Patent Application
20180212270
The present disclosure relates to lithium metal electrodes including a graphite-modified surface for a lithium battery, and the methods of making the same.
Various methods have been employed to make battery electrodes and/or deposit materials useful in battery electrodes. For example, methods are described in U.S. Pat. No. 5,720,780 (Liu et al.), U.S. Pat. No. 6,589,299 (Missling et al.), U.S. Pat. No. 6,939,383 (Eastin et al.), U.S. Patent Application Publication 2010/0055569 (Divigalpitiya et al.), and JP Pub. No. 2009 252629 (Toshiya et al.).
Briefly, in one aspect, the present disclosure describes an electrochemical device component including a lithium metal electrode having a first major surface, and a surface layer disposed on the first major surface of the lithium metal electrode. The surface layer has a composition including a compound of graphite and lithium. The surface layer is electrically conductive and lithium-ion conductive. The surface layer is chemically compatible with the lithium metal on a first side in contact with the lithium metal electrode, and chemically compatible with electrolyte environments on a second side of the surface layer.
In another aspect, the present disclosure describes a method of making an electrochemical device component. The method includes providing a lithium metal electrode having a first major surface and treating the first major surface of the lithium metal electrode to form a surface layer. The surface layer has a composition including a compound of graphite and lithium. The surface layer is electrically conductive and lithium-ion conductive. The surface layer is chemically compatible with the lithium metal on a first side in contact with the lithium metal electrode, and chemically compatible with electrolyte environments on a second side of the surface layer.
Various unexpected results and advantages are obtained in exemplary embodiments of the disclosure. One such advantage of exemplary embodiments of the present disclosure is that the surface layer is formed by the reaction between a graphite coating and a lithium metal electrode, and the interface between the surface layer and the lithium metal electrode can be a coherent, and substantially pinhole-free interface that exhibits superior electrical, electrochemical, and mechanical properties.
Various aspects and advantages of exemplary embodiments of the disclosure have been summarized. The above Summary is not intended to describe each illustrated embodiment or every implementation of the present certain exemplary embodiments of the present disclosure. The Drawings and the Detailed Description that follow more particularly exemplify certain preferred embodiments using the principles disclosed herein.
The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying figures, in which:
In the drawings like reference numerals indicate like elements. While the above-identified drawing, which may not be drawn to scale, sets forth various embodiments of the present disclosure, other embodiments are also contemplated, as noted in the Detailed Description. In all cases, this disclosure describes by way of representation of exemplary embodiments and not by express limitations. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of this disclosure.
For the following Glossary of defined terms, these definitions shall be applied for the entire application, unless a different definition is provided in the claims or elsewhere in the specification.
Certain terms are used throughout the description and the claims that, while for the most part are well known, may require some explanation. It should be understood that:
The term “coherent interface” used herein refers to an interface between a lithium metal electrode and a surface layer across which the microstructure thereof is continuous resulting from the reaction between the metal electrode surface and an initial material coating disposed thereon.
The term “graphite” refers to a crystalline form of carbon having a layered, planar structure. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of about 0.142 nm, and the distance between planes is about 0.335 nm. The graphite described herein is capable of reacting with lithium metal, and forming a lithium graphite intercalation compound (Li-GIC) where lithium metal ions are intercalated into the layered structure of graphite.
The term “lithium metal” refers to lithium element in the form of metal that is capable of reacting with a graphite coating in contact therewith at room temperatures to form a lithium graphite intercalation compound (Li-GIC).
As used herein, “buffing” refers to any operation in which a pressure normal to a subject surface (e.g., a major surface of a metallic substrate) coupled with movement (e.g., rotational, lateral, combinations thereof) in a plane parallel to said subject surface is applied.
The term “adjoining” with reference to a particular layer means joined with or attached to another layer, in a position wherein the two layers are either next to (i.e., adjacent to) and directly contacting each other, or contiguous with each other but not in direct contact (i.e., there are one or more additional layers intervening between the layers).
By using terms of orientation such as “atop”, “on”, “over,” “covering”, “uppermost”, “underlying” and the like for the location of various elements in the disclosed coated articles, we refer to the relative position of an element with respect to a horizontally-disposed, upwardly-facing substrate. However, unless otherwise indicated, it is not intended that the substrate or articles should have any particular orientation in space during or after manufacture.
By using the term “overcoated” to describe the position of a layer with respect to a substrate or other element of an article of the present disclosure, we refer to the layer as being atop the substrate or other element, but not necessarily contiguous to either the substrate or the other element.
By using the term “separated by” to describe the position of a layer with respect to other layers, we refer to the layer as being positioned between two other layers but not necessarily contiguous to or adjacent to either layer.
The terms “about” or “approximately” with reference to a numerical value or a shape means +/− five percent of the numerical value or property or characteristic, but expressly includes the exact numerical value. For example, a viscosity of “about” 1 Pa-sec refers to a viscosity from 0.95 to 1.05 Pa-sec, but also expressly includes a viscosity of exactly 1 Pa-sec. Similarly, a perimeter that is “substantially square” is intended to describe a geometric shape having four lateral edges in which each lateral edge has a length which is from 95% to 105% of the length of any other lateral edge, but which also includes a geometric shape in which each lateral edge has exactly the same length.
The term “substantially” with reference to a property or characteristic means that the property or characteristic is exhibited to a greater extent than the opposite of that property or characteristic is exhibited. For example, a substrate that is “substantially” transparent refers to a substrate that transmits more radiation (e.g. visible light) than it fails to transmit (e.g. absorbs and reflects). Thus, a substrate that transmits more than 50% of the visible light incident upon its surface is substantially transparent, but a substrate that transmits 50% or less of the visible light incident upon its surface is not substantially transparent.
As used in this specification and the appended embodiments, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to fine fibers containing “a compound” includes a mixture of two or more compounds. As used in this specification and the appended embodiments, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
As used in this specification, the recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.8, 4, and 5).
Unless otherwise indicated, all numbers expressing quantities or ingredients, measurement of properties and so forth used in the specification and embodiments are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached listing of embodiments can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings of the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claimed embodiments, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Electrochemical cells, such as lithium-ion electrochemical cells include a negative electrode and a positive electrode where lithium (Li) ions can be transferred between the positive and negative electrode through a lithium-ion-conducting electrolyte. In Li-ion batteries, Li intercalation compounds can be used as positive electrode (cathode) materials, and graphite can be used as negative electrode (anode) materials where Li ions can be intercalated into its layered structure. In Li metal batteries, the positive electrode (cathode) can include, for example, Li intercalation compounds, sulphur, or air electrodes, and the negative electrode (anode) is lithium (Li) metal. Li metal has specific properties such as a high specific capacity, low density, and a very negative electrochemical potential, all of which are advantageous for electrodes for rechargeable batteries or electrochemical cells. The main challenges for developing lithium metal batteries include, for example, the growth of Li dendrites during repeated charge/discharge processes (e.g., lithium plating and stripping), and excessive reactivity with organic electrolytes leading to the development of mossy lithium metal deposits. The problems can lead to dendritic electrical shorts in rechargeable batteries or cell and thermal runaway.
The present disclosure describes rechargeable batteries or electrochemical cells based on lithium (Li) metal electrodes including a graphite-modified surface that can overcome the challenges discussed above. In some embodiments, electrochemical device components are provided. The electrochemical device components can include a lithium metal electrode having a first major surface. A surface layer is disposed on the first major surface of the lithium metal. The surface layer can have a composition including a compound of graphite and lithium. The microstructure across the interface between the surface layer and the lithium metal can be continuous, which may result from a reaction between the lithium metal and a graphite coating thereon. The surface layer can be electrically conductive, and lithium-ion conductive. In addition, the surface layer can be chemically compatible with the lithium metal on a first side in contact with the lithium metal electrode, and chemically compatible with electrolyte environments on a second side opposite the first side.
Various exemplary embodiments of the disclosure will now be described with particular reference to the Drawings. Exemplary embodiments of the present disclosure may take on various modifications and alterations without departing from the spirit and scope of the disclosure. Accordingly, it is to be understood that the embodiments of the present disclosure are not to be limited to the following described exemplary embodiments, but are to be controlled by the limitations set forth in the claims and any equivalents thereof.
The negative electrode 10 includes a lithium (Li) metal electrode 110 having a first major surface 112, and a surface layer 120 disposed on the major surface 112. The surface layer 120 has a composition including a compound of graphite and lithium. In some embodiments, the composition of the surface layer 120 includes lithium graphite intercalation compound (Li-GIC) which can be formed by reaction of the lithium metal surface and a graphite coating disposed thereon. Li-GIC may have an approximate composition of LiC6. In Li-GIC, the graphite has a layered structure, and the lithium ions are intercalated into the layered structure of graphite. The microstructure across the interface between the surface layer 120 and the lithium metal electrode 110 can be continuous where the two sides at the interface are connected by interfacial chemical bonding such as lithium-carbon chemical bonding. At the side of the lithium metal electrode 110, lithium element is in the form of metal, while at the side of the surface layer, lithium ion is intercalated into the layered structure of graphite. It is to be understood that there may be an interface region where the lithium element is gradually changed from the metal form to the intercalated form. The interface region may have a thickness, for example, between a few nanometers and a few microns.
In some embodiments, the surface layer 120 may be substantially uniform with a thickness in the range of a few nanometers to microns, for example, from about 10 nm to about 100 microns, from about 10 nm to about 50 microns, from about 20 nm to about 50 microns, from about 50 nm to about 50 microns, from about 100 nm to about 50 microns, or from about one micron to about 50 microns. In some embodiments, the major surface 112 of the lithium metal electrode 110 may be patterned, and the surface layer 120 can conform to the surface morphology of the major surface 112.
The surface layer 120 is electrically conductive, and lithium-ion conductive. In some embodiments, the surface layer 120 has an electrical conductivity in the range, for example, from about 1×103 Siemens per centimeter (S/cm) to about 5×104 S/cm at room temperatures. The surface layer 120 can produce an electrically conductive layer on the lithium metal surface which can reduce or eliminate high current distribution areas on the lithium metal surface originating from, for example, a poor primary current distribution caused by a native passive surface layer on the cleaned lithium metal surface. In some embodiments, the surface layer 120 has a lithium-ion conductivity in the range, for example, from about 1×10−5 S cm2/mol to about 1×10−3 S cm2/mol at room temperatures.
By using the lithium metal electrode 110 with the surface layer 112 as a negative electrode (anode) for lithium rechargeable batteries, the lithium metal electrode 110 can serve as a primary lithium ion source during charging/discharging. The reaction of lithium metal electrode 110 can be expressed as the following equation:
Li→Li+e−
The surface layer 120 is lithium-ion conductive, and lithium ions can be transported between the electrolyte 30 and the major surface 112 of the lithium metal electrode 110 through the surface layer 120. In addition, with the surface layer 120 disposed thereon, the major surface 112 of the lithium metal electrode 110 is separated from the electrolyte environments 30 which may be corrosive to the lithium metal electrode 110. Thus, the surface layer 120 can serve as a barrier layer to provide protection for the lithium metal electrode 110. The surface layer 120 is chemically compatible with lithium metal on a first side in contact with the lithium metal electrode 120, and the surface layer 120 is chemically compatible with electrolyte environments such as the lithium-ion-conducting electrolyte 30 on a second side of the surface layer 120.
In some embodiments, the negative electrode 20 including the surface layer 120 can be prepared by applying a graphite coating onto the major surface 112 of the lithium metal electrode. The applied graphite coating may have a substantially uniform thickness in the range of a few nanometers to microns, for example, from about 10 nm to about 100 microns, or from about 20 nm to about 50 microns. In some embodiments, the graphite coating can be applied to a cleaned lithium metal surface by brushing an effective amount of dry, substantially solvent free graphite powders or flakes onto the cleaned lithium metal surface. In some embodiments, coating of the major surface of the lithium metal electrode can include buffing an effective amount of dry, substantially solvent free graphite composition onto the lithium metal surface. Exemplary buffing processes are described in US Patent Publication No. 2014/0302397 (Bommel et al.), which is incorporated herein by reference. It is to be understood that graphite coating can be applied onto the major surface 112 of the lithium metal electrode by any suitable methods as long as the applied graphite coating can react with the lithium metal to form the surface layer 120.
Buffing of the graphite coating composition may be carried out using any buffing apparatus known in the art (e.g., power sander, power buffer, orbital sander, random orbital sander) suitable for applying dry particles to a surface, or manually (i.e., by hand). An exemplary buffing apparatus may include a motorized buffing applicator (e.g., disc, wheel) which may be configured to apply a pressure normal to a subject surface as well as rotate in a plane parallel to said subject surface. The buffing applicator may include a buffing surface that contacts with, or is intended to contact with, the subject surface during a buffing operation. In some embodiments, the buffing surface may include metal, polymer, glass, foam (e.g., closed-cell foam), cloth, paper, rubber, or combinations thereof. In various embodiments, the buffing surface may be formed of a material having a Brinell hardness of at least 0.1 HB, at least 1 HB, at least 10 HB, at least 100 HB, or even at least 1000 HB.
In some embodiments, the buffing surface may include or otherwise be associated with (e.g., be fitted with) a metal foil (e.g., aluminum foil). That is, the provided methods may include buffing graphite compositions onto a major surface of lithium metal electrodes utilizing a metal foil as a buffing surface.
In some embodiments, the buffing applicator may be configured to move in a pattern parallel to the subject surface and to rotate about a rotational axis perpendicular to the subject surface. The pattern may include a simple orbital motion or random orbital motion. Rotation of the buffing applicator may be carried out as high as 100 orbits per minute, as high as 1,000 orbits per minute, or even as high as 10,000 orbits per minute. The buffing applicator may be applied in a direction normal to the subject surface at a pressure of a least 0.1 g/cm2, at least 1 g/cm2, at least 10 g/cm2, at least 20 g/cm2, or even at least 30 g/cm2.
In illustrative embodiments, adherence of the graphite coating composition to a major surface of lithium metal electrodes may be assisted by heating the metal electrodes prior to, during, or after the buffing operation to a temperature such that the adhesion of the coating is enhanced. Exemplary methods of heat input to the metal electrodes may include oven heating, heat lamp heating (e.g., infrared), or a heated platen in contact with the metallic substrate. Direct application of electrical currents to conductive substrates may also produce the desired heating affect.
The initially applied graphite coating may have an initial dark color. After being applied onto the cleaned lithium metal surface, the dark graphite coating may turn golden as solid state transport of lithium metal from the major surface 112 into the graphite coating to form a layer of lithium graphite intercalation compound (Li-GIC). Li-GIC may have an approximate composition of, for example, LiC6. The reaction between the graphite coating and the underlying lithium metal may take, for example, from several minutes to several hours at room temperatures. It is to be understood that other carbon materials (e.g., amorphous carbon) that do not have the graphite structure may not react with the lithium metal to form the surface layer including Li-GIC described herein.
The positive electrode 20 can include any suitable electrode materials such as, for example, LiCoO2, LiMn2O4, LiFePO4, MoS2, V2O5, elemental sulfur or oxygen, etc. In some embodiments, the positive electrode 20 may include Li intercalation compounds (e.g., LiCoO2, LiMn2O4, LiFePO4, etc.). In other embodiments, the positive electrode 20 may not include any Li intercalation compounds. The electrolyte 30 can include any suitable electrolyte that allows for lithium-ion movement. Suitable electrolyte can include, for example, lithium salts, such as LiPF6, LiBF4 or LiClO4 in an organic solvent, such as ethylene carbonate, dimethyl carbonate, and diethyl carbonate.
Graphite may be provided in form of flakes or powders with various shapes and dimensions. In some embodiments, graphite flakes or powders may have a dimension in the range, for example, of several nanometers to several tens microns. The graphite material described in this disclosure is distinguished from other carbon materials such as amorphous carbon for its high crystallinity and layered structure for lithium-intercalation. It is to be understood that amorphous carbon may not react with a lithium metal electrode at room temperatures to form a Li-GIC described herein.
In some embodiments, the graphite coating may include fillers mixed with graphite flakes or powders to increase its mechanical properties. The fillers may be polymeric fillers such as, for example, polyvinylidene difluoride (PVDF), styrene-butadiene rubber (SBR), carbomethoxy cellulose, etc. In some embodiments, the graphite coating may include no more than 30 wt %, no more than 20 wt %, or nor more than 10 wt % of fillers. In some embodiments, the graphite coating may include no less than 0.1 wt %, no less than 0.5 wt %, or nor more than 1 wt % of fillers. In some embodiments, the graphite coating may include, for example, from about 1 wt % to about 30 wt % of fillers. While fillers can be added to the surface layer 120 to improve its mechanical properties, it is to be understood that the primary force to connect the surface layer 120 and the lithium metal electrode 110 is not attributed to the fillers or other additives. Instead, the primary bonding force is the interfacial chemical bonding (e.g., lithium-carbon bonding) that is formed during the growth of Li-GIC on the lithium metal electrode 110.
The major surface 112 of the lithium metal electrode 110 can be cleaned before applying a graphite coating. In some embodiments, the lithium metal electrode 110 can be provided with a protective layer such as, for example, a lithium nitride layer, and the protective layer can be removed by, for example, mechanical polishing, before applying the graphite coating. In some embodiments, the cleaning process can be conducted in a dry environment of air. The major surface 112 of the lithium metal electrode 110 may include a layer of native oxide after cleaning. In some embodiments, the major surface 112 of the lithium metal electrode 110 may not have the layer of native oxide, the formed interface Li/Li-GIC may be free from a noticeable amount of oxygen (e.g., 0.1 wt % or less, 0.05 wt % or less, or 0.01 wt % or less).
Various unexpected results and advantages are obtained in exemplary embodiments of the disclosure. One such advantage of exemplary embodiments of the present disclosure is that the surface layer forms a coherent, substantially pinhole-free interface with the underlying lithium electrode, which results from reaction between a graphite coating and the underlying lithium metal surface. The surface layer provides protection for the active lithium metal surface, acts as an electrically conductive, and lithium-ion conductive layer, and exhibits superior electrical, electrochemical, and mechanical properties.
Exemplary embodiments of the present disclosure may take on various modifications and alterations without departing from the spirit and scope of the present disclosure. Accordingly, it is to be understood that the embodiments of the present disclosure are not to be limited to the following described exemplary embodiments, but is to be controlled by the limitations set forth in the claims and any equivalents thereof.
It is to be understood that any one of embodiments 1-12 and 13-30 can be combined.
Embodiment 1 is an electrochemical device component, comprising:
The operation of the present disclosure will be further described with regard to the following detailed examples. These examples are offered to further illustrate the various specific and preferred embodiments and techniques. It should be understood, however, that many variations and modifications may be made while remaining within the scope of the present disclosure.
These Examples are merely for illustrative purposes and are not meant to be overly limiting on the scope of the appended claims. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight. Table 1 provides abbreviations and a source for all materials used in the Examples below:
The following test methods have been used in evaluating some of the Examples of the present disclosure.
Isothermal heat flow calorimetry has enabled the direct investigation of the electrolyte reactivity and the development of mossy, dendritic lithium deposits. This technique has been used to characterize the thermal signal from lithium plating and stripping and the parasitic electrolyte reactions occurring in lithium ion cells. This allowed direct observation of the effects of electrolyte variations and surface modifications of the lithium metal electrode. The heat flow calorimeter was obtained from TA instruments (New Castle, Del.) as a TAM III (Thermally Activated Module) in which up to 12 calorimeters can be inserted. The temperature used throughout this work was 40° C. The TAM III is capable of controlling the bath temperature to within a few micro-degrees Centigrade. The calorimeters are equipped with stainless steel sample cans with an outside diameter of 27.6 mm and a length of 60 mm. The sample cans attach to lifters by which the sample can be raised or lowered into the measuring position. The electrochemical cell is inserted into the sample can.
The following illustrate Examples of the preparation of various surface layers according to the present disclosure, as well as Comparative Examples.
A lithium symmetric cell was prepared. Metallic lithium was first brushed with a nylon brush to remove tarnish due to lithium nitride formation to yield a bright metallic surface. Two circular pieces of metal lithium, 2 cm2 in area, were cut from the lithium metal foil. The electrodes were then coated with graphite by brushing a graphite powder onto the Li surface. After approximately 30 minute the initially black graphite surface layer turned a golden color. The two surface modified Li electrodes were then assembled into a 2325 coin cell using a porous polyolefin separator (Cellgard 3501) placed between the two lithium electrodes. The cell was then filled with 100 uL of 1M LiClO4 in a 50:50 by volume blend of ethylene carbonate and propylene carbonate. The coin cell was then placed into the isothermal heat flow calorimeter at 40° C. and electrically connected to a current/voltage source. The symmetric cell was then cycled under constant current conditions whereby each lithium electrode is being stripped of lithium metal while the opposing lithium electrode is being plated with Li metal. After approximately 7 cycles a small thermal peak (
A lithium symmetric cell was prepared as in example 1 but without a graphite surface modification
Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment,” whether or not including the term “exemplary” preceding the term “embodiment,” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the certain exemplary embodiments of the present disclosure. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the certain exemplary embodiments of the present disclosure. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
While the specification has described in detail certain exemplary embodiments, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. Accordingly, it should be understood that this disclosure is not to be unduly limited to the illustrative embodiments set forth hereinabove. In particular, as used herein, the recitation of numerical ranges by endpoints is intended to include all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). In addition, all numbers used herein are assumed to be modified by the term “about.”
Furthermore, all publications and patents referenced herein are incorporated by reference in their entirety to the same extent as if each individual publication or patent was specifically and individually indicated to be incorporated by reference. Various exemplary embodiments have been described. These and other embodiments are within the scope of the following claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/043291 | 7/21/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62197203 | Jul 2015 | US |
