Effect of solid-electrolyte pellet density on failure of solid-state batteries

Abstract

Despite the potentially higher energy density and improved safety of solid-state batteries (SSBs) relative to Li-ion batteries, failure due to Li-filament penetration of the solid electrolyte and subsequent short circuit remains a critical issue. Herein, we show that Li-filament growth is suppressed in solid-electrolyte pellets with a relative density beyond ~95%. Below this threshold value, however, the battery shorts more easily as the density increases due to faster Li-filament growth within the percolating pores in the pellet. The microstructural properties (e.g., pore size, connectivity, porosity, and tortuosity) of [Formula: see text] with various relative densities are quantified using focused ion beam-scanning electron microscopy tomography and permeability tests. Furthermore, modeling results provide details on the Li-filament growth inside pores ranging from 0.2 to 2 μm in size. Our findings improve the understanding of the failure modes of SSBs and provide guidelines for the design of dendrite-free SSBs.

Fig. 1. Effect of pellet density on cell resistance; error bars are defined as standard deviation.

a Relative density of LPS pellet for various fabrication pressures (theoretical LPS density of 1.88 g/cm³). b Nyquist plots from sequential EIS measurements (with time interval of 1.3 h) of Li|LPS|Li symmetric cell (with LPS pellet relative density of ~89.5%). All the intermediate curves (dashed lines) are enveloped by the initial (t = 0 h, blue solid line) and final (t = 12 h, blue solid line) curves. c Temporal evolution of bulk (SE + SEI) resistance of symmetric cells with four different LPS relative densities. d Ionic conductivity (blue curve) of LPS pellets and the initial overpotential (red curve) of the cell at current density of 0.2 mA/cm².

Fig. 2. Effect of pellet density on Li-filament growth; error bars are defined as standard deviation.

a Charging voltage of cells with different LPS pellet densities. b Cell-shorting time (“cell-shorting regime”) as a function of fabrication pressure and LPS relative density and threshold where the cell voltage increases rapidly (“no-short regime”). c Schematic of Li deposition within pores of LPS pellets with different pellet densities. Symmetric cells in the “percolating regime” have pore networks connecting two electrodes in the initial microstructure, whereas those in the “non-percolating regime” have no connecting pore network initially.

Fig. 3. Characterizations of micro- and macrostructure of LPS.

3D structure of pores within the LPS pellets at densities of (a) 89.2%, (b) 95.3%, and (c) 99.9%. d Flowrate of Ar gas flowing out of LPS pellet as a function of Ar pressure gradient across the pellet.

Fig. 4. Simulation of Li deposition in SE pores.

a Boundaries (dashed lines) of Li filament (gray area) at different charging times (in h) within a pore of 2-μm diameter. The Li metal on the top is the anode during charging. The maximum Li width at a specific time is the summation of the Li width on each side (W = w₁ + w₂). b Deposition current density along line A–B–E at different charging times. The labels “A”, “B”, and “E” along the x-axis correspond to locations A, B, and E in 4a. Note that location E changes at different time steps. c Maximum Li width in the pore when D_pore = 10 μm (dashed black line), D_pore = 2 μm (red line), D_pore = 1 μm (blue line), and D_pore = 0.5 μm (green line). d Length of Li filament in the pore when D_pore = 2 μm (red line), D_pore = 1 μm (blue line), and D_pore = 0.5 μm (green line).

Fig. 5. Schematics of Li-filament (Gray) growth in the solid electrolyte (Orange) at different conditions.

a Li (gray) deposit within the tortuous pore network at low current density with multiple branches and varying thickness. The yellow line represents the SEI layer formed due to the chemical reaction between LPS and the Li filament. b Li (Gray) deposit within the tortuous pore network and isolated pores at high current density. c Li deposit in isolated pores at low current density. d Li deposits in isolated pores and causes fracture at high current density. Four identified mechanisms in the symmetric SSB: percolating pores (Mechanism 1), chemical reaction (Mechanism 2), electronic conductivity (Mechanism 3), and SE fracture (Mechanism 4). Li metal, the SEI layer, and voids are colored gray, yellow, and white, respectively. The dashed lines for Mechanism 3 represent electron conduction due to electronic conductivity of the SE. The red arrows for Mechanism 4 represent the fracture directions of the SE due to the development of hydrostatic pressure P.