Origin of the lithium metal anode instability in solid-state batteries during discharge

Highlights

• Irregular pores form in LMA during discharge, resulting in rapidly increasing impedance

• Pore formation can be attributed to the dislocations

• Thermomechanical processing history of LMA is critical in SSBs

• A mechanism for pore formation is proposed

Summary

Enabling the lithium metal anode (LMA) in solid-state batteries (SSBs) would increase energy density and specific energy compared with lithium-ion batteries. However, pore formation in LMAs with irregular morphology, even at low current density, during discharge results in an unstable, high-impedance interface. Understanding and addressing this inherent anode instability is essential for increasing the power densities in SSBs. Herein, we suggest that the morphology of the stripped electrode is related to dislocations in the LMA. To investigate the influence of dislocations, symmetric cells, LiǀLi_6.25Al_0.25La₃Zr₂O₁₂(LLZO)ǀX-Li, are studied, where X-Li represents the microstructurally controlled LMA obtained via suitable thermomechanical processing. Operando impedance measurements are corroborated with SEM, confocal microscopy, and AFM data. Based on the experimental observations, a mechanism for pore formation is proposed. We show that the stack pressure required to maintain a stable interface is governed by the lithium microstructure and its thermomechanical processing history.

Introduction

The increasing trend toward renewable energy sources has accelerated research in secondary energy storage systems with high energy density, particularly in solid-state batteries (SSBs).^1,2,3 Lithium metal anodes (LMAs) combined with inorganic solid electrolytes are expected to increase both the energy density (3,860 mAh g⁻¹ and −3.04 V versus standard hydrogen electrode) and the power density of SSBs compared with conventional Li-ion batteries.^2,4 In addition, thermal runaway issues associated with flammable organic liquid electrolytes may be overcome, promising greater safety and longer life.^5,6,7,8

However, the rate capability of SSBs is generally still lower compared with cells with liquid electrolytes.^3,9 One of the contributing factors is the instability of Liǀsolid electrolyte (SE) interfaces, particularly the microstructure of the solid electrolyte (SE) and its influence on lithium electrodeposition (plating).^{10,11,12,13,14,15} Microstructural features of the Li|SE interface determine the maximum current density or the critical current density (CCD) that a system can withstand before a short circuit.¹⁶ Another critical factor is the rapid pore formation in the LMA, causing reduced electrode-electrolyte contact area during anodic stripping.^17,18,19 This leads to rapid overvoltage increase due to the buildup of local constriction resistances.^20,21,22 In fact, the CCD for stripping has been shown to be lower than the CCD for plating.²³

Inhomogeneous electrodissolution or stripping leads to a porous interface in LMAs.²⁴ As a result, the electrode surface gets rough. This undesirable factor increases the stack pressure thresholds in SSBs due to roughness-induced interfacial stress amplification and strain hardening.^25,26,27 In addition, different strain rates will occur at the depleted contacts depending on the area of each contact. This can result in a rapid and inhomogeneous ductile-to-brittle transition of the contacted lithium metal at the interface, as body-centered cubic (bcc) structures are sensitive to strain rates.²⁸ In the long term, this can manifest itself in an increase in pressure thresholds and thus contribute to technical challenges in SSBs.

Therefore, it is important to understand the basic mechanism of pore formation. The low self-diffusivity (D_Li ∼ 10⁻¹¹ cm² s⁻¹) of bulk lithium at room temperature, which cannot keep up with the imposed current (at or above a certain critical current threshold), is attributed to pore formation.¹⁷ This is confirmed by the more stable performance of alloy-based anodes, which increase the (effective) diffusivity of lithium by a factor of three for the case of Li_0.9Mg_0.1 compared to pure lithium.²⁹ However, the observed electrode morphology cannot be solely attributed to the low self-diffusivity of lithium alone. A spatially uniform Li diffusivity would lead to uniform stripping, i.e., layer-by-layer lithium removal at the interface. This suggests a complex mass transport (diffusion) coupled charge-transfer process in the LMA and at the LiǀSE interface, respectively.

Polycrystalline microstructures contain several structural defects such as grain boundaries (2D), dislocations (1D), and point defects (0D).³⁰ The structural defects are associated with individual (enhanced) charge-transfer and mass transport kinetics compared with the ideal or defect-free lattice, in particular in the case of 1D and 2D defects.^{31,32,33,34,35,36} It is well known that the self-diffusivities are different along each of these defects, typically with D_S ≥ D_GB ≥ D_D > D_V (S, grain surface; GB, grain boundary; D, dislocations; V, bulk).^28,37 Thus, highly inhomogeneous kinetics and resulting local fluxes are expected across the interface in response to the imposed current on account of the different local microstructures. Consideration of these defect types is even more indispensable in SSBs where thin Li electrodes (<30 μm)¹ will be employed. The lithium electrode undergoes significant microstructural changes (i.e., an increase in dislocation density) during mechanical or thermomechanical processing.³⁸ In effect, lithium anodes will have significantly different mechanical and chemical properties compared with bulk lithium depending on their treatment and their cycling history. The role of dislocations in pore formation at parent metal|SE anodes has already been demonstrated for single crystalline Ag anodes, where the pore density after stripping correlates well with the dislocation density and where complex oscillatory behavior results from the electrochemo-mechanical coupling.^39,40,41

Herein, we show that the dislocation density, topology (network structure and dislocation loops),²⁸ and distribution dictate the texture of the stripped LMA. Via appropriate mechanical and thermomechanical processing, we achieved control over the pristine lithium microstructure. The electrochemical performance of anodes with different microstructures was then evaluated in Li symmetric cells with Li_6.25Al_0.25La₃Zr₂O₁₂ (LLZO) as SE using operando impedance measurements under unidirectional current load of 0.1 mA cm⁻².

The extracted capacities and the temporal impedance evolution are then corroborated with the SEM, confocal microscopy, and AFM measurements. We then propose a mechanism based on the effect of enhanced kinetics at dislocations, dislocation topology and distribution, surface diffusion, adhesion, microstructural recovery/recrystallization, plastic anisotropy, and creep to account for the complex pore morphology observed during LMA stripping.