Structure and ionic conductivity of NASICON-type LATP solid electrolyte synthesized by the solid-state method

The increasing demand for safe and high-energy-density battery systems has led to intense research into solid electrolytes for rechargeable batteries. One of these solid electrolytes is the NASICON-type Li1+xAlxTi2-x(PO4)3 (LATP) material. In this study, different boron compounds (10% B2O3 doped, 10% H3BO3 doped, and 5% B2O3 + 5% H3BO3 doped) were doped at total 10 wt.% into the Ti4+ sites of an LATP solid electrolyte to investigate the structural properties and ionic conductivity of solid electrolytes using the solid-state synthesis method. Characterization of the synthesized samples was conducted using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS). The XRD patterns of the boron-doped LATP (LABTP) samples show that the samples have a rhombohedral phase with space group R3¯c together and low amounts of impurity phases. While all the LABTP samples exhibited similar ionic conductivity values of around 10-4 S cm-1, the LABTP2 sample doped with 10 wt.% H3BO3 demonstrated the highest ionic conductivity. These findings suggest that varying B3+ ion doping strategies in LATP can significantly advance the development of solid electrolytes for all-solid-state lithium-ion batteries.

The Electrolyte Frontier: A Manifesto

Stabilizing Solid Electrolyte-Anode Interface in Li-Metal Batteries by Boron Nitride-Based Nanocomposite Coating

Summary Here we created new composition of the lithium halide solid electrolyte (HSE) material with high lithium ionic conductivity applicable for negative electrode active materials reacting around lithium metal potential. From some of halide ions’ natures , e.g. monovalent and fairly large ionic radii, halide materials form weak bonding with cations, which is advantageous for lithium ionic conductivity but is disadvantageous to electrochemical stability of constituent cation against reduction. By strategically selecting containing elements and exploring various compositions, we find the materials, dubbed “stabilized HSE (s-HSE)” hereafter, overcoming the trade-off derived from halide ions’ natures. We demonstrated the excellent charge/discharge capability of bulk-type all-solid-state batteries (ASSBs), using only lithium halide materials as solid electrolytes (LiCoO2 / Li3YCl6 / s-HSE / graphite). The initial discharge capacity and coulomb efficiency reached as high as 118 mAh/g and 91 %. In this presentation, we will reveal the details of the material composition, estimation of the electrochemical stability, and the battery characteristics. Introduction The ASSBs are one of the most promising candidates for the post-lithium ion batteries. Towards realization of its commercialization, it is essential to improve solid state electrolytes. We have reported HSEs1 that exhibit high ionic conductivity and deformability required for low inter-grain resistance, etc. They are promising materials that have some excellent features distinct from sulfides2, oxides3, and several other material systems4. However, due to poor electrochemical reduction stability of HSEs, they have not been applicable for low redox potential negative electrode materials, e.g. graphite, silicon, and lithium metal. Owing to some of halide ion’s natures, e.g. monovalent and fairly large ionic radii, chemical bonds formed between halide anion and cation becomes relatively weak. While weak lithium-anion bonding leads to high lithium ionic conductivity, weak cation-anion bonding lower electrochemical reduction stability. This trade-off is a general issue for HSEs, not limited to our reported HSEs. In this study we create new composition of HSE material overcoming the aforementioned trade-off. One may consider to strengthen bonding between containing cations (M) and anion. Such a simple approach, however, easily fails because the ionic conductivity is severely sacrificed. Here to solve this conflicting issue, we strategically select constituting elements and tailor their compositions so that the advantageous characteristics of elements remain while the disadvantages are alleviated. Experiments, results and discussions The ionic conductivity of the s-HSE material was measured on pressed pellets using electrochemical impedance spectroscopy (EIS). The ionic conductivity at room temperature was as high as σ = 1.1 × 10-4 S/cm． Electrochemical stability against reduction was verified by cyclic voltammetry (CV) using the following cell configuration, stainless steel / s-HSE / Li3PS4 / Li. The reduction of s-HSE was not observed in the potential range from open circuit voltage to 0 V vs. Li+/Li. This clearly indicates that the s-HSE material is stable against lithium metal, and expected to be used in negative electrode reacting around lithium metal potential. We performed charge-discharge test for graphite / s-HSE / Li3PS4 / In-Li cell. The discharge capacity and coulomb efficiency were 262 mAh/g and 97 %, close to characteristics of cells using only Li3PS4, commonly studied electrolytes and known to work with graphite, as solid electrolyte. We also confirmed that the valence of M in the above cell at charged state was not reduced by X-ray absorption fine structure (XAFS). These results indicate that this halide material is applicable for negative electrode reacting around lithium metal potential. Furthermore, we consisted bulk-type ASSB using only HSE materials as solid electrolytes (LiCoO2 / Li3YCl6 / s-HSE / graphite). It showed the excellent characteristics; discharge capacity and coulomb efficiency were 118 mAh/g and 91 %. From all of the above characteristics of the s-HSE, we reveal that HSEs can be used for low redox potential negative electrode required for high battery voltage. By overcoming above trade-off derived from halide ion’s natures, high energy density batteries with HSEs can be realized. This study reveals remarkably high potential of HSEs for ASSBs. 1 T. Asano et al, Adv. Mater. 30, 1803075 (2018). 2 Y. Kato et al., Nat. Energy, 1, 16030 (2016). 3 X. Han et al, Nat. Materials 16, 572–579 (2017). 4 S. Kim et al, Nat. Commun. 10, 1081 (2019).

The research community is working hard on the development of solid-state batteries (SSB) as an alternative to commercialized lithium-ion batteries, due to their higher energy density and enhanced safety. One of the core research areas of SSBs is the solid-state electrolyte (SE), which should fit the demand for high ionic conductivity and electrochemical stability at the same time. Recently, halide SEs have emerged because they are mechanochemical processable, similar to sulfidic SEs, but halide SEs exhibit excellent electrochemical oxidation stability compared to sulfidic SEs [1,2].However, the relatively low ionic conductivity at room temperature still hinders the application of halide SE in SSBs. Aliovalent substitution is a very widely used method to improve the ionic conductivity of SEs [3,4]. We are also motivated by the application of earth-abundance elements to reduce raw material costs.Recently, significant focus has been gained by Zr-based compounds such as Na2ZrCl6. We investigate here such compounds, focusing on elemental substitutions to gain increased ionic conductivity and on gaining insight into the influence of synthesis on SE crystal structure and electrochemical properties.In our work, different amounts of elemental substitutions are investigated, and the related materials are synthesized at different temperatures. In particular, the influence of a series of subsequent heating temperatures on the structure of the binary system is studied through XRD diffraction analysis and further refinement. The Ionic diffusion paths are also estimated through the band valence sum approach, in order to validate the electrochemical impedance spectroscopy (EIS) results.In summary, we report our early results on the development of novel solid electrolytes for Na-based solid-state batteries.REFERENCES[1] Kwak, H.; Lyoo, J.; Park, J.; Han, Y.; Asakura, R.; Remhof, A.; Battaglia, C.; Kim, H.; Hong, S.-T.; Jung, Y. S. Na2ZrCl6 enabling highly stable 3 V all-solid-state Na-ion batteries. Energy Storage Mater. 2021, 37, 47−54.[2] Tong Zhao, Alexander N. Sobolev, Roman Schlem, Bianca Helm, Marvin A. Kraft and Wolfgang G. Zeier. Synthesis-Controlled Cation Solubility in Solid Sodium Ion Conductors Na2+xZr1 - xInxCl6. ACS Appl. Energy Mater. 2023, XXXX, XXX, XXX-XXX.[3] Schlem, R.; Banik, A.; Eckardt, M.; Zobel, M.; Zeier, W. G. Na3-xEr1-xZrxCl6-A Halide-Based Fast Sodium-Ion Conductor with Vacancy-Driven Ionic Transport. ACS Appl. Energy Mater. 2020, 3, 10164−10173.[4] Wu, E. A.; Banerjee, S.; Tang, H.; Richardson, P. M.; Doux, J.M.; Qi, J.; Zhu, Z.; Grenier, A.;Li, Y.; Zhao, E.; Deysher, G.; Sebti, E.;Nguyen, H.; Stephens, R.; Verbist, G.; Chapman, K. W.; Clement, R.J.; Banerjee, A.; Meng, Y. S.; Ong, S. P. A stable cathode-solid electrolyte composite for high-voltage, long-cycle-life solid-state sodium-ion batteries. Nat. Commun. 2021, 12, 1256.

Nanoscale Mapping of Extrinsic Interfaces in Hybrid Solid Electrolytes

Solid polymer electrolyte systems comprising of polyethylene oxide (PEO) and polymethyl methacrylate (PMMA) as blended polymer host, lithium trifluoromethanesulfonate (LiCF3SO3) as dopant salt, ethylene carbonate (EC) as plasticizer, and silicon dioxide (SiO2) as inorganic filler were prepared by solution casting method. PMMA–PEO–EC–LiCF3SO3–SiO2-blend solid polymer electrolytes were prepared to study the blending effect of PMMA into PEO solid polymer electrolytes. All the resulted solid polymer electrolytes were characterized using electrochemical impedance spectroscopy (EIS), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) methods. In the present study, the XRD, FTIR, SEM, and DSC results show clear evidence of the miscibility of PMMA and PEO. The incorporation of PMMA into the PMMA–PEO-blend solid polymer electrolytes shows significant improvement on ionic conductivity when small amount of PMMA is used. The highest ionic conductivity of 2.6301 × 10−4 S/cm is achieved for 5 wt% PMMA and 95 wt% PEO-blend solid polymer electrolytes at room temperature. The ionic conductivity values are found to be decreased when a higher wt% of PMMA is added. Upon incorporation of 20 wt% of PMMA, the ionic conductivity values are found in range of 10−6 to 10−7 S/cm. This finding is comparable with the reported results for gel-type polymer electrolytes. Hence, the amount of PMMA used in the PMMA–PEO-blend systems is crucial in the preparation of PMMA–PEO-blend solid polymer electrolyte.

Solid-state electrolytes (SSEs) are promising alternatives to conventional liquid organic electrolytes in lithium-ion batteries (LIBs), primarily due to their non-flammability and potential to reduce battery size. SSEs are categorized into three types: solid polymer electrolytes (SPEs), inorganic ceramic electrolytes (ICEs), and composite solid electrolytes (CSEs). Solid polymer electrolytes (SPEs) offer good interface contact with electrodes but suffer from low ionic conductivity and poor mechanical stability. In contrast, inorganic ceramic electrolytes (ICEs) exhibit high ionic conductivity and excellent mechanical stability but have weak electrode contact due to stiffness. Composite solid electrolytes (CSEs) aim to combine the advantages of both SPEs and ICEs, utilizing inorganic filler and polymer host to enhance conductivity, mechanical stability, and electrode contact. Understanding the mechanisms behind the enhanced ionic conductivity of CSEs is important to getting the key. Previous studies have investigated various factors, including the type and morphology of fillers, with active fillers showing promise in improving conductivity by facilitating extra Li-ion transport and maintaining the amorphous phase of the polymer. However, the exact cause of enhanced conductivity remains unclear.This study proposes the use of ionic liquids to exclude the amorphous effect of polymers and compares the ionic conductivity with active fillers. Additionally, nanoparticles are introduced to form a solid phase without additional solvents, enhancing filler-polymer contact and promoting thixotropy in the electrolyte, which improves safety and stability. To investigate the interphase effect between filler and matrix, our study focuses on controlling the vacancy concentration on the filler surface using garnet-type oxide as the filler material. X-ray photoelectron spectroscopy (XPS) analysis reveals a difference in oxygen vacancy (O-v) concentration between raw material and filled powder, indicating the influence of surface vacancies on ionic conductivity. Experimental results show that the addition of fillers increases the ionic conductivity of electrolytes, with lower oxygen vacancy concentrations leading to higher ionic conductivity. Distribution of relaxation time (DRT) analysis indicates low resistance in the electrolyte at most timescales, especially showing low diffusion resistance, suggesting improved ion transport properties. COMSOL Multiphysics simulations prove consistent electrolyte current density from the whole electrolyte, inducing uniform ion flux to electrodes. Laser-induced breakdown spectroscopy (LIBS) and XPS depth profiles confirm thin and uniform growth of the solid electrolyte interphase (SEI) layer, contributing to cycle stability.Overall, our study provides insights into the role of surface-controlled fillers in enhancing electrochemical properties and understanding ion-conduction mechanisms in composite solid electrolytes (CSEs). We explore the interphase effect between filler and matrix using Li conductive oxide. Vacancies on the filler surface impact electrolyte properties; fewer vacancies correlate with improved ionic conductivity and stability. Results show enhanced conductivity in surface-controlled electrolytes compared to host or composite electrolytes. Oxygen-filled electrolytes demonstrate superior thermal and electrochemical stability, promoting uniform SEI layer formation and enhancing specific capacity and cycle stability in Li metal half-cells. These findings contribute to developing safer, more efficient solid-state batteries for various applications. Figure 1

An effective solid-electrolyte interphase for stable solid-state batteries

Recent progress, challenges, and perspectives in the development of solid-state electrolytes for sodium batteries

Electrochemical Energy Storage with Mediator-Ion Solid Electrolytes

Poly(ethylene oxide) (PEO) was first developed as a conducting polymer over a half-century ago and is still the most fascinating electrolyte matrix to use in solid-state batteries. Although thousands of articles have been published on the conductivity of various PEO-based solid-state electrolytes, none of the studies involved a higher EO-to-salt ratio up to 85 to 1. In this study, solid polymer electrolytes (SPE) based on the PEO-PEG complex with Sodium(I) Bis(trifluoromethanesulfonyl)imide (NATFSI) were prepared using the solid-state synthesis methods. The measurement using the Electrochemical Impedance Spectroscopy (EIS) technique demonstrated that EO70Na - 9PEO:3PEG has the most optimum PEO-PEG-NATFSI electrolyte system with conductivities of 0.01 mS/cm and 0.13 mS/cm at 25 °C and 50 °C, respectively. As reported in published articles, the conductivity of the EO to Na ratio started to decline from 15:1 to 25:1 with the dropped in NATFSI concentration. Further decline of Na concentration with the addition of EO from 40 to 70 ratio indicates a new increasing trend for the conductivity series, which result in a bimodal graph distribution. Fourier transform infrared (FTIR) spectral studies for PEO-PEG-based SPE revealed that vibrational changes of intramolecular -OH change as the concentration of short-chain polymer backbone and salt varies. The Raman spectral studies for PEO-PEG-based SPE proposed that the percentage of bis(trifluoromethane)sulfonimide anion (TFSI- anions) increases with the increase in ratio from 15 to 70 of EO to Na. It is proposed that a higher percentage of both intramolecular -OH and TFSI- anions are crucial in providing high conductivity value. The structure of complexed PEO-PEG-based SPE from XRD suggested that the crystalline domain of SPE decreased with the smaller amount of NATFSI.

Sulfide solid state electrolytes (SSE) are among the most promising materials in the effort to replace liquid electrolytes, largely due to their comparable ionic conductivities. Among the sulfide SSEs, Argyrodites (Li6PS5X, X=Cl, Br, I) further stand out due to their high theoretical ionic conductivity (~1×10-2 S cm-1) and interfacial stability against reactive metal anodes such as lithium. Generally, solid state electrolyte pellets are pressed from powder feedstock at room temperature, however, pellets fabricated by cold pressing consistently result in low bulk density and high porosity, facilitating interfacial degradation reactions and allowing dendrites to propagate through the pores and grain boundaries. Here, we demonstrate the mechanical and electrochemical implications of hot-pressing standalone LPSCl SSE pellets with near-theoretical ionic conductivity, superior cycling performance, and enhanced mechanical stability. X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and x-ray diffraction spectroscopy (XRD) analysis reveal no chemical changes to the Argyrodite surface after hot pressing up to 250 °C. Moreover, we use electrochemical impedance spectroscopy (EIS) to understand mechanical stability of Argyrodite SSE pellets as a function of externally applied pressure, demonstrating for the first time pressed standalone Argyrodite pellets with near-theoretical conductivities at external pressures below 14 MPa.

The sodium-rich solid electrolyte, Na3SO4F (NSOF), holds promise for eco-friendly and resource-abundant energy storage. While the introduction of heterovalent dopants has the potential to enhance its suitability for battery applications by creating Na vacancies, the effect of vacancies and sodium concentrations on sodium conduction remains unclear. In this work, Mg2+ was introduced into Na+ sites in Na3SO4F, generating sodium vacancies with different contents by using solid-state synthesis method. Among the resulting materials, Na2.96Mg0.02SO4F exhibited an ionic conductivity that is two-order-of-magnitude higher than NSOF at 298 K. Notably, as the sodium concentration decreased, the ionic conductivity also declined, revealing an equilibrium between Na vacancies and concentrations. To further investigate the influence of sodium concentration, excess Na+ was introduced into NaMgSO4F, which inherently possesses a lower sodium content by using solid-state synthesis method. However, this adjustment only led to an approximately one-order-of-magnitude enhancement in optimal ionic conductivity at 298 K. Combined with an in situ X-ray diffraction analysis, our findings underscore the greater sensitivity of sodium conduction to variations in sodium vacancies. This study paves the way for the development of ultrafast sodium ion conductors, offering exciting prospects for advanced energy storage solutions.

Microstructural characterization and multiscale ionic conductivity in lithium and sodium-based solid state electrolytes