Patent Publication Number: US-6706447-B2

Title: Lithium metal dispersion in secondary battery anodes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to commonly owned copending provisional application Ser. No. 60/257,994, filed Dec. 22, 2000, and claims the benefit of the earlier filing date of this application under 35 U.S.C. §119(e). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to secondary batteries having high specific capacities and particularly to anodes for secondary batteries comprising a host material such as a carbonaceous material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. 
     BACKGROUND OF THE INVENTION 
     Lithium and lithium-ion secondary or rechargeable batteries have recently found use in certain applications such as in cellular phones, camcorders, and laptop computers, and even more recently, in larger power applications such as in electric vehicles and hybrid electric vehicles. It is preferred in these applications that the secondary batteries have the highest specific capacity possible but still provide safe operating conditions and good cycleability so that the high specific capacity is maintained in subsequent recharging and discharging cycles. 
     Although there are various constructions for secondary batteries, each construction includes a positive electrode (or cathode), a negative electrode (or anode), a separator that separates the cathode and anode, and an electrolyte in electrochemical communication with the cathode and anode. For secondary lithium batteries, lithium ions are transferred from the anode to the cathode through the electrolyte when the secondary battery is being discharged, i.e., used for its specific application. During this process, electrons are collected from the anode and pass to the cathode through an external circuit. When the secondary battery is being charged or recharged, the lithium ions are transferred from the cathode to the anode through the electrolyte. 
     Historically, secondary lithium batteries were produced using non-lithiated compounds having high specific capacities such as TiS 2 , MoS 2 , MnO 2  and V 2 O 5 , as the cathode active materials. These cathode active materials were coupled with a lithium metal anode. When the secondary battery was discharged, lithium ions were transferred from the lithium metal anode to the cathode through the electrolyte. Unfortunately, upon cycling, the lithium metal developed dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990&#39;s in favor of lithium-ion batteries. 
     Lithium-ion batteries typically use lithium metal oxides such as LiCoO 2  and LiNiO 2  as cathode active materials coupled with a carbon-based anode. In these batteries, the lithium dendrite formation on the anode is avoided thereby making the battery safer. However, the lithium, the amount of which determines the battery capacity, is totally supplied from the cathode. This limits the choice of cathode active materials because the active materials must contain removable lithium. Furthermore, the delithiated products corresponding to LiCoO 2  and LiNiO 2  that are formed during charging (e.g. Li x CoO 2  and Li x NiO 2  where 0.4&lt;x&lt;1.0) and overcharging (i.e. Li x CoO 2  and Li x NiO 2  where x&lt;0.4) are not stable. In particular, these delithiated products tend to react with the electrolyte and generate heat, which raises safety concerns. 
     SUMMARY OF THE INVENTION 
     The present invention is a secondary battery having a high specific capacity and good cycleability and that operates safely. In accordance with the invention, the freshly prepared, secondary battery includes an anode that is formed of a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material. Preferably, the lithium metal is a finely divided lithium powder and more preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials, Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. Preferably, the host material comprises a carbonaceous material and more preferably comprises graphite. 
     The freshly prepared, secondary batteries of the invention include a positive electrode including an active material, a negative electrode comprising a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material, a separator separating the positive electrode and the negative electrode and an electrolyte in communication with the positive electrode and the negative electrode. Preferably, the cathode active material is a compound that can be lithiated at an electrochemical potential of 2.0 to 5.0 V versus lithium. For example, the cathode active material can be MnO 2 , V 2 O 5  or MoS 2 , or a mixture thereof. The lithium metal in the anode is preferably a finely divided lithium powder and more preferably has a mean particle size of less than about 20 microns. The host material comprises one or more materials selected from the group consisting of carbonaceous materials, Si, Sn, tin oxides, composite tin alloys, transition metal oxides, lithium metal nitrides and lithium metal oxides. Preferably, the host material in the negative electrode comprises a carbonaceous material and, more preferably, comprises graphite. The amount of lithium metal present in the negative electrode is preferably no more than the maximum amount sufficient to intercalate in, alloy with, or be absorbed by the host material in the negative electrode. For example, if the host material is carbon, the amount of lithium is preferably no more than the amount needed to make LiC 6 . 
     The present invention also includes a method of preparing a freshly prepared anode for a secondary battery that includes the steps of providing a host material that is capable of absorbing and desorbing lithium in an electrochemical system, dispersing lithium metal in the host material and forming the host material and the lithium metal dispersed therein into an anode. The lithium metal and the host material is preferably mixed together with a non-aqueous liquid to produce a slurry and then applied to a current collector and dried to form the anode. Alternatively, the anode can be formed by chemical means by immersing the host material in a suspension of lithium metal in a non-aqueous liquid, and then formed into an anode. 
     The present invention further includes a method of operating a secondary battery. First, a freshly prepared, secondary battery is provided that includes a positive electrode including an active material, a negative electrode comprising a host material capable of absorbing and desorbing lithium in an electrochemical system and lithium metal dispersed in the host material, a separator for separating the positive electrode and the negative electrode, and an electrolyte in communication with the positive electrode and the negative electrode. In particular, the secondary battery is manufactured with lithium metal dispersed in the host material of the anode. The freshly assembled battery is in a charged state and more preferably is in a fully charged state (with all the removable lithium present in the anode of the freshly prepared battery). The freshly prepared secondary battery is initially discharged by transmitting lithium ions from the negative electrode to the positive electrode through the electrolyte. The secondary battery can then be charged or recharged by transmitting lithium ions from the positive electrode to the negative electrode through the electrolyte and then discharged again by transmitting lithium ions from the negative electrode to the positive electrode through the electrolyte. The charging and discharging steps can occur for numerous cycles while maintaining the high specific capacities of the cathode active materials and maintaining safe operating conditions. 
     These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawing, which describe both the preferred and alternative embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a simplified secondary battery construction including a cathode, anode, separator and electrolyte, in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     In the drawings and the following detailed description, preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawing. 
     As illustrated in FIG. 1, the present invention is a secondary battery  10  that comprises a positive electrode or cathode  12 , a negative electrode or anode  14 , a separator  16  for separating the positive electrode and the negative electrode, and an electrolyte in electrochemical communication with the positive electrode and the negative electrode. The secondary battery  10  also includes a current collector  20  that is in electrical contact with the cathode and a current collector  22  that is in electrical contact with the anode. The current collectors  20  and  22  are in electrical contact with one another through an external circuit (not shown). The secondary battery  10  can have any construction known in the art such as a “jelly roll” or stacked construction. 
     The cathode  12  is formed of an active material, which is typically combined with a carbonaceous material and a binder polymer. The active material used in the cathode  12  is preferably a material that can be lithiated at a useful voltage (e.g. 2.0 to 5.0 V versus lithium). Preferably, non-lithiated materials such as MnO 2 , V 2 O 5  or MoS 2 , or mixtures thereof, can be used as the active material, and more preferably, MnO 2  is used. However, lithiated materials such as LiMn 2 O 4  that can be further lithiated can also be used. The non-lithiated active materials are preferred because they generally have higher specific capacities than the lithiated active materials in this construction and thus can provide increased power over secondary batteries that include lithiated active materials. Furthermore, because the anode  14  includes lithium as discussed below, it is not necessary that the cathode  12  include a lithiated material for the secondary battery  10  to operate. The amount of active material provided in the cathode  12  is preferably sufficient to accept the removable lithium metal present in the anode  14 . For example, if MnO 2  is the cathode active material, then one mole of MnO 2  is preferably present in the cathode  12  per mole of lithium in the anode  14  to produce LiMnO 2  in the cathode upon discharge. 
     When cathode active materials are used that can be lithiated such as those described above, the removable lithium that is cycled in the battery is fully provided by the anode  14  and the battery is assembled or prepared in a fully charged state, as is preferred. Nevertheless, the cathode  12  can also include a minor amount of one or more lithiated active materials (e.g. LiCoO 2  or LiNiO 2 ) that do not further absorb lithium at a voltage between 2.0V and 5.0V and the battery can still be provided in a primarily charged state. In this event, the cathode preferably has less than 50% (molar) and more preferably less than 10% (molar) of the lithiated material (e.g. LiCoO 2  or LiNiO 2 ) as the active material. Because LiCoO 2  and LiNiO 2  do not further absorb lithium, the presence of these materials in the cathode  12  does not reduce the amount of cathode active material needed to accept the removable lithium from the anode  14 . 
     The anode  14  is formed of a host material  24  capable of absorbing and desorbing lithium in an electrochemical system with lithium metal  26  dispersed in the host material. For example, the lithium present in the anode  14  can intercalate in, alloy with or be absorbed by the host material when the battery (and particularly the anode) is recharged. The host material includes materials capable of absorbing and desorbing lithium in an electrochemical system such as carbonaceous materials; materials containing Si, Sn, tin oxides or composite tin alloys; transition metal oxides such as CoO; lithium metal nitrides such as Li 3−x Co x N where 0&lt;x&lt;0.5, and lithium metal oxides such as Li 4 Ti 5 O 12 . The lithium metal  26  is preferably provided in the anode  14  as a finely divided lithium powder. In addition, the lithium metal  26  preferably has a mean particle size of less than about 20 microns, more preferably less than about 10 microns. The lithium metal can be provided as a pyrophoric powder or as a stabilized low pyrophorosity powder, e.g., by treating the lithium metal powder with CO 2 . 
     The anode  14  is typically capable of reversibly lithiating and delithiating at an electrochemical potential relative to lithium metal of from greater than 0.0 V to less than or equal to 1.5. If the electrochemical potential is 0.0 or less versus lithium, then the lithium metal will not reenter the anode  14  during charging. Alternatively, if the electrochemical potential is greater than 1.5 V versus lithium then the battery voltage will be undesirably low. Preferably, the amount of lithium metal  26  present in the anode  14  is no more than the maximum amount sufficient to intercalate in, alloy with, or be absorbed by the host material in the anode  14  when the battery is recharged. For example, if the host material  24  is carbon, the amount of lithium  26  is preferably no more than the amount sufficient to make LiC 6 . In other words, the molar ratio of lithium to carbon in the anode is preferably no more than 1:6. 
     In accordance with the invention, the anode  14  can be prepared by providing a host material that is capable of absorbing and desorbing lithium in an electrochemical system, dispersing lithium metal in the host material, and forming the host material and the lithium metal dispersed therein into an anode. Preferably, the lithium metal and the host material are mixed with a non-aqueous liquid such as tetrahydrofuran (THF) and a binder, and formed into a slurry. The slurry is then used to form the anode  14 , for example, by coating the current collector  22  with the slurry and then drying the slurry. The lithium metal can also be provided in the anode by immersing the host material in a suspension containing lithium metal in a non-aqueous liquid such a hydrocarbon solvent (e.g. hexane). The lithium metal used in the suspension is preferably a finely divided lithium powder as discussed above. The host material can be formed into the shape of the anode and then dipped into the lithium metal suspension or it can be combined with the lithium metal suspension to form a slurry and then applied to the current collector and dried to form the anode. The non-aqueous liquid used to form the suspension can be removed by drying the anode (e.g. at an elevated temperature). No matter what method is used, the lithium metal is preferably distributed as well as possible into the host material. Accordingly, as discussed above, the lithium metal  26  preferably has a mean particle size of less than about 20 microns, more preferably less than about 10 microns. 
     The host material  24  in the anode  14  can include one or more materials capable of absorbing and desorbing lithium in an electrochemical system such as carbonaceous materials; materials containing Si, Sn, tin oxides or composite tin alloys; transition metal oxides such as CoO; lithium metal nitrides such as Li 3−x Co x N where 0&lt;x&lt;0.5; and lithium metal oxides such as Li 4 Ti 5 O 12 . Preferably, as mentioned above, the host material  24  preferably includes graphite. In addition, the host material  24  preferably includes a small amount of carbon black (e.g. less than 5% by weight) as a conducting agent. 
     As shown in FIG. 1, the cathode  12  is separated from the anode  14  by an electronic insulating separator  16 . Typically, the separator  16  is formed of a material such as polyethylene, polypropylene, or polyvinylidene fluoride (PVDF). 
     The secondary battery  10  further includes an electrolyte that is in electrochemical communication with the cathode  12  and anode  14 . The electrolyte can be non-aqueous liquid, gel or solid and preferably comprises a lithium salt, e.g., LiPF 6 . The electrolyte is provided throughout the battery  10  and particularly within the cathode  12 , anode  14  and separator  16 . Typically, the electrolyte is a liquid, and the cathode  12 , anode  14  and separator  16  are porous materials that are soaked in the electrolyte to provide electrochemical communication between these components. 
     As mentioned above, the battery  10  includes current collectors  20  and  22 , which are used to transmit electrons to an external circuit. Preferably, the current collector  20  is made of aluminum foil and current collector  22  is made of copper foil. 
     The battery  10  of the invention can be prepared by methods known in the art and preferably has a layer thickness within the following ranges (from left to right in FIG.  1 ): 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
               
                   
                 Layer 
                 thickness 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Current collector (20) 
                 20-40 μm 
                   
               
               
                   
                 Cathode (12) 
                 70-100 μm 
               
               
                   
                 Separator (16) 
                 25-35 μm 
               
               
                   
                 Anode (14) 
                 70-100 μm 
               
               
                   
                 Current collector (22) 
                 20-40 μm 
               
               
                   
                   
               
            
           
         
       
     
     The battery  10  also includes an electrolyte dispersed throughout the cathode  12 , anode  14  and separator  16 , and a casing (not shown). 
     In operation, the freshly prepared secondary battery  10  is initially in a charged state, more preferably a fully charged state, and is initially discharged by transmitting lithium ions from the anode  14  to the cathode  12  through the electrolyte. At the same time, electrons are transmitted from the anode  14  to the cathode  12  through the current collector  22 , the external circuit, and the current collector  20 . The secondary battery  10  can then be charged or recharged by transmitting lithium ions from the cathode  12  to the anode  14  through the electrolyte and then discharged again as discussed above. The charging and discharging steps can occur for numerous cycles while maintaining the high specific capacities of the cathode active materials and maintaining safe operating conditions. 
     The secondary battery  10  can be used for various types of applications. For example, the secondary battery can be used in portable electronics such as cellular phones, camcorders, and laptop computers, and in large power applications such as for electric vehicles and hybrid electric vehicles. 
     The present invention provides secondary batteries having a high specific capacity, safe operating conditions and good cycleability. In particular, because lithium metal is provided in the anode, non-lithiated materials can be used as the preferred cathode active material in the secondary battery. These non-lithiated materials have higher specific capacities than the lithiated materials presently used in lithium-ion batteries. Unlike traditional lithium secondary batteries having non-lithiated cathode active materials and metallic lithium anodes, it has been discovered that secondary batteries produced using non-lithiated cathode active materials combined with the anodes of the invention operate safely and do not generate lithium dendrites upon cycling. Furthermore, the secondary batteries of the present invention are safer to operate than lithium-ion batteries, which become unstable when lithium is removed from the cathode during charging. In particular, because the cathode active material in the secondary batteries of the invention is typically in a fully charged state when the battery is freshly prepared, it is more stable then the cathode materials used in lithium-ion batteries. Moreover, the batteries of the invention can be charged and discharged numerous times while maintaining safe operating conditions and the high specific capacities of the cathode active materials. 
     It is understood that upon reading the above description of the present invention and reviewing the accompanying drawings, one skilled in the art could make changes and variations therefrom. These changes and variations are included in the spirit and scope of the following appended claims.