Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-06-20
2003-04-29
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231900, C429S215000
Reexamination Certificate
active
06555271
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process for producing an anode for a lithium-ion battery and the anode itself. More particularly, the invention relates to the use of a generally continuous flexible graphite sheet as the anode of a lithium-ion battery. Moreover, a process is presented involving laminating particles of exfoliated graphite to a metallic substrate, so the particles of exfoliated graphite form a generally continuous sheet of graphite having a thickness of no more than about 350 microns, and the use of the resulting laminate as an anode for a lithium-ion battery.
BACKGROUND OF THE INVENTION
Lithium-ion electrochemical cells useful for electrical storage usually consist of a lithium anode and a cathode formed from an electrochemically active material that can take up ions of the metal. An electrolyte containing ions of the metal is placed in contact with the anode and the cathode. During discharge of the cell, metal ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in the release of electrical energy.
Provided that the reaction between the metal ions and the cathode-active material is reversible, applying electrical energy to the cell can reverse the process. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the electrochemical cell art as “secondary” cells.
Various proposals have been made for increasing the energy densities of electrochemical cells through the application of highly reactive metals (e.g., the alkali metals) as anodic materials. Lithium metal has received the most attention in this regard due to its low atomic weight and because it is the most electronegative of all the metals. Electrochemical cells containing lithium or other alkali metal anodes are generally provided with non-aqueous electrolytic solutions in which electrically conductive salts are dissolved in organic aprotic solvents. Among the numerous electrically conductive salts which have heretofore been employed in non-aqueous electrolytic solutions are the alkali metal salts of such anions as the halides, halates, perhalates, haloaluminates, haloarsenates, halophosphates, haloacetates, phosphates, thiocyanates, sulfides, sulfates, cyanides, picrates, acetylacetonates, fluoroborates, hydrides, borohydrides, and so forth. These electrically conductive salts have been dissolved in a wide variety of organic aprotic solvents including Lewis bases such as the tertiary amines; amides and substituted amides such as formamide; nitriles such as acetonitrile, propionitrile and benzonitrile; open chain and cyclic esters such as propylene carbonate, alkyl acylates and butyrolactone; oxysulfur compounds such as dimethylsulfoxide, dimethylsulfite and tetramethylene sulfone; and, open chain and cyclic ethers such as the poly(alkyleneoxy) glycols, dioxane and the substituted dioxanes, dioxolane, tetrahydrofuran and tetrahydropyran.
The use of non-aqueous electrolytic solutions does not eliminate all problems associated with the use of lithium metal in the anodes of electrochemical cells. For example, the problem of avoiding contact of the lithium metal with moisture during assembly of the cell remains. Hence, efforts have been made to develop a rechargeable lithium cell containing no metallic lithium. As a result of such efforts, cells have been developed using, instead of a lithium metal anode, an anode comprising lithium intercalated or inserted in a material (an “intercalation host”) that operates near the potential of lithium (see U.S. Pat. N
0
. 5,069,683).
Intercalation involves the formation of ionic bonds between the lithium atoms and the ions of the intercalation host. Graphite and coke are known intercalation hosts for lithium. In graphite, each lithium atom is associated with six carbon atoms of the crystalline graphite structure. Such “LiC
6
” combinations are regarded as the ideal for lithium intercalation in carbon. During cell operation, the lithium ion is released from the intercalation host and migrates into the cathode material (e.g., crystalline manganese dioxide) that can also reversibly incorporate lithium. Cells wherein both electrodes can reversibly incorporate lithium are known as “rocking chair” (or “lithium ion”) batteries. They have been called the rocking chair batteries because lithium ions “rock” back and forth between the electrodes during the charge/discharge cycles.
The output voltage of cells of the rocking chair type is determined by the difference between the electrochemical potential of lithium within the two electrodes. It is important to have, as the positive and negative electrodes, materials that can reversibly intercalate (or otherwise retain) lithium at high and low voltages, respectively. Among the host materials proposed for lithium-ion battery anodes are WO
2
, MoO
2
, Mo
6
Se
6
or carbon (e.g., coke or graphite), with the latter providing the best compromise between large specific capacity and reversible cycling behavior. However, in rocking chair cells a price is paid in terms of average output voltage and energy density when compared to a lithium metal cell; thus strongly oxidizing compounds (i.e., compounds which reversibly incorporate lithium above 4 volts) must be used as the positive electrode. LiNiO
2
, LiCoO
2
, and LiMn
2
O
4
satisfy this requirement. These lithium-bearing positive electrode (cathode) materials are not moisture sensitive and can be handled in ambient atmospheres as can lithium-free carbon anode materials. The rocking chair (or lithium-ion cell) is assembled in its discharged state, where the output voltage is close to zero, and needs to be charged prior to use.
Replacing lithium metal anodes with lithium ion intercalation host anodes removes the restrictions lithium metal places upon cell design and choice of electrolytes and also the adverse effect lithium metal places upon cycling performance and safety in the finished cell. In particular, highly graphitic carbonaceous materials are very suitable lithium intercalation hosts because highly graphitic carbonaceous materials (particularly graphite) are inexpensive, non-toxic and are capable of incorporation into electrochemical cells having relatively high specific capacities.
Rechargeable (also referred to as secondary) lithium-ion batteries are increasingly used for portable electronics, such as laptop computers and cellular phones. Typically, the anode of a lithium-ion battery is made by coating graphite powder on both sides of a thin copper foil, or expanded copper mesh material. The graphite coating can be applied to the copper foil by using a slurry consisting of an organic solvent, graphite powder and an inert, non-conductive binder. A typical formulation for the slurry utilizes an about 50/50 mixture of organic solvent and graphite powder along with up to about 10 parts or more by weight of binder. After evaporation of the solvent, the resulting coating of graphite can be as thin as about 100 microns. The resulting graphite-coated copper mesh is then itself laminated to separator membranes and the battery cathode to complete the battery. The overall laminate is then slit to width and cut to size prior to insertion in the battery case.
The anodes of this type for lithium-ion batteries have severe drawbacks, however. It is often found that loss of interparticle contact in the graphite layer during cycling of the battery leads to capacity fading. It is believed that the swelling and contraction of the graphite because of intercalation and deintercalation causes this contact loss, possibly because of the presence of binder material, which creates spacing between the graphite particles.
What is desired, therefore, is an anode for a lithium-ion battery which uses graphite as a lithium intercalation host, but which reduces or eliminates capacity fading due to contact loss. The preferred anode has superior permeability to lithium
Greinke Ronald Alfred
Krassowski Daniel Witold
Phillips Neal David
Cartiglia James R.
Chaney Carol
Graftech Inc.
Yuan Dah-Wei D.
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