High-Capacity Mn-Based Cation-Disordered Rocksalt Cathodes

The discovery of Li-excess cation-disordered rocksalt (DRX) cathodes greatly enlarges the design space beyond the layered NCM-type rocksalt chemistries. More importantly, this new strategy enables high capacity Co and Ni-free cathodes, which is a requirement for the Li-ion industry to grow to the Terra-Wh scale that is needed for the growing electric vehicle and utility applications. Mn-based DRX oxyfluorides are especially promising owing to their large energy densities and good cyclabilities as well as the low cost and earth abundance of Mn. Herein, we synthesize, characterize, electrochemically test and model four well-chosen compositions within the Li–Mn–O–F DRX chemical space with different Li content and F content: Li 1.333 Mn(III) 0.667 O 1.333 F 0.667 (HLF67), Li 1.333 Mn(III) 0.5 Mn(IV) 0.167 O 1.5 F 0.5 (HLF50), Li 1.333 Mn(III) 0.333 Mn(IV) 0.333 O 1.667 F 0.333 (HLF33), and Li 1.25 Mn(II) 0.167 Mn(III) 0.583 O 1.333 F 0.667 (LLF67). While all compositions tested achieve higher than 200 mAh/g initial capacity the material with high Li-excess (1.333 per formula unit, Li x Mn 2- x O 2- y F y ) and moderate fluorination (0.333 per formula unit) achieves 349 mAh/g initial capacity and 1068 Wh/kg specific energy. Higher fluorination (0.667 per formula unit) can activate Mn 2+ /Mn 4+ redox and thereby balance capacity with cycle life, achieving 256 mAh/g (822 Wh/kg) initial capacity (specific energy) with more than 85% retained after 30 cycles even upon charging to 5.0V. It can be concluded that the Li-site distribution plays a more important role than the metal-redox capacity in determining the initial capacity, whereas the metal-redox capacity is more closely related to the cyclability of the materials. We apply these insights and generate a capacity map of the Li–Mn–O–F chemical space, Li x Mn 2- x O 2- y F y (1.167 ≤ x ≤ 1.333, 0 ≤ y ≤ 0.667), which predicts both the accessible Li capacity and Mn-redox capacity. This map allows to design compounds which balance high capacity with good cyclability. Reference Lun, Zhengyan, et al. "Design principles for high-capacity Mn-based cation-disordered rocksalt cathodes.” Chem (2019), DOI: 10.1016/j.chempr.2019.10.001, in press.