Scholarly article on topic 'Searching for electrode materials with high electrochemical reactivity'

Searching for electrode materials with high electrochemical reactivity Academic research paper on "Materials engineering"

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{"Materials design" / "Electrode construction" / "Lithium ion battery" / Supercapacitor / "Integrated electrode"}

Abstract of research paper on Materials engineering, author of scientific article — Kunfeng Chen, Dongfeng Xue

Abstract The most key materials in electrochemical energy storage devices are electrode materials mainly including inorganic cathode and anode materials. However, inorganic electroactive materials often suffer from low conductivity, low capacity, low cycling life. In order to solve these problems, much research work focused on the design of electrode materials and the construction of novel electrode structures in the field of electrochemical energy storage. In this review, we reported the latest development of the design principles of the high-performance electrodes for lithium ion batteries and supercapacitors. We mainly discussed three kinds of examples, blended electrode, integrated electrode and in-situ formed electrode, to display the principles of electrode materials design and electrode construction. The new-developed integrated electrode and in-situ formed electrode maybe the promising candidates for next-generation high-performance energy storage devices. In addition, we conclude this review with personal perspectives on the directions toward which future research in this field might take.

Academic research paper on topic "Searching for electrode materials with high electrochemical reactivity"

Accepted Manuscript

Searching for electrode materials with high electrochemical reactivity Kunfeng Chen, Dongfeng Xue

PII: S2352-8478(15)00048-9

DOI: 10.1016/j.jmat.2015.07.001

Reference: JMAT 22

To appear in: Journal of Materiomics

Received Date: 19 May 2015 Revised Date: 24 June 2015 Accepted Date: 1 July 2015

Materiomics

Please cite this article as: Chen K, Xue D, Searching for electrode materials with high electrochemical reactivity, Journal of Materiomics (2015), doi: 10.1016/j.jmat.2015.07.001.

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Searching for electrode materials with high electrochemical reactivity

Kunfeng Chen and Dongfeng Xue* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China *Corresponding authors: dongfeng@ciac.ac.cn

The most key materials in electrochemical energy storage devices are electrode materials mainly including inorganic cathode and anode materials. However, inorganic electroactive materials often suffer from low conductivity, low capacity, low cycling life. In order to solve these problems, much research work focused on the design of electrode materials and the construction of novel electrode structures in the field of electrochemical energy storage. In this review, we reported the latest development of the design principles of the high-performance electrodes for lithium ion batteries and supercapacitors. We mainly discussed three kinds of examples, blended electrode, integrated electrode and in-situ formed electrode, to display the principles of electrode materials design and electrode construction. The new-developed integrated electrode and in-situ formed electrode maybe the promising candidates for next-generation high-performance energy storage devices. In addition, we conclude this review with personal perspectives on the directions toward which future research in this field might take.

Keywords: materials design; electrode construction; lithium ion battery; supercapacitor; integrated electrode

Received date: 20May 2015; Revised date: 28 June 2015; Accepted date: 10 July 2015

1 Introduction

Electrical energy storage is one of key routes to solve energy challenges that our society is facing, which can be used in transportation and consumer electronics [1,2]. The rechargeable electrochemical energy storage devices mainly include lithium-ion batteries, supercapacitors, sodium-ion batteries, metal-air batteries used in mobile phone, laptop, electric vehicles, etc.[3-5] In battery systems, the charge storage mechanisms include the insertion of secondary species into solid electrodes, alloying and conversion reaction with secondary species (H+, Li+, Na+, K+) [6]. Electrode materials with alloying and conversion mechanism often exhibit a much higher specific capacity than that of the intercalation electrode materials. Supercapacitors can provide larger power density and longer cycling ability than rechargeable batteries. The charge storage mechanisms of supercapacitors mainly originate from electrical double-layered capacitance (EDLC) at the electrode/electrolyte interface and surface Faradaic redox reactions at the surface of electrode materials (named pseudocapacitance) [7]. To increase the energy density and charge-discharge rate of these devices, the electrode materials should have high electrochemical activity, and can deliver more charge with fast electron and ion transfer/diffusion rate [8]. In order to achieve these goals, the design of active electrode materials with high surface area, short diffusion length, more active sites and high conductivity is more needed. For example, hollow, 1D, 2D, porous, nanosize structural materials have been designed to satisfy above demands [9-11]. However, the pure inorganic electrode materials often suffer from poor cyclability and low capacity, which is mainly caused by their low

conductivity and by the large volume change during the charge-discharge cycles [12,13]. In an attempt to solve this problem, powdered electrode materials are often blended with conductive carbon and polymer binders to coat on the metal current collector, which are the standard industrial electrode preparation methods [13,14]. However, the blended electrodes often suffer from phase-separation and the use of many non-active materials.

The design of electrode construction forms is another method to improve the electrochemical performance of these electrochemical energy storage devices. For example, the integrated active materials/current-collector electrode can show two main advantages: (1) the absence of both binder and carbon black is favorable for the direct study of their morphology and phase evolution upon electrochemical cycling (2) the integrated active materials on current collector can effectively enhance the electronic conductivity, and shorten electron transfer length [15,16]. The in-situ formed electrode in practical application environment can display high electrochemical activity because it can form active structured electrode materials compared with materials synthesis in the separate environment.

Although the scalable fabrication and the cost of electrode materials are also the key issues for the scale practical application of energy storage devices, in this review, we mainly focus on the study of the design principle of electrode material and electrode structure in electrochemical energy storage devices. Blended electrode, integrated electrode and in-situ formed electrode are used as three examples to review the recent progress about electrode material design and electrode construction for

high-performance electrochemical energy storage devices, mainly including lithium ion batteries and supercapacitors.

2 Blended electrode

The blended electrodes, which are often used in commercial electrochemical energy storage devices, are prepared by blending electroactive materials (i.e., MnO2, CuO, Fe2O3, LiMn2O4, LiFeO2) with conductive carbon and polymer binders and coating on the current collector [17-20]. The electrochemical performance of the blended electrode is mainly dependent on the electroactive materials. Recently, the design of high-performance electrode materials mainly focuses on the control of their morphologies, sizes and structures by different synthesis methods. In this section, we mainly discussed Mn-, Cu- and Fe-based blended electrodes for lithium ion batteries and supercapacitors.

2.1 Mn-based blended electrode

Mn-based materials are one kind of important electrode materials used in batteries and supercapacitors. For example, MnO2 materials are served as cathode in Zn-MnO2, Li-MnO2 primary batteries, while MnO2, Mn3O4, Mn2O3 materials are used as anode in lithium ion batteries [21,22]. LiMn2O4 materials are served as cathode in lithium ion batteries [23,24]. MnO2 are one of the most studied electrode materials for supercapacitors [20]. In order to improve their electrochemical performance, Mn-based materials with different structures, morphologies, sizes, are designed using

various methods, such as hydrothermal/solvothermal, sol-gel, electrochemical, CVD strategies [20].

It is known that MnCb can exist in different structural forms, a-, (3-, y-, 8-, £-, OMS-6, and todorokite-MnCh, when the basic structural unit ([MnOô] octahedron) is linked in different ways (Fig. 1) [17, 25]. Based on the different [MnOô] links, MnO? can be divided into three categories: the chain-like tunnel structure such as a-, (3-, and y-types, the sheet or layered structure such as ô-MnCh, and the 3D structure such as À-type. Thermodynamic morphologies of MnCb can be obtained according to chemical bonding theory of single crystal on the basis of their crystallographic characteristics. In addition, kinetic control can be used to synthesize various MnCb morphologies.

Fig. 1 Crystallographic structures of oc-, (3-, y-, 8-, £-, OMS-6, and todorokite-MnCb.

Reprinted with permission from Ref. [17]. Copyright 2015 Royal Society of Chemistry.

The crystal structures and morphologies of MnO2 can influence their electrochemical performances as lithium ion batteries or supercapacitors. Recently, polymorphs of a-, b-, g-, and 8-MnO2 nanostructures with sphere-, rod-, wire-, plate-and flower-like morphologies have been synthesized by a simple redox reaction between KMnO4 and NaNO2 aqueous solution [26]. Served as supercapacitor, a maximum specific capacitance of 200 F g-1 was obtained for poorly crystallized a-MnO2 at a current density of 1 A g-1. It should be noted that the columbic efficiency of MnO2 supercapacitor can approach 100%. For different crystallographic MnO2 phases, their specific capacitance values increase in the order: b < g < 8 < a, meanwhile, for any particular MnO2 phase, their capacitances decrease with increasing crystalline nature and particle size [26]. A proportional relationship between the specific capacitance of MnO2 and the percentage of effective Mn centers has been established on the basis of a tunnel structure-crystallization behavior correlation [25]. A quantitative relationship between the effective Mn centers at the surfaces and in the tunnels can distinguish the specific capacitance values that arise from the adsorption/desorption and insertion/extraction processes, respectively, of different MnO2 crystallographic forms in the Faradaic charge storage.

Table 1. Discharge capacities of MnO2 samples obtained by hydrothermal only, microwave only

and coupled microwave-hydrothermal method in the designed MnCl2-KMnO4 aqueous system as

lithium-ion batteries. Reprinted with permission from Ref. [27]. Copyright 2013 American Chemical Society.

Anode discharge capacity / Cathode discharge

Reaction conditions Composition mAh g 1 capacity / mAh g- 1

1st 2nd 15 th 1st 2nd 15th

Hydrothermal only at Y 1259.4 354.4 84.5 168.3 137.9 88.7

160°C for 1h

Microwave only for 10 Y 1244.2 256.4 26.9 181.9 175.1 129.2

Microwave-hydrother Y(dominant) 1379.7 636.3 286.7 151.1 111.9 77.6

mal at 160°C for

Microwave-hydrother ß(dominant) 1505.4 446.9 196.0 57.9 39.5 28.3

mal at 160°C for 1h

In addition, a-, b-, and y- MnO2 have been crystallized by a controllable redox process in MnCl2-KMnO4 aqueous solution with the use of coupled microwave-hydrothermal technique [27]. The microwave-driven redox reaction and hydrothermal-driven crystallization process can quicken the finding rate of high-performance electrode materials [28]. The as-crystallized a-, b-, and y-phase MnO2 samples can show interesting electrochemical performances for lithium-ion batteries (Table 1). The electrochemical performance data indicate that the mixture phase of b- and y-type MnO2 show the largest discharge capacities, such as 220.7, 205.9 and 167.8 mAh g-1 at different synthesis conditions after 30 discharge-charge cycles as shown in Table 1 and Fig. 2. The discharge capacities of pure b-MnO2 are less than 100 mAh g-1 after 30 cycles, while the capacity of a-MnO2 is 116.9 mAh g-1. After 5th discharge-charge cycles, the columbic efficiency of MnO2 electrode can approach 100%. The mix-phase of MnO2 can provide more grain boundary, phase

boundary for Li+ transfer, which can deliver high capacity than pure-phase MnO2. The present results prove that the conversion reactivity of these MnO2 anodes follows the order y- > a- > b-phase, and the mixed-phase MnO2 (b- and Y-MnO2) can provide better anode performances for lithium-ion batteries. Electrochemical measurements show that the intercalation-deintercalation reactivity of these MnO2 cathodes follows the order Y- > a- > b-phase, the as-crystallized MnO2 supercapacitors have Faradaic reactivity sequence a- > Y- > b-MnO2 upon their tunnel structures.

5 10 15 20 25 30 Cycle number (n)

5 10 15 20 25 30 Cycle number (n)

Fig. 2 Electrochemical performances of MnO2 anodes obtained by microwave-hydrothermal synthesis. Voltage profiles (a-d) and cycling retention (e,f)

of MnO2 were measured as Li-ion battery anode materials at a rate of 100 mA g

between 0.01 and 3.0 V. Insets show crystallographic structures of b- and g- MnO2.

Reprinted with permission from Ref. [27]. Copyright 2013 American Chemical Society.

The design of Mn-based cathode materials also play important role on the development of high-performance lithium ion batteries. Spinel cathode materials consisting of LiMn2O4@LiNi0.5Mn1.5O4 hollow microspheres have been synthesized by a facile solution-phase coating and subsequent solid-phase lithiation route in an atmosphere of air [29]. When used as the cathode of lithium-ion batteries, the double-shell LiMn2O4@LiNi0.5Mn15O4 hollow microspheres show a high specific capacity of 120 mAh g-1 at 1C rate, and excellent rate capability (90 mAh g-1 at 10C) over the range of 3.5-5 V versus Li/Li+ with a retention of 95% over 500 cycles [29]. Orthorhombic LiMnO2 (o-LiMnO2) material has been synthesized under hydrothermal conditions using sol-gel derived Mn2O3 and LiOH as precursors [30]. Galvanostatic charge-discharge test results showed that pure o-LiMnO2 had larger discharge capacity than the mixed phases of o-LiMnO2 and Li2MnO3. However, the cycling performance of the mixed phases of o-LiMnO2 with Li2MnO3 was found to be better than that of pure o-LiMnO2. Recently, the LiMn2O4 electrode materials were synthesized by the conventional-hydrothermal and microwave-hydrothermal methods, which can be served as supercapacitors in LiNO3 electrolyte and lithium-ion battery cathodes [23,24,31]. The capacitance of LiMn2O4 electrode increased with increasing crystallization time in conventional-hydrothermal route. In this case, LiMn2O4 supercapacitors had similar discharge capacity and potential window (1.2 V) as that of ordinary lithium-ion battery cathodes [24]. In LiNO3 aqueous electrolyte, the reaction

kinetics of LiMn2O4 supercapacitors was very fast. Even, at current densities of 1 and 5 A g-1, aqueous electrolyte gave good capacity compared with that in organic electrolyte at a current density of 0.05 A g-1 [24].

2.2 Cu-based blended electrode

Cu-based materials are often used as electrode materials for electrochemical energy storage, i.e., CuO, Cu2O, Cu3O4, Cu(OH)2 [32]. In order to improve their electrochemical performance, the hollow, polyhedral, wire-like, sheet-like Cu-based materials have been designed by various wet chemical methods [33].

Recently, Cu2O hollow octahedra and Cu2O core@shell structures were synthesized by room-temperature aqueous reduction reaction between CuAc2 and N2H4 [34]. As lithium-ion battery anodes, the Cu2O core@shell structures show a higher capacity and better cycling stability than the hollow octahedra because the core@shell structures can endure large volume changes during electrochemical reactions and nanoparticles with small sizes (on shell) can shorten the diffusion path of the Li+ ions and electrons. As supercapacitors, the Cu2O core@shell structures have a higher specific capacitance than that of the hollow octahedra. In addition, Cu2O solid octahedra and spherical aggregations were synthesized based on the hard-soft acid-base reactions between N2H4 and inorganic salts [35]. Electrochemical measurements indicated that Cu2O octahedra can show better cyclability than that of spherical aggregations. These studies open up a useful bottom-up strategy to design high performance electrode materials.

Micro- and nanocrystals with different facets may exhibit different chemical activities that are of great importance in practical applications. With a cubic crystal structure, Cu2O can crystallize into various polyhedral morphologies, such as cubes, octahedra, 26-facet polyhedra, and rhombic dodecahedra [36-38]. Our recent work comprehensively investigates the effect of the crystal plane on the electrochemical performance of Cu2O [39]. Firstly, Cu2O polyhedra with different morphologies, i.e., cube, octahedron and truncated octahedron, with different degrees of exposed {110} facets, were synthesized. Cu2O cubes show the highest capacity among these Cu2O polyhedra. The present results prove that the {100} facets of Cu2O display the highest electroactivities toward redox reactions. In addition, the columbic efficiency of Cu2O electrode can approach 100% after 5th discharge-charge cycles. The electroactivity sequence of the Cu2O crystal planes is {100} > {111} > {110} (Fig. 3). However, the capacity of as-obtained Cu2O samples is higher than theoretical value of Cu2O. The additional capacity of as-obtained Cu2O samples is due to the generation of CuO during electrochemical cycling, which can conduct a two-electron redox reaction. The anode performances of Cu2O polyhedra are dependent on its crystal planes, and this can provide a novel route to design high performance electrode materials for lithium-ion batteries.

10 20 30 40 Cycle number (n)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 Voltage (V vs. Li+/Li)

Fig. 3 Cycling performances (a) and CV curves (b) of Cu2O with different morphologies: cubes, octahedra and truncated octahedra with different degrees of exposed {110} facets. Comparison of reduction peaks (c) and oxidation peaks (d) in CV curves. The insets in (a) show the corresponding SEM images of the Cu2O polyhedra. The cycling stabilities of the Cu2O anodes were obtained at a current rate of 100 mA g 1. The CV curves were measured after 50 cycles at the scan rate of 0.3 mV s-1 and over the potential range of 0.01-3.0 V vs. Li+/Li. Reprinted with permission from Ref. [39]. Copyright 2015 Royal Society of Chemistry.

1D nanostructures allow for 1D electronic pathways allowing for efficient charge transport and better accommodation of the large volume changes without the initiation of fracture that can occur in bulk or micrometer-sized materials [40]. Recently, we demonstrated an efficient room-temperature chemical transformation route to CuO nanowires (NWs), from irregular particles to NWs coupled with a series of phase changes from CuCl, through Cu2(OH)3Cl, to Cu(OH)2, and finally to CuO [41]. The

room-temperature chemical transformation of Cu(OH)2 NW to CuO NW can reserve the initial NW morphology and made the as-synthesized CuO NW more active in electrochemical reactions. As the anode materials for lithium ion battery, these CuO NWs can exhibit a reversible capacity of 696.1 mAh g-1 after 40 discharge-charge cycles at the current rate of 100 mA g-1. In addition, after 2th discharge-charge cycles the columbic efficiency of CuO NW electrode can approach 100 %. The high lithium-storage capacity can be ascribed to the unique structure of these CuO NWs with size of ~10 nm and grain boundaries on the NWs surfaces, which show more active for the initial electrochemical reaction. CuO NWs and intermediate Cu(OH)2 NWs can also be fabricated as pseudocapacitor electrodes; in KOH electrolyte, their specific capacitances are 118 and 114 F g-1 at the current density of 1 A g-1. The room-temperature chemical transformation route is promising to produce advanced electrode materials for both lithium ion batteries and supercapacitors.

2D nanostructure can also increase the electrochemical performance of electrode materials. A chemical reaction controlled mechanochemical route was developed to synthesize mass CuO nanosheets by manual grinding in a mortar and pestle, which did not require any solvent, complex apparatus and techniques [42]. The microstructure and reaction route can be controlled by NaOH concentration according to the involved chemical reactions. The resultant CuO nanosheets materials with preferential nanoscale ribbon-like morphology can show large capacity and high cycle performance with a discharge capacity of 614 mAh g-1 after 50 cycles with capacity retention of 93%. The simple, economical, and environmentally friendly

mechanochemical route is of great interest in modern synthetic chemistry. 2.3 Fe-based blended electrode

Fe-based materials have been exploring for their wide applications in energy storage devices because of their abundance, low cost and non-toxic properties. Various synthetic methods have been carried out to produce Fe-based materials with different morphologies, such as the sol-gel method, hydrothermal, microwave synthesis, etc.[43-45] Recently, we have synthesized FeOOH nanorods via the hydrolysis of FeCl36H2O solution through a low-temperature hydrothermal route (100 °C) [46]. As shown in Fig. 4, the as-prepared FeOOH products have 1D rod morphologies and the lengths are increased with the increase of the concentration of FeCl36H2O solution. It is reported that the acidic solution can favor the crystallization of FeOOH bunches and FeOOH nanorods, in which dissolution and recrystallization processes can occur [47]. In this work, high FeCl3 concentration can produce a more acidic solution that helps to grow longer FeOOH rods.

200 nm

200 nm

Fig. 4 SEM images of the as-prepared FeOOH nanorods by using FeCl3 6H2O solution with different FeCl3-6H2O concentrations: 0.1 (a), 0.2 (b), 0.3 (c), 0.4 (d), 0.5

(e), 0.6 (f), 0.7 (g), 0.8 (h), 0.9 (i) and 1.0 M (j). Reprinted with permission from Ref. [46]. Copyright 2015 Royal Society of Chemistry.

O) 20 <

0) TS -£-10 a>

-0.6 IVI

-0.8 M

-0.9 M

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Potential (V)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Concentration (mol/L)

Fig. 5 (a) CV curves of the as-prepared FeOOH nanorods with 0.1-1.0 M FeCl3-6H2O precursor solutions at a scan rate of 100 mV s 1 in the potential range from -1.2 to 0.45 V. (b) Constant current discharge curves of a-FeOOH at a current density of 1 A

g 1 from -1.2 to 0 V. (c) The calculated specific capacitance of the as-prepared a-FeOOH. Reprinted with permission from Ref. [46]. Copyright 2015 Royal Society of Chemistry.

The electrochemical performance of the as-obtained FeOOH nanorods as supercapacitors is evaluated. The specific capacitances of FeOOH materials are 692.9, 714.8, 593.7, 589.3, 440.8, 520.8, 486.0, 345.4, 530.0 and 286.2 F g-1 prepared in 0.1-1.0 M FeCl36H2O precursor solutions, respectively (Fig. 5). The columbic efficiency of FeOOH electrode can approach 100 %. The specific capacitance

decreases with the increase in concentration of Fe3+. FeOOH electrode materials obtained in 0.2 M FeCl36H2O solution display the highest specific capacitance of 714.8 F g-1, which is higher than reported values for FeOOH electrode materials.

High Fe3+ concentration increases H+ concentration in the reaction solution. A strong acidic solution can damage the intact FeOOH structure by acidic etching. A large amount of defects can be formed in FeOOH nanorods, which can hinder the electron transfer in FeOOH nanorods, thus showing low electrochemical performance.

In addition, we designed a simple hydrolysis reaction to study the chemical roles of anions with different chemoaffinity abilities to iron ions on the crystallization of FeOOH [48]. FeOOH products with different morphologies and sizes can be crystallized by using commercial iron salts such as FeSO4, FeCl2, FeCl3 and Fe(NO3)3. Their crystallization mechanisms can be evidenced in terms of hard-soft acid-base theory, which can control the chemical reaction and crystallization process of FeOOH. FeOOH particles with high electroactivity were successfully synthesized at 80 °C via the use of a single-source of iron salt in each corresponding experiment. Served as electrode materials for supercapacitors, the as-obtained FeOOH samples exhibit high specific capacitances of 441.2, 385.7, 577.3 and 619.6 F g 1. An Ethylenediamine

(EN)-assisted hydrothermal route has been reported to produce high performance Fe2O3 microspindles as pseudocapacitor electrode materials [49]. A simple feasible nucleation-controlled approach to produce size tunable Fe2O3 microspindles was proved. It was found that a lower EN dose tailored smaller size of particles, leading to a higher specific capacitance of 558.7 F g-1. Moreover, this facile synthesis method provides a huge opportunity for future practical applications of supercapacitors. Iron oxide@C composites and lithium ferrites have been synthesized by a cotton-template method as anode materials for Li-ion batteries [50]. a-Fe2O3@C composites with 3D porous hollow secondary structures were prepared by directly burning the cotton containing the iron salt (FeCl3 or Fe(NO3)3) in air, and the Fe3O4@C composites with similar structures were obtained by annealing a-Fe2O3@C under a N2 atmosphere. a-LiFeO2 and a-LiFe5O8 particles with sizes of 100-500 nm were also prepared using a similar method. Electrochemical measurements showed that all these samples demonstrated good electrochemical performances as Li-ion battery anodes, especially a-Fe2O3@C derived from Fe(NO3)3, which delivers a high reversible capacity of 990 mA h g-1 at 100 mA g-1 after 50 cycles. Both the porous hollow secondary structure and the suitable amount of amorphous carbon are significant for the electrochemical performances of the iron oxide@C composites. Such a method is simple, rapid and inexpensive and may facilitate the preparation of other high performance electrode materials with porous hollow structures.

3 Integrated electrode

The synthesis of active materials directly integrated to the current collector can enhance the conductivity of electrode since the intimate contact with the current collector facilitates electron transfer processes, and avoids the use of binders and conductive carbon. Thus, these integrated electrodes can enhance the performance of the energy storage devices [51]. These integrated electrodes can be synthesized by hydrothermal route, electrodeposition, CVD, etc.[52] Most these methods need heterogeneous growth of active materials on the conductive substrate, which require the skillful design idea. Graphene materials can form self-support conductive film, serving as both active material and conductive substrate, which have received much interest to serve as integrated electrode for batteries and supercapacitors [53,54]. In this section, we focus on the self-support graphene paper electrode and heterogeneous grown integrated copper electrode.

3.1 Graphene paper electrode

The free-standing paper-like carbon-based electrodes show potential applications in flexible energy storage devices, such as wearable or rolling-up devices, which can display light weight, flexibility, and high conductivity [55]. Recently, we have designed folded structured graphene paper electrodes as high performance lithium ion batteries and supercapacitors [56]. Firstly, 3D graphene oxide (GO) aerogel was prepared by freeze-drying a homogeneous GO aqueous dispersion. Liquid water can crystallize into ice solids, which served as "spacer" between graphene sheets [54]. After the sublimation of ices, 3D porous structured GO aerogels were formed (Fig.

6a,b). Secondly, 3D graphene aerogel was formed by the thermal reduction of 3D GO aerogel (Fig. 6c,d). Thirdly, graphene paper electrode was formed by mechanical pressing of the graphene aerogel (Fig. 6e-h). Served as lithium ion battery, the first discharge and charge capacities of graphene paper are 1091 and 864 mAh g-1 (Fig. 7a,b). The second discharge and charge capacities are 815 and 806 mA h g-1. The corresponding reversible specific capacity of the as-prepared graphene paper can reach 557, 268, 169, and 141 mAh g-1 at the current densities of 200, 500, 1000, and 1500 mA g-1, respectively. After 100 discharge-charge cycles, the reversible capacity is still maintained at 568 mA h g-1 at 100 mA g-1 (Fig. 7a,b). It should be noted that the columbic efficiency of graphene electrode can approach 100 % after second discharge-charge cycle. Served as supercapacitor in 1M H2SO4 electrolyte, the graphene paper gives a specific capacitance of 172 F g-1 at a charge-discharge rate of 1 A g-1, and a capacitance of 110 F g-1 can be obtained even when the supercapacitor is operated at a fast rate of 100 A g-1 (Fig. 7c-e). These results show that the graphene paper electrode is a potential candidate for electrochemical energy storage devices with high reversible capacity, good cycle performance, and high rate discharge/charge capability.

Fig. 6 Digital image (a) and SEM image (b) of a GO aerogel. Digital image (c) and SEM image (d) of a graphene aerogel. Digital images (e-g) and SEM images (h) of graphene paper. Reprinted with permission from Ref. [56]. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

0 1000 2000 3000 4000 5000

Cycle number

Fig. 7 Electrochemical characterizations of graphene paper as a lithium-ion battery

anode (a and b) and a supercapacitor electrode (c-e). (a) Discharge-charge curves at a

current density of 100 mA g-1. (b) Cycle performance at different current densities. (c) CV curves of the graphene paper as a supercapacitor electrode at different scan rates.

(d) Specific capacitance of the graphene paper as a function of charge-discharge rate.

(e) Cycling test at a current density of 20 A g-1 up to 5000 cycles. Reprinted with permission from Ref. [56]. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The theoretical maximum specific capacitance of the graphene as supercapacitor electrode is calculated to be ~550 F g-1. However, the practical obtained specific capacitances (most less than 300 F g-1) of various synthetic graphene materials are lower than theoretical value. In order to further improve its capacitance, we introduced redox-electrolyte K3Fe(CN)6 into a graphene paper electrode and KOH electrolyte. 95-fold increase in the specific capacitance was obtained in KOH-K3Fe(CN)6 electrolyte compared with the value obtained in KOH only electrolyte [54].

Furthermore, we design a novel supercapacitor system with a graphene paper electrode in K3Fe(CN)6-Na2SO4 redox-electrolyte based on the system-level design principle (Fig. 8) [57]. In a neutral electrolyte, the capacitance values were 475 mF cm-2 in 0.3 M K3Fe(CN)6-1 M Na2SO4 redox-electrolyte and 93 mF cm-2 in 1 M

Na2SO4 electrolyte with the potential interval of 1.6 V and the current density of 20

mA cm . By combining electric double-layer capacitance and pseudocapacitance, the

specific capacitance could be increased 5-fold compared with the conventional

electrode-electrolyte system. More importantly, the potential interval could reach as

high as 1.6 V beyond the limited operating voltage of water (~1.23 V). After 5000

continuous cycles, 94% of the initial capacitance was retained. The superior electrochemical performance of the graphene-paper electrodes in the K3Fe(CN)6 electrolyte can be explained by the following factors: (1) additional pseudocapacitance contribution from the Fe(CN)63"/Fe(CN)64" couple, (2) high conductivity of the supercapacitor making electron and ion transfer more efficient and (3) no adverse effect from the binder as the system is binder-free. The designed graphene-paper-electrode/redox-electrolyte system can provide a versatile strategy for high capacitance supercapacitor systems.

Graphene Traditional electrolyte electrode

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j©2 © _ ©

+ Na* ©SOf

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.1 0.6 0.8

Potential (V vs. SCE)

Graphene Redox e|ectro, te electrode

|000 [Fe(CN)e]

!o"o o

< o.ooo

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Potential (V vs. SCE)

j©~0 I electrolyte]e o

^O^ 0 J

: aJ_ w

jo9o [Fe(CN)e]4

Fig. 8 Charge storage mechanisms of the graphene paper electrode in the traditional

electrolyte system (a) and the redox-electrolyte system (b). (a) Graphene paper electrode in the neutral Na2SO4 electrolyte, which displays electric double-layer capacitance with quasi-rectangular CV curves. (b) After adding redox-active K3Fe(CN)6 to the Na2SO4 electrolyte, this supercapacitor system combines two charge storage mechanisms, pseudocapacitance and electric double-layer capacitance. The novel electrode and electrolyte system can significantly increase the specific capacitance of supercapacitors. Reprinted with permission from Ref. [57]. Copyright 2015 Royal Society of Chemistry.

3.2 Cu-based integrated electrode

Copper foil is the often used commercial anode current collector for lithium ion batteries. The direct growth of Cu-based active materials on copper foil can provide a flexible method to construct integrated electrode. With copper foil as copper source, Cu-based materials can be formed by oxidizing copper foil in vapor or solution. Recently, we reported a facile vapor-phase strategy for the crystallization of copper oxide on Cu foil substrate in air and the further fabrication of integrated CuxO-Cu electrodes with CuxO readily on the Cu foil current collector, without using conductive carbon and binder [58]. The integrated CuxO-Cu electrodes can show better cycling stability and higher capability than those CuxO-C blended electrodes.

By wet chemical reaction route, CuO microsheets grown on the Cu current collector were also formed by the oxidation of metal Cu foil with (NH4)2S2O8 in alkaline aqueous solution (Fig. 9) [59]. As shown in Fig. 10, after 110 discharge-charge cycles, the discharge capacity of CuO/Cu integrated anode retains a large value of 706 mAh g-1, which is beyond the theoretical capacity of CuO

materials (674 mAh g-1). The columbic efficiency of CuO/Cu integrated anode can approach 100 % after second discharge-charge cycle. During electrochemical cycling, nanocrystallization of CuO microsheets via electrochemical reaction was occurred, which lead to the formation of CuO/Cu integrated anode with CuO nanoparticle-aggregated microsheets on Cu current collector (Fig. 9). Redox peaks in CV curves after 110 discharge-charge cycles show the creation of CuII1-xCuIxO1-x/2 (0 < x < 0.4) solid solution, Cu2O phase, and metal Cu. The excellent electrochemical performance of CuO/Cu integrated anode can be due to the following reasons: (1) the in-situ formed reactive nanoparticles under electrochemical reactions, (2) the active grains with size < 10 nm are interconnected and form a continuous network within CuO/Cu integrated anode, which provides a short pathway for local electron transport, (3) the abundant interface, formed between nanoparticles, provides a pathway for ionic transport and (4) the additional capacity of CuO anodes can be attributed to the release of Cu from Cu current collector by in-situ electrochemical redox reaction.

Cu substrate

Inorganic chemistry reaction synthesis

Cu + $20f + 20H"— CuO Chemical reaction in solution

CuO-Cu substrate-

CuO/Cu integrated unit

Electrochemical energy conversion

CuO + 2Li+ + 2e <—» Li20 + Cu Chemical reaction in battery

Active CuO

15 sss

••• »H

•«Î.V«ÎÎV

« _Ê«t-

Cu-collector —•

Converted CuO/Cu integrated anode Fig. 9 Schematic representation of the synthesis of integrated CuO/Cu anode via

chemical and electrochemical oxidation of the Cu current collector. The conversion

reactions occurred within the limit of initial CuO microsheets, which include

electrochemically driven nanocrystallization of CuO microsheets to CuO

nanoparticles. During the discharge/charge process, the release of Cu from the current

collector can maintain high capacity of CuO anode in the designed integrated anode

system. Reprinted with permission from Ref. [59]. Copyright 2015 Elsevier.

Fig. 10 Electrochemical properties of CuO/Cu integrated anode. (a) Charge-discharge curves and (b) Cycling curves at current density of 100 mA g-1 and potential range of 0.01-3.0 V. Dotted line in figure indicates theoretical capacity value of CuO anode materials. (c) Rate capability curves at the current densities from 0.1 to 1.5 A g-1. (d) CV curves of CuO/Cu anode after 110 charge-discharge cycles at the scan rate of 0.1 mV/s. (e) Electrochemical impedance spectra of CuO/Cu anode. Inset shows Randle's equivalent circuit. (f) Plot of impedance phase angle versus frequency. Reprinted with permission from Ref. [59]. Copyright 2015 Elsevier.

Fig. 11 Ex situ SEM images for the identification of Cu+ long-range diffusion path during the first charge-discharge cycles with different charge-discharge potentials. Discharge process: (a, b) 1.36 V vs. Li+/Li. Charge process: (c, d) 1.6 V vs. Li+/Li. The nanowire network was formed around CuO nanoparticles at 1.3-1.6 V vs. Li+/Li, which can provide ion channels for mass diffusion (exchange). Reprinted with permission from Ref. [60]. Copyright 2015 Royal Society of Chemistry.

In addition, the integrated electrode is also a good platform for studying

electrochemical reaction mechanism because the absence of negative influence of

binder and conductive carbon. With the use of CuO/Cu integrated materials as lithium

ion battery anodes, we found the existence of a Cu+ ion long-range transfer path at the

potential widow of 1.30-1.60 V during both charging and discharging processes (Fig.

11) [60]. The nanowires networks hanging between CuO nanoparticles provide a Cu+

diffusion path within the designed CuO/Cu integrated anode. It has been reported that

lithium can diffuse into a Cu current collector beyond the active material by

long-range diffusion [61]. The finding of the Cu+ ion diffusion path is helpful for displaying the real electrochemical reaction in lithium ion batteries. This work provides new insights into the conversion reaction of inorganic anode materials, and can favor the development of high-performance conversion anodes for lithium-ion batteries.

4 In-situ formed electrode

Traditionally, the synthesis environment and the testing environment of electrode materials are different, which lead to the change of electrode materials when they were transferred from synthesis environment to testing environment. It is believed that in-situ formed electrode materials in testing environment can display high electrochemical activity because it can form active structures in the in-situ growth process. Recently, we have developed the in-situ formation of supercapacitor electrode [62-70]. In these supercapacitor systems, multiple-valence metal salts can be in-situ transformed into electroactive colloids in an alkaline electrolyte and these colloids can fully participate in Faradaic reactions resulting in high electrochemical

2+ 3+ 2+ 21

performance. These multiple-valence metal cations include Cu, Fe, Co2+, Ni2+,

2+ 4+ 3+ 3+ 3+

Mn, Sn, Ce, Yb, and Er3+ ions with ionic electronegativity values in the range 1.2-1.5. These in-situ formed supercapacitor electrodes can show ultrahigh specific capacitance.

WlÊM • •

•vMîfe"

•viiWV®

MnCb salts electrode

Chemical coprecipitation

Pseudocapacitive reaction

Chemical coprecipitation MnCl2 + 20H —► Mn(OH)2 + 2C1

7Mn(OH)2 + 120H -» Mn70i3 5H2O + 8H2O + 12e

Pseudocapacitive reaction

Mn + e~

Mn + e ^ Mn

Electrochemical active materials

• MnCb salts • KOH electrolyte O MnyOisSHhO • Carbon

Fig. 12. Schematic drawing shows chemical processes of colloidal electrode in KOH aqueous electrolyte. Electrochemical measurement was performed to this electrode immediately (a). After undergoing chemical and electrochemical reactions, electroactive Mn7O135H2O colloids were formed in electrode (b,c). Faradaic reactions of electroactive Mn7O135H2O colloids occurred at the same time and electrode, with the corresponding cation reactions of Mn2+ « Mn3+ and Mn3+ « Mn4+ (d). Reprinted with permission from Ref. [71]. Copyright 2015 Elsevier.

As shown in Fig. 12, with the commercially available MnCl2 salts as starting materials and KOH as electrolyte, the electroactive colloids synthesis and subsequently integrating into practical electrode structures occur at the same spatial and temporal scale [71]. Highly electroactive Mn7O135H2O colloids are formed

in-situ by electric field assisted chemical coprecipitation in KOH solution. Fig. 13a and b show the discharge and charge curves. The specific capacitance values of Mn cations are 2082, 1610, 1174, 720, and 382 F g-1 at the current densities of 10, 20, 30, 40, 50 A g-1 and the potential interval of 0.8 V (Fig. 13c). The highest specific capacitance of 2518 F g-1 at the current density of 5 A g-1 and the potential interval of 0.8 V is obtained on the basis of the weight of Mn cations. Redox peaks in CV curves are corresponding to Faradaic redox reactions (Fig. 13d,e). A1 and C1 peaks represent oxidation and reduction of Mn3+ « Mn4+, while A2 and C2 peaks represent oxidation

and reduction of Mn « Mn . The long-term cycling stability of MnCl2 electrodes was measured with galvanostatic charge-discharge at the current density of 30 A g-1 for 5000 cycles (Fig. 13f). The capacitance retention can reach 67 % after 5000 cycles. These results confirm that the highly electroactive colloid was formed in the designed MnCl2-KOH system and the in-situ formed colloidal pseudocapacitor system is a novel route to engineer electrochemical performances of inorganic pseudocapacitors.

The in-situ formed colloidal electrode can be further extended to binary cations system. For example, a "combinatorial transition-metal cation pseudocapacitor" was

reported by designing combinatorial transition-metal cation pseudocapacitors with

2+ 2+ 2+ 2+ binary AxB1.x salt electrodes involving Mn , Fe , Co , and Fe cations in KOH

aqueous electrolyte [72]. Binary multi-valence cations were crystallized in colloid

form through an in-situ coprecipitation under electric field. Fig. 14 shows

electrochemical performance of binary FexCo1-x (0 < x < 1) salt electrodes in 2 M

KOH electrolyte. A pair of redox peaks is observed in the CV curves, showing the

pseudocapacitive characteristics, which correspond to redox reaction: Co « Co , Co3+ « Co4+, Fe2+ « Fe3+ (Fig. 14a). The Fe0.7Co0.3 electrode displays the highest value of 11789 F g-1 in the potential interval of 0.38 V and at the current density of 3 A g-1 (Fig. 14b-d). The specific capacitances of Fe0.6Co0.4 and Fe08Co02 electrodes are 6375 F g-1 and 8793 F g-1, respectively. As shown in Table 2, the Fe07Co03 electrode shows the highest capacitance among the reported values of various pseudocapacitive materials. In contrast to the reported electrode materials synthesized by traditional methods [79-81], our designed highly active materials were in-situ formed by electric field assisted chemical coprecipitation. The in-situ formed electroactive colloids can create short ion diffusion paths to enable the fast and reversible Faradaic reactions.

a b с

Fig. 13. Electrochemical performance of colloidal supercapacitors. (a) The discharge and (b) charge curves measured at various current densities and potential interval of 0.8 V. (c) The specific capacitance upon current density. (d) CV curves obtained at potential range of -0.6-0.45 V and scan rate of 5 mV s-1. (e) CV curves obtained at

different scan rates and potential range of -0.6-0.45 V. (f) The long-term cycling stability of MnCl2-4H2O colloidal supercapacitors was measured at a current density of 30 A g-1. Reprinted with permission from Ref. [71]. Copyright 2015 Elsevier.

0.0 0.1 0.2 0.3 0.4 Potential (V vs. SCE)

100 200 Time (s)

0 100 200 Time (s)

Fig. 14 Electrochemical performance of FexCo^x (0 < x < 1) salt electrodes in 2 M KOH electrolyte. (a) CV curves at a scan rate of 5 mV s-1 and the potential range of -0.1 to 0.45 V. (b) The discharge and (c) charge curves measured at a current density of 3 A g-1. (d) Variation of the specific capacitance as a function of the ratio of x (0 < x < 1) at a current density of 3 A g-1. Reprinted with permission from Ref. [72]. Copyright 2015 Royal Society of Chemistry.

Table 2 Comparison of the supercapacitor performances of metal oxides/hydroxides and colloidal electrode. Reprinted with permission from Ref. [72]. Copyright 2015 Royal Society of Chemistry. Material Specific Electrolyte Potential range Ion specific

capacitance (F g 1) (V) capacitance (F g-1)

Co3O4 [73] 1063 (10mA cm-2) 6M KOH 0-0.38 4343

Co(OH)2 [74] 1180 (4A g-1) 1M KOH -0.1-0.45 1881

a-Fe2O3 [75] 340.5 (1A g-1) 1M KOH -0.1-0.44 976

NiO/rGO [76] 1077 (1A g-1) 6M KOH -0.1-0.4 1371

Ni(OH)2/Ni [77] 3152 (4A g-1) 3% KOH -0.05-0.45 4978

MnO2/Au [16] 1145 (50mV s-1) 2 M Li2SO4 0-0.8 1812

Ni-Co oxide [78] 1846 (1A g-1) 2M KOH 0-0.37 7559

Fe0.?Co0.3 [72] --(3A g-1) 2M KOH 0-0.38V 11789

5. Conclusions and outlook

After so much intensive study on electrode materials, electrochemical energy storage field has been facing many progresses. Many new electroactive materials with high capacity, electrode materials with novel structures and new electrode construction forms are being found. The effects of phases, sizes and morphologies of electrode materials on electrochemical performance of energy storage devices have been explored by many research groups. These advances are being realized by the increase of the understanding of charge storage mechanism of electrode materials, which, in turn, can guide the design of new state-of-art electrode materials with high electrochemical performance. However, the finding of the exact physical and chemical process of charge storage mechanism is still challenging. The commercially used blended electrodes are easy for large-scale industrial manipulation, which have been used for long time since the battery commercialization. Therefore, the electrode materials design is needed to improve the electrochemical performance of lithium ion batteries and supercapacitors for a long time. The new optimized electrode structures are urgent to obtain fast electron transfer rate and the maximization ratio of active

materials in electrode. The integrated electrode and in-situ formed electrode should be put more attention, which maybe the promising candidates for next-generation high-performance energy storage devices. In addition, for the scale practical application of energy storage devices, the scalable fabrication and the cost of electrode materials are also need to take into account.

Acknowledgments

Financial support from the National Natural Science Foundation of China (Grant Nos. 51125009, 91434118), the National Natural Science Foundation for Creative Research Group (Grant No. 21221061), the External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121522KYS820150009) and the Hundred Talents Program of the Chinese Academy of Sciences is acknowledged.

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This review focuses on the latest development of the electrode materials design and electrode structure construction in the field of lithium ion batteries and

supercapacitors.

Authors's biography

Kunfeng Chen received his BE degree of inorganic non-metallic materials engineering in 2009 and PhD of inorganic chemistry in 2014 both from Dalian University of Technology under the supervision of Prof. Dongfeng Xue. He is working as assistant professor in Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CAS). His research interests focus on the controllable growth of functional materials such as graphene, metal oxide materials for electrochemical energy storage.

Professor Dongfeng Xue received his PhD from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (CIAC, CAS) in 1998. From 1999 to 2003, he worked in Universitaet Osnabrueck (AvH research fellow), University of Ottawa (Postdoc), and National Institute for Materials Science, Japan (JSPS Postdoc). In 2001, he was appointed as full professor in Dalian University of Technology. In 2011, he

joined CIAC as full professor. His research interests include crystallography, crystallization, calculation and simulation of functional materials, chemical synthesis of condensed matters, and electrochemical energy storage. His scientific contributions to the community include (i) Phillips-Van Vechten-Levine-Xue bond theory, (ii) chemical bonding theory of single crystal growth, (iii) ionic electronegativity scale of 82 elements in periodic table. He has published over 400 papers in peer-reviewed journals (with h = 50), and more than 20 invited book chapters. In 2003, he was elected as corresponding member of European Academy of Sciences, Arts and Humanities (Paris). He owned visiting professorship of Queen Mary University of London (2009.1-2010.12). In 2010, he was awarded Gledden Visiting Senior Fellowship of the University of Western Australia. He also received several prestigious domestic awards, e.g., China Youth Particuology Award issued by Chinese Society of Particuology in 2010. He serves as the editorial membership of more than 20 international journals such as Materials Research Bulletin, Materials Research Innovations, Science of Advanced Materials, Journal of Porous Materials, Nanoscale Research Letters, 3D Research, Energy and Environment Focus.