. Therefore, the Rion value can be reduced by utilizing carbonized electrode binders instead of conductive carbons as conductive agents. The Rion value of the carbonized Gr anode is smaller than that of the typical thick Gr anode (it decreased from 125.5 Ω to 64.3 Ω) due to the elimination of conductive carbons. For further improvement of ionic migration, the carbonized Gr anode is laser-structured to reduce ionic conduction length. As a result, the Rion value decreases to 47.7 Ω, increasing the capacity utilization of thick Gr electrodes at a current rate of 0.2C. The most efficient research method is to investigate the internal resistance components in LIBs, and then improve the largest resistance value. However, in the LIBs, the by-products from electrolyte decomposition called solid electrolyte interphase (SEI) affect the electrochemical behavior. The SEI layer functions to inhibit further decomposition of the electrolyte, but raises the electrical resistance and causes interfacial resistance. As the cycle number increase, the SEI layer gradually thickens and by-products cover the surface of anodes, raising internal resistance. Since SEI layers complicate the battery system, it is difficult to interpret electrochemical impedance spectroscopy (EIS) results and quantify resistance values. To further investigate internal resistance values such as Re and Rion values, used Gr anodes are employed to confirm the effect of SEI on ionic behaviors.