Abstract:
An electrochemical cell in one embodiment includes a first electrode, and a second electrode spaced apart from the first electrode, the second electrode including a substrate of active material formed with a plurality of interconnected chambers defined by a respective one of a plurality of inwardly curving walls, and 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 Ser. No. 12/437,774 entitled “Li-ion Battery with Anode Coating” by Boris Kozinsky et al., which was filed on May 8, 2009; U.S. Utility patent application Ser. No. 12/437,791 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 Ser. 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 Ser. 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 Ser. 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 , LiNiO 0.8 CoO 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]    Li/S batteries also have limitations. By way of example, the United States Advanced Battery Consortium has established a goal of &gt;1000 cycles for batteries used in powering an electric vehicle. Li/S batteries, however, exhibit relatively high capacity fade, thereby limiting the useful lifespan of Li/S batteries. 
         [0007]    One mechanism which may contribute to capacity fade of Li/S batteries is the manner in which the sulfur reacts with lithium. In general, sulfur reacts with lithium ions during battery discharge to form polysulfides (Li x S), which may be soluble in the electrolyte. These polysulfides react further with lithium (i.e., the value of x increases from ¼ to ⅓ to ½ to 1) until Li 2 S 2  is formed, which reacts rapidly to form Li 2 S. In Li/S batteries described in the literature, both Li 2 S 2  and Li 2 S are generally insoluble in the electrolyte. Hence, in a system in which intermediate polysulfides are soluble, each complete cycle consists of soluble-solid phase changes, which may impact the integrity of the composite electrode structure. 
         [0008]    Specifically, Li 2 S may deposit preferentially near the separator when the current through the depth of the positive electrode is non-uniform. Non-uniformity is particularly problematic at high discharge rates. Any such preferential deposition can block pores of the electrode, putting stress on the electronically conducting matrix and/or isolating an area from the composite electrode. All of these processes may lead to capacity fade or impedance rise in the battery. 
         [0009]    Moreover, soluble polysulfides are mobile in the electrolyte and, depending on the type of separator that is used, may diffuse to the negative electrode where the soluble polysulfides may becoming more lithiated through reactions with the lithium electrode. The lithiated polysulfide may then diffuse back through the separator to the positive electrode where some of the lithium is passed to less lithiated polysulfides. This overall shuttle process of lithium from the negative electrode to the positive electrode by polysulfides is a mechanism of self discharge which reduces the cycling efficiency of the battery and which may lead to permanent capacity loss. 
         [0010]    Some attempts to mitigate capacity fade of Li/S batteries rely upon immobilization of the sulfur in the positive electrode via a polymer encapsulation or the use of a high-molecular weight solvent system in which polysulfides do not dissolve. In these batteries, the phase change and self-discharge characteristics inherent in the above-described Li/S system are eliminated. These systems have a higher demonstrated cycle life at the expense of high rate capability and capacity utilization. 
         [0011]    Lithium in lithium ion batteries is also lost due to the formation of passivation layers on electrode materials. During the initial cycling of a cell, some of the lithium in the cell reacts with various cell components, e.g., electrolyte additives, to form a layer of material that is somewhat brittle and which exhibits low flexibility. The reaction creating this solid-electrolyte interface (SEI) is usually non-reversible. Accordingly, lithium consumed in forming passivation layers is no longer available for use in charging or discharging the cell. Various attempts to mitigate the loss of capacity resulting from passivation layer formation have been used. In one approach, additional lithium is charged to the cell after the formation of the passivation layers. In another approach, excess lithium is initially provided in the cell for use in forming the passivation layers. 
         [0012]    While the above approaches may be used to mitigate the effects of passivation layer formation, they do not address the systematic loss of lithium resulting from passivation layer formation. Specifically, in the case of a Li/S battery the sulfur active material increases in volume by ˜80% as it becomes lithiated during battery discharge. During charging, the process is reversed. As noted above, the passivation layer material is relatively non-resilient and brittle. Thus, as an electrode begins to expand, the passivation layer formed on a fully charged electrode is stressed, resulting in either separation from the underlying electrode material or in cracking of the passivation layer resulting exposed electrode material. 
         [0013]    As additional lithium ions come into contact with the exposed electrode material, new areas of passivation layer are formed. Thus, additional usable lithium is removed from the cell. 
         [0014]    Moreover, as the electrode contracts during the next portion of the cell cycle, the passivation layer cannot contract sufficiently. Accordingly, internal stresses generate additional fracturing of the passivation layer resulting in further passivation layer material flaking away from the electrode material. 
         [0015]    Thus, the removal of usable lithium by passivation layer formation continues over the life of the cell. Additionally, the build-up of passivation layer sediment causes reduced capacity. 
         [0016]    What is needed therefore is a battery that provides the benefits of materials that exhibit large volume changes during operation of the cell while reducing passivation layer generation and accumulation of passivation layer sediment. 
       SUMMARY 
       [0017]    In accordance with one embodiment, an electrochemical cell includes a first electrode, and a second electrode spaced apart from the first electrode, the second electrode including a substrate with active material and formed with a plurality of interconnected chambers defined by a respective one of a plurality of inwardly curving walls, and a form of lithium. 
         [0018]    In accordance with another embodiment, an electrochemical cell includes a first electrode, a second electrode spaced apart from the first electrode, the second electrode including a substrate with active material and formed with a plurality of interconnected chambers defined by a respective one of a plurality of inwardly curving walls, a form of lithium, and a separator layer positioned between the first electrode and the second electrode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  depicts a schematic of a battery system including an electrochemical cell with one electrode including a material that exhibits significant volume changes as the electrochemical cell cycles, the electrode formed with a number of small interconnected chambers with inwardly curving walls; and 
           [0020]      FIG. 2  depicts a schematic of a battery system including an electrochemical cell with one electrode including a material that exhibits significant volume changes as the electrochemical cell cycles, the electrode formed with a number of small interconnected chambers with inwardly curving walls which are more regularly shaped than the chambers of the electrochemical cell of  FIG. 1 . 
       
    
    
     DESCRIPTION 
       [0021]    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. 
         [0022]      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 a current collector  108  and a substrate  110  with active material which in this embodiment is a mixture of active materials into which lithium can be inserted and inert materials. The active materials may include silicon. Alternatively, the active material may include any other element that alloys with Li, such as Sn, Al, Mg, etc. 
         [0023]    The substrate  110  includes a number of small interconnected chambers  112  with inwardly curving walls  114 . The chambers  112  are connected by passages or narrowed areas  116 . In this embodiment, a fluid electrolyte  118  fills the chambers  112  and the passages  116 . In alternative embodiments, a solid electrolyte may fill the chambers  112  and the passages  116  or otherwise be in contact with the substrate  110 . 
         [0024]    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 . 
         [0025]    The positive electrode  104  includes active material  120  into which lithium can be inserted, inert material  122 , the electrolyte  118 , and a current collector  126 . The active material  120  includes a form of sulfur and may be entirely sulfur. 
         [0026]    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  136  of  FIG. 1 . 
         [0027]    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 from the substrate  110  of the negative electrode  102 , and the electrons are consumed at the positive electrode  104  because there is reduction of lithium ions into the active material  120  of the positive electrode  104 . During discharging, the reactions are reversed, with lithium and electrons moving in the direction of the arrow  138 . 
         [0028]    As lithium is inserted into the active substrate  110 , the volume of the substrate  110  increases. As the volume of the substrate  110  increases, the surface area of the chambers  112  may increase less for a given volume expansion compared to spherical particles, or may even decrease because of the inward curvature of the pore cavity walls  116 . Accordingly, a passivation layer (not shown) coating the active material is not stressed as much and may even be placed into compression. Additionally, the passivation layer within the passages  114  undergoes less deformation. The predominant effect, however, is the reduced change of surface area of the passivation layer due to any volume change which is in contradistinction to the effect in prior art configurations which place the passivation layer into significant tension upon large expansion thereby exposing the underlying substrate. 
         [0029]    Accordingly, when the volume of the substrate  110  is subsequently reduced, the stresses within the passivation layer are relaxed to the previous condition. Thus, there is little if any cracking or flaking of the passivation layer. To optimize the reduction in flaking of passivation layer material, the curvature of the inwardly curing walls  116  may be adjusted. Specifically, by increasing the “openness” of the substrate  108 , the surface area within the chambers  112  is increased thereby decreasing the amount of compression placed on the passivation layer as the chamber volume decreases. The amount of compression experienced by the substrate  110  may be increased by reducing the size of the chambers  112  resulting in a more “closed” substrate  110 . 
         [0030]    The amount of surface area change corresponding to a certain amount of volume change is governed not only by the geometry of the porous structure, but also by the surface energy. Therefore, the effect described above will depend on the particular properties of the materials comprising both the electrode and the electrolyte. Thus, the surface area change of the passivation coating can be tuned by adjusting the composition of the electrolyte. 
         [0031]    The amount of openness of a particular cell will depend upon the volume increase of the materials incorporated therein. U.S. patent application Ser. No. 11/935,721, filed on Nov. 6, 2007, the contents of which are herein incorporated in their entirety by reference, discloses a method of forming ceramic foam filters. As described therein, the “openness” of a ceramic filter may be controlled. Accordingly, a substrate  110  may be formed, for example, using the teachings of the &#39;721 application to form a substrate  110  of the desired openness for the particular battery cell chemistry. 
         [0032]    Moreover, while the chambers  112  are depicted as somewhat irregular in shape and size, the processes of the &#39;721 application along with other processes, including semiconductor chip forming processes such as chemical etching or anodization, may be used to provide extremely small and uniformly sized chambers. By way of example,  FIG. 2  depicts a lithium-ion cell  200  which includes a negative electrode  202 , a positive electrode  204 , and an electrolyte layer  206  between the negative electrode  202  and the positive electrode  204 . 
         [0033]    The negative electrode  202  includes a current collector  208  and a substrate  210  with active material which in this embodiment includes a form of silicon. The substrate  210  includes a number of small interconnected chambers  212  with inwardly curving walls  214 . The chambers  212  are connected by passages  216 . 
         [0034]    The electrolyte layer  206  provides a transfer path for lithium ions and serves as a physical and electrical barrier between the negative electrode  202  and the positive electrode  204  so that the electrodes are not electronically connected within the cell  200 . The positive electrode  204  includes active material  220  into which lithium can be inserted, inert material  222 , and a current collector  226 . The active material  220  includes a form of sulfur and may be entirely sulfur. 
         [0035]    The lithium-ion cell  200  is thus similar to the lithium-ion cell  100  with the exception of the provision of an electrolyte layer  206  rather than the electrolyte  118  of  FIG. 1 . Additionally, the chambers  212  are more uniformly shaped and positioned as compared to the chambers  112 . Accordingly, the stresses within the passivation layer formed on the substrate  210  are more uniform. 
         [0036]    An additional feature of the lithium-ion cell  200  and the lithium-ion cell  100  is that any negative effect caused by flaking or cracking of passivation layer material is localized. Specifically, migration of the passivation layer sediment within the cells  100  and  200  is “filtered” by the restricted diameter of the passages  116  and  216 . Accordingly, flaked passivation material is maintained within the particular chamber  112  or  212  that was generated the flake. Thus, passivation layer sediment build-up is contained within the chamber  112  or  212  that generated the sediment, thereby limiting the effect of sediment buildup to a local area. Moreover, the sediment build-up reduces the activity of the chamber  112  or  212  that generated the flakes, thereby reducing the rate of withdrawal of lithium within the system. 
         [0037]    Additionally, formation of dendrites occurs within a closed space of the chamber  112  or  212 . Thus, the potential for growth of dendrites into the separator layer  106  or the electrolyte layer  206  is reduced. 
         [0038]    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.