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

Electrospun Silicon Dioxide/poly(vinylidene fluoride) Nanofibrous Membrane Comprising a Skin Multicore-Shell Nanostructure as a New High-Heat-Resistant Separator for Lithium-Ion Polymer Batteries

Porous silicon dioxide (SiO2)/poly(vinylidene fluoride) (PVdF), SiO2/PVdF, and fibrous composite membranes were prepared by electrospinning a blend solution of a SiO2 sol-gel/PVdF. The nanofibers of the SiO2/PVdF (3/7 wt. ratio) blend comprised skin and nanofibrillar structures which were obtained from the SiO2 component. The thickness of the SiO2 skin layer comprising a thin skin layer could be readily tuned depending on the weight proportions of SiO2 and PVdF. The composite membrane exhibited a low thermal shrinkage of ~3% for 2 h at 200 °C. In the prototype cell comprising the composite membrane, the alternating current impedance increased rapidly at ~225 °C, and the open-circuit voltage steeply decreased at ~170 °C, almost becoming 0 V at ~180 °C. After being exposed at temperatures of >270 °C, its three-dimensional network structure was maintained without the closure of the pore structure by a melt-down of the membrane.

Partially Neutralized Polyacrylic Acid as an Efficient Binder for Aqueous Ceramic-Coated Separators for Lithium-Ion Batteries

A partially neutralized polyacrylic acid (Pn-PAA) is used for coating sub-micron-sized α-alumina on a conventional microporous polyolefin separator, fabricating a ceramic-coated separator (CCS). Pn-PAA acts as a dispersant and binder by adsorbing itself on alpha(α)-alumina surfaces under acidic condition through the columbic interaction, providing a repulsive force to disperse fine alumina in aqueous suspension, and binds alumina strongly on plasma-treated separator through hydrogen bonding. This CCS shows favorable wettability in carbonate-based liquid electrolyte and ionic conduction due to the high hydrophilicity of Pn-PAA and alumina. With that, this study found that Pn-PAA-made-CCS yields a substantial adhesion strength of ~106 N/m with enhanced cycle stability, a specific capacity of 145.0 mAh/g after 200 cycles at 1 C at room temperature in half cells (LFP/Li metal).

A Review of Nonaqueous Electrolytes, Binders, and Separators for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) are the most important electrochemical energy storage devices due to their high energy density, long cycle life, and low cost. During the past decades, many review papers outlining the advantages of state-of-the-art LIBs have been published, and extensive efforts have been devoted to improving their specific energy density and cycle life performance. These papers are primarily focused on the design and development of various advanced cathode and anode electrode materials, with less attention given to the other important components of the battery. The “nonelectroconductive” components are of equal importance to electrode active materials and can significantly affect the performance of LIBs. They could directly impact the capacity, safety, charging time, and cycle life of batteries and thus affect their commercial application. This review summarizes the recent progress in the development of nonaqueous electrolytes, binders, and separators for LIBs and discusses their impact on the battery performance. In addition, the challenges and perspectives for future development of LIBs are discussed, and new avenues for state-of-the-art LIBs to reach their full potential for a wide range of practical applications are outlined.

Graphic Abstract

Sequential Deposition of Integrated Cathode-Inorganic Separator-Anode Multilayers for High Performance Li-Ion Batteries

A porous, spray-deposited Al₂O₃-based separator was developed to enable the direct deposition of an electrode/separator/electrode Li-ion battery full cell assembly in a single operation. The optimized sprayed separator consisted of 50 nm Al₂O₃ particles, 1 wt % poly(acrylic acid), and 5 wt % styrene-butadiene rubber, deposited from an 80:20 vol % suspension of water and isopropanol. Separators between 5 and 22 μm thick had consistent and similar porosity of ∼58%, excellent wettability, thermal stability to at least 180 °C, adequate electrochemical stability and high effective ionic conductivity of ∼1 mS cm^-1 at room temperature in an EC/DMC electrolyte, roughly double that of a conventional polypropylene separator. A sequentially deposited three-layer LiFePO₄/Al₂O₃/Li₄Ti₅O₁₂ full cell, the first of its kind, showed similar rate performance to an identical cell with a conventional polypropylene separator, with a capacity of ∼50 mAh g^-1 at 30 C. However, after cycling at 2 C for 400 cycles, Al₂O₃ separator full cells retained 96.3% capacity, significantly more than conventional full cells with a capacity of 79.2% remaining.

Low-Cost Mass Manufacturing Technique for the Shutdown-Functionalized Lithium-Ion Battery Separator Based on Al2O3 Coating Online Construction during the β-iPP Cavitation Process

A shutdown-functionalized lithium-ion battery separator plays a pivotal role in preventing thermal runaway as cells experience electrical abuse, overcharge, and external short circuit. In this article, the trilayer separator endowed with shutdown function was fabricated by ingenious co-extrusion and bidirectional drawing based on the nano-Al₂O₃ coating online construction during the β-iPP cavitation process. The middle layer composed of nano-Al₂O₃, polyethylene, and polypropylene offers a shutdown temperature of 130 °C, and skin polypropylene layers with nano-Al₂O₃ coating hold optimized dimensional stability below the meltdown temperature. Crystal structure measurement and pore structure diagnosis disclose that nano-Al₂O₃ thins coarse fibrils and makes the porous structure uniform. De-bonding of nano-Al₂O₃/β-iPP interfaces retains nano-Al₂O₃ not only on the top surface of the separator but also on the pore intine to realize nano-Al₂O₃ coating online construction, consequently strengthening tensile capacity, dimensional stability to heating, and electrolyte affinity. Electrochemical tests further disclose that nano-Al₂O₃ coating stabilizes solid electrolyte interphase germination and heightens lithium-ion migration numbers, confining cell resistances and granting optimal high-rate performance and cycling ability. The proposed approach features simple technics, environment-friendly, continuous fabrication, and coating online construction, which can offer new ideas for the mass fabricating of the high-end separator.

Synergistic Flame Retardancy of Microcapsules Based on Ammonium Polyphosphate and Aluminum Hydroxide for Lithium-Ion Batteries

Flame retardants have important theoretical research and applied value for lithium-ion battery safety. Microcapsule flame retardants based on ammonium polyphosphate (APP) and aluminum hydroxide (ATH) were synthesized for application in lithium-ion batteries. First, the ATH-APP was prepared by coating a layer of ATH on the surface of the core APP. Then, the ATH-APP was encapsulated by poly(urea-formaldehyde) (PUF) to obtain en-ATH-APP. The structure and flame-retardant property of en-ATH-APP, the influence of en-ATH-APP on the thermal stability of the electrode, and the electrochemical performance of the battery were studied. The results of Fourier transform infrared and scanning electron microscope experiments indicated that APP was coated with ATH and PUF in turn. The results of differential scanning calorimetry and the fire extinguishing test for electrodes manifested that en-ATH-APP had better flame-retardant property than APP because of the synergistic effect between APP and ATH. Moreover, the flame-retardant efficiency of en-ATH-APP was comparable to that of ATH-APP, indicating that the presence of PUF had almost no effect on the flame-retardant property. The results of electrochemical experiments indicated that en-ATH-APP had the best electrochemical compatibility for the battery compared with APP and ATH-APP. The research lights the way to improve inherent safety of lithium-ion batteries by adding en-ATH-APP to the cathode.

Pristine and Modified Porous Membranes for Zinc Slurry-Air Flow Battery

The membrane is a crucial component of Zn slurry–air flow battery since it provides ionic conductivity between the electrodes while avoiding the mixing of the two compartments. Herein, six commercial membranes (Cellophane™ 350PØØ, Zirfon^®, Fumatech^® PBI, Celgard^® 3501, 3401 and 5550) were first characterized in terms of electrolyte uptake, ion conductivity and zincate ion crossover, and tested in Zn slurry–air flow battery. The peak power density of the battery employing the membranes was found to depend on the in-situ cell resistance. Among them, the cell using Celgard^® 3501 membrane, with in-situ area resistance of 2 Ω cm² at room temperature displayed the highest peak power density (90 mW cm⁻²). However, due to the porous nature of most of these membranes, a significant crossover of zincate ions was observed. To address this issue, an ion-selective ionomer containing modified poly(phenylene oxide) (PPO) and N-spirocyclic quaternary ammonium monomer was coated on a Celgard^® 3501 membrane and crosslinked via UV irradiation (PPO-3.45 + 3501). Moreover, commercial FAA-3 solutions (FAA, Fumatech) were coated for comparison purpose. The successful impregnation of the membrane with the anion-exchange polymers was confirmed by SEM, FTIR and Hg porosimetry. The PPO-3.45 + 3501 membrane exhibited 18 times lower zincate ions crossover compared to that of the pristine membrane (5.2 × 10⁻¹³ vs. 9.2 × 10⁻¹² m² s⁻¹). With low zincate ions crossover and a peak power density of 66 mW cm⁻², the prepared membrane is a suitable candidate for rechargeable Zn slurry–air flow batteries.

Revealing Anion Adsorption Mechanism for Coating Layer on Separator toward Practical Li Metal Batteries

Using a coating layer to modify the separator in a practical Li metal battery has attracted wide attention; however, its function on Li-ion diffusion and Li plating/stripping has not been systematically investigated. Herein, in situ electrochemical Raman characterization using modified coin cell configuration is employed to directly reveal the anion adsorption mechanism of the coating layer. The adsorption ability of the MOF-based coating layer on the commercial separator is able to preserve high concentration of anions near the electrolyte/Li interface, which generates high local Li-ion concentration that delays the drain of Li⁺ to uniform Li plating. The feasible and large-area fabrication of GO/ZIF-8-modified separator enables the assembly of pouch cell strictly following practical parameters. 0.4 Ah pouch cell (Li/NCM811) delivers stable capacity for over 100 cycles. The deep understanding of the mechanism of how a coating layer affects Li plating behavior is helpful for the designing and preparation of high-performance separators for Li metal batteries.

Anisotropic semi-aligned PAN@PVdF-HFP separator for Li-ion batteries

Compared with the common electrospun nanofibers, the alignment of the nanofibers exhibits interesting anisotropic mechanical properties and structural stability. In this paper, semi-aligned PAN@PVdF-HFP nanofiber separators were prepared by a modified electrospinning method. The composite separators exhibit anisotropic mechanical properties and enhanced electrochemical performance compared with electrospun PAN films. The PAN@PVdF-HFP nanofiber separator can deliver an ionic conductivity of 1.2 mSċcm^-1 with electrochemical stability up to 5.0 V. The LiFePO₄/Li cell with semi-aligned PAN@PVdF-HFP separator shows excellent cycling performance, good rate capability, as well as high discharge capacity.

Polydopamine Coated Lithium Lanthanum Titanate in Bilayer Membrane Electrolytes for Solid Lithium Batteries

The demand for solid lithium batteries with high energy density and safety boosts the development of solid-state electrolytes in which composite membrane electrolytes consisting of polymers and ceramic fillers are attractive. As the common ceramic filler, perovskite-structured Li_0.33La_0.557TiO₃ (LLTO) has great advantage on cost and environmental friendliness by using earth-abundant raw materials in the production. Nevertheless, the chemical instability of LLTO against Li-metal hinders its application. Herein, LLTO particles are coated by biodegradable polydopamine (PDA) layers and united with poly(vinylidene fluoride) (PVDF) to prepare composite electrolytes which perform superior stability against Li-metal. Besides, PVDF:LLTO membranes are assembled at cathode sides and show high voltage tolerance. The Li/Ni_0.6Mn_0.2Co_0.2O₂ cells with bilayer membrane electrolytes can deliver the specific capacity of 158.2 mAh g^-1 and maintain 83% capacity after 100 cycles at 0.1 C. Furthermore, based on the bilayer membranes with outstanding flexibility and stretchability, the cells can even survive under several extreme conditions, such as bending, twisting, crimping, and stretching. This study offers an environmentally friendly strategy to improve the stability of LLTO against Li and sheds light on the development of cost-effective solid electrolytes.

Preparation and performance of polypropylene separator modified by SiO2/PVA layer for lithium batteries

An aqueous silica (SiO2) dispersion was prepared by using silica as ceramic particles, polyvinyl alcohol (PVA) as a binder and deionized water as a dispersion medium. A SiO2 ceramic layer was applied to the surface of the polypropylene (PP) separator by dip coating. The separators before and after modification were characterized by XRD, SEM, DTA, lyophilic performance test, contact angle test and heat resistance test. The separators were assembled into lithium-ion batteries for electrochemical performance test. The results show that after the successful introduction of SiO2/PVA coating on the surface of PP separator, the lyophilic and heat resistance and electrochemical performance of PP separator have been improved significantly. The battery rate performance and cycle performance are significantly improved. Especially the capacity retention rate of the original separator was only 75.79% at 100 charge-discharge times, and that of the modified separator was as high as 87.18%.

A Review on Inorganic Nanoparticles Modified Composite Membranes for Lithium-Ion Batteries: Recent Progress and Prospects

Separators with high porosity, mechanical robustness, high ion conductivity, thin structure, excellent thermal stability, high electrolyte uptake and high retention capacity is today's burning research topic. These characteristics are not easily achieved by using single polymer separators. Inorganic nanoparticle use is one of the efforts to achieve these attributes and it has taken its place in recent research. The inorganic nanoparticles not only improve the physical characteristics of the separator but also keep it from dendrite problems, which enhance its shelf life. In this article, use of inorganic particles for lithium-ion battery membrane modification is discussed in detail and composite membranes with three main types including inorganic particle-coated composite membranes, inorganic particle-filled composite membranes and inorganic particle-filled non-woven mates are described. The possible advantages of inorganic particles application on membrane morphology, different techniques and modification methods for improving particle performance in the composite membrane, future prospects and better applications of ceramic nanoparticles and improvements in these composite membranes are also highlighted. In short, the contents of this review provide a fruitful source for further study and the development of new lithium-ion battery membranes with improved mechanical stability, chemical inertness and better electrochemical properties.

Adhesive Hybrid SiO2.01C0.23Hx Nanoparticulate Coating on Polyethylene (PE) Separator by Roll-to-Roll Atmospheric Pressure Plasma

For the ever-increasing demand for highly safe lithium-ion batteries (LIBs), the common sol-gel process provides heat-resistance to separators with an inorganic coating, where the adhesion to the separator is the key to safety and stability. In this paper, we present a SiO2.01C0.23Hx-coated polyethylene (PE) separator through a roll-to-roll atmospheric plasma-enhanced chemical vapor deposition (R2R-APECVD) of hexamethyldisiloxane (HMDSO)/Ar/O2. The adhesion strength of SiO2.01C0.23Hx-coated PE was tested by peel-off test and found to be higher than that of the commercial Al2O3-coated separator (0.28 N/mm vs. 0.06 N/mm). Furthermore, the SiO2.01C0.23Hx-coated PE separator showed better electrochemical performance in C-rate and long term cycle tests. FTIR, SEM, and XPS analysis indicate that the increased adhesion and electrochemical performance are attributed to the inner hybrid SiO2.01C0.23Hx coating with organic and inorganic components.

A Parallel Bicomponent TPU/PI Membrane with Mechanical Strength Enhanced Isotropic Interfaces Used as Polymer Electrolyte for Lithium-Ion Battery

In this work, a side-by-side bicomponent thermoplastic polyurethane/polyimide (TPU/PI) polymer electrolyte prepared with side-by-side electrospinning method is reported for the first time. Symmetrical TPU and PI co-occur on one fiber, and are connected by an interface transition layer formed by the interdiffusion of two solutions. This structure of the as-prepared TPU/PI polymer electrolyte can integrate the advantages of high thermal stable PI and good mechanical strength TPU, and mechanical strength is further increased by those isotropic interface transition layers. Moreover, benefiting from micro-nano pores and the high porosity of the structure, TPU/PI polymer electrolyte presents high electrolyte uptake (665%) and excellent ionic conductivity (5.06 mS·cm-1) at room temperature. Compared with PE separator, TPU/PI polymer electrolyte exhibited better electrochemical stability, and using it as the electrolyte and separator, the assembled Li/LiMn₂O₄ cell exhibits low inner resistance, stable cyclic and notably high rate performance. Our study indicates that the TPU/PI membrane is a promising polymer electrolyte for high safety lithium-ion batteries.

A core–shell structured polyacrylonitrile@poly(vinylidene fluoride-hexafluoro propylene) microfiber complex membrane as a separator by co-axial electrospinning

A novel and facile core–shell structured polyacrylonitrile@poly (vinylidene fluoride-hexafluoro propylene) (PAN@PVDF-HFP) microfiber complex membrane was designed and fabricated via co-axial electrospinning, which was used as a separator in lithium-ion batteries. Poly(vinylidene fluoride-co-hexafluoro propene) (PVDF-HFP) and polyacrylonitrile (PAN) were used as the shell (outer) layer and core (inner), respectively. Structure, surface morphology, porosity, and thermal properties of the core–shell structured microfiber membranes were investigated. Compared with the traditional commercial porous polyethylene (PE) separator, the PAN@PVDF-HFP microfiber complex membranes exhibited higher porosity, superior thermal stability, better electrolyte wettability and higher ionic conductivity. As a consequence, batteries assembled with the PAN@PVDF-HFP microfiber complex membrane display better cycling stability and superior rate performance compared to those with the PE separator.

A novel and facile core–shell structured polyacrylonitrile@poly (vinylidene fluoride-hexafluoro propylene) (PAN@PVDF-HFP) microfiber complex membrane was designed and fabricated via co-axial electrospinning.

QCM Biosensor Based on Polydopamine Surface for Real-Time Analysis of the Binding Kinetics of Protein-Protein Interactions

A quartz crystal microbalance (QCM) biosensor based on polydopamine (PDA) surface was developed for real-time analysis of the binding kinetics of protein-protein interactions. The biosensor was fabricated by simply immersing the gold sensor chip into an aqueous dopamine solution at pH 8.5 leading to a spontaneous deposition of PDA film onto the sensor chip surface, which was followed by incubation with the protein to immobilize it onto the PDA-coated sensor chip surface via Michael addition and/or Schiff base reactions. In this paper, the interaction between monoclonal anti-myoglobin 7005 antibody (IgG1) and its antigen human cardiac myoglobin was used as a model system for real-time analysis of biomolecule interactions on the biosensor surface. The kinetic parameters of the interaction between anti-myoglobin 7005 and myoglobin were studied on the biosensor surface, which were consistent with the results obtained via amine coupling. The biosensor based on PDA surface has excellent regenerability, reproducibility, and specificity. Compared with the most frequently/typically used amine coupling method for immobilization of proteins on carboxylated substrates, the modification methodology presented in this paper is simple, mild and is not subjected to the limitations of the isoelectric point (pI) of the protein. In addition, the PDA biosensor chip can be easily reused, which makes QCM biosensor analysis more efficient and cost effective.