Electrochemical cell

Abstract

Electrochemical cell (20) that cycles lithium ions, comprising: an electrochemical cell (20) comprising a silicon-containing electroactive material which undergoes volumetric expansion and contraction during the electrochemical cell cycle; and an electrolyte system (30) that promotes the passive formation of a flexible solid electrolyte interface (SEI) protective layer comprising a lithium fluoride (LiF) polymer composite on one or more exposed surface areas of the silicon-containing electroactive material, wherein the electrolyte system (30) comprises a lithium salt, one or more cyclic carbonate-containing co-solvents, and two or more linear carbonate-containing co-solvents, wherein at least one of the two or more linear carbonate-containing co-solvents is a linear fluorinated carbonate-containing co-solvential comprising fluorine, wherein the electrolyte system (30) accommodates the volumetric expansion and contraction of the silicon-containing electroactive material to promote long-term cycle stability; wherein at least one of the one or more cyclic carbonate-containing co-solvents of the electrolyte system (30) is fluoroethylene carbonate (FEC); and wherein the fluoroethylene carbonate (FEC) and the two linear carbonate-containing co-solvents are present in a volume ratio of 1:2:2; wherein the flexible SEI protective layer, which is promoted by the electrolyte system (30), has a thickness greater than or equal to 1 nm to less than or equal to 100 nm.

Description

[0001] The present description relates to an electrochemical cell with an electrolyte system that improves performance when an electrode contains a silicon-containing electroactive material, for example by promoting the passive formation of a flexible protective layer on one or more exposed surface areas of a silicon-containing electroactive material.

[0002] High-energy-density electrochemical cells, such as lithium-ion and lithium-sulfur batteries, can be used in a wide variety of consumer products and vehicles, including hybrid electric vehicles (HEVs) and electric vehicles (EVs). Typical lithium-ion and lithium-sulfur batteries include a primary electrode (e.g., a cathode), a secondary electrode (e.g., an anode), an electrolyte, and a separator. Often, a stack of lithium-ion battery cells is electrically connected to increase overall power. Conventional lithium-ion and lithium-sulfur batteries rely on the reversible flow of lithium ions between the negative and positive electrodes and back again. A separator and an electrolyte are positioned between the negative and positive electrodes. The electrolyte is conductive and can be in solid or liquid form.When the battery is charged, lithium ions move from a cathode (e.g., positive electrode) to an anode (e.g., negative electrode), and when the battery is discharged, they move in the opposite direction.

[0003] The contact of the anode and cathode materials with the electrolyte can generate an electrical potential between the electrodes. When an electron current is generated in an external circuit between the electrodes, the potential is maintained by electroactive reactions within the battery cells. Each of the negative and positive electrodes in a stack is connected to a current collector (typically a metal, such as nickel or copper, for the anode and aluminum for the cathode). During battery use, the current collectors connected to the two electrons are linked together via an external circuit, allowing the current generated by the electrons to be carried between the electrodes to compensate for the transport of lithium ions.

[0004] Many different materials can be used to manufacture components for a lithium-ion battery. As a non-limiting example, cathode materials for lithium batteries typically include an electroactive material that can be intercalated with lithium ions, such as lithium transition metal oxides or mixed oxides of the spinel type, for example, with spinel LiMn₂O₄, LiCoO₂, LiNiO₂, LiMn₂ 1.5 Ni 0.5 O4, LiNi (1-x-y) CO x M yO2 (where 0 <x<1, y<1 und M Al, Mn oder dergleichen sein können), oder Lithiumeisenphosphate. Der Elektrolyt enthält typischerweise ein oder mehrere Lithiumsalze, die in einem nichtwässrigen Lösungsmittel gelöst und ionisiert sein können. Die negative Elektrode beinhaltet typischerweise ein Lithium-Einlegematerial oder ein Legierungs-Wirtsmaterial. Typische elektrochemisch aktive Materialien zur Bildung einer Anode beinhalten zum Beispiel Lithium-Graphit-Interkalationsverbindungen, Lithium-Silicium-Interkalationsverbindungen, Lithium-Zinn-Interkalationsverbindungen, Lithiumlegierungen.

[0005] Certain anode materials offer particular advantages. While graphite compounds are the most common, anode materials with high specific capacity (compared to conventional graphite) have recently attracted increasing interest. For example, silicon has the highest known theoretical charge capacity for lithium, making it one of the most promising materials for rechargeable lithium-ion batteries. However, current silicon-based anode materials suffer from significant weaknesses. The large volume changes (e.g., expansion / contraction) of silicon-based materials during lithium insertion / extraction (e.g., insertion and removal) can lead to physical damage to the anode, including wrinkling, fracture, or cracking. Such volumetric expansion can thus result in a loss of electrical contact and electrode activity.This applies particularly to the charge density levels required for the commercial viability of the electrodes. Furthermore, a solid electrolyte interface layer (SEI) can form on the active material surface, causing continuous electrolyte consumption and lithium-ion loss, which can lead to irreversible capacity loss in a lithium-ion battery. The large volumetric expansion during the insertion / alloying of the silicon anode can thus result in a decrease in electrochemical cycle performance, reduced coulomb charge capacity (capacity loss), and an extremely limited lifespan.

[0006] US Patent 2016 O 233 513 A1 describes a polyimide binder for energy storage devices that can improve the properties of an energy storage device over a wide temperature range, as well as an electrode plate and an energy storage device that uses it. The polyimide binder for energy storage devices is a polyimide obtained by an imidation reaction of an aqueous solution of a polyamic acid consisting of a repeating unit, wherein the polyimide has a tensile modulus of 1.5 GPa or more and 2.7 GPa or less.

[0007] US 2009 0 253 048 A1 describes a non-aqueous electrolyte solution which has a linear carbonate.

[0008] The challenge lies in developing materials for use in high-energy lithium-ion batteries that overcome the current weaknesses preventing the widespread commercial use of silicon-based electrode materials, particularly in automotive applications. For long-term and effective use, silicon-containing anode materials would need to exhibit minimal capacity loss and optimal charge capacity for extended use in lithium-ion batteries.

[0009] An electrochemical cell according to the invention is described, which cycles lithium ions. The electrochemical cell comprises an electrode comprising an electroactive material that expands and contracts volumetrically during the electrochemical cell cycle. The electrochemical cell further comprises an electrolyte system that promotes the passive formation of a flexible, protective solid electrolyte interface layer (SEI) comprising a lithium fluoride (LiF) polymer composite on one or more exposed surface areas of the electroactive material. The electroactive material is a silicon-containing electroactive material. The electrolyte system comprises a lithium salt, one or more cyclic carbonate-containing co-solvents, and two or more linear carbonate-containing co-solvents. At least one of the two or more linear carbonate-containing co-solvents is a fluorinated carbonate-containing co-solvent.The electrolyte system accommodates the volumetric expansion and contraction of the electroactive material to promote long-term cycle stability. At least one of the cyclic carbonate-containing co-solvents in the electrolyte system is fluoroethylene carbonate (FEC). The fluoroethylene carbonate (FEC) and the two linear carbonate-containing co-solvents are present in a volume ratio of 1:2:2. The flexible SEI protective layer promoted by the electrolyte system (30) has a thickness greater than or equal to 1 nm to less than or equal to 100 nm.

[0010] In one embodiment, the cyclic carbonate-containing co-solvent of the electrolyte system is present in an amount greater than or equal to approximately 1 vol% to less than or equal to approximately 50 vol%.

[0011] In one embodiment, the linear fluorinated carbonate-containing co-solvent of the electrolyte system is present in an amount greater than or equal to approximately 2 vol% to less than or equal to approximately 90 vol%.

[0012] In one embodiment, the linear fluorinated carbonate-containing co-solvent of the electrolyte system is selected from the group consisting of: Methyl 2,2,2-trifluoroethyl carbonate; Ethyl 2,2,2-trifluoroethyl carbonate; bis-(2,2-difluoroethyl) carbonate; 2,2-Difluoroethyl-2,2,2-trifluoroethyl carbonate; 2,2-Difluoroethylhexafluoroisopropyl carbonate; bis-(2,2,2-trifluoroethyl) carbonate; 2,2,3,3,3-Pentafluoropropyl-2,2,2-trifluoroethyl carbonate; 2,2-Difluoroethylethyl carbonate (2F-DEC); 2,2-Difluoroethylmethyl carbonate (2F-EMC); and combinations thereof.

[0013] In one embodiment, the linear fluorinated carbonate-containing co-solvent of the electrolyte system is 2,2-difluoroethyl methyl carbonate (2F-EMC).

[0014] In one embodiment, the electrolyte system comprises an additional linear carbonate, such that at least one of the two or more linear carbonate-containing co-solvents is selected from the group consisting of: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and combinations thereof.

[0015] In one embodiment, the electrolyte system comprises an additional cyclic carbonate-containing co-solvent selected from the group consisting of: ethylene carbonate (EC), propylene carbonate (PC) and combinations thereof.

[0016] In one embodiment, the electrolyte system comprises fluoroethylene carbonate (FEC) in more than or equal to about 1 vol% to less than or equal to about 50 vol%, 2,2-difluoroethyl methyl carbonate (2F-EMC) in more than or equal to about 2 vol% to less than or equal to about 90 vol%, and a remainder of linear carbonate-containing co-solvents and cyclic carbonate-containing co-solvents, which cumulatively constitute more than or equal to about 1 vol% to less than or equal to about 98 vol%.

[0017] In one embodiment, at least one of the one or more cyclic, carbonate-containing co-solvents of the electrolyte system comprises fluoroethylene carbonate (FEC), wherein the linear, fluorinated, carbonate-containing co-solvential comprises 2,2-difluoroethylmethyl carbonate (2F-EMC) and a remaining linear, carbonate-containing co-solvential of the two or more linear, carbonate-containing co-solvents comprises dimethyl carbonate (DMC).

[0018] In one embodiment, fluoroethylene carbonate (FEC), 2,2-difluoroethylmethyl carbonate (2F-EMC) and dimethyl carbonate (DMC) are present in a volume ratio of approximately 1:2:2.

[0019] In one embodiment, the lithium salt comprises lithium hexafluorophosphate (LiPF6).

[0020] In one embodiment, the electrode comprises a silicon-containing electroactive material.

[0021] In one embodiment, the silicon-containing electroactive material of the electrode undergoes a volumetric expansion and contraction of more than or equal to approximately 300%.

[0022] In one embodiment, the electrode comprises SiO₂. x , where 0 ≤ x ≤ 2.

[0023] In one embodiment, the electrolyte system largely avoids precipitation at temperatures of less than or equal to -30 °C.

[0024] In one embodiment, the electrolyte system has an ionic conductivity greater than or equal to approximately 0.5 mS / cm at approximately -30 °C. Fig. Figure 1 is a schematic representation of an electrochemical battery cell; Fig. Figure 2 is a diagram showing the specific capacities of comparative, exemplary electrochemical cells; and Fig. Figure 3 is a diagram showing the specific capacities of comparative, exemplary electrochemical cells.

[0025] The same reference symbols in the different views of the drawings refer to the same parts.

[0026] Exemplary embodiments will now be described in more detail with reference to the accompanying drawings.

[0027] An exemplary representation of an electrochemical cell that cycles lithium ions (e.g., a lithium-ion battery) 20 times is shown in Fig. Figure 1 shows the lithium-ion battery 20, which includes a negative electrode 22, a positive electrode 24, and a separator 26 (e.g., a microporous polymer separator) located between the two electrodes 22 and 24. A negative electrode current collector 32 can be positioned on or near the negative electrode 22. A positive electrode current collector 34 can be positioned on or near the positive electrode 24. The negative electrode current collector 32 and the positive electrode current collector 34 each collect electrons and transport the free electrons to and from an external circuit 40. An interruptible external circuit 40 and load 42 connect the negative electrode 22 (via its current collector 32) and the positive electrode 24 (via its current collector 34).Each of the negative electrode 22, the positive electrode 24 and the separator 26 can further comprise an electrolyte 30 that can conduct lithium ions.

[0028] The separator 26, which serves both as an electrical insulator and for mechanical support, is inserted between the negative electrode 22 and the positive electrode 24 to prevent physical contact and thus avoid the occurrence of a short circuit. In addition to providing a physical barrier between the two electrodes 22 and 24, the separator 26 can provide a minimal resistance path for the internal passage of the lithium ions (and the associated anions) to facilitate the operation of the lithium-ion battery 20.

[0029] The lithium-ion battery 20 can generate an electric current during discharge through reversible electrochemical reactions that occur when the external circuit 40 is closed (to connect the negative electrode 22 and the positive electrode 24) when the negative electrode 22 contains a relatively larger amount of intercalated lithium. The chemical potential difference between the positive electrode 24 and the negative electrode 22 drives the electrons, which are generated by the oxidation of the intercalated lithium at the negative electrode 22, through the external circuit 40 to the positive electrode 24. Lithium ions, which are also formed at the negative electrode, are simultaneously transferred to the positive electrode 24 through the electrolyte system 30 and the separator 26.The electrons flowing through the external circuit 40 and the lithium ions migrating through the separator 26 in the electrolyte system 30 form lithium deposited at the positive electrode 24. The electric current flowing through the external circuit 18 can be used and conducted through the load 42 until the lithium deposited at the negative electrode 22 is depleted and the capacity of the lithium-ion battery 20 decreases.

[0030] The lithium-ion battery 20 can be recharged and made usable at any time by connecting an external power source to it, thus reversing the electrochemical reactions of battery discharge. Connecting an external power source to the lithium-ion battery 20 forces the otherwise non-spontaneous oxidation of intercalated lithium at the positive electrode 24 to generate electrons and lithium ions. The electrons, which flow back to the negative electrode 22 through the external circuit 40, and the lithium ions, which are transported back to the negative electrode 22 through the electrolyte system 30 and the separator 26, recombine at the negative electrode 22 and replenish it with stored lithium for use in the next battery discharge cycle.The external power source that can be used to charge the lithium-ion battery 20 can vary in size, design, and specific end-use application. Some notable and exemplary external sources include, but are not limited to, an AC wall outlet and a motor vehicle alternator.

[0031] In many lithium-ion battery configurations, the negative current collector 32, the negative electrode 22, the separator 26, the positive electrode 24 and the positive current collector 34 are each manufactured as relatively thin layers (for example, a few micrometers or a millimeter or less in thickness) and assembled in layers that are connected in electrical parallel to provide a suitable energy package.

[0032] Furthermore, the lithium-ion battery 20 can include a variety of other components (not shown). For example, the lithium-ion battery 20 can include a casing, seals, end caps, and any other conventional components or materials that may be located within the battery 20, including, as a non-limiting example, between or around the negative electrode 22, the positive electrode 24, and / or the separator 26. As mentioned above, the size and shape of the lithium-ion battery 20 can vary depending on the specific application for which it is designed. Battery-powered vehicles and portable consumer electronics are two examples where the lithium-ion battery 20 would likely have a different size, capacity, and power output.The lithium-ion battery 20 can also be connected in series or parallel with other, similar lithium-ion cells or batteries to produce a greater voltage output and power density if required by the consumer 42.

[0033] Accordingly, the lithium-ion battery 20 can generate electrical current for a load 42, which can be functionally connected to the external circuit 40. The load 42 can be fully or partially powered by the electrical current that flows through the external circuit 40 as the lithium-ion battery 20 discharges. While the load 42 can be any number of electrically powered devices, some specific examples of power-consuming devices include an electric motor for a hybrid vehicle or electric car, a laptop computer, a tablet computer, a mobile phone, and a cordless power tool or household appliances as non-limiting examples. However, the load 42 can also be a power-generating device that can charge the lithium-ion battery 20 for energy storage.

[0034] In various cases, the separator 26 in one embodiment can comprise a microporous, polymeric separator comprising a polyolefin. The polyolefin can be a homopolymer (derived from a single monomer component) or a heteropolymer (derived from more than one monomer component), which can be either linear or branched. If a heteropolymer is derived from two monomer components, the polyolefin can adopt any copolymer chain arrangement, including that of a block copolymer or a statistical copolymer. Likewise, a polyolefin that is a heteropolymer derived from more than two monomer components can also be a block copolymer or a statistical copolymer. In certain aspects, the polyolefin can be polyethylene (PE), polypropylene (PP), or a mixture of PE and PP.In various cases, the separator 26 can include a ceramic coating, which increases the safety of the lithium-ion battery 20. The ceramic coating can comprise aluminum oxide (Al2O3), silicon dioxide (SiO2), or combinations thereof.

[0035] If the separator 26 is a microporous polymeric separator, it can be a single layer or a multilayer laminate produced via either a dry or wet process. For example, in some cases, a single layer of the polyolefin can constitute the entire microporous polymeric separator 26. Alternatively, the separator 26 can be a fibrous membrane with numerous pores extending between opposing surfaces and can, for example, have a thickness of less than one millimeter. As another example, the microporous polymeric separator 26 can also be composed of several separate layers of the same or dissimilar polyolefins.The microporous polymeric separator 26 can comprise other polymers besides the polyolefin, such as, but not limited to, polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and / or a polyamide. The polyolefin layer and any other optional polymer layers can also be incorporated as a fibrous layer within the microporous polymeric separator 26 to provide it with suitable structural and porosity properties. Various common polymers and commercial products for forming the separator 26 are considered, as well as the numerous manufacturing processes that can be used to produce such a microporous polymeric separator 26.

[0036] In various cases, the positive electrode 24 can be formed from a lithium-based active material capable of undergoing sufficient lithium insertion and removal while serving as the positive pole of the lithium-ion battery 20. The positive electrode 24 may include a polymer binder to structurally hold the lithium-based active material together.

[0037] An exemplary common class of known materials that can be used to form the positive electrode 24 are layered lithium transition metal oxides. For example, in various cases the positive electrode 24 can comprise at least one spinel, such as lithium manganese oxide (Li₂). (1+x) Mn (2-x) O4, where 0 ≤ x ≤ 1,) (e.g., LiMn2O4); Lithium manganese nickel oxide (LiMn (2-x) Ni x O4, where 0 ≤ x ≤ 1) (e.g., LiMn 1.5 Ni 0.5O4); lithium cobalt oxide (LiCoO2); lithium manganese oxide (LiMn2O4); lithium nickel oxide (LiNiO2); a lithium-nickel-manganese-cobalt oxide (Li(Ni x Mn y Co z )O2, where 0 ≤ x ≤ 1, 0 ≤ y ≤ 1, 0 ≤ z ≤ 1, and x + y + z = 1); a lithium nickel cobalt metal oxide (LiNi (1-x-y) Co x M y O2, where 0 <x<1, y<1 und M Al, Mn oder dergleichen sein können); Lithium-Übergangsmetalloxide oder Mischoxide Lithiumeisenphosphate; oder ein Lithiumeisen-Polyanionoxid (e.g., Lithiumeisenphosphat (LiFePO4), Lithiumeisenfluorphosphat (Li2FePO4F)).

[0038] A variety of other known lithium-based active materials can also be used. As a non-restrictive example, alternative materials include lithium nickel oxide (LiNiO2) and lithium aluminum manganese oxide (Li). x Al y Mn (1-y)O2) and lithium vanadium oxide (LiV2O5). In certain variations, the positive electrode 24 comprises at least one of a spinel, such as lithium manganese oxide (Li (1+x) Mn (2-x) O4), lithium manganese nickel oxide, (LiMn (2-x) Ni x O4, where 0 ≤ x ≤ 1), lithium manganese nickel cobalt oxide, (e.g., LiMn 1 / 3 Ni 1 / 3 Co 1 / 3 O2) or lithium iron phosphate (LiFePO6).

[0039] The positive electrode 24 can also contain electrically conductive materials that facilitate the movement of electrons within the positive electrode 24. Examples include graphite, carbon-containing materials, or a conductive polymer. Carbon-containing materials can include, by way of a non-restrictive example, Ketchen black, Denka black, carbon black, carbon, and the like. Examples of conductive polymers include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. In certain applications, mixtures of conductive materials can be used.

[0040] The positive electrode 24 can also contain a polymeric binder to form a composite electrode. Thus, active materials and optionally electrically conductive materials can be mixed with at least one polymeric binder, for example by slip casting of active materials with these binders, such as polyvinylidene fluoride (PVDF), ethylene propylene diene monomer (EPDM) rubber, carboxymethoxylcellulose (CMC), styrene-butadiene rubber (SBR), or combinations thereof.

[0041] The positive current collector 34 can include a compound selected from the group consisting of: gold (Au), lead (Pb), niobium (Nb), palladium (Pd), platinum (Pt), silver (Ag), vanadium (V), tin (Sn), aluminum (Al), copper (Cu), tantalum (Ta), nickel (Ni), iron (Fe), and combinations thereof. As a non-restrictive example, the positive current collector 34 can be a continuous layer, such as a foil, or a grid or mesh.

[0042] In various cases, the negative electrode 22 incorporates an electroactive material as a lithium host material, which can serve as the negative terminal of a lithium-ion battery. The negative electrode 22 can thus include the electroactive lithium host material and, optionally, another electrically conductive material as well as one or more polymeric binding materials for structurally holding the lithium host material together. In various cases, the negative electrode 22 can also contain a graphite-containing electroactive material.

[0043] In various cases, the negative electrode 22 can contain an electroactive material that undergoes significant volumetric expansion and contraction (e.g., greater than or equal to approximately 300%) during the cycle of the electrochemical cell 20. In certain variations, such an electroactive material can contain a compound selected from the group consisting of: silicon (Si), tin (Sn), germanium (Ge), bismuth (Bi), zinc (Zn), tellurium (Te), lead (Pb), gallium (Ga), aluminum (Al), arsenic (As), lithium (Li), alloys, oxides, and combinations thereof. As mentioned above, in certain variations, the negative electrode 22 can contain an electroactive material that undergoes volumetric expansion and contraction of more than or equal to approximately 300% during the cycle of the electrochemical cell 20.The negative electrode 22 can, in certain variations, include an electroactive material that undergoes a volumetric expansion and contraction of more than or equal to approximately 100% during the cycle of the electrochemical cell 20.

[0044] For example, the negative electrode 22 can, in certain cases, contain a silicon-containing electroactive material. Silicon-containing electroactive materials are subject to significant volumetric expansion and contraction during the cycle of the electrochemical cell 20. Exemplary silicon-containing electroactive materials include lithium-silicon and silicon-containing binary and ternary alloys, such as Si-Sn; SiSnFe; SiSnAl; SiFeCo; SiO₂. x , where 0 ≤ x ≤ 2; and the like.

[0045] The negative electrode 22 can also contain electrically conductive materials that facilitate the movement of electrons within the negative electrode 22. Graphite, carbon-containing materials, or a conductive polymer can be used. Carbon-containing materials can include, by way of a non-restrictive example, Ketchen black, Denka black, carbon black, carbon, soot, and the like. Examples of a conductive polymer include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like. In certain aspects, mixtures of conductive materials can be used.

[0046] The electroactive materials comprising the negative electrode 22 can be mixed with at least one polymeric binder, for example by slurries of the electroactive materials with such binders as polyvinylidene fluoride (PVDF), ethylene propylene diene monomer (EPDM) rubber, carboxymethoxylcellulose (CMC), styrene-butadiene rubber (SBR) or combinations thereof.

[0047] The negative electrode 22 can comprise approximately 0% to approximately 95% by weight of the graphite-containing electroactive material. The negative electrode 22 can comprise approximately 0% to approximately 90% by weight of the silicon-containing electroactive material. The negative electrode 22 can comprise approximately 0% to approximately 30% by weight of an electrically conductive material and approximately 0% to approximately 30% by weight of a binder.

[0048] The negative electrode current collector 32 can comprise a compound selected from the group consisting of: gold (Au), lead (Pb), niobium (Nb), palladium (Pd), platinum (Pt), silver (Ag), vanadium (V), tin (Sn), aluminum (Al), copper (Cu), tantalum (Ta), nickel (Ni), iron (Fe), and combinations thereof. As a non-restrictive example, the negative current collector 32 can be a continuous layer, such as a foil, or a grid or mesh.

[0049] Each of the negative electrodes 22, the positive electrodes 24, and the separator 26 can contain an electrolyte 30 capable of conducting lithium ions. In particular, the electrolyte system 30 can conduct lithium ions between the negative electrode 22 and the positive electrode 24. Electrolyte systems 30 can contain at least one lithium salt, as discussed further below. The electrolyte system 30 promotes the passive formation of a flexible protective layer (not shown) in various aspects. The flexible protective layer can be a flexible solid electrolyte interface (SEI) protective layer. The protective layer can comprise lithium fluoride (LiF) polymer composites on one or more exposed surface areas of the electroactive material with the negative electrode 22. In certain cases, the lithium fluoride (LiF) polymer composites can serve as a lithium-ion conductive medium.In certain cases, the polymers of lithium fluoride (LiF) polymer composites can be one or more polyene polymers.

[0050] The term "flexible" means that the protective layer can accommodate the volumetric expansion and contraction of the electroactive materials (e.g., silicon-containing electroactive materials) in the negative electrode 22 during the long-term cycle (e.g., greater than 200 lithium delithation cycles) of the electrochemical cell 20 without damage, breakage, or significant electrolyte consumption. In certain cases, the flexible protective layer, applied to one or more exposed surface areas of the electroactive material, has a thickness of approximately 1 nm to 100 nm or less. The flexible protective layer protects the negative electrode 22 in various ways from reaction with the liquid electrolyte during prolonged use of the electrochemical cell, as described herein.

[0051] As explained above, many lithium-ion batteries can suffer from capacity degradation due to numerous factors, including the formation of a passive film known as a solid electrolyte intermediate (SEI) layer over the surface of the negative electrode (anode). This layer is often generated by reaction products of the anode material, electrolyte reduction, and / or lithium-ion reduction. SEI layer formation plays a significant role in determining electrode behavior and properties, including cycle life and irreversible capacity loss. For example, the large volume changes (e.g., volume expansion / contraction of more than 300%) of silicon-containing materials during lithium insertion and removal (e.g., during the charging and discharging processes) can significantly impact the SEI layer., Deposition / alloying and removal / alloy deposition / alloying) can lead to the formation of an SEI layer on silicon-containing electroactive materials, which can lead to continuous electrolyte consumption and lithium-ion loss, which in turn can lead to an irreversible capacity drop in a lithium-ion battery during the repeat cycle.

[0052] The passively formed flexible SEI protective layer, enabled by the electrolyte system 30 of this description, minimizes or prevents continuous electrolyte consumption and lithium ion loss, thereby minimizing or preventing the loss of charge capacity in the electrochemical cell and promoting long-term cycle stability. The flexible SEI protective layer can consist of a lithium fluoride (LiF) polymer composite layer, which particularly improves cycle performance during the volume expansion / contraction of the electroactive material (e.g., silicon). In certain cases, the flexible SEI protective layer can improve robustness at temperatures below or equal to -15 °C. In certain cases, the flexible SEI protective layer can improve robustness at temperatures below or equal to -30 °C.

[0053] While certain electrolyte systems have demonstrated good cycle performance, they often suffer from narrow operating windows at temperature. Therefore, the electrolyte system may function well at room temperature, but the performance of such an electrolyte formulation becomes problematic at low temperatures because co-solvents can precipitate out of solution. At low temperatures, for example, below approximately -15 °C, the electrolyte conductivity can thus drop abruptly as one or more solvents precipitate out of solution. However, the electrolyte system 30, provided by certain aspects of the present description, offers good cycle performance and electrolyte conductivity even at low temperatures and stabilizes the electroactive material by forming a flexible SEI protective layer, which can minimize or prevent undesirable SEI reactions and electrolyte consumption for long-term durability.

[0054] The electrolyte system 30 for promoting the passive formation of the flexible SEI protective layer of lithium fluoride (LiF) polymer composites comprises one or more lithium salts dissolved in one or more solvents. For example, only the lithium salt can be selected from the group consisting of lithium hexafluorophosphate (LiPF6); lithium perchlorate (LiClO4); lithium tetrachloroaluminate (LiAlCl4); lithium iodide (LiI); lithium bromide (LiBr); lithium thiocyanate (LiSCN); lithium tetrafluoroborate (LiBF4); lithium tetraphenylborate (LiB(C6H5)4); lithium hexafluoroarsenate (LiAsF6); lithium trifluoromethanesulfonate (LiCF3SO3); LiN(FSO2)2 (LIFSI); bis-(trifluoromethane)sulfonimide lithium salt (LiN(CF3SO2)2); lithium bis-(trifluoromethanesulfonyl)imide (C2F6LiNO4S2); LiB(C2O4)2(LiBOB); LiBF2(C2O4)(LiODFB); LiPF4(C2O4) (LiFOP); LiNO3; and combinations thereof. In certain variations, the lithium salt can be lithium hexafluorophosphate (LiPF6).The one or more lithium salts can constitute approximately 10% to less than or equal to approximately 20% by weight of the electrolyte system 30.

[0055] In certain variants, the solvent comprises one or more cyclic carbonate-containing co-solvents and two or more linear carbonate-containing co-solvents, wherein at least one of the two or more linear carbonate-containing co-solvents is a fluorinated carbonate-containing co-solvent. The at least one of the two or more linear carbonate-containing co-solvents is a linear, fluorinated carbonate-containing co-solvent comprising fluorine.

[0056] The at least one cyclic carbonate-containing co-solvent can be fluoroethylene carbonate (FEC). The linear fluorinated carbonate-containing co-solvent can be an acyclic, linear, organic carbonate-containing compound with fluorine (F) atom(s). The linear fluorinated carbonate-containing co-solvent can be, for example, methyl 2,2,2-trifluoroethyl carbonate; ethyl 2,2,2-trifluoroethyl carbonate; bis-(2,2-difluoroethyl) carbonate; 2,2-difluoroethyl-2,2,2-trifluoroethyl carbonate; 2,2-difluoroethylhexafluoroisopropyl carbonate; bis-(2,2,2-trifluoroethyl) carbonate; 2,2,3,3,3-pentafluoropropyl-2,2,2-trifluoroethyl carbonate; 2,2-difluoroethylethyl carbonate (2F-DEC); 2,2-difluoroethyl methyl carbonate (2F-EMC); or combinations thereof. In certain variations, the linear fluorinated carbonate-containing co-solvent can be 2,2-difluoroethylethyl carbonate (2F-DEC), 2,2-difluoroethylmethyl carbonate (2F-EMC), or combinations thereof.In certain cases, fluoroethylene carbonate (FEC) may be present in electrolyte system 30 in an amount greater than or equal to approximately 2 vol% to less than or equal to approximately 50 vol%, and the at least one linear fluorinated carbonate-containing co-solvent may be present in an amount greater than or equal to approximately 0 vol% to less than or equal to approximately 80 vol%.

[0057] Other cyclic and linear carbonate-containing co-solvents may be present cumulatively in the electrolyte system 30 in amounts ranging from more than 0% to less than or equal to approximately 98%, or possibly from more than or equal to approximately 1% to less than or equal to approximately 98% by volume. Other exemplary cyclic carbonate-containing co-solvents include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, and combinations thereof. Other exemplary linear carbonate-containing co-solvents include dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and combinations thereof.

[0058] In one variation, the electrolyte system comprises 30 fluoroethylene carbonate (FEC) at more than or equal to about 1 vol% to less than or equal to about 50 vol%, 2,2-difluoroethyl methyl carbonate (2F-EMC) at more than or equal to about 2 vol% to less than or equal to about 90 vol%, and a remainder of linear carbonate-containing co-solvents and cyclic carbonate-containing co-solvents, which cumulatively constitute more than or equal to about 1 vol% to less than or equal to about 98 vol%.

[0059] For example, electrolyte system 30 may, in certain cases, contain fluoroethylene carbonate (FEC) as a cyclic carbonate-containing co-solvent; 2,2-difluoroethyl methyl carbonate (2F-EMC) as a linear fluorinated carbonate-containing co-solvent; and dimethyl carbonate (DMC) as the remaining linear carbonate-containing co-solvent of the two or more linear carbonate-containing co-solvents. In certain variations, the fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and 2,2-difluoroethyl methyl carbonate (2F-EMC) may be present in a volume ratio of approximately 1:2:2. In electrolyte system 30, for example, only 2,2-difluoroethyl methyl carbonate (2F-EMC) may be present in an amount greater than or equal to approximately 1 vol% to less than or equal to approximately 90 vol%. Dimethyl carbonate (DMC) can be present in electrolyte system 30 in an amount greater than or equal to approximately 0 vol% to less than or equal to approximately 90 vol%.-%; and fluoroethylene carbonate (FEC) may be present in the electrolyte system 30 in an amount greater than or equal to about 1 vol% to less than or equal to about 30 vol%.

[0060] In certain cases, the electrolyte system 30 may also contain cyclic carbonate-containing co-solvent ethylene carbonate (EC) as one or more cyclic carbonate-containing co-solvents. In such cases, ethylene carbonate (EC) may be present in the electrolyte system 30 in an amount greater than or equal to approximately 0 vol% to less than or equal to approximately 50 vol%.

[0061] In certain cases, the electrolyte system 30 may also contain cyclic carbonate-containing co-solvent propylene carbonate (PC) as one or more cyclic carbonate-containing co-solvents. In such cases, propylene carbonate (PC) may be present in the electrolyte system 30 in an amount greater than or equal to approximately 0 vol% to less than or equal to approximately 50 vol%.

[0062] In certain cases, the electrolyte system 30 may, in addition to the at least one cyclic carbonate-containing co-solvent and two or more linear carbonate-containing co-solvent described above, also contain carbonate-containing co-solvenes. The electrolyte system 30 may further include, for example, alkyl carbonates such as aliphatic carboxylic acid esters (e.g., methyl formate, methyl acetate, methyl propionate), γ-lactones (e.g., γ-butyrolactone, γ-valerolactone), and mixtures thereof. The additional carbonate-containing co-solvenes may be present in the electrolyte system 30 in amounts greater than or equal to approximately 0 vol% to less than or equal to approximately 50 vol%.

[0063] In various aspects, the electrolyte system 30 comprises the lithium salt and the solvent, which includes one or more cyclic carbonate-containing co-solvents and two or more linear carbonate-containing co-solvents, wherein at least one of the two or more linear carbonate-containing co-solvents is a linear fluorinated carbonate-containing co-solvential that can substantially prevent precipitation at a temperature of less than or equal to about -30 °C. In various aspects, the electrolyte system 30 comprises the lithium salt and the solvent, which includes one or more cyclic carbonate-containing co-solvents and two or more linear carbonate-containing co-solvents, wherein at least one of the two or more linear carbonate-containing co-solvents is a linear fluorinated carbonate-containing co-solvential that can exhibit an electrical conductivity greater than or equal to about 0.5 mS / cm at about -30 °C.In various aspects, the electrochemical cell Cycling-Lithium-Ion 20 can exhibit a capacity loss of less than or equal to approximately 25% after 500 cycles.

[0064] The embodiments of the present technology are further illustrated by the following non-examples. Example 1

[0065] Fig. Figure 2 shows the charge and discharge profiles of silicon (Si) half-cell electrochemical cells, including comparative electrolyte systems. The y-axis (50) represents the specific capacity in mAh units, while the cycle number is shown on the x-axis (52).

[0066] The electrochemical cell 60 includes an electrolyte system prepared according to certain aspects of the present description. In particular, the electrolyte system of the electrochemical cell 60 includes 15 wt% lithium hexafluorophosphate (LiPF6) as the lithium salt and co-solvents fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and 2,2-difluoroethyl methyl carbonate (2F-EMC) in a volumetric ratio of approximately 1:2:2.

[0067] Electrochemical cell 70 comprises an electrolyte system with lithium hexafluorophosphate (LiPF6) and co-solvents fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volumetric ratio of approximately 1:2:2. Electrochemical cell 80 comprises an electrolyte system with lithium hexafluorophosphate (LiPF6) and co-solvents dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC) in a volumetric ratio of approximately 1:4.

[0068] As can be seen, electrochemical cell 80 exhibits improved performance compared to electrochemical cell 70. After 300 cycles, electrochemical cell 60 performs better than electrochemical cells 70 and 80. In particular, electrochemical cell 60 exhibits superior long-term stability compared to electrochemical cells 70 and 80. Accordingly, electrochemical cell 60, manufactured according to certain aspects of the present description, shows significantly improved cycle performance and reduced capacity decay. Example 2

[0069] Fig. Section 3 presents the charging and discharging profiles of electrochemical cells based on graphite / SiO₂. x -NMC, including comparative electrolyte systems. The y-axis 200 represents the specific capacity in mAh units, while the cycle number is displayed on the x-axis 202.

[0070] The electrochemical cell 220 comprises an electrolyte system prepared according to certain aspects of the present description. Specifically, the electrolyte system of the electrochemical cell 220 includes lithium hexafluorophosphate (LiPF6) as the lithium salt and co-solvents fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), and 2,2-difluoroethyl methyl carbonate (2F-EMC) in a volumetric ratio of approximately 1:2:2. The negative electrode of the electrochemical cell 220 contains approximately 70% graphite and approximately 20% SiO₂. x , where 0.5 ≤ x ≤ 1.5; approximately 2% carboxymethoxylcellulose (CMC), approximately 3% styrene-butadiene rubber (SBR) and 5% carbon black.

[0071] The electrochemical cell 240 contains an electrolyte system comprising lithium hexafluorophosphate (LiPF6), approximately 10 vol% fluoroethylene carbonate (FEC), and co-solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of approximately 1:2. The negative electrode of the electrochemical cell 240 contains 70% graphite and approximately 20% SiO₂. x , where 0.5 ≤ x ≤ 1.5; approximately 2% carboxymethoxylcellulose (CMC), approximately 3% styrene-butadiene rubber (SBR) and 5% carbon black.

[0072] As can be seen, the electrochemical cell 220 exhibits improved performance compared to the electrochemical cell 240. After 600 cycles, the electrochemical cell 220 shows better performance than the electrochemical cell 240. In particular, the electrochemical cell 220 exhibits superior long-term stability compared to the electrochemical cell 240. Accordingly, the electrochemical cell 220, manufactured according to certain aspects of the present description, shows significantly improved cycle performance and reduced capacity decay.

Claims

[1] An electrochemical cell (20) that cycles lithium ions, comprising: an electrochemical cell (20) comprising a silicon-containing electroactive material that undergoes volumetric expansion and contraction during the cycling of the electrochemical cell; and an electrolyte system (30) that promotes the passive formation of a flexible solid electrolyte interface (SEI) protective layer comprising a lithium fluoride (LiF) polymer composite on one or more exposed surface areas of the silicon-containing electroactive material, wherein the electrolyte system (30) comprises a lithium salt, one or more cyclic carbonate-containing co-solvents, and two or more linear carbonate-containing co-solvents, wherein at least one of the two or more linear carbonate-containing co-solvents is a linear fluorinated carbonate-containing co-solvent comprising fluorine, wherein the electrolyte system (30) accommodates the volumetric expansion and contraction of the silicon-containing electroactive material to promote long-term cycling stability; wherein at least one of the one or more cyclic carbonate-containing cosolvents of the electrolyte system (30) is fluoroethylene carbonate (FEC); and wherein the fluoroethylene carbonate (FEC) and the two linear carbonate-containing co-solvents are present in a volume ratio of 1:2:2; wherein the flexible SEI protective layer promoted by the electrolyte system (30) has a thickness greater than or equal to 1 nm to less than or equal to 100 nm. [2] The electrochemical cell (20) of claim 1, wherein the one or more cyclic carbonate-containing co-solvents are present in the electrolyte system (30) in an amount of greater than or equal to 1 vol% to less than or equal to 50 vol%; and wherein the linear fluorinated carbonate-containing co-solvent of the electrolyte system (30) is present in an amount of greater than or equal to 2 vol% to less than or equal to 90 vol%. [3] The electrochemical cell (20) of claim 1, wherein the linear fluorinated carbonate-containing co-solvent of the electrolyte system (30) is selected from the group consisting of: methyl 2,2,2-trifluoroethyl carbonate; ethyl 2,2,2-trifluoroethyl carbonate; bis-(2,2-difluoroethyl) carbonate; 2,2-difluoroethyl 2,2,2-trifluoroethyl carbonate; 2,2-difluoroethyl hexafluoroisopropyl carbonate; bis-(2,2,2-trifluoroethyl) carbonate; 2,2,3,3,3-pentafluoropropyl 2,2,2-trifluoroethyl carbonate; 2,2-difluoroethyl ethyl carbonate (2F-DEC); 2,2-difluoroethyl methyl carbonate (2F-EMC); and combinations thereof. [4] The electrochemical cell (20) of claim 1, wherein the linear fluorinated carbonate-containing co-solvent of the electrolyte system (30) comprises 2,2-difluoroethyl methyl carbonate (2F-EMC). [5] The electrochemical cell (20) of claim 4, wherein at least one of the two or more linear carbonate-containing cosolvents is selected from the group consisting of: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and combinations thereof. [6] The electrochemical cell (20) of claim 5, wherein the electrolyte system (30) further comprises an additional cyclic carbonate-containing co-solvent selected from the group consisting of: ethylene carbonate (EC), propylene carbonate (PC), and combinations thereof. [7] The electrochemical cell (20) of claim 6, wherein the electrolyte system (30) comprises fluoroethylene carbonate (FEC) at greater than or equal to 1 vol% to less than or equal to 50 vol%, 2,2-difluoroethyl methyl carbonate (2F-EMC) at greater than or equal to 2 vol% to less than or equal to 90 vol%, and a balance of linear carbonate-containing co-solvents and cyclic carbonate-containing co-solvents cumulatively greater than or equal to 1 vol% to less than or equal to 98 vol%. [8] The electrochemical cell (20) of claim 1, wherein at least one of the one or more cyclic carbonate-containing co-solvents of the electrolyte system (30) comprises fluoroethylene carbonate (FEC), the linear fluorinated carbonate-containing co-solvents comprises 2,2-difluoroethyl methyl carbonate (2F-EMC), and a remaining linear carbonate-containing co-solvent of the two or more linear carbonate-containing co-solvents comprises dimethyl carbonate (DMC); wherein the fluoroethylene carbonate (FEC), 2,2-difluoroethyl methyl carbonate (2F-EMC), and dimethyl carbonate (DMC) are present in a volume ratio of 1:2:

2. [9] The electrochemical cell (20) of claim 1, wherein the electrolyte system (30) avoids precipitation at a temperature less than or equal to -30°C and has an electrical conductivity greater than or equal to 0.5 mS / cm at -30°C.

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