Electrolyte of wide-temperature-range lithium metal battery and lithium metal battery
By using an electrolyte with a combination of carboxylic acid esters and fluorinated vinyl carbonate with different degrees of fluorination and fluorination sites, the problem of low efficiency and safety of lithium metal batteries in low temperature environments is solved, and high efficiency and high safety at low temperatures of -30°C are achieved.
Patent Information
- Application Number
- CN202510116020.3
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-01-24
- Publication Date
- 2025-05-16
AI Technical Summary
Lithium metal batteries are not efficient in low-temperature environments, and safety needs to be improved. In addition, existing electrolytes have problems such as lithium dendrites growth, low Coulomb efficiency and safety hazards under fast charging and discharging and low temperature conditions.
The prepared battery still maintains excellent performance at -30°C using an electrolyte composed of carboxylic acid esters and fluorinated vinyl carbonate with different fluorination degrees and fluorination sites.
The high ionic conductivity, long cycle life, good energy density and high safety of lithium metal batteries at low temperatures of -30℃ are achieved, which significantly improves the low-temperature performance and safety of the batteries.
Smart Images
Figure CN120015941A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a lithium ion battery, and in particular to an electrolyte for a wide temperature range lithium metal battery and a lithium metal battery. Background Art
[0002] Lithium-ion batteries have been widely used in portable electronic devices, electric vehicles, and energy storage systems due to their high energy density, long cycle life, and no memory effect. However, with the rapid development of society, the energy density of traditional lithium-ion batteries with graphite (theoretical capacity of 372 mAh / g) as the negative electrode has approached the theoretical limit. The theoretical capacity of lithium metal is as high as 3860 mAh / g, with the lowest electrochemical reduction potential (-3.04 V, relative to the standard hydrogen electrode) and a smaller density (0.53 g / cm 3 ), and nickel-rich ternary cathode materials such as LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.83 Co 0.11 Mn 0.06 O2(NCM83) and LiNi 0.90 Co 0.05 Mn 0.05 O2 (NCM90) combined with it is expected to achieve a high energy density of more than 400Wh / kg.
[0003] The electrolyte is regarded as the "blood" of the battery. However, active lithium metal reacts strongly with conventional electrolytes to form a fragile and uneven solid electrolyte interface (SEI). This leads to uneven lithium ion transport during the plating / stripping process, triggering lithium dendrite growth and low Coulomb efficiency (CE), etc., ultimately leading to short cycle life and safety issues. These problems are further exacerbated under extreme conditions such as fast charging / discharging and low temperatures. For example, classic electrolyte solvents such as ethylene carbonate (EC) and diethyl carbonate (DEC) will crystallize and precipitate at low temperatures due to their high freezing points, limiting their application in low-temperature fields. In addition, conventional carbonate electrolytes are highly flammable and can easily cause fires or even explosions when exposed to heat, high temperatures, or when the battery is subjected to other abuse conditions, posing a great safety hazard to the battery.
[0004] The commercialization of lithium metal batteries requires advanced electrolytes that must meet several key criteria, including 1) good lithium metal compatibility and high CE values; 2) high oxidative stability; 3) sufficient ionic conductivity over a wide temperature range; 4) flame retardancy; 5) moderate lithium salt concentration and low cost. Some promising strategies, including highly concentrated electrolytes and locally highly concentrated electrolytes, have been widely reported to reduce reactivity and promote the formation of a robust SEI. However, insufficient ionic conductivity and high cost limit their application. As a promising class of candidates, fluorinated electrolytes have attracted much attention due to their ability to form high-quality fluorine-rich SEI, high oxidative stability, and flame retardancy. Various fluorinated solvents are widely used and show good performance, including carbonates, carboxylates, ethers, sulfones, and silanes.
[0005] Unfortunately, over-fluorination often reduces ionic conductivity, increases cost, and is not conducive to sustainable energy development. In particular, the effects of fluorination degree and fluorination position on electrolyte properties and battery cycling remain poorly understood. Sustainable high-energy-density lithium metal batteries urgently need cost-effective electrolytes with adjustable fluorination degree, and further exploration and breakthroughs are of great significance, especially for low-temperature and fast charge and discharge applications. Summary of the invention
[0006] The present invention aims to solve the problems that the electrolyte of lithium metal batteries is not efficient in low-temperature environments and its safety needs to be improved. An electrolyte for lithium metal batteries with a wide temperature range is provided. The electrolyte is made of carboxylates and fluoroethylene carbonates with different fluorination degrees and fluorination sites. A battery prepared by the combined electrolyte can still maintain excellent performance at -30°C.
[0007] To achieve the above object, the technical solution adopted by the present invention is:
[0008] An electrolyte for a wide temperature range lithium metal battery, comprising an electrolyte lithium salt and an organic solvent, wherein the organic solvent comprises a fluorocarboxylate and fluoroethylene carbonate (FEC);
[0009] The fluorocarboxylic acid ester includes at least one of 2-fluoro-1-ethanol acetate (FEA), 2,2-difluoroethyl acetate (DFEA), 2,2,2-trifluoroethyl acetate (TFEA), ethyl 4,4,4-trifluorobutyrate (ETFB) and 2,2,2-trifluoroethyl n-butyrate (TFEB).
[0010] The present invention finds that for carboxylic acid ester solvents, fluorination is performed from the ethyl acetate (EA) part. In terms of physical and chemical properties, the electrolyte ion conductivity decreases with the increase of the fluorination degree. For isomers, ethoxy fluorination can reduce the electrolyte conductivity more than acetyl fluorination. In terms of lithium metal compatibility, EA compatibility is poor. With the increase of fluorination degree, CE will be improved. The CE and cycle life of ethoxy fluorinated electrolyte are much higher than those of acetyl fluorinated electrolyte. In terms of high voltage stability, oxidation stability increases with the increase of fluorination degree. Finally, it is found that organic solvents composed of FEA, DFEA, TFEA, ETFB and TFEB fluorinated carboxylates and FEC, the prepared lithium metal battery has excellent performance in conductivity, battery capacity, energy density, cycle performance and other properties at low temperature.
[0011] The volume content of the fluorocarboxylate in the organic solvent is 50-95%, and the volume content of fluoroethylene carbonate is 5-50%. FEC is beneficial to construct a strong SEI rich in LiF and thus beneficial to circulation, but too much FEC will cause the viscosity of the electrolyte to be higher, and the contact angle with the diaphragm is larger, which is not conducive to the transmission of lithium ions. Preferably, the volume content of the fluorocarboxylate in the organic solvent is 70-95%, and the volume content of fluoroethylene carbonate is 5-30%, and the volume content of fluoroethylene carbonate is further preferably 5-20%, and the volume content of fluoroethylene carbonate is most preferably 10%.
[0012] Preferably, the fluorocarboxylate is at least one of 2-fluoro-1-ethanol acetate (FEA), 2,2-difluoroethyl acetate (DFEA), and 2,2,2-trifluoroethyl acetate (TFEA). Batteries prepared from electrolytes obtained from these three fluorocarboxylates have better low temperature resistance and can be used in a wider temperature range.
[0013] The electrolyte lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium perchlorate, lithium difluorooxalatoborate and lithium trifluoromethanesulfonate.
[0014] The molar concentration of the electrolyte lithium salt in the electrolyte is 0.5 to 5.0 mol / L, and more preferably 1.0 mol / L.
[0015] The applicable temperature of the electrolyte is -40°C to 40°C.
[0016] The present invention also provides the use of the electrolyte of the wide temperature range lithium metal battery in the preparation of the lithium metal battery.
[0017] The present invention also provides a lithium metal battery, comprising a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte is the electrolyte of the wide temperature range lithium metal battery.
[0018] The negative electrode is metallic lithium, and the positive electrode includes at least one of lithium iron phosphate, lithium cobaltate, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, sulfur, and sulfurized polyacrylonitrile.
[0019] The separator includes at least one of a polypropylene separator, a polyethylene separator, a glass fiber separator, a polytetrafluoroethylene separator and a cellulose separator.
[0020] Preferably, the freezing point of the electrolyte of the present invention is lower than -100°C, and the ionic conductivity is greater than 1mS / cm at a low temperature of -30°C. The electrolyte can still achieve smooth bulk lithium deposition at low temperatures, with a coulombic efficiency of 95.95%. The Li||NCM811 battery using the electrolyte can maintain 67.8% of the room temperature capacity at -30°C, and has a capacity retention rate of 87.1% after 150 cycles at -20°C.
[0021] The electrolyte of the present invention has good compatibility with lithium metal negative electrode, and the coulombic efficiency of some batteries is 98.59%. It can achieve smooth and dense lithium deposition, and the cycle life of Li||Cu half-cell can reach 400 cycles. The polarization of Li||Li symmetric battery is stable in long-term cycle. The electrolyte has high oxidation stability (>5V), good compatibility with NCM811, NCM83 and NCM90 positive electrodes, excellent cycle performance at a high voltage of 4.4V, and the Li||NCM811 soft pack battery with a capacity of 1000mAh can achieve an energy density of more than 400Wh / kg under practical conditions.
[0022] Compared with the prior art, the present invention has the following beneficial effects:
[0023] (1) The electrolyte of the present invention has a low freezing point, and the ion conductivity is greater than 1 mS / cm at a low temperature of -30°C. The electrolyte can still achieve smooth large-block lithium deposition at low temperatures, with high coulombic efficiency and good battery cyclability;
[0024] (2) The electrolyte of the present invention has good compatibility with the lithium metal negative electrode, high coulombic efficiency, can achieve smooth and dense lithium deposition, long battery cycle life, and stable polarization in long-term cycles. The electrolyte has high oxidation stability, good compatibility with a variety of positive electrodes, excellent cycle performance under high voltage, high battery capacity, and high energy density.
[0025] (3) The electrolyte described in the present invention has good flame retardancy, and the prepared battery exhibits high safety in thermal runaway tests. BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Figure 1 is the molecular structural formula of the carboxylic acid ester solvent used in the examples and comparative examples.
[0027] Figure 2 The results of lithium salt dissolution in Examples 1-6 and Comparative Examples 1-2 are shown.
[0028] Figure 3 This is a graph showing the test results of electrolyte viscosity of Examples 1-8 and Comparative Example 3.
[0029] Figure 4 The electrolyte contact angle test diagram of Examples 1-8 and Comparative Example 3.
[0030] Figure 5 This is a test diagram of the electrolyte ion conductivity of Examples 1-6 and Comparative Example 3.
[0031] Figure 6 This is a graph showing the DSC test results of the electrolytes of Examples 1-6 and Comparative Example 3.
[0032] Figure 7 The graph is a graph showing the Aurbach Coulomb efficiency test results of lithium metal for Examples 1-8.
[0033] Figure 8 This is a graph showing the low-temperature lithium metal Aurbach Coulomb efficiency test results of Example 3 and Example 5.
[0034] Fig. 9 This is the Li||Cu half-cell cycle performance diagram of Examples 1-6.
[0035] Fig.10 This is the Li||Cu half-cell cycle performance diagram of Example 9 and Example 10.
[0036] Fig.11 The SEM surface morphology and cross-sectional morphology of lithium deposition of Examples 1-6.
[0037] Fig.12 These are the SEM test results of low-temperature lithium deposition of Example 3 and Example 5.
[0038] Fig.13 This is the Li||Li symmetric battery cycle performance diagram of Examples 4-6.
[0039] Fig.14 Graph showing the test results of electrolyte oxidation stability of Examples 1-6.
[0040] Fig.15 This is the high-rate long-cycle performance diagram of the Li||NCM811 full battery of Examples 1-8.
[0041] Fig.16 This is the low-rate long-cycle performance diagram of the Li||NCM811 full battery of Example 3-6.
[0042] Fig.17This is the high-rate long-cycle performance diagram of the Li||NCM83 full battery of Examples 4-6.
[0043] Fig.18 This is the low-rate long-cycle performance diagram of the Li||NCM83 full battery of Examples 4-6.
[0044] Fig.19 This is the low-rate long-cycle performance diagram of the Li||NCM90 full battery of Examples 4-6.
[0045] Fig. 20 This is the temperature-variable cycle performance diagram of the Li||NCM811 full battery of Example 3 and Example 5.
[0046] Fig.21 This is the low temperature and long cycle performance diagram of the Li||NCM811 full battery of Example 3 and Example 5.
[0047] Fig. 22 This is the cycle performance diagram of the Li||NCM811 soft-pack battery of Example 5.
[0048] Fig.23 This is a graph showing the flammability test results of the electrolytes of Example 1 and Example 5.
[0049] Fig.24 This is a diagram showing the safety test results of the Cu||NCM811 soft-pack battery of Example 1 and Example 5. DETAILED DESCRIPTION
[0050] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with embodiment. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Those skilled in the art can make modifications or equivalent substitutions based on the understanding of the technical scheme of the present invention without departing from the spirit and scope of the technical scheme of the present invention, and all should be included in the protection scope of the present invention.
[0051] The raw materials used in the following specific embodiments are all purchased from the market. The carboxylic acid ester structure used in the examples and comparative examples is as follows: Figure 1 shown.
[0052] Example 1
[0053] Preparation of electrolyte: LiPF6 is selected as the lithium salt, EA is selected as the carboxylate, the solvent FEC and EA are evenly mixed in a volume ratio of 1:9, and then a certain amount of lithium salt LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0054] Example 2
[0055] Preparation of electrolyte: LiPF6 is selected as lithium salt, EFA is selected as carboxylate, the solvent FEC and EFA are mixed evenly in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0056] Example 3
[0057] Preparation of electrolyte: LiPF6 is selected as the lithium salt, EDFA is selected as the carboxylate, the solvent FEC and EDFA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0058] Example 4
[0059] Preparation of electrolyte: LiPF6 is selected as lithium salt, FEA is selected as carboxylate, solvent FEC and FEA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0060] Example 5
[0061] Preparation of electrolyte: LiPF6 is selected as the lithium salt, DFEA is selected as the carboxylate, the solvent FEC and DFEA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0062] Example 6
[0063] Preparation of electrolyte: LiPF6 is selected as the lithium salt, TFEA is selected as the carboxylate, the solvent FEC and TFEA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0064] Example 7
[0065] Preparation of electrolyte: LiPF6 is selected as the lithium salt, DFEA is selected as the carboxylate, the solvent FEC and DFEA are evenly mixed in a volume ratio of 2:8, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0066] Example 8
[0067] Preparation of electrolyte: LiPF6 is selected as the lithium salt, DFEA is selected as the carboxylate, the solvent FEC and DFEA are evenly mixed in a volume ratio of 3:7, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0068] Example 9
[0069] Preparation of electrolyte: LiPF6 is selected as lithium salt, ETFB is selected as carboxylate, solvent FEC and ETFB are mixed evenly in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0070] Example 10
[0071] Preparation of electrolyte: LiPF6 is selected as the lithium salt, TFEB is selected as the carboxylate, the solvent FEC and TFEB are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to completely dissolve it to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0072] Comparative Example 1
[0073] Preparation of electrolyte: LiPF6 is selected as lithium salt, ETFA is selected as carboxylate, the solvent FEC and ETFA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to control the lithium salt concentration to 1 mol / L.
[0074] Comparative Example 2
[0075] Preparation of electrolyte: LiPF6 is used as lithium salt, TTFA is used as carboxylate, the solvent FEC and TTFA are evenly mixed in a volume ratio of 1:9, and then a certain amount of LiPF6 is added and stirred to control the lithium salt concentration to 1 mol / L.
[0076] Comparative Example 3
[0077] Preparation of electrolyte: LiPF6 is selected as the lithium salt, and the solvent ethylene carbonate EC and diethyl carbonate DEC are evenly mixed in a volume ratio of 5:5, and then a certain amount of LiPF6 is added and stirred to make it completely dissolved to obtain an electrolyte with a lithium salt concentration of 1 mol / L.
[0078] Application Examples
[0079] Electrolyte viscosity test: The viscosity of the electrolyte was tested at room temperature using a rotational rheometer. ETFA and TTFA have poor ability to dissolve lithium salts, resulting in the inability to prepare electrolytes for Comparative Examples 1 and 2 ( Figure 2 ).like Figure 3 Compared with Example 1, fluorination increases the viscosity of the electrolyte, but it is still much less than that of Comparative Example 3. Compared with Example 5, when the volume ratio of FEC is increased from 10% to 20% (Example 7) or 30% (Example 8), the viscosity of the electrolyte increases significantly. Therefore, we select the volume ratio of FEC as 10% for further performance testing.
[0080] Electrolyte contact angle test: Use a video contact angle meter to test the static contact angle of the electrolyte on the polypropylene separator ( Figure 4 ). Compared with Example 5, when the volume ratio of FEC is increased from 10% to 20% (Example 7) or 30% (Example 8), the contact angle between the electrolyte and the separator also increases, indicating that the wettability is insufficient.
[0081] Electrolyte conductivity test: Use an electrochemical workstation to obtain impedance data at different temperatures and calculate the ionic conductivity of the electrolyte ( Figure 5 ). When the temperature is below 0°C, the ionic conductivity of Comparative Example 3 drops sharply, indicating that conventional carbonate electrolytes are not suitable for use in low temperature environments. In contrast, Examples 1-6 can all achieve high conductivity of >1 mS / cm at -30°C.
[0082] Electrolyte freezing point test: Use differential scanning calorimetry (DSC) to detect the endothermic / exothermic behavior of the electrolyte between room temperature and -100°C ( Figure 6 ). Comparative Example 3 shows an obvious phase change at low temperature, indicating that the sudden drop in conductivity may be caused by the solidification of the electrolyte. In contrast, Examples 1-6 do not remain in liquid state over the entire temperature range.
[0083] Lithium metal Aurbach Coulomb efficiency test: Lithium metal was used as the negative electrode, copper foil as the positive electrode, and a single-layer polypropylene film as the separator to assemble a Li||Cu half-cell. The electrolyte dosage was 40 μL and the current was 0.5 mA cm -2 The current density is 5 mAh cm -2 The area capacity of the discharge / charge cycle is 1 cycle, and then the discharge is 5 mAh cm -2 , at 1mAh cm -2 The battery was charged and discharged for 10 cycles at an area capacity of 10000mAh / cm2, and the battery voltage was finally charged to 1V. The capacity of the last charge was recorded as xmAh / cm2. 2 The coulombic efficiency calculation method is: CE = (x + 10) / 15 * 100%.
[0084] like Figure 7 , the coulombic efficiency of Example 1 and Example 2 is <90%, and Example 3 is slightly better. In comparison, the five groups of electrolytes of Example 4-6, Example 7, and Example 8 designed with ethoxy fluorination can all achieve high coulombic efficiency of >98.5%. Compared with 98.59% of Example 5, the coulombic efficiency of Example 7 and Example 8 is slightly lower but very close, at 98.57% and 98.56% respectively. In addition, Figure 8 When tested at a low temperature of -20°C, compared with Example 3, Example 5 designed with ethoxy fluorination has excellent performance, with a coulombic efficiency of up to 95.95%.
[0085] Long cycle test of lithium copper half-cell: Li metal was used as negative electrode, copper foil as positive electrode, and single-layer polypropylene film as separator to assemble Li||Cu half-cell. The electrolyte dosage was 40 μL, and the current density of charge and discharge cycle was 1 mA cm -2 , the discharge capacity is 1 mAh cm -2 , the charging cut-off voltage is 1V.
[0086] like Fig. 9 The performance of Examples 1 and 2 is extremely poor, while that of Example 3 is slightly better, but the lifespan is only about 50 cycles. In contrast, Examples 4-6 designed with ethoxy fluorination can achieve a long cycle life of about 400 cycles. Fig.10 The performance of Example 10 is also significantly better than that of Example 9, which once again proves the advantage of ethoxy fluorination for the long-term cycle of lithium metal negative electrodes.
[0087] Morphological characterization of lithium metal deposits: Scanning electron microscopy (SEM) was used to observe the lithium metal (0.5 mA cm -2 , 5mAh cm -2 ).like Fig.11 , the lithium deposits in Examples 1 and 2 have a porous structure, dendritic morphology, and a thickness greater than 50 μm. The lithium deposit in Example 3 is slightly better, with an increased size but still a porous structure, and a thickness of about 35.7 μm. In contrast, Examples 4 and 5 have dense bulk lithium deposits with a thickness of less than 30 μm, which is conducive to reversible lithium plating / stripping and stable cycling. Example 6 forms uneven "island-like" clusters surrounded by dendritic protrusions. Fig.12 Even at a temperature as low as -20°C, the lithium metal deposits in the electrolyte of Example 3 present a dendritic morphology, which is not conducive to the long-term cycle of the battery. Compared with the slender and loose lithium dendrites in Example 3, the lithium metal in Example 5 is still a smooth large-block deposition morphology.
[0088] Lithium-Li symmetric battery long cycle test: Li metal was used as the positive and negative electrodes, and a single-layer polypropylene film was used as the separator to assemble a Li||Li symmetric battery. The electrolyte dosage was 40 μL, and the current density of the charge and discharge cycle was 1 mA cm -2 , the charge and discharge capacity is 1 mAh cm -2 .like Fig.13 , Example 5 has the best cycle stability and maintains the lowest polarization voltage during a cycle of up to 800 hours, which indicates that the effect of difluorination at the ethoxy group is better than monofluorination and trifluorination.
[0089] Electrolyte oxidation stability test: Lithium metal was used as the negative electrode, NCM811 (area capacity of 2.3 mAh cm-2) as the positive electrode, and a single-layer polypropylene film as the separator to assemble a Li||NCM811 half-cell. The battery was charged at a rate of 0.1C to 4.2V and maintained for 10 hours, and then increased by 0.1V for 10 hours until it reached 5.0V. Fig.14 , Example 1, Example 2 and Example 4 with a lower degree of fluorination all exhibited significant leakage current at 4.7V, while the difluorinated Example 3 and Example 5 and the trifluorinated Example 6 had an oxidation stability as high as 5V.
[0090] Li||NCM811 full battery high rate long cycle test: thin lithium metal (45μm) as negative electrode, NCM811 (area capacity 2.3mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for one cycle at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 1C / 1C.
[0091] like Fig.15 , Example 1 and Example 2 performed extremely poorly, and the cycle life of Example 3 was less than 40 cycles. In contrast, the three groups of ethoxyfluorinated Examples 4-6 had excellent cycle performance, among which Example 5 performed best, with a capacity retention rate of up to 80.3% after 170 cycles. In addition, the cycle performance of Example 7 and Example 8 ranked second and third, respectively, with capacity retention rates of 76.2% and 71.3%, respectively. This shows that compared with several other types of carboxylic acid esters, the electrolyte of the DFEA system has excellent long-term cycle stability.
[0092] Li||NCM811 full battery low rate long cycle test: thin lithium metal (45μm) as negative electrode, NCM811 (area capacity 2.3mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for one cycle at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 0.3C / 0.5C. Fig.16 When a smaller charge and discharge rate is used, the Li||NCM811 full battery of Example 5 still has the longest cycle life, which is higher than 200 cycles.
[0093] Li||NCM83 full battery high rate long cycle test: thin lithium metal (45μm) as negative electrode, NCM83 (areal capacity of 2.7mAh cm -2) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 1C / 1C. Fig.17 , the Li||NCM83 full battery cycle performance of Example 5 is the best, with a high capacity retention rate of 82% after 140 cycles, which is much higher than that of Example 4 and Example 6, showing the advantage of difluorination.
[0094] Li||NCM83 full battery low rate long cycle test: thin lithium metal (45μm) as negative electrode, NCM83 (areal capacity of 2.7mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 0.3C / 0.5C. Fig.18 The Li||NCM83 full battery of Example 5 has the best cycling performance at a small rate, with a high capacity retention rate of 83.8% after 240 cycles, which is much higher than that of Examples 4 and 6, showing the significant advantages of difluorination.
[0095] Li||NCM90 full battery low rate long cycle test: thin lithium metal (45μm) as negative electrode, NCM90 (area capacity 5.5mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 0.2C / 0.3C. Fig.19 Even with an ultra-high-load positive electrode, the Li||NCM90 full-battery cycle performance of Example 5 is still the best, with a cycle life of more than 120 cycles, which is much higher than that of Examples 4 and 6, showing the significant advantages of difluorination.
[0096] Li||NCM811 full battery variable temperature cycle test: thin lithium metal (45μm) as the negative electrode, NCM811 (area capacity of 2.5mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was cycled 5 times at 30℃, 10℃, 0℃, -10℃, -20℃, -30℃ and 30℃ at a charge and discharge rate of 0.1C / 0.1C. Fig. 20 The capacity of the battery of Example 3 decreases significantly when cycled at 10°C, while Example 5 has a high capacity at each temperature. The discharge capacity at -30°C is 67.8% of the room temperature capacity, and the low temperature performance is excellent.
[0097] Li||NCM811 full battery low temperature long cycle test: thin lithium metal (45μm) as negative electrode, NCM811 (area capacity 2.5mAh cm -2 ) as the positive electrode and a single-layer polypropylene film as the separator to assemble a Li||NCM811 full battery, with an electrolyte dosage of 40μL. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at room temperature at a charge and discharge rate of 0.1C / 0.1C, then activated and cycled for 3 cycles at a charge and discharge rate of 0.1C / 0.1C at -20℃, and finally a long cycle test was performed at a charge and discharge rate of 0.2C / 0.2C. Fig.21 At low temperatures, the capacity of the battery in Example 3 drops sharply after 30 cycles, while Example 5 has a high capacity retention rate of up to 87.1% after 150 cycles, showing excellent low-temperature performance.
[0098] Li||NCM811 soft pack battery long cycle test: thin lithium metal (20μm) as negative electrode, NCM811 (area capacity of 3.9mAh cm -2 ) as the positive electrode and a single-layer polyethylene film as the separator to assemble a Li||NCM811 soft-pack battery with a capacity of 1000mAh and an electrolyte dosage of 2g. On a battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at a charge and discharge rate of 0.1C / 0.1C, and then a long cycle test was performed at a charge and discharge rate of 0.2C / 0.3C. Fig. 22 After 50 cycles, the capacity retention rate of the soft-pack battery in Example 5 is as high as 92.4%, and the energy density is as high as 414.8Wh / kg, which is much higher than most existing lithium-ion batteries (<300Wh / kg).
[0099] Electrolyte flammability test: Take a certain amount of electrolyte into the battery case and ignite it with an igniter. Fig.23 , Example 1 still burned violently after the igniter was removed, while Example 5 showed excellent flame retardancy.
[0100] Cu||NCM811 soft pack battery safety test: using copper foil as negative electrode, NCM811 (area capacity 3.9mAh cm -2) as the positive electrode and a single-layer polyethylene film as the separator to assemble a Cu||NCM811 soft-pack battery with a capacity of 1000mAh and an electrolyte dosage of 3g. On the battery tester (Wuhan Blue Electric Company), the battery was activated and cycled for 2 cycles at a charge and discharge rate of 0.05C / 0.05C, and then charged to 4.4V at a rate of 0.05C for thermal runaway testing. Using an accelerating calorimeter, a typical thermal wait (HWS) mode was adopted during the test, with a heating increment of 5°C and a waiting time of 25 minutes to ensure that the chamber temperature reached equilibrium. Fig.24 Compared with Example 1, the abnormal heating temperature T1 of Example 5 increased from 75°C to 145°C and was delayed by 14.5 hours, indicating that the porous lithium deposition in Example 1 greatly increased the electrode-electrolyte contact area and aggravated the side reaction. For the trigger temperature T2 of thermal runaway, the temperature values are close, but the thermal runaway of Example 5 is delayed by 3.8 hours. For the highest temperature, T3 of Example 5 is 20.6% lower than that of Example 1. In summary, Example 5 significantly improves the safety of soft-pack batteries in thermal runaway.
Claims
1. An electrolyte for a wide temperature range lithium metal battery, comprising an electrolyte lithium salt and an organic solvent, characterized in that: The organic solvent includes fluorocarboxylic acid ester and fluoroethylene carbonate; The fluorinated carboxylic acid ester includes at least one of 2-fluoro-1-ethanol acetate, 2,2-difluoroethyl acetate, 2,2,2-trifluoroethyl acetate, ethyl 4,4,4-trifluorobutyrate, and 2,2,2-trifluoroethyl n-butyrate.
2. The electrolyte for a wide temperature range lithium metal battery according to claim 1, characterized in that: The volume content of the fluorocarboxylic acid ester in the organic solvent is 50-95%, and the volume content of the fluoroethylene carbonate is 5-50%.
3. The electrolyte for a wide temperature range lithium metal battery according to claim 1, characterized in that: The electrolyte lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(trifluoromethanesulfonyl imide), lithium bis(fluorosulfonyl imide), lithium perchlorate, lithium difluorooxalatoborate and lithium trifluoromethanesulfonate.
4. The electrolyte for a wide temperature range lithium metal battery according to claim 1, characterized in that: The molar concentration of the electrolyte lithium salt in the electrolyte is 0.5 to 5.0 mol / L.
5. The electrolyte for a wide temperature range lithium metal battery according to claim 1, characterized in that: The applicable temperature of the electrolyte is -40°C to 40°C.
6. The electrolyte for a wide temperature range lithium metal battery according to claim 1, characterized in that: The fluorocarboxylate is at least one of 2-fluoro-1-ethanol acetate, 2,2-difluoroethyl acetate and 2,2,2-trifluoroethyl acetate. The volume content of the fluorocarboxylate in the organic solvent is 70-95%, and the volume content of the fluoroethylene carbonate is 5-30%.
7. Use of the electrolyte for a wide temperature range lithium metal battery according to any one of claims 1 to 6 in the preparation of a lithium metal battery.
8. A lithium metal battery comprising a positive electrode, a negative electrode, a separator and an electrolyte, characterized in that: The electrolyte is the electrolyte of the wide temperature range lithium metal battery according to any one of claims 1 to 6.
9. The lithium metal battery according to claim 8, characterized in that The negative electrode is metallic lithium, and the positive electrode includes at least one of lithium iron phosphate, lithium cobaltate, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese iron phosphate, sulfur, and sulfurized polyacrylonitrile.
10. The lithium metal battery according to claim 8, characterized in that The separator includes at least one of a polypropylene separator, a polyethylene separator, a glass fiber separator, a polytetrafluoroethylene separator and a cellulose separator.