An anode material and an anode for a secondary cell

Abstract

An anode material for a secondary cell, characterized in that the anode material comprises a silicon dioxide matrix having carbon-encapsulated metal-doped silicon nanoparticles; a method to produce said anode material; an anode comprising said anode material; a secondary cell comprising said anode; and a vehicle comprising said secondary cell.

Description

The present invention relates to an anode material for a secondary cell. More particularly, the present disclosure relates to an anode material for a secondary cell, characterized in that the anode material comprises a silicon dioxide matrix having carbon-encapsulated metal-doped silicon nanoparticles; a method to produce said anode material; an anode comprising said anode material; a secondary cell comprising said anode; and a vehicle comprising said secondary cell.TECHNICAL BACKGROUNDSecondary cells, or rechargeable batteries, are powering everything from mobile phones to electric vehicles. The basic operation of a secondary cell involves reversible chemical reactions. During discharge, chemical energy is converted into electrical energy, which powers devices. During charging, an external electrical source reverses this chemical reaction, restoring the energy of the secondary cell. Key components of a secondary cell include anodes, cathodes, electrolytes, and separators. The performance, efficiency, and life span of secondary cells depend heavily on these components and their composition.Silicon anodes have a theoretical capacity approximately ten times higher than that of traditional graphite anodes, making them a significant area of research in the development of new secondary cells. The higher capacity enables the manufacture of secondary cells with longer life, alternatively, significantly smaller and lighter secondary cells with the same capacity. However, the implementation of silicon in anodes is challenging. The main issue is the large volume expansion that silicon undergoes during charging. This expansion leads to mechanical stress and may ultimately cause degradation and cracking of the anode, which would severely limit the lifetime of the secondary cell. In addition, the repeated expansion and contraction during charge / discharge cycling may cause the anode to lose electrical contact with the rest of the battery, the substrate in particular, reducing efficiency and lifespan even further. In addition, the solid electrolyte interface (SEI), formed during the first charge cycle, is easily damaged during discharge because of the shrinking of the anode material. This causes exposure of additional silicon species to the electrolyte, that in the next charging phase is available for formation of additional SEI. This drains the anode material of useful silicon, heavily hampering the capacity retention between cycles.Several attempts have been made to address these challenges. One approach involves using silicon nanoparticles embedded into another material, giving them space to swell, such as in Li, X., et al. "Mesoporous Silicon Sponge as an Anti-Pulverisation Structure for High-Performance Lithium-Ion Battery Anodes", Nature Communications, 5: 4105, 2014. However, this approach causes further challenges, as the silicon exposed directly to the electrolyte are prone to facilitate solid electrolyte interface (SEI) formation, permanently limiting the reacted silicon to accommodate lithium ions.Even though the integration of silicon based anodes in secondary cells may help to overcome some of the current limitations of secondary cell technology, additional challenges remain. It is the object of the present invention to provide a silicon based anode with an improved rate capability, mitigating some of these drawbacks.SUMMARY OF THE INVENTIONAn object of the present invention is to provide a silicon based anode material for a secondary cell. The anode material improves the maximum capacity compared to secondary cells comprising conventional graphite based anodes, without sacrificing capacity retention between charge / discharge cycles, usually associated with silicon based anodes. In addition, it is an object of the present invention to provide a method to produce such an anode material.The present invention provides an anode material for a secondary cell, characterized in that the anode material comprises a silicon dioxide matrix having carbon-encapsulated metal-doped silicon nanoparticles; a method to produce said anode material; an anode comprising said anode material; a secondary cell comprising said anode; and a vehicle comprising said secondary cell.LIST OF DEFINITIONSAs used herein, the term "about" refers to a value or parameter herein that includes (and describes) embodiments that are directed to that value or parameter per se. For example, description referring to "about 50" includes description of "50." Numeric ranges are inclusive of the numbers defining the range. Generally speaking, the term "about" refers to the indicated value of the variable and to all values of the variable that are within the experimental error of the indicated value (e.g., within the 95% confidence interval for the mean) or within 10 percent of the indicated value, whichever is greater.Figure 1 shows a schematic illustration of an anode material according to the invention, wherein 1 denotes a Mg doped nano-sized silicon particle, 2 denotes a carbon layer, and 3 denotes a silicon dioxide matrix.Figure 2 shows a SEM image of an anode of the present invention, 5000x enlargement.Figure 3 shows a SEM image of an anode of the present invention, 20000x enlargement.Figure 4 shows an X-ray diffraction (XRD) spectrum of an anode material according to the invention. Figure 5 shows the difference in specific capacity between two coin cells, comprising the anode material of the invention, and conventional silicon, respectively, over several charge / discharge cycles. Figure 6 shows the difference in capacity retention of between two coin cells, comprising the anode material of the invention, and conventional silicon, respectively, over several charge / discharge cycles.DETAILED DESCRIPTION OF THE INVENTIONIn a first aspect, the present invention relates to an anode material for a secondary cell, comprising a silicon dioxide matrix, wherein the silicon dioxide (SiO2) matrix comprises nano-sized silicon-particles doped with a metal, and wherein the metal doped Si-particles are encapsulated by a carbon layer.One of the largest issues with silicon based anodes is the significant volume change of the anode material during charging and discharging cycles, due to association of the silicon with up to four cations in a charged secondary cell. When full charging state is reached, the associated cations present in the silicon based anode material repel each other, causing the material to swell. Upon discharging, the opposite is true, and the silicon material loses the associated cations and returns to its original state. The anode material is adversely affected by repeated volume change, reducing the cycle life of the secondary cell. The anode material of the present invention mitigates this volume change by utilizing a SiO2 matrix comprising carbon-encapsulated silicon nanoparticles. The combination of the carbon encapsulation of the silicon nanoparticles and the SiO2 matrix provides structural support to the anode material, reducing swelling during charge / discharge cycles. In addition, swelling caused by the association of cations to the silicon in the anode material is even further reduced by the presence of SiO2. The oxygen of SiO2 blocks cations from being associated with the silicon nano-particle, thereby reducing the swelling as fewer cations are associated with the material. For a lithium ion secondary cell, the cations are lithium ions.The structural support provided by the anode material SiO2 matrix comprising carbon-encapsulated silicon nanoparticles, together with the reduced swelling thanks to the presence of SiO2, improve the capacity retention significantly, as demonstrated in example 2 and figures 5 and 6. In addition, the conductivity of the Si-nanoparticles, which are associated with very low conductivity, is improved by the carbon encapsulation in the SiO2 matrix.In one embodiment, the metal dopant is selected from Mg, Li, Ca, and Al, preferably the metal dopant is Mg. When magnesium is present in the material as dopant, it is in the form of magnesium silicate, SiO3Mg, while lithium would be in the form of lithium silicate, Li2iO3 or Li4SiO4. In one embodiment, the amount of metal dopant is less than about 12 wt%, preferably less than about 10 wt%, even more preferably the amount of metal dopant is about 7 wt%. Amorphous SiO2 is always present in some degree in Si matrixes. Lithium may react irreversible with amorphous SiO2 to form lithium silicate, Li4SiO4, negatively affecting the amount of available silicon in the anode material. The presence of a metal dopant reduces this reaction by the formation of metal silicate. Thereby, the coulombic efficiency, particularly during the first charging / discharging cycle, of the secondary cell is improved.In one embodiment, the diameter of the doped silicon particles is less than about 30 nm, preferably less than about 20 nm, as measured using Scanning Electron Microscopy (SEM). In one embodiment, the carbon encapsulation layer is less than about 100 nm thin, such as from 5 nm to 100 nm, or from 10 nm to 100 nm, as measured using SEM. The encapsulated silicon nanoparticles may be aggregated forming larger particles. In one embodiment, the anode material has a particle diameter size with a D90 value of the particle size distribution below about 50 µm.In a second aspect, the present invention relates to a method for preparing an anode material according to the first aspect of the invention. In one embodiment, the method comprises the steps of:a) heating a mixture of metallurgical silicon powder comprising a metal dopant, and silicon dioxide to a temperature from about 1200 °C to about 1600 °C, under vacuum pressure, in a reaction chamber;b) introducing an alkane gas;c) cooling the obtained mixture to below about 1100 °C, thereby forming a solid material; d) collecting all solid material from the reaction chamber;e) grinding and sieving the collected solid material.The duration time for step a) is not more than 10 hours, preferably 6 hours. The duration time for step b) is not more than 3 hours, preferably from 0.5 hours to 1 hour. The duration time for step c) is not more than 3 hours, preferably from 0.5 hours to 1 hour. In one embodiment, the metal dopant is selected from Mg, Li, Ca, and Al, preferably the metal dopant is Mg. In one embodiment, the temperature used in step a) is from about 1300 °C to about 1500 °C, preferably the temperature used in step a) is about 1400 °C.In one embodiment, the alkane gas in step b) is selected from methane, ethane, propane, and butane gas, a combination thereof, preferably the alkane gas used in step b) is methane gas. The presence of an alkane gas in the process is what creates the carbon encapsulation of the silicon particles. The gas also aids in the distribution of the metal dopant. The use of methane gas, as exemplified in example 1, distributes the carbon well in the SiO2 matrix, effectively encapsulating the small metal doped silicon nanoparticles inside the matrix.In order to avoid significant formation of silicon carbide, the temperature should not exceed 1100 °C at pressures higher than vacuum pressure. In one embodiment, the temperature of step c) is kept at a maximum of 1100 °C for at least 30 min and up to 2 hours before the pressure is released. In one embodiment, the solid material collected in step d) is grinded in step e) to a particle diameter size with a D90 value below 50 µm.In a third aspect, the present invention relates to an anode comprising the anode material according to the first aspect of the invention. In one embodiment, the anode comprises an anode material produced by the method according to the second aspect of the invention.In a further aspect, the present invention relates to a secondary cell comprising an anode according to the third aspect of the invention, a cathode, an electrolyte, and optionally a separator. The electrolyte used in the secondary cell according to the present invention is a liquid electrolyte comprising at least one lithium salt and at least one or more solvents selected from the group consisting of carbonate solvents and their fluorinated equivalents, diC1-4 ethers and their fluorinated equivalents and ionic liquids. The lithium salt is preferably one or more selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl) (trifluoromethanesulfonyl) imide (LiFTFSI), lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide (LiPTFSI), lithium trifluoromethanesulfonate (LiOTf), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorobis(oxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalato)phosphate (LiTFOP), lithium tetrafluoroborate (L1BF4), lithium nitrate (L1NO3) lithium 2-trifluoromethyl-4,5-dicyanoimidazole (LiTDI). In one embodiment, the solvent is selected from the group consisting of 1,2-dimethoxyethane (DME), / V-propyl- / V-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR13-FSI), N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13-TFSI), 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14-FSI), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14-TFSI), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EMIM-FSI), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM-TFSI). The solvent is preferably one or more selected from dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC), and propylene carbonate (PC), and their fluorinated equivalents.In yet a further aspect, the present invention relates to a vehicle comprising the secondary cell according to the previous aspect of the invention.All aspects and embodiments disclosed herein can be combined with any other aspect and / or embodiment disclosed herein.EXAMPLESExample 1 preparation of anode materialA mixture of metallurgical silicon powder comprising 7 weight% Mg silicate (2.4 g), silicon dioxide (2.4 g amount), was submitted to reduced pressure (add exact pressure) and heat (1400 °C). Methane gas (excess) was then introduced into the chamber using a flow rate of 0.1 liter / min at a pressure of 13.3 Pa (0.1 torr), while keeping the temperature at 1400 °C. The chamber was then allowed to cool to 1100 °C, at which point the gas precipitated on the inner surface of the chamber. The precipitated solid was collected from the inner surface of the chamber, and grinded to D50 particle diameter size of 25 pm. The grinded material was the subjected to sieving. The desired anode material was collected as a powder with a D90 particle diameter size of below 50 pm. The presence of both SiO2 and Si in the material was demonstrated using X-ray diffraction analysis, the results are shown in figure 4. The broad peak from around 26 to 30 shows the presence of silicon, and the sharper peak at 31 shows the presence of SiO2. The fact that silicon is in amorphous form with low crystallinity, makes the corresponding XRD peak become broad.The obtained anode material was used in Example 2.Example 2. Capacity testCapacity tests for two silicon-based anodes were performed. The density of each respective anode was 1.2 g / cc.Anode 1: SiO2 / flake graphite / Polyacrylic acid binder (PAA) in a weight ratio of 80 / 10 / 10.Anode 2: The silicon material produced according to Example 1 / flake graphite / PAA binder in a weight ratio of 80 / 10 / 10.Each anode was tested in a respective coin half-cell, with identical lithium metal counterparts. The half cells were soaked in electrolyte for 12 h prior the first charge / discharge cycle. 70 charge / discharge cycles were then performed with 10 min rest time between each cycle.The charging was performed using CC-CV, where the CC is 0.1C and CV is 5 mV with a cutoff current of 1 / 200C. The discharge was performed using 0.1C, with a cutoff CC at 1.5 V.The result after 70 cycles is shown in table 1 below. Figures 5 and 6 show the result after each cycle.Table 1

Claims

1. An anode material for a secondary cell, comprising a silicon dioxide matrix, wherein the silicon dioxide matrix comprises nano-sized silicon-particles doped with a metal, and wherein the metal doped Si-particles are encapsulated by a carbon layer.

2. The anode material according to claim 1, wherein the metal dopant is selected from Mg, Li, Ca, and Al, preferably the metal dopant is Mg.

3. The anode material according to claim 1 or 2, wherein the amount of metal dopant is less than about 12 wt%, preferably less than about 10 wt%, even more preferably the amount of metal dopant is about 7 wt%.

4. The anode material according to any one of the above claims, wherein the diameter of the doped silicon particles is less than about 30 nm, preferably less than about 20 nm.

5. The anode material according to any one of the above claims, wherein the thickness of the carbon encapsulation layer is less than about 100 nm.

6. A method for preparing an anode material according to any one of the above claims, comprising the steps of:a) heating a mixture of metallurgical silicon powder, a metal dopant, and silicon dioxide to a temperature from about 1200 °C to about 1600 °C, under vacuum pressure, in a reaction chamber;b) introducing an alkane gas;c) cooling the obtained mixture to below about 1100 °C, thereby forming a solid material;d) collecting all solid material from the reaction chamber;e) grinding and sieving the collected solid material.

7. The method according to claim 6, wherein the metal dopant is selected from Mg, Li, Ca, and Al, preferably the metal dopant is Mg.

8. The method according to any of claims 6-7, wherein the temperature used in step a) is about 1400 °C.

9. The method according to any one of claims 6-8, wherein the alkane gas in step b) is selected from methane, ethane, propane, and butane gas, or a combination thereof, preferably the alkane gas used in step b) is methane gas.

10. The method according to any one of claims 6-9, wherein the solid material collected in step d) is grinded in step e) to a particle diameter size with a D90 value below about 50 µm.

11. An anode comprising the anode material of any one of claims 1-5.

12. An anode comprising an anode material produced according to any one of claims 6-10.