Abstract
A silicon-carbon composite. In order to improve the cycle stability of a lithium cell equipped therewith, the silicon-carbon composite is produced by a condensation reaction of silicon particles surface-modified with a first condensation-capable group and carbon particles surface-modified with a second condensation-capable group, the silicon particles being covalently bonded to the carbon particles via the condensation reaction product of the first condensation-capable group and the second condensation-capable group. In addition, a method for the production thereof and to an electrode, an electrode material, and a lithium cell is described.
Claims
1. A method, comprising: forming a silicon-carbon composite by reacting, in a condensation reaction, silicon particles that are surface-modified with a first condensation-capable group and carbon particles that are surface-modified with a second condensation-capable group capable of condensing with the first condensation-capable group; wherein the silicon particles are covalently bonded to the carbon particles via a product of the condensation reaction in which the first condensation-capable group reacts with the second condensation-capable group.
2. The method as recited in claim 1, wherein the silicon particles are covalently bonded to the carbon particles via a product of the condensation reaction in which the first condensation-capable group reacts with the second condensation-capable group.
3. The method as recited in claim 1, wherein the condensation reaction is esterification, amidation, etherification, polycondensation, nucleotide formation, or aldol condensation.
4. The method as recited in claim 1, wherein the first condensation-capable group is a hydroxyl group, an amino group, or a carboxyl group, and the second condensation-capable group is a carboxyl group, a hydroxyl group, or an amino group.
5. The method as recited in claim 1, wherein the first condensation-capable group is a hydroxyl group and the second condensation-capable group is a carboxyl group.
6. The method as recited in claim 1, wherein the carbon particles have an average particle size which is smaller than an average particle size of the silicon particles.
7. The method as recited in claim 1, wherein the silicon particles have an average particle size in a range of 200 nm to 100 m.
8. The method as recited in claim 1, wherein the carbon particles have an average particle size in a range of 1 m to 50 m.
9. The method as recited in claim 1, wherein the carbon particles are graphite particles.
10. The method as recited in claim 1, further comprising: surface-modifying the silicon particles with the first condensation-capable group by at least one of: i) ultrasonic treatment in an optionally acidified water bath, ii) etching using hydrogen fluoride, and hydrolyzing, and iii) grafting.
11. The method as recited in claim 1, further comprising: surface-modifying the carbon particles with the second condensation-capable group by grafting.
12. The method as recited in claim 1, wherein the reacting takes place in the presence of a condensation agent, the condensation agent being at least one of dicyclohexylcarbodiimide, a molecular sieve, and sulfuric acid.
13. The method as recited in claim 1, further comprising: arranging the silicon-carbon composite as an anode of a lithium cell or battery.
14. The method as recited in claim 1, further comprising: arranging the silicon-carbon composite as one of an electrode material and an anode material of a lithium cell or battery.
15. The method of claim 1, further comprising: arranging the silicon-carbon composite in a lithium cell or battery as, or as part of, an electrode of the lithium cell or battery.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 shows a schematic representation illustrating a specific embodiment of a silicon-carbon composite according to the present invention manufactured using a condensation reaction.
(2) FIGS. 2a through 2c show Si2p XPS spectra for illustrating the surface modification and condensation reaction of the silicon particles.
(3) FIGS. 3a through 3c show C1s XPS spectra for illustrating the surface modification and condensation reaction of the carbon particles.
(4) FIG. 4 shows a graph for illustrating the variation of the specific capacity as a function of the number of cycles of one specific embodiment of a silicon-carbon composite according to the present invention and of simple physical mixtures of silicon particles and graphite particles.
DETAILED DESCRIPTION OF THE FIGURES SHOWING EXAMPLE EMBODIMENTS
(5) FIG. 1 illustrates a specific embodiment of a silicon-carbon composite manufactured according to the present invention via a condensation reaction.
(6) FIG. 1 shows that silicon particles 10 are subjected to a surface modification, in which condensation-capable hydroxyl groups 11 (OH) are formed on the surface of silicon particles 10 (SiOH).
(7) FIG. 1 shows that carbon particles 20, for example, graphite particles, are subjected to a surface modification, in which condensation-capable carboxyl groups 21 (COOH) are formed on the surface of carbon particles 20 (CCOOH).
(8) FIG. 1 further shows that condensation-capable hydroxyl groups 11 of silicon particles 10 and condensation-capable carboxyl groups 21 of carbon particles 20 are subjected to a condensation reaction, in which covalent bonds in the form of ester groups (OCO) are formed between silicon particles 10 and carbon particles 20 (SiOCOC) via the condensation reaction.
(9) FIGS. 2a through 2c show Si2p X-ray photoelectron spectra (XPS spectra) for illustrating the surface modification and condensation reaction of the silicon particles.
(10) FIGS. 3a through 3c show C1s X-ray photoelectron spectra (XPS spectra) for illustrating the surface modification and condensation reaction of the carbon particles;
(11) In FIGS. 2a through 2c and 3a through 3c, counts per second (C/s) are plotted against bond energy E.sub.B (eV). The relative quantities of different oxidation levels of silicon and carbon are ascertained and illustrated.
(12) FIG. 2a shows a Si2p X-ray photoelectron spectrum of the original and untreated silicon particles used in Example 1 as source material, i.e., before the surface modification with the aid of ultrasonics, which have an average particle size d.sub.50 of 82 m. A peak identified by reference numeral 41 was ascertained for elemental silicon (Si bulk), a peak identified by reference numeral 42 was ascertained for Si.sub.2O, and a peak identified by reference numeral 43 was ascertained for SiO.sub.2.
(13) FIG. 2b shows a Si2p X-ray photoelectron spectrum of the ultrasonically treated silicon particles (SiOH) from Example 1, i.e., after the surface modification with the aid of an ultrasonic bath and before the condensation reaction. A peak identified by reference numeral 41 was ascertained for elemental silicon (Si bulk), a peak identified by reference numeral 42 was ascertained for Si.sub.2O, a peak identified by reference numeral 43 was ascertained for SiO.sub.2 and a peak identified by reference numeral 44 was ascertained for Si.sub.2O.sub.3.
(14) A comparison of FIGS. 2a and 2b shows that the proportion of higher oxidation levels is obviously increased by the ultrasonic treatment. In particular, the 1+ or Si.sup.+ oxidation level represented by Si.sub.2O 42 shows that OH bonds were formed on the surface of silicon (SiOH) as a result of the ultrasonic treatment.
(15) FIG. 3a shows a C1s X-ray photoelectron spectrum of the original and untreated graphite particles used in Example 2 as source material, i.e., before surface modification with the aid of grafting. A peak identified by reference numeral 51 was ascertained for elemental carbon (CC). FIG. 3a shows that the untreated graphite particles are formed from elemental carbon.
(16) FIG. 3b shows a C1s X-ray photoelectron spectrum of the carboxyl group-grafted graphite particles (graphite-COOH) from Example 2, i.e., after the surface modification with the aid of grafting and before the condensation reaction. A peak identified by reference numeral 51 was ascertained for elemental carbon (CC), a peak identified by reference numeral 52 was ascertained for CN, a peak identified by reference numeral 53 was ascertained for COC and a peak identified by reference numeral 54 was ascertained for OCO.
(17) A comparison of FIGS. 3a and 3b shows that the proportion of the CC bonds normally present in pure graphite has been clearly reduced by the chemical modification, and the proportion of carboxyl groups has clearly increased, which indicates a successful chemical modification of graphite by COOH groups.
(18) FIG. 2c shows a Si2p X-ray photoelectron spectrum of the product from Example 3, i.e., after the condensation reaction, which was produced by the condensation reaction of ultrasonically treated silicon particles from Example 1 with the carboxyl group-grafted graphite particles from Example 2. The peak identified by reference numeral 41 was ascertained for elemental silicon (Si bulk), the peak identified by reference numeral 43 was ascertained for SiO.sub.2, and the peak identified by reference numeral 45 was ascertained for SiO.
(19) FIG. 2c shows that the binding state of silicon has been subjected to a further significant change as a result of the condensation reaction. In particular, FIG. 2c shows that no more Si.sup.+ (previously 42) is detected. The disappearance of the Si.sup.+ peak (previously 42) and the related reduction in hydroxyl groups (OH groups) may be explained by a successful reaction between dicyclohexylcarbodiimide (DCC) functioning as a water-removing condensation agent and the surface groups.
(20) FIG. 3c shows a C1s X-ray photoelectron spectrum of the product from Example 3, i.e., after the condensation reaction, which was produced by the condensation reaction of the ultrasonically treated silicon particles from Example 1 with the carboxyl group-grafted graphite particles from Example 2. The peak identified by reference numeral 51 was ascertained for elemental carbon (CC), the peak identified by reference numeral 52 was ascertained for CN, the peak identified by reference numeral 53 was ascertained for COC, and the peak identified by reference numeral 54 was ascertained for OCO.
(21) The portion of the spectrum circled in FIG. 3b shows that the proportion of carboxyl groups (COOH) 54 is significantly reduced by the condensation reaction. This also demonstrates a successful reaction with dicyclohexylcarbodiimide (DCC) as dehydrating agent or water-removing condensation agent. In this reaction dicyclohexylcarbodiimide (DCC) is hydrated with the formation of dicyclohexylurea, which is soluble in organic solvents and may be removed by filtration.
(22) The increase in oxygen bonds and simultaneous reduction in carboxyl groups proves a successful condensation reaction and surface modification by covalent bonds between the silicon and graphite particles.
(23) FIG. 4 depicts the results of the half-cell tests from Example 4. In FIG. 4 the variation of specific capacity C [mA/h] as a function of cycle count n of the surface-modified condensed silicon-carbon composite from Example 4.1 is compared with the simple physical mixtures of silicon particles and graphite particles from Examples 4.2 and 4.3 as comparative examples.
(24) The curve identified by reference numeral 4.1 depicts the results of a measurement at a C/10 rate of the surface-modified and condensed silicon-carbon composite from Example 4.1, which includes silicon particles having an average particle size d.sub.50 of 82 m.
(25) The curve identified by reference numeral 4.2 depicts the results of a measurement at a C/10 rate of the simple physical mixture of silicon particles having an average particle size d.sub.50 of 82 m and graphite particles from Example 4.2, used as a comparative example.
(26) The curve identified by reference numeral 4.3 depicts the results of a measurement at a C/20 rate of the simple physical mixture of silicon particles having an average particle size d.sub.50 of 82 m and graphite particles from Example 4.3, used as a comparative example.
(27) The curves identified by reference numerals 4.2 and 4.3 show that both at a C/10 rate and even at a lower C/20 rate, using which normally higher capacities may be measured than at a C/10 rate, the comparative cells barely withstand 25 cycles.
(28) The curve identified by reference numeral 4.1 shows that the surface-modified and condensed silicon-carbon composite according to the present invention from Example 4.1 has both a significantly better cycle stability, in particular, regarding a service life over multiple cycles and a lower capacity loss than both comparative examples 4.2 and 4.3, even at the higher C/10 rate. The jump in the 60 cycle count area is caused by an operating error of the BaSyTec instrument.
