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

Li, Na, K, Mg, Zn, Al, and Ca Anode Interface Chemistries Developed by Solid-State Electrolytes

Solid-state batteries (SSBs) have received significant attention due to their high energy density, reversible cycle life, and safe operations relative to commercial Li-ion batteries using flammable liquid electrolytes. This review presents the fundamentals, structures, thermodynamics, chemistries, and electrochemical kinetics of desirable solid electrolyte interphase (SEI) required to meet the practical requirements of reversible anodes. Theoretical and experimental insights for metal nucleation, deposition, and stripping for the reversible cycling of metal anodes are provided. Ion transport mechanisms and state-of-the-art solid-state electrolytes (SEs) are discussed for realizing high-performance cells. The interface challenges and strategies are also concerned with the integration of SEs, anodes, and cathodes for large-scale SSBs in terms of physical/chemical contacts, space-charge layer, interdiffusion, lattice-mismatch, dendritic growth, chemical reactivity of SEI, current collectors, and thermal instability. The recent innovations for anode interface chemistries developed by SEs are highlighted with monovalent (lithium (Li+ ), sodium (Na+ ), potassium (K+ )) and multivalent (magnesium (Mg2+ ), zinc (Zn2+ ), aluminum (Al3+ ), calcium (Ca2+ )) cation carriers (i.e., lithium-metal, lithium-sulfur, sodium-metal, potassium-ion, magnesium-ion, zinc-metal, aluminum-ion, and calcium-ion batteries) compared to those of liquid counterparts.

Enhanced Li-ion conductivity in LiBH4-ZrO2 nanocomposites and nanoscale Li imaging by energy-filtered transmission electron microscopy

A complementary solid-state nuclear magnetic resonance and transmission electron microscopy (TEM) analysis was performed for LiBH4-ZrO2 nanocomposites. As a result, amorphous LiBH4 films with thicknesses of less than 30 nm were observed covering the ZrO2 particles. Li imaging by energy-filtered TEM is useful for the real-space characterization of nanoscale LiBH4.

Ionic Conductivity of Nanocrystalline and Amorphous Li10GeP2S12: The Detrimental Impact of Local Disorder on Ion Transport

Solids with extraordinarily high Li⁺ dynamics are key for high performance all-solid-state batteries. The thiophosphate Li₁₀GeP₂S₁₂ (LGPS) belongs to the best Li-ion conductors with an ionic conductivity exceeding 10 mS cm^–1 at ambient temperature. Recent molecular dynamics simulations performed by Dawson and Islam predict that the ionic conductivity of LGPS can be further enhanced by a factor of 3 if local disorder is introduced. As yet, no experimental evidence exists supporting this fascinating prediction. Here, we synthesized nanocrystalline LGPS by high-energy ball-milling and probed the Li⁺ ion transport parameters. Broadband conductivity spectroscopy in combination with electric modulus measurements allowed us to precisely follow the changes in Li⁺ dynamics. Surprisingly and against the behavior of other electrolytes, bulk ionic conductivity turned out to decrease with increasing milling time, finally leading to a reduction of σ_20°C by a factor of 10. ³¹P, ⁶Li NMR, and X-ray diffraction showed that ball-milling forms a structurally heterogeneous sample with nm-sized LGPS crystallites and amorphous material. At −135 °C, electrical relaxation in the amorphous regions is by 2 to 3 orders of magnitude slower. Careful separation of the amorphous and (nano)crystalline contributions to overall ion transport revealed that in both regions, Li⁺ ion dynamics is slowed down compared to untreated LGPS. Hence, introducing defects into the LGPS bulk structure via ball-milling has a negative impact on ionic transport. We postulate that such a kind of structural disorder is detrimental to fast ion transport in materials whose transport properties rely on crystallographically well-defined diffusion pathways.

Designing Nanoconfined LiBH4 for Solid-State Electrolytes

Solid-state electrolytes are necessary for high-density and safe lithium-ion batteries. Lithium borohydride (LiBH₄) is one of the hydride compounds that shows promising candidates for solid-state electrolytes and enables all-solid-state batteries. LiBH₄ has good wetting properties and preferable mechanical properties when used in battery cells. The Li-ion conduction in LiBH₄ can be modified with nanoconfinement as a result of distinct properties on the interfaces. The ion conductivities can be modified further by choosing property support materials, i.e., composition, textural properties, and surface chemistry. The present work briefly reviews the Li-ion conduction in nanoconfined LiBH₄. A future perspective on the development of LiBH₄ as a solid-state electrolyte is further elaborated in the last section.

Extremely Fast Interfacial Li Ion Dynamics in Crystalline LiTFSI Combined with EMIM-TFSI

Materials providing fast transport pathways for ionic charge carriers are at the heart of future all-solid state batteries that completely rely on sustainable, nonflammable solid electrolytes. The mobile ions in fast ion conductors may take benefit from structural disorder, cation and anion substitution, or dimensionality effects. While these effects concern the bulk regions of a given material, one may also manipulate the surface or interfacial regions of a polycrystalline poorly conducting electrolyte to enhance its transport properties. Here, we used ⁷Li NMR to characterize interfacial effects in crystalline lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) to which a small amount of ionic liquid EMIM-TFSI (EMIM: 1-ethyl-3-methylimidazolium cation, C₆H₁₁N₂ ⁺) was added. We recorded longitudinal spin-lattice relaxation (SLR) curves M _z (t _d) that directly mirror the ⁷Li spin-fluctuations controlled by motional processes in such ionic-liquids-in-salt composites. Already at room temperature we observe strongly bimodal buildup curves M _z (t _d) leading to two distinct SLR rates differing by a factor of 100. While the slower rate does exactly reflect the temperature behavior expected for poorly conducting LiTFSI, the faster rate mirrors rapid motional processes that are governed by an activation energy as low as 73 meV. We attribute these fast processes to highly mobile Li⁺ ions in or near the contact area of crystalline LiTFSI and EMIM-TFSI. By using a method that characterizes motional processes from the atomic-scale point of view, we emphasize the importance of interfacial regions as reservoirs for fast Li⁺ ions in such solid composite electrolytes.