Since the previous research note · Earlier expectation not tested
Track continuity
Since the previous research note: How neutrons reveal the buried contact between lithium metal and a solid electrolyte
Previous expectation: Connecting buried-interface profiles with cell behavior could help engineers form contacts that remain effective during battery use.
What this study adds: Virtual-electrode imaging adds a dynamic view of lithium and sodium growth and a resolved sodium-removal sequence, including a persistent layer that changes subsequent microstructure. This supplies a distinct mechanism-focused milestone without testing the earlier forecast about contact-forming processes and cell behavior.
What changed in this study
An anode-free solid-state battery does not begin with a ready-made metal anode. The metal forms during charging at the boundary between a solid electrolyte and a current collector, then must be removed again during discharge. That boundary has to keep contact as the metal changes shape, while remaining strong enough to resist separation, intrusion and cracking.
The difficult question is not only whether lithium or sodium can be deposited. It is whether removing the metal restores the interface to its starting condition. A preprint dated March 1, 2026, reports nanoscale observations aimed at that problem. The study compares lithium and sodium growth and follows sodium removal in a controlled imaging system that makes buried-interface processes visible.
A virtual electrode makes hidden metal growth observable
The central technique is virtual-electrode low-energy electron microscopy, or VE-LEEM. In this setup, a directed electron beam acts as an electronic source of charge rather than relying on a conventional contacting current collector over the observation area. The beam induces metal formation at the surface. Ultraviolet light then drives the opposite process, allowing the researchers to watch the deposited metal dissolve.
That substitution matters because the real battery interface is difficult to observe directly while metal is forming inside a solid-state structure. Here, the experiment creates a controlled analogue of plating and stripping under ultrahigh-vacuum conditions. The electron beam energy was 6 eV. The experiments used a current of 0.3 μA for sodium and 1 μA for lithium, with deposited capacities reaching 100 μAh/cm² for sodium and 170 μAh/cm² for lithium.
Those values describe the conditions used to compare the two metals. They are not rankings of battery performance and are not direct conversions into device-scale current densities. The deposited amount was also estimated independently with atomic force microscopy, which provided an areal-capacity measure for comparing growth at a given amount of metal.
The electron images were combined with chemical and topographic information from synchrotron-based photoemission electron microscopy and atomic force microscopy. Together, these methods let the researchers follow not just whether a metal layer appeared, but how its surface structure and chemical signature changed as the experiment progressed.
Similar growth rules can produce different shapes
Lithium and sodium did not grow as identical films. The study reports distinct morphological pathways, linked to the different surface energetics of the two metals. In practical terms, the metal-specific interface energies influence how each deposit develops its visible structure.
Yet beneath those differences, the growth measurements followed a shared dynamic scaling regime. Scaling here means that measured features change in a related way as deposition proceeds. It does not mean that lithium and sodium have the same shape, the same surface texture or the same behavior in a working cell. Nor does it establish that the pattern applies to every solid electrolyte or every alkali metal.
This distinction is useful for anode-free battery research. A comparison based only on snapshots might suggest that two metals behave unrelatedly because their structures look different. A scaling relationship can reveal a common feature in how growth develops, even when the routes to that structure diverge. The result offers a quantitative way to compare the two chemistries while keeping their visible differences intact.
The comparison also shows why deposited capacity is a more meaningful reference than simply comparing elapsed experimental time. The researchers evaluated growth trajectories at controlled amounts of deposited material, with atomic force microscopy providing an independent estimate of the areal capacity. That helps separate the evolution of the deposit from the fact that the two experiments used different beam currents and capacity ranges.
Sodium removal unfolds in stages
The clearest evidence that deposition and removal are not mirror images comes from the sodium observations. During stripping, the metal does not simply shrink in the reverse order of its formation. The reported sequence begins with separation along grain boundaries. The separated clusters then shrink, while a residual layer persists at the interface with the electrolyte.
A grain boundary is a meeting line between crystalline regions in a polycrystalline metal. In this experiment, those boundaries become pathways along which the sodium structure opens. The connected deposit can therefore break apart before every part of it has disappeared. Cluster decay follows as the separated pieces diminish, but the interface is not returned to a blank starting state.
The persistent layer has a further consequence. It changes the topology of the grain boundaries and redirects later growth pathways, even without immediate capacity loss. That separates two ideas that can look equivalent in a coarse battery measurement. The visible shape of the metal may move toward its earlier appearance, while the energetic and structural condition of the interface has already changed.
The study therefore treats morphological recovery and complete energetic reversibility as different outcomes. A later deposit can grow on an interface whose local structure has been altered by the previous cycle. The metal has been removed, but the boundary retains a memory of what happened there.
The chemical identity of this residual interfacial layer remains unresolved in the reported work. Its presence is an important structural observation, but it does not by itself show irreversible capacity loss. Establishing what the layer is would require chemically resolved measurements that can identify its composition at the interface.
The observation is powerful but still an analogue
VE-LEEM addresses a difficult measurement problem: how to see nanoscale metal formation and removal at an interface that is normally buried inside a solid-state battery. Its strength is control. The electron beam supplies charge in a defined observation geometry, ultraviolet light initiates removal, and complementary instruments track morphology, chemical evolution and deposited amount.
That control also defines the boundary of the conclusion. The observations were made in an ultrahigh-vacuum virtual-electrode experiment, not as a demonstration that a complete anode-free cell will cycle for a specified number of cycles or retain a specified amount of energy. Real battery interfaces must also maintain contact while mechanical stresses and changing geometries act across the electrolyte and current-collector boundary.
The study provides a mechanism-focused view of how growth and removal can differ. It does not establish a quantified improvement in cell lifetime, energy density, charging speed, safety or manufacturing cost. The shared growth scaling is a comparison across the lithium and sodium experiments, not proof of equal device performance. Likewise, the detailed grain-boundary unzipping and cluster-decay sequence described here is specific to the sodium stripping observations and should not be assigned to lithium without corresponding evidence.
For engineers, the practical value is a sharper set of questions. Which interfacial energies encourage a deposit to remain connected during useful operation? Which grain-boundary structures allow removal without leaving a layer that changes the next cycle? And which of those behaviors persist when the interface is part of a mechanically constrained battery rather than a controlled microscopy analogue?
What this could change
If the observed growth and sodium-removal mechanisms persist under realistic contacts and mechanical loading, battery designers could use them to shape interfaces and metal microstructures that preserve usable contact through repeated anode formation and dissolution. Instead of treating deposition and removal as automatically reversible versions of one process, they could design around the different pathways each one takes.
That could make it easier to develop anode-free solid-state cells whose interfaces remain functional over repeated use. The possible payoff is more durable high-energy cells, but only if the nanoscale behavior translates into improved cycling in actual devices. The preprint supplies a way to investigate that connection; it does not yet demonstrate the durability itself.
Key Sources
- Primary source: Nanoscale imaging reveals critical plating and stripping mechanisms in anode-free lithium and sodium solid-state batteries
arXiv · Preprint · 2026-03-01