In Situ Measurement of the Plane-Strain Modulus of the Solid Electrolyte Interphase on Lithium-Metal Anodes in Ionic Liquid Electrolytes

Operando Spatial Pressure Mapping Analysis for Prototype Lithium Metal Pouch Cells Under Practical Conditions

Monitoring and diagnosing the battery status in real-time are of utmost importance for clarifying failure mechanism, improving battery performance, and ensuring safety, particularly under fast charging conditions. Recently, advanced operando techniques have been developed to observe changes in the microstructures of lithium deposits using laboratory-scale cell designs, focusing on understanding the nature of Li metal electrodes. However, the macroscopic spatial inhomogeneity of lithium electroplating/stripping in the prototype pressurized pouch cells has not been measured in real-time under practical conditions. Herein, a new noninvasive operando technique, spatial pressure mapping analysis, is introduced to macroscopically and quantitatively measure spatial pressure changes in a pressurized pouch cell during cycling. Moreover, dynamic spatial changes in the macroscopic morphology of the lithium metal electrode are theoretically visualized by combining operando pressure mapping data with mechanical analyses of cell components. Additionally, under fast charging conditions, the direct correlation between abrupt capacity fading and sudden increases in spatial pressure distribution inhomogeneity is demonstrated through comparative analysis of pouch cells under various external pressures, electrolyte species, and electrolyte weight to cell capacity (e/c) ratios. This operando technique provides insights for assessing the current battery status and understanding the complex origin of cell degradation behavior in pressurized pouch cells.

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

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

The Effect of the SEI Layer Mechanical Deformation on the Passivity of a Si Anode in Organic Carbonate Electrolytes

The solid electrolyte interphase (SEI) on a Si negative electrode in carbonate-based organic electrolytes shows intrinsically poor passivating behavior, giving rise to unsatisfactory calendar life of Li-ion batteries. Moreover, mechanical strains induced in the SEI due to large volume changes of Si during charge-discharge cycling could contribute to its mechanical instability and poor passivating behavior. This study elucidates the influence that static mechanical deformation of the SEI has on the rate of unwanted parasitic reactions at the Si/electrolyte interface as a function of electrode potential. The experimental approach involves the utilization of Si thin-film electrodes on substrates with disparate elastic moduli, which either permit or suppress the SEI deformation in response to Si volume changes upon charging-discharging. We find that static mechanical stretching and deformation of the SEI results in an increased parasitic electrolyte reduction current on Si. Furthermore, attenuated total reflection and near-field Fourier-transform infrared nanospectroscopy reveal that the static mechanical stretching and deformation of the SEI fosters a selective transport of linear carbonate solvent through, and nanoconfinement within, the SEI. These, in turn, promote selective solvent reduction and continuous electrolyte decomposition on Si electrodes, reducing the calendar life of Si anode-based Li-ion batteries. Finally, possible correlations between the structure and chemical composition of the SEI layer and its mechanical and chemical resilience under prolonged mechanical deformation are discussed in detail.

Chemomechanics of Rechargeable Batteries: Status, Theories, and Perspectives

Chemomechanics is an old subject, yet its importance has been revived in rechargeable batteries where the mechanical energy and damage associated with redox reactions can significantly affect both the thermodynamics and rates of key electrochemical processes. Thanks to the push for clean energy and advances in characterization capabilities, significant research efforts in the last two decades have brought about a leap forward in understanding the intricate chemomechanical interactions regulating battery performance. Going forward, it is necessary to consolidate scattered ideas in the literature into a structured framework for future efforts across multidisciplinary fields. This review sets out to distill and structure what the authors consider to be significant recent developments on the study of chemomechanics of rechargeable batteries in a concise and accessible format to the audiences of different backgrounds in electrochemistry, materials, and mechanics. Importantly, we review the significance of chemomechanics in the context of battery performance, as well as its mechanistic understanding by combining electrochemical, materials, and mechanical perspectives. We discuss the coupling between the elements of electrochemistry and mechanics, key experimental and modeling tools from the small to large scales, and design considerations. Lastly, we provide our perspective on ongoing challenges and opportunities ranging from quantifying mechanical degradation in batteries to manufacturing battery materials and developing cyclic protocols to improve the mechanical resilience.

Mechanical studies of the solid electrolyte interphase on anodes in lithium and lithium ion batteries

A stable solid electrolyte interphase (SEI) layer is key to high performing lithium ion and lithium metal batteries for metrics such as calendar and cycle life. The SEI must be mechanically robust to withstand large volumetric changes in anode materials such as lithium and silicon, so understanding the mechanical properties and behavior of the SEI is essential for the rational design of artificial SEI and anode form factors. The mechanical properties and mechanical failure of the SEI are challenging to study, because the SEI is thin at only ~10-200 nm thick and is air sensitive. Furthermore, the SEI changes as a function of electrode material, electrolyte and additives, temperature, potential, and formation protocols. A variety ofin situandex situtechniques have been used to study the mechanics of the SEI on a variety of lithium ion battery anode candidates; however, there has not been a succinct review of the findings thus far. Because of the difficulty of isolating the true SEI and its mechanical properties, there have been a limited number of studies that can fully de-convolute the SEI from the anode it forms on. A review of past research will be helpful for culminating current knowledge and helping to inspire new innovations to better quantify and understand the mechanical behavior of the SEI. This review will summarize the different experimental and theoretical techniques used to study the mechanics of SEI on common lithium battery anodes and their strengths and weaknesses.

Complementary sample preparation strategies (PVD/BEXP) combining with multifunctional SPM for the characterizations of battery interfacial properties

The cathode/anode-electrolyte interfaces in lithium/sodium ion batteries act as the “gate” for the ion exchange between the solid electrode and liquid electrolyte. Understanding the interfacial properties of these solid-liquid interfaces is essential for better design high-performance lithium/sodium ion batteries. Here, we provide a novel method for studying solid-liquid interfacial properties of battery materials through combining physical vapor deposition (PVD) and beam-exit cross-sectional polishing (BEXP) followed by controlled environment multifunctional Scanning Probe Microscope (SPM). In this method, commercial battery materials can be either directly grown on the current collector substrates, or polished by obliqued Ar-ion beams to get a nanoscale flat surface which allows the multifunctional SPM to study sample directly in the liquid electrolyte or in protective oxygen/H₂O free environment. This approach allows to investigate wide range of interfacial properties, including surface morphology, internal cracks, mechanical properties, electronic/ionic conductivity and surface potential, with nanoscale resolution in-operando during the battery cycles as well as post-mortem.•

PVD and novel BEXP methods were introduced to prepare battery powder materials as perfect specimens for nanoscale SPM characterization.

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Various physical/chemical properties of battery materials can be probed on the as-prepared specimens under liquid electrolyte using in situ/operando SPM techniques.

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Ex situ/post-mortem analyses based on the controlled environment multifunction SPM characterizations can be achieved in the BEXP polished degradation battery electrodes.

Highly Conductive, Flexible, and Nonflammable Double-Network Poly(ionic liquid)-Based Ionogel Electrolyte for Flexible Lithium-Ion Batteries

The solid-state lithium-ion battery is proposed as the ultimate form of battery and has rapidly become an updated attentive research field due to its high safety and extreme temperature tolerance. However, current solid-state electrolytes hardly meet the requirement in practical applications due to its low ionic conductivity, weak mechanical properties, and poor interfacial contact between the electrolyte and the electrode. In this work, we developed a double-network-supported poly(ionic liquid)-based ionogel electrolyte (DN-Ionogel) via a one-step method. Due to its compact cross-linking structure, the leakage-free DN-Ionogel electrolyte exhibits outstanding flexibility and favorable mechanical properties. In this ionogel electrolyte, the double network favors dissociation of lithium bis(trifluoromethanesulfony)imide (LiTFSI), further resulting in remarkable ionic conductivity (1.8 × 10^-3 S/cm, room temperature), wide electrochemical window (up to 5.0 V), and high lithium-ion transference number (0.33). Furthermore, the cell (LiFePO₄||DN-Ionogel||Lithium) delivers a discharge capacity as high as 150.5 mAh/g, stable cyclic performance (over 200 cycles), and superior rate behavior.

Highly efficient and stable ionic liquid-based gel electrolytes

Gel electrolytes are promising candidates for dye-sensitized solar cells (DSSCs) and other devices, but the ways to obtain stable gels always result in sacrifice of their ionic conductivity. This contradiction seriously limits the practical application of gel electrolytes. Herein, a new design strategy using rich carboxylic group-modified silica nanoparticles (COOH-SiO₂) with a branched, well-organized framework to develop ionic liquid-based gel electrolytes possessing high conductivity is demonstrated. The branched network of COOH-SiO₂ and the strong interaction in electrolytes result in the effective solidification of ionic liquids. Moreover, adding COOH-SiO₂ to ionic liquid electrolytes contributes to salt dissociation, decreases the activation energy, and improves the charge transport and recombination characteristics at the electrolyte/electrode interface. DSSCs fabricated with COOH-SiO₂ nanoparticles deliver a higher short-circuit photocurrent density (J_sc) than the reference cell. A maximum efficiency of 8.02% with the highest J_sc value of 16.60 mA cm^-2 is obtained for solar cells containing 6 wt% COOH-SiO₂.

Phosphazene-derived stable and robust artificial SEI for protecting lithium anodes of Li–O2 batteries

An artificial solid electrolyte interphase with high ionic conductivity and mechanical robustness was designed to suppresses the growth of Li dendrites.

Regulating electrodeposition morphology of lithium: towards commercially relevant secondary Li metal batteries

Rational approaches for achieving fine control of the electrodeposition morphology of Li are required to create commercially-relevant rechargeable Li metal batteries.

Recent Progress of Metal–Air Batteries—A Mini Review

With the ever-increasing demand for power sources of high energy density and stability for emergent electrical vehicles and portable electronic devices, rechargeable batteries (such as lithium-ion batteries, fuel batteries, and metal–air batteries) have attracted extensive interests. Among the emerging battery technologies, metal–air batteries (MABs) are under intense research and development focus due to their high theoretical energy density and high level of safety. Although significant progress has been achieved in improving battery performance in the past decade, there are still numerous technical challenges to overcome for commercialization. Herein, this mini-review summarizes major issues vital to MABs, including progress on packaging and crucial manufacturing technologies for cathode, anode, and electrolyte. Future trends and prospects of advanced MABs by additive manufacturing and nanoengineering are also discussed.