Patent Application
20230216058
The invention relates to electrodes and, high areal capacity electrodes, and the methods of making the same.
To meet trends such as the rise of electric vehicles, significant advances in the energy storage capability of batteries are urgently required. In the dominant field of lithium-ion batteries (LIBs), most research has focused on the development of high capacity electrode materials such as silicon for the anode or lithium sulfide for the cathode. An alternative strategy would be to optimize electrode architecture to maximize the electrode areal capacity (C/A). Maximising this parameter necessitates the ability to fabricate very thick electrodes (thicker than the 100 um maximum utilised in today's batteries) from high-performance active materials while retaining their high specific capacities at large mass loadings. For both anodes and cathodes, C/A=CSP×M/A, where CSP is the specific capacity (mAh/g) of the electrodes and M/A is its mass loading (mg/cm2). Thus, both CSP and M/A must be simultaneously maximized, leading to a number of challenges. First, high-performance electrode materials tend to be granular (i.e. particulate-based) and so display mechanical instabilities above the so-called critical cracking thickness (CCT), making it impossible to prepare very thick electrodes even with polymeric binders. Above a critical thickness, solution-processed particulate films display mechanical instabilities (for example, a mud-cracking effect), which lead to failure, even in the presence of polymeric binders. Secondly, poor conductivity of advanced electrode materials means conductive additives must be incorporated to allow charge distribution within the electrode. However, standard conductive additives such as carbon black (CB) yield low, inhomogeneous and unstable electrode conductivity, limiting electrochemical utilization, rate-behaviour and stability.
Various types of electrode composites exist e.g. Si nanowire composites, Si graphene composites (Anode); LiFePO4, NCM111, LiCoO2 etc. (Cathode) (as described in ‘Li-ion battery materials: present and future’, Materials Today. Volume 18, Number 5, pp. 252-264 (2015)). Various methods to produce electrodes that provide high areal capacity also exist, for example, vacuum filtration, CVD growth, slurry coating onto foam, magnetic templating etc. While a number of methods have been suggested to maximize C/A, none of them are industrially scalable. Many of the preparation methods require complex/non-scalable manufacturing techniques. Most of the scalable preparation methods achieve areal capacities of between 9 mAh cm−2 and 12.5 mAh cm−2. Commercial LIBs have electrodes of ˜50 μm effective thickness.
Hasegawa and Noda (Journal of Power Sources, vol. 321, pp. 155-162 (2016)) describe LIBs without binder or metal foils, based on a three-dimensional carbon nanotube (CNT) current collector using CNTs of 370 μm in length, and with a particle cathode (LiCoO2, 0.5 μm) and an anode (graphite, 10 μm). The full cells typically were 1 wt % CNT electrode-based cells. A discharge capacity of 353 mAh/ggraphite based on the anode weight at 0.1 C and 306 mAh/ggraphite at 1 C were achieved with a capacity retention of 65% even at the 500th cycle. Hasegawa and Noda conclude that a CNT content of ˜1 wt % is preferable because excess CNTs (i.e., 10 wt %) cause additional side reactions because of their large surface area. The thickness of the electrodes are 20-22 μm. Wu et al. (Nano Lett., vol. 14, pp. 4700-4706 (2014)) teaches embedding Li(Ni0.5Co0.2Mn0.3)—O2 in the single wall CNT network, creating a composite in which all components are electrochemically active. Long-term charge/discharge stability was obtained between 3.0 and 4.8 V, and both Li(Ni0.5Co0.2Mn0.3)—O2 and CNT contribute to the overall reversible capacity by 250 and 50 mAh/g, respectively. The electrode thickness was 44 μm.
One potential solution is to increase the areal capacity by increasing the thickness of the electrodes. The electrodes described above in Hasegawa and Noda (2016) and Wu (2014) are not suitable for thick electrodes over 250 μm and at the same time provide a higher areal capacity. However, as discussed above, when above a critical thickness, solution processed particulate films have mechanical instabilities (i.e. they can crack) which leads to failure.
US Patent Application No. 2016/036059 describes a battery including a binder-free cathode that is manufactured by vacuum filtration and which comprises a typical composite material. EP patent Application No. 3361537 describes a battery including an anode of SiO/carbon nanotubes, which includes binders and at least two types of conductors. US Patent Application No. 2016/028075 discloses a battery including a binder-free cathode composite, which is prepared by vacuum filtration. A paper by Hasegawa Kei et al. (Journal of Power Sources, vol. 321, pp. 155-162 (2016)) describes a battery including binder-free electrodes, which are prepared by vacuum filtration. US Patent Application No. 2016/028075 describes a battery including electrodes made of a composite of silicon and copper nanowires, which are prepared by using a sacrificial binder.
It is an object of the present invention to overcome at least one of the above-mentioned problems.
In brief, the invention lies in the use of a spontaneously formed segregated network of carbon nanotubes (CNT) (or the use of metallic nanowires in combination with or instead of the CNTs) to increase the maximum thickness of electrodes. This, in turn, allows for the increase of areal capacitance and therefore, improves energy storage device capacity. The inventors have developed a method to produce extremely thick and high areal capacity composite electrodes by the spontaneous formation of a segregated network composite of CNTs with either silicon or metal oxide particles. The spontaneously formed segregated CNT composite network increases the mechanical properties of the electrode dramatically, suppressing the mechanical instabilities by toughening the composite, and allowing the fabrication of electrodes with thicknesses of up to 2000 μm. Additionally, this spontaneously formed segregated composite CNT network provides very high conductivity, allowing fast charge distribution within the electrodes and enabling very high, record achieving areal capacities of up to 45 and 30 mAh/cm2 for anodes and cathode electrodes, respectively. Combining optimized composite anodes and cathodes yields full-cells with state-of-the-art areal capacities (29 mAh/cm2) and specific energies (540 Wh/kg).
The examples and discussion in the specification around lithium batteries is to provide an example only. The composite of the claimed invention can be formulated with active particulate materials/electrolyte combinations to provide electrodes that can be used in batteries selected from alkaline batteries (zinc manganese oxide, carbon), lithium battery, magnesium battery, mercury battery, nickel oxyhydroxide battery, silver-oxide battery, solid-state battery, zinc-carbon battery, zinc-chloride battery, lithium-ion battery, sodium-ion battery, magnesium ion battery, aluminium ion battery, carbon battery, vanadium redox battery, zinc bromide battery, zinc cerium battery, lead-acid battery, nickel cadmium battery, nickel ion battery, nickel metal hydride battery, nickel zinc battery, polymer-based battery, potassium ion battery, rechargeable alkaline battery, rechargeable fuel battery, silver-zinc battery, silver calcium battery, sodium-sulphur battery, zinc-ion battery, and the like.
According to the present invention there is provided, in one aspect, a composite for use as an electrode, the composition comprising a spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and a particulate active material, in which a polymeric binder or a conductive-additive are excluded.
According to the present invention, there is provided, as set out in the appended claims, a composite for use as an electrode, the composite comprising a spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and a particulate active material, without the need for additional binder, wherein the electrode remains crack free at a thickness of 50 μm or greater. The carbon nanotubes, metallic nanowires or a combination thereof, form a continuous two-dimensional membrane which wraps around the particulate active material and acts as a scaffold to hold the particulate active material in place to form said segregated network.
In one aspect, the electrode has a thickness of at least 100 μm.
In one aspect, the composite comprises from 0.1 wt % to 10 wt % of the spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof.
In one aspect, the carbon nanotubes, metallic nanowires or a combination thereof, are dispersed in either an organic solvent alone or an organic solvent water stabilised with 0.2 wt % to 2 wt % surfactant. Preferably, the surfactant is selected from sodium dodecyl sulfate (SDS), Sodium Dodecyl Benzene Sulphonate (SDBS), octyl-phenol-ethoxylate. Preferably, the solvent is selected from the group comprising n-methyl pyrrolidone (NMP), cyclohexylpyrrolidone, di-methyl formamide, Cyclopentanone (CPO), Cyclohexanone, N-formyl piperidine (NFP), Vinyl pyrrolidone (NVP), 1,3-Dimethyl imidazolidinone (DMEU), Bromobenzene, Benzonitrile, N-methyl-pyrrolidone (NMP), Benzyl Benzoate, N,N′-Dimethylpropylene urea, (DMPU), gamma-Butrylactone (GBL), Dimethylformamide (DMF), N-ethyl-pyrrolidone (NEP), Dimethylacetamide (DMA), Cyclohexylpyrrolidone (CHP), DMSO, Dibenzyl ether, Chloroform, Isopropylalcohol (IPA), Cholobenzene, 1-Octyl-2-pyrrolidone (N8P), 1-3 dioxolane, Ethyl acetate, Quinoline, Benzaldehyde, Ethanolamine, Diethyl phthalate, N-Dodecyl-2-pyrrolidone (N12P), Pyridine, Dimethyl phthalate, Formamide, Vinyl acetate, Acetone etc.
In one aspect, the metallic nanowires consist of silver, gold, platinum, palladium, nickel, or any metal nanowires coated with a thin layer of noble metal (for example, gold or platinum).
In one aspect, the particulate active material is selected from micron-sized silicon powder, lithium, sulphur, graphene, graphite, and lithium nickel manganese cobalt oxide (NMC, LiNixMnyCo0.2O2 where x+y+z=1), lithium cobalt oxide (LCO), lithium nickel cobalt aluminium oxide (NCA), lithium iron phosphate (LFP), lithium titanium oxide (LTO) alloying materials (Si, Ge, Sn, P etc.), chalcogenides (S, Se, Te), metal halides (F, CI, Br, I), and any other suitable battery material. Preferably, the particulate active material is Lithium Nickel Manganese Cobalt Oxide (NMC). Ideally, the NMC is LiNi0.8Mn0.1Co0.1O2 or Li Ni0.5Mn0.3C0.2O2.
In one aspect, the composite comprises from 90 wt % to 99.9 wt % of the particulate active material.
In one aspect, the ratio of the length of the carbon nanotubes, metallic nanowires or combination thereof, to the active material particles is at most 1:1.
In one aspect, the composite described above is for use as an electrode in an energy storage device such as a battery, a supercapacitor, an electrocatalyst, or a fuel cell.
In one aspect, there is provided a composite for use as a crack-free electrode having a thickness of at least 50 μm, the composite comprising a spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and a particulate active material, without the need for an additional binder, wherein the composite comprises from 0.1 wt % to 10 wt % of the spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and wherein the carbon nanotubes, metallic nanowires or a combination thereof, form a continuous two-dimensional membrane which wraps around the said particulate active material and acts as a scaffold to hold the particulate active material in place to form said segregated network. In one aspect, the crack-free electrode has a thickness of at least 100 μm.
In one aspect, there is provided a positive electrode comprising the composite described above. Preferably, the positive electrode has a composition wherein the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.01-25 wt %. Preferably, the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.05-20 wt %.
In one aspect, the positive electrode has a composition wherein the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.25-7.5 wt %.
In one aspect, the positive electrode has a thickness of between 50 μm to 2000 μm, suitably from between 100 μm to 1500 μm; ideally from between 200 μm to 1000 μm, or ideally from between 400 μm to 1000 μm. That is, a thickness selected from 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm, 250 μm, 275 μm, 300 μm, 325 μm, 350 μm, 375 μm, 400 μm, 425 μm, 450 μm, 475 μm, 500 μm, 525 μm, 550 μm, 575 μm, 600 μm, 675 μm, 700 μm, 725 μm, 750 μm, 775 μm, 800 μm, 825 μm, 850 μm, 875 μm, 900 μm, 925 μm, 950 μm, 975 μm, 1000 μm; 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm and 2000 μm.
In one aspect, there is provided a negative electrode comprising the composite described above.
In one aspect, the negative electrode has a composition wherein the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.001-15 wt %. Preferably, the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.01-10 wt %.
In one aspect, the carbon nanotube, metallic nanowires or combination thereof have a mass fraction (Mf) in the electrode of 0.25-7.5 wt %.
In one aspect, the negative electrode has a thickness from between 50 μm to 2000 μm, suitably from between 100 μm to 1500 μm; ideally from between 200 μm to 1000 μm; preferably from between 400 μm to 1000 μm. That is, a thickness selected from 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 125 μm, 150 μm, 175 μm, 200 μm, 250 μm, 275 μm, 300 μm, 325 μm, 350 μm, 375 μm, 400 μm, 425 μm, 450 μm, 475 μm, 500 μm, 525 μm, 550 μm, 575 μm, 600 μm, 675 μm, 700 μm, 725 μm, 750 μm, 775 μm, 800 μm, 825 μm, 850 μm, 875 μm, 900 μm, 925 μm, 950 μm, 975 μm, 1000 μm; 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm and 2000 μm.
In one aspect, there is provided a non-rechargeable battery comprising an anode material, a cathode material, and an electrolyte, wherein the anode material and the cathode material comprise the composite described above.
In one aspect, there is provided a rechargeable battery comprising an anode material, a cathode material, and an electrolyte, wherein the anode material and the cathode material comprise the composite described above.
In one aspect, there is provided a method for producing a positive or negative electrode, the method comprising mixing an aqueous dispersion of carbon nanotubes or metallic nanowires, or a combination thereof, with a particulate active material powder to form a mixture, and depositing the mixture onto a substrate to spontaneously form a segregated network that yields a (robust, flexible) electrode.
In one aspect, the mixture of the carbon nanotubes or metallic nanowires, or combination thereof, with the particulate active material has a viscosity of approximately 0.1 Pa·s at a shear rate of 100 s−1.
In one aspect, the mixture or slurry is dried to form the spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof.
In one aspect, the substrate is selected from glass, semi-conductors, metal, ceramic, Aluminium foil, Copper foil or other stable conductive foils or layers.
In one aspect, the mixture is deposited onto the substrate by any one or more of the following techniques: slurry casting, blade coating, filtration, screen printing, spraying (electrospray, ultrasonic-spray, conventional aerosol spray), printing (ink jet printing or 3D printing), roll-to-roll coating or processing, or drop casting.
There is also provided a use of the composite described above in the manufacture of electrodes.
There is also provided a high areal capacity battery comprising electrodes as described above.
There is also provided a composite for use as a crack-free electrode, the composite comprising a spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and a particulate active material, without the need for an additional binder, wherein the carbon nanotubes, metallic nanowires or a combination thereof, form a two-dimensional membrane dimensioned to wrap around the particulate active material and act as a scaffold to hold the particulate active material in place to form said segregated network.
There is also provided a composite for use as an electrode having a thickness greater than 50 μm, the composite consisting of a spontaneously formed segregated network of carbon nanotubes, metallic nanowires or a combination thereof, and a particulate active material. Preferably, the carbon nanotubes, metallic nanowires or a combination thereof, form a two-dimensional membrane dimensioned to wrap around particles of said particulate active material and act as a scaffold to hold the particulate active material in place to form said segregated network.
In the specification, the term “2-dimensional membrane” should be understood to mean a two-dimensional network of CNT which is very thin but of extremely large area.
In the specification, the term “areal capacity electrode” and “areal capacity battery” should be understood to mean the area-normalized specific capacity of an electrode or a battery, respectively.
In the specification, the term “composite” should be understood to mean a mixture of two or more components that, when combined, provide a mechanically stable and conductive network that have very high mechanical toughness. This toughness allows very thick electrodes to be made because it prevents crack formation (a process analogous to mud-cracking seen in drought conditions on dry river beds, for example, and which occurs during the manufacturing/drying of any particulate films thicker than some critical value) during charging/discharging, which improves stability. In a Li-ion battery, thick electrodes are able to store more Lithium ions and thus yield a higher energy density.
In the specification, the term “electrode active material”, “active material” or “active material particles”, all of which can be used interchangeably, should be understood to mean an oxide material with poor conductivity to which CNTs are added. Such electrode active materials that can be used for positive and/or negative electrodes include silicone, graphene, ceramics (SiO2, Al2O3, Li4Ti5O12, TiO2, CeO2, ZrO2, BaTiO3, Y2O3, MgO, CuO, ZnO, AlPO4, AlF, Si3N4, AlN, TiN, WC, SiC, TiC, MoSi2, Fe2O3, GeO2, Li2O, MnO, NiO, zeolite), LiNixMnyCozO2 (such as LiNi0.7Co0.3O2), LiCoO2, MnO2, LiMn2O4, LiFePO4, and LiMnPO4. When the active material is LiNixMnyCozO2, x+y+z=1. Any particulate material with a particle size greater than the nanotube length is suitable for use in the electrodes of the claimed invention.
In the specification, the term “carbon nanotubes” should be understood to mean single or multiple rolled layers of graphene nanosheets.
In the specification, the term “segregated network” should be understood to mean a network of CNTs (or metallic nanowires or a combination thereof) which spans the entire sample of electrode active material but is locally in the form of a porous, two-dimensional network, segregated on the surfaces of the active material particles, which can wrap the active material particles and act as a scaffold that both holds the active material particles in place and delivers charge to those particles. The segregated network can be likened to where the CNTs (or metallic nanowires or a combination thereof) are retained at the surface of the active material particles. That is, the dispersed CNTs (or metallic nanowires or a combination thereof) are being restricted to the space between the much larger active material particles in the composite. The segregated network spontaneously forms when the ratio of the length of the CNT (or metallic nanowires or combination thereof; for example, ˜1 μm) to the active material particles in the composite (such as micro-silicon, >1 μm) is in the order of 1:1. In other homogenous composites containing CNTs (or other nanotubes), the electrode active material particles (e.g. nanoparticles or polymer molecules) are much smaller than the CNT length. Under these circumstances, the CNTs form a homogeneous network (with roughly uniform spatial CNT density) and the electrode active material fits into the spaces between the CNTs. Thus, these two scenarios result in network structures which are quite different. While CNTs are used herein to illustrate an example of the claimed invention, CNTs can also be replaced by, or combined with, metallic nanowires to form the segregated network that spontaneously creates a two-dimensional membrane around the active material particles. Essentially, the segregated network composite consists of a disordered array of particles (the active material particles) wrapped in a continuous, locally 2D, network of CNT, metallic nanowires or combination thereof, which when collected together, form the architecture of an electrode.
In the specification, the term “metal nanowires” should be understood to mean a rod-like structure with diameter of typically 1-100 nm and length >10 times the diameter. To ensure stability in the electrolyte, the surface of the nanowires should consist of silver, gold, platinum, palladium or nickel.
In the specification, the term “battery” should be understood to mean an electric battery, consisting of cathode, anode, separator and electrolyte, which provided to power electrical devices such as flashlights, smartphones, and electric cars.
In the specification, the term “supercapacitor” should be understood to mean a high-capacity capacitor with capacitance values much higher than other capacitors, which are used in applications requiring many rapid charge/discharge cycles.
In the specification, the term “catalyst” should be understood to mean some special materials/composites, which are used to increase the rate of a chemical reaction but not be consumed in the catalysed reaction and can continue to act repeatedly.
In the specification, the term “fuel cell” should be understood to mean an electrochemical cell that converts the chemical energy from a fuel into electricity through an electrochemical reaction of hydrogen fuel with oxygen or another oxidizing agent.
In the specification, the term “uniformly distributed mixture” should be understood to mean that the CNTs, metallic nanowires or combination thereof and the active particulate material form a homogenous mixture in an aqueous solution. By that, it is meant the CNTs, metallic nanowires or combination thereof, are dispersed uniformly within the space between the active particulate materials. This homogenous mixture is present when the mixture is in the aqueous (slurry) state.
The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—
ϕCNT=ρcomposite·Mf,CNT/ρCNT (1)
where ϕc,e is the electrical percolation threshold (critical volume fraction at which the first conducting path is formed in the matrix). The line is a fit to the percolation scaling law,
σ=σ0(ϕ−ϕc,e)n
where σ0, and ne are the conductivity of the CNT film alone, and the electrical percolation exponent, respectively. The percolation equation fits very well, giving the values of ϕc,e=0.06 vol % (i.e., 0.054 wt %), and ne=1.06. The percolation threshold is much lower than the value observed from CB (ϕc,e=3-25 wt %), supporting well the reason for the much higher electrical conductivity than the conventional electrode even using a very small amount of CNTs.
The conductivities discussed above were measured in the plane of the electrode as is relatively common. However, it is known that out-of-plane conductivity is more relevant to the performance of battery electrodes. Out-of-plane conductivity is seldom reported as it is harder to measure than in-plane conductivity. Out-of-plane conductivity for a subset of 2 um Si/CNT composites. Out-of-plane conductivities of 0.2 S/m for Mf=1% and ˜1 S/m for Mf˜10% were measured. This result is important because once electrode out-of-plane conductivity reaches 1 S/m, performance is optimized in virtually all circumstances, even for very thick electrodes. Here, optimized means that performance is not degraded due to the additional electrical resistance associated with thick electrodes. This is also shown in
(C) Universal tensile stress, (D) Young's Modulus, (E) strain at break, and (F) tensile toughness (energy absorbed up to break) of the NMC samples versus CNT wt % (or PVDF/CB wt %). (G) Mechanical percolation plot showing toughness increase versus ϕ−ϕc,m where ϕc,m is the mechanical percolation threshold and T0 is the toughness of CNT-free film. The lines represent percolation-like scaling behavior via,
T=T
0
+T
net(ϕ−ϕc,m)n
where Tnet is the toughness of CNT film, and nt is the mechanical percolation exponent, respectively. Fitting the toughness data to percolation theory yields: ϕc,m=0.12 vol % (i.e. 0.1 wt %), nt=1.6. Similar to μ-Si/CNT composites, NMC/CNT composites show much higher mechanical properties than those for the conventional electrodes at the equivalent compositions. For example, all the mechanical properties for NMC/10 wt % CNT were >100 times greater than the traditional electrodes at the equivalent electrode compositions (NMC/5 wt % PVDF/5 wt % CB), or even ˜5 times higher at the very low CNT contents (0.2-0.5 wt %) used in electrodes. These results are important as increases in toughness such as these are required to enable the production of very thick (>100 um) crack free electrodes.
Here, it is shown that the production of very thick electrodes which retain almost all of their intrinsic capabilities can be achieved via simultaneously boosting the electrical/mechanical properties of granular electrode materials by using, for example, carbon nanotubes (CNT), specifically in the form of segregated networks, as both binder and conductive additive, without any additional polymer or CB. By adding nanotubes (or metallic nanowires or a combination thereof) to high-performance anode/cathode materials such as silicon and LiNixMnyCozO2(NMC), a new type of hierarchical composite is shown where the nanotubes arrange themselves into networked membranes that envelop the active material particles. These networks significantly improve the mechanical properties, allowing the fabrication of crack free, thick electrodes, and ensure their stability, even during repeated charge/discharge. At the same time the electrode conductivity is dramatically increased, facilitating fast charge distribution, even in thick electrodes. The production of extremely thick electrodes (up to 2000 μm) with CSP approaching theoretical values for each electrode material, enables the production of full cells with areal capacities and specific energies considerably higher than any previous reports. The production involves simple and scalable production techniques, which can be applied to both anodes and cathodes and is compatible with any high-performance electrode material.
Materials and Methods
Slurry-Casting
The hierarchical composite electrodes were prepared via a conventional slurry-casting method using a uniform CNT aqueous dispersion (0.2 wt % SWCNT in water, ˜0.2 wt % PVP as a surfactant, Tuball, OCSiAl) and battery active materials (AMs) without adding any additional polymeric binder or carbon black (CB).
For anodes, the uniform CNT dispersion was mixed with micron-sized Si powder (μ-Si) with a range of particle sizes (1-3 μm: denoted as 2 μm and 5, 10, 30, 45 μm size, all purchased from US Research Nanomaterials) and ground into a uniform slurry using a mortar and pestle. The CNT mass fraction, (Mf) in the resultant electrodes was controlled in the range of 0.05-20 wt % by simply changing the mass ratio between the μ-Si and CNT dispersion. For instance, 8 ml of CNT dispersion was mixed with 200 mg of μ-Si in order to obtain the electrode with 7.5 wt % CNT. Then the slurry was cast onto copper (Cu) foil using a doctor blade, then slowly dried at 40° C. for 2 hours and followed by vacuum drying at 100° C. for 12 hours. Alternatively, the doctor blade film formation step could be replaced by a vacuum filtration step. To remove a surfactant from the as-received CNT dispersion (˜0.2 wt % PVP), the dried electrodes were then heat-treated in Ar gas at 700° C. for 2 hours. By changing the height of doctor blade (200-2000 μm) while keeping the CNT Mf, electrodes with various thickness were obtained, ranging from 30-310 μm (M/A=0.9-21.5 mg/cm2).
For cathodes, NMC (LiNi0.8Mn0.1Co0.1O2, MTI Corp.) was employed to mix with the uniform CNT aqueous dispersion. Typically, 5 ml of CNT was ground with 2 g of NMC to obtain the electrode with 0.5 wt % CNT. The resultant slurry was cast onto aluminium (Al) foil and dried at the same manner. CNT Mf was controlled to be in the range between 0.01-10 wt % and varied electrode thickness, ranging from 40-800 μm (M/A=6-155 mg/cm2). The composite anode/cathode were denoted as “μ-Si/CNT” and “NMC/CNT”, respectively.
For the sake of comparison, nano-sized Si (n-Si) with different sizes (˜25 nm and ˜80 nm, US Research Nanomaterials) were employed to fabricate corresponding anodes by mixing with CNT dispersion in desired compositions and thicknesses.
In addition, traditional electrodes were also prepared using CB (Timical Super C65, MTI Corp.) and conventional binders, either PAA for Si anode or PVDF (EQ-Lib-PVDF, MTI Corp) for NMC cathode. Electrode thickness and composition were also controlled in the same manner.
Material Characterization
The morphology and micro-structure of Si/CNT anodes (or NMC/CNT cathodes) were examined by FE-SEM (Zeiss Ultra Plus, Zeiss) in a high vacuum mode with an acceleration voltage of 5 keV. The mass (M) and thickness (t) of the electrodes were determined using a microbalance (MSA6, Sartorious) and a digital micro-meter after subtracting the mass or thickness of the Al/Cu foils.
The structural properties of samples were characterized by X-ray diffraction (XRD, Bruker D5000 powder diffractometer) with a monochromatic Mo Kα radiation source (A=0.15406 nm). XRD patterns were collected between 10°<θ<80°, with a step size of 2θ=0.05° and a count time of 12 s/step.
Raman spectra of composite electrodes were acquired using a Witec Alpha 300 R with a 532 nm excitation laser and a spectral grating with 600 lines mm−1. Characteristic spectra were obtained by averaging 20 discrete point spectra for each sample. Raman maps were generated for AM/CNT composites by acquiring 120×120 discrete spectra over an area of 60×60 μm. Maps representing the presence of CNT and AM (either Si or NMC) were generated by mapping the intensity of the CNT G band and the AM's characteristic band (Si at 520 cm−1 or NMC A1g band).
The electrical conductivity of electrodes was measured using a four-point probe technique. The samples were prepared by the same slurry-casting method but they were coated onto a glass plate instead of Al/Cu foils to exclude the substrates' conductivity. Then, four parallel contact lines were deposited on the electrode surface using silver paint (Agar Scientific). The resistance of samples was measured using a Keithley 2400 source meter. The conductivity of samples was then calculated using the samples' geometric information (x-y length and thickness) obtained by digital caliper/micro-meter. The mechanical measurements were conducted from free-standing samples using a Zwick Z0.5 Pro-Line Tensile Tester (100 N Load Cell) at a strain rate of 0.5 mm/min. The films were prepared by simply peeling off the electrodes from the substrate. Each data point was obtained by averaging the results from four measurements.
Electrochemical Characterization
The electrochemical properties of the electrodes were investigated using 2032-type coin cells (MTI Corp.) assembled in an Ar-filled glovebox (UNIlab Pro, Mbraun). Each working electrode was punched into discs with diameter=12 mm. A Celgard 2320 was used as the separator for all coin cells.
For the half-cell electrochemical characterisation, the coin cells were assembled by pairing the working electrode with a Li-metal disc (diameter: 14 mm, MTI Corp.), the latter was used as the counter/reference electrode. 1.2 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate/diethyl carbonate/fluoroethylene carbonate (EC/DEC/FEC, 3:6:1 in v/v/v, BASF) with 2 wt % vinylene carbonate (VC, Sigma Aldrich) was used as the electrolyte for half-cell measurement.
The electrochemical properties of the Si anodes were measured within a voltage range of 0.005-1.2 V using Galvanostatic charge/discharge mode by a potentiostat (VMP3, Biologic). The NMC cathodes were measured within a voltage range of 3-4.3 V in the same manner. It should be noted that the terms between charging/discharging are based on the full-cell LiB system. For the anode, charging/discharging correspond to the lithiation/delithiation process, respectively. The areal capacities (C/A) of the electrodes were obtained by dividing the measured cell capacity by the geometric electrode area (1.13 cm2). To investigate the maximum accessible C/A of the electrodes, the cells were tested at a reasonably slow condition of 1/30 C-rate. The cyclabilities of the electrodes were evaluated at 1/15 C-rate after initial formation cycle at 1/30 C-rate. The discharge rate-capabilities of the electrodes were investigated using asymmetric charge/discharge conditions; the cells were charged at a fixed 1/30 C-rate then discharged at varied rates. For the post-mortem analysis, the cycled cells were carefully disassembled inside a glove box under inert atmosphere. The cycled electrodes were then rinsed with dimethyl carbonate (DMC) several times and dried inside the glove box at room temperature.
The full-cells were assembled by pairing our μ-Si/CNT anodes with NMC/CNT cathodes with various C/A (or M/A). The same sized cathode/anode disc (1.13 cm2) were used to match the C/A of both electrodes, which were previously determined in the half-cell experiments. For full-cells, 1.2 M LiPF6 in ethylene methyl carbonate/fluoroethylene carbonate (EMC/FEC, 95:5 in wt %, BASF) was used as the electrolyte. The N/P ratio, defined by the capacity ratio between the anode and cathode, was balanced to be ˜1.1 (See Table 5 for the details of cathodes/anodes in full-cells). The assembled full-cells were then cycled at 1/15 C within a voltage range of 2.5-4.3 V after the initial formation cycle at 1/30 C-rate. The total C/A of the full-cell, (full-cell C/A), was obtained by dividing the measured cell capacity by the geometric electrode area (1.13 cm2). The rate capabilities of the full cells were investigated using asymmetric charge/discharge conditions; the full cells were charged at ˜ 1/30 C-rate then discharged at varied discharge current densities. For the sake of comparison, a traditional full-cell (C/A=˜3.5 mAh/cm2) was also assembled and tested in the same manner by pairing the NMC cathode (C/A=3.5 mAh/cm2) and graphite anode (C/A=3.8 mAh/cm2) prepared by CB-binder combination.
Results
To produce high-performance electrodes at low cost, micron-sized silicon (μ-Si) particles were chosen as the active material (AM). Such particles combine the high specific capacity of silicon (3579 mAh/g, maximum metastable alloying composition of Li15Si4 at ambient temperature) with a cost that is much lower than silicon nanoparticles (n-Si). However, μ-Si is rarely used due to stability problems. Here it is shown that μ-Si, or indeed other electrode materials, can be fabricated into high-performance composite electrodes by replacing traditional combinations (i.e. polymeric binder and CB-conductivity enhancer) with low-loading networks of CNTs. Importantly, these composite electrodes are produced by a simple, industry-compatible slurry-casting technique, whereby mixing a single-wall CNT aqueous dispersion with μ-Si to form a uniform viscous slurry, allows direct casting onto Al/Cu substrates to yield robust composite films (
These composites contain CNT networks which are very different both to those found in polymer-nanotube composites and those typically reported in papers on battery electrodes. Since the AM particle size (>1 μm for μ-Si) is larger than the nanotube length (˜1 μm), such composites cannot form standard nanotube networks. Rather, the excluded-volume associated with the particles drives the spontaneous formation of a segregated network, where the CNTs spontaneously form networked 2-dimensional membranes which wrap and interconnect the AM particles (
Traditional polymer/CB-loaded battery electrodes combine moderate electrical conductivity with relatively poor mechanical toughness (toughness has been linked to electrode stability). Importantly, in Si/polymer/CB composite electrodes, the inventors found conductivity and toughness to be anti-correlated, making it impossible to prepare conductive yet tough electrodes (see
These composites were found to perform very well as lithium-ion battery anodes (
While all composite anodes were much more stable than traditional polymer/CB-loaded anodes, a significant fall-off in capacity with cycle number for larger μ-Si particle sizes was observed (
At this point, spontaneously formed segregated network composites of 2 μm Si particles mixed with CNT have been shown herein to display high conductivity, toughness and capacity without the high cost associated with nanoscale active materials. Here, these properties are utilized to achieve high-performance anodes. As described above, high areal capacity (C/A) requires large specific capacity (C/M) coupled with high mass loading (M/A). Importantly, the high toughness of the claimed composites should lead to significant increases in CCT. The measurements showed a traditional μ-Si/polymer/CB combination with low toughness to display a CCT of ˜98 μm, leading to severe cracking of thicker films (
This allows the production of extremely thick μ-Si/CNT anodes with record C/A of up to 45 mAh/cm2 (
μ-Si/CNT
μ-Si:CNT =
Total: 14.3 mg/cm
2
~3150 mAh/g @ 0.03 C
45.4
mAh/cm
2
composite by
92.5:7.5
slurry-casting
μ-Si/CNT
Total: 14.3 mg/cm
2
45.4
mAh/cm
2
composite by
slurry-casting
Crucially, this can be achieved while retaining the specific capacity close to its theoretical value (top in
The key to the performance enhancements described above lies in the development of the segregated nanotube/nanowire network to give a porous membrane which wraps the AM particles. Such network formation is not limited to μ-Si AM particles but can be achieved for any particulate material with particle size greater than the nanotube length. This has been demonstrated herein for LiNi0.8Mn0.1Co0.1O2 cathode material (denoted as NMC, see electrical and mechanical properties set out in
NMC/CNT
NMC811:CNT =
Total: 155 mg/cm
2
190 mAh/g @ 1/20 C
29.5
mAh/cm
2
composite by
99.5:0.5
slurry-casting
The ability to produce both anodes and cathodes with very high areal capacities allows one to produce high-performance full-cells. By keeping the anode/cathode thickness ratios at the level required to match areal capacities but increasing the total anode plus cathode mass loading, (M/A)A+c, full-cells were produced with record C/A up to 29 mAh/cm2 (
where V, I, t and A, are cell voltage, applied discharge current, discharge time and the cell electrode area, respectively.
where Mcathode, MAnode and MInactive are the mass for the cathode, anode, and inactive components (Al/Cu foils and separator), respectively. To calculate ESP in a practical way, the total mass of the electrodes as well as the inactive components, including Al/Cu foils and separator, were considered. Here M/A for Al, Cu and separator are 4, 9 and 2 mg/cm2, respectively, thus total MInactive/A is 15 mg/cm2. Then Esp can be calculated from the values for the full-cell areal energy and M/A for the cathode/anode. Considering the mass of both electrodes and inactive components, the specific energy, ESP, for the full-cells, along with high-C/A cells from other studies were calculated and plotted versus the corresponding full-cell areal capacity, (C/A)cell in
where V, CSP,Cathode, CSP,Anode and (M/A)Inactive are the average operating voltage, cathode/anode gravimetric capacity and the inactive components' M/A. Plotting equation 6 for each system shows the curves saturate at high (C/A)Cell at a value determined by CSP of the lower performing electrode, generally the cathode. As shown in
μ-Si/CNT
Total:
Total:
8~29
mAh/cm
2
441~542 Wh/kg
NMC/CNT
44~155 mg/cm
2
3~11.5 mg/cm
2
by slurry-casting
While nanotubes have been used before to improve electrode conductivity, segregated networks—which are much more appropriate to realistic electrode materials—have not been reported, nor have spontaneously formed segregated networks. In addition to unprecedented conductivity enhancement, these spontaneously formed segregated networks dramatically increase mechanical robustness, promoting stability and allowing the production of extremely thick electrodes. In addition, wrapping the AM particles in a flexible, porous 2-dimensional membrane facilitates stable, repeatable, expansion/contraction of the lithium-storing material without impeding lithium diffusion. In addition, these membranes should keep silicon fragments localized even after pulverization, further promoting stability.
Compared to electrodes prepared from traditional mixtures of active material and polymer-binder and conductive-additive, the spontaneously formed segregated networks of the composite of the claimed invention make an electrode comprising said composite having very high mechanical toughness, without cracking. This toughness allows very thick electrodes to be made because it prevents crack formation during charging/discharging (mud-cracking effect) and improves stability. These thick electrodes store more Li ions and yield higher energy density.
Compared to electrodes prepared from traditional mixtures of active material and polymer-binder and conductive-additive, these networks make the electrode have very high conductivity. This conductivity allows the electrode to reach its theoretical Li storage capacity (Li ions per unit mass). The very high conductivity facilitates this maximisation of capacity, even for very thick electrodes.
The segregated network of the composite of the claimed invention spontaneously forms a 2D membrane which acts as a scaffold to hold the active material particles in place.
All the above can be achieved using CNT contents of 0.25-7.5 wt % (or metallic nanowires or a combination thereof). This is much less than the 20 wt % or even 30 wt % of the polymer-binder and conductive-additive used in traditional batteries. The dispersion of the CNT in the composite provides the architecture for an electrode where the filler particles are segregated on the surfaces of the active particles instead of being randomly distributed throughout the bulk of the material. The CNTs spontaneously self-organise or self-arrange themselves around the particulate active materials to form the membranes which are locally 2-dimensional but extend throughout the electrode wrapping all particles. The 2D membranes wrap around the particles and fill space within the electrode architecture. The membranes can be considered to be a fractal object with fractal dimensions of between 2 and 3. The length of the particles are larger than the CNTs (or metallic nanowires) by a factor of, at most, 1:2. Without any (additional) polymeric binders or carbon black in the composite, more active material can be added to the composite to reach the theoretical maximum capacity when used in thick electrodes without cracking.
In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.
The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.
| Number | Date | Country | Kind |
|---|---|---|---|
| 19151026.2 | Jan 2019 | EP | regional |
This Application is a 35 U.S.C. § 371 National Phase Entry of International Patent Application No. PCT/EP2020/050468 filed Jan. 9, 2020, which designates the U.S. and claims benefit of foreign priority under 35 U.S.C. § 119(b) of EP Application Number 19151026.2 filed Jan. 9, 2019, the contents of which are incorporated herein in their entireties by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/050468 | 1/9/2020 | WO |
