WO2017192425A1

WO2017192425A1 - Electrolyte solutions for rechargeable batteries

Info

Publication number: WO2017192425A1
Application number: PCT/US2017/030351
Authority: WO
Inventors: Rémi PETIBON; Jian XIA; Jeffrey R. Dahn
Original assignee: 3M Innovative Properties Company
Priority date: 2016-05-05
Filing date: 2017-05-01
Publication date: 2017-11-09
Also published as: TW201817072A

Abstract

An electrolyte solution includes a linear alkyl carbonate; a passivating agent comprising vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methylene ethylene carbonate, succunic anhydride (SA), or prop-1-ene-1,3-sultone; and an electrolyte salt. Ethylene carbonate is present in the electrolyte solution in an amount of no more than 5 wt. %, based on the total weight of the electrolyte solution.

Description

ELECTROLYTE SOLUTIONS FOR RECHARGEABLE BATTERIES

FIELD

The present disclosure relates to compositions useful as electrolytes for

rechargeable batteries and methods for preparing and using the same.

BACKGROUND

Various electrolyte solutions have been introduced for use in secondary batteries. Such compositions are described, for example, in J. Electrochem. Soc, 2015, 162 (3), A330-A338; J. Electrochem. Soc, 2015, 162 (7), A1186-A1195; and J. Electrochem. Soc, 2015, 162 (7), A1170-A1174.

SUMMARY

In some embodiments, an electrolyte solution is provided. The electrolyte solution includes a linear alkyl carbonate; a passivating agent comprising vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methylene ethylene carbonate, succunic anhydride (SA), or prop-l-ene-l,3-sultone; and an electrolyte salt. Ethylene carbonate is present in the electrolyte solution in an amount of no more than 5 wt. %, based on the total weight of the electrolyte solution.

In some embodiments, an electrochemical cell is provided. The cell includes a positive electrode; a negative electrode; and the above-described electrolyte solution.

In some embodiments, a method of operating an electrochemical cell is provided. The method includes providing the above-described electrochemical cell; and operating the electrochemical cell such that the positive electrode reaches a voltage of at least 4.4 V versus Li/Li⁺.

The above summary of the present disclosure is not intended to describe each embodiment of the present disclosure. The details of one or more embodiments of the disclosure are also set forth in the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is an electrochemical impedance spectroscopy (EIS) spectrum, demonstrating how Ret is determined.

Figure 2 shows the cycle-hold experimental protocol for Frequency Response Analyzer (FRA) cycling.

Figure 3 shows typical profile of electrochemical cell voltage during smart storage experiments.

Figure 4 shows the results of ionic conductivity measurements for embodiments of the present disclosure.

Figure 5 shows the results of ionic conductivity measurements for embodiments of the present disclosure.

Figures 6A - 6F are plots of dQ/dV as a function of voltage for various embodiments of the present disclosure.

Figure 7 shows the amount of VC left (top) and the amount of EMC trans- esterification (bottom) as a function of initial VC content for electrochemical cells containing electrolytes of the present disclosure.

Figure 8A - 8D show the charge transfer impedance during FRA cycling of comparative examples and embodiments of the present disclosure.

Figure 9A - 9D show discharge capacity retention and voltage hysteresis after long-term cycling at 20 °C and 55 °C of comparative examples and embodiments of the disclosure.

DETAILED DESCRIPTION

The most extensively used lithium-ion battery electrolytes have limited thermal and high voltage stability. Thermal and electrochemical degradation of the electrolyte is considered a primary cause of reduced lithium-ion battery performance over time. Many of the performance and safety issues associated with advanced lithium-ion batteries are the direct or indirect result of undesired reactions that occur between the electrolyte and the highly reactive positive or negative electrodes. Such reactions result in reduced cycle life, capacity fade, gas generation (which can result in cell swelling or venting), impedance growth, and reduced rate capability. Stabilizing the electrode/electrolyte interface, therefore, is an important factor in controlling and minimizing these undesirable reactions and improving the cycle life and voltage and temperature performance limits of lithium- ion batteries.

Typically, driving the electrodes to greater voltage extremes (e.g., 4.4 V vs. Li/Li⁺) or exposing the cell to higher temperatures (e.g., > 45°C) accelerates these undesired reactions and magnifies the associated problems. Consequently, while typical electrode materials used in commercial lithium ion cells may be structurally stable at voltages higher than 4.4 V, such materials are being used with cut-off voltages below 4.4 V. For instance, a common electrode material, Li Nio.4Mno.4Coo.2]02, is employed with a potential cut-off of 4.28 V vs. Li/Li⁺ in Li-ion cells despite being structurally stable up to

4.78 V vs. Li/Li⁺. Such lower operating voltages, in turn, lead to electrochemical cells having significantly lower energy density than would otherwise be possible.

Consequently, there is an ongoing need for electrolyte additives and solvent blends that are capable of further improving the high temperature performance and stability (e.g., > 45°C) of lithium-ion cells, and provide electrolyte stability at high voltages (e.g. > 4.4 V) for increased energy density.

Ethylene carbonate is a primary electrolyte component used in nearly all lithium- ion electrochemical cells produced today, as it is generally believed in the industry that ethylene carbonate is essential to achieving optimum battery performance. Generally, in some embodiments, the present disclosure is directed to electrolyte formulations that include linear carbonate-based materials as a primary component (and may include little or no ethylene carbonate), and which enable cells that, when operated at high voltage, exhibit improved cycle life and calendar life relative to those containing conventional electrolyte formulations. As used herein, the singular forms "a", "an", and "the" include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended embodiments, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

As used herein, the recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.8, 4, and 5). Unless otherwise indicated, all numbers expressing quantities or ingredients, measurement of properties and so forth used in the specification and embodiments are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached listing of embodiments can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claimed embodiments, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

In some embodiments, the present disclosure is directed to an electrolyte solution for a rechargeable electrochemical cell (e.g., rechargeable lithium ion battery). In various embodiments, the electrolyte solution may include one or more linear alkyl carbonates and one or more passivating agents. In some embodiments, the electrolyte solution may include one or more alkyl carbonates, such as linear alkyl carbonates. In some embodiments, the electrolyte solution may include one or more linear alkyl carbonates having the formula: RiOC(0)OR2, where Ri and R2 are each independently an alkyl group, perfluoroalkyl group, or a

hydrofluoroalkyl group having 1 to 10, 1 to 6, or 1 to 4 carbon atoms. In some

embodiments, Ri and R2 may be the same alkyl group, perfluoroalkyl group, or hydrofluoroalkyl group.

In various embodiments, the alkyl carbonate may include one or more of

(including any combination of two or more of the materials) diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, methyl propyl carbonate, di propyl carbonate, and ethyl methyl carbonate. In various embodiments, alkyl carbonates may be present in the electrolyte solution as a major component (or solvent). For example, alkyl carbonates may be present in the electrolyte solution in an amount of at least 40 wt. %, at least wt. 50%, at least 60 wt. %, at least 70 wt. %, at least 80 wt. %, or at least 85 wt. %, based on the total weight of the electrolyte solution; or between 40 and 95 wt. %, between 50 and 90 wt. %, or between 60 and 85 wt. %., based on the total weight of the electrolyte solution. In some embodiments, the electrolyte solution may include one or more passivating agents. Generally, the passivating agents of the present disclosure serve to prevent undesirable reactions at the electrode-electrolyte interface by, for example, selectively reacting with, bonding to, or self-organizing at the electrode surface to form a solid electrolyte interface (SEI) between the electrode and electrolyte. It has been discovered that incorporation of a relatively small amount of certain passivating agents into an electrolyte solution having a linear alkyl carbonate as a primary component and including little or no ethylene carbonate produces an electrolyte solution that, when incorporated in an electrochemical cell, yields electrochemical cells that are capable of cycling to high voltages (e.g., > 4.4 V), and exhibit low impedance, low rates of electrolyte oxidation, good negative electrode passivation, low gas generation, and acceptable ionic conductivity. In some embodiments, the passivating agents may include one or more materials (individually or any combination of two or more of the materials) selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), difluoroethylene carbonate (di-FEC), methylene ethylene carbonate (MEC), succunic anhydride (SA), and prop-l-ene-l,3-sultone (PES).

In some embodiments, the amount of the passivating agent may be selected such that, on the one hand, the active material of the negative electrode (e.g., graphite, metal alloy) may be adequately passivated and, on the other hand, is not present in a quantity after electrochemical cell formation such that impedance is increased or that gassing occurs upon cell cycling. Factors contributing to the appropriate amount of the passivating agent may include the porosity of the negative electrode, specific surface area of the negative electrode particles, porosity of the separator, electrolyte fill volume, loading of active materials in the electrodes, and electrochemical cell design. In some embodiments, the passivating agents may be present in the electrolyte solution in an amount of between

0.3 and 10 wt. %, 0.5 and 8 wt. %, 1.0 and 5 wt. %, or 2.0 and 4.0 wt. %, based on the total weight of the electrolyte solution. VC may be present in the electrolyte solution in an amount of between 0.3 and 5 wt. %, 1 and 4 wt. %, 1.5 and 3.5 wt. %, or 2 and 3.5 wt. %, based on the total weight of the electrolyte solution. FEC may be present in the electrolyte solution in an amount of between 1 and 10 wt. %, 2 and 8 wt. %, 3 and 7 wt. %, or 3.5 and

7 wt. %, based on the total weight of the electrolyte solution. di-FEC may be present in the electrolyte solution in an amount of between 2 and 7 wt. %, 2.5 and 6 wt. %, 3 and 6 wt. %, or 3 and 5 wt. %, based on the total weight of the electrolyte solution. MEC may be present in the electrolyte solution in an amount of between 0.5 and 10 wt. %, 0.5 and 5 wt. %, or 1 and 3 wt. %, based on the total weight of the electrolyte solution. SA may be present in the electrolyte solution in an amount of between 0.5 and 4 wt. %, or 1.0 and 3 wt. %, based on the total weight of the electrolyte solution. PES may be present in the electrolyte solution in an amount of between 2 and 8 wt. %, 3 and 6 wt. %, or 4 and 5 wt. %, based on the total weight of the electrolyte solution.

In some embodiments, the electrolyte solution may include only small amounts of ethylene carbonate or be ethylene carbonate free. For example, the electrolyte solution may include less than 5 wt. %, less than 3 wt. %, less than 1 wt. %, or less than 0.1 wt. % ethylene carbonate, based on the total weight of the electrolyte solution.

In some embodiments, in addition to alkyl carbonates, the electrolyte solution may include one or more co-solvents. Suitable co-solvents may include one or more of (individually or any combination of two or more of the co-solvents) sulfolane, propylene carbonate, methyl acetate, methyl proprionate, methyl butyrate, ethyl acetate, ethyl propanoate, gamma butyrolactone. Additionally, or alternatively, any known electrolyte solvents known to those skilled in the art may be included as co-solvents. In various embodiments, the co-solvents may be present in the electrolyte solution in an amount of between 0.1 and 20 wt. %, 2 and 10 wt. %, or 2 and 5 wt. %, based on the total weight of the electrolyte solution.

In some embodiments, the electrolyte solution may further include one or more additives (individually or any combination of two or more of the additives) selected from triallyl phosphate (TAP), tris(trimethlysilyl) phosphate (TTSP), tris(trimethlysilyl) phosphite (TTSPi), pyridine-boron trifluoride (PBF), methylene methanedisulfonate (MMDS), l,3,2-dioxathiolane-2,2-dioxide (DTD) 1,3,2-dioxathiane 2,2-dioxide (TMS), pyridine phosphorus pentafluoride (PPF), diphenyl carbonate, and methyl phenyl carbonate. It is believed that such additives may further improve the electrolyte solution by lowering the resistance growth during high voltage cycling as well as contributing to improved safety of the electrochemical cells. For example, these additives may decompose on the positive electrode, thereby generating an interphase film and decreasing reactivity of electrolyte on the electrode surface. Consequently, electrolyte degradation and electrochemical cell swelling may be greatly suppressed. These additives may be present in the electrolyte solution in an amount of between 0.1 and 5 wt. %, 0.5 and 3 wt. %, 0.5 and 2 wt. %, or 1 and 2 wt. %, based on the total weight of the electrolyte solution. In some embodiments, any conventional electrolyte additives known to those skilled in the art may also be included in the electrolyte solutions of the present disclosure.

In some embodiments, the electrolyte solution may further include one or more electrolyte salts. In some embodiments, the electrolyte salts may include lithium salts and, optionally, minor amounts of other salts such as sodium salts (e.g., NaPF₆). Suitable lithium salts may include one or more of (including any combination of two or more of the materials) LiPF₆, LiBF₄, L1CIO4, lithium bis(oxalato)borate, LiN(SC"2CF₃)2,

LiN(S02C₂F₅)2, LiAsFe, LiC(S0₂CF₃)₃, LiN(S0₂F)₂, LiN(S0₂F)(S0₂CF₃),

LiN(S02F)(S02C₄F9), and lithium difluoro oxalate borate. In some embodiments, the lithium salts may include LiPF₆, lithium bis(oxalato)borate, LiN(SC"2CF₃)2, or

combinations thereof. In some embodiments, the lithium salts may include LiPF₆ and either or both of lithium bis(oxalato)borate and LiN(SC"2CF₃)2. The electrolyte salts may be present in the electrolyte solution in an amount of between 1 and 30 wt%, 2 and 25 wt%, or 5 and 20 wt%, based on the total weight of the electrolyte solution.

The present disclosure is further directed to electrochemical cells that include the above-described electrolyte solution. In some embodiments, the electrochemical cell may be a rechargeable electrochemical cell (e.g., a rechargeable lithium ion electrochemical cell) that includes a positive electrode, a negative electrode, and the electrolyte solution. In some embodiments, the electrochemical cell may be a rechargeable electrochemical cell in which the positive electrode is operated, or intended for operation, to potentials of 4.4 V vs. Li/Li⁺ and above. In some embodiments, the positive electrode may include a current collector having disposed thereon a positive electrode composition. The current collector for the positive electrode may be formed of a conductive material such as a metal. According to some embodiments, the current collector may include aluminum or an aluminum alloy. According to some embodiments, the thickness of the current collector may be 5 μιη to 75 μιη. It should also be noted that while the positive current collector may be described as being a thin foil material, the positive current collector may have any of a variety of other configurations according to various exemplary embodiments. For example, the positive current collector may be a grid such as a mesh grid, an expanded metal grid, a

photochemically etched grid, or the like. The current collector may include a layer of conductive agent coated on one or both sides, such as graphite flakes or carbon black, to improve electronic contact between the current collector and the active material.

In some embodiments, the positive electrode composition may include an active material. The active material may include a lithium metal oxide. In an exemplary embodiment, the active material may include lithium transition metal oxide intercalation compounds such as L1C0O2, LiCoo.2Nio.8O2, LiMmC^, LiFeP04, LiNiC , or lithium mixed metal oxides of manganese, nickel, and cobalt in any proportion, or of nickel, cobalt, and aluminum in any proportion. Blends of these materials can also be used in positive electrode compositions. Other exemplary cathode materials are disclosed in U.S. Patent No. 6,680,145 (Obrovac et al.) and include transition metal grains in combination with lithium-containing grains. Suitable transition metal grains include, for example, iron, cobalt, chromium, nickel, vanadium, manganese, copper, zinc, zirconium, molybdenum, niobium, or combinations thereof with a grain size no greater than about 50 nanometers. Suitable lithium-containing grains can be selected from lithium oxides, lithium sulfides, lithium halides (e.g., chlorides, bromides, iodides, or fluorides), or combinations thereof. The positive electrode composition may further include additives such as binders (such as polymeric binders (e.g., polyvinylidene fluoride), conductive diluents (e.g., carbon, carbon black, flake graphite, carbon nanotubes), fillers, adhesion promoters, thickening agents for coating viscosity modification such as carboxymethylcellulose, or other additives known by those skilled in the art. In some embodiments, the active material of the positive electrode composition may be structurally stable up to 4.4 V, 4.5 V, 4.6 V, 4.7 V, 4.8 V, 4.9 V, or 5.0 V or greater versus Li/Li⁺. For purposes of the present disclosure, the phrase structurally stable refers to a positive electrode composition that does not irreversibly change its crystal structure during long-term exposure (e.g., > 24 hours) to a particular potential. Positive electrode active materials known to be structurally stable at voltages of 4.7 V and above versus Li/Li⁺ include, for example, LifNio.4Mno.4Coo.23O2 as described in the paper (Jing Li, Remi Petibon, Stephen Glazier, Neeraj Sharma, Wei Kong Pang,

Vanessa K. Peterson and J.R. Dahn, In-situ Neutron Diffraction Study of High Voltage Li(Nio.42Mno.42Coo.i6)02/Graphite Pouch Cells, Electrochimica Acta 180, 234-240 (2015)). Most LiNixMnyCoi-x-yCh positive electrodes are structurally stable to 4.7 V versus Li/Li⁺. L1C0O2 shows a structural phase transition above 4.55 V versus Li/Li⁺ which is partially reversible but the material shows degradation during many cycles through this phase transition. For the purposes of the present disclosure, L1C0O2 is not considered structurally stable above 4.55 V versus Li/Li⁺.

The positive electrode composition can be provided on only one side of the positive current collector or it may be provided or coated on both sides of the current collector. The thickness of the positive electrode composition may be 0.1 μιη to 3 mm. According to some embodiments, the thickness of the positive electrode composition may be 10 μπι to 300 μιη. According to another embodiment, the thickness of the positive electrode composition may be 20 μιη to 90 μιη.

In various embodiments, the negative electrode may include a current collector and a negative electrode composition disposed on the current collector. The current collector for the negative electrode may be formed of a conductive material such as a metal.

According to some embodiments, the current collector includes copper or a copper alloy.

According to another exemplary embodiment, the current collector includes titanium or a titanium alloy. According to another embodiment, the current collector includes nickel or a nickel alloy. According to another embodiment, the current collector includes aluminum or an aluminum alloy. According to some embodiments, the thickness of the current collector may be 5 μιη to 75 μιη. It should also be noted that while the negative current collector has been described as being a thin foil material, the negative current collector may have any of a variety of other configurations according to various exemplary embodiments. For example, the negative current collector may be a grid such as a mesh grid, an expanded metal grid, a photochemically etched grid, or the like. The current collector may include a layer of conductive agent coated on one or both sides, such as graphite flakes or carbon black, to improve electronic contact between the current collector and the active material.

In some embodiments, the negative electrode composition may include an active material. The active material may include lithium metal, carbonaceous materials, or metal alloys (e.g., silicon alloy composition or lithium alloy compositions). Suitable

carbonaceous materials can include synthetic graphites such as mesocarbon microbeads (MCMB) (available from E-One Moli/Energy Canada Ltd., Vancouver, BC), SLP30 (available from TimCal Ltd., Bodio Switzerland), natural graphites, artificial graphites, and hard carbons. In some embodiments, the negative electrode compositions may include graphite in an amount of at least 50 wt. %, 60 wt. %, 70 wt. %, 80 wt. %, 90 wt. %, or 95 wt. %, based on the total weight of the negative electrode composition. Suitable alloys may include electrochemically active components such as silicon, tin, aluminum, gallium, indium, lead, bismuth, and zinc and may also include electrochemically inactive components such as iron, cobalt, transition metal silicides and transition metal aluminides. In some embodiments, the active material of the negative electrode may include a silicon alloy in an amount of at least 30 wt. %, 40 wt. %, 50 wt. %, 60 wt. %, or 70 wt. %, based on the total weight of the negative electrode composition.

In various embodiments, the negative electrode composition may further include an electrically conductive diluent to facilitate electron transfer from the composition to the current collector. Electrically conductive diluents include, for example, carbons, powdered metal, metal nitrides, metal carbides, metal silicides, and metal borides. Representative electrically conductive carbon diluents include carbon blacks such as Super P and Super S carbon blacks (both from MMM Carbon, Belgium), Shawanigan Black (Chevron

Chemical Co., Houston, Tex.), acetylene black, furnace black, lamp black, graphite, carbon fibers, carbon nanotubes (available from C-Nano Technology) and combinations thereof. In some embodiments, the amount of conductive diluent in the electrode composition may be between 0.1 wt. % and 10 wt.%, 0.1 wt. % and 5 wt. %, or 0.1 and 3 wt. %, based upon the total weight of the electrode composition. In embodiments in which the negative electrode composition includes a silicon alloy, the negative electrode composition may further include graphite in an amount of greater than 20 wt. %, greater than 50 wt. %, greater than 70 wt. %, or even greater, based upon the total weight of the negative electrode composition.

In some embodiments, the negative electrode compositions may include a binder. Suitable binders include oxo-acids and their salts, such as sodium carboxymethylcellulose, polyacrylic acid, and lithium polyacrylate. Other suitable binders include polyolefins such as those prepared from ethylene, propylene, or butylene monomers; fluorinated

polyolefins such as those prepared from vinylidene fluoride or vinylidene difluoride monomers; perfluorinated polyolefins such as those prepared from hexafluoropropylene monomer; perfluorinated poly(alkyl vinyl ethers); perfluorinated poly(alkoxy vinyl ethers); or combinations thereof. Other suitable binders include polyimides such as the aromatic, aliphatic or cycloaliphatic polyimides and polyacrylates. Further suitable binders include rubbers, such as synthetic rubbers (e.g., styrene-butadiene rubber). The binder may be crosslinked. The binder may also include a combination of any two or more of the preceding binders. In some embodiments, the amount of binder in the negative electrode composition may be between 1 wt. % and 30 wt. %, 1 wt. % and 20 wt. %, 1 wt. % and 10 wt. %, or between 1 wt. % and 5 wt. %, based upon the total weight of the electrode composition.

The negative electrode composition can be provided on only one side of the negative current collector or it may be provided or coated on both sides of the current collector. The thickness of the negative electrode composition may be 0.1 μιη to 3 mm. According to some embodiments, the thickness of the negative electrode composition may be 10 μπι to 300 μιη. According to another embodiment, the thickness of the negative electrode composition may be 20 μιη to 90 μιη. In some embodiments, the electrochemical cells of the present disclosure may include a separator (e.g., a polymeric microporous separator) provided intermediate or between the positive electrode and the negative electrode. The electrodes may be provided as relatively flat or planar plates or may be wrapped or wound in a spiral or other configuration (e.g., an oval configuration). For example, the electrodes may be wrapped around a relatively rectangular mandrel such that they form an oval wound coil for insertion into a relatively prismatic battery case. According to other exemplary

embodiments, the battery may be provided as a button cell battery, or any other know lithium ion battery configuration.

According to some embodiments, the separator can be a polymeric material such as a polypropylene/polyethelene copolymer, another polyolefin multilayer laminate, a polyester film, or a polyvinylidene fluoride film. The polymeric material may include micropores formed therein to allow electrolyte and lithium ions to flow from one side of the separator to the other. The polymeric material may be coated on one side or both sides with a ceramic layer of 0.2 to 5 μπι thickness. The ceramic can be AI2O3 (alumina).

AIOOH (boehmite) or other materials. The separator can also include ceramic particles dispersed in the porous polymer matrix. The thickness of the separator may be between approximately 8 micrometers (μιη) and 50 μιη according to an exemplary embodiment. According to another exemplary embodiment, the average pore size of the separator is between approximately 0.02 μιη and 0.1 μιη.

Electrochemical cells operating at high voltage and incorporating the electrolyte solutions of the present disclosure exhibit performance improvements (e.g., cycle life, calendar life) relative to such electrochemical cells employing known electrolytes (e.g., ethylene carbonate based electrolytes). Further, electrochemical cells operating at high voltage and incorporating the electrolyte solutions of the present disclosure exhibit low impedance, low rates of electrolyte oxidation, good graphite passivation, low gas generation, and acceptable conductivity, at low cost. Still further, the improved cycle life and calendar life at high voltage, enables operation of the cells at high voltage and, in turn, results in electrochemical cells having substantially increased energy densities.

The disclosed electrochemical cells can be used in a variety of devices including, without limitation, portable computers, tablet displays, personal digital assistants, mobile telephones, motorized devices (e.g., personal or household appliances and vehicles), instruments, illumination devices (e.g., flashlights), grid energy storage devices, and heating devices. One or more electrochemical cells of this disclosure can be combined to provide a battery pack.

The present disclosure further relates to methods of making an electrochemical cell. In various embodiments, the method may include providing any of the above- described negative electrodes, providing any of the above-described positive electrodes, and incorporating the negative electrode and the positive electrode into an electrochemical cell that includes any of the above-described electrolyte solutions.

The present disclosure further relates to methods of operating an electrochemical cell. In various embodiments, the method may include providing any of the above- described negative electrodes, providing any of the above-described positive electrodes, incorporating the negative electrode and the positive electrode into an electrochemical cell that includes any of the above-described electrolyte solutions, and operating the electrochemical cell such that the positive electrode reaches voltages of at least 4.4 V, 4.5 V, 4.6 V, 4.7 V, 4.8 V, 4.9 V, or 5.0 V, versus Li/Li⁺. Listing of Embodiments

1. An electrolyte solution comprising:

a linear alkyl carbonate;

a passivating agent comprising vinylene carbonate, fluoroethylene carbonate, difluoroethylene carbonate, methylene ethylene carbonate, succunic anhydride (SA), or prop-l-ene-l,3-sultone; and

an electrolyte salt;

wherein ethylene carbonate is present in the solution in an amount of no more than 5 wt. %, based on the total weight of the electrolyte solution.

2. The electrolyte solution of embodiment 1, wherein the linear alkyl carbonate comprises a carbonate having the following formula:

wherein Ri and R₂ are each independently an alkyl group having 1 to 10 carbon atoms.

3. The electrolyte solution of any one of embodiments 1-2, wherein the linear alkyl carbonate is present in the electrolyte solution in an amount of between 40 and 95 wt. %, based on the total weight of the electrolyte solution.

4. The electrolyte solution of any one of embodiments 1-3, wherein the linear alkyl carbonate comprises diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, methyl propyl carbonate, or di propyl carbonate.

5. The electrolyte solution of any one of embodiments 1-4, wherein the passivating agents are present in the electrolyte solution in an amount of between 0.3 and 10 wt. %, based on the total weight of the electrolyte solution.

6. The electrolyte solution of any one of embodiments 1-5, wherein:

i. vinylene carbonate is present in the electrolyte solution in an amount of between 0.3 and 5 wt. %, based on the total weight of the electrolyte solution; ii. fluoroethylene carbonate is present in the electrolyte solution in an amount of between 1 and 10 wt. %, based on the total weight of the electrolyte solution;

iii. difluoroethylene carbonate is present in the electrolyte solution in an

amount of between 2 and 7 wt. %, based on the total weight of the electrolyte solution;

iv. methylene ethylene carbonate is present in the electrolyte solution in an amount of between 0.5 and 10 wt. %, based on the total weight of the electrolyte solution;

v. succunic anhydride is present in the electrolyte solution in an amount of between 0.5 and 4 wt. %, based on the total weight of the electrolyte solution; or

vi. prop-l-ene-l,3-sultone is present in the electrolyte solution in an amount of between 2 and 8 wt. %, based on the total weight of the electrolyte solution.

7. The electrolyte solution of any one of embodiments 1-6, wherein the electrolyte salt comprises a lithium salt.

8. The electrolyte solution of any one of embodiments 1-7, wherein the lithium salt comprises LiPF₆, LiBF₄, L1CIO4, lithium bis(oxalato)borate, LiN(S02CF3)2,

LiN(S02C₂F₅)2, LiAsFe, LiC(S0₂CF₃)₃, LiN(S0₂F)₂, LiN(S02F)(S0₂CF₃),

LiN(S02F)(S02C₄F9), or lithium difluoro(oxalato)borate.

9. The electrolyte solution of any one of embodiments 1-8, wherein the electrolyte salt is present in the solution in an amount of between 5 and 40 wt.%, based on the total weight of the electrolyte solution.

10. The electrolyte solution of any one of embodiments 1-9, the electrolyte solution further comprising an additive comprising triallyl phosphate, tris(trimethlysilyl) phosphate, tris(trimethlysilyl) phosphite, or pyridine-boron trifluoride. methylene methanedi sulfonate, l,3,2-dioxathiolane-2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, diphenyl carbonate, or methyl phenyl carbonate 11. The electrolyte solution of embodiment 10, wherein the additive is present in the electrolyte solution in an amount of between 0.1 and 5 wt. %, based on the total weight of the electrolyte solution. 12. An electrochemical cell comprising:

a positive electrode;

a negative electrode; and

an electrolyte solution according to any one of embodiments 1-11. 13. The electrochemical cell according to embodiment 12, wherein the positive electrode comprises an active material, the active material comprising a lithium metal oxide or a lithium metal phosphate.

14. The electrochemical cell according to any one of embodiments 12-13, wherein the negative electrode comprises an active material, the active material comprising lithium metal, a carbonaceous material, or a metal alloy.

15. A method of operating an electrochemical cell comprising:

providing an electrochemical cell according to any one of embodiments 12-14; and operating the electrochemical cell such that the positive electrode reaches a voltage of at least 4.4 V versus Li/Li⁺.

The operation of the present disclosure will be further described with regard to the following detailed examples. These examples are offered to further illustrate various specific embodiments and techniques. It should be understood, however, that many variations and modifications may be made while remaining within the scope of the present disclosure.

EXAMPLES

The following examples and comparative examples are offered to aid in the understanding of the present disclosure and are not to be construed as limiting the scope thereof. Unless otherwise indicated, all parts and percentages are by weight. List of Materials

Preparation of electrochemical cells

Machine-made Li[Nio.4Mno.4Coo.2]02 ( MC442)/graphite pouch cells (402035 size, 220 mAh, balanced for 4.7V operation but used only at 4.2, 4.4 or 4.5 V operation) without electrolyte were supplied by Li-Fun Technology (Hunan Province, China). The MC442 was obtained from Umicore (Chonan, Korea). The pouch cells are 40 mm long x 20 mm wide x 3.5 mm thick. The electrode composition in the cells was as follows: Positive electrode - 96.2%: 1.8%:2.0% = Active Material: Carbon Black:PVDF Binder; Negative electrode - 95.4%: 1.3%: 1.1%:2.2% = Active Material :Carbon Black: CMC :SBR. Prior to electrolyte filling, cells were dried in a vacuum oven at 80°C for 14h. The cells were then transferred immediately to an argon-filled glove box for filling and vacuum sealing.

Electrochemical cell formation

1. All cells were filled with 0.75 mL of electrolyte. They were then put in a 40°C box and charged to 1.5 V using a Maccor charger, and held at that voltage for 24h. They were then charged to 3.5 V at C/20. The cell volume change was then measured using Archimedes' principle. The cell was then degassed. This is referred to as "formation 1. "

2. Cells operated up to 4.4 V were then put back on the Maccor charger at 40°C and charged to their upper voltage cut-off at C/20 and discharged to 3.8 V. Cells were held at 3.8 V until the current dropped below C/1000 and moved to a box at 10°C for their impedance to be measured.

3. Cells operated to 4.5 V were charged to 4.5 V at C/20 after formation 1 and held at 4.5 V for 1 h. They were then discharged to 3.8 V and held at that voltage until the current dropped below C/1000. The cell volume change was then measured using Archimedes' principle. Cells were then degassed. This is referred as "formation2. " Cells were then moved to a box at 10°C for their impedance to be measured.

Electrochemical Impedance Spectroscopy (EIS) measurements

EIS measurements were performed using a VMP3 electrochemical testing station (Biologic, France) unless stated otherwise. (See procedures for FRA cycling.) Cells were housed in a 10.0 °C ± 0.2°C box and connected to the VMP3. The EIS spectra were measured using a 10 mV excitation between 100 kHz - 20 mHz. Ret is defined as the real impedance distance between the two minima of the negative imaginary impedance of the EIS spectra. Figure 1 shows how Ret was determined.

Cell volume change

Using cell designs comprising soft enclosures such as pouch cells allows the volume of gas produced during cell operation to be measured. Cell volume changes are measured using Archimedes' principle. The weight of the cells when submerged in water is measured before and after cycling. The weight of the cells is measured by suspending the cell underneath a balance equipped with a weight sensor underneath its base using a thin wire. The weight of the submerged cell will then be equal to the tension on the wire and measured by the balance. The difference in wire tension before and after cell operation is then directly proportional to the volume change of the cell following:

F = (m— pV)g (1)

where F is the tension on the wire, m is the mass of the cell, g is the gravitational acceleration, p is the density of the water, V is the volume of the cell and the subscripts initial and final refer to before and after cell testing, respectively.

Dumb Storage

4.2 V dumb storage: After formation 7, cells were moved to the Maccor charger, charged and discharged two times between 2.8 V and 4.2 V. Then the cells were charged to 4.2 V and held at that voltage for 24 hours. Cells were then quickly (within 10 minutes) moved to the storage system where the open circuit voltage was measured every 6 hours for a total storage time of 500 h. Cells were finally charged to 3.8 V and held at that voltage until the current dropped below C/1000. Cells were then moved to a 10°C box for their impedance to be measured. The cell volume change was then measured using Archimedes' principle.

4.5 V dumb storage: After formation 2, cells were moved to the Maccor charger, discharged and charged two times between 2.8 V and 4.5 V. Then the cells were held at 4.5 V for 24 hours. Cells were quickly (within 10 minutes) moved to the storage system where the open circuit voltage was measured every 6 hours for a total storage time of 500 h. Cells were finally charged to 3.8 V and held at that voltage until the current dropped below C/1000. Cells were then moved to a 10°C box for their impedance to be measured. The cell volume change was then measured using Archimedes' principle as described above.

Ultra High Precision Charger (UHPC) "barn" cycling

UHPC at 4.2 V: After formation, cells were moved to the UHPC station. Cells were tested in a temperature controlled box set at 40. ±0.1 °C. Cells were first charged to 4.000 V using currents corresponding to C/15. Then the charging current was switched to C/40 and applied up to the 4.2 V. After that, cells were discharged at C/40 to 4.000 V and then discharged to 2.800 V using currents corresponding to C/15. This procedure was then repeated 15 times on the UHPC. Cells were finally charged to 3.8 V and held at that voltage until the current dropped below C/1000. Cells were then moved to a 10 °C box for their impedance to be measured. The cell volume change was then measured as described above using Archimedes' principle. All cells were cycled with clamps that applied about 0.5 atm pressure to the largest area faces of the cell.

UHPC at 4.4 V and 4.5 V: After formation, cells were moved to the UHPC station. Cells were first charged to 4.200 V using currents corresponding to C/15. Then the charging current was switched to C/40 and applied up to the upper cutoff potential. After that, cells were discharged at C/40 to 4.200 V and then discharged to 2.800 V using currents corresponding to C/15. This procedure was then repeated 15 times on the UHPC. Cells were finally charged to 3.8 V and held at that voltage until the current dropped below C/1000. Cells were then moved to a 10°C box for their impedance to be measured.

The cell volume change was then measured as described above. All cells were cycled with clamps that applied about 0.5 atm pressure to the largest area faces of the cell.

Frequency Response Analyzer (FRA) cycling

Cells were cycled between 2.8 V and an upper cut-off voltage of 4.4 or 4.5 V. All cells underwent two cycles between 2.8 V and the upper cut-off voltage at C/5 followed by three cycles between 2.8 V and the upper cut-off voltage with a C/5 charge and C/2.5 discharge. All five cycles included a 24 hour hold at the top of charge. The five cycles were followed by an "FRA cycle" consisting of a charge and discharge at C/20 between 2.8 V and the upper cut-off voltage while the FRA measured the cell impedance every 0.1 V between 3.6 and 4.4 or 4.5 V from 40 mHz to 100 kHz. After the FRA cycle, the cells were cycled again for 5 cycles and the protocol was repeated. After cycling cells were charged to 3.8 V and held at that voltage until the current dropped below C/1000. They were then moved to a 10°C box for their impedance to be measured. The cell volume change was then measured using as described above. Figure 2 shows the FRA cycling protocol. 4.4 V smart storage

Cells were formed according to the procedures described previously. Cells were moved to a smart storage station at 40°C. Cells were then discharged to 2.8 V at C/20. Cells were then charged to 4.4 V and discharged to 2.8 V four times at C/20. Cells were then charged to 4.4 V and stored at open circuit voltage for 500 h. After the 500 h storage, cells were then discharged to 2.8 V and charged to 4.4 V, followed by a last discharge to 2.8 Vat C/20. Cells were finally charged to 3.8 V and held at that voltage until the current dropped below C/1000. Cells were then moved to a 10°C box for their impedance to be measured. The cell volume change was then measured using Archimedes' principle. Figure 3 shows a typical profile of the cell voltage as a function of time profile during smart storage experiments.

The capacity losses during smart storage were calculated as follows:

Reversible capacity loss = Q_D2— Q_D1

Irreversible capacity loss = Q_D2— Q_DQ in which Do, Di, and D₂ are as shown in Figure 3.

Long term Constant-current Constant-voltage (CCCV) cycling at 4.4 V at Room Temperature (20°C) or 55°C

Cells were formed according to the procedures provided earlier. Cells cycled at 55 °C were put in a 55.0 ± 0.5 °C temperature controlled box and cells cycled at 20 °C were stored in a 20.0 ± 1 °C temperature controlled room. Cells were connected to a Neware battery testing cycler. Cells were cycled between 2.8 and 4.4 V using an 80 raA current (equivalent to C/2.2 rate). A constant voltage charge was applied at the top of charge and maintained until the current dropped below C/20. After cycling cells were charged to 3.8 V and held at that voltage until the current dropped below C/1000. They were then moved to a 10°C for their impedance to be measured. All cells were cycled with clamps that applied about 0.5 atm pressure to the largest area faces of the cell.

Long term CCCV cycling at 4.4 V or 4.5 V at 40 °C

After cycling on the UHPC at 4.5 V and 40 °C as described previously, cells were moved to a 40.0 °C (±0.5°C) box and connected to a Neware battery testing cycler. One pair cell was cycled up to 4.4 V and the other pair cell was cycled up to 4.5 V using the same cycling protocol as described in the previous section, except that the upper voltage cut-off was 4.5 V. After cycling cells were charged to 3.8 V and held at that voltage until the current dropped below C/1000. They were then moved to a 10°C box for their impedance to be measured. All cells were cycled with clamps that applied about 0.5 atm pressure to the largest area faces of the cell. Electrolyte conductivity measurement

Figure 4 and 5 show the results of conductivity measurements.

Passivating Agents

Figure 6 compares dQ/dV vs. Q of MC(442)/graphite cells containing electrolyte solutions of the present disclosure comprising various passivating agents that were charged for the first time at 40°C and C/20. VC, MEC, FEC, DiFEC, and SA demonstrate the passivation of graphite by the elimination of the large reduction peak associated with EMC.

Passivating Agent Concentration

Figure 7 shows the amount of VC remaining and the amount of EMC trans- esterification as a function of initial VC loading after the first cycle of cells containing 1M

LiPF₆ EMC:VC electrolytes. Figure 7 (top) shows that cells initially containing 1.7 - 3% VC have very little VC left after the first cycle. Without being bound by theory, it may be that cells filled with EMC :VC -based electrolytes with low initial VC loading have low gas evolution. Figure 7 (bottom) shows the amount of EMC that underwent trans-esterification producing dimethyl carbonate (DMC) and diethyl carbonate (DEC). These trans- esterification reactions are catalyzed by the presence of lithium alkoxides generated by the reduction of linear carbonates. Figure 7 shows that EMC reduction/transesterification is mostly suppressed at an initial VC loading between 1.8 - 3%.

Comparative Examples CE1 - CE13 and Examples 1-21 Formulations of the comparative and illustrative electrolyte solutions are provided in Table 1. These electrolyte formulations were used in the lithium ion pouch cells containing the NMC positive electrode and graphite negative electrode. Table 1. Electrolyte Solution Formulations

Lithium ion pouch cells containing the MC442 positive electrode and graphite negative electrode and the electrolyte examples were tested at 4.5 V at 40 °C using the UHPC, as described above. Coulombic inefficiency (CIE), charge endpoint capacity slippage, Ret, and gas evolution results were measured and the results are summarized in Table 2. The coulombic efficiency is the ratio of the discharge to charge capacity of a given cycle. CIE is equal to 1 minus Coulombic efficiency (that is, CIE = 1-CE). The charge endpoint capacity slippage is defined as the extent to which the top of charge endpoint slips to higher capacity with each charging cycle. It is typically measured by subtracting the cumulative cell capacity at the top of charge of a given cycle from the cumulative cell capacity at the top of charge of the previous cycle. Smaller CIE, charge slippage, Ret, and gas evolution can be indicators of fewer parasitic reactions at high voltage and longer cycle life of electrochemical cells. A Figure of Merit (FOM), which is defined in the equation below, combines the results of CIE, charge slippage, Ret, and gas evolution to show the collective benefits of electrolyte formulation in this disclosure.

FOM = (CIE x 250) + (CECS x 1.33) + (MAX x 2.0) + (R_ct x 0.003) (3) In which CECS is charge endpoint capacity slippage in units of (mAh/cycle), MAX is the amount of gas evolution in mL, and Ret is in units of Ω-cm². A smaller FOM is an indicator of longer cycle life and longer calendar life of electrochemical cells. The FOM values of Ex 1 - Ex 20 are less than 4 while comparative examples CE1-CE12 show FOM numbers higher than 4. These results clearly demonstrate that the electrolyte formulations of the current disclosure reduce CIE, charge slippage, Ret, and gas generation during UHPC cycling at elevated temperature and high voltage.

Table 2. NMC442 /Graphite Cell Performance in UHPC Cycling to 4.5V at 40 °C

Figure 8 compares the charge transfer impedance during FRA cycling of CE3 and CE13 with Examples 1, 1 1, 14, 17, 18, 20 and 21. It is obvious that Examples 1, 1 1, 14, 17, 18, 20 and 21 reduce impedance rise during cycling at 4.4V compared to Comparative examples 3 and 13. Figures 9a and 9b show long-term cycling at 20 °C of CE 3 and Examples 1, 12, 17. It is evident from Figures 9a and 9b that Examples 1, 12 and 17 improve cycle life at 4.4 V in long-term cycling tests at 20 °C compared to CE3. Figures 9c and 9d show long- term cycling at 55 °C of CE 3 and Examples 1, 12, 17. It is evident from Figures 9c and 9d that Examples 12 and 17 improve cycle life at 4.4 V in long-term cycling tests at 55 °C compared to CE3.

Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present 5 disclosure. This application is intended to cover any adaptations or variations of the preferred embodiments discussed herein. Therefore, it is manifestly intended that this disclosure be limited only by the claims and the equivalents thereof.

Claims

What is Claimed is:

1. An electrolyte solution comprising:

a linear alkyl carbonate;

an electrolyte salt;

2. The electrolyte solution of claim 1, wherein the linear alkyl carbonate comprises a carbonate having the following formula:

3. The electrolyte solution of claim 2, wherein the linear alkyl carbonate is present in the electrolyte solution in an amount of between 40 and 95 wt. %, based on the total weight of the electrolyte solution.

4. The electrolyte solution of claim 2, wherein the linear alkyl carbonate comprises diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, methyl propyl carbonate, or di propyl carbonate.

5. The electrolyte solution of claim 2, wherein the passivating agents are present in the electrolyte solution in an amount of between 0.3 and 10 wt. %, based on the total weight of the electrolyte solution.

6. The electrolyte solution of claim 2, wherein:

iii. difluoroethylene carbonate is present in the electrolyte solution in an

7. The electrolyte solution of any one of claim 2, wherein the electrolyte salt comprises a lithium salt.

8. The electrolyte solution of claim 7, wherein the lithium salt comprises LiPF₆, L1BF4, L1CIO4, lithium bis(oxalato)borate, LiN(S0₂CF₃)₂, LiN(S0₂C₂F₅)2, LiAsFe,

LiC(S0₂CF₃)₃, LiN(S0₂F)₂, LiN(S0₂F)(S0₂CF₃), LiN(S02F)(S0₂C4F₉), or lithium difluoro(oxal ato)b orate .

9. The electrolyte solution of claim 8, wherein the electrolyte salt is present in the solution in an amount of between 5 and 40 wt.%, based on the total weight of the electrolyte solution.

10. The electrolyte solution of claim 2, the electrolyte solution further comprises an additive comprising triallyl phosphate, tris(trimethlysilyl) phosphate, tris(trimethlysilyl) phosphite, or pyridine-boron trifluoride. methylene methanedi sulfonate, 1,3,2- dioxathiolane-2,2-dioxide, 1,3,2-dioxathiane 2,2-dioxide, diphenyl carbonate, or methyl phenyl carbonate

11. The electrolyte solution of claim 10, wherein the additive is present in the electrolyte solution in an amount of between 0.1 and 5 wt. %, based on the total weight of the electrolyte solution.

12. An electrochemical cell comprising:

a positive electrode;

a negative electrode; and

an electrolyte solution according to claim 2.

13. The electrochemical cell according to claim 12, wherein the positive electrode comprises an active material, the active material comprising a lithium metal oxide or a lithium metal phosphate.

14. The electrochemical cell according to claim 13, wherein the negative electrode comprises an active material, the active material comprising lithium metal, a carbonaceous material, or a metal alloy.

15. A method of operating an electrochemical cell comprising:

providing an electrochemical cell according to claim 12; and

operating the electrochemical cell such that the positive electrode reaches a voltage of at least 4.4 V versus Li/Li⁺.