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Fluorine compounds as energy conversion materials
Journal of Fluorine Chemistry 149 (2013) 104–111
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Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Review
Fluorine compounds as energy conversion materials Tsuyoshi Nakajima * Department of Applied Chemistry, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2012 Received in revised form 1 February 2013 Accepted 1 February 2013 Available online 14 February 2013
Fluorine compounds are quite useful as energy conversion materials, in particular as primary and secondary lithium battery materials. Primary lithium battery with graphite fluoride cathode was commercialized in 1973, based on the research on graphite fluoride. Since then, it has been shown that fluorine compounds are useful and attractive materials for lithium batteries. The present account paper reports the historical aspects and update topics on fluorine compounds such as graphite fluorides, fluorine–graphite intercalation compounds and organo-fluorine compounds as energy conversion materials. Surface modification by fluorine is also reported. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon-fluorine compound Graphite fluoride Fluorine–graphite intercalation compound Surface modification Organo-fluorine compound Lithium battery
Contents 1. 2. 3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis, structures and properties of graphite fluorides, and their application to primary lithium battery New graphite anode for electrolytic production of F2 gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface modification of natural graphite and metal oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safety improvement of lithium ion battery by organo-fluorine compounds . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Fluorine compounds such as graphite fluoride, LiBF4, LiPF6 and PVdF (polyvinylidenefluoride) have been used for primary and secondary (rechargeable) lithium batteries for many years. It was after 1960 that research and development of high energy density batteries (energy density: Wh kg1) became very active. One of the targets was a new high energy density battery for space development. Since aprotic solvents used for lithium batteries have wide potential windows of 3–4 V, it is possible to manufacture lithium battery with high energy density. Primary lithium battery consists of metallic Li anode, cathode with high electron affinity and aprotic solvents containing inorganic electrolyte such as LiBF4. If F2 is used as a cathode material, the electromotive force of Li/F2 cell is 6.09 V at 25 8C under 1 105 Pa. Since it is difficult to use F2 gas as a cathode material, many kinds of fluorides, chlorides,
* Tel.: +81 565 48 8121; fax: +81 565 48 0076. E-mail addresses: [email protected], [email protected] 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.02.007
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104 105 108 108 109 110 110
oxides and sulfides were examined [1]. After many investigations, it was found that graphite fluoride is the best material as a cathode of primary lithium battery [1–3]. Graphite fluoride is prepared by the fluorination of carbon materials using F2 gas between 350 8C and 600 8C. It is an electric insulator having covalent C–F bond and gray to white color, being stable under usual environment but easily accepting electrochemical reduction. Primary Li/(CF)n battery was commercialized by Matsushita Battery Co. Ltd. in Japan in 1973. After several years, another primary Li battery, i.e. Li/MnO2 battery was also commercialized by replacing (CF)n cathode to MnO2. It was reported in the early date of 1980 that fluorine is inserted into graphite at room temperature, yielding fluorine–graphite intercalation compound [3,4]. This compound is an electric conductor different from graphite fluoride, being applied to new graphite anode for electrolytic production of F2 gas using KF2HF melt [5,6]. After the commercialization of primary lithium batteries, research interest moved to the development of secondary (rechargeable) lithium battery. It was in 1991 that lithium ion secondary battery was sold by Sony Co. Ltd. Since then, research
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111
activity on lithium ion battery materials has been very high. For the application of lithium ion batteries to the electric sources of hybrid cars and electric vehicles, high safety of the batteries is one of the most important issues because lithium ion batteries use flammable organic solvents. High temperature, overcharging, short circuit formation and so on may cause firing and/or explosion of the batteries. To avoid these dangerous accidents, it is necessary to reduce the flammability of organic solvents and the reactivity of organic solvents with Li. For this purpose, organo-fluorine compounds with high stability would play important role.
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(4) Above 600 8C, solid product, (CF)n decreases and fluorocarbon gases such as CF4 and C2F6 increase. Composition and structures of graphite fluorides change depending on the crystallinity of raw carbon materials and reaction temperatures. (1) Graphite fluorides synthesized from natural graphite with high crystallinity 350 8C–400 8C: (C2F)n 400 8C–570 8C: (CF)n + (C2F)n
2. Synthesis, structures and properties of graphite fluorides, and their application to primary lithium battery F2 gas is produced by electrolysis of KF2HF melt at ca. 90 8C using carbon anode. High overpotential is usually observed on carbon anode due to the formation of hydrophobic fluorinated carbon (CF) film. The wettability of KF2HF melt with carbon anode is therefore reduced with increasing CF film having a low surface energy, which causes the sudden decrease in electrolytic current and simultaneously increase in cell potential. This phenomenon finally leads to a so-called ‘‘anode effect’’ that arc or spark is observed between anode and cathode and electrolysis cannot be continued [2]. To investigate the CF film on carbon anode, graphite fluoride was synthesized by the direct fluorination of carbon materials using F2 gas and the reaction, composition, structure and physicochemical properties of graphite fluoride were extensively studied [2,3]. Carbon materials react with F2 gas in a wide range of temperature, yielding several different products depending on reaction temperature and crystallinity of raw carbon materials. The most important factor is the reaction temperature. (1) Below ca. 100 8C, fluorine–graphite intercalation compound, CxF is formed in the presence of a Lewis acid. CxF has ionic – covalent C–F bonds, being electric conductor – semi-conductor with black color. (2) Between ca. 100 8C and 350 8C, surface region of carbon materials is fluorinated. Surface fluorinated layers have covalent C–F bonds. (3) Between 350 8C and 600 8C, graphite fluorides, (CF)n and (C2F)n with C–F covalent bonds are formed. (CF)n and (C2F)n are both electric insulators with low surface energies, having graywhite and black colors, respectively.
570 8C–600 8C: (CF)n (2) Graphite fluoride prepared from petroleum coke 350 8C–590 8C: (CF)n (3) Graphite fluorides prepared from petroleum coke heat-treated at 2700 8C 337 8C–570 8C: (CF)n + (C2F)n 570 8C–600 8C: (CF)n High crystalline natural graphite yields (C2F)n at relatively low temperatures and (CF)n at about 600 8C. In the intermediate temperatures, product is a mixture of (CF)n and (C2F)n. On the other hand, only (CF)n is prepared from low crystalline petroleum coke. Petroleum coke graphitized by high temperature treatment gives the similar result to the case of natural graphite. Fig. 1 shows the examples of graphite and graphite fluoride. Graphite fluoride has two crystal structures, (CF)n and (C2F)n. As shown in Fig. 2, (CF)n has a single puckered graphene layer to which fluorine atoms are covalently bonded from above and below while (C2F)n consists of double puckered graphene layers [2,3,7,8]. Graphene layer of graphite fluoride is not planar but cyclohexane type zigzag structure due to sp3 bonding. (C2F)n having double puckered graphene layers is synthesized only from high crystalline graphite at 350 8C–400 8C. The structure of double puckered graphene layers of (C2F)n and stacking structures of CF layers of (CF)n and (C2F)n were recently studied by neutron diffraction [8]. The double puckered graphene layers of (C2F)n is AB-type and stacking of CF layers takes two types for both (CF)n and (C2F)n. One is that the same CF layer is translated along c-axis and another one is that stacked CF layers have mirror symmetry to each other (Fig. 2). Graphite fluorides are hydrophobic materials due to their covalent C–F bonds. Contact angles of liquids and surface free
Fig. 1. Graphite (A) and graphite fluoride (B).
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Fig. 2. Structures of (A) (CF)n and (B) (C2F)n.
energies calculated from the contact angles are listed in Table 1 [2,3,9,10]. Flaky (CF)n gives the similar surface free energy to that of PTFE (polytetrafluoroethylene), and (CF)n tablet prepared from powdery (CF)n has the smaller value because of a large number of surface CF2 and CF3 groups. Graphite fluorides having layered structures also show good lubricating properties [3]. (CF)n and (C2F)n are stable under usual environment and against oxidation, however, easily accepting electrochemical reduction. Fig. 3 shows discharge curves of some carbon–fluorine compounds [2,3,11]. Discharge capacity of graphite fluoride changes depending on the crystallinity of a raw carbon material. (CF)n prepared from natural graphite with a large basal plane has the larger discharge capacity than (CF)n obtained from petroleum coke with a narrower basal plane. Coronene fluoride, C24F36 has the much smaller capacity and PTFE is not electrochemically reduced. This result shows that fluorine atoms bonded to basal plane are easily discharged accompanying the recovery of sp2 carbons, but those bonded to edge plane are stable against electrochemical reduction [12].
OCV (open circuit voltage) of Li/(CF)n battery is in the range of 3.3–3.5 V, being slightly different depending on the organic solvent, that is, solvation energy towards to Li+ ion [2,3,13]. If the cell reaction is the following, CF þ Li ! C þ LiF;
(1)
the electromotive force calculated by thermodynamic data is 4.57 V at 25 8C, having a large difference from the observed OCV values [2,3,13]. It was revealed by the measurements of electrode potentials of graphite fluorides in various organic solvents and analyses of discharge products that OCV of Li/(CF)n battery is determined by the discharge product containing solvent molecules (S) [2,3,13]. CF þ Li þ xS ! CF Liþ yS þ ðxyÞS
(2)
This discharge product is finally decomposed to carbon, LiF and solvent molecules, generating heat of crystallization of LiF. CF Liþ yS ! C þ LiF þ yS
(3)
The activity of graphite fluoride cathode is unity and discharge potential is therefore flat because electrode surface of graphite fluoride moves from the surface of graphite fluoride particles to inside with discharge except the end of discharge where the electrode area is reduced. Since the discharge of graphite fluoride
Table 1 Contact angles of liquids and surface free energies of graphite fluoride samples [9,10]. Sample
Fig. 3. Discharge curves of fluorocarbons [11]. (A) PTFE (polyterafluoroethylene), (B) C24F36 (coronene fluoride), (C) (CF)n prepared from petroleum coke, (D) (CF)n prepared from natural graphite.
Graphite sheet (CF)n flake (CF)n tablet PTFE tablet
Contact angle (8) at 30 8C Water
Formamide
95 117 143 109
81 97 128 95
Surface free energy (mJ m2)
44 18 2 20
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111 Table 2 BET surface areas of carbon materials and graphite fluoride samples. Carbon material
Petroleum coke (original) Petroleum coke (2800 8C) Natural graphite (290–883 mm) Natural graphite (61–74 mm)
Fluorination temperature
380–500 8C 560 8C 475–600 8C 460–555 8C
Surface area (m2 g1) Carbon material
Graphite fluoride
6.4 2.4 0.6 1.4
206–240 98 95–115 102–105
proceeds with insertion of Li+ ion into CF layers, discharge characteristics are governed by surface area and crystallite size along c-axis (direction of stacking of CF layers) of graphite fluoride. BET surface areas are highly increased by fluorination as given in Table 2. The reactivity of F2 gas is high due to its small dissociation energy, 155 kJ mol1. This would cause the large increase in the surface areas by carbon-carbon bond breaking. The large surface area facilitates the electrochemical discharge of graphite fluoride as a cathode of lithium battery. CF layers of graphite fluoride are expanded along c-axis since the discharge reaction proceeds with insertion of solvated Li+ ion into CF layers. Therefore discharge overpotential decreases as the c-axis crystallite size of graphite fluoride is reduced. Thermal decomposition of graphite intercalation compounds with covalent bonds between host graphite and guests, that is, graphite oxide and graphite fluorides gives new type of carbons having small c-axis crystallite sizes [2,3,14,15]. Fluorination of this submicronic layered carbon similar to ‘‘graphene’’ gives graphite fluoride, (CF)n having high energy
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density [2,3,14,15]. Fig. 4 shows SEM images of submicronic layered carbons obtained by thermal decomposition of graphite fluoride and graphite oxide and (CF)n samples prepared from those submicronic layered carbons [14]. Discharge curve of (CF)n prepared from submicronic layered carbon is shown in Fig. 5 in comparison with those for (CF)n and (C2F)n prepared by direct fluorination of the same raw natural graphite [14]. The (CF)n prepared from submicronic layered carbon shows excellent discharge behavior, that is, high discharge potential and large capacity because the insertion of Li+ ion into CF layers is very easy. The good discharge characteristics are maintained at high current densities as shown in Fig. 6 [14]. Li/(CF)n battery has high stability and long life because graphite fluoride cathode is a stable compound under severe conditions. Recent topic is that Li/(CF)n battery was used for the collection of a capsule carried by a small planet explorer, ‘‘Hayabusa’’, which travelled for 7 years in the space. Graphite oxide is another graphite intercalation compound with covalent bond between graphite and oxygen. Smooth discharge of graphite oxide is difficult probably because reaction products such as Li2O and LiOH are not as stable as LiF. Li+ ion diffusion in graphite oxide is slow, which therefore gives a low discharge capacity. However, fluorination of graphite oxide by F2 at low temperatures between 100 and 150 8C well modifies the discharge properties [16,17]. Fluorinated graphite oxides are smoothly discharged, giving similar discharge capacities to those of (CF)n samples. Stage 1 CxF prepared at room temperature partly contains C–F covalent bonds, being also examined as a cathode of primary lithium battery [17–19]. It gives the higher discharge potential, but the current is smaller.
Fig. 4. SEM images of submicronic layered carbons and (CF)n samples [14]. (A) Submicronic layered carbon obtained by decomposition of (C2F)n, (B) (CF)n obtained from (A), (C) submicronic layered carbon obtained by decomposition of graphite oxide, (D) (CF)n obtained from (C).
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Fig. 7. X-ray diffraction pattern of stage 3 fluorine–graphite intercalation compound formed in graphite anode in well dehydrated KF2HF melt containing 3–6 wt% LiF [5].
4. Surface modification of natural graphite and metal oxide Fig. 5. Discharge curves of graphite fluorides (0.5 mA cm2, 1 mol L1 LiClO4-PC, 25 8C) [14]. (A) (CF)n prepared from natural graphite at 600 8C, (B) (C2F)n prepared from natural graphite at 350 8C, (C) (CF)n prepared from submicronic layered carbon via (C2F)n at 450 8C, (D) (CF)n prepared from submicronic layered carbon via graphite oxide at 400 8C.
3. New graphite anode for electrolytic production of F2 gas Fluorine–graphite intercalation compound, CxF is prepared below ca. 100 8C, usually at room temperature in the presence of a Lewis acid such as HF, being an electric conductor (stage 2 or higher stage) or semi-conductor (stage 1) [3,4]. An electroconductive CxF was applied to graphite anode for electrolytic production of F2 gas [5,6,20]. Anode effect easily takes place by the formation of hydrophobic CF film when graphite with high crystallinity is used as anode in KF2HF melt. It was, however, found that electro-conductive CxF is formed in graphite anode without occurrence of anode effect when dried solid LiF is added to well dehydrated KF2HF melt as shown in Figs. 7 and 8 [5]. Fig. 7 shows X-ray diffraction pattern of stage 3 CxF formed in graphite sheet anode in well dehydrated KF2HF melt with solid LiF. It is found from Fig. 8(B) that anode effect does not occur when solid LiF is added to well dehydrated KF2HF melt. High stage CxF is formed in graphite anode even in well dehydrated melt without LiF as shown in Fig. 8(A). Wettability of KF2HF melt with graphite anode may be improved by the formation of stage 2 or higher stage CxF with high electrical conductivity because of its large polar component of surface free energy [10]. The effect of LiF addition to KF2HF melt may be catalytic intercalation of fluorine into graphite by LiFHF or dehydration of KF2HF melt by LiF. Based on this finding, new LiF-impregnated graphite anode was developed and commercialized [6].
Fig. 6. Discharge curves of (CF)n prepared from submicronic layered carbon via graphite oxide at 400 8C at different current densities (1 mol L1 LiClO4-PC, 25 8C) [14].
Fluorine compounds and fluorination techniques are useful for secondary (rechargeable) lithium batteries. One example is LiPF6 used as an electrolyte, which also contributes to the safety of the battery by depositing LiF on graphite anode at high temperatures. Li-graphite intercalation compound is mainly used as anode of lithium ion battery because it shows flat discharge potential curve and constant capacity. For graphite with high crystallinity, ethylene carbonate (EC)-based electrolyte solution should be used to make protective surface film on graphite by decomposition of a small amount of EC. However, EC has a disadvantage of the high melting point, 36 8C. If possible, it is desirable to use propylene carbonate (PC) having a low melting point, 55 8C. Unfortunately PC cannot be used for graphite anode because electrochemical reduction of PC continues for a long time on graphite anode without formation of surface film. This gives a large irreversible capacity. Since natural graphite powder is prepared by mechanical pulverization of large particles, many lattice defects, i.e. dangling bonds detected by Electron paramagnetic resonance (EPR) measurement would exist at the surface. Based on the concept that the surface lattice defects act as active sites for the electrochemical reduction of PC, surface fluorination and chlorination of natural graphite were attempted to reduce surface lattice defects by forming C–F and C–Cl bonds [21,22]. Surface fluorine concentrations were between 10 and 20 at% under the fluorination conditions of F2 pressure: 3 104 Pa, temperature: 200 and 300 8C, and time: 2 min [21]. Surface fluorination well suppressed the reduction of PC, increasing first coulombic efficiencies by 10–20% as given in Table 3 [21]. Cl2 gas is a milder reagent than F2, also being effective for surface passivation of natural graphite powder. Surface chlorine concentrations were in the range of 0.3–2.3 at%
Fig. 8. Stage numbers of fluorine–graphite intercalation compounds formed in graphite anode in KF2HF melt [5]. (A) well dehydrated KF2HF melt without LiF, (B) well dehydrated KF2HF melt with 3–6 wt% LiF, (C) KF2HF melt without LiF, (D) KF2HF melt with 3 wt% LiF. (G) graphite, (CF) graphite fluoride film, (A.E.) anode effect.
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111
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Table 3 First coulombic efficiencies for original and surface-fluorinated natural graphite samples in 1 mol L1 LiClO4-EC/DEC/PC [21]. Sample
Current density
Original
200 8C
300 8C (%)
NG10 mm
60 mA g1 150 mA g1 60 mA g1 150 mA g1
66 59 52 44
78 70 72 66
76 72 75 64
NG15 mm
Table 4 First coulombic efficiencies for original and surface-chlorinated NG10 mm and NG15 mm samples in 1 mol L1 LiClO4-EC/DEC/PC (1:1:1 vol.) at 60 mA g1 [22]. Sample
60 mA g1
Sample
60 mA g1
NG10 mm 10 min 20 min 30 min
58.9 (%) 79.5 79.8 81.0
NG15 mm 10 min 20 min 30 min
49.5 (%) 53.3 82.4 82.4
Cl2 gas pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min.
under the conditions of Cl2 pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min [22]. Surface oxygen also increased by chlorination probably because HClO, generated by the reaction of Cl2 with a trace of H2O, reacted with surface lattice defects, yielding C5 5O bonds. First coulombic efficiencies increased by 10–30% in 1 mol L1 LiClO4-EC/DEC(diethyl carbonate)/PC (1:1:1 vol.) as given in Table 4 [22]. Improvement of first coulombic efficiencies was also observed in 1 mol L1 LiPF6-EC/EMC(ethyl methyl carbonate)/PC (1:1:1 vol.) (Table 5) [22]. Surface passivation of natural graphite powder by fluorine and chlorine is thus effective for the increase in first coulombic efficiencies, i.e. decrease in irreversible capacities. Surface fluorination of cathode and anode oxide materials such as lithium manganate and titanate was also performed [23–26]. Light fluorination by F2 or NF3 increased the capacities of oxide electrodes. The effect of fluorination may be dehydration of oxide surfaces and/or reduction of surface lattice defects. Thermal stability of mixtures of electrolyte solution and oxide cathode or anode is also improved by surface fluorination.
5. Safety improvement of lithium ion battery by organofluorine compounds For the application of lithium ion batteries to hybrid cars and electric vehicles, high safety of the batteries is urgently requested. To improve the thermal stability, phosphorus compounds such as phosphates are often added to electrolyte solutions. However, addition of phosphorus compounds sometimes lowers the battery performance. A new idea to improve the safety is to make use of organo-fluorine compounds as solvents because fluorine-containing organic compounds usually have high stability against oxidation. Fluorine-containing ethers and esters were used for the first time as additives to improve low temperature characteristics of Table 5 First coulombic efficiencies for original and surface-chlorinated NG10 mm and NG15 mm samples in 1 mol L1 LiPF6-EC/EMC/PC (1:1:1 vol.) at 60 and 300 mA g1 [22]. Sample
60 mA g1
300 mA g1
Sample
60 mA g1
300 mA g1
NG10 mm 10 min 20 min 30 min
75.0 78.0 74.0 77.0
66.4 (%) 71.3 74.6 74.2
NG15 mm 10 min 20 min 30 min
63.8 70.1 77.2 78.9
58.3 (%) 64.7 76.7 76.3
Cl2 gas pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min.
Fig. 9. DSC curves for mixtures of metallic Li and EC/DMC (1:1 vol.) or fluorine compound (A, B or C) [33]. : EC/DMC, : fluoro-ether (A), : fluoro-ether (B), : fluoro-carbonate (C).
graphite anode [27,28]. It was shown in this study that added fluoroethers and -esters improve electrode characteristics at low temperatures of 0–10 8C, and fluoro-ethers are more stable than fluoro-esters against electrochemical reduction [27,28]. Recent interest is to improve the thermal stability of lithium ion battery by organo-fluorine compounds [29–35]. Diffrential scanning calorimetry (DSC) is often used for the investigation of thermal stability at high temperatures. Fig. 9 shows the examples of the reactions of metallic Li with organic solvents and organo-fluorine compounds A, B and C without electrolyte [33]. HCF2CF2CH2OCF2CF2H 3-(1,1,2,2-tetrafluoroethoxy)A: 1,1,2,2-tetrafluoropropane B: HCF2CF2CH2OCF2CFClH 3-(2-chloro-1,1,2-trifluoroethoxy)1,1,2,2-tetrafluoropropane O C: O
O
4-(2,2,3,3,3-pentafluoropropoxy-
CH2OCH2CF2CF3 methyl)-[1,3]dioxolan-2-one No peak is found below 180 8C, at which an endothermic peak showing the melting of Li is observed. As soon as surface oxide film is broken by melting of Li at 180 8C, EC/DMC(dimethyl carbonate) mixture readily reacts with Li, yielding a broad exothermic peak. The reaction of Li with fluoro-carbonate C shifts to 230 8C because electron density of oxygen atom in C5 5O bond is reduced by the effect of fluorine having an electron withdrawing ability. Fluoroethers A and B are more stable, giving exothermic peaks between 250 and 300 8C, which are close to the thermal decomposition temperatures of organic compounds. Thus fluorine compounds have the higher stability against the reactions with Li. Fig. 10 shows the examples of DSC study on mixtures of electrolyte solution and delithiated or lithiated graphite (Figs. 10a and 10c) and Li-intercalated graphite (Fig. 10b) [33]. Fig. 10a was obtained for mixtures of delithiated graphite and electrolyte solution, in which exothermic peaks indicating thermal decomposition of surface film on graphite and electrolyte solution were observed at 244–264 8C in addition to weak exothermic peaks at 83–103 8C due to the reaction of PF5 generated by the dissociation of LiPF6. In Fig. 10b for only lithiated graphite, weak exothermic curves corresponding to the reaction of deintercalated Li with surface film on graphite started above ca.130 8C and exothermic peaks indicating thermal decomposition of surface film were found at 275–297 8C. In the case of the mixtures of lithiated graphite and electrolyte solution (Fig. 10c), the reaction with deintercalated Li is more suppressed in fluoro-carbonate C-containing electrolyte
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Fig. 11. Linear sweep voltammograms for glassy carbon electrode in fluoro-ethermixed electrolyte solutions [33]. (a) 1 mol L1 LiPF6-EC/DMC (1:1 vol.) and 1 mol L1 LiPF6-EC/DMC/(A or B) (1:1:1 vol.), and (b) magnified figure between 5.5 and 8.0 V. () EC/DMC, (A) EC/DMC/A, (B) EC/DMC/B.
Fig. 10. DSC curves for mixtures of 1 mol L1 LiPF6-EC/DMC (1:1 vol.) or 1 mol L1 LiPF6-EC/DMC/(A, B or C) (1:1:1 vol.) and delithiated graphite with SEI film (a), only lithiated graphite (Li0.90–0.98C6) (b), and 1 mol L1 LiPF6-EC/DMC (1:1 vol.) or 1 mol L1 LiPF6-EC/DMC/(A, B or C) (1:1:1 vol.) and lithiated graphite (Li0.90–0.98C6) (c) [33]. : EC/DMC, : EC/DMC/A, : EC/DMC/B, : EC/DMC/C.
solution than in fluoro-ether A or B-containing solution. This may be ascribed to the difference in the liquid structures on molecular level, i.e. that fluoro-carbonate C with the higher dielectric constant than fluoro-ethers is randomly mixed with EC in the solution, however, fluoro-ether A and B with lower dielectric constant mainly interact with hydrocarbon groups of EC. Therefore the reactions of EC with Li deintercalated from LiC6 preferentially take place in the solutions containing fluoro-ethers. Electrochemical oxidation stability is well improved by mixing of organo-fluorine compounds. Fig. 11 shows the cases of fluoroethers A and B [33]. Oxidation currents due to decomposition of the solvents are smaller in the fluorine compound-mixed electrolyte solutions than in original one. Since fluorine substitution of organic compounds elevates their electrochemical reduction potentials, it is necessary to investigate charge/discharge behavior of graphite anode in the low potential region. Many fluoro-carbonates and fluoro-ethers can be used as solvents because they facilitate the formation of surface film on graphite. These compounds are also useful as additives for the formation of surface film. Fig. 12 shows the examples of charge/discharge curves at 1st cycle in fluoro-ether A or B-containing solution [33]. Electrochemical decomposition of PC is well suppressed and first coumobic efficiencies are increased by mixing of fluoro-ethers. As shown in these data, fluoro-ethers and fluoro-carbonates are good candidates as new solvents having high stability against oxidation and reaction with Li.
Fig. 12. First charge/discharge curves of NG15 mm in 1 mol L1 LiPF6-EC/EMC/PC (1:1:1 vol.) and 1 mol L1 LiPF6-EC/EMC/PC/(A or B) (1:1:1:1.5 vol.) at 60 mA g1 [33]. : EC/EMC/PC, : EC/EMC/PC/A, : EC/EMC/PC/ B.
6. Conclusions Reactions of F2 with carbon materials yield several different types of carbon-fluorine compounds such as graphite fluorides and fluorine–graphite intercalation compounds. These compounds are useful as cathode of primary lithium battery and new graphite anode for electrolytic production of F2, respectively. Graphite fluorides show excellent properties in electrochemical reduction reactions. Fluorination techniques are also useful for surface modification of electrode materials. Organo-fluorine compounds have high stability against the reactions with Li and electrochemical oxidation, effectively improving the safety of lithium ion batteries. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jfluchem.2013.02.007. References [1] M. Fukuda, T. Iijima, National Tech. Rep. 20 (1974) 35–50. [2] N. Watanabe, T. Nakajima, H. Touhara, Graphite Fluorides, Elsevier, Amsterdam, 1988.
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111 [3] T. Nakajima, N. Watanabe, Graphite Fluorides and Carbon-Fluorine Compounds, CRC Press, Boca Raton, FL, 1991. [4] T. Nakajima, Synthesis, structure, and physicochemical properties of fluorine– graphite intercalation compounds, in: T. Nakajima (Ed.), Fluorine-Carbon and Fluoride-Carbon Materials – Chemistry, Physics and Applications, Marcel Dekker, New York, NY, 1995 (Chapter 1). [5] T. Nakajima, T. Ogawa, N. Watanabe, J. Electrochem. Soc. 134 (1987) 8–11. [6] T. Tojo, Metal Fluoride-Impregnated Carbon Electrode for Fluorine Production, in: T. Nakajima (Ed.), Fluorine–Carbon and Fluoride–Carbon Materials – Chemistry, Physics and Applications, Marcel Dekker, New York, NY, 1995 (Chapter 9). [7] H. Touhara, K. Kadono, Y. Fujii, N. Watanabe, Z. Anorg. Allg. Chem. 544 (1987) 7–20. [8] Y. Sato, K. Itoh, R. Hagiwara, T. Fukunaga, Y. Ito, Carbon 42 (2004) 2897–2903. [9] N. Watanabe, T. Nakajima, N. Ohsawa, Bull. Chem. Soc. Jpn. 55 (1982) 2029–2033. [10] T. Nakajima, M. Touma, J. Fluorine Chem. 57 (1992) 83–91. [11] N. Watanabe, T. Nakajima, R. Hagiwara, J. Power Sources 20 (1987) 87–92. [12] K. Naga, Y. Ohzawa, T. Nakajima, Electrochim. Acta 51 (2006) 4003–4010. [13] N. Watanabe, R. Hagiwara, T. Nakajima, H. Touhara, K. Ueno, Electrochim. Acta 27 (1982) 1615–1619. [14] T. Nakajima, R. Hagiwara, K. Moriya, N. Watanabe, J. Electrochem. Soc. 133 (1986) 1761–1766. [15] T. Nakajima, A. Mabuchi, R. Hagiwara, N. Watanabe, F. Nakamura, J. Electrochem. Soc. 135 (1988) 273–277. [16] T. Nakajima, Y. Matsuo, Y. Morino, Electrochemistry 61 (1993) 594–599. [17] R. Yazami, Electrochemical Properties of Graphite Fluorides, Metal Fluorides, and Oxide-Fluoride-GICs, in: T. Nakajima (Ed.), Fluorine-Carbon and Fluoride-Carbon Materials – Chemistry, Physics and Applications, Marcel Dekker, New York, NY, 1995 (Chapter 7). [18] R. Hagiwara, M. Lerner, N. Bartlett, T. Nakajima, J. Electrochem. Soc. 135 (1988) 2393–2394. [19] T. Nakajima, M. Koh, V. Gupta, B. Zˇemva, K. Lutar, Electrochim. Acta 45 (2000) 1655–1661.
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[20] T. Nakajima, T. Ogawa, N. Watanabe, J. Fluorine Chem. 40 (1988) 407–417. [21] T. Achiha, T. Nakajima, Y. Ohzawa, J. Electrochem. Soc. 154 (2007) A827– A833. [22] S. Suzuki, Z. Mazej, B. Zˇemva, Y. Ohzawa, T. Nakajima,. Acta Chimica Slovenica, in press. [23] S. Yonezawa, M. Yamasaki, M. Takashima, J. Fluorine Chem. 125 (2004) 1657–1661. [24] T. Nakajima, A. Ueno, T. Achiha, Y. Ohzawa, M. Endo, J. Fluorine Chem. 130 (2009) 810–815. [25] X. Kang, H. Utsunomiya, T. Achiha, Y. Ohzawa, T. Nakajima, Z. Mazej, B. Zˇemva, M. Endo, J. Electrochem. Soc. 157 (2010) A437–A442. [26] H. Utsunomiya, T. Nakajima, Y. Ohzawa, Z. Mazej, B. Zˇemva, M. Endo, J. Power Sources 195 (2010) 6805–6810. [27] T. Nakajima, K. Dan, M. Koh, J. Fluorine Chem. 87 (1998) 221–227. [28] T. Nakajima, K. Dan, M. Koh, T. Ino, T. Shimizu, J. Fluorine Chem. 111 (2001) 167–174. [29] J. Yamaki, Thermally stable fluoro-organic solvents for lithium ion battery, in: T. Nakajima, H. Groult (Eds.), Fluorinated Materials for Energy Conversion, Elsevier, Oxford, 2005 (Chapter 12). [30] T. Achiha, T. Nakajima, Y. Ohzawa, M. Koh, A. Ymauchi, M. Kagawa, H. Aoyama, J. Electrochem. Soc. 156 (2009) A483–A488. [31] T. Achiha, T. Nakajima, Y. Ohzawa, M. Koh, A. Yamauchi, M. Kagawa, H. Aoyama, J. Electrochem. Soc. 157 (2010) A707–A712. [32] Y. Matsuda, T. Nakajima, Y. Ohzawa, M. Koh, A. Yamauchi, M. Kagawa, H. Aoyama, J. Fluorine Chem. 132 (2011) 1174–1181. [33] N. Ohmi, T. Nakajima, Y. Ohzawa, M. Koh, A. Yamauchi, M. Kagawa, H. Aoyama, J. Power Sources 221 (2013) 6–13. [34] K. Naoi, E. Iwama, N. Ogihara, Y. Nakamura, H. Segawa, Y. Ino, J. Electrochem. Soc. 156 (2009) A272–A276. [35] K. Naoi, E. Iwama, Y. Honda, F. Shimodate, J. Electrochem. Soc. 157 (2010) A190–A195.
Contents lists available at SciVerse ScienceDirect
Journal of Fluorine Chemistry journal homepage: www.elsevier.com/locate/fluor
Review
Fluorine compounds as energy conversion materials Tsuyoshi Nakajima * Department of Applied Chemistry, Aichi Institute of Technology, Yakusa, Toyota 470-0392, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2012 Received in revised form 1 February 2013 Accepted 1 February 2013 Available online 14 February 2013
Fluorine compounds are quite useful as energy conversion materials, in particular as primary and secondary lithium battery materials. Primary lithium battery with graphite fluoride cathode was commercialized in 1973, based on the research on graphite fluoride. Since then, it has been shown that fluorine compounds are useful and attractive materials for lithium batteries. The present account paper reports the historical aspects and update topics on fluorine compounds such as graphite fluorides, fluorine–graphite intercalation compounds and organo-fluorine compounds as energy conversion materials. Surface modification by fluorine is also reported. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Carbon-fluorine compound Graphite fluoride Fluorine–graphite intercalation compound Surface modification Organo-fluorine compound Lithium battery
Contents 1. 2. 3. 4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthesis, structures and properties of graphite fluorides, and their application to primary lithium battery New graphite anode for electrolytic production of F2 gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface modification of natural graphite and metal oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Safety improvement of lithium ion battery by organo-fluorine compounds . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction Fluorine compounds such as graphite fluoride, LiBF4, LiPF6 and PVdF (polyvinylidenefluoride) have been used for primary and secondary (rechargeable) lithium batteries for many years. It was after 1960 that research and development of high energy density batteries (energy density: Wh kg1) became very active. One of the targets was a new high energy density battery for space development. Since aprotic solvents used for lithium batteries have wide potential windows of 3–4 V, it is possible to manufacture lithium battery with high energy density. Primary lithium battery consists of metallic Li anode, cathode with high electron affinity and aprotic solvents containing inorganic electrolyte such as LiBF4. If F2 is used as a cathode material, the electromotive force of Li/F2 cell is 6.09 V at 25 8C under 1 105 Pa. Since it is difficult to use F2 gas as a cathode material, many kinds of fluorides, chlorides,
* Tel.: +81 565 48 8121; fax: +81 565 48 0076. E-mail addresses: [email protected], [email protected] 0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jfluchem.2013.02.007
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oxides and sulfides were examined [1]. After many investigations, it was found that graphite fluoride is the best material as a cathode of primary lithium battery [1–3]. Graphite fluoride is prepared by the fluorination of carbon materials using F2 gas between 350 8C and 600 8C. It is an electric insulator having covalent C–F bond and gray to white color, being stable under usual environment but easily accepting electrochemical reduction. Primary Li/(CF)n battery was commercialized by Matsushita Battery Co. Ltd. in Japan in 1973. After several years, another primary Li battery, i.e. Li/MnO2 battery was also commercialized by replacing (CF)n cathode to MnO2. It was reported in the early date of 1980 that fluorine is inserted into graphite at room temperature, yielding fluorine–graphite intercalation compound [3,4]. This compound is an electric conductor different from graphite fluoride, being applied to new graphite anode for electrolytic production of F2 gas using KF2HF melt [5,6]. After the commercialization of primary lithium batteries, research interest moved to the development of secondary (rechargeable) lithium battery. It was in 1991 that lithium ion secondary battery was sold by Sony Co. Ltd. Since then, research
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111
activity on lithium ion battery materials has been very high. For the application of lithium ion batteries to the electric sources of hybrid cars and electric vehicles, high safety of the batteries is one of the most important issues because lithium ion batteries use flammable organic solvents. High temperature, overcharging, short circuit formation and so on may cause firing and/or explosion of the batteries. To avoid these dangerous accidents, it is necessary to reduce the flammability of organic solvents and the reactivity of organic solvents with Li. For this purpose, organo-fluorine compounds with high stability would play important role.
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(4) Above 600 8C, solid product, (CF)n decreases and fluorocarbon gases such as CF4 and C2F6 increase. Composition and structures of graphite fluorides change depending on the crystallinity of raw carbon materials and reaction temperatures. (1) Graphite fluorides synthesized from natural graphite with high crystallinity 350 8C–400 8C: (C2F)n 400 8C–570 8C: (CF)n + (C2F)n
2. Synthesis, structures and properties of graphite fluorides, and their application to primary lithium battery F2 gas is produced by electrolysis of KF2HF melt at ca. 90 8C using carbon anode. High overpotential is usually observed on carbon anode due to the formation of hydrophobic fluorinated carbon (CF) film. The wettability of KF2HF melt with carbon anode is therefore reduced with increasing CF film having a low surface energy, which causes the sudden decrease in electrolytic current and simultaneously increase in cell potential. This phenomenon finally leads to a so-called ‘‘anode effect’’ that arc or spark is observed between anode and cathode and electrolysis cannot be continued [2]. To investigate the CF film on carbon anode, graphite fluoride was synthesized by the direct fluorination of carbon materials using F2 gas and the reaction, composition, structure and physicochemical properties of graphite fluoride were extensively studied [2,3]. Carbon materials react with F2 gas in a wide range of temperature, yielding several different products depending on reaction temperature and crystallinity of raw carbon materials. The most important factor is the reaction temperature. (1) Below ca. 100 8C, fluorine–graphite intercalation compound, CxF is formed in the presence of a Lewis acid. CxF has ionic – covalent C–F bonds, being electric conductor – semi-conductor with black color. (2) Between ca. 100 8C and 350 8C, surface region of carbon materials is fluorinated. Surface fluorinated layers have covalent C–F bonds. (3) Between 350 8C and 600 8C, graphite fluorides, (CF)n and (C2F)n with C–F covalent bonds are formed. (CF)n and (C2F)n are both electric insulators with low surface energies, having graywhite and black colors, respectively.
570 8C–600 8C: (CF)n (2) Graphite fluoride prepared from petroleum coke 350 8C–590 8C: (CF)n (3) Graphite fluorides prepared from petroleum coke heat-treated at 2700 8C 337 8C–570 8C: (CF)n + (C2F)n 570 8C–600 8C: (CF)n High crystalline natural graphite yields (C2F)n at relatively low temperatures and (CF)n at about 600 8C. In the intermediate temperatures, product is a mixture of (CF)n and (C2F)n. On the other hand, only (CF)n is prepared from low crystalline petroleum coke. Petroleum coke graphitized by high temperature treatment gives the similar result to the case of natural graphite. Fig. 1 shows the examples of graphite and graphite fluoride. Graphite fluoride has two crystal structures, (CF)n and (C2F)n. As shown in Fig. 2, (CF)n has a single puckered graphene layer to which fluorine atoms are covalently bonded from above and below while (C2F)n consists of double puckered graphene layers [2,3,7,8]. Graphene layer of graphite fluoride is not planar but cyclohexane type zigzag structure due to sp3 bonding. (C2F)n having double puckered graphene layers is synthesized only from high crystalline graphite at 350 8C–400 8C. The structure of double puckered graphene layers of (C2F)n and stacking structures of CF layers of (CF)n and (C2F)n were recently studied by neutron diffraction [8]. The double puckered graphene layers of (C2F)n is AB-type and stacking of CF layers takes two types for both (CF)n and (C2F)n. One is that the same CF layer is translated along c-axis and another one is that stacked CF layers have mirror symmetry to each other (Fig. 2). Graphite fluorides are hydrophobic materials due to their covalent C–F bonds. Contact angles of liquids and surface free
Fig. 1. Graphite (A) and graphite fluoride (B).
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T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111
Fig. 2. Structures of (A) (CF)n and (B) (C2F)n.
energies calculated from the contact angles are listed in Table 1 [2,3,9,10]. Flaky (CF)n gives the similar surface free energy to that of PTFE (polytetrafluoroethylene), and (CF)n tablet prepared from powdery (CF)n has the smaller value because of a large number of surface CF2 and CF3 groups. Graphite fluorides having layered structures also show good lubricating properties [3]. (CF)n and (C2F)n are stable under usual environment and against oxidation, however, easily accepting electrochemical reduction. Fig. 3 shows discharge curves of some carbon–fluorine compounds [2,3,11]. Discharge capacity of graphite fluoride changes depending on the crystallinity of a raw carbon material. (CF)n prepared from natural graphite with a large basal plane has the larger discharge capacity than (CF)n obtained from petroleum coke with a narrower basal plane. Coronene fluoride, C24F36 has the much smaller capacity and PTFE is not electrochemically reduced. This result shows that fluorine atoms bonded to basal plane are easily discharged accompanying the recovery of sp2 carbons, but those bonded to edge plane are stable against electrochemical reduction [12].
OCV (open circuit voltage) of Li/(CF)n battery is in the range of 3.3–3.5 V, being slightly different depending on the organic solvent, that is, solvation energy towards to Li+ ion [2,3,13]. If the cell reaction is the following, CF þ Li ! C þ LiF;
(1)
the electromotive force calculated by thermodynamic data is 4.57 V at 25 8C, having a large difference from the observed OCV values [2,3,13]. It was revealed by the measurements of electrode potentials of graphite fluorides in various organic solvents and analyses of discharge products that OCV of Li/(CF)n battery is determined by the discharge product containing solvent molecules (S) [2,3,13]. CF þ Li þ xS ! CF Liþ yS þ ðxyÞS
(2)
This discharge product is finally decomposed to carbon, LiF and solvent molecules, generating heat of crystallization of LiF. CF Liþ yS ! C þ LiF þ yS
(3)
The activity of graphite fluoride cathode is unity and discharge potential is therefore flat because electrode surface of graphite fluoride moves from the surface of graphite fluoride particles to inside with discharge except the end of discharge where the electrode area is reduced. Since the discharge of graphite fluoride
Table 1 Contact angles of liquids and surface free energies of graphite fluoride samples [9,10]. Sample
Fig. 3. Discharge curves of fluorocarbons [11]. (A) PTFE (polyterafluoroethylene), (B) C24F36 (coronene fluoride), (C) (CF)n prepared from petroleum coke, (D) (CF)n prepared from natural graphite.
Graphite sheet (CF)n flake (CF)n tablet PTFE tablet
Contact angle (8) at 30 8C Water
Formamide
95 117 143 109
81 97 128 95
Surface free energy (mJ m2)
44 18 2 20
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111 Table 2 BET surface areas of carbon materials and graphite fluoride samples. Carbon material
Petroleum coke (original) Petroleum coke (2800 8C) Natural graphite (290–883 mm) Natural graphite (61–74 mm)
Fluorination temperature
380–500 8C 560 8C 475–600 8C 460–555 8C
Surface area (m2 g1) Carbon material
Graphite fluoride
6.4 2.4 0.6 1.4
206–240 98 95–115 102–105
proceeds with insertion of Li+ ion into CF layers, discharge characteristics are governed by surface area and crystallite size along c-axis (direction of stacking of CF layers) of graphite fluoride. BET surface areas are highly increased by fluorination as given in Table 2. The reactivity of F2 gas is high due to its small dissociation energy, 155 kJ mol1. This would cause the large increase in the surface areas by carbon-carbon bond breaking. The large surface area facilitates the electrochemical discharge of graphite fluoride as a cathode of lithium battery. CF layers of graphite fluoride are expanded along c-axis since the discharge reaction proceeds with insertion of solvated Li+ ion into CF layers. Therefore discharge overpotential decreases as the c-axis crystallite size of graphite fluoride is reduced. Thermal decomposition of graphite intercalation compounds with covalent bonds between host graphite and guests, that is, graphite oxide and graphite fluorides gives new type of carbons having small c-axis crystallite sizes [2,3,14,15]. Fluorination of this submicronic layered carbon similar to ‘‘graphene’’ gives graphite fluoride, (CF)n having high energy
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density [2,3,14,15]. Fig. 4 shows SEM images of submicronic layered carbons obtained by thermal decomposition of graphite fluoride and graphite oxide and (CF)n samples prepared from those submicronic layered carbons [14]. Discharge curve of (CF)n prepared from submicronic layered carbon is shown in Fig. 5 in comparison with those for (CF)n and (C2F)n prepared by direct fluorination of the same raw natural graphite [14]. The (CF)n prepared from submicronic layered carbon shows excellent discharge behavior, that is, high discharge potential and large capacity because the insertion of Li+ ion into CF layers is very easy. The good discharge characteristics are maintained at high current densities as shown in Fig. 6 [14]. Li/(CF)n battery has high stability and long life because graphite fluoride cathode is a stable compound under severe conditions. Recent topic is that Li/(CF)n battery was used for the collection of a capsule carried by a small planet explorer, ‘‘Hayabusa’’, which travelled for 7 years in the space. Graphite oxide is another graphite intercalation compound with covalent bond between graphite and oxygen. Smooth discharge of graphite oxide is difficult probably because reaction products such as Li2O and LiOH are not as stable as LiF. Li+ ion diffusion in graphite oxide is slow, which therefore gives a low discharge capacity. However, fluorination of graphite oxide by F2 at low temperatures between 100 and 150 8C well modifies the discharge properties [16,17]. Fluorinated graphite oxides are smoothly discharged, giving similar discharge capacities to those of (CF)n samples. Stage 1 CxF prepared at room temperature partly contains C–F covalent bonds, being also examined as a cathode of primary lithium battery [17–19]. It gives the higher discharge potential, but the current is smaller.
Fig. 4. SEM images of submicronic layered carbons and (CF)n samples [14]. (A) Submicronic layered carbon obtained by decomposition of (C2F)n, (B) (CF)n obtained from (A), (C) submicronic layered carbon obtained by decomposition of graphite oxide, (D) (CF)n obtained from (C).
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Fig. 7. X-ray diffraction pattern of stage 3 fluorine–graphite intercalation compound formed in graphite anode in well dehydrated KF2HF melt containing 3–6 wt% LiF [5].
4. Surface modification of natural graphite and metal oxide Fig. 5. Discharge curves of graphite fluorides (0.5 mA cm2, 1 mol L1 LiClO4-PC, 25 8C) [14]. (A) (CF)n prepared from natural graphite at 600 8C, (B) (C2F)n prepared from natural graphite at 350 8C, (C) (CF)n prepared from submicronic layered carbon via (C2F)n at 450 8C, (D) (CF)n prepared from submicronic layered carbon via graphite oxide at 400 8C.
3. New graphite anode for electrolytic production of F2 gas Fluorine–graphite intercalation compound, CxF is prepared below ca. 100 8C, usually at room temperature in the presence of a Lewis acid such as HF, being an electric conductor (stage 2 or higher stage) or semi-conductor (stage 1) [3,4]. An electroconductive CxF was applied to graphite anode for electrolytic production of F2 gas [5,6,20]. Anode effect easily takes place by the formation of hydrophobic CF film when graphite with high crystallinity is used as anode in KF2HF melt. It was, however, found that electro-conductive CxF is formed in graphite anode without occurrence of anode effect when dried solid LiF is added to well dehydrated KF2HF melt as shown in Figs. 7 and 8 [5]. Fig. 7 shows X-ray diffraction pattern of stage 3 CxF formed in graphite sheet anode in well dehydrated KF2HF melt with solid LiF. It is found from Fig. 8(B) that anode effect does not occur when solid LiF is added to well dehydrated KF2HF melt. High stage CxF is formed in graphite anode even in well dehydrated melt without LiF as shown in Fig. 8(A). Wettability of KF2HF melt with graphite anode may be improved by the formation of stage 2 or higher stage CxF with high electrical conductivity because of its large polar component of surface free energy [10]. The effect of LiF addition to KF2HF melt may be catalytic intercalation of fluorine into graphite by LiFHF or dehydration of KF2HF melt by LiF. Based on this finding, new LiF-impregnated graphite anode was developed and commercialized [6].
Fig. 6. Discharge curves of (CF)n prepared from submicronic layered carbon via graphite oxide at 400 8C at different current densities (1 mol L1 LiClO4-PC, 25 8C) [14].
Fluorine compounds and fluorination techniques are useful for secondary (rechargeable) lithium batteries. One example is LiPF6 used as an electrolyte, which also contributes to the safety of the battery by depositing LiF on graphite anode at high temperatures. Li-graphite intercalation compound is mainly used as anode of lithium ion battery because it shows flat discharge potential curve and constant capacity. For graphite with high crystallinity, ethylene carbonate (EC)-based electrolyte solution should be used to make protective surface film on graphite by decomposition of a small amount of EC. However, EC has a disadvantage of the high melting point, 36 8C. If possible, it is desirable to use propylene carbonate (PC) having a low melting point, 55 8C. Unfortunately PC cannot be used for graphite anode because electrochemical reduction of PC continues for a long time on graphite anode without formation of surface film. This gives a large irreversible capacity. Since natural graphite powder is prepared by mechanical pulverization of large particles, many lattice defects, i.e. dangling bonds detected by Electron paramagnetic resonance (EPR) measurement would exist at the surface. Based on the concept that the surface lattice defects act as active sites for the electrochemical reduction of PC, surface fluorination and chlorination of natural graphite were attempted to reduce surface lattice defects by forming C–F and C–Cl bonds [21,22]. Surface fluorine concentrations were between 10 and 20 at% under the fluorination conditions of F2 pressure: 3 104 Pa, temperature: 200 and 300 8C, and time: 2 min [21]. Surface fluorination well suppressed the reduction of PC, increasing first coulombic efficiencies by 10–20% as given in Table 3 [21]. Cl2 gas is a milder reagent than F2, also being effective for surface passivation of natural graphite powder. Surface chlorine concentrations were in the range of 0.3–2.3 at%
Fig. 8. Stage numbers of fluorine–graphite intercalation compounds formed in graphite anode in KF2HF melt [5]. (A) well dehydrated KF2HF melt without LiF, (B) well dehydrated KF2HF melt with 3–6 wt% LiF, (C) KF2HF melt without LiF, (D) KF2HF melt with 3 wt% LiF. (G) graphite, (CF) graphite fluoride film, (A.E.) anode effect.
T. Nakajima / Journal of Fluorine Chemistry 149 (2013) 104–111
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Table 3 First coulombic efficiencies for original and surface-fluorinated natural graphite samples in 1 mol L1 LiClO4-EC/DEC/PC [21]. Sample
Current density
Original
200 8C
300 8C (%)
NG10 mm
60 mA g1 150 mA g1 60 mA g1 150 mA g1
66 59 52 44
78 70 72 66
76 72 75 64
NG15 mm
Table 4 First coulombic efficiencies for original and surface-chlorinated NG10 mm and NG15 mm samples in 1 mol L1 LiClO4-EC/DEC/PC (1:1:1 vol.) at 60 mA g1 [22]. Sample
60 mA g1
Sample
60 mA g1
NG10 mm 10 min 20 min 30 min
58.9 (%) 79.5 79.8 81.0
NG15 mm 10 min 20 min 30 min
49.5 (%) 53.3 82.4 82.4
Cl2 gas pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min.
under the conditions of Cl2 pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min [22]. Surface oxygen also increased by chlorination probably because HClO, generated by the reaction of Cl2 with a trace of H2O, reacted with surface lattice defects, yielding C5 5O bonds. First coulombic efficiencies increased by 10–30% in 1 mol L1 LiClO4-EC/DEC(diethyl carbonate)/PC (1:1:1 vol.) as given in Table 4 [22]. Improvement of first coulombic efficiencies was also observed in 1 mol L1 LiPF6-EC/EMC(ethyl methyl carbonate)/PC (1:1:1 vol.) (Table 5) [22]. Surface passivation of natural graphite powder by fluorine and chlorine is thus effective for the increase in first coulombic efficiencies, i.e. decrease in irreversible capacities. Surface fluorination of cathode and anode oxide materials such as lithium manganate and titanate was also performed [23–26]. Light fluorination by F2 or NF3 increased the capacities of oxide electrodes. The effect of fluorination may be dehydration of oxide surfaces and/or reduction of surface lattice defects. Thermal stability of mixtures of electrolyte solution and oxide cathode or anode is also improved by surface fluorination.
5. Safety improvement of lithium ion battery by organofluorine compounds For the application of lithium ion batteries to hybrid cars and electric vehicles, high safety of the batteries is urgently requested. To improve the thermal stability, phosphorus compounds such as phosphates are often added to electrolyte solutions. However, addition of phosphorus compounds sometimes lowers the battery performance. A new idea to improve the safety is to make use of organo-fluorine compounds as solvents because fluorine-containing organic compounds usually have high stability against oxidation. Fluorine-containing ethers and esters were used for the first time as additives to improve low temperature characteristics of Table 5 First coulombic efficiencies for original and surface-chlorinated NG10 mm and NG15 mm samples in 1 mol L1 LiPF6-EC/EMC/PC (1:1:1 vol.) at 60 and 300 mA g1 [22]. Sample
60 mA g1
300 mA g1
Sample
60 mA g1
300 mA g1
NG10 mm 10 min 20 min 30 min
75.0 78.0 74.0 77.0
66.4 (%) 71.3 74.6 74.2
NG15 mm 10 min 20 min 30 min
63.8 70.1 77.2 78.9
58.3 (%) 64.7 76.7 76.3
Cl2 gas pressure: 1 105 Pa, temperature: 400 8C, and time: 10–30 min.
Fig. 9. DSC curves for mixtures of metallic Li and EC/DMC (1:1 vol.) or fluorine compound (A, B or C) [33]. : EC/DMC, : fluoro-ether (A), : fluoro-ether (B), : fluoro-carbonate (C).
graphite anode [27,28]. It was shown in this study that added fluoroethers and -esters improve electrode characteristics at low temperatures of 0–10 8C, and fluoro-ethers are more stable than fluoro-esters against electrochemical reduction [27,28]. Recent interest is to improve the thermal stability of lithium ion battery by organo-fluorine compounds [29–35]. Diffrential scanning calorimetry (DSC) is often used for the investigation of thermal stability at high temperatures. Fig. 9 shows the examples of the reactions of metallic Li with organic solvents and organo-fluorine compounds A, B and C without electrolyte [33]. HCF2CF2CH2OCF2CF2H 3-(1,1,2,2-tetrafluoroethoxy)A: 1,1,2,2-tetrafluoropropane B: HCF2CF2CH2OCF2CFClH 3-(2-chloro-1,1,2-trifluoroethoxy)1,1,2,2-tetrafluoropropane O C: O
O
4-(2,2,3,3,3-pentafluoropropoxy-
CH2OCH2CF2CF3 methyl)-[1,3]dioxolan-2-one No peak is found below 180 8C, at which an endothermic peak showing the melting of Li is observed. As soon as surface oxide film is broken by melting of Li at 180 8C, EC/DMC(dimethyl carbonate) mixture readily reacts with Li, yielding a broad exothermic peak. The reaction of Li with fluoro-carbonate C shifts to 230 8C because electron density of oxygen atom in C5 5O bond is reduced by the effect of fluorine having an electron withdrawing ability. Fluoroethers A and B are more stable, giving exothermic peaks between 250 and 300 8C, which are close to the thermal decomposition temperatures of organic compounds. Thus fluorine compounds have the higher stability against the reactions with Li. Fig. 10 shows the examples of DSC study on mixtures of electrolyte solution and delithiated or lithiated graphite (Figs. 10a and 10c) and Li-intercalated graphite (Fig. 10b) [33]. Fig. 10a was obtained for mixtures of delithiated graphite and electrolyte solution, in which exothermic peaks indicating thermal decomposition of surface film on graphite and electrolyte solution were observed at 244–264 8C in addition to weak exothermic peaks at 83–103 8C due to the reaction of PF5 generated by the dissociation of LiPF6. In Fig. 10b for only lithiated graphite, weak exothermic curves corresponding to the reaction of deintercalated Li with surface film on graphite started above ca.130 8C and exothermic peaks indicating thermal decomposition of surface film were found at 275–297 8C. In the case of the mixtures of lithiated graphite and electrolyte solution (Fig. 10c), the reaction with deintercalated Li is more suppressed in fluoro-carbonate C-containing electrolyte
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Fig. 11. Linear sweep voltammograms for glassy carbon electrode in fluoro-ethermixed electrolyte solutions [33]. (a) 1 mol L1 LiPF6-EC/DMC (1:1 vol.) and 1 mol L1 LiPF6-EC/DMC/(A or B) (1:1:1 vol.), and (b) magnified figure between 5.5 and 8.0 V. () EC/DMC, (A) EC/DMC/A, (B) EC/DMC/B.
Fig. 10. DSC curves for mixtures of 1 mol L1 LiPF6-EC/DMC (1:1 vol.) or 1 mol L1 LiPF6-EC/DMC/(A, B or C) (1:1:1 vol.) and delithiated graphite with SEI film (a), only lithiated graphite (Li0.90–0.98C6) (b), and 1 mol L1 LiPF6-EC/DMC (1:1 vol.) or 1 mol L1 LiPF6-EC/DMC/(A, B or C) (1:1:1 vol.) and lithiated graphite (Li0.90–0.98C6) (c) [33]. : EC/DMC, : EC/DMC/A, : EC/DMC/B, : EC/DMC/C.
solution than in fluoro-ether A or B-containing solution. This may be ascribed to the difference in the liquid structures on molecular level, i.e. that fluoro-carbonate C with the higher dielectric constant than fluoro-ethers is randomly mixed with EC in the solution, however, fluoro-ether A and B with lower dielectric constant mainly interact with hydrocarbon groups of EC. Therefore the reactions of EC with Li deintercalated from LiC6 preferentially take place in the solutions containing fluoro-ethers. Electrochemical oxidation stability is well improved by mixing of organo-fluorine compounds. Fig. 11 shows the cases of fluoroethers A and B [33]. Oxidation currents due to decomposition of the solvents are smaller in the fluorine compound-mixed electrolyte solutions than in original one. Since fluorine substitution of organic compounds elevates their electrochemical reduction potentials, it is necessary to investigate charge/discharge behavior of graphite anode in the low potential region. Many fluoro-carbonates and fluoro-ethers can be used as solvents because they facilitate the formation of surface film on graphite. These compounds are also useful as additives for the formation of surface film. Fig. 12 shows the examples of charge/discharge curves at 1st cycle in fluoro-ether A or B-containing solution [33]. Electrochemical decomposition of PC is well suppressed and first coumobic efficiencies are increased by mixing of fluoro-ethers. As shown in these data, fluoro-ethers and fluoro-carbonates are good candidates as new solvents having high stability against oxidation and reaction with Li.
Fig. 12. First charge/discharge curves of NG15 mm in 1 mol L1 LiPF6-EC/EMC/PC (1:1:1 vol.) and 1 mol L1 LiPF6-EC/EMC/PC/(A or B) (1:1:1:1.5 vol.) at 60 mA g1 [33]. : EC/EMC/PC, : EC/EMC/PC/A, : EC/EMC/PC/ B.
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