Sodium battery electrolyte and sodium metal / negative-electrode-free sodium battery

Abstract

The invention belongs to the technical field of sodium secondary batteries, and particularly relates to a sodium battery electrolyte and a sodium metal / negative-electrode-free sodium battery, the sodium battery electrolyte comprises the following components: a sodium salt and an ether solvent; the ether solvent is a multi-chain ether solvent or a cyclic ether solvent; wherein the multi-chain ether solvent is chain ether containing a branched chain; according to the invention, the multi-chain ether solvent or the cyclic ether solvent is introduced, and the ability of the multi-chain ether solvent or the cyclic ether solvent to form a multipoint coordination configuration with sodium ions can be hindered on the basis that branched chains in the multi-chain ether and a cyclic structure of the cyclic ether have a relatively large steric hindrance effect, so that the dissociation degree of the sodium salt is slightly reduced, and the dissociation efficiency of the sodium salt is improved. According to the present invention, the sodium ion desolvation difficulty is significantly reduced, the sodium ion rapid desolvation is achieved, and particularly the desolvation advantage in the low temperature environment of-70-60 DEG C is significant, such that the efficient reversible deposition-stripping of the sodium metal at the low temperature is achieved, the formation of the sodium dendrite is avoided, and the application of the sodium metal battery under different environmental conditions is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium secondary batteries, and in particular relates to a sodium battery electrolyte and a sodium metal / negative electrode-free sodium battery. Background Art

[0002] With the continuous growth of energy demand, sodium batteries, as a highly promising battery system, have received widespread attention. Sodium is abundant and widely distributed in nature, giving sodium batteries the advantages of cost and sufficient supply. However, compared with lithium batteries, their energy density is relatively low, which limits their application in many occasions with high energy density requirements, such as electric vehicles. In terms of battery materials, unlike the graphite electrodes used in lithium batteries, the negative electrode of sodium batteries usually uses hard carbon, such as bio-based hard carbon or coal-based hard carbon. However, whether it is bio-based hard carbon or coal-based hard carbon, the cost of large-scale production and manufacturing remains high, making the cost reduction task of sodium batteries still arduous.

[0003] To overcome the limitations of traditional sodium batteries in terms of energy density and cost, sodium metal / anode-free sodium batteries based on the sodium metal deposition-stripping mechanism have emerged as a new sodium battery configuration. Their anode consists of sodium metal or only the current collector, which can significantly reduce the cost of sodium batteries while significantly improving the battery's energy density. However, sodium metal / anode-free sodium batteries generally suffer from the problem of limited cycle life in practical applications. The reason is that during the sodium metal deposition-stripping process, the surface passivation layer (Solid-electrolyte interphase, SEI) formed by the sodium metal and the electrolyte continuously breaks down and reforms due to large volume changes. The continuous rupture and reformation of the surface passivation layer continuously consumes sodium metal, resulting in a low sodium deposition-stripping efficiency, which greatly affects the battery's cycle life. Therefore, in order to improve the sodium deposition-stripping efficiency and extend the cycle life, designing an electrolyte system that is stable to sodium metal and ensuring that the electrolyte can operate stably over a wide temperature range, such as high and low temperatures, has become an effective and feasible technical route.

[0004] From the perspective of electrochemistry, there is a difference in electrode potential between sodium metal and lithium metal. The electrode potential of sodium metal is 0.3V higher than that of lithium metal, which leads to significant differences in the redox behavior, decomposition path and decomposition products of solvents and salt molecules in the electrolyte on the surface of sodium metal and those of lithium metal. At the same time, the radius of sodium ions is larger than that of lithium ions, which makes the solvation and desolvation behavior of sodium ions in the electrolyte quite different from those of lithium ions. Based on the above differences, the electrolyte design of sodium metal batteries and lithium metal batteries needs to follow different design criteria. For example, the fluorocarbonates commonly used in lithium metal batteries can increase the deposition-stripping efficiency of lithium to more than 99%, but it is difficult to achieve the same effect in sodium metal batteries.

[0005] At present, the electrolyte system commonly used in existing sodium metal batteries is an ether electrolyte system; however, commonly used ether solvent molecules usually have strong coordination ability; especially straight-chain ether solvents, such as diethylene glycol dimethyl ether (G2) and tetraethylene glycol dimethyl ether (G4), which have more than two ether oxygen groups on the straight chain. The ether oxygen groups will form a multi-coordination structure with sodium ions, making the desolvation process of sodium ions difficult, resulting in the rapid consumption of sodium ions in the Helmholtz layer on the sodium metal surface at high current density or low temperature, which on the one hand triggers the formation of sodium dendrites, and on the other hand causes the battery to be unable to work at low temperatures, seriously limiting the application of sodium metal batteries under different environmental conditions.

[0006] In summary, in order to solve the above-mentioned problems of sodium metal / anode-free sodium batteries in the electrolyte system, it is urgent to optimize the design of the electrolyte system to improve the performance of sodium metal / anode-free sodium batteries and promote their widespread promotion in practical applications. Summary of the Invention

[0007] In response to the technical problems existing in the prior art, the present invention provides a sodium battery electrolyte and a sodium metal / anode-free sodium battery to solve the technical problem that the electrolyte system commonly used in existing sodium metal batteries makes the desolvation process of sodium ions difficult, easily triggers the formation of sodium dendrites, and causes the battery to be unable to operate at low temperatures, which seriously limits the application of sodium metal batteries under different environmental conditions.

[0008] In order to achieve the above object, the technical solution adopted by the present invention is:

[0009] The present invention provides a sodium battery electrolyte, comprising the following components: a sodium salt and an ether solvent; the ether solvent is a multi-chain ether solvent or a cyclic ether solvent;

[0010] Wherein, the multi-chain ether solvent is a chain ether containing branches, and the structural formula of the multi-chain ether solvent is:

[0011]

[0012] Among them, the molecular formula of R1, R2, R3 and R4 is C x H y , x=1, 2 or 3, y is determined according to the unsaturation degree of the C atom in R1, R2, R3 or R4;

[0013] The structural formula of the cyclic ether solvent is:

[0014]

[0015] Among them, the molecular formula of R5 is O a C b H c, a=0 or 1, b=0, 1 or 2, c is determined according to the unsaturation of the C atom in R5; the molecular formula of R6 and R7 is C n H m , n=0 or 1, m is determined according to the unsaturation degree of the C atom in R5.

[0016] Furthermore, the following components are also included: conventional solvent;

[0017] Wherein, the conventional solvent is one or more of tetrahydrofuran, 2-methyltetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0018] Furthermore, the concentration of the sodium salt is 0.2-2.0M.

[0019] Furthermore, the mass of the ether solvent is 5% to 95% of the total mass of the electrolyte.

[0020] Furthermore, the sodium salt is one or more of NaPF6, NaClO4, NaCF3SO3, Na(CF3SO2)2N, Na(FSO2)2N, NaBF4, NaC2BF2O4, NaPF2O2, NaB(C6H5)4 and NaC4BO8.

[0021] The present invention also provides a sodium metal / negative electrode-free sodium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and the sodium battery electrolyte.

[0022] Furthermore, the positive electrode sheet includes a positive electrode current collector and a positive electrode body containing a positive electrode active material adhered to the positive electrode current collector; wherein the positive electrode active material is one or more of sodium layered transition metal oxides, polyanions, Prussian blue and iron sulfide.

[0023] Furthermore, the negative electrode sheet is a negative electrode current collector, or the negative electrode sheet includes a negative electrode current collector and sodium metal loaded on the negative electrode current collector.

[0024] Furthermore, the diaphragm is polypropylene, polyethylene, PP / PE composite film or ceramic coated diaphragm.

[0025] Furthermore, the application temperature of the sodium metal / anode-free sodium battery is -70°C to 60°C.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The sodium battery electrolyte provided by the present invention introduces a multi-chain ether solvent or a cyclic ether solvent, wherein the multi-chain ether solvent is a chain ether containing a branch, and the cyclic ether solvent is a cyclic ether having a cyclic structure. Based on the large steric hindrance effect of the branch in the multi-chain ether and the cyclic structure of the cyclic ether, the ability of the multi-chain ether solvent or the cyclic ether solvent to form a multi-point coordination configuration with the sodium ion is hindered, so that the difficulty of sodium ion desolvation is significantly reduced while slightly reducing the degree of dissociation of the sodium salt, so that the sodium ion is rapidly desolvated, especially in a low temperature environment of -70°C to 60°C. The desolvation advantage is significant, thereby achieving efficient reversible deposition-stripping of sodium metal at low temperature, avoiding the formation of sodium dendrites, and improving the performance of sodium metal batteries under different environmental conditions. application; in addition, there are more than two ether oxygen groups on the straight chain in the multi-chain ether, which can form a multi-point coordination configuration with the sodium ion in space to promote the full dissociation of the sodium salt; secondly, by regulating the length and structure of the side chain in the chain ether, an asymmetric molecular structure or a molecular structure with large steric hindrance is formed to reduce the lattice energy and thus the melting point of the molecule, thereby ensuring the mobility of sodium ions under low temperature conditions, thereby ensuring the dissociation effect of the sodium salt; the present invention is simple, easy to operate and low in cost; in the manufacturing process of the sodium metal / anode-free sodium battery, there is no need to change any existing battery manufacturing process and no additional process cost is added, which is of great significance for achieving sodium metal / anode-free batteries with high specific energy, long life and wide temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0029] Figure 1 is the structural formula of the multi-chain ether solvent in the embodiment of the present invention;

[0030] Figure 2 is the structural formula of the cyclic ether solvent in the embodiment of the present invention. DETAILED DESCRIPTION

[0031] In order to make the technical problems, technical solutions, and beneficial effects solved by this application more clearly understood, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application; it is obvious that the described embodiments are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of this application.

[0032] The invention provides a sodium battery electrolyte comprising the following components: a sodium salt and an ether solvent.

[0033] The concentration of the sodium salt is 0.2-2.0M; preferably, the sodium salt is one or more of NaPF6, NaClO4, NaCF3SO3, Na(CF3SO2)2N, Na(FSO2)2N, NaBF4, NaC2BF2O4, NaPF2O2, NaB(C6H5)4 and NaC4BO8.

[0034] The ether solvent is a multi-chain ether solvent or a cyclic ether solvent; wherein the multi-chain ether solvent is a chain ether containing branches, and the structural formula of the multi-chain ether solvent is:

[0035]

[0036] Among them, the molecular formula of R1, R2, R3 and R4 is C x H y , x = 1, 2 or 3, y is determined according to the unsaturation degree of the C atom in R1, R2, R3 or R4.

[0037] The structural formula of the cyclic ether solvent is:

[0038]

[0039] Among them, the molecular formula of R5 is O a C b H c , a=0 or 1, b=0, 1 or 2, c is determined according to the unsaturation of the C atom in R5; the molecular formula of R6 and R7 is C n H m , n=0 or 1, m is determined according to the unsaturation degree of the C atom in R5.

[0040] Optionally, the following components are also included: conventional solvents.

[0041] The ether solvent is one or more of tetrahydrofuran, 2-methyltetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

[0042] Optionally, the mass of the ether solvent is 5% to 95% of the total mass of the electrolyte.

[0043] The electrolyte design principle is explained as follows:

[0044] The sodium battery electrolyte of the present invention forms a branched chain ether by adding side chains to a conventional linear ether molecular structure. By regulating the length and structure of the branches in the linear ether, an asymmetric molecular structure or a molecular structure with significant steric hindrance is formed. By reducing the lattice energy, the melting point of the molecule is lowered, which can effectively lower the freezing temperature of the electrolyte, ensure the mobility of sodium ions at low temperatures, and thus ensure the dissociation of sodium salts. By introducing a multi-chain ether solvent or a cyclic ether solvent, the difficulty of sodium ion desolvation is significantly reduced while slightly reducing the degree of sodium salt dissociation. Specifically, the linear chain of the linear ether contains two or more ether oxygen groups, which can form multi-point spatial coordination with sodium ions to promote the full dissociation of sodium salts. The branches in the multi-chain ether and the cyclic structure of the cyclic ether have a significant steric hindrance effect, which can hinder the ability of the multi-chain ether solvent and the cyclic ether solvent to form a multi-point coordination configuration with sodium ions. Although this can slightly reduce the degree of sodium salt dissociation, it greatly reduces the difficulty of sodium ion desolvation, thereby achieving rapid sodium ion desolvation.

[0045] It should be noted that in ether solvents, the coordination sites on the straight chain with sodium ions will be replaced by other chain ethers or cyclic ethers to form a co-coordinated solvation structure, which reduces the desolvation barrier of sodium ions and enables rapid desolvation of sodium ions. This is especially advantageous at low temperatures of -70°C to 60°C, thereby achieving efficient and reversible deposition-stripping of sodium metal at low temperatures.

[0046] Optionally, the degree of dissociation of the sodium salt can be effectively improved by introducing a conventional solvent.

[0047] The present invention also provides a sodium metal / negative electrode-free sodium battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode body containing a positive electrode active material adhered to the positive electrode current collector; wherein the positive electrode active material is one or more of sodium layered transition metal oxides, polyanions, Prussian blue and iron sulfide; the negative electrode sheet is a negative electrode current collector, or the negative electrode sheet comprises a negative electrode current collector and sodium metal loaded on the negative electrode current collector; the separator is arranged on the positive electrode sheet and the negative electrode sheet, and the separator is polypropylene, polyethylene, PP / PE composite film or ceramic coated separator; the electrolyte adopts the sodium battery electrolyte described in the present invention.

[0048] Preferably, the application temperature of the sodium metal / negative electrode-free sodium battery is -70°C to 60°C.

[0049] The sodium battery electrolyte and sodium metal / anode-free sodium battery described in the present invention can form a co-coordinated solvation structure by introducing branched chain ethers and cyclic ethers, thereby reducing the desolvation barrier of sodium ions and enabling rapid desolvation of sodium ions, thereby achieving efficient reversible deposition-stripping of sodium metal at low temperatures. The raw material cost of the electrolyte of the present invention is low and the preparation process is simple and easy. For the production process of the sodium metal / anode-free sodium battery, there is no need to change any existing battery cell manufacturing process and no additional process cost is added, which is of great significance for achieving sodium metal / anode-free batteries with high specific energy, long life, and a wide temperature range.

[0050] The sodium battery electrolyte provided by the present invention is further explained below with reference to some specific examples.

[0051] Example 1-101

[0052] Taking Example 1 as an example, this Example 1 provides a sodium battery electrolyte, including a sodium salt, a polyether solvent, and diethylene glycol dimethyl ether. The sodium salt is NaPF6, and the concentration of the sodium salt is 1M; the structural formula of the polyether solvent is as follows:

[0053]

[0054] The preparation process of the sodium battery electrolyte comprises:

[0055] In a glove box filled with argon, the pre-weighed multi-chain ether solvent and diethylene glycol dimethyl ether are mixed, and then the pre-weighed sodium salt is added, shaken and dissolved to obtain the sodium battery electrolyte.

[0056] In order to study the performance of the sodium battery electrolyte, this embodiment 1 also provides a sodium metal / negative electrode-free sodium battery, including a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the positive electrode sheet includes a positive electrode current collector and a positive electrode body containing a positive electrode active material adhered to the positive electrode current collector; wherein the positive electrode active material is one or more of sodium layered transition metal oxides, polyanions, Prussian blue and iron sulfide; the negative electrode sheet is a negative electrode current collector, or the negative electrode sheet includes a negative electrode current collector and sodium metal loaded on the negative electrode current collector; the separator is arranged on the positive electrode sheet and the negative electrode sheet, and the separator is polypropylene, polyethylene, PP / PE composite film or ceramic coated separator; the electrolyte uses the sodium battery electrolyte described in this embodiment 1.

[0057] The manufacturing process of the sodium metal / anode-free sodium battery includes:

[0058] The positive electrode sheet, separator and negative electrode sheet are encapsulated into a CR2032 button battery in the order of negative electrode shell-spring-gasket-negative electrode-diaphragm-electrolyte-positive electrode-positive electrode shell to obtain a sodium metal / negative electrode-free sodium battery.

[0059] Sodium ferric phosphate pyrophosphate (SFPP) was used as the positive electrode, and the electrolytes described in Examples 1-101 and Comparative Examples 1-5 were added to produce various sodium metal / anode-free sodium batteries. The cycling performance and results of all of these sodium metal / anode-free sodium batteries were evaluated. The following details the compositional characteristics of the sodium battery electrolytes used in Examples 1-101 and Comparative Examples 1-5, as well as the battery performance test results, as shown in Table 1.

[0060] Table 1 Component characteristics of sodium battery electrolyte and battery performance test results

[0061]

[0062]

[0063]

[0064]

[0065] It should be noted that in Table 1, M represents the molar concentration of sodium salt; A1 to A9 represent different types of polyether solvents, and the structural formulas of A1 to A9 are shown in the attached Figure 1 As shown; B1 to B13 represent different types of cyclic ether solvents respectively; THF represents tetrahydrofuran; MTHF represents 2-methyltetrahydrofuran; DME represents ethylene glycol dimethyl ether; G2 represents diethylene glycol dimethyl ether, and G4 represents tetraethylene glycol dimethyl ether.

[0066] As can be seen from Table 1, when the electrolyte contains only diethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether, or contains both diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether, the room temperature discharge capacity is >90 mAh / g, but the capacity at a low temperature of -20°C is <15 mAh / g, which shows that the discharge capacity at low temperature is very low; the reasons are analyzed as follows: since the linear ether has more than two ether oxygens, it will form multiple coordination with the sodium ions, making the desolvation process of the sodium ions difficult, resulting in the inability of the sodium ions to desolvate and nucleate at low temperatures; from the results of Examples 1-1010, it can be seen that when a chain ether or a cyclic ether with a branched chain is used, the discharge capacity of the sodium metal / anode-free sodium battery at low temperature is greatly improved.

[0067] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention.

Claims

1. A sodium battery electrolyte, characterized in that The invention comprises the following components: sodium salt and ether solvent; the ether solvent is a multi-chain ether solvent or a cyclic ether solvent; Wherein, the multi-chain ether solvent is a chain ether containing branches, and the structural formula of the multi-chain ether solvent is: Among them, the molecular formula of R1, R2, R3 and R4 is C x H y , x=1, 2 or 3, y is determined according to the unsaturation degree of the C atom in R1, R2, R3 or R4; The structural formula of the cyclic ether solvent is: Among them, the molecular formula of R5 is O a C b H c , a=0 or 1, b=0, 1 or 2, c is determined according to the unsaturation of the C atom in R5; the molecular formula of R6 and R7 is C n H m , n=0 or 1, m is determined according to the unsaturation degree of the C atom in R5.

2. A sodium battery electrolyte according to claim 1, characterized in that: It also includes the following components: conventional solvents; Wherein, the conventional solvent is one or more of tetrahydrofuran, 2-methyltetrahydrofuran, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether and tetraethylene glycol dimethyl ether.

3. The sodium battery electrolyte according to claim 1, characterized in that: The concentration of the sodium salt is 0.2-2.0M.

4. The sodium battery electrolyte according to claim 1, characterized in that: The mass of the ether solvent is 5% to 95% of the total mass of the electrolyte.

5. The sodium battery electrolyte according to claim 1, characterized in that: The sodium salt is one or more of NaPF6, NaClO4, NaCF3SO3, Na(CF3SO2)2N, Na(FSO2)2N, NaBF4, NaC2BF2O4, NaPF2O2, NaB(C6H5)4 and NaC4BO8.

6. A sodium metal / negative electrode-free sodium battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator arranged between the positive electrode sheet and the negative electrode sheet, and the sodium battery electrolyte according to any one of claims 1 to 5.

7. A sodium metal / negative electrode-free sodium battery according to claim 6, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode body containing a positive electrode active material adhered to the positive electrode current collector; wherein the positive electrode active material is one or more of sodium layered transition metal oxides, polyanions, Prussian blue and iron sulfide.

8. The sodium metal / negative electrode-free sodium battery according to claim 6, characterized in that: The negative electrode sheet is a negative electrode current collector, or the negative electrode sheet includes a negative electrode current collector and sodium metal loaded on the negative electrode current collector.

9. The sodium metal / negative electrode-free sodium battery according to claim 6, characterized in that: The diaphragm is polypropylene, polyethylene, PP / PE composite film or ceramic coated diaphragm.

10. The sodium metal / negative electrode-free sodium battery according to claim 6, characterized in that: The application temperature of the sodium metal / negative electrode-free sodium battery is -70°C to 60°C.

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