A novel de-coupling solid polymer electrolyte via semi-interpenetrating network for 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.

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).

ABSTRACT

The deployment of safe and high‐energy density lithium metal polymer batteries (LMPBs) still requires further advances in the quest for new solid polymer electrolytes (SPEs). In this regard, salt anions have a decisive role in the overall SPE performance. While lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) was chosen earlier to have a highly flexible sulfonimide center and an extensively delocalized negative charge, it still suffers from several drawbacks ascribed to its poor interfacial compatibility with the lithium metal (LiM) anode and the fact that it is a PFAS. In this work, a novel lithium salt is cunningly designed, aiming to combine the advantages of previously reported lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(difluoromethanesulfonyl)imide (LiDFSI) to overcome the limitations of the state‐of‐the‐art SPE based on LiTFSI/poly(ethylene oxide) (PEO). The SPE containing the developed (difluoromethanesulfonyl)(fluorosulfonyl)imide (LiDFFSI) salt presented reduced interfacial resistance and improved compatibility with the lithium metal (LiM) anode compared with LiTFSI/PEO, enabled by the formation of a stable, uniform, and ionically conductive solid–electrolyte interphase (SEI). In addition, LiDFFSI‐based SPEs demonstrated a prolonged cycling stability, achieving over 125 cycles at C/10 with minimal capacity fading in LiM||LiFePO4 cell configuration. These findings evidence how a rational design of the lithium salt chemistry allows tuning the formed SEI, directly impacting the overall SPE performance. Thus, LiDFFSI is presented as a promising alternative lithium salt to improve electrochemical performance and interfacial stability in next‐generation LiM batteries.

The anion chemistry of lithium salts plays a pivotal role in dictating the physicochemical and electrochemical performance of solid polymer electrolytes (SPEs), thus affecting the cyclability of all-solid-state lithium metal batteries (ASSLMBs). The bis(trifluoromethanesulfonyl)imide anion (TFSI- ) has long been studied as the most promising candidate for SPEs; however, the Li-ion conductivities of the TFSI-based SPEs still remain low (Li-ion transference number: ca. 0.2). In this work, we report new hydrogen-containing anions, conceived based on theoretical considerations, as an electrolyte salt for SPEs. SPEs comprising hydrogen-containing anions achieve higher Li-ion conductivities than TFSI-based ones, and those anions are electrochemically stable for various kinds of ASSLMBs (Li-LiFePO4 , Li-S, and Li-O2 batteries). This opens up a new avenue for designing safe and high-performance ASSLMBs in the future.

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

Lithium metal has generated significant interest as an anode material because of its high theoretical capacity. However, issues such as dendrite growth and lithium loss during cycling make this material incompatible with liquid electrolytes. Solid polymer electrolytes (SPE) have been proposed as replacements as they are non-flammable, resist dendrite growth, have decent ionic conductivity, and have low resistance with lithium metal. Passivation layers, which form on the lithium metal surface and are hence intrinsic to its chemical composition, are often overlooked. Residual quantities of atmospheric gases are present in lithium metal storage environments, making surface modification and its subsequent impact on anode reactivity inevitable. Moreover, the impact of this phenomenon in a realistic lithium metal anode (LMA) environment with SPE has not yet been extensively investigated. In this study, the impact of gas exposure on an LMA was investigated by exposing freshly cut lithium rods to O2, CO2, and N2. Passivation layers were characterized via X-ray photoelectron spectroscopy. The effect of passivation layer formation on LMA reactivity toward SPE was measured by exposing passivated samples to common SPE materials. The resultant interface was characterized using Raman spectroscopy. SPE-passivation layer reactivity was correlated to ageing by electrochemical impedance spectroscopy and kinetic charge transfer via galvanostatic linear polarization at the LMA-SPE interface in symmetric Li─SPE─Li stacks. This study revealed that the chemical composition of the passivation layer affects LMA reactivity toward SPE and electrochemical performance. A thorough characterization of the lithium metal passivation layer is essential to understanding the fundamental factors affecting solid-state lithium metal battery performance.

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

Solid polymer electrolytes (SPEs) are promising candidates for usage in rechargeable lithium metal batteries (LMBs) as they possess high mechanical, thermal, and chemical stability. However, the poor ionic conductivity of SPEs in comparison to liquid electrolytes hampers the commercialization of SPE-based LMBs. In the case of poly[bis(methoxy-ethoxy-ethoxy-)phosphazene] (MEEP), one explanation for the low ionic conductivity is the trapping of lithium cations in backbone coordination sites, hindering lithium ion movement through the electrolyte membrane. Herein, modelling the ion coordination in MEEP using DFT calculations reveals that, compared to lithium, heavier alkali cations are more likely to be complexed at the backbone coordination sites. With other alkali cations masking these coordination sites, enhanced lithium ion mobility through the SPE is expected. Experimental data proves these expectations: doping MEEP-based LiBOB-containing SPE membranes with small amounts of in-house synthesized potassium bis(oxalato)borate (KBOB) increases the lithium ion transference number from 0.08 to 0.18. Also, the partial lithium ion conductivity of the salt-in-MEEP electrolyte is boosted to outstanding 0.08 mS cm−1, far exceeding state-of-the-art literature values for this material. A cross-check using SPEs based on the structurally similar poly(ethylene oxide) (PEO) validates the proposed cation displacement model. The obtained insights may aid the development of highly effective poly(phosphazene)-based SPEs.

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.

The practical application of lithium (Li) metal battery is impeded by the Li dendrite growth and unstable solid electrolyte interphase (SEI) layer. Herein, an ultra‐stretchable and ionic conducting chemically crosslinked pressure‐sensitive adhesive (cPSA) synthesized via the copolymerization of 2‐ethylhexyl acrylate and acrylic acid with poly(ethyleneglycol)dimethacrylate as crosslinker (short for 70cPSA), is developed as both artificial SEI layer and solid polymer electrolyte (SPE) for stable Li‐metal electrode, enabling all‐solid‐state Li metal batteries with excellent cycling performance. As an artificial SEI layer, the 70cPSA‐modified electrodes exhibit excellent electrochemical performance in Li|70cPSA@Cu half cells and 70cPSA@Li|70cPSA@Li symmetric cells. In full cells with LiFePO4 (LFP) as cathode, the 70cPSA@Li|LFP cell exhibits stable cycling performance over 250 cycles. Utilized as SPE, the all‐solid‐state Li|SPE|LFP cell delivers excellent cycling stability with a capacity retention of 86% over 500 cycles. With high‐voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) as cathode, the Li|SPE|NMC811 cell exhibits a discharge capacity of 124.3 mAh g−1 with a capacity retention of 71% after 200 cycles. The rational design of PSAs and investigation of their dual role for stable and safe Li‐metal batteries may shed a light on adhesive polymers for battery applications.

Covalent organic frameworks (COFs)/poly(ethylene oxide) (PEO) solid electrolytes offer bright application prospects in all-solid-state lithium–metal batteries (ASSLBs), while they struggle with low ionic conductivity due to poor interfacial Li+ transport kinetics. This work employs acetamide (EA) as a shuttle to transfer Li+ at COF/PEO interfaces via the synergistic EA-PEO, EA-COF, and EA-Li+ interactions, thereby constructing continuous channels for fast Li+ transport. The resulting EA-modified COF/PEO solid electrolyte delivers an ionic conductivity of 1.01 × 10–4 S·cm–1 at 30 °C. In addition, the synergistic interactions induced by EA promote the dissociation of lithium salt, leading to a LiF- and Li3N-rich solid electrolyte interface (SEI) layer and thus enhanced interfacial stability. As a result, the LiFePO4-based ASSLB assembled with the PEO/LiTFSI-COF/EA solid electrolyte demonstrates exceptional cycling stability with 70.34% capacity retention over 1200 cycles at 1 C and 60 °C. Our work provides a powerful strategy to facilitate interfacial ion transport and reinforce long-term stability in solid polymer electrolytes.

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.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are favorable for all-solid-state lithium metal batteries (ASSLBs) to ensure safety and enhance energy density. However, their narrow work windows and unstable electrode/electrolyte interfaces hinder their practical application in high-voltage ASSLBs. Although introducing additives in SPEs has been proven to be effective to address the above issues, it could hardly optimize both cathode and anode interfaces by an individual additive. Herein, heterogeneously double-layer SPEs are constructed with two typical additives (LiPO2F2 and LiFSI), which are used to modify the LiNi0.6Co0.2Mn0.2O2 (NCM)-cathode/electrolyte interface (CEI) and lithium-anode/solid electrolyte interface (SEI), and further understand their respective mechanism in enhancing the capacity and cycling stability of ASSLBs. Specifically, LiPO2F2 not only leads to a uniform CEI layer to prevent the oxidation decomposition of PEO and LiTFSI but also ensures fast Li+ diffusion at high voltage (>3.9 V), improving the rate performances and life spans of the cells. The LiFSI contributes to a stable SEI layer with rich LiF, suppressing the growth of lithium dendrites and maximizing the specific capacity for ASSLBs. Integrating the advantages of the two functional molecules, the optimized ASSLB displays an excellent capacity of 141.4 mAh g-1 at 1C and an outstanding capacity retention of 81.6% after 400 cycles when using the NCM cathode, even reaching 154.2 mAh g-1 at 0.1 mA cm-2 with a high mass loading (6.4 mg cm-2). Additionally, the bilayer SPEs also match well with a LiFePO4 electrode with a high mass loading of 11.0 mg cm-2, displaying a high capacity of 155.7 mAh g-1 at 0.1 mA cm-2.

Lithium metal batteries have the potential to enhance the energy and power density of commercial rechargeable batteries. A lithium metal anode may increase the energy density by 30% relative to current lithium-ion battery technologies. The promise of these improvements hinges on a successful solid electrolyte that is thin (< 30 μm). Inorganic ceramic electrolytes can offer reasonable ionic conductivities but are typically bottlenecked by intensive processing. Solid polymer electrolytes offer the advantages of flexibility, low-cost processing, and a greater adhesion to the electrodes than a ceramic. The biggest drawback to solid polymer electrolytes is their low ionic conductivity. A composite electrolyte consisting of a hard ceramic phase and a processible polymer phase aims to utilize the advantages of each as a solution for solid-state electrolytes. Here we introduce an approach to fabricate a solid composite electrolyte film that is thin, highly ionically conductive, and mechanically robust with good manufacturability. Without the use of plasticizers, our composite film has an ionic conductivity of approximately 1.0 × 10-4 S/cm at 30°C with an activation energy of ca. 0.44 eV. The high conductivity was achieved by creating a 3D, interconnected structure of the ceramic with greatly reduced interparticle resistance. Ion conduction path in the composite electrolyte is investigated by finite element analysis. Acknowledgements: This research at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE), under contract DE-AC05-00OR22725, was primarily sponsored by the Office of Energy Efficiency and Renewable Energy for the Vehicle Technologies Office’s Advanced Battery Materials Research program and partially sponsored by the Laboratory Directed Research and Development Program (LDRD) of ORNL.