Unusual Li2O Sublimation in Promoting Single Crystal Growth, Conversion and Recycling

Li2O is rarely used for cathode material synthesis due to its high melting point of 1438 °C. However, this work has discovered an unusual Li2O sublimation that occurs at high temperatures of 800-1000 °C, even under 1 atm pressure. Utilizing Li2O vapor sublimed directly from solids, we propose and validate a fundamentally new reaction mechanism that may find broad applications. This mechanism enables the synthesis of single crystals such as LiNi0.8Mn0.1Co0.1O2 (NMC811) or LiNi0.9Mn0.05Co0.05O2 (NMC90) without direct contact with Li2O salts. Moreover, Li2O vapor successfully converts spent polycrystalline NMC811 into well-segregated single crystals without any milling or post-treatment, further confirming the proposed Li2O sublimation mechanism. Li2O vapor, derived from Li2O solids, diffuses rapidly and reacts with other precursors, mimicking a molten salt environment without actual melting, which facilitates single crystal growth. The chemical lithiation process continuously drives Li2O to sublime, sintering crystals, as confirmed experimentally and theoretically. Single crystals derived from Li2O salts, using either fresh precursors or spent polycrystals, demonstrate outstanding cycling stability after 1000 cycles in full cells. The discovery of the Li2O sublimation phenomenon and its universal role in promoting single crystal growth provides a novel direction for broad single crystal synthesis, scale-up, and recycling.