The applications of carbon nanotubes and graphene

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Advanced rechargeable lithium batteries are desired energy storage devices for electric vehicles. These batteries ... graphene in advanced lithium batteries, and update their most recent research ...... Sci., 2011, 4, 4023–4030. 32 Q. Wang, P.

Volume 4 Number 23 21 June 2016 Pages 8917–9320

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Materials Chemistry A Materials for energy and sustainability www.rsc.org/MaterialsA

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REVIEW ARTICLE Yani Zhang et al. The applications of carbon nanotubes and graphene in advanced rechargeable lithium batteries

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Materials Chemistry A

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Cite this: J. Mater. Chem. A, 2016, 4, 8932

The applications of carbon nanotubes and graphene in advanced rechargeable lithium batteries Wenyu Yuan,ab Yani Zhang,*ab Laifei Cheng,ab Heng Wu,ab Lianxi Zhengc and Donglin Zhaoab Advanced rechargeable lithium batteries are desired energy storage devices for electric vehicles. These batteries require their electrodes to have high electrical and thermal conductivity, an appropriate high specific surface area, an outstanding hierarchical architecture, high thermal and chemical stability and to be relatively low cost and environmentally benign. Carbon nanotubes (CNTs) and graphene are two candidate materials that could meet these requirements, and thus have been widely studied. The present paper reviews the applications of CNTs and graphene in batteries, with an emphasis on the particular roles (such as conductive, active, flexible and supporting roles) they play in advanced lithium batteries. We will summarize the unique advantages of CNTs and graphene in battery applications, update the

Received 21st February 2016 Accepted 29th March 2016

most recent progress, and compare the prospects and challenges of CNTs and graphene for future full utilization in energy storage applications. The effects and mechanisms of heteroatoms doping, the

DOI: 10.1039/c6ta01546h www.rsc.org/MaterialsA

distribution of pore sizes, different architectures (anchored, sandwich-like and wrapped hybrid architecture) are discussed in detail.

1. Introduction Oil represents 34% of the world's total energy source, and results in over 40% of the world's CO2 emissions,1 which is contributing to future energy-depletion issues and raising a lot of environmental concerns. Electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid EV (PHEVs) will be among the rst batch of immediate applications that could signicantly reduce our energy dependence on fossil fuels. The further development of these applications, however, requires advanced energy storage devices that could offer high energy density, high power density, a long cycle life and high temperature resistance, etc.2 Among several main energy conversion and storage devices, advanced lithium batteries, including lithium-ion batteries (LIBs), lithium–sulfur (Li–S) batteries and lithium–oxygen (Li–O2) batteries, are potential candidates to meet the requirements for EV and HEV applications,3 and have thus also attracted great scientic interest. In general, LIBs have high energy density, a small selfdischarge current and high current charge/discharge cycles;

while Li–S batteries offer low-cost, are environmentally benign, have high gravimetric/volumetric energy density, and lower operation voltage; and Li–O2 batteries have very high specic energy density. The energy and power density of LIBs, Li–S batteries and Li–O2 batteries are shown in Fig. 1. At the same time, these batteries face many obstacles, such as safety issues, low active material utilization, and a capacity fade in LIBs, volume expansion in Li–S batteries and poor cycle life in Li–O2 batteries.

a

Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, 710072, Xi'an, China. E-mail: [email protected] hotmail.com; [email protected]

b

State Key Laboratory of Solidication Processing, Northwestern Polytechnical University, 710072, Xi'an, China

c

Department of Mechanical Engineering, Khalifa University, 127788, Abu Dhabi, UAE

8932 | J. Mater. Chem. A, 2016, 4, 8932–8951

Distribution of the different electrochemical energy storage performances.

Fig. 1

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Currently, the realization of their potentials and elimination of the challenges/concerns largely depend on the selection of electrode materials and the design of electrode structures, with considerations including the conductivity, specic surface area (SSA), development of a hierarchical architecture and structural stability. Nanomaterials, especially carbon nanotubes (CNTs), and graphene have been widely studied for battery applications owing to their outstanding electrical conductivity, excellent mechanical properties and good structural stability. When used as the electrodes in lithium batteries, these nanostructured materials can offer: (1) fast transport of mobile species; (2) enhanced surface reactivity; (3) mechanical robustness; (4) enhanced ionic and electronic conductivity; (5) high SSA.4 This review article will summarize the unique roles of CNTs and graphene in advanced lithium batteries, and update their most recent research progress. Specically, CNTs and graphene, respectively, as conductive, active, supporting and mechanical materials will be discussed in detail.

2. Challenges for advanced lithium batteries Fig. 2 presents the principle of LIBs, Li–S and Li–O2 batteries.5 For a LIB, the process can be represented as: ()-Cn|1 mol L1 LiPF6–EC + DEC|LiMOx (+), in which C means carbon materials and M indicates a metal. During the charging process, Li+ ions are de-intercalated from the cathode and intercalate into anode materials. During the discharging process, Li+ ions are deintercalated from the anode and intercalate back to cathode materials. The overall reaction is: LiMOx þ Cn )

charge

* Li1y MOx þ Liy Cn :

discharge

For a Li–S battery, sulfur, one of the most abundant and cheap ($150 per ton) elements on earth, is used as a cathode material, which has a highest theoretical capacity of 1675 mA h g1 among the solid elements.6 As a result, Li–S batteries have

Fig. 2

a high theoretical energy density of 2600 W h kg1. The overall reaction is:7 discharge

2Li þ S )

charge

* Li2 S:

During the discharging process, high order lithium polysuldes Li2Sx (6 < x < 8) are produced rst, then lower order lithium polysuldes Li2Sx (2 < x < 6) and Li2S at the end of discharging. For a Li–O2 battery, the theoretical energy density can reach 5200 W h kg1, 10 times higher than that of a LIB. In a non-aqueous system, the overall reaction is thought to be: discharge

2Liþ þ O2 þ 2e )

charge

* Li2 O2 :

During the discharging process, the lithium metal releases Li+ and combines with O2 from the air electrode, and nally Li2O2 is formed. During the charging process, the Li2O2 is oxidized to reform Li+ and O2.8 In LIBs, the cathode materials mainly include layered LiMO2 (e.g. LiCoO2, LiNi0.5Mn0.5O2, LiNi1/3Co1/3Mn1/3O2 (NCM),9 LiCo0.2Ni0.4Mn0.4,10 LiNi0.8Co0.15Al0.05O2 (NCA)), spinel LiM2O4 (e.g. LiMn2O4, LiNi0.5Mn1.5O4x) and olivine LiMPO4 (e.g. LiFePO4, LiMnPO4, LiCoPO4) and silicate compounds Li2MSiO4 (e.g. Li2FeSiO4, Li2MnSiO4); while the anode materials mainly include graphite, Si,11,12 SnO2 and MnO2. The key challenges with LIBs are believed to be in the following areas: (1) the side reactions, which occur at the interface, between the electrolyte and electrode during normal operation of the batteries or under abuse conditions, as these not only consume electrode materials, but also deposit impurities on the surface of active materials, resulting in capacity fade and thermal instability (re or explosion);13–16 (2) the low conductivity of some cathode materials, such as LiCoO2 and LiFePO4 (LFP), which causes their low utilization and results in a low energy (power) density of the batteries;17,18 (3) the heat generation due to the irreversibility in batteries and from the electrochemical

Schematic of Li-ion, non-aqueous Li–O2 and Li–S batteries. Reproduced with permission from ref. 5. Copyright 2012 Nature Publishing

Group.

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J. Mater. Chem. A, 2016, 4, 8932–8951 | 8933

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Journal of Materials Chemistry A Table 1

Advantages and disadvantages of the common electrode materials

Cathode materials

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Review

LiFePO4

LiNiO2 LiCoO2 LiMn2O4

S Li2S

Anode materials

Li Graphite Fe3O4

Sn

Si MnO2 ZnO

Advantages

Disadvantages

Relatively high capacity, thermal stability, excellent reversibility, low cost, environmental benign Good specic capacity High specic capacity Naturally abundant, environmentally friendly, good safety High specic capacity (1675 mA h g1), non-toxic, low cost Improved safety with a theoretical capacity of 1166 mA h g1

Lower electronic conductivity, low Li-ion diffusion coefficient

Low conductivity, poor cyclic performance, volume change (80%) Low electronic and ionic conductivities, sensitivity to moisture and oxygen Unstable, poor safety

Low potential and high capacity (3860 mA h g1) Low cost, good conductivity High theoretical capacity (z1000 mA h g1), non-toxic, improved safety, and low cost High theoretical capacity (993 mA h g1), high electronic conductivity, safety High theoretical specic capacity (4200 mA h g1) Low cost and rich resources Abundant, low cost and eco-friendly

reactions and the consequent slow thermal runaway, which can lead to serious safety issues; (4) the volume changes during the intercalation and de-intercalation of Li+ in some anode materials, such as Si, Ge,19 and Sn, which can cause drastic capacity fading and lead to a poor cycle life. In Li–S batteries, sulfur and Li2S are the common cathode materials, meanwhile lithium metal is used as the anode material. Overall, Li–S batteries offer substantially higher specic energy compared with LIBs, but they have low active material utilization, and suffer from capacity degradation, selfdischarge, poor coulombic efficiency, a poor cycle life, and electrode volume expansion. On the cathode side, the low conductivity of sulfur (5  1030 S cm1) leads to a low capacity and fast capacity fading, while the volume change of sulfur (up to 76% during charging and discharging processes) leads to structural deterioration and break down in the morphology, and the formation of polysulde anions, which can cause structural damage and surface reactions. On the lithium anode side, the instability of lithium metal raises some safety concerns and the surface coating results in low cycling efficiency.5,6 Li–O2 batteries display the highest theoretical specic energy among all the lithium batteries, but the achieved performance is still far from the theoretical capacity and the batteries suffer from poor cycling stability because of the following issues: (1) the pores of the air cathodes can become increasingly blocked by Li2O2 precipitates as the discharge proceeds; (2) an overpotential develops during discharging and charging processes due to the deposition of discharge products and blockage of the transport pathway at the cathode;20 and (3) the Li metal anode

8934 | J. Mater. Chem. A, 2016, 4, 8932–8951

Poor safety, poor cycle life High cost, poor conductivity Low capacity, capacity fading

Limited gravimetric capacity Poor cycling performance and low rate capability, large specic volume changes High volume change (300%), poor cyclability Huge volume change (>300%), poor conductivity Poor conductivity, instability Huge volume change (163%)

usually suffers low cycling efficiency and has safety issues because of the interfacial carbonate formation.21 Table 1 summarizes the advantages and disadvantages of the common electrode materials, which could be used as guidance in battery improvement. Many issues could be resolved through appropriate electrode design.

3. Requirements in electrodes for advanced lithium batteries As mentioned above, most challenges in the batteries are directly related to the electrode materials, and even the realization of the general requirements of high energy density and high power density could also be linked to the specic properties of electrode materials. Here, we discuss in detail the material requirements in terms of conductivity, SSA, micro- and nano-structures, and cost and safety related properties. 3.1

Electrical conductivity

One of the basic requirements for electrode materials is high electric conductivity, which is helpful to improve energy density and power density. There are three intrinsic negative aspects of poor electrical conductivity: (1) it reduces the operating voltage, the specic capacity, the energy density and the power density. Usually, U ¼ E  IR, C ¼ It ¼ Ut/R

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or C¼

ðt

Idt ¼

0

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1 R0

According to a simplied equation for the capacitance of electrical double-layer capacitors (EDLCs):

ðt Udt;



0

CUaverage ; m

The specic capacitance is directly proportional to the SSA. Similarly, the capacitance of batteries can be calculated from this equation:

W ¼ IUaverage ¼ IðE  IR0 Þ t

We can see that the voltage, capacity, energy density and power density decrease with the increase of resistance; (2) Li+ cannot be fully extracted/inserted at room temperature, because of the difficulty in transport. Several materials have failed to fully utilize the redox sites, and no pathways exist for electron transport between the redox site and external circuit;22 (3) the mass of electrodes is increased if conductive additives have to be used. In order to increase the conductivity, conductive additives, such as carbon black, are usually added to active materials to increase their electrochemical performance. The electrical conductivity data of the common electrode materials is shown in Table 2. The electrical conductivity of a powder material is generally lower than that of the theoretical bulk material, since the interface between the particles offers extra resistance to charge transport.23 It is found that most currently used electrode materials (e.g. LFP, Si, MnO2, S, etc.) have poor electrical conductivity, and therefore, the overall performance of the batteries is limited. Enhancing the conductivity of current electrode materials or utilizing new electrode materials that have high conductivity as well as other electrochemical properties will be a key step for further applications in EVs, HEVs and PHEVs. 3.2

Specic surface area

An appropriately high SSA of electrode materials increases the contact area between the electrodes and the electrolytes and provides more active sites, which reduces the polarization loss and improves the specic energy, power density, cycle stability and utilization of the active materials.

Table 2

30 3r A : d

Cb ¼ 26:8n

m : M

where m is the mass of active materials involved in the reaction, n is the electron number in the equation and M is the molar mass of active materials. In general, a higher SSA means more atoms (M) are involved in the reaction and this then yields a higher battery capacitance. A high SSA usually results in a smaller current density under the same discharge current, which is benecial for the decrease of electrochemical polarization and improvement of the utilization of active materials, especially decreasing the concentration polarization. The distribution of current density under non-concentration polarization can be described by the following equation: ia;0  ia;L ic;0  ic;L ia 2 nF :  ¼ 2RTksS a b where ia,0, ic,0, ia,L, ic,L express the oxidized current density and reduced current density of the surface outside (x ¼ 0) and at depth (x ¼ L), a, b are the transfer coefficients, S means SSA and k means the electrical conductivity. From this equation we can see that a higher SSA results in a more uniform distribution of current density. A high SSA is also helpful to improve the rapid charge performance. Because of the strong electrostatic repulsion between lithium ions and the cations in active materials, a high rate capability is usually difficult to expect.24 The mean diffusion (or storage) time seq is determined by the diffusion coefficient (D) and the diffusion length (L) according to the following formula: seq ¼ L2/2D.

Electrical and thermal properties of the electrode materials

Materials

Theoretical capacity (mA h g1)

Electrical conductivity (S cm1)

Thermal conductivity (W m1 K1)

Density (g cm3)

LiCoO2 LiMnO2 LiFePO4 LiMn2O4 LiNiO2 Graphene CNT Graphite Si MnO2 Sx

274 285 170 148 275 1116 1116 372 4200 2500 1675

0.5

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