Precursor for cathode material for rechargeable lithium ion batteries
阅读说明：本技术 用于可再充电锂离子电池的阴极材料的前体 (Precursor for cathode material for rechargeable lithium ion batteries ) 是由 金大铉 延斯·鲍森 吴振杜 马克西姆·布朗吉诺 于 2018-02-07 设计创作，主要内容包括：本发明涉及一种用于可再充电锂离子电池的阴极材料的前体。具体地,本发明涉及一种用于制造钴基氢氧化碳酸盐化合物的方法,该钴基氢氧化碳酸盐化合物具有孔雀石-斜方绿铜锌矿矿物结构,该方法包括以下步骤：提供包含Co源的第一水性溶液,提供包含Na<Sub>2</Sub>CO<Sub>3</Sub>的第二水性溶液,在高于70℃的温度下在沉淀反应器中混合两种溶液,从而沉淀出钴基氢氧化碳酸盐化合物,同时从所述反应器中抽空由沉淀反应形成的任何CO<Sub>2</Sub>,以及回收所述钴基氢氧化碳酸盐化合物。钴基氢氧化碳酸盐化合物用作锂钴基氧化物的前体,所述锂钴基氧化物可用作锂离子电池中的活性正极材料。(The present invention relates to a precursor for a cathode material for a rechargeable lithium ion battery. In particular, the present invention relates to a method for manufacturing a cobalt-based carbonate hydroxide compound having a malachite-orthorhombic shapeA aurichalcite mineral structure, the method comprising the steps of: providing a first aqueous solution comprising a Co source, providing a second aqueous solution comprising Na 2 CO 3 At a temperature above 70 ℃, in a precipitation reactor, thereby precipitating the cobalt-based bicarbonate compound, while evacuating from the reactor any CO formed by the precipitation reaction 2 And recovering the cobalt-based carbonate hydroxide compound. The cobalt-based carbonate hydroxide compound is used as a precursor of a lithium cobalt-based oxide, which can be used as an active positive electrode material in a lithium ion battery.)
1. A process for the manufacture of a cobalt-based bicarbonate compound, the process comprising the steps of:
-providing a first aqueous solution comprising a Co source,
-providing a composition comprising Na2CO3Of the second aqueous solution of (a) to (b),
-mixing the two solutions in a precipitation reactor at a temperature above 70 ℃ to precipitate the cobalt-based bicarbonate compound, while evacuating any CO formed by the precipitation reaction from the reactor2Wherein the residence time of the compound in the reactor is between 1 hour and 4 hours, and
-recovering the cobalt-based carbonate hydroxide compound.
2. The method of claim 1, wherein the second aqueous solution consists of any one of:
-at least 2N Na2CO3A solution of (A), or
By between 0.5 and 3mol/L of Na2CO3And NaOH between 1mol/L and 6mol/L, wherein the Na2CO3The Na content in (b) is as high as or higher than twice the Na content in the NaOH.
3. The method of claim 1, wherein the first aqueous solution further comprises a source of any one or more of Ni, Mn, Al, Mg and Ti.
4. The method of claim 1, wherein the solution comprising a Co source comprises CoSO4And further comprises MgSO4、Al2(SO4)3、NiSO4And MnSO4Any one or more of Mg, Al, Ni and Mn with respect toA molar ratio between 0.2 mol% and 5 mol% of the Co content is present.
5. The method of claim 1, wherein during the step of mixing the two solutions, a solution of TiO is added2MgO and Al2O3Any one or more of the above.
6. The process of claim 1, wherein the step of recovering the cobalt-based bicarbonate compound comprises the substep of transferring the compound to a settling reactor coupled to the precipitation reactor and then recycling the settled compound from the settling reactor to the precipitation reactor.
7. A process for the manufacture of lithiated cobalt-based oxides, the process comprising the steps of any one of claims 1 to 6, and subsequently comprising the steps of:
-mixing the cobalt-based carbonate hydroxide compound with a Li source, and
-sintering the mixture in an oxygen-containing atmosphere at a temperature above 950 ℃.
8. The process of claim 7, wherein the precipitated cobalt-based bicarbonate hydroxide compound comprises between 0.1 wt.% and 0.3 wt.% Na as an impurity, and wherein:
-during the step of mixing the cobalt-based carbonate hydroxide compound with a Li source, or
-during said step of sintering said mixture,
adding a sulfate compound, whereby SO4Is equal to or higher than the molar content of Na, and then comprises the steps of washing the lithiated cobalt-based oxide with water and drying the lithiated cobalt-based oxide.
9. The method of claim 8, wherein the sulfate compound is Li2SO4、NaHSO4、CoSO4And Na2S2O8Any one of the above.
10. Cobalt-based hydroxide carbonate compound having a malachite-rosasite mineral structure, obtained by the process according to any one of the preceding claims.
11. The cobalt-based bicarbonate compound of claim 10, having the general formula [ Co1-aAa]2(OH)2CO3A is any one or more of Ni, Mn, Al, Ti, Zr and Mg, wherein a is less than or equal to 0.05.
12. The cobalt-based bicarbonate compound of claim 11, wherein the cobalt-based bicarbonate compound further comprises up to 0.3 wt.% Na as an impurity.
13. The cobalt-based bicarbonate compound of any one of claims 10-12, wherein the cobalt-based bicarbonate compound has a particle size distribution with a D50 between 15 μ ι η and 25 μ ι η and a span < 0.80.
14. The cobalt-based bicarbonate compound of any one of claims 10-13, wherein the compound has a spherical morphology and>1.8g/cm3the tap density of (1).
15. The cobalt-based bicarbonate compound according to any one of claims 11 to 14, wherein A is either or both of Al and Mg, wherein 0.002 ≦ a ≦ 0.020, and wherein either or both of Al and Mg are homogeneously doped in the compound.
16. A cathode active material powder manufactured from the cobalt-based bicarbonate compound according to any one of claims 10 to 15.
Technical Field
The present invention relates to a precursor for a cathode material for a rechargeable lithium ion battery. In particular, the present invention relates to a powdered cobalt-based compound useful as a precursor for a cathode material in a rechargeable lithium ion battery, and a method for preparing a cathode material for a rechargeable lithium ion battery using the same. More specifically, the precursor compound is a cobalt-based carbonate hydroxide based compound, which is prepared by a precipitation process using sodium carbonate. In one embodiment, the precursor compound is additionally doped with elements such as Al, Mg, Mn, Ni, etc., and preferably the compound has a spherical morphology, which provides the benefits of improved electrochemical performance and higher energy density.
Background
Doped withLithium cobalt oxide (LiCoO)2Hereinafter referred to as LCO) or undoped lithium cobalt oxide has been used as a cathode material in rechargeable batteries for most commercial portable electronic applications such as mobile phones, tablet computers, laptop computers and digital cameras due to its high energy density and good cycle life LCO has hexagonal α -NaFeO2Type structure (space group of R-3 m) in which lithium ion layer is located at CoO6Between the octahedral blocks. As the demand for smaller and lighter cells with high energy density and good electrochemical performance increases, many research and development groups are working on developing or improving cathode materials, especially LCO.
There are several approaches for increasing the volumetric energy density of lithium ion batteries, such as applying thinner current collectors and separators, and using cathode and anode materials with higher packing densities. The bulk density of the cathode material depends primarily on two components:
first, the particle size distribution (hereinafter referred to as PSD) of LCO is directly related to the bulk density, as it determines how close the particles can be packed in a limited volume. In general, higher values of D50 (particle size of the median particles in the normal distribution) tend to enable higher bulk densities. Furthermore, the value of D100 (or D99) should be as low as possible, since large particles result in poor quality electrode coatings and damage to the current collector. The span as the value of (D90-D10)/D50) is a useful criterion for identifying how similar the particle size is, and defines the relative particle size of large particles compared to D50. The smaller the span, the less large particle problems are expected, even though D50 is large to achieve large density.
Secondly, the porosity in the individual particles should be as low as possible to obtain a maximum density of the individual particles.
Using lithium (Li) precursors (typically Li)2CO3) And a cobalt (Co) precursor (usually Co)3O4) And (4) synthesizing LCO. There are two possibilities to obtain the target D50 value for LCO. One is to adjust the synthesis conditions such as sintering temperature, sintering time, and Li to Co ratio. For sufficiently high Li to Co ratios and sintering temperatures, inter-particle sintering occurs and D50 is significantAnd (4) increasing. This allows the use of non-shaped cobalt precursors as their size is determined by the sintering conditions. The disadvantage of this method is that high sintering temperatures are required, which increases the process cost and/or the obtained LCO has a high Li to Co stoichiometry, which is detrimental to the electrochemical performance. This dilemma is discussed in detail in WO 2009-003573.
Another approach is to use a preformed cobalt precursor, such as spherical Co3O4As disclosed in US 2015/0221945. The term "shaped" herein refers to a precursor that already resembles the desired shape of the final LCO. This relaxes the sintering requirements. This method is preferred because when using a non-shaped cobalt precursor, the particle size of the LCO is expected to be irregular-with a wider span. In addition, high temperatures or high Li to Co ratios are necessary to shape the LCO from the non-shaped cobalt precursor, which requires high energy consumption or additional heat treatment steps to obtain stoichiometric LCO. The method to obtain the preferred morphology of LCO is to start the synthesis from a shaped cobalt precursor with high D50 and narrow span. Having a high density and low porosity is beneficial because it further increases the bulk density of the final LCO and reduces sintering effort. The shaped cobalt precursor should also have sufficient mechanical hardness so as not to crack during processing, such as mixing with a lithium precursor.
Cathode materials are one of the most critical components that determine the electrochemical properties of lithium ion batteries. One way to increase the energy density of a lithium ion battery is to increase its operating voltage by applying a higher charging voltage. However, as the state of charge is increased by increasing the charging voltage, less lithium ions remain in the crystal structure, resulting in thermodynamically unstable CoO2. Thus, due to the reaction of the delithiated cathode material with the electrolyte, cobalt may slowly dissolve in the electrolyte at high voltage, which is referred to as cobalt dissolution, resulting in battery failure. Great efforts have been made to reduce the dissolution of cobalt by doping LCO. Preferably, the dopant is already present and well distributed in the cobalt precursor prior to sintering, such as in CN102891312A, CN105731551A and CN 102583585B. This is because it is difficult to achieve a solid state process by which a shaped cobalt precursor is blended with a dopantGood doping is obtained in large particles. Complete diffusion of the dopant into the shaped particles of the LCO requires long sintering times or very high sintering temperatures. If the particle size is large, e.g.>10 μm, which is particularly suitable for use in a synthesis process where the dopant is added during blending of the lithium precursor and cobalt precursor.
Cobalt precursors for LCO can be prepared by a precipitation method. For example, will have a certain concentration of CoSO4The solution of (a) and the solution with a concentration of NaOH are mixed in a reactor at a controlled pH, with the impeller rotating at a certain RPM. Thus, solid cobalt hydroxide (Co (OH) will precipitate2) It may be a cobalt source for LCO. However, when Co (OH) is used2There are disadvantages in that it is difficult to obtain large D50 hydroxide due to certain particle growth limitations.
And Co (OH)2CoCO, a precipitation method3Precipitation allows to more easily obtain large spherical and dense cobalt precursors. For precipitation, the cobalt salt may be selected from CoSO4、CoCl2、Co(NO3)2Or other water soluble cobalt salt, and the alkali can be selected from Na2CO3、K2CO3、NaHCO3、KHCO3、NH4HCO3Or other soluble carbonates or bicarbonates. Na (Na)2CO3、NaHCO3And NH4HCO3Are the three most widely used for CoCO3The precipitating agent of (1).
Today, CoCO3Typically by co-precipitation of a bicarbonate solution with a cobalt salt solution. If CoSO is selected4As a cobalt salt, a typical reaction equation is:
CoSO4+2AHCO3→CoCO3+A2SO4+H2CO3wherein A ═ H or NH4(EQ 1)
For example, if CoCl is used2As a cobalt salt, the reaction equation will accordingly be:
CoCl2+2AHCO3→CoCO3+2ACl+H2CO3wherein A ═ Na, K or NH4(EQ 2)
In addition, when H2CO3Dissociation into H2O+CO2When, or for a ═ NH4By NH3There will be side reactions that precipitate. The bicarbonate process according to EQ1 and EQ2 described above has the disadvantage of using only 50% of the available-CO in the product3. 50% of-CO3Remaining in solution or as CO2And (4) evaporating.
In the typical NH4HCO3In the process, when CoSO is used4The basic reaction is:
CoSO4+2NH4HCO3→CoCO3+(NH4)2SO4+CO2+H2O(EQ 3)
in the course of precipitation, CO2Continuously released from the reactor and produced (NH)4)2SO4As a by-product. 1kg of CoCO can be calculated3The product required 1.329kg of NH4HCO3. Sulfur (S) and nitrogen (N) are CoCO obtained in the process3The main impurities in the product. Due to (NH)4)2SO4Cannot be released to the environment and therefore requires treatment of the wastewater to remove ammonia-preferably recycle ammonia. These ammonia recycling facilities are expensive and add significantly to the capital investment, as well as the operating costs for waste treatment, particularly due to higher energy requirements.
In the presence of NaHCO3When CoSO is used4When, the reaction is:
CoSO4+2NaHCO3→CoCO3+Na2SO4+CO2+H2O(EQ 4)
1kg of CoCO can be calculated3The product required 1.413kg of NaHCO3. And NH4HCO3Compared with the process, an ammonia recovery system is not required to be installed. However, NaHCO3The throughput of the process is an issue. And NH4HCO3High solubility (216 g/L at 20 ℃ C.) comparison of NaHCO3The solubility of (B) was relatively low (96 g/L at 20 ℃). Thus, lower concentrations of NaHCO are required for precipitation3Higher flow rates of (C), which results in CoCO3Relatively low throughput of production. On the other hand, due to NaHCO3The dissolution of (a) is very slow and therefore a separate dissolution/storage facility is required for large scale production.
Another obvious method is to use carbonate for precipitation. Typical reaction equation (if MSO is used)4As salts) are:
MSO4+A2CO3→MCO3+A2SO4wherein A ═ Na, K or NH4(EQ 5)
However, WO2016-055911 teaches that if M is Ni-Mn-Co-and a ═ Na or K, MCO with high content of alkali metal impurities precipitates3。
It is an object of the present invention to provide an improved cobalt-based precursor compound for a cathode material and a manufacturing method that achieves a low impurity content in the final cathode material, which improves the electrochemical stability and increases the energy density of the cathode material at cheaper process costs.
Disclosure of Invention
Viewed from a first aspect, the present invention may provide the use of a cobalt-based hydroxide carbonate compound having a malachite-rosasite mineral structure as a precursor for a lithium cobalt-based oxide useful as an active positive electrode material in a lithium ion battery. The compound may have the general formula [ Co1-aAa]2(OH)2CO3A is any one or more of Ni, Mn, Al, Zr, Ti and Mg, wherein a is less than or equal to 0.05. In one embodiment, A is Al or Mg, wherein 0.002. ltoreq. a.ltoreq.0.020, and Al or Mg is uniformly doped in the compound. The compound may have an XRD pattern wherein the peak ratio P has a value<1, wherein P-P1/P2, P1 is the maximum peak intensity at 32 to 33 degrees, and P2 is the maximum peak intensity at 34 to 35 degrees. P may also be<0.8 or even is<0.2 to yield excellent compounds.
The same value for P can also be reached if the compound is part of a mixture that also comprises cobalt carbonate. The cobalt carbonate may have a rhombohedral structure. The compounds of the foregoing embodiments may also beContains Na as an impurity in an amount of at most 0.3% by weight. In addition, the compounds may have a D50 of between 15 μm and 25 μm or between 20 μm and 25 μm and a span<A particle size distribution of 0.80. Another PSD-related characteristic may be D99/D50<2. In addition, the cobalt-based bicarbonate compound can have a spherical morphology and>1.8g/cm3the tap density of (1).
Viewed from a second aspect the invention may provide a process for the manufacture of a cobalt-based carbonate hydroxide compound according to the first aspect of the invention, the process comprising the steps of:
-providing a first aqueous solution comprising a Co source,
-providing a composition comprising Na2CO3Of the second aqueous solution of (a) to (b),
-mixing the two solutions in a precipitation reactor at a temperature above 70 ℃ to precipitate the cobalt-based bicarbonate compound, while evacuating any CO formed by the precipitation reaction from the reactor2Wherein the residence time of the compound in the reactor is between 1 and 4 hours, and
-recovering the cobalt-based carbonate hydroxide compound. In one embodiment, the step of mixing the two solutions may be performed in an open precipitation reactor to promote CO2While in another embodiment, the open reactor may be exposed to air.
The residence time may be further limited to between 1 and 3 hours, and the temperature may be limited to between 80 and 95 ℃: the maximum reaction temperature may be limited to 95 deg.c in view of water evaporation. In a method embodiment, the second aqueous solution may consist of any one of:
-at least 2N Na2CO3A solution of (A), or
By between 0.5 and 3mol/L of Na2CO3And NaOH between 1mol/L and 6mol/L, wherein Na2CO3The Na content in (b) is as high as or higher than twice the Na content in NaOH. In one embodiment, the solution is composed of between 1.5mol/L and 2.5mol/L of Na2CO3And NaOH between 3mol/L and 5mol/L, wherein Na2CO3The Na content in (b) is as high as or higher than twice the Na content in NaOH. In one embodiment, the solution is prepared from>50 vol% of 2mol/L Na2CO3Solutions and<50 vol% of 4mol/L NaOH solution. It is also possible that the first aqueous solution further comprises a source of any one or more of Ni, Mn, Al, Mg and Ti. In another embodiment, the Co source-containing solution comprises CoSO4And further comprises MgSO4、Al2(SO4)3、NiSO4And MnSO4Any one or more of Mg, Al, Ni and Mn, wherein any one or more of Mg, Al, Ni and Mn is present in a molar ratio of between 0.2 mol% and 5 mol% relative to the Co content. Another way of adding the dopant A may be to add the dopant A from TiO during the step of mixing the two solutions2MgO and Al2O3Any one or more of the above. In a particular process embodiment, the step of recovering the cobalt-based bicarbonate compound comprises the substeps of transferring the compound to a settling reactor coupled to the precipitation reactor and then recycling the settled compound from the settling reactor to the precipitation reactor.
The invention may also provide a method of making a lithiated cobalt-based oxide, including the steps of any of the preceding method embodiments, and subsequently including the steps of:
-mixing a cobalt-based carbonate hydroxide compound with a Li source, and
-sintering the mixture in an oxygen-containing atmosphere at a temperature above 950 ℃. In the process, the precipitated cobalt-based bicarbonate hydroxide compound may comprise between 0.1 wt% and 0.3 wt% Na as an impurity, and wherein:
during the step of mixing the cobalt-based carbonate hydroxide compound with the Li source, or
-during the step of sintering the mixture,
adding a sulfate compound, whereby SO4Is equal to or higher than the molar content of Na, and subsequently comprisesA step of washing the lithiated cobalt-based oxide with water and drying the lithiated cobalt-based oxide. Here, the sulfate compound may be Li2SO4、NaHSO4、CoSO4And Na2S2O8Any one of the above.
Drawings
FIG. 1 shows typical results of the float test.
FIG. 2 is based on Na2CO3Schematic representation of the coprecipitation apparatus of (1).
Figure 3 is an XRD pattern of cobalt hydroxide carbonate based cobalt precursor.
FIG. 4 shows Co2(OH)2CO3The ratio of phases versus the amount of sodium impurities.
FIGS. 5a and 5b are EDS plots of EX 3-P-3.
Detailed Description
A cobalt compound is disclosed which can be used as a cobalt precursor for a cathode material of a rechargeable lithium ion battery. More specifically, the precursor is a cobalt hydroxy (or hydroxide) carbonate based compound. In the drawings and the following detailed description, preferred embodiments are described in detail to practice the invention. While the invention has been described with reference to these specific embodiments, it will be understood that the invention is not limited to these preferred embodiments. On the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the detailed description and accompanying drawings.
The present invention provides:
1) cobalt precursor compounds for cathode materials in rechargeable lithium ion batteries,
2) a process for producing such a cobalt compound, and
3) a method of preparing cathode materials with low impurity levels, which facilitates improved electrochemical performance.
In the field of precursors for cathodes for rechargeable lithium batteries, precipitation is widely used. Ammonia is generally added as a chelating agent (e.g.to prepare M (OH)2) Or as precursor salts (e.g. in CoCO)3In the case of (2) is hydrogen carbonateAmmonium) is used. Ammoniated solutions are unstable, especially at higher temperatures and higher pH, they decompose rapidly and produce NH3A gas. Therefore, standard practice is (1) to use a closed reactor and (2) to avoid high temperatures to avoid ammonia contamination of the air in the plant.
Assuming these process conditions, CoCO3The precursor may also pass through Na2CO3The process precipitates, and when CoSO is used4The theoretical reaction is:
CoSO4+Na2CO3→CoCO3↓+Na2SO4(EQ 6)
1kg of CoCO can be calculated3The product only needs 0.892kg of Na2CO3This is compared to the use of NaHCO3And NH4HCO3Much less often. Due to Na2CO3Binahco3And NH4HCO3Is inexpensive, and therefore from the viewpoint of cost, Na2CO3The precipitation process is more attractive. Na (Na)2CO3The main problem of the process is usually the MCO finally obtained3High sodium impurity in the product. In the typical Na2CO3In the process, sodium is in the CoCO finally obtained3Up to thousands of ppm, sometimes even 10000ppm in the precursor. High impurity levels will result in poor electrochemical performance of the obtained LCO, especially low reversible capacity. New strategies need to be adopted to address CoCO3Sodium impurities in the precursor product. A particular problem, apart from the usual impurities, is that in standard precipitation, no high quality precipitate can be obtained. In general, CoSO4+Na2CO3The precipitated precipitate has a poor morphology and a very low density.
The present invention discloses that cobalt-based carbonate hydroxide compounds can be prepared by using Na under stirring at high temperature2CO3Precipitation process and ensuring any CO formed2Is prepared by evacuating from the reactor mixture. The invention combines the following aspects:
1) based on cost-effective Na2CO3Precipitation process for producing compounds having narrow spanSpherical dense cobalt compound: the median particle size (D50) of the precipitated cobalt hydroxide carbonate can easily be above 20 μm, spanning less than 0.8. The cathode material (LCO) may also have a higher density and narrower span due to the characteristics of spherical dense cobalt compounds with narrow span.
2) The dopant (Ni, Mn, Nb, Al, Mg, Ti, Zr, etc.) can be uniformly distributed in the crystal structure of the cobalt precursor compound having an atomic scale distribution because the dopant is added during the precipitation process. Surprisingly, 3-valent aluminum can be doped into the structure of the cobalt compound. In addition, Mg may also be doped, contrary to other processes. In addition to atomic scale distribution, nanoparticle doping may also be applied (e.g. for TiO)2). Since aluminum suppresses structural changes at high voltages, the effect of cobalt dissolution on the electrochemical properties of the final lithiated cobalt-based oxide can be mitigated. Doping the cobalt precursor with manganese may stabilize the crystal structure of the LCO, resulting in improved cyclability and power performance of the LCO. Nickel doping can increase the capacity of LCO.
3) Sodium impurities are generally suppressed in cobalt hydroxide carbonate compounds. However, under certain conditions, the Na impurities remain after precipitation and can be removed by an intermediate washing step followed by a drying step after addition of the sulfur or chlorine compound.
Surprisingly, under these conditions, a high quality precipitate can be achieved, which is associated with the surprising finding that the high quality precipitate is not a cobalt-based carbonate but a cobalt hydroxide carbonate. To get from Na2CO3Cobalt hydroxide carbonate is precipitated from the alkali feed (as will be described in detail later), and CO is continuously evaporated from solution2. If the temperature during precipitation is sufficiently high and when appropriate CO is foreseen2Evacuation, such as by using an open reactor, then CO2Only evaporates. If CO is present2Insufficient evaporation rate precipitates cobalt carbonate instead of the desired cobalt hydroxide carbonate and a high quality precursor cannot be obtained.
Typically, for the precipitation reaction, Na will be included under normal to vigorous stirring2CO3Of a stream comprising CoSO4Or CoCl2And another dopant source stream are fed into the reactor. Typically, agitation is achieved by rotating impellers or circulating streams. The precipitation reaction may be a batch process or a continuous process, with the overflow being recycled back to the reactor.
Under normal stirring, the precipitation process is mainly controlled by the following parameters:
temperature of
Residence time
-pH
Metal concentration
-CO3Molar ratio of/Co (or base/acid)
After precipitation, the obtained cobalt-containing precursor is separated from the liquid by a suitable separation technique, such as filtration, and then washed with deionized water. Washing with deionized water can remove a portion of the sodium impurities from the resulting cobalt-containing precursor, but significant amounts of impurities remain even after washing with large amounts of deionized water. The obtained cobalt-containing precursor, which still contains some water, is dried in a drying oven at elevated temperature.
It was observed that the precipitated material comprised a material which could be represented by the general formula Co according to the precipitation process parameters described above2(OH)2CO3Cobalt hydroxide carbonate represented according to the reaction:
2CoSO4+2Na2CO3+H2O→Co2(OH)2CO3↓+2Na2SO4+CO2↑(EQ7)
in addition to this cobalt hydroxide carbonate, the precipitated material may also contain CoCO according to the aforementioned theoretical reaction (EQ 6)3. The nucleation and growth of precipitated particles is related to the kinetics of the precipitation reaction. Since large spherical particles with narrow span are available in the process, this means that during precipitation existing particles grow and no or only a small amount of new particles are produced. Therefore, the main method is to mix Co2(OH)2CO3Precipitating to existing Co2(OH)2CO3On the particles, the desired particle growth occurs. The inventors speculate that the precipitation reaction is carried out in two steps. CoCO3Can be used as intermediateThe stable compound precipitates, which then reacts with the liquid and produces a carbonate hydroxide compound by an ion exchange process according to the following reaction scheme:
2CoCO3+2H2O→Co2(OH)2CO3+H2CO3(EQ 8)
H2CO3→H2O+CO2↑(EQ 9)
the inventors hypothesized that the rate-limiting step was an ion exchange reaction (EQ 8). Thus if the temperature is too low or CO2Without being evacuated effectively, the ion exchange kinetics are negatively affected and residual CoCO remains3And (5) reserving. If residual CoCO3Above 50% by weight, the sodium impurity level increases severely and the preferred morphology is no longer obtained. High quality precursors can only be obtained when the cobalt hydroxide carbonate phase is predominant. The reaction in (EQ 7) is promoted by high temperatures during precipitation, preferably in a well-stirred open reactor, so that the CO produced can be easily removed from the system2. It has therefore been found that if the base is Na2CO3The precipitation needs to be carried out at a temperature of at least 70 ℃, preferably at about 90 ℃, preferably in an open reactor, to allow CO2And (4) discharging.
During precipitation, the alkaline stream contains Na2CO3And (3) solution. Alternatively, the base may be Na2CO3And NaOH, wherein Na2CO3Up to 50% of the Na present in (a) may be replaced by NaOH. After the reaction, 50% molar solution (Na)2CO3+2NaOH) does not require CO2Followed by the following reaction:
2CoSO4+Na2CO3+2NaOH→Co2(OH)2CO3+2Na2SO4(EQ 10)
as the NaOH content in the caustic stream increases, less CO needs to be evaporated2And the precipitation temperature can be lowered.
To achieve complete precipitation of cobalt, a ratio of base to acid of greater than 1 is suggested. If it is too low, not invertedThe corresponding cobalt remains in solution. For example, if 100% Na is used2CO3Then CO is generated3The molar ratio to Co should be at least 1. If 50/50% Na is used2CO3NaOH mixture, then (CO)3The ratio of +2OH) to Co should be at least 1. Na in alkali2CO3The content should not be less than 50% (CO)3>2 OH). In this case, too much NaOH is present, and some of the precipitate will be undesirable Co (OH)2。
The base concentration and acid concentration may be high enough to achieve low nucleation rates and high reactor throughput. If Na is used2CO3Then, the typical concentration is at least 2N, which corresponds to 1mol Na2CO3Preferably it is at least 3N, and most preferably at least 4N. The acid solution is generally at least 2N, corresponding to 1mol of CoSO4L, more preferably at least 3N, and most preferably at least 4N.
Residence time is the time required to fill the reactor: this time is the reactor volume divided by the sum of the feed flow rates. The residence time should be high enough to grow the particles into the desired shape. It should not be too high as this would result in low reactor throughput and more carbonate-type precursor formed than the desired hydroxycarbonate.
Uniformly distributed dopants can play an important role in the cathode material. For example, aluminum significantly suppresses changes in the crystal structure of the cathode material when charged to a high voltage, resulting in better stability at such high voltages. The invention discloses a method for preparing a sodium-based zinc oxide2CO3The dopant may be co-precipitated by using a sulfate solution such as aluminum sulfate and magnesium sulfate, or a suspension of nanoscale powder. The doping is preferably applied during the precipitation reaction. One embodiment applies nanoparticle doping. The nanoparticles may be added as a powder or dispersed in a separate feed stream to the reactor. Alternatively, the nanoparticles may be dispersed within the acid feed stream or the base feed stream. Suitable nanoparticles are embedded within the growing precipitate particles. Typical examples of nanoparticle doping are TiO2、Al2O3MgO, and the like. In generalAt least 500mol ppm (amount of metal dopant/transition metal) and not more than 2 mol% by doping with nanoparticles. In another embodiment, the doping is performed by adding a dopant solution to the reactor. The salts of Ni, Mn, Mg or Al may be added as separate feed streams or they may be part of the acid feed. Typical dopant salts are sulfates, nitrates, and the like. Typical doping amounts are at least 0.2 mol% (amount of metal dopant/transition metal) and not more than 5 mol%.
In general, the possibility of doping an aluminum solution into a cobalt carbonate or bicarbonate compound is surprising, since not only does
By Na-based under industrial precipitation process conditions2CO3The process of (a) yields a cobalt precursor compound containing high sodium impurity levels of 1000ppm to 3000 ppm. This sodium impurity cannot be removed from the precursor even if it is not washed excessively with water. Undesirable phases that are not electrochemically active may be produced due to the high sodium impurities in the cobalt precursor, which may be responsible for the poor electrochemical performance of the cathode material in the final lithium ion battery. Doped or undoped LCO, which is the cathode material in lithium ion batteries, can be synthesized by a lithiation process based on precursors of cobalt hydroxide carbonate. First, the precursor is mixed with a lithium source (such as lithium carbonate or lithium hydroxide) and certain additives, and then heated at an elevated temperature in an oxygen-containing atmosphere with a suitable heating profile. Finally, the sintered material is crushed and sieved. The charge capacity and discharge, which are one of the most important electrochemical characteristics in lithium ion batteries, are expectedCapacity decreases with increasing sodium impurity levels. The present invention discloses that sodium impurities can be effectively removed by adding specific additives before or during lithiation followed by heating and washing steps. These additives may be sulfates such as sodium bisulfate, lithium sulfate, cobalt sulfate, ammonium sulfate, and the like. It is important to add enough sulfate so that almost all of the sodium is removed as alkali metal sulfate. For example, by washing away Na2SO4，1mol NaHSO41mol of Na can be removed. Analogously, by adding 1mol of Li2SO4Can be used as LiNaSO41mol of Na was removed. When adding CoSO4Etc. can be used as Na2SO42mol of Na are removed.
Since sodium lithium sulfate is thermodynamically stable, if sulfate is added to a cobalt compound and then heat treated, sodium lithium sulfate is formed during lithiation at elevated temperatures. Thus, the sulfate may be added in the blending step of the cobalt precursor and the lithium source, and the sodium lithium sulfate may be removed by a washing step after firing. Although lithium sulfate and cobalt sulfate are preferred because lithium and cobalt are the main elements in the cathode material, other sulfates such as aluminum sulfate and magnesium sulfate or bisulfates, persulfates, etc. may also be a good choice because some additives produce a positive effect in the cathode material. Alternatively, if a sodium sulfate compound is formed, it is water soluble and can be easily removed by a simple washing step with water.
By applying the technique of the present invention: by being based on Na2CO3The process of (a) precipitates a cobalt hydroxide carbonate based precursor and removes sodium impurities before or during the lithiation step, high quality cathode materials with improved electrochemical stability and higher energy density can be obtained.
The invention is further illustrated in the following examples:
description of the analytical methods
Data on Particle Size Distribution (PSD) and span such as D50, D99 are preferably obtained by laser PSD measurement methods. In the present invention, after the powder is dispersed in an aqueous medium, the laser PSD is measured using a Malvern Mastersizer2000 with a Hydro 2000MU wet dispersion attachment. In order to improve the dispersion of the powder in the aqueous medium, sufficient ultrasonic irradiation and stirring are performed, and an appropriate surfactant is introduced. Note that since narrow span is an indicator of apparent sphericity of the particle, the span value is used in the examples to measure sphericity.
Specific surface area was measured using a Micromeritics Tristar 3000 using the Brunauer-Emmett-Teller (BET) method. Before the measurement, 3g of a powder sample was dried under vacuum at 300 ℃ for 1 hour in order to remove adsorbed substances before the measurement.
An Inductively Coupled Plasma (ICP) method is used to measure the contents of elements such as lithium, cobalt, sodium, aluminum, and magnesium by using agilent ICP 720-ES. A2 g sample of the powder was dissolved in 10mL of high purity hydrochloric acid in an Erlenmeyer flask. The flask was covered with glass and heated on a hot plate to completely dissolve the precursor. After cooling to room temperature, the solution was transferred to a 100mL volumetric flask and rinsed 3 to 4 times with distilled water (DI). After filling the flask with solution, the volumetric flask was filled with DI water until 100mL was marked and then fully homogenized. A 5mL pipette is used to withdraw 5mL of the solution and transfer it to a 50mL volumetric flask for a second dilution, which is filled with 10% hydrochloric acid until 50mL is marked and then homogenized. Finally, the 50mL solution was used for ICP measurement.
Tap Density (TD) measurements were performed by mechanically tapping a graduated cylinder (100ml) containing the sample (having a mass W of about 60g to 120 g). After observing the initial powder volume, the cylinder was mechanically tapped 400 times so that there was no further volume (in cm)3The V) or mass (W) change of the meter is observed. TD is calculated as TD ═ W/V. TD measurement at
XRD measurements were performed using a Rigaku X-ray diffractometer (D/MAX-2200/PC) using Cu K α, the scan speed was set to 1 degree per minute for consecutive scans, the step size was 0.02 degrees, the scans were performed between 15 and 85 degrees, quantitative phase analysis was performed using TOPAS software, for the purpose of quantitative phase analysisFor purposes of the present invention, peak intensity P1 is defined as being 34 to 35 degrees (corresponding to Co) without background removal2(OH)2CO3The (021) peak of the structure), and the peak intensity P2 is defined as the maximum intensity at 32 to 33 degrees (corresponding to the (104) peak of the CoCO3 structure) without background subtraction. The peak ratio P is the ratio of P1 to P2.
The cross-sectional analysis was performed by a focused ion beam instrument, which was a JEOL (IB-0920 CP). The instrument used argon as the light beam source. A small amount of powder was mixed with the resin and hardener and the mixture was then heated on a hot plate for 10 minutes. After heating, it was placed in an ion beam instrument and the settings were adjusted in a standard procedure, setting the voltage to 6kV for 3 hours. Scanning Electron Microscopy (SEM) was performed using a JEOL JSM 7100F scanning electron microscope. Electron microscope was equipped with 50mm from Oxford instrument2An X-MaxN EDS (energy dispersive X-ray spectrometer) sensor.
Button cells for float tests and for performing general electrochemical tests were assembled by:
step 1): preparing a positive electrode: slurry containing solids: an electrochemically active material, a conductor (Super P, Timcal) and a binder (KF #9305, Kureha) in a weight ratio of 90:5: 5; and the solvent (NMP, Sigma-Aldrich) was prepared in a high speed homogenizer. The homogenized slurry was coated on one side of an aluminum foil using a knife coater with a 230 μm gap. It was dried in an oven at 120 ℃, pressed using a calendering tool, and dried again in a vacuum oven to completely remove the solvent.
Step 2): assembling the button cell: the button cell was assembled in a glove box filled with an inert gas (argon). For general electrochemical testing, a separator (Celgard) is positioned between the positive electrode and a sheet of lithium foil that serves as the negative electrode. For the float test, two sheets of separator were positioned between the positive electrode and the negative electrode material consisting of graphite. 1M LiPF6 in EC/DMC (ratio 1:2) was used as the electrolyte and was spotted between the separator and the electrode. The button cells were then completely sealed to prevent electrolyte leakage.
The float test analyzes the stability of the cathode material when charged at high voltage at elevated temperature. The prepared coin cells were tested according to the following charging protocol: the coin cells were first charged to 4.5V in constant current mode and at a C/20 rate (1C ═ 160mAh/g) in a 60 ℃ chamber, and then kept at constant voltage (4.5V) for 5 days (120 hours), which is a very stringent condition. The maximum current was 1 mA. Fig. 1 shows the results of a typical float test. First, the cathode is charged in a CC (constant current) mode (data not shown). When the final voltage is reached, the battery is continuously charged in a Constant Voltage (CV) mode. The figure shows the recorded current, where t-0 is the time where CV mode charging begins. Once side reactions or metal dissolution occur, a voltage drop occurs. The electrochemical instrument will automatically compensate for the (lost) current to keep the voltage constant. The recorded current is therefore a measure of the ongoing side reactions. As shown in fig. 1, time (in hours) starts from the start of constant voltage charging, and recording voltage (V-right axis) and current (mA/g-left axis) are represented by a dotted line and a solid line, respectively. From the change in current, the deterioration of the tested coin cells at high voltage and high temperature was observed. Finally, the current of the coin cell reached the maximum current (1mA) and the voltage dropped due to a short circuit, at which point the "time to failure" (denoted FT in the figure) was recorded, which is a measure of the high voltage stability and cobalt solubility of the cathode material. After the float test, the button cells were disassembled. The anode and the separator near the anode are analyzed by ICP (inductively coupled plasma) for metal dissolution analysis, since the prior art describes that if metal dissolution occurs, the dissolved metal will be deposited on the surface of the anode in the form of a metal or metal alloy. The measured cobalt content is normalized by the time to failure and the total amount of active material in the electrode, so that a specific cobalt dissolution value can be obtained.
A typical electrochemical test for button cells consists of the following two parts: (see also Table 1). Part I is a rate performance evaluation at 0.1C, 0.2C, 0.5C, 1C, 2C, and 3C over a 4.3 to 3.0V/Li metal window. The first charge capacity and discharge capacity (CQ1 and DQ1) were measured by multiplying at 0.1C in constant current mode, where 1C is defined as 160 mAh/g. At each timeRelaxation times of 30 minutes for the first cycle and 10 minutes for all subsequent cycles were allowed between sub-charging and discharging. Irreversible capacity Qirr.Expressed as%:
the rate performance at 0.2C, 0.5C, 1C, 2C and 3C is expressed as the ratio of the retained discharge capacity DQn, where n is 2, 3, 4, 5 and 6 for nC ═ 0.2C, 0.5C, 1C, 2C and 3C, respectively, as follows:
for example,
part II is an evaluation of cycle life. The charge cut-off voltage was set to 4.6V/Li metal. The discharge capacity at 4.6V/Li metal was 31 measured at 0.1C with 7 cycles and 32 measured at 1C with 8 cycles. The capacity fade at 0.1C and 1C was calculated as follows and expressed as% per 100 cycles:
Table 1: test scheme for button cell analysis
The invention is further illustrated in the following examples: