All-solid-state Li-ion Batteries Research Articles

An all coupled electrochemical-mechanical model for all-solid-state Li-ion batteries considering the effect of contact area loss and compressive pressure

A fully coupled electrochemical-mechanical theoretical model for all-solid-state Li-ion batteries (ASSLBs) is established considering the effects of contact areas and compressive pressure. The influences of contact area loss and compressive pressure on the electrical and mechanical properties of the ASSLB are numerically studied in the charging process. The results indicate that the loss of contact area may result in poor electrical performance. The contact area loss of cathode/SE interface can lead to the lithium concentration reaching the minimum value earlier, which brings about the decreases of both voltage and capacity, while that of anode/SE interface enables the ASSLB to attain the charging cut-off voltage in advance. Therefore, a lower capacity of ASSLB is found. The battery performance can be reduced for both higher (more than 1 MPa) and lower (0.05–0.4 MPa) compressive pressures, while the optimal charging performance can be obtained for medium (0.4–1 MPa) compressive pressures. Meanwhile, the stress-drop at the electrode/SE interfaces can reach the minimum. The charge and discharge experiments with 0.4 MPa compressive pressure are carried out. The maximum error between the experimental results and simulation results is 8.12%, which verified the accuracy of the simulated method.

Insight into the activation energy for the interfacial stability evaluation in all-solid-state Li-ion batteries

While all-solid-state Li-ion batteries (ASSLIBs) by using non-flammable and superionic solid electrolytes (SEs) exhibit improved safety and higher energy density, a major challenge that remains to be overcome is the dissatisfactory rate performance and capacity retention due to the undesirable reaction at electrode/SE interfaces. Reaction energy is generally taken as the indicator of interfacial reactivity, i.e. reaction degree and reaction rate, although it only represents whether the reaction is thermodynamically favorable. In the physical origin, activation energy (reaction barrier) instead of reaction energy should be the determinant of reaction rate. Here, first principles calculations with the Arrhenius relationship show a difference of several orders of magnitude in reaction rates due to the distinct activation energies although the reaction energies are similar. Then, by combining the activation energy with the reaction energy and conductive ability of products, which representing kinetic feasibility, thermodynamic favorability, and continuity of a reaction respectively, we propose an evaluation strategy for interfacial stability and correspondingly classify the interfaces. This study manifests that the usually unheeded activation energy can provide a kinematic perspective for the interfacial reactions.

Current Trends in Nanoscale Interfacial Electrode Engineering for Sulfide‐Based All‐Solid‐State Li‐Ion Batteries

All‐solid‐state Li‐ion batteries (ASSLBs) have been a topic of interest in the recent two decades for use in energy storage technologies. Among various types of solid electrolytes, sulfide‐based solid electrolytes (SSEs) have been regarded as the most promising electrolytes for practical ASSLBs due to their high ionic conductivity (>1 mS cm−1) and the possibility of high energy density cells (>500 Wh kg−1) using a currently available high capacity oxide cathode and a Li‐metal anode, as well as their nonflammable nature. However, SSEs are known to suffer from: 1) low electrochemical stability window; 2) parasitic interfacial reactions at oxide cathodes; 3) electrochemomechanical degradation at the interface; and 4) dendritic growth at bare Li‐metal anodes. Herein, a comprehensive review of specific strategies for high energy density ASSLBs is provided, focusing on the nanoscale interfacial engineering on oxide‐based cathode materials and Li‐metal anodes. Recent progresses in in situ‐based complex interfacial coatings, deposition techniques for synthesizing complex core–shell structures at oxide‐based cathode active materials, and lithiophilic seeding followed by dendrite protective coatings at anodes are mainly summarized. Finally, the perspectives for the development of high energy density sulfide‐based ASSLBs are highlighted.

Enhancing purity and ionic conductivity of NASICON-typed Li1.3Al0.3Ti1.7(PO4)3 solid electrolyte

Increasing demand for safe energy storage and portable power sources has led to intensive investigation for all-solid state Li-ion batteries and particularly to solid electrolytes for such rechargeable batteries. One of the most promising types of solid electrolytes is NASICON-structured Li1.3Al0.3Ti1.7(PO4)3 (LATP) due to its relatively high ionic conductivity and stability towards air and moisture. Here, the work is aimed on implementing the steps to hinder formation of impurity phases reported for various synthesis routes. Consequently, the applied modifications in the preparation strategies alter a crystal shape and size of prepared material. These two parameters have an enormous impact on properties of LATP. Fabrication of larger particles with a cubic shape significantly improves its ionic conductivity. As a result, LATP preparation methods such as a solution chemistry and molten flux resulted in the highest ionic conductivity samples with the value of ~10−4 S cm−1 at room temperature. Other LATPs obtained by solid-state reaction, sol-gel and spray drying methods depicted the ionic conductivity of ~10−5 S cm−1. The activation energy of lithium ion transfer in LATP varied in a range of 0.25–0.4 eV, which is in well agreement with the previously reported data.

Cold sintering, enabling a route to co-sinter an all-solid-state lithium-ion battery

All-solid-state Li-ion batteries (ASSB) are one of the most attractive next generation batteries for large scale application due to improved safety and higher energy density. However, the high temperature process required for densification of the solid-state electrolytes and for co-sintering of the multilayered ASSB is still a major challenge for large scale fabrication. In this study, a low temperature process, named cold sintering process, is applied to co-sinter all the layers in the ASSB at a low temperature. The cold sintered ASSB, a full-cell of Li4Ti5O12/Li7La3Zr2O12/LiFePO4, has densified microstructures and exhibits impressive electrochemical performance. The ASSB delivers high capacity of 140 mAh g−1 at 0.1 C, rate capability of up to 2 C with 85 mAh g−1, and 90% capacity retention over 100 cycles at room temperature under a current density of 0.2 C.

Interfacial Engineering in a Cathode Composite Based on Garnet‐Type Solid‐State Li‐Ion Battery with High Voltage Cycling

AbstractGarnet‐type solid electrolyte is a promising candidate for the fabrication of high energy all‐solid‐state Li‐ion batteries (ASSLIBs), but its use is hampered by a large interfacial resistance. Herein, we propose a surface modification and subsequent sintering to enhance the interfacial connection between the cathode and the solid electrolyte. The ASSLIB prepared by this method delivered an initial discharge capacity of ∼66 mAh g−1 (80 °C) at a rate of 0.1 C. However, the poor contact between the cathode and electrolyte triggered the increase of the interfacial resistance, which caused severe capacity decay upon cycling. The encapsulation of LiCoO2 particles with LiBO2 using a single‐step sintering process dramatically increased the interfacial contact, resulting in a higher discharge capacity of 116 mAh g−1 with good cycling behavior. Therefore, surface modification of the cathode offers a reduction of resistance and promotes efficient Li‐ion transfer pathways across the cathode/solid‐electrolyte interface.

Synthesis and interface modification of oxide solid-state electrolyte-based all-solid-state lithium-ion batteries: Advances and perspectives

All-solid-state Li-ion batteries (ASSLiBs) are considered as promising next-generation energy storage devices, and the one that is based on oxide ceramic solid-state electrolyte (SSE) has attracted much attention for its high safety and stability in ambient conduction compared with that of used sulfur and polymer SSEs. However, the undeformable nature of the ceramic SSEs brings new issues such as poor interface bonding, limited contact area and limited cathode utilization for the ASSLiBs. In addition, the interface reaction and resistance are also obstacles for ASSLiBs application. In this review, we focus on the synthesis and electrochemical properties, interface modification and failure mechanism of ASSLiBs. Finally, perspectives of future researches on the ceramic SSEs-based ASSLiBs are discussed.

Particle Size Effects of the Ni-Rich Cathode Material in All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries (ASLBs) may be promising candidate to solve the problem because it uses non-flammable inorganic solid electrolytes as an electrolyte and separator. However, ASLBs has serious drawback such as low ionic conductivity compared to liquid electrolyte and have a large interfacial resistance between positive electrode and solid electrolyte in positive electrode. In these regards, reducing particle size of the cathode material can be positive affect to increase the contact area between cathode material and solid electrolyte in the positive electrode to decrease the interfacial resistance. In addition, high capacity Ni-rich cathode material generally was used for high energy density in ASLBs.In this work, we report on the particle size effects of the Ni-rich cathode material synthesized by co-precipitation method in all-solid-state Li-ion batteries. For comparison of the size effect, the physical, chemical, and electrochemical properties were evaluated by X-ray diffraction, field-emission scanning electron microscopy (FE-SEM), electrochemical impedance spectroscopy (EIS) and galvanostatic tests.

Mechanical Property Anisotropy Behavior Analysis of Inorganic Solid Electrolytes Used in All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries (ASSLIBs) are now being intensively researched within the energy storage community as next-generation battery technologies1,2. In contrast to organic liquid electrolytes used in current Li-ion batteries, solid electrolytes used in ASSLIBs can exhibit a high safety performance, which means a much less possibility for the thermal runaway to occur3,4. In recent years, many studies have indicated that the mechanical property of solid electrolytes could play a significant role in affecting the cycling performance and longevity of ASSLIBs5-8. However, efforts on investigating the mechanical property of solid electrolytes are not sufficient to understand its characteristics and effects on battery performance. In this presentation, firstly we will introduce models used in analyzing the anisotropy behavior of mechanical properties of solid electrolytes with different crystal structures. Secondly, we will present our quantitative results on analyzing the spatial distributions of mechanical properties (including Bulk, shear and Young’s modulus) for several different types of inorganic solid electrolytes (including garnet, NASICON, Phosphate, Thio-phosphate and Argyrodite) used in ASSLIBs. Finally, we will provide suggestions on which type of inorganic solid electrolytes might be more beneficial to the ASSLIB performance with respect to the anisotropy of their mechanical properties. Acknowledgements This work was partially supported by the Leonard Case Jr. Endowment Fund and Case Western Reserve University.

Electron and Ion Modulation at the Nasicon Solid-Electrolyte/Electrode Interfaces at Static and Operating Conditions in All-Solid-State Batteries: A DFT Study

High interfacial resistance has been one of the main obstacles that limit the application of all-solid-state Li-ion batteries (ASSLB), which relates to Li+ transport and distribution around the interfaces. Sulfide solid-electrolytes (SEs), many of which have higher ionic conductivity in bulk, usually showed significant deterioration of the resistance or interface states. In this respect, oxide SEs, even with lower ionic conductivity in bulk, is a promising route for acquiring better interface performance. Recently, several studies have shown the stability of phosphate moiety (PO4 3-) [1]. In this work, we examined the characteristics of interfaces of Li1+xAlxTi2-x(PO4)3 (LATP) SE with LiCoO2 (LCO) cathode and Li metal anode as a representative model system, for better understanding of the interfacial electron and ion modulations, and Li+ transport.We used Density Functional Theory (DFT)-based first-principles calculation to evaluate the electronic states and the Li chemical potential around the Li/LATP/LCO interfaces. We especially examined the interfacial band alignments and redox processes as well as the possible Li+ movement. To explore the probable interface structures, we built the pristine atomic interface structures at various displacements via the CALYPSO technique [2], and then explored the exchange of cations for the LATP/LCO interface.Single and double exchanges of cation pairs, including Ti-Co, P-Co, and Al-Co, at LATP/LCO interface, and all the 288 possible configurations are considered. The calculation results show that the interface with the Ti-Co exchange is more thermodynamically favorable than the pristine interface and the other exchange combinations, which agrees with the previous experiments [3, 4]. This indicates that the mutual diffusion of Ti-Co may occur around the LATP/LCO interface upon contact of LATP and LCO. Though the P-Co exchange was expected according to the previous studies, it does not seem to be the case at the LATP/LCO interface, indicating the strong and stable PO4 3- moiety in LATP.We also calculated the partial density of states (PDOS) to evaluate the band alignment at the LATP/LCO and LATP/Li interfaces and electron modulation. The results reveal that there is a possible electron transfer from LCO to LATP upon contact through thermal excitation and/or interfacial states induced by oxygen vacancies, as the valence band maximum of LCO is higher than LATP and close to conduction band minimum of LATP. It would cause the reduction of Ti in LATP that has been observed by the experiments [3]. On the other hand, the LATP/Li interface model shows the Fermi level that lies in the conduction bands of LATP, indicating the electron transfer from Li to LATP, and it would likely cause the reduction of LATP at the interface.By calculating the formation energy of Li vacancy (EV) in bulk and at interfaces [2], which relates to taking electron and Li+ altogether, we can correlate with Li chemical potential. Upon contact of LATP and LCO, Li+ is likely to move from LCO to LATP due to the lower Li chemical (-4.7 eV) in bulk LATP compared with bulk LCO (-4.2 eV), which can be compensated by negative charges that are from LCO to keep the neutrality. The EV of Li at the probable LATP/LCO interface (3.7 eV) is smaller than that of bulk LCO and bulk LATP. Thus, while charging, the Li+ will be removed primary from the LATP/LCO interface toward the bulk LATP. This removing tendency will cause the Li-depletion around the interface while charging, which was also observed by the experiments [5].This work provides a modeling method and scenario that captures the electrons and ions modulation at the SE/electrode interfaces, which relates to the interfacial resistance, and agrees with experimental observations. It can be further applied to investigate other all-solid-state battery systems in future works.This work was partly supported by MEXT “Fugaku Battery & Fuel Cell Project” and JSPS KAKENHI “Interface IONICS” Grant Number JP19H05815.[1] F. Walther, J. Janek et al., Chem. Mater., 31, 3745–3755 (2019).[2] B. Gao, R. Jalem, Y. Tateyama et al., Chem. Mater., 32, 85–96 (2020).[3] Y. Yamamoto, Y. Iriyama, S. Muto, J. Am. Ceram. Soc., 103, 1454–1462 (2020).[4] Y. Liu, X. Sun et al., ACS Appl. Mater. Interfaces, 12, 2293–2298, (2020).[5] B. Tsuchiya, Y. Iriyama et al., Adv. Mater. Interfaces, 6, 1900100 (2019).

LiBO2-modified LiCoO2 as an efficient cathode with garnet framework Li6.75La3Zr1.75Nb0.25O12 electrolyte toward building all-solid-state lithium battery for high-temperature operation

Garnet-based all-solid-state Li-ion batteries (ASSLIBs) suffer from high interfacial resistance at the cathode/electrolyte interface. Herein, we propose a LiBO2 (LBO) modification over a LiCoO2 (LCO) cathode to improve the interfacial contact and subsequently, the electrochemical performance of the cell. The High-Resolution Transmission Electron Microscopy (HR-TEM) images of the cathode particle show that the LiBO2 (LBO) content on the LCO surface effectively improves the electrode/electrolyte interface. In the cut-off voltage range of 3 – 4.3 V vs. Li, the LBO-LCO maintains 50% of its initial capacity (60 mAh g–1) after the 40th cycle, which is much higher than bare LCO in the ASSLIB configuration. The LBO layer over LCO enhances the Li-ion diffusion kinetics and eventually improves the electrochemical performance of the cell. To suppress the mechanical degradation of the electrode/electrolyte interface, we have reduced the working potential window to 3 – 4.1 V vs. Li, where the LBO-modified LCO provides an initial discharge capacity of 101 mAh g–1 and assists in realizing improved cyclic performance in comparison with the conventional potential window.

Understanding Degradation Mechanism between Garnet Solid Electrolyte Li7La3Zr2O12 and LiNi0.6Mn0.2Co0.2O2 Cathode

All-solid-state Li-ion batteries (ASSLBs) using oxide based solid electrolytes have been considered as a promising candidates for the next generation batteries due to their excellent thermal and electrochemical stability. Especially, garnet-type Li7La3Zr2O12 (LLZO) has gained much attention due to its good conductivity and large electrochemical window. Since the electrochemical window of LLZO allows usage of high voltage cathodes such as NMC (LiNixMnyCo1-x-yO2) and Li anode, it is possible to design high voltage, high capacity and safe energy storage medium using the eletrolyte.However, current attempts on exploiting LLZO as a solid electrolyte have been hindered by high resistance at the interface, especially between cathode and the solid electrolyte. Even though the importance of the issue is well known, there has not been a comprehensive understanding yet on the topic. The lack of understanding comes from the difficulty of characterizing buried interface. It is difficult to study thin interfacial region between cathode and solid electrolyte with conventional techniques since the signal from the interface is often overwhelmed by the bulk, and destructive characterization techniques ruin the signal itself.We developed model system by sputtering thin-film cathode on top of pre-prepared solid state electrolyte pellets to overcome this problem. Samples were annealed afterwards to simulate sintering process. Since the interfacial region is near surface, we could use XANES and EXAFS which are surface-sensitive and non-destructive. Those techniques gave us valuable information regarding oxidation state and local chemical environment of each element, which were used to evaluate migration tendency. Moreover, those results can be used to precisely characterize secondary phases by complementing complex XRD results. Results obtained from those characterization techniques were correlated with electrochemical analysis techniques such as EIS and chronovoltammetry to evaluate the effect of the interfacial degradation on cell performance.In this work, we aimed to obtain fundamental understanding on the instability of cathode|electrolyte interface during two different stages: manufacturing stage and operation stage. We systematically varied variables related to manufacture (Sintering temperature, sintering time, gas environment during sintering), and operation (Electrochemical potential, current density). We chose NMC622 (LiNi0.6Mn0.2Co0.2O2) as a cathode, and Al-doped cubic LLZO (Li7La3Zr2O12) as a solid electrolyte.To study degradation of NMC622|LLZO during manufacturing stage, we varied temperature condition (300°C, 500°C, 700°C for 4h respectively) and gas environment (air, O2, humidified O2, N2, CO2) during annealing. Samples annealed in air showed high interfacial resistance and poor performance, which are signs for interfacial degradation. At 700°C, formation of detrimental phase such as La2Zr2O7 and La(Ni,Co)O3 led to complete blockage of Li ions. XANES and EXAFS showed severe change of oxidation state and chemical environment of Ni and Co, which indicated that those species escaped out of the cathode and participate in the secondary phase formation. In contrast, NMC622|LLZO interface remained chemically stable up to 700°C with no formation of detrimental phase when the sample was annealed in O2. On the other hand, when the sample was annealed in humidified O2 at 700°C, detrimental phase(La2Zr2O7) could be found. In addition, we could characterize substantial amount of Li2CO3 from samples annealed in humidified O2, which could have formed by reaction between LiOH and CO2 during sample characterization done in air. Annealing in CO2 condition led to the crystallization of detrimental phases such as Li2CO3, La2O2CO3, NiCO3 at 500°C. In addition, we could see formation of La2Zr2O7 and La2(Ni,Co)O4 from the sample annealed at 700°C. In both air and CO2 environment, we could find La-(Ni,Co)-O secondary phase from annealed samples, which indicates interdiffusion at the interface. However, extent of the degradation was much higher in CO2 environment. This could be seen by the intensity of XRD peaks corresponding to secondary phases, which overwhelmed the ones corresponding to bulk LLZO. Co L-edge XANES and XRD data for samples annealed at 700°C in different gas conditions could be seen in the attached figure.These findings suggest that interfacial degradation is strongly dependent on the sintering temperature and gas environment. Sintering temperature should be kept as low as possible to avoid formation of detrimental phase. In terms of gas environment, O2 environment is the most promising since the sample remained stable up to 700°C. On the other hand, we found that CO2 and H2O(vapor) are detrimental to the sample. This could be because that they could both form phases which could extract Li out of the system, which led to formation of delithiated secondary phases such as La2Zr2O7. Findings from the work can be used to design optimal conditions to manufacture and operate all solid Li-batteries using LLZO with minimal interfacial degradation. Figure 1

High-Voltage Superionic Halide Solid Electrolytes for All-Solid-State Li-Ion Batteries

All-solid-state Li-ion batteries (ASSBs), considered to be potential next-generation energy storage devices, require solid electrolytes (SEs). Thiophosphate-based materials are popular, but these s...

Evaluation of The Electrochemo-Mechanically Induced Stress in All-Solid-State Li-Ion Batteries

The mechanical degradation of all-solid-state Li-ion batteries (ASSLBs) is expected to be more severe than that in traditional Li-ion batteries with liquid electrolytes due to the additional mechanical constraints imposed by the solid electrolyte on the deformation of electrodes. Cracks and fractures could occur both inside the solid electrolyte (SE) and at the SE/electrode interfconce. A coupled electrochemical-mechanical model was developed and solved by the Finite Element Method (FEM) to evaluate the stress development in ASSLBs. Two sources of volume change were considered, namely the expansion/shrinkage of electrodes due to lithium concentration change and the interphase formation at the SE/electrode interface due to the decomposition of SEs. The most plausible solid electrolyte decomposition reactions and their associated volume change were predicted by density functional theory (DFT) calculations. It was found that the stress associated with a volume change due to solid electrolyte decomposition can be much more significant than that of electrode volumetric changes associated with Li insertion/extraction. This model can be used to design 3D ASSLB architectures to minimize their internal stress generation.

Sr Doped Amorphous LLTO as Solid Electrolyte Material

Amorphous Li0.35La0.55TiO3 (LLTO) shows great promise as solid electrolyte material in all-solid-state Li-ion batteries (ASSLiB). Amorphous LLTO thin films with high stability were successfully synthesized by sol-gel process in our previous work. The ionic conductivity can reach up to 1.88*10−5 S cm−1 at 30 °C. In order to further increase the ionic conductivity to meet the requirements of ASSLiB, cation doping is applied in this study. Specifically, Strontium (Sr) is introduced as dopant and the ionic conductivity reaches 8.38 × 10−5 S cm−1 at 30 °C with 5% of Sr doping, which is about one order of magnitude higher than that of undoped LLTO. It is also confirmed that amorphous LLSTO is stable in direct contact with Lithium and with stability window up to 10 V. Furthermore, the relationship between the structure change along with Sr ratio and ionic conductivity is identified. This could be critical for further study of solid electrolyte materials.

Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

AbstractRechargeable Li‐ion batteries (LIBs) are electrochemical storage device widely applied in electric vehicles, mobile electronic devices, etc. However, traditional LIBs containing liquid electrolytes suffer from flammability, poor electrochemical stability, and limited operational temperature range. Replacement of the liquid electrolytes with inorganic solid‐state electrolytes (SSEs) would solve this problem. However, several critical issues, such as poor interfacial compatibility, low ionic conductivity at ambient temperatures, etc., need to be surmounted before the commercialization of all‐solid‐state Li‐ion batteries (ASSLIBs). In this review, a brief historical context for the inorganic SSEs is described first. Then, two critical issues in the ASSLIBs are highlighted: interfacial incompatibility of the electrodes and SSEs and internal stresses. For the interfacial incompatibility, the discussion is focused on the dynamic characterization of the electrode/SSE interfaces, the origin and evolution of the interfacial resistance, and interface engineering to minimize the interfacial resistance. The internal stresses in the ASSLIBs are another major concern because rigid contacts are introduced. Stress generation, stress evolution during battery cycling, stress measurement/simulation, and ways to alleviate the stresses are outlined in detail. Finally, current challenges and perspectives for future development of the inorganic SSEs and ASSLIBs are outlined.

Multi-ion Conduction in Li3OCl Glass Electrolytes.

Antiperovskite glasses such as Li3OCl and doped analogues have been proposed as excellent electrolytes for all-solid-state Li ion batteries (ASSB). Incorporating these electrolytes in ASSBs results in puzzling properties. This Letter describes a theoretical Li3OCl glass created by conventional melt-quench procedures. The ion conductivities are calculated using molecular dynamics based on a polarizable force field that is fitted to an extensive set of density functional theory-based energies, forces, and stresses for a wide range of nonequilibrium structures encompassing crystal, glass, and melt. We find high Li+ ion conductivity in good agreement with experiments. However, we also find that the Cl- ion is mobile as well so that the Li3OCl glass is not a single-ion conductor, with a transference number t + ≈ 0.84. This has important implications for its use as an electrolyte for all-solid-state batteries because the Cl could react irreversibly with the electrodes and/or produce glass decomposition during discharge-charge.

A new approach for synthesizing bulk-type all-solid-state lithium-ion batteries

All-solid-state Li-ion batteries (ASSLiB) have been considered to be the next generation energy storage devices that can overcome safety issues and increase the energy density by replacing the organic electrolyte with inflammable solid electrolyte.

Fundamental Relationship of Microstructure and Ionic Conductivity of Amorphous LLTO as Solid Electrolyte Material

All-solid-state Li-ion batteries (ASSLiB) attracted lots of attention mainly due to their higher safety compared with commercial Li-ion batteries. The research of solid electrolyte is critical for the development of ASSLiB, and among all the candidates of solid electrolyte materials, amorphous Lithium Lanthanum Titanate Oxide (LLTO) is promising because of its outstanding electrochemical stability up to 12V. However, the fundamental relationship among the ionic conductivity, microstructure, and mechanism of Li ion transport is still unrevealed. In this study, both amorphous LLTO thin film and LLTO powder were successfully prepared by sol-gel process. In order to determine the relationship between microstructure and ionic conductivity, various annealing time at 500°C were applied. This work shows that the ionic conductivity increases from 2.32 × 10−8 S/cm to 9.01 × 10−6 S/cm with the annealing time before LLTO starts to crystallize and decreases back to ∼1.6 × 10−9 S/cm after the formation of crystal phase. The absolute value of difference is close to 3 orders of magnitude. Furthermore, the morphology of the thin film changes accordingly, solvent evaporation, surface refinement and crystallization phases would occur sequentially, and this indicates that there is an optimal synthesis condition for the LLTO film. However, the changes of structure, morphology and ionic conductivity does not affect activation energy. This will help researchers to find optimized synthesis condition of amorphous LLTO and potentially search for and design other solid electrolyte materials.

(Invited) Insights from Computational Modeling and Experiments on the Li-Ion Dynamics and Electrochemical Stability of Garnet-Based Solid Electrolytes

Solid electrolytes are crucial for the development of next-generation Li-ion batteries; they can drastically improve safety during work operation as they are essentially non-flammable unlike conventional liquid- or organic-based electrolytes. In this regard, garnet-type Li-based oxide compounds are amongst the most promising ones and they are now being intensively optimized to push their conductivity to levels that would make them practical for use. However, technical challenges such as poor scattering due to Li (which makes diffraction techniques measurements tedious) and inherent variability in synthesis, material processing, and powder handling have led to ionic conductivity values differing by several factors even for the same nominal composition (thus, the scatter of data in literature), thereby making it difficult to study the exact nature of the Li ion dynamics and distribution at the Li sub-lattice in these materials, especially when considering dopant incorporation. Another important property is electrochemical stability which can have profound impacts on battery performance, this is in relation to Li ion transport across electrolyte-electrode interface. Interfacial impedance can change significantly depending on the various decomposition products formed at the interface, this is also technically challenging to study by direct approaches. In this presentation, several fundamental issues related to garnet-type compounds will be tackled that are critical towards achieving high-performance all-solid state Li ion batteries: Li ion dynamics,[1,2,3] electrochemical stability behavior,[4] and Li and dopant distribution[1,2,3]. Results from various computational techniques will be discussed (DFT, augmented plane-wave approach, classical force-field method, and molecular dynamics). Specifically, special attention will be given to the study of the distribution, dynamics, and ion transport of Li, vacancy, and dopants in the Li sub-lattice of the garnet structure. Two dopants of interest will be highlighted: Ga and proton. Doping by Ga has been reported to show one of the highest conductivity so far (on order of 10-3 S/cm) for garnet solid electrolytes while proton incorporation via exchange reaction with Li is associated to interfacial resistance increase (due to formation of resistive phases) and variability in ionic conductivity. On the other hand, experimental data from XRD, SEM, FTIR, and cyclic voltammetry will also be presented in relation to the capacity fading phenomena at the first-cycle charge process (related to interfacial impedance increase) of a garnet solid electrolyte in a solid-state battery cell. Acknowledgements The authors acknowledge the financial support from JST PRESTO program (JST PRESTO: Japan Science and Technology, “Promoting Individual Research to Nurture the Seeds of Future Innovation and Organizing Unique, Innovative Network”) and National Institute for Materials Science (NIMS) for the financial support.