Designing Ionic Pathways to Improve Overall Kinetics of Lithium ion Batteries

Abstract

Lithium-ion batteries (LIBs) have been considered as the most attractive energy storage systems. They have been used in a wide range of applications from portable devices to large-scale stationary energy storage systems (ESSs) due to their merits superior to other energy storage systems such as leadacid, Na-S, and redox flow batteries. They have higher cell voltages at > 3.6 V, higher efficiencies at >90 % and longer cycle life when compared with their alternatives. However, more research and development activities would be required before large-scale LIBs are widely spread for automotive and stationary fields. Power densities as well as energy densities should be improved higher with cost reduction. Many efforts have been devoted to not only discovering high-performance active materials but also enhancing ionic and electric conductivities. In order to increase ionic conductivities, smaller particles in a nanoscale level are favored to give larger surface area and shorter ionic diffusion length. Electric conductivities are enhanced by coating active materials with conductive materials such as metallic and carbonaceous compounds. The main topic of this thesis is how to design ionic pathways for improving overall kinetics of LIBs even if electric conductivity enhancement is involved. As the simplest strategy which can be used as a startup, nanosizing in the primary particle level was tried for LiMnPO4 olivine(LMP) to overcome its poor electric and ionic conductivities. By confining Mn3(PO4)2 precipitation on surface of a precursor seed of Li3PO4, the size of LMP particles is limited to less than 100 nm for a smaller dimension. Larger active area and shorter ionic transport length resulting from the nanosizing improved kinetic properties of LMP as a cathode material for LIB cells. When compared with LMP particles synthesized by a conventional co-precipitation method, the performances are recognized to be considerably enhanced. As the next strategy, the primary-particle-level nanosizing was evolved to the secondary-structure level of morphology control. Hollow structures with porous shells were designed for a conversionreaction-based anode material Fe3O4. The structure was chosen because hollow particles benefit from larger surface area on which active materials meet electrolyte, shorter pathways for lithium ions to pass through and voids within hollow shell providing buffer space during lithiation. The hollow structure was proved more beneficial in terms of electrochemical performances when compared with its nonhollow counterpart. Hollow void of ~80 nm diameter accommodated volume expansion during lithiation while the porous shell structure allowed lithium ions move through in a facile manner and enhanced accessibility to surface of the active materials. As the third strategy of morphological control following primary- and secondary-structure levels, higher level structures were designed for another conversion-reaction-based anode materials, Co3O4. Two different morphologies of Co3O4 (plate-like and rod-like) were achieved through pseudomorphic conversion, depending on macroscopic morphologies of parent metal-organic-frameworks (MOFs). Both Co3O4 nanostructures were composed of almost identical 10 nm-sized primary nanocrystals. These Co3O4 nanomaterials were utilized as an electrode in lithium ion batteries (LIBs), and their electrochemical properties were comparatively investigated. It was revealed that the different cyclability and rate capability are attributed to their different microstructures. The pseudo-monolithic integration of primary and secondary structures at higher level was the governing factor, which determined the electrochemical performances of the Co3O4 electrode. In addition to the morphology controls in nanoscales, crystallographic parameters of graphite as an anode material were controlled for the same purpose of improving ionic conduction or transport during faradaic reactions. To widen the ionic pathways inside active materials, the d-spacing of graphite increased from 0.3359 nm to 0.3395 nm by oxidizing natural graphite under a mild condition. Oxygencontaining functional groups were developed not only at edges but also on planes of graphite. Subsequent thermal reduction of the oxidized graphite eliminated a portion of the functional groups, but did not change d-spacing significantly. The enlargement of d-spacing reduced kinetic hindrance of lithium ion movement within the expanded graphite by reserving more space for the ionic transport route. In addition, the activation energy of lithium ion intercalation in expanded graphite are reduced by surface charge polarization of graphite induced by hydrogen bonds between oxygen atoms of carbonates in electrolytes and hydrogen atoms of surface functional groups. The expanded graphite showed higher delithiation capacities especially at high currents. By designing ionic pathways of electroactive materials, overall kinetics was enhanced, resulting in a much better improved electrochemical storage system.