A Multilayer Ceramic Electrolyte for All‐Solid‐State Li Batteries

Robust and Adhesive Laminar Solid Electrolyte with Homogenous and Fast Li-Ion Conduction for High-Performance All-Solid-State Lithium Metal Battery

Constructing composite solid electrolytes (CSEs) integrating the merits of inorganic and organic components is a promising approach to developing high-performance all-solid-state lithium metal batteries (ASSLMBs). CSEs are now capable of achieving homogeneous and fast Li-ion flux, but how to escape the trade-off between mechanical modulus and adhesion is still a challenge. Herein, a strategy to address this issue is proposed, that is, intercalating highly conductive, homogeneous, and viscous-fluid ionic conductors into robust coordination laminar framework to construct laminar solid electrolyte with homogeneous and fast Li-ion conduction (LSE-HFC). A 9 µm-thick LSH-HFC, in which poly(ethylene oxide)/succinonitrile is adsorbed by coordination laminar framework with metal-organic framework nanosheets as building blocks, is used here as an example to determine the validity. The Li-ion transfer mechanism is verified and works across the entire LSE-HFC, which facilitates homogeneous Li-ion flux and low migration energy barriers, endowing LSE-HFC with high ionic conductivity of 5.62 × 10-4 S cm-1 and Li-ion transference number of 0.78 at 25 °C. Combining the outstanding mechanical strength against punctures and the enhanced adhesion force with electrodes, LSE-HFC harvests uniform Li plating/stripping behavior. These enable the realization of high-energy-density ASSLMBs with excellent cycling stability when being assembled as LiFePO4/Li and LiNi0.6Mn0.2Co0.2O2/Li cells.

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.

Different-grain-sized boehmite nanoparticles for stable all-solid-state lithium metal batteries

PEO is one of the common composite polymer electrolyte vehicles; however, the presence of crystalline phase at room temperature, high interface impedance, and low oxidation resistance (<4.0 V) limit its application in stable all-solid-state lithium metal batteries. Herein, we designed a PEO-based solid polymer electrolyte (SPE) by adding boehmite nanoparticles to address the above-mentioned issues. Different-grain-sized boehmite nanoparticles were synthesized by adjusting the hydrothermal temperature. Moreover, the impacts of these distinct grain-sized boehmite nanoparticles used to fabricate boehmite/PEO polymer electrolytes (BPEs) on the performance of all-solid-state lithium metal batteries were investigated. It was found that with the increase in boehmite's grain size, BPEs show better performance. The best BPE exhibited an improved Li+ transference number (0.59), high ionic conductivity (1.25 × 10-4 S m-1), and wide electrochemical window (∼4.5 V) at 60 °C. The assembled lithium symmetric battery can stably undergo 500 hours of lithium plating/stripping at 0.1 mA cm-2. At the same time, the LiFePO4/BPE/Li battery exhibits excellent cycling stability after 100 cycles at 0.5C. This reasonable design strategy with a superior capacity retention rate (86%) demonstrates great potential in achieving high ionic conductivity and good interface stability for all-solid-state lithium metal batteries simultaneously.

Single-Ion Conductive Polymer-Based Composite Electrolytes for High-Performance Solid-State Lithium Metal Batteries

Flexible composite polymer electrolytes (CPEs) with inorganic electrolyte fillers dispersed in polymer electrolytes integrate the merits of the polymer and inorganic electrolytes and have attracted much attention in recent years. In order to increase the electrochemical performance, especially the low lithium (Li)-ion transference number in traditional dual-ion Li salt-containing CPEs, single-ion conductive CPEs are synthesized with a single-ion polymer conductor (SIPC) as the matrix and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) particles as the active fillers. The single-ion conductive CPEs show a high Li-ion transference number (up to 0.96), high room-temperature (RT) ionic conductivity (>1.0 × 10-4 S cm-1 ), wide electrochemical stability window (>5.0 V, vs Li/Li+ ), and excellent long-term cycling stability with Li metal at RT (3200 h). Based on the SIPC-LLZTO CPE, the solid-state lithium metal batteries with LiFePO4 - and LiCoO2 -based cathodes deliver average discharge capacities of 159 mAh g-1 for 600 cycles and 119 mAh g-1 for 200 cycles at RT, respectively. This study sheds light on the design of high-performance CPEs for next-generation solid-state lithium metal batteries.

Enabling high rate capability and stability all-solid-state batteries via cationic surfactant modification of composite electrolyte

The garnet-type solid electrolyte Li6.4La3Zr1.4Ta0.6O12 (LLZTO) was modified with a cationic surfactant Cetyltrimethylammonium Bromide (CTAB) to improve the dispersion of LLZTO inorganic particles in Poly (ethylene oxide) (PEO) electrolyte (PEO-LLZTO@CTAB) by a liquid phase casting method. During fabrication, the cationic modifier CTAB is uniformly adsorbed on the surface of LLZTO particles, which can effectively reduce their surface energy and lead to a thin CTAB surface coating layer. This fabricated PEO-LLZTO@CTAB can avoid the aggregation of LLZTO particles in the composite solid-state electrolyte (CSSE) and improve the interfacial contact at the PEO/LLZTO interface, thus reducing the overall resistance of PEO-LLZTO@CTAB/Li half-cell and inhibiting the dendrite growth during cycling. The all-solid-state batteries (ASSBs) with LiFePO4 (LFP) as the cathode, PEO-LLZTO@CTAB as the electrolyte and Li as the anode exhibit a high initial discharge capacity of 146.6 mAh-g-1, excellent rate performance and high-capacity retention of 91.0 % after 300 cycles at 0.2 C multiplier and 60 °C within the voltage range of 2.7-4.0 V.

Titanium-Oxygen Clusters Brazing Li with Li6.5La3Zr1.5Ta0.5O12 for High-Performance All-Solid-State Li Batteries

Garnet-based solid-state electrolytes are considered crucial candidates for solid-state Li batteries due to their high Li+ conductivity and nonflammability; however, poor interfacial contact with the Li anode and growth of Li dendrites limit their application. Herein, a high-activity titanium-oxygen cluster is used as a brazing filler to braze the Li6.5La3Zr1.5Ta0.5O12 (LLZTO) with an Li anode into the whole unit. The brazing layer leads to a significantly lower interfacial impedance of 8.32 Ω cm2. Furthermore, the brazing layer is an isotropic amorphous ion-electron hybrid conductive layer, which significantly promotes Li+ transport and regulates the distribution of the electric field, therefore inhibiting the growth of Li dendrites. The cell exhibits an ultrahigh critical current density of 2.3 mA cm-2 and stable cycling of over 4000 h at 0.5 mA cm-2 (25 °C).

Synergized Tricomponent All-Inorganics Solid Electrolyte for Highly Stable Solid-State Li-Ion Batteries

Garnet-type oxide Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) features superior ionic conductivity and good stability toward lithium (Li) metal, but requires high-temperature sintering (≈1200 °C) that induces high fabrication cost, poor mechanical processability, and high interface resistance. Here, a novel high-performance tricomponent composite solid electrolyte (CSE) comprising LLZTO-4LiBH4 /xLi3 BN2 H8 is reported, which is prepared by ball milling the LLZTO-4LiBH4  mixture followed by hand milling with Li3 BN2 H8 . Green pellets fabricated by heating the cold-pressed CSE powders at 120 °C offer ultrafast room-temperature ionic conductivity (≈1.73 × 10-3  S cm-1  at 30 °C) and ultrahigh Li-ion transference number (≈0.9999), which enable the Li|Li symmetrical cells to cycle over 1600 h at 30 °C with only 30 mV of overpotential. Moreover, the Li|CSE|TiS2  full cells deliver 201 mAh g-1  of capacity with long cyclability. These outstanding performances are due to the low open porosity in the electrolyte pellets as well as the high intrinsic ionic conductivity and easy deformability of Li3 BN2 H8 .

Interface Engineering of All-Solid-State Batteries Based on Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes (SEs) are one of the most promising strategies for next-generation energy storage systems and electronic devices due to the higher energy density and intrinsic safety. However, the poor solid-solid contact and restricted chemical/electrochemical stability of inorganic SEs both in cathode and anode SE interfaces cause contact failure and the degeneration of SEs during prolonged charge-discharge processes. As a result, the increasing interface resistance significantly affects the coulombic efficiency and cycling performance of ASSBs. Herein, we present a fundamental understanding of physical contact and chemical/electrochemical features of ASSB interfaces based on mainstream inorganic SEs and summarize the recent work on interface modification. SE doping, optimizing morphology, introducing interlayer/coating layer, and utilizing compatible electrode materials are the key methods to prevent side reactions, which are discussed separately in cathode/anode-SE interface. We also highlight the constant extra stack pressure applied during ASSB cycling, which is important to the electrochemical performance. Finally, our perspectives on interface modification for practical high-performance ASSBs are put forward.

PVDF-HFP/PAN/PDA@LLZTO Composite Solid Electrolyte Enabling Reinforced Safety and Outstanding Low-Temperature Performance for Quasi-Solid-State Lithium Metal Batteries

Lithium-ion batteries (LIBs) have achieved a triumph in the market of portable electronic devices since their commercialization in the 1990s due to their high energy density. However, safety issue originating from the flammable, volatile, and toxic organic liquid electrolytes remains a long-standing problem to be solved. Alternatively, composite solid electrolytes (CSEs) have gradually become one of the most promising candidates due to their higher safety and stable electrochemical performance. However, the uniform dispersity of ceramic filler within the polymer matrix remains to be addressed. Generally, all-solid-state lithium metal batteries without any liquid components suffer from poor interfacial contact and low ionic conductivity, which seriously affect the electrochemical performance. Here we report a CSE consisting of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), polydopamine (PDA) coated Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) (denoted as PDA@LLZTO) microfiller, polyacrylonitrile (PAN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Introducing only 4 μL of liquid electrolyte at the electrode|electrolyte interface, the CSE-based cells exhibit high ionic conductivity (0.4 × 10^-3 S cm^-1 at 25 °C), superior cycle stability, and excellent thermal stability. Even under low temperatures, the impressive electrochemical performance (78.8% of capacity retention after 400 cycles at 1 C, 0 °C, and decent capacities delivered even at low temperature of -20 °C) highlights the potential of such quasi-solid-state lithium metal batteries as a viable solution for the next-generation high-performance lithium metal batteries.

High Lithium Salt Content PVDF-Based Solid-State Composite Polymer Electrolyte Enhanced by h-BN Nanosheets

Due to the unique safety qualities, solid composite polymer electrolyte (SCPE) has achieved considerable attentions to fabricate high-energy-density lithium metal batteries, but its overall performance still has to be improved. Herein, a high lithium salt content poly(vinylidene fluoride) (PVDF)-based SCPE was developed, enhanced by hexagonal boron nitride (h-BN) nanosheets, presenting perfect electrochemical performance, fast ion transport, and efficient inhibition of lithium dendrite growth. The optimized SCPE (PVDF-L70-B5) could deliver high ionic conductivity (2.98×10^-4  S cm^-1 ), ultra-high Li⁺ ion transfer number (0.62), wide electrochemical stability window (5.24 V), and strong mechanical strength (3.45 MPa) at room temperature. Density functional theory calculation further confirmed that the presence of h-BN could promote the dissociation of bis(trifluoromethanesulfonyl)imide lithium (LiTFSI) and the rapid transfer of Li⁺ ions. As a result, the assembled symmetric Li/Li battery and asymmetric Li/LiFePO₄ battery using PVDF-L70-B5 SCPEs both exhibited high reversible capacity, long-term cycle stability, and high-rate performance when cycled at 60 or 30 °C. The designed SCPEs will open up a new route to synthesize solid-state lithium batteries with high energy density and high safety.

Combating Li metal deposits in all-solid-state battery via the piezoelectric and ferroelectric effects

All-solid-state Li-metal batteries (ASSLBs) are highly desirable, due to their inherent safety and high energy density; however, the irregular and uncontrolled growth of Li filaments is detrimental to interfacial stability and safety. Herein, we report on the incorporation of piezo-/ferroelectric BaTiO3 (BTO) nanofibers into solid electrolytes and determination of electric-field distribution due to BTO inclusion that effectively regulates the nucleation and growth of Li dendrites. Theoretical simulations predict that the piezoelectric effect of BTO embedded in solid electrolyte reduces the driving force of dendrite growth at high curvatures, while its ferroelectricity reduces the overpotential, which helps to regularize Li deposition and Li+ flux. Polarization reversal of soft solid electrolytes was identified, confirming a regular deposition and morphology alteration of Li. As expected, the ASSLBs operating with LiFePO4/Li and poly(ethylene oxide) (PEO)/garnet solid electrolyte containing 10% BTO additive showed a steady and long cycle life with a reversible capacity of 103.2 mAh g-1 over 500 cycles at 1 C. Furthermore, the comparable cyclability and flexibility of the scalable pouch cells prepared and the successful validation in the sulfide electrolytes, demonstrating its universal and promising application for the integration of Li metal anodes in solid-state batteries.

Stabilizing the Halide Solid Electrolyte to Lithium by a β-Li3N Interfacial Layer

As a new class of solid electrolytes, halide solid electrolytes have the advantages of high ionic conductivity at room temperature, stability to high-voltage cathodes, and good deformability, but they generally show a problem of being unstable to a lithium anode. Here, we report the use of Li₃N as an interface modification layer to improve the interfacial stability of Li₂ZrCl₆ to the Li anode. We found that commercial Li₃N can be easily transformed into an α-phase and a β-phase by ball-milling and annealing, respectively, in which β-phase Li₃N simultaneously has high room-temperature ionic conductivity and good stability to both Li and Li₂ZrCl₆, making it a good choice for an artificial interface layer material. After the modification of the β-Li₃N interfacial layer, the interfacial impedance between Li₂ZrCl₆ and the Li anode decreased from 1929 to ∼400 Ω. At a current density of 0.1 mA cm^-2, the overpotential of the Li symmetric cell decreased from 250 to ∼50 mV, which did not show an obvious increase for at least 300 h, indicating that the β-Li₃N interface layer effectively improves the interfacial stability between Li₂ZrCl₆ and Li.

Comparative Study of Stability against Moisture for Solid Garnet Electrolytes with Different Dopants

The cubic garnet Li7La3Zr2O12 (c-LLZO) is one of the most promising solid electrolytes due to its high ionic conductivity and large electrochemical window. However, the critical issue of Li2CO3 formation on the c-LLZO surface when exposed to air is problematic, which is detrimental to the ionic conductivity and storage. Herein, comparative studies were carried out on the air stability of Al-doped Li7La3Zr2O12 (Al-LLZO), Al-Ta-doped Li7La3Zr2O12 (Al-LLZTO), and Al-Nb-doped Li7La3Zr2O12 (Al-LLZNO). It was found that Al-LLZTO and Al-LLZNO are less reactive with air than Al-LLZO. The morphology of Li2CO3 on Al-LLZTO micro-sized powders after air exposure was island-like with ~1.5 μm in thickness. The interfacial resistance of Li/Al-LLZTO was also a factor of ~3 smaller than that of Li/Al-LLZO, leading to the improved cycle stability of Li/Al-LLZTO/Li symmetric cells. The first-principles calculations based on density functional theory (DFT) verified that the decomposition energy of Al-LLZTO was larger than that of Al-LLZO, inhibiting the reaction product of Li2O and, thus, the next step product of Li2CO3 following the reactions of Li2O + H2O → LiOH and LiOH + CO2 → Li2CO3.

SnF2 -Catalyzed Formation of Polymerized Dioxolane as Solid Electrolyte and its Thermal Decomposition Behavior

Polymerized-dioxolane(P-DOL) is of potential as a solid-polymer-electrolyte(SPE) due to its high Li⁺ -conductivity, good compatibility with Li-metal and desired preparation method of in situ polymerization in cells. In this study, SnF₂ was demonstrated not only to be an efficient catalyst for the polymerization of DOL at room temperature, but also an effective additive for improving interfacial wettability and suppressing dendrite through the reaction with Li-metal and the formation of LiF/Li_x Sn based composite solid electrolyte interlayer(SEI). Using the SnF₂ polymerized P-DOL containing 1 M LiTFSI as SPE(P-DOL-SPE), obviously denser Li-deposition was obtained, and the all-solid-state(ASS) Li/LiFePO₄ cell delivered stable cycling over 350 cycles at 45 °C. At the same time, the irreversible decomposition of P-DOL-SPE into formaldehyde and small molecule epoxides are observed at 110 °C, which is even initiated at lower temperature of 40 °C under vacuum. This thermal decomposition of P-DOL-SPE in pouch cell causes huge volume swell, and therefore putting a strict limitation on the operating temperature window for the P-DOL based electrolytes.

Exchange of Li and AgNO3 Enabling Stable 3D Lithium Metal Anodes with Embedded Lithophilic Nanoparticles and a Solid Electrolyte Interphase Inducer

Three-dimensional (3D) current collectors can effectively mitigate the volumetric expansion of working lithium metal anodes (LMAs). However, the practical utilization of 3D current collectors for lithium metal batteries remains unsatisfactory because of inhomogeneous deposition of lithium ions and an unstable solid electrolyte interphase (SEI). Herein, a facile method based on the exchange reaction between Li and AgNO₃ is exploited to embed Ag nanoparticles (NPs) and LiNO₃ in a carbon paper (ALCP@Li). The Ag NPs act as a seed for even lithium deposition inside the carbon matrix by virtue of their excellent lithiophilicity. Simultaneously, LiNO₃ plays an effective role in stabilizing LMAs by evolving a robust N-rich SEI. As a result, such 3D LMAs show a high Coulombic efficiency in half-cells (200 cycles, 99% at 1 mA cm^-2, 1 mAh cm^-2) and a low overpotential (60 mV). When paired with commercial thick NCM622 and LiFePO₄ cathodes, the 3D LMA-based full cells exhibit stable cycling in carbonate electrolytes.

Fluorinated Poly-oxalate Electrolytes Stabilizing both Anode and Cathode Interfaces for All-Solid-State Li/NMC811 Batteries

The relatively narrow electrochemical steady window and low ionic conductivity are two critical challenges for Li⁺ -conducting solid polymer electrolytes (SPE). Here, a family of poly-oxalate(POE) structures were prepared as SPE; among them, POEs composed from diols with an odd number of carbons show higher ionic conductivity than those composed from diols with an even number of carbons, and the POE composed from propanediol (C5-POE) has the highest Li⁺ conductivity. The HOMO (highest occupied molecular orbital) electrons of POE were found located on the terminal units. When using trifluoroacetate as the terminating unit (POE-F), not only does the HOMO become more negative, but also the HOMO electrons shift to the middle oxalate units, improving the antioxidative capability. Furthermore, the interfacial compatibility across the Li-metal/POE-F is also improved by the generation of a LiF-based solid-electrolyte-interlayer(SEI). With the trifluoroacetate-terminated C5-POE (C5-POE-F) as the electrolyte and Li⁺ -conducting binder in the cathode, the all-solid-state Li/LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811) cells showed significantly improved stability than the counterpart with poly-ether, providing a promising candidate for the forthcoming all-solid-state high-voltage Li-metal batteries.

The Electrolysis of Anti-Perovskite Li2 OHCl for Prelithiation of High-Energy-Density Batteries

Anti-perovskite type Li₂ OHCl was previously studied as a solid-state Li⁺ conductor. Here, we report that the Li₂ OHCl can be electrolyzed at 3.3 V or 4.0 V, with the creation of O₂ /HCl gases and the release of 2 equiv. Li⁺ via two different decomposition routes, depending on the acidity of electrolyte. In the electrolyte with trace acid, the Li₂ OHCl is oxidized at a constant voltage of 3.3 V. In neutral electrolyte, the oxidization of Li₂ OHCl starts at 4.0 V, but the produced HCl will increase the acidity of electrolyte and lead to a voltage drop to 3.3 V for the electrolysis of Li₂ OHCl. The electrolysis of Li₂ OHCl delivers a lithium releasing capacity as high as 810 mAh g^-1 , with an equivalent Li-deposition or Li-intercalation on anode, making it a promising candidate as a Li reservoir for prelithiation of anode. Using Li₂ OHCl as the lithium source, silicon-carbon (Si@C) composite anode can be effectively prelithiated. The full cells composed of LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811) cathode and prelithiated Si@C anode exhibited increased capacities with the increment of prelithiation dosages.

Designing Polymer-in-Salt Electrolyte and Fully Infiltrated 3D Electrode for Integrated Solid-State Lithium Batteries

Solid-state lithium batteries (SSLBs) are promising owing to enhanced safety and high energy density but plagued by the relatively low ionic conductivity of solid-state electrolytes and large electrolyte-electrode interfacial resistance. Herein, we design a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP)-based polymer-in-salt solid electrolyte (PISSE) with high room-temperature ionic conductivity (1.24×10^-4  S cm^-1 ) and construct a model integrated TiO₂ /Li SSLB with 3D fully infiltration of solid electrolyte. With forming aggregated ion clusters, unique ionic channels are generated in the PISSE, providing much faster Li⁺ transport than common polymer electrolytes. The integrated device achieves maximized interfacial contact and electrochemical and mechanical stability, with performance close to liquid electrolyte. A pouch cell made of 2 SSLB units in series shows high voltage plateau (3.7 V) and volumetric energy density comparable to many commercial thin-film batteries.