Lithium- and manganese-rich (LMR) layered oxides are promising high-energy cathodes for next-generation lithium-ion batteries, yet their commercialization has been hindered by a number of performance issues. While fluorination has been explored as a mitigating approach, results from polycrystalline-particle-based studies are inconsistent and the mechanism for improvement in some reports remains unclear. In the present study, we develop an in situ fluorination method that leads to fluorinated LMR with no apparent impurities. Using well-defined single-crystal Li_1.2Ni_0.2Mn_0.6O₂ (LNMO) as a platform, we show that a high fluorination level leads to decreased oxygen activities, reduced side reactions at high voltages, and a broadly improved cathode performance. Detailed characterization reveals a particle-level Mn³⁺ concentration gradient from the surface to the bulk of fluorinated-LNMO crystals, ascribed to the formation of a Ni-rich Li_zNi_xMn_2–xO_4–yF_y (x > 0.5) spinel phase on the surface and a “spinel-layered” coherent structure in the bulk where domains of a LiNi_0.5Mn_1.5O₄ high-voltage spinel phase are integrated into the native layered framework. This work provides fundamental understanding of the fluorination effect on LMR and key insights for future development of high-energy Mn-based cathodes with an intergrown/composite crystal structure.

中文翻译：

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氟化对富锂和富锰层状氧化物阴极的影响

富锂和富锰（LMR）层状氧化物是下一代锂离子电池的有前途的高能阴极，但其商业化受到许多性能问题的阻碍。虽然氟化已被探索作为一种缓解方法，但基于多晶颗粒的研究结果并不一致，并且一些报告中的改进机制仍不清楚。在本研究中，我们开发了一种原位氟化方法，可产生不含明显杂质的氟化 LMR。使用明确的单晶Li _1.2 Ni _0.2 Mn _0.6 O ₂ (LNMO)作为平台，我们发现高氟化水平可以降低氧活性，减少高电压下的副反应，并广泛改善阴极性能。详细的表征揭示了从氟化 LNMO 晶体表面到本体的颗粒级 Mn ^{3+浓度梯度，这归因于富镍 Li} _z Ni _x Mn _{2– x} O _{4– y} F _y ( x > 0.5) 表面上的尖晶石相和本体中的“尖晶石层状”相干结构，其中LiNi _0.5 Mn _1.5 O ₄高压尖晶石相的域被集成到原生层状框架中。这项工作提供了对 LMR 氟化效应的基本了解，并为未来开发具有共生/复合晶体结构的高能锰基阴极提供了关键见解。