DOI:10.30919/es.180318

Received: 06 Mar 2018
Accepted: 18 Mar 2018
Published online: 18 Mar 2018

Electrodes with High Conductivities for High Performance Lithium/Sodium Ion Batteries

Litao Yan,1,† Haizhen Wang,1,† Di Huang1 and Hongmei Luo1,*

1Department of Chemical and Materials Engineering, New Mexico State University, New Mexico 88003, USA

† Those authors contribute equally.   

* Email: hluo@nmsu.edu (H. Luo)

 

Abstract

Electrochemical devices including lithium/sodium-ion batteries and supercapacitors, as the promising solution to energy storage and environmental issues in the future, have attracted great attention recently due to the rapidly growing demand for non-renewable resources. The common electrode materials, including metal oxides, suffer from low conductivities, which lead to low charge transfer rate during the charge/discharge process and thus poor battery performance such as low specific capacitance/capacity, low rate capability, and poor cycling stability. Therefore, researchers have been searching for various strategies to enhance the conductivity of the electrode materials and thus the performance of lithium/sodium-ion batteries and supercapacitors. In this mini-review, different strategies to enhance the conductivities of the electrode materials would be discussed which will provide researchers various routes to improve the conductivity of the electrode materials in order to improve the performance of electrochemical device.


Table of Content

Different strategies to enhance the conductivities of the electrode materials would be discussed which will provide researchers various routes to improve the conductivity of the electrode materials in order to improve the performance of electrochemical device.

 

 

Keywords

Lithium/Sodium batteries      Supercapacitors      Conductivity       Electrode materials      Electrochemical devices


1. Introduction

The combustion of fossil fuel leads to the emission of global warming gas including carbon dioxide and other gases, resulting in various environmental issues. The utilization of renewable energy resources including solar, wind and hydropower energy can not only effectively reduce the greenhouse gas emission but also decrease the rapidly growing demand for non-renewable resources.13 The critical energy storage issue for the utilization of renewable energy resource during the on-peak period limits the extensive applications of renewable technologies.4,5 Electrochemical devices including batteries and supercapacitors, as the promising solution to energy storage and environmental issues in the future, have attracted great attention since they could average out the production of renewable resources and fill the gap of energy shortage during the on-peak period.57
Batteries and supercapacitors, the typical electrochemical energy storage devices, consist of two electrodes and ion conducting electrolyte.8 The charge/discharge process associated with ions intercalation-de-intercalation between two electrodes can realize the energy storage and release process. The electrochemical properties of two electrodes could determine the performance of batteries and supercapacitors. Therefore, it is expected that the energy and power density of batteries could be increased by the optimization of the electrode materials. Up to now, graphite, lithium titanate (Li4Ti5O12), silicon, transition metal oxides and metal sulfides have been employed as anode materials for batteries and supercapacitors.9–21 However, among all the anode materials, only the conductivity of commercial graphite and porous carbon can meet the requirements as electrodes for lithium ion batteries (LIBs) and supercapacitors, respectively. The poor electronic conductivities of lithium titanate, silicon, transition metal oxides and metal sulfides will reduce the charge transfer during the discharge/charge processes for supercapacitors and batteries, which limits their further performance improvement.10,22–26

 To enhance the electrode conductivities in those electrochemical devices in order to optimize their electrochemical performance, ongoing research focuses on improving the charge transfer and ion diffusion by incorporation of conductive materials into electrodes including graphene based materials,27,28 high surface area mesoporous carbon,13,29 and carbon nanotubes;30 and introduction of conductive layer on active materials such as carbon coating;31,32 reducing the size of the electrode materials to nanoscale.5 With the unique structural, mechanical and electrical properties of the critical conductive materials, the composite electrodes can enhance the electron transport, realize fast charge transfer and ion diffusion, resulting in improved capacity and cyclability and consequently high electrochemical performance, since the conductive materials could function as buffering components and serve as the backbone in the electrode materials to prevent the detachment and agglomeration of metal oxides. Therefore, it is of great importance to familiarize researchers with different approaches to improve the conductivities of the electrode materials. This review will not cover all the information of LIBs with high conductive electrode materials. Instead, the recent developments of different strategies to create highly conductive composite electrode materials in LIBs especially in our group will be summarized. It is expected that this mini review will be of great benefit to scientists and researchers who are interested in the high-performance LIBs with highly conductive electrode materials.


2. Electrochemical energy storage devices

Batteries and supercapacitors (SCs), two typical electrochemical energy storage devices, have been regarded as the dominant power sources for our personal electronics and electrical vehicles.4,33,34 A typical battery or supercapacitor system consists of a negative electrode (anode, e.g., graphite for LIBs, hard carbon for sodium ion batteries (SIBs) or active carbon for SCs), an ion conducting electrolyte, and a positive electrode (cathode, e.g., LiMn1/3Co1/3Ni1/3O2 and LiCoO2 for LIBs or active carbon for SCs),5 as shown in Fig. 1. The working principle of batteries and SCs involves the cations (Li+, Na+, and H+) and anions (OH-) diffusion from one electrode to the other under the voltage-driven force and the redox reactions will take place at two electrodes. Moreover, electrons will flow toward the external circuit in order to maintain the system charge neutrality. Although LIBs with graphite as anodes are commercially available, the low theoretical capacity of graphite still limits its application in the next-generation high performance LIBs. Besides, the most promising electrodes including metal oxides suffer from low electrical conductivities which also limit their commercialization. Therefore, it is valuable to develop electrodes with high conductivities in order to realize high performance LIBs. Except for LIBs, SIBs (Fig. 1c) have also been investigated extensively due to rich sodium resources with low cost. However, the sluggish kinetics of sodium ions, resulted from large radius of sodium ion (compared with lithium ions), lead to low energy density and poor electrochemical performance of the batteries.15,24,35 Therefore, electrode materials with high conductivities will also be necessary for SIBs to achieve high energy/power density and cycle stability. Meanwhile, the hybrid supercapacitors (Fig. 1b) with metal oxides (pseudocapactive behavior) as positive electrodes can be an efficient way to increase the energy density of SCs.36,37 Similar to LIBs or SIBs with metal oxides as electrodes, the low conductivities of most metal oxides lead to low rate performance and low power densities in SCs as well. Only hybrid supercapacitor with high conductive noble metal oxide RuO2 as electrode has delivered superior specific capacitance and cycling stability. However, the high cost of RuO2 motivates us to seek for alternative cheaper metal oxides as electrodes in hybrid supercapacitors.38 As discussed above, metal oxides with high theoretical capacity generally suffer from the low conductivities. Thus, developing electrodes with high conductivities plays a vital role in achieving the high performance electrochemical energy storage devices including both batteries and supercapacitors.

Fig. 1 Schematic illustration of (a) a lithium ion battery; (b) an electrochemical supercapacitor and (c) sodium ion battery. Adapted and reproduced with the permission from Ref. 39. Copyright 2015; Ref. 40. Copyright 2015; Ref. 41. Copyright 2012, Royal Society of Chemistry.


3 Strategies to improve electrode conductivities

3.1 Graphene or reduced graphene oxide (rGO)-based composites

Graphene and reduced graphene oxide (rGO) have been good candidates for constructing graphene-based composite materials with metal oxides to enhance the conductivities of the electrodes due to their specific open porous two-dimensional structure, large surface area and excellent electronic conductivity.3,14,42–46 The conductivity of electrode materials could be significantly improved after combining with graphene or rGO.26,47–49 In addition, the high surface area of graphene or rGO can balance the volume change stress during charge and discharge processes, especially for high capacity and high rate electrode materials.47,50–52 It has been widely reported that incorporation of graphene or rGO with active electrode materials (e.g. metal oxides, metal sulfides and silicon) results in excellent battery performance compared to the corresponding pure active electrode materials.3,28,47,53 The functional groups with negative charge on the surface of rGO nanosheets provide the platform that the transition metal ions with positive charge can be easily absorbed by and uniformly distributed onto the GO nanosheets. The stabilized metal ions on GO nanosheets can interact with those negative functional groups to form active electrode materials/GO nanosheets composites. Highly conductive rGO could be obtained after the post heat treatment of GO under reduction atmosphere. Recently, a new nanocomposite formulation of the FeS-based anode for LIBs has been proposed, where FeS nanoparticles embedded in rGO have been produced via a facile direct-precipitation approach14 and employed as high performance electrode materials for LIBs. During synthesis process, the functional groups (e.g., -OH, -COOH) with negative charges on the surface of GO have strong columbic interaction with the Fe3+ ions and the introduction of Na2S leads to the formation of the FeS precipitate. Finally, small FeS nanoparticles are wrapped by rGO nanosheets, which can increase their electronic conductivity and prevent the polysulfide dissolution, resulting in the enhanced electrochemical performance.

In addition, graphene with high surface area and high electronic conductivity would also enhance the charge transfer and ion diffusion, resulting in high performance batteries and supercapacitors.51,54–58 For example, Nb2O5/graphene paper electrode could be prepared by hydrothermal reaction of niobium precursor and graphene,57 as shown in Fig. 2. The ion diffusion could be enhanced due to the channels between two compact layers, as can be seen from the SEM images (Fig. 2 a-f ). TEM images (Fig. 2 g-i) clearly showed that ultrafine Nb2O5 nanoparticles (average size of 1-3 nm) were uniformly distributed on the graphene nanosheets.

TiO2/graphene also has been studied as promising anode materials for lithium ion batteries with fairly good performance.27,59 For example, self-assembled TiO2/graphene showed enhanced lithium ion insertion/extraction kinetics in TiO2, and thus delivered high charge/discharge rate performance.27 Besides, rGO–TiO2 nanospheres as electrodes for sodium ion battery demonstrated a capacity of 300 mA h g-1 and excellent cycling stability with a capacity retention of 208 mA h g-1 over 300 cycles,35 which further confirmed that uniform conductive network coating on metal oxide can efficiently increase the conductivities of metal oxide electrode materials and thus the electrochemical performance of the batteries.

Fig. 2 SEM images of Nb2O5/rGO (a-c); T-Nb2O5/graphene composite papers (d-f); TEM image of Nb2O5/rGO (g-h); HRTEM and electron diffraction pattern (i) of T-Nb2O5/graphene. Adapted and reproduced with the permission from Ref. 57. Copyright 2015, American Chemical Society.

Fig. 3 (a) The TEM image of niobium hydroxide and graphene oxide after coating process; (b, c) HRTEM images of Nb2O5 NCs/rGO; and (d) SAED pattern of Nb2O5 NCs/rGO. Adapted and reproduced with the permission from Ref. 61. Copyright 2016, American Chemical Society.

Fig. 4 (a) CV curves of Nb2O5 NCs/rGO at a scan rate of 0.1 mV s-1; (b) galvanostatic discharge/charge profiles of Nb2O5 NCs/rGO at 0.2 A g-1; (c) rate capability of Nb2O5 NCs/rGO and the mixture of Nb2O5 and rGO at various current densities from 0.2 to 10 A g-1; and (d) cycling performance and corresponding Coulombic efficiency of Nb2O5 NCs/rGO at 0.2 A g-1 (voltage range: 0-3 V). Adapted and reproduced with the permission from Ref. 61. Copyright 2016, American Chemical Society.

Ultrafine TiO2 and Nb2O5 nanocrystals uniformly anchored on reduced graphene oxide by controllable hydrolysis of titanium and niobium precursors, respectively, have been investigated as promising anode materials for high performance lithium ion batteries and sodium ion batteries by our group. The interactions between the functional groups (e.g. OH-, and COO- with negative charge on the surface of GO) and niobium or titanium precursor (positive charge) lead to the amorphous niobium/titanium hydroxide thin layer uniformly anchoring on the surface of GO sheets at step 1. During step 1, introducing proper amount of water can result in the controllable hydrolysis of the precursors which can prevent the aggregation of amorphous niobium/titanium hydroxide nanoparticles and result in metal hydroxides uniformly coating on GO nanosheet. After annealing at 450 °C under forming gas, amorphous hydroxide layers would convert to TiO2 or Nb2O5 nanocrystals with particle size of around 5 nm or 3 nm, respectively, and GO reduced to rGO simultaneously with enhanced conductivity. The architecture of ultrafine TiO260 or Nb2O561 nanocrystals anchored on highly conductive rGO network can not only enhance charge transfer and buffer the volume change during lithiation/delithiation or sodiation/ desodiation processes, but also provide more active surface area for lithium or sodium ion storage, resulting in superior rate and cycle performance.

Fig. 3a displays the typical TEM image of ultrafine Nb2O5 NCs and the nanocrystals stayed on the surface of rGO. The rate capability of Nb2O5 NCs/rGO could achieve as high as 85 mA h g-1 at a current density of 10 A h g-1 while almost no capacity could be observed for mixture of Nb2O5 and rGO, as shown in Fig. 4c. The poor performance of the mixture can be ascribed to the large particle size and non-robust combination of Nb2O5 in the M-Nb2O5-rGO (Fig. 5a). It is noted that the covalent bonding between GO and niobium precursor can produce uniform coating on GO, so that ultrafine Nb2O5 on rGO could be obtained after heat treatment under the forming gas. However, larger particle size of Nb2O5 was obtained when no GO was added into the solution during the hydrolysis process.

The cycle durability at a current density of 0.2 A g-1 for NCs/rGO also has been examined (Fig. 4c). The first discharge capacity delivered around 1200 mAh g-1, and then stable reversible capacity as high as 242 mAh g-1 could be achieved after 30 cycles. The Coulombic efficiency (CE) is nearly 100 %. The reversible capacity could maintain around 60 mAh g-1 after 5000 cycles (Fig. 5b), indicating fairly long-term cycling stability. The high rate performance and long-term cycling performance could be ascribed to the ultrafine Nb2O5 and high conductive rGO network in which the ultrafine Nb2O5 NCs provide more active surface area and the high conductive rGO can enhance the charge transfer. Metal oxide nanoparticles anchored on the surface of rGO platelets act as spacers between these platelets and rGO not only acts as a conductive additive but also offers sites for lithium insertion/desertion during charge and discharge process thus improve the battery performance. However, for the M-Nb2O5-rGO, the capacity decreased to 30 mAh g-1 after 100 cycles, as shown in Fig. 5c. These examples have further confirmed the feasibility of using both graphene and rGO to enhance the electrode conductivities of electrochemical devices.

In addition to graphene and rGO, graphite also has been proposed to be a good candidate to improve the conductivity of electrode materials. Fei et al developed62 a Co3O4/graphite composite material, in which the graphite as an additive material, significantly enhanced the battery performance compared to pristine Co3O4 due to its high conductivity and structural stability. The Co3O4 was synthesized by hydrothermal method shown as follows:

In order to study the effect of graphite, three groups of samples have been prepared. Group A is the pristine Co3O4. In group B, 0.1g graphite was added before the hydrothermal step and removed after post annealing step. And in group C, the same amount of graphite was added and retained after post annealing step. XRD patterns proved Co3O4 with cubic structure has been successfully synthesized. No impurity phases were detected in group A and B. However, in group C, two extra peaks which can be indexed as graphite were detected and 21 wt % of graphite in group C was then quantitatively measured and confirmed by TGA. TEM images revealed the remarkable morphology change from nanowire to nanoparticle due to the addition of graphite. Possible mechanism for morphology change has been proposed. It is noted that crystals are likely to grow along the same direction to form nanowires as a result of the hydrogen bonding reaction. Nevertheless, when graphite was introduced into hydrothermal reaction, hydroxyl group on graphite surface would attract the Co and further disorganize its morphology, thereby forming nanoparticles instead of nanowires. Eventually, the half-cell test stated that Co3O4/graphite with a capacity of 1082 mAh g-1 at low current density of 100 mA g-1 presents an excellent cyclic performance and rate capability, indicating that additional graphite extraordinarily improves the pure Co3O4 battery performance.

3.2 Carbon nanotubes-based composites

Carbon nanotubes (CNTs) with one-dimensional (1D) nanostructures and high electronic conductivities make them the promising conductive agents in the application of electrochemical energy storage devices since they take the advantage of directional electronic and ionic transportation, leading to high electronic and ion conductivities.30,36,63–67 For the composites based on CNTs and transition metal oxides, CNTs are ideal conducting path to ensure a good charge propagation. Besides, they serve as a perfect skeleton with stretchable properties that significantly improves the cyclability of electrochemical devices through decreasing the volumetric changes of material during charging/discharging. Recently, highly porous MnO2 nanoflakes/carbon nanotube (CNT) nanocomposites were employed as the promising anode materials for high performance LIBs, delivering a large reversible capacity of 801 mAh g-1,high rate and high durability.68 The excellent electrochemical performance of the MnO2/CNT nanocomposite electrode can be attributed to its unique hierarchy architecture (Fig. 6a-b) and high conductivities of electrode after combining with CNTs, providing fast lithium ion and electron transport. Except for MnO2/CNT composite, Fe3O4 sheath (particle size of 5−7 nm)/aligned carbon nanotube (CNT) scaffolds (Fig. 6c-d) composite electrode also showed a high reversible capacity of 800 mAh g-1 and high rate capability.69 The superior electrical conductivity of CNTs maintains the charge transfer of Fe3O4 sheath/CNT composite electrode during charge/discharge process. Hybridization of CNTs with TiO2 and LiNbO3 is considered to be the most facile and effective approach since CNTs can facilitate faster electron transfer and also provide more Li+ diffusion paths, which would increase the performance of LIBs.70,71

Fig. 5 (a) TEM image of mechanically mixed Nb2O5 and rGO; (b) Cycle performance of Nb2O5 NCs/rGO at a current density of 10 A g-1; (c) The cycling stability of the mechanically mixed Nb2O5 and rGO. Adapted and reproduced with the permission from Ref. 61. Copyright 2016, American Chemical Society.

Similar to the uniform TiO2 coating on graphene, TiO2 coating on CNTs also has been proposed and utilized as electrode in lithium ion batteries to increase the electrochemical performance. TiO2, a promising material with high energy density and durability, has been widely used in supercapacitors or LIBs. Meanwhile CNTs as 3D material, not only function as a template to control the morphologies, but also strengthen the electrochemical performance of TiO2 due to its high conductivity. Recently, we have developed a hybrid material consisting of uniform porous TiO2 coating on CNTs prepared by sol-gel method. The porous TiO2/CNTs hybrids were synthesized via hydrolysis reaction of titanium isopropoxide on the surface of CNTs. TEM images show that continuous porous TiO2 was uniformly coated on CNTs with a thickness of ~20 nm. Anatase TiO2 phase was confirmed by XRD pattern and Raman spectraindicate the existence of anatase TiO2 and CNTs, respectively. High resolution TEM was also used to verify the porous structure with pore size < 10 nm. Moreover, to study the importance of the contact between TiO2 and CNTs, a controlled group was established, in which the same mass ratio of non-functionalized CNTs were added. It has been reported that, compared with the mixture of TiO2 and non-functionalized CNTs, the porous TiO2/CNTs shows a comparable initial discharge/charge capacity (268 and 207 mAh g-1, respectively), superior durability and stability. The retention of 97 % among the current density increased from 0.2 A g-1 to 1.6 A g-1 and decreased back to 0.2 A g-1, suggesting that the strong interaction between TiO2 and CNTs has a dominate impact on electrical performance. In conclude, it has been stated that the superior electrical performance of the porous TiO2/CNTs can be attributed to following factors: 1) the porous structure boosts lithium ions diffusion by providing a shorter diffusion path; 2) CNTs as highly orderedmaterials with good conductivity not only benefit the current collecting, but also form strong interaction with TiO2 which plays a vital role in maintaining the stability of electrode during charge/discharge process.

Fig. 6 TEM images of MnO2 nanoflakes/carbon nanotube (a-b), adapted and reproduced with the permission from Ref. 68 Copyright 2010, Royal Society of Chemistry; and Fe3O4 sheath/CNT (c-d), adapted and reproduced with the permission from Ref. 69. Copyright 2013, American Chemical Society.

3.3 3D mesoporous carbon-based composites

Recently, the instant gelation method has been proposed to prepare 3D mesoporous MoS2@C composite.72 The jelly-like semisolid would form after combining silicon based macromolecules and other macromolecules such as sugar since they are rich in hydroxyl (-OH) groups and can bind with polyatomic anions such as MoS42-.The subsequent calcination process can lead to the formation of SiO2 network and 3D carbon network after the carbonization process of sugar. SiO2 network could be removed using NaOH solution, resulting in final product with mesoporous structure and high surface area. Therefore, it can be used to prepare numerous 3D carbon composites with metal oxides, metal nitrides and metal sulfides. The LIBs with 3D MoS2@C composite as anode materials delivered enhanced electrochemical performance compared with pure MoS2.The conductive 3D carbon matrix not only promotes charge transfer but also prevents structural collapse of the electrode materials due to huge volume changes during the charging/discharging cycles. Moreover, numerous micropores and mesopores in the 3D carbon matrix can facilitate the ion diffusion to active sites with less resistance, resulting in enhanced charge transfer and thus better electrochemical performance. Recently, we have also successfully designed and prepared hierarchical MoS2–C microspheres by a facile and scalable ultrasonic nebulization route.73 The space between few-layer MoS2 nanosheets is filled and bridged with mesoporous carbon, as shown in Fig. 7a-d. The hierarchical MoS2–C microspheres exhibited excellent electrochemical properties including low resistance, high rate capacity (a capacity of 730 mAh g-1 at high current density of 3200 mA g-1), and stable cycling capacity (maintain 800 mAh g-1 over long cycles). The excellent electrochemical performance of hierarchical MoS2–C microspheres may be ascribed to their hollow structures which can provide more space for fast mass transport and the bridged carbon between few-layer MoS2 nanosheet which can enhance the electronic conductivity. Mesoporous Nb2O5/carbon (m-Nb2O5-C) nanocomposite has been employed as SIB anode.29 Besides, ordered mesoporous carbon TiO2 materials which were synthesized by evaporation-induced selfassembly and in-situ crystallization delivered high rate and high cycle durability.13 The microporous carbide-derived carbon composite (CDC) takes the advantage of the highest specific surface area of more than 3000 m2 g-1.74 It is expected that the electrode materials and high surface carbide-derived carbon composite exhibit high electrochemical performance for supercapacitors. For example, the composite consisting of carbide-derived carbon composite and niobium pentoxide delivered high capacitance after employing this composite as electrode for supercapacitors.75

3.4 Carbon coated metal oxide composites

Carbon is an abundant element and could be obtained by many adaptable and cheap precursors such as biomass, polymers and glucose. Our group has successfully developed a versatile method to obtain carbon coated materials with high conductivity via polymer assistance chemical solution (PACS) method.76 For PACS method, water soluble metal salts (e.g metal nitrate, metal chloride and metal hydroxide) bind with a polymer such as polyethyleneimine (PEI). The lone electron pairs on the nitrogen atoms of the PEI provide the ability to bind with some metal ions through covalent bonding, e.g. Ni and Co.77,78 Other metal ions (e.g. Mn) that cannot bind to PEI directly could bind to ethylenediaminetetraacetic acid (EDTA) to form an EDTA-metal complex via covalent bonding.16 The binding between the EDTA complexes and PEI could be further strengthened via a combination of hydrogen bonding and electrostatic attraction. Fig. 8 displays the molecular structure of EDTA, PEI, and the bonding with transition metal ions. Foam-like powders of carbon coated metal oxide nanoparticles with high surface area could be achieved after thermal treatment of the metal-polymer complex precursors in furnace under the air. Comparing with other solution methods, PACS owns the unique advantage of preventing the hydrolysis of some metal precursors (e.g niobium chloride and titanium chloride) due to the strong binding between polymers and metal ions.

Fig. 7 SEM image (a), TEM images (b, c) and high resolution TEM image (d) of hierarchical MoS2–C microspheres; (e) rate capacity and cycling performance (f) of the hierarchical MoS2–C microspheres, Adapted and reproduced with the permission from Ref. 73. Copyright 2016, Royal Society of Chemistry.

It is noted that certain amount of carbon coexists with metal oxides when the metal-polymer complex solutions are heated at relatively low temperatures.79 The carbon generated from the incomplete depolymerization of PEI and EDTA could be in-situ coated on the metal oxides. Therefore, the nanoparticle size and the carbon coating thickness can be easily controlled by tuning the heating temperature, time, as well as the concentration of the solutions. The carbon coating also can prevent the agglomeration of metal oxides under heat treatment. More importantly, the carbon coating could enhance the electronic conductivities of pristine metal oxides. Our recent work has showed that two-dimensional V2O5 sheet network, which was synthesized by a one-step PACS method, was employed as electrode materials for LIBs.80 From Fig. 9a, a network-like sheet structure could be observed and the thin carbon layer was uniformly coated on the V2O5 nanoparticles. The thin carbon coating layer remained after the incomplete decomposition of the polymer. The specific network-like sheet structure can enhance charge transfer and lithium ion diffusion speed, resulting in high capacity of about 300 mAh g-1 (cathode) and 600 mAh g-1 (anode). Carbon coated transition metal oxides (TMOs) nanostructures were firstly proposed to grow on nickel foam by a facile PACS method for binder-free LIB anodes.79 Fig. 9b presents the growth mechanism of TMOs on nickel foam by PACS method. The polymers can serve as a binder in the growth process. The long polymer chains with strong adsorption of metal ions can tangle on the skeleton of the nickel foam during the synthesis process. The metal ions can be in-situ fixed on the skeleton of the nickel foam and further be oxidized to metal oxides under the subsequent annealing process. Fig. 9c displays the SEM images of Co3O4 nanostructures on the skeleton of nickel foam. We can see from this figure that the nickel foam was uniformly covered by Co3O4 sheet, which can retain the same shape as the skeleton of nickel foam. The thickness of Co3O4 sheet is around 100 nm (The cross-section of the Co3O4 sheet is shown in the inserted picture of Fig. 9c). The Co3O4 sheets have strong binding with the surface of nickel foam and without any detachment after the strong sonication process. TEM images of Fig. 9d shows that Co3O4 nanoparticles with diameter of around 30 nm were uniformly coated by thin carbon layers. Similar to carbon coated V2O5 nanosheet network, the existence of thin carbon layer on the Co3O4 nanoparticles can serve as binder between Co3O4 and nickel foam. The interconnected structure and conductive carbon thin layer on Co3O4 nanoparticles provide a continuous pathway and fast path for lithium ions diffusion and charge transfer during charge and discharge processes. The carbon coated nickel doped LiMn2O4 cathode has also been synthesized via PACS method. The battery with carbon coated nickel doped LiMn2O4 cathode exhibited the best durable rate capability, with 100 % of the capacity retained (∼100 mAh g−1) after 400 cycles at 10 C current rate.76 The excellent cyclability and rate capability originate from the thin carbon layer formed from incomplete depolymerization of polymers and improved conductivity.76

Fig. 8 The molecular structure of EDTA and PEI, and the bonding with transition metal ions to form metal complexes.

Fig. 9 HRTEM images of carbon coated V2O5 (a), schematic mechanism for the growth process of TMOs on nickel foam. The photographs of the original nickel foam (left) and Co3O4 covered on nickel foam (right) (b); SEM image of Co3O4 on nickel foam prepared from PACS, the insert is the cross-section of Co3O4 sheet (c); and TEM image of carbon coated Co3O4 (d). Adapted and reproduced with the permission from Ref. 80. Copyright 2014, American Chemical Society; Ref. 79, Copyright 2013, Elsevier.

3.5 Special structure design of carbon coated metal oxide composites

Carbon coated metal oxide composites can not only increase the electronic conductivity but also stabilize the structure of the electrodes during charge/discharge process. Special structure design (e.g. York shell81 and peapod-like structure with carbon coating82,83 of the electrode materials can significantly increase the cycle durability and capacity of batteries. Recent study has reported that MnO nanoparticles confined in carbon shell with peapod-like structure provided 20.6 % void space inside the carbon shell.82 The carbon shell resulted from self-polymerization of dopamine was uniformly coated on MnO nanowires. The MnO@C core-shell with numerous voids could balance the huge volume change and stabilize the electrode structure, especially under high rate test. However, the high rate performance of MnO@C core-shell structures is also strongly dependent on the highly conductive carbon shell, which can enhance the electronic conductivity to satisfy the requirement of fast charge transfer under high rate test. Ex-situ TEM (Fig. 10) demonstrated that there is no obvious morphology change after 1000 cycles at high current densities of 2000 mA g-1, indicating that MnO@carbon core-shell composites with peapod structure are promising candidate as electrode materials in superior stable energy storage electrochemical devices.83 Another successful design was to make FeM3O4@Fe3C core@shell nanoparticles confined into hollow conductive carbon shell.81 The specific York-shell structure with sufficient internal void space can introduce more space to maintain the integrity of the electrode structure for high rate and long cycle performance. Moreover, the double shells of both highly conductive Fe3C and carbon shell not only prevent the dissolution of Fe3O4, but also increase the conductivity of the active electrode materials. The LIBs with Fe3O4@Fe3C-C yolk–shell nanospindles as anode materials delivered superior electrochemical performance including high rate capacity and high cycle performance. The void space in carbon shell for huge volume change and contraction as well as the significant charge transfer enhancement of carbon shell can lead to high electrochemical performance.

Fig. 10 The performance of MnO@C core@shell (a) and the cycle durability of Fe3O4@Fe3C-C yolk–shell nanospindles (b). Adapted and reproduced with the permission from Ref. 81 and Ref. 83. Copyright 2016 and 2014, American Chemical Society.

3.6 Electronic structure modification of electrode materials

Most metal oxides suffer poor electronic conductivity and thereby result in the lower electrochemical performance, especially for titanium or niobium-based oxides with large band gap. Take the intercalation-type TiNbxO2+2.5x (x = 2, 5 and 24) anode materials as the example, the empty 3d/4d orbitals in Ti4+/Nb5+ ions of TiNbxO2+2.5x lead to their low electronic conductivity (~ 10-9 S cm-1), greatly limiting their practical applications.84 Various strategies to enhance the conductivities of TiNbxO2+2.5x have been explored in order to take the advantage of their large theoretical capacities (388–402 mAh g-1). In addition to incorporation with conductive carbon-based materials85–87 or nitridation88 of surface layer on TiNbxO2+2.5x, modification of the electronic structures via doping other elements and generating oxygen vacancy to improve their conductivities could also be employed. Recently, highly conductive Cr0.5Nb24.5O62 (C2 space group) nanowires with open ReO3 crystal structure have been reported as a class of high performance anode materials for LIBs.89 The total/partial density of state (DOS) of Cr0.5Nb24.5O62 is displayed in Fig. 11a. Fermi level of Cr0.5Nb24.5O62 is inside of some band and moves to high energy end, indicating the conductor behavior of Cr0.5Nb24.5O62. The 3d orbital of Cr is slightly hybridized with the 2p orbital of oxygen, making the contribution of the bands below the Fermi level. The unpaired 3d electrons in the Cr3+ ions provide the free transport electron in crystal structure, resulting in Cr0.5Nb24.5O62 with high electronic conductive behavior. High electronic conductivity (3.6 × 10-2 S cm-1) of Cr0.5Nb24.5O62 nanowire leads to large Li+ ion diffusion coefficient (2.19 × 10-13 cm2 s-1), and thus outstanding electrochemical properties. The reversible capacity of 344 mAh g-1 at 0.1 C was the largest capacity among all the reported Nb-based anode materials. Excellent rate capability of 209 mAh g-1 at 30 C and superior cycling stability, makes Cr0.5Nb24.5O62 high performance anode materials for LIBs with improved safety since high operation potential (> 1.65 V vs Li/Li+) can prevent the formation of lithium dendrites, as shown in Fig. 11b and 11c.

Fig. 11 (a) Calculated DOS of Cr0.5Nb24.5O62; (b) (i) First-cycle discharge–charge curves at 0.1 C and second-cycle discharge–charge curves at different rates of Cr0.5Nb24.5O62-M and Cr0.5Nb24.5O62-N; and (c) Cycling stability of Cr0.5Nb24.5O62 -M and Cr0.5Nb24.5O62-N cells at 10 C. Adapted and reproduced with the permission from Ref. 89. Copyright 2017, American Chemical Society.

Fig. 12 XPS spectra of pristine and vacuum-treated TiO2 nanocrystals (a); the capacities of pristine and vacuum-treated TiO2 nanocrystals for the first 200 cycles (b). Adapted and reproduced with the permission from Ref. 93. Copyright 2011, Royal Society of Chemistry.

Creating oxygen vacancy is another typical technique to improve the conductivity of metal oxides.90 The formation of the oxygen vacancy in metal oxides can be described by the Krgoer-Vink equation91:

where OO and V..O represent the lattice oxygen and oxygen vacancy, respectively. The formation of oxygen vacancy is also associated with the reduction of valence state of metal based on the defect equation91,92 (taking vacuum-treated TiO2 as example):

Fig.12a displays the high resolution XPS spectrum of titanium in pristine TiO2 and vacuum treated TiO2 (with generation of oxygen vacancy). A shoulder around 457 eV in vacuum treated TiO2 could be assigned to the characteristic binding energy of Ti3+. The partially reduced titanium can provide excessive free electrons in TiO2 crystal structure, leading to highly conductive TiO2. The reversible capacity (140 mAh g-1 at 1 C rate) of TiO2 nanocrystals with oxygen vacancy is 34.6 % higher than that of pristine TiO2 nanocrystals (104 mAh g-1), as shown in Fig. 12b.

Oxygen vacancy generation in TiO2 nanotubes has been demonstrated as an effective way to enhance the capacity of titania.93 The strategy of oxygen vacancy has been also introduced to enhance the performance of SIBs. Recent study has reported that MoO3-x with rich oxygen vacancy can be significantly beneficial to the high electrochemical performance of SIBs94 since oxygen vacancy can increase the electronic conductivity and ion diffusion coefficient, leading to high reversible capacity and excellent rate capacity as well as high cycle durability. Our group also reported that stoichiometric Nb2O5-x@carbon matrix exhibited high rate and high durability after employing it as anode materials for LIBs (Ref. 84). In our work, the transformation from the stoichiometric Nb2O5 insulator to Nb2O5-x semiconductor as well as the uniform distribution of Nb2O5-x nanocrystals in the carbon matrix can effectively enhance the electronic conductivity and ion diffusion coefficient, leading to the enhanced battery performance by using Nb2O5-x@carbon matrix as electrode material.

3.7 Nitridation layer on the pristine electrode materials

Titanium and niobium-based oxides have been developed as promising anode materials for LIBs. Among them, titanium niobium oxide (TiNb2O7) takes the advantages of high theoretical capacity of 387 mAh g−1, high rate performance and improved safety issues since the high operation potential can prevent the formation of lithium dendrites.95 However, the wide band gap energy (> 2 eV) of TiNb2O7 leads to poor Li+ ion diffusivity and low electronic conductivity which can significantly decrease the battery performance. Recent study has shown that conductive Ti1−xNbxN layer on the surface of porous TiNb2O7 microspheres could be introduced by ammonia gas nitridation heat treatment,96 as shown in Fig. 13a. The formation of conductive nitridation layer can reduce the band gap of TiNb2O7 from 2.35 to 1.3 eV (Fig. 13b), leading to the enhancement of the mass and charge transport kinetics associated with Li+ ions (ion diffusion coefficient ≈1.4×10-12 cm2 S-1) and electrons. Consequently, the nitridation layer coated porous TiNb2O7 microspheres delivered a high discharge capacity of around 265 mAh g−1, superior rate capability of about 143 mAh g−1 at 100 C and high cycle durability of 91 % capacity retention after 1000 cycles, as shown in Fig. 13c and 13d.

Fig. 13 (a) High resolution TEM images of nitridation layer on TiNb2O7; (b) schematic illustration of surface energy band diagram of TiNb2O7 and nitridation layer on TiNb2O7; (c) rate capability of TiNb2O7 and nitridation layer on TiNb2O7; and (d) cycling performance and their Coulombic efficiencies at 5C. Adapted and reproduced with the permission from Ref. 96. Copyright 2015. WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Therefore, nitridation process has been also utilized to increase lithium ion diffusion and electrocatalytic activity of other positive electrode materials for high performance LIBs. For example, one study showed that the nitridated TiO2 hollow fibers delivered two times higher rate capacity compared to pristine TiO2 nanofibers.97 Besides, nitridation-driven anatase TiO2@graphene composite, Li4Ti5O12 and TiO2 nanowires have been recognized as anode materials for high performance LIBs.98 The incorporation of conductive nitridation layer on Li-rich layered Li[Li0.17Ni0.25Mn0.58]O2 oxide results in high rate (164.7 mAh g-1 at the current density of 1500 mA g-1) and high cycle performance (capacity retention of 256 mAh g-1 after 60 cycles).99 Considering that the easy handling of nitridation process and enhanced conductivity resulted from it,nitridation could be introduced to TiO2/CNTs to further enhance the charge transfer. It is expected that the excellent electronic conductivity of TiON and introducing nitrogen into CNTs could both increase the charge and mass transfer, resulting in the improved capacitance performance.

3.8 Electrode materials with dual conductive network

It has been discussed above that electrode materials with surface carbon coating can enhance the conductive behavior and ion diffusion rate. However, the poor electronic conductivity or disconnection between the separated particles still has limited the performance improvement, especially in micro-sized electrode materials. Carbonbased material including mesoporous carbon and graphene with two-dimension structure is usually identified as a conductive media to connect separated particles and thus improve the battery performance by improving charge transfer and maintaining electrical contact in electrodes.100,101 Thus, it is expected that the active electrode materials sandwiched by dual conductive layers will further enhance the electrode conductivity due to the decrease of the contact resistance of the electrode. Recently, a novel micro-sized graphene@silicon@carbon composite with dual conductive network (Fig. 14a) has been reported as high performance anode materials for LIBs.102 The electrical resistance of silicon has been significantly decreased by the incorporation of dual conductive network where carbon was coated onto micro-sized silicon and graphene could be used to connect separated micro-size silicon with carbon coating, resulting in better battery performance. Carbon coated SnO2/graphene nanosheet is another example of using dual conductive networks to enhance the electronic conductivity.103 The illustration of the formation of carbon coated SnO2/graphene nanosheet is displayed in Fig. 14b. The dual conductive network in which SnO2 nanoparticles were wrapped by graphene nanosheets and carbon layer can serve as electron conductor, leading to superior battery performance with large reversible capacity and high cycle durability. Besides, this carbon coated SnO2/graphene nanosheet not only increases the conductivity, but also can maintain the structure and avoid the detachments of SnO2 nanoparticles during cycling. Similar design of 2D core-shell graphene@silicon@carbon has been proposed as the high-performance electrode materials as well.104 Except for graphene as conductive media, CNTs also serve as the conductive chain to enhance the electronic conductivity. Recently, carbon-coated Fe2O3 hollow nanohorns uniformly grew on CNTs backbone (carbon@Fe2O3@CNTs),105 as shown in Fig. 14c. The combination of highly conductive CNTs backbone and carbon coated layer on Fe2O3 makes carbon@Fe2O3@CNTs a durable electrode material for LIBs. Dual conductive network (carbon layer and CNTs or graphene composite) served the same function of conductive matrix in which electrode material is homogenously immersed or distributed into the conductive network. This specific conductive network with superior conductivity can enhance the ion diffusion, charge transfer and maintain the material structure during high rate charge/discharge process.

Fig. 14 Preparation process of G/Si–C. (a); the formation of carbon-coated SnO2/graphene nanosheets (b); and TEM images of carbon@Fe2O3@CNTs (c-e); Figure a and b is adapted and reproduced with the permission from Ref. 103. Copyright 2014 and Ref. 104, Copyright 2012, Elsevier, respectively; Figure c-d is adapted and reproduced with the permission from Ref. 105. Copyright 2012. Royal Society of Chemistry.

3.9 The incorporation of metal particles or metal coating with electrode materials

Incorporation of metal nanoparticle or coating metal thin layer on electrode is another strategy to increase its conductivity due to the enhanced ion diffusion and the high conductivity of the nanostructured metal architecture. Recently, nanoporous gold/MnO2 thin films with thickness of 100 nm have been successfully prepared by combination of chemical de-alloying and electrodeless plating,106 as shown in Fig. 15a. The nanoporous gold was prepared by de-alloying Ag65Au35 using HNO3. TEM images (Fig. 15b) showed that the pores were filled with MnO2 nanoparticles. The excellent contact between the gold ligaments and nanocrystalline MnO2 could be confirmed since that MnO2 nanoparticles grew epitaxially on the gold surfaces. The formation of chemically bonded metal/oxide interface caused the manganese L2,3 edges shift to the low-energy side in the gold/MnO2 interface region, resulting in enhanced charge transfer between the gold substrate and MnO2 nanocrystals. The nanoporous gold acts as the good electronic/ionic conductor to enhance the electrochemical performance of MnO2 nanoparticles which are immersed in the nanoporous gold. The nanoporous gold could provide the electron transport path to MnO2 and thus increase ion diffusion rate between the electrolyte and MnO2. The hybrid structure can not only maintain the high specific capacity of MnO2 but also enhance the conductivity of MnO2 (poor conductivity of 10-5–10-6 S cm-1), resulting in high specific capacitance of 1145 mAh g-1 that is almost close to the theoretical value (1232 mAh g-1)107 of MnO2.

Incorporation of silver nanoparticles on electrode is another typical example to enhance the conductivity. Recent study has found that the electronic conductivity of the Ag/TiNb2O7 and Ag/ Li4Ti5O12 (LTO) electrode materials have been significantly increased, leading to fast charge transfer, electron transport and enhancement of electrochemical performance including high cycle and high rate performance. 108, 109 Besides, addition of uniformly dispersed metal in cathode materials has been also demonstrated to increase the conductivity and electrochemical performance. Corce et al showed that silver or copper dispersion on phosphor-olivine LiFeO4 did not change the structure of LiFePO4 but decreased the interparticle resistance. LIBs with metal modified LiFePO4 electrodes delivered the enhanced capacity comparing with pristine LiFePO4.110

Fig. 15 The fabrication process for nanoporous gold/MnO2 hybrid materials (a); bright-field TEM image of the nanoporous gold/MnO2 hybrid (b); HRTEM image of nanoporous gold/MnO2 (c); high-angle annular dark-field STEM image taken from the interface between gold/MnO2 (d); and EELS profile image from a region across the interface between gold/MnO2 (e). Adapted and reproduced with the permission from Ref. 106. Copyright 2012. Nature Publish Group.

3.10 Conductive polymers on electrode materials

Conductive polymers, such as poly(3,4-ethylenedioxythiophene (PEDOT), due to its excellent conductivity, high stability and mechanical flexibility, could be used as conductive media to improve the charge transfer.111,112 Recently, it has been reported that 1D rod-like layer manganese oxide could be uniformly distributed on 3D graphene matrix with the assistance of PEDOT. The specific structure with PEDOT coating on MnO2 and 3D graphene matrix provides more surface area for mass transport and enhances the charge transfer to get high rate performance.111 MnO2/PEDOT coaxial nanowires with 1-D structure have been developed as high performance electrochemical energy storage materials. Highly conductive and porous PEDOT shell increases the mass and charge transfer to core MnO2, leading to that the coaxial nanowires with PEDOT shell coating exhibit high rate performance and excellent cycle durability.113 Rhombohedral LiNbO3 with poor conductivity is considered to be a promising anode material in LIBs since it owns a large theoretical capacity (363 mAh g-1). 3D conductive network consisting of CNTs and conductive polymer polypyrrole (PPy) was introduced to synthesize self-weaving CNT-PPy-LiNbO3 electrodes.70 The special 3D structure with superior conductivities is composed of LiNbO3 nanoplates, CNTs backbone and PPy coating. This structure can facilitate the mass and charge transfer, yielding larger reversible capacities of around 250 mAh g-1 at a current density of 0.4 A g-1, an excellent rate capability of 150 mAh g-1 at a high rate of 4 A g-1 and high cycle durability with a high capacity of 220 mAh g-1 which could be maintained after 500 cycles.


4 Summary and future perspectives

In this mini review, we have summarized the recent development of highly conductive electrodes for high performance LIBs. Various strategies employed for enhancing the conductivity of the electrode materials have been discussed. Carbon based materials (e.g. graphene, CNTs and porous carbon), which were regarded as the light and high conductive materials, have been the first choice to be utilized to combine with the electrode materials in order to increase their conductivities and subsequently improve the battery performance. Besides, introduction of nitridation layer, conductive polymer layer, oxygen vacancy on electrode materials (usually metal oxides) as well as electrodes being sandwiched by dual conductive networks and so on, could also be utilized as alternatives to improve the conductivities of the electrode materials and subsequently the battery performance.

To make full utilization of carbon-based materials, we have proposed a sol-gel strategy to uniformly coat electrode materials on graphene oxide or functional carbon nanotubes. The strong affinity between metal precursors and functional groups on graphene/ reduced graphene oxide or carbon nanotubes can result in the uniform coating. However, the introduction of functional groups could decrease the conductivity and subsequent annealing process under the reduced atmosphere at high temperature is required, which involves complicated and tedious work. Moreover, the synthesis process requires high oxidant reagents and strong acids, leading to some critical environmental issues. It is highlighted that the porous MoS2/C micro-sphere has delivered excellent electrochemical performance including high durability and rate performance. However, considering the sacrifice of volume capacity of battery, the balance between the porous structures of the metal oxides and electrochemical performance should be optimized although the porous structure of electrode materials will be beneficial for ion diffusion. All the aforementioned strategies involve more than two steps, which might introduce extra safety issues and reduce the electrode production efficiency and thus limit the practical applications of LIB/SIBs. Therefore, it would be more convenient to prepare metal oxides on CNTs or graphene as well as other carbon composites using one-step procedure. Moreover, the battery performance with electrode materials growing on nickel foam by PACS method has demonstrated that PACS will be an effective method to prepare electrode materials on substrate or support directly. Therefore, the future work of our group will focus on uniformly coating electrode on carbon-based materials by a versatile PACS method, through which highly conductive electrode materials could be prepared via one-step reaction.


Acknowledgements

Dr. Luo thanks the support from New Mexico EPSCoR with NSF-1301346 and USDA National Institute of Food and Agriculture, HSI Collaboration: Integrating Food Science/Engineering and Education Network (IFSEEN, award number: 2015-38422-24059).


Authors' Biography

Litao Yan

Litao Yan is currently a postdoctoral at Pacific Northwest National lab. He got Ph.D degree in chemical Engineering at New Mexico State University. His research interest lies in lithium ion batteries, fuel cells and flow batteries. He received his B. S. degree in Materials Physics from Lanzhou University in 2007 and M.S. in Materials Science from the University of Science and Technology of China in 2010.

Haizhen Wang

Haizhen Wang is a Ph.D. student in Department of Chemical and Materials Engineering at New Mexico State University. She received her B.S. degree in Chemistry from Liaocheng University and M.S. degree in Inorganic chemistry from University of Science and Technology of China (USTC). Her research interest mainly focuses on the design of transition metalbased bifunctional electrocatalysts as well as anode materials for lithium ion batteries.

 

Di Huang

Di Huang is a Ph.D. student in Department of Chemical and Materials Engineering at New Mexico State University with a research interest in the development of novel cathode materials for lithium ion batteries. He received his B.S. degree in Chemical Engineering from East China University of Science and Technology in 2013 and M.S. degree in Chemical Engineering at New Mexico State University in 2015.

Hongmei Luo

Hongmei Luo is currently a Luke Barry Shires Endowed Associate Professor in Chemical Engineering at New Mexico State University. She received her B.S. (1992) in chemistry from Fuyang Normal University, China, and Ph.D. (2006) in chemical engineering from Tulane University. She was a postdoctoral research associate at Los Alamos National Laboratory from 2006 to 2009 and she joined NMSU in fall, 2009. Her group research focuses on epitaxzial oxide thin films and nanostructured materials for magnetic, optoelectronics, catalysis, and renewable energy applications.

 

 

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