3D Asymmetric Bilayer Garnet-Hybridized High-Energy-Density Lithium-Sulfur Batteries

Recent advances in garnet-based electrolytes for solid-state lithium metal batteries: interfacial challenges and engineering strategies

Solid-state lithium metal batteries (SSLMBs) are recognized as the most efficient and promising energy storage devices due to their enhanced safety and high energy density. As a key component in solid-state lithium metal batteries, solid-state electrolytes (SSEs) are critical for the development of batteries. Garnet-type SSEs have been employed as the most critical component of inorganic electrolytes and fillers for composite electrolytes. To date, significant research on garnet-based electrolytes has been carried out and numerous breakthrough results have been achieved. However, poor interfacial contacts between garnet-based electrolytes and electrodes still lead to Li dendrite growth and unsatisfactory electrochemical performance of batteries. Herein, recent developments of the interfaces for garnet based SSLMBs, including the strategies of garnet types and composite electrolytes as well as Li metal anode stabilizations are presented. Moreover, interfacial engineering strategies between electrolytes and cathodes are also discussed. Meanwhile, advanced techniques and electrochemical approaches for interfacial characterization are also introduced. This work provides insights into the interfacial engineering of garnet-based electrolytes, contributing to the understanding of interfaces in solid-state batteries.

Operando Discovery of LiNO3-Induced Ultrathick Solid Electrolyte Interphase Formation on Li Metal Anodes

Solid electrolyte interphases (SEIs) play essential roles in lithium metal batteries by dictating the deposition morphology and Coulombic efficiency (CE) of lithium metal anodes. However, the understanding of the formation and evolution of SEIs remains elusive so far. Herein, we present an operando investigation of the formation and evolution dynamics of LiNO3-induced SEI under liquid electrolytes using high-resolution electrochemical atomic force microscopy (EC-AFM). For the first time, we discovered the in situ formation of an ultrathick SEI (∼648 nm) under liquid electrolytes, which far exceeded the thicknesses observed via ex situ characterization techniques. This ultrathick SEI formed through a sequential reduction process, wherein an inorganic-rich inner layer was initially formed, followed by the deposition of a soft outer layer with a modulus of approximately 24 MPa. We also observed that the outer SEI layer was metastable and susceptible to dissolution in liquid electrolytes. Our findings provide a comprehensive investigation of the formation and evolution processes of the LiNO3-induced SEI, deepening the understanding of the in situ structure of SEIs under liquid electrolytes.

Constructing a Li2O/LiZn Mixed Ionic Electron Conductive Layer by Ultrasonic Spraying to Enhance Li/Garnet Solid Electrolyte Interface Stability for Solid-State Batteries

Garnet-type Li6.25Ga0.25La3Zr2O12(LGLZO) is believed to be a promising solid electrolyte for solid-state batteries due to its high ionic conductivity, safety, and good stability toward Li. However, one of the most challenges in practical application of LGLZO is the poor contact between Li and LGLZO. Herein, a ZnO layer is prepared on the surface of LGLZO pellet by ultrasonic spraying. Then, a Li2O/LiZn mixed ionic and electron conductive (MIEC) layer is formed at the Li/LGLZO interface via the conversion reaction between ZnO and molten Li. Experiments and theoretical calculations reveal that this MIEC layer can simultaneously improve interface contact, optimize interfacial kinetics, and guide homogeneous Li deposition. As a result, the interface impedance decreases significantly to 36 Ω cm2, the critical current density (CCD) can reach as high as 2.15 mA cm-2, and the Li/ZnO@LGLZO/LiFePO4 all-solid-state cells stably cycle 100 times at 0.5 C. This simple and effective approach may provide a viable strategy for improving the cycle performance of solid lithium metal batteries.

On the interfacial phenomena at the Li7La3Zr2O12 (LLZO)/Li interface

Research on the Li7La3Zr2O12 (LLZO)/Li interface is essential for improving the performance of LLZO-based solid-state batteries. In this comment, the authors present an analysis of the key interfacial phenomena at the LLZO/Li interface, highlighting recent developments and unresolved issues.

Progresses and Perspectives of Carbon-Free Metal Compounds-Modified Separators for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have the advantages of high theoretical specific capacity, excellent energy density, abundant elemental sulfur reserves. However, the LSBs is mainly limited by shuttling of lithium polysulfides (LiPSs), slow reaction kinetics of sulfur cathode. For solving the above problems, by developing high-performance battery separators, the reversible capacity, Coulombic efficiency (CE) and cycle life of LSBs can be effectively enhanced. Carbon-free based metal compounds are expected to be highly efficient separator modifiers for a new generation of high-performance LSBs by virtue of superior chemical adsorption capacity, strong catalytic properties and excellent lithophilicity to a certain extent. They can give play to the synergistic effect of their "adsorption-catalysis" sites to accelerate the redox kinetics of LiPSs, and their good lithophilicity can accelerate the Li+ transport kinetics, thus showing more remarkable electrochemical performances. However, a comprehensive summary of carbon-free metal compounds-modified separators for LSBs is still lacking. Here, this review systematically summarizes the researching progresses and performance characteristics of carbon-free-based metal compounds modified materials for separators of LSBs, and summarizes the corresponding mechanisms of using carbon-based separators to enhance the performance of LSBs. Finally, the review also looks forward to the prospects of LSBs using carbon-free metal compounds separators.

Progress on critical cell fabrication parameters and designs for advanced lithium-sulfur batteries

Since 1990, commercial lithium-ion batteries have made significant strides, approaching their theoretical performance limits, albeit with escalating costs. To address these challenges, attention has shifted toward lithium-sulfur batteries, which offer higher theoretical energy densities and cost-effectiveness. However, lithium-sulfur cells face challenges such as active-material loss, excessive electrolyte usage, and rapid degradation of lithium-metal anodes. To overcome these issues, research has focused on optimizing cell configurations and fabrication parameters while exploring novel electrolytes and electrode materials. This feature article delves into the intrinsic material challenges and extrinsic engineering issues in current lithium-sulfur research and explores the development of advanced lithium-sulfur cells with crucial progress on high-loading sulfur cathodes, lean-electrolyte cells, and solid-state electrolytes. Moreover, it outlines the fundamental principles, structures, performances, and developmental trajectories indicated in research articles published after 2020, highlighting future research directions aimed at resolving key challenges for the practical application of lithium-sulfur cells.

Solvent-Free Method of Polyacrylonitrile-Coated LLZTO Solid-State Electrolytes for Lithium Batteries

Solid-state electrolytes (SSEs), particularly garnet-type Li6.4La3Zr1.4Ta0.6O12 (LLZTO), offer high stability and a wide electrochemical window. However, their grain boundaries limit ionic conductivity, necessitating high-temperature sintering for improved performance. Yet, this process results in brittle electrolytes prone to fracture during manufacturing. To address these difficulties, solvent-free solid-state electrolytes with a polyacrylonitrile (PAN) coating on LLZTO particles are reported in this work. Most notably, the PAN-coated LLZTO (PAN@LLZTO) electrolyte demonstrates self-supporting characteristics, eliminating the need for high-temperature sintering. Importantly, the homogeneous polymeric PAN coating, synthesized via the described method, facilitates efficient Li+ transport between LLZTO particles. This electrolyte not only achieves an ionic conductivity of up to 2.11 × 10-3 S cm-1 but also exhibits excellent interfacial compatibility with lithium. Furthermore, a lithium metal battery incorporating 3% PAN@LLZTO-3%PTFE as the solid-state electrolyte and LiFePO4 as the cathode demonstrates a remarkable specific discharge capacity of 169 mAh g-1 at 0.1 °C. The strategy of organic polymer-coated LLZTO provides the possibility of a green manufacturing process for preparing room-temperature sinter-free solid-state electrolytes, which shows significant cost-effectiveness.

AC Electric Conductivity of High Pressure and High Temperature Formed NaFePO4 Glassy Nanocomposite

Olivine-like NaFePO4 glasses and nanocomposites are promising materials for cathodes in sodium batteries. Our previous studies focused on the preparation of NaFePO4 glass, transforming it into a nanocomposite using high-pressure-high-temperature treatment, and comparing both materials' structural, thermal, and DC electric conductivity. This work focuses on specific features of AC electric conductivity, containing messages on the dynamics of translational processes. Conductivity spectra measured at various temperatures are scaled by apparent DC conductivity and plotted against frequency scaled by DC conductivity and temperature in a so-called master curve representation. Both glass and nanocomposite conductivity spectra are used to test the (effective) exponent using Jonscher's scaling law. In both materials, the values of exponent range from 0.3 to 0.9, with different relation to temperature. It corresponds to the electronic conduction mechanism change from low-temperature Mott's variable range hopping (between Fe2+/Fe3+ centers) to phonon-assisted hopping, which was suggested by previous DC measurements. Following the pressure treatment, AC conductivity activation energies were reduced from EAC≈0.40 eV for glass to EAC≈0.18 eV for nanocomposite and are lower than their DC counterpart, following a typical empirical relation with the value of the exponent. While pressure treatment leads to a 2-3-orders-of-magnitude rise in the AC and apparent DC conductivity due to the reduced distance between the hopping centers, a nonmonotonic relation of AC power exponent and temperature is observed. It occurs due to the disturbance of polaron interactions with Na+ mobile ions.

A Comparatively Low Cost, Easy-To-Fabricate, and Environmentally Friendly PVDF/Garnet Composite Solid Electrolyte for Use in Lithium Metal Cells Paired with Lithium Iron Phosphate and Silicon

Solid electrolytes may be the answer to overcome many obstacles in developing the next generation of renewable batteries. A novel composite solid electrolyte (CSE) composed of a poly(vinylidene fluoride) (PVDF) base with an active nanofiber filler of aluminum-doped garnet Li ceramic, Li salt lithium bis-(trifluoromethanesulfonyl)imide (LiTFSI), Li fluoride (LiF) stabilizing additive, and plasticizer sulfolane was fabricated. In a Li|CSE|LFP cell with this CSE, a high capacity of 168 mAh g-1 with a retention of 98% after 200 cycles was obtained, representing the best performance to date of a solid electrolyte with a PVDF base and a garnet inorganic filler. In a Li metal cell with Si and Li, it yielded a discharge capacity of 2867 mAh g-1 and was cycled 60 times at a current density of 100 mAh g-1, a significant step forward in utilizing a solid electrolyte of any kind with the desirable Si anode. In producing this CSE, the components and fabrication process were chosen to have a lower cost and improved safety and environmental impact compared with the current state-of-the-art Li-ion battery.

In Situ Fabrication of High Ionic and Electronic Conductivity Interlayers Enabling Long-Life Garnet-Based Solid-State Lithium Batteries

Garnet-type Li6.75La3Zr1.75Ta0.25O12 (LLZTO) is a promising solid-state electrolyte (SSE) because of its fast ionic conduction and notable chemical/electrochemical stability toward the lithium (Li) metal. However, poor interface wettability and large interface resistance between LLZTO and Li anode greatly restrict its practical applications. In this work, we develop an in situ chemical conversion strategy to construct a highly conductive Li2S@C layer on the surface of LLZTO, enabling improved interfacial wettability between LLZTO and the Li anode. The Li/Li2S@C-LLZTO-Li2S@C/Li symmetric cell has a low interface impedance of 78.5 Ω cm2, much lower than the 970 Ω cm2 of a Li/LLZTO/Li cell. Moreover, the Li/Li2S@C-LLZTO-Li2S@C/Li cell exhibits a high critical current density of 1.4 mA cm-2 and an ultralong stability of 3000 h at 0.1 mA cm-2. When used in a LiFePO4 battery, the Li/Li2S@C-LLZTO/LiFePO4 battery exhibits a high initial discharge capacity of 150.8 mA h g-1 at 0.2 C without lithium storage capacity attenuation during 200 cycles. This work provides a novel and feasible strategy to address interface issues of SSEs and achieve lithium-dendrite-free solid-state batteries.

Electrospun MoS2-CNTs-PVA/PVA Hybrid Separator for High-Performance Li/FeS2 Batteries

As a promising candidate for high-energy-density rechargeable lithium metal batteries, Li/FeS2 batteries still suffer from the large volume change and severe shuttle effect of lithium polysulfides during cycling. To improve the electrochemical performance, great efforts have been made to modify FeS2 cathodes by constructing various nanocomposites. However, energy density is sacrificed, and these materials are not applicable at a large scale. Herein, we report that the electrochemical performance of commercial FeS2 can be greatly enhanced with the application of a double-layer MoS2-CNTs-PVA (MCP)/PVA separator fabricated by electrospinning. The assembled Li/FeS2 batteries can still deliver a high discharge capacity of 400 mAh/g after 200 cycles at a current density of 0.5 C. The improved cycling stability can be attributed to the strong affinity towards lithium polysulfides (LiPSs) of the hydroxyl-rich PVA matrix and the unique double-layer structure, in which the bottom layer acts as an electrical insulation layer and the top layer coupled with MoS2/CNTs provides catalytic sites for LiPS conversion.

A Review on Engineering Design for Enhancing Interfacial Contact in Solid-State Lithium-Sulfur Batteries

The utilization of solid-state electrolytes (SSEs) presents a promising solution to the issues of safety concern and shuttle effect in Li-S batteries, which has garnered significant interest recently. However, the high interfacial impedances existing between the SSEs and the electrodes (both lithium anodes and sulfur cathodes) hinder the charge transfer and intensify the uneven deposition of lithium, which ultimately result in insufficient capacity utilization and poor cycling stability. Hence, the reduction of interfacial resistance between SSEs and electrodes is of paramount importance in the pursuit of efficacious solid-state batteries. In this review, we focus on the experimental strategies employed to enhance the interfacial contact between SSEs and electrodes, and summarize recent progresses of their applications in solid-state Li-S batteries. Moreover, the challenges and perspectives of rational interfacial design in practical solid-state Li-S batteries are outlined as well. We expect that this review will provide new insights into the further technique development and practical applications of solid-state lithium batteries.

Surface Chemistry of LLZO Garnet Electrolytes with Sulfur in Electron Pair Donor Solvents

Conventional Li-S batteries rely on liquid electrolytes based on LiNO3/DOL/DME mixtures that produce a quasistable interface with the lithium anode. Electron pair donor (EPD) solvents, also known as high donor number solvents, provide much higher polysulfide solubility and close-to-ideal sulfur utilization, making them solvents of choice for lean electrolyte Li-S cells. However, their instability to reduction requires incorporation of an ion-conductive membrane that is stable with Li-such as garnet LLZO and also stable with sulfur/polysulfides. We report that even trace amounts of LiOH on a LLTZO surface trigger a complex reaction with sulfur dissolved in typical EPD solvents (i.e., N,N-dimethylacetamide, DMA) to produce a highly resistive impedance layer that quickly grows with time from 1000 to 10,000 Ω cm2 over a few hours, thus impeding Li+ transport across the interface. Decorating the LLZO with protective phosphate groups to produce a modified surface provides a very low charge-transfer resistance of 40 Ω cm2 that is maintained over time and inhibits the reaction of LiOH and dissolved sulfur. Hybrid liquid-solid electrolyte cells constructed on this concept result in a high sulfur utilization of 1400 mAh g-1 which is 85% of theoretical and remains constant over cycling even with conventional, unoptimized carbon/sulfur cathodes.

Influence of Molecular Weight and Lithium Bis(trifluoromethanesulfonyl)imide on the Thermal Processability of Poly(ethylene oxide) for Solid-State Electrolytes

New energy systems such as all-solid-state battery (ASSB) technology are becoming increasingly important today. Recently, researchers have been investigating the transition from the lab-scale production of ASSB components to a larger scale. Poly(ethylene oxide) (PEO) is a promising candidate for the large-scale production of polymer-based solid electrolytes (SPEs) because it offers many processing options. Hence, in this work, the thermal processing route for a PEO-Lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) SPE in the ratio of 20:1 (EO:Li) is investigated using kneading experiments. Here, we clearly show the sensitivity of PEO during thermal processing, especially for high-molecular-weight PEO (Mw = 600,000 g mol-1). LiTFSI acts as a plasticizer for low-molecular-weight PEO (Mw = 100,000 g mol-1), while it amplifies the degradation of high-molecular-weight PEO. Further, LiTFSI affects the thermal properties of PEO and its crystallinity. This leads to a higher chain mobility in the polymer matrix, which improves the flowability. In addition, the spherulite size of the produced PEO electrolytes differs from the molecular weight. This work demonstrates that low-molecular-weight PEO is more suitable for thermal processing as a solid electrolyte due to the process stability. High-molecular-weight PEO, especially, is strongly influenced by the process settings and LiTFSI.