Method for manufacturing an Fe-Si alloy material, Related material for Li-ion battery electrode.

Abstract

Method for manufacturing an Fe-Si alloy material, related material for a Li-ion battery electrode. The invention relates to a method for manufacturing an alloy with the atomic composition Si85Fe15, comprising the following steps: i / mixing silicon pieces and iron granules in stoichiometric proportions of 85% and 15%, respectively; ii / arc-melting the mixed pieces and granules to form a liquid mixture; iii / pouring the liquid mixture into a cooled crucible to form an ingot; iv / grinding the ingot into a micron-sized powder. Figure for the abstract: Fig. 1

Description

Title of the invention: Method for manufacturing an Fe-Si alloy material, Related material for Li-ion battery electrode. technical field

[0001] The present invention relates to the field of electrochemical accumulators, and more particularly to Li-ion accumulators.

[0002] The present invention aims primarily to provide a solution for improving the initial coulombic efficiency, defined as the ratio between the amount of lithium inserted in the active material and that disinserted at the first charge / discharge cycle of a battery, the capacity and the lifespan of Li-ion batteries by trapping the active silicon with respect to the lithium and protecting it from contact with the electrolyte in order to limit degradation phenomena at the interfaces. Previous technique

[0003] Lithium-ion batteries are the most widely used electrochemical storage systems for portable electronic devices and electric vehicles due to their performance in terms of energy density, operating voltage and lifespan.

[0004] Graphite is conventionally used as an active material, enabling the storage of lithium, for the manufacture of the negative electrodes or anodes of Li-ion batteries. Graphite is distinguished by its long lifespan and its safety compared to other anode materials such as lithium metal. However, the disadvantage of graphite lies in its high specific capacity, theoretically equal to 372 mAh / g, which is much lower than that of lithium metal, theoretically equal to 3870 mAh / g.

[0005] The development of new materials for the negative electrode of Li-ion batteries has therefore been the subject of much research for over two decades. This research has led to the development of silicon-based negative electrode materials, which make it possible to improve energy densities.

[0006] Silicon has a high theoretical specific capacity, equal to 3579 mAh / g, corresponding to the formation of the Lii5Si4 alloy. However, the formation of this alloy generates a strong volumetric expansion of the silicon-based materials and can lead to the mechanical spraying of the silicon-containing particles and to the instability of the interfaces between the silicon and the liquid electrolyte used for the transport of lithium ions.

[0007] In order to limit the adverse consequences of the change in volume, different strategies are studied.

[0008] One of these methods involves preparing silicon-containing alloys by grinding together iron and silicon powders, as well as manganese powder: [1]. The aim is to limit the contact area between the silicon and the electrolyte and to reduce the size of the silicon domains. The FeSi2 alloy as prepared in publication [1] exhibits very limited performance, with a reversible capacitance on the first cycle of around 700 mAh / g, which decreases very rapidly in just a few cycles.

[0009] More interesting performances were obtained:[2] and [3], with binary alloys of respective atomic compositions Si85Fei5 and SiseFe^ prepared by grinding.

[0010] The alloy obtained Si85Fei5 is composed of Si, of phases [3-FeSi2 and a-FeSi2, or a-Fe 2Si5 depending on the versions of the iron-silicon phase diagram, and has a specific capacity of the order of 1600 mAh / g with a capacity retention of more than 90% after a number of 50 cycles.

[0011] The alloy obtained Si86Fei4 is mainly composed of the Si and a-FeSi2 phases and has a specific capacity of the order of 1500 mAh / g with a capacity retention of less than 50% after 50 cycles.

[0012] The difference in performance between these two alloys may be related to the phase differences present, but also to the parameters of the electrochemical tests with a voltage window of 5 mV to 0.9 V in [2] and of 20 mV to 1.5 V in [3]. Power performance is not specified in [2] and [3].

[0013] The use of other elements and synthesis techniques has also been explored, for example, silicon-titanium-iron alloys [4] or silicon-copper-aluminum-iron alloys [5], prepared by arc melting and wheel quenching. The resulting alloys are composed of silicon nanocrystals, with sizes on the order of 100 nm, embedded in a TiFeSi2 matrix or a mixture of Al4Cu9 and AlFe, according to studies [4], [5]. The performance obtained is promising, with capacities on the order of 1200 mAh / g for silicon-titanium-iron alloys and 900 mAh / g for silicon-copper-aluminum-iron, stable over 50 cycles. In both cases, the metallic matrix, inactive with respect to lithium, limits the atomization of the silicon particles and ensures electron flow through its electrical conductivity, as well as the diffusion of lithium ions.

[0014] US patent application 2005 / 0031957A1 proposes an alloy of at least three such elements, free from crystallites larger than 100 nm.

[0015] WO2010 / 132146A1 discloses a synthesis method associated with alloys obtained free of crystallites larger than 50 nm.

[0016] The performance obtained with alloys made of silicon nanocrystals in a metallic matrix seems sufficient to consider commercialization.

[0017] However, the wheel quenching synthesis step, which allows obtaining the expected microstructure with small crystallites, could be a hindrance to industrialization, due to the low productivity.

[0018] There is a need to further improve silicon-based materials for the negative electrode of Li-ion batteries, in order to overcome the aforementioned drawbacks and in particular to enable their industrialization.

[0019] The object of the invention is to meet at least partially this need. Description of the invention

[0020] To this end, the invention relates, in one of its aspects, to a process for manufacturing an alloy of atomic composition Si85Fei5, comprising the following steps consisting of:

[0021] i / mix in stoichiometric proportions of 85% and 15% respectively, pieces of silicon and iron granules;

[0022] ii / to perform an arc fusion of the mixed pieces and granules, so as to form a liquid mixture;

[0023] iii / pour the liquid mixture into a cooled crucible, so as to form an ingot;

[0024] iv / grind the ingot into a micron powder.

[0025] Advantageously, step ii / is carried out under an argon atmosphere.

[0026] According to an advantageous embodiment, step iv / is carried out so as to obtain that the micron powder has an average diameter between 0.05 and 6 microns.

[0027] The invention also relates to a method for making a negative electrode for a Li-ion battery comprising the following steps:

[0028] v / mix a micron-sized powder of alloy with atomic composition Si85Fei5 obtained in step iv / of the manufacturing process as described above with an electrically conductive material and a polymeric binder, so as to obtain an ink;

[0029] vi / deposition of ink, in particular by coating on a current collector, preferably made of copper.

[0030] Preferably, the electrically conductive material is made of black and / or carbon fibers.

[0031] Preferably, the polymeric binder is carboxymethylcellulose (CMC).

[0032] According to an advantageous embodiment, the composition of the ink obtained according to step v / comprising 70%m of the Si85Fei5 alloy, 9%m of carbon fibers, 9%m of carbon black and 12%m of CMC.

[0033] The invention further relates to an alloy material of atomic composition Si85 Feis in the form of a micron powder with an average diameter between 0.05 and 6 microns.

[0034] The invention also relates to a negative electrode for a Li-ion battery comprising the alloy material of atomic composition Si85Fei5tel as described above.

[0035] The invention also relates to a Li-ion accumulator in which: - the negative electrode material(s) is that as described previously;

[0036] The positive electrode material(s) is chosen from the group comprising LiFePO4, LiCoO2, LiNi0.33Mn0.33Co0.33O2

[0037] Thus, the invention essentially consists of a process for manufacturing a material in powder form composed of silicon-iron alloy particles comprising an active silicon phase and an inactive a-Fe2Si5 phase. The process comprises arc melting, followed by crucible casting, preferably cooled, and grinding to obtain a micron-sized powder.

[0038] The material obtained in the form of a micron-sized powder of Si85Fei5 alloy exhibits a higher electrical conductivity than silicon thanks to the a-Fe2Si5 phase which also makes it possible to limit volume changes in the electrode and thus mitigate their harmful consequences.

[0039] The performance achieved by this material is high with a specific capacity greater than 1000 mAh / g, an initial coulombic efficiency greater than 85%, a capacity retention of more than 80% of the initial capacity after 50 cycles and high power performance with more than 60% of the capacity recovered at 5C.

[0040] Other advantages and features of the invention will become clearer from the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0041] [Fig. 1] [Fig. 1] is a reproduction of an image obtained by SEM (Scanning Electron Microscope) of a Si85Fei5 alloy powder according to the invention.

[0042] [Fig.2] the [Fig.2] illustrate X-ray diffractograms of different Siioo xFex alloy powders, including Si85Fei5 according to the invention.

[0043] [Fig.3] the [Fig.3] are reproductions of SEM images as well as a chemical map by EDX and EDX spectra of a cross-section of an ingot of atomic composition Si8oFe2o.

[0044] [Fig. 4] [Fig. 4] shows in the form of curves the normalized specific charge capacity and the coulombic efficiency of different compositions, namely respectively ground silicon (BM Si), a mixture of ground silicon and a Si7 powder i.5Fe28.5 (BM Si / a Fe2Si5 mix) and material according to the invention (AM Si85 Fe15).

[0045] [Fig.5] [Fig.5] illustrates in the form of curves the power performance of Siioo xFex alloy powders with x respectively equal to 5, 15 and 25.

[0046] [Fig.6] [Fig.6] is an X-ray diffractogram of an alloy material of the same binary shape as that of the invention but obtained according to publication [2] (BM Si85Fei5).

[0047] [Fig.7] [Fig.7] illustrates in the form of curves the power performance of alloy powders according to the invention (AM Si85Fe) and according to publication [2] (BM Si85Fe is)- Detailed description

[0048] Example 1 £ according to the invention ): Si85Fei5 material

[0049] An alloy powder with the atomic composition Si85Fei5 is synthesized by arc melting from pieces of silicon (purity 6N) and iron (purity 3N) mixed in stoichiometric proportions. The Si and Fe pieces are marketed by the company Alfa Aesar.

[0050] More specifically, these precursors are placed on a copper mold inside the melting zone and three vacuum / fill cycles are carried out with argon (purity 5N) until a partial pressure of argon of 0.4 bar is reached.

[0051] An electric discharge is applied between a tungsten tip and the copper mold, producing an electric arc that melts the precursors by Joule heating. The process is complete when a uniform droplet is formed, which takes less than a minute, and the samples are cooled by a system of water passing through the copper mold.

[0052] The ingot obtained is pre-ground for 5 minutes with a high-energy mill in a 65 mL steel grinding bowl and 10 mm diameter steel balls with a mass ratio between balls and ingot equal to 6. The high-energy mill used is the one marketed under the brand SPEX CertiPrep™ 8000M-230.

[0053] This first grinding material is ground with another grinder, for 10 minutes at 350 rpm in a 50 mL steel grinding bowl filled with hexane, agate balls of 10 mm diameter.

[0054] The mass ratio between the balls and the first ground material is equal to 10, in order to reduce the particle size. The grinder used is the one marketed under the brand name Retsch PM-100 CM.

[0055] The alloy powder obtained with this second grinding is a material with an atomic composition of Si85Fe5 and is in the form of a micron-sized powder (D50 = 0.4 pm), as illustrated in [Fig. 1]. This material is composed of 43 wt% pure silicon and 57 wt% a-Fe2Si5, as shown in [Fig. 2].

[0056] For comparison, an example of the microstructure of a Si80 Fe20 composition ingot with single-phase domains of several tens of microns is shown in [Fig.3].

[0057] The Si85Fei5 alloy powder according to the invention is then incorporated into a mixture of carbon powders (fibre and carbon black) and carboxymethylcellulose (CMC) (Mw = 250 kg mol1, degree of substitution Ds = 0.7), used as a polymeric binder, as follows.

[0058] The carbon fibers used are marketed under the brand name VGCF-H by Showa Denko. The carbon black used is brand name C65 from Timcal. The CMC used is marketed by Sigma-Aldrich.

[0059] The alloy powder and carbon powders are placed in an agate mortar and manually ground for 15 minutes. The CMC is dissolved in deionized water with a concentration of 3 wt%.

[0060] The powders and the CMC are mixed until a homogeneous ink is obtained. The composition of the resulting ink is 70% w of the alloy, 9% w of carbon fibers, 9% w of carbon black and 12% w of CMC.

[0061] The ink is then deposited on a 15 µm thick copper strip as a current collector.

[0062] After drying, electrodes of 14 mm in diameter are cut and dried under vacuum at 80°C for 24 hours before being placed in a glove box under argon.

[0063] Next, CR2032 format accumulators or batteries are assembled in the glove box. Each accumulator or battery comprises a stack of the following layers: - a negative electrode obtained from the alloy powder as described above, - a glass microfiber filter as a separator, - a lithium foil as a positive electrode.

[0064] The separator used is marketed under the brand name Whatman GF / D and is impregnated with an electrolyte consisting of LiPF6 and ethylene carbonate / diethyl carbonate (1:1 by volume) with 2% vinylene carbonate and 10% fluoroethylene carbonate.

[0065] The batteries are tested in CCCV charging mode (acronym for "Constant Current -Constant Voltage") between 50 mV and 1.5 V at a C / 5 regime, i.e. a complete charge or discharge in 5 hours, and at C / 50 for the constant potential stage in lithiation, except for the first cycle at a C / 20 regime between 10 mV and 1.5 V.

[0066] Comparative samples with different compositions were prepared under the same conditions as Example 1 according to the invention.

[0067] Example 2 (comparative): Si95Fe5 material

[0068] The powder with atomic composition Si95Fe5 is in the form of a micron-sized powder, composed of 79 wt% of pure silicon and 21 wt% of a-Fe2Si5, as shown in [Fig.2].

[0069] Example 3 (comparative); material Si75Fe25

[0070] The powder with atomic composition Si75Fe25 is in the form of a micron-sized powder, composed of 12 wt% of pure silicon and 88 wt% of a-Fe2Si5, as shown in [Fig.2].

[0071] Example 4 (comparative); material Si7 i.5Fe28.5

[0072] The powder with atomic composition Si7 i.5Fe28 5se is in the form of a micron-sized powder, mainly composed of a-Fe2Si5, as shown in [Fig.2]. Comparison of examples

[0073] The Si85Fei5 material according to the invention has been compared to:

[0074] - of ground silicon, with the same mass proportion of active silicon in the electrode; and

[0075] - a mixture of ground silicon and the comparative material according to example 4 for in the same proportions as the material of the invention (43 wt% of pure Si and 57 wt% of a-Fe2Si5).

[0076] The performance of these samples has been compared and is illustrated in [Fig.4].

[0077] It appears from these curves that the Si85Fei5 material according to the invention has the best retention capacity and coulombic efficiency.

[0078] The performance obtained by the Si85Fei5 material according to the invention is summarized in Table 1 below.

[0079] [Tables 1] Cycle Q (mAh.g') Qret (%) CE (%) 1st 1336 104.9 88.8 2nd 1272 100 98.2 3rd 1250 98.3 99.2 20th 1200 94.3 99.2 50th 1008 79.2 98.9

[0080] It is specified here that the acronym Qret designates the retention capacity which is normalized with the capacity of the second cycle.

[0081] The power performance of the material of the invention has been compared to the materials according to examples 3 and 4.

[0082] Figure 5 illustrates these performance evaluations with a regime varying from C / 10 to 5C with a constant potential step up to a C / 50 regime in lithiation.

[0083] It also appears that the material according to the invention has better power performance than the material of example 2 but slightly lower than the material according to example 3. However, the material according to example 3 has a much lower reversible capacity, on the order of 300 mAh / g.

[0084] The power performance of the material according to the invention was also compared to a material of the same composition, but prepared by grinding as described in [2]. To do this, silicon sold by Sigma Aldrich, with a particle size of 45 µm and 99% purity, and iron powder sold by Alfa Aesar, with a particle size of 74 µm and 99% purity, were mixed in the proportions specified under an argon atmosphere, and then the Si85Fei5 alloy was mechanically synthesized by high-energy ball milling with a SPEX 8000M mill.

[0085] The sample was ground in 7 cycles of 100 min with 10 min breaks, using 15 mm diameter stainless steel balls and a 65 ml jar, and a mass ratio between the balls and the powder equal to 12.

[0086] Fig. 6 shows the diffractogram of the material obtained with a mixture of phases, noted BM Si85Fei5, Si, [3-FeSi2, a-Fe2Si5 and traces of SiFe and / or Fe.

[0087] Figure 7 presents a comparison of power performance and shows that The Si85Fei5 material according to the invention is more efficient than the Si85Fei5 material obtained by grinding, with in particular more than 60% of the capacity recovered at 5C for the material according to the invention versus less than 50% for that obtained by grinding.

[0088] The inventors also measured the electrical conductivity of the Si85Fei5 material according to the invention.

[0089] Table 2 below compares this conductivity with those of other materials.

[0090] [Tables2] Example Effective electrical conductivity (S.m1) Si 7.7E-05 Ground Si 5.7E-03 Fe 1712 FeSi2 716 a Fe2Si5 2208 BM Si85Fei5 (invention) 974 Si85Fei5 according to publication [2] 53

[0091] It appears from this table 2 that the Si85Fei5 material according to the invention has a much better electrical conductivity than the Si85Fei5 material obtained by grinding.

[0092] Other variants and embodiments may be envisaged without departing from the scope of the invention. List of cited references

[0093] [1]: Jayaprakash et al., Intermetallics 15 (2007) 442 - 450.

[0094] [2]: Cao et al., Journal of the Electrochemical Society, 166 (2) A21-A26 (2019).

[0095] [3]: Ruttert et al., ACS Appl.EnergyMater.2020, 3, 743-758.

[0096] [4]: ​​Kim et al., Journal of Electroanalytical Chemistry 687 (2012) 84-88.

[0097] [5]: Yu et al., Electrochimica Acta 130 (2014) 583-586.

Claims

Demands

1. A process for manufacturing an alloy of atomic composition Si85Fei5, comprising the following steps of: i / mixing in stoichiometric proportions of 85% and 15% respectively, pieces of silicon and iron granules; ii / performing an arc melting of the mixed pieces and granules, so as to form a liquid mixture; iü / pouring the liquid mixture into a cooled crucible, so as to form an ingot; iv / grinding the ingot into a micron powder.

2. Method according to claim 1, step ii / being carried out under an argon atmosphere.

3. A method according to claim 1 or 2, step iv / being carried out so as to obtain that the micron powder has an average diameter between 0.05 and 6 microns.

4. Method of making a negative electrode for a Li-ion battery comprising the following steps of: v / mixing a micron-sized powder of alloy of atomic composition Si85Fei5 obtained in step iv / of the manufacturing process according to one of the preceding claims with an electrically conductive material and a polymeric binder, so as to obtain an ink; vi / deposition of the ink, in particular by coating on a current collector, preferably made of copper.

5. Method according to claim 4, the electrically conductive material being made of black and / or carbon fibers.

6. Method according to claim 4 or 5, the polymeric binder being carboxymethylcellulose (CMC).

7. Process according to any one of claims 4 to 6, the ink composition obtained according to step v / comprising 70%m of Si85 Feis alloy, 9%m of carbon fibers, 9%m of carbon black and 12%m of CMC.

8. Alloy material of atomic composition Si85Fei5 in the form of a micron powder with an average diameter between 0.05 and 6 microns.

9. Negative electrode for Li-ion battery comprising the alloy material of atomic composition Si85Fei5 according to claim 8.

10. Li-ion battery in which: - the negative electrode material(s) is that of claim 8; The positive electrode material(s) is chosen from the group including LiFePO4, LiCoO2, LiNi0.33Mn0.33Co0.33O2.

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