Neutrons catch lithium in motion inside a solid-state battery

For the first time, neutron diffraction experiments at the ILL have tracked lithium movement in real time inside a working solid-state battery, revealing hidden structural complexity and new clues for smarter battery design

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Energie

Batteries are part of everyday life, powering everything from phones and laptops to electric cars. Most rechargeable batteries use a liquid to help lithium ions move during charging and discharging. But this liquid can create safety issues and limit how far battery performance can improve.

Solid-state batteries replace the liquid with a solid material. They could lead to safer, more compact and more powerful batteries. But making them work reliably remains a challenge. Unlike in a liquid, lithium ions must move through solid materials and if this movement is uneven, some parts of the battery may charge faster than others, reducing performance.

For the first time, a team at the Institut Laue-Langevin and its partners have watched the lithium movement in real time using neutron diffraction, the only technique sensitive enough to track lithium deep inside a thick, working battery.

Their results revealed unexpected structural complexity in the electrode, showing that lithium movement inside the battery was not as uniform as expected. Understanding this hidden behaviour could point the way toward smarter battery designs.

The challenge of looking inside solid-state batteries

Lithium-ion batteries have become essential to modern life, powering everything from portable electronics to electric vehicles. During charge and discharge, lithium ions move between the two electrodes of the battery. In most commercial lithium-ion batteries, this movement takes place through a liquid electrolyte.

All-solid-state batteries replace this liquid with a solid material. This makes them a promising route toward safer batteries with higher energy density and better performance under demanding conditions such as extreme temperatures. However, replacing a liquid by a solid also brings new challenges. Lithium ions must now move through solid materials and across solid-solid interfaces, where transport can be more difficult and less uniform.

To design better solid-state batteries, scientists need to understand what happens inside them while they are actually operating. This is not straightforward. The key processes take place inside a sealed device, often within relatively thick layers of material. They also involve lithium, a light element that is difficult to follow with many experimental techniques.

This makes it challenging to connect the battery’s performance with the structural changes taking place inside it during charge and discharge.

Why neutrons are the right probe for studying solid-state batteries

Neutron diffraction offers a uniquely powerful way to overcome this challenge. Unlike X-rays, which interact mainly with electrons, neutrons interact directly with atomic nuclei. This makes them particularly sensitive to lithium, the very element responsible for charge transfer in lithium-based batteries. Neutrons can also help distinguish elements with similar atomic numbers, such as the transition metals found in many positive electrode materials.

Another important advantage is penetration. Neutrons can travel deep into matter, allowing researchers to probe the full bulk of a thick battery pellet rather than only its surface. They can do this without damaging the sample, which is essential when the goal is to follow a battery while it is working.

This makes operando neutron powder diffraction especially valuable: it allows structural changes to be tracked in real time as the battery charges and discharges.

A working battery designed for neutron experiments

This is the approach taken in a recent study published in Advanced Energy Materials, where researchers from the ILL and their partners followed a working all-solid-state battery during cycling using operando neutron powder diffraction.

The battery studied combined LiNi_0.6Mn_0.2Co_0.2O₂, known as NMC622, as the positive electrode; a mixed-halide argyrodite solid electrolyte, Li_5.4PS_4.4BrCl_0.6 and a Li_0.5In alloy as the negative electrode. One of the main experimental obstacles was the need for a large quantity of active material, at least 140 mg of NMC622, to generate to obtain a clear neutron diffraction signal. This required the team to build a 2.5 mm-thick battery pellet, which introduced new electrochemical challenges, notably increased internal resistance.

The key to overcoming this challenge was the use of a newly developed high-conductivity electrolyte, Li_5.4PS_4.4BrCl_0.6, whose ionic conductivity is six times higher than that of conventional Li₆PS₅Cl. This allowed the team to extract approximately 55% of the lithium from the active material, comparable to the performance of standard laboratory-scale cells, despite the much thicker battery pellet required for the neutron experiment.

Unexpected complexity inside the positive electrode revealed by neutrons

The experiment was carried out on the D20 diffractometer at the ILL using the newly designed ILLBAT#5 electrochemical cell. This cell allowed the team to collect neutron diffraction data while the all-solid-state battery was charging and discharging.

During the first charge-discharge cycle at room temperature, neutron diffraction patterns were collected every hour. These operando measurements were combined with ex situ diffraction data collected at selected states of charge, giving the researchers a detailed picture of how the battery evolved during cycling.

The results revealed several key findings:

Lithium extraction was not as uniform as expected. During charging, two distinct structural phases of NMC622, known as H1 and H2, coexisted through most of the charge. This was unexpected at the very slow cycling rate used in the experiment, C/60, where the electrode would normally be expected to follow a smoother single-phase pathway. The team attributed this behaviour to a non-uniform current distribution within the thick positive electrode, causing different regions to reach different states of charge simultaneously.
Temperature made the reaction more homogeneous. When the experiment was repeated at 100°C, the two-phase behaviour disappeared and the electrode followed a clean single-phase pathway. This confirmed that improved ionic conductivity at higher temperature homogenizes the lithiation.
The solid electrolyte remained structurally stable. Throughout the entire first cycle the solid electrolyte remained structurally stable with no significant changes in the peak positions or intensities observed in the neutron diffraction data. This is an encouraging result for the viability of sulfide-based all-solid-state batteries.

Together, these findings highlight the unique value of operando neutron diffraction for studying working solid state batteries. By following phase fractions, lattice parameters and lithium occupancy in real time, the technique reveals processes that ex situ measurements alone cannot capture.

The study provides a clearer picture of how lithium extraction occurs inside a thick working solid-state battery, and shows how electrode design, ionic conductivity and operating conditions can influence the uniformity of the battery reaction. These insights could help guide smarter designs for future all-solid-state batteries.

Reference:

S. A. Kumar, D. Shanbhag, O. Korjus, et al. “Investigation of the Lithium Extraction Mechanism from LiNi0.6Mn0.2Co0.2O2 by Using Operando Neutron Diffraction in an All-Solid-State Battery.” Advanced Energy Materials 16, no. 14 (2026) https://doi.org/10.1002/aenm.202506600

ILL Instruments: D20

ILL Contact Person: Emmanuelle Suard

Institutions involved in the research: Université de Picardie Jules Verne, Umicore, University of Bordeaux, TIAMAT, RS2E, Institut Universitaire de France