Additives for improving cycle life of non-aqueous rechargeable lithium batteries

The loss in delivered capacity (fade rate) after cycling non-aqueous rechargeable lithium batteries can be reduced by incorporating a small amount of an improved additive in the battery. Improved additives include boron trifluoride (BF.sub.3), fluoboric acid (HBF.sub.4), or complexes thereof. The invention is particularly suited to lithium ion batteries. Complexes comprising BF.sub.3 and dietyl carbonate or ethyl methyl carbonate can be prepared which are particularly effective additives. Preferably, the additive is dissolved in the electrolyte.

FIELD OF THE INVENTION 
This invention pertains to non-aqueous rechargeable lithium batteries and 
to methods for improving the performance thereof. Specifically, it 
pertains to the use of boron trifluoride (BF.sub.3) or complexes 
containing BF.sub.3 as an electrolyte additive in order to improve the 
capacity delivered from lithium ion batteries after extended cycling. 
BACKGROUND OF THE INVENTION 
Many varied types of non-aqueous rechargeable lithium batteries are used 
commercially for consumer electronics applications. Typically, these 
batteries employ a lithium insertion compound as the active cathode 
material, a lithium containing material of some sort (eg. pure lithium 
metal, lithium alloy, lithium insertion compound) as the active anode 
material, and a non-aqueous electrolyte. An insertion compound is a 
material that can act as a host solid for the reversible insertion of 
guest atoms (in this case, lithium atoms). 
Lithium ion batteries use two different insertion compounds for the active 
cathode and anode materials. Presently available lithium ion batteries are 
high voltage systems based on LiCoO.sub.2 cathode and coke or graphite 
anode electrochemistries. However, many other lithium transition metal 
oxide compounds are suitable for use as the cathode material, including 
LiNiO.sub.2 and LiMn.sub.2 O.sub.4. Also, a wide range of carbonaceous 
compounds is suitable for use as the anode material. These batteries 
employ non-aqueous electrolytes comprising LiBF.sub.4 or LiPF.sub.6 salts 
and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl 
carbonate, and the like. Again, numerous options for the choice of salts 
and/or solvents in such batteries are known to exist in the art. 
The excellent reversibility of this insertion makes it possible for lithium 
ion batteries to achieve hundreds of battery cycles. However, a gradual 
loss of lithium and/or buildup of impedance can still occur upon such 
extended cycling for various reasons and particularly at higher operating 
voltages. This in turn typically results in a gradual loss in delivered 
capacity with cycle number. Researchers in the art have devoted 
substantial effort to reducing this loss in capacity. For instance, 
co-pending Canadian patent application serial number 2,150,877, filed Jun. 
2, 1995, and titled `Use of P.sub.2 O.sub.3 in Non-aqueous Rechargeable 
Lithium Batteries` discloses a means for reducing this loss which involves 
exposing the electrolyte to P.sub.2 O.sub.5. However, P.sub.2 O.sub.5 
shows at best only limited solubility in typical non-aqueous electrolytes 
and can be somewhat awkward to use in practice. Alternatives which are 
soluble may be more convenient, but it is unclear why such exposure is 
effective and hence what compounds might serve as effective alternatives. 
B.sub.2 O.sub.3 is a common chemical that is extensively used in the glass 
industry, and its properties are well known. B.sub.2 O.sub.3 has also been 
used in the lithium battery industry for a variety of reasons. In most 
cases, the B.sub.2 O.sub.3 is used as a precursor or reactant to prepare 
some other battery component. However, Japanese published patent 
application 07-142055 discloses that lithium batteries can show improved 
stability characteristics to high temperature storage when using lithium 
transition metal oxide cathodes which contain B.sub.2 O.sub.3. Also, 
co-pending Canadian patent application serial number 2,175,755, filed May 
3, 1996, and titled `Use of B.sub.2 O.sub.3 additive in Non-aqueous 
Rechargeable Lithium Batteries` discloses that B.sub.2 O.sub.3 additives 
can be used to reduce the rate of capacity loss with cycling in 
rechargeable lithium batteries and that this advantage can be obtained by 
having the additive dissolved in the electrolyte. However, the reason that 
the additive resulted in an improvement with cycling was not understood. 
In a like manner, Japanese published patent application 09-139232 also 
discloses that the use of B.sub.2 O.sub.3, or possibly certain other B 
containing compounds, can improve the cycling and storage characteristics 
of lithium rechargeable batteries. 
Certain other compounds containing boron, oxygen, carbon, and hydrogen have 
been used historically in battery and/or fuel cell applications. For 
instance, trimethyl borate has been used as a precursor in a process to 
make an electrode substrate (as in Japanese laid-open patent application 
07-105955, a precursor B-containing compound was kneaded in with the other 
electrode components before heat treating the mixture to 1000 degrees C.). 
Boron-oxygen-carbon-hydrogen containing compounds have also been used in 
the preparation of lithium haloboracite (a lithium-boron-oxygen-halogen 
containing material) solid electrolyte films for battery usage (as in 
Japanese laid-open patent application 06-279195). 
Recently, researchers have discovered that certain compounds containing 
boron, oxygen, carbon, and hydrogen can serve as improved electrolyte 
additives in rechargeable lithium batteries. For instance, in Canadian 
patent application serial no. 2,196,493, filed Jan. 31, 1997 by a common 
applicant and having the same title as the instant application, fade rate 
reducing additives for rechargeable lithium battery electrolytes are 
disclosed. The fade rate reducing additives comprised a (BO).sub.3 
boroxine ring. 
Further, in international patent application WO 97/16862 by C. A. Angell et 
al., improved electrolytes for lithium rechargeable batteries are 
disclosed wherein the solvent of the electrolyte consists predominantly of 
a liquid boron electrolyte solvent. The disclosed boron electrolyte 
solvents all comprise boron atoms which are bonded to two or three oxygen 
atoms. The electrolytes showed a wider electrochemical stability window 
than other conventional electrolytes. 
Also, Japanese published patent application number 09-120825 of Sanyo 
discloses the use of various boronate esters and/or borinate esters in 
lithium secondary batteries in order to suppress self discharge during 
storage. 
For various historical reasons, BF.sub.3 and complexes containing BF.sub.3 
have also been employed in primary or non-rechargeable batteries before. 
(Herein, the term `complex` is defined as a `complex substance in which 
the constituents are more intimately associated than in a simple mixture` 
in accordance with the definition in Webster's Ninth New Collegiate 
Dictionary, 1984, Merriam-Webster Inc.) In U.S. Pat. No. 3,915,743, Varta 
discloses a primary battery having a lithium metal anode, a sulfur 
cathode, and which operates below about 2.5 V. The battery comprised a 
BF.sub.3 adduct of organic solvents (such as dimethyl carbonate or 1,2 
dimethoxy ethane) to prevent the formation of polysulphides. 
Further, Japanese published patent application number 02-158059 of Tokai 
Carbon Co. discloses primary or non-rechargeable batteries comprising an 
aromatic nitrogen compound dissolved in the electrolyte. In examples in 
this application, BF.sub.3 was used as an electrolyte additive. 
BF.sub.3 has also been employed in the assembly of rechargeable lithium 
batteries before. In Japanese published patent application number 
59-154767, Hitachi Maxell discloses a rechargeable lithium battery 
containing a Li halide salt and BF.sub.3 wherein the BF.sub.3 reacts with 
the lithium halide salt to form a product which has advantages over 
LiBF.sub.4 salt. In this disclosure, residual BF.sub.3 is removed prior to 
assembling the battery. Thus, unreacted BF.sub.3 does not remain in the 
electrolyte. The resulting electrolyte is more stable at high temperature. 
In the disclosure, it was mentioned that the complex DME.BF.sub.3 might be 
used instead of BF.sub.3. 
While the preceding prior art may employ BF.sub.3 and/or complexes 
containing BF.sub.3 in primary batteries or in the assembly of secondary 
batteries, it appears that BF.sub.3 and/or complexes containing BF.sub.3 
have not been used for purposes of improving the fade rate of rechargeable 
lithium batteries. 
SUMMARY OF THE INVENTION 
Rechargeable batteries exhibit a loss in delivered capacity as a function 
of the number of charge/discharge cycles. Herein, the fractional loss of 
capacity per cycle is referred to as the capacity fade rate. The instant 
invention includes non-aqueous rechargeable lithium batteries having 
improved fade rates and methods for achieving the reduced fade rate. 
Non-aqueous rechargeable lithium batteries generally comprise a lithium 
insertion compound cathode, a lithium or lithium compound anode, and a 
non-aqueous electrolyte comprising a lithium salt dissolved in a 
non-aqueous solvent. Incorporating a small amount of an additive 
comprising a boron fluorine compound selected from the group consisting of 
BF.sub.3, BF.sub.3 complexes, HBF.sub.4, and HBF.sub.4 complexes in the 
batteries can result in improved fade rate characteristics. Preferably, 
the additive is dissolved in the electrolyte. Such additives therefore 
serve to function as fade rate reducing compounds. 
In additive complexes of the invention, the component complexed with 
BF.sub.3 or HBF.sub.4 is desirably relatively inert or is itself 
advantageous to use with respect to the components and function of the 
non-aqueous rechargeable lithium battery. Thus, the complexed component 
can itself be a suitable non-aqueous battery solvent such as a linear or 
cyclic organic carbonate (eg. diethyl carbonate or ethyl methyl 
carbonate), an ether (eg. diethyl ether), a lactone, or the like. However, 
in small quantities, complexed components such as phosphoric acid, which 
is generally considered to be undesirable, may still be employed. 
Improved fade rates can be achieved for batteries employing conventional 
lithium ion battery electrochemistries. Thus, the cathode can be a lithium 
transition metal oxide, in particular the layered compound LiCoO.sub.2. 
The anode can be a carbonaceous insertion compound anode, in particular 
graphite. The electrolyte can contain a lithium salt having a fluorine 
containing anion, such as LiPF.sub.6, dissolved in an organic carbonate 
solvent, in particular mixtures containing ethylene carbonate, propylene 
carbonate, ethyl methyl carbonate, and/or diethyl carbonate solvents. The 
electrolyte can contain other additives, included for other functions, 
without interfering with the additive of the invention. For instance, the 
electrolyte can additionally comprise biphenyl for safety related reasons. 
In principle, the fade reducing additive may be incorporated in the battery 
in a variety of ways. However, the additive is preferably dispersed inside 
the battery. Also, the additive may be hygroscopic which makes it more 
difficult to deal with during battery manufacture. For these reasons, the 
additive is preferably dissolved in the electrolyte. 
Conventional assembly methods can be used to prepare a battery of the 
invention, except that an additional step is required wherein an amount of 
one of the aforementioned fade reducing additive compounds is incorporated 
in the battery as well. A preferred method for accomplishing this is 
simply to dissolve a suitable amount of additive into the electrolyte 
solvent prior to using the electrolyte during assembly of the battery. 
Incorporating an amount of fade reducing additive in the range from greater 
than about 1% to about 5% of the weight of the electrolyte can be 
effective in improving capacity fade rate. Preferably however, a 
sufficiently small amount of fade reducing additive is incorporated such 
that other desirable bulk properties of the battery are not adversely 
affected, eg. such that the thermal stability threshold of the battery 
remains essentially unchanged. In this way, other bulk properties such as 
the relative safety of the battery are not compromised by the inclusion of 
the additive. For certain choices of fade reducing additives, 
incorporating an amount less than about 2.5% of the weight of the 
electrolyte can be effective in improving capacity fade rate without 
compromising fundamental battery safety. 
The additives of the invention are particularly suitable for use in lithium 
ion batteries which operate at very high voltages (operating potentials 
circa 4.2 volts or greater) wherein the electrolytes are subjected to 
oxidation and reduction extremes.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
Several compounds with boron-oxygen bonding in their structure have already 
been identified in the art as additives which can improve the capacity 
fade rate of non-aqueous lithium rechargeable batteries in general. We 
have discovered that a fade rate improvement can also be achieved using 
boron-fluorine additive compounds selected from the group consisting of 
BF.sub.3, BF.sub.3 complexes, HBF.sub.4, and HBF.sub.4 complexes. 
Typically, this type of battery employs a lithium insertion compound as the 
cathode and one of a variety of lithium containing materials as the anode. 
Possible lithium containing materials include lithium metal, lithium 
alloys, and lithium insertion compounds. Preferred embodiments are lithium 
ion batteries wherein the anode is also a lithium insertion compound. 
Presently, the majority of commercial lithium ion batteries employ 
transition metal oxide cathodes (either LiCoO.sub.2, LiNiO.sub.2, or 
LiMn.sub.2 O.sub.4) and carbonaceous anodes (either coke or graphite). 
Preferred electrolytes for lithium ion batteries comprise a lithium salt 
(typically having a fluorine containing anion) dissolved in a mixture of 
non-aqueous organic carbonate solvents (such as ethylene carbonate, 
propylene carbonate, ethyl methyl carbonate, and/or diethyl carbonate). 
LiPF.sub.6 is a typical choice for the lithium salt since it can result in 
a safer, more stable electrolyte than would some other salt choices. 
Generally, only a small amount (circa 1% by weight of the electrolyte) of 
additive compound is incorporated in the battery, and so the other bulk 
characteristics of the electrolyte can remain largely unaffected. In 
principle, the additive may be incorporated into the battery in various 
ways (eg. as a solid dispersed in an electrode). Preferably however, the 
additive is dissolved in the electrolyte before assembly. 
As a result, the additive is well dispersed throughout the battery 
immediately after assembly. Also, this method can make it easier to handle 
the additive during manufacture if the additive is hygroscopic or 
difficult to incorporate into either electrode for some reason. 
BF.sub.3 is functionally a preferred additive but it is somewhat difficult 
to work with since it is a hazardous gas under ambient conditions. In 
principle, a gas can be introduced into the battery container under 
pressure and then sealed inside. Alternately, if a gas is soluble enough 
in the liquid electrolyte, it might be dissolved therein and added in that 
way. (BF.sub.3 may be dissolved in halogenated or saturated hydrocarbons 
and/or aromatic compounds.) However, a typical electrolyte filling process 
for lithium batteries involves exposing the electrolyte to vacuum for 
brief periods whereupon a variable, unknown, and undesirable amount of 
dissolved gaseous additive may be lost. 
For handling and manufacturing purposes therefore, it is preferred to 
employ a suitable complex of BF.sub.3 instead. Such complexes can be solid 
under ambient conditions. We have found that solid BF.sub.3 complexes can 
be formed with some of the solvents employed in conventional lithium 
rechargeable batteries (eg. linear or cyclic organic carbonates). For 
instance, both BF.sub.3 -diethyl carbonate and BF.sub.3 -etliyl methyl 
carbonate complexes can be formed. Thus, complexes which are relatively 
much easier to handle can be used as additives without introducing any 
additional foreign chemicals other than BF.sub.3 itself. 
Ideally, it seems that the additive should be completely chemically 
compatible with the battery components (i.e. relatively inert with respect 
to the cathode, anode, and electrolyte and therefore should not 
significantly interfere with the normal functioning of the battery). 
However, we have found that other additives might also be employed to 
advantage even though they contain certain chemical groups that are not 
generally considered to be compatible with the battery components. For 
example, protons or hydrogen ions are generally avoided in these otherwise 
aprotic non-aqueous batteries. However, if only a small amount of additive 
is employed, the net effect of using an additive with an undesirable 
chemical group can still be positive. As an indication of this, additives 
which include phosphoric acid or HBF.sub.4 and complexes thereof have been 
observed to improve fade rate as well. 
Nonetheless, not every additive can be expected to be suitable for use in 
all circumstances. For instance, certain complex additives may work well 
within a given range in the operating potential of a battery but not 
perhaps over a wider range in operating potential. Further, it should be 
noted that the presence of additive compound can result in an increase in 
the irreversible capacity loss experienced during the first charging of 
such batteries. Also, the use of too much additive compound can adversely 
affect the thermal stability threshold of such batteries. And, an 
excessive amount of dissolved additive compound could be expected to 
adversely affect electrolyte conductivity and hence battery rate 
capability. Thus, it is important not only to determine the capacity fade 
rate as a function of amount of additive in any particular embodiment, but 
also to determine the effects of amount of additive on these other 
important battery characteristics. Some non-inventive characterization 
trials must therefore be performed in order to arrive at a sensible trade 
off between fade rate improvement and these other characteristics. 
The invention relates to battery constructions with one of the 
aforementioned additive compounds dissolved in the electrolyte. Various 
battery configurations are suitable, including prismatic formats or 
miniature coin cells. A preferred conventional construction for a lithium 
ion type product is depicted in the cross-sectional view of a spiral-wound 
battery in FIG. 1. A jelly roll 4 is created by spirally winding a cathode 
foil 1, an anode foil 2, and two microporous polyolefin sheets 3 that act 
as separators. 
Cathode foils are prepared by applying 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 onto a thin aluminum foil. 
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 
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 like manner except that a powdered (also 
typically about 10 micron size) carbonaceous insertion compound 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. 
The jelly roll 4 is inserted into a conventional battery can 10. A header 
11 and gasket 12 are used to seal the battery 15. The header may include 
safety devices if desired such as a combination safety vent and pressure 
operated disconnect device. Additionally, a positive thermal coefficient 
device (PTC) may be incorporated into the header to limit the short 
circuit current capability of the battery. The external surface of the 
header 11 is used as the positive terminal, while the external surface of 
the can 10 serves as the negative terminal. 
Appropriate cathode tab 6 and anode tab 7 connections are made to connect 
the internal electrodes to the external terminals. Appropriate insulating 
pieces 8 and 9 may be inserted to prevent the possibility of internal 
shorting. 
Lithium ion batteries of the invention have a fade reducing additive 
compound incorporated therein in order to improve the fade rate. 
Preferably, the additive is dissolved in the electrolyte which can be 
accomplished in a variety of ways. However, the most straightforward and 
thus the preferred method simply involves dissolving a suitable amount of 
an additive in the electrolyte solvent before filling the battery with the 
electrolyte. Then, prior to crimping the header 11 to the can 10 and 
sealing the battery, the electrolyte 5 comprising the fade reducing 
additive is added to fill the porous spaces in the jelly roll 4. 
At this point, the battery is in a fully discharged state. Generally, an 
electrical conditioning step, involving at least a single complete 
recharge of the battery, is performed as part of the overall assembly. One 
of the reasons for so doing is that some initial irreversible processes 
take place on this first recharge. For instance, a small amount of lithium 
is irreversibly lost during the first lithiation of the carbonaceous 
anode. 
Advantages of the invention can be achieved using modest amounts of fade 
reducing additive compound. In the examples to follow, desirable results 
were obtained using of order of 1% additive by weight in the electrolyte. 
As mentioned above, some tradeoffs in other desirable battery 
characteristics can be expected if excessive amounts of additive compound 
are employed. For instance, care must be taken not to unacceptably alter 
the thermal stability threshold of the battery by using the additive. 
Also, care must be taken not to unacceptably increase the irreversible 
capacity loss experienced in lithium ion batteries by using the additive. 
Some straightforward quantification trials usually would be required in 
order to select an appropriate amount of additive compound to use. 
Certain additives of the invention can be obtained commercially (eg. boron 
trifluoride diethyl etherate). However, some preferred additives, 
particularly the BF.sub.3 -linear organic carbonate complex compounds, 
presently are not available and these must be prepared. Solid BF.sub.3 
-linear organic carbonate complexes of the invention can be prepared 
simply by passing BF.sub.3 gas through the linear organic carbonate in the 
liquid phase to form a suspension. Then, the suspension is separated from 
the excess carbonate liquid, leaving a BF.sub.3 -linear organic carbonate 
complex solid. 
It can be advantageous to cool the liquid while passing BF.sub.3 gas 
therethrough since the reaction is exothermic and the heat generated might 
lead to the formation of undesirable by-products or to the evaporation of 
the liquid carbonate. After forming a suspension of the BF.sub.3 -linear 
organic carbonate, it can be advantageous to heat the suspension in order 
to recrystallize the product complex. The suspended solids can then be 
removed by conventional means, such as filtration. This method is suitable 
for preparing BF.sub.3 -diethyl carbonate and BF.sub.3 -ethyl methyl 
carbonate complexes. 
At this time, the reason for the fade rate improvement using such additive 
compounds is unclear. Without being bound by theory, a possible 
explanation is that the presence of these additive compounds in the 
electrolyte affects the passivation/decomposition reactions which occur at 
one or both of the electrodes. (These additives are also good scavengers 
of water. However, the mere removal of water is not believed to lead to 
the long term fade rate improvement obtained with these additives.) 
Passivation films can be initially formed as a result of these reactions 
which can then inhibit the further decomposition of electrolyte. Further 
decomposition may consume active lithium, and also result in the formation 
of decomposition products which, in turn, may coat the electrode material 
or otherwise adversely impede ionic transport thereby resulting in an 
increase in battery impedance (and hence result in a loss of deliverable 
capacity at a given rate). The presence of the additive compounds may 
result in the production of a chemically different passivation film and/or 
affect the rate of further decomposition reactions. 
In the aforementioned Canadian patent application serial no. 2,196,493, it 
was disclosed that trimethoxyboroxine was a preferred fade reducing 
additive. In the Illustrative Example following, it was however determined 
that this additive is decomposed early on during cycling testing of a 
battery and that HBF.sub.4 and BF.sub.3 were suspected of being 
decomposition products thereof. The trimethoxyboroxine in these batteries 
may react perhaps with the small amount of water present and/or the 
lithium salt. Since the scavenging of the small amount of water itself was 
not expected to significantly improve the fade rate and the fade rate was 
nonetheless improved, it was speculated that the presence of decomposition 
products of the trimethoxyboroxine found in the electrolyte might be 
responsible for the fade rate improvement. Thus, decomposition products 
like HBF.sub.4 and BF.sub.3 may themselves be fade rate reducing 
additives. 
If the anion of the lithium salt is involved in an advantageous 
decomposition reaction, the choice of salt may thus influence the extent 
of the fade rate improvement observed. Further, the additives of the 
invention may play a role in other chemical reactions too. For instance, 
in the following Examples, an optional biphenyl additive was employed to 
activate a safely device during overcharge abuse. The additives of the 
invention may also help to stabilize the slow degradation of the biphenyl 
additive and thereby also contribute to fade rate improvement. 
The following Examples are provided to illustrate certain aspects of the 
invention but should not be construed as limiting in any way. 18650 size 
cylindrical batteries (18 mm diameter, 650 mm height) were fabricated as 
described in the preceding and shown generally in FIG. 1. Cathodes 1 
comprised a mixture of LiCoO.sub.2 powder, a carbonaceous conductive 
dilutant, and polyvinylidene fluoride (PVDF) binder that was uniformly 
coated on both sides of a thin aluminum foil. Anodes 2 were made using a 
mixture of a spherical graphitic powder plus Super S (trademark of 
Ensagri) carbon black and PVDF binder that was uniformly coated on thin 
copper foil. Either Setela.RTM. or Celgard.RTM. 2300 microporous 
polyethylene film was used as the separators 3. The electrolytes 5 
employed were solutions of 1M LiPF.sub.6 salt dissolved in a solvent 
mixture of organic carbonates. The choice of LiPF.sub.6 salt can result in 
a safer, more stable electrolyte than would other salt choices. 
To protect against hazardous conditions on overcharge of the battery, the 
header of these batteries included a pressure operated electrical 
disconnect device. The electrolytes employed also contained 2.5% biphenyl 
additive by weight to act as a gassing agent for purposes of activating 
the electrical disconnect device (in accordance with the disclosure in 
co-pending Canadian Patent Application Serial No. 2,163,187, filed Nov. 
17, 1995, titled `Aromatic Monomer Gassing Agents for Protecting 
Non-aqueous Lithium Batteries Against Overcharge`, by the same applicant). 
Finally, the electrolytes 5 employed in examples of the invention also 
contained certain fade reducing additive compounds in amounts ranging up 
to about 5% by weight of the electrolyte. Approximately 4 cc of 
electrolyte was used in each battery. 
For electrical testing, batteries were thermostatted at a constant 
temperature (.+-.1.degree. C.) as indicated below. Cycling was performed 
using a current limited (1 or 1.5 A maximum as indicated below), constant 
voltage charge (either 4.1 or 4.2 volts as indicated below) for 2.5 hours 
and a constant 1 or 1.5 A current discharge to a 2.5 volt cutoff. (Note: 
For purposes of observing changes in battery impedance, a prolonged, low 
rate charging or discharging was performed every 20 cycles. Subsequent 
discharge capacities may then be significantly different from than the 
previous ones. Many of these points have been omitted from the data 
presented below for purposes of clarity. However, this type of testing can 
introduce a noticeable discontinuity in the capacity versus cycle number 
data curves.) 
COMATIVE EXAMPLES 
Several 18650 batteries were constructed as described above with an 
electrolyte comprising ethylene carbonate (EC), propylene carbonate (PC), 
diethyl carbonate (DEC) solvents in a volume ratio of 30/20/50 
respectively and no fade reducing additive. The batteries were then 
electrically conditioned and tested in various ways (in sets of two or 
more to check reproducibility) for purposes of comparing performance and 
safety results with those of batteries comprising fade reducing additives. 
FIG. 2 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 40.degree. C. to a 4.2 V 
upper cutoff. Discharge and charge currents were 1 A and limited to 1 A 
maximum respectively. FIG. 3a shows discharge and charge profiles for 
cycle numbers 10 and 200 of this representative battery. A significant 
increase in impedance has occurred in this battery with cycling. 
FIG. 5 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 21.degree. C. to a 4.1 V 
upper cutoff at similar currents. FIG. 7 shows the discharge capacity 
versus cycle number data for a representative battery which was cycle 
tested at 21.degree. C. to a 4.2 V upper cutoff. Here, the discharge and 
charge currents were 1.5 A and limited to 1.5 A maximum respectively. 
Some tests involved storing batteries at elevated temperature after the 
first ten or so cycles. This high temperature storage can adversely affect 
subsequent cycling performance. FIG. 4 shows the discharge capacity versus 
cycle number data for a representative battery which was cycle tested at 
21.degree. C. to a 4.2 V upper cutoff and had been stored at 60.degree. C. 
for 7 days. (Discharge and charge currents here were 1 A and limited to 1 
A maximum respectively.) FIG. 6 shows the discharge capacity versus cycle 
number data for a representative battery which was cycle tested at 
21.degree. C. to a 4.2 V upper cutoff and had been stored at 85.degree. C. 
for 10 hours. (Here, discharge and charge currents were 1.5 A and limited 
to 1.5 A maximum respectively.) In both cases, the high temperature 
storage has significantly affected subsequent cycling. 
As shown in co-pending Canadian patent application serial number 2,175,755, 
the use of a B.sub.2 O.sub.3 additive can adversely affect the thermal 
threshold stability of such batteries. Consequently, it may be important 
not to use an excessive amount of additive. For purposes of comparison, 
several batteries were electrically conditioned, charged to 4.2 V, and 
then exposed to a temperature of 150.degree. C. in a convection oven (a 
`hot box` thermal stability test). Since the batteries were not heat sunk 
to the oven, exothermic chemical reactions can be triggered within the 
batteries which, in turn, can result in further heating and potential 
thermal run away. The thermal response of each battery was monitored. In 
this `hot box` test, the safety vent of the Comparative batteries 
activated due to pressure buildup but no fire nor violent venting was 
observed. Thermal run away was thus avoided. 
INVENTIVE EXAMPLES 
i) BF.sub.3 -diethyl Carbonate Complex Additive 
A BF.sub.3 -diethyl carbonate complex compound was prepared in the 
following manner. BF.sub.3 gas was passed through liquid diethyl carbonate 
under nitrogen at about 0.degree. C. for about 20 minutes. The resulting 
cloudy saturated solution was then placed in a 40.degree. C. incubator for 
about 1/2 hour and afterwards it was allowed to cool to ambient 
temperature. The solids were separated from the resulting suspension by 
filtration to give solid crystalline BF.sub.3 -diethyl carbonate complex. 
Based on titration results of a sample of the filtrate and assuming the 
filtrate is BF.sub.3 -diethyl carbonate plus residual diethyl carbonate, 
the yield of BF.sub.3 -diethyl carbonate complex was about 80%. 
A series of 18650 batteries was then constructed as in the Comparative 
Examples above except that varying amounts of the fade reducing additive 
BF.sub.3 -diethyl carbonate complex were dissolved in the electrolyte 
prior to assembly. The amounts employed were 1.5%, 2%, 2.5%, and 5% by 
weight in the electrolyte. The batteries were then electrically 
conditioned and underwent several similar performance and safety tests 
(again in sets of two or more). 
FIG. 2 shows the discharge capacity versus cycle number data for 
representative batteries having 1.5%, 2.5%, and 5% additive which were 
cycle tested at 40.degree. C. to a 4.2 V upper cutoff. Again, discharge 
and charge currents were 1 A and limited to 1 A maximum respectively. A 
substantial improvement in the fade rate is achieved in batteries 
comprising the additive. However, the battery with 5% BF.sub.3 -diethyl 
carbonate complex additive shows noticeably less capacity than the 
Comparative battery at early cycle numbers. 
FIGS. 3b and c show discharge and charge profiles for cycle numbers 10 and 
200 of representative batteries having 1.5% and 5% BF.sub.3 -diethyl 
carbonate complex additive respectively. The impedance increase with 
cycling seen in a Comparative battery (FIG. 3a) is progressively reduced 
as the amount of additive is increased. 
FIG. 4 shows the discharge capacity versus cycle number data for a 
representative battery having 1.5% BF.sub.3 -diethyl carbonate complex 
additive which was cycle tested at 21.degree. C. to a 4.2 V upper cutoff 
and had been stored at 60.degree. C. for 7 days. (Again, discharge and 
charge currents were 1 A and limited to 1 A maximum respectively.) The 
fade rate is better than that of a Comparative battery with no additive. 
To determine what amount of additive might be excessive with regards to the 
thermal stability threshold, `hot box` safety tests as in the Comparative 
Example above were performed on sets of batteries comprising varied 
amounts of BF.sub.3 -diethyl carbonate complex additive. In batteries with 
1.5% and 2% additive, the safety vent activated but there was no fire nor 
violent venting. However, of two batteries tested with 2.5% additive, one 
vented with significant smoke and the other caught fire. Thus, a 2.5% 
level of BF.sub.3 -diethyl carbonate complex additive seemed to adversely 
affect the thermal threshold stability of these particular batteries. 
ii) BF.sub.3 -ethyl Methyl Carbonate Complex Additive 
A BF.sub.3 -ethyl methyl carbonate complex compound was prepared in a 
manner similar to that used to prepare the BF.sub.3 -diethyl carbonate 
complex in Inventive Example i) above. The filtrate of the prepared 
suspension was solid crystalline BF.sub.3 -ethyl methyl carbonate complex. 
Again, a yield of about 80% was obtained. 
A series of 18650 batteries was then constructed as in the Comparative 
Examples above except that the electrolyte comprised a solvent mixture of 
ethylene carbonate (EC) and ethyl methyl carbonate (EMC) solvents in a 
volume ratio of 30/70 respectively and varying amounts of the fade 
reducing additive BF.sub.3 -ethyl methyl carbonate complex were dissolved 
in the electrolyte prior to assembly. The amounts employed were 1.5% and 
2.5% by weight in the electrolyte. The batteries were then electrically 
conditioned and are undergoing cycle testing at both 21.degree. C. and 
40.degree. C. to a 4.2 V upper cutoff. To date, the fade rate is similar 
to that of the batteries of Inventive Example i) above. 
iii) BF.sub.3 -phosphoric Acid Complex Additive 
18650 batteries were constructed as in the Comparative Example except that 
1.4% by weight of boron trifluoride phosphoric acid complex additive 
(obtained from the Aldrich chemical company in Wis., USA) was dissolved in 
the electrolyte prior to assembly. 
The batteries were then electrically conditioned and underwent several 
cycle life performance tests. FIG. 5 shows the discharge capacity versus 
cycle number data for a representative battery which was cycle tested at 
21.degree. C. to a 4.1 V upper cutoff. Again, discharge and charge 
currents were 1 A and limited to 1 A maximum respectively. The battery 
comprising the BF.sub.3 -phosphoric acid additive has a significantly 
improved fade rate over that of the comparative battery. 
FIG. 6 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 21.degree. C. to a 4.2 V 
upper cutoff and had been stored at 85.degree. C. for 10 hours. Her and 
charge currents were 1.5 A and limited to 1.5 A maximum respectively. 
Again, the battery comprising the BF.sub.3 -phosphoric acid additive has a 
significantly improved fade rate over that of the comparative battery. 
iv) BF.sub.3 -diethyl Etherate Complex Additive 
18650 batteries were constructed as in the Comparative Example except that 
1.2% by weight of boron trifluoride diethyl etherate complex additive 
(obtained from the Aldrich chemical company) was dissolved in the 
electrolyte prior to assembly. 
The batteries were then electrically conditioned and underwent several 
cycle life performance tests. FIG. 5 shows the discharge capacity versus 
cycle number data for a representative battery which was cycle tested at 
21.degree. C. to a 4.1 V upper cutoff. Again, discharge and charge 
currents were 1 A and limited to 1 A maximum respectively. The battery 
comprising the BF.sub.3 -diethyl etherate additive has a significantly 
improved fade rate over that of the Comparative battery. (Note that the 
data for the battery containing BF.sub.3 -phosphoric acid above and for 
this battery containing BF.sub.3 -diethyl etherate overlap one another and 
cannot be distinguished in this Figure.) 
FIG. 6 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 21.degree. C. to a 4.2 V 
upper cutoff and had been stored at 85.degree. C. for 10 hours. Discharge 
an charge currents were 1.5 A and limited to 1.5 A maximum respectively. 
Again, the battery comprising the BF.sub.3 -diethyl etherate additive has 
a significantly improved fade rate over that of the Comparative battery. 
However, the fade rate is not as good as that of the battery of Inventive 
Example iii) comprising the BF.sub.3 -phosphoric acid additive. 
FIG. 7 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 21.degree. C. to a 4.2 V 
upper cutoff. (Discharge and charge currents here were 1.5 A and limited 
to 1.5 A maximum respectively.) Surprisingly here, the battery comprising 
the BF.sub.3 -diethyl etherate additive actually has a fade rate which is 
substantially inferior to that of a Comparative battery. As will be shown 
below, another etherate additive also performs poorly under these 
conditions. However, batteries comprising etherate additives consistently 
show improved fade rates when the upper cutoff voltage is 4.1 V. 
Additionally, as shown in FIG. 6, a battery with an etherate additive can 
show better fade rate than a Comparative battery, even with a 4.2 V upper 
cutoff, if the batteries were subjected to extreme high temperature 
storage conditions. It is speculated that the etherate component may not 
be compatible with battery chemistries at the slightly higher 4.2 V 
operating potential. Nonetheless, the benefits of the BF.sub.3 component 
of the additive, which can improve performance after high temperature 
storage, may outweigh the negative effect of the etherate component at a 
4.2 V cutoff and explain the results observed in FIG. 6. (Note that FIG. 7 
also shows the discharge capacity versus cycle number data for a 
representative battery having 1.5% BF.sub.3 -diethyl carbonate complex 
additive which was cycle tested without having been stored at high 
temperature. The cycle testing of this battery differed in that discharge 
and charge currents were 1.5 A and limited to 1.5 A maximum respectively. 
The fade rate is slightly better but similar to that of the Comparative 
battery with no additive. Note however, that this battery was cycled 
harder and that only about 110 cycles are shown. A fade rate improvement 
may not be observed in batteries cycled at 21.degree. C. until after 200 
or more cycles (see FIG. 5)). 
Thus, the etherate additive can provide improved fade rates but not under 
all circumstances. Etherate additives might be unsuitable overall for use 
in lithium ion batteries with operating potentials of 4.2 V or greater. 
v) HBF.sub.4 -diethyl Etherate Complex Additive 
1 8650 batteries were constructed as in the Comparative Example except that 
1.4% by weight of tetrafluoroboric acid diethyl ether complex additive 
(obtained from the Aldrich chemical company) was dissolved in the 
electrolyte prior to assembly. 
The batteries were then electrically conditioned and underwent several 
cycle life performance tests. FIG. 5 shows the discharge capacity versus 
cycle number data for a representative battery which was cycle tested at 
21.degree. C. to a 4.1 V upper cutoff. (Again, discharge and charge 
currents were 1 A and limited to 1 A maximum respectively.) The battery 
comprising the HBF.sub.4 -diethyl etherate additive has a significantly 
improved fade rate over that of the comparative battery. 
FIG. 7 shows the discharge capacity versus cycle number data for a 
representative battery which was cycle tested at 21.degree. C. to a 4.2 V 
upper cutoff. (Here, discharge and charge currents were 1.5 A and limited 
to 1.5 A maximum respectively.) As with the battery comprising BF.sub.3 
-diethyl etherate additive of Inventive Example iv), the battery 
comprising the HBF.sub.4 -diethyl etherate additive also has a fade rate 
which is substantially inferior to that of a Comparative battery. 
Again, this etherate additive can provide improved fade rates but not under 
all circumstances. 
Etherate additives might be unsuitable overall for use in lithium ion 
batteries with operating potentials of 4.2 V or greater. 
The preceding examples demonstrate that various BF.sub.3 and HBF.sub.4 
complex additives can be effective fade reducing additives in lithium ion 
batteries. Complexes comprising linear organic carbonates seem preferred 
since the linear organic carbonate component can be a desired bulk 
electrolyte solvent component and these additives can be effective in 
batteries with operating potentials of 4.2 V. Etherate and phosphoric acid 
based complexes can also be effective but these additives may limit 
performance under certain conditions and/or be unsuitable if used in 
larger amounts. ILLUSTRATIVE EXAMPLE 
Reaction Products of Trimethoxyboroxine in Batteries 
In the aforementioned Canadian patent application serial no. 2,196,493, 
trimethoxyboroxine was disclosed as being a preferred fade rate reducing 
additive in lithium ion batteries. In an attempt to determine what the 
additive did functionally to improve the fade rate, lithium ion batteries 
were constructed as described in CA 2,196,493 with trimethoxyboroxine 
additive dissolved in the electrolyte. After electrical conditioning, the 
batteries were disassembled and the electrolyte analyzed by atomic 
absorption. No significant quantity of trimethoxyboroxine remained. Thus, 
it appeared that the additive had reacted away early on in the life of the 
battery. It was speculated that the trimethoxyboroxine may have reacted 
with the small amount of water in the battery or with the lithium salt in 
the electrolyte. Thus, test solutions of trimethoxyboroxine in electrolyte 
solvent and either water or lithium salt were prepared to determine what 
the reaction products in these test solutions were. Impurity FTIR analysis 
of the test solutions suggested that the boron, which was added to the 
batteries in the form of trimethoxyboroxine, now existed as BF.sub.4 
.sup.- anions and thus as a HBF.sub.4 decomposition product. BF.sub.3 
would be an expected intermediate in any reaction which produced 
HBF.sub.4. 
Those skilled in the art will be aware that the specific embodiments 
disclosed in the preceding are merely representative of the invention and 
that many other variations are possible within the scope of the invention. 
For example, the aforementioned additives may be expected to provide 
cycling benefits in lithium metal or polymer electrolyte based batteries 
as well as in conventional lithium ion batteries. Accordingly, the scope 
of the invention should be construed by the following claims.