New anions for lithium battery electrolytes
Doctoral thesis, 2011
Energy storage is crucial to realize a new global energy paradigm based on renewable energy sources. Batteries are well suited this need, with the advantages of being mobile and potentially environmentally benign. Li-ion batteries have revolutionized the market of portable electronics and are now being implemented as back up electricity for grid storage, and for transportation – powering hybrid and electric vehicles. However, for large scale applications, the safety of current Li-ion batteries is an obstacle. The safety problems of the Li-ion cell is inherent the reactivity of the choice of materials. Of particular concern, is the flammable organic electrolyte with the thermally unstable lithium hexafluorophosphate (LiPF6) salt. New salt alternatives must, in addition to high thermal stability, combine high electrochemical stability with facile Li+ transport.
In this thesis, alternatives to LiPF6 are explored by a combination of computational and spectroscopic techniques. The vertical transition energy, ΔEv, and ion pair dissociation energy, Ed, are computational approaches to the electrochemical stability of anions and the Li+–anion interaction strength, respectively. From computationally predicted structures of anions and ion pair configurations, simulated vibration spectra can be compared with experimental Raman spectroscopic results to probe the molecular level environment of electrolytes. The lithium salts investigated can be categorized according to 1) their approximate geometric characteristics; linear, planar, or spherical, and 2) their substituents; –F, –CF3, or –C≡N groups.
The approach taken here is extremely idealized compared to the complex nature of real battery electrolytes; this is both a weakness and a strength. It will be further evaluated and modified based on future experimental results – implementation in lithium battery electrolytes.