Great progress has been made on the cyclability and material utilization in recent development of lithium–sulfur (Li–S) batteries; however, most of the sulfur electrodes reported so far have a considerable low loading of sulfur (60%), which causes a substantial decrease in energy density and is therefore difficult for application in batteries. To deal with this issue, we fabricate a novel sulfur composite with a coaxial three-layered structure, in which sulfur is deposited on carbon fibers and coated with poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), thus enabling a high sulfur loading of 70.8 wt % without the expense of its electrochemical performance. Benefiting from the rigid conductive framework of carbon fibers and flexible buffering matrix of the polymer for blocking the diffusion loss of discharge intermediates, the as-fabricated composite electrode exhibits a high initial reversible capacity of 1272 mA h g–1 (based on the total mass of the composite), a stable cyclability with a retained capacity of 807 mA h g–1 after 200 cycles, and a high Coulombic efficiency of ∼99% upon extended cycling, offering a new selection for practical application in Li–S batteries.Keywords: coaxial nanofibers; conductive polymer; high sulfur loading; lithium−sulfur batteries; sulfur cathode;

Lithium ion batteries have now been used as a power source for electric vehicles; however, their safety still remains a serious concern as the accidents reported increase with the rapid increase of electric vehicles in transportation markets. To address this issue, we describe herein a novel temperature-responsive cathode by coating an ultra-thin layer of poly(3-octylthiophene) (P3OT) with a thickness less than 1 μm in between the Al substrate and cathode-active LiCoO2 layer to form a sandwiched Al/P3OT/LiCoO2 cathode (LCO-PTC). This LCO-PTC cathode demonstrates almost the same electrochemical performance as the conventional LiCoO2 cathode at ambient temperature but a strong PTC behavior to switch off the cell reaction in a high temperature range of 90–100 °C, thus protecting the cell from thermal runaway. Because of its easy fabrication, cost effectiveness and particularly good compatibility with the current battery technology, this new type of PTC electrode can be conveniently extended to other Li-insertion cathodes for building safer Li-ion batteries.

Covalent organic frameworks (COFs) represent an emerging class of porous crystalline materials and have recently shown interesting applications in energy storage. Herein, we report the construction of a cycle-stable sulfur electrode by embedding sulfur into a 2D COF. The designed porphyrin-based COF (Por-COF), featuring a relatively large pore volume and narrow pore size distribution, has been employed as a host material for sulfur storage in Li–S batteries. With a 55% sulfur loading in the composite, the thus-prepared cathode delivers a capacity of 633 mA h g−1 after 200 cycles at 0.5C charge/discharge rates. Therefore, embedding sulfur in the nanopores of the Por-COF significantly improves the performance of the sulfur cathode. Considering the flexible design of COFs, we believe that it is possible to synthesize a 2D COF host with a suitable pore environment to produce more stable Li–S batteries, which may help in exploration of the structure–property relationship between the host material and cell performance.

Silicon (Si) has been regarded as a promising high-capacity anode material for developing advanced lithium-ion batteries (LIBs), but the practical application of Si anodes is still unsuccessful mainly due to the insufficient cyclability. To deal with this issue, we propose a new route to construct a dual core–shell structured Si@SiOx@C nanocomposite by direct pyrolysis of poly(methyl methacrylate) (PMMA) polymer on the surface of Si nanoparticles. Since the PMMA polymers can be chemically bonded on the nano-Si surface through the interaction between ester group and Si surface group, and thermally decomposed in the subsequent pyrolysis process with their alkyl chains converted to carbon and the residue oxygen recombining with Si to form SiOx, the dual core–shell structure can be conveniently formed in a one-step procedure. Benefiting from the strong buffering effect of the SiOx interlayer and the efficient blocking action of dense outer carbon layer in preventing electrolyte permeation, the obtained nanocomposite demonstrates a high capacity of 1972 mA h g–1, a stable cycling performance with a capacity retention of >1030 mA h g–1 over 500 cycles, and particularly a superiorly high Coulombic efficiency of >99.5% upon extended cycling, exhibiting a great promise for practical uses. More importantly, the synthetic method proposed in this work is facile and low cost, making it more suitable for large-scale production of high capacity anode for advanced LIBs.Keywords: dual core−shell; Li-ion battery; poly(methyl methacrylate) (PMMA); pyrolytic synthesis; Si anode

Sulfur/carbon (S/C) nanocomposite-filled polyacrylonitrile (PAN) nanofibers (denoted as S/C/PAN) are synthesized as a long life and high capacity cathode material for lithium–sulfur (Li–S) batteries. In the S/C/PAN nanofibers, the sulfurized PAN matrix acts not only as ionic and electronic channels to allow Li+ and electrons to arrive at and react with the S/C nanoparticles, but also as a protective barrier to prevent the S/C nanoparticles from contacting the electrolyte, thus avoiding the discharge intermediates of sulfur to dissolve in and react with the organic carbonate electrolyte. Since the redox reaction of sulfur in the nanofibers occurs mostly at the interior S/C interface through a solid state reaction mechanism, the microstructures and electrochemical interfaces in the nanofiber cathode remain stable during repeated cycles. As a consequence, the S/C/PAN cathode demonstrates a high reversible capacity of 1179 mA h g−1 at a current rate of 200 mA g−1, a high Coulombic efficiency of ∼100% after a few cycles, a good rate capability with 616 mA h g−1 at 4.0 A g−1 and a long cycling stability with 60% capacity retention over 400 cycles, showing great prospect for Li–S battery applications.

Great efforts have been devoted to developing nano-Si anodes for next-generation lithium ion batteries (LIBs); however, the reversible capacity and cycling stability of all Si anodes developed so far still need to be improved for battery applications. In this work, we propose a new strategy to develop a cycling-stable Si anode by embedding nano-Si particles into a Li+-conductive polymer matrix, in which a stable Si/polymer interface is established to avoid the contact of the Si surface with the electrolyte and to buffer the volume change of the Si lattice during cycles, thus promoting the capacity utilization and long cycle life of nano-Si particles. The nano-Si/polybithiophene composite synthesized in this work demonstrates a high Li-storage capacity of >2900 mA h g−1, a high-rate capability of 12 A g−1 and a long-term cyclability with a capacity retention of >1000 mA h g−1 over 1000 cycles, possibly serving as a high capacity anode for lithium battery applications. In addition, the fabrication technique for this type of composite material is facile, scalable and easily extendable to other Li-storable metals or alloys, opening up a new avenue for developing high capacity and cycling-stable anodes for advanced Li-ion batteries.

Safety issues have severely retarded the commercial applications of high-capacity and high-rate lithium ion batteries (LIBs) in electric vehicles and renewable power stations. Thermal runaway is a major cause for the hazardous behaviors of LIBs under extreme conditions. In this paper, a new thermal shutdown separator with a more reasonable shutdown temperature of ∼90 °C is developed by coating thermoplastic ethylene-vinyl acetate copolymer (EVA) microspheres onto a conventional polyolefin membrane film and tested for thermal protection of lithium-ion batteries (LIBs). The experimental results demonstrate that owing to the melting of the EVA coating layer at a critical temperature, this separator can promptly cut off the Li+ conduction between the electrodes and thus shut down the battery reactions, so as to protect the cell from thermal runaway. In addition, this type of the separator has no negative impact on the normal battery performance, therefore providing an internal and self-protecting mechanism for safety control of commercial LIBs.

Commercial development of lithium–sulfur (Li–S) batteries is severely hindered by their insufficient cyclability, which is due to the loss of soluble lithium polysulfide intermediates generated during the discharge processes. To overcome this problem, considerable efforts have been devoted to designing novel micro- or nano-structured host materials, aiming to trap soluble polysulfide within the network. Herein, we report a new approach to construct a sulfur electrode by impregnating sulfur into the nanopores of covalent-organic frameworks (COFs). Our results clearly demonstrate that by using a 2D COF as a host material, e.g. CTF-1 (CTF: covalent triazine-based framework), the thus-prepared cathode can show a remarkable positive effect on the capacity retention of Li–S batteries. Considering the unique features of COFs, such as highly flexible molecular design and a controllable pore size, this proof-of-principle study provides new opportunities for materials scientists for tailoring cathode materials in Li–S batteries.

Si has been considered as a promising alternative anode for next-generation lithium ion batteries (LIBs), but the commercial application of Si anodes is still limited due to their poor cyclability. In this paper, we propose a new strategy to enhance the long-term cyclability of Si anode by embedding nano-Si particles into a Li+-conductive polymer to form a Si/polymer composite with core-shell structure, in which nano-Si cores act as active Li-storage phase and the polymeric matrix serves not only as a strong buffer to accommodate the volume change, but also as a protection barrier to prevent the direct contact of Si surface with electrolyte, so as to maintain the mechanical integrity of Si anode and suppress the repeated destruction and construction of solid electrolyte interphase (SEI) on the Si surface. To realize this strategy, we synthesize a Si/PPP (polyparaphenylene) composite simply by ball-milling the Si nanoparticles with PPP polymer that has n-doping activity. Our experimental results demonstrate that the thus-prepared Si/PPP composite exhibits a high capacity of 3184 mA h g-1 with an initial coulombic efficiency of 78%, an excellent rate capability with a considerably high capacity of 1670 mA h g–1 even at a very high rate of 16 A g-1, and a long-term cyclability with 60% capacity retention over 400 cycles, showing a great prospect for battery application. In addition, this structural design could be adopted to other Li-storable metals or alloys for developing cycle-stable anode materials for Li-ion batteries.Keywords: anode material; Li+-conductive polymer; lithium-ion batteries; polyparaphenylene (PPP); silicon;

•A potential-sensitive separator is prepared by incorporating an electroactive poly (4-methoxytriphenylamine) (PMOTPA) into the micropores of a commercial porous polyolefin film.•This separator can be used as an internal and self-actuating voltage control device to provide overcharge protection for LiFePO4/Li4Ti5O12 lithium ion batteries.•This type of the separators works reversibly and has no any discernable impact on the battery performances.A potential-sensitive separator is prepared by incorporating an electroactive poly (4-methoxytriphenylamine) (PMOTPA) into the micropores of a commercial porous polyolefin film and tested as an internal voltage control device for overcharge protection of LiFePO4/Li4Ti5O12 lithium ion batteries. The experimental results demonstrate that the PMOTPA polymer embedded in the separator can be electrochemically p-doped at overcharged voltages into an electrically conductive state, producing an internal conducting bypass for shunting the charge current to maintain the charge voltage of LiFePO4/Li4Ti5O12 cells at a safety value less than 2.6 V, thus protecting the cell from voltage runaway. Since this type of the separators works reversibly and has no any discernable impact on the battery performances, it may offer a self-protection mechanism for development of safer lithium ion batteries.

In this paper, we propose a new strategy to develop high performance sulfur electrode by impregnating sulfur into the micropores of a Li+-insertable carbon matrix with the simultaneous use of a carbonate electrolyte, which does not dissolve polysulfides, to restrain the solution of the reaction intermediates of sulfur. To proof this concept, we prepared a Li+-insertable microporous carbon–sulfur composite by vaporizing sulfur into the micropores of the nanofiber-wired carbon microspheres. The experimental results demonstrate that, in the carbonate electrolyte of 1 M LiPF6/PC-EC-DEC, such S/C composite electrode exhibits not only stable cycling performance with a reversible capacity of 720 mAh g−1 after 100 cycles, but also superior high coulombic efficiency of ∼100% upon extended cycling (except the first three cycles). The structural and electrochemical analysis indicates that the improved electrochemical behaviors of the S/C composite arise from a new reaction mechanism, in which Li+ ions and electrons transport through the carbon matrix into the interior of the cathode and then react with the embedded sulfur in the S/C solid–solid interfaces, avoiding the solution of the intermediates into the bulk electrolyte. More significantly, the structural design and working mechanism of such a sulfur cathode could be extended to a variety of poorly conductive and easily soluble redox-active materials for battery applications.Graphical abstractHighlights► A carbon–sulfur composite was prepared by vaporizing sulfur into the nanopores of Li+-conductive carbon microspheres. ► The redox reaction of S8 molecules embedded in the nanopores of carbon microspheres proceeds through a solid–solid mechanism at the S/C interfaces. ► The carbon–sulfur composite exhibits a stable cycling performance and a superior high coulombic efficiency of 100%.

Si-based alloy materials have received great attention as an alternative anode for high capacity and safe Li-ion batteries, but practical implementation of these materials is hindered by their poor electrochemical utilization and cyclability. To tackle this problem, we developed a core–shelled FeSi2/Si@C nanocomposite by a direct ball-milling of Fe and Si powders. Such a nanostructured composite can effectively buffer the volumetric change by alloying active Si phase with inactive FeSi2 matrix in its inner cores and prevent the aggregation of the active Si particles by outer graphite shells, so as to improve the cycling stability of the composite material. As a result, the FeSi2/Si@C composite exhibits a high Li-storage capacity of ∼1010 mA g–1 and an excellent cyclability with 94% capacity retention after 200 cycles, showing a great promise for battery applications. More significantly, the synthetic method developed in this work possesses several advantages of low cost, zero emission, and operational simplicity, possibly to be extended for making other Li-storage alloys for large-scale applications in Li-ion batteries.Keywords: ball milling; core−shell structure; FeSi2/Si alloy; FeSi2/Si@C Nanocomposite; lithium ion battery; Si anode;

A thermally polymerizable monomer, 1,1′-(methylenedi-4,1-phenylene) bismaleimide (BMI), is investigated as a safety electrolyte additive for thermal protection of lithium ion batteries. The experimental results demonstrate that the BMI additive can polymerize to cause the rapid solidification of electrolyte at 110 °C, which can effectively block off the ion transport between electrodes, thus generating a thermal shutdown of the electrode reactions for safe control of lithium ion batteries. Meanwhile, the addition of BMI additive has no significant influence on the normal charge–discharge performance of rechargeable lithium batteries, showing a great prospect for battery applications.Highlights► A polymerizable monomer, BMI, can serve as safety additive for Li-ion batteries. ► The BMI additive could shut down the battery reactions at 110 °C. ► The BMI additive only has an indiscernible impact on normal battery performance.

A new positive-temperature-coefficient (PTC) material was prepared simply by blending of conductive Super P carbon black (CB) with insulating poly(methyl methacrylate) (PMMA) polymer matrix, which was empolyed as a coating layer on the aluminium foil substrate to fabricate a sandwiched Al/PTC/LiCoO2 cathode. The experimental results from cyclic voltammetry, charge-discharge measurements and impedance spectroscopy demonstrated that the PTC electrode has a normal electrochemical performance at ambient temperature, but shows an enormous increase in the resistance at the temperature range of 80–120°C. This PTC behavior greatly restrains the reaction current passing through the electrode at elevated temperatures, capable of acting as a self-actuating safety mechanism to prevent the battery from thermal runaway.

A novel temperature-sensitive cathode material, LiCoO2@P3DT, exhibits not only improved cycling performance at ambient temperature, but also a thermal shutdown action at an elevated temperature of 110 °C, providing a self-activating thermal protection for lithium ion batteries.

A voltage-sensitive separator is prepared simply by impregnating electroactive poly(3-decylthiophene) (P3DT) polymer into a commercial porous separator and tested for a self-actuating control of overcharge voltage of LiFePO4/C lithium-ion batteries. The experimental results demonstrate that this type of separator can be reversibly p-doped and dedoped to maintain the cell's voltage at a safe value of ≤4 V even at high rate overcharge of 3 C current, effectively protecting the batteries from voltage runaway. Since this P3DT-modified separator has no obvious negative impact on the normal charge–discharge performance of the batteries, it may be adopted for practical application in commercial lithium ion batteries.

A polymerizable monomer, diphenylamine (DPAn), is reported to act as a safety electrolyte additive for overcharge protection of 3.6 V-class lithium ion batteries. The experimental results demonstrated that the DPAn monomer could be electro-polymerized to form a conductive polymer bridging between the cathode and anode of the battery, and to produce an internal current bypass to prevent the batteries from voltage runaway during overcharge. The charge–discharge tests of practical LiFePO4/C batteries indicated that the DPAn additive could clamp the cell's voltage at the safe value less than 3.7 V even at the high rate overcharge of 3 C current, meanwhile, this monomer molecule has no significant impact on the charge–discharge performance of the batteries at normal charge–discharge condition.

Lithium ion batteries have now been used as a power source for electric vehicles; however, their safety still remains a serious concern as the accidents reported increase with the rapid increase of electric vehicles in transportation markets. To address this issue, we describe herein a novel temperature-responsive cathode by coating an ultra-thin layer of poly(3-octylthiophene) (P3OT) with a thickness less than 1 μm in between the Al substrate and cathode-active LiCoO2 layer to form a sandwiched Al/P3OT/LiCoO2 cathode (LCO-PTC). This LCO-PTC cathode demonstrates almost the same electrochemical performance as the conventional LiCoO2 cathode at ambient temperature but a strong PTC behavior to switch off the cell reaction in a high temperature range of 90–100 °C, thus protecting the cell from thermal runaway. Because of its easy fabrication, cost effectiveness and particularly good compatibility with the current battery technology, this new type of PTC electrode can be conveniently extended to other Li-insertion cathodes for building safer Li-ion batteries.

Commercial development of lithium–sulfur (Li–S) batteries is severely hindered by their insufficient cyclability, which is due to the loss of soluble lithium polysulfide intermediates generated during the discharge processes. To overcome this problem, considerable efforts have been devoted to designing novel micro- or nano-structured host materials, aiming to trap soluble polysulfide within the network. Herein, we report a new approach to construct a sulfur electrode by impregnating sulfur into the nanopores of covalent-organic frameworks (COFs). Our results clearly demonstrate that by using a 2D COF as a host material, e.g. CTF-1 (CTF: covalent triazine-based framework), the thus-prepared cathode can show a remarkable positive effect on the capacity retention of Li–S batteries. Considering the unique features of COFs, such as highly flexible molecular design and a controllable pore size, this proof-of-principle study provides new opportunities for materials scientists for tailoring cathode materials in Li–S batteries.

Sulfur/carbon (S/C) nanocomposite-filled polyacrylonitrile (PAN) nanofibers (denoted as S/C/PAN) are synthesized as a long life and high capacity cathode material for lithium–sulfur (Li–S) batteries. In the S/C/PAN nanofibers, the sulfurized PAN matrix acts not only as ionic and electronic channels to allow Li+ and electrons to arrive at and react with the S/C nanoparticles, but also as a protective barrier to prevent the S/C nanoparticles from contacting the electrolyte, thus avoiding the discharge intermediates of sulfur to dissolve in and react with the organic carbonate electrolyte. Since the redox reaction of sulfur in the nanofibers occurs mostly at the interior S/C interface through a solid state reaction mechanism, the microstructures and electrochemical interfaces in the nanofiber cathode remain stable during repeated cycles. As a consequence, the S/C/PAN cathode demonstrates a high reversible capacity of 1179 mA h g−1 at a current rate of 200 mA g−1, a high Coulombic efficiency of ∼100% after a few cycles, a good rate capability with 616 mA h g−1 at 4.0 A g−1 and a long cycling stability with 60% capacity retention over 400 cycles, showing great prospect for Li–S battery applications.

Covalent organic frameworks (COFs) represent an emerging class of porous crystalline materials and have recently shown interesting applications in energy storage. Herein, we report the construction of a cycle-stable sulfur electrode by embedding sulfur into a 2D COF. The designed porphyrin-based COF (Por-COF), featuring a relatively large pore volume and narrow pore size distribution, has been employed as a host material for sulfur storage in Li–S batteries. With a 55% sulfur loading in the composite, the thus-prepared cathode delivers a capacity of 633 mA h g−1 after 200 cycles at 0.5C charge/discharge rates. Therefore, embedding sulfur in the nanopores of the Por-COF significantly improves the performance of the sulfur cathode. Considering the flexible design of COFs, we believe that it is possible to synthesize a 2D COF host with a suitable pore environment to produce more stable Li–S batteries, which may help in exploration of the structure–property relationship between the host material and cell performance.

Great efforts have been devoted to developing nano-Si anodes for next-generation lithium ion batteries (LIBs); however, the reversible capacity and cycling stability of all Si anodes developed so far still need to be improved for battery applications. In this work, we propose a new strategy to develop a cycling-stable Si anode by embedding nano-Si particles into a Li+-conductive polymer matrix, in which a stable Si/polymer interface is established to avoid the contact of the Si surface with the electrolyte and to buffer the volume change of the Si lattice during cycles, thus promoting the capacity utilization and long cycle life of nano-Si particles. The nano-Si/polybithiophene composite synthesized in this work demonstrates a high Li-storage capacity of >2900 mA h g−1, a high-rate capability of 12 A g−1 and a long-term cyclability with a capacity retention of >1000 mA h g−1 over 1000 cycles, possibly serving as a high capacity anode for lithium battery applications. In addition, the fabrication technique for this type of composite material is facile, scalable and easily extendable to other Li-storable metals or alloys, opening up a new avenue for developing high capacity and cycling-stable anodes for advanced Li-ion batteries.