Transition metal fluorides as lithium ion cathodes

Thesis

More energy dense lithium ion batteries are essential for the production of long-range electric vehicles and all-electric aircraft. These next generation batteries will require high capacity cath- ode materials that move beyond the lithium intercalation chemistry of current electrodes. On paper, transition metal fluorides are among the most promising cathode materials, combining high electrode potentials (2.6 V to 3.5 V vs. Li+/Li) with three to six times the theoretical capacity of conventional cathode materials. However, their commercial application has been hindered by their inability to cycle in a truly energy dense format (i.e. at high active material concentra- tion) and ultimately the inadequate understanding of their electrochemical capabilities and limitations. Both of these issues have been largely resolved as a result of this DPhil project. First I design a materials system that is practically ideal for reversible high-capacity cycling as well as detailed mechanistic study. This is achieved through the development of a new colloidal synthesis method for single-crystalline, monodisperse transition metal fluoride (FeF2, CoF2) nanorods. Size and shape control are demonstrated, and a chemical reaction pathway and nu- cleation and growth mechanism are proposed. Next I present an ionic liquid electrolyte (1 molal lithium bis(fluorosulfonyl)imide in N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide) that forms a stable solid electrolyte interphase, prevents the fusing of particles, and consequently precludes the major failure mechanisms associated with transition metal fluoride cathodes. The use of this electrolyte with the iron(II) fluoride (FeF2) nanorod system results in near theoretical capacity (570 mA h g−1) on discharge and charge, with extraordinarily stable cycling (>90% ca- pacity retention after 100 cycles). This stability is maintained for over 200 cycles at relatively high cycling rates (C/2) and temperatures (50 ◦C). By combining the unique vantage point afforded by this novel materials system with the morphology preserving qualities of our new electrolyte, I am further able to characterize the discharge/charge mechanism with an unprecedented level of detail. High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that that provide a new and comprehensive understanding of the discharge/charge (conversion) reaction mechanism. This new mechanistic understanding defines the structural and chemical origins of reversible cycling and cell failure in transition metal fluorides and unearths new concepts such as an inherently high discharge rate capability in FeF2.

Authors: Xiao, AW

• Alternative title: Understanding battery mechanisms and performance using monodisperse nanocrystal samples
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: transition metal fluoride; nanoparticle; transmission electron microscopy; reaction mechanism; lithium ion battery; conversion cathode; ionic liquid electrolyte
• Subjects: Electrochemistry; Energy Storage; Materials Science