Berge Astrid H, Vema Sundeep, O'Keefe Christopher A, Grey Clare P
Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Chem Mater. 2025 May 13;37(10):3842-3852. doi: 10.1021/acs.chemmater.5c00807. eCollection 2025 May 27.
An understanding of the nature of the grain boundaries and impurity phases contained in complex mixed metal oxide solid electrolytes is key to the development of improved and more stable solid-state batteries with reduced grain boundary resistances and higher ionic conductivities of the bulk sample. The Li-ion solid electrolyte LiLaZrO (LLZO) is one of the most researched electrolytes in the field due to its high ionic conductivity, thermal stability, and wide voltage stability window. Despite its potential, the nature of the impurity and surface phases formed during the synthesis of LLZO and their role and influence on LLZO's performance when used as an electrolyte remain poorly understood and controlled. In addition, there are limited characterization methods available for detailed studies of these impurity phases, particularly if these phases are buried in or close to the grain boundaries of a dense sintered material. Here, we demonstrate a solid-state nuclear magnetic resonance (ssNMR) and dynamic nuclear polarization (DNP) approach that exploits both endogenous and exogenous dopants to select for either specific impurities or separate bulk vs surface/subsurface phases. Specifically, the location of Al-containing phases within an Al doped LLZO and the impurity phases that form during synthesis are mapped: by doping LLZO with trace amounts of paramagnetic metal ions (Fe and Gd), DNP is used to selectively probe Al- and La-containing impurity phases, respectively, allowing us to enhance the signals arising from the LiAlO and LaAlO impurities and to confirm their identity. A O DNP experiment using Gd doped LLZO is performed to identify further La-containing impurities (specifically LaZrO and LaO). Finally, a Li DNP irradiated Li-Al dipolar-based heteronuclear multiple quantum correlation experiment is performed by using the radical TEKPol as the polarization agent. This experiment demonstrates that the poorly crystalline LiAlO that is found close to the surfaces of the LLZO composite is coated by a thin Li-containing impurity layer and thus not directly present at the surface.
了解复杂混合金属氧化物固体电解质中晶界和杂质相的性质，是开发改进型、更稳定的固态电池的关键，这类电池具有降低的晶界电阻和更高的本体样品离子电导率。锂离子固体电解质LiLaZrO（LLZO）因其高离子电导率、热稳定性和宽电压稳定窗口，是该领域研究最多的电解质之一。尽管LLZO有潜力，但在其合成过程中形成的杂质和表面相的性质，以及它们在用作电解质时对LLZO性能的作用和影响，仍知之甚少且难以控制。此外，用于详细研究这些杂质相的表征方法有限，特别是当这些相埋在致密烧结材料的晶界中或靠近晶界时。在这里，我们展示了一种固态核磁共振（ssNMR）和动态核极化（DNP）方法，该方法利用内源性和外源性掺杂剂来选择特定杂质，或区分本体相与表面/次表面相。具体来说，绘制了Al掺杂LLZO中含Al相的位置以及合成过程中形成的杂质相：通过用痕量顺磁性金属离子（Fe和Gd）掺杂LLZO，DNP分别用于选择性探测含Al和含La的杂质相，使我们能够增强来自LiAlO和LaAlO杂质的信号并确认它们的身份。使用Gd掺杂的LLZO进行了一个¹⁷O DNP实验，以识别更多含La杂质（特别是LaZrO和LaO）。最后，使用自由基TEKPol作为极化剂进行了Li DNP辐照的基于Li-Al偶极的异核多量子关联实验。该实验表明，在LLZO复合材料表面附近发现的结晶性差的LiAlO被一层薄的含Li杂质层覆盖，因此并非直接存在于表面。
