Atomistic Insight into Ion Transport and Conductivity in Ga/Al-Substituted Li₇La₃Zr₂O₁₂ Solid Electrolytes

Abstract

is a promising solid electrolyte for next-generation solid-state Li batteries. However, sufficiently fast Li-ion mobility required for battery applications only emerges at high temperatures, upon a phase transition to cubic structure. A well-known strategy to stabilize the cubic phase at room temperature relies on aliovalent substitution; in particular, the substitution of Li + by Al 3+ and Ga 3+ ions. Yet, despite having the same formal charge, Ga 3+ substitution yields higher conductivities (10 −3 S/cm) than Al 3+ (10 −4 S/cm). The reason of such difference in ionic conductivity remains a mystery. Here we use molecular dynamic simulations and advanced sampling techniques to precisely unveil the atomistic origin of this phenomenon. Our results show that Li + vacancies generated by Al 3+ and Ga 3+ substitution remain adjacent to Ga 3+ and Al 3+ ions, without contributing to the promotion of Li + mobility. However, while Ga 3+ ions tend to allow limited Li + diffusion within their immediate surroundings, the less repulsive interactions associated with Al 3+ ions lead to a complete blockage of neighboring Li + diffusion paths. This effect is magnified at lower temperatures, and explains the higher conductivities observed for Ga-substituted systems. Overall this study provides a valuable insight into the fundamental ion transport mechanism in the bulk of Ga/Al-substituted Li 7 La 3 Zr 2 O 12 and paves the way for rationalizing aliovalent substitution design strategies for enhancing ionic transport in these materials.

Exploring Li-ion conductivity in cubic, tetragonal and mixed-phase Al-substituted Li7La3Zr2O12 using atomistic simulations and effective medium theory

“…Coulombic repulsion. Similar tendencies are reported by Jalem et al 22 and García et al 53 for Ga substituted LLZO. The analogous trend was observed in the histograms corresponding to t-Al x -LLZO (not shown).…”

“…The GSHMC method has been successfully employed in the study of rare events in biological systems, 58,[65][66][67] and ion mobility in solid battery materials. 49,50,53 A theoretical description of the GSHMC method and details of its implementation have been published elsewhere. 48,58,62,67 To compare sampling efficiency of MD and GSHMC methods, we measured the integrated autocorrelation functions (IACF ) of potential energy U…”

“…We determined the GSHMC parameters following our previous work on Al/Ga co-substituted LLZO systems. 53 The validated GSHMC parameters resulted in L = 150, ∆t = 4.0 fs, φ = 0.3 and the 4 th order modified Hamiltonian for GSHMC. We combined GSHMC with the two-stage M-BCSS integrator specifically derived for GSHMC, 68 which allowed us to use the time step as long as 4.0 fs.…”

Lithium Garnet Li7La3Zr2O12 Electrolyte for All‐Solid‐State Batteries: Closing the Gap between Bulk and Thin Film Li‐Ion Conductivities

“…[37] The difference in ionic conduction between Al-and Ga-doped LLZO has been recently investigated employing molecular dynamic simulations and attributed to different interaction strengths of the dopant with the neighboring Li + vacancies. [38] When comparing to the state-of-the-art submicron LLZO thin films, the effective ionic conductivity of the here reported Ga-doped films outperforms by about one order of magnitude the record value of 2.9 × 10 −5 S cm -1 reported by Pfenninger et al in Al and Ta co-doped LLZO films deposited by PLD and post-annealed at 660 °C. [16] These new conductivity values also exceed by one order of magnitude the previous values reported by our group for sputtered Al-doped LLZO [13] and Ga-doped LLZO thin films, [18] which were prepared using a multilayer deposition approach.…”

crystal-torture: A crystal tortuosity module

“…The long-ranged diffusion of mobile ions now depends on the proportion and arrangement of blocked sites, and the degree to which the mobile ions can access percolating paths through the crystal structure [ Fig. 1] (Deng, Radhakrishnan, & Ong, 2015;García Daza, Bonilla, Llordés, Carrasco, & Akhmatskaya, 2018;Lee et al, 2014;. If the proportion of blocked sites, p b , exceeds 1 − p, where p is the site-percolation threshold for that crystal lattice, then no continuous paths exist and the diffusion coefficient and ionic conductivity for the mobile ions are zero.…”