Lithium ion (Li-ion) batteries are currently the best performing batteries and already became the standard for portable electronic devices. In addition, these batteries already penetrated and rapidly gain ground in other industries such as automotive and electrical storage. Enabling advantages of such batteries are a high energy density combined with a good power performance.
A Li-ion battery typically contains a number of so-called Li-ion cells, which in turn contain a positive (cathode) electrode, a negative (anode) electrode and a separator which are immersed in an electrolyte. The most frequently used Li-ion cells for portable applications are developed using electrochemically active materials such as lithium cobalt oxide or lithium nickel manganese cobalt oxide for the cathode and a natural or artificial graphite for the anode.
It is known that one of the important limitative factors influencing batteries' performance and in particular batteries' energy density is the active material in the anode. Therefore, to improve the energy density, newer electrochemically active materials based on e.g. tin, aluminium and silicon were investigated and developed during the last decades, such developments being mostly based on the principle of alloying said active material with Li during Li incorporation therein during use.
The best candidate seems to be silicon as theoretical capacities of 4200 mAh/g (gravimetric) or 2200 mAh/cm3 (volumetric) can be obtained and these capacities are far larger than that of graphite (372 mAh/g) but also those of other candidates.
Note that throughout this document silicon is intended to mean the element Si in its zerovalent state. The term Si will be used to indicate the element Si regardless of its oxidation state, zerovalent or oxidised.
However, one drawback of using a silicon based electrochemically active material in an anode is its large volume expansion during charging, which is as high as 300% when the lithium ions are fully incorporated, e.g. by alloying or insertion, in the anode's active material—a process often called lithiation. The large volume expansion of the silicon based materials during Li incorporation may induce stresses in the silicon, which in turn could lead to a mechanical degradation of the silicon material.
Repeated periodically during charging and discharging of the Li-ion battery, the repetitive mechanical degradation of the silicon electrochemically active material may reduce the life of a battery to an unacceptable level.
In an attempt to alleviate the deleterious effects of the volume change of the silicon, many research studies showed that by reducing the size of the silicon material into submicron or nano-sized Si domains, typically with an average size smaller than 500 nm and preferably smaller than 150 nm, and using these as the electrochemically active material may prove a viable solution.
In order to accommodate the volume change, the silicon domains are usually used as composite particles in which the silicon domains are mixed with a matrix material, usually a carbon based material, but possibly also a Si-based alloy or SiO2.
Further, a negative effect of silicon is that a thick SEI, a Solid-Electrolyte Interface, may be formed on the anode. An SEI is a complex reaction product of the electrolyte and lithium, and therefore leads to a loss of lithium availability for electrochemical reactions and therefore to a poor cycle performance, which is the capacity loss per charging-discharging cycle. A thick SEI may further increase the electrical resistance of a battery and thereby limit the achievable charging and discharging rates.
It is known from U.S. Pat. No. 6,589,696 and US 2006/0134516 that in theory reactions between an active anode material and the electrolyte may be avoided by putting a coating material on the active particles of the anode material.
In practice this was attempted in these documents by mixing particles of the anode material with a polyvinyl alcohol (PVA) solution, evaporating the solvent and pyrolising the obtained product to decompose the PVA to carbon.
This will only give, at best, a partial and defective coating however, offering insignificant shielding of the anode material from the electrolyte.
The reasons for this are probably related to one or more of the following factors:                The amounts of PVA were too low to form a complete coating.        In the disclosed process a significant proportion of the PVA will end up some distance from the active anode material and is not available to form a coating.        The carbon yield of PVA decomposition is only 10-20%, so that very significant shrinkage of a carbon layer during its formation will occur, leading to cracks of the carbon layer while it is being formed and to uncoated areas.        Escaping decomposition gasses, 80-90% by weight, will create channels for themselves in the decomposing PVA layer during conversion to carbon, creating porosities in the carbon layer thereby reducing its protective capabilities.        
In addition, it is suspected that the oxygen molecules in PVA will, during thermal decomposition, react with silicon to form SiO2, thereby rendering at least part of the silicon inert for electrochemical applications.
Despite the advances in the art of negative electrodes and electrochemically active materials contained therein, there is still a need for yet better electrodes that have the ability to further optimize the performance of Li-ion batteries. In particular, for most applications, negative electrodes having improved capacities and coulombic efficiencies are desirable.
In order to reduce the abovementioned and other problems, the invention concerns a composite powder for use in an anode of a lithium ion battery, whereby the particles of the composite powder comprise silicon-based domains in a matrix, whereby the matrix is carbon or a precursor material that can be converted into carbon by thermal treatment, whereby the individual silicon-based domains are either                free silicon-based domains that are not or not completely embedded in the matrix        or are fully embedded silicon-based domains that are completely surrounded by the matrix,whereby the percentage of free silicon-based domains is lower than or equal to 4 weight % of the total amount of Si in metallic or oxidized state in the composite powder whereby the silicon-based domains have a weight based size distribution with a d50 of 200 nm or less and a d90 of 1000 nm or less.        
Free silicon-based domains are hereby defined as those silicon-based domains that are not or not completely shielded by the matrix material and are therefore freely accessible from outside the composite particle.
By a silicon-based domain is meant a cluster of mainly silicon having a discrete boundary with the matrix. The silicon content in such a silicon-based domain is usually 80 weight % or more, and preferably 90 weight % or more.
In practice, such a silicon-based domain can be either a cluster of mainly silicon atoms in a matrix made from different material or a discrete silicon particle. A plurality of such silicon particles is a silicon powder.
The composite powder is in other words a carbon-based composite, in which a separately produced silicon nano powder is agglomerated with separately produced carbon and/or a carbon precursor acting as the matrix. In this case the silicon-based domains are formed by the actual discrete silicon particles from the silicon nano powder.
The silicon-based domains may have a thin surface layer of silicon oxide.
Such a composite powder according to the invention will have a strongly reduced tendency for SEI formation compared to traditional composite powders with silicon-based domains, and therefore will have a better cycle performance and will be more apt to be used with high currents.
Without being bound by theory the inventors speculate that this is related to a lower possible contact surface between the electrolyte and the silicon-based domains than in traditional powders, even though Si is usually not a significant component in SEIs.
A further advantage is that less stringent requirements can be put on the water content of the electrolyte. This is because of the following reason: water in the electrolyte can react with LiPF6 in the electrolyte to form HF. This HF can corrode the silicon, leading to a silicon loss and to the formation of Li2SiF6 which reduces the electrical conductivity of the electrolyte. To avoid this, the water content in the electrolyte is kept extremely low, often 50 ppm or less. However, expensive raw materials and/or expensive processing facilities are needed to obtain this.
With the low level of free silicon of the powder of the invention, this problem is much reduced, so that the stringent water limitation requirements of the electrolyte can be relaxed and overall cost reduced.
Preferably the percentage of free silicon-based domains is lower than 3 weight % and preferably lower than 2 weight %, and more preferably lower than 1 weight % of the total amount of Si in metallic or oxidised state in the composite powder, so that the advantages are obtained to a higher degree.
In a preferred embodiment the matrix is pitch or thermally decomposed pitch.
Such a product has been shown to give good performance in a battery.
Preferably the composite powder contains less than 3 weight %, more preferably less than 2 weight % and most preferably less than 1% of oxygen.
The silicon-based domains may have any shape, e.g. substantially spherical but also whiskers, rods, plates, fibers and needles, etc.
In a preferred embodiment the percentage of free silicon-based domains is the percentage as determined by placing a sample of the composite powder in an alkaline solution for a specified time, determining the volume of hydrogen that has evolved after the specified time, calculating the amount of silicon needed for evolving this amount of hydrogen based on a production of two moles of hydrogen for every mole of silicon reacted and dividing this by the total amount of Si in metallic or oxidised state present in the sample.
Such a calculation can simply be done by the skilled person based on the well-known ideal gas law.
The specified time is optimally the time that is needed to fully complete a reaction of nano silicon powder, which is not part of a composite, in the alkaline solution, but not longer. This will of course depend on the temperature chosen and the concentration of the alkaline solution. By choosing these conditions, all free silicon is measured, but fully embedded silicon is not measured incorrectly, which might happen if a longer period or more severe conditions are chosen, due to diffusion/penetration of the alkaline solution through the matrix.
An example of a specified time is 48 hrs, at a temperature of 45° C. and using a 1.2 g/l KOH solution. It was determined that these conditions are sufficient for completion of the reaction of pure silicon nano powder, but not longer than needed.
For measuring a gas amount various easy methods are available. A particularly practical method is to use a gas burette.
The total amount of Si in metallic or oxidised state is in many cases known from the amount of Si-containing material used to prepare the composite, or can alternatively be determined by a standard chemical analysis.
In a preferred embodiment the silicon-based domains are silicon-based particles, meaning that they were, before forming the composite, individually identifiable particles that existed separately from the matrix, since they were not formed together with the matrix.
In another preferred embodiment the silicon-based particles are preferably free from other elements than Si and O, so consist of silicon and oxidised Si, not taking inevitable impurities into account.
In a further preferred embodiment the silicon-based domains have a weight based size distribution with a d50 of 200 nm or less and a d90 of 1000 nm or less.
In a further preferred embodiment the ratio d90/d50 is lower than 10, and more preferably lower than 7.
The d50 value is defined as the size of a silicon-based domain corresponding to 50 weight % cumulative undersize domain size distribution. In other words, if for example the silicon-based domain size d50 is 93 nm, 50% of the total weight of domains in the tested sample are smaller than 93 nm. Analogously d90 is the domain size compared to which 90% of the total weight of domains is smaller.
In the case that the silicon-based domains are or were individual loose particles, such size distribution may be simply determined by laser diffraction of these particles. As is well known to the skilled person, particular care has to be taken to de-agglomerate agglomerates in order to reliably determine the particle size.
Aggregates of silicon-based domains may be formed during their synthesis. In the context of this invention, an aggregate is to be understood as a group of domains which are coalesced together in a structure with such an intergrowth degree that said structure can be divided into the individual domains only partially, if at all.
The degree of intergrowth of the aggregates can be influenced by the parameters of the synthesis process of forming said domains which may, for example during their formation, coalesce and grow subsequently together to form the aggregates. Thus a characteristic of an aggregate may be that when attempting to divide it into individual constituent domains, destruction of some or all of the domains will occur.
For simplicity, the definition of domains in accordance with the present invention also includes aggregates of domains which are fused together so that they may not be separated without risk of destruction of the domains.
The domains may also agglomerate due to Van der Waals forces and other electromagnetic forces acting between them to form agglomerates. In contrast to the aggregates, agglomerates are understood in the context of this invention as meaning only a loose association of domains which can readily disintegrate into the constituent domains and are not considered as domains in their own right.
Alternatively, such a size distribution may be determined optically from SEM and/or TEM images by measuring at least 200 silicon-based domains. This method is appropriate if the silicon-based domains are present in a matrix from which they cannot be separated, but may also be used for a silicon based powder. It should be noted that by domain is meant the smallest discrete domain that can be determined optically from SEM or TEM images. The size of a silicon based domain is then determined as the largest measurable line distance between two points on the periphery of the domain.
Such an optical method will give a number-based domain size distribution, which can be readily converted to a weight based size distribution via well-known mathematical equations.
The invention further concerns a method for preparing composite powder whereby the particles of the composite powder comprise silicon-based domains in a matrix comprising a mixing step in which silicon-based particles, preferably nano silicon powder are mixed with matrix material in the molten state, preferably without additional solvent, followed by a size reduction step of the obtained mixture and/or of the product obtained by thermally treating the obtained mixture.
Preferably the matrix material in the molten state is molten pitch.
Preferably the mixing step is entirely done with the matrix in the molten state.
Preferably the mixing step is performed in an extruder.
This method allows to produce easily a good composite powder for use in anodes, presumably due to the fact that the matrix materials will cover the entire surface of the silicon-based domains.
In a specific variant the method is a method for preparing a composite powder according to the invention as defined above.
Both in the composite powder according to the invention as well as the method according to the invention the matrix is preferably lithium-ion conducting and electron conducting or is made from a precursor material that can be made lithium-ion conducting and electron conducting by thermal decomposition.
The invention further concerns a method for determining, on a composite powder having particles of composite powder comprising silicon-based domains in a matrix, the weight percentage of silicon-based domains that are not fully embedded in the matrix, comprising the following steps in order:                A: placing an amount of the composite powder in an alkaline solution for a specified time;        B: determining the volume of hydrogen that has evolved after the specified time;        C: calculating the amount of silicon needed for evolving this amount of hydrogen based on a production of two moles of hydrogen for every mole of silicon reacted and dividing this by the total amount of Si in metallic or oxidised state present in the sample.        
The invention further concerns the use of a composite powder according to the invention in a lithium ion battery for limiting or avoiding SEI formation.
The manufacture and characterisation of a powder according to the invention is described in the following examples and counter examples.