Abstract:
In at least one embodiment, a method of scavenging hydrogen in a lithium-ion battery is provided. The method may comprise including an atomic intermetallic material in at least one of a positive electrode or a negative electrode of a lithium-ion battery and reacting hydrogen present inside the lithium-ion battery with the atomic intermetallic material to form a metal hydride. The method may include preparing a positive electrode slurry and a negative electrode slurry, each slurry including an active material and a binder, mixing an atomic intermetallic material including a proton absorbed state into at least one of the slurries, and casting the slurries to form a positive electrode and a negative electrode. The method may alternately include applying an atomic intermetallic material including a proton absorbed state to a surface of at least one of a lithium-ion battery positive electrode or negative electrode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/155,740 filed Jan. 15, 2014, the disclosure of which is hereby incorporated in its entirety by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to compositions for reducing moisture in battery electrolytes, for example, in lithium-ion batteries. 
       BACKGROUND 
       [0003]    Enhancement of battery performance is a focal point in the advancement of portable electronics, power grid regulation and electrified vehicles. Rechargeable or secondary batteries generally include positive and negative electrodes, a separator and an electrolyte. Current collectors are typically attached to each electrode in order to extract current from the battery. Lithium-ion (Li-ion) batteries are used in electric and hybrid-electric vehicles due to their relatively high voltage, high specific energy, high energy density, low self-discharge rate, long cycle life and/or wide temperature operational range. A separator is generally placed between an anode and a cathode of the Li-ion battery. The separator prevents physical contact of the two electrodes (e.g., internal short circuits), while still allowing for rapid transportation of ionic charge carriers between the cathode and anode. The electrolyte of the Li-ion batteries may generally include a conductive lithium salt and an organic solvent. A commonly used lithium salt for Li-ion batteries is lithium hexafluorophosphate (LiPF 6 ). 
       SUMMARY 
       [0004]    In at least one embodiment, a lithium-ion battery is provided comprising a positive electrode, a negative electrode, a separator situated between the electrodes. At least one of the electrodes may include an atomic intermetallic material including a proton absorbed state. In one embodiment, the positive electrode may include 0.01 to 5 wt. % of the atomic intermetallic material. The atomic intermetallic material may have a hydrogen absorbency from 0.0% to 3.5% by weight of the atomic intermetallic material. 
         [0005]    In one embodiment, the atomic intermetallic material reacts with a proton to form a metal hydride in the proton absorbed state. The atomic intermetallic material may include an A x B y  alloy, wherein A is a first metal or metal alloy, B is a second metal or metal alloy different from the first, and x and y are integers greater than or equal to 1. In one embodiment, the atomic intermetallic material is included in both the positive electrode and the negative electrode. 
         [0006]    The battery may include an electrolyte including LiPF6. The atomic intermetallic material may be formed as a powder or as a film. The atomic intermetallic material may be coated on a surface of at least one of the electrodes adjacent to the separator. The atomic intermetallic material may also be mixed throughout a bulk of at least one of the electrodes. 
         [0007]    In at least one embodiment, a lithium-ion battery is provided comprising an anode, a cathode, and a separator situated between the anode and cathode. The cathode may include from 0.01 to 5 wt. % of an atomic intermetallic material including a proton absorbed state. The atomic intermetallic material may react with a proton to form a metal hydride in the proton absorbed state. In one embodiment, the atomic intermetallic material includes an AxBy alloy, wherein A is a first metal or metal alloy, B is a second metal or metal alloy different from the first, and x and y are integers greater than or equal to 1. The cathode may include from 0.05 to 1 wt. % of the atomic intermetallic material. 
         [0008]    In at least one embodiment, a lithium-ion battery is provided comprising an anode, a cathode, and a separator situated between the anode and cathode. The cathode may have a first layer including a cathode active material and a second layer including a proton absorbing material that is different from the cathode active material and has a hydrogen absorbency from 0.75% to 3.5% by weight of the proton absorbing material. The proton absorbing material may include an atomic intermetallic material including a proton absorbed state. The atomic intermetallic material may react with a proton to form a metal hydride in the proton absorbed state. In one embodiment, the atomic intermetallic material includes an AxBy alloy, wherein A is a first metal or metal alloy, B is a second metal or metal alloy different from the first, and x and y are integers greater than or equal to 1. The cathode may include from 0.01 to 5 wt. % of the proton absorbing material. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic of a lithium-ion battery including a proton absorbing material, according to an embodiment; 
           [0010]      FIG. 2  is a schematic of a lithium-ion battery including an electrode having a layer of proton absorbing material, according to an embodiment; 
           [0011]      FIG. 3  is a charge-discharge curve of a full coin cell including a proton absorbing material in the anode and cathode; and 
           [0012]      FIG. 4  shows two cyclic voltammogram curves at first cycle, one of a half coin cell including a proton absorbing material and one of a half coin cell without a proton absorbing material. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
         [0014]    Although lithium hexafluorophosphate (LiPF 6 ) is relatively stable in dry inert atmosphere up to 107° C., it may suffer from degradation upon exposure to water and moisture. While efforts are made to reduce the moisture level in the electrolyte, trace amounts of water may be found (e.g., up to about 100 ppm). The mechanism of the degradation may include that the anion of this salt (PF 6   − ) undergoes a reversible reaction: 
         [0000]      LiPF 6 (sol.)         LiF(s)+PF 5 (sol.)  (eq. 1)
 
         [0015]    where the strong Lewis acid PF 5  tends to further react with organic solvents and thus move the reaction of equation 1 toward products. Furthermore, labile P—F bonds are highly susceptible to hydrolysis through reacting with trace amounts of moisture in the electrolyte solvent: 
         [0000]      LiPF 6 (sol.)+H 2 O→POF 3 (sol.)+LiF(s)+2HF(sol.)  (eq. 2)
 
         [0000]      PF 5 (sol.)+H 2 O→POF 3 (sol.)+2HF(sol.)  (eq. 3)
 
         [0016]    As a consequence, the formed hydrofluoric acid (HF) may continually react with the positive electrode (cathode) materials (e.g., LiMOx, wherein M=Mn, Co, Al, Ni, Fe, Fe m P x , or others or a combination thereof). This reaction may result in the formation of water (H 2 O) molecules. This LiPF 6  decomposition cycle may continue until all of the LiPF 6  is consumed, which may cause performance deterioration of the Li-ion cell, significantly reduce the calendar and/or cycle life time of the cell or other problems. Therefore, it may be beneficial to remove and/or prevent moisture inside the cell and/or prevent runaway LiPF 6  decomposition if moisture is present. While the reactions and description in the disclosure recite LiPF 6  as the lithium salt, they may be applicable to other lithium salts, such as lithium hexafluoroarsenate monohydrate (LiAsF 6 ), lithium perchlorate (LiClO 4 ), lithium tetrafluoroborate (LiBF 4 ) and lithium triflate (LiCF 3 SO 3 ) and others known in the art. Accordingly, the lithium ion batteries disclosed herein are not limited to LiPF 6 , but rather its use is merely exemplary. 
         [0017]    With reference to  FIG. 1 , a cross-section of a battery  10  is shown, which may be a rechargeable battery (e.g., a lithium-ion battery). The battery  10  includes a negative electrode (anode)  12 , a positive electrode (cathode)  14 , a separator  16 , and an electrolyte  18 , which may be disposed within the electrodes  12 ,  14  and separator  16 . However, the battery  10  may include additional components or may not require all the components shown, depending on the battery type or configuration. In addition, a current collector  20  may be disposed on one or both of the anode  12  and cathode  14 . The current collector  20  may be formed of any suitable material. For example, the current collector  20  for the anode  12  may be copper and the current collector  20  for the cathode  14  may be aluminum. 
         [0018]    In at least one embodiment, the anode  12  may include graphite (e.g., natural, artificial, or surface modified) as the active material. However, any suitable active material may be used, for example, hard carbon, soft carbon, lithium titanate oxide (LTO), silicon or tin-enriched graphite or carbonaceous compounds. In at least one embodiment, the cathode  14  may include a lithium nickel cobalt manganese oxide (NCM) active material. However, any suitable active material may be used, for example, lithium nickel cobalt aluminum oxide (NCA), lithium manganese spinel oxide (Mn Spinel or LMO), and lithium iron phosphate (LFP) and its derivatives lithium mixed metal phosphate (LFMP). In addition, mixtures of any of two or more of these materials may be used, for example a mixture of NCM and LMO. 
         [0019]    The separator  16  may be formed of any suitable material, for example, a polyolefin, such as polyethylene or polypropylene. The electrolyte  18  may include a liquid electrolyte including a lithium salt and an organic solvent. The lithium salt may include, but is not limited to, LiPF 6 , LiBF 4 , LiClO 4 , LiCF 3 SO 3  or combinations thereof. The organic solvent may include, but are not limited to, ethylene carbonate (EC), ethylene-methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC) or combinations thereof. The concentration of the lithium salt and the volume/weight ratios of the organic solvents may be any suitable values, which are known by one of ordinary skill in the art. For example, the electrolyte may include 1 M LiPF 6  and EC:EMC (3:7 weight ratio). 
         [0020]    With further reference to  FIG. 1 , the battery  10  may include a proton absorbing or scavenging material  22 . The proton absorbing material (PAM)  22  may be a material that reacts with a proton, thereby preventing it from reacting with other materials. In at least one embodiment, the PAM  22  is an inorganic material. The PAM  22  may be a metal or a metal alloy, such that when it reacts with a proton a metal hydride (MHx) is formed. The PAM  22  may therefore have proton desorbed and proton absorbed states. The reaction may be described as follows: 
         [0000]      H + +M+e − →MHx  (eq. 4)
 
         [0021]    In at least one embodiment, the PAM  22  is an atomic intermetallic material. The atomic intermetallic material may be represented as A x B y , where A is a first metal or metal alloy, B is a second metal or metal alloy that is different from the first, and x and y are integers that are greater than or equal to one. Non-limiting examples of suitable intermetallic materials include AB 5 -type alloys, AB 2 -type alloys, zirconium-nickel (Zr—Ni)-based alloys, and vanadium (V)-based BCC type alloys. The PAM  22  may have a hydrogen absorbency of at least 0.5% hydrogen by weight of the PAM  22 . For example, the PAM  22  may absorb at least 0.75, 1, 2, 3, 4, or 5% hydrogen by weight of the PAM  22 . In one embodiment, the PAM  22  may have a hydrogen absorbency of 0.5 to 4.0% hydrogen by weight of the PAM  22 . In another embodiment, the PAM  22  may have a hydrogen absorbency of 0.75 to 3.5% hydrogen by weight of the PAM  22 . In another embodiment, the PAM  22  may have a hydrogen absorbency of 1 to 3% hydrogen by weight of the PAM  22 . For example, LaNi 5  (AB 5 ) may absorb about 1.4 wt. % hydrogen (forming LaNi 5 H 6 ) and ZrV 2  (AB 2 ) may absorb about 3.0 wt. % hydrogen (forming ZrV 2 H 5.5 ). While the PAM  22  may absorb hydrogen in the amounts disclosed, it is to be understood that if the concentration of hydrogen ions in the electrolyte is below the saturation point of the PAM  22 , then the PAM  22  may absorb less hydrogen by weight than the amounts disclosed. 
         [0022]    AB 5  alloys may generally combine a hydride forming metal, A, with a non-hydride forming metal, B. The A element may be a rare earth metal, for example, lanthanum (La), cerium (Ce), neodymium (Nd), praseodymium (Pr), samarium (Sm), yttrium (Y), or others. The A element may also be a mixture of rare earth metals, which may be known as a “mischmetal” (Mm). A mischmetal may be a natural mixture of rare earth elements, mostly including Ce (30-52%), La (13-25 wt. %), and one or more of Nd, Pr, and Sm (13-57%), where the amounts may depend on the place of origin. The B element may be Ni, which may be alloyed with other metals. If the Ni is alloyed, the alloying metal may be, for example, cobalt (Co), tin (Sn), aluminum (Al), iron (Fe), or others. The AB 5  alloy may be any combination of the A and B elements above, for example, LaNi 5 , MmNi 5 , Mm(Ni—Co—Al—Mn) 5 , or others. One of ordinary skill in the art will recognize that numerous specific alloy compositions may be formulated using the alloying elements described. In addition, the formula may deviate from AB 5  slightly, such that the composition is, for example, A 1.1 B 5  or AB 5.1 . For example, the AB 5  alloy may be MmNi 3.55 Mn 0.4 Al 0.3 Co 0.75 , LaNi 4.7 Sn 0.3 , LaNi 2.5 Co 2.5 , or other formulations. 
         [0023]    AB 2  alloys (also known as Laves phases) may generally combine titanium (Ti), Zr, hafnium (Hf), or alloys thereof as element A and a transition metal as element B. Non-limiting examples of suitable B elements may include manganese (Mn), Ni, chromium (Cr), V, Fe, or others. The AB 2  alloy may be any combination of the A and B elements above, for example, ZrMn 2 , TiCr 2 , HfNi 2 , or others. In addition, one of ordinary skill in the art will recognize that numerous specific alloy compositions may be formulated using the alloying elements described. In addition, the formula may deviate from AB 2  slightly, such that the composition is, for example, A 1.1 B 2  or AB 2.1 . For example, the AB 2  alloy may be ZrFe 1.5 Cr 0.5 , TiMn 1.4 V 0.6 , (Ti 0.9 Zr 0.1 ) 1.1 CrMn, Mn 0.9 V 0.1 Fe 0.5 Ni 0.5 , ZrMn 0.9 V 0.1 Fe 0.5 Co 0.5 , or other formulations. 
         [0024]    The PAM  22  may also be a Zr—Ni-based alloy. The Zr—Ni alloy may include additional alloying elements, for example, Co, Mn, Ti, Cr, Nb, Sn, Si, or others. One of ordinary skill in the art will recognize that numerous specific alloy compositions may be formulated using the alloying elements described. In another embodiment, the PAM  22  may be a V-based BCC-type alloy, wherein BCC refers to the crystal structure of the alloy (body-centered cubic). In at least one embodiment, the vanadium alloy may include titanium. The vanadium and/or V—Ti alloy may also be alloyed with other metals, for example, Al, silicon (Si), Zr, Cr, Fe, Co, Ni, niobium (Nb), tantalum (Ta), Mn, or others. The V-based alloy may include any combination of the above elements. Non-limiting examples of suitable alloys may include V—Ti—Cr, V—Ti—Mn, V—Ti—Ni, V—Ti—Cr—Mn, or others. One of ordinary skill in the art will recognize that numerous specific alloy compositions may be formulated using the alloying elements described. 
         [0025]    As discussed above, HF may be formed in electrolytes including LiPF 6  (or other lithium salts) when the LiPF 6 (sol.) and/or PF 5 (sol.) react with trace amounts of water in the electrolyte. The HF, in turn, may react with the cathode material (e.g., LiMOx, wherein M may be Mn, Co, Al, Ni, Fe, FemPx, or others, or a combination thereof), forming water molecules as a product. The water produced may then react with the remaining LiPF 6 (sol.) and/or PF 5 (sol.), which creates more HF, which then may react with the cathode materials, continuing the cycle of degradation of the cathode materials and consuming the LiPF 6  in the electrolyte. If the cycle is not slowed or stopped, all of the LiPF 6  may be consumed, causing significant performance reduction in the Li-ion cell and reducing the calendar life and cycle time of the cell. 
         [0026]    Without being held to any particular theory, it is believed that the PAM  22  (e.g., an intermetallic material), absorbs protons in the electrolyte by capturing H +  ions in the electrolyte. By capturing the H +  ions, they may be prevented from reacting with the electrode materials (e.g., the cathode active material). If the PAM  22  prevents at least some of the H +  ions from reacting with the electrode materials, then the production of water can be reduced or eliminated. The reduction or elimination of water production may therefore reduce or eliminate the production of additional HF, thereby further reducing or eliminating reactions with the electrode materials. Accordingly, by absorbing a portion, substantially all, or all of the H +  ions in the electrolyte, the cycle of electrode material degradation and LiPF 6  consumption may be slowed or stopped. The inclusion of PAM  22  may therefore suppress or eliminate the side reactions of the electrode materials and/or the electrolyte, reduce or eliminate the consumption of LiPF 6 , stabilize the electrolyte, reduce or eliminate HF formation, and/or stabilize the inner pressure of the Li-ion cell. As a result, an increase in electrical resistance of the electrodes may be prevented, as well as increased gas evolution, both of which may improve the performance and life cycle of the battery. 
         [0027]    While the reaction between the hydrogen ions and the PAM  22  may be reversible, it may be possible to prevent desorption of the hydrogen, for example, by selecting appropriate PAM materials and operating conditions of the battery. Desorption is favored at high temperatures and low pressures. Therefore, for a certain PAM material chosen, the temperature and pressure of the battery may be maintained at values that do not favor desorption. Alternatively, if the battery conditions cannot be adjusted, materials that do not favor absorption at those conditions may be chosen. In addition, desorption may be favored at high potentials. Therefore, by not allowing the battery to deep discharge, desorption may be mitigated or prevented. 
         [0028]    The PAM  22  may be present in the battery in any suitable form, such as a powder or a film. In  FIG. 1 , the PAM  22  is shown as a powder  24 , which may be present within the anode  12  and/or the cathode  14 . In one embodiment, the powder  24  may be combined with the electrode materials as they are prepared. For example, as is known in the art, a cathode active material may be combined with a binder (e.g., PVDF) to form a slurry, which is then cast onto a current collector or separator and then dried. Accordingly, the powder  24  may be included in the slurry during the electrode production such that it is present in the electrode when it dries. 
         [0029]    In another embodiment, the powder  24  may be coated or otherwise applied to an already fabricated anode and/or cathode. The anode  12  and/or cathode  14  may therefore have two layers in some embodiments, as shown in  FIG. 2 . The first layer  26  may include the anode/cathode active material and the second layer  28  may include the PAM  22 . Regardless of how the powder  24  is incorporated into/on the electrode, it may be included in a portion of the electrode surface or it may be included in the entire electrode surface. The surface including the powder  24  (e.g., the second layer  28 ) may be a surface that is configured to be adjacent to the separator  16  of the battery cell  10 . In another embodiment, the surface including the powder  24  may alternatively be a surface that is configured to be adjacent to a current collector  20 . In another embodiment, both the surface configured to be adjacent to the separator  16  and the surface adjacent to the current collector  20  may include the powder  24 . In addition, the powder  24  may be present throughout a portion or all of the bulk of the anode  12  and/or cathode  14 , not only at one or more surfaces. 
         [0030]    The PAM  22  may be applied to the anode and/or cathode using any suitable method, for example, spin coating, dip coating, slurry painting, tape casting, slot die coating, micro gravure coating, sputtering, or others. The resulting coating (e.g., the second layer  28 ), may have a thickness of 0.1 to 10 μm, or any sub-range therein. For example, the coating may have a thickness of 0.5 to 5 μm, 0.5 to 3 μm, 0.7 to 2 μm, or others. In one embodiment, the PAM  22  is a powder  24  having a particle size of about 1 μm. The coating may be one to several particles thick, and therefore may have a thickness of about 1, 2, or 3 μm. 
         [0031]    In at least one embodiment, the anode  12  and/or cathode  14  may include 0.01 to 5 wt. % PAM  22 , or any sub-range therein. For example, the anode  12  and/or cathode  14  may include 0.05 to 1 wt. %, 0.05 to 0.5 wt. %, 0.07 to 0.5 wt. %, 0.1 to 0.5 wt. %, 0.1 to 0.3 wt. %, or other sub-ranges within 0.05 to 5 wt. %. The PAM  22  may be present in amounts greater than 5 wt. %, however, battery performance may begin to decline without significant additional proton absorption. The PAM  22  may also be present in amount less than 0.05 wt. %, however, the proton absorption may not be adequate to significantly slow or stop the LiPF 6  decomposition cycle. Since the anode  12  and/or cathode  14  include the PAM  22  in relatively small amounts (e.g., 5 wt. % or less), the initial performance of the battery  10  may be substantially unaffected. In addition, the addition of the PAM  22  may improve battery performance over time, relative to the same battery without the PAM, since the electrode materials will not be degraded to the same extent or at all. 
       EXAMPLES 
       [0032]    A 2.5 Ah 18650 lithium-ion battery (18 mm in diameter, 65 mm in height) may include 16 g of NCM cathode material. A LaNi 5  PAM has a hydrogen absorbency of about 1.4% hydrogen by weight of the LaNi 5 . The battery may require about 5 g of electrolyte, which may have a moisture content of 30 to 50 ppm. Accordingly, about 0.2 g of LaNi 5  may be required to absorb the hydrogen ions from the moisture content. Therefore, the cathode includes about 0.125% of LaNi 5  by weight. 
         [0033]    A full coin cell Li-ion battery was constructed, including a PAM powder of LaNi 5  incorporated into the anode and the cathode. The anode active material was graphite and the cathode had an NCM active material. The electrolyte included 1 M LiPF 6  and EC:EMC (3:7 weight ratio). A full charge and discharge curve of the full coin cell is shown in  FIG. 3 . The curve shows that the cell including the LaNi 5  operates as normal, with no apparent side reaction between the LaNi 5  and the electrolyte. 
         [0034]    A half coin cell was constructed, including a PAM powder of MmNi 3.6 Al 0.4 Mn 0.3 Co 0.7  applied to the cathode surface. The cathode had an NCM active material and the electrolyte included 1 M LiPF6 and EC:EMC (3:7 weight ratio). Two cyclic voltammetry tests were conducted: one on the half coin cell including the PAM powder and one on a half coin cell without it, as shown in  FIG. 4 . The results show that no side reactions occurred between the proton absorbing material and the electrolyte, indicating that the PAM acted as a proton absorber with no significant adverse impacts on cell performance. 
         [0035]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.