升级回收；单晶结构；富锂正极；废旧高镍正极

近年来锂离子电池产业飞速发展，伴随而来的是大量废旧电池的产生。从环境保护和资源可持续性的角度来看，废旧锂离子电池的回收处理具有重要的现实意义。然而，传统火法、湿法冶金回收方法一般工序复杂、成本高、污染严重，而直接回收方法可以简化回收工艺流程、减少污染排放、降低能耗和成本，但对原料分类有较高要求，而且电化学性能较差，总体产出价值较低，限制了其商业化的发展。另外，已报道的直接再生回收方式主要聚焦于中低镍的三元正极材料，对于高能量密度的高镍三元材料涉及较少。此外，如何实现不同锂损失程度、不同三元材料混合回收、如何进一步提高回收产物附加值仍然存在较大挑战。

基于此，中南大学陈根副教授和周江教授合作，通过共晶LiOH-Na₂SO₄熔盐策略，将循环过的多晶LiNi_0.88Co_0.095Al_0.025O₂（S-NCA）直接回收并升级为单晶和富锂NCA正极（R-NCA）。在LiOH-Na₂SO₄体系中，LiOH起着补充损失Li的主要作用，Na₂SO₄作为离子迁移和颗粒重结晶的助熔剂。变温原位XRD和一系列并行实验记录了单晶相变过程，证明过量的Li占据了过渡金属（TM）层。得益于单晶和富锂特性，R-NCA在循环、倍率、极化和抑制氧释放方面表现出显著增强的电化学性能。

该方法还可以成功扩展到其他正极材料，例如LiNi_xCo_yMn_zO₂（NCM）和具有不同程度Li损失量的混合废旧NCA正极，展现出对批量废旧正极材料的普遍性和直接升级回收的巨大潜力。相关工作以“A Universal Molten Salt Method for Direct Upcycling of Spent Ni-rich Cathode towards Single-crystalline Li-rich Cathode”为题，发表在化学权威期刊《Agnew. Chem. Int. Ed.》。

回收过程如图1a所示：初级球形商业NCA（C-NCA）包含大量具有随机晶体取向的二次颗粒。经过长期循环后，球体整体结构被破坏，晶间裂纹开始生长，锂严重缺失，杂质相大量生成。然而，在所提出的LiOH-Na₂SO₄熔盐的有利环境下，有严重缺陷的二次颗粒熔化并重塑为单晶和富锂晶粒。图1b描述了再生的动力学过程，其中LiOH中的Li⁺可以通过将岩盐NiO相氧化恢复为层状相，重新插入Li晶格并占据单晶表面的过渡金属 (TM) 层（因为Ni²⁺和Ni³⁺之间的能垒最低）。

Figure 1. (a) Schematic of LiOH-Na₂SO₄ eutectic molten salt approach to regenerate S-NCA. (b) The evolution of surface structure during the treatment. (c) The change of Ni chemical state during regeneration and the energy band structures in NCA.

Figure 2. (a) Contour plot of the in situ XRD patterns recorded during the LiOH-Na₂SO₄ eutectic molten salt treatment. (b) Phase diagram of LiOH-Na₂SO₄. (c) DSC and TGA curves of S-NCA and LiOH-Na₂SO₄ salt from 30 to 750 °C. Rietveld refinement results of (d) S-NCA and (e) R-NCA. (f) XRD patterns of S-NCA and mixture salt at different temperature.

Figure 3. FFT images of the (a) rock-salt and (b) layered areas. (c) HR-TEM image of S-NCA obtained along the [100] zone axis. The images on the right of (c) are the inverse FFT images of the corresponding areas in S-NCA. (d) The interplanar spacings of rock-salt and layered phases with corresponding schematic drawings of atomic arrangements on the right. (e) Schematic of the microscopic structure evolution from S-NCA to R-NCA. HR-TEM image of bulk (f) and boundary (g) in R-NCA. Inserts are the FFT images of dashed boxes. The right images (I and II) are the corresponding inverse FFT images with the schematic graph of interplanar spacing below. STEM of (h) R-NCA and (i) C-NCA with the EDS mapping distribution of O, Ni, Co, and Al elements.

Figure 4. (a) Contour plot of the in situ XRD patterns collected from 17.5−20.5° and 42−46° during the relithiation process. SEM images of R-NCA treated with holding time of (b) 5 h and (c) 10 h at the temperature of 750 °C, respectively. (d) Li content from the ICP-OES and (e) particle size measured by Image J of R-NCA at different holding times. Rietveld refinement results of (f) R-NCA-5h and (g) R-NCA-10h. XPS spectra for (h) O 1s and (i) Li 1s of C-NCA, R-NCA and S-NCA.

Figure 5. The in situ XRD measurement for (a) R-NCA and (c) C-NCA with selected contour plot during the initial cycle in the voltage range of 3.0−4.4 V. The variation of the c-axis parameter during charging for (b) R-NCA and (d) C-NCA. CV curves and SEM images of (e) R-NCA and (f) C-NCA measured after 1st, 100th and 250th cycles.

Figure 6. Evolution of HF (a) and O₂ (b) via operando DEMS measurements of R-NCA and C-NCA during the first charge at 0.1C, respectively. Half-cell electrochemical tests of the R-NCA cathode in comparison with the C-NCA between 3.0 and 4.3 V. (c) Rate performance. (d) Long-term cycling test at 1C (1C=150 mA g⁻¹). (e) Nyquist plots measured from 106 Hz to 0.01 Hz before cycling. Comparisons of particle size (f) and Li content (g) before, and after NCM regeneration. (h) CV curves of R-NCM in the range of 3.0−4.3 V measured after 1, 100 and 250 cycles. (i) Sustaining rate performance tests for R-NCM and C-NCM. (j) Over 300 cycle tests for R-NCM and 220 cycles for C-NCM at 1C, respectively.

陈根教授简介：中南大学材料科学与工程学院特聘副教授，入选湖南省第十一批“百人计划”，湖湘高层次人才聚集工程创新人才，主持湖南省杰出青年基金、国家自然科学基金等。主要从事新型高容量负极材料界面化学研究与开发（金属锂负极、硅基负极）、镍钴锰正极材料再生回收等。在国际知名期刊发表100余篇学术论文，包括Agnew. Chem. Int. Ed.，Adv. Mater.，Adv. Energy Mater.，Adv. Funct. Mater.，Nano Energy，Small等，被引用6200余次，H-index 39。目前担任《中国化学快报》、《物理化学学报》青年编委。