WO2019091667A1

WO2019091667A1 - High tap density lithium positive electrode active material, intermediate and process of preparation

Info

Publication number: WO2019091667A1
Application number: PCT/EP2018/076886
Authority: WO
Inventors: Jonathan HØJBERG; Jakob Weiland HØJ; Lars Fahl LUNDEGAARD; Jon Fold VON BÜLOW; Søren Dahl
Original assignee: Haldor Topsøe A/S
Priority date: 2017-11-13
Filing date: 2018-10-03
Publication date: 2019-05-16

Abstract

The invention relates to a lithium positive electrode active material intermediate comprising less than 10 wt% spinel phase and a net chemical composition of LixMn2O4-δ, wherein: 0 ≤ x ≤ 1.1; and 0.88 ≤ δ, and has been heat treated in an atmosphere with less than 500 ppm O2 at a temperature of between 300ºC and 1200ºC. The invention 5 further relates to a process for the preparation of a lithium positive electrode active material for a secondary battery where the cathode is fully or partially operated at voltages between 3.5 V and 4.3 V vs. Li/Li+, said process comprising the steps of: a) Heat treat- ing starting materials comprising lithium and manganese in an atmosphere with less than 500 ppm O2 at a temperature of between 300ºC and 1200ºC to obtain a lithium 10 positive electrode active material intermediate; b) Heat treating the lithium positive electrode active material intermediate of step a. in an oxidizing atmosphere at a temperature of between 500ºC and 1200ºC. The mass of the product of step b. increases by at least 0.25 % compared to the mass of the product of step a.

Description

High tap density lithium positive electrode active material, intermediate and process of preparation

Background

The present invention relates to a novel process of preparation and an intermediate for the preparation of lithium positive electrode active materials for use in lithium secondary batteries. Lithium positive electrode active materials may be characterized by the formula

LiyMn204, wherein 0.9 < y < 1.1. Such materials may be used for e.g.: portable equipment; electric vehicles, energy storage systems, auxiliary power units (APU) and uninterruptible power supplies (UPS).

Lithium positive electrode active materials may be prepared from precursors obtained by mechanically mixing starting materials to form a homogenous mixture. Lithium positive electrode active materials may also be prepared from mechanically mixed starting materials. One or more of the starting materials may be precipitated, so that a lithium source may be mixed with a manganese composition prepared by precipitation (see Journal of Power Sources (2013) 238, 245-250 and Electrochimica Acta (2014) 1 15, 290 - 296).

It is desirable to increase the tap density of battery materials as an increase in tap density may increase the energy density of the battery. Additionally, it is desirable to produce a spinel phase material corresponding to the formula Li_yMn204, where 0.9 < y < 1 .1 , which material requires a smaller excess expensive starting materials, such as ma- terials comprising lithium, that may have fewer process steps (heating steps), and/or is applicable to any starting material regardless of the process of its preparation.

It is an object of the present invention to provide a lithium positive electrode active material intermediate and a process for preparing a spinel lithium positive electrode active material corresponding to the formula Li_yMn204, 0.9 < y < 1.1 , that has a high tap density equal to or greater than 1.5 g cm^-3. It is additionally desirable that this material has a high capacity equal to or greater than 100 mAhg^"1 at a current of 30 mAg^"1, and high stability wherein the capacity of the material decreases by no more than 8 % over 100 cycles between from 3.5 to 4.3 V at 55°C.

Disclosure of the Invention: It has been discovered that when a lithium positive electrode active material intermediate having a composition ϋχΜη2θ4-δ, 0 < x < 1 .1 , 0.88< δ, is to be prepared it is necessary that it undergoes a specific heating step in an atmosphere with very little or none oxygen. An aspect of the present invention relates to a lithium positive electrode active material intermediate comprising less than 10 wt% spinel phase and a net chemical composition of Ι_ίχΜη2θ4-δ, 0 < x < 1 .1 , 0.88< δ, and has been heat treated in an atmosphere with less than 500 ppm O2 at a temperature of between 300°C and 1200°C. It has been shown that more than 500 ppm O2 in the atmosphere results in a material that has not sintered well, and thus to a material having a lower tap density and less dense secondary particles. Therefore, surprisingly the oxygen content of the atmosphere has to be as low as less than 500 ppm, preferably less than 200 ppm and more preferably less than 10 ppm. This is denoted an atmosphere with very low oxygen content. The temperature of the heat treatment in atmosphere with very low oxygen content preferably lies between 400°C and 1 100°C, more preferably between 500°C and 900°C, such as about 650°C. The value of x lies between 0 and 1.1 (both included); preferably, x lies between 1 .0 and 1 .1 (both included), and a particularly preferred value is about 1 .04.

Even though no upper limit to δ is indicated above, it is self-evident that an upper limit to δ exists. Such an upper limit is less than 4; preferably, an upper limit to δ is about 2; preferably 1 < δ < 2; most preferably δ is between 1 and 1.1 . As an average across a batch or a receptacle of lithium positive electrode active material intermediate, it comprises between from 0 to 10 wt% spinel phase, such as between from 0 to 5 wt% spinel phase. Preferably, the lithium positive electrode active material intermediate comprises about 0% spinel phase.

"Spinel phase" means a crystal lattice where the oxide anions are arranged in a cubic close-packed lattice and the cations are occupying some or all of the octahedral and tethahedral sites in the lattice. The symmetry of the spinel lattice is described by the space group Fd-3m with lattice constant a at around 8.2 A.

"Phase composition" is determined based on X-ray diffraction patterns acquired using an X'pert PRO MPD diffractometer, using Cu Ka radiation. Structural analysis is per- formed using Rietveld refinement in the Bruker software TOPAS. The phase composition is given in wt% with a typical uncertainty of 1 -2 percentage points, and represents the relative composition of all crystalline phases. Any amorphous phases are thus not included in the phase composition. The term that the lithium positive electrode active material intermediate has been "heat treated in an atmosphere less than 500 ppm O2 at to a specified temperature" is meant to denote that the lithium positive electrode active material intermediate has been heated, in an atmosphere with less than 500 ppm O2, for a sufficient time range for the intermediate to at least substantially reach the specified temperature in the atmosphere concerned.

The lithium positive electrode active material intermediate may comprise products formed from starting materials that are partly or fully decomposed in the heat treatment in an atmosphere with less than 500 ppm O2 at a temperature of from 300°C to 1200°C. These products may comprise from 30 to 100 wt% rock salt phase; between from 35 to 80 wt% rock salt phase and from 0 to 70 wt% LiMnC>2; between from 10 wt% to 65 wt% LiMn0₂.

"Rock salt phase" means a cubic close-packed crystal lattice where the oxide anions are arranged in a cubic close-packed lattice and the cations are occupying the octahedral sites in the lattice. The symmetry of the rock salt lattice is described by space group of Fm-3m with a lattice constant a at around 4.4 A. Herein, the "rock salt phase" denotes a lattice where the cations are mainly manganese, but also some lithium may be present. Thus, a "rock salt phase" will have the composition Li_xMni-_xO, where 0 < x < 0.05.

"LiMnCV means a crystal lattice described by the space group Pmnm. Li¹⁺ and Mn³⁺ cations exist in the molar ratio 1 :1. Li and Mn cations are arranged in an ordered pattern breaking the cubic symmetry resulting in the orthorhombic space group (Pmnm) with lattice constants a at around 2.8 A, b at around 5.8 A and c at around 4.7 A.

It should be noted that the indication of an atmosphere with less than 500 ppm O2 is an indication of the overall atmosphere within the receptacle housing the starting materials during the heat treatment, even though higher levels of O2 might be measured in the oven. For example, the atmosphere of the oven may comprise up to e.g. 1 % O2, whilst the atmosphere within the bed is less than 500 ppm. Thus, the term "an atmosphere with less than 500 ppm O2" is meant to indicate that substantially none of the starting materials have experienced more than 500 ppm O2 during the heat treatment in the at- mosphere concerned.

In an embodiment, the tap density of the intermediate is equal to or greater than 1 .5 g cm^-3. Preferably, the tap density of the intermediate is equal to or greater than 1.8 g cm^"3, equal to or greater than 2.0 g cm^"3; equal to or greater than 2.2 g cm^"3; or equal to or greater than 2.4 g cm^"3.

In an embodiment, the lithium positive electrode active material intermediate has an average oxidation state of manganese between 2.0+ and 3.0+. Preferably, the lithium positive electrode active material intermediate has an average oxidation state of man- ganese between 2.0+ and 2.6+. If the lithium positive electrode active material to be prepared from the lithium positive electrode active material intermediate is Lio.9Mn2C>4, the average oxidation state of manganese will be about 2.48 or lower and δ be at least 1 .07. If the lithium positive electrode active material to be prepared from the lithium positive electrode active material intermediate is Lii.i Mn2C>4, the average oxidation state of manganese will be about 2.57 or lower and δ be at least 0.88. If Li has not reacted into a manganese phase and thus become a part of the lithium positive electrode active material intermediate, the average oxidation state of manganese will be 2 and δ will be 2, i.e. MnO. After heat treating of the lithium positive electrode active material intermediate in an oxidizing atmosphere, e.g. air, manganese in the resultant lithium positive electrode active material has an average oxidation state of about 3.5+. It should be noted that the "average oxidation state of manganese" is an indication of the molar average oxidation state of any manganese; thus, LiMn³⁺02 and Mn²⁺0 will also be included together with LiMn₂0₄.

In an embodiment, the lithium positive electrode active material intermediate has been produced from two or more starting materials and where the starting materials have been partly or fully decomposed by said heat treatment. The decomposition of the starting materials into the intermediate takes place at the atmosphere with less than 500 ppm O2 at a temperature between 300°C and 1200°C. The starting materials may e.g. be MnCC , L12CO3. Different reaction paths of the starting materials are conceivable; for the embodiments of the invention the important feature is that the starting mate- rials undergo the heat treatment in an atmosphere with less than 500 ppm O2 at a temperature of between 300°C and 1200°C.

In the case where one or more of the starting materials comprises carbonate, such as MnCC>3, the starting materials will give off CO2 during the heat treatment and thus fur- ther reduce the O2 content of the atmosphere in the close vicinity of the intermediate.

In an embodiment, a Li containing starting material has been added prior to the heat treatment in an atmosphere with less than 500 ppm O2. Substantially all the Li containing starting material needed may have been added before the heat treatment in the at- mosphere with less than 500 ppm O2, or some Li containing starting material may be added before heat treatment in an atmosphere with less than 500 ppm O2, and some Li containing starting material may be added after the heat treatment in an atmosphere with less than 500 ppm O2. Such starting materials are e.g. a manganese carbonate and a lithium carbonate, or a manganese carbonate and a lithium hydroxide, or a man- ganese hydroxide and a lithium hydroxide, or a manganese hydroxide and a lithium carbonate, or a manganese oxide and a lithium carbonate.

It should be noted, that the material that comprise the lithium positive electrode active material intermediate may also comprise separate phases containing only Li as cation such as, but not limited to, LiO, L12O, L1O2, L12CO3 and LiOH, that have been added before the heat treatment in an atmosphere with less than 500 ppm O2 and that are not fully decomposed, or that is a decomposition product with lithium as the only cation. Any such Li phase is not included in the net chemical composition of the intermediate, since such Li phases are not part of the active material intermediate. As an example, the positive electrode active intermediate

may have x=0 even though lithium has been added to the synthesis in a molar Li:Mn ratio of 1 :2, if the lithium has not reacted with the manganese phase. Moreover, these separate phases containing only Li as cation can be XRD amorphous phases. The term "Li phase" is meant to denote a distinct phase where the only metal/cation is Li.

In an embodiment, the lithium positive electrode active material intermediate according to the invention has composition of about 50 wt% LiMn02, and about 50 wt% rock salt phase. This corresponds to an overall composition of the lithium positive electrode active material intermediate of Lio.84Mn2O2.87. In this embodiment, some Li is present as a distinct Li phase and therefor does not form part of the lithium positive electrode active material.

According to an embodiment, if the lithium positive electrode active material intermediate is heat treated in an oxidizing atmosphere at a temperature of between 500°C and 1200°C, the resultant positive electrode active material comprises at least 95% of spinel phase

wherein 0.9 < y < 1 .1. Such a high phase purity provides for a high capacity of the lithium positive electrode active material produced from the lithium positive electrode active material intermediate.

It is well-known in the art to dope a lithium positive electrode active material intermediate, with e.g. Al, excess Li or other components. This is also the case for the lithium positive electrode active material intermediate of this invention. Thus, in an embodiment, the lithium positive electrode active material intermediate comprises up to 5 mol% of one or more other elements than Li, Mn and O. Such elements may for example be one or more of the following: B, N, F, Mg, Al, Si, P, S, Ca, Ti, Cr, Fe, Co, Cu, Zn, Zr, Mo, Sn, W, any mixture thereof or any chemical composition containing one or more of these compounds. The other elements, also denoted "dopants", may originate from addition or from impurities in starting materials.

Another aspect of the invention relates to a process for the preparation of a lithium pos- itive electrode active material for a medium voltage secondary battery, where the cathode is fully or partially operated at voltages between 3.5 V and 4.3 V vs. Li/Li+, said process comprising the steps of: a. Heat treating starting materials comprising lithium and manganese in an atmosphere with less than 500 ppm C>2 at a temperature of between 300°C and 1200°C to obtain a lithium positive electrode active material intermediate; b. Heat treating the lithium positive electrode active material intermediate of step a. in an oxidizing atmosphere at a temperature of between 500°C and 1200°C.

Whilst the average oxidation state of manganese in the lithium positive electrode active material intermediate is between 2.0+ and 3.0+, the average oxidation state of manganese in the lithium positive electrode active material is at about 3.5+.

The lithium positive electrode active material of the invention is a material for a medium voltage secondary battery, where the cathode is fully or partially operated at voltages between 3.5 V and 4.3 V vs. Li/Li+, primarily at about from 3.9 V to 4.2 V vs. Li/Li+. For a lithium positive electrode active material of the invention, having with no dopants, there will be no activity at voltages above 4.3 V vs. Li/Li+; however, if the lithium positive electrode active material id doped, e.g. with some Ni, some activity at voltages above 4.3 V vs. Li/Li+ may exist.

The product of steps a. and b. may optionally be cooled after heating and, once cooled, they may be isolated. Alternatively, the process comprising the steps: a. and b. may be carried out consecutively, without cooling and the products of step a. or b. are not isolated. Steps a. and b. may be repeated as required.

"Heat treating" means treating a material at a temperature or temperature range in order to obtain the desired crystallinity. The temperature or temperature range is in- tended to represent the temperature of the material being heat treated. Typical heat treatment temperatures are about 500°C, about 600°C, about 700°C, about 800°C, about 900°C, about 1000°C and temperature ranges are from about 300°C to about 1200°C; from about 500 to about 1000°C; from 650°C to 950°C. The term "heat treatment at a temperature of between X °C and Y°C" is not meant to be limiting to one specific temperature between X and Y; instead, the term also encompasses heat treatment to a range of temperatures within the temperature span from X to Y during the time of the heating.

"Cooling" means treating a material at a temperature or temperature range that is gradually lowered in order to reduce the temperature of the material. Typical cooling conditions are cooling at between 1 °C and 5°C per minute when lowering the temperature from 900°C to 700°C. Optionally, the material may be cooled to, for example, 600°C, 500°C, 400°C, 300°C, 200°C, 100°C, 50°C, room temperature (i.e. about 25 °C).

In an embodiment of the process, the temperature of step a. is between 300°C and 650°C, preferably between 400°C and 650°C, more preferably between 500°C and 650°C. In an embodiment of the process, the temperature of step b. is between 800°C and 1200°C, preferably between 900°C and 1200°C, more preferably between 900°C and 1 100°C.

In an embodiment of the process, it is ensured that substantially all the lithium starting material has reacted in step a. before step b. is initiated. In the case where the lithium starting material is lithium carbonate, this may be detected by measurement of the partial pressure of CO2 within the receptacle during the heating of step a. In an embodiment, an intermediate step a2. is carried out between step a. and step b., where step a2 comprises a2. Heat treating a lithium positive electrode active material intermediate obtained by step a. in an oxidizing atmosphere at a temperature between 300°C and 500°C. The temperature of step a2. is preferably between 350°C and 500°C, more preferably be- tween 400°C and 500°C.

"Starting materials" may be mechanically mixed or precipitated to obtain a homogenous mixture (Journal of Power Sources (2013) 238, 245 - 250); or a lithium source may be mixed with a manganese composition prepared by precipitation (Electrochimica Acta (2014) 1 15, 290 - 296). Starting materials are selected from one or more compounds selected from the group consisting of metal oxide, metal carbonate, metal oxalate, metal acetate, metal nitrate, metal sulfate, metal hydroxide and pure metals; wherein the metal is selected from the group consisting of manganese (Mn) and lithium (Li) and mixtures thereof. Preferably, the starting materials are selected from one or more compounds selected from the group consisting of manganese oxide, manganese carbonate, nickel carbonate, man- ganese sulfate, manganese nitrate, lithium hydroxide, lithium carbonate and mixtures thereof. Metal oxidation states of starting materials may vary; e.g. MnO, Mn3<D₄, Μη2θ3, Mn0₂, Mn(OH), MnOOH.

"Intermediate" means a product of a step that is not the final step of the process. For example, the product of step a. is an intermediate of the process comprising steps a. and b. as the product of step a. is used in the subsequent step b.

In an embodiment, the atmosphere with less than 500 ppm O2 is a gaseous composition selected from the group of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and mixtures thereof. The atmosphere with less than 500 ppm O2 may be created by adding a substance to the precursor composition, by decomposition of the precursor or by adding a gaseous composition to the atmosphere in order to remove all or part of any oxidizing species present in the atmosphere. Preferably no ambient air can enter the reaction vessel.

"An atmosphere with less than 500 ppm O2" is meant to encompass any atmosphere with less than 500 ppm O2. Such atmospheres keep the average valence state of Mn below 3+ after the relevant heat treatment temperature. Preferably, the average oxidation state of Mn is less than or equal to 3+, and more preferably the average oxidation state of Mn is between from about 2+ to about 3+. The atmosphere with less than 500 ppm O2 may be provided by the type of gas present within the reaction vessel during heating. This gas may be one or more gases selected from the group of: hydrogen; carbon monoxide; carbon dioxide; nitrogen; less than 500 ppm oxygen in an inert gas; and mixtures thereof. The term "less than 500 ppm oxygen in an inert gas" is meant to cover the range from 0 ppm oxygen, corresponding to an inert gas without oxygen, up to 500 ppm oxygen in an inert gas. Preferably, the amount of oxygen in the atmosphere with less than 500 ppm O2 is very low, such as below 200 ppm and preferably below 10 ppm. Typically, oxygen would not be added to the atmosphere; however, oxygen may be formed during the heating. In the case where one or more of the starting materials comprises carbonate, such as MnCC , the starting materials will give off CO2 during the heat treatment and thus further reduce the oxygen concentration of the atmosphere in the close vicinity of the intermediate. The amounts of oxygen in the atmosphere with less than 500 ppm O2 is meant to denote the oxygen experienced by substantially all the starting materials and/or the lithium positive electrode active material intermediate during the heat treatment in the atmosphere with less than 500 ppm O2. Thus, the oxygen content in the oven may be higher than the oxygen content indicated above, due to the possible flushing out of oxygen from a bed of starting materials/lithium positive electrode active material intermediate by CO2 given off by the starting materials during the heat treatment.

"Inert gas" means a gas that does not participate in the process. Examples of inert gas- ses comprise one or more gases selected from the group of: argon; nitrogen; helium; and mixtures thereof. Additionally, an "atmosphere with less than 500 ppm O2" may be obtained by adding a substance to the precursor composition or by adding a gaseous composition to the atmosphere in order to remove all or part of any oxidizing species present in the atmosphere of the reaction vessel during heating. The substance may be added to the precursor either during the preparation of the precursor or prior to heat treatment. The substance may be any material that can be oxidized and preferably comprising carbon, for example, the substance may be one or more compounds selected from the group consisting of graphite, acetic acid, carbon black, oxalic acid, wooden fibers and plastic materials. "Oxidizing atmosphere" means an atmosphere that shifts the thermodynamic equilibrium of the solid towards the spinel phase when the temperature is below 900°C; an example is that an oxidizing atmosphere below 900°C may increase the average oxidation state of Mn to a value of +3.5 in the case where x=1 in the formula Ι_ίχΜη2θ4-δ. In an embodiment, the atmosphere with less than 500 ppm O2 is a gaseous composition selected from the group consisting of air, and a composition comprising at least 5 vol% oxygen in an inert gas. The oxidizing atmosphere may be provided by the type of gas present within the reaction vessel during heating. Preferably, the oxidizing atmosphere is air. The lithium positive electrode active material has an initial specific discharge capacity of equal to or greater than 100 mAhg^"1 when discharged using a 30 mAg^"1 current. Discharge capacities and discharge currents in this document are stated as specific values based on the mass of the active material.

The specific capacity of the lithium positive electrode active material decreases by no more than 8% over 100 charge-discharge cycles between from 3.5 to 4.3 V; it decreases by no more than 5 % over 100 charge-discharge cycles between from 3.5 to 4.3 V; it decreases by no more than 3 % over 100 charge-discharge cycles between from 3.5 to 4.3 V when cycled at room temperature with charge and discharge currents of 74 mAg^"1 and 148 mAg^"1, respectively. Cell types and testing parameters are provided in the Examples. In an embodiment of the process, the resultant lithium positive electrode active material has a tap density of equal to or greater than 1 .5 g cm^"3; from 1 .5 to 3.5 g cm^"3, from 1.8 to 3.5 g cm^"3, from 2.0 to 3.5 g cm^"3, such as e.g. 2.2 g cm^"3.

"Tap density" is the term used to describe the bulk density of a powder (or granular solid) after consolidation/compression prescribed in terms of 'tapping' the container of powder a measured number of times, usually from a predetermined height. The method of 'tapping' is best described as 'lifting and dropping'. Tapping in this context is not to be confused with tamping, sideways hitting or vibration. The method of measurement may affect the tap density value and therefore the same method should be used when comparing tap densities of different materials. The tap densities of the present invention are measured by weighing a measuring cylinder before and after addition of at least 10 g of powder to note the mass of added material, then tapping the cylinder on the table for some time and then reading of the volume of the tapped material. Typically, the tapping should continue until further tapping would not provide any further change in volume. As an example only, the tapping may be about 120 or 180 times, carried out during a minute.

In an embodiment, the starting materials are mechanically mixed to obtain a homogenous mixture. An embodiment of the process of the invention relates to a process resulting in a lithium positive electrode active material comprising at least 95 wt% of the spinel phase Li_yMn₂04; 0.9 < y < 1.1 . Another aspect of the invention relates to positive electrode active materials prepared according to the process of the present invention. An aspect of the invention relates to lithium positive electrode active materials prepared via a novel lithium positive electrode active material intermediate of the invention.

Yet another aspect of the invention relates to the use of the positive electrode active material prepared according to the process of the present invention for a secondary battery.

Examples:

In the following, exemplary and non-limiting embodiments of the invention are de- scribed. Example A describes a method of electrochemical testing, whilst Examples 1 -3 relate to methods of preparation, and characterization of the intermediate.

Short description of the Figs:

Fig. 1 shows the discharge capacity of the two materials produced according to Example 1 (triangles) and 2 (circles) as a function of cycling according to the description in Example A,

Fig. 2 shows the voltage curve of the two materials produced according to Example 1 (triangles) and 2 (circles) of the third charge and discharge cycle from the test described in Example A,

Figs. 3a and 3b show SEM micrographs of the lithium positive electrode active material produced according to a) Example 1 and b) Example 2, respectively,

Fig. 4 shows the first and the last XRD scans of two samples calcined in atmospheres with an O2 content of 0 ppm and 200 ppm, respectively,

Fig. 5 shows the first and the last XRD scans of the two samples calcined in atmos- pheres an O2 content of 0.2% and 20%, respectively, and Fig. 6 shows the observed variation in the peak broadening of the (1 1 1 ) peak of the LMO lithium positive electrode active material produced from the samples represented in Figs. 4 and 5. Example A: Method of Electrochemical Testing of lithium positive electrode active materials prepared according to Examples 1 and 2:

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 lithium posi- tive electrode active material (prepared according to Examples 1 and 2) 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 100 μηη gap and dried for 12 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.

Coin cells were assembled in argon filled glove box (< 1 ppm O2 and H2O) using two polymer separators (Toray V25EKD and Freudenberg FS2192-1 1 SG) and electrolyte containing 1 molar LiPF6 in EC:DMC (1 :1 in weight). Two 135 μηη thick lithium disks were used as counter electrodes and the pressure in the cells were regulated with two stainless steel disk spacers and a 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.

A power test was programmed to run the following cycles: 3 cycles 0.2C/0.2C

(charge/discharge), 3 cycles 0.5C/0.2C, 5 cycles 0.5C/0.5C, 5 cycles 0.5C/1 C, 5 cycles 0.5C/2C, 5 cycles 0.5C/5C, 5 cycles 0.5C/10C, and then 0.5C/1 C cycles with a

0.2C/0.2C cycle every 20^th cycle. C-rates were calculated based on the theoretical specific capacity of the material of 148 mAhg^"1 so that e.g. 0.2C corresponds to 29.6 mAg^"1 and 10C corresponds to 1 .48 Ag^"1.

The C-rate is an indication on the rate of charge/discharge. In general x C indicates that the charge/discharge takes place at a rate so that the battery is fully charged/discharged in x hours. Thus, 1 C indicates that the charge/discharge takes place at a rate so that the battery is fully charged/discharged in 1 hour, and 0.5 C indicates that the charge/discharge takes place at a rate so that the battery is fully charged/discharged in 0.5 hours.

Example 1 : Method of preparing lithium positive electrode active material.

Precursors in the form of 1500.26 g co-precipitated Mn-carbonate and 250.54 g U2CO3 (corresponding to an Li:Mn ratio of 1 : 1 .04) were mixed with ethanol to form a viscous slurry. The slurry was shaken in a paint shaker for 3 min. in order to obtain full de-ag- glomeration and mixing of the particulate materials. The slurry was poured into trays and left to dry at 90°C. The dried material was further de-agglomerated by shaking in a paint shaker for 1 min in order to obtain a free flowing homogeneous powder mix.

The powder mix was sintered in a muffle furnace with nitrogen flow. The heating profile is given in Table 1 .

Table 1 : Step a and a2 of two step calcination procedure.

In tables 1 and 2, "RT" denotes Room Temperature.

This intermediate product was distributed in alumina crucibles and sintered in a standard furnace in air according to the heating profile given in Table 2.

Table 2: Steb b of two step calcination procedure. The powder was de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45-micron sieve resulting in a lithium positive electrode active material consisting of 96.1 wt% spinel phase, 2.3 wt% O3 phase and 1.7 wt% birnesite phase. The tap density was determined to be 1 .5 g cm^-3.

Example 2: Comparative Example

The material of the comparative Example was prepared as in Example one, but without step a described in Table 1 . The calcination profile and gases in the furnace is thus de- scribed in Table 2.

The powder was de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45-micron sieve resulting in a lithium positive electrode active material consisting of 92.8 wt% spinel phase, 5.1 wt% O3 phase and 2.0 wt% birnesite phase. The tap density was determined to be 1 .3 g cm^-3.

Fig. 1 shows the discharge capacity of the two materials produced according to Example 1 (triangles) and Example 2 (circles) as a function of cycling according to the description in Example A. It is seen that the degradation is significantly improved in the material of Example 1 compared to the material of Example 2. The degradation in the material of Example 2 is from 8 % per 100 cycles whilst the degradation in the material of Example 1 is only 3 % per 100 cycles. This is due to the introduction of step a with a step at an atmosphere with less than 500 ppm O2 in the heat treatment.

Fig. 2 shows that voltage curve of the two materials produced according to Example 1 (triangles) and 2 (circles) of the third charge and discharge cycle from the test described in Example A. It is seen that the two voltage curves are substantially identical, which shows that the electrochemistry is not affected by introducing step a with an atmosphere with less than 500 ppm O2 in the heat treatment.

Figs. 3a and 3b show SEM micrographs of the lithium positive electrode active material produced according to a) Example 1 (Fig. 3a) and b) Example 2 (Fig. 3b). It is seen that the particles produced by introducing step a with a step of heat treatment in an atmosphere with less than 500 ppm O2 seem denser and have larger primary particle sizes. Example 3: ln-situ XRD measurements during synthesis

The samples were investigated using a PanAlytical X'PertPro diffractometer (Cu Ka) equipped with an Anton Paar XRK 900 in situ cell. Four different in situ calcinations were performed in atmospheres with the following compositions: pure N2, 200ppm O2 in N2, 2000ppm O2 in N2 and finally dry technical air (20% O2 in N2). The temperature profile for the calcination consists of a ramp to 650°C (5°C/min) and a hold time of 8 hours at 650°C. The temperature profile was the same for all four calcinations. Half way through the 650°C plateau (after 4 hours) the original gas composition was changed to dry technical air. For the fourth experiment dry technical air was thus used for the entire experiment. XRD data were collected continuously throughout the 8 hours at 650°C with a time resolution of 15 minutes per scan. Fig. 4 shows the first and the last XRD scans of the two samples calcined in atmospheres with lowest O2 content, viz. at atmospheres of 0 ppm O2 and 200 ppm O2, while Fig. 5 shows the first and the last XRD scans of the two samples calcined in atmospheres with highest O2 content, viz. at atmospheres of 0.2% O2 and 20% O2. From these two figures it is clear that all samples end up as LMO, but the path to the final LMO phase is very different, depending on the initial O2 content. At low O2 content (Fig. 4), LMO formation happens via an intermediate rock salt (RS) phase, while at high O2 content (Fig. 5) the LMO phase has already formed at the first scan at 650°C.

Fig. 6 shows the observed variation in the peak broadening of the (1 1 1 ) peak. Using the Scherrer equation the following particle sizes were calculated: 218nm (0 ppm), 217nm (200 ppm), 72nm (2000 ppm), 59nm (20%). This shows that calcination route at low O2 partial pressure via the RS structure results in larger crystallites.

A thorough analysis of the XRD scans in Fig. 4 that shows in situ scans of the intermediate, shows several observations of the two calcinations with lowest O2 content: 1 . MnCOs forms MnO rock salt before the 650°C.

2. At 650°C, Li is slowly reacting with MnO to form a second rock salt Li_xMni-_xO, which saturates at Mn/Li = 19/1 , corresponding to (Lio.o5,Mn³⁺o.o5, Mn²⁺o.9)0. 3. After saturation, the rock salt reacts further with lithium to form LiMn³⁺02. The molar ratio between (Lio.o5,Mn³⁺o.o5,Mn²⁺o.9)0 and LiMn³⁺02 approaches 1 :1 with time in the case where the molar Li:Mn ratio in the starting materials is 1 :2 corresponding to a positive electrode active material described by

with y=1 . By weight this corre- sponds to 35wt% (Li₀.o5,Mn³⁺o.o5,Mn²⁺o.9)0 and 65wt% LiMn³O₂. In the intermediate phase, it is thus shown that the maximum oxidation state of Mn is 3+ and the average oxidation state is less than or equal to 2.5+. In the final lithium positive electrode active material, half of the Mn is in oxidation state 3+ and the other half is in oxidation state 4+, thus the average oxidation state is 3.5+. This difference can be quantified using a number of techniques, including the x-ray absorption near edge structure (XANES).

"U2CO3" means a crystal lattice described by space group C2/c with the lattice parameters a, b, c and β around 8.4 A, 5.0 A, 6.2 A and 1 15°, respectively. "ΜηΟΟβ" means a crystal lattice described by space group R-3c with the lattice parameters a and b around 4.8 A and 15.5 A, respectively.

"LixMn204" means a crystal lattice described by the space group Fd-3m for the cation ordered and disordered phase, respectively, with the lattice parameter a around 8.2 A.

"LixMO" means a crystal lattice described by space group Fm-3m with the lattice parameter a around 4.2 A. In some cases, two Li_xMO phases are identified with slightly different values of the lattice parameter a. "LiMnCV means a crystal lattice described by the space group Pmnm. Li¹⁺ and Mn³⁺ cations exist in the molar ratio 1 :1. Li and Mn cations are arranged in an ordered pattern breaking the cubic symmetry resulting in the orthorhombic space group (Pmnm).

The precursor is constituted by "U2CO3" and "ΜηΟΟβ", the key phases of the intermedi- ate are "LiMnCV and "Li_xMO" and the product is the LMO spinel phase "LixM^CU"

Claims

Claims:

A lithium positive electrode active material intermediate comprising less than 10 wt% spinel phase and having a net chemical composition of Li_xMn204-5, wherein:

0 < x < 1 .1 ; and

0.88 < δ

and has been heat treated in an atmosphere with less than 500 ppm O2 at a temperature of between 300°C and 1200°C.

A lithium positive electrode active material intermediate according to claim 1 , where said intermediate has been produced from two or more starting materials and where said starting materials have been partly or fully decomposed by said heat treatment.

A lithium positive electrode active material intermediate according to claim 1 or 2, wherein the composition of the lithium positive electrode active material intermediate is about 50 wt% LiMn02, and about 50 wt% rock salt phase.

A lithium positive electrode active material intermediate according to any of the claims 1 to 3, wherein the average oxidation state of manganese is between 2.0+ and 3.0+.

A lithium positive electrode active material Intermediate according to any of the claims 1 to 4, where if the lithium positive electrode active material intermediate is heat treated in an oxidizing atmosphere at a temperature of between 500°C and 1200°C, the resultant positive electrode active material comprises at least 95% of spinel phase

wherein 0.9 < y < 1.1 .

A lithium positive electrode active material intermediate according to any of the claims 1 to 5, further comprising up to 5 mol% of one or more other elements than Li, Mn and O.

7. A process for the preparation of a lithium positive electrode active material for a medium voltage secondary battery, where the cathode is fully or partially operated at voltages between 3.5 V and 4.3 V vs. Li/Li+, said process comprising the steps of: a. Heat treating starting materials comprising lithium and manganese in an atmosphere with less than 500 ppm C>2 at a temperature of between 300°C and 1200°C to obtain a lithium positive electrode active material intermediate; b. Heat treating the lithium positive electrode active material intermediate of step a. in an oxidizing atmosphere at a temperature of between 500°C and 1200°C.

8. A process according to claim 7, wherein the temperature of step a. is between 300°C and 650°C, preferably between 400°C and 650°C, more preferably between 500°C and 650°C.

9. A process according to claim 7 or 8, wherein the temperature of step b. is between 800°C and 1200°C, preferably between 900°C and 1200°C, more preferably between 900°C and 1 100°C.

10. A process according to any of the claims 7 to 9, wherein the atmosphere with less than 500 ppm O2 is a gaseous composition selected from the group of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and mixtures thereof.

1 1 . A process according to any of the claims 7 to 10, wherein the oxidizing atmosphere is a gaseous composition selected from the group consisting of air, and a composition comprising at least 5 vol% oxygen in an inert gas.

12. A process according to any of the claims 7 to 1 1 , wherein the capacity of the lithium positive electrode active material decreases by no more than 8 % over 100 cycles between from 3.5 to 4.3 V at 55°C.

13. A process according to any of the claims 7 to 12, wherein the product of step b. has a tap density equal to or greater than 1 .5 g cm^-3.

14. A process according to any of the claims 7 to 13, wherein the lithium positive electrode active material comprises at least 95 wt% of spinel phase

wherein 0.9 < y < 1 .1 .

15. A lithium positive electrode active material prepared according to any of the claims 7 to 14.

16. Use of the positive electrode active material according to claim 15 for a secondary battery.