Structure-Property Relationships in LiFePO4 as a Cathode Material for Lithium-Ion-Batteries
For lithium ion batteries used in numerous mobile electrochemical energy storage devices currently graphite in various forms is used as anode material, as cathode material LiCoO2 is still most popular. Because of limited resources of the raw materials, safety issues and not yet perfect performance alternative materials are actively searched. Sulfur appears to be the ultimate goal, poor electrical conductivity of the active material and solubility issues of the reduced form have so far prevented widespread application.
Dioxygen (in a lithium-air-accumulator) has been suggested years ago as an even more promising cathode material from a theoretical point of view, numerous practical issues are very far from being resolved. Instead various lithium metal polyanions of the type LiMeXO4 have been proposed, LiFePO4 among them. An electrode potential of lithium intercalation (during discharge) around 3.5 V vs. Li/Li+ and a gravimetric energy density of about 170 mA·h-1 make this material rather attractive. Raw material is in ample supply, numerous methods for preparation have been proposed. Unfortunately this material is a very poor electrical conductor. Thus substantial amounts of conducting agents like e.g. graphite are similar materials have to be added. As an alternative or in support of these additional structural modifications like surface coating (core-shell-structure) with non-stoichiometric compounds with higher ionic conductivity or nanostructuring limiting the length of ionic pathways or with intrinsically conducting polymers have been suggested.
PLANNED RESEARCH
In the research project proposed here these approaches will be evaluated quantitatively and systematically. Particular attention will be paid to relationships between type and mass fraction of added graphitic material and measurable conductivity and the respective relationship between type, thickness and composition of coating and obtained conductivity. The same approach will be employed with polymer-coated materials. In the former case percolation thresholds will be established.
The degree of crystallinity of carbon and related graphitic materials will affect the electrochemical performance of LiFePO4. This can be evaluated by Raman spectroscopy since there are two bands for the carbon or graphite materials. Following the effect of this structural detail of the carbon materials on the LiPO4 particles on reversible capacity, rate capability and cycling behavior of LiFePO4 will be systematically studied. The relationship between carbon or graphite content and the electrochemical performance of LiFePO4 is also very important factor. How to detect or analyze efficiently the carbon amount in the composite of LiFePO4 with carbon is another challenging problem since the carbon materials will exist at different crystallization degree. If the correlation can be understood, the inherent factors on electrochemical performance of LiFePO4 as a cathode material for lithium ion batteries can be understood better.
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