Cathode composition

Abstract

A cathode material for a battery wherein the cathode material has an anti-fluorite crystal structure and belongs to the group represented by the formula Li5Fe (1-x) AlxO4 wherein x is greater than 0 and less than or equal to 1.

Description

Technical Field

[0001] The present invention relates to cathode materials for batteries and methods of making cathodes for batteries. Background Art

[0002] Modern society is characterized by an ever-increasing proliferation of small electrified devices such as mobile phones, tablets, and smart watches, as well as the progressive electrification of larger machines such as cars. Across all electrified devices, a common goal is to produce higher capacity rechargeable (secondary) batteries, for example to support greater processing power in devices or to increase the range of electric vehicles. As secondary batteries become increasingly important for the functioning of many everyday products, there is a growing concern for sustainability. This manifests itself in many ways, from seeking to improve the electrochemical stability of batteries over multiple charging cycles to improve the battery's lifespan, to evaluating the materials used to make batteries (with an eye towards using materials that are less energy-intensive, more abundant, or less environmentally toxic when extracted from their ores).

[0003] Current high performance secondary batteries are typically referred to as "lithium ion batteries". The cathode materials used to make lithium ion batteries are usually lithiated transition metal oxide compounds, such as NMC (LiNi x Mn y Co z O2). These are made up of metals such as cobalt and nickel, which are expensive and becoming scarce, especially given projected future battery demand. They are also inherently toxic and pose an environmental risk.

[0004] Therefore, it is desirable to find cheaper and less toxic metals for use as alternative cathode materials to provide greener solutions for future energy storage applications that can match or exceed current performance. It is against this background that the present invention has been proposed. Summary of the Invention

[0005] In a first aspect, the present invention provides a cathode material for a battery. The cathode material has an anti-fluorite crystal structure and is of the formula Li5Fe (1-x) Al x The group represented by O4, where x is greater than 0 and less than or equal to 1.

[0006] The cathode material has a substituted Li2O-based inverse fluorite structure, in which iron and aluminum ions replace lithium ions within the structure to improve the structural stability of the cathode material through charging cycles. The addition of iron and aluminum ensures a high specific capacity because the redox of anions and cations can occur simultaneously through the redox of iron and aluminum and oxygen species. Finally, the high abundance, low cost and non-toxicity of both iron and aluminum make the cathode composition significantly more environmentally friendly than existing cathode materials that often contain cobalt and / or nickel.

[0007] The x value of the cathode material may be less than or equal to 0.5, and preferably may be greater than or equal to 0.1 and less than or equal to 0.25. These x values provide an optimal compromise between many factors important for good practical electrochemical performance, such as specific capacity, conductivity, and the integrity of the crystal structure under repeated charge and discharge cycles.

[0008] The cathode material may adopt a β-polymorph structure, which is an orthorhombic structure in the Pbca space group.

[0009] In a second aspect, the present invention provides a method for making a cathode for a battery. The method comprises: mixing Li2O, LiFeO2, and Al2O3 powders in a stoichiometric ratio; ball milling the powders; heating the powders to consolidate them into a cathode material; and cooling the cathode material to room temperature. The stoichiometric ratio of the powders is such that the cathode material has the formula Li5Fe (1-x) Al x O4, where x is greater than 0 and less than or equal to 1.

[0010] The method may comprise heating the powder to between 650° C. and 750° C., preferably to 700° C. The method may comprise heating the powder to between 850° C. and 1000° C. The method may comprise heating the powder for at least 15 hours.

[0011] The cathode material can be cooled to room temperature at a rate of at least 5°C / minute. Cooling the cathode material in this manner, combined with heating the powder to between 850°C and 1000°C and the presence of aluminum in the crystal structure, stabilizes the Li5Fe (1-x) Al x β-polymorph of O4.

[0012] The value of x may be less than or equal to 0.5, and preferably may be greater than or equal to 0.1 and less than or equal to 0.25.

[0013] In a third aspect, the present invention provides a cathode comprising the above cathode material. In a fourth aspect, the present invention provides an electrochemical cell comprising such a cathode, an electrolyte and an anode. The cell may be a secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will now be described with reference to the following drawings, in which:

[0015] Figure 1 A super-cell of Li2O is shown, which adopts the anti-fluorite crystal structure and α-Li5Fe 0.5 Al 0.5 Unit cell of O4;

[0016] Figure 2a and 2b The general formula Li5Fe (1-x) Al x X-ray diffraction patterns using Cu Kα radiation of members of the material group represented by O4;

[0017] Figure 3 shows that α-Li5Fe 0.5 Al 0.5 Rietveld refinement of the X-ray diffraction pattern of the synthetic phase of O4;

[0018] Figure 4a shows that α-Li5Fe 0.8 Al 0.2 Charge-discharge curve of O4 in the first cycle, where the voltage was limited to 1.5 V during discharge;

[0019] Figure 4b shows that α-Li5Fe 0.8 Al 0.2 Charge-discharge curves of O4 during the first and twentieth cycles, where the voltage was limited to 3.6 V during charge and 1.5 V during discharge;

[0020] Figure 5a β-Li5Fe 0.75 Al 0.25 The charge-discharge curve of O4 in the first cycle, wherein the discharge is limited to 1 V during discharge; and

[0021] Figure 5b β-Li5Fe 0.75 Al 0.25 Charge-discharge curves of O4 in the first, second, third, and twentieth cycles, where the voltage was limited to 3.6 V during charge and 1.5 V during discharge. DETAILED DESCRIPTION

[0022] Research on promising lithium-rich cathode compositions is extensive. For example, KR20130079109A discloses many possible materials for cathodes of secondary batteries. The present invention relates to a lithium-rich cathode having the general formula Li5Fe (1-x) Al x A cathode material of O4 with an inverse fluorite crystal structure, wherein the value of x is greater than 0 and less than or equal to 1.

[0023] As is known to all, batteries generate electricity by using chemical reactions that occur at the electrodes. In secondary batteries, the reactions are reversible redox reactions. During the discharge of the battery, a substance is oxidized at the anode, which produces lithium ions (Li + ) and electrons (e - ), while another species is reduced at the cathode to incorporate the generated lithium ions into the cathode material. During charging of the battery, the two reactions at each electrode are reversed. In NMC cathodes, for example, the species reduced during discharge are nickel, manganese, and cobalt, which are transition metals. The specific capacity of current lithium-ion batteries is limited by the number of lithium ions available for reversible insertion into the cathode structure and the high atomic weight of the transition metal cations, as the battery's specific capacity is normalized by mass.

[0024] A recent development in cathode materials for secondary batteries has been the discovery of cathode oxygen redox. This differs from current transition metal redox at the cathode because it relies on anionic redox of oxygen ligands at the cathode, as opposed to cationic redox where transition metals form redox centers (oxygen-based ions are typically negatively charged, while transition metal ions are positively charged).

[0025] Among the materials proposed to utilize the redox of cathode oxygen, Li2O has the highest possible theoretical specific capacity because it does not contain heavy transition metals and therefore relies solely on the redox of oxygen. The redox reaction is given by the following equation:

[0026] 2Li2O Li2O2 + 2Li + + 2e -

[0027] Relative to the Li + / Li redox reaction, the potential of which is about 3.0 V. However, despite the high theoretical specific capacity of Li2O-based cathodes, both Li2O and Li2O2 exhibit low electrochemical activity and poor conductivity, and therefore require a catalyst. Another problem is that Li2O-based cathodes release oxygen (O2) due to the metastability of delithiated Li2O, and therefore their electrochemical performance is further adversely affected by their structural instability.

[0028] One way to address the poor performance of Li2O-based cathodes is to improve the ionic and electrical conductivity of the cathode. One promising approach to achieve this is by partially replacing lithium with transition metals to produce lithium-rich defective inverse fluorite compounds. One such compound is Li5FeO4. If one considers four unit cells of Li2O (effectively a single cell of Li8O4), then one Fe 3+ ions and two lithium vacancies replace three Li + ions produce Li5FeO4. Figure 1 The left hand side of FIG shows a super cell of Li2O with the lithium ions labeled 10 and the oxygen ions labeled 20, while the right hand side shows the same cell with the lithium ions substituted as described above, with the potential iron ion / vacancy sites collectively labeled 30.

[0029] Due to the presence of vacancies in the structure, Li5FeO4 provides enhanced ionic conductivity and electrical conductivity compared to Li2O. The specific capacity of Li5FeO4 is theoretically at least 700 mAh / g, which is due to the high lithium content and the addition of transition metal Fe 3+ The simultaneous redox of cations and anions can occur because O 2- / O - The redox potential of the reaction is similar to that of Fe 3+ / Fe 4+ In addition to its high theoretical specific capacity, the Li5FeO4 cathode uses cheaper and more abundant materials than current alternatives and, being cobalt- and nickel-free, is more environmentally friendly.

[0030] However, Li5FeO4 cathodes still face many challenges caused by poor electrochemical performance, mainly due to structural instability and loss of oxygen from the lattice under repeated cycling. One reason for this is the Fe 3+ / Fe 4+ The redox reaction results in a gradual amorphization of the crystal structure of Li5FeO4. This is because the repeated insertion and removal of lithium ions during the charge and discharge cycles of the cathode requires the removal of iron ions from the Fe 3+ The +3 oxidation state occupied by the ion is repeatedly changed from Fe 4+ ions occupy the +4 oxidation state. However, this results in the coordination around the iron ion shifting from Fe 3+ The tetrahedral coordination around the ion changes to Fe4+ As charge and discharge cycles are repeated, the incomplete reconstruction of the oxygen coordination around each iron ion in each redox cycle eventually leads to the amorphization of the crystal structure and the subsequent loss of oxygen from the lattice.

[0031] The present inventors have found that these problems can be at least partially alleviated by partially replacing iron with other trivalent ions (e.g., aluminum in Li5FeO4). In the case of partial substitution with aluminum, the resulting family of materials has the general formula Li5Fe (1-x) Al x O4. Depending on the value of x, different proportions of iron ions are replaced by aluminum ions. Figure 1 The right-hand unit cell shows the iron ion / vacancy site 30 with a half-and-half shading pattern to indicate that the aluminum ion has replaced half of the iron ion. Thus, this unit cell represents Li5Fe 0.5 Al 0.5 O4 (i.e., x=0.5).

[0032] Li5Fe (1-x) Al x The O4 material family largely enjoys the same benefits as the Li5FeO4 cathode, as the high lithium content and the simultaneous redox of cations and anions ensure a high specific capacity while using abundant, cheap, non-toxic materials. However, the addition of aluminum helps to keep the crystal structure intact because it is a redox-inactive element and therefore does not undergo any changes in oxidation state. The addition of aluminum also increases the theoretical specific capacity due to its lower weight, however, at the same time it reduces the amount of active material involved in the redox process, which has been substituted into the structure instead of the redox-active iron, and also reduces the conductivity of the crystal structure. In particular, as the value of x increases, the reduced amount of redox-active material ultimately reduces the actual specific capacity of the cathode material, since redox processes are required for the insertion and deinsertion of lithium. Therefore, increasing the value of x too high effectively reduces the number of lithium ions that can be inserted into and deinserted from the cathode.

[0033] Therefore, there is a balance that needs to be struck between many factors, including theoretical and actual specific capacity, conductivity, and the amount of redox-active species, in order to achieve optimal performance with a reasonable and practical level of structural integrity.

[0034] Furthermore, the present inventors have found that Li5Fe (1-x) Al x The material family of O4 has two polymorphs, represented here as α-Li5Fe (1-x) Al x O4 and β-Li5Fe(1-x) Al x O4, which are in the orthorhombic Pmmn and Pbca space groups, respectively. Generally, the α-polymorph is the low-temperature polymorph, while the β-polymorph is the high-temperature polymorph. However, as will be described below, the high-temperature β-Li5Fe (1-x) Al x The O4 polymorph (except for Li5FeO4, where x = 0) can be successfully stabilized at room temperature. In other words, the presence of aluminum helps stabilize the β-polymorph at room temperature. Both the α- and β-polymorphs have an inverse fluorite crystal structure, but the β-polymorph exhibits a greater degree of symmetry in the crystal structure, likely due to improved lithium distribution.

[0035] A range of materials from this group with different values of x have been synthesized. The cathode materials can be synthesized by typical powder processing routes. In one method, the precursor powders are initially placed in a ball mill in stoichiometric amounts and then ball milled and finally heated to react and obtain Li5Fe (1-x) Al x The final powder of the O4 composition can be obtained by ball milling in a stainless steel tank using stainless steel balls at 200 rpm for four 30-minute periods, with a 15-minute break between each period to allow the powder to cool. It should be understood that any ball milling process or similar process can be used to produce a suitable fine powder for consolidation.

[0036] To make Li5Fe (1-x) Al x O4 composition, using a mixture of Li2O, LiFeO2, and Al2O3 powders, where the exact stoichiometric ratio of the three powders depends on the value of x in the final composition. To synthesize the α-polymorph, the ball-milled precursor powder mixture can be heated to between 650°C and 750°C, preferably to 700°C, for at least 10 hours, preferably 15 hours, while the β-polymorph is synthesized by heating the ball-milled precursor powder mixture or the synthesized α-polymorph powder to between 850°C and 1000°C for at least 10 hours, preferably 15 hours, followed by quenching or rapid cooling at a cooling rate of at least 5°C / minute until the sample reaches room temperature. This cooling rate, combined with the presence of substituted aluminum in the crystal structure, allows the β-polymorph to be room temperature stable. However, naturally, it is also necessary to heat the powder to a sufficiently high temperature to first form the β-polymorph.

[0037] Figure 2a Li5FeO4, α-Li5Fe 0.75 Al 0.25 O4、α-Li5Fe 0.5 Al 0.5On the other hand, Figure 2b β-Li5Fe 0.75 Al 0.25 O4、β-Li5Fe 0.5 Al 0.5 XRD patterns of O4 and β-Li5AlO4 (ie, values of x are 0.25, 0.5, and 1).

[0038] The purity of the synthesized phases was assessed by Rietveld refinement of the XRD patterns of the as-synthesized samples. Representative analyses are shown in Figure 3 In the case of α-Li5Fe 0.5 Al 0.5 O4 (x = 0.5) sample, in which a suitable crystal structure was determined and the XRD pattern was refined as shown in Figure 2. A satisfactory fit was seen between the refinement result and the measured XRD pattern, and all peaks in the XRD pattern could be indexed, indicating that the synthesized phase was pure.

[0039] Among the substituted cathode materials studied by the present inventors, the strongest electrochemical performance was observed for values of x greater than zero but below 0.5, with particularly good performance observed for values of x between 0.1 and 0.25. At values of x above 0.5, the composition begins to become inactive, as aluminum is redox inactive, and further reduces the conductivity of the composition. Values of x between 0.1 and 0.25, such as x = 0.1 and x = 0.2, show the best compromise between improving the structural stability of the cathode composition under cycling and maintaining acceptable electrochemical performance.

[0040] α-Li5Fe 0.8 Al 0.2 The electrochemical performance of O4 (x = 0.2) is shown in Figure 4a and 4b middle. Figure 4a It was shown that a specific capacity of over 700 mAh / g was achieved in the first charge, although only about 220 mAh / g could be extracted in the first discharge when the discharge was limited to 1.5 V. Figure 4b shows that α-Li5Fe 0.8 Al 0.2The performance of O4 in the first and twentieth cycles and shows that the specific capacity is stable between 100 and 150 mAh / g during cycling, with the voltage limited to 3.6 V during charge and 1.5 V during discharge.

[0041] β-Li5Fe 0.75 Al 0.25 The electrochemical performance of O4 (x = 0.25) is shown in Figure 5a and 5b In. Figure 5a As can be seen in Figure 3, a specific capacity of over 800 mAh / g is achieved in the first charge. If the discharge is limited to 1.5 V, only about 220 mAh / g can be extracted in the first discharge, although this increases to about 410 mAh / g if the discharge is instead limited to 1 V. Figure 5b β-Li5Fe 0.75 Al 0.25 Performance of O4 in the first, second, third, and 20th cycles. While only about 80 mAh / g was withdrawn in the first discharge during this test, subsequent cycles showed that the specific capacity stabilized at about 150 mAh / g upon cycling, with the voltage limited to 3.6 V on charge and 1.5 V during discharge.

[0042] It will be appreciated that various modifications may be made to the invention as described herein without departing from the scope of protection defined by the appended claims.

Claims

1. A cathode material for a battery, wherein the cathode material has an inverse fluorite crystal structure and is of the formula Li5Fe (1-x) Al x The group represented by O4, where x is greater than 0 and less than or equal to 1.

2. The cathode material according to claim 1, wherein x is less than or equal to 0.

5. The cathode material according to claim 2 , wherein x is greater than or equal to 0.1 and less than or equal to 0.

25.

4. The cathode material according to any one of the preceding claims, wherein the crystal structure of the cathode material occupies the orthorhombic Pbca space group.

5. A method for preparing a cathode for a battery, the method comprising: Mix Li2O, LiFeO2 and Al2O3 powders in a stoichiometric ratio; ball milling the powder; heating the powder to consolidate it into a cathode material; and allowing the cathode material to cool to room temperature; wherein the stoichiometric ratio is such that the cathode material has the formula Li5Fe (1-x) Al x O4, where x is greater than 0 and less than or equal to 1.

6. The method of claim 5, comprising heating the powder to between 650°C and 750°C.

7. The method of claim 6, comprising heating the powder to 700°C.

8. The method of claim 5, comprising heating the powder to between 850°C and 1000°C.

9. The method of any one of claims 5 to 8, comprising heating the powder for at least 15 hours.

10. The method of any one of claims 5-9, wherein the cathode material is cooled to room temperature at a rate of at least 5°C / minute.

11. The method according to any one of claims 5 to 10, wherein x is less than or equal to 0.

5. The method according to claim 11 , wherein x is greater than or equal to 0.1 and less than or equal to 0.

25.

13. A cathode comprising the cathode material according to any one of claims 1 to 4.

14. An electrochemical cell comprising the cathode according to claim 13, an electrolyte and an anode.

15. The electrochemical cell of claim 14, wherein the cell is a secondary battery.

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