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Clarifying the Dopant Local Structure and Effect on Ionic Conductivity in Garnet Solid-State Electrolytes for Lithium-Ion Batteries.

Published version
Peer-reviewed

Repository DOI


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Abstract

The high Li-ion conductivity and wide electrochemical stability of Li-rich garnets (Li7La3Zr2O12) make them one of the leading solid electrolyte candidates for solid-state batteries. Dopants such as Al and Ga are typically used to enable stabilization of the high Li+ ion-conductive cubic phase at room temperature. Although numerous studies exist that have characterized the electrochemical properties, structure, and lithium diffusion in Al- and Ga-LLZO, the local structure and site occupancy of dopants in these compounds are not well understood. Two broad 27Al or 69,71Ga resonances are often observed with chemical shifts consistent with tetrahedrally coordinated Al/Ga in the magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of both Al- and Ga-LLZO, which have been assigned to either Al and/or Ga occupying 24d and 96h/48g sites in the LLZO lattice or the different Al/Ga configurations that arise from different arrangements of Li around these dopants. In this work, we unambiguously show that the side products γ-LiAlO2 and LiGaO2 lead to the high frequency resonances observed by NMR spectroscopy and that both Al and Ga only occupy the 24d site in the LLZO lattice. Furthermore, it was observed that the excess Li often used during synthesis leads to the formation of these side products by consuming the Al/Ga dopants. In addition, the consumption of Al/Ga dopants leads to the tetragonal phase formation commonly observed in the literature, even after careful mixing of precursors. The side-products can exist even after sintering, thereby controlling the Al/Ga content in the LLZO lattice and substantially influencing the lithium-ion conductivity in LLZO, as measured here by electrochemical impedance spectroscopy.

Description

Publication status: Published

Keywords

40 Engineering, 4016 Materials Engineering, 34 Chemical Sciences, 3406 Physical Chemistry

Journal Title

Chem Mater

Conference Name

Journal ISSN

0897-4756
1520-5002

Volume Title

35

Publisher

American Chemical Society (ACS)
Sponsorship
European Commission Horizon 2020 (H2020) ERC (835073)
Royal Society (RP/R1/180147)
Cambridge Commonwealth European and International Trust Faraday Institution (SOLBAT, FIRG007) Royal Society (RP/R1/180147) EU ERC (EC H2020 835073) Science and Engineering Research Board (Government of India) - Newton-Bhabha International Fellowship (NIF/R1/180075) Diamond Light Source, Oxford, under rapid access (CY34151) and BAG proposal (CY28349) EPSRC and BBSRC (EP/T015063/1) (EP/R029946/1)