Mn-Based Olivine Cathode Materials for High-Performance Li-Ion Batteries

Abstract

Energy sources are important for the way of life in modern society, but most of the energy demand now depends on the power of nuclear and fossil fuels. This will eventually accelerate global warming and seriously deplete natural resources. As a result, it is important to develop efficient, environmentally friendly, and safe energy sources such as fuel cells and solar cells, and the development of efficient energy storage systems for storing these eco-friendly energy sources is also becoming an important issue. Among the various energy storage systems, lithium-ion batteries are attracting attention as the most realistic energy source because they have the charm of high energy density and durability. Because the performance of a battery is usually determined by electrode materials, people have been looking for a breakthrough challenge to overcome the limitations of the known materials. Conventional cathode materials such as lithium transition metal oxides (LiMO2, M = transition metals) possess intrinsic chemical instability at overcharged state. They release oxygen from the crystal structure or experience irreversible phase transformation at elevated temperature, which consequently raises safety concerns during operation. In this respect, numerous studies have been carried out in order to find a safe and stable cathode material. Among many candidates, phosphate materials have been considered as the best candidate of energy storage system for large-scale applications due to its high structural stability and safety by strong P-O covalent bonding, potentially low production cost, high energy density, and excellent cyclability. Olivine structured lithium iron phosphate (LiFePO4) has been extensively studied as a promising candidate for cathode materials of lithium-ion batteries due to its high theoretical capacity, superior structural stability, environmental benignity, and low cost. However, the LiFePO4 has relatively low redox potential (3.4 V vs. Li+/Li), which results in low energy density limiting its wider application to the market. For this reason, isostructural LiMnPO4 with higher redox potential (4.1 V vs. Li+/Li) has emerged as an alternative material for LiFePO4. Therefore, in my thesis, I focused on design of novel Mn-based olivine cathode materials (LiMn0.8Fe0.2PO4) and comprehensive analysis of the reaction mechanism of Mn and Fe in LiMn0.8Fe0.2PO4 electrodes during battery operation. In Chapter 1, the issues to overcome the limitation of olivine cathode materials for practical application are briefly introduced, mainly dealing with the development of Mn-based olivine cathode materials. In Chapter 2, electrochemically efficient micro/nano-structured LiMn0.8Fe0.2PO4 electrodes were designed by controlling synthesis parameters. I demonstrated that control of the size and shape of the LiMn0.8Fe0.2PO4 crystals as well as of the particles tendency toward oriented agglomeration (mesocrystal) is possible by applying synthesis route. Furthermore, performance enhancement of LiMn0.8Fe0.2PO4 has been realized by a morphology tailoring from ellipsoidal-shaped mesocrystals into flake-shaped mesocrystals. The origin of the enhanced electrochemical performance is investigated in terms of the primary particle size, porosity, anti-site defect concentration, and secondary particle shape. I believe that this work provides one of the routes to design electrochemically-favorable meso/nano-structures, which is of great potential for improving the battery performance by tuning the morphology of particles at the multi-length scale. A thorough understanding on the electronic structure of LiMn0.8Fe0.2PO4 can provide a guide to design high performance multi-transition-metal olivine materials, since the electronic structure comprises the electrochemical potential and structural stability of cathodes during battery operation. Thus, in Chapter 3, in order to investigate the electronic-structure effects of each transition metal (Mn and Fe) on the electrochemical performance, I performed synchrotron-based soft and hard x-ray absorption spectroscopy (sXAS and XAS), and quantitatively analyzed the changes of the transition-metal redox states in the carbon-coated LiMn0.8Fe0.2PO4 electrodes during the electrochemical reaction. I believe that our comprehensive as well as complementary analyses using ex situ sXAS and in situ XAS can provide clear experimental evidence on the reaction mechanism of LiMn0.8Fe0.2PO4 electrodes during battery operation. In chapter 4, the kinetic processes during lithiation/delithiation reaction of LixMn0.8Fe0.2PO4 were investigated through in situ x-ray diffraction (XRD) and in situ electrochemical impedance spectroscopy (EIS) combined with galvanostatic intermittent titration technique (GITT), by which unprecedented insights on the phase propagation and sluggish kinetics of LiMn0.8Fe0.2PO4 (LMFP) cathode materials are delivered. In situ analyses on the carbon-coated LMFP mesocrystal disclosed that the phase-propagation mechanism of LMFP differs during lithiation/delithiation process, and the sluggish kinetics of LMFP mesocrystal and resultant limitation of obtainable discharge capacity is featured from significant reduction of apparent Li+ diffusivity during cycling through the region governed by Mn redox reaction. Being an in-depth characterization on the in operando kinetics of LMFP mesocrystal, I believe that this work provides fundamental understandings needed for proceeding to high-performance Mn-based olivine cathodes. Finally, in Appendix 1, the graphene-wrapped LiFePO4 (LiFePO4/G) was introduced as a cathode material for Li-ion battery with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO4 and a lithium precursor resulted in highly conducting LiFePO4/G composites, which are featured by ~70-nm sized LiFePO4 crystallites with robust connection to external graphene network. This unique morphology enables all LiFePO4 particles to be readily accessed by electrons during battery operation, leading to remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO4/G.