Microstructure of Lithium Dendrites Revealed by Room-Temperature Electron Microscopy

A Large-Scale Fabrication of Flexible, Ultrathin, and Robust Solid Electrolyte for Solid-State Lithium-Sulfur Batteries

All-solid-state lithium metal batteries (ASSLMBs) are considered as the most promising candidates for the next-generation high-safety batteries. To achieve high energy density in ASSLMBs, it is essential that the solid-state electrolytes (SSEs) are lightweight, thin, and possess superior electrochemical stability. In this study, a feasible and scalable fabrication approach to construct 3D supporting skeleton using an electro-blown spinning technique is proposed. This skeleton not only enhances the mechanical strength but also hinders the migration of Li-salt anions, improving the lithium-ion transference number of the SSE. This provides a homogeneous distribution of Li-ion flux and local current density, promoting uniform Li deposition. As a result, based on the mechanically robust and thin SSEs, the Li symmetric cells show outstanding Li plating/stripping reversibility. Besides, a stable interface contact between SSE and Li anode has been established with the formation of an F-enriched solid electrolyte interface layer. The solid-state Li|sulfurized polyacrylonitrile (Li|SPAN) cell achieves a capacity retention ratio of 94.0% after 350 cycles at 0.5 C. Also, the high-voltage Li|LCO cell shows a capacity retention of 92.4% at 0.5 C after 500 cycles. This fabrication approach for SSEs is applicable for commercially large-scale production and application in high-energy-density and high-safety ASSLMBs.

Morphology characterization of dendrites on lithium metal electrodes by NMR spectroscopy

Despite the considerable potential offered by lithium metal's high capacity for rechargeable batteries, challenges such as dendrite formation and safety concerns persist. As strategies continue to advance in dendrite management, the demand for efficient monitoring tools becomes increasingly pronounced. In this study, we delve into the characterization of dendrites, elucidating the influence of microstructure morphology on the NMR spectrum using a combination of simulations and experiments. Systematic variations in various geometrical parameters highlight dendrite density as a pivotal distinguishing feature. Furthermore, the investigation explores the effectiveness of a pulse sequence in selectively exciting microstructures over the bulk, providing valuable insights into mitigating dendrite-related challenges in lithium-metal batteries.

Quantifying the Growth Kinetics of Lithium Metal Reduced from Solid Ionic Conductors

Investigating the growth kinetics of Li metal in solid-state batteries is crucial to both a fundamental understanding and practical application. Here, by directly observing the formation of Li metal from Ta-doped Li6.4La3Zr1.4Ta0.6O12 (LLZTO) in a transmission electron microscope, the growth kinetics is analyzed quantitatively. The growth kinetics of Li deposits shows a cubic-curve characteristic for LLZTO with Li-source-free. Instead, a linear growth process is observed with Li-source supplied. The impact of the illuminating electron dose rate on the growth kinetics is clarified, indicating that even low dose rates (1-3 e-/Å2/s) could affect Li growth, highlighting the significance of controlling dose rates. Furthermore, a new pathway for the formation of Li metal from Li-containing materials utilizing the field-emission effect is reported. This work has implications on the failure mechanism in solid batteries by using limited Li anodes and opens pathways for regulating Li growth in LLZTO at various scenarios, which can also extend to other ionic conductors.

Interface Modifications of Lithium Metal Anode for Lithium Metal Batteries

Lithium metal batteries (LMBs) enable much higher energy density than lithium-ion batteries (LIBs) and thus hold great promise for future transportation electrification. However, the adoption of lithium metal (Li) as an anode poses serious concerns about cell safety and performance, which has been hindering LMBs from commercialization. To this end, extensive effort has been invested in understanding the underlying mechanisms theoretically and experimentally and developing technical solutions. In this review, we devote to providing a comprehensive review of the challenges, characterizations, and interfacial engineering of Li anodes in both liquid and solid LMBs. We expect that this work will stimulate new efforts and help peer researchers find new solutions for the commercialization of LMBs.

Revealing the Distribution of Lithium Compounds in Lithium Dendrites by Four-Dimensional Electron Microscopy Analysis

Characterizing the microstructure of radiation- and chemical-sensitive lithium dendrites and its solid electrolyte interphase (SEI) is an important task when investigating the performance and reliability of lithium-ion batteries. Widely used methods, such as cryogenic high-resolution transmission electron microscopy as well as related spectroscopy, are able to reveal the local structure at nanometer and atomic scale; however, these methods are unable to show the distribution of various crystal phases along the dendrite in a large field of view. In this work, two types of four-dimensional electron microscopy diffractive imaging methods, i.e., scanning electron nanodiffraction (SEND) and scanning convergent beam electron diffraction (SCBED), are employed to show a new pathway on characterizing the sensitive lithium dendrite samples at room temperature and in a large field of view. Combining with the non-negative matrix factorization (NMF) algorithm, orientations of different lithium metal grains along the lithium dendrite as well as different lithium compounds in the SEI layer are clearly identified.

Atomic-Resolution Electron Microscopy Unravelling the Role of Unusual Asymmetric Twin Boundaries in the Electron-Beam-Sensitive NASICON-Type Solid Electrolyte

An atomic-scale understanding of the role of nonperiodic features is essential to the rational design of highly Li-ion-conductive solid electrolytes. Unfortunately, most solid electrolytes are easily damaged by the intense electron beam needed for atomic-resolution electron microscopy observation, so the reported in-depth atomic-scale studies are limited to Li0.33La0.56TiO3- and Li7La3Zr2O12-based materials. Here, we observe on an atomic scale a third type of solid electrolyte, Li1.3Al0.3Ti1.7(PO4)3 (LATP), through minimization of damage induced by specimen preparation. With this capability, LATP is found to contain large amounts of twin boundaries with an unusual asymmetric atomic configuration. On the basis of the experimentally determined structure, the theoretical calculations suggest that such asymmetric twin boundaries may considerably promote Li-ion transport. This discovery identifies a new entry point for optimizing ionic conductivity, and the method presented here will also greatly benefit the mechanistic study of solid electrolytes.

Directing (110) Oriented Lithium Deposition through High-flux Solid Electrolyte Interphase for Dendrite-free Lithium Metal Batteries

Controlling lithium (Li) electrocrystallization with preferred orientation is a promising strategy to realize highly reversible Li metal batteries (LMBs) but lack of facile regulation methods. Herein, we report a high-flux solid electrolyte interphase (SEI) strategy to direct (110) preferred Li deposition even on (200)-orientated Li substrate. Bravais rule and Curie-Wulff principle are expanded in Li electrocrystallization process to decouple the relationship between SEI engineering and preferred crystal orientation. Multi-spectroscopic techniques combined with dynamics analysis reveal that the high-flux CF3 Si(CH3 )3 (F3 ) induced SEI (F3 -SEI) with high LiF and -Si(CH3 )3 contents can ingeniously accelerate Li+ transport dynamics and ensure the sufficient Li+ concentration below SEI to direct Li (110) orientation. The induced Li (110) can in turn further promote the surface migration of Li atoms to avoid tip aggregation, resulting in a planar, dendrite-free morphology of Li. As a result, our F3 -SEI enables ultra-long stability of Li||Li symmetrical cells for more than 336 days. Furthermore, F3 -SEI modified Li can significantly enhance the cycle life of Li||LiFePO4 and Li||NCM811 coin and pouch full cells in practical conditions. Our crystallographic strategy for Li dendrite suppression paves a path to achieve reliable LMBs and may provide guidance for the preferred orientation of other metal crystals.

Ant-Nest-like Lithiophilic Host for Long-Life Lithium Metal Anodes

The rational design of confined host to tutor Li nucleation and deposition behavior remains a key challenge for the long stability of lithium metal anodes (LMAs), while the scalability and feasibility of the method need to be taken into concern. Herein, a biomimic strategy is designed for tutoring in-depth nucleation and bottom-up Li deposition by composing ant-nest-like lithiophilic hosts for LMAs with light-weight flexible and conductive carbon nanotubes (CNTs) as the framework, table salt (sodium chloride, NaCl) as the washable porous creator, and homogeneously dispersed nano-Si as the nucleation site. It possesses similar optimized structure as ant nests in nature and can provide large and conductive inner volume for Li storage. Combining with the interconnected passways can ensure effective ion compensation like food transport channels for ants, and the well-designed host can take effect as an individual Li anode (5 mA h cm-2 area Li loading for demonstration) and the record-long stable LMA host can be achieved for over a 2200 h lifespan with minimum volume expansion. Therefore, this biomimic strategy is developed with all commercialized battery materials, and all industry compatible production methods can provide a feasible technical path for the stable, long-cyclability, and reliable host design for LMAs.

Origin of Heterogeneous Stripping of Lithium in Liquid Electrolytes

Lithium metal batteries suffer from low cycle life. During discharge, parts of the lithium are not stripped reversibly and remain isolated from the current collector. This isolated lithium is trapped in the insulating remaining solid-electrolyte interphase (SEI) shell and contributes to the capacity loss. However, a fundamental understanding of why isolated lithium forms and how it can be mitigated is lacking. In this article, we perform a combined theoretical and experimental study to understand isolated lithium formation during stripping. We derive a thermodynamic consistent model of lithium dissolution and find that the interaction between lithium and SEI leads to locally preferred stripping and isolated lithium formation. Based on a cryogenic transmission electron microscopy (cryo TEM) setup, we reveal that these local effects are particularly pronounced at kinks of lithium whiskers. We find that lithium stripping can be heterogeneous both on a nanoscale and on a larger scale. Cryo TEM observations confirm our theoretical prediction that isolated lithium occurs less at higher stripping current densities. The origin of isolated lithium lies in local effects, such as heterogeneous SEI, stress fields, or the geometric shape of the deposits. We conclude that in order to mitigate isolated lithium, a uniform lithium morphology during plating and a homogeneous SEI are indispensable.

Microstructure of the Li-Al-O Second Phases in Garnet Solid Electrolytes

The microstructure of the Li₇La₃Zr₂O₁₂ (LLZO) garnet solid electrolyte is critical for its performance in all-solid-state lithium-ion battery. During conventional high-temperature sintering, second phases are generated at the grain boundaries due to the reaction between sintering aids and LLZO, which have an enormous effect on the performances of LLZO. However, a detailed structure study of the second phases and their impact on physical properties is lacking. Here, crystal structures of the second phases in LLZO pellets are studied in detail by transmission electron microscopy. Three different crystal structures of Li-Al-O second phases, γ-LiAlO₂, α-Li₅AlO₄, and β-Li₅AlO₄ were identified, and atomic-scale lattice information was obtained by applying low-dose high-resolution imaging for these electron-beam-sensitive second phases. On this basis, the structure-property relationship of these structures was explored. It was found that sintering aids with a higher Li/Al ratio are beneficial to form Li-rich second phases, which result in more highly ionic conductive LLZO.