Patent Application
20240014438
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The present invention relates generally to solid-state batteries and methods of making solid-state batteries and, in particular to, solid-state batteries that utilize Li-garnet solid-state electrolyte.
Solid-state batteries (SSBs) which utilize solid-state electrolyte (SSE) have attracted recent attention because they address the energy density and safety issues of traditional Li-ion batteries. By replacing the flammable organic electrolyte with a safer SSE, it has been found that one can minimize the safety issues associated with traditional Li-ion batteries. In particular, ceramic SSEs have attracted more and more attention for their applications in SSBs to improve energy density, decrease manufacturing costs, and fundamentally address the safety concerns in traditional Li-ion batteries.
High-entropy ceramics (>1.5 R) are formed from the theory of high-entropy alloys. The basic arrangement of high-entropy ceramics has at least four different equimolar elements on a single binding site. The entropy-stabilized oxide (Mg, Ni, Co, Cu, Zn)O exhibits a stabilized single phase with homogenous elements distributed at the atomic scale. Examples of high-entropy or medium-entropy ceramics include oxide, carbide, boride, and the like.
Processed ceramic materials show unique properties such as ultra-low thermal conductivity useful for the application of environmental barrier coatings, high mechanical properties useful for structural applications, and superior energy storage properties useful for battery applications (either cathode or anode). However, there are also unique structures or properties of high-entropy ceramic materials such as the sluggish diffusion effects which influence the grain growth and morphology, various vacancies/deficiencies which modify the matter transport, and lattice distortions due to the different atomic radii.
For this reason, the mechanical properties of the SSE play an important role in preventing lithium dendrite penetration and maintaining the integrity of the electrolyte for fast charging and long cycle life of the SSB. For example, during the charging and discharging cycles in SSBs, there are stresses applied to the SSEs, which originate from the lithium dendrite penetration. To withstand the high current and long-term cycling, it is desired that the SSEs be strong enough to ensure their integrity. However, ceramic solid-state electrolyte is normally brittle and easy to fracture under load, so it is necessary to improve the corresponding mechanical properties of these materials.
Due to the unique structures of certain high-entropy ceramics, it is believed that high-entropy Li-garnet may have promising properties. The present invention has also found that using a highly specific Li metal cathode or anode can significantly improve the energy density of the battery.
Referring to
It has been found that the properties of the Li metal anode are highly dependent on the doped elements, processing conditions, and microstructures. Therefore, for desirable applications of Li-garnet in SSBs, it is desirable to optimize such properties through doping engineering.
By introducing various dopants, the sintering behaviors, microstructures, electrochemical properties, and mechanical properties of Li-garnet can be modified to fulfill the requirements for the applications in SSBs. For instance, the introduction of Ta can increase the ionic conductivity to about 1.0×10−3 S/cm compared with undoped LLZO by stabilizing the cubic structure. The introduction of Ga can further increase the ionic conductivity to about 1.46×10−3 S/cm. The mechanisms of increasing the ionic conductivity include the stabilization of the cubic structure of Li-garnet, increasing Li occupancy at the distorted octahedral site, and expanding the bottleneck to create extra Li-ion transporting pathways.
The present invention provides a high-entropy solid-state electrolyte whereby the ability to dope multi-elements in the solid-state electrolyte can improve the properties, modify the microstructures, and eventually expand the design and processing of such materials.
The present inventors have found that the type of dopant along with the amount of dopant influences the phases, densification behaviors, microstructures, and eventually the properties (both electrochemical and mechanical properties) of the SSB.
The present invention provides a novel high-entropy Li-garnet electrolyte with the chemical formula of Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12, in which Zr, Nb, Ta, and Hf of equimolar amounts are on the Zr site in LLZO. The present invention also provides a novel method of manufacturing said high-entropy Li-garnet electrolyte.
The present invention shows that Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12(LLZNTH) high-entropy Li-garnet has a fine microstructure and improved mechanical properties compared to Ta-doped Li-garnet (Li6.75La3Zr1.75Ta0.25O12, LLZT).
In one embodiment of the present invention is a solid-state battery comprising a Li-garnet ceramic electrolyte comprising a powder with a chemical composition of Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12.
The powder may have an entropy greater than 1.5 R.
The powder can be densified to a density of about 93-94% at a temperature of about 1100° C.
The powder may have a fine grain size of about 1-10 μm.
Nb and Ta may be of equimolar amounts on the Zr site. Further, Zr and Hf may be of equimolar amounts on the Zr site.
The powder may have a single cubic garnet phase (space group: Ja
The powder can be densified to have an ionic conductivity of about 4.67×10−4 S cm−1 at room temperature.
The powder can be densified to have an activation energy of about 0.25 eV or less.
The powder can be densified to have an electronic conductivity in the order of about 10−8 S cm−1 or less.
The powder can be densified to have a flexural strength of at least 84.8±6.9 MPa.
The powder can be densified to have a hardness of at least 8.5±0.8 GPa.
One embodiment of the present invention provides a method of forming a solid-state battery comprising a Li-garnet ceramic electrolyte comprising: ball milling LiOH·H2O (with 10% excess), La2O3, ZrO2, Nb2O5, Ta2O5, and HfO2 using milling media and liquid media to produce a mixture; drying the mixture at a low temperature; grounding the mixture; molding the mixture into pellets; and sintering the pellets at a high temperature. The ball milling may be for at least 24 hours. Drying the mixture may be at about 105° C. for 8 hours to 20 hours. Sintering of the mixture may be at about 1100° C. for 8 hours to 20 hours.
The method may further comprise forming a powder with the chemical composition of Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12.
The milling media may be zirconia and the liquid media may be isopropyl alcohol (IPA).
The method may further comprise calcinating the pellets in an alumina crucible at about 900° C. for at least 12 hours. The method may further comprise ball milling using zirconia milling media in IPA after the calcinating step to reduce a particle size of the pellets.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.
The introduction of Al can stabilize the cubic phase Li-garnet to improve the ionic conductivity of the ceramic material. Other dopants in the Zr site, such as Ta, Nb, Te, and W, can also stabilize the cubic phase as well as produce a high ionic conductivity. However, prior art studies have only focused on single element doping. For example, the prior art focuses on single element doping on either the A site or B site, or A and B site co-doping with a single element on each site. There are few studies focused on doping multi elements on a single site.
Besides electrochemical properties, the type of dopant can influence the microstructures and mechanical properties of the ceramic material. For example, Ga-LLZO shows a mechanical strength of about 143 MPa and a fracture toughness of 1.22 MPa·m1/2, which is higher than Al- and Ta-doped LLZO. Further studies indicate that the dopant type changes the microstructures such as grain size and grain boundary conditions and consequently the mechanical properties of the ceramic material.
A fine microstructure is anticipated to have higher hardness and flexural strength. The type of dopant also influences the stability against lithium metal. For example, Ta doped LLZO has much better stability against Li metal than Nb doped LLZO due to the reduction reaction of Nb dopant in the interface.
Besides the dopant types, the amount of dopant can also influence the structures and properties of the ceramic material. For example, studies have shown that the effects of Ta doping in Li7-xLa3Zr2-xTaxO12 (LLZO) indicate that the optimal amount of Ta in LLZO is between 0.4 and 0.6.
To further improve the ceramic material's properties, novel doping strategies, such as co-doping, are necessary to continue to improve and tune the properties.
Therefore, the present inventors have found that it is desirable to optimize doping to further improve the ionic conductivity, enhance the stability against Li metal, and refine its microstructures for better mechanical properties.
High-entropy ceramics (HECs), initiated from high-entropy alloys, contain at least four different equimolar or near equimolar elements on the same site and show various unique properties. For instance, HECs show lower thermal conductivities than regular ceramics (including oxide and non-oxide ceramics) and have potential applications as thermal barrier coating and ultra-high temperature ceramics.
Carbide-based HECs show very promising oxidation resistance under elevated temperatures for high-temperature applications. HE carbide also has very good mechanical properties including hardness, flexural strength, and fracture toughness, which can be attributed to the unique microstructures including solid solution strengthening mechanism and nanoplate pullout along with microcrack deflection toughening mechanisms. It has been found that HE carbide has a lower grain growth rate than traditional carbide.
The present invention provides a novel Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 high-entropy Li-garnet ceramic electrolyte having desirable electrochemical properties as well as unique super fine microstructures. The equimolar doping of Ta and Nb (Li64La3Zr1.4Ta0.3Nb0.3O12) has a higher ionic conductivity than non-equimolar ones (Li64La3Zr1.4Ta0.5Nb0.1O12 and Li6.4La3Zr1.4Ta0.4Nb0.2O12). The equimolar ratio of the dopants (Zr, Nb, Ta, and Hf) leads to desirable electrochemical properties, and the desirable fine microstructures can be attributed to the sluggish effects of high-entropy compounds. The unique fine microstructures with high grain boundary strength can alter and improve the mechanical properties of the ceramic material including hardness, fracture toughness, and flexural strength. For example, the high-entropy Li-garnet has a higher hardness than the traditional Li-garnet.
The high-entropy Li-garnet shows very promising electrochemical properties, which indicates its potential applications in SSBs. Computational research indicates the possibility to result in a higher ionic conductivity through the high-entropy approach. Electrochemical characterizations indicate that the higher density sample has an adequate ionic conductivity of 4.67×10−4 S cm−1 at room temperature, a low activation energy of 0.25 eV, and a low electronic conductivity in the order of 10−8 S cm−1.
Mechanical properties of Li-garnet play vital roles in preventing Li penetration during charge and discharge in SSBs. The mechanical properties of Li-garnet are highly dependent on the microstructures. For instance, finer grain size and strong grain boundaries can lead to higher mechanical properties. HECs have abnormal grain growth behaviors and consequently unique microstructures. Thus, the HE Li-garnet may have unique sintering behaviors and consequently different mechanical properties as well.
The present invention also provides for the synthetization of LLZNTH Li-garnet powders which has a single cubic garnet phase (space group: Ia
Therefore, the present invention provides for the processing of single phase Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 high-entropy Li-garnet electrolyte with the following desirable properties:
Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12(LLZNTH) high-entropy Li-garnet has a fine microstructure and improved mechanical properties compared to Ta-doped Li-garnet (Li6:75La3Zr1.75Ta0.25O12, LLZT). The formation, sintering, and electrochemical properties indicate that the LLZNTH sample has a finer particle size than LLZT after calcination and ball milling. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) regarding the solid-state reaction process indicate that the LLZNTH forms at a lower temperature than LLZT. Both samples can be densified to a relative density up to about 93-94% at a temperature of about 1100° C., however, they show significantly different sintering and grain growth behaviors. The LLZNTH sample takes 16 hours to reach the maximum relative density while the LLZT sample only needs 12 hours.
LLZNTH sample has a lower grain growth parameter due to the sluggish effects of high-entropy compounds so that it maintains fine microstructures (grain size about 10 μm) than the LLZT sample (grain size over 100 μm). Due to the fine microstructures, the LLZNTH sample shows both higher flexural strength (84.8±6.9 MPa compared with 47.9±10.1 MPa) and hardness (8.5±0.8 GPa compared with 7.7±0.4 GPa) than the LLZT sample. Ionic conductivity characterizations indicate that the LLZNTH sample shows a moderate conductivity of 4.67×10−4 S/cm at room temperature and a low activation energy of 0.25 eV.
The sintering, microstructures, mechanical properties, and electrochemical properties of high-entropy Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 were compared to the results with Li6:75La3Zr1:75Ta0.25O12 and show:
The following non-limiting examples illustrate the compositions, methods, and applications of the present teachings.
1. Experiment
Powder Synthesis
The powders (high-entropy LLZNTH powders with the chemical composition of Li7La3Zr0.5Nb0.5Ta0.5Hf0.5O12 and Ta doped Li-garnet powders, LLZT, with the chemical composition of Li6.75La3Zr1.75Ta0.25O12) were prepared through a solid-state synthesis method described herein.
Stoichiometric amounts of LiOH·H2O with 10% excess (Li is easy to evaporate during synthesis and sintering therefore excess amount was added, 98% minimum purity, Thermo Scientific), La2O3 (99.9% purity, Tokyo Chemical Industry America, dried at 950° C.), ZrO2 (99.9% purity, Inframat Advanced Materials), Nb2O5 (99.9% purity, Alfa Aesar), Ta2O5 (99.99% purity, Inframat Advanced Materials), and HfO2 (99%, Thermo Scientific) were ball milled for about 24 hours using zirconia milling media and isopropyl alcohol (IPA, 99.9% purity, Alliance Chemical) as the liquid media. The mixture was dried at about 105° C., ground, and pressed into pellets, as further described below.
To study the formation behaviors of the two different samples (LLZNTH and LLZT), the mixed precursors were subjected to TGA and DSC tests from room temperature to about 1050° C. with a heating rate of about 10° C./min under argon flowing (Perkin Elmer Thermal Analyzer 6000, Waltham, MA). The microstructures of the powders before and after the ball mill were characterized by a scanning electron microscope (SEM, SU1510, Hitachi) equipped with energy-dispersive X-ray spectroscopy (EDS, QUANTAX 100, Bruker). Phases and particle size of the powders were characterized by X-ray diffraction (XRD, D8 Discovery, Bruker) and a particle size analyzer (Mastersizer 2000, Malvern Instruments Limited), respectively.
Sample Densification and Characterizations
The produced powders were uniaxially pressed into pellets with a diameter of either about 13 mm (for microstructure and electrochemical characterizations) or about 30 mm (for mechanical properties tests) under pressure of about 120 MPa. The green pellets were surrounded by the same powder and placed in an alumina crucible with a lid to minimize the evaporation of Li during sintering. To find out the appropriate sintering conditions, the samples were sintered at about 1100° C. for about 8, 12, 16, and 20 hours in the air atmosphere. The heating and cooling rates were about 3° C./min.
Calcination occurred in an alumina crucible at about 900° C. for about 12 hours. Subsequent about 24 hours ball milling using zirconia milling media in IPA after the calcination was employed to reduce the particle size.
The samples were ground into powders and subjected to XRD and Raman (DXRxi Raman Imaging Microscope, Thermo Fisher Scientific) characterizations. The fracture surface of the sample was observed under SEM. The samples were also polished down to a finishing surface of 0.5 pin to observe the polished surface along with elements distribution/mapping under SEM equipped with EDS.
The sintered samples were ground to remove any coved powder bed. The density of the sintered sample was determined through Archimedes' method using IPA as the immersing medium. Relative density was calculated based on the theoretical density which was obtained from the molecular weight and lattice parameter. The sintered samples were also broken and ground into powders for XRD characterizations. The fracture surface of the sample was observed under SEM. The samples were also polished down to a finishing surface of 0.5 μm to observe the polished surface along with elements distribution/mapping under SEM equipped with EDS. The polished samples were also thermally etched at a temperature between about 800 to 1000° C. for a duration of about 5 to 30 minutes for different samples to observe the grain size. The grain size was determined through the intercept method using the ImageJ software package.
Mechanical Properties Characterizations
The samples were subjected to the 3-point bending test following ASTM C1161. The sintered large pellets were cut and then polished down to the desired dimension (2×1.5×25 mm) by controlling the polishing time with a surface finishing of 1 μm using diamond paste. The tests were performed on a mechanical tester (C43.504, MTS Systems Corporation) utilizing an attached 3-point bending fixture with a support span size of 20 mm. The crosshead speed was 0.2 mm/min. The flexural strength a was calculated as below:
where P is the break force, L is the support span size (20 mm), b is the sample width (2 mm), and d is the sample thickness (1.5 mm).
The samples were also subjected to the Vickers hardness test. The samples were polished down to a finishing surface of 0.5 μm and cleaned with IPA to remove any residual contaminations. The hardness tests were performed using a Buehler Vickers hardness tester under a load of 500 g. At least 10 tests were performed on each sample to obtain mean and standard deviation values.
Electrochemical Properties Characterizations
Li-ion blocking silver paste was applied on the polished samples and fired at about 700° C. for about 0.5 hours to ensure a good interface. The ionic conductivity of each sample was determined by electrochemical impedance spectroscopy (EIS, Reference 3000, Gamry Instruments) in a frequency range of about 1 MHz to 1 Hz using the potentiostatic EIS model with an about 100 mV amplitude. The results were fitted using a proper equivalent circuit model to obtain the ionic conductivity.
2. Results and Discussion
Characterization of the Synthesized Powders and the Formation Behaviors
Particle size analysis on the ball-milled powders in
The formation temperatures of both samples were compared through TGA/DSC. As shown in
Sintering Properties of the Samples
The density from Archimedes' method and the corresponding relative density (percentage of the theoretical density) is shown in
At a short sintering time (about 8 hours), the densification starts, and minimal grain growth can be observed for both samples, compared with the original particle size. The pores are mainly near the grain boundaries for both samples (
Comparing between the two samples, the differences in the sintering behaviors and microstructures can both be attributed to the sluggish diffusion effects of the high-entropy compounds (LLZNTH). The LLZT sample needs a shorter time to reach the peak density than the LLZNTH sample (about 12 hours against about 16 hours). Although the finer particle size of LLZNTH can increase the driving force of densification, the lower rate of diffusion hinders the densification process therefore a longer sintering time is necessary to reach the peak density. The sluggish diffusion effect also detracts the grain growth so no abnormal grains form after densification. However, for the LLZT sample, some grains significantly grew during the sintering process possibly due to the high rates of diffusion and grain boundary migration. Therefore, some significantly grown grains can be observed when the sintering time is over about 12 hours. The differences in the grain growth behaviors lead to completely different microstructures including grain size, grain boundary strength, fracture model, etc. Such differences can lead to different mechanical and electrochemical properties.
Mechanical and Electrochemical Properties
Flexural strength has a similar trend, as shown in
The LLZNTH sample shows higher mechanical properties than the LLZT samples, which can be attributed to the very fine grain size than that of the LLZT sample due to the sluggish effects of high-entropy compounds. The sluggish effect of the high-entropy compounds can slow the grain growth as discussed above, thus improving the mechanical properties including hardness and flexural strength.
Mechanical properties of the Li-garnet play vital roles in the development of SSBs. For instance, higher hardness and strength can more efficiently prevent lithium penetration during charge and discharge which can improve the durability of the battery for long-term use. Higher strength can also ensure the structural integrity of the electrolyte during the processing and assembling of the batteries. It is necessary to maintain the high mechanical properties of the SSEs. Meanwhile, different grain sizes may also influence the resistance to lithium penetration. The finer the grain size, the longer the penetration path along the grain boundaries, therefore higher resistance to lithium penetration.
It is hypothesized that the LLZNTH samples with finer grain sizes and higher mechanical properties may also have a higher resistance to lithium penetration. Furthermore, the high-entropy concept in ceramic SSEs can modify the ionic conductivity, the electrochemical window against Li metal, and interface stabilities, thus, high-entropy ceramic electrolytes may have more potential applications in SSBs.
As indicated in
Electrochemical Properties of LLZNTH
Electrochemical characterizations including AC impedance spectra (dimensional nominalized) for ionic conductivity, Arrhenius plot for activation energy, and DC polarization plot for electrical conductivity of the sintered samples are shown in
Referring to
An equivalent circuit, (R1Q1)Q2, where R is the resistance and Q is the constant phase element, was used to obtain the total ionic conductivity. As indicated in
Such conductivity results are consistent with the microstructure results as discussed above: (1) longer sintering time leads to higher relative density (increased from about 79% to 94%) and consequently higher ionic conductivity; (2) glass-like Li-rich compounds near the grain boundaries may significantly influence the ionic conductivity therefore the samples sintered at about 8 hours and about 12 hours have similar conductivity although the relative density increases (as well as the shape of the spectrum, thus, the disappearing of the semicircle may be due to the sufficient reaction/sintering); and (3) longer sintering above 16 hours may lead to further loss of the lithium which decreases the ionic conductivity.
The LLZNTH sample shows a promising ionic conductivity compared with prior art results. For instance, the most typical Li7-xLa3Zr2-xTaxO12 (or similar level of Li content along with Al and/or Ta dopant) has an ionic conductivity in the range of about 3-9×10−4 S cm−1. Such a range of ionic conductivity is sufficient for the lithium battery application. The current composition, which results in the relatively high ionic conductivity (about 4.67×10−4 S cm−1), can be attributed to: (1) the four equimolar elements Zr, Nb, Ta, and Hf with different valences and atomic radii can maximize the Li-ion vacancy/deficiency and disordered Li sublattice, which can provide extra lithium ion transport pathways and consequently the high conductivity; and (2) the good stability of the cubic phase (at least 20 hours at a temperature of about 1100° C.), which was due to the proper selection of the dopants (consider supervalent doping of Nb(V) and Ta(V) to Zr(IV) and Hf(IV), which can stabilize the cubic phase), can maintain the high conductivity.
The impedance of the sample sintered at about 16 hours was further tested in the temperate range between room temperature (about 23° C.) and about 105° C. The Arrhenius plot of the results shown in
The present invention of a high-entropy Li-garnet with multi elements in either the La site and/or the Zr site is significant for the application of lithium batteries. For example, the modification of the dopant may further increase the ionic conductivities by introducing more vacancies. Further, the variation of the dopants may modify the grain boundaries to prevent lithium penetration, increase the stability window between the electrolyte and electrode, and stabilize the interface between electrolyte and electrode. Also, the effects of high-entropy compounds, which means the introduction/modification of a single element, may significantly influence the final material properties, and can expand the capability of searching ideal solid-state electrolyte to fulfill the requirement of both electrochemical properties as well as other properties such as mechanical properties.
Thus, the high-entropy Li-garnet electrolyte of the present invention can expand the categories of such materials, improve the properties, and eventually the design of the materials with desired properties by modifying the dopants with the help of computational modeling.
Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.
When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.
It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.
To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.
This application claims the benefit of U.S. Provisional Application No. 63/358,731, filed Jul. 6, 2022, hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63358731 | Jul 2022 | US |
