Electrolyte and lithium ion battery
By adding organic magnesium salt and organic calcium salt to the electrolyte of the lithium-ion battery, a SEI film with a mixed phase of Li/Mg/Ca/Si is formed, which solves the SEI film problem caused by the expansion of the silicon-based negative electrode, and improves the cycling stability and ionic conductivity of the battery.
Patent Information
- Application Number
- CN202411587908.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2024-11-08
- Publication Date
- 2025-03-14
AI Technical Summary
The silicon-based negative electrode has a high expansion rate in lithium-ion batteries, which leads to repeated growth of SEI film, damage to the electrode structure and a decrease in ionic conductivity, affecting the long cycle stability of the battery.
By introducing organic magnesium salt and organic calcium salt into the electrolyte, it participates in the reduction reaction of the silicon-based negative electrode surface to form a SEI film protective layer with a mixed phase of Li/Mg/Ca/Si, thereby improving the stability and ionic conductivity of the SEI film.
The formed SEI film is denser and stable, reducing repeated growth phenomena, and improving the cycle stability and charge and discharge rate of lithium-ion batteries.
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Figure CN119627235A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the technical field of silicon-based negative electrode system lithium-ion batteries, and in particular to an electrolyte and a lithium-ion battery. Background Art
[0002] The rapid development of new energy vehicles has put forward new demands on the energy density of lithium-ion batteries. High energy density batteries need to break through the original graphite negative electrode system and adopt a silicon-based negative electrode system with higher gram capacity. The gram capacity of silicon-based negative electrodes is 10 times that of graphite, and they are widely available and inexpensive, so they have been widely used.
[0003] The SEI film (Solid-Electrolyte Interface) is mainly formed on the surface of lithium-ion battery electrodes (especially negative electrodes). It is a layer of solid electrolyte interface film formed by a series of complex electrochemical reduction reactions of the electrolyte on the electrode surface during the first charge and discharge process of the battery. The performance of the SEI film (stability and ionic conductivity) is closely related to the long-cycle performance of the battery cell. However, the silicon-based negative electrode has the disadvantage of a high expansion rate, which will lead to problems such as particle pulverization, electrode structure destruction, repeated growth of the SEI film, and decreased ionic conductivity, which seriously affects the long-cycle stability of the lithium-ion battery cell. Summary of the invention
[0004] The present application provides an electrolyte and a lithium-ion battery to solve the following technical problem: how to improve the performance of the SEI film on the surface of a silicon-based negative electrode.
[0005] In a first aspect, the present application provides an electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, and the raw materials of the electrolyte include: an organic magnesium salt, an organic calcium salt and a basic electrolyte; wherein,
[0006] The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt and the weight M3 of the basic electrolyte satisfy a first relationship: (M1+M2) / M3=3% to 8%.
[0007] Optionally, the first relationship is: (M1+M2) / M3=8%.
[0008] Optionally, the organic magnesium salt includes at least one of the following: magnesium bis(difluorosulfonyl imide), magnesium bis(trifluoromethylsulfonyl imide), and magnesium trifluoromethanesulfonate.
[0009] Optionally, the organic calcium salt includes at least one of the following: calcium bis(difluorosulfonyl)imide, calcium bis(trifluoromethylsulfonyl)imide, and calcium trifluoromethanesulfonate.
[0010] Optionally, the molar ratio of the organic magnesium salt to the organic calcium salt is (2:3) to (3:2).
[0011] Optionally, the weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy a second relationship: M5 / (M4+M5)=12%-18%.
[0012] Optionally, the basic electrolyte includes a carbonate solvent, a lithium salt and fluoroethylene carbonate.
[0013] Optionally, the carbonate solvent includes: ethylene carbonate, propylene carbonate, diethyl carbonate and ethyl methyl carbonate; wherein, by mass fraction,
[0014] The content of the ethylene carbonate is 20% to 30%, the content of the propylene carbonate is 0% to 5%, the content of the diethyl carbonate is 40% to 60%, and the content of the ethyl methyl carbonate is 20% to 30%.
[0015] Optionally, the lithium salt includes a conductive lithium salt and a film-forming lithium salt, wherein:
[0016] The weight ratio of the conductive lithium salt to the film-forming lithium salt is (9-11):1; and / or,
[0017] The conductive lithium salt includes lithium hexafluorophosphate; and / or,
[0018] The film-forming lithium salt includes at least one of the following: lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide), and lithium bis(trifluoromethylsulfonylimide).
[0019] In a second aspect, the present application provides a lithium-ion battery, comprising the electrolyte described in any one of the embodiments of the first aspect, and a silicon-based negative electrode.
[0020] The above technical solution provided by the embodiment of the present application has the following advantages compared with the prior art:
[0021] The electrolyte provided in the embodiment of the present application can be used to participate in the reduction reaction on the surface of the silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode; an organic magnesium salt and an organic calcium salt are introduced into the basic electrolyte, and both the organic magnesium salt and the organic calcium salt can participate in the reduction of the silicon-based negative electrode to participate in the formation of the SEI film, the organic magnesium salt is reduced to form a MgF2 component in the SEI film, and the organic calcium salt is reduced to form a CaF2 component in the SEI film, so that the original pure lithium phase SEI film is converted into a Li / Mg / Ca / Si mixed phase SEI film protective layer, and the formed MgF2 component can improve the ionic conductivity of the SEI film; the formed CaF2 component can improve the stability of the SEI film, thereby forming a dense and stable SEI film; therefore, the MgF2 component and the CaF2 component balance the stability and ionic conductivity of the SEI film, and the SEI film containing MgF2 / CaF2 compounds formed by the organic magnesium salt and the organic calcium salt is more stable and has high ionic conductivity, thereby improving the performance of the SEI film on the surface of the silicon-based negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.
[0023] In order to more clearly illustrate the embodiments of the present application 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, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.
[0024] Figure 1 A schematic diagram of the principle of an electrolyte participating in the reduction reaction on the surface of a silicon-based negative electrode to form a stable SEI film provided in an embodiment of the present application. DETAILED DESCRIPTION
[0025] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application.
[0026] Various embodiments of the present application may be presented in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be understood as a rigid limitation on the scope of the present application; therefore, the range description should be considered to have specifically disclosed all possible sub-ranges and single numerical values within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, which apply regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.
[0027] In the present application, in the absence of any contrary description, the directional words used, such as "upper" and "lower", are specifically the directions of the drawings in the accompanying drawings. In addition, in the description of the specification of the present application, the terms "include", "comprise", etc. refer to "including but not limited to". In this article, "and / or" describes the association relationship of the associated objects, indicating that there may be three relationships, for example, A and / or B, which can represent: A exists alone, A and B exist at the same time, and B exists alone. Wherein A and B can be singular or plural. In this article, "at least one" refers to one or more, and "multiple" refers to two or more. "At least one", "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can all represent: a, b, c, ab (i.e., a and b), ac, bc or abc, where a, b, c can be single or multiple, respectively.
[0028] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.
[0029] In a first aspect, the present application provides an electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, and the raw materials of the electrolyte include: an organic magnesium salt, an organic calcium salt and a basic electrolyte; wherein,
[0030] The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt and the weight M3 of the basic electrolyte satisfy a first relationship: (M1+M2) / M3=3% to 8%.
[0031] In some embodiments, the first relationship is: (M1+M2) / M3=8%.
[0032] In the embodiment of the present application, the electrolyte is used to participate in the reduction reaction on the surface of the silicon-based negative electrode to form a SEI film (full name: solid electrolyte interface film, which is a passivation layer covering the surface of the electrode material formed during the first charge and discharge process of a lithium-ion battery) on the surface of the silicon-based negative electrode; Figure 1 A schematic diagram of the principle of an electrolyte provided in an embodiment of the present application participating in the reduction reaction on the surface of a silicon-based negative electrode to form a stable SEI film; see Figure 1 , organic magnesium salt and organic calcium salt are introduced into the basic electrolyte, and both organic magnesium salt and organic calcium salt can participate in the formation of SEI film by being reduced by silicon-based negative electrode, and organic magnesium salt is reduced to form MgF2 component in SEI film, and organic calcium salt is reduced to form CaF2 component in SEI film, so that the original pure lithium phase SEI film is converted into a Li / Mg / Ca / Si mixed phase SEI film protective layer, and the formed MgF2 component can improve the ion conductivity of SEI film; the formed CaF2 component can improve the stability of SEI film, so as to form a dense and stable SEI film; therefore, MgF2 component and CaF2 component balance the stability and ion conductivity of SEI film, and the SEI film containing MgF2 / CaF2 compound formed by organic magnesium salt and organic calcium salt is more stable and has high ion conductivity, and finally improves the performance of SEI film on the surface of silicon-based negative electrode.
[0033] During the battery charging process, the expansion of the silicon-based negative electrode will cause the SEI film originally covering its surface to be stretched. Frequent stretching may cause cracks, ruptures, or separation of the SEI film from the surface of the silicon-based negative electrode. As the charge and discharge cycle proceeds, the continuous expansion of the silicon-based negative electrode will continuously destroy and change the structure of the SEI film, which may cause the ionic conductivity of the SEI film to decrease. Therefore, the expansion of the silicon-based negative electrode seriously affects the stability and ionic conductivity of the SEI film. The weight of the organic magnesium salt M1, the weight of the organic calcium salt M2, and the weight of the basic electrolyte M3 can satisfy the first relationship: (M1+M2) / M3=3%~8%, which can fully convert the original pure lithium phase SEI film into a Li / Mg / Ca / Si mixed phase SEI film. If the value of M1+M2 is too high and exceeds 8%, it means that too much Mg / Ca is introduced into the SEI film, which will reduce the ionic conductivity of the SEI film to a certain extent; if the value of M1+M2 is too low and is lower than 3%, it will cause the SEI film to be unstable to a certain extent. Exemplarily, the value of (M1+M2) / M3 can be 3%, 4%, 5%, 6%, 7%, 8%, etc. Considering the synergistic effect and economic cost of organic magnesium salt and organic calcium salt, the value of (M1+M2) / M3 can be 8%.
[0034] In some embodiments, the organic magnesium salt includes at least one of: magnesium bis(difluorosulfonyl)imide, magnesium bis(trifluoromethylsulfonyl)imide, and magnesium trifluoromethanesulfonate.
[0035] In the embodiments of the present application, the organic magnesium salt may be a combination of one or more of magnesium bis(difluorosulfonyl)imide, magnesium bis(trifluoromethylsulfonyl)imide, and magnesium trifluoromethanesulfonate. These organic magnesium salts can be reduced by the negative electrode to participate in the formation of the SEI film, and are reduced to form a MgF2 component in the SEI film. These organic magnesium salts are common organic magnesium salt additives and have good compatibility with the carbonate solvent used.
[0036] In some embodiments, the organic calcium salt includes at least one of the following: calcium bis(difluorosulfonyl)imide, calcium bis(trifluoromethanesulfonyl)imide, and calcium trifluoromethanesulfonate.
[0037] In the embodiments of the present application, the organic magnesium salt may be a combination of one or more of calcium bis(difluorosulfonyl)imide, calcium bis(trifluoromethanesulfonyl)imide, and calcium trifluoromethanesulfonate. These organic calcium salts can be reduced by the negative electrode to participate in the formation of the SEI film, and are reduced to form a CaF2 component in the SEI film. These organic calcium salts are common organic calcium salt additives and have good compatibility with the carbonate solvent used.
[0038] In some embodiments, the molar ratio of the organic magnesium salt to the organic calcium salt is (2:3) to (3:2).
[0039] In the embodiment of the present application, from the stability of the SEI film, CaF2>MgF2>LiF, but from the ionic conductivity, CaF2<MgF2<LiF. If only the organic magnesium salt is reduced to form MgF2, the stability of the SEI film is insufficient, especially when the silicon material expands, it may rupture or reconstruct, resulting in obstruction of lithium ion conduction; if only the organic calcium salt is reduced to form CaF2, the ionic conductivity of the SEI film is insufficient, and it is difficult to make up for the gap in the decrease in the ionic conductivity of the SEI film caused by the expansion of the silicon-based negative electrode. The molar ratio of the organic magnesium salt to the organic calcium salt can be (2:3) to (3:2), which can better balance the stability and ionic conductivity of the SEI film. Exemplarily, the molar ratio of the organic magnesium salt to the organic calcium salt can be 3:2, 2:3, 1:1, etc., and the molar ratio of the organic magnesium salt to the organic calcium salt can be 1:1.
[0040] In some embodiments, the base electrolyte includes a carbonate solvent, a lithium salt, and fluoroethylene carbonate (FEC).
[0041] In the embodiments of the present application, the carbonate solvent can dissolve organic magnesium salts, organic calcium salts and lithium salts; the lithium salt can improve the ionic conductivity of the electrolyte and promote the formation of an inorganic phase in a stable SEI film on the surface of the silicon-based negative electrode; fluoroethylene carbonate can be used as a solvent additive to promote the formation of an organic phase and an inorganic phase in a stable SEI film on the surface of the silicon-based negative electrode.
[0042] In some embodiments, the weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%-18%.
[0043] In the embodiment of the present application, the weight of the carbonate solvent M4 and the weight of the fluoroethylene carbonate M5 can satisfy the second relationship: M5 / (M4+M5)=12% to 18%, so that the fluoroethylene carbonate can fully assist in the formation of a stable SEI film on the surface of the silicon-based negative electrode. Exemplarily, M5 / (M4+M5) can be 12%, 13%, 14%, 15%, 16%, 17%, 18%, etc.
[0044] In some embodiments, the carbonate solvent includes: ethylene carbonate, propylene carbonate, diethyl carbonate and ethyl methyl carbonate; wherein, by mass fraction,
[0045] The content of the ethylene carbonate is 20% to 30%, the content of the propylene carbonate is 0% to 5%, the content of the diethyl carbonate is 40% to 60%, and the content of the ethyl methyl carbonate is 20% to 30%.
[0046] In the embodiment of the present application, the carbonate solvent may include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC), and the mass fraction of each carbonate solvent is reasonably designed, so that the electrolyte can have certain high temperature performance and low temperature performance, and have both cost economy. Exemplarily, the content of ethylene carbonate can be 20%, 22%, 24%, 26%, 28%, 30%, etc., the content of propylene carbonate can be 1%, 2%, 3%, 4%, 5%, etc., the content of diethyl carbonate can be 40%, 45%, 50%, 55%, 60%, etc., and the content of ethyl methyl carbonate can be 20%, 22%, 24%, 26%, 28%, 30%, etc.
[0047] In some embodiments, the lithium salt includes a conductive lithium salt and a film-forming lithium salt, wherein:
[0048] The weight ratio of the conductive lithium salt to the film-forming lithium salt is (9-11):1; and / or,
[0049] The conductive lithium salt includes lithium hexafluorophosphate; and / or,
[0050] The film-forming lithium salt includes at least one of the following: lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide), and lithium bis(trifluoromethylsulfonylimide).
[0051] In the embodiment of the present application, the conductive lithium salt in the lithium salt can improve the ionic conductivity of the electrolyte, and the film-forming lithium salt in the lithium salt can promote the formation of an inorganic phase in the stable SEI film on the surface of the silicon-based negative electrode, thereby improving the stability of the SEI film. The weight of the conductive lithium salt and the film-forming lithium salt is reasonably distributed to give play to the respective advantages of the conductive lithium salt and the film-forming lithium salt. Exemplarily, the weight ratio of the conductive lithium salt to the film-forming lithium salt can be 9:1, 10:1, 11:1, etc., and the film-forming lithium salt can be a combination of one or more of lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide), and lithium bis(trifluoromethylsulfonylimide). The concentration of the above lithium salt can be 1 mol / L to 1.25 mol / L.
[0052] In a second aspect, the present application provides a lithium-ion battery, comprising the electrolyte described in any one of the embodiments of the first aspect, and a silicon-based negative electrode.
[0053] In the embodiment of the present application, the reduction reaction of lithium salt, organic magnesium salt and organic calcium salt in the electrolyte on the surface of silicon-based negative electrode can obtain a stable SEI film of Li / Mg / Ca / Si mixed phase, which has a compact and uniform structure and can conduct lithium ions more effectively. The SEI film has high stability and ionic conductivity, reduces the phenomenon of repeated growth of SEI film, and constructs a highly stable and highly conductive SEI film layer on the silicon surface, which will not only reduce the continuous consumption of active lithium, but also solve the problems of increased internal resistance and flatulence of battery cells to a large extent, and help improve the long cycle stability of battery cells, thereby improving the charge and discharge rate and cycle efficiency of lithium-ion batteries. The above-mentioned silicon-based negative electrode can be a bare silicon-based negative electrode coated with carbon, including one of porous silicon, nano-silicon particles, silicon nanowires and silicon nanosheets. This electrolyte is applied to a lithium-ion battery system with silicon-based materials as negative electrodes. Compared with electrolytes without organic magnesium salts and calcium salts, lithium-ion batteries show better cycle stability.
[0054] The lithium-ion battery is realized based on the above-mentioned electrolyte. The specific raw materials of the electrolyte can refer to the above-mentioned embodiments. Since the electrolyte adopts part or all of the technical solutions of the above-mentioned embodiments, it has at least all the beneficial effects brought by the technical solutions of the above-mentioned embodiments, which will not be described one by one here.
[0055] The present application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are intended only to illustrate the present application and are not intended to limit the scope of the present application. The experimental methods for which specific conditions are not specified in the following examples are usually measured according to national standards. If there is no corresponding national standard, then the conditions recommended by the manufacturer are followed.
[0056] Example 1
[0057] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: an organic magnesium salt, an organic calcium salt and a basic electrolyte; wherein:
[0058] The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt, and the weight M3 of the basic electrolyte satisfy the first relationship: (M1+M2) / M3=5%, the molar ratio of the organic magnesium salt to the organic calcium salt is 1:1, the organic magnesium salt is magnesium bis(trifluoromethylsulfonyl imide), and the organic calcium salt is calcium bis(trifluoromethylsulfonyl imide);
[0059] The basic electrolyte includes carbonate solvent, lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, PC, DEC and EMC; EC accounts for 30% by weight, PC accounts for 5% by weight, DEC accounts for 45% by weight, and EMC accounts for 20% by weight;
[0060] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%;
[0061] The concentration of the lithium salt is 1 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium difluorooxalatoborate.
[0062] Example 2
[0063] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: an organic magnesium salt, an organic calcium salt and a basic electrolyte; wherein:
[0064] The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt, and the weight M3 of the basic electrolyte satisfy the first relationship: (M1+M2) / M3=8%, the molar ratio of the organic magnesium salt to the organic calcium salt is 1:1, the organic magnesium salt is magnesium bis(difluorosulfonyl)imide, and the organic calcium salt is calcium trifluoromethanesulfonate;
[0065] The basic electrolyte includes carbonate solvent, lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, DEC and EMC; EC accounts for 30% by weight, DEC accounts for 45% by weight, and EMC accounts for 25% by weight;
[0066] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=18%;
[0067] The concentration of the lithium salt is 1.25 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium bis(trifluoromethylsulfonyl)imide.
[0068] Example 3
[0069] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: an organic magnesium salt, an organic calcium salt and a basic electrolyte; wherein:
[0070] The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt, and the weight M3 of the basic electrolyte satisfy the first relationship: (M1+M2) / M3=6%, the molar ratio of the organic magnesium salt to the organic calcium salt is 1:1, the organic magnesium salt is magnesium bis(difluorosulfonyl)imide, and the organic calcium salt is calcium trifluoromethanesulfonate;
[0071] The basic electrolyte includes carbonate solvent, lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, DEC and EMC; EC accounts for 30% by weight, DEC accounts for 45% by weight, and EMC accounts for 25% by weight;
[0072] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=15%;
[0073] The concentration of the lithium salt is 1.25 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium bis(trifluoromethylsulfonyl)imide.
[0074] Comparative Example 1
[0075] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: a carbonate solvent, a lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, PC, DEC and EMC; EC accounts for 30% by weight, PC accounts for 5% by weight, DEC accounts for 45% by weight, and EMC accounts for 20% by weight;
[0076] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%;
[0077] The concentration of the lithium salt is 1 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium difluorooxalatoborate.
[0078] Comparative Example 2
[0079] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: an organic calcium salt and a basic electrolyte; wherein:
[0080] The weight M2 of the organic calcium salt and the weight M3 of the basic electrolyte satisfy the first relationship: M2 / M3=5%, and the organic calcium salt is calcium bis(trifluoromethylsulfonyl)imide;
[0081] The basic electrolyte includes carbonate solvent, lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, PC, DEC and EMC; EC accounts for 30% by weight, PC accounts for 5% by weight, DEC accounts for 45% by weight, and EMC accounts for 20% by weight;
[0082] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%;
[0083] The concentration of the lithium salt is 1 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium difluorooxalatoborate.
[0084] Comparative Example 3
[0085] An electrolyte, which can be used to participate in the reduction reaction on the surface of a silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode, wherein the raw materials of the electrolyte include: an organic magnesium salt and a basic electrolyte; wherein:
[0086] The weight M1 of the organic magnesium salt and the weight M3 of the basic electrolyte satisfy the first relationship: M1 / M3=5%, and the organic magnesium salt is magnesium bistrifluoromethylsulfonyl imide;
[0087] The basic electrolyte includes carbonate solvent, lithium salt and fluoroethylene carbonate; wherein the carbonate solvent includes: EC, PC, DEC and EMC; EC accounts for 30% by weight, PC accounts for 5% by weight, DEC accounts for 45% by weight, and EMC accounts for 20% by weight;
[0088] The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%;
[0089] The concentration of the lithium salt is 1 mol / L; the lithium salt includes a conductive lithium salt and a film-forming lithium salt; wherein the weight ratio of the conductive lithium salt to the film-forming lithium salt is 9:1, the conductive lithium salt is lithium hexafluorophosphate, and the film-forming lithium salt is lithium difluorooxalatoborate.
[0090] The electrolytes obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were respectively assembled with a lithium metal counter electrode, a carbon-coated porous silicon-based negative electrode and a separator into button-type lithium-ion batteries for lithium desorption capacity cycle testing. The results are shown in Table 1.
[0091] Table 1 Lithium removal capacity cycle test results
[0092] Serial number First week lithium removal capacity (mAh / g) 50-week lithium removal capacity (mAh / g) 100-week lithium removal capacity (mAh / g) Example 1 2998 1994 1316 Example 2 3027 2214 1548 Example 3 3003 2149 1401 Comparative Example 1 3068 1345 643 Comparative Example 2 2876 1679 924 Comparative Example 3 3098 1534 876
[0093] Note: X-week lithium removal capacity refers to the lithium removal capacity of the electrode material after X cycles.
[0094] As shown in Table 1, in Examples 1 to 3, organic magnesium salt and organic calcium salt are added to the electrolyte, and through reasonable matching with FEC and film-forming lithium salt, a SEI film protective layer rich in Li / Mg / Ca / Si mixed phase is generated on the surface of the silicon-based negative electrode, which improves the density and stability of the SEI film, and rarely cracks appear. The SEI film also has good ionic conductivity. Through the preparation of the electrolyte and the improvement of the SEI film, the problem of repeated growth of the SEI film is greatly improved, and the cycle life of the high specific energy battery is greatly improved.
[0095] In Comparative Example 1, the electrolyte contains no organic magnesium salt and organic calcium salt, which is reflected in a significant decrease in the 100-week lithium removal capacity; in Comparative Example 2, the electrolyte contains no organic magnesium salt, the stability of the SEI film is insufficient, the internal resistance of the lithium ion battery increases, and the lithium removal capacity after the cycle is lower than that of the embodiment; in Comparative Example 3, the electrolyte contains no organic calcium salt, the ionic conductivity of the SEI film is insufficient, and the 100-week lithium removal capacity is lower than that of the embodiment.
[0096] One or more technical solutions in the embodiments of the present application also have at least the following technical effects or advantages:
[0097] (1) Rationally prepare the electrolyte to generate a dense and stable SEI film, reduce the repeated growth of the SEI film and the destruction of the SEI film structure caused by the expansion of the silicon-based negative electrode, and improve the cycle stability of the lithium-ion battery;
[0098] (2) Adding appropriate proportions of FEC, film-forming lithium salts, organic magnesium salts, and calcium salts to the electrolyte, in situ generating a SEI film protective layer rich in Li / Mg / Ca / Si phases on the surface of the silicon-based negative electrode, thereby improving the long-cycle performance of the silicon-based negative electrode;
[0099] (3) By adding a small amount of organic magnesium salt and organic calcium salt to the basic electrolyte, a SEI membrane with both high stability and high ionic conductivity can be achieved, which is highly feasible.
[0100] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features applied for herein.
Claims
1. An electrolyte, characterized in that: The electrolyte can be used to participate in the reduction reaction on the surface of the silicon-based negative electrode to form a SEI film on the surface of the silicon-based negative electrode. The raw materials of the electrolyte include: organic magnesium salt, organic calcium salt and basic electrolyte; wherein, The weight M1 of the organic magnesium salt, the weight M2 of the organic calcium salt and the weight M3 of the basic electrolyte satisfy a first relationship: (M1+M2) / M3=3% to 8%.
2. The electrolyte according to claim 1, characterized in that The first relation is: (M1+M2) / M3=8%.
3. The electrolyte according to claim 1, characterized in that The organic magnesium salt includes at least one of the following: magnesium bis(difluorosulfonyl)imide, magnesium bis(trifluoromethylsulfonyl)imide, and magnesium trifluoromethanesulfonate.
4. The electrolyte according to claim 1, characterized in that The organic calcium salt includes at least one of the following: calcium bis(difluorosulfonyl)imide, calcium bis(trifluoromethylsulfonyl)imide, and calcium trifluoromethanesulfonate.
5. The electrolyte according to any one of claims 1 to 4, characterized in that The molar ratio of the organic magnesium salt to the organic calcium salt is (2:3) to (3:2).
6. The electrolyte according to claim 1, characterized in that The basic electrolyte includes a carbonate solvent, a lithium salt and fluoroethylene carbonate.
7. The electrolyte according to claim 6, characterized in that The weight M4 of the carbonate solvent and the weight M5 of the fluoroethylene carbonate satisfy the second relationship: M5 / (M4+M5)=12%-18%.
8. The electrolyte according to claim 6, characterized in that The carbonate solvents include: ethylene carbonate, propylene carbonate, diethyl carbonate and ethyl methyl carbonate; wherein, by mass fraction, The content of the ethylene carbonate is 20% to 30%, the content of the propylene carbonate is 0% to 5%, the content of the diethyl carbonate is 40% to 60%, and the content of the ethyl methyl carbonate is 20% to 30%.
9. The electrolyte according to claim 6, characterized in that The lithium salt includes a conductive lithium salt and a film-forming lithium salt, wherein: The weight ratio of the conductive lithium salt to the film-forming lithium salt is (9-11):1; and / or, The conductive lithium salt comprises lithium hexafluorophosphate; and / or, The film-forming lithium salt includes at least one of the following: lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium bis(difluorosulfonylimide), and lithium bis(trifluoromethylsulfonylimide).
10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the electrolyte according to any one of claims 1 to 9, and a silicon-based negative electrode.