Lithium Difluorophosphate‐Based Dual‐Salt Low Concentration Electrolytes for Lithium Metal Batteries

The safety hazards and low Coulombic efficiency originating from the growth of lithium dendrites and decomposition of the electrolyte restrict the practical application of Li metal batteries (LMBs). Inspired by the low cost of low concentration electrolytes (LCEs) in industrial applications, dual‐salt LCEs employing 0.1 m Li difluorophosphate (LiDFP) and 0.4 m LiBOB/LiFSI/LiTFSI are proposed to construct a robust and conductive interphase on a Li metal anode. Compared with the conventional electrolyte using 1 m LiPF₆, the ionic conductivity of LCEs is reduced but the conductivity decrement of the separator immersed in LCEs is moderate, especially for the LiDFP–LiFSI and LiDFP–LiTFSI electrolytes. The accurate Coulombic efficiency (CE) of the Li||Cu cells increases from 83.3% (electrolyte using 1 m LiPF₆) to 97.6%, 94.5%, and 93.6% for LiDFP–LiBOB, LiDFP–LiFSI, and LiDFP–LiTFSI electrolytes, respectively. The capacity retention of Li||LiFePO₄ cells using the LiDFP–LiBOB electrolyte reaches 95.4% along with a CE over 99.8% after 300 cycles at a current density of 2.0 mA cm^?2 and the capacity reaches 103.7 mAh g^?1 at a current density of up to 16.0 mA cm^?2. This work provides a dual‐salt LCE for practical LMBs and presents a new perspective for the design of electrolytes for LMBs.     

Solvent‐Free Synthesis of Thin,Flexible, Nonflammable Garnet‐Based Composite Solid Electrolyte for All‐Solid‐State Lithium Batteries

Thin solid‐state electrolytes with nonflammability, high ionic conductivity, low interfacial resistance, and good processability are urgently required for next‐generation safe, high energy density lithium metal batteries. Here, a 3D Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) self‐supporting framework interconnected by polytetrafluoroethylene (PTFE) binder is prepared through a simple grinding method without any solvent. Subsequently, a garnet‐based composite electrolyte is achieved through filling the flexible 3D LLZTO framework with a succinonitrile solid electrolyte. Due to the high content of garnet ceramic (80.4 wt%) and high heat‐resistance of the PTFE binder, such a composite electrolyte film with nonflammability and high processability exhibits a wide electrochemical window of 4.8 V versus Li/Li⁺ and high ionic transference number of 0.53. The continuous Li⁺ transfer channels between interconnected LLZTO particles and succinonitrile, and the soft electrolyte/electrode interface jointly contribute to a high ambient‐temperature ionic conductivity of 1.2 × 10^?4 S cm^?1 and excellent long‐term stability of the Li symmetric battery (stable at a current density of 0.1 mA cm^?2 for over 500 h). Furthermore, as‐prepared LiFePO₄|Li and LiNi_0.5Mn_0.3Co_0.2O₂|Li batteries based on the thin composite electrolyte exhibit high discharge specific capacities of 153 and 158 mAh g^?1 respectively, and desirable cyclic stabilities at room temperature.     

Flexible Ion‐Conducting Composite Membranes for Lithium Batteries

The use of metallic lithium anodes enables higher energy density and higher specific capacity Li‐based batteries. However, it is essential to suppress lithium dendrite growth during electrodeposition. Li‐ion‐conducting ceramics (LICC) can mechanically suppress dendritic growth but are too fragile and also have low Li‐ion conductivity. Here, a simple, versatile, and scalable procedure for fabricating flexible Li‐ion‐conducting composite membranes composed of a single layer of LICC particles firmly embedded in a polymer matrix with their top and bottom surfaces exposed to allow for ionic transport is described. The membranes are thin (<100 μm) and possess high Li‐ion conductance at thicknesses where LICC disks are mechanically unstable. It is demonstrated that these membranes suppress Li dendrite growth even when the shear modulus of the matrix is lower than that of lithium. It is anticipated that these membranes enable the use of metallic lithium anodes in conventional and solid‐state Li‐ion batteries as well as in future Li? S and Li? O₂ batteries.     

Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

Rechargeable Li‐ion batteries (LIBs) are electrochemical storage device widely applied in electric vehicles, mobile electronic devices, etc. However, traditional LIBs containing liquid electrolytes suffer from flammability, poor electrochemical stability, and limited operational temperature range. Replacement of the liquid electrolytes with inorganic solid‐state electrolytes (SSEs) would solve this problem. However, several critical issues, such as poor interfacial compatibility, low ionic conductivity at ambient temperatures, etc., need to be surmounted before the commercialization of all‐solid‐state Li‐ion batteries (ASSLIBs). In this review, a brief historical context for the inorganic SSEs is described first. Then, two critical issues in the ASSLIBs are highlighted: interfacial incompatibility of the electrodes and SSEs and internal stresses. For the interfacial incompatibility, the discussion is focused on the dynamic characterization of the electrode/SSE interfaces, the origin and evolution of the interfacial resistance, and interface engineering to minimize the interfacial resistance. The internal stresses in the ASSLIBs are another major concern because rigid contacts are introduced. Stress generation, stress evolution during battery cycling, stress measurement/simulation, and ways to alleviate the stresses are outlined in detail. Finally, current challenges and perspectives for future development of the inorganic SSEs and ASSLIBs are outlined.     

Perovskite Membranes with Vertically Aligned Microchannels for All‐Solid‐State Lithium Batteries

Perovskite‐type solid‐state electrolytes exhibit great potential for the development of all‐solid‐state lithium batteries due to their high Li‐ion conductivity (approaching 10^?3 S cm^?1), wide potential window, and excellent thermal/chemical stability. However, the large solid–solid interfacial resistance between perovskite electrolytes and electrode materials is still a great challenge that hinders the development of high‐performance all‐solid‐state lithium batteries. In this work, a perovskite‐type Li_0.34La_0.51TiO₃ (LLTO) membrane with vertically aligned microchannels is constructed by a phase‐inversion method. The 3D vertically aligned microchannel framework membrane enables more effective Li‐ion transport between the cathode and solid‐state electrolyte than a planar LLTO membrane. A significant decrease in the perovskite/cathode interfacial resistance, from 853 to 133 Ω cm², is observed. It is also demonstrated that full cells utilizing LLTO with vertically aligned microchannels as the electrolyte exhibit a high specific capacity and improved rate performance.     

Stack Pressure Considerations for Room‐Temperature All‐Solid‐State Lithium Metal Batteries

All‐solid‐state batteries are expected to enable batteries with high energy density with the use of lithium metal anodes. Although solid electrolytes are believed to be mechanically strong enough to prevent lithium dendrites from propagating, various reports today still show cell failure due to lithium dendrit growth at room temperature. While cell parameters such as current density, electrolyte porosity, and interfacial properties have been investigated, mechanical properties of lithium metal and the role of applied stack pressure on the shorting behavior are still poorly understood. Here, failure mechanisms of lithium metal are investigated in all‐solid‐state batteries as a function of stack pressure, and in situ characterization of the interfacial and morphological properties of the buried lithium is conducted in solid electrolytes. It is found that a low stack pressure of 5 MPa allows reliable plating and stripping in a lithium symmetric cell for more than 1000 h, and a Li | Li₆PS₅Cl | LiNi_0.80Co_0.15Al_0.05O₂ full cell, plating more than 4 µm of lithium per charge, is able to cycle over 200 cycles at room temperature. These results suggest the possibility of enabling the lithium metal anode in all‐solid‐state batteries at reasonable stack pressures.     

High Capacity All‐Solid‐State Lithium Batteries Enabled by Pyrite‐Sulfur Composites

As the theoretical limit of intercalation material‐based lithium‐ion batteries is approached, alternative chemistries based on conversion reactions are presently considered. The conversion of sulfur is particularly appealing as it is associated with a theoretical gravimetric energy density up to 2510 Wh kg^?1. In this paper, three different carbon‐iron disulfide‐sulfur (C‐FeS₂‐S) composites are proposed as alternative positive electrode materials for all‐solid‐state lithium‐sulfur batteries. These are synthesized through a facile, low‐cost, single‐step ball‐milling procedure. It is found that the crystalline structure (evaluated by X‐ray diffraction) and the morphology of the composites (evaluated by scanning electron microscopy) are greatly influenced by the FeS₂:S ratio. Li/LiI‐Li₃PS₄/C‐FeS₂‐S solid‐state cells are tested under galvanostatic conditions, while differential capacity plots are used to discuss the peculiar electrochemical features of these novel materials. These cells deliver capacities as high as 1200 mAh g_(FeS2+S)^?1 at the intermediate loading of 1 mg cm^?2 (1.2 mAh cm^?2), and up to 3.55 mAh cm^?2 for active material loadings as high as 5 mg cm^?2 at 20 °C. Such an excellent performance, rarely reported for (sulfur/metal sulfide)‐based, all solid‐state cells, makes these composites highly promising for real application where high positive electrode loadings are required.     

Ultrathin,Flexible Polymer Electrolyte for Cost‐Effective Fabrication of All‐Solid‐State Lithium Metal Batteries

All‐solid‐state batteries are promising candidates for the next‐generation safer batteries. However, a number of obstacles have limited the practical application of all‐solid‐state Li batteries (ASSLBs), such as moderate ionic conductivity at room temperature. Here, unlike most of the previous approaches, superior performances of ASSLBs are achieved by greatly reducing the thickness of the solid‐state electrolyte (SSE), where ionic conductivity is no longer a limiting factor. The ultrathin SSE (7.5 µm) is developed by integrating the low‐cost polyethylene separator with polyethylene oxide (PEO)/Li‐salt (PPL). The ultrathin PPL shortens Li⁺ diffusion time and distance within the electrolyte, and provides sufficient Li⁺ conductance for batteries to operate at room temperature. The robust yet flexible polyethylene offers mechanical support for the soft PEO/Li‐salt, effectively preventing short‐circuits even under mechanical deformation. Various ASSLBs with PPL electrolyte show superior electrochemical performance. An initial capacity of 135 mAh g^?1 at room temperature and the high‐rate capacity up to 10 C at 60 °C can be achieved in LiFePO₄/PPL/Li batteries. The high‐energy‐density sulfur cathode and MoS₂ anode employing PPL electrolyte also realize remarkable performance. Moreover, the ASSLB can be assembled by a facile process, which can be easily scaled up to mass production.     

High‐Energy Aqueous Lithium Batteries

Owing to the high voltage of lithium‐ion batteries (LIBs), the dominating electrolyte is non‐aqueous. The idea of an aqueous rechargeable lithium battery (ARLB) dates back to 1994, but it had attracted little attention due to the narrow stable potential window of aqueous electrolytes, which results in low energy density. However, aqueous electrolytes were employed during the 2000s for the fundamental studies of electrode materials in the absence of side reactions such as the decomposition of organic species. The high solubility of lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) in water has introduced new opportunities for high‐voltage ARLBs. Nonetheless, these ideas are somehow overshadowed by the common perception about the essential limitation of the aqueous electrolyte. The electrochemical behaviour of conventional electrode materials can be substantially tuned in the water‐in‐salt electrolytes. The latest idea of utilising a graphite anode in the aqueous water‐in‐salt electrolytes has paved the way towards not only 4‐V ARLB but also a new generation of Li?S batteries with a higher operating voltage and energy efficiency. Furthermore, aqueous electrolytes can provide a cathodically stable environment for Li?O₂ batteries. The present paper aims to highlight these emerging opportunities possibly leading to a new generation of LIBs, which can be substantially cheaper and safer.     

Low‐Cost Hollow Mesoporous Polymer Spheres and All‐Solid‐State Lithium,Sodium Batteries

A practical, low‐cost synthesis of hollow mesoporous organic polymer (HMOP) spheres is reported. The electrochemical properties of Li⁺/Na⁺‐electrolyte membranes with these spheres substituting for oxide filler particles in poly(ethylene oxide) (PEO)‐filler composite are explored. The electrolyte membranes are mechanically robust, thermally stable to over 250 °C, and block dendrites from a metallic‐lithium/sodium anode. The Li⁺/Na⁺ transfer impedance across the lithium/sodium–electrolyte interface is initially acceptable at 65 °C and scavenging of impurities by the porous‐spheres filler lowers this impedance relative to that with Al₂O₃. All‐solid‐state Li/LiFePO₄ and Na/NaTi₂(PO4)₃ cells give stable discharge capacity of ≈130 and 80 mAh g^?1, respectively, at 0.5 C and 65 °C for 100 cycles.     

Dynamic Visualization of Cathode/Electrolyte Evolution in Quasi‐Solid‐State Lithium Batteries

Solid‐state lithium–sulfur batteries (SSLSBs) are highly appealing for electrochemical energy devices because of their promising theoretical energy density. An intensive acquaintance of SSLS interfacial behavior is of importance in gaining fundamental knowledge of working/failure mechanisms and clarifying further optimized design of advanced batteries. Herein, a direct visualization of the evolution of both component and structure is present inside a working SSLSB. In situ Raman spectroscopy clearly sheds light on the potential‐dependent evolution of sulfur speciation via subtly fabricating the electrochemical cell. Moreover, the real‐time optical microscopic views show that the irreversible structure deformation of solid‐state electrolytes (SSEs), which results from the decomposition of dissolved polysulfides (PSs) and gas generation inside the SSE, directly causes the fracture of sulfur cathode with the cycling times increasing. Furthermore, by an atomic force microscopy study, the evolving structure and dynamic behavior of SSEs are directly captured at the nano/microscale and further elucidate the PS shuttling determining the mechanism stability of electrolyte. This work provides a straightforward monitoring of the compositional and morphological evolution, which contributes one to exploring the failure mechanisms and interfacial reactions for the cell performance enhancement.     

Facilitating Interfacial Stability Via Bilayer Heterostructure Solid Electrolyte Toward High‐energy,Safe and Adaptable Lithium Batteries

Solid‐state electrolytes are widely anticipated to enable the revival of high energy density and safe metallic Li batteries, however, their lower ionic conductivity at room temperature, stiff interfacial contact, and severe polarization during cycling continue to pose challenges in practical applications. Herein, a dual‐composite concept is applied to the design of a bilayer heterostructure solid electrolyte composed of Li⁺ conductive garnet nanowires (Li_6.75La₃Zr_1.75Nb_0.25O₁₂)/polyvinylidene fluoride‐co‐hexafluoropropylene (PVDF‐HFP) as a tough matrix and modified metal organic framework particles/polyethylene oxide/PVDF‐HFP as an interfacial gel. The integral ionic conductivity of the solid electrolyte reaches 2.0 × 10^?4 S cm^?1 at room temperature. In addition, a chemically/electrochemically stable interface is rapidly formed, and Li dendrites are well restrained by a robust inorganic shield and matrix. As a result, steady Li plating/stripping for more than 1700 h at 0.25 mA cm^?2 is achieved. Solid‐state batteries using this bilayer heterostructure solid electrolyte deliver promising battery performance (efficient capacity output and cycling stability) at ambient temperature (25 °C). Moreover, the pouch cells exhibit considerable flexibility in service and unexpected endurance under a series of extreme abuse tests including hitting with a nail, burning, immersion under water, and freezing in liquid nitrogen.     

A Lithium Amide‐Borohydride Solid‐State Electrolyte with Lithium‐Ion Conductivities Comparable to Liquid Electrolytes

High ionic conductivity of up to 6.4 × 10^?3 S cm^?1 near room temperature (40 °C) in lithium amide‐borohydrides is reported, comparable to values of liquid organic electrolytes commonly employed in lithium‐ion batteries. Density functional theory is applied coupled with X‐ray diffraction, calorimetry, and nuclear magnetic resonance experiments to shed light on the conduction mechanism. A Li₄Ti₅O₁₂ half‐cell battery incorporating the lithium amide‐borohydride electrolyte exhibits good rate performance up to 3.5 mA cm^?2 (5 C) and stable cycling over 400 cycles at 1 C at 40 °C, indicating high bulk and interfacial stability. The results demonstrate the potential of lithium amide‐borohydrides as solid‐state electrolytes for high‐power lithium‐ion batteries.     

Safety‐Reinforced Poly(Propylene Carbonate)‐Based All‐Solid‐State Polymer Electrolyte for Ambient‐Temperature Solid Polymer Lithium Batteries

An integrated preparation of safety‐reinforced poly(propylene carbonate)‐based all‐solid polymer electrolyte is shown to be applicable to ambient‐temperature solid polymer lithium batteries. In contrast to pristine poly(ethylene oxide) solid polymer electrolyte, this solid polymer electrolyte exhibits higher ionic conductivity, wider electrochemical window, better mechanical strength, and superior rate performance at 20 °C. Moreover, lithium iron phosphate/lithium cell using such solid polymer electrolyte can charge and discharge even at 120 °C. It is also noted that the solid‐state soft‐package lithium cells assembled with this solid polymer electrolyte can still power a red light‐emitting diode lamp without suffering from internal short‐circuit failures even after cutting off one part of the battery. Considering the aspects mentioned above, the solid polymer electrolyte is eligible for practical lithium battery applications with improved reliability and safety. Just as important, a new perspective that the degree of amorphous state of polymer is also as critical as its low glass transition temperature for the exploration of room temperature solid polymer electrolyte is illustrated. In all, this study opens up a kind of new avenue that could be a milestone to the development of high‐voltage and ambient‐temperature all‐solid‐state polymer electrolytes.     

Solid‐State Prelithiation Enables High‐Performance Li‐Al‐H Anode for Solid‐State Batteries

Lithium alanates exhibit high theoretical specific capacities and appropriate lithiation/delithiation potentials, but suffer from poor reversibility, cycling stability, and rate capability due to their sluggish kinetics and extensive side reactions. Herein, a novel and facile solid‐state prelithiation approach is proposed to in situ prepare a Li₃AlH₆‐Al nanocomposite from a short‐circuited electrochemical reaction between LiAlH₄ and Li with the help of fast electron and Li‐ion conductors (C and P6₃mc LiBH₄). This nanocomposite consists of dispersive Al nanograins and an amorphous Li₃AlH₆ matrix, which enables superior electrochemical performance in solid‐state cells, as much higher specific capacity (2266 mAh g^?1), Coulombic efficiency (88%), cycling stability (71% retention in the 100th cycle), and rate capability (1429 mAh g^?1 at 1 A g^?1) are achieved. In addition, this nanocomposite works well in the solid‐state full cell with LiCoO₂ cathode, demonstrating its promising application prospects. Mechanism analysis reveals that the dispersive Al nanograins and amorphous Li₃AlH₆ matrix can dramatically enhance the lithiation and delithiation kinetics without side reactions, which is mainly responsible for the excellent overall performance. Moreover, this solid‐state prelithiation approach is general and can also be applied to other Li‐poor electrode materials for further modification of their electrochemical behavior.