Core-Shell Materials as Positive Electrodes in Lithium-Ion Batteries
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Electric vehicles with lithium-ion batteries as their energy storage technology have been sold by major automotive manufacturers for the past few years, but with limited sales. Sales are poor because of the high purchase price, short driving range and limited lifetime of the battery pack. Current electric vehicles have a driving range of 30 to 265 miles, battery warranty of 5 to 8 years and a price of 30,000 to 75,000 USD. The choice of electrolyte and positive electrode materials impact cost, lifetime and energy density. In this work the synthesis and characterization of core-shell positive electrode materials were explored. Core-shell materials have a different composition at the surface of the particles, where oxidative degradation of the electrolyte occurs, than the bulk. This structure was previously purposed for increasing lifetime and energy density without significant cost. Several core-shell and gradient composition materials were synthesized by first making a core-shell mixed transition metal hydroxide precursor in a continuously stirred tank reactor and then blending with Li2CO3 and sintering at high temperatures. It was necessary to use multiple characterization techniques (XRD, SEM and EDS) to verify that the precursors had the desired core-shell structure and that the core and shell had the proper compositions. It was demonstrated that the composition of the core and shell phases and the thickness of the shell could easily be controlled. A technique using absorption in X-ray diffraction patterns was developed to estimate the thickness of the shell layer, which was validated with spatial EDS measurements and a constant growth model. The core-shell structure was maintained after sintering except that cobalt diffused from the shell phase into the core creating an approximately homogeneous cobalt concentration throughout the particles. Lithium uptake into the core and shell phases during sintering was different than what was observed when those phases were synthesized as single-phases not as core-shell samples. Electrochemical data was collected using ultra high precision chargers which showed that the core-shell materials had less charge endpoint capacity slippage than the core only material, suggesting less electrolyte oxidation at the positive electrode.