Charging method and charging device

Abstract

To provide a charging method and a charging device that can improve the full charging capacity of a secondary battery at the end of charging.SOLUTION: A method for charging a secondary battery using a lithium compound with an olivine crystal structure as a positive electrode active material includes a first step of charging the secondary battery to a full charge voltage, a second step of discharging the secondary battery, and a third step of charging the secondary battery to the fully charged voltage in this order, and in the second step, discharging is performed until the SOC of the secondary battery decreases to a predetermined target value within a range of 70 to 99%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a charging method and a charging device. [Background technology]

[0002] As a method for charging a secondary battery such as a lithium ion secondary battery, WO 2011 / 074169 (Patent Document 1) discloses a method for charging a secondary battery that contains composite oxide particles having an olivine crystal structure as a positive electrode active material. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2011 / 074169 Summary of the Invention [Problem to be solved by the invention]

[0004] As a positive electrode active material, for example, LFP (LiFePO 4 In a battery containing LiFePO, the Li desorption reaction (LiFePO 4 →FePO 4 ) occurs.

[0005] In batteries containing LFP (LFP cells), the positive electrode active material particles (LFP particles) that make up the positive electrode generally contain reactive particles (relatively small particles A) and less reactive particles (relatively large particles B). With conventional charging methods (for example, constant current-constant voltage charging), the Li desorption reaction is less likely to occur with these less reactive particles, so Li is not completely removed, and this amount is not used as battery capacity, resulting in a problem of reduced battery capacity.

[0006] An object of the present disclosure is to provide a charging method and charging device that can improve the full charge capacity of a secondary battery when charging is completed. [Means for solving the problem]

[0007] (1) A method for charging a secondary battery using a lithium compound having an olivine crystal structure as a positive electrode active material, comprising the steps of: a first step of charging the secondary battery to a full charge voltage; a second step of discharging the secondary battery; a third step of charging the secondary battery to the full charge voltage, In the second step, the secondary battery is discharged until its SOC falls to a predetermined target value within a range of 70 to 99%.

[0008] According to the charging method (1) above, by charging in the first step, discharging in the second step, and then charging again in the third step, it is possible to improve the full charge capacity of the secondary battery at the end of charging. This is believed to be due to the following reasons.

[0009] After the first charging step is completed, only particle B (the less reactive particle) remains unreacted and becomes LiFePO 4 In this state, the activation energy of the Li desorption reaction of particle B is high, making it difficult to further desorb Li from particle B. However, by discharging in the second step, particle A (the easily reactive particle) is converted to LiFePO 4 Back to LiFePO 4 By allowing particles A and B in this state to coexist, it is possible to reduce the activation energy in the Li desorption reaction during charging in the positive electrode material. Therefore, in the recharging step in the third step, the Li desorption reaction occurs not only in particles A but also in particles B, and LiFePO 4 The remaining particle B is treated as FePO 4 As a result, more of the less reactive particles are available for charging.

[0010] The charging method of the present disclosure, which takes into account the charge / discharge behavior specific to lithium compounds having an olivine crystal structure such as LFP, allows a larger amount of the positive electrode material to be released from Li compared to conventional methods, thereby increasing the battery capacity.

[0011] In addition, when a large amount of the positive electrode material is desorbed from the secondary battery after charging, the voltage drop after charging can be suppressed. This makes it possible to confirm from the voltage value that the battery is fully charged.

[0012] (2) The lithium compound is LiFePO 4 , LiMnPO 4 , LiCoPO 4 , LiCuPO 4 , LiNiPO 4 , LiVPO 4 and a lithium compound having an olivine crystal structure in which a part of the transition metal element in these compounds is replaced with another element.

[0013] In the charging method described in (2) above, the full charge capacity of the secondary battery at the end of charging can be improved more reliably.

[0014] (3) A charging device having a function for carrying out the charging method described in (1) or (2).

[0015] According to the charging device described in (3) above, the full charge capacity of the secondary battery at the time of completion of charging can be improved. [Brief description of the drawings]

[0016] [Figure 1] FIG. 2 is a flow diagram of a method for charging a secondary battery according to an embodiment. [Diagram 2] 5 is a diagram showing an example of a charging profile in the charging method for the secondary battery of the embodiment. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present disclosure will be described, however, the present disclosure is not limited thereto.

[0018] In this specification, "SOC (State Of Charge)" refers to the percentage of the charge capacity at that point in time relative to the fully charged capacity.

[0019] (Charging method) The method for charging a secondary battery in this embodiment is a method for charging a secondary battery that uses a lithium compound having an olivine crystal structure as a positive electrode active material.

[0020] As shown in FIG. 1, the charging method of the present embodiment includes the following steps: A first step (charging step: S10) of charging a secondary battery to a full charge voltage; A second step (discharging step: S20) of discharging the secondary battery; and a third step (recharging step: S30) of charging the secondary battery (again) to a full charge voltage.

[0021] In the above-mentioned second step, discharging is carried out until the SOC of the secondary battery decreases to a predetermined target value within a range of 70 to 99% (preferably 90 to 98%).

[0022] That is, in this embodiment, the SOC of the battery after discharge in the second step is 70 to 99%, and more preferably 90 to 98%. For example, the inventors' tests have shown that if the battery is discharged until the SOC drops to 50%, even if the battery is recharged in the third step, the effect of increasing the capacity after charging and the effect of suppressing the voltage drop after charging are hardly obtained.

[0023] The above lithium compound is LiFePO 4 , LiMnPO 4 , LiCoPO 4 , LiCuPO 4 , LiNiPO 4 , LiVPO 4and a lithium compound having an olivine crystal structure in which a part of the transition metal element in these compounds is replaced with another element, and LiFePO 4 It is more preferable that the secondary battery is a LFP. In this case, the full charge capacity of the secondary battery at the end of charging can be improved more reliably.

[0024] The "Many-particle-model" is known as one of the models that express the characteristics of lithium compounds with the olivine crystal structure of LFP. Among the several models that have been proposed, this model is considered to be the one that can correctly explain the actual phenomenon. According to this model, LiFePO 4 and FePO 4 The ratio of these phases changes depending on the charging state. In a fully discharged state, 4 It exists as.

[0025] In addition, the smaller the particle size of LFP particles, the more likely they are to react. Therefore, when the battery is charged, the Li desorption reaction (LiFePO 4 →FePO 4 In addition, the Li desorption reaction also occurs on the less reactive particles, although it occurs less frequently than on the more reactive particles.

[0026] As the battery continues to charge and approaches full charge, only the less reactive (larger) particles are released into the LiFePO 4 In this state, a high activation energy is required to promote the Li desorption reaction, causing the battery voltage to rise rapidly. This causes the battery to be deemed fully charged and charging to be terminated even if rechargeable Li-containing particles remain.

[0027] In contrast, in the charging method of the present disclosure, in the second step (discharging step) after the first step, the Li insertion reaction (FePO 4 → LifePO 4 As a result, the positive electrode again separates reactive and non-reactive particles into LiFePO 4 They will coexist in this state.

[0028] In this state, if charging is performed again, the reactive particles and the non-reactive particles will form LiFePO 4 While the two particles coexist in this state, the voltage does not rise sharply, and Li desorption reactions occur in both the reactive and non-reactive particles. As a result, more of the non-reactive particles are transported to the charged state (FePO 4 ) can be changed to

[0029] For these reasons, it is believed that the charging method disclosed herein can improve the full charge capacity of a secondary battery at the end of charging (References: Journal of the Electrochemical Society, 165, A2047 (2018), Nature Materials, 9,448 (2010)).

[0030] In the conventional charging method, the FePO 4 There was also a problem that the battery voltage would decrease after a certain period of time had passed after charging due to the presence of positive electrode active material particles that had not completely reacted.

[0031] In contrast, in the charging method of the present disclosure, the FePO 4 Since the amount of positive electrode active material particles that have not yet reacted with the positive electrode is reduced, the voltage drop after a certain time has elapsed since the completion of charging is suppressed. This makes it possible to confirm from the voltage value that the battery is fully charged.

[0032] In the first, second and third steps, the current value during charging or discharging is not particularly limited, but in order to more reliably obtain the above-mentioned effects, it is preferable that the current value be 1 C or less.

[0033] Fig. 2 is a diagram showing an example of a charging profile in the charging method for a secondary battery of this embodiment. In Fig. 2, in the third step, constant current charging is performed followed by constant voltage charging, so the current decreases halfway through. However, in the third step, constant voltage charging does not necessarily have to be performed after constant current charging, and only constant current charging may be performed. In the first step, only constant current charging may be performed, or constant current charging and constant voltage charging may be performed in combination. In the second step, only constant current discharging may be performed, or constant current discharging and constant voltage discharging may be performed in combination.

[0034] From the viewpoint of enhancing the effect of this embodiment, it is preferable to use a combination of constant current charging (or constant current discharging) and constant voltage charging (or constant voltage discharging) in at least any one of the first step, the second step, and the third step.

[0035] The second step (discharging) and the third step (recharging) may be performed once each, or the second step and the third step may be performed repeatedly. The more the number of repetitions, the greater the effect of increasing the capacity after charging and the effect of suppressing the voltage drop after charging. However, from the viewpoint of preventing the overall charging time from becoming long, the number of repetitions is preferably three or less.

[0036] (charging device) The charging device of this embodiment has the functions (control device, etc.) for implementing the above-mentioned charging method. According to the charging device of this embodiment, the full charge capacity of the secondary battery at the end of charging can be improved.

[0037] The charging device of the present embodiment may include, for example, an outlet, an external terminal, a power supply circuit unit, a charging control circuit unit, a resistor, a switch element, etc. In this case, the charging device may be connected to a power supply such as a household AC power supply via the outlet, and may be connected to an external terminal of the secondary battery via the external terminal. EXAMPLES

[0038] (Lithium-ion secondary battery under test) Capacity: 80Ah Cathode active material: LFP (LiFePO 4 ) Negative electrode active material: Carbon material Voltage when fully charged (SOC=100%) (fully charged voltage): 3.8V

[0039] <Tests 1-10> As Tests 1 to 10, the first step (charging), the second step (discharging), and the third step (recharging) were carried out under the conditions shown in Table 1 on the above-mentioned lithium ion secondary battery in a fully discharged state.

[0040] At the end of the first and third steps, the battery voltage reached the full charge voltage (3.8 V). In Table 1, the magnitude of the time rate of current during charging or discharging is represented by the symbol "C." A current of 1C will discharge the rated capacity of the battery in 1 hour. In Test 8, the charging was switched from constant current charging to constant voltage charging in the first and third steps when the voltage reached 3.8 V. The discharging was switched from constant current discharging to constant voltage discharging in the second step when the SOC reached 90%.

[0041] After each test, the battery capacity was measured immediately after the end of step 3, and the battery voltage was measured one hour after the end of step 3. The measurement results are shown in Table 1.

[0042] [Table 1]

[0043] The results shown in Table 1 show that in Test 1, in which the second and third steps were not performed, and Test 2, in which the first, second and third steps were performed but the SOC after discharge in the second step (achieved SOC) was 50%, Tests 3 to 10, in which the first, second and third steps were performed (achieved SOC: in the range of 70 to 99%) and the third step were performed, showed a higher final battery capacity after charging and a less significant decrease in voltage after charging.

[0044] In addition, in Tests 9 and 10, in which the second and third steps were repeated, it is believed that the effect of increasing the battery capacity after charging and suppressing the voltage drop after charging was further enhanced.

[0045] The embodiments and examples disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is not limited to the above description, but to the claims. It is intended that all modifications within the meaning and scope of the appended claims be embraced therein.

Claims

1. A method for charging a secondary battery using a lithium compound having an olivine crystal structure as a positive electrode active material, comprising the steps of: a first step of charging the secondary battery to a full charge voltage; a second step of discharging the secondary battery; a third step of charging the secondary battery to the full charge voltage, In the second step, the secondary battery is discharged until its SOC falls to a predetermined target value within a range of 70 to 99%.

2. The lithium compound is LiFePO ４ , LiMnPO ４ , LiCoPO ４ , LiCuPO ４ , LiNiPO ４ , LiVPO ４ and a lithium compound having an olivine crystal structure in which a part of the transition metal element in these compounds is replaced with another element.

3. A charging device having a function for carrying out the charging method according to claim 1 or 2.

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