Note: Descriptions are shown in the official language in which they were submitted.
~ ~4~42~
~ .TC RESIN PREC~RSOR PRE-GRAPHITIC
RO~ T~OUs lN~ . COMPOUNDS AND
USE AS ANODES IN RT!t'TTl~R~:R~RT.T~ BATTERIES
S FIE~D OF TRl~ T~vl3NTIoN
The invention pertaine to the f ield of carbonaceou3
materialA and, in particular, to phenolic reein precureor
pre-graphitic carbonaceous ineertion materials.
Additionally, the invention pertains to the field of
rechargeable batterieA and, in particular, to rechargeable
batteriee comprieing ~ ArhonAceoue anode materiale .
BACK~RO~ D OF T~ INVENTION
The group of pre-graphitic compounds includeA
carbonaceous materiale that are generally prepared at low
temperaturee (eg: leAe than about 2000C) from various
organic eourceA and that tend to graphitize when annealed
at higher temperatures. There are however both hard and
eoft pre-graphitic carbon compounde, the former being
difficult to graphitize Aubetantially even at temperature_
of order of 3000C, and the latter, on the other hand, being
virtually completely graphitized around 3000C.
The af~,L~ n~d _et of compounds has been of great
intereAt for use ae anode materialA in lithium-ion or
rocking chair type batteriee. These batteries repreAent
the Atate of the art in Amall rechargeable power AourceA
for coneumer electronicA applications. These batterieA
have the greatest energy denAity (wh/r~) of conv~n~ nAl
rechargeable syeteme (ie. NiCd, NiMH, or lead acid
batterieA). Additionally, lithium ion batterieA operate
around 33~ volts which is often sufficiently high such that
a eingle cell can suffice for many electronice
applicatiOn-A.
Lithium ion batteriee uee two different insertion
compound_ f or the active cathode and anode materials .
InAertion compounds are thoAe that act as a ho3t solid for
the reverAible insertion of gueet atoms (in thie caee,
. _ . . .. . . . .. . . . . . . _ _ _ _ _ _ _ _ _ . . . _
~ 21~42~
-- 2 --
lithium atoms). The structure of the insertion compound
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
S 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 ~t,orn;~l 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 I,iCoO2 / pre-
graphitic carbon electrochemistry is commercially available
(produced by Sony Energy Tec. ) wherein the carbonaceous
anode can reversibly insert about 0 . 65 ~i 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 ~T.R. Dahn et
al., ~.ithium ~3atteries, New Materials and New
Perspectives", edited by G. Pigtoia, 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 a8 the specific capacity for that material.
Graphitized carbon8 and/or graphite itself can be employed
under certain conditions (as for example in the
presentation by Matsushita, 6th Inter~ational Lithium
~` 2146~2~
- 3 -
Battery Conference, Muenster, Germany, May 13, 1992, or in
U.S. Patent No. 5,130,211) .
Other alternatives for increasing the specific
capacity of carbonaceous anode materials have included the
5 addition of other Pl PnlPn~ to the carbonaceous compound.
For example, t~nA~11 An Patent Application Serial No .
2,098,248, Jeffrey R. Dahn et al., 'Electron Acceptor
Substituted Carbons for Use as Anodes in Rechargeable
Lithium Batteries', filed June 11, 1993, discloses a means
10 for PnhAnr; ng 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 a3 high as 440
mAh/g were obtained with boron substituted carbons.
'AnA(1;An Patent Application Serial No. 2,122,770, Alfred M.
Wilson et al., '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
r.,ntAin;nrJ gilicon as disclosed in ~nA~l;An Patent
Application Serial No. 2,127,621, Alfred M. Wilson et al.,
' Carbonaceous Insertion Compounds and Use as Anodes in
Rechargeable Batteries', filed July 8, 1994.
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. have demonstrated that pyrolyzed coal tar
pitch can have specific capacities as high as 750 mAh/g at
pyrolysis temperatures about 700C. ~. Sato et al. in
Science 264, 556, (1994) disclosed a similar carbonaceous
material prepared by heating polyparaphenylene at 700C
which has a reversible capacity of 680 mAh/g. S. Yata et
,
:
`- 21~642
- 4 --
al, Synthetic Metals 62, 153 (1994) also discloee a similar
material made in a similar way. These values are much
greater than that of pure graphite. The afur~ t;oned
materials can have a very large irreversible capacity as
5 evidenced by f irst di8charge 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
~l0 lithium ion battery using such a material as an anode, this
would result in a similar signlficant hysteresis in battery
voltage between discharge and charge with a resulting
undesirable energy inefficiency.
It is not understood why the afor~ml~nt;r~n
15 carbonaceous materials have very high specif ic capacity .
All were prepared at temperatures of about 700C and are
crystalline enough to exhibit x-ray patterns from which the
parameters doo2, Lc, a, and L~, can be determined. (The
definition and determination of these parameters can be
20 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
25 pyrolyzing at higher temperature degrades the specific
capacity substantially with a concurrent reduction in the
hydrogen content. (In the aforementioned reference by
~abuchi et al, pyrolyzing the pitch above about 800C
results in a specific capacity decrease to under 450 mAh/g
30 with a large reduction in H/C. Similar results were found
in the afor~m~nt; on~(l reference by Yata et al . )
The prior art also discloses carbonaceous compounds
with specif ic capacitie8 higher than that of pure graphite
made from precursors that form hard carbons on pyrolysis.
35 However, the very high specific capacities of the
aforementioned materials pyrolyzed at about 700C were
apparently not at~ained. A. Omaru et al, Paper #25,
~ 21~2~
s
Extended Abetracts of Battery Division, p34, Meeting of the
~lectrochemical Society, Toronto, Canada (1992), disclose
the preparation of a hard carbonaceous compound Cont;~;n;n~
phosphorus with a specific capacity of about 450 mAh/g by
5 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 discloee a hard carbonaceous compound comprising
2.4~6 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 suostantially less
than o.1). These hard carbonaceous compounds neither
exhibited the very high specif ic capacities nor the same
serious hysteresis in voltage of the aforementioned
materials pyrolyzed at about 700C.
Additionally, other high capacity carbonaceous
materials have recently been prepared which show high
capacity for lithium and little or no voltage hysteresiH.
In Paper 2B05 at the 35th Battery Symposium in Nagoya,
Japan, Nov. 14-16, 1994, Y. T~k~h~Rh; et al. describe
materials with reversible capacities of about 480 mAh/g,
but do not give the details of their preparation. In paper
2BO9 at the same Symposium, N. Sonobe et al. deecribe hard
carbon materials made from petroleum pitch with reversible
capacites near 500 mAh/g. The synthesis procedure therein
was not given.
In ~'An~ n Patent Application Serial No. 2,138,360,
Y. Liu and J. Dahn, of the same title, filed Dec. 16, 1994,
carbonaceous insertion compounds also having high capacity
for lithium and little voltage hysteresis were disclosed.
Therein, the carbonaceous insertion compounds comprised a
pre-graphitic carbonaceous host wherein i) the empirical
parameter R, determined from an x-ray diffraction pattern
and defined as the {002} peak height divided by the
- `
-- 214642~
- 6 -
background level, i8 less than about 2.2; ii) the H/C
atomic ratio is less than about 0.1; and iii) the methylene
blue ab60rption capacity of the pre-graphitic carbonaceous
host is less than about 4 micromoles per gram of host.
These carbonaceous insertion compounds were prepared by
pyrolyzing suitable organic precursors. Specifically shown
in the Examples were insertion compound3 prepared from
different epoxy precursors.
SI~ aRY OF T~IE LNV~ N
The instant invention pertains to phenolic resin
precursors which can be employed to produce carbonaceous
insertion compounds with a high capacity for lithium and
little voltage hysteresis. Thus, carbonaceous insertion
compounds derived from phenolic resins, methods of
preparing said compounds, and the use of said compounds as
electrode materials in electrochemical devices comprise the
subject matter of the instant invention.
Car3~onaceous insertion compounds of the invention
comprise a pre-graphitic r~rh~n~reous host prepared by
pyrolyzing a phenolic resin precursor and atoms of an
alkali metal inserted therein. The alkali metal inserted
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 aoout O.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
metal, R can pref erably be less than about 1. 6 .
Hydrogen can be pr~ ;ni~ntly removed from the
phenolic resin precursor by pyrolysis at a temperature
above about 800C. However, the pyrolysis temperature
cannot be too high in order that the empirical parameter R,
.
~ 642~
- 7 -
determined from an x-ray diffraction pattern of the host
and defined as the {002} peak height divided by the
background level, i8 leBs than about 2 . 2 .
The phenolic resin precursor can be of the novolac or
5 the resole type. The latter can be preferably pyrolyzed at
a temperature in the range from about 900C to about
1100C. Both types can be cured at about 150C before
pyrolysis. The pyrolysis temperature for both types can be
maintained for about one hour.
Methods of the invention include processes f or
preparing suitable pre-graphitic carbonaceous hosts for the
afc~ nt;oned compounds. Such hosts can be prepared by
pyrolyzing a phenolic resin precursor at a temperature
above about 800C such that the empirical parameter R,
15 determined from an x-ray diffraction pattern and defined as
the {002} peak height divided by the background level, is
less than about 2.2. The phenolic resin employed can be of
either the novolac or the resole type. Alkali metal atoms
can be inserted into the ho~t thereafter by convf~nt; nni~l
20 chemical or electrochemical means to make insertion
compounds of the invention.
Compounds of the invention can be used as a portion of
an electrode in various electrochemical devices based on
insertion materials (eg. supercapacitors, electrochromic
25 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
30 a mixture of non-aqueous solvents; and an anode comprising
the carbonaceous insertion compound of the iLvention.
BRIEF DES~ ~r~ , OF ~I'TTR nR~-wrl-".c
Figure 1 shows a cross-sectional view of a
conv~nt;nn~l lithium ion spiral-wound type battery.
.
` ~14~42~
- 8 -
Figure 2 depicts an exploded view of the laboratory
coin cell battery used in the Examples.
Figures 3a and 3b show the voltage versus capacity
5 plots for the first and second cycles respectively for
batteries comprising samples prepared from the A type
precursor in Inventive Example 1. The curves have been
offset sequentially for clarity. (In both Figures, the
shifts are 0.0, 0.15, 0.3, 0.45, and 0.7 volts for sample
A700, A800, A900, A1000, and A1100 respectively.)
Figures 4a and ~b show the voltage versus capacity
plots for the first and second cycles respectively for
batteries comprising samples prepared from the B type
15 precursor in Inventive Example 1. The curves have been
offset sequentially for clarity. (In Figure 4a, the
shifts are 0.0, 0.1, 0.25, 0.3, and 0.4 volts for sample
B700, B800, B900, B1000, and B1100 respectively. In
Figure 4b, the shifts are 0.0, O.I, 0.3, 0.5, and 0.8
volts for sample B700, B800, B900, B1000, and B1100
respectively )
Figures 5a and 5b show the voltage versus capacity
plots for the irst and second cycles respectively for
25 batteries comprising samples prepared from the C type
precursor in Inventive Example 1. The curves have been
offset se~]~nt;Ally for clarity. (In both Figures, the
shifts are 0.0, 0.15, 0.3, and 0.45 volts for sample
C800, C900, C1000, and C1100 respectively.)
Figure 6 shows the capacity ver~us cycle number for
the battery comprising sample B1000 of Inventive ~xample 1.
Figure 7 shows the voltage versus capacity plots for
35 the second cycle of batteries comprising samples prepared
from the B type precursor in Inventive Example 2. The
plots have been se~l~nt.Ally offset ~y O.lV for clarity.
` ~14~425
DET~TT.T.'TI DES' ~TV-l~JN OF TT~R SPT~'rTT~IC
~ OF TTTT~ lNV~N~ ~VN
Compounds of the invention comprise hard pre-graphitic
5 carbonaceous hoste 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 phenolic resin precursors. The pyrolysis of epoxy
10 novolac resins (eg. DEN 438, trademark of DOW) gives
product yields near 30~. It is well known however that
phenolic resins (or phenol-formaldehyde resins) can also be
pyrolysed to give hard carbons with high yield (as for
example mentioned in E. Fitzer et al., Carbon 7, 643
(1969). Since the former can cost about $5 per pound
versus about $1. 00 per pound for the latter at the time of
this writing, a cost advantage might be expected for
phenolic resin precursors.
Suitable phenolic resin precursors are those that,
20 when pyrolyzed at temperatures above about 800C, do not
graphitize to such an extent that the empirical parameter
R as determined by the x-ray diffraction pattern exceeds
about 2.2. R is defined as the {002} graphite peak height
divided by the background level. (The detailed method for
25 this determination is described again 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 dif fraction patterns .
Pyrolyzing suitable phenolic resin precursors above
30 800C provides pre-graphitic carbonaceous hosts that do not
exhibit severe hysteresis in voltage upon in~ertion 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
35 with high specific capacities for lithium. Based on the
Examples to follow, R appears to be preferably less than
about 1. 6 .
` ~14642S
- 10 -
The pyrolysis should be performed under a controlled
atmo3phere to prevent formation of undesired oxides of
carbon. A suitable reaction system could consist of a
reaction tube (quartz for example) installed in a
5 conv~nt~n:~l tube furnace wherein the tube has sealed inlet
and outlet connections for purposes of controlling the
atmosphere therein. The phenolic resin precursor/s could
thus be pyrolyzed in the reaction tube under an inert gas
flow or even under reduced or elevated pre~sure.
10 Additionally, controlled partial reduction or oxidation, if
desired, can be achieved by admitting controlled amounts of
an d~ L iate gas .
To ensure good product yields, ideally the phenolic
resin precursor/s should substantially pyrolyze rather than
15 simply evaporate. This issue must be considered in the
selection of preferred precursor/s. It can therefore be
advantageous to cure, or cross-link, the precursor before
pyrolysis. Such curing may be a significant variable
affecting the desired ultimate properties of the pyrolyzed
20 precursor/s. It may therefore 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 precursor/s prior to a
f inal heating to the pyrolysis temperature . Alternately,
25 the heating rate might be varied to control the extent of
the curing prior to pyroly~3is.
The product of pyrolysis can have relatively large
surface areas, of order of 200 m~/g, as determined by
conventional nitrogen adsorption methods (eg. BET).
30 However, the surface area that is actually accessible to
common non-aqueous electrolyte solutions is relatively
small. This is especially important for application in
lithium ion batteries. Electrolyte reactions that con~ume
lithium occur at the anode surface in such batteries.
35 Thus, use of an anode having a large surf ace area
accessible to electrolyte results in substantial
irreversible capacity 1O8B and electrolyte loss. These
~ 21~642G
11
loeeee are avoided if the anode 3urface ie not acceseible
to the electrolyte.
The aforementioned product hae no alkali metal
inserted ae prepared. Alkali metal atome, in particular
Li, can be ineerted thereafter via conv~n~;-)ri~l chemical or
electrochemical means (such ae in a lithium or lithium ion
battery) .
Generally, powdered forme of euch compounde are used
in electrode applicatione and thue a grinding of the
pyrolyzed product i8 ueually required. A variety of
embodiments, in particular varioue battery conf iguration3,
are poesible using electrode material prepared by the
method of the invention. Miniature laboratory batteriee
employing a lithium metal anode are deecribed in the
examplee to follow. However, a preferred conetruction for
a lithium ion type product ie that depicted for a
conventional spiral-wound type battery in the croee-
eectional view of Figure 1. A j elly roll 4 ie created by
epirally winding a cathode foil (not 3hown), an anode foil
(not ehown), and two microporoue polyolefin sheete (not
shown) that act as eèparatore.
Cathode foils are prepared by applying a mixture of a
suitable powdered (about 10 micron eize typically) cathode
material, euch ae a lithiated traneition metal oxide,
poeeibly other powdered cathode material if deeired, a
binder, and a conductive dilutant onto a thin aluminum
foil. Typically, the application method firet involvee
di3solving the binder in a suitable liquid carrier. Then,
a elurry i8 prepared using thie eolution plue the other
powdered eolid components. The ~lurry ie then coated
uniformly onto the eub3trate foil. Afterwarde, the carrier
solvent ie evaporated away. Often, both eide3 of the
aluminum foil substrate are coated in thie manner and
eubeequently the cathode foil ie calendered.
Anode foils are prepared in a like manner except that
a powdered (aleo typically about 10 micron size)
carbonaceoue ineertion compound of the invention i8 ueed
` 214~2S
- 12 -
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.
S 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 1.
The j elly roll 4 is inserted into a conventional
battery can 3. A header 1 and gasket 10 are used to seal
10 the battery 16. The header may include safety devices if
desired. A combination safety vent and pressure operated
disconnect device may be employed. Figure 1 shows one such
combination that is described in detail in t~;3nA~ n Patent
Application No. 2,099,657, Alexander H. Rivers-Bowerman,
15 'Electrochemical Cell and Method of Manufacturing Same',
filed June 25, 1993. Additionally, a positive thermal
coefficient device (PTC) may be incorporated into the
header to limit the short circuit current capability of the
battery. The extf~rnill surface of the header 1 is used as
20 the positive terminal, while the external surface of the
can 3 serves as the negative ter~inal.
Appropriate cathode tab 5 and anode tab 6 connections
are made to connect the ;ntGrni~l electrodes to the ex~rn~l
terminals. Appropriate insulating pieces 2 and 7 may be
25 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
30 types of and amounts of the component materials must be
chosen based on component material properties and the
desired performance and safety requirements. The compounds
prepared in the Examples to follow can have somewhat
increased irreversible capacity for lithium along with an
35 increased reversible capacity over that of many typical
commercial carbonaceous anode materials. Also, Example
compounds typically have somewhat lower density than that
` 21~6425
- 13 -
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, i3 part of the assembly process.
5 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
10 the batteries of the invention (eg. prismatic format). 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.
Without wishing to be bound by theory, adversely or
otherwise, the inventor3 offer the following view of the
prior art and of the instant invention to explain how
certain prior art carbonaceous materials can have specific
capacities that significantly exceed that of graphite.
The presence of substantial hydrogen in carbonaceous
materials of the prior art prepared by pyrolysis at low
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 ~urface~ oE each side of the
layers. These surfaces are found within the carbon
particlec, on the atomic ~cale. In graphite, the layers
are well stacked in a parallel fashion and intercalation of
lithium to a composition of ~iC6 is pos~ible (corresponding
to about 370 mAh/g and one intercalated layer of lithium
per graphene sheet). In materials with poorly stacked
.
.-- 214642~
- 14-
layers, unshared lithium layer3 might possibly be found on
each side of the graphene ~heets, resulting in compositions
up to almost Li~C6 (corresponding to about 740 mAh/g).
Thus, the number of single layer graphene ~heets in the
5 carbonaceous material may be important vi~ a vis specific
capacity .
X-ray diffraction can be used to learn about the
average number, N, of 3tacked graphene sheets in a carbon
in between serious stacking mistakes. Thi3 number N,
lO 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
15 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
20 a house of cards. ) A8 N increases (beginning to stack the
deck of cards), the {002} peak increases in height and
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
25 describing such structures and is determined by dividing
the {002} peak height by an estimate of the background
level at the Bragg angle corresponding to the position of
the {002} peak. The background e~timate is that value
given by the intersection of a line tangential to the
30 background in the immediate vicinity of the {002} peak and
a line positioned at 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
35 would have significantly 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
~` ~146~2~
- 15 -
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. 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. Carbonaceous materials
were prepared from cured 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 for pyrolysis.
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
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 80 that none o~ the x-ray beam missed
the sample in the angular range from 10 to 35 in
scattering angle. The slit width was ~ixed during the
mea~UL. t. This ensured reproducibility in the measured
values of R.
Carbon, 1IYdL~JY~ and nitrogen content was determined
on samples using a standard CHN analysis (gas
chromatographic analysis after combustion of the samples in
air) . The weight percents 80 determined had a standard
deviation of ::0.39~. In every case, the carbon content was
greater than 90~ of the sample weight and the hydrogen
content was less than 29~. The H/C atomic ratio was
estimated by taking the ratio of the hydrogen and carbon
weight percentages and multiplying by 12 (the mass ratio o~
carbon to hydrogen). The oxygen content oE the samples was
not analyzed.
The sample surface area accessible to common non-
` 214~2~
- 16 -
aqueous electrolytes wa3 not directly measurable. In3tead,
the adsorption capacity for methylene blue (MB) was u3ed to
provide a related mea~uL~ -n~. [In the literature (see for
example, Active Carbon by H. Jankowska, A. Swiatkowski, J.
5 Choma, tran31ated by T.J. Kemp, publi3hed by Elli3 Horwood,
New York, 1991), methylene blue (MB) i3 con3idered to have
an equivalent minimum linear dimen3ion of 1.5 nm. That i3,
MB is expected to penetrate into pores having diameters
greater than 1.5 nm. Although certain specific non-aqueou3
lO electrolyte 301utions can have equivalent linear dimension3
3maller than this, generally tho3e of interest for
commercial application3 might be of that order in 3ize or
greater. Thus, it wa3 e3timated that if certain area3 of
a 3ample were not acce33ible to MB, then the3e 3ame area3
15 would al30 not be acce33ible to electrolyte. ]
The method for det~orm;nln~ ad30rption capacity for MB
i3 a modif ication of conventional method3 . A 3ample wa3
dried prior to te3ting at 130C. About 0.1 grams of sample
wa3 placed in a fla3k along with 1-2 ml of 0.296 3urfactant
20 301ution (prepared u3ing Micro-Liquid Laboratory Cleaner
(trademark), a 3tandard laboratory detergent) plu3 about 5
ml of deionized water. A titration wa3 then performed
u3ing a 1. 5 g/~ titrating solution of hydrated MB in
discrete step3. An initial 0.1 ml amount of 301ution wa3
25 added followed by 5 minute3 of vigorou3 3haking. (The
initial amount wa3 either a minimum O .1 ml or 1. 0 ml
depending on the e3timated ad30rption capacity of the
3ample. ) The re3ulting mixture wa3 then visually compared
to a 0.4 mg/~ reference solution of MB. If the mixture wa3
30 clearer than the reference, another 1. 0 ml of titrating
301ution would be added and the 3tep3 repeated. If the
mixture wa3 not clearer than the reference, ad30rption wa3
allowed to ~ nt;nll~ for a maximum of 3 day3. If the
mixture again became clearer than the reference, the
35 di3crete titrating would be c~n~;n~ l. Otherwi3e, the
mea3urement wa3 f ini3hed and the ad30rption capacity wa3
taken to be that amount of MB titrated ju3t before the la3t
` ~14~426
- 17-
stepwise addition.
Laboratory coin cell batteries were used to determine
electrochemical 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 Acta, 38, 1179 (1993) . Figure 2 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 cr~nt~;npr and also serve as negative and
positive t~rm;n;~l~ 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 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 mic~ us polypropylene film was used as the
separator 26. The 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
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 f oil . The powdered sample and the carbon
black were initially added to a solution of 2096 PVDF in N-
methylpyrol 1; 1; n~n~ (NMP) to form a slurry such that 5~ of
the f inal 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 cm~ in area) using a spreader,
and the NMP was evaporated of f at about 9 0C in air . Once
14~426
- 18-
the sample cathode stock was dried, it was compressed
between flat plates at about 25 bar pressure. These
electrodes were then weighed and the weight of the foil,
the PVDF, and the carbon black were subtracted to obtain
5 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 i 1C, and
then charged and discharged using constant current cyclers
10 with i 19~i current stability. Data was logged whenever the
battery voltage changed by more than 0 . 005 V. Currents
were adjusted to be 18.5 m~/g of active material for the
initial two cycles of the battery and 37mA/g of active
~aterial thereafter for ~tPn~lP~l cycle testing. Much of
lS 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
20 such that the battery voltage would fall below zero volts
(versus I.i) and such that lithium plating on the carbon
electrode would occur. Note that the plating of lithium
does not occur immediately after the battery voltage goes
below zero volts due to the overvoltage caused by the
25 f inite constant current used. ~owever, plating does begin
shortly thereaf ter (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
30 lithium plating is clearly and easily ~tF~rm; nf~d 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
35 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
` 2146~2~
- 19 -
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
not used for this calculation because irreversible
processes occur on the f irst discharge .
Inveative Example 1
A series of samples was prepared using three different
phenolic resins as a precursor. Two are base-catalysed or
resole types and one is an acid catalyzed or novolac type.
The three dif f erent precursors used were:
A) resole type, product # 11760 of Plenco, Plastics
Engineering Company, Sheboygan, WI, 53082-0758 U.S.A.;
B) resole type, product # 29217 of Oxychem, of ~ nt~l
Chemical Corp, Durez Engineering Materials, 5005 LBJ
freeway, Dallas, Texas 75244, U.S.A.; and
C) novolac type, product # 12116 of Plenco, supra.
The phenolic resin precursors were all supplied in
powder form. In each case, the powder was cured at from
about 150C to 160C for 30 minutes prior to pyrolysis. At
the end of the curing step, a solid lump was obtained. The
lump was next reduced to powder in an autogrinder. The
powdered cured resin was then pyrolyzed in a tube furnace
under f lowing argon . The samples were ramped f rom room
temperature to the desired pyrolysis temperature over 3
hours and held there for 1 hour. The furnace power was
then turned of f and the samples were cooled to near room
temperature within the furnace tube under flowing argon.
Cooling took several hours.
Pyrolysis was performed at temperatures varying from
700C to 1100C. Afterwards, the samples were ground into
a powder. R, H/C (by CH~analysis), and specific capacity
value~ (by coin cell battery tests) were obtained for most
samples in the series as described in the preceding. The
MB adsorption capacity was also obtained ~or sample B1000
~` 2146~2~
-20 -
and was found to be about 1. 6 micromoles per gram of host .
Yield was determined from the weights of the samples before
and after pyrolysis. The results of these mea~uL~ t~ is
given in Table 1. (Two batteries of each sample were made
5 and the results from each experiment were within 20 mAh/g.
The values given in Table 1 represent the average values
~.hti~; n~rl . )
2146~2~
- 21 -
T~ble 1. Data ïor the samples oS Inventive Example 1
Sample Pyrolysis Weight Weight Weight H/C Yield R Reversible h~versible
IDTemp. % C % H % N (%) Capacity Capacity
(C) (mAhlg) (mAh/g)
(i20) (i20)
A700 700 91.2 1.5 1.2 0.19 57 1.37 500 440
A800 800 93.1 1.0 1.3 0.13 55 1.56 510 280
A900 900 92.3 0.6 1.2 0.07 55 1.63 510 210
A10001000 94.2 0.4 1.9 0.05 54 1.68 450 160
AllO01100 96.7 0.3 0.8 0.04 52 1.79 330 70
B700 700 94.7 1.8 0.4 o.n 58 1.33 630 260
B800 800 95.8 0.9 0.7 0.11 57 1.39 540 210
B900 900 94.8 0.5 0.5 0.06 57 1.32 410 300
B10001000 95.6 0.3 0.6 0.04 56 1.34 560 200
BllO01100 97.4 0.4 1.4 0.05 56 1.64 340 110
C800 800 95.7 0.9 0.6 0.11 64 1.53 530 210
C900 900 95.1 0.4 0.7 0.05 57 1.63 450 180
C10001000 96.5 0.3 0.8 0.04 58 1.54 450 130
CllO01100 97.0 0.3 1.3 0.03 56 1.64 330 120
2~642~
- 22 -
Figure 3a shows the first discharge-charge cycle for
the series of pyrolyzed A type precursors. The samples
heated at 700C and 800C show significant hysteresis in
the voltage prof ile (Li is inserted near OV but is removed
5 near 1. OV) . Thi3 has been ascribed to the large hydrogen
content in the samples. Upon heating to 900C, the
hysteresis is prP~~;n~ntly eliminated and the samples show
substantial capacity at low voltage. Figure 3b shows the
second cycle of the same series. The vertical lines
10 indicate the onset of lithium plating during discharge and
the t~rTn;n~t;on of lithium stripping during charge. The
batteries prepared from material heated to 900C and
1000C appear most promising for this series. Their
reversible capacities are about 510 and 450 mAh/g
15 respectively.
Figures 4a and 4b show the first and second cycle
voltage prof iles for the series of pyrolyzed B type
precursors. The sample made at 1000C gives a reversible
capacity of about 560 mAh/g and an irreversible capcity of
20 only about 200 mAh/g. This is a very attractive material
for use as a lithium ion battery anode . Figures 5a and 5b
show the f irst and second cycle voltage prof iles for the
series of pyrolyzed C type precursors. The samples made
at 900C and 1000C give reversible capacities near 450
25 mAh/g. The latter has an irrever3ible capacity of only 130
mAh/g .
Exterlded cycling was carried out on a battery
comprising sample B1000. Figure 6 shows the capacity
versus cycle number for this battery. There is little
30 capacity loss upon cycling.
Insertion compounds of the invention can theref ore
have high reversible specif ic capacity coupled with
acceptable associated hysteresis in voltage and acceptable
associated irreversible capacity.
~14~ 42S
- 23 -
IAveAtive Example 2
The serie3 of 3ample3 made from the B type precur30r
were 3hown to have the highe3t rever3ible capacitie3 in the
5 preceding Example. In order to determine how the
rever3ible and irrever3ible capacities varied in the
narrower temperature range between 900C and 1100C, an
additional 3erie3 of 3amples using thi3 precur30r wa3
prepared. The 3ample3 were te3ted in coin cell batteries
10 a3 de3cribed earlier and voltage profile3, irrever3ible
capacitie3, and rever3ible capacities were mea3ured. Two
batteries of each were made and the result3 from each
experiment were within 2 0 mAh/g .
Table 2 summarize3 the average ~pecific capacity
15 re3ults for all the 3amples prepared from the B type
precur30r. Figure 7 show3 representative second cycle
voltage prof iles for the batterie3 made with the3e
sample3 .
2 0 Table 2. Data for the ssrnples of InYentive Example I
Sample ID F~eversible Capacity Irreve~sible Capacity
(mAh/g) (~t20) (mAh/g) (~ 20)
s900 4 l0 300
B940 470 160
2 5 B970 550 160
s1000 560 200
slO30 540 140
B1060 450 200
Bl100 340 llO
` ' 214642~
24
Appropriate selection of the pyrolysis temperature
appears to be important in order to optimize the properties
of these insertion compounds.
As will be apparent to those skilled in the art in the
5 light of the foregoing disclosure, rnany alteration~ and
modif ications are possible in the practice of this
invention without departing from the spirit or scope
thereof. For example, mixture8 of more than one precursor
may be used to prepare compounds. Accordingly, the scope
10 of ~he invention is to be construed in accordance with the
substance defined by the following claims.
