Quantifying the Structural Properties of the Lithium-Solid Electrolyte Interface

Solid-state batteries utilizing lithium metal anodes have the potential to enable batteries with a specific energy of >500 Wh/kg and an energy density of >1,500 Wh/L for thousands of cycles. When optimized they will improve the energy efficiency, operating temperature range, sustainability, and safety at a lower cost compared to projections for advanced Li-ion batteries. This improved performance is critical for the widespread adoption of electric vehicles and may enable future applications such as electric aviation. Expectations for solid-state batteries are high, there are significant materials and processing challenges that need to be overcome. Some of these challenges are well known; others are more subtle and are just becoming known. The challenges and their solutions must be clearly identified to realize high-energy solid-state Li metal batteries. In the United States, the U.S. Department of Energy (DOE) funds the majority of energy storage research and development, including solid state battery research. Web of Science publication analysis shows that worldwide DOE is second largest funder. Work on solid state batteries is distributed across different arms of DOE and is coordinated at headquarters to ensure a broad national portfolio for advanced batteries, including lithium batteries and solid-state battery devices. We note that university and national laboratory researchers in the US with a long-standing interest in solid state batteries is a small, integrated community with recognized impact for publications, patents and startup companies. The opportunity for this group to engage for research with international colleagues is growing, fostered by periodic bilateral meetings supported by DOE and by the recent joint US German research program for lithium battery interface studies. These are valuable opportunities to speed solid-state battery development. The ORNL led virtual workshop, overviewed here, confirms the community’s shared vision of the exciting advances, opportunities, and challenges. Further, we are seeing that this workshop has spawned several new informal collaborations and draft proposals. This encourages us to propose future discussion. On May 11, 2020, Oak Ridge National Laboratory (ORNL) hosted a 6-hour, on-line national workshop to discuss recent advances and most the prominent obstacles to realizing solid-state Li metal batteries. The workshop included more than 30+ experts from national laboratories, universities, and companies, all of whom have worked on solid-state batteries for multiple years. The participants shared recent advances, many not yet in print, illustrating that the community has gained significant new insights for solid-state Li battery materials over the last 5 years. In this report, the major outcomes of the workshop are organized to identify the gaps in our scientific knowledge for four core materials science areas: (1) Li metal anodes, (2) the solid electrolyte in contact with Li metal, (3) active cathode materials and solid-state composite cathodes, and (4) solid electrolytes. Illustrative examples and discussions are reported with the more comprehensive issues in the report. Discussion of additional challenges related to processing of solid-state battery materials and to the designs and architectures for mechanically robust, long-lived batteries received less attention due to time constraints. These are good themes for deeper discussion at a follow-on workshop, where a complete list of critical research topics can be identified.

The development of solid-state batteries (SSBs) with a Li metal anode (LMA) requires the exploration of various solid electrolytes (SEs), including sulfides, halides, and oxides. Oxide-based SEs, like LLZTO, allow the use of a Li metal anode (LMA) and high-voltage cathodes[1]. However, the limited compatibility of oxide electrolytes with existing cathode chemistries arises from high-temperature processes like co-sintering, which can induce degradation reactions between the materials and negatively impact performance.Recent investigations into hybrid cell concepts, utilizing dual solid electrolytes, aim to combine the benefits of different SE types. In these systems, LLZTO acts as a separator layer in contact with the LMA, while another SE is used as the catholyte mixed with the CAM. Softer SEs, such as sulfides and argyrodites, can be used to achieve the conformal contact at the heterotonic interface (SE1|SE2) as shown in previous studies.[2][3]Achieving interfacial resistance below 50 ohm.cm2 at low stack pressures (<1-2 MPa) remains crucial for the practical viability of hybrid cells[4].Herein, we investigate the hetero-ionic interface of recently discovered oxyhalide-based Li catholytes - LiNbOCl4 (LNOC), LiTaOCl4 (LTOC), and LiAlCl4-2x O x (LAOC) in contact with LLZTO in a symmetric cell configuration (i.e., oxyhalide | LLZTO | oxyhalide). Our findings highlight the importance of achieving conformal contact at the nanoscale level under practically achievable stack pressures. The charge transfer process across the hetero-ionic interface is shown to depend significantly on the passivation layer (Li2CO3/LiOH/LiCl) present on the LLZTO surface. For instance, the activation energy at the hetero-ionic interface is lower for heat-treated LLZTO compared to acid-treated LLZTO. Furthermore, the pressure-dependent interfacial resistance measurements (1-60 MPa) support the current-constriction concept.[5] The interfacial resistance of LLZTO | LAOC is lower than that of LLZTO | LNOC or LTOC, likely due to differences in the mechanical properties of the oxyhalides.This study also incorporates an in-depth investigation of such interfaces, combining techniques like impedance spectroscopy, nanoindentation, FIB-SEM, and XPS. Additionally, phase-field based computational methodology is employed to analyze the densification differences between argyrodite and oxyhalides at different fabrication pressures, based on their experimental hardness and Young’s modulus values.Our findings will be presented in this talk that emphasize the significance of softer Li-ion oxyhalide catholytes in enhancing the long-term stability of hetero-ionic interfaces under minimal stack pressures, positioning garnet | oxychloride combinations as promising candidates for the bilayer cell concept in Li metal SSBs. References : Balaish, M., Gonzalez-Rosillo, J. C., Kim, K. J., Zhu, Y., Hood, Z. D., & Rupp, J. L. (2021). Processing thin but robust electrolytes for solid-state batteries. Nature Energy, 6(3), 227-239.Eckhardt, J. K., Kremer, S., Merola, L., & Janek, J. (2024). Heteroionic Interfaces in Hybrid Solid-State Batteries─ Current Constriction at the Interface between Different Solid Electrolytes. ACS Applied Materials & Interfaces, 16(14), 18222-18235.Hüttl, J., Seidl, C., Auer, H., Nikolowski, K., Görne, A. L., Arnold, M., & Michaelis, A. (2021). Ultra-low LPS/LLZO interfacial resistance–towards stable hybrid solid-state batteries with Li-metal anodes. Energy Storage Materials, 40, 259-267.Janek, J., & Zeier, W. G. (2023). Challenges in speeding up solid-state battery development. Nature Energy, 8(3), 230-240.Merola, L., Singh, V. K., Palmer, M., Eckhardt, J. K., Benz, S. L., Fuchs, T., & Janek, J. (2024). Evaluation of Oxide| Sulfide Heteroionic Interface Stability for Developing Solid-State Batteries with a Lithium–Metal Electrode: The Case of LLZO| Li6PS5Cl and LLZO| Li7P3S11. ACS Applied Materials & Interfaces, 16(40), 54847-54863.

Silicon is one of the promising anode materials for next-generation all-solid-state batteries (ASSBs). With a high theoretical capacity of 3590 mAh∙g−1, it is an alloy-type anode active material (AAM) that is both non-toxic and widely available in nature.1 However, its practical application is hindered by severe volume changes taking place during cycling and the formation of an unstable solid-electrolyte interphase (SEI). These issues lead to mechanical degradation, loss of contact, and poor long-term cycling stability, thus novel approaches are needed. Silicon is often used combined with argyrodite solid electrolytes which have good ionic partial conductivity (σion ≈ 10−3 S cm−1) and their softness facilitates the adaptation to volume changes taking place during cycling.2 Silicon nitrides (SiN x ) are promising AAMs that could overcome these limitations. By forming silicon clusters in a matrix phase consisting of silicon, lithium, and nitrogen, SiN x offers enhanced chemical stability, potentially mitigating the volume expansion and SEI instability.3 However, the electrochemical investigation of SiN x in ASSBs remains in its early stages, with the matrix formation in the first cycle impacting the initial Coulomb efficiency (ICE).This work investigates the electrochemical performances of SiN x in ASSBs and the parameters affecting capacity decay during cycling. The electrochemical test performed (e.g., long-term cycling and c-rate tests) revealed the enhanced performance of SiN x -based anodes. We investigate the role of SEI and its formation in contact with an argyrodite solid electrolyte. Further analytical methods (e.g., FIB-SEM and EDX) are performed to examine the impact of the nitrogen incorporation. References 1 Huo, H.; Janek, J. Silicon as Emerging Anode in Solid-State Batteries. ACS Energy Lett. 2022, 7 (11), 4005–4016. DOI:10.1021/acsenergylett.2c01950. 2 Zheng, F.; Kotobuki, M.; Song, S.; Lai, M. O.; Lu, L. Review on Solid Electrolytes for All-Solid-State Lithium-Ion Batteries. Journal of Power Sources 2018, 389, 198–213. DOI:10.1016/j.jpowsour.2018.04.022. 3 Kilian, S. O.; Wankmiller, B.; Sybrecht, A. M.; Twellmann, J.; Hansen, M. R.; Wiggers, H. Active Buffer Matrix in Nanoparticle-Based Silicon-Rich Silicon Nitride Anodes Enables High Stability and Fast Charging of Lithium-Ion Batteries. Advanced Materials Interfaces 2022, 9 (26), 2201389. DOI:10.1002/admi.202201389.

Lithium (Li) metal anodes have become an important component of the next generation of high energy density batteries. However, the Li metal anode still has problems such as Li dendrite growth and unstable solid electrolyte interface layer. Herein, we present a functional electrolyte additive (PANHF) successfully synthesized from acrylonitrile and hexafluorobutyl methacrylate via a polymerization reaction. With extensive analytical characterization, it is found that the PANHF can improve the reversibility and Coulombic efficiency of the Li deposition/dissolution reaction and prevent the growth of Li dendrites by forming a solid electrolyte interphase rich in organic matter on the outer layer and LiF on the inner layer. The results show that the cycling performance of the Li/Li cell was greatly improved in the electrolyte containing 0.5 wt % PANHF. Specifically, the cycling stability of more than 700 cycles was achieved at a current density of 1.0 mA cm-2. Moreover, the Li/NCM811 cell with 0.5 wt % PANHF has a higher capacity of 137.7 mA h g-1 at 1.0 C and a capacity retention of 83.41% after 200 cycles. This work highlights the importance of protecting the Li metal anode with functional bipolymer additives for next-generation Li metal batteries.

Innovation in the design of Li-ion rechargeable batteries is necessary to overcome safety concerns and meet energy demands. In this regard, a new generation of Li-ion batteries (LIBs) in the form of all-solid-state batteries (ASSBs) has been developed, attracting a great deal of attention for their high-energy density and excellent mechanical-electrochemical stability. This review describes the current state of research and development on ASSB technology. To this end, study of the literature and patents as well as market analysis over the last two decades were carried out, highlighting how scientific achievements have informed the application of commercially profitable ASSBs. Analyzing the patents registered over the past 20 years revealed that the number of them had increased exponentially-from only few per year in early 2000 to more than 342 in 2020. Published literature and patents on the topic declare a solid-state electrolyte (SSE) to be the main component of ASSBs, and most patented examples are referred to as solid inorganic electrolytes (SIEs), followed by solid polymer electrolytes (SPEs) and solid hybrid electrolytes (SHEs) in popularity. Investigation of company websites, social media profiles, reports, and academic publications identified 93 companies associated with ASSBs. A list of leading businesses in the solid-state battery sector was compiled, out of which 36 provided information on the ASSB units in their product portfolio for detailed analysis.

The sulfide-type solid electrolyte (SE) is a promising candidate for realizing all-solid-state batteries (ASSBs) due to its good mechanical properties and high ionic conductivity. However, challenges such as limited electrochemical stability, inadequate solid-solid contact with the electrode, and reactivity with lithium often lead to dendrite growth and electrolyte reduction. A simple, solvent-free method was developed to form a robust artificial interphase between lithium metal and the solid electrolyte to address these issues. This was achieved by incorporating a composite electrolyte composed of Li6PS5Cl, polyethylene glycol (PEG), and lithium bis(fluorosulfonyl)imide (LiFSI), leading to the in-situ formation of a LiF-rich interfacial layer. This interphase effectively suppresses electrolyte reduction and enhances lithium-ion diffusion. Notably, the PEG additive improves the mechanical strength of the electrolyte by enhancing adhesion between sulfide particles. It also increases the flexibility of the Li6PS5Cl solid electrolyte, improving physical contact with the lithium anode and acting as a protective barrier to prevent direct SE-Li interaction, thus reducing electrolyte decomposition. As a result, Li/Li symmetric cells exhibited excellent cycling stability, maintaining performance for over 3000 hours at a current density of 0.1 mA cm-2, while the composite solid electrolyte achieved a critical current density of 4.75 mA cm-2. This study underscores the synergistic benefits of combining an artificial solid electrolyte interphase (SEI) with composite solid electrolytes, paving the way for high-performance all-solid-state lithium metal batteries (ASSLBs).

Lithium metal batteries (LMBs) have generally become potential candidates for energy storage devices because of their high energy density. However, the practical applications of LMBs have been limited due to the low safety of liquid electrolytes. Compared to liquid electrolytes, solid-state polymer electrolytes (SPEs) have excellent mechanical strength. However, there is room for improvement because traditional PEO-based SPEs need more mechanical properties and electrode contacts for flexible energy device applications. Self-healing solid polymer electrolytes (SHSPEs) have excellent flexibility and healing ability and can also suppress dendrite formation to increase the safety of LMBs. In this study, a novel SHSPE is designed by a combination of the zwitterionic copolymer, SBMA ([2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide)-co-BA (Butyl Acrylate), and a lithium salt, LiTFSI. We explored their solvation structures and reactivity on the Li metal anode using Density functional theory (DFT) and ab initio molecular dynamics (AIMD) methods. Besides, we also studied the reactivity of the considered SHSPE on the Cu (100) surface to explore the SEI formation mechanisms. The cationic NR4 + and anionic SO3 - groups of SBMA interact electrostatically and form cross-linked network structures within the electrolyte framework, providing the designed SHSPE with prominent self-healing capacity. The decomposition reactions of SHSPE on the Li metal anode are explored, and the resultant decomposition products that contribute to the formation of the solid electrolyte interphase (SEI) are identified. Furthermore, AIMD results indicate that highly concentrated SO3 - groups in the SHSPE promote Li ion diffusion, which increases Li ionic conductivity. The present findings provide design criteria for developing highly conductive self-healing SPEs.

Objective: Despite recent global efforts to establish all-solid-state batteries (ASSB) as potentially safe and stable high-energy and high-speed storage technology, long-term performance, specific power, and economic viability remain challenging. However, efficient and short ionic and electronic transport pathways and optimized interfacial contacts between the cell materials and compounds are essential. This poses a significant challenge for the microstructure of cathodes and a careful design and tailoring of anodes and separators in ASSB, which rely solely on solid-state contacts. Here, we present our recent work on ASSB with thiophosphate electrolytes conducted within the German Competence Cluster for Solid-State Batteries (FestBatt). Our work aims to investigate relevant parameters in different cell components on material, electrode, and cell level, such as the importance of particle size distributions, microstructures, and kinetic factors to optimize ASSB cathodes, separators, and anodes. Methodology: The work will be presented in four sections, which showcase key challenges as well as optimization strategies mainly focusing on cathode composites, but also discussing selected examples for separators and anodes in ASSB with thiophosphate electrolytes. Further, an outlook for relevant production steps of ASSB electrodes and cells will be given. For these studies, we mainly conducted focused ion beam scanning electron microscopy tomography, electrochemical impedance spectroscopy, cycling experiments on ASSB cells with thiophosphate solid electrolytes. Results: Fast conducting solid electrolytes are necessary to ensure sufficient transport in composite cathodes but does not represent the only key factor for high-quality cathode composites in ASSB. In our studies, we aimed to optimize the microstructure of composite cathodes consisting of intercalation-type active material and a thiophosphate-based solid electrolyte. We varied the particle size distribution of the solid electrolyte to create various cathode microstructures and assessed the effectiveness of electronic and ionic conduction pathways using partial conductivities. We used both electronically and ionically blocking electrodes for electrochemical impedance spectroscopy measurements to quantify the respective partial conductivities. Our analysis revealed that the particle size of the solid electrolyte significantly influences the charge transport and electrochemical performance of the cathodes. Additionally, using FIB-SEM tomography, we created detailed 3D reconstructions of the cathode microstructure and correlated the obtained partial conductivities with microstructural descriptors such as tortuosity factors, identifying possible kinetic bottlenecks. We found that minimizing residual porosity, which blocks ion and electron transport, is necessary to optimize cathode microstructures and enhance ASSB performance. Moreover, high-performance anodes, along with protective concepts, are crucial for establishing dense, high-energy ASSB cells. Lithium metal anodes may not be the ultimate solution, and silicon anodes can pose advantages, e. g. in terms of electrochemical stability for thiophosphate separators. Discussion: Our studies demonstrate the importance of optimizing the microstructure of cathodes and carefully designing Si anodes for improved ASSB cells. By manipulating the particle size distribution of the solid electrolyte, we achieved more efficient electronic and ionic transport pathways, leading to improved battery performance. The presented work is primarily published in references 1-5, detailing key challenges and showcasing optimization strategies for these topics. Beyond that, diversity in materials, research teams, and approaches is crucial for long-term successful development of ASSB. Acknowledgement: Scientific and administrative coordination of the BMBF Competence Cluster for Solid-State Batteries (FestBatt), FB2-Koord (03XP0431), and cell platform Thiophosphates, FB2-Thio (03XP0430A).

Argyrodite-type Li6 PS5 Cl (LPSCl) has attracted much attention as a solid electrolyte for all-solid-state batteries (ASSBs) because of its high ionic conductivity and good mechanical flexibility. LPSCl, however, has challenges of translating research into practical applications, such as irreversible electrochemical degradation at the interface between LPSCl and cathode materials. Even for Li-ion batteries (LIBs), liquid electrolytes have the same issue as electrolyte decomposition due to interfacial instability. Nonetheless, current LIBs are successfully commercialized because functional electrolyte additives give rise to the formation of stable cathode-electrolyte interphase (CEI) and solid-electrolyte interphase (SEI) layers, leading to supplementing the interfacial stability between electrolyte and electrode. Herein, inspired by the role of electrolyte additives for LIBs, trimethylsilyl compounds are introduced as solid electrolyte additives for improving the interfacial stability between sulfide-based solid electrolytes and cathode materials. 2-(Trimethylsilyl)ethanethiol (TMS-SH), a solid electrolyte additive, is oxidatively decomposed during charge, forming a stable CEI layer. As a result, the CEI layer derived from TMS-SH suppresses the interfacial degradation between LPSCl and LiCoO2 , thereby leading to the excellent electrochemical performance of Li | LPSCl | LiCoO2 , such as superior cycle life over 2000 cycles (85.0% of capacity retention after 2000 cycles).

Due to their superior oxidative stability, high ionic conductivity and excellent chemical compatibility with uncoated 4 V-class cathode active materials, halide compounds, particularly those with a general formula of Li3MCl6 (M = Sc, Zr, In, Y, Er, and Yb etc.), have attracted much attention as solid electrolytes (SEs) for all-solid-state batteries (ASSBs).1,2 While a great deal of effort has been devoted to the discovery of new halide SEs,3–5 fundamental understanding of their properties, such as the mechanism of ionic conductivity, chemical stability, and the interfacial reactivities at the cathode and the anode are still lacking. Here we use Li3YCl6 (LYC) as an example to investigate the key parameters that impact their performances in ASSBs. Hopping frequency analysis is used to understand how synthesis methods and chemical compositions affect mobile carrier concentration and ionic conductivity in the Li–Y–Cl series.6 Synchrotron X-ray absorption (XAS) and diffraction (XRD) are used to investigate electrochemical and interfacial chemistry of LYC in full ASSB cells with an uncoated nickel-rich cathode, a Li metal anode or a lithium-metal alloy anode. Our strategies in developing halide SEs for higher capacity and high energy density ASSBs will be presented.

Anode-free all solid-state batteries (AF-ASSBs) employ "empty" current collector with three active interfaces that determine electrochemical stability; lithium metal - Solid electrolyte (SE) interphase (SEI-1), lithium - current collector interface, and collector - SE interphase (SEI-2). Argyrodite Li6PS5Cl (LPSCl) solid electrolyte (SE) displays SEI-2 containing copper sulfides, formed even at open circuit. Bilayer of 140nm magnesium/30nm tungsten (Mg/W-Cu) controls the three interfaces and allows for state-of-the-art electrochemical performance in half-cells and fullcells. AF-ASSB with NMC811 cathode achieves 150 cycles with Coulombic efficiency (CE) above 99.8%. With high mass-loading cathode (8.6 mAh cm-2), AF-ASSB retains 86.5% capacity after 45 cycles at 0.2C. During electrodeposition of Li, gradient Li-Mg solid solution is formed, which reverses upon electrodissolution. This promotes conformal wetting/dewetting by Li and stabilizes SEI-1 by lowering thermodynamic driving force for SE reduction. Inert refractory W underlayer is required to prevent ongoing formation of SEI-2 that also drives electrochemical degradation. Inert Mo and Nb layers likewise protect Cu from corroding, while Li-alloying layers (Mg, Sn) are less effective due to ongoing volume changes and associated pulverization. Mechanistic explanation for observed Li segregation within alloying LixMg layer is provided through mesoscale modelling, considering opposing roles of diffusivity differences and interfacial stresses.

Benefiting from the significantly improved energy density and safety, all-solid-state lithium batteries (ASSLBs) are considered to be one of the most promising next-generation energy technologies. Their practical applications, however, are strongly impeded by Li dendrite formation. Despite this recognized challenge, a comprehensive understanding of the Li dendrite nucleation and formation mechanism remains elusive. In particular, the initial locations of Li dendrite formation are still ambiguous: do Li clusters form directly at the Li anode surface, inside the bulk solid electrolyte (SE), or within the solid-electrolyte interphase (SEI)? Here, based on the deep-potential molecular dynamics simulations combined with enhanced sampling techniques, we investigate the atomic-level mechanism of Li cluster nucleation and formation at the Li anode/SE interface. We observe that an isolated Li cluster initially forms inside the SEI between the Li6PS5Cl SE and the Li metal anode, located ∼1 nm away from the Li anode/SEI boundary. The local electronic structure of the spontaneously formed SEI is found to be a key factor enabling the Li cluster formation within the SEI, in which a significantly decreased band gap could facilitate electronic conduction through the SEI and reduce Li+ ions to metallic Li atoms therein. Our work provides atomic-level insights into Li-dendrite nucleation at anode/SE interfaces in ASSLBs and could guide future design for developing Li-dendrite-inhibiting strategies.

All solid-state batteries (ASSBs) are the next generation battery technology, in which the classical liquid electrolyte is replaced by an ion-conducting solid electrolyte. Materials that are used as solid-state electrolytes are generally divided into three material classes: polymers, oxides and sulphides. Electrolyte materials for ASSBs have been extensively studied regarding their electrochemical performance and their stability towards the electrode materials. Nevertheless, the commercialization of ASSBs is still hindered by the material availability in case of oxide and sulphides and the knowledge of scaled production routes for all kinds of separators. Polymers are readily available on the global market. It is thus possible to focus more on the processing routes of polymer based solid electrolytes. The use of thermoplastics as matrix for solid electrolytes (SE) offers the prospect of facile upscaling of the process from lab-scale to industrial series production by the implementation of a solvent-free thermal extrusion process. A twin-screw extruder offers the possibility of continuously mixing the SE components to produce homogeneous polymer based SEs. With two feeding units, the addition of the components takes place separately. This eliminates the time-consuming premixing step to obtain a homogeneous mixture.Thermoplastic poly(ethylene oxide) (PEO) is one of the most investigated materials for polymer based SEs due to its capability to dissolve alkali salts like Lithium(bistrifluoromethanesulfonyl)imide (LiTFSI). The thermal process route of PEO and its effect on the polymeric properties are currently under investigation. Previous works from our research group show that the processability of PEO not only depends on the conductive salt (LiTFSI), but also on its molecular weight and applied process parameters. [1] With increasing molecular weight, PEO becomes more sensitive towards thermal and mechanical load that leads to chain scissoring and degradation even at small load. Hence, the use of low molecular weight PEO (Mw= 100.000 g mol-1) is preferable for the dry production of PEO based SEs.The aim of this work is to optimize the extrusion process using a co-rotating twin-screw extruder for the solvent free production of solid PEO/LiTFSI-electrolytes in a molar ratio of 20:1 (EO:Li). Here, we investigate the effects of several extruder set-ups on the mixing behaviour of PEO and LiTFSI. Therefore, type of dosing and screw design are varied. The screw can be designed with conveying, kneading and mixing elements. We compare the dry extrusion of a premixed PEO/LiTFSI (20:1) batch with dry extrusion of PEO/LiTFSI (20:1) using two feeding units, one for each component. Process parameters, such as pressure at the extruder die, actual screw speed, torque and barrel temperature are continuously measured during the production. Process temperature and screw speed are optimized to avoid degradation of PEO. PEO/LiTFSI-electrolytes are produced with a process temperature of 90 °C across the whole extruder barrel and with screw speeds of 5 – 15 rpm. The products are evaluated by their homogeneity and quality of the extruded electrolytes. Optical microscopy is used for macroscopic evaluation and the Li-salt distribution is checked with SEM/EDX. Conductivity and rheology measurements are performed to determine the lithium concentration within the electrolyte and loss of molecular weight, respectively. For the determination of the ionic conductivity, we examine electrochemical impedance spectroscopy (EIS). The characterization of the extruded electrolytes helps to find optimal process parameters for the production of polymer based SEs.

Solid-state batteries have seen a dramatic increase in research in recent years because of their ability to address safety challenges associated with flammable liquid electrolytes, and the potential to enable Li metal anodes. However, all solid-state interfaces present unique challenges, including high interfacial impedances, accommodation of mechanical stresses due to solid-solid interfacial contact, and (electro)chemical instabilities that can evolve during dynamic cycling conditions1.To address these challenges, our group focuses on gaining new fundamental insights into the coupled phenomena occurring at interfaces, and applies this knowledge to rationally design interfacial composition and structure to address the root cause of performance limitations. In this talk, I will first present a multi-modal in situ/operando characterization approach to study Li metal-solid electrolyte interfaces during cycling2-4. Mechanistic insight will be provided into solid-electrolyte interphase (SEI) evolution, as well as nucleation and growth dynamics of the Li metal anode during cycling. In addition to these coupled morphological/chemical/electrochemical phenomena, the unique mechanical properties of Li metal will be discussed in the context of solid-state battery interfaces4-5.Equipped with this fundamental knowledge, I will describe the rational design of interlayers at the Li metal-SE interface using Atomic Layer Deposition (ALD). Examples will be presented in both bulk solid-state battery interfaces, and thin film electrolytes deposited by ALD3,6-7. Through this interdisciplinary approach of fundamental materials chemistry and applied engineering, strategies to address future interfacial challenges will be addressed, pointing towards rational design and manufacturing of optimized interfaces.1) K. B. Hatzell, X. C. Chen, C. L. Cobb, N. P. Dasgupta, M. B. Dixit, L. E. Marbella, M. T. McDowell, P. P. Mukherjee, A. Verma, V. Viswanathan, A. S. Westover, W. G. Zeier “Challenges in Lithium Metal Anodes for Solid State Batteries” ACS Energy Lett. 5, 922 (2020).2) Gupta, E. Kazyak, N. Craig, J. Christensen, N. P. Dasgupta, J. Sakamoto, “Evaluating the Effects of Temperature and Pressure on Li/PEO-LiTFSI Interfacial Stability and Kinetics” J. Electrochem. Soc. 165, A2801 (2018).3) A. L. Davis, R. Garcia-Mendez, K. N. Wood, E. Kazyak, K.-H. Chen, G. Teeter, J. Sakamoto, N. P. Dasgupta “Electro-Chemo-Mechanical Evolution of Sulfide Solid Electrolyte/Li Metal Interfaces: Operando Analysis and ALD Interlayer Effects” J. Mater. Chem. A 8, 6291 (2020).4) E. Kazyak, R. Garcia-Mendez, W. S LePage, A. Sharafi, A. L. Davis, A. J. Sanchez, K.-H. Chen, C. Haslam, J. Sakamoto, N. P. Dasgupta “Li Penetration in Ceramic Solid Electrolytes: Operando Microscopy Analysis of Morphology, Propagation, and Reversibility” Matter 2, 1 (2020).5) W. S. LePage, Y. Chen, E. Kazyak, K.-H. Chen, A. J. Sanchez, A. Poli, E. M. Arruda, M. D. Thouless, N. P. Dasgupta “Lithium Mechanics: Roles of Strain Rate and Temperature and Implications for Lithium Metal Batteries” J. Electrochem. Soc. 166, A89 (2019).6) E. Kazyak, K.-H. Chen, K. N. Wood, A. L. Davis, T. Thompson, A. J. Sanchez, X. Wang, C. Wang, J. Sakamoto, N. P. Dasgupta, “Atomic Layer Deposition of the Solid Electrolyte Garnet Li7La3Zr2O12” Chem. Mater. 29, 3785 (2017).7) E. Kazyak, K.-H. Chen, A. L. Davis, S. Yu, A. J. Sanchez, J. Lasso, A. R. Bielinski, J. Sakamoto, D. J. Siegel, N. P. Dasgupta, “Atomic Layer Deposition and First Principles Modeling of Glassy Li3BO3-Li2CO3 Electrolytes for Solid-State Li Metal Batteries” J. Mater. Chem. A 6, 19425 (2018).

The solid electrolyte interphase (SEI) on a Li metal anode is important in protecting the Li from further oxidation. Previous work has demonstrated that the SEI formed in a lithium-air battery (LAB) can be tailored depending on the gas species present. Oxygen helps create an oxide-rich SEI and promotes uniform Li metal deposition as opposed to dendritic Li which are formed when only argon is present. In order to compare the protective quality of the SEI formed under O2 or under Ar, we used TEMPO, (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl, as a probe. First, we studied the possible reduction products of TEMPO. We found that TEMPO initially reduces to TEMPO-, then undergoes further reductive degradation such as ring opening or formation of amines. Next, we exposed a Li metal electrode with SEI formed either under O2 or Ar to an electrolyte solution containing TEMPO. We characterized the ability of the SEI to prevent TEMPO reduction using operando Electron Paramagnetic Resonance (EPR) spectroscopy as well as ex situ solid-state and solution Nuclear Magnetic Resonance (NMR) spectroscopy. We show that the concentration of TEMPO in the electrolyte decreases more slowly and fewer TEMPO reduction products are formed when the Li metal has an oxide-rich SEI. This suggests that the oxide-rich SEI is better at protecting the Li metal from oxidation and that TEMPO is a suitable probe for SEI quality.