Optimization of Cation−Anion Dynamics in Cationic Polymer Electrolytes for Boosting the Performance of All‐Solid‐State Lithium−Metal Battery

<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.

Implementation of solid polymer electrolytes (SPEs) is believed to be one pathway forward in the development of lithium-metal batteries (LMBs) that are both safe and possesses high capacities. This combination is possible since the free-standing SPE maintains its mechanical integrity during electrochemical and thermal exposure while utilizing lithium metal as the negative electrode, thus accessing the extremely high theoretical capacity of lithium metal. The main drawback for SPEs are however the poor ionic conductivity compared to the liquid electrolytes used in lithium-ion batteries today.[1] The low ionic conductivity for SPEs stems from the conduction mechanism of the ions, which is a slower process in SPEs compared to in liquid electrolytes. In general, the conductivity in SPEs are confined to the amorphous regions, where segmental motion of the polymer chains facilitates the ion transport; while the crystalline regions restrict the movements of the polymer chains and impairs the ion transport.[2] The highly crystalline polyketone poly(1-oxoheptamethylene) (POHM) is a material that has shown interesting characteristics for implementation as a SPE in batteries. In earlier studies, the polyketone revealed an extensive improvement of the ionic conductivity upon increasing the salt concentration, although the crystallinity remained with a high melting point.[3] Herein this study, the effect of the crystallinity on the characteristics and the performance was investigated further. To understand its effect, a second less hydrogenated version of POHM was synthesized, POHM-75, which is fully amorphous at high salt concentrations. The characteristics of the polyketones were in addition also compared with the more well-studied SPEs, that is polyethylene oxide (PEO) and poly(trimethylene carbonate) (PTMC). Focus of the presentation is on the interplay between the ionic conductivity and mechanical stability for the polyketones, for which something different to the normally observed tradeoff between the two properties is observed. The electrochemical performance of the polyketones are furthermore evaluated when implemented as SPEs in LMBs at extreme temperature environments, a possible application area for mechanically stable SPEs.[1] D. Zhou, D. Shanmukaraj, A. Tkacheva, M. Armand, G. Wang, Chem 2019, 5, 2326-2352.[2] M. A. Ratner, P. Johansson, D. F. Shriver, MRS Bulletin 2011, 25, 31-37.[3] T. Eriksson, H. Gudla, Y. Manabe, T. Yoneda, D. Friesen, C. Zhang, Y. Inokuma, D. Brandell, J. Mindemark, Macromolecules 2022, 55, 10940-10949. Figure 1

Lithium metal has generated significant interest as an anode material because of its high theoretical capacity (3860 mAh/g). However, issues such as dendrite growth and lithium loss during cycling make this material incompatible with current liquid electrolytes. Solid polymer electrolytes (SPE) have been proposed as a potential replacement as they are non-flammable, able to resist dendrite growth, have decent ionic conductivity and good interfacial contact with lithium metal.One aspect that is often overlooked is the fact that passivation layers, initially reported by Naudin et Al (1), often form on the lithium metal surface and are hence intrinsic to the chemical composition of the lithium surface (2, 3). According to Otto et Al (4), residual quantities of atmospheric gases are often present in lithium metal storage environments, thus electrode surface modification is inevitable. This passivation layer is expected to impact the reactivity of the lithium metal anode (LMA) towards SPE. Moreover, such phenomenon was originally described by Peled et Al as the solid electrolyte interface (SEI) (5). Lithium metal reactivity towards PEO, suspected by Galluzo et Al (6), was recently reported by Liu et Al (7), under optimized conditions (ultra-thin neat materials), but the impact of this phenomenon in a realistic LMA environment has not been extensively investigated.In this study, the impact of gas exposure on a LMA was investigated by exposing freshly cut lithium rods to O2, CO2 and N2. The resultant passivation layers were characterized via XPS. The effect of passivation layer formation on LMA reactivity towards SPE was measured by exposing samples with pre-characterized passivation layers to common SPE materials. The resultant interface was characterized using Raman spectroscopy. SPE-passivation layer reactivity was correlated to ageing properties by EIS and kinetic charge transfer characteristics in galvanostatic linear polarization at the LMA-SPE interface, in a symmetric Li – SPE – Li stack.This study revealed that the chemical composition of the lithium metal passivation layer affects its reactivity towards SPE as well as the electrochemical performance of a Li-SPE-Li symmetric cell. A thorough characterization of the lithium metal passivation layer is essential for the understanding of some of the fundamental factors affecting all solid-state lithium metal battery performance and is therefore consubstantial with the development of modified LMA. C. Naudin et al., Characterization of the lithium surface by infrared and Raman spectroscopies. Journal of Power Sources 124, 518-525 (2003).S.-K. Otto et al., In-Depth Characterization of Lithium-Metal Surfaces with XPS and ToF-SIMS: Toward Better Understanding of the Passivation Layer. Chemistry of Materials 33, 859-867 (2021).P. R. G. Zhuang, F. Kong, F. McLarnon The reaction of clean Li surfaces with small molecules in ultrahigh vacuum: II. Journal of The Electrochemical Society, 159-164 (1998).S.-K. Otto et al., Storage of Lithium Metal: The Role of the Native Passivation Layer for the Anode Interface Resistance in Solid State Batteries. ACS Applied Energy Materials 4, 12798-12807 (2021).E. Peled, Film forming reaction at the lithium/electrolyte interface. Journal of Power Sources 9, 253-266 (1983).M. D. Galluzzo et al., Dissolution of Lithium Metal in Poly(ethylene oxide). ACS Energy Letters 4, 903-907 (2019).P. Liu et al., Increasing Ionic Conductivity of Poly(ethylene oxide) by Reaction with Metallic Li. Advanced Energy and Sustainability Research 3, 2100142 (2022).

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) incorporating lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offer excellent potential for solid-state lithium metal batteries (SSLMBs) due to their wide electrochemical stability window and high lithium salt solubility. LiTFSI supplies Li+ ions and also acts as a plasticizer, promoting the formation of the amorphous phase, which is vital for ionic conduction. Reduced ionic conductivity can result from very high crystallinity, spherulitic morphology, and secondary crystallization disrupting the ion pathways. The [EO:Li] molar ratio critically governs crystallization behavior and ionic transport; a ratio of 10:1, close to the stable amorphous eutectic composition (9:1), promotes amorphous phase formation and high ionic conductivity while limiting excessive crystallization. However, high salt content at this ratio leads to membrane stickiness, low mechanical strength, and handling difficulties. Here, we present a parafilm-assisted peeling technique that enables the fabrication of freestanding PEO/LiTFSI membranes while preserving their structural integrity. SEM analysis shows that conventional Teflon-mold peeling yields cracked, stretched surfaces, whereas parafilm-assisted films are smooth and defect-free. FTIR confirms Li+–PEO coordination with no parafilm–electrolyte interaction. Both methods deliver similar electrochemical performance: ∼4.30 V stability window, ionic conductivity 3.96 × 10−3 S cm−1 at 50 ° C (E a ∼ 0.898 eV), and Li+ transference number ∼0.14–0.15. Symmetric Li|SPE|Li cells cycle stably for 300 h at 50 ° C , while LiFePO4 full cells retain ∼161 mAh g−1 and >98% Coulombic efficiency over 30 cycles, confirming parafilm-assisted peeling as an effective fabrication method. This approach presents an effective means to address processing difficulties, thereby extending the viability of PEO based SPEs in next-generation energy storage devices.

Jeffamine® based polymers as highly conductive polymer electrolytes and cathode binder materials for battery application

Electrolyte plays a pivotal role in Li-ion battery (LIB). Generally, it has been considered as a liquid phase material that allows for reasonable dissolution ability of inorganic salt and ion transportability across the electrodes. Since conventional liquid electrolytes faced limitations to improve battery performance, the solid-state electrolyte has raised rapidly to replace the liquid electrolyte. Solid polymer electrolyte (SPE) is one of the most promising solid-state electrolytes, it has attracted attention due to its strength as an electrolyte such as high accessibility, tolerance against volume change of electrodes, and reasonable chemical/thermal stability. Especially poly(ethylene oxide) (PEO) or PEO-based electrolytes have been widely developed, because of their outstanding Li salt dissolution ability. PEO forms a unique helical complex structure with Li salt, the complex is highly stable. It allows PEO to dissolve Li salt much easier than other polyethers, such as poly(propylene oxide) or poly(oxymethylene). However, the high stability of the complex could disturb fast ionic conduction in electrolyte medium, it is the reason why PEO electrolyte has poor ionic conductivity at ambient temperature (10-7 to 10-6S/cm at room temperature). Crown ethers, the macrocycles composed of ethylene oxide, also attract attention for SPEs. Even though the monomer is the same (ethylene oxide) for both PEO and Crown ethers, the properties of each complex are different. Crown ether electrolyte shows higher ionic conductivity than PEO electrolyte at ambient temperature, it is assumed that the crown ether electrolyte is an amorphous phase while PEO electrolyte has high crystallinity below the melting point (60~80%). Crown ether is also promising for Li metal batteries. Crown ether is the hosting material that has a preorganized acceptor structure (also well-known as macrocyclic effect), therefore it can adjust kinetics of reduction on the surface of Li metal anode. Even though promising abilities of crown ether electrolytes have been reported, their ion conduction mechanisms are yet understood well. Therefore, it is worth studying about complex structure and ion conduction mechanism of crown ether electrolyte for a better understanding of SPE, and further, it can be a milestone to design new SPEs. To understand the complex structure and ion conduction mechanism in crown ether electrolytes, we performed both experimental and theoretical analysis. We synthesized the complexes of different sizes of crown ethers (12-crown-4, 15-crown-5, and 18-crown-6) and Li salts (LiTFSI, LiClO4, LiBF4, and LiI). The complexes are characterized by Fourier transformed infrared (FTIR), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), and AC impedance analysis. DFT calculation and molecular dynamics for each complex were performed to obtain an atomistic description of the complexes.

Cubic garnet solid polymer electrolyte for room temperature operable all-solid-state-battery

The electrolyte of lithium secondary batteries is an important component. Among lithium secondary batteries, the lithium polymer battery, which has similar performance to the lithium secondary battery, is made of a solid polymer electrolyte. Lithium ions in a solid polymer electrolyte exist in the form of a solution by a polar group in a polymer matrix. Lithium ions in the solid polymer electrolyte migrate via the segmental motion of the polymer. That is, the properties of a polymer matrix in a solid polymer electrolyte can affect the conduction of lithium ions. Therefore, this study focused on the electrochemical properties of poly(ethylene oxide)-block-poly(propylene oxide)-blockpoly( ethylene oxide) copolymer-based solid polymer electrolyte. For this, poly(ethylene oxide)-blockpoly( propylene oxide)-block-poly(ethylene oxide) copolymer-based solid polymer electrolytes, which have spherical micelles, and various sizes of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer micelle, were prepared. Amino acids were added to the poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) copolymer-based solid polymer electrolyte as an ion dissociator to assist in the dissociation of a lithium salt and increase the ionic conductivity of the solid polymer electrolyte. The copolymer-based solid polymer electrolytes was characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, impedance analysis, cyclic voltammetry, and linear sweep voltammetry.

Asbestos-functionalized solid polymer electrolyte for uniform Li deposition in lithium metal batteries

A novel de-coupling solid polymer electrolyte via semi-interpenetrating network for lithium metal battery

Liquid electrolytes (LEs) commonly show severe side reactions at the electrode-electrolyte interface, especially with alkali metal anodes, leading to rapid capacity fade of metal-ion batteries. Solid polymer electrolytes (SPEs), however, contribute to the suppression of side reactions due to their inherent inertness and high mechanical strength, providing long-term stable battery operation. Herein, we investigated physical and electrochemical properties of SPEs based on our previously reported microphase-separated poly(vinyl benzyl methoxy poly(ethylene oxide) ether)-block-polystyrene block copolymer (PVBmPEO-b-PS) with different alkali metal ions (A+ = Li+, Na+ or K+) and their use in the respective metal batteries, showing the potential for the transition from lithium to post-lithium batteries. Rheological and thermal properties as well as ion transport in the SPEs with different bis(trifluoromethanesulfonyl)imide (TFSI) salts concentrations revealed similar shear storage moduli (G’) for the investigated SPEs, while the lowest glass transition temperatures (Tg) were found for KTFSI-based films. By contrast, the highest total ionic conductivity was found for the LiTFSI-based SPEs. To quantify the A+ transference numbers (TA+), the Bruce-Vincent method and pulsed-field gradient (PFG) NMR were conducted, revealing significant challenges for TA+ determination of post-Li systems. Further, the examination of the interfacial stability of SPE/A interfaces by conducting plating/stripping experiments revealed significantly higher resistances for sodium- and potassium-based systems in comparison to their lithium-based counterpart. Nonetheless, A-metal/SPE/cathode cells with PVBmPEO-b-PS-based Na- and K-SPEs with the Prussian Blue analogues (PBAs, Na2-xFe[Fe(CN)6] and K2-xFe[Fe(CN)6]) positive electrodes and the respective alkali metal negative electrodes enabled cycling at elevated temperature of 55 °C. Herein, both sodium and potassium metal batteries exhibited stable cycling with capacity retentions of 73% over 100 cycles for the Na-cell, and 94% over the same cycle number for the K-cell, (and a high coulombic efficiency (CE) of 98% at the 100th cycle).

We discuss recent progresses on anisotropic ion transport in solid polymer electrolytes.

In-situ polymerized acrylate-based polymers are very appealing as solid polymer electrolytes (SPEs) in lithium (Li) batteries due to the drop-in compatibility of such in-situ polymerization with conventional battery production, the ease of acrylate polymerization and their inexpensive, facile SPE chemistry. We identified, however, that such SPEs suffer from either a low cationic transference number with dual ion conducting salts (0.12-0.2) or a low ionic conductivity with single Li-ion conducting (SLIC) salts (6·10-6 S cm-1). In this project we overcame these limitations and developed significantly improved SPE with ionic conductivity of up to 2·10-4 Scm-1 and a relatively high Li-ion transference number of 0.4. With a significantly reduced fraction of mobile anions in the hybrid SPE, in-situ polymerized SPE cells with an LiFePO4 (LFP) cathode achieve a stable performance for over 100 cycles at temperatures as high as 100 °C, which is unattainable with conventional Li salts or electrolytes. Furthermore, the motional narrowing observed in the line-shapes of solid-state nuclear magnetic resonance (SS-NMR) spectra provided additional insights of the differences in Li nucleus environments and revealed a reduced activation energy for the hybrid salt SPEs due to their more open structure. This study opens the path for the fabrication of high-performance solid polymer lithium batteries capable of operating at high temperatures using commercial battery fabrication equipment. We expect that further tuning of the acrylate based SPE composition may allow further increases in its conductivity without sacrifices in its electrochemical stability or mechanical properties.

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.

The development of metallic lithium anode is restrained by lithium dendrite growth during cycling. The solid polymer electrolyte with high mechanical strength and lithium ion conductivity could be applied to inhibit lithium dendrite growth. To prepare the high‐performance solid polymer electrolyte, the environment‐friendly and cheap bacterial cellulose (BC) is used as filler incorporating with PEO‐based electrolyte owing to good mechanical properties and Li salts compatibility. PEO/LiTFSI/BC composite solid polymer electrolytes (CSPE) are prepared easily by aqueous mixing in water. The lithium ion transference number of PEO/LiTFSI/BC CSPE is 0.57, which is higher than PEO/LiTFSI solid polymer electrolyte (SPE) (0.409). The PEO/LiTFSI/BC CSPE exhibits larger tensile strength (4.43 MPa) than PEO/LiTFSI SPE (1.34 MPa). The electrochemical window of composite electrolyte is widened 1.43 V by adding BC. Density functional theory calculations indicate that flex of PEO chains around Li atoms is suppressed, suggesting the enhanced lithium ion conductivity. Frontier molecular orbitals results suggest that an unfavorable intermolecular charge transfer lead to achieve higher potential for BC composite electrolyte. All solid‐state Li metal battery with PEO/LiTFSI/BC CSPE delivers longer cycle life for 600 cycles than PEO/LiTFSI SPE battery (50 cycles). Li symmetrical battery using PEO/LiTFSI/BC CSPE could be stable for 1160 h.