Understanding and mitigating degradation in Li-ion batteries

Abstract

The market for Li-ion batteries has seen unprescedented growth in recent years due to the adoption of electric vehicles (EVs) and growth of grid-level energy storage. For these applications to be sustainable, inexpensive and long-lasting Li-ion batteries are required. This thesis considers LiFePO4 (LFP) as a positive electrode material for use in long-lifetime Li-ion batteries. Already a commercially used material, LFP is seeing a renewed interest in many applications due to the cost and relative scarcity of commonly used transition metals in Li-ion batteries, Ni and Co.

Initial studies of LFP/graphite cells considered the impact of water contamination and different electrolyte additives on lifetime, and an optimal electrolyte composition was determined. Isothermal microcalorimetry techniques were used to rank the lifetime of cells with different electrolyte additives.

Next, different approaches were taken to improve the lifetime of LFP/graphite cells, including considering the surface area of LFP, different Li salts in the electrolyte, and different graphite materials. Combining the results of these studies led to an LFP cell with greatly improved capacity retention. Isothermal microcalorimetry techniques were developed to observe parasitic reactions separately at the positive and negative electrodes, and to infer the degree of “cross-talk” reactions in the cell.

Finally, the storage performance, gas evolution, and parasitic heat flow for Li-ion cells with different positive electrodes, negative electrodes, and electrolytes were studied. The results of these experiments highlighted the complex interactions that occur between different components of the cell. In LFP cells, capacity loss was correlated with the reactivity of the negative electrode.

The results presented in this thesis demonstrate significant lifetime improvements for LFP/graphite cells by targeting different cell components. Additional insights into the role of parasitic reactions on the lifetime of Li-ion cells have been developed. This work should contribute to the future development of Li-ion cells with extremely long lifetimes.