Method of controlling charge and discharge of non-aqueous electrolyte secondary cell

1. Field of the Invention

The present invention relates to methods of controlling charge and discharge of non-aqueous electrolyte secondary cells such as lithium secondary cells.

2. Description of Related Art

A cell that has in recent years drawn attention as having a high energy density is a non-aqueous electrolyte secondary cell in which the negative electrode active material is composed of metallic lithium, an alloy or carbon material that is capable of intercalating and deintercalating lithium ions and the positive electrode active material is composed of a lithium-transition metal complex oxide represented by the chemical formula LiMO 2 (where M is a transition metal).

A representative example of the lithium-transition metal complex oxide is lithium cobalt oxide (LiCoO 2 ), which has already been in commercial use as a positive electrode active material for non-aqueous electrolyte secondary cells.

Materials containing Mn as a transition metal and those containing Ni as a transition metal have also been researched, and materials containing all of Mn, Ni, and Co have been actively researched. (See, for example, Japanese Patent Nos. 2561556 and 3244314; and Journal of Power Sources, 90 (2000) pp. 176-181).

It has also been reported in Electrochemical and Solid-State Letters, 4(12) A200-A203 (2001), for example, that among the lithium-transition metal complex oxides containing Mn, Ni, and Co, a material represented by the chemical formula LiMn x Ni x Co (1-2x) O 2 , in which the contents of Mn and Ni are equal, shows exceptionally high thermal stability even in a charged state (high oxidation state).

Japanese Unexamined Patent Publication No. 2002-42813 reports that a complex oxide containing Ni and Mn at substantially equal amounts shows a voltage of about 4 V, which compares with LiCoO 2 , and exhibits high capacity and good charge-discharge efficiency.

A cell employing a positive electrode using as its main material (50 weight % or more) such a lithium-transition metal complex oxide containing Mn, Ni, and Co and having a layered structure (for example, the one represented by the chemical formula Li a Mn b Ni b Co (1-2b) O 2 (0≦a≦1.2, 0<b≦0.5)) is expected to improve cell reliability remarkably because such a cell has high thermal stability during charge.

Nevertheless, through a study on cell charge-discharge performance, it was found that a cell in which the positive electrode active material is such a lithium-transition metal complex oxide containing Mn, Ni, and Co and having a layered structure and the negative electrode is made of a material having a higher initial charge-discharge efficiency than that of the positive electrode active material, such as graphite, shows a significantly inferior performance to a conventionally-used cell adopting lithium cobalt oxide for the positive electrode.

As for the charge-discharge conditions for non-aqueous electrolyte secondary cells using the above-noted lithium-transition metal complex oxide as a positive electrode active material and a carbon material as a negative electrode active material, Japanese Unexamined Patent Publication No. 2002-42813, for example, discloses that the end-of-charge voltage should be 4.2 V and the end-of-discharge voltage should be 2.5 V.

Also, Japanese Unexamined Patent Publication No. 2003-17052 discloses that the end-of-charge voltage should be 4.2 V and the end-of-discharge voltage should be 2.0 V. Further, Japanese Unexamined Patent Publication No. 7(1995)-153494 describes in paragraph [0015] that generally the end-of-discharge voltage should be 2.0 V for a non-aqueous electrolyte secondary cell adopting a similar complex oxide containing Ni and Co as the positive electrode active material and a carbon material for the negative electrode. From these descriptions, it is thought that the end-of-discharge voltage for cells that use a lithium-transition metal complex oxide containing Ni as the positive electrode and a carbon material as the negative electrode is about 2.0-2.5 V.

To date, as an example of a method for improving the charge-discharge cycle performance of a non-aqueous electrolyte secondary cell that uses as the positive electrode active material a complex oxide containing Li, Ni, and Co and having a layered structure, Japanese Unexamined Patent Publication No. 5(1993)-290890, for example, has proposed that the capacity ratio of the positive electrode and the negative electrode during charge should be set so that the range of x in Li x MO 2 (where M is Co and/or Ni) of the positive electrode falls within the range 0.35≦x≦0.9. (Note that the value x decreases during charge and increases during discharge.) Paragraph [0009] of Japanese Unexamined Patent Publication No. 5(1993)-290890 describes that in this case, the use of Li x MO 2 having such a composition range can suppress destruction of the structure of the positive electrode active material resulting from charge/discharge cycles.

Nevertheless, it has been found that cells that use the lithium-transition metal complex oxide containing Mn, Ni, and Co as the positive electrode active material show remarkably poor charge-discharge cycle performance although the value x (lithium content) during charge and discharge falls within the range specified in Japanese Unexamined Patent Publication No. 5(1993)-290890, and this method is incapable of attaining sufficient advantageous effects.

Further, Japanese Unexamined Patent Publication No. 7(1995)-153494 discloses that although using a complex oxide containing Ni and Co and having a layered structure as a positive electrode active material, an increase in the potential of the negative electrode in the final stage of discharge of a cell is suppressed by reducing the initial charge-discharge efficiency of the positive electrode to be lower than the initial charge-discharge efficiency of the negative electrode, thereby suppressing decomposition of the electrolyte solution on the negative electrode surface. However, with the above-described cell that uses a lithium-transition metal complex oxide containing Mn, Ni, and Co as a positive electrode active material, the initial charge-discharge efficiency of its positive electrode is lower than the initial charge-discharge efficiency of the graphite negative electrode. Therefore, although the potential of the negative electrode does not increase in the final stage of discharge of the cell, its charge-discharge cycle performance is remarkably poor, and it has been found that a sufficient effect cannot be attained even when degradation in the negative electrode side is suppressed.

Accordingly, it is an object of the present invention to provide a method of controlling charge and discharge of a non-aqueous electrolyte secondary cell comprising a positive electrode containing, as a positive electrode active material, a lithium-transition metal complex oxide having a layered structure and containing at least Ni and Mn as transition metals, and a negative electrode containing a carbon material as a negative electrode active material and having an initial charge-discharge efficiency higher than that of the positive electrode, or an assembled battery having a plurality of cells each being the non-aqueous electrolyte secondary cell, the method being capable of suppressing capacity degradation due to charge-discharge cycles.

In order to accomplish the foregoing and other objects, the present invention provides a method of controlling charge and discharge of either a non-aqueous secondary cell comprising a positive electrode including a positive electrode active material containing a lithium-transition metal complex oxide having a layered structure and containing at least Ni and Mn as transition metals, and a negative electrode containing a carbon material as a negative electrode active material and having an initial charge-discharge efficiency higher than that of the positive electrode, or an assembled battery having a plurality of cells each being the secondary cell, the method comprising: controlling discharge of the secondary cell or each of the cells in the assembled battery so that the end-of-discharge voltage of the secondary cell or each of the cells is 2.9 V or higher.

According to the present invention, capacity degradation due to charge/discharge cycles can be suppressed. Consequently, charge-discharge cycle performance can be improved.

As will be shown in the later-described Experiment 2, with a non-aqueous electrolyte secondary cell using as a positive electrode active material a lithium-transition metal complex oxide having a layered structure and containing Ni and Mn, the XRD (X-ray diffraction) pattern of the positive electrode taken out of a cell after charge/discharge cycles showed no substantial change that indicates capacity degradation; thus, it was found that no crystal destruction occurred in the bulk of the positive electrode active material. In addition, neither Ni nor Co was detected from the negative electrode taken out of the cell after charge/discharge cycles, but a large amount of Mn was detected therefrom. From these facts, it is presumed that deterioration of active material developed because of the mechanism described below.

Since the lithium-transition metal complex oxide containing Ni and Mn and having a layered structure shows lower initial charge-discharge efficiency than the carbon negative electrode, the potential of the positive electrode greatly reduces in the final stage of discharge of the cell. Generally, it is believed that in a compound represented by the formula Li a Mn b Ni b Co (1-2b) O 2 (where 0≦a≦1.2 and 0<b≦0.5), the oxidation number of Mn is +4 (Mn 4+ ) and the oxidation state of Mn does not change during charge and discharge (cf. Proceedings of the 42nd Battery Symposium in Japan, pp. 51-52, etc.). However, it is believed that when the potential of the positive electrode greatly reduces as described above, part of Mn 4+ in the positive electrode is reduced into Mn 3+ , which is unstable, and Mn 3+ causes disproportionation, forming Mn 4+ and Mn 2+ . It is thought that because Mn 2+ tends to form a complex compound with, and dissolve into, the solvent that constitutes the electrolyte solution easily, it dissolves from the positive electrode surface into the solvent and deposits on the negative electrode. It is presumed that due to such dissolution of Mn from the positive electrode surface, the crystal structure of the surface deteriorates, and as a result, the resistance at the interface between the positive electrode and the electrolyte solution increases, causing the capacity degradation.

It is thought that by controlling the end-of-discharge voltage of the secondary cell or each of the cells in the assembled battery to be 2.9 V or higher in accordance with the present invention, the decrease of the positive electrode potential in the final stage of discharge is suppressed; as a result, the dissolution of Mn can be suppressed and the capacity deterioration can be lessened, leading to a remarkable improvement in charge-discharge cycle performance.

In the present invention, it is possible that with the non-aqueous electrolyte secondary cell or the assembled battery having a plurality of cells each being the secondary cell, a control circuit that is incorporated in the secondary cell or each of the cells in the assembled battery, or in an apparatus using the cell/battery, monitors the voltage of the secondary cell or each of the cells in the assembled battery and controls the charge and discharge of the cell/battery so that the end-of-discharge voltage of each cell is 2.9 V or higher.

Generally, charging and discharging of a battery is controlled by a control circuit. As described above, the control circuit may be incorporated either in the battery, or in an apparatus using the battery as its energy source. Meanwhile, in order to ensure safety at the time of malfunctions on the apparatus side, the battery is usually provided with a protection circuit. The protection circuit monitors voltage, temperature, and the like of the battery. The protection circuit has a function to protect the battery from overdischarging, so it prevents the battery from being discharged excessively. In many conventional lithium secondary batteries that use a lithium cobalt oxide as the positive electrode active material and a graphite material as the negative electrode active material, the overdischarge-detection voltage is usually set at about 2.0 V to 2.5 V.

The present invention also provides a charge-discharge controlling device for controlling charge and discharge of either a non-aqueous secondary cell including a positive electrode composed of a positive electrode active material containing a lithium-transition metal complex oxide having a layered structure and containing at least Ni and Mn as transition metals, and a negative electrode containing a carbon material as a negative electrode active material and having an initial charge-discharge efficiency higher than that of the positive electrode, or an assembled battery having a plurality of cells each being the secondary cell, the charge-discharge controlling device comprising: a charge control switch for starting or ending charge of the secondary cell or each of the cells in the assembled battery; a discharge control switch for starting or ending discharge of the secondary cell or each of the cells in the assembled battery; and a control circuit, incorporated in the secondary cell or assembled battery, or in an apparatus using the secondary cell or assembled battery, for supplying a signal to the discharge control switch such that an end-of-discharge voltage of the secondary cell or each of the cells in the assembled battery is 2.9 V or higher.

It is preferable that the charge-discharge controlling device of the present invention be provided with a protection circuit for preventing overdischarge of the secondary cell or the assembled battery. In addition, it is preferable that with the protection circuit, an overdischarge-detection voltage for the secondary cell or for each of the cells in the assembled battery is set to be a value equal to or lower than the end-of-discharge voltage controlled by the control circuit and equal to or higher than 2.9 V.

It is preferable that the control circuit control the discharge of each of the cells when the control circuit monitors the voltage of each of the cells in the assembled battery.

If the control circuit is the one that monitors an overall voltage of the assembled battery, the overdischarge-detection voltage of the protection circuit for each of the cells in the assembled battery may be set at 2.9 V or higher so that the end-of-discharge voltage of each cell results in 2.9 V or higher.

In the present invention, it is preferable that the lithium-transition metal complex oxide further contain cobalt. Specifically, it is preferable that the lithium-transition metal complex oxide contain Ni, Mn, and Co as transition metals. It is preferable that such a lithium-transition metal complex oxide be represented by the chemical formula Li a Mn x Ni y Co z O 2 , wherein a, x, y, and z satisfy 0≦a≦1.2,x+y+z=1, 0<x≦0.5, 0<y≦0.5, and z≧0.

Furthermore, in the present invention, it is preferable that the lithium-transition metal complex oxide contain Ni and Mn at substantially equimolar amounts. The term “substantially equimolar amounts” herein means that x and y in the foregoing chemical formula satisfy the following equations:

0.45 ≦x /( x+y )≦0.55; and

0.45 ≦y /( x+y )≦0.55.

Nickel has such properties that lead to a large cell capacity but low thermal stability during charge, whereas manganese has such properties that lead to a small capacity but high thermal stability during charge. Accordingly, when a lithium-transition metal complex oxide contains these elements at substantially equimolar amounts, it is possible to obtain their properties in a desirable balance.

In the present invention, it is preferable that the carbon material serving as a negative electrode active material be a graphite material. In the present invention, even when a graphite material is used as the negative electrode material, the potential of the negative electrode does not increase because the voltage of the cell decreases as the positive electrode potential decreases in the final stage of discharge when ending the discharge. Use of a graphite material in such a condition can make full use of excellent reversibility of the graphite material.

The solvent of the non-aqueous electrolyte used in the present invention may be any solvent that has conventionally been used as a solvent for an electrolyte in non-aqueous electrolyte secondary batteries. Examples of such a solvent include: cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; and chain carbonates such as dimethyl carbonate, methylethyl carbonate, and diethyl carbonate. In particular, a mixed solvent of the cyclic carbonate and chain carbonate is preferable.

In the present invention, the solute of the non-aqueous electrolyte may be any lithium salt that is generally used as a solute in non-aqueous electrolyte secondary batteries. Examples of such a lithium salt include LiPF 6 , LiBF 4 , LiCF 3 SO 3 , LiN(CF 3 SO 2 ) 2 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 )(C 4 F 9 SO 2 ), LiC(CF 3 SO 2 ) 3 , LiC(C 2 F 5 SO 2 ) 3 , LiAsF 6 , LiClO 4 , Li 2 B 10 Cl 10 , Li 2 B 12 Cl 12 , and mixtures thereof.

According to the present invention capacity degradation due to charge-discharge cycles can be reduced and charge-discharge cycle performance improved.

FIG. 1 is a schematic view illustrating a protection circuit and a control circuit of a non-aqueous electrolyte secondary cell;

FIG. 2 is a schematic view illustrating a protection circuit and a control circuit for an assembled battery having a plurality of cells each being the non-aqueous electrolyte secondary cell;

FIG. 3 is a graph showing charge-discharge cycle performance in Examples 1 and 2 as well as Comparative Examples 1 to 3;

FIG. 4 is a graph illustrating X-ray diffraction patterns of the positive electrodes of Examples 1 and 2 as well as Comparative Examples 1 to 3; and

FIG. 5 is a schematic view illustrating a three-electrode beaker cell.

Hereinbelow, preferred embodiments of the present invention are described by way of examples thereof. However, the present invention is not limited to the following examples, but various changes and modifications are possible unless such changes and variations depart from the scope of the invention.

FIG. 1 is a schematic view illustrating a control circuit and a protection circuit in the case where a non-aqueous electrolyte secondary cell is used alone. A non-aqueous electrolyte secondary cell 1 is provided with a protection circuit 2 . The protection circuit 2 monitors the voltage and temperature of the non-aqueous electrolyte secondary cell 1 to prevent the non-aqueous electrolyte secondary cell 1 from being discharged excessively. A control circuit 3 is provided within the secondary cell or within an apparatus using the secondary cell. The control circuit 3 monitors the voltage of the secondary cell 1 and controls its charging and discharging by starting or ending the charge and discharge of the non-aqueous electrolyte secondary cell using a charge control switch 4 and a discharge control switch 5 .

FIG. 2 is a schematic view illustrating a protection circuit and a control circuit provided for an assembled battery 6 in which a plurality of the non-aqueous electrolyte secondary cells are combined to serve as cells. The assembled battery 6 is provided with a protection circuit 2 . The protection circuit 2 has functions to monitor the voltage and temperature of each of the cells in the assembled battery 6 and to protect the cells from overdischarging and the like. A control circuit 3 is incorporated within the assembled battery or within an apparatus using the assembled battery 6 . The control circuit 3 monitors the voltage of the assembled battery 6 and controls charge and discharge of the assembled battery 6 using a charge control switch 4 and a discharge control switch 5 . The method of controlling discharge of the assembled battery 6 using the control circuit 3 may be either a control method in which discharge of all the cells is stopped when the voltage of one of the cells in the assembled battery 6 reaches a predetermined voltage, or a control method in which, by monitoring an overall voltage of the assembled battery 6 , discharge of all the cells is stopped when the voltage reaches a predetermined voltage. In the case where the overall voltage of the assembled battery is monitored, the voltage of one of the cells may become lower than the predetermined voltage, i.e., the end-of-discharge voltage before the overall voltage of the assembled battery reaches the predetermined voltage since the performance of the cells may vary from one to another. If this happens, controlling is carried out by the protection circuit 2 so that the discharge voltage of each cell does not become less than 2.9 V. Specifically, in the case where the control circuit 3 monitors the overall voltage of the assembled battery 6 , the overdischarge-detection voltage of the protection circuit 2 for each cell is set to be equal to or higher than 2.9 V.

Control circuits useful in the present invention are known in the art and include those described in U.S. Pat. Nos. 6,396,246 and 6,492,791, which are incorporated herein by reference. Protection circuits useful in the present invention are also known in the art and include those described in U.S. Pat. Nos. 6,051,955 and 6,340,880, which are incorporated herein by reference.

EXPERIMENT 1