Eungje Lee

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.

Insights into the dual-electrode characteristics of layered Na0.5Ni0.25Mn0.75O2 materials for sodium-ion batteries

Sodium-ion batteries are now close to replacing lithium-ion batteries because they provide superior alternative energy storage solutions that are in great demand, particularly for large-scale applications. To that end, the present study is focused on the properties of a new type of dual-electrode material, Na0.5Ni0.25Mn0.75O2, synthesized using a mixed hydroxy-carbonate route. Cyclic voltammetry confirms that redox couples, at high and low voltage ranges, are facilitated by the unique features and properties of this dual-electrode, through sodium ion deintercalation/intercalation into the layered Na0.5Ni0.25Mn0.75O2 material. This material provides superior performance for Na-ion batteries, as evidenced by the fabricated sodium cell that yielded initial charge-discharge capacities of 125/218 mAh g-1 in the voltage range of 1.5-4.4 V at 0.5 C. At a low voltage range (1.5-2.6 V), the anode cell delivered discharge-charge capacities of 100/99 mAh g-1 with 99% capacity retention, which corresponds to highly reversible redox reaction of the Mn4+/3+ reduction and the Mn3+/4+ oxidation observed at 1.85 and 2.06 V, respectively. The symmetric Na-ion cell, fabricated using Na0.5Ni0.25Mn0.75O2, yielded initial charge-discharge capacities of 196/187 μAh at 107 μA. These results encourage the further development of new types of futuristic sodium-ion-battery-based energy storage systems.

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.