Abstract:
The invention relates to doped nickelate-containing compounds comprising A a M 1   V M 2   W M 3   X M 4   y M 5   Z O 2-c  wherein A comprises either sodium or a mixed alkali metal in which sodium is the major constituent; M 1  is nickel in oxidation state greater than 0 to less than or equal to 4+, M 2  comprises a metal in oxidation state greater than 0 to less than or equal to 4+, M 3  comprises a metal in oxidation state 2+, M 4  comprises a metal in oxidation state greater than 0 to less than or equal to 4+, and M 5  comprises a metal in oxidation state 3+ wherein 0≦a&lt;1, v&gt;0, at least one of w and y is &gt;0 x≧0, z≧0 wherein c is determined by a range selected from 0&lt;c≦0.1 and wherein (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to certain novel doped nickelate-containing compounds, to a novel process for making doped nickelate-containing compounds, to electrodes containing doped nickelate-containing compounds, to the use of doped nickelate-containing compounds, for example in energy storage devices, to a process for storing electrical charge using a device comprising doped nickelate-containing compounds, and to energy storage devices, for example rechargeable batteries, comprising doped nickelate-containing compounds. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
         [0004]    NaNi 0.5 Mn 0.5 O 2  is a known Na-ion material in which the nickel is present as Ni 2+  while the manganese is present as Mn 4+ . The material is ordered with the Na and Ni atoms residing in discrete sites within the structure. The nickel ions (Ni 2+ ) are a redox element which contributes to the reversible specific capacity and the manganese ions (Mn 4+ ) play the role of a structure stabilizer. Compound NaNi 0.5 Ti 0.5 O 2  is analogous to NaNi 0.5 Mn 0.5 O 2  in that the Ni 2+  ions provide the active redox centre and the Ti 4+  ions are present for structure stabilization. There is plenty of literature describing the preparation of NaNi 0.5 Mn 0.5 O 2  (and to a lesser extent NaNi 0.5 Ti 0.5 O 2 ) as the precursor for making LiNi 0.5 Mn 0.5 O 2  and LiNi 0.5 Ti 0.5 O 2  by Na→Li ion exchange for Li-ion applications. A direct synthesis method to make these Li materials yields undesirable disordered materials, for example, as a result of the lithium and nickel atoms sharing structural sites. However, recent electrochemical studies reported by Komaba et al Adv. Funct. Mater. 2011, 21, 3859 describe the sodium insertion performance of hard-carbon and layered NaNi 0.5 Mn 0.5 O 2  electrodes in propylene carbonate electrolyte solutions. The results obtained show that although NaNi 0.5 Mn 0.5 O 2  exhibits some reversible charging and discharging ability, the capacity of the material fades by 25% or more, after only 40 cycles. 
         [0005]    It is typically possible to predict the maximum charge capacity for sodium and lithium nickelate compounds based on the Ni 2+  to Ni 4+  redox process, however as Zhonghua Lu and J. R. Dahn, J. Electrochemical Society, 149 (7) A815-A822 (2002) explain, the electrochemical behaviour of cells made using lithium containing compounds, e.g. Li[Ni x Li (1/3−2x/3) Mn (2/3−x/3) ]O 2 , where x=⅙, ¼, ⅓ and 5/12 do not always follow this conventional wisdom. These workers report that when cells containing lithium-nickelate materials are charged at voltages up to 4.45V, this causes lithium to be removed until the Mn oxidation state reaches 4+; thus giving an expected charge capacity of 2×. However, when lithium cells where x&lt;½ are charged to higher voltages, e.g. between 4.5 and 4.7V, they exhibit a long plateau approximately corresponding to 1-2× and subsequent to this plateau, these materials reversibly cycle at capacities over 225 mAh/g. Put simply, lithium-containing compounds of the formula Li[Ni x Li (1/3−2x/3) Mn (2/3−x/3) ]O 2  where the amount of nickel is less than 0.5, exhibit a significantly higher charge capacity than would be expected from conventional theoretical calculation. By contrast, Lu and Dahn note that a similar increase in charge capacity is not observed when x=0.5, i.e. LiNi 0.5 Mn 0.5 O 2 , as there is sufficient nickel present to remove all of the lithium. To explain the higher than expected charge capacity phenomenon, Lu and Dahn demonstrated that their lithium materials undergo a non-reversible loss of oxygen when they are charged to higher voltages, and these oxygen deficient materials then react reversibly with lithium. Notwithstanding this, although this increased charge capacity is a potentially interesting discovery, the commercial utility of such compounds is hindered by the high cost of lithium, as discussed above. 
         [0006]    US2007/0218361 teaches a sodium ion secondary battery which comprises a positive electrode active material which includes a sodium-containing transition metal oxide Na a Li b M x O 2±α . The M includes at least two of manganese, iron, cobalt and nickel. In such materials it is apparently extremely important that the amount of sodium is neither too large (otherwise excess sodium oxide or sodium hydride is produced which causes the positive electrode to become highly moisture absorptive) nor too small. In the latter case the amount of sodium ions that can be intercalated and de-intercalated is said to be reduced and this results in not being able to obtain a high discharge capacity. This prior art describes that the optimum amount of sodium, i.e. the value of a, is preferably 0.6 to 1.1, further preferably 0.9 to 1.1 and more preferably 1. Meanwhile the amount of oxygen is also described as critical to performance. Too much oxygen is said to occupy transition metal and/or alkaline metal sites, and presumably this will hinder re-intercalation of the sodium ions during charge/discharge. Too little oxygen is said to produce material with a crystal structure with lots of defects. The optimum range for a is from 0 to 0.1. Another feature of the specific examples described in US2007/0218361 is that they all contain manganese in oxidation state +3. This is to allow sodium extraction by a manganese oxidation process (Mn +3 &gt;Mn 4+ ). 
         [0007]    The present invention aims to provide novel doped nickelate-containing compounds that are capable of achieving a considerably higher specific charge capacity than would be expected from conventional theoretical calculations. The doped nickelate-containing compounds of the present invention may be used to provide an electrode which is able to be recharged multiple times without significant loss in charge capacity. In particular the doped nickelate-containing compounds of the present invention will provide an energy storage device for example a battery for use in a sodium-ion cell or a sodium metal cell. The present invention also provides a process for storing electrical charge using electrodes comprising one or more of the doped nickelate-containing compounds of the present invention. Further, the present invention provides a process for making doped nickelate-containing compounds, which is extremely straightforward and cost effective to carry out. 
         [0008]    The doped nickelate-containing compounds of the present invention comprise the general formula: 
         [0000]      A a M 1   v M 2   w M 3   x M 4   y M 5   z O 2-c          wherein   A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;   M 1  is nickel in oxidation state greater than 0 to less than or equal to 4+,   M 2  comprises a metal in oxidation state greater than 0 to less than or equal to 4+,   M 3  comprises a metal in oxidation state 2+,   M 4  comprises a metal in oxidation state greater than 0 to less than or equal to 4+, and   M 5  comprises a metal in oxidation state 3+   wherein   0≦a&lt;1, preferably 0≦a&lt;0.6, and further preferably 0.2≦a&lt;1, or 0.2≦a&lt;0.6   v&gt;0, preferably 0&lt;v&lt;0.5   at least one of w and y is &gt;0   x≧0, preferably x&gt;0   z≧0   wherein c is determined by a condition selected from 0&lt;c≦0.1 and 0.05≦c≦0.1   wherein (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.         
         [0024]    Preferred doped nickelate-containing compounds of the present invention comprise the general formula: 
         [0000]      A a M 1   v M 2   w M 3   x M 4   y M 5   z O 2-c          wherein   A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;   M 1  is nickel in oxidation state less than 4+,   M 2  comprises a metal in oxidation state greater than 0 to less than or equal to 4+,   M 3  comprises a metal in oxidation state 2+,   M 4  comprises a metal in oxidation state greater than 0 to less than or equal to 4+, and   M 5  comprises a metal in oxidation state 3+   wherein   0≦a&lt;1, preferably 0≦a&lt;0.6, further preferably 0≦a≦0.5   v&gt;0, preferably 0&lt;v&lt;0.5   at least one of w and y is &gt;0   x≧0, preferably x&gt;0   z≧0   wherein c is determined by a condition selected from 0&lt;c≦0.1 and 0.05≦c≦0.1   wherein (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.         
         [0040]    Particularly preferred doped nickelate-containing compounds of the present invention comprise the formula: 
         [0000]      A a M 1   v M 2   w M 3   x M 4   y M 5   z O 2-c          wherein   A comprises either sodium or a mixed alkali metal in which sodium is the major constituent;   M 1  is nickel in oxidation state 4+,   M 2  comprises a metal in oxidation state 4+,   M 3  comprises a metal in oxidation state 2+,   M 4  comprises a metal in oxidation state 4+, and   M 5  comprises a metal in oxidation state 3+   wherein   0≦a&lt;1, preferably 0≦a&lt;0.6, further preferably 0≦a≦0.5   v&gt;0, preferably 0&lt;v&lt;0.5   at least one of w and y is &gt;0   x≧0, preferably x&gt;0   z≧0   wherein c is determined by a condition selected from 0&lt;c≦0.1 and 0.05≦c≦0.1   wherein (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.         
         [0056]    Metals M 2  and M 4  may be the same or a different metal in oxidation state greater than 0 to less than or equal to 4+. Moreover M 2  and M 4  are interchangeable with each other. When M 2 =M 4  then the doped nickelate-containing compounds of the present invention may be written either as: 
         [0000]      A a M 1   V M 2   W M 3   X M 4   Y M 5   Z O 2-c , 
         [0000]      or 
         [0000]      A a M 1   V M 2   W+Y M 3   X M 5   Z O 2-c , 
         [0000]      or 
         [0000]      A a M 1   V M 3   X M 4   Y+W M 5   X O 2-c ,       and all of these forms of the equation are to be regarded as equivalent.         
         [0058]    Especially preferred doped nickelate-containing compounds of the present invention comprise the formula: 
         [0000]      A a M 1   v M 2   x M 3   x M 4   y O 2-c    
         [0000]    wherein
 
0≦a&lt;1; preferably 0≦a&lt;0.6, further preferably 0≦a≦0.5, advantageously 0≦a≦0.4, highly preferably 0.1≦a&lt;0.6 and conveniently 0.2≦a&lt;1;
 
v&gt;0, preferably 0&lt;v&lt;0.5;
 
at least one of w and y is in the range greater than 0 to less than or equal to 0.5 (i.e. 0&lt;w and/or y≦0.5);
 
0≦x&lt;0.5; preferably 0&lt;x&lt;0.5;
 
z≧0; and
 
wherein c is determined by a condition selected from 0&lt;c≦0.1 and 0.05≦c≦0.1,
 
wherein a, v, w, x, y, z and c are chosen to maintain electroneutrality.
 
         [0059]    The doped nickelate-containing compounds of the present invention preferably comprise sodium alone as the chosen alkali metal. 
         [0060]    Also, in further preferred doped nickelate-containing compounds of the present invention, M 2  comprises a metal selected from one or more of tin, manganese, titanium and zirconium; M 3  comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt; M 4  comprises a metal selected from one or more of tin, manganese, titanium and zirconium; and M 5  comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium. 
         [0061]    In a highly preferred doped nickelate-containing compound of the present invention, (v+w)&lt;0.8, especially when M 2 =Mn and/or Ti. 
         [0062]    Advantageous doped nickelate-containing compounds include: 
         [0000]    Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2-c  
 
Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2-c  
 
Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2-c  
 
Na 0.2 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2-c  
 
Na 0.2 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2-c  
 
Na 0.2 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2-c  
 
Ni 0.325 Mn 0.325 Mg 0.1625 Ti 0.1875 O 2-c  
 
Na 0.2 Ni 0.325 Mn 0.325 Mg 0.1625 Ti 0.1875 O 2-c .
 
Na 0.2 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2-c  
 
and
 
Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2-c .
 
         [0063]    The most preferred doped nickelate-containing compounds are: 
         [0000]    Ni 0.3167 Mn 0.3167 M 3   0.1583 Ti 0.2083 O 2-c ,
 
Na 0.2 Ni 0.3167  Mn 0.3167 M 3   0.1583 Ti 0.2083 O 2-c ,
 
Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2-c  
 
and
 
Na 0.2 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2-c .
 
         [0064]    In all of the above compounds of the present invention, c is in the range 0&lt;c≦0.1. 
         [0065]    The doped nickelate-containing compounds of the present invention may be made using any convenient method such as a chemical process. For example, appropriate starting materials may be mixed together and heated in the presence or absence of a solvent (the latter being a solid state process). The choice and amount of each starting material is preferably selected to provide the elements present in the required doped nickelate-containing compound, and preferably the required doped nickelate-containing compound is formed substantially in the absence of side reaction products and unreacted starting materials. 
         [0066]    Alternatively, doped nickelate-containing compounds according to the invention may be formed using a process comprising the step of charging an electrochemical cell containing one or more compounds of Formula 1 (described below) beyond their conventional theoretical specific capacity as determined by the Ni 2+ /Ni 4+  redox couple, wherein Formula 1 is defined as 
         [0000]      A′ a′ M 1′   v′ M 2′   w′ M 3′   x′ M 4′   y′ M 5′   z′ O 2   (Formula 1)
       wherein   A′ comprises either sodium or a mixed alkali metal in which sodium is the major constituent;   M 1′  is nickel in oxidation state 2+,   M 2′  comprises a metal in oxidation state 4+,   M 3′  comprises a metal in oxidation state 2+,   M 4′  comprises a metal in oxidation state 4+, and   M 5′  comprises a metal in oxidation state 3+   0&lt;a′&lt;1; preferably 0.5&lt;a′&lt;1; conveniently 0.8&lt;a′&lt;1, advantageously 0.833≦a′&lt;1 and most preferably 0.85≦a′&lt;1;   0&lt;v′&lt;0.5;   at least one of w′ and y′ is &gt;0; preferably the range 0&lt;(w′ and/or y′)≦0.5;   0≦x′≦0.5;   z′≧0;   and wherein a′, v′, w′, x′, y′ and z′ are all chosen to maintain electroneutrality.       
 
         [0080]    Compounds of Formula 1 preferably comprise sodium alone as the chosen alkali metal. 
         [0081]    Also in further preferred compounds of Formula 1, M 2′  comprises a metal selected from one or more of tin, manganese, titanium and zirconium; M 3′  comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt; M 4′  comprises a metal selected from one or more of tin, manganese, titanium and zirconium; and M 5′  comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium. 
         [0082]    A′, M 1′ , M 2′ , M 3′ , M 4′ , M 5′ , a′, v′, w′, x′, y′ and z′ in a compound of Formula 1 correspond with A, M 1 , M 2 , M 3 , M 4 , M 5 , a, v, w, x, y and z respectively in a doped nickelate compound produced by the process of the present invention. 
         [0083]    Especially preferred compounds of Formula 1 include:
       Na 0.975 Ni 0.325 Mn 0.325 Mg 0.1625 Ti 0.1875 O 2      Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2      Na 0.925 Ni 0.308 Mn 0.308 Mg 0.1541 Ti 0.229 O 2      Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2      Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2      Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2      Na 0.8 Ni 0.0.267 Mn 0.267 Mg 0.133 Ti 0.333 O 2      Na 0.75 Ni 0.25 Mn 0.25 Mg 0.125 Ti 0.375 O 2      Na 0.7 Ni 0.233 Mn 0.233 Mg 0.117 Ti 0.417 O 2      Na 0.667 Ni 0.222 Mn 0.2222 Mg 0.111 Ti 0.444 O 2          
 
         [0094]    In preferred embodiments, such as in the above compounds according to Formula 1, all of the Ni is in oxidation state 2+ and all of the Mn is in oxidation state 4+. 
         [0095]    The step of charging an electrochemical cell containing one or more compounds of Formula 1 beyond the conventional theoretical specific capacity as determined by the Ni 2+ /Ni 4+  redox couple is typically accompanied by the loss of oxygen from one or more of the compounds of Formula 1, thus the resulting product i.e. the doped nickelate-containing compounds produced thereby may be described to be “oxygen deficient” relative to the compounds of Formula 1. The compounds of Formula 1 are also referred to herein as “active cathode compounds”. 
         [0096]    Thus, a desirable process of the present invention comprises the step of causing at least a partial loss of oxygen from a compound of Formula 1 by overcharging them, that is, by charging them to a specific capacity greater than the conventional theoretical capacity based on the oxidation of Ni 2+  to Ni 4+ . Typically this means charging an electrochemical cell (e.g. the Na-ion electrochemical cell) containing one or more compounds of Formula 1, to at least 4.0V but the actual voltage required will vary slightly for each material. Preferably the overcharging step is the first charge performed on the compounds of Formula 1. However, it is also possible that the overcharging step, and the consequent at least partial oxygen loss, may be realised over several charging steps in order to achieve the required oxygen-deficient doped nickelate-containing compounds of the present invention. In a further embodiment of the present invention, the compounds of Formula 1 may have the structure A′ a′ M 1′   v′ M 2′   w′ M 3′   x′ M 4′   y′ M 5′   z′ O 2±δ , in which A′, a′, M 1′ , v′, M 2′ , w′, M 3′ , x′ M 4′ , y′, M 5′  and z′ are the same as defined above, and O 2±δ  means that this Formula 1 compound is either oxygen rich or oxygen deficient before it is overcharged in the process of the present invention and becomes oxygen deficient or yet more oxygen deficient (respectively depending on whether we start from O 2+δ  or O 2−δ ). The amount of oxygen in A′ a′ M 1′   v′ M 2′   w′ M 3′   x′ M 4′   y′ M 5′   z′ O 2±δ  will be higher than the amount of oxygen present in the compounds of the present invention i.e. A a M 1   v M 2   w M 3   x M 4   y M 5   z O 2-c . 
         [0097]    In line with what would expected from the prior art, charging doped sodium nickelate compounds of Formula 1 up to their conventional theoretical capacity causes the oxidation of Ni 2+  to Ni 4+  and the removal of at least a portion of the sodium ions from the lattice; this process is reversible and the compounds of Formula 1 will be obtained again upon discharge. By contrast, the process of the present invention which comprises charging such compounds above their conventional theoretical capacity, i.e. “overcharging” them, is an irreversible process which produces doped nickelate-containing compounds including the doped nickelate-containing compounds of the present invention. 
         [0098]    Not wishing to be bound by any particular theory, the Applicant currently understands that the enhanced charge capacity (or anomalous charge capacity) observed when compounds of Formula 1 are “overcharged” during the process of the present invention, is due to several changes that occur as the overcharging is taking place. Broadly speaking this is a combination of i) the conventional (or “normal”) charge process that uses just the oxidation of Ni 2+  to Ni 4+ , together with ii) other structural changes that occur exclusively at the higher voltages, such as the removal of a higher than expected (from conventional theory) number of alkali metal ions, e.g. sodium ions, which is typically accompanied by an irreversible partial loss of oxygen to yield O 2-c -containing active materials. 
         [0099]    The present invention therefore provides a process of producing doped nickelate-containing compounds with an increased specific charge capacity than would be expected (as explained above), comprising the steps of i) forming an electrochemical cell (e.g. a Na-ion electrochemical cell) comprising at least one compound of Formula 1, and ii) charging the resulting electrochemical cell above the conventional theoretical capacity based on the oxidation of Ni 2+ /Ni 4+  to thereby produce at least one doped nickelate-containing compound of the present invention with a specific charge capacity that is higher than expected based on the conventional theoretical charge capacity. 
         [0100]    As described above the overcharging process is typically accompanied by at least a partial loss of oxygen, therefore the present invention also provides a process for providing oxygen deficient doped nickelate-containing compounds of the present invention comprising:
       i) forming an electrochemical cell, e.g. a Na-ion electrochemical cell, comprising at least one compound of Formula 1; and   ii) charging the resulting electrochemical cell (Na-ion electrochemical cell) above the theoretical capacity based on Ni 2+ /Ni 4+  to cause the loss of oxygen from the compound of Formula 1 and thereby form adoped nickelate-containing compound that is oxygen deficient.       
 
         [0103]    The mechanism by which oxygen is lost from the compounds of Formula 1 during an overcharging process operates most efficiently when the compounds of Formula 1 comprise layered structures. Layered A x MO 2  materials are known to adopt several, very similar structural types, the most well-known of which have been categorised by Delmas et al.  Physica B+C,  91, (1980), 81, as “O3, P2 and P3”. These notations describe the stacking of the layers; the letter refers to the way in which the oxygen ions are coordinated to the alkali ions, with the letter O denoting an octahedral coordination and the letter P denoting a prismatic coordination. The number refers to the number of MO 2  layers contained in the unit cell. In the case of the uncharged active cathode compounds of Formula 1, it is preferred that they generally adopt a layered α-NaFeO 2  type structure (space group R-3m, no. 166), which has an O3-type crystal structure, although the precise structure will be influenced by the composition of the compound being formed as well as the synthesis conditions. Following charging, these materials are thought to adopt O3, P2 or P3 or modified versions of these crystal structures, although it is also possible that other crystal structures may be adopted. 
         [0104]    The Applicant has demonstrated in Example 7 in the Specific Examples below that when the layered active cathode compounds of Formula 1, are overcharged in an electrochemical cell (e.g. a Na-ion electrochemical cell), they undergo a structural change, which produces a reduction in the volume of the unit cell as compared with both the pristine precursor material, e.g. compounds of Formula 1 before charging, and after charging to their conventional theoretical capacities, e.g. based on Ni 2+  to Ni 4+ . It is believed that such unit cell volume reduction is consistent with the loss of alkali metal-ions and at least partial oxygen loss. Another characteristic of overcharged oxide-containing cathode compounds is that they appear to lose crystallinity (compared with the uncharged material). 
         [0105]    A determination of when a material is said to be “overcharged” is made by observing the voltage curve for the particular material. At capacities less than or close to the conventional theoretical maximum charge capacity (i.e. the Ni 2+ /Ni 4+  redox couple) a first voltage feature is seen. However the curve continues to rise to a second voltage feature upon further or “over” charging. The materials are said to be “overcharged” from the beginning of this second voltage feature. 
         [0106]    Therefore the present invention provides a process of increasing the specific charge capacity of a compound according to Formula 1 comprising forming an electrochemical cell, e.g. a sodium ion electrochemical cell; charging the electrochemical cell (sodium ion cell) and compound according to Formula 1 onto at least a portion of a voltage feature which is observed as a result of the compound of Formula 1 being charged above the conventional theoretical maximum charge capacity. “Charging” in the process of the present invention may be carried out as a single charging process in which the compound of Formula 1 undergoes a single charge to beyond the theoretical maximum charge capacity based on Ni 2+  to Ni 4+  oxidation, or alternatively may involve two or more consecutive stages. In the latter case “charging” may include one or more earlier or first stages in which one or more compounds of Formula 1 are charged in one or more earlier or first charging operations to a charge capacity below or equivalent to its conventional theoretical maximum charge capacity based on Ni 2+  to Ni 4+  oxidation, followed by one or more later or second stages in which the compound from the earlier or first stage is charged in one or more later of second charging operations to beyond the theoretical maximum charge capacity based on Ni 2+  to Ni 4+  oxidation. Alternatively, the compound of Formula 1 may be charged in one or more charging operations to beyond the theoretical maximum charge without first undergoing one or more first charging operations to a charge capacity below or equivalent to its theoretical maximum. 
         [0107]    In a further embodiment, the present invention also provides a process of increasing the specific charge capacity of an oxide-containing cathode compound according to Formula 1 in an electrochemical cell, e.g. a Na-ion electrochemical cell, comprising:
       a) charging the electrochemical cell (sodium-ion electrochemical cell) comprising one or more compounds of Formula 1 beyond the conventional theoretical capacity of the cathode based on the Ni 2+ /Ni 4+  redox couple; and optionally   b) degassing the electrochemical cell (Na-ion electrochemical cell) to remove gasses formed during the charging process.       
 
         [0110]    As described above, the charging step may be performed as a single- or a multi-stage process. 
         [0111]    A doped nickelate of Formula 1 that is charged up to its theoretical specific capacity limit based on the Ni 2+ /Ni 4+  redox couple, will re-yield a compound with no oxygen loss when discharged; i.e. at this level, charging is seen to be reversible. However, discharging compounds of Formula 1 that have been “overcharged” in accordance with the process of the present invention typically results in producing doped nickelate compounds of the present invention in which
       M 1  is nickel in oxidation state greater than 0 to less than 4+,   at least one of M 2  and M 4  comprises a metal in oxidation state greater than 0 to less than or equal to 4+,   0&lt;a&lt;1   0&lt;v&lt;0.5 and   0&lt;c≦0.1 and preferably 0.05≦c≦0.1.       
 
         [0117]    Consequently, during the discharge of over-charged compounds of Formula 1 at least some of the nickel is reduced from oxidation state 4+ to oxidation state 2+, some or all of the sodium-ions are re-inserted into the lattice and the lost oxygen atoms are not replaced. Moreover, although the metals M 2′  and/or M 4′  do not contribute anything to the initial overcharging process, upon discharge of the overcharged material it is found that at least one of the metals M 2′  and/or M 4′  can also be reduced to an oxidation state greater than 0 to less than 4+. This allows some or all of the sodium ions that were removed during the first “overcharge” to be reinserted into the material, and therefore produces a material with a higher discharge capacity. It is clear from this description that the intended role of the Mn 4+  (or Ti 4+ ) in the uncharged materials of the present invention is to allow for the reduction (Na insertion) process during cell discharge. This is in direct contrast to the role of the manganese in US2007/0218361 which is to allow Na extraction by the Mn oxidation process Mn 3+ →Mn 4+  during the first charge process. 
         [0118]    The role of the M 3′  and M 5′  metals is to reduce the amount of nickel in the doped nickelate materials of Formula 1, thereby allowing the “overcharge” mechanism to take place as oxygen is released from the structure. The presence of the M 3′  and M 5′  metals also reduces the relative formula mass (formula weight) of these materials, thereby further improving the cost effectiveness of the electrode materials. In order to achieve anomalous capacity, there must be over twice the amount of sodium as there is nickel in the pristine materials (e.g. Formula 1) so that, when all Ni 2+  has oxidised to Ni 4+ , there is still some Na left in the material, which can then be removed when the material is overcharged. This explains why anomalous capacity is not observed for NaNi 0.5 Mn 0.5 O 2 ; in this case there is enough Ni 2+  present to remove all of the Na, by conventional Ni 2+  to Ni 4+  oxidation. In each of the compounds of Formula 1 a′ is &lt;1, and following the requirement above, v′ (i.e. the amount of nickel) has to be less than half the value of a′ (i.e. the amount of alkali metal) and the overall material must be electroneutral. Satisfaction of these two conditions is facilitated by the presence of an M 3′  metal, and/or an M 5′  metal. 
         [0119]    The doped nickelate-containing compounds made by the process of the present invention described above, may be cycled in an electrochemical cell (e.g. a Na-ion electrochemical cell) between normal voltage limits for the Ni 2+ /Ni 4+  redox couple, typically this might be expected to be between 1 and 4 V, and no further unconventional voltage plateaux are formed. However, once formed, cycling the doped nickelate-containing compounds of the present invention at normal voltage limits maintains a higher specific capacity than would be expected from conventional theoretical calculations, i.e. the doped nickelate-containing compounds of the present invention perform better than the pristine compounds of Formula 1 when cycled using the same normal voltage limits. 
         [0120]    Thus the present invention provides a process of using one or more doped nickelate-containing compounds of the present invention, in an electrochemical cell (e.g. a Na-ion electrochemical cell) said process comprises the step of cycling the electrochemical cell (Na-ion electrochemical cell) containing the one or more doped nickelate-containing compounds of the present invention, over a voltage range within the normal voltage limits of the Ni 2+ /Ni 4+  redox couple; wherein the electrochemical cell (Na-ion electrochemical cell) is optionally degassed to remove gasses formed during the charging process. 
         [0121]    The doped nickelate-containing compounds of the present invention are suitable for use in many different applications including electrochemical cells, e.g. sodium ion electrochemical cells, which may be widely used for example in energy storage devices, such as batteries, rechargeable batteries, electrochemical devices and electrochromic devices. 
         [0122]    The process of the present invention for making doped nickelate-containing compounds is conveniently performed in situ in an energy storage device, a battery, a rechargeable battery, an electrochemical device or an electrochromic device. 
         [0123]    The present invention also provides the use of one or more doped nickelate-containing compounds of the present invention in energy storage devices including batteries, particularly rechargeable batteries, in electrochemical devices and electrochromic devices. Further, the present invention provides a process of storing electrical charge using energy storage devices including batteries, rechargeable batteries, electrochemical devices and electrochromic devices, comprising one or more doped nickelate-containing compounds of the present invention. 
         [0124]    The present invention also provides energy storage devices, including batteries, rechargeable batteries, electrochemical devices and electrochromic devices comprising one or more doped nickelate-containing compounds of the present invention, and also includes methods of making and using such energy storage devices. 
         [0125]    In a further alternative process for the preparation of doped nickelate-containing compounds of the present invention, at least a partial loss of oxygen from the materials of Formula 1 may be facilitated using an agent such as NO 2 BF 4 , NO 2 PF 6 , bromine in methanol or persulfates such as potassium persulfate or ammonium sulphate. 
         [0126]    Advantageously, electrodes comprising one or more doped nickelate-containing compounds of the present invention are 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). 
         [0127]    It is also possible to convert sodium-ion derivatives into mixed lithium-ion/sodium-ion materials using an ion exchange process. 
         [0128]    Typical ways to achieve Na to Li-ion exchange include: 
         [0000]    1. Mixing the sodium-ion material with a lithium-containing material e.g. LiNO 3 , heating to above the melting point of LiNO 3  (264° C.), cooling and then washing to remove the excess LiNO 3  and side-reaction product
 
2. Treating the Na-ion material with an aqueous solution of lithium salts, for example 1M LiCl in water; and
 
3. Treating the Na-ion material with a non-aqueous solution of lithium salts, for example LiBr in one or more aliphatic alcohols such as hexanol, propanol etc.
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0129]    The present invention will now be described with reference to the following figures in which: 
           [0130]      FIG. 1(A)  is the XRD profile for the compound NaNi 0.5 Mn 0.5 O 2  used in comparative Example 1; 
           [0131]      FIG. 1(B)  shows the Charge-Discharge Voltage Profile for the first 4 cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell, cycled between 1.5 to 4.0V at 30° C.; 
           [0132]      FIG. 1(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell, cycled between 1.5 to 4.0V at 30° C.; 
           [0133]      FIG. 1(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell, cycled between 1.5 to 4.0V at 30° C.; 
           [0134]      FIG. 1(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell, cycled between 1.5 to 4.0V at 30° C.; 
           [0135]      FIG. 2(A)  is the XRD profile for the compound Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  used in Example 2; 
           [0136]      FIG. 2(B)  shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0137]      FIG. 2(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na 0.95 Ni 0.0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0138]      FIG. 2(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0139]      FIG. 2(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0140]      FIG. 3(A)  is the XRD profile for the compound Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  used in Example 3; 
           [0141]      FIG. 3(B)  shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0142]      FIG. 3(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0143]      FIG. 3(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0144]      FIG. 3(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0145]      FIG. 4(A)  is the XRD profile for the compound Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  used in Example 4; 
           [0146]      FIG. 4(B)  shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0147]      FIG. 4(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0148]      FIG. 4(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0149]      FIG. 4(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0150]      FIG. 5(A)  is the XRD profile for the compound Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  used in Example 5; 
           [0151]      FIG. 5(B)  shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0152]      FIG. 5(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0153]      FIG. 5(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0154]      FIG. 5(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0155]      FIG. 6(A)  is the XRD profile for the material Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  used in Example 6; 
           [0156]      FIG. 6(B)  shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0157]      FIG. 6(C)  shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0158]      FIG. 6(D)  shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; 
           [0159]      FIG. 6(E)  shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell, cycled between 1.0 to 4.2V at 30° C.; and 
           [0160]      FIG. 7  is a comparison of XRD profiles for a) the precursor starting material, Na 0.95 Ni 0.3167 Ti 0.3167 Mg 0.1583 Mn 0.2083 O 2 , b) an electrode, originally containing Na 0.95 Ni 0.3167 Ti 0.3167 Mg 0.1583 Mn 0.2083 O 2 , recovered from a cell after charging to a specific capacity of 139 mAh/g and c) an electrode, originally containing Na 0.95 Ni 0.3167 Ti 0.3167 Mg 0.1583 Mn 0.2083 O 2  recovered from a cell after charging to a specific capacity of 192 mAh/g. 
       
    
    
     DETAILED DESCRIPTION 
       [0161]    Any convenient process may be used to make the compounds of Formula 1 described above. For example, the following general method may be used: 
       General Method: 
       [0000]    
       
         
           
             1) Intimately mix together the starting materials in the correct stoichiometric ratio, to ensure that the required product is formed and ideally with the minimum amount of side reactions and unreacted starting materials, and press into a pellet. 
             2) Heat the resulting mixture in a furnace using either an ambient air atmosphere, or a flowing inert atmosphere (e.g. argon or nitrogen), at a furnace temperature of between 400° C. and 1500° C. until reaction product forms. 
             3) Allow the product to cool before grinding it to a powder. 
           
         
       
     
         [0165]    Table 1 below lists the starting materials and heating conditions used to prepare a comparative compound NaNi 0.5 Mn 0.5 O 2  (Example 1) and compounds of Formula 1 (Examples 2 to 6). 
         [0000]    
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 STARTING 
                   
               
               
                 COMPOUND 
                   
                 MATERIALS 
                 FURNACE CONDITIONS 
               
               
                   
               
             
             
               
                 1 
                 COMPARATIVE 
                   
                   
               
               
                   
                 NaNi 0.5 Mn 0.5 O 2   
                 0.5 Na 2 CO 3   
                 1) Air/900° C., dwell time of 8 
               
               
                   
                   
                 0.5 NiCO 3   
                 hours. 
               
               
                   
                   
                 0.5 MnO 2   
                 2) Air/900° C., dwell time of 8 
               
               
                   
                   
                   
                 hours. 
               
               
                   
                 COMPOUNDS OF FORMULA 1 
               
               
                 2 
                 Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2   
                 0.475 Na 2 CO 3   
                 1) Air/900° C., dwell time of 8 
               
               
                   
                   
                 0.3167 NiCO 3   
                 hours. 
               
               
                   
                   
                 0.3167 MnO 2   
               
               
                   
                   
                 0.1583 
               
               
                   
                   
                 Mg(OH) 2   
               
               
                   
                   
                 0.2083 TiO 2   
               
               
                 3 
                 Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2   
                 0.463 Na 2 CO 3   
                 1) Air/900° C., dwell time of 10 
               
               
                   
                   
                 0.308 NiCO 3   
                 hours. 
               
               
                   
                   
                 0.308 MnO 2   
               
               
                   
                   
                 0.154 Mg(OH) 2   
               
               
                   
                   
                 0.229 TiO 2   
               
               
                 4 
                 Na 0.9 Ni 0.30 Mn 0.30 Mg 0.150 Ti 0.25 O 2   
                 0.45 Na 2 CO 3   
                 1) Air/900° C., dwell time of 10 
               
               
                   
                   
                 0.3 NiCO 3   
                 hours 
               
               
                   
                   
                 0.3 MnO 2   
               
               
                   
                   
                 0.15 Mg(OH) 2   
               
               
                   
                   
                 0.25 TiO 2   
               
               
                 5 
                 Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2   
                 0.425 Na 2 CO 3   
                 1) Air/900° C., dwell time of 10 
               
               
                   
                   
                 0.283 NiCO 3   
                 hours 
               
               
                   
                   
                 0.283 MnO 2   
               
               
                   
                   
                 0.142 Mg(OH) 2   
               
               
                   
                   
                 0.292 TiO 2   
               
               
                 6 
                 Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2   
                 0.833 Na 2 CO 3   
                 1) Air/900° C., dwell time of 10 
               
               
                   
                   
                 0.317 NiCO 3   
                 hours 
               
               
                   
                   
                 0.467 TiO 2 , 
               
               
                   
                   
                 0.1 Mg(OH) 2   
               
               
                   
                   
                 0.117 MnO 2   
               
               
                   
               
             
          
         
       
     
         [0166]    It is desirable for electrode materials to be safe during charge and discharge in an energy storage device. Li-ion batteries in common use today undergo safety/abuse testing, the results of such tests revealing that lithium oxide-based cathode materials are liable to liberate oxygen, which is a major contributing factor to an undesirable process known as thermal runaway. Prior to the present invention it might have been expected that a similar thermal runaway process would also be observed for sodium oxide-based materials, and that such sodium oxide-based materials would be unsafe and/or rendered completely useless by overcharging in rechargeable battery applications. However, the present invention has surprisingly demonstrated that when these sodium-based materials are overcharged they yield materials which are highly thermodynamically stable and which are extremely effective and safer when used as cathode materials in electrochemical cells (e.g. Na-ion electrochemical cells). 
       Product Analysis Using XRD 
       [0167]    Analysis by X-ray diffraction techniques was conducted using a Siemens D5000 powder diffractometer to confirm that the desired target materials 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 unit cell lattice parameters. 
         [0168]    The general XRD operating conditions used to analyse the materials are as follows: 
         [0000]    Slits sizes: 1 mm, 1 mm, 0.1 mm 
       Range: 2θ=5°-60° 
     X-ray Wavelength=1.5418 Å (Angstroms) (Cu Kα) 
       [0169]    Speed: 1.0 seconds/step 
       Increment: 0.025° 
     Electrochemical Results 
       [0170]    The target materials were tested using a Na-ion test cell using a hard carbon anode. Cells may be made using the following procedures: 
         [0171]    A Na-ion electrochemical test cell containing the active material is constructed as follows: 
       Generic Procedure to Make a Hard Carbon Na-Ion Cell 
       [0172]    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-2-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 120° C. The electrode film contains the following components, expressed in percent by weight: 88% active material, 6% Super P carbon, and 6% PVdF binder. 
         [0173]    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-2-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 120° C. The electrode film contains the following components, expressed in percent by weight: 89% active material, 2% Super P carbon, and 9% PVdF binder. 
       Cell Testing 
       [0174]    The cells are tested as follows, using Constant Current Cycling techniques. 
         [0175]    The cell is cycled at a given current density between pre-set voltage limits. A commercial battery cycler from Maccor Inc. (Tulsa, Okla., 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. 
       Convenient Method for Overcharging the Electrochemical Cells 
       [0176]    The cell is charged under constant current conditions to a pre-set cell voltage limit, typically 4.2V. At the end of this constant current stage, the cell is further charged at a constant voltage (CV—potentiostatic) (typically 4.2V) until the cell current drops to less than 1% of the constant current value. This CV step ensures that the cell is fully charged. Following this charging process the cell is discharged to a lower pre-set voltage limit (typically 1.0V). 
       Example 1 
     Comparative Compound NaNi 0.5 Mn 0.5 O 2    
       [0177]    In this comparative Example, it is shown that no anomalous capacity behaviour is observed when the process of the present invention is performed on NaNi 0.5 Mn 0.5 O 2 . This is because the [Ni] activity is sufficient to allow (theoretically) all of the Na to be extracted using the Ni 2+ /Ni 4+  oxidation process. As  FIGS. 1(B) to 1(E)  illustrate and as explained below, no anomalous capacity is observed for this compound when an electrochemical cell containing it is charged beyond the conventional theoretical specific capacity as determined by the Ni 2+ /Ni 4+  redox couple. 
         [0178]    A Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell was prepared as follows, charged to 4V and held potentiostatically (constant voltage step) at 4V. The data shown in  FIGS. 1(B) to 1(E)  are derived from the constant current cycling data for a NaNi 0.5 Mn 0.5 O 2  compound in a Na-ion cell (Cell#204056) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.1 mA/cm 2  between voltage limits of 1.50 and 4.00 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.0 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0179]      FIG. 1(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell (Cell#204056). During the first charge process a cathode specific capacity of 162 mAh/g is achieved for the NaNi 0.5 Mn 0.5 O 2  active material. At cell voltages lower than about 3.7 V, a sloping profile is evident reflecting the Na extraction process proceeding based on the Ni 2+  to Ni 4+  redox couple from the NaNi 0.5 Mn 0.5 O 2  compound. The first discharge process is equivalent to a cathode specific capacity of 86 mAh/g. This discharge process is based on the Ni 4+  to Ni 2+  reduction process within the cathode material. 
         [0180]      FIG. 1(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell (Cell#204056). The cell shows good reversibility with the delivered cathode specific capacity being around 85 mAh/g after 4 cycles. 
         [0181]      FIG. 1(D)  shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell (Cell#204056). The cathode specific capacity in this cycle corresponds to 86 mAh/g. 
         [0182]      FIG. 1(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//NaNi 0.5 Mn 0.5 O 2  cell (Cell#204056). These symmetrical data demonstrate the reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0183]    In conclusion, this comparative Example shows that NaNi 0.5 Mn 0.5 O 2 , although containing more Ni than any of the experimental examples, delivers far lower reversible specific capacity. The Na:Ni ratio (2:1) is set at a level where all of the redox processes that are occurring during cell charge and discharge are solely based on the Ni 2+  to Ni 4+  redox couple in the cathode active material. There is no anomalous capacity behaviour, and this limits the cathode reversible capacity. 
       Example 2 
     The Effect of Charging Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  beyond the Conventional Theoretical Capacity as Determined by the Ni 2+  to Ni 4+  Redox Couple, in Accordance with the Process of the Present Invention 
       [0184]    A Hard Carbon#Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention. 
         [0185]    The data shown in  FIGS. 2(B) to 2(E)  are derived from the constant current cycling data for a Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  compound in a Na-ion cell (Cell#311033) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm 2  between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0186]      FIG. 2(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell (Cell#311033). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 214 mAh/g is achieved—a figure which is significantly larger than the theoretical specific capacity (based on the Ni 2+  to Ni 4+  redox couple) for the Na 0.95 Ni 0.0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  compound. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident reflecting the conventional Na extraction process based on the Ni 2+  to Ni 4+  redox couple from the Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  compound. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni 2+  to Ni 4+  redox couple) occurring for the Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  compound. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 158 mAh/g. 
         [0187]      FIG. 2(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na 0.95 Ni 0.0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell (Cell#311033). The cell shows good reversibility with the delivered cathode specific capacity being around 150 mAh/g after 4 cycles. 
         [0188]      FIG. 2(D)  shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell (Cell#311033). The cathode specific capacity in this cycle corresponds to 152 mAh/g. 
         [0189]      FIG. 2(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cell (Cell#311033). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0190]    In conclusion, the electrochemical data obtained in Example 2 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher. 
       Example 3 
     The Effect of Charging Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  beyond the Conventional Theoretical Capacity as Determined by the Ni 2+  to Ni 4+  Redox Couple, in Accordance with the Process of the Present Invention 
       [0191]    A Hard Carbon#Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention. 
         [0192]    The data shown in  FIGS. 3(B) to 3(E)  are derived from the constant current cycling data for a Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  compound in a Na-ion cell (Cell#312001) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm 2  between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0193]      FIG. 3(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell (Cell#312001). During the first charge process above approximately 4 V an anomalously high charge capacity is realized—a cathode specific capacity of 199 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni 2+  to Ni 4+  redox couple) for the Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  active material. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident reflecting the conventional Na extraction process based on the Ni 2+  to Ni 4+  redox couple from the Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  active material. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni 2+  to Ni 4+  redox couple) occurring for the Na 0.925 Ni 0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 150 mAh/g. 
         [0194]      FIG. 3(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na 0.925 Ni 0.308  Mn 0.308  Mg 0.154 Ti 0.229 O 2  cell (Cell#312001). The cell shows good reversibility with the delivered cathode specific capacity being around 145 mAh/g after 4 cycles. 
         [0195]      FIG. 3(D)  shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.925 Ni 0.308  Mn 0.308  Mg 0.154 Ti 0.229 O 2  cell (Cell#312001). The cathode specific capacity in this cycle corresponds to 147 mAh/g. 
         [0196]      FIG. 3(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na 0.925 Ni 0.308  Mn 0.308 Mg 0.154 Ti 0.229 O 2  cell (Cell#312001). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0197]    In conclusion, the electrochemical data obtained in Example 3 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher. 
       Example 4 
     The Effect of Charging Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  Beyond the Conventional Theoretical Capacity as Determined by the Ni 2+  to Ni 4+  Redox Couple, in Accordance with the Process of the Present Invention 
       [0198]    A Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention. 
         [0199]    The data shown in  FIGS. 4(B) to 4(E)  are derived from the constant current cycling data for a Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  active material in a Na-ion cell (Cell#311069) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm 2  between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0200]      FIG. 4(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell (Cell#311069). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 197 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni 2+  to Ni 4+  redox couple) for the Na 0.925 Ni 0.0.308 Mn 0.308 Mg 0.154 Ti 0.229 O 2  active material. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident reflecting the conventional Na extraction process based on the Ni 2+  to Ni 4+  redox couple from the Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  active material. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni 2+  to Ni 4+  redox couple) occurring for the Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 140 mAh/g. 
         [0201]      FIG. 4(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell (Cell#311069). The cell shows good reversibility with the delivered cathode specific capacity being around 133 mAh/g after 4 cycles. 
         [0202]      FIG. 4(D)  shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell (Cell#311069). The cathode specific capacity in this cycle corresponds to 134 mAh/g. 
         [0203]      FIG. 4(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na 0.9 Ni 0.3 Mn 0.3 Mg 0.15 Ti 0.25 O 2  cell (Cell#311069). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0204]    In conclusion, the electrochemical data obtained in Example 4 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher. 
       Example 5 
     The Effect of Charging Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  Beyond the Conventional Theoretical Capacity as Determined by the Ni 2+  to Ni 4+  Redox Couple, in Accordance with the Process of the Present Invention 
       [0205]    A Hard Carbon//Na 0.85 Ni 0.0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention. 
         [0206]    The data shown in  FIGS. 5(B) to 5(E)  are derived from the constant current cycling data for a Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  active material in a Na-ion cell (Cell#311068) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm 2  between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0207]      FIG. 5(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell (Cell#311068). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 187 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni 2+  to Ni 4+  redox couple) for the Na 0.85 Ni 0.0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  active material. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident reflecting the conventional Na extraction process based on the Ni 2+  to Ni 4+  redox couple from the Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  active material. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni 2+  to Ni 4+  redox couple) occurring for the Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 134 mAh/g. 
         [0208]      FIG. 5(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell (Cell#311068). The cell shows good reversibility with the delivered cathode specific capacity being around 131 mAh/g after 4 cycles. 
         [0209]      FIG. 5D ) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell (Cell#311068). The cathode specific capacity in this cycle corresponds to 131 mAh/g. 
         [0210]      FIG. 5(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na 0.85 Ni 0.283 Mn 0.283 Mg 0.142 Ti 0.292 O 2  cell (Cell#311068). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0211]    In conclusion, the electrochemical data obtained in Example 5 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher. 
       Example 6 
     Effecting Oxygen Loss from Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  in Accordance with the Process of the Present Invention 
       [0212]    A Hard Carbon/Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention. 
         [0213]    The data shown in  FIGS. 6(B) to 6(E)  are derived from the constant current cycling data for a Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  active material in a Na-ion cell (Cell#311063) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO 4  in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm 2  between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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. 
         [0214]      FIG. 6(B)  shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell (Cell#311063). During the first charge process a cathode specific capacity of 160 mAh/g is achieved for the Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  active material. The first discharge process is equivalent to a cathode specific capacity of 122 mAh/g. 
         [0215]      FIG. 6(C)  shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell (Cell#311063). The cell shows good reversibility with the delivered cathode specific capacity being around 122 mAh/g after 4 cycles. 
         [0216]      FIG. 6D ) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1 Ti 0.117 O 2  cell (Cell#311063). The cathode specific capacity in this cycle corresponds to 122 mAh/g. 
         [0217]      FIG. 6(E)  shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na 0.833 Ni 0.317 Mn 0.467 Mg 0.1  Ti 0.117 O 2  cell (Cell#311063). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell. 
         [0218]    The Applicant does not wish to be rigidly bound to the following explanation but it is their current belief that the process of overcharging is capable of oxidising O 2−  in the lattice, thus releasing this from the structure as O 2 , and this is accompanied by a removal of some of the remaining sodium-ions. The loss of both sodium and oxygen in this way appears to explain an observed reduction in both the a- and c-parameters of the unit cell, and the resulting overall reduction in unit cell volume. 
       Example 7 
     The Preparation of Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454  Using the Process of the Present Invention i.e. from a Cell Showing Anomalous Capacity Behaviour Fully Charged 
       [0219]    Active precursor doped nickelate-containing compound Na 0.95 Ni 0.0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  (compound 2 in Table 1), was prepared using the conditions outlined in Table 1 and produced the XRD shown in  FIG. 7 . The asterisks in  FIG. 7  indicate peak positions for aluminium, as the aluminium current collector may contribute some intensity to XRD patterns (b) and (c). 
         [0220]    This compound was made into a cell (#412004) comprising hard carbon (Carbotron P(J)) anode//Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cathode and using an electrolyte comprising 0.5M NaPF 6  in EC/DEC/PC (1:1:1). The Active Mass of the cathode in the as-prepared cell=53.6 mg. Following a charge process to a specific capacity of 192 mAh/g (i.e. incorporating the anomalous capacity process of the present invention) (XRD shown in  FIG. 7 c   ) produced), the cathode electrode was removed from the cell, washed several times in clean dimethyl carbonate to remove the electrolyte and then dried at 70° C. The Active Mass of the washed cathode after this first charge process=44.2 mg. 
         [0221]    Looking at  FIG. 7 c   ), it is clear that the material obtained following the charge process to a specific capacity of 192 mAh/g is not the same as that shown in either  FIG. 7 b   ) (charged to a specific capacity of 139 mAh/g) which does not include an anomalous capacity process, only a conventional Ni 2+  to Ni 4+  charging process, or that shown by the original active precursor doped nickelate-containing material Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  (shown in  FIG. 7 a   )). The proposed composition for the product obtained following the overcharge process to a specific capacity of 192 mAh/g is Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454 , as determined by the following mass loss experiment. 
         [0000]    Mass Loss Experiment to Evidence the Formation of Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454 . 
         [0222]    Using the information above, the Active Mass Loss following overcharging the above cell (#412004) containing Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cathode material=(53.6 mg 44.2 mg)=9.4 mg. 
         [0223]    Thus, the Measured % Mass Loss=17.5% 
         [0224]    If the charge process was just by Na-ion extraction then mass loss should be: 
         [0000]    Starting compound Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 = 
       Molecular Weight=103.6 
       [0225]    Based on only Ni 2+  to Ni 4+  redox process, on cell charge it is only possible to extract 0.6333Na ions per formula unit, i.e. 
         [0000]      Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 →Na 0.3167 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 +0.6333 Na + +0.6333 e   −   (1)
 
         [0226]    Therefore, the theoretical capacity based on this reaction (1) may be given by the following: 
         [0000]    Theoretical Specific Capacity=(0.6333×96485)/(103.6×3.6)=164 mAh/g
 
Thus the Expected % Mass loss for reaction (1)=(14.6/103.6)×100=14.0%
 
         [0227]    The charge process for Cell#412004 corresponds to an actual cathode specific capacity of 192 mAh/g i.e. far in excess of the expected theoretical specific capacity of 164 mAh/g. 
         [0228]    Thus it is proposed that the following overall charge mechanism is: 
         [0000]      Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 →Na 0.3167 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 +0.6333 Na + +0.6333 e   −   (1)
 
         [0000]      followed by: 
         [0000]      Na 0.3167 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 →Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454 +0.1091Na + +0.0273O 2 +0.1 e   −   (2)
 
         [0229]    Looking at the overall process (by the Applicants mechanism) for the extraction of Na: 
         [0000]      Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 →Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454 +0.7425Na + +0.0273O 2 +0.7425 e   −   (3)
 
         [0230]    Therefore, the theoretical capacity based on this reaction (3) may be given by: 
         [0000]    Theoretical Specific Capacity=(0.7425×96485)/(103.6×3.6)=192 mAh/g 
         [0231]    Also, the Expected mass loss for reaction (3)=((103.6−85.7)/103.6)×100%=17.3%. This percentage mass loss is extremely close to the 17.5% (as detailed above) which is observed. 
         [0232]    In conclusion, on the basis that there is very close correspondence between theoretical and actual results for both Specific Capacity and Expected Mass Loss, the Applicant has been able to determine with a high degree of certainty that Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454  is obtained when Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  is charged to the end of the second unconventional voltage plateau. 
         [0233]    From reaction (3) above, it is proposed that the anomalous capacity arises as a result of the net loss of Na 2 O, i.e. the production of active Nations plus the liberation of O 2 , and this produces a new layered oxide active material Na 0.2075 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 1.9454 . This is surprising because it is not a usual charging mechanism i.e. a simple Na +  extraction from the cathode, but is a structural change that releases Na +  and oxygen from the material to produce a new compound. 
       Example 8 
     Charging Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  Up to the Conventional Theoretical Maximum for the Ni 2+  to Ni 4+  Redox Process, ie. From a Cell Showing No Anomalous Capacity Behaviour Partially Charged 
       [0234]    Active precursor doped nickelate-containing compound Na 0.96 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  (compound 2 in Table 1), was prepared using the conditions outlined in Table 1 and produced the XRD shown in  FIG. 7 a   )). This compound was made into a cell (#412003) comprising hard carbon (Carbotron P(J)) anode//Na 0.96 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cathode and using an electrolyte comprising 0.5M NaPF 6  in EC/DEC/PC (1:1:1). The Active Mass of the cathode in the as-prepared cell=51.0 mg. 
         [0235]    Following a charge process to a specific capacity of 139 mAh/g (i.e. not incorporating the anomalous capacity process of the present invention) instead just using a conventional Ni 2+  to Ni 4+  charging process (XRD shown in  FIG. 7 b   ) produced), the cathode electrode was removed from the cell, washed several times in clean dimethyl carbonate to remove the electrolyte and then dried at 70° C. The Active Mass of the washed cathode after this first charge process=45.0 mg. 
         [0236]    Looking at  FIG. 7 b   ), it is clear that the material obtained following the charge process to a specific capacity of 139 mAh/g is not the same as that shown in either  FIG. 7 c   ) (charged to a specific capacity of 192 mAh/g) which includes an anomalous capacity process, or that shown by the original active precursor doped nickelate-containing material Na 0.96 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  (shown in  FIG. 7 a   )). The proposed composition for the product obtained following the overcharge process to a specific capacity of 139 mAh/g is Na 0.4125 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 , as determined by the following mass loss experiment. 
         [0000]    Mass Loss Experiment to Evidence the Formation of Na 0.4125 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 . 
         [0237]    Using the information above, the Active Mass Loss following overcharging the above cell (#412003) containing Na 0.96 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  cathode material=(51.0 mg 45.0 mg)=6 mg. 
       Mesaured % Mass Loss=11.8% 
       [0238]    If charge process was just by Na ion extraction then mass loss should be: 
         [0000]    Starting compound=Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583  Ti 0.2083 O 2  
 
Molecular weight=103.6
 
         [0239]    Based on only the Ni 2+  to Ni 4+  redox process, the charge process may be given as: 
         [0000]      Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 →Na 0.4125 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2 +0.5375Na + +0.5375 e   −   (1)
 
       Expected % Mass Loss for Reaction (1)=(103.6−91.3)/103.6×100=11.9% 
       [0240]    In conclusion, on the basis that there is very close correspondence between theoretical and actual results for the Expected % Mass Loss 11.8% v 11.9%, the Applicant has been able to determine with a high degree of certainty that Na 0.4125 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  is obtained when Na 0.95 Ni 0.3167 Mn 0.3167 Mg 0.1583 Ti 0.2083 O 2  is charged to the convention theoretical charge process i.e. the Ni 2+  to Ni 4+  redox reaction takes place under these conditions.