All‐solid‐state Li‐metal batteries (ASLMBs) represent a significant breakthrough in the quest to overcome limitations associated with traditional Li‐ion batteries, particularly in energy density and safety aspects. However, widespread implementation is stymied due to a lack of profound understanding of the complex mechano‐electro‐chemical behavior of Li metal in the ASLMBs. Herein, operando neutron imaging and X‐ray computed tomography (XCT) are leveraged to nondestructively visualize Li behaviors within ASLMBs. This approach offers real‐time observations of Li evolutions, both pre‐ and post‐ occurrence of a “soft short”. The coordination of 2D neutron radiography and 3D neutron tomography enables charting of the terrain of Li metal deformation operando. Concurrently, XCT offers a 3D insight into the internal structure of the battery following a “soft short”. Despite the manifestation of a “soft short”, the persistence of Faradaic processes is observed. To study the elusive “soft short” , phase field modeling is coupled with electrochemistry and solid mechanics theory. The research unravels how external pressure curbs dendrite growth, potentially leading to dendrite fractures and thus uncovering the origins of both “soft” and “hard” shorts in ASLMBs. Furthermore, by harnessing finite element modeling, it dive deeper into the mechanical deformation and the fluidity of Li metal.

Li-Mg alloys are promising as positive electrodes (anodes) for Li-ion batteries due to the high Li storage capacity and the relatively lower volume change during the lithiation/delithiation process. They also present a unique opportunity to image the Li distribution through the electrode thickness at various delithiation states. In this work, spatial distributions of Li in electrochemically delithiated Li-Mg alloy electrodes have been quantitatively determined using neutron tomography. Specifically, the Li concentration profiles along thickness direction are determined. A rigorous analytical model to quantify the diffusion-controlled delithiation, accompanied by phase transition and boundary movement, has also been developed to explain the delithiation mechanism. The analytical modeling scheme successfully predicted the Li concentration profiles which agreed well with the experimental data. It is demonstrated that during discharge Li is removed by diffusion through the solid solution Li-Mg phases and this proceeds with β→α phase transition and the associated phase boundary movement through the thickness of the electrode. This is also accompanied by electrode thinning due to the change in molar volume during delithiation. Following the approaches developed here, one can develop a rigorous and quantitative understanding of electrochemical delithiation in electrodes of electrochemical cells, similar to that in the present Li-Mg electrodes.

Lithium-metal (Li⁰) anodes potentially enable all-solid-state batteries with high energy density. However, it shows incompatibility with sulfide solid-state electrolytes (SEs). One strategy is introducing an interlayer, generally made of a mixed ionic-electronic conductor (MIEC). Yet, how Li behaves within MIEC remains unknown. Herein, we investigated the Li dynamics in a graphite interlayer, a typical MIEC, by using operando neutron imaging and Raman spectroscopy. This study revealed that intercalation-extrusion-dominated mechanochemical reactions during cell assembly transform the graphite into a Li-graphite interlayer consisting of SE, Li⁰, and graphite-intercalation compounds. During charging, Li⁺ preferentially deposited at the Li-graphite|SE interface. Upon further plating, Li⁰-dendrites formed, inducing short circuits and the reverse migration of Li⁰. Modeling indicates the interface has the lowest nucleation barrier, governing lithium transport paths. Our study elucidates intricate mechano-chemo-electrochemical processes in mixed conducting interlayers. The behavior of Li⁺ and Li⁰ in the interlayer is governed by multiple competing factors.

The surge in electric vehicle demand has propagated the extensive application of lithium-ion batteries (LIBs) in recent years. Gaining significant traction due to their promising high-energy density and elevated safety over traditional LIBs, all-solid-state Li batteries (ASLBs) have nonetheless been faced with hurdles relating to battery performance. These include concerns with interfacial compatibility, structural stability, Li dendrite inhibition, and large-scale manufacturing. To tackle these issues, it is necessary to employ advanced characterization methods to comprehend the intrinsic mechanisms within ASLBs. In this article, we advocate for the use of neutron imaging as a nondestructive approach for the operando visualization of ASLBs. We draw comparisons with other operando visualization strategies, underline the benefits of neutron imaging, and discuss its potential applicability in the scrutiny of all-solid-state Li metal batteries and all-solid-state Li-sulfur batteries. Neutron imaging provides valuable insights into the dynamics of Li concentration, reaction mechanisms, and transport constraints in ASLBs. These insights are pivotal in contributing to the evolution of high-performance all-solid-state batteries. Graphical abstract This article discusses the application of neutron imaging for operando characterization of all-solid-state batteries. It compares neutron imaging to other techniques and highlights its advantages in visualizing light elements such as lithium. It also covers recent progress in using neutron imaging to investigate reaction mechanisms, lithium dynamics, and failure modes in all-solid-state lithium metal and lithium sulfur batteries. It also analyzes the future outlook for neutron imaging as a powerful nondestructive tool to gain insights into interfacial phenomena in all-solid-state batteries.

High-mass-loading cathodes are crucial for achieving high energy density in all-solid-state batteries from the lab scale to industry. However, as mass-loading increases, electrochemical performance is significantly compromised due to sluggish kinetics. In this work, operando neutron imaging is deployed on a high-mass-loading NMC 811 cathode of 33 mg/cm² (5.0 mAh/cm²) and directly visualizes the lithiation prioritization of the cathode active material (CAM) from the solid electrolyte membrane side to the current collector side. In addition to the tortuosity, another key limitation on ion transfer in the cathode arises from the mismatch between the uniform distribution of the solid electrolyte (catholyte) in the conventional composite cathode and the non-uniform Li⁺ flux generated by the faradaic reaction of CAMs. Therefore, we engineer a gradient in the catholyte concentration to match the Li⁺ flux distribution as a means of eliminating the ion transfer obstacle. This approach demonstrates enhanced rate performance, even with high-mass-loading cathodes. A LiCoO₂ composite cathode with 100 mg/cm² high-mass-loading exhibits an areal capacity of 10.4 mAh/cm² at a current density of 2.25 mA/cm². This work provides insight into the ion-transport limitation in thick cathodes and demonstrates an effective gradient design to overcome the kinetic barrier and achieve high battery performance.