What is claimed is:
1. A method for preparing a cathode material, comprising the steps of:
modifying a surface of a chemically lithiated H 2 V 3 O 8 by adding a hydrated aluminum salt to the chemically lithiated H 2 V 3 O 8 , and
providing a surface modification of the chemically lithiated H 2 V 3 O 8 in the form of a coating.
2. The method according to claim 1 , the method further comprising adding a base after the modifying, wherein the hydrated aluminum salt is an Al(III) salt, and the base is a Brønsted and a Lewis base, and wherein the ratio of Al(III):base is about 1:1.
3. The method according to claim 1 , wherein the chemically lithiated H 2 V 3 O 8 has the formula Li x H 2-x V 3 O 8 , wherein x ranges from 0.1 to 1.5.
4. The method according to claim 1 , wherein x in Li x H 2-x V 3 O 8 is about 1.5.
5. The method according to claim 1 , wherein after adding the hydrated aluminum salt to chemically lithiated H 2 V 3 O 8 , the method further comprising the steps of adjusting the pH to a range between 7 and 8, and then allowing the pH to drop to a range between 6 and 7.
6. The method according to claim 1 , wherein the amount of aluminum as a wt % with respect to H 2 V 3 O 8 is between 0.5 wt % and 10 wt %.
7. The method according to claim 1 , wherein the amount of aluminum as a wt % with respect to H 2 V 3 O 8 is between 1 wt % to 3 wt %.
8. The method according to claim 1 , wherein the amount of aluminum as a wt % with respect to H 2 V 3 O 8 is between 1 wt % to 2 wt %.
9. The method according to claim 1 , wherein the amount of aluminum as a wt % with respect to H 2 V 3 O 8 is about 1.5 wt %.
10. The method according to claim 1 , wherein the surface modification step is performed in aqueous media at a temperature below 100° C.
11. The method according to claim 1 , wherein the surface modification step is performed in aqueous media at a temperature of 70 to 90° C.
12. The method according to claim 1 , wherein the surface modification step is performed in aqueous media at a temperature of about 80° C.
13. A Li x H 2-x V 3 O 8 surface with a surface modification being an Al(OH) 3 coating or Al(OH) 3 decoration, wherein x has a value between 0.1 to 1.5.
14. The surface modified Li x H 2-x V 3 O 8 according to claim 13 , wherein the surface modification is an Al(OH) 3 decoration.
15. The surface modified Li x H 2-x V 3 O 8 of claim 13 , wherein the Li x H 2-x V 3 O 8 surface comprises carbon black.
16. The surface modified Li x H 2-x V 3 O 8 according to claim 13 , wherein the Li x H 2-x V 3 O 8 surface further comprises a binder.
17. A cathode comprising a cathode material, wherein the cathode material is a modified Li x H 2-x V 3 O 8 surface with a surface modification being an Al(OH) 3 coating or Al(OH) 3 decoration, the cathode material being disposed on a current collector.
18. An electrochemical cell comprising an assembled stack of the cathode of claim 17 , a separator, an electrolyte layer, and an anode.
19. A battery comprising at least one electrochemical cell of claim 18 .
20. The method according to claim 1 , wherein the cathode material is incorporated into a cathode for use in rechargeable lithium ion batteries.
21. The method according to claim 2 , wherein the base is ammonia.
22. The method according claim 2 , wherein the Al(III) is provided by Al(NO 3 ) 3 .9H 2 O, AlCl 3 .6H 2 O or Al 2 (SO 4 ) 3 .18H 2 O, preferably AlCl 3 .6H 2 O or Al(NO 3 ) 3 .9H 2 O.
23. The method according claim 22 , wherein the Al(III) is provided by AlCl 3 .6H 2 O.
24. The method according claim 22 , wherein the Al(III) is provided by Al(NO 3 ) 3 .9H 2 O.
25. The method according to claim 2 , wherein the base is ammonia.
This application claims priority from European Patent Application No. 13161096.6 filed Mar. 3, 2013 and European Patent Application No. 13176170.2 of Jul. 11, 2013, the entire disclosure of which is incorporated herein by reference.
FIELD OF THE INVENTION
The invention relates to the field of high energy density lithium ion batteries. It especially concerns improved cathode materials, in particular surface modified Li x H 2-x V 3 O 8 cathode material.
BACKGROUND OF THE INVENTION
Vanadium oxides have attracted great interest for the positive electrode in lithium ion batteries over the past few decades. Among them, V 2 O 5 was initially pointed as a very promising cathodic electro active material due to its low atomic mass, high oxidation number of vanadium (V) and easiness of synthesis in an industrial scale . While close to three lithium equivalents can be inserted into the V 2 O 5 crystal lattice when the battery is first discharged down to 2 V, less than two lithium equivalents can actually be quasi-reversibly intercalated into the structure . This fact hindered the dissemination of this compound as a standard for mass production cathodes so far. However, research continued on other vanadium oxide structures like Li x V 3 O 8 and H 2 V 3 O 8 . Both of these cathodic materials are promising candidates for commercial lithium ion batteries but stability issues on long term cycling are still to be solved [3, 4].
It has already been shown that up to four lithium equivalents (ca. 400 Ah/kg) can be intercalated in H 2 V 3 O 8 between 4.2 V and 1.5 V vs. Li/Li + [3, 5] with a mean potential close to 2.7 V. This leads to a specific energy density higher than 1 kWh/kg which shows by its own the great potential associated to this compound for lithium ion batteries. For instance, the theoretical energy density associated to commercially available lithium ion cathodes like iron, cobalt and/or manganese oxides or phosphates is in the range of 0.5 kWh/kg. The practical capacity measured in such systems, however, is lower than the reported theoretical value.
In a previous co-owned published patent application , the synthesis of H 2 V 3 O 8 and electrode preparation with graphene oxide was explored. The present invention represents a new approach to increase the cathodic charge retention during cycling by surface decoration with aluminum hydroxide.
Conversely to the majority of the current generation cathodes for lithium ion batteries, H 2 V 3 O 8 shows low thermal stability. It is known that structural water present in this compound (also written as V 3 O 7 .H 2 O) is released when heated above 200° C. [2, 7]. It is generally accepted that the anhydrous material is obtained at a temperature close to 350° C. This property limits the choice of possible compounds to coat the surface of H 2 V 3 O 8 . Several metal oxides like MgO, Al 2 O 3 , SiO 2 , TiO 2 , ZnO, SnO 2 , ZrO 2 , glasses and phosphates, for example, have been extensively studied as surface coatings for lithium ion cathodes. It has been reported that these coatings prevent the direct contact from the intercalation compound with the electrolytic solution, suppress undesirable phase transitions, improve the structural stability, and decrease the disorder of cations in crystal sites. As a result, side reactions and heat generation during cycling can be decreased . However, either due to processing limitations related to solvent and pH compatibility or thermal stability of H 2 V 3 O 8 , the common wet and solid state chemistry methods used to coat or decorate the surface of cathodic materials with the referred metal oxides are not suited for H 2 V 3 O 8 .
Thus, the object of the invention was to increase the capacity retention of the lithium intercalation under charge and discharge cycling of the H 2 V 3 O 8 compound as lithium ion battery cathode.
SUMMARY OF THE INVENTION
The present invention provides a cathode material for rechargeable lithium ion battery with improved cycling stability obtained by surface modification with Al(OH) 3 , performed at low temperature in aqueous media. Dependent on its thickness, this surface modification with Al(OH) 3 is termed coating or decoration. In the scope of this invention, a coating is a thick layer covering the entire surface of a particle of electro active material while a decoration is a (usually thinner) layer that forms a partial coating only, i.e. does not cover the entire surface.
Thus, one object of the present invention is a method for preparing an at least partially stabilized electro active cathode material (EAM) for rechargeable lithium ion batteries based on Li x H 2-x V 3 O 8 by applying a coating/decoration of Al(OH) 3 .
Usually the coating/decorating step is performed in that a soluble aluminum source, preferably an aluminum salt hydrate, is added to a suspension comprising chemically lithiated H 2 V 3 O 8 , followed by addition of a base up to a pH of 7 to 8. A suitable base is simultaneously a Brønsted and a Lewis base, i.e. is simultaneously able to raise the pH and to complex the Lewis acid Al (III), in particular aqueous ammonia. This step preferably is performed in aqueous media at low temperature, i.e. below 100° C., such as 70 to 90° C., preferably about 80° C.
In general the suspension comprising chemically lithiated H 2 V 3 O 8 , is formed in situ from H 2 V 3 O 8 , a lithium source and a reducing agent as described in  and is used without any separation and/or purification step. It is also within the scope of the present invention to provide the Al(OH) 3 coated/decorated EAM with a graphene layer as described in .
The rechargeable lithium ion cell is formed by an assembled stack composed of a cathode, a separator, an electrolytic layer and an anode. Suitable anode materials include e.g. lithium, lithium alloys, Li x C 6 , silicides and nano-silicon-based materials. The electrolytic layer preferably includes a spacer and an aprotic solvent or a mixture of aprotic solvents with dissolved lithium salt. A suitable spacer is a foam filled with electrolyte.
A battery may be made up of several stacks and is sealed in a container.
The cathode material is essentially composed of the modified Li x H 2-x V 3 O 8 and carbon black. The use of a binder is possible, but not compulsory. A cathode is formed by coating a current collector with such cathode material.
Li x H 2-x V 3 O 8 retains the H 2 V 3 O 8 structure (Pnam). In this compound x is ranging from 0.1 to 1.5 and preferably is close to 1.5. The lithiation process is advantageously performed immediately before the coating/decorating reaction.
H 2 V 3 O 8 is lithiated using at least one Li + source and a reducing agent. Such Li + source and reducing agent preferably consists of or contains a reducing lithium salt, much preferred lithium ascorbate.
The Al(OH) 3 coating is preferably produced in a one pot multistep reaction starting with phase pure H 2 V 3 O 8 as starting material, followed by a chemical lithiation of the starting material. Subsequently surface coating or decorating with aluminum hydroxide is performed.
In a preferred embodiment, Al(OH) 3 is generated from an aluminum salt hydrate, e.g. a salt selected from Al(NO 3 ) 3 .9H 2 O, AlCl 3 .6H 2 O or Al 2 (SO 4 ) 3 .18H 2 O, preferably AlCl 3 .6H 2 O or Al(NO 3 ) 3 .9H 2 O. The hydroxide is generated in situ by addition of a base that is simultaneously a Brønsted and a Lewis base, preferably aqueous ammonia (NH 3 ). Usually the ratio of Al(III):NH 3 is about 1:1.
The amount of aluminum used should be comprised between 0.5 wt % and 10 wt %, preferably between 1 and 3 wt %, more preferred between 1 and 2%, and most preferred about 1.5 wt % referred to H 2 V 3 O 8 .
Since the reaction is preferably performed such that the Al(OH) 3 is deposited on the EAM and not precipitated in the solution, the amount used essentially corresponds to the amount of the coating/decoration and the thickness of the coating can be adjusted by the amount of aluminum used for performing the reaction.
Since the coating/decorating preferably is performed in a one pot method, it is assumed that the Al(OH) 3 coating/decoration may comprise some impurities like ascorbate (if Li ascorbate is used for lithiation) or NH 3 (if NH 3 is used as base) or AlO(OH).
A lithium ion battery assembled with such cathodic intercalation material shows higher stability over 200 cycles compared to the uncoated Li x H 2-x V 3 O 8 . For instance, after 200 charge and discharge cycles between 4.05 and 2.2 V vs. Li/Li + at a rate of 100 A/kg, the Al(OH) 3 coated Li x H 2-x V 3 O 8 with an Al amount of 1.5 wt. % referred to H 2 V 3 O 8 shows a capacity retention of 89% compared to 67% of the uncoated Li x H 2-x V 3 O 8 .
Without wanting to be bound by any attempt to interpret the effect, it is assumed that the Al(OH) 3 coating or decoration inhibits vanadium dissolution during cycling either by increasing the structural stability of the fibers and/or by surface passivation.
BRIEF DESCRIPTION OF THE DRAWINGS
To better understand the disclosure of the invention, a series of drawings is presented.
FIG. 1 a : Helium ion microscopy micrograph of the uncoated H 2 V 3 O 8 fibers.
FIG. 1 b : Helium ion microscopy micrograph of the Li x H 2-x V 3 O 8 fibers coated with 1.5 wt. % Al.
FIG. 1 c : Helium ion microscopy micrograph of the Li x H 2-x V 3 O 8 fibers coated with 3 wt. % nominal Al.
FIG. 2 a : HRTEM micrograph of the Li x H 2-x V 3 O 8 fibers coated with 0.5 wt. % nominal Al.
FIG. 2 b : HRTEM micrograph of the Li x H 2-x V 3 O 8 fibers coated with 1.5 wt. % nominal Al
FIG. 2 c : HRTEM micrograph of the Li x H 2-x V 3 O 8 fibers coated with 3 wt. % nominal Al
FIG. 3 a : HRTEM micrograph of a representative zone of the Li x H 2-x V 3 O 8 coated with 1.5 wt. % nominal aluminum sample.
FIG. 3 b : Fast Fourier Transform of the HRTEM micrograph shown if FIG. 3 a showing amorphous background superimposed to the H 2 V 3 O 8 crystal structure.
FIG. 3 c : EELS on the Al L2/3 edge of the HRTEM shown if FIG. 3 a . Left Y axis indicates the I/10 measured signal. Right Y axis represents the measured signal with subtracted background. It clearly shows the Al L2/3 edge appearing at ca. 70 eV.
FIG. 3 d : EDX spectrum of the HRTEM shown if FIG. 3 a . The Cu is a trace element coming from the Cu grids used for TEM sample preparation. 1.2 at. % aluminum was measured.
FIG. 4 : XRD of coated, decorated and uncoated Li x H 2-x V 3 O 8 fibers. The only crystalline structure detected is the one of H 2 V 3 O 8 , indicating an amorphous Al(OH) 3 surface film.
FIG. 5 : Normalized DSC of coated, decorated and uncoated Li x H 2-x V 3 O 8 . Influence of the Al(OH) 3 surface coating on the H 2 V 3 O 8 fibers dehydration temperature: dehydration temperature increases with increased amount of aluminum. Coating has a direct influence on the thermal stabilization of the Li x H 2-y V 3 O 8 fibers. Experimental conditions: heating ratio: 7.5° C. min −1 ; synthetic air: 50 mL min −1 .
FIG. 6 : Charge and discharge curves on the 20 th and 200 th cycle between 4.05 V and 2.2 V vs. Li/Li + at a rate of 100 A/kg of the cell assembled with uncoated Li x H 2-x V 3 O 8 cathode, prepared as described in example 3.
FIG. 7 : Charge and discharge curves on the 20 th and 200 th cycle between 4.05 V and 2.2 V vs. Li/Li + at a rate of 100 A/kg of the cell assembled with Li x H 2-x V 3 O 8 cathode Al(OH) 3 coated with 1.5 wt. % Al, prepared as described in example 3.
FIG. 8 : Differential capacity (specific charge) curves calculated from the galvanostatic charge and discharge curves on the 100 th cycle of the coated and uncoated samples of the example 3. Small polarization induced by the surface coating is present on samples containing 1.5 and 3 wt. % Al.
FIG. 9 : Shows the charge and discharge curves on the 20 th and 200 th cycle between 4.05 V and 2.2 V vs. Li/Li + at a rate of 100 A/kg of the cell assembled with Li x H 2-x V 3 O 8 cathode Al(OH) 3 coated with 3 wt. % Al, prepared as described in example 3.
FIG. 10 : HRTEM micrograph of the Li x H 2-x V 3 O 8 fibers Al(OH) 3 coated with 3 wt. % Al of the example 3. Imaging of the aluminum hydroxide coating after 200 charge and discharge cycles, showing that the coating remains at the surface during the electrochemical test.
FIG. 11 : Plot showing the evolution of the specific charge of the cells assembled as described in the example 3, with different Al(OH) 3 contents, as a function of the number of cycles.
DETAILED DESCRIPTION OF THE INVENTION
The deposition of an amorphous Al(OH) 3 coating film on the surface of the EAM in aqueous media at low temperature is an additional step during the chemical lithiation of H 2 V 3 O 8 using a Li + source together with a reducing agent, in particular a reducing agent with a redox potential close to the one of ascorbic acid/ascorbate. A presently much preferred lithium source and reduction agent is lithium ascorbate. The whole process consists of a multistep one pot reaction where the H 2 V 3 O 8 fibers are first at least partially and preferably partially lithiated and then coated/decorated. A full surface coverage with the Al(OH) 3 thin film can be achieved with higher aluminum contents while surface decorations are obtained with lower aluminum contents. For the coating/decorating step, a soluble aluminum source (an aluminum salt hydrate) is added to the suspension prepared in situ from H 2 V 3 O 8 , a lithium source and a reducing agent. The coating/decorating reaction is then induced by the addition of a diluted ammonia solution and takes place at pH comprised between 7 and 8. However, it was noticed that either the presence of remaining organic species from the lithiation step or the reduced EAM surface, showing higher concentration of vanadium (IV) species compared to bare temperature stabilized EAM, have a considerable influence on the homogeneity of the Al(OH) 3 surface coverage and consequently on the electrochemical performance.
Synthesis of H 2 V 3 O 8
The phase pure H 2 V 3 O 8 fibers may be prepared following the method described in the co-owned published United States patent application US 2012/0100402 A1 : 3 g of VOSO 4 .5H 2 O were dissolved in 50 mL deionized water. 2 mL of 25 wt. % ammonia (NH 4 OH) were then added to the previous suspension. The dense precipitate which formed immediately was filtered and the grey wet solid product collected, put in a TEFLON® vessel of an autoclave and dispersed in 400 mL distilled water. After addition of 1 mL 12 M HCl, the autoclave was sealed and the suspension treated hydrothermally for 48 h at 220° C. At the end of the reaction, the green to green-yellow solid product was filtered, washed with water and isopropanol and dried overnight at 100° C. in air. 750 mg of H 2 V 3 O 8 were obtained by such method. The vanadium yield equaled 70%.
One Pot Chemical Lithiation and Al(OH) 3 Surface Coating
The chemical lithiation of the H 2 V 3 O 8 fibers prepared as described in the example 1 was performed using lithium ascorbate as a reducing and complexation agent in aqueous media at ambient temperature.
At first, a solution containing ascorbic acid and the lithium salt, preferably LiOH, was prepared. 30 mg (0.17 mmol) of ascorbic acid (Sigma Aldrich L-Ascorbic acid 99%) were dissolved in 15 mL MilliQ water (18.2 MΩ cm) and 15 mg LiOH.H 2 O (Alfa Aesar monohydrate 56.5% min.) (0.20 mmol min.) were then added. For sufficient proton exchange, the solution was stirred until the pH stabilized, usually for about 30 minutes. This solution will be called S1, onwards.
A second suspension, named S2, containing 100 mg of the green H 2 V 3 O 8 fibers dispersed in 50 mL MilliQ water was prepared. The pH of this suspension was close to 4.
The lithium ascorbate solution (S1) was subsequently added to the suspension containing the H 2 V 3 O 8 compound (S2). Thereby the pH of S2 was neutralized due to the excess of lithium hydroxide in S1. After a few minutes under stirring, the pH decreased from 10 to a value close to 7. The suspended fibers now showed a bluish color (suspension S3) which indicates that to some extent the vanadium oxidation state was reduced from vanadium (V) to vanadium (IV). Considering the charge balance in the compound, this fact is an indication that Li + was incorporated in the H 2 V 3 O 8 structure. This color change—or rather the absence of further color change—is an indication that the lithiation reaction has been completed. To this reaction media (S3), the desired amount of a hydrated aluminum salt, e.g. AlCl 3 .6H 2 O or Al(NO 3 ) 3 .9H 2 O, was added as solid. The amount of aluminum used in the synthesis of the different coatings/decorations is indicated in the Table 1.
Amount of aluminum used for the different coating compositions.
H 2 V 3 O 8 (mg)
Li x H 2−x V 3 O 8
Li x H 2−x V 3 O 8 0.5 wt. % Al
1.9 × 10 −5
Li x H 2−x V 3 O 8 1.5 wt. % Al
5.6 × 10 −5
Li x H 2−x V 3 O 8 3 wt. % Al
1.1 × 10 −4
Upon addition of solid aluminum salt to the suspension S3, the pH dropped to a value comprised between 4.5 and 6 depending on the amount of aluminum salt added. Afterwards, this suspension was heated up to 80° C. and a 0.1 M ammonia solution, having a (aluminum (III)/NH 3 ) molar ratio equal to 1, was added drop wise. The pH increased to a value between 7 and 8. After 2 h under stirring, the pH of the suspension reached a stable value between 6 and 7. The suspension was then filtered and washed once with water. The filtered compound was finally dried at 85° C. before being heat treated for 1 h at 180° C. in air.
FIGS. 1 a , 1 b and 1 c show the surface roughness of the uncoated fibers and those decorated/coated with 1.5 wt. % and 3 wt. % aluminum. The images were obtained using a Helium ion microscope. Although only a minor change in surface roughness can be discerned between the uncoated and 1.5 wt. % Al samples, a clear homogenous coverage of the surface of the fibers was identified in the 3 wt. % Al sample ( FIG. 1 c ). High magnification images of the surface obtained by HRTEM are shown in FIGS. 2 a , 2 b and 2 c . It can be seen that the surface of the 0.5 and 1.5 wt. % aluminum compounds is rough but no clear coated layer can be identified from HRTEM images on the lower content aluminum coatings. To further understand the presence of the aluminum on the Li x H 2-x V 3 O 8 surface, EELS and EDX analysis were conducted on the Li x H 2-x V 3 O 8 —1.5 wt. % Al compound. The results are shown on FIG. 3 c and FIG. 3 d . The analysis performed on the zone shown in FIG. 3 a , allowed to detect the Al L 2,3 edge in the EELS spectra revealing the presence of aluminum. This result is confirmed by the EDX analysis where ca. 1.2 wt. % Al could be measured. A clear coverage of the Li x H 2-x V 3 O 8 surface by amorphous Al(OH) 3 can be seen on the 3 wt. % aluminum composition. The morphology of this coating is uniform as confirmed by the images present in FIG. 1 c and FIG. 2 c . A rough thin layer covers the surface, with the Al(OH) 3 film thickness ranging from 3 to 10 nm. The influence of the coating thickness and surface coverage on the electrochemical performance of the EAM will be discussed later in this text. No identifiable crystalline structure changes of the EAM was induced by the coating, independently of the amount of aluminum used. No additional crystalline phase could be detected by X-ray diffraction, although 3 wt. % of a crystalline Al phase should give a signal. Results are show in FIG. 4 . The crystalline structure of the EAM was identifiable in all samples, but no ordered structure could be seen by transmission electron microscopy of the aluminum decoration. This is a strong indication that the aluminum structure is amorphous. Also, the electron diffraction pattern of the Li x H 2-x V 3 O 8 —1.5 wt. % Al compound shown in FIG. 3 b revealed a strong amorphous contribution superimposed to the EAM crystal structure, which is absent without Al coating. This fully agrees with the XRD data.
To further comprehend the aluminum distribution on the samples, low magnification SEM/EDX allows characterizing the bulk composition of the compounds. Also, XPS analysis on the uncoated and 3 wt. % aluminum compound was performed. Results are summarized in Table 2. EDX results indicate that the yield of the coating reaction is close to 40%. A very high aluminum surface concentration is confirmed by XPS on the 3 wt. % aluminum compound.
XPS and SEM-EDX surface and bulk element composition
of coated and uncoated samples.
(bulk 5k x mag.)
Li x H 2−x V 3 O 8
Li x H 2−x V 3 O 8 0.5 wt. % Al
Li x H 2−x V 3 O 8 1.5 wt. % Al
Li x H 2−x V 3 O 8 3 wt. % Al
Only aluminum traces were detected on the lower aluminum content coatings presumably indicating a not full surface coverage. This assumption is supported by the local EDX analysis of the FIG. 3 d where a higher content could be measured compared to the bulk. However, it seems that even a small amount of aluminum has a considerable effect on the thermal stability. FIG. 5 shows the normalized differential scanning calorimetry curves obtained with pristine H 2 V 3 O 8 , Li x H 2-x V 3 O 8 and Al(OH) 3 coated Li x H 2-x V 3 O 8 compounds. A clear shift towards higher temperatures for the dehydration reaction was observed on the coated samples. This reaction can be described as [2, 7]:
H 2 V 3 O 8 →V 3 O 7 +H 2 O
Under synthetic air the temperature in which the dehydration reaction occurs increased for approximately 50° C. for the materials coated with 1.5 wt. % and 3 wt. % aluminum amount used, confirming the influence of the surface modification on the structural stability of the material.
The degree of aluminum hydroxide surface coverage and film thickness can be adjusted in a nonlinear manner by the amount of aluminum in the deposition reaction.
Electrochemical Performance of Coated/Decorated and Uncoated Samples
Batteries were made to characterize electrochemically the stability of the cathodic storage materials. To prepare each battery, 20 mg of coated/decorated or uncoated Li x H 2-x V 3 O 8 compound were first dispersed ultrasonically in 5 mL THF (Alfa Aesar 99%). 10 mg of Carbon black (Super P® from TIMCAL) were then added and ultrasonically mixed for a few minutes until a dark homogenous suspension was obtained allowing the carbon and the EAM to be intimately mixed. Half of the suspension volume was transferred to a mortar and gently stirred with a pestle. The THF evaporation resulted in a thick paste. This paste containing the EAM was then transferred to an aluminum current collector, pressed with paper sheet to absorb remaining THF and allowed to dry in air before performing a heat treatment at 180° C. for 10 minutes under air. Close to 4 mg of EAM were deposited on the current collector by this process. The batteries were assembled in an argon filled glove box (<0.1% H 2 O and <0.1% O 2 ) by building a vertical stack composed by the aluminum current collector containing the EAM, a polypropylene Celgard separator, a silica foam spacer filled with Merck® LP 50 electrolyte (1M LiPF 6 in EC/EMC 1:1), and a metallic lithium anode. The stack is contained inside a well tighten stainless steel cell.
The batteries assembled as described were subjected to galvanostatic charge/discharge cycles between 4.05 V and 2.2 V vs. Li/Li + at a current rate of 100 A/kg.
FIG. 6 shows the variation in the battery voltage as a function of the intercalated lithium in the uncoated Li x H 2-x V 3 O 8 cathode for the 20 th and 200 th cycles. On the 20 th cycle, a specific charge of 240 Ah/kg was measured for this compound. Close to 2.5 mole of lithium atoms were intercalated per mole of EAM during the discharge between 4.05 V and 2.2 V vs. Li/Li + . Typical voltage profiles of H 2 V 3 O 8 were registered under charge/discharge where several voltage plateaus can be seen. The coulombic efficiency between discharge and charge was close to 98.7% on 20 th cycle and increased with the number of cycles to 99.4% on the 200 th cycle. However, the increase in the number of cycles led to a degradation of the electrode performance and on the 200 th cycle, a capacity of 162 Ah/kg remained. This value indicates that only 1.7 mole of lithium atoms are now intercalated per mole of EAM representing a capacity retention of 67% between the 20 th and 200 th cycles. Also, the decrease in the specific charge was uniformly distributed over the entire potential range. Interestingly, the capacity retention in the high potential region, above 2.7 V, was identical to the one measured between 2.7 and 2.2 V vs. Li/Li + . In both cases, 67% of the capacity measured on the 20 th cycle was available on the 200 th .
FIG. 7 is similar to FIG. 6 showing the voltage profiles registered on Li x H 2-x V 3 O 8 decorated with nominal 1.5 wt % aluminum. On the 20 th cycle, a lower specific charge (ca. 203 Ah/kg) was measured for this compound compared to the uncoated EAM, which means that around 15% less lithium atoms were intercalated in the structure. However, the capacity retention after 200 cycles compared to the 20 th cycle was close to 89% which was much higher than for the uncoated fibers. After 200 cycles, the Li x H 2-x V 3 O 8 —1.5 wt. % Al compound could still intercalate 2 mole of lithium atoms per mole of H 2 V 3 O 8 compared to 1.7 of the uncoated compound. Almost no difference was noticed in the voltage profiles of the uncoated and 1.5 wt. % aluminum decorated compounds as can be seen in FIG. 6 and FIG. 7 . Nevertheless, by analyzing the calculated differential capacity curves presented in FIG. 8 , it can be seen that a small polarization was induced by the coating. Such polarization is revealed by the shift towards higher potentials of the lithium de-intercalation reaction while charging and vice-versa on discharging, comparing the uncoated and decorated compounds.
The charge and discharge voltage profiles on the 20 th and 200 th cycles on Li x H 2-x V 3 O 8 coated with nominal 3 wt % aluminum are shown in FIG. 9 . It is noticed on this plot that the voltage plateaus are less well defined than in the case of the uncoated and lower aluminum content materials possibly due to the increased complexity of the lithium intercalation reaction induced by the thicker coating. Moreover, the capacity involved in the charge and discharge reactions was also lower than that measured on the uncoated Li x H 2-x V 3 O 8 . Only close to 1 mole of lithium atoms per mole of H 2 V 3 O 8 were intercalated in the structure. However, after 200 cycles a 5% increase in the specific charge was registered. Also, the voltage profile curves became better defined indicating that lithium percolation on this compound improved under cycling, especially in the higher potential region.
These results show that such a thick coating considerably affects the specific charge available for the Li intercalation in the EAM structure but it allows for a very high stability under cycling compared to the bare EAM.
The stability of the amorphous Al(OH) 3 coating under electrochemical stress, can be seen from a comparison of the HRTEM images of the 3 wt. % aluminum coated fibers before cycling ( FIG. 2 c ) and after cycling ( FIG. 10 ). These pictures clearly show that the surface coating is stable under the cycling conditions.
FIG. 11 shows the variation of the capacity as a function of the number of cycles in the batteries assembled with uncoated and coated/decorated compounds. The stability under charge and discharge cycles of Li x H 2-x V 3 O 8 fibers can be increased by means of an Al(OH) 3 surface coating/decoration performed in aqueous media at low temperature. The presence of the aluminum hydroxide coating increases the capacity retention of Li x H 2-x V 3 O 8 under cycling.
Vanadium dissolution and a change in the fibers morphology due to the fracture of the long single crystal nanobelts have been reported as the most likely mechanisms for loss of capacity during cycling on the compound . The decorations/coatings reported in the present invention increases the capacity retention under cycling by influencing the bulk compound in two possible major ways.
1) Increasing structural stability;
2) Limiting vanadium dissolution.
Thermo-gravimetric data corroborates the electrochemical results indicating that the structural stability of Li x H 2-x V 3 O 8 improves with the coating.
Both the decoration/coating thickness and the surface coverage play an important role for the electrode stability. It is worth notice that these variables also influence the lithium percolation of the electrode which is related to the kinetics of the intercalation/de-intercalation reactions. The fine tuning of the Al(OH) 3 decoration/coating morphology, namely the surface coverage and thickness is important to ensure high cycling stability while keeping high lithium percolation and limiting polarization side effects.
—U.S. Pat. No. 5,219,677.
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