High-voltage lithium ion battery electrolyte containing cluster-cage-shaped hydroboron and application of high-voltage lithium ion battery electrolyte

Abstract

The invention belongs to the technical field of battery energy storage, and particularly relates to a high-voltage lithium ion battery electrolyte containing cluster-cage-shaped hydroboron and application of the high-voltage lithium ion battery electrolyte. The electrolyte provided by the invention is based on a lithium ion battery electrolyte, and is formed by taking a cluster-cage-shaped hydroboron or a salt thereof as an additive; the cluster cage shaped hydroboron is selected from Li2B10H10, Li2B12H12, Li2CB11H12 and Li2CB9H10, the cluster cage shaped hydroboron is selected from Li2B10H10, Li2B12H12 The salt containing the cluster cage-shaped borohydride group is a lithium salt containing [B10H10] 2-, [B12H12] 2-or [CB11H12] 2-. The electrolyte can be assembled to obtain a high-voltage lithium ion battery. In the formation process of the battery, a compact CEI layer with a stable structure is formed on the surface of a positive electrode before solvolysis of cluster-cage-shaped hydroboron and fluorine-containing lithium salt; the CEI layer can prevent the continuous decomposition of the electrolyte, relieve the collapse of the positive electrode structure and prevent the escape of oxygen ions and transition metal ions. According to the invention, the cycling stability of the lithium ion battery under high-voltage operation can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery energy storage, and in particular relates to an electrolyte for a high-voltage lithium-ion battery and applications thereof. Background Art

[0002] With the development of the times, energy storage devices are required to have higher energy density. However, due to the decomposition of electrolytes, the current mainstream energy storage matrix, lithium-ion batteries, still use low-potential cathodes (typically <4.3V). Developing high-voltage cathode materials with higher energy density and electrolytes suitable for high-voltage lithium batteries is an urgent goal and challenge.

[0003] For positive electrodes such as layered oxides, as the operating potential increases, the transition metal in the positive electrode material changes valence more, and lithium ions are more inserted / extracted, thereby achieving higher capacity and energy. However, this also brings its biggest disadvantage, that is, the crystal structure of the transition metal collapses during the valence change process, and the positive electrode is transformed into a structure that cannot insert lithium ions. In addition, it is accompanied by the precipitation of transition metal ions and the release of lattice oxygen, which ultimately leads to the attenuation of positive electrode capacity and a decrease in energy density.

[0004] In the study of how to alleviate structural distortion and the precipitation of transition metal ions, the cathode-electrolyte interface (CEI) has attracted extensive attention from researchers. The CEI layer is formed by the decomposition of solutes and solvents in the electrolyte, and has the function of conducting lithium ions, blocking electrons, and preventing the continued decomposition of the electrolyte. However, the naturally formed CEI layer has disadvantages such as poor uniformity and low mechanical strength. It is difficult to effectively maintain the stability of the positive electrode structure, and it is also unable to prevent the escape of transition metal ions and lattice oxygen. At present, researchers are starting from the formation process of CEI and trying to enhance the CEI function from the electrolyte end. The specific methods are mainly divided into the selection and concentration adjustment of solutes, solvents, and additives. Among these three modification entities, additives change the CEI formation mechanism and final structure by adding a small amount of key components, which has a more economical advantage compared to the first two.

[0005] The materials used as additives need to meet the following three conditions: (1) The amount added is extremely small, usually less than 5wt%; (2) There is no side reaction with other components of the electrolyte; (3) It can significantly improve battery performance. The current mainstream high-voltage electrolyte additives are divided into unsaturated carbon, fluorinated, phosphorus-containing and boron-containing types. The mechanism of action of unsaturated carbon additives (such as VC, etc.) is to form a dense protective film on the surface of the positive electrode before the electrolyte decomposes, thereby alleviating the occurrence of decomposition reactions. Fluorinated additives improve the stability of the positive electrode by generating more LiF in CEI. The mechanism of action of phosphorus-containing additives is to capture H radicals in the electrolyte, prevent the generation of HF acid, and protect the battery components from corrosion. Boron-containing additives have a higher HOMO energy level than the electrolyte, and decompose before the electrolyte to form a protective film. In addition, B compounds can inhibit PF6 - The hydrolysis of boride reduces the LiF content in the CEI and increases its ionic conductivity. However, while reducing the LiF content improves the battery's rate performance, it hinders operation at higher potentials, affecting the battery's energy density. Maintaining both film-forming ability and high voltage stability is key to the development of next-generation boride additives. Summary of the Invention

[0006] The object of the present invention is to provide a high-voltage lithium-ion battery electrolyte and its application that can improve the battery cycle performance.

[0007] The lithium-ion battery electrolyte provided by the present invention is composed of a common lithium-ion battery electrolyte and an additive; the additive can be a cluster cage borohydride or a salt containing a cluster cage borohydride group; wherein:

[0008] The cluster cage borohydride includes but is not limited to Li2B 10 H 10 、Li2B 12 H 12 、Li2CB 11 H 12 、Li2CB9H 10 .

[0009] The salt containing cluster caged boron hydride groups is but not limited to [B 10 H 10 ] 2- 、[B 12 H 12 ] 2- 、[CB 11 H 12 ] 2- Lithium salts of isoclustered caged borohydride groups.

[0010] Specifically, in the lithium-ion battery electrolyte, the components of the solvent include but are not limited to ester solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and ether solvents such as 1,2-dimethoxyethane (DME), dioxolane (DOL), dimethyl sulfoxide (DMSO), tetrahydrofuran (THF); the components of the solute include but are not limited to lithium salts such as lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium peroxyborate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0011] In the lithium ion battery electrolyte, the amount of the additive is 0.1-10 wt % of the total mass of the lithium ion battery electrolyte.

[0012] Preparation of lithium-ion battery electrolyte: Mix the above solvent, solute and additives according to the required weight ratio and stir for 20-25 hours to ensure uniform mixing.

[0013] The electrolyte is then injected into a high-voltage lithium-ion battery and packaged to obtain a high-voltage lithium-ion battery.

[0014] The lithium-ion battery comprises a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes, but is not limited to, a layered oxide positive electrode, a spinel positive electrode, and an olivine positive electrode. The negative electrode includes, but is not limited to, lithium metal, graphite, and a silicon-carbon composite material.

[0015] In lithium-ion batteries, the ratio of electrolyte content to positive electrode capacity is 0.5-10g / Ah.

[0016] The upper limit of the operating voltage of the lithium-ion battery is 4.3-5.0V.

[0017] In the present invention, the introduction of the additive can effectively improve the film-forming uniformity of CEI and achieve uniform lithium ion conduction. In addition to the film-forming ability common to boron-containing additives, the additive has a unique decomposition mechanism: before the organic solvent decomposes (during the first charge), it decomposes together with the solvent salt to form a cathode-electrolyte interface (CEI) layer, significantly increasing the LiF content in the CEI. The CEI layer has high electrochemical / chemical stability and can effectively maintain the structural stability of the positive electrode. Lithium-ion batteries using this electrolyte additive can achieve excellent cycling performance at high operating potentials.

[0018] Compared with the prior art, the present invention has the following outstanding substantive features and significant advantages:

[0019] (1) The present invention introduces a cluster cage borohydride with a unique structure. The cluster cage borohydride includes but is not limited to [B 10 H 10] 2- 、[B 12 H 12 ] 2- 、[CB 11 H 12 ] 2- The additives include but are not limited to cluster cage borohydride single substance and a mixture of multiple cluster cage borohydrides.

[0020] (2) The additives used in the present invention can interact with other components in the electrolyte to achieve effects different from those of existing boron-based additives. In addition to forming a uniform film on the positive electrode surface, they can also increase the inorganic content in the CEI. This type of CEI layer not only has electrochemical / chemical stability, which can prevent the decomposition of the electrolyte under high voltage, but also has high mechanical strength, which can maintain the stability of the positive electrode structure and alleviate the collapse of the positive electrode structure.

[0021] (3) The additive of the present invention can effectively prolong the cycle stability of lithium-ion batteries at high operating potentials, thereby realizing lithium-ion batteries with high energy density. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Transmission electron microscopy images of the cathode material after cycling with different electrolytes, along with energy-dispersive elemental analysis data corresponding to the CEI of the cathode surface. In the figure, (a) the former uses base solution 1; (b) the former uses electrolyte 1; (c) the latter uses base solution 1; and (d) the latter uses electrolyte 1.

[0023] Figure 2 Atomic force microscopy images of the cathode material after cycling with different electrolytes, along with measured surface roughness. (a) Base solution 1; (c) Electrolyte 1; (b) Base solution 1; (d) Electrolyte 1.

[0024] Figure 3 Images of the modulus of the cathode surface after cycling with different electrolytes, measured using atomic force microscopy. (a) Base solution 1; (b) Electrolyte 1; (c) Comparison of modulus distribution. DETAILED DESCRIPTION

[0025] The present invention is further described below with reference to the accompanying drawings and examples. The present invention is not limited to the following examples.

[0026] Unless otherwise specified, the solvents and lithium salts in the examples were purchased from commercial sources.

[0027] Preparation of electrolyte base solution A: In a glove box filled with high-purity argon, weigh ethylene carbonate (EC) and dimethyl carbonate (DMC) to a mass ratio of EC:DMC = 1:1. After mixing thoroughly, add lithium hexafluorophosphate (LiPF6) at a concentration of 1 mol / L and mix thoroughly before use.

[0028] Preparation of electrolyte base solution B: In a glove box filled with high-purity argon, weigh dimethyl ether (DME) and 1,3-dioxolane (DOL) to a mass ratio of DME:DOL = 1:1. After mixing thoroughly, add 1 mol / L lithium salt lithium bis(fluorosulfonyl)imide (LiFSI) and mix thoroughly until ready for use.

[0029] Example 1, Preparation of electrolyte 1: Take a certain amount of electrolyte base liquid A, add Li2B 12 H 12 Additives, mix well, and add 1% of the base liquid mass ratio.

[0030] Example 2, Preparation of Electrolyte 2: Li2B 12 H 12 :Li2B 10 H 10 Mix well and use as additives. Take a certain amount of electrolyte base liquid A and add the above additives, the amount added is 2% of the base liquid mass ratio.

[0031] Example 3, Preparation of Electrolyte 3: Li2B 12 H 12 :Li2CB 11 H 12 Mix well and use as additives. Take a certain amount of electrolyte base liquid A and add the above additives, the amount added is 2% of the base liquid mass ratio.

[0032] Example 4, Preparation of Electrolyte 4: Take a certain amount of electrolyte base liquid B, add Li2B 12 H 12 Add the additives and mix well, the amount added is 1% of the base liquid mass ratio.

[0033] Example 5, Preparation of Electrolyte 5: Li2B 12 H 12 :Li2B 10 H 10 Mix well and use as additives. Take a certain amount of electrolyte base liquid B and add the above additives, the amount added is 2% of the base liquid mass ratio.

[0034] Example 6, Preparation of Electrolyte 6: Li2B 12 H 12:Li2CB 11 Mix H1 evenly as an additive. Take a certain amount of electrolyte base liquid B and add the above additives, the amount added is 2% of the base liquid mass ratio.

[0035] Example 7, assembly of button-type lithium-ion batteries:

[0036] Lithium-rich manganese oxide (LRM) was used as the positive electrode active material. LRM, Super P, and PVDF were mixed in a mass ratio of 95:2.5:2.5 and dissolved in NMP to prepare a slurry. The resulting slurry was coated on aluminum foil and vacuum-dried at 80°C for 12 hours. The surface density of the active material was 25 mg cm -2 After coating and drying, the film was cut into discs with a diameter of 12 mm and used as the positive electrode.

[0037] The negative electrode material used was a lithium anode with a diameter of 14 mm and a thickness of 60 μm, and the separator used a polypropylene separator with a diameter of 16 mm. CR2032 button cells were assembled using base fluid A, base fluid B, electrolyte 1, electrolyte 2, electrolyte 3, electrolyte 4, electrolyte 5, and electrolyte 6, respectively. The assembled batteries are labeled Battery A, Battery B, Battery 1, Battery 2, Battery 3, Battery 4, Battery 5, and Battery 6, respectively.

[0038] Example 8, testing of button-type lithium-ion batteries:

[0039] The LAND system was used to perform electrochemical performance tests on battery A, battery B, battery 1, battery 2, battery 3, battery 4, battery 5, and battery 6 respectively.

[0040] Specifically, a constant current charge and discharge activation was performed using a charge / discharge current density of 0.2 / 0.2C (1C=280mA / g). Subsequently, a constant current charge and discharge test was performed using a charge / discharge current density of 0.2 / 0.5C. The charge and discharge voltage window was 2.0-4.6V, and the battery was left to stand for one minute after each charge / discharge. The cycle capacity retention rate was calculated based on the first cycle discharge capacity, and the battery was considered to have failed when the capacity retention rate fell below 80% for the first time in the cycle performance test. The test results are shown in Table 1:

[0041] Table 1. Lithium-ion battery cycle test table

[0042]

[0043]

[0044] As can be seen from Table 1, the electrolyte provided by the present invention can effectively improve the cycle stability of lithium ions under a high cut-off voltage working environment.

[0045] As can be seen from Table 1, the electrolyte provided by the present invention is applicable to both ether and ester electrolytes.

[0046] Example 9, Assembly of soft-pack lithium-ion battery cells:

[0047] Lithium-rich manganese oxide (LRM) was used as the positive electrode active material. LRM, Super P, and PVDF were mixed in a mass ratio of 95:2.5:2.5 and dissolved in NMP to prepare a slurry. The resulting slurry was coated on aluminum foil and vacuum-dried at 80°C for 12 hours. The surface density of the active material was 25 mg cm -2 Cut the aluminum foil into a rectangle with a length of 12.2 cm and a width of 5.4 cm, and weld a tab on the narrow side as the positive electrode.

[0048] A 20-micron lithium foil was cut into a rectangle with a length of 12.5 cm and a width of 5.6 cm, and a tab was welded to the narrow side to serve as the negative electrode. A polypropylene separator with a width of 5.9 cm was used.

[0049] The cells are stacked in the order of "negative electrode - separator - positive electrode - separator - negative electrode". Each battery uses 3 layers of positive electrode and 4 layers of negative electrode. The stacked cells are sealed in aluminum plastic film.

[0050] Next, base solution A and electrolyte 1 were injected into the dry cells, respectively, and labeled as cell A and cell 1. The cells were sealed and left to stand for 12 hours for aging.

[0051] Example 10, Testing of Soft-Pack Lithium-Ion Batteries:

[0052] The electrochemical charge and discharge process is carried out on the Xinwei test system. First, the aged battery cell is charged and discharged at a constant current with a charge / discharge current density of 0.2 / 0.2C. Then, the battery cell is sealed under negative pressure. Figure 1 As shown. Next, constant current charge and discharge tests were performed using a charge / discharge current density of 0.2 / 0.5C. The charge and discharge voltage window was 2.0-4.6V, and each charge / discharge was followed by a 0.5-hour rest. The cycle capacity retention rate was calculated based on the first cycle discharge capacity. During the cycle performance test, the battery was considered failed when the capacity retention rate fell below 80% for the first time. The test results are shown in Table 2:

[0053] Table 2. Lithium battery cycle test table

[0054] Electrolyte type First cycle discharge capacity First-cycle Coulomb efficiency First cycle discharge energy Failure cycles Base fluid A 3.1Ah 90％ 401Wh / kg 5 Electrolyte 1 3.0Ah 83％ 392Wh / kg 62

[0055] As can be seen from Table 2, the electrolyte provided by the present invention can effectively improve the cycling stability of lithium ions under a high cut-off voltage working environment. In addition, the excellent practical value of the present invention is demonstrated.

[0056] After 10 cycles, battery A and battery 1 were dismantled and the positive electrode materials were observed using a transmission electron microscope (TEM). Figure 2 As shown in (a) and (b), the electrolyte with the additive can form a more uniform CEI layer. Energy dispersive spectroscopy (EDX) coupled with transmission electron microscopy shows that the electrolyte without the additive can only form a CEI layer with a 1.0% fluorine content. After adding the additive, the fluorine content in the CEI layer increases from 1% to 6.1%, as shown in Figure 2. Figure 2 As shown in (c) and (d).

[0057] After 10 cycles, battery A and battery 1 were dismantled and their positive electrodes were observed using an atomic force microscope (AFM). Figure 3 As shown in Figure 2, the electrolyte with additives can form a more uniform CEI layer, and the surface roughness of the positive electrode decreases from 123nm to 10.7nm after cycling. Figure 3 As shown in Figure 2, the modulus of the cathode surface after cycling also increased from 4.22 GPa to 8.77 GPa with the use of additives. This is attributed to the increase in fluorine content in CEI, which is consistent with the transmission electron microscopy results.

[0058] Combined with the above characterization, the electrolyte additive described in the present invention can form a fluoride-rich CEI layer on the surface of the positive electrode, thereby enhancing the structural strength of the CEI, maintaining the stability of the electrode during the cycle, and extending the cycle life of the lithium-ion battery at high operating voltage.

[0059] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above specific embodiments. Any improvement, adjustment, substitution, combination or simplification based on the technical spirit and basic principles of the present invention, as long as it achieves the purpose of the present invention and does not deviate from the technical core and creative concept of the present invention, shall be regarded as an equivalent replacement method and fall within the scope of protection of the present invention.

Claims

1. A lithium ion battery electrolyte, characterized in that The invention is composed of a common lithium-ion battery electrolyte and an additive; the additive is a cluster cage boron hydride compound, or a salt containing a cluster cage boron hydride group; wherein: The cluster cage borohydride is selected from Li2B 10 H 10 、Li2B 12 H 12 、Li2CB 11 H 12 、Li2CB9H 10 ; The salt containing cluster caged boron hydride groups is a salt containing [B 10 H 10 ] 2- 、[B 12 H 12 ] 2- or [CB 11 H 12 ] 2- of lithium salts.

2. The lithium-ion battery electrolyte according to claim 1, wherein In the lithium-ion battery electrolyte, the solvent is selected from ester solvents and ether solvents; the solute is selected from lithium salts; and the amount of the additive is 0.1-10 wt% of the mass of the lithium-ion battery electrolyte.

3. The lithium-ion battery electrolyte according to claim 2, wherein The ester solvent is selected from ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate; the ether solvent is selected from 1,2-dimethoxyethane, dioxolane, dimethyl sulfoxide, and tetrahydrofuran; the solute is a lithium salt selected from lithium hexafluorophosphate, lithium tetrafluoroborate, lithium peroxyborate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

4. A lithium-ion battery, characterized in that: include: A positive electrode, a negative electrode and a lithium-ion battery electrolyte as described in claim 1, 2 or 3; wherein the positive electrode is a layered oxide positive electrode, a spinel positive electrode or an olivine positive electrode; and the negative electrode is lithium metal, graphite or a silicon-carbon composite material.

5. The lithium-ion battery according to claim 4, characterized in that The ratio of electrolyte content to positive electrode capacity is 0.5-10 g / Ah.

6. The lithium-ion battery according to claim 5, characterized in that The operating voltage limit is 4.3-5.0 V.

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