Low Temperature Synthesis of Ni- and Mn-Based Cathode Active Materials

Le résumé fourni par la source

Layered cathodes, such as LiCoO 2 (LCO) and LiNiO 2 (LNO), are usually synthesized at high temperatures ≥ 700 °C by conventional solid-state process. We recently proposed a practical synthesis approach called the “hydroflux process” to synthesize a highly crystalline layered LCO at low temperatures of ≤ 300 °C within a short duration of ≤ 1 h. [1] A series of XRD studies revealed that the layered LiCoO 2 formation occurs even at 150 °C. We also confirmed that the water molecules in the molten mixed hydroxides significantly accelerate the crystal growth of layered LCO. Under strong alkaline conditions, the soluble HCoO 2 – anion species enable the low-temperature fast crystal growth of LiCoO 2 without spinel Co 3 O 4 formation. On the other hand, crystal growth of the layered LNO does not proceed under the hydroflux condition. Since the stable region of the Ni 3+ species exists above the oxygen line in the Pourbaix Diagram, the soluble HNiO 2 – cannot be oxidized to Ni 3+ species. Hence, we conducted a hydroflux process for the layered LNO synthesis at 300 °C using a nickel oxyhydroxide (NiOOH) as an alternative Ni source. Even though the layered LNO was successfully obtained as a primary product, the XANES spectra suggested that the Ni 3+ species are partially reduced to Ni 2+ . The result shows that the water molecules in the system simultaneously reduce the Ni 3+ species to Ni 2+ . The Mn system behaves differently than the Co or Ni system. Rocksalt MnO provides the zigzag-layered orthorhombic-LiMnO 2 (o-LMO), monoclinic LiMnO 2 (m-LMO), and layered Li 2 MnO 3 mixed phase with poor crystallinity. The o-LMO is obtained under an inert atmosphere using Mn 2 O 3 as a Mn source. We also found that the o-LMO formation proceeds even below 100 °C. Since the o-LMO synthesized at 80 °C showed higher crystallinity than that of 300 °C, we think the water content in the starting materials is the primary parameter for the crystal growth process of o-LMO, rather than temperature. We also synthesized o-LMO at 300 °C under Ar flow containing 30% moisture. The obtained o-LMO showed significant improvement in crystallinity, as we expected.

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