We combined advanced TEM (HRTEM, HAADF, EELS) with solid-state (SS)MAS NMR and electroanalytical techniques (GITT, etc.) to understand the site-specific sodiation of selenium (Se) encapsulated in a nanoporous carbon host. The architecture employed is representative of a wide number of electrochemically stable and rate-capable Se-based sodium metal battery (SMB) cathodes. SSNMR demonstrates that during the first sodiation, the Se chains are progressively cut to form an amorphous mixture of polyselenides of varying lengths, with no evidence for discrete phase transitions during sodiation. It also shows that Se nearest the carbon pore surface is sodiated first, leading to the formation of a core-shell compositional profile. HRTEM indicates that the vast majority of the pore-confined Se is amorphous, with the only localized presence of nanocrystalline equilibrium Na2Se2 (hcp) and Na2Se (fcc). A nanoscale fracture of terminally sodiated Na-Se is observed by HAADF, with SSNMR, indicating a physical separation of some Se from the carbon host after the first cycle. GITT reveals a 3-fold increase in Na+ diffusivity at cycle 2, which may be explained by the creation of extra interfaces. These combined findings highlight the complex phenomenology of electrochemical phase transformations in nanoconfined materials, which may profoundly differ from their "free" counterparts.

中文翻译：

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硒在微孔碳基质中的位点特异性吸附机理。

我们将先进的TEM（HRTEM，HAADF，EELS）与固态（SS）MAS NMR和电分析技术（GITT等）结合使用，以了解封装在纳米多孔碳基质中的硒（Se）的位点特异性固溶。所采用的体系结构代表了众多电化学稳定且具有速率能力的Se基钠金属电池（SMB）阴极。SSNMR表明，在第一次糖化过程中，Se链逐渐被切割以形成各种长度的聚硒化物的无定形混合物，没有证据表明在糖化过程中存在离散的相变。这也表明最接近碳孔表面的硒首先被胶化，导致形成核-壳组成轮廓。HRTEM表明，大部分的孔隙封闭硒都是无定形的，仅存在纳米晶平衡Na2Se2（hcp）和Na2Se（fcc）的局部存在。HAADF用SSNMR观察到了末端钠化的Na-Se的纳米级断裂，表明在第一个循环后，一些Se从碳主体中分离出来。GITT揭示了在第2个循环中Na +扩散率增加了3倍，这可以通过创建额外的界面来解释。这些综合的发现突显了纳米受限材料中电化学相变的复杂现象，这可能与它们的“自由”对应物有很大的不同。GITT揭示了在第2个循环中Na +扩散率增加了3倍，这可以通过创建额外的界面来解释。这些综合的发现突显了纳米受限材料中电化学相变的复杂现象，这可能与它们的“自由”对应物有很大的不同。GITT揭示了在第2个循环中Na +扩散率增加了3倍，这可以通过创建额外的界面来解释。这些综合的发现突显了纳米受限材料中电化学相变的复杂现象，这可能与它们的“自由”对应物有很大的不同。