Carbon Alloys part 13 pdf

to explain the negative magnetoresistance of turbostratic carbons, because of the different type of field dependence of the negative magnetoresistance and the different type of dependenc

404 Chapter 24

polyimide molecules, PMDAPPD is the most flat, as shown in Fig 4 This molecule

does not contain ether oxygen but its film is very brittle A PMDA/PPD film, named

PPT, was prepared at the Research LaboratoIy of Toho Rayon Co Ltd with a small

amount of additive to keep the film flexible (Fig 4)

Rectangular specimens, approximately 26 mm long and 10 mm wide, were cut from

a 45 pm thick PPT film, sandwiched between polished artificial graphite plates and

heated by infrared radiation to 900°C at a rate of 2°C mind in a flow of nitrogen for 1

h These carbonized films were then stepwise heat-treated, 1800-3200°C in flowing

argon The residence times at final temperatures were 30 min between 1800 and

3000°C, and 10 min at 3200°C

Figure 5 shows changes in the X-ray 006 diffraction profiles (using Cu K a

radiation) of the specimen films with increasing HTI' For each film specimen, in the

Fig 5 '006' Diffraction profiles of PFT-derived carbon films heat-treated at various temperatures

(reprinted with the permission of Elsevier Science from Ref [lo])

Fig 6 d,,,,, for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted

with the permission of Elsevier Science from Ref [lo])

' 1800 2dOO 22bO 24bO B O O 28100 3d0(

HTT("C)

Fig 7 Mosaic spread for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C

(reprinted with the permission of Elsevier Science from Ref [lo])

Fig 8 ( A ~ / P ) ~ ~ ~ measured in a field of 1 T for PPT-derived carbon films heat-treated at temperatures

between 1800 and 3000°C (reprinted with the permission of Elsevier Science from Ref [lo])

The values of (AP/P"),,,~~ for the 2200°C specimen, at 77 K, are positive at all

magnetic fields up to 1 T, but are slightly negative for ( A ~ / P ~ ) ~ , , , ~ ~ and (Ap/po)rLmin,

-0.01% and -0.009%, respectively This dependence of magnetic field orientation at 1

T results from the superposition of two magnetoresistance components, one compo-

nent with positive magnetoresistance and the other with negative magnetoresistance

Because the negative magnetoresistance relates to the turbostratic structures and the

positive to graphite, the 2200°C specimen consists, electrically, of two phases But no

direct evidence for such a composite structure has been found in X-ray diffraction

studies The negative magnetoresistance decreases its absolute value more rapidly

with increasing temperature of measurement than the positive magnetoresistance

[12] Hence, the value of ( A ~ / P ~ ) , , , ~ of the 2200°C specimen, when measured at room

temperature, should be larger than that at 77 K Results in a field of 1 T confirm this as

being 1.03% at room temperature and 0.400% at 77 K, as indicated by open circles in

Fig 8a Also, the 2250°C specimen gives nearly the same values, 1.33 and 1.37%,

respectively Further increases of HTT up to 2300 and 2400°C cause reversal of the

relative magnitudes as indicated by changes to arrows from solid to open circles in

Fig 8a With a decreasing contribution of the negative component, the net magneto-

resistance is reduced with increasing temperature This is the case for the specimen of

HTT 2400°C (Fig 8a), and less markedly for the specimens of HTT 2250 and 2300°C

Magnetoresistance 407

HTT ("C )

Fig 9 r for PPT-derived carbon films heat-treated at temperatures between 1800 and 3000°C (reprinted

with the permission of Elsevier Science from Ref [lo])

Figure 8b shows the dependence of ( A ~ / P ~ ) , , , ~ ~ on H'IT between 2250 and 3000°C

(Ap/p,Jrnax exceeds 900% for the specimen of H l T 3000°C and reaches 1206% for the specimen of H'IT 3200°C (held for 10 min) It must be noted that values of ( A P / ~ ~ ) , , , ~ ~ ,

observed in regular quality HOPG, are in the range of 1100-1300% [SI

The textures of the carbon films are similar to that of ideal graphite but are less

perfect The values of rT and rTL of each heat-treated film are close to zero, but are not identical with each other, because of a small inhomogeneity (heterogeneity) in

texture Therefore, the mean value of rT and rTL, r = (rT + rTL)/2, was used as a parameter for the degree of orientation In Fig 9, r is plotted as a function of HTT

The trend in Fig 9 can be interpreted as follows

The orientation of the specimen of HTT 1800°C is turbostratic and with a relatively high degree of orientation of the graphene layers along the film surface The degree

of orientation was improved by successive heat treatment up to 2000°C even though the transformation from turbostratic structure to graphite was initiated A further increase in HTT caused slight deformation of the film, probably due to the growth of the graphite crystallites At higher values of Hl'T, the crystallites continue to grow

and, as a consequence, r increased gradually It seems that at a certain H l T an annealing effect of lattice imperfections takes place in addition to the crystal growth

A critical HTT appears at -2600°C when the maximum value of r was obtained

However, the annealing effect might granulate graphite crystallites, so that r begins to

level off changing little on heating to 2900°C The r value at 3200°C is 0.0333 (not shown in Fig 9) The resistivity ratio pRT/p4.2K of this carbon specimen is 3.45, pRT and

p4,2K being the resistivities at room and liquid helium temperatures with the crystallinity corresponding to that of an HOPG

Kaburagi et al [ll] obtained highly graphitic films from PPT-derived carbon films

by heat treatment at 3200°C Rectangular strips of film, 3 mm wide and 20 mm in length, were cut from the PPT film and carbonized by infrared heating The

Fig 10 (Ap/pJmar at liquid helium temperature for PPT3200-2, together with those KG4.65, KG11.9,

HOPG3600 and PPT3200-1 (reprinted with the permission of Material Research Society from Ref [ll])

carbonized film-strips were then heat treated using two different procedures Each of the carbonized films was sandwiched between two polished artificial graphite plates One, called PPT3200-1, was heated up 3200°C in a graphite resistance furnace in a flow of argon (soak time 10 min) The second strip, called PPT3200-2, was heated to

3100°C (soak time 40 min) and then at 3200°C (soak time 23 min) The d,,, values for

these specimens were identical, i.e 0.3354 nm

The resistivity ratio pRT/p4,2Kwas 3.45 for PPT3200-1 and 4.90 for PPT3200-2, while

( A P / ~ , ) , , , ~ in a field of 1 T at 77 K was 1206% for PPT3200-1 and 1621 % for

PPT3200-2, indicating higher crystallinity of PP3200-2 This is supported by a

measurement of the Shubnikov-de Haas oscillation observed of ( A P / ~ ~ ) , , , ~ ~ at 4.2 K for PPT3200-2, as shown in Fig 10 Here, the results for KG4.65, KG11.9, HOPG3600 and PPT3200-1 are shown for comparison (KG is kish graphite) followed by its

resistivity ratio pRT/p4,2K, the number after HOPG is the annealing temperature The r

values for PPT3200-1 and PPT-3200-2 are 0.0170 and 0.0051, and that of HOPG3600

is 0.0051 These results indicate the extensive graphitization of PPT3200-2

Magnetoresistance 409

5 Negative Magnetoresistance in Boron-doped Graphites

Boron substitutes carbon atoms in graphene layers, the maximum solid solubility being 2.35 at% at 2350°C [13] Hishiyama et al [14-16] and Sugihara et al [17] found

a weak negative transverse magnetoresistance for three kinds of boron-doped graph- ites with the field perpendicular to the specimen surface, and characterized by a field dependence proportional to B1" at temperatures below 4.2 K The boron-doped natural graphite compacts, boron-doped Grafoil materials and boron-doped graphite films were studied, the boron-doped graphite films being Kapton-derived highly graphitic graphite films (HOGF) All of the boron-doped carbons have interlayer

spacing, doOz, lower than for single crystal graphite, 0.3345 nm, and electrical resistivities which exhibit weak temperature dependence but which, at substantially lower temperatures, increase with decreasing temperature expressed as TI" This type

of negative magnetoresistance is not due to increases in carrier density, as for turbo- stratic carbons [4], nor to two-dimensional weak localization which was used by Bayot

et al to explain the negative magnetoresistance of turbostratic carbons, because of the different type of field dependence of the negative magnetoresistance and the different type of dependence of the resistivity on temperature [6] It is probably explained by a three-dimensional weak localization in graphites as proposed by Hishiyama et al [16,17] Hishiyama et al [16] related the negative magnetoresistance

of boron-doped graphite to a disordered structure created by the substitution by

boron atoms An X-ray study of the boron-doped HOGFs (B-HOGFs) was carried out as well as a cross-sectional study by scanning electron microscopy (SEM) and Raman scattering

Hishiyama et al [16] used HOGF, about 11 pm in thickness, for boron doping The high level of orientation of the graphene layers is slightly disturbed by boron doping The disturbance is seen in cross-sectional SEM micrographs of HOGF and B-HOGF The lattice constant c,, was determined from the 002, 004 and 006 diffraction lines

measured in the reflection mode with a,) being measured from the 100 and 110

diffraction lines in the transmission mode The atomic fraction of dissolved boron x,

for B-HOGFs was estimated from X-ray diffraction measurements from relationships

of the lattice constants a,, and c,) vsx, given by Lowel [13] The values of a,, and c,,,x,

and the full width at half maximum of the peak intensity of the 002 diffraction (PI!? for

the samples are listed in Table 1

The X-ray diffraction results for the B-HOGF indicated well-crystallized materials, but appear to be rather disordered as indicated by the Raman spectra Figure 11 shows the first and second order Raman spectra of HOPG, HOGF, B-HOGFs and glass-like carbon heat-treated at 1600°C (GC-1600) The Raman spectra of HOPG and HOGF are those of well-crystallized graphite materials and show a G band at 1585 cm-I, an overtone band at 2441 cm-', a doublet at 2680 cm-' (GI' band) and 2725 cm-' ( G i band) and an overtone band at 3249 cm-I The spectra of B-HOGF-0.4 and B-HOGF-2.2 are similar to those of carbons with small crystallites [18,19] The spectra show the relatively strongD band (1367 cm-I), the D'band (1623

410 Chapter 24

Table 1

The lattice constants a and co, atomic fraction of boron dissolved into latticexB, the full width at the half-maximum of the peak intensity recording of the 002 diffraction I$,,2, room temperature resistivity

p300K, resistivity ratio p300K/p3K and transverse magnetoresistance measured at 3 K i n a field of 1 T

(Ap/po)3K,IT for HOGF and B-HOGFs [16]

Magnetoresistance 411

cm-') at the high frequency side of the G band (1584 cm-'), the weak 2441 cm-' band, and the single unsymmetrical G' band (2720 cm-') which is a merged band of G' and

G bands observed in HOPG and HOGF, the D" band (2962 cm-I), and the band at

3246 cm-I The occurrence of D, D' and D" bands is a characteristic of a disordered graphite structure and is related to the substituted boron atoms in the graphene layers With increasingx,, the relative intensities of the D andD' bands to the G band increase and that of the G' band to the G band decreases The Raman spectrum of GC-1600 is similar to that of B-HOGF-2.2, showing a higher disorder

The room temperature electrical resistivity, p300K, the resistivity ratio p300K/p3K,

where p3K is the resistivity at 3.0 K and transverse magnetoresistance at 3.0 K in a

magnetic field of 1 T ( A P / & ~ , ' ~ for HOGF and B-HOGFs are listed in Table 1

x, value, while keeping the weak dependence for all of the B-HOGF carbons This fact indicates that scattering from the substituted boron atoms dominates over the lattice scattering [20] Values of p7/p30nK for B-HOGF carbons are plotted as a function of T112 in Fig 12 where the number represents thevalue ofx, With increasing

TI", starting from the lowest temperature, the resistivity decreases gradually and linearly with to about 20 K, then decreases a little gradually to pass through a shallow minimum and then finally increases The temperature of minimum resistivity increases with increasing xB, exceeding 300 K for B-HOGF-1.4 and -2.2 Because the temperature dependence of the resistivity for B-HOGF-1.4 and -2.2 is very weak, the linear TIi2 dependence at low temperature is related to an additional resistivity superimposed onto the Boltzmann contribution The additional resistivity could be attributed to the quantum correction of the resistivity due to the 3-D weak localiza-

tion 6p which is obtained by extending the Kawabata's theory to the SWMcC band

[ 16,171

Figure 13 shows the transverse magnetoresistance Ap/pn measured at 3.0 K for B-HOGF-1.4 and -2.2 plotted as a function ofB112 and indicating that Ap/pO is negative

Magnetoresistance 413

and decreases with increasing B’” For B-HOGF-0.9, Ap/po is negative in low fields,

decreases linearly with B’” in fields below 0.41 T, passes through a minimum and then increases with a change in sign with a further increase of B Similarities among the

curves of the field dependence of Ap/po for B-HOGF-0.4,0.5, and 0.9, and the linear

B’” dependence of Aplp,, in low fields for these samples is qualitatively explained by Hishiyama et al [16] and Sugihara et al [17] For B-HOGF-1.4 and -2.2, Ap/p,,

decreases linearly with B’” in fields above about 0.8 T

References

1 Y Hishiyama, Y Kaburagi and M Inagaki, Characterization of structure and microtexture

of carbon materials by magnetoresistance technique In: P.A Thrower (Ed.), Chemistry and Physics of Carbon, Vol 23, pp 1-68 Marcel Dekker, New York, 1991

2 D.E Soule, Magnetic field dependence of the Hall effect and magnetoresistance in graph-

ite single crystals Phys Rev., 112 698-707, 1958

3 Y Hishiyama, Negative magnetoresistance in soft carbons and graphite Carbon, 8:

4 P Delhaes, P de Kepper and M Uhlich, A study of the negative magnetoresistancc in

5 A.A Bright, Negative magnetoresistance of pregraphitic carbons Phys Rev., B20:

6 V Bayot, L Piraux, J.-P Michenaud, J.-P Issi M Lelaurain and A Moore, Two-

7 M Inagaki, Microtextures of carbon materials Tanso (No 122): 114-121,1985

8 M Inagaki, New Carbons Elsevier Science, Oxford, 2000

9 Y Hishiyama, Y Kaburagi, M Inagaki, T Imamura and H Honda, Graphitization of ori- ented coke made from coal tar pitch in magnetic field Carbon, 13: 54&542,1975

10 Y Hishiyama, M Nakamura, Y Nagata and M Inagaki, Graphitization behavior of carbon film prepared from high modulus polyimide film: synthesis of high-quality graphite film

Carbon, 3 2 645-650,1994

11 Y Kaburagi, A Yoshida and Y Hishiyama, Microtexture of highly crystallized graphite as studied by galvano-magnetic properties and electron channeling contrast effect J Mater

Res., 11: 769-778,1996

12 Y Kaburagi, R.H Bragg and Y Hishiyama, Electrical resistivity, transverse magneto-

resistance and Hall coefficient in pyrolytic carbon: correlation with interlayer spacing d,”,>

Phil Mag B., 63: 417-436, 1991

13 C.E Lowell, Solid solution of boron in graphite Am Ceram SOC., 50: 142-144, 1967

14 Y Hishiyama, Y Kaburagi, K Kobayashi and M Inagaki, Structure and properties of

boronated graphite Molec Cryst Liquid Cryst., 310 279-284, 1998

15 Y Hishiyama, Y Kaburagi and K Sugihara, Negative magnetoresistance and of magnetic susceptibility of boronated graphite Molec Cryst Liquid Cryst., 340: 337-342,2000

16 Y Hishiyama, H Irumano, Y Kaburagi and Y Soneda, Structure, Rarnan scattering and

transport properties of boron-doped graphite Phys Rev B, 63: 245406-1-24506-11,2001

17 K Sugihara, Y Hishiyama and Y Kaburagi, Electronic and transport properties of boronated graphite: 3D-weak localization effect Molec Cryst Liquid Cryst., 340: 367-371,

2000

414 Chapter 24

18 R.J Nemanich and S.A Solin, First and second-order Raman scattering from finite-size

crystals of graphite Phys Rev B, 20: 392401,1979

19 R.P Vidano, D.B Fischbach, L.J Wills andT.M Loehr, Observation of Raman band shift- ing with excitation wavelength for carbons and graphites Solid State Commun., 39:

20 D.E Soule, The effect of boron on the electronic properties of graphite In: Proceedings of

341-344,1981

the Fifth Conference on Carbon, Vol 1, pp 13-21 Pergamon Press, Elmsford, N.Y., 1962

Part 5 Function Developments and Application

Potentials

417

Chapter 25

Applications of Advanced Carbon Materials to the

Lithium Ion Secondary Battery

Morinobu Endo and Yoong Ahm Kim

Faculty of Engineering, Shimhu Univemiy, Nagano-shi 380-8553, Japan

Abstract: Lithium ion secondary batteries are currently the best portable energy storage device for the consumer electronics market Recent developments of the lithium ion secondary batteries have been achieved by the use of selected carbon and graphite materials as anodes The performance of lithium ion secondary batteries depends significantly on the micro- structure of the anode materials made from carbon and graphite Due to the contribution of the carbon materials used in the anode for last ten years, the capacity of the typical Li-ion battery has been improved almost double However, active investigations continue to identify the key parameters of carbons that provide improved anode properties Carbon and graphite materials have a wide variety of microstructure, texture, crystallinity and morphology, depending on their preparation processes and precursor materials, as well as various forms such as powders, fibers and spherules This chapter describes correlations between microstructural parameters and electrochemical properties of conventional and novel types of carbon materials for Li-ion batteries Anode carbons include graphitizable carbons such as milled mesophase pitch-based carbon fibers, non-graphitizable carbon such as polyparaphenylene-based carbon heat-treated

at low temperatures Market demand and the trends in lithium-ion secondary batteries are commented upon

Kevw0rd.c Carbon, Microstructure, Anode material, Lithium ion battery

1 Introduction

This chapter discusses recent achievements of carbon anode materials and their structural design for better performances of Li-ion secondary batteries Metallic lithium is very promising as an electrode material of batteries that can bring the lightest weight with high voltage and high energy density Because lithium possesses the lowest electronegativity of the standard cell potential -3.045 V in existing metals,

it is the anode material which gives electrons most easily to form positive ions [l]

However, the negative electrode of lithium metal has serious problems in secondary battery use, because its cyclic life is too short and there are safety factors caused by dendrite formation on surfaces of lithium metal electrode during charge/discharge

418 Chapter 25

cycles [2-51 To solve these problems a locking-chair concept was established in which intercalation phenomena were used for the anode reaction of lithium ion secondary batteries [6-lo] The intercalation of lithium into graphite through vapor transport was first synthesized as a graphite intercalation compound (GIC) with stage structure [ 111 Since then, extensive studies have investigated both staging and charge transfer phenomena of the Li-GICs, of composition Li& with 0 I x 5 1, andx = 3 under high pressure, in carbon materials having a wide range of structures [12-151

In the rechargeable Li-ion batteries, based on the rocking chair or shuttle cock concept, lithium ions shift back and forth easily between the intercalation hosts of cathode and anode Thus, lithium ion secondary batteries consist of a carbonaceous anode and a lithium transition metal oxide such as LiCoO,, LiNiO, and LiMn,O, as the cathode, as shown in Fig la The anode on Cu foil and cathode on Al foil are formed into spiral or plate-folded shapes which give a US18650 cylindrical type (18

Insulator -

Fig 1 (a) Charging/discharging mechanism of Li-ion secondary battery; (b) cylindrical cell

Lithium Zon Secondary Batteries 419

Fig 2 SEM photographs of the carbon anode sheets formed on both sides of a Cu foil lead

mm diameter and 650 mm height, Fig lb), and prismatic cells Between these two electrodes, a porous polymer separator of polyolefin with about 25 pm thickness, made by polyethylene (PE) and polypropylene (PP) is placed (Fig lb) [16,17] Figure

2 shows SEM photographs of anodes in which carbon sheets are formed on both sides

of a copper-foil lead The electrolyte is an organic liquid such as PC, EC+DEC or a

recently developed gel type polymer stable under high voltages A lithium salt such as

LiClO,, LiBF, and LiPF, is dissolved in the electrolyte

A theoretical lithium storage capacity of a graphite anode for a Li ion secondary battery could be 372 mAh g-' and corresponding to the 1st stage GIC LiC, The charge/discharge total reactions and anode reaction based on Li+ intercalation and de-intercalation are as follows [MI:

Charge

Li,-,CoO, + Li,C,

Charge

yc+xLi+xe- < Discharge ' Li,C,

On the other hand, disordered carbons with lithium storage capacities exceeding the theoretical capacity have been reported This phenomenon is still difficult to explain

by the GIC formation mechanism mentioned above, and new explanations are required

Several carbon types from highly ordered graphite to disordered carbons have been investigated experimentally and theoretically for applications as anode materials with an emphasis on specific capacity, cyclic efficiency and cyclic lifetime

As anodes in lithium-ion batteries, carbon microstructure and morphology must be controlled It is already established that the performance of lithium-ion batteries depends strongly on the thermal history and morphology of carbon and graphite materials used [19] As carbon and graphite materials vary widely in their

420 Chapter 25

microstructure, texture, crystallinity and morphology, it has been important to choose

an anode material to give the best battery performance [20-221 Two types of carbon material have been used, that is highly ordered graphites heat-treated to 3000°C and non-graphitizable carbon heat-treated only to 1100°C Precursor materials include cokes, polymers and fibers The insertion behavior and mechanism of lithium ions into carbon and graphite hosts have been extensively studied [23-281 In particular, lithium insertion and resultant electrochemical properties of low temperature carbons are not yet fully understood Low temperature carbons are promising for the Li-ion battery because of their superior capacity Also, low temperature forms of carbons could save production energy because graphites need to be heated to

-3000°C with a production level of - 200 ton month-'

2 Characteristics of Li-ion Secondary Battery

The basic features of Li-ion secondary batteries are summarized as follows [16]:

1 High energy density: 135 Wh kg-' and 300 Wh L-I

2 High operating output voltage: the operating output voltage is 3.6 V, three times higher than that of Ni-Cd or Ni-MH cells

3 High charging characteristics: there is no memory effect as may occur with Ni-Cd batteries with repeated weak discharges

4 Long cycle life: superb recharging properties allow more than 500 repeated charge/discharged cycles

5 Minimal self-discharge: self-discharge is less than 10% per month

6 Remaining capacity display: use of a discharge curve easily shows remaining capacity

Therefore, Li-ion secondary batteries, currently, are the best of available energy storage devices for portable consumer electronics They were first developed and commercialized in 1992 and are used in computer notebooks, cellular phones, digital video cameras, personal computers with color liquid crystal device (LCD) and high speed CPU [22,29,30] As shown in Fig 3, Iithium-ion secondary batteries are superior to Ni-Cd batteries (1.2 V) They have the advantages of 1.5 times in volume and 1.5-2 times in energy density (on a weight basis), and have -3 times the voltage

(-3.6V) [29] There exists a strong consumer demand from the electronics market for further improvements in energy density, out-put current, safety and cost High performance batteries in personal computers and cellular phones may eliminate battery charging Li-ion polymer type batteries, described later, may be the next challenge in this market

3 Carbon and Graphite Host Materials

Carbon materials exhibit many relevant and diverse characteristics including their crystallinity, morphology and texture Structure is the most important characteristic

in terms of electrochemical performance Structure in a carbon is a function of the