A method for replenishing lithium in negative electrode

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular relates to a method for replenishing negative electrode lithium, comprising the following steps: step S1, performing a charge and discharge test on a button cell containing a lithium replenisher to obtain a first spectrum; step S2, determining an addition amount of the lithium replenisher; step S3, performing a charge and discharge test on the button cell containing the lithium replenisher in the determined amount and a positive electrode active material to obtain capacity utilization in different voltage segments; step S4, comparing the capacity utilization in step S3 with the first spectrum in step S1 to obtain a second spectrum; step S5, converting and calculating the amount of active lithium required for replenishment in each cycle based on the capacity loss during each cycle; step S6, calculating the discharge cutoff voltage of the entire cell in each cycle based on the electrode potentials of the positive and negative electrode active materials; and step S7, controlling the discharge cutoff voltage of the cell according to the needs of the charge and discharge cycle to slowly release the active lithium in the negative electrode.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to a method for replenishing lithium at a negative electrode. Background Art

[0002] During the first charge and discharge process of lithium-ion batteries, the side reaction of forming the SEI film consumes the active lithium in the positive electrode active material, resulting in a decrease in the first charge and discharge efficiency of the lithium-ion battery.

[0003] To improve the initial charge and discharge efficiency of lithium-ion batteries, the industry is researching methods to replenish lithium at both the positive and negative electrodes. Because replenishing the negative electrode is technically difficult and carries high safety risks, replenishing the positive electrode is more common. This involves adding lithium-replenishing additives to the positive electrode to replace the active lithium lost during the initial charge and discharge process.

[0004] However, during the subsequent charging and discharging process, the negative electrode graphite expands and contracts, the SEI film ruptures and repairs, the positive electrode active lithium is consumed, the capacity of the lithium-ion battery decays, and the cycle life is reduced. Summary of the Invention

[0005] One of the purposes of the present invention is to address the deficiencies of the prior art and provide a method for replenishing lithium at a negative electrode, which can achieve initial lithium replenishment and can also meet the requirements of the cycle attenuation process to achieve precise lithium replenishment without the occurrence of lithium plating during the cycle.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A method for replenishing lithium at a negative electrode comprises the following steps:

[0008] Step S1: preparing a button battery containing a lithium supplement material, and performing a charge and discharge test to obtain a first spectrum of the capacity of the lithium supplement material in different voltage ranges;

[0009] Step S2: Preparing a preliminary amount of lithium supplement agent based on lithium supplementation needs;

[0010] Step S3: Mixing a predetermined amount of lithium supplement with the positive electrode active material to prepare a button-type battery containing the predetermined lithium supplement, and performing charge and discharge tests to determine the capacity utilization at different voltage ranges;

[0011] Step S4, comparing the capacity utilization in step S3 with the first spectrum in step S1 to obtain a second spectrum of the capacity utilization of the lithium supplement agent in different voltage ranges;

[0012] Step S5: obtaining the capacity loss during each cycle of the lithium-ion battery according to the charge and discharge cycles, and converting the data into the capacity loss per gram, so as to know how much active lithium needs to be replenished in each cycle;

[0013] Step S6: Obtain the electrode potentials of the positive and negative active materials at different gram capacities through the charge-discharge curves of the positive and negative electrodes, thereby calculating the discharge cut-off voltage of the full battery at each cycle;

[0014] Step S7: Mix the lithium replenishing additive with the positive electrode active material to prepare a battery, and perform charge and discharge cycles. During the charge and discharge cycles, the discharge cut-off voltage of the entire battery is controlled to release the lithium replenishing additive to replenish the active lithium lost in the first cycle, and the excess active lithium is stored in the negative electrode. The discharge cut-off voltage of the battery is controlled according to the needs of the charge and discharge cycles of the second graph so that the available active lithium in the lithium replenishing agent is slowly released from the negative electrode and the active lithium lost in each cycle is replenished in a timely manner.

[0015] The lithium replenishment requirement in step S2 is to replenish one or both of the initial charge and discharge efficiency and the cycle attenuation life.

[0016] The full battery discharge cut-off voltage V in step S7 is equal to the positive electrode potential V during discharge. 正 - Negative electrode potential during discharge V 负 .

[0017] The negative electrode potential V 负 =(capacity of 1g positive electrode active material before lithium replenishment ÷ first effect + cycle loss capacity) × N / P ratio.

[0018] The N / P ratio is between 1 and 1.5. The N / P ratio affects the effectiveness of the lithium supplementation method. Setting a certain N / P ratio within a certain range makes lithium supplementation more effective. The N / P ratio can be 1, 1.1, 1.2, 1.3, 1.4, or 1.5.

[0019] Wherein, the N / P ratio is 1.10.

[0020] The positive electrode potential V 正 =Capacity of 1g positive electrode active material before lithium replenishment ÷ first effect.

[0021] The test voltage of the charge and discharge test in step S1 is 3.65-4.45 V. The test voltage can be 3.65 V, 3.7 V, 3.8 V, 3.9 V, 4.0 V, 4.1 V, 4.2 V, 4.3 V, 4.4 V, or 4.45 V.

[0022] The number of cycles in step S7 is 100 to 500, and can be 100, 150, 200, 250, 300, 350, 400, 450, or 500.

[0023] The lithium supplement agent is one of nickel-based lithium supplement agents, iron-based lithium supplement agents, and manganese-based lithium supplement agents. Nickel-based lithium supplement agents include common Li2NiO2, containing ≥2 Li atoms, iron-based lithium supplement agents include Li5FeO4, and manganese-based lithium supplement agents include Li 1+ x Ni 0.5 Mn 1.5 O4, as well as binary lithium compound lithium supplements, including Li2O2 and Li3N.

[0024] Compared with the prior art, the beneficial effect of the present invention is that the negative electrode lithium replenishment method of the present invention can achieve the first lithium replenishment, and can also meet the cycle attenuation process to achieve precise lithium replenishment, and lithium plating will not occur during the cycle process. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the discharge curve of the positive electrode lithium iron phosphate button.

[0026] Figure 2 It is the charge and discharge curve of the negative electrode graphite button.

[0027] Figure 3 It is the positive electrode discharge curve and the negative electrode discharge curve before and after lithium replenishment. DETAILED DESCRIPTION

[0028] The present invention will be described in further detail below with reference to specific implementation methods and the accompanying drawings, but the implementation methods of the present invention are not limited thereto.

[0029] Method or steps for storing lithium at the negative electrode and slowly releasing lithium:

[0030] 1) The lithium supplement material is made into a button battery and charged and discharged at different voltages to obtain a graph of the capacity of the lithium supplement material in different voltage ranges.

[0031] 2) Based on the needs of lithium supplementation, such as whether to supplement the first effect, or to supplement the first effect and cycle attenuation at the same time, and how much lithium is needed for cycle attenuation, so as to fully utilize the capacity of the lithium supplement material at different voltages, the amount of lithium supplement added can be preliminarily determined.

[0032] 3) The lithium supplement agent, in the specified amount, is mixed with the positive electrode active material and manufactured into button cells according to the normal process. Charge and discharge are then performed at different voltages to determine the capacity utilization in these different voltage ranges. Based on the capacity of the positive electrode active material in these different voltage ranges, the capacity utilization profile of the lithium supplement agent in these different voltage ranges is further refined and accurately determined.

[0033] 4) The capacity loss during each cycle of the lithium-ion battery is obtained based on the charge and discharge cycles, and the data on the capacity loss per gram is converted to determine how much active lithium needs to be replenished in each cycle.

[0034] 5) Obtain the electrode potentials of the positive and negative active materials at different gram capacities through the charge-discharge curves of the positive and negative electrodes. Calculate the discharge cutoff voltage of the full battery at each cycle.

[0035] This method is to release all the available active lithium (i.e., irreversible gram capacity) in the lithium replenisher during the first charging process, and store the excess lithium in the negative electrode after replenishing the active lithium lost in the first effect. Then, the discharge cut-off voltage of the lithium-ion battery is controlled according to the needs of the cycle, so that the available active lithium in the lithium replenisher can be slowly released from the negative electrode and the active lithium loss in each cycle during the cycle can be replenished in time.

[0036] Another advantage of this method is that the charging voltage during the cycle can be controlled according to normal use, and the high voltage resistance of the electrolyte is not required to be high.

[0037] Specifically, taking the lithium iron phosphate plus negative electrode artificial graphite system, and lithium ferrite (Li5FeO4) as the lithium replenisher, the lithium replenishment amount is calculated based on the initial replenishment effect and the active lithium consumption after 500 cycles, as well as the charging cut-off voltage spectrum when the positive electrode stores lithium and the discharge cut-off voltage when the negative electrode slowly releases lithium.

[0038] 1. Lithium iron phosphate plus negative electrode artificial graphite system, the relevant information of the full battery before lithium replenishment is shown in Table 1 below:

[0039] Table 1

[0040]

[0041] The capacity loss during the cycle is shown in Table 2 below:

[0042] Table 2

[0043] Number of cycles Capacity retention rate Capacity loss rate 100 98.86％ 1.14％ 200 97.62％ 2.38％ 300 96.90％ 3.10％ 400 96.35％ 3.65％ 500 96.31％ 3.69％

[0044] As shown in Table 1 above, the full-cell initial capacity loss is 7.5%, corresponding to a gram capacity loss of approximately 11.35 mAh / g. The full-cell capacity loss after 500 cycles is 3.69%, corresponding to a gram capacity loss of approximately 5.12 mAh / g. To compensate for both initial and cyclic capacity losses, a total of 16.47 mAh / g of gram capacity is required.

[0045] 2. The gram capacity of lithium supplement lithium ferrite (Li5FeO4) at different voltages is shown in Table 3 below.

[0046] Table 3

[0047]

[0048] From the above, we can know that according to the gram capacity required to be supplemented and the gram capacity performance under the corresponding voltage of the lithium supplement, the amount of lithium supplement to be added can be calculated as follows.

[0049] Taking the lithium supplement agent at 4.2V as an example, its available gram capacity is 544.5mAh / g, so about 3.02% of lithium supplement agent needs to be added. Similarly, if taking the voltage at 4.45V as an example, the available gram capacity is 606.9mAh / g, then about 2.71% of lithium supplement additive needs to be added.

[0050] This article uses 4.2V as an example for calculation, at which point 3.02% lithium supplement needs to be added.

[0051] 3. Modification of the gram capacity utilization chart of lithium supplements.

[0052] 3.02% lithium supplement (LFO) and 92.48% lithium iron phosphate (LFP) were mixed to make a button battery. The battery was then charged and discharged at different voltages. Based on the gram capacity of pure lithium iron phosphate at different voltages, the available gram capacity of the lithium supplement at different voltages was inferred. The data is shown in Table 4 below:

[0053] Table 4

[0054]

[0055] As can be seen from the above, the gram capacity of the lithium supplement LFO at different voltages, calculated by reverse engineering the mixture of LFO and lithium iron phosphate (LFP), is consistent with the gram capacity of pure LFO measured by withholding electricity, with only a small difference. Therefore, the amount of LFO added meets the requirements. If the gram capacity of the lithium supplement LFO at different voltages, calculated by reverse engineering the mixture of LFO and lithium iron phosphate (LFP), is lower than the gram capacity of pure LFO measured by withholding electricity, the amount of lithium supplement LFO added should be corrected based on the gram capacity of the lithium supplement LFO at different voltages calculated by reverse engineering the mixture of LFO and lithium iron phosphate (LFP).

[0056] 4. Calculation of full battery voltage control after lithium replenishment.

[0057] (1) Combination Figure 1 The positive electrode lithium iron phosphate button discharge curve and Figure 2 The negative electrode graphite button charge and discharge curve shown:

[0058] (2) Taking 1g lithium iron phosphate as an example, the N / P ratio is designed to be 1.10, and the amount of artificial graphite required is 0.453g; when replenishing the loss of active lithium in the first effect and the loss of active lithium consumed in 500 cycles, the amount of artificial graphite required is 0.517g. According to the above (1), the positive discharge curve and the negative electrode discharge curve before and after lithium replenishment can be obtained, as shown in Figure 3 As shown:

[0059] (3) Methods for storing lithium at the negative electrode and for slow release of lithium.

[0060] During the cycle, by controlling the discharge cut-off voltage, the active lithium temporarily stored in the negative electrode is slowly released after the active lithium in the lithium replenishment is replenished for the first time, so as to replenish the active lithium consumed during the cycle.

[0061] Calculation of full battery cut-off voltage during discharge:

[0062] Full cell cut-off voltage V = positive electrode LFP potential V during discharge - negative electrode AG potential V during discharge.

[0063] Because the method of storing lithium in the negative electrode is to store the active lithium after the first effect is replenished in the negative electrode, for the positive electrode, during the cyclic charge and discharge process, the active lithium consumed and the active lithium stored in the negative electrode can be fully replenished, so the electrode potential of the positive electrode remains unchanged.

[0064] The positive LFP electrode potential V is the potential corresponding to the capacity of the lithium iron phosphate per 1g after lithium replenishment. At this point, capacity = capacity of the lithium iron phosphate per 1g after lithium replenishment = capacity of the lithium iron phosphate per 1g before lithium replenishment / initial efficiency = 140 / 92.5% = 151.35 mAh. Based on the curve of the positive LFP discharge voltage and capacity at 1g of lithium iron phosphate in (2), it can be seen that the positive electrode potential corresponding to a capacity of 151.35 mAh is 3.3027 V.

[0065] Calculated based on the active lithium lost in the first effect and the active lithium consumed in the cycle, the capacity of the negative electrode AG = (1g lithium iron phosphate capacity before lithium replenishment ÷ first effect + cycle loss capacity) × N / P ratio.

[0066] After 100 cycles, the capacity loss rate is 1.14%, corresponding to a capacity loss of approximately 1.73 mAh per 1g of lithium iron phosphate. At this time, the capacity of the negative electrode AG = (140 ÷ 92.5% + 1.73) × 1.1 = 168.39 mAh. Based on the negative electrode discharge curve data after lithium replenishment (3), the corresponding negative electrode electrode potential at this time is 0.3311 V. From this, the full battery discharge cut-off voltage V is calculated as: the positive electrode LFP electrode potential V during discharge - the negative electrode AG electrode potential V during discharge = 3.3027 - 0.3311 ≈ 2.972 V.

[0067] After 200 cycles, the capacity loss rate is 2.38%, which corresponds to a capacity loss of approximately 3.60 mAh per 1g of lithium iron phosphate. At this time, the capacity of the negative electrode AG = (140 ÷ 92.5% + 3.6) × 1.1 = 170.45 mAh, and the corresponding negative electrode AG electrode potential is 0.3927 V. From this, the full battery discharge cut-off voltage is calculated as 3.3027 - 0.3927 = 2.91 V.

[0068] Similarly, the full battery discharge cut-off voltage at 300, 400, and 500 cycles can be calculated in turn, as shown in Table 5 below.

[0069] Table 5

[0070]

[0071]

[0072] Finally, the lithium replenishing additive is mixed with the positive electrode active material to prepare a battery, and a charge and discharge cycle is performed. During the charge and discharge cycle, the discharge cut-off voltage of the whole battery is controlled to release the lithium replenishing additive to replenish the active lithium lost in the first effect, and the excess active lithium is stored in the negative electrode. The discharge cut-off voltage of the battery is controlled according to the needs of the charge and discharge cycle in Table 5, so that the available active lithium in the lithium replenishing agent is slowly released from the negative electrode and the active lithium loss in each cycle during the cycle is replenished in time.

[0073] Based on the disclosure and teachings of the above description, those skilled in the art will be able to make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments. Any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention fall within the scope of protection of the present invention. In addition, although certain specific terms are used in this description, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for replenishing lithium at a negative electrode, characterized in that: The following steps are involved: Step S1: preparing a button battery containing a lithium supplement material, and performing a charge and discharge test to obtain a first spectrum of the capacity of the lithium supplement material in different voltage ranges; Step S2: Preparing a preliminary amount of lithium supplement agent based on lithium supplementation needs; Step S3: Mixing a predetermined amount of lithium supplement with the positive electrode active material to prepare a button-type battery containing the predetermined lithium supplement, and performing charge and discharge tests to determine the capacity utilization at different voltage ranges; Step S4, comparing the capacity utilization in step S3 with the first spectrum in step S1 to obtain a second spectrum of the capacity utilization of the lithium supplement agent in different voltage ranges; Step S5: obtaining the capacity loss during each cycle of the lithium-ion battery according to the charge and discharge cycles, and converting the data into the capacity loss per gram, so as to know how much active lithium needs to be replenished in each cycle; Step S6: Obtain the electrode potentials of the positive and negative active materials at different gram capacities through the charge-discharge curves of the positive and negative electrodes, thereby calculating the discharge cut-off voltage of the full battery at each cycle; Step S7: Mix the lithium replenishing additive with the positive electrode active material to prepare a battery, and perform charge and discharge cycles. During the charge and discharge cycles, the discharge cut-off voltage of the entire battery is controlled to release the lithium replenishing additive to replenish the active lithium lost in the first cycle, and the excess active lithium is stored in the negative electrode. The discharge cut-off voltage of the battery is controlled according to the needs of the charge and discharge cycles of the second graph so that the available active lithium in the lithium replenishing agent is slowly released from the negative electrode and the active lithium lost in each cycle is replenished in a timely manner.

2. The negative electrode lithium replenishment method according to claim 1, characterized in that: The lithium replenishment requirement in step S2 is to replenish one or both of the initial charge and discharge efficiency and the cycle attenuation life.

3. The negative electrode lithium replenishment method according to claim 1, characterized in that: In step S7, the full battery discharge cut-off voltage V = the positive electrode potential V during discharge 正 - Negative electrode potential during discharge V 负 .

4. The negative electrode lithium replenishment method according to claim 3, characterized in that: The negative electrode potential V 负 =(capacity of 1g positive electrode active material before lithium replenishment ÷ first effect + cycle loss capacity) × N / P ratio.

5. The negative electrode lithium replenishment method according to claim 4, characterized in that: The N / P ratio is 1 to 1.

5.

6. The negative electrode lithium replenishment method according to claim 5, characterized in that: The N / P ratio was 1.

10.

7. The negative electrode lithium replenishment method according to claim 3, characterized in that: The positive electrode potential V 正 =Capacity of 1g positive electrode active material before lithium replenishment ÷ first effect.

8. The negative electrode lithium replenishment method according to claim 1, characterized in that: The test voltage of the charge and discharge test in step S1 is 3.65-4.45V.

9. The negative electrode lithium replenishment method according to claim 1, characterized in that: The number of cycles in step S7 is 100 to 500.

10. The negative electrode lithium replenishment method according to claim 1, characterized in that: The lithium supplement agent is one of a nickel-based lithium supplement agent, an iron-based lithium supplement agent, and a manganese-based lithium supplement agent.

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