A Perovskite Electrolyte That Is Stable in Moist Air for Lithium‐Ion Batteries

Solid‐oxide Li⁺ electrolytes of a rechargeable cell are generally sensitive to moisture in the air as H⁺ exchanges for the mobile Li⁺ of the electrolyte and forms insulating surface phases at the electrolyte interfaces and in the grain boundaries of a polycrystalline membrane. These surface phases dominate the total interfacial resistance of a conventional rechargeable cell with a solid–electrolyte separator. We report a new perovskite Li⁺ solid electrolyte, Li_0.38Sr_0.44Ta_0.7Hf_0.3O_2.95F_0.05, with a lithium‐ion conductivity of σ_Li=4.8×10^?4 S cm^?1 at 25 °C that does not react with water having 3≤pH≤14. The solid electrolyte with a thin Li⁺‐conducting polymer on its surface to prevent reduction of Ta⁵⁺ is wet by metallic lithium and provides low‐impedance dendrite‐free plating/stripping of a lithium anode. It is also stable upon contact with a composite polymer cathode. With this solid electrolyte, we demonstrate excellent cycling performance of an all‐solid‐state Li/LiFePO₄ cell, a Li‐S cell with a polymer‐gel cathode, and a supercapacitor.     

Hybrid Polymer/Garnet Electrolyte with a Small Interfacial Resistance for Lithium‐Ion Batteries

Li₇La₃Zr₂O₁₂‐based Li‐rich garnets react with water and carbon dioxide in air to form a Li‐ion insulating Li₂CO₃ layer on the surface of the garnet particles, which results in a large interfacial resistance for Li‐ion transfer. Here, we introduce LiF to garnet Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT) to increase the stability of the garnet electrolyte against moist air; the garnet LLZT‐2 wt % LiF (LLZT‐2LiF) has less Li₂CO₃ on the surface and shows a small interfacial resistance with Li metal, a solid polymer electrolyte, and organic‐liquid electrolytes. An all‐solid‐state Li/polymer/LLZT‐2LiF/LiFePO₄ battery has a high Coulombic efficiency and long cycle life; a Li‐S cell with the LLZT‐2LiF electrolyte as a separator, which blocks the polysulfide transport towards the Li‐metal, also has high Coulombic efficiency and kept 93 % of its capacity after 100 cycles.     

An Adjustable‐Porosity Plastic Crystal Electrolyte Enables High‐Performance All‐Solid‐State Lithium‐Oxygen Batteries

The limited triple‐phase boundaries (TPBs) in solid‐state cathodes (SSCs) and high resistance imposed by solid electrolytes (SEs) make the achievement of high‐performance all‐solid‐state lithium‐oxygen (ASS Li‐O₂) batteries a challenge. Herein, an adjustable‐porosity plastic crystal electrolyte (PCE) has been fabricated by employing a thermally induced phase separation (TIPS) technique to overcome the above tricky issues. The SSC produced through the in‐situ introduction of the porous PCE on the surface of the active material, facilitates the simultaneous transfer of Li⁺/e^?, as well as ensures fast flow of O₂, forming continuous and abundant TPBs. The high Li⁺ conductivity, softness, and adhesion of the dense PCE significantly reduce the battery resistance to 115 Ω. As a result, the ASS Li‐O₂ battery based on this adjustable‐porosity PCE exhibits superior performances with high specific capacity (5963 mAh g^?1), good rate capability, and stable cycling life up to 130 cycles at 32 °C. This novel design and exciting results could open a new avenue for ASS Li‐O₂ batteries.     

Lithium Ion Pathway within Li7La3Zr2O12‐Polyethylene Oxide Composite Electrolytes

Polymer–ceramic composite electrolytes are emerging as a promising solution to deliver high ionic conductivity, optimal mechanical properties, and good safety for developing high‐performance all‐solid‐state rechargeable batteries. Composite electrolytes have been prepared with cubic‐phase Li₇La₃Zr₂O₁₂ (LLZO) garnet and polyethylene oxide (PEO) and employed in symmetric lithium battery cells. By combining selective isotope labeling and high‐resolution solid‐state Li NMR, we are able to track Li ion pathways within LLZO‐PEO composite electrolytes by monitoring the replacement of ⁷Li in the composite electrolyte by ⁶Li from the ⁶Li metal electrodes during battery cycling. We have provided the first experimental evidence to show that Li ions favor the pathway through the LLZO ceramic phase instead of the PEO‐LLZO interface or PEO. This approach can be widely applied to study ion pathways in ionic conductors and to provide useful insights for developing composite materials for energy storage and harvesting.     

A Highly Efficient All‐Solid‐State Lithium/Electrolyte Interface Induced by an Energetic Reaction

The energetic chemical reaction between Zn(NO₃)₂ and Li is used to create a solid‐state interface between Li metal and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) electrolyte. This interlayer, composed of Zn, ZnLi_x alloy, Li₃N, Li₂O, and other species, possesses strong affinities with both Li metal and LLZTO and affords highly efficient conductive pathways for Li⁺ transport through the interface. The unique structure and properties of the interlayer lead to Li metal anodes with longer cycle life, higher efficiency, and better safety compared to the current best Li metal electrodes operating in liquid electrolytes while retaining comparable capacity, rate, and overpotential. All‐solid‐state Li||Li cells can operate at very demanding current–capacity conditions of 4 mA cm^?2–8 mAh cm^?2. Thousands of hours of continuous cycling are achieved at Coulombic efficiency >99.5 % without dendrite formation or side reactions with the electrolyte.     

Fluorine‐Doped Antiperovskite Electrolyte for All‐Solid‐State Lithium‐Ion Batteries

A fluorine‐doped antiperovskite Li‐ion conductor Li₂(OH)X (X=Cl, Br) is shown to be a promising candidate for a solid electrolyte in an all‐solid‐state Li‐ion rechargeable battery. Substitution of F^? for OH^? transforms orthorhombic Li₂OHCl to a room‐temperature cubic phase, which shows electrochemical stability to 9 V versus Li⁺/Li and two orders of magnitude higher Li‐ion conductivity than that of orthorhombic Li₂OHCl. An all‐solid‐state Li/LiFePO₄ with F‐doped Li₂OHCl as the solid electrolyte showed good cyclability and a high coulombic efficiency over 40 charge/discharge cycles.     

Cycling Performance at Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript>|Li Interface

Cyclability of the Li|Li₇La₃Zr₂O₁₂ interface was tested by voltammetry under externally applied potential difference. It was found that the solid electrolyte synthesized in the study contains a minor amount of an impurity in the form of lithium carbonate. This impurity forms, when brought in contact with metallic lithium, carbon that pierces the whole volume of the ceramic separator and produces a channel for a flow of electrons through the material, which leads to a poor cyclability of the solid electrolyte. A possible way to solve the given problem is via a purposeful replacement of the carbonate in the intergrain space of Li₇La₃Zr₂O₁₂ with another crystalline or glassy plasticizer that possesses an acceptable unipolar lithium conductivity (no less than 10^–6 S cm^–1) and forms, when brought in contact with metallic lithium, no electrically conducting compound or a compound capable of reversibly intercalating/deintercalating lithium.     

Recent progress in liquid electrolytes for lithium metal batteries

Li metal batteries are revived as the next-generation batteries beyond Li-ion batteries. The Li metal anode can be paired with intercalation-type cathodes LiMO₂ and conversion-type cathodes such as sulfur and oxygen. Then, energy densities of Li/LiMO₂ and Li/S,O₂ batteries can reach 400 Whkg^?1 and more than 500 Whkg^?1, respectively, which surpass that of the state-of-the-art LIB (280 Whkg^?1). However, replacing the intercalation-type graphite anode with the Li metal anode suffers from low coulombic efficiency during repeated Li plating/stripping processes, which leads to short cycle lifetime and potential safety problems. The key solution is to construct a stable and uniform solid electrolyte interphase with high Li⁺ transport and high elastic strength on the Li metal anode. This review summarizes recent progress in improving the solid electrolyte interphase by tailoring liquid electrolytes, a classical but the most convenient and cost-effective strategy.     

Synergistic Dual‐Additive Electrolyte Enables Practical Lithium‐Metal Batteries

A rechargeable Li metal anode coupled with a high‐voltage cathode is a promising approach to high‐energy‐density batteries exceeding 300 Wh kg^?1. Reported here is an advanced dual‐additive electrolyte containing a unique solvation structure and it comprises a tris(pentafluorophenyl)borane additive and LiNO₃ in a carbonate‐based electrolyte. This system generates a robust outer Li₂O solid electrolyte interface and F‐ and B‐containing conformal cathode electrolyte interphase. The resulting stable ion transport kinetics enables excellent cycling of Li/LiNi_0.8Mn_0.1Co_0.1O₂ for 140 cycles with 80 % capacity retention under highly challenging conditions (≈295.1 Wh kg^?1 at cell‐level). The electrolyte also exhibits high cycling stability for a 4.6 V LiCoO₂ (160 cycles with 89.8 % capacity retention) cathode and 4.95 V LiNi_0.5Mn_1.5O₄ cathode.     

A Versatile Halide Ester Enabling Li‐Anode Stability and a High Rate Capability in Lithium–Oxygen Batteries

Li‐O₂ batteries are promising candidates for next‐generation high‐energy‐density battery systems. However, the main problems of Li–O₂ batteries include the poor rate capability of the cathode and the instability of the Li anode. Herein, an ester‐based liquid additive, 2,2,2‐trichloroethyl chloroformate, was introduced into the conventional electrolyte of a Li–O₂ battery. Versatile effects of this additive on the oxygen cathode and the Li metal anode became evident. The Li–O₂ battery showed an outstanding rate capability of 2005 mAh g^?1 with a remarkably decreased charge potential at a large current density of 1000 mA g^?1. The positive effect of the halide ester on the rate capacity is associated with the improved solubility of Li₂O₂ in the electrolyte and the increased diffusion rate of O₂. Furthermore, the ester promotes the formation of a solid–electrolyte interphase layer on the surface of the Li metal, which restrains the loss and volume change of the Li electrode during stripping and plating, thereby achieving a cycling stability over 900 h and a Li capacity utilization of up to 10 mAh cm^?2.     

In Situ Construction of an Ultra‐Stable Conductive Composite Interface for High‐Voltage All‐Solid‐State Lithium Metal Batteries

The garnet electrolyte presents poor wettability with Li metal, resulting in an extremely large interfacial impedance and drastic growth of Li dendrites. Herein, a novel ultra‐stable conductive composite interface (CCI) consisting of Li_ySn alloy and Li₃N is constructed in situ between Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) pellet and Li metal by a conversion reaction of SnN_x with Li metal at 300 °C. The Li_ySn alloy as a continuous and robust bridge between LLZTO and Li metal can effectively reduce the LLZTO/Li interfacial resistance from 4468.0 Ω to 164.8 Ω. Meanwhile, the Li₃N as a fast Li‐ion channel can efficiently transfer Li ions and give their uniform distribution at the LLZTO/Li interface. Therefore, the Li/LLZTO@CCI/Li symmetric battery stably cycles for 1200 h without short circuit, and the all‐solid‐state high‐voltage Li/LLZTO@CCI/LiNi_0.5Co_0.2Mn_0.3O₂ battery achieves a specific capacity of 161.4 mAh g^?1 at 0.25 C with a capacity retention rate of 92.6 % and coulombic efficiency of 100.0 % after 200 cycles at 25 °C.     

Advanced High‐Voltage Aqueous Lithium‐Ion Battery Enabled by “Water‐in‐Bisalt” Electrolyte

A new super‐concentrated aqueous electrolyte is proposed by introducing a second lithium salt. The resultant ultra‐high concentration of 28 m led to more effective formation of a protective interphase on the anode along with further suppression of water activities at both anode and cathode surfaces. The improved electrochemical stability allows the use of TiO₂ as the anode material, and a 2.5 V aqueous Li‐ion cell based on LiMn₂O₄ and carbon‐coated TiO₂ delivered the unprecedented energy density of 100 Wh kg^?1 for rechargeable aqueous Li‐ion cells, along with excellent cycling stability and high coulombic efficiency. It has been demonstrated that the introduction of a second salts into the “water‐in‐salt” electrolyte further pushed the energy densities of aqueous Li‐ion cells closer to those of the state‐of‐the‐art Li‐ion batteries.     

Molten salt synthesis and characterization of fast ion conductor Li<Subscript>6.75</Subscript>La<Subscript>3</Subscript>Zr<Subscript>1.75</Subscript>Ta<Subscript>0.25</Subscript>O<Subscript>12</Subscript>

The limited electrochemical stability and the flammability of the liquid electrolytes presently used in Li-ion batteries stimulates the search for alternatives including ceramic solid electrolytes. Moreover, solid electrolytes also fulfil crucial functions in various large-scale energy storage systems, e.g. as anode-protecting membranes in aqueous Li-air batteries. Here, the processing of the solid electrolytes Li₇La₃Zr₂O₁₂ is studied for applications in Li-air batteries. Molten salt method (MSM) was adopted previously on synthesis of simple oxides; to the best of our knowledge, we report for the first time the adaptation of the MSM to prepare this class of solid electrolytes. As a model compound, we prepared the garnet-related Li_6.75La₃Zr_1.75Ta_0.25O₁₂. It has been prepared by using stoichiometric amounts of La₂O₃, ZrCl₄, and Ta₂O₅ in excess 0.88 M LiNO₃:0.12 M LiCl molten salt. Subsequently, samples were heated to various temperatures in the range 600–900 °C for 6 h in air in a recrystallized alumina crucible and finally washed with distilled water to remove excess salts. The obtained Li_6.75La₃Zr_1.75Ta_0.25O₁₂ electrolyte powder was characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Raman, and impedance spectroscopy as well as surface area measurements. The cubic single phase was obtained for samples prepared at temperatures ≥700 °C. The effects of washing with water or aqueous LiOH solution on the structure and conductivity of the phases will be discussed.     

Understanding LiOH Chemistry in a Ruthenium‐Catalyzed Li–O2 Battery

Non‐aqueous Li–O₂ batteries are promising for next‐generation energy storage. New battery chemistries based on LiOH, rather than Li₂O₂, have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru‐catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e⁻ oxygen reduction reaction, the H in LiOH coming solely from added H₂O and the O from both O₂ and H₂O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li₂O₂, LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long‐lived battery. An optimized metal‐catalyst–electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.     

DABCOnium: An Efficient and High‐Voltage Stable Singlet Oxygen Quencher for Metal–O2 Cells

Singlet oxygen (¹O₂) causes a major fraction of the parasitic chemistry during the cycling of non‐aqueous alkali metal‐O₂ batteries and also contributes to interfacial reactivity of transition‐metal oxide intercalation compounds. We introduce DABCOnium, the mono alkylated form of 1,4‐diazabicyclo[2.2.2]octane (DABCO), as an efficient ¹O₂ quencher with an unusually high oxidative stability of ca. 4.2 V vs. Li/Li⁺. Previous quenchers are strongly Lewis basic amines with too low oxidative stability. DABCOnium is an ionic liquid, non‐volatile, highly soluble in the electrolyte, stable against superoxide and peroxide, and compatible with lithium metal. The electrochemical stability covers the required range for metal–O₂ batteries and greatly reduces ¹O₂ related parasitic chemistry as demonstrated for the Li–O₂ cell.     

Ionic Liquid Electrolyte with Weak Solvating Molecule Regulation for Stable Li Deposition in High-Performance Li−O2 Batteries

Li−O₂ batteries with bis(trifluoromethanesulfonyl)imide-based ionic liquid (TFSI-IL) electrolyte are promising because TFSI-IL can stabilize O₂⁻ to lower charge overpotential. However, slow Li⁺ transport in TFSI-IL electrolyte causes inferior Li deposition. Here we optimize weak solvating molecule (anisole) to generate anisole-doped ionic aggregate in TFSI-IL electrolyte. Such unique solvation environment can realize not only high Li⁺ transport parameters but also anion-derived solid electrolyte interface (SEI). Thus, fast Li⁺ transport is achieved in electrolyte bulk and SEI simultaneously, leading to robust Li deposition with high rate capability (3 mA cm⁻²) and long cycle life (2000 h at 0.2 mA cm⁻²). Moreover, Li−O₂ batteries show good cycling stability (a small overpotential increase of 0.16 V after 120 cycles) and high rate capability (1 A g⁻¹). This work provides an effective electrolyte design principle to realize stable Li deposition and high-performance Li−O₂ batteries.     

Integrated lithium metal anode protected by composite solid electrolyte film enables stable quasi-solid-state lithium metal batteries

Lithium (Li) metal, possessing an extremely high theoretical specific capacity (3860 mAh/g) and the most negative electrode potential (−3.040 V vs. standard hydrogen electrode), is one the most favorable anode materials for future high-energy-density batteries. However, the poor cyclability and safety issues induced by extremely unstable interfaces of traditional liquid Li metal batteries have limited their practical applications. Herein, a quasi-solid battery is constructed to offer superior interfacial stability as well as excellent interfacial contact by the incorporation of Li@composite solid electrolyte integrated electrode and a limited amount of liquid electrolyte (7.5 μL/cm²). By combining the inorganic garnet Al-doped Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZO) with high mechanical strength and ionic conductivity and the organic ethylene-vinyl acetate copolymer (EVA) with good flexibility, the composite solid electrolyte film could provide sufficient ion channels, sustained interfacial contact and good mechanical stability at the anode side, which significantly alleviates the thermodynamic corrosion and safety problems induced by liquid electrolytes. This innovative and facile quasi-solid strategy is aimed to promote the intrinsic safety and stability of working Li metal anode, shedding light on the development of next-generation high-performance Li metal batteries.     

Reversing the dendrite growth direction and eliminating the concentration polarization via an internal electric field for stable lithium metal anodes

Lithium (Li) dendrite growth is a long-standing challenge leading to short cycle life and safety issues in Li metal batteries. Li dendrite growth is kinetically controlled by ion transport, the concentration gradient, and the local electric field. In this study, an internal electric field is generated between the anode and Au-modified separator to eliminate the concentration gradient of Li⁺. The Li–Au alloy is formed during the first cycle of Li plating/stripping, which causes Li⁺ deposition on the Au-modified side and lithium anode electrode, reversing the lithium dendrite growth direction. The electrically coupled Li metal electrode and Au-modified film create a uniform electric potential and Li⁺ concentration distribution, resulting in reduced concentration polarization and stable Li deposition. As a result, the Au-modified separator improves the lifespan of Li‖Li batteries; the Li‖LiFePO₄ cells show excellent capacity retention (>97.8% after 350 cycles), and Li‖LiNi_0.8Co_0.1Mn_0.1O₂ cells deliver 75.1% capacity retention for more than 300 cycles at 1C rate. This strategy offers an efficient approach for commercial application in advanced metallic Li batteries.

An internal electric field is built between the anode and the Au-modified separator to eliminate the concentration gradient of Li⁺ and reverse the dendrite growth direction.     

A 3D Nanostructured Hydrogel‐Framework‐Derived High‐Performance Composite Polymer Lithium‐Ion Electrolyte

Solid‐state electrolytes have emerged as a promising alternative to existing liquid electrolytes for next generation Li‐ion batteries for better safety and stability. Of various types of solid electrolytes, composite polymer electrolytes exhibit acceptable Li‐ion conductivity due to the interaction between nanofillers and polymer. Nevertheless, the agglomeration of nanofillers at high concentration has been a major obstacle for improving Li‐ion conductivity. In this study, we designed a three‐dimensional (3D) nanostructured hydrogel‐derived Li_0.35La_0.55TiO₃ (LLTO) framework, which was used as a 3D nanofiller for high‐performance composite polymer Li‐ion electrolyte. The systematic percolation study revealed that the pre‐percolating structure of LLTO framework improved Li‐ion conductivity to 8.8×10^?5 S cm^?1 at room temperature.     

Anchoring an Artificial Solid–Electrolyte Interphase Layer on a 3D Current Collector for High‐Performance Lithium Anodes

The application of Li anodes is hindered by dendrite growth and side reactions between Li and electrolyte, despite its high capacity and low potential. A simple approach for this challenge is now demonstrated. In our strategy, the garnet‐type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO)‐based artificial solid–electrolyte interphase (SEI) is anchored on Cu foam by sintering the Cu foam coated with LLZTO particles. The heat treatment leads to the interdiffusion of Cu and Ta₂O₅ at the Cu/LLZTO interface, through which LLZTO layer is fixed on Cu foam. 3D structure lowers the current density, and meanwhile the SEI reduces the contact of Li and electrolyte. Furthermore, the anchoring construction can endure Li‐deposition‐induced volume change. Therefore, LLZTO‐modified Cu foam shows much improved Li plating/stripping performance, including long lifespan (2400 h), high rate (maximum current density of 20 mA cm^?2), high areal capacity (8 mA h cm^?2 for 100 cycles), and high efficiency (over 98 %).