Abstract
The invention relates to a lithium positive electrode active material for a high voltage secondary battery, where the cathode is fully or partially operated above 4.4 V vs. Li/Li+. The lithium positive electrode active material comprises at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, 0.02≤z≤0.2, wherein D is a dopant chosen between the following elements: Co, Cu, Ti, Zn, Mg, Fe or combinations thereof. The lithium positive electrode active material is a powder composed of secondary particles formed by primary particles, wherein said lithium positive electrode active material has a tap density of at least 1.9 g/cm.sup.3. The invention also relates to process for preparing the lithium positive electrode active material of the invention and a secondary battery comprising the lithium positive electrode active material of the invention.
Claims
1. A lithium positive electrode active material for a high voltage secondary battery, where the cathode is fully or partially operated above 4.4 V vs. Li/Li+, said lithium positive electrode active material comprising at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, 0.02≤z≤0.2, wherein D is a dopant chosen between the following elements: Co, Cu, Ti, Zn, Mg, Fe or combinations thereof, wherein the lithium positive electrode active material is a powder composed of secondary particles formed by primary particles, wherein said lithium positive electrode active material has a tap density of at least 1.9 g/cm.sup.3.
2. A lithium positive electrode active material according to claim 1, wherein the dopant D is distributed substantially uniformly throughout the lithium positive electrode material.
3. A lithium positive electrode active material according to claim 1, wherein at least 90% of said dopant D is incorporated in the spinel of said lithium positive electrode material.
4. A lithium positive electrode active material according to claim 1, wherein 0.96≤x≤1.0 in the composition Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4.
5. A lithium positive electrode active material according to claim 1, wherein said lithium positive electrode active material is cation disordered.
6. A lithium positive electrode active material according to claim 1, wherein the BET surface area of the secondary particles is below 0.25 m.sup.2/g.
7. A lithium positive electrode active material according to claim 1, wherein the secondary particles are characterized by an average circularity higher than 0.55 and simultaneously an average aspect ratio lower than 1.60.
8. A lithium positive electrode active material according to claim 1, wherein D50 of the secondary particles is between 3 and 50 μm.
9. A lithium positive electrode active material according to claim 8, wherein the distribution of the agglomerate size of the secondary particles is characterized in that the ratio between D90 and D10 is smaller than or equal to 4.
10. A lithium positive electrode active material according to claim 1, wherein the diameter or the volume equivalent diameter of the primary particles larger than D5 is between 100 nm and 2 μm and where the diameter or the volume equivalent diameter of the secondary particles is between 1 μm and 25 μm.
11. A lithium positive electrode active material according to claim 1, wherein at least 90% of the dopant D is part of the spinel.
12. A lithium positive electrode active material according to claim 1, the capacity of the lithium positive electrode active material is above 120 mAh/g.
13. A lithium positive electrode active material according to claim 1, wherein the separation between the two Ni-plateaus around 4.7 V of the lithium positive electrode active material is at least 50 mV.
14. A process for preparing a lithium positive electrode active material comprising at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, and 0.02≤z≤0.2 wherein D is a dopant chosen between the following elements: Co, Cu, Ti, Zn, Mg, Fe or combinations thereof, wherein the lithium positive electrode active material is composed of particles, wherein said lithium positive electrode active material has a tap density of at least 1.9 g/cm.sup.3 and wherein said lithium positive electrode active material comprises at least 95 wt % spinel phase, said process comprising the steps of: a) providing a lithium positive electrode active material comprising at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-yO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, b) mixing the lithium positive electrode active material of step a) with a dopant precursor of the dopant D, c) heating the mixture of step b) to a temperature of between 600° C. and 1000° C.
15. A process for preparing a lithium positive electrode active material comprising at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, and 0.02≤z≤0.2, wherein D is a dopant chosen between the following elements: Co, Cu, Ti, Zn, Mg, Fe or combinations thereof, wherein the lithium positive electrode active material is composed of particles, wherein said lithium positive electrode active material has a tap density of at least 1.9 g/cm.sup.3 and wherein said lithium positive electrode active material comprises at least 95 wt % spinel phase, said process comprising the steps of: a) providing precursors for preparing a lithium positive electrode active material comprising at least 95 wt % spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, wherein 0.9≤x≤1.1, 0.4≤y≤0.5, and 0.02≤z≤0.2, said precursors comprising Ni, Mn, Li and the dopant D, and b) heating the precursors of step a) to a temperature of between 600° C. and 1000° C.
16. A method according to claim 15, wherein the precursors comprise both lithium carbonate and either nickel carbonate and manganese carbonate or nickel manganese carbonate.
17. A secondary battery comprising a positive electrode which comprises the lithium positive electrode active material according to claim 1.
Description
SHORT DESCRIPTION OF THE FIGURES
[0047] The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.
[0048] FIG. 1 shows X-ray diffraction (XRD) pattern of an LMNO material doped with Co;
[0049] FIG. 2 shows elemental distribution of multiple secondary particles of a lithium positive electrode active material according to the invention;
[0050] FIG. 3 shows elemental mapping of a single primary particle from a lithium positive electrode active material according to the invention;
[0051] FIG. 4 shows two representative SEM images of a lithium positive electrode active material according to the invention;
[0052] FIG. 5 shows the effect of doping on stability for an undoped lithium positive electrode active material and similar but doped lithium positive electrode active materials according to the invention;
[0053] FIG. 6a shows the result of an electrochemical cycling test at 55° C. as described in Example 1;
[0054] FIG. 6b shows the discharge capacity of six doped lithium positive electrode active materials shown in FIG. 6a;
[0055] FIG. 7 shows voltage curves of 3.sup.rd discharge at 0.2 C and 55° C. for reference and doped samples; and
[0056] FIG. 8 shows capacity between 4.4 V and 4.2 V during 3.sup.rd discharge at 0.2 C and 55° C. for reference and doped samples.
[0057] FIG. 9a shows the relationship between circularity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;
[0058] FIG. 9b shows the relationship between roughness and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;
[0059] FIG. 9c shows the relationship between average diameter and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;
[0060] FIG. 9d shows the relationship between aspect ratio and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;
[0061] FIG. 9e shows the relationship between solidity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry; and
[0062] FIG. 9f shows the relationship between porosity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry.
DETAILED DESCRIPTION OF THE FIGURES
[0063] FIG. 1 shows an X-ray diffraction (XRD) pattern of a lithium positive electrode active material in the form of a LMNO material doped with Co. The sample has the composition Li.sub.0.96Ni.sub.0.44Mn.sub.1.47Co.sub.0.09O.sub.4. Marked peaks refer to the spinel phase of the LMNO material. Rietveld refinement shows that the doped lithium positive electrode active material has 96 wt % spinel phase and a primary particle size of 220 nm.
[0064] FIG. 2 shows elemental distribution of a lithium positive electrode active material according to the invention. The lithium positive electrode active material is a Co doped LMNO material having the nominal composition Li.sub.0.96Ni.sub.0.44Mn.sub.1.47Co.sub.0.09O.sub.4. The pictures of FIG. 2 show elemental analysis by wavelength-dispersive X-ray spectroscopy so that FIG. 2a shows the distribution of Mn within the secondary particles. FIGS. 2b and 2c show the distribution of Co and Ni, respectively, within the secondary particles of the lithium positive electrode active material. FIG. 2d shows the secondary electron image. Prior to the x-ray spectroscopy, the lithium positive electrode active material has been embedded into an epoxy material and ground in order to reveal the inside of the lithium positive electrode material. From FIG. 2b it is clear that the dopant, in this case cobalt, is uniformly distributed inside the secondary particles of the lithium positive electrode active material.
[0065] FIG. 3 shows elemental mapping of a single primary particle from a lithium positive electrode active material according to the invention. The mapping of elements across the single primary particle is STEM-EDS mapping. FIG. 3A has four individual images, where the image with the indication “HAADF” is a high-angle annular dark-field image of the primary particle, and the images with indication “Mn”, “Ni”, and “Co”, respectively, are mapping across the primary particles of manganese, nickel, and cobalt, respectively. The primary particle is of a lithium positive electrode active material composition Li.sub.0.96Ni.sub.0.44Mn.sub.1.47Co.sub.0.09O.sub.4. From the Co map of FIG. 3A, it is clear that the dopant distribution, viz. the Co distribution, is uniform across the primary particle. This is also seen by the line profile of FIG. 3B. The line profile is measured along the path marked with two black lines in the HAADF map of FIG. 3A.
[0066] FIG. 4 shows two representative SEM images of a lithium positive electrode active material according to the invention. The lithium positive electrode active material has the composition Li.sub.0.96Ni.sub.0.44Mn.sub.1.47Co.sub.0.09O.sub.4. FIG. 4 shows secondary particles of the material and it is seen from FIG. 4 that the secondary particles are spherical and have a diameter in the range from about 6 to about 10 μm. Primary particles are seen as the facetted objects in the surface of the secondary particles.
[0067] FIG. 5 shows the effect of doping on stability for an undoped lithium positive electrode active material and similar but doped lithium positive electrode active materials according to the invention. The effect of doping on stability is shown as the degradation after 100 cycles at 55° C. in 2032 type coin cell half cells. This is described more thoroughly in Example 1 below.
[0068] All doped LMNO materials shown in FIG. 5 have the nominal composition Li.sub.0.96Ni.sub.0.44Mn.sub.1.47D.sub.0.09O.sub.4, where D is the dopant, viz. Co, Cu, Mg, Ti, Zn, or Fe. From FIG. 5 it is seen that each of the doped materials has a reduced degradation compared to the undoped material. Whilst Li.sub.0.96Ni.sub.0.44Mn.sub.1.47Ti.sub.0.09O.sub.4 shows a 1 C degradation of about 3.3%, Fe shows 1 C degradation of less than 3%, Zn a 1 C degradation of less than 2%, Co a 1 C degradation of about 1%, whilst Mg and Cu have the lowest 1 C degradation, viz. of about 0.3% and 0.1%, respectively.
[0069] FIG. 6a shows the result of an electrochemical cycling test following an electrochemical power test (cycle 1 in the Figure corresponds to cycle 32 in Example 1) at 55° C. To ease comparison between the different samples, the discharge capacities have been normalized to 1 in the first 1 C cycle (cycle 2 in the graph). In FIG. 6a, a reference material and six lithium positive electrode active materials according to the invention and prepared as described in Example 2 have been tested. The lithium positive electrode active materials of the invention have a nominal composition of Li.sub.0.96Ni.sub.0.44Mn.sub.1.47D.sub.0.09O.sub.4, where D is the dopant, viz. Co, Cu, Mg, Ti, Zn or Fe, whilst the reference material is the undoped lithium positive electrode active material described in Example 2, i.e. Li.sub.1.0Ni.sub.0.46Mn.sub.1.54O.sub.4.
[0070] It is seen from FIG. 6a that the six doped lithium positive electrode active materials have increased stability, in that the capacity of the lithium positive electrode active materials of the invention decrease by no more than 3.3% over 100 cycles between from 3.5 to 5.0 V at 55° C. as described in Example 1. This is significantly better than the stability of the reference material as shown in FIGS. 5 and 6a.
[0071] FIG. 6b shows the discharge capacity of six doped lithium positive electrode active materials shown in FIG. 6a. From FIG. 6b it can be seen that even though doping of the lithium positive electrode active material has benefits in relation to decreasing the degradation, this benefit may be accompanied by a lowering of the discharge capacity for some of the dopants. The choice of dopant and the amount thereof can be optimized in order to obtain a lithium positive electrode active material having both a high discharge capacity and a low degradation.
[0072] FIG. 7 shows voltage curves of 3.sup.rd discharge at 0.2 C and 55° C. for a reference sample and for doped samples of the material according to the invention. The capacity is normalized to the total discharge capacity. Clear differences are seen, between the reference sample and the doped sample of a material according to the invention, in the final part of the discharge, where the voltage drops below 4.6 V. It is seen that all doped samples have a higher relative amount of capacity at voltage values below 4.6V compared to the reference sample.
[0073] FIG. 8 shows capacity between 4.4 V and 4.2 V during 3.sup.rd discharge at 0.2 C and 55° C. for a reference sample and for doped samples of the material according to the invention. This capacity between 4.4 V and 4.2 V during the discharge is a measure of the slope of the voltage curve when moving between Mn-redox activity around 4 V and Ni-redox activity around 4.7 V. A steep slope of this voltage curve, and thus small value of the capacity between 4.2 V and 4.4 V, seems to indicate a material with a relatively high degradation. It seems that a steep slope of the voltage curve correlates to a high strain which may give rise to an increase of the degradation of the material. This is especially the case at high discharge rates. Comparing with FIG. 5, it is supported that a high capacity between 4.2 V and 4.4 V decreases degradation.
[0074] FIGS. 9a-9f show the relationship between degradation and a range of parameters for the four samples of lithium positive electrode active materials have differing degradations values, but very similar spinel stoichiometries. Of the four samples shown in FIG. 9a-9f, the spinel of three of the samples has the spinel stoichiometry LiNi.sub.0.454Mn.sub.1.546O.sub.4, whilst the spinel of the fourth sample has the spinel stoichiometry LiNi.sub.0.449Mn.sub.1.551O.sub.4. The four samples are all prepared based on co-precipitated precursors and the particles are secondary particles. Even though these four samples are non-doped, viz. z=0 in the formula Li.sub.xNi.sub.yMn.sub.2-y-zD.sub.zO.sub.4, the impact of circularity, roughness, average diameter, aspect ratio, solidity and internal porosity on degradation correspond is the same as the impact of these factors on a similar material with doping, viz. 0.02≤z≤0.2. However, the doping of the material further assists in decreasing the degradation.
[0075] FIG. 9a shows the relationship between circularity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The circularity of a secondary particle is measured from the area and the perimeter of the particle shape as 4π*[Area]/[Perimeter].sup.2. Circularity describes both overall shape and surface roughness, where a higher value means more circular shape and smoother surface. A circle with a smooth surface has circularity 1. Average circularity is the arithmetic mean of the circularities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 9a it is seen that higher value of circularity corresponds to lower degradation.
[0076] FIG. 9b shows the relationship between roughness of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The roughness of a secondary particle is measured as the ratio between the perimeter and the perimeter of an ellipse fitted to the particle shape. Roughness describes how rough the surface is, where a higher value means rougher surface. Average roughness is the arithmetic mean of the roughnesses of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagei.nih.gov). In FIG. 9b it is seen that lower value of roughness corresponds to lower degradation.
[0077] FIG. 9c shows the relationship between average diameter of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The diameter of a secondary particle is measured as the equivalent circle diameter, i.e. the diameter of a circle with the same area as the particle. Average diameter is the arithmetic mean of the diameters of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 9c it is seen that a lower average diameter to lower degradation. The average diameter of secondary particles is given in μm.
[0078] FIG. 9d shows the relationship between aspect ratio of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The aspect ratio of a secondary particle is measured from an ellipse fitted to the particle shape. The aspect ratio is defined as [Major axis]/[Minor Axis] where Major axis and Minor Axis are the major and minor axes of the fitted ellipse. Average aspect ratio is the arithmetic mean of the aspect ratios of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagei.nih.gov). In FIG. 9d it is seen that a lower aspect ratio in general corresponds to lower degradation.
[0079] FIG. 9e shows the relationship between solidity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The solidity of a secondary particle is defined as the ratio between the particle area and the area of the convex area, i.e. [Area]/[Convex Area]. The convex area can be thought of as the shape resulting from wrapping a rubber band around the particle. The more concave features in a particle's surface, the higher is the convex area and the lower is the solidity. Average solidity is the arithmetic mean of the solidities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagei.nih.gov). In FIG. 9e it is seen that higher values of solidity correspond to lower degradation.
[0080] FIG. 9f shows the relationship between porosity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The porosity of a secondary particle is the percentage of the internal area that appears with dark contrast in the SEM image, where dark contrast is interpreted as a porosity, i.e. a hole inside the particle. Average porosity is the arithmetic mean of the porosities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagei.nih.gov). In FIG. 9f it is seen that a lower value of porosity in general corresponds to lower degradation.
EXAMPLES
Example 1
[0081] Electrochemical tests have been realized in 2032 type coin cells, using thin composite positive electrodes of doped lithium positive electrode active material according to the invention and metallic lithium negative electrodes (half-cells). The thin composite positive electrodes were prepared by thoroughly mixing 84 wt % of lithium positive electrode active material (prepared as described in Example 2) with 8 wt % Super C65 carbon black (Timcal) and 8 wt % PVdF binder (polyvinylidene difluride, Sigma Aldrich) in NMP (N-methyl-pyrrolidone) to form a slurry. The slurry was spread onto a carbon coated aluminum foil using a doctor blade with a 160 μm gap and dried for 2 hours at 80° C. to form a film.
[0082] Electrodes with a diameter of 14 mm and a loading of approximately 7 mg of lithium positive electrode active material were cut from the dried film, pressed in a hydraulic pellet press (diameter 20 mm; 3 tonnes) and subjected to 10 hours drying at 120° C. under vacuum.
[0083] Coin cells were assembled in argon filled glove box (<1 ppm O.sub.2 and H.sub.2O) using two polymer separators (Toray V25EKD and Freudenberg FS2192-11SG) and 100 μL electrolyte containing 1 molar LiPF.sub.6 in EC:DMC (1:1 in weight). Two 250 μm thick lithium disks were used as counter electrodes and the pressure in the cells were regulated with a stainless steel disk spacer and disk spring on the negative electrode side.
[0084] Electrochemical lithium insertion and extraction was monitored with an automatic cycling data recording system (Maccor) operating in galvanostatic mode. A power test was programmed to run the following cycles: 3 cycles 0.2 C/0.2 C (charge/discharge), 3 cycles 0.5 C/0.2 C, 5 cycles 0.5 C/0.5 C, 5 cycles 0.5 C/1 C, 5 cycles 0.5 C/2 C, 5 cycles 0.5 C/5 C, 5 cycles 0.5 C/10 C, and then 0.5 C/1 C cycles with a 0.2 C/0.2 C cycle every 20.sup.th cycle. C-rates were calculated based on the theoretical specific capacity of the material of 148 mAhg.sup.−1 so that e.g. 0.2 C corresponds to 29.6 mAg.sup.−1 and 10 C corresponds to 1.48 Ag.sup.−1. Degradation per 100 cycles is measured from after the power test, i.e. from cycle 33 to cycle 133.
Example 2
[0085] Preparation of doped lithium positive electrode active material can be made by heating a lithium positive electrode active material, i.e. Li.sub.xNi.sub.yMn.sub.2-yO.sub.4 (LNMO), with a dopant precursor. In this example, Li.sub.1.0Ni.sub.0.46Mn.sub.1.54O.sub.4 has been used as undoped starting material and DNO.sub.3 has been used as dopant precursor, where D is the dopant, viz. Co, Cu, Mg, Ti, Zn, or Fe.
[0086] D-nitrate (e.g. CoNO.sub.3) is dissolved 1:1 by weight in water and added to 20 g LNMO material in stoichiometric ratio in order to obtain an average composition of Li.sub.0.96Ni.sub.0.44Mn.sub.1.47D.sub.0.09O.sub.4 in the doped lithium positive electrode active material. The slurry is dried at 80° C. and calcined at 700° C. 4 h.
