Optimizing the Microstructure of Composite Cathodes in Garnet-Based All Solid-State Batteries By Continuum Modeling and Simulation

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

All-solid-state batteries (ASSBs) using inorganic solid electrolytes are expected to achieve high safety and high energy density as alternatives to conventional lithium-ion batteries using liquid electrolytes. However, in many cases, rate capability of ASSBs is poor compared with that of the conventional liquid system. This is because the contact interface between solid active materials and solid electrolyte is insufficient, resulting in high resistance for electrochemical properties. The solid electrode-electrolyte interface in ASSBs is related to the morphological structure such as porosity and mixed state. It has been confirmed that pressurization reduces the grain boundaries of solid electrolytes and improves conductivity1). From this improvement, it is generally known that applying high pressure when assembling ASSBs improves the contact area of the solid-solid interface, including electrode and electrolyte interface. However, the effect of pressure on particle-particle contact and battery performance is not understood enough. In this study, we investigated the effect of pressure-induced three-dimensional structure on the electrochemical properties of ASSBs by using X-ray computed tomography (CT). ASSBs were prepared at various stack pressures using a composite cathode consisting of oxide active materials LiNi1/3Co1/3Mn1/3O2 (NCM), sulfide solid electrolyte Li10GeP2S12 (LGPS), and acetylene black. Electrochemical properties were investigated in ASSBs of NCM composite cathode|LGPS solid-electrolyte|In-Li anode. The charge–discharge measurements were performed using various stack pressure, which indicates that the capacity increases as the stack pressure increases. To analyze the factors behind this result in terms of the internal structure of the ASSBs, we performed X-ray CT with synchrotron radiation at 20 keV using a homemade operando measurement cell that electrode diameter is 1 mm. This measurement system allows us to analyze the three-dimensional structure of composite cathode during monitoring applied pressure, electrochemical measurements, and X-ray CT measurements. From the X-ray CT data, we succeeded in quantifying the changes in the porosity of the composite cathode and the contact state between the active material and electrolytes by changing the pressure. Quantitative analysis revealed that the porosity and contact state correlate with the charge–discharge properties. Additionally, CT analysis revealed that the voids are preferentially presented near active materials and these voids spread the horizontal direction (not pressurization direction), resulting in different porosities of composite cathode and solid electrolyte layer. Since it was confirmed that voids remained in the vicinity of the active material even when the pressurization was continued, controlling these voids decreases the voids of the entire cathode layer. Pressure dependencies on AC impedance during X-ray CT measurements showed that the apparent conductivity of ASSBs increases linearly as the confinement stack pressure increases. This result is caused by suggesting that the decrease in porosity and tortuosity in cathode layer, which is detected by CT analysis. Additionally, date of tortuosity shows over-pressurization may bypass Lithium conduction path. The improved internal structure such as contact state due to pressurization enhances the apparent conductivity and charged-discharge characteristics of ASSBs. With the latest synchrotron radiation measurement technology and image processing technology, high-precision analysis has become possible.1) Jean-Marie Doux, Yangyuchen Yang, Darren H. S. Tan, Han Nguyen, Erik A. Wu, a Xuefeng Wang, Abhik Banerjeea and Ying Shirley Meng, J. Mater. Chem. A, 8, 5049, (2020).

Enabling stable and high areal capacity solid state battery with Ni-rich cathode via failure mechanism study

1. Purpose Solid-state lithium-ion batteries (SSLIBs) have attracted extensive attentions because of their potentials to improve safety and to achieve higher power and energy densities compared with the conventional lithium-ion batteries. In composite electrodes for SSLIBs, particles of active material (AM) and solid electrolyte (SE) are three-dimensionally distributed and form complicated Li-ion and electron pathways. Then, especially during high rate charging/discharging, the inhomogeneous reaction distribution may be formed in the electrode. The reaction distribution can substantially deteriorate the battery performances of SSLIBs such as capacity, power output, rate capability, and cyclability. Therefore, understanding how the reaction distribution affects the battery performances is crucial for the development of high-performance SSLIBs. In this study, we performed operando three-dimensional (3D) observation of reaction distributions in composite electrodes for SSLIBs using CT-XAFS1 to elucidate the influence of reaction distributions on the battery performances of SSLIBs. 2. Experimental An SSLIB cell with a configuration of Li x CoO2 (LCO)-Li2.2C0.8B0.2O3 (LCBO) composite electrode|LCBO electrolyte|poly (ethylene oxide) (PEO)-based polymer electrolyte|Li metal electrode was fabricated based on the literature2. The composite electrode comprised 8 mg of LCO and 2 mg of LCBO. The SSLIB cell was charged to 4.5V and discharged to 2.0 V with a current of 240 μA (0.3 C) for 3 cycles at 100 °C. The reaction distribution in the composite electrode was observed every 40 minutes during charging and discharging using operando CT-XAFS. The CT-XAFS measurements were carried out in the energy range between 7725.5-7730 eV with an energy step of 0.1 eV under an exposure time of 20 ms per energy and projection angle from -65 to 65° with an angle step of 0.1°. The observation area was 517 × 517 × 49 μm. The spatial and time resolutions were 3.1 μm and 40 min., respectively. 3. Results and Discussion Figure 1(a) shows the charge-discharge curves of the SSLIB cell. The SSLIB cell exhibited the initial charge and discharge capacities of 84 and 46 mAh·g−1, respectively. Those degraded to 24 and 13 mAh·g−1 in the 2nd cycle, and 12 and 12 mAh·g−1 in the 3rd cycle, respectively. Figure 1(b) shows the 3D charging state (Li content) maps of the composite electrode and the representative 2D cross-sectional charging state maps in the in-plane direction at the end of 1st, 2nd, and 3rd charge. Colored/uncolored areas stand for the regions where the AMs exist or not, respectively. The red/blue areas stand for the charged (x = 0.6) /discharged (x = 1.0) AMs, respectively. After the 1st charge, the AMs generally showed a high charging state (red). However, the regions with high charging state significantly decreased after the 2nd charge, and most of the AMs showed a charging state corresponding to x = 0.7 ~ 0.8 (green). The regions with high charging state further decreased after the 3rd charge, and AMs with a low charging state were increased (blue). To more precisely investigate where the less-charged AMs existed after the 2nd and 3rd charge, we subtracted the charging state map after the 1st discharge from that after the 2nd charge, and also the charging state map after the 2nd discharge from that after the 3rd charge (Fig. 1(c)). The 2D maps in Fig. 1(c) thus indicate to what extent the charge reaction progressed during the 2nd or 3rd charge compared with the end of the 1st or 2nd discharge, respectively. The red/blue areas in Fig. 1(c) represent the AM regions where the reaction progressed (Δx = 0.4) or not (Δx = 0) from the end of the 1st or 2nd discharge, respectively. As shown in the left map in Fig. 1(c), the inner parts of the aggregated AMs were significantly less charged compared to the regions near AM/SE interfaces during the 2nd charge. Similarly to the 2nd charge, the charge reaction preferentially progressed near the AM/SE interfaces, while the reaction insufficiently progressed at the inner parts of the aggregated AMs during the 3rd charge. Such a variation in the charging state distribution during cycling could be explained by considering the microstructural change due to the expansion/shrinkage of AM particles. The above results suggest that the expansion/shrinkage of AM particles reduced the connections between AM particles but did not reduce the AM/SE interfaces. We further discuss the mechanism of the capacity degradation during cycling in the presentation.

The sulfide-based all-solid-state batteries (ASSBs) has guaranteed having excellent safety by using non-combustible inorganic solid-electrolyte (SE). SE separator can transport Li ion selectively and there is no concentration gradient in theory, enabling fast charging. In addition, due to high ionic conductivity of sulfide-based SE comparable to liquid based electrolyte, high capacity and high energy density ASSBs of 5mAh cm-2 have been developed to a stage close to commercialization.So far, among the many issues of ASSBs, the performance improvement has been achieved by focusing on interfacial deterioration issues. However, recent several studies have analyzed and identified the cause of inhomogeneous reaction in composite cathode, which lead to capacity degradation quickly. It had been confirmed that the inhomogeneous reaction was caused by not only loosely contact and isolated active materials but also restriction Li conductivity in nano-crystal boundaries of active materials, resulting in localizing overcharge-discharge and decreasing in capacity and rate capability. As one of intrinsic properties of ASSBs, the solid-solid contact induce the limitation of lithium ion mobility and it also triggered insufficient electronic conduction pathway increasing charge transfer resistance.In order to develop high-energy density and high power ASSB, the ability of the conductivity for electron and ion must be balanced through the formation of a well-developed electronic percolating pathway. In this study, the inhomogeneous reactivity that can be maximized in thick electrodes is alleviated by improving the electron transfer path in the composite cathode. To uniformly transfer electrons in the composite cathode, we introduced argyrodite type SE coating with 2D conductive material under solvent environment. This approach is effective on forming a continuous electronic pathway for homogeneous reactivity as well as reduce SE oxidation by preventing direct contact between SE and cathode active material. As a result, we obtained a high initial capacity of 206 mAh g-1 using a single LiNi0.8Co0.1Mn0.1O2 cathode and SE coated with 2D conductive material, and confirmed that approximately 90% rapacity maintained after 200th cycle.

Lithium-sulfur (Li–S) batteries are among the most promising beyond conventional Li–ion technologies. These are based on conversion–type cathode active materials (CAMs) which can provide high energy and power densities. In this context, iron disulfide (FeS2) is one of the most favorable candidates due to its high theoretical capacity of 894 mAh·g–1, natural abundance, non–toxicity, and cost–effectiveness.1 However, its (de–)lithiation leads to significant volume changes, causing chemo–mechanical degradation. This is particularly critical in All–Solid–State Batteries (ASSBs), where the electrode structure significantly impacts the electrochemistry and performances of the cells.2 Therefore, understanding the morphological and structural evolution of FeS2 is essential to advancing this technology.The morphological transformations of conversion–type CAMs can profoundly affect the electrode properties. While these phenomena have gained growing attention, their study remains in the early stages. Recent studies have shown the volume changes of Li2S active material and the associated mechanical failures during cycling.3 For example, copper sulfide (CuS) was proven to form micrometer-sized copper crystals that create a continuous conductive network in the lithiated state.4 To our knowledge, systematic studies on FeS2 are still scarce.This study explores the structural evolution of FeS2 during (de–)lithiation in Li–S ASSBs (FeS2/Li5.5PS4.5Cl1.5/carbon black | Li5.5PS4.5Cl1.5 | In/InLi) at different states of charge. Using electrochemical and analytical techniques, including ex situ SEM–EDS of cathode cross–sections, we observe that FeS2 structural evolution depends on particle size and exhibits no preferential directional changes. These findings provide crucial insights into the design of strategies to mitigate chemo-mechanical degradation and improve the performance and durability of FeS2–based Li–S ASSBs.References 1 Bai, Xiaojie; Jiang, Di; Yin, Song; Hao, Yao; Zheng, Jiahan; Hao, Huiying; Xing, Jie; Dong, Jingjing; Liu, Hao; Liao, Libing. Pyrite-Based Solid-State Batteries: Progresses, Challenges, and Perspectives. Advanced Functional Materials 2024,2416808. DOI: 10.1002/adfm.202416808. 2 Dewald, Georg F.; Ohno, Saneyuki; Kraft, Marvin A.; Koerver, Raimund; Till, Paul; Vargas-Barbosa, Nella M. Experimental Assessment of the Practical Oxidative Stability of Lithium Thiophosphate Solid Electrolytes. Chemistry of Materials 201931, 8328–8337. DOI: 10.1021/acs.chemmater.9b01550. 3 Fujita, Yushi; Münch, Konrad; Asakura, Taichi; Motohashi, Kota; Sakuda, Atsushi; Janek, Jürgen; Hayashi, Akitoshi. Dynamic Volume Change of Li2S-Based Active Material and the Influence of Stacking Pressure on Capacity in All-Solid-State Batteries. Chemistry of Materials 202436, 7533–7540. DOI: 10.1021/acs.chemmater.4c01514. 4 Santhosha, Aggunda L.; Nazer, Nazia; Koerver, Raimund; Randau, Simon; Richter, Felix H.; Weber, Dominik A.; Kulisch, Joern; Adermann, Torben; Janek, Jürgen; Adelhelm, Philipp. Macroscopic Displacement Reaction of Copper Sulfide in Lithium Solid-State Batteries. Advanced Energy Materials 2020, 10, 2002394. DOI: 10.1002/aenm.202002394.

All-solid-state batteries (ASSBs) are expected to have less safety concern than Lithium-ion batteries with organic liquid electrolyte (LIBs) because flammable liquid electrolytes are not used in ASSBs. The thermal behaviors in LIBs have widely discussed in many literatures1. However, thermal behaviors of ASSBs have not investigated much.In this study, to clarify the self-heating phenomenon of our prototype ASSB with Argyrodite-type solid electrolyte we have performed calorimetric studies and gas analyses. In the calorimetric measurement, an accelerated rate calorimetry (ARC) was used to compare the starting temperature of self-heating in ASSB to that in LIB. It was found that the self-heating of ASSB takes place at 200-300ºC which was significantly higher than that in LIB.To identify the endothermic and exothermic peaks in the calorimetric profile and to estimate the amount of heat generation in ASSB, a Calvet-type calorimeter (C600) and a differential scanning calorimeter (DSC) were used with a heating rate of 1 ℃/min under Ar atmosphere for a full cell and electrode layers of the ASSB, respectively. The exothermic peaks at 200- 300℃ for charged ASSB (at ca. 4.1 V) mostly corresponded to those for the cathode layer which was extracted from the charged ASSB. The crystal structure change of the cathode layer charged at ca. 4.1 V associated with temperature change was analyzed by X-ray diffraction (XRD) to understand the reaction mechanism. The XRD peaks for the active material and the solid electrolyte in the cathode layer were disappeared at 200-300ºC. It was suggested that the heat generation at 200- 300ºC were mainly caused by the reaction between the cathode active material and the solid electrolyte.We also analyzed the evolved gas during the heating of ASSB using a temperature-programmed desorption-mass spectrometry (TPD-MS). The detailed mechanism will be discussed on-site.AcknowledgmentsThis study was carried out in SOLiD-EV project supported by NEDO (New Energy and Industrial Technology Development Organization).References Xuning Feng, Minggao Ouyang, Xiang Liu, Languang Lu, Yong Xi, Xiangming He, Energy Storage Materials 10 (2018) 246-267.

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

All-solid-state batteries (ASSBs) using sulfide solid electrolytes with high room-temperature ionic conductivity are expected as promising next-generation batteries, which might solve the safety issues and enable the utilization of lithium metal as the anode to further increase the energy density of cells. Most researchers in the academic community currently focus on developing novel sulfide solid electrolytes, clarifying the interface issues between sulfide electrolytes and solid electrodes and mechanism of lithium dendrite growth in ASSB. However, there is a lacking in the technical route analysis about the commercialization of ASSBs based on sulfide solid electrolytes. This review mainly introduces the specific preparation methods of various parts in sulfide-based ASSBs, including the preparation methods of sulfide solid electrolyte particles, sulfide-based composite electrolyte membranes, composite cathodes and anodes, and analyzes the advantages and disadvantages of these methods. In addition, several schemes of ASSB assembly are also introduced. Finally, a perspective of large-scale production of sulfide-based ASSBs is provided, which is expected to accelerate the commercialization of sulfide-based ASSBs.

All solid-state batteries (ASSBs) play an indispensable role in modern electronic devices, ranging from cellphones and automobiles to energy storage systems. The performance and lifespan of these devices are highly dependent on the condition of the installed battery. During charge-discharge cycles, batteries undergo microstructural changes, such as void formation, solid electrolyte interphase formation, and dendritic growth at the anode and solid electrolyte (SE) interface. These changes can lead to a reduction in energy storage capacity or even device failure. Therefore, developing stable ASSBs that prevent abrupt microstructural evolution by suppressing dendritic growth is crucial to ensuring safe service during charge/discharge cycles. Meanwhile, utilizing computational methods can provide additional insights and directions regarding the microstructure evolution of ASSBs for further experimental validation.In this study, we explored the use of phase-field modeling to investigate Na dendrite growth behavior in Na and Na-Sb alloy anodes. We find that the dead Na inside grain boundaries of SEs from previous plating/stripping cycles can facilitate further Na dendrite penetration during subsequent cycles. In comparison, the Na₃Sb second phase in the Na-Sb alloy anode can decrease the dendrite penetration depth during cycling. We anticipate that this approach can be applied to showcase various microstructure evolutions in ASSBs, providing insights for suppressing dendrite growth via anode composition and microstructure design.

A model-based understanding can assist and accelerate developing all-solid-state batteries (ASSB). In addition to chemo-mechanical influences within electrode particles (e.g., NMC) [1-2], a solid electrolyte (e.g., argyrodites) introduces additional interfacial interactions between electrode and electrolyte phases [3-5]. The present research derives and implements a coupled multi-physics finite-element model that captures electrochemical, transport, and structural behaviors of composite electrode structures. The models incorporate concentration-dependent and anisotropic material properties that are based on previously published combinations of experiment and density functional theory (DFT). These include stiffness and fracture toughness, porosity and crystallographic orientation, and operating conditions such as charge/discharge rates and external pressure.Figure 1 illustrates predicted stresses developed during electrode manufacturing. The relatively complex cathode microstructure is based on replicating scanning electron microscopy (SEM) images [6]. The composite electrode consists of electrode, electrolyte, and pore phases (Fig. 1a). As illustrated in Fig. 1b, the ASSB synthesis process involves applying and removing high compressive pressure, which causes plastic deformation and introduces residual stresses. Figure 1c shows residual von Mises stresses near electrode-electrolyte interfaces that can be on the order of a gigapascal. The synthesis-generated residual stresses serve as initial conditions for modeling the chemo-mechanics during battery cycling.During cell operation, spatially varying Li concentrations cause material deformation and associated stresses. Figure 1d shows predicted crack nucleation and growth during operation. Depending on the stress levels, crack nucleation and growth leads to cell degradation and capacity fade. The models predict interfacial fracture and phase separations using phase-field fracture theory. In phase-field formulation, the sharp cracks are approximated as diffuse cracks using a process-zone. High values of the phase parameter ξ (Fig. 1d) represent cracked surfaces. The structural disintegration and loss of active surface areas increase the tortuous path for Li/Li-ion transport, which eventually manifests as capacity-fade.The simulations are validated using published experimental work. The modeling approach, which combines phase-field and finite-element algorithms, is implemented using the COMSOL Multiphysics software. The models are expected to inform microstructure/manufacturing design and optimal operating conditions that improve cycling performance and limit/prevent mechanical damage.[1] K. Taghikhani, P.J. Weddle, J.R. Berger, and R.J. Kee. Modeling coupled chemo-mechanical behavior of randomly oriented NMC811 polycrystalline Li-ion battery cathodes. J. Electrochem. Soc., 168(8):080511, 2021.[2] R. Xu, Y. Yang, F. Yin, P. Liu, P. Cloetens, Y. Liu, F. Lin, and K. Zhao, Heterogeneous damage in Li-ion batteries: experimental analysis and theoretical modeling. J. Mech. Phys. Solids, 129, 160, 2019.[3] K. Taghikhani, P.J. Weddle, R.M. Hoffman, J.R. Berger, and R.J. Kee. Electro-chemo-mechanical finite-element model of single-crystal and polycrystalline NMC cathode particles embedded in an argyrodite solid electrolyte. Electrochim. Acta, 460:142585, 2023.[4] A. Bielefeld, D.A. Weber, R. Rueß, V. Glavas, and J. Janek. Influence of lithium ion kinetics, particle morphology and voids on the electrochemical performance of composite cathodes for all-solid-state batteries. J. Electrochem. Soc., 169(2):020539, 2022.[5] P. Minnmann, F. Strauss, A. Bielefeld, R. Ruess, P. Adelhelm, S. Burkhardt, S.L. Dreyer, E. Trevisanello, H. Ehrenberg, T. Brezesinski, F.H. Richter, and J. Janek. Designing cathodes and cathode active materials for solid-state batteries. Adv. Energy Mater., 12(35):2201425, 2022.[6] C. Doerrer, I. Capone, S. Narayanan, J. Liu, C.R.M. Grovenor, M. Pasta, and P.S. Grant. High energy density single-crystal NMC/Li6PS5Cl cathodes for all-solid-state lithium-metal batteries. ACS Appl. Mater. & interfaces, 13(31):37809–37815, 2021. Figure 1

Recently, the demand for the middle- or large-sized lithium batteries with safety has increased. To solve this problem, all-solid state batteries (ASSBs) are actively researched. However, it is known that the ASSBs has a higher interfacial resistance than the conventional lithium batteries. Also, the ASSBs are operated at a high temperature of 60 to 80 °C, resulting in the degradation of the cycle performance due to the collapse of the structure of the active material. In this study, NCM materials were coated with TiO2 via sol-gel process. The crystal structure and surface morphology were observed according to TiO2 content and calcination temperature using X-ray diffraction (XRD), Scanning electron microscope (SEM), and Transmission electron microscope (TEM). Moreover, ASSBs were fabricated by using the TiO2-coated NCM as cathode, composite electrolyte (CE) and lithium metal as anode. Electrochemical characteristics were evaluated on the all-solid cells at 70°C, 0.1C. As a result, it has been found that the lattice parameters and crystal size of the NCM bulk are affected by the sintering temperature and the TiO2 coating content, which also affects the discharge capacity and cycle characteristics of the cells. We describe about the electrochemical behavior of NCM coatings by TiO2 in ASSBs in more detail.

In order to make garnet-based all-solid-state batteries (ASSBs) attractive for industrial applications, their rate capability has to be significantly improved. Recently, cubic Li<sub>6.4</sub>Ga<sub>0.2</sub>La<sub>3</sub>Zr<sub>2</sub>O<sub>12</sub> (LLZO:Ga) was found to have the highest total ionic conductivity of any oxide solid-state electrolyte by far, reaching up to 2 × 10<sup>-3</sup> S/cm at room temperature. Since the rate performance of composite cathodes is directly linked to their ionic conductivity, LLZO:Ga is an ideal solid-state electrolyte for high-performance ASSBs. However, careful material selection is required for the fabrication of such ceramic composite cathodes at elevated temperatures in order to avoid incompatibility issues that could lead to low electrochemical performance. We therefore systematically studied the co-sintering behavior of cubic LLZO:Ga in combination with common cathode active materials, including LiCoO<sub>2</sub> (LCO), LiNi<sub>1/3</sub>Mn<sub>1/3</sub>Co<sub>1/3</sub>O<sub>2</sub> (NCM111), and LiNi<sub>0.8</sub>Mn<sub>0.1</sub>Co<sub>0.1</sub>O<sub>2</sub> (NCM811). The analyses were performed using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The experimental conditions were chosen to enable a direct comparison with our previous study on Li<sub>6.45</sub>La<sub>3</sub>Zr<sub>1.6</sub>Ta<sub>0.4</sub>Al<sub>0.05</sub>O<sub>12</sub> (LLZO:Ta). For the first time, we were thus able to elucidate the impact of different LLZO compositions on material compatibility. While most of the observed secondary phases were similar to those found for LLZO:Ta-based composites, a more severe degradation of the cubic LLZO:Ga structure itself was observed, reducing its conductivity and thus limiting the performance of the final cell. Consequently, the processing window for producing LLZO:Ga-based composite cathodes is even narrower than for LLZO with other dopants, thus requiring careful tailoring and tight control over the processing conditions when manufacturing garnet-based ASSBs.

Garnet-type Li7La3Zr2O12 (LLZO) with its competitive ionic conductivity (~ 1 mS cm-1), wide electrochemical stability (above 5.0 V vs. Li/Li+), and compatibility with Li-metal, is a promising electrolyte for solid-state Li-metal batteries. Despite the promise, manufacturing all-solid-state battery architectures faces challenges, mainly due to missing robust contact and high interfacial resistance on the cathode-electrolyte interface due to the evolution of secondary phases during processing. Simultaneously achieving high cathode active material loadings and low interfacial impedances remain to be shown in devices with relevant architectures. In this project, we investigate the scalable powder-to-device fabrication for a cathode/electrolyte half- and full-cell with LiCoO2 as the active material and LLZO as the ion conducting electrolyte phase, combined with a Li-metal anode. We report the electrical and ionic resistance of LLZO-LiCoO2 composite cathodes being reduced by three orders of magnitude by manipulating the sintering atmosphere. Raman spectroscopy mapping reveals the secondary phase evolution inside the LLZO due to Cobalt diffusion being absent in composite cathodes sintered in a lithium-rich atmosphere as compared to ambient conditions. Further investigations lead to the conclusive assumption that the formation of secondary phases can be blocked by stopping lithium volatilization during high-temperature co-firing. Testing the dense composite cathodes in a liquid electrolyte battery, very low initial impedances of >30 Ω and high areal capacities of 3.5 mAh cm-2 with almost full utilizations can be achieved. Such batteries can be operated at 0.25 mA cm-2 at room temperature for 50 cycles without failing. Furthermore, we demonstrate tape-cast composite cathodes with thicknesses of < 100 µm being built into such battery setups, showcasing their mechanical integrity and density. Combining these results with recently developed dense-porous Li-garnet bilayers, we give an outlook to manufacture an all-solid-state, co-fired battery architecture that operates without the need of stack pressure.

Li-rich layered oxide (LLO) cathodes possess high theoretical capacity and voltage, but constructing cathodes with high active material fractions to achieve high energy density in all-solid-state batteries (ASSBs) remains challenging. In this work, it is demonstrated that the main limitation to electrochemical performance lies not in interfacial instability but in sluggish Li+ transport arising from intra- and interparticle voids. Intraparticle voids are found to hinder ion diffusion within polycrystalline LLO, while extensive interparticle voids are introduced in reduced-particle-size LLO due to particle aggregation under limited solid-state electrolyte content. In these aggregated domains, Li+ conduction is restricted to isolated LLO regions and the intrinsically low ionic conductivity of LLO further aggravates transport limitations. To address these issues, a composite cathode with a Li3InCl6 coating was designed to improve the interfacial contact and Li+ conduction pathways. The resulting ASSB delivers 245.2 mAh/g at 0.1 C, retains 80.9% capacity after 250 cycles at 0.3 C, and maintains 75.9 mAh/g at 5 C with only 27.5 wt % Li3InCl6. This work provides a practical cathode design strategy for realizing high-energy-density LLO-based halide ASSBs.