USOO7749641B2
`
`(12) United States Patent
`Ren et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,749,641 B2
`Jul. 6, 2010
`
`(54) SECONDARY LITHIUM ION CELL OR
`BATTERY, AND PROTECTING CIRCUIT,
`ELECTRONIC DEVICE, AND CHARGING
`DEVICE OF THE SAME
`
`1/1996 Tanaka ....................... 429,332
`5,487.960 A
`2f1997 Andrieu et al.
`5,604,418 A
`6.558,848 B1* 5/2003 Kobayashi et al. .......... 429,241
`2003/0113613 A1* 6/2003 Takeuchi et al. .............. 429/60
`
`(*) Notice:
`
`(76) Inventors: Xiaoping Ren, 17A No. 4 Building,
`Shijijiayuan, 45Xiaoguanbeili, Anwai,
`Beijing, 100029 (CN); Jie Sun, 17A, No.
`4 Building, Shijijiayuan, 45
`Xiaoguanbeili, Anwai, Beijing, 100029
`(CN)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1561 days.
`10/491,134
`Sep. 28, 2002
`PCT/CNO2/OO696
`
`(21) Appl. No.:
`(22) PCT Filed:
`(86). PCT No.:
`
`S371 (c)(1),
`(2), (4) Date: May 6, 2004
`(87) PCT Pub. No.: WO03/030293
`PCT Pub. Date: Apr. 10, 2003
`
`(65)
`
`Prior Publication Data
`US 2004/0209 156A1
`Oct. 21, 2004
`
`Foreign Application Priority Data
`(30)
`Sep. 28, 2001
`(CN) ................................ O1 141615
`
`(51) Int. Cl.
`(2006.01)
`HIM ID/244
`(2006.01)
`HIM ID/242
`(2010.01)
`HOLM 4/58
`(2006.01)
`H02. 700
`(52) U.S. Cl. ........................ 429/50: 429/49; 429/231.6;
`320/137
`(58) Field of Classification Search ................... 429/60,
`429/231.95, 49,50; 320/137
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`
`FOREIGN PATENT DOCUMENTS
`
`CN
`CN
`JP
`
`1135103 A 11, 1996
`1185860 A
`6, 1998
`2000-270491 A
`9, 2000
`
`OTHER PUBLICATIONS
`K. Mizushima et al., “LiCo (0<x<1): A New Cathode Material for
`Batteries of High Energy Density'. Mat. Res. Bull., vol. 15, pp.
`T83-789 1980.
`G. Pistoia et al., Synthesis of Mn spinels from different polymorphs
`of MnOJournal of Power Sources, 56 (1995) 37-43.
`Lei Yongquan, “Materials for New Energy', 2000, p. 136.
`Office Action dated Jun. 9, 2009, in corresponding Japanese Appli
`cation No. 2003-533374, English Translation 3 pages.
`Patent Abstracts of Japan Publication No. 2001-176559 dated Jun.
`29, 2001, * Abstract.
`Patent Abstracts of Japan Publication No. 09-259928 dated Oct. 3,
`1977, * Abstract.
`International Search Report, Dated: May 6, 2003.
`* cited by examiner
`Primary Examiner Patrick Ryan
`Assistant Examiner—Muhammad Siddiquee
`(74) Attorney, Agent, or Firm—Stephen A. Bent; Foley &
`Lardner LLP
`
`(57)
`
`ABSTRACT
`
`The present invention provides a new method for improving
`capacity, average operating Voltage and specific energy of a
`secondary lithium ion cell or battery. This method is achieved
`by means of properly adjusting the ratio between a positive
`material and negative material, which is calculated by theo
`retical specific energy, and properly increasing charge cut-off
`Voltage. The present method can greatly increasing specific
`energy and average operating Voltage of a secondary lithium
`ion cell without influence on recycle property of the cell. The
`present invention also provides a secondary lithium ion cellor
`battery practicing the method, a protecting circuit adapted for
`the secondary lithium ion cell or battery, a electronic device
`using said protecting circuit and said secondary lithium ion
`cell or battery, and a charging device for the secondary
`lithium ion cell or battery.
`
`5,260,148 A * 1 1/1993 Idota .......................... 429/307
`
`18 Claims, No Drawings
`
`APPLE 1001
`
`1
`
`

`

`US 7,749,641 B2
`
`1.
`SECONDARY LITHIUM ION CELL, OR
`BATTERY AND PROTECTING CIRCUIT,
`ELECTRONIC DEVICE, AND CHARGING
`DEVICE OF THE SAME
`
`TECHNICAL FIELD
`
`The present invention relates to a new method for improv
`ing capacity, average operating Voltage and specific energy of
`a secondary lithium ion cell or battery, and to a secondary
`lithium ion cell or battery prepared by using the method, a
`protecting circuit adapted for the secondary lithium ion cellor
`battery, a electronic device using the secondary lithium ion
`cell or battery, and a charging device for the secondary
`lithium ion cell or battery.
`
`10
`
`15
`
`BACKGROUND ART
`
`2
`of the theoretical capacity, although many documents indi
`cate that the charge cut-off voltage can be over 4.2V for a test
`cell using metallic lithium as counter electrode. In fact,
`according to MIZUSHIMAK et al., “A new cathode material
`for batteries of high energy density'. Mater. Res. Bull., 1980.
`15:783, the quantity of dedoped lithium ion increases with the
`increase of charge Voltage, and the electrochemical capacity
`of lithium cobalt oxides increases accordingly. However, the
`study deems that the reversible charge-discharge Voltage is
`about 4.3 V when metallic lithium is used as counter elec
`trode, and when said voltage is higher than 4.3 V, the structure
`of lithium cobalt oxides changes and the lattice parameter C
`decreases from 4.4 nm to 4.0 nm, and thus the recycle life of
`cell is affected.
`G. PISTOIA et al., J. Power Source, 56(1995), 37-43,
`deems that the structure the lithium cobalt oxides changes
`with the charge voltage, and the coexistence of monoclinic
`phase and hexagonal phase will appear when the charge Volt
`age is over a certain value, which will spoil the recycle prop
`erty of cell. The results of experiments showed that as to a test
`button cell having metallic lithium as negative electrode, the
`capacity of lithium cobalt oxides reaches 159 mAh/g when
`the charge cut-off voltage is 4.35 V, but it drops to 135 mAh/g
`after several cycles; and the capacity attenuates quickly when
`the charge cut-off voltage is 4.25 V. This document takes the
`opinion that lithium cobalt oxides maintains excellent recycle
`property and a capacity about 130 mAh/g only when the
`charge cut-off voltage is 4.15 V, and the corresponding Volt
`ages of monoclinic phase and hexagonal phase separately is
`4.05 V and 4.17 V, i.e., both of them are below 4.2 V.
`In addition, Lei Yongquan, “Materials for New Energy” (in
`Chinese), 2000, p.136, discloses that the decomposition volt
`age of electrolyte solution using LiPF as electrolyte and
`EC/DMC as mixture solvent is 4.2V, and thus deems that the
`electrolyte solution will be decomposed and the recycle life
`will be affected when the charge cut-off voltage is above 4.2
`V.
`In 1990, Sony Corporation issued the lithium ion cell using
`coke as negative electrode, which has a charge cut-off voltage
`of not more than 4.20 V, and it is accepted as a common
`technical requirement of lithium ion cells thereafter.
`The prior art deems:
`1. The increase of charge cut-off voltage will change the
`structure of positive electrode material, which mainly
`exhibits at the following two aspects: one aspect is that
`the phase change, i.e., the coexistence of monodinic
`phase and hexagonal phase and the conversion between
`them may seriously affect the recycle life of lithium ion
`cell; and another aspect is that the change of lattice
`parameter may narrow the channel for passing lithium
`ions, squeeze the space occupied by lithium ions, jam
`the channel of lithium ions, and decrease the recycle
`property of lithium ion cell.
`2. The elevated charge cut-off voltage may decompose the
`electrolyte solution, and the loss of electrolyte solution
`renders the transportation of lithium ion more difficult,
`and thus the recycle life of cell is seriously affected.
`Therefore, it can be seen that the limitation of charge Volt
`age restricts the actual utilization of active electrode materi
`als. Under this condition, even if new positive electrode mate
`rial and negative electrode material having higher specific
`energy are developed, the lithium ion cell cannot exhibit the
`best performance. Hence, it is urgently needed to provide a
`method that can improve the efficacy of active substances of
`lithium ion cell, consequently increase the capacity and aver
`age operating Voltage, and maintain the better cell perfor
`mance simultaneously.
`
`The industry of lithium ion cell develops quickly since
`SONY corporation of Japan invented and commercialized a
`secondary lithium ion cell. Up to 2000, the manufactures of
`lithium ion battery around the world compete allsidedly for
`improving the competitive power of their products mainly
`around the key issue, the capacity of lithium ion battery. At
`present, the improvement of capacity of commercialized sec
`ondary lithium ion battery generally depends on the increase
`of loading quantities of active substances (positive electrode
`materials and negative electrode materials). However, the
`limitation of the volume of lithium ion battery greatly restricts
`the increase of the battery capacity. For notably raising the
`capacity, the researches for the development of active Sub
`stances (positive electrode materials and negative electrode
`materials) having higher specific energy are been conducting
`around the world, but so far, there is no notable breakthrough
`in this aspect for various technical difficulties.
`In fact, the positive electrode materials and negative elec
`trode materials used in the current secondary lithium ion
`battery have relatively higher theoretical capacity, and the
`problem merely lies in the lower actual utilization rate of said
`capacity. For example, lithium cobalt oxides as a positive
`electrode material of secondary lithium ion cell has a theo
`retical capacity of 248 mAh/g, while the actually used capac
`ity of it is merely about 140 mAh/g, i.e., about half of said
`theoretic capacity is not utilized. This is mainly caused by the
`limitation of charge cut-off voltage commonly used in the art.
`At present, the charge cut-off voltage of single secondary
`lithium ion cell is limited to no more than 4.2 V, and this is
`well accepted as a technical requirement in the industry of
`manufacture of secondary lithium ion battery. Further, all
`lithium ion batteries in the markets around the world are
`manufactured under this technical requirement For example,
`the charge cut-off voltage is limited to below 4.2 V and the
`overcharge release voltage of its protection circuit is con
`trolled below 4.15 V during the formation of single lithium
`ion cell. The reasons that the charge cut-off Voltage being
`limited to below 4.2V lie in the following opinions in the prior
`research results and documents: although the capacity and
`average operating voltage are improved by increasing the
`charge cut-off voltage, the positive electrode materials and
`the negative electrode materials will undergo structure
`change, the electrolyte may decompose, and the recycle prop
`erty of the cell will be adversely affected when the charge
`cut-off voltage is greater than 4.2 V.
`For instance, as to lithium cobalt oxides that is used as
`positive electrode material in the most commercial lithium
`ion batteries, the charge cut-off voltage is limited to below 4.2
`V and the actual capacity is 120-140 mAh/g, i.e., about 50%
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`
`

`

`3
`SUMMARY OF INVENTION
`
`US 7,749,641 B2
`
`4
`cell, or a single lithium ion cell comprising a protecting
`circuit, or a battery comprising a number of single lithium ion
`cells, or a battery comprising a number of single lithium ion
`cell and protecting circuits. For briefness, sometimes it may
`also be termed as “lithium ion cell'. In addition, “theoretical
`capacities of positive electrode and negative electrode
`means the capacities of the positive electrode and the negative
`electrode calculated with an charge cut-off Voltage set at 4.2
`V.
`The present inventor studied the relation between the
`charge cut-off Voltage and the cell properties of secondary
`lithium ion cell by gradually elevating the charge cut-off
`Voltage. For example, in the formation test for the commercial
`secondary single lithium ion cells and the self made single
`secondary lithium ion cells, the inventor elevated the charge
`cut-off voltage from 4.2V to 4.30 V.4.35 V.4.40 V, 4.45V and
`4.6 V, and the results showed that when the charge cut-off
`voltage is, 4.3 V, 4.35 V and 4.40 V, the specific energy of cell
`increases by 6-20% than that under the charge cut-off voltage
`of 4.2V, and the cells still maintain excellent recycle property,
`e.g., the capacity maintains more than 95% after 50 cycles,
`and more than 80% after 300 cycles. However, when the
`charge cut-off voltage is 4.45 V or high, the specific energy
`increases by about 30%, but the cell exhibits inferior recycle
`property, e.g., the capacity maintains merely 83.9% after 6
`cycles.
`For investigating the reasons of poor recycle property when
`the charge cut-off voltage is 4.45 V or high, the inventor
`adjusted the ratio of positive electrode material to negative
`electrode material, i.e., said ratio was adjusted within the
`range from 1:1.3 to 1:2.5 calculated according to the theoreti
`cal capacity under the charge cut-off voltage of 4.2V, and then
`the single lithium ion cell having the adjusted ratios were
`tested by charging at an charge cut-off Voltage of 4.45 V, 4.6
`V, 4.8 V, 5.0 V. 5.2 V. 5.4 V, 5.6 V and 5.8 V, preferably said
`charging tests were conducted during the formation and test
`of the cells. Experimental results show that the specific
`energy was greatly increased under the charge cut-off voltage
`of 4.45 V or higher, and the corresponding recycle property
`was essentially not affected, when appropriate ratios of posi
`tive electrode material to negative electrode material were
`adopted.
`In the present method, the elevating of the charge cut-off
`Voltage is advantageously carried out during the formation
`and examination of the cell. The use of charge cut-off voltage
`of 4.20 V or high could increase the specific energy of cell, the
`capacity of positive electrode active material and the average
`operating Voltage, and activate the cell so that the cell reaches
`its optimal working State with simultaneously achieving the
`effect of the formation, substantial improvement of the spe
`cific energy of commercial lithium ion cells, and imparting
`the competitiveness of lithium ion cells without altering the
`original process.
`The mechanism of the above experimental results needs
`more researches. Without limited by anytheory, the presumed
`reasons are as follows.
`1. After the elevation of charge cut-off voltage, the amount
`of lithium ion dedoped from the positive electrode greatly
`increases, which renders an unmatched State to the original
`negative electrode material, and deposition of excessive
`lithium ions on the Surface of negative electrode as metallic
`lithium, which jam part of the channel for passing lithium
`ions and result in the reduction of capacity and the deteriora
`tion of recycle property. The increase of the amount of nega
`tive electrode material meets the requirement of doping the
`relatively excessive lithium ions, and avoids the deposition of
`lithium ions on the Surface of negative electrode as metallic
`
`5
`
`10
`
`15
`
`25
`
`30
`
`35
`
`As to the common opinion in the art that increasing the
`charge cut-off Voltage above 4.2 V may greatly shorten the
`recycle life, the inventor of the present invention conducted a
`large number of experiments and studies to elevate the charge
`cut-off voltage and the capacity of cell. Contrary to this opin
`ion, the inventor unexpectedly found that the efficacy of elec
`trode active materials is greatly increased by increasing the
`charge cut-off Voltage and property adjusting the ratio of
`positive electrode material to negative electrode material of
`single lithium ion cell. Consequently, the specific energy,
`capacity and average operating Voltage of secondary lithium
`ion cellor battery are improved, while the performance of cell
`is substantially not changed. The present invention is fulfilled
`based on the aforesaid discovery.
`It is one object of the present invention to provide a new
`method for improving capacity, average operating Voltage
`and energy density of a secondary lithium ion cell or battery,
`wherein the charge cut-off Voltage of the singe cell is greater
`than 4.2 V, and less than 5.8 V, and the ratio of positive
`electrode material to negative electrode material of the single
`cell is from 1:1.0 to 1:2.5, preferably from 1:1.16 to 1:2.5, as
`calculated by the specific capacity with the charge Voltage
`limited to 4.2 V.
`Another object of the present invention is to provide a
`secondary lithium ion cell or battery, wherein the single sec
`ondary lithium ion cell has an charge cut-off Voltage of
`greater than 4.2V but less than 5.8 V, and the ratio of positive
`electrode material to negative electrode material of the single
`cell is from 1:1.0 to 1:2.5, preferably from 1:1.15 to 1:2.5, as
`calculated by the specific capacity with the charge voltage
`limited to 4.2 V.
`Yet another object of the present invention is to provide a
`protecting circuit adapted for the secondary lithium ion cellor
`battery, said protecting circuit having a first overcharging
`protection Voltage of greater than 4.35 V, and an overcharge
`release voltage of greater than 4.15 V.
`Yet another object of the present invention is to provide an
`electronic device using the secondary lithium ion cell or
`battery as power Supply, said electronic device comprising a
`protecting circuit having a first overcharging protection Volt
`age of greater than 4.35 V, and an overcharge release Voltage
`of greater than 4.15 V.
`Yet another object of the present invention is to provide a
`charging device for the secondary lithium ion cell or battery,
`said charging device controlling an charge cut-off voltage for
`the single lithium ion cell within the range of greater than 4.3
`V but less than 5.8 V, preferably within the range from 4.3 V
`to 5.2 V, and more preferably from 4.3 V to 4.8V.
`50
`
`40
`
`45
`
`EMBODIMENTS FOR CARRYING OUT THE
`PRESENT INVENTION
`
`Being contrary to the common opinion in the art that the
`charge cut-off voltage shall be controlled below 4.2 V, the
`present inventor unexpectedly found that the capacity, spe
`cific energy and average operating Voltage of the secondary
`lithium ion cell or battery were notably improved with little
`cost and with the other properties Substantially unchanged,
`when investigating the effect of the elevated charge cut-off
`Voltage on the cell performances with a large number of
`experiments of gradually elevating the charge cut-off voltage
`and appropriately adjusting the ratio of the positive electrode
`to the negative electrode of the single lithium ion cell. In the
`present invention, “lithium ion cell or battery' means that the
`present invention can be applied to either a single lithium ion
`
`55
`
`60
`
`65
`
`3
`
`

`

`US 7,749,641 B2
`
`5
`lithium, and thus the maintaining properties (self-discharge
`properties) and recycle property are not affected. The suitable
`ratio of positive electrode material to negative electrode mate
`rial will avoid the deposition of metallic lithium on the surface
`of negative electrode and the obstruction of channel for pass
`ing lithium ions, and consequently avoid the attenuation of
`capacity of cell. In particular, when the charge cut-off voltage
`of lithium ion cell is greater than 4.45 V, the negative elec
`trode material of the ordinary commercial lithium ion cell is
`still more deficient, and the excessive lithium ions will
`deposit on the Surface of negative electrode and form metallic
`lithium, which jams the channel for passing lithium ions and
`attenuates the capacity of cell. The great increase of content of
`negative electrode could reduce the attenuation of capacity of
`lithium ion cell caused by the increase of recycle times.
`According to this viewpoint, the inventor performed a lot of
`experiments, and the results prove the aforesaid assumption
`(see the Examples).
`2. Within a certain range of charge cut-off Voltage, the
`decomposition of little electrolyte Solution brings about neg
`ligible effect on the recycle property of cell. The electrolyte
`having a high decomposition potential or an additive increas
`ing the decomposition potential of electrolyte solution may
`bring about better performance. The decomposition of elec
`trolyte Solution mainly occurs on the positive electrode.
`Although prior documents disclosed that the decomposition
`Voltage of the electrolyte solution comprising LiPF as elec
`trolyte and the mixture of EC/DMC as solvent on the surface
`of aluminum foil is 4.2 V, according to the results of experi
`ments, this factor essentially does not affect the recycle life of
`the currently commercialized lithium ion cell. Namely, even
`though the electrolyte solution decomposes under a voltage
`higher than 4.2V, the electric energy is mainly converted into
`chemical energy and the electric energy involved in the
`decomposition of electrolyte is very little, thus, this decom
`position of electrolyte can hardly affect the recycle life of the
`lithium ion cell. As to the Voltages above the decomposition
`voltage, such as above 5.0 V, the substance A can be added
`into the electrolyte solution, or the electrolyte solution B
`having a higher decomposition Voltage can be used. The
`decomposition potentials of the components of electrolyte
`solution commonly used in the art are depicted in Table 1. It
`can be seen that the lowest decomposition Voltage of solvent
`is above 4.5 V.
`
`10
`
`15
`
`25
`
`30
`
`6
`lithium ions is more than the space that should be occupied by
`the lithium ions, i.e., the space available to the lithium ions
`exceeds about 10% of the space needed by the activated
`lithium ions, thus, although the elevation of the charge Volt
`age changes the structure of positive electrode material, i.e.,
`reduces the lattice parameter, the recycle life is not affected
`because the space occupied by the actually deintercalation
`and intercalation lithium ions is less than the space actually
`possessed by the positive electrode material, and thus the
`change of structure in a certain extent will not obstruct the
`dedoping and doping of lithium ions and will not affect the
`recycle life of cell. Using positive material more stable in
`structure with respect to the change of charge Voltage exhibit
`better performance. Although the positive electrodegenerates
`inert Substance poor in conductivity when overcharged,
`according to the results of experiments, it occurs only when
`the lithium ions completely dedoped. For example, as to
`lithium cobalt oxides, lithium nickel oxides, and doped
`lithium cobalt oxides and lithium nickel oxides, when they
`are charged with 3C5A current, the experimental data show
`that only when the voltage is about 6.20 V the lithium ions
`completely dedoped, with releasing a lot of oxygen, and
`forming an insulator. As to lithium manganese oxides and
`doped lithium manganese oxides, the lithium ions completely
`dedoped under the same charge current when the cell poten
`tial is about 6.50 V. Generally, the specific energy of positive
`electrode active Substance actually used in the present com
`mercialized lithium ion cell is far less than the theoretical
`capacity thereof, and even if the charge cut-off Voltage is
`elevated up to 5.8 V, the theoretical capacity cannot be
`achieved under the proviso that suitable formulation is used,
`thus, the cell is not overcharged. Therefore, the present
`method still enjoys satisfactory safety.
`4. Besides the aforementioned factors, the self-discharge
`of cell, the selection of current collector, and the formation of
`passive film all affect the recycle life of lithium ion cell, while
`these factors mainly depend on the preparation of lithium ion
`cell. It is believable that if the optimal processes are used, the
`capacity, average operating Voltage and specific energy can
`be greatly improved, while the recycle life and other proper
`ties of the cell are not affected.
`Hence, the present invention is to provide a novel method
`for effectively improving the specific energy and average
`operating Voltage of secondary lithium ion cell or battery.
`Contrary to the prior art, the present method elevates the
`charge cut-off voltage to greater than 4.2V but less than 5.8V.
`and control the ratio of positive electrode material to negative
`electrode material of single lithium ion cell at 1:1.0 to 1:2.5
`calculated by theoretical specific energy, so as to increase the
`efficacy of electrode active materials, to improve the capacity,
`energy density and output Voltage of cell, and to maintain the
`performance of cell. For achieving the better effect, the
`charge cutoff voltage used in the present invention is prefer
`ably 4.3-5.2V, and more preferably 4.2–4.8V. In addition, the
`ratio of positive electrode material to negative electrode mate
`rial of cell is 1:1.0 to 1:2.5 calculated by the theoretical
`capacity under the charge cut-off voltage of 4.2V. The experi
`ments showed that when the process parameters go beyond
`the aforesaid ranges, the properties of cell are deteriorated
`and unsuitable for use. When the ratio of positive electrode
`material to negative electrode material is less than 1.0 calcu
`lated by theoretical capacity, the recycle life of cell is exces
`sively reduced, while when said ratio is greater than 2.5, the
`volume efficiency of cell is notably reduced. Further, when
`the charge cut-off voltage is greater than 5.8 V, the cell has
`inferior properties and is unsuitable for use.
`
`35
`
`40
`
`TABLE 1.
`
`Decomposition potentials of various mixture electrolyte solutions
`
`Solute
`
`LiCIO
`Decomposition
`potentials (V)
`
`LiASF
`Decomposition
`potentials (V)
`
`LiPF
`Decomposition
`potentials (V)
`
`4.51
`4.5
`
`4.5
`
`4.72
`4.5
`4.5
`4.62
`4.52
`4.9
`
`4.8
`4.9
`5.9
`
`Mixture
`solvent
`
`PC:DME
`PC:DEC
`PC:EC
`PC:EC:DME
`PC:DEC:2MLF
`EC:DEC
`EC:DMC
`B
`
`3. As to the reason that the lithium ion cell does not appear
`attenuation of capacity caused by the change of structure as
`mentioned in the prior documents when the charge cut-off
`voltage is above 4.2 V, it may be due to that: after the ithium
`ions first intercalaton into anode, there is about 10% lithium
`ions which form a SEI film, so that the actual space of the
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4
`
`

`

`7
`It can be seen that the capacity, specific energy and average
`operating Voltage of secondary lithium ion cell can be greatly
`improved by elevating the charge cut-off Voltage and by
`appropriately adjusting the ratio of positive electrode material
`to negative electrode material calculated by the theoretical
`capacity, while the recycle property of cell is not affected,
`which renders the lithium ion cell possess more commercial
`value and broader application range. Hence, it can also be
`seen that the present method further optimizes the utilization
`of positive electrode material, and thus is an economical
`method. The inventors believe that the method can still be
`used to positive and negative electrode materials having
`higher specific energy developed in future, and achieves the
`optimal effect.
`In addition, it is worthy to be noted that the used in the
`present method is not such that the cell works in extreme
`conditions. After repetitive experiments, the inventor never
`found that the present method increased the probability of
`damage of cell. Hence, the present method is also a safe
`method.
`Further, the present invention further provides a novel sec
`ondary cell or battery having improved specific energy and
`average operating Voltage. Contrary to the prior art, the
`charge cut-off voltage of the single cell of said secondary cell
`or battery is greater than 4.2V but less than 5.8V, and the ratio
`of positive electrode material to negative electrode material
`calculated by the theoretical capacity under the charge cut-off
`voltage of 4.2 V is 1:1.0 to 1:2.5. As compared to the prior
`secondary lithium ion cell or battery cell, the capacity, energy
`density and output voltage of the present secondary lithium
`ion cell or battery are greatly improved, while its recycle life
`is equivalent to that of the prior art cell. For achieving better
`effect, the aforesaid charge cut-off voltage used in the present
`invention is preferably in the range from 4.3 V to 5.2 V. more
`preferably in the range from 4.3 V to 4.8 V. In addition, the
`ratio of positive electrode material to negative electrode mate
`rial of said singe lithium ion cell, which is calculated by the
`theoretical capacity under the charge cut-off voltage of 4.2 V.
`40
`is 1:1.5 to 1:2.5. Experiments showed that when the process
`parameters go beyond the aforesaid ranges, the properties of
`cell are deteriorated and unsuitable for use. When the ratio of
`positive electrode material to negative electrode material is
`less than 1.0 calculated by theoretical capacity, the recycle
`life of cell is excessively reduced, while when said ratio is
`greater than 2.5, the volume efficiency of cell is notably
`reduced. Further, when the charge cut-off Voltage is greater
`than 5.8V, the cell has inferior properties and is unsuitable for
`SC.
`Moreover, it is important that the present method not only
`can be used to the secondary lithium ion cell or battery of the
`present invention, but also can be used to the secondary
`lithium ion cellor battery prepared according to the method of
`the prior art, Such as the present commercialized secondary
`lithium ion cell or battery.
`Without limitation, the following contents more concretely
`introduce the secondary lithium ion cell used in the present
`method. Generally, a secondary lithium ion cell comprises a
`positive electrode, a negative electrode, a non-aqueous elec
`trolyte, and a separator the positive electrode and the negative
`electrode. The non-aqueous electrolyte can be obtained by
`dissolving lithium-containing metal salt, such as LiPF, as
`electrolyte into a non-aqueous solvent, such as ethylene car
`bonate or dimethyl carbonate. The separator can be insoluble
`in said non-aqueous solvent, and is a porous membrane made
`of polyethylene or polypropylene resin. The ratio of positive
`
`50
`
`8
`electrode material to negative electrode material is calculated
`by theoretical capacity under the charge cut-off voltage of 4.2
`V.
`
`Positive Electrode
`The positive electrode is prepared e.g., by dispersing posi
`tive electrode active material, conducting agent and binder in
`a Suitable solvent to form a Suspension, coating said Suspen
`sion on a current collector, such as aluminum foil, then drying
`and pressing the coated current collector by rollers.
`The positive electrode active substance used in the present
`invention is lithium-containing compound. Although the
`examples use lithium cobalt oxides (lithium cobalt composite
`oxides), lithium manganese oxides and lithium nickel oxides
`as positive electrode material, it is understood that the prac
`tice of the present invention is not limited to the specific
`properties of said lithium-containing composite oxides,
`rather a wide range of positive electrode active Substance can
`be used in the present invention. The common feature of these
`oxides is that their specific energy increases with the increase
`of Voltage, and the experiments (see the examples) prove that
`the capacity of cell is greatly elevated when the charge cut-off
`voltage is above 4.20 V, while the other properties of cell are
`not affected. The present invention can also be used to lithium
`ion cells having doped lithium-containing compound as posi
`tive electrode active material. Such as various positive elec
`trode active materials containing various oxides and Sulfides,
`Such as lithium cobalt composite oxides, lithium manganese
`composite oxides, lithium nickel composite oxides, lithium
`nickel cobalt composite oxides, lithium manganese cobalt
`composite oxides, and Vanadium oxides. Among these posi
`tive electrode materials, lithium cobalt composite oxides
`(such as LiCoO), lithium manganese composite oxides
`(such as LiMnO), lithium nickel composite oxides (such as
`LiNiO), lithium nickel cobalt composite oxides (such as
`LiNiCo,O), and lithium manganese cobalt composite
`oxides (such as LiMn, CoO), which have higher cell Volt
`age, are preferably used. In addition, the present invention can
`use conventional conducting agent and binder, and the mix
`ture ratio for each components in the positive electrode active
`material can be those well known in the art.
`Separator
`The separator used in the present invention is a separator
`well known in the art. For example, it can be a non-woven
`fabric made of synthetic resin, polyethylene porous mem
`brane or polypropylene porous membrane, and a material
`formed by like materials.
`Negative Electrode
`The negati