Accelerating the Development of LLZO in Solid-State Batteries Toward Commercialization: A Comprehensive Review.

Solid-state batteries (SSBs) are under development as high-priority technologies for safe and energy-dense next-generation electrochemical energy storage systems operating over a wide temperature range. Solid-state electrolytes (SSEs) exhibit high thermal stability and, in some cases, the ability to prevent dendrite growth through a physical barrier, and compatibility with the "holy grail" metallic lithium. These unique advantages of SSEs have spurred significant research interests during the last decade. Garnet-type SSEs, that is, Li7La3Zr2O12 (LLZO), are intensively investigated due to their high Li-ion conductivity and exceptional chemical and electrochemical stability against lithium metal anodes. However, poor interfacial contact with cathode materials, undesirable lithium plating along grain boundaries, and moisture-induced chemical degradation greatly hinder the practical implementation of LLZO-based SSEs for SSBs. In this review, the recent advances in synthesis methods, modification strategies, corresponding mechanisms, and applications of garnet-based SSEs in SSBs are critically summarized. Furthermore, a comprehensive evaluation of the challenges and development trends of LLZO-based electrolytes in practical applications is presented to accelerate their development for high-performance SSBs.

Metal-Free Polymer-Based Current Collector for High Energy Density Lithium-Metal Batteries.

The energy density of lithium-metal batteries (LMBs) relies substantially on the thickness of the lithium-metal anode. However, a bare, thin lithium foil electrode is vulnerable to fragmentation due to the inhomogeneity of the lithium stripping/plating process, disrupting the electron conduction pathway along the electrode. Accordingly, the current collector is an integral part to prevent the resulting loss of electronic conductivity. However, the common use of a heavy and lithiophobic Cu current collector results in a great anode mass increase and unsatisfactory lithium plating behavior, limiting both the achievable specific energy and the cycle life of LMBs. Herein, a metal-free polymer-based current collector is reported that allows for a substantial mass reduction, while simultaneously extending the cycle life of the lithium-metal anode. The specific mass of the ultra-light, 10 µm thick polymer-based current collector is only 1.03 mg cm-2, which is ≈11% of a 10 µm thick copper foil (8.96 mg cm-2). As a result, LMB cells employing this novel current collector provide a specific energy of 448 Wh kg-1, which is almost 18% higher than that of LMBs using the copper current collector (378 Wh kg-1), and a greatly enhanced cycle life owing to a more homogeneous lithium deposition.

Layered Oxide Cathodes for Sodium-Ion Batteries: Storage Mechanism, Electrochemistry, and Techno-economics.

ConspectusLithium-ion batteries (LIBs) are ubiquitous in all modern portable electronic devices such as mobile phones and laptops as well as for powering hybrid electric vehicles and other large-scale devices. Sodium-ion batteries (NIBs), which possess a similar cell configuration and working mechanism, have already been proven as ideal alternatives for large-scale energy storage systems. The advantages of NIBs are as follows. First, sodium resources are abundantly distributed in the earth's crust. Second, high-performance NIB cathode materials can be fabricated by using solely inexpensive and noncritical transition metals such as manganese and iron, which further reduces the cost of the required raw materials. Recently, the unprecedented demand for lithium and other critical minerals has driven the cost of these primary raw materials (which are utilized in LIBs) to a historic high and thus triggered the commercialization of NIBs.Sodium layered transition metal oxides (NaxTMO2, TM = transition metal/s), such as Mn-based sodium layered oxides, represent an important family of cathode materials with the potential to reduce costs, increase energy density and cycling stability, and improve the safety of NIBs for large-scale energy storage. However, these layered oxides face several key challenges, including irreversible phase transformations during cycling, poor air stability, complex charge-compensation mechanisms, and relatively high cost of the full cell compared to LiFePO4-based LIBs. Our work has focused on the techno-economic analysis, the degradation mechanism of NaxTMO2 upon cycling and air exposure, and the development of effective strategies to improve their electrochemical performances and air stability. Correlating structure-performance relationships and establishing general design strategies of NaxTMO2 must be considered for the commercialization of NIBs.In this Account, we discuss the recent progress in the development of air-stable, electrochemically stable, and cost-effective NaxTMO2. The favorable redox-active cations for NaxTMO2 are emphasized in terms of abundance, cost, supply, and energy density. Different working mechanisms related to NaxTMO2 are summarized, including the electrochemical reversibility, the main structural transformations during the charge and discharge processes, and the charge-compensation mechanisms that accompany the (de)intercalation of Na+ ions, followed by discussions to improve the stability toward ambient air and upon cycling. Then the techno-economics are presented, with an emphasis on cathodes with different chemical compositions, cost breakdown of battery packs, and Na deficiency, factors that are critical to the large-scale implementation. Finally, this Account concludes with an overview of the remaining challenges and new opportunities concerning the practical applications of NaxTMO2, with an emphasis on the cost, large-scale fabrication capability, and electrochemical performance.

Locally Concentrated Ionic Liquid Electrolytes for Wide-Temperature-Range Aluminum-Sulfur Batteries.

Aluminum-sulfur (Al-S) batteries are promising energy storage devices due to their high theoretical capacity, low cost, and high safety. However, the high viscosity and inferior ion transport of conventionally used ionic liquid electrolytes (ILEs) limit the kinetics of Al-S batteries, especially at sub-zero temperatures. Herein, locally concentrated ionic liquid electrolytes (LCILE) formed via diluting the ILEs with non-solvating 1,2-difluorobenzene (dFBn) co-solvent are proposed for wide-temperature-range Al-S batteries. The addition of dFBn effectively promotes the fluidity and ionic conductivity without affecting the AlCl4 - /Al2 Cl7 - equilibrium, which preserves the reversible stripping/plating of aluminum and further promotes the overall kinetics of Al-S batteries. As a result, Al-S cells employing the LCILE exhibit higher specific capacity, better cyclability, and lower polarization with respect to the neat ILE in a wide temperature range from -20 to 40 °C. For instance, Al-S batteries employing the LCILE sustain a remarkable capacity of 507 mAh g-1 after 300 cycles at 20 °C, while only 229 mAh g-1 is delivered with the dFBn-free electrolyte under the same condition. This work demonstrates the favorable use of LCILEs for wide-temperature Al-S batteries.

Locally Concentrated Ionic Liquid Electrolytes for Lithium-Metal Batteries.

Non-flammable ionic liquid electrolytes (ILEs) are well-known candidates for safer and long-lifespan lithium metal batteries (LMBs). However, the high viscosity and insufficient Li+ transport limit their practical application. Recently, non-solvating and low-viscosity co-solvents diluting ILEs without affecting the local Li+ solvation structure are employed to solve these problems. The diluted electrolytes, i.e., locally concentrated ionic liquid electrolytes (LCILEs), exhibiting lower viscosity, faster Li+ transport, and enhanced compatibility toward lithium metal anodes, are feasible options for the next-generation high-energy-density LMBs. Herein, the progress of the recently developed LCILEs are summarised, including their physicochemical properties, solution structures, and applications in LMBs with a variety of high-energy cathode materials. Lastly, a perspective on the future research directions of LCILEs to further understanding and achieve improved cell performances is outlined.

Single-Ion Conducting Polymer Electrolyte for Superior Sodium-Metal Batteries.

Sodium-metal batteries (SMBs) are considered a potential alternative to high-energy lithium-metal batteries (LMBs). However, the high reactivity of metallic sodium towards common liquid organic electrolytes renders such battery technology particularly challenging. Herein, we propose a multi-block single-ion conducting polymer electrolyte (SIPE) doped with ethylene carbonate as suitable electrolyte system for SMBs. This novel SIPE provides a very high ionic conductivity (2.6 mS cm-1 ) and an electrochemical stability window of about 4.1 V at 40 °C, enabling stable sodium stripping and plating and excellent rate capability of Na||Na3 V2 (PO4 )3 cells up to 2 C. Remarkably, such cells provide a capacity retention of about 85 % after 1,000 cycles at 0.2 C thanks to the very high Coulombic efficiency (99.9 %), resulting from an excellent interfacial stability towards sodium metal and the Na3 V2 (PO4 )3 cathode.

Locally Concentrated Ionic Liquid Electrolytes Enabling Low-Temperature Lithium Metal Batteries.

Lithium metal is a promising anode material for next-generation high-energy-density batteries but suffers from low stripping/plating Coulombic efficiency and dendritic growth particularly at sub-zero temperatures. Herein, a poorly-flammable, locally concentrated ionic liquid electrolyte with a wide liquidus range extending well below 0 °C is proposed for low-temperature lithium metal batteries. Its all-anion Li+ solvation and phase-nano-segregation solution structure are sustained at low temperatures, which, together with a solid electrolyte interphase rich in inorganic compounds, enable dendrite-free operation of lithium metal anodes at -20 °C and 0.5 mA cm-2 , with a Coulombic efficiency of 98.9 %. As a result, lithium metal batteries coupling thin lithium metal anodes (4 mAh cm-2 ) and high-loading LiNi0.8 Co0.15 Al0.05 O2 cathodes (10 mg cm-2 ) retain 70 % of the initial capacity after 100 cycles at -20 °C. These results, as a proof of concept, demonstrate the applicability of locally concentrated ionic liquid electrolytes for low-temperature lithium metal batteries.

Evaluation and Improvement of the Stability of Poly(ethylene oxide)-based Solid-state Batteries with High-Voltage Cathodes.

Solid-state batteries (SSBs) with high-voltage cathode active materials (CAMs) such as LiNi1-x-y Cox Mny O2 (NCM) and poly(ethylene oxide) (PEO) suffer from "noisy voltage" related cell failure. Moreover, reports on their long-term cycling performance with high-voltage CAMs are not consistent. In this work, we verified that the penetration of lithium dendrites through the solid polymer electrolyte (SPE) indeed causes such "noisy voltage cell failure". This problem can be overcome by a simple modification of the SPE using higher molecular weight PEO, resulting in an improved cycling stability compared to lower molecular weight PEO. Furthermore, X-ray photoelectron spectroscopy analysis confirms the formation of oxidative degradation products after cycling with NCM, for what Fourier transform infrared spectroscopy is not suitable as an analytical technique due to its limited surface sensitivity. Overall, our results help to critically evaluate and improve the stability of PEO-based SSBs.

Zinc-Ion Hybrid Supercapacitors Employing Acetate-Based Water-in-Salt Electrolytes.

Halide-free, water-in-salt electrolytes (WiSEs) composed of potassium acetate (KAc) and zinc acetate (ZnAc2 ) are investigated as electrolytes in zinc-ion hybrid supercapacitors (ZHSs). Molecular dynamics simulations demonstrate that water molecules are mostly non-interacting with each other in the highly concentrated WiSEs, while "bulk-like water" regions are present in the dilute electrolyte. Among the various concentrated electrolytes investigated, the 30 m KAc and 1 m ZnAc2 electrolyte (30K1Zn) grants the best performance in terms of reversibility and stability of Zn plating/stripping while the less concentrated electrolyte cannot suppress corrosion of Zn and hydrogen evolution. The ZHSs utilizing 30K1Zn, in combination with a commercial activated carbon (AC) positive electrode and Zn as the negative electrode, deliver a capacity of 65 mAh g-1 (based on the AC weight) at a current density of 5 A g-1 . They also offer an excellent capacity retention over 10 000 cycles and an impressive coulombic efficiency (≈100%).

Enabling High-Stability of Aqueous-Processed Nickel-Rich Positive Electrodes in Lithium Metal Batteries.

Lithium batteries occupy the large-scale electric mobility market raising concerns about the environmental impact of cell production, especially regarding the use of poly(vinylidene difluoride) (teratogenic) and N-methyl-2-pyrrolidone (NMP, harmful). To avoid their use, an aqueous electrode processing route is utilized in which a water-soluble hybrid acrylic-fluoropolymer together with sodium carboxymethyl cellulose is used as binder, and a thin phosphate coating layer is in situ formed on the surface of the nickel-rich cathode during electrode processing. The resulting electrodes achieve a comparable performance to that of NMP-based electrodes in conventional organic carbonate-based electrolyte (LP30). Subsequently, an ionic liquid electrolyte (ILE) is employed to replace the organic electrolyte, building stable electrode/electrolyte interphases on the surface of the nickel-rich positive electrode (cathode) and metallic lithium negative electrode (anode). In such ILE, the aqueously processed electrodes achieve high cycling stability with a capacity retention of 91% after 1000 cycles (20 °C). In addition, a high capacity of more than 2.5 mAh cm-2 is achieved for high loading electrodes (≈15 mg cm-2 ) by using a modified ILE with 5% vinylene carbonate additive. A path to achieve environmentally friendly electrode manufacturing while maintaining their outstanding performance and structural integrity is demonstrated.

Photo-Cross-Linked Single-Ion Conducting Polymer Electrolyte for Lithium-Metal Batteries.

Polymer electrolytes are considered potential key enablers for lithium-metal batteries due to their compatibility with the lithium-metal negative electrode. Herein, cross-linked self-standing single-ion conducting polymer electrolytes are obtained via a facile UV-initiated radical polymerization using pentaerythritol tetraacrylate as the cross-linker and lithium (3-methacryloyloxypropylsulfonyl)-(trifluoromethylsulfonyl)imide as the ionic functional group. Incorporating propylene carbonate as charge-transport supporting additive allowed for achieving single-ion conductivities of 0.21 mS cm-1 at 20 °C and 0.40 mS cm-1 at 40 °C, while maintaining a suitable electrochemical stability window for 4 V-class positive electrodes (cathodes). As a result, this single-ion polymer electrolyte featured good cycling stability and rate capability in Li||LiFePO4 and Li||LiNi0.6 Mn0.2 Co0.2 O2 cells. These results render this polymer electrolyte as potential alternative to liquid electrolytes for high-energy lithium-metal batteries.

The Emergence of Aqueous Ammonium-Ion Batteries.

Aqueous ammonium-ion (NH4 + ) batteries (AAIB) are a recently emerging technology that utilize the abundant electrode resources and the fast diffusion kinetics of NH4 + to deliver an excellent rate performance at a low cost. Although significant progress has been made on AAIBs, the technology is still limited by various challenges. In this Minireview, the most recent advances are comprehensively summarized and discussed, including cathode and anode materials as well as the electrolytes. Finally, a perspective on possible solutions for the current limitations of AAIBs is provided.

Block copolymers as (single-ion conducting) lithium battery electrolytes.

Solid-state batteries are considered the next big step towards the realization of intrinsically safer high-energy lithium batteries for the steadily increasing implementation of this technology in electronic devices and particularly, electric vehicles. However, so far only electrolytes based on poly(ethylene oxide) have been successfully commercialized despite their limited stability towards oxidation and low ionic conductivity at room temperature. Block copolymer (BCP) electrolytes are believed to provide significant advantages thanks to their tailorable properties. Thus, research activities in this field have been continuously expanding in recent years with great progress to enhance their performance and deepen the understanding towards the interplay between their chemistry, structure, electrochemical properties, and charge transport mechanism. Herein, we review this progress with a specific focus on the block-copolymer nanostructure and ionic conductivity, the latest works, as well as the early studies that are fr"equently overlooked by researchers newly entering this field. Moreover, we discuss the impact of adding a lithium salt in comparison to single-ion conducting BCP electrolytes along with the encouraging features of these materials and the remaining challenges that are yet to be solved.

On the nanoscopic structural heterogeneity of liquid n-alkyl carboxylic acids.

Herein we report the first in-depth structural characterisation of simple linear carboxylic acids with alkyl tail length ranging from one to six carbon atoms. By means of the SWAXS technique, a pronounced nanoscopic heterogeneity evolving along the aliphatic portion of the molecule is highlighted. Via classical molecular dynamics, the origin of such heterogeneity is unambiguously assigned to the existence of aliphatic domains resulting from the self-segregation of the polar and apolar portions of the molecules. Furthermore, the structural correlation of aliphatic-separated polar domains is responsible for observing the so-called "pre-peak" in the SAXS region.

Influence of Carbonate-Based Additives on the Electrochemical Performance of Si NW Anodes Cycled in an Ionic Liquid Electrolyte.

Addition of electrolyte additives (ethylene or vinylene carbonate) is shown to dramatically improve the cycling stability and capacity retention (1600 mAh g-1) of Si nanowires (NWs) in a safe ionic liquid (IL) electrolyte (0.1LiTFSI-0.6PYR13FSI-0.3PYR13TFSI). We show, using postmortem SEM and TEM, a distinct difference in morphologies of the active material after cycling in the presence or absence of the additives. The difference in performance is shown by postmortem XPS analysis to arise from a notable increase in irreversible silicate formation in the absence of the carbonate additives. The composition of the solid electrolyte interphase (SEI) formed at the active material surface was further analyzed using XPS as a function of the IL components revealing that the SEI was primarily made up of N-, F-, and S-containing compounds from the degradation of the TFSI and FSI anions.

Challenges and Strategies for High-Energy Aqueous Electrolyte Rechargeable Batteries.

Aqueous rechargeable batteries are becoming increasingly important to the development of renewable energy sources, because they promise to meet cost-efficiency, energy and power demands for stationary applications. Over the past decade, efforts have been devoted to the improvement of electrode materials and their use in combination with highly concentrated aqueous electrolytes. Here the latest ground-breaking advances in using such electrolytes to construct aqueous battery systems efficiently storing electrical energy, i.e., offering improved energy density, cyclability and safety, are highlighted. This Review aims to timely provide a summary of the strategies proposed so far to overcome the still existing hurdles limiting the present aqueous batteries technologies employing concentrated electrolytes. Emphasis is placed on aqueous batteries for lithium and post-lithium chemistries, with potentially improved energy density, resulting from the unique advantages of concentrated electrolytes.

Overcoming the Interfacial Limitations Imposed by the Solid-Solid Interface in Solid-State Batteries Using Ionic Liquid-Based Interlayers.

Li-garnets are promising inorganic ceramic solid electrolytes for lithium metal batteries, showing good electrochemical stability with Li anode. However, their brittle and stiff nature restricts their intimate contact with both the electrodes, hence presenting high interfacial resistance to the ionic mobility. To address this issue, a strategy employing ionic liquid electrolyte (ILE) thin interlayers at the electrodes/electrolyte interfaces is adopted, which helps overcome the barrier for ion transport. The chemically stable ILE improves the electrodes-solid electrolyte contact, significantly reducing the interfacial resistance at both the positive and negative electrodes interfaces. This results in the more homogeneous deposition of metallic lithium at the negative electrode, suppressing the dendrite growth across the solid electrolyte even at high current densities of 0.3 mA cm-2 . Further, the improved interface Li/electrolyte interface results in decreasing the overpotential of symmetric Li/Li cells from 1.35 to 0.35 V. The ILE modified Li/LLZO/LFP cells stacked either in monopolar or bipolar configurations show excellent electrochemical performance. In particular, the bipolar cell operates at a high voltage (≈8 V) and delivers specific capacity as high as 145 mAh g-1 with a coulombic efficiency greater than 99%.

Structural Investigation of Quaternary Layered Oxides upon Na-Ion Deinsertion.

Na-ion batteries are emerging alternatives to Li-ion chemistries for large-scale energy storage applications. Quaternary layered oxide Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 offers outstanding electrochemical performance in Na-ion batteries compared to pure-phase layered oxides because of the synergistic effect of the P/O-phase mixing. The material is indeed constituted by a mixture of P3, P2, and O3 phases, and a newly identified Na-free phase, i.e., nickel magnesium oxide phase, which improves heat removal and enhances the electrochemical performance. Herein, we structurally investigate, through synchrotron-radiation X-ray diffraction, the modifications occurring after full desodiation, detailing the material structural rearrangement upon Na removal and revealing the effect of two different charging protocols, i.e., constant current (CC) and constant current-constant voltage (CCCV). While the Na-free phase is electrochemically inactive, likely helping in homogenization of the thermal gradient in the electrode during cycling, O-P intergrown phases appear during the extraction of Na ions from interslab layers, and they are dependent on the desodiation level. The application of a constant voltage step at the end of the galvanostatic charge is responsible for a shortening of the interslab distance and a significant volume contraction (-11.9%).

From Solid-Solution Electrodes and the Rocking-Chair Concept to Today's Batteries.

Lithium-ion batteries (LIBs) have become ubiquitous power sources for small electronic devices, electric vehicles, and stationary energy storage systems. Despite the success of LIBs which is acknowledged by their increasing commodity market, the historical evolution of the chemistry behind the LIB technologies is laden with obstacles and yet to be unambiguously documented. This Viewpoint outlines chronologically the most essential findings related to today's LIBs, including commercial electrode and electrolyte materials, but furthermore also depicts how the today popular and widely emerging solid-state batteries were instrumental at very early stages in the development of LIBs.

New Electrode and Electrolyte Configurations for Lithium-Oxygen Battery.

Cathode configurations reported herein are alternative to the most diffused ones for application in lithium-oxygen batteries, using an ionic liquid-based electrolyte. The electrodes employ high surface area conductive carbon as the reaction host, and polytetrafluoroethylene as the binding agent to enhance the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) reversibility. Roll-pressed, self-standing electrodes (SSEs) and thinner, spray deposited electrodes (SDEs) are characterized in lithium-oxygen cells using an ionic liquid (IL) based electrolyte formed by mixing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt and N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethanesulfonyl)imide (DEMETFSI). The electrochemical results reveal reversible reactions for both electrode configurations, but improved electrochemical performance for the self-standing electrodes in lithium-oxygen cells. These electrodes show charge/discharge polarizations at 60 °C limited to 0.4 V, with capacity up to 1 mAh cm-2 and energy efficiency of about 88 %, while the spray deposited electrodes reveal, under the same conditions, a polarization of 0.6 V and energy efficiency of 80 %. The roll pressed electrode combined with the DEMETFSI-LiTFSI electrolyte and a composite Lix Sn-C alloy anode forms a full Li-ion oxygen cell showing extremely limited polarization, and remarkable energy efficiency.