Lithium Ion Transference Research Articles

Polyimide/cellulose composite membrane with excellent heat-resistance and fast lithium-ion transport for lithium-ion batteries.

Polyimide membranes have long been of great interest in the battery industries due to their outstanding thermal stability and flame retardancy. However, the preparation of polyimide membranes with ideal pore structure and excellent lithium-ion transference remains a challenge. In this study, we reported for the first time, that a nano-porous fluorinated and partially carboxylated polyimide/cellulose composite membrane was successfully synthesized by selected monomers and prepared by thermal imidization, phase separation, and alkaline hydrolysis method. Particularly, an appropriate addition of cellulose acetate (CA) during the synthesis process can optimize the pore structure of the membrane. Besides, CA was converted to cellulose after alkaline hydrolysis, further enhancing the electrolyte affinity and lithium-ion transference of the membrane. Hence, this composite membrane exhibited distinct heat-resistance, high porosity (78 %), electrolyte absorption (344 %), and lithium-ion transfer number (0.84). Most importantly, thanks to the above characteristics of the membrane, the assembled LiFePO4/Li cells demonstrated excellent cycling stability compared with the cell with PP membrane, showing a capacity retention rate of as high as 93 % after 500 cycles at 1C. We anticipate that this composite membrane with superior physical and electrochemical properties would shed light on the development of next-generation membranes for high-power and high-safety batteries.

Beneficial Effects of FEC on an In-Situ Polymerized Deep Eutectic Electrolyte for Solid-State Batteries.

Eutectic-based polymer electrolytes have emerged as promising solid electrolytes because of their ionic liquid-like properties, while modifications are essential to further increase their ionic conductivity at room temperature and solve their compatibility with lithium anode. In this work, an in situ polymerized composite electrolyte is modified by the addition of fluoroethylene carbonate (FEC) whose beneficial effect is systematically investigated in different contents. Poly(ethylene glycol) diacrylate (PEGDA), deep eutectic solvent (LiTFSI:N-methylacetamide = 1:3), and alumina fiber work as the monomer, solvent, and three-dimensional skeleton, respectively. In adjusting FEC content, ionic conductivity at room temperature is dramatically raised by three times to 8.93 × 10-4 S cm-1, with a 4-fold increase in lithium-ion transference number to 0.405. Meanwhile, the electrochemical window is widened from 3.5 to 4.8 V. The FEC addition also helps in improving the stability with Li anode, which comes from LiF-rich interphases formed at interfaces. The dynamics of LiFePO4 is significantly enhanced with higher reversibility in full cells, so that fast capacity decay is inhibited with a specific capacity of 124.1 mAh g-1 obtained after 300 cycles at 1 C. These results provide an effective modification for the deep eutectic electrolyte, which will boost its development in solid-state batteries.

Enhanced Electrochemical Performance of Dual-Ion Batteries with T-Nb2O5/Nitrogen-Doped Three-Dimensional Porous Carbon Composites.

Niobium pentoxide (T-Nb2O5) is a promising anode material for dual-ion batteries due to its high lithium capacity and fast ion storage and release mechanism. However, T-Nb2O5 suffers from the disadvantages of poor electrical conductivity and fast cycling capacity decay. Herein, a nitrogen-doped three-dimensional porous carbon (RMF) was prepared for loading niobium pentoxide to construct a composite system with excellent electrochemical performance. The obtained T-Nb2O5/RMF composites have a well-developed pore structure and a high specific surface area of 1568.5 m2 g-1, which could effectively increase the contact area between the material and electrolyte, improving the electrode reaction and lithium-ion transfer diffusion. Nitrogen doping increased surface polarity, creating more active sites and accelerating the electrode reaction rate. The introduction of T-Nb2O5 imparted high power density and excellent cycling stability to the battery. The composites exhibited good electrochemical performance when used as dual-ion battery anode, with a stable cycle life of 207.2 mA h g-1 at 1 A g-1 current density after 650 cycles and great rate performance of 181.5 mA h g-1 at 5A g-1 was also obtained. This work provides the possibility for applying T-Nb2O5/RMF as an anode for a high-performance dual-ion battery.

Enhancing performance of lithium metal batteries through acoustic field application

The graphic illustrates how an external acoustic field stabilizes the SEI layer by enhancing lithium-ion mass transfer at slip lines and kinks, reducing pit formation and promoting a more uniform SEI, ultimately improving battery performance.

Molecular-Level designed gel polymer electrolyte with ultrahigh lithium transference number for high-performance lithium metal batteries

Molecular-Level designed gel polymer electrolyte with ultrahigh lithium transference number for high-performance lithium metal batteries

Enhancing Quasi-Solid-State Lithium-Metal Battery Performance: Multi-Interlayer, Melt-Infused Lithium and Lithiophilic Coating Strategies for Interfacial Stability in Li||VS2-DSGNS-LATP|PEO-PVDF||NMC622-Al2O3 Systems.

The development of quasi-solid-state lithium metal batteries (QSSLMBs) is hindered by inadequate interfacial contact, poor wettability between electrodes and quasi-solid-state electrolytes, and significant volume changes during long-term cycling, leading to safety risks and cataclysmic failures. Here, we report an innovative approach to enhance interfacial properties through the in situ construction of QSSLMBs. A multilayer design integrates a microwave-synthesized Li1.3Al0.3Ti1.7(PO4)3 (LATP) ceramic electrolyte, which is surface-coated with a lithiophilic conductive ink comprising VS2 and disulfonated functionalized graphene nanosheets (VS2-DSGNS) using a low-cost nail-polish binder. Subsequently, a few drops of LiPF6 in EC/DMC liquid electrolyte (LE) are impregnated into the uncoated side of the LATP surface. The quasi-solid-state electrolyte pellet of LATP-VS2-DSGNS surface was allowed to be in contact with the molten Li and held until Li flowed into the LATP-VS2-DSGNS surface completely via a "melt-infusion strategy" as an anode side. Additionally, a heterogeneous polymer matrix consisting of poly(ethylene oxide) (PEO) and poly(vinylidene difluoride) (PVDF) as a polymer interlayer is fabricated using a solution casting technique for improving the wettability between the LE impregnated side of LATP and cathode, to enhance overall charge transfer kinetics. The assembled symmetric cells, Li||LATP-VS2-DSGNS||Li and Li||PEO-PVDF/LE-LATP||Li, demonstrate high lithium-ion conductivities of 3.69 × 10-4 and 1.02 × 10-3 S cm-1, respectively, with impressive lithium-ion transfer numbers of 0.84 and 0.93 at 25 °C. Both cells exhibit a highly reversible lithium stripping/plating cycling process for over 600 h, with minimal voltage polarization of 10 and 31.6 mV, across a broad redox window (-1 to 6 V), effectively inhibiting lithium dendrite formation. Furthermore, the combination of a surface-modified Al2O3 dry-coated, high-nickel NMC622 cathode with the PEO-PVDF|LATP-VS2-DSGNS||Molten-Li architecture in a CR2032 coin-type full-cell delivers a galvanostatic discharge capacity of 130.6 mAh g-1 at a 1C rate after 200 cycles, achieving 84.3% capacity retention, thereby demonstrating substantial reduction in interfacial resistance and enhanced stable battery performance of QSSLMBs.

Regulating Crystalline Phase/Plane of Polymer Electrolyte for Rapid Lithium Ion Transfer.

Electronic-rich functional groups and flexible segments have long been perceived to be the decisive factors influencing lithium-ion transfer in polymer electrolytes, while crystallinity is regarded as the great scourge. Actually, the research on the influence of crystalline phase and crystalline plane is still in scarcity. Herein, taking poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP) as an example, new (111/201) crystal planes (belonged to β-phase) are regulated by dissolving process and clarified by Synchrotron radiation X-ray diffraction and X-ray diffraction. Density functional theory calculation indicates that the newly exposed (111/201) crystal planes provide higher binding energy with lithium ions and are conducive to provide more ion transport channels. 7Li nuclear magnetic resonance of new crystalline planes contained PVDF-HFP based electrolyte shows lower field and sharper peak intensity, further proves the rapid lithium ion transfer. Therefore, a high ionic conductivity of 6.42×10-4 S cm-1 and a large lithium-ion transfer number of 0.7 are achieved. This study offers a new insight into the influence of crystalline phase and crystalline plane on the transfer of lithium ion for polymer electrolytes.

Regulating Crystalline Phase/Plane of Polymer Electrolyte for Rapid Lithium Ion Transfer

Electronic‐rich functional groups and flexible segments have long been perceived to be the decisive factors influencing lithium‐ion transfer in polymer electrolytes, while crystallinity is regarded as the great scourge. Actually, the research on the influence of crystalline phase and crystalline plane is still in scarcity. Herein, taking poly(vinylidene fluoride)‐hexafluoropropylene (PVDF‐HFP) as an example, new (111/201) crystal planes (belonged to β‐phase) are regulated by dissolving process and clarified by Synchrotron radiation X‐ray diffraction and X‐ray diffraction. Density functional theory calculation indicates that the newly exposed (111/201) crystal planes provide higher binding energy with lithium ions and are conducive to provide more ion transport channels.7Li nuclear magnetic resonance of new crystalline planes contained PVDF‐HFP based electrolyte shows lower field and sharper peak intensity, further proves the rapid lithium ion transfer. Therefore, a high ionic conductivity of 6.42×10−4 S cm−1 and a large lithium‐ion transfer number of 0.7 are achieved. This study offers a new insight into the influence of crystalline phase and crystalline plane on the transfer of lithium ion for polymer electrolytes.

Polymerizable Ionic Liquid-Based Gel Polymer Electrolytes Enabled by High-Energy Electron Beam for High-Performance Lithium-Ion Batteries.

Polymerizable ionic liquid-based gel polymer electrolytes (PIL-GPEs) were developed for the first time using high-energy electron beam irradiation for high-performance lithium-ion batteries (LIBs). By incorporating an imidazolium-based ionic liquid (PIL) into the polymer network, PIL-GPEs achieved high ionic conductivity (1.90 mS cm-1 at 25 °C), a lithium transference number of 0.62, and an electrochemical stability exceeding 5 V. E-beam irradiation enabled rapid polymer network formation within a metal-cased battery structure, eliminating the need for initiators and improving the process efficiency. In the NCM811/PIL-GPE/Li cells, PIL-GPE (8:2) delivered an initial discharge capacity of 198.8 mAh g-1 with 82% retention at 100 cycles, demonstrating enhanced thermal stability and cycling performance compared to traditional GPEs. The demonstrated PIL-GPEs demonstrate strong potential for high-stability, high-performance LIB applications.

Solvate Complex LiAlCl4·SO2: Synthesis and Physical-Chemical Properties.

This work describes the synthesis of the LiAlCl4·SO2 solvate complex and its properties. Its composition was determined using thermogravimetric analysis, Raman spectroscopy, and UV-vis spectroscopy. The specific ionic conductivity of LiAlCl4·SO2 is 4.7 × 10-2 S·cm-1. The dynamic viscosity is 19 cP at 25 °C, and the lithium transference number is 0.78. The melting point (+2.5 °C) and decomposition temperature (+60 °C) of LiAlCl4·SO2 were determined by simultaneous thermogravimetric analysis and differential scanning calorimetry. The structure of the solvate complex LiAlCl4·SO2 has been studied using a molecular dynamics method. A lithium cation is coordinated by one atom of Cl of the anion AlCl4- and one oxygen atom of SO2. The coordination number of lithium ions of LiAlCl4·SO2 is 4. The lithium cation is coordinated by three anions of AlCl4- and one molecule of SO2. Anion AlCl4- acts as a bridging ligand and binds different lithium cations.

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.

Electrospun PAN membrane encapsulated in PEO as a polymer electrolyte for lithium metal batteries

The novel two-step fabrication method of polymer electrolyte membrane was proposed. In the first step, the polyacrylonitrile nano fibrous membrane was prepared with the help of electrospinning. In the second step, the polyethylene oxide solution was prepared along with lithium salt and plasticizers. The solution was poured on the electrospun polyacrylonitrile to get polymer electrolyte membrane in the form of PAN membrane encapsulated in PEO. The polymer electrolyte membrane was boosted by the liquid LiPF6 before assembled in the lithium metal batteries. The polymer electrolyte membrane was compared with the commercial Celgard separator when assembled in coin cell for the application of lithium metal batteries. The polymer electrolyte membrane outperformed and exhibited a good ultimate tensile strength of 20 MPa and the better thermal stability of 97 % at 305 °C. Moreover, the polymer electrolyte membrane also exhibited excellent ionic conductivity of 1.2 mS cm−1, good lithium-ion transference number of 0.57 and uniform lithium plating/stripping cycles for up to 500 cycles without internal short circuiting. The polymer electrolyte membrane represented outstanding rate capability and exhibited the good discharge capacity of 141 mA h g−1 after 100 cycles at 0.5 C rate.

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.

ETPTA polymer network confined amide-based eutectic electrolyte for safe and long- life lithium metal battery

Serious safety issues and uncontrollable lithium dendrites hinder the commercial application of high-voltage lithium metal batteries (LMBs) that employ carbonate-based electrolytes. Herein, we propose a eutectogel electrolyte, an amide-based eutectic electrolyte consisting of lithium bis (trifluoromethyl sulfonyl) imide (LiTFSI) and N-methyl-2,2,2-trifluoroacetamide (3F-NMA) confined by trimethylolpropane ethoxylate triacrylate (ETPTA) polymer network. Compared with the carbonate-based electrolyte, the non-flammable eutectogel electrolyte facilitates the formation of LiF/LixN-rich solid electrolyte interfaces, therefore expands the electrochemical window, enhances the interfacial compatibility, and improves the cycling stability of LMB. Besides, it shows a high lithium-ion transference number of 0.63 and a stable long-term lithium stripping/plating (>2300 h) in a Li||Li symmetric cell, at a current density of 0.2 mA cm−2. Additionally, the Li||LiFePO4 full cell with eutectogel electrolyte exhibits a commendable cycling stability, achieving a capacity retention of 77.4 % after 1000 cycles at 5 C (30 °C) and maintaining 82 % capacity following 400 cycles at 1 C (60 °C). When used as the electrolyte in Li||LiCoO2 full cell, it renders the battery appealing performance including a notable initial discharge capacity of 170.3 mAh g−1 and a retained capacity of 86.3 % after 300 cycles, at a high charge voltage of 4.45 V. This research may provide insight into the design of eutectogel electrolytes for high-performance lithium metal batteries.

Composite Polymer Electrolyte Based on PAN/TPU for Lithium-Ion Batteries Operating at Room Temperature.

Lithium-ion batteries have garnered significant attention owing to their exceptional energy density, extended lifespan, rapid charging capabilities, eco-friendly characteristics, and extensive application potential. These remarkable features establish them as a critical focus for advancing next-generation battery technologies. However, the commonly used organic liquid electrolytes in batteries are explosive, volatile, and possess specific toxic properties, resulting in persistent safety concerns that remain to be addressed. Composite polymer electrolytes (CPEs) exhibit enhanced safety and stable electrochemical performance, emerging as one of the most promising alternatives. However, single polymers often need to meet the multifaceted performance requirements of batteries. In this study, a composite polymer electrolyte was prepared using solution casting, consisting of a blend of polyurethane (TPU) and polyacrylonitrile (PAN), along with the ceramic filler Li1.3Al0.3Ti1.7(PO4)3 (LATP) and lithium perchlorate (LiClO4). The optimal formulation, which included 40 wt% TPU, 60 wt% PAN, and 10 wt% LATP, exhibited a commendable ionic conductivity of 2.1 × 10-4 S cm-1, a lithium-ion transference number (tLi+) of 0.60, and notable electrochemical stability at 30 °C. The LiFePO4/Li battery assembled with this CPE demonstrated excellent cycling stability and rate capability at room temperature. It delivered a discharge specific capacity of 130 mAh g-1 at 1C. Under a charge-discharge rate of 0.2C, the battery achieved a discharge specific capacity of 168 mAh g-1, retaining 98% of its capacity after 100 cycles at 25 °C. Additionally, the CPE exhibited robust safety performance. Consequently, this composite polymer electrolyte holds significant promise for application in lithium-ion batteries.

Practical Li-S Battery Design with Lean Electrolyte Using NiCo/NiCoO2 Nanoparticles as Polysulfide Mediator

Lithium-sulfur (Li-S) batteries are promising candidates for energy storage systems that can achieve high energy density.1 However, reducing the electrolyte volume in Li-S full batteries is generally unsatisfactory due to limited electrochemical reaction of Li-S. Also, the challenges of shuttle effect and sluggish sulfur conversion reaction impede towards practical application.2 Herein, I propose a practical-level lithium-sulfur battery system by introducing a method to promote the lithium-sulfur conversion reaction and suppressed polysulfide dissolution.A noteworthy designed with NiCo / NiCoO2 nanoparticles anchored in CNTs-embedded self-standing carbon structure is manufactured as host for sulfur. Through experimental techniques and first-principle calculation, the functions of NiCo/NiCoO2 nanoparticles in carbon structure are performed as follow; (ⅰ) NiCo/NiCoO2 affords polysulfide scavenging and chemisorption ability, suppressing the shuttle effect. (ⅱ) NiCo/NiCoO2 is an efficient polysulfide mediator by giving a electrocatalytic site for the Li-S redox reaction, promoting conversion kinetics and reversibility. (ⅲ) The high conductivity of host structure and sufficient porosity in carbon structure exhibit considerable improvement in the transfer of lithium ions and electrons in Li-S batteries. Even with high sulfur loading, the nanostructured NiCo/NiCoO2 enable to have stable cycling performance with lean electrolyte using a graphite anode in full-cell batteries, showing potential as a practical battery. A. Manthiram, Y. Fu, S.-H Chung, C Zu, Y.-S Su. Rechargeable Lithium–Sulfur Batteries. Chem Rev. 23 (2014)11751-11787.D. Bresser, S. Passerini, B. Scrosati. Recent progress and remaining challenges in sulfur-based lithium secondary batteries – a review. Chem Commun. 49 (2013)10545-10562.

Sulfide Solid Electrolyte Coated Cathode in All-Solid-State Batteries

Using a sulfide solid electrolyte, the all-solid-state batteries emerge as a promising candidate for next generation batteries, having significant advantages such as high lithium ionic conductivity and wide electrochemical stability window. These characteristics pave the way for the realization of elevated power and energy densities. Nonetheless, this cutting-edge technology is not without its hurdles; indeed, there are pressing issues that demand attention and refinement.In contrast to conventional lithium-ion batteries, which rely on organic liquid electrolytes with high wettability having uniform lithium ion transfer pathways within electrodes, the ASSBs encounter constraints in establishing solid-solid interfaces owing to its intrinsic solid nature. This limitation becomes particularly apparent during the cycling, wherein the fluctuating volume of the active material, prompted by the (de)intercalation of lithium ions, precipitates the degradation of the interface between active material and solid electrolyte within the composite electrode. This phenomenon leads up to loss of the lithium ionic pathway, precipitating a deterioration in cell performanceThe key point of this study lies in the endeavor to enhance battery performance through the application of a solid electrolyte coating onto the surface of the active material. By doing so, the aim is to preserve a uniform pathway for lithium ion transfer within the composite electrode. It is noteworthy that the solid electrolyte coating layer exerts profound influence on the electrochemical dynamics of the cell, important in adapting the formation of homogeneous lithium ionic pathway and the mitigation of void formation within the composite electrode. Thus, the main thrust of this research is to validate the formation of meticulously uniform solid electrolyte coating layer, depend upon the size of the solid electrolyte particles.In pursuit of this objective, a multifaceted approach was adopted. Through the utilization of liquid-phase process and a dispersant, the particle size of the solid electrolyte was deliberately manipulated, thereafter subjecting it to a dispersion process for the application of solid electrolyte coating. Intriguingly, it was observed that as the particle size of the solid electrolyte diminished, a more uniform solid electrolyte coating layer ensured, thereby engendering better electrochemical performance. Furthermore, it was corroborated that the lithium ionic pathway persevered even after long cycling, attributable to the presence of the solid electrolyte coating layer. Concomitantly, the incidence of undesirable side reactions was appreciably mitigated, with the homogeneous electrochemical reactions within composite electrode.

Electrochemical Properties of Lif-Coated Graphite Negative Electrode Modelized By Atomic Layer Deposition

Introduction In recent years, carbon dioxide emissions are rapidly increasing, and electric vehicles (EVs) are expected to play a significant role towards achieving a carbon-neutral society with most of them utilizing Lithium-Ion Batteries (LIBs). The negative electrode active material of LIB is typically made of graphite, and during initial charging, the electrolyte undergoes reductive decomposition at the graphite/electrolyte interface, forming a Solid-Electrolyte Interphase (SEI). SEI exhibits lithium-ion conductivity while being electronically insulating, facilitating highly efficient charging and discharging by suppressing further electrolyte decomposition. On the other hand, the formation of SEI consumes lithium ions, resulting in irreversible capacity loss[1]. Therefore, the nature of SEI has a significant impact on LIB performance. Regarding the composition of SEI, it is known to comprise clusters of inorganic compounds such as LiF, Li2CO3, Li2O and organic decomposition products like polyolefins. Each component has different properties such as LiF was reported to have lower lithium-ion diffusion barrier and higher chemical stability than other inorganic components[2]. However, many aspects of the individual electrochemical roles are still unclear, and forming an excellent SEI is an important issue. Objective In this work, we focused on LiF, which is considered to be a crucial component in SEI[3], and modelized the LiF-coated graphite by employing atomic layer deposition (ALD) on highly oriented pyrolytic graphite (HOPG). We investigated the influence of LiF on the lithium-ion transfer reaction behavior at the graphite/electrolyte interface. Experimental Highly oriented pyrolytic graphite (HOPG) served as the graphite negative electrode, and atomic layer deposition (ALD) was used for the LiF coating. Regarding ALD, LiN(Si(CH3)3)2 and WF6 were used as the Li and F sources, respectively. The deposition temperature was set at 300 °C, and HOPG was deposited at various numbers of cycles (10, 30, 60, and 90 times). X-ray photoelectron spectroscopy (XPS) was performed to analyze the electrode after the ALD coating. Regarding electrochemical measurements, the setup utilized a three-electrode cell with HOPG as the working electrode, lithium metal as the counter and the reference electrodes. 1.0 mol dm−3 LiClO4/EC+DEC (1:1 vol.%) was used as the electrolyte. Cyclic voltammetry (CV) was performed at a scanning rate of 0.1 mV s−1 and a scanning range of 0.005 – 3 V (vs Li+/Li). Electrochemical impedance spectroscopy (EIS) was performed at an applied AC voltage of 5 mV and a frequency range of 100 kHz – 10 mHz. EIS measurement was also performed at different temperature with a holding potential of 0.2 V to obtain activation energy. Results and discussion For the HOPG electrode coated with LiF, XPS analysis was performed and it was confirmed that the LiF coating layer was thickening as the number of coating cycle increased. In the CV measurement results, redox peaks were observed in the range of 0 – 0.6 V, confirming that the intercalation-deintercalation reaction of lithium ion was progressing. From the Nyquist plot obtained by EIS measurements, semicircles derived from SEI were observed in the high frequency region, and semicircles derived from charge transfer resistance were observed in the medium and low frequency regions. The activation energy of the charge transfer resistance was calculated by fitting the Nyquist plot (Figure 1) obtained from the temperature-dependent measurement of EIS, and revealed that the values were 54.8 kJ mol−1 for the uncoated HOPG electrode, 55.1 kJ mol−1 for 10 coatings, and 52.8 kJ mol−1 for 30 coatings, respectively, showing no significant change. On the other hand, the activation energy slightly decreased with increasing number of depositions, which was found to be 49.7 kJ mol−1 and 50.0 kJ mol−1 in the case of 60 and 90 coatings, respectively. This suggests that when the LiF coating layer reaches a certain thickness, lithium-ion charge transfer between the electrolyte and the electrode becomes easier. Details will be discussed in the conference.

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.