Joong Sun Park

Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes

Surface coating of cathode materials with Al2O3 has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al2O3-coated LiNi0.5Co0.2Mn0.3O2 and LiCoO2 cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al2O3-coated LiNi0.5Co0.2Mn0.3O2, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al2O3 coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al2O3-coated LiCoO2, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from surface coatings to dopants, which is not observed for LiNi0.5Co0.2Mn0.3O2. As a result, Al2O3-coated LiCoO2 annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.

Role of Cr3+/Cr6+ redox in chromium-substituted Li2MnO3·LiNi1/2Mn1/2O2 layered composite cathodes: electrochemistry and voltage fade

The effect of redox-active Cr substitution on the electrochemistry and voltage fade of a lithium-rich “layered–layered” composite cathode material has been investigated. A series of Cr-substituted 0.5Li2MnO3·0.5LiNi1/2Mn1/2O2 powder samples (i.e., Li1.2Ni0.2−2/xMn0.6−2/xCrxO2, where x = 0, 0.05, 0.1, and 0.2) was synthesized via the sol–gel method. X-ray diffraction data confirmed the incorporation of Cr ions into the lattice structure. While similar initial charge capacities (∼300 mA h g−1) were obtained for all of the cathode samples, the capacity contribution from the Li2MnO3 activation plateau (at 4.5 V vs. Li) decreased with increasing Cr content. This finding suggests suppressed oxygen loss that triggers cation migration and voltage fade in subsequent cycles. Continued investigation revealed that the Cr substitution mitigates the voltage fade on charge but not discharge. The resulting insignificant effect of Cr substitution on mitigating voltage fade, in spite of decreased Li2MnO3 activation, is attributed to the additional instability caused by Cr6+ migration to a tetrahedral site, as evidenced by ex situ X-ray absorption spectroscopy. Our results provide the framework for a future redox active cation substitution strategy by highlighting the importance of the structural stability of the substituent itself.

Exploring Lithium-Cobalt-Nickel Oxide Spinel Electrodes for ≥3.5 V Li-Ion Cells

Recent reports have indicated that a manganese oxide spinel component, when embedded in a relatively small concentration in layered xLi2MnO3·(1-x)LiMO2 (M = Ni, Mn, or Co) electrode systems, can act as a stabilizer that increases their capacity, rate capability, cycle life, and first-cycle efficiency. These findings prompted us to explore the possibility of exploiting lithiated cobalt oxide spinel stabilizers by taking advantage of (1) the low mobility of cobalt ions relative to that of manganese and nickel ions in close-packed oxides and (2) their higher potential (∼3.6 V vs Li0) relative to manganese oxide spinels (∼2.9 V vs Li0) for the spinel-to-lithiated spinel electrochemical reaction. In particular, we revisited the structural and electrochemical properties of lithiated spinels in the LiCo1-xNixO2 (0 ≤ x ≤ 0.2) system, first reported almost 25 years ago, by means of high-resolution (synchrotron) X-ray diffraction, transmission electron microscopy, nuclear magnetic resonance spectroscopy, electrochemical cell tests, and theoretical calculations. The results provide a deeper understanding of the complexity of intergrown layered/lithiated spinel LiCo1-xNixO2 structures when prepared in air between 400 and 800 °C and the impact of structural variations on their electrochemical behavior. These structures, when used in low concentrations, offer the possibility of improving the cycling stability, energy, and power of high energy (≥3.5 V) lithium-ion cells.

The quest for manganese-rich electrodes for lithium batteries: strategic design and electrochemical behavior

Manganese oxides, notably γ-MnO2 and modified derivatives, have played a major role in electrochemical energy storage for well over a century. They have been used as the positive electrode in primary (single discharge) Leclanche dry cells and alkaline cells, as well as in primary and secondary (rechargeable) lithium cells with non-aqueous electrolytes. Lithiated manganese oxides, such as LiMn2O4 (spinel) and layered lithium–nickel–manganese–cobalt (NMC) oxide systems, are playing an increasing role in the development of advanced rechargeable lithium-ion batteries. These manganese-rich electrodes have both cost and environmental advantages over their nickel counterpart, NiOOH, the dominant cathode material for rechargeable nickel–cadmium and nickel–metal hydride batteries, and their cobalt counterpart, LiCoO2, the dominant cathode material in lithium-ion batteries that power cell phones. An additional benefit is that tetravalent manganese can be used as a redox-active and/or stabilizing ‘spectator’ ion in lithiated mixed-metal oxide electrodes. This paper provides an overview of the historical development of manganese-based oxide electrode materials and structures, leading to advanced systems for lithium-ion battery technology; it updates a twenty-year old review of manganese oxides for lithium batteries. The narrative emanates largely from strategies used to design manganese oxide electrode structures at the Council for Scientific and Industrial Research, South Africa (1980–1994), Oxford University, UK (1981–1982), and Argonne National Laboratory, USA (1994–2017); it highlights the worldwide evolution of ideas and recent trends to improve the design, stability, and electrochemical capacity of structurally integrated, manganese-rich electrode materials.