SECONDARY BATTERIES – LITHIUM RECHARGEABLE SYSTEMS | Negative Electrodes: Lithium Metal

SECONDARY BATTERIES – LITHIUM RECHARGEABLE SYSTEMS | Negative Electrodes: Lithium Metal

Negative Electrodes: Lithium Metal K Kanamura, Tokyo Metropolitan University, Hachioji, Japan & 2009 Elsevier B.V. All rights reserved. Introduction ...

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Negative Electrodes: Lithium Metal K Kanamura, Tokyo Metropolitan University, Hachioji, Japan & 2009 Elsevier B.V. All rights reserved.

Introduction Lithium metal anode has been widely used in primary batteries for various types of portable and stationary electronic applications. Lithium is an alkaline metal that has the lowest atomic number of all elements, except for hydrogen and helium. The electrochemical potential of lithium metal is about  3.0 V vs NHE. When lithium metal is used as the anode material of a battery, a high cell voltage can be achieved because of its very low electrode potential. However, this low electrode potential does not allow us to use aqueous electrolytes that have been used in lead–acid battery, manganese oxide–zinc dry cell, Ni–Cd battery, and so on. As aqueous electrolytes can be easily reduced by lithium metal, they cannot be used in a battery with lithium metal anode. Fortunately, nonaqueous electrolytes have been used in the electrochemistry of organic synthesis. Nonaqueous electrolytes consist of organic solvents and lithium salts. In general, an organic solvent is electrochemically more stable than water. The theoretical decomposition voltage of water is 1.23 V. On the contrary, some of the organic solvents have higher decomposition voltages; hence, they can be used in batteries with lithium metal. Thus, lithium batteries have been developed using nonaqueous electrolytes. Various types of cells, such as coin type and cylindrical type, have been commercialized for power sources of several types of electronic devices. Figure 1 shows the photographs of commercialized batteries. Although lithium metal has been investigated as the anode material for rechargeable lithium batteries, it is not used in rechargeable batteries because of the safety issue of the battery. An electrode reaction of lithium metal anode is described as follows:

moisture and oxygen; therefore, its surface is always covered with some lithium compounds. In addition, lithium metal also reacts with nonaqueous electrolytes to form surface film. This surface film has Li þ ion conductivity and prevents severe chemical reactions of lithium metal with the electrolyte. This was found by E. Peled and the surface film on lithium metal is called ‘solid electrolyte interface’ (SEI). In this article, the electrochemistry and the surface chemistry of lithium metal are discussed.

Surface Film on Lithium Metal Lithium foils have been used in practical primary batteries, which are produced from lithium ingot by using equipment as shown in Figure 2. This preparation process is performed under a controlled atmosphere to maintain the surface state of lithium foil. Lithium foil is pushed out from the preparation equipment to the chamber and it simultaneously reacts with the components present in the chamber. Then, the surface film of

Liþ þ e $ Li

This is a simple electrochemical dissolution and deposition of lithium metal. In the course of discharge process, lithium ion dissolves in the electrolyte and transfers to the cathode. At the cathode, lithium ion is consumed by the electrode reaction, such as the insertion process into cathode materials. For example, the cathode reaction of MnO2 is expressed by the following equation: MnO2 þ Liþ þ e $ LiMnO2

Another important aspect of lithium metal anode is the surface chemistry. Lithium metal is very sensitive to

Figure 1 Photographs of primary lithium batteries. Cited from http://www.tachibana.co.jp/jigyo/handotai_device/device_list/ d_08/index.html.

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

lithium foil, that is, the so-called ‘native surface film’, is formed on the lithium foil. The atmosphere contains oxygen, water, carbon dioxide, and nitrogen. These compounds may react with fresh lithium metal surface to

Li ingot

produce several kinds of lithium compounds. The compounds consisting of the native film on lithium metal foil have been analyzed by X-ray photoelectron spectroscopy (XPS). Figure 3 shows the XPS spectra for lithium foil surface used in primary lithium batteries. From O 1 s, C 1 s, and Li 1 s XPS spectra, it can be seen that the lithium foil surface is covered with LiOH, Li2O, and Li2CO3, respectively. The upper layer contains mainly Li2CO3 and LiOH and the lower layer consists of Li2O, as shown in Figure 4. The thickness and the chemical composition of this native surface film depend on the atmospheric conditions in the preparation equipment, as shown in Figure 2. When the surface film is very thin, lithium metal still has a high reactivity with oxygen, water, and nitrogen. The production of lithium batteries is

Controlled atmosphere

Li2CO3, LiOH, … 100 Å Li sheet

Li2O, LiOH, … 600 Å

Li metal

Figure 4 Schematic illustration of native surface film on lithium metal foil.

Figure 2 Preparation equipment of lithium foil from ingot.

53.7 eV (Li2O) 55.3 eV (Li–O–)

290.2 eV (Carbonate)

52.3 eV

532.1 eV

528.5 eV

(Li)

(Li–O–)

(Li2O)

285.0 eV (Hydrocarbon)

282.4 eV (Li carbide)

0 min

Etching time

10 min

40 min

90 min 63

48 540

524

292

278

Binding energy (eV) (a)

(b)

(c)

Figure 3 X-ray photoelectron microscopy (XPS) spectra for lithium metal foil: (a) Li 1 s, (b) O 1 s, and (c) C 1 s. From Kanamura K, Tamura H, and Takehara Z (1992) XPS analysis of a lithium surface immersed in propylene carbonate solution containing various salts. Journal of Electroanalytical Chemistry 333: 127.

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

performed under dry-air conditions. Therefore, the lithium foil with a very thin surface film may react with nitrogen to form lithium nitride. The production of lithium nitride is not good for the production of primary lithium batteries, due to its mechanically hard nature. Therefore, the surface film on lithium metal anode should be controlled very carefully. In order to obtain a good surface film on lithium metal, the atmospheric conditions of the preparation equipment should be precisely controlled.

Surface Reaction of Lithium Metal in Nonaqueous Electrolyte As discussed earlier, several lithium compounds exist on the lithium metal surface as components of the native surface film. Of course, lithium metal foil is used in nonaqueous electrolytes. When lithium metal is immersed in a nonaqueous electrolyte, either the solvent or the salt reacts with lithium metal to form various reaction products. If these undesirable chemical reactions proceed for a long time, lithium metal dissolves into the electrolyte completely and the electrolyte is reduced to produce various organic products. This is a self-discharge of battery, and therefore, lithium battery using lithium metal anode cannot be established. However, the native surface film on lithium metal prevents a direct reaction with the electrolytes, as shown in Figure 4. In fact, this surface film is sometimes very stable in nonaqueous electrolytes and acts as an SEI. Figure 5 shows the XPS spectra of lithium metal surface after immersion in nonaqueous electrolytes. When lithium perchlorate (LiClO4) is used as the electrolyte salt, the chemical composition of the surface film on lithium metal is not changed significantly. A slight formation of lithium chloride is observed. This is due to the chemical reaction between the native film and some components of the nonaqueous electrolytes. In the case of lithium hexafluorophosphate (LiPF6), the native film is extremely changed. A large amount of lithium fluoride is observed on the lithium metal surface after immersing the lithium foil in an electrolyte containing LiPF6. When lithium tetrafluoroborate (LiBF4) is used as the electrolyte salt, lithium fluoride is also formed on the lithium foil surface. The native film is almost converted to a new surface film. Depending on the structure and the chemical composition of the surface film formed as a result of the chemical reactions between the native surface film and components of the nonaqueous electrolytes, the surface film may or may not act as an SEI. Figure 6 shows the structural model for surface film before and after the immersion of lithium metal in nonaqueous electrolytes. These structural models are obtained from the depth profile of each elements obtained by the XPS analysis with argon ion etching process. When LiPF6 is used as the

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electrolyte salt, the native film is significantly changed by the surface reaction; lithium fluoride can be formed by a chemical reduction of PF6  ion by lithium metal. In addition, a nonaqueous electrolyte with LiPF6 contains hydrogen fluoride as an impurity; hydrogen fluoride is usually involved in LiPF6 salt as a result of the decomposition of PF6  ion through chemical reaction with water. In general, a small amount of water is added to both the electrolyte salt and the solvent. Hydrogen fluoride reacts with basic compounds, such as lithium oxide, lithium hydroxide, and lithium carbonate. These reactions result in the formation of lithium fluoride on lithium metal surface. Possible chemical reactions for the formation of LiF are described as follows: Decomposition of salt: LiPF6 - LiF þ PF5 LiBF4 - LiF þ BF3

Acid–base reaction with HF: LiOH þ HF- LiF þ H2 O Li2 CO3 þ 2HF- 2LiF þ H2 CO3 Li2 O þ 2HF- 2LiF þ H2 O

In the case of a nonaqueous electrolyte with LiClO4, a small amount of lithium chloride is formed on the native film of lithium metal. The formation of lithium chloride is possibly due to the chemical reaction between ClO4  ion and lithium metal or due to an acid–base reaction between hydrogen chloride and basic lithium compounds present in the native film. If the anion is directly reduced by the lithium metal to generate lithium chloride, a large amount of lithium chloride is formed in the surface film on lithium metal. As shown in Figure 6, the amount of lithium halides strongly depends on the anions in the electrolyte salt. This indicates that the main reaction for the formation of lithium halides is explained by the acid– base reaction between basic lithium compounds and the acid generated from electrolyte salt or existing in electrolyte as impurities. Thus, a small amount of chemical species plays an important role in the surface reaction of lithium metal. On the contrary, solvents in electrolyte are important. They can penetrate into the SEI (surface film) and react with lithium metal to form some organic compounds. Therefore, the surface film on lithium metal sometimes contains organic species, such as lithium alkoxides and alkyl carbonates. These organic chemical reactions have been investigated by in situ or ex situ Fourier transform infrared spectroscopy. The penetration of organic solvent depends on the physicochemical properties of solvents. Dimethoxyethane and tetrahydrofuran have been used as the solvent for lithium batteries. In comparison with propylene carbonate and ethylene carbonate, these

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

Li2CO3 LiOH

Li2O Li

Li 1s

Li2CO3

Etching time

Hydrocarbon

Li2CO3

C 1s

O 1s

LiOH

Li2O

LiCI CI 2p

0 min

Etching time Etching time

5 min

0 min

0 min 25 min

5 min

Etching time 45 min

0 min 5 min 25 min 45 min 65 min 85 min 105 min 125 min

65 min 85 min 105 min 125 min 145 min

60

56

52

48

145 min

292

Binding energy (ev)

288

284

280

5 min 25 min

45 min

45 min

65 min

65 min

85 min

85 min

105 min 125 min

105 min 125 min

145 min

538

Binding energy (ev)

(a) LiF Li2CO3 LiOH Li2O Li

534

145 min

530

526

LiOH

Li2O

194

LiF

O 1s

C 1s

Etching time

198

(d)

Li2CO3

Hydrocarbon

202

Binding energy (ev)

(c)

Li2CO3

206

Binding energy (ev)

(b)

Li 1s

25 min

F 1s Etching time

Etching time 0 min

0 min

5 min

5 min

0 min 25 min 45 min

0 min 5 min

65 min

105 min 125 min 145 min

60

56

52

48

292

Binding energy (ev) (a)

288

284

105 min 125 min 145 min 280

45 min

65 min

65 min

85 min

85 min

105 min

105 min

125 min

125 min 145 min

145 min

538

Binding energy (ev) (b)

25 min

45 min

25 min 45 min 65 min 85 min

85 min

5 min

25 min

Etching time

534

530

526

696

Binding energy (ev) (c)

692

688 684 680

Binding energy (ev) (e)

Figure 5 X-ray photoelectron microscopy (XPS) spectra for lithium metal foil immersed in propylene carbonate containing 1 mol dm3  LiClO4 (top row) and LiPF6 (bottom row): (a) Li 1s, (b) C 1s, (c) O 1S, (d) Cl 2p, and (e) F 1s. Reproduced with permission from Kanamura K and Shiraishi S (2002) In: Kumagai N and Komaba S (eds.) Materials Chemistry in Lithium Batteries, pp. 107–147. Kerala, India: Research Signpost.

compounds are more stable against chemical reactions with lithium metal. However, more severe chemical reactions are sometimes observed in electrolytes consisting of dimethoxyethane and tetrahydrofuran, which are lowviscosity solvents. Such a difference in chemical reactions of lithium is due to the penetration process depending on the viscosity of solvent. Hence, the surface film on lithium metal in nonaqueous electrolytes is very complicated and changes with different kinds of solvents. At least, lithium metal can be used in practical batteries when lithium metal surface has a stable surface film acting as an SEI. In practical primary lithium batteries, the surface film has been already stabilized by a proper selection of electrolyte.

Discharge and Charge Behavior of Lithium Metal Anode Lithium metal as anode of primary batteries is very successful. However, lithium metal anode is not used in secondary (rechargeable) batteries. This is due to the safety issue of lithium metal anode and low cyclability. As shown in Figure 7, lithium metal deposited in nonaqueous electrolytes sometimes exhibits a dendrite shape. In general, this dendrite of lithium is so active that an explosion of secondary lithium batteries may occasionally occur. The dendrite of lithium metal is formed during the charging process of battery. When this dendrite lithium is discharged, lithium metal dissolves into nonaqueous

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

31

Li2CO3, LiOH, LiCl Li2O, LiOH

Surface layer

High resistivity Li metal

Low resistivity

(a)

Li metal LiF, Li2CO3, LiOH Li2O, LiOH Li metal

(b)

Figure 6 Schematic illustration of lithium metal surface after immersion in propylene carbonate containing 1 mol L1 LiClO4 (a) and LiPF6 and LiBF4 (b).

Figure 7 Scanning electron micrographs of lithium dendrite. Reproduced with permission from Kanamura K (1997) Denki Kagaku 65: 722.

electrolytes. The dissolution process takes place near the current collector because of the large current distribution. As a result of this nonuniform dissolution, some dendrite lithium metal is isolated from the current collector and remains in the anode. This lithium is chemically still active, but is electrochemically inactive. In addition, it has a large surface area to react with electrolytes. In some cases, chemical reaction between dendrite lithium and electrolyte occurs, resulting in an explosion of batteries. In order to use lithium metal

Current flow

Figure 8 Schematic illustration of surface film on lithium metal surface and current distribution.

anode in secondary batteries, this dendrite formation should be suppressed. Therefore, a various studies have been done based on the surface chemistry of lithium metal anode. The dendrite formation of lithium is related to current distribution in the course of the deposition process of lithium metal (charging process of battery). In the case of lithium metal anode in nonaqueous electrolytes, the current distribution is influenced by the surface film on lithium metal, which acts as an SEI with some resistance. In general, the surface film on lithium metal is not so uniform and has a low ionic conductivity. If the surface film on lithium metal has some defects as shown in Figure 8, the current is concentrated on such defective parts (pores in the surface film). Thus, the current density at local sites on lithium metal surface is increased by the presence of surface film. Therefore, in order to suppress the formation of lithium dendrite, the surface reaction of lithium metal should be controlled to establish a uniform surface film on lithium metal. So far, many surface modifications have been proposed by using some additives, for example, hydrogen fluoride, carbon dioxide, and other organic compounds. Figure 9 shows the scanning electron micrographs of lithium metal deposited on nickel substrate in nonaqueous electrolyte with or without hydrogen fluoride additive. The morphologies of the deposited lithium metals in both electrolytes are completely different. In the electrolyte with hydrogen fluoride, lithium deposit has an extremely smooth surface. On the contrary, lithium metal surface prepared in the electrolyte without hydrogen fluoride shows a typical dendrite morphology. The morphology of lithium metal depends on current density: with decreasing current density, the surface becomes smoother. At high current density, lithium dendrite is often observed. Figure 10 shows the XPS spectra and depth profiles of each element

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

Figure 9 Scanning electron micrographs of lithium metal in propylene carbonate containing 1 mol L1 LiClO4 (a) with small amount of HF and (b) without HF. Reproduced with permission from Kanamura K (1997) Denki Kagaku 65: 722.

consisted in the surface film for lithium metals deposited in electrolytes with or without HF additive. From the XPS spectra of Li 1 s and F 1 s for lithium metal prepared with hydrogen fluoride, the presence of lithium fluoride is confirmed. From the XPS spectra of C 1 s and O 1 s, it can be seen that Li2CO3 and lithium oxide are components for the surface film, but they are minor species consisted in the surface film. On the contrary, the XPS spectra for lithium metal prepared in the electrolyte without hydrogen fluoride show that the surface film on lithium metal does not involve lithium fluoride. In addition, the thickness of the surface film on lithium metal deposited in the electrolyte with hydrogen fluoride additive is smaller than that on lithium metal deposited

in the electrolyte without hydrogen fluoride additive. Thus, the surface film on lithium metal is strongly influenced by a small amount of additives in electrolytes. A similar phenomenon has been observed while using standard electrolytes. In fact, the deposition of lithium metal in electrolyte containing PF6  ions provides a similar surface film on lithium deposited in electrolyte with hydrogen fluoride additive. This means that the electrolyte with LiPF6 contains a small amount of hydrogen fluoride, which is due to the decomposition of PF6  ions. Figure 11 shows the scanning electron micrograph of lithium metal deposited on lithium foil that has the native film. The particle of lithium metal is different from dendrite morphology but similar to spherical particles observed in Figure 9. These particles were locally observed on the surface of lithium metal foil. Figure 12 shows mass change during lithium deposition and dissolution at constant current in propylene carbonate containing LiClO4, which was measured with quartz crystal microbalance (QCM). The theoretical line is calculated from mass change corresponding to ideal lithium deposition (without any chemical reactions). Chemical reactions of lithium metal can be observed during lithium deposition and dissolution processes from the mass change on electrode measured by QCM method. In the case of electrolyte without hydrogen fluoride additive, the mass change during deposition and dissolution of lithium metal is not in agreement with the theoretical one, indicating that chemical reactions between lithium metal and electrolyte take place. On the contrary, the mass change of electrode during lithium deposition in electrolyte with hydrogen fluoride additive is different from that in electrolyte without hydrogen fluoride. In this case, the mass change is more similar to the theoretical one. However, the mass change of the dissolution process is not in agreement with the theoretical curve. This is due to chemical reactions taking place during the lithium dissolution process. Thus, surface reactions of lithium metal with electrolyte exhibit a very unique behavior, which strongly depends on the surface film on lithium that is formed during lithium deposition. When the electrolyte contains no hydrogen fluoride, surface reactions occur during both lithium deposition and dissolution processes. This means that the surface film on lithium metal formed as a result of chemical reactions between electrolyte and lithium metal does not perfectly act as an SEI film. In this case, the surface film on lithium metal mainly consists of some organic compounds. When the electrolyte contains hydrogen fluoride, the surface film acts as an SEI. This surface film consists of lithium fluoride, as discussed earlier. During the charging process, the surface film grows according to morphology change of deposited lithium metal. On the contrary, this surface is broken in the course of

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal LiF

Li2CO3

LiOH

Li2O

Li 1s

Li

Li2CO3

LiCl

LiOCO2R

Cl 2p

33

Hydrocarbon

C 1s

0 min 0 min 1 min

0 min

1 min 11 min

1 min

11 min

11 min

21 min

60

56

31 min

31 min

41 min

41 min

52

48

Binding energy (ev) Li2CO3

LiOH

21 min

206

202

31 min 41 min 292

194

Binding energy (ev)

Li2O

O 1s

198

21 min

288

284

280

Binding energy (ev)

LiF

Depth profile

F 1s

Li 0 min 0 min 1 min 1 min 11 min

11 min F

21 min 31 min

O

21 min C

Cl

31 min

0

10

20

30

40

Etching time (min) 41 min 538 534 530 526 Binding energy (ev)

41 min 696 692 688 684 680 Binding energy (ev)

(a)

Figure 10 X-ray photoelectron microscopy (XPS) spectra of propylene carbonate containing 1 mol L1 LiClO4 (a) with small amount of HF and (b) without one. Reproduced with permission from Kanamura K and Shiraishi S (2002) In: Kumagai N and Komaba S (eds.) Materials Chemistry in Lithium Batteries, pp. 107–147. Kerala, India: Research Signpost.

dissolution process of lithium metal. Therefore, the mass change during the lithium deposition process is different from that during the dissolution process. Figure 13 schematically illustrates the deposition and dissolution processes of lithium metal and surface reaction of lithium metal, which are expected from QCM experiments. From these results, it can be concluded that the stability of surface film not only depends on chemical compositions of the surface film, but is also influenced by morphology change during lithium metal deposition and dissolution processes. In future, a dynamic control of surface film on lithium metal is needed, or the structure of lithium metal anode has to be optimized to suppress the morphology change of lithium metal.

Discharge and Charge Cycle of Lithium Metal Electrode Lithium metal electrodes have been studied as anode for rechargeable lithium batteries. In fact, some groups tried to commercialize lithium metal electrode in rechargeable lithium batteries. However, the cyclability (rechargeability or reversibility of deposition and dissolution processes) of lithium metal electrode is not sufficient for practical use; therefore, an excess amount of lithium metal is used to realize high cyclability of practical lithium batteries. If the rechargeability of lithium metal anode is 99.9%, 0.1% discharge capacity is always lost at each discharge and charge cycle. During 500 cycles, the discharge capacity of

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal Li2CO3

LiOH

Li2O

LiCl Cl 2p

Li 1s

0 min 0 min

1 min

1 min 11 min

11 min

21 min

21 min

31 min

31 min

41 min

41 min

60 56 52 48 Binding energy (ev) Li2CO3 LIOCO2R

206 202 198 194 Binding energy (ev) Li2CO3

Hydrocarbon C 1s

LiOH

Li2O

Figure 11 Scanning electron micrograph of lithium metal on lithium metal foil in propylene carbonate containing 1 mol L1 LiPF6. Reproduced with permission from Kanmura K, Shiraishi S, and Takehara Z (1995) Morphology control of lithium deposited in nonaqueous media. Chemistry Letters 24: 209.

O 1s 0 min

0 min 1 min 1 min 11 min

11 min

21 min

21 min

31 min

31 min

41 min

41 min

292 288 284 280 Binding energy (ev)

538 534 530 526 Binding energy (ev)

Depth profile Li

O C Cl 0 (b)

10 20 30 Etching time (min)

40

Figure 10 (Continued).

lithium metal anode decreased to 50% of the original capacity. In the case of rechargeable lithium batteries for cell phones, a lifetime of 2–3 years is needed. In such cases, two times the amount of lithium metal is to be used in practical batteries. Such an excess amount of lithium metal may lead to lowering of safety. In addition, in the course of discharge and charge cycles, some of the lithium is isolated from lithium metal bulk or current collector, as discussed earlier. This lithium is so active that chemical reactions between lithium metal and electrolyte can occur in rechargeable lithium batteries. In some cases, batteries

explode during the charging process, due to increasing temperature of the battery. The chemical activity of lithium metal strongly depends on the morphology of lithium metal deposited during the charging process. Lithium dendrite has a high surface area, and therefore, it is chemically very active. Another serious problem of lithium dendrite formation is the internal contact between anode and cathode, which is a very dangerous situation. When batteries with internal connection between cathode and anode are charged, charging current flows beyond the standard capacity of batteries, which is an overcharged state. When batteries are overcharged, the temperature of the battery is increased. This is the worst case for a battery. In order to suppress the formation of lithium dendrites or control morphology of lithium metal, various works have been done throughout the world. Several types of new electrolyte systems and additives have been proposed to improve the cyclability of lithium metal anode. Some of the investigated electrolyte systems and additives for lithium metal anode are HF, 2MeF, CO2, Al3 þ , benzene, and other surfactants. Using these new materials, the morphology of lithium is changed from the dendrite type, so that some of electrolytes or additives improve the cyclability of lithium metal anode. However, the performance of lithium metal anode is not adequate for practical use. In most of the new electrolytes, lithium dendrite formation is suppressed during initial cycles. However, the morphology of lithium metal gradually changes with discharge and charge cycles, resulting in a low homogeneous nature of lithium metal surface. This morphological state of lithium metal surface leads to concentrated current distribution to form lithium dendrite after discharge and charge cycles of battery.

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

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5 Deposition

Dissolution

4

⫺⌬F (kHz)

Without HF (a) 3 Observed curves 2

With HF (b)

1 m.p.e. ⫽ 7 (Li) 0 0.0

0.1

0.2 0.0

0.1

0.2

Qc (C cm⫺2)

Figure 12 Mass change during lithium metal deposition on Ni substrate in propylene carbonate containing 1 mol L1 LiClO4 (a) with and (b) without HF additive, which is measured with quartz crystal microbalance (QCM). From Kanamura K, Shiraishi S, and Takehara Z (2000) Journal of Electrochemical Society 147: 2070. Reproduced by permission of The Electrochemical Society.

Before

Deposition

Attack by electrolyte

Dissolution

Figure 13 Schematic illustration for deposition and dissolution of lithium metal and behavior of surface film on lithium metal surface.

Polymer Battery with Lithium Metal Anode As discussed earlier, lithium metal used in electrolytes is basically active; therefore, it has a safety problem. In order to overcome this disadvantage, a solid electrolyte

has been used in rechargeable lithium batteries with lithium metal anode. Many types of solid electrolytes have been developed. Solid polymer electrolyte is one of the promising materials for rechargeable lithium batteries. Polyethylene oxide with lithium salt has been proposed as the solid polymer electrolyte with high ionic conductivity. Figure 14 schematically illustrates the solid polymer electrolyte and Li þ ion movement inside the polymer electrolyte. Table 1 lists some of the polymer electrolytes that have been used in lithium batteries. Most of the solid polymer electrolytes have 104– 103 S cm1 ionic conductivity. Figure 15 shows the discharge and charge curves of a battery with lithium metal anode and LiCoO2 cathode. Even at room temperature, theoretical discharge capacity is obtained by using solid polymer electrolyte, indicating that both electrochemical interfaces between solid polymer electrolyte and cathode or anode are well established. In some cases, this interface involves some pores and defects, resulting in poor contact between the lithium metal surface and the solid polymer electrolyte. At present, the ionic conductivity and the preparation process of electrodes are not adequate for practical battery use. In addition, the lithium dendrite problem is still present even when the liquid electrolyte is replaced by a solid one. In fact, an internal short of batteries sometimes takes place during charging process. Of course, its possibility is much lower than that in liquid electrolyte system. If the solid polymer electrolyte had enough high mechanical strength, an internal short problem could be prevented. However, a high mechanical strength of solid polymer

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

E (V) (Li/Li+)

4.5

Li⫹

4.0 3.5 3.0 2.5

0

20

40

80 100 120 60 Capacity (mA h g−1)

140

160

Positive electrode: LiCoCO2:Acetylene black:PVdF = 60:20:20 Negative electrode: Li Electrolyte: Star-MES (0.05 mol% LiBETI)

Figure 15 Discharge and charge curves of lithium batteries with lithium metal anode and LiCoO2 cathode, electrolyte: polyethylene oxide base solid polymer electrolyte.

polymer electrolytes with nanophase separation structure have been prepared by some research groups and applied to rechargeable lithium batteries with lithium metal anode. The discharge and charge curves in Figure 15 are obtained by using such polymer electrolyte. Figure 16 schematically illustrates the solid polymer electrolyte with nanophase separation structure. The polystyrene part provides a high mechanical strength to the polymer electrolyte and the polyethylene oxide part provides a high ionic conductivity. Similar new solid polymer electrolytes have been synthesized by some research groups; therefore, in the near future, lithium metal anodes will be commercialized by using polymer electrolytes.

Figure 14 Schematic illustration of solid polymer electrolyte and movement of Li þ ion in it. Table 1 Solid conductivities

polymer

electrolytes

and

their

ionic

Ionic conductivity at 298 K (S cm  1) Polymer Dry electrolyte

PEO based

106–104

PAN based Gel

PEO based PVdF based PAN based PMMA based

>103

PEO, poly(ethylene oxide); PAN, polyacrylonitrile; PVdF, poly(vinylidene fluoride); PMMA, poly(methyl methacrylate).

electrolyte leads to low ionic conductivity of polymer electrolyte. This is due to ionic conduction mechanism that has been explained by the Williams–Landel–Ferry (WLF) theory, as shown in Figure 14. Recently, solid

Highly Stable Electrolyte for Lithium Metal Anode Solid polymer electrolytes are one of the possibilities for establishment of rechargeable batteries with lithium metal anode. Other possible electrolytes for lithium metal anode are ceramic materials and ionic liquid with lithium-ion conductivity. So far, some of the oxide and sulfide electrolytes have been developed as lithium-ion conductive materials, which have 104–103 S cm1 ionic conductivity and high transference number of Li þ ion. In the case of oxide materials, the stability of oxide against lithium metal is a problem. For example, Li0.35La0.55TiO3 has high Li þ ion conductivity, but it can be reduced by lithium metal. Therefore, lithium metal anode cannot be used. Sulfide materials react with lithium metal to form Li2S or other products that still have high Li þ ion conductivity. Further chemical reactions are suppressed by the formation of Li2S. Therefore, sulfide materials can be used in batteries with lithium metal anode. In other words, Li2S layer acts as an SEI. Another interesting solid electrolyte is called lithium phosphorous oxynitride

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

37

PEO polymer chain O

O

O

St polymer Li⫹ n

O

O

O

O

Li⫹

O

O

O

Figure 16 Schematic illustration of nanophase separation structure for solid polymer electrolyte. PEC, poly(ethylene oxide). Published by Elsevier Ltd.: Niitani T, Shimada M, Kawamura K, and Kanamura K (2005) Journal of Power Sources 146: 386.

4.3 4.2 4.1

E (V)

4.0 3.9 3.8

1 ␮A

3.7

5 ␮A 10 ␮A

3.6 3.5 0

1000

2000

3000 4000 5000 Capacity (␮As)

6000

7000

Figure 17 Discharge and charge curves of lithium batteries with lithium metal anode and LiCoO2 cathode, electrolyte: LiPON. Published by Elsevier Ltd.: Iriyama Y, Kako T, Yada C, Abe T, and Ogumi Z (2005) Solid State Ionics 176: 2371.

Pr

(LIPON), which is lithium phosphate containing a small amount of nitrogen. This electrolyte has a relatively low ionic conductivity. However, using a very thin film of LIPON, rechargeable lithium batteries with lithium metal can be prepared. Figure 17 shows the discharge and charge curves of batteries with LiCoO2 cathode and lithium metal anode. The cyclability of this battery is excellent, indicating the presence of an interface between lithium metal and solid electrolyte containing phosphates. Recently, a new electrolyte system consisting of only anions and cations has been extensively studied by many researchers. Such electrolyte is called ‘ionic liquid’. Figure 18 schematically illustrates the ionic liquid. This electrolyte system does not contain solvent; therefore, the stability of electrolyte is limited by anions and cations. In general, an ionic liquid is more stable than a standard electrolyte containing organic solvent. At present, most

Et

Me ⫹ N

N⫹

Am

N Am

Me PP13 N-methyl-N-propyl piperidinium

EMI 1-ethyl-3-methylimidazolium

⫺ N

SO2CF3 SO2CF3

TFSI bis(trifluoromethane sulfonyl)imide

⫺ N

⫹ N

Am Am

TeAA tetraamyl(pentyl) ammonium

SO2CF3 COCF3

TSAC 2,2,2-trifluoro-N(trifluoromethyl sulfonyl)acatamide

Figure 18 Schematic illustration of ionic liquid. Based on Sakaebe H, Matsumoto H, and Tatsumi K (2005) Journal of Power Sources 146: 693.

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Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal 4.5

Nomenclature

Cell voltage (V)

4.2

3rd

1st

2nd 4th

3.9 3.6 4th

3.3 3

3rd 0

20

40

60

80

1st 2nd 100

120

140

160

Specific capacity (mAh g⫺1)

Figure 19 Discharge and charge curves of lithium batteries with lithium metal anode and LiCoO2 cathode (electrolyte: ionic liquid). Published by Elsevier Ltd.: Sakaebe H and Matsumoto H (2003) Electrochemistry Communications 5: 594.

ionic liquids do not have Li þ ion conductivity even though they have a high ionic conductivity. In order to realize ionic liquids with high Li þ ion conductivity, lithium salts, such as LiBF4, are to be added to the ionic liquids. Figure 19 shows the discharge and charge curves of rechargeable lithium batteries with lithium metal anode, in which ionic liquid is used as the electrolyte. From this result, it can be concluded that ionic liquids are good candidates for lithium metal anode in the future.

Conclusions Lithium metal anode has the highest energy density among several kinds of anode materials. Unfortunately, it is so active that chemical reactions of lithium metal with electrolyte take place in rechargeable lithium batteries. These chemical reactions form various products on the lithium surface. In the case of primary lithium batteries, these compounds result in the formation of a stable surface film on lithium metal, leading to a very low selfdischarge performance of battery. On the contrary, lithium metal anode in rechargeable lithium batteries poses a big problem, which is the formation of lithium dendrite during the charging process. This is caused by a large distribution of surface film nature. This distribution is enhanced by the low stability of surface film on lithium metal during the discharging process. Using an excess of lithium metal anode, rechargeable lithium batteries can be constructed with long cycle life. As lithium dendrite formation occurs during cycles and accumulates in battery, such batteries become dangerous during discharge and charge cycles. The lithium dendrite is extremely active and violently reacts with the electrolyte. Therefore, the safety aspect of rechargeable lithium metal anode is the most important target of research. In the future, lithium dendrite formation will be suppressed by using new electrolytes, new structures, and so on; hence, high-energy-density batteries can be realized.

Abbreviations and Acronyms LiPON NHE PAN PEO PMMA PP13 PVdF QCM SEI WLF XPS

lithium phosphorous oxynitride normal hydrogen electrode polyacrylonitrile poly(ethylene oxide) poly(methyl methacrylate) N-methyl-N-propylpiperidinium poly(vinylidene fluoride) quartz crystal microbalance solid electrolyte interface Williams–Landel–Ferry X-ray photoelectron spectroscopy

See also: Electrolytes: Gel; Ionic Liquids; Polymer; Secondary Batteries – Lithium Rechargeable Systems: Electrolytes: Nonaqueous; Secondary Batteries – Lithium Rechargeable Systems – LithiumIon: Overview; Positive Electrode: Lithium Cobalt Oxide.

Further Reading Arakawa M, Tobishima S, Nemoto Y, Ichikawa M, and Yamaki J (1993) Thermal stability of lithium anodes in an amorphous V2O5/Li battery system. Journal of Power Sources 43–44: 27. Armand MB, Chabagno JM, and Dulclot MJ (1979) Fast ion transport in solids. In: Vashishta P, Mundy JN, and Shenoy GK (eds.) Fast Ion Transport in Solids, p. 131. New York: Elsevier. Aurbach D and Zaban A (1993) Impedance spectroscopy of lithium electrodes. Part 1. General behavior in propylene carbonate solutions and the correlation to surface chemistry and cycling efficiency. Journal of Electroanalytical Chemistry 348: 155. Aurbach D and Zaban A (1994) Journal of Electroanalytical Chemistry 365: 41. Aurbach D and Zaban A (1994) Journal of Electroanalytical Chemistry 367: 15. Aurbach D, Daroux ML, Faguy PW, and Yeager E (1987) Journal of Electrochemical Society 134: 1611. Aurbach D, Gofer Y, Ben-Zion M, and Aped P (1992) Journal of Electroanalytical Chemistry 339: 451. Aurbach D, Weissman I, Zaban A, and Chusid O (1994) Correlation between surface chemistry, morphology, cycling efficiency and interfacial properties of Li electrodes in solutions containing different Li salts. Electrochimica Acta 39: 51. Gofer Y, Ben-Zion M, and Aurbach D (1992) Solutions of LiAsF6 in 1,3dioxolane for secondary lithium batteries. Journal of Power Sources 39: 163. Kanamura K, Okagawa T, and Takehara Z (1995) Journal of Power Sources 57: 119. Kanamura K, Shiraishi S, and Takehara Z (1994) Electrochemical deposition of uniform lithium on an Ni substrate in a nonaqueous electrolyte. Journal of Electrochemical Society 141: L108. Kanamura K, Shiraishi S, and Takehara Z (1996) Journal of Electrochemical Society 143: 2187. Kanamura K, Shiraishi S, Tamura H, and Takehara Z (1994) Journal of Electrochemical Society 141: 2379. Kanamura K, Takezawa H, Shiraishi S, and Takehara Z (1997) Journal of Electrochemical Society 144: 1900. Kanamura K, Tamura H, Shiraishi S, and Takehara Z (1995) Electrochimica Acta 40: 913.

Secondary Batteries – Lithium Rechargeable Systems | Negative Electrodes: Lithium Metal

Kanamura K, Tamura H, Shiraishi S, and Takehara Z (1995) Journal of Electroanalytical Chemistry 142: 340. Kanamura K, Tamura H, Shiraishi S, and Takehara Z (1995) Journal of Electrochemical Society 142: 340. Kanamura K, Tamura H, and Takehara Z (1992) XPS analysis of a lithium surface immersed in propylene carbonate solution containing various salts. Journal of Electroanalytical Chemistry 333: 127. Kanamura K, Toriyama S, Shiraishi S, and Takehara Z (1995) Journal of Electrochemical Society 142: 1383. Kanamura K, Toriyama S, Shiraishi S, and Takehara Z (1996) Journal of Electrochemical Society 143: 2548. Malik Y and Aurbach D (1991) The electrochemical behaviour of 2-methyltetrahydrofuran solutions. Journal of Electroanalytical Chemistry 282: 73. Matsumoto H, Sakaebe H, Tatsumi K, Kikuta M, Ishiko E, and Kono M (2006) Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]. Journal of Power Sources 160: 1308. Mizuno F, Hayashi A, Tadanaga K, and Tatsumisago M (2005) Effects of conductive additives in composite positive electrodes on charge– discharge behaviors of all-solid-state lithium secondary batteries. Journal of Electrochemical Society 152: A1499.

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Naoi K, Mori M, Naruoka Y, Lamanna WM, and Atanasoski R (1999) The surface film formed on a lithium metal electrode in a new imide electrolyte, lithium Bis(perfluoroethylsulfonylimide) [LiN(C2F5SO2)2]. Journal of Electrochemical Society 146: 462. Osaka T, Momma T, Tajima T, and Matsumoto Y (1994) Protection against H2O impurity for cyclability of lithium anode in propylene carbonate electrolyte by an existence of CO2. Denki Kagaku 62: 450. Peled E (1979) Journal of Electrochemical Society 126: 2047. Peled E, Golodnitsky D, Ardel G, and Eshkenazy V (1995) The sei model-application to lithium-polymer electrolyte batteries. Electrochimica Acta 40: 2197. Shiraishi S, Kanamura K, and Takehara Z (1995) Journal of Applied Electrochemistry 25: 584. Shiraishi S, Kanamura K, and Takehara Z (1997) Study of the surface composition of highly smooth lithium deposited in various carbonate electrolytes containing HF. Langmuir 13: 3542. Yoshimatsu I, Hirai T, and Yamaki J (1988) Lithium electrode morphology during cycling in lithium cells. Journal of Electrochemical Society 135: 2422.