Guiding Growth of Solid-Electrolyte Interphases via Gradient Composition

An electrolyte structure for a battery cell with a lithium metal anode has a first side configured to contact the anode and a second side facing opposite the first side. The electrolyte structure includes a first region that is adjacent to the first side and extends towards the second side and a second region disposed between the first region and the second side. The first region has a first composition of materials that is electronically insulating such that the electrolyte is stable against the lithium metal anode. The second region has a second composition of materials that is different than the first composition and has typical electrolyte properties such as mechanical strength, stability against a cathode, and ionic conductivity. The first region and the second region define a compositional gradient across a thickness of the electrolyte structure. The compositional gradient is continuum-fabricated at one point via a gradient growth method.

FIELD

The disclosure relates to batteries and more particularly to a thin film electrolyte with a gradient composition for use in batteries.

BACKGROUND

Batteries are a useful source of stored energy that can be incorporated into a number of systems. Rechargeable lithium-ion (“Li-ion”) batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. In particular, batteries with a form of lithium (“Li”) metal incorporated into the negative electrode or anode afford exceptionally high specific energy (measured in Wh/kg) and energy density (measured in Wh/L) compared to batteries with conventional carbonaceous negative electrodes.

In Li-ion battery applications with Li metal anodes, unintended side reactions occur at the electrode/electrolyte interface. This side reaction consumes the electrode material, produces a solid-electrolyte interphase (SEI) layer, and consequently reduces the battery energy capacity. However, the SEI layer often does not substantially inhibit battery operation if it satisfies two criteria: (1) the SEI layer must be a Li ionic conductor, which allows the transport of Li ions for normal battery charging/discharging operations; and (2) the SEI layer must be a poor electronic conductor, preventing electron transport across the SEI layer. In contrast, if the material is both ionically and electronically conducting, the SEI layer may grow rather than passivate. Furthermore, Li metal is sometimes unintendedly deposited on top of the SEI layer rather than below the SEI layer, potentially leading to stranded Li upon cycling.

In view of the successful use of LiPON as a thin film electrolyte in battery applications, related lithium oxides and nitrides such as LiSiO, LiSiON, LiPSiON, LiPSiBON, and others may also be useful in such applications. However, stability calculations suggest that these materials can form SEI layers that are electronically and ionically conductive, which may inhibit their use in batteries. Stability calculations are performed using the convex hull analysis in multidimensional space. For example, suppose an objective is to find the lowest-energy materials at a given composition, such as Li4SiO4. Depending on the physics, it could be that the lowest total energy is achieved by one compound (e.g. Li4SiO4) or the phase separation of multiple compounds (e.g. 2(Li2O)+SiO2).

Typically, these total-energy calculations are performed via ab-initio computations in the density-functional theory formalism. A database is constructed with many such computations, and the “convex hull” is mapped out of lowest-energy compounds or groups of compounds. Projecting any point (such as Li4SiO4) onto the convex hull will give the decomposition products that provide the lowest energy. The “formation energy” of those products is the energy cost to form those products as opposed to keep them at some other endpoints. Importantly, the actual formation of these products depends on the kinetics of the reaction and is not easily predictable. Thus, the primary use of the analysis is to determine what products could possibly form.

During the cycling of a battery, the electrochemical potential of Li will change, so to find all SEI compositions that may form, the line between the electrolyte and the Li metal is examined, and that line is projected onto the convex hull. The decomposition products (intermediate vertices along the projected line) are further examined in order to test whether any of the products are electronic conductors (metals) by computing the predicted bandgap. A zero bandgap means there is a metal, whereas a nonzero bandgap indicates a semiconductor or insulator. This determination is subject to the usual constraints and accuracy of density-functional theory, but it is expected to be reasonably reliable to predict whether LiPSiBON compounds are insulators or metals, even if the bandgap is slightly inaccurate.

FIG. 6includes three charts that show the convex hull of LiPON, LiSiON, and LiBON compounds, respectively. The data was taken from the Materials Project, an open-source, web-based database of computed information on known and predicted materials. The data was analyzed with the MulPhaD module written by Georgy Samsonidze and plotting scripts written by Mordechai Kornbluth. The x-axis contains the composition ratio between the electrolyte and Li. The electrolyte for each line is given by the legend to the left of the plot. The y-axis contains the energy of formation in eV/atom. The curves describe the straight lines between electrolyte and Li projected onto the convex hull. The points describe the decomposition products, and are annotated by the product with the smallest bandgap, and are colored according to the magnitude of that smallest bandgap where zero bandgap (in red color) is not desirable and nonzero bandgap (in green color) is desirable.

LiPON contains products that are electronic insulators (nonzero bandgap), usually Li3P+Li3N+Li2O (not shown). Therefore, the SEI layer is an electronic insulator, leading to good performance of a LiPON electrolyte with a stable SEI layer. These characteristics enable LiPON to be useful in thin film batteries. However, LiSiO and LiBO form LiSi and LiB compounds (possibly with trace oxygenation), respectively, which are metallic. The typical decomposition may be Li5SiN3+Li2O+Li21Si5, where the last product is a metal. This characteristic poses a challenge for using LiSiO/LiBO against Li metal since the equipotential of Li on all sides of LiSi/LiB compounds will cause more LiSiO/LiBO to decompose into LiSi/LiB compounds such that the SEI layer will continuously grow.

What is needed, therefore, is a thin film electrolyte with a compositional gradient in which the side of the electrolyte that adheres to the anode has greater compositions of materials that are electronically insulating. The remainder of the electrolyte has other compositions of materials that exhibit traditional electrolyte properties, such as mechanical strength, stability against the cathode material, ionic conductivity, and others. It would be advantageous to provide the compositional gradient of the electrolyte by use of a single, gradient-producing process. A thin film electrolyte with a compositional gradient comprising multiple, independently fabricated layers of different compositions would also be advantageous.

SUMMARY

A battery cell in one embodiment includes a positive electrode, a negative electrode that includes lithium metal, and an electrolyte structure disposed between the negative electrode and the positive electrode, the electrolyte structure including a first side configured to contact the negative electrode and a second side spaced from the first side and facing the positive electrode, the first side and the second side defining a thickness, a first region disposed adjacent to the first side and extending towards the second side, the first region having a first composition of materials that is electronically insulating, and a second region disposed between the first region and the second side, the second region having a second composition of materials that is different than the first composition, the first region and the second region defining a compositional gradient across the thickness of the electrolyte structure.

A thin film electrolyte structure for a battery cell in one embodiment includes a first side configured to contact a lithium metal anode of the battery cell and a second side facing opposite the first side, the first side and the second side defining a thickness, a first region disposed adjacent to the first side and extending towards the second side, the first region having a first composition of materials that is configured to be stable against the lithium metal anode, and a second region disposed between the first region and the second side, the second region having a second composition of materials that is different than the first composition, the first region and the second region defining a compositional gradient across the thickness of the electrolyte structure.

DETAILED DESCRIPTION

FIG. 1depicts an electrochemical cell100. The electrochemical cell100includes an anode102, a cathode104with an aluminum (“Al”) current collector106, and an electrolyte with a compositional gradient110(hereinafter the “gradient-composition electrolyte110” or “GCE110”). The anode102, the cathode104, the Al current collector106, and the gradient-composition electrolyte110are referred to collectively as “cell components” or “components of the cell” hereinafter for efficiency. The anode102includes Li metal or some other Li-insertion material that can reversibly insert and extract Li ions electrochemically. The anode102is sized such that it has at least as much capacity as the associated cathode104, and preferably at least 10% excess capacity and up to greater than 50% capacity in some embodiments. The Al current collector106is typically less than 30 μm in width and preferably less than 15 μm. In some embodiments, the Al current collector106has a surface treatment.

The cathode104includes a mixture of at least an active material and a matrix configured to conduct the primary ions of relevance to the cell100. The active material in various embodiments includes a sulfur or sulfur-containing material (e.g., PAN-S composite or Li2S); an air electrode; Li-insertion materials such as NCM, LiNi0.5Mn1.5O4, Li-rich layered oxides, LiCoO2, LiFePO4, LiMn2O4; Li-rich NCM, NCA, and other Li intercalation materials, or blends thereof; or any other active material or blend of materials that react with and/or insert Li cations and/or electrolyte anions.

The matrix in various embodiments includes Li-conducting liquid, gel, polymer, or other solid electrolyte. Solid electrolyte materials in the cathode104may further include lithium conducting garnets, lithium conducting sulfides (e.g., Li2S—P2S) or phosphates, Li3P, LIPON, Li-conducting polymer (e.g., polyethylene oxide (PEO) or polycaprolactone (PCL)), Li-conducting metal-organic frameworks, Li3N, Li3P, thio-LISiCONs, Li-conducting NaSICONs, Li10GeP2S12, lithium polysulfidophosphates, or other solid Li-conducting material. Other materials in the cathode104may include electronically conductive additives such as carbon black, binder material, metal salts, plasticizers, fillers such as SiO2, or the like. The cathode materials are selected to allow sufficient electrolyte-cathode interfacial area for a desired design. The cathode104may be greater than 1 m in thickness, preferably greater than 10 μm, and more preferably greater than 40 μm. In one embodiment, the composition of the cathode104includes approximately 60 to 85 weight percent active material, approximately 3 to 10 weight percent carbon additive, and 15 to 35 weight percent catholyte.

The gradient-composition electrolyte110in the embodiment shown inFIG. 1has an anode side112and a cathode side114spaced from anode side112in a first direction116. The anode side112of the gradient-composition electrolyte110is configured to adhere to or otherwise contact the anode102. The cathode side114of the gradient-composition electrolyte110is configured to adhere to or otherwise contact the cathode104. The anode side112of the gradient-composition electrolyte110defines an anode-facing surface120that faces an anode surface122of the anode102. The cathode side114of the gradient-composition electrolyte110defines a cathode-facing surface124that faces a cathode surface126of the cathode104.

The first direction116(viewed leftward or rightward inFIG. 1) corresponds generally to a respective thickness of the gradient-composition electrolyte110and respective thicknesses of the other cell components. A second direction118(viewed upward or downward inFIG. 1) corresponds generally to a respective height of the gradient-composition electrolyte110and respective heights of the other cell components. A third direction (not shown, but viewed perpendicular to the viewing plane ofFIG. 1) corresponds generally to a respective width of the gradient-composition electrolyte110and respective widths of the other cell components.

The gradient-composition electrolyte110ofFIG. 1generally contains one or more of lithium, silicon, phosphorous, boron, oxygen, fluorine, and nitrogen. The gradient-composition electrolyte110is grown via a gradient sputtering process or any other gradient-composition formation process as described below with reference toFIG. 4. These formation processes guide the growth of the anode-facing layer to a desired composition with desired properties.

The gradient-composition electrolyte110has greater compositions of materials that are electronically insulating on the anode-side112than on the cathode-side114so that the gradient-composition electrolyte110is stable against the anode102. Such electronically insulating compositions generally include lithium, phosphorous, oxygen, fluorine, and nitrogen, which are known enhance electric resistivity. Such electronically insulating compositions more particularly include compositions that are closer to one or more of Li3P and Li3N. The greater compositions of materials that are electronically insulating are encompassed in a first region113of the gradient-composition electrolyte110with a thickness of about 500 nm or thinner measured from the anode-side112. In other embodiments, the first region113containing the greater compositions of materials that are electronically insulating has a thickness of about 100 nm measured from the anode-side112. As used herein, a thickness measured “from” an indicated side of an element or feature means that the thickness is measured from that indicated side in a direction of shortest extent towards the opposite of side of the element or feature. For example, since the anode side112and the cathode114side of the of the gradient-composition electrolyte110illustrated inFIG. 1are roughly parallel, a thickness measured from the anode side112means the thickness is measured from the anode side112in a direction perpendicular to anode side112and extending towards the cathode side114. More specifically, the thickness is measured in the first direction116.

The gradient-composition electrolyte110has a second region115starting from an approximate end or boundary of the first region113and moving away from the anode102in the first direction116towards the cathode side114. In the second region115, the gradient-composition electrolyte110has other compositions that exhibit more typical electrolyte properties, such as mechanical strength, stability against the cathode material, ionic conductivity, and other properties. The thickness of the second region115of the gradient-composition electrolyte110varies depending on the thickness of the first region113and the total thickness of the gradient-composition electrolyte110. For instance, in an embodiment of the gradient-composition electrolyte110with a total thickness of 5,000 nm and a first region113thickness of 500 nm, the second region115will have a thickness of approximately 4,500 nm. In another embodiment of the gradient-composition electrolyte110with a total thickness of 25,000 nm and a first region113thickness of 100 nm, the second region115will have a thickness of approximately 24,000 nm. In the embodiment ofFIG. 1, the gradient-composition electrolyte110is the primary and sole electrolyte and separator.

In the gradient-composition electrolyte110shown inFIG. 1, the compositional gradient from the anode side112to the cathode side114is discrete through a transition117from the first region to the second region such that the quantity of a given material composition in one region changes abruptly and distinctly for a given increment in the first direction to another region. In other embodiments of the gradient-composition electrolyte110such as that shown inFIG. 1A, the compositional gradient from the anode side112to the cathode side114is smooth, continuous, and/or gradual through the transition117from the first region to the second region.

FIG. 2depicts an electrochemical cell200. The cell200is similar to the cell100ofFIG. 1in that the cell200includes the anode102, the cathode104with the Al current collector106, and a gradient-composition electrolyte210. The cell200also includes an auxiliary electrolyte211disposed between the gradient-composition electrolyte210and the cathode104. The gradient-composition electrolyte210ofFIG. 2is essentially identical to the gradient-composition electrolyte110ofFIG. 1. The gradient-composition electrolyte210has an anode side212that is configured to adhere to or otherwise contact the anode102. The anode side212defines an anode-facing surface220that faces the anode surface122of the anode102. A first region213of the gradient-composition electrolyte210includes greater compositions of materials that are electronically insulating. The first region213is disposed adjacent to the anode side212.

One difference between the cell100and the cell200is that the gradient-composition electrolyte210has an auxiliary side214that is configured to adhere to or otherwise contact the auxiliary electrolyte211. The auxiliary side214of the gradient-composition electrolyte210defines an auxiliary-facing surface224that faces an auxiliary surface226of the auxiliary electrolyte211. The gradient-composition electrolyte210has a second region215disposed between the first region213and the auxiliary side. In the second region215, the gradient-composition electrolyte210has other compositions that exhibit more typical electrolyte properties, such as mechanical strength, stability against the cathode material, ionic conductivity, and other properties. Another difference between the cell100and the cell200is that the total thickness of the gradient-composition electrolyte210is approximately 1,000 to 5,000 nm, which is smaller than the total thickness of the gradient-composition electrolyte110ofFIG. 1.

The auxiliary electrolyte211is configured to have high ionic conductivity. The thickness of the auxiliary electrolyte211is in the range of 10 to 20 μm. The auxiliary electrolyte211in one embodiment is configured as a liquid electrolyte in pores of conventional polyolefin separator. The auxiliary electrolyte211in another embodiment is configured as a polymer separator. In yet another embodiment, the auxiliary electrolyte211is configured as a ceramic separator such as sulfide with approximately 1e-3 S/cm or higher.

FIG. 3depicts an electrochemical cell300with an alternative gradient-composition electrolyte310. The cell300is similar to the cell100(FIG. 1) and the cell200(FIG. 2) in that the cell300includes the anode102, the cathode104with the Al current collector106, and a gradient-composition electrolyte310. One difference in the cell300is that the gradient-composition electrolyte310includes multiple independent layers that are not grown in a single gradient process. The gradient-composition electrolyte310includes a first layer312positioned adjacent to the anode102, a second layer314positioned adjacent to the first layer312, and a third layer316positioned adjacent to and between the second layer314and the cathode104.

The first layer312has a first anode-facing side318and a first cathode-facing side320spaced from the first anode-facing side318in the first direction. The first anode-facing side318of the gradient-composition electrolyte310is configured to adhere to or otherwise contact the anode102. The second layer314has a second anode-facing side322and a second cathode-facing side324spaced from the second anode-facing side322in the first direction. The second anode-facing side322of the second layer314is configured to adhere to or otherwise contact the first cathode-facing side320of the first layer312. The third layer316has a third anode-facing side326and a third cathode-facing side328spaced from the third anode-facing side326in the first direction. The third anode-facing side326of the third layer316is configured to adhere to or otherwise contact the second cathode-facing side324of the second layer314. The third cathode-facing side328of the third layer316is configured to adhere to or otherwise contact the cathode104.

The first layer312has a thickness of approximately 50 nm and contains LiPON or another electrolyte that has relatively poor ionic conductivity (i.e., approximately 1e-6 S/cm) and resistivity of approximately 5 Ωcm2and desirable properties for an SEI such as electronic resistivity. The second layer314has a thickness of approximately 0.5 μm and contains LiSiPON or another glass that is electronically conducting but has moderate ionic conductivity (i.e., approximately 1e-5 S/cm) and resistivity of 5 Ωcm2. The third layer316has a thickness of approximately 20 μm and constitutes a separator with high-conductivity. The third layer316in one embodiment is configured as a liquid electrolyte in pores of conventional polyolefin separator. The third layer316in another embodiment is configured as a polymer separator. In yet another embodiment, the third layer316is configured as a ceramic separator such as sulfide with approximately 1e-3 S/cm or higher and resistivity of 2 Ωcm2. Assuming negligible interfacial impedance, the entire gradient-composition electrolyte310has a resistivity of approximately 12 Ωcm2.

FIG. 4depicts a process400of forming the gradient-composition electrolyte110ofFIG. 1or the gradient-composition electrolyte210ofFIG. 2. A first region of a gradient-composition electrolyte110,210is formed using a gradient sputtering process, such as disclosed in Maibach et al., J. Phys. Chem. Lett. 2016 https://dx.doi.org/10.1021/acs.jplett.6b00391 and Phuoc and Ong, IEEE Trans. Mag. 2014 https://doi.org/10.1109/TMAG.2013.2296936, or a similar gradient-composition forming process (block402). The first region contains a first composition of materials that in some embodiments includes one or more of lithium, phosphorous, oxygen, fluorine, and nitrogen. The first composition of materials is electronically insulating and in some embodiments includes compositions that are closer to one or both of Li3P and Li3N. A second region of the gradient-composition electrolyte110,210is then continuum-fabricated on the first region using the gradient sputtering process or the similar gradient-composition forming process (block404). The second region contains a second composition of materials that has compositions that exhibit typical electrolyte properties, such as mechanical strength, stability against the cathode material, ionic conductivity, and other properties.

The process400has a number of technical advantages: (1) It is expected to be easier and cheaper to produce the gradient-composition electrolyte than multiple layers of electrolyte stacked upon each other, because the entire electrolyte is grown with a single process. (2) The anode-facing layer has a composition and thickness than can be controlled easier than a naturally-forming SEI layer. (3) The adhesion between different parts of the electrolyte is expected to be better (causing lower interfacial resistance) because they are grown as one unit.

FIG. 5depicts a process500of forming the gradient-composition electrolyte310ofFIG. 3. A first layer of a gradient-composition electrolyte310is discretely formed using a deposition process (block502). The first layer has a first composition of materials that is electronically insulative and has relatively poor ionic conductivity. A second layer of the gradient-composition electrolyte310is discretely formed using the deposition process (block504). The second layer has a second composition of materials that has some electronic conductivity and has moderate ionic conductivity. A third layer of the gradient-composition electrolyte310is discretely formed using the deposition process (block506). The gradient-composition electrolyte310is formed by stacking the first layer, the second layer, and the third layer upon each other. In one embodiment of the process500, the layers are independently fabricated and then attached with a joining process. In another embodiment of the process500, each layer forms a substrate for processing of the next layer.

The gradient-composition electrolyte disclosed herein as well as batteries and devices which include the gradient-composition electrolyte can be embodied in a number of different types and configurations. The following embodiments are provided as examples and are not intended to be limiting.

Embodiment 1: An electrolyte with a compositional gradient (henceforth GCE, gradient-composition electrolyte), where one side has the property of being stable against a Li-metal anode, and the rest has other desired properties such as mechanical strength and ionic conductivity.

Embodiment 3: Where some or all of the GCE is fabricated via sputtering.

Embodiment 4: Where the compositional gradient is continuum-fabricated at one point (such as a substrate surface) via a gradient-growth method, such as gradient sputtering.

Embodiment 5: Where the compositional gradient is discrete, formed by stacking multiple electrolyte layers upon each other, whether independently fabricated or by each layer forming a substrate for processing the next layer.

Embodiment 6: Where the entire GCE is placed upon another supporting structure; or is directly applied to the cathode.

Embodiment 7: Where the GCE next to the anode contains any or all of: lithium, phosphorous, oxygen, fluorine, and nitrogen; which are known enhance electric resistivity.

Embodiment 8: Where the electronically-insulating component of the GCE (anode-facing layer) has thickness of 500 nm or less, and ideally less than 100 nm.

Embodiment 9: Where the sputtered part of the GCE is 5 μm or less, and ideally less than 1 μm.