Nanosizing of LGPS to Enhance its Li-Ion Conductivity

In a recent study published in the journal ACS Materials Letters, researchers analyzed the effect of downsizing a lithium superionic conductor, Li10GeP2S12 (LGPS), single-crystal solid-state electrolyte on lithium (Li)-ion diffusion conductivity by bringing it down to a single-digit nanometer (nm) size (i.e., 2-10 nm). 

Study: A Nanoscale Design Approach for Enhancing the Li-Ion Conductivity of the Li10GeP2S12 Solid Electrolyte. Image Credit: petrmalinak/Shutterstock.com

LGPS with the smallest particle size of 2.15 nm, i.e., an average volume of 10 nm3, exhibited the highest conductivity value of 15.10 mS cm−1 at 300 K, which was higher than that of the previously achieved maximum value of 12 mS cm−1 at the same temperature. The primary reason behind the increase in ion diffusion conductivity is due to the transformation of the one-dimensional Li-ion conduction mechanism into a three-dimensional mechanism with a decrease in size, especially below 100 nm.

LGPS as Electrolyte in Solid-state Li-ion Batteries

Since the discovery of LGPS solid electrolytes by Kamaya et al. in 2011, solid-state Li-ion batteries (LiBs) have garnered significant attention owing to their high energy density and associated safety during storage and transportation. Its Li-ion conductivity of 12 mS cm−1 at room temperature is significantly higher than many liquid electrolyte-based LiBs. The contributing factor to such high conductivity is its 3D framework crystal structure consisting of negatively charged (GeS4)4− and (PS4)3− tetrahedra, and positively charged Li-ions in corresponding tetrahedral and octahedral coordination.

Furthermore, several studies have shown a decrease in grain boundary or interfacial resistance of sulfide and oxide-based solid electrolytes led to enhanced metal ion transportation. For instance, nanoporous Li3.25P0.95S4 crystal with average particle size reduced from 80-100 nm to ~5 nm exhibits significant improvement in Li-ion conductivity. Similarly, a Li-argyrodite solid electrolyte of particle size ~20 nm has also demonstrated significant Li-ion conductivity. Based on a similar concept, a nanocrystalline LGPS with reduced interfacial resistance may facilitate a 3D Li-ion diffusion conduction mechanism by allowing intergranular ion diffusion.

About the Study

In this study, researchers used molecular dynamics (MD) simulation to analyze the conductivity performance of an LGPS single-crystal after downsizing to 2-10 nm. The simulation was performed on three crystal cells consisting of ~100,000 ions, each with a different particle volume of 10, 100, and 1000 nm3, and for 10 ns with a time step of 1 fs and temperature range of 300-700 K.

The Li-ion self-diffusion data obtained from a three-directional mean square displacement (MSD) analysis was converted into corresponding conductivity values using the Nernst-Einstein equation. A modified Morse potential was used to analyze Ge-S interaction in LGPS, and the atomic charge was evaluated using Bader analysis.

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Observations

The downsizing of LGPS nanocrystals resulted in shortened Li-ion diffusion path along the normal direction (i.e., the primary direction in large LGPS single-crystals) of the LGPS tetrahedra and a reduction in interfacial resistance that facilitated intergranular diffusion in the other two directions. This three-directional isotropic conduction mechanism enhanced Li-ion conductivity to 15.10 mS cm1 for the least particle volume of 10 nm3 at 300 K. Moreover, this conductivity value was almost three times that of larger single-crystal systems with a conductivity value of 5.87 mS cm−1. The activation energies for all three particle volumes were between 0.21-0.24 eV.

Furthermore, the ion conductivity had a direct correlation with the surface area of the LGPS owing to the presence of high concentrations of defects and microstrains at the surface. The localized cation-cation disorder and a reduction in Li-S coordination due to decreased particle volume also facilitated the enhanced Li-ion conductivity.

Conclusions

In summary, the researchers of this study analyzed the effect of downsizing the single-crystal size of LGPS solid-state electrolyte on Li-ion diffusion conductivity for the potential enhancement of solid-state LiBs. The MD simulation model demonstrated the highest conductivity value of 15.10 mS cm1 for the smallest particle volume of 10 nm3 at 300, which was higher than the previously recorded value of 12 mS cm1 and even higher than many liquid electrolyte-based LiBs.

The increase in the conductivity was attributed to two factors resulting as a consequence of downsizing LGPS particle size i.e., formation of three-directional isotropic ion conduction path, localized ionic disorder, and the resulting decrease in Li-S coordination. This finding opens up a new field of study for high-performance solid-state electrolytes at the nanometer level.

Source

Dawson, J., Islam, M., A Nanoscale Design Approach for Enhancing the Li-Ion Conductivity of the Li10GeP2S12 Solid Electrolyte, ACS Materials Letters, 2022, 4, 424-431, https://pubs.acs.org/doi/10.1021/acsmaterialslett.1c00766

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Bismay Prakash Rout

Written by

Bismay Prakash Rout

Bismay is a technical writer based in Bhubaneshwar, India. His academic background is in Engineering and he has extensive experience in content writing, journal reviewing, mechanical designing. Bismay holds a Masters in Materials Engineering and BE in Mechanical Engineering and is passionate about science & technology and engineering. Outside of work, he enjoys online gaming and cooking.

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