Lithium Batteries

Nanostructured materials are widely used as electrodes in rechargeable lithium batteries.19 Elongated titanates also attract attention as possible negative electrode material for lithium cells due to their open, meso-porous structure, efficient transport of lithium ions, and effective ion-exchange properties. Such properties result in these electrodes having a high charge/ discharge capacity (<300mAhg-1), and fast kinetics, together with very good robustness and effective safety characteristics.20,21 These new electrodes can replace commercial, carbon-based negative electrodes, which suffer from safety concerns (due to the directional electrodeposition of lithium) and the formation of a solid-electrolyte interface (SEI) layer which leads to charge loss.

Figure 5.3 a shows the principle of lithium storage in nanotubular titanates. During charging, lithium ions from the electrolyte solution intercalate between layers in the wall along the axis of nanotubes,22 followed by cathodic reduction to form the LixTiO2 phase:

where x is the lithium insertion coefficient. The simplified product formula does not represent the crystal structure of TiO2 or titanates, but it is convenient to express the value of x. The nature of LixTiO2 intercalate is unclear (e.g. oxi-dative state of Ti and Li), but it is suggested that lithium atoms accommodate the positions of ion exchanged protons in titanate nanostructures or the channels in TiO2-B nanofibres.23,24 The charge capacity of lithium batteries depends on the availability of lithium reduction sites, and the power characteristics of the batteries are often dominated by the kinetics of lithium intercalation.

There are three consecutive steps in the process of charging/discharging the electrode, namely: (i) the diffusion of lithium ions in the electrolyte; (ii) the diffusion of intercalated ions/atoms; and (iii) the electrochemical reaction. Each of these stages, as well as electron transport, can be limiting in the overall process. The use of nanostructured titanates and TiO2 significantly improves the rate of the diffusion of intercalated lithium ions due to their small size and the featured crystal structure of the material, which provides sufficient space (interlayer spacing in walls) for ionic transport.

At the present time, the following elongated nanostructures have been studied o c ^n no ^ i for lithium storage: titanate

(TiNF), TiO2-(B) nanotubes,32,33 nanofibres34 36 and TiO2 (anatase) nano-rods.33,37 The last three nanostructures were obtained by calcination of the protonated forms of corresponded titanates (see Chapter 4, Figure 4.10). All of these structures demonstrate an impressive ability to store lithium ions (see Table 5.1).

Titanate nanotubes are characterised by a high initial specific capacity which can rapidly decrease from approximately 300 to ca. 180mA h g-1 within several

Figure 5.3 The principle of lithium storage in TiO2 nanotubes: a) the principle and b) the first discharge curve for (1) TiNT, (2) TiO2-(B) NT and (3) TiO2 NR (adapted from ref. 33).

cycles. The charge/discharge curves are characterised by the absence of a plateau in which the voltage of the cell is constant. A slightly improved stability on cycling was observed using titanate nanofibres, which exhibited similar values for lithium storage capacity. Although these nanostructures have different

Table 5.1 Summary of electrodes containing elongated titanate or TiO2 nanoparticles used for lithium storage. [P25: spheroidal TiO2 nanoparticles (Degussa)].

Electrode

Discharge capacitya / mA hg

Specific current density / mA g

Merits for lithium storage

Ref.

TiNT

220-250

110, 200

Higher capacity than P25

25

170

2000

TiNF

220

110

Higher capacity than P25

28,29,30

190

300

130

2500

TiO2-(B) NT

240

50

Higher capacity than P25

33

TiO2-(B) nf

200

200

Higher capacity than P25

34,35

100

2000

TiO2 (anatase) NR

190

50

Plateau in current vs.

33,37

potential curve

240

36

NiO/TiO2-(B) NT

240

100

Durability, lower electrical

40

resistance

170

2000

C-TiO2 (anatase)

204

70

Lower resistance, plateau in

38

NR

current vs. potential curve

Co-TiNF

350

50

Intercalated Li affects mag-

39

netic properties

Co-TiO2 (anatase)

140

50

NF

Ag/TiNT

190

50

Higher cycling stability at

41

higher discharge rate

160

600

Sn/TiNT

312

30

SnLi alloying in pores of

42

TiNT

TiO2-(B) NT

296

25

Effect of electrode thickness

43

on the discharge kinetics

TiO2 (anatase) NR

215

25

"After 10 cycles.

"After 10 cycles.

morphologies and typical sizes, the rate of lithium ion intercalation is relatively fast in both as seen by their pseudocapacitive, faradaic behaviour.26,30 This apparent inconsistency can be explained by taking into account the directions of lithium ion movement in nanotubes and nanofibres. It was suggested that in titanate nanotubes alkaline ions diffuse along their length, whereas in nanofibres they diffuse in a direction perpendicular to the length.22 The typical length of nanotubes and width of nanofibres are several hundreds of nanometres, resulting in a characteristic diffusion time for intercalated lithium ion transport in both nanostructures of tens of minutes

The mechanism of lithium ion diffusion in TiO2-(B) nanotubes and nano-fibres is probably different to that in titanates; instead of diffusion between the layers of titanates, the lithium ions diffuse inside the smaller tunnels of theTiO2-(B) crystals.35 Due to an absence of ion-exchanged protons, hydrolysis of the electrolyte is suppressed. As a result, the initial discharge capacity does not deteriorate rapidly and the cycling stability is slightly better than that for nanostructured titanates. After a few cycles, the lithium storage capacity is similar to that of nanostructured titanates and the pseudocapacitive current behaviour also suggests that diffusion of lithium (external in electrolyte and internal intercalated) is not the limiting stage.

Anatase nanorods, produced by the calcination of TiNT, are characterised by a lower discharge capacity (see Table 5.1), a characteristic plateau on the charge/discharge curve (see Figure 5.3 b) and good reversibility. For all elongated nanostructures, the coulombic efficiency can approach 100%.

The power output demand for new generation lithium batteries (including power cells) is stimulating their use at higher current densities, which place additional requirements on the electrical conductivity of the nanostructured titanates and TiO2 electrodes. Recent approaches to improve the conductivity of elongated structures include doping of TiO2 nanorods with carbon;38 doping TiNF and TiO2NF with cobalt;39 deposition of NiO particles on the surface of TiO2-(B) nanotubes;40 coating the surface of TiNT with silver nano-particles;41 and co-precipitation of a tin phase in the pores of TiNT. 42 All of these methods allow a reduced electrical resistance of the electrode, improve durability and decrease charge capacity degradation during high rate charge/ discharge trials.

Another important issue for high current lithium batteries is the electrical resistance due to the limited mass transport of lithium in the electrolyte, resulting in a reduced voltage of the cell at high current densities for thick electrodes.43 Potentially, this issue can be resolved by using 3-D electrodes of controlled hierarchy, which can be achieved, for example, by using TiO2 nanotube arrays decorated with titanate nanotubes deposited inside the channels. The large pores would provide efficient and rapid transport of lithium ions from the cathode to the anode, while titanate nanotubes would supply sufficient sites for lithium intercalation.

Further improvements in lithium cell performance will probably focus on improvements in (i) the electrical conductivity of the elongated nanostruc-tures;(ii) the mass transport of lithium ions in electrolyte by an optimal design of electrode porosity; (iii) the charge capacity of the electrode; and (iv) the stability of the cell with repeated cycling.

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