Field

This disclosure relates to electrolyte additives for secondary batteries.

Background

The use of fluorinated polymers in the construction of lithium-ion electrochemical cells is well known in the art. In general these are high molecular weight polymers comprising vinylidene fluoride (VDF) or a mixture of VDF with hexafluoropropylene (HFP) for use either as composite electrode binders or in the preparation of polymer gel electrolytes. For such applications, dissolution of the fluorinated polymers in the liquid electrolyte would be detrimental to their intended function which is the binding together of electrode particles when the fluorinated polymers are used in binders or maintaining gel cohesion and physical separation between positive and negative electrode when the fluorinated polymers are used in polymer gel electrolytes.

Fluoropolymers used in these applications are chosen to have a sufficiently high molecular weight and a composition that renders them essentially insoluble in the liquid electrolyte - generally a -1.0M solution of lithium salt (usually LiPF ₆ ) in a mixture of cyclic and acyclic organic carbonate solvents. In the case of polymer gel electrolytes, the fluoropolymer is a major component of the electrolyte (not just a minor additive) and is designed to be swollen by, but not dissolve in, the liquid electrolyte in order to maintain the cohesive strength of the polymer and provide a robust physical barrier between cathode and anode (to prevent shorting), while providing channels for facile Li-ion transport.

While commercial lithium- ion batteries (LIBs) perform satisfactorily for most home electronics applications, currently available LIB technology does not satisfy some of the more demanding performance goals for Hybrid Electric Vehicles (HEV) or Plug- in Hybrid Electric Vehicles (PHEV), or Pure Electric Vehicles (EV). In particular, currently available LIB technology does not meet the 10- 15 year calendar life requirement set by the Partnership for a New Generation of Vehicles (PNGV). The most extensively used LIB electrolytes are composed of LiPF ₆ dissolved in organic carbonates or esters. However, these commonly used electrolytes have limited thermal and high voltage stability. Thermal and electrochemical degradation of the electrolyte can be a primary cause of reduced Li- 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, gassing (which can result in cell venting), impedance growth and reduced rate capability. Typically, driving the electrodes to greater voltage extremes or exposing the cell to higher temperatures accelerates these undesired reactions and magnifies the associated problems. Under extreme abuse conditions, uncontrolled reaction exotherms may occur that result in thermal runaway and catastrophic disintegration of the cell.

Stabilizing the electrode/electrolyte interface is important for controlling and minimizing these undesirable reactions and improving the cycle life and voltage and temperature performance limits of Li ion batteries. Electrolyte additives designed to selectively react with, bond to, or self organize at the electrode surface in a way that passivates the interface represents one of the simplest and potentially most cost effective ways of achieving this goal. The effect of common electrolyte solvents and additives, such as ethylene carbonate (EC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), and lithium bisoxalatoborate (LiBOB), on the stability of the negative electrode solid-electrolyte interface (SEI) layer is well documented. Evidence suggests that VC and LiBOB, for example, react on the surface of the anode to generate a more stable SEI. Stabilizing the SEI and inhibiting the detrimental thermal and redox reactions that can cause electrolyte degradation at the electrode interface (both cathode and anode) will lead to extended calendar life and enhanced thermal stability of LIBs.

Lithium bis(trifluoromethanesulfonyl) imide (HQ-1 15, available from 3M, St. Paul, MN) has been used as an electrolyte additive in commercial lithium ion batteries. When used as an electrolyte additive, lithium bis(trifluoromethanesulfonyl) imide improves cycle life in

graphite/LiCo0 ₂ cells at high temperature. Similar results can be obtained in graphite/lithium manganese nickel cobalt oxide (MNC cells). Cycle life improvements achieved by adding lithium bis(trifluoromethanesulfonyl) imide correlates with reduced cell impedance. Lithium

bis(trifluoromethanesulfonyl) imide also can reduce gassing at the negative electrode and can prevent shorting under high temperature float test conditions with single layer PE separator. Thus cell life and safety are improved using lithium bis(trifluoromethanesulfonyl) imide as additives in standard electrolyte. Fluorocarbon electrolyte additive that include multifunctional anions have also been disclosed, for example, in U.S. S.N. 61/494,094 (Lamanna et al.) filed on June 7, 201 1 and entitled "Lithium-ion Electrochemical Cells That Include Fluorocarbon Electrolyte Additives".

However, there is an ongoing need within Li-ion battery industry for improved additives that provide reduced cost of ownership, better performance, and novel functionality. There is a need for electrolyte additives that: 1) are capable of improving the high temperature performance and stability (e.g. > 55°C) of lithium- ion cells, 2) can provide electrolyte stability at high voltages (e.g. > 4.2V) for increased energy density, and 3) can enable new state of the art high capacity electrode materials (both new cathodes and new anodes).

Summary

We have discovered a new family of fluorinated compounds that provide significant performance benefits in Li-ion cells when used at relatively low loadings in the electrolyte compared to more conventional additives. The new partially fluorinated additives are low molecular weight, soluble fluoropolymers containing at least one monomer selected from tetrafluoroethylene (TFE), vinylidene fluoride (VDF) and hexafluoropropylene (HFP). The provided partially fluormated polymer additives are soluble in common lithium-ion battery electrolyte formulations and are capable of enhancing lithium- ion cell performance (cycle life, calendar life, impedance) at elevated temperatures or high voltage.

In one aspect, an electrochemical cell is provided that includes a positive electrode that includes at least one electrochemically active material, a negative electrode; and a charge-carrying liquid electrolyte, wherein the charge-carrying liquid electrolyte includes at least one organic solvent, an electrolyte salt, and at least one partially fluorinated polymer additive that is soluble in the liquid electrolyte. The electrochemically active material can be a lithium transition metal oxide and can include cobalt, manganese, or nickel. The at least one organic solvent can include ethylene carbonate, dimethyl carbonate, or methyl ethyl carbonate. The at least one partially fluorinated polymer additive can be the product of polymerization of a monomer mixture that includes tetrafluoroethylene, vinylidene fluoride, or hexafluoropropylene.

In another aspect, a method of stabilizing an electrochemical cell is provided that includes providing an electrochemical cell having a positive electrode that includes at least one

electrochemically active material, a negative electrode, and at least one organic solvent, wherein the charge-carrying liquid electrolyte includes at least one organic solvent, and an electrolyte salt; and dissolving a partially fluorinated polymer additive in the charge-carrying liquid electrolyte.

In the present disclosure:

"active" or "electrochemically active" refers to a material that can undergo lithiation and delithiation by reaction with lithium;

"inactive" or "electrochemical inactive" refers to a material that does not react with lithium and does not undergo lithiation and delithiation;

"lithium mixed metal oxide" refers to a lithium metal oxide composition that includes one or more transition metals in the form of an oxide;

"loading" refers to the amount (in weight) of partially fluorinated polymer additive that is placed in the electrolyte whether it is soluble or not;

"negative electrode" refers to an electrode (often called an anode) where electrochemical oxidation and delithiation occurs during a discharging process;

"partially fluorinated polymeric additive" refers to a polymer comprising at least one of VDF (CH ₂ =CF ₂ ), HFP (CF ₂ =CFCF ₃ ), or TFE (CF ₂ =CF ₂ ) monomers and includes a hydrocarbon skeleton in which most (greater than about 10% to about 90%) of the hydrogen atoms, but not all, have been replaced by fluorine atoms; and

"positive electrode" refers to an electrode (often called a cathode) where electrochemical reduction and lithiation occurs during a discharging process; and "soluble" or "solubility" refers to the amount of partially fluorinated polymer additive that can dissolve in the liquid electrolyte at room temperature— if the "solubility" of the additive is greater than 1 weight percent in the liquid electrolyte, the additive is considered "soluble".

Typically, the solubility of the partially fluorinated polymer additive is greater than 5 weight percent in the liquid electrolyte.

The provided partially fluorinated polymer additives are soluble in common lithium-ion battery electrolyte formulations and are capable of enhancing lithium- ion cell performance (cycle life, calendar life, impedance) at elevated temperatures (e.g., > 55°C) or high voltage (e.g., > 4.2 V vs. Li/Li ⁺ ). They are low molecular weight and are soluble (at 1 weight percent or greater, typically 5 weight percent or greater) in liquid electrolytes.

The above summary is not intended to describe each disclosed embodiment of every implementation of the present invention. The brief description of the drawings and the detailed description which follows more particularly exemplify illustrative embodiments. Brief Description of the Drawings

Fig. 1 is the ¹⁹ F NMR spectrum of the supernatant solution obtained by combining 1.0M LiPF ₆ in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by weight) electrolyte with 2 wt% of LFC- 1 from Preparatory Example 1 at room temperature.

Fig. 2 is a schematic diagram showing how voltage drop, percent reversible, irreversible, and total capacity loss is calculated from high temperature thermal storage data in full cells.

Detailed Description

In the following description, reference is made to the accompanying set of drawings that form a part of the description hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense.

Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims 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 claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. The use of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5) and any range within that range.

Electrochemical cells, typically, lithium-ion electrochemical cells, are provided that include a positive electrode that includes at least one electrochemically active material. Provided electrochemically active materials include, for example, LiFeP0 ₄ , LiMnP0 ₄ , LiMn ₂ 0 ₄ , L1C0PO ₄ , and L1C0O ₂ ; lithium transition metal oxides as disclosed in U. S. Pat. Nos. 5,858,324; 5,900,385; 6,143,268 (all to Dahn et al.); and 6,680, 145 (Obrovac et al.); U. S. Pat. Nos. 6,964,828 and 7,078, 128 (Lu et al.); U. S. Pat. Appl. Publ. No. 2008/0032185 (Le); U. S. Pat. No. 7,21 1,237 (Eberman et al.); U. S. Pat. Appl. Publ. No. 2006/0046144 (Obrovac); combinations thereof and other materials that will be familiar to those skilled in the art.

Additionally electrochemically active materials that are useful in positive electrodes for the provided electrochemical cells can include, for example, LiNio.5Mn1.5O4 and L1VPO ₄ F. In some embodiments the provided lithium- ion electrochemical cells can include a positive electrode comprising a mixed transition metal oxide having the formula, Li _a [Mn _x Ni _y Co _z ]0 ₂ , wherein x+y+z=1.0. In some embodiments, the lithium mixed metal oxide compositions can adopt an 03 or -NaFe0 ₂ -type layered structure that can be desirable for efficient lithiation and delithiation. These materials are well known in the art and are disclosed, for example, in U. S. Pat. Nos.

5,858,324; 5,900,385 (both Dahn et al.); and 6,964,828 (Lu et al.). In some embodiments, the provided cathode compositions can include transition metals selected from manganese (Mn), nickel (Ni), and cobalt (Co). The amount of Mn can range from 0 to about 50 mole percent (mol%), from 0 to about 40 mol%, or from greater than zero to about 10 mol% based upon the total mass of the cathode composition, excluding lithium and oxygen. The amount of Ni can range from 0 to about 50 mol%, from 0 to about 40 mol%, or from 0 to about 10 mol% based upon the total mass of the cathode composition, excluding lithium and oxygen. The amount of Co can range from greater than about 10 mol% to about 95 mol%, from greater than about 15 mol% to about 70 mol%, or even from greater than about 20 mol% to about 50 mol% of the composition excluding lithium and oxygen. In some embodiments, the lithium metal oxide can include additional metals. For example, the lithium mixed metal oxide can include one or more additional metals as dopants. Exemplary metals include Al, Mg, Zr, Fe, Cu, Zn, V, or Ti. It has been shown by Jouanneau et al., J. Electrochem. Soc, 150 (10), A1299 (2003) and Jiang et al., J. Electrochem. Soc, 152 (3), A566 (2005) that the addition of a small amount of transition metals such as manganese and nickel into the LiCo0 ₂ structure significantly increases the thermal stability. Both of these papers show that lithium mixed metal oxides with about 90 mole percent cobalt have much higher thermal stability than pure LiCo0 ₂ .

In some other embodiments, the lithium mixed metal oxides can be aluminum-doped lithium transition metal oxides as disclosed, for example, in U. S. Pat. No. 7,709,149 (Paulsen et al.) or lithium transition metal oxides with a gradient of metal compositions as disclosed, for example, in U. S. Pat. No. 7,695,649 (Paulsen et al.) Other mixed metal oxide disclosures include U. S. Pat. Nos. 7,648,693; 7,939,049; and 7,939,203 (all Paulsen et al.)

Lithium transition metal oxides can be in the form of particles having a single phase having an 03 ( -NaFe0 ₂ ) crystal structure. The particles may have a maximum average dimension that is no greater than 60 micrometers, no greater than 40 micrometers, or no greater than 20 micrometers. The powders may for example have a maximum average particle diameter that is submicron, at least 1 micrometer, at least 2 micrometers, at least 5 micrometers, or at least 10 micrometers. For example, suitable powders often have a maximum average dimension of 1 to 60 micrometers, 10 to 60 micrometers, 20 to 60 micrometers, 40 to 60 micrometers, 1 to 40 micrometers, 2 to 40 micrometers, 10 to 40 micrometers, 5 to 20 micrometers, or 10 to 20 micrometers. The powdered materials may contain optional matrix formers within powder particles. Each phase originally present in the particle (i.e., before a first lithiation) may be in contact with the other phases in the particle. In some embodiments the average diameter of particles of the mixed metal oxide materials can be from about 2 μηι to about 25 μηι. In other embodiments, the average particle size can be less than about 1000 nm, less than about 500 nm, less than about 250 nm, less than about 100 nm, or less than about 50 nm.

The theoretical crystal density of Li[Mn _x Ni _y Co _z ]0 ₂ for x=y has been calculated from the lattice constant data in a reference by Lu et al., J. Electrochem. Soc, 149 (10), A1332 (2002). The calculations show that lithium mixed metal oxides with manganese, nickel, and cobalt have crystal densities that increase with increasing cobalt content. They range from about 4.62 g/cm ³ for Li[Mno _.5 Nio _.5 Coo _. o]0 ₂ to about 5.05 g/cm ³ for LiCo0 ₂ . Thus, lithium mixed metal oxides with higher cobalt content will have higher energy density.

The cathode compositions may be synthesized by jet milling or by combining precursors of the metal elements (e.g., hydroxides, nitrates, and the like), followed by heating to generate the cathode composition. Heating is preferably conducted in air at temperatures of at least about 600°C, more preferably at least 800°C. In general, higher temperatures are preferred because they lead to materials with increased crystallinity. The ability to conduct the heating process in air is desirable because it obviates the need and associated expense of maintaining an inert atmosphere. Accordingly, the particular metal elements are selected such that they exhibit appropriate oxidation states in air at the desired synthesis temperature. Conversely, the synthesis temperature may be adjusted so that a particular metal element exists in a desired oxidation state in air at that temperature.

The provided lithium-ion electrochemical cells include a negative electrode capable of intercalating lithium or alloying with lithium. The lithium metal oxide positive electrodes described above can be combined with an anode and an electrolyte to form a lithium-ion electrochemical cell or a battery pack from two or more electrochemical cells. Examples of suitable anodes can be made from compositions that include lithium, carbonaceous materials, silicon alloy compositions, tin alloy compositions and lithium alloy compositions. Exemplary carbonaceous materials can include synthetic graphites such as mesocarbon microbeads (MCMB) (available from Osaka Gas Co., Japan), SLP30 (available from TimCal Ltd., Bodio Switzerland), natural graphites and hard carbons. Useful anode materials can also include alloy powders or thin films. Such alloys may include electrochemically active components such as silicon, tin, aluminum, gallium, indium, lead, bismuth, and zinc and may also comprise electrochemically inactive components such as iron, cobalt, transition metal silicides and transition metal aluminides. Useful alloy anode compositions can include alloys of tin or silicon such as Sn-Co-C alloys, and Si ₇ oFeioTi ₁₀ Cio where Mm is a Mischmetal (an alloy of rare earth elements). Metal alloy compositions used to make anodes can have a nanocrystalline or amorphous microstructure. Such alloys can be made, for example, by sputtering, ball milling, rapid quenching, or other means. Useful anode materials also include metal oxides such as Li ₄ Ti ₅ 0i ₂ , W0 ₂ , and tin oxides. Other useful anode materials include tin-based amorphous anode materials such as those disclosed in U. S. Pat. No. 7,771,876 (Mizutani et al.).

Exemplary silicon alloys that can be used to make suitable anodes include compositions that comprise from about 65 to about 85 mol% Si, from about 5 to about 12 mol% Fe, from about 5 to about 12 mol% Ti, and from about 5 to about 12 mol% C. Additional examples of useful silicon alloys include compositions that include silicon, copper, and silver or silver alloy such as those discussed in U. S. Pat. Appl. Publ. No. 2006/0046144 (Obrovac et al.); multiphase, silicon- containing electrodes such as those discussed in U. S. Pat. No. 7,498,100 (Christensen et al.); silicon alloys that contain tin, indium and a lanthanide, actinide element or yttrium such as those described in U. S. Pat. Nos. 7,767,349, 7,851,085, and 7,871,727 (all to Obrovac et al.);

amorphous alloys having high silicon content such as those discussed in U. S. Pat. No. 7,732,095 (Christensen et al.); and other powdered materials used for negative electrodes such as those discussed in U. S. Pat. Appl. Publ. No. 2007/0269718 (Krause et al.) and U. S. Pat. No. 7,771,861 (Krause et al.). Anodes can also be made from lithium alloy compositions such as those of the type described in U. S. Pat. Nos. 6,203,944 and 6,436,578 (both to Turner et al.) and in U. S. Pat. No. 6,255,017 (Turner).

The provided electrochemical cells include a charge-carrying liquid electrolyte that includes at least one organic solvent, an electrolyte salt, and at least one partially fluorinated polymer additive that is soluble in the liquid electrolyte. A variety of electrolytes can be employed. Representative electrolytes can contain one or more lithium salts and a charge-carrying medium in the form of a liquid or gel. Exemplary lithium salts are stable in the electrochemical window and temperature range (e.g. from about -30°C to about 70°C) within which the cell electrodes can operate, are soluble in the chosen charge-carrying media, and perform well in the chosen lithium-ion cell. Exemplary lithium salts include LiPF ₆ , LiBF ₄ , L1CIO ₄ , lithium bis(oxalato)borate, LiN(CF ₃ S0 ₂ ) ₂ , LiN(C ₂ F ₅ S0 ₂ ) ₂ , LiAsF ₆ , LiC(CF ₃ S0 ₂ ) ₃ , and combinations thereof. Exemplary liquid electrolytes include at least one organic solvent such as, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl-methyl carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate, γ-butyrolactone, methyl difluoroacetate, ethyl difluoroacetate, dimethoxyethane, diglyme (bis(2-methoxyethyl) ether), tetrahydrofuran, dioxolane, combinations thereof and other media that will be familiar to those skilled in the art. Exemplary electrolyte gels include those described in U.S. Pat. Nos. 6,387,570 (Nakamura et al.) and 6,780,544 (Noh).

The electrolyte can include other additives that will be familiar to those skilled in the art. For example, other additives, such as redox chemical shuttles, can also be added to the electrolyte of the provided lithium-ion electrochemical cells. Redox chemical shuttles can impart overcharge protection to rechargeable lithium-ion electrochemical cells. Redox chemical shuttles have been disclosed, for example, in U. S. Pat. Nos. 7, 585,590 (Wang et al.) and in U. S. Pat. Nos.

7,615,312; 7,615,317; 7,648,801 ; and 7,811,710 (all to Dahn et al.). Redox chemical shuttles for high voltage cathodes have been disclosed for example, in U. S. Pat. Appl. Publ. No.

2009/0286162 (Lamanna et al).

Partially fluorinated polymer additives are provided that are soluble in common lithium ion battery electrolyte formulations and are capable of enhancing lithium ion cell performance (cycle life, calendar life, impedance) at elevated temperatures or high voltage. The partially fluorinated polymer additives are the product of polymerization of a monomer mixture that includes at least one monomer selected from tetrafluoroethylene (TFE), vinylidene fluoride (VDF), and hexafluoropropylene (HFP). In some embodiments, the partially fluorinated polymer additives are the reaction product of vinylidene fluoride and hexafluoropropylene. Other fluorinated and non- fluorinated comonomers may also optionally be present, including but not limited to fluorinated vinyl ethers (such as CF ₂ =CFOCF ₃ , CF ₂ =CFOC ₃ F ₇ , CF ₂ =CFO(CF ₂ ) ₃ OCF ₃ , CF ₂ =CFO(CF ₂ ) ₅ CN, CF ₂ =CFO(CF ₂ ) ₄ S0 ₃ Li, CF ₂ =CFOCF ₂ CF(CF ₃ )OCF ₂ CF ₂ S0 ₃ Li, and

CF ₂ =CFO-R _f -COOLi) and hydrocarbon comonomers (such as ethylene and propylene). Generally, the solubility of the partially fluorinated polymer additives in the base electrolyte formulation is greater than 1.0% by weight, greater than 5.0% by weight, or even greater than 10% by weight at room temperature. A typical loading of the partially fluorinated polymer additive in the base electrolyte formulation is less than 5.0% by weight, less than 3.0% by weight, or even less than 1.0% by weight. Partially fluorinated polymer additive loadings in the base electrolyte formulation of at least 0.2 weight percent or from about 0.2 weight percent to about 2.0 weight percent are typical, with loadings of from about 0.5 weight percent to about 1.5 weight percent being most typical. The partially fluorinated polymer additive typically has a molecular weight (Mn) < 50,000 atomic mass units (amu), < 20,000 amu, or even < 10,000 amu.

Partially fluorinated polymer additives have the same structure as their fully hydrogenated analogs with 10 percent to 90 percent of the hydrogen atoms of their fully hydrogenated analog replaced by fluorine substituents. In some embodiments, the partially fluorinated polymer additives have at least 60% of the hydrogen atoms of their fully hydrogenated analogs replaced by fluorine substituents. The provided partially fluorinated polymer additives can be made using firee- radical polymerization of ethylenically unsaturated monomers in the presence of chain-transfer agents as disclosed, for example, in U. S. Pat. No. 5,208,305 (Grootaert).

A representative example of a useful partially fluorinated polymer additive is described in the current disclosure. The material, referred to as LFC-1, is a copolymer of VDF and HFP with a molecular weight of -10,000 amu. The cycle life and thermal storage test results described herein, using lithium ion coin cells as test vehicles, demonstrate that the LFC- 1 additive provides significant benefits to lithium ion cell performance. Specifically, when employed as a soluble electrolyte additive at low concentrations in Li ion cells, LFC- 1 can improve cycle life performance, reduce irreversible capacity loss during high temperature storage, and decrease the buildup of cell resistance at elevated temperature (> 55°C) and high voltage (> 4.2V vs. Li/Li+).

While not wishing to be bound by theory, we believe this and related fluoropolymer additives of this invention are able to passivate the electrode-electrolyte interface through loss of HF and subsequent reaction of the unsaturated fluoropolymer at the electrode (anode and/or cathode) surface, as described in Scheme (I).

-HF Reacts at

-CF ₂ -CH ₂ -CF ₂ -CF(CF ₃ -CF ₂ -CH=CF-CF(CF ₃ - Pass ⁱ vatmg

Electrode ^Fllm

(Scheme I)

The provided electrochemical cells can contain additives such as will be familiar to those skilled in the art. The electrode composition can include an electrically conductive diluent to facilitate electron transfer between the composite electrode particles and from the composite to a current collector. Electrically conductive diluents can include, but are not limited to, carbon black, 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 (both from MMM Carbon, Belgium), SHAWANIGAN BLACK (Chevron Chemical Co., Houston, TX), acetylene black, furnace black, lamp black, graphite, carbon fibers and combinations thereof.

The electrode composition can include an adhesion promoter that promotes adhesion of the composition and/or electrically conductive diluent to the binder. The combination of an adhesion promoter and binder can help the electrode composition better accommodate volume changes that can occur in the composition during repeated lithiation/delithiation cycles.

Alternatively, the binders themselves can offer sufficiently good adhesion to metals and alloys so that addition of an adhesion promoter may not be needed. If used, an adhesion promoter can be made a part of the binder itself (e.g., in the form of an added functional group), can be a coating on the composite particles, can be added to the electrically conductive diluent, or can be a combination of such measures. Examples of adhesion promoters include silanes, titanates, and phosphonates as described in U. S. Pat. No. 7,341,804 (Christensen).

A method of stabilizing an electrochemical cell is provided that includes providing an electrochemically active positive electrode, a negative electrode, and a charge-carrying electrolyte that includes at least one organic solvent and an electrolyte salt. The method further includes dissolving at least 0.2 weight percent of a partially fluorinated polymer additive in the charge- carrying liquid electrolyte.

Detailed electrochemical and battery test results obtained with the LFC- 1 additive that illustrate the performance benefits of this additive in Li ion cells are summarized below (see Examples). Those benefits include improved cycle life performance, high voltage stability, high temperature resiliency, and reduced impedance buildup in cells. The new additives may be used alone or in combination with other additives, including additives that are known in the art.

The disclosed cells may be used in a variety of devices, including 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) and heating devices. The disclosed cells may have particular utility in low-cost mass market electrical and electronic devices such as flashlights, radios, CD players and the like,

Objects and advantages of this invention are further illustrated by the following examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.

Examples

Preparation of LFC- 1 Fluoropolymer Additive.

LFC- 1 partially fluorinated polymer additive was prepared by copolymerization of vinylidene fluoride and hexafluoropropylene according to the procedure disclosed in

Example 1 of U. S. Pat. No. 5,208,305 (Grootaert). The resulting latex was purified by extraction into methyl ethyl ketone, evaporation to dryness at elevated temperature, reextraction of the resulting gum into dimethyl carbonate, filtration by suction to remove insoluble impurities, and then final evaporation to dryness under vacuum at 25-100°C. The polymer had a number average molecular weight, M _n , of approximately 10,000 amu as determined by standard gel phase chromatography (GPC) techniques. According to NMR analysis ( ^l H and ¹⁹ F) the polymer had a vinylidene fluoride:hexafluoropolypropylene ratio of 3.38: 1 and contained 2.5 wt% dimethyl carbonate solvent.

Measurement of LFC- 1 Solubility in Electrolyte Solution.

Most fluorinated polymers are not soluble in the baseline electrolyte, so it was necessary to understand the solubility of LFC- 1 in the electrolyte after initial mixing. The concentration of dissolved LFC- 1 was measured using ¹⁹ F NMR spectroscopy. 2 wt% LFC- 1 was charged to the baseline electrolyte formulation, 1.0 M LiPF ₆ in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 by volume) (available from Novolyte, Independence, OH). After mixing to dissolve, an aliquot of this mixture was filtered and transferred into a sealed NMR tube. The NMR sample was analyzed on a Bruker 500MHz NMR spectrometer. Fig. 1 is the ¹⁹ F NMR spectrum of the electrolyte solution. In Fig. 1, the doublet at -73.8 ppm results from the resonance of LiPF ₆ . The peaks between -76.2 and -76.6 ppm are attributed to fluorine atom A in CF3 side group in HFP of LFC-1.

-CF ₂ -CF(CF ₃ )-CH ₂ -CF ₂ - A

All peaks were integrated and normalized to 1M LiPF ₆ peak. The molarity of HFP was obtained using the peak areas of CF ₃ side group divided by the peak area of LiPF ₆ . Since the actual mole ratio of VDF/HFP in this LFC-1 sample is 77.2/22.8, it is easy to obtain the molarity of VDF. From the HFP and VDF molarity, the wt% concentration of LFC-1 in the electrolyte was readily calculated assuming an electrolyte density of 1.17 g/ml as illustrated in Eq. 1. The calculated concentration of LFC- 1 in solution was 1.78 wt%, confirming that all the additive charged to the electrolyte dissolved (within error limits) and the solubility of LFC-1 in our standard electrolyte formulation is ample for use as an electrolyte additive.

Equation (1)

Electrochemical Cell Preparation.

Preparation of Electrodes

95% by weight of LiNio. ₄ Mno. ₄ Coo.2O2 (positive electrode active material, available from 3M, St. Paul, MN), 2.5% by weight of Super P carbon (conductive agent available from Timcal Graphite and Carbon, Bodio, Switzerland) and 2.5% by weight of polyvinylidene fluoride binder (KYNAR RX PVDF available from Arkema Inc., King of Prussia, PA) were mixed in l-methyl-2- pyrrolidinone (NMP available from Honeywell) as a solvent. The solid content of above solution was 58.3 wt. % and slurry wet density was 1.91 g/cm ³ . Then the resulting slurry was coated on an aluminum foil and dried at 120°C to prepare a positive electrode (cathode). The resulting cathodes were then calendered to 2.91 g/cm ³ (30% porosity) before use. Similarly, 92% by weight of MAGE graphite (negative electrode active material available from Hitachi) and 8 % by weight of PAA-Li binder (prepared from PAA (Polyacrylic acid available from Sigma-Aldrich) by neutralization with LiOH in water) were mixed in water as a solvent. The resulting mixture was applied to a copper foil and dried to generate a negative electrode. The anodes were calendared to 1.61 g/cm ³ (25% porosity) before cell assembly.

Preparation of Electrolyte

A non-aqueous solvent mixture comprising ethylene carbonate (EC):ethyl methyl carbonate (EMC) (both available from Novolyte) having a ratio of 3:7 by volume was prepared. Lithium salt, LiPF ₆ (available from Novolyte), was dissolved in above solvent mixture to prepare a 1.0 M electrolyte stock solution. Various amounts of additives were added to the l .OM electrolyte solution, as indicated in the Examples below. All electrolytes were prepared in an Ar purged glove box with water content less than 5 ppm. The above formulated electrolytes were filtered just prior to injection into the lithium ion cells.

Preparation of Coin Cells

Coin cells were fabricated with the resulting cathodes and anodes in 2325-size (23 mm diameter and 2.5 mm thickness) stainless steel coin-cell hardware in a dry room. Two layers of CELGARD #2335 (available from Celgard, Charlotte, NC) were used as a separator. 100 μΐ electrolyte prepared as described above was injected into the coin cells manually. Finally the cells were sealed by crimping. Coin Cell Cycling.

Coin cell test conditions (voltage limits, temperature and rate) were chosen to stress cells and cause significant capacity fade in control cells over the course of 200 cycles to allow differentiation of performance with and without additives. Additive testing was conducted at two different temperatures (room temperature and 60°C) using the testing protocol described below. For any given cell, formation and cycling were conducted at the same temperature.

1) Standard formation step (constant current charge at C/8 to 4.4V with constant voltage trickle to C/30 limit - Rest 15 min at open circuit voltage - constant current discharge at C/8 to 2.5V - Rest 15 min at open circuit voltage).

2) Constant current charge at C/2 rate to 4.4V with constant voltage trickle to C/20 limit . 3) Constant current discharge at 1C rate to 2.5V - rest 15 min at open circuit voltage.

4) Repeat steps 2 to 3 for 200 more cycles.

Comparative Examples 1 and 2 and Examples 1 and 2

Coin cells were prepared with cathodes and anodes as described above. The additives shown in Table 1 were added to the formulated electrolyte stock solution containing l .OM LiPF ₆ , described above. Table 1

Additives to 1M Electrolyte Stock Solution

The coin cells for Comparative Examples 1 -2 and Examples 1 -2 were cycled according to the protocol detailed above. Table 2 includes the discharge capacity and capacity retention at 200 ^th cycle of coin cells held at room temperature. The cells with added VC (Comparative Example 2) when cycled at room temperature show obvious capacity fade after 200 cycles. However the control and 2% LFC-1 cells (Comparative Example 1 and Example 1) deliver similar eye lability and 1% LFC-1 cells (Example 2) show the best performance without significant capacity loss after 200 cycles. Table 2 also indicates the voltage hysteresis of coin cells held at room temperature. The voltage hysteresis measures the total polarization and impedance of coin cells which can be calculated by Eq. 2.

VoltageHysteresis = V _{chw%e(average)} — V _{dischw%e(average)}

The cells with added VC (Comparative Example 2) when cycled at room temperature show highest voltage hysteresis at the 200 ^th cycle. However the control and 2% LFC-1 cells (Comparative Example 1 and Example 1) deliver similar polarization and 1% LFC-1 cells (Example 2) show the best performance without significant impedance rise after 200 cycles.

Table 2

Discharge Capacity and Voltage Hysteresis at 200 ^th Cycle Under Room Temperature Cycling

Comparative Examples 3 and 4 and Examples 3 and 4.

High Temperature Cycling of Coin Cells at 60°C: Coin cells were prepared with

LiNio. ₄ Mno. ₄ Coo.2O2, cathodes and MAGE graphite anodes, as described above. A non-aqueous solvent mixture comprising EC:EMC having a ratio of 3:7 by volume was prepared. The lithium salt, LiPF ₆ , was dissolved in above solvent mixture to prepare a 1.0 M electrolyte solution. The additives shown in Table 3 were added to the 1.0 M LiPF ₆ electrolyte stock solution described above. To separate samples of this baseline (or control) electrolyte solution was added, 2.0 wt% of vinylene carbonate (VC), 2.0 wt% of LFC-1, and a mixture of 2.0 wt% VC + 2.0 wt% LFC-1, respectively. All electrolytes were prepared in an Ar purged glove box with water content less than 5 ppm.

Table 3

Additives to 1M Electrolyte Stock Solution

Coin cell test conditions (voltage limits, temperature and rate) were chosen to stress cells and cause significant capacity fade in control cells over the course of 200 cycles to allow differentiation of performance with and without additives. Additive testing was conducted at 60°C using the same testing protocol as described above for Examples 1-2. For any given cell, formation and cycling were conducted at the same temperature.

Under these extreme test conditions (60°C cycling), the cells with LFC- 1 (Examples 3) display higher discharge capacity retention than control cells (Comparative Example 3), as indicated in Table 4. The binary mixture of VC + LFC- 1 (Example 4) provides even better cycling performance (higher discharge capacity and capacity retention) than VC or LFC- 1 alone

(Comparative Example 4 and Example 3) at 60°C.

Table 4

Discharge capacity and voltage hysteresis at 200 ^th cycle under 60°C cycling

Table 4 also shows the voltage hysteresis of coin cells held at high temperature. The voltage hysteresis measures the total polarization and impedance of coin cells which can be calculated by Eq. 2. The cells with added LFC-1 (Example 3) when cycled at high temperature show lowest voltage hysteresis at 200 ^th cycles. Additionally 2% VC and 2% LFC-1 cells

(Example 4) deliver less impedance rise than VC alone cells (Comparative Example 4) after 200 cycles. Comparative Examples 5 and Examples 5.

Thermal Storage Test of Coin Cells Having LiNio. ₄ Mno. ₄ Coo.2O2 Cathodes

Coin cells were prepared with LiNio. ₄ Mno. ₄ Coo.2O2, cathodes and MAGE graphite anodes, as described above. The additives shown in Table 5 were added to the formulated electrolyte stock solution containing 1.0M LiPF ₆ , described above.

Table 5

Additives to 1M Electrolyte Stock Solution

Coin cells were charged and discharged seven times at C/10 rate at room temperature between 4.2V and 2.8V. Subsequently, the batteries were charged to a terminal voltage of 4.2 V at 100% State of Charge (SOC). Then all coin cells were stored in a 60°C oven for one week. After that the battery was discharged and charged four times at room temperature. The discharge capacity of the cell before and after thermal storage was collected. The voltage drop during storage, reversible capacity loss, irreversible capacity loss, and total capacity loss of the battery after storage was calculated based on the schematic diagram in Fig. 2. Table 6 clearly shows that the LFC- 1 additives reduces the voltage drop, reversible, irreversible, and total capacity loss of cells stored at elevated temperature compared to the control cells with no additive.

Table 6

Voltage Drop and Capacity Loss on 60°C Storage For 7 Days

Various modifications and alterations to this invention will become apparent to those skilled in the art without departing from the scope and spirit of this invention. It should be understood that this invention is not intended to be unduly limited by the illustrative embodiments and examples set forth herein and that such examples and embodiments are presented by way of example only with the scope of the invention intended to be limited only by the claims set forth herein as follows. All references cited in this disclosure are herein incorporated by reference in their entirety.