The correlation between the electrochemical performance and the electronic structure and chemical composition of the cathode/electrolyte-and anode/electrolyte-interfaces in all-solid-state batteries (ASSB) is systematically studied. ASSBs are composed of LiCoPO 4 (LCP) or LiCoO 2 (redox potential ~4.8 V and ~3.8 V vs Li + /Li, respectively) thin-film cathode materials, Li 1+x Al x Ti 2-x (PO 4 ) 3 (LATP) as the solid-state electrolyte and lithium metal as the anode. X-ray photoelectron spectroscopy (XPS) interface experiments on LCP thin-film deposition on the LATP surface reveal a partial reduction of Ti 4+ ions attributed to electronic charge transfer from LCP to LATP without the involvement of the PO 4 polyanion in the process. Electrochemical activity of the ASSBs is primarily limited by the anode/electrolyte interface rather than the cathode/electrolyte interface. Post-cycling XPS analysis of the Li∣LATP interface indicates lateral and in-depth chemical inhomogeneity with a strong change in PO 4 polyanionic chemical environment and Li + accumulation at the areas with a better electrolyte/anode contact. Engineering the Li∣LATP interface via coating of the LATP surface with lithium oxynitride (LiPON) or a LiTFSI-PEO polymer electrolyte, or by wetting the LATP with the liquid electrolytes, significantly improves battery cycling stability even without artificial interface modification at the cathode side. The optimized Li∣LiTFSI-PEO∣LATP∣LCP cells showed excellent cycling performance between 3.0 V and 5.0 V at a charging time of 12 min (≈ 5C rate). Stable cycling was sustained over 10 cycles, followed by 30 and 50 additional cycles at reduced charging rates (≈ 4C and 1C, respectively). Good electrochemical cyclability was further demonstrated with an upper cutoff voltage of 5.3 V vs. Li + /Li.

NaSICON-type Na1+xZr2SixP3-xO12 (0 <= x <= 3, NZSP) were synthesized via a solution-assisted solid-state reaction method, and the effect of Na3LaP2O8 (NLP) addition on their microstructure, mechanical properties, and electrical performance was systematically investigated. NLP incorporation refined the grain structure, yielding a more uniform grain size distribution, reduced porosity, and suppressed microcrack formation, which collectively enhanced densification and mechanical performance. The optimized composition (2.5 mol% NLP) exhibited substantial increases in elastic modulus, hardness and fracture toughness compared to original NZSP. However, excessive NLP addition hindered ionic transport due to the insulating nature of NLP, revealing a trade-off between mechanical robustness and ionic conductivity. This work establishes a quantitative correlation between the mechanical and electrical properties of NLP-modified NZSP and provides a design strategy for mechanically reinforced, high-performance solid-electrolytes for all-solid-state sodium batteries.

Ceramic-based lithium metal batteries widespread application is limited by the persistent difficulty in achieving stable performance under high c-rate conditions. Herein, we designed flat, thin (similar to 200 mu m) Li6.45Al0.05La3Zr1.6Ta0.4O12 single-layer 3D porous scaffolds by the tape-casting technique. Leveraging a meticulously engineered single-layer scaffold, lithium metal is uniformly infiltrated to an average thickness of 35 mu m, ensuring seamless interfacial contact. Concurrently, a solid polymer electrolyte is integrated, facilitating the formation of a robust composite solid polymer-garnet separator with a precise thickness of 165 mu m, all within a unified structural framework. The integrated single-framework not only reinforces structural and interfacial stability of the metallic anode but also facilitates rapid Li+ transport, markedly enhancing ionic conductivity. This synergistic effect enables exceptionally high-rate performance, paving the way for more efficient and reliable electrochemical applications. Full cells with lithium iron phosphate cathode and the Li-infiltrated single-layer ceramic/polymer electrolyte cycled at record current rates of 2C and 5C at room temperature and achieved 100 % capacity retention for 30 cycles at 0.1C. The discharge specific capacities at 2C and 5C are 106.4 and 76.7 mAh g(-1), respectively. This innovative single-layer framework is designed to enable high-current-density, room-temperature solid-state lithium-metal batteries while eliminating the need for stacking pressure.

Abstract Alkali metal solid-state batteries (Li, Na, K) suffer from contact loss at the metal|solid electrolyte interface during stripping (discharge), driven by vacancy accumulation and pore formation. This reduces contact area, increases resistance, and promotes dendrite growth during subsequent plating, eventually leading to battery failure. While lithium has been studied extensively, sodium and potassium remain comparatively underexplored despite their high abundance. Their lower yield strength, larger molar volume, and higher homologous temperature strongly affect interfacial contact evolution. Operando electrochemical dilatometry is used to determine the formed pore volume in sodium and potassium metal anodes, enabling evaluation of the vacancy injection ratio (VIR) and comparison with lithium. At low stack pressure, pores form immediately upon stripping for all three metals. The formed pore volume scales with the molar volume across lithium, sodium, and potassium. Increasing current density enhances contact loss and leads to a weaker increase in the formed pore volume in sodium and potassium, with a fraction of the formed pore volume in potassium located inside the metal bulk. In contrast, lithium shows an initial reduction in the formed pore volume (partial pore collapse), followed by a stronger increase at higher current densities. These findings reveal systematic trends among lithium, sodium, and potassium, suggesting lower yield strengths and smaller molar volumes as favorable material properties. Based on these results, a generalized mechanism for pore formation in alkali metal anodes is proposed.

Development of solid-state electrolytes (SSE) is the basis to establish safe and reliable sodium batteries on the market. Despite their advantages, including non-flammable components and reduced reactivity with metallic sodium, many crystalline SSEs suffer from increased charge transfer resistance, sodium dendrite formation, and lower sodium ion conductivity compared to liquid electrolytes. To optimize SSE properties, comprehensive studies of the SSE surface in its pristine state and after contact with metallic sodium during battery operation are desirable. In this work, the surface of pellets with the NaSICON-type Na3.4Zr2Si2.4P0.6O12 composition, which is considered as a promising SSE for sodium batteries, was investigated in detail using conventional surface analytical techniques. Grazing-incidence X-ray diffraction revealed that the subsurface region of the as-prepared pellets consists of several crystalline NaSICON phases and amorphous ternary oxides, extending to a depth of approximately 1 mu m. In contrast, after grinding the pellet, only one NaSICON phase could be detected using the transmission XRD technique. The chemical composition of the surface appeared to change after contact with metallic sodium, particularly for the Si-, P-and O-species. Furthermore, we observed surface instability under electron beam exposure during electron microscopy and spectroscopy measurements over 30-60 min, leading to the formation of metallic sodium on the irradiated surface area. The irradiation with X-rays during photoelectron spectroscopy measurements for several hours induced sodium depletion within the irradiated spot while depositing metallic sodium radially around the irradiated area. These findings demonstrate the restricted application of surface analytical techniques for studying ternary oxides in the Na-Zr-Si-P-O system, especially when using NaSICON as a solid electrolyte.

In this study, we investigated the impact of sintering temperature on the densification, phase formation, microstructure, crystallinity, and ionic conductivity of NaSICON materials with varying nominal Zr deficiency and a varying Si/P ratio. Several powder batches were synthesized and resulted in substantially different sintering abilities using conventional sintering. For most of the powder batches, the conventionally sintered specimens reached maximum ionic conductivities between 2 and 3 mS cm-1 after sintering at 1200-1300 degrees C. Cold sintering was explored using one of the powder batches with different sintering additives. After cold sintering, an annealing step at 900 degrees C yielded similar conductivities. Without postannealing, a maximum ionic conductivity of 0.55 mS cm-1 was reached at temperatures as low as 275 degrees C. There is clear evidence that (a) the densification temperature can be significantly reduced with increasing glass fraction in the specimens and (b) the total conductivity increases with increasing sintering temperatures due to increasing density and crystallinity.

Conductive ceramics currently play a vital role in human life. In practical applications, most conductive ceramics are polycrystalline, and their overall conductivity (sigma total) is influenced by both bulk and grain boundary resistances (Rbulk and Rgb, respectively). While Rbulk is mainly of academic interest, Rgb often determines the quality of a conductive ceramic component. Currently, studies discussing the influence of specific methods on grain boundary resistances are typically related to individual ceramics. In this study, it is discovered that the addition of 0.5-3 mol% nominal LaNbO4 significantly reduces the Rgb of several well-known conductive ceramics, such as rhombohedral NaSICON-type Na+-ion-conducting Na3.4Zr2Si2.4P0.6O12 and Li+-ion conducting Li1.5Al0.5Ti1.5P3O12, Li+-ion-conducting tetragonal perovskite Li0.34La0.56TiO3, oxygen-ion-conducting cubic fluorite 8 mol% Y2O3 stabilized ZrO2, and electron-conducting perovskite SrTiO3 (sintered in a reducing atmosphere). In particular, for NZSP and LATP, the enhanced sigma total reaches 9.3 x 10-3 S cm-1 and 2.1 x 10-3 S cm-1 at 25 degrees C, surpassing previously published results. Detailed investigations reveal that the microstructure of the grain boundaries in all the ceramics undergoes significant improvements. The findings elevate the importance of research on grain boundaries, inspiring the development of conductive ceramics with higher sigma total for superior applications.

This study investigates the feasibility and limitations of almost-solid-state sodium batteries (Na-aSSBs) as novel energy storage solutions. The cell concept comprises a sodium metal anode, a tape-cast Na3.4Zr2Si2.4P0.6O12 solid electrolyte, and a Na3V2(PO4)(3) cathode with liquid electrolyte. The impact of the Na3.4Zr2Si2.4P0.6O12 separator and sodium electrode on total cell resistance is evaluated in symmetric Na| Na3.4Zr2Si2.4P0.6O12|Na cells, demonstrating an ultra-low Ohmic resistance below 10 Omega cm2. The Na-aSSB achieved (85 +/- 1) % of the theoretical cathode capacity and energy densities up to (239 +/- 10) Wh/l at the cell level, among the highest reported for similar concepts. Cycling stability shows a Coulombic efficiency exceeding 99% over 70 cycles at a 2-h discharge rate. Five performance-limiting factors were identified: initial cathode resistance, degrading cell resistance during cycling, insufficient mechanical strength of the separator, dendrite formation, and non-optimized energy density. Suggested approaches to address these limitations highlight the technological potential of Na-aSSBs.

Developing highly conductive and reliable solid-electrolytes (SEs) is still important for the advancement of solid-state sodium batteries. NaSICON-type polycrystalline SEs exhibit the dominance of grain-boundary resistance to the total resistance, which is mainly due to the thermal expansion anisotropy of NaSICON-type lattices. In this study, we modify the grain boundaries of NaSICON-type Na3.4Zr2Si2.4P0.6O12 (NZSP) by adding 2.5 mol% Na3LaP2O8 (NLP) to counteract the effect of thermal expansion anisotropy. NLP does not serve as a sintering aid for NZSP because the sintering temperature and relative density of NZSP is not changed. The total conductivity of modified NZSP increases to 7.1 mS cm(-1) at 25 degrees C, surpassing other reported polycrystalline oxide SEs. The critical current density of Na | modified NZSP | Na symmetric cells increases to 22 mA cm(-2). The cells can survive under long-term galvanostatic cycling up to 10 mA cm(-2), indicating the unprecedented dendrite tolerance. Remarkably, the main failure mode in these cells shifts from Na-dendrite short-circuiting to the loop of substantial polarizations and short-circuits.

Two composition series of Zr‐deficient NaSICON materials are investigated with respect to their ionic and thermal transport properties. The bulk conductivity varies between 1.4 and 6.6 mS cm −1 . The total conductivity showdecreasing values with increasing Zr deficiency due to the impact of the increasing fraction of glass phase. The calculated grain boundary conductivity is about two orders of magnitude lower than the total conductivity but does not correspond to the conductivity of any known glass composition of sodium silicates/phosphates. Nuclear magnetic resonance reveals three 23 Na relaxation rates, the fastest of which is attributed to the NaSICON phase and the two slower relaxation rates to sodium orthophosphates and the glass phase. Thermal conductivity varies between 0.9 and 1.0 W m −1 K −1 at 25 °C. At elevated temperatures, a clear trend is observed toward lower thermal conductivity with a higher glass fraction. In addition, atom probe tomography is applied to precisely quantify the composition of specific microstructural regions found within the glassy phase. A scanning electron microscopy study of the surfaces of sintered pellets shows an increasing amount of glass phase between the NaSICON particles with increasing Zr deficiency. Furthermore, a time‐dependent phase separation is observed in relation to the dynamic formation and dissolution of Na 3 PO 4 domains.

Selective laser -induced etching enables the high -precision fabrication of three-dimensional components from transparent materials. First, ultrashort pulsed, tightly focused laser radiation is used to create a modification within the bulk of the transparent material. By selectively moving the focus, entire contiguous surfaces and volumes can be modified. Subsequently, the modified areas are etched free using an etching liquid. Thus, the desired three-dimensional component is exposed and separated from the surrounding material. Using the SLE process, molding tools made of fused silica are fabricated with functional structures of less than 150 gm in size. By molding the solid-state electrolyte foil of a sodium -based battery cell, the contact areas between the chemical components are increased to enhance the power density. Different structural geometries and dimensions are compared to increase this effect.

This work focuses on a very narrow region in the quaternary system Na2O-P2O5-SiO2-ZrO2 to explore the occasionally proposed deficiency in zirconium and oxygen content of Na+ super-ionic conductor (NaSICON) materials. In addition, this region is known for the formation of glass-ceramics, but a systematic study of such materials has not been carried out yet. For this purpose, 2 series of compositions were defined and synthesized: Na3.4Zr2-3x/4Si2.4-x/4P0.6+x/4O12-11x/8 and Na3.4Zr2-3x/4Si2.4+x/4P0.6+1.5x/4O12-x/16. They only differ in the silicate and phosphate content. In the first series the molar content is constant, nSi + nP = 3. The latter series allows an excess of the 2 cations to meet the composition Na3.1Zr1.55Si2.3P0.7O11 or alternatively re-written as Na3.4Zr1.7Si2.52P0.77Ol2, which was formerly regarded as a superior material to the frequently reported composition Na3Zr2Si2POl2.Several characterization techniques were applied to better understand the relationships between phase formation, processing, and properties of the obtained glass ceramics in the context of the quasi-quaternary phase diagram. The investigations gave clear evidence that a glass phase is progressively formed with increasing x. Therefore, compounds with x > 0.2 have to be regarded as glass-ceramic composites. The resulting NaSICON materials revealed a very limited Zr deficiency with charge compensation by Na ions and a non-detectable amount of oxygen vacancies verified by neutron scattering and atomistic simulations.Hence, this work is the first systematic investigation of pretended Zr-deficient NaSICON materials, which clearly show the chemistry of a 2-phase region. The 2 investigated series are directed toward a region that is orthogonal to the series Na3Zr3-ySi2PyO11.5+y/2 reported in the first part of this series of publications.

Sodium vanadium triphosphate (Na3V2(PO4)3, NVP) is a promising cathode material for Na-ion batteries. Due to its intrinsically low electronic conductivity, it is usually mixed or coated with carbon. However, so far there have been no systematic studies on the ionic and electronic conductivity of carbon-coated NVP particles. In this work, NVP with varying carbon contents are prepared. The powders are sintered as single pellets or sandwiched between a solid electrolyte for measurements in an ion blocking and non-ion blocking configuration. In these two different configurations, two different electrodes are attached and several electrochemical characterization techniques are applied such as impedance spectroscopy, chronoamperometry, and four-point measurements. The NVP/C composites with carbon content >0.1 wt% show a high degree of densification and an amorphous carbon network. The conductivity of NVP in composites with carbon content <0.1 wt% shows dominating ionic conduction with an average value of ∼2 × 10−6 S cm−1. NVP/C samples with carbon contents >0.1 wt% show a dominance of electronic conduction in the range of 0.01–0.2 mS cm−1 because of the percolated carbon network at the grain boundaries. The ionic conductivity, however, remains almost constant in the same order of magnitude (∼6 × 10−6 S cm−1).

The recent development of solid-state batteries brings them closer to commercialization and raises the need for heat management. The NASICON material class (Na1+x Zr2P x Si3-x O12 with 0 <= x <= 3) is one of the most promising families of solid electrolytes for sodium solid-state batteries. While extensive research has been conducted to improve the ionic conductivity of this material class, knowledge of thermal conductivity is scarce. At the same time, the material's ability to dissipate heat is expected to play a pivotal role in determining efficiency and safety, both on a battery pack and local component level. Dissipation of heat, which was, for instance, generated during battery operation, is important to keep the battery at its optimal operating temperature and avoid accelerated degradation of battery materials at interfaces. In this study, the thermal conductivity of NaZr2P3O12 and Na4Zr2Si3O12 is investigated in a wide temperature range from 2 to 773 K accompanied by in-depth lattice dynamical characterizations to understand underlying mechanisms and the striking difference in their low-temperature thermal conductivity. Consistently low thermal conductivities are observed, which can be explained by the strong suppression of propagating phonon transport through the structural complexity and the intrinsic anharmonicity of NASICONs. The associated low-frequency sodium ion vibrations lead to the emergence of local random-walk heat transport contributions via so-called diffusons. In addition, the importance of lattice dynamics in the discussion of ionic transport as well as the relevance of bonding characteristics typical for mobile ions on thermal transport, is highlighted.

"Anode-free" solid-state battery concepts are explored extensively as they promise a higher energy density with less material consumption and simple anode processing. Here, the homogeneous and uniform electrochemical deposition of alkali metal at the interface between current collector and solid electrolyte plays the central role to form a metal anode within the first cycle. While the cathodic deposition of lithium has been studied intensively, knowledge on sodium deposition is scarce. In this work, dense and uniform sodium layers of several microns thickness are deposited at the Cu|Na3.4Zr2Si2.4P0.6O12 interface with high reproducibility. At current densities of approximate to 1 mA center dot cm-2, relatively uniform coverage is achieved underneath the current collector, as shown by electrochemical impedance spectroscopy and 3D confocal microscopy. In contrast, only slight variations of the coverage are observed at different stack pressures. Early stages of the sodium metal growth are analyzed by in situ transmission electron microscopy revealing oriented growth of sodium. The results demonstrate that reservoir-free ("anode-free") sodium-based batteries are feasible and may stimulate further research efforts in sodium-based solid-state batteries. The cathodic deposition of sodium at the Cu|Na3.4Zr2Si2.4P0.6O12 interface aiming for "reservoir-free" sodium solid-state batteries is studied systematically. Dense and roughly 10 mu m thick sodium layers are formed underneath the copper current collector. By increasing the current density j during deposition, a higher coverage is obtained, while the stack pressure has only a minor influence on the coverage.image

The phosphate lithium-ion conductor Li1.5Al0.5Ti1.5(PO4)3 (LATP) is an economically attractive solid electrolyte for the fabrication of safe and robust solid-state batteries, but high sintering temperatures pose a material engineering challenge for the fabrication of cell components. In particular, the high surface roughness of composite cathodes resulting from enhanced crystal growth is detrimental to their integration into cells with practical energy density. In this work, we demonstrate that efficient free-standing ceramic cathodes of LATP and LiFePO4 (LFP) can be produced by using a scalable tape casting process. This is achieved by adding 5 wt % of Li2WO4 (LWO) to the casting slurry and optimizing the fabrication process. LWO lowers the sintering temperature without affecting the phase composition of the materials, resulting in mechanically stable, electronically conductive, and free-standing cathodes with a smooth, homogeneous surface. The optimized cathode microstructure enables the deposition of a thin polymer separator attached to the Li metal anode to produce a cell with good volumetric and gravimetric energy densities of 289 Wh dm-3 and 180 Wh kg-1, respectively, on the cell level and Coulombic efficiency above 99% after 30 cycles at 30 °C.

In recent years, many efforts have been made to introduce reversible alkali metal anodes using solid electrolytes in order to increase the energy density of next-generation batteries. In this respect, Na3.4Zr2Si2.4P0.6O12 is a promising solid electrolyte for solid-state sodium batteries, due to its high ionic conductivity and apparent stability versus sodium metal. The formation of a kinetically stable interphase in contact with sodium metal is revealed by time-resolved impedance analysis, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. Based on pressure- and temperature-dependent impedance analyses, it is concluded that the Na|Na3.4Zr2Si2.4P0.6O12 interface kinetics is dominated by current constriction rather than by charge transfer. Cross-sections of the interface after anodic dissolution at various mechanical loads visualize the formed pore structure due to the accumulation of vacancies near the interface. The temporal evolution of the pore morphology after anodic dissolution is monitored by time-resolved impedance analysis. Equilibration of the interface is observed even under extremely low external mechanical load, which is attributed to fast vacancy diffusion in sodium metal, while equilibration is faster and mainly caused by creep at increased external load. The presented information provides useful insights into a more profound evaluation of the sodium metal anode in solid-state batteries.

Inorganic all-solid-state batteries with oxide electrolytes show improved safety compared to conventional lithium-ion batteries due to the application of a non-flammable solid electrolyte. However, the currently applied production methods are unsuitable for creating oxide composite cathodes with a good interfacial contact between the solid electrolyte and the cathode active material, which limits the accessible discharge capacity. Thus, solid electrolyte matrix-supported all-solid-state batteries, for which a porous scaffold is filled with cathode active material, have recently seen increasing research interest. This publication introduces a scalable production route for a matrix-supported cell concept with a three-dimensionally-structured oxide-based composite cathode. Directed microstructures with different geometries were introduced into NASICON-type Li1.5Al0.5Ti1.5(PO4)(3) oxide solid electrolyte layers via laser ablation. The obtained porous scaffold was infiltrated with various cathode slurries containing cathode active material and an ion-conducting polymer electrolyte to fabricate hybrid composite cathodes with an improved electrode-electrolyte interface. Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed a high pore filling degree. A promising specific discharge capacity of 120.1 mAh.g(-1) was achieved during electrochemical testing of a prototype all-solid-state battery with a LiNi0.6Mn0.2Co0.2O2 composite cathode and a lithium metal anode. Overall, this work serves as a proof-of-concept for the novel, matrix-supported cell design and provides insights into the production processes involved.

This paper presents a suitable combination of different sodium solid electrolytes to surpass the challenge of highly reactive cell components in sodium batteries. The focus is laid on the introduction of ceramic Na3.4Zr2Si2.4P0.6O12 serving as a protective layer for sulfide-based separator electrolytes to avoid the high reactivity with the sodium metal anode. The chemical instability of the anode|sulfide solid electrolyte interface is demonstrated by impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Na3.4Zr2Si2.4P0.6O12 disk shows chemical stability with the sodium metal anode as well as the sulfide solid electrolyte. Impedance analysis suggests an electrochemically stable interface. Electron microscopy points to a reaction at the Na3.4Zr2Si2.4P0.6O12 surface toward the sulfide solid electrolyte, which does not seem to affect the performance negatively. The results presented prove the chemical stabilization of the anode-separator interface using a Na3.4Zr2Si2.4P0.6O12 interlayer, which is an important step toward a sodium all-solid-state battery. Due to the applied pressure that is mandatory for battery cells with sulfide-based cathode composite, the use of a brittle ceramic in such cells remains challenging.