GB2588254A - Li/Na-ion battery anode materials - Google Patents

Li/Na-ion battery anode materials Download PDF

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GB2588254A
GB2588254A GB2002487.3A GB202002487A GB2588254A GB 2588254 A GB2588254 A GB 2588254A GB 202002487 A GB202002487 A GB 202002487A GB 2588254 A GB2588254 A GB 2588254A
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active electrode
electrode material
active
material according
anode
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GB2588254B (en
GB202002487D0 (en
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S Groombridge Alexander
De La Verpilliere Jean
Santhanam Sumithra
Zhang Wanwei
E Houck Maurits
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Echion Technologies Ltd
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Echion Technologies Ltd
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Publication of GB202002487D0 publication Critical patent/GB202002487D0/en
Priority to GB2008352.3A priority Critical patent/GB2588264B/en
Priority to GB2011681.0A priority patent/GB2595745B/en
Priority to PCT/GB2020/052486 priority patent/WO2021074593A1/en
Priority to EP20793069.4A priority patent/EP4046215A1/en
Priority to PCT/GB2020/052485 priority patent/WO2021074592A1/en
Priority to EP20793068.6A priority patent/EP4046214A1/en
Priority to US17/769,716 priority patent/US20220384797A1/en
Priority to BR112022007307A priority patent/BR112022007307A2/en
Priority to CA3157162A priority patent/CA3157162A1/en
Priority to KR1020227016652A priority patent/KR20220105637A/en
Priority to CN202080072727.4A priority patent/CN114930570A/en
Priority to CA3157452A priority patent/CA3157452A1/en
Priority to JP2022522980A priority patent/JP2023501888A/en
Priority to US17/769,717 priority patent/US20220380226A1/en
Priority to BR112022007117A priority patent/BR112022007117A2/en
Priority to US17/769,720 priority patent/US20220384798A1/en
Priority to EP20793067.8A priority patent/EP4046213A1/en
Priority to CN202080071972.3A priority patent/CN114946047A/en
Priority to KR1020227016654A priority patent/KR20220105638A/en
Priority to PCT/GB2020/052487 priority patent/WO2021074594A1/en
Priority to CA3166174A priority patent/CA3166174A1/en
Priority to JP2022523003A priority patent/JP2023501778A/en
Priority to JP2022521991A priority patent/JP2023501077A/en
Priority to CN202080072794.6A priority patent/CN114868278A/en
Priority to KR1020227016651A priority patent/KR20220103946A/en
Priority to BR112022007323A priority patent/BR112022007323A2/en
Publication of GB2588254A publication Critical patent/GB2588254A/en
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    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
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Abstract

An active electrode material disclosed in the application is expressed by the general formula [M1]x[M2](1-x)[Nb]y[O]z, wherein M1 and M2 are different. In this one particular aspect the material is selected from the group consisting of [M1xMo(1-x)]Nb12O(33-33α), [M1xW(1-x)]Nb12O(33-33α), [M1xV(1-x)]Nb9O(25-25α), [M1xZr(1-x)]Nb24O(62-62α), [M1xW(1-x)]7Nb4O(31-31α) and [M1xW(1-x)]9Nb8O(47-47α), wherein M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Si, Sb, Y, La, Ta, Zn or Cd, and wherein 0 < x < 0.5 (i.e. M1 comprises the minor proportion in the mixed cation) and 0 ≤ α ≤ 0.05 (i.e. the material may or may not be oxygen depleted). Another disclosed aspect relates to an oxygen-depleted mixed niobium oxide selected from the group consisting of MoNb12O(33-33α),WNb12O(33-33α), VNb9O(25-25α), ZrNb24O(62-62α), W7Nb4O(31-31α) and W9Nb8O(47-47α), wherein 0 < α ≤ 0.05 (i.e. the material must be oxygen depleted). It is disclosed that adapting known mixed niobium oxides to either include the mixed cations of the first aspect or to be oxygen-depleted as in the second aspect improves electrochemical behaviour. Such materials can be used as electrode active materials in lithium-ion or sodium-ion batteries.

Description

Li/Na-ion battery anode materials
Field of the Invention
The present invention relates to electrode active materials and to methods for the manufacture of electrode active materials. Such materials are of interest for example as electrode active materials in lithium ion or sodium ion batteries.
Background
Lithium ion (Li-ion) batteries are a commonly used type of rechargeable battery with a global market estimated at $40bn in 2018 and predicted to grow to $200bn by 2030. This large market is divided between various applications, ranging from transport and utility-scale energy storage to consumer electronics and power tools. Accordingly, rechargeable (secondary) Li-ion batteries are currently under intense research and development to improve their performance to reach industrial demands of the technology [Goodenough and Park (2013)]. In particular, Li-ion batteries are the technology of choice for electric vehicles that have multiple demands across technical performance to environmental impact, providing a viable pathway for a green automotive industry.
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 if necessary with an electrically conductive additive (such as carbon) and a polymeric binder. A slurry of these components is coated as a thin film on a current collector (typically a thin foil of copper or aluminium), thus forming the electrode upon drying.
In the known Li ion battery technology, the poor rate capability of graphite anodes upon battery charging is a serious impediment to its application in high-power electronics, automotive and industry. Among a wide range of potential alternatives proposed recently, Si, Si alloys and lithium titanate oxide (LTO), and niobium oxide-based materials are the main contenders to replace graphite as the active material of choice for high power applications.
Battery charge rate is usually expressed as a "C-rate". 1 C charge rate means a charge current such that the battery is fully charged in lh, 10 C charge means that the battery is fully charged in 1/10th of an hour (6 minutes).
Batteries relying on a graphitic anode are fundamentally limited in terms of charging rate. Under nominal conditions, lithium ions are inserted into the anode active material upon charging. When charging rate increases, typical graphite voltage profiles are such that there is a high risk that overpotentials lead to the potential of the anode to become < 0 V vs. Li/Li+, which leads to a phenomenon called lithium dendrite electroplating, whereby lithium ions instead deposit at the surface of the graphite electrode as lithium metal. This leads to irreversible loss of active lithium and hence rapid capacity fade of the cell. In some cases these dendritic deposits can grow to such large sizes that they pierce the battery separator and lead to a short-circuit of the cell. This can trigger a catastrophic failure of the cell leading to a fire or an explosion. Accordingly, the fastest-charging batteries having graphitic anodes are limited to charging rates of 5-70, but often less. Despite this, graphitic anodes accounted for >90% of the Li-ion battery market in 2018.
Si and Si alloys offer large specific capacities but suffer from poor lifetime when charged and discharged at high rates, as well as poor capacity retention when increasing rates from low rates (e.g. 0.5 C) to higher rates (e.g. 5 C). This is due to inhomogeneous lithiation of the active material particles upon charging due to limited diffusion rates of lithium ions within the particles. The core of the active material particles (typically 1-20 pm spheres) may not have time to be lithiated upon fast charge as lithium ions need to diffuse to it from the particle surface, hence poor capacity retention when increasing charge rate.
Moreover, Si and Si alloys active materials physically expand up to 400% by volume upon lithiation.
Inhomogeneous particle lithiation therefore leads to internal mechanical stresses within the particles, which can lead to their break up and electrode pulverisation, hence the poor cycle life of these electrodes upon fast charging.
Lithium titanate oxide (LTO) anodes do not suffer from dendrite electroplating at high charging rate thanks to their high potential (1.6 V vs. Li/Li+), and have excellent cycle life as they do not suffer from volume expansion upon lithiation. LTO cells are typically regarded as high safety cells for these two reasons. However LTO is a relatively poor electronic and ionic conductor, which leads to limited capacity retention at high rate, unless the material is nanosized to increase specific surface area, and carbon-coated to increase electronic conductivity. This particle-level material engineering increases material particle cost, and decreases the tapped-density of the active material LTO powders. This is significant because it leads to low density electrodes and a higher fraction of electrochemically inactive material (e.g. binder, carbon additive).
A key measure of anode performance is its volumetric capacity (mAh/cms), that is, the amount of electric charges (that is lithium ions) that can be stored per unit volume of the anode. This is an important factor to determine the overall battery energy density on a volumetric basis (VVh/L). Volumetric capacity can be approximated as the product of electrode density, active material specific capacity, and fraction of active material in the electrode. LTO anodes typically have relatively low specific capacities (c. 170 mAh/g, to be compared with c. 330 mAh/g for graphite), which, combined with their low electrode densities (typically 1.9 g/cms) and low active material fractions (<87%) discussed above, lead to very low volumetric capacities (<300 mAh/cm3) and therefore low battery energy density and high $/kWh cost. As a result, LTO batteries/cells are generally limited to specific niche applications, despite their long cycle life, fast-charging capability, and high safety.
Mixed niobium oxides (MNO) were first identified as potential battery materials in the academic literature in the 1980's [Cava et al. (1983); Cava et al. (1984)], but generated limited interest at the time because of the lack of commercially available cathodes to match their rate performance.
Interest in MNO anodes was revived in the early 2010's, with the demonstration of a practical cell combining a TiNb207 anode and a commercially-available LNMO cathode showing promising performance in terms of rate capability, cycle life, and energy density [Goodenough and Park (2013)]. Selected MNO anodes such as TiNb207 offer characteristics that are similar to LTO in terms of high operating potential vs. Li/Li+ (1.6V) and low volume expansion (<5%) leading to safe fast-charge and long cycle life (>10,000 cycles). A key advantage of MNO anodes is that specific capacities significantly higher than LTO can be achieved (e.g. c. 300 mAh/g for TiNb204, which improves cell energy density.
However, electronic conductivities are typically too low in MNO materials such as TiNb207 to sustain fast charge rates without particle engineering and carbon coatings, which is a limitation similar to that of LTO.
On the other hand, it was recently shown that other MNOs such as Nb2O5, also studied in the 1980's for batteries, which typically have a so-called "Wadsley-Roth" or "bronze" crystal structure, can offer extremely fast lithium ion diffusion rates 10-14_10-10 sm2 s-1 (LTO typically 10-17 cm2s-1) [Griffith et al. (2016)]. This can potentially improve on electrode density (i.e. >2.5 g/cm3), and thus in volumetric capacity (>600 mAh/cms) and cell energy density. However, several challenges limit the commercial deployment of these materials, such as low electronic conductivity, lifetime issues as a "micromaterial" (crystals on the order of 1-10 pm in size), and "sloping" lithiation voltage profiles. A lithiation voltage profile refers to the shape of the anode potential vs. amount of lithium inserted in the anode. LTO and TiNb207 have a "flat" voltage profile whereas materials such as Nb205 typically have a "sloping" voltage profile. Voltage profiles that are too sloping lead to large voltage windows which make full cell balancing challenging in a commercial cell.
TiNb207 in particular has further limitations to its application in high power battery technologies. It has a limited Li-ion diffusion rate (8.0x1 0-16mc 2 s-1, ) as compared to other MNOs (eg. Nb12Mo033= 4.0x1 0-14 CM2 5-1) [Zhu 2019], which limits its performance at high power. In particular this limits the utilisation of the pseudocapacitive charge storage mechanism, a key benefit to the use of MNOs for high power [Yang 2017].
US915319B2 discusses TiNb207 and contemplates but does not exemplify modifications of this material.
However the feedstocks and processes used in this disclosure are expensive (furnace treatments to 1500°C up to 50 h), and the materials produced show low initial Coulombic efficiency (84.7%, 86.5%).
The present invention has been devised in light of the above considerations.
Summary of the Invention
The present inventors have realised that, despite the apparent challenges presented by the prior art, it is possible to provide an active electrode material which overcomes some or all of the problems presented by prior art materials discussed above.
Accordingly, in a first aspect, the present invention provides an active electrode material expressed by the general formula [M1].[M2](1,0[Nb]y[O]z, wherein: M1 and M2 are different; M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; M2 represents one or more of Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; and wherein x satisfies 0< x < 0.5; y satisfies 0.5 y 5 49 z satisfies 4 z 124 A material in which M1 and M2 are different can also be referred to as a mixed cation active materials, or a complex oxide active material. These terms are used interchangeably in the present disclosure to refer to a material of the general formula as set out above. Such materials may offer improved electrochemical properties in comparison to non-mixed cation active materials (e.g. materials having the general formula [M]x[Nb][0]1, where M represents a single ion.
As set out above, M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd. M2 represents one or more of Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd. By 'represents one or more of, it is intended that either M1 or M2 may each represent two or more elements from their respective lists. An example of such a material is Tioo5W025Moo 5oNbi2035. Here, M1 represents Ti.Wx. (where x' + x" = x), M2 represents Mo, x=0.3, y=12, z=33. Another example of such a material is Tioo5Zr005W0 25 MOO 65Nb12033. Here, M1 represents TixZr>eWx. (where x' + x" + x-= x), M2 represents Mo, x=0.35, y=12, z=33.
M2 does not represent Ti. In other words, preferably, Ti is not the major non-Nb cation in the active electrode material. Where M1 represents Ti alone, preferably x is 0.05 or less. Where M1 represents one or more cations including Ti, preferably the amount of Ti relative to the total amount of non-Nb cations is 0.05:1 or less.
Preferably, M2 is selected from one or more of Mo, W, V, or Zr. The present inventors have found that when M2 is selected from one of these elements, the active electrode material may have improved electrochemical properties.
As x satisfies 0 < x < 0.5, M2 is the major non-Nb cation in the active electrode material. Preferably x satisfies 0.01 x 0.4, more preferably x satisfies 0.05 x 0.25, for example, x may be about 0.05.
The precise values of y and z within the ranges defined may be selected to provide a charge balanced, or substantially charge balanced, crystal structure. Additionally or alternatively, the precise values of y and z within the ranges defined may be selected to provide a thermodynamically stable, or thermodynamically metastable, crystal structure.
In some cases, z may be defined in the format z=(z'-z'a), where a is a non-integer value less than 1, for example where a satisfies 0 5 a 0.05. a may be greater than 0, i.e. a may satisfy 0 <a 5 0.05. When a is greater than 0, the active electrode material is an oxygen deficient material, i.e. the material has oxygen vacancies. Such a material would not have precise charge balance, but is considered to be "substantially charge balanced" as indicated above. Altemafively, a may equal 0, in which case the material is not an oxygen deficient material.
When a is 0.05, the number of oxygen vacancies is equivalent to 5% of the total oxygen in the crystal structure. In some embodiments, a may be greater than 0.001 (0.1% oxygen vacancies), greater than 0.002 (0.2% oxygen vacancies), greater than 0.005 (0.5% oxygen vacancies), or greater than 0.01 (1% oxygen vacancies). In some embodiments, a may be less than 0.04 (4% oxygen vacancies), less than 0.03 (3% oxygen vacancies), less than 0.02 (2% oxygen vacancies), or less than 0.1 (1% oxygen vacancies). For example, a may satisfy 0.001 a 0.05. When the material is oxygen deficient, the electrochemical properties of the material may be improved, for example, resistance measurements may show improved conductivity in comparison to equivalent non-oxygen deficient materials. As will be understood, the percentage values expressed here are in atomic percent.
The oxygen deficiency (e.g. expressed as the percentage of oxygen vacancies) in a material can be measured by e.g. thermogravimetric analysis (TGA) in an oxygen-rich atmosphere, by measurement of how the mass of the sample changes over time due to re-inclusion of oxygen in the oxygen vacancies. Alternatively or additionally, the oxygen deficiency can be qualitatively measured by assessing the colour of a material relative to a non-oxygen deficient sample of the same material. For example, non-oxygen deficient MoNb12033 has a white, off-white, or yellow colour. Oxygen-deficient MoNb120,33 has a purple colour. On production of an oxygen deficient crystal of MoNbi20,33 a colour change from white/offwhite/yellow to purple can be observed.
Preferably, the active electrode material is a material selected from the group consisting of: M1 xM0(1-x)N b120(33-33 a) Ml x1A/(1-x)N b120(33-33a) M1 xV(1-x)N b90(25-25 a) Ml xZr(1-x)N b240(62-62 a) Ml.W(l_x)Nbo 570(4 43-4.43 a) MlxW(1_x)Nbo 890(5 22-5.22 a) where M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd: and wherein x satisfies 0< x <0.5; and a satisfies 0 5a 0.05.
In a second aspect, the present invention provides an active electrode material expressed by the general formula [M].[Nb]y[0](z'_z..), selected from the group consisting of: MoNb120(33-33 WNbi 20(33-33a) VNb90(25-25 a) ZrNb240 (62-62 a) W7Nb40(31-31 0) WON b80(47-47 a) wherein a satisfies 0 <a 0.05 Materials according to the second aspect of the invention are oxygen deficient analogues of known 'base' materials MoNbi2033, WNID12033, ZrNb24062, VNID9025,W7N134031, and W9Nb8047. The comments set out above in relation to materials according to the first aspect specifying possible ranges for a when z is defined as z=(z'-z'a) also apply here to materials of the second aspect of the invention. For example, a may satisfy 0.001 So S 0.05.
The present inventors have found that by modifying materials such as MoNbi2033, VVNb12033, ZrNb24062, VNID9025, W7Nb403i, and W9Nb8047 by either incorporating multiple non-Nb cations to form mixed cation active materials/ complex oxide active materials (as per the first aspect of the invention), and/or by creating an oxygen deficiency (as per the second aspect of the invention), it is possible to create active electrode materials having improved electrochemical properties, and in particular improved electrochemical properties when used as an anode material.
The specific capacity/reversible delithiation capacity of active electrode materials according to the invention may be 200 mAh/g or more, 225 mAh/g or more, 250 mAh/g or more, up to about 300 mAh/g or more. Here, specific capacity is defined as that measured in the 2nd cycle of a half cell galvanostatic cycling test at a rate of 0.050 with a voltage window of 1.1-3.0V vs Li/Lit. It may be advantageous to provide materials having a high specific capacity, as this can provide improved performance in an electrochemical device comprising the active electrode material.
Furthermore, active electrode materials according to the invention may have an appropriate voltage profile as measured in the 2" cycle of a half cell galvanostatic cycling test at a rate of 0.050 with a voltage window of 1.1-3.0V vs Li/Li+. That is, the capacity of the material may be between 2.0 V and 1.1 V upon lithiation >180 mAh/g, and the capacity of the material may be between 1.1 V and 2.0V upon delithiation >180 mAh/g.
When formulated or coated as an electrode (optionally with conductive carbon additive and binder materials), the bulk resistivity of active electrode materials according to the invention, may be 5 kf2.cm or less, more preferably 2 kacm or less. Bulk resistivity can be a useful proxy measurement of the electronic conductivity of such materials. It may be advantageous to provide materials having a suitably low bulk resistivity, as this can provide improved performance in an electrochemical device comprising the active electrode material.
The direct current internal resistance (DCIR) and resultant area specific impedance (ASI) of the active electrode material when measured in a Li-ion half coin cell with the described electrode may be 900 or less (for DCIR) and 170 0cm2 or less (for ASI). It may be advantageous to provide materials having a suitably low DCIR and/or ASI, as this can provide improved performance in an electrochemical device comprising the active electrode material. However, further improvements in DCIR/ASI values may be seen for e.g. carbon coated active electrode materials, or where the active electrode material is incorporated in a commercial power cell with a cathode, with an electrode which has been calendared and prepared in a typical known manner. When measured in such an arrangement in a coin cell, the inventors theorise that the ASI may be as low as e.g. 26 0cm2 or less.
Active electrode materials according to the invention may have a lithium diffusion rate of greater than 1 0-14 cm2 s-1. It may be advantageous to provide materials having a suitably high lithium diffusion rate, as this can provide improved performance in an electrochemical device comprising the active electrode material.
Active electrode materials according to the invention may have an electrode density of 2.5 g/cms or more after calendaring. For example, electrode densities of up to 3.0 g/cm3 or more after calendaring have been achieved. It may be advantageous to provide materials having such an electrode density, as this can provide improved performance in an electrochemical device comprising the active electrode material.
Specifically, when the electrode density is high, high volumetric capacities can be achieved, as gravimetric capacity x electrode density x active material fraction = volumetric capacity.
The initial coulombic efficiency of active electrode materials according to the invention may be greater than 88%, more preferably greater than 90%. In some cases the initial coulombic efficiency of the active electrode material may be as high as 92% or more, 93% or more or 94% or more. It may be advantageous to provide materials having a suitably high initial coulombic efficiency, as this can provide improved performance in an electrochemical device comprising the active electrode material.
Further optional features of the first and second aspects of the invention are set out below.
Preferably, the crystal structure of the active electrode material, as determined by X-ray diffraction analysis, corresponds to the crystal structure of one or more of: MoNb12033 WNbi 2033 ZrNb24062 VNI39025 W7Nb4031 W9Nb8047.
Here the term 'corresponds' is intended to reflect that peaks identified by X-ray diffraction analysis of the active electrode material may be shifted by no more than 0.5 degrees (preferably shifted by no more than 0.2 degrees, more preferably shifted by no more than 0.1 degrees) from corresponding peaks in an X-ray diffraction analysis of one or more of MoNb12033 VVNI312033,ZrNb24062,VNID9025,W7N134031, and/or W9Nb8047. Preferably the crystal structure of the active electrode material does not correspond to the crystal structure of TiNb207, for example, preferably the measured XRD diffraction pattern of the active electrode material does not correspond to the JCPDS crystallography database entry database 00-0391407, for TiNb207.
At least some of the active electrode material may have a Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze (TTB) crystal structure. Preferably, the majority of the active electrode material has a Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze (TTB) crystal structure, for example, at least 50%, at least 60%, at least 70%, at least 80% or at least 90% of the active electrode material by volume may have a Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze (TTB) crystal structure. In preferred embodiments, substantially all of the active electrode material may have a Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze (TTB) crystal structure. When the material has such a crystal structure it may have improved electrochemical properties.
The crystal formula of a charge balanced and thermodynamically stable Wadsley-Roth crystal structure obeys the following formula: (1) (M1, M2, M3, *** )rnnp+103ninp-O-n+n)p+4 In this formula, 0 is oxygen (the anion) and M (the cation) is any combination of elements selected from Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd. In materials according to the invention, at least one of (M1, M2, M3...) comprises Nb.
Formula (1) is based on crystal topography: m and n are the dimensions of the formed edge sharing superstructure blocks, ranging from 3 -5 (integers). At the corner, blocks are connected into infinite ribbons (p=co) only by edge-sharing, into pairs (p=2) by partly edge-sharing and partly tetrahedra or into isolated blocks only by tetrahedra (p=1). When p is infinity the formula becomes: (2) (M1, M2, M3, *** )mn°3mn-(m+n) Together, formula (1) and (2) define the full composition samples for Wadsley-Roth crystal structures. Preferably the total crystal composition should also be charge neutral and thermodynamically favourable. More information can be found in work by Griffith et al. (2017).
Reference to tetragonal tungsten bronze (TTB) crystal structures (or simply 'bronze' structures) in the present disclose refers to tetragonal tungsten bronze (TTB) structures with partially filled tunnels. As described in Montemayor 1998, such phases consist in a framework of NbOs octahedra sharing corners linked in such a way that three, four and five sided tunnels are formed. A number of 5-sided tunnels are filled with W, Nb, 0, or a suitable metal cation to form the structure.
The active electrode material may further comprise Li and/or Na. In other words, the active electrode material may be a lithiated and/or a sodiated active electrode material. The active electrode material may be expressed by the general formula [Li]A[M1]x[M2]0,0[Nb]y[0]z or [Na]x[M1]x[M2]0_,0[Nb]y[0]z, where x, y, and z satisfy the ranges discussed above, and wherein A is selected to provide a charge balanced, or substantially charge balanced, crystal structure, and/or a thermodynamically stable, or thermodynamically metastable, crystal structure.
The active electrode material may comprise a plurality of primary crystallites (sometimes referred to as microcrystals or microcrystallites). The average diameter of the primary crystallites may be from 10 nm to pm, preferably from 100 nm to 5 pm, although the most desirable diameter for the primary crystallites may depend on their intended use. For example, where the active electrode material is intended for use in ultra-high power products, it may be advantageous for the primary crystallite size to be low, e.g. 50 nm or less, or 30 nm or less. Where the active electrode material is intended for use in developing "high energy power cells", it may be advantageous for the crystallite size to be higher, e.g. 5 pm or more, or 7 pm or more.
Some or all of these primary crystallites may be agglomerated into secondary particles. Alternatively, the primary crystallites may be substantially non-agglomerated. Where some or all of these primary crystallites are agglomerated into secondary particles, the average diameter of the secondary particles is from 1 pm to 30 pm, preferably from 2 pm to 15 pm, although the most desirable diameter for the secondary particles may depend on their intended use. For example, where the active electrode material is intended for use in ultra-high power products, it may be advantageous for the secondary particle size to be low, e.g. 4 pm or less, 2 pm or less, or 1.5 pm or less. VVhere the active electrode material is intended for use in developing "high energy power cells", it may be advantageous for the secondary particle size to be higher, e.g. 8 pm or more, 12 pm or more, or 15 pm or more. The secondary particles may be porous.
The average diameter of the primary crystallites and/or the secondary particles may be measured using any conventional known technique, for example using SEM imaging to examine a sample of the material, selecting a number (n) of primary crystallites and/or secondary particles, and calculating the average diameter as the mean diameter of the n primary crystallites/secondary particles measured, e.g. where n equals 30.
An alternative method for measuring the secondary particle size is using solid state powder laser diffraction, for example using a Horiba laser diffraction particle analyser for dry powder with air pressure maintained at 0.3 MPa.
The active electrode material may comprise a carbon coating formed on the surface of the primary crystallites and/or secondary particles. Some suitable methods for forming carbon coatings on the surface of the primary crystallites and/or secondary particles are set out in literature e.g. Zhou (2012). Other suitable methods are described below. The carbon coating may be present in an amount of up to 5 w/w°/0, based on the total weight of the active electrode material. The carbon coating may comprise graphitic carbon.
Where the active electrode has a morphology of a plurality of primary crystallites where some or all of these primary crystallites are agglomerated into porous secondary particles, the secondary particles may comprise a coating of carbon formed at least at the surfaces of pores in the secondary particles.
In a third aspect, the present invention provides an electrochemical device comprising an anode, a cathode and an electrolyte disposed between the anode and the cathode, wherein the anode comprises an electrode active material according to the first or second aspect of the invention.
The electrolyte may be a liquid electrolyte. Alternative or additionally the electrolyte may be a solid state electrolyte.
The anode may further comprise a conductive additive and/or a binder. For example, the anode may have a composition of about 80 wt% active material, about 10 wt% conductive additive, and about 10 wt% binder. Alternatively the anode may have a composition of about 91 wt% active material, about 5 wt% conductive additive, and about 4 wt% binder. The amount of active electrode material in the anode may be in a range from 70 wt% to 99 wt%, more preferably in a range from 75 wt% to 98 wt%, even more preferably in a range from 85 wt% to 96 wt%.
In a fourth aspect, the present invention provides a use of an electrode active material according to the first or second aspect of the invention as an anode active material, or a component of an anode active material, in an anode in conjunction with a cathode and an electrolyte in: (i) a lithium ion battery for charging and discharging of the lithium ion battery; or (ii) a sodium ion battery for charging and discharging of the sodium ion battery.
In a fifth aspect, the present invention provides a method for processing an electrode active material according to the first or second aspects of the invention as or in an anode active material for: (i) a lithium ion battery, wherein the method includes diffusing lithium ions into the anode active material; or for (ii) a sodium ion battery, wherein the method includes diffusing sodium ions into the anode active material.
In a sixth aspect, the present invention provides a method of making an active electrode material according to the first aspect of the invention, the method comprising steps of providing one or more precursor materials; mixing said precursor materials to form a precursor material mixture; and heat treating the precursor material mixture in a temperature range from 400°C -1350 °C to form the active electrode material.
Where it is desired to make a material according to the first aspect of the invention, preferably the one or more precursor materials includes an M1 ion source (i.e. a source of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd), an M2 ion source (i.e. a source of Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd), and a source of Nb.
Where it is desired to make a material according to the second aspect of the invention, preferably, the one or more precursor materials includes a source of Mo, W, Zr, or V, and a source of Nb.
The phrase Ml ion source' is used herein to describe a material comprising M1 ions/atoms. The phrase M2 ion source' is used herein to describe a material comprising M2 ions/atoms. The phrase 'a source of Mo/W/Zr/V/Nb' is used herein to describe a material comprising Mo/W/ZrN/Nb ions/atoms, as appropriate.
The precursor materials may include one or more metal oxides, metal hydroxides, metal salts or oxalates. For example, the precursor materials may include one or more metal oxides of different oxidation states and/or of different crystal structure. Examples of suitable metal oxide precursor materials include but are not limited to: Nb205, Nb02, W03, Ti02, Mo03 V205, Zr02, and MgO. However, the precursor materials may not comprise a metal oxide, or may comprise ion sources other than oxides. For example, the precursor materials may comprise metal salts (e.g. NO3-, SO3-) or other compounds (e.g. oxalates). Preferably the one or more precursor materials includes one or more of a Nb source, Mo source, a W source, a Zr source, and/or a V source.
Some or all of the precursor materials may be particulate materials. Where they are particulate materials, preferably they have an average particle size of <20 pm in diameter. The average particle size may be in a range from e.g. 10 nm to 20 pm. Providing particulate materials with such an average particle size can help to promote more intimate mixing of precursor materials, thereby resulting in more efficient solid-state reaction during the heat treatment step. However, it is not essential that the precursor materials have an initial particle size of <20 pm in diameter, as the particle size of the one or more precursor materials may be mechanically reduced during the step of mixing said precursor materials to form a precursor material mixture.
The step of mixing/milling the precursor materials to form a precursor material mixture may be performed by a process selected from (but not limited to): dry or wet planetary ball milling, rolling ball milling, high shear milling, air jet milling, and/or impact milling. The force used for mixing/milling may depend on the morphology of the precursor materials. For example, where some or all of the precursor materials have larger particle sizes (e.g. an average particle size of greater than 20 pm in diameter), the milling force may be selected to reduce the average particle size of the precursor materials such that the such that the average particle size of the precursor material mixture is reduced to 20 pm in diameter or lower. When the average particle size of particles in the precursor material mixture is 20 pm or less, this can promote a more efficient solid-state reaction of the precursor materials in the precursor material mixture during the heat treatment step.
The step of heat treating the precursor material mixture may be performed for a time of from 1 hour to 24 hours, more preferably from 3 hours to 14 hours. For example, the heat treatment step may be performed for 1 hour or more, 2 hours or more, 3 hours or more, 6 hours or more, or 12 hours or more.
The heat treatment step may be performed for 24 hours or less, 18 hours or less, 14 hours or less, or 12 hours or less.
In some methods it may be beneficial to perform a two-step heat treatment. For example, the precursor material mixture may be heated at a first temperature for a first length of time, follow by heating at a second temperature for a second length of time. Preferably the second temperature is higher than the first temperature. Performing such a two-step heat treatment may assist the solid state reaction to form the desired crystal structure.
The step of heat treating the precursor material mixture may be performed in a gaseous atmosphere. The gaseous atmosphere may be an inert atmosphere, or may be a reducing atmosphere. Where it is desired to make an oxygen-deficient material, preferably the step of heat treating the precursor material mixture is performed in an inert or reducing atmosphere. Suitable gaseous atmospheres comprise: air, N2, Ar, He, CO2, CO, 02, H2, and mixtures thereof.
The method may include one or more post-processing steps after formation of the active electrode material.
In some cases, the method may include a post-processing step of heat treating the active electrode material, sometimes referred to as 'annealing'. This post-processing heat treatment step may be performed in a different gaseous atmosphere to the step of heat treating the precursor material mixture to form the active electrode material. The post-processing heat treatment step may be performed in an inert or reducing gaseous atmosphere. Such a post-processing heat treatment step may be performed at temperatures of above 500 °C, for example at about 900 °C. Inclusion of a post-processing heat treatment step may be beneficial to e.g. form deficiencies or defects in the active electrode material, for example to form oxygen deficiencies.
In some cases, the method may include a post-processing step of mixing the active electrode material with a carbon source, and thereby forming a carbon coating on the active electrode material. Optionally, the mixture of the active electrode material and the carbon source may be heated to thereby form the carbon coating on the active electrode material. Suitable carbon sources include but are not limited to: carbohydrate materials (e.g. sugars, polymers); conductive carbons (e.g. carbon black); and/or aromatic carbon materials (e.g. pitch carbon).
One preferred method of forming a carbon coating includes a step of milling the active electrode material with a carbon source, followed by pyrolysis of the active electrode material and carbon source (e.g. in a furnace) under an inert or reducing atmosphere.
Another preferred method of forming a carbon coating includes mixing of the active electrode material with a carbon source, dispersion of the active electrode material and carbon source in an aqueous slurry, followed by spray drying. The resulting powder may optionally be pyrolysed. Where the carbon source is e.g. conductive carbon black, it is not necessary to pyrolyse the material post spray-drying.
In some cases, the method may include a post-processing step of milling the active electrode material to modify the active electrode material particle size. For example, the active electrode material may be treated by one or more processes including air jet milling, impact milling, high shear milling, sieving, or ball milling. This may provide a more suitable particle size for use in desired applications of the active electrode material.
The invention includes the combination of the aspects and preferred features described above except where such a combination is clearly impermissible or expressly avoided.
Summary of the Figures
Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows XRD diffraction patterns of samples 1,4, 14, 2, 5 15, 16, 18 and 22; Figure 2 shows XRD diffraction patterns of samples Band 9; Figure 3 shows XRD diffraction patterns of samples 6, 7, 17, 19 and 20; Figure 4 shows XRD diffraction patterns of samples 10, 11 and 21; Figure 5 shows XRD diffraction patterns of samples 12 and 13; Figure 6 shows TGA characterisation in air of sample 3; Figure 7 shows the particle size distribution of samples 1, 2, 15, and 16; Figure 8 shows the particle size distribution of sample 3; Figure 9 is an SEM image of sample 3 before pyrolysis and coated with conductive Au for imaging; Figure 10 is an SEM image of sample 3 after pyrolysis (no conductive coating); Figures 11 are SEM images of samples 1 and 2; Figure 12 shows representative lithiation and delithiation voltage profiles obtained by galvanostatic cycling in half cell configuration, 1.1-3.0 V voltage window, first 2 cycles at 0.050 rate for samples 1 and 16; Figure 13 shows representative lithiation and delithiation voltage profiles obtained by galvanostatic cycling in half cell configuration, 1.1-3.0 V voltage window, first 2 cycles at 0.050 rate for samples 6 and 25 7; Figure 14 shows lithiation and delithiation capacity obtained by galvanostatic cycling in half cell configuration, 1.1-3.0 V voltage window, at current densities of 0.50, 10, 20, 50 (seen as step-changes in the data) for samples 1,4, and 16; Figure 15 shows Lithiafion capacity obtained by galvanostatic cycling in half cell configuration, 1.1-3.0 V voltage window at current densities of 0.50, 10, 20, 5C, 0.50 (seen as step changes in the data) for samples 6, 7, and 17; Figures 16 (a) and (b) show EIS measurements of samples 1, 7, and 16 at different axes scales.
Figure 17 shows the particle size distributions of sample 16 before and after post-processing; Figure 18 is an SEM image of the surface of an electrode made from sample 22, focused on the surface of an active material particle; Figure 19 shows representative lithiation and delithiation voltage profiles obtained by galvanostatic cycling in half cell configuration, 1.1-3.0 V voltage window, first 2 cycles at 0.050 rate for samples 12 and 13.
Detailed Description of the Invention
Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference.
A number of different materials were prepared and characterised, as summarised in Table 1, below. Broadly, these samples can be split into a number of groups: Samples 1, 2, 3, 4, 5, 14, 15, 16, 18, and 22 belong to the same family of Wadsley-Roth phases based on MoNbi2033. Sample 1 is the base crystal structure, which is modified to a mixed metal cation structure by exchanging one or multiple cations in samples 2 to 4, and/or in a mixed crystal configuration (blending with isostructural WN1312033) in samples 14, 15, 16, 18, and 22. Oxygen deficiencies are created in the base crystal in sample Sand in the mixed metal cation structure 18. Sample 3 is a spray-dried and carbon-coated version of the crystal made in sample 2, and sample 22 is a spray-dried and carbon-coated version of the crystal made in sample 16.
Samples 6,7, 17, 19,20 belong to the same family of Wadsley-Roth phases based on ZrNb24062. Samples 8 and 9 belong to the same family of Wadsley-Roth phases based on VVNID12033. Samples 10, 11 and 21 belong to the same family of Wadsley-Roth phases based on VNI39025. Samples 12 and 13 belong to the same family of tetragonal bronzes based on VV7N1b4031.
Sample No. Composition Material Synthesis 1 * MoNb12033 Solid state 2 Tioo5Mo095Nb12033 Solid state 3 Tio osMoo 95Nb 1 2033 + C Solid state, spray dry, carbon pyrolysis 4 Zroo5Mo095Nb12033 Solid state MoNb120<33 Solid state 6 " ZrNb24052 Solid state 7 VoosZro 95N b24062 Solid state 8 " WN1312033 Solid state 9 Tio 05W0 ooNbi2033 Solid state 10* VNI39025 Solid state 11 Tio 05V0.95Nbo025 Solid state 12' VWNID4031 (VVNbo.5704 43) Solid state 13 Tio os1No 95N bo 5704 43 (Tio 35W6.65Nb4031) Solid state 14 Wo.25Moo.75N b12033 Solid state Tio osWo 2sMooloN bi2033 Solid state 16 Tio osZro o5Wo.25Moo e5NI:h2033 Solid state 17 Tio 05.Zro 95N b24062 Solid state 18 Tio osZro osWo.25Moo esN bi 20<as Solid state 19 Moo.o5Zro.95N b24052 Solid state Moo.05V0021-0.95N b24062 Solid state 21 Mo0.05Vo 95N b9025 Solid state 22 Tio 05Zro 05W0.25Moo ooNbi2Oso + C Solid state, spray dry, carbon pyrolysis Table 1: A summary of different compositions synthesised. Samples indicated with * are comparative samples.
Material Synthesis Samples listed in Table 1 were synthesised using a solid-state route. In a first step, metal oxide precursor commercial powders (Nb205, Nb02, Mo03, Zr02, Ti02, W03, V205, Zr02, and/or MgO) were mixed in stochiometric proportions and planetary ball-milled at 550 rpm for 3h in a zirconia jar and milling media with a ball to powder ratio of 10:1. The resulting powders were then heated in a static muffle furnace in air in order to form the desired crystal phase. Samples 1 to Sand 12 to 16, 18 and 22 were heat-treated at 900°C for 12h; samples 6 to 9, 17, 19, and 20 were heat-treated at 1200°C for 12h, with samples 6, 7, 17, 19 and undergoing a further heat treatment step at 1350 °C for an additional 4h; samples 10, 11 and 21 were heat-treated at 1000°C for 12h. Sample 3 and 22 were further mixed with a carbohydrate precursor (such as sucrose, maltodextrin or other water-soluble carbohydrates), dispersed in an aqueous slurry at concentrations of 5, 10, 15, or 20 wiw% with ionic surfactant, and spray-dried in a lab-scale spray-drier (inlet temperature 220°C, outlet temperature 95°C, 500 mL/h sample introduction rate). The resulting powder was pyrolyzed at 600°C for Shin nitrogen. Sample Sand 18 were further annealed in nitrogen at 900°C for 4 hours.
Elemental analysis of Samples Elemental analysis was carried out by Inductively-Coupled Plasma-Optical Emission Spectroscopy (ICPMS/OES). The measurements were carried out on a Thermo Scientific ICP-OES Duo iCAP 7000 series.
The samples were digested using 5 ml Nitric acid and 1 ml I-IF acid and an internal standard was used to account for any instrumental variation. In this process the plasma is used to vaporise the material into its atomic/ionic state of elements. The atoms are in excited state due to high temperature and the decay to normal state through energy transitions. The characteristic radiation emitted by each excited ion is measured for analysis. The results are set out in Table 2, below.
Sample Composition Elemental ratio Expected Measured 1* moNb12033 Nb/Mo 12 12 2 Tio ooMoo osN bi 2033 Mo/Ti 19 18 4 Zro.05Moo.95N1312033 Mo/Zr 19 18 14 Wo.25Mo0.75Nbi2033 Mo/W 3 3.1 3 Tio o5Moo 95N13120o3 + C Mo/Ti 19 18 16 Tio osZro cm \No.2sMOo.00N bi 20so Mo/Zr; Mo/Ti 13; 13 11.4; 13.5 9 Tio oolNo osNb12033 W/Ti 19 18 11 Tio osVo.o5N13902s V/Ti 19 19 17 Tio 0oZro osNb24062 Zr/Ti 19 19 Table 2: Summary of ICP-OES elemental analysis results for samples 1, Z 4, 14, 3, 16, 9, 11, and 17 This table of elemental analysis demonstrates that substantially the expected cation ratio has been achieved for each composition tested.
XRD Characterisation of samples The phase purity of some samples was analysed using Rigaku Miniflex powder X-ray diffractometer in 20 range (10-70°) at 1°/min scan rate.
Figure 1 shows the measured XRD diffraction patterns for samples 1, 4, 14, 2, 5, 15, 16, 18, 22 which are relevant to Comparative Study A. All diffraction patterns have peaks at the same locations (within instrument error, that is 0.1°), and match JCPDS crystallography database entry database JCPDS 73-1322, which corresponds to MoNbi2033. There is no amorphous background noise and the peaks are sharp and intense. This means that all samples are phase-pure and crystalline, with crystallite size -200 nm according to the Scherrer equation and crystal structure matching MoNb12033.
Figure 2 shows the measured XRD diffraction patterns for samples 8 and 9. All diffraction patterns have peaks at the same locations (within instrument error, that is 0.1°), and match JCPDS crystallography database entry database JCPDS 73-1322, which corresponds to VVNID12033. There is no amorphous background noise and the peaks are sharp and intense. This means that all samples are phase-pure and crystalline, with crystallite size -200 nm according to the Scherrer equation and crystal structure matching VVNID12033.
Figure 3 shows the measured XRD diffraction patterns for samples 6, 7, 17, 19, 20 which are relevant to Comparative Study B. All diffraction patterns have peaks at the same locations (within instrument error, that is 0.1°), and match JCPDS crystallography database entry database JCPDS 01-072-1655, which corresponds to ZrNb24062. There is no amorphous background noise and the peaks are sharp and intense.
This means that all samples are phase-pure and crystalline, with crystallite size -200 nm according to the Scherrer equation and crystal structure matching ZrNb24062.
Figure 4 shows the measured XRD diffraction patterns for samples 10, 11, 21. All diffraction patterns have peaks at the same locations (within instrument error, that is 0.1°), and match JCPDS crystallography database entry database JCPDS 00-049-0289, which corresponds to VNI39025. There is no amorphous background noise and the peaks are sharp and intense. This means that all samples are phase-pure and crystalline, with crystallite size -200 nm according to the Scherrer equation and crystal structure matching VNID9025.
Figure 5 shows the measured XRD diffraction patterns for samples 12 and 13. All diffraction patterns have peaks at the same locations (within instrument error, that is 0.1°), and match JCPDS crystallography database entry database JCPDS 00-020-1320, which corresponds to W7N1b4031. There is no amorphous background noise and the peaks are sharp and intense. This means that all samples are phase-pure and crystalline, with crystallite size -200 nm according to the Scherrer equation and crystal structure matching W7N134031 TGA Characterisation of Samples Thermogravimetric Analysis (TGA) was performed on some samples using a Perkin Elmer Pyris 1 system in a synthetic air atmosphere. Samples were first held for 15 min at 30°C, then heated from 30°C to 950°C at 5°C/min, and finally held for 30 min at 950°C. TGA was performed on sample 3 to quantify carbon content, and on sample 5 to show mass increase as oxygen vacancies are filled.
Figure 6 shows TGA characterisation in air of sample 3. The sharp drop in mass between -400°C and 500°C is attributed to the decomposition of the carbon coating. The decomposition temperature corresponds to a mixture of amorphous and graphitic carbon. The amount of mass loss indicates that sample 3 includes 1.1 w.% of carbon coating, which is in line with the amount expected from the stoichiometry of the precursors.
Qualitative assessment of oxygen deficiency As discussed above, sample 5 and 18 were heat-treated at 900°C for 12h to form the active electrode material, and was then further annealed in nitrogen (a reducing atmosphere) at 900°C, in a post-processing heat treatment step. A colour change from white to dark purple was observed after the post-processing heat treatment in nitrogen, indicating change in oxidation states and band structure of the material, as a result of oxygen deficiency of the sample.
Particle Size Distribution analysis of samples Particle Size Distributions were obtained with a Horiba laser diffraction particle analyser for dry powder.
Air pressure was kept at 0.3 MPa. The results are set out in Table 3, below.
Sample Dio [Pm] Dm [Pm] D90 [pm] 1* 3.8 11.2 50.0 2 2.6 10.9 87.4 3.6 21.2 55.3 16 4.7 31.2 82.9 18 5.1 57.7 176 3 before pyrolysis 4.2 8.2 16.3 3 after pyrolysis 6.7 12.7 51.1 16 after impaction milling 1.0 2.6 4.8 18 after impaction milling 1.4 4.4 9.6 Table 3: Summary of particle size distribution statistics for samples 1, 2, 15, 16, 18, 3 before pyrolysis, 3 after pyrolysis, 16 and 18 after post-processing.
Figure 7 shows particle size distributions (measured particle size being secondary particle size, not crystal or crystallite size) for samples 1,2, 15, and 16, as a representative example of particle size distributions obtained by solid state routes in this study without further processing or size optimisation. The particle size distributions are typically bi-modal, with a first mode -10 pm, and a second mode -90 pm. Sample 3 presents significant differences in terms of particle size distribution, as shown in Figure 8 due to the spray-drying and pyrolysis post-processing step.
All particle size distributions can also be refined with further processing steps, for example spray drying, ball milling, high shear milling, jet milling or impact milling to reduce the particle size distribution to the desired range (e.g. d90 <20 pm, <10 pm or <5 pm) as shown in Fig. 17 and Table 3. Typically the particle size distributions are tuned by optimising the phase formation process (i.e. solid state synthesis route) and post-processing steps for the target application. For example, for a Li ion electrode with high power, one would typically target lower average particle sizes, amongst other considerations.
SEM Characterisation of Samples The morphology of some samples was analysed by Scanning Electron Microscopy (SEM).
Figures 9 and 10 show SEM images of sample 3 before and after pyrolysis. A porous microsphere morphology with carbon coating is observed, with primary crystallites organised into secondary particles.
It can be seen that the material has with homogeneous porous particles that can pack efficiently to form a high-density electrode. Qualitatively the conductivity is vastly improved as a conductive coating does not need to be applied for SEM imaging to be carried out, implying an order of magnitude improvement in material surface conductivity. Figure 18 is an SEM image of the surface of a particle in an electrode of sample 22, where conductive carbon black particles contained in the electrode can also be seen in the right side of the image. This visibly shows evidence of a conformal carbon coating around the MNO material.
Figure 11 shows SEM images of samples 1 and 2, and corroborates XRD and PSD data, showing compact secondary particle micron-size particles composed of -200 nm primary crystallites.
Electrochemical Testing of Samples Electrochemical tests were carried out in half-coin cells (CR2032 size) for initial analysis. In half-coin tests, the material is tested in an electrode versus a Li metal electrode to assess its fundamental performance. In the below examples, the active material composition to be tested was combined with N-Methyl Pyrrolidone (NMP), carbon black acting as a conductive additive, and poly(vinyldifluoride) (PVDF) binder and mixed to form a slurry using a lab-scale centrifugal planetary mixer (although it is also possible to form aqueous slurries by using water rather than NMP). The non-NMP composition of the slurries was w.% active material, 10 w.% conductive additive, 10 w.% binder. The slurry was then coated on an Al foil current collector to the desired loading of 1 mg/cm2 by doctor blade coating and dried in a vacuum oven for 12 hours. Electrodes were punched out at the desired size and combined with a separator (Celgard porous PP/PE), Li metal, and electrolyte (1 M LiPF6 in EC/DEC) inside a steel coin cell casing and sealed under pressure. Formation cycling was then carried out at low current rates (C/20) for 2 full charge and discharge cycles. After formation, further cycling can be carried out at a fixed or varied current density as required. These tests have been termed "half-cell galvanostatic cycling" for future reference.
Homogeneous, smooth coatings on current collector foil, the coatings being free of visible defects were also prepared as above with a centrifugal planetary mixer to a composition of 94 w.% active material, 4 w.% conductive additive, 2 w.% binder. The coatings were calendared at 80 °C to a density of up to 3.0 g/cms at loadings of 1.3 -1.7 mAh/cm2 in order to demonstrate possible volumetric capacities > 700 mAh/cms in the voltage range 0.7 -3.0 Vat C/20, and >640 mAh/cms in the voltage range 1.1 -3.0 Vat C/5. This is an important demonstration of these materials being viable in a commercially focussed electrode power cell formulation, where retaining performance after calendaring to a high electrode density allows for high volumetric capacities. Loadings of up to and including 1.0, 1.5, 2.0, 2.5, or 3.0 mAh/cm2 may be useful for Li-ion cells focussed on power performance; loadings greater than 3.0 mAh/cm2 are useful for energy-focussed performance in Li ion cells.
Electrical conductivity of electrodes made with the samples listed in Table 1 was measured using a 4-point probe thin film resistance measurement apparatus. Slurries were formulated according to the procedure described above and coated on a dielectric mylar film at a loading of 1 mg/cm2. Electrode-sized discs where then punched out and resistance of the coated-film was measured using a 4-point probe. Bulk resistivity can be calculated from measured resistance using the following equation: (3) Bulk resistivity (p)=2-rrs(V/1); R=V/I; s=0.1cm = 2-rrx0.1xR (n) The results of this test are shown in Table 4, below: Sample Resistance [kO] Bulk resistivity [kfl.cm] 1" 8.5 5.3 2 1.7 1.1 4 3.2 2.0 0.52 0.33 6* 0.37 0.23 7 0.52 0.33 13 0.45 0.28 14 2.7 17 1.2 0.75 16 1.3 0.82 17 0.34 0.21 18 0.89 0.56 19 0.18 0.11 0.20 0.13 22 0.33 0.21 Table 4 -Summary of 4-point probe resistivity measurement results for samples 1, 2, 4, 5, 6, 7, 13 to 20, and 22.
The direct current internal resistance (DCIR) and the resultant area specific impedance (ASI) is a key measurement of internal resistance in the electrode in a Li-ion cell. In a typical measurement, a cell that has already undergone formation will be cycled at C/2 for 3 cycles. With the electrode in its delithiated state a C/2 discharge current is applied for 1 h to achieve -50% lithiafion. The cell is rested for 30 mins to equilibrate at its OCV (open circuit voltage), and then a 5C current pulse is applied for 10 s, followed by a 30 mins rest to reach the OCV. During the 10 s pulse the voltage response is sampled at a higher frequency to determine the average internal resistance accurately. The resistance is then calculated from V = IR, using the difference between the OCV (the linear average between the initial OCV before the pulse and afterwards) and the measured voltage. The resistance is then multiplied by the area of the electrode to result in the ASI.
The results of this test are shown in Table 5, below: Sample ASI / acm2 1" 141 2 125 4 120 6* 126 7 162 13 67 14 99 16 74 17 162 18 75 19 164 22 121 Table 5 -Summary of DCIR/ASI measurement results for samples 1, 2, 4, 7, 14, 16, and 17.
The reversible specific capacity C/20, initial coulombic efficiency, nominal lithiation voltage vs Li/Li* at C/20, 5C/0.5C capacity retention, and 10C/0.5C capacity retention for a number of samples were also tested, the results being set out in Table 6, below. Nominal lithiation voltage vs Li/Li+ has been calculated from the integral of the V/Q curve divided by the total capacity on the 2nd cycle C/20 lithiation. Capacity retention at 10C and 5C has been calculated by taking the specific capacity at 10C or 5C, and dividing it by the specific capacity at 0.5C. It should be noted that the capacity retention was tested with symmetric cycling tests, with equivalent C-rate on lithiation and de-lithiation. Upon testing with an asymmetric cycling program, 10C/0.5C capacity retention greater than 89% is routinely observed.
Sample Reversible specific capacity C/20 [mAh/g] Initial Nominal lithiation voltage vs Li/Li + [V] 5C/0.5C capacity retention [%] 10C/0.5C capacity retention [%] coulombic efficiency [%] 1* 214 87.8 1.61 62 35 2 240 90.9 1.61 64 45 3 203 84.9 1.58 79 68 4 286 90.7 1.59 68 54 253 86.0 1.60 63 43 6* 224 93.5 1.57 61 38 7 263 93.6 1.58 74 67 8* 192 82.0 1.60 54 36 9 188 86.8 1.61 64 54 10* 172 74.3 1.55 64 54 11 176 71.6 1.59 56 45 12* 164 93.9 1.77 86 81 13 184 95.4 1.75 86 80 14 278 91.0 1.59 228 89.2 1.59 16 281 90.8 1.58 72 58 17 203 94.6 1.58 18 228 90.1 1.59 84 68 19 193 87.0 1.56 63 44 21 169 70.9 1.59 67 56 22 267 86.9 1.57 71 62 Table 6-Summary of electrochemical testing results from Li-ion half coin cells using a number of samples. In general (although not exclusively) it is beneficial to have a higher capacity, a higher /CE, a lower nominal voltage, and higher capacity retentions.
Figures 12, 13, and 19 show representative lithiation/delithiation curves for unmodified and modified MoNbi2033 (Fig. 12-samples 1 and 6) ZrNb24052 (Fig. 13-samples 6 and 7), and W7N1D4031 (Fig. 19 -samples 12 and 13) in their first two formation cycles at C/20 rate. In Fig. 12, approximately 90% of the specific capacity for sample 16 demonstrated is shown to be in a narrow voltage range of ca. 1.2 -2.0 V, and in Fig. 13 approximately 90% of the capacity for sample 7 demonstrated is shown to be in a narrow range of ca. 1.25-1.75 V; these data highlight the attractive voltage profiles achievable with MNO crystals based upon Wadsley-Roth crystal structures. In Fig. 19, approximately 90% of the specific capacity for sample 13 is shown to be in a narrow range of ca. 1.2 -2.2 V; this demonstrates that attractive voltage profiles are achieved with MNO crystals based upon a tetragonal bronze crystal structure. Secondly, the complex metal oxide samples 7, 16, and 13 demonstrate improved specific capacity as compared to their unmodified crystals samples 1, 6 and 12. This is due to the cations that are included in the complex structures increasing the number of sites in the crystal that Li ions can accommodate due to their differing ionic radii and oxidation states, thus increasing capacity. An increase in ICE was observed between samples 1 and 16, and samples 12 and 13, which further demonstrates that Li ions intercalated in the modified crystal structure can be more efficiently delithiated as the Li ion sites are modified to enable their de-intercalation.
Across all materials tested, each material according to the invention demonstrates an improvement versus the unmodified 'base' crystal structure. This is inferred from measurements of resistivity/impedance by two different methods, and also electrochemical tests carried out in Li-ion half coin cells, particularly the capacity retention at increased current densities (c.f. rates, Table 6, Fig. 14 and 15). Without wishing to be bound by theory, the inventors suggest that this is a result of increased ionic and electronic conductivity of the materials as defects are introduced, or by alterations to the crystal lattice by varying ionic radii; also evidenced by DCIR/ASI (Table 5) and EIS (Fig. 16) measurements to show decreased resistance or impedance upon material modification. Li-ion diffusion rates likely also increase in materials according to the invention, as compared with the unmodified 'base' materials.
Specific capacities themselves may also increase in some cases as shown in Table 6, as doping/exchange with metal ions of different sizes can expand or contract the crystal lattice and allow for more intercalation or more reversibility of intercalation of Li-ions than possible in the unmodified structure.
The data in Table 4 show a large reduction in the resistivity between sample 1 (comparative) and samples 2, 4, 5, 14, 15, 16, 18, 22, demonstrating the effect of embodiments of the present invention on improving electrical conductivity of the crystal structures through both cation exchange, oxygen deficiencies, and carbon coating. Samples 17, 19, and 20 also show a similarly low resistivity versus sample 6. The resistivity slightly increased upon incorporation of 0.05 equivalents of V species in the base crystal in sample 7, however an improvement in specific capacity was observed due to the changes in available Li-ion sites in the crystal lattice likely as a result of the differing ionic radius of V over Zr (see
Table 6).
The data in Table 5 shows a large reduction in the DCIR/ASI from sample 1 (comparative) to samples 2, 4, 14, 16, 18 and 22, reflecting the trends shown in Table 4. Samples 7, 17, and 19 demonstrate a higher than these by DCIR, however these relate to a different base crystal structure. Without wishing to be bound by theory, the inventors hypothesise that samples 7, 17, and 19 demonstrate an increase in DCIR/ASI as compared with the comparative material of sample 6 (ZrNb24062) due to the changes in the crystal lattice with the introduced cations of different ionic radii. However, it remains beneficial in terms of conductivity for these structures for samples 17 and 19 as the electrical resistivity is decreased as shown in Table 4, thereby minimising joule heating and enabling a more uniform current distribution across the material, which in turn can enable improved safety and lifetime of a Li ion system. For sample 7, whilst there is no demonstrated improvement utilising V to exchange with Zr, there is an increase in specific capacity, as discussed above.
In Table 6, across most samples there is a trend for improved specific capacities, initial Coulombic efficiencies (ICE), nominal lithiation voltage vs Li/Li, and importantly capacity retention at 5C and 10C vs 0.5C for materials according to the invention versus the comparative 'base' materials (e.g. samples 1, 6, 8, 10, 12). For example samples 2,3, 4, 5, 14, 15, 16, 18,22 all demonstrate improvements in one or more of these parameters vs sample 1. This is also the case for samples 7, 17, 19 versus sample 6 across multiple parameters; sample 11 and 21 versus 10 where an improvement in specific capacity or capacity retention is observed; sample 9 versus 8 where ICE and capacity retention are improved; and sample 13 versus 11 where there are improvements in all parameters.
Figs. 14 and 15 demonstrate improved capacity retention at higher cycling rates for materials according to the invention (samples 4, 16,7, 17) versus the comparative materials (samples 1 and 6).
Electrochemical impedance spectroscopy (EIS) measurements were also carried out to gain a further understanding on the impedance present in the electrode in a Li-ion cell. In a typical measurement, the cell is prepared as for DCIR measurements to -50% lithiation and then the frequency of alternating charge/discharge current pulses is varied whilst measuring the impedance. By plotting the real and imaginary components as the axes, and varying the AC frequency, a Nyquist plot is generated. From this plot for a Li-ion cell different types of impedance in the cell can be identified, however it is typically complex to interpret. For example, Ohmic resistance can be partially separated from electrochemical double layer effects and also separated from diffusion effects.
Fig. 16(a) and (b) show EIS spectra for (comparative) sample 1 and samples 16 and 7 (samples according to the invention). ***
The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof 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.
For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations.
Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.
Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and "include", and variations such as "comprises", "comprising", and "including" will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.
It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment. The term "about" in relation to a numerical value is optional and means for example +/-10%.
References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.
Goodenough and Park, The Li-Ion Rechargeable Battery: A Perspective", Journal of the American Chemical Society 2013 135(4), 1167-1176, DOI: 10.1021/ja3091438 Griffith et al., "High-Rate Intercalation without Nanostructuring in Metastable Nb205 Bronze Phases, Journal of the American Chemical Society 2016 138 (28), 8888-8899, DOI: 10.1021/jacs.6b04345 Griffith et al., "Structural Stability from Crystallographic Shear in TiO2Nb205 Phases: Cation Ordering and Lithiation Behavior of TiNb24062"Inorganic Chemistty (2017), 56, 7, 4002-4010 Montemayor et al., "Lithium insertion in two tetragonal tungsten bronze type phases, M8W9047 (M = Nb and Ta)", Journal of Material Chemistry (1998), 8,2777-2781 Zhou et al., "Facile Spray Drying Route for the Three-Dimensional Graphene Encapsulated Fe2O3 Nanoparticles for Lithium Ion Battery Anodes", Ind. Eng. Chem. Rea (2013), 52, 1197-1204 Zhu et al., "MoNb12033 as a new anode material for high-capacity, safe, rapid and durable Li* storage: structural characteristics, electrochemical properties and working mechanisms", J Mater. Chem. A. (2019),7, 6522-6532 Yang et al., "Porous ZrNb24062 Nanowires with Pseudocapacitive Behavior Achieve High-Performance Lithium-Ion Storage". J. Mater. Chem. A. (2017) 5. 10.1039/C7TA07347J.

Claims (1)

  1. Claims.1. An active electrode material expressed by the general formula [M1]x[M2]0,0[Nb]y[O]z, wherein: M1 and M2 are different; M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; M2 represents one or more of Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; and wherein x satisfies 0 <x < 0.5; y safisfies 0.5 y5 49 z satisfies 4 z 124 2. The active electrode material according to claim 1, wherein M2 is selected from one or more of Mo, W, V, or Zr.3. The active electrode material according to claim 2 wherein the [M1]x[M2]0_,0[Nb]y[0]z is a material selected from the group consisting of: M1xMo0_x)N bi20(33-33 a) Ml xW(1-x)Nb120(33-33a) MlxV0_4Nb90(25-25 a) MlxZro.x)Nb240(132-82 a) Ml xW(1-x)Nb0.570(4 43-4.43 a) Ml.V1/(l-.)N b0.890(5 22-5.22 a) where M1 represents one or more of Ti, Mg, V, Cr, W, Zr, Nb, Mo, Cu, Fe, Ga, Ge, Ca, K, Ni, Co, Al, Sn, Mn, Ce, Te, Se, Si, Sb, Y, La, Hf, Ta, Re, Zn, In, or Cd; and wherein x satisfies 0< x< 0.5; and a satisfies 0 0.05.4. The active electrode material according to any one of the preceding claims wherein the active electrode material is oxygen deficient.5. An active electrode material expressed by the general formula [M]x[Nb]y[0](z -in), selected from the group consisting of: MoNb120(33-33 a) WNb120(33 33a) VNb90(25-25 a) ZrNb240(52-62 a) W7Nb40(31-31 a) W9Nb80(47-47 a) wherein a satisfies 0 <a 0.05.6. The active electrode material according to any one of the preceding claims wherein at least some of the material has a Wadsley-Roth crystal structure and/or a tetragonal tungsten bronze crystal structure.7. The active electrode material according to any one of the preceding claims wherein the active electrode material comprises a plurality of primary crystallites, some or all of the primary crystallites optionally being agglomerated into secondary particles.8. The active electrode material according to claim 7, wherein the average diameter of the primary crystallites is from 10 nm to 10 pm.9. The active electrode material according to claim 7 or claim 8, wherein some or all of the primary crystallites are agglomerated into secondary particles, and the average diameter of the secondary particles is from 1 pm to 30 pm.10. The active electrode material according to any one of the preceding claims wherein the active electrode material comprises a carbon coating formed on the surface of the primary crystallites and/or secondary particles.11. The active electrode material according to claim 10 wherein the carbon coating is present in an amount of up to 5 w/w%, based on the total weight of the active electrode material.12. An active electrode material according to any one of the preceding claims wherein the crystal structure of the active electrode material, as determined by X-ray diffraction analysis, corresponds to the crystal structure of one or more of MoNbi2033 VVNbi2033 ZrNb24062 VN139025 W7N134031 W9Nb8047.13. An active electrode material according to any one of the preceding claims, further comprising Li and/or Na.14. An electrochemical device comprising an anode, a cathode and an electrolyte disposed between the anode and the cathode, wherein the anode comprises an electrode active material according to any one of claims 1 to 13.15. A use of an electrode active material according to any one of claims 1 to 13 as an anode active material, or a component of an anode active material, in an anode in conjunction with a cathode and an electrolyte in: (i) a lithium ion battery for charging and discharging of the lithium ion battery; or (ii) a sodium ion battery for charging and discharging of the sodium ion battery.16. A method for processing an electrode active material according to any one of claims 1 to 13 as or in an anode active material for (i) a lithium ion battery, wherein the method includes diffusing lithium ions into the anode active material; or for (ii) a sodium ion battery, wherein the method includes diffusing sodium ions into the anode active material.17. A method of making an active electrode material according to any one of claims 1 to 13, the method comprising steps of: providing one or more precursor materials; mixing said precursor materials to form a precursor material mixture; and heat treating the precursor material mixture in a temperature range from 400 °C -1350°C to form the active electrode material.18. The method of making an active electrode material according to claim 17 wherein the one or more precursor materials includes a source of Mo, W, Zr, or V, and a source of Nb.19. The method of making an active electrode material according to claim 17 or claim 18 wherein the one or more precursor materials includes an M1 ion source, an M2 ion source, and a source of Nb, and wherein the resulting active electrode material is a material as defined in any one of claims 1 to 4, or claims 6 to 13 as dependent from claim 1.20. The method of making an active electrode material according to claim 17 wherein the precursor materials include one or more metal oxides, metal hydroxides, metal salts or oxalates.21. The method according to any one of claims 17 to 20 wherein the one or more precursor materials are particulate materials, optionally having an average particle size of <20 pm in diameter.22. The method according to any one of claims 17 to 21 wherein the step of mixing said precursor materials to form a precursor material mixture is performed by a process selected from dry or wet planetary ball milling, rolling ball milling, high shear milling, air jet milling, and/or impact milling.23. The method according to any one of claims 17 to 22 wherein the step of heat treating the precursor material mixture is performed for a time of from 1 to 14 h. 24. The method according to any one of claims 17 to 23 wherein the step of heat treating the precursor material mixture is performed in a gaseous atmosphere, the gas being selected from air, N2, Ar, He, CO2, CO, 02, H2, and mixtures thereof.25. The method according to any one of claims 17 to 24 wherein the method includes one or more post-processing steps selected from: (i) heat treating the active electrode material; (ii) mixing the active electrode material with a carbon source, and, optionally, further heating the mixture, thereby forming a carbon coating on the active electrode material; (iii) spray-drying the active electrode material; and/or (iv) milling the active electrode material to modify the active electrode material particle size.
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