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Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH₄/Al₂O₃.

ABSTRACT: Synthesizing Li-ion-conducting solid electrolytes with application-relevant properties for new energy storage devices is a challenging task that relies on a few design principles to tune ionic conductivity. When starting with originally poor ionic compounds, in many cases, a combination of several strategies, such as doping or substitution, is needed to achieve sufficiently high ionic conductivities. For nanostructured materials, the introduction of conductor-insulator interfacial regions represents another important design strategy. Unfortunately, for most of the two-phase nanostructured ceramics studied so far, the lower limiting conductivity values needed for applications could not be reached. Here, we show that in nanoconfined LiBH₄/Al₂O₃ prepared by melt infiltration, a percolating network of fast conductor-insulator Li⁺ diffusion pathways could be realized. These heterocontacts provide regions with extremely rapid ⁷Li NMR spin fluctuations giving direct evidence for very fast Li⁺ jump processes in both nanoconfined LiBH₄/Al₂O₃ and LiBH₄-LiI/Al₂O₃. Compared to the nanocrystalline, Al₂O₃-free reference system LiBH₄-LiI, nanoconfinement leads to a strongly enhanced recovery of the ⁷Li NMR longitudinal magnetization. The fact that almost no difference is seen between LiBH₄-LiI/Al₂O₃ and LiBH₄/Al₂O₃ unequivocally reveals that the overall ⁷Li NMR spin-lattice relaxation rates are solely controlled by the spin fluctuations near or in the conductor-insulator interfacial regions. Thus, the conductor-insulator nanoeffect, which in the ideal case relies on a percolation network of space charge regions, is independent of the choice of the bulk crystal structure of LiBH₄, either being orthorhombic (LiBH₄/Al₂O₃) or hexagonal (LiBH₄-LiI/Al₂O₃). ⁷Li (and ¹H) NMR shows that rapid local interfacial Li-ion dynamics is corroborated by rather small activation energies on the order of only 0.1 eV. In addition, the LiI-stabilized layer-structured form of LiBH₄ guarantees fast two-dimensional (2D) bulk ion dynamics and contributes to facilitating fast long-range ion transport.

SUBMITTER: Zettl R

PROVIDER: S-EPMC8287566 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Several "Beyond Li-Ion Battery" concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to "non-garnet" (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10-4 S cm-1 to 1.2 × 10-3 S cm-1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure-properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.

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Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH₄/Al₂O₃.

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Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH4/Al2O3.

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Conductor-Insulator Interfaces in Solid Electrolytes: A Design Strategy to Enhance Li-Ion Dynamics in Nanoconfined LiBH₄/Al₂O₃.