Controlling Dendrite Growth in Graphite Anodes Using Potassium Electrolyte Additives in Li-Ion Batteries

• Main Author: Moharana, Sanghamitra
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2021
• Subjects: Electrolytes; Graphite; Lithium; Potassium

Fast charging of lithium-ion batteries is a critical requirement in the endeavour towards efficient operation of highly demanding electric vehicles. However, fast charging is one of the conditions that can provoke metallic lithium deposition and its subsequent growth to dendrites on graphite anodes, giving rise to potential safety risks. The likelihood for lithium dendrite growth becomes greater when the operating temperature falls below ambient temperature ranges. Additionally, the exfoliation of the graphite is another degradation mechanism in the presence of low temperature electrolyte solvents -such as propylene carbonate - leading to capacity fade. In this regard, an electrolyte additive needs to be employed to mitigate the dendritic growth and graphite exfoliation, hence enhancing the performance and lifetime of the battery. This research work implements an inorganic electrolyte additive, KPF6 as a mitigation strategy to enable the graphite anode to endure fast charging and low temperature operation. The key aspect of this work is to determine the optimised electrolyte and the underlying mechanism behind the mitigation process of Li dendrite growth - as well as graphite exfoliation. The electrochemical characterisation in collaboration with the post-mortem analysis provides an insight into the microstructural and compositional evolution of the anode, with respect to electrolyte composition. In order to obtain qualitative and/or quantitative information regarding the cycled graphite anode and the developed electrode/electrolyte interface, post-mortem characterisation techniques such as SEM, EDX, XRF, XRD, XPS, and SIMS are employed. This work also investigates the internal resistance developed by the additive incorporation throughout using an EIS study. This research work reports the formation of a robust SEI layer by introducing a KPF6 electrolyte additive. The optimal 0.001M KPF6 concentration effectively inhibits the growth of Li dendrite at 2C charging rate in comparison with commercial RD281 electrolyte. Firstly, KPF6 addition produces a thin LiF-rich SEI layer on graphite, which blocks the electron transfer at defect sites. Secondly, K+ resides at the defect sites such as particle boundaries due to its fast diffusion rate and blocks the incoming Li+, thereby restricting the growth of Li dendrites. The combined processes obstruct the incoming Li+ and electrons, hence inhibiting the reduction of Li+ to Li0 for further Li deposition. This improves the performance of the cell, which better directs the transport of Li+ through the thin, durable, and low resistance LiF-rich SEI layer. This study reveals the presence of metallic potassium dendrites along with decreased LiF concentration in the SEI layer, which fails to inhibit the dendritic growth as the concentration of KPF6 additive is increased in the electrolyte. The development of low temperature ternary electrolyte containing PC as co-solvent is also investigated together with a KPF6 additive. An optimised concentration of 10 vol % PC with 0.1M KPF6 enhances the electrochemical performance by suppressing the exfoliation of graphite and the growth of the dendrite. 0.1M KPF6 generates a strong LiF-rich SEI layer on graphite, which reduces the developed over-potential, thereby decreasing the likelihood for dendrite growth. The fast diffusion of K+ assists in suppressing PC decomposition, hence ensuring unexfoliated, dendritic-free graphite. This comprehensive in-depth study of the impact of KPF6 additive shows the beneficial effects of electrolyte additives for improving electrochemical performance and advancing the safety of LIBs.