The role of grain boundaries in solid-state Li-metal batteries

doi: 10.1088/2752-5724/aca703

Emily Milan¹,
Mauro Pasta^{1, 2
,}

1.
Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom
2.
The Faraday Institution, Didcot OX11 0RA, United Kingdom

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Corresponding author: E-mail: mauro.pasta@materials.ox.ac.uk

摘要

Abstract: Despite the potential advantages promised by solid-state batteries, the success of solid-state electrolytes has not yet been fully realised. This is due in part to the lower ionic conductivity of solid electrolytes. In many solid superionic conductors, grain boundaries are found to be ionically resistive and hence contribute to this lower ionic conductivity. Additionally, in spite of the hope that solid electrolytes would inhibit lithium filaments, in most scenarios their growth is still observed, and in some polycrystalline systems this is suggested to occur along grain boundaries. It is apparent that grain boundaries affect the performance of solid-state electrolytes, however a deeper understanding is lacking. In this perspective, the current theories relating to grain boundaries in solid-state electrolytes are explored, as well as addressing some of the challenges which arise when trying to investigate their role. Glasses are presented as a possible solution to reduce the effect of grain boundaries in electrolytes. Future research directions are suggested which will aid in both understanding the role of grain boundaries, and diminishing their contribution in cases where they are detrimental.
- solid electrolytes /
- grain boundaries /
- lithium growth /
- glasses
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Figure 1. (a) The calculated polycrystalline (red solid line) and bulk (black dashed line) conductivity of Li₃OCl at 300 K as a function of grain size. The blue band represents the polycrystalline upper and lower limits based on different densification behaviours. Reprinted with permission from [4]. Copyright (2018) American Chemical Society. (b) Na-Na and Na-P Radial Distribution Functions calculated for bulk and polycrystalline (grain volumes of 108 and 2.16 nm³) Na₃PS₄ (left) and Na₃PO₄ (right) at 400 K. Reprinted with permission from [16]. Copyright (2019) American Chemical Society. (c) Ionic conductivities for the bulk and grain boundaries of Li₃OCl (top), Li₃PS₄ (middle) and Li₃InCl₆ (bottom). Corresponding activation energies, E_a, for each system are also shown. Reproduced from [21]. CC BY 4.0.

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Figure 2. (a) SEM image of the fractured surface of cycled LLZO, showing apparent lithium deposition along grain boundaries. Reprinted from [1], Copyright (2017), with permission from Elsevier. (b) Example calculation of the elastic modulus across a grain boundary in LLZO carried out by Yu and Siegel and used to explain why lithium accumulates along grain boundaries. Reprinted with permission from [32]. Copyright (2018) American Chemical Society. (c) Suggested mechanism of intergranular lithium deposition in which grain boundaries act as fast-diffusion pathways, resulting in a pile up’ of lithium at the anode-electrolyte interface. Reprinted with permission from [32]. Copyright (2018) American Chemical Society. (d) Schematic showing how electrons tunnelling into the solid electrolyte can reduce lithium ions and result in lithium deposits which interconnect over time. This is proposed to occur preferentially along grain boundaries thanks to their reduced band gaps. Reproduced from [33], with permission from Springer Nature.

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Figure 3. Finite element analysis of the current density distribution at Li $|$ SG-LPSCl and Li $|$ LG-LPSCl interfaces obtained via digitisation of FIB-SEM images. Sharper and deeper flaws are observed in the LG case. (SGsmall grain, LGlarge grain). Reprinted from [41], Copyright (2022), with permission from Elsevier.

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Figure 4. CCD and fracture toughness of LLZO samples as a function of grain size. Reprinted from [44] with permission from the Royal Society of Chemistry.

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Figure 5. Critical research fronts necessary to gain a complete understanding of the role of grain boundaries in solid-state electrolytes, and reduce their detrimental effects.

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Table 1. Summary of microstructural, mechanical and electrochemical properties of different LLZO samples investigated by Sharafi et al A-1300 ^{C refers to a HP-1100 ^{C sample which was subsequently annealed at 1300 ^{C for 50 h. Reprinted from [44] with permission from the Royal Society of Chemistry.}}}

	Microstructural properties				Mechanical properties		Electrochemical properties
Pellet	Phase purity	Relative density (%)	d $_{a v e}$ ( $μ m$ )	Misorientation angle (⁾	H (GPa)	K $_{I C}$ (MPa m $^{- 1 / 2}$ )	_total (mS cm^-1)	CCD (mA cm^-2)
HP-1100 ^C	Cubic LLZO	96.0 0.5	5 2	20	9.88 0.49	0.82 0.07	0.46	0.3
HP-1200 ^C	3 vol% pyrochlore	97.7 0.5	40 13	35	8.05 0.52	0.61 0.05	0.52	0.4
HP-1250 ^C	1 vol% pyrochlore	98.1 0.5	60 20	40	7.74 0.46	0.60 0.06	0.54	0.4
HP-1300 ^C	Cubic LLZO	99.4 0.5	80 20	40	7.42 0.48	0.61 0.04	0.56	0.5
A-1300 ^C	Cubic LLZO	99.4 0.5	600 200	41	6.80 0.49	0.60 0.05	0.57	0.6

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