Multifunctional Nanocomposites for Energy and Environmental Applications

By Yuan Chen

Focusing on real applications of nanocomposites and nanotechnologies for sustainable development, this book shows how nanocomposites can help to solve energy and environmental problems, including a broad overview of energy-related applications and a unique selection of environmental topics.
Clearly structured, the first part covers such energy-related applications as lithium ion batteries, solar cells, catalysis, thermoelectric waste heat harvesting and water splitting, while the second part provides unique perspectives on environmental fields, including nuclear waste management and carbon dioxide capture and storage.
The result is a successful combination of fundamentals for newcomers to the field and the latest results for experienced scientists, engineers, and industry researchers.

• Language: English
• Publisher: Wiley
• Release date: Jan 2, 2018
• ISBN: 9783527342495

Book preview

Chapter 1

Introduction to Nanocomposites

Xingru Yan and Zhanhu Guo

University of Tennessee, Chemical and Biomolecular Engineering Department, Integrated Composites Laboratory (ICL), 1512 Middle Dr, Knoxville, TN, 37916, USA

Nanocomposites are materials that are composed of two or more constituents, with different physical and chemical properties, which remain separate and distinct at the microscopic level but collectively comprise a single physical material possessing any phase dimension of less than 100 nm [1–3]. In the broadest sense, the primary motivation of making a nanocomposite is to integrate one or more discontinuous nano-dimensional phases into a single continuous macrophase to generate synergistic properties; that is, the physical/chemical properties of the combined entity are inherently different from, and hopefully superior to, those of the individual material constituents. In this context, one of the combined material's constituents is generally in much greater concentration and forms a continuous matrix surrounding the others, which assume the role of a nanofiller or reinforcement. During the nanocomposite formation process, each of the distinct phases is structure- and property-integrated to fabricate hybrid materials that possess multifunctionalities in terms of both structures and material properties. As scientific and societal needs in the late twentieth century drove the demand for higher-performance, sustainable, and multifunctional nanomaterials, more innovative nanotechnology and nanocomposites were increasingly investigated. The advent of new materials and characterization tools in the nanotechnology domain has been paving the way for the latest design of next-generation nanocomposites that not only are easily controllable but also possess multiple intrinsic engineering functionalities.

Based on the different types of matrix materials, nanocomposites can be generally divided into four categories including polymer-, carbon-, metal-, and ceramic-based nanocomposites. Polymers are large molecules or macromolecules composed of repeating structural units, typically connected by covalent chemical bonds; these are generally excellent host matrixes for nanocomposite materials due to their light weight, ease of processing, low-cost manufacturing, and good adhesion to substrates [4, 5]. Polymer nanocomposites (PNCs) are polymer composites using nanostructured materials as reinforcements. Depending on the type of reinforcement material, different properties can be achieved for PNCs. The introduction of inorganic or organic nanofillers into polymer systems has resulted in PNCs exhibiting multifunctional, high-performance polymer characteristics beyond those possessed by traditionally filled polymeric materials. Meanwhile, through control of the filler at the nanoscale level, PNCs are able to maximize property enhancement of selected polymer systems or have an exceptional potential to generate new physical phenomena to meet the requirements of military, aerospace, and commercial applications. The properties of PNCs not only are determined by their individual components but also depend on their morphology and interfacial characteristics. In multifunctional PNCs, the reinforcements impart their special mechanical, optical, electrical, and magnetic properties to the composites, whereas the polymer matrix provides support for the reinforcements and retains the properties of the constituent polymer.

Extensive research on PNC materials has already resulted in the development of many various PNCs with multifunctionality that shows substantial improvement in physicochemical properties of the combined material. For example, highly efficient electromagnetic (EM) field absorption at gigahertz frequency (GHz) was reported by He et al. in novel magnetic PNCs with in situ synthesized Fe@Fe2O3 core–shell nanoparticles (NPs) or their decorated multiwall carbon nanotubes (MWCNTs) dispersed in a polypropylene (PP). At the same time, in situ-formed nanofillers significantly reduced the flammability of PP for a wide range of potential applications [6]. Carbon-based structures as a type of matrix for nanocomposites may contain carbon nanotubes, carbon nanofibers, carbon nanoplates, and graphene. The unique chemical and electronic structures of carbon have made carbon-based nanocomposites noteworthy in a wide variety of applications, such as anticorrosion in electronic devices and sensors, magnetic data storage and magnetic imaging, and environmental remediation for heavy metals and other toxic species. For example, Cao et al. reported that newly designed fluorine-doped magnetic carbon was used as an adsorbent for Cr(VI) removal, in which as high as 48.78 and 1423.4 mg/g removal capacities in the neutral and acidic solution were achieved, higher than the state-of-the-art adsorbents such as magnetic carbons, activated carbon, and surface-modified adsorbents in environmental remediation [7].

Metal-based nanocomposites refer to materials consisting of a ductile metal or alloy matrix in which a nanoscale reinforcement material is implanted. Metal-based nanocomposites can possess a wide range of matrix materials, including aluminum, titanium, copper, nickel, iron, and so on. Reinforcement materials can be carbides (e.g., SiC, B4C), nitrides (e.g., Si3N4, AlN), oxides (e.g., Al2O3, SiO2), and elemental materials (e.g., C, Si). The excellent physical and mechanical properties that can be obtained from metal-based nanocomposites, such as enhanced specific modulus, strength, and thermal stability, have been investigated extensively. Recently, Li reported that an in situ nano-TiB2-decorated AlSi10Mg composite (NTD-Al) powder was fabricated by gas atomization for selective laser melting (SLM). This metal-based nanocomposite showed a very high ultimate tensile strength (∼530 MPa), excellent ductility (∼15.5%), and high microhardness (∼191 HV0.3), all of which were higher than most conventionally fabricated wrought and tempered Al alloys [8].

Ceramics materials generally display good wear resistance and high thermal and chemical stability; however, they are typically very brittle. In order to overcome this limitation, ceramic-based nanocomposites have been gaining attention, primarily due to the significant enhancement of the mechanical properties that can be achieved by the incorporation of energy-dissipating components, such as whiskers, fibers, platelets, or particles within the ceramic matrix. For example, fully dense, isotropic Al2O3/10% MWCNT nanocomposites containing sharp starter cracks of various lengths (DC) show modest toughening, with the steady-state toughness reaching 4.6 MPa √m.

In order to produce nanocomposites with multiple functionalities, the method in which the nanofiller is dispersed within the matrix and the subsequent processing are crucial in obtaining the targeted properties for advanced applications. The emerging subfield of nanochemical engineering is aimed at the development of a core set of technology enabling the manufacture of large-scale chemical production of multifunctional nanocomposites [9]. The traditional direct mixing method is not able to attain uniform dispersion and retain the strong connection between the filler and the matrix, thus failing to achieve the desired properties. However, the nanochemical engineering and nano-processes to multifunctional nanocomposites are targeted depending on the types of materials as well as the functions of the nanocomposites. For example, the electrochemical polymerization techniques are generally applied for synthesizing conductive nano thin films for electrochromic devices [10] or anticorrosion to protect metal substrate [11], whereas chemical oxidative polymerization techniques are expected to achieve powder-form nanocomposites for electrochemical energy storage [12] or giant magnetoresistance sensor applications [13]. Therefore, the nanochemical engineering and nano-processes can bring innovative products or improved performances, which have been very well explained in the following chapters. Due to the inherent synergistic properties that can be designed into multifunctional nanocomposites, including catalytic, electrical, magnetic, mechanical, optical, sterical, and biological, these relatively new materials have wide applications spanning broad areas such as energy storage, environmental remediation, EM absorption, sensing and actuation, transportation and safety, defense systems, novel catalysts, biomedicine, and so on.

This book is divided into two parts that coincide with how nanocomposites have affected the two main application areas in energy and environment science since the dawn of the twenty-first century. The first part describes the achievements that polymer- and/or carbon-based nanocomposites have precipitated in various segments of energy applications, such as lithium ion batteries, electrochemical capacitors, solar cells, electrochromic devices, electrocatalysts, photocatalysts, biofuels production, and so on. This direction of research has motivated many researchers to explore more innovative and durable nanocomposites to meet or even exceed present requirements of materials and to help broaden their applications. The second part is centered on the effects of nanocomposites on environmental applications, such as vehicle NOx emission control, nuclear waste management, EM absorption, and wastewater treatment. This research has motivated the development of novel and sustainable nanocomposites to solve increasingly serious global environmental problems against concordant ecological conditions. All the following chapters in this book present recent developments in each relevant field.

Finally, this book also includes with our general assessment and perspectives of multifunctional nanocomposites, especially their potential for energy and environmental applications. Of course, this book does not extensively cover every aspect of multifunctional nanocomposites but rather selectively reports what have been achieved to date.

References

1 Guo, Z., Wei, S., Shedd, B., Scaffaro, R., Pereira, T., and Hahn, H.T. (2007) Particle surface engineering effect on the mechanical, optical and photoluminescent properties of Zno/vinyl-ester resin nanocomposites. J. Mater. Chem., 17, 806–813.

2 Zhu, J., Wei, S., Ryu, J., Sun, L., Luo, Z., and Guo, Z. (2010) Magnetic epoxy resin nanocomposites reinforced with core − shell structured Fe@ Feo nanoparticles: fabrication and property analysis. ACS Appl. Mater. Interfaces, 2, 2100–2107.

3 He, Q., Yuan, T., Wang, Y., Guleria, A., Wei, S., Zhang, G., Sun, L., Liu, J., Yu, J., and Young, D.P. (2016) Manipulating the dimensional assembly pattern and crystalline structures of iron oxide nanostructures with a functional polyolefin. Nanoscale, 8, 1915–1920.

4 Schadler, L.S. (2003) Polymer-Based and Polymer-Filled Nanocomposites, Wiley Online Library.

5 Koo, J.H. (2006) Polymer Nanocomposites, McGraw-Hill Professional Pub.

6 He, Q., Yuan, T., Zhang, X., Yan, X., Guo, J., Ding, D., Khan, M.A., Young, D.P., Khasanov, A., and Luo, Z. (2014) Electromagnetic field absorbing polypropylene nanocomposites with tuned permittivity and permeability by nanoiron and carbon nanotubes. J. Phys. Chem. C, 118, 24784–24796.

7 Cao, Y. et al. (2017) Poly(vinylidene fluoride) derived fluorine-doped magnetic carbon nanoadsorbents for enhanced chromium removal. Carbon, 115, 503–514.

8 Li, X.P., Ji, G., Chen, Z., Addad, A., Wu, Y., Wang, H.W., Vleugels, J., Van Humbeeck, J., and Kruth, J.P. (2017) Selective laser melting of nano-TiB2 decorated AlSi10mg alloy with high fracture strength and ductility. Acta Mater., 129, 183–193.

9 Denn, M. (2011) Chemical Engineering: An Introduction, Cambridge University Press.

10 Wei, H., Zhu, J., Wu, S., Wei, S., and Guo, Z. (2013) Electrochromic polyaniline/graphite oxide nanocomposites with endured electrochemical energy storage. Polymer, 54, 1820–1831.

11 Wei, H., Wang, Y., Guo, J., Shen, N.Z., Jiang, D., Zhang, X., Yan, X., Zhu, J., Wang, Q., and Shao, L. (2015) Advanced micro/nanocapsules for self-healing smart anticorrosion coatings. J. Mater. Chem. A, 3, 469–480.

12 Gu, H., Wei, H., Guo, J., Haldolaarachige, N., Young, D.P., Wei, S., and Guo, Z. (2013) Hexavalent chromium synthesized polyaniline nanostructures: magnetoresistance and electrochemical energy storage behaviors. Polymer, 54, 5974–5985.

13 Gu, H., Guo, J., Wei, H., Huang, Y., Zhao, C., Li, Y., Wu, Q., Haldolaarachchige, N., Young, D.P., and Wei, S. (2013) Giant magnetoresistance in non-magnetic phosphoric acid doped polyaniline silicon nanocomposites with higher magnetic field sensing sensitivity. Phys. Chem. Chem. Phys., 15, 10866–10875.

Chapter 2

Advanced Nanocomposite Electrodes for Lithium-Ion Batteries

Jiurong Liu¹, Shimei Guo², Chenxi Hu¹, Hailong Lyu¹,³, Xingru Yan³ and Zhanhu Guo³

¹Shandong University, Ministry of Education and School of Materials Science and Engineering, Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, No. 17923, Jinshi Road, Jinan, Shandong, 250061, China

²Qujing Normal University, College of Physics and Electronic Engineering, Center for Magnetic Materials and Devices & Key Laboratory for Advanced Functional and Low Dimensional Materials of Yunnan Higher Education Institute, Sanjing Road, Qujing, Yunnan, 655011, China

³University of Tennessee, Department of Chemical & Biomolecular Engineering, Integrated Composites Laboratory (ICL), Knoxville, TN, 37996, USA

2.1 Introduction

Since the Sony Corporation first successfully marketed a commercial rechargeable lithium-ion battery (LIB) that functioned on the principle of simultaneous lithium-ion extraction and insertion in its electrode materials in 1991 [1], as an alternative energy storage device to lead-acid or nickel-metal hydride batteries, it has attracted remarkable attention due to its available high voltage, high energy density, memory-free effect, and so on. A typical LIB consists of a cathode and an anode, together with an electrolyte-filled separator that allows lithium (Li)-ion transfer but prevents electrodes from direct contact, as shown in Figure 2.1 [2, 3]. The two electrodes have different electrochemical potentials [4]. In general, Li transitional metal oxides, such as LiCoO2, LiNiO2, LiMn2O4, and LiFePO4, offering a high electrode potential of 4 V versus metal lithium, are used as cathode materials. Correspondingly, due to a lower electrode potential of <1 V versus lithium, graphitic carbon is the most common choice for anode material in commercial LIBs. When the battery is charging, Li ions are extracted from the cathode, then transported through the electrolyte, and finally inserted into the anode materials. The driving force of such intercalating transport is the potential provided by the external power source, which donates electrons and makes them flow in the external circuit from the cathode toward the anode to maintain charge balance. During the discharging process, Li ions are released from the anode and reinserted into cathode materials via electrolyte, accompanied by a flow of electrons through the external circuit. The overall electrochemical reactions between two electrodes can be described as follows:

2.1

equation

2.2

equationScheme for commercial lithium-ion battery where graphite and LiCoO2 are used as anode and cathode, respectively.

Figure 2.1 Schematic illustration of a commercial lithium-ion battery where graphite and LiCoO2 are used as anode and cathode, respectively.

(Liu et al. 2011 [2]. Reproduced with permission of Royal Society of Chemistry.)

Typically, the active materials employed for Li storage in the electrodes determine the most part of the electrochemical performance of LIBs, and thus the selection of anode and cathode materials becomes key factor during the optimization of battery systems [4, 5]. In the past few decades, improvements in the capacity, cycling and rate performances, lifespan, and safety characteristics of rechargeable LIBs have continue to be made by developing new-type anode and cathode materials and enhancing electronic conductivities and ionic diffusivities in solid electrode materials. Especially, the advent and comprehensive application of nanocomposite electrodes made it possible to obtain modern rechargeable LIBs with superior electrochemical performance.

2.2 Advanced Nanocomposites as Anode Materials for LIBs

In general, the anode materials can be mainly summarized into three categories: intercalation materials (including graphite, Li4Ti5O12, TiO2, etc.), alloying-reaction materials (e.g., Si, Ge, Sn, Pb, Al, and Bi), and conversion-type compounds (such as transition metal oxides, nitrides, sulfides, and phosphides), respectively. Unfortunately, to date, any of the aforementioned materials can hardly meet the ever-growing comprehensive demands for high capacity, high power density, and no security risks due to each intrinsic drawback, including low theoretical capacity for intercalation materials, huge initial capacity loss and fast capacity fading for alloying-reaction anodes, and low initial coulombic efficiency and poor rate capability for conversion-type compounds. In fact, the attempted development of materials over the past few years suggests that one component alone will not suffice [1]. As a result, tremendous research efforts have been on developing new-type nanocomposite anodes because nanocomposites offer special benefits of integrating two or more functional components while preserving small particle size. At present, the attractive nanocomposite anodes can be divided into two main types, that is, carbonaceous and carbon-free nanocomposites. In this section, some potential nanocomposites anode materials will be discussed in detail.

2.2.1 Carbonaceous Nanocomposites

Carbon materials, such as graphite, coke, and graphitic carbons, are the most commonly used anodes in commercial LIBs due to their excellent reversibility for lithium insertion, relatively low inherent cost, and immiscibility with many electrolyte solutions. However, the limited theoretical capacity of 372 mAh/g, corresponding to a maximum uptake of lithium intercalation by forming an intercalation compound of LiC6 (Figure 2.2a) during charge, has seriously hindered their application in large-capacity LIBs. Therefore, to overcome the capacity limits, some carbonaceous nanocomposite anodes, especially for graphene-based, carbon nanotube (CNT)-based, and porous carbon-based nanocomposites, have obtained intensive attention owing to their unique structure and electrochemical properties.

Illustration of structure schematic of LiC6 (a) and graphene (b) [6].

Figure 2.2 The structure schematic of LiC6 (a) and graphene (b) [6].

Graphene (Figure 2.2b) is a one atomic layer of graphite comprising sp²-hybridized carbon atoms arranged in a honeycombed network, which has many promising characteristics, for example, high electrical and thermal conductivity, large surface area(∼2600 m²/g), light weight, and both high mechanical strength and flexibility [7, 8]. As an anode material for LIBs, sheet morphology and large surface area of graphene can increase the interface for Li+ ion interactions and provide a twice theoretical capacity (744 mAh/g) of graphite with maximum Li intercalation to two Li per six-membered carbon ring (Li2C6) during charging [9]. In addition, the unique two-dimensional (2D) conductive network and superior mechanical strength and flexibility also make graphene a desirable component of composite electrode materials in recent years. At present, the graphene-based nanocomposite anodes for LIBs have attracted remarkable attention and demonstrated excellent application prospect. For example, Kung reported a composite anode of Si nanoparticles highly dispersed in graphene sheets, which exhibited high lithium-ion storage capacities and cycling stability, as well as retained a high capacity of more than 2200 mAh/g even after 50 charge–discharge cycles [10]. Liu et al. demonstrated a three-dimensional (3D) macroporous graphene framework (GF)/SnO2 composite (3D SnO2/GFs) by a self-assembly process, which displayed a capacity of 1244 mAh/g for 50 cycles at 100 mA/g and a rate capability of 753 mAh/g for up to 200 cycles at 1000 mA/g [11]. Wei et al. reported a Fe3O4@GS/GF nanocomposite consisted of Fe3O4 nanospheres, three-dimensional graphene (3DG), and GF, which delivered a high reversible capacity of 1059 mAh/g over 150 cycles and good rate capability, exhibiting potential as an anode material for LIBs [11]. Yang et al. constructed an anode electrode based on graphene-encapsulated Co3O4 nanoparticles via electrostatic interaction, which exhibited a high specific capacity of over 1000 mAh/g for more than 130 cycles [12]. Additionally, other novel composites, including CuO–graphene [13], NiO–graphene [13], MoO2–graphene [13], MnO–graphene [13], Li4Ti5O12–graphene [13], MoS2–graphene [14], graphene–SnS2 [14], and so on, also attracted considerable attention as anode materials for LIBs. In these graphene-based composite anode materials, graphene matrix provides flexible and elastic supports to accommodate volume expansion, shortens diffusion length of Li ions, enhances electrical conductivity of the overall electrode, and affords good dispersion of small metalloid, metal, or metal oxide nanoparticles with a strong anchoring effect, ultimately leading to improved reversible capacity, rate capability, and cycling lifespan.

CNT is an allotrope of graphite, which has a one-dimensional (1D) tubular structure with a nanometer scale diameter and typically micrometer scale length [15]. At the molecular level, CNTs can be viewed as a graphene sheet rolled up into a single-walled carbon nanotube (SWCNT) (Figure 2.3a) or a stack of graphene sheets rolled up into a coaxial multiwalled carbon nanotube (MWCNT) (Figure 2.3b). CNTs possess many fascinating properties, including unique 1D tubular structure, large surface area, good thermal and chemical stability, excellent mechanical properties, and high electrical conductivity. Therefore, CNTs have been found attractive in many fields, such as sensors, photocatalysts, and electronic devices. In LIBs, although unique morphology and stable structure make CNTs exhibit a higher capacity (400 mAh/g for MWNTs and 500 mAh/g for SWCNTs) than graphite (maximum theoretical capacity of 372 mAh/g), it is rarely found to utilize pure CNTs as a sole anode material for practical application because of their high irreversible capacity loss in the first cycle and lack of a stable voltage as the battery discharges. Indeed, CNTs are generally used as anode additives to form graphite/CNT composite electrodes in commercial LIBs. At present, more than 50% of cellphones and notebooks are using batteries that contain CNTs [16]. In addition, CNTs also have been explored as an intercalation host or flexible matrix to construct nanocomposite electrodes directly, in which CNTs restrain the volume expansion or pulverization of electrode materials by dispersing other active nanoparticles while retaining high reversible capacities based on synergistic effect. To date, different CNT-based nanocomposite systems, such as metal (Sn, Sb, Bi, etc.)–CNT, metal oxide (SnO, SnO2, MnO2, Fe2O3, Fe3O4, CuO, etc.)–CNT, Si–CNT, and alloy (SnSb, SnCo, SnMn, SnFe, AgFeSn, etc.)–CNT, have been reported as promising anode materials with high charge capacities and good durability for LIBs [17]. For example, SnS2 nanoflake decorated MWCNT (SnS2/MWCNT) hybrid structures exhibited excellent lithium storage performance with a large reversible capacity of 510 mAh/g at current density of 100 mA/g up to 50 cycles, superior cycling performance, and good rate capability as compared with pure SnS2 nanoflakes [18]. 1D hierarchical NiCoO2 nanosheet (NS)@amorphous CNT (NiCoO2@CNT) composites based on the templates and carbon source of polymeric nanotubes (PNTs) delivered an ultrahigh discharge capacity of 1309 mAh/g even after 300 cycles at a current density of 400 mA/g [19]. The hybrid Si–CNTs exhibited high reversible capacity of 2000 mAh/g with a very little fade in capacity of 0.15% per cycle over 25 cycles [17]. According to these aforementioned researches, as crucial carbon materials, CNTs have been playing a significant role in exploiting novel anode materials for LIBs, but achieving hybrid materials with safe, stable, and high capacity characters is still a realistic problem, which needs more intensive researches to verify.

Scheme for carbon nanotube formation by rolling up a 2D graphene sheet (a) or a stack of graphene sheets (b).

Figure 2.3 Schematic representation of the carbon nanotube formation by rolling up a 2D graphene sheet (a) or a stack of graphene sheets (b).

Porous carbons have been considered as improving anode materials for LIBs because they show higher capacities (400–700 mAh/g) than those of graphite (372 mAh/g). Porous carbons not only have higher electronic conductivity (0.1–3 S/cm) but also increase the electrode–electrolyte interfacial area and provide efficient ion transport paths to improve Li+ ion transport kinetics [20]. As an anode material alone, porous carbon suffers from large irreversible capacity during the first cycle. Therefore, in general, porous carbon is incorporated with some active materials (e.g., Fe2O3, Co3O4, NiO, Si, Sn, SnO2, etc.), forming composite electrode, in which it serves as a conductive matrix. For example, Lee's group synthesized Fe3O4 nanocrystals impregnated with mesoporous carbon foams (CFs) using Fe(NO3)3 as raw material that showed more than 780 mAh/g after 50 cycles [20]. Jeong et al. prepared Si nanoparticles embedded in porous nitrogen-doped carbon spheres, which exhibited a high capacity of 1579 mAh/g at 0.1 C rate based on the mass of both Si and C and a capacity kept at 94% after 300 cycles [20]. Li et al. prepared a composite anode of Mn3O4 nanoparticles embedded within ordered mesoporous carbon (CMK-3) through an impregnating route, which displayed a specific capacity of 802 mAh/g and a high coulombic efficiency of up to 99.2% after 50 cycles at a high current density of 100 mA/g [21]. The improving electrochemical performances of these nanocomposite electrodes were attributed to their ordered mesoporous structure, which buffered well against the local volume change during the charge–discharge processes, shortened the transport length for Li+ ion diffusion, and improved the electrolyte infiltration. Simultaneously, electrons can fast transport in the pore walls, and Li+ ions can fast diffuse along the channels, as shown in Figure 2.4.

Scheme for e- and Li+ transports in the composite electrode materials of porous carbons and other active materials.

Figure 2.4 Schematic illustration of e− and Li+ transports in the composite electrode materials of porous carbons and other active materials.

(Zhang et al. 2013 [20]. Reproduced with permission of Elsevier.)

Besides these aforementioned materials, carbonaceous nanocomposite anodes also include other composite material systems consisting of amorphous carbon coating or carbon nanofibers. These carbonaceous nanocomposite anodes for LIBs also demonstrate interesting application prospect. For example, the carbon-coated TiO2 and MnTiO3 (TiO2/MnTiO3@C) ternary hybrid composites as anodes for LIBs exhibited superior cycling and rate performances, as shown in Figure 2.5a–c, arising from the synergistic effect that was created by little volume variation of the TiO2 matrix, high capacity of MnTiO3, and good electrical conductivity of the carbon coating during the charge–discharge processes [22]. The c-GeNW-CNF hybrid materials (carbon-coated single-crystal 1D Ge nanowires on the surface of carbon nanofibers) as flexible and self-supported electrodes for LIBs showed excellent lithium-ion storage properties with a high reversible specific capacity (≈1520 mAh/g after 100 cycles at 0.1 C), superior rate capacity (484 mAh/g at 10 C), and long cycling stability (≈840 mAh/g after 200 cycles at 2 C) [23]. The TiO2/SnO2/carbon hybrid nanofibers synthesized by a simple method based on thermal pyrolysis and oxidation of an as-spun titanium–tin/polyacrylonitrile nanoweb composite in an argon atmosphere displayed a reversible capacity of 442.8 mAh/g for up to 100 cycles and exhibited excellent rate capability [24]. A core–shell nanocomposite composed of a Si core and NiSi2/Ni shell with a carbon layer coating as the outer surface (Si@NiSi2/Ni/C) showed a reversible charge capacity of 1194 mAh/g and 98% retention at a charge–discharge rate of100 mA/g after 105 cycles, which are superior to its Si/C and NiSi2/Si counterparts [25]. A mesoporous Co3O4–carbon nanowire array nanocomposite exhibited an enhanced lithium storage performance with a higher reversible capacity, better cycling stability, and rate capability compared with bare Co3O4 nanowires (Co3O4 NWs) and Co3O4 nanoparticles (Co3O4 NPs), as shown in Figure 2.6 [26]. The ZnFe2O4/carbon nanocomposites composed of ZnFe2O4 nanoparticles with an average size of 16 ± 5 nm encapsulated within the continuous carbon network as anode materials for LIBs exhibited an excellent rate performance with high capacities of 1238, 1198, 1136, 1052, 926, and 521 mAh/g at specific currents of 100, 200, 500, 1000, 2000, and 5000 mA/g, respectively. Moreover, their cycling performance at specific currents of 200 mA/g delivered an outstanding prolonged cycling stability for several hundred cycles [27].

Image described by caption and surrounding text.

Figure 2.5 (a) Long-term cycling performance and coulombic efficiency of TiO2/MnTiO3@C at a current density of 100 mA/g; (b) cycling performance of TiO2, TiO2@C, MnTiO3@C, TiO2/Mn3O4, and TiO2/MnTiO3@C at a current density of 100 mA/g; and (c) rate capabilities of TiO2, TiO2@C, MnTiO3@C, and TiO2/MnTiO3@C.

(Guo et al. 2015 [22]. Reproduced with permission of John Wiley and Sons.)

Plots for Cycling performance of the Co3O4/carbon nanowire/belt array at a current density of 100 mA/g (a) and the rate capability test (b) with bare Co3O4 nanowires and NPs included for comparison.

Figure 2.6 Cycling performance of the Co3O4/carbon nanowire/belt array at a current density of 100 mA/g (a) and the rate capability test (b) with bare Co3O4 nanowires and NPs included for comparison.

(Peng et al. 2016 [26]. Reproduced with permission of Elsevier.)

2.2.2 Carbon-Free Nanocomposites

Carbon-free nanocomposite anodes for LIBs mainly include metal oxide (sulfides, phosphides, etc.)-based and Si (Sn, Al, Ga, Ge, Pb, Sb, etc.)-based materials. For these matrix material themselves, each of them usually has a higher theoretical capacity and security than graphite but suffers from a significant capacity fading or poor rate capability. Therefore, based on these aforementioned materials, to fabricate each nanocomposite by combining with complementary materials in physical and chemical properties is a promising alternative to improving their electrochemical performances. What follows is a brief overview of some researches for these carbon-free nanocomposite anodes based on different composite systems.

Metal oxides (sulfides, phosphides, etc.) include insertion-type (such as Li4Ti5O12, TiO2) [28], alloying-type (such as SnO2, SnO) [28], and conversion-type (such as Co3O4, Fe2O3, SnS2, MnP4) materials [28]. In the past few years, these metal oxide (sulfides, phosphides, etc.)-based composites also have been explored and evaluated as anode materials for LIBs. For example, Wu et al. synthesized a Co3O4/α-Fe2O3 core–shell structured nanowire array by a two-step hydrothermal method. In the nanocomposites, Fe2O3 branches not only serve as volume spacers between neighboring Co3O4 nanowires to maintain electrolyte penetration but also reduce aggregation during Li+ intercalation. When examined in LIBs, the Co3O4/α-Fe2O3 core–shell structured nanowire anode exhibited an almost constant capacity from the second cycle and retained a value of 980 mAh/g after 60 cycles at 100 mA/g [29]. Shi and Lu fabricated crystalline echinus-like SnO2@SnS2 shell–shell structured nanospheres (SSNs) by a hydrothermal method based on nanoscale Kirkendall effect. When tested as anode materials for LIBs, the echinus-like SnO2@SnS2 SSN displays an initial capacity of 1558 mAh/g and retains a reversible capacity of 548 mAh/g after 100 cycles at a current density of 100 mA/g, which are much higher than that of pure SnO2 (403 mAh/g of 60th cycle) at the same current rate. Meanwhile, SnO2@SnS2 nanocomposites also display excellent rate capability with a reversible capacity of 443.4 mAh/g even at the current rate of 5 C (about 5 A/g) [30]. Ji et al. designed nanostructured SnO2@TiO2 core–shell composites where the thin crystalline TiO2 has been converted into conductive LiyTi1−yO2 shell on a core aggregated from SnO2 nanoparticles, producing reversible Li-ion storage hosts furnished with their own current collectors. The core–shell nanocomposite exhibited high electrical conductivity even after expansion and contraction of the active material during cycling. It also delivered a charge (delithiation) capacity of 735 mAh/g at 1 A/g in the first cycle with 69% capacity retention after 30 cycles without any conductive additives in the voltage range of 0.005–2.0 V. By increasing the current density to 3 A/g, the charge capacity was 649 mAh/g in the first cycle with 58% capacity retention after 30 cycles, indicating the efficacy in minimizing the capacity loss in cycling at high current densities [31].

The anode materials (Si, Sn, Al, Ga, Ge, Pb, Sb, etc.) based on alloying-reaction mechanism also display some higher theoretical capacities than graphite; for instance, the theoretical capacity of metal tin is 994 mAh/g corresponding to the formation of Li22Sn5. The theoretical capacity of silicon can reach up to 4200 mAh/g based on the fully alloyed form of Li22Si4, which is the highest capacity among all investigated anode materials to date. Therefore, Si has been recognized as one of the most promising anode candidates for LIBs. At present, Si-based nanocomposite anodes for LIBs have gained extensive attention and research. Besides incorporating with carbonaceous materials, Si also can fabricate carbon-free nanocomposites composed of other components, such as metal, metal oxides, metal nitrides, and metal sulfides. For example, Zhou et al. synthesized a unique heteronanostructure that consists of 2D TiSi2 nanonets and particulate Si coating (Figure 2.7a) [31]. When tested as the anode material for lithium-ion storage at a charge–discharge rate of 8400 mA/g, this heteronanostructure delivered specific capacities of over 1000 mAh/g and showed only an average of 0.1% capacity fade per cycle between the 20th and the 100th cycles (Figure 2.7b). Park et al. reported a simple synthesis of TixSiy-coated Si nanopowders via a silicothermic reduction process, in which Si acted as the reducing agent and titanium oxide as a source material of Ti [32]. The TixSiy-coated Si nanocomposite anode for LIBs demonstrated a high reversible capacity (1470 mAh/g) and high rate capability (1150 mAh/g at 20 C rate) and even showed significantly improved high thermal stability compared with bare Si electrodes. Sakaguchi and coworkers fabricated a series of thick-film nanocomposite electrodes by coating Si particles with Ni, Ni–Sn, and Ni–P, respectively [33]. Among them, the Ni-coated and Ni3P-coated Si electrodes exhibited discharge capacities of 580 and 750 mAh/g at the 1000th cycle, respectively. Except for the aforementioned materials, some nanocomposite anodes, such as Si/TiN [31], TiO2/Si [31], LaSi2/Si [31], and Si/Ge [31] nanocomposites, Ni-core/Si-shell [31] and Cu-wrapped Si [31] nanowires, and so on, have also been reported for their excellent electrochemical performances and promising application prospect.

Image described by caption and surrounding text.

Figure 2.7 (a) Schematic of the Si/TiSi2 heteronanostructure. SiNPs are deposited on highly conductive TiSi2 nanonets to act as the active component for Li storage; (b) a low-magnification TEM picture manifests the particulate nature of the Si coating on TiSi2 nanonets; and (c) charge capacity and coulombic efficiency of the Si/TiSi2 heteronanostructure with 8400 mA/ g charge–discharge rate tested between 0.15 and 3.00 V.

(Liang et al. 2014 [31]. Reproduced with permission of Elsevier.)

Summarizing the aforementioned research studies, the enhanced electrochemical performances of carbon-free nanocomposite anodes also can be obtained arising from their stable nanostructure and the synergistic effect of different functional components. Therefore, for exploring new-type anodes with high capacity, high power density, and no security risks, it is a major development direction to fabricate nanocomposite anodes composed of materials with complementary properties and stable nanostructure.

2.3 Advanced Nanocomposites as Cathode Materials for LIBs

With the increasing consumption of energy and its associated price and the environmental deterioration, significant progress has been made in the development of renewable energy technologies, which used to be marginalized. This new technology makes alternative energy more practical and price competitive with fossil fuel; it is expected that the coming decades will usher in a long expected transition away from oil and gasoline as our primary fuel [34]. Efficient and environmentally friendly energy conversion and storage devices have attracted great attention in recent years. At present, rechargeable LIBs that have already been intensively studied as power supplies of electric vehicles (EVs) and hybrid electric vehicles (HEVs) have been considered as one of the most competitive energy storage devices due to their high energy density, long lifetime, and sustainable and renewable characteristics. Among them, the electrode materials are typically intercalation materials, which can undergo oxidation to higher valences when lithium is removed. Intercalation refers to the reversible intercalation of mobile guest species (atoms, molecules, or ions) into a crystalline host lattice that contains an interconnected system of empty lattice sites of appropriate size, while the structural integrity of the host lattice is formally conserved [35]. The principal concept of LIBs could be described as combination of anode with another lithium intercalation material (cathode) having a more positive redox potential [34]. And anode and cathode are separated by electrolyte. During the charge process, the lithium ions are released by the cathode and intercalated at the anode. According to previous researches, the electrode materials must fulfill three fundamental requirements: (i) a high specific charge and discharge density, (ii) a high cell voltage, and (iii) a high reversibility of electrochemical reactions at both cathodes and anodes. And the reaction in cathode can be described as follows:

equation

Cathode materials are typically oxides of transition metals, which can undergo oxidation to higher valences when lithium is removed [35]. While oxidation of the transition metal can maintain charge neutrality in the compound, large compositional changes often lead to phase changes, so the stability of crystal structures is one of the issues. Normally, during the rapid charge–discharge process, exchange of lithium ions with the electrolyte occurs at the electrode–electrolyte interface, so cathode performance depends critically on the electrode microstructure and morphology, as well as on the inherent electrochemical properties of the cathode material. Among the LIB components, the cathode materials have attracted increasing attention due to their significant impact on battery capacity, cycle life, safety, and cost structure [36]. However, the power density is relatively low because of a large polarization during the rapid charge–discharge process. The phenomenon leads to the destruction of anode structure and rapid degrading of electrochemical performances. To improve the cycling and rate performances, various strategies have been proposed and tried out containing downsizing particles, carbon coating, and constructing hybrid nanocomposites. In essence, benefiting from increasing the electrical conductivity and buffering the volume expansion, these strategies have brought about evident effects on the improvement of cycling stability. Nanostructures could offer huge surface area, short mass and charge diffusion distance, and freedom for volume change during charge–discharge cycles and thus are expected to improve the lithium-ion intercalation properties [34]. Another strategy relies mainly on improving the electrical conductivity of the electrode materials. Coating and doping with supervalent cations are all methods that are beneficial to samples' electrical conductivity or bulk conductivity. Coating with Ag, Cu, carbon, or conducting polymers have all been explored by previous researches. Although metal additives may improve electrodes' conductivity effectively, it is very difficult to achieve a uniform metal dispersion on the surface. By comparison, carbon coating has been attractive with respect to its high conductivity, its low cost, and its simplicity of introduction.

2.3.1 Traditional Cathode

Ever since the idea of a rechargeable lithium cell based on Li intercalation reactions was initiated in the early 1970s, numerous lithium intercalation electrodes are much less developed than anodes [37]. There are two main types of cathode materials. One comprises layered compounds with an anion close-packed lattice; transition metal cations occupy alternate layers between the anion sheets, and lithium ions are intercalated into remaining empty layers (i.e. LiCoO2, LiMnO2, and LiNixMnxCo1–2xO2). Because of their more compact lattices, this class of materials has the inherent advantage of high energy density. Due to their high capacity, LiCoO2 has been widely used in batteries for portable electronics since its introduction to the market by Sony Inc. in 1991 [36]. The other group of cathode materials has more open structures, such as transition metal phosphates (e.g., LiFePO4) and vanadium oxides.

2.3.1.1 Lithium Transition Metal Oxides

As mentioned in the following, LiCoO2 has been widely used since 1991 and forms a distorted rock-salt structure where the cations order in alternating (1 1 1) planes. Although it is a successful choice for cathode material and has been largely used in small batteries, alternatives are being developed to lower cost and improve stability. Meanwhile, its use in large-size batteries has been vetoed by safety concerns. All of these concerns prohibit the large-scale applications of LiCoO2 in EV or HEV. LiNiO2 and LiMn2O4 are also types of intercalation materials. Specifically, this material can be easily synthesized using both solid-state and chemical approaches [38, 39]. LiCoO2 system has been studied extensively far. LixCoO2 exhibits excellent cyclability at room temperature for 1 > x > 0.5. Therefore, the specific capacity of the material is limited to the range of 137–140 mAh/g [34]. In addition, LiCoO2 is not as stable as other potential electrode materials and can undergo performance degradation or failure when overcharged [40]. However it is lower in cost and has a higher energy density (15% higher by volume, 20% higher by weight) [41], but is less stable and ordered, as compared with LiCoO2. And the reversible capacity of LixNiO2 is higher than that of LixCoO2, since the amount of lithium that can be extracted/intercalated during redox cycles is around 0.55, in comparison with 0.5 for LiCoO2, allowing the specific capacity to be more than 150 mAh/g with appropriate cyclability [34]. LiMn2O4 is the third most popular cathode material for LIBs. In comparison with other two electrodes, it possesses essential advantages of less toxicity and having an abundant material source. In principle, it permits the intercalation/extraction of lithium ions in the range of 0 < x < 2 [34]. Simultaneously the intercalation of lithium ions effectively decreases the average valance of manganese ions and leads to a pronounced cooperative Jahn–Teller effect, in which the cubic spinel crystal becomes distorted tetragonal with a c/a ≈ 1.16 and the volume of the unit cell increases by 6.5%. This high c/a ratio causes a low capacity, restricted to 120–125 mAh/g. So it has lower capacity as compared with the cathode of LiNiO2 and LiCoO2.

2.3.1.2 Vanadium Oxide

Vanadium oxide forms layered compounds and vanadium can have multiple valences, so vanadium oxides have been used as electrode materials. For Li-ion intercalation applications, vanadium oxide offers the essential advantages of low cost, an abundant source, easy synthesis, and high energy density. Orthorhombic crystalline V2O5 consists of layers of VO5 square pyramids that share edges and corners [42]. The reversible electrochemical lithium intercalation into V2O5 at room temperature was first reported in 1975 [35]. Electrochemical lithium intercalation of this type of materials can be described as follows:

equation

In addition to crystalline V2O5, high Li intercalation capacity has been reported for hydrated vanadium pentoxides (V2O5·nH2O). V2O5·nH2O xerogels are composed of ribbonlike particles and display lamellar ordering, with water molecules intercalated between the layers [43]. These water molecules expand the distance between the layers, and the intercalation capacities of V2O5·nH2O xerogels are enhanced as a result. Like V2O5, the monoclinic LiV3O8 [44–48] has also been used as cathode. These electrodes have high capacities but relatively low voltages (typically 3 V or less) as compared with compounds discussed previously.

2.3.1.3 Lithium Phosphates

Another promising class of cathode materials is phosphates with olivine structure (Pnma). Since their discovery in 1997 [49], great progress has been made in improving and understanding the structure, electrochemical performance, and synthesis techniques of LiFePO4. The olivine structure of LiFePO4 crystal consists of a polyoxyanionic framework containing LiO6 octahedra, FeO6 octahedra, and PO4 tetrahedra [000]. Strong P–O covalent bonds in (PO4)³− polyanion stabilize the oxygen when fully charged and avoid O2 release at high states of charge, making LiFePO4 an excellent, stable, and safe cathode material (Figure 2.8) [50].

Scheme for olivine structure is stable during Li-ion insertion and extraction.

Figure 2.8 The olivine structure is stable during Li-ion insertion and extraction.

(Wang and Sun 2012 [36]. Reproduced with permission of Royal Society of Chemistry.)

In LiFePO4, the cation arrangement differs from that in layered LiCoO2 or spinel LiMn2O4. The divalent Fe²+ ions occupy the corner-shared octahedral. The P⁵+ is located in tetrahedral sites and Li+ resides in chains of edge-shared octahedral. The skeleton of PO4 polyanions is very stable thermally. LiFePO4 is stable up to 400 °C, while LiCoO2 starts to decompose at 250 °C [51, 52]. The high lattice stability results in excellent cyclic performance and cycling safety for LiFePO4. However, the strong covalent oxygen bonds also lead to low ionic diffusivity and poor electronic conductivity (∼10−9 S/cm) [53], which is much lower than those of LiCoO2 (∼10−3 S/cm) and LiMn2O4 (∼10−9 S/cm) [36]. Other phosphates used for cathodes in LIBs include LiMnPO4 [54], LiCoPO4 [55], and Li3V2(PO4)3 [56]. They have higher open-circuit voltages than LiFePO4 but with lower capacities.

Another material having an olivine structure like LiFePO4 is the orthosilicate Li2MSiO4 (M = Fe, Mn, etc.). Besides the high safety arising from the strong Si–O covalent bond and environmental benignancy and low cost form the characteristics of Fe, Mn, and Si elements, Li2MSiO4 may theoretically allow two-electron exchange per formula unit to meet the high energy density demands [57]. All of these made them a new research direction.

2.3.2 Advanced Nanocomposites as Cathode Materials

As mentioned in the following text, the current commercial cathode material comes across a series of challenges that need to be solved with the development of modern technology. Here we would like to introduce several methods using technology to improve the electrochemical performance of traditional cathode materials.

2.3.2.1 Coating

Nowadays, coating is regarded as a promising way to improving the electrochemical performance of the electrodes. Including metal, conductive polymers, and carbon, a large number of works have been done.

Kim et al. made an extensive study on the effect of the MPO4 (M = Al, Fe, SrH, and Ce) nanoparticle coating on LiCoO2 cathode materials [58]. They found that the extent of the coating coverage is affected by the nanoparticle size and morphology despite having the same coating concentration and annealing temperature. Smaller nanoparticles of AlPO4 or FePO4 with a size less than 20 nm fully encapsulate LiCoO2, whereas CePO4 particles with a size larger than 150 nm or whisker-shaped SrHPO4 only partially cover LiCoO2. Not surprisingly, LiCoO2 fully covered by AlPO4 or FePO4 exhibits the highest intercalation capacity of 230 mAh/g in a voltage range of 4.8 and 3 V at a rate of 0.1 °C. The AlPO4-coated LiCoO2 also shows the best capacity retention. Nevertheless, CePO4- and SrHPO4-coated cathodes show better capacity retention than the FePO4-coated cathode at 90 °C, which is attributed to the continuous Fe metal ion dissolution at this temperature. The improvement in the electrochemical performance of the coated cathode is ascribed to the suppression of cobalt dissolution and the nonuniform distribution of local strain by the coating layer. In a further investigation of AlPO4-coated LiCoO2, the electrochemical properties of AlPO4 nanoparticle-coated LiCoO2 at various cutoff voltages were found to depend on the annealing temperature [59]. The AlPO4-coated cathodes exhibit excellent electrochemical performance with high cutoff voltages, larger than 4.6 V, when annealed at 600 and 700 °C, while such cathodes annealed at 400 °C show a lower capacity and poorer rate capability. However, the AlPO4-coated LiCoO2 annealed at 400 °C showed optimal capacity retention [60]. Figure 2.9 shows typical TEM images of AlPO4-coated LiCoO2 deposited at room temperature, 400, and 700 °C. A continuous layer of AlPO4 with thickness of about 100 nm is coated on the surface of LiCoO2, as shown in Figure 2.9a. The coating layer deposited at room temperature is amorphous (Figure 2.9b). The coating deposited at 400 °C is composed of nanocrystals with size in the range of 3–5 nm (Figure 2.9c), and the coating deposited at 700 °C consists of nanocrystals about 20–30 nm in size (Figure 2.9d). The dependence of electrochemical properties on the annealing temperature can be explained by the effect of temperature on the nanostructures of the coating layer and the interdiffusion at the interface between the coating layer and the LiCoO2 cathode.

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Figure 2.9 (a) Cross-sectional TEM images of AlPO4-coated LiCoO2. A about 100 nm thick AlPO4 continuous layer is coated on LiCoO2. High-resolution images of the AlPO4-coated LiCoO2 at (b) room temperature, (c) 400 °C, and (d) 700 °C.

(Adapted with permission from Ref. [60].)

Meanwhile, carbon coating is also one of the most important techniques used to improve the electrodes' capacity. The main role of carbon here is to enhance the electronic conductivity and somehow keep the structure stability of the electrode. Jiang et al. [61] synthesized carbon-coated Li3V2(PO4)3 through a poly(vinyl alcohol) (PVA)-assisted sol–gel method. Figure 2.10 shows the HRTEM image of an individual Li3V2(PO4)3 particle, from which a surface film could be clearly seen. This surface film could be divided into two sections. The first section with a thickness of 60 Å is attributed to the surface carbon layer, while the second section with a thickness of 20 Å could be regarded as the interface region between the Li3V2(PO4)3 body and the surface carbon. In the body part of the particle, the clear lattice fringes with interplanar distance of 5.4 Å is in accordance with the d-spacing of the (1 1 1) planes of Li3V2(PO4)3. No lattice fringes are observed from the surface carbon layer, indicating its amorphous nature. And after 80 cycles at the current rate of 1 C, the samples still exhibited a capacity of around 100 mAh/g. Ren et al. [62] also prepared a core–shell Li3V2(PO4)3@C cathode materials via a sol–gel route followed by hydrothermal treatment. TEM images of Li3V2(PO4)3@C are shown in Figure 2.11. The Li3V2(PO4)3 particle is encapsulated with amorphous carbon shell, similar to carbon-coated LiMn2O4 or silicon [63, 64]. It can be seen that the thickness of the carbon shell is about 10 nm.

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Figure 2.10 HRTEM image of the material.

(Jiang et al. 2010 [61]. Reproduced with permission of Elsevier.)

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Figure 2.11 TEM image of Li3V2(PO4)3@C composite (a) and the corresponding high-resolution TEM image (b) [62].

The thickness of the carbon shell can be controlled by the hydrothermal reaction time and quantity of glucose. The high-resolution TEM image in Figure 2.11b clearly demonstrates the coexistence of two phases, that is, Li3V2(PO4)3 and amorphous carbon. The discharge capacity of Li3V2(PO4)3@C is 125.9 mAh/g after 50 cycles at a current density of 28 mA/g, much higher than pure Li3V2(PO4)3 (68.1 mAh/g after 30 cycles), so the retention rate in discharge capacities is 98.5% for Li3V2(PO4)3@C after 50 cycles but only 62.8% for pure Li3V2(PO4)3 after 30 cycles. At a higher current density (140 mA/g, 1 C rate), the retention rate still attains 96.2% for Li3V2(PO4)3@C. Electronically conducting RuO2 was used as an oxidic nanoscale interconnect for carbon-containing porous LiFePO4 to improve electrode performance [64]. By using a low-temperature solution infiltration approach, RuO2 was successfully deposited on carbon-containing porous LiFePO4 with an average size of 50 nm. As shown in Figure 2.12, nanometer-sized RuO2 with a particle size of about 5 nm was directly deposited on the bare surface of LiFePO4, as was observed in extensive HRTEM studies. In addition, some RuO2 nanoparticles grew in a coherent manner on the exposed LiFePO4 facets. Nanosized RuO2 as an oxide adheres well with oxides such as LiFePO4 while simultaneously assuring good contact with carbon. Hence, RuO2 repairs incomplete carbon network in porous LiFePO4 and thus improves the kinetics and rate capability of the composite.

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Figure 2.12 (a, b) Typical HRTEM images of C-LiFePO4 after RuO2 coating. (c) Scheme showing the repair of the electronically conducting network of carbon on porous LiFePO4 by nanometer-sized RuO2.

2.3.2.2 Composite with Carbon Nanotubes of Graphene

Recently, an increasing attention has been paid on CNTs and graphene due to their low density, high tensile strength, high electrical conductivity, and so on. Until now, various researches have been reported on CNTs or graphene composites with each cathode. For example, CNTs and LiFePO4 composites with a 3D network wiring were synthesized via a novel preparation in order to improve electronic conducting and rate capacity [65]. The web structure can improve electron transport and electrochemical activity effectively. The initial discharge capacity was increased to 155 mAh/g at C/10 rate (0.05 mA/cm²) and 146 mAh/g at 1 C rate. The comparative investigation on MWCNTs and acetylene black as a conducting additive in LiFePO4 proved that MWCNT addition was an effective way to increase rate capability and cycle efficiency. Jin et al. [66] reported LiFePO4–MWCNT composites prepared by a hydrothermal method followed by ball-milling and heat treating. Luo et al. developed an ultrasonication and co-deposition technique and fabricate binder-free electrodes using super-aligned carbon nanotubes (SACNTs) [55]. Figure 2.13 illustrates the 3D conductive and flexible structural network in which active materials can be embedded. All the cathodes with 1–5 wt% SACNT composites showed high specific capacities (144.1–151.4 mAh/g at 0.1 °C) and excellent cycling performance with capacity retention more than 95% after 50 cycles.

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Figure 2.13 Schematic of the structures of (a) the binder-free LiCoO2-SACNT cathode and (b) the classical LiCoO2-super P cathode. In the binder-free LiCoO2-SACNT cathode, LiCoO2 particles are uniformly distributed in the continuous SACNT network. Without the hinder of insulating binder, both electron and ion transfer are greatly improved. While for the LiCoO2-super P cathode, super P powders agglomerate heavily and are separated from LiCoO2 particles by the binder (polytetrafluoroethylene, PTFE).

(Luo et al. 2012 [55]. Reproduced with permission of John Wiley and Sons.)

Meanwhile, Yang et al. developed a novel 3D hierarchical LiFePO4–graphene hybrid cathode with a porous structure through a facile template-free sol–gel route [67]. After 100 cycles, LFP/G still delivered a capacity of 146 mAh/g, which is more than 1.4 times the capacity (104 mAh/g) of LFP, demonstrating that the incorporated graphene greatly enhances the specific capacity throughout the cycle process, as shown in Figure 2.14 [68]. After 2 h of annealing (Figure 2.14a), very fine LFP nanoparticles were dispersed homogeneously on unfolded graphene NSs. As annealing time progressed to 6 h (Figure 2.14b), the nanoparticles had grown larger and were uniformly dispersed on the unfolded graphene. When the annealing time was increased to 12 h (Figure 2.14c), the graphene NSs were crimped and connected to form a conducting 3D network, and spherical-shaped LFP nanoparticles were anchored to the graphene matrix. The nanoparticles had grown larger, with sizes up to 100 nm. A further increase of the annealing time to 24 h (Figure 2.14d) resulted in larger and irregular particles. As shown by TEM images (insets in Figure 2.14a–d), with an increase in the annealing time, the particle size increased as follows: 3 nm for 2 h of annealing (LFP–UG-2), 5 nm for 6 h of annealing (LFP–UG-6), 70 nm for 12 h of annealing (LFP–UG-12), and 200 nm for 24 h of annealing (LFP–UG-24). The results presented earlier reveal that the morphology and the size of the LFP nanoparticles can be tailored by adjusting the annealing time. The longer the annealing time, the larger the LFP particle size.

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Figure 2.14 SEM and TEM (inset) images of LiFePO4–unfolded graphene nanocomposites obtained with different annealing time: (a) 2 h, (b) 6 h, (c) 12 h, and (d) 24 h.

(Yang et al. 2013 [68]. Reproduced with permission of Royal Society of Chemistry.)

2.3.2.3 Doping

The performance of cathode materials can be improved by doping, but the interpretation of doping effects can be complicated by the interrelations between doping and microstructure and morphology, since the microstructure formed can be affected by the dopant additions. Some examples in which the effects of doping on the electrochemical properties of the electrode are attributed to the effects of the dopant on the cathode microstructure or morphology rather than the effects on the material properties include cesium doping of LiMn2O4 [69], chromium doping of LiFePO4 [70] and Li3V2(PO4)3 [71], copper doping of phosphates [72, 73], and aluminum doping of LiCoO2 [74]. With that caveat, the effects of dopant additions on the performance of cathode materials are discussed as follows. Figure 2.15 shows the powder X-ray diffraction (XRD) patterns of the Li2Ru1−yMnyO3 solid solution [75]. All XRD look similar except for a splitting of the peak at 2θ = 45° for the Ru-rich compositions (y = 0 and y = 0.2 in Li2Ru1−yMnyO3), resulting from a distortion of the rock-salt oxygen framework. This peak corresponds to the (104) Bragg reflection in the averaged R3̅m cell (shown in Figure 2.8) and is a good sensor to see which structural model (Li2MnO3, C2/m space group or Li2RuO3, C2/c space group) is best suited to describe the structure. As a result, ruthenium substitution in Li2MnO3 drastically improves its electrochemical performance, and one believes this positive effect to be related to the increased electronic conductivity of the substituted compounds, hence enabling better kinetics of the Li uptake removal process.

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Figure 2.15 Powder X-ray diffraction patterns of the Li2Ru1−yMnyO3 solid solution. Compositions with 100% Ru (y = 0) and 80% Ru (y = 0.2) present a distortion of the rock-salt oxygen framework and were refined in C2/c.

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