Newly Synthesized Lithium Salt Chemistries for Improved Solid-State Batteries´ Performances

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.

Initially limited to portable consumer electronics, the field of lithium ion batteries (LIBs) is rapidly expanding toward performance-demanding applications such as electric vehicles and load levelling of electric grids. The success of LIBs is owed to high energy density, lightweight, rapid charge/discharge, and long lifetime. However, safety issues deriving from the use of flammable liquid organic electrolytes are at present one of the major drawbacks of this technology. Solid polymer electrolytes (SPEs), representing a lithium salt associated with a polar neutral polymer or with an ion-conducting polymer matrix have been proposed to replace liquid electrolytes in LIBs. Among other benefits, SPEs offer inherent thermal stability, nonflammability and good mechanical stability. Moreover, they do not require liquid electrolyte confinement and, therefore, enable the production of flexible and thinner batteries. Despite the mentioned advantages, the intrinsic low ionic conductivity of polymer electrolytes have precluded their use in real devices so far, and significant research efforts are still required to address this open issue. Considering such a scenario, the research work during this Ph.D. career has been focused on the development of novel high-performance polymer electrolytes for applications in LIBs. The goal has been pursued exploiting a series of smart engineering strategies and synthetic pathways. All of the newly designed materials were characterized in terms of their physicochemical and electrochemical properties, and their performance evaluated in lab-scale lithium cell prototypes. In the first part of this Ph.D. work, UV-induced crosslinking has demonstrated to be a versatile tool for preparing different families of quasi-solid polymer electrolytes based on polyethylene oxide (PEO). In the past decades, this polymer has been intensively studied since its ability to complex and transport alkali metal cations. At ambient temperature, the ionic conductivity of lithium salt complexes in PEO is limited by the semicristalline domains, and ion conduction is limited to the amorphous phase. Recently, combinations of high molecular weight PEO, lithium salts and low molecular weight plasticizer have been explored as polymer based electrolyte. Despite an increase of ionic conductivity, the mechanical stability of the composite was poor when the content of plasticizer exceeded a certain limit. In this thesis a solution to this problem was proposed: highly conductive PEO based polymer electrolytes were prepared via UV induced crosslinking in the presence of a lithium salt (lithium bis(trifluoromethanesulfonyl) imide, LiTFSI) and various high boiling point liquid plasticizers. A room temperature ionic liquid, namely 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMITFSI) was used in the former case, whereas tetraethylene glycol dimethyl ether (TEGDME) was employed in the latter. In both systems, ionic conductivity substantially increased upon incorporation of the plasticizer (up to 10-4 S cm-1 at 25 °C). Noteworthy, the obtained crosslinking assured the mechanical properties to be well retained despite the high plasticizer content (up to 45% wt). Moreover, the prepared SPE showed outstanding characteristics in terms of thermal stability (> 170 °C) and electrochemical stability window (>4.5 V in both cases). Finally, the prepared materials were successfully tested in lithium cells prototypes. An in situ polymerization method was developed to obtain an improved interfacial adhesion between the polymer electrolyte and the cells electrodes. Lab-scale lithium cells were assembled and tested up to hundred cycles of full charge and discharge, showing excellent performance at different operating temperatures and applied current rates. Given the promising prospect of the developed materials, along with the easiness of the proposed process, the newly developed preparati

Rechargeable lithium-ion batteries (LIBs) involving lithium metal oxides, liquid electrolyte and graphite have been widely used in portable electronic devices due to their relatively high energy density and long cycle life. These desirable features make LIBs very attractive as the power source for electronic devices, hybrid electric vehicles (HEVs) and electric vehicles (EVs) applications [1, 2]. For future EV applications, higher energy density of LIBs up to 360 Wh kg-1 is required. Currently, the energy density of the state-of-the-art LIBs using conventional graphite anode, LiFePO4 (denoted as LFP) or LiNi0.5Co0.2Mn0.3O2 (NCM523) cathodes and 1-1.2 M LiPF6 in organic carbonate electrolytes provide practically achievable energy densities of up to around 200-260 Wh kg−1 [3]. When commercial graphite anodes are used, LiNi0.8Co0.15Al0.05O2 (NCA), LiNi0.8Co0.1Mn0.1O2 (NCM811), LiNi0.5Mn1.5O4 (LNMO) and LiNiPO4 (LNP) cathode based batteries with high-voltage provide energy densities of 354, 338, 351 and 414 Wh kg-1, respectively. However, LIBs using these high-voltage cathode materials and the organic carbonate electrolytes exhibit quite low thermal stability and tend to catch fire or even explode when abnormal charge/discharge cycling or accidental penetration of cells occurs, which greatly limits the automotive applications. When replacing graphite with a Li metal anode, the energy densities of all battery systems can be enhanced significantly due to the highest theoretical specific energy density (3860 mAh g-1) among all anode materials for rechargeable LIBs. Nevertheless, commercial LIBs are prone to cause safety problems due to the safety concern arising from Li dendrite growth in liquid organic electrolytes [4-6].The promising solid-state LIBs offer high thermal stability (i.e., low risk in catching fire), high energy density, wide electrochemical stability window and less environmental impact. A competent electrolyte is the key component of solid-state LIBs. The solid-state electrolyte materials are mainly classified as solid polymer electrolytes (SPEs), inorganic solid electrolytes (ISEs), and organic/inorganic composite electrolytes. ISEs include oxide-based and sulfide-based materials [7, 8], which show very high ionic conductivity (10-2 – 10-3 S cm-1). Furthermore, the lithium ion transference number is close to 1. However, the major limitation factors of practical solid-state LIB applications are the large interfacial impedance between electrode and ISE and the difficulty of processing [9]. Considering processability, mechanical flexibility, interfacial compatibility and electrochemical stability, one prefers SPEs to the inorganic ceramic electrolytes. Nevertheless, SPEs have low ion conductivities (10−7 − 10−5 S cm−1 near room temperature) and most of the Li+ transference numbers are lower than 0.5 [10, 11]. The major requirements for SPEs include high ionic conductivity and transference number at room temperature, wide electrochemical potential window, high mechanical strength and excellent thermal stability. However, the ion conductivity is the most important (> 10-4 S cm-1 at room temperature desired) and should be considered first. The coordinating groups of a good polymeric host are expected to interact with Li+ and facilitate dissociation.In this study, we prepared various novel acrylonitrile-based polymers (e.g., acrylonitrile/acrylate copolymer and polymer with two pendant groups b-cyano ethyl ether (-O-CH2CH2-CN) sulfonate alkyl ether (-O-(CH2)3SO3Li). The corresponding SPEs comprising acrylonitrile-based polymer and ca. 50 wt.% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) with high ionic conductivity (up to 10-3 S cm-1) at room temperature, high ion transfer number (up to 0.45) and large electrochemical potential window (oxidation stability > 5 V vs. Li+/Li) achieved. The selected SPEs were used as the separator in solid-state batteries with LiFePO4 as the cathode and Li foil as the anode; and long-term cycle stability of solid-state LIB was achieved. The polymers and corresponding SPEs were characterized by using DSC, SEM, XRD and FTIR measurements. Ionic conductivities of SPEs were determined from electrochemical impedance spectroscopy results. The linear sweep voltammetry technique was adopted to measure the oxidation stability window of SPE, and the Evans-Vincent-Bruce method was used to determined ion transfer number.

In the current technological landscape, lithium metal batteries have significantly surpassed the energy density of conventional lithium-ion batteries. Therefore, The advent of anode-free lithium metal batteries (AFLMBs) offers higher energy densities.. They also provide advantages such as reduced process complexity and lower costs due to their design that eliminates the need for anode materials. Unlike the lithium-ion insertion and extraction reactions in traditional lithium-ion batteries. This makes them easier to lithium dendrite formation, which can puncture the separator or solid electrolytes, among other issues. In this work, we developed anin-operando electron microscopic technique with a unique vacuum square-cell design to ensure air-tight transfer of in-operandocells with vacuum protection into the SEM chamber. We significantly improved the imaging resolution with a windowless design, enabling the direct electron beam to focus on the electrode interfaces. Thus, real-time imaging of Li growth could be achieved via a stable current supply and controlled temperature. we systematically analyzed the Li growth and dendrite propagation behavior in AFLMBs using solid polymer electrolyte (SPE). In particular, we used polyethylene oxide (PEO)-based SPEs containing different lithium salts, namely 25wt% Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), Lithium difluoro (oxalateo)borate (LiDFOB), and Lithium bis(oxalate)borate (LiBOB), within a LiFePO4||Cu anode-free cell configuration. Under a fixed current density, Li deposition and stripping were conducted, in which the PEO-LiTFSI SPE exhibited uniform deposition with a mossy lithium dendrite morphology. In the subsequent deposition process, some branching lithium dendrites were observed. Meanwhile, PEO-LiBOB showed uneven Li deposition with needle-like dendritic formation, potentially disrupting the SPE/electrode interface and leading to safety hazards. Lastly, PEO-LiDFOB initially showed mossy lithium deposition. In contrast, a few dendritic lithium structures appeared as the deposition process continued into the intermediate and later stages. The study successfully established a novel in-situ electron microscopy observation platform isolating water and oxygen. This platform recreated the actual battery operating environment, enhancing the credibility of experimental results. Additionally, we successfully elucidated the growth mechanisms of lithium dendrites in SPE with different lithium salts in the LFP||Cu configuration. This in-situ microscope technology aims to observe the relevant mechanisms behind dynamic deposition. Understanding how this deposition affects battery performance and safety and combining it with advanced operational and ex-situ characterization techniques can further promote the development of all-solid -state batteries in the future.

Lithium ion batteries (LIB) are representing a milestone in electrochemical energy storage and are still the state-of-the-art battery system for various mobile and stationary energy storage applications. However, the practical energy density of LIBs starts to reach an asymptotic limit. Beside LIBs, an auspicious variety of battery systems comprising a better option for specific applications in terms of e.g. energy density, so establishing a diversity of specific battery systems for specific applications is a good strategy.[1 ] After initially paving the way for the LIB, the lithium metal battery (LMB) experiences a revival due to an outstanding theoretical specific capacity (3 860 mAh g−1) and low electrochemical potential (−3.04 V vs. SHE). However, continuous electrolyte consumption, the formation of an inhomogeneous SEI and high surface area lithium (HSAL), whose growth is induced by the heterogeneous and fragile structure of the SEI film, are still dominant challenges that need to be overcome. The liquid electrolytes also deal with safety issues like risk of leakage and flammability. The combination of Li metal with solid polymer electrolytes (SPE) could supress HSAL formation and avoid those safety hazards. However, SPEs deal with poor ionic conductivity at room temperature (10−8 S cm−1 ≤ σ ≤ 10−5 S cm−1) and, additionally, it is necessary to control the Li morphology during electrodeposition/dissolution to realize high-energy all-solid-state batteries (ASSB) based on Li metal anodes.[2 ,3 ] Several artificial protective coatings have been proposed to improve the LMA/SPE interface by facilitating the Li ion flux, promoting a homogeneous Li electrodeposition/dissolution and protecting the LMA against electrolyte degradation as well as enhancing the Li wetting interface. The SPE induces a more flexible interphase that withstands the volume change. Recently, metal oxides coated by atomic layer deposition (ALD) have gained attention due to a great thickness control, the possibility of monolayer deposition as well as a consequential homogeneity of the deposited protection layer. Furthermore, ALD is suitable for roll-to-roll coatings which is feasible for industrial application.[3,4 ] Herein, the setup of Li-metal-polymer batteries (LMP® technology) commercialized by Blue Solutions and applied in their “blue cars” (30 kWh, 100 Wh kg-1) was modified in several points. Li metal was coated with a metal oxide via atomic layer deposition (ALD) to form an intermetallic phase as protective layer and to improve the Li+ flux. The artificial protective coating at Li metal was combined with a PEO- and/or polyether-based SPE and the effect of the modifications on the electrochemical performance in different ASSB setups was investigated and characterized.[1] Placke, T.; Kloepsch, R.; Dühnen, S.; Winter, M. Lithium ion, lithium metal, and alternative rechargeable battery technologies: the odyssey for high energy density. Journal of Solid State Electrochemistry 2017, 21, 1939-1964.[2] Cheng, X.-B.; Zhang, R.; Zhao, C.-Z.; Zhang, Q. Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review. Chemical Reviews 2017, 117, 10403-10473.[3] Han, Z.; Zhang, C.; Lin, Q.; Zhang, Y.; Deng, Y.; Han, J.; Wu, D.; Kang, F.; Yang, Q. H.; Lv, W. A Protective Layer for Lithium Metal Anode: Why and How. Small Methods 2021, 5, 2001035.[4] Han, Y.; Liu, B.; Xiao, Z.; Zhang, W.; Wang, X.; Pan, G.; Xia, Y.; Xia, X.; Tu, J. Interface issues of lithium metal anode for high‐energy batteries: Challenges, strategies, and perspectives. InfoMat 2021, 3, 155-174.

Electric vehicles (EVs) have become the dominant choice in the automotive industry, displacing internal combustion engine vehicles (ICEVs) to some extent. The development of high-efficiency batteries is essential to meet these market demands. Conventional lithium-ion batteries (LIBs), which consist of a cathode, anode, separator, and liquid electrolyte (such as 1M LiPF6 in carbonate solvents), have been widely utilized in portable electronic devices, hybrid electric vehicles (HEVs), and EVs. This popularity is attributed to their relatively high energy density and extended cycle life [1]. However, LIBs with energy density exceeding 240 Wh kg-1 face challenges in terms of thermal stability, potentially leading to fire and explosions by overcharging or accidental cell penetration. This drawback significantly hinders their application in automotive settings [2, 3]. In response to these safety concerns, there is a shift towards incorporating solid-state electrolytes (SSEs) as a replacement for liquid electrolytes. SSEs include solid polymer electrolyte (SPE), inorganic solid electrolyte (ISE), and composite polymer electrolyte (CPE).Solid-state lithium metal batteries (SSLMBs) incorporating SPE hold significant promise for advancing energy storage technologies due to their high energy density and improved safety features. However, the low ionic conductivity and ionic transference number of SPEs present challenges in their application for solid-state batteries. This work focuses on designing novel copolymers with polar soft unit and zwitterionic unit to address these challenges.In this study, poly(ethylene glycol) methyl ether acrylate (PEGMEA) and sulfobetaine methacrylate (SBMA) were selected as the polar soft segment and zwitterionic unit, respectively. Various ratios of PEGMEA/SBMA in copolymers were synthesized and characterized, and the resultant SPEs consisting of resultant copolymers and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were prepared. Subsequently, their electrochemical, thermal, mechanical, and morphological properties were systematically investigated. The copolymers-based SPEs exhibited improved electrochemical properties. The optimized SPE comprising the copolymer with the molar ratio of PEGMEA/SBMA = 1/3 and 50 wt% LiTFSI exhibited the ionic conductivity of approx. 2×10–4 S cm–1, lithium-ion transference number of approx. 0.3, Li+ diffusion coefficient of 15×10-12 cm² s-1, and oxidation stability of 5.2 V (vs. Li/Li+) at 25 oC. Additionally, the mechanical properties of such SPEs were assessed, showing improved tensile strength (up to approx. 6 MPa) and Young’s modulus (up to approx. 83 MPa) with increasing SBMA content. The selected SPE with the best electrochemical properties was sandwiched between a Li metal electrode and a LiFePO4 electrode to assemble a SSLMB. As a result, the discharge capacity (DC) at 0.1 C-rate and room temperature was approx. 170 mA h g–1. Furthermore, the DC at a 0.5 C-rate was approx. 146 mA h g–1, and the capacity retention of 70% obtained after 470 cycles. In summary, this work demonstrates the potential of tailored copolymers with zwitterionic moiety-based SPEs for SSLMB. The comprehensive characterization and performance assessments provide valuable insights into the design and optimization of polymer electrolytes for next-generation energy storage systems.The copolymers and corresponding SPEs underwent characterization through various techniques such as DSC, SEM, XRD, and FTIR. The Ionic conductivity of the SPEs was determined by analyzing the results obtained from EIS measurement. The lithium-ion diffusion coefficient (DLi+) for SPE in a symmetric SSLMB cell, Li/SPE/Li, was also calculated based on the EIS results [4]. The oxidation stability window of the SPE was measured using the linear sweep voltammetry technique, and the ion transfer number was determined using the Evans-Vincent-Bruce method [5].

Flowable polymer electrolytes for lithium metal batteries

Currently, lithium-ion batteries (LIBs) are considered to be one of the most popular energy storage systems for electronic devices supported by high energy density, high operating voltage, and favorable cycling performance. However, commercial LIBs with the organic liquid electrolyte and lithium (Li) salts are associated with critical safety issues such as uncontrollable side reactions, toxic liquid electrolyte leakage, flammability of electrolytes, and poor thermal stability. Therefore, replacing the liquid electrolyte with solid electrolytes is quite necessary. Among several solid ion conductors, solid polymer electrolytes (SPEs) can offer excellent flexibility, interfacial compatibility with electrodes, good processibility, low cost, and light weights that can overcome the limitations of ceramic ion conductors. However, current SPEs often encounter limitations such as poor mechanical strength and dimensional thermal stability, inferior electrochemical stability, and low Li+ ion conductivity at room temperature (~10-5 S cm-1 at 25 ℃).Here we present a multifunctional solid polymer electrolyte based on zwitterionic polyurethanes (zPU-SPE) for all-solid-state LIBs (SLBs). Our zPU-SPE exhibits a great potential to overcome current technical limitations of conventional SPE materials in SLB applications (e.g., low Li+ ion conductivity, inferior electrochemical/mechanical stabilities, unsatisfactory suppression of Li dendrite growth). We designed and synthesized a series of zPU [i.e., poly ((diethanolamine ethyl acetate)-co-poly(tetrahydrofuran)-co-(1,6-diisocyanatohexane))]. Our zPU-SPE can host an equal amount of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) without phase separation (up to 90 wt% of LiTFSI loading). The Li-ion conductivity value of zPU exponentially increases with the addition of LiTFSI and reaches 7.4 × 10-4 S/cm at 25° C, almost 14 times higher than that of poly(ethylene oxide) (PEO) SPE with ethylene oxide/Li+ ratio = 16. In addition, its superior adhesion energy (487.5 J/m2 of zPU-SPE vs. 150 J/m2 of commercial 3M Scotch Tape) can minimize interfacial resistance between electrode and SPE, and thus cell resistance of 100-µm-thick zPU SPE is as low as 280 W/cm2 compared to 1230 W/cm2 of the cell with a similar thickness of PEO SPE. Our zPU-SPE also showed an excellent elastic property with a tensile break of 1700% owing to its high density of inter- and intra-molecular hydrogen bonding in polymer matrix. The SLB battery performance of PEO and zPU-SPEs was evaluated using a solid-state Li/SPE/LiFePO4 cell; the assembled SLB cell was cycled at a constant current rate of 1 C at 25 °C. After a discharge capacity of the cell with zPU-SPE stabilized at 100 mAh g-1 after 15 cycles, there was a negligible capacity decrease (only 3% capacity decrease after 500 cycles), delivering a discharge capacity of 97 mAh g-1 with stable Coulombic efficiency (100%) over entire cycles. However, the discharge capacity of Li/PEO/LiFePO4 cells drops rapidly to 3 mAh g-1 after 100 cycles. These results demonstrate that the SLB cell assembled with PCB-PTHFU shows high discharge/charge capacity and excellent capacity retention with stable Coulombic efficiency. The good electrochemical performance of the zPU SPE can be attributed to good compatibility with electrodes, low charge transfer resistance at the interface of electrode/electrolyte, and high Li-ion conductivity, strongly suggesting that zPU SPEs are potential candidates for development of high performance of SLBs. Figure 1

With the increase of energy demand, the depletion of oil reserves and the development of intermittent renewable energy sources, the storage and the production of energy are one of the main concerns of the XXI century. Lithium Ion Battery (LIB) are among the best choices because of their high energy and power densities, longer lifespan and lighter than other energy storage systems. [1]LIB are composed of two intercalation electrodes, electronically isolated by an ions conductive electrolyte. The early LIB were composed of an organic liquid electrolyte. However, they had many disadvantages, the most worrying of which is their safety concern related to short-circuits and potential leakages of the flammable liquid solvent [2]. In this context, the development of new solid materials for electrolyte (such as ceramics and polymers) are a central issue. Solid Polymer Electrolyte (SPE) are among the most promising solution to deal with safety problems. They are ionically conducting solid material in which lithium salts are dissolved thanks to polar groups [3]. In LIBs, polymer electrolyte has a dual role. It plays both the role of separator between electrodes and binder in the composite electrodes. Chemical and electrochemical stabilities, absence of solvents intercalation and good interfacial contact between active material and electronic components are among the advantages of SPE.The conduction mechanisms in polymer are quite different from the processes in low molecular weight solvents and are still not fully understood. Cation transport in polymer involves two steps which are considered to be dissociated or not, depending on the model chosen (Arrhenius or Vogel-Fulcher-Tammann, respectively) [4]. Above the glass transition temperature, segmental motions of polymer chains allow ions hopping between coordinating sites on the same polymer chain or between two neighbouring chains. According to previous studies, ionic conduction takes place mainly in the amorphous phase with high segmental motions. That’s why, polymer electrolytes often suffer from poor ionic conductivity at room temperature (less than 10-5 S/cm) compared to their liquid counterparts. Lots of research tried to improve this factor by decreasing the crystalline part and the glass transition, melting and crystallisation temperatures, but to the detriment of mechanical, thermal or electrochemical stabilities.Our team made an effort to preserve a good ionic conductivity, and at the same time, improve other stability properties, by using a blend of polymers. The existence of functional polar groups makes them ideal candidates to dissolve lithium salts (such as Lithium bis(trifluoromethanesulfonyl)imide LiTFSI) and Lithium bis(fluorosulfonyl)imide (LiFSI)) and form stable ion-polymer complexes. In this work, thermal, mechanical and electrochemical properties of our solid polymer electrolyte based on different ratios of each polymer and salt were characterized thanks to Differential Scanning Calorimetry (DSC), Atomic Force Microscopy (AFM) and other electrochemical techniques (such as Electrochemical Impedance Spectroscopy and Cyclic Voltammetry). Conduction mechanisms and salt/polymers interactions are characterised thanks to Scanning electron microscopy, Energy Dispersive X-ray Spectroscopy (SEM/EDX) and Fourier-transform Infrared Spectroscopy (FTIR). Half-cells with high voltage cathodes (LiNiMnCoO2 (NMC), LiFePO4 (LFP)) and low voltage anodes (Graphite, Li4Ti5O12 (LTO)) showed high charge/discharge capacity at 0.1C (at 60°C and 50°C). These first results indicate that this blend of polymers is a promising candidate as SPE for Lithium-Ion Batteries.References : J. B. Goodenough and K. S. Park, American Chemical Society, 2013, 135, 1167-1176. D. Baril, C. Michot and M. Armand, Solid State Ionics, 1997, 94, 35-47. F. M. Gray, Solid polymer electrolytes, VCH New Tork, 1991. R. C. Agrawal and G. P. Pandey, Journal of Physics D: Applied Physics, 2008, 41, 223001.

Solid-state electrolytes are receiving increasing attention in lithium metal batteries due to the advantage of high energy density. Poly(ethylene oxide) (PEO) electrolyte possesses good compatibility with lithium salts. However, PEO suffers from a low lithium-ion transference number and poor high-voltage resistance, which significantly hinder its application in lithium metal batteries. Herein, a perfluoropolyether-terminated single-ion polymer (PFPE-polymer) is designed and synthesized in this contribution. By incorporating the PFPE-polymer and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) into PEO via casting, the polymer electrolyte (PFPE-SE) is successfully prepared. Compared to PEO-based electrolytes, PFPE-SE forms a solid electrolyte interface (SEI) layer and inhibits the growth of lithium dendrites on the anode. At 70°C, the lithium-ion transference number and the electrochemical window reach 0.72 and 4.7V, respectively. When tested at a discharge rate of 0.5 C, the Li|PFPE-SE|LFP cell exhibits a specific capacity of 156.0 mAh g-1, with a capacity retention of 74.3% after 230 cycles, superiority the performance of the electrolyte prepared by mixing PEO with LiTFSI. This work presents a promising polymer electrolyte strategy for achieving high-performance lithium metal batteries, leveraging the in situ construction of the SEI layer and the utilization of a single-ion polymer.

Solid polymer electrolytes (SPE) have generated an extensive and sustained interest for its application in lithium metal batteries.[ 1 ] Compared with liquid electrolytes, solid polymer electrolytes have higher safety and thermal stability, since they can provide a physical barrier layer to prevent efficiently lithium dendrite growth and avoid thermal runaway under high temperature or impact. Despite the substantial benefits, some limitations remain to be improved, such as a low ionic conductivity at room temperature and low transference number. Several studies are being conducted to overcome these weaknesses and develop new generation of solid polymer electrolyte lithium metal batteries.[ 2 ] Most of the research on SPEs is focused on polyethylene oxide (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and its derivative. In general, PEO-based solid electrolytes have shown good dimensional stability, good safety, and can mechanically prevent dendritic growth. However, they exhibit low ionic conductivity and rather low cationic transference number (t+) of ca. 0.15. To achieve sufficient conductivity at low temperatures, polymers that are difficult to crystallize are proposed. Ionic liquids (IL) are poor crystal formers; therefore, polymers of IL are potentially good alternatives to PEO for lower temperature conductivity. An Iongel membrane based on the poly(dimethyldiallylammonium) polyDADMA-TFSI poly(ionic liquid) has gained attention in polymer metal batteries due to its favorable mechanical properties and stability against Li-metal.[ 3 ] The present study provides an insight into the properties of free-standing membranes using poly (ionic liquid)-glyme mixtures compared to typical polymer salt blends (PEO-LiTFSI) and PDADMA-LiTFSI. The benefit of tetraethylene glycol dimethyl ether (G4) is highlighted and appears to be the key component to obtain higher ionic conductivity regardless on the type of polymer used. The impact of the increase in LiG4TFSI/poly ionic liquid ratio within the electrolyte on the transference number, ionic conductivity and mechanical properties was further analyzed. For 33-66 wt% composition for PDADMAT-LiG4TFSI and PEO-LiG4TFSI membranes, a good compromise between the physicochemical and electrochemical properties was achieved.

Solid polymer electrolytes in solvent-free lithium batteries may overcome some of the disadvantages of liquid electrolytes such as flammability and instability.1 Thereby, the polymer electrolyte acts as both ion transport medium and electrical separator between the electrodes. Compared to rigid separators (e.g. fiber glass) a higher shape flexibility is a further advantage, as these SPEs can compensate volume changes of the electrodes by elastic and plastic deformation.2 Poly(ethylene glycol) (PEG) possesses one of the highest ionic conductivities among solvent-free SPEs but suffers from a conductivity drop below its melting temperature about 50-60°C due to high crystallinity depending on the molecular weight.1 In addition to linear PEG polymers, ion-conducting bottlebrush graft copolymers can be obtained by attaching PEG side chains to a polymer backbone in order to reduce the crystallinity maintaining very high molecular weight.In this work we synthesized five new bottlebrush polymers using free radical polymerization as well as ring-opening metathesis polymerization (ROMP). These brush polymers contain different lengths of PEG side chains (1 kg mol-1 and 2 kg mol-1) and two different backbones, poly(methacrylate) and poly(norbornene). We present the influence of the polymer architecture on mechanical stability, ionic conductivity, Li-ion transport number and electrochemical stability of a series of SPEs obtained thereof by mixing with different amounts of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). We also compare the results to the respective linear PEG counterparts. The differences in ionic conductivity were analyzed and correlated with respect to thermal properties such as T g and ΔH melt in order to understand the fundamental factors which influence the properties of solvent-free PEG-containing bottlebrush SPEs.We also examined the implications of changing a linear polymer system to a brush architecture for potential applications in batteries and correlate the occurring processes in the cell with the distribution of relaxation times (DRT). Detailed comparative measurements under similar cell configurations for diverse O/Li ratios in a temperature range of 25 to 80 °C for different SPEs were carried out to elucidate structure-property relationships. The interesting findings are that by applying a brush architecture, we suppress the crystallinity of PEG and improve the mechanical strength without losing ionic conductivity. We obtained conductivities in the range of 10-3 to 10-4 S cm-1 for solvent-free SPEs. Furthermore, the best ionic conductivities for any system correlate strongly with their respective T g.3 Nevertheless, there are still several unresolved questions regarding these bottlebrushes compared to linear PEG in terms of interfacial as well as bulk processes in both blocking steel (ionic conductivity) and lithium (lithium plating/stripping, interfacial resistance) electrode setups. In addition to the polymer component, the lithium salt has a major effect on the properties of the electrolyte. LiTFSI is probably the most common Li-ion source in SPEs.4 Besides that, lithium borate salts have gained high interest due to their high thermal stability, cost-effectiveness, favorable solid electrolyte interface (SEI) formation and ionic conductivities in the same range as LiTFSI.5 For example, Lithium bis(oxalate)borate (LiBOB) and its asymmetric counterpart Lithium difluoro(oxalate) borate (LiDFOB) are stable in organic solvents and the electrochemical stability is higher than 4.5 V vs. Li/Li+.6 Different salts in an electrolyte can influence the Li-ion transport as well as the processes at the interfaces or the formation of the SEI. This again requires a detailed comparative analysis.For this, we prepared promising bottlebrush polymer electrolytes (1 kg mol-1 PEG sidechain) containing LiBOB and LiDFOB and subsequently analyzed them electrochemically by impedance spectroscopy. The measurement data was finally interpreted by the extended Distribution of Relaxation Times (eDRT). Both SPEs showed similarities in the lithium-ion conducting process in possessing one major, resistive-capacitive bulk conductivity mechanism. The resulting, temperature-dependent conductivities were evaluated and are in the range of 10-4 to 10-5 S cm-1.During cycling, the SPEs showed increased interface resistances over time, which are higher than respective bulk resistances. By applying eDRT, the time-dependent formation of an interphase layer in the SPEs is identified, separated from the slower charge transfer process and quantified. Thus, the electrolytes cannot be considered electrochemically stable against metallic Li, which is similar for liquid electrolytes.7 References (1) Scrosati, B.; Energy Environ. Sci. 2011, 4, 3287.(2) Janek, J.; Nat. Energy 2016, 1, 16141.(3) Rosenbach, D.; ACS Appl. Energy Mater. 2019, 2, 3373–3388.(4) Etacheri, V.; Energy and Environmental Science 2011, pp 3243–3262.(5) Xu, K. Chem. Rev. 2014, 114, 11503–11618.(6) Liu, Z.; Coord. Chem. Rev. 2015, 292, 56–73.(7) Hahn, M.; Electrochim. Acta 2020, 344, 136060. Figure 1

Solid polymer electrolytes (SPEs) have emerged as promising candidates for solid-state lithium metal batteries (LMBs) due to their inherent safety advantages and potential to facilitate high energy density devices [1]. Poly(ethylene oxide) (PEO) has been the most prominent representative polymer host in SPEs since 1970s because of their excellent ability to solvate Li ions and support ion transport [2,3]. However, it suffers from limited oxidation stability (<4 V vs. Li) and low ambient temperature conductivity (up to 10−6 S cm−1), thus hindering its practical application in high voltage (>4V) high energy density LMBs. In this study, a new type of polymer-in-“high concentrated ionic liquid” solid electrolyte is designed with high molecular weight PEO, N-propyl-N-methylpyrrolidinium bis(fuorosulfonyl) imide (C3mpyrFSI) ionic liquid and lithium bis(fluorosulfonyl)imide (LiFSI). The EO: [LiFSI/C3mpyrFSI] ratio has been widely varied and the resulting physicochemical and electrochemical properties have been explored. The optimal electrolyte provides promisingly high oxidative stability exceeding 5V and ambient temperature conductivity of 5.6 x10-4 S cm-1. It induces stable solid electrolyte interface with Li metal and demonstrates prolonged reversible Li plating-stripping in Li symmetrical cells even at high temperatures (up to 70°C). All solid-state Li metal cells assembled with LFP cathode show excellent rate capability and cycling performance (see Figure 1). The discharge capacity can be maintained at 47.3% when the current density increases 20 times from 0.1 C to 2 C (areal capacity ~0.4 mAh/cm2), and long-term cycling of Li||LFP cells showed 97% capacity retention after 100 cycles at 0.2 C. Based on the findings, an ion conduction network consisting of interconnected anion-rich clusters and pores is highlighted to help understand the structural change, ion dynamics and good battery performance of the designed SPE. Therefore, the novel approach of polymer-in-“high concentrated ionic liquid” solid electrolyte enables a new pathway to design high-performing SPEs for high energy density all-solid-state LMBs. Figure 1

Solid state electrolytes are the key components for high energy density lithium ion batteries and especially for lithium metal batteries where lithium dendrite growth is an inevitable obstacle in liquid electrolytes. Solid polymer electrolytes based on a complex of polymers and lithium salts are intrinsically advantageous over inorganic electrolytes in terms of processability and film‐forming properties. But other properties such as ionic conductivity, thermal stability, mechanical modulus, and electrochemical stability need to be improved. Herein, for the first time, 2D additives using few‐layer vermiculite clay sheets as an example to comprehensively upgrade poly(ethylene oxide)‐based solid polymer electrolyte are introduced. With clay sheet additives, the polymer electrolyte exhibits improved thermal stability, mechanical modulus, ionic conductivity, and electrochemical stability along with reduced flammability and interface resistance. The composite polymer electrolyte can suppress the formation and growth of lithium dendrites in lithium metal batteries. It is anticipated that the clay sheets upgraded solid polymer electrolyte can be integrated to construct high performance solid state lithium ion and lithium metal batteries with higher energy and safety.

A new class of solid polymer electrolytes (SPEs) is successfully developed by employing poly(ethylene oxide) (PEO) as a polymer host, N-ethyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (P12FSI) as an ionic plastic crystal, and lithium bis(fluorosulfonyl)imide (LiFSI) as a lithium salt. Their properties, including thermal behavior and electrochemical and mechanical characteristics are evaluated. For the as-obtained SPE system, P12FSI combines with PEO-LiFSI uniformly and no phase separation occurs. Such SPEs exhibit high thermal stability, good electrochemical stability, satisfactory ionic conductivity, acceptable mechanical strength and flexibility, as well as the potential to inhibit the lithium dendrite growth. Furthermore, Li/LiFePO4 cells containing the as-prepared SPE at 0.2C rate could exhibit a discharge capacity of about 157 mAh g–1 at 50 °C, with high cycling stability and favorable rate capability. Such SPEs are expected to have great potential for all-solid-state lithium batteries.