Organic Electrolyte Research Articles

Pyrene-TEMPO-Modified Carbon Electrodes for Organic Radical Lithium-Ion Batteries

Organic radical batteries (ORBs) are a promising alternative to traditional lithium-ion batteries (LIBs) because of their environmentally friendly materials and potential to be low-cost, light weight, and mechanically flexible. Among ORB materials, stable organic radical polymers have great potential as high energy density and high power density cathode materials due to their small molecular weight and fast, reversible redox reactions. A stable radical polymer for LIB applications contains an organic radical unit with redox potentials suitable for the LIB anode or cathode that is immobilized on an electrode via a main chain polymer. Conductive and light-weight polymer main chains are necessary to ensure fast electron transfer to the current collector, without adding significant weight to the cell. Despite extensive research, organic radical materials present some key challenges which limit their present use in electrochemical cells. Many polymer backbones are non-conductive, leading to increased electrode resistance and poor cell performance at high charge/discharge rates. Many organic radical materials also have relatively low thermal stability and low packing density. Moreover, many organic radicals are soluble in typical organic battery electrolytes, resulting in capacity fading.In this research, we apply pyrene as a polymer backbone to 2,2,6,6-tetramethyl-piperidine-1-oxyl radical (TEMPO) for the first time in LIB applications to reduce the solubility of the active organic radical and enable π−π stacking interactions with aromatic carbons. We prepared a surface-immobilized p–TEMPO on fullerene C60 and graphene foam (GF) electrodes via π−π stacking interactions to increase electrode conductivity. Pyrene is used to modify certain carbon allotropes via π−π stacking interactions. It was hypothesized that this approach would enable strong bond formation of the TEMPO radical with the electrode, while providing a non-destructive and simple modification approach of conductive carbon surfaces. The successful conjugation of pyrene and TEMPO molecules was established by NMR spectroscopy. The redox activity of p-TEMPO in LIB electrolytes was validated in solution. The non-covalent attachment of p-TEMPO to fullerene C60 and GF electrodes was established by XPS. The mechanisms and reversibility of the TEMPO redox reaction was then explored for p-TEMPO immobilized on the carbon electrodes. While both electrodes showed diffusion-limited current, and quasi-reversible reactions, C60-p-TEMPO has better reversibility and less resistance than GF-p-TEMPO. C60-p-TEMPO electrodes exhibit exceptional cycling stability over 600 cycles. These results are outstanding as the C60 has a high tendency to dissolve in LIB electrolytes. LIB half-cell testing identified challenges and opportunities for the p-TEMPO-modified electrodes. C60-p-TEMPO discharge capacity at ~5C rate is comparable to PTMA-based literature, however no distinct TEMPO response was observed in the charge or discharge curves. GF-p-TEMPO electrodes showed a TEMPO response in cycle 1 only, suggesting possible p-TEMPO dissolution in the electrolyte. Overall, the results show that the use of a noncovalent immobilization method allows facile preparation of p-TEMPO functionalized electrodes and demonstrates the potential of p-TEMPO noncovalent bonding in organic LIB applications. Figure 1

Gas Additives for Silicon Anode Lithium-Ion Batteries

While many organic liquid and electrolyte salt formulations have been explored to improve the stability of lithium-ion batteries, less research has been done to determine the role of evolved gasses in the performance of commercial Li-ion battery chemistries, let alone novel combinations of gas additives that may affect the solid-electrolyte interphase (SEI) formation and evolution.[1,2] Many gases are generated during Li-ion battery cycling, any of which could have possible beneficial or harmful effects on performance. Currently, only the effect of carbon dioxide (CO2) gas addition to Li-ion batteries with Si anodes has been studied.[3-4] In addition to CO2, ethylene gas (C2H4) is a promising additive for Si anode systems because it is a precursor in polyethylene polymerization reactions, nCH2=CH2 --> [-CH2-CH2-]n. In addition, polyethylene oxide (PEO) has been observed in the Si-anode SEI and contributes to its passivation while maintaining flexibility.[5] Therefore, creating polyethylene and increasing PEO concentration in situ via ethylene gas doping may yield an improved SEI and more-stable battery.This work investigates the potential of gas-phase additives, including CO2 and C2H4, to improve the performance of Si anodes for Li-ion batteries. The pressure decay of the gasses at different starting pressures dissolving in GenF3 electrolyte (1.2 M LiPF6 in 3:7 wt:wt ethylene carbonate to ethyl methyl carbonate + 3 wt% fluoroethylene carbonate) was monitored to determine the saturation concentration of dissolved gas. The experimental pressure decay curve was fit to a model and extrapolated to predict the final pressure at equilibrium.[6] The relationship between partial pressure and concentration of dissolved gas in GenF3 at equilibrium was plotted and a curve was drawn to determine the Henry’s law constant. Varying volumes of gas were injected into battery pouch cells containing Si nanoparticle anodes and LiFePO4 (LFP) cathodes to pressurize them to different pressures. Electrochemical cycling and subsequent multi-phase analyses, including vibrational spectroscopy and X-ray characterization of the SEI surface layer and GC-MS/FID were conducted to determine the impact of gas doping on the capacity, SEI, and gas phase reaction products.

A New Class of Lithium Conductive Polymers Based on Electrochemically Stable Carbon Rings

Among all the various secondary battery technologies, lithium ion batteries (LIBs) have received a significant amount of attention in the last decades and turned into the most successful commercial batteries. LIBs are currently extensively used in portable electronics, electric vehicles, and energy storage stations. In recent years, the elevated requirements for high energy density and increased safety have driven the research to develop the next generation battery technology, and solid state batteries are one of the most potential alternatives for the mainstream LIBs. Polymer based solid state electrolyte (SSE) have been widely studied due to their nature of softness and flexibility that relieve the interface problems between electrodes and separator, especially compared with inorganic solid electrolyte. However, most of the SSEs developed so far show either limited Li+ conductivity or narrow electrochemical windows (< 4 V), which limits the area of applications of these electrolytes significantly. In order to overcome the above limitations, Piersica Inc. developed a new type of polymer (PG17) that can be used as SSEs and applied to the mainstream organic electrolyte based LIBs. The optimized SSE exhibits impressive conductivity (0.1 mS/cm) at room temperature and extremely high oxidized stability (approx. 4.9 V). When assembled using NMC811 cathodes and commercial graphite anodes, both the coin cells and the pouch cells demonstrated excellent capacity and cycling stability. In the near future, the PG17 will be optimized for applications in all solid state batteries.

(Invited) Enabling Next-Generation High-Performing Sodium Metal Batteries

The increasing energy demands of society today have led to pursue alternative energy storage systems that can fulfill rigorous requirements like cost-effectiveness and high storage capacities. Based fundamentally on earth-abundant sodium sources, sodium-based batteries are a promising solution in applications where existing lithium-ion technology remains less economically viable, particularly in large-scale stationary systems such as grid-level storage. Although simply replacing lithium with sodium in current lithium-ion batteries is not a viable solution due to deteriorating electron transfer in conventional intercalation cathodes, sodium is highly attractive when leveraging other battery chemistries due to its unique redox reactions. Sodium metal holds great promise as an attractive anode material for sodium-based batteries owing to its high theoretical specific capacity, low electrochemical potential, low cost and high natural abundance. However, its highly reactive nature with organic electrolyte leads to the formation of unstable solid electrolyte interphase (SEI) and dendritic/mossy sodium growth. In this talk, I will present a series of versatile strategies to realize diverse high-energy sodium metal battery systems through multi-faceted tactical regulations of the electrode-electrolyte interface, electrode structural design, and electrolyte engineering.

Bulk SEI: Investigating the Instability of Li6PS5Cl at the Lithium Metal Anode

Among the multitude of lithium ion conducting materials, thiophosphate-based solid electrolytes (thio-SEs) attract great attention from academia and industry alike. Due to their high ionic conductivities and increased safety, thio-SEs – such as lithium argyrodite Li6PS5Cl – are considered suitable to be employed in solid-state batteries (SSBs). Thus, by substituting liquid organic electrolytes of lithium-ion batteries (LIBs), higher energy and power densities (in combination with lithium metal anodes (LMAs)) as well as simplified cell designs are promoted.[1] Unfortunately, one major drawback of thio-SEs is the inadequate reduction stability in contact with LMAs,[2] which impedes device-level applications and has to be overcome.Due to its thermodynamic instability, Li6PS5Cl is reduced upon contact with highly reactive lithium metal to a mixture of compounds, i.e., Li2S, Li3P, and LiCl. Thus, a so-called solid electrolyte interphase (SEI) is formed, which is predominantly ion-conducting. Its electronically insulating nature is suspected to limit the interphase formation and further growth.[3] Additionally, the chemical and structural properties differ from that of pure SE. Unfortunately, the SEI only evolves at the Li | Li6PS5Cl interface and its thickness is in the nanometer range.[4] Suitable methods to examine the characteristics of SEI layers are therefore scarce and mostly limited to surface sensitive characterization techniques.Here, we employ a novel approach to gain further insights into the nature of SEI layers of thiophosphate-based SE in contact with lithium metal, demonstrated with Li6PS5Cl as model solid electrolyte. Bulk material corresponding to the composition of the SEI is synthesized from SE and Li-metal in a direct manner and hence, examined in respect to different properties. The successful synthesis of bulk-phase SEI is confirmed by X-ray photoelectron spectroscopy measurements, validating the chemical composition. This is complemented by structural analysis. Electrochemical characterization (electrochemical impedance spectroscopy and Wagner-Hebb) is utilized to quantify the ionic and electronic conductivity of the obtained bulk material, respectively, and to further determine transport properties and SEI growth at the Li | Li6PS5Cl interface. The combined results of our studies allow comprehensive understanding of the SEI nature and offer boundary conditions to approximate the nanoscale properties of the SEI at the Li | Li6PS5Cl interface from bulk material synthesis and analysis.

Multifunctional Zeolite Coated Separators for Improved Performance and Safety of Lithium Metal Batteries

Lithium (Li) metal batteries are attractive due to their high gravimetric and volumetric energy densities. However, they can fail catastrophically due to dendritic nucleation, growth, and penetration through the polypropylene (PP) or polyethylene (PE) separators. Poor electrolyte wetting and non-uniform Li ion flux are known to affect Li dendrite formation, especially since the PP/PE separators have broad gaussian pore size distribution and typically organic electrolytes do not wet them well. In this work, we demonstrate that a multifunctional zeolite coating on a commercial PP separator (Z-PP) can improve electrolyte wettability and in-plane ionic conductivity, giving rise to more uniform Li flux. Consequently, Z-PP can effectively delay dendrite penetration and enhance cell performance and safety. Firstly, electrochemical impedance spectroscopy (EIS) was used to establish that the thin zeolite coating on PP separator had little effect on through-plane ionic conductivity. For electrochemical cycling behavior, PP and Z-PP separator based symmetric cells and half-cells were studied by monitoring the cell overpotential (symmetric cell) and discharge capacity (half-cell). For the Z-PP separator, lower overpotential and better discharge capacity retention were observed in the respective symmetric and half-cell studies, suggesting improved electrolyte wetting and relatively uniform Li flux. Scanning electron microscopy (SEM) equipped with energy dispersive x-ray spectroscopy (EDS) studies helped in understanding coating microstructure and cycled electrode morphologies. SEM images of cycled Li metal harvested from the Z-PP cells show much smoother Li surface for Z-PP cell as compared to PP cells. This work suggests that overall cell performance and safety can be improved by employing Z-PP separators.

High-Performance K Metal Batteries Enabled by Regulating an Electrolyte Solvation Structure

Batteries using a potassium metal (K-metal) anode are attention to a new type of low-cost and high-energy storage devices. However, the thermodynamic instability of the K-metal anode in organic electrolyte solutions causes uncontrolled growth of dendrites and parasitic reactions, which leads rapid capacity loss and low Coulombic efficiency.[1-4] In this presentation, I will introduce the advanced electrolyte using dual-salt for regulated solvation chemistry showing an improved the K-metal anode stability. Experimental and theoretical studies supported the details of solvation chemistry and results. Incorporating dual-salt into the electrolyte solution decreased the number of free solvent molecules surrounding the K+ leading to facile K+ desolvation as well as lowering the exchange current density improving the uniform K electrodeposition. Moreover, the modified electrolyte mitigates decomposition of the solvent molecules and leads to form enhanced mechanical rigidity of the SEI layer on the surface of the K–metal electrode with larger fraction of KF compound. Compared to previous studies, the K-metal anode using modified electrolyte shows long cycling stability at fast charge-discharge capability in symmetrical K | K cells. Even under high current density, the modified electrolyte ensures high areal capacity and an unprecedented lifespan with stable Coulombic efficiency for the K-metal full battery employing a sulfurized polyacrylonitrile cathode. M. Park, Y. Jeong, M. H. Alfaruqi, Y. Liu, X. Xu, S. Xiong, M.–G. Jung, H.–G. Jung, J. Kim, J.–Y. Hwang, Y.–K. Sun. Stable Solid Electrolyte Interphase for Long–Life Potassium Metal Batteries, ACS Energy Lett. 7 (2022) 401–409.Kim, W. Song, D.–Y. Son, L. K. One, Y. Qi. Lithium–Ion Batteries: Outlook on Present, Future, and Hybridized Technologies. J. Mater. Chem. A 7 (2019) 2942–2964.–H. Kim, G.–T. Park, B.–K. Son, G. W. Nam, J. Liu, L.–Y. Kuo, P. Kaghazchi, C. S. Yoon, Y.–K. Sun. Heuristic Solution for Achieving Long–Term Cycle Stability for Ni–Rich Layered Cathodes at Full Depth of Discharge, Nat. Energy 5 (2020) 860.–Y. Hwang, S.–T. Myung, Y.–K. Sun. Recent Progress in Rechargeable Potassim Batteries, Adv. Funct. Mater. 28 (2018) 1802938.

Rethinking the Role of Formerly Sub-Sufficient Industrial/Synthesized SEI Additive Compounds - a New Perspective

In order to improve the performance of lithium-ion batteries (LIBs), novel electrolytes are of primary importance. Recently, fluorinated cyclic phosphazene derivatives in combination with fluoroethylene carbonate (FEC) are mentioned in the literature as a promising electrolyte additive combination, which can decompose to form a dense, uniform, and thin protective layer on the surface of the anode and cathode electrode.[1,2] Additionally, suppressing further electrolyte decomposition and electrode corrosion, thus protecting the structural destruction of the electrodes, are mentioned within this electrolyte composition.[1–3] Furthermore, galvanostatic charge and discharge experiments with different cell composition materials demonstrate that fluorinated cyclic phosphazene compounds as additional additive material tend to improve cycling stability.[1,3,4] Although the electrochemical aspects of cyclic fluorinated phosphazene compounds combined with FEC are briefly introduced, it is still not fully clear how these two compound classes interact constructively during operation mode. Thus, the positive synergistic effect of FEC/Hexafluorocyclotriphosphazene (HFPN)-derivatives on the electrochemical performance during cell operation is not enlightened. The focus of this study is to investigate the complementary effect of FEC and ethoxy(pentafluoro)cyclotriphosphazene (EtPFPN) as additive compounds in an aprotic organic electrolyte in LiNi0.5Co0.2Mn0.3O (NCM523) SiOx/C full cells. Furthermore, the formation mechanism of lithium ethyl methyl carbonate (LEMC)-EtPFPN interfacial products and the reaction mechanism of lithium alkoxide with EtPFPN are proposed and supported with DFT measurements. Additionally, a new effect of FEC regarding the SEI formation will be introduced. The EtPFPN decomposition compounds in the electrolyte after the SEI formation have been investigated via gas chromatography-mass spectrometry (GC-MS) and gas chromatography-high resolution mass spectrometry (GC-HRMS). The electrode electrolyte interface investigation of the SEI has been performed via in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and scanning electron microscopy (SEM). Constant current cycling is conducted, and in-situ Raman measurements characterize the deposition of electrolyte components and LEMC-EtPFPN traces on the SiOx/C anode material during the SEI formation. Finally, the interplay between EC, EMC, Li-alkoxide, LEMC, FEC, and EtPFPN has been visualized schematically via a reaction mechanism postulated based on analytical data of the electrolyte.[1] A. Ghaur, C. Peschel, I. Dienwiebel, L. Haneke, L. Du, L. Profanter, A. Gomez‐Martin, M. Winter, S. Nowak, T. Placke, Adv Energy Mater 2023, 2203503.[2] J. Liu, X. Song, L. Zhou, S. Wang, W. Song, W. Liu, H. Long, L. Zhou, H. Wu, C. Feng, Z. Guo, Nano Energy 2018, 46, 404–414.[3] Q. Liu, Z. Chen, Y. Liu, Y. Hong, W. Wang, J. Wang, B. Zhao, Y. Xu, J. Wang, X. Fan, L. Li, H. bin Wu, Energy Storage Mater 2021, 37, 521–529.[4] Y.-H. Liu, M. Okano, T. Mukai, K. Inoue, M. Yanagida, T. Sakai, J Power Sources 2016, 304, 9–14.

Exploring Surfactant Electrolyte-Porous Electrode Interactions

Microemulsions are an alternative to aqueous organic electrolytes for redox flow batteries (RFB) that can be modified without synthesis. Water, oil, and emulsifiers, when combined using optimized chemistries and compositions, produce stable mixtures with aggregate structures (microemulsions). Design limitations imposed by the interdependency of redox solubility and electrolyte conductivity are circumvented by solubilizing redox active molecules in the oil phase and supporting ions in the aqueous phase. While macroscopically homogeneous, microemulsions are biphasic at the nanometer scale, effectively decoupling solubility from conductivity. Recently, the first microemulsion RFB was demonstrated using ferrocene solubilized in an anionic surfactant system. Polarization curve analysis revealed performance limitations seemingly arising from electrolyte interactions with the cell components including porous electrodes. Further optimization requires a deeper understanding of microemulsion-porous electrode interactions.Due to the complexity of microemulsion electrochemistry, a first step towards understanding these interactions is made by investigating the relationship between surfactants and porous electrodes. Vanadium redox flow battery (VRFB) electrolytes with surfactant additives are used as a model system to explore fundamental interactions, in situ, between a surfactant-containing electrolyte and a porous electrode. Transport and kinetics are evaluated as a function of electrode, electrolyte, and surfactant properties. Redox species transport is quantified through mass transfer coefficients, using a generalization of an existing porous electrode model fit to experimental polarization curve data. Electrochemical impedance spectroscopy, interpreted using existing kinetic models, is also employed to gain insight into the effects of surfactant on electron transfer rates.

Usage of Co-Solvent for Fabricating the Composite Cathode for Sulfide-Based All-Solid-State Lithium Batteries

In recent decades, lithium-ion batteries (LIBs) have been widely used in electric vehicles and large-scale energy storage systems. However, current LIBs using the flammable organic electrolytes have a safety issue. Therefore, all-solid-state lithium batteries (ASSLBs) with enhanced safety and high energy density are in the spotlight. Among many solid electrolytes, sulfide-based solid electrolytes with high ion conductivity and ductile property are the most promising. In order to develop the ASSLBs, the composite cathode with high mass loading is imperative, which can be fabricated by the wet process suitable for mass production. Polar solvents can disperse a binder with polar groups well, but it may cause severe dissolution of sulfide solid electrolytes in the composite cathode. Therefore, we tried to use co-solvent that can disperse a polymer binder well and prevent the dissolution of sulfide solid electrolyte. Cycling performance of ASSLBs employing the wet-type composite cathode was evaluated and will be reported.

High Energy Density Porous RuO2 micro-Supercapacitors Using Protic Ionic Liquid Electrolytes

The rapid development of the Internet of Things (IoT) demands reliable and long-term energy supply to microelectronic devices distributed over the network with high power performance and less maintenance required.[1] Micro-supercapacitors (MSCs) have come to the foreground as miniaturized energy storage devices showing outstanding power density and long cycle life. However, the low cell voltage and low energy density remain major bottleneck that prevents their adoption in real device applications. To this end, several studies have been devoted to the engineering of MSC electrode materials and structural architecting of current collectors within the limited available footprint. This approach could offer opportunities to enhance the electrochemically active surface and mass loading of active materials with fast ion diffusion kinetics, leading to high areal energy density performance.[2] Although several efforts have been put forth in this direction, the complex synthesis route, unfavourable interfacial and mechanical stability of the electrode, electrolyte compatibility issues, etc. remain an arduous challenge.[3] The low cell voltage is another major issue preventing from achieving high energy density values in MSCs as it is directly linked to the electrochemical stability window (ESW) of the electrolytes used.[4] In addition, liquid-state electrolytes currently employed are inappropriate for the microfabrication route as it is prone to evaporation, leakage, and potential safety issues. Hence, a lot of research attention has been given to developing solid-state electrolytes able to afford large operational windows and help promote the application of on-chip MSCs. In this work, we have demonstrated the use of protic ionic liquid (PIL)-based electrolytes able to provide pseudocapacitance in hydrous ruthenium dioxide (RuO2) electrodeposited on interdigitated MSC substrates with extended operational cell voltage. As a pseudocapacitive material, RuO2 exhibits excellent conductivity, high electrochemical reversibility, and cycling stability.[5] On the other hand, as room temperature molten salts, PILs help to overcome evaporation and encapsulation problems associated with the conventional aqueous electrolytes and flammability and safety issues linked to common organic electrolytes.[6-9] We explored pyrrolidinium-based PILs with varying alkyl substitutions, their structure-property, and electrochemical studies for RuO2 MSCs. To further expand the use of these PILs in real devices, 3D MSCs with higher active material mass loading was realised using interdigitated porous Au current collector substrates. The PIL-based porous RuO2 MSCs showed superior charge storage and higher energy density performance as compared to conventional aqueous electrolytes. To envision the practical application of RuO2 MSCs and their subsequent integration with microelectronic devices, ionogel-based solid-state electrolytes were developed. This work opens pathways to develop micro-supercapacitors exhibiting high energy and power density by combining pseudocapacitive metal oxide-based active materials and protic ionic-liquid-based non-aqueous electrolytes and help their integration with on-chip IoT devices.

Enhanced Electrochemical Performance of Lithium Metal Batteries with Fluorine Doped SO2 Based Nonflammable Inorganic Electrolytes

With the demands for high energy density of lithium ion batteries(LIBs), replacing conventional anode of LIBs with lithium metal provide a great opportunity to overcome the limits of batteries because of their high-energy-density. However, utilizing lithium metal in battery system has problems like uncontrollable lithium dendrite growth during lithium plating/stripping and unstable solid electrolyte interface(SEI) on lithium metal which can cause short circuit of batteries. Fluorine is one of the well-known components to stabilize SEI layer in the battery system using commercial organic electrolytes. In this presentation, we introduce lithium metal batteries(LMBs) based fluorine doped LiAlCl4-xFx-3SO2 nonflammable inorganic liquid electrolyte to achieve stable SEI and uniform deposition of lithium. Compare to non-doped LiAlCl4-3SO2 electrolyte, the fluorine doped SO2 based inorganic electrolyte show much alleviated overpotential during cycling. We will show how fluorine doping in electrolytes impact the electrochemical performance of LMBs with SO2 based inorganic liquid electrolytes. Detailed reaction mechanism of our battery system with fluorine doped LiAlCl4-xFx-3SO2 nonflammable inorganic liquid electrolyte will be discussed in the presentation.

Exploring Unconventional Electrolytes to Tackle the Problem of Shuttling in Precipitation-Dissolution Battery Chemistries

A1-Triple I: Any galvanic cell functions by compartmentalizing, via the electron-blocking electrolyte, an exothermic chemical reaction into two redox half-reactions to force release of the reaction free energy as electrical energy. The conventional electrolyte is an ion-conductor shuttling the operative ions and preventing any other chemical exchange to maintain the integrity and reversibility of the electrodes. In overcoming the capacity ceiling of layered intercalation hosts, conversion reactions between low-cost chalcogens (sulfur, selenium, and oxygen) and alkali metals (lithium and sodium) have been investigated for decades. However, parasitic crossover of soluble polychalcogenides limit battery cycle life. We conceptualize that selective solute transport at the interface of partially miscible solvents can be utilized in designing organic electrolytes to suppress parasitic crossover in precipitation-dissolution battery chemistries. This talk will discuss successful concept-demonstration experiments and highlight intriguing physical phenomena within such electrolyte system under the dynamics of electrochemistry.

Operando Hard X-Ray Photoelectron Spectroscopy for Observing Interfacial Electrochemical Reactions in All-Solid-State Batteries

All-solid-state lithium-ion batteries (ASS-LIBs) have been attracting wide attention for their potential superior performance, such as high safety and long cycle life compared with the conventional lithium-ion batteries with liquid organic electrolytes. However, one of the critical problems of the ASS-LIBs for wide spread applications, especially for electric vehicles, is the large interfacial resistance across the electrode/solid electrolyte (SE)1. When an electrode and SE come into contact, some mobile ions are redistributed at the interface according to their potential difference2,3. In addition, the distribution of the mobile ions and electric potential are also changed during the battery operations. Therefore, dynamic observation of their distribution across the electrode/SE interface is essential to identify and control the origin of the interfacial resistance.Hard x-ray photoelectron spectroscopy (HAXPES) is capable of nondestructive analysis of the elemental composition, oxidation states and electronic structure of sample surfaces and interfaces including those buried with over layers at depth > 10 nm[4].In this study, we developed a laboratory-based operando HAXPES system equipped with a Cr-Kα source (5414.9 eV), which enables angle-resolved measurements of and bias applications to the sample transferred without air-exposure, and applied it to the charging/discharging reactions of a LiCoO2 (LCO) positive electrode deposited on a Li6.6La3Zr1.6Ta0.4O12 (LLZT) solid electrolyte in a half cell configuration. HAXPES measurements were performed while the LCO electrode was electrically grounded to a hemispherical analyzer.The galvanostatic charge/discharge potential profiles exhibited a characteristic plateau due to delithiation of a O3-LiCoO2 at ~3.9 V vs. Li+/Li5. In the Co 2p 3/2 region of the pristine cell, a sharp intense main peak and a broad satellite peak typical to a LCO were observed. At the charged states, however, the main peak was asymmetrically broadened to the higher binding energy. The main peak was deconvoluted into two peaks at 780 eV and ~781 eV, corresponding to Co3+ ions and Co4+ ions, respectively. The full width half maximums (FWHMs) of the Co4+ ions peak increased at charged states, while that of the Co3+ ions peak was remained unchanged. Furthermore, the relative areas of satellite peak decreased at the charged states. These results suggest that Co3+ ions were oxidized to the Co4+ ions by delithiation of the LCO electrode6,7. After the subsequent discharging, the component of higher binding energy of the main peaks decreased, and the satellite peak increased. In addition, the FWHMs of the Co4+ ions peak decreased, suggesting that the Co4+ ions were reduced by lithiation into the LCO electrode during discharging. Thus, the lithiation/delithiation of LCO was successfully observed by operando HAXPES.References N. Ohta, K. Takada, L. Zhang, R. Ma, M. Osada and T. Sasaki, Advanced Materials, 18, 2226 (2006) K. Yamamoto, Y. Iriyama, T. Asaka, T. Hirayama, H. Fujita, C. A. Fisher, K. Nonaka, Y. Sugita and Z. Ogumi, Angew Chem Int Ed Engl, 49, 4414 (2010) J. Haruyama, K. Sodeyama, L. Han, K. Takada and Y. Tateyama, Chemistry of Materials, 26, 4248 (2014) N. K. F. Curran Kalha, Prajna Bhatt, et al., Journal of Physics: Condensed Matter, 33, 233001 (2021) J. N. Reimers and J. R. Dahn, The Electrochemical Society, 139, 2091 (1992) L. Dahéron, R. Dedryvère, H. Martinez, M. Ménétrier, C. Denage, C. Delmas and D. Gonbeau, Chemistry of Materials (2008) H. Kiuchi, K. Hikima, K. Shimizu, R. Kanno, F. Toshiharu and E. Matsubara, Electrochemistry Communications, 118, 106790 (2020)

Combining electrochemical, molecular simulation and operando techniques to investigate the stability of electrodes and organic electrolytes used in EDLCs

A comprehensive study involving Electrochemical, Mass Spectrometry, Raman & FT-IR operando analyses, and Molecular Dynamics Simulations is present to evaluate the stability of electrodes and electrolytes used in organic-based EDLCs. In this sense, we present some directions to correctly determine the maximum stable cell voltage, specific capacitance, energy, and power, as well as the equivalent series resistance. Multi-walled carbon nanotubes and activated carbon electrodes are contrasting in symmetric devices filled with 1.0 M LiTFSI and TEABF4 salts dissolved in AN, PC, and EC:DMC electrolyte. We observed anomalous transport caused by adsorption effects in the electrical double-layer in all electrochemical studies due to the defective nature of the ionic channels in the presence of displacement of finite-sized charge carriers. This behavior is unequivocally verified from the impedance analysis using a double-channel transmission line model representing the macrohomogenous behavior of the electrochemical system. The residual gas analysis suggested a maximum stable voltage of 3 V to avoid electrolyte degradation. The carbon-based electrodes remain stable in all cases, as evidenced by the Raman & FT-IR operando studies. We verified that the AC/TEABF4-PC/AC system is auspicious for further studies aiming to develop EDLC prototypes with high specific capacitance, energy, and power characteristics. The derivative analysis method involving the galvanostatic findings permitted us to verify the different dynamics regarding the charge-storage process in macro and microporous structures as a function of the applied current. Computational simulations indicated that [TEA]+ has a high affinity for the carbon electrodes, which is related to the higher electrochemical stability of TEABF4-PC.

Multicomponent Covalent Organic Framework Solid Electrolyte Allowing Effective Li-Ion Dissociation and Diffusion for All-Solid-State Batteries.

Organic solid electrolytes compatible with all-solid-state Li metal batteries (LMBs) are essential to ensuring battery safety, high energy density, and long-term cycling performance. However, it remains a challenge to develop an approach to provide organic solid electrolytes with capabilities for the facile dissociation of strong Li-ion pairs and fast transport of ionic components. Herein, a diethylene glycol-modified pyridinium covalent organic framework (DEG-PMCOF) with a well-defined periodic structure is prepared as a multicomponent solid electrolyte with a cationic moiety of high polarity, an additional flexible ion-transporter, and an ordered ionic channel for all-solid-state LMBs. The DEG-containing pyridinium groups of DEG-PMCOF allow a lower dissociation energy of Li salts and a smaller energy barrier of Li-ion transport, leading to high ion conductivity (1.71 × 10-4 S cm-1) and a large Li-ion transfer number (0.61) at room temperature in the solid electrolyte. The DEG-PMCOF solid electrolyte exhibits a wide electrochemical stability window and effectively suppresses the formation of Li dendrites and dead Li in all-solid-state LMBs. Molecular dynamics and density functional theory simulations provide insights into the mechanisms for the enhanced Li-ion transport driven by the integrated diffusion process based on hopping motion, vehicle motion, and free diffusion of DEG-PMCOF. The all-solid-state LMB assembled with a DEG-PMCOF solid electrolyte displays a high specific capacity with a retention of 99% and an outstanding Coulombic efficiency of 99% at various C-rates during long-term cycling. This DEG-PMCOF approach can offer an effective route to design various solid-state Li batteries.

Electrochemical Li+ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces.

Redox reactions of the Li+ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li+. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li+/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li+ adsorption/desorption at the LiPON/Gr interface. Moreover, Li+ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li+. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.

Recent Advances of LATP and Their NASICON Structure as a Solid‐State Electrolyte for Lithium‐Ion Batteries

The organic electrolyte in commercial liquid lithium‐ion batteries is volatile, prone to low‐temperature failure, has a declining safety performance at high temperatures, and is susceptible to serious side reactions with electrodes. The current research hotspots are solid‐state electrolytes with high energy densities and high safety performance. The next‐generation lithium metal solid‐state battery electrolyte is expected to be Li1+xAlxTi2−x(PO4)3 (LATP) with a sodium superionic conductor structure due to its high ionic conductivity, high energy density, and good stability in air. In this article, a review of the crystal structure of LATP, lithium‐ion diffusion channels, synthesis methods, factors affecting high ionic conductivity, and regulation and application of interfacial stability is presented. This effectively addresses the problems of LATP in current applications and facilitates the promotion of all‐solid‐state batteries in future applications.

Engineering Cathode-Electrolyte Interface of High-Voltage Spinel LiNi0.5Mn1.5O4 via Halide Solid-State Electrolyte Coating.

The high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode material with high energy density, low cost, and excellent rate capability has grabbed the attention of the field. However, a high-voltage platform at 4.7 V causes severe oxidative side reactions when in contact with the organic electrolyte, leading to poor electrochemical performance. Furthermore, the contact between the liquid electrolyte and LNMO leads to Mn dissolution during cycles. In this work, we applied the sol-gel method to prepare Li3InCl6-coated LNMO (LIC@LNMO) to address the mentioned problems of LNMO. By introducing a protective layer of halide-type solid-state electrolyte on LNMO, we can prevent direct contact between LNMO and electrolyte while maintaining good ionic conductivity. Thus, we could demonstrate that 5 wt % LIC@LNMO exhibited a good cycle performance with a Coulombic efficiency of 99% and a capacity retention of 80% after the 230th cycle at the 230th cycle at 1C at room temperature.

Enhancing gravimetric and volumetric capacitance in supercapacitors with nanostructured partially graphitic activated carbon

The volumetric and gravimetric capacitance of supercapacitors are primarily influenced by the specific surface area and porosity required for the formation of electric double layers in the electrode material, and these capacitance are known to have a trade-off relationship. To address this issue, additional capacitance enhancement mechanisms are needed not only at the electrode material surface but also within the limited mass and volume, which can be achieved through structural improvements of the material. In this study, we produce partially graphitic activated carbon with low specific surface area but nano-sized crystalline regions from petroleum pitch, a soft carbon precursor. This approach allows for improvements in gravimetric capacitance. The resulting activated carbon with a specific surface area of 1122 m2 g−1 exhibits volumetric specific capacitance of 27.9 F cc−1 and gravimetric specific capacitance of 38 F g−1 in an organic electrolyte, indicating a new direction for electrode materials in supercapacitors.