The subject matter of the present invention is a silicon-carbon composite material (Si/C composite
material) and a method for producing the silicon-carbon composite material. The composite
material can be used as an active material for the negative electrode of lithium-ion batteries on
a silicon basis or processed further to form such an active material.
In the case of use as a lithium store, the composite material is characterized by a particularly
high specific capacity and a charging and discharging cycle-dependent life span which is long
for such materials.
The proposed method is able to produce a cost-efficient active material for storing lithium ions in
a lithium-ion battery on an industrial scale. This material can be used as a “drop-in-replacement”
for materials used according to the current prior art, in particular for graphite, in existing production
plants for lithium-ion batteries. Since, as a result of the stated material, the production costs
of lithium-ion batteries can be reduced, alongside simultaneously increasing both the volumetric
and gravimetric energy density of the battery, all of the known applications, in which stores for
electrical energy are used, in particular in the case of mobile applications as in electromobility or
in portable electronic devices of any type, profit from this.
The aim of the invention is, by providing a new material for the negative electrode of lithium-ion
battery cells and by means of a new method for the production thereof, to make a significant
contribution to reducing the cost of lithium-ion batteries while simultaneously increasing the energy
stored in the battery for each unit of weight or volume. Silicon is in principle very well suited
as a material for the negative electrode, however, during operation of the battery cell, the silicon
is chemically and mechanically changed so that it is available to a decreasing extent to take up
lithium in the event of multiple charging and discharging of the battery cell.
Traditionally, lithium ions are incorporated in the graphite during charging of lithium-ion batteries.
In this manner, up to 372 mAh charge per gram graphite can be stored in the battery. On
the search for new materials which make it possible to increase the energy density of batteries,
the attention of battery manufacturers in recent years has focused on silicon as a suitable replacement
for graphite. Silicon opens up the possibility of incorporating more than ten times the
quantity of lithium ions in proportion with its mass. The theoretical limit for the specific gravimetric
capacity of the active material lies in this case around 4200 mAh per gram silicon. It was
possible to approximately achieve this value in practice, but the usable capacity drops significantly
after only a few cycles. The reason for this is the very significant expansion in volume of
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the silicon-lithium alloy during incorporation of the lithium ions into the silicon structure (Zhang L
et al: Si-containing precursors for Si-based anode materials of Li-ion batteries: A review, in: Energy
Storage Materials 4 (2016) S.92-102). This process leads to ever further progressive mechanical
breaking apart of the silicon particles. It was indeed possible to significantly improve
these properties of the material by developing a metallurgic silicon alloy with an aluminum basis,
but the degradation of the material is still comparatively pronounced.
Prior art
A method is known from US 2015/0295233 A1 with the help of which, among other things, silicon
particles are coated with carbon by thermal decomposition of saccharose. The material produced
in this manner should be suitable for use as an active material in lithium-ion batteries. In
this case, carbon particles are mixed into the starting mixture. Moreover, a carboxylic acid must
be added. Very high proportions of graphite particles are used in the method described there
and the coating method takes place in a single step. The composite material obtained achieves
a discharge capacity of less than 500 mAh/g.
Li Y et al.: Growth of conformal graphene cages on micrometre-sized silicon particles as stable
battery anodes, in Nature Energy 1, 15029 (2016) deals with the problem of the breakdown of
silicon microparticles as a result of lithium absorption. To solve this, it is proposed to surround
the silicon microparticles with a graphene cage which has a cavity in order to tolerate an expansion
of the microparticle. In this case, breakdown of the microparticle is not prevented, but rather
fragments are retained in the cage.
One criterion which is decisive for the function of lithium-ion batteries is the formation of a suitable
passive layer on the surface of the active material of the negative electrodes. Were one to
use silicon as a host to store lithium ions without further measures, an inexpedient layer, above
all composed of lithium silicate, would form on its surface. As a result of the breaking part of the
silicon particular due to the volume expansion during incorporation of lithium, new Si surfaces
would furthermore continuously arise which in turn in subsequent charging cycles allow the generation
of new inexpedient layers on the new Si surfaces. Their increasing formation during
each charging process consumes lithium which is subsequently no longer actively available for
the charge carrier transport and thus devours energy. The number of charge carriers which can
be used in the battery is reduced and the transport of lithium ions into the silicon particles is obstructed.
The battery thus loses storage capacity with increasing numbers of cycles until it potentially
can no longer be used for its application. This must be prevented during the intended
life span (expected number of cycles) of a battery and for it to have the minimum charging capacity
at all times.
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Description of the invention
The method according to the invention provides preventing as much as possible the progressive
formation of the stated inexpedient passive layers and thus also the progressive removal of lithium
which is required for the charge carrier transport by virtue of the fact that the silicon particles,
prior to use in the battery, are covered with a suitable coating of carbon and are thus protected.
If this carbon coating is suitably selected, a direct chemical reaction of electrolyte with silicon
can be avoided. Instead, what are known as SEI (solid electrolyte interphase) layers are formed
only on the surface of the carbon coating which comes into direct contact with the electrolyte.
The boundary layer can therefore be very limited in terms of its extent (initial growth) in a similar
manner to the prior art for graphite-based anode materials so that from then on stable conditions
in terms of charge carrier transport through these layers can be enabled without allowing
the internal resistance of the battery to further increase continuously with increasing cycle numbers.
Comparatively stable conditions arise after only a few cycles, in the case of which conditions
the electrolyte is not further decomposed and noteworthy amounts of lithium are not continually
consumed for the formation of growing passive layers which are then no longer available
to the battery as storage capacity. In contrast to SEI layers of silicon particles which are not
coated with carbon, advantageous conditions can thus be realized by the suitable coating for
battery cycle stability. As a result of this, a passive layer which has an expedient effect on the
long life span of the battery is generated on the surface of the carbon coating ideally on a oneoff
basis. Such stabilized SEI layers are known from the prior art for graphite anodes and they
are largely composed of lithium carbonate, lithium methyl carbonate and lithium ethylene dicarbonate
(Decomposition Reactions of Anode Solid Electrolyte Interphase (SEI) Components with
LiPF6, J.Phys Chem. C2017, 121, pp22733-22738).
The stabilization of these SEI boundary layers between carbon and electrolyte is performed by
a selection of suitable electrolyte additives which are known from the prior art.
At the same time, the carbon coating of the silicon particles brings about that the electric conductivity
between the individual composite particles of the stated material is durably maintained
and not continuously reduced by progressive SEI growth, as a result of which the energy efficiency
of a battery produced therefrom is improved. The possibility may even arise to dispense
with electrically conductive additives in the electrode of the battery, as a result of which the
overall energy density of the battery is even further increased. The carbon layer which is composed
in particular at least partially of structured carbon such as graphene or graphene-type
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compounds is permeable for lithium ions so that the operation of the battery cell is enabled
while the silicon is protected from chemical attack.
The object of the invention thus lies in providing an improved material for the anode of a lithiumion
battery which makes it possible to obtain more efficient batteries with a longer life span. A
further object lies in providing a simple and particularly low-cost method for the production of silicon-
carbon composite material for battery applications. The object is achieved by the subject
matters of this invention.
One criterion which is important for the function of lithium-ion battery cells is the formation of a
suitable passive layer on the surface of the active material of the negative electrode, known as
“SEI” (Solid Electrolyte lnterphase). Were one to use silicon at the stated point as a host for
storing Li-ions without taking further measures, a layer which is inexpedient for the function of
the battery cell, comprising lithium silicates and other reaction products, would thus be formed
on its surface in interaction with the electrolyte. Since, as a result of the significant expansion of
volume during the incorporation of lithium into silicon, silicon particles would break up or cracks
would form therein, the further formation of these inexpedient layers led during each charging
process to an ever increasing consumption of the silicon and lithium for the growth of these undesirable
layers. The number of charge carriers which can be used in the battery cell and the
proportion of active material are reduced as a result of this. The transport of lithium ions into the
silicon particles and back to the cathode side would furthermore be inhibited by the growing inexpedient
layers and additionally the electronic conductivity between the particles would be reduced
considerably. As a result, the battery would form an increasingly higher internal resistance.
The method according to the invention provides in particular largely prevented the formation of
this inexpedient passive layer on silicon surfaces by virtue of the fact that the silicon particles
are covered with a suitable layer of carbon prior to use in a battery electrode and are thus protected.
As a result of this, a passive layer which has an expedient effect on the cycle durability
or the maintenance of the capacity of the battery cell over a large number of cycles is then generated
on the surface of the carbon ideally on a one-off basis when the battery is charged for
the first time, similarly to the use of graphite instead of silicon. The carbon coating additionally
brings about that the electrical conductivity between the individual particles of the stated material
is maintained over the increased life span of the battery and the charge carrier transport between
the battery electrodes over significantly more charging and discharging cycles remains
adequately ensured. As a result of this, the energy efficiency of a battery cell produced from this
composite material of silicon particles coated with carbon is significantly improved in comparison
with a battery cell produced only from silicon particles. The battery cell can consequently
5
also be charged significantly more rapidly. The carbon layer (coating), which can be composed
in particular at least partially of structured carbon such as graphene or graphene-type structures
and can have a scaly arrangement on the silicon surface, is permeable to lithium ions so that
the operation of the battery cell is enabled, while the silicon is protected from chemical attack.
As a result, a composite material with one or more silicon particles can be generated, which silicon
particles are embedded into the matrix from the described carbon material.
The specific size of the active surface is furthermore relevant for the usability of this composite
material in battery cells since it also determines the quantity of the passive layer which is created
and is thus also a key factor for the Coulombic efficiency of the battery. The term active
surface in this context means the surface of the composite particles which interacts with the
electrolyte of the battery cell. This size can be determined, for example, by a BET measurement
(adsorption/desorption properties). In the present method according to the invention, the specific
size of the active surface can be influenced via the control of the process parameters. The
person skilled in the art understands the term specific surface of a body as the quotient of the
surface of the body and its mass. As a result of this, it is possible to generate composite particles
which have a lower specific active surface than the specific surface of the silicon particles
contained in the inside of the composite particles in their initial state. As a result of this, sufficiently
small silicon particles can also be used which no longer break during lithium take-up
without the comparatively large specific surface (BET measurement) of the small particles in the
interior of the composite having a negative effect on the Coulombic efficiency of the battery cell.
In one embodiment, the composite material has a specific surface which is no more than twice
as large, in particular less than 50% larger, in particular smaller than the specific surface of the
silicon particles in the composite material.
One preferred feature of this invention is that the active Si surface of the composite particles
which in a battery cell is in exchange with the electrolyte is reduced at least by a factor of 10 in
comparison with the case where no carbon coatings are applied around the silicon particles.
The silicon particles used are preferably approximately spherical. In particular, the ratio of the
largest diameter to the smallest diameter of a particle is at most 1.5:1, preferably at most 1.3:1
and particularly preferably at most 1.2:1 or at most 1.1:1. This applies in particular to a majority
of the particles, i.e. to more than half of the particles or even to more than two-thirds or more
than 90% of the particles.
The composite material is generated by mixing silicon particles with a carbon compound (preferably
a carbohydrate or in another preferred embodiment a liquid or solid hydrocarbon) and the
subsequent controlled thermal conversion or carbonizing of the carbon compound. The term
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“thermal conversion” refers to the fact that the carbon compound by way of the heat treatment,
in particular in step A, undergoes one or more of the following changes: polymerization, change
in the mutarotation, inversion, caramelization, oxidation, splitting off of H2O, splitting off of OH
groups, condensation reaction, formation of intramolecular covalent bonds, redistributions,
isomerizations, partial pyrolysis, decomposition. The terms heat treatment and temperature
treatment are used synonymously. The “transition temperature” is the lowest temperature at
which a compound undergoes this conversion under the conditions of the method according to
the invention. Depending on the initial composition of the components for the composite material,
there are various possible temperature ranges for the selection of the temperature of temperature
treatment step A. After the completion of the conversion of the carbon compound in a
heat treatment step A, usually with a loss of mass in relation to the carbon compound used, the
thermally processed intermediate product is in a different chemical and/or mechanical state for
a second thermal treatment step B. This other state also influences the reactivity of the initial
components after heat treatment step A in interaction with (other) components in the interior of
the systems and tools used for the temperature conversion. The term “carbonization” refers to
the fact that the carbon-containing intermediate product generated from the thermal conversion
of the carbon compound by way of the heat treatment, in particular in step B, undergoes one or
more of the following changes: pyrolysis, splitting off of water vapor, splitting off of OH groups,
splitting off of CO, splitting off of CO2, splitting off of H2, splitting off of hydrocarbon compounds.
It can be advantageous for heat treatment step B that arising or escaping reaction gases from
heat treatment step A are discharged and/or actively removed. It is furthermore advantageous if
the converted components, i.e. the silicon particles and at least one carbon compound, after
heat treatment step A no longer interact with the containers or transport means from heat treatment
step A in second heat treatment step B. It can be advantageous to convey the thermally
processed intermediate product (after heat treatment step A) into other containers or conveying
devices which have different interaction properties for heat treatment step B. In particular, it can
be desirable and advantageous that no or only minimal material reactions of the resultant silicon-
carbon composite material are performed in heat treatment step B with objects or solid bodies
with which the arising silicon-carbon composite material comes into contact during heat
treatment step B. It is thus furthermore possible to avoid that arising or escaping reaction gases
which are damaging or disadvantageous for these materials precipitate on the walls which enclose
or separate off the heated space for thermal conversion or are used for the transport of
the material.
Thermogravimetric measurements with downstream mass spectroscopic analysis of the arising
or escaping reactions gases can be used to adjust the suitable temperature ranges for heat
7
treatment steps A and B. Moreover, the gas atmosphere can be adjusted in a targeted manner
during the method via the studied temperature and heat treatment.
The process temperature in the second heat treatment step of the thermal synthesis process as
well as optional processes for generating the desired particle size distribution (milling, de-agglomeration,
rolling, breaking, fragmentation, mixing) influence the size of the specific active
surface. It is furthermore possible via the selection of the synthesis temperature to reduce any
oxides (SiOx) present on the surface of the silicon particles carbothermally (e.g. as a result of
arising carbon monoxide) or by selecting another reducing atmosphere. Irrespective of whether
this is desired, carbides can be generated on the surface of the silicon particles. Evidence of the
formation of carbides could be provided by means of XRD in the case of an elevated synthesis
temperature of 1300 °C. The carbon can optionally also assume the structure of synthetic
graphite. A further optional measure of the method according to the invention provides comminuting
the intermediate product of the first heat treatment step already prior to the second heat
treatment step (also: high-temperature process step) to the defined particle size of the end
product (or to a suitable intermediate size). This has advantages when performing the high-temperature
process step:
- prior to the high-temperature process step, the material is less hard and is easier to
grind; this also applies in particular in the case of materials with carbohydrates as a carbon
source which tend to form very hard composite particle agglomerates after both heat
treatment steps;
- as a result of a particle size distribution which can be predefined in the milling process,
the subsequent high-temperature process step becomes more reproducible and the
choice of suitable production systems for production methods suited to mass production
becomes larger; in particular subsequent printing or slotted nozzle coating methods require
a suitable starting particle size distribution in the pastes, slurries, hot melt composites
or inks to be printed;
- in particular when using a rotary furnace, the temperature-time profile can be better and
more reproducibly controlled in a through-feed process; it is furthermore possible to
avoid by means of the first heat treatment step A (i.e. the conversion) that undesirable
splitting-off products concentrate in the gas atmosphere and negatively influence the result
of the high-temperature treatment; it can thus also be avoided that residues increasingly
accumulate on the inner tube wall of the furnace;
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- the surfaces of the comminuted particles can be more uniformly and better flushed by
flushing/processing gases in the high-temperature process step. Splitting-off products
during thermal conversion can be better and more reproducibly extracted and transported
away and undesirable secondary reactions with these splitting-off products can
be avoided or minimized.
Milling processes after the second heat treatment step could, depending on the method, undesirably
form new open silicon surfaces and negatively change the fabric or structure of the Si/C
composite particle and as a result impair the function in a battery. This can be suppressed or
minimized by suitable comminuting of the Si/C composite material after step A.
When using hydrocarbons as a carbon source, on one hand, an oxidation of silicon in both heat
treatment steps can be minimized or suppressed, possibly even existing surface oxides can be
removed. Moreover, in the case of suitable selection of hydrocarbons, such as, for example,
paraffins, the formation of very hard and larger, compactly adhering particle agglomerates can
be avoided. A milling step between first and second temperature treatment is thus no longer
necessary. However, it may be necessary to feed the intermediate product in other containers
or via a conveying process, for example, into a rotary furnace as bulk material to the second
temperature treatment step. In this case, de-agglomeration or comminution of the composite
bulk material preferably and automatically occurs when using hydrocarbons as a carbon source
and a dispersant.

We Claim:
6. Method according to Claim 5, wherein the temperature in step B is adjusted such that substantially
no silicon carbide formation takes place.
7. Method according to at least one of the preceding claims, wherein the thermally processed
intermediate product is comminuted, in particular to a particle size D90 of less than 50 μm
or less than 35 μm.
8. Method according to at least one of the preceding claims, wherein the mixture of silicon and
carbon compound additionally contains structure-giving and/or catalytically acting additives,
in particular selected from graphene, graphene oxide, graphite, fullerenes, nanotubes and
combinations thereof.
9. Method according to at least one of the preceding claims, wherein the solids content in the
mixture is at least 9 wt.-% and/or the silicon proportion in the silicon-carbon composite material
is at least 20 wt.-%, at least >40 wt.-% or at least 51 wt.-%.
10. Method according to at least one of the preceding claims, wherein the lowest temperature
in step B is higher than the highest temperature in step A.
11. Method according to at least one of the preceding claims, wherein
- the indicated temperature in step A is maintained for a period of at least 1 minute, in particular
of 5 minutes to 1000 minutes; and/or
- the indicated temperature in step B is maintained for a period of at least 1 minute, in particular
of 5 minutes to 600 minutes.
12. Method according to at least one of the preceding claims, wherein the carbon compound is
a carbohydrate, in particular a saccharide.
13. Method according to at least one of the preceding claims, wherein particles of the product
after temperature step A or of the intermediate product between temperature steps A and B
smaller than 500 nm and/or particles larger than 35 μm are largely removed by filtering.
14. Method according to at least one of the preceding claims, wherein the mixture of silicon particles,
at least one carbon compound, and optionally at least one dispersant has a viscosity
of greater than 5000 mPaꞏs, preferably greater than 15000 mPaꞏs, more preferably greater
than 25000 mPaꞏs, measured with a rotational viscometer in the case of opposite rotation, a
shear rate of 100/s and a temperature of 21.5°C.
49
15. Method according to at least one of the preceding claims, wherein the carbon compound
alternatively or additionally comprises at least one carbon compound selected from the list
of lignin, waxes, plant oils, fats, oils, fatty acids, rubber and resins.
16. Method according to at least one of the preceding claims, wherein the carbon compound
and/or the dispersant is a paraffin or paraffin oil, wherein in particular a paraffin or paraffin
oil is the single carbon compound.
17. Method according to at least one of the preceding claims, wherein the mixture of silicon particles,
at least one carbon compound, and optionally at least one dispersant, further comprises
lithium or a starting material which contains lithium.
18. Method according to at least one of the preceding claims, wherein silicon is comminuted
prior to the thermal processing, in particular prior to step A, wherein paraffin is used as a
dispersing agent.
19. Method according to Claim 18, wherein the silicon has a particle size D90 of less than 500
nm and/or a particle size D90 of at least 50 nm.
20. Method according to at least one of the preceding claims, wherein the method has a step of
the removal of silicon dioxide from the surface of the silicon by means of etching, wherein
the etching for the removal of the silicon dioxide takes place using one of the substances
from HF, KOH, NH4F, NH4HF2, LiPF6, H3PO4, XeF2, SF6, preferably HF.
21. Silicon-carbon composite material, which can be obtained according to a method of the preceding
claims, with an average Coulombic efficiency over 1000 charging/discharging cycles
of at least 99.5% in the half cell test with a specific charging capacity of at least 1000 mAh/g
relative to the silicon mass in the composite,
wherein the composite material has a silicon proportion of 40 to 99 wt.-% and/or a carbon
proportion of 1 to 60 wt.-%,
wherein the composite material is present in the form of composite particles with a particle
size D90 of less than 50 μm.
22. Composite material according to Claim 21, with a particle size D10 of more than 500 nm
relative to the mass distribution of the particles.
23. Composite material according to at least one of Claims 21 and 22, wherein the majority or
all the composite particles have at least two silicon particles per composite particle.
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24. Composite material according to at least one of Claims 21 to 23, with a specific surface of
at most 300 m2/g, in particular in the range from 40 to 300 m2/g and particularly advantageously
in the range from 10 to 100 m2/g.
25. Composite material according to at least one of Claims 21 to 24, with a specific surface
which is no more than twice as large, in particular less than 50% larger, in particular smaller
than the specific surface of the silicon particles in the composite material.
26. Composite material according to at least one of Claims 21 to 25, with a specific discharging
capacity of at least 1000 mAh/g relative to the mass proportion of the silicon in the composite
material over more than 1000 charging/discharging cycles in the half cell test.
27. Use of a composite material according to at least one of Claims 21 to 26, as an anode material
in a battery cell.
28. Battery cell comprising an anode, which is composed at least partially of the composite material
according to at least one of Claims 21 to 26.