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Optimization of Li/metal ratio in LiNiCoMn oxyde cathode material

IP.com Disclosure Number: IPCOM000199438D
Publication Date: 2010-Sep-03
Document File: 5 page(s) / 48K

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Optimization of Li/metal ratio in LiNiCoMn oxyde cathode materials

The investigation relates to a powderous lithium transition metal oxide, used as active cathode material in rechargeable lithium batteries. More particularly, in Li(Mn-Ni-Co)O2 type compounds the electrochemical characteristics of the cathode material can be optimized by adapting the Li/(Mn‑Ni‑Co) ratio to the Ni/Mn ratio.

LiCoO2 is the most widely applied cathode material for rechargeable batteries. However, there exists a strong pressure to replace it by other materials for particular reasons. Currently, scarce resources of cobalt and fear of high prices accelerate this trend. Besides LiFePO4 and Li-Mn-spinel, which both suffer from much lower energy density, LiNiO2 based layered cathode materials and Li(Mn-Ni-Co)O2 based layered cathode materials are the most likely candidates to replace LiCoO2 in commercial battery applications. Today it is basically known that any composition Li[LixM1-x]O2 with M=Mn, Ni, Co within the quarternary system Li[Li1/3Mn2/3]O2 – LiCoO2 – LiNiO2 – LiNi0.5Mn0.5O2 exists as a layered phase, and in most cases is electrochemically active.

Even this quarternary system is to be seen as a simplified model because it does not take into account  further phenomena like the possibility of cation mixing. One type of cation mixing is known from LiNiO2 where some nickel is misplaced on lithium sites of the r-3m layered crystal structure,  a more realistic formula is approximated as {Li1-xNix}[Ni]O2. It is also known that Li1+xM1-xO2 with M=Mn1/3Ni1/3Co1/3 is better written as {Li1+yMy}[LizM1-z]O2.

As a result, layered Li(Mn-Ni-Co)O2 phases which are of interest for battery cathode materials belong to the quarternary (according Gibbs phases rule) subspace of the 5 dimensional thermodynamic system LiNiO2 – {Li1-aNia}NiO2 - Li[Li1/3Mn2/3]O2 – LiCoO2 – LiNiO2. Most of the phases within this triangle are electrochemically active.

Since many years it is known that the layered structure of LiNiO2 can be stabilized, and electrochemical properties can be improved if Ni is replaced by Mn or Co, resulting in LiNi1-xMnxO2 and LiNi1-xCoO2. Quite soon it was discovered that Mn and Co can be co-doped, resulting in layered Li(Ni-Mn-Co)O2 phases LiNi1-x-yMnxCoyO2. So JP3244314 (Sanyo) claims LiaMbNicCodOe covering a wide range of metal compositions.

It can be summarized that at the mid 90ties prior art were compositions within the Ni rich corner of the solid state solution between LiCoO2 – LiMn1/2Ni1/2O2 – {Li1-xNix}NiO2 , including further dopants (like Al). During the 90ties there was put little focus on the Li stoichiometry. So the patents above just claim LiMO2 , or a range of Li stoichiometries, but it has generally not been understood that the Li:M ratio is an important variable needing optimization. Li1M1 was typically seen as a desired stoichiometry which only can be obtained if a small lithium excess is used.

In the late 90ties slowly understandi...