Patent ID: 12188116

DETAILED DESCRIPTION

Lithium phosphorus oxynitride (LiPON) is a popular amorphous solid electrolyte used in thin-film solid-state lithium ion batteries. An electrolyte in a battery serves as a medium between an anode and a cathode of the battery, which provides electrical isolation between the anode and cathode while allowing ions (e.g., lithium ions) to migrate between the anode and cathode during the battery operation—e.g., charging and discharging. Thin-film solid-state batteries includes multiple layers of various materials formed by thin-film deposition techniques, such as RF sputtering, pulsed laser deposition, or atomic layer deposition. Typically, such thin-film deposition techniques require rigid substrates (e.g., substrates including silicon, silicon-oxide, alumina, or the like), on which the thin-films of various materials are deposited. Accordingly, the thin-film solid-state batteries are built on rigid substrates.

In some cases, the rigid substrates in the thin-film solid-state batteries can hinder progress of proliferating applications of the thin-film solid-state batteries (e.g., due to form factor limitations) or of establishing fundamental scientific knowledge of the thin-film materials used in the thin-film solid-state batteries. For example, LiPON is an amorphous solid-state lithium ion conductor displaying superb cyclability against lithium metal anodes. But there is no definitive explanation for this stability due to the limited understanding of the structure of LiPON. In some cases, the amorphous nature of LiPON may make it difficult to analyze LiPON deposited on the substrate by the thin-film deposition techniques. Additionally, or alternatively, presence of the substrate (typically a few hundred micrometers (μm) thick) carrying the material of interest (e.g., LiPON thin films, typically a few μm thick) may dominate analytical signals to make scientific studies difficult to establish fundamental material properties—e.g., LiPON's mechanical properties and glassy transition behavior.

The present technology disclosed in this patent document facilitates production of a LiPON thin film in a novel free-standing form (a free-standing LiPON thin film) without a rigid substrate. The free-standing LiPON thin films synthesized in accordance with the present technology exhibit a high degree of flexibility highlighting the unique mechanical properties of glassy materials. Moreover, the free-standing LiPON thin film can be at least partially transparent to visible light.

As described in more detail herein, the present technology modifies a surface of a substrate prior to depositing a layer of LiPON (e.g., via RF sputtering a Li3PO4target) on the modified surface in contrast to conventional techniques that directly deposit LiPON on the surface of the substrate without modifying or treating the surface. In some embodiments, modifying the surface of the substrate includes depositing a sacrificial layer on the surface of the substrate. The sacrificial layer can be configured to have (or selected to have) a removal rate that is greater than that of the layer of LiPON. In this manner, the sacrificial layer can be selectively removed (e.g., etched, dissolved) without significantly affecting the layer of LiPON, thereby separating (e.g., loosening, releasing) the layer of LiPON from the substrate to obtain the free-standing LiPON thin films.

This patent document also describes a variety of analytical results that confirm the free-standing LiPON thin films (i.e., LiPON deposited on the sacrificial layer) include consistent material properties to those of conventional LiPON thin films (i.e LiPON deposited on solid substrates). By way of examples for the conventional LiPON thin films that are directly deposited on solid substrates, a study by Bates et al. reported that LiPON thin film can be synthesized by RF sputtering using Li3PO4target in a N2atmosphere with an ionic conductivity of 2×10−6S/cm (see J. B. Bates, N. J. Dudney, G. R. Gruzalski, R. A. Zuhr, A. Choudhury, C. F. Luck, Electrical properties of amorphous lithium electrolyte thin films. Solid State Ionics. 53-56 (1992)647-654); a study of Kozen et al. illustrated that a LiPON thin film can be synthesized by atomic layer deposition using tert-butoxide, H2O, trimethylphosphate and plasma N2as precursors with an ionic conductivity of 3×10−7S/cm (see A. C. Kozen, A. J. Pearse, C. F. Lin, M. Noked, G. W. Rubloff, Atomic Layer Deposition of the Solid Electrolyte LiPON. Chem. Mater. 2015, 27 (15), 5324-5331); and a study of Zhao et al. reported that pulsed laser deposited LiPON thin film using Li3PO4target in a N2gas atmosphere exhibits an ionic conductivity of 1.6×10−6S/cm (see S. Zhao, Z. Fu, Q. Qin, A Solid-State Electrolyte Lithium Phosphorus Oxynitride Film Prepared by Pulsed Laser Deposition. Thin Solid Films. 2002, 415 (1-2), 108-113).

The free-standing LiPON thin films can largely motivate and ease additional characterization methods to potentially achieve a scientific breakthrough in understanding this material. Moreover, the free-standing LiPON thin films can increase the flexibility of geometries for devices including the free-standing LiPON thin films (e.g., no longer requiring rigid substrates that tend to impose restrictive form factor requirements), and may be applied to a variety of alternative architectures. It is envisioned that the free-standing LiPON thin films can be used as solid electrolytes in all-solid-state battery systems or as separator materials in liquid-electrolyte-based battery systems—e.g., placed between the anode and cathode of the thin-film solid-state lithium ion batteries. Moreover, the free-standing LiPON thin films can also be used as a flexible electronically insulating layer. In some embodiments, the free-standing LiPON thin films can be used in electrochromic window applications in view of its transparent nature to the visible light, in which transparency of glass windows can be electrically controlled—e.g., smart windows applications.

Also disclosed in this patent document are novel material structures of the free-standing LiPON thin films determined via experimental and computational spectroscopic methods. As described in more details herein, the material structures of the free-standing LiPON thin films include representations of a structural model of RF-sputtered LiPON. The material structures are determined based on extensive experimental studies performed on the free-standing LiPON thin films. Moreover, computational spectroscopic methods are used to compare the experimental results with theoretical results that are deduced based on first principles and ab initio approaches.

For example, 1-dimensional (1D) and 2-dimensional (2D) solid-state nuclear magnetic resonance (NMR) experiments are performed on the layer of free-standing LiPON thin films to investigate chemical shift anisotropy and dipolar interactions. The NMR experiments generate information about the short-range structure of the free-standing LiPON thin films, which have been confirmed to be in an amorphous phase as described herein. Further, chemical shielding calculations of Li—P—O/N structures are carried out based on first principles and ab initio molecular dynamics-generated amorphous LiPON models. Subsequently, the information generated by the NMR experiments and the chemical shielding calculations are compared to identify the glassy structure of the free-standing LiPON thin film as primarily isolated phosphate monomers with nitrogen (N) incorporated in both apical and as bridging sites in phosphate dimers. The structural model suggests that LiPON's stability is a result of its glassy character. Further details of characterizing material structures and properties of the free-standing LiPON thin films are described in the above-identified U.S. Provisional Application No. 63/067,801.

FIGS.1A and1Billustrate example process steps and photographs depicting certain process steps in accordance with embodiments of the present technology.FIGS.2through5describe various analytical results confirming that the free-standing LiPON thin films include material properties consistent with the conventional LiPON thin films deposited on solid substrates.FIGS.6and7are flowcharts of methods of synthesizing the free-standing LiPON thin films in accordance with embodiments of the present technology.

FIG.1Ashows schematic diagrams illustrating example process steps to synthesize free-standing LiPON thin films in accordance with embodiments of the present technology.FIG.1Aincludes diagrams101through106depicting various process steps to synthesize free-standing LiPON thin films.

Diagram101illustrates a substrate110. In some embodiments, the substrate110include silicon, silicon oxide, alumina, glass, or a suitable component to provide a rigid surface for a layer of LiPON to be deposited.

Diagram102illustrates a surface of the substrate110, which has been modified (e.g., altered, treated). In some embodiments, modifying the surface of the substrate110includes depositing a sacrificial layer115on the surface of the substrate110. Further, the sacrificial layer115may include a first removal rate greater than a second removal rate of a layer of LiPON to be deposited on the sacrificial layer115. In some embodiments, the sacrificial layer115includes a photoresist, and modifying the surface of the substrate includes coating the surface of the substrate110with the photoresist.

In some embodiments, coating the surface of the substrate110with the photoresist includes applying the photoresist on the surface of the substrate, and rotating the substrate with the photoresist at speeds ranging from 500 revolutions per minutes (RPM) to 2,000 RPM for durations ranging from 40 seconds to 80 seconds—e.g., a spin coating process. In some embodiments, the photoresist is AZ1512 and the substrate110is a glass substrate. Table 1 shows example spin coating recipes for coating the AZ1512 photoresist on the glass substrate.

TABLE 1Spin coating recipeRamp timeSpin time500 RPM20 s20 s1000 RPM20 s20 s2000 RPM20 s60 s

In some embodiments, modifying the surface of the substrate110includes exposing the photoresist that has been coated (e.g., spin coated) to ultraviolet (UV) light—e.g., in a chamber generating the UV light. In some embodiments, the photoresist may be exposed to the UV light for 5 minutes. In some embodiments, the photoresist may be exposed to the UV light of the sun light.

Diagram103illustrates that a layer of LiPON120has been formed on the modified surface of the substrate110—e.g., on the surface of the sacrificial layer115. In some embodiments, forming the layer of LiPON includes depositing LiPON by RF sputtering a target of Li3PO4in an environment including nitrogen (e.g., a N2atmosphere). Moreover, the RF sputtering the Li3PO4target may include setting the RF power to 50 W with minimal reflected power, setting the N2gas pressure at 15 mTorr, or maintaining a distance between the target and the substrate110at 5 cm. In some embodiments, the Li3PO4target is 2 inch in diameter. In some embodiments, a deposition rate of the LiPON thin film corresponds to approximately 3 nm per minutes (e.g., 3.28 nm/min). The substrate110carrying the sacrificial layer115and the layer of LiPON120may be referred to as a stack125.

Diagram104illustrates that the stack125has been immersed in a solution135of a container130. The solution135may be configured to selectively remove the sacrificial layer115without affecting the layer of LiPON120. For example, the sacrificial layer115has a first removal rate in the solution135that is greater than a second removal rate of the layer of LiPON120in the solution135. In some embodiments, the sacrificial layer115corresponds to a photoresist (e.g., AZ1512) and the solution135includes organic solvent (e.g., dimethyl carbonate (DMC)). In such embodiments, selectively dissolving the AZ1512 photoresist includes immersing the substrate with the layer of LiPON (i.e., the stack125) in the solution135including DMC for twelve (12) hours.

Diagram105illustrates that that the stack125has been immersed in the solution135sufficiently long enough (e.g., 12 hours or more) such that the sacrificial layer115(e.g., AZ1512 photoresist) is removed (e.g., dissolved in the solvent including DMC). As a result of selectively removing the sacrificial layer115, the layer of LiPON120may separate from the substrate110.

Diagram106illustrates that that the layer of LiPON120can be lifted (e.g., picked up, peeled off from the substrate110) from the solution135using a tweezer140(e.g., a flat-tip tweezer). The layer of LiPON120, after being picked up from the container130, is not in contact with the substrate110. In this manner, free-standing LiPON thin films can be synthesized.

In some embodiments, a layer of LiPON includes a first surface and a second surface opposite to the first surface, where both the first and second surfaces are not in contact with an external structure. In some embodiments, the first and second surfaces, absent any external structure attached thereto, are configured to enable experiments to determine a microscopic structure of the LiPON layer, such as solid-state nuclear magnetic resonance experiments. In some embodiments, at least a portion of the LiPON layer is a free-standing and flexible thin-film layer. In some embodiments, at least a portion of the LiPON layer has a thickness of approximately 3.7 μm (e.g., 3.7±0.18 μm, 3.7±0.37 μm, 3.7±0.55 μm, or the like). In some embodiments, at least a portion of the LiPON layer is at least partially transparent to visible light In some embodiments, at least a portion of the LiPON layer is flexible to bend in response to external force applied thereto.

FIG.1Bshows photographs depicting example process steps to synthesize free-standing LiPON thin films in accordance with embodiments of the present technology.FIG.1Binclude diagrams107,108a/b, and109a/bshowing photographs taken during the fabrication of the free-standing LiPON thin films.

Diagram107is a photograph showing the substrate110(e.g., a glass slide) with its surface modified (e.g., AZ1512 photoresist has been spin-coated), which corresponds to the substrate110with the sacrificial layer115formed on the substrate110as shown in diagram102.

Diagram108ais a photograph showing the stack125(e.g., the substrate110carrying the sacrificial layer115and the layer of LiPON120) immersed in the solution135. The photograph of diagram108ashows the layer of LiPON120starting to peel-off (e.g., lifted up) at the edges of the layer of LiPON120as indicated by two arrows pointing to the edges where different contrasts start to develop. Then, the layer of LiPON120gradually peels off from the substrate110. Diagram108bis a photograph showing the layer of LiPON120separated from the substrate110-i.e., the sacrificial layer115(e.g., AZ1215 photoresist) no longer holding the layer of LiPON120attached to the substrate110.

Diagram109ais a photograph showing the layer of LiPON120picked up from the solution135(as indicated by an arrow) using a flat-tip tweezer—e.g., a free-standing LiPON thin film. Diagram109bis another photograph showing another free-standing LiPON thin film (as indicated by a dotted rectangle pointed by an arrow) held by a flat-tip tweezer. As shown in diagrams109aand109b, the free-standing LiPON thin films can be synthesized to have different shapes or areas, and thicknesses (by controlling the RF sputtering time, for example).

Although the example process steps with reference toFIGS.1A and1Bdescribe removing the sacrificial layer115in a liquid solution (e.g., the solution135) by completely submerging the stack125in the liquid solution, the present technology is not limited thereto. For example, the sacrificial layer115can be removed in a process chamber configured to provide a gaseous etching environment. Further, the layer of LiPON120may be patterned (e.g., using photolithography and etching process steps) to expose the sacrificial layer115underneath to facilitate the gaseous etching process (or the liquid etching process). Such patterning of the layer of LiPON120may provide for additional degrees of freedom to synthesize the free-standing LiPON thin film having certain predetermined shapes.

FIG.2shows X-ray diffraction (XRD) results from free-standing LiPON thin films synthesized in accordance with embodiments of the present technology. XRD analyses can be used to determine crystallographic structures of solid materials.FIG.2includes diagrams201aand201bshowing the XRD results from an as-obtained free-standing LiPON thin film synthesized in accordance with embodiments of the present technology.

Diagram201ashows no diffraction spots present confirming that no crystallization has occurred in the free-standing LiPON thin film—e.g., by forming Li2CO3. Further, diagram201plotting integrated signals during XRD analyses shows an amorphous feature (“hump”) at around 23 degree (denoted by an arrow). This amorphous feature is regarded as a typical sign indicating that the material under the XRD analysis is in an amorphous phase. Accordingly, the XRD results from the free-standing LiPON thin film indicates that the free-standing LiPON thin film is amorphous, which is consistent with the conventional LiPON thin films deposited on rigid substrates.

FIG.3shows X-ray photoelectron spectroscopy (XPS) plots comparing spectra from a LiPON reference sample (a conventional LiPON thin film deposited on a solid substrate) and from a free-standing LiPON thin film synthesized in accordance with embodiments of the present technology. XPS analyses can be used to determine chemical environments of the material at its surface.FIG.3includes diagrams301aand301bshowing the XPS spectra from the LiPON reference sample and the free-standing LiPON thin film, respectively.

The XPS results of diagrams301aand301bshow similar spectra at O 1s, N 1s, P 2p and Li 1s regions, which indicates that the free-standing LiPON thin film has the same (or similar) chemical bonding environment as the LiPON reference sample. Accordingly, the XPS results confirm that the chemical bonding configurations of the free-standing LiPON thin films are consistent with those of the conventional LiPON thin films deposited on rigid substrates.

FIG.4Ashows a cross-section image and energy dispersive X-ray spectroscopy (EDS) results from free-standing LiPON thin films synthesized in accordance with embodiments of the present technology. Focused ion beam (FIB) technique is used to create a trench on the free-standing LiPON thin film to examine cross-sectional morphology of the free-standing LiPON thin film using a scanning electron microscope (SEM). Moreover, EDS mapping on the sidewall surface of the free-standing LiPON thin film provides 2-dimensional (2D) analyses that identify chemical elements included in the free-standing LiPON thin film and their distribution across its thickness.FIG.4Aincludes diagrams401aand401bshowing the EDS mapping and the cross-sectional SEM image of the free-standing LiPON thin film, and an EDS spectrum from the sidewall of the free-standing LiPON thin film, respectively.

The cross-sectional SEM image of diagram401ashows that the free-standing LiPON thin film has a thickness of approximately 3.7 μm. The SEM image shows fully dense structural morphology without any sign of anomaly (e.g., holes, hairlines cracks). Further, the EDS mapping of diagram401adisplays the N, O, and P signals uniformly distributed across the entire thickness indicating that those primary elements of LiPON are distributed uniformly across the free-standing LiPON thin film. It should be noted that EDS mapping may be insensitive to detect the Li signal. Also, the C signal can be attributed to contamination from C contents of atmospheric air—e.g., forming carbonates. Further, the Ga signal is regarded as an artifact of the FIB technique, which is distributed out of the region of interest.

The EDS spectrum of diagram401bcorroborates with the 2D EDS results indicating that the primary elements (e.g., N, O, and P) of LiPON are detected throughout the sidewall surface of the free-standing LiPON thin film. The EDS results and the cross-sectional SEM image confirm that the free-standing LiPON thin films have fully dense morphology and chemical components uniformly distributed throughout the entire thickness of the free-standing LiPON thin film, which are consistent with the conventional LiPON thin films deposited on solid substrates.

FIG.4Bshows EDS results obtained from free-standing LiPON thin films synthesized in accordance with embodiments of the present technology. The EDS results fromFIG.4Bare obtained from a surface of the free-standing LiPON thin film to confirm morphology and chemical components and their distribution from the surface.FIG.4Bincludes diagrams401c,401d, and401eshowing the 2D EDS mapping of the surface, the EDS spectrum collected from the surface, and stoichiometric information of the free-standing LiPON thin film determined from the EDS analyses, respectively.

Diagram401cshows 2D EDS mapping results that displays uniform distribution of the N, O, and P signals across the entire surface area indicating the primary elements of LiPON are distributed uniformly across the entire surface area. It should be noted that the EDS mapping of diagram401ccovers a much wider surface area when compared to the sidewall surface area examined in the 2D EDS mapping shown in diagram401a. The 2D EDS spectrum shown in diagram401d, which is collected from the surface area, facilitates quantitative analyses of the primary constituents of LiPON (e.g., N, O, and P) based on the EDS spectrum. Diagram401eshows the stoichiometry of the free-standing LiPON thin films based on the 2D EDS spectrum analyses, namely LixPO3.66N0.27.

FIG.5shows a plot depicting electrochemical impedance spectroscopy (EIS) measurements from free-standing LiPON thin films synthesized in accordance with embodiments of the present technology. EIS is a multifrequency AC electrochemical measurement technique measuring the electrical resistance (impedance) of the metal/solution interface over a wide range of frequencies (e.g., from 1 mHz to 1 MHz). As shown inFIG.5, the free-standing LiPON thin films provides an ionic conductivity of 2×10−6S/cm, which agrees well with the ionic conductivity value reported from a conventional LiPON thin film deposited on a solid substrate by RF sputtering techniques.

FIG.6is a flowchart600of a method of synthesizing free-standing LiPON thin films in accordance with embodiments of the present technology. The flowchart600includes aspects of the methods described with reference toFIGS.1A and1B.

The method comprises modifying a surface of a substrate (box610). The method further comprises forming a layer of LiPON on the modified surface of the substrate (box615). The method further comprises separating the layer of LiPON from the substrate (box620).

In some embodiments, the layer of LiPON separated from the substrate is a free-standing and flexible thin-film layer. In some embodiments, modifying the surface of the substrate comprises depositing a sacrificial layer on the surface of the substrate, the sacrificial layer including a first removal rate greater than a second removal rate of the layer of LiPON. In some embodiments, separating the layer of LiPON from the substrate includes selectively removing the sacrificial layer between the surface of the substrate and the layer of LiPON. In some embodiments, the sacrificial layer comprises a photoresist, and modifying the surface of the substrate comprises coating the surface of the substrate with the photoresist.

In some embodiments, coating the surface of the substrate with the photoresist comprises applying the photoresist on the surface of the substrate, and rotating the substrate with the photoresist at speeds ranging from 500 RPM to 2,000 RPM for durations ranging from 40 seconds to 80 seconds. In some embodiments, the method further comprises exposing, after rotating the substrate with the photoresist, the photoresist to ultraviolet light. In some embodiments, separating the layer of LiPON comprises dissolving the photoresist using a solvent that selectively removes the photoresist. In some embodiments, the photoresist includes AZ1512 and the solvent includes DMC. In some embodiments, dissolving the photoresist includes immersing the substrate with the layer of LiPON in a solution including DMC for twelve (12) hours.

In some embodiments, forming the layer of LiPON comprises depositing a LiPON thin film by RF sputtering a target of Li3PO4in a nitrogen (N2) environment. In some embodiments, the RF sputtering the target of Li3PO4comprises at least one of setting the RF power to 50 W with minimal reflected power, setting the N2gas pressure at 15 mTorr, or maintaining a distance between the target and the substrate at 5 cm. In some embodiments, a deposition rate of the LiPON thin film corresponds to approximately 3 nm per minutes (e.g., 3±0.15 nm per minutes, 3±0.3 nm per minutes, 3±0.45 nm per minutes, or the like).

FIG.7is a flowchart700of a method of synthesizing free-standing LiPON thin films in accordance with embodiments of the present technology. The flowchart700includes aspects of the methods described with reference toFIGS.1A and1B.

The method comprises coating a surface of a glass substrate with a layer of photoresist (box710). The method further comprises depositing a layer of LiPON on the layer of photoresist (box715). The method further comprises immersing the glass substrate carrying the layer of photoresist and the layer of LiPON in a solution configured to selectively dissolve the layer of photoresist (box720). The method further comprises lifting the layer of LiPON from the solution (box725).

In some embodiments, coating the surface of the glass substrate with the layer of photoresist includes applying the photoresist on the surface of the glass substrate, rotating the substrate with the photoresist at speeds ranging from 500 RPM to 2,000 RPM for durations ranging from 40 seconds to 80 seconds, and exposing, after rotating the glass substrate with the photoresist, the photoresist to ultraviolet light.

In some embodiments, depositing the layer of LiPON on the layer of photoresist includes RF sputtering a target of Li3PO4in a nitrogen (N2) environment based on setting the RF power to 50 W with minimal reflected power, setting the N2gas pressure at 15 mTorr, and maintaining a distance between the target and the glass substrate at 5 cm.

In some embodiments, the photoresist comprises AZ1512. In some embodiments, the solution comprises a solvent including DMC. In some embodiments, immersing the glass substrate carrying the layer of photoresist and the layer of LiPON in the solution comprises totally submerging the glass substrate carrying the layer of photoresist and the layer of LiPON in the solution for 12 hours.

It is intended that the specification, together with the drawings, be considered exemplary only, where exemplary means an example. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Additionally, the use of “or” is intended to include “and/or”, unless the context clearly indicates otherwise.

While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub combination or variation of a sub combination.

Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.

Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.