Thermal Stability of Materials in Lithium-Ion Cells

Thermal Stability of Materials in Lithium-Ion Cells

20 Thermal Stability of Materials in Lithium-Ion Cells Jun-ichi Yamaki OFFICE OF SOCI ETY-ACADE MI A C OLLABORATION FOR INNOVATION, KYOTO UNIVERSITY, ...

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20 Thermal Stability of Materials in Lithium-Ion Cells Jun-ichi Yamaki OFFICE OF SOCI ETY-ACADE MI A C OLLABORATION FOR INNOVATION, KYOTO UNIVERSITY, CENTER FOR ADVAN CED SCIENCE AND INNOVATION, G OKASHO, U JI, JAPAN [email protected]

CHAPTER OUTLINE 1. Introduction ................................................................................................................................... 461 2. Basic Consideration on Cell Safety .............................................................................................. 462 3. Chemical Reduction of the Electrolyte by the Negative Electrode .......................................... 463 3.1. Graphite Electrode ............................................................................................................... 463 3.2. Silicon/Li Alloy ........................................................................................................................ 466 4. Thermal Decomposition of the Electrolyte................................................................................. 468 4.1. LiPF6/Alkyl Carbonate Mixed-Solvent Electrolytes.............................................................. 468 4.2. LiPF6/Methyl Difluoroacetate Electrolyte ............................................................................ 469 5. Electrolyte Oxidation at the Positive Electrode ......................................................................... 475 5.1. LiCoO2 ..................................................................................................................................... 475 5.2. FeF3 ......................................................................................................................................... 476 6. Safety Evaluation by Abuse Tests ............................................................................................... 478 6.1. Safety Devices ........................................................................................................................ 479 7. Conclusions .................................................................................................................................... 480 Acknowledgments ............................................................................................................................. 481 References........................................................................................................................................... 481

1. Introduction Li-ion batteries have been used in a number of portable electronic devices. Due to their high energy densities, these batteries have been considered as possible power sources in electric vehicles, artificial satellites and load leveling. However, before Li-ion batteries can be used in large-scale applications, their performance still needs to be improved with regard to battery cycle life, rate capability and safety. Among them, the safety issues should be alleviated to make these batteries serve as reliable power sources. Larger batteries have a serious safety problem, i.e. the risk of explosion resulting from either shorting or an external temperature increase. Therefore, safety improvements to Lithium-Ion Batteries: Advances and Applications. http://dx.doi.org/10.1016/B978-0-444-59513-3.00020-0 Ó 2014 Elsevier B.V. All rights reserved.

461

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

the large batteries are indispensable for their practical applications. It is generally considered that “thermal runaway” of Li cells occurs if the heat output exceeds thermal diffusion. Therefore, it is difficult to design large cells that can pass safety-test criteria.

2. Basic Consideration on Cell Safety

Heat generation rate or heat dissipation rate

Li-ion cells may smoke when abused and can ignite when the abuse is extreme. Thermal stability is a basic problem as regards cell safety. Several exothermic reactions occur inside a cell as its temperature increases. It is generally considered that “thermal runaway” occurs if heat output exceeds thermal diffusion. The possible exothermic reactions are: (1) chemical electrolyte reduction at the negative electrode, (2) thermal electrolyte decomposition and (3) electrolyte oxidation at the positive electrode [1–5]. In this last case, a high-voltage metal oxide electrode releases oxygen at elevated temperatures. It should also be noted that, when a separator melts as a result of the temperature exceeding its melting point (w125  C for polyethylene and w155  C for polypropylene), this frequently triggers a large heat output induced by an internal short. First, we consider the mechanism causing a cell to ignite. Generally, combustion is defined as resulting from a reaction which causes materials to generate heat and light. This reaction is usually oxidation and sometimes halogenation. Fire can be seen if a material is heated to a high temperature and as a result the thermal radiation wave becomes visible. Therefore, a fire is caused by an exothermic reaction that provides a sufficient increase in the temperature of the materials. For combustion to continue, the heat generation and dissipation rates must be equal, as shown in Figure 20.1. The expression “thermal runaway” is often used to describe the situation when cells catch fire; however, the expression is unsuitable. If T is higher than T1, the materials catch fire. T1 is called the ignition point, and T2 the fire point. Therefore, the ignition and fire points are not physical values of the materials but depend on conditions around the materials. A good method for preventing this unsafe situation is to increase T1 in one of the

Heat generation rate (

Heat dissipation rate (

T0

)

)

T2 T1 Ignition point Fire point Temperature

FIGURE 20.1 The balance between heat generation and dissipation rates, which describes combustion.

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

463

following two ways: (1) reduce the heat generation rate, or (2) increase the heat dissipation rate. Heat is generated by the thermal decomposition and/or reaction of materials in the cell. The heat dissipation rate depends strongly on the size and shape of the cell. Recent studies on exothermic reactions in Li-ion cells are introduced in the following session.

3. Chemical Reduction of the Electrolyte by the Negative Electrode 3.1.

Graphite Electrode [6]

The results reported here refer to a graphite electrode prepared by mixing 95 wt.% of natural graphite with 5 wt.% of poly(vinylidene fluoride) (PVdF)-binder. Graphite electrodes without PVdF-binder were also fabricated. The electrolyte was 1 M LiPF6/ethylene carbonate (EC) þ dimethyl carbonate (DMC) (1:1 v/v) and the counter electrode a Li metal sheet. The cells were cycled between 0.01 and 1.5 V with a relaxation period of 60 min at the end of charge, at a constant current of 0.2 mA/cm2. After two cycles in this condition, the cells were charged to 0 V with the time limit of 372 mAh/g to obtain a fully charged negative electrode. Figure 20.2 shows DSC curves for fully lithiated or delithiated graphite (a–d) and the electrolyte (e). Sample (a) shows a mild heat generation starting at 130  C with a small peak at 140  C. The mild heat generation continued until a sharp exothermic peak appeared at 280  C. From our experiments, the small peak at 140  C is caused by SEI formation. There is already SEI on the sample (lithiated graphite), which is formed during cycling for sample preparation. This original SEI protects the reaction of the electrolyte and Li in graphite at a lower temperature, and there is no heat generation. However, at w140  C, the protection effect of the original SEI is not sufficient, and a new, thicker SEI is formed. When this becomes thick enough, its formation speed decreases, and a small exothermic heat peak is observed at 140  C. The mild heat generation continued until a sharp exothermic peak appeared at 280  C, because the SEI formation continues with increase in temperature even if there is a protection effect of the SEI. If the original SEI formed during cycling is thick enough, the small peak at 140  C does not appear because the protection effect of the original SEI is enough even at this temperature. Sample (b) is charged at a very low current density because PVdF-binder is not used to make the electrode. Therefore, SEI of sample (b) is very thick, and no peak appeared at 140  C. Samples (c) and (d) did not show the small peak at 140  C. Therefore, the lithiated graphite and the electrolyte are necessary to show the small peak at 140  C. This fact also supports that the small peak at 140  C is the formation of SEI. This peak is sometimes large and the peak temperature is different from 140  C, because the thickness of original SEI is different (the charge current density is different).

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8

Heat flow [mW/mg]

6

(a) 4 (b) (c) 2 (d)

(e) 0

100

200

300

400

500

Temperature (°C) FIGURE 20.2 DSC curves of (a) fully lithiated graphite with the electrolyte and PVdF (the usual graphite anode), (b) fully lithiated graphite with the electrolyte, (c) fully delithiated graphite with the electrolyte and PVdF (the usual graphite electrode), (d) fully lithiated graphite with PVdF (the usual graphite electrode), and (e) the electrolyte (1 M LiPF6/EC þ DMC) [6].

There is evidence [7] that the peak at 280  C in Figure 20.2 is caused by decomposition of SEI with reaction of Li in graphite. DSC curves of charged electrode powder (without electrolyte) obtained after the 2nd charge are shown in Figure 20.3, together with that of discharged electrode powder (without electrolyte) after the 2nd discharge. No exothermic peak was seen at around 100–130  C. The heat values, which were evaluated by integrating DSC curves, were proportional to the amount of charged electrode powder. These results suggest that SEI formed on graphite during charging would react with charged graphite at w280  C accompanied by exothermic heat. As shown in Figure 20.3 no exothermic peak was visible at 100–160  C for charged graphite only, thus the electrolyte should be directly involved in the exothermic reaction at this temperature. To identify the effect of solvent and LiPF6 in the electrolyte separately, the thermal behavior of the charged graphite in solvent was studied firstly [8]. Figure 20.4(a) shows DSC curves for 4 mg of Li0.92C6 mixed with a given amount of the EC þ DMC solvent (from 0.25 to 4 ml). When the amount of the solvent was 0.25 ml, an exothermic peak was observed at w160  C. When the amount of solvent increased from

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

465

25 285 °C 0.29 J

20

(a) 284 °C 0.66 J

15 (mW)

(b) 284 °C 1.4 J

10 (c) 5 (d) 0

–5 50

100

150

200

250

300

350

400

Temperature (°C) FIGURE 20.3 DSC curves of (a–c) charged and (d) discharged graphite-electrode powder. The weight of the graphiteelectrode powder in the pan was (a) 1 mg, (b) 2 mg, and (c, d) 4 mg [7].

0.25 to 2 ml, the heat values of the peak increased significantly. However, the heat value was almost constant when the amount of the coexisting solvent increased to 3 and 4 ml. Therefore, the exothermic peak at w160  C is caused by the reaction between solvent and intercalated Li. The protection effect of original SEI, which was formed during sample

(a)

(b)

FIGURE 20.4 (a) DSC curves for mixtures of 4 mg Li0.92C6 and given amounts of EC þ DMC solvent; (b) DSC curves for mixtures of 4 mg Li0.48C6 and given amounts of EC þ DMC solvent [8]. (For color version of this figure, the reader is referred to the online version of this book.)

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

preparation, has to be considered. The heat value of the peak increased with the increase of the solvent until the amount of solvent became to 3 ml. All the coexisting solvent was used for the reaction, and some of intercalated Li remained, because the reaction was limited by the amount of the solvent. With 4 ml, the heat value did not increase too much from that of 3 ml solvent, because the reaction was limited by the amount of intercalated Li. All the intercalated Li was consumed, and excess solvent remained after the reaction. To confirm the above supposition of exothermic peak at around 160  C, half-charged graphite (Li0.48C6) with solvent was also quantitatively studied by DSC. Figure 20.4(b) shows DSC curves for 4 mg of Li0.48C6 mixed with a given amount of EC þ DMC solvent (from 0.25 to 4 ml). Compared with the DSC curves of the mixtures of Li0.92C6 and solvent (Figure 20.4(a)), it was easy to find that the dominant peak was quite similar to that obtained for Li0.92C6 in the solvent, including peak position and peak shape. At the same time, similar tendency of the heat value was visible. The heat values in both cases increased with the increasing of the amount of solvent, and then remained almost constant when all the intercalated Li was consumed with excess solvent. The heat value became almost constant when the solvent was about 3 ml with 4 mg of Li0.92C6, and the solvent was from 1 to 2 ml with 4 mg of Li0.48C6. Furthermore, the largest heat value of Li0.48C6 was almost half the value of Li0.92C6. Based on these results, it was clear that the amount of solvent limits the reaction when its amount is small, and the amount of intercalated Li limits the reaction when the amount of solvent is large. LiPF6 in the electrolyte is needed to form SEI (with protection effect) on the charged graphite.

3.2.

Silicon/Li Alloy

A silicon/disordered-carbon powder was applied as the negative electrode material in Liion batteries. The thermal characteristics of the Si/C electrode (with or without the electrolyte) were investigated by DSC [9]. Both lithiated (with capacity of 1120 mAh/g) and delithiated Si/C electrode powders showed exothermic peaks in DSC curves due to the reactions between the Li in the electrode and the SEI. The lithiated electrode caused much larger exothermic heat than the delithiated one, especially when they were mixed with the electrolyte. As the thermal risk of Si/C electrode is mainly related to the Li in the electrode, the thermal behaviors of the mixture of lithiated electrode (Si/C) and the electrolyte (1 mol/dm3 LiPF6/EC-DMC) were studied in detail. Figure 20.5 shows variations in DSC curves with the amount of lithiated electrode while the amount of coexisting electrolyte was fixed at 0.5 ml. Although the curves showed significant variation with regard to the amount of electrode, an exothermic peak at about 140  C was observed in each curve. Moreover, the peak intensity was found to have a direct relationship to the amount of electrode. So it was reasonable to attribute this peak to the formation of an SEI as discussed on graphite above. The peak appeared because a thicker SEI was formed and then further SEI formation was prevented, which is the same mechanism as that of graphite electrode. During the reaction, no limitation deriving from the amount of coexisting

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

467

FIGURE 20.5 DSC curves of mixtures of 0.5 ml electrolyte and (a) 0 mg, (b) 0.25 mg, (c) 0.37 mg, (d) 0.5 mg, (e) 0.65 mg, (f) 0.85 mg, (g) 1 mg, (h) 1.5 mg, and (i) 2 mg lithiated Si/C electrode [9].

electrolyte was observed. Based on these data, the effect of Li in Si/C on the formation of SEI was investigated. The exothermic heat at around 140  C was limited by the amount of lithiated electrode. Compared with the stable peak group at 140  C, the peaks at higher temperature were more remarkable in Figure 20.5 as shown by the dotted circles. The peak temperature became lower and the intensity became greater with the increase of the electrode amount. The most drastic peak was the exothermic one, shown in Figure 20.5(d), caused by direct reactions between lithiated electrode and electrolyte because of the decomposition of SEI. From Figure 20.5(b–d), the development of this exothermic peak is clear. Based on these data, we could make some hypotheses about the reactions between lithiated electrode and electrolyte. The main peak at w290  C in Figure 20.5(d) disappeared when the electrode amount was increased from 0.5 mg to 0.65 mg as shown in Figure 20.5(e). A larger ratio of electrode/electrolyte could induce milder heat generation. As already explained for lithiated carbon electrode, it was found that overabundant lithiated electrode induced much stronger exothermic heat. The mechanism of the special phenomenon of the Si/C electrode has not been clarified. It can be supposed that the Si/C electrode has a large specific capacity and could consume a large part of the coexisting electrolyte during SEI formation. As shown in Section 3.1, LiPF6 in the electrolyte is needed to form SEI with protection effect. Moles of LiPF6 are less than moles of solvent in the electrolyte. Therefore, all the LiPF6 is consumed during the SEI formation, and only the solvent remains. At this temperature, exothermic heat of solvent reaction with lithiated electrode is observed, because there is no protection effect as shown in Section 3.1. The peaks shown by dotted circles are the reactions of solvent and lithiated

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

12

10

Heat flow (mw/mg)

8

LiPF6(1M)/PC + DMC(1:1)

6

LiPF6(1M)/PC + DEC(1:1) 4

2

LiPF6(1M)/EC + DMC(1:1)

0 without H2O

LiPF6(1M)/EC + DEC(1:1) –2 200

220

240

with H2O 260

280

300

Temperature (°C)

FIGURE 20.6 DSC profiles of 1 M LiPF6/EC þ DEC (1:1), 1 M LiPF6/EC þ DMC (1:1), 1 M LiPF6/PC þ DEC (1:1), and 1 M LiPF6/PC þ DMC (1:1) electrolytes with/without water.

electrode. As the ratio of electrode/electrolyte increases (from Figure 20.5(e–i)), the peak temperature decreases, because larger amount of SEI is formed on the larger surface area of the electrode (All the LiPF6 is consumed at a lower temperature when the ratio of electrode/electrolyte is large). In Figure 20.5(e), only a smaller peak (shown by dotted circle) was observed at w250  C. It was supposed that all the LiPF6 is consumed and SEI formation stopped. The thickness of the SEI was not enough to protect the Li in the electrode when the temperature became higher, in which case the Li in Si/C probably reacted with the remained solvent and showed a peak at w250  C. The tendency of the peaks shown by dotted circles in Figure 20.5 from (e) to (i) could be explained by the stopping of SEI formation.

4. Thermal Decomposition of the Electrolyte 4.1.

LiPF6/Alkyl Carbonate Mixed-Solvent Electrolytes

The thermal stability of some mixed-solvent electrolytes used in Li-ion cells was also measured by DSC [10]. The electrolytes used were EC þ diethyl carbonate (DEC), EC þ DMC, propylene carbonate (PC) þ DEC, and PC þ DMC in which was dissolved 1 M LiPF6. As shown in Figure 20.6, the exothermic peak of LiPF6 electrolytes containing DEC was found at 255  C, and the peak temperature of the electrolytes containing DEC was

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

469

20

LiPF6(1M)/PC + DMC(1:1)

Heat flow (mw/mg)

16

12

LiPF6(1M)/PC + DEC(1:1)

8

LiPF6(1M)/EC + DMC(1:1) 4

LiPF6(1M)/EC + DEC(1:1) 0

100

150

200

250

300

350

Temperature (°C) FIGURE 20.7 DSC profiles of 1 M LiPF6/EC þ DEC (1:1), 1 M LiPF6/EC þ DMC (1:1), 1 M LiPF6/PC þ DEC (1:1), and 1 M LiPF6/PC þ DMC (1:1) electrolytes with lithium metal and with both lithium metal and water. The solid line is electrolytes with lithium metal, and the dotted line is electrolytes with lithium metal and water.

15–20  C lower than that of LiPF6 electrolytes containing DMC. DEC was found to be more unstable than DMC. The thermal behavior of various kinds of LiPF6 electrolytes with lithium metal was measured by DSC (Figure 20.7). The exothermic reaction of 1 M LiPF6/EC þ DEC, 1 M LiPF6/EC þ DMC, and 1 M LiPF6/PC þ DMC with lithium metal began at the melting point of lithium metal. The temperature was approximately 180  C, whereas the self-heating of 1 M LiPF6/PC þ DEC occurred before the melting point of lithium metal. The temperature at which the self-exothermal reaction began was 140  C. Therefore, the lithium metal in this electrolyte was found to be thermally unstable.

4.2.

LiPF6/Methyl Difluoroacetate Electrolyte

Organic compounds containing fluorine species are nonflammable and have unique properties. Therefore, many kinds of fluorinated organic solvents have been studied as potential cosolvents of electrolytes in order to improve the flammability and lowtemperature performance of graphite anode and Li-ion cells [11–13].

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

Yamaki et al. [14–16] investigated the thermal stability of fluorinated esters, which are the same fluorinated esters used as additives to improve the cycling performance reported by Nakajima et al. [12,13] prior to our study. In this study, partially fluorinated carboxylic acid esters (Table 20.1) were used as the electrolyte solvent and LiPF6 as the salt. LiPF6 was dissolved in methyl difluoroacetate (MFA) and ethyl difluoroacetate (EFA) to a salt concentration of 1 M. In the other fluorinated esters, however, LiPF6 salt could be dissolved to a salt concentration of less than 0.2 M. Therefore, the solutions of fluorinated esters (10 and 20 ) with 0.2 M of LiPF6 were used, and the other fluorinated esters were saturated with LiPF6. For comparison, the solutions of corresponding esters with 0.2 M LiPF6 were prepared. Similar measurements were performed employing the conventional electrolyte solution used in lithium batteries: 1 M LiPF6/EC þ DMC (1:1 by vol). The thermal stability of fluorinated esters (Table 20.2) was examined using a TG-DSC. In some cases, a piece (weighing several milligrams) of lithium metal or a charged LiCoO2 pellet was packed and sealed along with a sample in the stainless-steel case. LiCoO2 pellet was prepared by mixing LiCoO2, acetylene black, and a polytetrafluoroethylene binder. The mixture was packed in a coin cell with lithium metal anode and 1 M LiPF6/ EC þ DMC electrolyte, and then charged to Li0.5CoO2. Table 20.2 shows that the MFA electrolyte is the most stable one. The thermal stability of the LiPF6/fluorinated esters was similar to those of the LiPF6/corresponding esters, except for esters 3, 4, 5, and 7 (Table 20.2). It should be noted that the fluorinated esters contained smaller amounts of LiPF6 than the corresponding esters, except for 1 and 2. In LiPF6 electrolytes, ionic dissociation of LiPF6 is not high, and LiPF6 is in equilibrium with LiF and PF5. PF5 is a strong Lewis acid, which reacts with a small amount of water in electrolytes following the reaction: PF5 þ H2O / POF3 þ 2HF [17]. Based on the analogy with this reaction, organic solvents may have reacted with PF5 at a high temperature. The thermal decomposition of LiPF6 electrolytes is probably caused by PF5 in the electrolytes. It has been reported that the direct reaction of PF5 with EC/EMC is similar to the thermal decomposition of LiPF6 in EC/EMC [18]. PF5 may attack the carbonyl oxygen of nonfluorinated solvents. Table 20.1

Esters Used as Solvents [14]

Nonfluorinated Solvent Sample No.

Solvent

1 2 3 4 5 6 7 8 9

CH3COOCH3 (MA) CH3COOCH2CH3 (EA) CH3CH2COOCH3 CH3CH2COO CH2CH3 H(CH3)2CCOOCH3 H(CH2)3COOCH3 H(CH2)3COOCH2CH3 H(CH2)4COOCH2CH3 H(CH2)7COOCH2CH3

Fluorinated Solvent Sample No. 10 20 30 40 50 60 70 80 90

Solvent CHF2COOCH3 (MFA) CHF2COOCH2CH3 (EFA) CF3CF2COOCH3 CF3CF2COO CH2CH3 F(CF3)2CCOOCH3 F(CF2)3COOCH3 F(CF2)3COOCH2CH3 H(CF2)4COOCH2CH3 F(CF2)7COOCH2CH3

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

Table 20.2

1 2 3 4 5 6 7 8 9

471

Initial Peak Temperatures in DSC Curves [14] Nonfluorinated Solvent

Fluorinated Solvent

Initial Peak Temperature ( C)

Initial Peak Temperature ( C)

Electrolyte

Electrolyte D Li

Electrolyte D Li0.5CoO2

280 210 260 210 250 260 210 240 250

220 110 90 90 70 90 90 120 120

230 240 200 200 250 170 180 170 170

10 20 30 40 50 60 70 80 90

Electrolyte

Electrolyte D Li

Electrolyte D Li0.5CoO2

290 210 280 250 270 260 260 250 230

290 180 330 180 290 300 170 300 160

310 210 210 240 180 230 220 230 220

The reaction mechanism and the stabilities of PF5-solvent complexes are not changed significantly by the fluorination of the esters, since the thermal stabilities of LiPF6/fluorinated esters were similar to those of the LiPF6/corresponding esters. The DSC curves of electrolytes with 1 M LiPF6 and 1 M lithium-imide salts are shown in Figure 20.8. The electrolytes with the lithium-imide (Li-imide) salt were not stable compared with LiPF6/MFA. However, the Li-imide electrolytes showed better stability than LiPF6/EC:DMC (l:1). There was no exothermic peak at 300  C for LiPF6/MFA, although solid LiPF6 decomposes to LiF and PF5 at w300  C. The cycling efficiency of lithium metal electrode was estimated by a cycle test using a coin cell. The cycling efficiency of the lithium metal electrode with 1 M LiPF6/MA (methyl acetate), MFA, EA (ethyl acetate), or EFA is 30, 84, 0, or 50%, respectively. The DSC curves of electrolytes in the presence of Li metal are shown in Figure 20.9 [19]. The Li-imide salt electrolytes were stable; however, their SEI was found to be slightly different

FIGURE 20.8 DSC curves of electrolyte (3 ml) with 1 M LiPF6 and 1 M lithium-imide salts.

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

FIGURE 20.9 DSC curves of 1 M LiPF6 and 1 M lithium-imide salt electrolytes (3 ml) with 0.56 mg of Li metal [19].

(a)

3.0 1st cycle 2nd cycle

2.5

Voltage (V)

2.0 Discharge 1.5 Charge 1.0 0.5

(b)

0.0 3.0 2.5

Voltage (V)

2.0 1.5 1.0 0.5

(c)

0.0 3.0 2.5

Voltage (V)

2.0 1.5 1.0 0.5 0.0 0

100

200

300

400

500

Capacity (mAh/g)

FIGURE 20.10 The initial two discharge/charge profiles of graphite electrodes in (a) 1 mol/dm3 LiPF6/MFA electrolyte, (b) 1 mol/dm3 LiPF6/MFA þ 3%VC electrolyte, and (c) 1 mol/dm3 LiPF6/EC-DMC electrolyte [20].

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

473

from that of LiPF6/MFA by FT–IR measurement. A main component of SEI formed between LiPF6/MFA and Li metal is CHF2COOLi [16], which is a reaction product of Li and MFA. An MFA-based LiPF6 solution was applied as an electrolyte to improve the thermal stability of Li-ion batteries. However, when a graphite anode is used, large irreversible capacity and poor cycling efficiency were obtained with 1 M LiPF6/MFA electrolyte, and these results were attributable to the poor passivation effect of the SEI formed solely with MFA solvent. When 3% vinylene carbonate (VC) was added to the electrolyte as an SEI modification additive, the electrochemical characteristics of the electrolyte were improved significantly [20] (Figures 20.10 and 20.11). With natural graphite as the negative electrode material, reversible capacities of 356 and 346 mAh/g were obtained after the first and the tenth cycles, respectively. The thermal stability of the electrolytes was investigated with DSC. At a DSC heating rate of 5  C/min, whether or not additive was used, the exothermic decomposition temperature of the electrolytes was more than 200  C higher than that of the 1 M LiPF6/EC-DMC electrolyte. Moreover, the thermal behavior of the 1 M LiPF6/MFA þ 3% VC electrolyte mixed with the fully lithiated

(a)

500 100

(b)

90 300 80 200 70 100

Discharge capacity Charge capacity Efficiency

0 500

Efficiency (%)

Capacity (mAh/g)

400

60 50 100

(c)

90 300 80 200 70 100

Efficiency (%)

Capacity (mAh/g)

400

60 0 500

50 100 90

300 80 200 70 100

Efficiency (%)

Capacity (mAh/g)

400

60 0 50 0

2

4

6

8

10

Cycle number

FIGURE 20.11 Cycling performance of graphite electrodes in (a) 1 mol/dm3 LiPF6/MFA electrolyte, (b) 1 mol/dm3 LiPF6/MFA þ 3%VC electrolyte, and (c) 1 mol/dm3 LiPF6/EC-DMC electrolyte [20].

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

120

100

Heat flow (mW)

80

60

4 mg

40

2 mg 20

1 mg 0.5 mg

0 50

150

250

350

450

550

Temperature (°C) FIGURE 20.12 DSC curves of 1 ml 1 mol/dm3 LiPF6/MFA þ 3%VC electrolyte mixed with 0.5, 1, 2, and 4 mg lithiated graphite electrode powder [20].

graphite electrode was studied in detail (Figure 20.12). The ratio between electrolyte and electrode was found to be a dominant factor in the heat generation of the mixture. A sharp exothermic peak at about 330  C was observed when the electrode was superabundant, but the heat value was much smaller than that obtained with 1 mol/dm3 LiPF6/EC-DMC electrolyte under the same conditions (Figure 20.13). When the electrolyte was superabundant, a mild exothermic decomposition of the electrolyte became 10

8

Heat (J)

EC-DMC electrolyte 6

4 MFA + VC electrolyte 2

0 0

1

2 3 Electrode weight (mg)

4

FIGURE 20.13 Comparison of the heat generation from the mixtures of lithiated graphite and different electrolytes [20].

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

475

the dominant reaction in the mixture. The effect of VC additive on SEI modification was also investigated by XPS. CHF2COOLi was expected to be the main component of SEI even when VC was added to the MFA electrolyte. VC-added MFA-based electrolyte was considered to be a good candidate in the development of safer Li-ion batteries.

5. Electrolyte Oxidation at the Positive Electrode 5.1.

LiCoO2

The thermal stability of electrolytes with LixCoO2 cathode or lithiated carbon anode was reviewed, including our results [21,22]. From our experiments, it was found that LixCoO2, delithiated by a chemical method using H2SO4, showed two exothermic peaks, one beginning at 190  C and the other beginning at 290  C. From high-temperature XRD, it was found that the first peak, from 190  C, was the phase transition from a monoclinic (R-3m) to a spinel structure (Fd3m). The spinel structure LixCoO2 showed a very small cycling capacity. Probably, cation mixing was induced by the heat treatment. DSC measurements of Li0.49CoO2 with 1 M LiPF6/EC þ DMC showed two exothermic peaks (Figure 20.14). The peak starting at 190  C probably resulted from the decomposition of solvent due to an active electrode surface, and the peak starting at 230  C was electrolyte oxidation caused by released oxygen from Li0.49CoO2. The exothermic heat from 190 to 230  C based on electrode weight was 420  120 J/g, and that from 230 to 300  C was 1000  250 J/g. From DSC profiles of chemically delithiated Li0.49CoO2 and 1 M PC electrolytes with various Li salts, it was found that the inhibition effect of the surface reaction starting at 190  C was large when LiBF4, LiPF6 and LiClO4 were used.

50

Heat flow (mW)

40

Weight of Li0.49CoO2

30

3.6 mg 20

2.7 mg 2.5 mg 2.1 mg 0.8 mg 0 mg

10

0 200

250

300

Temperature (°C) FIGURE 20.14 DSC profiles of chemically delithiated Li0.49CoO2 with electrolyte for various cathode weights in 3 ml electrolyte [22].

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

FeF3

Voltage (V)

The thermal stability of a FeF3 cathode via a conversion reaction was quantitatively studied using DSC [23]. The charge and discharge profiles of the FeF3 electrodes via a 2Li or 3Li conversion reaction are shown in Figure 20.15. For the 2Li conversion reaction, both the discharge and charge capacities were set as 474 mAh/g, which is twice the theoretical capacity of FeF3 (237 mAh/g). For the 3Li conversion reaction, the discharge and charge capacities were set as 711 mAh/g, which is triple the theoretical capacity of FeF3. Hence, we could assume a stoichiometry of 2Li (Figure 20.15(b)) and 3Li (Figure 20.15(a)) for the FeF3 electrode in its discharged state, whereas almost no Li-ion remained in the charged state. Figure 20.16 shows DSC curves for mixtures of 1 mg discharged FeF3 electrode and 0.5–4 ml of electrolyte. When the amount of electrolyte was above 2 ml, a sharp exothermic peak at around 290  C was observed, and the heat values, which were evaluated by integrating DSC curves in the range of 250–350  C, increased with the amount of electrolyte. The exothermic behavior was quite similar to that of the electrolyte alone, including the peak position and shape [24,25]. Thus, the exothermic peak at around 290  C might be associated with the electrolyte decomposition. When the amount of coexisting electrolyte was 2 ml, a small exothermic peak at around 320  C was observed in the DSC curves. It could be referred to the reaction between the discharged electrode and electrolyte, rather than the thermal decomposition of electrolyte. The heat values of the mixtures (discharged FeF3 and electrolyte) were much smaller than that of the electrolyte itself, although the heat values increased with the amount of electrolyte. Therefore, the discharged FeF3 electrode via the 2Li conversion reaction exhibited great thermal stability in the electrolyte. Why do the mixtures exhibit a small heat generation? The amount of Fe metal in the electrode was estimated to be c. 0.3 mg based on 1 mg of discharged FeF3 electrode in the conversion state. Moreover, the Fe metal generated in the discharged electrode after the conversion reaction has a rather small particle size. Therefore, 0.3 mg of nanosized Fe powder was used for the reference analysis. Figure 20.17 shows the DSC curves of 5 4 3 2 1 0 4 3 2 1 0

(b)

(a)

0

100

200

300

400

500

600

700

800

Capacity (mAh/g) FIGURE 20.15 Charge and discharge curves of a FeF3 electrode in a 1 M LiPF6/EC þ DMC electrolyte [23].

Chapter 20 • Thermal Stability of Materials in Lithium-Ion Cells

477

20

1.2 J

Heat flow (mW)

15

(f) 0.74 J

(e) 10

0.46 J

(d)

0.35 J

(c) 5

0.21 J

(b)

0.16 J

(a)

0 50

100

150

200

250

300

350

400

450

500

Temperature (°C) FIGURE 20.16 DSC curves for mixtures of 1 mg discharged FeF3 electrode via a 2Li conversion reaction and (a) 0.5, (b) 1, (c) 2, (d) 2.5, (e) 3, and (f) 4 ml of electrolyte [23].

FIGURE 20.17 DSC curves of (a) 0.3 mg of Fe powder and of mixtures of 0.3 mg Fe powder and (b) 0.5 ml, (c) 1 ml, (d) 2 ml, (e) 2.5 ml, (f) 3 ml, (g) 4 ml, and (h) 5 ml of 1 M LiPF6/EC þ DMC electrolyte [26].

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

mixtures of 0.3 mg Fe powder and given amounts of electrolyte (ranging from 0.5 to 5 ml) [26]. When the amount of coexisting electrolyte was 0.5–1 ml, no obvious exothermic heat was observed even up to 500  C; only some small thermal fluctuations were detected at w300  C. When the amount of electrolyte increased from 2 to 3 ml, an overlapped peak at around 300  C became distinct and slightly shifted to a lower temperature. Compared to the results of the electrolyte [4,5], the peak temperature was much higher and the heat value was much less. In addition, no noticeable peak was found in the DSC curve of pristine Fe powder as shown in Figure 20.17(a). Hence, the exothermic peak at around 300  C was connected to the reaction between Fe powder and electrolyte. With a further increase of the coexisting electrolyte from 3 to 4 and 5 ml, the two overlapped peaks became closer, and shifted downwards to 290  C. Consequently, it was clear that Fe metal generated in the electrode at the conversion state had suppressed heat generation from the discharged FeF3 electrode.

6. Safety Evaluation by Abuse Tests The safety of lithium cells are evaluated by the cell users under an established guideline before placing on the market. The test items, listed in Table 20.3 [27], are programmed to include comprehensive abuses of the end user. These tests are performed by using practically produced cells. In particular, the following abuse tests are indispensable for the safety evaluations of Li-ion cells, because their tolerance level is rather low.

Table 20.3

Main Abuse Tests [27] Test Items

Electrical abuse tests

Mechanical abuse tests

Thermal abuse tests

1. Overcharging 2. Forced discharge 3. External short circuit 4. Abnormal voltage charging 5. Abnormal current charging 6. Nail penetration (internal short) 1. Crush 2. Drop 3. Vibration 4. Pressure 5. Vacuum 1. Heating 2. High and low-temperature cycling 3. Fire exposure 4. Hot plate 5. Oil bath

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Heating test [27–32]. This test is very useful for evaluating the thermal stability of a cell. The test cell is in an incubator that is heated from room temperature to a certain temperature (T) at the heating rate of 5 C/min; the cell is then kept at this temperature for at least 2 h. If the cell does not fire, T is set at a little higher temperature. Repeating this process, the highest T without cell firing (Tc) is determined. During heating, as mentioned before, exothermic decomposition reactions occur in the cell. If the heat generated by the exothermic decomposition reactions is large enough, the cell fires. In “Guideline for the Safety Evaluation of Secondary Lithium Cells (Japan Battery Association, 1997)”, the heating condition is 130  C for 1 h (Tc is 130  C). However NTT DOCOMO, a Japanese cellular phone company, sets Tc at 150  C. Overcharging [27,28,32–34]. There is a possibility that a cell is excessively overcharged when the charger breaks down or is irregularly used, or when the charging system incorrectly controls the cell voltage. These charging failures mostly cause electrolyte oxidation and lithium deposition at the positive and negative electrodes, respectively. The overcharging test observes the cell temperature and voltage behavior at a constant current. It provides important information such as electrolyte decomposition and separator meltdown during the thermal runaway process. Nail penetration [27,28,32,34,35]. An internal cell short can be simulated by this test and all cells need to pass it, because there is no safety device to prevent it. In this test, a nail of adequate diameter (e.g. 2.5 mm) is forced into the battery at a prescribed speed, followed by a hard current flow at the pass point, thereby generating the joule heat violently. Smoke and fire can be observed, concomitantly with the voltage and temperature monitoring. The worst case occurs when the cell internal resistance is the contact resistance of the short point. The voltage and temperature behavior on these abuse tests is shown in Figure 20.18 [28,35]. Crush. This test is done to check the damage caused by external mechanical stress. There are two types of crush tests: one uses a plate while the other uses a bar. I recommend crush with a bar because a cell fires more frequently with this the test vs. the one with a plate.

6.1.

Safety Devices

To increase the safety, commercial Li-ion cells are usually equipped with safety devices that catch an abnormal behavior and shutdown or limit the current. The current interrupt device (CID) and the positive temperature coefficient (PTC) are representative of such safety devices. The CID functions as a circuit breaker for the overcharge mainly, disconnecting the positive lead from the circuit by using a concave and movable aluminum disk, when the internal pressure of a cell suddenly increases [36,37]. The PTC is a fuse-like device based on materials whose resistance increases dramatically with an increase in temperature. When a large current flow in the circuit and a violent temperature rise due to the Joule heat occurs as a result of external shorting, the resistance of the PTC element can rapidly increase by several orders and limit the current to a relatively low and safe

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LITHIUM-ION BATTERIES: ADVANCES AND APPLICATIONS

FIGURE 20.18 Voltage and thermal behavior on (a) heating test, (b) overcharging test, and (c) nail penetration test [27,34].

level [37,38]. Besides, there are simple safety vents allowing gases to escape, or circuit breakers such as magnetic switches, bimetallic thermostat and electronic protection circuit unit. The shutdown separator is also one of safety devices because it cuts off the ionic current in the circuit as a result of abuses and excessive temperature [39–41].

7. Conclusions When a cell is heated (for example, by an internal short), heat will be generated by thermal decomposition and/or reaction of the materials in the cell. Therefore, it is very

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important to evaluate thermal stability of all materials; this can be accomplished by using DSC. Thermal stability of graphite and silicon/Li alloy negatives, LiPF6/Alkyl carbonate mixed-solvent electrolytes, LiPF6/methyl difluoroacetate electrolyte, LiCoO2 and FeF3 positives with 1 M LiPF6/EC þ DMC were investigated using DSC. Charged graphite and silicon/Li alloy showed almost the same heat output based on the amount of Li in each anode. LiPF6/methyl difluoroacetate electrolyte and FeF3 showed a very small heat output.

Acknowledgments This work was financially supported by the Li-EAD and RISING project of the New Energy and Industrial Technology Development Organization (NEDO), Japan.

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