HIGH ENERGY ELECTROLYTE FORMULATIONS

BACKGROUND OF THE INVENTION

[0001] The present invention is in the field of battery technology and, more particularly, in the area of additive compounds for use with high-energy electrodes in electrochemical cells.

[0002] An electrolyte serves to transport ions and prevent electrical contact between electrodes in a battery. Organic carbonate-based electrolytes are most commonly used in lithium-ion batteries and, more recently, efforts have been made to develop new classes of electrolytes based on sulfones, silanes, and nitriles. In many commercial lithium and lithium- ion batteries, the electrolyte is unstable to oxidation and/or reduction by the electrodes when the cells are charged.

[0003] As described in more detail below, solvents, salts, or additives have been incorporated into the electrolyte to decompose to form a protective film called a solid electrolyte interphase (SEI). Depending on the exact chemical system, this film can be composed of organic or inorganic lithium salts, organic molecules, oligomers, or polymers. Often, several components of the electrolyte are involved in the formation of the SEI (e.g. lithium salt, solvent, and additives). As a result, depending on the rate of decomposition of the different components, the SEI can be more or less homogenous.

[0004] In past research, organic compounds containing polymerizable functional groups such as alkenes, furan, thiophene, and pyrrole had been reported to form an SEI on the cathode of lithium ion batteries. See, e.g., Y.-S. Lee et al., Journal of Power Sources 196 (2011) 6997- 7001. These additives likely undergo polymerization during cell charging to form passivation films on the electrodes. The improvement in cell performance using these materials was slight.

[0005] Further, certain organic polymers have also been used as bulk electrolyte for lithium ion batteries due to the generally superior chemical stability of polymeric-based solvents as compared to smaller organic molecules, such as organic carbonates. However, practical application of such systems has been limited due to poor ionic conductivity. [0006] Despite extensive work across the battery industry to develop electrolyte components, there is still significant room for improvement in electrolyte stability. The embodiments disclosed herein provide some of those improvements. Certain of the shortcomings of known electrolyte formulations are addressed by embodiments of the invention disclosed herein by, for example, improving power performance at low temperature without substantially decreasing high temperature stability on storage.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0007] Figure 1 illustrates a lithium-ion battery implemented in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0008] The following definitions apply to some of the aspects described with respect to some embodiments of the invention. These definitions may likewise be expanded upon herein. Each term is further explained and exemplified throughout the description, figures, and examples. Any interpretation of the terms in this description should take into account the full description, figures, and examples presented herein.

[0009] The singular terms "a," "an," and "the" include the plural unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

[0010] The terms "substantially" and "substantial" refer to a considerable degree or extent. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation, such as accounting for typical tolerance levels or variability of the embodiments described herein.

[0011] The term "about" refers to the range of values approximately near the given value in order to account for typical tolerance levels, measurement precision, or other variability of the embodiments described herein.

[0012] The term "specific capacity" refers to the amount (e.g., total or maximum amount) of electrons or lithium ions a material is able to hold (or discharge) per unit mass and can be expressed in units of mAh/g. In certain aspects and embodiments, specific capacity can be measured in a constant current discharge (or charge) analysis, which includes discharge (or charge) at a defined rate over a defined voltage range against a defined counterelectrode. For example, specific capacity can be measured upon discharge at a rate of about 0.1C (e.g., about 18 mA/g) from 4.35 V to 3.0 V versus a Li/Li+ counterelectrode. Other discharge rates and other voltage ranges also can be used, such as a rate of about 0.1C (e.g., about 18 mA/g), or about 0.5C (e.g., about 90 mA/g), or about 1.0 C (e.g., about 180 mA/g).

[0013] A rate "C" refers to either (depending on context) the discharge current as a fraction or multiple relative to a "1 C" current value under which a battery (in a substantially fully charged state) would substantially fully discharge in one hour, or the charge current as a fraction or multiple relative to a "1 C" current value under which the battery (in a substantially fully discharged state) would substantially fully charge in one hour.

[0014] The term "rated charge voltage" refers to an upper end of a voltage range during operation of a battery, such as a maximum voltage during charging, discharging, and/or cycling of the battery. In some aspects and some embodiments, a rated charge voltage refers to a maximum voltage upon charging a battery from a substantially fully discharged state through its (maximum) specific capacity at an initial cycle, such as the 1 ^st cycle, the 2 ^nd cycle, or the 3 ^rd cycle. In some aspects and some embodiments, a rated charge voltage refers to a maximum voltage during operation of a battery to substantially maintain one or more of its performance characteristics, such as one or more of coulombic efficiency, retention of specific capacity, retention of energy density, and rate capability.

[0015] The term "rated cut-off voltage" refers to a lower end of a voltage range during operation of a battery, such as a minimum voltage during charging, discharging, and/or cycling of the battery. In some aspects and some embodiments, a rated cut-off voltage refers to a minimum voltage upon discharging a battery from a substantially fully charged state through its (maximum) specific capacity at an initial cycle, such as the 1 ^st cycle, the 2 ^nd cycle, or the 3 ^rd cycle, and, in such aspects and embodiments, a rated cut-off voltage also can be referred to as a rated discharge voltage. In some aspects and some embodiments, a rated cut-off voltage refers to a minimum voltage during operation of a battery to substantially maintain one or more of its performance characteristics, such as one or more of coulombic efficiency, retention of specific capacity, retention of energy density, and rate capability.

[0016] The "maximum voltage" refers to the voltage at which both the anode and the cathode are fully charged. In an electrochemical cell, each electrode may have a given specific capacity and one of the electrodes will be the limiting electrode such that one electrode will be fully charged and the other will be as fully charged as it can be for that specific pairing of electrodes. The process of matching the specific capacities of the electrodes to achieve the desired capacity of the electrochemical cell is "capacity matching."

[0017] The term "NMC" refers generally to cathode materials containing LiNi _x Mn _y CozO _w and includes, but is not limited to, cathode materials containing LiNio.33Mno.33Coo.33O2.

[0018] The term "polymer" refers generally to a molecule whose structure is composed of multiple repeating units. The structure can be linear or branched. They may contain various functional groups attached to the polymer chain. They may contain routine chemical substitutions or modifications in one or more of the repeating units.

[0019] The term "copolymer" refers generally to a molecule whose structure is composed of at least two different repeating units. The structure can be alternating, periodic, statistical, random, block, linear, branched, combinations thereof, or other structure. They may contain various functional groups attached to the polymer chain. They may contain routine chemical substitutions or modifications in one or more of the repeating units.

[0020] To the extent certain battery characteristics can vary with temperature, such characteristics are specified at room temperature, unless the context clearly dictates otherwise.

[0021] Ranges presented herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values 1 and 3 as well as intermediate values.

[0022] Figure 1 illustrates a lithium-ion battery 100 implemented in accordance with an embodiment of the invention. The battery 100 includes an anode 102, a cathode 106, and a separator 108 that is disposed between the anode 102 and the cathode 106. In the illustrated embodiment, the battery 100 also includes an electrolyte formulation 104, which is disposed between the anode 102 and the cathode 106 and should remain stable during battery cycling.

[0023] The operation of the battery 100 is based upon reversible intercalation and de- intercalation of lithium ions into and from host materials of the anode 102 and the cathode 106. Other implementations of the battery 100 are contemplated, such as those based on conversion chemistry. Referring to Figure 1, the voltage of the battery 100 is based on redox potentials of the anode 102 and the cathode 106, where lithium ions are accommodated or released at a lower potential in the former and a higher potential in the latter. [0024] Without being bound by a particular theory not recited in the claims, the formation of the cathode SEI can occur through one or more of the following mechanisms: (1) the additive compound(s) can decompose to form the cathode SEI, which inhibits further oxidative decomposition of electrolyte components; (2) the additive compound(s) or its decomposed product(s) form or improve the quality of a passivation film on the cathode or anode; (3) the additive compounds can form an intermediate product, such as a complex with LiPF6 or a cathode material, which intermediate product then decomposes to form the cathode SEI that inhibits further oxidative decomposition of electrolyte components; (4) the additive compounds can form an intermediate product, such as a complex with LiPF6, which then decomposes during initial charging. The resulting decomposition product can then further decompose during initial charging to form the cathode SEI, which inhibits further oxidative decomposition of electrolyte components; (5) the additive compounds can stabilize the cathode material by preventing metal ion dissolution.

[0025] Other mechanisms of action of the electrolyte formulation are contemplated, according to an embodiment of the invention. For example, and in place of, or in combination with, forming or improving the quality of the cathode SEI, one or more additives or a derivative thereof (e.g., their decomposition product) can form or improve the quality of the anode SEI, such as to reduce the resistance for Li ion diffusion through the anode SEI. As another example, one or more additives or a derivative thereof (e.g., their decomposition product) can improve the stability of the electrolyte formulation by chemically reacting or forming a complex with other electrolyte components. As a further example, one or more additives or a derivative thereof (e.g., their decomposition product) can scavenge decomposition products of other electrolyte components or dissolved electrode materials in the electrolyte formulation by chemical reaction or complex formation. Any one or more of the cathode SEI, the anode SEI, and the other decomposition products or complexes can be viewed as derivatives, which can include one or more chemical elements corresponding to, or derived from, those present in one or more additives, such as a heteroatom included in the additives.

[0026] Certain embodiments are related to a class of polymer additives for non-aqueous electrolyte formulations. Such embodiments include several electrolyte additives that improve the oxidative stability of the electrolyte formulation and the cycle life and coulombic efficiency of electrochemical cells containing these additives.

[0027] A high temperature electrolyte formulation according to some embodiments of the invention can be formed with reference to the formula: base electrolyte + additive compound(s)→ high temperature electrolyte (1)

[0028] In formula (1), the base electrolyte can include a set of solvents and a set of salts, such as a set of lithium-containing salts in the case of lithium ion batteries. Examples of suitable solvents include nonaqueous electrolyte solvents for use in lithium ion batteries, including carbonates, such as ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, methyl propyl carbonate, and diethyl carbonate; sulfones; silanes; nitriles; esters; ethers; and combinations thereof.

[0029] Referring to formula (1), an amount of a particular compound can be expressed in terms of a weight percent of the compound relative to a total weight of the electrolyte formulation (referred to herein as "weight percent" or "wt.%"). For example, an amount of a compound can be in the range of about 0.01 wt.% to about 30 wt.%, such as from about 0.05 wt.% to about 30 wt.%, from about 0.01 wt.% to about 20 wt.%, from about 0.2 wt.% to about 15 wt.%, from about 0.2 wt.% to about 10 wt.%, from about 0.2 wt.% to about 5 wt.%, or from about 0.2 wt.% to about 1 wt.%, and, in the case of a combination of multiple compounds, a total amount of the compounds can be in the range of about 0.01 wt.% to about 30 wt.%, such as from about 0.05 wt.% to about 30 wt.%, from about 0.01 wt.% to about 20 wt.%, from about 0.2 wt.% to about 15 wt.%, from about 0.2 wt.% to about 10 wt.%, from about 0.2 wt.% to about 5 wt.%, or from about 0.2 wt.% to about 1 wt.%. An amount of a compound also can be expressed in terms of a ratio of the number of moles of the compound per unit surface area of either, or both, electrode materials. For example, an amount of a compound can be in the range of about 10 ^"7 mol/m ² to about 10 ^"2 mol/m ² , such as from about 10 ^"7 mol/m ² to about 10 ^"5 mol/m ² , from about 10 ^"5 mol/m ² to about 10 ^"3 mol/m ² , from about 10 ^"6 mol/m ² to about 10 ^"4 mol/m ² , or from about 10 ^"4 mol/m ² to about 10 ^"2 mol/m ² . As further described below, a compound can be consumed or can react, decompose, or undergo other modifications during initial battery cycling. As such, an amount of a compound can refer to an initial amount of the compound used during the formation of the electrolyte formulations according to formulas (1) or (2), or can refer to an initial amount of the additive within the electrolyte formulation prior to battery cycling (or prior to any significant amount of battery cycling).

[0030] Resulting performance characteristics of a battery can depend upon the identity of a particular compound used to form the high temperature electrolyte formulation according to formulas (1) or (2), an amount of the compound used, and, in the case of a combination of multiple compounds, a relative amount of each compound within the combination.

Accordingly, the resulting performance characteristics can be fine-tuned or optimized by proper selection of the compounds and adjusting amounts of the compounds in formulas (1) or (2).

[0031] The formation according to formulas (1) or (2) can be carried out using a variety of techniques, such as by mixing the base electrolyte and the additives, dispersing the additives within the base electrolyte, dissolving the additives within the base electrolyte, or otherwise placing these components in contact with one another. The additives can be provided in a liquid form, a powdered form (or another solid form), or a combination thereof. The additives can be incorporated in the electrolyte formulations of formulas (1) or (2) prior to, during, or subsequent to battery assembly.

[0032] The electrolyte formulations described herein can be used for a variety of batteries operated at high temperatures. In particular, these additives are useful for Li-ion batteries containing NMC cathode materials.

[0033] Batteries including the electrolyte formulations can be conditioned by cycling prior to commercial sale or use in commerce. Such conditioning can include, for example, providing a battery, and cycling such battery through at least 1, at least 2, at least 3, at least 4, or at least 5 cycles, each cycle including charging the battery and discharging the battery at a rate of 0.1C (e.g. , a current of 18 mA/g) between 4.35 V and 2.75 V (or another voltage range) versus a reference counterelectrode, such as a graphite anode. Charging and discharging can be carried out at a higher or lower rate, such as at a rate of 0.1 C (e.g., a current of 18 mA/g), at a rate of 0.5C (e.g. , a current of 80 mA/g), or at a rate of 1C (e.g. , a current of 180 mA/g). Typically a battery is conditioned with 1 cycle by charging at 0.1 C rate to 4.35 V followed by applying constant voltage until the current reaches 0.05C, and then discharging at 0.1 C rate to 2.75V.

[0034] The polymer-based additives according to embodiments herein are molecules formed from numerous repeated monomer units, as is conventionally understood in the art. Such polymer-based additives may contain various functional groups attached to the polymer chain. Certain properties are preferred in polymer-based additives for use in batteries. For example, the additives preferably are: (i) soluble in the electrolyte solvent (that is, they are sufficiently polar as compared to the solvent and sufficiently low molecular weight); (ii) either chemically resistant to oxidation and/or reduction under the cell conditions or, if not chemically resistant to oxidation and/or reduction, then the additives should decompose to intermediates or products that form a stable SEI film on the anode, cathode, or both; and (iii) sufficiently low molecular weight to be soluble in electrolyte formulation at room temperature and to make the electrolyte formulation viscosity not worse than without the additive.

[0035] Polymeric additives disclosed herein have a number of potential benefits: (i) due to their unique functional groups pre-arranged in the backbone, polymeric additives will strongly and evenly adsorb on to the surface of the electrodes before decomposition, potentially improving the quality /stability of the resulting SEI; (ii) the pre-formed polymer chain may be more mechanically and chemically stable than the organic oligomers and short-chain polymers formed from conventional solvents and additives; and (iii) by incorporating functional groups into the polymer, those functional groups (or their decomposition products, including salts) can be homogenously dispersed throughout the SEI film and/or undergo the more simultaneous decomposition reactions, thus resulting in a more uniform, mechanically and chemically more stable SEI.

[0036] Certain polymer-based additives demonstrate the properties listed above, including: poly(methyl vinyl ether-alt-maleic anhydride) (PMVMA); poly(pyrrolidone-co-vinyl acetate) (PPVA) poly(9-vinylcarbazole); poly(oxy-l,4-phenylenesulfonyl-l,4-phenylene); poly(l- hexadecene-sulfone); poly(hexafluoropropylene oxide); poly(bis(4-

(ethoxycarbonyl)phenoxy)phosphazene); polyacrylonitrile and combinations thereof. Included in the scope of this disclosure are routine chemical modifications of these listed polymers provided that the modifications do not substantially diminish the desired properties of the polymers or substantially interfere with their performance as additives.

[0037] In some embodiments, the additive is a polymer having at least one functional group selected from the group consisting of carbazoles, anhydrides, sulfones, fluorinated ethers, phosphazenes, esters, nitriles, amides, and combinations thereof. These functional groups can include routine chemical substitutions or modifications.

[0038] These polymer additives are soluble in the conventional electrolyte and have functional groups that contribute to stable SEI formation. These polymer additives can form more mechanically and chemically stable SEI films compared with molecular or short-chain oligomers. These polymer additives significantly improve cycle life in a full cell configuration. Importantly, these polymer additives show superior performance when compared to their monomer analogues, which indicates that incorporation of the functional groups into a polymer background is responsible for the improved performance. That is, the functional groups alone, in a non-polymer structure, do not deliver the levels of performance achieved using their polymer equivalents. [0039] Polymeric additives can be combined with traditional additives. This combination demonstrates synergies in performance improvement at high temperatures and high voltage. The results below use polymer compounds as additives for a battery cell including a graphite anode and an NMC cathode cycled at high voltage. The results indicate that the combination of polymer additives with conventional electrolyte additives improve battery performance over the conventional additive itself.

[0040] Conventional additives appropriate for use in the combinations disclosed herein include, but are not limited to fluorinated ethylene carbonate, vinylene carbonate, vinyl ethylene carbonate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, maleic anhydride, succinic anhydride, lithium tetrafluoroborate, lithium bis- trifluoromethanesulfonimide, lithium bis(fluorosulfonyl)imide, 1,3-propene sultone, 1,4- butane sultone, ethylene sulfite, succinimide, γ-butyrolactone, triallyl isocyanurate, 2- vinylpyridine, vinyl acetate, triphenyl phosphate, triallyl phosphate, 2,4,6- tris(trifluoromethyl)-l,3,5-triazine, succino nitrile, biphenyl, and combinations thereof. Conventional additives such as those described above which contain routine chemical substitutions or modifications are also included in the scope of the invention.

[0041] To demonstrate the performance of the classes of polymer based additives disclosed herein, two polymeric additives were screened: (poly(methyl vinyl ether-alt-maleic anhydride) (PMVMA) and poly(pyrrolidone-co-vinyl acetate) (PPVA)) combined with either fluorinated ethylene carbonate (FEC) or 1,3-propane sultone (PS) in Graphite//NMC full cells cycled from 3-4.35 V.

[0042] Tests performed included the following: (i) room temperature cycle life (1C/1C) to evaluate capacity fade; (ii) high temperature (45 degrees Celsius) cycle life (1C/1C) to evaluate capacity fade; and (iii) high temperature (70 degrees Celsius) storage for 7 days to evaluate capacity fade and resistance growth.

[0043] In lithium ion batteries, low temperature performance is characterized by measuring the area specific impedance (ASI), which includes contributions due to the electrode materials, the SEI layers formed on those materials, and the bulk electrolyte properties. As this is a measure of impedance, low ASI values are desirable.

[0044] High temperature performance is characterized by measuring the change in ASI after storage at elevated temperature. Again, small changes in the ASI after storage are desirable as such small changes would indicate stability of the cell while it is stored at elevated temperature.

[0045] At high temperature, stability of the battery cell can become compromised. Instability at high temperature is believed to be due to: 1) increased reactivity of the electrolyte with an active material; 2) accelerated decomposition of LiPF6, which generates decomposition products that can be reactive with the both the electrolyte and the electrode active materials. Parasitic reactions driven by the decomposition products can result in loss of cell capacity and further decomposition of any SEI.

[0046] The following examples describe specific aspects of some embodiments of the invention to illustrate and provide a description for those of ordinary skill in the art. The examples should not be construed as limiting the invention, as the examples merely provide specific methodology useful in understanding and practicing some embodiments of the invention.

EXAMPLES

[0047] Battery cells were formed in a high purity Argon filled glove box (M-Braun, C and humidity content < 0.1 ppm). The electrodes were prepared by the following methods, (i) For the cathode, a commercial LiNio.5Mno.3Coo.2O2 cathode material was mixed with poly(vinylidene fluoride) (Sigma Aldrich) and carbon black (Super P Li, TIMCAL) with 1- methyl-2-pyrrolidinone (Sigma Aldrich) as solvent. The resulting slurry was deposited on an aluminum current collector and dried to form a composite cathode film, (ii) For the anode, a commercial graphitic carbon was mixed with poly(vinylidene fluoride) (Sigma Aldrich) and carbon black (Super P Li, TIMCAL) with l-methyl-2-pyrrolidinone (Sigma Aldrich) as solvent. The resulting slurry was deposited on a copper current collector and dried to form a composite cathode film.

[0048] Each battery cell included the composite cathode film, a polypropylene separator, and the composite anode film. A conventional electrolyte was mixed with a given electrolyte additive and added to the battery cell. The battery cell was sealed and pre-formed at 30 degrees Celsius and cycled at ambient temperature using 0.05C charge followed by constant voltage hold until the current drops to C/100 and then discharged to 3V using 0.05C constant current. The cycle was repeated one more time prior to high temperature cycling.

[0049] For high temperature cycling, batteries were cycled at 45 degrees Celsius using 1C charge and discharge currents with a voltage range of 3.0 to 4.35V.

[0050] For high temperature storage, batteries were charged to 4.35V at C/10 (100% state of charge "SOC") at room temperature. The cells were then heated to 70 degrees Celsius at open circuit potential for 7 days. The remaining capacity at 1C was measured (3.0V cutoff). The cells were then recharged at 1C to 4.35V and the recovered discharge capacity was measured at 1C (3.0V cutoff).

[0051] Cell resistance (Ohms) was calculated using the formula:

AY/AC

where AY is measured by the voltage change of 0.1 seconds before discharge and after discharge and AC is measured by the current change of 0.1 seconds before discharge and after discharge at any given cycle.

[0052] The change in area-specific impedance ("Delta ASF) (Ohm ^» cm ² ) was calculated using the formula:

(Resistance of Cycle 200 * area of cathode) - (Resistance of initial 1C cycle * area of cathode)

RESULTS

[0053] For all results, concentration effects are observed.

[0054] Tables 1 and 2 show that the combination of a polymer additive and a conventional additive has similar first cycle capacity and coulombic efficiency as compared to the conventional additive by itself.

[0055] Tables 1 and 2 show that the high temperature (45 degrees Celsius) cycle life (180 cycles) is improved by addition of a polymer additive to a conventional additive over the conventional additive by itself.

[0056] Tables 3 and 4 show that the combination of a polymer additive and a conventional additive has similar initial cell resistance, but decreases the growth in resistance during high temperature cycle life relative to the conventional additive by itself.

[0057] Tables 5 and 6 show that the combination of a polymer additive and a conventional additive can improve high temperature storage performance by increasing the capacity retention during storage, the recoverable capacity after storage, and reducing the impedance growth during storage relative to the conventional additive by itself. Table 1 : Summary of performance of additive combinations with FEC

Table 3: Summary of performance of additive combinations with FEC

Polymer Second Initial Resistance Additive Additive resistance (Ohms, after

(Ohms) 200 cycles at

45°C)

3% FEC 11.4 31.3 none

3% FEC 12.3 32.3

0.5% PPVA

3% FEC 11.7 36.4

2.0% PPVA

3% FEC 11.2 23.6

0.5% PMVMA

3% FEC 12.3 21.7

2.0% PMVMA Table 4: Summary of performance of additive combinations with FEC and PS

Table 5: Summary of performance of additive combinations with FEC

Table 6: Summar of erformance of additive combinations with FEC and PS

[0058] While the invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention as defined by the appended claims. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, or process to the objective, spirit and scope of the invention. All such modifications are intended to be within the scope of the claims appended hereto. In particular, while the methods disclosed herein have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the invention. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not limitations of the invention.