Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies

Operando Discovery of LiNO3-Induced Ultrathick Solid Electrolyte Interphase Formation on Li Metal Anodes

Solid electrolyte interphases (SEIs) play essential roles in lithium metal batteries by dictating the deposition morphology and Coulombic efficiency (CE) of lithium metal anodes. However, the understanding of the formation and evolution of SEIs remains elusive so far. Herein, we present an operando investigation of the formation and evolution dynamics of LiNO3-induced SEI under liquid electrolytes using high-resolution electrochemical atomic force microscopy (EC-AFM). For the first time, we discovered the in situ formation of an ultrathick SEI (∼648 nm) under liquid electrolytes, which far exceeded the thicknesses observed via ex situ characterization techniques. This ultrathick SEI formed through a sequential reduction process, wherein an inorganic-rich inner layer was initially formed, followed by the deposition of a soft outer layer with a modulus of approximately 24 MPa. We also observed that the outer SEI layer was metastable and susceptible to dissolution in liquid electrolytes. Our findings provide a comprehensive investigation of the formation and evolution processes of the LiNO3-induced SEI, deepening the understanding of the in situ structure of SEIs under liquid electrolytes.

Rational Design of a Bilayer Interface for Long-Term Stability of Zn Anodes and MnO2 Cathodes

Understanding the composition-characteristics-performance relationship of the electrolyte-electric double layer-electrode-electrolyte interface (EEI) is crucial to construct stable EEIs for high-performance aqueous Zn-MnO2 batteries (AZMBs). However, the interaction mechanisms in AZMBs remain unclear. This work introduces sodium thioctate (ST) into ZnSO4 electrolyte to construct a stable bilayer EEI on both Zn and MnO2 electrodes. First, zincophilic ST regulates the solvation structure of hydrated Zn2+, suppressing corrosion and the hydrogen evolution reaction. Second, the specific adsorption of ST reconstructs the inner Helmholtz plane, facilitating the desolvation of hydrated Zn2+ and homogenizing charge distribution. Finally, ST molecules undergo reversible polymerization at the interface, forming a stable bilayer EEI with a poly(zinc thioctate) outer layer and a ZnS-organic amorphous inner layer, which ensures uniform zinc-ion flux and enhances mechanical stability. Additionally, the dynamic disulfide bonds in ST further enable self-regulation and self-healing of the interface, mitigating damage during cycling. As a result, the ST-enhanced Zn symmetric battery achieves 7800 cycles at 60 mA cm-2, while the AZMB exhibits only 0.0014% capacity decay over 10 000 cycles at 2000 mA g-1. This bilayer EEI engineering strategy offers effective guidance for the rational design of safe and long-life aqueous zinc-ion batteries.

Two-Stage Growth of Solid Electrolyte Interphase on Copper: Imaging and Quantification by Operando Atomic Force Microscopy

The solid electrolyte interphase (SEI) plays a key role in the aging of lithium-ion batteries. The engineering of advanced negative electrode materials to increase battery lifetime relies on accurate models of SEI growth, but quantitative measurement of SEI growth rates remains challenging due to their nanoscale heterogeneity and environmental sensitivity. In this work, using operando electrochemical atomic force microscopy, we track the growth of SEI on copper in a carbonate electrolyte. From operando measurements of SEI thickness and irreversible electrochemical capacity, we directly visualize the dual growth regimes of the SEI, observing an early-stage primary SEI approximately ten times more "electrochemically compact" than later-stage secondary SEI, as quantified via the incremental thickness per charge passed. While primary SEI is responsible for about half of the irreversible capacity lost in a 24 h period, it accounts for only a tenth of thickness. We also show that nanoscale defects on the copper substrate play a key role in determining the nonuniform growth morphology of the SEI, thus providing direct evidence that initial SEI growth is not purely transport-limited. Our experiments reveal that SEI grows by two modes: first reaction-limited nucleation and growth of a dense, passivating primary SEI layer, governed by ion-coupled electron transfer kinetics; and subsequently by diffusion-limited growth of a porous secondary SEI layer, once the primary SEI fully passivates the electrode surface.

Thin Zinc Electrodes Stabilized with Organobromine-Partnered H2O-Zn-MeOH Cluster Ions for Practical Zinc-Metal Pouch Cells

The utilization of thin zinc (Zn) anodes with a high depth of discharge is an effective strategy to increase the energy density of aqueous Zn metal batteries (ZMBs), but challenged by the poor reversibility of Zn electrode due to the serious Zn-consuming side reactions at the Zn||electrolyte interface. Here, we introduce 2-bromomethyl-1,3-dioxolane (BDOL) and methanol (MeOH) as electrolyte additive into aqueous ZnSO4 electrolyte. In the as-formulated electrolyte, BDOL with a strong electron-withdrawing group (-CH2Br) tends to pair with the H2O-Zn-MeOH complex, leading to the formation of organobromine-partnered H2O-Zn-MeOH cluster ions. During the Zn electrodeposition process, the formed ZnO-dominated by-products induce the polymerization of BDOL monomers, which are previously adsorbed on the electrode. As a result, a uniform dual-layer SEI with ZnO-dominated outer layer and polyether-dominated inner layer is built on the surface of Zn electrode. With such an in situ formed dual-layer SEI, the Zn||Mg0.9Mn3O7 ⋅ 2.7H2O pouch cell using a 10-um Zn anode (corresponding to a low negative to positive areal capacity ratio of 3.56) successfully operated for 300 cycles with a high capacity retention of 86 %, promising a high practical energy density of >120 Wh/kg (based on the total mass of Zn and Mg0.9Mn3O7 ⋅ 2.7H2O).

Selective Methylation of Cyclic Ether Towards Highly Elastic Solid Electrolyte Interphase for Silicon-based Anodes

Silicon (Si)-based anodes offer high theoretical capacity for lithium-ion batteries but suffer from severe volume changes and continuous solid electrolyte interphase (SEI) degradation. Here, we address these challenges by selective methylation of 1,3-dioxolane (DOL), thus shifting the unstable bulk polymerization to controlled interfacial reactions and resulting in a highly elastic SEI. Comparative studies of 2-methyl-1,3-dioxolane (2MDOL) and 4-methyl-1,3-dioxolane (4MDOL) reveal that 4MDOL, with its larger ring strain and more stable radical intermediates due to hyperconjugation effect, promotes the formation of high-molecular-weight polymeric species at the electrode-electrolyte interface. This elastic, polymer-rich SEI effectively accommodates volume changes of Si and inhibits continuous side reactions. Our designed electrolyte enables Si-based anode to achieve 85.4 % capacity retention after 400 cycles at 0.5 C without additives, significantly outperforming conventional carbonate-based electrolytes. Full cells also demonstrate stable long-term cycling. This work provides new insights into molecular-level electrolyte design for high-performance Si anodes, offering a promising pathway toward next-generation lithium-ion batteries with enhanced energy density and longevity.

Anion-Dominated Solvation in Low-Concentration Electrolytes Promotes Inorganic-Rich Interphase Formation in Lithium Metal Batteries

While the formation of an inorganic-rich solid electrolyte interphase (SEI) plays a crucial role, the persistent challenge lies in the formation of an organic-rich SEI due to the high solvent ratio in low-concentration electrolytes (LCEs), which hinders the achievement of high-performance lithium metal batteries. Herein, by incorporating di-fluoroethylene carbonate (DFEC) as a non-solvating cosolvent, a solvation structure dominated by anions is introduced in the innovative LCE, leading to the creation of a durable and stable inorganic-rich SEI. Leveraging this electrolyte design, the Li||NCM83 cell demonstrates exceptional cycling stability, maintaining 82.85% of its capacity over 500 cycles at 1 C. Additionally, Li||NCM83 cell with a low N/P ratio (≈2.57) and reduced electrolyte volume (30 µL) retain 87.58% of its capacity after 150 cycles at 0.5 C. Direct molecular information is utilized to reveal a strong correlation between solvation structures and reduction sequences, proving the anion-dominate solvation structure can impedes the preferential reduction of solvents and constructs an inorganic-rich SEI. These findings shed light on the pivotal role of solvation structures in dictating SEI composition and battery performance, offering valuable insights for the design of advanced electrolytes for next-generation lithium metal batteries.

Preformation of Insoluble Solid-Electrolyte Interphase for Highly Reversible Na-Ion Batteries

A stable solid-electrolyte interphase (SEI) is crucial for cycling reversibility of Na-ion batteries by mitigating continuous side reactions. So far, the severe SEI dissolution leads to low Coulombic efficiency (CE) and short cycle life. Meanwhile, the quantified relationship between SEI components and their solubility remains unclear. In this work, we establish the direct correlation between SEI components and SEI solubility, and quantify that the solubility of organic-rich SEI is 3.26 times of inorganic-rich SEI. We further propose a feasible strategy to preform inorganic-rich insoluble SEI and demonstrate a practical hard carbon (HC)||NaMn0.33Fe0.33Ni0.33O2 full cell in a commercial electrolyte of 1 M NaPF6 in propylene carbonate (PC) with 80.0 % capacity retention for 900 cycles, and achieve a record-high average CE of 99.95 % for a practical Na-ion full cell. This study provides an effective strategy of preforming insoluble SEI to suppress its dissolution towards highly reversible Na-ion batteries.

Plasma Coupled Electrolyte Additive Strategy for Construction of High-Performance Solid Electrolyte Interphase on Li Metal Anodes

High-performance lithium metal anodes are crucial for the development of advanced Li metal batteries. Herein, this work reports a novel plasma coupled electrolyte additive strategy to prepare high-quality composite solid electrolyte interphase (SEI) on Li metal to achieve enhanced performance and stability. With the guidance of calculations, this work selects diethyl dibromomalonate (DB) as an additive to optimize the solvation structure of electrolytes to modify the SEI. Meanwhile, this work groundbreakingly develops DB plasma technology coupled with DB electrolyte additive to construct a combinatorial SEI: inner plasma-induced SEI layer composed of LiBr and Li2CO3 plus additive-reduced SEI containing LiBr/Li2CO3/organic lithium compounds as an outer compatible layer. The optimized hybrid SEI has strong affinity toward Li+ and good mechanical properties, thereby inducing horizontal dispersion and uniform deposition of Li+ and keep structure stable. Accordingly, the symmetrical cells exhibit enhanced cycling stability for 1200 h at an overpotential of 23.8 mV with average coulombic efficiency (99.51%). Additionally, the full cells with LiNi0.8Co0.1Mn0.1O2 cathode deliver a capacity retention of 81.7% after 300 cycles at 0.5 C, and the pouch cell achieves a volumetric specific energy of ≈664 Wh L‒1. This work provides new enlightenment on plasma technology for fabrication of advanced metal anodes for energy storage.

Insights into the SiO2 Stress Effect on the Electrochemical Performance of Si anode

Silicon (Si) is regarded as the most potential anode material for next-generation lithium-ion batteries (LIBs). However, huge volume expansion hinders its commercial application. Here, a yolk-shell structural nitrogen-doped carbon coated Si@SiO2 is prepared by SiO2 template and HF etching method. The as-prepared composite exhibits superior cycling stability with a high reversible capacity of 577 mA h g-1 at 1 A g-1 after 1000 cycles. The stress effect of SiO2 on stabilizing the electrochemical performance of Si anode is systematically investigated for the first time. In situ thickness measurement reveals that the volume expansion thickness of Si@SiO2 upon charge-discharge is obviously smaller than Si, demonstrating the electrode expansion can be effectively inhibited to improve the cyclability. The density functional theory (DFT) calculation further demonstrates the moderate young's modulus and enhanced hardness after SiO2 coating contribute significantly to the mechanical reinforcement of overall Si@SiO2@void@NC composite. Various post-cycling electrode analyses also address the positive effects of inner stress from the Si core on effectively relieving the damage to electrode structure, facilitating the formation of a more stable inorganic-rich solid electrolyte interphase (SEI) layer. This study provides new insights for mechanical stability and excellent electrochemical performance of Si-based anode materials.

Achieving Stable Lithium Anodes through Leveraging Inevitable Stress Variations via Adaptive Piezoelectric Effect

Unleashing the potential of lithium-metal anodes in practical applications is hindered by the inherent stress-related challenges arising from their limitless volume expansion, leading to mechanical failures such as electrode cracking, solid electrolyte interphase damage, and dendritic growth. Despite the various protective strategies to "combat" stress in lithium-metal anodes, they fail to address the intrinsic issue fundamentally. Here, a unique strategy is proposed that leverages the stress generated during the battery cycling via the piezoelectric effect, transforming to the adaptive built-in electric field to accelerate lithium-ion migration, homogenize the lithium deposition, and alleviate the stress concentration. The mechanism of the piezoelectric effect in modulating electro-chemomechanical field evolution is further validated and decoupled through finite element method simulations. Inspired by this strategy, a high sensitivity, fast responsive, and strength adaptability polymer piezoelectric is used to demonstrate the feasibility and the corresponding protected lithium-metal anode shows cycling stability over 6000 h under a current density of 10 mA cm-2 and extending life in a variety of coin and pouch cell systems. This work effectively tackles the stress-related issues and decoupling the electro-chemomechanical field evolution also contributes to developing more stable lithium anodes for future research.

Electrochemical Hydrophobic Tri-layer Interface Rendered Mechanically Graded Solid Electrolyte Interface for Stable Zinc Metal Anode

The aqueous zinc-ion battery is promising as grid scale energy storage device, but hindered by the instable electrode/electrolyte interface. Herein, we report the lean-water ionic liquid electrolyte for aqueous zinc metal batteries. The lean-water ionic liquid electrolyte creates the hydrophobic tri-layer interface assembled by first two layers of hydrophobic OTF- and EMIM+ and third layer of loosely attached water, beyond the classical Gouy-Chapman-Stern theory based electrochemical double layer. By taking advantage of the hydrophobic tri-layer interface, the lean-water ionic liquid electrolyte enables a wide electrochemical working window (2.93 V) with relatively high zinc ion conductivity (17.3 mS/cm). Furthermore, the anion crowding interface facilitates the OTF- decomposition chemistry to create the mechanically graded solid electrolyte interface layer to simultaneously suppress the dendrite formation and maintain the mechanical stability. In this way, the lean-water based ionic liquid electrolyte realizes the ultralong cyclability of over 10000 cycles at 20 A/g and at practical condition of N/P ratio of 1.5, the cumulated areal capacity reach 1.8 Ah/cm2 , which outperforms the state-of-the-art zinc metal battery performance. Our work highlights the importance of the stable electrode/electrolyte interface stability, which would be practical for building high energy grid scale zinc-ion battery.

High-Performance Silicon-Rich Microparticle Anodes for Lithium-Ion Batteries Enabled by Internal Stress Mitigation

Si is a promising anode material for Li ion batteries because of its high specific capacity, abundant reserve, and low cost. However, its rate performance and cycling stability are poor due to the severe particle pulverization during the lithiation/delithiation process. The high stress induced by the Li concentration gradient and anisotropic deformation is the main reason for the fracture of Si particles. Here we present a new stress mitigation strategy by uniformly distributing small amounts of Sn and Sb in Si micron-sized particles, which reduces the Li concentration gradient and realizes an isotropic lithiation/delithiation process. The Si8.5Sn0.5Sb microparticles (mean particle size: 8.22 μm) show over 6000-fold and tenfold improvements in electronic conductivity and Li diffusivity than Si particles, respectively. The discharge capacities of the Si8.5Sn0.5Sb microparticle anode after 100 cycles at 1.0 and 3.0 A g-1 are 1.62 and 1.19 Ah g-1, respectively, corresponding to a retention rate of 94.2% and 99.6%, respectively, relative to the capacity of the first cycle after activation. Multicomponent microparticle anodes containing Si, Sn, Sb, Ge and Ag prepared using the same method yields an ultra-low capacity decay rate of 0.02% per cycle for 1000 cycles at 1 A g-1, corroborating the proposed mechanism. The stress regulation mechanism enabled by the industry-compatible fabrication methods opens up enormous opportunities for low-cost and high-energy-density Li-ion batteries.

The Critical Role of Fillers in Composite Polymer Electrolytes for Lithium Battery

With excellent energy densities and highly safe performance, solid-state lithium batteries (SSLBs) have been hailed as promising energy storage devices. Solid-state electrolyte is the core component of SSLBs and plays an essential role in the safety and electrochemical performance of the cells. Composite polymer electrolytes (CPEs) are considered as one of the most promising candidates among all solid-state electrolytes due to their excellent comprehensive performance. In this review, we briefly introduce the components of CPEs, such as the polymer matrix and the species of fillers, as well as the integration of fillers in the polymers. In particular, we focus on the two major obstacles that affect the development of CPEs: the low ionic conductivity of the electrolyte and high interfacial impedance. We provide insight into the factors influencing ionic conductivity, in terms of macroscopic and microscopic aspects, including the aggregated structure of the polymer, ion migration rate and carrier concentration. In addition, we also discuss the electrode-electrolyte interface and summarize methods for improving this interface. It is expected that this review will provide feasible solutions for modifying CPEs through further understanding of the ion conduction mechanism in CPEs and for improving the compatibility of the electrode-electrolyte interface.