A potassium-based dual-ion battery with dual graphite electrodes and its preparation method

Abstract

The present invention belongs to the field of electrochemistry technology, and particularly relates to a potassium-based dual-ion battery with dual graphite electrodes and a preparation method thereof. The potassium-based dual-ion battery includes a graphite electrode, an electrolyte, and the compatibility between the two. The binder used in the electrode is sodium carboxymethyl cellulose, and the electrolyte is a phosphate-based concentrated salt electrolyte, which is characterized by flame retardancy and a relatively wide electrochemical stability window, effectively improving the battery safety, specific capacity, and cycle life compared with traditional organic carbonate-based electrolytes. The potassium-based dual-ion battery composed of dual graphite electrodes provided by the present invention provides feasibility for the commercialization of high-safety and low-cost dual-ion batteries.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemistry technology, and particularly relates to a potassium-based dual-ion battery with dual graphite electrodes and a preparation method thereof. Background Art

[0002] Lithium-ion batteries (LIBs) have been widely used in portable electronic products, electric vehicles, military industries, etc. due to their advantages such as high energy density, long cycle life, low self-discharge phenomenon, and no memory effect. However, lithium resources are unevenly distributed globally and have a low content in the earth's crust. Moreover, the cathode materials usually contain heavy metals such as iron, cobalt, and nickel, with a high cost, and long-term accumulation will have an adverse impact on the environment. At present, the recycling technology of LIBs has not been popularized in actual production applications. If the dependence on LIBs continues to increase, it may deplete the lithium resources on the earth. In addition, in recent years, electric vehicle explosion incidents have occurred frequently, and it is crucial to ensure the safe operation of the battery. Therefore, the development of new secondary batteries with low cost, environmental friendliness, and high safety has great scientific significance and commercial value.

[0003] As a new type of battery system, dual-ion batteries (DIBs) rely on the working mechanism of anions and cations intercalating / deintercalating into / from the positive and negative electrodes respectively, and usually use inexpensive graphite as the positive electrode, making DIBs have advantages such as high working voltage, high power density, environmental friendliness, and low cost, and are expected to become substitutes for LIBs. However, anions generally exist in the form of complex coordination ions (such as PF6ˉ, BF4ˉ, TFSIˉ, FSIˉ, ClO4ˉ, etc.), with a relatively large ionic radius. The volume expansion generated during the intercalation process is likely to cause the collapse of the graphite structure, resulting in a large irreversible capacity, which limits the energy density of DIBs. In addition, the anionic coordination ions in the organic electrolyte need a relatively high potential to complete the intercalation into the graphite positive electrode. The concentration of traditional organic carbonate electrolytes is relatively low, and the electrolyte is prone to oxidative decomposition and gas generation at high potentials, reducing the Coulombic efficiency and cycle life of DIBs, and its volatility and flammability bring safety hazards to the battery. Therefore, developing an electrolyte with strong antioxidant ability and non-flammability is an important means to improve the safety of DIBs.

[0004] Since the standard electrode potential of potassium (-2.93 V) is closest to that of lithium (-3.04 V), potassium has a relatively high abundance in the earth's crust (1.5 wt%), and the use of dual-carbon electrodes further reduces the manufacturing cost of potassium-based dual-ion batteries (KDIBs), which is conducive to large-scale applications. Therefore, the research on KDIBs composed of dual-carbon electrodes has received increasing attention. Although concentrated salt carbonate and sulfone electrolytes have been applied to KDIBs composed of dual-carbon electrodes, significantly improving the reversible capacity and cycle life of the batteries, there is still a risk of combustion and explosion. Therefore, in this invention, commercial graphite powder is used as the active material in the positive and negative electrodes, a binder regulation strategy is adopted to improve the mechanical structure properties of the graphite electrodes, and a phosphate ester solvent with flame retardant effect is used as an important component of the concentrated salt electrolyte to eliminate potential battery safety hazards. The above synergistic effects alleviate the volume expansion of graphite and the peeling of graphite layers during the intercalation of anionic counterions, enabling the dual-graphite electrode-based KDIBs to exhibit excellent comprehensive performance including high working voltage, long cycle life, and high safety. Summary of the Invention

[0005] The object of this invention is to provide a potassium-based dual-ion battery (KDIBs) with dual-graphite electrodes and its preparation method. An inexpensive graphite is used as the active material for the positive and negative electrodes, and a phosphate-based concentrated salt electrolyte is used to construct a KDIBs with high working voltage, long life, low cost, and high safety. This invention proposes to use sodium carboxymethyl cellulose (CMC) as the binder to improve the mechanical structure stability of the graphite positive electrode, thereby increasing the specific capacity and Coulomb efficiency of the graphite positive electrode; the designed phosphate-based concentrated salt electrolyte has a flame retardant effect and can form a stable electrode / electrolyte interface film to inhibit the decomposition of the electrolyte and the volume expansion of graphite, enabling the dual-graphite electrode-based KDIBs to exhibit excellent comprehensive performance.

[0006] This invention provides a potassium-based dual-ion battery with dual-graphite electrodes and its preparation method, which is realized through the following technical solutions:

[0007] (1) Preparation of the electrolyte: Potassium bis(fluorosulfonyl)imide (KFSI) and triethyl phosphate (TEP) are mixed in different molar ratios and stirred until dissolved evenly in a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm) to prepare a concentrated salt electrolyte.

[0008] (2) Preparation of the graphite electrodes: Graphite, conductive carbon black, and sodium carboxymethyl cellulose (CMC) binder are mixed in a mass ratio of 80%:10%:10%, ground evenly, and then deionized water is added to make a slurry, which is respectively coated on aluminum foil and copper foil, and then vacuum dried to obtain the graphite positive electrode and graphite negative electrode accordingly.

[0009] (3) Design of dual graphite electrode-based KDIBs: Inside a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), KDIBs based on graphite positive electrode, graphite negative electrode, and concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) were assembled, and the capacity ratio of the negative electrode to the positive electrode (N / P value) was controlled within 1.1 to 1.2.

[0010] The advantages and positive effects of the present invention are:

[0011] Through a simple binder and electrolyte optimization strategy, the present invention designs a kind of KDIBs based on non-flammable, phosphate ester-based concentrated salt electrolyte and dual graphite electrodes, which has the advantages of relatively simple modification means and low cost. Compared with polyvinylidene fluoride (PVDF) and sodium alginate (SA), the use of CMC binder significantly improves the specific capacity and Coulomb efficiency of the graphite positive electrode because it enhances the mechanical structure stability of the graphite positive electrode. Compared with traditional organic carbonate electrolytes, the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) not only has a flame retardant effect, but also has a high ionic conductivity and a wide electrochemical stability window, and can form an anion-derived interfacial film on the graphite positive electrode to alleviate electrolyte loss and graphite volume expansion, thereby greatly improving the specific capacity and cycle stability of the graphite positive electrode. In addition, this electrolyte has good compatibility with metallic potassium and the graphite negative electrode. Due to the above-mentioned synergistic effects, the constructed dual graphite electrode-based KDIBs have a high working voltage, long cycle life, and high safety, which helps to promote the commercialization process of the DIBs system. Description of the Drawings

[0012] Figure 1 It is the scanning electron microscope (SEM) image of the graphite powder used in Example 1;

[0013] Figure 2 It is the linear sweep voltammetry (LSV) diagram of the K / / Al battery based on electrolytes with different molar ratios of KFSI / TEP in Example 1 and the combustion experiment diagrams of the traditional organic carbonate electrolyte 1 mol L -1 KPF6 / ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethyl methyl carbonate (EMC) and the high-concentration KFSI / TEP electrolyte (molar ratio 1:1.3);

[0014] Figure 3 It is the cycle performance curve of the half-cell assembled with a graphite positive electrode containing different binders and a potassium metal negative electrode in the KFSI / TEP electrolyte with a molar ratio of 1:1.5, in the voltage range of 3.0 - 5.0 V (vs. K + / K) and at a current density of 100 mA g -1 ;

[0015] Figure 4The cyclic performance curves of the half-cell assembled with a graphite positive electrode containing CMC binder and a potassium metal negative electrode in Example 1 in KFSI / TEP electrolytes with different molar ratios, different voltage ranges (3.0 - 5.2 V, 3.0 - 5.3 V vs. K + / K), and at a current density of 200 mA g -1 ;

[0016] Figure 5 The cyclic performance curves of the KDIBs assembled with a graphite positive and negative electrode containing CMC binder and a KFSI / TEP electrolyte with a molar ratio of 1:1.3 in Example 1 in the voltage range of 2.5 - 5.0 V and at a current density of 500 mA g -1 ;

[0017] Figure 6 The charge-discharge curves of the 50th and 100th cycles of the KDIBs assembled with a graphite positive and negative electrode containing CMC binder and a KFSI / TEP electrolyte with a molar ratio of 1:1.3 in Example 1 in the voltage range of 2.5 - 5.0 V and at a current density of 500 mA g -1 ; Detailed implementation manners

[0018] The present invention will be further described in detail through specific embodiments below. The following embodiments can enable those skilled in the art to understand the present invention more comprehensively, but do not limit the present invention in any way.

[0019] Example 1:

[0020] (1) Prepare electrolytes with different potassium salt concentrations. The specific process is as follows: In a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), potassium bis(fluorosulfonyl)imide (KFSI) and a phosphate ester solvent - triethyl phosphate (TEP) are mixed at molar ratios of 1:2.0, 1:1.5, and 1:1.3 respectively, and stirred for 24 h to obtain a series of KFSI / TEP electrolytes.

[0021] (2) Prepare graphite electrodes. The specific process is as follows: Graphite, conductive carbon black, and binders (including polyvinylidene fluoride (PVDF), sodium alginate (SA), and sodium carboxymethyl cellulose (CMC)) are mixed at a mass ratio of 80%:10%:10%, ground for 30 min, and then a dispersant is added to make a slurry, which is evenly coated on an aluminum sheet and vacuum dried at 70 °C for 24 h to obtain a graphite positive electrode. In addition, CMC is selected as the binder and a copper sheet is used as the current collector, and a graphite negative electrode is obtained using the same preparation process. Specifically, when the binder used in the electrode is PVDF, the solvent used for preparing the slurry is N-methylpyrrolidone; when the binder used in the electrode is SA or CMC, the solvent used for preparing the slurry is deionized water.

[0022] (3) Half-cell assembly and performance testing were carried out as follows: Using the KFSI / TEP electrolyte with a molar ratio of 1:1.5 obtained above, in a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), half-cells based on graphite cathodes with different binders and potassium metal anodes were assembled, and their cycling performance was tested in the voltage range of 3.0 - 5.0 V (vs. K + / K) and at a current density of 100 mA g -1 to screen the binder. In addition, half-cells based on KFSI / TEP electrolytes with different molar ratios and graphite cathodes with the screened binder were assembled, and their cycling performance was tested in the voltage range of 3.5 - 5.2 or 3.5 - 5.3 V (vs. K + / K) and at a current density of 200 mA g -1 .

[0023] (4) Assembly and performance testing of dual-graphite electrode-based KDIBs were carried out as follows: In a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), KDIBs based on a graphite cathode containing CMC binder, a graphite anode, and a KFSI / TEP electrolyte with a molar ratio of 1:1.3 were assembled. The capacity ratio of the anode to the cathode (N / P value) was controlled at 1.1, and their cycling performance was tested in the voltage range of 2.5 - 5.0 V and at a current density of 500 mA g -1 .

[0024] Example 2:

[0025] (1) Preparation of electrolytes with different potassium salt concentrations was carried out as follows: In a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), potassium bis(fluorosulfonyl)imide (KFSI) and a phosphate ester solvent - trimethyl phosphate (TMP) were mixed at molar ratios of 1:2.0 and 1:1.5 respectively, and stirred for 24 h to obtain a homogeneous electrolyte.

[0026] (2) Preparation of graphite electrodes was carried out as follows: Graphite was mixed with carbon black and binders (including polyvinylidene fluoride (PVDF), sodium alginate (SA), and sodium carboxymethyl cellulose (CMC)) at a mass ratio of 80%:10%:10%, ground for 30 min, and then a dispersant was added to make a slurry, which was evenly coated on the surface of an aluminum sheet and vacuum dried at 70 °C for 24 h to obtain a graphite cathode. In addition, CMC was selected as the binder and a copper sheet as the current collector, and a graphite anode was obtained using the same preparation process. Specifically, when the binder used in the electrode was PVDF, the solvent used for preparing the slurry was N-methylpyrrolidone; when the binder used in the electrode was SA or CMC, the solvent used for preparing the slurry was deionized water.

[0027] (3) Half-cell assembly and performance testing were carried out as follows: Using the KFSI / TMP electrolyte with a molar ratio of 1:1.5 obtained above, in a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), half-cells based on graphite cathodes and potassium metal anodes with different binders were assembled, and their cycling performance was tested in the voltage range of 3.0 - 5.0 V (vs. K + / K) and at a current density of 100 mAg -1 to screen the binder. In addition, half-cells based on KFSI / TMP electrolytes with different molar ratios and graphite cathodes containing the screened binder were assembled, and their cycling performance was tested in the voltage range of 3.5 - 5.2 V or 3.5 - 5.3 V (vs. K + / K) and at a current density of 200 mAg -1 .

[0028] (4) Assembly and performance testing of dual-graphite electrode-based KDIBs were carried out as follows: In a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), KDIBs based on graphite cathodes containing CMC binder, graphite anodes, and KFSI / TMP electrolyte with a molar ratio of 1:1.5 were assembled, where the capacity ratio of the anode to the cathode (N / P value) was controlled at 1.1, and their cycling performance was tested in the voltage range of 2.5 - 5.0 V and at a current density of 100 mA g -1 .

[0029] The commercial graphite powder used in the present invention was characterized by scanning electron microscopy (SEM) to observe its morphology. Galvanostatic charge-discharge tests were used to evaluate the cycling performance of graphite cathode-based half-cells and dual-graphite electrode-based KDIBs, and relevant data were collected using a Neware CT-4008 battery tester. An Autolab electrochemical workstation was used to collect linear sweep voltammetry (LSV) curves of K / / Al cells based on KFSI / TEP electrolytes with different potassium salt concentrations.

[0030] Figure 1 is the SEM image of the graphite powder used in Example 1. It can be seen from the figure that the commercial graphite has an irregular flaky morphology with micron-sized dimensions. Without any modification and treatment, it is directly used as the active material of the positive and negative electrode sheets, greatly reducing the manufacturing cost of KDIBs.

[0031] Figure 2 is the linear sweep voltammetry (LSV) curve of K / / Al cells based on KFSI / TEP electrolytes with different molar ratios in Example 1 at 1.0 mV s -1 and the traditional carbonate-based electrolyte of 1 mol L -1Combustion experiment diagrams of KPF6 / ethylene carbonate (EC)-dimethyl carbonate (DMC)-ethyl methyl carbonate (EMC) and concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3). It can be seen from the LSV curves that the initial voltages of the KFSI / TEP electrolytes with molar ratios of 1:2.0, 1:1.5, and 1:1.3 for the oxidation decomposition reaction are 5.37 V, 5.62 V, and 5.98 V respectively. Thus, it can be seen that as the potassium salt concentration increases, the oxidation stability of the phosphate ester-based electrolyte increases accordingly, and among them, the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) has the strongest antioxidant ability. The combustion experiment shows that when the outer flame of the fire touches the toilet paper soaked with 1 mol L -1 the toilet paper soaked with KPF6 / EC-DMC-EMC electrolyte is instantly ignited, while the toilet paper soaked with the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) shows obvious flames after 34 s in the flame, indicating that compared with the traditional low-concentration carbonate electrolytes, the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) has better flame retardancy, providing guarantee for the safe operation of KDIBs.

[0032] Figure 3 is the cyclic performance curve of the half-cell assembled with the KFSI / TEP electrolyte with a molar ratio of 1:1.5, the graphite positive electrode containing different binders, and the potassium metal negative electrode in Example 1 in the voltage range of 3.0 - 5.0 V (V vs. K + / K) and at a current density of 100 mA g -1 When using PVDF, SA, and CMC as binders, the Coulombic efficiencies of the graphite positive electrode after 100 cycles are 96.7%, 79.9%, and 93.8% respectively, and the specific capacities are 17.3 mAh g -1 、14.6 mAh g -1 、29.3 mAh g -1 respectively. After comprehensive comparison, when CMC is selected as the binder, the graphite positive electrode shows relatively remarkable FSIˉ anion storage performance.

[0033] Figure 4 is the cyclic performance curve of the half-cell assembled with the KFSI / TEP electrolytes with different molar ratios, the graphite positive electrode containing CMC binder, and the potassium metal negative electrode in Example 1 at different voltage ranges and a current density of 100 mA g -1 In the voltage range of 3.5 - 5.2 V (V vs. K + / K), when using the KFSI / TEP electrolytes with molar ratios of 1:2.0, 1:1.5, and 1:1.3 respectively, after 50 cycles, the mass specific capacities of the graphite positive electrode reach 32.0 mAh g -1 、43.0 mAh g -1, 44.4 mAh g -1 , the Coulombic efficiencies are 35.9%, 70.6%, and 84.6% respectively. After comprehensive comparison, when using the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3), the storage performance of FSIˉ anions in the graphite positive electrode is relatively excellent. On this basis, the charging cut-off voltage is further increased to 5.3 V. After 50 cycles, although the specific capacity of the graphite positive electrode increases to 57.0 mAh g -1 , the Coulombic efficiency is only 56.3%. This may be due to the serious oxidative decomposition of the electrolyte caused by the high operating voltage of 5.3 V, which exacerbates the irreversible reaction during the cycling process. It can be seen that the graphite positive electrode containing CMC binder has relatively excellent comprehensive performance when using the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) and in the voltage range of 3.5 - 5.2 V (V vs. K + / K).

[0034] Figure 5 is the cycling performance curve of the KDIBs assembled with the concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3), the graphite negative electrode containing CMC binder, and the graphite positive electrode (N / P value is 1.1) in Example 1 in the voltage range of 2.5 - 5.0 V and at a current density of 500 mA g -1 . Calculated according to the active mass of the graphite positive electrode, the specific capacity of the double-graphite electrode-based KDIBs remains at 26.5 mAh g after 350 cycles -1 , and the Coulombic efficiency is 91.8%. This is mainly attributed to the use of CMC binder, which enhances the mechanical structure stability of the graphite electrode. The concentrated salt electrolyte KFSI / TEP (molar ratio 1:1.3) can form an efficient and stable FSIˉ anion-derived interfacial film on the graphite electrode, effectively inhibiting the further decomposition of the electrolyte and the volume expansion and graphite layer peeling caused by the intercalation of FSIˉ anions during the cycling process. These synergistic effects enable the double-graphite electrode-based KDIBs to have a long lifespan and good cycling stability.

[0035] Figure 6 is the charge-discharge curves of the double-graphite electrode-based KDIBs assembled in Example 1 at the 50th and 100th cycles. It can be seen from this figure that the average working voltage of this battery is as high as 4.2 V, ensuring the high power density of the KDIBs.

Claims

1. A potassium-based dual-ion battery with dual graphite electrodes, characterized in that: A flame-retardant concentrated salt electrolyte composed of potassium bis(fluorosulfonyl)imide and triethyl phosphate solvent is adopted. The molar ratio of potassium bis(fluorosulfonyl)imide to triethyl phosphate solvent in the concentrated salt electrolyte is controlled within the range of 1:2 to 1:1.

3. Commercial graphite powder is selected as the active material for the positive and negative electrode plates, and sodium carboxymethyl cellulose is used as the binder to construct a potassium-based dual-ion battery based on dual graphite electrodes.

2. A preparation method of a potassium-based dual-ion battery with double graphite electrodes, characterized in that The preparation process includes the following steps: (1) Inside a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm), potassium bis(fluorosulfonyl)imide and triethyl phosphate solvent are mixed and stirred evenly to obtain a flame-retardant concentrated salt electrolyte; (2) Graphite is mixed with conductive carbon black and sodium carboxymethyl cellulose binder, ground evenly, and then deionized water is added to make a slurry, which is uniformly coated on an aluminum sheet and a copper sheet respectively, and vacuum dried at 70 °C for 24 h to obtain a graphite positive electrode and a graphite negative electrode accordingly; (3) The flame-retardant concentrated salt electrolyte obtained in step (1), the graphite positive electrode and the graphite negative electrode obtained in step (2) are assembled into a potassium-based dual-ion battery based on dual graphite electrodes inside a glove box under a high-purity argon atmosphere (O2 < 0.01 ppm, H2O < 0.01 ppm).

3. The preparation method of a potassium-based dual-ion battery with double graphite electrodes according to claim 2, characterized in that: In step (2), the graphite electrode uses sodium carboxymethyl cellulose as the binder, and the mass ratio of the binder is 5 wt.% to 10 wt.%.

4. A preparation method of a potassium-based dual-ion battery with double graphite electrodes according to claim 2, characterized in that: In step (3), the potassium-based dual-ion battery with dual graphite electrodes is a button cell or a soft-pack battery, and the capacity ratio of the negative electrode plate to the positive electrode plate is 1.1:1 to 1.2:1.

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