Disordered Cubic Spinel Structure in the Delithiated Li2MnO3 Revealed by Difference Pair Distribution Function Analysis
Disordered Cubic Spinel Structure in the Delithiated Li2MnO3 Revealed by Difference Pair Distribution Function Analysis
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差对分布函数分析揭示脱锂Li2MnO3中的无序立方尖晶石结构
DOI：
10.1021/acs.jpcc.0c07124
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发表时间：
2020
期刊：
影响因子：
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通讯作者：
Uchimoto Yoshiharu
中科院分区：
文献类型：
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作者：
Oishi Masatsugu;Shimoda Keiji;Ohara Koji;Kabutan Daiki;Kawaguchi Tomoya;Uchimoto Yoshiharu
典型的富锂层状氧化物 Li2MnO3 显示出约 350 mAh g-1 的大初始充电容量，且成分 Mn 离子的氧化很少；然而，脱锂Li2MnO3的晶体结构仍不清楚，因为大量Li提取引起的结构无序使得分析具有挑战性。 X 射线对分布函数 (PDF) 分析是通过实验阐明无序相结构的强大工具。在这里，我们进行了以 PDF 分析为重点，结合 X 射线粉末衍射 (XRPD)、透射电子显微镜 (TEM) 和 X 射线吸收光谱 (XAS) 的综合分析，揭示了电化学脱锂 Li2MnO3 的无序晶体结构。 XRPD 和 TEM 分析根据平均结构阐明了低结晶度相的形成。 XAS和PDF分析进一步表明，初始充电后，MnO6基框架发生了重新排列，并保持了MnO6八面体配位。因此，采用差分对分布函数（d-PDF）技术来提取低结晶度无序相的结构信息。发现脱锂相具有类似于立方尖晶石 LiMn2O4 的结构，而不是脱锂 LiMn2O4(λ-MnO2) 的结构。此外，充电后脱锂相的中程有序度恶化，表明相干域尺寸减小至单个纳米量级。因此，第一次充电后形成的复合结构由无序立方尖晶石结构和未反应的 Li2MnO3 组成。复合结构的形成在结构上“激活”了电极材料，最终导致该材料具有大容量特性。
An archetypical Li-rich layered oxide, Li2MnO3, shows a large initial charge capacity of ∼350 mAh g–1with little oxidation of the constituent Mn ions; yet, the crystal structure of delithiated Li2MnO3is still unclarified because the structural disorder induced by the considerable Li extraction makes the analysis challenging. X-ray pair distribution function (PDF) analysis is a powerful tool to experimentally elucidate the structure of the disordered phase. Here, we conducted a comprehensive analysis with a focus on PDF analysis in combination with X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) to reveal the disordered crystalline structure of the electrochemically delithiated Li2MnO3. The XRPD and TEM analyses clarified the formation of a low-crystallinity phase in the light of the average structure. The XAS and PDF analyses further revealed that the MnO6-based framework was rearranged with maintenance of the MnO6octahedral coordination after the initial charge. The difference pair distribution function (d-PDF) technique was therefore employed to extract the structural information of the low-crystallinity disordered phase. The delithiated phase was found to have a structure similar to that of the cubic spinel, LiMn2O4, rather than that of delithiated LiMn2O4(λ-MnO2). In addition, the middle-range order of the delithiated phase deteriorated after the charge, indicating a decrease of coherent domain size to a single nm order. The composite structure formed after the first charge, therefore, consists of the disordered cubic spinel structure and unreacted Li2MnO3. The formation of the composite structure “activates” the electrode material structurally and eventually induces characteristic large capacity of this material.