CROSS-REFERENCE TO RELATED APPLICATION^ )

[0001] This application claims priority to and the benefit of U.S. [Provisional] Patent Application Serial No. 63/172,655, filed April 8, 2021 the entire disclosure of which is hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

[0002] This invention was made with government support under DE-EE0008853 awarded by the U.S. Department of Energy. The government has certain rights in the invention.

BACKGROUND

[0003] The present invention is in the field of battery technology, and, more particularly, in the area of lithium metal battery cells.

[0004] Conventional lithium ion batteries include a positive electrode, or cathode, a negative electrode, or anode, an electrolyte, and typically a separator. The anode in many lithium ion batteries is carbon-based, such as graphite. However, a lithium metal anode can provide several advantages over graphite and other carbon-based anode materials. Lithium metal has very high theoretical specific capacity (3860 mAh/g), low density (0.59 g cm ^-3 ) and very low negative electrochemical (e.g., redox) potential (-3.04 V vs. the standard hydrogen electrode). These attributes of lithium metal could enable rechargeable lithium metal batteries to significantly increase the cell-level energy of state-of-the-art lithium ion batteries.

[0005] However, lithium metal also has several drawbacks that have limited the commercialization of lithium metal batteries. The drawbacks may include high reactivity of lithium metal anode with conventional organic solvents in the electrolyte, formation of an unstable solid electrolyte interphase (SEI), growth of lithium dendrites, evolution of inactive lithium during lithium plating and stripping, and/or volume change during battery operation. For example, the dendrites can penetrate the battery separator, resulting in short circuiting of the battery and volatilization of the liquid electrolyte due to increased temperature. Due to these issues, experimentally-tested lithium metal batteries have been observed to have low columbic efficiency (CE), relatively short battery life, safety concerns, and sluggish electrode kinetics.

[0006] It is desirable to enhance the performance of lithium metal batteries by remedying the drawbacks associated with using lithium metal anodes.

BRIEF SUMMARY

[0007] An anode comprises a lithium metal core material and one or more passivation layers coated on the lithium metal core material. An anode comprises a lithium metal core material; and a passivation layer coated on the lithium metal core material, the passivation layer comprises a first material, comprised of one or more of lithium nitrate and a reaction product of the lithium nitrate and the lithium metal core material and a second material comprised of one or more of the second material and a reaction product of the second material. The reaction product of the second material may be a reaction product of the second material and a material present in the passivation layer or battery such as electrolytes, other additives in the battery and the like including the lithium core material or lithium nitrate. The first material is comprised of one or more of lithium nitrate and reaction product of the lithium nitrate and lithium core material. The second material is comprised of one or more of a lithium borate salt such as lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalate)borate (LiBOB), lithium tetrafluoroborate (LiBF4) or polyvinylidene fluoride (PVDF), (vinylbenzyl) trimethylammonium chloride (VBTC) and a reaction product of one of these (i.e., lithium borate salt, PVDF and VBTC).

[0008] The passivation layer or layers may be formed by coating the first material and second material on the lithium metal core. The coating may be performed by any known coating method known in the art that realizes a uniform thin coating on the lithium metal core material. Exemplary methods include chemical vapor deposition, precipitation from solution, and other well known coating methods such as spraying, brushing dipping into a slurry or solution containing the first and second materials. It is understood that the first and second materials may react when subjected to the operating battery conditions and other components within the battery or upon coating the lithium metal core as described herein. An exemplary method includes forming a solution that comprises the first material and an organic solvent. The first material is comprised of lithium nitrate. The second material is comprised of one or more of a lithium borate salt such as lithium difluoro(oxalato)borate (LiDFOB), lithium bis(oxalate)borate (LiBOB), lithum (L1BF4) or polyvinylidene fluoride (PVDF), and (vinylbenzyl) trimethylammonium chloride (VBTC). The method coating a surface of a lithium metal core material with the materials such as from a solution of the first and second materials, which may then be followed by removal of the solvent from the solution leaving the first and second material deposited upon the lithium metal core material forming the passivation layer.

[0009] The anode may be used to make a battery having a cathode, an anode, separator and a liquid electrolyte and other additives to impart one or more desirable attributes. The anode may comprise a lithium metal core material, an inner passivation layer that coated the lithium metal core material, and an outer passivation layer that coats the inner passivation layer. The lithium metal core material comprises at least 50% lithium by weight. One of the inner passivation layer or the outer passivation layer comprises at least one of lithium nitrate as a first additive or a first reaction product between the lithium nitrate and the lithium metal core material. The other of the inner passivation layer or the outer passivation layer comprises at least one of a second material or a second reaction product between the second material and the lithium metal core material or other component present in the battery. The liquid electrolyte may be comprised of one or more carbonate solvents and a lithium salt.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a graph showing the discharge capacity over total cycles for multiple lithium metal batteries that have composite cathodes and different lithium metal anodes.

[0011] Figure 2 is a graph showing voltage over time of four tested Li/Li symmetric battery cells.

[0012] Figure 3 is a flow chart of a method for forming a lithium metal battery according to a first embodiment.

[0013] Figure 4 is a flow chart of a method for forming a lithium metal battery according to a second embodiment. DETAILED DESCRIPTION

[0014] The following definitions apply to aspects described with respect to one or more embodiments of the inventive subject matter. These definitions may likewise be expanded upon herein. Each term is further explained and exemplified throughout the description, figures, and examples. Any interpretation of the terms in this description should take into account the full description, figures, and examples presented herein.

[0015] The singular terms “a,” “an,” and “the” include the plural unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.

[0016] A rate “C” refers to either (depending on context) the discharge current as a fraction or multiple relative to a “1 C” current value under which a battery (in a substantially fully charged state) would substantially fully discharge in one hour, or the charge current as a fraction or multiple relative to a “1 C” current value under which the battery (in a substantially fully discharged state) would substantially fully charge in one hour.

[0017] To the extent certain battery characteristics can vary with temperature, such characteristics are specified at 30 degrees C, unless the context clearly dictates otherwise.

[0018] Ranges presented herein are inclusive of their endpoints. Thus, for example, the range 1 to 3 includes the values 1 and 3 as well as the intermediate values.

[0019] Embodiments of the inventive subject matter provide a lithium metal battery. The lithium metal battery includes a cathode, an anode, an electrolyte, and optionally a separator. The electrolyte and the separator are disposed between the cathode and the anode. The lithium metal battery may be a secondary battery, such that the battery is rechargeable. Discharging and charging of the battery may be accomplished by reversible intercalation and de-intercalation, respectively, of lithium ions into and from the host material of the cathode and plating/stripping of lithium on the anode. The voltage of the battery may be based on redox potentials of the anode and the cathode, where lithium ions are accommodated or released at a lower potential in the former and a higher potential in the latter. [0020] The anode of the lithium metal battery has a lithium metal core material. The lithium metal may define at least 50% by weight of the core material. The lithium metal may be pure lithium, or at least 95% lithium by weight, at least 97% lithium by weight, at least 99% lithium by weight, or intermediary percentages of lithium by weight. Alternatively, the lithium metal core material may be an alloy of lithium with at least one other metal. For example, the lithium metal anode may be a lithium-indium (Li-In) alloy, a lithium-magnesium (Li-Mg) alloy, a lithium- aluminum (Li-Al) alloy, or the like. In the alloy compositions, the lithium component may represent between about 50 wt.% and about 90 wt.% of the alloy. The other metal or metals may represent about 10 wt.% to about 50 wt.% of the alloy.

[0021] In one or more embodiments, the cathode comprises any suitable intercalation compound such as those known in lithium ion battery art. The intercalation-type cathode material may include phosphates, fluorophosphates, fluorosulfates, fluorosilicates, spinels, lithium-rich layered oxides, composite layered oxides, and the like. Further examples of suitable cathode materials include: spinel structure lithium metal oxides, layered structure lithium metal oxides, lithium-rich layered structured lithium metal oxides, lithium metal silicates, lithium metal phosphates, metal fluorides, metal oxides, sulfur, and metal sulfides. Non-limiting examples of specific cathode active materials include lithium cobalt oxide (e.g., LiCoC ), lithium nickel manganese cobalt oxides (“NMC”), and lithium nickel cobalt aluminum oxides (e.g., LiNi0.8Co0.15Al0.05O2). The formula for NMC may be LiNixMn _y Co _z Ow, where 0 < x < 1, 0 < y < 1, 0 < z < l, x + y + z = l, and 0 < w < 2. NMC cathode materials include, but are not limited to, electrically active materials containing LiNio.33Mno.33Coo.33O2, LiNio.6Mno.2Coo.2O2 (referred to herein as NMC 622), LiNio.8Mno.1Coo.1O2, and LiNio.5Mno.3Coo.2O2.

[0022] The electrolyte may be any suitable electrolyte such as those known in the art and may be a liquid or solid electrolyte or combination thereof. The electrolyte of the lithium metal battery may be a liquid electrolyte such that the electrolyte has a liquid, gel, or other non-solid and non gas phase. In at least one embodiment, the liquid electrolyte includes one or more organic solvents, such as carbonates, and a lithium salt. The one or more organic solvents may include ethylene carbonate (EC), ethyl methyl carbonate (“EMC”), diethyl carbonate (“DEC”), fluoroethylene carbonate (“FEC”), trifluoropropylene carbonate (“TFPC”), propylene carbonate (“PC”), and/or the like. The lithium salt may include lithium hexafluorophosphate (LiPFe). lithium bis(fluorosulfonyl)imide (“LiFSI”), lithium difluoro(oxalato)borate (“LiDFOB”) ( LiBF ₂ (C ₂ 0 ₄ ), and/or the like. In an alternative embodiment, the liquid electrolyte according to an embodiment may be added to a solid state battery which has a solid electrolyte. Adding the liquid electrolyte to the solid electrolyte could enhance the performance of the battery.

[0023] The lithium metal batteries herein have a formulated coating on the lithium metal anode that stabilizes the anode during cycling. The result is improved performance by reducing capacity fade. The anode is formulated to have at least one passivation layer that coats the lithium metal core or base. The passivated lithium metal anode significantly improves the cycle life of the lithium metal battery relative to a lithium metal battery that has a non-pas sivated lithium metal anode of the same composition as the lithium metal core material.

[0024] The passivation layer includes multiple materials (i.e., first and second materials). The materials may be reagents that are present in a solution during the deposition process that may react at least partially with the underlying lithium metal core material. After deposition, the passivation layer(s) may include reaction products between the materials (first and second), between the materials and other components in the battery, and between the materials and the lithium metal core material. A passivation layer may be a coating which represents the deposited coating material alone or a reaction product of the deposited coating material with the lithium metal core. For example, a passivation coating may include the first or second material itself and/or a reaction product of said material with lithium and/or another element in the core or other component in the battery. In one example, a single passivation layer includes the first and second material. In another example, the passivation layer of may be comprised two or more layers with each layer being of a different composition. For example, the passivation layer may be comprised of one layer having the first material and a second layer having the second material. It is understood that such layers may have differing concentrations of the first and second materials. The passivation layer may reduce capacity fade by preventing, or at least reducing, liquid electrolyte decomposition on the lithium metal anode in secondary batteries, such as NMC/Li cells. The passivation layer is applied to the lithium metal anode prior to cell assembly. For example, the lithium metal anode may be pre-coated with the passivation layer(s) prior to the anode being inserted into a cell package or housing and prior to introducing the liquid electrolyte into the cell package. [0025] The number, arrangement, and composition of the one or more passivation layers are selected based on application- specific parameters. For example, different materials (first and second) provide different aspects of protection to the lithium metal anode. Inorganic materials may enhance mechanical robustness of the passivation layer(s) which may inhibit or prohibit dendrite formation. Polymer materials may enhance elasticity of the passivation layer(s) which may suppress volumetric expansion of the anode [may enhance the mechanical integrity of the anode by elastically compressing the anode’s volume changes occurring during cycling of the battery. The use of multiple different materials in combination can remedy or postpone multiple forms of cell failure, therefore increasing the cell life.

[0026] The material in the the passivation layer which may have multiple layers of differing composition includes a first material comprised of lithium nitrate (LiNCL), and a second material comprised of one or more a lithium borate salt such as LiDFOB, polyvinylidene fluoride (“PVDF”) (CFhCF2)n, and/or (vinylbenzyl) trimethylammonium chloride (“VBTC”) (H ₂ C=CHC ₆ H ₄ CH ₂ N(CH ₃ ) ₃ C1) as previously described. Lithium nitrate is an example of an inorganic additive. PVDF is an example of a polymer additive. In an embodiment, the passivation layer includes lithium nitrate as a first material and one of LiDFOB, PVDF, or VBTC as a second material. Both the lithium nitrate and the second material may be present in a common passivation layer surrounding the lithium metal core material. In another embodiment, the lithium nitrate and the second material are present in different layers within the passivation layer. For example, an inner passivation layer that is applied directly on the surface of the lithium metal may include lithium nitrate and/or a reaction product between lithium nitrate and the lithium core material. The inner passivation layer may lack or have less of the second material and any reaction product of the second material. An outer passivation layer that is applied directly on the inner passivation layer may include the second material and/or a reaction product between the second material and the lithium core material. The outer passivation layer may lack or have less of the lithium nitrate and any reaction product of lithium nitrate. In another example, the aforementioned arrangement is inverted such that the inner passivation layer is characterized by the presence of the second material and lacking or having less of lithium nitrate, and the outer passivation layer is characterized by the presence of lithium nitrate and lacking or having less of the second material. [0027] In an embodiment, the passivation layer or layers of the passivation layer may be formed on the lithium metal core material by a coating process. For example, a solution is formed that includes at least one material (first or second material) in an organic solvent. When multiple materials are incorporated into the same passivation layer, each of the additives may be dissolved into an organic solvent forming a solution. For example, to form a passivation layer that includes lithium nitrate, the lithium nitrate is mixed with the organic solvent to form a solution. The organic solvent may be any suitable organic solvent capable of dissolving the additive or additives at the desired concentration for depositing onto the lithium core material. Typically, the organic solvent is a polar aprotic solvent that does not react with the lithium metal of the lithium metal core material at the coating conditions such as an ether. For example, the organic solvent may be tetrahydrofuran (“THF”) (C4H8O). The solution may be applied to the lithium metal core material in an inert environment to coat the surface. The concentration of the materials applied to the lithium metal may be controlled by controlling the additive concentration in additive solution and/or by controlling the volume of additive solution coated onto the surface of the lithium metal. The coated lithium metal anode is subsequently dried to remove the organic solvent while retaining the first and/or second material on the surface of the lithium metal. During the drying process, the deposited materials may react with or otherwise intermolecularly bond to the lithium metal to form a chemically-derived passivation layer on the lithium metal core material. The drying process may have a duration of hours, such as from 1 hour to 10 hours. The drying process may include passive, atmospheric drying and/or active drying in the form of vacuum drying, heating, or the like. In a non-limiting example, the coated lithium metal core material is dried for a period of 3-4 hours using a combination of both atmospheric drying and vacuum drying.

[0028] The passivation layer may be comprised of 2 or more layers having different compositions. For example, a first or inner layer coats the surface of the lithium metal core material, and a second or outer layer is subsequently applied to coat the first or inner layer of the passivation layer. The first layer, which is between the lithium metal core and the second passivation layer, is referred to herein as an inner passivation layer. The second passivation layer is referred to herein as an outer passivation layer. The passivation layer may include more than two layers coated upon the lithium metal core material. [0029] The order of addition of multiple layers and their composition within the passivation layer may have an effect on the resulting passivation. Characteristics of the layering, such as the composition of the layers, the order of the layers, and the thickness of the layers, can be controlled to fine tune the nature of the passivation layers and therefore better control the subsequent cell cycle life.

[0030] In an embodiment, the inner passivation layer is formed from a first solution or dispersion that only has lithium nitrate and the outer passivation layer is formed from a second solution or dispersion that only has the second material (e.g., LiDFOB, PVDF, or VBTC), where the first solution is first applied forming the inner passivation layer that is then coated with the second solution. Or, the order of the deposition of the first and second solution is reversed.

[0031] After the solvent of the first solution is removed forming the inner passivation layer coated upon the lithium metal core material, the second additive may be applied to the coated Li metal core material by coating it with a second solution form the outer passivation layer after removal of the solvent. Optionally, the drying period may be abbreviated when a subsequent passivation layer is to be formed, such as from about 30 minutes to 1 hour instead of about 3-4 hours. The second solution may be applied to the lithium metal core material to coat the inner passivation layer. Then, the lithium metal anode is dried again to remove the organic solvent of the second solution and form the outer passivation layer. The outer passivation layer is formed directly on the inner passivation layer such that the inner passivation layer is disposed between the surface of the lithium metal core material and the outer passivation layer.

[0032] The specific formulations of the lithium metal batteries disclosed herein were identified and confirmed via experimental testing. For the testing, battery cells were formed in a high purity argon filled glove box (M-Braun, O2 and humidity content < 0.1 ppm). A first experiment involved constructing four lithium metal batteries that include a metal oxide cathode active material. For example, the cathode active material comprised nickel, manganese, and cobalt, and more specifically was NMC 622. The cathode active material was formed into a cathode film. The anode active material in each of the four tested batteries was a 20 mhi lithium metal electrode formed into an anode film. The only differences between the four battery cells was in the anode, specifically the materials in the passivation layer and their arrangement in the passivation layer surrounding the lithium metal anode. [0033] In a first, reference battery cell, the lithium metal core material was untreated. The other three tested cells were formed according to embodiments of the inventive subject matter. In all three example cells, the respective anode was coated by a passivation layer that included a combination of the first and second material. For example, the passivation layer in a second battery cell included lithium nitrate and LiDFOB. A third battery cell included lithium nitrate and in the passivation layer. A fourth battery cell includes lithium nitrate and VBTC in the passivation layer. The second, third, and fourth battery cells differed from each other only in the type of material combined with lithium nitrate in the respective passivation layer. In the formation of the treated, passivated anodes for the second, third, and fourth battery cells, the two material were applied to the lithium metal during the same time period. For example, the lithium nitrate and the LiDFOB for the first battery cell were combined in a solution with at least one organic solvent (e.g., THF), and then the solution was coated or otherwise applied to the surface of the lithium metal in a casting process.

[0034] Each battery cell included the composite cathode film, a polypropylene separator (Celgard 2400, Celgard LLC. Charlotte, NC), and the lithium metal anode film. Electrolyte components were formulated and added to the battery cells, which were then sealed. A conventional carbonate-based electrolyte (1.2 M LiPF ₆ in EC (ethylene carbonate)/EMC (ethyl methyl carbonate) 3/7 wt/wt) with no additional additives was used in these experiments. The four NMC622/Li battery cells were cycled at 25° C between 3.0 to 4.3 V at a 0.3 C rate (1.0 mAh/cm ² current density).

[0035] Figure 1 is a graph 100 showing the discharge capacity over total cycles for multiple lithium metal batteries that have composite cathodes and different lithium metal anodes. Discharge capacity is measured in units of mAh/g. The total cycles refer to charging and discharging cycles. Figure 1 shows data lines for the four different lithium metal batteries tested in the first experiment. As described above, each of the four tested batteries has the same lithium metal anode active material and the same NMC 622 cathode active material. The four tested batteries also have the same electrolyte, which includes 1.2 M LiPF ₆ salt in a solvent mixture of EC and EMC. The four tested batteries differ only in the treatment of the anode active material, so the variation in discharge capacity over total cycles is attributable to the different anode compositions resulting from different treatments. For example, the first plot line 102 represents the reference battery cell in which the lithium metal is untreated. The lithium metal core material in each of the other three cells is coated with a respective passivation layer that includes two additives commonly disposed in the passivation layer. For example, two additives were simultaneously deposited (e.g., coated) on the surface of the lithium metal core in each of the second, third, and fourth test cells. The second plot line 104 represents the second battery cell in which the lithium metal is coated with a passivation layer that includes lithium nitrate and LiDFOB as a second material. The third plot line 106 represents the third battery cell in which the lithium metal is coated with a passivation layer that includes lithium nitrate and PVDF. The fourth plot line 108 represents the fourth battery cell in which the lithium metal is coated with a passivation layer that includes lithium nitrate and VBTC.

[0036] The data in the graph 100 shows that the first battery 102 having the non-treated, baseline lithium metal anode experienced significant capacity fade at around 40 cycles and lost significant capacity by the 65 ^th cycle. All three of the other battery cells 104, 106, 108 that included passivated lithium metal anodes showed substantially better cycling performance than the first battery 102. For example, all three battery cells 104, 106, 108 experienced capacity fade of no more than about 20 mAh/g (or no more than about 12% fade) from the initial capacity at the 100 ^th cycle, while the reference battery 102 faded about 160 mAH/g or over 90%. This data indicates that the combinations of lithium nitrate and the differing second materials in the passivation layer were successful at stabilizing the lithium metal in the anode and limiting the undesired processes that produce high resistance layers on the lithium metal anode surface and accelerate battery capacity fade. All three of the battery cells with passivated lithium metal anodes showed favorable performance and represent different embodiments of the lithium metal batteries disclosed herein. Between the three, the second battery 104 that included a combination of lithium nitrate and LiDFOB in the passivation layer performed the best at limiting capacity fade in the first 100 cycles, as the capacity fade was very limited (e.g., less than 5% of the initial capacity). The third battery 106 with the combination of lithium nitrate and PVDF performed the worst of the three preferred embodiments in the first 100 cycles.

[0037] A second experiment was performed that included constructing four lithium-lithium (Li/Li) symmetric battery cells. The Li/Li cells include lithium metal for both the anode active material and the cathode active material. The Li/Li cells were constructed similarly to the NMC composite/Li cells in the first experiment, except for the different cathode active materials. For example, the electrolytes used in the Li/Li battery cells was 1.2 M LiPF ₆ salt in a solvent mixture of EC and EMC. The lithium metal anode, lithium metal cathode, electrolyte, and separator were assembled into a cell package and then sealed. After sealing the Li/Li batteries were cycled at 25° C between -3.0 to 3.0 V using an alternating current of 1.0 mAh/cm ² for one hour periods.

[0038] The four Li/Li cells differed from each other only in the treatment of the lithium metal anodes, so performance differences are attributable to the different anode treatments. The first Li/Li battery cell is a reference or baseline cell in which the lithium metal of the anode is untreated (e.g., non-pas sivated). The anode of the second battery cell includes a single passivation layer that includes both lithium nitrate and LiDFOB in combination. The anode of the third battery cell includes an inner passivation layer that has lithium nitrate and an outer passivation layer that has LiDFOB. The lithium nitrate is not present in the outer passivation layer, and the LiDFOB is not present in the inner passivation layer. The inner passivation layer is disposed between the surface of the lithium metal and the outer passivation layer. For example, the inner passivation layer is formed directly on the surface of the lithium metal, and the outer passivation layer is subsequently formed directly on the surface of the inner passivation layer. The anode of the fourth battery cell is essentially the reverse of the anode of the third battery cell. The anode of the fourth battery cell has an inner passivation layer that includes LiDFOB and an outer passivation layer that includes lithium nitrate, such that the lithium nitrate passivation layer is applied subsequent to the LiDFOB passivation layer.

[0039] Figure 2 is a graph 200 showing voltage over time of the four tested Li/Li symmetric battery cells. The voltage is measured in units of volts (V), and the time is in hours (hr). The graph 200 includes markers 202, 204, 206, 208 that represent times at which an overpotential of the four tested Li/Li cells increases dramatically. The dramatic increase in cell overpotential is a sign of cell failure, and may be due to resistance increase and/or electrolyte consumption. As such, cells that experienced dramatic overpotential increases later in time performed better than cells that experienced the dramatic overpotential increases earlier because the cells took longer to fail.

[0040] The reference, first Li/Li battery cell that had an untreated lithium metal anode is associated with marker 202, meaning that overpotential increase occurred at about 55 hours. The second battery cell that included a single passivation layer with both lithium nitrate and LiDFOB experienced dramatic overpotential increase at about 320 hours, as indicated by marker 204. The third battery cell that had lithium nitrate in the inner passivation layer and LiDFOB in the outer passivation layer experienced dramatic overpotential increase at about 360 hours, as indicated by marker 206. The fourth battery cell that had LiDFOB in the inner passivation layer and lithium nitrate in the outer passivation layer experienced dramatic overpotential increase at about 405 hours, as indicated by marker 208.

[0041] The data in the graph 200 shows that each of the three Li/Li battery cells with passivated (e.g., treated) lithium metal anodes operated for over 250 hours longer than the reference Li/Li battery cell before experiencing a dramatic increase in the overpotential, or more than six times the duration of the reference Li/Li battery cell. All three passivated Li/Li battery cells represent preferred embodiments of the inventive subject matter. Among the three preferred embodiments, the third and fourth cells that include multiple, stacked passivation layers performed better than the second cell that has a single passivation layer. The experimental results may indicate that multiple layers within the passivation having different compositions better stabilize the lithium metal in the anode and better limit the undesired processes that produce high resistance layers on the lithium metal anode surface and accelerate battery capacity fade, relative to a passivation layer of uniform composition throughout.

[0042] Furthermore, the fourth battery cell 208 which had LiDFOB in the inner passivation layer and lithium nitrate in the outer passivation layer performed the best of the preferred embodiments, even better than the third battery cell which had the inverse passivation layer arrangement. Stated differently, the only difference between the third battery cell 206 and the fourth battery cell 208 is in the stacking order of the passivation layers. Both cells 206, 208 have one layer characterized by lithium nitrate and another layer characterized by LiDFOB . Although both cells represented by the 206 and 208 in Figure 2 with stacked passivation layers performed better than the cell 204 with a single passivation layer that has multiple additives, and vastly superior to the control cell 202, the fact that the fourth cell performed better than the third cell indicates that the order of deposition affects cell performance. Without being bound by a particular theory, having an inorganic additive, such as lithium nitrate, in the outer passivation layer may enhance the mechanical robustness of the anode and limit dendrite formation to a greater extent than if the inorganic additive is in the inner passivation layer and covered by the LiDFOB in the outer passivation layer. It was not expected that merely varying the order of deposition of the same two additives would have such an affect on the cell cycling performance.

[0043] Figure 3 is a flow chart 300 of a method for forming a lithium metal battery according to a first embodiment. At 302, a first solution is formed that includes with a desired dissolved material and an organic solvent such as THF. The dissolved material may be for example lithium nitrate, lithium borate salt (e.g., LiDFOB), PVDF, or VBTC.

[0044] At 304, a first solution is applied to coat the surface of a lithium metal anode. For example, the lithium metal anode may be dipped into the first solution or the first solution may be deposited, for example, by painting, spraying or any known method of depositing a liquid on a film or surface, onto the lithium metal anode. The first solution may coat a portion (e.g., one face of a lithium metal sheet or anode) or all of the surface of the lithium metal anode.

[0045] At 306, the lithium metal anode coated by the first solution is dried to remove the organic solvent and form a passivation layer on the lithium metal anode. In an embodiment, the passivation layer with the first material is an inner passivation layer that is applied directly on the surface of the lithium metal.

[0046] At 308, a second solution is formed that includes another dissolved material and an organic solvent. The second solution has a different dissolved material from the first solution. For example, the second solution may have one or more of lithium nitrate, LiDFOB, PVDF, or VBTC that is not the same as the dissolved material in the first solution (e.g., differing compound(s) or concentrations of the same compound(s)) of the first solution. For example, if the first solution has lithium nitrate, the second solution may have one of LiDFOB, PVDF, or VBTC dissolved therein. Alternatively, if the first solution has LiDFOB, PVDF, or VBTC dissolved therein, the second solution has lithium nitrate dissolved therein. The organic solvent in the first and second solution may be THF.

[0047] At 310, after the lithium metal anode is dried at step 306 to form the inner passivation layer, the second solution is applied to coat the inner passivation layer of the lithium metal anode. The second solution may be applied similarly to the application of the first solution to the surface of the lithium metal. At 312, the lithium metal anode is dried after applying the second solution, which removes the organic solvent and forms an outer passivation layer on the inner passivation layer.

[0048] At 314, the lithium metal battery is assembled to include the lithium metal anode as treated, a cathode, and an electrolyte in a cell package or housing. The electrolyte may include one or more carbonate solvents and a lithium salt. The cathode may have a composite active material that includes nickel, manganese, and cobalt (e.g., NMC).

[0049] Figure 4 is a flow chart 400 of a method for forming a lithium metal battery according to a second embodiment. At 402, a solution is formed that includes a first additive, a second additive, and an organic solvent. In an embodiment, the first additive is lithium nitrate, and the second additive is LiDFOB, PVDF, or VBTC. In a first example, the second additive is LiDFOB. In a second example, the second additive is PVDF. In a third example, the second additive is VBTC. Optionally, the organic solvent is THF.

[0050] At 404, the additive solution is applied to coat the surface of a lithium metal anode. For example, the lithium metal anode may be dipped into the additive solution or the additive solution may be deposited onto the lithium metal anode. The additive solution may coat the lithium metal anode along an entire surface area of the lithium metal anode.

[0051] At 406, the lithium metal anode coated by the additive solution is dried to remove the organic solvent and form a passivation layer on the surface of the lithium metal. The passivation layer includes a combination of the first and second additives. For example, the passivation layer may include both lithium nitrate and one of LiDFOB, PVDF, or VBTC.

[0052] At 408, the lithium metal battery is assembled to include the lithium metal anode as treated, a cathode, and an electrolyte in a cell package or housing. The electrolyte may include one or more carbonate solvents and a lithium salt. The cathode may have a composite active material that includes nickel, manganese, and cobalt (e.g., NMC).

[0053] As used herein, value modifiers such as “about,” “substantially,” and “approximately” inserted before a numerical value indicate that the value can represent other values within a designated threshold range above and/or below the specified value, such as values within 5%, 10%, or 15% of the specified value. [0054] It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) can be used in combination with each other. In addition, many modifications can be made to adapt a particular situation or material to the teachings of the various embodiments of the disclosure without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments of the disclosure, the embodiments are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments of the disclosure should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims and the detailed description herein, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Moreover, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. § 112(f), unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure.

[0055] This written description uses examples to disclose the various embodiments of the disclosure, including the best mode, and also to enable any person skilled in the art to practice the various embodiments of the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments of the disclosure is defined by the claims, and can include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or if the examples include equivalent structural elements with insubstantial differences from the literal language of the claims.