JP5920613B2 - Secondary battery - Google Patents

Description

  The present invention relates to a secondary battery whose charge state can be easily detected.

  Lithium secondary batteries characterized by high energy density have been used in small consumer devices such as mobile phones and notebook computers. In recent years, application to large devices such as stationary power storage systems, hybrid vehicles, and electric vehicles has been studied, and basic characteristics required for lithium secondary batteries are diversifying.

Among them, a lithium secondary battery that employs a positive electrode active material in which a two-phase coexistence type charge / discharge reaction proceeds, such as LiFePO _4, has less changes in input density and output density due to changes in charge state, and has input / output characteristics. It becomes possible to provide a stable secondary battery.

Here, not only LiFePO ₄ but also a secondary battery having a small potential change accompanying a change in the charged state may require improvement from the viewpoint of detecting the charged state from the potential change. That is, the potential change accompanying the change in the charging state during charging / discharging becomes small, and it becomes difficult to determine the charging / discharging state, abnormality occurrence, etc. from the detected battery potential.

Conventional techniques for enabling detection of potential changes accompanying changes in the state of charge include a negative electrode using soft carbon as a negative electrode active material, and an olivine-type lithium compound represented by LiFe _0.8 Mn _0.2 PO ₄ A secondary battery is disclosed in which a positive electrode having a positive electrode active material is disposed via a non-aqueous electrolyte (Patent Document 1).

  In addition, in order to realize a secondary battery that can exhibit stable output characteristics and easily detect the state of charge, charging and discharging is performed by mixing two or more types of positive electrode active materials having different lithium ion diffusion coefficients. A battery capable of adding a step to the voltage change at the time is disclosed (Patent Document 2).

  Furthermore, the positive electrode active material includes at least one of a lithium-containing transition metal composite oxide having an olivine crystal structure, a lithium-containing transition metal composite oxide having a layered crystal structure, and a lithium-containing transition metal composite oxide having a spinel crystal structure. A non-aqueous electrolyte secondary battery is disclosed (Patent Document 3).

JP 2009-104983 A JP 2010-27409 A JP 2007-250299 A

  However, although the battery disclosed in Patent Document 1 has a high charging potential in a region where the charging depth is deep as is apparent from FIGS. It cannot be used for depth estimation. Further, in the battery disclosed in Patent Document 2, the charge potential and the discharge potential are changed by changing the diffusion coefficient of lithium ions. However, it is difficult to control the diffusion coefficient to a desired size, and the diffusion coefficient Small active materials are difficult to charge during high-speed charging, and the capacity may be wasted. The battery disclosed in Patent Document 3 employs at least one of a lithium-containing transition metal composite oxide having a spinel crystal structure and a layered crystal structure, and therefore, as shown in FIG. There is a region where the accompanying potential change is large, and the output stability may not be sufficient.

  The present invention has been completed in view of the above circumstances, and an object to be solved is to provide a secondary battery having a structure different from that of the prior art and capable of detecting the state of charge from the battery potential.

The secondary battery according to claim 1 includes an active material A that is LiFe _X Mn _(1-X) PO ₄ (0 ≦ X ≦ 1), and Li ₂ Fe _Y Mn _(1-Y) SiO ₄ (0 ≦ Y <1) and a positive electrode including an active material B (excluding a combination of active material A and active material B in which active material A is LiFePO ₄ and active material B is Li ₂ MnO ₃ ),
In the active material A alone, when the charge / discharge capacity in the use potential range of the secondary battery is 100%, the voltage fluctuation in a continuous region corresponding to the capacity exceeding 80% is within 0.2V,
The active material B alone has both a charging voltage higher by 0.2 V or more and a discharging voltage lower by 0.2 V or more, based on the average voltage of the active material A in the region, and a charging voltage at a charge / discharge depth of 50% The difference from the discharge voltage is 0.5V or more,
The use potential range has a discharge end voltage in a range of 1.5 to 2.0 V and a charge end voltage in a range of 4.0 to 4.5 V,
Furthermore, the secondary battery characterized by satisfying the following (i) .
(I) The content of the active material B is ΔV (at the charging / discharging depth of 80% to 90% at the time of charging at a voltage higher than the average voltage and charging when the charging / discharging capacity within the working potential range is 100%. The absolute value of the voltage change per 1% charge / discharge capacity (the same applies hereinafter) can reach 0.0025V for the first time, and at a voltage lower than the average voltage and at the discharge depth of 15% to 25% during the discharge , ΔV This is the amount that the absolute value can reach 0.0025V for the first time.

  The secondary battery of the present invention is superior in output characteristics by combining an active material A having a region with a small potential change due to charge / discharge and an active material B having a large region, and is charged by a potential (SOC: other than SOC) Although there are terms that are strictly different from each other in terms of the state of charge of the secondary battery, such as the depth of charge, the depth of discharge, etc., in this specification, representatives of similar words that indicate the state of the secondary battery during charging and discharging In other words, the term SOC is used as an index for determining how much charge / discharge is performed during charging or discharging and how much charge / discharge can be performed later. A secondary battery that enables detection and further reduces capacity loss can be provided. In other words, it is possible to provide a secondary battery that can exhibit a stable charge / discharge potential in a wide charge state and has a large change in charge / discharge potential in a state with a margin rather than immediately before overcharge or overdischarge. it can. In addition, the SOC at which a change in charge / discharge potential occurs can be controlled.

In the secondary battery according to claim 1 , the active material A is LiFe _X Mn _(1-X) PO ₄ (0 ≦ X ≦ 1). LiFe _X Mn _(1-X) PO ₄ has a small potential change over a wide range of SOC and exhibits stable output characteristics.

In the secondary battery according to claim 1 , the active material B is Li ₂ Fe _Y Mn _(1-Y) SiO ₄ (0 ≦ Y ≦ 1). Since Li ₂ Fe _Y Mn _(1-Y) SiO ₄ has a large hysteresis of charge / discharge potential, the charge / discharge potential changes greatly before full charge occurs. Therefore, by detecting the change in the charge / discharge potential, it becomes easy to detect that the overcharge state is approached or the overdischarge state is approached. Since the voltage at (or during charging) is not significantly different from the normal discharge voltage (or charge potential), the input / output characteristics during charging and discharging can be stabilized.

The secondary battery according to claim 2 is the secondary battery according to claim 1, wherein the content of the active material B is 5 based on the sum of masses of the active material A and the active material B. It is not less than 35% by mass.

By adopting these active materials A and B and adopting this range as their abundance ratio,
It is possible to provide a secondary battery that can exhibit a stable charge / discharge potential in a wide range of SOC and that has a charge / discharge potential that changes more than before just before overcharge or overdischarge.

It is sectional drawing of the coin-type battery used in the Example. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 1. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 2. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 3. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 4. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 5. It is a figure which shows the charge / discharge depth-potential curve of the test secondary battery of Test Example 6. It is a figure explaining the detection of t1 and t2 from the charging / discharging depth-potential curve of a secondary battery.

  The secondary battery of the present invention will be described in detail below based on the embodiment. The positive electrode of the secondary battery of the present invention includes an active material A and an active material B. Moreover, a positive electrode contains another element as needed. The configuration of the negative electrode is not particularly limited. The following description is based on a lithium secondary battery. The lithium secondary battery includes a positive electrode, a negative electrode, an electrolyte interposed between the positive and negative electrodes, and other necessary members.

(Positive electrode)
When the characteristics of the active material A are evaluated singly, the potential fluctuation in the region exceeding 80% of the charge / discharge capacity within the usable potential range of the secondary battery is within 0.2V. Examples of the active material A include olivine type lithium compounds. By adopting a positive electrode active material such as LiFePO _{4 in which} a charge / discharge reaction of a two-phase coexistence proceeds, a change in input density and output density due to a change in charge state is reduced, and a secondary battery with stable input / output characteristics It becomes possible to provide. In particular, it is desirable to employ a material represented by LiFe _X Mn _(1-X) PO ₄ (0 ≦ X ≦ 1).

  The active material A and the active material B are used in such a ratio that the voltage fluctuation in the charge / discharge region, which is at least 80% of the charge / discharge capacity of the entire secondary battery, is 0.2 V or less. The ratio will be described in detail later. The charge / discharge capacity of the entire secondary battery can be determined by setting the use potential range of the secondary battery.

The active material B has a charge potential higher by 0.2 V or more and a discharge potential lower by 0.2 V or more with reference to the average potential in the above-described charge / discharge region when the characteristics are evaluated independently. The active material B desirably has a large potential hysteresis during charging and discharging. Due to the large hysteresis, even if the SOC does not reach 100% during charging, a large potential change occurs before that. On the other hand, even if the SOC does not reach 0% during discharging, a large potential change occurs before that. Will occur. That is, since a large potential change occurs in the SOC at the previous stage that adversely affects the secondary battery, the SOC control of the secondary battery can be performed better. Here, having a hysteresis in the potential during charging / discharging means that when the SOC-charging / discharging potential curve is measured for the active material B alone, the difference between the charging potential and the discharging potential at 50% SOC is 0.5 V or more. Say something. Desirable active material B can be exemplified by silicate, and particularly Li ₂ Fe _Y Mn _(1-Y) SiO ₄ (0 ≦ Y ≦ 1). Y is not particularly limited, but is preferably 0 or more and less than 1, more preferably 0.25 or more and 0.75 or less, and more preferably 0.25 or more and 0.5 or less. .

  The form of active material A and active material B, and the state of existence are not particularly limited. As the form, a powder form is usually adopted. The presence state can be mixed after powdering both, or can be arranged separately (overlapping in layers, or in different parts). In the case where the active material A and the active material B are disposed in an overlapping manner, the active material A and the active material B may be stacked in any manner as long as the mixing ratio of the active material A and the active material B becomes a set value as a whole. For example, the order of stacking may be either, the same active material may be stacked a plurality of times, or a mixture of both may be stacked (the mixing ratio does not matter). When formed in a layer form, the abundance ratio of the active material A and the active material B in the thickness direction of the layer may be changed.

  The abundance ratio between the active material A and the active material B is determined so that the potential fluctuation in the charge / discharge region described above is 0.2 V or less, but the desirable abundance ratio is that of the active material A and the active material B. The active material B is preferably present in an amount of about 5 to 35% by mass, more preferably about 15 to 30% by mass, based on the sum of masses.

  In addition, the range of the charge / discharge region can be adjusted by changing the abundance ratio between the active material A and the active material B. Hereinafter, the relationship between the abundance ratio of the active material A and the active material B and the characteristic values of the active material A and the active material B will be described.

  The following values are adopted as characteristic values. For the active material A, when calculating the electric capacity Cx (mAh / g: t1 to be described later at the time of charging, the electric capacity of the charge / discharge region and the region showing a higher potential, and t2 to be described later at the time of discharging Is the charge / discharge region and the electric capacity of the region showing a potential lower than that), and the existence ratio rx (mass standard: 0 <rx <1). For the active material B, when calculating the electric capacity Cy (mAh / g :: t1 described later at the time of charging, the electric capacity of the charging / discharging region and the region showing a potential higher than that, and t2 described later at the time of discharging are calculated. In this case, the electric capacity of the active material B in the electric potential range higher (or lower) than the average electric potential in the charging / discharging area is Cz (mAh / g). The ratio is ry (mass standard: 0 <ry <1, ry + rx = 1).

  When the capacitance of the active material B in a potential range higher than the average potential in the charge / discharge region is set as Cz, a range t1 (0 <t1 <1 is set in a range where the SOC change during charge can be detected by the potential change: For example, if t1 is 0.1, a large potential change occurs when the SOC is 90% (100% -100% × t1) or more, and the relationship between the other characteristic values can be expressed by equation (1): By solving simultaneous equations of rx = (Cz−t1 · Cy) / (Cz−t1 · Cy + t1 · Cx), Equation (2): ry = (t1 · Cx) / (Cz−t1 · Cy + t1 · Cx) Can be calculated.

  When the electric capacity of the active material B in a potential range lower than the average potential in the charge / discharge region is set as Cz, a range t2 in which the SOC change during discharge can be detected by the potential change (set in a range of 0 <t2 <1). For example, if t2 is 0.1, a large potential change occurs when the SOC becomes 10% (100% × t2) or less, and the relationship between the other characteristic values is expressed by equation (3): rx = (Cz−t2 · Cy) / (Cz−t2 · Cy + t2 · Cx), Formula (4): Calculated by solving simultaneous equations of ry = (t2 · Cx) / (Cz−t2 · Cy + t2 · Cx) can do.

  For example, when Cx = 160, Cy = 200, Cz = 140, and t1 (or t2) = 0.1, rx = (140−0.1 × 200) / (140−0.1 × 200 + 0.1 ×) 160) = 120/136 = 0.88, ry = (0.1 × 160) / (140−0.1 × 200 + 0.1 × 160) = 16/136 = 0.12 or ry = 1−rx = 0 .12 can be calculated as an abundance ratio (amount of active material A and active material B present).

  In addition, although it increases or decreases to some extent depending on what potential range is set as the charge / discharge region in the secondary battery, the capacitance (Cz) in the potential range higher (or lower) than the average potential in the active material B is the active material. If A and the active material B are determined, the values are determined almost uniquely. For this reason, it is considered that it is normal that the combination of t1 and t2 cannot be arbitrarily set. Therefore, it is desirable to set the values of t1 and t2 appropriately in consideration of both.

  As materials that can be used as the active material A and the active material B, lithium-containing transition metal oxides (transition metals such as Co, Mn, Ni, and Fe) can also be used.

  Other elements that can be included include a conductive material, a binder, and a current collector. The active materials A and B can form an active material layer formed in layers on the surface of the current collector in a state of being mixed with a conductive material, a binder, and the like. For example, the positive electrode active material, the binder, the conductive material, and the like can be formed by mixing on a current collector after mixing them in a solvent such as water or NMP.

  The conductive material is a material that exchanges electrons generated from the active material A and the active material B, and may be any material that has conductivity. Examples thereof include a carbon material and a conductive polymer material. Examples of the carbon material include ketjen black, acetylene black, carbon black, graphite, carbon nanotube, and amorphous carbon. Examples of the conductive polymer material include polyaniline, polypyrrole, polythiophene, polyacetylene, and polyacene.

  The binder is a material that forms an electrode by combining components such as the active material A and the active material B. Various polymer materials can be adopted, and those having high chemical and physical stability are desirable. Examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, ethylene propylene rubber (EPDM), styrene butadiene rubber (SBR), nitrile rubber (NBR), and fluorine rubber. Further, when a conductive polymer material is employed as the conductive material, the action of the binder can be expressed in addition to the action of the conductive material. The current collector may be a metal foil formed from a metal such as aluminum.

(Negative electrode)
Although the structure of a negative electrode is not specifically limited, It can have a suitable negative electrode active material. Depending on the type of the negative electrode active material, a binder or a current collector may be used. The binder can be the same as that described for the positive electrode. The current collector may be a metal foil formed from a metal such as copper.

  In the case of constituting a lithium secondary battery, a compound capable of inserting and extracting lithium ions can be used alone or in combination as the negative electrode active material. Examples of compounds that can occlude and release lithium ions include metal materials such as lithium, alloy materials containing silicon, tin, etc., graphite, titanium oxide, coke, organic polymer compound fired bodies, or carbon such as amorphous carbon. Materials. These active materials can be used not only alone but also as a mixture of two or more thereof.

  For example, when a lithium metal foil is used as the negative electrode active material, it can be formed by pressure bonding the lithium foil to the surface of a current collector made of a metal such as copper. When an alloy material or a carbon material is used as the negative electrode active material, the negative electrode active material and a binder are mixed in a solvent such as water or NMP, and then applied onto a current collector made of a metal such as copper. Can be formed.

(Electrolytes)
An electrolyte is a medium that transports charge carriers such as ions between a positive electrode and a negative electrode, and is not particularly limited, but is physically, chemically, and electrically stable in an atmosphere in which a lithium secondary battery is used. desirable.

For example, as the electrolyte, LiBF ₄ , LiPF ₆ , 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 An electrolytic solution in which at least one selected from ₂ ) is used as a supporting electrolyte and dissolved in an organic solvent is preferable. Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran and the like and mixtures thereof. Among them, an electrolytic solution containing a carbonate solvent is preferable because of its high stability at high temperatures. In addition, a solid polymer electrolyte containing the above electrolyte in a solid polymer such as polyethylene oxide, a polymer material having lithium ion conductivity, a solid electrolyte such as ceramic or glass can also be used.

(Other necessary components)
As other necessary members, a separator, a case, an electrode terminal, and the like are selected according to the configuration and usage of the secondary battery.

  It is desirable to interpose a separator that is a member that achieves both electrical insulation and ion conduction between the positive electrode and the negative electrode. When the electrolyte is liquid, the separator also serves to hold the liquid electrolyte. Examples of the separator include porous synthetic resin films, particularly porous films made of polyolefin polymers (polyethylene, polypropylene) and glass fibers, and nonwoven fabrics. Furthermore, it is preferable that the separator has a larger size than the positive electrode and the negative electrode for the purpose of ensuring the insulation between the positive electrode and the negative electrode.

  In general, the positive electrode, the negative electrode, the electrolyte, the separator, and the like are housed in some case. The case is not particularly limited and can be made of a known material and form. The case is provided with electrode terminals for transmitting and receiving electric power inside and outside the case.

(Charging / discharging state detection method)
As a method for detecting the SOC of the secondary battery of the present invention, the charge / discharge potential at the time of charge / discharge is measured, and the SOC of the secondary battery is estimated from the measured value. The estimation of the SOC is to detect that the SOC corresponding to the point corresponding to t1 or t2 described above has been reached by detecting the point where the slope of the charge / discharge potential-SOC curve becomes larger than a certain value (FIG. 8). reference). It can be determined as appropriate how much the inclination is to determine that t1 or t2 has been reached. For example, it can be determined that the SOC has reached t1 or t2 at a point where the slope of the charge / discharge potential-SOC curve has become larger than a certain level. For example, when the voltage change ΔV per 1% of SOC is ΔV> 0.0025V (0.2V / SOC80%), or ΔV> 0.005V as a criterion for determining whether or not it has become larger than a certain level. It can be a point or a point where ΔV continuously becomes a certain level or more in a SOC range (for example, SOC 1% or 2%). This criterion can be determined flexibly. That is, if the criterion is changed, the SOC value that is determined to have approached overcharge (or overdischarge) changes even if the charge / discharge potential-SOC curve is the same, but the object of the present invention is to overcharge or overcharge the SOC. The point is how reliably it can be detected that the discharge is approaching. Therefore, even if the SOC size that can be detected is changed by changing the determination criterion, the object of the present invention can be sufficiently achieved by controlling the secondary battery in accordance with the change in the SOC.

  As a device used to detect the charge / discharge state, a means for measuring the charge / discharge potential, a means for estimating the current rough SOC, and the slope of the charge / discharge potential-SOC curve derived from these means are constant. There should be a means for obtaining a point that becomes larger. Examples of the means for measuring the charge / discharge potential include an electrometer, and examples of the means for estimating the SOC include a current integrator and a combination of an ammeter and a time measuring means. A computer or the like can be employed as a means for obtaining a point at which the inclination increases.

Hereinafter, the secondary battery of the present invention will be described in detail based on examples. Needless to say, the present invention is not limited to the scope of the following examples.
(Creation of test secondary battery)
The positive electrode active material of each test example having the composition shown in Table 1 and acetylene black as a conductive material were uniformly mixed and suspended in N-methyl-2-pyrrolidone. Further, polyvinylidene fluoride as a binder was added to obtain a black slurry. This was applied to a current collector (aluminum foil) and vacuum dried to produce a positive electrode. In the column of “active material A” and “active material B” in Table 1, the mass ratio (%) of active material A and active material B contained in each test example is shown.

  Here, in Test Examples 1 to 3, the active material A and the active material B were mixed and suspended. For Test Example 4, active material A and active material B are separately suspended to prepare a slurry. First, a slurry containing active material A is applied and dried on a current collector, and then a slurry containing active material B. Was applied and dried. For Test Example 5, the active material A alone was suspended, and for Test Example 6, the active material B alone was suspended.

FIG. 1 shows a cross-sectional view of the coin-type battery created. The positive electrode prepared by the above method was used as the positive electrode 1, and lithium metal was used for the negative electrode 2. As the electrolyte, a non-aqueous solvent electrolyte in which LiPF ₆ was added at a concentration of 1.0 mol / L to an organic solvent obtained by mixing ethylene carbonate and diethyl carbonate was used.

  A power generating element in which these positive and negative electrodes 1 and 2 are laminated via a separator 7 (polyethylene porous membrane) is housed in a stainless steel case (consisting of a positive electrode case 4 and a negative electrode case 5). A coin-type battery was manufactured. The positive electrode case 4 and the negative electrode case 5 serve as a positive electrode terminal and a negative electrode terminal. A gasket 6 made of polypropylene is interposed between the positive electrode case 4 and the negative electrode case 5 to ensure sealing and insulation between the positive electrode case 4 and the negative electrode case 5.

  An example of a method for synthesizing the active material A and the active material B is given below. Other general methods such as the following methods, for example, a solid phase synthesis method, a sol-gel method, a coprecipitation method or a hydrothermal synthesis method can also be used for synthesis.

Synthesis Example of Active Material A Lithium Dihydrogen Phosphate LiH ₂ PO ₄ as Li Source and PO ₄ Source, Iron (II) Oxalate Dihydrate FeC ₂ O ₄ · 2H ₂ O as Fe Source, Mn Source Manganese acetate (II) Mn (CH ₃ COO) ₂ was used as a sample, and precisely weighed so that the molar ratio of Li: Fe: Mn: PO ₄ was 1: 0.25: 0.75: 1. After thoroughly mixing the raw materials with ethanol using a ball mill, the dried and crushed powder in a mortar was baked at 600 ° C. for 12 hours in an argon gas atmosphere. The obtained fired product was pulverized again with a ball mill, mixed with a polyvinyl alcohol aqueous solution, dried, and fired in an argon gas atmosphere at 600 ° C. for 12 hours. (Adjusted so that the mass ratio of olivine to carbon is 97: 3).

Synthesis Example of Active Material B Lithium hydroxide LiOH as the Li source, iron (II) oxalate dihydrate FeC ₂ O ₄ .2H ₂ O as the Fe source, manganese acetate (II) Mn (CH ₃ COO) ₂ was precisely weighed so that SiO ₂ was used as the Si source and Li: Fe: Mn: Si was in a molar ratio of 2: 0.25: 0.75: 1. After thoroughly mixing the raw materials with ethanol using a ball mill, the dried and crushed powder in a mortar was baked at 650 ° C. for 12 hours in an argon gas atmosphere. The obtained fired product was subjected to ball milling and firing at 1100 ° C. for 12 hours in an argon gas atmosphere twice. The obtained powder and acetylene black were mixed at a mass ratio of 98: 2 using a planetary ball mill. Furthermore, the mixture was baked at 600 ° C. for 12 hours in an argon gas atmosphere. The fired product was crushed in a mortar.

(Charge / discharge test)
A charge / discharge test was conducted for the working potential range shown in Table 1. The results are shown in FIG. 2 (Test Example 1) to FIG. 7 (Test Example 6). The state of the charge / discharge depth-potential curve at the time of charging / discharging was calculated | required from FIGS. The results are also shown in Table 1. The charging time in Table 1 indicates the charging depth at the point where the slope of the charging / discharging depth-potential curve when charging continues, and the discharging time indicates the charging / discharging depth when discharging continues. -Indicates the depth of charge (discharge depth) at the point where the slope of the potential curve increases. In the test secondary batteries of Test Examples 1 to 5, stable output was obtained in the SOC range (charge / discharge region) indicated by the charge and discharge. For the test secondary battery of Test Example 6, the potential change is large over the entire charging / discharging depth, and it is easy to estimate the charging / discharging depth from the potential change. A stable output with a large potential change in the entire region could not be obtained.

  The test secondary battery of Test Example 5 shows stable output characteristics in a wide range of 10% to 95%, but at the time of charging, the potential change cannot be clearly detected until 95%, and there is only 5% margin. I understood that. It was found that there was only a 10% margin at the time of discharging as in the case of charging.

  On the other hand, the test secondary batteries of Test Examples 1 to 4 have a margin of 15% to 20% (25%) both at the time of charging and at the time of discharging, and the part where the SOC of the battery is to be detected (overcharge or overdischarge) Can be detected, and the risk of overdischarge and overcharge can be reduced.

(Control of position (charge / discharge region) where slope in charge / discharge depth-potential curve increases)
Table 2 shows the result of calculating the position of the charge / discharge region based on the above-described equations (1) to (4) and the result obtained by the above test. The values actually measured for t1 and t2 (without correction) are values read from the drawings, and are read from the potential-charge / discharge depth curves in FIGS.

  Specifically, t1 is a portion from the point where the voltage change during charging increases to SOC 100%, and t2 is the portion where the voltage change during discharge increases to SOC 0%. Where the voltage change is large, the voltage change per 1% SOC is ΔV, and it is a part where ΔV> 0.0025 V (0.2 V / SOC 80%).

  “With correction” is a value calculated by subtracting the electric capacity derived from the active material A. Specifically, for the electrode containing the active material A alone, the position where the slope in the charge / discharge depth-potential curve becomes large is specified by the same method as described above (a specific example is shown in FIG. 6). The capacity after increasing the slope at the time of charging is represented by t1A, the capacity after the increasing slope at the time of discharging is represented by t2A), and the magnitude corresponding to the capacity is measured by t1 (no correction) or t2 By subtracting from (without correction), t1 (with correction) and t2 (with correction) derived from the active material B were calculated. Therefore, the value decreased by the correction varies depending on the type of the active material A.

  As is apparent from Table 2, the actually measured value and the calculated value after correction are within the fluctuation range of 10 to 20% except for the value at the time of charging in Test Example 4. That is, it is possible to realize the targeted values for t1 and t2. In Test Example 4, the difference between the calculated value at the time of charging and the actually measured value (with correction) is considered to be because the active material layer was formed by separately superposing the active material A and the active material B. .

That is, in Test Example 4, and painted on top of Li ₂ MSiO ₄ on the LiMPO _4, since Li ₂ MSiO ₄ is in the portion far from the current collector, and is likely to polarization increases during charging situation it is conceivable that. As a result, the upper limit voltage of charging is reached at an earlier stage than theoretically by the magnitude of the generated polarization. Therefore, the actual t1 is considered to be smaller than the theoretical value t1.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode 1a ... Positive electrode collector 2 ... Negative electrode 2a ... Negative electrode collector 3 ... Electrolyte solution 4 ... Positive electrode case 5 ... Negative electrode case 6 ... Gasket 7 ... Separator 10 ... Coin type battery

Claims (2)

An active material A that is LiFe _X Mn _(1-X) PO ₄ (0 ≦ X ≦ 1) and an active material B that is Li ₂ Fe _Y Mn _(1-Y) SiO ₄ (0 ≦ Y <1) A positive electrode containing (excluding the combination of active material A and active material B in which active material A is LiFePO ₄ and active material B is Li ₂ MnO ₃ ),
In the active material A alone, when the charge / discharge capacity in the use potential range of the secondary battery is 100%, the voltage fluctuation in a continuous region corresponding to the capacity exceeding 80% is within 0.2V,
The active material B alone has both a charging voltage higher by 0.2 V or more and a discharging voltage lower by 0.2 V or more, based on the average voltage of the active material A in the region, and a charging voltage at a charge / discharge depth of 50% The difference from the discharge voltage is 0.5V or more,
The use potential range has a discharge end voltage in a range of 1.5 to 2.0 V and a charge end voltage in a range of 4.0 to 4.5 V,
Furthermore, the secondary battery characterized by satisfying the following (i) .
(I) The content of the active material B is ΔV (at the charging / discharging depth of 80% to 90% at the time of charging at a voltage higher than the average voltage and charging when the charging / discharging capacity within the working potential range is 100%. The absolute value of the voltage change per 1% charge / discharge capacity (the same applies hereinafter) can reach 0.0025V for the first time, and at a voltage lower than the average voltage and at the discharge depth of 15% to 25% during the discharge , ΔV This is the amount that the absolute value can reach 0.0025V for the first time. The secondary battery according to claim 1, wherein the content is 35 wt% or less than 5 mass% before Kikatsu material A and the active material sum the active material B as a reference of mass B.

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