Abstract:
A method of extending the life of a battery, including positioning a dendrite seeding material in an electrolyte solution disposed between a metal-containing electrode and an electrolyte permeable separator membrane, growing metal dendrites from the lithium dendrite seeding material toward the lithium-containing electrode, and contacting metal dendrites extending from the metal containing electrode with metal dendrites extending from the metal dendrite seeding material, wherein the electrolyte contains metal ions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This utility patent application claims priority to co-pending U.S. provisional patent Ser. No. 61/486,946, filed on May 17, 2011, to co-pending U.S. provisional patent application Ser. No. 61/498,192, filed Jun. 17, 2011, and to co-pending U.S. provisional patent application Ser. No. 61/565,101, filed on Nov. 30, 2011, which are all incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The use of Lithium metal as an anode to build a rechargeable Lithium cell or battery system with the highest anode-specific capacity has long been desired. However, the growth of Li-metal dendrites gives rise to serious technical barriers for developing such a battery. Recently, modified versions of the Li metal battery, such as the Lithium ion battery, have been introduced with some success. However, the current modified versions possess limitations and inefficiencies that would not arise with a cell that uses Lithium metal as an anode. 
         [0003]    Typically, a Lithium metal cell includes an anode and a cathode separated by an electrically insulating barrier or ‘separator’ and operationally connected by an electrolyte solution. During the charging process, the positively charged lithium ions move from the cathode, through the permeable separator, to the anode and reduce into Li metal. During discharge, the Li metal is oxidized to positively charged lithium ions which move from the anode, through the separator, and onto the cathode, while electrons move through an external load from the anode to the cathode, yielding current and providing power for the load. During repeated charges and discharges, Lithium dendrites begin to grow from on the surface of the anode. Dendritic lithium deposits, sometimes called mossy lithium, eventually tear through the separator and reach the cathode causing an internal short and rendering the cell inoperable. Lithium dendrite formation is inherently unavoidable during the charging and discharging processes of Li-metal cells. Thus, there remains a need for a lithium electrode cell system that does not suffer the effects of dendrite growth while simultaneously maintaining the cycle ability, ionic conductivity, voltage and specific capacity of the cells. The present novel technology addresses these needs. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  is schematic view of a lithium ion cell according to a first embodiment of the present novel technology. 
           [0005]      FIG. 2A  is a perspective view of the separator of  FIG. 1 . 
           [0006]      FIG. 2B  is an exploded view of the separator surface of  FIG. 2 . 
           [0007]      FIG. 3A  is a first perspective view of a composite electrode of  FIG. 1 . 
           [0008]      FIG. 3B  is a second perspective view of a composite electrode of  FIG. 1 . 
           [0009]      FIG. 3C  is a third perspective view of a composite electrode of  FIG. 1 . 
           [0010]      FIG. 3D  is a fourth perspective view of a composite electrode of  FIG. 1 . 
           [0011]      FIG. 4  is a perspective view of a second embodiment coin cell implementation of the present novel technology. 
           [0012]      FIG. 5  is an enlarged elevation view of a dendrite growing from an electrode surface of  FIG. 1 . 
           [0013]      FIG. 6  is an exploded view of the surface of the separator of  FIG. 1  as partially coated with FNC. 
           [0014]      FIG. 7  is a process diagram a third embodiment of the present novel technology, showing of a method to form dendrite seeding material. 
           [0015]      FIG. 8  is a process diagram a fourth embodiment of the present novel technology, showing of a method of controlling metal dendrite growth. 
           [0016]      FIG. 9  is a process diagram a fifth embodiment of the present novel technology, showing of a method of extending the life a cell. 
           [0017]      FIG. 10  is a process diagram a sixth embodiment of the present novel technology, showing of a method of producing an FNC-coated separator. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    For the purposes of promoting and understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated novel technology and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates. 
         [0019]    As shown in  FIGS. 1-10 , the present novel technology relates to a rechargeable lithium metal electrochemical storage cell  10  having lithium metal electrodes  20 . Referring to  FIG. 1 , a rechargeable lithium electrode cell  10  is shown with a Li metal cathode portion  12  and Li-metal anode portion  14 . Separator  50  is positioned between the anode  14  and cathode  12 . Separator  50  is typically coated with a layer  80  of functionalized nanocarbons particles  40 . Separator  50  includes an anode facing side  53  and a cathode facing side  52 , and is typically coated with a thin or very thin film  80  of the functionalized nanocarbon (FNC) particles  40 , more typically about 0.1 μm thick, and typically oriented facing the surface  70  of the Li-metal electrode  20 . Gap  26  is filled with an electrolyte  25  positioned between the Li-metal electrode  20  and the FNC-coated separator  60 . The functionalized nanocarbon particles  40  typically have Li+ ions immobilized on the surface  65  of the layer  80  of nanocarbon particles  40 . The FNC film  80  is electrically connected to the Li-metal electrode  20 . When the Li-metal electrode  20  is charged, Li dendrites  11  extend from the surface  70  of the Li metal electrode  20  toward the FNC-coated separator  60 . Simultaneously, dendrites  55  extend from the surface  65  of the FNC film  80  toward the surface  70  of the Li-metal electrode  20 . The dendrites  55  grow in the through plane direction  94  of the Li metal electrode  20  and FNC coated separator  60 . 
         [0020]    Referring to  FIG. 5 , growth of dendrites  11 ,  55  is driven by the potential difference (ΔE) between the tip (Et)  59  and the base (Eb)  57  of the respective dendrites  11 ,  55 . With cycling, dendrites  11 ,  55  continue extending toward each other; eventually, the dendrites  11 , 55  touch each other and the potential difference (ΔE) dendrite  11 ,  55  is approximately zero because the FNC film  80  and the Li-metal electrode  20  have the same potential. Consequently, dendrite  11 ,  55  growth is retarded or stopped along the through plane direction  94 . In the subsequent cycles, dendrites  11 ,  55  may grow in a direction perpendicular to the major axis of the respective dendrite  11 ,  55  and parallel to the plane of the Li-metal electrode  20 , also referred as the in-plane direction  84 , which prevents dendrites  11 ,  55  from piercing through permeable or selectively permeable membrane  50 , as shown in  FIGS. 3A-3D . Eventually, a Li secondary surface  70  may form, from the intersection of the Li dendrites  11 ,  55 . Thus, a composite Li metal electrode  20  is formed in which an Li electrode  20  is assembled with the thin carbon layer  80 . 
         [0021]    While the lithium is typically specifically discussed herein as the electrode metal, the storage cell  10  may alternately include other alkaline earth and/or alkaline metal elements and combinations thereof as the electrode materials. 
         [0022]    Two types of cell exemplary configurations for exploiting the Li-metal dendrite/electrode system include a symmetric cell  400  in which a Li-metal electrode  420  is used as both the anode  414  and the cathode  412 , having the configuration of Li/polymer/Li (anode/electrolyte/cathode=A/E/C), enabling Li-dendrite mechanism study or Li-polymer battery systems; and an asymmetric cell  500  in which Li metal is the anode  514  and a different material is selected for the cathode  512 , such as Li/polymer electrolyte/V2O5, Li/liquid electrolyte/graphite, Li/polymer electrolyte/graphite, and Li/polymer electrolyte/FePO4. The symmetric cell  400  provides a better medium for Li-metal dendrite growth and can accelerate the cycle testing, while the asymmetric cell  500  better approximates field applications. 
         [0023]    Dendrite growth, as shown in  FIG. 5 , is fundamentally unavoidable because the metallurgic characteristics of Li-metal surfaces result in surface imperfections of Li-metal electrodes after the application of either mechanical stress or the plating/stripping cycles. While configurations known in the art focus solely on stopping dendrite  11  growth, the novel cell design  10  focuses on controlling the direction of the Li-metal dendrite  11 , 55  growth. 
         [0024]    As described in  FIG. 9 , one implementation  800  of the novel electrode  20  may have a carbon-coated layer of functionalized nanocarbon particles (FNC)  80  on a separator  50  that is positioned  801  in an electrolyte  25  and grows  803  Li dendrites  11 ,  55  simultaneously from the surface  51  of the Li metal electrode  20  and the surface  65  of the FNC coated separator  60 . An electrolyte  25  is placed  802  in the gap  26  the between the electrode  20  and FNC-coated separator  60 . The dendrites  11 ,  55  grow  803  after repeated charging and discharging  804  of the cell  10 . Dendrites  11 , 55  contact each other  805  and when contact occurs, the dendrites  11 ,  55  stop extending in the through plane direction  94  due to the zero potential difference that results from contact. The control of dendrite growth direction  800  occurs by contact  805  between the FNC coated separator dendrites  55  and the electrode dendrites  11 . After multiple combinations of dendrites  11 ,  55  the formation  806  of a Li-secondary Li surface  70  results. 
         [0025]    The establishment of a zeroing potential difference gives the rechargeable Li-metal electrode  20  a high specific capacity, high cycle ability, and high safety. Accordingly, the rechargeable lithium metal electrode system  10  may be implemented in many kinds of Li batteries including Li-polymer, Li-air and Li-metal oxide cells and battery systems as well as any other cells or battery systems in which Li metal anodes  14  are used, and yield benefits for electronics, electric vehicles and hybrid electric vehicles, large-scale energy storage and the like. 
         [0026]    Typically, a challenge for developing a high specific capacity and rechargeable Lithium metal electrode  20  for different Li batteries (i.e. Li polymer, Li-air and Li-ion, etc), has been stopping electrode dendrite  11  growth during the cycling  803 . The Li-metal electrode  20  has an inherent metallurgic tendency to form dendrites  11 , and dendrite  11  growth is driven by the potential difference between the base  57  and the dendrite tip  59 . Thus, Li electrode dendrite  11  growth is unavoidable. However, the instant system  800  incorporates, rather than avoids, the dendrite growth mechanism. 
         [0027]    In one embodiment, a rechargeable Li-metal electrode  220  is used in other Li battery systems, such as Li-polymer and Li-air and may be fabricated by coating the FNC layers  280  on the polymer electrolyte membranes  200 , which are used as the electrolyte  225  in both Li-polymer batteries and Li-air batteries. These FNC-coated polymer electrolytes  225  are typically incorporated as the interlayer  280  and assembled into a soft packed Li-air cell  285 . Such polymer electrolyte membranes  260  may include those of poly(ethylene oxide) (PEO), poly(vinylidene fluoride) (PVdF), poly(acrylonitrile) (PAN), and the other polymer electrolytes, which are widely used for both Li-polymer batteries and Li-air batteries. 
         [0028]    Additionally, many modes of producing the FNC coated separator  60  are available. The FNC layer  80  plays a role in the novel Li-metal electrode  20  because the immobilized Li+ ions  30  in the FNC layer  80  serve as ‘seeds’  31  for Li-metal dendrite  55  formation on the FNC layer  80 . The FNC layer  80  is typically porous, allowing the FNC aggregates to be bonded  605  together by the binder network  604  to form a rigid structure  606  to hold  607  the integrity of the layer  80 . The layer  80  is typically very thin with four main properties: 1) good pore structure to facilitate the passage of Li+ ions therethrough, 2) high electric conductivity to reduce internal impedance, 3) high coverage of Li+ ions  30  over the nanocarbon surface  65  for easy formation of Li metal dendrites  55 , and 4) good adhesion to a polymer separator  50  or a polymer electrolyte membrane. All of these properties are similar to those for the catalyst layer in the fuel cell, (i.e. a porous layer for gas and water diffusion, electric conductivity necessitated for gas reactions, SO 3 — coverage for proton conduction, and good adhesion of the catalyst layer on the polymer electrolyte membrane for durability). The thinner the FNC layer  80 , the less the loss of specific capacity of the Li-metal electrode  20 . 
         [0029]    The morphology of the FNC layer  80  depends on how the layer is fabricated  601 . Such techniques of applying  609  the layer  80  include (1) spraying, (2) machine blade-coating, (3) brush hand-painting, and the like. Carbons may be selected from sources including carbon blacks, nanographites, graphenes, and the like. It has been found that the higher the degree of graphitization, the higher the chemical stability. The nanocarbon particles  40  may be made from carbon black, which is inexpensive, but is an amorphous structure rather than a graphite structure. Graphene may also be used and possesses unique properties such as high electronic conductivity, high modulus, and high surface area. 
         [0030]    The morphology of the FNC layer  80  is also influenced by the ink formulation. To make a thin carbon layer, the first step is to mix  600  the carbon source with solvents to make a uniformly dispersed suspension  603 . To form such a well-dispersed carbon ink, solvent type is carefully selected based on polarity (i.e. dielectric constant) and their hydrophobicity in order to match those of the carbon aggregates and the binders. This mixture  602  is also called ‘ink formulation’. The type of carbons and solvents in an ink will affect the morphology of the thin FNC layer  80 . The type of binder  33  also affects the adhesion of the carbon layer  80  on the separator  50 . Typically, the binder  33  has a similar chemical structure to the separator/electrolyte membrane  50  so that they can be fused together  605  through hot pressing or other techniques to form a well-bonded interface  62  between the carbon layer  80  and the separator/electrolyte membrane  50 . 
         [0031]    The immobilized Li+ ions  30  over the surface of nanocarbon particles  40  serve as the ‘seeds’  31  for Li dendrite  55  formation on the FNC-coated separator  60 . Immobilization of the Li+ ions  30  is carried out by formation  900  of a dendrite seeding material  61 , such as by diazonium reaction or similar means  902  on an appropriate  901  carbon separator  50  to chemically attach an SO 3 H group  902  onto the carbon surface  65 , allowing the carbon separator  50  to become functionalized  903 . Then, attached SO 3 H exchanges  906  with Li+ ions  30  to immobilize the Li+ ions  30  onto the surface  65 . Thus, a dendrite seeding material  61  is formed  907 . The dendrite seeding material  61  is typically carbonaceous, but may also be a metal substrate, such as Li, Na, K, Al, Ni, Ti, Cu, Ag, Au, and combinations thereof. The seeding material  61  may also be a functionalized metal substrate, such as a self-assembled monolayer structure comprised of Au with a thiol-terminated organic molecule that contains at least one function group, such as SO3-M+, COO-M+, and NR3+X—, an electrically conductive organic polymer, such as polyacetylene, polyphenylene vinylene, polypyrrole, polythiophene, polyaniline, and plypohenylene sulfide, or a functionalized electrically conductive organic polymer, wherein the functional groups are chemically bound to the polymer. These materials  61  may be deposited using conventional physical deposition techniques, such as mechanical layering, or physical vapor deposition techniques, such a sputtering, or the like. 
         [0032]    The novel technology allows attachment  903  of different functional groups to the carbon surface  65 , such as through the diazonium reaction and the like. In this reaction, the functional group Y is attached  903  onto the carbon surface  65  through the introduction  904  of a diazonium salt XN 2 C 6 H 4 —Y (wherein Y=Sulfonate, SO3-M+, Carboxylate, COO-M+; and Tertiary amine, NR3+X—; etc.). The attachment of different chemical groups not only provides a platform for immobilizing Li+ ions  30  at the FNC surface  65 , but also changes the surface energy of the carbon particles which can be used as a tool for adjusting the surface hydrophobicity of the carbon film  80 , and is helpful for ink formulation  603 . The adhesion  609  of the FNC layer to a separator/polymer electrolyte  50  influences the cycle life of the novel Li-metal electrode  20 . A good interface  62  between the FNC layer  80  and the separator/electrolyte membrane  50  is typically formed  608 . This mainly depends on the network of binders  33  in the FNC layer  80  and the techniques for the formation of the interface  62 . Such a catalyst layer can withstand several thousand hours of long-term durability testing due, in part to the binder  33  in maintaining  607  the FNC layer  80  bound to the separator/electrolyte membrane  50 . A TEM observation of such this catalyst/membrane interface  62  would show little or no delamination after approximately 2000 hours of durability testing. Hot pressing is one of techniques for fabrication, and the parameters of the hot pressing technique (i.e. temperature, pressure, and time) allow systematic control of the process. 
         [0033]    The morphology (i.e. surface area, pore structure, and geometry) of the FNC layer  80  on the membrane  50  has a significant impact on the performance of the novel metal electrode  20 . The FNC layer  80  porosimetry  81  (i.e. pore size, pore size distribution and pore volume) is a factor in controlling the direction of dendrite growth  700  because it influences the presence  705  of metal cations  30  on the FNC membrane surface  65  and the addition  703  of the dendrite seeding material  61 . The pore structure typically allows metal ions  30  to pass through smoothly during cycling  704 , but not to form dendrites inside the pores that would block the diffusion of the metal ions  30 . Thus, determining  701  and production  702  of an appropriate FNC layer  80  with porosimetry  81  is useful in allowing for dendrite  11 ,  55  presence  706  and eventual formation  707  of a secondary metal layer  70 . On the other hand, the FNC layer  80  has to adhere to a separator/electrolyte membrane  50  and the diffusion barrier (if there is any) from the formed interface  62  should be minimized. 
         [0034]    Typically, the specific capacity of the rechargeable metal electrode  20  may be affected by varying the thickness  89  of the FNC film  80  against the thickness  29  of the Li metal electrode  20 . The examples herein relate to the novel technology and various embodiments, and are not intended to limit the scope of the present novel technology to those modes and embodiments discussed herein. 
       Example 1 
       [0035]    The effect of the different carbon-coated layers on the specific capacity of the Li metal composite electrode  20  was approximately calculated and is shown in Table 1. For instance, for the carbon-coated layer  80  with the 0.1 μm thickness, the corresponding specific capacity loss of Li metal electrode  20  is only 0.026%. Even for the thick FNC film  80 , 4 μm, the corresponding loss of specific capacity is only 0.53%. Thus, the effects of the carbon-coated layer  80  on the specific capacity of the Li metal electrode  20  are negligible. The thin carbon-coated layer  80  retains the advantage of the high specific capacity of Li metal electrodes. 
         [0000]                                          TABLE 1                       Reduction of Li Metal       Thickness of Carbon   Thickness of Li Metal   Electrode Specific       Film (μm)   Electrode(mm)   Capacity (%)                                0.1   0.75   0.0133       1   0.75   0.1332       2   0.75   0.1332       3   0.75   0.1332       4   0.75   0.5305               Effect of thickness of carbon film on the Li metal electrode specific capacity.            
Therefore, carbon has been proven to be very stable in a wide potential window. The composite Li electrode having a very thin carbon film is very stable. Carbon black may be used in many battery systems (i.e. Zn/MnO 2 ,), in particular, Li-ion batteries (as the anode) and Li—SOCl 2  batteries (as the carbon cathode).
 
         [0036]    Referring to  FIG. 4 , The Li metal anode  14  was assembled together with a separator  350  (thickness=25 μm) coated with a thin nanocarbon layer  80  of functionalized carbon nanoparticles  340  (δ=3.2 μm) and a LiPFeO 4  cathode  312  into a coin cell  300  configuration using the electrolyte of 1.2 M LiPF 6  in ethylene carbonate/ethyl-methyl carbonate (EC:EMC=3:7). A coin cell using the same components, but without the nanocarbon coating layer  380 , was used as a baseline for the comparison. One concern for using such a carbon coating layer  380  is whether the addition of the FNC layer  380  on the separator  350  would result in the increased internal impedance from the carbon layer  380  blocking the pores of the separator  350 , thus hindering the diffusion of Li+ ions  330  through and, consequently, reducing the power performance of the cell  300 . However, it is clear that coating the carbon layer  380  on the separator  350  did not cause an increase in the internal impedance of the cell  300 , but instead gave rise to a slight impedance reduction. The Li/FNC cell  300  possesses a slightly higher discharge voltage than the baseline Li cell. Even after five hundred cycles, the same trend was observed. Noise was observed for the baseline cell, which was attributed to the formation of dendrites  355 . In addition, the same phenomenon of reduction of internal impedance has been observed during the charging process. 
         [0037]    The cell  300  was not balanced for capacity, and the capacity of the cell  300  was limited by the LiPFeO 4  cathode  312 ; a much higher capacity of the cell  300  is expected if an appropriate high energy density cathode is used (such as a V 2 O 5  aerogel or an air cathode). The Li metal electrode  314  using an FNC layer  380  showed excellent cycleablity, approximately 84% capacity after 500 cycles. The estimated capacity decay rate of the novel Li metal electrode cell  300  after the first 45 cycles is only 0.026%/cycle. Based on this decay rate, the cycle life of such a cell can typically achieve at least 500, more typically at least 725 cycles, and still more typically at least 1000 cycles, with 80% capacity (death definition of a battery in electric vehicle (EV) applications). This decay rate (0.026%/cycle) of the novel Li metal electrode  320  in the coin cell  300  may be caused by the degradation of the LiFePO 4  cathode  312  because the coin cells  300  are sealed in ambient atmospheric pressure, which may allow the introduction of moisture into the cell  300 . The moisture reacts with LiPF 6  to produce HF, which can react with LiFePO 4 , causing the degradation. Therefore, the true decay rate of the novel Li metal electrode  320  should be much lower than 0.026%/cycle if the coin cell  300  is sealed, such as inside an Argon filled glovebox. 
       Example 2 
       [0038]    Referring to  FIG. 6 , an FNC-coated separator  60  was examined via SEM analysis after repeated cycling. Li metal dendrites  55  were observed on the surface  65  of the FNC-coated separator  60  facing the surface of the Li metal electrode  20 . Moreover, the Li dendrites  55  formed a unitary layer instead of aggregating as loosely arranged dendrites. The thickness  89  of the FNC layer  80  was measured to be about 3 μm, while the Li dendrite  70  layer was around 20 μm thick. Referring to  FIG. 6 , and to further illustrate the function of the FNC layer  80  for inducing Li metal dendrite  55  formation, the separator  50  was coated with an FNC layer  80  on half the area of the surface, while the other half was not coated. No dendrites  55  formed on the non-coated region of the separator  50 . No Li dendrites  55  were found on the opposite side of the FNC-coated separator  50 . Some large size particles (50 μm or more) were observed seen underneath the separator  50 ; these large particles likely originated from the SEM conducting paste used to adhere the sample of the separator  50  on the SEM aluminum disc. 
         [0039]    In another embodiment, the layer  80  formed over the electrochemical separator  50  to enable dendritic growth toward the metal anode  14  is a thin metallic layer  80 . The dendrites  55  growing from the separator  50  contact dendrites  11  growing from the metal anode  14 , shorting the circuit and thus preventing the dendrites  11  growing from the anode  14  toward the separator  50  to reach and pierce the separator  50 . The anode  14  is typically lithium, but may likewise be sodium or the like. The metal layer  80  on the separator  50  is typically lithium, but may also be sodium or another electrically conductive metal, electrically conducting polymer, an organometallic matrix, functionalized electrically conducting polymer, or the like. More typically, the layer  80  is a non-reactive metal, such as Ni. The metal layer  80  on the separator  50  is typically formed thin enough such that its electrical resistivity is high, typically high enough such that the layer  80  is not easily electrically or otherwise degraded. Optionally, the thin metal layer  80  may be functionalized after deposition onto the separator  50 . 
         [0040]    While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected.