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 current collector, an electrically conducting rigid support frame electrically connected to the current collector, and an active material coated to the rigid support frame.

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. 12/437,822 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 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 , 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]    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]    In the case of a Li/S battery, however, the sulfur active material increases in volume by ˜80% as it becomes lithiated during battery discharge. Thus, an all solid-state cathode, composed of sulfur (or lithiated sulfur) and a mixed conducting material, particularly if the latter is a ceramic, is susceptible to fracture due to the volume change upon battery cycling. Fracture of the cathode can result in capacity fade and is a potential safety hazard due to venting of the cell. Other materials which exhibit desired capabilities when incorporated into a battery also exhibit significant increases in volume. By way of example, LiSi, typically used as an anode material, exhibits a large increase in volume during operation. 
         [0012]    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 the likelihood of fracture of material or internal shorts within the cell. 
       SUMMARY 
       [0013]    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 current collector, an electrically conducting rigid support frame electrically connected to the current collector, and an active material coated to the rigid support frame. 
         [0014]    In accordance with another embodiment, an electrochemical cell includes a first electrode, and a second electrode spaced apart from the first electrode, the second electrode including, a current collector, an electrically conducting first support wall electrically connected to the current collector, an electrically conducting second support wall spaced apart from the first support wall and electrically connected to first support wall, and an active material coated to the first support wall and the second support wall. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  depicts a schematic of a lithium ion cell including a cathode and an anode with a rigid framework of nanowires; 
           [0016]      FIG. 2  depicts a schematic of a lithium ion cell including a cathode and an anode with a rigid framework configured to provide directional lithium ion coating of the framework; 
           [0017]      FIG. 3  depicts a schematic of another embodiment of a lithium ion cell including a cathode and an anode with a rigid framework configured to provide directional lithium ion coating of the framework; and 
           [0018]      FIG. 4  depicts a schematic of another embodiment of a lithium ion cell including a cathode and an anode with a rigid framework configured to provide directional lithium ion coating of the framework. 
       
    
    
     DESCRIPTION 
       [0019]    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. 
         [0020]      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 . A first support wall  110  is attached to the current collector  108  on one side while the other side of the support wall  1   10  faces and is spaced apart from another support wall  112 . 
         [0021]    The first support wall  110  and the second support wall  112  are formed from nanotubes, nanowires, or conducting fibers such as carbon  114  which in this embodiment are formed as a grid from a material onto which lithium  116  plates, although other materials such as graphite particles may be used. The first support wall  110  and the second support wall  112  are connected such that lithium ions can migrate between the support walls  110  and  112 . In one embodiment, a single ply of woven material is folded to provide facing surfaces of the support walls  110  and  112 . In one alternative embodiment, a solid LI-ion conductor, such as lithium phosphate, lisicon, a lithium-conducting polymer or glass, is used to create connections between support structures for lithium migration. In a further embodiment, an electrolyte is used within the electrode  102  to provide a transfer path. The support walls  110  and  112  may be spaced apart as depicted in  FIG. 1  or they may be in contact with the other support wall  110  or  112  along the facing surfaces. 
         [0022]    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 . 
         [0023]    The positive electrode  104  includes a current collector  120  and an active portion  122  into which lithium can be inserted. The active portion  122  may include a form of sulfur and may be entirely sulfur. If desired, the positive electrode  104  may include a support structure similar to the support walls  110  and  112 . 
         [0024]    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 . 
         [0025]    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 lithium  116  plated on the nanowires  114  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  138 . 
         [0026]    As lithium ions are inserted into the active portion  122 , the volume of the active portion  122  increases. As the volume of the active portion  122  increases, the pressure within the positive electrode  104  increases. The increased pressure in the positive electrode  104 , in embodiments incorporating a fluid such as a fluid electrolyte, causes the fluid to flow toward the negative electrode  102 . Because the nanowires  114  do not fill the entire negative electrode  102 , the fluid can flow into the negative electrode  102 . The rigid support walls  110  and  112  thus provide an expansion volume within the negative electrode  102 . Additionally, the rigidity of the support structures  110  and  112  protects the active material in the electrode  102  from the volume change in the positive electrode  104 . 
         [0027]    As lithium plates onto the nanowires  114 , the lithium  116  may plate predominantly in directions toward another nanowire  114 . Specifically, since one side of the rigid wall  110  is mounted to the current collector  108 , lithium will not plate onto that surface portion. Thus, plating of lithium on the support wall  110  occurs predominantly on the sides of the nanowires  114  facing other nanowires  114  within the support wall  110  or nanowires  114  in the opposing support wall  112 . Likewise, since one side of the nanowires  114  in the rigid wall  112  is attached to the separator layer  106 , lithium will not plate onto that surface portion. Thus, plating of lithium on the support wall  112  occurs predominantly on the sides of the nanowires  114  facing other nanowires  114  within the support wall  112  or nanowires  114  in the opposing support wall  110 . 
         [0028]    Accordingly, any deformation of the lithium layer  116  on the support wall  110  will typically occur in a direction that is not directly toward the current collector  108  and any deformation of the lithium layer  116  on the support wall  110  will typically occur in a direction that is not directly toward the separator layer  106 . Thus, deformation or dendrites must extend for a significantly longer distance before any significant deleterious effects on the current collector  108  or the separator layer  106  are generated. 
         [0029]    The benefits of providing a rigid support wall that provides protection of active material from volume changes within the cell and which may promote directional plating of lithium can be increased by modifying the shape of the rigid support structure members. By way of example,  FIG. 2  depicts a lithium-ion cell  200  which includes a negative electrode  202 , a positive electrode  204 , and a separator layer  206  between the negative electrode  202  and the positive electrode  204 . The negative electrode  202  includes a current collector  208 . A first support wall  210  is attached to the current collector  208  on one side while the other side of the support wall  210  faces and is spaced apart from another support wall  212 . 
         [0030]    The first support wall  210  and the second support wall  212  are formed from shaped nanowires  214  which in this embodiment are formed as a grid from a material onto which lithium  216  plates. The first support wall  210  and the second support wall  212  are connected such that lithium ions can migrate between the support walls  210  and  212 . 
         [0031]    The separator layer  206  is a lithium conductor 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  while allowing transfer of lithium ions between the negative electrode  202  and the positive electrode  204 . The positive electrode  204  includes a current collector  220  and an active portion  222  into which lithium ions can be inserted. 
         [0032]    The lithium-ion cell  200  operates in a manner similar to the lithium-ion battery cell  100 . The nanowires  214  of the lithium-ion cell  200  are formed, however, to increase the effect of directional plating as compared to the nanowires  114  of the lithium-ion cell  100 . To this end, the nanowires  214  include a mounting surface  230  attached to either the current collector  208  or to the separator layer  206 , and a plating surface  232  on which lithium is allowed to plate. The plating or active surface  232  is configured such that the plating surface  232  does not face the surface on which the nanowire  214  is mounted. Thus, plating of lithium  216  onto the nanowires  214  occurs in a direction away from the surface on which the nanowires  214  are mounted. 
         [0033]    The inclusion of a rigid framework may increase the necessary volume for a particular cell. The increased volume may be minimized by selective spacing and shaping of the members used to form the support walls.  FIG. 3 , for example, depicts a lithium-ion cell  300  which includes a negative electrode  302 , a positive electrode  304 , and an electrolyte layer  306  between the negative electrode  302  and the positive electrode  304 . The negative electrode  302  includes a current collector  308 . A first support wall  310  is attached to the current collector  308  on one side while the other side of the support wall  310  faces and is spaced apart from another support wall  312  which is connected to the electrolyte layer  306 . 
         [0034]    The first support wall  310  is formed as a solid base portion  314  from which shaped protrusions  316  extend. The second support wall  312  is formed from shaped wires  318  which in this embodiment are formed into a grid which may include openings to the electrolyte layer  306 . Shaped plating surfaces  320  are supported by the wires  318  and extend toward the support wall  310 . Both the support wall  310  and the support wall  312  are formed from a material onto which lithium  322  plates. The support wall  310  and the support wall  312  may be shaped using semiconductor chip forming processes or other manufacturing processes. The first support wall  310  and the second support wall  312  are connected such that lithium ions can migrate between the support walls  310  and  312 . 
         [0035]    The electrolyte layer  306  includes an electrolyte with a lithium cation and serves as a physical and electrical barrier between the negative electrode  302  and the positive electrode  304  so that the electrodes are not electronically connected within the cell  300  while allowing transfer of lithium ions between the negative electrode  302  and the positive electrode  304 . The positive electrode  304  includes a current collector  324  and an active portion  326  into which lithium can be inserted. 
         [0036]    The lithium-ion cell  300  operates in a manner similar to the lithium-ion battery cell  200 . The support wall  310  and the support wall  312  of the lithium-ion cell  300  are formed to increase the effect of directional plating like the nanowires  214  of the lithium-ion cell  200 . The controlled manufacturing process used to form the support walls  310  and  312 , however, ensure a more uniform distance between the plating surfaces of support wall  310  and the support wall  312  and any other surface in the cell  300 . Accordingly, usefulness of the space within the electrode  302  is optimized. Moreover, the support wall  310  and the support wall  312  are formed to allow interlacing of plating surfaces to further minimize space requirements. 
         [0037]    While the foregoing embodiments incorporate wall structures which are highly ordered, a wall structure with randomly oriented support members may be used. To this end,  FIG. 4  depicts a lithium-ion cell  400 , which includes a negative electrode  402 , a positive electrode  404 , and a separator region  406  between the negative electrode  402  and the positive electrode  404 . The negative electrode  402 , the positive electrode  404 , and the separator region  406  are located within a pouch  408 . 
         [0038]    The negative electrode  402  includes a support structure  410  which is in electrical contact with a current collector  412  and an electrolyte  414 . The support structure  410  is made from nanotubes or nanowires  416  which in this embodiment are a conductive mixture onto which lithium or some other active material can plate. The conductive material in the nanowires  416  may include carbon. The nanowires  416  are randomly oriented. Random orientation may be obtained, for example, by drying of a slurry incorporating a plurality of nanowires  416 . Once dried, the nanowires  416  may be compacted to reduce porosity. 
         [0039]    The separator region  406  in one embodiment includes an electrolyte with a lithium cation and serves as a physical and electrical barrier between the negative electrode  402  and the positive electrode  404  so that the electrodes are not electronically connected within the cell  400  while allowing transfer of lithium ions between the negative electrode  402  and the positive electrode  404 . 
         [0040]    The positive electrode  404  includes active material  420  into which lithium can be inserted, inert material  422 , the electrolyte  414 , and a current collector  426 . The active material  420  includes a form of sulfur and may be entirely sulfur. 
         [0041]    The lithium-ion cell  400  operates in a manner similar to the lithium-ion battery cells  100 ,  200 , and  300 . Although the nanowires  416  are randomly oriented, most of the active surfaces of the nanowires  416  are either spaced away from the outer walls of the nanowire support structure  410  or oriented away from an immediately adjacent structure such as the current collector  412 . Accordingly, while providing a substantial volume occupied only by electrolyte  414  which can easily be displaced as active material plates onto the support structure  410 , any dendrite formation is less likely to establish an internal short. 
         [0042]    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.