High-Performance Sulfide-based Solid-state Electrolytes and Interfaces for All-solid-state Li Batteries

(Oxalato)borate: The key ingredient for polyethylene oxide based composite electrolyte to achieve ultra-stable performance of high voltage solid-state LiNi0.8Co0.1Mn0.1O2/lithium metal battery

Lithium-metal batteries are considered one of the most promising energy-storage systems owing to their high energy density, but their practical applications have long been hindered by significant safety concerns and poor cycle stability. Solid-state electrolytes (SSEs) are expected to improve not only the safety but also the energy density of Li-metal batteries. The key challenge for solid-state Li-metal batteries lies in the low ionic conductivity of the SSEs and moreover the interface contact between the electrode and SSE. To achieve feasible solid-state Li-metal batteries, it is imperative that the ionic conductivity is improved, especially at the electrode-SSE interface. Herein, recent advances in interface engineering for solid-state Li-metal batteries are reported, mainly focusing on garnet-type SSEs. Various materials to modify the cathode-garnet and Li-garnet interfaces by intermediate layers, alloys, and polymer electrolytes are analyzed. Structural innovations for SSEs including composite electrolytes and multilayer SSE frameworks are reviewed, along with advanced characterization approaches to probe the interfaces, which will provide further insights for garnet-based solid-state batteries. Future challenges and the great promise of garnet-based Li-metal batteries are discussed to close.

Sulfide‐based solid‐state electrolytes (SSEs) for all‐solid‐state Li metal batteries (ASSLMBs) are attracting significant attention due to their high ionic conductivity, inherently soft properties, and decent mechanical strength. However, the poor incompatibility with Li metal and air sensitivity have hindered their application. Herein, the Sn (IV) substitution for P (V) in argyrodite sulfide Li6PS5I (LPSI) SSEs is reported, in the preparation of novel LPSI‐xSn SSEs (where x is the Sn substitution percentage). Appropriate aliovalent element substitutions with larger atomic radius (R<Sn> > R<P>) provides the optimized LPSI‐20Sn electrolyte with a 125 times higher ionic conductivity compared to that of the LPSI electrolyte. The high ionic conductivity of LPSI‐20Sn enables the rich I‐containing electrolyte to serve as a stabilized interlayer against Li metal in sulfide‐based ASSLMBs with outstanding cycling stability and rate capability. Most importantly, benefiting from the strong Sn–S bonding in Sn‐substituted electrolytes, the LPSI‐20Sn electrolyte shows excellent structural stability and improved air stability after exposure to O2 and moisture. The versatile Sn substitution in argyrodite LPSI electrolytes is believed to provide a new and effective strategy to achieve Li metal‐compatible and air‐stable sulfide‐based SSEs for large‐scale applications.

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.

With increasing demands for safe, high capacity energy storage to support personal electronics, newer devices such as unmanned aerial vehicles, as well as the commercialization of electric vehicles, current energy storage technologies are facing increased challenges. Although alternative batteries have been intensively investigated, lithium (Li) batteries are still recognized as the preferred energy storage solution for the consumer electronics markets and next generation automobiles. However, the commercialized Li batteries still have disadvantages, such as low capacities, potential safety issues, and unfavorable cycling life. Therefore, the design and development of electromaterials toward high-energy-density, long-life-span Li batteries with improved safety is a focus for researchers in the field of energy materials. Herein, recent advances in the development of novel organic electrolytes are summarized toward solid-state Li batteries with higher energy density and improved safety. On the basis of new insights into ionic conduction and design principles of organic-based solid-state electrolytes, specific strategies toward developing these electrolytes for Li metal anodes, high-energy-density cathode materials (e.g., high voltage materials), as well as the optimization of cathode formulations are outlined. Finally, prospects for next generation solid-state electrolytes are also proposed.

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.

Solid-state batteries enabled by solid-state polymer electrolytes (SPEs) are under active consideration for their promise as cost-effective platforms that simultaneously support high-energy and safe electrochemical energy storage. The limited oxidative stability and poor interfacial charge transport in conventional polymer electrolytes are well known, but difficult challenges must be addressed if high-voltage intercalating cathodes are to be used in such batteries. Here, ether-based electrolytes are in situ polymerized by a ring-opening reaction in the presence of aluminum fluoride (AlF3 ) to create SPEs inside LiNi0.6 Co0.2 Mn0.2 O2 (NCM) || Li batteries that are able to overcome both challenges. AlF3 plays a dual role as a Lewis acid catalyst and for the building of fluoridized cathode-electrolyte interphases, protecting both the electrolyte and aluminum current collector from degradation reactions. The solid-state NCM || Li metal batteries exhibit enhanced specific capacity of 153 mAh g-1 under high areal capacity of 3.0 mAh cm-2 . This work offers an important pathway toward solid-state polymer electrolytes for high-voltage solid-state 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.

An ultrathin polymer-based solid-state electrolyte membrane with high conductivity for lithium metal batteries operable at -25°C.

Suppressing lithium dendrites within inorganic solid-state electrolytes

Current lithium-ion batteries cannot meet the requirement of higher energy density with further large-scale application of electrical vehicles. Lithium metal batteries combined with Ni-rich layered oxides cathode are expected as the one of promising solutions, while the poor electrode and electrolyte interface impedes the commercial development of lithium metal batteries. A new double-salts super concentrated (DSSC) carbonate electrolyte is proposed to improve the electrochemical performance of LiNi0.90Co0.05Mn0.05O2 (NCM9055)||Li metal battery which exhibits stable cycling performance with the capacity retention of 93.04% and reversible capacity of 173.8 mAh g-1 after 100 cycles at 1 C, while cells with conventional 1m diluted electrolyte remains only 60.55% and capacity of 114.2 mAh g-1. The double salts synergistic effect in super concentrated electrolyte promotes the formation for more balanced stable cathode electrolyte interface (CEI) inorganic compounds of CFx, LiNOx, SOF2, Li2SO4, and less LiF by X-ray photoelectron spectroscopy (XPS)test, and the uniform 2-3nm rock-salt phase protection layer on the cathode surface by transmission electron microscope (TEM) characterization, improving the cycling performance of the Ni-rich NCM9055 layered oxide cathode. The DSSC electrolyte also can relief the Li dendrite growth on Li metal anode, as well as exhibit better flame retardance, promoting the application of more safety Ni-rich NCM9055||Li metal batteries.

3D lithium metal anodes hosted in asymmetric garnet frameworks toward high energy density batteries

Regulating interfacial reactions via quasi-solid polymer electrolyte to enable high-voltage lithium metal batteries

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.

Solid‐state electrolytes (SSEs) are pivotal to advancing lithium metal batteries (LMBs). Electrochemically active LiCoO 2 (LCO) has high ionic conductivity (1.45 × 10 −3 S cm −1 at 35 °C), but its inherent electronic conductivity limits its application as an SSE. To address this limitation, a sandwich‐structured composite solid electrolyte (SSCSE) is presented by integrating an LCO layer between non‐electron‐conducting porous polypropylene (PP). This design decouples the electronic and ionic conductivities of LCO, enabling LCO to be an SSE for LMBs for the first time. Meanwhile, the intermediate fast‐ion conductor layer homogenizes Li⁺ flux and enhances mechanical strength, suppressing lithium dendrites growth and improving safety. Li||Li symmetric cell and Li||LFP full cell using LCO as the SSE achieve excellent performance (800 h at 0.5 mA cm −2 and 0.5 mAh cm −2 ; 93.6% capacity retention after 800 cycles with 99.8% coulombic efficiency). This breakthrough in utilizing LCO with high ionic conductivity as an SSE promotes the development of safer and higher energy‐density batteries.