Self-Catalyzed Growth of Co4N and N-Doped Carbon Nanotubes toward Bifunctional Cathode for Highly Safe and Flexible Li-Air Batteries

The practical application of high-energy density lithium-oxygen (Li-O2) batteries is severely impeded by the notorious cycling stability and safety, which mainly comes from slow kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at cathodes, causing inferior redox overpotentials and reactive lithium metal in flammable liquid electrolyte. Herein, a bifunctional electrode, a safe gel polymer electrolyte (GPE), and a robust lithium anode are proposed to alleviate above problems. The bifunctional electrode is composed of N-doped carbon nanotubes (N-CNTs) and Co4N by in situ chemical vapor deposition self-catalyzed growth on carbon cloth (N-CNTs@Co4N@CC). The self-supporting, binder-free N-CNTs@Co4N@CC electrode has a strong and stable three-dimensional (3D) interconnected conductive structure, which provides interconnectivity between the active sites and the electrode to promote the transfer of electrons. Furthermore, the N-CNT-intertwined Co4N ensures efficient catalytic activity. Hence, the electrode demonstrates improved electrochemical properties even under a large current density (2000 mA g-1) and long cycling operation (250 cycles). Moreover, a highly safe and flexible rechargeable cell using the 3D N-CNTs@Co4N@CC electrode, GPE, and robust lithium anode design has been explored. The open circuit voltage is stable at ∼3.0 V even after 9800 cycles, which proves the mechanical durability of the integrated GPE cell. The stable cable-type Li-air battery was demonstrated to stably drive the light-emitting diodes (LEDs), highlighting the reliability for practical use.

An Electrode-Crosstalk-Suppressing Smart Polymer Electrolyte for High Safety Lithium-Ion Batteries

Electrode crosstalk between anode and cathode at elevated temperatures is identified as a real culprit triggering the thermal runaway of lithium-ion batteries. Herein, to address this challenge, a novel smart polymer electrolyte is prepared through in situ polymerization of methyl methacrylate and acrylic anhydride monomers within a succinonitrile-based dual-anion deep eutectic solvent. Owing to the abundant active unsaturated double bonds on the as-obtained polymer matrix end, this smart polymer electrolyte can spontaneously form a dense crosslinked polymer network under elevated temperatures, effectively slowing down the crosstalk diffusion kinetics of lithium ions and active gases. Impressively, LiCoO2/graphite pouch cells employing this smart polymer electrolyte demonstrate no thermal runaway even at the temperature up to 250 °C via accelerating rate calorimeter testing. Meanwhile, because of its abundance of functional motifs, this smart polymer electrolyte can facilitate the formation of stable and thermally robust electrode/electrolyte interface on both electrodes, ensuring the long cycle life and high safety of LIBs. In specific, this smart polymer electrolyte endows 1.1 Ah LiCoO2/graphite pouch cell with a capacity retention of 96% after 398 cycles at 0.2 C.

In-Situ Cross-linked F- and P-Containing Solid Polymer Electrolyte for Long-Cycling and High-Safety Lithium Metal Batteries with Various Cathode Materials

The practical application of lithium metal batteries (LMBs) has been hindered by limited cycle-life and safety concerns. To solve these problems, we develop a novel fluorinated phosphate cross-linker for gel polymer electrolyte in high-voltage LMBs, achieving superior electrochemical performance and high safety simultaneously. The fluorinated phosphate cross-linked gel polymer electrolyte (FP-GPE) by in-situ polymerization method not only demonstrates high oxidation stability but also exhibits excellent compatibility with lithium metal anode. LMBs utilizing FP-GPE realize stable cycling even at a high cut-off voltage of 4.6 V (vs Li/Li+) with various high-voltage cathode materials. The LiNi0.6Co0.2Mn0.2O2|FP-GPE|Li battery exhibits an ultralong cycle-life of 1200 cycles with an impressive capacity retention of 80.1 %. Furthermore, the FP-GPE-based batteries display excellent electrochemical performance even at practical conditions, such as high cathode mass loading (20.84 mg cm-2), ultrathin Li (20 μm), and a wide temperature range of -25 to 80 °C. Moreover, the first reported solid-state 18650 cylindrical LMBs have been successfully fabricated and demonstrate exceptional safety under mechanical abuse. Additionally, the industry-level 18650 cylindrical LiMn2O4|FP-GPE|Li4Ti5O12 cells demonstrate a remarkable cycle-life of 1400 cycles. Therefore, the impressive electrochemical performance and high safety in practical batteries demonstrate a substantial potential of well-designed FP-GPE for large-scale industrial applications.

Interconnected Three-Dimensional Porous Alginate-Based Gel Electrolytes for Lithium Metal Batteries

Lithium batteries have been widely used in our daily lives for their high energy density and long-term stability. However, their safety problems are of paramount concern for consumers, which restricts their scale applications. Gel polymer electrolytes (GPEs) compensate for the defects of liquid leakage and lower ionic conductivity of solid electrolytes, which have attracted a lot of attention. Herein, a 3D interconnected highly porous structural gel electrolyte was prepared with alginate dressing as a host material, poly(ethylene oxide) (PEO), and a commercial liquid electrolyte. With rich polar functional groups and (CH2-CH2-O) segments on the polymer matrix, the transportation of Li+ is faster and uniform; thus, the formations of lithium dendrite were significantly inhibited. The cycle stability of symmetrical Li||Li batteries with modified composite electrolytes (SAA) is greatly improved, and the overpotential remains stable after more than 1000 h. Meanwhile, under the same conditions, the cycle performance of batteries with unmodified electrolytes is inferior and overpotentials are nearly 1 V after 100 h. Additionally, the capacity retention of Li||LiFePO4 with SAA is more than 95% after 200 cycles, while those of the others declined sharply. The alginate dressing-based GPEs can greatly enhance the mechanical and thermal stability of PEO-based GPEs, which provides an environmentally friendly avenue for gel electrolytes' applications in lithium batteries.

Yolk-Shell Structured ST@Al2 O3 Enables Functional PE Separator with Enhanced Lewis Acid Sites for High-Performance Lithium Metal Batteries

Commercial polymer separators usually have limited porosity, poor electrolyte wettability, and poor thermal and mechanical stability, which can deteriorate the performance of battery, especially at high current densities. In this work, a functional polyethylene (PE) separator is prepared by surface engineering a layer of Ti-doped SiO2 @Al2 O3 particles (denoted as ST@Al2 O3 -PE) with strong Lewis acid property and uniform porous structure on one side of the PE separator. On the other hand, ST@Al2 O3 particles with abundant pore structures and large cavities can store a large amount of electrolyte, providing a shortened pathway for lithium-ion transport, and the Lewis acid sites and porous structure of the ST@Al2 O3 can tune Li plating/stripping behavior and stabilize the lithium metal anode. The ST@Al2 O3 -PE separators exhibit better ionic conductivity (5.55 mS cm-1 ) and larger lithium-ion transference number (0.62). At a current density of 1 mA cm-2 , Li/Li symmetric cells with ST@Al2 O3 -PE separator can be stably cycled for more than 400 h, and both lithium iron phosphate /Li cells and lithium cobaltate/Li cells with ST@Al2 O3 -PE separator have good cycling and rate performance. This work provides a new strategy for developing functional separators and promoting the application of lithium metal batteries.

A Magnesium Carbonate Hydroxide Nanofiber/Poly(Vinylidene Fluoride) Composite Membrane for High-Rate and High-Safety Lithium-Ion Batteries

Simultaneously high-rate and high-safety lithium-ion batteries (LIBs) have long been the research focus in both academia and industry. In this study, a multifunctional composite membrane fabricated by incorporating poly(vinylidene fluoride) (PVDF) with magnesium carbonate hydroxide (MCH) nanofibers was reported for the first time. Compared to commercial polypropylene (PP) membranes and neat PVDF membranes, the composite membrane exhibits various excellent properties, including higher porosity (85.9%) and electrolyte wettability (539.8%), better ionic conductivity (1.4 mS·cm-1), and lower interfacial resistance (93.3 Ω). It can remain dimensionally stable up to 180 °C, preventing LIBs from fast internal short-circuiting at the beginning of a thermal runaway situation. When a coin cell assembled with this composite membrane was tested at a high temperature (100 °C), it showed superior charge-discharge performance across 100 cycles. Furthermore, this composite membrane demonstrated greatly improved flame retardancy compared with PP and PVDF membranes. We anticipate that this multifunctional membrane will be a promising separator candidate for next-generation LIBs and other energy storage devices, in order to meet rate and safety requirements.

In Situ Polymerized Flame Retardant Gel Electrolyte for High-Performance and Safety-Enhanced Lithium Metal Batteries

A flame retardant gel electrolyte (FRGE) is deemed as one of the most promising electrolytes to relieve the problems of safety hazards and interfacial incompatibility of Li metal batteries. Herein, a novel solvent triethyl 2-fluoro-2-phosphonoacetate (TFPA) with outstanding flame retardancy is introduced in the polymer skeleton synthesized by in situ polymerization of the monomer polyethylene glycol dimethacrylate (PEGDMA) and the cross-linker pentaerythritol tetraacrylate (PETEA). The FRGE exhibits superb interfacial compatibility with Li metal anodes and inhibits uncontrolled Li dendrite growth. This can be ascribed to the restriction of free phosphate molecules by the polymer skeleton, thus realizing a stable cycling performance over 500 h at 1 mA cm^-2 and 1 mAh cm^-2 in the Li||Li symmetric cell. In addition, the high ionic conductivity (3.15 mS cm^-1) and Li⁺ transference number (0.47) of the FRGE further enhance the electrochemical performance of the correspondent battery. As a result, the LiFePO₄|FRGE|Li cell exhibits excellent long-term cycling life with a capacity retention of 94.6% after 700 cycles. This work points to a new pathway for the practical development of high-safety and high-energy-density Li metal-based batteries.

TinO2n-1/MXene Hierarchical Bifunctional Catalyst Anchored on Graphene Aerogel toward Flexible and High-Energy Li-S Batteries

The development of lithium-sulfur (Li-S) batteries with high-energy density, flexibility, and safety is very appealing for emerging implantable devices, biomonitoring, and roll-up displays. Nevertheless, the poor cycling stability and flexibility of the existing sulfur cathodes, flammable liquid electrolytes, and extremely reactive lithium anodes raise serious battery performance degradation and safety issues. Herein, a metallic 1T MoS₂ and rich oxygen vacancies Ti_nO_2n-1/MXene hierarchical bifunctional catalyst (Mo-Ti/Mx) anchored on a reduced graphene oxide-cellulose nanofiber (GN) host (Mo-Ti/Mx-GN) was proposed to address the above challenges. By applying a directional freezing process, the hierarchical architecture of a flexible GN scaffold composed of waved multiarch morphology with long-range alignment is achieved. The synergetic effects of 1T MoS₂ and Ti_nO_2n-1/MXene are beneficial to suppress the shuttling behavior of lithium polysulfides (LiPSs), expedite the redox kinetics of sulfur species, and promote the electrocatalytic reduction of LiPSs to Li₂S. The electrode demonstrates improved electrochemical properties with high sulfur-mass loading (8.4 mg_s cm^-2) and lean electrolyte (7.6 μL mg_s^-1) operation. We also explored the feasibility of producing pouch cells with such flexible electrodes, gel polymer electrolytes, and a robust lithium anode, which exhibited reversible energy storage and output, wide temperature adaptability, and good safety against rigorous strikes, implying the potential for practical applications.

A High-Energy and Safe Lithium Battery Enabled by Solid-State Redox Chemistry in a Fireproof Gel Electrolyte

Recent years have witnessed thriving efforts in pursuing high-energy batteries at an unaffordable cost of safety. Herein, a high-energy and safe quasi-solid-state lithium battery is proposed by solid-state redox chemistry of polymer-based molecular Li₂ S cathode in a fireproof gel electrolyte. This chemistry fully eliminates not only the negative effect of extremely reactive Li metal and oxygen species on cell safety but also the damage of electrode reversibility by soluble redox intermediates. The molecular Li₂ S cathode exhibits an exceptional lifetime of 2000 cycles, 100% Coulombic efficiency, high capacity of 830 mA h g^-1 with ultralow capacity loss of 0.005-0.01% per cycle and superior rate capability up to 10 C. Meanwhile, it shows high stability in the carbonate-involving electrolyte for maximizing the compatibility with carbonate-efficient Si anode. The optimized cell chemistry exerts high energy over 750 W h kg^-1 for 500 cycles with fast rate response, high-temperature adaptability, and no self-discharge. A fire-retardant composite gel electrolyte is developed to further strengthen the intrinsic safe redox between the Li₂ S cathode and the Si anode, which secures remarkable safety against extreme abuse of overheating, short circuits, and mechanical damage in air/water or even when on fire.

Thermally Stable PVDF-HFP-Based Gel Polymer Electrolytes for High-Performance Lithium-Ion Batteries

The development of gel polymer electrolytes (GPEs) for lithium-ion batteries (LIBs) has paved the way to powering futuristic technological applications such as hybrid electric vehicles and portable electronic devices. Despite their multiple advantages, non-aqueous liquid electrolytes (LEs) possess certain drawbacks, such as plasticizers with flammable ethers and esters, electrochemical instability, and fluctuations in the active voltage scale, which limit the safety and working span of the batteries. However, these shortcomings can be rectified using GPEs, which result in the enhancement of functional properties such as thermal, chemical, and mechanical stability; electrolyte uptake; and ionic conductivity. Thus, we report on PVDF-HFP/PMMA/PVAc-based GPEs comprising poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) and poly(methyl methacrylate) (PMMA) host polymers and poly(vinyl acetate) (PVAc) as a guest polymer. A physicochemical characterization of the polymer membrane with GPE was conducted, and the electrochemical performance of the NCM811/Li half-cell with GPE was evaluated. The GPE exhibited an ionic conductivity of 4.24 × 10-4 S cm-1, and the NCM811/Li half-cell with GPE delivered an initial specific discharge capacity of 204 mAh g-1 at a current rate of 0.1 C. The cells exhibited excellent cyclic performance with 88% capacity retention after 50 cycles. Thus, this study presents a promising strategy for maintaining capacity retention, safety, and stable cyclic performance in rechargeable LIBs.

Thermal-Responsive and Fire-Resistant Materials for High-Safety Lithium-Ion Batteries

As one of the most efficient electrochemical energy storage devices, the energy density of lithium-ion batteries (LIBs) has been extensively improved in the past several decades. However, with increased energy density, the safety risk of LIBs becomes higher too. The frequently occurred battery accidents worldwide remind us that safeness is a crucial requirement for LIBs, especially in environments with high safety concerns like airplanes and military platforms. It is generally recognized that the catastrophic thermal runaway (TR) event is the major cause of LIBs related accidents. Tremendous efforts have been devoted to coping with the TR concerns in LIBs, and thus enhance battery safety. This review first gives an introduction to the fundamentals of LIBs and the origins of safety issues. Then, the authors summarize the recent advances to improve the safety of LIBs with a unique focus on thermal-responsive and fire-resistant materials. Finally, a perspective is proposed to guide future research directions in this field. It is anticipated this review will stimulate inspiration and arouse extensive studies on further improvement in battery safety.

Facile and Powerful In Situ Polymerization Strategy for Sulfur-Based All-Solid Polymer Electrolytes in Lithium Batteries

All-solid-state polymer electrolytes can improve the safety of lithium batteries. However, the common Bellcore polymer electrolyte technology faces several issues such as wasting a mass of solvent, high manufacturing cost, and poor interfacial compatibility between polymer electrolytes and electrodes. Herein, we propose an in situ polymerization technique to synthesize all-solid-state polymer electrolytes by a thiol-Michael addition click reaction. The alternating copolymer is made from the Michael addition reaction of ethylene glycol dimethacrylate (EGDMA) and 1,2-ethane dithiol (EDT). At ambient temperature, the obtained composite polymer electrolyte displays an ionic conductivity of 3.02 × 10^-5 S/cm, an electrochemical window of 4.5 V, and a lithium-ion transference number of 0.45. In light of this unique polymerization process, the traditional fabrication method of liquid electrolyte-based lithium batteries can be adopted in the current study for the preparation of all-solid-state Li/LiFePO₄ batteries. It was found that the assembled all-solid-state Li/LiFePO₄ batteries exhibited superior charging/discharging performance and preferable safety. Thus, this facile and powerful in situ polymerization strategy may open up a new approach for the design and fabrication of all-solid-state batteries with desirable performances.

Advances and Perspectives of Cathode Storage Chemistry in Aqueous Zinc-Ion Batteries

Rechargeable aqueous zinc-ion batteries (AZIBs) have captured a surge of interest in recent years as a promising alternative for scalable energy storage applications owing to the intrinsic safety, affordability, environmental benignity, and impressive electrochemical performance. Despite the facilitated development of this technology by many investigations, however, its smooth implementation is still plagued by inadequate energy density and undesirable life span, which calls for an efficient and controllable cathode storage chemistry. Here, this review focuses on the key bottlenecks by offering a comprehensive summary of representative cathode materials and comparatively analyzing their structural features and electrochemical properties. Then, we critically present several feasible electrode design strategies to guide future research activities from a fundamental perspective for high-energy-density and durable cathode materials mainly in terms of interlayer regulation, defect engineering, multiple redox reactions, activated two-electron reactions, and electrochemical activation and conversion. Finally, we outline the remaining challenges and future perspectives of developing high-performance AZIBs.

Hierarchical NiSe2 Nanosheet Arrays as a Robust Cathode toward Superdurable and Ultrafast Ni-Zn Aqueous Batteries

Zn-based aqueous batteries are enjoying the hotspots of worldwide research as their significant merits in economic cost and safety. However, the lack of a robust cathode (positive electrode) owning excellent rate ability, high capacity, and stability challenges their practical application. Herein, we propose hierarchical NiSe₂ nanosheet arrays as a robust cathode toward high-performance Ni-Zn aqueous batteries. Attributed to in situ anion exchange and Kirkendall effects, the nanosheet arrays are hierarchically constructed by NiSe₂ nanoparticles and abundant mesopores, which fully expose the active sites and accelerate the electrode kinetics. This unique structure endows the NiSe₂ electrode with remarkable specific capacity (245.1 mAh g^-1) and extraordinary high-rate ability (maintains 58% at 72.8 A g^-1) together with 10,000 cycles without any obvious capacity degeneration. As a result, based on the total active weight, our NiSe₂//Zn battery is capable of record-high power density (91.22 kW kg^-1/639.1 mW cm^-2), imposing energy density (328.8 Wh kg^-1/2.303 mWh cm^-2), and ultralong lifespan (only 8.3% capacity loss after 10,000 cycles), surpassing most of the aqueous batteries and supercapacitors recently reported. Moreover, this NiSe₂//Zn battery is also affordable (US$40 per kWh) and safe. These results open a new avenue for developing superdurable and ultrafast high-energy Ni-Zn batteries toward affordable and practical energy storage.

Composition Modulation of Ionic Liquid Hybrid Electrolyte for 5 V Lithium-Ion Batteries

Electrolyte is a key component in high-voltage lithium-ion batteries (LIBs). Bis(trifluoromethanesulfonyl)imide-based ionic liquid (IL)/organic carbonate hybrid electrolytes have been a research focus owing to their excellent balance of safety and ionic conductivity. Nevertheless, corrosion of Al current collectors at high potentials usually happens for this kind of electrolyte. In this study, this long-standing problem is solved via the modulation of the IL/carbonate ratio and LiPF₆ concentration in the hybrid electrolyte. The proposed electrolyte suppresses Al dissolution and electrolyte oxidation at 5 V (vs Li⁺/Li) and thus allows for ideal lithiation/delithiation performance of a high-voltage LiNi_0.5Mn_1.5O₄ (LNMO) cathode even at 55 °C. The underlying mechanism is examined in this work. Excellent cycling stability (97% capacity retention) for an LNMO cathode after 300 cycles is achieved. This electrolyte shows good wettability toward a polyethylene separator and low flammability. In addition, satisfactory compatibility with both graphite and Si-based anodes is confirmed. The proposed electrolyte design strategies have great potential for applications in high-voltage LIBs.

Multilayer Nanofiber Composite Separator for Lithium-Ion Batteries with High Safety

An original Von Koch curve-shaped tipped electrospinneret was used to prepare a polyimide (PI)-based nanofiber membrane. A multilayer Al2O3@polyimide/polyethylene/Al2O3@polyimide (APEAP) composite membrane was tactfully designed with an Al2O3@ polyimide (AP) membrane as outer shell, imparting high temperature to the thermal run-away separator performance and a core polyethylene (PE) layer imparts the separator with a thermal shut-down property at low temperature (123 °C). An AP electrospun nanofiber was obtained by doping Al2O3 nanoparticles in PI solution. The core polyethylene layer was prepared using polyethylene powder and polyterafluoroethylene (PTFE) miniemulsion through a coating process. The addition of PTFE not only bonds PE power, but also increases the adhesion force between the PE and AP membranes. As a result, the multilayer composite separator has high safety, outstanding electrochemical properties, and better cycling performance as a lithium-ion battery separator.

High-Safety All-Solid-State Lithium-Metal Battery with High-Ionic-Conductivity Thermoresponsive Solid Polymer Electrolyte

Lithium-metal batteries (LMB) are very attractive owing to their high theoretical energy density, but significant challenges such as low ionic conductivity and safety risks prevent their widespread application. Herein, we report a new design of high-safety all-solid-state LMB by using high-ionic-conductivity thermoresponsive solid-polymer electrolyte (TSPE), providing a smart and active approach to realize thermally induced autonomic shutdown of LMBs by efficiently inhibiting the ionic conduction between electrodes beyond an unsafe temperature. The as-obtained TSPE exhibits a high ionic conductivity (2 × 10^-4 S cm^-1 at 30 °C), which enables a significantly improved capacity of 160 mA h g^-1 at 0.2 C and outstanding high rate capability up to 5 C as well as a super-long cycle life of over 400 cycles for the constructed all-solid-state Li||LiFePO₄ batteries. The present study opens up a new avenue for the fabrication of self-protective all-solid-state batteries with inherent intelligent thermal management to ensure battery-series safety.

Flexible, High-Wettability and Fire-Resistant Separators Based on Hydroxyapatite Nanowires for Advanced Lithium-Ion Batteries

Separators play a pivotal role in the electrochemical performance and safety of lithium-ion batteries (LIBs). The commercial microporous polyolefin-based separators often suffer from inferior electrolyte wettability, low thermal stability, and severe safety concerns. Herein, a novel kind of highly flexible and porous separator based on hydroxyapatite nanowires (HAP NWs) with excellent thermal stability, fire resistance, and superior electrolyte wettability is reported. A hierarchical cross-linked network structure forms between HAP NWs and cellulose fibers (CFs) via hybridization, which endows the separator with high flexibility and robust mechanical strength. The high thermal stability of HAP NW networks enables the separator to preserve its structural integrity at temperatures as high as 700 °C, and the fire-resistant property of HAP NWs ensures high safety of the battery. In particular, benefiting from its unique composition and highly porous structure, the as-prepared HAP/CF separator exhibits near zero contact angle with the liquid electrolyte and high electrolyte uptake of 253%, indicating superior electrolyte wettability compared with the commercial polyolefin separator. The as-prepared HAP/CF separator has unique advantages of superior electrolyte wettability, mechanical robustness, high thermal stability, and fire resistance, thus, is promising as a new kind of separator for advanced LIBs with enhanced performance and high safety.

A Modified Ceramic-Coating Separator with High-Temperature Stability for Lithium-Ion Battery

In this work, the ceramic coating separator (CCS-CS) prepared with polyethylene (PE) separator, Al₂O₃ inorganic particles, carboxymethyl cellulose sodium (CMC) and styrene-butadiene rubber (SBR) mix binders is further modified by coating with a thin polydopamine (PDA) layer through a simple chemical deposition method. Compared with the bare ceramic coating separator, the PDA-modified CCS-CS (CCS-CS-PDA) exhibits excellent thermal stability, which shows no thermal shrinkage after storing at 200 °C for 30 min. Compared with the PE separator, both the uptake and wettability with the electrolyte and water of CCS-CS-PDA are improved significantly. Meanwhile, when saturated with liquid electrolyte, the CCS-CS-PDA also shows enabled high ionic conductance. Furthermore, the test of the electrochemical impedances changing with the temperatures suggests that only the PE separator exhibits no thermal shutdown behaviors, and the CCS-CS separator only has a shutdown temperature range from 138 to 160 °C, while the CCS-CS-PDA shows a shutdown temperature range from 138 to more than 200 °C. The cells prepared with the CCS-CS-PDA also show stable repeated cycling performance and good rate capacity at room temperature.