The structural and electrochemical dynamics of the electrode-electrolyte interphase of metal fluoride nanocomposite positive electrodes for lithium batteries.

摘要

Metal fluorides are attractive for use as positive electrodes in Li and Li-ion batteries because of their high gravimetric and volumetric energy densities. When synthesized into nanocomposites, these materials undergo conversion reactions and exhibit near theoretical specific capacity and good rate capability. Despite these positive attributes, metal fluorides nanocomposites generally exhibit unacceptable rates of capacity loss during cycling. This stands as a significant barrier to their realization as a viable battery technology. This thesis explored a candidate material, BiF3, and for the first time, the mechanisms by which metal fluoride nanocomposite positive electrode materials fail during cycling have been investigated. The chemistry of the electrode / electrolyte interface and its influence on the BiF3 material were of greatest interest.;Early in the course of study, it was discovered that the Bi0 metal produced through the discharge reaction of BiF3 was a catalytically active site for the electrochemical reduction of ethylene carbonate (EC) at potentials exceeding 2 V vs. Li/Li+. This potential range is well above the values typically observed on carbonaceous negative electrodes on which preferential reduction of electrolyte species yields insoluble phases. These ionically conducting layers are deemed solid-electrolyte interphases (SEI), and in the case of carbonaceous materials, they are necessary for enabling functionality of the electrode and preventing deleterious interactions with the electrolyte. Thorough electrochemical and spectroscopic examinations identified Li2CO3 as the predominant SEI species formed on Bi0 from EC. In stark contrast to carbonaceous materials, the presence of SEI on Bi0 was detrimental to the cycling performance of BiF3. Elaboration of this topic identified instability of the SEI during the charging process of the BiF3 and the formation of BiOxF3-2x in the fully charged state.;Electrolytes composed of linear organic carbonates, as opposed to cyclic organic carbonates, did not exhibit SEI formation, and a distinct improvement in the cycling performance of BiF3 nanocomposites was observed. Extending this concept, other straight-chained solvents including dinitriles and 3-alkoxypropionitriles were formulated into novel electrolytes with low additive concentrations. After proving their stability and functionality in a 4 V Li-ion configuration, these nitrile electrolytes were investigated with BiF3 nanocomposites. To date, the best long-term cycling performance of a BiF3 nanocomposite has been achieved using a dinitrile electrolyte.;The findings of this dissertation merit consideration of SEI formation in other metal fluoride conversion systems. The experimental designs serve as a platform for the exploration of the potentially complex and dynamic interactions of the electrolyte with metal fluoride nanocomposite electrodes during cycling.