Pre-lithiation reagent and application thereof

Abstract

The invention discloses a pre-lithiation reagent and application thereof. The pre-lithiation reagent provided by the invention is prepared by mixing the following raw materials: cyclodextrin, biphenyl and lithium metal, the molar ratio of the cyclodextrin to the biphenyl to the lithium metal is (1-2): 1: (0.8-1). The pre-lithiation reagent provided by the invention has relatively low reduction potential, can improve the pre-lithiation effect of the negative electrode of the lithium ion battery, and improves the first effect and the cycle life of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a pre-lithiation reagent and its application. Background Technology

[0002] Using high-specific-capacity anode materials is one of the important measures to improve the energy density of lithium-ion batteries. However, during the first charge and discharge of the battery, the formation of the SEI film on the anode consumes active lithium, and the binder groups in the positive and negative electrode sheets combine with active lithium, which will cause irreversible capacity loss. On the other hand, emerging high-specific-capacity anode materials, such as silicon-based materials, have large volume changes during lithium insertion and extraction, resulting in poor cycle performance. Therefore, researchers have adopted pre-lithiation technology to directly add extra active lithium to the anode, or to add extra active lithium to the positive electrode first, and then release the active lithium to the anode during the first charge. Based on this, pre-lithiation technology can achieve: (1) compensating for the loss of active lithium and improving the first coulombic efficiency and energy density of the battery; (2) causing the anode to expand in volume in advance, avoiding structural collapse due to large volume changes of the anode during subsequent charge and discharge cycles, and improving the cycle life of the battery.

[0003] Chemical pre-lithiation technology has attracted much attention due to its advantages such as uniform pre-lithiation and simple operation. Pre-lithiation is achieved by immersing the electrode in a solution of lithium-containing reducing agents, such as lithium-naphthalene or lithium-biphenyl compounds. This strategy is currently mainly used for the pre-lithiation of negative electrodes. However, the reported potentials of lithium-containing reducing compounds are higher than the lithium intercalation potential of the negative electrode active material; for example, the potential of lithium-naphthalene is approximately 0.37V vs. Li. + The potential of lithium-biphenyl is approximately 0.33V vs. Li. + / Li, while the lithium intercalation potential of silicon-based anodes is approximately 0.2V vs. Li. + The lithium intercalation potential of graphite is approximately 0.01-0.2V compared to Li. + / Li causes SEI to form only on the negative electrode during the pre-lithiation process, resulting in low initial efficiency and short cycle life of lithium-ion batteries. Summary of the Invention

[0004] The purpose of this invention is to provide a pre-lithiation reagent and a chemical pre-lithiation method for lithium-ion battery anode sheets. The potential of the pre-lithiation reagent is lower than the lithium intercalation potential of the anode active material, thereby improving the first efficiency and cycle life of lithium-ion batteries.

[0005] To achieve the above objectives, the following technical solution is adopted: In a first aspect, the present invention provides a pre-lithiation reagent, which is composed of the following raw materials: cyclodextrin, biphenyl, and lithium metal; wherein the molar ratio of the cyclodextrin, the biphenyl and the lithium metal is (1-2):1:(0.8-1).

[0006] In the organic solution of the present invention, cyclodextrin forms a complex with biphenyl; after the addition of lithium metal, the cyclodextrin and biphenyl dissociate first, and then the biphenyl can be reduced by lithium metal. That is, the reduction process of biphenyl is restricted, reducing the potential of the lithium-biphenyl compound, thereby obtaining a pre-lithiation reagent with low reduction potential.

[0007] The cyclodextrin is one or more selected from α-cyclodextrin, α-cyclodextrin derivatives, β-cyclodextrin, β-cyclodextrin derivatives, γ-cyclodextrin, and γ-cyclodextrin derivatives.

[0008] The hydrogen atoms at the 2-, 3-, or 4-substitution positions on the benzene ring of the biphenyl are substituted by one or more of alkyl, alkoxy, amino, and acyloxy groups.

[0009] In the pre-lithiation reagent, the concentration of biphenyl is 0.2-0.5M.

[0010] Secondly, the present invention provides a method for preparing the above-mentioned pre-lithiation reagent, comprising the following steps: S1. Under an inert atmosphere, cyclodextrin and biphenyl are dissolved in an organic solvent to obtain a cyclodextrin-biphenyl reagent; S2. Dissolve lithium metal in the cyclodextrin-biphenyl reagent to obtain a pre-lithiation reagent.

[0011] In step S1, the organic solvent is one or more of tetrahydrofuran, 2-methyltetrahydrofuran, and 2-ethyltetrahydrofuran.

[0012] Thirdly, the present invention provides a pre-lithiation method for a lithium-ion battery negative electrode sheet, wherein the negative electrode sheet is soaked in the above-mentioned pre-lithiation reagent.

[0013] The soaking time is 1.5h-3h.

[0014] Fourthly, the present invention provides a lithium-ion battery, including a negative electrode sheet; the negative electrode sheet is an electrode sheet obtained by the above-mentioned pre-lithiation reagent treatment.

[0015] The negative electrode sheet uses one or more of the following as the negative electrode active material: graphite, hard carbon, silicon, silicon oxide, and silicon carbon.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are: The cyclodextrin-biphenyl lithium reagent provided by this invention has a lower potential than that of lithium-biphenyl, and the potential of cyclodextrin-biphenyl lithium is in the lithium intercalation potential range of graphite. In the pre-lithiation process, lithium can be replenished not only by generating SEI in advance, but also by intercalating lithium in graphite in advance, which can make up for the consumption of active lithium during the first charge and discharge process of the battery. Therefore, it can significantly improve the first efficiency and cycle performance of the battery and achieve 1000 cycles without degradation. Detailed Implementation

[0017] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0018] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0019] Unless otherwise specified, all reagents, materials, instruments, etc. used in the following examples are commercially available.

[0020] Example 1 Under an argon atmosphere, α-cyclodextrin (CAS: 10016-20-3) and biphenyl (CAS: 92-52-4) were dissolved in tetrahydrofuran solvent, with concentrations of 0.5 M for cyclodextrin and 0.5 M for biphenyl, to obtain a cyclodextrin-biphenyl reagent. Lithium metal was then dissolved in the cyclodextrin-biphenyl reagent at a concentration of 0.5 M to obtain a cyclodextrin-lithium biphenyl reagent. The potential of the cyclodextrin-lithium biphenyl reagent was approximately 0.08 V vs. Li. + / Li.

[0021] The graphite anode sheet was soaked in cyclodextrin-biphenyl lithium reagent for 2 hours. After soaking, the graphite anode sheet was removed and washed with tetrahydrofuran solvent for 30 minutes. After drying at room temperature, the overlithiated graphite anode sheet was obtained.

[0022] Example 2 The difference from Example 1 is that the cyclodextrin used is γ-cyclodextrin (CAS: 17465-86-0), and the potential of cyclodextrin-lithium biphenyl is approximately 0.1V vs. Li. + / Li. The graphite anode sheet was soaked in cyclodextrin-lithium biphenyl reagent for 3 h.

[0023] Example 3 The difference from Example 1 is that the cyclodextrin used is β-cyclodextrin (CAS: 7585-39-9), and the potential of cyclodextrin-lithium biphenyl is approximately 0.05V vs. Li. + / Li. The graphite anode sheet was soaked in cyclodextrin-biphenyl lithium reagent for 1.5 h.

[0024] Example 4 The difference from Example 3 is that the concentration of biphenyl used is 0.3M, the concentration of lithium metal used is 0.27M, and the potential of cyclodextrin-lithium biphenyl is approximately 0.03V vs. Li. + / L. The graphite anode sheet was immersed in cyclodextrin-lithium biphenyl reagent for 2 hours. After immersion, the graphite anode sheet was removed and washed with tetrahydrofuran solvent for 20 minutes.

[0025] Example 5 The difference from Example 3 is that the concentration of biphenyl used is 0.3M, the concentration of cyclodextrin used is 0.6M, the concentration of lithium metal used is 0.27M, and the potential of cyclodextrin-lithium biphenyl is approximately 0.04V vs. Li. + / L. The graphite anode sheet was immersed in cyclodextrin-lithium biphenyl reagent for 2 hours. After immersion, the graphite anode sheet was removed and washed with tetrahydrofuran solvent for 20 minutes.

[0026] Example 6 The difference from Example 3 is that the concentration of biphenyl used is 0.2M, the concentration of cyclodextrin used is 0.3M, the concentration of lithium metal used is 0.16M, and the potential of cyclodextrin-lithium biphenyl is approximately 0.04V vs. Li. + / L. The graphite anode sheet was soaked in cyclodextrin-lithium biphenyl reagent for 3 hours. After soaking, the graphite anode sheet was removed and washed with tetrahydrofuran solvent for 10 minutes.

[0027] Example 7 The difference from Example 1 is that the cyclodextrin used is methyl-β-cyclodextrin (CAS: 128446-36-6), the biphenyl used is 2-methylbiphenyl (CAS: 643-58-3), and the potential of cyclodextrin-lithium biphenyl is approximately 0.05V vs. Li. + / Li. The graphite anode sheet was soaked in cyclodextrin-biphenyl lithium reagent for 1.5 h.

[0028] Example 8 The difference from Example 1 is that the cyclodextrin used is hydroxypropyl-β-cyclodextrin (CAS: 128446-35-5), the biphenyl used is 3-aminobiphenyl (CAS: 2243-47-2), and the potential of cyclodextrin-lithium biphenyl is approximately 0.05V vs. Li. + / Li. The graphite anode sheet was soaked in cyclodextrin-biphenyl lithium reagent for 1.5 h.

[0029] Example 9 The difference from Example 1 is that the cyclodextrin used is hydroxypropyl-β-cyclodextrin (CAS: 128446-35-5), the biphenyl used is 4,4′-dimethylbiphenyl (CAS: 613-33-2), and the potential of cyclodextrin-lithium biphenyl is approximately 0.04V vs. Li. + / Li. The graphite anode sheet was soaked in cyclodextrin-biphenyl lithium reagent for 1.5 h.

[0030] Comparative Example 1 Under an argon atmosphere, biphenyl (CAS: 92-52-4) was dissolved in tetrahydrofuran solvent at a concentration of 0.5 M; lithium metal was dissolved in the cyclodextrin-biphenyl reagent at a concentration of 0.5 M to obtain a lithium-biphenyl reagent. The reduction potential of lithium-biphenyl was 0.33 V vs. Li. + / Li. The graphite anode sheet was immersed in lithium-biphenyl reagent for 2 hours. After immersion, the graphite anode sheet was removed and washed with tetrahydrofuran solvent for 30 minutes. After drying at room temperature, an overlithiated graphite anode sheet was obtained.

[0031] Examples 1-9 and Comparative Example 1 were compared using pre-lithiated graphite anode sheets (material composition and weight ratio: graphite 96.5%, conductive agent Super P 0.5%, dispersant CMC 1.2%, binder SBR 1.8%, coating single-sided areal density 81.5 g / m²). 2 The compacted density is 1.6 g / cm³. 3 The lithium iron phosphate anode sheet (material composition and weight ratio: LiFePO4 96.8%, conductive agent Super P and graphene conductive paste 1.2%, binder PVDF 2%, coating single-sided areal density of 170 g / m²) 2 The compacted density is 2.40 g / cm³. 3 The pre-lithiated stacked cells are made by combining PP separator with aluminum-plastic film, and then packaged with electrolyte (composition and weight ratio of LiPF6 12.5%, VC 2.5%, EC 32.0%, PC 4.2%, EMC 48.8%) to form a pre-lithiated soft pack battery.

[0032] Comparative Example 2 Using the un-pre-lithiated graphite anode sheets from Examples 1-9 and Comparative Example 1, along with the same lithium iron phosphate cathode sheets, PP separators, and electrolytes, a comparative soft-pack battery was fabricated according to the same procedures.

[0033] Battery performance testing: The batteries of Examples 1-9 and Comparative Examples 1 and 2 were subjected to capacity testing at 25°C, and the initial coulombic efficiency of the batteries was calculated. The results are shown in Table 1. The batteries of Examples 1-9 and Comparative Examples 1 and 2 were subjected to 1000-cycle performance testing at 25°C, and the capacity retention rate of the batteries was calculated. The results are shown in Table 1.

[0034] Table 1. Initial Coulomb Efficiency and Capacity Retention

[0035] As shown in Table 1, the test results indicate that pre-lithiating the graphite anode with lithium-biphenyl and cyclodextrin-biphenyl lithium reagents before assembling it into a pre-lithiated pouch cell significantly improves the initial efficiency of the battery. Since the potential of cyclodextrin-biphenyl lithium is lower than that of lithium-biphenyl, and the potential of cyclodextrin-biphenyl lithium falls within the lithium intercalation potential range of graphite, the pre-lithiation process can compensate for lithium consumption during the first charge and discharge cycle not only by generating SEI in advance but also by pre-intercalating lithium into graphite. Therefore, the initial efficiency improvement is more significant than that of lithium-biphenyl. Furthermore, the pouch cell pre-lithiated with cyclodextrin-biphenyl lithium exhibits significantly improved cycle performance, achieving 1000 cycles without degradation, while lithium-biphenyl does not show a significant improvement in cycle performance.

[0036] Meanwhile, among the three cyclodextrins (α-cyclodextrin, β-cyclodextrin, and γ-cyclodextrin), β-cyclodextrin resulted in the lowest potential and best pre-lithiation effect for cyclodextrin-biphenyl lithium. Using cyclodextrin derivatives or auxiliary groups on biphenyl had little effect on the potential of cyclodextrin-biphenyl lithium. Increasing the molar ratio of cyclodextrin to biphenyl slightly decreased the potential of the prepared cyclodextrin-biphenyl lithium, possibly due to the better complexation effect between cyclodextrin and biphenyl. Within the 0.2-0.5 M range, reducing the concentration of the cyclodextrin-biphenyl lithium reagent could achieve the same pre-lithiation effect by extending the pre-lithiation time.

[0037] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A pre-lithiation reagent, characterized in that, It is composed of a mixture of the following raw materials: cyclodextrin, biphenyl, and lithium metal; The molar ratio of the cyclodextrin, the biphenyl, and the lithium metal is (1-2):1:(0.8-1).

2. The pre-lithiation reagent according to claim 1, characterized in that, The cyclodextrin is one or more selected from α-cyclodextrin, α-cyclodextrin derivatives, β-cyclodextrin, β-cyclodextrin derivatives, γ-cyclodextrin, and γ-cyclodextrin derivatives.

3. The pre-lithiation reagent according to claim 1 or 2, characterized in that, The hydrogen atoms at the 2-, 3-, or 4-substitution positions on the benzene ring of the biphenyl are substituted by one or more of alkyl, alkoxy, amino, and acyloxy groups.

4. The pre-lithiation reagent according to claim 3, characterized in that, In the pre-lithiation reagent, the concentration of biphenyl is 0.2-0.5M.

5. The method for preparing the pre-lithiation reagent according to any one of claims 1-4, characterized in that, Includes the following steps: S1. Under an inert atmosphere, cyclodextrin and biphenyl are dissolved in an organic solvent to obtain a cyclodextrin-biphenyl reagent; S2. Dissolve lithium metal in the cyclodextrin-biphenyl reagent to obtain a pre-lithiation reagent.

6. The preparation method according to claim 5, characterized in that, In step S1, the organic solvent is one or more of tetrahydrofuran, 2-methyltetrahydrofuran, and 2-ethyltetrahydrofuran.

7. A method for pre-lithiation of a lithium-ion battery negative electrode, characterized in that, The negative electrode sheet is soaked with the pre-lithiation reagent according to any one of claims 1-4.

8. The pre-lithiation method according to claim 7, characterized in that, The soaking time is 1.5h-3h.

9. A lithium-ion battery, characterized in that, Includes a negative electrode sheet; the negative electrode sheet is an electrode sheet obtained by pre-lithiation reagent treatment according to any one of claims 1-4.

10. The lithium-ion battery according to claim 9, characterized in that, The negative electrode sheet uses one or more of the following as the negative electrode active material: graphite, hard carbon, silicon, silicon oxide, and silicon carbon.

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