Abstract:
An electrochemical cell in one embodiment includes a negative electrode including a form of lithium, a positive electrode spaced apart from the negative electrode, an electrolyte, a separator positioned between the negative electrode and the positive electrode, and a current collector in the negative electrode, the current collector including a substrate material and a coating material on the surface of the substrate material, wherein the coating material does not include a form of lithium.

Description:
[0001]    Cross-reference is made to U.S. Utility patent application Ser. No. 12/437,576 entitled “Li-ion Battery with Selective Moderating Material” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,592 entitled “Li-ion Battery with Blended Electrode” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,606 entitled “Li-ion Battery with Variable Volume Reservoir” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,622 entitled “Li-ion Battery with Over-charge/Over-discharge Failsafe” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,643 entitled “System and Method for Pressure Determination in a Li-ion Battery” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,745 entitled “Li-ion Battery with Load Leveler” by John F. Christensen et al., which was filed on May 8, 2009; U.S. Utility Patent Application Serial No. [Attorney Docket No. 1576-0304] entitled “Li-ion Battery with Anode Expansion Area” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility Patent Application Serial No. [Attorney Docket No. 1576-0305] entitled “Li-ion Battery with Porous Silicon Anode” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility Patent Application Serial No. [Attorney Docket No. 1576-0306] entitled “Li-ion Battery with Rigid Anode Framework” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility Patent Application Serial No. [Attorney Docket No. 1576-0308] entitled “System and Method for Charging and Discharging a Li-ion Battery” by Nalin Chaturvedi et al., which was filed on May 8, 2009; and U.S. Utility Patent Application Serial No. [Attorney Docket No. 1576-0310] entitled “System and Method for Charging and Discharging a Li-ion Battery Pack” by Nalin Chaturvedi et al., which was filed on May 8, 2009, the entirety of each of which is incorporated herein by reference. The principles of the present invention may be combined with features disclosed in those patent applications. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to batteries and more particularly to lithium-ion batteries. 
       BACKGROUND 
       [0003]    Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium metal incorporated into the negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. 
         [0004]    When high-specific-capacity negative electrodes such as lithium are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. Conventional lithium-intercalating oxides (e.g., LiCoO 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , Li 1.1 Ni 0.3 Co 0.3 Mn 0.3 O 2 ) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g. In comparison, the specific capacity of lithium metal is about 3863 mAh/g. The highest theoretical capacity achievable for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li 2 S and Li 2 O 2 . Other high-capacity materials including BiF 3  (303 mAh/g, lithiated) and FeF 3  (712 mAh/g, lithiated) are identified in Amatucci, G. G. and N. Pereira, Fluoride based electrode materials for advanced energy storage devices. Journal of Fluorine Chemistry, 2007. 128(4): p. 243-262. All of the foregoing materials, however, react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy. The theoretical specific energies of the foregoing materials, however, are very high (&gt;800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes). 
         [0005]    Lithium/sulfur (Li/S) batteries are particularly attractive because of the balance between high specific energy (i.e., &gt;350 Wh/kg has been demonstrated), rate capability, and cycle life (&gt;50 cycles). Only lithium/air batteries have a higher theoretical specific energy. Lithium/air batteries, however, have very limited rechargeability and are still considered primary batteries. 
         [0006]    While generally safe, the amount of energy stored within a battery as well as the materials used to manufacture the battery can present safety issues under different scenarios. Safety is particularly an issue when a battery is subjected to increased temperatures either as a result of internal processes or as a result of the environment in which the battery is located. 
         [0007]    By way of example, when batteries are charged or discharged, they typically generate heat due to a finite internal resistance that includes ohmic, mass-transfer, and kinetic contributions. Exothermic side reactions can also generate heat within the battery. This heat generation can pose a safety risk if it is large and rapid. For instance, commercial Li-ion cells generally go into thermal runaway if the internal cell temperature climbs above the decomposition temperature of the cathode (˜180 to 220° C., depending upon the chemistry and the state of charge). Often the events that lead to a temperature rise above this critical temperature are triggered at much lower temperatures. For example, exothermic anode film decomposition can occur at ˜120° C., providing enough energy to raise the battery temperature above 180° C. Excessive temperature in a battery may lead to venting of gases, smoke, flames, and, in rare cases, explosion. 
         [0008]    Undesired amounts of heat may also be generated in a battery due to undesired physical changes in the battery. By way of example, formation of an electronically conducting phase between the two electrodes (i.e., internal shorting) of the battery can lead to excessive internal discharge. Internal shorting may be caused by dendrite formation, separator melting, separator cracking, separator tearing, pinholes, or growth of some conductive material through the separator. Thus, in addition to safety concerns, dendrite formation can significantly shorten the lifespan o an electrochemical cell. 
         [0009]    What is needed therefore is a battery that is less susceptible to dendrite formation. 
       SUMMARY 
       [0010]    In accordance with one embodiment, an electrochemical cell includes a negative electrode including a form of lithium, a positive electrode spaced apart from the negative electrode, an electrolyte, a separator positioned between the negative electrode and the positive electrode, and a current collector in the negative electrode, the current collector including a substrate material and a coating material on the surface of the substrate material, wherein the coating material does not include a form of lithium. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  depicts a schematic of an electrochemical cell with one electrode including a form of lithium and having a coating applied to the current collector to assist in forming a smooth lithium layer. 
       
    
    
     DESCRIPTION 
       [0012]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
         [0013]      FIG. 1  depicts a lithium-ion cell  100 , which includes a negative electrode  102 , a positive electrode  104 , and a separator region  106  between the negative electrode  102  and the positive electrode  104 . The negative electrode  102  includes electrolyte  112  and a current collector  114 . A coating  116  is provided on the current collector  114 . 
         [0014]    The negative electrode  102  may be provided in various alternative forms. The negative electrode  102  may incorporate a dense form of Li metal or a in a porous composite electrode. Incorporation of Li metal is desired since the Li metal affords a higher specific energy than graphite. 
         [0015]    The separator region  106  includes an electrolyte with a lithium cation and serves as a physical and electrical barrier between the negative electrode  102  and the positive electrode  104  so that the electrodes are not electronically connected within the cell  100  while allowing transfer of lithium ions between the negative electrode  102  and the positive electrode  104 . 
         [0016]    The positive electrode  104  includes active material  120  into which lithium can be inserted, inert materials  124 , the electrolyte  112  and a current collector  126 . The active material  120  may include a form of sulfur and may be entirely sulfur. The active material  120  may incorporate a form of lithium such as a Li—SI alloy or a Li—Sn alloy. 
         [0017]    The lithium-ion cell  100  operates in a manner similar to the lithium-ion battery cell disclosed in U.S. patent application Ser. No. 11/477,404, filed on Jun. 28, 2006, the contents of which are herein incorporated in their entirety by reference. In general, electrons are generated at the negative electrode  102  during discharging and an equal amount of electrons are consumed at the positive electrode  104  as lithium and electrons move in the direction of the arrow  130  of  FIG. 1 . 
         [0018]    In the ideal discharging of the cell  100 , the electrons are generated at the negative electrode  102  because there is extraction via oxidation of lithium ions as lithium is plated on the coating  116  of the negative electrode  102 , and the electrons are consumed at the positive electrode  104  because metal cations or sulfur ions change oxidation state in the positive electrode  104 . During charging, the reactions are reversed, with lithium and electrons moving in the direction of the arrow  132 . 
         [0019]    The physical characteristics of the lithium layer that is formed on the current collector  114  is influenced by the coating  116 . Specifically, use of pure forms of Li can result in shortened lifespan of a cell because Li is highly reactive. Accordingly, upon repeated cycling of a Li-anode cell, the anode undergoes significant morphology changes. For example, the initially dense metal, after a number of cycles, develops surface roughness and a sponge-like morphology. This morphology is dangerous due to increased surface area which increases the chance and severity of runaway reactions, and due to growth of metallic dendrites that can puncture the separator and cause an internal short of the cell. 
         [0020]    The inventors believe that surface roughness develops partly because Li deposition onto the current collector during cell charging happens non-uniformly. This non-uniformity is caused in part by roughness and defects on the atomic level of the anodic current collector  114  (typically Cu metal). Li metal plating nucleates at these defect sites and the subsequent growth pattern of Li is determined by these initial sites. 
         [0021]    The coating  116 , however, encourages the growth of a smooth layer of lithium on the collector  114  regardless of surface imperfections in the substrate material. In one embodiment, this is accomplished by providing a coating  116  that exhibits a smoother surface for lithium adherence as compared to the substrate material. Accordingly, the lithium coats more uniformly onto the current collector  114 . 
         [0022]    Thus, by making the surface of the coating  116  very smooth, the anode morphology is improved thereby extending the cycle life and safety of the cell. The coating  116  may be provided in the form of pure metals and alloys, conducting oxides such as indium oxide or zinc oxide, or sulfides, etc. The coating  116  can be applied by a sputtering process or chemical deposition onto the current collector  114  during the assembly of the cell  100 . 
         [0023]    Preferably, the coating  116  is very thin to reduce cost and effects on electronic conductivity. The coating  116  need only be sufficiently thick to provide a very smooth surface on which Li metal can be electrochemically deposited with minimal initial development of roughness. 
         [0024]    In another embodiment, the coating  116  is in the form of a thin electronically conductive coating that it has high chemical affinity for Li metal. Accordingly, the coating  116  functions as a wetting agent so that during cell charge Li does not form isolated islands or beads but rather spreads uniformly, “wetting” the entire surface of the current collector. By selecting a material with a sufficiently high affinity for Li, such as tin, magnesium, aluminum, or graphite, Li will form a uniform layer even if the coating  116  exhibits a surface roughness similar to the surface roughness of a Cu anode. 
         [0025]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.