Formation Of Solid Electrolyte Interphase Layer Research Articles

Simultaneous Realization of Multilayer Interphases on a Ni-Rich NCM Cathode and a SiOx Anode by the Combination of Vinylene Carbonate with Lithium Difluoro(oxalato)borate.

Ni-rich NCM and SiOx electrode materials have garnered the most attention for advanced lithium-ion batteries (LIBs); however, severe parasitic reactions occurring at their interfaces are critical bottlenecks in their widespread application. In this study, an effective additive combination (VL) composed of vinylene carbonate (VC) and lithium difluoro(oxalato)borate (LiDFOB) is proposed for both Ni-rich NCM and SiOx electrode materials. The LiDFOB additive individually delivers inorganic-rich cathode-electrolyte interphase (CEI) and solid-electrolyte interphase (SEI) layers in anodic and cathodic polarizations before the VC additive. Subsequently, the VC additive is capable of the formation of additional CEI and SEI layers composed of relatively organic-rich components through an electrochemical reaction; thus, inorganic-organic hybridized CEI and SEI layers are simultaneously formed at the Ni-rich NCM and SiOx electrodes. Accordingly, the VL-assisted electrolyte exhibits remarkably prolonged cycling retention for the Ni-rich NCM cathode (86.5%) and SiOx anode (72.7%), whereas the standard electrolyte shows a substantial decrease in cycling retention for the Ni-rich NCM cathode (59.2%) and SiOx anode (18.1%). Further systematic analyses prove that VL-assisted electrolytes form effective interphases for Ni-rich NCM and SiOx electrodes simultaneously, thereby leading to stable and prolonged cycling behaviors of LIBs that offer high energy densities.

Utilizing High-Capacity Spinel-Structured High-Entropy Oxide (CrMnFeCoCu)3O4 as a Graphite Alternative in Lithium-Ion Batteries

In the realm of advanced anode materials for lithium-ion batteries, this study explores the electrochemical performance of a high-entropy oxide (HEO) with a unique spinel structure. The equiatomic composition of CrMnFeCoCu was synthesized and subjected to a comprehensive materials characterization process, including X-ray diffraction and microscopy techniques. The multicomponent alloy exhibited a multiphase structure, comprising two face-centered cubic (FCC) phases and an oxide phase. Upon oxidation, the material transformed into a spinel oxide with a minor presence of CuO. The resulting high-entropy oxide demonstrated excellent electrochemical behavior when utilized as an anode material. Cyclic voltammetry revealed distinctive reduction peaks attributed to cation reduction and solid electrolyte interphase (SEI) layer formation, while subsequent cycles showcased high reversibility. Electrochemical impedance spectroscopy indicated a decrease in charge transfer resistance during cycling, emphasizing the remarkable electrochemical performance. Galvanostatic charge/discharge tests displayed characteristic voltage profiles, with an initial irreversible capacity attributed to SEI layer formation. The HEO exhibited promising rate capability, surpassing commercial graphite at higher current densities. The battery achieved 80% (275 mAh g−1) of its initial stable capacity at a current density of 500 mA g−1 by the 312th cycle. Post-mortem analysis revealed structural amorphization during cycling, contributing to the observed electrochemical behavior. This research highlights the potential of HEOs as advanced anode materials for lithium-ion batteries, combining unique structural features with favorable electrochemical properties.

Low-Temperature and Fast-Charge Sodium Metal Batteries.

Low-temperature operation of sodium metal batteries (SMBs) at the high rate faces challenges of unstable solid electrolyte interphase (SEI), Na dendrite growth, and sluggish Na+ transfer kinetics, causing a largely capacity curtailment. Herein, low-temperature and fast-charge SMBs are successfully constructed by synergetic design of the electrolyte and electrode. The optimized weak-solvation dual-salt electrolyte enables high Na plating/stripping reversibility and the formation of NaF-rich SEI layer to stabilize sodium metal. Moreover, an integrated copper sulfide electrode is in situ fabricated by directly chemical sulfuration of copper current collector with micro-sized sulfur particles, which significantly improves the electronic conductivity and Na+ diffusion, knocking down the kinetic barriers. Consequently, this SMB achieves the reversible capacity of 202.8mAhg-1 at -20°C and 1C (1C = 558mAg-1 ). Even at -40°C, a high capacity of 230.0mAhg-1 can still be delivered at 0.2C. This study is encouraging for further exploration of cryogenic alkali metal batteries, and enriches the electrode material for low-temperature energy storage.

Elastic MXene conductive layers and electrolyte engineering enable robust potassium storage.

The precisely engineered structures of materials greatly influence the manifestation of their properties. For example, in the process of alkali metal ion storage, a carefully designed structure capable of accommodating inserted and extracted ions will improve the stability of material cycling. The present study explores the uniform distribution of self-grown carbon nanotubes to provide structural support for the conductive and elastic MXene layers of Ti3C2Tx-Co@NCNTs. Furthermore, a compatible electrolyte system has been optimized by analyzing the solvation structure and carefully regulating the component in the solid electrolyte interphase (SEI) layer. Mechanistic studies demonstrate that the decomposition predominantly controlled by FSI- leads to the formation of a robust inorganic SEI layer enriched with KF, thus effectively inhibiting irreversible side reactions and major structural deterioration. Confirming our expectations, Ti3C2Tx-Co@NCNTs exhibits an impressive reversible capacity of 260 mA h g-1, even after 2000 cycles at 500 mA g-1 in 1 M KFSI (DME), surpassing most MXene-based anodes reported for PIBs. Additionally, density functional theory (DFT) calculations verify the superior electronic conductivity and lower K+ diffusion energy barriers of the novel superstructure of Ti3C2Tx-Co@NCNTs, thereby affirming the improved electrochemical kinetics. This study presents systematic evaluation methodologies for future research on MXene-based anodes in PIBs.

In Situ Raman Spectroelectrochemical Investigation of Composite Si Nanoparticle-Based Anode for Li-Ion Batteries during (de)Lithiation Process

The key degradation processes in the composite anode for the Li-ion batteries (LIBs) prepared using Si nanoparticles (SiNPs) with two types of a conductive carbon-based matrix [carbon black (CB) and carbonized polypyrrole (CPPy)] were studied by in situ Raman spectroelectrochemistry (SEC). This combined technique provides non-destructive and real-time monitoring of the chemical and structural changes that occur during battery operation. These processes, such as the crystal lattice changes (expansion/contraction) and possible degradation/amorphization of silicon, the solid electrolyte interphase (SEI) layer formation on the electrode surface during the charge/discharge reactions, and the stability of the carbon-based matrix, significantly affect the performance of Si-based LIBs in many ways. Especially, the large silicon volume expansion (more than 300%) and associated stress cause mechanical instability resulting in rapid capacity fading. To overcome this challenge, various strategies have been proposed, including the use of CPPy as a conductive matrix to prevent large volumetric changes and also provide a pathway for electron transfer.This study investigates the initial degradation mechanisms such as the formation of the SEI layer, as well as possible structural changes caused by the volume expansion of SiNPs. The shift of the first-order Raman peak of Si at 521 cm-1 which is assigned to crystalline silicon is related to the stress evolution in nanoparticles caused by the (de)lithiation-induced stress (tensile-to-compressive transition) in SiNPs and the native oxide on their surface. Additionally, the detailed in situ Raman measurement of the first lithiation cycle allowed us to detect the decomposition of the electrolyte (LiPF6 in EC/DMC) close to the Si surface which is associated with SEI layer formation. A decrease in the intensity of the Raman vibrational modes (700–1050 cm−1) of EC/DMC corresponding to the decomposition of the electrolyte was observed at reduction potentials of 0.8 V and 0.3 V vs. Li/Li+ corresponding to the same potentials determined by cyclic voltammetry. The Si-based anode containing CPPy as active carbon showed better electrochemical properties (higher charge/discharge capacity and cycling stability) in Li-ion batteries than Si/CB despite the higher conductivity of the latter. The correlation between the chemical structure of the active carbon and the electrochemical properties of the resulting Si-based anodes will be discussed.Acknowledgment: This work was supported by the Czech National Foundation, Project No. 21–09830S.

Enhancing Cycle Life of Lithium Metal Batteries By Regulating Solid-Electrolyte Interphase Using Gel Polymer Electrolyte

Lithium metal with high theoretical capacity and low redox potential is one of the promising anode materials for high energy density batteries. However, the lithium metal has safety problems and poor cycling performance, due to the growth of Li dendrite and side reactions between Li and electrolytes. One of the most effective strategies to stabilize Li metal is forming robust solid-electrolyte interphase (SEI). Recently, anion-derived and inorganic-rich SEI is known to be stable and ionic conductive, leading to good cycling stability. One of the most popular strategies to form durable SEI is to use the localized high-concentration electrolytes (LHCE) which construct an anion-derived SEI layer on Li metal. In this work, we synthesized LHCE-based gel polymer electrolytes to form a stable SEI layer and enhance cycling stability. Small amount of cross-linking agent was added into dimethoxyethane (DME)-based LHCE when synthesizing gel polymer electrolyte. The functional groups in cross-linking agents increase Li+ and anion interaction in the gel polymer electrolytes, resulting in formation of the stable SEI layer. The solvation structure and chemical composition of the SEI layer were investigated by density functional theory (DFT) calculation and spectroscopic analyses such as FT-IR, Raman, and XPS. The Li/NCM811 cells with LHCE-based gel polymer electrolyte were assembled for the cycling test, and they exhibited superior cycling performance than liquid electrolyte-based cells.

Dynamic Molecular Investigation of the Solid-Electrolyte Interphase of an Anode-Free Lithium Metal Battery Using In Situ Liquid SIMS and Cryo-TEM.

We use in situ liquid secondary ion mass spectroscopy, cryogenic transmission electron microscopy, and density functional theory calculation to delineate the molecular process in the formation of the solid-electrolyte interphase (SEI) layer under the dynamic operating conditions. We discover that the onset potential for SEI layer formation and the thickness of the SEI show dependence on the solvation shell structure. On a Cu film anode, the SEI is noticed to start to form at around 2.0 V (nominal cell voltage) with a final thickness of about 40-50 nm in the 1.0 M LiPF6/EC-DMC electrolyte, while for the case of 1.0 M LiFSI/DME, the SEI starts to form at around 1.5 V with a final thickness of about 20 nm. Our observations clearly indicate the inner and outer SEI layer formation and dissipation upon charging and discharging, implying a continued evolution of electrolyte structure with extended cycling.

Interface Controls Large-Volume Change Anodes Pulverization

Micron sized alloys have been considered as the most attractive candidates for advanced lithium or sodium ion battery anodes due to their high capacity, cost effectiveness, and ease of mass production. However, their large volume variation during (de)alloying reaction with Li or Na ions causes severe mechanical degradation of anode materials, resulting in particle pulverization and loss of electrical interparticle contact. In addition, this substantial volumetric fluctuation leads to newly exposed anode surface to electrolyte and instability of solid-electrolyte interphase (SEI), which can bring about continuously formation of SEI layers. To mitigate the large volume change issues of micron sized alloys, various approaches have been studied such as introducing various electrolytes and additives for stabilizing the SEI layers. However, these modified SEI layers are still not enough to suppress the large volume fluctuation of micron sized alloys.Here, we demonstrate that the effect of interface between electrolyte and anode surface on anode pulverization. With micron sized alloying anodes for sodium-ion batteries (Bi and Sn), we observed substantially different pulverization behavior as a choice of electrolyte solvents leading to different anode-electrolyte interface. This different interface reactivity between solvents has a big impact on not only pulverization behavior of alloying anodes but also formation and growth of SEI layers on the surface of anode. Thus, the electrochemical performances of the alloying anodes also can be influenced by its interface characteristics. We thoroughly analyzed the effect of interface of alloying anode from electrode-level to nanoscale by various characterization tools such as Scanning electron microscopy (SEM), X-ray computed tomography (X-ray CT), X-ray photoelectron spectroscopy (XPS), Atom probe tomography (APT), and cryo-transmission electron microscopy (TEM). Moreover, we clarified the mechanism of the interface reaction and pulverization pathway by theoretical simulations. We believe this study can help researchers to understand the degradation behavior of large volume change alloy anodes.

Synergistic dual electrolyte additives for fluoride rich solid-electrolyte interface on Li metal anode surface: Mechanistic understanding of electrolyte decomposition

Improving the quality of the solid-electrolyte interphase (SEI) layer is highly imperative to stabilize the Li-metal anodes for the practical application of high-energy–density batteries. However, controllably managing the formation of robust SEI layers on the anode is challenging in state-of-the-art electrolytes. Herein, we discuss the role of dual additives fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2, LiPF) within the commercial electrolyte mixture (LiPF6/EC/DEC) considering their reactivity with Li metal anodes using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Synergistic effects of dual additives on SEI formation mechanisms are explored systematically by invoking different electrolyte mixtures including pure electrolyte (LP47), mono-additive (LP47/FEC and LP47/LiPF), and dual additives (LP47/FEC/LiPF). The present work suggests that the addition of dual additives accelerates the reduction of salt and additives while increasing the formation of a LiF-rich SEI layer. In addition, calculated atomic charges are applied to predict the representative F1s X-ray photoelectron (XPS) signal, and our results agree well with the experimentally identified SEI components. The nature of carbon and oxygen-containing groups resulting from the electrolyte decompositions at the anode surface is also analyzed. We find that the presence of dual additives inhibits undesirable solvent degradation in the respective mixtures, which effectively restricts the hazardous side products at the electrolyte-anode interface and improves SEI layer quality.

Ti3CNTx MXene/rGO scaffolds directing the formation of a robust, layered SEI toward high-rate and long-cycle lithium metal batteries

The practical usage of lithium (Li) metal anodes has been hindered by the inherent uncontrolled growth of Li dendrites during stripping/plating. Approaching regulation of uniform Li deposition and robust solid-electrolyte interphase (SEI) is of significance. Herein, we utilized the unique surface chemistry of two-dimensional titanium carbonitride (Ti3CNTx) and the three-dimensional reduced graphene oxide (rGO)-based conductive scaffold architecture, to regulate the Li nucleation and SEI. The lithiophilic functional groups on Ti3CNTx improve the charge-transfer-promoted decomposition of LiN(CF3SO2)2 and resulting in the formation of a uniform SEI layer decorated with evenly distributed LiF and ordered layered Li2O, as evidenced by cryogenic transmission electron microscopy. The Ti3CNTx-rGO-Li electrodes based full cells with LiFePO4 guarantee a superior cycling lifespan over 900 cycles with a capacity retention of 77.7% at 30 C. This work demonstrates that through surface engineering and multiscale architecture design, the suppression of Li dendrites and formation of robust SEI layer can be achieved simultaneously.

An Inorganic-Rich SEI Layer by the Catalyzed Reduction of LiNO3 Enabled by Surface-Abundant Hydrogen Bonding for Stable Lithium Metal Batteries.

Lithium (Li) metal anodes (LMAs) are promising anode candidates for realizing high-energy-density batteries. However, the formation of unstable solid electrolyte interphase (SEI) layers on Li metal is harmful for stable Li cycling; hence, enhancing the physical/chemical properties of SEI layers is important for stabilizing LMAs. Herein, thiourea (TU, CH4 N2 S) is introduced as a new catalyzing agent for LiNO3 reduction to form robust inorganic-rich SEI layers containing abundant Li3 N. Due to the unique molecular structure of TU, the TU molecules adsorb on the Cu electrode by forming CuS bond and simultaneously form hydrogen bonding with other hydrogen bonds accepting species such as NO3 - and TFSI- through its NH bonds, leading to their catalyzed reduction and hence the formation of inorganic-rich SEI layer with abundant Li3 N, LiF, and Li2 S/Li2 S2 . Particularly, this TU-modified SEI layer shows a lower film resistance and better uniformity compared to the electrochemically and naturally formed SEI layers, enabling planar Li growth without any other material treatments and hence improving the cyclic stability in Li/Cu half-cells and Li@Cu/LiFePO4 full-cells.

Greatly recovered electrochemical performances of regenerated graphite anode enabled by an artificial PMMA solid electrolyte interphase layer

Recycling valuable elements, such as Li, Co and Ni, from spent cathode materials, is attracting considerable attention. However, the perspective on the regeneration of spent graphite (SG) anode is still being overlooked. Actually, low-cost recyclization and regeneration are highly feasible due to the intrinsic stable crystal structure of graphite. Herein, a facile sustainable way is proposed to repair the broken surface structure of SG and recover its depressed electrochemical performances. Polymethyl methacrylate (PMMA) is rationally coated onto the SG via a low-temperature polymerization as an artificial solid electrolyte interphase (SEI) layer. The study indicates that the formation of the outer natural SEI layer in the continuous lithiation/delithiation process can be effectively modulated by the inner artificial SEI, which will lead to the formation of a homogeneous bilayer structure. The bilayer SEI configuration can effectively alleviate the decomposition of electrolytes and the loss of the inventory of lithium storage, which is also supported by simulation study. The full cell constructed using regenerated graphite possesses a high capacity of 149 mAh g–1 at 1 C with a retention of 86.7% after 500 cycles. This work provides an economic strategy to regenerate spent graphite anode, paving a way for sustainable battery technology.

Cycle-dependent morphology and surface potential of germanium nanowire anode electrodes.

Germanium nanowire (GeNW) electrodes have shown great promise as high-power, fast-charging alternatives to silicon-based electrodes, owing to their vastly improved Li ion diffusion, electron mobility and ionic conductivity. Formation of the solid electrolyte interphase (SEI) on the anode surface is critical to electrode performance and stability but is not completely understood for NW anodes. Here, a systematic study characterizing pristine and cycled GeNWs in charged and discharged states with SEI layer present and removed is performed using Kelvin probe force microscopy in air. Correlating changes in the morphology of the GeNW anodes with contact potential difference mapping at different cycles provides insight into SEI layer formation and growth, and the effect of the SEI on battery performance.

Fluorine- and maleimide-functionalized electrolyte additive as an interface modifier for Ni-rich LNMC/SiOx batteries

SiOx anodes and Ni-rich LiNixCoyMnzO2 (LNMC) cathodes are considered practical electrodes for electric vehicles; however, these advanced electrode materials suffer from poor prolonged cycling. In this study, N-(4-fluorophenyl)maleimide (FPMI) is used as an electrolyte additive that simultaneously enhances the interfacial stability of each electrode through the formation of solid electrolyte interphase (SEI) or cathode electrolyte interphase (CEI) layers. The electrochemical reduction and oxidation of FPMI promote the formation of SEI and CEI layers on the SiOx anode and Ni-rich LNMC cathode, respectively. FPMI addition also considerably increases the cycling retention because the FPMI-derived SEI and CEI layers effectively inhibit electrolyte decomposition during cycling, thereby increasing the cell lifespan. Additional spectroscopic analyses indicate that the SEI and CEI layers developed on each electrode effectively prevent parasitic reactions, such as electrolyte decomposition with transition metal dissolution in the cell; thus, the surface stability of the cell is also improved.

Solid Electrolyte Interphase Layer Formation on the Si-Based Electrodes with and without Binder Studied by XPS and ToF-SIMS Analysis

The formation and evolution of the solid electrolyte interphase (SEI) layer as a function of electrolyte and electrolyte additives has been extensively studied on simple and model pure Si thin film or Si nanowire electrodes inversely to complex composite Si-based electrodes with binders and/or conductive carbon. It has been recently demonstrated that a binder-free Si@C-network electrode had superior electrochemical properties to the Si electrode with a xanthan gum binder (Si-XG-AB), which can be principally related to a reductive decomposition of electrolytes and formation of an SEI layer. Thus, here, the Si@C-network and Si-XG-AB electrodes have been used to elucidate the mechanism of SEI formation and evolution on Si-based electrodes with and without binder induced by lithiation and delithiation applying surface analytical techniques. The X-ray photoelectron spectroscopy and time-of-flight ion mass spectrometry results demonstrate that the SEI layer formed on the surface of the Si-XG-AB electrode during the discharge partially decomposes during the subsequent charging process, which results in a less stable SEI layer. Contrarily, on the surface of the Si@C-network electrode, the SEI shows less significant decomposition during the cycle, demonstrating its stability. For the Si@C-network electrode, initially, the inorganic and organic species are formed on the surface of the carbon shell and the silicon surface, respectively. These two parts of species in the SEI layer gradually grow and then fuse when the electrode is fully discharged. The behavior of the SEI layer on both electrodes corroborates with the electrochemical results.

Stabilizing Interface pH by Mixing Electrolytes for High-Performance Aqueous Zn Metal Batteries.

Aqueous zinc metal batteries with mild acidic electrolytes are considered promising candidates for large-scale energy storage. However, the Zn anode suffers from severe Zn dendrite growth and side reactions due to the unstable interfacial pH and the absence of a solid electrolyte interphase (SEI) protective layer. Herein, a novel and simple mixed electrolyte strategy is proposed to address these problems. The mixed electrolytes of 2M ZnSO4 and 2M Zn (CF3 SO3 )2 can efficiently buffer the interfacial pH and induce the in situ formation of the organic-inorganic SEI layer, which eliminates dendrite growth and prevents side reactions. As a result, Zn anodes in mixed electrolyte exhibit a lifespan enhancement over 400 times, endure stable cycling over 270h at a high DOD of 62% and achieve high Zn plating/stripping reversibility with an average CE of 99.5% for 1000 cycles at 1mA cm-2 . The findings pave the way for developing practical electrolyte systems for Zn batteries.

A Thermal Tanks-in-Series Model for Capacity Fade Validation Studies in Lithium-Ion Batteries

Lithium ion batteries are widely adopted due to their high energy density, low self-discharge rate and fast charging capabilities1. However, during their operation, capacity fade occurs as a result of various parasitic reactions causing loss of cyclable lithium ions. A dominant and widely studied fade mechanism is the formation of the Solid Electrolyte Interphase (SEI) layer caused by the reduction of electrolyte at the anode-electrolyte boundary. Temperature plays a crucial role on the electrochemical performance of individual cells, affecting the diffusivities of ions, kinetics of reactions and rate of degradation of the battery. Thermal battery models allow us to study the thermal behavior of cells and are often used for cell configuration optimization and thermal management system design2.In literature, thermal models are incorporated with empirical and physics-based models to capture battery dynamics influenced by temperature. Empirical thermal models are simple and fast to solve but are applicable only for specific operating conditions and cannot capture electrochemical subtleties. Conversely, physics-based models use equations that incorporate transport phenomena and electrochemical reactions occurring in the cell. Common physics-based models are the Single Particle Model (SPM) and the Psuedo-2D (P2D) model. The P2D model is robust but computationally expensive3 and the SPM model does not capture electrolyte transport which has a non-linear temperature dependence.The Thermal Tank-In-Series4,5 model is a systematically volume-averaged form of the P2D model with energy balance equations for the current collectors, cathode, separator and anode. This reduces the number of equations in the model and improves computational speed while maintaining accuracy. This model can be ideal to predict capacity fade during fast charging.Extending on our previous work, we take the Thermal Tanks-in-Series model coupled with SEI layer formation for a single-cell sandwich and apply it to analyze capacity fade under different temperature cycling conditions. Experimental validation of the model will facilitate understanding of diffusive, kinetic and temperature effects on the growth of the SEI layer and cell capacity fade. References M. Pathak, S. Kolluri, and V. R. Subramanian, J. Electrochem. Soc., 164, A973–A986 (2017).H. Liu, Z. Wei, W. He, and J. Zhao, Energy Convers. Manag., 150, 304–330 (2017).V. Ramadesigan et al., J. Electrochem. Soc., 159, R31–R45 (2012).A. Subramaniam, S. Kolluri, S. Santhanagopalan, and V. R. Subramanian, J. Electrochem. Soc., 167, 113506 (2020).A. Subramaniam et al., J. Electrochem. Soc., 167, 013534 (2020).

Gyrification structure of Si–Al thin film anodes with high-rate performance

Si–Al thin films with the different sputtering power and time were prepared by radio frequency (RF) magnetron sputtering. Si–Al thin film with deposition time of 40 min at sputtering power of 100 W has a better initial discharge specific capacity of 2186 mAhg−1 and initial coulombic efficiency (ICE) of 89.5%. The method of hydrochloric acid (HCl) etching can be used to obtain both higher capacity and cyclic stability in Si–Al thin film anodes. The solid electrolyte interphase (SEI) layer is formed by etching Si–Al thin film with 1 M HCl for 40 min (1M-40min). It shows a robust gyrification structure with plentiful folds, which can provide abundant reaction sites for electrochemical reactions, and exhibit the adaptability to the high current density. The mechanisms involved in the formation and constitution of SEI layer have been discussed in details. The 1M-40min Si–Al thin film anode shows a high initial discharge specific capacity of 2854 mAhg−1 at 0.8 Ag-1, which reaches to 2561 mAhg−1 at 5.0 Ag-1. The specific capacity of this anode is kept above 1459 mAhg−1 at 20 Ag−1 and can recover to 1981 mAhg−1 when the current density is restored to 0.5 Ag−1.

Structure and Property Changes in Sulfide Solid Electrolytes with Lithiation: A First-Principles Study

Sulfide-based lithium-ion conductors such as Li3PS4 (LPS) and Li10GeP2S12 (LGPS) have received considerable attention for use as solid electrolytes for rechargeable batteries because of their high ionic conductivity, wide electrochemical window, and appropriate mechanical properties. However, their wide application in all-solid-state lithium batteries is hindered by spontaneous decomposition and solid electrolyte interphase layer formation when in contact with Li metal. To better understand the structure and property changes of LPS and LGPS with lithiation, we have systematically analyzed lithiated LPS/LGPS with varying Li contents by calculating their structural, electronic, and mechanical properties with density functional theory. The variation of Li-ion conductivity in these materials with the lithiation degree is also evaluated using ab initio molecular dynamic simulations. Our results unequivocally show that Li incorporation leads to decomposition of PS4 3- and GeS4 4- tetrahedra, resulting in the formation of small P and GeP clusters, along with Li2S production. The P and GeP clusters are eventually converted to Li3P and Li15Ge4 when LPS and LGPS are fully lithiated. The structural evolution gives rise to volume expansion and band gap narrowing. Over-lithiated LPS/LGPS tends to show metallic character, which could be partly responsible for the electrolyte failure. Despite the significant structural changes, the Li ion diffusivity and conductivity in both LPS and LGPS remain at the same order of the magnitude. This work provides a better understanding of the reaction behavior of LPS and LGPS with Li metal and also insight into how to analyze the reaction at the Li metal/sulfide electrolyte interface leading to an inorganic interphase layer.

Mechanistic Insight into Li–Host Interactions in Carbon Hosts for Reversible Li Metal Storage

In recent years, the industrial requirements for rechargeable batteries in terms of energy density have continuously increased owing to the widespread adoption of portable electronic devices and electric vehicles. Li metal has been identified as a promising anode material to meet the ever-increasing demand for rechargeable batteries with high energy densities because of its low redox potential (−3.04 V vs. standard hydrogen electrode)and high specific capacity (3860 mAh g−1) [1]. Despite these advantages, the commercial use of Li metal anodes has been hindered by unresolved critical issues, such as the uncontrollable growth of Li dendrites and the severe volume changes during cycling. In practice, the dendritic growth of Li deposits causes the formation of dead Li species and triggers internal short-circuiting of batteries during cycling, leading to a significant loss of reversibility and raising safety concerns. Moreover, large volume changes in the Li metal anode during Li plating–stripping, which originate from the “hostless” character of the Li metal, impose severe stresses on the solid–electrolyte interphase (SEI) layers, and thus reducing the mechanical stability of SEIs. Once the SEI layers are cracked or fractured, bare Li metal is exposed to the electrolyte, leading to the continuous formation of thick SEI layers on the anode surface. This is mainly responsible for the continuous, irreversible loss of active Li, i.e., low Coulombic efficiency and short cycle lifetimes.3D “host” architectures are beneficial for securing the dimensional stability of Li metal anodes by utilizing internal pores (i.e., free-spaces) as reservoirs for Li storage. Moreover, they provide large active surface areas and hence reduce the effective current densities, suppressing uneven Li plating (dendritic growth) during high-rate operations. Metal-organic framework (MOF)-derived carbon offers many advantages over conventional carbon materials in terms of specific surface area, pore volume, and structural tunability. Furthermore, the critical role of atomically dispersed heteroatoms has been verified in reducing the energy barriers for the nucleation and growth of Li [2].Herein, we report MOF-derived porous carbon hosts with strong Li–host interactions achieved by galvanically displaced lithiophilic seeds. Owing to the distinctive chemical and structural nature of zeolitic imidazolate frameworks (ZIF-8), atomically dispersed Zn can be spontaneously replaced by Ag with a strong lithiophilicity via a galvanic displacement (GD) reaction. Based on a comparative study of carbon hosts with different sizes, morphologies, and distributions of Ag, in particular, we assess the efficacy of lithiophilic Ag in promoting reversible Li storage inside porous carbon hosts. The strong Li–host interactions enabled by lithiophilic Ag effectively reduce the overpotentials for nucleation and growth of Li, and more importantly, the role of lithiophilic Ag in governing the reversibility of Li plating–stripping is discussed in terms of the spatial distribution characteristics of Ag. The atomically dispersed Ag triggers the outward growth of Li from the internal pores of the carbon host, showing a stable cycling performance (>745 cycles). Contrarily, the surface-anchored Ag nanoparticles induce uneven Li plating on the outer surface of the carbon host, resulting in a rapid performance drop (~305 cycles) during cycling. This work provides new insights into the development of advanced 3D host materials for reversible Li metal anodes by utilizing strong Li–host interactions.