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Improved Positive Electrode Materials for Li-ion Batteries


The introduction of the first commercially produced Li-ion battery by Sony in 1990 sparked a period of unprecedented growth in the consumer electronics industry. Now, with increasing efforts to move away from fossil-fuel-derived energy sources, a substantial amount of current research is focused on the development of an electrified transportation fleet. Unfortunately, existent battery technologies are unable to provide the necessary performance for electric vehicles (EV's) and plug-in hybrid electric vehicles (PHEV's) vehicles at a competitive cost. The cost and performance metrics of current Li-ion batteries are mainly determined by the positive electrode materials. The work here is concerned with understanding the structural and electrochemical consequences of cost-lowering mechanisms in two separate classes of Li-ion cathode materials; the LiMO2 (M = Ni, Mn, Co) layered oxides and the LiMPO4 olivine materials; with the goal of improving performance.

Al-substitution for Co in LiNizMnzCo1-2zO2 ("NMC") materials not only decreases the costly Co-content, but also improves the safety aspects and, notably, enhances the cycling stability of the layered oxide electrodes. The structural and electrochemical effects of Al- substitution are investigated here in a model NMC compound, LiNi0.45Mn0.45Co0.1-yAlyO2. In addition to electrochemical measurements, various synchrotron-based characterization methods are utilized, including high-resolution X-ray diffraction (XRD), in situ X-ray diffraction, and X-ray absorption spectroscopy (XAS). Al-substitution causes a slight distortion of the as-synthesized hexagonal layered oxide lattice, lowering the inherent octahedral strain within the transition metal layer. The presence of Al also is observed to limit the structural variation of the NMC materials upon Li-deintercalation, as well as extended cycling of the electrodes.

Various olivine materials, LiMPO4 (M=Fe,Co) are produced using a custom-built spray pyrolysis system. Spray pyrolysis is a simple, inexpensive, and scalable method used to produce highly uniform and phase-pure particle materials. The materials are synthesized here as porous, carbon-coated spherical particles with micron-sized diameters and nanoscale primary particles. The LiMPO4 (M=Fe,Co) olivine electrodes display exceptional electrochemical properties, in terms of high discharge capacities, rate capability, and cycling stability. The excellent performance is due to the particle morphologies that include a hierarchical pore structure and conductive carbon network throughout the particles. This allows liquid electrolyte penetration into the particle interiors, thus limiting the necessary solid-state diffusion distances, as well as efficient charge transfer and collection.

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