Layered lithium metal oxides free of localized cubic spinel-like structural phases and methods of making same

Field of the Invention

The present invention relates to lithium metal oxides for use as positive electrode materials for lithium and lithium-ion secondary batteries, and to methods of making lithium metal oxides.

Background of the Invention

Lithium metal oxides of the formula LiMO₂, wherein M is a transition metal, are important cathode (positive electrode) materials for rechargeable lithium and lithium-ion batteries. Examples of LiMO₂ compounds include LiCoO₂, LiNiO₂, and LiMnO₂. Presently, LiCoO₂ is used in most commercial lithium and lithium-ion batteries as a cathode material.

LiMO₂ compounds can have different crystal structures and phases, even within the same compound. For example, LiCoO₂ synthesized at greater than 700°C has a hexagonal layered structure analogous to α-NaFeO₂. LiCoO₂ synthesized at around 400°C, however, has a cubic spinel-like structure analogous to Li₂Ti₂O₄. Both structures have essentially the same FCC (face centered cubic) closed packed arrangement for oxygen except the layered structure has a small distortion in the direction perpendicular to the layers. Additionally, the two structures differ in cation arrangement.

It has been determined that the cubic spinel-like LiCoO₂ turns into hexagonal layered LiCoO₂ when heated to temperatures above 700°C. Therefore, phase transformation between the two structures is possible and the layered structure is energetically favored only at high temperatures. Layered LiCoO₂ also has an energetically favored tendency of changing into spinel LiCo₂O₄ when 50% of the lithium ions are removed from the LiCoO₂ during electrochemical charging. See<nplcit id="ncit0001" npl-type="s"><text> A. van der Ven et al, Phys, Rev. B 58, 2975 (1998</text></nplcit>); and<nplcit id="ncit0002" npl-type="s"><text> H. Wang et al., J. Electrochem. Soc, 146, 473 (1999</text></nplcit>). The spinel-like LiCoO₂ and spinel LiCo₂O₄ also have essentially the same atom arrangement except that lithium is at the octahedral 16c site in spinel-like LiCoO₂ and at tetrahedral 8a site in spinel LiCo₂O₄.

The tendency of the phase transformation from hexagonal layered LiMO₂ to cubic spinel-like LiMO₂ is not unique to LiCoO₂. Layered LiMnO₂ also turns into spinel-like LiMnO₂ only after a few cycles in an electrochemical cell. Although a cubic spinel-like LiNiO₂ has not been experimentally observed, Li₀₅NiO₂ (50% delithiated LiNiO₂) will indeed turn into LiNi₂O₄ spinel.

The electrochemical performance of LiMO₂ compounds having a cubic spinel-like structure has been found to be particularly poor, especially compared to layered structures. Moreover, the mere presence of the cubic spinel-like structural phase within the layered phase or on the surface of the layered phase has also been found to be detrimental to battery performance. In particular, the presence of cubic spinel-like phases within the layered crystal structure impedes the diffusion of lithium ions during the charge and discharge cycles of the rechargeable lithium or lithium-ion battery. Furthermore, because the cubic spinel-like phase is energetically favored and only kinetic limitations prevent large scale phase transformation, the presence of localized cubic spinel-like structures can act as a seed for phase transformation to readily occur in the LiMO₂ compound. Therefore, even the minor presence of cubic spinel-like phases, even at levels that cannot be detected by bulk techniques, such as powder x-ray diffraction (XRD), can cause problems in battery cycling.

Summary of the Invention

The present invention provides lithium metal oxides that are substantially single-phase compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases. Therefore, the lithium metal oxides of the invention have more consistent electrochemical performance than prior art compounds. In addition, the lithium metal oxide compounds of the invention have good structural stability and maintain their structure through cycling. Therefore, the lithium metal oxides of the invention are useful for rechargeable lithium and lithium ion secondary batteries.

The lithium metal oxides of the invention have the formula Li_αM_βAγO₂, wherein M is Co, A is one or more dopants selected from Ti, Zr, Mg, Ca, Sr, Ba, Al, Ga, Si, Ge, Sn and combinations thereof having an average oxidation state N such that +2.5 ≤ N ≤ +3.5, 0.90 ≤ α ≤ 1.10, γ &gt; 0 and β + γ = 1, said compound having a substantially single phase, hexagonal layered crystal structure and being substantially free of localized cubic spinel-like structural phases, wherein in the powder x-ray diffraction pattern, there are no diffraction peaks at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003), said compound obtained by uniformly cooling the compound from a temperature of at least about 600°C at a rate of between 8°C/min and 140°C/min. In addition, the ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (110) to the integrated intensity of the diffraction peak corresponding to Miller indices (108) using powder x-ray diffraction is preferably greater than or equal to 0.7, more preferably greater than or equal to 0.8. The ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (102) to the integrated intensity of the diffraction peak corresponding to Miller indices (006) using powder x-ray diffraction is preferably greater than or equal to 1.0, more preferably greater than or equal to 1.2. The average oxidation state of the dopants N is preferably about +3.

In addition to the Li_αM_βA_γO₂ compounds above, the present invention is also directed to the dilithiated forms of these compounds resulting from the electrochemical cycling of these compounds. Specifically, the present invention includes Li_α-xM_βA_γO₂ compounds wherein 0≤x≤α that are derived by electrochemically removing x Li per formula unit from a compound having the formula Li_αM_βA_γO₂, wherein M is Co, A is one or more dopants selected from the elements mentioned hereinbefore having an average oxidation state N such that +2.5 ≤ N ≤ +3.5, 0.90 ≤ α ≤ 1.10, γ &gt; 0 and β + γ = 1. The Li_α-xM_βA_γO₂ compounds are substantially single-phase lithium metal oxide compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases.

The present invention further includes lithium and lithium ion secondary batteries including a positive electrode comprising the compound having the formula Li_αM_βA_γO₂ and described hereinbefore. The Li_αM_βA_γO₂ compound used in the positive electrode has a substantially single phase, hexagonal layered crystal structure and is substantially free of localized cubic spinel-like structural phases.

The present invention further includes a method of preparing compounds having a substantially single phase, hexagonal layered crystal structure that are substantially free of localized cubic spinel-like structural phases. The lithium metal oxide having the formula Li_αM_βA_γO₂ and described hereinbefore is provided at a temperature of at least about 600°C, and preferably of greater than 800°C. The lithium metal oxide is then cooled at a rate of between 8°C/min and 140°C/min, more preferably between 10°C/min and 100°C/min. The lithium metal oxide can be synthesized at a temperature of at least about 600°C, and preferably of greater than 800°C, and then cooled at these rates, or the lithium metal oxide can be previously synthesized, heated to a temperature of at least about 600°C, and preferably of greater than 800°C, and then cooled at these rates. The lithium metal oxide is preferably uniformly cooled to provide homogeneity throughout the material being produced.

These and other features and advantages of the present invention will become more readily apparent to those skilled in the art upon consideration of the following detailed description and accompanying drawings, which describe both the preferred and alternative embodiments of the present invention.

Brief Description of the Drawings

<ul id="ul0001" list-style="none"><li><figref idref="f0001">Fig. 1</figref> is a graph comparing the cycle performance between two samples (sample 1 and sample 2).</li><li><figref idref="f0002">Fig. 2</figref> is a graph illustrating the electron paramagnetic resonance (EPR) spectrum of a weak pitch standard sample with a correction factor of 1.14.</li><li><figref idref="f0003">Fig. 3</figref> is a graph illustrating the EPR spectrum of sample 1.</li><li><figref idref="f0004">Fig. 4</figref> is a graph illustrating the EPR spectrum of sample 2.</li><li><figref idref="f0005">Fig. 5</figref> is a graph illustrating thermogravimetric analysis (TGA) curves for sample 1 and sample 2.</li><li><figref idref="f0006">Fig. 6</figref> is a powder x-ray diffraction pattern for sample 2 using Cu K_α radiation.</li><li><figref idref="f0007">Fig. 7</figref> is a graph comparing the cycle performance of two samples (sample 3 and sample 4).</li></ul>

Detailed Description of the Preferred Embodiments of the Invention

In the following detailed description, preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

The present invention is directed to substantially single-phase lithium metal oxide compounds having hexagonal layered crystal structures that are substantially free of localized cubic spinel-like structural phases on the surface of the crystal or within the crystal. The lithium metal oxides of the invention have the formula Li_αM_βA_γO₂, wherein M is Co, A is one or more dopants selected from Ti, Zr, Mg, Ca, Sr, Ba, Al, Ga, Si, Ge, Sn and combinations thereof having an average oxidation state N such that +2.5 ≤ N ≤ +3.5, 0.90 ≤ α ≤ 1.10, γ &gt; 0 and β + γ = 1, said compound having a substantially single phase, hexagonal layered crystal structure and being substantially free of localized cubic spinel-like structural phases, wherein in the powder x-ray diffraction pattern, there are no diffraction peaks at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003), said compound obtained by uniformly cooling the compound from a temperature of at least about 600°C at a rate of between 8°C/min and 140°C/min.

The dopants A are elements other than M selected to produce an oxidation state N wherein +2.5 ≤ N ≤ +3.5, and preferably N is about 3. As would be readily understood by those skilled in the art, the average oxidation state N is based on the molar amounts of the dopants used and the valences of the dopants used. For example, if the dopants are 40% Ti⁴⁺ and 60% Mg²⁺, on a molar basis, the average oxidation state N would be (0.4)(+4) + (0.6)(+2) = +2.8.

As defined above, the dopants A are used to replace the transition metal M and are not used to take the place of lithium ions in the lithium metal oxide, i.e., β = 1 - γ. Therefore, the reversible capacity is maximized in the intercalation compounds of the invention. Dopants for use in the invention are Ti, Zr, Mg, Ca, Sr, Ba, Al, Ga, Si, Ge, Sn and combinations thereof. For example, A can include equal amounts of dopants Ti⁴⁺ and Mg²⁺. In the compounds of the invention, γ is greater than 0 and preferably less than about 0.5.

The substantially single-phase, hexagonal layered structures of the compounds of the invention are characterized by their powder x-ray diffraction patterns. As measured using powder x-ray diffraction, the Li_αM_βA_γO₂ compounds according to the invention have no diffraction peaks at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003) thereby demonstrating that the compounds of the invention are substantially single phase. In addition, the ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (110) to the integrated intensity of the diffraction peak corresponding to Miller indices (108) using powder x-ray diffraction is preferably greater than or equal to 0.7, more preferably greater than or equal to 0.8. The ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (102) to the integrated intensity of the diffraction peak corresponding to Miller indices (006) using powder x-ray diffraction is preferably greater than or equal to 1.0, more preferably greater than or equal to 1.2. The integrated intensities for these measurements is based on the area measured below the respective peaks. Alternatively, the heights of the peaks can be used to provide a rough comparison of the integrated intensities and because the widths of the peaks are relatively uniform, the ratios of peak heights are approximately equal to the ratios of the integrated intensities for the two peaks being compared.

The present invention further includes a method of preparing compounds having a substantially single phase, hexagonal layered crystal structure that are substantially free of localized cubic spinel-like structural phases. In accordance with this method, a lithium metal oxide is provided having the formula Li_αM_βA_γO₂ and described hereinbefore at a temperature of at least about 600°C, and preferably of greater than 800°C. The lithium metal oxide can be provided at these temperatures by either synthesizing the material at these temperatures or by heating previously synthesized material.

The lithium metal oxide compounds of the invention can be prepared or synthesized by mixing together stoichiometric amounts of source compounds containing lithium, M and A to give the desired molar ratio for the formula Li_αM_βA_γO₂ described above. The source compounds (raw materials) can be the pure elements but are typically compounds containing the elements such as oxides or salts thereof. For example, the source compounds are typically hydrated or anhydrous oxides, hydroxides, carbonates, nitrates, sulfates, chlorides or fluorides, but can be any other suitable source compound that will not cause elemental defects in the resulting lithium metal oxide compound. The elements for the lithium metal oxide compound can each be supplied from separate source compounds or at least two of the elements can be supplied from the same source compounds. In addition, the source compounds can be mixed in any desirable order.

Although the lithium metal oxide compounds are preferably prepared by solid state reactions, it can be advantageous to react the raw materials using wet chemistry such as sol-gel type reactions or spray drying techniques, alone or in combination with solid state reactions. For example, the source compounds comprising the M and A can be prepared as a solution in a solvent such as water and the M and A precipitated out of solution as an intimately mixed compound such as a hydroxide. The mixed compound can then be blended with a lithium source compound. The reaction mixture can also be prepared by suspending source compounds in a solution of other source compounds and spray drying the resulting slurry to obtain an intimate mixture. Typically, the selection of reaction methods will vary depending on the raw materials used and the desired end product.

In a preferred method embodiment of the invention, the lithium metal oxide is prepared using a lithium source compound and a cobalt source compound. In particular, the preferred lithium source compound is selected from the group consisting of Li₂CO₃ and LiOH and the preferred cobalt source compound is selected from the group consisting of Co₃O₄ and Co(OH)₂.

The mixture once prepared can be reacted to form the lithium metal oxide. Preferably, the mixture is reacted by firing the mixture at a temperature between 600°C and 1000°C for sufficient time to produce the lithium metal oxide compound in a single phase. The mixture is generally fired for a total of between about 4 and about 48 hours in one or more firing steps. Any suitable apparatus can be used for firing the mixture, such as a rotary calciner, a stationary furnace or a tunnel furnace, that uniformly heats the source compounds to produce the lithium metal oxide.

Once the lithium metal oxide is at its final preparation temperature or after previously synthesized lithium metal oxide has been reheated, the lithium metal oxide is cooled at a rate of between 8°C/min and 140°C/min, more preferably between 10°C/min and 100°C/min. It has been discovered that cooling at a rate of greater than 140°C/min results in a structure with high crystalline stress and strain that does not have the strength of lithium metal oxides cooled at a rate of between 8°C/min and 140°C/min. Moreover, it has been discovered that cooling at a rate of less than 8°C/min results in the formation of localized cubic spinel-like structural phases on the surface of the crystal or within the crystal and thus decreased electrochemical performance. With the lithium metal oxides of the invention, the lack of localized hetero-structural phases, e.g., cubic spinel-like phases, within the crystal and on the crystal surface does not induce further phase transformation that impedes the diffusion of the Li+ ions during the charge and discharge cycles. Thus, the hexagonal layered compounds of the invention have better and more consistent electrochemical performance than prior art compounds that are cooled at slower rates.

The lithium metal oxide is uniformly cooled (quenched) in accordance with the invention. In particular, the lithium metal oxide material is preferably cooled at approximately the same rate. For example, the variation between the mean cooling rate and the cooling rate for any specific portion of the material should be less than about 10 percent. In a preferred embodiment of the invention, uniform cooling can be accomplished using a rotary calciner, or a stationary furnace or tunnel furnace with smaller bed depths. The uniformly cooled material prepared according to the invention has greater homogeneity and less variance in its material properties than material that is not uniformly cooled.

The present invention further includes lithium and lithium ion secondary batteries that include a positive electrode comprising the lithium metal oxides of the invention. Typically, the lithium metal oxide compound of the invention is combined with a carbonaceous material and a binder polymer to form a cathode. The negative electrode of the lithium battery can be lithium metal or alloys, or any material capable of reversibly lithiating and delithiating at an electrochemical potential relative to lithium metal between about 0.0 V and 0.7 V. Examples of negative electrode materials include carbonaceous materials containing H, B, Si and Sn; tin oxides; tin-silicon oxides; and composite tin alloys. The negative electrode is separated from the positive electrode material in the cell using an electronic insulating separator. The electrochemical cell further includes an electrolyte. The electrolyte can be non-aqueous liquid, gel or solid and preferably comprises a lithium salt, e.g., LiPF₆. Electrochemical cells using the lithium metal oxide compounds of the invention as positive electrode material can be combined for use in portable electronics such as cellular phones, camcorders; and laptop computers, and in large power applications such as for electric vehicles and hybrid electric vehicles.

The lithium metal oxide compounds of the invention allow lithium ions to readily diffuse during both the charge and discharge cycles of the battery. In particular, in the discharge cycle for these lithium metal oxides wherein x Li per formula unit are electrochemically removed per formula unit, the lithium metal oxide takes the formula Li_αM_βA_γO₂, wherein 0 ≤ x ≤ α.

The lithium metal oxide compounds of the invention have been found to have good initial specific capacities and good cycleability as is desired in the art. For example, the initial specific capacity of the LiCoO₂ is greater than 140 mAh/g, preferably greater than 150 mAh/g. In addition, the capacity loss over 100 cycles for the lithium metal oxides of the invention is less than 25%, preferably less than 20%, with a constant current of C/3 (3 hours for complete charge or discharge) when cycled between 3.0 and 4.3 V versus lithium.

EXAMPLE 1

(not according to the invention)

A commercial LiCoO₂ sample (sample 1) was heated to 950°C for 1 hour and then quench cooled by taking the sample directly from the hot zone and spreading the sample onto a stainless steel pan at room temperature. The cooling time was estimated at about 10 minutes from 950°C to room temperature. Sample 1 and the quenched sample (sample 2) were used as positive electrode materials for different electrochemical cells, each cell using a coin cell configuration with Li metal as the negative electrode. NRC 2325 coin cell hardware and Celgard 3501 separators were used. The electrolyte was 1 M LiPF₆ in a 50:50 mixture of ethylene carbonate and dimethyl carbonate solvents. The positive electrode consisted of 85% active material (by weight), 10% super S™ carbon black and 5% polyvinylidene fluoride (PVDF) as a binder polymer, coated on aluminum foil. The cycle tests were conducted between 3.0 and 4.3 V using a constant current of C/3 (3 hours for complete charge or discharge) in both charge and discharge.

<figref idref="f0001">Fig. 1</figref> compares the cycle performance of sample 1 and sample 2. As shown in <figref idref="f0001">Fig. 1</figref>, sample 2 retains more capacity upon cycling than sample 1 and has much improved cycle performance over sample 1.

In addition, electron paramagnetic resonance (EPR) spectra of sample 1 and sample 2 were obtained using a Bruker Instruments EMX system. The sweep of the magnetic field was from 100 to 5100 Gauss, and the microwave frequency was fixed at 9.85 GHz. A Bruker Instruments' weak pitch standard (0.0035% pitch in KCl) with a correction factor of 1.14 was used to calibrate the intensity. <figref idref="f0002">Fig. 2</figref> shows the EPR spectrum from this standard. The intensity of the carbon feature from this standard, as shown in <figref idref="f0002">Fig. 2</figref>, is defined as 1.14 standard weak pitch units.

The LiCoO₂ samples (sample 1 and sample 2) were directly packed into EPR tubes without dilution for the measurement. The resulting EPR spectra of samples 1 and 2 are shown in <figref idref="f0003">Figs. 3</figref> and <figref idref="f0004">4</figref>, respectively. The sharp feature in both <figref idref="f0003">Figs. 3</figref> and <figref idref="f0004">4</figref> at around g = 2.14 is due to nickel impurities. The broad feature from about g = 14 to about g = 2.5 in <figref idref="f0004">Fig. 4</figref> is due to the high spin cobalt that is characteristic of the LiCoO₂.

Thermogravimetric analysis (TGA) of samples 1 and 2 were also conducted. As shown in <figref idref="f0005">Fig. 5</figref>, neither sample 1 nor sample 2 has any significant weight loss in the range of 650 to 900°C.

Sample 2 was further tested using powder x-ray diffraction with Cu Kα radiation to determine if this material had a substantially single-phase, hexagonal layered structure. As shown in <figref idref="f0006">Fig. 6</figref>, sample 2 has a ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (110) to the integrated intensity of the diffraction peak corresponding to Miller indices (108) using powder x-ray diffraction greater than or equal to 0.7, a ratio of the integrated intensity of the diffraction peak corresponding to Miller indices (102) to the integrated intensity of the diffraction peak corresponding to Miller indices (006) using powder x-ray diffraction greater than or equal to 1.0, and no diffraction peaks using powder x-ray diffraction at a smaller scattering angle than the diffraction peak corresponding to Miller indices (003).

EXAMPLE 2

(not according to the invention)

Stoichiometric amounts of Li₂CO₃ and Co₃O₄ were mixed and then heated at a rate of 3.75°C/min from room temperature to 950°C, held at 950°C for 5 hours, and then cooled to room temperature at a rate of about 3.7°C/min (total cooling time slightly longer than 4 hours). The resulting compound is sample 3.

Stoichiometric amounts of Li₂CO₃ and Co₃O₄ were mixed and then heated at a rate of 3.75°C/min from room temperature to 950°C, held at 950°C for 5 hours, and then cooled to room temperature at a rate of about 8°C/min (total cooling time just under 2 hours). The resulting compound is sample 4.

Samples 3 and 4 were cycle tested according to the method described in Example 1. <figref idref="f0007">Fig. 7</figref> compares the cycle performance of sample 3 and sample 4. As shown in <figref idref="f0007">Fig. 7</figref>, sample 4 prepared according to the invention has better cycling performance than sample 3.