KR20140099956A - Double-shell core lithium nickel manganese cobalt oxides - Google Patents

Abstract

A lithium transition metal oxide powder for use in a rechargeable battery is disclosed wherein the surface of the primary particles of the powder is coated with a first inner layer and a second outer layer and the second outer layer comprises a fluorine- The first inner layer consists of the reaction product of the primary particle surface and the fluorine-containing polymer. An example of the reaction product is LiF, and lithium is derived from the surface of the primary particles. Also, by way of example, the fluorine-containing polymer is PVDF, PVDF-HFP or PTFE. Examples of the lithium transition metal oxide include any one of the following:
- LiCo _d M _e 0 ₂ , where M is at least one of Mg and Ti, e <0.02, d + e = 1;
_{- Li 1 + a M '1} - a 0 2 ± b M 1 k S m ( where, -0.03 <a <0.06, b <0.02, M' is a transition metal compound, at least 95% by weight of this group is Ni , Mn, Co, Mg and Ti, M ¹ is at least one element selected from the group Ca, Sr, Y, La, Ce and Zr, 0? K? 0.1 and 0? M? 0.6, and m is mol%); And
_{- Li a 'Ni x Co y} M "z 0 2 ± e A f ( where, 0.9 <a'<1.1, 0.5≤x≤0.9, 0 <y≤0.4, 0 <z≤0.35, e <0.02, 0 Wherein M is at least one element selected from the group consisting of Al, Mg, and Ti; and A is at least one of S and C). &Lt; / RTI &gt; In order to prepare the coated powder, an illustrative method comprises the steps of preparing a non-coated lithium transition metal oxide powder, mixing the powder with a fluorine-containing polymer, and mixing the resultant powder-polymer mixture with the fluorine-containing polymer Heating at a temperature of not less than 50 DEG C but not more than 140 DEG C higher than the melting temperature.

Description

{DOUBLE-SHELL CORE LITHIUM NICKEL MANGANESE COBALT OXIDES}

The present invention relates to a cathode material for a rechargeable lithium battery, particularly to a lithium nickel manganese cobalt oxide coated with a fluorine-containing polymer and then heat-treated.

Previously, LiCoO ₂ was the most commonly used cathode material for rechargeable lithium batteries. However, in recent years, LiCoO ₂ has been replaced with a lithium nickel oxide-based cathode and lithium nickel manganese cobalt oxide. In the alternative material, depending on the selection of the metal composition, different limits may appear or need to be addressed. Briefly, the term "lithium nickel oxide based cathode" is hereinafter referred to as "LNO &quot;,and" lithium nickel manganese cobalt oxide "

One example of a material LNO has _{LiNi 0 .80 Co 0 .15 Al 0} .05 O 2. Although this has high capacity, it is generally difficult to manufacture because a free atmosphere (oxygen) is required and a special carbonate-free precursor, such as lithium hydroxide, is used instead of lithium carbonate. Therefore, this manufacturing limitation tends to significantly increase the cost of the material. LNO is a very sensitive cathode material. This is not sufficiently stable in air, making the production of large-scale batteries more difficult and, due to its low thermodynamic stability, causes poor safety records in actual cells. Finally, it is difficult to produce lithium nickel oxide having a low soluble base content.

"Soluble base" means lithium which is located close to the thermodynamically unstable surface and becomes a solution, while lithium in the bulk is thermodynamically stable but can not be dissolved. Hence, the gradient of Li stability exists between low stability at the surface and high stability at the bulk. The presence of "soluble bases" is disadvantageous because high base content is often associated with problems during the manufacture of batteries: high amounts of salt during slurry preparation and coating cause deterioration of the slurry (slurry instability, gelling) The amount of salt also causes poor high temperature properties such as excessive gas evolution (cell swelling) during exposure to high temperatures. By ion exchange reaction is measured ^{_{(LiMO 2 + δH + ← →}} Li 1 -δ H δ M0 2 + δLi +) "base-soluble" content by pH titration is based on, there are Li gradient can be set. The degree of the reaction is surface characteristics.

In US2009 / 0226810A1, the problem of soluble base was added to the soil: LiM0 ₂ cathode material is prepared by using a mixed transition metal hydroxide as the precursor. This is obtained by coprecipitation of transition metal sulfates and industrial grade bases such as NaOH, which is the least expensive industrial method for the preparation of LiMO ₂ precursors. The base ² C0 ₃ in the form of Na ₂ C0 ₃ ^- comprises an anion, which is a mixed hydroxide trapping (trapping) the hydroxide, the mixed-hydroxy lifting typically of 0.1-1% by weight C0 ₃ ^{2 -} including do. In addition to the transition metal precursor, a lithium precursor Li ₂ CO _3, or of at least 1% by weight of technical grade containing _{Li 2 CO 3 LiOH * H 2} 0 are used. Lithium and transition metal precursors react at high temperatures, generally above 700 ° C. In the case of cathode LNO with a high nickel content, Li ₂ CO ₃ impurities remain on the resulting lithium transition metal oxide powder, especially on its surface. If higher purity materials are used, less Li ₂ CO ₃ impurities are found, but there are always some LiOH impurities that react with CO _{2 in} air to form Li ₂ CO ₃ . Although the above solution is proposed in JP2003-142093, it is not desirable to use expensive precursors of very high purity.

Examples of LMNCO is Li, and _{1 + x M 1 -x 0 2} ( _{where, M = Mn 1/3 Ni} 1/3 Co 1/3 O 2) is well known, manganese and nickel content is substantially the same. The "LMNCO" cathode tends to be very stable, easy to manufacture, and generally has a low cost since it has a relatively low content of cobalt. The main disadvantage of these is that the reversible capacity is relatively low. Typically, between 4.3 V and 3.0 V, the capacity is less than about 160 mAh / g, compared to 185-195 mAh / g for the LNO cathode. A further disadvantage of LMNCO compared to LNO is the relatively low crystallographic density, which also has a small volumetric capacity; And relatively low electronic conductivity.

A nickel-rich lithium nickel manganese cobalt oxide Li _{1 + x} M _{1 -x} 0 ₂ (where M = Ni _{1 -x- y} Mn _x Co _y or M = Ni _{1 -xy -} the position _z Mn _x Co _y Al _z), and, Ni: Mn is more than 1, typically Ni: the values for Mn is 1.5 to 3, Co content (y) is generally from 0.1 to 0.3. Briefly, we call this class of substances "LNMO". An example is _{M = Ni 0 .5 Mn 0 .3} Co 0 .2, M = Ni 0 .67 Mn 0 .22 Co 0 .11 and _{M = Ni 0.6 Mn 0.2 Co 0.2} .

Compared to LNO, LNMO can be prepared by standard methods (using Li ₂ CO ₃ precursors) and no special gases (such as oxygen as mentioned above) are required. Compared to LMNCO, LNMO has a low propensity to react with electrolytes at high temperatures (which are typically characterized by dissolution of Mn) and a much higher intrinsic capacity. Therefore, it became clear that LNMO plays an important role in the substitution of LiCoO ₂ . In general, the base content increases and safety performance tends to deteriorate with increasing Ni: Mn ratio. On the other hand, high Mn content is widely recognized as contributing to safety.

High base content is associated with moisture sensitivity. In this regard, LNMO is less sensitive to moisture than LNO but more sensitive than LMNCO. Immediately after manufacture, well-prepared LNMO samples have a relatively low surface base content and, if well-formed, most surface bases are not Li ₂ CO ₃ type bases. However, in the presence of moisture, airborne CO ₂ or organic radicals react with LiOH-type bases to form Li ₂ CO ₃ type bases. Similarly, since the consumed LiOH is slowly reformed by Li from bulk, the total amount of base (total base = Li ₂ CO ₃ + moles of LiOH-type base) increases. At the same time, moisture (ppm H ₂ O) increases. The above process is not very good for battery production. Li ₂ CO ₃ and water are known to cause serious swelling and poor slurry stability. It is therefore desirable to lower the moisture sensitivity of the LNMO and LNO materials.

In US2009 / 0194747A1 a method of improving the environmental stability of LNO cathode materials has been described. The patent describes a polymer coating of a nickel-based cathode material in the form of a single layer of undissociated polymer. The polymer (e.g., PVDF) is selected from binders commonly used in the manufacture of lithium ion batteries (slurry preparation for electrode coating).

Thermal stability (safety) is related to the interfacial stability between the electrolyte and the cathode material. A common approach to improve surface stability is to coat. A number of different coating examples can be found in the literature, especially in the patent literature. There are different ways to classify coatings. For example, it is possible to differentiate between the ex-situ and in-situ coatings. The layer is coated on the particles in the x-drill coating. The coating can be obtained by dry or wet coating. Generally, the coating is applied to an individual process comprising at least a coating step and generally an additional heating step. Therefore, the overall process cost is high. Alternatively, in some cases, phosphorus-borne coatings or self-organizing coatings are possible. In this case, if the coating material is added to the mixture before cooking, the individual phase is formed during the cooking and preferably the coating phase is liquid, and the wetting between LiMO ₂ and the coating phase is strong, a thin, dense coating phase Finally, LiMO ₂ phase, which is electrochemically active, is coated. Obviously, the phosphorus-borne coating is only effective when the coating phase wets the core.

The difference between the cationic coating and the anionic coating can also be distinguished. An example of a cationic coating is an Al ₂ O ₃ coating. Examples of anionic coatings include fluorides, phosphates, silicate coatings, and the like. The fluoride coating is particularly preferred because a protective film of LiF is formed. Since LiF is thermodynamically very stable and does not react with the electrolyte, the LiF coating can be expected to obtain good stability at high temperatures and voltages. The usual method, Croguennec et al. in Journal of The Electrochemical Society , 156 (5) A349-A355 (2009) is to add LiF as a lithium transition metal oxide to obtain a protected LiF film. However, due to the high melting point of LiF and poor wetting properties, it is impossible to obtain a thin and densely packed LiF film. Croguennec reported that small particles or 'sheets' could be found at the grain boundaries of LiMO ₂ particles instead of coatings. An additional possible method is to use MgF ₂ , AlF ₃ or lithium cryolite.

In addition, the difference between the inorganic coating and the organic coating can be distinguished. An example of an organic coating is a polymer coating. One advantage of the polymer coating is that an elastic coating can be obtained. On the other hand, problems arise from poor electrical conductivity, and sometimes the movement of lithium across the polymer is poor. Generally, the polymer coating is somewhat attached to the surface, but does not chemically alter the surface.

Conventional experimental data showing that the approach described above is effective in improving the cited problems of LNO and LNMO materials is not found.

The summary is as follows:

(1) LMNCO is a robust material but has severe capacity limitations.

(2) it is required to reduce the base content of LNO and increase the thermal stability.

(3) It is required to reduce the base content of LNMO and increase the thermal stability.

It is an object of the present invention to remedy or overcome the above-mentioned problems and to provide a high-capacity alternative to LMNCO materials.

According to a first aspect, the present invention provides a lithium transition metal oxide powder having a surface of a primary particle of a powder coated with a first inner layer and a second outer layer and used in a rechargeable battery, The outer layer comprises a fluorine-containing polymer, wherein the first inner layer consists of the reaction product of the fluorine-containing polymer and the primary particle surface. In one embodiment, the reaction product is LiF, and the lithium is derived from the primary particle surface. In another embodiment, the fluorine in the reaction product LiF is derived from some degraded fluorine-containing polymer present in the outer layer.

In certain embodiments, the first inner layer consists of a LiF film having a thickness of at least 0.5 nm, or at least 0.8 nm, even at least 1 nm. In another particular embodiment, the fluorine-containing polymer is any one of PVDF, PVDF-HFP, or PTFE. The fluorine-containing polymer may be composed of agglomerated primary particles having an average particle size of 0.2-0.5 μm. The particle size is believed to be beneficial to the wetting properties of the molten fluorine-containing polymer.

An example of a lithium transition metal oxide may be one of the following:

- LiCo _d M _e 0 ₂ , where M is at least one of Mg and Ti, e <0.02, d + e = 1;

_{- Li 1 + a M '1} - a 0 2 ± b M 1 k S m ( where, -0.03 <a <0.06, b <0.02, M' is a transition metal compound, at least 95% by weight of this group is Ni, Mn, Co, Mg and Ti; M ¹ is at least one element selected from the group Ca, Sr, Y, La, Ce and Zr; 0? K? , and m is expressed as mol%); And

_{- Li a 'Ni x Co y} M "z 0 2 ± e A f ( where, 0.9 <a'<1.1, 0.5≤x≤0.9, 0 <y≤0.4, 0 <z≤0.35, e <0.02, 0 Wherein M is at least one element selected from the group consisting of Al, Mg, and Ti; and A is at least one of S and C). &Lt; / RTI &gt;

In one _{embodiment, M '= Ni a "Mn} b" Co c ", here, a"> 0, b "> 0, c"> 0 , and a "+ b" + c " = 1; and a" / b "> 1. In another embodiment, 0.5? a?? 0.7, 0.1 <c? <0.35, and a" + b "+ c" = 1. a "/ b"> 1 embodiment is particularly suitable for use in a lithium-ion prismatic battery or polymer cell.

The pristine polymer applied to the initial coating contains fluorine. In one embodiment, it comprises at least 50% by weight of fluorine. Typical examples of pristine polymers are PVDF homopolymers or PVDF copolymers (e.g., HYLAR® or SOLEF® PVDF, both from Solvay SA, Belgium). Other known PVDF-based copolymers include, for example, PVDF-HFP (hexa-fluoropropylene). These polymers are often known as "Kynar ". Teflon or PTFE can also be used as the polymer.

According to a second aspect, the present invention provides a method of coating a lithium transition metal oxide powder with a fluorine-containing double-layered coating,

Preparing a bare lithium transition metal oxide powder;

Mixing the powder with a fluorine-containing polymer; And

A double layer coating is formed on the surface of the metal oxide powder by heating the obtained powder-polymer mixture at a temperature higher than the melting temperature of the fluorine-containing polymer by 50 ° C or more and 140 ° C or less, An outer layer and an inner layer consisting of the reaction product of the powder surface and the polymer.

In one embodiment, the amount of fluorine-containing polymer in the powder-polymer mixture is 0.1-2 wt%, and in other embodiments 0.2-1 wt%. Further, it is preferable that the inner layer is made of LiF. The inner layer of the example has a thickness of at least 0.5 nm, or at least 0.8 nm, even at least 1 nm.

One example method uses a fluorine-containing polymer, such as PVDF, and the powder-polymer mixture is heated at a temperature of 220 ° C to 325 ° C for at least 1 hour. In certain embodiments, it is heated at a temperature of 240 ° C to 275 ° C for at least 1 hour.

An example of the lithium transition metal oxide used in the above process is one of the following:

- LiCo _d M _e 0 ₂ , where M is at least one of Mg and Ti, e <0.02, d + e = 1;

_{- Li 1 + a M '1} - a 0 2 ± b M 1 k S m ( where, -0.03 <a <0.06, b <0.02, M' is a transition metal compound, at least 95% by weight of this group is Ni, Mn, Co, Mg and Ti; M ¹ is at least one element selected from the group Ca, Sr, Y, La, Ce and Zr; 0? K? , and m is expressed as mol%); And

_{- Li a 'Ni x Co y} M "z 0 2 ± e A f ( where, 0.9 <a'<1.1, 0.5≤x≤0.9, 0 <y≤0.4, 0 <z≤0.35, e <0.02, 0 Wherein M is at least one element selected from the group consisting of Al, Mg, and Ti; and A is at least one of S and C). &Lt; / RTI &gt;

In embodiments of the _{examples, M '= Ni a "Mn} b" Co c " , where a"> 0, b "> 0, c"> 0 , and a "+ b" + c " = 1; and a" / b "> 1. In another embodiment, 0.5? a?? 0.7, 0.1 <c? <0.35, and a" + b "+ c" = 1.

The double-shell according to the invention has the following functions: the outer layer of the partially degraded polymer prevents moisture absorption, while the thin LiF based inner layer replaces the reactive surface base layer thereby reducing the base content and enhancing safety .

1 is the unit cell volume (lower), the base content (medium, μmol / g) and the moisture content (ppm) of the LNMO / 1% PVDF mixture versus heating temperature.
Figure 2a shows the rate of LNMO / 1% PVDF mixture (lower, 1C vs. 0.1C (%)) versus heating temperature; Irreversible capacity (medium, mAh / g) and reversible capacity (upper, mAh / g).
Figure 2b is the energy fade (%) (measured in% per 100 cycles) measured at 0.1 C rate (upper) or 1 C rate (lower) of the LNMO / 1% PVDF mixture versus heating temperature.
3 is an SEM of an LNMO / 1% PVDF mixture heated at 200 &lt; 0 &gt; C.
4 is an FESEM micrograph of an LNMO / 1% PVDF mixture heated at 250 &lt; 0 &gt; C.
5 is a SEM of an LNMO / 1% PVDF mixture heated at 350 &lt; 0 &gt; C.
6, the upper part is the moisture content after exposure of the LNMO / 0.3% PVDF mixture to water temperature, and the lower part is the water content before the water exposure (*) and after (Δ) of the LNMO / 0.3% It is the content.
Figures 7a and 7b are SEMs of an LCO / 1% PVDF mixture heated at 300/600 ° C.
8 is the discharge voltage profile: voltage (V) profile for the cathode capacity (mAh / g) of the LCO / 1% PVDF mixture heated at different temperatures.
Figure 9 is a SEM photograph of a Kynar® 2801 sample.
Figure 10 is a DSC measurement of a mixture of Kynar (R) 2801 and Kynar and LiOH 占₂ 2O, which represents the heat flow (W / g) against temperature.
11 is an X-ray diffraction pattern (arbitrary unit for scattering angle (°)) of a control standard PVDF (lower) treated at 250 ° C and an LNO / PVDF mixture (upper).
Figure 12 is an X-ray diffraction pattern of a control standard PVDF (bottom) and LNO / PVDF mixture (top) treated at 175 ° C (top curve) / 150 ° C (bottom curve).
13, the upper part is the fluorine (g / g sample) detected by chromatography on the heat treatment temperature of the LNMO / PVDF mixture and the lower part is the calculated fluorine (g / g sample) calculated from the chromatographic data mixture for the heat treatment temperature of LNMO / PVDF (G of fluorine to fluorine in PVDF).
14A-14C are XPS narrow scan spectra of the F 1s subregion showing deconvolution to two F contributions: Organic F at 687.5 eV and LiF at 684.7 eV F ^- within.

In simple terms, the structure of the cathode material in the first aspect of the present invention may be described, for example, as a double-shell-core design. The double-shell is not obtained by repeated coating but by phosphorus-drilling reaction between the surface of the material core and the initial coating. The reaction takes place at certain heating temperatures as described below. The outermost portion of the double-shell is a thin polymer layer. The polymer is partially degraded to contact the very thin inner layer, which is basically lithium fluoride, again coating the LNO or LNMO core. The LiF layer is derived from the reaction of the decomposing polymer with the lithium-containing surface base of LNO or LNMO. On the other hand, conventional fluoride-containing polymers such as Kynar® (see below) can be melted upon heating to initiate a chemical reaction by contacting with the Li base at the surface of the transition metal oxide to induce decomposition of the polymer. The decomposition eventually produces a gas that will evaporate, and at sufficiently high temperatures, the residual carbon can surprisingly react with the particles and decompose without reforming the Li ₂ CO ₃ type base. It can be assumed that the LiF film protects Li in the particles by preventing reaction of carbon forming Li ₂ CO ₃ . It is clear that this "complete" decomposition occurs only when sufficient heat is applied in contrast to some decomposition in the present invention. Depending on the amount of polymer coated on the transition metal oxide, the outer shell contains some pristine (unreacted) polymer in addition to the partially degraded polymer. In that sense, the term &quot; partly decomposed &quot; includes both:

A mixture of degradation polymer and pristine polymer, and

- a mixture of polymers that have a different composition from the original pristine polymer, but which are somewhat degraded and can still be considered polymers.

Indeed, the term 'double-shell' can cover an outer shell composed of some degraded polymer besides an internal LiF shell, and can also be covered by a layer of less degraded polymer or pristine polymer. The double-shell has the following function: the outer layer of the partially degraded polymer prevents moisture absorption, while the thin LiF based inner layer replaces the reactive surface base layer thereby reducing the base content and enhancing safety.

Examples of surface-coated lithium transition metal oxides do not correspond to the coating classes described in the Background of the Invention: In the Examples we have observed the presence of reaction products from the degradation polymers and the formation of double-shells. Therefore, this is not a polymer coating as described in US2009 / 0194747A1. This is not the case with anionic coatings because (a) some degraded polymers play an important role and (b) LiF is crystallized at high temperatures, so coating with LiF is produced at low temperatures. In conclusion, this is not an in-drilling coating or an x-drilling coating, and in fact is something in between.

A method of coating a lithium transition metal oxide comprises the steps of:

1) mixing the LNO or LNMO cathode with a small amount of pristine polymer.

2) heating the mixture to a temperature above the melting point of the polymer and continuing to heat the polymer until it reacts with the cathode powder.

3) cooling when the polymer is completely decomposed.

In the exemplary process, the mixing step may also consist of (1) wet coating or (2) dry coating. In the wet coating method, after the polymer is dissolved in the solvent, the powder is dipped in the solution, and the slurry (or the wet powder) is dried. In the dry coating process, the polymer powder is mixed with the powder, and after heating to a temperature above the melting point of the polymer, the molten polymer wetts the surface. In one embodiment of the dry coating, a polymer having a small primary particle size, for example, a size much smaller than 1 micron, is used to obtain a good surface coating.

In this example method, the LNO / LNMO cathode material is encapsulated in a very thin film. If the film is thick, it is difficult for lithium to permeate through the film, resulting in a loss of electrochemical performance (low capacity and low rate performance). If the LNO / LNMO cathode has a high porosity, it is difficult to encapsulate without filling the pores, so more polymer is required to coat the surface with LiF.

In an exemplary embodiment, the amount of polymer is from 0.1 wt% to 2 wt%. If the polymer loading is less than 0.1%, it is difficult to obtain a good film. If it exceeds 2%, the capacity of the powder may be lowered. An amount of polymer of 0.2-0.5 wt% is used in another embodiment of the embodiment.

In certain embodiments, the exemplary polymer coating may be a temporary coating with a polymer. The polymer is very soluble in the solvent used by the cell manufacturer for slurry preparation. In the production of the final positive electrode, the polymer is dissolved during the slurry preparation step, but the LiF interface remains. Therefore, the polymeric type outer shell protects the LNMO or LNO cathode powder from its preparation until the slurry production, which is a temporary coating. The protection mechanism is determined by the strong hydrophobicity of the polymer coating to prevent moisture from adhering to the surface of the cathode powder, and (1) a considerable amount of water absorption by the powder, (2) Li ₂ CO ₃ type of LiOH type base A significant transformation rate to the base, and (3) an increase in the total base content induced by water.

The coin cell preparation includes a slurry production step. The solvent commonly used by battery manufacturers for slurry production is N-methyl pyrrolidone (NMP). Therefore, the polymer of the example used for coating can be dissolved in NMP. It is also beneficial that the polymer is compatible with Li battery chemistry. Therefore, the polymers of the other examples are basically the same as the binder used by the battery manufacturer. Battery manufacturers use PVDF-based polymers as binders. Therefore, the coating polymer is a PVDF-based polymer in the above example. During the preparation of the slurry, the polymer coating dissolves, but the LiF film protecting the surface remains.

As described above, certain embodiments of the coating step are subjected to a drying coating step after a heating step to a temperature significantly higher than the melting temperature of the polymer. Only when the melting temperature is well exceeded, the molten polymer reacts with the surface base and effectively wet the surface of the LNO / LNMO particles. In another specific embodiment, the powder mixture of LNMO or LNO and PVDF based polymer powders is heat treated at a temperature in excess of 220 캜, which is at least 50 캜 higher than the melting temperature of the PVDF (other PVDFs have melting temperatures of 135 캜 to 170 캜 Temperature range). In another embodiment, the PVDF based polymer powder is heat treated at a temperature of from 225 캜 to 320 캜. Wetting in this temperature range not only has a physical effect (due to the low viscosity of the polymer), but also the reaction between the polymer and the surface base of LNO / LNMO also plays an important role. If the temperature is lower than 220 캜, the polymer melts but is not excellent in wetting. As a result, a poor surface coating is obtained. If the temperature exceeds 320 DEG C, the polymer is completely decomposed. The temperature at which the chemical reaction with the Li base occurs is lower than the temperature at which Kynar or PVDF begins to decompose by briefly heating in air and is about 350-375 ° C. Since PTFE has a melting temperature of about 330 캜, it is clear that the heating temperature for obtaining the LiF layer is at least 380 캜 when PTFE is used as the polymer.

In US2009 / 0194747 A1 (assigned INCO) it is appropriate to state that the PVDF binder material is applied at a temperature below its decomposition temperature so that the LiF film is not formed and that all applied polymers remain and do not chemically alter.

The INCO patent carries out the polymer coating step in a liquid phase in a high temperature or (preferably) dissolved form. The INCO patent has observed poor adhesion between the polymer and the cathode powder, thus adding Lewis acids such as oxalic acid to improve adhesion and neutralize the LiOH in particular to avoid reaction with PVDF on the cathode material surface.

Embodiments of the coating methods described above follow different concepts. First, the mixture of the polymer and the cathode is generally carried out in solid form at room temperature. The mixture is then heated to a temperature at which decomposition of the polymer begins through reaction with the cathode powder surface. On the other hand, the heat treatment time is limited and the polymer is not completely decomposed while it must be long enough to allow the polymer to react sufficiently at the polymer-cathode interface to form a LiF-based interface film. Second, it does not require the addition of Lewis acid. Surprisingly, it has been found that poor adhesion between the cathode and the polymer is caused by the low heating temperature. As the temperature rises, the polymer and cathode surfaces begin to chemically react and very strong adhesion is obtained. In fact, excellent wetting of the molten polymer is observed on the surface of the cathode powder particles. Good wetting is believed to be evidence of degradation of the polymer at the cathode surface.

Of course, the LNMO cathode material is useful in a cylindrical cell. This is because of their high capacity, and the disadvantage of LNMCO, where gas is generated (believed to be related to the base content), can be handled in a cylindrical cell (the cylindrical cell has a very rugged case). At present, implementation into a square cell is more difficult and is impossible for a polymer cell because the expansion can not be easily treated. The LNMO cathode material according to the present invention has a lower base content because the LiF film can replace the surface base. In addition, since they have improved safety, the cathode can also be implemented in a square cell or a polymer cell.

The present invention can be practiced, for example, by different embodiments described below.

Example  One:

This example demonstrates the effect of coating with a fluorine-containing polymer after thermal treatment:

(1) the reaction between the cathode and the polymer at high temperatures, and

(2) Formation of LiF protective film.

The present example also examined the effect of temperature on a sample coated with a polymer having a LiF interface. This example shows the results for a sample made by adding 1% polymer. LNMO mass production samples are used as cathode precursors (precursors = uncoated or uncoated samples). The composition is Li _{1 + x} M _{1 -x} 0 ₂ where M = Ni _0.5 Mn _0.3 Co _0.2 and x is about 0.00. The precursor further contains 0.145 mol% of S and 142 ppm of Ca.

100 g of cathode precursor and 10 g of PVDF powder are carefully premixed using a coffee grinder. Then 110 g of the intermediate mixture and 900 g of the remaining cathode precursor are mixed at medium energy using a Hensel type mixer. The precursor-PVDF mixture is sampled in a batch of 100 g each. The batch is heat treated at a temperature in the range of 150-350 C for 5 hours. 1% PVDF represents the addition of 1 g of PVDF per 100 g of sample used as a precursor, since the mass of the sample is changed during the heat treatment (some or all of the polymer is degraded). The exact amount per gram of final sample may be slightly lower, for example, if there is no loss of mass during the heat treatment, the correct value is 0.99%. The obtained powders are sieved. Two series of experiments are conducted. The initial series is carried out at 150, 200, 250, 300 and 350 ° C; T = 25, 150, 180, 200, 225, 250, 275, 300, 325, 350 ° C. For the two series, an additional 'blank' sample without PVDF is added.

The powders of the selected samples are analyzed as follows:

(1) X-rays and Rietveld refinement to obtain accurate lattice parameters;

(2) Coin cell test for measuring electrochemical performance (1st series only),

(3) a scanning electron microscope (SEM) and / or a field emission total scanning electron microscope (FESEM), and

(4) Water Exposure Test (5 days, 50% humidity, 30 ° C):

   (A) Measurement of water content before and after exposure

   (B) Adjusted pH titration of soluble bases before and after exposure

A summary of the test results is provided in Table 1 and Figures 1-5.

In this and all the following examples, the electrochemical performance was tested in a coin-type cell having a Li foil as a counter electrode in a lithium hexafluoride (LiPF ₆ ) type electrolyte at 25 ° C. The cell is charged to 4.3V and discharged to 3.0V to measure rate performance and capacity. Capacity retention during extended cycling is measured at a 4.5V charge voltage. A specific capacity of 160 mAh / g assumes the measurement of the discharge rate. For example, for discharging at 2C, a specific current of 320 mA / g is used. The following is a summary of the test:

The following definitions are used for data analysis: (Q: capacity, D: discharge, C: charge)

The irreversible capacity Q (irr) is (QC1-QD1) / C1.

Fade rate (0.1 C) for 100 cycles: (1-QD31 / QD7) * 100/23

Fade rate (1.0 C) for 100 cycles: (1-QD32 / QD8) * 100/23

Energy fade: Discharge energy (capacity × average discharge voltage) is used instead of discharge capacity (QD).

In pH titration: PVDF coated samples are often highly hydrophobic, making pH titration difficult in aqueous solutions. Therefore, a sample of 7.5 g is first wetted with 10 g of acetone, then 90 g of water is added, and then stirred for 10 minutes. After filtration, the content of soluble base in the clear filtrate is titrated by standard pH titration using 0.1 M HCl.

[Table 1] Samples, manufacture and list of results

The series indication is shown in Figs. 1 and 2

Figure 1 shows:

(1) Lower: Results for unit cell volume (Series 1b: ∇, 2a: Δ) for one unit of formula (LiM0 ₂ ) obtained by Rietveld refinement of powder diffraction data,

(2) Medium: The content of the soluble base obtained by the pH titration method before the water exposure (series 1a: ★, 2a: Δ) and thereafter (series 1b:

(3) upper part: moisture content after water exposure (series 1b: ▽)

Figure 2 shows the electrochemical performance of the samples without water exposure.

The following can be observed in Figure 1:

300-325 ℃에서 단계적 증가. 격자 상수의 증가는 거의 확실하게 부분 반리튬화(delithiation)에 의해서 야기된다. 반리튬화는 불소 함유 폴리머의 분해에 의해서 유도되며, 여기서 리튬은 폴리머와 반응하여 LiF를 형성한다. 단위 셀 부피는 180 ℃ 미만에서 PVDF와 캐소드 사이에서 반응이 일어나지 않았다는 것을 나타내며, 이는 가열 처리되지 않은 전구물질의 부피도 또한 33.8671 ų이기 때문이다. 약 200 ℃에서만 반응이 개시되고, 약 300 ℃에서 주요 반응이 일어난다. 본 발명자들은 LiF 필름이 200 ℃ 이상의 온도에서 존재할 것이라고 결론지었다. Unit cell volume ( unit cell volume ): a sustained increase in unit cell volume at T &gt; 175 [deg.] C and a T

Increase stepwise at 300-325 ° C. The increase in the lattice constant is most likely caused by partial semi-lithiation (delithiation). Semi-lithiation is induced by the decomposition of the fluorine-containing polymer, where lithium reacts with the polymer to form LiF. The unit cell volume indicates that no reaction occurred between the PVDF and the cathode at less than 180 ° C, since the volume of the unheated precursor is also 33.8671 Å ³ . The reaction starts only at about 200 ° C, and the main reaction occurs at about 300 ° C. We conclude that the LiF film will be present at temperatures above 200 ° C.

Base : Lower solubility base at higher processing temperatures. Optimum (lowest base) is observed at approximately 275-325 ° C. Soluble base is located at the surface, it dissolves in water to form LiOH or Li ₂ C0 _3. Soluble bases are the most reactive form of lithium. Therefore, the lithium in LiF formed by the reaction of the surface with the fluorine-containing polymer will be derived from a soluble base. In fact, LiF films replace films of soluble bases. We have observed that at least 250 [deg.] C is required to reduce soluble bases by 50%. At higher temperatures (> 325 ° C), a new soluble base can be reformed from the bulk and replaces the base consumed by LiF film formation.

Moisture : Very low moisture content with good moisture stability at> 200 ° C to about 325 ° C. At temperatures above 325 DEG C, the polymer gradually decomposes completely and the surface is no longer protected against moisture absorption. At temperatures below 200 ° C, the polymer does not completely cover the surface. At a sufficiently high but not too high temperature, the surface is covered by a partially degraded polymer film which prevents water absorption. It is clear that good coverage (= good wetting properties) relates to the reaction of the polymer with the soluble base at the surface.

Figure 2a shows that the reversible capacity (C1: cycle 1) of the (top) coated powder decreases and the (intermediate) irreversible capacity (Qirr = [discharge-charge] / charge,%) increases significantly at temperatures above 300 ° C . At the same time (lower) rate performance (2C, C for 0.1C) drops. There are two reasons for this observation:

(1) Li is lost from the cathode to form LiF. When the stoichiometry of the oxygen is balanced, the loss of Li results in Li deficient-Li _{1 - x} M _{1 + x} 0 ₂ . One percent by weight of PVDF contains about 6000 ppm of fluorine, corresponding to a loss of about 3 mol% of lithium. In general, the lithium-deficient _{-Li 1 - x M 1 + x} 0 2 has a low rate and increased irreversible capacity;

(2) the surface is covered by an electronically and ionically insulated LiF film, which is thicker than desired, resulting in poor rate performance.

Figure 2b shows the results of energy fades (capacity × average discharge voltage, measured at 0.1 C (upper) or 1 C (lower)) after cycling for 23 cycles between 3.0 and 4.5. Figure 2b shows that cyclic stability is increased by increasing the temperature to 250 &lt; 0 &gt; C. Possible improved cycling stability almost certainly contributes to the formation of the protective LiF film.

FIGS. 3 to 5 show micrographs of samples prepared at 200 ° C. (SEM - FIG. 3), 250 ° C. (FESEM - FIG. 4) and 350 ° C. (FESEM - FIG. Figure 3 shows the SEM of a sample prepared at 200 캜: the particles are represented by a number of small "droplets" at the surface. The droplet is a possibly fused PVDF particle. Obviously, PVDF does not wet the surface. At 250 ° C (see FIG. 4), the droplet disappeared and the surface was covered smoothly by the PVDF film, and the surface structure represents the formation of a LiF plate under the film. At 350 DEG C (FIG. 5), the polymer is completely decomposed and the surface is covered by a small crystal plate of lithium fluoride.

Conclusion: Example 1 demonstrates that the polymer film covers the particles at a temperature greater than 200 ° C and less than 350 ° C, and the interface between the polymer and the cathode surface is a LiF film. The LiF film replaces the soluble surface base of the cathode.

Example  2:

Example 1 investigated the coating with 1% PVDF. However, at a treatment temperature of T> 275 ° C, especially> 300 ° C, the degradation polymer extracted more Li from the cathode, which was observed to cause a reduction in reversible capacity. This indicates that the obtained LiF film can be unnecessarily thickened. Therefore, this example shows the present invention in which a heat treatment is performed using only a small amount of polymer of 0.3 wt% PVDF. In the above example, the influence of temperature was examined in the production of a sample coated with a polymer having a LiF interface. LNMO mass production samples are used as cathode precursors. The composition thereof is Li _{1 + x} M _{1 -x} 0 ₂ , where M = Ni _{0 .5} Mn _{0 .3} Co _{0 .2} and x is about 0.00. The precursor further contains 0.145 mol% of S and 142 ppm of Ca.

100 g of cathode precursor and 3 g of PVDF powder are carefully premixed using a coffee grinder. Then, 103 g of the mixture was mixed with the remaining 900 g of the cathode precursor and mixed at the intermediate energy using a Hansel type mixer. The mixture is sampled in batches of 100 g each. The batch is heat treated at a temperature in the range of 225-350 C for 5 hours. Samples were prepared at 225, 250, 275, 300, 325 and 350 ° C for 'black' samples without PVDF. The powders obtained were sieved and analyzed in a similar manner as in Example 1. Table 2 summarizes the samples, manufacture and results:

[Table 2]

Figure 6 shows the pH titration results before (lower) water exposure (★) and after (Δ), as well as the moisture content after (upper) water exposure (5 days, 30 ° C., 50%). Similar to Example 1, the PVDF treatment significantly lowers the soluble base at temperatures above 250 ° C, protects against moisture absorption (but has a lower effect), and the optimum is 250-275 ° C. Good coin cell test results are obtained over the entire temperature range. It is believed that decreasing the base content forms a LiF layer, which is beneficial for improving high voltage stability and safety performance in a full cell.

Compared with Example 1, the optimum base content was observed at 275-350 ° C, the moisture content was lowest at a limited range of about 250 ° C, and the electrochemical test results were found to be excellent over the entire temperature range. Although several effects can already be observed using 0.1 wt.% PVDF, 0.3 wt.% PVDF in combination with a heating temperature of 200-300 DEG C appears close to the lower limit for obtaining the desired result, The% PVDF can be the upper limit. This analysis was further investigated in the following Examples 5a to 5d. The optimum equilibrium between the desired effects on water uptake and base content was found in 0.5-0.8 wt.% PVDF irrespective of the lithium transition metal oxide composition tested, without negatively affecting the electrochemical results.

Example  3:

This example investigated the effect of temperature on the preparation of a LiCoO ₂ sample coated with a polymer having a LiF interface. This example discusses the microstructure and voltage profile of a suitable LiCoO ₂ to provide further evidence of the conclusions of Examples 1-2. An important conclusion is that LiF films are formed at 200-350 [deg.] C similarly to Example 1-2. The thickness increases with temperature. Otherwise, the LiF film can not be maintained at a higher temperature.

This example shows the results of a sample made by adding 1% PVDF polymer. The lithium cobalt oxide mass production sample is used as a cathode precursor. Its composition is 1 mol% Mg doped LiCoO ₂ and has an average particle size of 17 μm. 1000 g of the precursor powder and 10 g of PVDF powder are carefully mixed using a Henschel type mixer. The mixture is sampled in batches of 150 g each. The batch is heat treated at a temperature in the range of 150-600 C for 9 hours. The obtained powder is sieved. The powder was analyzed by a coin cell test, SEM and conductivity.

The SEM analysis shows an irregular coating of the polymer at 150 캜, and an increase in the temperature to 250 캜 increases the smoothness and homogeneity. At 300 캜, the surface layer began to change, and at 350 캜, the surface film was observed to have inorganic characteristics instead of polymer coating. At 600 캜, the surface film is damaged and a well-formed crystallizable LiF is formed. The formation of the crystals proved that LiF did not wet the surface at high temperatures. It seems impossible to obtain a LiF film by direct high-temperature synthesis. FIG. 7A is a SEM graph of the sample at 300.degree. C., and FIG. 7B is a SEM graph of the sample at 600.degree. Indicating the presence of well-formed LiF crystals.

Table 3 provides a summary of the electrochemical test measurements.

Table 3: Charge (QC), discharge (QD) and irreversible capacity (Q irr) of samples treated at different temperatures.

Figure 8 shows the discharge voltage profile (4.3-3.0V, 0.1C rate) of Table 3 samples made with 1% PVDF at different temperatures. Samples manufactured at low temperatures (150 ° C, 200 ° C) exhibit exactly the same discharge voltage profile. The profile is similar to the reference (the data not shown), which is an untreated sample used as a precursor, but has a slightly lower capacity (less than about 1%). The capacitance value represents the actual mass of the sample (and thus includes the weight of the polymer coating). The low T samples (150 캜, 200 캜) include a 1% PVDF coating layer, which illustrates a 1% lower capacity. The voltage profile is typical for LiCoO ₂ with high Li: Co ratios and no phase transition at 4.1 V was detected. The 250 캜 samples exhibit different voltage profiles and are common in LiCoO ₂ with poor rate performance. The polarization is larger (voltage drop) and the discharge end is much less square (more rounded). This contributes to the LiF interfacial layer formed between the polymer coating and the LiCoO ₂ surface. The LiF layer completely covers the surface and has low ion and electrical conductivity, resulting in a low rate voltage profile.

As the temperature increases (300 ° C, 350 ° C), the capacity drops sharply. This clearly shows the formation of a resistive LiF layer as the thickness increases and apparently covers the entire surface. However, if the fabrication temperature is further increased, at 600 ° C, the inventor has observed nearly full charge, improved rate performance (not shown), and a clear phase transition at 4.1 V (typically 4.1 V phase transition is Li- Observed only in stoichiometric LiCoO ₂ ).

At 600 ° C the data show that the resistive LiF surface layer is absent. Obviously, at high temperatures, the homogeneous LiF surface layer is destroyed and a large fraction of the surface is no longer covered by the LiF layer. The data is in full agreement with the SEM, which represents the formation of damaged surfaces and larger LiF crystals.

Example 3 demonstrates the formation of a homogeneous polymer surface film at temperatures above the melting point (140-170 DEG C) of PVDF. However, the temperature needs to be increased above 200 DEG C, preferably above 250 DEG C before the reaction between the polymer and the cathode surface is carried out, which forms the desired interfacial LiF film. However, if the temperature is too high, the protective layer is no longer active. Thereafter, the LiF surface film is peeled from the surface to form LiF crystals. Example 3 also shows that the results obtained in 1% Mg doped LiCoO ₂ samples are comparable to those in the LNMO samples of Example 1.

Example  4:

Example 4 investigated the influence of temperature on the production of LNO type samples coated with a polymer having a LiF interface.

This example shows the results for a sample made by adding 0.3% and 1% PVDF polymer. _{LiNi 0 .8 Co 0 .15 Al 0} .05 O 2 sample (where 0.15 having a mol% of S and C of 500-1000 ppm) is LiOH and alumina-containing mixed transition under oxygen flow from 5 kg scale in a pilot plant Metal precursors. The PVDF treatment is carried out similarly as in Examples 1 and 2. [ Table 4 summarizes the samples, manufacture and results.

Table 4: Samples for high nickel content cathode materials, preparation and results obtained

Table 4 shows that the PVDF treatment improves water stability and the initial base content is significantly lower at T = 250 ° C. No base reduction was observed compared to no PVDF at 150 ° C, but the base content was reduced by consumption of the base to form LiF at a higher T. For the LNO composition, the moisture content is its lowest at 350 캜 for treatment. Compared to the untreated sample, the base ratio increases while water exposure is reduced. Similar to Examples 1 to 3, the use of 0.3% PVDF results in good electrochemical results over the entire temperature range, while 1% PVDF is used, the capacity and rate at the higher T is reduced.

Example  5a to 5d

This example reproduces the results of Example 1 and Example 2 in a large-scale sample. The sample was further tested in a polymer type complete cell. In all embodiments, high-volume manufacturing _{LNMO (M = Ni 0 .5 Mn} 0 .3 Co 0 .2) ( wherein, Li of about 1.0: M) is used as precursor. The precursor further contains 0.145 mol% of S and 142 ppm of Ca.

Example 5a: 1% by weight PVDF

200 g of mass produced LNMO and 18 g of PVDF powder are pre-mixed in four batches using a coffee grinder. The mixture is added as 1.6 kg of LNMO and mixed continuously using a Henschel type mixer using a 2 L vessel. The mixture is sieved after heat treatment at 25O &lt; 0 &gt; C in a convection oven for 5 hours.

Example 5b: 1% by weight PVDF (large amount of sample)

15 kg of the cathode precursor powder and 150 g of PVDF powder are carefully mixed using a pilot plant ribbon blender. The powder mixture is heated at 250 &lt; 0 &gt; C for 5 hours, then pulverized and sieved.

Example 5c: 0.3% PVDF at &lt; RTI ID = 0.0 &gt; 300 C &

It is basically similar to the 1.8 kg sample of Example 5a, but the heat treatment temperature is 300 占 폚, and less PVDF (5.4 g) is used. Premixing is carried out in two batches of 2.7 g PVDF and 50 g sample.

Example 5d: 0.3% PVDF at &lt; RTI ID = 0.0 &gt; 350 C &

Similar to Example 5c, but with a heat treatment temperature of 350 占 폚.

The test is carried out in a similar manner as in Examples 1 to 3, and wound pouch type cells with an additional 800 mAh wound are assembled and tested (cells of this type are described, for example, in US 7,585,589, Technology). Table 5 summarizes the above results.

[Table 5] Samples of larger size, manufacture and test results

QD: discharge capacity; Rate:% against 0.1 C, base: before and after exposure to the water chamber

The following results were obtained from the table:

(1) 1% sample at 250 ° C: highest moisture stability. The base is not increased during water exposure and the water content after water exposure is very low. However, the LiF film is thin, and the base content is reduced to only about 30%.

(2) 0.3% at 300 ° C Sample: The water stability caused by the thinner polymer film is worse at 1 ° C than at 1 ° C at 250 ° C, otherwise the overall salt content is lower and less than 50% of the reference standard. This indicates that the LiF is better developed and most of the base is consumed by decomposition of the polymer. The present inventors have observed a slight reduction in unit cell volume consistent with the extraction of some lithium from the bulk.

(3) 0.3% at 350 ° C Sample: Moisture content is better than at 300 ° C.

Table 6 summarizes the pouch cell test results. It was observed that swelling greatly decreased after storage at high temperature (4 hours, 90 ° C). The expansion is the ratio of the cell thickness after 4 hours measured when the cell is still hot (90 [deg.] C) compared to the thickness measured before (cooling). Several additional tests with differently treated samples were performed, but only PVDF treated samples showed much less swelling than the values typically obtained at 40-50%. The inventors have further observed that all PVDF treated cells pass the overcharge test, indicating improved safety performance. Overcharge is performed at 700 mA until it reaches 5.5V. Passage means that there is no fire or smoke. The nailing test is carried out using a sharp nail of 2.5 mm diameter at a speed of 6.4 mm per second. Passage means no smoke or fire.

[Table 6] Results of complete cell test using LNMO

Example  6:

Example 6 is a so-called &quot; black &quot; embodiment, simulating possible reactions between molten PVDF covering the particle surface and LiOH type base present on the particle surface. Using the differential scanning calorimetry (DSC) method, this example shows that the polymer reacts with the lithium containing base at a temperature about 50 캜 higher than the melting point of Kynar. This reaction is necessary to form the desired internal LiF layer.

The Kynar® 2801 samples from Arkema (fine powders, having a melting point of 142 ° C as reported by the manufacturer) and LiOH * H ₂ O samples were each jetted Milled (jet milled). Figure 9 provides a SEM image of a Kynar sample and shows that it consists of agglomerated spherical primary particles having an average particle size of 0.2-0.5 [mu] m.

The obtained fine Kynar powder and the fine particles of LiOH * H ₂ O are then mixed at a mass ratio of 2: 1. This corresponds to a molar ratio F: Li of about 2.62 to fluorine in Kynar for Li in the hydroxide. Therefore, even though all Li reacts with the polymer, there is still an excess of unreacted polymer. The mixture is heated to 150 캜, 200 캜 and 250 캜. The mass loss is recorded and X-ray diffraction is measured for the heated mixture.

The mixture and the Kynar control standard were examined by DSC. The samples are placed in a sealed stainless steel DSC can. The heat flow is measured while heating from room temperature to 350 캜 using a temperature rate of 5K / min.

10 is a result obtained DSC (heat flow versus temperature; top: a mixture of Kynar and LiOH * H ₂ 0; bottom: pure Kynar): minimum heat flow to the Kynar (maximum heat-absorption) are obtained in 142 ℃, which 142 Lt; 0 &gt; C.

The curve obtained while heating the mixture is completely different. First, a sharp endothermic event is observed with minimal heat flow at 109.1 ° C. This is the release of water. _{(LiOH * H 2 0 → LiOH} + H 2 0). A strong exotherm is then observed. The maximum heat flow is observed at 186.2 ° C. It is believed that at this temperature, the PVDF in contact with the Li base and high pressure moisture is decomposed and LiF (and possible carbons) are formed. The DSC can was sealed so that no further reaction occurred. However, at higher temperatures in air, the polymer will continue to decompose, which will be demonstrated in Example 7.

Example  7:

This example is another 'black' embodiment and simulated possible reactions between molten PVDF covering the particle surface and LiOH type base present on the particle surface. This example shows that a reaction takes place between the base and PVDF at a temperature of at least 200 &lt; 0 &gt; C in the air and the decomposed polymer and possible carbon are formed.

Exemplary sample mixture as in Example 6 (2: 1 weight ratio in a jet mill LiOH * H ₂ 0 and Kynar® of Arkema the 2801 samples) is used. The mixture is heated in air at 150 DEG C, 200 DEG C and 250 DEG C for 5 hours. The mass loss is recorded and X-ray diffraction is measured for the heated mixture. Example 6 is a closed system (high pressure wet) whereas Example 7 is an open system (most of the water is evaporated).

Table 7 summarizes the results and 'X-ray' lists the observed compounds.

[Table 7] Heat treatment of LiOH-Kynar mixture in air

At 150 캜 and 175 캜, the mixture basically does not react. The color is yellowish white and the electrical conductivity is zero. 13-15% by weight of the mass loss is observed, usually resulting from the reaction _{LiOH * H 2 0 + PVDF →} LiOH + PVDF. The mixture is completely dissolved in NMP. X- ray diffraction pattern represents the LiOH and _{LiOH * H 2 0, Li 2} C0 3, the polymer and a very small amount of LiF.

The mixture reacts at 200 ° C. The color obtained is black. Much more mass loss is observed. Conductivity can not be measured (too low). The mixture can not be completely dissolved in acetone and black particles remain. The X-ray diffraction pattern represents LiF and a polymer. The polymer has a diffraction pattern different from that of pure PVDF.

A stronger reaction takes place at 250 ° C. The mass loss is 58.7% by weight. The mixture exhibits increased conductivity of 3 * 10 &lt;&quot; ⁷ &gt; S / cm. The mixture can not be completely dissolved in acetone, and black particles remain. The X-ray diffraction pattern represents LiF and a polymer, which has a diffraction pattern different from that of pure PVDF.

Figures 11 and 12 show a portion of the collected X-ray diffraction pattern: Figure 11 shows a control standard Kynar (PVDF) sample (bottom) and a sample (top) Bottom curve of the top view), 175 [deg.] C (top curve of the top view), and the control standard PVDF in the bottom view. In Fig. 11, in the top view, the two high intensity peaks (10% of the full scale disclosed) are LiF. A wide hump at 15-30 ° is an 'undefined' polymer and has an X-ray pattern that remains in PVDF but clearly different from the control standard. The pattern of the mixture (not shown) after heat treatment at 200 占 폚 is very similar. In FIG. 12, the X-ray diffraction pattern of the PVDF precursor (= control standard) and mixture after heat treatment at 150 ° C and 175 ° C basically indicates that PVDF did not react and the pattern of PVDF remained. At 150 캜, a trace amount of LiF can be detected. At 175 캜, LiF becomes a small amount of impurities. The diffraction peaks of LiF are indicated by arrows. Other peaks can be indexed with lithium salts such as Li ₂ CO ₃ and LiOH.

Example 7 shows that a reaction between LiOH and PVDF takes place at about 200 ° C in air, for example, at a temperature higher than about 50K above the melting point, to form a modified polymer with LiF. This example confirmed that the formation of LiF layer and decomposition of PVDF in the lithium transition metal oxide powder should be caused by the reaction of lithium base and PVDF.

Example  8:

This experiment was designed to demonstrate that:

(1) At low heating temperatures, there is no LiF layer (PVDF covers the particles but no LiF reactive layer is formed)

(2) At the heating temperature according to the invention, the reaction between the PVDF and the cathode is initiated (a thin interface LiF layer is obtained)

(3) At too high a temperature, a thick LiF film is formed (all PVDF is consumed by reaction with the cathode to form LiF)

LiF has a small solubility in water (about 1.5 grams per liter). On the other hand, PVDF is insoluble. Therefore, it is expected that after immersing the heat-treated product, LiF is dissolved and dissolved fluorine ions can be detected by liquid chromatography. However, since the sample containing PVDF is hydrophobic, it is not certain that all LiF will be accessible by water. Since PVDF has high solubility in acetone or NMP but not LiF, a sample can be prepared in which PVDF is dissolved and dissolved in NMP or acetone so that water can approach and dissolve LiF.

The following samples were tested:

(1) a sample prepared with the same or similar sample as that described or analyzed in Example 1 without washing,

(2) A sample washed with a small amount of acetone and decanted,

(3) A sample washed and decanted with NMP.

The liquid chromatography (LC) procedure is as follows:

(1) Place 1 g of sample in a 300 mL glass Erlenmeyer flask;

(2) adding 100 mL of double deionized water;

(3) stirring with a glass stirring bar for 1 hour;

(4) Microfilter Filtration on Millipore 0.45 μm;

(5) Determination of the filtrate in ion chromatography (with procedure blanks).

Table 8 summarizes the samples, manufacture and results.

[Table 8] Summary of Samples, Manufacture and Results

Percentage%:% of PVDF reacted and can be estimated from the actual amount of F. The F (-) analysis results show three different concentrations in the wash water: 0.006 wt% to 0.010 wt% indicate that there is little LiF present; 0.054% to 0.064% by weight indicates the presence of a substantially the same thickness of LiF layer; Finally, 0.158 wt.% To 0.162 wt.% Indicates that almost all of the PVDF has reacted, which will be described below.

The results are also shown in Fig. The top figure shows the% by weight of fluorine detected by chromatography. The lower graph shows the fraction of dissolved F detected by chromatography calculated from the data.

First, the inventors have observed that cleaned samples (in NMP or acetone) and uncleaned samples provide the same results. For example, EX0121 and EX0121C are compared. PVDF, on the other hand, provides an effective protection against moisture absorption in water exposure tests, while allowing immersion of base LiF in water immersion.

Secondly, it has clearly been demonstrated by ion chromatography that LiF is not actually present (in some cases insufficiently present) at 150 &lt; 0 &gt; C. For example, see EX0121 and EX0121C. The polymer therefore does not react with the surface of the treated cathode product. The PVDF film covers the particles but no protective LiF film is present. At 250 DEG C, the fraction of PVDF reacts, for example, to EX0194 and EX0194C. The total amount of LiF formed (= the amount of PVDF reacted) is almost independent of the amount of initial PVDF, which is estimated by comparing EX0194C and EX0126C. The inventors conclude that the rate of reaction is limited by the availability of the surface area of the cathode and the surface base. A large amount of unreacted PVDF covers the particles, but an interfacial layer of LiF is formed.

At 350 ° C, all PVDF react. In an ideal experiment, we detected as much fluorine as LC added to the sample in the form of PVDF. Assuming that the results obtained for the fraction of fluorine (84-88%) detected were within the experimental system error, we concluded that all PVDF was decomposed at 350 ° C.

In the treatment at 250 캜, the amount of F detected is limited by the surface, for example, there is a limit depending on the amount of the base Li, and at 350 캜 the amount probably limits the PVDF, It depends.

Example  9:

This example describes the irradiation of PVDF-treated cathode materials using X-ray photoelectron spectroscopy (XPS) to investigate the formation of LiF and the decomposition of PVDF as a function of temperature. This example shows the results for selected samples (EX0124, EX0127, EX0161) of Example 1 prepared by adding 1% PVDF and treating at three different temperatures of 200 캜, 250 캜 and 350 캜.

This experiment was designed to demonstrate that:

(1) Complete decomposition of the PVDF coating is obtained by prolonged heating at high temperature (~ 350 ° C).

(2) As the temperature increases, a LiF layer having an increased thickness is formed. The fluorine in the layer comes out of the PVDF, and Li in the layer comes out of the surface base present on the surface of the cathode particles.

The results of the C, F and Li spectra are summarized in Table 9 below.

[Table 9] Summary of apparent atom concentration (at%) measured at the surface after deconvolution of the C 1s, F 1s and Li 1s spectra at different contributions

Results for Table 9:

1. C 1s:

1.1. The disappearance (= decomposition) of PVDF at 350 ° C due to a decrease in the CF ₂ -CF ₂ peak at 291.1 eV. PVDF (pristine or partially degraded) is present at a temperature below this temperature.

1.2. Li ₂ CO ₃ was observed at the particle surface by CO ₃ -peak at 289.2 eV. At 350 캜, Li ₂ CO ₃ is removed. This can be explained by the formation of LiF, and Li ₂ CO ₃ present on the particle surface is used as a source of Li.

2. F 1s:

2.1. The disappearance (= decomposition) of PVDF at 350 ° C caused by a decrease in F-org-peak at 686.7 eV. PVDF (pristine or partially degraded) is present at a temperature below this temperature.

2.2. Formation of LiF at 350 ° C by LiF-peak at ~ 685 eV.

2.3. The formation of LiF is directly related to the reduction in Li ₂ CO ₃ , which indicates the use of Li ₂ CO ₃ during its formation. It can not be concluded that the formation of LiF at lower temperatures is due to the masking of the LiF layer by the PVDF overlay (XPS is known to have a limited penetration depth). Therefore, in Example 8, the PVDF over layer is removed by solvent cleaning.

3. Li 1s:

3.1. When the temperature is increased, the Li ⁺ / LiF ratio decreases to 1 and more LiF is formed. This clearly shows that the formation of LiF at 350 캜 is complete and that all Li on the surface exists as LiF.

XPS data clearly supports the following model:

1. Complete decomposition of the PVDF coating is obtained by prolonged heating at high temperature (~ 350 ° C).

2. As the temperature increases, a thick LiF layer is formed. F in the layer comes out of PVDF and Li in the layer comes out of the surface base present on the cathode particle surface. (Wherein said base surface is to be monitored by the consists of a lithium salt such as Li ₂ CO ₃ and LiOH. The Li ₂ CO ₃ is the main part of the base surface XPS). Especially:

2.1. At low T (150-200 占 폚), PVDF is still present in the coating and almost no LiF is present. All surface bases (Li ₂ CO ₃ ) are still present on the surface of the cathode material.

2.2. At high T (250 캜), the reaction of PVDF with Li ₂ CO ₃ is initiated (a thin interface LiF layer is obtained). PVDF still exists as a coating.

2.3. At high T (350 캜), a thick LiF film is formed: over time PVDF is completely decomposed and its F is consumed by reacting with the available Li ₂ CO _{3 on the} particle surface to form LiF.

Example  10:

This example investigated PVDF-coated cathode materials using X-ray photoelectron spectroscopy (XPS) to investigate the formation of LiF and the decomposition of PVDF as a function of temperature. This example provides results for treatment at 0.3% PVDF and three different temperatures of 150 캜, 250 캜 and 350 캜. EX0120, EX0126 and EX0160, which are the selected samples of Example 2, were investigated.

XPS is a surface sensitivity technique with a limited penetration depth. The underlying LiF interface developed in Example 9 can be detected only in high T samples that are masked by the polymer surface and in which the polymer is degraded. In this embodiment, a cleaning step may be applied to remove the remaining PVDF so that the primary LiF layer can be seen more clearly.

Samples EX0120, EX0126 and EX0160 are cleaned using the following method:

(1) Shake 5 g in 20 ml NMP for 1 hour;

(2) diluted with 40 ml of acetone;

(3) 2 decanting, drying.

Since the polymer is soluble in NMP and acetone but LiF is substantially insoluble, the polymer is removed and the basic LiF is considered acceptable for XPS analysis.

The results of the C, F and Li spectra are summarized in Table 10 below.

Figure 14a shows the F1s spectrum at 150 ° C, Figure 14b shows the spectrum at 250 ° C, and Figure 14c shows the spectrum at 350 ° C. The counts per second (CPS) is plotted against the coupling energy (eV).

TABLE 10 Summary of apparent atomic concentrations (at%) measured at the surface after deconvolution of the C 1s, F 1s and Li 1s spectra at different contributions. "/" Indicates absence of XPS peak.

Results for Table 10:

1. C 1s:

1.1. Based on the absence of the CF ₂ -CF ₂ peak, we conclude that most of the PVDF is removed by solvent rinsing. Especially at T = 350 ° C, no PVDF was observed (due to complete decomposition and conversion to LiF).

1.2. Li ₂ CO ₃ was observed at the particle surface by CO ₃ - peak at 289.7 eV. The direct relationship between the removal of Li ₂ CO ₃ and the increase in temperature is explained by the PVDF being converted to LiF. In this method, Li ₂ CO ₃ present on the particle surface is used as a source of Li.

2. F 1s:

2.1. The increased formation of the LiF layer with increasing temperature is evident by an increase in the typical LiF peak at 684.7 eV (see FIG. 13).

2.2. The formation of LiF is directly related to the reduction in Li ₂ CO ₃ , which indicates the use of Li ₂ CO ₃ during its formation.

3. Li 1s:

3.1. When the temperature is increased, the Li ⁺ / LiF ratio decreases to 1 and more LiF is formed. This clearly shows that the formation of LiF is completed at 350 DEG C and that all Li on the surface is present as LiF. At 250 ° C, there are still some other Li-species such as Li ₂ CO ₃ . At 150 ℃, other Li-species exist mainly and there is almost no LiF.

LiF  thickness:

The LiF thickness calculation is described by van der Marel et al. in Journal of Vacuum Science and Standards A, 23 (5) 1456-1470 (2005), which is based on standard exponential attenuation of photoelectron intensity as a function of travel distance. Equal to the layer structure of the sample LiF is assumed to form a uniform layer: bulk _{MnO x, CoO x, NiO x} , from -CO ₃ C and Li ⁺ _remaining / Li and LiF in the F ^- / organic C, Organic F and O-org.

The LiF thickness increased as a function of temperature: an initial thin layer of 0.2 nm at 150 캜 was formed. At 250 占 폚, the LiF thickness was approximately 1 nm or more, reaching its total thickness. At 350 캜, the LiF layer reached its full thickness and PVDF was completely consumed. The results correspond to the thickness obtained from fluorine ion chromatography. Example 10 provides strong evidence that the polymer starts to react with the surface base at a sufficiently high temperature (about 50 캜 higher than the melting point) and the protective LiF film consumes and replaces the surface base.

Based on the results of Examples 1 to 10, it is concluded that an effective LiF film should have a thickness of at least 0.5 nm (extrapolated value at> 200 ° C), preferably 0.8 nm (extrapolated value at> 225 ° C) .

The invention may optionally be described by the following terms, but the serial numbers of the following terms and claims do not necessarily match the claims:

1. A lithium transition metal oxide powder for use in a rechargeable battery,

Wherein the surface of the primary particles of the powder is coated with a first inner layer and a second outer layer, the second outer layer comprises a fluorine-containing polymer, the first inner layer comprises a primary particle surface, A lithium transition metal oxide powder comprising a reaction product of a fluorine-containing polymer.

2. The method of claim 1,

Wherein the reaction product is LiF, and lithium is derived from the surface of the primary particles.

3. The method of claim 2,

Wherein the fluorine in the reaction product LiF is derived from a partially decomposed fluorine-containing polymer present in the outer layer.

4. The method according to any one of claims 1 to 3,

Wherein the fluorine-containing polymer is selected from the group consisting of PVDF, PVDF-HFP, and PTFE.

5. The method according to any one of claims 1 to 4,

Wherein the fluorine-containing polymer is comprised of agglomerated primary particles having an average particle size of from about 0.2 占 퐉 to about 0.5 占 퐉.

6. The method according to any one of claims 1 to 5,

Wherein the lithium transition metal oxide is selected from the group consisting of lithium transition metal oxide powder:

- LiCo _d M _e 0 ₂ , where M is at least one of Mg and Ti, e <0.02, d + e = 1;

_{- Li 1 + a M '1} - a 0 2 ± b M 1 k S m ( where, -0.03 <a <0.06, b <0.02, M' is a transition metal compound, at least 95% by weight of M 'is Ni , Mn, Co, Mg and Ti; M ¹ is at least one element selected from the group Ca, Sr, Y, La, Ce and Zr; 0? K? , and m is expressed as mol%); And

_{- Li a 'Ni x Co y} M "z 0 2 ± e A f ( where, 0.9 <a'<1.1, 0.5≤x≤0.9, 0 <y≤0.4, 0 <z≤0.35, e <0.02, 0 Wherein M is at least one element selected from the group consisting of Al, Mg, and Ti; and A is at least one of S and C). &Lt; / RTI &gt;

7. The method of claim 6,

_{M '= Ni a "Mn b} " Co c " , wherein a"> 0, b "> 0, c"> 0 , and a "+ b" + c " = 1; and a" / b "> 1 of Lithium transition metal oxide powder.

8. The method of claim 7,

Lithium transition metal oxide powder wherein 0.5? A?? 0.7, 0.1 <c? <0.35, and a "+ b" + c "= 1.

9. The method according to any one of claims 1 to 8,

Wherein the first inner layer comprises a LiF film having a thickness of at least 0.5 nm.

10. The method according to any one of claims 1 to 9,

Wherein the first inner layer comprises a LiF film having a thickness of at least 0.8 nm.

11. The method according to any one of claims 1 to 10,

Wherein the first inner layer comprises a LiF film having a thickness of at least 1 nm.

12. A method of coating a lithium transition metal oxide powder with a fluorine-containing double-layered coating,

Preparing a bare lithium transition metal oxide powder;

Mixing the non-coated lithium transition metal oxide powder with a fluorine-containing polymer to form a powder-polymer mixture; And

- forming a double layer coating on the surface of the metal oxide powder by heating the powder-polymer mixture at a temperature greater than or equal to the melting temperature of the fluorine-containing polymer from 50 캜 to 140 캜, And an inner layer composed of an outer layer composed of a fluorine-containing polymer and a reaction product of the powder surface and the polymer.

13. The method of claim 12,

Wherein the amount of the fluorine-containing polymer in the powder-polymer mixture is 0.1 wt% to 2 wt%.

14. The method according to claim 12 or 13,

Wherein the amount of the fluorine-containing polymer in the powder-polymer mixture is 0.2 wt% to 1 wt%.

15. The method according to claim 12 or 13,

Wherein the inner layer is composed of LiF.

16. The method according to any one of claims 12 to 15,

Wherein the fluorine-containing polymer is PVDF and the powder-polymer mixture is heated at a temperature of from 220 캜 to 325 캜 for at least one hour.

17. The method according to any one of claims 12 to 15,

Wherein the fluorine-containing polymer is PVDF and the powder-polymer mixture is heated at a temperature of from 240 캜 to 275 캜.

18. The method according to any one of claims 12 to 17,

Wherein the lithium transition metal oxide is any one of the following: a method of coating a lithium transition metal oxide powder;

- LiCo _d M _e 0 ₂ , where M is at least one of Mg and Ti, e <0.02, d + e = 1;

_{- Li 1 + a M '1} - a 0 2 ± b M 1 k S m ( where, -0.03 <a <0.06, b <0.02, M' is a transition metal compound, M is at least 95% by weight of 'the group Mn, Co, Mg and Ti; M ¹ is at least one element selected from the group Ca, Sr, Y, La, Ce and Zr; 0? K? 0.1; &Lt; / = 0.6, and m is mol%); And

_{- Li a 'Ni x Co y} M "z 0 2 ± e A f ( where, 0.9 <a'<1.1, 0.5≤x≤0.9, 0 <y≤0.4, 0 <z≤0.35, e <0.02, 0 Wherein M is at least one element selected from the group consisting of Al, Mg, and Ti; and A is at least one of S and C). &Lt; / RTI &gt;

19. The method of claim 18,

_{M '= Ni a "Mn b} " Co c " , wherein a"> 0, b "> 0, c"> 0 , and a "+ b" + c " = 1; and a" / b "> 1 of , A method of coating a lithium transition metal oxide powder.

20. The method of claim 19,

0.5? A?? 0.7, 0.1 <c? <0.35, and a "+ b" + c "= 1. A method for coating a lithium transition metal oxide powder,

21. The method according to any one of claims 12 to 20,

Wherein the inner layer has a thickness of at least 0.5 nm.

22. The method according to any one of claims 12 to 21,

Wherein the inner layer has a thickness of at least 0.8 nm.

23. The method according to any one of claims 12 to 22,

Wherein the inner layer has a thickness of at least 1 nm.

24. A double-shell core lithium transition metal oxide powder comprising primary particles having a surface,

Shell core lithium transition metal oxide powder wherein the surface of the primary particles is coated with an inner layer and an outer layer.

25. The method of claim 24,

Shell core lithium transition metal oxide powder, wherein the outer layer comprises a fluorine-containing polymer.

26. The method of claim 24,

Wherein the inner layer comprises the reaction product of a primary particle surface and a fluorine-containing polymer.

27. The method of claim 24,

Shell core lithium transition metal oxide powder wherein the surface of said primary particles is completely coated.

28. The method of claim 24,

Wherein the primary particles are formed after precipitation and firing of a lithium transition metal oxide powder.

29. The method of claim 24,

A dual-shell core lithium transition metal oxide powder further comprising secondary particles and coated with both primary and secondary particles.

30. The method of claim 29,

Wherein the primary particles are formed before secondary particles are formed.

31. A method for coating a lithium transition metal oxide powder with a fluorine-containing double layer coating,

Mixing a non-coated lithium transition metal oxide powder with a fluorine-containing polymer to form a powder-polymer mixture; And

And heating the powder-polymer mixture,

A method for coating a lithium transition metal oxide powder, wherein a bilayer coating is formed on the surface of the metal oxide powder.

32. The method of claim 31,

Wherein the bilayer coating comprises an outer layer and an inner layer.

33. The method of claim 31,

Wherein the outer layer comprises a fluorine-containing polymer.

34. The method of claim 31,

Wherein the inner layer comprises a reaction product of a polymer and a powder surface.

35. The method of claim 31,

Wherein the powder-polymer mixture is heated at a temperature higher than the melting point of the fluorine-containing polymer by 50 占 폚 or more and less than 140 占 폚.

36. The method of claim 31,

A method of coating a lithium-transition metal oxide powder, wherein the double-shell core lithium transition metal oxide powder according to claim 24 is formed by heating the powder-polymer mixture.

37. The method of claim 24,

Shell core lithium transition metal oxide powder wherein the double-shell core lithium transition metal oxide powder is used in a lithium-ion prismatic cell or a polymer battery.

38. An LNMO / LNO cathode material,

Comprising primary LNMO / LNO particles comprising an LNMO / LNO core,

The core is coated with a fluorine containing polymer and a partially degraded polymer substrate in contact with the fluorine containing polymer.

39. The method of claim 38,

Wherein the fluorine-containing polymer is LiF.

40. The method of claim 38,

LNMO / LNO cathode material wherein no carbon is present on the surface of the primary LNMO / LNO particles.

41. A method of making a dual-shell coated LNMO / LNO cathode material,

Blending the LNMO / LNO powder material with a fluorine-containing polymer to form a powder-polymer mixture;

Heating the powder-polymer mixture to a temperature above the melting point of the fluorine-containing polymer;

Reacting the fluorine-containing polymer with an LNMO / LNO powder material; And

And forming a bilayer coating on the LNMO / LNO powder material. &Lt; Desc / Clms Page number 20 &gt;

It is to be understood that certain embodiments and / or details of the present invention have been described above to illustrate the application of the principles of the present invention and that the present invention may be embodied in other specific forms without departing from the spirit or scope of the present invention. As is known to those of ordinary skill in the art.

Claims (18)

A double-shell core lithium transition metal oxide powder comprising a primary particle having a surface,
Shell core lithium transition metal oxide powder wherein the surface of the primary particles is coated with an inner layer and an outer layer. The method according to claim 1,
Shell core lithium transition metal oxide powder, wherein the outer layer comprises a fluorine-containing polymer. The method according to claim 1,
Wherein the inner layer comprises the reaction product of a primary particle surface and a fluorine-containing polymer. The method according to claim 1,
Shell core lithium transition metal oxide powder wherein the surface of said primary particles is completely coated. The method according to claim 1,
Wherein the primary particles are formed after precipitation and firing of a lithium transition metal oxide powder. The method according to claim 1,
A dual-shell core lithium transition metal oxide powder further comprising secondary particles and coated with both primary and secondary particles. The method according to claim 6,
Wherein the primary particles are formed before secondary particles are formed. The method according to claim 1,
Shell core lithium transition metal oxide powder wherein the double-shell core lithium transition metal oxide powder is used in a lithium-ion prismatic cell or a polymer battery. A method for coating a lithium transition metal oxide powder with a fluorine-containing double layer coating,
Mixing a non-coated lithium transition metal oxide powder with a fluorine-containing polymer to form a powder-polymer mixture; And
And heating the powder-polymer mixture,
A method for coating a lithium transition metal oxide powder, wherein a bilayer coating is formed on the surface of the metal oxide powder. 10. The method of claim 9,
Wherein the bilayer coating comprises an outer layer and an inner layer. 11. The method of claim 10,
Wherein the outer layer comprises a fluorine-containing polymer. 11. The method of claim 10,
Wherein the inner layer comprises a reaction product of a polymer and a powder surface. 10. The method of claim 9,
Wherein the powder-polymer mixture is heated at a temperature higher than the melting point of the fluorine-containing polymer at 50 占 폚 to 140 占 폚. 10. The method of claim 9,
A method of coating a lithium-transition metal oxide powder, wherein the powder of the powder-polymer mixture forms the double-shell core lithium transition metal oxide powder of claim 1. As the LNMO / LNO cathode material,
Comprising primary LNMO / LNO particles comprising an LNMO / LNO core,
The core is coated with a fluorine containing polymer and a partially degraded polymer substrate in contact with the fluorine containing polymer. 16. The method of claim 15,
Wherein the fluorine-containing polymer is LiF. 16. The method of claim 15,
LNMO / LNO cathode material wherein no carbon is present on the surface of the primary LNMO / LNO particles. A method of making a dual-shell coated LNMO / LNO cathode material,
Blending the LNMO / LNO powder material with a fluorine-containing polymer to form a powder-polymer mixture;
Heating the powder-polymer mixture to a temperature above the melting point of the fluorine-containing polymer;
Reacting the fluorine-containing polymer with an LNMO / LNO powder material; And
And forming a bilayer coating on the LNMO / LNO powder material. &Lt; Desc / Clms Page number 20 &gt;

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