Patent Application
20190341617
The work leading to this invention has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement number 337739.
The present invention relates to materials and methods for their manufacture and processing, the materials being of particular (but not necessarily exclusive) interest as anode materials for lithium ion batteries. The invention also relates to lithium ion batteries and methods for their operation.
As background to this technical field, we refer in particular to the review provided in Glaize & Genies (2013). Some parts of that disclosure are set out below, to provide the context for the disclosure of the present invention.
Lithium ion (Li-ion) batteries are a commonly used type of rechargeable battery with a global market estimated at $11 bn in 2010 and predicted to grow to $50 bn by 2020. This huge market is divided between a number of applications:
The main outlet for Li-ion batteries is consumer electronics, both in terms of number of units sold and turnover. Electric vehicles, if they develop as they are predicted to, will ultimately represent the dominant market.
Each of these applications have very different requirements in terms of battery performances. For instance electrical vehicle batteries need to be able to provide a large electric current without degrading to sustain vehicle acceleration phases, whereas consumer electronics batteries would rather benefit from the capability to be flexed, folded, or stretched. Ultimately these specific requirements lead to different technological choices in terms of battery design, especially with regards to the choice of the electrochemically active materials that store the lithium ions during charge and discharge. For this reason, one technology—e.g. silicon anodes—cannot dominate the entire market. Battery technologies rather need to be examined in the light of how well they perform on a number of metrics, the combination of which will ultimately give one technology a competitive advantage for one specific application. Examples are given in Table 1. These metrics are described below.
A typical lithium-ion battery is composed of multiple cells connected in series or in parallel. Each individual cell is usually composed of an anode (negative polarity electrode) and a cathode (positive polarity electrode), separated by a porous, electrically insulating membrane (called a separator), immersed into a liquid (called an electrolyte) enabling lithium ions transport.
In most systems, the electrodes are composed of an electrochemically active material—meaning that it is able to chemically react with lithium ions to store and release them reversibly in a controlled manner—mixed with an electrically conductive additive (such as graphitic carbon) and a polymeric binder. This slurry is coated as a thin film on a current collector (typically a thin foil of copper or aluminium, or a carbon nanotube mat in emerging applications), thus forming the electrode.
In the known Li ion battery technology, the low theoretical capacity (about 370 mA g−1) of graphite anodes is a serious impediment to its application in high-power electronics, automotive and industry. Among a wide range of potential alternatives proposed recently, Si, MxSy and FexOy are the main contenders to replace graphite as the active material of choice. Si has about 10 times more theoretical capacity than graphite but its dramatic volume expansion (up to about 400%) severely limits high-power applications. Although this problem can be partially tackled by carbon coating [Liu et al (2014)], implementation of these in large scale is still problematic. Similarly, metal sulphide (MxSy) electrodes, despite their high theoretical capacity not only suffer from volume expansion but dissolution of polysulfides that form during charge/discharge [Liang et al (2015)] in battery electrolytes. On the other hand, FexOy-nanocarbon [Tuek et al (2014)] has now emerged as a promising anode material platform because of its higher (600-1000 mAh/g, or 600-800 mAh/g sustained) capacity than graphite, good capacity retention at high rates, environmental-benignity, high corrosion resistance, low-cost, non-flammability and high-safety. However, FexOy based anodes have some drawbacks, operating via conversion or conversion alloying, as explained in Loeffler et al (2015).
Ren et al (2015) reported the formation of a composite material of carbon fibre with CoFe2O4 binary metal oxide particles. The performance of this material as an anode material for a lithium ion battery was investigated. After 20 cycles the capacity reported in Ren et al (2015) is 400 mAh/g. Further improvements in the performance of candidate anode materials would be desirable.
Tuek et al (2014) and Ren et al (2015) are two examples of conversion batteries, meaning that the chemical mechanisms leading to lithium ions storage and release is a conversion reaction. The conversion mechanism can be generally described as follows:
TMxOy+z e−+zLi+->xTM(0)+LizOy
where TM is a transition metal and TM(0) refers to is elemental form. Upon battery charging, lithium ions diffuse and react into these materials, and nanoscale metallic domains of TM(0) are formed, embedded in an amorphous matrix of LizOy. The reaction is reversed during battery discharge.
Conversion anodes have recently been branded as the next generation anodes [Loeffler et al (2015)]. As explained in Loeffler et al (2015), an appealing feature of conversion materials is their ability to store more equivalents of lithium (two to eight per unit formula of the starting material) than any insertion compound (up to two), resulting in substantially higher specific capacities. However, conversion materials exhibit a series of severe drawbacks which necessarily need to be overcome before they can be seriously considered for commercial applications [Cabana et al (2010)]. These drawbacks are also explained in Loeffler et al (2015). The conversion reaction inherently causes a massive structural reorganisation, which potentially leads to a loss of electrical contact and electrode pulverisation. Moreover, conversion materials suffer from a very high reactivity towards commonly used electrolytes and a marked (dis-)charge voltage hysteresis, considerably affecting the energy storage efficiency of such electrodes. The elevated operational potentials of many conversion materials also limit the achievable energy density and the large first-cycle irreversible capacity is considered to be unacceptable for practical applications.
Electrode pulverisation refers to the loss of electrode mechanical integrity after charge and discharge cycling. Upon active material lithiation and delithiation, the active material swells and contracts, creating internal stresses that can ultimately lead to structural damage.
EP-A-0825153 discloses cathode active materials for Li ion batteries. In one embodiment, the cathode active material is Li0.95Fe0.95A0.05O2.
The present inventors have realised that FexOy-nanocarbon structures provide a particularly advantageous basis for the development of new anode materials for lithium ion batteries. The invention has been devised based on a realisation that a known nanostructured material can be used as an anode material for lithium ion batteries. Further developments of the invention are based on additional insights into modifications of the material, to form new materials, and the development of additional uses for such materials.
U.S. Pat. No. 8,628,747 discloses a CVD process for the bulk production of carbon nanotubes (CNTs). First, metal composite Fe-Ai particles are generated by spray pyrolysis by spraying a solution consisting of water, iron nitrate and aluminium nitrate and subsequently carrying out pyrolytic conversion of the free floating metal nitrate particles by heating in a furnace at about 1000° C. in hydrogen. The aerosol (free floating) metal composite particles are then reacted with a suitable hydrocarbon compound (e.g. acetylene) in a suitable thermal reactor, with an inert carrier gas and hydrogen, at about 750° C., to facilitate growth of CNTs on the surface of the metal composite particles. The present inventors consider that having the particles free-floating here means that very large flowrates of carrier gas/H2/precursor gas need to be used in order to maintain the particles suspended/aerosolised. This is expensive and disadvantageously limits residence time in the reactor.
The resultant nanostructures have a core particle of Fe-Al-O with an array of CNTs anchored at the surface of the core particle. Due to their particular morphology, these nanostructures are referred to in the academic literature and in U.S. Pat. No. 8,628,747 as having a “sea urchin” structure.
To the knowledge of the inventors, the proposed applications of these Fe-Al-O/CNT sea urchin nanostructures has been limited so far to the bulk production of carbon nanotubes [Kim et al (2011)], nanofluid coolants additives [Han et al (2007)], solar cells [Park et al. (2010)] and thermite materials [Kim et al (2014)].
The present invention has been devised in order to address at least one of the problems identified above. Preferably, the present invention reduces, ameliorates, avoids or overcomes at least one of the above problems.
Accordingly, in a first preferred aspect, the present invention provides an electrochemical device comprising an anode, cathode and electrolyte, wherein the anode and/or cathode comprises an active material comprising core particles and carbon nanotubes, the core particles are electrochemically active in the device and the carbon nanotubes are anchored on the core particles to form nanostructures.
In a second preferred aspect, the present invention provides an anode active material for a lithium ion battery, the anode active material comprising particles of Fe-Al-Li-O, wherein Fe is present in an amount of at least 10 wt % to at most 90 wt %, Al is present in an amount of at least 0.1 wt % to at most 90 wt %, and Li is optionally present, in an amount of 0 wt % or higher, wherein wt % is expressed in terms of the total mass of the particles of Fe-Al-Li-O.
In a third preferred aspect, the present invention provides a lithium ion battery comprising an anode, cathode and electrolyte, wherein, either:
(i) the lithium ion battery comprises an electrochemical device according to the first aspect; or
(ii) the anode comprises an anode active material according to the second aspect.
In a fourth preferred aspect, the present invention provides a use of an anode active material according to the second aspect in an anode in conjunction with a cathode and an electrolyte in a lithium ion battery for charging and discharging of the lithium ion battery.
In a fifth preferred aspect, the present invention provides a method for processing an anode active material for a lithium ion battery, the method including:
In a sixth preferred aspect, the present invention provides a layer of material comprising particles of Fe-Al-Li-O and carbon nanotubes, wherein Fe is present in an amount of at least 10 wt % to at most 90 wt %, Al is present in an amount of at least 0.1 wt % to at most 90 wt %, and Li is optionally present, in an amount of 0 wt % or higher, wherein wt % is expressed in terms of the total mass of the particles of Fe-Al-Li-O, wherein the particles of Fe-Al-Li-O are core particles and the carbon nanotubes are anchored at one end on the core particles to form nanostructures.
In a seventh preferred aspect, the present invention provides a method for the manufacture of nanostructures comprising core particles and carbon nanotubes, wherein the carbon nanotubes are anchored at one end on the core particles, the method comprising:
providing a solution of at least one metal salt and a combustible component; spray drying the solution to form precursor particles comprising the at least one metal salt and the combustible component;
subjecting the precursor particles to combustion heat treatment to combust the combustible component and to convert the at least one metal salt to metal oxide, metal, or a mixture of metal and metal oxide, thereby forming core particles; and subjecting the core particles to carbon nanotube growth conditions, to grow carbon nanotubes from the core particles, thereby forming the nanostructures.
The first, second, third, fourth, fifth, sixth and/or seventh aspect of the invention may have any one or, to the extent that they are compatible, any combination of the following optional features.
Preferably, the carbon nanotubes are grown from the core particles. The carbon nanotubes may be, for example, covalently bonded to the core. The core particles may have protrusions extending from the core particle, the protrusions being formed integrally with the core particle, wherein respective protrusions protrude into respective carbon nanotubes to anchor the carbon nanotubes with respect to the core. Note that this arrangement differs from the situation where a core particle has heterogeneous catalyst particles disposed at the surface of the core particle and the CNTs grow from those catalyst particles. In that case, the catalyst particles are not integral with the core. The difference between these situations can be considered in relation to the electrochemically active material. In the preferred embodiment of the present invention, the core particle is formed of the electrochemically active material, including the integral protrusion.
Preferably, Al is present in an amount of at least 5 wt %. Furthermore, preferably, Al is present in an amount of at most 70 wt %. The amount of Al is chosen in order to promote the formation of a suitable array of CNTs at the core particle surface, as explained in more detail below.
Preferably, Li is present in the particles of Fe-Al-Li-O in an amount of at least 0.1 wt %. In the context of Li ion batteries, such a state would exist, for example, where the anode active material has been used for charging and/or discharging a Li ion battery. Furthermore, in view of the conversion reaction and/or conversion alloying that takes place during charging, the core particles preferably contain lithium oxide and metallic iron.
The anode active material may further comprise an electrically conductive additive. The electrically conductive additive preferably comprises elemental carbon. For example, the electrically conductive additive may comprise carbon nanotubes. In a particularly preferred embodiment, the particles of Fe-Al-Li-O are core particles and the carbon nanotubes are anchored at one end on the core particles to form nanostructures. As can be noted on careful inspection of the nanostructures, preferably the carbon nanotubes are grown from the core.
The core particles may have, on average, at least 1011 carbon nanotubes per m2 anchored on the core particles. The core particles may have, on average, at most 1017 carbon nanotubes per m2 anchored on the core particles. These values may be determined by SEM inspection of the nanostructures, by assessing the diameter of the core particle to determine the surface area of the core particle (not including porosity, where present) and counting the anchored carbon nanotubes.
The anode active material may comprise at least 0.1 wt % by weight of carbon nanotubes, expressed in terms of the total weight of the core particles and the carbon nanotubes. The anode active material may comprises not more than 99% by weight of carbon nanotubes, expressed in terms of the total weight of the core particles and the carbon nanotubes. The benefits of the carbon nanotube content for the material are explained in more detail below.
The particles may have a diameter in the range 30 nm to 50 μm. Within this range, it is found that the material provides suitable performance in particular as an active material for Li ion batteries. It is further preferable that the particles have a diameter in the range 30 nm to 10 μm.
The particles may include a matrix of amorphous Al-Fe-O. Furthermore, Al-Fe-O crystallites may be embedded in the matrix of amorphous Al-Fe-O. The Al-Fe-O crystallites may comprise a solid solution of hercynite into magnetite. Where carbon nanotubes are anchored to the core particles, it is preferred that the carbon nanotubes are attached to the core particles at Al-Fe-O crystallites. It is considered by the inventors, without wishing to be bound by theory, that the Al-Fe-O crystallites provide nucleation and growth sites for the carbon nanotubes.
Preferably, the anode active material has an average discharge potential, when measured against Li/Li+ in a half cell, of at most 1.8 V.
Preferably, during the use of the material in the third aspect of the invention, during charging, lithium ions diffuse into the particles, and a conversion reaction and/or a conversion alloying reaction takes place in which lithium oxide and metallic iron are formed.
Capacity, rate capability, cyclability (i.e. durability), safety, cost can potentially be improved by the use of conversion anodes. However, as stated above, fundamental problems have so far impeded the use of conversion anodes in commercial systems. The present inventors consider that the preferred embodiments of the present invention address these problems, as now explained.
One problem which has been considered to be present for conversion anodes is loss of electrical contacts and electrode pulverisation. The present inventors, without wishing to be bound by theory, consider that the sea-urchin structure promotes the so-called buffer effect of carbon nanotubes. This means that the stiff carbon nanotubes act as a structural reinforcement helping to preserve the integrity of active material particles. Also the nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, limits the proportion of active material that may become inaccessible to electrons or lithium-ions during battery operation. Furthermore, the volume expansion/structural changes occurring to the active material seem to be fundamentally mitigated by the use of the Al—Fe—O alloy, limiting electrode pulverisation for both the Al-Fe-O particles and the full Al-Fe-O sea-urchins.
Another problem which has been considered to be present for conversion anodes is a perceived high reactivity towards commonly used electrolytes. However, in the preferred embodiments of the present invention it is considered that the Al—Fe—O alloy forms a stable solid electrolyte interphase (SEI) with the electrolyte, ensuring stable battery operation.
A further problem which has been considered to be present for conversion anodes is a marked (dis-)charge voltage hysteresis. This is addressed by the sea urchin structures. The nature of the carbon nanotube network resulting from the sea-urchin structures is that each active core is essentially a node of the network. This limits the proportion of active material that may become inaccessible to electrons or lithium-ions during battery operation.
With respect to the elevated operational potentials of many conversion materials being considered also to limit the achievable energy density, this problem is considered to be addressed by the composition of the core particles. Furthermore, high voltage cathodes are now available.
It is considered that the Al—Fe—O alloy enables higher rates (high current density charge/discharge) and life (cycle number) than pure iron oxide based electrodes. This is at present thought to be due to a stabilising effect of Al.
The nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, ensures good electrode electrical conductivity, which contributes to good battery performances at high current rates.
Similarly, the nature of the carbon nanotube network resulting from the sea-urchin structures, each active core being essentially a node of the network, ensures good electrode thermal conductivity, which contributes to enhanced battery thermal management and ultimately battery safety
The layer of material according to the sixth aspect may be capable of self support. For example, the layer of material may have a tensile strength, measured on the layer without the presence of a supporting substrate, of at least 1 MPa.
In the method according to the seventh aspect, the precursor particles are preferably subjected to combustion heat treatment in a furnace set to a temperature of not more than 500° C. More preferably, the precursor particles are subjected to combustion heat treatment in a furnace set to a temperature of not more than 400° C., not more than 300° C. or not more than 250° C.
Preferably, the precursor particles are supported together in the furnace on a combustion heat treatment solid substrate. In some embodiments, the solid substrate may move, e.g. rotate. Consequently, the particles may flow to some extent. However, the precursor particles at this stage of the process are not free-floating.
Similarly, preferably the core particles are supported together during carbon nanotube growth on a carbon nanotube growth solid substrate. In some embodiments, the solid substrate may move, e.g. rotate. Consequently, the core particles may flow to some extent. However, the core particles at this stage of the process are not free-floating.
The core particles contain one or more voids, generated during drying or during combustion of the combustible component. There may be provided networks of nanopores/nanochannels of diameter about 1 nm extending throughout the particle. Additionally or alternatively there may be hollow/multishell particles with thickness of about 10-100 nm for each shell, the remainder of the particle being hollow.
The combustible component may be a carbon-based material, soluble in the solution. For example, the combustible component is a carbohydrate material, such as sucrose or maltodextrin.
Further optional features of the invention are set out below.
The indication “Fe-Al-Li-O” is intended to indicate a composition which includes Fe, Al, O and optionally Li. The presence of other elements is not necessarily excluded. However, in some preferred embodiments, the indication “Fe-Al-Li-O” may designate a composition which consists of Fe, Al, O and optionally Li, and optionally up to 10 wt % of further components, including incidental impurities.
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
As explained above, it is preferred to consider battery technologies in the light of how well they perform on a number of metrics, the combination of which will ultimately give one technology a competitive advantage for one specific application. Examples are given in Table 1, which is taken from Glaize & Genies (2013).
Some terminology relating to battery technology is now explained, in order better to understand the context of comparing the performance of different batteries. This is again taken primarily from Glaize & Genies (2013).
Energy Density
The total energy stored in a battery is the product of its capacity by its voltage. It is expressed in kilo watt hours (kWh). The capacity, expressed in ampere-hour Ah (same dimension as coulomb C) is the amount of charge that the battery can store in the active material in the form of lithium ions. The voltage, expressed in volts (V), refers to the electrical potential difference between the anode and the cathode. It is the driving force that enables the lithium ions to shuttle from one electrode to the other.
A distinction must be made between the total stored energy and the deliverable energy, which is the quantity that matters for the consumer. There are inevitable losses in the battery system (poor conductivity of the active material that is not entirely accessible electrically, etc.), and in practice some restrictions must be enforced on the levels of lithiation/delithiation that the active materials/electrolyte can sustain without degrading irreversibly. In other words the depth of charge and discharge should be actively limited. The deliverable energy density depends on the operating current, the allowed depth of discharge, the temperature, and the target cycle life of the battery. Although by definition the total energy of the battery does not depend on the battery operating conditions, the deliverable energy does. This is the reason why the energy density of a battery should be stated given a precisely defined operating conditions context. However, this is often not the case for figures found on the existing battery manufacturers' website or in the media, making general one-to one comparison of batteries rather difficult.
This concept of deliverable energy, as opposed to total energy, transfers to the notions of capacity and voltage. Theoretical capacity and open circuit voltage are different to nominal capacity and useful voltage. Nominal capacity and useful voltage are linked to a specific context, and their product gives the deliverable energy in this context. For example, the theoretical capacity of silicon is 10 times higher than that of graphite (the commercial standard for anode active material), and this is the figure often quoted out of context, but this is meaningless as only a fraction of that capacity can be used in practice, silicon being extremely prone to degradation at high currents or prolonged battery operation. Transition metal oxides, upon which the present technology is based, have a lower theoretical capacity (only 3 to 4 times that of graphite), but a much larger fraction of that capacity can be used in practical operations as this material is more robust, leading to levels of nominal capacity that can be greater or lower than silicon depending on the operating conditions, and therefore the end application. Likewise the open circuit voltage depends solely on the electrochemistry of the anode and cathode active materials, but the useful voltage is smaller and depends on the engineering constraints dictated by the battery operating conditions.
Additionally, some applications have requirements in terms of gravimetric energy density, expressed in kWh/kg, whereas others have requirements in terms of volumetric energy density, expressed in kWh/L, depending on whether weight or volume of the battery is the most important parameter to optimise for a given application. Here battery energy is normalised by the weight or the volume of the battery, respectively. This depends primarily on the properties of the active materials, but also on the overall battery design.
Power Density
As shown above, the charge and discharge currents are key operating parameters defining the deliverable energy stored in the battery. This can also be understood in terms of power density. Power is expressed in kW and power density is expressed in kW/kg, or kW/L. It can be understood as the maximum achievable power (e.g. that achieved in starter batteries for a short duration), or the maximum acceptable power over a given duration, that is actively limited for reasons of heat generation or active material degradation for instance. It is the product of the battery voltage by the charge or discharge current. Properties of the battery active material dictate an upper electrical current that can be extracted from the battery, either because of concerns about the cycle life of the battery, or because the current is physically limited by the amount of lithium ions that can be extracted from the active material in a given time (diffusion-limited operation for instance). To further complicate the matter, in the case of a battery the voltage is inversely proportional to the current being drawn, with a proportionality factor that depends on the composition of the active material (its electrical conductivity in particular).
Linked to the notion of power density is the notion of acceptance current. This is the maximum current that can be used to recharge the battery, and is therefore related to the battery charging time. As above it is limited either by the kinetics of the electrochemical reactions in the active material, or by safety/cycle life considerations.
Battery Lifetime
Cyclability is the ability of the battery to maintain nominal or dose to nominal performances in terms of energy and power density over a large number of charge-discharge cycles for given operating conditions. Over cycles the battery energy density and power density degrade—this phenomenon is called aging—eventually leading to battery failure. This is routinely experienced by smartphone users keeping the same phone for more than a few months. It is an important requirement in the automotive industry as the battery accounts for a significant portion of the total cost of vehicles, and must withstand daily charge and discharge for years. It is beyond the scope of this document to discuss the mechanisms of aging, but suffice it to say that it is dependent on the chemistry of the electrode active materials and on the operating conditions. Higher currents, higher operating temperatures, and deeper charges/discharges lead to accelerated aging. In terms of materials chemistry, the main drawback of silicon is its very poor cyclability.
Other metrics such as faradaic efficiency, shelf life, self-discharge may be of importance for some applications but are usually not the most constraining factors for battery lifetime. These concepts will be understood by the skilled person, with reference for example to Glaize, C., & Genies, S. (2013).
Safety
The weak point preventing the widespread introduction of Li-ion batteries to high-energy onboard or stationary applications is its lack of tolerance to abusive operation such as over-charging or high temperatures. Thermal runaway is the catastrophic failure of the battery whereby self-sustained temperature rises in the system lead to electrolyte evaporation that causes risks of hazardous gas leakages, combustion, or explosion.
Susceptibility to thermal runaway depends on the operating conditions, the inherent electrochemistry of the active materials (their propensity to generate temperature rises), and the overall battery design (the capability of the battery to safely dissipate any excessive heat being generated).
Versatility
Versatility can be understood in a number of ways. In some contexts this refers to the capability to operate the battery in high temperature environments, which is typically a requirement for cars, due to the concerns over energy density, power density, cyclability, and safety losses associated with high temperatures. In other context this refers to the capability of the battery to operate in low temperature environments. This is a requirement for cars that operate in a cold climate, and for aeronautics and space applications. It is a problem as the kinetics of lithium ion electrochemical reactions are slowed down by low temperatures leading to dramatic decreases in performances unless special measures are taken.
Versatility can refer to additional mechanical capabilities of the battery, such as being transparent, being folded, flexed, weaved, stretched, rolled, cut open, etc. These are typical requirements for the next generation of consumer electronics wearables. Again, versatility depends on the operating conditions requirements, the overall battery design, and the choice of active materials for the electrodes.
Cost and Sustainability
A distinction should be made between the cost of the entire battery, the cost of one cell, and the cost of the active material itself. A detailed description of typical battery manufacturing processes is out of the scope of this disclosure. However, in a typical battery manufacturing process active materials are generally supplied to the battery assembly line as a powder that is roll-coated onto current collectors. This powder can be synthesised in house on a separate manufacturing line, or most often bought from an external supplier. Ideally, active materials must be synthesised from available, affordable compounds, and the process itself should not be costly.
Glaize & Genies, 2013 estimate the purchasing cost of Li-ion batteries in 2013 to lay between €400/kWh and €2000/kWh. As will be understood, battery prices change quickly with evolving technology and improvements in manufacturing efficiency. However, note that the price per kilowatt-hour is only one metric, which does not take into account the other parameters mentioned above. Excluding processing, the cost of battery-grade graphite delivering a capacity of 325 mAh/g can be estimated to be $2.2/kg in 2013, that is about 0.07$/Ah. Accounting for the other components of the electrode and the processing cost, the total anode cost is about 0.13$/Ah in 2013. Anode material cost accounts for about 50% of the anode cost, but that is only about 8% of the total battery cost. For this reason, there is room for significantly more expensive anode materials, especially if they enable higher specific capacities (mAh/g) as this translates to longer electric vehicle range.
How can we Compare Batteries?
From the explanation above it is clear that one figure only, such as the energy density, or even the price per kilowatt-hour, cannot be used to compare the relative merits of different batteries.
An alternative approach is to use web-type diagrams such as the one shown in
However this methodology is not necessarily perfect. As explained above the different metrics are all interdependent. For instance an energy density figures makes sense only if the required power density/discharge current and cyclability performances are specified. For this reason the only rigorous way to compare batteries is to fully specify the operating conditions for which the comparison is being made. This is what the US Advanced Battery Consortium (USABC) recommends (see http://www.uscar.org/guest/article_view.php?articles_id=85 accessed 21 Sep. 2016). An example of their benchmark for electrical vehicles is given in table 2. Comparisons are then only valid for a given context, which is chosen as being best representative of the requirement for a specific application. Every major electric vehicle research project funded by the US government uses this as a benchmark.
The goals listed in Table 2 are far out of reach for commercially available batteries at the time of writing.
Next we disclose processes for the manufacture of suitable anode materials and details of the characterization of such materials.
Processes for the Manufacture of Materials According to Embodiments of the Invention, and the Characterization of Such Materials
In the present disclosure, we focus on a continuous gas-phase carbon nanotube (CNT) production process that relies on a three-dimensional organisation of CNTs around a central particle in a structure named Carbon Nanotube Sea Urchin (CNTSU).
Here CNTSUs are broadly defined as microscale structures whereby CNTs are grown radially from a central nano or micro particle, which is usually referred to as the core of the sea urchin. Early accounts of such architecture include magnetic hollow nickel microspheres covered with oriented CNTs [Han et al (2006)], boron nitride/CNT composite particles synthesised using a spray-pyrolisis route [Nandiyanto et al (2009)], CNT forests grown on spherical alumina microparticles via a CVD method [He et al (2010 and 2011)], or boundary layer CVD synthesis of radial filled CNT structures [Boi et al (2013)].
This disclosure focuses on the most studied process to date, which was first reported in Kim et al (2011) whereby an aerosol of CNTSUs with bimetallic nanoscale cores is continuously synthesised in the gas-phase. As shown in
This disclosure provides new characterisation data showing that contrary to what was assumed so far in the literature, cores are composed of an amorphous AlxFeyOz alloy which composition is quantified. This leads to the proposition of a new mechanism for catalytic site formation, whereby catalytic sites nucleate at the surface of the cores upon cooling in the downstream temperature gradient of the calcination furnace, effectively decoupling catalytic site formation and CNT growth from the initial metal nitrate precipitation stage. Building on that finding, which indicates that the process is more versatile than what was originally thought as it does not rely on the initial structure created upon droplet precipitation, we show that CNTSU morphology can be tuned independently of core size and composition by changing the operating parameters of the CVD growth furnace, and that CNT length, density, and quality can be increased to unprecedented levels with a novel hybrid aerosol-substrate CVD growth process that results in the formation of CNTSU carpets.
Experimental Methods
CNTSU Synthesis
The apparatus for CNTSU synthesis is illustrated in
X-Ray Diffraction. X-Ray Photoemission Spectroscopy, and Scanning Electron Microscopy
For the purpose of X-ray diffraction (XRD) studies, about 50 mg of core particles were collected downstream of furnace 2 with the system operated at nominal conditions except that no acetylene was introduced in furnace 2. Particles were filtered using a PTFE membrane filter (1.2 μm pore size, Cole Parmer). The powder was loaded on a 10 mm diameter sample holder and analysed using a Bruker D8 theta/theta diffractometer operated with a Cu Kα radiation source with 2θ=10-80° at a scan rate of 0.3°/min (2θ) and a step size of 0.019° (2θ). Diffraction data was interpreted using the PANalytical X'Pert HighScore software operating with standard background and peak fitting techniques.
X-ray photoemission spectroscopy (XPS) was performed on particles collected at various locations in the system listed in Table 3, on a silicon wafer using the thermophoretic precipitator described below. Collection time was about 1 h, enough time to coat the wafer with a layer of particles of thickness >10 μm as confirmed by scanning electron microscopy (SEM). Samples were analysed using a Thermo Scientific Escalab 250Xi UPS/XPS photoelectronic spectrometer. Scans were recorded with a monochromatic Al Kα anode X-ray source with a power of 210 W, 650 μm spot size and the adventitious carbon Is peak at 284.8 eV as a reference marker to detect sample charging, which was neutralised with an electron flood gun. An Ar+ ion gun was used for sample cleaning and depth profiling at an ionization energy of 1000 eV with a beam current of 2.5 μA for 0, 60, or 920 s prior to measurement. The etch rate was estimated to be about 0.2 nm s−1. XPS spectra were collected from an area of 300 μm2 on the substrate, with an interaction depth estimated to be about 10 nm. XPS data was interpreted using the Thermo Scientific Avantage software. For each sample a survey scan was first carried out, followed by high resolution scans on the spectral regions of interest. Fe2p multiplet peaks were fitted using a Shirley background substraction and 75% Gaussian 25% Lorentzian line shapes, as recommended in Lin et al (1997).
All Scanning Electron Microscope (SEM) images were obtained by collecting particles, either cores or CNTSUs, on silicon wafers using the thermophoretic precipitator described below, and imaging them using a LEO Gemini 1530VP SEM.
Energy Dispersive X-Ray Spectroscopy was performed by collecting core particles on silicon wafers using the thermophoretic precipitator described below, and imaging them using a LEO Gemini 1530VP SEM equipped with an Oxford Instrument EDS detector.
Scanning Mobility Particle Sizer and Centrifugal Particle Mass Analyser
The Scanning Mobility Particle Sizer (SMPS) is a common in-line aerosol characterization tool used to measure particle size distribution based on their mobility equivalent diameter. As shown in
The Centrifugal Particle Mass Analyser (CPMA) 126 shown in
Thermophoretic Precipitator
A thermophoretic precipitator, as shown in
where ν is the gas kinematic viscosity and T [K] the absolute temperature of the gas, that curves particle trajectories towards the cold plate 136 where they are collected on a substrate (typically a silicon wafer 132). This design was inspired by Gonzalez et al (2005). The coated silicon wafer can then be taken to a CVD furnace for CNT growth, or for ex-situ characterisation.
Substrate-Based CNTSU CVD Growth
Cores were collected downstream of first furnace 110 with the system operated at nominal conditions onto a silicon wafer of area about 1×1 cm using the thermophoretic precipitator described above. Collection time was about 60 min to ensure full coverage of the wafer and >10 μm coating thickness as confirmed by SEM. The wafer was then transferred to a horizontal tube furnace where it underwent thermal CVD at atmospheric pressure, with flows of 100/400/100 sccm C2H4/H2/He, at 800° C. with a 20 min growth time. The resulting CNTSUs were rapidly cooled in the growth atmosphere before purging the CVD chamber with helium. This approach is described in Ahmad et al (2016).
Modified Process for Manufacture of CNTSUs
In stage 1, an aqueous solution of aluminium nitrate, iron nitrate, and sucrose is spray-dried using a commercially available spray-drier 200 to form a dry precursor powder of aluminium nitrate+ iron nitrate+sucrose microparticles.
Suitable iron and aluminium nitrate concentrations are typically about 40 wt. %, although the concentrations can be from 1 wt. % up to the solubility limit for the salts used in the relevant solvent at the temperature range of interest. Note that other solvents can be used, such as ethanol or acetone, provided that the precursors are soluble. Note also that other iron and aluminium salts can be used. The present inventors have confirmed for example that citrates, tartrate, sulfates can be used. Furthermore, the carbon source is not necessarily sucrose. In fact, in one preferred embodiment, sucrose is not used and maltodextrin is used instead. Any suitable soluble carbon source can be used, such as glucose. The sucrose (in this example) is dissolved/solvated. Where maltodextrin is used, a typical concentration is 8 wt. %. However, other embodiments may use concentrations in the range 2-30 wt. %.
For stage 2, the precursor powder collected from stage 1 is then taken out of the collection chamber and into a calcination reactor (furnace 202). Here, it is not free floating. Instead, it is held in a container such as a ceramic crucible 204, or on a substrate such as a ceramic substrate, or on a silicon wafer or metal strip. The atmosphere in the calcination reactor is an inert atmosphere (e.g. nitrogen or helium). It is possible to use an oxidising atmosphere, such as air. In stage 2, the precursor powder is heated gently to start a combustion reaction between the nitrates and the sucrose, which results in forming mixed metal oxides particles (i.e. the core particles). The temperature in the calcination reactor is set to about 200° C., for example. A suitable range for the temperature in the calcination reactor is 100-1000° C. In an alternative embodiment, the precursor powder may be held on a wall of the furnace 202, such as on the inner surface of the tube insert into the furnace 202. This tube may be rotated around its principal axis.
In stage 3, the core particles are then transferred to another furnace 206 for growing CNTs. In the CNT growth furnace, the core particles are once again not free floating but are held in a container such as a ceramic crucible, or on a substrate such as a ceramic substrate, or on a silicon wafer or metal strip. Carbon nanotubes are then grown in accordance with the carbon nanotube growth conditions described above. Alternative carbon nanotube growth conditions can be used, as will be understood by the skilled person, such as replacing acetylene by ethylene, and/or with different flowrates and relative concentrations. In a similar manner as for stage 2, in an alternative embodiment, the core particles may be held on a wall of the furnace 206, such as on the inner surface of the tube insert into the furnace 206. This tube may be rotated around its principal axis.
Results and Discussion
Chemical Composition of Cores
XRD results are shown in
Quantification of crystallite size ds via the Scherrer equation is shown in Table 4, which corresponds to
A scanning electron microscope image, elemental maps and an EDX spectrum, are reported in
XPS spectra of the sample reported in
To conclude on the question of core composition downstream of the second furnace 114, it can be said firstly that cores are composed of small (<10 nm) crystalites of a solid solution of hercynite into magnetite with a proportion of hercynite comprised between 50% and 80%, dispersed in an amorphous matrix that represents the majority of the sample. Secondly, neither XRD nor XPS suggest that Al and Fe are segregated into distinct phases. If that were the case it is likely that Fe203 would be the main Fe phase, which is not coherent with the significant proportion of Fe(II) identified via XPS. Moreover no Al2O3 phase could be detected via XRD. This is not surprising as Tumock and Eugster (1962) has shown that hercynite and magnetite form full solid solutions above 850° C. (see also Golla-Schindler (2005)). Thirdly, overall atomic ratio of Al/Fe/O was estimated to be 1.4/1/4.1. Fourthly, there are indications that core composition is not uniform along core radius and that this composition evolves in the second furnace 114.
Investigation of CNT Growth
In the light of this new information on core composition, a new hypothesis on the mechanism of catalytic site formation and CNT growth can be formulated. We propose that small crystalline domains that nucleate in the downstream temperature gradient of the first furnace 110 and are separated from each other by an amorphous matrix act as catalytic sites for the growth of CNTs in the second furnace 114. The crystalline domains in questions would be those identified in the XRD study reported above. Their size (<10 nm) is compatible with their role as catalytic sites for CNT growth and with the diameter of resulting CNTs (see
In-Line Monitoring of CNT Growth
Optimisation of nominal conditions as shown on
Discussion of Results for Modified CNTSU Manufacturing Process
Following the manufacturing process set out in
Furthermore, in U.S. Pat. No. 8,628,747, it is necessary to use hydrogen in the furnace for the formation of the core particles. Hydrogen is expensive and is typically the most important operational cost in such processes. Not using hydrogen also makes the process safer and easier to implement industrially.
In the spray dryer, the present inventors have found that it is possible to recycle the carrier gas (e.g. hot nitrogen) used in the spray-drier. This improves efficiency. It is notable that it is would be very difficult to recycle the spray dryer carrier gas in U.S. Pat. No. 8,628,747.
It is found that the combination of spray drying and combustion enables the creation of voids within the core particles, in the form of porosity and/or hollow structures within the core particles. This is considered to be advantageous from the point of view of applying the material in a battery. Methods for producing hollow structures in this manner are disclosed in WO2014183169, although not in the context of CNTSUs.
The present inventors have found that they are able to increase their throughput by about 1000 times (from 100 mg/day to 100 g/day) compared with the throughput possible based on U.S. Pat. No. 8,628,747, but still at the laboratory scale, using apparatus of similar cost.
Conclusion
XRD, EDX, and XPS characterisation showed that cores of CNTSUs produced using the floating substrate process with equal concentrations of aluminium nitrate and iron nitrate precursors by weight are mostly composed of an amorphous alloy of Al/Fe/O in the molar proportions 1.4/1/4.1 downstream of the CNT growth furnace following exposition to the atmosphere. Small crystalline domains (Scherrer size <8 nm) composed of a solid solution of hercynite into magnetite are embedded into this amorphous matrix. Our insight, based on existing literature, is that these crystallites act as catalytic sites for the growth of CNTs from the surface of the cores in the second furnace. We further hypothesise that these crystal nucleate in the calcination furnace downstream temperature gradient, effectively decoupling catalytic site formation and CNT growth from the initial structure created upon droplet precipitation. Demonstration that CNT growth can be controlled independently of core composition by tuning system operating parameters is in accordance with this assumption. Inherent limitation to CNT length and quality resulting from the limited residence time for CNT growth in the gas-phase process are overcome with the demonstration of a new substrate-based CVD growth of CNTSUs enabling unprecedented CNT length, quality and density coverage of CNTSUs. We report the formation of CNTSU carpets resulting from this process. Finally we propose a novel in-line characterisation technique combining differential mobility analysis and centrifugal mass classification to continuously monitor the growth of CNTs in the gas-phase process.
Further Details and Discussion of Preferred Embodiments of the Invention
The protrusion of the material of the core particle into the carbon nanotube itself has the effect of increasing the contact area between the carbon nanotube and the active material. For example, the protrusion may protrude into the carbon nanotube for a distance which is greater than the internal diameter of the carbon nanotube. Considering another example, the protrusion may have an aspect ratio of protrusion length (along the principal axis of the carbon nanotube) to diameter (perpendicular to the principal axis of the carbon nanotube) of greater than 1, more preferably greater than 1.5. In some embodiments, this aspect ratio may be greater than 2, greater than 5 or even greater than 10.
The protrusion of the material of the core particle into the carbon nanotube is found to enable enhanced electricity conduction between carbon nanotube and active material of the core particle. It is considered that this is due to a reduction of interfacial resistance compared to the case where active material and carbon nanotube are connected via a foreign intermediate (e.g. a catalyst particle), which introduces an extra interfacial resistance, which is large because the surface area of the connection is small. Achieving a reduced interfacial resistance is considered to be a key point for improved battery performance.
The protrusion of the material of the core particle into the carbon nanotube is found to provide enhanced heat conduction between the carbon nanotube and the active material of the core particles. Similar reasons for this apply as for electrical conduction described above. This feature also is considered to be key to battery performance.
The protrusion of the material of the core particle into the carbon nanotube is found to lead to enhanced mechanical integrity of the overall structure, for similar reasons as described above. This is again considered to be key to battery performance, especially with conversion materials that have large volume expansion upon cycling. Additionally, this is considered to assist in maintaining the integrity of the sea-urchin structure during battery manufacturing, which typically involve high shear mixing/pestle and mortar of a sea urchin slurry.
As explained above, a particular preferred application for the CNTSUs is in the fabrication of lithium-ion battery electrodes. In such an application, the CNTSUs are arranged as an assembly, either as a film or layer deposited on a substrate.
The material reported here outperforms existing lithium-ion battery anode materials for a number of reasons:
Referring now to
In a lithium ion battery as shown schematically in
Within the realm of lithium-ion batteries, the current technology that is set to dominate the market for another ten years at least, these performances are primarily determined by the choice of the active material for the cathode and anode. The current challenge is to find materials that largely exceed the energy density and rate performances of commercially available batteries, without sacrificing the other performance parameters.
The material is an assembly of individual carbon nanotube-metal oxide nanostructures (
The nanostructured film thickness can range from less than one monolayer to several hundreds of micrometres. The packing density can be varied according to the deposition process, from a very porous film, to a highly compact one.
In addition to the use as an anode material for lithium ion batteries, there are other uses for the material: active material for other electrochemical devices (alternative battery types, supercapacitors, fuel cells, electrochemical water filters, electrocatalysis, photocatalytic water splitting, etc.), filler additive for enhanced mechanical, thermal, and electrical properties of composites, or production of large area carbon nanotube mat or fibres for a number of applications (composites, actuators, heat sinks, electric cables, electromagnetic shielding, etc.).
In the context of lithium ion batteries, the present invention combines both material advantage and nanostructure engineering to design an anode structure that optimises ion diffusion, electron transport, mechanical stability and Li ion kinetics, as these are the most stringent conditions needing a balance in high energy/power batteries. One main problem impeding the commercialisation of metal oxide electrodes [Cabana et al (2010)] is voltage hysteresis associated with their charge/discharge cycles. This originates from multiple pathways of the conversion reactions between metal oxide and Li ions and complexities in phase transformation of active particles, resulting in unregulated reaction pathways during battery charge-discharge cycles. Thus, maintaining an uninterrupted ion/electron transport across electrodes together with a high mechanical resilience as battery is charged and discharged, remains a challenging task till to date.
Rationale Behind Our Metal Oxide-CNT Electrode Design
One way to mitigate poor ion/electron transport and electrode stability that causes capacity decay is to design electrodes that can couple the active particles (iron oxide) and conductive additive (carbon) at a single particle level with adequate control over particle morphology and carbon-metal oxide interface. In our urchin like structures (FexAl1-xOy-MWCNT), the core is made of an alloy of Al-Fe-O that acts as active particles while CNTs play a dual role by acting as in situ conductive additive and mechanical spacer for active particles that typically tend to swell and disintegrate during charge-discharge cycles. The major advantages of our design are that (i) it in principle can offer high electrical accessibility of active particles for effective conversion reactions, during which the volume expansion of particles is efficiently accommodated by void space and the mechanical buffer effect of CNTs, (ii) the compaction of urchins in electrodes results in a porous network conducive to better electrolyte immersion, and (iii) CNTs are efficient heat dissipaters that can potentially avoid thermal runway of batteries.
Electrical Characterization
Method:
Swagelok-type coin cells were fabricated using standard procedures to test the performances of the invented material versus lithium metal (half cell configuration) and versus a common commercially available cathode material (full cell configuration). In both cases the CNTSU electrode was fabricated according to the steps shown in
We found that the 2nd cycle capacity was almost retained when the current density was doubled.
We carried out cycling voltammetry (CV), which can give information on the actual electrodes processes (i.e. conversion reactions between urchins and Li+ ions that produce current/voltage). In
Lastly, we assembled full cells in which urchins served as anodes and LNCO (a commercial cathode material) served as cathodes.
In conclusion of this section, sea urchin-like (FexAl1-x Oy)-MWCNT structures were tested in coin type half cells with Li metal and in full cells with lithium nickel cobalt oxide (Sigma-Aldrich). Urchin electrodes operate through a main voltage plateau around about 1V vs. Li+/Li which delivered a capacity of about 1800 mAh/g in the first cycle. Although this capacity was faded to almost 50% due to the commonly attributed SEI, it quickly stabilises in successive cycles yielding a reversible capacity (about 700 mAh/g) that is as twice as the capacity of the commercial graphite anodes. Constant current charge/discharge profiles show that urchin electrodes exhibit good cyclability and CV data confirm the reversible capacity.
It is possible therefore to produce anode electrode materials permitting the following performance:
Capacity: up to 2800 mAh·g−1 (including 1st cycle)
Rate (charge/discharge): upto 30 C (1 C=˜1000 mA/g)
Operation V Vs. Li+/Li: 1-3 V
Full cell voltage range vs. LNCO and vs. LiFePO4: 1-4.2 V
In preferred embodiments, the full cell has the following construction:
Anode: as described above.
Cathode: LNCO (lithium nickel cobalt oxide) and LiFePO4 (commercial), or NCA (lithium nickel cobalt aluminium oxide) or NMC (lithium nickel cobalt manganese oxide) or LCO (lithium cobalt oxide)
Electrolyte: LiFP6 in polycarbonates and LiTFSI in polycarbonates (commercial)
Binder: PDF, carboxymethyl cellulose, binder-free
Separator: Poly propylene (Cell guard), Whatman glass microfiber (commercial), paper, ceramic
A control experiment was also carried out, in which the anode material consisted only the core particles (not the sea urchin structures) mixed with CNTs as conductive additive.
The present inventors consider that the irreversible capacity loss in the first cycle, as shown in
In terms of the scaled-up costs for the battery manufacture, the fact that CNTSUs can be synthesised via an aerosol process is a definite cost advantage.
Safety and extreme temperature range have not been measured at the time of writing, but in principle this should be better than graphite and silicon anodes owing to the fundamental nature of conversion reactions taking place in the core particles.
Overall, based on our experimental data and our understanding of the behaviour of conversion-type active materials, one can expect the current performances of our CNTSU material to be as depicted in
The present inventors have carried out further investigations into the spray-combustion process for forming the core particles, and the characterisation of CNTSUs formed using such as process, leading to further developments from the process schematically illustrated in
For stage 1 of the process (spray drying), a commercial lab scale spray-dryer was used to spray-dry aqueous solutions of metal nitrates and maltodextrin (rather than sucrose) and to collect a dry powder of the same material. Briefly, the liquid feed is atomized into a drying chamber with a flow of hot nitrogen where droplets dry to form solid metal nitrate microparticles. Particles are then continuously entrained into a cyclone where they are collected into a vial. Nitrogen gas is then filtered and dried by a condenser, re-heated in a heat exchanger, and recirculated to the atomizer. A small amount of gas is bled-off to an exhaust and replaced with fresh nitrogen from a compressed gas bottle. At the end of the run (about 15 minutes), the spray-drier is stopped and the powder collected.
Operating parameters were as follows:
This resulted in a nitrogen flowrate of about 50 sL/min
This resulted in an exhaust temperature of about 110° C.
The liquid feed composition was as follows:
In other embodiments, nitrates are replaced by citrates, and/or maltodextrin is replaced by sucrose.
The small amount of water soluble carbon precursor (e.g. maltodextrin) was added to the feed solution with the effect of reducing the hygroscopicity of the dried particles.
For stage 2 of the process (combustion), 300 mg of spray-dried powder was loaded in a ceramic boat, placed in a tube furnace set at T=300° C. with a 1 L/min flowrate of nitrogen gas at atmospheric pressure for 2 hours.
For stage 3 of the process (growing CNTs), 200 mg of the combusted powder was loaded in a ceramic boat, placed in a tube furnace where CNT growth by CVD is performed at T=750° C. Typical CVD growth parameters were:
The resulting sea urchin powder was used as the active material in a Li ion coin cell and tested.
In order to carry out electrochemical testing of sea-urchins with different Al-Fe-O ratios and with different CNT ratios, the core particles were produced using the spray combustion process and subsequently CNTSUs were produced as described above except that:
For comparison, sea-urchins with ZnFe2O4 cores and sea urchins with NCA cores were manufactured.
Commercially available ZnFe2O4 nanoparticle powder was purchased from Sigma Aldrich (<100 nm diameter characterised by BET). 200 mg of the powder was loaded in a ceramic crucible and CVD growth of CNTs was performed in the substrate-based growth CVD furnace as described above. After growth powder was collected and used as active material to be characterised in half cells.
For morphological comparison, of sea-urchins with NCA cores were manufactured. Commercially available Lithium Nickel Cobalt Aluminium Oxide (NCA) powder was purchased. 200 mg of the powder was loaded in a ceramic crucible and CVD growth of CNTs was performed in the substrate-based growth CVD furnace as described above.
Note that this material is a cathode material.
While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.
All references referred to above and/or below are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1618934.2 | Nov 2016 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/078732 | 11/9/2017 | WO | 00 |
