Cathode Active Material For High Voltage Secondary Battery

Abstract

The invention relates to a cathode active material for a high voltage secondary battery with a cathode arranged for being fully or mainly operated above 4.4 V vs. Li/Li.sup.+, wherein the cathode active material is an oxide that comprises sulfate as a capacity fade reducing compound. The invention also relates to a cathode active material for a high voltage secondary battery having the composition Li.sub.xM.sub.yMn.sub.2yO.sub.4v(SO.sub.4).sub.z, where 0.9x1.1, 0.4y0.5, 0<z0.1, 0vz and M is a transition metal chosen from the group consisting of Ni, Mg, Ti, V, Cr, Fe, Co, Cu, Zn, Al, Ga, Rb, Ge, Mo, Nb, Zr, Si and combinations thereof, wherein the cathode active material comprises sulfate as a capacity fade reducing compound.

Furthermore, the invention relates to a secondary battery comprising the cathode active material according to the invention, and to a method for preparing the cathode active materials of the invention.

Claims

1. A cathode active material for a high voltage secondary battery with a cathode arranged for being fully or mainly operated above 4.4 V vs. Li/Li.sup.+, wherein the cathode active material is an oxide that comprises sulfate as a capacity fade reducing compound.

2. The cathode active material according to claim 1, wherein the sulfur content in the cathode active material is between 1000 and 16000 ppm.

3. The cathode active material according to claim 1, wherein the cathode active material comprises lithium.

4. The cathode active material according to claim 1, said cathode active material having the composition Li.sub.xM.sub.yMn.sub.2yO.sub.4v(SO.sub.4).sub.z, where 0.9x1.1, 0.4y0.5, 0<z0.1, 0vz and M is a transition metal chosen from the group consisting of Ni, Mg, Ti, V, Cr, Fe, Co, Cu, Zn, Al, Ga, Rb, Ge, Mo, Nb, Zr, Si and combinations thereof.

5. The cathode active material according to claim 4, wherein the transition metal M is Ni.

6. The cathode active material according to claim 1, wherein the mean primary particle size is above 50 nm.

7. The cathode active material according to claim 1, wherein d.sub.50 of the cathode active material secondary particles is between 1 and 50 m, and wherein the particle size distribution of the secondary particles is characterized by the ratio of d.sub.90 to d.sub.10 of less than 8.

8. The cathode active material according to claim 1, wherein the surface area of the cathode active material is less than 0.5 m.sup.2/g.

9. The cathode active material according to claim 1, wherein the tap density of the cathode active material is above 2 g/cm.sup.3.

10. The cathode active material according to claim 1, wherein the surface of the secondary particles is enriched in sulfate compared to the average composition of the material.

11. A secondary battery comprising the cathode active material according to claim 4 wherein the cathode is fully or mainly operated above 4.4 V vs. Li/Li.sup.+.

12. A method for preparing a cathode active material for a high voltage secondary battery having the composition Li.sub.xM.sub.yMn.sub.2yO.sub.4v(SO.sub.4).sub.z, where 0.9x1.1, 0.4y0.5, 0<z0.1, 0vz and M is a transition metal chosen from the group consisting of Ni, Mg, Ti, V, Cr, Fe, Co, Cu, Zn, Al, Ga, Rb, Ge, Mo, Nb, Zr, Si and combinations thereof, wherein the cathode active material comprises sulfate as a capacity enhancing compound, the process comprising the steps of: (a) mixing and/or co-precipitating starting materials containing metals and sulfur in appropriate molar ratios determined by the molar ratios between metals and sulfate in the final product; and (b) carrying out heat treatment at a temperature between 700 C. and 1200 C. of the mixture of step (a) to provide the cathode active material.

13. The method according to claim 12, wherein step (a) comprises the steps of: (a1) mixing and/or co-precipitating starting materials in the form of metal precursors; (a2) carrying out heat treatment at a temperature between 300 C. and 1200 C. of the mixture of step (a1), resulting in an intermediate, (a3) mixing the intermediate of step (a2) with a sulfate precursor to provide the mixture of step (a).

14. The method according to claim 13, wherein the starting materials comprises metal precursors in the form of one or more oxides, one or more hydroxides, one or more carbonates, one or more nitrates, one or more acetates, one or more oxalates or a combination thereof.

15. The method according to claim 12, wherein the sulfate precursor comprises a metal sulfate, where the metal is either Li, Ni or Mn or a combination thereof, or the sulfate precursor is a compound comprising SO.sub.4 and only leaving SO.sub.4.sup.2 behind in the final product.

16. The method according to any of the claims 12 to 15 claim 12, wherein step (b) is carried out at a temperature of between about 700 C. and about 1200 C. in an oxygen rich atmosphere.

17. The method according to claim 12, wherein step (a2) is carried out at a temperature of between about 300 C. and about 1200 C. in a reducing atmosphere.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0045] Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0046] FIG. 1 is a graph showing the calibrated S2p spectra obtained using XPS of two cathode active materials with sulfate doping, prepared as described in Examples 1 and 2, and Li.sub.2SO.sub.4 as a reference;

[0047] FIG. 2 is a graph of the amount of Li2SO4 in the sulfur doped cathode active material with the amount of sulfur added to the synthesis as described in Examples 1-3;

[0048] FIG. 3 are scanning electron micrographs (a and b) and energy-dispersive X-ray spectrograms (c, d and e) of a representative sulfur doped cathode active material particle as prepared in Example 2 with 8000 ppm S;

[0049] FIG. 4 is a graph showing the voltage profile of constant current charge and discharge of cathode active materials with and without sulfate doping, prepared as described in Example 1;

[0050] FIG. 5 is a graph showing the voltage profile of constant current discharges of cathode active materials with and without sulfate doping, prepared as described in Example 1;

[0051] FIG. 6 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 1;

[0052] FIG. 7 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 2;

[0053] FIG. 8 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 3; and

[0054] FIG. 9 is a graph showing the relative change in battery material parameters: discharge capacity, power capability, 0.2 C degradation and 1 C degradation as a function of sulfate doping in the cathode active material.

DETAILED DESCRIPTION OF THE FIGURES

[0055] In the following, reference is made to embodiments of the invention. However, it should be understood that the invention is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice the invention.

[0056] Moreover, in the following, the terms cathode active material is meant to denote a LNMO material with the formula Li.sub.xNi.sub.yMn.sub.2yO.sub.4v(SO.sub.4).sub.z, where 0.9x1.1, 0.4y0.5, 0<z0.1, 0vz. In addition, the term cathode active material is meant to cover reference samples with 0 ppm sulfur corresponding to z=0.

[0057] FIG. 1 is a graph showing the calibrated S2p spectra obtained using XPS of two cathode active materials with sulfate doping, prepared as described in Examples 1 and 2, and Li.sub.2SO.sub.4 as a reference. The binding energy is around 169 eV in all three cases, showing that the sulfur present is in the form of sulfate rather than sulfide in which the binding energy is around 161.5 eV. This is also evident by direct comparison with the spectrum of Li.sub.2SO.sub.4. The spectra are calibrated according to the C1s peak, predominantly from the carbon tape, and the peak heights and baselines are autoscaled.

[0058] FIG. 2 is comparing the amount of Li.sub.2SO.sub.4 in the sulfur doped cathode active material with the amount of sulfur added to the synthesis as described in Examples 1-3. The amount of Li.sub.2SO.sub.4 is determined by Rietveld refinement of XRD spectra acquired of the sulfur doped cathode active materials.

[0059] From FIG. 2 it is seen that too much sulfur leads to significant formation of Li.sub.2SO.sub.4. In the current example, more than 4000 ppm S will lead to the formation of Li.sub.2SO.sub.4. The presence on Li.sub.2SO.sub.4 is not desired because it does not contribute to the capacity of the cathode active material. Furthermore, Li.sub.2SO.sub.4 may be unstable in batteries operated mainly or partly above 4.4 V vs. Li/Li+.

[0060] FIG. 3 is scanning electron micrographs (a and b) and energy-dispersive X-ray spectrograms (c, d and e) of a representative sulfur doped cathode active material particle as prepared in Example 2 with 8000 ppm S. The grey substance on the particle in FIG. 3a is enlarged in FIG. 3b and analysed with EDX in FIGS. 3c-3e. From FIGS. 3c-3e it can be seen, that the grey substance contains S, but not Ni and Mn, and it is thus most likely excess sulfur in the form of Li2.sub.sO.sub.4.

[0061] FIG. 4 is a graph showing the voltage profile of constant current charge and discharge of cathode active material with and without sulfate doping, prepared as described in Example 1. The electrochemical measurements are performed in half cells at 50 C. with a current corresponding to 0.2 C. It is seen that the discharge capacity and the shape of the voltage profile are unchanged by sulfur doping. This indicates that the bulk properties of the material are unchanged.

[0062] FIG. 5 is a graph showing the voltage profile of constant current discharges of cathode active materials with and without sulfate doping, prepared as described in Example 1. The electrochemical measurements are performed in half cells at 50 C. with discharge currents corresponding to 0.5 C, 2 C and 10 C. The three uppermost curves correspond to 0.5 C, whilst the three curves in the middle correspond to 2 C and the three lowermost curves correspond to 10 C. The curve in full line corresponds to 0 ppm sulfur, the broken line corresponds to 2000 ppm sulfur and the dotted curve corresponds to 4000 ppm.

[0063] From FIG. 5 it is seen, that for 0.5 C, the curves for 0 ppm, 2000 ppm and 4000 ppm substantially follow each other and end in substantially the same discharge capacity value. For 2 C, the curve for 0 ppm is a bit distanced from the curves for 2000 ppm and 4000 ppm, and the curve for 0 ppm ends in a lower discharge capacity value than the curves for 2000 ppm and 4000 ppm. For rapid discharging, viz. for 10 C, the curve for 0 ppm is a somewhat distanced from the curves for 2000 ppm and 4000 ppm, and the curve for 0 ppm ends in a somewhat lower discharge capacity value than the curves for 2000 ppm and 4000 ppm. Moreover, it is seen that the material comprising 4000 ppm has both lower resistance (as seen by the higher voltage measurements) and higher discharge capacity than the material comprising 2000 ppm. Thus, in conclusion, as the current is increased, the over-potential increases and the discharge capacity decreases, but it is seen that an increased amount of sulfur decreases the over-potential at high rates and thereby increases the discharge capacity.

[0064] FIG. 6 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 1. The electrochemical measurements are performed in half cells at 50 C. between 3.5 V and 5 V with charge and discharge currents corresponding to 0.5 C and 1 C, respectively. It is seen that sulfate doping of cathode active material decreases the degradation significantly.

[0065] FIG. 7 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 2. The electrochemical measurements are performed in half cells at 50 C. between 3.5 V and 5 V with charge and discharge currents corresponding to 0.5 C and 1 C, respectively. It is seen that sulfate doping of cathode active material precursors decreases the degradation significantly.

[0066] FIG. 8 is a graph showing the relative degradation during constant current electrochemical cycling of cathode active materials with and without sulfate doping, prepared as described in Example 3. The electrochemical measurements are performed in half cells at 50 C. between 3.5 V and 5 V with charge and discharge currents corresponding to 0.5 C and 1 C, respectively. It is seen that sulfate doping even at only 500 ppm, and as a result of impurities in the cathode active material precursors, decreases the degradation significantly.

[0067] FIG. 9 is a graph showing the relative change in battery material parameters: initially measured discharge capacity, power capability, 0.2 C degradation and 1 C degradation as a function of sulfate doping in the cathode active material. The cathode active materials have been prepared in different ways and include the materials described in Examples 1, 2 and 3 among others. It is seen that sulfate doping does not change the discharge capacity; moreover, it increases the power by up to 40% and decreases degradation by up to 70%.

[0068] The relevant amount of Sviz. a sulfur content in the cathode active material is between 1000 and 16000 ppmis thus an optimization between obtaining good performance as described in FIGS. 4-9, while avoiding Li.sub.2SO.sub.4 as shown in FIGS. 2-3.

Example A: Method of Electrochemical Testing of Battery Materials Prepared According to Examples 1, 2 and 3

[0069] Electrochemical tests have been realized in 2032 type coin cells, using thin composite positive electrodes and metallic lithium negative electrodes (half-cells). The thin composite positive electrodes were prepared by thoroughly mixing 84 wt % of cathode active material (prepared according to Examples 1, 2 and 3) with 8 wt % Super C65 carbon black (Timcal) and 8 wt % PVdF binder (polyvinylidene difluoride, Sigma Aldrich) in NMP (N-methyl-pyrrolidone) to form a slurry. The slurries were spread onto carbon coated aluminum foils using a doctor blade with a 160 m gap and dried for 2 hours at 80 C. to form films. 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 films, pressed in a hydraulic pellet press (diameter 20 mm; 3 tonnes) and subjected to 10 hours drying at 120 C. under vacuum in an argon filled glove box.

[0070] 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 electrolyte containing 1 molar LiPF.sub.6 in EC:DMC (1:1 in weight). Two 250 m 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. Electrochemical lithium insertion and extraction was monitored with an automatic cycling data recording system (Maccor) operating in galvanostatic mode.

[0071] A standard 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 discharge 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.

[0072] The performance parameter discharge capacity, power capability, 0.2 C degradation and 1 C degradation are extracted from the standard test in the following way. The discharge capacity is the initial discharge capacity at 0.5 C, measured in cycle 7. The power capability is the relative decrease in the measured discharge capacity at 10 C compared to 0.5 C, measured at cycles 29 and 7 respectively. The 0.2 C degradation is the relative loss of discharge capacity at 0.2 C over 100 cycles, measured between cycles 32 and 132. The 1 C degradation is the relative loss of discharge capacity at 1 C over 100 cycles, measured between cycles 33 and 133.

Example 1: Method of Preparing Sulfate Doped Cathode Active Material

[0073] Precursors in the form of 1162.47 g co-precipitated Ni,Mn-carbonate (Ni:0.5, Mn: 1.5) and 190.65 g Li.sub.2CO.sub.3 are mixed with ethanol to form a viscous slurry. The slurry is shaken in a paint shaker for 3 min. in order to obtain full de-agglomeration and mixing of the particulate materials. The slurry is poured into trays and left to dry at 80 C. The dried material is further de-agglomerated by shaking in a paint shaker for 1 min. in order to obtain a free flowing homogeneous powder mix.

[0074] The powder mix is sintered in a muffle furnace 2.5 hours at 700 C. with nitrogen flow.

[0075] This product is de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45 micron sieve. The powder is distributed in a 10-25 mm layer in alumina crucibles and sintered in air 14 hours at 900 C. and 4 hours at 700 C.

[0076] The powder is again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45 micron sieve resulting in 866 g cathode active material consisting of 95.4% LNMO, 3.6% 03 and 1.1% Rock salt.

[0077] Three 50 g portions are taken from the produced cathode active material. Two are mixed with 0.3434 g and 0.6868 g Li.sub.2SO.sub.4, respectively, to obtain sulfur content in the final product of 2000 ppm and 4000 ppm. The mixing is performed by solution of Li.sub.2SO.sub.4 in 10 g H.sub.2O and 8 g ethanol and mixing this with the cathode material. The three powder samples, including the powder without sulfur doping, are sintered 4 hours at 900 C. and 4 h at 700 C. in air. The powder is again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 38 micron sieve. The phase purity of all samples are 95 wt % or above. The electrochemical performances of the three samples are compared in FIGS. 4, 5 and 6.

[0078] The actual sulfur contents in the products corresponding to 0 ppm sulfur and 2000 ppm sulfur was determined to be 40 ppm and 2090 ppm, respectively, using ICP.

Example 2: Method of Preparing Sulfate Doped Cathode Active Material

[0079] Precursors in the form of 2258.66 g co-precipitated Ni,Mn-carbonate (Ni:0.5, Mn: 1.5) and 394.78 g LiOH are dry-mixed for 1 hour.

[0080] Two portions of 50 g are taken from the dry-mixed precursor: One is mixed with Li.sub.2SO.sub.4 to obtain sulfur content in the final product of 2000 ppm. The two powder portions are sintered in a muffle furnace 3 hours at 700 C. with nitrogen flow.

[0081] The products are de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45 micron sieve. The powder is distributed in a 10-25 mm layer in alumina crucibles and sintered in air 14 hours at 900 C. and 2 hours at 700 C.

[0082] The powder is again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45 micron sieve. The phase purity of both samples are 95 wt % or above. The electrochemical performances of the two samples are compared in FIG. 7.

[0083] To determine the chemical identity of the sulfur at the surface, XPS measurements were conducted on the cathode active materials with 2000 ppm sulfur doping from Examples 1 and 2. FIG. 1 shows the calibrated S2p spectra of these materials and Li.sub.2SO.sub.4 as a reference. It is seen that the binding energy is around 169 eV in all three cases, showing that the sulfur present is in the form of sulfate rather than sulfide in which the binding energy is around 161.5 eV. This is also evident by direct comparison with the spectrum of Li.sub.2SO.sub.4.

[0084] The XPS measurement can also reveal any radial distribution of the sulfate in the cathode active material particles. Table 1 shows the relative atomic ratios of the relevant compounds O, Mn, Ni and S in the cathode active materials from Examples 1 and 2 containing 2000 ppm sulfur.

TABLE-US-00001 TABLE 1 Concentration of sulfur in the surface of sulfate doped cathode active material. Target O Mn Ni S O/(Mn + Ni) (Mn + Ni)/S Z.sub.surface sulfur 2000 ppm 72% 24% 1.8% 1.5% 2.8 18 0.11 2.0 wt % (Example 1) 2000 ppm 71% 25% 3.1% 0.54% 2.5 53 0.038 0.67 wt % (Example 2)

[0085] O/(Mn+Ni) is the atomic ratio between oxygen and the transition metals in the LNMO spinel, i.e. Mn and Ni. The bulk value of this is 2, but deviations from bulk values are often found at the surface. (Mn+Ni)/S is the atomic ratio between the transition metals in the LNMO spinel and sulfur. This is used to calculate the value of z in the surface, z.sub.surface. by using the formula Li.sub.xM.sub.yMn.sub.2yO.sub.4v(SO.sub.4).sub.z. A calculation of the relative amount of sulfur by weight corresponding to the z-value shows that the sulfur content is 10 times higher than the bulk value when the material is prepared as described in Example 1, and 3 times higher than the bulk value when the material is prepared as described in Example 2. This shows that the sulfate is preferentially found in the surface of the particles, when either one of the methods described in Examples 1 or 2 are used.

Example 3: Method of Preparing Sulfate Doped Cathode Active Material

[0086] Two cathode active materials based on precursors with different sulfur impurity levels in the Ni,Mn-carbonate are prepared identically: Precursors in the form of 30 g co-precipitated Ni,Mn-carbonate (Ni:0.5, Mn: 1.5) and 5.1 g LiOH are mixed dry in order to obtain a free flowing homogeneous powder mix. The two powder mixes are sintered in a muffle furnace 3 hours at 730 C. with nitrogen flow.

[0087] The products are de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45 micron sieve. The powder is distributed in a 10-25 mm layer in alumina crucibles and sintered in air 4 hours at 900 C. and 12 hours at 715 C.

[0088] The powders are again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 20 micron sieve. The phase purity of both samples is 98 wt %. The electrochemical performances of the two samples are compared in FIG. 8.

[0089] The two precursors have different amounts of sulfur impurities. One is 100 ppm and the other is 500 ppm. It was shown by ICP that the sulfur to NiMn ratio is constant throughout the entire preparation of the sulfate doped cathode active material such that different amounts of sulfur impurities in the precursor will give battery cathode materials with correspondingly different amounts of sulfate doping.

[0090] Comparison of the electrochemical performance of the cathode materials produced in Examples 1, 2 and 3 is shown in FIGS. 4-9. FIG. 9 furthermore includes additional experiments showing the same trend that the discharge capacity is unchanged, the power capability increases with sulfate doping and the degradation decreases with sulfate doping.

[0091] While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.