Interfacial Atomistic Mechanisms of Lithium Metal Stripping and Plating in Solid-State Batteries

Atomic mechanism of lithium dendrite penetration in solid electrolytes

Lithium dendrite penetration through ceramic electrolytes is known to result in mechanical failure and short circuits, which has impeded the commercialization of all-solid-state lithium anode batteries. However, the underlying mechanism still remains under debate, due in part to a lack of in situ atomic-level observations of the dendrite penetration process. Here, we employ molecular dynamics simulations to reproduce the dynamic process of dendrite nucleation and penetration. Our findings reveal that dynamically generated lithium depositions lead to a continuous accumulation of internal stress, culminating in fracture of the solid electrolyte at dendrite tips. We demonstrate that the classical Griffith theory remains effective in assessing this fracture mode, but it is necessary to consider the electrochemical impact of local lithium ion concentration on the fracture toughness. Additionally, in polycrystalline solid electrolytes, we observe that dendrite nuclei within grains typically deflect towards and propagate along grain boundaries. Simulations and experimental evidence both identify that dendrite induced fractures at grain boundaries exhibit a mixed Mode I and Mode II pattern, contingent on their fracture toughness and the angle between dendrites and grain boundaries. These insights deepen our understanding of dendrite penetration mechanisms and may offer valuable guidance for improving the performance of solid electrolytes.

Hopping-Phase Ion Bridge Enables Fast Li+ Transport in Functional Garnet-Type Solid-State Battery at Room Temperature

Composite polymer electrolytes (CPEs) containing Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is widely regarded as leading candidate for high energy density solid-state lithium-metal batteries due to its exceptional ionic conductivity and environmental stability. However, Li2CO3 and LiOH layers at LLZTO surface greatly hinder Li+ transport between LLZTO-polymer and the electrode-electrolyte interface. Herein, the surface of LLZTO is boronized to obtain functionalized LLZTO, and its conversion mechanism is clarified. By dissolving the crystal structure of cellulose to obtain hopping-phase ion bridge (HPIB), which release the Li+ transport activity of its oxygen-containing polar functional group (─OH, ─O─). Therefore, a high-throughput ion transporter (HTIT-37) with high ion transfer number (0.86) is prepared by introducing the HPIB into functionalized LLZTO and polyvinylidene fluoride interface by intermolecular hydrogen bond interaction, and it is demonstrated that the HPIB acts as a "highway" for the Li+ across this heterogeneous interface. Moreover, the HPIB is found to self-adsorb on the SEI surface, leading to fast Li+ transport kinetics at anode-CPE interface. Thus, the lifespan of Li|HTIT-37|Li is over 8000 h, and the critical current density exceeds 2.3 mA cm-2. The LiNi0.5Co0.2Mn0.3O2|Li and Li1.2Ni0.13Co0.13Mn0.54O2|Li battery remains stable with the HPIB-enhanced electrode process, proving the application potential of LLZTO-based CPE in high energy density SSLMB.

Dendrite formation in solid-state batteries arising from lithium plating and electrolyte reduction

All-solid-state batteries offer high-energy-density and eco-friendly energy storage but face commercial hurdles due to dendrite formation, especially with lithium metal anodes. Here we report that dendrite formation in Li/Li7La3Zr2O12/Li batteries occurs via two distinct mechanisms, using non-invasive solid-state nuclear magnetic resonance and magnetic resonance imaging. Tracer-exchange nuclear magnetic resonance shows non-uniform Li plating at electrode-electrolyte interfaces and local Li+ reduction at Li7La3Zr2O12 grain boundaries. In situ magnetic resonance imaging reveals rapid dendrite formation via non-uniform Li plating, followed by sluggish bulk dendrite nucleation from Li+ reduction, with an intervening period of stalled growth. Formation of amorphous dendrites and subsequent crystallization, the defect chemistry of solid electrolytes and battery operating conditions play a critical role in shaping the complex interplay between the two mechanisms. Overall, this work deepens our understanding of dendrite formation in solid-state Li batteries and provides comprehensive insight that might be valuable for mitigating dendrite-related challenges.

Dual-Seed Strategy for High-Performance Anode-Less All-Solid-State Batteries

Interest in all-solid-state batteries (ASSBs), particularly the anode-less type, has grown alongside the expansion of the electric vehicle (EV) market, because they offer advantages in terms of their energy density and manufacturing cost. However, in most anode-less ASSBs, the anode is covered by a protective layer to ensure stable lithium (Li) deposition, thus requiring high temperatures to ensure adequate Li ion diffusion kinetics through the protective layer. This study proposes a dual-seed protective layer consisting of silver (Ag) and zinc oxide (ZnO) nanoparticles for sulfide-based anode-less ASSBs. This dual-seed-based protective layer not only facilitates Li diffusion via multiple lithiation pathways over a wide range of potentials, but also enhances the mechanical stability of the anode interface through the in situ formation of a Ag-Zn alloy with high ductility. The capacity retention during full-cell evaluation is 80.8% for 100 cycles when cycled at 1 mA cm-2 with 3 mAh cm-2 at room temperature. The dual-seed approach provides useful insights into the design of multi-seed concepts in which, from a mechanochemical perspective, various lithiophilic materials synergistically impact upon the anode-less interface.

Compositional Engineering of Lithium Metal Anode for High-Performance Garnet-Type Solid-State Lithium Battery

Garnet-type solid-state lithium batteries (SSLBs) possess excellent potential owing to their safety and high energy density. However, fundamental barriers are deficient cycling stability and poor rate capability. The main concern lies in generating voids at the Li|garnet interface during Li stripping, stemming from the sluggish diffusion of Li atoms inside the bulk Li metal. Herein, a composite anode (AN@Li) containing Li-Al alloy, Li3N, and LiNO2 is designed by introducing aluminum nitrate into molten Li. The lower interfacial formation energies exhibited by Li-Al alloy, Li3N, and LiNO2 with garnet solid-state electrolyte (SSE) enhance the wettability of AN@Li toward SSE. Meanwhile, it affords efficient conductive pathways that facilitate Li+ diffusion in the bulk anode (not just on the surface). Impressively, the resulting symmetric cell with AN@Li electrodes achieves high critical current density (1.95 mA cm-2) and long cycle life (6000 h at 0.3 mA cm-2). The SSLB coupled with LiFePO4 cathode and AN@Li anode enables stable cycling for 200 cycles at a high rate of 1 C with a retention of 96% and exhibiting outstanding rate capability (145.9 mAh g-1 at 2 C). This work provides practical insights for producing high-performance lithium metal anode for advanced garnet-type SSLBs.

Theoretical calculations and simulations power the design of inorganic solid-state electrolytes

Using solid-state electrolytes (SSEs) to build batteries helps improve the safety and lifespan of batteries, making it crucial to deeply understand the fundamental physical and chemical properties of SSEs. Theoretical calculations based on modern quantum chemical methods and molecular simulation techniques can explore the relationship between the structure and performance of SSEs at the atomic and molecular levels. In this review, we first comprehensively introduce theoretical methods used to assess the stability of SSEs, including mechanical, phase, and electrochemical stability, and summarize the significant progress achieved through these methods. Next, we outline the methods for calculating ion diffusion properties and discuss the advantages and limitations of these methods by combining the diffusion behaviors and mechanisms of ions in the bulk phase, grain boundaries, and electrode-solid electrolyte interfaces. Finally, we summarize the latest research progress in the discovery of high-quality SSEs through high-throughput screening and machine learning and discuss the application prospects of a new mode that incorporates machine learning into high-throughput screening.

Subnanoscale Ion Beam Modification-Assisted Smoothing of Heterostructure Surfaces

Glass ceramic (GC) is the most promising material for objective lenses for extreme ultraviolet lithography that must meet the subnanometer precision, which is characterized by low values of high spatial frequency surface roughness (HSFR). However, the HSFR of GC is typically degraded during ion beam figuring (IBF). Herein, a developed method for constructing molecular dynamics (MD) models of GC was presented, and the formation mechanisms of surface morphologies were investigated. The results indicated that the generation of the dot-like microstructure was the result of the difference in the erosion rate caused by the difference in the intrinsic properties between ceramic phases (CPs) and glass phases (GPs). Further, the difference in the microstructure of the IBF surface under different beam angles was mainly caused by the difference in the two types of sputtering. Quantum mechanical calculations showed that the presence of interstitial atoms would result in electron rearrangement and that the electron localization can lead to a reduction in CP stability. To obtain a homogeneous surface, the effects of beam parameters on the heterogeneous surface were systematically investigated based on the proposed MD model. Then, a novel ion beam modification (IBM) method was proposed and demonstrated by TEM and GIXRD. The range of ion beam smoothing parameters that could effectively converge the HSFR of the modified surface was determined through numerous experiments. Using the optimized beam parameters, an ultrathin homogeneous modified surface within 3 nm was obtained. The HSFR of GC smoothed by ion beam modification-assisted smoothing (IBMS) dropped from 0.348 to 0.090 nm, a 74% reduction. These research results offer a deeper understanding of the morphology formation mechanisms of the GC surfaces involved in ion beam processing and may point to a new approach for achieving ultrasmooth heterostructure surfaces down to the subnanometer scale.

Bridging the gap between academic research and industrial development in advanced all-solid-state lithium-sulfur batteries

The energy storage and vehicle industries are heavily investing in advancing all-solid-state batteries to overcome critical limitations in existing liquid electrolyte-based lithium-ion batteries, specifically focusing on mitigating fire hazards and improving energy density. All-solid-state lithium-sulfur batteries (ASSLSBs), featuring earth-abundant sulfur cathodes, high-capacity metallic lithium anodes, and non-flammable solid electrolytes, hold significant promise. Despite these appealing advantages, persistent challenges like sluggish sulfur redox kinetics, lithium metal failure, solid electrolyte degradation, and manufacturing complexities hinder their practical use. To facilitate the transition of these technologies to an industrial scale, bridging the gap between fundamental scientific research and applied R&D activities is crucial. Our review will address the inherent challenges in cell chemistries within ASSLSBs, explore advanced characterization techniques, and delve into innovative cell structure designs. Furthermore, we will provide an overview of the recent trends in R&D and investment activities from both academia and industry. Building on the fundamental understandings and significant progress that has been made thus far, our objective is to motivate the battery community to advance ASSLSBs in a practical direction and propel the industrialized process.

Regenerative Solid Interfaces Enhance High-Performance All-Solid-State Lithium Batteries

The performance of all-solid-state lithium batteries (ASSLBs) is significantly impacted by lithium interfacial instability, which originates from the dynamic chemical, morphological, and mechanical changes during deep Li plating and stripping. In this study, we introduce a facile approach to generate a conductive and regenerative solid interface, enhancing both the Li interfacial stability and overall cell performance. The regenerative interface is primarily composed of nanosized lithium iodide (nano-LiI), which originates in situ from the adopted solid-state electrolyte (SSE). During cell operation, the nano-LiI interfacial layer can reversibly diffuse back and forth in synchronization with Li plating and stripping. The interface and dynamic process improve the adhesion and Li+ transport between the Li anode and SSE, facilitating uniform Li plating and stripping. As a result, the metallic Li anode operates stably for over 1000 h at high current densities and even under elevated temperatures. By using metallic Li as the anode directly, we demonstrate stable cycling of all-solid-state Li-sulfur batteries for over 250 cycles at an areal capacity of >2 mA h cm-2 and room temperature. This study offers insights into the design of regenerative and Li+-conductive interfaces to tackle solid interfacial challenges for high-performance ASSLBs.

Design of High-Entropy Tape Electrolytes for Compression-Free Solid-State Batteries

Advanced solid electrolytes with strong adhesion to other components are the key for the successes of solid-state batteries. Unfortunately, traditional solid electrolytes have to work under high compression to maintain the contact inside owing to their poor adhesion. Here, a concept of high-entropy tape electrolyte (HETE) is proposed to simultaneously achieve tape-like adhesion, liquid-like ion conduction, and separator-like mechanical properties. This HETE is designed with adhesive skin layer on both sides and robust skeleton layer in the middle. The significant properties of the three layers are enabled by high-entropy microstructures which are realized by harnessing polymer-ion interactions. As a result, the HETE shows high ionic conductivity (3.50 ± 0.53 × 10-4 S cm-1 at room temperature), good mechanical properties (toughness 11.28 ± 1.12 MJ m-3, strength 8.18 ± 0.28 MPa), and importantly, tape-like adhesion (interfacial toughness 231.6 ± 9.6 J m-2). Moreover, a compression-free solid-state tape battery is finally demonstrated by adhesion-based assembling, which shows good interfacial and electrochemical stability even under harsh mechanical conditions, such as twisting and bending. The concept of HETE and compression-free solid-state tape batteries may bring promising solutions and inspiration to conquer the interface challenges in solid-state batteries and their manufacturing.

Fusion of Ni Plating on CP-Titanium by Electron Beam Single-Track Scanning: Toward a New Approach for Fabricating TiNi Self-Healing Shape Memory Coating

The limited wear resistance of commercially pure titanium (CP-Ti) hinders its use in abrasive and erosive environments, despite its good strength-weight ratio and corrosion resistance. This paper reports the first study proposing a novel method for wear-resistant TiNi coating through Ni plating and electron beam (EB) irradiation in an in situ synthetic approach. Single-track melting experiments were conducted using the EB to investigate the feasibility of forming a TiNi phase by fusing the Ni plate with the CP-Ti substrate. Varying beam powers were employed at a fixed scanning speed to determine the optimal conditions for TiNi phase formation. The concentration of the melt region was found to be approximate as estimated from the ratio of the Ni-plate thickness to the depth of the melt region, and the region with Ni-48.7 at.% Ti was successfully formed by EB irradiation. The study suggests that the mixing of Ti atoms and Ni atoms was facilitated by fluid flow induced by Marangoni and thermal convections. It is proposed that a more uniform TiNi layer can be achieved through multi-track melting under appropriate conditions. This research demonstrates the feasibility of utilizing EB additive manufacturing as a coating method and the potential for developing TiNi coatings with shape memory effects and pseudoelasticity.

Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

High performance alkali metal anode solid-state batteries require solid/solid interfaces with fast ion transfer that are morphologically and chemically stable upon electrochemical cycling. Void formation at the alkali metal/solid-state electrolyte interface during alkali metal stripping is responsible for constriction resistances and hotspots that can facilitate dendrite propagation and failure. Both externally applied pressures (35-400 MPa) and temperatures above the melting point of the alkali metal have been shown to improve the interfacial contact with the solid electrolyte, preventing the formation of voids. However, the extreme pressure and temperature conditions required can be difficult to meet for commercial solid-state battery applications. In this review, we highlight the importance of interfacial adhesion or 'wetting' at alkali metal/solid electrolyte interfaces for achieving solid-state batteries that can withstand high current densities without cell failure. The intrinsically poor adhesion at metal/ceramic interfaces poses fundamental limitations on many inorganics solid-state electrolyte systems in the absence of applied pressure. Suppression of alkali metal voids can only be achieved for systems with high interfacial adhesion (i. e. 'perfect wetting') where the contact angle between the alkali metal and the solid-state electrolyte surface goes to θ=0°. We identify key strategies to improve interfacial adhesion and suppress void formation including the adoption of interlayers, alloy anodes and 3D scaffolds. Computational modeling techniques have been invaluable for understanding the structure, stability and adhesion of solid-state battery interfaces and we provide an overview of key techniques. Although focused on alkali metal solid-state batteries, the fundamental understanding of interfacial adhesion discussed in this review has broader applications across the field of chemistry and material science from corrosion to biomaterials development.

Role of Interfacial Defects on Electro-Chemo-Mechanical Failure of Solid-State Electrolyte

High-stress field generated by electroplating of lithium (Li) in pre-existing defects is the main reason for mechanical failure of solid-state electrolyte because it drives crack propagation in electrolyte, followed by Li filament growth inside and even internal short-circuit if the filament reaches another electrode. To understand the role of interfacial defects on mechanical failure of solid-state electrolyte, an electro-chemo-mechanical model is built to visualize distribution of stress, relative damage, and crack formation during electrochemical plating of Li in defects. Geometry of interfacial defect is found as dominating factor for concentration of local stress field while semi-sphere defect delivers less accumulation of damage at initial stage and the longest failure time for disintegration of electrolyte. Aspect ratio, as a key geometric parameter of defect, is investigated to reveal its impact on failure of electrolyte. Pyramidic defect with low aspect ratio of 0.2-0.5 shows branched region of damage near interface, probably causing surface pulverization of solid-state electrolyte, whereas high aspect ratio over 3.0 will trigger accumulation of damage in bulk electrolyte. The correction between interfacial defect and electro-chemo-mechanical failure of solid-state electrolyte is expected to provide insightful guidelines for interface design in high-power-density solid-state Li metal batteries.

Lithium crystallization at solid interfaces

Understanding the electrochemical deposition of metal anodes is critical for high-energy rechargeable batteries, among which solid-state lithium metal batteries have attracted extensive interest. A long-standing open question is how electrochemically deposited lithium-ions at the interfaces with the solid-electrolytes crystalize into lithium metal. Here, using large-scale molecular dynamics simulations, we study and reveal the atomistic pathways and energy barriers of lithium crystallization at the solid interfaces. In contrast to the conventional understanding, lithium crystallization takes multi-step pathways mediated by interfacial lithium atoms with disordered and random-closed-packed configurations as intermediate steps, which give rise to the energy barrier of crystallization. This understanding of multi-step crystallization pathways extends the applicability of Ostwald's step rule to interfacial atom states, and enables a rational strategy for lower-barrier crystallization by promoting favorable interfacial atom states as intermediate steps through interfacial engineering. Our findings open rationally guided avenues of interfacial engineering for facilitating the crystallization in metal electrodes for solid-state batteries and can be generally applicable for fast crystal growth.

The void formation behaviors in working solid-state Li metal batteries

The fundamental understanding of the elusive evolution behavior of the buried solid-solid interfaces is the major barrier to exploring solid-state electrochemical devices. Here, we uncover the interfacial void evolution principles in solid-state batteries, build a solid-state void nucleation and growth model, and make an analogy with the bubble formation in liquid phases. In solid-state lithium metal batteries, the lithium stripping-induced interfacial void formation determines the morphological instabilities that result in battery failure. The void-induced contact loss processes are quantified in a phase diagram under wide current densities ranging from 1.0 to 10.0 milliamperes per square centimeter by rational electrochemistry calculations. The in situ-visualized morphological evolutions reveal the microscopic features of void defects under different stripping circumstances. The electrochemical-morphological relationship helps to elucidate the current density- and areal capacity-dependent void nucleation and growth mechanisms, which affords fresh insights on understanding and designing solid-solid interfaces for advanced solid-state batteries.

Self-Healing Mechanism of Lithium in Lithium Metal

Li is an ideal anode material for use in state-of-the-art secondary batteries. However, Li-dendrite growth is a safety concern and results in low coulombic efficiency, which significantly restricts the commercial application of Li secondary batteries. Unfortunately, the Li-deposition (growth) mechanism is poorly understood on the atomic scale. Here, machine learning is used to construct a Li potential model with quantum-mechanical computational accuracy. Molecular dynamics simulations in this study with this model reveal two self-healing mechanisms in a large Li-metal system, viz. surface self-healing, and bulk self-healing. It is concluded that self-healing occurs rapidly in nanoscale; thus, minimizing the voids between the Li grains using several comprehensive methods can effectively facilitate the formation of dendrite-free Li.

Interfacial Defect of Lithium Metal in Solid-State Batteries

All-solid-state battery with Li metal anode is a promising rechargeable battery technology with high energy density and improved safety. Currently, the application of Li metal anode is plagued by the failure at the interfaces between lithium metal and solid electrolyte (SE). However, little is known about the defects at Li-SE interfaces and their effects on Li cycling, impeding further improvement of Li metal anodes. Herein, by performing large-scale atomistic modeling of Li metal interfaces with common SEs, we discover that lithium metal forms an interfacial defect layer of nanometer-thin disordered lithium at the Li-SE interfaces. This interfacial defect Li layer is highly detrimental, leading to interfacial failure such as pore formation and contact loss during Li stripping. By systematically studying and comparing incoherent, coherent, and semi-coherent Li-SE interfaces, we find that the interface with good lattice coherence has reduced Li defects at the interface and has suppressed interfacial failure during Li cycling. Our finding discovered the critical roles of atomistic lithium defects at interfaces for the interfacial failure of Li metal anode, and motivates future atomistic-level interfacial engineering for Li metal anode in solid-state batteries.

The carrier transition from Li atoms to Li vacancies in solid-state lithium alloy anodes

The stable cycling of energy-dense solid-state batteries is highly relied on the kinetically stable solid-state Li alloying reactions. The Li metal precipitation at solid-solid interfaces is the primary cause of interface fluctuations and battery failures, whose formation requires a clear mechanism interpretation, especially on the key kinetic short board. Here, we introduce the lithium alloy anode as a model system to quantify the Li kinetic evolution and transition from the alloying reaction to the metal deposition in solid-state batteries, identifying that there is a carrier transition from Li atoms to Li vacancies during lithiation processes. The rate-determining step is charge transfer or Li atom diffusion at different lithiation stages.

Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries

With the low redox potential of -3.04 V (vs SHE) and ultrahigh theoretical capacity of 3862 mAh g-1 , lithium metal has been considered as promising anode material. However, lithium metal battery has ever suffered a trough in the past few decades due to its safety issues. Over the years, the limited energy density of the lithium-ion battery cannot meet the growing demands of the advanced energy storage devices. Therefore, lithium metal anodes receive renewed attention, which have the potential to achieve high-energy batteries. In this review, the history of the lithium anode is reviewed first. Then the failure mechanism of the lithium anode is analyzed, including dendrite, dead lithium, corrosion, and volume expansion of the lithium anode. Further, the strategies to alleviate the lithium anode issues in recent years are discussed emphatically. Eventually, remaining challenges of these strategies and possible research directions of lithium-anode modification are presented to inspire innovation of lithium anode.