Symposium EN09—Metal Sulfides for High Performance Electrochemical Batteries

Transition-metal dichalcogenides are gaining much interest in the energy storage sector due to the two-dimensional (2D) nature and conductivity of the materials. However, single transition-metal dichalcogenides are not stable, preventing their practical use in real devices. Herein, we demonstrate the synthesis of binary metal dichalcogenides (Co–W–S) via carbonization of zeolitic imidazolate framework (ZIF-67), a subclass of metal-organic frameworks, encapsulated with phosphotungstic acid (PTA@ZIF-67). The morphology and surface functional groups of the as-synthesized Co–W–S composite are characterized via field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and Fourier transform infrared (FTIR) spectroscopy. Furthermore, the crystal structure and elemental composition of the fabricated Co–W–S composite are elucidated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. Upon testing its electrochemical performance as a supercapacitor electrode, the fabricated Co–W–S@N, S-codoped porous carbon (N, S-PC) shows exceptional specific capacitance (1929 F g^–1 at 5 mV s^–1). Furthermore, the constructed asymmetric supercapacitor device using Co–W–S@N, S-PC, and activated carbon as positive and negative poles, respectively, displays superior energy density and power density of 32.9 Wh kg^–1 and 700.2 W kg^–1, respectively, with high Columbic efficiency over 10 000 charge/discharge cycles at 10 A g^–1.

In this presentation I will give an overview of the mechanism by which Li ions achieve fast ion conductivity in sulfides and contrast it to oxide solid electrolytes. While screening of the electrostatic interaction through the large sulfur ion is very effective in providing a flat energy landscape for lithium, oxygen anions do not provide effective screening, rationalizing why sulfides and oxides require different structure types and compositions to achieve high ionic conductivity. I will also discuss claimed mechanisms for high conductivity such as paddle wheels etc. Finally, I will review the limitations on the anodic and cathodic stability of sulfide-based conductors and discuss their breakdown products.

Transition metal sulfides are appealing for next generation electrochemical energy storage as they can serve multiple roles as sulfur-equivalent electroactive materials and species promoting effective polysulfide trapping. Chemically and electrochemically based synthesis approaches providing control of sulfide composition and defect structure will be described in this presentation. Materials to mesoscale characterization and correlation with electrochemical function will also be reviewed.

The key challenge for the Li-metal electrode is to maintain the smooth surface at the microscopic scale during cycling. The same crystalline structure of Li₂O and Li₂S but different chemistry serve as model systems for the fundamental understanding of the difference of oxides and sulfides during Li stripping and plating processes. To simulate the vacancy formation and accumulation at the Li-metal surface, a combined density functional theory (DFT) and kinetic Monte Carlo (KMC) simulation was developed. It was found that the lithiophilic interface repels vacancies into the bulk Li, so Li atoms can quickly fill the Li vacancies near the interface and maintain a smooth Li surface. In contrast, the lithiophobic interface traps Li vacancies toward the interface, and the accumulated Li vacancies form voids and roughen the surface. While Li/Li₂O interface adhesion is larger than the Li/Li2S interface, it is predicted that it is easier to maintain a smooth Li surface during delithiation at Li/Li₂O interface. During lithiation platting, it was found the bandgap, tunneling barriers, and electron localization on internal defects, such as pores and crack surfaces and grain boundaries play a more important role for Li-dendrite nucleation and growth. A combined DFT and phase-field method was developed to demonstrate these effects. Several oxides (cubic Li7La3Zr2O12 (c-LLZO), Li1.17Al0.17Ti1.83(PO4)3 (LATP), and Li2PO2N) and sulfide (β-Li3PS4) solid electrolytes and interlayer materials (LiF, Li₂O, and Li₂S) were compared. All of them showed smaller bandgaps on the surface and grain boundaries than the corresponding bulk materials. While LLZO showed the most significant excess electrons on the surface and grain boundaries, β-Li3PS4 showed no such behavior. This is consistent with the observed trend and morphology of Li dendrite growth in different solid electrolyte materials.

The family of thiophosphate solid-state electrolytes (SSEs) is one of the most promising candidates for future all-solid-state lithium metal batteries due to their high ionic conductivity at room temperature (>1 mS cm^-1), even exceeding conventional liquid electrolytes. However, high-ionic-conductive solid electrolytes such as Li₁₀GePS₁₂ are not stable against lithium metal, leading to fast failure of the cell. This talk will first present the synthesis of a new Li₂S-P₂S₅-Li₃N-Al₂S₃ glass-ceramic SSE system through a solid-state reaction approach. The synthesized SSE demonstrated a high ionic conductivity of ~ 5.19 mS cm^-1 at room temperature and superior stability against lithium metal anode. And stable Li electrodeposition over 1000 h was achieved at a high current rate of 0.6 mA cm^-2 and a high areal capacity of 0.6 mAh cm^-2. Second, a solution synthesis of argyrodite-type SSEs with low annealing temperatures below 200 ^oC will be present. The synthesized SSE demonstrated a high ionic conductivity of ~ 4.01 mS cm^-1 at room temperature and superior stability against lithium metal anode. At room temperature, Li|SSE|Li symmetric cell demonstrated a high critical current density of 1 mA cm^-2 and low areal interfacial resistance of 6 ~ 7 Ω cm². Last, I will present the use of highly conductive SSEs in fabrications of all-solid-state Li-S batteries to achieve high utilization of sulfur and high energy density.

Beyond the current lithium ion technologies, sodium-sulfur batteries stand out because of the high theoretical specific energy attributed to the multi-electron redox reaction, which overcomes the capacity limitation of intercalation-based chemistry. Major challenges in realizing the full potential of a sodium-sulfur battery are associated with the formation of undesirable intermediates and irreversible precipitation of solid products. This work presents a new series of sodium phosphorothioate complexes that offer superior battery performance across a wide temperature range from -60°C to ambient temperature, showing great promise for enabling next-generation high energy and low cost energy storage systems. The complexes are designed by taking advantage of the chemical affinity of phosphorus with both sulfur and sodium. The complex structures and the electrochemical reaction mechanisms are revealed through coupled experimental characterization and theoretical calculations. The desirable electrochemical properties are attributed to the ability of the complexes to prevent the formation of undesirable precipitates over a fairly wide range of voltages.

Understanding the thermal stability of nanowires is of key importance for the performance and longevity of nanowire-containing devices, particularly where resistive heating may occur. With growing interest in sodium-ion batteries (NIBs) as a more economical and lower environmental impact alternative to lithium-ion batteries (LIBs), and layered transition-metal dichalcogenides (TMDs) as energy storage materials, it is important to determine both the advantages and limitations of replacing lithium-ion conductors with their sodium analogs. Here, we report an in situ heating study to investigate the thermal stability of sodium-intercalated titanium (IV) sulfide (TiS₂) nanowires, performed in the transmission electron microscope (TEM). Both sodiated (Na_xTiS₂) and non-sodiated nanowires were studied using a specialized heating holder, heating between room temperature and 700 °C. Control areas were also investigated to distinguish between heating and electron-beam effects. We observed chemical, morphological, and crystalline changes with increased temperature, solely in the sodiated-TiS₂ samples, demonstrating that sodiation profoundly affects the thermal stability of TiS₂ nanowires.

Sodium-Sulfur (Na-S) batteries are considered a promising candidate for stationary energy storage techniques due to their low cost, long cycle life, and high energy density. For example, Na/β''-alumina/S battery has already been developed as stationary energy storage systems. However, the requirement of high temperature (>300 °C) to maintain the molten state of Na anode and S cathode causes extra power consumption and potential safety issues, restricting its broader applications. All-solid-state Na-S batteries operating at a lower temperature (e.g., < 100 °C) have attracted significant attention because of potentially improved safety features while maintaining long cycling life due to no generation of soluble polysulfide. One of the main challenges in developing such all-solid-state Na-S batteries operating at lower temperatures is its poor interfacial stability between Na metal anode and solid electrolyte, resulting in increased impedance, dendrite formation/penetration, and poor cycling life. Herein, we investigated Na alloy anodes and their applications in Na-S batteries. The alloy anode materials (e.g., Na-Sn and Na-Sb alloys) were synthesized by a high-energy ball-milling procedure. Electrochemical performance of the alloy anodes was evaluated, including comparing overpotential and critical current densities in Na-alloy symmetric cells, studying effects of different mass loading and N/P ratios in Na-S batteries. It is found that Na-Sb alloy is a more stable alloy anode with low overpotentials at a wide range of current densities. By pairing with TiS₂ cathode, Na-Sb alloy anode exhibits stable cycle life than Na-Sn alloy anode. All-solid-state Na-S battery cells were also fabricated with high mass loading of the sulfur cathode. The all-solid-state Na-S battery cell can deliver a high initial discharge capacity and maintain a specific capacity above 1000 mAh/g after long-term cycling operating at 60 °C. This finding shows a promising stable Na alloy anode for all-solid-state Na metal batteries to achieve long cycle life at a lower temperature for practical applications in stationary energy storage.

Although room temperature sodium-sulfur (Na-S) batteries have attracted huge attention due to its promise for large-scale applications, severe shuttle effect of polysulfide and the sluggish reaction kinetics of non-conductive sulfur in sulfur cathodes, as well as dendritic Na growth in Na metal anodes are the main challenges of this technology. Notably, most of the previous published works usually devote to solving the issues related to sulfur cathode or sodium metal anode separately. There are rare works of simultaneously enhancing the electrochemical performance of both cathode and anode with two-in-one materials. Herein, high-rate and long-cycling room-temperature Na-S batteries are achieved by employing covalent sulfur confined in mesoporous hollow carbon spheres (S@MHCS) as stable sulfur cathodes and solid additives for metallic sodium anode in a carbonate electrolyte to overcome the above obstacles simultaneously. As the cathode, MHCS can not only provide sufficient electrical contact and accessible Na⁺ diffusion to the encapsulated sulfur copolymers for fast kinetics and high capacity, but effectively confine formed short-chain sodium polysulfides for prolonged cycle life. Additionally, the direct addition of the S@MHCS into bulk Na contributes to the homogenous Na plating by inducing Na nucleation and growth within the inner cavity of carbon spheres, and minimizes volume change during cycling, enabling extremely stable Na metal anodes at a ultrahigh current density (up to 40 mA/cm²) along with an ultrahigh capacity (up to 40 mAh/cm²). As a result, a Na-S full cell constructed with S@MHCS exhibits excellent rate capability and outstanding cycling performance.

A facile one-step method was demonstrated for the electrodeposition of manganese–nickel sulfide (Mn–Ni–S) 3D interconnected sheets on nickel foam substrates. The as-synthesized materials were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) techniques. Upon their use as supercapacitor electrodes, the electrodeposited Mn–Ni–S showed exceptionally high specific capacitance (2849 and 1986 F/g at 1 and 5 A/g, respectively) and an excellent rate capability. Using Fe₃O₄-GR as the negative electrode and the Mn–Ni–S 3D interconnected sheets as the positive electrode to assemble an asymmetric supercapacitor device revealed high power density (800 W kg^–1) and energy density (40.44 Wh kg^–1) with 90% capacitance retention and a Columbic efficiency of 100% after 11 000 cycles, indicating the high potential of the fabricated materials for practical energy storage devices.

Sulfide-based superionic sodium (Na) conductors show promising applications as solid electrolytes (SEs) in solid-state Na batteries to achieve the high energy dense, safe, and economic features. For solid-state ionic conductors, the synthesis process strongly influences their crystal structures/phase purity, and thus affect their ionically conductive properties. In situ neutron diffraction (ND) is a powerful tool to track the structural changes during solid-state synthesis process. Among sulfide-based SEs, Na₃SbS₄ family conductors have attractive intense attentions due to their high ionic conductivity (10^-4~10^-3 S cm^-1) at room temperature and impressive chemical stability in ambient environment. In this work, we performed in situ neutron diffraction on pristine Na₃SbS₄ without and with doping (i.e. Se) to track the phase change and structural evolution during the synthesis and heat treatment (heating or cooling). The solid-state reaction mechanism during the synthesis process was revealed. In addition, the doping effects on the solid-state reactions, lattice parameters and structure stability were studied. This work presents a detailed structural study using in situ NDs for the synthesis of sulfide-based Na conductors, which provide useful information for the design and synthesis of new solid-state conductors.

All-solid-state Lithium metal batteries (ASSLMBs) cannot only maximize the energy density, but also show improved safety compared with the conventional liquid electrolyte-based Li-ion batteries. Solid-state electrolytes (SSEs), as the most important component in the ASSLMBs, can determine the electrochemical performance. High ionic conductivity is one of the most basic requirements for an excellent SSE. Sulfide-based SSEs exhibit ultrahigh ionic conductivity (> 10^-2 S cm^-1 at room temperature) that can be comparable to the liquid electrolytes, making this kind of SSEs popular and promising. However, air sensitivity and Li metal incompatibility are two major obstacles, which are together with the problematic cathode interface, hindering the development of sulfide SSEs for practical ASSLMBs.^{1, 2}
In our work, we employed the strategy of element substitution to design F-substituted Li₆PS₅Cl, Sn-substituted Li₆PS₅I, and Sn-substituted Li₃PS₄ as high-performance Li-metal compatible sulfide SSEs.^4-6 Using these element doping SSEs can derive functional interphases that contain LiF, LiI, and Li-Sn alloy, respectively. These interfacial contents were demonstrated as important to achieve stable Li plating/stripping. Importantly, we found that replacing P with Sn could significantly improve the air stability and ionic conductivity at the same time. The high reaction energy between the Sn-substituted PS₄ tetrahedra and moisture was regarded as the main reason for the improved air stability, while the expanded unit cell and increased Li⁺ concentration led to superior ionic conductivity. Our modifications on various common-used sulfide SSEs with multi-function elements provide more choices of sulfide SSEs for practical ASSLMBs. At the cathode side,^7-11 we used interface modification (coating strategy) to improve the interfacial Li⁺ transport and chemical inertness. The electrode and electrochemical performance will be discussed. ¹²
References
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2. C. Yu, F. Zhao, X. Sun*, et al. Recent Development of lithium argyrodite solid-state electrolytes for solid-state batteries: synthesis, structure, stability and dynamics. Nano Energy, 2021, 83, 105858.
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5. F. Zhao, X. Sun*, et al. A Versatile Sn-Substituted Argyrodite Sulfide Electrolyte for All-Solid-State Li Metal Batteries, Adv. Energy Mater. 2020, 10, 1903422.
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It is clear that a deeper understanding of metal sulfide materials for solid state batteries requires greater insights into their underlying structure-property relationships. This talk will highlight recent studies in two related areas: (i) Na₃PS₄ is one of the most promising Na-ion conductors, and our studies indicate a new high temperature polymorph showing fast-ion conduction associated with rotational motion of the thiophosphate polyanions; (ii) nanoscale effects on sulfide-based lithium-ion conductors are investigated with the results indicating enhancement in ion conductivity due to changes in local structure and ion diffusion pathways. A combination of materials modelling and experimental methods has helped us gain new insights into sulfide materials, which are valuable in developing strategies to optimize their solid electrolyte properties.

T. Famprikis et al., J. Am. Chem. Soc, 142, 18422 (2020); ACS Mater. Lett., 1, 641 (2019); Nature Mater., 18, 1278 (2019).

Site-disordered thiophosphates of the formula Li₆PS₅X (X = Br, Cl) are considered as the most promising solid electrolytes for all-solid-state lithium batteries with respect to their superior Li⁺ diffusion properties [1]. Also for Li₆PS₅I, with its ordered anion and cation sublattice, ⁷Li (and ³¹P) nuclear magnetic resonance (NMR) measurements suggest rapid localized Li⁺ exchange processes. However, the missing site disorder of Li₆PS₅I does only result in poor long-range ion transport as verified by conductivity spectroscopy [1]. Here, we compare results from ³¹P spin-lock NMR measurements with recent results from 1/T_1ρ ⁷Li NMR experiments [1] to shed light on the pronounced bimodal diffusion behaviour seen in Li₆PS₅I. The observation of ³¹P via spin-lock NMR offers a another perspective to look at ion diffusion within solids. The influence of rotational jump processes of the PS₄^3- polyanions on Li⁺ dynamics (and vice versa) is discussed as well [2,3]. In conclusion, site disorder in the Li₆PS₅X family is the key property to understand the extraordinarily high Li⁺ self-diffusivities seen by NMR for the samples with X = Br, Cl [4]. In addition, we will discuss the influence of nanostructuring on Li diffusion in high-energy ball milled Li₆PS₅I [5] and Li₁₀GeP₂S₁₂ (LGPS). In structurally disordered Li₆PS₅I an increase in ion conductivity by 2 orders of magnitude is observed. Hence, Li₆PS₅I turned out to be a suitable model system to study the influence of local disorder by both broadband conductivity spectroscopy and NMR relaxation techniques [5].

References

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Highly conductive solid electrolytes are necessary to build well performing All Solid-State Batteries with high energy densities.^1,2 However, the discovery and optimization of suitable solid electrolyte materials has been limited, in part due to the lack of a fundamental understanding of the connection between the crystal structure, lattice dynamics and the diffusion process.³ Many of the established connections have been investigated through one-dimensional solid-solution type substitution experiments, where one atom is exchanged for another and corresponding changes in ionic transport are monitored.^4-6 However, the simultaneous alterations of the crystal structure and lattice dynamics in these substitution series makes it extremely challenging to discern different influences on the transport properties. Herein we utilize a two-dimensional solid solution approach to show that the correlation of structure-based metrics with ionic transport can be misleading, whereas dynamical descriptors (e.g. bond strengths) may be more pertinent to the activation barrier in some fast ionic conductors. Specifically, we use the model Na₃P_1−xSb_xS_4−ySe_y system, comprised of the four substitution series Na₃P_1−xSb_xS₄, Na₃P_1−xSb_xSe₄, Na₃PS_4−ySe_y and Na₃SbS_4−ySe_y to systematically decouple static and dynamic contributions to the activation barrier. This study indicates that lattice dynamical properties cannot be overlooked in assessing trends of the activation barrier in solid electrolyte substitution series.

(1) Culver, S. P.; Koerver, R.; Krauskopf, T.; Zeier, W. G., Designing Ionic Conductors: The Interplay between Structural Phenomena and Interfaces in Thiophosphate-Based Solid-State Batteries. Chem. Mater. 2018, 30 (13), 4179–4192. https://doi.org/10.1021/acs.chemmater.8b01293.

(2) Janek, J.; Zeier, W. G., A Solid Future for Battery Development. Nat. Energy 2016, 1 (16141). https://doi.org/10.1038/nenergy.2016.141.

(3) Ohno, S.; Banik, A.; Dewald, G. F.; Kraft, M. A.; Krauskopf, T.; Minafra, N.; Till, P.; Weiss, M.; Zeier, W. G., Materials Design of Ionic Conductors for Solid State Batteries. Prog. Energy 2020, 2, 022001. https://doi.org/10.1088/2516-1083/ab73dd.

(4) Krauskopf, T.; Pompe, C.; Kraft, M. A.; Zeier, W. G., Influence of Lattice Dynamics on Na+Transport in the SolidElectrolyte Na₃PS_4−xSe_x. Chem. Mater. 2017, 29, 8859–8869.

(5) Schlem, R.; Ghidiu, M.; Culver, S. P.; Hansen, A. L.; Zeier, W. G., Changing the Static and Dynamic Lattice Effects for the Improvement of the Ionic Transport Properties within the Argyrodite Li₆PS_5−xSe_xI. ACS Appl. Energy Mater. 2020, 3 (1), 9–18. https://doi.org/10.1021/acsaem.9b01794.

(6) Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H.-H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y., Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem. Rev. 2015, 116 (1), 140–162. doi:10.1021/acs.chemrev.5b00563.

The high theoretical specific capacity of 893 mAh/g and energy density of 1273 Wh/kg of FeS₂ make it an attractive choice to pair with a lithium anode for next generation batteries. Development and implementation of secondary FeS₂/Li batteries has been hampered for a variety of reasons including soluble polysulfide formation which causes capacity loss and cell death. Another factor has been a lack of suitable electrolyte for cycling both the FeS₂ cathode and Li anode, with high coulombic efficiency. This talk will focus on the synthesis of FeS₂ nanoparticles, choice of a suitable electrolyte, and cycling protocol to enabling stable cycling of a FeS₂/Li cell. By cycling the nano FeS₂ in a concentrated bisalt electrolyte with a charging voltage cutoff of 2.4 V we have achieved 80 cycles (at the time of this submission) with an average discharge capacity of 421 mAh/g, well above 80% capacity retention under this condition. Using Cu/Li cells we show the plating/stripping of Li in the bisalt electrolyte has an average coulombic efficiency of 98.8% over 50 cycles and 99.1% for the last 40 cycles. Furthermore, we show that the high performance cycling of the cathode and anode is attributed not only to use of a bisalt electrolyte, but also by ensuring a submicron domain size of the FeS₂ particles to mitigate polysulfide generation.

This work was Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. The views expressed herein do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Solid electrolytes (SEs) are at the heart of all-solid-state batteries (ASSBs), which promise higher energy density along with safer operation compared to commercial Li ion batteries. As the ASSB technology matures, research in the field needs to respond to questions pertaining to the upscaling of solid electrolytes, their stability, and the integration of the individual components into ASSBs with extended cycle life. As a consequence, new research areas have been emerging lately, including interfacial engineering and microstructure design, and questions such as the role of stack pressure on battery performance increasingly enter the focus of ASSB design.
This presentation will discuss the development and optimization of lithium^1,2 and sodium³ thiophosphate SEs based on three complementary approaches, which address both the atomistic level and microstructure of SEs:
(i) We highlight the importance of phase engineering and hybridization in thiophosphate solid electrolytes, exemplified by the discovery of two new polymorphs of Na₄P₂S₆ and the formation of composite thiophosphate SEs.⁴ Our results showcase how synthetic control and post-synthetic treatment of solid electrolytes can significantly impact the materials properties, even though structural differences may be minute or even hidden by the glassy ceramic character of many thiophosphates.⁵
(ii) The development of solution processing routes for the thiophosphate Li₇SiPS₈ is demonstrated, revealing that aprotic solvents with donor numbers smaller than 15 kcalmol^-1 are suitable for wet processing, since both crystal structure of the SE and high ionic conductivities (> 1 mScm^-1) are retained.⁶
(iii) Finally, we pinpoint the role of moderate pressure on the microstructure of SEs and demonstrate its complex impact on the Li ion conductivity of LGPS-type SEs.
Taken together, our findings reinforce the notion that a holistic picture of a SE – including its atomistic structure, defect- and disorder-related phenomena, as well as its microstructure – is needed to understand and accurately predict its function in a realistic ASSB environment.

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Solid electrolytes (SEs) exhibit complex atomic dynamics, featuring simultaneous facile hopping of mobile species and soft lattice dynamics of the host lattice. Importantly, fast diffusion involves large-amplitude motions on strongly anharmonic potential energy surfaces, and such anharmonic effects need to be assessed when investigating phonons and their coupling with the stochastic hopping underlying the diffusion process. Phonons describe the atomic dynamics in crystalline materials and provide a basis to encode possible minimum energy pathways for ion migration. Identifying and controlling the pertinent phonon modes coupled most strongly with ionic conductivity could pave the way for discovering and designing new SEs via phonon engineering. This talk will report on investigations of phonons in Na3PX4 and Na3SbX4 (X=S, Se), highlighting strong anharmonic effects and their coupling to fast Na diffusion, using a combination of neutron scattering, ab-initio molecular dynamics (AIMD) and machine-learning accelerated MD. We identify specific anharmonic soft-modes that strongly couple with the Na diffusion process, and show how these modes enable Na-ions to hop along the minimum energy pathways. Further, our quasi-elastic neutron scattering (QENS) measurements, supplemented with large-scale molecular dynamics simulation, provide the Na diffusion constant and the diffusion characteristics. These results offer detailed microscopic insights into the dynamic mechanism of fast Na diffusion and provide an avenue to search for further Na solid electrolytes.

The big market of energy storage systems is mainly dominated by the widely available and conventional lithium-ion batteries (LiBs). Low earth-abundance of lithium and the use of not sustainable cathode materials, like lithium cobalt oxide’s, are currently bottlenecks of the commercially available LiBs. In quest of new cathode materials, metal sulfide getting more and more attention to be next-generation cathode materials. Nevertheless, achieving high capacity and good cycling stability are remaining challenges. Especially, pyrite (FeS₂), also known as fools’ gold, is a promising candidate to overcome the issues of availability (iron = 5.6 %; sulfur = 0.035 %) and specific capacity (894 mAh g^-1). Moreover, by changing the battery setup from lithium to magnesium the energy storage technology can be even more sustainable (Li = 0.002 %; Mg = 2.4 %). Crucial to achieving good energy storage performance of the metal sulfides is the size and shape of the particles in the electrode which affects the thermodynamic properties.
Different analogs of iron sulfides (FeS, Fe₃S₄, and Fe₇S₈) were investigated as cathode material in combination with a magnesium metal anode and characterized by XPS, SEM, and Raman spectroscopy. The influences of the particle size and additives were studied, and demonstrators were successfully manufactured. Additionally, the effect of carbon additives, like CNF and rGO, on the performance of the battery was investigated.

There is a need for safe, reliable, and inexpensive batteries with high capacity for grid storage applications. The high capacity (1675 mAh/g) and low cost (~0.1 $/kg) of sulfur make Li-S batteries an excellent candidate for this; however, S has poor electrical conductivity and the proximity of Li to the cathode in conventional batteries can present a safety hazard at grid scale. We address these problems by designing a highly scalable, membrane free redox-mediated Li-S flow battery. Benefits of this design include: 1) The presence of soluble redox mediators enable the reduction/oxidation of the S without the need for conductive carbon additives. 2) The physical separation of the Li anode from the cathode reduces the risk of short circuiting and propagating battery fires. 3) The use of electrolyte additives enables stable Li plating on the anode which improves the cell efficiency, extends the cycle life, and removes the need for expensive solid state separators. We chose decamethyl ferrocene (DmFc, 2.86 V vs Li/Li⁺) and cobaltocene (CoCp₂, 2.06 V vs Li/Li⁺) as redox mediators for their proximity to the redox potential of the Li-S reaction. During cycling, these mediators help chemically oxidize or reduce the S without the need for electrical contact between the S and the electrode. Instead, the soluble mediators are electrochemically oxidized or reduced at the electrode. In order to address the non-uniform deposition of Li on Li metal anodes, we presoaked the Li metal in a LiI solution to form a more stable solid electrolyte interphase (SEI) and used LiNO₃ in the electrolyte to improve the stability of the Li deposition. To examine the Li surface chemistry and explain the cycling performance, we conducted pre- and post-cycling characterization of the system. Characterization with x-ray photoelectron spectroscopy and energy dispersive x-ray spectroscopy revealed the presence of uniformly dispersed LiI in the SEI on the Li before cycling, which improves the uniformity of Li deposition on the anode. Scanning electron microscopy revealed globular, rather than filament-like, Li deposition, which is attributed to the presence of LiNO₃ in the electrolyte and explains the high coulombic efficiency and cycle life. UV-vis spectroscopy revealed a decrease in the amount of polysulfides present in the electrolyte between 50 and 200 cycles, indicating that the observed capacity loss is likely due to the loss of available sulfur. The combination of the redox mediators and additives enabled our cells to cycle stably for 200 cycles with an average coulombic efficiency of 99.0% and a voltage efficiency of 82.0%.
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

All-solid-state batteries using ceramic solid electrolyte is promising next-generation battery technology. To enable a high-performance solid-state battery, the solid electrolytes need to exhibit high ionic conductivity, wide electrochemical window, interfacial compatibility with anode and cathode, multiple mechanical properties, and air/moisture stability. However, no single solid electrolyte material can satisfy all these desired attributes. For example, sulfides have the highest ionic conductivity but exhibit narrow electrochemical windows and notoriously poor air stability, whereas oxides generally exhibit better stability but lower ionic conductivity. In order to develop all-around solid electrolytes, our first-principles computation will provide guiding principles and design strategies for improving ionic conductivity, electrochemical window, air stability in solid electrolyte materials. We will also show new solid electrolyte materials systems discovered and designed by computation through an accelerated fashion.

Glassy, glass–ceramic, and crystalline lithium thiophosphates have attracted interest in their use as solid electrolytes in all-solid-state batteries. Despite similar structural motifs, including PS₄^3–, P₂S₆^4–, and P₂S₇^4– polyhedra, these materials exhibit a wide range of possible compositions, crystal and amorphous structures, as well as ionic conductivities. Calculations based on density functional theory can be a helpful tool for understanding diffusion pathways and Li⁺ ionic conductivity and interface stabilities.
In this contribution I will first discuss recent results on the defect chemistry and conductivity of the solid electrolyte Li₄P₂S₆ as well as its interfacial instability with respect to Li. Then, molecular dynamics simulations of crystalline and amorphous Li₄PS₄I will be shown, which unravel the diffusion mechanism and can be explained by a rate-equation model based on superbasins. Finally, results on the Lithium argyrodites of the type Li₆PS₅X (X = Cl, Br, I) are presented, where the influence of S₂-/Br- site-disorder was studied. The simulations reveal that local “Li cages” trap Li ions in the ordered material. At higher degrees of site-disorder the cage structures dissolve and long-range low energy pathways are established. The analysis of pair distribution functions (PDF) and Li-density maps elucidates the correlation between structural disorder and ionic conductivity.
[1] C Dietrich, M Sadowski, S Sicolo, DA Weber, SJ Sedlmaier, KS Weldert, et al, Chemistry of Materials 2016, 28, 8764-8773
[2] M Sadowski, S Sicolo, K Albe, Solid State Ionics 2018, 319, 53-60
[3] S. Sicolo, C. Kalcher, S. Sedlmaier, J. Janek, K. Albe, Solid State Ionics, 2018, 319, 83-91
[4] A Gautam, M Sadowski, N Prinz, H Eickhoff, et al., Chemistry of Materials, 2019, 31, 10178-10185
[5] M Sadowski, K. Albe, J. Power Sources 2020, 478, 229041
[6] M. Sadowski, K. Albe, Phil. Trans. A, 2021

High performance solid state ionic conductors are much desired as the solid electrolyte for all-solid-state-batteries. The solid electrolyte materials are required to have high room temperature ionic conductivity, low electronic conductivity, good chemical and electrochemical stabilities, and good mechanical properties, and low cost. With these expectations, in recent years we have design and discovered a number of new compounds with promising Li-ion and Na-ion conduction properties, including oxide, sulfide and halide compounds. In this MRS conference we will report some of our recent findings, including sulfide Li-ion conductors with argyrodite structure, halides with layered structures, and Na-ion conductors with modified cubic structures. These compounds all show very high room temperature ionic conductivities and good performance in the cycling test of all solid state batteries. High resolution synchrotron X-ray and neutron scattering techniques, solid state NMR and electron microscopy were complementarily used to reveal the structure-property relationships in these compounds. The impacts of synthesis condition, chemical composition and processing to the ionic conductivity and battery performances will be discussed.

Metal chalcogenides such as SnS₂ and SnS are promising anode materials for lithium-ion batteries as their theoretical reversible capacities (644 mAh/g for SnS₂ and 755 mAh/g for SnS) are significantly higher than that of graphite (372 mAh/g). SnS₂ exhibits a layered, hexagonal CdI₂-type crystal structure, in which each Sn⁴⁺ ion is octahedrally coordinated by S ions with S–Sn–S stacking across the crystallographic c-direction. On the other hand, α-SnS can be considered as a “pseudo-2D” orthorhombic structure in which each Sn²⁺ ion is coordinated to three S ions within the layer. The complete lithiation of SnS takes place via successive conversion and alloying reactions. In the conversion reaction, an electrochemically inactive but ionically conductive Li₂S matrix is formed, which buffers the volume change of Sn (~ 300% for formation of Li₁₇Sn₄) during the alloying reaction. On the other hand, due to the higher +4 oxidation state of the Sn ions in SnS₂ as well as the presence of vacant interstitial sites in the crystal lattice, the lithiation of SnS₂ starts with the intercalation of lithium ions into the octahedral sites between the S–Sn–S layers. This is followed by the conversion and alloying reactions as for SnS.
In the literature, the electrochemical performance properties of SnS₂ and SnS have been investigated separately from each other, so that there is a lack of comparative data for SnS and SnS₂ anodes with comparable particle morphologies and subjected to the same electrochemical test conditions. Therefore, the motivation of this work is to establish the structure–processing–property relations of SnS₂ and SnS anode active materials with similar particle morphologies, understand the fundamental lithiation mechanisms of both, and elucidate their degradation mechanisms under long-term cycling. In all our experiments, a single, commercially available SnS₂ powder with ~ 10 µm particle size was used as the starting material.
In the first step, XRD, SEM and laser diffraction were performed on the pristine SnS₂ micro-powder to characterize its crystal structure, particle morphology and particle size distribution. Differential thermal analysis, thermogravimetric analysis and heat-treatment experiments in flowing argon followed by XRD with Rietveld refinement of the measured diffraction patterns were conducted to understand the phase transformation kinetics of pristine SnS₂ to SnS. Based on these results, phase pure SnS was synthesized from pristine SnS₂ by subjecting it to heat treatments at T > 700°C in Argon atmosphere.
To produce the anode active materials, pristine SnS₂ and SnS synthesized by the thermal decomposition of SnS₂ were subjected to high-energy ball-milling in ZrO₂ bowls at 200 rpm in an argon atmosphere. This processing yields SnS₂ and SnS nanoparticles with similar morphologies. The anode active materials were subjected to electrochemical testing including galvanostatic cycling, cyclic-voltammetry, impedance spectroscopy, galvanostatic titration and in-situ dilatometry to quantify their structure–performance properties. Furthermore, ex-situ XRD was performed on electrodes at different states of charge to characterize the discharge reaction mechanism, and SEM and XRD were performed on electrodes after 100 discharge/charge cycles to assess the change in structure of the active anode materials after cycling.
Our results show that although the theoretic reversible capacity of SnS is larger than that of SnS₂, SnS₂ performs better during long term cycling. This is attributed to the intercalation of lithium into the SnS₂ crystal structure before the conversion mechanism. This crucial step generates a finer distribution of Li₂S, which is better suited to buffer the volume changes of the Sn particles during successive lithiation/de-lithiation steps.

The lithium–sulfur battery is a promising candidate for next-generation electrical energy storage. The inexpensive, naturally abundant, and environmentally benign sulfur cathode offers an order of magnitude higher theoretical charge-storage capacity (1,672 mAh g^-1) than the those of currently used insertion-compound cathodes. However, as a new energy-storage battery system, the lithium-sulfur battery cathodes suffer both material and electrochemical challenges. The sulfur cathode and its end-discharge sulfide products are both good insulators. The high cathode resistance causes a low and irreversible electrochemical utilization of the cell. The conversion between sulfur and sulfides generates liquid polysulfides, which readily diffuse out of the cathode and migrate in the cell. The irreversible migration of polysulfides within the cells results in the fast capacity fade and poor discharge/charge efficiency. Moreover, the low electrochemical utilization and stability challenge the design and fabrication of a high-performance sulfur cathode with a necessary high amount of sulfur and a desired low amount of electrolyte in a cell. The inappropriate cathode design impacts the sulfur cathode electrochemistry, which impedes the development of sulfur cathodes with high energy density and the commercialization of the lithium–sulfur technology.

In our presentation, we present innovations on metal-sulfur nanocomposite possessing a conductive metal nanoshell coated on sulfur particles to improve the conductivity of sulfur cathode, the stabilization of polysulfides, and the reaction kinetics of lithium-sulfur batteries. We adopt the electroless-nickel plating method to synthesize the nickel/sulfur nanocomposite features a multi-functional nickel nanoshell. The metallic coating provides the metal/sulfur nanocomposite with high conductivity. Moreover, nickel has the high polysulfide trapping capability and strong electrocatalytic capability. As a result of these critical advantages in sulfur cathode, the metal/sulfur nanocomposite exhibits excellent lithium-sulfur battery performance, showing high electrochemical utilization and kinetics with high discharge capacity of above 1,000 mAh g^-1 and excellent rate capability of C/20–C/2 rates with stable cyclability for 200 cycles. These improved electrochemical performances are realized by the high-loading cathode with a high sulfur content of 74 wt% and a high sulfur loading of 14 mg cm^-2 in a lean-electrolyte lithium-sulfur battery with a low electrolyte-to-sulfur ratio of 7 µL mg^-1. The simultaneous improvement in the outstanding material and electrochemical characteristics results in high areal and gravimetric capacities along with a high energy density of 28 mWh cm^-2. In conclusion, the superior material design and electrochemical characteristics obtained by our nickel/sulfur nanocomposite as the high-loading sulfur cathode in a lean-electrolyte lithium-sulfur cells make its advanced cathode designs for the development of high-energy-density lithium-sulfur batteries.

Metal-sulfur batteries offer tremendous advantages compared to lithium-ion batteries in terms of cost and energy density as sulfur is abundant, inexpensive, and environmentally benign, and both sulfur cathode and metal anodes offer much higher capacities. However, the practical viability of metal-sulfur batteries is hampered by the low energy density and poor cycle life in practical cells. These challenges originate from the poor electronic and ionic conduction in sulfur and its discharged products, polysulfide shuttle, and poor cyclability of metal anodes. To overcome some of these difficulties, generally a large amount of conductive carbon and liquid electrolyte are added to the cell; they enhance the cycle life, but drastically lower the practical energy density and make the metal-sulfur batteries uncompetitive with lithium-ion batteries. On the other hand, it is challenging to manufacture and handle thin lithium-metal or sodium-metal foils, drastically increasing their cost.

To overcome the above challenges, this presentation will first focus on anode-free cells, in which lithium sulfide (Li₂S) cathode is paired with an anode current collector without any free lithium at the anode. The anode-free cells eliminate the handing and manufacturing issues associated with thin lithium-metal foils, increase the cell energy density, and suppress self-discharge before deploying the cells as the cells are assembled in the discharged state similar to the currently used lithium-ion cells. To stabilize lithium-metal plating and stripping, a small amount of additive is incorporated into the Li₂S cathode. The presentation will first present the effect of the incorporation of a small amount of tellurium as a case study. Being in the same group as sulfur in the periodic table, but with a metalloid behavior, tellurium substitutes into polysulfides to form polytellurosulfides, which migrate to the anode and get reduced to form Li₂TeS₃ as a thin, lithium-ion conducting solid-electrolyte interphase (SEI) layer on the plated lithium metal. This enables a uniform deposition of lithium and mitigates the severe electrolyte decomposition, enabling the assembly of cells with less amount of electrolyte. As a result, remarkable improvement in cyclability and capacity is achieved in energy-dense, anode-free full cells and lean-electrolyte pouch cells with high sulfur loading. The mechanisms involved in the drastic improvement in the performance is elucidated with in-depth characterization methodologies, such as time-of-flight secondary ion spectrometry (TOF-SIMS), x-ray photoelectron spectroscopy (XPS), nuclear magnetic resonance (NMR) spectroscopy, etc. The speciation with TOF-SIMS is particularly valuable to delineate the dynamics involved in lithium plating and stripping as well as to understand the chemistry involved.

The presentation will then focus on the incorporation of other additives to engineer the behavior of dissolved polysulfides in order to stabilize both the sulfur cathode and lithium-metal plating and stripping. The role of additives on the electrochemical performances will be illustrated with pouch cells fabricated with high sulfur loading and low electryte/sulfur ratio to be practically viable. Then, the presentation will transition to employ some of the understanding gained with lithium-sulfur batteries to sodium-sulfur batteries. Long-life, dendrite free sodium-sulfur cells will be presented, including pouch cell performances, with appropriate engineering of the cell chemistry.

Solid state Li-S battery promises high energy density and low cost. Multiple challenges remain for the sulfur cathode. S is insulating and experiences large volume changes during cycling. In this talk, we will report our recent effort in developing polymer-S composite materials to address these challenges. The first class of material is based on sulfurized polyacrylonitrile (SPAN). SPAN contains ~ 45% of S and demonstrates excellent cycling stability due to small volume change. Its capacity is limited to ~ 600 mAh/g with a discharge voltage of 1.8 V. In order to further improve materials energy density, we have investigated polymer-S materials with higher S content. SPAN has an approximate formula of C3NS. The carbon content can be reduced to a unit formula of CNS in poly (SCN). The material can be synthesized with free radical oxidation of SCN salts. The material demonstrates an average voltage of 2.1 V with a capacity of 700 mAh/g, thus a much higher energy density than SPAN. Further optimization of materials morphology and cell fabrication will pave the way towards a long-life solid state Li-S battery.

Sulfide solid-state electrolytes (SEs) featured with nonflammability and superior ionic conductivity (>10^-3 S cm^-1) enable all-solid-state lithium batteries (ASLBs) to deliver safer and more reliable energy storage. However, current sulfides-based ASLBs exhibit far below expected energy densities at the cell level (<50 Wh kg^-1, <100 Wh L^-1), due to the employment of SE membranes with extreme high thickness (>500 μm), large weight (>150 mg cm^-2), and high areal resistance (>100 Ω cm²). The binder-assisted solution method is a promising strategy to fabricate a thin and robust sulfide SE membrane. However, the high sensitivity to polar solvents and natural brittleness of sulfide SE challenge the fabrication of thin, light-weight SE membranes with no sacrifice in ionic conductivity. To successfully fabricate a thin SE membrane with high ionic conductivity and mechanical strength, a critical step is to select a binder satisfying the following requirements: 1) excellent solubility and stability in the nonpolar solvent; 2) high stability with sulfide SE; 3) high thermal stability; 4) high binding strength. This work will report an advanced binder and a freestanding SE membrane with an ultralow thickness of 47 μm, ultralow areal resistance of 5.10 Ω cm2, and superior ionic conductivity of 1.08 mS cm^-1, and the ultrahigh ion conductance of 190.11 mS. The ASLB utilizing this advanced SE membrane delivered cell-level high gravimetric and volumetric energy densities of 175 Wh kg^-1 and 675 Wh L^-1, individually.

Under the carbon reduction policy, the use of internal combustion engines and fossil fuels has been reduced and electric vehicles and renewable energy have begun to be widely distributed. Batteries are drawing attention as a means of using electric vehicles as electricity and storing renewable energy. However, due to the limitations of the capacity and stability of existing lithium-ion batteries, research is underway on next-generation batteries that can replace the above-mentioned applications.
One of the next-generation batteries that has close performance with existing lithium-ion batteries is sulfide based All Solid State Batteries (ASSBs). In general, the higher the ionic conductivity of used solid electrolytes, the greater the capacity. Sulfide solid electrolyte has relatively higher ionic conductivity than other solid electrolytes. In addition, sulfide solid electrolytes have soft property, making it easy to manufacture batteries with smooth inter-material contacts. It reduce internal resistance of batteries and improves batter performance. For this reason, many developments are underway for sulfide solid electrolytes with high ionic conductivity and soft properties. However, sulfide solid electrolyte is reactive because materials used in sulfide solid electrolyte is P₂S₅ precursor and P-S bond is relatively weak. Therefore the phosphorus in sulfide solid electrolyte react with water, making the electrolyte vulnerable to moisture. Recent studies have been developing chemically stable and high ionic conductive solid electrolytes by replacing or doping the solid electrolyte.
In this study, argyrodite structure sulfide solid electrolyte is synthesized using mechanical milling method. In addition, to improve moisture-weak properties and ionic conductivity, argyrodite sulfide solid electrolytes with other composition are synthesized using antimony and halogen elements. The electrochemical analysis was performed using synthesized solid electrolytes.

The expectations on the solid-state batteries are growing fast. The employment of solid electrolyte instead of liquid not only provides a much better safety, but also enables the use of lithium metal anode, leading to a higher cell energy density [1]. Although it is widely accepted that Li dendrite can penetrate through the solid electrolyte layer causing internal short circuits [2], we have demonstrated that lithium metal anode can be successfully cycled with a proper design of the interfacial layer on the solid electrolyte [3].
High internal resistance is one of the remaining challenges that has to be addressed in the development of solid-state batteries. The ionic conductivity of some sulfide-based solid electrolytes (1 ~ 10 mS/cm) is comparable to that of liquid electrolytes with counting the cation transference number near unity. Nevertheless, the internal resistance of solid-state batteries is considered high due mainly to the interfacial behaviors between solid phases [4, 5]. The charge transfer resistances at the both electrodes and the bulk and grain boundary resistances of the electrolyte are the factors that contribute to the overall cell impedance.
In this work, the internal resistance of a sulfide-based solid-state battery is analyzed component by component. For example, the bulk resistance and the grain boundary resistance for the electrolyte inside the cathode are extracted separately with an AC method employing a transmission line model [6]. In addition, the influences of cell design and process will be discussed.

References
[1] Y. Su et al., Energy Environ. Sci., 2020, 13, 908-916.
[2] E. Kazyak et al., Matter, 2020, 2, 1025-1048.
[3] Y.-G. Lee et al., Nat. Energy, 2020, 5, 299-308.
[4] Y. Xiao et al., Joule, 2019, 3, 1252-1275.
[5] C. Yu et al., Nat. Commun., 2017, 8, 1086.

Li-S batteries have witnessed considerable progress in the past decades, mainly in terms of cycle life. The shuttling process, the main capacity fading mechanism, was addressed by a variety of approaches targeting all of sulfur cathode, separator, and electrolyte. Nevertheless, the main progress was based on ether-based electrolytes with compatibility with Li metal counter electrode. To achieve more competitive energy density, electrolyte amount should be minimized thus satisfying the necessity of lean electrolyte conditions. In this talk, I will introduce recent progress in my lab in identifying high donor electrolytes. High solubility of polysulfides results directly in boosting the volumetric energy density. I will also discuss how to secure the compatibility with Li metal anode so that high donor electrolytes can be adopted in practically viable conditions.

The application of lithium-ion batteries (LIBs) has been expanded from portable electronic devices to battery-driven electric vehicles and grid-scale energy storage systems. Accordingly, considerations about safety and cost issues have become a top priority. Specifically, the safety concerns originating from the use of flammable organic liquid electrolytes and the cost issues related to Li are serious obstacles. In this regard, the replacement of flammable liquid electrolytes with inorganic Na⁺ superionic conductors are considered as an ideal solution. Among various materials for Na⁺-conducting solid electrolytes (SEs), sulfide materials have drawn significant attraction with their high ionic conductivities and mechanical deformability, which enables scalable cold-pressing-based fabrication protocols. Unfortunately, due to the intrinsically narrow electrochemical windows, the sulfide SE materials undergo severe side reactions. They do not exhibit sufficient stability with the 3 V-class oxide cathode materials, such as NaCrO₂.
Recently, through the reinvestigations on halide SEs, Asano and coworkers reported that trigonal Li₃YCl₆ and monoclinic Li₃YBr₆ showed high Li⁺ conductivities of 0.51 and 1.7 mS cm^-1, respectively. More importantly, halide SEs, especially chloride compounds, exhibit the combined advantages of sulfide and oxide SEs: deformability and excellent (electro)chemical oxidation stability. Despite these advances in Li⁺-conducting halide SEs, reports on Na⁺-conducting halide SEs have been scarce.
In this presentation, our recent results on the development of new Na⁺-conducting halide SE, Na₂ZrCl₆, and their application to all-solid-state batteries will be presented.

Li-free cathodes (e.g. FeSx, FeFx) demonstrate high operational voltages and high specific capacities, offering high energy densities for rechargeable lithium metal batteries. However, they suffer from severe capacity decay. Here, in this work, we synthesized a series of transition metal sulfides (M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu). We find that FeSx demonstrates the best cycling stability and rate performances. Its electrochemical reaction mechanism is correspondingly revealed. Pouch cell using FeSx as cathodes demonstrates a specific energy density of higher than 350 Wh kg^-1. This study reveals that Li-free cathodes will receive broad attentions, especially with the mature of Li metal and electrolyte.

An important strategy for beyond Li-ion rechargeable battery chemistries has been to use cheaper earth-abundant and sustainable electroactive materials. In this line of thought, sulfur is an important example as it is available in considerable abundance and is non-toxic. Sulfur delivers a high theoretical capacity of 1672 mAh g^-1, nearly one order higher compared to the best of the intercalation cathodes (IOCs) used in Li/Na-ion batteries. Metal sulfur-based batteries function based on conversion reactions with the possibility of two exchangeable Li-ions per S-atom as compared to approximately one Li-ion/formula unit of IOC. The underlying electrochemistry in a liquid electrolyte-based metal-S battery poses several challenges. The formation of intermediate polysulfides during the reversible conversion of elemental S₈ to metal-sulfide is a major challenge. The majority of the published work focuses on the design of suitable conducting matrices targeted towards high S-loading and efficient entrapment of polysulfides. The initial part of the lecture will discuss a few of our contributions from this perspective for Li/Na/Mg-S batteries. Our work highlights the critical role of materials design and in operando studies for the development of highly stable metal-S batteries (Gomes et al. ACS Sustainable Chem. Eng. 2020; Bhardwaj et al ACS Appl. Materials Interfaces. Eng. 2020; ACS Appl. Energy Mater. 2018). The studies reveal several interesting fundamental insights related to the mechanism of storage which eventually has a strong bearing on the metal-sulfur battery performance. For the success of metal-sulfur batteries at the commercial level, drastic new and alternate strategies are necessary. The subsequent part of the talk focuses on this aspect and discusses alternative configurations of metal-sulfur batteries. An important example in this regard is our recent demonstration of a Li-metal free sulfur battery employing a lithiated anatase TiO₂ anode and free-standing Li₂S-carbon cathode (Bhardwaj et al, 2021).

The increasing demand for advanced rechargeable batteries featuring a high energy density and high electrochemical efficiency has driven the development of new electrode materials with a higher charge-storage capacity than those of traditional intercalation cathodes. In this regard, the conversion lithium-sulfur battery is the most promising inexpensive, high-capacity novel rechargeable battery system because of the low cost, wide availability, low environmental impact, and high energy density of sulfur. However, the fast progress in the lithium-sulfur technology has encountered the inability to develop a high-loading sulfur cathode with a promising electrochemical performance in a lean-electrolyte lithium-sulfur battery. A result of the insulating nature of solid active materials and the diffusion problem of the liquid active materials, the lithium-sulfur literature usually reports performance of devices by utilizing a sulfur loading and sulfur content of less than 4 mg cm^-2 and 60 wt%, respectively, in a cell using an excess amount of liquid electrolyte with a high electrolyte-to-sulfur ratio of over 11–20 μL mg^-1. Unfortunately, this might result in a challenge in the development and investigation of the lean-electrolyte lithium-sulfur cell with high-loading sulfur cathode and with high electrochemical utilization of sulfur.

In our presentation, we present the development of a lean-electrolyte lithium-sulfur with low electrolyte-to-sulfur ratios of 4–7 μL mg^-1. In the lean-electrolyte lithium-sulfur, a high-loading sulfur cathode with a high sulfur content of 68 wt% and a high sulfur loading of 8.64 mg cm^-2 demonstrates high electrochemical utilization, excellent reaction kinetics, and stable cyclability. The cells exhibit discharge capacities of 630–870 mAh g^-1 and 580–631 mAh g^-1 at C/10 and C/5 cycling rates for 200 cycles. The corresponding energy density values attain 11–16 mWh cm^-2. Moreover, we investigate the possible failure mechanism of a lean-electrolyte lithium-sulfur cell with a high-loading sulfur cathode. Our study indicates that the consumption of electrolyte during cycling leads to the slow ion diffusion and the deposition of insulating solid active material on the electrode, which causes the decreased Coulombic efficiency and irreversible capacity loss. Therefore, it would be necessary studies of the lithium-sulfur battery cathode at a slow rate for a long cycle life to guarantee the electrochemical stability and with the Coulombic efficiency to ensure the cyclability and the electrochemical efficiency during long cycles.

In conclusion, our cathode and cell design exhibit an outstanding cathode performance featuring high energy density and long cyclability with a high amount of sulfur at a lean electrolyte condition. Moreover, we explore the challenges that might be encountered by other high-loading cathodes under research at a low electrolyte-to-sulfur ratio, and propose the possible analytical tools and solutions to address the issues found.

Owing to the high energy density, lithium-ion batteries (LIBs) have expanded their applications from mobile electronic devices to electric vehicles and energy storage systems. In terms of high energy density and safety, all-solid-state lithium batteries (ASLBs) are promising next-generation batteries. Ni-rich layered lithium metal oxides, LiMO₂ (M = Co, Ni, Mn, Al), and inorganic solid electrolytes are considered as the crucial components for high-performance ASLBs. Especially, sulfide SEs (e.g., Li₁₀GeP₂S₁₂, Li_5.5PS₄Cl_1.5) that exhibit the maximum Li⁺ conductivity of 10^-2 S cm^-1 and mechanical deformability that enables simple cold-pressing-based fabrication of ASLBs, are expected to achieve commercialization of ASLBs.
However, sulfide SEs suffer from serious interfacial degradation between LiMO₂ and SEs, which originates from their low electrochemical oxidation stability (< 3 V vs. Li/Li⁺). Specifically, the high reactivity of Ni⁴⁺ of LiMO₂ induces high interfacial resistances of cathodes. Despite unsatisfactory ionic conductivity (max. 10^-3 S cm^-1), halide SEs have been newly emerging due to their excellent electrochemical oxidation stability (> 4 V vs. Li/Li⁺) and thus good compatibility with LiMO₂^[1][2].
In this work, we propose a multi-use of sulfide and halide SEs to compromise their drawbacks for the application to LiMO₂ cathodes in ASLBs. The partial replacement of catholytes with Li₃YCl₆ in LiNi_0.88Co_0.11Al_0.01O₂ electrodes using Li₆PS₅Cl, which leads to the improved electrochemical performances, will be presented.
References
[1] Y. Han, S.H. Jung, H. Kwak, S. Jun, H.H. Kwak, J.H. Lee, S.T. Hong, Y.S. Jun Adv. Energy Mater. 2021, 11, 2003190
[2] H. Kwak, D. Han, J. Lyoo, J. Park, S.H. Jung, Y. Han, G. Kwon, H. Kim, S.-T. Hong, K.- W. Nam, Y.S. Jung Adv. Energy Mater. 2021, 11, 2100126

Electrification of vehicles boosted research regarding Li ion batteries (LIBs) with higher energy. However, energy density of conventional LIBs has approached its theoretical limit. Applying alternative electrode such as Li metal anode is one promising strategy. Unfortunately, it is hindered by internal short circuit and severe reaction with liquid electrolyte. All-solid-state Li metal batteries (ASLMBs) using non-flammable inorganic solid electrolytes have thus emerged to resolve the problems related with liquid electrolytes and the safety concerns as well. Especially, sulfide solid electrolytes have been one of the most promising candidates thanks to their high ionic conductivities comparable to that of liquid electrolyte (~ 2.0 × 10^-2 S cm^-1) and mechanically ductile property. However, sulfide SEs also do not fully stabilize Li metal anodes. They are reductively decomposed to form byproducts, such as Li₂S and Li₃P, in contact with Li metal. Moreover, cycling upon high current density leads to the void formation which promotes the dendritic Li growth, which calls for the need to pressurize the ASLMBs. Thus far, in most previous reports, ASLMB cells have been tested using thick Li metal foils (~1 mm).
Herein, we report on our recent results of ASLMB cells employing thin (~20 um) Li metal anodes that are prepared by scalable electroplating process. Furthermore, cycling performances affected by the application of protective layers will be presented.

FeS₂ is a promising conversion cathode material, due to its earth abundance, low toxicity, and high energy density. Despite these advantages, commercialization of rechargeable Li-FeS₂ batteries has been inhibited by rapid capacity fade. One factor believed to play a critical role in the origin of this capacity fade is polysulfide shuttling. However, mechanistic understanding of polysulfide shuttling in Li-FeS₂ batteries is lacking. Herein, we use in-situ UV-vis spectroscopy to study the formation and shuttling of polysulfides in Li-FeS₂ batteries. We find two key results. First, both our chemical and electrochemical lithiation studies show that polysulfides form during the initial lithiation step. This surprising finding is in contrast with the previously proposed mechanism for Li-FeS₂ batteries. Second, polysulfide shuttling decreases by a factor of 2-4x when an ionic liquid electrolyte is used in place of a conventional organic electrolyte. Furthermore, we show that careful choice of the electrolyte anion can lead to nearly complete polysulfide mitigation. Overall, our results provide important insights for understanding and thus prevention of polysulfide shuttling in Li-FeS₂ batteries.