Ionic Transference Number Research Articles

Challenges and Strategies for Zinc Metal Anode

Aqueous zinc‐ion batteries (AZIBs) are received intensive attention for their inherent safety, economic feasibility, competitive electrochemical performance, and environmental friendliness. However, the rampant side reactions such as dendrite growth, hydrogen evolution, corrosion, and passivation of zinc metal anode seriously hinder the stability of AZIBs. This concept is aim to enhancing the practical viability of AZIBs, with a particular focus on the stability of zinc metal anodes. Here, we summeriazed the strategies for achieving high reversibility zinc metal anode, and points out the shortcomings that still exist in current research. Then the crucial role of regulation electric double layer (EDL) layer, zinc ion transfer number and electrolyte design is summerized in detail. Finally, we consider that the design of zinc anode should focus more on practicality and commercialization, and provide a perspective for the future development direction of zinc anode. We hope to provide an inspiration for the design of stable zinc metal anodes in AZIBs from this concept.

Abnormal Freezing Effect on Electron Conduction of Ball‐milled Lanthanum Trihydride for Superionic Conductor

AbstractA recent finding shows that lattice deformation could transform the mixed electronic/hydride (H−) conducting lanthanum trihydride (LaH3) to a superionic H− conductor. Such a feature would enable the development of a brand‐new all‐solid‐state hydride ion battery. It is essential to elucidate the mechanism of such a phenomenon. Here, we disclose an abnormal freezing effect on the electronic conductivity (σe) of the ball‐milled LaH3; that is, σe can be lowered by over 2 orders of magnitude upon a low‐temperature treatment. Low‐temperature Raman reveals that at low temperatures, lattice deformation has a noticeable influence on the interaction of hydrogen at octahedral (Ho) and tetrahedral (Ht) sites, which may play a crucial role in hindering electron conduction. This freezing effect can significantly improve the ionic transfer number of rare earth‐based H− conductors and provide a new direction to the development of solid electrolytes for low‐temperature all‐solid‐state batteries.

Abnormal Freezing Effect on Electron Conduction of Ball-milled Lanthanum Trihydride for Superionic Conductor.

A recent finding shows that lattice deformation could transform the mixed electronic/hydride (H-) conducting lanthanum trihydride (LaH3) to a superionic H- conductor. Such a feature would enable the development of a brand-new all-solid-state hydride ion battery. It is essential to elucidate the mechanism of such a phenomenon. Here, we disclose an abnormal freezing effect on the electronic conductivity (σe) of the ball-milled LaH3; that is, σe can be lowered by over 2 orders of magnitude upon a low-temperature treatment. Low-temperature Raman reveals that at low temperatures, lattice deformation has a noticeable influence on the interaction of hydrogen at octahedral (Ho) and tetrahedral (Ht) sites, which may play a crucial role in hindering electron conduction. This freezing effect can significantly improve the ionic transfer number of rare earth-based H- conductors and provide a new direction to the development of solid electrolytes for low-temperature all-solid-state batteries.

Realization of Hydrogel Electrolytes with High Thermoelectric Properties: Utilization of the Hofmeister Effect.

Ionic thermoelectric materials, renowned for their high Seebeck coefficients, are gaining prominence for their potential in harvesting low-grade waste heat. However, the theoretical underpinnings for enhancing the performance of these materials remain underexplored. In this study, the Hoffmeister effect was leveraged to augment the thermoelectric properties of hydrogel-based ionic thermoelectric materials. A series of PAAm-x Zn(CF3SO3)2, PAAm-x ZnSO4, and PAAm-x Zn(ClO4)2 hydrogels were synthesized, using polyacrylamide (PAAm) as the matrix and three distinct zinc salts with varying anion volumes to impart the Hoffmeister effect. Exceptionally, the most cost-effective ZnSO4 yielded the highest ionic Seebeck coefficient among the hydrogels, with PAAm-1 ZnSO4 achieving a remarkable value of -3.72 mV K-1. To elucidate the underlying mechanism, we conducted an innovative analysis correlating the Seebeck coefficient with the zinc ion transfer number. Additionally, the hydrogel materials demonstrated outstanding mechanical properties, including high elongation at break (>1400% at its peak), exceptional resilience (virtually no hysteresis loops), and robust fatigue resistance (overlapping cyclic tensile curves). This work not only advances the understanding of ionic thermoelectric materials but also showcases the potential of hydrogels for practical waste heat recovery applications.

Flexible and Compact PVDF/PMMA-Based Gel Polymer Electrolytes for High-Performance Sodium Metal Batteries.

Sodium is gaining recognition as a promising alternative to lithium for battery applications, particularly in sodium metal batteries (SMBs), where the performance is critically influenced by the choice of electrolyte. Although conventional organic liquid electrolytes are widely used, they pose significant issues. Gel membrane (GM)-based electrolytes have demonstrated enhanced reliability and stability. Nevertheless, there remains a pressing need to improve their electrochemical and mechanical properties to fulfill the demands of next-generation batteries, which require extended lifespan, rapid charging capabilities, and excellent flexibility. To address these challenges, a compact and flexible GM is developed by gelating a cast Polyvinylidene fluoride (PVDF)/Polymethyl methacrylate (PMMA) blend film. The resulting GM possesses a suitable sodium ionic conductivity of 0.507 mS cm-1 and an impressive ion transference number of 0.47, enabling dendrite-free reversible sodium plating and stripping for up to 300 h at a current density of 0.8 mA cm-2. Furthermore, the effectiveness of sodium dendrite suppression is demonstrated in Na/GM/Na3V2(PO4)3 cells, which exhibit a high discharge capacity and excellent cycling stability. The flexible SMB retains stable voltage output and continues to function even when bent or twisted. This study introduces a novel strategy for creating gel electrolytes for high-performance flexible SMBs.

Single ion conducting gel polymer electrolytes containing polybenzimidazole-based ionomers with abundant Li+ transport channels for dendrite-free lithium metal batteries

Single ion conducting polymer electrolytes (SIPEs) have attracted much attention due to their ability to effectively inhibit the growth of lithium dendrites, but still confront the problems of low ionic conductivity and poor mechanical properties. In this study, a new polybenzimidazole-based ionomer (M-OPBI) is synthesized by chemically modifying aromatic poly(2,2′-(p-oxydiphenylene)-5,5′-bibenzimidazole) (OPBI), and two series of novel SIPEs (i.e. M-OPBI/PEO and M-OPBI/PVDF) are achieved after blending with different polymer matrices (i.e. poly(ethylene oxide) (PEO) and poly(vinylidene fluoride) (PVDF)). Afterwards, some single ion conducting gel polymer electrolytes (SIGPEs) with abundant Li+ transport channels are fabricated by absorbing appropriate amount of plasticizers. The imidazole anions and sulfonylimide anions in the ionomer present low binding energies with Li+, which is beneficial for the dissociation and transport of Li+. Moreover, in view of the presence of rigid aromatic structure in the main chain of the ionomer and the interactions between the polymers, polybenzimidazole-based SIPEs all exhibit excellent thermal and mechanical properties. Most importantly, in comparison with PVDF matrix, PEO is approved to be more compatible with the ionomer and provide additional Li+ conduction sites, and this will enhance the interfacial compatibility and facilitate the construction of abundant Li+ transport channels in M-OPBI/PEO membranes. As a result, M-OPBI/PEO-0.4 membrane shows the lowest activation energy of 0.098 eV, the highest room-temperature ionic conductivity of 9.9 × 10-5 S cM-1, and a satisfactory lithium ion transference number of 0.88. Thanks to these merits, the symmetric Li||Li cell assembled with M-OPBI/PEO-0.4 membrane exhibits an excellent reversible lithium plating/stripping performance without short circuit over 2160 h. This study provides a new design strategy for the development of SIGPEs for application in dendrite-free lithium metal batteries.

A Solid Electrolyte Based on Sodium-Doped Li4-xNaxTi5O12 with PVDF for Solid State Lithium Metal Battery.

Solid-state batteries (SSBs) present a potential pathway for advancing next-generation lithium batteries, characterized by exceptional energy density and enhanced safety performance. Solid-state electrolytes have been extensively researched, yet an affordable option with outstanding electrochemical performance is still lacking. In this work, Li4-xNaxTi5O12 (LNTO)-based composite solid electrolytes (CSEs) were developed to enhance the interface stability and electronic insulation. The CSE is composed of Li3.88Na0.12Ti5O12 (LNTO3) and poly (vinylidene fluoride) (PVDF) with a proportion of 20 wt % exhibited high ionic conductivity (4.49×10-4 S cm-1 at a temperature value equal to 35 °C), high ionic transfer number (equal to 0.72), low activation energy (equal to 0.192 eV), and favorable compatibility with the Li metal anode. The Li|LNTO3|LiFePO4 cell, tested at a 0.5 C current density, demonstrated 154.5 mAh g-1 of outstanding cycling stability for 200 cycles, capacity retention of 97.6 % along with a Coulombic efficiency of over 99 %, as well as a significant average specific capacity of 127.8 mAh g-1 over 400 cycles at 5 C. The Li|LNTO3|LiNi0.8Co0.1Mn0.1O2 (NCM811) cell could also operate over 100 cycles at 1 C. This study offers an effective method for preparing commercial CSEs for SSBs.

Biocompatible hydrogel electrolyte with high ionic conductivity and transference number towards dendrite-free Zn anodes

Biocompatible hydrogel electrolyte with high ionic conductivity and transference number towards dendrite-free Zn anodes

Li-ion batteries with poly[poly(ethylene glycol) methyl ether methacrylate]-grafted oxidized starch solid and gel polymer electrolytes

Polymer electrolytes are considered in lithium-ion batteries because of their high safety and properties such as flexibility, easy moldability, etc. Starch is one of these polymers from renewable resources. Considering the semi-crystal structure of starch and ion conduction in amorphous phase, herein starch is oxidized and then modified with poly[poly(ethylene glycol) methyl ether methacrylate]. Solid polymer electrolytes (SPEs) are prepared by dissolution of lithium salt within polymer while gel polymer electrolytes (GPEs) as crosslinked structures are swollen in lithium salt solution. After validation of successful syntheses, all SPEs and GPEs with different oxidation state and various PEGMA/oxidized starch are evaluated in Li-ion battery performance. The synthesized GPEs and SPEs show the highest ionic conductivity of 5.5 × 10−3 and 2.19 × 10⁻⁴ S cm⁻1, respectively at room temperature. Lithium ion transfer number (t+) of 0.6–0.9 and electrochemical stability window of 4.4–4.9 V are obtained for SPEs and GPEs. The discharge capacity is ∼180 mAh g−1 at 0.2 C with capacity retention of 75 % after 100 cycles.

A PET-enhanced PEO-ionic liquid-based gel polymer electrolyte for solid state lithium metal batteries

Gel polymer electrolytes (GPEs) are attractive for their promising prospect in lithium metal batteries (LMBs) owing to their outstanding thermal stability and remarkable electrochemical performance. Nevertheless, the practical application of traditional GPEs is significantly impeded by their limited mechanical strength. In order to address this issue, this study introduces an ultrathin and mechanically-strong poly(ethylene terephthalate) (PET) nonwoven fabric (PET-NW) as a rigid support layer for the GPEs composed of poly (ethylene oxide) (PEO) and ionic liquid (IL) EMIM-TFSI. The resulting PEO-IL/PET-NW-based GPE (NW GPE) demonstrates an excellent lithium ionic conductivity of 4.21 × 10−4 S cm−1, a lithium ion transference number of 0.20, and an ultrahigh mechanical strength of 53.2 MPa. These characteristics collectively contribute to the suppression of lithium dendrites growth and improved electrochemical performance of Li|NW GPE|LiFePO4 full cells which exhibit enhanced rate capability and cycling stability in comparison with Li|GPE-I30|LiFePO4 full cells. The employment of PET-NW in the fabrication of GPEs can pave a viable way for significant advancements in high-performance solid state LMBs for various practical applications.

(Invited) Low-Temperature Processing of Mixed Conducting Oxides: Control of Defects, Transport, and Reactions Away from Equilibrium

Low-to-intermediate temperature processing and use of mixed conducting oxides targets the following potential benefits: ultra-fine nanostructuring with avoidance of coarsening, control of surface chemistry, lower energy consumption, integration with thermally sensitive components, and slower degradation vs. conventional high temperature synthesis and use. In this lower temperature regime, we have pursued two routes for non-equilibrium control of defect chemistry and therefore properties of mixed ionic/electronic conductors (MIECs) in the ferrite perovskite family: crystallization and illumination.Room-temperature growth followed by mild anneals to induce crystallization has proven to yield superior oxygen surface exchange kinetics, attributed to development of facile charge transfer and pristine surface chemistry in our prior work. The close link between crystallinity and oxygen stoichiometry appears to play an important role in the structural evolution and electrical response. However the effect of evolving crystallinity on the ionic conductivity has not been studied in-depth. Therefore we recently pursued an approach combining in-situ synchrotron grazing-incidence X-ray diffraction with ac and dc electrical methods on thin-film blocking electrode cells to evaluate evolution of transference numbers during crystallization. In certain ferrite films we observe a ~2 orders-of-magnitude increase in ionic conductivity during crystallization, while the electronic conductivity increases much more. Through the process, we demonstrate that it is possible to turn a predominantly ionic conductor into a predominantly electronic conductor - and to access almost any ionic transference number in between - simply by controlling the degree of crystallinity. [1]Oxygen stoichiometry can alternatively be controlled at low-to-intermediate temperatures through application of low fluence above-gap illumination. We have shown in certain non-dilute ferrite films that UV light causes oxygen to enter the films from the gas phase with kinetics limited by the oxygen surface-exchange reaction, which is largely unchanged by the presence of excited carriers. We explain the response through simulations of excited state defect equilibria and transient absorption spectroscopy measurements. [2]Together these processing strategies indicate that low-temperature control of oxygen stoichiometry is feasible in mixed conductors, enabling wide tailoring of properties and potential for use in low-to-intermediate temperature solid-state ionic devices.[1] H.B. Buckner, J. Simpson-Gomez, A. Bonkowski, K. Rubartsch, H. Zhou, R.A. De Souza, N.H. Perry (in review)[2] E. Skiba, H.B. Buckner, G. McKnight, C. Lee, R. Wallick, R. van der Veen, E. Ertekin, N.H. Perry (in review)We gratefully acknowledge funding from the US Department of Energy, Basic Energy Sciences, under DOE Early Career grant DE-SC-0018963 and from the US National Science Foundation, through the University of Illinois at Urbana-Champaign Materials Research Science and Engineering Center DMR-2309037.

Effects of Polymer Side Chains on Ion Transport Properties of Gel Electrolytes Containing High Concentration Li Salt Solution

Highly concentrated Li salt electrolyte solutions have attracted attention recently due to the unique physicochemical and electrochemical properties.1 Recently, our group reported that highly concentrated Li salt/sulfolane electrolytes exhibit Li+ ion hopping conduction mechanism.2 Gel electrolytes, which incorporate liquid electrolytes within polymer network generally maintain ion transport properties derived from liquid electrolytes. The gelation of liquid electrolytes can prevent the leakage of liquids and improve the safety of batteries. However, certain gel electrolytes containing high concentration Li salt solution have been reported that the ion transport properties can be affected by the participation of the polymer matrix in the solvation structure.3 In this study, we focused on the effects of polymer side chains on ion transport properties of gels. We prepared novel gel electrolytes using polymers with different side chains as a polymer matrix containing high concentration Li salt/sulfolane solutions. Gel electrolytes incorporating lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) and sulfolane (SL) at 1:3 molar ratio, which exhibits a high Li+ ion transference number (t Li+ ), were prepared by free radical polymerization using four types of methacrylate monomers (methyl methacrylate: MMA, propyl methacrylate: PMA, 2,2,3,3-tetrafluoropropyl methacrylate: TFPMA, diethylene glycol monomethyl ether methacrylate: DEGMA). Ionic conductivity and of PMMA gel were comparable to those of PPMA gel with extended alkyl side chains, while in PDEGMA gel with multiple coordination sites for Li+ showed lower t Li+ because Li+ ions were trapped by the ether moiety of the side chain. On the other hand, PTFPMA gel with partially fluorinated propyl side chains showed a higher t Li+ . The self-diffusion coefficients of Li+, TFSA−, and SL in PTFPMA gel measured with pulsed field gradient (PFG) NMR suggested lower diffusivity of TFSA− compared to that in PPMA gel. Moreover, 19F NMR revealed that the signal derived from TFSA− shifted to downfield side in PPMA gel and upfield shift in PTFPMA gel compared to that in the liquid electrolyte of [LiTFSA]/[SL] =1:3, indicating a change of solvation structure due to the polymer side chain. Finally, we demonstrated rate capability tests for gel electrolytes with LiCoO2/Li cells and found that the cell with PTFPMA gel having high t Li+ exhibited better rate performance than one with PPMA gel. Acknowledgements This study was partially supported by Japan Science and Technology Agency (JST) GteX Program (Grant No. JPMJGX23S0) and JSPS KAKENHI (Grant No. 22H00340) from the Japan Society for the Promotion of Science (JSPS). References Yamada and A. Yamada, Review—Superconcentrated Electrolytes for Lithium Batteries, J. Electrochem. Soc., 2015, 162, A2406–A2423.Dokko, D. Watanabe, Y. Ugata, M. L. Thomas, S. Tsuzuki, W. Shinoda, K. Hashimoto, K. Ueno, Y. Umebayashi and M. Watanabe, Direct Evidence for Li Ion Hopping Conduction in Highly Concentrated Sulfolane-Based Liquid Electrolytes, J. Phys. Chem. B, 2018, 122, 10736–10745.Tasaki, Y. Ugata, K. Hashimoto, H. Kokubo, K. Ueno, M. Watanabe and K. Dokko, Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers, Phys. Chem. Chem. Phys., 2023, 25, 17793–17797.

(Invited) A Flexible and Sustainable Gel Electrolyte Toward Safe Zn-MnO2 Alkaline Rechargeable Batteries

Zn-MnO2 alkaline batteries form inactive compounds during charging and discharging that hinder their ability to be utilized in rechargeable settings. Design and development of functional gel polymer electrolyte (GPE) to replace traditional electrolyte have become an effective way to inhibit irreversible compound formations and Zn dendrite growth, avoid leakage issues and metallic packaging. However, many challenges such as reliable and reproducible ionic conductivity and ion transference number of free-standing electrolyte films at lower thickness (250 µm), and interfacial contact resistance between electrolyte and electrode layer still exist. Our assumption is producing reliable and consistent ionic conductivity and ion transference number of electrolytes at lower thickness (250 µm) will help in providing the faster redox reaction kinetics at electrolyte-electrode interface. Moreover, by reducing the interfacial contact resistance between electrolyte and electrode interface will help in reducing the charge transfer resistance and zincate ion movements and results in improving the reversible life cycle of batteries. Additionally, by improving the mechanical strength and flexibility of free-standing GPE layer will be helpful to suppress the dendrite formation. Our assumption is well-arranged continuous porous structure of GPE will result in easy movement of ions and well absorption and retention of alkaline or acidic salt solution. It will also provide mechanical strength to the overall gel films at lower thickness to reduce dendrite formation. To fabricate well-arranged continuous porous GPE structure, we propose to use freeze casting method.Here, we propose to use naturally occurring sustainable chitosan, with different additives like PVA, PAAK to form GPE using freeze casting method. The different thickness electrolytes were soaked into alkaline solution e.g. Hydroxide (KOH) and fluorinate compounds and compared on their Ionic Conductivity, Ionic Conductance, Ion Transference Number, PH, and Swelling Ratio. To reduce the interfacial contact resistance between electrolyte and electrode, free standing electrolyte layer will be dipped into Chitosan solution and then put in contact with electrodes. We will also perform SEM to observe the arrangement of polymer chain using freeze casting method. These electrolytes were then used in the fabrication of Zn-MnO2 batteries and measure their performance using cyclic voltammetry and galvanostatic charge-discharge.

Gel Polymer Electrolytes Composed of Highly Concentrated Li Salt/Sulfolane Electrolytes and Homogeneous Polymer Network Having Weakly Coordinating Ability

Lithium-ion batteries (LIBs) use flammable organic electrolytes, and there are issues of ignition and leakage of liquid electrolytes. Sulfolane solvent is a non-flammable solvent. We previously reported that the sulfolane-based highly concentrated electrolytes exhibit unique transport properties and high Li+ ion transference numbers. 1) Gelation of liquid electrolyte is useful in preventing leakage and improves the safety of LIBs. Conventional gel polymer electrolytes (GPEs) have low mechanical reliability due to the structural defects such as loop chains and dangling chains. The mechanical toughness of GPE depends on the homogeneity of the polymer network. To achieve good mechanical toughness of GPE, the use of tetra-arm poly(ethylene glycol) (Tetra-PEG) has been proposed.2) The end-coupling reaction of two symmetrical tetra-PEGs can form homogeneous network structure, and tetra-PEG gels exhibit good mechanical toughness.In this work, we synthesized tetra-arm poly(2,2,3,3-tetrafluoropropyl acrylate) (Tetra-PTFPA), and GPEs composed of a sulfolane-based highly Li salt-concentrated electrolyte and tetra-PTFPA. Tetra-PTFPA was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization using a tetrafunctional chain transfer agent in THF solvent. This polymerization solution was mixed with a highly concentrated electrolyte, LiN(SO2CF3)2 (LiTFSA) : sulfolane (SL) = 1:3 (molar ratio), then the THF was removed. Then, GPE with a uniform network structure were prepared by adding 1,3-propanediyl diacrylate (PDA) and further RAFT polymerization of both ends of PDA to tetra-PTFPA in the electrolyte to crosslink the different tetra-polymers. A self-standing GPE membrane could be prepared by RAFT polymerization at a polymer concentration of 20 wt%, while self-standing membrane could not be obtained through free radical polymerization at the same crosslinking density. This suggests that the homogeneous network structure in the tetra-PTFPA gel was effective in improving the mechanical reliability of GPE. The Li ion transference number (t Li+) of the tetra-PTFPA gel was evaluated by DC polarization and determined to be 0.64. This value is higher than t Li+ of the mother liquid electrolyte [LiTFSA]/[SL] =1/3 (t Li+ = 0.55). This is probably due to the interaction between TFSA anion and the acidic protons surrounded by fluorine atoms in the polymer side chains of tetra-PTFPA.TFSA anions are assumed to be trapped by polymer side chains in the gel, while Li+ ion less interact with the side chains. This results in a higher transference number of Li+ in the gel electrolytes compared to that in the mother liquid electrolyte. Further results including mechanical and ion transport properties of the tetra-PTFPA gel will be presented. Acknowledgements This study was partially supported by Japan Science and Technology Agency (JST) GteX Program (Grant No. JPMJGX23S0) and JSPS KAKENHI (Grant No. 22H00340) from the Japan Society for the Promotion of Science (JSPS).References1) Dokko K., et al, J. Phys. Chem. B, 2018, 122, 10736-10745.2) Sakai T., et al, Macromolecules, 2008, 41, 5379–5384.

Hypercrosslinked Metal-Organic Polyhedra Electrolyte with High Transference Number and Fast Conduction of Li Ions.

Solid-state electrolytes (SSEs) with high Li-ion transference numbers and fast ionic conductivity are urgently needed for technological innovations in lithium-metal batteries. To promote the dissociation of ion pairs and overcome the mechanical brittleness and interface defects caused by traditional fillers in polymeric electrolytes, we designed and fabricated a cationic hypercrosslinking metal-organic polyhedra (HCMOPs) polymer as SSE. Benefiting a three-component synergistic effect: cationic MOPs, branched polyethyleneimine macromonomer and polyelectrolyte units, the Li-HCMOP electrolyte possesses a high Li-ion conductivity, a high Li-ion transference number and a low activation energy. The LiFePO4/Li battery exhibits high capacity with superior rate performance and cycling stability. Moreover, soluble MOPs serve as high crosslinking nodes to provide excellent mechanical strength for electrolytes and good compatibility with polymers. This work highlights an effective idea of high-performance MOP-based solid-state electrolytes applied in LMBs.

Ion Transport Properties in Glyme-Based Highly Concentrated Electrolyte with Asymmetric Li Imide Salts

The spread of electric vehicles is being expanded around the world with the aim of reducing carbon dioxide emissions toward carbon neutrality by 2050. This trend requires even higher performance for Li secondary batteries. Previous research has focused on increasing ionic conductivity of the electrolyte. It is now clear that the Li ion transference number is equally important to further improve the performance of batteries for automotive applications[1]. However, the relationship between the structure of Li salts and ion transport properties is not understood well, and there is no established method to systematically increase the Li ion transference number. Here we show how the structure of the Li salt affects the ion transport properties for an equimolar mixture of a novel Li salt and tetraglyme (G4). We have synthesized a new Li imide salt, Li (N-(trifluoromethyl)sulfonyl)acetamide) (Li[TfNAc]), having both sulfonyl and carbonyl groups. For the novel highly concentrated electrolyte [Li(G4)][TfNAc], we observed a high Li ion transference number of 0.65 at the cost of a low ionic conductivity of 0.21 mS cm−1. This trade-off between ionic conductivity and transference number is in line with previous empirical observations made in our group[2][3]. Ab initio molecular orbital calculations of lithium affinity in comparison with anions in established electrolytes revealed the order [TFSA]− < [BF4]− < [TfNAc]− < [TFA]−, consistent with the qualitative order observed in ionic transport properties. Thus, ion transport is affected significantly by small changes to the anion structure. We observed that the solvate structure of [Li(G4)][TfNAc] is dominated by correlated motion in ion pairs and larger aggregates, in line with previous studies.[3] Based on the concepts of ionicity (inverse Haven ratio) and Onsager transport coefficients[4], we were able to quantify the impact of ion interactions on the macroscopic transport properties.[1]K. M. Diederichsen et al, ACS Energy Lett., 2017, 2, 2563.[2]K. Shigenobu, et al., Phys. Chem. Chem. Phys., 2020, 22, 15214.[3] K. Shigenobu, et al., Phys. Chem. Chem. Phys., 2021, 23, 2622.[4] Dong, D.; Sälzer, F.; Roling, B.; Bedrov, D., Phy. Chem. Chem. Phy., 2018, 20, 29174.

Synthesis and Characterization of Dicationic Ionic Liquids with the Difluorophosphate Anion

Dicationic ionic liquids (DILs) can be diversely designed by tuning spacer and cationic-center structure. Their features possibly superior to conventional ionic liquids include thermal stability, wide potential window, and high Li ion transference number, which make them good electrolyte candidates for secondary batteries. However, there are few reports of DILs and many unknown factors about the relationship between their structure and physical properties.Although Li[PO2F2] is known as an additive in lithium secondary batteries that improves the negative electrode performance, [1] PO2F2 - itself forms ionic liquids with a variety of onium cations. [2] In this study, we attempted syntheses of DILs with PO2F2 -, especially the ones with ether oxygen in the spacer and evaluated the physical properties of the resulting DILs.The synthesis of DILs was carried out in three steps. Initially, ethylene glycol was reacted with paraformaldehyde and trimethylsilyl chloride in dichloromethane at room temperature to prepare 1,2-bis(chloromethoxy)ethane. In the second step, dihalide reacted with two equivalents of amines at room temperature and converted to the corresponding chloride salts. Finally, the chloride salts and K[PO2F2] were reacted in dichloromethane at room temperature to exchange anions.According to the results of differential scanning calorimetry, the dication salts with ether spacer exhibit lower melting points than the corresponding dication salts without ether spacer owing to the flexibility around the ether group. Comparison of densities between pyrrolidinium- and linear alkylammonium-based DILs revealed the higher density of the former salt, which arises from the rotational freedom of the alkyl chain in the latter. The higher viscosity of the DIL with BF4 - than that of the DIL with PO2F2 - is due to the high dipole moment and thus polarity of PO2F2 - consisting of two oxygen and two fluorine atoms. High ionic conductivity is an important factor as electrolyte. The ion conductivity of the DIL with PO2F2 - is higher than that of the DIL with BF4 -, which implies the potential of PO2F2 - DILs in battery applications.

Linear Ether-Based Molten Li Salt Solvate Electrolytes for Li-Metal Batteries

Li metal batteries have attracted much attention as the next-generation rechargeable batteries owing to the exceptionally high theoretical capacity (3860 mAh g⁻¹) and the lowest electrode potential of Li metal electrode. Molten Li salt solvate electrolytes or highly concentrated electrolytes (HCEs) are an emerging class of electrolytes having a similarity to ionic liquids (ILs) in high ionic nature, and indeed some of which are found to behave like an IL.1 This type of the electrolytes have attracted considerable attention as prospective candidates for electrolyte materials in future Li metal batteries owing to their favorable properties both in the bulk and at the electrochemical interface. In this study, linear ether-based molten Li salt solvates were investigated for potential effects of weakly coordinating linear ethers on transport properties and battery performance. We focused on the electrolytes using lithium bis(trifluoromethanesulfonyl)amide (LiTFSA) or lithium bis(fluorosulfonyl)amide (LiFSA) as the salt and linear chain monoethers such as methylpropyl ether (MPE), n-butylmethyl ether (BME), and ethyl propyl ether (EPE) as the solvent for lithium-sulfur and lithium metal batteries. Owing to the low lithium polysulfide solubility, low viscosity, relatively high ionic conductivity, high Li ion transference number, high reductive stability, and low specific gravity, all of which are favorable for achieving high-energy density batteries, linear ether-based molten Li salt solvates were found to be effective electrolytes. The correlation between Li ion solvation and ionic transport properties of the linear ethers with different alkyl chain length were further studied to optimize the electrolyte compositions. The chemical structure of the ethers had a strong impact on the Li ion coordination structure and ionic conductivity: the monomethyl ether such as MPE and BME showed more-pronounced Li ion coordination and higher ionic conductivity whereas steric hinderance of longer alkyl chains in ethers with longer alkyl chain length such as ethyl propyl ether (EPE) resulted in lower solvation number, enhanced ion pairing and lower ionic conductivity. The improved Li ion transport property of the linear ether-based electrolytes led to the better rate performance. Furthermore, a pouch-type cell demonstrated an energy density exceeding 300 Wh kg-1 under lean electrolyte conditions.2 Shigenobu, T. Sudoh, J. Murai, K. Dokko, M. Watanabe, K. Ueno, Chem. Rec., 2023, 23, e202200301.Ishikawa, S. Haga, K. Shigenobu, T. Sudoh, S. Tsuzuki, W. Shinoda, K. Dokko, M. Watanabe, K. Ueno, Faraday Discuss., 2024, Accepted Manuscript. DOI: 10.1039/D4FD00024B

In situ preparation of zincophilic covalent–organic frameworks with low surface work function and high rigidity to stabilize zinc metal anodes

Zinc-ion batteries (ZIBs) are inexpensive and safe, but side reactions on the Zn anode and Zn dendrite growth hinder their practical applications. In this study, 1,3,5-triformylphloroglycerol (Tp) and various diamine monomers (p-phenylenediamine (Pa), benzidine (BD), and 4,4′’-diamino-p-terphenyl (DATP)) were used to synthesize a series of two-dimensional covalent-organic frameworks (COFs). The resulting COFs were named TpPa, TpBD, and TpDATP, respectively, and they showed uniform zincophilic sites, different pore sizes, and high Young’s moduli on the Zn anode. Among them, TpPa and TpBD showed lower surface work functions and higher ion transfer numbers, which were conducive to uniform galvanizing/stripping zinc and inhibited dendrite growth. Theoretical calculations showed that TpPa and TpBD had wider negative potential region and greater adsorption capacity for Zn2+ than TpDATP, providing more electron donor sites to coordinate with Zn2+. Symmetric cells protected by TpPa and TpBD stably cycled for more than 2300 h, whereas TpDATP@Zn and the bare zinc symmetric cells failed after around 150 and 200 h. The full cells containing TpPa and TpBD modification layers also showed excellent cycling capacity at 1 A/g. This study provides comprehensive insights into the construction of highly reversible Zn anodes via COF modification layers for advanced rechargeable ZIBs.

Accelerating the Gelation of in-Situ Gel Polymer Electrolyte by Lithium Salts-Anion-like Initiator for Lithium-ion Batteries

The demand for high-energy-density batteries is expanding with the rise of EV markets. Moreover, the benefits of solid-state electrolytes are emerging to achieve high energy density. However, the technology of solid-state electrolytes needs to mature further. In the present state, a gel polymer electrolytes (GPEs) could act as a bridge between liquid (LEs) and solid-state electrolytes (SSEs). GPEs under certain conditions can offer the advantages of both LEs and SSEs. Firstly, GPEs should be feasible to create a gel structure with minimal polymer content to ensure high ionic conductivity. Ideally, they should also exhibit a high lithium ion transference number. Secondly, GPEs should ensure safety from the risk of fire. It is essential not only to prioritize the non-flammability of polymer matrix but also to suppress the ignition of the LEs contained within GPEs. Lastly, in the current manufacturing process, the injection of liquid-state GPEs without additional steps should be possible. Also, in-situ gelation is necessary to mitigate interfacial problems. The non-flammable GPEs developed in the previous study satisfied above three conditions but had the disadvantage of long gelation time about one day at 60 oC. The gelation reaction was hindered by the bulky size of the functional groups substituted to enhance the non-flammability. Therefore, the amount of substituted functional groups was inevitably limited.In this study, fully substituted polymers were used to form the GPEs rapidly at room temperature or low temperatures (45 oC) with the addition of a lithium salts-anion-like initiator. The strong CN bonds were allowed to form carbocation due to easily ionized initiators. As a result, generated highly conjugated C=N-C bond great help to the formation of non-flammable polymer matrix compared with conjugated C=N-C bond from LiPF6. Furthermore, thermal degradation of the batteries was minimized because of rapid gelation at room temperature or lukewarm temperature. As the GPEs can undergo gelation without LiPF6, which was employed as an initiator in the previous method, stable operation at 60 oC or higher became possible. In the previous method, the decomposition of LipF6 into hydrogen fluoride at 60 oC resulted in a side reaction at the interphase between the electrolyte and the electrode. However, with the use of an initiator consisting of two anions instead of LiPF6, a stable lithium salt could be employed at high temperatures, ensuring stable operation. Moreover, the absence of hydroxyl groups enables the operation of a lithium metal battery that was previously inoperable when using PVA-CN. Due to the highly potent oxidizing power of the initiator, the gelation could rapidly progress even within a bulky substitute structure. Therefore, bulky flame-retardant structures could be fully incorporated into the polymer instead of the hydroxyl group (OH). Consequently, the side reaction of OH was eliminated. Specifically, the maximized presence of the fluorine functional group with radical scavenging ability ensured enhanced non-flammability of the entire electrolyte compared to previous studies.In summary, the addition of the initiator enabled rapid gelation with minimal energy consumption, facilitating the development of a lithium metal battery suitable for high-temperature operation. Furthermore, complete substitution of the functional group enhanced various functions and the flame retardancy of the electrolyte.