Solid-state aluminum ion conductors

Abstract

The present invention relates to new solid-state conductors as electrolytes for use in aluminum batteries.

Description

[0001] "Solid-state aluminum ion conductors"

[0002] DESCRIPTION

[0003] The present invention relates to a family of solid-state ionic conductors of aluminum ions to be used, for example, as electrolytes for Al ion batteries .

[0004] Technical field of the invention

[0005] Solid-state ionic conductors are a class of materials that enable the operation of various energy storage and conversion devices , such as fuel cells , batteries , electrolyzers and solar cells ; they are also used in areas such as water puri fication, sensors , microelectronics , micro-nano actuation, etc .

[0006] With reference to applications in the field of energy production and storage , solid-state ionic conductors are typically used as electrolytes and / or additives for the production of anodes and cathodes for batteries . In fact , these have the undoubted advantage that , being solid, they are inherently much safer than the corresponding liquid materials , because they typically have lower flammability and vapor pressure . At the device level , this class of materials allows , for example , safer batteries to be obtained, which can be used in a wider temperature range and with higher energy and power density than those based on liquid electrolytes . With particular reference to the automotive sector, the use of batteries that include solid-state ionic conductors allows for greater autonomy ( dri ving range) and greater safety in the event of an accident . For example , in the event of an accident involving battery perforation, the use of a solid electrolyte prevents the leakage of flammable and toxic liquid materials, resulting in a significant increase in the safety of the entire vehicle.

[0007] There are different types of solid-state ionic conductors based on different classes of materials such as: a) oxides; b) sulfides; c) polymers (including composites) ; d) halides; e) organic-inorganic hybrids; e) ceramic-polymer hybrids; f) semi-solids. Each of these classes of electrolytes is characterized by well-defined physical and chemical characteristics such as: ionic conductivity (o) , chemical / electrochemical stability, thermal stability, etc. that make each material more or less suitable for a given application.

[0008] One of the preferred fields of use for solid-state ionic conductors is that of batteries, in particular as electrolytes. Their study is, for example, driving the development of fully solid lithium batteries with high energy density. The demand for safe, high-energy- density and sustainable batteries, however, has pushed battery research towards the development of new cells based on metals other than lithium, such as sodium, magnesium, calcium and aluminum.

[0009] Among these, aluminum shows many competitive advantages over lithium, especially in terms of volumetric capacity (which is four times greater) and natural abundance (8.13% compared to 0.01% of the earth ' s crust ) .

[0010] Furthermore, aluminum is not harmful to humans or the environment and enjoys well-established technology for both its production and recycling.

[0011] Aluminum, then, does not suffer from the problems related to mining and lithium extraction. It is more stable in air than lithium, offering a considerable advantage in terms of safety.

[0012] Although aluminum is very promising, there are several problems that severely limit the diffusion of aqueous and non-aqueous aluminum batteries; these limits are mostly related to the availability of practical electrolytes and cathodes.

[0013] The total number of liquid electrolytes capable of allowing the oxidation and reduction of aluminum ions at the interface with an electrode is extremely limited; in fact, there are about a dozen compounds, the most promising of which are based on chloroaluminate ionic liquids.

[0014] Despite the excellent electrochemical properties, the compounds of this class of materials: 1) are extremely sensitive to air and are hygroscopic; 2) are strongly acidic and therefore corrosive with respect to the other components of the battery (e.g. anode, cathode, battery casing, etc.) ; 3) release dangerous gaseous products in the event of battery failure; 4) have numerous safety limitations typical of liquid electrolytes such as: leaks in the event of battery perforation, limited operating temperature range, etc.

[0015] At present, therefore, it seems clear that liquid electrolytes for aluminum batteries have impractical characteristics, which do not make them suitable for industrial or large-scale applications or for large-scale diffusion.

[0016] Solid electrolytes show clear advantages over liquid electrolytes, especially in terms of safety, workability, high temperature operation, chemical / electrochemical stability, etc. However, the trivalent nature of Al3+ions , which is typically associated with poor ionic conductivity and mobility in the solid state , makes the development of solid-state aluminum ion conductors for application in aluminum batteries particularly challenging .

[0017] To date , the only solid electrolytes developed in the literature are based on low molecular weight polymers , liquids or gels , or they are semi-solid systems composed of a solid matrix soaked in a liquid electrolyte .

[0018] Unlike lithium batteries , fully solid-state ceramic materials have never been reported .

[0019] It is clear that in the case of Al batteries ( and similarly to lithium batteries ) solid-state inorganic conductors would represent a breakthrough in terms of safety, thermal stability, electrochemical stability window and chemical compatibility with other battery elements .

[0020] The development of solid-state ionic conductors , however, faces signi ficant challenges , mostly associated with the typically lower conductivity of solid materials compared to liquids and the poor mobility of solid-state Al3+.

[0021] Despite the high need for solid ionic conductors for aluminum ion batteries , their investigation appears extremely challenging and could lead to a signi ficant step forward in the field of batteries beyond lithium ions (beyond Li -i on ba t teri es') .

[0022] The prior art document JP 2006228605 A describes a solid-state electrolyte based on A12 (WO4 ) 3 intended for use in aluminum-air batteries ; this document does not report any information about the conductivity, structure or conductivity mechanism of the electrolyte , which is probably based on so-called channeling, making the material substantially di f ferent from the present invention .

[0023] Summary of the invention

[0024] The inventors of the present invention have surprisingly developed a family of fully solid-state inorganic materials to be used as ionic conductors of Al3+ions .

[0025] Obj ect of the invention

[0026] In a first obj ect , the present invention describes a process for preparing solid-state ionic conductors .

[0027] The solid-state conductors obtained represent a second obj ect of the invention .

[0028] In a third obj ect , the present invention describes aluminum ion batteries comprising said solid electrolytes .

[0029] In a fourth obj ect , the present invention describes the use of solid compounds as electrolytes for aluminum ion batteries .

[0030] Brief description of the figures

[0031] Figure 1 shows the XRD profiles of the precursors Pl , P2 , P3 and P4 with their phase assignments .

[0032] Figure 2 shows the XRD profiles of electrolytes El , E2 , E3 and E4 and the phase assignments .

[0033] Figure 3 shows the Raman spectrum of the precursors .

[0034] Figure 4 shows the Raman spectrum of the electrolytes . AICI3 is reported to show that it has reacted completely with the precursors and there are no traces in the final electrolytes .

[0035] Figure 5 shows the XANES spectrum ( X-ray absorpti on near edge structure) of the electrolytes. The interpolation and deconvolution of the observed peaks are also shown.

[0036] Figure 6 shows the trend of the ratio between the XANES peaks indicated as A2 and A3 and the conductivity (o) measured at 25°C as a function of the aluminum content in the electrolytes.

[0037] Figure 7 shows the conductivity of precursors and electrolytes as a function of temperature. The linear interpolation of the conductivity of the electrolytes is shown with a dashed line.

[0038] Figure 8 shows the comparison between the conductivity of different precursors and electrolytes.

[0039] Figure 9 shows the conductivity of: a) P3 (Al = 0% by weight) , El (Al = 0.7% by weight) , E2 (Al = 1.1% by weight) , E3 (Al = 3.1% by weight) and E4 (Al = 3.8% by weight) as a function of the aluminum content at fixed temperatures (T = 25, 50, 80°C) ; b) El (F = 5.6% by weight) , E2 (F = 8.2% by weight) , E3 (F = 19.6% by weight) and E4 (F = 21.1% by weight) as a function of the fluorine content at fixed temperatures (T = 25, 50, 80°C) .

[0040] Figure 10 shows a schematic representation of the conductivity mechanism of the proposed electrolytes.

[0041] Detailed description of the invention

[0042] In accordance with a first object, the present invention describes a process for the preparation of solid-state ionic conductive compounds.

[0043] The ionic conductive compounds of the invention will also be referred to herein as "electrolytes" or "solid electrolytes" or "solid state electrolytes". In particular, the process of the invention comprises the step of reacting an ammonium oxofluorotitanate precursor of the formula : (NH4) aTlObFc wherein the molar ratios a, b and c can be :

[0044] 0<ad3 l<b<2

[0045] 0<cd5 .

[0046] Advantageously, both the ammonium oxofluorotitanate precursor and aluminum chloride are in solid form (powder ) .

[0047] Moreover, the electrolyte compound obtained is also in solid form (powder ) .

[0048] For the purposes of the present invention, the reaction is optionally performed at a temperature of about 200-500 ° C .

[0049] According to one aspect of the present invention, the reaction is optionally performed at a temperature of about 300 ° C .

[0050] For the purposes of the present invention, the reaction is optionally performed for a period of time from about 30 minutes to 5 hours .

[0051] According to one aspect of the present invention, the reaction is optionally performed for a period of time of about 2 hours .

[0052] For the purposes of the present invention, the reaction is performed in an inert atmosphere , for example , nitrogen and / or argon .

[0053] According to one aspect of the present invention, the reaction is performed for a period of 2 hours at a temperature of 300 ° C under nitrogen or argon .

[0054] Under the reaction conditions described, the absence of unreacted products, such as aluminum chloride (AICI3) , has been demonstrated .

[0055] More specifically, the absence of aluminum chloride (AICI3) as an unreacted product (reagent) has been demonstrated.

[0056] According to one aspect of the invention, the electrolytes do not contain aluminum chloride (AICI3) .

[0057] According to one aspect, in the conductors obtained according to the invention, aluminum Al3+is present on the surface of the fluorinated anatase particles and, in particular, only on the surface of the fluorinated anatase particles.

[0058] In accordance with a second object, the conductive compounds obtained as described above are described.

[0059] These compounds have the following general formula:

[0060] AlxTlOyFz where the molar ratios x, y and z are as follows:

[0061] 0.02<x<0 .15

[0062] 1.80<y<2 .20

[0063] 0.26<z<1.18

[0064] With the process of the present invention, at least the following four electrolytes El, E2, E3, E4 were obtained from four different precursors Pl, P2, P3, P4, whose elemental compositions in % by weight are shown in the following Table:

[0065]

[0066] As it can be seen, the four electrolytes obtained are characteri zed by a di f ferent content of fluorine and aluminum .

[0067] The electrolytes obtained do not include chlorine (understood as chloride ) in quantities > 0 . 1 % .

[0068] According to one aspect of the invention, the best performing precursor is the P2 precursor and correspondingly the electrolyte with the highest ionic conductivity is E2 .

[0069] The conductors obtained by the process described above represent a second obj ect of the invention .

[0070] In a third obj ect , the present patent application describes aluminum ion batteries comprising the solid conductive compounds ( electrolytes ) obtained according to the process described above .

[0071] In a fourth obj ect , the present patent application describes the use of solid ionic conductive compounds obtained according to the process described above as electrolytes in aluminum ion batteries .

[0072] The present invention will be further described by the following experimental part . STRUCTURE (XRD)

[0073] The phase composition of the precursors (Pl, P2, P3 and P4) and electrolytes (El, E2, E3, E4) was studied using X-ray diffraction. The results are shown in Figure 1 for the precursors (P) and Figure 2 for the electrolytes obtained (E) .

[0074] The phase composition of the precursors and electrolytes is shown in the following tables:

[0075] As can be seen from the data in the tables above, the precursors P are composed of: a) anatase as the main constituent; b) traces of ammonium oxofluorotitanates with general formula (NH4)xTiOyFz; and c) traces of NH4F not detectable quantitatively . The presence of both (NH4 ) xTiOFxand NH4F can be attributed to the synthetic route necessary to obtain the precursors Pl , P2 , P3 and P4 .

[0076] The family of solid electrolytes obtained shows the typical structure of anatase with minor traces of NJhTiOFs, NH4F, rutile and (NH4 ) 2TiOF4 . The presence of these compounds is due to the presence of residual impurities of the precursors and / or high temperature synthesis that induce chemical reactions that modi fy the phase composition of the materials .

[0077] Taken together, it is possible to conclude that the main structure of the electrolytes is that of anatase and there are no interstitial or substitutional compounds based on aluminum . This is a fundamental point since the formation of covalent or ionic aluminum-based phases would lead to poorly conductive electrolytes . On the contrary, the materials show a structure in which the main material is essentially Ti02 ( in the form of anatase ) and the aluminum ions are present only between the anatase particles . In this regard, it seems reasonable to deduce that the Al3+ions are present on the anatase particles and neutrali ze the coordination sites based on fluoride ions ( F~ ) and / or Ti3+. The presence of defective titanium atoms , i . e . having a coordination number other than 6 , was investigated by XANES spectroscopy ( see dedicated paragraph) .

[0078] STRUCTURE (RAMAN)

[0079] The structure of the precursors and electrolytes was studied using Raman spectroscopy to further support the conclusions obtained using X-ray di f fraction and to propose a reasonable structure and conduction mechanism for the electrolytes studied.

[0080] It is possible to observe that both the precursors (see Figure 3) and the electrolytes show the typical structure of anatase with the presence of defective TixO2-yFyphases. No residual AICI3 (see Figure 4) or aluminum-based species are found in the electrolytes. The data are in strong agreement with the XRD results and suggest that the main structure of the materials is that of anatase and no covalent or ionic aluminum-based compounds have been identified.

[0081] These results further support the evidence that Al3+ions are present only on the surface of the anatase particles. Furthermore, the presence of TixO2-yFy(see references: A) Kazuo Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds: Part A: Theory and Applications in Inorganic Chemistry, John Wiley and Sons, Inc, 2008; and B) N. Louvain et al. Fluorination of anatase HO2 towards titanium oxyfluoride T1OF2: a novel synthesis approach and proof of the Li-insertion mechanism, J. Mater. Chem. A, 2014,2, 15308-15315) suggests that fluorine is mainly on the surface of defective particles of HO2 and acts as a coordination site for Al3+.

[0082] STRUCTURE (XANES)

[0083] The medium-short range structure of titanium, with particular reference to its oxidation state, was investigated by means of X- ray absorption measurements (XANES, X-ray absorption near edge structure) using synchrotron light. The results for the ionic conductors El, E2, E3, E4 are shown in Figure 5. In accordance with the literature, it is possible to observe that all electrolytes (El, E2 E3, E4) : A) show XANES profiles corresponding to the anatase phase; and B) are attributable to titanium with oxidation state +4. These results are in excellent agreement with the XRD and Raman measurements and confirm: a) that the bulk structure of the materials is that of anatase; b) that in no case were aluminum intercalation products obtained in the bulk structure of anatase.

[0084] According to the literature, the ratio between the intensities of the A2 and A3 peaks of the XANES profiles can be correlated with the degree of anatase defectiveness. In particular, with reference to Figure 6, the A2 peak is associated with the presence of titanium- based species with a coordination number of 5 located at the surface of the crystallites.

[0085] In detail, the analysis of the ratio of the intensities of the peaks I [A2] / I [A3] , after appropriate deconvolution of the XANES spectra, has allowed to demonstrate that the degree of defectiveness is closely correlated with the measured ionic conductivity. In particular, the most conductive sample E2 is also the most defective. It is therefore clear that the proposed electrolytes are characterized by the presence of Al3+located at coordination sites on the surface of defective anatase particles.

[0086] CONDUCTIVITY

[0087] The conductivity of the electrolytes obtained (o) , in the form of solid-state pellets , was evaluated using electrochemical impedance spectroscopy (EIS) measurements. The results are summarized in Figure 7 and Figure 8.

[0088] It is shown that the conductivity of the precursors Pl, P2, P3 and P4 is in the range 10~9-10~7S / cm for all the temperatures investigated, demonstrating the insulating nature of the precursors (P) . This level of conductivity ( 10~9- 10~7S) can probably be associated with the migration of H+, NH4+and F~ in the precursors. On the contrary, the conductivity is greatly improved for electrolytes (El, E2, E3, E4) . In particular, the conductivity values are in the range 1.2xl0~7-2.7xl0~5S / cm at 25°C and 3.5xl0~7-5.4xl0~5S / cm at 80°C (see Figure 8) . In all cases, following the reaction with AICI3, the conductivity (o) increases significantly (up to 2-3 orders of magnitude) .

[0089] The electrolytes show a linear dependence of logo vs. the inverse of the temperature (1 / T) , thus supporting the ionic nature of the measured conductivity. The best results in terms of conductivity are obtained for E2 which always shows values >10~5S / cm throughout the investigated temperature range.

[0090] It can be concluded that the increase in conductivity (o) from the precursors to the electrolytes is associated with the functionalization with Al3+of the fluorinated anatase particles. In particular, the Al3+mobile ions are located at the grain boundaries between the anatase particles and the linear trend of log(o) with respect to 1 / T suggests that the high conductivity obtained is associated with a long-range migration mechanism of Al3+ions based on the hopping of Al3+between different particles of defective fluorinated anatase.

[0091] Figure 9 shows the dependence of conductivity on the aluminum and fluorine content. For an aluminum content of ~ 1% by weight, a maximum conductivity is measured, while increasing amounts of Al lead to a reduction in conductivity values . A similar trend can be appreciated for fluorine with a peak around 9% by weight .

[0092] These data are particularly interesting because they indicate that by modulating the fluorine and aluminum content of the electrolytes it is possible to regulate : a ) the abundance of fluorinated coordination sites ; b ) the hopping distance for Al3+; and c ) the conductivity of the final electrolytes .

[0093] Overall , the functionali zation o f the ammonium oxofluorotitanate type precursors after the synthesis process described in the preceding paragraphs , gives a satis factory ionic conductivity to the resulting solid state electrolytes (El , E2 , E3 , E4 ) . An aluminum content of 1 % by weight is particularly advantageous from the point of view of conductivity, representing an optimal compromise between the number of charge carriers and their mobility .

[0094] CORRELATION BETWEEN STRUCTURE AND CONDUCTIVITY MECHANISM

[0095] Summari zing all the results , it is possible to propose a reasonable ionic conductivity mechanism for the electrolytes obtained, which is based on the following points :

[0096] XRD and Raman techniques show that during synthesis , no intercalation compounds or ionic or covalent compounds are formed . This evidence supports the fact that Al3+is only on the surface of the fluorinated anatase particles .

[0097] The presence of species based on TixO2-yFy, as detected by Raman spectroscopy, suggests that Al3+can be weakly coordinated by fluoride ions or Ti3+coordination sites . In all cases , there is no evidence of a strong bond ( e . g . covalent ) between Al3+and F or Ti . This is the reason for the high ionic conductivity measured;

[0098] The conductivity increases considerably after functionali zation with Al3+, thus supporting the fact that the conductivity in the electrolyte is ionic and associated with the migration of aluminum ions .

[0099] In this scenario , it can be concluded that Al3+ions can migrate ef fectively between fluorinated and defective anatase particles that act as coordination sites . The proposed mechanism is illustrated schematically in Figure 10 .

[0100] The performance level of the best electrolyte E2 ( conductivity between 10~5e 10~4S / cm in the range 25 a 80 ° C ) is close to that of polymeric electrolytes for lithium batteries and : a ) demonstrates the great promise of this new class of materials that has no similarities in literature ; b ) for the first time , paves the way for the development of a future aluminum battery completely in the solid state .

[0101] Bibliography

[0102] [1] Review— Progress in Electrolytes for Rechargeable Aluminum Batteries Journal of The Electrochemical Society, 2021 168 056509

[0103] [2] Flexible Stable Solid-State Al-Ion Batteries Advanced Functional Materials, 2019 29 1806799

[0104] [3] Recent developments on electrode materials and electrolytes for aluminum-ion batteries Journal of Energy Storage, 2024 86 111287

[0105] [4] A Flexible Solid-State Ionic Polymer Electrolyte for Application in Aluminum Batteries ACS Applied Energy Materials, 2023 6 2914-2923

Claims

CLAIMS1 . A process for the preparation of solid-state ionic conductive compounds comprising the step of reacting an ammonium oxof luorotitanate-type precursor of the formula :(NH4) aTiObFcwherein the molar ratios a, b and c are :0<ad3 l<b<20<cd5 .2 . The process for the preparation of solid-state ionic conductive compounds according to the preceding claim, wherein both the aluminum chloride and the ammonium oxofluorotitanate precursor are in solid form .

3. The process for the preparation of solid-state ionic conductor compounds according to claim 1 or 2 , wherein the reaction is carried out at a temperature of approximately 200-500 ° C .4 . The process for the preparation of solid-state ionic conductor compounds according to any one of the preceding claims , wherein the reaction is carried out for a period of time from about 30 minutes to 5 hours .5 . A solid-state ionic conductive compound obtained according to the process of any of the preceding claims .

6. The solid-state ionic conductive compound according to the preceding claim having the following general formula :AlxTiOyFzwhere the molar ratios x, y and z are as follows :0 . 02<x<0 . 151 . 8<y<2 . 200 . 26<z<1 . 18 .7 . The solid-state ionic conductive compound according to the preceding claim 5 or 6 which does not comprise aluminum chloride .8 . The solid-state ionic conductor compound according to any one of the preceding claims 5 to 7 , wherein Al3+aluminum is present on the surface of the fluorinated anatase particles .

9. An aluminum ion battery comprising the solid-state ionic conductor compounds according to any one of claims 5 to 8 .10 . Use of the solid-state ionic conductor compounds obtained according to the process of any one of claims 1 to 4 as electrolytes in aluminum ion batteries .

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