All-solid-state batteries are identified as the next generation of batteries. The advantage of these technologies lies in the solid nature of the electrolyte, which could enhance safety and energy density of the systems. The current state of the art is still far from the anticipated expectations, and ongoing studies identify several challenges responsible for this gap. Firstly, all-solid-state technology requires careful management of the pressure applied. It must take into account the mechanical properties of the components and be adapted to ensure good ionic conductivity¹. Secondly, the fabrication of components must be homogeneous in composition and densification. The distribution and contact of active material and electrolyte particles are fundamental to achieving the expected capacity. Finally, these materials must be chemically and electrochemically compatible to avoid accelerated degradation of the system². In our research, we aim to develop all-solid-state prototype references through the coating process. The objective is to study the performance during prolonged cycling and to investigate the failures mechanisms. Optimization of formulations, processes, and cycling conditions has enabled us to achieve five-centimeter-square pouch-cell format prototypes with reproducible performance over several hundred cycles. The electrochemical test was conducted at a low current density of 70 µA·cm⁻² under 60°C with a relatively low pressure of 3 MPa. The work presented will focus on the development of prototypes. The design of the separator, with a thickness of 400 µm obtained by coating, will be particularly detail, as it is the critical component of this technology³. The use of the coating process creates porosity during the solvent evaporation step. This porosity is not suitable because it could affect ionic conductivity and create preferential pathways for lithium dendrite growth. However, the main advantage of this process is that it is widely known and used in current battery manufacturing plants. Therefore, the transition to all-solid-state battery production would be facilitated and accelerated. In this presentation, various optimizations and strategies to overcome the porosity issues will also be discussed.

Rechargeable batteries can contribute to powering electric transportation and storing electrical energy generated from intermittent renewable sources. Li ion batteries (LIBs) are the current battery of choice and new types of batteries such as solid-state Li metal batteries (SSLMBs) are also under active research. There are constant demands for further improving rate capability and energy density of batteries. Thick electrodes can increase energy densities at the cell level, but one of the limiting factors is slow ion transport that reduces capacity at increasing (dis)charge rates. This problem is exacerbated in SSLMBs as the ion transport coefficient is usually lower in solids than in liquids. Here, two novel processing technologies have been developed to optimise the battery electrode microstructure and improve ion diffusion kinetics: (i) directional ice templating (DIT) for fabricating thick (900 µm) cathodes with vertical pore arrays and porosity gradient for LIBs [1]; and (ii) directional freezing and polymerisation (DFP) for fabricating cathodes with vertical arrays of solid polymer electrolyte (SPE) directly incorporated in the cathode microstructure during processing for SSLMBs [2]. Both techniques reduced the tortuosity of ion diffusion pathways through electrode thickness from ~3.3 for conventional electrodes to 1.5.

A new operando correlative imaging technique of combining X-ray Compton scattering imaging (XCS-I) and computed tomography (XCT) has also been developed that allows pixel-by-pixel mapping of Li⁺ chemical stoichiometry variations in a LiNi_0.8Mn_0.1Co_0.1O₂ cathode in an SSLMB [3]. This technique shows how the anisotropic electrode microstructure improved Li⁺ ion diffusivity, homogenised Li⁺ ion concentrations, and improved energy storage performance.

Nowadays, commercially available lithium batteries are based on liquid electrolytes, due to their high ionic conductivity at room temperature. But the search for new battery technologies that can substitute the existing ones has become very important in the last decades, mainly due to issues related to safety, durability and manufacturing complexity. Hence, all-solid-state lithium-metal batteries (employing solid electrolytes) have gained attention because of their higher energy density, while being safer, and more chemically and thermally stable than conventional lithium-ion batteries [1,2]. When it comes to solid electrolytes, ceramics offer higher chemical and thermal stability, better mechanical properties and wider electrochemical window. However, their low performances and interfacial detrimental interactions with the electrodes still need to be improved for their implementation  into final devices.

A possible strategy to address these drawbacks and improve battery performance involves the employment of innovative manufacturing processes, as 3D printing and ultra-fast high temperature sintering (UHS). Both techniques are explored in this work for NASICON-type ceramic electrolytes and Co-free cathodes.

Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolytes were printed by stereolithography (SLA) in complex geometries generally impossible to produce with conventional ceramic manufacturing techniques. Up to date, only a few studies have reported the production of full ceramic electrolytes by 3D printing [5-7].  Here we present the development of LAGP full-ceramic electrolytes by SLA, exploring different complex geometries to improve the electrolytes behaviour (i.e. area specific resistance, critical current density, available capacity in a full battery). Crack-free dense (>80% of theoretical density) LAGP electrolytes were succesfully prepared, showing conductivities in good agreement with LAGP manufactured by conventional techniques (10⁻⁴ S cm⁻¹), without detrimental reactions or degradation due to the debinding step. Additionally, plating/stripping galvanostatic tests were carried out on symmetrical cells Li/3DPrinted-LAGP/Li up to 200h.

In parallel to the development of electrolytes, Co-free composite cathodes have also been studied. LiFePO₄ (LFP) was chosen as active material to be combined with LAGP in the composites. The cathodes were attached by an optimized conventional thermal treatment (650 ºC, 4h, N₂ atmosphere) onto LAGP sintered pellets. The compatibility of the involved materials was investigated showing no degradation phenomena or unwanted reactions. Finally, a full cell Li/LAGP/LFP-LAGP(50:50) was mounted, tested by EIS and cycled at constant currents. Despite the limited capacity detected, these first results represent a promising step forward towards full ceramic batteries based on these combination of materials.

UHS was also developed for both electrolytes and cathodes. This technique was presented in 2020 as a sintering technique capable of densifying ceramics in seconds, to reduce both the manufacturing times and energy consumption respect to conventional thermal processes, while avoiding degradation phenomena [8-10]. In the case of materials containing lithium, UHS can also be helpful to minimize lithium losses [8-10]. UHS evaluation on sintering of electrolytes and cathodes will be presented. LAGP was sintered by UHS in less than 2 minutes reaching very high densification (88%) and ionic conductivities comparable with the ones obtained by conventional sintering. Furthermore, LFP-LAGP composite cathodes were successfully attached on LAGP electrolytes by UHS avoiding detrimental interfacial interactions. As last step UHS was successfully applied for debinding and sintering of 3D printed LAGP electrolytes, lowering the times needed for these processes from days to minutes.

Finally, advanced characterization techniques as Grazing-Incidence Wide-Angle X-ray Scattering using Synchrotron radiation (GIWAXS) and Glow Discharge Emission Spectrometry (GDOES) were employed to study possible side reactions between these materials, and lithium content differences comparing conventional thermal treatments and UHS.

All Solid-State Batteries (ASSBs) are emerging as the next-generation energy storage devices due to their superior safety and performance potential compared to traditional lithium-ion batteries (LiBs). The presence of a solid electrolyte enhances safety by minimizing flammability and mitigates dendrite propagation, enabling the use of high-capacity lithium metal anodes (3860 mA h/g). Combined with high-voltage cathodes like LMNO, ASSBs promise significant improvements in energy density, both gravimetric and volumetric beyond incumbent LIBs. Ceramic materials show mechanical properties that can reduce the propagation of dendrites and interesting ionic conductivity for lithium ions.

Oxides exhibit high stability with lithium metal and mechanical properties to prevent dendrite propagation, but face challenges in room-temperature ionic conductivity and require high sintering temperatures to achieve the mechanical and structural qualities to be used as electrolytes.

Sulfides achieve high ionic conductivities (up to 10 mS/cm) at room temperature and high relative densities (>90%) without sintering. However, their air sensitivity delayed their entrance into the battery market.

Halides, with moderate ionic conductivities (1-2 mS/cm), bridge the gap between oxides and sulfides. They offer room temperature processability and operation with less toxic byproducts, despite their sensitivity to atmospheric exposure.

Across all ceramic electrolyte types, optimizing interfaces, particularly with lithium metal, remains a key challenge for achieving higher performance in ASSBs. This work focuses on the intrinsic behavior and reactivity of these electrolytes with individual cell components and examines the synergies formed by combining different chemistries. Advanced characterization techniques such as NMR, EIS, and DRT are employed to investigate the phenomena occurring during the operation of solid-state batteries. We present here practical cases on how material design, processing techniques, and interface engineering play a significantly effect on the performance of the cell.

The performance of high energy density solid-state Li-metal batteries is typically also limited due to unstable interface between solid electrolytes and the Li metal. This is particularly critical when employing NASICON-based solid electrolytes, where the transition metal tends to reduce in the presence of Li-metal, together with the solid-electrolyte decomposition. In this work,  the interface stability of the Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) with Lithium metal has been improved by coating of the electrolyte with binary metal oxides, such as Alumina and Zirconia, as a function of thickness  employing the atomic layer deposition (ALD) technique. Furthermore, our studies suggest that after coating the as deposited Alumina and Zirconia layer with specific metal, the assembled coin cells can run with low overpotentials and high stability even at medium current densities. For these studies, the coin cell CR2032 have been assembled in symmetrical mode (Li/interlayers/solid-electrolyte/interlayer/Li) and cycled with current density of 6.4, 12.8 and 25.6 μA cm^-2 for 2 h platting and stripping conditions (50 cycles at each current density). Impedance studies also suggest an improved interface with low contact resistance before and even after the cycling at high current densities as compared to the pristine LATP as well as LAGP.  During this presentation, we will discuss the electrochemical performance of our surface-modified cells and present structural characterization to address the mechanisms for the observed improvements.

Li_1-xAl_xTi_2-x (PO₄)₃ (LATP) ceramics are claimed to be one of the most attractive for Solid State Electrolytes (SSEs) ceramics for the next generation of Li batteries.[1] This is due to its advantages, such as non-flammability, excellent electrochemical stability, no leakage risk, non-volatile materials, and low production costs. As the most important component of All Solid-State Lithium Battery (ASSLB), the high ionic conductivity, good interfacial compatibility and stability are the basic requirements, but some of them still remain a challenging. In addition, the search for suitable electrolytes with sufficiently high ionic conductivity does also includes the development of new eco-friendly processing techniques. 

As an alternative, solid electrolyte LATP ceramics have been successfully sintered by Cold Sintering Process (CSP) using transient liquid phase (TLP) and solid modifiers. This process allows samples to be sintered below 200 ºC, meaning that composites such as ceramics in polymer can be sintered.  And, of course, to reduce CO₂ emissions in line with the European Climate Law, which requires a reduction in net greenhouse gas emissions of at least 55% by 2030.[2]

Here, we have studied the CSP to sinter ceramic electrolytes at 150 ºC and ~700 MPa, to boost a competitive ionic conductivity. A screening upon the LAPT solid electrolyte, from morphology and particle size, the nature and content of TLP, including the effect of polymer addition, shows that SE show relative density in the range of 85 – 90 % of the theoretical value of LATP, and the total ionic conductivity achieved by CSP is competitive with those sintered by high temperature treatments. Electrochemical impedance spectroscopy (EIS) technique is employed during the sintering process as a control tool to monitor the densification.[3]

Moreover, EIS is also used during the cycling of the half-cells to identify limiting factors of the cold-sintered electrolyte kinetics and to predicts the overall electrochemical behaviour. The results suggest that ionic conductivity fading can occur in the prepared samples due to the low grain boundary ionic conductivity and secondary phases in the intergranular regions, and that additives used in the processing are critical to the final microstructure.  

The viability of a simple and innovative method of densifying ceramic powders at low temperatures by applying uniaxial pressure and using a transient acid solution is demonstrated.

Enabling scalable processing of halides with high Li conduction

R. Artal¹, R.del Olmo², N. Stankiewicz², S. Daubner³, R. Jimenez-Rioboo¹, S. Cooper³, I. Villanueva², A. Aguadero^1,3

¹ Instituto de Ciencia de Materiales de Madrid, CSIC, Madrid, Spain

² POLYMAT, University of Basque Country, San Sebastian, Spain

³ Imperial College, London, United Kingdom

The development of all-solid-state batteries (ASSBs) has the potential to be transformational leading to safer, longer life and higher energy and power density performance to that of conventional based on liquid organic electrolytes. To achieve this goal, solid electrolytes that possess high cation conductivity combined with the adequate electro-chemo-mechanic stability and sustainable, low-cost and large scale processability have to be developed.

In this work, we focus the study on families of chlorines using non-critical elements that can act catholytes due to their high stability at high voltages (>4V). These chlorines have been reported to have very high Li conductivities when disorder is enforced by high energy mechanical milling leading to metastable phases high low thermal stability. This imposes a limitation in the scalability of the production of these systems and in their integration in full devices. 

Here, we analyze a range of inorganic and polymer-inorganic solid electrolytes with a combination of experimental and computational techniques, to qualitatively understand the structural and chemical factors affecting the ion mobility in these systems in correlation with their processability.  We use a combination of high frequency range (3GHz-mHz) electrochemical testing with solid-state NMR, synchrotron and neutron diffraction studies and FIB-SEM analysis with multiscale simulations to correlate local and long range structural and chemical features with the electrochemical performance. Finally, we propose a highly disorder spinel based on non-critical materials as a potential electrolyte high combined enhanced electrochemical performance and processability.

As the demand for safer and more sustainable energy storage systems continues to rise, all solid-state batteries (ASSBs) have emerged as a promising alternative to traditional lithium-ion batteries. By replacing liquid electrolytes with solid-state ones, the resulting solid batteries offer the potential for enhanced safety operation and higher energy density, as well as the elimination of harmful and flammable organic compounds. In this line, the development of a green solid-state electrolyte is a fundamental part of the transition to a new generation of solid-state batteries, since solid electrolytes can prevent and minimize the failure mechanisms derived from dendrite growth in lithium-metal batteries, while reducing the amount of critical organics and liquids. Among all the types of solid electrolytes within the inorganic families (NaSICON, LiSICON, garnet, perovskites…), the Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) a NaSICON-like phosphate, stands out due to its theoretical high ionic conductivity (3 mS cm^-1), air and humidity stability and lack of scarce transition metals. ^[1]

However, ceramic bodies like the LATP have traditionally been processed following solid-state reactions involving  high-temperature routes, which result in a high energy consumption and large CO₂ emissions. To assess these problems, the Cold Sintering Process (CSP) has emerged as a new sintering route that enables the obtention of dense ceramics like LATP employing a low processing temperature (less than 300 ºC), mid-to-large pressures (hundreds of MPa) and a small amount of a transient liquid phase (TLP).^[2] Furthermore, with the CSP it is possible to incorporate polymeric additives to form composites with enhanced electrochemical properties taking advantage of the low operating temperatures. In this way, this new processing technique opens the door for the obtention of new hybrid materials acting as solid electrolytes for the new ASSBs.

In this work, authors present a comprehensive study of the application of the Cold Sintering Process in the manufacture of composite solid electrolytes (CSEs) based on LATP and a 10% PEO₂-LiTFSI polymer matrix additive, by focusing on some of the most important processing variables such as the TLP content, the ceramics particle size, the nature of the dopants and the sintering pressure, which will determine the final behavior of the electrolyte in a lithium-metal battery. It is worth mentioning that by optimizing the above-mentioned variables, CSEs with a maximum ionic conductivity of 0.5 mS cm^-1, a low activation energy (0.296 eV) and good cycling stability (160.1 mAh g^-1_LFP at C/10 and 75.8 mAh g^-1_LFP at 1C) are obtained at a maximum pressure and temperature of 300 MPa and 150 ºC, respectively. Furthermore, in operando electrochemical impedance spectroscopy (EIS) is employed as a powerful characterization technique, since it is not only used during the sintering process as a control tool to monitor the densification, but also during the cycling of CSP-obtained half-cells (with an LFP|CSE|Li structure) to identify potential failure mechanisms derived from the stacking of the layers.

Thin film solid ionic conductors have been identified as a key component for miniaturised all-solid-state sensors and batteries since the 1980’s. Among them, amorphous Li-based materials prepared by vacuum deposition techniques are particularly attractive as electrolytes for making lithium microbatteries and integrating the latter into smart autonomous microsystems. In that respect, lithium phosphorus oxynitride, prepared by reactive sputtering of a Li₃PO₄ target under a pure nitrogen atmosphere, exhibits a combination of interesting properties such as an ionic conductivity of ~ 2.10^-6 S.cm^-1 at room temperature, a very low electronics conductivity < 10^-14 S.cm^-1, and passivating behaviour on lithium metal, which led to the market launch of first microbatteries. Since then, energy density specifications for IoT and medical applications have led to changes in conventional cell design with more stringent requirements for ionic conductivity. In recent years, there has also been renewed interest in these thin-film amorphous electrolytes as part of the development of next-generation batteries for automotive applications. Used in the form of coatings, they can solve reactivity problems at the electrode/electrolyte interface and improve the cycling behaviour of metallic lithium [1].

Amorphisation and then partial nitridation of lithium orthophosphate, leading to the formation of a disordered material containing condensed units with bridging nitrogen [2, 3], was found to be an effective means of boosting ionic conductivity. The introduction of a second glass former, such as in Li-Si-P-O-N, can also provide an additional tenfold increase through a mixed former effect [4], while maintaining exceptional mechanical properties [5]. This evolution illustrates the need for research into materials that are more and more complex in their composition, and not confined to ‘simple’ systems, to identify the best ionic conductors. In this respect, high-throughput experimentation is an attractive approach for screening multi-element compositions [6]. It usually involves a combinatorial synthesis method capable of rapidly producing a large number of samples with different compositions, i.e. material libraries, combined with automated characterisation methods that allow various specific properties to be rapidly measured on all the samples produced.

The development of a specific experimental approach for the high-throughput screening of thin film solid electrolytes based on the synthesis of material libraries by magnetron co-sputtering on 4’’ wafers and the rapid characterisation of their thickness (stylus profilometry), composition (LIBS, ICP-OES), structure (Raman spectroscopy) and conduction properties (Impedance spectroscopy) will be presented [7]. Mapping the composition of these thin films containing low-Z elements, particularly Li, on large substrates is undoubtedly the most difficult stage in this process. Laser-induced breakdown spectroscopy (LIBS) appears to be the most appropriate technique for achieving this objective, as it meets the key requirements of speed, spatial resolution and sensitivity [8]. However, further developments are required to achieve elemental quantification. To this end, combined ICP-OES, LIBS and ion beam (RBS, NRA) analyses were carried out.

Ultimately, screening large parts of the compositional space of Li(Si,P)(O,N) reveals general relationships between composition, local structure and ion transport properties, but also raises questions about the influence of synthesis conditions on the degree of disorder in these amorphous materials

The nanoscale structure and chemistry of materials and interfaces largely govern the performance of (solid‑state) batteries.[1] Thin film solid-state batteries (TFSSB) have well-defined geometry (parallel layers of known thickness and area) and compact size, making them ideal model systems for studying material and interface properties.[2,3] The thin-film architecture allows for relatively easy investigations of (buried) interfaces and materials that would be challenging to measure in bulk cells that have multi-phase components with often random structure. Here, I will show recent highlights from our work on transition metal fluoride conversion cathodes (TM = Fe, Cr, etc.) for TFSSBs. I will focus on scanning transmission electron microscopy investigations into the nanostructure-performance relationship and its evolution during cycling. Briefly, the performance of TM fluoride cathodes is dependent not only on the choice of the transition metal but also on its nanostructure and mixing of the constituent phases. For example, in Fe-LiF cathodes, we observe electrochemical activation, wherein the particle size of the cathode coarsens and the elemental distribution changes during cycling which is associated with a gradual increase in discharge capacities. I will also talk about the challenges associated with this approach in terms of sample preparation and handling. This work uses the TFSSB system as a model to study fundamental mechanisms and electrochemistry that will ultimately improve bulk conversion cathodes based on transition metal fluorides.

To decrease carbon dioxide emissions, unprecedented efforts are being undertaken toward the development of efficient and inexpensive electric vehicles and stationary energy-storage systems. Lithium-ion batteries represent an efficient solution, that have transformed personal electronics and enabled the market introduction of electric vehicles. However, the ever-growing energy storage industry imposes great demands that current lithium-ion batteries could hardly satisfy. In this perspective, the use of metallic lithium as anode, both in Li-ion cells and in the so-called “post Li-ion technologies”, would represent the “holy grail” of battery research thanks to its extremely high theoretical specific capacity (3860 mA h g^-1), the lowest redox potential (-3.040 V vs the standard hydrogen electrode) and a low gravimetric density (0.534 g cm^-3).

However, metallic Li also presents many challenges derived primarily from dendrite formation upon cycling causing both safety issues and poor cycling performance. In addition, liquid electrolytes contain combustible organic solvents that can cause leakage and fire risks during overcharge or abused conditions, especially in large-scale operation. Therefore, replacement of liquid electrolytes with a solid electrolyte has been recognized as a fundamental approach to effectively address the above problems.

Among the solid-state systems under study, polymer-based electrolytes represent a good compromise in term of room temperature ionic conductivity, thermal and electrochemical stability, and above all, interfacial contact. In lithium metal batteries, the preparation of methacrylate-based polymer matrix, in a one pot, solvent free, UV or thermally induced, radical polymerization, is an inexpensive and quick method to obtain versatile membranes [1]. Meanwhile, eventual activation with small amount of ionic liquids can allow to obtain composite electrolytes with outstanding room-temperature conductivities, while preserving the non-flammability of the system thus enhancing the safety [2]. The simplicity of the formulation and the preparation method open the road to highly versatile electrolytes, adaptable in function of the final application [3]. Additionally, the insertion of particular groups in the matrix can introduce self-healing abilities (for example through dynamic hydrogen bonding), further improving the safety features of the cells [4]. Last but not least, bio-derived macromolecules can be functionalized with methacrylate groups to be then directly used as oligomers in the same kind of procedures, obtaining interesting quasi solid state electrolyte systems directly from waste [5, 6, 7], and fully integrating the final device in a circular economy approach.

The development of high-performing garnet based all-solid-state batteries requires intensive knowledge of the materials and processes throughout the whole cycle life of the battery to ensure ecological and economical battery production. Life-cycle analysis of the battery production line uncovered the production of raw materials and the temperature treatment steps as the crucial factors. Therefore, a better understanding of the garnet-based ceramic cathodes is needed.
3D analysis and reconstruction of the cells allowed the modelling of realistic microstructures and connects them to experimental data. This realistic cell model facilitates a deeper understanding of the limiting factors towards higher energy densities. Based on these findings, an optimized microstructure was derived. With the introduction of sequential tape-casting and an optimized sintering strategy to mitigate stresses in the material during high-temperature treatments, cathodes with this optimized microstructure were produced. These cathodes show a significant increase in the available areal specific capacity of ceramic batteries.
The introduction of advanced processing methods, as well as significant improvement of the anode side, an analysis of the degradation mechanisms in the cathodes during operation helped uncover the nature of capacity losses during cycling. Based on these findings, a strategy for direct recycling of the ceramic cells was derived, regaining over 40% of the initial capacity after complete degradation.

Perspective on Warm Isostatic Pressing for Mass Production of Solid-State Batteries

T. Rabe, Västerås / SE-721 66, M. Dixit, Oak Ridge / TN 37830, C. Beamer, Columbus / OH 43035, J. Shipley Västerås / SE-721 66, J. Fischer Västerås / SE-721 66, Ilias Belharouak / Oakridge National Laboratory 5200, 1 Bethel Valley Rd, Oak Ridge, TN 37830, United States

In recent years, there have been serious commercial advancements with regards to solid-state batteries (SSBs), in which liquid electrolyte is replaced by a solid-state electrolyte (SSE).[1,2,3] Cell concepts with functional layers realizing zero-excess lithium metal anodes were developed, combining overall reduced weight and lithium metal anodes, achieving superior energy density (>1000 Wh/l).[4,5] The need for external pressure, also called “stack pressure”, of SSBs due to cell volume change in operation and subsequent particle contact loss was reduced to a minimum (~2 MPa) with cells based on highly conductive sulfide SSEs densified by warm isostatic pressing (WIP).[5,6] Considering mass production, it seems that the common, uniaxial methods for calendering lead to insufficient composite density and lower electrochemical performance.[7] Most individuals working in battery production are critical towards WIP, because of its batch process characteristic. Ironically, the knowledge about WIPs scaling potential is unknown to the community.[8] Vessel volumes for presses which are already employed in mass production have reached 2000 l and above. With this presentation Quintus Technologies will give the audience at the “MATSUS conference” a summarizing perspective of basics for (warm) isostatic pressing and the status for industrial production of SSBs from an equipment suppliers view.

Literature: [1] J. Janek, W. Zeier, Nat. Energy 2016, 1, 16141. [2] D. H. S. Tan, S. Meng et al., Joule 2022, 6, 1755-1769. [3] J. Janek, W. Zeier, Nat. Energy 2023, 8, 230-240. [4] Y.-G. Lee et al., Nat. Energy 2020, 5, 299-308. [5] W. Choi et al., ACS Appl. Mater. Interfaces 2024, 16, 26066-266078. [6] Y.-T. Chen, S. Meng et al., Adv. Energy Mater. 2024, 2304327. [7] C. A. Heck, Batteries & Supercaps 2024, 7, e202300487. [8] M. Dixit, T. Rabe et al., Device 2024, 100370.

High-temperature sintering of composite cathodes triggers lithium loss and secondary phase formation, one of the major challenges in oxide-based solid-state batteries. However, the interdiffusion during sintering in the different partial pressures of lithium and oxygen remains poorly understood. Here, we study the reaction mechanism for the formation of impurity phases during the co-firing of Li-garnet (Li7La3Zr2O12) and LiCoO2 at high temperature and propose an alternative processing strategy that enables the production of impurity-free all-oxide composite cathodes. By using Li-rich atmospheres, the secondary phase of LaCoO3 is avoided and the composite cathodes exhibit >95% relative density. The electrical resistance of the composite cathode is increased by 3-4 orders of magnitude. Electronic conductivity of over 1 mS cm-1 is achieved at room temperature. An unprecedented areal discharge capacity of 3.48 mAh cm-2 is realised at current densities of 0.25 mA cm-2, demonstrating the high performance of the cathodes. Our results encourage further demonstration of all oxide solid state battery prototypes in combination with thin oxide electrolytes and lithium metal anodes.