Electrolyte for lithium metal / lithium iron phosphate battery system and lithium metal / lithium iron phosphate battery

Abstract

The present application relates to the technical field of lithium metal battery, especially to an electrolyte for lithium metal / lithium iron phosphate battery system and a lithium metal / lithium iron phosphate battery. The electrolyte for lithium metal / lithium iron phosphate battery system comprises lithium salt, solvent and additive; the additive comprises 1,4-butylenesulfone and fluoroethylene carbonate, and the solvent comprises ether solvent. The electrolyte for lithium metal / lithium iron phosphate battery system has good compatibility with lithium metal, can form a stable solid-state electrolyte interface film on the surface of lithium metal negative electrode, and the addition of 1,4-butylenesulfone and fluoroethylene carbonate can improve the stability and ionic conductivity of the electrolyte film, thereby ensuring the coulomb efficiency and cycle stability of the battery.

Description

Electrolytes for lithium metal / lithium iron phosphate battery systems and lithium metal / lithium iron phosphate batteries Technical Field

[0001] This invention relates to the field of lithium metal battery technology, and in particular to an electrolyte for a lithium metal / lithium iron phosphate battery system and a lithium metal / lithium iron phosphate battery. Background Technology

[0002] Lithium metal batteries are highly anticipated due to lithium's negative electrode potential (-3.04V vs. SHE) and high theoretical specific capacity (3860mAh / g). However, the current problem is that the highly reactive lithium metal undergoes many side reactions with the electrolyte, resulting in low coulombic efficiency, poor cycle performance, and poor low-temperature performance. Furthermore, the uneven lithium deposition that forms "lithium dendrites" can easily cause short circuits or micro-short circuits in the battery, leading to battery failure or even safety accidents.

[0003] Since the electrolyte is in contact with both the positive and negative electrodes of the battery, it significantly affects the coulombic efficiency and cycle stability of lithium metal batteries. In existing lithium metal / lithium iron phosphate battery systems, the ester-based electrolytes used have poor compatibility with lithium metal and insufficient electrolyte membrane stability; moreover, the electrolyte viscosity is high, resulting in low battery efficiency and poor cycle performance at low temperatures. Therefore, developing electrolytes that can improve the coulombic efficiency and cycle stability of batteries at both room temperature and low temperatures is of great significance for the practical application of lithium metal / lithium iron phosphate battery systems.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] One object of the present invention is to provide an electrolyte for lithium metal / lithium iron phosphate battery systems to solve the technical problems of low battery efficiency and poor cycle stability of lithium metal / lithium iron phosphate battery systems at low temperatures in the prior art.

[0006] Another object of the present invention is to provide a lithium metal / lithium iron phosphate battery.

[0007] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted:

[0008] Electrolytes for lithium metal / lithium iron phosphate battery systems, including lithium salts, solvents, and additives;

[0009] The additives include 1,4-butyrolactone and fluoroethylene carbonate, and the solvents include ether solvents.

[0010] The electrolyte for the lithium metal / lithium iron phosphate battery system of the present invention uses an ether solvent, which has good compatibility with lithium metal, and can form a stable solid electrolyte interface film on the surface of the lithium metal anode. Furthermore, the addition of 1,4-butyrolactone and fluoroethylene carbonate can improve the stability and ionic conductivity of the electrolyte interface film, thereby ensuring both the coulombic efficiency and cycle stability of the battery.

[0011] In a specific embodiment of the present invention, the ether solvent is tetrahydrofuran and / or ethylene glycol dimethyl ether.

[0012] In a specific embodiment of the present invention, the solvent includes solvent A and solvent B; solvent A is tetrahydrofuran and / or ethylene glycol dimethyl ether, and solvent B is tetrafluoroethyl tetrafluoropropyl ether and / or dichloromethane.

[0013] In a specific embodiment of the present invention, the volume fraction of solvent A is 30% to 50%, and the volume fraction of solvent B is 50% to 70%.

[0014] In a specific embodiment of the present invention, the lithium salt is a composite lithium salt. Further, the composite lithium salt includes lithium salt I and lithium salt II; lithium salt I includes any one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate, lithium hexafluorophosphate, lithium difluorooxalateborate, and lithium dioxalateborate; and lithium salt II is lithium nitrate.

[0015] In a specific embodiment of the present invention, the molar ratio of lithium salt I to lithium salt II in the electrolyte is 1:(0.033~0.15).

[0016] In a specific embodiment of the present invention, the concentration of lithium salt I in the electrolyte is 1 to 1.5 M; and the concentration of lithium salt II is 0.05 to 0.15 M.

[0017] In a specific embodiment of the present invention, the mass of 1,4-butyryl lactone in the electrolyte accounts for 0.5% to 2% of the mass of the electrolyte.

[0018] In a specific embodiment of the present invention, the mass of the fluoroethylene carbonate in the electrolyte accounts for 1% to 5% of the mass of the electrolyte.

[0019] The present invention also provides a lithium metal / lithium iron phosphate battery, including an electrolyte for any of the lithium metal / lithium iron phosphate battery systems described above.

[0020] In a specific embodiment of the present invention, the active material of the positive electrode of the lithium metal / lithium iron phosphate battery is lithium iron phosphate, and the negative electrode is lithium metal.

[0021] In a specific embodiment of the present invention, the battery further includes a separator. More specifically, the separator comprises a polyethylene separator or a polyethylene-polypropylene composite separator.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) The electrolyte of the lithium metal / lithium iron phosphate battery system of the present invention uses an ether solvent which has good compatibility with lithium metal and can form a stable solid electrolyte interface film on the surface of the lithium metal anode; and the mixed use of lithium salt I and lithium salt II of the present invention can form an organic-inorganic composite electrolyte interface film on the lithium metal anode of the battery, further improving the stability of the interface film, reducing the occurrence of side reactions, and improving the coulombic efficiency of the battery.

[0024] (2) The electrolyte of the lithium metal / lithium iron phosphate battery system of the present invention further adopts tetrafluoroethyl tetrafluoropropyl ether or dichloromethane to reduce the overall viscosity of the electrolyte; and the addition of 1,4-butyric acid lactone and fluoroethylene carbonate can improve the stability and ionic conductivity of the electrolyte interface film, thereby ensuring the coulombic efficiency and cycle stability of the battery.

[0025] (3) The lithium metal / lithium iron phosphate battery obtained by using the electrolyte of the present invention has high coulombic efficiency (>99.7%) at room temperature and low temperature and good cycle stability. Detailed Implementation

[0026] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are some embodiments of the present invention, but not all embodiments, and are only used to illustrate the present invention, and should not be regarded as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0027] It should be noted that the "lithium metal / lithium iron phosphate battery system" referred to in this invention refers to a battery system composed of lithium iron phosphate material as the positive electrode active material and lithium metal material as the negative electrode active material. The lithium iron phosphate material can be unmodified and / or modified; the lithium metal material can be unmodified and / or modified. The modification treatment refers to treatments performed to improve one or more aspects of the performance of the lithium iron phosphate material or lithium metal material, thereby contributing to improved battery performance.

[0028] Electrolytes for lithium metal / lithium iron phosphate battery systems, including lithium salts, solvents, and additives;

[0029] The additives include 1,4-butyrolactone and fluoroethylene carbonate, and the solvents include ether solvents.

[0030] In a specific embodiment of the present invention, the ether solvent is tetrahydrofuran and / or ethylene glycol dimethyl ether.

[0031] The electrolyte for the lithium metal / lithium iron phosphate battery system of the present invention utilizes the good stability of tetrahydrofuran and / or ethylene glycol dimethyl ether at low voltages (less than 4V), small viscosity changes at low temperatures, and minimal influence of temperature on conductivity.

[0032] In a specific embodiment of the present invention, the solvent includes solvent A and solvent B; solvent A is tetrahydrofuran and / or ethylene glycol dimethyl ether, and solvent B is tetrafluoroethyl tetrafluoropropyl ether and / or dichloromethane.

[0033] In different embodiments, the solvent can be combined among various solvents of solvent A and solvent B. For example, in the first case, the solvent includes any one of solvent A and any one of solvent B; in the second case, the solvent includes two of solvent A and any one of solvent B; in the third case, the solvent includes two of solvent A and two of solvent B; and in the fourth case, the solvent includes any one of solvent A and two of solvent B.

[0034] Specifically, for the first scenario, the solvent can be: tetrahydrofuran + tetrafluoroethyl tetrafluoropropyl ether, tetrahydrofuran + dichloromethane, ethylene glycol dimethyl ether + tetrafluoroethyl tetrafluoropropyl ether, or ethylene glycol dimethyl ether + dichloromethane; for the second scenario, the solvent can be tetrahydrofuran + ethylene glycol dimethyl ether + tetrafluoroethyl tetrafluoropropyl ether, or tetrahydrofuran + ethylene glycol dimethyl ether + dichloromethane; for the third scenario, the solvent can be tetrahydrofuran + ethylene glycol dimethyl ether + tetrafluoroethyl tetrafluoropropyl ether + dichloromethane; for the fourth scenario, the solvent can be tetrahydrofuran + tetrafluoroethyl tetrafluoropropyl ether + dichloromethane, or ethylene glycol dimethyl ether + tetrafluoroethyl tetrafluoropropyl ether + dichloromethane.

[0035] In a specific embodiment of the present invention, the volume fraction of solvent A is 30% to 50%, and the volume fraction of solvent B is 50% to 70%.

[0036] In different embodiments, the volume fraction of solvent A can be 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, etc.; and the volume fraction of solvent B can be 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70%, etc.

[0037] In a specific embodiment of the present invention, the lithium salt is a composite lithium salt. Further, the composite lithium salt includes lithium salt I and lithium salt II; lithium salt I includes any one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate, lithium hexafluorophosphate, lithium difluorooxalateborate, and lithium dioxalateborate; and lithium salt II is lithium nitrate.

[0038] The combined use of lithium salt I and lithium salt II can form an organic-inorganic composite electrolyte interface film on the lithium metal anode of the battery, further improving the stability of the interface film, reducing the occurrence of side reactions, and improving the coulombic efficiency of the battery. Specifically, lithium nitrate can undergo a reduction reaction on the lithium metal anode of the battery at low potentials, generating Li3N and LiN. x O y The resulting interfacial film helps to ensure uniform deposition of lithium ions and slows down the formation of lithium dendrites.

[0039] Furthermore, the composite lithium salt, when used in conjunction with solvents A and B, reduces the overall viscosity of the electrolyte while ensuring a high concentration of lithium ions in the solvent at the microscopic level, thereby improving low-temperature performance.

[0040] In a specific embodiment of the present invention, the molar ratio of lithium salt I to lithium salt II in the electrolyte is 1:(0.033~0.15).

[0041] In different embodiments, the molar ratio of lithium salt I to lithium salt II in the electrolyte can be 1:0.033, 1:0.05, 1:0.08, 1:0.1, 1:0.12, 1:0.15, etc.

[0042] In a specific embodiment of the present invention, the concentration of lithium salt I in the electrolyte is 1 to 1.5 M; and the concentration of lithium salt II is 0.05 to 0.15 M.

[0043] In different embodiments, the concentration of lithium salt I in the electrolyte can be 1M, 1.1M, 1.2M, 1.3M, 1.4M, 1.5M, etc.; and the concentration of lithium salt II can be 0.05M, 0.1M, 0.11M, 0.12M, 0.13M, 0.14M, 0.15M, etc.

[0044] In a specific embodiment of the present invention, the mass of the 1,4-butyryl lactone in the electrolyte accounts for 0.5% to 2% of the mass of the electrolyte, such as 1.5% to 2%.

[0045] In different embodiments, the mass of the 1,4-butyryl lactone in the electrolyte may be 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, etc., of the electrolyte.

[0046] In a specific embodiment of the present invention, the mass of the fluoroethylene carbonate in the electrolyte accounts for 1% to 5% of the mass of the electrolyte, such as 4% to 5%.

[0047] In different embodiments, the mass of the fluoroethylene carbonate in the electrolyte may be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc., of the electrolyte.

[0048] The present invention also provides a lithium metal / lithium iron phosphate battery, including an electrolyte for any of the lithium metal / lithium iron phosphate battery systems described above.

[0049] The lithium metal / lithium iron phosphate battery of the present invention is suitable for low-temperature conditions and has excellent coulombic efficiency and cycle stability.

[0050] In a specific embodiment of the present invention, the lithium metal / lithium iron phosphate battery has a coulombic efficiency of >99.7% after 300 cycles at room temperature (25±3℃) and a coulombic efficiency of >99.7% after 80 cycles at low temperature (-10±3℃).

[0051] In a specific embodiment of the present invention, the active material of the positive electrode of the lithium metal / lithium iron phosphate battery is lithium iron phosphate, and the negative electrode is lithium metal.

[0052] In a specific embodiment of the present invention, the battery further includes a separator. More specifically, the separator comprises a polyethylene separator or a polyethylene-polypropylene composite separator.

[0053] Examples 1-5

[0054] Examples 1-5 provide electrolytes for lithium metal iron phosphate battery systems, the specific composition of which is shown in Table 1. The preparation process of the electrolyte is as follows: lithium salt and additives are added to solvent A in proportion and mixed evenly, then solvent B is added and mixed evenly to obtain the electrolyte.

[0055] Table 1 Electrolyte composition information for different embodiments

[0056]

[0057]

[0058] Notes: In Table 1, the amounts of lithium salt I and lithium salt II correspond to their respective molar concentrations in the final electrolyte; the amounts of solvent A and solvent B correspond to their respective volume fractions relative to the total amount of solvent A and solvent B; the amounts of additives BS and FEC correspond to their respective mass percentages relative to the mass of the electrolyte; in Example 2, the molar concentrations of LiFSI and LiBOB in lithium salt I are 1.05M and 0.05M, respectively.

[0059] The abbreviations in Table 1 are as follows: LiFSI - lithium bis(trifluoromethanesulfonylimide); LiNO3 - lithium nitrate; LiBOB - lithium dioxarate borate; DME - ethylene glycol dimethyl ether; TTE - tetrafluoroethyl tetrafluoropropyl ether; THF - tetrahydrofuran; DCM - dichloromethane; BS - 1,4-butyric acid lactone; FEC - fluoroethylene carbonate.

[0060] Examples 6-9

[0061] Examples 6-9 refer to Example 2, differing only in the amount of FEC and / or BS used. Table 2 shows the amounts of FEC and / or BS used in different examples (the proportion of their respective masses to the corresponding electrolyte mass).

[0062] Table 2. Composition information of additives in the electrolytes of different embodiments.

[0063] Composition Examples 6, 7, 8, and 9: BS 0.1%, 0.1%, 1.5%, 1.5%; FEC 2%, 0.5%, 0.5%, 4%. surface

[0064] Examples 10-13

[0065] Examples 10-13 refer to Example 2, differing only in the composition and / or amount of lithium salt. Information on the composition and / or amount of lithium salt (molar concentration of each in the final electrolyte) of the different examples is shown in Table 3. Specifically, in lithium salt I of Example 10, the molar concentrations of LiFSI and LiBOB are 1.05M and 0.05M, respectively; in lithium salt I of Example 11, the molar concentrations of LiFSI and LiBOB are 0.45M and 0.05M, respectively; and in lithium salt I of Example 12, the molar concentrations of LiFSI and LiBOB are 2.95M and 0.05M, respectively.

[0066] Table 3. Composition information of lithium salts in the electrolytes of different embodiments.

[0067]

[0068] Examples 14-17

[0069] Examples 14-17 refer to Example 2, except that the composition and / or amount of solvent A and solvent B are different. Information on the composition and / or amount of solvent A and solvent B (volume fraction of solvent A or solvent B in the total of solvent A and solvent B) of different examples is shown in Table 4.

[0070] Table 4. Composition information of solvent A and solvent B in the electrolytes of different embodiments.

[0071]

[0072] Comparative Example 1

[0073] Comparative Example 1 refers to Example 1, except that BS and FEC are not added, and solvent A and solvent B are ethylene carbonate and dimethyl carbonate, respectively, each with a volume fraction of 50%.

[0074] Comparative Example 2

[0075] Comparative Example 2 refers to Example 1, except that BS and FEC are not added.

[0076] Comparative Example 3

[0077] Comparative Example 3 refers to Example 1, except that BS is not added.

[0078] Comparative Example 4

[0079] Comparative Example 4 refers to Example 1, except that FEC is not added.

[0080] Comparative Example 5

[0081] Comparative Example 5 refers to Example 1, except that solvent A and solvent B are ethylene carbonate and dimethyl carbonate, respectively.

[0082] Experimental Example

[0083] To compare and illustrate the differences in electrolytes used in lithium metal / lithium iron phosphate battery systems of different embodiments and comparative examples, batteries were assembled using each electrolyte and tested. The test results are shown in Table 5, and the specific test methods are as follows.

[0084] Test method:

[0085] The positive electrode preparation process in this test is as follows: lithium iron phosphate material, conductive agent (Super-P) and binder (PVDF) are added to solvent (N-methylpyrrolidone) at a mass ratio of 96:2:2 and mixed thoroughly. The mixture is then coated onto a current collector (12μm aluminum foil), dried and rolled to obtain a positive electrode sheet, which is then cut to obtain the positive electrode sheet.

[0086] The battery separator uses polyethylene or polyethylene-polypropylene composite separator, specifically Celgard 2320 separator.

[0087] Battery assembly: The prepared positive electrode, separator, and lithium metal negative electrode are assembled in sequence, and an appropriate amount of electrolyte is added to assemble a button cell.

[0088] Battery cycle testing: At 25°C, the prepared battery was charged at a constant current rate of 0.2C to 3.9V, and then discharged at a constant current rate of 0.5C to 2.8V, constituting one charge-discharge cycle. The lithium metal battery was cycled using this method, and the charging capacity, discharging capacity, and charge-discharge efficiency were recorded for each cycle. At 0°C, the prepared battery was charged at a constant current rate of 0.2C to 3.9V, and then discharged at a constant current rate of 0.2C to 2.8V, constituting one charge-discharge cycle. The lithium metal battery was cycled using this method, and the charging capacity, discharging capacity, and charge-discharge efficiency were recorded for each cycle.

[0089] In this test, the capacity retention rate (%) in week x = discharge capacity in week x / discharge capacity in the first cycle * 100 (%).

[0090] In this test, charge / discharge efficiency refers to the average charge / discharge efficiency over cycles 1 to x.

[0091] Table 5 Cycle test results of electrolyte-assembled batteries from different embodiments and comparative examples

[0092]

[0093]

[0094] The test results above show that the lithium metal / lithium iron phosphate battery system obtained by using the electrolyte of the present invention has high coulombic efficiency (>99.7%) at room temperature and low temperature and good cycle stability.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrolyte for lithium metal / lithium iron phosphate battery systems, characterized in that, The electrolyte comprises a lithium salt, a solvent, and additives; the additives are 1,4-butyrolactone and fluoroethylene carbonate, wherein the fluoroethylene carbonate accounts for 4% to 5% of the mass of the electrolyte; the solvent consists of solvent A and solvent B; solvent A is tetrahydrofuran and / or ethylene glycol dimethyl ether; solvent B is tetrafluoroethyl tetrafluoropropyl ether and / or dichloromethane; wherein the volume fraction of solvent A is 30% to 50%, and the volume fraction of solvent B is 50% to 70%; the lithium salt is a composite lithium salt; the composite lithium salt includes lithium salt I and lithium salt II; lithium salt I includes any one or more of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate, lithium hexafluorophosphate, lithium di(fluorooxalate-borate), and lithium di(oxalate-borate); and lithium salt II is lithium nitrate.

2. The electrolyte for a lithium metal / lithium iron phosphate battery system according to claim 1, characterized in that, In the electrolyte, the molar ratio of lithium salt I to lithium salt II is 1:(0.033~0.15).

3. The electrolyte for a lithium metal / lithium iron phosphate battery system according to claim 1, characterized in that, In the electrolyte, the concentration of lithium salt I is 1~1.5M; the concentration of lithium salt II is 0.05~0.15M.

4. The electrolyte for a lithium metal / lithium iron phosphate battery system according to any one of claims 1 to 3, characterized in that, In the electrolyte, the mass of 1,4-butyryl lactone accounts for 0.5% to 2% of the mass of the electrolyte.

5. A lithium metal / lithium iron phosphate battery, characterized in that, The electrolyte for the lithium metal / lithium iron phosphate battery system as described in any one of claims 1 to 4 is included.

6. The lithium metal / lithium iron phosphate battery according to claim 5, characterized in that, The positive electrode of the lithium metal / lithium iron phosphate battery is made of lithium iron phosphate, and the negative electrode is lithium metal.

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