Charging method of non-aqueous electrolyte secondary battery

Abstract

The charging method is a charging method for a non-aqueous electrolyte secondary battery containing a lithium-excess type positive electrode active material. After constant current charging to a prescribed voltage V2 above a set voltage V1, constant current discharging is performed to a prescribed voltage V3, where V3 < V1 ≤ V2, and the battery capacity C1 at V1, the battery capacity C2 at V2, and the battery capacity C3 at V3 satisfy 0.99C1 ≤ C3 < C2.

Description

Technical Field

[0001] The present disclosure relates to a charging method for a non-aqueous electrolyte secondary battery, and particularly to a charging method for a non-aqueous electrolyte secondary battery including a lithium-excess type positive electrode active material. Background Art

[0002] Hitherto, as a positive electrode active material for secondary batteries such as lithium ion batteries, lithium transition metal composite oxides have been widely used, and as a secondary battery for high capacity, a lithium-excess type positive electrode active material containing a large amount of lithium has attracted attention. In Patent Document 1, in order to suppress the swelling of a battery including a lithium-excess type positive electrode active material and to achieve a good discharge capacity and a shortening of the initial charging time, a charging method of performing constant voltage charging after constant current charging is disclosed.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2019-61874 Summary of the Invention

[0006] In a positive electrode including a lithium-excess type positive electrode active material, not only transition metals but also redox of anions such as oxygen are utilized, and thus unstable oxygen (O - ) may sometimes exist near the surface. After charging to a high voltage of a certain level or more, this tendency becomes significant, and the positive electrode and the electrolyte are likely to deteriorate. Since the secondary battery is repeatedly charged and discharged, every time the charging maintains a high potential, the battery deteriorates and the durability deteriorates. The charging method disclosed in Patent Document 1 still has room for improvement in terms of the durability of the battery.

[0007] A charging method according to one aspect of the present disclosure is a charging method for a non-aqueous electrolyte secondary battery including a lithium-excess type positive electrode active material. It is characterized in that after constant current charging to a prescribed voltage V2 above a set voltage V1, constant current discharge is performed to a prescribed voltage V3, where V3 < V1 ≤ V2, and the battery capacity C1 at V1, the battery capacity C2 at V2, and the battery capacity C3 at V3 satisfy 0.99C1 ≤ C3 < C2.

[0008] By the charging method according to one aspect of the present disclosure, high capacity and high durability of the non-aqueous electrolyte secondary battery can be achieved concurrently. Brief Description of the Drawings

[0009] Figure 1 It is a configuration block diagram of a secondary battery and a charge and discharge control device in an example of an embodiment.

[0010] Figure 2 It is a diagram showing a charging method in an example of an embodiment.

[0011] Figure 3 A diagram showing a charging method as another example of an embodiment.

[0012] Figure 4 is Figure 2 A process flow chart of the shown charging method. Detailed implementation manners

[0013] First, the basic principle of this embodiment will be described. Conventionally, when charging a secondary battery, a method of performing constant current charging and stopping charging at the time point when the set voltage is reached has been used. However, when charging a secondary battery having a positive electrode containing a lithium-excess type positive electrode active material to a high voltage state where the voltage with respect to the lithium electrode reference is greater than 4V, unstable oxygen (O - ) is generated, which deteriorates the positive electrode and the electrolyte. Therefore, repeated charge and discharge leads to a decrease in battery capacity. The inventors of the present invention have conducted in-depth research on this problem and focused on the charge-discharge of a secondary battery containing a lithium-excess type positive electrode active material. Compared with other NCA (Ni-Co-Al) series positive electrode active materials, the hysteresis characteristic of the voltage (V) with respect to the battery capacity (mAh / g) is relatively large. As a result, a charging method in the following manner that can achieve both high capacity and high durability has been conceived.

[0014] A charging method as one aspect of the present disclosure is a charging method for a non-aqueous electrolyte secondary battery containing a lithium-excess type positive electrode active material. It is characterized in that after constant current charging to a specified voltage V2 above a set voltage V1, constant current discharge is performed to a specified voltage V3, where V3 < V1 ≤ V2, and the battery capacity C1 at V1, the battery capacity C2 at V2, and the battery capacity C3 at V3 satisfy 0.99C1 ≤ C3 < C2.

[0015] Next, the configuration of this embodiment will be described. Figure 1 A block diagram of the secondary battery 10 and the charge-discharge control device 12 in an example of the embodiment.

[0016] The secondary battery 10 is a lithium-ion secondary battery in which lithium ions move between the positive electrode and the negative electrode through a non-aqueous electrolyte to perform charge and discharge. It includes: a positive electrode having a positive electrode mixture layer containing a positive electrode active material, and a negative electrode having a negative electrode mixture layer containing a negative electrode active material. The outer shape of the secondary battery 10 is not particularly limited. For example, it can be cylindrical, square, coin-shaped, etc., or a battery case composed of a laminate including a metal layer and a resin layer.

[0017] The positive electrode included in the secondary battery 10 is composed of, for example, a positive electrode current collector such as a metal foil, and a positive electrode mixture layer formed on the positive electrode current collector. In the positive electrode current collector, a foil of a metal stable within the potential range of the positive electrode such as aluminum, or a thin film of the metal disposed on the surface layer can be used. The positive electrode mixture layer contains, for example, a positive electrode active material, a binder, a conductive agent, and the like. The positive electrode can be produced, for example, by coating a positive electrode mixture slurry containing a positive electrode active material, a binder, a conductive agent, etc. on the positive electrode current collector and drying it, and after forming the positive electrode mixture layer, rolling the positive electrode mixture layer.

[0018] As the positive electrode active material, a lithium-excess type positive electrode active material can be used. The lithium-excess type positive electrode active material can contain a lithium transition metal composite oxide represented by the general formula Li x Mn y Ni z Me 2-x-y-z O a F b (where 1 ≤ x ≤ 1.2, 0.4 ≤ y ≤ 0.8, 0 ≤ z ≤ 0.4, 0 < b ≤ 0.2, 1.9 ≤ a + b ≤ 2.1, and Me is at least one element selected from Co, Ti, Al, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi). For the secondary battery 10 containing the lithium-excess type positive electrode active material, the hysteresis characteristic of the voltage with respect to the battery capacity during charge and discharge is larger compared with other NCA (Ni-Co-Al) series positive electrode active materials, etc. Therefore, if the charging method which is one aspect of the present disclosure described later is used, high capacity and high durability can be achieved simultaneously.

[0019] Examples of the conductive agent include carbon-based particles such as carbon black (CB), acetylene black (AB), Ketjen black, and graphite. These can be used alone or two or more of them can be used in combination. Examples of the binder include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF), polyacrylonitrile (PAN), polyimide-based resins, acrylic-based resins, and polyolefin-based resins. These can be used alone or two or more of them can be used in combination.

[0020] The negative electrode included in the secondary battery 10 includes, for example, a negative electrode current collector such as a metal foil, and a negative electrode mixture layer formed on the negative electrode current collector. In the negative electrode current collector, a foil of a metal stable within the potential range of the negative electrode such as copper, or a thin film of the metal disposed on the surface layer can be used. The negative electrode mixture layer contains, for example, a negative electrode active material, a thickening material, a binding material, and the like. The negative electrode can be produced, for example, by coating a negative electrode mixture slurry containing a negative electrode active material, a thickening material, a binding material, etc. on the negative electrode current collector and drying it, and after forming the negative electrode mixture layer, rolling the negative electrode mixture layer.

[0021] The negative electrode active material is not particularly limited as long as it is a material that can store and release lithium ions. For example, carbon materials such as graphite, non-graphitizable carbon, graphitizable carbon, fibrous carbon, coke, and carbon black can be used. In addition, non-carbon materials such as silicon, tin, and alloys and oxides thereof can also be used as the negative electrode active material.

[0022] As a binder, for example, PTFE can be used as in the case of the positive electrode, or styrene-butadiene copolymer (SBR) or its modified body can be used. As a thickener, for example, carboxymethyl cellulose (CMC) or its modified body (including salts such as sodium salt) can also be used.

[0023] The non-aqueous electrolyte contained in secondary battery 10 includes a non-aqueous solvent and an electrolyte salt. Examples of the non-aqueous solvent (organic solvent) include carbonates, lactones, ethers, ketones, and esters, and two or more of these solvents may be mixed. Examples of the electrolyte salt include LiPF6, LiBF4, LiCF3SO3, and mixtures thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L.

[0024] Figure 1 In the present invention, the discharge and charge control device 12 measures the voltage and battery capacity of the secondary battery 10 and controls the charging and discharging of the secondary battery 10. The discharge and charge control device 12 includes a processor and a memory, and includes a measuring unit 120, a recording unit 122, and a control unit 124 as functional blocks.

[0025] The measuring unit 120 measures the voltage and battery capacity of the secondary battery 10. The measuring unit 120 may measure the time from the start of charging.

[0026] The recording unit 122 records the voltage and battery capacity of the secondary battery 10 measured by the measuring unit 120. The recording unit 122 may also measure the time from the start of charging.

[0027] The control unit 124 controls the charging and discharging of the secondary battery 10. When the voltage of the secondary battery 10 measured by the measuring unit 120 reaches a predetermined voltage, the control unit 124 stops constant current charging and starts constant current discharging. Subsequently, when the voltage reaches the predetermined voltage, the control unit 124 stops constant current discharging.

[0028] Next, a specific charging method will be described. Figure 2 A diagram showing a charging method as an example of an embodiment.

[0029] The secondary battery 10 before charging is in the state of P0 with a constant voltage and a constant battery capacity. When the constant current charging starts, from the state of P0, the battery capacity of the secondary battery 10 increases and the voltage rises, reaching the state of P1. In the state of P1, the voltage is the set voltage V1 and the battery capacity is C1. Near the set voltage V1, due to the high voltage, it is in a state where unstable oxygen (O - ) is likely to be generated.

[0030] V1 can be 4.4V or more based on the lithium electrode reference (hereinafter, sometimes the potential based on the lithium electrode reference is represented by vsLi). Thus, since the battery capacity C1 increases, a secondary battery 10 in a state with a large capacity (C3) and a reduced voltage (V3) can be obtained.

[0031] Next, after reaching the set voltage V1, constant current charging is performed up to a prescribed voltage V2 above the set voltage V1, and the secondary battery 10 reaches the state of P2. In the state of P2, the battery capacity is C2.

[0032] V2 can be 4.9V or less based on the lithium electrode reference. When V2 > 4.9V vsLi, damage remains in the positive electrode and the electrolyte, and sometimes the durability deteriorates.

[0033] Furthermore, constant current discharge is performed up to a prescribed voltage V3, and the secondary battery 10 reaches the state of P3. In the state of P3, the battery capacity is C3. According to the above relationship, V1, V2, and V3 satisfy V3 < V1 < V2. Since V3 < V1, generation of unstable oxygen (O - ) in the secondary battery 10 in the state of P3 can be suppressed compared to the secondary battery 10 in the state of P1. It is preferable to start the constant current discharge as soon as possible after the secondary battery 10 reaches the state of P2, and the time for maintaining the state of P2 can be set to 1 minute or less, preferably 30 seconds or less, more preferably 10 seconds or less.

[0034] In addition, during the charge and discharge of a secondary battery containing a lithium-excess type positive electrode active material, the voltage with respect to the battery capacity has a large hysteresis. Therefore, the battery capacity C3 of the secondary battery 10 in the state of P3 can be substantially the same as or higher than the battery capacity C1 of the secondary battery 10 in the state of P1. In other words, C1, C2, and C3 can satisfy 0.99C1 ≤ C3 < C2. Thus, the battery capacity of the secondary battery 10 at the end of charging (state of P3) can be increased and the voltage can be reduced, so the durability of the battery is improved.

[0035] C3 is preferably 90% SOC or more, and further preferably 95% SOC or more. The closer the secondary battery 10 at the end of charging (state of P3) is to full charge, the more remarkable the above effect is.

[0036] Figure 3 A diagram showing a charging method as another example of an embodiment.

[0037] The secondary battery 10 before charging is in Figure 2 Similarly, it is in the state of P0 with a constant voltage and a constant battery capacity, and is charged with a constant current to reach the state of P1 with a voltage of the set voltage V1 and a battery capacity of C1. Near the set voltage V1, due to the high voltage, it is in a state where unstable oxygen (O - ) is likely to be generated. Figure 3 In the illustrated method, different from the case of Figure 2 , a constant current discharge is performed until a specified voltage V3 instead of charging the secondary battery 10 from the set voltage V1 to V2 with a constant current, so that the secondary battery 10 reaches the state of P3. That is, V1, V2, and V3 satisfy V3 < V1 = V2. Since V3 < V1, the generation of unstable oxygen (O - ) in the secondary battery 10 in the state of P3 can be suppressed compared to the secondary battery 10 in the state of P1. Here, it can also be 4.4V vs Li ≤ V1 (V2) ≤ 4.9V vs Li.

[0038] In addition, Figure 3 the method illustrated in Figure 2 is the same as the case of

[0039] Figure 4 It is possible to make C1, C2, and C3 satisfy 0.99C1 (C2) ≤ C3 < C1 (C2). Thus, the battery capacity of the secondary battery 10 at the end of charging (state of P3) can be increased and the voltage can be reduced, so that the durability of the battery is improved. Here, C3 is preferably 90% or more of SOC, and more preferably 95% or more of SOC.

[0039] Figure 4 It is Figure 2 a flowchart of the charging method shown. <00001​​Then, while measuring the voltage and battery capacity using the measuring unit 120, the control unit 124 discharges the secondary battery 10 with a constant current to a predetermined voltage V3 (S108). Furthermore, the battery capacity C3 of the secondary battery 10 when the voltage reaches V3 is recorded in the recording unit 122 (S110). The time t3 from the start of charging may also be recorded in the recording unit 122.

[0043] Through the above steps, the secondary battery 10 in state P3 is obtained. After the secondary battery 10 in state P3 is used (discharged) and restored to the initial state P0, the above steps are repeated to obtain the secondary battery 10 in state P3. In this case, the secondary battery 10 can be charged using t1, t2, and t3 recorded by the recording unit 122 instead of the voltage and battery capacity measurements by the measuring unit 120.

[0044] for Figure 3 The charging method shown can be used with Figure 4 The same flowchart is used for processing.

[0045] <Example>

[0046] Hereinafter, the present disclosure will be further described with reference to examples, but the present disclosure is not limited to these examples.

[0047] <Example 1>

[0048] [Production of positive electrode]

[0049] Mixed with the general formula Li in a mass ratio of 92:5:3 1.1166 Mn 0.556 Ni 0.278 O 1.94 F 0.06 A positive electrode mixture slurry is prepared using the positive electrode active material, acetylene black, and polyvinylidene fluoride (PVdF) of the composition shown in the figure, using N-methyl-2-pyrrolidone (NMP) as a dispersion medium. This positive electrode mixture slurry is then applied to the surface of a positive electrode core formed of aluminum foil. The coating is dried, compressed, and then cut into a specified electrode size to produce a positive electrode having a positive electrode mixture layer formed on the positive electrode core.

[0050] [Preparation of non-aqueous electrolyte]

[0051] LiPF6 was dissolved at a concentration of 1 mol / L in a mixed solvent of fluoroethylene carbonate (FEC) and methyl 3,3,3-trifluoropropionate (FMP) at a mass ratio of 1:3 to prepare a non-aqueous electrolyte.

[0052] [Fabrication of test battery cells]

[0053] Leads were attached to the positive electrode and the lithium metal counter electrode, and the positive electrode and counter electrode were placed opposite each other with a polyolefin separator interposed therebetween to form an electrode assembly. This electrode assembly and the nonaqueous electrolyte were enclosed in an outer casing composed of an aluminum laminate film to produce a test cell. The test cell was charged at a constant current of 0.5 It at a temperature of 25°C until the cell voltage reached 3.0 V vs Li (V0). This state was used as the initial state.

[0054] [Evaluation of Energy Density Retention Rate after Cycle Test]

[0055] The following cycle test was performed on the test battery cell in the initial state. The discharge capacity of the first cycle and the discharge capacity of the 25th cycle of the cycle test were obtained, and the capacity retention rate was calculated using the following formula: the capacity retention rate was multiplied by the average voltage V during discharge. ave , calculate the energy density maintenance rate.

[0056] Capacity retention rate (%) = (25th cycle discharge capacity ÷ 1st cycle discharge capacity) × 100

[0057] Energy density retention rate (%) = capacity retention rate × average voltage V ave

[0058] <Cyclic test>

[0059] First, in an environment with a temperature of 25°C, the test battery cell in its initial state was charged with a constant current until the battery voltage reached 4.75 V vs Li (V2), which is higher than the set voltage of 4.7 V vs Li (V1). Then, the battery was discharged with a constant current of 0.5 It until the battery voltage reached 4.5 V vs Li. Thereafter, the battery was discharged with a constant current of 1 It until the battery voltage reached 3.0 V vs Li (V0). This charge and discharge cycle was repeated 25 times.

[0060] <Examples 2 to 4>

[0061] The test cell was evaluated in the same manner as in Example 1 except that V1, V2, and V3 were changed as shown in Table 1.

[0062] <Comparative Example 1>

[0063] The battery was charged at a constant current until the battery voltage reached the set voltage of 4.7 V vs Li (V1). Thereafter, the battery was discharged at a constant current of 1 It until the battery voltage reached 3.0 V vs Li (V0). This charge-discharge cycle was repeated 25 times.

[0064] <Comparative Example 2>

[0065] The test cell was evaluated in the same manner as in Comparative Example 1 except that V1 was changed to 4.6 VvsLi.

[0066] Table 1 summarizes the energy density maintenance results for the test battery cells of the Examples and Comparative Examples. The energy density maintenance rates for Examples 1 and 2 are shown relative to the energy density maintenance rate of Comparative Example 1, and the energy density maintenance rates for Examples 3 and 4 are shown relative to the energy density maintenance rate of Comparative Example 2. Table 1 also summarizes the voltages (V1, V2, V3) and battery capacities (C1, C2, C3) of the test battery cells in states P1, P2, and P3 for the Examples and Comparative Examples.

[0067] [Table 1]

[0068]

[0069] The energy density maintenance rates of the test cells of Examples 1 and 2 were higher than those of the test cell of Comparative Example 1, and the energy density maintenance rates of the test cells of Examples 3 and 4 were higher than those of the test cell of Comparative Example 2.

[0070] Description of Reference Numerals

[0071] 10 Secondary Batteries

[0072] 12. Discharge and charge control device

[0073] 120 Measurement Department

[0074] 122 Records Department

[0075] 124 Control Department

Claims

1. A charging method for a non-aqueous electrolyte secondary battery containing a lithium-excess positive electrode active material, After constant current charging to a predetermined voltage V2 which is higher than the set voltage V1, constant current discharge is performed to a predetermined voltage V3. V3 <V1≤V2, The battery capacity C1 at V1, the battery capacity C2 at V2, and the battery capacity C3 at V3 satisfy 0.99C1≤C3 <C2, The lithium-excess type positive electrode active material contains a lithium transition metal composite oxide represented by the general formula Li x Mn y Ni z Me 2-x-y-z O a F b In the formula, 1≤x≤1.2, 0.4≤y≤0.8, 0≤z≤0.4, 0<b≤0.2, 1.9≤a + b≤2.1, and Me is at least one element selected from Co, Ti, Al, Si, Sr, Nb, W, Mo, P, Ca, Mg, Sb, Na, B, V, Cr, Fe, Cu, Zn, Ge, Zr, Ru, K, and Bi.

2. The charging method according to claim 1, wherein: V1 is 4.4 V or higher based on the lithium electrode.

3. The charging method according to claim 1 or 2, wherein: V2 is 4.9 V or less based on the lithium electrode standard.

4. The charging method according to claim 1 or 2, wherein: C3 has an SOC of 90% or more.

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