DE102023104776A1 - Process for the production of iron-containing layered oxides - Google Patents

Abstract

The invention relates to a process for iron-containing layered oxides of the formula LiM1-x-yFexMeyO2where 0.01 ≤ x ≤ 0.99 and 0 ≤ y ≤ 0.99, M is a transition metal selected from the group comprising Ni, Mn, Ti and Co and Me is a metal selected from the group comprising Mn, Ni, Ti, Cu, Co, Sn and Zn, comprising the following steps: a) mixing an iron salt and a salt of the transition metal and optionally a further metal salt in a solvent; b) co-precipitating an iron-containing precursor as a hydroxide, separating and drying the iron-containing precursor M1-x-yFexMey(OH)2; c) Calcining the iron-containing precursor M1-x-yFexMey(OH)2from step b) with LiOH or a lithium salt, wherein the reaction vessel for the calcination is heated to a temperature at or above the melting point of LiOH or lithium salt and LiOH or the lithium salt and the iron-containing precursor M1-x-yFexMey(OH)2 are introduced into the heated reaction vessel or LiOH, or wherein the lithium salt is initially introduced into the reaction vessel and the iron-containing precursor M1-x-yFexMey(OH)2 is introduced into the reaction vessel at a temperature at or above the melting point of LiOH or the lithium salt.

Description

The invention relates to a process for producing iron-containing layered oxides.

Layered oxides LiMeO ₂ with R-3m structure, where the metal can be cobalt, nickel, manganese, aluminum or magnesium in various stoichiometric ratios, are currently the most commonly used cathode material in lithium-ion batteries due to their energy density (> 600 Wh/kg). Especially for mobile applications such as in electrified vehicles, the energy density combined with excellent cycle stability is the decisive factor for a cathode material. The only serious alternative to layered oxides is LiFePO ₄ with Pnma structure, which has a lower energy density (~ 300 Wh/kg) but does not contain the transition metals nickel and cobalt. The use of cobalt in particular is controversial and the automotive industry is trying to reduce the proportion of cobalt.

Cobalt-free layered oxides are known and investigated for applications in battery technology. JR Mueller-Neuhaus et al. (Journal of The Electrochemical Society, 147 (10) 3598-3605 (2000) ) the use of Li _x Ni _1-y Fe _y O ₂ with 0.05 ≤ y ≤ 0.10 (space group R-3m) as cathode material. The production by means of solid-state synthesis by mixing a stoichiometric amount of Ni(OH) ₂ , FeNO ₃ and LiOH and subsequent calcination at 700°C for 100 hours. In this way, phase-pure LiNi _0.95 Fe _0.05 O ₂ layered oxides with R-3m structure and low iron content (<10 %) could be produced, but due to the morphology of the particles, which cannot be controlled in solid-state synthesis, these could only be covered with current rates < C/50 (ie more than 50 h per charge/discharge cycle).

US 11,233,239 B2 describes low-cobalt and cobalt-free cathode materials for lithium-ion batteries of the formula Li _a Ni _1-bc Co _b M _c O _d with R-3m structure, where a = 0.9 to 1.1, b = 0 to 0.05, c = 0 to 0.67, d = 1.9 to 2.1, and M can be iron, among other things. The oxides are generally produced by calcination, but the production of an iron-containing material is not described. To date, it has not been possible to synthesize iron-containing layered oxides with a phase-pure R-3m structure in an industrially relevant process, since iron oxides form as impurities in the usual calcination process.

US2012148920 A1 describes lithium nickel iron oxides of the formula Li ₂ Ni _1-x Fe _x O ₂ with 0<x<approx. 0.2 as cathode material for lithium-ion batteries, in particular Li ₂ Ni _0.975 Fe _0.025 O ₂ , Li ₂ Ni _0.95 Fe _0.05 O ₂ , and Li ₂ Ni _0.925 Fe _0.075 O ₂ . These are produced under an inert gas (N ₂ ) atmosphere at about 550°C, whereby the described structure does not represent a layered oxide with the space group R-3m. Bixian Ying et al. (2023) (https://doi.org/10. 102 1/acs.chemmater.2c02639 ) describe the preparation of nickel-poor (NCM111, LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) and nickel-rich (NCM811 LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) lithium transition metal oxides of the space group R-3m from hydroxide precursors by calcination using a precalcination step.

CN103797621A describes lithium nickel oxides of the general formula LiNi _1-x M _x O ₂ where M can be cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), or gallium (Ga) and x = 0.01 to 0.3. Lithium nickel iron oxides in the form of a layered oxide with R-3m structure or their preparation are not described.

JPH 0950810 A describes the preparation of an active material for electrodes, wherein compounds of transition metals M (Co, Mn and/or Fe) and a lithium compound are dissolved or suspended in a solvent, a lithium-nickel mixed oxide of the general formula Li _x Ni _y N _z O ₂ (N stands for elements other than Li, Ni and O; 0.8<z<1.2;0.8<y+z<1.2;0≤z<0.2) is added to obtain a slurry which is dried and calcined. The crystallographic structure or the crystallographic composition of the materials is not specified. JPH09213330 A and JPH 09167636 A describe a process for producing a lithium nickel oxide of the formula Li _x Ni _y N _z O ₂ (0.8<x<1.2, 0.8<y+z<1.2, 0≤z<0.2; N is an element other than Li, Ni, O) by calcining a mixture of a lithium compound, a nickel compound and a N-containing compound. Lithium nickel iron oxides in the form of a layered oxide with R-3m structure or their production are not described.

The present invention was therefore based on the object of providing a process which is suitable for the production of iron-containing layered oxides with a phase-pure R-3m structure in an industrially relevant process.

1. a) Mischen eines Eisensalzes und eines Salzes des Übergangsmetalls (M = Ni, Co) und optional eines weiteren Metallsalzes in einem Lösungsmittel;
2. b) Co-Präzipitieren eines eisenhaltigen Präkursors als Hydroxid (M_1-x-yFe_xMe_y(OH)₂), Abtrennen und Trocknen des eisenhaltigen Präkursors M_1-x-yFe_xMe_y(OH)₂;
3. c) Kalzinieren des eisenhaltigen Präkursors M_1-x-yFe_xMe_y(OH)₂ aus Schritt b) mit LiOH oder einem Lithiumsalz,

wobei man den Reaktionsbehälter für die Kalzinierung auf eine Temperatur oberhalb des Schmelzpunktes von LiOH oder des Lithiumsalzes temperiert und LiOH oder das Lithiumsalz und den eisenhaltigen Präkursor M_1-x-yFe_xMe_y(OH)₂ in den temperierten Reaktionsbehälter einbringt, oder wobei man LiOH oder das Lithiumsalz im Reaktionsbehälter vorlegt und den eisenhaltigen Präkursor M_1-x-yFe_xMe_y(OH)₂ bei einer Temperatur oberhalb des Schmelzpunktes von LiOH oder des Lithiumsalzes in den Reaktionsbehälter einbringt.This object is achieved by a process for the preparation of iron-containing layered oxides of the formula LiM _1-xy Fe _x Me _y O ₂ where 0.01 ≤ x ≤ 0.99 and 0 ≤ y ≤ 0.99, M is a transition metal selected from the Group comprising Ni, Mn, Ti or Co and Me a metal is selected from the group comprising Mn, Ni, Ti, Cu, Co, Sn and Zn, comprising the following steps:

1. a) mixing an iron salt and a salt of the transition metal (M = Ni, Co) and optionally another metal salt in a solvent;
2. b) co-precipitating an iron-containing precursor as hydroxide (M _1-xy Fe _x Me _y (OH) ₂ ), separating and drying the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ ;
3. c) calcining the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ from step b) with LiOH or a lithium salt,

wherein the reaction vessel for the calcination is heated to a temperature above the melting point of LiOH or the lithium salt and LiOH or the lithium salt and the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ are introduced into the heated reaction vessel, or wherein LiOH or the lithium salt is initially introduced into the reaction vessel and the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ is introduced into the reaction vessel at a temperature above the melting point of LiOH or the lithium salt.

It was surprisingly found that the calcination process according to the invention allows the production of iron-containing layered oxides with an R-3m structure and an iron content x > 0.02 in an industrially applicable process. It was found that the formation of iron oxide contamination is avoided if the reaction vessel is preheated above the melting point of lithium hydroxide (LiOH) (> 446 °C) and the precursor/LiOH or precursor/lithium salt mixture is introduced at these temperatures or if the iron-containing precursor is only introduced into the reaction vessel when molten LiOH or lithium salt is already present. Without being bound to a specific theory, it is assumed that lithium ions can be immediately intercalated into the precursor structure in the melt and that this process takes place faster than the formation of iron oxide contamination and thus stabilizes the formation of a rhombohedral layered oxide structure from the iron-containing precursor. Energy dispersive X-ray spectroscopy images of prepared lithium nickel iron oxides, for example LiNi _0.6 Fe _0.4 O ₂ , LiNi _0.7 Fe _0.3 O ₂ , LiNi _0.8 Fe _0.2 O ₂ , LiNi _0.9 Fe _0.1 O ₂ or LiNi _0.98 Fe _0.01 O ₂ , showed that iron was contained homogeneously in the obtained particles. Thus, a process for the synthesis of LiNi _1-x Fe _x O ₂ (LNFO) and other iron-containing oxides LiM _1-xy Fe _x Me _y O ₂ (M = Ni, Mn, Ti, Co) with an R-3m structure can be provided, which allows control of particle size and morphology for the layered oxides, but suppresses the formation of iron oxide impurities. In particular, an industrially applicable process can be provided that allows the morphology of the active material to be controlled.

The lower temperature limit of the calcination, which is determined by the melting point of LiOH or the lithium salt used, is essential. The calcination is carried out at a temperature above the melting point of LiOH or the lithium salt. For this purpose, the reaction vessel, for example a tube furnace, can be preheated to a temperature above the melting point of LiOH (approx. 446 °C) or the lithium salt. The iron-containing precursor compound is introduced into the temperature-controlled reaction vessel. This can be done together with the lithium compound, or LiOH or lithium salt and iron-containing precursor are introduced separately.

In preferred embodiments, the reaction vessel is preheated and LiOH or the lithium salt and the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ are introduced together into the temperature-controlled reaction vessel. This variant of parallel introduction has the advantage that LiOH or lithium salt and the iron-containing precursor can be better homogenized before the reaction. In other embodiments, LiOH or lithium salt is introduced first, then the reaction vessel containing LiOH or the lithium salt is preheated, and the iron-containing precursor is introduced into the molten LiOH or lithium salt. According to this embodiment, too, the precipitation of iron oxide impurities can be avoided.

LiOH or a lithium salt can serve as a source of lithium ions. Water-soluble lithium salts are preferred. In embodiments, the lithium salt is selected from the group comprising lithium acetate, lithium carbonate, lithium nitrate and lithium sulfate. LiOH is particularly preferably used.

The calcination is carried out at a temperature above the melting point of LiOH or the lithium salt. In preferred embodiments, the calcination is carried out using LiOH at a temperature in the range of ≥ 500 °C to ≤ 1000 °C, preferably in the range of ≥ 600 °C to ≤ 900 °C, preferably in the range of ≥ 700 °C to ≤ 900 °C, in particular in the range of ≥ 750 °C to ≤ 800 °C. Calcination is particularly preferably carried out using LiOH at a temperature around or equal to 800 °C. Calcination at 800 °C (20 h) yielded iron-containing layered oxides with an R-3m structure and a very low metal/lithium disorder, for example < 2%, independent of the Ni:Fe ratio.

When using lithium carbonate, the calcination is preferably carried out at a temperature in the range of ≥ 800 °C to ≤ 1000 °C. When using lithium nitrate, the calcination is preferably carried out at a temperature in the range of ≥ 255 °C to ≤ 1000 °C.

In embodiments, the calcination is carried out in two steps, with a precalcination being carried out before the calcination at a temperature that is lower than the temperature of the calcination but above the melting point of LiOH or the lithium salt. A precalcination is particularly advantageous when using metal and transition metal salts in which the metal or transition metal is present in a lower oxidation state than in the iron-containing layered oxide to be produced and is oxidized during the reaction. An example of this are nickel-rich layered oxides (LiNi _1-xy Fe _x Me _y O ₂ with x + y ≤ 0.2 where Ni has to be converted from a 2+ oxidation state to a 3+ state. Oxidation proceeds better at low temperatures, while at high temperatures the lattice structure proceeds more quickly. Thus, the purity of the iron-containing layered oxide to be produced can be increased by precalcining these salts. Commonly used Co and Mn (cobalt and manganese salts, where Co and Mn are each in the +2 oxidation state) are oxidized very quickly even at temperatures below 250 °C. In embodiments for the production of iron-manganese layered oxides, no increase in the purity of the oxides obtained was found by precalcining.

In embodiments, the precalcination is carried out using LiOH at a temperature in the range of ≥ 460 °C to ≤ 500 °C. In these embodiments, the subsequent calcination is preferably carried out in the range of ≥ 500 °C to ≤ 1000 °C, preferably in the range of ≥ 600 °C to ≤ 900 °C, preferably in the range of ≥ 700 °C to ≤ 900 °C, in particular in the range of ≥ 750 °C to ≤ 800 °C.

The duration of the precalcination is determined in particular by the oxidation state of the metal ion present in the metal salt and the oxidation state to be achieved. In embodiments, the precalcination is carried out in a period in the range of ≥ 30 min to ≤ 8 h, preferably in the range of ≥ 2 h to ≤ 5 h. If the difference is small and/or the metal ion can be oxidized quickly, a period of 30 minutes may be sufficient. A period of 2 hours is usually considered sufficient.

In embodiments, the calcination is carried out in a period in the range from ≥ 5 h to ≤ 20 h, preferably in the range from ≥ 10 h to ≤ 20 h. The duration of the calcination step can have an effect on the metal/lithium disorder of the iron-containing layered oxide to be produced. A high or very high order is particularly advantageous for the production of layered oxides to be used in lithium-ion batteries, as this increases the efficiency of the battery. A calcination time of 10 hours leads to layered oxides with a high purity, which show good efficiency in applications in lithium-ion batteries. Calcination times of less than 5 hours are also suitable for the production of iron-containing layered oxides, but these can show impurities in the layer structure or layer lattice (metal/lithium disorder), which is at least not very advantageous for use in lithium-ion batteries. A calcination time of 20 hours resulted in iron-nickel, iron-nickel-manganese, and iron-nickel-titanium layered oxides with very high order (metal/lithium disorder < 5%), which showed very high efficiency in a lithium-ion battery.

In preferred embodiments, the calcination is carried out in a period of time in the range of ≥ 8 h to ≤ 10 h or in a range of ≥ 10 h to ≤ 20 h. For example, in the production of iron-nickel layered oxides, it was found that a calcination time of 20 hours at a temperature of 800 °C showed the lowest Li/metal disorder, regardless of the Ni:Fe ratio. Increasing the calcination time from 10 h to 20 h in the production of iron-nickel layered oxides leads to an improvement in the Li/metal order of up to 50%, depending on the stoichiometry of the transition metals.

In embodiments, the precalcination is carried out in a period in the range of ≥ 30 min to ≤ 8 h, preferably in the range of ≥ 2 h to ≤ 5 h, and the calcination in a period in the range of ≥ 5 h to ≤ 20 h, preferably in the range of ≥ 10 h to ≤ 20 h.

In embodiments, the heating rate between precalcination and calcination is 4°C/min or slower, for example 2°C/min. It is preferred that the heating rate between precalcination and calcination is slow, since this can also increase the order of the resulting layered oxides. It was found that with a heating rate between precalcination and calcination of 4°C/min or slower, iron-containing layered oxides with a high order in the layer structure or layer lattice could be produced.

Layered oxides can be produced by calcination by first precipitating a transition metal as a hydroxide precursor from a transition metal salt solution. Depending on the type of precipitation, the size and morphology of the secondary and primary particles can be adjusted. The resulting hydroxide precursor can be separated from the solvent such as water by filtration or centrifugation and dried. This precipitation reaction works for transition metals whose solubility product in water is sufficiently low at a given pH. The precursor is then calcined with lithium hydroxide (LiOH) or another lithium salt.

To produce iron-containing layered oxides, in step a) an iron salt and a salt of a transition metal (M = Ni, Mn, Ti, Co) and optionally another metal salt (Me = Mn, Ni, Ti, Cu, Co, Sn, Zn) are first mixed in a solvent. Dissolving is preferably carried out in an aqueous solution, in particular in water. The solvent is therefore preferably water. The metal or transition metal salt should dissolve in water or an aqueous solution. The salt of the metal or transition metal is therefore preferably a sulfate, acetate or carbonate. The salts are preferably mixed in the corresponding stoichiometric ratio of the iron-containing layered oxide to be produced. Complexing agents such as ammonium hydroxide (NH ₄ OH) or ethylenediaminetetraacetate (EDTA) can be added if necessary.

In step b), the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ is co-precipitated from the mixture from step a), separated and then dried. In embodiments, the co-precipitation in step b) is carried out at ≥ pH 8, preferably in the range from ≥ pH 9 to ≤ pH 12, preferably in the range from ≥ pH 11 to ≤ pH 12. The pH can be adjusted, for example, by adding a NaOH and/or NH ₃ solution. The addition is preferably carried out with stirring or the solutions are pumped into a reactor. The precipitation can be carried out, for example, in a continuous stirred tank reactor or a Taylor reactor. After the precipitation has ended, the resulting suspension can be stirred, for example, for a further period of 1 to 6 hours. The size and morphology of the layered oxides to be produced can be adjusted by the stirring speed, the pumping speed when adding the metal salt and NaOH/NH ₃ solution and the duration of stirring after precipitation has ended. Controlling the size and shape of the secondary and primary particles (morphology) is very advantageous for use as cathode material because it minimizes overvoltages during battery operation and allows the materials to be charged with relatively high current rates.

The hydroxide precursor can then be separated from the solvent, in particular water, for example by filtration or centrifugation, and dried. The separated hydroxide precursor can be washed, for example by means of an aqueous solution with a pH value in the range from pH 7 to pH 11. After separation and before calcination, the precursor is dried. Preferably, the hydroxide precursor is dried at a temperature in the range from 60°C to 80°C.

In step c), the resulting iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ is calcined with LiOH or a lithium salt. Preferably, the precursor is calcined with an excess of lithium hydroxide (LiOH) or lithium salt, in particular nitrate, sulfate, acetate or carbonate. The excess of lithium hydroxide (LiOH) or lithium salt is preferably in the range of 3 to 10 mol%, based on the stoichiometric ratio of lithium to metal.

Preferably, the precursor is ground with lithium hydroxide (LiOH) or another lithium salt such as lithium nitrate, lithium sulfate or lithium carbonate, for example in a mortar, depending on the desired subsequent morphology of the material to be produced. Alternatively, lithium hydroxide (LiOH) or lithium salt and the precursor can also be mixed in an acetone suspension, with the acetone being removed again by drying, for example at 80 °C, before calcination.

The calcination process according to the invention allows the production of iron-containing layered oxides with R-3m structure with a high iron content in high purity and order. Results of powder diffraction measurements indicated a phase-pure R-3m layer structure of produced iron-nickel layered oxides LiNi _1-x Fe _x O ₂ (LNFO) with x = 0.1, 0.2, 0.4, 0.5 and 0.6. Furthermore, manganese-hal Titanium-containing iron-nickel layered oxides LiNi _1-xy Fe _x Mn _y O ₂ and LiNi _1-xy Fe _x Ti _y O ₂ can be produced in pure phases.

In particular, the process can be used to produce iron-containing layered oxides LiM _1-xy Fe _x Me _y O ₂ with R-3m structure with both a low and, in particular, a high or very high iron content 0.02 ≤ x ≤ 0.99. The iron content of the iron-containing layered oxides LiM _1-xy Fe _x Me _y O ₂ is preferably in the range of 0.02 ≤ x ≤ 0.6.

M is a transition metal selected from the group comprising Ni, Mn, Ti and Co. M can be selected from Ni or Co. Preferably, M is a transition metal selected from the group comprising Ni, Mn and Ti. The production of low-cobalt or cobalt-free layered oxides is particularly preferred for use as cathode materials for lithium-ion batteries. Preferably, M is nickel. Iron-nickel layered oxides with R-3m structure could be produced with variable iron content in very good purity and order.

A preferred iron-containing precursor is Ni _1-x Fe _x (OH) ₂ with 0.02 ≤ x ≤ 0.9, preferably 0.02 ≤ x ≤ 0.9, more preferably 0.02 ≤ x ≤ 0.6, more preferably 0.02 ≤ x ≤ 0.4. In preferred embodiments, an iron-containing layered oxide of the formula LiNi _1-x Fe _x O ₂ is produced, where 0.02 ≤ x ≤ 0.9, preferably 0.02 ≤ x ≤ 0.9, preferably 0.02 ≤ x ≤ 0.6, more preferably 0.1 ≤ x ≤ 0.5, particularly preferably x = 0.1, 0.2, 0.4, 0.5 or 0.6, with R-3m structure. In embodiments for producing iron-containing layered oxides of the formula LiNi _1-x Fe _x O ₂ , precalcination can preferably be provided. Preferred nickel-iron layered oxides Ni _1-xy Fe _x O ₂ that can be produced using the described process are selected from LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64), and LiNi _0.3 Fe _0.3 O ₂ (LNFO55).

Iron-containing layered oxides LiNi _1-x Fe _x O ₂ with x = 0.1, 0.2, 0.4 and 0.5 with R-3m structure were produced with high purity and order and tested as cathode material in lithium-ion batteries. It was shown that even after numerous charging cycles, the capacity was comparable to conventional NCMs. The LiNi _1-x Fe _x O ₂ produced can therefore be used analogously to the widely used NCMs.

Electrochemical half-cell measurements showed that the capacity of the materials was comparable to that of the cathode materials already in commercial use, with the average discharge voltage being even higher for some layered oxides. Furthermore, tests have shown that for certain compositions the capacity was even higher than that of conventional cobalt-containing materials with comparable cycle stability.

Iron-nickel layered oxides doped with other transition metals could also be produced in very good purity and order. The additional metal Me contained can be selected from the group comprising Mn, Ni, Ti, Cu, Co, Sn and Zn. The additional metal Me contained is preferably selected from the group comprising Mn, Ti and Cu. The production of cobalt-free layered oxides is particularly preferred for use as cathode materials for lithium-ion batteries. Me is preferably selected from Mn and Ti.

Preferably, M is selected from Ni or Co and Me is selected from the group comprising Mn, Ti, Cu, Sn and Zn. Preferably, M is nickel and Me is selected from the group comprising Mn, Ti, Cu, Sn and Zn. Particularly preferably, M is nickel and Me is selected from the group comprising Mn, Ti and Cu.

In preferred embodiments, M is Ni and Me is Mn or Ti. Manganese-containing and titanium-containing iron-nickel layered oxides with R-3m structure could be produced with variable iron content in very good purity and order. Further preferred iron-containing precursors are selected accordingly from Ni _1-xy Fe _x Mn _y (OH) ₂ and Ni _1-xy Fe _x Ti _y (OH) ₂ with 0.02 ≤ x ≤ 0.99, preferably 0.02 ≤ x ≤ 0.9, preferably 0.02 ≤ x ≤ 0.6, further preferably 0.02 ≤ x ≤ 0.4, more preferably 0.02 ≤ x ≤ 0.2 and 0.02 ≤ y ≤ 0.99, preferably 0.02 ≤ y ≤ 0.9, preferably 0.02 ≤ y ≤ 0.6, further preferably 0.02 ≤ y ≤ 0.4, more preferably 0.02 ≤ y ≤ 0.2.

In further preferred embodiments, an iron-containing layered oxide of the formula LiNi _1-xy Fe _x Mn _y O ₂ where 0.02 ≤ x ≤ 0.6 and 0.02 ≤ y ≤ 0.6, preferably 0.02 ≤ x ≤ 0.4 and 0.02 ≤ y ≤ 0.4, preferably 0.02 ≤ x ≤ 0.2 and 0.02 ≤ y ≤ 0.2, with R-3m structure is prepared. Manganese-containing nickel-iron layered oxides Ni _1-xy Fe _x Mn _y O ₂ with x = 0.02, 0.1 and 0.2 and y = 0.02, 0.1 and 0.2 with R-3m structure could be produced with high purity and tested as cathode material in lithium-ion batteries. It was shown that Ni _1-xy Fe _x Mn _y O ₂ in particular showed excellent properties as a cathode material. Preferred manganese Gan-containing nickel-iron layered oxides Ni _1-xy Fe _x Mn _y O ₂ that can be produced using the described process are selected from LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622).

In further preferred embodiments, an iron-containing layered oxide of the formula LiNi _1-xy Fe _x Ti _y O ₂ where 0.02 ≤ x ≤ 0.6 and 0.02 ≤ y ≤ 0.6, preferably 0.02 ≤ x ≤ 0.4 and 0.02 ≤ y ≤ 0.4, preferably 0.02 ≤ x ≤ 0.2 and 0.02 ≤ y ≤ 0.2, with R-3m structure is produced. Titanium-containing nickel-iron layered oxides Ni _1-xy Fe _x Ti _y O ₂ with x = 0.02, 0.1 and 0.2 and y = 0.02, 0.1 and 0.2 with R-3m structure were produced with high purity and tested as cathode material in lithium-ion cells. It was shown that the phase-pure Ti-substituted layered oxides produced form another cathode material that exhibits promising electrochemical properties. Good cycle stability was found for these materials. Preferred titanium-containing nickel-iron layered oxides Ni _1-xy Fe _x Ti _y O ₂ that can be produced using the described process are selected from LiNi _0.96 Fe _0.02 Ti _0.02 O ₂ (NFT9622), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811) and LiNi _0.6 Fe _0.2 Ti _0.2 O ₂ (NFT622).

Examples and figures serving to illustrate the present invention are given below.

The figures show:

• 1 shows in the 1a) , 1b) and 1c ) X-ray diffractograms of layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82) and LiNi _0.6 Fe _0.4 O ₂ (LNFO64) prepared according to the invention as well as the respective Rietveld refinement with the R-3m space group.
• 2 shows in the 2a) , 2b) and 2c ) Powder diffraction data and refinements of layered oxides LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622) prepared according to the invention, synthesized at 800 °C for 20 h.
• 3 shows in the 3a) , 3b) and 3c ) Powder diffraction data and refinements of LiNi _0.96 Fe _0.02 Ti _0.02 O ₂ (NFT9622), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811) and LiNi _0.6 Fe _0.2 Ti _0.2 O ₂ (NFT622), calcined at 800 °C for 20 h.
• 4 shows X-ray diffractograms of the nickel-iron layered oxide LiNi _0.8 Fe _0.2 O ₂ (LNFO82) as a function of the calcination temperature.
• 5 shows powder diffractograms of LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64) and LiNi _0.5 Fe _0.5 O ₂ (LNFO55) calcined at 500 °C for 20 h in comparison to LNFO82, synthesized without preheating with a precalcination at 500 °C (2 h) and a calcination at 800 °C (20 h).
• 6 shows the Me/Li disorder (Me=Ni and Fe) as a function of the calcination temperature of LiNi _x Fe _1-x O ₂ (x=0.9 (LNFO91), 0.8 (LNFO82), 0.6 (LNFO64) and 0.5 (LNFO55)).
• 7 shows powder diffractograms of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) calcined at 800 °C for 10 h in 7a) and 20 hrs in 7b) .
• 8 Powder diffractograms of LiNi _0.9 Fe _0.1 O ₂ (LNFO91) in 8a) , LiNi _0.8 Fe _0.2 O ₂ (LNFO82) in 8b) , LiNi _0.6 Fe _0.4 O ₂ (LNFO64) in 8c ) and LiNi _0.5 Fe _0.5 O ₂ (LNFO55) in 8d ), synthesized with preheating to 800 °C and calcination at 800 °C for 20 h compared to preheating to 500 °C, precalcination at 500 °C for 2 h and subsequent calcination at 800 °C for 20 h.
• 9 in 5a) the capacity behavior of electrodes containing NCM811 (LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) as reference, LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811), LiNiO ₂ (LNO) as reference, LiNi _0.98 Fe _0.02 O ₂ (LNFO982), LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82); in 9b) the voltage profile, and in 9c ) the differential capacity of the first charge and discharge cycle.
• 10 in 10a) the capacity behavior of electrodes containing LNO (LiNiO ₂ ) as reference, LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622). In 10b) the voltage profile, and in 10c ) the differential capacitance of the first charge and discharge cycle of the cathodes.
• 11 shows in 11a) Powder diffraction data of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) during charging and discharging. 11b) shows near-edge X-ray absorption spectroscopy (NEXAFS) spectra of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) at 100%, 90%, 85%, 75%, 40%, 25% and 0% charge state.

Example 1

Production of nickel-iron layered oxides LiNi _1-x Fe _x O ₂

Co-precipitation and drying of the precursor Ni _1-x Fe _x (OH) ₂

First, a 1.5 molar aqueous solution (100 mL) of a stoichiometric amount of iron sulfate and nickel sulfate in deionized water (Millipore) was prepared. In addition, 110 mL of a 4.875 molar NaOH solution and 100 ml of a 12 wt.% NH ₃ solution were added together. 10 mL of a 4.875 molar NaOH solution were placed in a round-bottomed flask. Oxygen was removed from this solution with stirring by flowing gaseous nitrogen through the solution for 1 h. With a further nitrogen flow and continuous stirring, the metal salt and the NaOH/NH ₃ solution were then pumped into the solution over a period of 2 h in a ratio of 1:1.2. The metal salts precipitated as hydroxide precursors (Ni _1-x Fe _x (OH) ₂ ). During the entire addition of the metal salt and the NaOH/NH ₃ solution, the pH was kept constant at pH 11 by adding a 4.875 molar NaOH solution as required. After the addition of the metal salt and the NaOH/NH ₃ solution had been completed, the suspension was stirred for a further 4 h under a stream of nitrogen. The precursor was then separated from the aqueous phase by vacuum filtration and dried overnight at 80 °C in a drying oven (Binder E53).

Calcination

The precursor was suspended in acetone with 3% to 10% excess, mol% based on the stoichiometric ratio of Li to M + Me, of lithium hydroxide (LiOH). The suspension was dried overnight in a fume hood. A tube furnace (Carbolite™) was preheated to a temperature of 800°C for 2.5 h. The mixture of LiOH and precursor was then introduced into the furnace and calcined for 20 h under oxygen flow.

The following Table 1 summarizes the metal salts used to produce the layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64), and LiNi _0.5 Fe _0.5 O ₂ (LNFO55). Table 1: Initial weight of the metal salts NiSO ₄ * 6 H ₂ O [mg] FeSO ₄ * 7 H ₂ O [mg] LiOH* H ₂ O [mg] LiNi _0.9 Fe _0.1 O ₂ 38.64 0.83 1.17 LiNi _0.8 Fe _0.2 O ₂ 35.48 4.17 1.17 LiNi _0.6 Fe _0.4 O ₂ 31.54 8.34 1.17 LiNi _0.5 Fe _0.5 O ₂ 23.66 16.68 1.18

During calcination, the morphology of the secondary particles was preserved, resulting in a fine powder. The stoichiometry of the cathode materials after calcination was determined by energy dispersive X-ray spectroscopy (EDX) using a scanning electron microscope (SEM, ZEISS Auriga) equipped with an EDX detector (INCAPentaFETx3, Oxford Instruments) and by inductively coupled plasma optical emission spectrometry (ICP-OES, Spectro ARCOS ICP-OES, Spectro Analytical Instruments, Kleve, Germany). Both measurements confirmed that iron was homogeneously distributed in the particles and that the Li:Fe:Ni ratio corresponded to the stoichiometric ratio (±0.02%). The phase purity of the materials was determined by powder diffraction (Bruker Cu Kα _1.2 radiation, λ = 1.54059 Å/ 1.54443 Å and synchrotron source, Beamline I11, DLS UK, λ = 0.489951 Å).

1 shows in the 1a) , 1b) and 1c ) the X-ray diffractograms of the obtained layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82) and LiNi _0.6 Fe _0.4 O ₂ (LNFO64), as well as the refinement of the data with an R-3m space group (Rietveld refinement with Fullprof). How to 1 The iron-containing layered oxides produced showed a phase-pure R-3m structure. The following Table 2 summarizes the results of the refinements of the layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), and LiNi _0.6 Fe _0.4 O ₂ (LNFO64). The a,b lattice parameter, the c lattice parameter, the Me/Li disorder (with Me=Ni and Fe) and the R _Bragg value of the refinement are listed. Table 2: Results of refinements R-3m a,b [Å] c [Å] Me/Li [%] R _Bragg LiNi _0.9 Fe _0.1 O ₂ 2.8855(5) 14,239(3) 3.9(5) 4.21 LiNi _0.8 Fe _0.2 O ₂ 2,891(1) 14,29(1) 2.1(3) 3.59 LiNi _0.6 Fe _0.4 O ₂ 2.8857(5) 14,246(2) 2.9(3) 6.28

It was found that both the lattice parameters and the Li/Me disorder were of the order of magnitude of NCMs and other known layered structures.

Example 2

Production of nickel-iron-manganese layered oxides

Manganese-containing nickel-iron layered oxides of the formulas LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622) were prepared as described in Example 1, with the exception that 1.5 molar aqueous solutions of a stoichiometric amount of iron sulfate, nickel sulfate and manganese sulfate were prepared.

The following Table 3 summarizes the results of the refinements of the produced layered oxides LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622). The a,b lattice parameter, the c lattice parameter, the Me/Li disorder (with Me=Ni and Fe) and the R _Bragg value of the refinement are listed. Table 3: Results of the refinements R-3m a,b [Å] c [Å] Me/Li [%] RBragg LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ 2,880(2) 14.21(2) 2.01(3) 2.11 LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ 2,885(2) 14,26(1) 2.31(3) 2.73 LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ 2,895(2) 14.32(1) 7.9(5) 2.84

2 shows in the 2a) , 2b) and 2c ) the X-ray diffractograms of the obtained layered oxides LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFM9622), LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFM622), as well as the refinement of the data with an R-3m space group (Rietveld refinement with Fullprof). How to 2 The refinement showed phase-pure R-3m layered oxides.
It was found that both the lattice parameters and the Li/Me disorder were of the order of magnitude of NCMs and other known layered structures.

Example 3

Production of nickel-iron-titanium layered oxides

Titanium-containing nickel-iron layered oxides of the formulas LiNi _0.96 Fe _0.2 Mn _0.2 O ₂ (NFT9622), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811) and LiNi _0.6 Fe _0.2 Ti _0.2 O ₂ (NFT622) were prepared as described in Example 1, except that a 1.5 molar aqueous solution of a stoichiometric amount of iron sulfate, nickel sulfate and titanium sulfate was prepared.

The following Table 4 summarizes the results of the refinements of the produced layered oxides LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFT9622), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811) and LiNi _0.6 Fe _0.2 Ti _0.2 O ₂ (NFT622). The a,b lattice parameter, the c lattice parameter, the Me/Li disorder (with Me=Ni and Fe) and the R _Bragg value of the refinement are listed. Table 4: Results of the refinements R-3m a,b [Å] c [Å] Me/Li [%] RBragg LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ 2,881(1) 14,22(1) 3.9(2) 3.01 LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ 2,889(1) 14,29(1) 5.3(3) 2.36 LiNi _0.6 Fe _0.2 Ti _0.2 O ₂ 2,904(2) 14,32(2) 13.7(5) 2.72

3 shows in the 3a) , 3b) and 3c ) the X-ray diffractograms of the obtained layered oxides LiNi _0.96 Fe _0.02 Mn _0.02 O ₂ (NFT9622), LiNi _0.8 Fe _0.1 Ti _0.1 O ₂ (NFT811) and LiNi _0.6 Fe _0.2 Mn _0.2 O ₂ (NFT622), as well as the refinement of the data with an R-3m space group (Rietveld refinement with Fullprof). How to 3 The refinement showed phase-pure R-3m layered oxides.

It was found that both the lattice parameters and the Li/Me disorder were of the order of magnitude of NCMs and other known layered structures.

Comparison example 4

Investigation of calcination without preheating at different calcination temperatures

Co-precipitation and drying of the precursor for nickel-iron layered oxide LiNi _0.8 Fe _0.2 O ₂ (LNFO82) was carried out as described in Example 1. The precursor was also suspended in acetone with a 3% to 10% excess, by weight, of lithium hydroxide (LiOH) based on the stoichiometric ratio, and the suspension was dried overnight in a fume hood. Alternatively, the dried precursor and LiOH were introduced into the tube furnace (Carbolite™) at room temperature (20±2°C) and then the tube furnace and the LiOH/Ni _0.8 Fe _0.2 (OH) ₂ mixture contained therein were brought to a temperature of 100 °C, 200 °C, 300 °C or 500 °C at a heating rate of 2 °C/min. After the temperature was reached, the tube furnace was cooled to room temperature.

Furthermore, LiNi _0.8 Fe _0.2 O ₂ (LNFO82) was prepared by introducing the dried precursor and LiOH into the tube furnace at room temperature (20±2°C) as described above and then heating the tube furnace to a temperature of 800°C with the LiOH/Ni _0.8 Fe _0.2 (OH) ₂ mixture contained therein. The mixture was then calcined for 20 h.

X-ray diffractograms were recorded from the lithium nickel iron oxides produced. 4 shows the respective powder diffractograms as a function of the calcination temperature in the lower part. The measurements show that iron oxides are formed below 500°C, i.e. below the melting point of LiOH. This results in lithium nickel iron oxides LiNi _x Fe _1-x O ₂ · Fe _y O _z .

4 also shows in the upper part the powder diffractogram of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) produced by conventional calcination at 800°C (solid line) and that produced according to Example 1 after preheating the tube furnace to 800°C with LNFO82 (dotted line). The comparison of the diffractograms of both samples confirms that the iron oxide impurities formed below 500°C can still be found in the final material even after 20 hours of calcination at 800°C if conventional calcination is used. The formation of iron oxides could be prevented by preheating the tube furnace to 800°C and then introducing the LiOH/Ni _0.8 Fe _0.2 (OH) ₂ mixture into the furnace.

Example 5

Investigation of calcination at 500 °C

Nickel-iron layered oxides of the formulas LiNi _0.9 Fe _0.1 Mn _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64) and LiNi _0.5 Fe _0.5 O ₂ (LNFO55) were prepared using a 1.5 molar aqueous solution of a stoichiometric amount of iron sulfate, nickel sulfate and, if necessary, manganese sulfate as described in Example 1. In contrast, the tube furnace was preheated to a temperature of 500°C for 2.5 h, the mixture of LiOH and precursor was introduced into the furnace and calcined at 500°C for 20 h under oxygen flow.

In a comparison approach, LiNi _0.8 Fe _0.2 O ₂ (LNFO82) was synthesized without preheating the tube furnace, whereby the precursor and LiOH were introduced into the tube furnace at room temperature (20±2°C) and the tube furnace was then heated to a temperature of 500°C with the LiOH/Ni _0.8 Fe _0.2 (OH) ₂ mixture contained therein. and precalcined for 2 h at 500 °C. Then the tube furnace was heated to 800 °C at a heating rate of 2 °C/min and calcined for a further 20 h.

X-ray diffractograms were recorded from the lithium nickel iron oxides produced. 5 shows the respective powder diffractograms of the obtained layered oxides LiNi _0.9 Fe _0.1 Mn _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64) and LiNi _0.5 Fe _0.5 O ₂ (LNFO55), as well as the comparison sample of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) without preheating. The latter showed iron oxide impurities Fe _x O _y .

Example 6

Investigation of the influence of calcination temperature above 500°C

Nickel-iron layered oxides LiNi _1-x Fe _x O ₂ were prepared as described in Example 1, whereby for each of the layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64), and LiNi _0.5 Fe _0.5 O ₂ (LNFO55) the calcination was carried out for 20 hours at a calcination temperature of 500°C, 600°C, 700°C, 800°C or 900°C.

X-ray diffraction patterns were recorded from the nickel-iron layered oxides produced and the Me/Li disorder (Me=Ni and Fe) was refined. The 6 shows the Li/Me disorder as a function of the calcination temperature of LiNi _x Fe _1-x O ₂ (x=0.9, 0.8, 0.6 and 0.5). 6 As can be seen, the Me/Li disorder in nickel-iron layered oxides with a higher iron content decreased significantly when calcined at 600 °C compared to 500 °C. Calcination at 800 °C resulted in iron-containing layered oxides with R-3m structure and very low lithium/metal disorder, independent of the Ni:Fe ratio.

Example 7

Investigation of different calcination times

Nickel-iron layered oxides LiNi _0.8 Fe _0.2 O ₂ (LNFO82) were prepared as described in Example 1, whereby calcination was carried out at 800°C for 10 h or 20 h.

X-ray diffractograms were recorded from the produced nickel-iron layered oxides and the Me/Li disorder (Me=Ni and Fe) was determined. 7 shows the powder diffractograms of LNFO82 (LiNi _0.8 Fe _0.2 O ₂ ), synthesized at 800 °C for 10 hours in 7a) and 20 Hours in 7b) . How to 7 The Me/Li order increased with a calcination time of 20 hours compared to 10 hours.

Example 8

Production of nickel-iron layered oxides with precalcination

Co-precipitation and drying of the precursor for nickel-iron layered oxides of the formulas LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64), LiNi _0.3 Fe _0.3 O ₂ (LNFO55) from a 1.5 molar aqueous solution of a stoichiometric amount of iron sulfate and nickel sulfate was carried out as described in Example 1. The precursor was also suspended in acetone with a 3% to 10% excess, weight percent based on the stoichiometric ratio, of lithium hydroxide (LiOH) and the suspension was dried overnight in a fume hood.

A sample of the mixture of the respective precursor and LiOH was calcined for 20 hours at 800°C as described in Example 1. Another sample was first precalcined by preheating the tube furnace to 500°C, adding the precursor/LiOH mixture and precalcining it for 2 hours at 500°C. The tube furnace was then heated to 800°C at a heating rate of 2°C/min and calcined for a further 20 hours.

X-ray diffractograms were recorded from the lithium nickel iron oxides produced. 8 shows the respective powder diffractograms of the obtained layered oxides LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82), LiNi _0.6 Fe _0.4 O ₂ (LNFO64), LiNi _0.5 Fe _0.5 O ₂ (LNFO55). The ratio of the reflections R003 to R104 corresponds to an estimate of the disorder, whereby a large R003/R104 value means a lower Li/metal disorder. How to 8 the ratio of the R003 to R104 reflection increased by a precalcination step at 500 °C in the case of LNFO82 and LNFO91. This shows that that in the case of nickel-rich layered oxides (LiNi1 _1-x Fe _x O ₂ with x ≤ 0.2) an increase in order can also be achieved by a precalcination step at 500 °C. Without being bound to a specific theory, it is assumed that the precalcination at 500 °C allows the slow oxidation of Ni ²⁺ to Ni ³⁺ to occur on a larger scale and that the oxidation remains incomplete if the temperature is increased too quickly for these materials.

Example 9

Electrochemical investigations

Electrode production

For the electrode production, the nickel-iron layered oxides LiNi _0.98 Fe _0.02 O ₂ (LNFO982), LiNi _0.9 Fe _0.1 O ₂ (LNFO91), LiNi _0.8 Fe _0.2 O ₂ (LNFO82) and LiNiO ₂ (LNO) produced according to Example 1 were used as reference and the nickel-iron-manganese layered oxides LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) produced according to Examples 2 and 3, nickel-iron-titanium layered oxides LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ (NFM811) and as reference LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811, self-synthesized) were coated with conductive carbon (Super C65, Imerys Graphite & Carbon) and Polyvinylidene difluoride (Kynar flex) was used as a binder in a weight ratio of 80:10:10. N-methyl pyrrolidone (Sigma-Aldrich, 99.5% purity) was added to the mixture of layer oxide, carbon and binder as a solvent, which dissolves the binder when mixed with a solder paste mixer (Thinky SR-500, 20 min, 1800 rpm). The paste thus obtained was applied to aluminum foil (20 µm thick, Goodfellow) using a doctor blade with a wet layer thickness of 200 µm. The electrodes thus produced were dried overnight at 80°C in a drying cabinet (Binder E53). Round electrodes with a diameter of 12 mm, or an area of 1.13 cm ² , were then punched out and dried overnight at 120°C under vacuum. The surface loading was approximately 6 to 7 mg cm ^-2 . The surface loading was determined by weighing the pure foil and the punched-out electrodes.

The electrochemical investigations were carried out in 2032 button cells with lithium metal foils (Albemarle, battery grade) as counter electrodes, Celgard2500 (16 mm Ø) as separator and 35 µL LP572 (1 molar LiPF ₆ in ethylene carbonate/ethyl methyl carbonate in a weight ratio of 3:7, with 2 wt.% vinylene carbonate, BASF, battery grade) as electrolyte. The voltages given refer to the Li ⁺ /Li counter electrode. All electrochemical investigations were carried out at a temperature of 20°C ± 2°C. A Maccor 4300 battery test system was used as potentiostat/galvanostat.

The cells were cycled between 3.0 V and 4.6 V (4.4 V), depending on the voltage at which redox (charge exchange) processes take place. The cells were cycled in the first five cycles with a constant current density of 9 mA/g (C/20, a capacity of 180 mAh/g was used as the nominal capacity regardless of the material). This was followed by five cycles each with 18 mA/g (C/10), 36 mA/g (C/5), 90 mAh/g (C/2), 180 mAh/g (1C) and 20 cycles with 360 mAh/g (2C). This procedure was repeated once and finally 5 more charge and discharge cycles were carried out with a current of 9 mA/g (C/20).

9 and 10 show the results of the electrochemical investigations. 9a) and 10a) the capacity of the first discharge cycle and its efficiency (discharge capacity/charge capacity * 100%), 9b) and 10b) the voltage profile of the first cycle and 9c ) and 10c ) the differential capacitance (charge exchange processes as a function of voltage). 9 shows in the left column Fe/Mn and Fe/Ti substituted cathodes with 80% Ni (LiNi _0.98 Fe _0.02 Me _0.1 O ₂ with Me= Mn, Ti, reference material was NCM811, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) and in the right column Fe- and Ni- containing cathodes LiNi _1-x Fe _x O ₂ with x = 0.02, 0.1 and 0.2. LNO, LiNiO ₂ , served as reference material.

Peaks in the differential capacity ( 9c and 10c ) mark the voltage at which charge exchange processes take place, with positive peaks corresponding to charging currents and negative peaks to discharging currents. The great similarity of the differential capacities, which are characteristic of the stoichiometry (x and y in LiM _1-xy Fe _x Me _y O ₂ ) of the layered oxides, shows that the mechanism (the redox behavior) of the Fe-containing cathodes does not differ from the reference materials LNO (LiNiO ₂ ) and the lithium nickel cobalt manganese oxides (NCMs) (NCM811 (LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ) was used as a reference here:). This was confirmed by synchrotron powder diffraction data of LNFO82 (LiNi _0.8 Fe _0.2 O ₂ ) during charging and discharging and near-edge X-ray absorption spectroscopy (NEXAFS) spectra of LNFO82 at 0%, 25%, 50%, 75%, 80%, 85%, 90% and 100% charge state ( 11 ): The Crystallographic changes (c lattice parameter increased, a,b lattice parameter decreased) correspond to the changes in NCMs. The NEXAFS measurements suggest that analogous to NCMs, charge exchange occurs by oxidation/reduction of nickel, while electron density is shifted from/to oxygen to/from nickel.

How to 9a) The electrochemical measurements of the Fe-containing layered oxides showed that the performance of the materials was comparable or in some cases better than that of the conventional NCM811 or the LNO. NFT811 showed an 8% higher discharge capacity in the first cycle, NFM811 a 3% higher discharge capacity. The discharge curves ( 9b) ) or the peaks in the differential capacity ( 9c )) of NFT811 and NFM811 is 0.2-0.5 V higher compared to NCM811, which is why the end-of-discharge voltage was increased from 4.4 V (NCM811) to up to 4.6 V for NFT811 and NFM811 (LiNi _0.8 Fe _0.1 Mn _0.1 O ₂ ). This also applies to LNFO91 (LiNi _0.9 Fe _0.1 O ₂ ) and LNFO82 (LiNi _0.8 Fe _0.2 O ₂ ), 9 (right column). Since the energy density is equal to the product of capacity and voltage, this means that for comparable capacity, the Fe-containing cathodes have a higher energy density.

While the cycle stability of the cathodes with 80% Ni ( 9a) , right column), despite the higher voltage window of the Fe-containing cathodes, was comparable to that of NCM811 ( 9a) , left column), LNFO82 (LiNi _0.8 Fe _0.2 O ₂ ) showed a lower cycle stability at higher voltages (LNFO82: 58% capacity loss, LNO: 39% capacity loss after 50 cycles). The cycle stability of LNFO91 (LiNi _0.9 Fe _0.1 O ₂ ) is comparable to that of LNO. LNFO982 (LiNi _0.98 Fe _0.02 O ₂ ) cycled much more stable than LNO with only 10% capacity loss after 50 cycles.

The underlying reactions of the layered oxides were investigated using near-edge X-ray absorption spectroscopy (NEXAFS). 11 shows NEXAFS spectra of LiNi _0.8 Fe _0.2 O ₂ (LNFO82) at 100%, 90%, 85%, 75%, 40%, 25% and 0% state of charge (OK NEXAFS spectra → left, Ni L NEXAFS spectra → right). As stated above, the oxidation/reduction of Ni takes place between 0% and 75% state of charge.

The NEXAFS measurements ( 11 , below) showed that the OO dimer formation (peak at > 4.1 V) was more pronounced in the Fe-containing cathodes compared to conventional layered oxides, comparable to the known OO dimer formation in Li-rich layered oxides. At the same time, a more pronounced peak at > 4.1 V was observed in the differential capacity ( 11c ), right side). This is considered to be the reason for the lower cycling stability of the nickel-iron oxides LNFO91 (LiNi _0.9 Fe _0.1 O ₂ ) and LNFO82 (LiNi _0.8 Fe _0.2 O ₂ ) compared to NFM811, NFT811 and NCM811, since charge exchange processes can be partially irreversible at these voltages.

Without being committed to a theory, the reason for the more pronounced redox processes at high voltages is assumed to be the discrepancy between the formal charge and the real charge of the transition metals and oxygen, which leads to the formation of π-bands and ultimately to O-O redox activity. This assumption is confirmed by the fact that the introduction of tetravalent cations such as Mn and Ti improved the cycling stability compared to LNFO91 and LNFO82.

Overall, iron-containing layered oxides of the formula LiM _1-xy Fe _x Me _y O ₂ with a high degree of purity were produced by calcination. These are therefore suitable as cathode material for lithium-ion batteries.

The invention underlying this patent application was developed in a project funded by the BMBF under the funding code 03XP0231.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• US 11233239 B2 [0004]
• US 2012148920 A1 [0005]
• CN103797621A [0006]
• JP H0950810 A [0007]
• JP-H09213330 [0007]
• JP H09167636 A [0007]

Cited non-patent literature

• J.R. Mueller-Neuhaus et al. (Journal of The Electrochemical Society, 147 (10) 3598-3605 (2000) [0003]
• Bixian Ying et al. (2023) (https://doi.org/10. 102 1/acs.chemmater.2c02639 [0005]

Claims (10)

Process for producing iron-containing layered oxides of the formula LiM _1-xy Fe _x Me _y O ₂ where 0.01 ≤ x ≤ 0.99 and 0 < y ≤ 0.99, M is a transition metal selected from the group comprising Ni, Mn, Ti and Co and Me is a metal selected from the group comprising Mn, Ni, Ti, Cu, Co, Sn and Zn, comprising the following steps: a) mixing an iron salt and a salt of the transition metal and optionally a further metal salt in a solvent; b) co-precipitating an iron-containing precursor as a hydroxide, separating and drying the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ ; c) Calcining the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ from step b) with LiOH or a lithium salt, wherein the reaction vessel for the calcination is heated to a temperature above the melting point of LiOH or the lithium salt and LiOH or the lithium salt and the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ are introduced into the heated reaction vessel, or wherein LiOH or the lithium salt is initially introduced into the reaction vessel and the iron-containing precursor M _1-xy Fe _x Me _y (OH) ₂ is introduced into the reaction vessel at a temperature above the melting point of LiOH or the lithium salt. Procedure according to Claim 1 , characterized in that the calcination is carried out using LiOH at a temperature in the range of ≥ 500 °C to ≤ 1000 °C, preferably in the range of ≥ 700 °C to ≤ 900 °C. Procedure according to Claim 1 or 2 , characterized in that the calcination is carried out in two steps, wherein, before the calcination, a precalcination is carried out at a temperature which is lower than the calcination temperature. Process according to one of the preceding claims, characterized in that the precalcination is carried out using LiOH at a temperature in the range of ≥ 460 °C to ≤ 500 °C. Process according to one of the preceding claims, characterized in that the lithium salt is selected from the group comprising lithium acetate, lithium carbonate, lithium nitrate and lithium sulfate. Process according to one of the preceding claims, characterized in that the precalcination is carried out in a period in the range from ≥ 30 min to ≤ 8 h, preferably in the range from ≥ 2 h to ≤ 5 h, and/or the calcination is carried out in a period in the range from ≥ 5 h to ≤ 20 h, preferably in the range from ≥ 10 h to ≤ 20 h. Process according to one of the preceding claims, characterized in that the heating rate between the precalcination and the calcination is 4°C/min or slower. Process according to one of the preceding claims, characterized in that an iron-containing layered oxide of the formula LiNi _1-x Fe _x O ₂ where 0.02 ≤ x ≤ 0.99, preferably 0.02 ≤ x ≤ 0.9, more preferably 0.02 ≤ x ≤ 0.6, more preferably x = 0.02, 0.1, 0.2, 0.4, 0.5 or 0.6 is produced with R-3m structure. Process according to one of the preceding claims, characterized in that an iron-containing layered oxide of the formula LiNi _1-xy Fe _x Mn _y O ₂ where 0.02 ≤ x ≤ 0.6 and 0.02 ≤ y ≤ 0.6, preferably 0.02 ≤ x ≤ 0.4 and 0.02 ≤ y ≤ 0.4, preferably 0.02 ≤ x ≤ 0.2 and 0.02 ≤ y ≤ 0.2, with R-3m structure is produced. Process according to one of the preceding claims, characterized in that an iron-containing layered oxide of the formula LiNi _1-xy Fe _x Ti _y O ₂ where 0.02 ≤ x ≤ 0.6 and 0.02 ≤ y ≤ 0.6, preferably 0.02 ≤ x ≤ 0.4 and 0.02 ≤ y ≤ 0.4, preferably 0.02 ≤ x ≤ 0.2 and 0.02 ≤ y ≤ 0.2, with R-3m structure is produced.

Images