Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-01-07
2003-07-29
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S231100, C429S231900, C429S231500, C429S217000
Reexamination Certificate
active
06599662
ABSTRACT:
BACKGROUND OF THE INVENTION
Metals that form alloys or intermetallic compounds with lithium have been considered for use as alternatives to lithium metal or carbon electrodes in lithium electrochemical devices such as lithium ion batteries. Examples of such metals include Al, B, Bi, Cd, In, Ga, Pb, Sb, Si, Sn, Zn, or mixtures of these metals, as reviewed by R. A. Huggins (J. Power Sources, 26 109-120 (1989) and D. Fateaux and R. Koksbang (
J. Appl. Electrochemistry,
23, p. 1 (1993)).
Even those metals that do alloy with lithium to high concentrations, for example reaching a lithium to metal molar ratio of at least one, have severe limitations as an electrode material, as discussed in the article by D. Fateaux and R. Koksbang. A major limitation is the fact that the formation of a highly lithiated alloy or compound from a metal is accompanied by a large volume expansion. The volume expansion of the alloy or compound relative to the molar volume of the starting metal increases greatly with lithium concentration, such that the compositions that are most desirable due to having the highest lithium storage have the largest expansion. Amongst numerous metals that can alloy with lithium to a high concentration, the lithiated alloy can easily expand to a molar volume that is 1.5 to 5 times that of the starting metal. It is widely recognized by those skilled in the art that this volume expansion, and the subsequent volume contraction which occurs when lithium is electrochemically removed, is undesirable as it often causes mechanical fracture of the metal electrode, resulting in decreased lithium storage capability. The lithium-concentration-dependent volume change is particularly detrimental in electrochemical devices that undergo cycling, in which lithium is repeatedly removed from and inserted into the metal electrode. A rechargeable lithium battery is one such device.
Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, (
Science,
276 1395 (1997)), and T. Kubota and M. Tanaka (U.S. Pat. No. 5,654,114) have described lithium battery anodes based on metal oxides, primarily tin oxide, that are capable of reversible charge capacity of approximately 600 mAh/g (2200 mAh/cm
3
). The primary drawback of this type of material has been the high first-cycle irreversible capacity loss, typically 400-600 mAh/g, due to the consumption of lithium in oxide-forming reactions during the first lithium insertion. These anodes are prepared as an oxide, not as a metal or a metal-oxide composite, and do not demonstrate the advantageous features which result from the processes of partial oxidation or reduction as described herein below prior to being assembled as a battery.
O Mao et al., (
Electrochem. Sol. St. Lett.,
2[2]3-5 (1999)) have described Sn/Fe/C composites of several nanometer individual phase particle size produced by mechanical alloying. The composites demonstrate a lower first-cycle irreversible capacity loss and also lower reversible capacity on a weight and volume basis (200 mAh/g, 1700 mAh/cm
3
) than the tin oxides. These electrode materials have an inactive phase of high density, detracting from their capacity, and rely upon fine size alone for their improved properties relative to bulk metals. They do not utilize the beneficial methods of partial reduction or oxidation in their preparation.
Thus, the electroactive metal oxides and metals of the prior art do not provide a mechanically robust material having the desired reversible charge capacity for use in cyclically operated electrochemical devices. New materials and methods of their manufacture are needed.
SUMMARY OF THE INVENTION
One objective of the invention is to provide an electroactive material having a robust microstructure that can accommodate the large volume changes associated with electrochemical insertion or removal of lithium.
Another object of the invention is to provide an electrode and other electrochemical devices such as rechargeable batteries which contain a mechanically robust electroactive material and which are resistant to mechanical failure of the electrode.
It is also an object of the invention to provide an electroactive material having a high reversible charge capacity for us in cyclically operated electrochemical devices such as a rechargeable battery.
Still another object of the invention is to provide a method of making an electroactive material which is sufficiently mechanically robust to withstand the internal stresses experienced during the large volume changes which occur during electrochemical insertion or removal of lithium.
These and other objectives are realized in the materials and methods of preparation comprising the invention, as is substantially described herein.
In one aspect of the invention, a composite material is provided having a first material that is an elemental metal, metal alloy, metal oxide, or other metal compound, selected so that it is able to alloy with lithium or other element such as hydrogen, potassium, sodium and the like, and prepared in a dispersed one-, two- or three-dimensional form. The first material is intimately mixed with or dispersed within a second material that provides mechanical support for the first material and, optionally, is substantially conductive to electrons or electron holes or lithium ions. The composite material may possess void spaces, or it may experience internal stress. Either feature may be advantageous in increasing the reversible charge capacity of the material.
By the term “dispersed,” it is understood that the smallest dimension of the metal or metal compound that is less than 10 micrometers, preferably less than 5 micrometers, and more preferably still less than 1 micrometer. The dispersed material is substantially surrounded by the supportive second material.
By the term “electroactive,” it is understood to mean that during operation of the device the metal, alloy, or compound stores electrical charge by forming an alloy with lithium or other element such as hydrogen, potassium, sodium and the like.
In a preferred embodiment of the invention, the composite material includes a first material that is an elemental metal or metal alloy, selected so that it is able to alloy with lithium or other element such as potassium, sodium and the like, and prepared in a dispersed one-, two- or three-dimensional form. The first material is intimately mixed with or dispersed within a metal oxide that provides mechanical support for the first material and, optionally, is substantially conductive to electrons or electron holes or lithium ions. The metal oxide may be glassy or crystalline. Crystalline metal oxide systems are currently preferred since they simplify the selection process, their ionic and electronic transport properties being generally known. However, glassy oxides can have equally advantageous transport properties. In preferred embodiments, the crystalline metal oxide is a normal spinel, inverse spinel, or disordered spinel structure compound Me
II
c
X
d
where 0.5<c/d<1, or a rutile structure compound Me
II
x
O
y
where 0.5<x/y<1 or an ordered or disordered derivative of this structure type, or a corundum or ilmenite structure compound Me
II
x
O
y
where 0.5<x/y<1 or an ordered or disordered derivative of this structure type, or a perovskite structure compound Me
II
x
O
y
where 0.5<x/y<1 or an ordered or disordered derivative of this structure type.
In other preferred embodiments, the crystal structure is selected to promote ion, e.g., lithium ion, transport and electronic conductance.
In preferred embodiments, the composite may be prepared by the process hereafter referred to as “partial reduction,” “preferential reduction,” or “internal reduction,” or the process hereafter known as “partial oxidation.” These methods of preparation confer numerous benefits as described herein. Partial reduction or oxidation refers to processes in which only a portion of the material is reduced or oxidized, respectively. By internal reduction, the term is understood to mean a process in which the reduction product
Ceder Gerbrand
Chiang Yet-Ming
Limthongkul Pimpa
Chaney Carol
Dove Tracy
Massachusetts Institute of Technology
Wolf Greenfield & Sacks P.C.
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