Cathode material for lithium-ion rechargeable batteries
Disclosed are a cathode active material and a method to produce the same at low cost. The cathode powder comprises modified doped LiCoO 2 carrying a secondary phase having either one of space groups Fm-3m or Fd-3mS. The modified LiCoO 2 is Ni and Mn bearing and has regions of low and high manganese content, where regions with high manganese content are located in islands on the surface. The cathode material has high cycling stability, a very high rate performance and good high temperature storage properties.
1. A lithium metal oxide powder for use as a cathode material in a rechargeable battery, consisting of a core and a surface layer, the lithium metal oxide having the general formula Li x (Co a M b A c ) 2−x O 2±δ , with x≦0.995, b>0, c≧0, a+b+c=1, |δ|≦0.1, M=(Mn m Ni n ) with m+n=1, A is a dopant, wherein the core and surface layer are either:
a pristine Li cobaltite core and a surface layer comprising a lithium metal oxide compound wherein at least 98 mol % of the metal consists of the elements Li, Mn, Ni and Co; or
a doped Li cobaltite core, wherein the dopant A is either one or more of Mg, Ti, Al and Zr, and a surface layer comprising a lithium metal oxide compound wherein at least 98 mol % of the metal consists of the elements Li, Mn, Ni, Co and dopant A, and
wherein in both alternatives the surface layer comprises:
a phase with rhombohedral symmetry with space group R-3m, and
either one or both of a cubic phase oxide having a crystal structure with a Fm-3m space group, and a cubic phase oxide having a crystal structure with a Fd-3mS space group, and
wherein the surface layer comprises Mn and Ni enriched islands, the islands having a Mn and Ni concentration that is higher than in the bulk of the particles.
2. The lithium metal oxide powder of claim 1 , wherein x≧0.970.
3. The lithium metal oxide powder of claim 1 , wherein 0.974≦x≦0.993.
4. The lithium metal oxide powder of claim 1 , wherein 0<c≦0.03, and A is either one or more dopants selected from the group consisting of Mg, Ti, Al and Zr.
5. The lithium metal oxide powder of claim 1 , wherein the cubic phase oxide having a crystal structure with a Fm-3m space group has a lattice constant “a” with 4.05<=a<=4.30, and the cubic phase oxide having a crystal structure with a Fd-3mS space group has a lattice constant “a” with 8.055<=a<=8.150, “a” being expressed in Å.
6. The lithium metal oxide powder of claim 1 , wherein the cubic phase oxide having a crystal structure with a Fm-3m space group has a lattice constant “a” with 4.13<=a<=4.20, and the cubic phase oxide having a crystal structure with a Fd-3mS space group has a lattice constant “a” with 8.055<=a<=8.110, “a” being expressed in Å.
7. The lithium metal oxide powder of claim 1 , wherein the amount of cubic phase oxide in the powder is between 0.1 and 3 wt %.
8. The lithium metal oxide powder of claim 1 , wherein the powder has an electrical conductivity of less than 10 −5 S/cm when pressed with 63.7 MPa at 25° C.
9. The lithium metal oxide powder of claim 1 , wherein the powder has an electrical conductivity of less than 10 −8 S/cm, when pressed with 63.7 MPa at 25° C.
10. The lithium metal oxide powder of claim 1 , wherein the surface layer comprises more Mn than the core, and wherein the surface layer comprises less Co than the core.
11. The lithium metal oxide powder of claim 1 , wherein the islands comprise at least 5 mol % of Mn.
12. The lithium metal oxide powder of claim 1 , wherein n≧m, and wherein at the surface of the powder the molar ratio Ni:Mn>1, and said surface Ni:Mn molar ratio being greater than n/m.
13. An electrochemical cell comprising a cathode comprising as active material the lithium metal oxide powder according to claim 1 .
14. A method for preparing a lithium metal oxide powder according to claim 1 , comprising the steps of:
providing a mixture of a doped or undoped LiCoO 2 powder, a Ni—Mn—Co comprising powder, and a Li-comprising compound, the mixture comprising more than 90 wt % of the doped or undoped LiCoO 2 powder, and
sintering the mixture at a temperature T of at least 910° C., for a time t between 1 and 48 hours,
wherein the quantity of the Li-comprising compound in the mixture is selected to obtain a Li over metal ratio in the lithium metal oxide powder with Li:M≦0.99.
15. The method according to claim 14 , wherein the mixture further comprises a compound comprising the dopant A, or wherein the Ni—Mn—Co comprising powder further comprises the dopant A.
16. The method according to claim 14 , wherein the quantity of the Li-comprising compound in the mixture is selected to obtain a Li over metal ratio in the lithium metal oxide powder with 0.95≦Li:M≦0.99.
17. The method according to claim 14 , wherein the Ni—Mn—Co comprising powder is either a Ni—Mn—Co hydroxide, a Ni—Mn—Co oxyhydroxide or a mixture of both.
18. The method according to claim 14 , wherein the doped LiCoO 2 powder comprises either one or more of Al, Mg and Ti and is prepared by sintering a mixture of Li 2 CO 3 and either Co(OH) 2 or Co 3 O 4 , the Co(OH) 2 or Co 3 O 4 being doped with either one or more of Al, Mg and Ti.