Electrolyte for lithium metal battery and lithium metal battery

Abstract

The present invention provides an electrolyte for a lithium metal battery and a lithium metal battery thereof. The electrolyte for a lithium metal battery comprises a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises a cyclic phosphate compound as shown in structural formula I and / or a cyclic phosphate compound as shown in structural formula II. Compared with the prior art, the additive of the present invention comprises a cyclic phosphate compound as shown in structural formula I or structural formula II. This cyclic phosphate compound can form a bond with lithium metal and form a stable protective film on the surface of the lithium metal negative electrode, which is rich in LiP x O y 、Li x SiO y The organic components can significantly inhibit lithium dendrites and help inhibit the oxidative decomposition of the electrolyte under the 4.55V high voltage system, thereby improving the first coulombic efficiency and cycle performance of lithium metal batteries. #imgabs0#

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to an electrolyte for a lithium metal battery and a lithium metal battery thereof. Background Art

[0002] Since lithium metal has an excellent theoretical capacity of 3860 mAh / g and a low potential of -3.045 V relative to the standard hydrogen electrode (SHE), which enables the production of batteries with high capacity and high energy density, there has been much research on lithium metal batteries (LMBs) using lithium metal as the negative electrode active material of lithium secondary batteries.

[0003] Although lithium metal batteries are expected to become a rising star in the field of energy storage in the future, safety issues such as coulombic efficiency limitations and lithium dendrite growth in the lithium insertion-extraction process are the main challenges they face. Specifically, in the case of lithium metal batteries, lithium metal is easy to react with electrolytes, impurities and lithium salts due to its high chemical / electrochemical reactivity, and forms a solid electrolyte interphase (SEI) on the electrode surface, and such a solid electrolyte interphase causes a local current density difference, thereby forming dendrites on the surface of the lithium metal. Lithium dendrites not only shorten the life of lithium secondary batteries, but also cause short circuits and dead lithium in the battery, thereby increasing the physical and chemical instability of lithium secondary batteries, reducing the capacity of the battery, shortening the cycle life, and adversely affecting the stability of the battery. Summary of the Invention

[0004] In order to solve the above problems, the purpose of the present invention is to provide an electrolyte for a lithium metal battery and a lithium metal battery thereof, wherein the additive in the electrolyte can improve the electrochemical properties of the lithium metal battery, especially the first coulombic efficiency and cycle performance of the high-voltage lithium metal battery.

[0005] To achieve the above objectives, the first aspect of the present invention provides an electrolyte for a lithium metal battery, comprising a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises a cyclic phosphate compound as shown in structural formula I and / or a cyclic phosphate compound as shown in structural formula II.

[0006]

[0007] Compared with the prior art, the additive of the present invention includes a cyclic phosphate compound as shown in structural formula I or structural formula II. This cyclic phosphate compound can form a bond with lithium metal and form a stable protective film on the surface of the lithium metal negative electrode. The film is rich in LiP x O y 、Li x SiO yThe organic components can significantly inhibit lithium dendrites and help inhibit the oxidative decomposition of the electrolyte under the 4.55V high voltage system, thereby improving the first coulombic efficiency and cycle performance of lithium metal batteries.

[0008] Preferably, the additive accounts for 0.1-5% of the sum of the mass of the lithium salt, the non-aqueous organic solvent and the additive, and more preferably 0.5-2%. The cyclic phosphate compound may be, but is not limited to, 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5% of the sum of the mass of the lithium salt, the non-aqueous organic solvent and the additive. When its content is less than 0.1%, the SEI film cannot be uniformly formed on the surface of the lithium metal negative electrode, and thus the desired effect cannot be obtained. On the contrary, when the content exceeds 5%, unnecessary reactions may occur when the lithium metal battery is driven, which may deteriorate the performance of the lithium metal battery.

[0009] Preferably, the lithium salt accounts for 6-15% of the total mass of the lithium salt, the non-aqueous organic solvent, and the additive, and specifically may be, but is not limited to, 6%, 7%, 7.5%, 8%, 8.5%, 9%, 10%, 11%, 12%, 13%, 14%, or 15%. The lithium salt is selected from at least one of LiClO4, LiBF4, LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi, and (CF3SO2)3Cli. Preferred are (CF3SO2)2NLi (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) and (SO2F)2NLi (lithium bis(fluorosulfonyl)imide, LiFSI). Since the anion -SO2F3 of LiTFSI and the anion -SO2F of LiFSI are highly reactive with lithium, the decomposition product is mainly LiF. The high surface energy of LiF and lithium metal is conducive to inhibiting the growth of lithium dendrites. At the same time, LiF has low electronic conductivity and high electrochemical stability, which can effectively passivate the surface of the lithium metal negative electrode, thereby protecting the lithium metal negative electrode.

[0010] Preferably, the organic solvent is at least one of a linear carbonate, a cyclic carbonate, a carboxylate and a lactone. Among them, the linear carbonate can be at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (PMC) and ethyl propyl carbonate (PEC). The cyclic carbonate can be at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate and pentylene carbonate. The carboxylate can be at least one of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and propyl propionate. The lactone can be at least one of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone and ε-caprolactone.

[0011] Preferably, the composition further comprises an auxiliary agent, which accounts for 0.1-10% of the total mass of the lithium salt, non-aqueous organic solvent, additives, and auxiliary agents, and specifically may be, but is not limited to, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or 10%. Furthermore, the auxiliary agent may be at least one of ethylene sulfite (ES), fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), and diethylene sulfate (DTD).

[0012] A second aspect of the present invention provides a lithium metal battery comprising a positive electrode material, a negative electrode material, and an electrolyte, wherein the electrolyte is the aforementioned electrolyte, the negative electrode material is lithium metal or a lithium alloy, and the maximum charging voltage is 4.55V. The electrolyte additive of the lithium metal battery of the present invention includes a cyclic phosphate compound represented by Structural Formula I or Structural Formula II, which has excellent coulombic efficiency and cycle performance and is particularly suitable for lithium metal batteries in a 4.55V high-voltage system.

[0013] Preferably, the positive electrode material is lithium cobalt oxide, nickel cobalt manganese oxide or nickel cobalt aluminum oxide. Preferably, the lithium cobalt oxide material is lithium cobalt oxide modified by doping and coating, and the chemical formula of nickel cobalt manganese oxide is LiNi x Co y Mn z M (1-x-y-z) O2, the chemical formula of nickel cobalt aluminum oxide is LiNi x Co y Al z N (1-x-y-z) O2, wherein M is at least one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V and Ti, N is at least one of Mn, Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V and Ti, 0.6≤x<1, 0 <y<1,0<z<1,x+y+z≤1。 DETAILED DESCRIPTION

[0014] The following specific examples further illustrate the purpose, technical solutions and beneficial effects of the present invention, but do not constitute any limitation of the present invention. Where specific conditions are not specified in the examples, the experiments can be carried out according to conventional conditions or conditions recommended by the manufacturer. Where the manufacturer of the reagents or instruments is not specified, they are all conventional products available on the market.

[0015] Example 1

[0016] (1) Preparation of electrolyte: In a vacuum glove box with an argon atmosphere and a moisture content of <1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of EC:EMC = 3:7. Then, various additives were added, dissolved, and stirred thoroughly, and then lithium salt was added and mixed uniformly to obtain an electrolyte.

[0017] (2) Preparation of positive electrode: LiNi 0.6 Mn 0.2 Co 0.2 O2, adhesive PVDF and conductive agent SuperP are evenly mixed in a mass ratio of 95:2:3 to form a lithium metal battery positive electrode slurry with a certain viscosity. The mixed slurry is coated on both sides of the aluminum foil, dried and rolled to obtain the positive electrode sheet.

[0018] (3) Preparation of separator: Polyethylene (PE) with a thickness of about 15 μm was used as the separator.

[0019] (4) Preparation of the negative electrode: Metallic lithium is composited onto a current collector copper foil with a thickness of approximately 10 μm by physical roller pressing. The roller pressure is adjusted to coat the copper current collector with lithium on both sides. The thickness of the coated lithium is controlled to be approximately 35 μm, thus obtaining a lithium-copper composite tape negative electrode. The negative electrode is then cut and slit, and placed in a dry argon atmosphere glove box for storage.

[0020] (5) Preparation of lithium metal battery: The positive electrode, separator, and lithium copper composite strip negative electrode are stacked in order, and then stacked as needed. After the tabs are welded, they are placed in the aluminum plastic film of the battery outer packaging. The prepared electrolyte is injected into the dried bare battery cell, and vacuum packaging, static, formation (0.05C constant current charging to 3.6V, then 0.1C constant current charging to 3.9V), shaping, capacity testing and other processes are carried out in sequence to finally obtain a 1Ah soft-pack lithium metal battery.

[0021] The electrolyte formulations of Examples 2 to 10 and Comparative Examples 1 to 6 are shown in Table 1. The steps for preparing the electrolytes and manufacturing the batteries are the same as those of Example 1.

[0022] Table 1 Electrolyte components of various examples

[0023]

[0024]

[0025]

[0026] The lithium metal batteries prepared in Examples 1 to 10 and Comparative Examples 1 to 6 were subjected to initial coulombic efficiency and cycle tests, respectively. The specific test conditions are as follows. The performance test results are shown in Table 2.

[0027] First coulombic efficiency:

[0028] The lithium metal battery was placed in a high-temperature, high-pressure formation cabinet and subjected to three-step formation at 45°C and a pressure of 0.28 MPa. The first step was a 0.05C constant current for 60 minutes, and the charge capacity C1 was recorded. The second step was a 0.1C constant current for 120 minutes, and the charge capacity C2 was recorded. The third step was a 0.2C constant current for 240 minutes, and the charge capacity C3 was recorded. The upper limit voltage was 3.95V. The battery was then sealed twice using a rotary sealing machine. Then, at room temperature, a 0.5C constant current was used to charge the battery to a voltage of 4.55V, followed by a 4.55V constant voltage charge to a current of 0.05C, and then a 1C constant current discharge to a voltage of 2.8V. The first discharge capacity C0 was recorded, and the first coulombic efficiency = C0 / (C1+C2+C3)×100%.

[0029] Normal temperature cycle performance:

[0030] The lithium metal batteries of Examples 1 to 10 and Comparative Examples 1 to 6 were charged and discharged once at 0.5C / 0.5C at 25°C (the battery discharge capacity is C0) with an upper limit voltage of 4.55V, and then charged and discharged at 0.5C / 0.5C for 300 cycles at room temperature (the battery discharge capacity is C1). The capacity retention rate was calculated according to the following formula.

[0031] Capacity retention rate = (C1 / C0)*100%

[0032] Table 2 Performance test results of various lithium metal batteries

[0033]

[0034] From the results in Table 2, it can be seen that the first coulombic efficiency and cycle performance of Examples 1 to 10 are better than those of Comparative Examples 1 to 6. This is mainly because the additives of the present invention include cyclic phosphate compounds as shown in Structural Formula I or Structural Formula II. This cyclic phosphate compound can form a bond with lithium metal and form a stable protective film on the surface of the lithium metal negative electrode. The film is rich in LiP x O y 、Li x SiO yThe organic components can significantly inhibit lithium dendrites and help inhibit the oxidative decomposition of the electrolyte under the 4.55V high voltage system, thereby improving the first coulombic efficiency and cycle performance of the lithium metal battery. Comparative Examples 2 and Comparative Examples 5-6 use chain phosphate compounds, which cannot form a stable LiP-rich x O y 、Li x SiO y Protective films made of organic components are difficult to suppress lithium dendrites, so they cannot improve coulombic and cycle performance.

[0035] By comparing Examples 1 and 2, it can be seen that when the additive is a cyclic phosphate compound of structural formula II, the first coulombic efficiency and cycle performance are better, which may be because the effective component formed by the eight-membered ring cyclic phosphate compound is more stable and can better inhibit the formation of lithium dendrites.

[0036] By comparing Examples 2, 9-10 and Comparative Examples 3-4, it can be seen that when additives such as VC and PS are added to the cyclic phosphate compounds, the cycle and coulombic efficiency of the lithium metal battery are better.

[0037] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium metal battery comprising a positive electrode material, a negative electrode material and an electrolyte, characterized in that: The negative electrode material is lithium metal or lithium alloy, and the maximum charging voltage is 4.55V. The electrolyte includes a lithium salt, a non-aqueous organic solvent and an additive, wherein the additive includes a cyclic phosphate compound shown in structural formula I and / or a cyclic phosphate compound shown in structural formula II.

2. The lithium metal battery according to claim 1, wherein The additive accounts for 0.1 to 5% of the total mass of the lithium salt, the non-aqueous organic solvent and the additive.

3. The lithium metal battery according to claim 2, wherein The additive accounts for 0.5-2% of the total mass of the lithium salt, the non-aqueous organic solvent and the additive.

4. The lithium metal battery according to claim 1, wherein The lithium salt accounts for 6-15% of the total mass of the lithium salt, the non-aqueous organic solvent and the additive.

5. The lithium metal battery according to claim 1, wherein The lithium salt is selected from at least one of LiClO4, LiBF4, LiPF6, LiCF3SO3, LiCF3CO2, LiC4BO8, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, (C2F5SO2)2NLi, (SO2F)2NLi and (CF3SO2)3Cli.

6. The lithium metal battery according to claim 1, wherein The non-aqueous organic solvent is at least one of a chain carbonate, a cyclic carbonate, a carboxylate, and a lactone.

7. The lithium metal battery according to claim 1, wherein The invention also includes an auxiliary agent, which accounts for 0.1 to 10% of the total mass of the lithium salt, the non-aqueous organic solvent, the additive and the auxiliary agent.

8. The lithium metal battery according to claim 7, wherein The auxiliary agent is at least one of vinyl sulfite, fluoroethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, 1,3-propane sultone and vinyl sulfate.

9. The lithium metal battery according to claim 1, wherein The positive electrode material is lithium cobalt oxide, nickel cobalt manganese oxide or nickel cobalt aluminum oxide, and the chemical formula of the nickel cobalt manganese oxide is LiNi x Co y Mn z M (1-x-y-z) O2, the chemical formula of the nickel cobalt aluminum oxide is LiNi x Co y Al z N (1-x-y-z) O2, wherein M and N are each independently selected from at least one of Mg, Cu, Zn, Sn, B, Ga, Cr, Sr, V and Ti, 0 <x<1,0<y<1,0<z<1,x+y+z≤1。

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