Microcapsules containing additional lithium salt for libs

Abstract

Microcapsules containing additional lithium salts for Lithium-ion batteries (LIBs) are presented here. The microcapsules are comprised of outer hollow shell such as porous SiO2 and inner core of a lithium salt such as lithium squarate. Further, processes of preparing the microcapsules are provided.

Description

[0001] Microcapsules containing Additional Lithium Salt for LIBs

[0002] Microcapsules containing additional lithium salts for Lithium-ion batteries (LIBs) are presented here. The microcapsules are comprised of outer hollow shell such as porous SiC>2 and inner core of a lithium salt such as lithium squarate (Li2C4O4). Further, processes of preparing the microcapsules are provided.

[0003] In LIBs, it is extremely necessary to retain a longer life cycle that delivers nonstop lithium ion intercalation and de-intercalation. At the time of intercalation and de-intercalation processes, there are very prominent volume changes in the electrode materials, which leads to continuous re-formation of the solid electrolyte interphase (SEI) and consumption of active lithium (up to 70% consumption in first few cycles) ions. Therefore, it is very important to have additional lithium ions source which can fulfil the requirement of loss of active lithium ions in LIB full-cells and improve their life span.

[0004] DESCRIPTION OF INVENTION

[0005] In this work, porous SiO2-based microcapsules containing lithium salt (such as ^2 404) is investigated for the first time as an extra lithium source for the cathode in lithium metal battery cells and in LIB cells with a graphite based negative electrode. Although the concept of microcapsules is very old term in medical and cosmetic fields but in case of energy storage application it can be a revolution.

[0006] Thus, the present invention provides a microcapsule comprising.

[0007] (i) an outer shell comprising or consisting of SiC>2, and

[0008] (ii) an inner core comprising or consisting of one or more organic or inorganic lithium salts.

[0009] As synthesized microcapsules are air stable, cost-effective and compatible with standard industrial electrode formulation processing protocol.

[0010] The material of the outer shell (i) comprises or consists of SiC>2.

[0011] Porous SiC>2 shells are applicable as a shell material due to their high thermochemical and mechanical stability, controlled permeability, high surface area, and porosity, making them valuable in various research fields.Porous SiC>2 is superior to other shell materials and is advantageous for the controlled or slow release of ions or molecules from the core. The porous SiC>2 shell prevents unwanted catalytic reactions, ensuring sustained lithium availability throughout cycling, when the microcapsules of the invention are used in a lithium-ion battery.

[0012] The inner core (ii) preferably comprises or consists of a lithium salt selected from Li2C4O4, U2S, LiaS LiNOa, LiaN, U2MOO3, LisFeOt, LiaNiOa and combinations thereof. More preferably, the lithium salt is U2C4O4.

[0013] In a particularly preferred embodiment of the invention, the outer shell comprises or consists of porous SiC>2, and the core comprises or consists of U2C4O4, U2S, or LiNOa. Most preferred microcapsules comprise an outer shell comprising or consisting of SiOa and a core comprising or consisting of U2C4O4.

[0014] In the present disclosure, microcapsules with a porous SiOa-based shell and U2C4O4 as the core are used for the first time as an additional lithium supplement in Li-ion batteries. Encapsulation of a lithium salt, in particular U2C4O4, with SiC>2 is beneficial due to activation at higher voltage, which can be technically implemented in graphite / / LFP (Lithium Iron Phosphate or LiFePCL) cells.

[0015] The amount of lithium salt in a microcapsule may be in a range of 1-25 wt% based on the total weight of the microcapsule, more preferably 10-15 wt%.

[0016] Encapsulation in a SiOa shell enables a controlled release of Li+ ions from the microcapsules in an LFP-cathode-based system. In an NMC-cathode-based system (NMC = nickel manganese cobalt oxides), there is a problem with the overlapping of redox peaks with the microcapsules. Therefore, functionalisation (e.g. with groups like -NH2, or-COOH) at the outer surface of the microcapsules allows controlled release or triggering of Li+ ions on demand. Triggering the release of Li+ on demand can be achieved by a polymer coating on the SiOa shell. Thus, in a particular embodiment, the microcapsule of the invention further comprises a polymer coating on the outer shell. Coating with a thermoresponsive polymer, such as a PEGMEMA-based polymer (PEGMEMA = poly(ethylene glycol) monomethyl ether monomethacrylate), is preferred. When the temperature rises, the thermoresponsive polymer melts and Li+ can increasingly be released from the microcapsule. At lower temperatures, the polymer blocks the release.Another aspect of the invention is an electrode, comprising the microcapsule described above. Particularly advantageous is the use of the microcapsules according to the invention in electrodes for LIBs where they serve as an additional source of lithium ions.

[0017] Still another aspect of the invention is a lithium-ion battery comprising the above electrode of the invention. Preferably, the electrode of the invention serves as a cathode. In a lithium ion battery, the microcapsule of the invention included in the electrode serves as an additional source of lithium ions for the electrode which can compensate for the loss of active lithium ions in LIB full-cells and improve their life span. An increase in cycle life of NMC622+Li2C4C>4@porous SiC>21| graphite full-cells is reported, which grows linearly with the initial amount of lithium salt due to extra Li+ions but decrease at higher amount due to parasitic reactions.

[0018] Further, the invention provides a method for preparing the microcapsule of the invention, comprising the steps

[0019] (i) providing a material to be encapsulated comprising or consisting of one or more organic or inorganic lithium salts,

[0020] (ii) preparing a hollow porous shell comprising or consisting of SiC>2, and

[0021] (iii) encapsulating the material of (i) inside the hollow porous shell,

[0022] wherein steps (ii) and (iii) can be carried out simultaneously or sequentially.

[0023] Steps (ii) and (iii) can be carried out simultaneously or successively. That is, it is possible to introduce the material to be encapsulated through a hollow porous shell or the shell can be formed directly around a material to be encapsulated.

[0024] The material of the hollow porous shell prepared in step (ii) comprises or consists of SiC>2.

[0025] According to a preferred embodiment, step (ii) of preparing the hollow porous SiC>2 shell comprises adding tetraethyl orthosilicate (TEOS) to a solution comprising sodium polyacrylate, ammonia and EtOH and isolating the hollow porous SiC>2 shell.

[0026] The material to be encapsulated is preferably selected from U2C4O4, U2S, LiaS UNO3, LisN, U2MOO3, LisFeCL, LiaNiCh and combinations thereof. More preferably, the material comprises or consists of Li2C4C>4.

[0027] In a preferred embodiment wherein the material to be encapsulated is U2C4O4, step (i) comprises synthesizing lithium squarate by a method comprising reacting squaric acid and lithium carbonate, preferably in a mixture of water and ethanol, and isolating lithium squarate.Encapsulating U2C4O4 inside the hollow porous SiC>2 shell can be accomplished by contacting the hollow porous SiC>2 shell obtained from step (ii), preferably as a dry powder, with an aqueous solution of Li2C4C>4 to yield impregnation of Li2C4C>4 inside the porous SiC>2 shell. Subsequently, the resulting microcapsule is dried, preferably by vacuum drying.

[0028] In an alternative embodiment, steps (ii) and (iii) are carried out simultaneously, preferably by in-situ hydrolysis and condensation of TEOS on an organic lithium salt. In this procedure, the shell is formed directly around the material to be encapsulated.

[0029] Step (ii) may comprise a miniemulsion polymerization process.

[0030] In a preferred embodiment, steps (ii) and (iii) are carried out simultaneously and comprise an in-situ hydrolysis of TEOS and polymerization with ionic based polymer (such as PEGMEMA) containing -NH and -COOH functional groups on a lithium salt to be encapsulated.

[0031] Optionally, the method of preparing microcapsules further comprises a step

[0032] (iv) coating the microcapsules obtained in (iii) with a polymer coating.

[0033] The polymer coating preferably comprises a thermoresponsive coating, such as a PEGMEMA-based polymer. Methods for applying such a polymer coating are known in the field. For example, a solution of the polymer can be applied to the capsules, which then forms a solid polymer layer around the capsule.

[0034] A further aspect of the invention is the use of the microcapsule of the invention as an additive in the preparation of an electrode, in particular a cathode of a lithium-ion battery.

[0035] A still further aspect of the invention is a method for preparing the electrode of the invention, comprising a step of providing a slurry comprising electrode materials and the microcapsule of the invention, and preparing an electrode from the slurry. The electrode materials are those that are commonly used for the manufacture of electrodes in the technical field. They may vary depending on the intended use of the electrode. Suitable manufacturing processes for electrodes and corresponding materials are known to the skilled person; the special feature of the production according to the invention is the use of the novel microcapsules as an additive.

[0036] The invention will be further described by the following Figures and Examples which are not meant to limit the scope of the invention defined by the appended claims.FIGURES

[0037] Fig. 1 Powder X-ray diffraction (XRD) patterns for as-synthesized materials porous SiC>2, U2C4O4, and microcapsules (Li2C4C>4@porous SiCh). A single broad peak at about 23° indicates the amorphous nature of SiC>2, main peaks in U2C4O4 represents its monoclinic phase and crystalline nature, the microcapsules displayed the combination of both SiC>2 and U2C4O4 XRD patterns.

[0038] Fig. 2 FT-IR spectrum for as-synthesized materials porous SiC>2, U2C4O4, and microcapsules (Li2C4O4@porous SiCh)

[0039] Fig. 3 SEM and TEM images confirm the hollow porous shell of SiC>2 and presence of U2C4O4 inside it.

[0040] Fig. 4 Electrochemical evaluation of the influence of different amounts of microcapsules (Li2C4C>4@porous SiCh) with different amount of U2C4O4 within the cathode in NMC622+Li2C4C>4@porous SiCh / Zgraphite LIB cells.

[0041] Fig. 5 General schematic illustration of electrodes and pouch cell’s preparation.

[0042] Fig. 6 Electrochemical response related to degradation of U2C4O4 in with and without protective shell

[0043] Fig. 7 Electrochemical characterisation of graphite / / LFP cells without U2C4O4 with admixed U2C4O4 and with Li2C4O4@SiC>2 shells

[0044] Fig. 8 Schematic illustrations of synthesis of dilithium squarate and Li2C4O4@SiO2@DIM : PEGM EMA.

[0045] Fig. 9 Electrochemical characterisation of graphite / / NMC cell with Li2C4O4@SiO2 covered with thermoresponsive polymer showing activation after the cell was exposed to 40°C.EXAMPLES

[0046] Experimental details

[0047] Materials: Tetraethoxysilane (TEOS), Absolute Ethanol (EtOH), Ammonia solution (NH4OH), Sodium Polyacrylate (CsHsNaC^n, 3, 4-dihydroxy-3-cyclobutene-1, 2-dione (Squaric acid), Lithium Carbonate (Li2CO3), and de-ionized water.

[0048] Synthesis of microcapsules (Li2C4O4@SiO2): The synthesis of microcapsules has done in three steps. In step 1, synthesis of hollow porous SiC>2 shell have done by hydrolysis and condensation process of TEOS under the control of ammonia. In step 2, synthesis of lithium squarate (U2C4O4) have done by simple wet chemical method using squaric acid (3,4-dihydroxy-3-cyclobutene-1, 2-dione) and lithium carbonate as an initial reaction precursor. In step 3, encapsulation of U2C4O4 inside the hollow porous SiO2 shell have done by wet impregnation method as shown in scheme 1. The optimization of U2C4O4 inside hollow porous SiC>2 shell have been done by varying the percentage of U2C4O4 from 1-25 wt%. The products in all three steps have been characterized by FT-IR, XRD, SEM and TEM analysis. The details about synthesis processes and characterization is given below.

[0049] 1. Synthesis of Hollow Porous SiO2 Shell: The hollow porous SiC>2 shell is prepared at room temperature by making clear solution of 0.08 g of sodium polyacrylate in 1.5 ml of ammonia solution, then added 30 ml EtOH in it, which turned solution into turbid white. After 30 min, TEOS (0.75 ml) was added in the above solution (at time interval of 2h) under vigorous stirring. The white colloids were obtained after 12 hours continuous stirring. Finally, the hollow porous SiO2 shells were obtained by removing the sodium polyacrylate from white colloids by washing it several times with de-ionized water and dried at 80°C overnight.

[0050] 2. Synthesis of Lithium Squarate (Li2C4O4): The synthesis of lithium squarate (U2C4O4) has done by simple wet chemical method using squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) and lithium carbonate as an initial reaction precursor. Squaric acid and Li2CO3 were dissolved in de-ionized water and EtOH (1:1) at room temperature by continuous stirring for 4 h. Later, EtOH and de-ionized water was evaporated by rotary evaporator at 50°C. The resulting white powders were grinded with mortar pestle, then vacuum dried at 50°C overnight.

[0051] 3. Encapsulation of Li2C4O4 inside the hollow porous SiO2 shell (Li2C4O4@SiO2): The microcapsules of Li2C4O4@SiO2 have synthesized by wet impregnation method as shown in scheme 1. The optimization of U2C4O4 inside hollow porous SiC>2 shell have been done by varying the percentage of U2C4O4 from 1-25 wt%. The microcapsules of Li2C4O4@porousSiO2 were prepared by making the solution of U2C4O4 in water, then added it on dry power of SiO2 dropwise until whole power become wet (1ml / 1g of SiC>2 power), then leave it for almost 6 h to complete impregnation of U2C4O4 inside porous SiC>2 shell. The hydrate water was then removed by complete drying at 80°C overnight. The vacuum drying at 50°C was performed afterward to recrystallize the U2C4O4 inside the porous SiC>2 shell.

[0052] Characterizations: As synthesized materials were characterized for structure, compositional and morphological characterizations. The phase purity and function groups identification have done by power X-ray diffractometer and FT-IR techniques. Surface morphologies of the synthesized materials were investigated by scanning electron microscopy and transmission electron microscopy techniques.

[0053] Formulation of cathode composite with microcapsules: The formulation of cathode composite with microcapsules have done by simple industrial standard slurry processing as shown in Figure 5. The influence of different amounts of U2C4O4 (1-25 wt%) and microcapsules (1-2 wt%) within the cathode was systematically evaluated in half and full pouch cell fabrications.

[0054] Results and discussions

[0055] Structural and morphological characterizations: As synthesized materials were characterized by XRD, FT-IR, and SEM and TEM analysis for their structural, compositional and morphological identifications. The powder X-ray diffraction (XRD) patterns for as-synthesized materials shown in Figure 1 confirm the successful synthesis of porous SiC>2, U2C4O4, and microcapsules (Li2C4C>4@porous SiCh). A single broad peak at about 23° indicates the amorphous nature of SiC>2, main peaks in U2C4O4 represents its monoclinic phase and crystalline nature, the microcapsules displayed the combination of both SiC>2 and U2C4O4 XRD patterns as shown in figure 1. All the main bands in FT-IR spectrum is attributed to SiC>2 and U2C4O4 as shown in figure 2. SEM and TEM images are shown in figure 3, TEM images confirm the hollow porous shell of SiC>2 and presence of U2C4O4 inside it.

[0056] Electrochemical characterization: Electrochemical investigations were conducted in two-electrode configuration pouch cells, assembled in glove box (0.1 ppm O2 and H2O). A Celgard 2500 membrane served as a separator and LP57 (1M LiPF6 in 3:7 vol% ethylene carbonate / ethyl methyl carbonate, EC / EMC) as electrolyte. At least two to three cells per sample were assembled to ensure reproducible results. The electrochemical investigationshave been done for half and full cells (Li metal || U2C4O4, Li metal || Li2C4C>4@porous SiC>2, Li metal || NMC622+Li2C4C>4@porous SiC>2, and graphite || NMC622+Li2C4C>4@porous SiC>2 cells). The microcapsules (Li2C4C>4@porous SiCh) were evaluated as additional lithium ion source for the positive electrode to compensate for active lithium losses due to SEI formation at anode surface. The microcapsule holds great promise as additive due to its compatibility with industrial standard processing of the NMC-based cathodes. The influence of different amounts of microcapsules (Li2C4C>4@porous SiCh) with different amount of U2C4O4 within the cathode was systematically evaluated in NMC622+Li2C4C>4@porous SiO2 / / graphite LIB cells. (Figure 4) The U2C4O4 in microcapsules was irreversibly oxidized in the first few charge cycles, increasing the attainable charge capacities and providing a surplus of active lithium within the cell. However, the discharge capacities were gradually decreased when increasing the amount of U2C4O4 in microcapsules.

[0057] The use of porous SiC>2 shells is advantageous for the controlled or slow release of ions or molecules from the core. The porous SiC>2 shell prevents unwanted catalytic reactions, ensuring sustained lithium availability throughout cycling. This is demonstrated in Figure 6, where the release of lithium from the shell starts at 4.2 V vs Li+ / Li reference, whereas adding U2C4O4 to the battery composite electrode without SiC>2 shell protection results in much earlier release, and lithium is not released in a controlled manner. This approach can be directly applied in graphite / / LFP cells (>40% of the world market), where SiC>2 shells protect U2C4O4 within the voltage range of graphite / / LFP batteries (typically cycled up to 3.6 V). The release of additional lithium can be triggered by a simple increase of the upper cut-off voltage to 4.3 V, which can contribute to increasing the lithium inventory in the cell and restoring capacity.

[0058] Figure 7 shows three cells: one without shells, one with shells, and one where U2C4O4 was added without protection. The figure demonstrates the possibility of using Li2C4O4@SiC>2 with graphite / / LFP cells as an additional source of lithium, which can be activated when the battery capacity drops.

[0059] Polymer coatings

[0060] Encapsulation of U2C4O4 with SiC>2 is beneficial due to activation at higher voltage, which can be technically implemented in graphite / / LFP cells. For graphite / / NMC battery cells, the same approach cannot be used since the operating voltage range fits within the activation voltage. Therefore, we propose a polymer coating that can be melted at elevated temperature to enable activation of the encapsulated lithium salt, in particular U2C4O4.SiC>2-based microcapsules can be easily modified. We have demonstrated the controlled release of Li+ ions from microcapsules in an LFP-cathode-based system. However, in an NMC-cathode-based system, there is a problem with the overlapping of redox peaks with the microcapsules. Therefore, functionalisation (-NH2, -COOH) of SiC>2 can be achieved using certain polymers, allowing the release or triggering of Li+ ions on demand after melting the polymer on the surface. In this context, we have synthesised microcapsules coated with a thermoresponsive polymer (for example, P EGM EMA), as shown in Figure 8. The encapsulation of U2C4O4 is achieved by in situ hydrolysis and condensation of TEOS and the PEGMEMA polymer. Initially, U2C4O4 is dispersed in absolute ethanol and TEOS is added, then the mixture is stirred for 1 h to promote the adsorption of TEOS on U2C4O4. Subsequently, ammonium hydroxide mixed with absolute ethanol is added and the mixture is stirred for 12 h to promote the hydrolysis of TEOS. After 12 h of hydrolysis, PEGMEMA-based polymer dissolved in ethanol (1:1 ratio) is added and stirred for 12 h. The final product is centrifuged, washed, and dried. The experimental schematic illustration of the synthesis is shown in Fig. 8.

[0061] Figure 9 shows the cycling of the graphite / NMC cell at room temperature (25°C). The battery was stopped after 12 cycles and then heated to 40°C to melt the polymer covering the Li2C4O4@SiO2 shells (as shown in Fig.8). After treatment at 40°C, the capacity increased due to the release of lithium from the porous structure. This was not possible in earlier cycles because the polymer blocked the release.

Claims

CLAIMS1. Microcapsule comprising.(i) an outer shell comprising or consisting of SiC>2, and(ii) an inner core comprising or consisting of one or more organic or inorganic lithium salts, preferably selected from U2C4O4, U2S, U3S LiNOs, LisN, U2MOO3, LisFeCU, Li2NiC>2 and combinations thereof, more preferably U2C4O4.

2. The microcapsule of claim 1, wherein the outer shell comprises or consists of porous SiC>2, and the core comprises or consists of U2C4O4, U2S, or LiNOs.

3. The microcapsule of claim 1 or 2, wherein the amount of the lithium salt is in a range of 1-25 wt% based on the total weight of the microcapsule, more preferably 10-15 wt%.

4. The microcapsule of any one of claims 1-3, further comprising a polymer coating on the outer shell, in particular coating comprising a PEGMEMA based-polymer.

5. Electrode, comprising the microcapsule of any one of claims 1-4.

6. Lithium-ion battery comprising the electrode of claim 5, preferably as a cathode.

7. Method for preparing the microcapsule of any one of claims 1-4, comprising the steps (i) providing a material to be encapsulated comprising or consisting of one or more organic or inorganic lithium salts preferably selected from U2C4O4, U2S, U3S UNO3, LisN, U2MOO3, LisFeCU, Li2NiC>2 and combinations thereof, more preferably U2C4O4, (ii) preparing a hollow porous shell comprising or consisting of SiC>2, and(iii) encapsulating the material of (i) inside the hollow porous shell,wherein steps (ii) and (iii) can be carried out simultaneously or sequentially, wherein the method optionally comprises a step(iv) coating the microcapsules obtained in (iii) with a polymer coating, wherein the coating is preferably obtained by applying a polymer solution, e.g. a solution of a PEGMEMA-based polymer, to the microcapsules obtained in (iii).

8. The method of claim 7 for preparing a microcapsule, wherein step (ii) of preparing the hollow porous SiO2 shell comprises adding tetraethyl orthosilicate (TEOS) to a solution comprising sodium polyacrylate, ammonia and EtOH and isolating the hollow porous SiO2 shell.

9. The method of claim 7 or 8, wherein the material to be encapsulated comprises or consists of U2C4O4, wherein step (i) comprises synthesizing lithium squarate by a method comprising reacting squaric acid and lithium carbonate, preferably in a mixture of water and ethanol, and isolating lithium squarate.

10. The method of any one of claim 7-9, wherein step (iii) comprises encapsulating Li2C4C>4 inside the hollow porous SiC>2 shell by contacting the hollow porous SiC>2 shell obtained from step (i), preferably as a dry powder, with an aqueous solution of U2C4O4 to yield impregnation of U2C4O4 inside the porous SiC>2 shell, and drying, preferably vacuum drying.

11. The method of claim 7, wherein steps (ii) and (iii) are carried out simultaneously, preferably by in-situ hydrolysis and condensation of TEOS on an organic lithium salt.

12. The method of any one of claims 7-11, wherein step (ii) comprises a polymerization process.

13. The method of any one of claims 7 and 11-12, wherein steps (ii) and (iii) are carried out simultaneously and comprise a polymerization process of one or more organic materials, in particular ionic polymer such as PEGMEMA on a lithium salt to be encapsulated.

14. Use of the microcapsule of any one of claims 1-4 as an additive in the preparation of an electrode, in particular a cathode of a lithium-ion battery.

15. Method for preparing the electrode of claim 5, comprising a step of providing a slurry comprising electrode materials and the microcapsule of any one of claims 1-4, and preparing an electrode from the slurry.

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