Patent Publication Number: US-2013230777-A1

Title: Lithium based anode with nano-composite structure and method of manufacturing such

Description:
TECHNICAL FIELD 
     This invention relates generally to batteries, and more particularly to the anode of a battery and the method of manufacturing such. 
     BACKGROUND OF THE INVENTION 
     Batteries typically include a cathode, an anode and an electrolyte. One problem associated with lithium batteries has been that high storage capacity lithium battery anode materials expand over 100% when fully lithiated. This expansion of the anode causes disintegration of anode structure by cracking. The cracking severely reduces the performance of the anode and the associated batteries that contain such anode, thus limiting commercial applicability of the lithium based battery technology. An additional problem is associated with the use of lithium based anodes in combinations with liquid electrolytes. As lithium plates at the anode during recharge of a conventional electrochemical cell that employs liquid electrolyte, lithium appearing at the surface of active materials within the anode can react with the liquid electrolyte before being intercalated. Such parasitic reactions can result in not only consumption of the lithium and thereby a reduction in storage capacity of the cell because less lithium is available for cycling between the anode and cathode; but it can also result in a passivation coating on the surface of the active material which can result in an increase in cell impedance. 
     It thus is seen that a need remains for a battery anode which overcomes problems associated with those of the prior art. Accordingly, it is to the provision of such that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     In a preferred form of the invention, a battery anode comprises a base made of a first anodic material having a lithium intercalation potential, and a plurality of particles made of a second anodic material having a—lithium intercalation potential that is different from the intercalation potential of the first material. Each particle of the plurality of particles being encapsulated within a cavity within the base. The first anodic material has high electronic conductivity and high lithium diffusivity. 
     In another preferred form of the invention, a method of producing a battery anode comprises the steps of preparing a quantity of a first anodic material having a first lithium intercalation potential, preparing a quantity of a particulated second anodic material having a second lithium intercalation potential that is different from said first lithium intercalation potential of said base first anodic material, coating the particles of the second anodic material with a removable coating, mixing the second anodic material into the first anodic material to form a mixture of anodic materials, forming an anodic layer with the mixture of anodic materials, and removing the coating from the particles of the second anodic material within the anodic layer so as to form a cavity about the particles of the second anodic material in the area once occupied by the removed coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an anode in a preferred form of the invention. 
         FIG. 2  is a table showing properties of anodic materials. 
     
    
    
     DETAILED DESCRIPTION 
     With reference next to the drawings, a dimensionally stable lithium based anode is disclosed having a composite structure that includes internal space for expansion and contraction of electrochemically active electrode material. This invention is targeted particularly to lithium based anodes. 
     Lithium has a very high columbic capacity at 2047 mAh/cm 3 . To avoid plating and stripping of pure lithium, lithium reactive anode materials have been identified that have comparably high volumetric capacity. A selection of such materials is listed in  FIG. 2 . Considering two examples, magnesium has a lithium capacity of 4355 mAh/cm 3  with a volumetric expansion change of 100% with the intercalation of lithium (lithiated). The fully lithiated volumetric capacity of magnesium is 2178 mAh/cm 3 . On the other hand germanium has a capacity of 8623 mAh/cm 3  with a volumetric expansion change of 270% fully lithiated resulting in the germanium fully lithiated volumetric capacity of 2331 mAh/cm 3 . 
     The active anode  10  of the present invention consists of two anodic materials. One of the materials will be implemented as a structural matrix for supporting the second material. The second material is preferably in powder form and contained within oversized cavities within the first material. The first material should have good electronic conductivity and high lithium diffusivity. Magnesium and germanium are believed to be the preferred materials of the present invention. The anode  10  consists of a base or framework  11  of a first anodic material, preferably magnesium, which contains cavities  12  that include micron or submicron sized powder or particles (particulated)  13  of a second anodic material (which may be referred to herein as nano-powder), preferably germanium, although magnesium of a particle size of 30 to 40 microns is available today from Afla Aesar and US Research Nanomaterials, Inc. which is believed to be capable to working. The cavities should be large enough so that a fully lithiated nano-particle of the germanium material fits into or within the cavity  12  that contains it and does not apply significant stress to the framework. The first anodic material (magnesium) is the material with a lower lithium intercalation potential with respect to lithium, while the second anodic material (germanium) has a higher lithium intercalation potential. Magnesium has a potential of 0.1V while germanium has a potential of 0.3 to 0.4V. The magnesium is the material in contact with the electrolyte  16 . It is partially lithiated so in order to achieve enhanced lithium diffusion rates. The magnesium and germanium materials are selected because of their high electronic conductivities and a high lithium diffusivities. A high electronic conductivity is believed to be one wherein the electronic resistivity lower than 50 Ohm×cm. Materials such as silicon and aluminum have low diffusion rates; however, they are suitable for use as the second anodic material in the present invention because they can be employed as nano sized particles that would require minimal diffusion distance. Silicon in particular has low electronic conductivity; however, it works as the second anodic material when implemented as small size particles, less than on micron. 
     An essentially dimensionally stable anode structure is made possible based on the difference in intercalation potentials between the two materials. Germanium (second material) has an intercalation potential of approximately 0.3V to 0.4V, whereas magnesium (first material) has an intercalation potential of only 0.1 volts. Because of the difference in intercalation potentials, lithium will preferentially intercalate into the germanium. Given the high lithium diffusion rate in the range of 5×10 −7  cm 2 /sec of magnesium, the lithium will readily diffuse through the magnesium to reach the higher intercalation potential germanium. Because the process avoids extended residence time of lithium in the magnesium due to the higher intercalation potential of germanium, the level of lithiation of the magnesium remains essentially unchanged by lithium that passes through the magnesium on its was to the germanium and thereby the magnesium maintains relatively stable dimensions within the design capacity limit of the anode electrode. 
     To produce the anode  10 , the nano-particles of the germanium material are coated with an organic or polymer coating such as the polymer ethylene carbonate, i.e., the nano-particles of germanium material are embedded or encapsulated within an organic coating. Other polymer coatings may include PMMA and low molecular weight PEO materials. The volume and diameter of the organic coating mimics the volume expansion of the particle when fully lithiated, i.e., the diameter of the coated particle will generally equal the diameter of the cavity  12  and the diameter of the fully lithiated germanium nano-particle. It should be noted that preferably the cavity size is equal to or greater than the size of the coated particle to prevent stresses upon the framework during expansion. Germanium expands by approximately 270% when fully lithiated. The germanium material nano-particles have a preferred diameter size of approximately 0.07 to 5 microns, thus once lithiated the particles will expand along the diameter by approximately 60%. Germanium of a 0.07 micron size is available from Sky Spring Nanomaterials, inc. Accordingly, a 0.1 size particle should have a coating of approximately 0.03 (0.1 micron+two coatings of 0.03 along the diameter for a total diameter of 0.16 microns). The larger the particle size the thicker the coating material will need to be to provide for the volumetric expansion associated with being lithiated. The organic material coating may be produced by immersing the germanium particles within a melted polymer bath (maintained at about 35 degrees celsius for ethylene carbonate). The resulting material mixture is then solidified by freezing it at the temperature of liquid nitrogen and subsequently ground using a mortar and pestle or other suitable grinding technique to break the frozen contiguous solid into separately coated germanium nanoparticles. A milling process is used to separate the coated nano-particles from each other. 
     The nano-particles of the germanium material still coated by the solidified polymer are then mixed with particles of the magnesium powder material to form a mixture or composite active anode structure. The resulting composite anode structure is first pressed and then rolled through a roller to tightly bind the particles in order to form a composite layer or slab. The pressed composite layer/slab is then heated at approximately 400 degrees celsius in a vacuum so that the polymer coating is removed by sublimation/evaporation from the germanium nano-particle. The heating is done in a vacume, or alternatively an inert atmosphere, and below temperatures that support significant alloying between the germanium and magnesium materials. The removal of the polymer leaves a space or cavity  12  in the area previously occupied by the polymer coating. As previously stated, the resulting cavity  12  is sized to approximate the enlarged size of the germanium nano-particle once it increases volumetrically as a result of being lithiated. 
     Alternatively, the anode  10  consists of a base or framework  11  of a third anodic material, preferably germanium, which contains cavities  12  that include micron or submicron sized powder or particles (particulated)  13  of a fourth anodic material (which may be referred to herein as nano-powder), preferably magnesium. The cavities should be large enough so that a fully lithiated nano-particle of the magnesium material fits into or within the cavity  12  that contains it and does not apply significant stress to the framework. The third anodic material (germanium in this case) is the material with a higher lithium intercalation potential with respect to lithium, while the second anodic material (magnesium in this case) has a lower lithium intercalation potential. Magnesium has a potential of 0 to 0.1V with a plateau at about 0.5V while germanium has a potential of 0 to 0.4V with a plateau around 0.35V. The germanium is the material in contact with the electrolyte 16. It is fully lithiated so in order to achieve enhanced lithium diffusion rates and a lithium reaction potential at 0.1V or less. In this configuration, the anode can be cycled between 0.01V and 0.1V with very little change in volume of the germanium because it already fully lithiated. Lithium will diffuse through the germanium to the magnesium particles with in the pores of the germanium. 
     An essentially dimensionally stable anode structure is made possible based on the difference in intercalation potentials between the two materials. Germanium (third material) is fully lithiated well beyond its 0.35V plateau down to a range of 0.05V, whereas magnesium (fourth material) has a an intercalation plateau in the 0.5V range where it has significant intercalation capacity. During cycling, the germanium can accommodate only a small amount of additional lithium as the anode is cycled between about 0.02 and 0.1 volts where a, magnesium has a large capacity in this voltage range. The lithium will diffuse through the germanium and intercalate into the magnesium. Given the high lithium diffusion rate in the range of 5×10 −7  cm 2 /sec of germanium, the lithium will readily diffuse through the germanium under the intercalation potential of the magnesium. Because the level of lithiation of the germanium remains essentially unchanged by lithium that passes through the germanium on its was to the magnesium, the germanium, thereby, maintains relatively stable dimensions within the design capacity limit of the anode electrode. 
     Under this alternative construction, the nano-particles of the magnesium material are coated with an organic coating such as the polymer ethylene carbonate, i.e., the nano-particles of magnesium material are embedded or encapsulated within an organic coating. The volume and diameter of the organic coating mimics the volume expansion of the particle when fully lithiated, i.e., the diameter of the coated particle will generally equal the diameter of the cavity  12  and the diameter of the fully lithiated magnesium nano-particle. It should be noted that preferably the cavity size is equal to or greater than the size of the coated particle to prevent stresses upon the framework during expansion. Magnesium expands by approximately 100% when fully lithiated. 
     The nano-particles of the magnesium material still coated by the solidified polymer are then mixed with particles of the germanium powder material to form a mixture or composite active anode structure. The resulting composite anode structure is then pressed by being forced through a roller to tightly bind the particles in order to form a composite layer or slab. The pressed composite layer/slab is then heated to remove the polymer coating from the magnesium nano-particles. The heating is done in an inert atmosphere and below temperatures that support significant alloying between the germanium and magnesium materials. The removal of the polymer leaves a space or cavity  12  in the area previously occupied by the polymer coating. As previously stated, the resulting cavity  12  is sized to approximate the enlarged size of the magnesium nano-particle once it increases volumetrically as a result of being lithiated. 
     Once the anode is produced, it is incorporated into a battery cell having a cathode  15 , an electrolyte  16 , a cathode anode current collector and an anode current collector. The cathode is made of a lithium intercalation compound. The electrolyte is preferably made of either a solid lithium ion conducting electrolyte such as lithium phosphorus oxynitride, Li x PO y N z , a lithium lanthanum zirconium oxide (LiLaZrO), a polymer based lithium ion conducting electrolyte, or a liquid lithium ion conducting electrolyte. Finally, an anode current collector and cathode current collector are preferably made of copper or nickel. 
     It should be understood that as used herein the term particle or each particle may include more than one particle and is not intended to be limited to only one particle, as particles may stick together to form a conglomerate, a particle comprised of multiple pieces or particles, or simply two or more particles in close proximity to each other. 
     It thus is seen that an anode and method of producing an anode is now provided which restricts the damage associated with the anode being lithiated. It should of course be understood that many modifications may be made to the specific preferred embodiment described herein, in addition to those specifically recited herein, without departure from the spirit and scope of the invention as set forth in the following claims.