Patent Publication Number: US-2022231286-A1

Title: Composite Lithium-metal Anodes for Enhanced Energy Density and Reduced Charging Times

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application No. 62/865,015 filed on Jun. 21, 2019, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Batteries can be used to temporarily provide electrical power to various devices when those devices are not connected to an external power source, for example. For a variety of masons, batteries (and specifically rechargeable batteries) have become increasingly prevalent in many technology areas. The lithium-ion battery is an example of a rechargeable battery. 
     In order to improve performance in applications that make use of rechargeable batteries, it may be desirable to miniaturize such batteries, reduce the charging time for such batteries, and/or increase capacity of such batteries. 
     SUMMARY 
     The specification and drawings disclose embodiments that relate to composite lithium-metal anodes for enhanced energy density and reduced charging times. The composite anode may include a graphite layer, a hard carbon layer, and a lithium-metal layer, each of which may have various thicknesses. The composite of layers of various materials may produce an anode that exhibits a blend of various qualities associated with each of the constituent materials. For example, the composite anode may have enhanced storage at a low state of charge substantially due to the graphite layer, reduced charging time substantially due to the hard carbon layer, and enhanced overall storage capacity substantially due to the lithium-metal layer. The composite anode may also include a protective layer that minimizes side reactions between the anode and an electrolyte of a battery that includes the anode. 
     In a first aspect, an electrode is disclosed. The electrode includes a protective layer. The electrode also includes a current collecting layer. Further, the electrode includes an active layer disposed between the protective layer and the current collecting layer. The active layer includes a graphite layer. The active layer also includes a hard carbon layer. Further, the active layer includes a lithium-metal layer. 
     A thickness of the graphite layer may be between 20% and 30% of a thickness of the active layer. 
     A thickness of the hard carbon layer may be between 40% and 60% of a thickness of the active layer. 
     A thickness of the lithium-metal layer may be between 20% and 30% of a thickness of the active layer. 
     A thickness of the protective layer may be between 1.0 μm and 5.0 μm. 
     The protective layer may comprise an ex-situ ceramic layer. 
     The protective layer may comprise an ex-situ layer of Li 3 N, Li 3 AlN 2 , AlN, or SiN. 
     The protective layer may comprise an in-situ LiF layer. 
     The protective layer may comprise a composite of an ex-situ ceramic layer, an ex-situ layer of LiN, Li 3 AlN 2 , AlN, or SiN, and an in-situ LiF layer. 
     The electrode may further comprise an additional active layer disposed between an additional protective layer and the current collecting layer, wherein the additional active layer is on a side of the current collecting layer opposite the active layer and wherein the additional active layer comprises: an additional graphite layer; an additional hard carbon layer; and an additional lithium-metal layer. 
     In a second aspect, a lithium-ion battery is disclosed. The lithium-ion battery includes a cathode that includes a cathode current collecting layer. The lithium-ion battery also includes an anode. The anode includes a protective layer. The anode also includes an anode current collecting layer. Further, the anode includes an active layer disposed between the protective layer and the anode current collecting layer. The active layer includes a graphite layer. The active layer also includes a hard carbon layer. Further, the active layer includes a lithium-metal layer. In addition, the lithium-ion battery includes an electrolyte disposed between the cathode and the anode. 
     The cathode may comprise LiCoO 2 , LiNiCoMnO 2 , or LiNiCoAlO 2 . 
     The lithium-ion battery may be a pouch cell or a prismatic cell. 
     The electrolyte may be a solution comprising: a salt of lithium bis(fluorosulfonyl)imide (LiFSI); and an ether or a fluorinated ether. 
     In a third aspect, a method of fabrication is disclosed. The method includes applying a graphite layer onto a current collecting layer. The method also includes applying a hard carbon layer onto the graphite layer. Further, the method includes applying a lithium-metal layer onto the hard carbon layer. In addition, the method includes applying a protective layer onto the lithium-metal layer. 
     The graphite layer may be applied onto the current collecting layer using a web-coating process. 
     The hard carbon layer may be applied onto the graphite layer using a web-coating process. 
     The lithium-metal layer may be applied onto the hard carbon layer using an electrochemical deposition process. 
     The protective layer may be applied onto the lithium-metal layer using an atomic layer deposition (ALD) process or a web-coating process. 
     The current collecting layer, the graphite layer, the hard carbon layer, the lithium-metal layer, and the protective layer may collectively form an anode, and the method may further comprise: positioning a cathode adjacent to and separated from the anode; encapsulating the cathode and the anode within a casing; and inserting an electrolyte into an interstice defined by a separation between the cathode and the anode. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the figures and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is an illustration of a battery, according to example embodiments. 
         FIG. 1B  is an illustration of a battery, according to example embodiments. 
         FIG. 2A  is a cross-section of an anode, according to example embodiments. 
         FIG. 2B  is a cross-section of an anode, according to example embodiments. 
         FIG. 2C  is a cross-section of an anode, according to example embodiments. 
         FIG. 2D  is a cross-section of an anode, according to example embodiments. 
         FIG. 3A  is a front-view illustration of a battery, according to example embodiments. 
         FIG. 3B  is a side-view illustration of a battery, according to example embodiments. 
         FIG. 4  is a flow chart illustrating a method, according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods and systems are described herein. Any example embodiment or feature described herein is not necessarily to be construed as preferred or advantageous over other embodiments or features. The example embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed systems and methods can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein. 
     Furthermore, the particular arrangements shown in the figures should not be viewed as limiting. It should be understood that other embodiments might include more or less of each element shown in a given figure. In addition, some of the illustrated elements may be combined or omitted. Similarly, an example embodiment may include elements that are not illustrated in the figures. 
     I. Overview 
     Example embodiments relate to composite lithium-metal anodes for enhanced energy density and reduced charging times. 
     Lithium-ion batteries are the battery of choice for a number of applications, such as consumer electronics and electric vehicles. It continues to be desirable to improve the volumetric energy density and gravimetric energy density of lithium-ion batteries, while reducing the charging time for such batteries. One technique used to improve the energy density/capacity of lithium-ion batteries is to employ a lithium-metal anode. Batteries with lithium-metal anodes may have enhanced capacities when compared to batteries having more conventional anodes (e.g., graphite anodes). However, anodes made solely of lithium metal can present their own challenges, such as reduced coulombic efficiencies (e.g., low charge/discharge efficiency), dendritic growth (which can lead to a short circuit within the battery between the cathode and the anode), inadequate charging rate (e.g., maximum charging rate of 0.1 C, which corresponds to a charging time between 10 hours and 20 hours in some cases), and/or challenges in fabrication. 
     Other anode materials may have alternate benefits when used within a lithium-ion battery. For example, graphite anodes may be adept at energy storage for low states of charge and hard carbon anodes may exhibit fast charging and enhanced lithium plating. Embodiments described herein attempt to combine all these benefits by including each of these materials in a single anode. For example, one embodiment includes an anode that has a graphite layer, a hard carbon layer, and a lithium-metal layer. Such an anode may provide enhanced volumetric energy density (e.g., a 30%-50% increase over lithium-ion batteries with alternate lithium-metal anodes) while maintaining an acceptable charging rate. The relative thicknesses of each layer may be designed so as to emphasize and/or optimize the advantage of one of the materials more than another. For instance, in an embodiment where storage capacity is of paramount importance, the thickness of the lithium-metal layer may be greater than the thickness of the hard carbon layer and the thickness of the graphite layer. 
     In addition to the three layers described above (e.g., the graphite layer, the hard carbon layer, and the lithium-metal layer), the anode may also include a protective layer (e.g., an ex-situ ceramic protective layer, an in-situ LiF protective layer, or a protective layer that includes an ex-situ layer of Li 3 N, Li 3 AlN 2 , AlN, or SiN). The protective layer may minimize side reactions between the anode and an electrolyte of the corresponding battery, which may reduce or prevent the formation of dendrites within the battery. Further, in some embodiments, the anode may include a current collecting layer (e.g., a copper current collector). The current collecting layer may provide a connection between the anode and outside circuitry (e.g., a connection to a load that is being powered by the battery). 
     Anodes that include the graphite layer, the hard carbon layer, the lithium-metal layer, the protective layer, and the current collecting layer may be practical for mass production using industry manufacturing techniques. For example, the graphite layer may be coated onto the current collecting layer using a web-coating process, the hard carbon layer may be coated onto the graphite layer using a web-coating process, the lithium-metal layer may be deposited onto the hard carbon layer using an electrochemical deposition process, and the protective layer may be deposited onto the lithium-metal layer using an atomic layer deposition process or coated onto the lithium-metal layer using a web-coating process. 
     The composite anode described above may be combined with a high-voltage cathode (e.g., a LiCoO 2  (LCO) cathode, a LiNiCoMnO 2  (NCM) cathode, or a LiNiCoAlO 2  (NCA) cathode) within a battery. Such a battery may also include a separator and an electrolyte. Depending on the intended application, a battery that includes the anode described above may have a variety of form factors. For example, such a battery may be a pouch cell or a prismatic cell. 
     II. Example Devices 
       FIG. 1A  is an illustration of a battery  100  (e.g., a single-celled battery). The battery  100  may be a rechargeable lithium-ion battery, for example. The battery  100  may include an anode  102 , a cathode  104 , a separator  106 , and free lithium ions  108  within an electrolyte  110 . The elements of the battery  100  are not necessarily illustrated to scale (e.g., the free lithium ions  108  may be significantly smaller than illustrated in the figure). Further, as illustrated in  FIG. 1A , the battery  100  may be chargeable by an electrical power source  112  (e.g., a rectified alternating current (AC) signal, a separate charged battery, or a charged capacitor). In some embodiments, multiple cells of cathode, anode, separator, and electrolyte may be electrically arranged in series and/or parallel to form a composite battery. Such cell arrangements may enhance the capacity and/or voltage of the composite battery. The battery  100  may provide electrical power to one or more devices (e.g., consumer-electronic devices). 
     Charging may include electrons flowing from the cathode  104  to the anode  102  through circuitry external to the battery  100 . In addition, charging may include free lithium ions  108 , within the electrolyte  110 , flowing from the cathode  104  to the anode  102  through the separator  106 . Further, charging may include the free lithium ions  108  being intercalated into the anode  102 . Such a process is illustrated in  FIG. 1A  by the lithium ions that are sitting on “shelves” of the anode  102 . The charging may represent a first formation charging process, in some embodiments. The first formation charging process may last between 10 hours and 20 hours, in some embodiments. Additionally, the battery  100  may be configured to undergo repeated charge/discharge cycles during a lifetime of the battery  100 . For example, the battery  100  may be a rechargeable battery configured to be charged by an external voltage between 4.20 volts and 4.50 volts or between 4.40 volts and 4.60 volts. It will be understood that other external charge voltage values and/or ranges are possible and contemplated herein. 
     In various embodiments, various charging/recharging schemes may be used. For example, a constant voltage (CV) scheme may be used, where a constant voltage is applied across the terminals of the battery, resulting in a decreasing current as the battery charges, until the current reaches 0.0 Amps (or within a threshold current of 0.0 Amps), at which point the voltage source charging the battery is removed. In other embodiments, a constant current (CC) scheme may be used, where the voltage applied across the terminals of the battery by a charging device is varied such that the current is maintained at a constant rate. Once the battery voltage reaches a threshold value to maintain the continuous current, the battery may be determined to be charged, and the voltage source charging the battery may be removed. 
     Alternatively, in some embodiments, a hybrid constant current/constant voltage (CC/CV) charging mode may be used to charge the battery. The CC/CV charging mode may have two stages. In a first stage (a CC stage), the voltage may be increased continuously to maintain a constant current charging the battery. Then, once the voltage reaches a certain maximum charging voltage threshold, the second stage of the CC/CV charging mode may begin. In the second stage (a CV stage), the voltage may be maintained at the maximum charging voltage threshold, and the charging current may be allowed to decrease. Once the charging current reaches a threshold level, indicating the battery is charged, the CC/CV charging mode may cease. 
     The anode  102  may be the negative terminal (electrode) of the battery  100 . For example, the anode  102  may include one or more external electrical contacts (e.g., current collectors) on the side of the anode  102  facing away from the separator  106 . The external electrical contact(s) may allow an electrical connection between the anode  102  and the power source  112  or a load to be made. The anode  102  may include graphite, Li. Li 4 TiO 12 , a lithium-metal composite, a hard carbon, and/or Si, in various embodiments. Further, as described below with reference to  FIGS. 2A-2D , the anode  102  may include a multilayered composite structure. 
     The cathode  104  may be the positive terminal (electrode) of the battery  100 . For example, the cathode  104  may include one or more external electrical contacts (e.g., current collectors) on the side of the cathode  104  facing away from the separator  106 . The external electrical contact(s) may allow an electrical connection between the cathode  104  and the power source  112  or a load to be made. The cathode  104  may include LiCoO 2  (LCO), LiMn 2 O 4 , a vanadium oxide, LiNiCoMnO 2  (NCM). LiNiCoAlO 2  (NCA), an olivine (e.g., LiFePO 4 ), or a composite of two or more of such materials, in various embodiments. LCO may be used in applications where enhanced volumetric energy density is valued (e.g., consumer-electronic devices, such as mobile devices). Additionally or alternatively, NCM may be used in applications where enhanced gravimetric energy density is valued (e.g., electric vehicles). Other lithium-containing cathode materials are possible and contemplated herein. 
     The separator  106  may prevent a short circuit of the cathode  104  to the anode  102  within the battery  100 . For example, the separator  106  may include a semi-permeable membrane (e.g., permeable to the free lithium ions  108 ). To achieve such semi-permeability, the separator  106  may include micropores that are sized to selectively allow the passage of the free lithium ions  108  during charging or discharging processes. The semi-permeable membrane of the separator  106  may also have an amorphous or a semi-crystalline structure. Further, the semi-permeable membrane of the separator  106  may be polymeric (e.g., fabricated from cellulose acetate, nitrocellulose, cellulose esters, polysulfone, polyether sulfone, polyacrilonitrile, polyamide, polyimide, polyethylene, polypropylene, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylchloride, and/or aramid). In addition, the separator  106  may be chemically and electrochemically stable for use within the battery  100  during charging and discharging processes. In some embodiments, the separator  106  may include a multi-layered structure. 
     In some embodiments, the separator  106  may be a non-standard separator having an increased mechanical stability, which can prevent dendrites from piercing the separator  106 . Further, the separator  106  may also include compounds that are chemically and/or electrochemically stable for use within the battery  100  during charging or discharging processes. Such compounds may enhance the lifetime of the battery  100 , for example. 
     In some embodiments, the battery  100  may be a thin-film battery. In such embodiments, the battery  100  may not include a separator  106 . Further, in such embodiments, the electrolyte  110  may be solid (e.g., rather than liquid), thereby satisfying purposes of both the electrolyte  110  and the separator  106  (e.g., transporting ions and preventing a short circuit of the cathode  104  to the anode  102 ). In such embodiments, a discrete separator may not be needed. 
     The free lithium ions  108  may transfer between the anode  102  and the cathode  104  during charging/discharging processes of the battery  100 . In some embodiments, the free lithium ions  108  may originate from the cathode  104 . For example, the cathode  104  may include LiCoO 2 , which may be a source of free lithium during the chemical reactions occurring during the charging process (e.g., during the first formation charging process). Other sources of free lithium ions are also possible. For example, the anode  102  may provide free lithium ions and/or lithium salts (e.g., LiPF 6 , LiBF 4 , LiBC 4 O 8 , Li[PF 3 (C 2 F 5 ) 3 ], LiClO 4 , or LiC 2 F 6 NO 4 S 2  (i.e., lithium bis(fluorosulfonyl)imide (LiFSI))) dissolved within the electrolyte  110  may provide free lithium ions. 
     The electrolyte  110  may be a medium through which the free lithium ions  108  travel during charging and discharging processes of the battery  100 . The electrolyte  110  may be a gel or a liquid, in various embodiments and/or at various temperatures. For example, the electrolyte  110  may be an organic solvent (e.g., ethylene carbonate, dimethyl carbonate, diethyl carbonate, an ether, or a fluorinated ether). Additives may be included within the electrolyte  110  to enhance the effectiveness of the electrolyte  110 . In some embodiments, for instance, ionic liquids may be included within the electrolyte to reduce volatility of the electrolyte solution. 
     As described above, in some embodiments (e.g., embodiments where the battery  100  is a thin-film battery), the electrolyte  110  may be a solid (e.g., rather than a liquid or gel). For example, in some embodiments, the electrolyte  110  may include one or more amorphous glassy layers deposited on the cathode  104  (e.g., deposited using sputtering or vapor deposition). One type of amorphous glassy material that may be used is lithium phosphorous oxynitride (LiPON). 
       FIG. 1B  is another illustration of the battery  100 . The battery  100  illustrated in  FIG. 1B  may be discharging across a load  122 . Discharging the battery  100  may include electrons flowing from the anode  102  to the cathode  104 , across the load  122 , through circuitry external to the battery  100 . Discharging the battery  100  may also include the free lithium ions  108  within the electrolyte  110  flowing from the anode  102  to the cathode  104  through the separator  106  (in embodiments having a discrete separator). Further, discharging the battery  100  may include the free lithium ions  108  being intercalated into the cathode  104 . Such a scenario is illustrated in  FIG. 1B  by the lithium ions that are sitting on “shelves” of the cathode  104 . 
     The load  122  may be a device powered by the battery  100 , such as an electric vehicle, a hybrid electric vehicle, a mobile device, a tablet computing device, a laptop computing device, a light source, television remote, headphones, etc. The load  122  may be powered by the flow of electrons through the circuitry external to the battery  100  during the discharging process, for example. 
       FIG. 2A  is a cross-section of an anode  200 , according to example embodiments. The anode  200  includes a current collecting layer  202 , an active layer (which includes a graphite layer  204 , a hard carbon layer  206 , and a lithium-metal layer  208 ), and a protective layer  210 . 
     The current collecting layer  202  may be used to connect the anode  200  to one or more components external to the battery. For example, the current collecting layer  202  may be used to connect the anode  200  to a charging circuit (e.g., a wall charger) or to a load across which the battery may be discharged to provide power (e.g., an electric vehicle or a mobile device). The current collecting layer  202  may include one or more metallic layers (e.g., one or more metallic foil sheets). For example, the current collecting layer  202  may include copper. Additionally or alternatively, the current collecting layer  202  may include aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, or an alloy thereof. The thickness of the current collecting layer  202  (e.g., dimension of the current collecting layer  202  measured along the x-direction illustrated in  FIG. 2A ) may be between 5 μm and 15 μm (e.g., between 8 μm and 12 μm), in some embodiments. 
     The graphite layer  204  may be located adjacent to the current collecting layer  202  and/or the hard carbon layer  206 , in some embodiments. For example, in some embodiments, the graphite layer  204  may have been applied onto the current collecting layer  202  using a web-coating process. Graphite (as in the graphite layer  204 ) is a hexagonally organized crystalline form of carbon. The graphite layer  204  may be a portion of the active layer configured to store energy for use at low states of charge. Further, the graphite layer  204  portion of the active layer may be chargeable using an intermediate charging current (e.g., a maximum charging current between 0.7 C and 1.0 C). For reference, the “C-rate” (i.e., 1.0 C) of a battery is equal to the current at which the total capacity of the battery (in terms of charge) flows by a given point in the span of an hour. For example, a 1.0 C charging current for a 2,500 mAh battery equals 2.5 A. Likewise, a 0.5 C discharging current for a 3,000 mAh battery equals 1.5 A. Further, a 2.0 C charging current for a 1,700 mAh battery is 3.4 A. 
     The thickness  222  of the graphite layer  204  is illustrated in  FIG. 2A . As illustrated, the thickness  222  of the graphite layer  204  may be the dimension of the graphite layer  204  measured along the x-axis. The thickness  222  of the graphite layer  204  may be different in various embodiments. For example, the thickness  222  of the graphite layer  204  may be between 20% and 30% of an overall thickness  232  of the active region of the anode  200  (as measured in the x-direction, as illustrated). Further, in some embodiments, the graphite layer  204  may store between 12.5% and 37.5% of an energy capacity of the anode  200  (e.g., about 25%). 
     The hard carbon layer  206  may be located adjacent to the graphite layer  204  and/or the lithium-metal layer  208 , in some embodiments. For example, in some embodiments, the hard carbon layer  206  may have been applied onto the graphite layer  204  using a web-coating process. Hard carbon (as in the hard carbon layer  206 ) is an irregular and disordered form of carbon (e.g., synthesized by pyrolysis of polymers). The hard carbon layer  206  may be a portion of the active layer configured to be charged quickly (e.g., due to efficient lithium plating inside voids within the hard carbon crystal structure). For example, the hard carbon layer  206  portion of the active layer may be chargeable using a relatively large charging current (e.g., a maximum charging current between 1.0 C and 1.5 C). 
     The thickness  224  of the hard carbon layer  206  is illustrated in  FIG. 2A . As illustrated, the thickness  224  of the hard carbon layer  206  may be the dimension of the hard carbon layer  206  measured along the x-axis. The thickness  224  of the hard carbon layer  206  may be different in various embodiments. For example, the thickness  224  of the hard carbon layer  206  may be between 40% and 60% of an overall thickness  232  of the active region of the anode  200  (as measured in the x-direction, as illustrated). Further, in some embodiments, the hard carbon layer  206  may store between 37.5% and 62.5% of an energy capacity of the anode  200  (e.g., about 50%). 
     The lithium-metal layer  208  may be located adjacent to the hard carbon layer  206  and/or the protective layer  210 , in some embodiments. For example, in some embodiments, the lithium-metal layer  208  may have been applied onto the hard carbon layer  206  using an electrochemical deposition process. The lithium-metal layer  208  may be a portion of the active layer configured to exhibit a high energy storage capacity. Further, the lithium-metal layer  208  portion of the active layer may be chargeable using a relatively small charging current (e.g., a maximum charging current between 0.1 C and 0.2 C). 
     The thickness  226  of the lithium-metal layer  208  is illustrated in  FIG. 2A . As illustrated, the thickness  226  of the lithium-metal layer  208  may be the dimension of the lithium-metal layer  208  measured along the x-axis. The thickness  226  of the lithium-metal layer  208  may be different in various embodiments. For example, the thickness  226  of the lithium-metal layer  208  may be between 20% and 30% of an overall thickness  232  of the active region of the anode  200  (as measured in the x-direction, as illustrated). Further, in some embodiments, the lithium-metal layer  208  may store between 12.5% and 37.5% of an energy capacity of the anode  200  (e.g., about 25%). 
     In some embodiments, the maximum charging rate of a battery that includes the composite anode (e.g., a maximum charging current in terms of C-rate) may be determined empirically. Further, such a maximum charging rate may depend on: the size, shape, and/or materials used to fabricate a corresponding cathode; the size, shape, and/or materials used to fabricate the corresponding electrolyte; and/or the size, shape, and/or materials used to fabricate the corresponding separator. Additionally or alternatively, such a maximum charging rate may depend on the mass ratio or thickness ratio of the layers within the active layer of the anode  200  (e.g., the mass ratio of the graphite layer  204  to the hard carbon layer  206  to the lithium-metal layer  208 ). 
     The protective layer  210  may be used to protect the anode  200  from other components of the battery. For example, the protective layer  210  may be used to minimize side reactions between the anode  200  and an electrolyte (e.g., the electrolyte  110  of the battery  100  illustrated in  FIGS. 1A and 1B ). The protective layer  210  may include various materials. For example, the protective layer  210  may include an ex-situ ceramic layer (i.e., a ceramic layer that was formed separately from the anode  200  and then incorporated into the anode  200 ). Additionally or alternatively, the protective layer  210  may include an in-situ LiF layer (i.e., a LiF layer that was formed in place within the anode  200 ). Further, the protective layer  210  may include an ex-situ layer of LiN, Li 3 AlN 2 , AlN, and/or SiN. In still other embodiments, the protective layer  210  may include a composite of an ex-situ ceramic laver; an ex-situ layer of Li 3 N, Li 3 AlN 2 , AlN, and/or SiN; and/or an in-situ LiF layer. Even further, in some embodiments, the protective layer  210  may include a solid polymer layer. 
     The protective layer  210  may be disposed within the anode  200  adjacent to the active layer (e.g., adjacent to the lithium-metal layer  208  within the active layer). For example, in some embodiments, the protective layer  210  may have been applied onto the lithium-metal layer  208  using an atomic layer deposition (ALD) process or a web-coating process. In addition, the thickness of the protective layer  210  (e.g., dimension of the protective layer  210  measured along the x-axis) may be between 1.0 μm and 5.0 μm (e.g., between 2.0 μm and 3.0 μm), in some embodiments. 
     It is understood that the anode  200  illustrated in  FIG. 2A  is provided only as an example, and that other embodiments are also possible. For example, in some embodiments, the relative thicknesses of the current collecting layer  202 , the graphite layer  204 , the hard carbon layer  206 , the lithium-metal layer  208 , and the protective layer  210  may be different than illustrated in  FIG. 2A  (e.g., the relative thickness of the hard carbon layer  206  could be increased to enhance the maximum charging/discharging rate of a battery that includes the anode  200  and/or the relative thickness of the lithium-metal layer  208  could be increased to enhance the volumetric energy density of a battery that includes the anode  200 ). Additionally or alternatively, the relative positions of the layers within the active layer (e.g., the graphite layer  204 , the hard carbon layer  206 , and the lithium-metal layer  208 ) may also be different than illustrated in  FIG. 2A . For example, in other embodiments, the bottom layer of the active layer may be the hard carbon layer  206 , the middle layer of the active layer may be the lithium-metal layer  208 , and the top layer of the active layer may be the graphite layer  204 . 
     Additionally or alternatively, in some embodiments, there may be layers of the anode on both sides of the current collecting layer  202  (e.g., to enhance the energy storage density of the anode). For example, as in the anode  240  illustrated in  FIG. 2B , some anodes may include multiple active layers and multiple protective layers (e.g., where such active layers and protective layers are positioned symmetrically about the current collecting layer  202 ). As illustrated,  FIG. 2B  is a cross-section of an anode  240 , according to example embodiments. The anode  240  includes a current collecting layer  202 , an active layer (which includes a graphite layer  204 , a hard carbon layer  206 , and a lithium-metal layer  208 ), a protective layer  210 , an additional active layer (which includes an additional graphite layer  244 , an additional hard carbon layer  246 , and an additional lithium-metal layer  248 ), and an additional protective layer  250 . The additional active layer may be positioned on a side of the current collecting layer  202  that is opposite the active layer, as illustrated. Layers of the additional active layer and/or the additional protective layer  250  may be fabricated using the respective techniques described above with respect to the active layer and the protective layer  210  (e.g., a web-coating process, an electrochemical deposition process, and/or an ALD process). 
     As illustrated in  FIG. 2B , the active layer/the protective layer  210  and the additional active layer/the additional protective layer  250  may be symmetric within the anode  240 . In alternate embodiments, other arrangements are also possible. For example, in some embodiments, the additional active layer may be thicker or thinner than the active layer. Similarly, in some embodiments, thicknesses of individual layers within the additional active layer may be different from thicknesses of individual layers within the active layer. For example, a thickness  262  of the additional graphite layer  244  may be different than the thickness  222  of the graphite layer  204 , a thickness  264  of the additional hard carbon layer  246  may be different than the thickness  224  of the hard carbon layer  206 , and/or a thickness  266  of the additional lithium-metal layer  248  may be different than the thickness  226  of the lithium-metal layer  208 . 
     Additionally or alternatively, in some embodiments, the additional protective layer  250  may be thicker or thinner than the protective layer  210 . Still further, in some embodiments, the relative positions of the additional graphite layer  244 , the additional hard carbon layer  246 , and/or the additional lithium-metal layer  248  within the additional active layer may be different from the relative positions of the graphite layer  204 , the hard carbon layer  206 , and/or the lithium-metal layer  208  within the active layer. 
     Additional embodiments (e.g., other than those illustrated in  FIGS. 2A and 2B ) are also possible. In some embodiments, an anode (e.g., the anode  270  illustrated in  FIG. 2C ) may include an active layer that has more than three layers. For example, the anode  270  of  FIG. 2C  includes an active layer that has nine layers (e.g., three graphite layers  204 , three hard carbon layers  206 , and three lithium-metal layers  208 ). It is understood that other numbers of layers are also possible (e.g., four, five, six, seven, eight, ten, etc.). The design of  FIG. 2C  may provide for a thicker anode while making use of the same fabrication techniques described above. For example, a web-coating process may be capable of producing layers of a predefined thickness. Hence, in order to achieve an increased mass of a given material, applying multiple web-coated layers may be more practical than attempting to apply a single web-coated layer of an increased thickness. In some embodiments, for example, the mass ratio of graphite to hard carbon to lithium metal in the active layer may be the same in  FIG. 2C  as in  FIG. 2A , even though the numbers of layers of each are different. In other embodiments, the nine layers of  FIG. 2C  may be arranged differently (e.g., all of the similar layers may be adjacent to one another, such as each of the three graphite layers  204  being positioned adjacent to one another). 
     Additionally, the anode  270  of  FIG. 2C  could include additional active layer(s) on an opposite side of the current collecting layer  202  (e.g., similar to the anode  240  illustrated in  FIG. 2B ). Any additional active layer(s) on an opposite side of the current collecting layer  202  may have the same or different numbers of layers than the active layer illustrated in  FIG. 2C . Further, any additional active layers(s) on an opposite side of the current collecting layer  202  may have a different arrangement and or relative thickness of the layers interior to the additional active layer(s) than the active layer pictured in  FIG. 2C . 
     Even further, in some embodiments, there may not be equal numbers of graphite layers  204 , hard carbon layers  206 , and lithium-metal layers  208 . For example, as illustrated in the anode  280  of  FIG. 2D , there may be multiple graphite layers  204  (e.g., three graphite layers  204 ), multiple hard carbon layers  206  (e.g., three hard carbon layers  206 ), and a single lithium-metal layer  208 . It is understood that other combinations are also possible and contemplated herein (e.g., one graphite layer  204 , two hard carbon layers  206 , and three lithium-metal layers  208 ). Also as illustrated in  FIG. 2D , in some embodiments, different layers of the same material may have different thicknesses. For example, each of the graphite layers  204  in  FIG. 2D  has a different thickness. Similarly, as illustrated in  FIG. 2D , each of the hard carbon layers  206  has a different thickness. 
     Alternatively, in some embodiments, an anode may not include discrete layers (e.g., as illustrated in  FIGS. 2A-2D ). Instead, an anode may include an amorphous active layer that is an alloy of different materials (e.g., with a given mass ratio to exhibit desired properties of the anode, such as a mass ratio of graphite to hard carbon to lithium-metal similar to the active layers of  FIGS. 2A and 2B ). 
     Anodes described here (e.g., the anodes  200 / 240 / 270 / 280  illustrated in  FIGS. 2A-2D ) may be components of lithium-ion batteries (e.g., along with cathodes, electrolytes, and/or separators, as in the battery  100  illustrated in  FIG. 1A ). For example, the anodes described herein may be paired with a high-voltage cathode (e.g., a LCO cathode, a NCM cathode, a NCA cathode, or a cathode that is a composite of LCO, NCM, and/or NCA) within a lithium-ion battery. Such lithium-ion batteries may have a maximum charged voltage between 2.5 volts and 4.4 volts (e.g., depending on the composition of the cathode). 
     In various embodiments, the lithium-ion batteries described herein may take a variety of form factors. For example, a lithium-ion battery  300  that include the anodes  200 / 240 / 270 / 280  illustrated in  FIGS. 2A-2D  is illustrated in  FIGS. 3A and 3B .  FIG. 3A  is a front-view illustration (e.g., from a perspective perpendicular to the x-axis) of the lithium-ion battery  300  and  FIG. 3B  is a side-view illustration (e.g., from a perspective perpendicular to the y-axis) of the lithium-ion battery  300 . As illustrated, the lithium-ion battery  300  may be a pouch cell. Also as illustrated, the lithium-ion battery  300  may have a positive terminal with a positive lead  302  (e.g., connected to the cathode of the lithium-ion battery  300 ) and a negative terminal with a negative lead  304  (e.g., connected to the anode of the lithium-ion battery  300 ) defined within the pouch cell. 
     In alternate embodiments, the lithium-ion battery  300  may take other forms. For example, the lithium-ion battery  300  may be a prismatic cell, a coin cell (e.g., a CR2032 coin cell), or a jellyroll cell. Such a jellyroll conformation may be encapsulated in a metallic or plastic cylindrical casing (e.g., to prevent leakage of electrolyte solution and/or to enhance safety in the case of battery failure). In some embodiments, the lithium-ion battery  300  may have an enhanced gravimetric energy density when in the pouch form factor as when compared to the jellyroll form factor because no cylindrical casing is used in the pouch conformation. 
     In some embodiments, the lithium-ion battery  300  may supply electrical power to components of a device (e.g., a consumer-electronic device). For example, the lithium-ion battery  300  may be connected to circuitry within the device (e.g., electrically coupled to a motherboard within the device). Further, the lithium-ion battery  300  may be connectable to an external power source (e.g., a wall socket) in order to recharge the lithium-ion battery  300 . Alternatively, the lithium-ion battery  300  could be charged via wireless charging (e.g., using inductive coupling with an external power source). 
     III. Example Processes 
       FIG. 4  is a flow chart illustrating a method  400  of fabrication. The method  400  may be performed to fabricate a battery (e.g., a battery that includes the anode  200  illustrated in  FIG. 2A ). 
     At block  402 , the method  400  may include applying a graphite layer onto a current collecting layer. In some embodiments, the graphite layer may be applied onto the current collecting layer using a web-coating process. Web-coating processes may include passing a flexible material (e.g., a film) over multiple rollers and printing or otherwise depositing an additional material onto the flexible material. For example, the current collecting layer may be a flexible copper layer passed over multiple rollers onto which the graphite layer is coated. 
     At block  404 , the method  400  may include applying a hard carbon layer onto the graphite layer. In some embodiments, the hard carbon layer may be applied onto the graphite layer using a web-coating process. For example, a composite of the graphite layer and the current collecting layer may be passed over multiple rollers, and the hard carbon layer may be coated onto the composite. 
     At block  406 , the method  400  may include applying a lithium-metal layer onto the hard carbon layer. In some embodiments, the lithium-metal layer may be applied onto the hard carbon layer using an electrochemical deposition process (e.g., an electroplating process). For example, a composite of the current collecting layer, the graphite layer, and the hard carbon layer may be submerged or partially submerged in a solution that has dissolved lithium-metal ions. After submerging or partially submerging such a composite, an electric current may be applied such that a reduction reaction actions, thereby plating the lithium-metal on the hard carbon layer. 
     At block  408 , the method  400  may include applying a protective layer onto the lithium-metal layer. In some embodiments, protective layer may be applied onto the lithium-metal layer using an ALD process or a web-coating process. For example, a composite of lithium-metal layer, the hard carbon layer, the graphite layer, and the current collecting layer may be passed over multiple rollers, and the protective layer may be coated onto the composite (e.g., in embodiments where the protective layer includes an ex-situ ceramic layer or an ex-situ layer of Li 3 N, Li 3 AlN 2 , AlN, and/or SiN). Alternatively, LiF (e.g., a thin film of LiF) may be applied (e.g., in situ) to the composite using ALD by introducing precursors (e.g., lithd and TiF 4 ) near the surface of the lithium-metal layer. 
     In some embodiments, the current collecting layer, the graphite layer, the hard carbon layer, the lithium-metal layer, and the protective layer may collectively form an anode. Further, the method  400  may include positioning a cathode adjacent to and separated from the anode. The method  400  may also include encapsulating the cathode and the anode within a casing. Further, the method  400  may include inserting an electrolyte into an interstice defined by a separation between the cathode and the anode. 
     IV. Conclusion 
     The above detailed description describes various features and functions of the disclosed systems, devices, and methods with reference to the accompanying figures. While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent. The various aspects and embodiments disclosed herein are for purposes of illustration only and are not intended to be limiting, with the true scope being indicated by the following claims. 
     The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. 
     The above detailed description describes various features and functions of the disclosed systems, devices, and methods with reference to the accompanying figures. In the figures, similar symbols typically identify similar components, unless context dictates otherwise. The example embodiments described herein and in the figures are not meant to be limiting. Other embodiments can be utilized, and other changes can be made, without departing from the scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     The particular arrangements shown in the figures should not be viewed as limiting. It should be understood that other embodiments can include more or less of each element shown in a given figure. Further, some of the illustrated elements can be combined or omitted. Yet further, an example embodiment can include elements that are not illustrated in the figures. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.