Double-network composite solid electrolyte with stable interface for dendrite-free Li metal anode

In situ polymerization of cyclic ethers is a promising strategy to construct solid-state lithium (Li) metal batteries with high energy density and safety. However, their practical applications are plagued by the unsatisfactory electrochemical properties of polymer electrolytes and the unstable solid electrolyte interphase (SEI). Herein, organic perfluorodecanoic acid (PFDA) is proposed as a new initiator to polymerize 1,3-dioxolane electrolyte (PDOL), which enables the as-obtained PDOL electrolyte to deliver greatly enhanced ionic conductivity and broadened electrochemical window. Besides, the experimental data and theoretical calculations demonstrate a dual-layered SEI with PFDA-derived organic component on the top and LiF at the bottom constructed on the surface of Li metal, which can provide enough mechanical strength to suppress Li dendrite growth and high flexibility to accommodate volume fluctuations during the repeated cycling. As a result, the Li symmetric cells with PFDA-induced PDOL electrolyte (P-PDOL) can achieve a superior plating/stripping cycle for 1400h at 0.3mA cm-2. Additionally, the Li||P-PDOL||LiFePO4 (LFP) full cells maintain stable cycling over 300 times at 0.5 C. This work offers a potential strategy to simultaneously prepare high-performance PDOL electrolytes and stabilize the Li metal/PDOL interface, providing new research insights to advance solid-state Li metal batteries toward practical applications.

In vacuo XPS investigation of surface engineering for lithium metal anodes with plasma treatment

High-voltage lithium (Li) metal batteries (LMBs) are promising next-generation high-energy-density rechargeable batteries. Siloxane electrolytes exhibit excellent performance in high-voltage LMBs. Herein, the mechanisms responsible for the Li metal compatibility and high-voltage resistance of siloxane electrolytes were probed by classical molecular dynamics (MD) simulations, first-principles calculations, and experimental characterizations. Siloxane electrolytes have been demonstrated to deliver anion-rich solvation structures, which are induced by weak Li ion (Li+)-solvent interactions and strong Li+-anion interactions. The silicon (Si)─oxygen (O) bond energy of siloxane is larger than that of carbon (C)─O of C-siloxane (replacing Si atoms in siloxane with C atoms) because the atomic radius of Si is larger than that of C, and the Pauli exclusion of Si is smaller than that of C. Additionally, ab initio molecular dynamics (AIMD) simulations revealed that the decomposition of siloxane produces substances containing Si─O fragments on Li metal surfaces, which is beneficial for interfacial stability. This work reveals the mechanism of interfacial stability and intrinsic stability of siloxane electrolytes, providing a theoretical basis for the practical application of siloxane electrolytes in high-voltage LMBs.

<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.

Development of solid inorganic lithium (Li) ion conducting materials for the use as solid electrolytes is indispensable for the realization of next-generation all-solid-state Li batteries with high safety and reliability. Among various oxide-based solid electrolyte materials, a garnet-type oxide with the formula of Li7La3Zr2O12 (LLZO) has attracted much attention because of its high Li ion conductivity at room temperature, excellent thermal performance, and high stability against Li metal.1,2) However, the formation of a solid-solid interface between LLZO and the Li metal anode is challenging. Poor interfacial connection causes non-uniform Li plating and intergranular penetration of Li dendrite in polycrystalline LLZO when the cell is cycled particularly at high current densities, resulting in internal short-circuit failure.3–5) There is no doubt that the establishment of prevention technology for short-circuit failures is a top priority issue for the development of all-solid-state Li metal batteries.5) On the other hand, from the viewpoint of effective use of material resources, the possibility of reusing LLZO extracted from a solid-state battery after a short-circuit failure occurred is worth considering. In this work, we investigated the reusability of a Ta-doped Li6.55La3Zr1.55Ta0.45O12 (Ta-LLZO) solid electrolyte shorted by Li dendrite growth during electrochemical Li plating/stripping testing for a Li/Ta-LLZO/Li symmetric cell. Ta-LLZO was taken out of a tested cell after the degradation by Li dendrite growth occurred, and then annealed at 700 ºC in air. The annealing temperature was set to suppress possible excess Li loss from Ta-LLZO during post-annealing.6) In the first Li plating/stripping test, the cell was shorted at 0.85 mA cm-2 and the dark gray area with possible Li dendrite growth was confirmed on the surface of Ta-LLZO. This dark gray area turned white but slightly different from the original color of Ta-LLZO by post-annealing. The ionic conductivity of as-synthesized and post-annealed Ta-LLZO was measured and compared. Post-annealed Ta-LLZO retained high room temperature ionic conductivity of 0.82 mS cm-1, which is slightly lower than the conductivity of as-synthesized one (= 0.90 mS cm-1). We also prepared a symmetric cell with the post-annealed Ta-LLZO and Li metal electrodes and the second Li plating/stripping test was carried out. Symmetric cell with post-annealed Ta-LLZO showed stable voltage response. This indicates the possibility of reusing the degraded garnet-type solid electrolyte by Li dendrite growth for an another solid-state Li battery.

Li10GeP2S12 (LGPS) solid electrolyte with extremely high ionic conductivity is a promising candidate for all‐solid‐state lithium batteries. However, continued LGPS reduction at Li/LGPS interface is huge challenge. Thus, developing a simple and effective strategy to improve Li/LGPS interfacial stability is highly urgent. Herein, LiF‐rich multifunctional interfaces on Li metal (Li@LiF), consisting of three functional components of LiF, carbon particles, and thin CF bond top‐layer, are in situ constructed on the Li metal surface by spontaneous reaction between Li metal and poly(tetrafluoroethylene) at room temperature. LiF, as the main component, significantly suppresses the interfacial reaction due to low electronic conductivity and promotes smooth lithium plating on the Li metal surface arising from high interfacial energy against Li. Meanwhile, carbon particles with excellent Li+ affinity and thin strong polar CF bond top‐layer enable homogenous Li+ flux as well as fast Li+ migration across the LiF‐rich interface, respectively. Consequently, the LGPS‐based symmetric lithium cell possesses significantly prolonged cycling stability and increased critical current density. The LGPS‐based all‐solid‐state lithium cells employing LiCoO2 cathode and Li@LiF anode exhibit excellent rate capability and impressive cycling stability, achieving a high capacity retention of 80.9% over 300 cycles at 0.1 C under 25 °C.

Li metal batteries (LMBs) containing cross‐linked polymer electrolytes (PEs) are auspicious candidates for next‐generation batteries. However, the wetting behavior of PEs on uneven Li metal surfaces has been neglected in most studies. Herein, it is shown that microscale defect sites with curved edges play an important role in a wettability‐dependent electrodeposition. The wettability and the viscoelastic properties of PEs are correlated, and the impact of wettability on the nucleation and diffusion near the Li|PE interface is distinguished. It is found that the curvature of the edges is a key factor for the investigation of wetting phenomena. The appearance of microscale defects and phase separation are identified as main causes for erratic nucleation. It is emphasized that the implementation of stable and consistent long‐term cycling performance of LMBs using PEs requires a deeper understanding of the “soft‐solid”–solid contact between PEs and inherently rough Li metal surfaces.

Na super‐ionic conductor (NASICON)‐structured solid electrolyte Li 1.3 Al 0.3 Ti 1.7 (PO 4 ) 3 (LATP) excels in mechanical strength, ionic conductivity, and commercialization prospect, yet its incompatibility against Li metal anode typically leads to interfacial failure, significantly reducing the overall electrochemical performance of fabricated solid‐state Li metal batteries (SSLMBs). Here, this issue is tackled by interposing an in situ cured poly(ethylene oxide) (PEO)‐LiTFSI@BN interfacial layer between LATP and Li metal. This layer exhibits an ionic conductivity of 7.76 × 10 −5 S cm −1 at room temperature, robust thermostability, as well as satisfactory stability against Li metal (critical current density: 1.5 mA cm −2 ). A Li|LATP|Li symmetric cell employing the interfacial layer demonstrates long cyclic stability over 1200 cycles with a low overpotential of ± 100 mV. Furthermore, this interfacial layer enables a LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)|LATP|Li metal SSLMB to sustain 120 cycles of operation at a cutoff voltage of 4.2 V. This work provides a practical strategy for integrating the LATP solid electrolyte into SSLMBs.

The development of high-performance solid polymer electrolytes is crucial for producing all-solid-state lithium metal batteries with high safety and high energy density. However, the low ionic conductivity of solid polymer electrolytes and their unstable electrolyte/electrode interfaces have hindered their widespread utilization. To address these critical challenges, a strong Lewis acid (aluminum fluoride (AlF3)) with dual functionality is introduced into polyethylene oxide) (PEO)-based polymer electrolyte. The AlF3 facilitates the dissociation of lithium salt, increasing the iontransfer efficiency due to the Lewis acid-base interaction; further the in-situ formation of lithium fluoride-rich interfacial layer is promoted, which suppresses the uneven lithium deposition and continuous undesired reactions between the Li metal and PEO matrix. Benefiting from our rational design, the symmetric Li/Li battery with the modified electrolyte exhibits much longer cycling stability (over 3600 h) than that of the pure PEO/lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte (550 h). Furthermore, the all-solid-state LiFePO4 full cell with the composite electrolyte displays a much higher Coulombic efficiency (98.4% after 150 cycles) than that of the electrolyte without the AlF3 additive (63.3% after 150 cycles) at a large voltage window of 2.4–4.2 V, demonstrating the improved interface and cycling stability of solid polymer lithium metal batteries.

Bifunctional composite separator with a solid-state-battery strategy for dendrite-free lithium metal batteries

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.

Li metal batteries are attracting attention as next-generation high energy density energy storage devices for the realization of a carbon-neutral society. The practical utilization of Li metal in Li-metal batteries faces challenges due to its high reactivity with the electrolytes. In order to suppress such high reactivity, it is necessary to form a stable protection film known as the solid electrolyte interphase (SEI) at the interface based on rational electrolyte design. However, the SEI formation mechanism on the Li metal remains unclear. In this study, the lithium bis(fluorosufonyl)imide (LiFSI)/ethylene carbonate (EC) electrolyte decomposition process on Li metal surface was investigated with molecular dynamics simulations using a machine-learning force field. A newly developed automatic program successfully obtained the main pathways among the numerous reaction events during the simulations. Notably, the main products in the SEI formation were identified as LiF and Li2O, which is consistent with the experimental tendency.

Carbonate-based electrolytes show distinct advantages in high-voltage cathodes but generate nonuniform and mechanically fragile solid-electrolyte interphase (SEI) in lithium (Li) metal batteries. Herein, we propose a LiF-rich SEI incorporating an in situ polymerized poly(hexamethylene diisocyanate)-based gel polymer electrolyte (GPE) to improve the homogeneity and mechanical stability of SEI. Fluoroethylene carbonate (FEC) as a fluorine-based additive for building LiF-rich SEI on Li metal electrodes. With this strategy, the assembled Li symmetric batteries cycled stably for 700 h, and the formation of byproducts on the Li electrode surface was significantly inhibited. The Li/LiFePO4 battery delivered significant capacity retention (91% retention after 800 cycles) at 1 C. With high-voltage LiNi0.8Co0.1Mn0.1O2 (NCM811) as cathode, the Li/GPE-FEC/NCM811 cell delivered a discharge capacity of 168.9 mAh g-1 with a capacity retention of 82% after 300 cycles at 0.5 C. From the above, the work could assist the rapid development of high-energy-density rechargeable Li metal batteries toward remarkable performance.

Lithium (Li) metal is an ideal anode material for rechargeable batteries due to its extremely high theoretical specific capacity (3860 mAh/g), low density (0.59 g/cm3) and the lowest negative electrochemical potential (-3.04 V vs. the standard hydrogen electrode, SHE). Li battery based on Li metal anode is expected to have an order of magnitude higher energy density than today's Li-ion battery. This will enhance energy and power capability of battery devices used in portable electronics, hybrid electric cars, aerospace, telecommunication, military, etc.). The Li metal battery will fulfill the high energy demand that cannot be met through the current Li-ion batteries (e.g. all electric transportation or many other applications which require light weight, high energy density, high power density batteries with small battery footprint and safest). But, to date, uncontrolled dendritic Li growth and limited Coulombic efficiency (CE) during Li deposition/stripping of a bulk Li anode, are the main reason behind low cycle life, which has limited their practical applications. Furthermore, the rate of dendrite growth gets amplified when the cell is cycled at high charge-discharge current density, high charge voltage, and high areal discharge. Increasing anode surface area would reduce the effective charge/discharge current. Additionally, thin films and high surface area Li anodes exhibit high ionic and electronic conductivity contributing to the high efficiency of the current collection required for high capacity battery. Although low CE can be partially compensated by an excess amount of Li (at the expense of lower useable Li energy), fire and other hazards associated with the dendrite growth have limited the development of rechargeable Li metal batteries. Most approaches to dendrite prevention focus on improving the stability and uniformity of the solid-electrolyte interface (SEI) on the bulk Li surface by adjusting electrolyte components and optimizing SEI formation additives. It is very difficult to achieve sufficient passivation on Li electrode due to the thermodynamic instability of Li with electrolytes [1,2]. As an alternative, various mechanical barriers have been proposed to block dendrite penetration [3]. These approaches rely on a strong mechanical barrier provided by an SEI film or a separator to suppress Li dendrite penetration but do not change the fundamental, self-amplifying behavior of the dendrite growth. Also, it is hard to find a mechanical barrier material with required shear modulus (required > 109 Pa) with desired conductivity. In other words, these methods did not prevent Li dendrites from growing during long-term cycling at high charge-discharge current density, high charge voltage, and high areal discharge, or hardly improved the CE and safety in Li deposition/stripping which is not suitable for practical rechargeable Li batteries [1]. We have developed a novel ionic-electronic-mechanical membrane (an artificial SEI), which allows long-term Li cycleability and improved safety by reducing dendrite formation. The new artificial SEI is comprised of three different materials to fulfill the desired SEI criteria such as: (a) facilitate uniform Li ion dispersion and deposition on entire Li surface, (b) desired SEI conductivity (c) mechanical separation of Li and electrolyte and flexibility to contain anode volume change during Li stripping and deposition. The combined effect of the invented SEI result into more than 300% increase in number of cycles and 3-4 times increase in usable Li energy density (i.e. high Coulombic efficiency (CE)) of a wet battery composed of protected Li metal anode, liquid electrolyte (e.g. lithium hexafluoro arsenate, LiAsF6) and Li-ion cathode (e.g. lithium cobalt oxide, LCO) for the advancement of a battery beyond the state-of-the-art Li-ion battery. The added advantage of the invented SEI is improved safety due to suppression of dendrite growth by mechanically strong/flexible SEI and controlled, uniform deposition/stripping of Li from Li anode surface. The protected Li anode may be incorporated into wet cell batteries or solid state batteries having transition metal oxide cathodes (more specifically the state-of-the-art Li-ion cathodes) or Sulfur cathode (Li-S battery) or Oxygen cathode (Li-O2 battery). Moreover, similar methodologies can be applied to other rechargeable metal anodes (Na, Al, Mg, etc.) used in batteries.

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.