Lithium-ion batteries power the devices we depend on. But their liquid electrolytes bring risks—leakage, fire, explosion. Solid-state batteries promise to eliminate these dangers while packing more energy into less space. The catch? Making them work reliably has proven fiendishly difficult.

One reason is deceptively simple. Nobody knew what lithium and sodium metal actually looked like inside these batteries at the microscopic level. Not the surface morphology—researchers have studied that extensively—but the internal grain structure: how the metal atoms organize themselves into crystalline domains, how those domains meet at boundaries, and how all of this affects performance when the battery charges and discharges.

That ignorance just ended.

Researchers have developed a protocol to image the grain structure of lithium and sodium metal electrodes in solid-state batteries using a technique called electron backscatter diffraction. What they found challenges assumptions and suggests new strategies for improving battery performance.

The Anode-Free Advantage

Traditional solid-state batteries use a foil of lithium or sodium metal as the anode. But handling reactive alkali metals during manufacturing is dangerous and expensive. It also adds unnecessary weight.

Enter the reservoir-free cell, misleadingly called "anode-free." Instead of pre-installing a metal foil, these cells deposit the metal onto a current collector during the first charge cycle. This approach increases energy density by eliminating excess metal. It simplifies manufacturing. It enhances safety during storage and transport.

The challenge? Controlling how that deposited metal behaves.

When lithium or sodium plates onto a current collector inside a solid-state battery, it must form a smooth, dense layer that maintains good contact with the solid electrolyte separator. If the morphology goes wrong—if dendrites form, if the metal grows unevenly, if voids appear during discharge—the battery fails prematurely.

Various strategies have emerged to guide metal deposition: specialized plating protocols, engineered current collector materials, seed layers, applied pressure. But all of these efforts proceeded without knowing what the metal's internal grain structure actually looked like. Researchers were flying blind.

The Homologous Temperature Problem

Lithium and sodium present a peculiar challenge. At room temperature, lithium sits at 61 percent of its melting point on the absolute temperature scale. Sodium reaches 80 percent. Materials scientists call this the homologous temperature.

Metals typically undergo microstructural changes—grain growth, recrystallization—at homologous temperatures above 40 to 60 percent. By that rule of thumb, lithium and sodium should constantly rearrange their grain structures at room temperature, always seeking equilibrium. If true, this would make characterizing their microstructure pointless. The structure you measure today might differ from tomorrow's.

Prior work on lithium suggested its microstructure could indeed be tuned through thermal processing, implying slow grain coarsening. Electrodeposited silver—a metal often studied as a model system—behaves this way despite a lower homologous temperature of only 23 percent at room temperature. Silver starts nanocrystalline when plated but anneals during the first hours until grain growth stops around 40 micrometers.

Do lithium and sodium follow similar rules? Or do they stabilize? Without answering this question, any characterization protocol would produce unreliable results.

Proving the Protocol

The research team began by preparing lithium and sodium foils with different thermal histories. Reference foils were used as-received. Quenched foils were created by melting the metals and rapidly cooling them in liquid nitrogen.

Scanning electron microscopy of freshly prepared surfaces revealed lines and triple junctions—features indicating grain boundaries. The reference lithium foil showed apparent grain sizes of 100 to 300 micrometers. Quenched lithium displayed smaller grains of 10 to 50 micrometers. Sodium showed even larger grains: millimeter-scale in the reference foil, 200 to 600 micrometers in the quenched version.

Crucially, quenched samples stored at room temperature for days showed no visible grain growth. This was the first critical finding: despite high homologous temperatures, both lithium and sodium remain microstructurally stable during room temperature storage.

Electron backscatter diffraction confirmed these observations. The technique analyzes diffraction patterns created when an electron beam strikes a crystalline surface at a steep angle. Each grain orientation produces a distinct pattern, allowing researchers to map grain size and orientation across a sample.

The challenge? Achieving a sufficiently clean, crystalline surface. Even thin passivation layers—the degradation films that form on reactive metals exposed to trace impurities—mask the diffraction signal. The researchers solved this by working entirely under inert atmosphere or high vacuum, carefully cutting fresh surfaces, and using cryogenic focused ion beam milling to polish cross-sections without inducing grain growth through heating.

Cross-sectional analysis provided the second critical finding. Grain structures observed in cross-sections matched those seen at surfaces. No microstructural changes occurred during cryogenic focused ion beam preparation. The protocol worked reliably.

Interestingly, grain boundaries in the foil cross-sections ran predominantly vertical. This probably reflects texturing that occurs when metal ingots are pressed into thin foils, or simply that grains exceed the foil thickness.

Inside the Deposited Films

With the protocol validated, the team turned to electrodeposited films. They prepared three different configurations representing the most investigated solid electrolytes: copper current collectors paired with a garnet-type lithium conductor, stainless steel with a sulfide-type lithium conductor, and carbon-coated aluminum with a sodium conductor.

Each cell was charged to deposit metal onto the current collector, then carefully fractured or peeled to expose a cross-section perpendicular to the interface. Cryogenic focused ion beam milling polished these cross-sections for electron backscatter diffraction analysis.

The results were striking. Grain sizes in electrodeposited films ranged from 10 to 150 micrometers—surprisingly large compared to other electrodeposited metals, which typically start nanocrystalline. More importantly, grain boundaries in all three systems ran perpendicular to the current collector interface, creating a columnar "row of teeth" microstructure distinctly different from the random grain boundary orientations in metal foils.

This geometry matters. When the battery discharges, lithium or sodium is stripped away from the solid electrolyte interface. That process should create voids—gaps where metal once existed. But where do those voids form? At grain boundaries, where defects concentrate? Or within grain interiors?

Watching Grains Evolve

To answer this question, the researchers performed in situ electron backscatter diffraction—imaging the microstructure while simultaneously charging or discharging the battery inside the microscope chamber.

For lithium deposition, they used microelectrodes prepared via focused ion beam to ensure changes occurred within the field of view. Initially, the freshly deposited lithium layer was too thin to produce quality diffraction patterns. But after depositing 10 to 15 additional micrometers, several grains 10 to 30 micrometers wide appeared.

Then something unexpected happened. After depositing another 5 micrometers, the grain structure changed. Two small grains with similar crystallographic orientations had merged with neighboring grains. After another deposition step, the trend continued. Multiple grains fused together.

Eventually, after depositing roughly 50 micrometers total, one enormous grain over 100 micrometers wide dominated the cross-section. Grain boundaries had moved laterally during electrodeposition, consuming smaller grains. This dynamic grain coarsening resembles a process called Ostwald ripening or secondary recrystallization, where interface and grain boundary energies drive microstructural evolution.

Critically, when the sample was stored for two weeks after deposition and then re-examined, the microstructure remained unchanged. No grain growth occurred during storage. The evolution happened exclusively during active electrodeposition.

For sodium dissolution, the team started with a sodium anode and stripped metal away while periodically stopping to map the microstructure. As expected, the voltage profile showed signatures of pore formation—voids opening at the interface as metal dissolved.

The electron backscatter diffraction maps revealed where those pores formed: predominantly within grain interiors, not at grain boundaries. In one large grain, dark regions indicating porosity appeared near the interface while the vertical grain boundaries on both sides remained intact. As stripping continued, pores nucleated within additional grains and grew into the metal bulk.

This preferential pore formation within grains is counterintuitive. Grain boundaries—two-dimensional defects with higher free energy than the bulk crystal—should thermodynamically favor metal dissolution and therefore void nucleation. But the observations suggest otherwise.

The explanation likely involves kinetics rather than thermodynamics. Computational work by other researchers predicts that vacancies—the atomic-scale voids created when metal atoms are stripped away—diffuse much faster along grain boundaries than through grain interiors. If vacancies can quickly move away from the interface along grain boundaries, they never accumulate sufficiently to form macroscopic pores there. Within grains, slower vacancy diffusion allows accumulation and pore nucleation.

Implications for Battery Design

These findings open new pathways for optimizing solid-state batteries. The microstructure of electrodeposited metal is neither random nor fixed. It evolves during deposition and influences subsequent performance—particularly pore formation during discharge, which affects how long the battery maintains good interfacial contact.

If smaller grains reduce pore formation by providing more grain boundaries for vacancy diffusion, then maximizing grain boundary density becomes a design goal. That density depends on initial nucleation—how many sites begin growing metal when deposition starts. Nucleation density in turn depends on current density, temperature, and the surface properties of the solid electrolyte and current collector.

Interestingly, the deposited metal microstructure showed no correlation with the underlying microstructures of the current collector or solid electrolyte. Even lithium plated onto a lithium foil exhibited the same columnar growth. The microstructure appears governed primarily by the electrochemical deposition conditions: current density, capacity deposited, and applied pressure.

But grain boundaries also move during deposition. Controlling that motion could further refine the microstructure. One approach: introduce impurities that pin grain boundaries in place. Even low concentrations of certain impurities dramatically reduce grain boundary mobility in metals. Seed layers or particles at the metal-electrolyte interface might serve this role. Indeed, previous work showed enhanced stripping performance when gold particles were present during initial lithium deposition—possibly by constraining grain growth.

Another strategy involves geometric confinement. Three-dimensional solid electrolyte host structures with sub-micrometer features could mechanically prevent lateral grain boundary motion, forcing the deposited metal to maintain a fine-grained structure.

These approaches remained speculative before because researchers lacked the tools to observe what actually happened inside the battery. Now they can test hypotheses directly.

The Broader Significance

Solid-state batteries represent a critical frontier in energy storage technology. Eliminating flammable liquid electrolytes addresses safety concerns that have plagued lithium-ion batteries. Using lithium or sodium metal anodes promises higher energy density than current graphite anodes. Reservoir-free configurations that deposit metal in situ rather than pre-installing reactive foils could simplify manufacturing and reduce costs.

But realizing these benefits requires solving fundamental materials challenges. How do you ensure uniform, dense metal deposition? How do you prevent dendrite formation? How do you minimize interfacial resistance? How do you maintain contact as the battery cycles?

These questions ultimately trace back to microstructure. The size, shape, and orientation of metal grains affect mechanical properties, diffusion pathways, nucleation sites, and electrochemical behavior. Without seeing and measuring that microstructure, optimization remained guesswork.

The protocol developed here—combining cryogenic focused ion beam preparation with electron backscatter diffraction under inert atmosphere—now provides that missing capability. It reveals that electrodeposited lithium and sodium form unexpectedly large columnar grains, that those grains dynamically coarsen during deposition but stabilize afterward, and that pore formation during discharge preferentially occurs within grains rather than at boundaries.

Each of these observations challenges prior assumptions and suggests new experimental directions. Researchers can now systematically vary deposition parameters—current density, temperature, pressure, electrolyte composition, surface treatments—and directly measure the resulting microstructural changes. They can correlate microstructure with electrochemical performance. They can test whether seed layers, three-dimensional hosts, or controlled impurities produce finer-grained structures that resist pore formation.

The work also extends beyond solid-state batteries. Understanding how metal microstructure evolves during electrochemical deposition has implications for electroplating, electroforming, and any application where metal films grow at interfaces. The finding that grain boundaries can move during electrodeposition—creating ever-larger grains—adds a dynamic dimension often overlooked in favor of studying initial nucleation alone.

For sodium, this marks the first detailed grain-level characterization. Sodium-ion batteries have attracted increasing attention as a lower-cost alternative to lithium-ion for grid storage and other applications where weight matters less than price. If sodium solid-state batteries follow similar development paths as their lithium counterparts, the insights gained here about microstructure and performance relationships will prove directly transferable.

Looking Forward

This research does not solve solid-state battery challenges. It provides the foundation for solving them. The next steps are clear: systematically map how every processing parameter affects microstructure, then connect microstructure to performance metrics like cycle life, rate capability, and interfacial stability.

Some questions remain open. Why do certain grains grow faster than others during deposition? What determines the initial nucleation density? How does stack pressure affect grain boundary mobility? Can post-deposition annealing at elevated temperatures further refine the microstructure?

The answers will emerge through experiments now made possible by this characterization protocol. What was once invisible—the hidden architecture of metal electrodes—can now be seen, measured, and ultimately controlled.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1038/s41563-024-02006-8