Low-temperature potassium ion battery electrolyte with weak solvation structure

Abstract

The invention discloses a low-temperature potassium ion battery electrolyte with a weak solvation structure and application of the low-temperature potassium ion battery electrolyte in a potassium ion battery. The electrolyte comprises carbonic ester serving as a solvent and fluorobenzene serving as a diluent. According to the invention, carbonic ester and a fluorobenzene diluent are combined for use, on one hand, interaction between FB and PC can weaken the coordination strength of K + and PC, and the desolvation behavior of an electrode interface is improved, so that the electrochemical compatibility of PC and an electrode is improved; and on the other hand, the concentration of the electrolyte is reduced and the low-temperature viscosity is improved by introducing the FB, so that the low-temperature interface impedance is improved and the low-temperature performance of the battery is favorably exerted.

Description

Technical Field

[0001] The present invention relates to the technical field of potassium ion batteries, and in particular to an organic solvent and a diluent for a potassium ion battery electrolyte which have a weak solvation structure and take both normal temperature cycle and low temperature cycle into consideration, as well as a potassium ion battery electrolyte and a potassium ion battery comprising the solvent and the diluent. Background Art

[0002] Rechargeable lithium-ion batteries (LIBs) are widely used in mobile electronic devices and electric vehicles due to their long life and high energy density. However, the rising cost of lithium due to its scarcity and uneven geographical distribution may limit its sustainable application in the near future. Therefore, next-generation secondary batteries based on the earth-abundant elements Na, K, and Mg are considered as promising alternatives to LIBs in future energy storage systems. + / K has low redox potential, small K+ Stokes radius and K + The development of low-cost potassium-ion batteries with reversible deintercalation in graphite has attracted widespread attention in recent years, making them an ideal alternative to lithium-ion batteries. However, the slow desolvation process, unstable interface and severe capacity degradation of potassium-ion batteries at extreme temperatures have hindered their application as an alternative technology to lithium-ion batteries. In addition to improving the electrochemical performance of potassium-ion batteries at room temperature, their operating temperature should also be expanded to adapt to extreme conditions, such as cold winters or hot summers. At low temperatures, the chemical and electrochemical reaction kinetics of the battery are slow, charging is difficult, and the capacity decays rapidly. When operating at high temperatures, there are always problems such as severe electrolyte decomposition, unstable solid electrolyte interface and thermal runaway, which seriously limit the practical performance of potassium-ion batteries.

[0003] Since the electrolyte must meet the requirements of compatibility with positive and negative electrode materials, high dielectric constant, wide liquid range, high conductivity and low viscosity during operation, cyclic carbonates and chain carbonate compounds are generally selected to form a combined solvent. Usually, a mixture of a cyclic carbonate (ethylene carbonate EC, propylene carbonate PC) and one or more linear carbonates constitutes a good solvent system, among which PC is the main choice of cyclic carbonate due to its wide liquid range (-49 to 240°C), strong solvation ability and high anode stability. However, the high dielectric properties of PC have a strong polarity, which makes it sensitive to K + It has a strong coordination effect, which undoubtedly poses a huge challenge to regulating the interaction between potassium ions and solvent molecules. In addition, the melting point of cyclic carbonates is high, and carbonate-based electrolytes are very easy to solidify at low temperatures, which makes PC-based electrolytes difficult to use in low-temperature batteries.

[0004] At present, the methods used to improve the low-temperature performance of potassium ion battery electrolytes mainly include the optimization of the solvent system and the development of low-impedance additives. The existing low-temperature electrolyte strategies mainly include liquefied gas electrolytes, weakly solvated electrolytes, locally high-concentration electrolytes, high-entropy electrolytes, etc.

[0005] Among the existing technologies, the literature Energy & Environmental Science, 2024, 17, 274-283 developed a high concentration electrolyte that weakened the PC and K + The affinity between them forms a more favorable contact ion pair and aggregate solvation structure, which enables the graphene∥K half-cell to achieve up to 220 mA hg at 0 °C. -1 The reversible capacity does not decay for more than 6 months.

[0006] Fluorobenzene (FB) is a low-cost and industrially available co-solvent that is very suitable for low-temperature electrolytes. Reference Advanced Energy Materials, 2022, 12(48): 2201801 FB was used to weaken the PC and Li at a relatively high lithium salt concentration (2.3 M). + The affinity between them was improved, and the primary solvation structure of the FB-PC interaction was retained, so that the PC-based electrolyte remained liquid in the range of -40 to 60°C, and the PC-based electrolyte remained liquid in the range of -90 to 90°C, so that the graphite electrode still retained 80% of its initial specific capacity after 500 cycles at 1C, and the NCM811 positive electrode also achieved long-term stability for 400 cycles.

[0007] Up to now, in the prior art, due to K + -PC is strong and the electrolyte structure is poorly understood, ideal co-solvents are rare. In addition, it is difficult to form a passivation film on the electrode surface without changing the solvation structure, which inhibits the electrochemical stability of the PC electrolyte on the negative electrode side. Summary of the invention

[0008] In order to solve the above technical problems, the present invention proposes a low-temperature potassium ion battery electrolyte with a weak solvation structure and a potassium ion battery containing the electrolyte. By using propylene carbonate PC as an organic solvent and FB as a diluent, the room temperature cycle, high temperature performance and low temperature performance of the battery can be improved at the same time, and the cycle stability of the potassium ion battery under a wide temperature condition (-20 to 60°C) can be improved.

[0009] The objective of the present invention is achieved through the following technical solutions:

[0010] A low-temperature potassium ion battery electrolyte with a weak solvation structure, comprising a potassium salt, a non-aqueous solvent and a diluent, wherein the non-aqueous solvent comprises a carbonate solvent, and the potassium salt is potassium hexafluorophosphate (KPF 6 ), potassium perchlorate (KClO 4 ), potassium tetrafluoroborate (KBF 4 ), potassium trifluoromethanesulfonate (KCF 3 SO 3 ), potassium bis(fluorosulfonyl)imide (KFSI) and potassium bis(trifluoromethanesulfonyl)imide (KTFSI) or more; the diluent is one or more of fluorobenzene (FB), o-difluorobenzene, 1,3,5-trifluorobenzene;

[0011] In particular, the carbonate solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC)

[0012] Particularly, the molar ratio of carbonate solvent to electrolyte is (1-5):9.

[0013] Particularly, the molar ratio of the carbonate-containing solvent to the electrolyte is 4:9.

[0014] In particular, the potassium salt comprises potassium hexafluorophosphate (KPF 6 ), potassium perchlorate (KClO 4 ), potassium tetrafluoroborate (KBF 4 ), potassium trifluoromethanesulfonate (KCF 3 SO 3 ), at least one of potassium bis(fluorosulfonyl)imide (KFSI) and potassium bis(trifluoromethanesulfonyl)imide (KTFSI).

[0015] Particularly, the molar ratio of the potassium-containing salt to the electrolyte is 4:9.

[0016] In particular, the diluent is at least one of fluorobenzene (FB), o-difluorobenzene, and 1,3,5-trifluorobenzene;

[0017] Particularly, the molar ratio of the diluent to the electrolyte is 1:9.

[0018] The present invention also provides a potassium ion battery, which comprises the low-temperature potassium ion battery electrolyte with the weak solvation structure.

[0019] In particular, the positive electrode material of the potassium ion battery is KVPO 4 F. KFeSO 4 F.K xFe[Fe(CN) 6 ], K 0.6 CoO 2 , K 2 Ni 2 TeO 6 One of the following, wherein 0≤x≤2; the negative electrode material is one of metallic potassium, potassium-carbon composite material, carbon-based material or potassium titanate.

[0020] In particular, the upper limit of the working voltage of the potassium ion battery is 4.3V, the lower limit of the discharge voltage is 2.0V, and the operating temperature range is -20 to 60°C.

[0021] The present invention uses PC-based electrolyte as a non-aqueous solvent and fluorobenzene as a diluent. The cyclic carbonate and fluorobenzene diluent are used together. On the one hand, the interaction between FB and PC can weaken K + The coordination strength with PC improves the desolvation behavior of the electrode interface, thereby improving the electrochemical compatibility of PC and the electrode; on the other hand, the introduction of FB reduces the concentration of the electrolyte, improves the low-temperature viscosity, and thus improves the low-temperature interface impedance, which is beneficial to the low-temperature performance of the battery.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. Compared with the prior art, the present invention introduces FB molecules into high-concentration electrolytes and utilizes the interaction between FB and PC to weaken K + -PC coordination strength promotes the K + -Desolvation of PC clusters, thereby improving the electrochemical compatibility of PC and graphite.

[0024] 2. On the other hand, the introduction of FB diluent in this electrolyte not only maintains the stability of the KF-rich interface, but also utilizes the traction effect of FB on PC to achieve a rapid desolvation process of the interface, accelerates ion migration, and thus achieves stable operation of the battery at high rates and in a wide temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0026] Figure 1 The viscosity and conductivity curves of the electrolytes in Example 1 of the present invention and Comparative Examples 1-2 at different temperatures.

[0027] Figure 2It is a room temperature (25°C) cycle performance diagram of Example 1 of the present invention and Comparative Examples 1-2.

[0028] Figure 3 The room temperature (25°C) cycle performance diagram of Example 2 of the present invention and Comparative Examples 3-4.

[0029] Figure 4 60°C high temperature cycle performance diagram of Example 1 of the present invention and Comparative Examples 1-2

[0030] Figure 5 60°C high temperature cycle performance diagram of Example 2 of the present invention and Comparative Examples 3-4

[0031] Figure 6 The -10°C low temperature cycle performance diagram of Example 1 of the present invention and Comparative Examples 1-2

[0032] Figure 7 The -20°C low temperature cycle performance diagram of Examples 3-4 of the present invention and Comparative Example 3 DETAILED DESCRIPTION

[0033] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0034] Unless otherwise defined, all professional terms used hereinafter have the same meaning as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. The preferred implementation methods and materials described herein are for demonstration purposes only. Several improvements and modifications may be made without departing from the principles of the embodiments of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

[0035] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0036] Example 1

[0037] K-FeHCFe cathode material preparation

[0038] The Prussian white sample (K-FeHCFe, PW) was synthesized at 0°C by double salt coprecipitation method. Specifically, 1.67 g (4 mmol) of FeSO 4 7H 2 O was dissolved in 100 mL of deionized water to form solution A. 1.69 g (4 mmol) K 4 Fe(CN) 6 ·3H 2O was dissolved in 100 mL of deionized water to form solution B. Then, 16 mmol K 3 C 6 H 5 O 7 ·H 2 O and 0.16 mol KCl were added to solution A. After the above salts were completely dissolved, solution B was slowly added dropwise to solution A under stirring to gradually form a milky white precipitate. The mixture was stirred for 2 h, aged for 12 h, then washed thoroughly with deionized water and alcohol, the precipitate was collected by centrifugation, and finally dried for 12 h to obtain the product.

[0039] Electrode preparation

[0040] The graphite slurry is prepared by mixing 85wt% natural graphite powder, 10wt% sodium alginate and 5wt% Ketjen black in deionized water and stirring with a magnetic stirrer for 12h. The resulting slurry is then evenly coated on a copper foil, transferred to a vacuum oven at 80°C for drying, and cut into 12mm discs for later use. The mass loading of the graphite pole piece is about 1.5–2.0mg cm -2 Similarly, K-FeHCFe slurry was prepared by mixing 70 wt% K-FeHCFe, 20 wt% Ketjen black, and 10 wt% PVDF in NMP solvent. The slurry was then coated on carbon-coated aluminum foil, dried at 110 °C under vacuum for 12 h, and cut into 12 mm discs for use. The loading of each pole piece was about 1.0–2.0 mg cm -2 .

[0041] Preparation of electrolyte

[0042] Before preparing the electrolyte, use PC Molecular sieves were dried for 24 hours. Potassium salt KFSI: propylene carbonate PC: fluorobenzene FB were mixed in a molar ratio of 1:3:2 (hereinafter, this electrolyte was named KPF132), and stirred with a magnetic stirrer for 12 hours.

[0043] The steps are as follows

[0044] (1) Battery assembly: The graphite pole pieces were cut into discs with a diameter of 12 mm using a punching machine, dried in an oven at 90°C for 12 h, and then transferred to a glove box for standby use. Button cells were assembled in the order of “positive electrode shell-graphite pole piece-70 μL electrolyte-diaphragm-70 μL electrolyte-metal potassium sheet-stainless steel gasket-spring-negative electrode shell” and sealed using an automatic battery packaging machine.

[0045] (2) After the battery has been left to rest for 12 hours, the electrochemical performance is tested, including: variable temperature charge and discharge curves and cycle performance.

[0046] Comparative Example 1

[0047] In the preparation process of the electrolyte, potassium salt KFSI: propylene carbonate PC: fluorobenzene FB are mixed in a molar ratio of 1:4:4 (hereinafter, this electrolyte is named KPF144), and the rest is the same as Example 1.

[0048] Comparative Example 2

[0049] In the process of preparing the electrolyte, potassium salt KFSI:propylene carbonate PC is mixed in a molar ratio of 1:3 (hereinafter this electrolyte is named KP13), and the rest is the same as Example 1.

[0050] Example 2

[0051] The metal potassium sheet in Example 1 was replaced with K-FeHCFe positive electrode material to assemble a K-FeHCFe / / graphite full battery.

[0052] Comparative Example 3

[0053] The metal potassium sheet in Example 2 was replaced with K-FeHCFe positive electrode material to assemble a K-FeHCFe / / graphite full battery.

[0054] Comparative Example 4

[0055] The metal potassium sheet in Example 1 was replaced with K-FeHCFe positive electrode material to assemble a K-FeHCFe / / graphite full battery.

[0056] The above-mentioned Examples 1-2 and Comparative Examples 1-4 were applied to various performance tests of the battery, including:

[0057] 1. Cycling performance at room temperature (25°C)

[0058] The test box was adjusted to 25°C, and the graphite half-cell was discharged to 0.01V at 0.5C, and then charged to 2.0V at 0.5C. This cycle was repeated 300 times, and the charging capacity of the first cycle and the charging capacity of the 300th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0059] Capacity retention rate = 300th charge capacity / 1st charge capacity*100%.

[0060] The K-FeHCFe / / graphite full battery was charged to 4.3V at 5C and then discharged to 2.0V at 5C. This cycle was repeated 1000 times, and the discharge capacity of the first cycle and the discharge capacity of the 1000th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0061] Capacity retention rate = 1000th discharge capacity / 1st discharge capacity*100%.

[0062] Table 1 Normal temperature cycle performance test results

[0063] Example / Comparative Example Cycle times at 25℃ Capacity retention rate Example 1 300 times 91.6％ Comparative Example 1 300 times 63.3％ Comparative Example 2 300 times 68.1％ Example 2 1000 times 77.4％ Comparative Example 3 1000 times 51.6％ Comparative Example 4 1000 times 59.9％

[0064] 2. High temperature (60℃) cycle performance

[0065] The test chamber was adjusted to 60°C, and the graphite half-cell was discharged at 4C to 0.01V, and then charged at 4C to 2.0V. This cycle was repeated 400 times, and the charging capacity of the first cycle and the charging capacity of the 400th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0066] Capacity retention rate = 400th charge capacity / 1st charge capacity*100%.

[0067] The test box was adjusted to 60°C, and the K-FeHCFe / / graphite full battery was charged to 4.3V at 10C, and then discharged to 2.0V at 10C. This cycle was repeated 1000 times, and the discharge capacity of the first cycle and the discharge capacity of the 1000th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0068] Capacity retention rate = 1000th discharge capacity / 1st discharge capacity*100%.

[0069] Table 260℃ cycle performance test results

[0070] Example / Comparative Example 60℃ Cycle times Capacity retention rate Example 1 400 times 96.3％ Comparative Example 1 400 times 0.8％ Comparative Example 2 400 times 97.1％ Example 2 1000 times 96.2％

[0071] 3. Low temperature cycle performance

[0072] The test box was adjusted to -10°C, and the graphite half-cell was discharged at 0.2C to 0.01V, and then charged at 0.2C to 2.0V. This cycle was repeated 100 times, and the charging capacity of the first cycle and the charging capacity of the 100th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0073] Capacity retention rate = 100th charge capacity / 1st charge capacity*100%.

[0074] The test box was adjusted to -20°C, and the K-FeHCFe / / graphite full battery was charged to 4.3V at 0.2C, and then discharged to 2.0V at 0.2C. This cycle was repeated 300 times, and the discharge capacity of the first cycle and the discharge capacity of the 300th cycle were recorded. The capacity retention rate of the battery cycle was calculated as follows:

[0075] Capacity retention rate = 300th discharge capacity / 1st discharge capacity*100%.

[0076] Table 3 Low temperature cycle performance test results

[0077] Example / Comparative Example Capacity retention rate Example 1 88.6％ Comparative Example 1 16.0％ Comparative Example 2 48.2％ Example 2 91.2％

[0078] Figure 1 It shows that the electrolytes in Example 1 and Comparative Examples 1 and 2 remain in liquid state at temperatures between -20 and 60°C, and maintain a relatively high ionic conductivity (~1 mS cm at -20°C). -2 ), which ensures rapid ion transfer in the electrolyte liquid phase. The introduction of FB can significantly reduce the viscosity at low temperatures, thereby improving the wettability of the electrolyte at low temperatures, which is beneficial to improving the low temperature performance.

[0079] from Figure 2 It can be seen that Example 1 maintains 91.6% (249.9 mAh g) after 300 cycles at a rate of 0.5C. -1 ) has a high capacity retention rate, which is significantly ahead of the electrolytes of Comparative Example 1 (68.1%) and Comparative Example 2 (63.3%).

[0080] From the comparison between Example 1 and Comparative Example 1, it can be seen that the addition of FB diluent improves the long cycle stability of the graphite half-cell, and increases the capacity retention rate of 100 cycles from 63.3% in Comparative Example 2 to 91.6% in Example 1.

[0081] From the comparison between Example 1 and Comparative Example 2, it can be seen that the FB diluent can play its role when the molar ratio content is between 3-4, and the best mass ratio is KFSI:PC:FB=1:3:2.

[0082] from Figure 3 It can be seen that, compared with Comparative Example 4, the addition of diluent significantly improved the long cycle stability of the K-FeHCFe / / graphite full battery, increasing the capacity retention rate of 1000 cycles from 51.6% in Comparative Example 4 to 77.4% in Example 2.

[0083] Figure 4 and Figure 6 This reflects the beneficial effect of FB diluent on the cycling of graphite half-cells under high and low temperature environments. Figure 4 It can be seen that at 60°C, the capacity of the battery decays severely with the low-temperature cycle. Compared with Comparative Example 1, the low-temperature cycle of Example 1 is relatively stable, which shows that the introduction of FB can effectively improve the low-temperature cycle performance and can also exert a considerable capacity level in a low-temperature environment. Figure 6 It can be seen that in a high temperature environment of -10°C, Example 1 still maintains stable capacity within 400 cycles, and FB can also greatly improve the high temperature cycle performance.

[0084] Figure 5 and Figure 7 The results show that FB diluent can effectively enhance the stable cycling of K-FeHCFe / / graphite full battery under high and low temperature conditions.

[0085] In summary, the present invention can effectively improve the wide temperature cycle stability of high-voltage batteries by adding FB diluent pairs to conventional electrolytes, and the electrolyte has a certain universality for common high-voltage positive and negative electrodes, thereby developing a high-voltage potassium ion battery that can operate in a wide temperature range of -20°C to 60°C.

[0086] The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Therefore, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the content of the technical solution of the present invention shall fall within the scope of protection of the technical solution of the present invention.

Claims

1. A low-temperature potassium ion battery electrolyte with a weak solvation structure, comprising a potassium salt, a non-aqueous solvent and a diluent, characterized in that: The non-aqueous solvent includes a carbonate solvent, the potassium salt is one or more of potassium hexafluorophosphate (KPF6), potassium perchlorate (KClO4), potassium tetrafluoroborate (KBF4), potassium trifluoromethanesulfonate (KCF3SO3), potassium bis(fluorosulfonyl)imide (KFSI) and potassium bis(trifluoromethanesulfonyl)imide (KTFSI); the diluent is one or more of fluorobenzene (FB), o-difluorobenzene and 1,3,5-trifluorobenzene.

2. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to claim 1, characterized in that The carbonate solvent is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC) and diethyl carbonate (DEC).

3. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to claim 1 or 2, characterized in that: The molar ratio of the carbonate solvent to the electrolyte is (1-5):

9.

4. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to any of claims 1 to 3, characterized in that: The molar ratio of the carbonate solvent to the electrolyte is 4:

9.

5. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to any of claims 1 to 4, characterized in that: The potassium salt includes at least one of potassium hexafluorophosphate (KPF6), potassium perchlorate (KClO4), potassium tetrafluoroborate (KBF4), potassium trifluoromethanesulfonate (KCF3SO3), potassium bis(fluorosulfonyl)imide (KFSI) and potassium bis(trifluoromethanesulfonyl)imide (KTFSI).

6. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to any of claims 1 to 5, characterized in that: The diluent is at least one of fluorobenzene (FB), o-difluorobenzene and 1,3,5-trifluorobenzene; the molar ratio of the diluent to the electrolyte is 20% to 50%.

7. The low-temperature potassium ion battery electrolyte with a weak solvation structure according to any of claims 1 to 6, characterized in that: The molar ratio of the diluent to the electrolyte is 44%.

8. A potassium ion battery, characterized in that: A low-temperature potassium ion battery electrolyte comprising the weak solvation structure according to any one of claims 1 to 7.

9. The potassium ion battery according to claim 8, characterized in that The positive electrode material of the potassium ion battery is KVPO4F, KFeSO4F, K x Fe[Fe(CN)6]、K 0.6 One of CoO2, K2Ni2TeO6, where 0≤x≤2; the negative electrode material is one of metallic potassium, potassium-carbon composite material, carbon-based material or potassium titanate.

10. The potassium ion battery according to claim 8 or 9, characterized in that: The upper limit of the working voltage of the potassium ion battery is 4.3V, the lower limit of the discharge voltage is 2.0V, and the operating temperature range is -20 to 60°C.

Images