Building Solid-State Batteries: Insights from Swiss Research Labs.

This review article delves into the growing field of solid-state batteries as a compelling alternative to conventional lithium-ion batteries. The article surveys ongoing research efforts at renowned Swiss institutions such as ETH Zurich, Empa, Paul Scherrer Institute, and Berner Fachhochschule covering various aspects, from a fundamental understanding of battery interfaces to practical issues of solid-state battery fabrication, their design, and production. The article then outlines the prospects of solid-state batteries, emphasizing the imperative practical challenges that remain to be overcome and highlighting Swiss research groups' efforts and research directions in this field.

Eliminating Flooding-related Issues in Electrochemical CO2-to-CO Converters: Two Lines of Defense.

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO2 reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO2 in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm-2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO2-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO2-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO2 from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO2 electrolysis.

Quantifying Degradation Parameters of Single-Crystalline Ni-Rich Cathodes in Lithium-Ion Batteries.

Single-crystal LiNix Coy Mnz O2 (SC-NCM, x+y+z=1) cathodes are renowned for their high structural stability and reduced accumulation of adverse side products during long-term cycling. While advances have been made using SC-NCM cathode materials, careful studies of cathode degradation mechanisms are scarce. Herein, we employed quasi single-crystalline LiNi0.65 Co0.15 Mn0.20 O2 (SC-NCM65) to test the relationship between cycling performance and material degradation for different charge cutoff potentials. The Li/SC-NCM65 cells showed >77 % capacity retention below 4.6 V vs. Li+ /Li after 400 cycles and revealed a significant decay to 56 % for 4.7 V cutoff. We demonstrate that the SC-NCM65 degradation is due to accumulation of rock-salt (NiO) species at the particle surface rather than intragranular cracking or side reactions with the electrolyte. The NiO-type layer formation is also responsible for the strongly increased impedance and transition-metal dissolution. Notably, the capacity loss is found to have a linear relationship with the thickness of the rock-salt surface layer. Density functional theory and COMSOL Multiphysics modeling analysis further indicate that the charge-transfer kinetics is decisive, as the lower lithium diffusivity of the NiO phase hinders charge transport from the surface to the bulk.

Assessing Long-Term Cycling Stability of Single-Crystal Versus Polycrystalline Nickel-Rich NCM in Pouch Cells with 6 mAh cm-2 Electrodes.

Lithium-ion batteries based on single-crystal LiNi1- x - y Cox Mny O2 (NCM, 1-x-y ≥ 0.6) cathode materials are gaining increasing attention due to their improved structural stability resulting in superior cycle life compared to batteries based on polycrystalline NCM. However, an in-depth understanding of the less pronounced degradation mechanism of single-crystal NCM is still lacking. Here, a detailed postmortem study is presented, comparing pouch cells with single-crystal versus polycrystalline LiNi0.60 Co0.20 Mn0.20 O2 (NCM622) cathodes after 1375 dis-/charge cycles against graphite anodes. The thickness of the cation-disordered layer forming in the near-surface region of the cathode particles does not differ significantly between single-crystal and polycrystalline particles, while cracking is pronounced for polycrystalline particles, but practically absent for single-crystal particles. Transition metal dissolution as quantified by time-of-flight mass spectrometry on the surface of the cycled graphite anode is much reduced for single-crystal NCM622. Similarly, CO2 gas evolution during the first two cycles as quantified by electrochemical mass spectrometry is much reduced for single-crystal NCM622. Benefitting from these advantages, graphite/single-crystal NMC622 pouch cells are demonstrated with a cathode areal capacity of 6 mAh cm-2 with an excellent capacity retention of 83% after 3000 cycles to 4.2 V, emphasizing the potential of single-crystalline NCM622 as cathode material for next-generation lithium-ion batteries.

The Hydrotropic Effect of Ionic Liquids in Water-in-Salt Electrolytes*.

Water-in-salt electrolytes have successfully expanded the electrochemical stability window of aqueous electrolytes beyond 2 V. Further improvements in stability can be achieved by partially substituting water with either classical organic solvents or ionic liquids. Here, we study ternary electrolytes composed of LiTFSI, water, and imidazolium ionic liquids. We find that the LiTFSI solubility strongly increases from 21 mol kg-1 in water to up to 60 mol kg-1 in the presence of ionic liquid. The solution structure is investigated with Raman and NMR spectroscopy and the enhanced LiTFSI solubility is found to originate from a hydrotropic effect of the ionic liquids. The increased reductive stability of the ternary electrolytes enables stable cycling of an aqueous lithium-ion battery with an energy density of 150 Wh kg-1 on the active material level based on commercially relevant Li4 Ti5 O12 and LiNi0.8 Mn0.1 Co0.1 O2 electrode materials.

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides.

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of ∼ 100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

The Origin of the Catalytic Activity of a Metal Hydride in CO2 Reduction.

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time-of-flight secondary ion mass spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.

MoS2 P-type transistors and diodes enabled by high work function MoOx contacts.

The development of low-resistance source/drain contacts to transition-metal dichalcogenides (TMDCs) is crucial for the realization of high-performance logic components. In particular, efficient hole contacts are required for the fabrication of p-type transistors with MoS2, a model TMDC. Previous studies have shown that the Fermi level of elemental metals is pinned close to the conduction band of MoS2, thus resulting in large Schottky barrier heights for holes with limited hole injection from the contacts. Here, we show that substoichiometric molybdenum trioxide (MoOx, x < 3), a high work function material, acts as an efficient hole injection layer to MoS2 and WSe2. In particular, we demonstrate MoS2 p-type field-effect transistors and diodes by using MoOx contacts. We also show drastic on-current improvement for p-type WSe2 FETs with MoOx contacts over devices made with Pd contacts, which is the prototypical metal used for hole injection. The work presents an important advance in contact engineering of TMDCs and will enable future exploration of their performance limits and intrinsic transport properties.

Strain-induced indirect to direct bandgap transition in multilayer WSe2.

Transition metal dichalcogenides, such as MoS2 and WSe2, have recently gained tremendous interest for electronic and optoelectronic applications. MoS2 and WSe2 monolayers are direct bandgap and show bright photoluminescence (PL), whereas multilayers exhibit much weaker PL due to their indirect optical bandgap. This presents an obstacle for a number of device applications involving light harvesting or detection where thicker films with direct optical bandgap are desired. Here, we experimentally demonstrate a drastic enhancement in PL intensity for multilayer WSe2 (2-4 layers) under uniaxial tensile strain of up to 2%. Specifically, the PL intensity of bilayer WSe2 is amplified by ∼ 35× , making it comparable to that of an unstrained WSe2 monolayer. This drastic PL enhancement is attributed to an indirect to direct bandgap transition for strained bilayer WSe2, as confirmed by density functional theory (DFT) calculations. Notably, in contrast to MoS2 multilayers, the energy difference between the direct and indirect bandgaps of WSe2 multilayers is small, thus allowing for bandgap crossover at experimentally feasible strain values. Our results present an important advance toward controlling the band structure and optoelectronic properties of few-layer WSe2 via strain engineering, with important implications for practical device applications.

Hole selective MoOx contact for silicon solar cells.

Using an ultrathin (∼ 15 nm in thickness) molybdenum oxide (MoOx, x < 3) layer as a transparent hole selective contact to n-type silicon, we demonstrate a room-temperature processed oxide/silicon solar cell with a power conversion efficiency of 14.3%. While MoOx is commonly considered to be a semiconductor with a band gap of 3.3 eV, from X-ray photoelectron spectroscopy we show that MoOx may be considered to behave as a high workfunction metal with a low density of states at the Fermi level originating from the tail of an oxygen vacancy derived defect band located inside the band gap. Specifically, in the absence of carbon contamination, we measure a work function potential of ∼ 6.6 eV, which is significantly higher than that of all elemental metals. Our results on the archetypical semiconductor silicon demonstrate the use of nm-thick transition metal oxides as a simple and versatile pathway for dopant-free contacts to inorganic semiconductors. This work has important implications toward enabling a novel class of junctionless devices with applications for solar cells, light-emitting diodes, photodetectors, and transistors.

Amorphous Si thin film based photocathodes with high photovoltage for efficient hydrogen production.

An amorphous Si thin film with TiO2 encapsulation layer is demonstrated as a highly promising and stable photocathode for solar hydrogen production. With platinum as prototypical cocatalyst, a photocurrent onset potential of 0.93 V vs RHE and saturation photocurrent of 11.6 mA/cm(2) are measured. Importantly, the a-Si photocathodes exhibit impressive photocurrent of ~6.1 mA/cm(2) at a large positive bias of 0.8 V vs RHE, which is the highest for all reported photocathodes at such positive potential. Ni-Mo alloy is demonstrated as an alternative low-cost catalyst with onset potential and saturation current similar to those obtained with platinum. This low-cost photocathode with high photovoltage and current is a highly promising photocathode for solar hydrogen production.

A bridge between trust and control: computational workflows meet automated battery cycling.

Compliance with good research data management practices means trust in the integrity of the data, and it is achievable by full control of the data gathering process. In this work, we demonstrate tooling which bridges these two aspects, and illustrate its use in a case study of automated battery cycling. We successfully interface off-the-shelf battery cycling hardware with the computational workflow management software AiiDA, allowing us to control experiments, while ensuring trust in the data by tracking its provenance. We design user interfaces compatible with this tooling, which span the inventory, experiment design, and result analysis stages. Other features, including monitoring of workflows and import of externally generated and legacy data are also implemented. Finally, the full software stack required for this work is made available in a set of open-source packages.

Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells.

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 µm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

Elucidating the pressure-induced enhancement of ionic conductivity in sodium closo-hydroborate electrolytes for all-solid-state batteries.

Hydroborates are an emerging class of solid electrolytes for all-solid-state batteries. Here, we investigate the impact of pressure on the crystal structure and ionic conductivity of a close-hydroborate salt consisting of Na2B10H10 and Na2B12H12. Two Na2B10H10:Na2B12H12 ratios were studied, 1:1 and 1:3. The anions of the as-synthesized powder with 1:1 ratio crystallize in a single face-centered cubic phase, while the anions of the powder with 1:3 ratio crystallize in a single monoclinic phase. After applying pressure to densify the powder into a pellet, a partial phase transformation into a body-centered cubic (BCC) phase is observed for both ratios. The BCC content saturates at 50 weight percent (wt%) at 500 MPa for the 1:1 ratio and at 77 wt% at 1000 MPa for the 1:3 sample. The room temperature sodium-ion conductivity follows an analogous trend. For the 1:1 ratio, it increases from 2 × 10-4 Scm-1 at 10 wt% BCC content to about 1.0 × 10-3 Scm-1 at 50 wt% BCC content. For the 1:3 ratio, it increases from 1.3 × 10-5 Scm-1 at 11.9 wt% BCC to 8.1 × 10-4 Scm-1 at 71 wt% BCC content. Our results show that pressure is a prerequisite to achieve high sodium-ion conductivity by formation of the highly conductive BCC phase. Supplementary Information: The online version contains supplementary material available at 10.1007/s10853-022-08121-8.

Operando Electrochemical Liquid Cell Scanning Transmission Electron Microscopy Investigation of the Growth and Evolution of the Mosaic Solid Electrolyte Interphase for Lithium-Ion Batteries.

The solid electrolyte interphase (SEI) is a key component of a lithium-ion battery forming during the first few dischage/charge cycles at the interface between the anode and the electrolyte. The SEI passivates the anode-electrolyte interface by inhibiting further electrolyte decomposition, extending the battery's cycle life. Insights into SEI growth and evolution in terms of structure and composition remain difficult to access. To unravel the formation of the SEI layer during the first cycles, operando electrochemical liquid cell scanning transmission electron microscopy (ec-LC-STEM) is employed to monitor in real time the nanoscale processes that occur at the anode-electrolyte interface in their native electrolyte environment. The results show that the formation of the SEI layer is not a one-step process but comprises multiple steps. The growth of the SEI is initiated at low potential during the first charge by decomposition of the electrolyte leading to the nucleation of inorganic nanoparticles. Thereafter, the growth continues during subsequent cycles by forming an island-like layer. Eventually, a dense layer is formed with a mosaic structure composed of larger inorganic patches embedded in a matrix of organic compounds. While the mosaic model for the structure of the SEI is generally accepted, our observations document in detail how the complex structure of the SEI is built up during discharge/charge cycling.

Enhancing C≥2 product selectivity in electrochemical CO2 reduction by controlling the microstructure of gas diffusion electrodes.

We fabricate polymer-based gas diffusion electrodes with controllable microstructure for the electrochemical reduction of CO2, by means of electrospinning and physical vapor deposition. We show that the microstructure of the electrospun substrate is affecting the selectivity of a Cu catalyst, steering it from H2 to C2H4 and other multicarbon products. Specifically, we demonstrate that gas diffusion electrodes with small pores (e.g. mean pore size 0.2 µm) and strong hydrophobicity (e.g. water entry pressure >1 bar) are necessary for achieving a remarkable faradaic efficiency of ∼50% for C2H4 and ∼75% for C≥2 products in neutral 1M KCl electrolyte at 200 mA cm-2. We observe a gradual shift from C2H4 to CH4 to H2 during long-term electrochemical reduction of CO2, which we ascribe to hygroscopic carbonate precipitation in the gas diffusion electrode resulting in flooding of the Cu catalyst by the electrolyte. We demonstrate that even with minimal electrolyte overpressure of 50 mbar, gas diffusion electrodes with large pores (mean pore size 1.1 µm) lose selectivity to carbon products completely, suddenly, and irreversibly in favor of H2. In contrast, we find that gas diffusion electrodes with small pore size (mean pore size 0.2 µm) and strong hydrophobicity (water entry pressure ∼5 bar) are capable of resisting up to 1 bar of electrolyte overpressure during CO2RR without loss of selectivity. We rationalize these experimental results in the context of a double phase boundary reactivity, where an electrolyte layer covers the Cu catalyst and thus governs local CO2 availability. Our results emphasize the pivotal role of microstructure and hydrophobicity in promoting high C≥2 product selectivity and long-term stability in CO2RR flow cells.

Mitigating First-Cycle Capacity Losses in NMC811 via Lithicone Layers Grown by Molecular Layer Deposition.

Nickel-rich LiNi1-x-yMnxCoyO2 (NMC, 1 - x - y ≥ 0.8) is currently considered one of the most promising cathode materials for high-energy-density automotive lithium-ion batteries. Here, we show that capacity losses occurring in balanced NMC811||graphite cells can be mitigated by lithicone layers grown by molecular layer deposition directly onto porous NMC811 particle electrodes. Lithicone layers with a stoichiometry of LiOC0.5H0.3 as determined by elastic recoil detection analysis and a nominal thickness of 20 nm determined by ellipsometry on a flat reference substrate improve the overall NMC811||graphite cell capacity by ∼5% without negatively affecting the rate capability and long-term cycling stability.

Nanoimprint lithography for high-efficiency thin-film silicon solar cells.

We demonstrate high-efficiency thin-film silicon solar cells with transparent nanotextured front electrodes fabricated via ultraviolet nanoimprint lithography on glass substrates. By replicating the morphology of state-of-the-art nanotextured zinc oxide front electrodes known for their exceptional light trapping properties, conversion efficiencies of up to 12.0% are achieved for micromorph tandem junction cells. Excellent light incoupling results in a remarkable summed short-circuit current density of 25.9 mA/cm(2) for amorphous top cell and microcrystalline bottom cell thicknesses of only 250 and 1100 nm, respectively. As efforts to maximize light harvesting continue, our study validates nanoimprinting as a versatile tool to investigate nanophotonic effects of a large variety of nanostructures directly on device performance.

Li7La3Zr2O12 Protonation as a Means to Generate Porous/Dense/Porous-Structured Electrolytes for All-Solid-State Lithium-Metal Batteries.

Ceramic Li7La3Zr2O12 (LLZO) represents a promising candidate electrolyte for next-generation all-solid-state lithium-metal batteries. However, lithium-metal batteries are prone to dendrite formation upon fast charging. Porous/dense and porous/dense/porous LLZO structures were proposed as a solution to avoid or at least delay the formation of lithium-metal dendrites by increasing the electrode/electrolyte contact area and thus lowering the local current density at the interface. In this work, we show the feasibility of producing porous/dense/porous LLZO by a new and scalable method. The method consists of LLZO chemical deep protonation in a protic or acidic solvent, followed by thermal deprotonation at high temperatures to create the porous structure by water and lithium oxide elimination. We demonstrate that the produced structure extends the lifetime of Li/LLZO/Li symmetric cells by a factor of 8 compared to a flat LLZO at a current density of 0.1 mA/cm2 and with a capacity of 1 mAh/cm2 per half-cycle. We also show clear improvement of the Li/LLZO/LiFePO4 full cell performance with a thermally deprotonated LLZO.

Elucidating the Rate-Limiting Processes in High-Temperature Sodium-Metal Chloride Batteries.

Sodium-metal chloride batteries are considered a sustainable and safe alternative to lithium-ion batteries for large-scale stationary electricity storage, but exhibit disadvantages in rate capability. Several studies identify metal-ion migration through the metal chloride conversion layer on the positive electrode as the rate-limiting step, limiting charge and discharge rates in sodium-metal chloride batteries. Here the authors present electrochemical nickel and iron chlorination with planar model electrodes in molten sodium tetrachloroaluminate electrolyte at 300 °C. It is discovered that, instead of metal-ion migration through the metal chloride conversion layer, it is metal-ion diffusion in sodium tetrachloroaluminate which limits chlorination of both the nickel and iron electrodes. Upon charge, chlorination of the nickel electrode proceeds via uniform oxidation of nickel and the formation of NiCl2 platelets on the surface of the electrode. In contrast, the oxidation of the iron electrodes proceeds via localized corrosion attacks, resulting in nonuniform iron oxidation and pulverization of the iron electrode. The transition from planar model electrodes to porous high-capacity electrodes, where sodium-ion migration along the tortuous path in the porous electrode can become rate limiting, is further discussed. These mechanistic insights are important for the design of competitive next-generation sodium-metal chloride batteries with improved rate performance.