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Conductive polymer composites (CPCs) with a positive temperature coefficient (PTC) have received considerable attention in the intelligent electric heating field due to their low cost, chemical stability, flexibility, and formability compared to conventional inorganic ceramic PTC composites. To fabricate high-performance PTC composites for application, considerable efforts have been made to improve the overall PTC performance of CPCs in both academia and industry. However, few reviews are focusing on the systematic summary and discussion regarding the research strategies to improve the PTC performance of CPCs. In this review, the conductive mechanism and PTC effect mechanism of CPCs with the PTC effect are first summarized. Then, the research strategies to regulate the PTC performance of CPCs are systematically reviewed in terms of the polymer matrix, conductive fillers, and preparation processes. CPCs with a crystalline polymer and an amorphous polymer as the matrix are reviewed separately, especially for the former due to their current widespread application in PTC composites. Meanwhile, the various research strategies for improving the PTC performance are analyzed and summarized. The current challenges and future outlook in this field are also discussed. The research strategies reviewed here for improving the PTC performance of CPCs can provide a comprehensive guide for researchers seeking high-performance PTC composites. [ABSTRACT FROM AUTHOR]

Zinc sulfide (ZnS) is considered as a candidate anode material to replace commercial graphite anodes in high-performance lithium-ion batteries (LIBs). However, the significant volume change during the lithiation/delithiation process leads to poor cycle and rate performance, severely limiting the application of the ZnS anode in LIBs. Here, core–shell ZnS@C microrods (ZnS@C MRs) were synthesized via a simple hydrothermal method, followed by in situ polymerization and a sulfidation process. Due to the rationally designed structure, the outer carbon shell can avoid the volume expansion of ZnS, thus maintaining the structural stability. Moreover, the carbon shell can not only enhance the conductivity, but also inhibit direct contact between ZnS and the electrolyte during the repeated lithiation/delithiation process. Accordingly, ZnS@C MRs exhibit outstanding electrochemical performance in LIBs. Specifically, a capacity of 392 mA h g−1 is obtained after 500 cycles at 1.0 A g−1. Moreover, a capacity of 162 mA h g−1 is maintained at 3.0 A g−1. [ABSTRACT FROM AUTHOR]

Elimination of interfacial thermal resistance is a critical issue to improve the thermal conductivity of graphene-based metal matrix composites. Here, thiophenol groups were employed to construct an interfacial molecular junction between graphene and copper to improve the thermal conduction performance of the graphene/copper composite. The highly delocalized aryl properties of thiophenol groups can facilitate electron tunneling at the interface, thereby increasing the electron thermal conduction and reducing the interfacial thermal resistance between graphene and copper nanoparticles. The resulting thiophenol group linked graphene and copper composite shows a high thermal conductivity of 500.6 W m−1 K−1, which is 1.43 and 1.30 times higher than those of copper (Cu) and pristine graphene–copper composites (Gr–Cu), respectively, and also higher than most reported values via a single phonon thermal conduction route. Placed on a heat source heated from 26 °C to 213 °C within 5 min, the top surface temperature of TP–Gr–Cu increases to 184.2 °C rapidly, whereas those of Gr–Cu and Cu only increase slowly to 61.4 °C and 51.6 °C. This work demonstrates that an interfacial electron thermal conduction route can be built through electron tunneling by constructing a molecular junction between graphene and copper, which can significantly reduce the interfacial thermal resistance and improve the interfacial thermal conductivity. [ABSTRACT FROM AUTHOR]

Lithium iron phosphate (LFP) has become one of the current mainstream cathode materials due to its high safety and low price. Most modification methods applied (e.g. ion doping, carbon coating and particle size restriction) are used to overcome its poor electronic and ionic conductivity. Here, the Y–F co-doped LFP/carbon (LFP/C) precursor was successfully synthesized using a high temperature solid phase method. The electronic conductivity of the material is enhanced by doping with F which induces the rearrangement of the PO43+ electron cloud, while the doping with Y introduces Li+ vacancies, thereby reducing the space resistance of Li ion diffusion, resulting in an overall enhancement of the ionic conductivity of the material. In addition, XRD refinement results show that Y and F doping leads to a weakening of the Li–O bond while also widening the lithium ion diffusion tunnel, thereby increasing the lithium ion diffusion rate. Therefore, this work has produced LFP/C-YF-2, which exhibits an ultra-high discharge specific capacity of 135.8 mAh g−1 at 10C, and a discharge specific capacity of 148.6 mA h g−1 without attenuation after 700 cycles at 5C. It is hoped that this high-capacity and high-rate cycling stability material will become a promising cathode material for applications in high-power electric vehicles and other equipment. [ABSTRACT FROM AUTHOR]

Tin sulfide (SnS) has attracted much attention as an anode material for sodium ion batteries (SIBs) because of its various advantages, including high capacity and unique 2D structure. However, SnS has poor electrochemical performance caused by the large volume change and low intrinsic electric conductivity, which seriously limited its practical application in SIBs. Herein, we successfully constructed bimetallic sulfide SnS/MoS2 heterostructures decorating N, S co-doped carbon nanosheets (SnS/MoS2/NS-CNs) as anode materials for SIBs. The designed SnS/MoS2 heterostructures induce an electric field within the nanocrystals, which lead to lower ion-diffusion resistance and facilitate interfacial electron transport. Moreover, the N, S co-doped carbon nanosheets can buffer the volume change of SnS/MoS2, avoiding the direct contract between SnS/MoS2 and electrolyte, as well as favorable transport kinetics for electrons and ions. Accordingly, benefiting from these merits, the as-prepared SnS/MoS2/NS-CNs exhibit outstanding rate capability (372.9 mA h g−1 at 5.0 A g−1) and long-term cycling performance (287.2 mA h g−1 after 800 cycles at 1.0 A g−1). [ABSTRACT FROM AUTHOR]

The nickel-rich layered material LiNi0.8Co0.1Mn0.1O2 (NCM) is a promising candidate cathode material for next generation lithium-ion batteries due to its high energy density and relatively low cost. Unfortunately, NCM suffers from capacity fading upon cycling and poor rate capability that seriously limit its wide application. To overcome these drawbacks, we present an effective inorganic coating strategy for a complex LixNi1−yFeyO2&NiFe2O4 (LNFO&NFO) in situ surface modified NCM cathode material by a sample wet-chemical method, which could effectively stabilize the layer structure and eliminate lithium impurities. As a result, LNFO&NFO surface modified NCM shows excellent electrochemical performance. For example, the optimized NFO-2 cathode not only shows an excellent rate performance with a discharge capacity of 140.56 mA h g−1 at 10C, but also exhibits outstanding cycling stability with an improved capacity retention of 87.43% and 74.32% at 1C after 250 and 100 cycles at room and elevated temperatures of 25 and 55 °C. The superior electrochemical performance is attributed to the dual functions of NiFe2O4 modification. The formed LNFO&NFO coating layer bonds with the surface oxygen vacancies of the bulk material which could effectively stabilize the surface structure and bulk structure associated with capacity degradation as well as the electrical and ionic conductivity at the interface. Therefore, ingenious design of the LNFO&NFO surface coating layer is a key approach for improving the durability of the NCM cathode material during the cycling process. [ABSTRACT FROM AUTHOR]

Si has received much attention as an anode material for high performance LIBs due to its high theoretical capacity. Nevertheless, the serious volume changes during the lithiation/delitaiation process and poor electrical conductivity result in fast capacitative decay and poor rate performance, which seriously limit its practical applications. Herein, a 2D Si@graphitic carbon (GC) nanosheets composite was synthesized through the facile ball-milling method using NaCl as a bifunctional template, which not only served as a hard template but also stabilized the Si nanoparticles during the fabrication process. In this rationally designed architecture, Si nanoparticles are embedded into carbon nanosheets and encapsulated by a graphitic carbon wall. Therefore, the 2D Si@GC nanosheets composite as an anode for LIBs with outstanding cycling and rate performance, delivered a high capacity of 1450.7 mA h g−1 after 400 cycles at 0.5 A g−1, and maintained high reversible capacity of 877.4 mA h g−1 even at 3.0 A g−1. This outstanding electrochemical performance can be ascribed to the carbon nanosheets effectively buffering the volume change of the Si nanoparticles, graphitic carbon wall improving the electrical conductivity and 2D nanosheets facilitating the transport of Li+ during the charge/discharge process. Thus, the Si@GC nanosheet composite exhibited an outstanding rate and cycling performance. [ABSTRACT FROM AUTHOR]

Iron sulfide (Fe1−xS) has been considered as a promising anode material for sodium ion batteries (SIBs) because of its high theoretical capacity. However, Fe1−xS suffers from large volume change and sulfur dissolution issues during the sodiation/desodiation process, which lead to a poor cycle performance. Here, heterostructured Fe1−xS@S-doped carbon (Fe1−xS@SC) hollow spheres (HSs) have been prepared as SIB anode material. The Fe1−xS@SC HSs show a high reversible capacity of 776.2 mA h g−1 at 0.2 A g−1 and 454.3 mA h g−1 at 1.0 A g−1 after 500 cycles. The constructed Fe1−xS/SC heterojunctions play a key role in alleviating the large volume variation and stabilizing the reaction products Fe0 and Na2S, thus enhancing the conversion reaction kinetics of Fe0 and Na2S and ultimately resulting in a highly reversible conversion reaction. This work provides a possibility to solve the reversibility issue of conversion-type anode materials. [ABSTRACT FROM AUTHOR]

Fe-based sulfides have received tremendous attention as promising anode materials for sodium ion batteries (SIBs) owing to their advantages of abundant resources, high theoretical specific capacity, and environmental benignity. However, unsatisfactory electronic conductivity and severe volume change during the cycling process limit their applications. Here, an FeS/N-doped carbon composite (FeS/NC) is designed and prepared through an in situ chemical transformation method via pyrolyzing a mixture of Fe(NO3)3 and urea followed by sulfidation. The individual FeS nanoparticles are coated by the in situ formed nitrogen-doped carbon and then embedded in the ultrathin carbon sheets. When evaluated as an anode material for SIBs, the composite delivered a high charge capacity of 511 mA h g−1 after 100 cycles at 200 mA g−1, 326 mA h g−1 after 500 cycles at 1.0 A g−1, and exhibited a superior rate performance of 260 mA h g−1 at 4.0 A g−1. This good electrochemical performance of the FeS/NC can be ascribed to the in situ carbon-coating effectively alleviating the volume change of FeS particles during the cycling process and the interconnected ultrathin carbon sheets enhancing the electronic conductivity of the ensemble. [ABSTRACT FROM AUTHOR]

Potassium-ion batteries (KIBs) are considered as a promising competitor to other metal-ion battery systems due to their low-cost and large-scale energy storage. However, their development is hindered by poor intercalation property of K ions in electrodes due to the heavier weight and larger size of K ion than those of Li ion. Therefore, achieving an ultra-long cycle life with high capacity and excellent rate capability remains a significant challenge for KIB electrodes. Herein, activated hollow carbon nanospheres (AHCSs), with enlarged interlayer spacing, oxygen-containing functional groups (OCFGs) on the surface and high specific surface area, are proposed as a new anode electrode for high-performance KIBs. Ex situ XRD and TEM results demonstrated that the enlarged interlayer spacing allow more reversible K ion intercalation into the carbon layer and readily accommodated large (de)potassiation strain without fracture, while OCFGs on AHCSs enhanced pseudocapacitance type behavior and specific capacity. Due to the synergistic effect of these structural features, the AHCSs served as anode for KIBs, exhibiting impressive electrochemical properties with high initial charge capacity of 370.2 mA h g−1 at 0.2 A g−1, ultra-long cycling life for 5000 cycles at a high current density of 2.0 A g−1, as well as prominent rate capability of 137.0 mA h g−1 at 4.0 A g−1. This study highlights the significant role of interlayer spacing, surface oxygen functionalization and hierarchical porosity of carbon-based anodes in potassium storage. [ABSTRACT FROM AUTHOR]

Potassium ion batteries (PIBs) have been regarded as promising energy storage devices for large-scale energy storage owing to the abundance of potassium resources. In this work, high pyridine N-doped porous carbon synthesized at 600 °C (NPC-600) derived from metal–organic frameworks (MOFs) has been fabricated and employed as the anode material for PIBs. NPC-600 can deliver a high reversible specific capacity (587.6 mA h g−1 at 50 mA g−1), outstanding rate properties (186.2 mA h g−1 at 2 A g−1) and cycling performance (231.6 mA h g−1 at 500 mA g−1 after 2000 cycles). This excellent electrochemical performance could be attributed to the increased amounts of pyridine N and neglectable change of the interlayer space during the potassiation/depotassiation process of NPC-600, which can provide additional adsorption sites to “capture” more K ions and ensure structural stability. This simple synthesis approach and unique structure make NPCs a promising candidate for next generation rechargeable PIBs. [ABSTRACT FROM AUTHOR]

Potassium ion batteries (KIBs) have attracted tremendous attention because of the abundance of potassium resources and the applicability of carbonaceous materials for use as anodes, which indicates that the manufacturing techniques of lithium ion batteries can be directly transferred to KIBs. However, the huge volume change during the potassiation/depotassiation process, and the poor kinetics of the large K+ ions seriously restrict the electrochemical performance of graphite because the K+ ions are squeezed into the restricted interlayer spacing. Compared with well-crystallized graphite, amorphous carbon has a more flexible structure, but its capacity contribution is mainly dependent on a capacitive storage mechanism. Therefore, finding a carbon material with a suitable degree of graphitization and structure is necessary to achieve superior electrochemical properties. In the research reported in this paper, nitrogen-doped bamboo-like carbon nanotubes composed of amorphous carbon and discontinuous graphene layers with a porous hollow structure were prepared. Benefiting from its unique structure, this material delivered a high, reversible capacity of 204 mA h g−1 at 500 mA g−1 after 1000 cycles, and exhibited a remarkable rate capability of 186 mA h g−1 at 1000 mA g−1. [ABSTRACT FROM AUTHOR]

A designed hierarchical nanostructure consisting of SnS nanosheets and ultrathin MoS2 nanosheets was achieved by a facile hydrothermal process with the assistance of fluoride and glucose. In this unique architecture, on the one hand, SnS and MoS2 nanosheets can greatly reduce the Li-ion and electron diffusion distance in the electrode. On the other hand, MoS2 nanosheets and amorphous carbon can not only prevent the direct exposure of SnS to the electrolyte but also maintain the structural stability of the electrode. In addition, the MoS2 nanosheets can offer more active sites for hosting lithium ions, resulting in higher capacity. When evaluated as anode material for lithium-ion batteries (LIBs), this SnS/MoS2–C composite exhibited stable cycling performance (989.7 mA h g−1 at 0.2 A g−1 after 60 cycles), superior rate capability (675 mA h g−1 even at 5.0 A g−1) and a long cycle life (718 mA h g−1 at 2.0 A g−1 after 700 cycles). Therefore, this SnS/MoS2–C composite is a promising candidate as anode material for next-generation high-performance LIBs. [ABSTRACT FROM AUTHOR]

Carbon-coated VPO4 nanoparticles embedded into a porous carbon matrix were synthesized via a facile sol–gel approach and investigated as a novel polyanion anode material for sodium-ion batteries. The VPO4@carbon anode demonstrates excellent rate capability and superior cyclic stability (245.3 mA h g−1 at 1000 mA g−1 after 200 cycles). [ABSTRACT FROM AUTHOR]

The goal of this research was to study the effect of molecular structure surfactant on the performance of LiFePO4/C composite. The solid state reaction method was applied to prepare a series of LiFePO4/C materials by adding various surfactants. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Infrared spectrum analysis (FTIR), Raman spectral analysis and electrochemical methods. The results confirm the existence of interactions between the surfactant molecules and the precursor that contribute to the formation of nanoparticles and a homogeneous carbon coating layer on the surface. The structure of the surfactants played an important role in improving the performance of LiFePO4/C composites. The structures of surfactants affect the particle size and the amount of graphite-like carbons of LiFePO4/C composites. The surfactant with longer alkyl C–C chain length could effectively prevent particle growth, while the surfactant with shorter alkyl C–C chain length form more graphene-like carbon during pyrolysis. The as-prepared LiFePO4/C particles using various surfactants show different electrochemical performances. According to Raman spectroscopy and EIS analysis, these composites have different ID/IG peak ratios and charge transfer resistance. Especially, LiFePO4/C synthesis with composite surfactant (weight ratio of Tween80 to Tween20 equal to 1.5) showed an excellent electrochemical performance with a discharge capacity of 167.3 mA h g−1 at 0.1 C rate and 129.4 mA h g−1 at 5 C rate. [ABSTRACT FROM AUTHOR]

Layered graphene and molybdenum disulfide have outstanding sodium ion storage properties that make them suitable for sodium-ion batteries (SIBs). However, the easy and large-scale preparation of graphene and molybdenum disulfide composites with structural stability and excellent performance face enormous challenges. In this study, a self-supporting network-structured MoS 2 /heteroatom-doped graphene (MoS 2 /NSGs-G) composite is prepared by a simple and exercisable electrochemical exfoliation followed by a hydrothermal route. In the composite, layered MoS 2 nanosheets and heteroatom-doped graphene nanosheets are intertwined with each other into self-supporting network architecture, which could hold back the aggregation of MoS 2 and graphene effectively. Moreover, the composite possesses enlarged interlayer spacing of graphene and MoS 2 , which could contribute to an increase in the reaction sites and ion transport of the composite. Owing to these advantageous structural characteristics and the heteroatomic co-doping of nitrogen and sulfur, MoS 2 /NSGs-G demonstrates greatly reversible sodium storage capacity. The measurements revealed that the reversible cycle capacity was 443.9 mA h g -1 after 250 cycles at 0.5 A g -1 , and the rate capacity was 491.5, 490.5, 453.9, 418.1, 383.8, 333.1, and 294.4 mA h g -1 at 0.1, 0.2, 0.5, 1, 2, 5 and 10 A g -1 , respectively. Furthermore, the MoS 2 /NSGs-G sample displayed lower resistance, dominant pseudocapacitive contribution, and faster sodium ion interface kinetics characteristic. Therefore, this study provides an operable strategy to obtain high-performance anode materials, and MoS 2 /NSGs-G with favorable structure and excellent cycle stability has great application potential for SIBs., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Carbon-coated VPO 4 nanoparticles embedded into a porous carbon matrix were synthesized via a facile sol-gel approach and investigated as a novel polyanion anode material for sodium-ion batteries. The VPO 4 @carbon anode demonstrates excellent rate capability and superior cyclic stability (245.3 mA h g -1 at 1000 mA g -1 after 200 cycles).