Brian W. Sheldon

Alkali metal solid-state batteries (Li, Na, K) suffer from contact loss at the metal|solid electrolyte interface during stripping (discharge), driven by vacancy accumulation and pore formation. This reduces contact area, increases resistance, and promotes dendrite growth during subsequent plating, eventually leading to battery failure. While lithium has been studied extensively, sodium and potassium remain comparatively underexplored despite their high abundance. Their lower yield strength, larger molar volume, and higher homologous temperature strongly affect interfacial contact evolution. Operando electrochemical dilatometry is used to determine the formed pore volume in sodium and potassium metal anodes, enabling evaluation of the vacancy injection ratio (VIR) and comparison with lithium. At low stack pressure, pores form immediately upon stripping for all three metals. The formed pore volume scales with the molar volume across lithium, sodium, and potassium. Increasing current density enhances contact loss and leads to a weaker increase in the formed pore volume in sodium and potassium, with a fraction of the formed pore volume in potassium located inside the metal bulk. In contrast, lithium shows an initial reduction in the formed pore volume (partial pore collapse), followed by a stronger increase at higher current densities. These findings reveal systematic trends among lithium, sodium, and potassium, suggesting lower yield strengths and smaller molar volumes as favorable material properties. Based on these results, a generalized mechanism for pore formation in alkali metal anodes is proposed.

Lithium dendrite propagation in solid electrolytes can be mitigated by engineering nanoscale, multifunctional interlayers. This work establishes a general electro–chemo–mechanical design framework for multilayer and composite solid electrolytes.

Solid electrolytes (SE) such as Li _6.5 La ₃ Zr _1.5 Ta _0.5 O ₁₂ (LLZTO) have the potential to enable the production of safe, high‐capacity lithium (Li) metal batteries, but remain vulnerable to Li filaments. Applying in‐plane residual compressive stress to suppress these dendrites provides a possible solution. Herein, we quantify the effectiveness of compressive surface layers over an appropriate thickness and demonstrate that stresses between 300 MPa and 1 GPa can be introduced with a multi‐step process using AgNO ₃ to ion‐exchange (IX) smaller Li ⁺ ions for larger Ag ⁺ ions. This process requires balancing the diffusion kinetics (to create stress) against the thermodynamic factors which cause decomposition of the SE. Surface‐sensitive analysis confirms the presence of silver in the LLZTO, which maintains its cubic structure after high temperature processing while critical current density (CCD) measurements show a threefold increase. These findings demonstrate that dendritic fracture can be inhibited using IX to introduce internal compressive stress at sufficient depths.

We address dendrite growth resistance in solid electrolytes in all-solid-state Li-ion batteries. First, we introduce a dendrite-growth model in the electrical, ionic-flow, temperature, and stress fields in the solid-state electrolyte. The model analysis shows that the applied electrode potential generates field singularities at the dendrite/crack tip, driven by electrochemical ion deposition pressure and the thermal conductivity jump at the dendrite/electrolyte interface. An extended J-integral is introduced to characterize the stress and temperature-gradient singularities and associated driving forces of the dendrite/crack tip in the heat-generating medium. These singularities are then used to construct universal threshold-current-density diagrams of dendrite/crack growth in solid electrolytes. From the universal diagram, the resistance to dendrite/crack growth is defined. This resistance depends on the dendrite/crack length, film thickness, and the properties of the electrochemically active solid-state electrolyte. These properties include ion conductivity, thermal conductivity, thermal expansion coefficient, elastic moduli, and toughness against dendrite/crack growth. The analyses show that the driving force from thermal stresses induced by Joule heating of the ionic current in the electrolyte is particularly significant for short dendrite/crack structures. The polarity of the temperature gradient in the electrolyte is a primary factor in determining the threshold current density for short-dendrite/crack growth. The analysis offers methods for testing the dendrite/crack toughness of the electrochemically active solid-state electrolyte. It also guides the design of battery-cell packing sequences to maximize resistance to dendrite/crack formation and growth along the cooling path.

Charging rates, cycling performance and safety of solid-state batteries using metal negative electrodes are often limited by dendrites^1,2,3, the growth of which depends on coupling between electrochemical and mechanical driving forces. Previously, it has been assumed that dendrites propagate when plating-induced stresses reach the fracture stress of the solid electrolyte. Here we show that dendrites can propagate at far lower stresses. Using operando birefringence microscopy⁴, we directly measure stresses around growing dendrites in garnet Li_6.6La₃Zr_1.6Ta_0.4O₁₂, a highly stable solid electrolyte^5,6,7. Plating-induced stresses are present throughout growth and approach the mechanical fracture stress for the slowest-growing dendrites. As current densities and dendrite velocities increase, the stresses accompanying dendrite growth surprisingly decrease, with dendrite propagation occurring at stresses up to 75% lower than under mechanical load alone. Cryogenic scanning transmission electron microscopy (STEM) of dendrites propagated at high current reveals electrolyte decomposition to new phases, associated with which is a net molar volume contraction. The electrochemically induced mode of embrittlement may be mitigated through understanding and control of the nature of phase transitions accompanying instability.

Fast charging of high capacity electrodes is an engineering challenge due to lithium plating and electrolyte depletion, which can lead to poor battery life and safety issues. Therefore, accurately predicting thresholds for safe and efficient operation is crucial for robust battery design. While the most commonly used porous electrode theory (PET)-based models (e.g., the pseudo-2D model) can predict detailed behavior of the local electrochemical environment, they are notoriously complex and difficult to calibrate. Scaling-based models provide a simplified means to understand these complex systems, reduce a vast parameter space, and generalize experimental findings. Here, a scaling framework is derived from PET to predict the coupled behavior of electrolyte transport, reaction distribution, and ohmic losses during fast charging. Limits in system operation, including lithium plating onset and electrolyte depletion, are predicted by critical values of four non-dimensional numbers. These critical values provide theoretical limits on C-rates for safe operation from electrode mass loading, and material properties of the electrode and electrolyte. Theoretical predictions are in excellent agreement with experimentally validated measurements of plating onset and a rigorously calibrated fast-charging pseudo-2D model of graphite half-cells. The scaling behavior derived here may be generalized to a range of electrodes, electrolytes and charging conditions.

Ion exchange offers a pathway to impose residual compressive stresses to mitigate the electro-chemo-mechanical cracking of solid-state electrolytes such as lithium lanthanum zirconium oxide. This study uses a coupled multiscale framework (integrating density functional theory (DFT), molecular dynamics (MD), and continuum modeling) to examine how exchange ion size influences stress, diffusion, fracture toughness, and electronic properties. Larger isovalent ions (Na⁺, Ag⁺, K⁺) were exchanged with Li⁺, with DFT confirming their preference for octahedral 96h sites and a linear relationship between ion size and chemical free expansion coefficient. MD simulations reveal stress and concentration effects on exchange ion diffusivity at elevated temperatures, with Na⁺ and Ag⁺ maintaining favorable mobility while K⁺ showing concentration-dependent clustering. Continuum modeling predicts the range of fracture strength improvements and the required ion exchange concentration profile. It was shown that a 5% surface exchange concentration can induce ∼0.6 GPa of surface compressive stress using Na⁺ and ∼1.0 GPa of surface compressive stress using Ag⁺. On the other hand, larger ion exchange species may penalize Li⁺ diffusivity by increasing the activation volume and activation energy. Interestingly, Na⁺ has a negligible penalty on Li-ion diffusivity. The room temperature Li⁺ ion diffusivity is reduced by ∼40% with Ag⁺ ion exchange. Electronic band structure analysis shows no size-dependent change in the bandgap, though Ag⁺ introduces localized defect states near the valence band maximum. This study highlights ion size as a key factor in optimizing LLZO properties, offering a framework to improve the solid-state battery performance.