Patent Description:
Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.

Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today; however lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality.

Metal oxides with the general formula AxMO<NUM> (where A represents one or more alkali metal ions and M represents one or more metal ions at least one of which has several oxidation states, for example a transition metal) are known to crystallise in a number of different layered structures. This is described in detail by <NPL>. In summary, the structures are all made up of MO<NUM> edge sharing octahedra which form (MO<NUM>)n sheets. These sheets are stacked one on top of the other and are separated by the alkali metal atoms and the exact position of the alkali metal will dictate whether the overall structure of the metal oxide is to be described as octahedral (O), tetrahedral (T) or prismatic (P). In a lattice made up of hexagonal sheets, there are three possible positions for the oxygen atoms, conventionally named A, B and C. It is the order in which these sheets are packed together that leads to the O, T and P environments. The number <NUM> or <NUM> is also used to describe the number of alkali metal layers in the repeat unit perpendicular to the layering. For example, when the layers are packed in the order ABCABC, an O3 structure is obtained. This translates to <NUM> alkali metal layers in the repeat unit and each alkali metal being in an octahedral environment. Such materials are characterised by the alkali metal ions being in octahedral orientation and typical compounds of this structure are AxMO<NUM> (x≤<NUM>). The order ABAB with the alkali metal ions in tetrahedral orientation will yield a T1 structure which is typified by A<NUM>MO<NUM> compounds. Packing the sheets in ABBA order gives a P2 structure in which one half of the prism shares edges with MO<NUM> octahedra and the other half shares faces and typical compounds are A≈<NUM>MO<NUM>. And finally, packing in ABBCCA order results in a P3 structure type in which all prisms share one face with one MO<NUM> octahedron and three edges with three MO<NUM> octahedra of the next sheet. A≈<NUM>MO<NUM> compounds are found to adopt the P3 structure. It will be noted that the amount of alkali metal present in the AxMO<NUM> material has a direct bearing on the overall structure of the metal oxide.

Further, <NPL>, the preparation and structural properties of layer-type oxides NaxNix/<NUM>Ti<NUM>-x/<NUM>O<NUM>, in which x is in the range <NUM> ≤ x ≤ <NUM>. In particular, these workers disclose that rhombohedral (type O) is observed when <NUM> < x ≤ <NUM> and hexagonal lattice (type P) is observed when <NUM> ≤ x ≤ <NUM>, and that both structure types O and P are present as a mixture when the product is made in a solid state process at around <NUM> (approximately <NUM>).

Over the last ten years, numerous workers have investigated the electrochemical properties of single phase metal oxides with either P2 or O3 structures. For example, C. Delmas et al report the phase transformations and electrochemical behaviour of P2-NaxCoO<NUM>, see for example <NPL> and <NPL>. Further, Delmas et al have reported that although layered O3 type materials NaxVO<NUM>, NaxCrO<NUM>, NaxMnO<NUM> and NaxFeO<NUM> are able to host Na-ions upon charge and discharge and have excellent specific capacity performance, they nevertheless suffer significant capacity fading. <NPL>, demonstrate that the P2-layered oxide Na<NUM>/<NUM>[Ni<NUM>/<NUM>Mn<NUM>/<NUM>]O<NUM> can reversibly exchange Na-ions in sodium half cells however, these oxide compounds are expected to show poor cycling ability, especially between <NUM> - <NUM> V at C/<NUM>.

More recently, <NPL> report that the presence of lithium in single phase P2 lithium substituted compounds such as Na<NUM>Li<NUM>Ni<NUM>Mn<NUM>O<NUM>, provides some improvement in the structural stability during cycling, but the reversible capacity of these compounds is still too low due to the limited amount (<NUM>%) of redox active divalent Ni. An attempt to increase the capacity to be closer to the theoretical value of 180mAhg-<NUM> is reported by <NPL>, and involves using Na<NUM>-xLixNi<NUM>Mn<NUM>O<NUM> (Na/Li=<NUM>). During the course of this work, Kim et al note the presence of an intergrowth of P2 and O3 layered phases in this material which they hypothesize, stabilises the crystal structure and leads to improved reversible capacity. The best capacity results are reported for the x=<NUM> compound, which also corresponds as being the compound with the highest percentage of P2. The x=<NUM> material which is O3 stacked, is the lowest performer. In another recent paper by <NPL>, P2-Na<NUM>/<NUM>[Ni<NUM>/<NUM>Mn<NUM>/<NUM>]O<NUM> is reported to exhibit excellent cycling and a high rate capability, however these results are only achieved when the material is charged below <NUM>. 22V; above <NUM>. 22V, the charge capacity in not maintained during cycling due to the phase transformation from P2 to O2.

In conclusion, the metal oxides studied discussed above are hampered either by low specific charge capacity or poor cycling stability especially across a wide range of charge voltages, and as a consequence the commercial application of these compounds in Na ion cells is limited.

The current workers have developed novel electrodes comprising particular doped-nickelate-containing compositions that are capable of delivering excellent specific capacity performance, in conjunction with little or no fading on cycling. Moreover, the doped-nickelate-containing compositions used in the electrodes of the present invention have been found to achieve these excellent results under voltage conditions that would typically result in their phase transformation from P2 to O2; this is a significant improvement over compounds used in the electrodes described in the prior art. Thus the present invention may be used to provide electrodes which are able to be recharged multiple times without significant loss in charge capacity. Advantageously, these electrodes may be used in batteries, especially rechargeable batteries, electrochemical devices and electrochromic devices.

The present invention therefore provides electrodes comprising doped nickelate-containing compositions comprising a first component-type comprising one or more components with an O3 structure together with one or more component types selected from a second component-type comprising one or more components with a P2 structure, and a third component-type comprising one or more components with an P3 structure, and with a weighted average formula represented by the general formula:.

A‴a‴ M<NUM>‴V‴ M<NUM>‴W‴ M<NUM>‴X‴ M<NUM>‴y‴ M<NUM>‴Z‴O<NUM>.

A‴ is preferably selected from either sodium or a mixed alkali metal in which sodium is the major constituent.

M<NUM>‴ comprises one or more metals in oxidation state <NUM>+ selected from manganese, titanium and zirconium; M<NUM>‴ comprises one or more metals in oxidation state <NUM>+ selected from magnesium, calcium, copper, zinc and cobalt; M<NUM>‴ comprises one or more metals in oxidation state <NUM>+ selected from manganese, titanium and zirconium; and M<NUM>‴ comprises one or more metals in oxidation state <NUM>+ selected from aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.

Metals M2'" and M<NUM>‴ may be the same or different metal(s) in oxidation state <NUM>+. Moreover M2'" and M<NUM>‴ are interchangeable with each other.

The doped nickelate-containing compositions used in the electrodes of the present invention are conveniently described by a formula that uses a weighted average of the first component-type, together with one or more of the second and third component-types. For example a doped nickelate-containing composition with a first component-type comprising an O3 compound such as O3- NaNi<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>, and a second component-type comprising a P2 compound such as P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (whereO3:P2 is in the ratio <NUM>:<NUM>) can be described by the following weighted average formula: Na<NUM>N<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>.

It is worth noting that when the doped nickelate-containing compositions are made by chemical mixing, it is likely that the exact structure of each of the components of the first, second and third component-types will, in practice, be determined by whichever is the most thermodynamically stable structure for the O3, P2 and P3 phases, and this will be based on the ratio of the precursor materials used. Thus, in the above example, the O3 and P2 phases may be represented by Na<NUM>-εNi<NUM>±εMn<NUM>±εMg<NUM>±εTi<NUM>±εO<NUM> and Na<NUM>±εNi<NUM>±εMn<NUM>±εMg<NUM>±εTi<NUM>±εO<NUM> respectively, where ε refers to an unknown quantity.

Preferred doped nickelate-containing compositions used in the electrodes of the present invention are described by the following weighted average formulae:.

<NUM>/P2-N<NUM>N<NUM>Mr<NUM>Mg<NUM>T<NUM><NUM><NUM>,.

O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>,.

O3/P2-Na<NUM>Ni<NUM>Mn<NUM>mg<NUM>Ti<NUM>O<NUM>,.

<NUM>/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>,.

<NUM>/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>, and.

<NUM>/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>.

The electrodes of the present invention are suitable for use in many different applications including sodium ion and/or lithium ion and/or potassium ion cells which may be widely used for example in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices. Preferably the electrodes of the present invention may be used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s).

Advantageously, the electrodes of the present invention are cathode electrodes.

In a second aspect, the present invention provides for the use of electrodes that comprise doped nickelate-containing compositions with a weighted average formula represented by the general formula described above, in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices.

In a third aspect, the present invention provides energy storage devices such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices that comprise an electrode comprising doped nickel-containing compositions as described above.

The compositions used in the electrodes of the present invention will be either i) a single compound comprising discrete areas containing one or more components with an O3 structure, together with discrete areas of components with one or both of P2 and P3 structures, or ii) it will be a physical mixture comprising one or more compounds with an O3 structure together with one or more compounds with a P2 and/or a P3 structure, or iii) it will be a mixture of i) and ii).

When making doped nicklate-containing compositions it is possible to convert sodium-ion derivatives into mixed lithium-ion/sodium-ion materials using an ion exchange process.

Typical ways to achieve Na to Li-ion exchange include:.

The present invention will now be described with reference to the following figures in which:.

Any convenient process may be used to make the doped nickelate-containing compositions of the present invention and as described above they may be prepared directly using a chemical reaction between one or more ready-made components of one or more first, second and third component-types. Alternatively, precursors for the one or more components of the first, second and third component types can be caused to react together. Further alternatively a combination of one or more ready-made components for the first, second and third component-types, together with one or more precursors therefor, may be used.

A convenient chemical reaction may use the following general method:.

Alternatively, the doped nickelate-containing compositions may be made with no chemical reaction between the first, second and third component-types, by physically admixing the components (i.e. the ready-made components) of the first, second and third component-types described above. Each of the separate components may be pre-made using the general method described above, and used directly as made from step <NUM>) or step <NUM>) by admixing to produce the doped nickelate-containing compositions used in the electrodes of the present invention.

Table <NUM> below lists the starting materials and heating conditions used to prepare the doped nickelate-containing compositions.

Analysis by X-ray diffraction techniques was conducted using a Siemens D5000 powder diffractometer to confirm that the desired target doped nickelate-containing compositions had been prepared, to establish the phase purity of the product material and to determine the types of impurities present. From this information it is possible to determine the lattice parameters of the unit cells.

The general XRD operating conditions used to analyse the materials are as follows:.

The target doped nickelate-containing compositions were tested using a Na-ion test cell using a hard carbon anode. Cells may be made using the following procedures:
A Na-ion electrochemical test cell containing the active material is constructed as follows:.

The positive electrode is prepared by solvent-casting a slurry of the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P (Timcal). PVdF is used as the binder, and N-methyl-<NUM>-pyrrolidone (NMP) is employed as the solvent. The slurry is then cast onto aluminium foil and heated until most of the solvent evaporates and an electrode film is formed. The electrode is then dried under dynamic vacuum at about <NUM>. The electrode film contains the following components, expressed in percent by weight: <NUM>% active material (doped nickelate-containing composition), <NUM>% Super P carbon, and <NUM>% PVdF binder.

The negative electrode is prepared by solvent-casting a slurry of the hard carbon active material (Carbotron P/J, supplied by Kureha), conductive carbon, binder and solvent. The conductive carbon used is Super P (Timcal). PVdF is used as the binder, and N-Methyl-<NUM>-pyrrolidone (NMP) is employed as the solvent. The slurry is then cast onto aluminium foil and heated until most of the solvent evaporates and an electrode film is formed. The electrode is then dried further under dynamic vacuum at about <NUM>. The electrode film contains the following components, expressed in percent by weight: <NUM>% active material, <NUM>% Super P carbon, and <NUM>% PVdF binder.

The cells are tested as follows, using Constant Current Cycling techniques.

The cell is cycled at a given current density between pre-set voltage limits. A commercial battery cycler from Maccor Inc. (Tulsa, OK, USA) is used. On charge, alkali ions are extracted from the cathode active material. During discharge, alkali ions are re-inserted into the cathode active material.

<FIG> shows the X-ray diffraction pattern of the known material Na<NUM>Ni<NUM>Mn<NUM>O<NUM> (sample number X1657). The pattern shows that this material conforms to a layered P2-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a Na<NUM>Ni<NUM>Mn<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaCIO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

During the cell charging process, sodium ions are extracted from the cathode active material, and inserted into the Hard Carbon anode. During the subsequent discharge process, sodium ions are extracted from the Hard Carbon and re-inserted into the cathode active material.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // Na<NUM>Ni<NUM>Mn<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is large indicating the relatively poor kinetic reversibility of the Na-ion extraction-insertion reactions in this cathode material.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// Na<NUM>Ni<NUM>Mn<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates relatively poor capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1659). The pattern shows that the sample conforms to a layered P2-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate (PC). The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

During the cell charging process, sodium ions are extracted from the cathode active material, and inserted into the Hard Carbon anode. During the subsequent discharge process, sodium ions are extracted from the Hard Carbon and re-inserted into the cathode active material. <FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represent a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1663). The pattern shows that the sample conforms to a layered P2-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a P2-Na<NUM>Ni<NUM>Ti<NUM>Mg<NUM>Mn<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate (PC). The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // P2-Na<NUM>Ni<NUM>Ti<NUM>Mg<NUM>Mn<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// P2-Na<NUM>Ni<NUM>Ti<NUM>Mg<NUM>Mn<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of Na<NUM>Ni<NUM>Mg<NUM>Mn<NUM>O<NUM> (sample number X1684). The pattern shows that the sample conforms to a layered P2-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a P2-Na<NUM>Ni<NUM>Mg<NUM>Mn<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaCIO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // P2-Na<NUM>Ni<NUM>Mg<NUM>Mn<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// P2-Na<NUM>Ni<NUM>Mg<NUM>Mn<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1713). The pattern shows that the sample conforms to a layered P2-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of the known material Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1714). The pattern shows that the sample conforms to a layered O3-type structure.

The data shown in <FIG> are derived from the constant current cycling data for a O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test demonstrates reasonable capacity retention behaviour.

The data shown in <FIG> are derived from the constant current cycling data for a physically mixed active cathode comprising (<NUM> mass % P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> and <NUM> mass % O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>) in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // (<NUM> mass % P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> and <NUM> mass % O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>) cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon//(<NUM> mass % P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> and <NUM> mass % O3-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM>). For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represents a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1682). The pattern shows the presence of both P2-type and O3-type structures.

The data shown in <FIG> are derived from the constant current cycling data for a mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaClO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the first <NUM> charge/discharge cycles of the Hard Carbon // mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. The cathode specific capacity has improved by around <NUM> % over the first <NUM> cycles. The cathode material under test clearly demonstrates outstanding capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1692). The pattern shows the presence of both P2-type and O3-type structures.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1696C). The pattern shows the presence of both P2-type and O3-type structures.

The data shown in <FIG> are derived from the constant current cycling data for a mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cathode active material in a Na-ion cell (Cell#<NUM>) where this cathode material was coupled with a Hard Carbon (Carbotron P(J)) anode material. The electrolyte used was a <NUM> solution of NaCIO<NUM> in propylene carbonate. The constant current data were collected at an approximate current density of <NUM> mA/cm<NUM> between voltage limits of <NUM> and <NUM> V. To ensure that the Na-ion cell was fully charged, the cell was potentiostatically held at <NUM> V at the end of the constant current charging process until the current density dropped to <NUM>% of the constant current value. The testing was carried out at <NUM>.

<FIG> shows the constant current cycle life profile (i.e. the relationship between Cathode Specific Capacity for Discharge [mAh/g] and cycle number for the Hard Carbon// mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. For cycle <NUM> the discharge specific capacity for the cathode is about <NUM> mAh/g. This represent a capacity fade of about <NUM> % over <NUM> cycles or an average of <NUM> % per cycle. The cathode material under test clearly demonstrates excellent capacity retention behaviour.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number X1700). The pattern shows the presence of both P2-type and O3-type structures.

<FIG> shows the cell voltage profile (i.e. Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for <NUM> charge/discharge cycles of the Hard Carbon // mixed phase O3/P2-Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> cell. These data demonstrate that the level of voltage hysteresis (i.e. the voltage difference between the charge and discharge processes) is small, indicating the excellent kinetic reversibility of the Na-ion extraction-insertion reactions. In addition, the generally symmetrical nature of the charge/discharge voltage profile confirms the excellent reversibility of the extraction-insertion reactions.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number S0842). The pattern shows the presence of both P3-type and P2-type structures.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number S1430A). The pattern shows the presence of both P3-type and P2-type structures.

<FIG> shows the X-ray diffraction pattern of the weighted average formula Na<NUM>Ni<NUM>Mn<NUM>Mg<NUM>Ti<NUM>O<NUM> (sample number S1458B). The pattern shows the presence of P3-type, P2-type and O3-type structures.

Claim 1:
An electrode comprising a mixed-phase doped nickelate-containing composition comprising a first component-type comprising one or more components with an O3 structure together with one or more component types selected from a second component-type comprising one or more components with a P2 structure, and a third component-type comprising one or more components with a P3 structure, represented by a weighted average formula:

        A‴a‴ M<NUM>‴V‴ M<NUM>‴W‴ M<NUM>‴X‴ M<NUM>‴y‴ M<NUM>‴Z‴ O<NUM>

wherein
A‴ comprises one or more alkali metals selected from sodium, lithium and potassium;
M<NUM>‴ is nickel in oxidation state <NUM>+,
M<NUM>‴ comprises one or more metals in oxidation state <NUM>+,
M<NUM>‴ comprises one or more metals in oxidation state <NUM>+,
M<NUM>‴ comprises one or more metals in oxidation state <NUM>+, and
M<NUM>‴ comprises one or more metals in oxidation state <NUM>+
wherein
<NUM> ≤ a‴ < <NUM>;
<NUM> < v‴ < <NUM>;
at least one of w‴ and y‴ is > <NUM>;
x‴ ≥ <NUM>;
z‴ ≥ <NUM>;
wherein a‴, v"', w"', x"', y‴ and z‴ are chosen to maintain electroneutrality.