New Regime for Lithium-Manganese-Rich Layered Oxides—Enhancing Oxygen Redox Reversibility and Longevity via Strategic Voltage Window Regulation

Lithium-manganese-rich layered oxide (LLO) has received considerable attention for its high energy density at a low cost, owing to its competitive price due to high manganese content and the participation of both oxygen and manganese in redox reactions over a wide voltage window.¹ However, the rapid fading in both capacity and voltage significantly diminishes these advantages, posing formidable challenges to the practical application of LLO. This rapid degradation in performance has been primarily attributed to the irreversibility of oxygen redox and structural rearrangement, both of which are closely associated with the operating voltage window.²
To mitigate the rapid energy density fade driven by the irreversibility of oxygen redox, we adopted the practical and effective strategy of regulating oxygen redox by confining the voltage window. The upper voltage limit was lowered to 4.3 V vs. graphite to reduce the maximum or average oxidation state of lattice oxygen during cycle, as an increase in the fraction of oxidized oxygen makes the structure more prone to structural rearrangement and degradation. Conversely, the lower voltage limit was extended to 2.0 V vs. graphite to prevent the incomplete reduction of oxidized oxygen that may result from substantial shift of oxygen reduction to lower potential. We discovered that differences in the fraction of oxidized oxygen remaining within the lattice, driven by the operating voltage range, lead to varying levels of oxygen redox reversibility, which predominantly influences the extent of nanovoid formation and structural densification.
Our refined protocol (Protocol 4320, 2.00-4.30V vs. graphite) unprecedentedly achieved a volumetric energy density of 550 Wh/L with 93.2% retention after 400 cycles in large-format battery packs that exceeds 40 Ah. Notably, despite the partial loss of its capacity, voltage controlled LLO still retains its ability to provide sufficiently high energy density at a reduced cost. This solidifies the positioning of LLO as an eminent candidate for volume market applications, which are essential for bridging the polarized gap between high-performance premium models and cost-effective budget models. Consequently, voltage window regulation not only effectively addresses the challenge of rapid degradation in performance but also highlights its potential to contribute to a well-structured price range and lineup tailored to the specific market demands. This work demonstrates the potential of voltage window regulation to extend the lifespan and reliability of LLO, positioning it as a commercially viable solution for volume market applications.


References
G. Assat et al. Nat Energy, 3, 373–386 (2018).
M. Zhang et al. Nat Rev Mater, 7, 522-540 (2022).