Sanja Tepavcevic

Electrolytes composed of mixed solvents are of significant scientific and technological importance in lithium-ion and beyond-lithium-ion batteries, and more broadly across physical chemistry. Their emergence has highlighted the importance of multiscale phenomena, particularly the development of nanoscale solution heterogeneities. It is therefore essential to assess how these inhomogeneities and nanoscale aggregates influence solvation, transport, and electronic properties. Here we investigate the solution structure and transport properties of aqueous zinc sulfate electrolytes with acetonitrile (ACN) additives, where the addition of ACN leads to improved cyclability. Using complementary anomalous small-angle X-ray scattering, X-ray pair distribution function, electrochemistry, and NMR, we identify the presence of salt-rich-water-rich and salt-poor-ACN-rich nanoheterogeneities and, at higher ACN concentrations, liquid–liquid phase separation. We find that the size of zinc sulfate ion clusters in the water domains increases slightly upon ACN addition, while the ACN-rich domains grow much larger as phase separation is approached. The inverse Haven ratio, determined using the ionic conductivity and diffusivity, is much less than unity and decreases slightly upon addition of ACN, which shows ion–ion interactions. We speculate that the increased ion–ion interactions with increased ACN are a result of a reduction in free water, the increase in zinc sulfate cluster size, and/or an increase in solution tortuosity. This work highlights key phenomena to consider when engineering new water-organic-salt electrolyte systems and establishes a phase diagram for water–ACN–ZnSO4 that can guide advances, such as self-adaptive electrolytes.

Replacements for traditional graphite anodes are needed to meet growing demands for energy-dense batteries used in electric vehicles and portable electronics. Compared to graphite, lithium metal has a higher specific capacity (3,860 mAh/g) and lower redox potential (−3.040 V vs. SHE), which makes it one of the most promising anode candidates for next-generation batteries including lithium-ion, lithium-metal, lithium-sulfur/air, and solid-state systems. However, practical adoption of reversible lithium metal anodes has been hindered by performance and safety issues related to unstable solid electrolyte interphase (SEI) layers, volume changes during electroplating/stripping, and dendrite formation. This seminar series highlights recent progress tackling these challenges with specific emphasis on: (i) applying artificial SEI layers, (ii) modifying anode/electrolyte interfaces through material design, and (iii) developing lithium-based alloy anodes. The seminar will also present fundamental studies on dendrite growth mechanisms through experimental and modeling work. Overall, thie discussion will aim to provide new insights on designing lithium metal anodes to improve performance and reliability for next-generation battery technologies. Determination of the Exchange Current Density With Polymer Electrolytes and Its Correlation With Coulombic Efficiency Polymer electrolytes are promising materials to enable practical lithium-metal batteries for high-energy-density energy storage. Yet, parasitic side reactions at the lithium│polymer interface and inhomogeneous lithium deposition, compromising cycling reversibility and safety, remain a challenge. Despite the key role of the interfacial reactivity, there is a lack of quantitative and comparable information on interfacial charge-transfer kinetics with polymer electrolytes and their impact on the lithium plating/stripping behavior remains unclear. Therefore, in this work, we apply a validated method for quantifying interfacial charge transfer kinetics via the exchange current density j0 to a series of polyethylene oxide (PEO)-based polymer electrolytes to clarify how the electrolyte composition influences interfacial kinetics. By combining the j0 analyses with measurements of the solid electrolyte interphase resistance and lithium cycling reversibility, we identify a consistent trend in which smaller j0 and higher RSEI values correlate with higher Coulombic efficiencies. This indicates that moderated interfacial kinetics and a sufficiently passivating interphase favor higher plating/stripping reversibility. The results provide a coherent dataset of interfacial charge-transfer parameters for polymer electrolytes and highlight the importance of deliberate electrolyte and interphase design for achieving highly reversible lithium-metal cycling in polymer-based electrolyte systems. Nanoscale metal and metal oxide interlayers for anode-free solid-state lithium metal batteries: reactivity and lithium plating mechanisms Anode-less lithium-metal batteries potentially offer further increased energy densities. However, the Coulombic efficiency (CE) of lithium plating and stripping, as a classical measure of Li inventory reversibility, is commonly still insufficient for achieving long-lasting rechargeable batteries. Herein, the potential benefits of employing thin (20 nm) metal interlayers of Ag, Pt, and Au on Cu to alloy with Li in Cu||Li half-cells and induce homogeneous Li plating with poly(ethylene oxide)-based electrolytes are investigated. Interestingly, not all alloying interlayers enable a higher CE compared to neat Cu foil with 84%—specifically Ag@Cu with only 81%, while the best performing one, Au@Cu, provides a substantially increased CE of 91%. While generally the formation of “dead lithium” is found to be the major source of CE, this appears to be less pronounced in the case of Au@Cu, indeed. Further improvement can be achieved by carefully adjusting the cell voltage to a region in which the continuous de-/alloying is suppressed, yielding a further enhanced CE of 94%, thus highlighting the need for a comprehensive approach to design suitable electrode chemistries and designs beyond a “simple” material improvement. Effects of Cu Surface Area and Microtexture on Lithium Plating in Anode-Less Batteries Anode-less lithium metal batteries (ALLMBs) replace the lithium anode with a bare copper current collector, boosting energy density by 40–60% [1,2]. Uniform lithium plating on Cu remains however a key challenge, as bare surfaces promote dendritic growth and dead lithium formation [3]. Laser texturing - producing either nanoscale periodic structures (LIPSS, 1 µm pitch) or micrometric grooves (GR5, 5 µm depth) - was explored to increase electroactive surface area and homogenize lithium nucleation. We systematically compare these three architectures in anode-less Cu||LiFePO₄ coin cells over a range of plated capacities (0.5–2.5 mAh cm⁻²). Counterintuitively, both 3D collectors underperformed planar Cu2D in cycle life and coulombic efficiency. EIS and ex situ SEM reveal that laser processing generates a thicker Cu₂O/CuO interlayer that amplifies parasitic reactions proportionally to surface area, accelerates SEI degradation, ultimately masking the geometric benefits of texturing. To decouple surface architecture from surface chemistry, the oxide layer was selectively removed via glucose-based chemical reduction. The deoxidized LIPSS (D_LIPSS) achieves 99.5% CE and sustains stable capacity for up to 180 cycles, confirming that the nanostructured surface intrinsically promotes dense, dendrite-free lithium deposition. These findings establish that Cu₂O is not a suitable interlayer for 3D copper current collectors and highlight the critical interplay between surface chemistry and microstructural design, calling for rationally engineered alternatives. Ultrasonic Welding of Garnet Solid Electrolytes to Lithium Metal: Achieving Intimate Interfacial Contact in Seconds Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a promising solid electrolyte for solid-state lithium metal batteries, yet forming intimate Li–LLZO contact remains challenging because of surface roughness, interfacial voids, and insulating surface species such as Li₂CO₃. Here, we introduce ultrasonic welding (USW), a mature technique conventionally used for joining metals, to rapidly form Li–LLZO interfaces at room temperature. High-frequency vibration combined with controlled pressure disrupts surface contaminants and promotes plastic deformation of Li metal, enabling it to conform closely to the rigid LLZO surface without melting. USW alone reduces the Li–LLZO interfacial resistance to approximately 225 Ω·cm². With a thin Au interlayer, the resistance is further reduced to1.5 Ω·cm², among the lowest values reported for room-temperature-processed Li–LLZO interfaces. This work demonstrates a rapid and effective approach to engineering metal–ceramic interfaces for oxide-based solid-state batteries.

Hybrid solid electrolytes (HSEs) represent a promising material system for lithium metal batteries by combining high ionic conductive and mechanically strong inorganic with soft organic solid electrolytes. However, for HSEs consisting of ceramic fillers, lithium hopping between the polymer and ceramic phases remains unproven, emphasizing the tortuosity inside the polymer phase as a pivotal factor. Herein, we report an in‐plane aligned Li _6.6 La ₃ Zr _1.6 Ta _0.4 O ₁₂ (Ta‐LLZO) ceramic fiber network structure for HSEs and compare its morphological nature to typical LLZO‐based fillers. Simulations reveal a high tortuosity inside the polymer phase with a parallel to the electrodes aligned network structure, enabling dendritic structure blocking behavior, but still reaching decent ionic transport (0.44 mS cm ⁻¹ , 60°C). Post‐mortem microscopy analyses confirmed the dendrite blocking mechanism of the network, ultimately leading to long cycling lifetime of > 1100 h in symmetric lithium metal cells operated with 0.1 mA cm ⁻² at 60°C. Overall, the ceramic network HSE represents a superior morphology by combining decent ionic conductivity, low resistance and long cycling life in symmetric and full cells with LiFePO ₄ cathodes. The comparison of different filler characteristics with a special focus on tortuosity eventually facilitates the evaluation of these properties for the purpose of optimizing HSEs.

Lithium‐metal batteries with solid electrolytes can deliver higher energy density and improved safety than conventional Li‐ion batteries. Among solid electrolyte candidates, polymer/ceramic composite electrolytes are attractive because they combine polymer flexibility with the high ionic conductivity of ceramics. However, whether ceramic fillers synergistically reduce polarization losses in the polymer matrix remains unclear. A central unknown is the critical polymer/ceramic interfacial resistance ( R _int,crit ), below which adding ceramics lowers electrolyte overpotential. Here, we present the first macroscale model framework to quantify R _int,crit for composite electrolytes based on polyethylene oxide (PEO) and Ta‐doped Li ₇ La ₃ Zr ₂ O ₁₂ (LLZO). A 1D model for DC‐polarization of tri‐layer cells (PEO‐LiTFSI/LLZO/PEO‐LiTFSI) shows that LLZO surface functionalization reduces the PEO/LLZO interfacial resistance, consistent with electrochemical impedance measurements. Extending to a 2D composite model, we show notably that R _int,crit scales linearly with LLZO particle diameter and shifts toward experimentally accessible values (e.g., 28.8 Ωcm ² ) as particle size increases. At fixed ceramic volume fraction, larger LLZO particles reduce the number of interfacial crossings, driving more current through the ceramic phase and lowering concentration polarization. In contrast, R _int,crit is largely independent of ceramic volume fraction. These results demonstrate that ceramic filler‐size engineering can enable synergistic, energy‐efficient transport in polymer/ceramic composite electrolytes.

Silane-functionalized LLZO nanoparticles enhance polymer–particle interfacial Li ⁺ transport, enabling stable lithium cycling at higher current densities through improved interfacial dynamics and modulated Li ion behavior.

Ongoing efforts to design stable, ionically conductive solid-state electrolytes (SSEs) for next-generation solid-state batteries make it clear that both long and short-range structural order strongly influence materials performance. However, clear structure–property relationships are generally lacking, making it difficult to develop design rules for improving the (electro) chemical stability of SSEs. In this work, we synthesize epitaxial, single-crystal lithium lanthanum titanium oxide (LLTO) films and demonstrate that the kinetics of Ti4+ reduction and lithium intercalation depend sensitively on the crystal orientation, with electrochemical stability increasing as LLTO (001) < (110) ∼ (112) < (100). However, thermodynamic stability is ultimately unaffected─all orientations fully reduce after extended contact with Li metal. In contrast, amorphous LLTO films exhibit minimal, self-limiting reactivity that results in an interface that is stable to extended contact with Li metal. The results demonstrate the potential to engineer crystal lattice strain and long-range order to differentially tune the stability of the solid electrolyte toward reactive lithium metal and cathode materials, suggesting strategies for enabling the wider deployment of LLTO and other ionically conductive ceramic films in advanced energy storage technologies.

This study investigates the formation mechanism of non-doped cubic lithium lanthanum zirconium oxide (c-LLZO) nanofibers using in situ synchrotron X-ray scattering techniques. Electrospun polymer precursor nanofibers were annealed at temperatures up to 800 °C, enabling real-time tracking of phase transitions via simultaneous small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and evolved CO₂ gas analysis. The results reveal a three-step transformation pathway: polymer decomposition, formation of La₂Zr₂O₇ (LZO), and direct conversion of LZO to c-LLZO without intermediate tetragonal phases detected within the sensitivity of our in situ WAXS measurement. Cryo-electron energy loss spectroscopy (EELS) further elucidates the role of lithium diffusion, showing Li enrichment at fiber surfaces and Li deficiency in the interior, which stabilizes the cubic phase. This Li segregation effect in nanostructured LLZO materials extends beyond the previously reported size effect. This work advances the understanding of c-LLZO formation mechanisms and provides practical insights for optimizing synthesis routes to achieve phase-pure c-LLZO for solid-state battery applications.

Reversible lithium metal anodes require interphases that are chemically stable, mechanically robust, and compatible with scalable processing. Here, we use slot‐die‐coated, salt‐rich PEO interlayers to design SEI nanolayers on Cu current collectors and isolate how salt anion chemistry controls interphase composition and lithium plating/stripping behavior in architectures relevant to Li plating and anode‐free cells. Mesoscale modeling shows that dendrite initiation at SEI defects is governed by a tradeoff among interphase thickness, ionic conductivity, and stiffness through defect‐driven current focusing, motivating controlled formation from thin precursor layers. Using UHV‐transferred XPS and ToF‐SIMS depth profiling after controlled Li deposition, we find that fluorinated sulfonylimide salts (LiTFSI, LiFSI) form mixed inorganic/organic interphases containing LiF and PEO‐derived alkoxides, with LiFSI producing a more LiF‐dominant Li‐facing surface than LiTFSI under identical conditions. In contrast, LiNO ₃ ‐containing interlayers yield a comparatively thick nitrate‐derived interphase that reduces upon Li contact to an N‐rich inorganic layer (Li–N/N ³⁻ , LiN _x O _y ). In Cu||Li cells, these designed interlayers reduce interfacial resistance and improve Coulombic efficiency (CE) and critical current density (CCD) relative to uncoated controls, with LiFSI‐ and LiNO ₃ ‐based interlayers providing the highest CE/CCD. Overall, the results demonstrate a manufacturing‐relevant approach to engineer SEI nanolayers by salt chemistry, providing pathways to improve reversible lithium metal anodes while reducing reliance on LiTFSI.

Formation of a stable solid electrolyte interphase (SEI) between lithium electrodes and electrolyte upon multiple charge/discharge cycles is crucial to a long-term lithium-ion battery performance. Addition of LiNO3 to lithium bis (fluorosulfonyl) imide/poly(ethylene oxide) (LiFSI/PEO) electrolyte leads to a durable SEI that is electrically insulating yet highly conductive to Li ions, chemically and electrochemically stable, physically uniform, and mechanically robust. ToF-SIMS was used here in combination with sputtering by a gaseous cluster ion beam (GCIB) to examine how the addition of a small proportion of LiNO3 to the LiFSI/PEO electrolyte affects the SEI composition. Negative ion ToF-SIMS spectra of the cycled samples display an intense m/z 26 peak associated with the SEI. Exact mass assignments and isotopic ratios indicate that this peak should be assigned as 12CN−, with little to no negative secondary ion signal arising from 7LiF−. This CN− signal appears to arise from an N-rich portion of the SEI adjacent to the Li electrode that is depleted in LiF relative to the bulk electrolyte. The dearth of LiF− (and LiF+ from the positive ion spectra) is unexpected because LiF has been identified in the SEI in similar samples. Finally, GCIB sputtering indicates that the SEI adheres more strongly to the Li electrode than to the LiFSI/PEO electrolyte.

In this study, we provide valuable insights into the fast rate preconditioning effects on optimization of a zinc ion (Zn–MnO ₂ ) battery performance for long duration.