Note: Descriptions are shown in the official language in which they were submitted.
213836P
PRE-GRAPHITIC CARBONACEOUS INSERTION COMPOUNDS
AND USE AS ANODES IN REC~Rq~RT~ BATTERIES
FIELD OF THE lNV~NLlON
The invention pertains to the field of carbonaceous
materials and, in particular, to pre-graphitic carbonaceous
insertion materials. Additionally, the invention pertains
to the field of rechargeable batteries and, in particular,
to rechargeable batteries comprising carbonaceous anode
materials.
R~KGROUND OF THE lNV~NllON
The group of pre-graphitic compounds includes
carbonaceous materials that are generally prepared at low
temperatures (eg: less than about 2000C) from various
organic sources and that tend to graphitize when annealed
at higher temperatures. There are however both hard and
soft pre-graphitic carbon compounds, the former being
difficult to graphitize substantially even at temperatures
of order of 3000C, and the latter, on the other hand, being
virtually completely graphitized around 3000C.
The aforementioned set of compounds has been of great
interest for use as anode materials in rechargeable
lithium-ion or rocking chair type batteries. These
batteries represent the state of the art in small
rechargeable power sources for consumer electronics
applications. These batteries have the greatest energy
density (Wh/L) of conventional rechargeable systems (ie.
NiCd, NiMH, or lead acid batteries). Additionally, lithium
ion batteries operate around 3~ volts which is often
sufficiently high that a single cell can suffice for many
electronics applications.
Lithium ion batteries use two different insertion
compounds for the active cathode and anode materials.
Insertion compounds are those that act as a host solid for
the reversible insertion of guest atoms (in this case,
lithium atoms). The structure of the insertion compound
213836~
host is not significantly altered by the insertion. In a
lithium ion battery, lithium is extracted from the anode
material while lithium is concurrently inserted into the
cathode on discharge of the battery. The reverse processes
occur on recharge of the battery. Lithium atoms travel or
"rock" from one electrode to the other as ions dissolved in
a non-aqueous electrolyte with the associated electrons
travelling in the circuit external to the battery.
The two electrode materials for lithium ion batteries
are chosen such that the chemical potential of the inserted
lithium within each material differs by about 3 to 4
electron volts thus leading to a 3 to 4 volt battery. It
is also important to select insertion compounds that
reversibly insert lithium over a wide stoichiometry range
thus leading to a high capacity battery.
A 3.6 V lithium ion battery based on a LiCoO2 / pre-
graphitic carbon electrochemistry is commercially available
(produced by Sony Energy Tec.) wherein the carbonaceous
anode can reversibly insert about 0.65 Li per six carbon
atoms. (The pre-graphitic carbon employed is a disordered
form of carbon which appears to be similar to coke.)
However, the reversible capacity of lithium ion battery
anodes can be increased by using a variety of alternatives
mentioned in the literature. For example, the crystal
structure of the carbonaceous material affects its ability
to reversibly insert lithium (as described in J.R. Dahn et
al., "Lithium Batteries, New Materials and New
Perspectives", edited by G. Pistoia, Elsevier North-
Holland, pl-47, (1993)). Graphite, for instance, can
reversibly incorporate one lithium per six carbon atoms
which corresponds electrochemically to 372 mAh/g. This
electrochemical capacity per unit weight of material is
denoted as the specific capacity for that material.
Graphitized carbons and/or graphite itself can be employed
under certain conditions (as for example in the
presentation by Matsushita, 6th International Lithium
Battery Conference, Muenster, Germany, May 13, 1992, or in
21~8~50
U.S. Patent No. 5,130,211).
Other alternatives for increasing the specific
capacity of carbonaceous anode materials have included the
addition of other elements to the carbonaceous compound.
For example, Canadian Patent Application Serial No.
2,098,248, Jeffrey R. Dahn, "Electron Acceptor Substituted
Carbons for Use as Anodes in Rechargeable Lithium
Batteries", filed June 11, 1993, discloses a means for
enhancing anode capacity by substituting electron acceptors
(such as boron, aluminum, and the like) for carbon atoms in
the structure of the carbonaceous compound. Therein,
reversible specific capacities as high as 440 mAh/g were
obtained with boron substituted carbons. Canadian Patent
Application Serial No. 2,122,770, Alfred M. Wilson,
"Carbonaceous Compounds and Use as Anodes in Rechargeable
Batteries", filed May 3, 1994, discloses pre-graphitic
carbonaceous insertion compounds comprising nanodispersed
silicon atoms wherein specific capacities of 550 mAh/g were
obtained. Similarly, specific capacities of about 600
mAh/g could be obtained by pyrolyzing siloxane precursors
to make pre-graphitic carbonaceous compounds containing
silicon as disclosed in Canadian Patent Application Serial
No. 2,127,621, Alfred M. Wilson, "Carbonaceous Insertion
Compounds and Use as Anodes in Rechargeable Batteries",
filed July 8, 1994.
Most recently, practitioners in the art have prepared
carbonaceous materials with very high reversible capacity
by pyrolysis of suitable starting materials. At the
Seventh International Meeting on Lithium Batteries,
Extended Abstracts Page 212, Boston, Mass. (1994), A.
Mabuchi et al. demonstrated that pyrolyzed coal tar pitch
can have specific capacities as high as 750 mAh/g at
pyrolysis temperatures about 700C. K. Sato et al. in
Science 264, 556, (1994) disclosed a similar carbonaceous
material prepared by heating polyparaphenylene at 700C
which material has a reversible capacity of 680 mAh/g. S.
Yata et al, Synthetic Metals 62, 153 (1994) also disclose
213~360
a similar material made in a similar way. These values are
much greater than that of pure graphite. The
aforementioned materials can have a very large irreversible
capacity as evidenced by first discharge capacities that
can exceed 1000 mAh/g. Additionally, the voltage versus
lithium of all the aforementioned materials has a
significant hysteresis (ie. about 1 volt) between discharge
and charge (or between insertion and extraction of
lithium). In a lithium ion battery using such a
carbonaceous material as an anode, this would result in a
similar significant hysteresis in battery voltage between
discharge and charge with a resulting undesirable energy
inefficiency.
It is not understood why the aforementioned
carbonaceous materials have very high specific capacity.
All were prepared at temperatures of about 700C and are
sufficiently crystalline to exhibit x-ray patterns from
which the parameters doo2l Lc, a, and La can be determined.
(The definition and determination of these parameters can
be found in K. Kinoshita, "Carbon - Electrochemical and
Physicochemical Properties", John Wiley & Sons 1988.)
Also, all have substantial amounts of incorporated hydrogen
as evidenced by H/C atomic ratios that are greater than
0.1, and often near 0.2. Finally, it appears that
pyrolyzing at higher temperature degrades the specific
capacity substantially with a concurrent reduction in the
hydrogen content. (In the aforementioned reference by
Mabuchi et al, pyrolyzing the pitch above about 800C
results in a specific capacity decrease to under 450 mAh/g
with a large reduction in H/C. Similar results were found
in the aforementioned reference by Yata et al.)
The prior art also discloses carbonaceous compounds
with specific capacities higher than that of pure graphite
made from precursors that form hard carbons on pyrolysis.
However, the anticipated very high specific capacities of
the aforementioned materials pyrolyzed at about 700C were
apparently not attained. A. Omaru et al, Paper #25,
213~36û
-- 5
Extended Abstracts of Battery Division, p34, Meeting of the
Electrochemical Society, Toronto, Canada (1992), disclose
the preparation of a hard carbonaceous compound containing
phosphorus with a specific capacity of about 450 mAh/g by
pyrolyzing polyfurfuryl alcohol. The polyfurfuryl alcohol
in turn had been prepared from the monomer polymerized in
the presence of phosphoric acid. In Japanese Patent
Application Laid Open number 06-132031, Mitsubishi Gas
Chemical disclose a hard carbonaceous compound comprising
2.4~ sulfur with a specific capacity of about 500 mAh/g.
These hard carbonaceous compounds have additional elements
incorporated therein and have all been pyrolyzed at
sufficient temperature such that they contain little
hydrogen (ie. the H/C atomic ratio is substantially less
than 0.1). These hard carbonaceous compounds neither
exhibited the very high specific capacities nor the same
serious hysteresis in voltage of the aforementioned
materials pyrolyzed at about 700C.
SUMMARY OF THE lN V~N'l'lON
The subject invention includes carbonaceous insertion
compounds, methods of preparing said compounds, and the use
of said compounds as electrode materials in electrochemical
devices in general.
Carbonaceous insertion compounds of the invention
comprise a pre-graphitic carbonaceous host and atoms of an
alkali metal inserted therein. The inserted alkali metal
can be lithium as would be the case for use in lithium ion
batteries. The empirical parameter R, determined from an x-
ray diffraction pattern of the host and defined as the
{002} peak height divided by the background level, is less
than about 2.2. The H/C atomic ratio of the host is less
than about 0.1. Additionally, the host has an adsorption
capacity for methylene blue that is less than about 4
micromoles per gram of host. To achieve a large
stoichiometry range for reversible insertion of alkali
21~36~
metal, R is preferably less than about 2, and most
preferably less than about 1.5.
The pre-graphitic carbonaceous host can be prepared by
pyrolyzing an epoxy precursor at a temperature above about
700C, thereby predominantly removing hydrogen from the
precursor. However, the pyrolysis temperature cannot be
too high in order that the empirical parameter R,
determined from an x-ray diffraction pattern of the host
and defined as the {002} peak height divided by the
background level, is less than about 2.2.
The epoxy precursor can be an epoxy novolac resin and
can comprise a hardener in a range from zero to about 40
by weight. The hardener can be phthallic anhydride and the
epoxy can be cured at about 120C before pyrolysis. The
maximum pyrolysis temperature can be attained by ramping at
from about 1C/min to about 20C/min. A possible
embodiment of the invention can be prepared by pyrolyzing
an epoxy novolac resin having the formula
0-CH2-CH-CH2 0-CHz-CH-CH2 0-C~2-CH-CHz
~ CHz ~ CH
Epoxy Novolac Resin n = l.
at a maximum temperature below about 1100C.
Alternatively, the epoxy precursor can be a bisphenol
A epoxy resin. The maximum pyrolysis temperature can be
attained by ramping at about 30C/min. A possible
embodiment of the invention can be prepared by pyrolyzing
a bisphenol A epoxy resin having the formula
2138360
CH2~H--CH2--o~3C~o-CH2-CH-CH2--O~CH~O-CH2-CH-CH2
CH3 n CH., .-
5 Bisphenol-A Epoxy Resin
n = 12
at a temperature about 800C.
Methods of the invention include processes for
preparing suitable pre-graphitic carbonaceous hosts for the
aforementioned compounds. Such hosts can be prepared by
pyrolyzing an epoxy precursor at a temperature above about
700C such that the empirical parameter R, determined from
15 an x-ray diffraction pattern and defined as the {002} peak
height divided by the background level, is less than about
2.2.
Alkali metal atoms can be inserted into the host
thereafter by conventional chemical or electrochemical
means to make insertion compounds of the invention. The
epoxy employed in the method of the invention can be an
epoxy novolac resin having a formula
O _ O _ O
2 50--CH2--CH--C~20--CH2--CH--CH2 0--CH2--Cl I--CH2
~ CH, ~ CH2
_ --n
Epoxy Novolac Resin n = 1.6
35 wherein the pyrolysis is performed at a maximum temperature
below about 1100C. Alternately, the epoxy employed in the
method of the invention can be a bisphenol A epoxy resin
2138360
having a formula
CH2~-cH2-o ~ CH3 0-CH2-CH-CH2-O ~ CH3 0-CH2-CH-CH2
Bisphenol-AEpoxyResin
n = 12
wherein the pyrolysis is performed at a temperature about
800C.
Compounds of the invention can be used as a portion of
an electrode in various electrochemical devices based on
insertion materials (eg. supercapacitors, electrochromic
devices, etc.). A preferred application for these
compounds is use thereof as an electrode material in a
battery, in particular a non-aqueous lithium ion battery
comprising a lithium insertion compound cathode; a non-
aqueous electrolyte comprising a lithium salt dissolved in
a mixture of non-aqueous solvents; and an anode comprising
the carbonaceous insertion compound of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In drawings which illustrate specific embodiments of
the invention, but which should not be construed as
restricting the spirit or scope of the invention in any
way:
Figure 1 shows the definition of R on an almost
featureless x-ray diffraction pattern of a pre-graphitic
carbon in the region around the {002} peak.
Figure 2 shows a cross-sectional view of a
conventional lithium ion spiral-wound type battery.
213836()
Figure 3 depicts an exploded view of the laboratory
coin cell battery used in the Examples.
Figure 4 shows the H/C atomic ratio versus pyrolysis
temperature for the samples of Comparative Example 2 and of
Inventive Example 1.
Figure 5 shows the x-ray diffraction patterns in the
vicinity of the {002} peak for some of the samples of
Comparative Example 2. The patterns have been offset
vertically by 2000 counts for clarity.
Figures 6a and 6b show the voltage versus capacity
plots for some of the batteries of Comparative Example 2.
Figure 6a is an expanded version of Figure 6b in the region
near zero volts. The points at which lithium plating and
stripping occur are indicated by arrows for the battery
comprising the 550C pyrolyzed sample. The plots in each
Figure are offset sequentially by 0.05 volts and 0.1 volts
respectively for clarity.
Figure 7 shows the x-ray diffraction patterns in the
vicinity of the {002} peak for the M20E activated carbon
samples of Comparative Example 3.
Figure 8 shows the second cycle voltage versus
capacity plot for the battery containing M30 activated
carbon pyrolyzed at 1000C of Comparative Example 3.
Figure 9 shows the first cycle voltage versus capacity
plot for the battery containing M30 activated carbon
pyrolyzed at 1000C of Comparative Example 3.
Figure 10 compares the second cycle voltage versus
capacity plots of sample no. I of Inventive Example 1 to
that of the 700C pyrolyzed sample of Comparative Example
213~360
- 10 -
Figure 11 shows the x-ray diffraction patterns in the
vicinity of the {002} peak for samples I, II, and III of
Inventive Example 1. The patterns have been offset
vertically by 1600 counts for clarity.
Figures 12a and 12b show the voltage versus capacity
plots of samples I, II, III, and IV of Inventive Example 1.
Figure 12a is an expanded version of Figure 12b in the
region near zero volts. The points at which lithium
plating and stripping occur are indicated by arrows for the
battery comprising sample IV. The plots in each Figure are
offset sequentially by 0.05 volts and 0.1 volts
respectively for clarity.
Figures 13a and 13b show the voltage versus capacity
plots of samples V, VI, VII, and IX of Inventive Example 1
and illustrates the relation between R and specific
capacity for samples pyrolyzed at 1000C to 1100C. Figure
13a is an expanded version of Figure 13b in the region near
zero volts. The points at which lithium plating and
stripping occur are indicated by arrows for the battery
comprising sample VII. The plots in each Figure are offset
sequentially by 0.05 volts and 0.1 volts respectively for
clarity.
Figure 14 shows the x-ray diffraction pattern in the
vicinity of the {002} peak for the samples of Figures 13a
and b. The patterns have been offset vertically by 3000
counts for clarity.
Figure 15 shows a summary plot of specific capacity
versus R for sample nos. III to IX inclusive of Inventive
Example 1.
Figure 16 shows the voltage versus capacity plot of
2138~0
the first discharge and charge of the battery comprising
sample no. VII of Inventive Example 1.
Figures 17a and 17b show the voltage versus capacity
plots of a battery of Inventive Example 2. Figure 17a is
an expanded version of Figure 17b in the region near zero
volts.
DET~TT.T~n DESCRIPTION OF THE SPECIFIC
EMBODIMENTS OF THE lNV~L.llON
Compounds of the invention comprise hard pre-graphitic
carbonaceous hosts having very poorly stacked graphene
layers, little hydrogen content, and a low adsorption
capacity for common non-aqueous electrolyte solutions.
Said compounds can be derived from pyrolysis products of
suitable epoxy precursors. Herein, epoxy precursor refers
to that group of thermosetting resins based on the
reactivity of the epoxide group (as per the definition in
The Condensed Chemical Dictionary, Ninth Ed., Van Nostrand
Reinhold, 1977). Common members of this group include
bisphenol A-based epoxies and epoxy novolac resins.
Suitable epoxy precursors are those that, when pyrolyzed at
temperatures above about 700C, do not graphitize to such
an extent that the empirical parameter R as determined by
x-ray diffraction pattern exceeds about 2.2. Herein, R is
defined as the {002} graphite peak height divided by the
background level. (The detailed method for this
determination is described later in the specification.) R
provides a convenient empirical means of quantifying the
degree of graphitization of these compounds which have
almost featureless x-ray diffraction patterns. Figure 1
illustrates the definition of R on a representative, almost
featureless x-ray diffraction pattern of a pre-graphitic
carbon in the region around the {002} peak.
Pyrolyzing suitable epoxy precursors above 700C
provides pre-graphitic carbonaceous hosts that do not
2138350
- 12 -
exhibit severe hysteresis in voltage upon insertion or
extraction of lithium. Hosts prepared in this way are also
characterized by low H/C atomic ratios. Pyrolyzing at
temperatures such that R is below 2.2 provides for hosts
with very high specific capacities for lithium. The
specific capacity for lithium increases as R decreases.
Based on the Examples to follow, R appears to be preferably
less than about 2 and most preferably less than about 1.5.
The pyrolysis should be performed under a controlled
atmosphere to prevent formation of undesired oxides of
carbon. A suitable reaction system could consist of a
reaction tube (quartz for example) installed in a
conventional tube furnace wherein the tube has sealed inlet
and outlet connections for purposes of controlling the
atmosphere therein. The epoxy precursor/s could thus be
pyrolyzed in the reaction tube under an inert gas flow or
even under reduced or elevated pressure. Additionally,
controlled partial reduction or oxidation, if desired, can
be achieved by admitting controlled amounts of an
appropriate gas.
To ensure good product yields, ideally the epoxy
precursor/s should substantially pyrolyze rather than
simply evaporate. This issue must be considered in the
selection of preferred precursor/s. Also, in certain
cases, the extent of the curing or cross-linking may be a
significant variable affecting the desired ultimate
properties of the pyrolyzed epoxy. Thus, it may be
advantageous to consider incorporating soaking periods at
several temperatures as part of the heat treatment. For
example, a low temperature soak might be used for curing
the epoxy prior to a final heating to the pyrolysis
temperature. Alternately, the heating rate might be varied
to control the extent of the curing prior to pyrolysis.
The product of pyrolysis can have relatively large
surface areas, of order of 200 m2/g, as determined by
conventional nitrogen adsorption methods (eg. BET).
However, the surface area that is actually accessible to
2138360
- 13 -
common non-aqueous electrolyte solutions is relatively
small. This is especially important for application in
lithium ion batteries. Electrolyte reactions that consume
lithium occur at the anode surface in such batteries.
Thus, use of an anode having a large surface area
accessible to electrolyte results in substantial
irreversible capacity loss and electrolyte loss. These
losses are avoided if the anode surface is not accessible
to the electrolyte.
10The aforementioned product has no alkali metal
inserted as prepared. Alkali metal atoms, in particular
Li, can be inserted thereafter via conventional chemical or
electrochemical means (such as in a lithium or lithium ion
battery).
15Generally, powdered forms of such compounds are used
in electrode applications and thus a grinding of the
pyrolyzed product is usually required. A variety of
embodiments, in particular various battery configurations,
are possible using electrode material prepared by the
method of the invention. Miniature laboratory batteries
employing a lithium metal anode are described in the
examples to follow. However, a preferred construction for
a lithium ion type product is that depicted for a
conventional spiral-wound type battery in the cross-
sectional view of Figure 2. A jelly roll 4 is created byspirally winding a cathode foil (not shown), an anode foil
(not shown), and two microporous polyolefin sheets (not
shown) that act as separators.
Cathode foils are prepared by applying onto a thin
aluminum foil a mixture of a suitable powdered (about 10
micron size typically) cathode material, such as a
lithiated transition metal oxide, possibly other powdered
cathode material if desired, a binder, and a conductive
dilutant. Typically, the application method first involves
dissolving the binder in a suitable liquid carrier. Then,
a slurry is prepared using this solution plus the other
powdered solid components. The slurry is then coated
2138~60
- 14 -
uniformly onto the substrate foil. Afterwards, the carrier
solvent is evaporated away. Often, both sides of the
aluminum foil substrate are coated in this manner and
subsequently the cathode foil is calendered.
Anode foils are prepared in a similar manner except
that a powdered (also typically about 10 micron size)
carbonaceous insertion compound of the invention is used
instead of the cathode material and thin copper foil is
usually used instead of aluminum. Anode foils are
typically slightly wider than the cathode foils in order to
ensure that anode foil is always opposite cathode foil.
This feature is illustrated with the cathode upper edge 13,
cathode lower edge 14, anode upper edge 12, and anode lower
edge 15 depicted in Figure 2.
The jelly roll 4 is inserted into a conventional
battery can 3. A header 1 and gasket 10 are used to seal
the battery 16. The header may include safety devices if
desired. A combination safety vent and pressure operated
disconnect device may be employed. Figure 2 shows one such
combination that is described in detail in Canadian Patent
Application No. 2,099,657, Alexander H. Rivers-Bowerman,
"Electrochemical Cell and Method of Manufacturing Same",
filed June 25, 1993. Additionally, a positive thermal
(PTC) coefficient device may be incorporated into the
header to limit the short circuit current capability of the
battery. The external surface of the header 1 is used as
the positive terminal, while the external surface of the
can 3 serves as the negative terminal.
Appropriate cathode tab 5 and anode tab 6 connections
are made to connect the internal electrodes to the external
terminals. Appropriate insulating pieces 2 and 7 may be
inserted to prevent the possibility of internal shorting.
Prior to crimping the header 1 to the can 3 in order to
seal the battery, electrolyte 8 is added to fill the porous
spaces in the jelly roll 4.
Those skilled in the art will understand that the
types of and amounts of the component materials must be
2138360
- 15 -
chosen based on component material properties and the
desired performance and safety requirements. The compounds
prepared in the Examples that follow have increased
irreversible capacity for lithium along with an increased
reversible capacity over that of many typical commercial
carbonaceous anode materials. Also, the Example compounds
typically have lower density than that of typical
commercial anode materials. This must be taken into
account in the battery design. Generally an electrical
conditioning step, involving at least the first recharge of
the battery, is part of the assembly process. Again, the
determination of an appropriate conditioning step along
with the setting of the battery operating parameters (eg.
voltage, current, and temperature limits) would be required
of someone familiar with the field.
Other configurations or components are possible for
the batteries of the invention. For example, a prismatic
format is considered highly desirable and possible. A
miniature embodiment, eg. coin cell, is also possible and
the general construction of such cells is described in the
laboratory coin cell examples to follow.
There is no known quantitative model which can
explain how certain prior art carbonaceous materials can
have specific capacities that significantly exceed that of
graphite. (However, J. Dahn et al, Electrochimica Acta,
Vol. 3, No.9, p 1179-1191, 1993 speculated on the
possibility of certain unorganized carbons exceeding the
capacity of graphite via lithium adsorption on single
graphite layers contained within. Also, in the
aforementioned reference by K. Sato et al, Li dimer
formation was proposed as an explanation for the very high
specific capacity of their carbonaceous material.) Without
wishing to be bound by theory, adversely or otherwise, the
inventors offer the following view of the prior art and of
the instant invention.
The presence of substantial hydrogen in carbonaceous
materials of the prior art prepared by pyrolysis at low
213836~
- 16 -
temperatures (between 550C and 750C) correlates with very
high specific capacity. Certain hard carbonaceous
materials of the prior art however have little hydrogen but
still exhibit high specific capacities that exceed that of
graphite. The graphene sheets in the precursors for these
hard carbonaceous materials are cross-linked and this
prevents the ordered stacking of layers in the graphite
structure as the precursors are pyrolyzed. When poorly
stacked graphene layers are present, it may be possible to
adsorb lithium onto the surfaces of each side of the
layers. These surfaces are found within the carbon
particles, on the atomic scale. In graphite, the layers
are well stacked in a parallel fashion and intercalation of
lithium to a composition of LiC6 is possible (corresponding
to about 370 mAh/g and one intercalated layer of lithium
per graphene sheet). In materials with poorly stacked
layers, unshared lithium layers might possibly be found on
each side of the graphene sheets, resulting in compositions
up to almost Li2C6 (corresponding to about 740 mAh/g).
Thus, the number of single layer graphene sheets in the
carbonaceous material may be important vis a vis specific
capacity.
X-ray diffraction can be used to learn about the
average number, N, of stacked graphene sheets in a carbon
in between serious stacking mistakes. This number N,
multiplied by the average layer spacing is commonly given
the name, Lc. It may therefore be desirable to make
carbonaceous materials with N about 1 and with very small
Lc (eg. less than about 5A). The {002} Bragg peak measured
in a powder x-ray diffraction experiment is normally used
to determine Lc and N (see for example, the aforementioned
reference by K. Kinoshita). For N=1, there is no {002}
peak since there are no stacked parallel graphene layers to
create interferences. (Such a carbon sample might be
thought of as having single graphene sheets arranged as in
a house of cards.) As N increases (beginning to stack the
deck of cards), the {002} peak increases in height and
213~36~
- 17 -
decreases in width. Simultaneously, the background on the
low angle side of the peak decreases, as N increases.
Herein, the empirical parameter R is used for purposes of
describing such structures and is determined by dividing
5 the {002} peak height (B in Figure 1) by an estimate of the
background level at the Bragg angle corresponding to the
position of the { 002 } peak (A in Figure 1). The background
estimate (A in Figure 1) is that value given by the
intersection of a line tangential to the background in the
immediate vicinity of the {002} peak and B (the position of
the { 002 } peak). R can thus be used to distinguish the
stacking order in very disorganized materials. Materials
having very small R values (about 1) would have N values
near 1. Materials having R near 5 would have significantly
15 larger N, possibly with N about 10. Thus, increases in R
can be interpreted as increases in the average N in the
sample. To quantitatively measure R reproducibly, all of
the x-ray beam of the diffractometer must be confined to
the carbon sample in the angular range of interest (ie.
20 from 10 to 35 when a copper target x-ray tube is used).
The following examples are provided to illustrate
certain aspects of the invention but should not be
construed as limiting in any way. In general, carbonaceous
materials were prepared from hydrocarbon or polymer
25 precursors by pyrolysis under inert gas. Weighed amounts
of the precursors were placed in alumina boats and inserted
within a stainless steel or quartz furnace tube. The tube
was flushed with inert gas for about 30 minutes and then it
was inserted into a tube furnace. The furnace and hence
30 the sample temperature was raised to the final pyrolysis
temperature and held there for one hour. The heating rate
was sometimes deemed to be important, and in those cases
the rate was carefully controlled using a programmable
temperature controller.
Powder x-ray diffraction was used to characterize
samples using a Seimens D5000 diffractometer equipped with
a copper target x-ray tube and a diffracted beam
213~36~
- 18 -
monochromator. The diffractometer operates in the
Bragg-Brentano pseudofocussing geometry. The samples were
made by filling a 2mm deep well in a stainless steel block
with powder and levelling the surface. The incident slits
used were selected so that none of the x-ray beam missed
the sample in the angular range from 10 to 35 in
scattering angle. The slit width was fixed during the
measurement. This ensured reproducibility in the measured
values of R.
Carbon, hydrogen, and nitrogen content was determined
on samples using a standard CHN analysis (gas
chromatographic analysis after combustion of the samples in
air). The results are reported in weight percent of the
sample made up by each element and have a standard
deviation of +0.3~. In every case, the carbon content was
greater than 90~ of the sample weight and the hydrogen
content was less than 3.3~. The H/C atomic ratio was
estimated by taking the ratio of the hydrogen and carbon
weight percentages and multiplying by 12 (the approximate
mass ratio of carbon to hydrogen). The nitrogen content of
all the samples was low and has not been reported. The
oxygen content of the samples was not analyzed.
Conventional BET methods were used to determine the
surface area of some samples based on the adsorption of
nitrogen. The surface area of samples of the invention
could not be determined reliably in this way however.
During analysis, adsorption continued slowly over long
periods of time (hours). It seemed therefore that the
samples had substantial surface area that was difficult,
but possible, to access with nitrogen.
The sample surface area accessible to common non-
aqueous electrolytes was not directly measurable. Instead,
the adsorption capacity for methylene blue (commonly used
for activated carbons) was determined to provide a related
measurement. In the literature (see for example, Active
Carbon by H. Jankowska, A. Swiatkowski, J. Choma,
translated by T.J. Kemp, published by Ellis Horwood, New
213836~
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York, 1991), methylene blue (MB) is considered to have an
equivalent minimum linear dimension of 1.5 nm. That is, MB
is expected to penetrate into pores having diameters
greater than 1.5 nm. Although certain specific non-aqueous
electrolyte solutions can have equivalent linear dimensions
smaller than this, generally those of interest for
commercial applications might be of that order in size or
greater. Thus, it was estimated that if certain areas of
a sample were not accessible to MB, then these same areas
would also not be accessible to electrolyte.
The adsorption capacity for MB was determined using a
modification of conventional methods (as in the
aforementioned reference Active Carbon). Samples were
dried prior to testing at 130C. About 0.1 grams of sample
was placed in a flask along with 1-2 ml of 0.2~ surfactant
solution (prepared using Micro-Liquid Laboratory Cleaner
(trademark), a standard laboratory detergent) plus about 5
ml of deionized water. A titration was then performed
using a 1.5 g/L titrating solution of hydrated MB in
discrete steps. An initial amount of solution was added
followed by 5 minutes of vigorous shaking. (The initial
amount was either a minimum 0.1 ml or 1.0 ml depending on
the estimated adsorption capacity of the sample.) The
resulting mixture was then visually compared to a 0.4 mg/L
reference solution of MB. If the mixture was clearer than
the reference, another 1.0 ml of titrating solution was
added and the steps repeated. If the mixture was not
clearer than the reference, adsorption was allowed to
continue for a maximum of 3 days. If the mixture again
became clearer than the reference, the discrete titrating
continued. Otherwise, the measurement was finished and the
adsorption capacity was taken to be that amount of MB
titrated just before the last stepwise addition. For the
samples tested, generally the titrated MB was adsorbed in
the 5 minute interval periods with the exception of the
last few stepwise additions. Laboratory coin cell
batteries were used to determine electrochemical
2138360
- 20 -
characteristics of the samples including specific capacity
for lithium. These were assembled using conventional 2325
hardware and with assembly taking place in an argon filled
glove box as described in J.R. Dahn et al, Electrochimica
5 Acta, 38, 1179 (1993). Figure 3 shows an exploded view of
the coin cell type battery. For purposes of analysis, the
samples were used as cathodes in these batteries opposite
a lithium metal anode. A stainless steel cap 21 and
special oxidation resistant case 30 comprise the container
and also serve as negative and positive terminals
respectively. A gasket 22 is used as a seal and also
serves to separate the two terminals. Mechanical pressure
is applied to the stack comprising lithium anode 25,
separator 26, and sample cathode 27 by means of mild steel
15 disc spring 23 and stainless disc 24. The disc spring was
selected such that a pressure of about 15 bar was applied
following closure of the battery. 125 ~m thick metal foil
was used as the lithium anode 25. Celgard 2502 microporous
polypropylene film was used as the separator 26. The
20 electrolyte 28 was a solution of lM LiPF6 salt dissolved in
a solvent mixture of ethylene carbonate and diethyl
carbonate in a volume ratio of 30/70.
Sample cathodes 27 were made using a mixture of
powdered sample compound plus Super S (trademark of
25 Ensagri) carbon black conductive dilutant and
polyvinylidene fluoride (PVDF) binder (both in amounts of
about 5~ by weight to that of the sample) uniformly coated
on thin copper foil. The powdered sample and the carbon
black were initially added to a solution of 20~ PVDF in N-
30 methylpyrollidinone (NMP) to form a slurry such that 5~ ofthe final electrode mass would be PVDF. Excess NMP was
then added until the slurry reached a smooth syrupy
viscosity. The slurry was then spread on small preweighed
pieces of Cu foil (about 1. 5 cm2 in area) using a spreader,
35 and the NMP was evaporated off at about 90C in air. Once
the sample cathode stock was dried, it was compressed
between flat plates at about 25 bar pressure. These
~13836~
- 21 -
electrodes were then weighed and the weight of the foil,
the PVDF, and the carbon black were subtracted to obtain
the active electrode mass. Typical electrodes were 100
micrometers thick and had an active mass of 15 mg.
After construction, the coin cell batteries were
removed from the glove box, thermostatted at 30 + 1C, and
then charged and discharged using constant current cyclers
with + 1~ current stability. Data was logged whenever the
cell voltage changed by more than 0.005 V. Currents were
adjusted to be either 7.4 mA/g, 18.5 mA/g, or 37mA/g of
active material, depending on the desired test. Much of
the discharge capacity of the example carbons is very close
to the potential of lithium metal. Thus, special testing
methods were required to determine the full reversible
capacity. Coin cell batteries were therefore discharged at
constant current for a fixed time, the time being chosen
such that the battery voltage would fall below zero volts
(versus Li) and such that lithium plating on the carbon
electrode would occur. It should be noted that the plating
of lithium does not occur immediately after the battery
voltage goes below zero volts due to the overvoltage caused
by the finite constant current used. However, plating does
begin shortly thereafter (usually around -0.02V) and is
characterized by a region where the voltage of the battery
rises slightly once plating is initiated followed by a
constant or nearly constant voltage region. The onset of
lithium plating is clearly and easily determined as shown
in the following examples. The plating of lithium on the
carbon electrode was continued for a few hours and then the
current was reversed. First, the plated lithium is
stripped, and then inserted lithium is removed from the
carbon. The two processes are easily distinguished
provided that the charge rates are small (ie. less than 37
mA/g of active material). The reversible capacity was
calculated as being the average of the second discharge and
second charge capacities of the battery, excluding lithium
plating and stripping. The first discharge capacity was
213~36~
- 22 -
not used for this calculation because irreversible
processes occur on the first discharge.
Comparative Example 1.
Several samples were made by preparing a thermoset
polymer from furfuryl alcohol in the presence of either
phosphoric, oxalic, or boric acid followed by pyrolysis at
various temperatures up to 1100C according to the methods
of the aforementioned A. Omaru reference. R values for all
these samples were determined as mentioned above and the
results are listed in Table 1.
Table 1 Data for the samples of Comparative Example 1.
P~c~;ul~olPolylllcli~hlg Acid Pyrolysis R
Lt;~ dLul~i
(C)
Polyrulrulyl Alcohol Pho~ph~rir 600 2.30
Polyrulrulyl Alcohol Phosphoric 1100 2.45
Polyrulrulyl Alcohol Oxalic 900 2.56
2 o POlyrulrulyl Alcohol Phosphoric 1000 2.74
Polyfurfuryl Alcohol Boric 900 4.9
The high capacity, hard carbon samples of the prior
art appear to have R values that exceed 2.2.
Comparative Example 2
KSRAW grade (trademark) petroleum pitch was obtained
from Kureha Company of Japan in order to replicate the
prior art material of Mabuchi et al. A series of soft
carbon samples was made by pyrolysing said pitch at
213836~
- 23 -
temperatures between 550C and 950C. The H/C atomic
ratios for this series was determined as mentioned above
and are shown in Figure 4 (also shown are H/C ratios for
samples of Inventive Example 1 to follow). The x-ray
diffraction pattern in the vicinity of the {002} peak is
shown in Figure 5 for some of these samples along with the
pattern of the precursor itself. (Note that the patterns
have been offset vertically by 2000 counts for clarity.)
R values and H/C data for this series are presented in
Table 2. None of the samples have both Rc2.2 and H/C~0.1.
Table 2 Data for the samples of Comparative Example 2.
Pyrolysis T~ ldtul~ H/C R
(C)
550 0.382.67
600 0.2352.14
700 0.1832.33
900 0.0803.33
Laboratory coin cell batteries were prepared using
some of these samples as described previously. Figure 6b
shows the voltage versus capacity plot for the second cycle
of these batteries. (The plots have been shifted upwards
sequentially by 0.05 V for clarity in Figure 6b.) Figure
6a shows an expanded version of Figure 6b near 0 volts to
better indicate the onset of lithium plating and completion
of lithium stripping (indicated by arrows for the 550C
data) during cycling. (The data have been shifted upwards
sequentially by 0.1 V for clarity in Figure 6a.)
Each of the samples pyrolyzed at 700C or less show a
maximum specific capacity (calculated as described
previously) of about 650 mAh/g. Samples pyrolyzed above
700C had significantly less capacity (down to about 400
213836~
- 24 -
mAh/g for the sample pyrolyzed at 900C). Substantial
hysteresis in the voltage plots can be seen, especially for
samples pyrolyzed at the lower temperatures.
The very high capacity carbon samples of the prior art
appear to lose their very high capacity characteristics
when pyrolyzed at temperatures above about 700C. There
seems to be a correlation between larger specific capacity
and larger H/C ratio for these samples.
Comparative Example 3
M20E and M30 (trademarks) grade activated carbons were
obtained from Spectracorp, MA, U.S.A.. Some of each
activated carbon sample was analyzed as is and some was
pyrolyzed at 1000C prior to analysis. Additionally,
polyvinylidene fluoride (PVDF, obtained from Aldrich
Chemical company, U.S.A.) was pyrolyzed at 1000C. R, H/C,
CHN, and specific capacity values were obtained as
described in the preceding discussion for each of these
samples. For each activated carbon sample, R was about 1.1
and the H/C atomic ratio was very small (~0.03). Figure 7
shows the x-ray diffraction pattern in the vicinity of the
{002} peak for the M20E sample as received and after
pyrolysis to 1000C. For the pyrolyzed PVDF sample, R was
about 1.3 and the H/C atomic ratio was 0.053.
The BET surface areas for all these samples are
relatively high (~100 m2/g). Also, the adsorption capacity
for MB is also relatively high. For M20E and M30 activated
carbons as supplied, the MB adsorption capacity exceeded
400 micromoles/g. (It was deemed to be unnecessary to
continue the titration.) The pyrolyzed PVDF carbon sample
adsorbed about 200 micromoles of MB per gram.
All samples exhibited high specific capacities but
also substantial hysteresis in the voltage plot and
substantial irreversible capacity on the first discharge.
For instance, Figure 8 shows the second cycle voltage
versus capacity plot for the battery containing M30
213836~
- 25 -
activated carbon pyrolyzed at 1000C. The specific
capacity is about 550 mAh/g and there is substantial
hysteresis. Figure 9 shows the first cycle voltage versus
capacity plot for the same battery containing M30 activated
carbon pyrolyzed at 1000C. The first discharge capacity
is enormous at about 2000 mAh/g and thus there is
substantial irreversible capacity.
This example shows that some hard carbons, derived
from precursors other than epoxies, when pyrolyzed at
temperatures above 700C can have R<2.2 and H/C<0.1 and yet
not provide the low hysteresis and irreversible capacity
advantages of the invention. Such hard carbons have high
BET surface areas and also have relatively high adsorption
capacities for MB ( ~>4 micromoles/g carbon).
Inventive Example 1
A series of samples was prepared using Dow 438
(trademark of Dow Chemical Co., U.S.A.) epoxy novolac resin
as a precursor. The resin was usually mixed with
different amounts of phthallic anhydride hardener which was
cured at about 120C to a hard plastic state prior to
pyrolysis. Pyrolysis was performed at temperatures varying
from 700C to 1100C. Afterwards, R, H/C, CHN, and
specific capacity values were obtained for most samples in
the series as described in the preceding discussion. BET
and MB adsorption capacities were also obtained for some
representative samples in the series. A summary of samples
prepared with these corresponding values is shown in Table
3.
2138360
- 26 -
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213836~
- 27 -
The voltage versus capacity plots for sample no. I
pyrolyzed at 700C is compared to that of the pitch sample
of Comparative Example 2 pyrolyzed at the same temperature
in Figure 10. These two plots show almost identical
behaviour (although the battery using sample no. I was
allowed to plate more lithium). Figure 4 indicates that
the two samples in Figure 10 have almost the same H/C
ratio. Figure 11 shows the x-ray diffraction patterns of
samples no. I, II, and III (offset by 1600 counts).
Therein, it can be seen that sample no. I has a
substantially smaller R than the corresponding pitch sample
in Figure 5. There are very few stacked graphene layers in
sample no. I as evidenced by the {002} peak amounting to
only a shoulder on the low angle background. Figures 11
and 5 also show that these structural differences persist
at higher pyrolysis temperatures.
Figures 12a and b show the voltage versus capacity
plots for samples no. I, II, III, and V (plots are offset
by 0.05 and 0.1 volts in Figures a and b respectively).
These samples all have R~2.2. Sample I shows considerable
hysteresis in the voltage plot. At higher pyrolysis
temperatures, the capacity available near 1.0 V during the
charge of sample no. I is shifted down near 0 V, so that
around 900C to 1000C reversible cycling with little
hysteresis is obtained. Furthermore, high specific
capacity is maintained in samples no. III and V at
pyrolysis temperatures of 900C to 1000C, unlike that of
the pyrolyzed pitch of Comparative Example 2.
Figures 13a and b show the voltage versus capacity
plots for samples no. V, VI, VII, and IX (plots are offset
by 0.05 and 0.1 volts in Figures a and b respectively).
These Figures also illustrate the relation between R and
specific capacity for samples pyrolyzed at 1000C to
1100C. As R increases, the specific capacity decreases.
Figure 14 shows the x-ray diffraction patterns in the
vicinity of the {002} peak for the samples of Figures 13a
- 21383~
- 28 -
and b. (The patterns have been offset upwards sequentially
by 3000 counts for clarity.) Figure 15 is provided to show
a summary plot of specific capacity versus R for samples
III to IX inclusive which were all pyrolyzed between 900C
and 1100C. The samples therein all exhibited voltage
curves with little hysteresis and all had H/CcO.1. Again,
as R increases, the specific capacity decreases.
Figure 16 shows the first discharge and charge of the
laboratory coin cell battery employing sample no. VII. The
battery shows a first discharge capacity of about 625 mAh/g
and a first recharge capacity of about 465 mAh/g. The
irreversible capacity of sample VII is therefore only about
160 mAh/g, which is considered to be in an acceptable range
for practical lithium ion batteries. The surface area
measured by the BET method for sample VII was 217 m2/g. If
this area were all accessible to electrolyte, such low
values for the irreversible capacity would not be expected
(for example, based on the disclosure of U.S. Patent No.
5,028,500). However, the MB adsorption capacity is
relatively low (<5 micromoles/g) for this and all the other
inventive samples tested.
Insertion compounds of the invention can therefore
have very high specific capacity coupled with acceptable
associated hysteresis in voltage and acceptable associated
irreversible capacity.
Inventive Example 2
A sample was prepared using Dow D.E.R. 667 (trademark
of Dow Chemical Co., U.S.A.) bisphenol A type epoxy resin
as a precursor. No hardener was used in this preparation.
Pyrolysis was performed by heating first at 250C for 2
hours followed by ramping at 30C/min to 800C and
thereafter holding for 2 hours. R for this sample was
about 1.52. Laboratory coin cell batteries were then
prepared and specific capacity values were obtained.
The voltage versus capacity plot for one of these
2138360
- 29 -
batteries is shown in Figures 17a and b (plots are offset
by 0.05 and 0.1 volts in Figures a and b respectively).
Therein, the specific capacity was 410mAh/g. The
irreversible capacity is only about 160 mAh/g and the
hysteresis in the voltage is considered acceptable.
It thus appears possible to make insertion compounds
of the invention using bisphenol A type epoxy resin.
As will be apparent to those skilled in the art in the
light of the foregoing disclosure, many alterations and
modifications are possible in the practice of this
invention without departing from the spirit or scope
thereof. For example, mixtures of more than one precursor
may be used to prepare compounds. Accordingly, the scope
of the invention is to be construed in accordance with the
substance defined by the following claims.
