Ferric hydroxynitrate with a surface for adsorbing hydrogen phosphate ions, a method for preparing the same, a positive electrode for a lithium secondary battery including the ferric hydroxynitrate, and a lithium secondary battery

Abstract

The present invention relates to iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on its surface, a method for preparing the same, a positive electrode for a lithium secondary battery including the iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on its surface as a positive electrode additive, and a lithium secondary battery including the positive electrode.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0098681, filed on August 13, 2019, Korean Patent Application No. 10-2019-0098694, filed on August 13, 2019, and Korean Patent Application No. 10-2020-0078154, filed on June 26, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on its surface, a method for preparing the same, a positive electrode for a lithium secondary battery including the iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on its surface as a positive electrode additive, and a lithium secondary battery including the positive electrode. Background Art

[0003] A lithium secondary battery is basically composed of materials such as a positive electrode, an electrolyte, and a negative electrode. Since the materials of the positive electrode and the negative electrode determine the capacity of the battery, the capacity of the lithium ion secondary battery is limited due to the limitations of the materials of the positive electrode and the negative electrode. In particular, since a secondary battery used for applications such as electric vehicles and plug-in hybrid electric vehicles (PHEVs) must be used for as long a time as possible after a single charge, the discharge capacity of the secondary battery is very important. The biggest limitation in the sale of electric vehicles is that the distance that can be traveled after a single charge is much shorter than that of a vehicle with a conventional gasoline engine.

[0004] Despite much effort, it is difficult to completely overcome such a capacity limitation of a lithium secondary battery due to the limitations of the structure and materials of the lithium secondary battery. Therefore, in order to fundamentally solve the capacity problem of a lithium secondary battery, it is necessary to develop a new concept secondary battery that goes beyond the concept of existing secondary batteries.

[0005] A lithium-sulfur battery is a new high-capacity and low-cost battery system that transcends the capacity limitation determined by the insertion / extraction reaction of lithium ions into / from layered metal oxides and graphite, which is the basic principle of existing lithium ion secondary batteries. This can lead to the replacement of transition metals and cost savings.

[0006] The theoretical capacity of a lithium-sulfur battery is 1675 mAh / g, which is obtained from the conversion reaction of lithium ions and sulfur in the positive electrode (S8 + 16Li + + 16e -→ 8Li2S), and the negative electrode uses lithium metal, enabling the battery system to have a very high capacity (theoretical capacity: 3860 mAh / g). In addition, since the discharge voltage is about 2.2 V, the theoretical energy density is 2600 Wh / kg based on the amounts of the positive and negative electrode active materials. These values are 6 to 7 times the theoretical energy density of 400 Wh / kg of a commercially available lithium secondary battery (LiCoO2 / graphite) using a layered metal oxide and graphite.

[0007] However, the main problem associated with lithium-sulfur batteries is that polysulfide lithium dissolves in the electrolyte, which is an intermediate product of sulfur generated during the discharge process. As the discharge progresses, sulfur (S8) continuously reacts with lithium ions, and thus its phase continuously changes to S8 → Li2S8 → (Li2S6) → Li2S4 → Li2S2 → Li2S, etc. Among them, Li2S8, Li2S4 (polysulfide lithium), etc., which are long chains of sulfur, have the property of being easily soluble in the common electrolytes used in lithium-ion batteries.

[0008] When these reactions occur, not only is the reversible capacity of the positive electrode greatly reduced, but the dissolved polysulfide lithium diffuses into the negative electrode and causes various side reactions. Especially during the charging process, the polysulfide lithium causes a shuttle reaction, resulting in a continuous increase in the charging capacity and a rapid deterioration of the charge / discharge efficiency. Recently, in order to solve this problem, as one of various methods, a method of improving the characteristics of the positive electrode has been proposed.

[0009] The method of improving the characteristics of the positive electrode is a method of forming a coating on the surface of the positive electrode particles to prevent the dissolution of polysulfides, adding a porous material capable of capturing the dissolved polysulfides, etc. Typically, the following methods have been proposed: a method of coating the surface of a positive electrode structure containing sulfur particles with a conductive polymer; a method of coating the surface of the positive electrode structure with a metal oxide on which lithium ions can transfer; a method of adding a porous metal oxide having a large specific surface area and large pore diameter to the positive electrode, the porous metal oxide being capable of adsorbing a large amount of polysulfide lithium; a method of attaching a functional group capable of adsorbing polysulfide lithium to the surface of a carbon structure; a method of using graphene or graphene oxide to wrap sulfur particles, etc.

[0010] Although such attempts are being made, these methods are not only somewhat complex, but also have the problem that the amount of the active material sulfur that can be added is limited. In particular, in the case of the method of adding a metal oxide to the positive electrode, due to the interaction with a binder having a carboxylate group (-COO - ) functional group, agglomeration may occur between the binder and the metal oxide, which may cause problems such as uneven distribution of the metal oxide, deterioration of the processability and storage properties of the positive electrode, and weakening of the adhesion force of the electrode.

[0011] [Prior Art Documents]

[0012] [Patent Document]

[0013] Japanese Patent Publication No. 2002 - 248348

[0014] Korean Patent Publication No. 10 - 2006 - 0054515 Summary of the Invention

[0015] [Technical Problem]

[0016] The inventors of the present invention have conducted various studies to solve the above problems, and as a result, it has been confirmed that when hydrogen phosphate ions are adsorbed on the surface of high - purity iron oxyhydroxide nitrate used as a positive electrode additive, the hydrogen phosphate ions (HPO4 2- ) repel the carboxylate groups (-COO - ) of the binder, thereby alleviating the agglomeration phenomenon between the binder and the additive, and enhancing the adhesion of the positive electrode for a lithium secondary battery, while having no adverse effect on the life characteristics and discharge capacity of the lithium secondary battery.

[0017] Therefore, an object of the present invention is to provide iron oxyhydroxide nitrate having hydrogen phosphate ions adsorbed on its surface and a method for preparing the same.

[0018] In addition, another object of the present invention is to provide a positive electrode for a lithium secondary battery, which comprises iron oxyhydroxide nitrate having hydrogen phosphate ions adsorbed on its surface as a positive electrode additive.

[0019] In addition, still another object of the present invention is to provide a lithium secondary battery, which comprises the positive electrode for a lithium secondary battery.

[0020] [Technical Solution]

[0021] To achieve the above object, the present invention provides iron oxyhydroxide nitrate having hydrogen phosphate ions (HPO4 2- ) adsorbed on its surface, and the iron oxyhydroxide nitrate has the following formula 1:

[0022] [Formula 1]

[0023] FeO(NO3) x (OH) 1-x (where 0 < x < 1).

[0024] In addition, the present invention provides a method for preparing iron oxyhydroxide nitrate having hydrogen phosphate ions (HPO4 2- ) adsorbed on its surface, and the method comprises the following steps:

[0025] (1) Prepare an Fe(NO3)3·9H2O solution by dissolving Fe(NO3)3·9H2O in a mixed solvent of an aqueous solvent and an organic solvent;

[0026] (2) Dry the Fe(NO3)3·9H2O solution to obtain iron oxyhydroxide nitrate having the following formula 1;

[0027] (3) Prepare an aqueous phosphoric acid solution with a pH of 6 to 12;

[0028] (4) Prepare a mixed solution by mixing the obtained iron oxyhydroxide nitrate having formula 1 with the aqueous phosphoric acid solution having a pH of 6 to 12; and

[0029] (5) After the reaction of the mixed solution is completed, recover the solid material and dry it,

[0030] [Formula 1]

[0031] FeO(NO3) x (OH) 1-x (where 0 < x < 1).

[0032] In addition, the present invention provides a positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising: a positive electrode current collector; and a positive electrode active material layer located on at least one surface of the positive electrode current collector,

[0033] wherein the positive electrode active material layer comprises a positive electrode active material, a conductive material, a binder, and an additive,

[0034] wherein the binder contains a carboxylate group, and

[0035] wherein the additive comprises the above-mentioned iron oxyhydroxide nitrate.

[0036] In addition, the present invention provides a lithium secondary battery, the lithium secondary battery comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte,

[0037] wherein the positive electrode is the positive electrode of the present invention.

[0038] [Beneficial effects]

[0039] The iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on the surface of the present invention can reduce the agglomeration phenomenon between the binder and the additive, because the hydrogen phosphate ions on the surface of the iron oxyhydroxide nitrate repel the carboxylate group (-COO-) functional groups of the binder, whereby the iron oxyhydroxide nitrate can be uniformly distributed in the positive electrode when used as a positive electrode additive, and can improve the adhesion of the positive electrode active material layer to the positive electrode current collector.

[0040] In addition, a positive electrode for a lithium secondary battery containing iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface and a binder containing a carboxylic acid ester group functional group has excellent manufacturing processability and storage performance, and has an excellent effect in terms of adhesion to a current collector. Description of the Drawings

[0041] Figure 1 Results of SEM-EDS analysis for Preparation Example 1.

[0042] Figure 2 FT-IR diagrams for Preparation Examples 1 to 7 and Comparative Preparation Example 1.

[0043] Figure 3 ζ potential diagrams for Preparation Example 1 and Comparative Preparation Example 1.

[0044] Figure 4 Particle size distribution diagram for Experimental Example 2.

[0045] Figure 5 Particle size (D 50 ) diagram showing changes over time for Experimental Example 2.

[0046] Figure 6 Particle size (D 90 ) diagram showing changes over time for Experimental Example 2.

[0047] Figure 7 Particle size diagram showing changes over time for Experimental Example 2.

[0048] Figure 8 Diagram derived from cyclic voltammetry (CV) for Experimental Example 3.

[0049] Figure 9 Diagram of the discharge capacity of the lithium-sulfur battery for Experimental Example 5.

[0050] Figure 10 and 11 Diagram of the discharge capacity of the lithium-sulfur battery for Experimental Example 6. Detailed Description of the Invention

[0051] Hereinafter, the present invention will be described in more detail.

[0052] Iron oxyhydroxide nitrate is used as a positive electrode additive for a lithium secondary battery, preferably a lithium-sulfur battery.

[0053] Iron oxyhydroxide nitrate can adsorb polysulfide lithium, thereby reducing the problem of polysulfide lithium transferring to the negative electrode and thus reducing the life of the lithium secondary battery, and can suppress the reduction in reactivity caused by polysulfide lithium, thereby increasing the discharge capacity of the lithium secondary battery and improving the life characteristics.

[0054] In addition, the binder for the positive electrode of a lithium secondary battery is a compound mainly containing a carboxylate group (-COO - ), and the carboxylate group can interact with the surface of iron oxyhydroxide nitrate as a transition metal oxide. Therefore, when forming a positive electrode active material layer by coating a slurry composition for forming a positive electrode active material layer in the form of a slurry on at least one surface of a current collector, there are the following problems: Agglomeration occurs between the binder and iron oxyhydroxide nitrate, whereby the distribution of iron oxyhydroxide nitrate in the positive electrode is uneven, and the manufacturing process and storage stability of the positive electrode deteriorate. In addition, because agglomeration occurs between iron oxyhydroxide nitrate and the binder, there are the following problems: It is difficult for the binder to function as a binder, whereby the adhesion between the current collector and the positive electrode active material layer is weakened, and the positive electrode active material layer peels off, etc.

[0055] Therefore, in the present invention, in order to solve the above problems, it is intended to adsorb hydrogen phosphate ions on the surface of iron oxyhydroxide nitrate and weaken the interaction by causing the surface of the hydrogen phosphate ion (HPO4 2- ) to repel the carboxylate group (-COO - ) of the binder, thereby solving the agglomeration phenomenon between the binder and the additive, and thereby improving the adhesion of the positive electrode active material layer to the current collector without reducing the life characteristics and discharge capacity of the lithium secondary battery.

[0056] Hydroxy oxidized ferric nitrate adsorbed with hydrogen phosphate ions (HPO4 2- )

[0057] That is, the present invention relates to iron oxyhydroxide nitrate on the surface of which hydrogen phosphate ions (HPO4 2- ) are adsorbed, and the iron oxyhydroxide nitrate has the following formula 1:

[0058] [Formula 1]

[0059] FeO(NO3) x (OH) 1-x (where 0 < x < 1).

[0060] On the surface of the iron oxyhydroxide nitrate having Formula 1, hydrogen phosphate ions are adsorbed as a monolayer, which means that the surface of the iron oxyhydroxide nitrate is modified. In addition, since the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxide nitrate in the form of a monolayer, it can be seen that a coating containing hydrogen phosphate ions is formed on the surface of the iron oxyhydroxide nitrate. If the hydrogen phosphate ions are formed as a monolayer instead of being composed of nanometer-thick multilayers, when the iron oxyhydroxide nitrate on the surface of which hydrogen phosphate ions are adsorbed is applied as an (positive electrode) additive to a lithium-sulfur battery, polysulfides may easily approach the surface of the iron oxyhydroxide nitrate, whereby polysulfides can be easily adsorbed.

[0061] The hydrogen phosphate ion can be H2PO4 - 、HPO4 2- and PO4 3- Among them, HPO4 2- .

[0062] Among several anions, the hydrogen phosphate ion has a strong binding force with iron ions exposed on the surface of ferric nitrate hydroxide as an iron oxide-based material, and thus can remove the sites capable of binding carboxylic ester groups (contained in the binder). In addition, the isoelectric point (IEP) of the surface of ferric nitrate hydroxide adsorbed with hydrogen phosphate ions on the surface is 5 or less, so that the surface is negatively charged in neutral and alkaline aqueous solutions, and thus can repel carboxylic ester groups. Therefore, the agglomeration phenomenon associated with the binder containing carboxylic ester groups can be solved, thereby improving the adhesion between the current collector and the positive electrode active material layer.

[0063] The average particle size of ferric nitrate hydroxide having the formula 1 can be 1 μm to 200 μm, preferably 10 μm to 100 μm, more preferably 20 μm to 40 μm. In this range, as the average particle size of the particles decreases, it is more suitable to be used as a positive electrode additive for a lithium secondary battery, preferably a lithium-sulfur battery. If the average particle size of the particles exceeds the above range, the particle size may be too large to be suitable as a positive electrode additive for a lithium-sulfur battery.

[0064] As described later, ferric nitrate hydroxide adsorbed with hydrogen phosphate ions on the surface can be an additive for the positive electrode of a lithium secondary battery, preferably an additive for the positive electrode of a lithium-sulfur battery. Therefore, the agglomeration phenomenon between the binder and the additive can be solved because the carboxylic ester groups of the binder containing carboxylic ester groups in the positive electrode and the hydrogen phosphate ions on the surface of ferric nitrate hydroxide repel each other. Therefore, ferric nitrate hydroxide adsorbed with hydrogen phosphate ions on the surface can be uniformly distributed in the positive electrode, and improve the manufacturing processability and storage performance of the positive electrode and the adhesion to the current collector.

[0065] Method for preparing iron hydroxyoxynitrate with hydrogen phosphate ions adsorbed on the surface

[0066] In addition, the present invention relates to a method for preparing ferric nitrate hydroxide adsorbed with hydrogen phosphate ions (HPO4 2- ) on the surface, and the method includes the following steps:

[0067] (1) Prepare a Fe(NO3)3·9H2O solution by dissolving Fe(NO3)3·9H2O in a mixed solvent of an aqueous solvent and an organic solvent;

[0068] (2) Dry the Fe(NO3)3·9H2O solution to obtain ferric nitrate hydroxide having the following formula 1;

[0069] (3) Prepare an aqueous phosphoric acid solution with a pH of 6 to 12;

[0070] (4) Prepare a mixed solution by mixing the obtained iron hydroxy nitrate of Formula 1 with the aqueous phosphoric acid solution having a pH of 6 to 12; and

[0071] (5) After the reaction of the mixed solution is completed, recover the solid material and dry it.

[0072] [Formula 1]

[0073] FeO(NO3) x (OH) 1-x (where 0 < x < 1).

[0074] Step (1) is a step of preparing an Fe(NO3)3·9H2O solution by dissolving Fe(NO3)3·9H2O in a mixed solvent of an aqueous solvent and an organic solvent.

[0075] The aqueous solvent may be water, preferably secondary distilled water (DW) or tertiary distilled deionized water (DIW). In addition, the organic solvent may be at least one selected from the following organic solvents: methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, and tert-butanol, preferably ethanol.

[0076] The aqueous solvent and the organic solvent may be mixed in a weight ratio of 30:70 to 70:30, specifically in a weight ratio of 40:60 to 60:40, preferably in a weight ratio of 50:50.

[0077] If the proportion of the aqueous solvent exceeds the above range, iron hydroxy nitrate represented by Formula 1 may not be formed. Therefore, the aqueous solvent and the organic solvent should be mixed and used within the above range.

[0078] In addition, the concentration of the Fe(NO3)3·9H2O solution may be 0.5 M to 2.5 M, preferably 1.5 M to 2 M. If the concentration of the solution is less than 0.5 M, the evaporation rate of the solution is slow, and the resulting iron hydroxy nitrate crystals may be large or the yield may decrease. If the concentration of the solution exceeds 2.5 M, the resulting iron hydroxy nitrate may agglomerate together, and thus it is not preferred to use it as an additive for the positive electrode of a lithium secondary battery.

[0079] Step (2) is a step of preparing iron hydroxy nitrate represented by Formula 1 by drying the Fe(NO3)3·9H2O solution prepared in step (1). The term "drying" is carried out under conditions higher than a specific temperature and may include the meaning of "heat treatment", etc.

[0080] Drying can be carried out at 70 °C to 90 °C, preferably 75 °C to 85 °C. Additionally, drying can be carried out for 18 hours to 36 hours, preferably 20 hours to 30 hours, within the above temperature range. If the drying temperature is lower than the above temperature or the drying time is short, because excessive moisture may remain in the reactant Fe(NO3)3·9H2O, and then the moisture will evaporate unevenly or residual reaction residues will remain during drying, thus it may not be possible to synthesize iron oxyhydroxide nitrate represented by Formula 1 according to the present invention.

[0081] Furthermore, if the drying temperature exceeds the above temperature or the drying time is long, after all the water in Fe(NO3)3·9H2O as a reactant has been evaporated, an oxidation reaction may partially occur due to drying. In this case, an uneven oxidation reaction may occur during the drying process, and the size of the generated particles may increase and appear in a lump form, thus it may not be possible to synthesize iron oxyhydroxide nitrate according to Formula 1 with the desired physical properties in the present invention.

[0082] For example, when preparing iron oxyhydroxide nitrate according to the present invention, if an aqueous solvent exceeding the above range is used and drying is carried out at a temperature exceeding the above range (e.g., 140 °C to 160 °C), Fe x O3 (where 1.7 < x < 2.0) may be generated instead of iron oxyhydroxide nitrate represented by Formula 1. Additionally, if an organic solvent exceeding the above range is used and drying is carried out at a temperature exceeding the above range (e.g., 140 °C to 160 °C), since Fe2O3 may be generated instead of iron oxyhydroxide nitrate represented by Formula 1, the drying temperature is appropriately adjusted within the above drying temperature range.

[0083] The drying pretreatment step can be carried out using a convection oven in an environment with sufficient air introduced.

[0084] Fe(NO3)3·9H2O undergoes a drying step to manufacture the material represented by Formula 1.

[0085] In Formula 1, x can vary with the drying time and temperature. Preferably, x can be 0.5 ≤ x < 1, more preferably 0.7 ≤ x < 1. In Formula 1, the smaller the value of x, the lower the stability of the generated iron oxyhydroxide nitrate. As the temperature increases during the drying step, the functional group (OH) contained in the iron oxyhydroxide nitrate undergoes thermal decomposition and is converted into water (H2O), and the structure of the iron oxyhydroxide nitrate may collapse, and when applied to a lithium secondary battery, preferably a lithium-sulfur battery, this is not ideal because water (H2O) will be electrolyzed during the charge / discharge process of the battery, thereby generating hydrogen gas (H2(g)).

[0086] The average particle size of the prepared iron oxyhydroxynitrate particles can be 1 μm to 200 μm, preferably 10 μm to 100 μm, and more preferably 20 μm to 40 μm. Within this range, as the average particle size of the particles decreases, it is suitable for use as an additive for the positive electrode of a lithium secondary battery, preferably a lithium-sulfur battery. If the average particle size of the particles exceeds the above range, the particle size may be too large to be suitable as an additive for the positive electrode of a lithium-sulfur battery.

[0087] Step (3) is a step of preparing an aqueous phosphoric acid solution with a pH of 6 to 12, that is, a step of preparing an aqueous phosphoric acid solution and adjusting the pH of the aqueous phosphoric acid solution to 6 to 12.

[0088] The concentration of the aqueous phosphoric acid solution can be 1×10 -5 M to 1×10 -1 M, preferably 1×10 -4 M to 5×10 -2 M. If the concentration of the aqueous phosphoric acid solution is lower than 1×10 -5 M, adsorption may not occur because the amount of hydrogen phosphate ions adsorbed on the surface is too small. If the concentration of the aqueous phosphoric acid solution exceeds 1×10 -1 M, there may be an excessive amount of unwanted hydrogen phosphate ions, which may be uneconomical.

[0089] An alkaline aqueous solution is added to the prepared aqueous phosphoric acid solution to adjust the pH of the aqueous phosphoric acid solution to 6 to 12, preferably 8 to 10. When the pH of the aqueous phosphoric acid solution is adjusted to 6 to 12, it may be possible to adsorb hydrogen phosphate ions on the surface without modifying the iron oxyhydroxynitrate having Formula 1 prepared in Step (2).

[0090] For example, if the pH is acidic, that is, less than 6, the following problems may occur: when using iron oxyhydroxynitrate adsorbed with hydrogen phosphate ions on the surface as an additive for the positive electrode of a lithium secondary battery, the iron oxyhydroxynitrate dissolves, and the pH of the slurry for the positive electrode containing the iron oxyhydroxynitrate may decrease, thereby potentially causing stability problems.

[0091] Specifically, hydrogen phosphate ions have different forms such as H2PO4 - and HPO4 2- according to the pH of the aqueous phosphoric acid solution. If the pH is less than 6, the hydrogen phosphate ions exist in the form of H2PO4 - . If the pH is 6 to 12, the hydrogen phosphate ions exist in the form of HPO4 2- . Therefore, if the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxynitrate at a pH less than 6, H2PO4 - exists on the surface of the iron oxyhydroxynitrate, and H2PO4 - is more than HPO42- contains more hydrogen ions (H + ). Therefore, if the iron hydroxy nitrate adsorbed with H2PO4 on the surface is redispersed in an aqueous solution, the pH of the aqueous solution may decrease. -

[0092] When the pH is 6 - 12, the slurry for forming the positive electrode active material layer containing the binder with a carboxylic acid ester group has a higher repulsive force as the negative charge of the hydrogen phosphate ions on the surface of the carboxylic acid ester group and the iron hydroxy nitrate further develops, resulting in high slurry stability. However, when the pH is less than 6, the repulsive force weakens, and agglomeration may occur between the binder and the additive.

[0093] Therefore, the hydrogen phosphate ions adsorbed on the surface of the iron hydroxy nitrate of the present invention can be HPO4 2- .

[0094] There is no particular limitation on the alkaline aqueous solution, but it can be at least one selected from the following: NaOH, LiOH, NH4OH, LiH2PO4, Li2HPO4, Li2SO4, and KOH, preferably LiOH.

[0095] Step (4) is a step of mixing the iron hydroxy nitrate having the formula 1 obtained in step (2) with the phosphoric acid aqueous solution having a pH of 6 - 12 prepared in step (3) to prepare a mixed solution. In step (4), specifically, the mixed solution can be prepared by adding the iron hydroxy nitrate in solid form to the phosphoric acid aqueous solution.

[0096] At this time, based on the total weight of the mixed solution, the content of the iron hydroxy nitrate having the formula 1 can be 10 wt% - 80 wt%, preferably 30 wt% - 50 wt%. If the content of the iron hydroxy nitrate is less than 10 wt%, excessive wastewater will be generated during the process of recovering the iron hydroxy nitrate adsorbed with hydrogen phosphate ions on the surface, which is inefficient. If the content of the iron hydroxy nitrate exceeds 80 wt%, the viscosity of the mixed solution increases, and the hydrogen phosphate ions may not be adsorbed evenly on the surface.

[0097] After step (4) and before performing step (5), it may further include a step of readjusting the pH of the mixed solution prepared in step (4) to 6 - 12.

[0098] When adding the iron hydroxy nitrate having the formula 1 to the phosphoric acid aqueous solution having a pH of 6 - 12 in step (4), since the pH of the solution may change, in order to maintain the pH, an alkaline aqueous solution is added to adjust the pH of the mixed solution back to 6 - 12. The alkaline aqueous solution is the same as that in step (3) above.

[0099] Step (5) is a step of recovering and drying the solid material after the reaction of the mixed solution prepared in step (4), that is, preparing iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface.

[0100] In addition, preferably, the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxide nitrate in a monolayer form, which means that the surface of the iron oxyhydroxide nitrate is modified by the hydrogen phosphate ions. In addition, since the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxide nitrate in a monolayer form, it can be seen that a coating containing hydrogen phosphate ions is formed on the surface of the iron oxyhydroxide nitrate.

[0101] The reaction of the mixed solution can be to stir the mixed solution with a magnetic stirrer for 1 hour to 24 hours, preferably stir with a magnetic stirrer for 13 hours to 18 hours. During the reaction time, the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxide nitrate having formula 1 prepared in step (2) to obtain iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on its surface.

[0102] The reaction can be carried out at room temperature. If the reaction time is less than 1 hour, the hydrogen phosphate ions may not be completely adsorbed on the surface of the iron oxyhydroxide nitrate. If the reaction time exceeds 24 hours, it may be uneconomical because it no longer reacts.

[0103] After the stirring is completed by the magnetic stirrer, the excess hydrogen phosphate ions can be removed to obtain iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on its surface. The present invention has no particular limitation on the method for removing the excess hydrogen phosphate ions, but the centrifugation method can be preferably adopted.

[0104] The iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on its surface is vacuum dried at 40°C to 80°C for 8 hours to 24 hours, preferably vacuum dried at 50°C to 70°C for 10 hours to 15 hours to finally obtain iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on its surface. If the temperature and time are lower than the above temperature and time range, a large amount of moisture may remain during the drying process, resulting in a decrease in purity. If the temperature and time exceed the above temperature and time range, agglomeration may occur between the particles, resulting in the formation of large particles or phase transformation.

[0105] Positive electrode for lithium secondary battery comprising iron hydroxyoxynitrate with hydrogen phosphate ions adsorbed on the surface

[0106] In addition, the present invention is a positive electrode for a lithium secondary battery, and the positive electrode for a lithium secondary battery includes: a positive electrode current collector; and a positive electrode active material layer located on at least one surface of the positive electrode current collector,

[0107] Wherein the positive electrode active material layer contains a positive electrode active material, a conductive material, a binder, and an additive,

[0108] wherein the binder contains a carboxylate group, and

[0109] wherein the additive contains iron hydroxyoxynitrate having hydrogen phosphate ions (HPO4 2- ) adsorbed on the surface (the above-mentioned), and the iron hydroxyoxynitrate has the following formula 1:

[0110] [Formula 1]

[0111] FeO(NO3) x (OH) 1-x (where 0 < x < 1).

[0112] The additive is the same as the iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on the surface, as described in "iron hydroxyoxynitrate having hydrogen phosphate ions adsorbed on the surface" above. Therefore, the detailed description of the additive is omitted here.

[0113] Relative to 100 parts by weight of the base solid material contained in the active material layer for a lithium secondary battery, the content of the additive can be 0.1 to 40 parts by weight, preferably 1 to 15 parts by weight, more preferably 5 to 10 parts by weight. If the additive is less than 0.1 part by weight, the effect of improving the reactivity of the additive with the positive electrode active material is insufficient. If the additive exceeds 40 parts by weight, the capacity of the positive electrode decreases, which is not desirable.

[0114] The base solid material contained in the active material layer for a lithium secondary battery refers to a solid material including a positive electrode active material, a binder, and a conductive material.

[0115] The binder is used to prevent the detachment of the positive electrode active material layer by increasing the adhesion between the positive electrode current collector and the positive electrode active material layer. The binder can be a material that is usually soluble in a solvent and can well form a conductive network between the positive electrode active material and the conductive material. Preferably, a compound containing a carboxylate group functional group is used as the binder because it can adhere to various substrates to increase the adhesion between the positive electrode active material and the current collector.

[0116] There is no particular limitation on the type of the binder as long as it contains a carboxylate group functional group. Preferably, poly(vinyl acetate), poly(methyl methacrylate), poly(ethyl acrylate), carboxymethyl cellulose, ethylene glycol series substances such as polyethylene glycol diacrylate and its derivatives, blends thereof, and copolymers thereof can be used, but are not limited thereto.

[0117] The content of the binder may be about 1 to 10% by weight, preferably about 3 to 7% by weight, based on the total weight of the base solid material. If the content of the binder is less than 1% by weight, the physical properties of the positive electrode are deteriorated, so that the positive electrode active material and the conductive material may be separated. If the content of the binder exceeds 10% by weight, the ratio of the active material and the conductive material in the positive electrode may be relatively reduced, thereby reducing the battery capacity.

[0118] The positive electrode active material may be at least one selected from the following: elemental sulfur (S8), an organic sulfur compound and a sulfur-carbon composite material, preferably a sulfur-carbon composite material. Therefore, the positive electrode for the lithium secondary battery of the present invention may be preferably a positive electrode for a lithium sulfur battery. Since the sulfur material alone is not conductive, it can be used in combination with a conductive material. The oxyhydroxide ferric nitrate with hydrogen phosphate ions adsorbed on the surface according to the present invention does not affect the retention of the sulfur-carbon composite structure.

[0119] Based on 100 parts by weight of the sulfur-carbon composite material, the sulfur-carbon composite material may contain 60 to 80 parts by weight of sulfur, preferably 70 to 75 parts by weight of sulfur. If the sulfur content is less than 60 parts by weight, the content of carbon material in the sulfur-carbon composite material is relatively increased. As the carbon content increases, the specific surface area increases, so when preparing the slurry, the amount of binder added should be increased. This increase in the amount of binder added may ultimately increase the sheet resistance of the electrode, and may act as an insulator to prevent electrons from passing through, thereby deteriorating battery performance. If the sulfur content exceeds 80 parts by weight, sulfur or sulfur compounds that are not combined with the carbon material may aggregate with each other or dissolve back into the surface of the carbon material, and may be difficult to directly participate in the electrode reaction because it is difficult to accept electrons. Therefore, the sulfur content is appropriately controlled within the above range.

[0120] The carbon in the sulfur-carbon composite material may have a porous structure or a high specific surface area, and may be any one of the porous carbon materials conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the following: graphite; graphene; carbon black such as Deco black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and activated carbon, and its shape can be used without restriction, as long as it is generally used in lithium secondary batteries in the form of spheres, rods, needles, plates, tubes, or blocks.

[0121] Based on the total weight of the base solid material, the content of the positive electrode active material can be 50 to 95% by weight, preferably 60 to 80% by weight. If the content of the positive electrode active material is less than 50% by weight, it is difficult to fully exhibit the reaction of the positive electrode. If the positive electrode active material exceeds 95% by weight, the content of the conductive material and the binder is relatively insufficient, making it difficult to fully exhibit the reaction of the positive electrode.

[0122] The conductive material is a material that electrically connects the electrolyte to the positive electrode active material and serves as a path for electrons to move from the current collector to the positive electrode active material, and there is no particular limitation as long as it has porosity and conductivity and does not cause chemical changes in the battery. For example, the following substances are used alone or in combination: graphite materials such as KS6; carbon black materials such as Super-P, carbon black, Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon derivatives such as fullerenes; conductive fibers such as carbon fibers and metal fibers; fluorocarbons, metal powders such as aluminum powder and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0123] Based on the total weight of the base solid material, the content of the conductive material can be 1 to 10% by weight, preferably 3 to 7% by weight. If the content of the conductive material is less than 1% by weight, the unreacted portion of the positive electrode active material increases, resulting in a decrease in battery capacity. If the conductive material exceeds 10% by weight, this is undesirable because it has a negative impact on the high-efficiency discharge characteristics and charge / discharge cycle life.

[0124] In order to form a positive electrode active material layer in the positive electrode for a lithium secondary battery, it is necessary to make a slurry state of the base solid material containing the positive electrode active material, the conductive material, the binder, and the additive. To make a slurry state, an additional solvent is included, and most preferably, the solvent is easily dried and can dissolve the binder well, but keeps the positive electrode active material, the conductive material, and the unreacted fine particles in a dispersed state without dissolving them.

[0125] The solvent according to the present invention can be water or an organic solvent. The organic solvent can be an organic solvent including at least one selected from the following: dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran.

[0126] By a conventional method using a conventional mixer such as a paste mixer, a high-speed shear mixer, or a homogenizer, it is possible to mix the positive electrode active material, the binder, the conductive material, iron hydroxyoxynitrate having the formula 1 adsorbed on the surface, and the solvent.

[0127] In addition to the iron hydroxyoxide nitrate with hydrogen phosphate ions adsorbed on its surface, the positive electrode active material layer of the present invention may additionally contain, as needed, additives commonly used in the relevant art to improve its functions. For example, a viscosity regulator, a fluidizing agent, a filler, etc. may be additionally contained.

[0128] The positive electrode can be manufactured by methods known in the art. For example, the positive electrode can be prepared by coating a slurry on a positive electrode current collector and roll-pressing to manufacture a positive electrode for a lithium secondary battery, and the current collector can be coated at an appropriate thickness according to the thickness of the positive electrode to be formed.

[0129] There is no limitation on the method of coating the slurry on the current collector. For example, coating methods such as blade coating, dip coating, gravure coating, slot die coating, spin coating, comma coating, bar coating, reverse roll coating, screen coating, and cap coating can be used.

[0130] The positive electrode current collector can generally be made to have a thickness of 3 to 500 μm, and there is no particular limitation as long as it has high conductivity while carrying the positive electrode active material and does not cause chemical changes in the battery. For example, a conductive metal such as stainless steel, aluminum, copper, or titanium can be used as the positive electrode current collector, and an aluminum current collector is preferably used. The positive electrode current collector can be formed in various forms such as a film, sheet, foil, mesh, porous body, foam, or non-woven fabric.

[0131] Lithium secondary battery

[0132] In addition, the present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0133] The positive electrode can be the positive electrode for the lithium secondary battery of the present invention as described above, and the lithium secondary battery can be a lithium-sulfur battery.

[0134] The negative electrode can be composed of a current collector and a negative electrode active material layer formed on one or both of its surfaces. Additionally, the negative electrode can be a lithium metal plate.

[0135] The current collector is used to carry the negative electrode active material, and there is no particular limitation as long as it is electrochemically stable within the voltage range of the lithium secondary battery while having excellent conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; or copper or stainless steel with a surface treated with carbon, nickel, silver, etc.; or an aluminum-cadmium alloy, etc. can be used.

[0136] The negative electrode current collector can enhance the bonding force with the negative electrode active material by having fine concavities and convexities on its surface, and can be formed in various forms such as a film, sheet, foil, sieve, mesh, porous body, foam, or non-woven fabric.

[0137] The negative electrode active material may include a material capable of reversibly inserting or extracting lithium ions, a material capable of reacting with lithium ions to reversibly form a lithium-containing compound, or lithium metal or a lithium alloy.

[0138] The material capable of reversibly inserting or extracting lithium ions may be, for example, crystalline carbon, amorphous carbon, or a mixture thereof.

[0139] The material capable of reacting with lithium ions to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon.

[0140] The lithium alloy may be, for example, an alloy of lithium (Li) and a metal selected from the following: sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0141] A separator is further included between the positive electrode and the negative electrode. The separator can allow lithium ions to be transmitted between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. The separator may be made of a porous non-conductive or insulating material. The separator may be an independent member such as a film or a coating added to the positive electrode and / or the negative electrode.

[0142] The materials constituting the separator include, but are not limited to, for example: polyolefins such as polyethylene and polypropylene; glass fiber filter paper; and ceramic materials, and the thickness thereof may be about 5 μm to about 50 μm, preferably about 5 μm to about 25 μm.

[0143] The electrolyte is composed of a lithium salt and an electrolyte as a non-aqueous electrolyte containing the lithium salt. As the electrolyte, a non-aqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

[0144] The lithium salt can be used without limitation as long as it is commonly used in the electrolyte of a lithium-sulfur battery. For example, at least one selected from the following may be used: LiSCN, LiBr, LiI, LiPF6, LiBF4, LiB 10 Cl 10 , LiSO3CF3, LiCl, LiClO4, LiSO3CH3, LiB(Ph)4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiFSI, lithium chloroborane, lithium lower aliphatic carboxylate, etc.

[0145] In addition, the concentration of the lithium salt in the electrolyte can be 0.2 M to 2 M, preferably 0.6 M to 2 M, and more preferably 0.7 M to 1.7 M. If the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte may decrease, and thus the performance of the electrolyte may deteriorate. If the concentration of the lithium salt exceeds 2 M, the viscosity of the electrolyte may increase, and thus the mobility of lithium ions may decrease.

[0146] The non-aqueous organic solvent should dissolve the lithium salt well, and the non-aqueous organic solvents of the present invention may include, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxane, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate, and these organic solvents can be used alone or as a mixed solvent of two or more of them.

[0147] As the organic solid electrolyte, for example, polyethylene derivatives, poly(ethylene oxide) derivatives, poly(propylene oxide) derivatives, phosphate polymers, poly(alginate-lysine), polyester sulfides, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ionic dissociation groups can be used.

[0148] As the inorganic solid electrolyte, for example, nitrides, halides, sulfates, etc. of Li such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, Li3PO4-Li2S-SiS2 can be used.

[0149] In order to improve the charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (poly)glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and trifluoroethylene can also be added, and in order to improve the storage characteristics at high temperatures, carbon dioxide gas can also be included, and FEC (fluoroethylene carbonate), PRS (propylene sulfonic acid lactone), FPC (fluoropropyl carbonate), etc. can also be included.

[0150] The electrolyte can be used as a liquid electrolyte or also in the form of a solid electrolyte separator. When used as a liquid electrolyte, it also includes a separator made of porous glass, plastic, ceramic, or polymer as a physical separator having the function of physically separating the electrodes.

[0151] Lithium secondary batteries can be classified into cylindrical, square, coin-shaped, and pouch-shaped according to shape, and can be classified into block type and thin film type according to size. The structures and manufacturing methods of these batteries are well known in the art, so their detailed descriptions are omitted.

[0152] The lithium secondary battery of the present invention includes iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions on the surface as an additive to the positive electrode, without affecting the initial discharge capacity and life characteristics of the battery. Therefore, the adhesion between the current collector and the positive electrode active material layer can be improved while maintaining the initial discharge capacity and life characteristics of the battery.

[0153] Examples

[0154] Hereinafter, the present invention will be described in detail with reference to examples for a specific illustration of the present invention. However, the embodiments according to the present invention can be modified in many different forms, and the scope of the present invention should not be construed as limited to the examples described below. The examples of the present invention are provided to more comprehensively describe the present invention to those skilled in the art.

[0155] [Preparation Example 1] Preparation of iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions on the surface

[0156] An 1.8 M solution was prepared by dissolving 75 g of Fe(NO3)3·9H2O (Sigma-Aldrich company) in a mixed solvent of 50 g of DIW (deionized water) and 50 g of ethanol. The prepared solution was placed in a glass bath, air was sufficiently injected in a convection oven, and dried at 80 °C for 24 hours to obtain iron hydroxyoxynitrate having the formula FeO(NO3) x (OH) 1-x (where 0.5 ≤ x < 1).

[0157] Mix 568 μL of an 85 wt% aqueous solution of phosphoric acid (H3PO4) with 250 mL of distilled water to prepare an aqueous phosphoric acid solution with a concentration of 3.3×10 -2 M.

[0158] Adjust the pH of the aqueous phosphoric acid solution to 9.7 by adding an aqueous solution of lithium hydroxide (LiOH) with a concentration of 1.25 M.

[0159] Add 1 g of iron(III) hydroxide nitrate to the aqueous phosphoric acid solution with a pH of 9.7 to prepare a mixed solution, and adjust the pH of the mixed solution to 9.7 by adding an aqueous solution of lithium hydroxide with a concentration of 1.25 M.

[0160] Magnetically stir the mixed solution with a pH of 9.7 for 16 hours to adsorb hydrogen phosphate ions on the surface of iron(III) hydroxide nitrate.

[0161] Thereafter, remove the remaining unreacted excess hydrogen phosphate ions by centrifugation and recover the solid material. Vacuum dry the recovered solid material at 60 °C for 12 hours to prepare monolayer iron(III) hydroxide nitrate with hydrogen phosphate ions (HPO4 2- ) adsorbed on its surface.

[0162] The average particle size of iron(III) hydroxide nitrate is 25 μm.

[0163] Perform SEM-EDS analysis to confirm whether hydrogen phosphate ions (HPO4 2- ) are adsorbed on the surface of iron(III) hydroxide nitrate. As a result, it is confirmed that phosphorus elements are distributed on the surface of iron(III) hydroxide nitrate particles. Therefore, it can be seen that hydrogen phosphate ions are adsorbed on the surface of iron(III) hydroxide nitrate ( Figure 1 ).

[0164] [Preparation Example 2] Preparation of iron(III) hydroxide nitrate with hydrogen phosphate ions adsorbed on its surface

[0165] Except for using an aqueous phosphoric acid solution with a concentration of 8.2×10 -3 M instead of the 3.3×10 -2 M aqueous phosphoric acid solution used in Preparation Example 1, iron(III) hydroxide nitrate with hydrogen phosphate ions (HPO4 2- ) adsorbed on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the iron(III) hydroxide nitrate was 25 μm.

[0166] [Preparation Example 3] Preparation of iron(III) hydroxide nitrate with hydrogen phosphate ions adsorbed on its surface

[0167] Except for using an aqueous phosphoric acid solution with a concentration of 6.2×10 -3 M instead of the 3.3×10 -2Except for using an aqueous phosphoric acid solution of M, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the hydroxyl iron nitrate was 25 μm.

[0168] [Preparation Example 4] Preparation of hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on its surface

[0169] Except for using an aqueous phosphoric acid solution of 4.1×10 -3 M instead of the aqueous phosphoric acid solution of 3.3×10 -2 M used in Preparation Example 1, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the hydroxyl iron nitrate was 25 μm.

[0170] [Preparation Example 5] Preparation of hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on its surface

[0171] Except for using an aqueous phosphoric acid solution of 1.2×10 -3 M instead of the aqueous phosphoric acid solution of 3.3×10 -2 M used in Preparation Example 1, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the hydroxyl iron nitrate was 25 μm.

[0172] [Preparation Example 6] Preparation of hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on its surface

[0173] Except for using an aqueous phosphoric acid solution of 8.2×10 -4 M instead of the aqueous phosphoric acid solution of 3.3×10 -2 M used in Preparation Example 1, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the hydroxyl iron nitrate was 25 μm.

[0174] [Preparation Example 7] Preparation of hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on its surface

[0175] Except for using an aqueous phosphoric acid solution of 4.1×10 -4 M instead of the aqueous phosphoric acid solution of 3.3×10 -2 M used in Preparation Example 1, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on its surface was prepared in the same manner as in Preparation Example 1, and the average particle size of the hydroxyl iron nitrate was 25 μm.

[0176] [Comparative Preparation Example 1] Preparation of iron hydroxyoxynitrate

[0177] An 1.8 M solution was prepared by dissolving 75 g of Fe(NO3)3·9H2O (Sigma-Aldrich company) in a mixed solvent of 50 g of DIW (deionized water) and 50 g of ethanol. The prepared solution was placed in a glass bath, air was thoroughly introduced in a convection oven, and it was dried at 80 °C for 24 hours to obtain iron hydroxyoxynitrate with the formula FeO(NO3) x (OH) 1-x (where 0.5 ≤ x < 1). The average particle size of the iron hydroxyoxynitrate was 25 μm.

[0178] [Comparative Preparation Example 2] Preparation of iron oxide adsorbed with hydrogen phosphate ions on the surface

[0179] An aqueous phosphoric acid solution with a concentration of 8.2×10 -4 M was prepared by mixing an 85 wt% aqueous solution of phosphoric acid (H3PO4) with distilled water.

[0180] The pH of the aqueous phosphoric acid solution was adjusted to 9.7 by adding a 1.25 M aqueous solution of lithium hydroxide (LiOH).

[0181] 1 g of iron oxide was added to the aqueous phosphoric acid solution with a pH of 9.7 to prepare a mixed solution, and the pH of the mixed solution was adjusted to 9.7 again by adding a 1.25 M aqueous solution of lithium hydroxide.

[0182] The mixed solution with a pH of 9.7 was magnetically stirred for 16 hours to adsorb hydrogen phosphate ions on the surface of the iron oxide.

[0183] Thereafter, the remaining unreacted excess hydrogen phosphate ions were removed by centrifugation, and the solid material was recovered. The recovered solid material was vacuum dried at 60 °C for 12 hours to prepare iron oxide adsorbed with a monolayer of hydrogen phosphate ions (HPO4 2- ).

[0184] [Comparative Preparation Example 3] Preparation of iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions on the surface

[0185] Except that the pH of the aqueous phosphoric acid solution and the mixed solution was adjusted to 4.7, iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions on the surface was prepared in the same manner as in Preparation Example 1, and the hydrogen phosphate ion was H2PO4 - . The average particle size of the iron hydroxyoxynitrate was 25 μm.

[0186] [Experimental Example 1] Measurement of the Adsorption of Hydrogen Phosphate Ions on the Surface of Iron Hydroxynitrate

[0187] FT-IR (Agilent Technologies, Cary-630) was used to confirm whether hydrogen phosphate ions were adsorbed on the surface of the iron hydroxynitrate prepared in Preparation Examples 1 to 7 and Comparative Preparation Example 1.

[0188] As a result of FT-IR, in the case of Preparation Examples 1 to 7, a broad band in the range of 900 cm -1 to 1150 cm -1 was observed ( Figure 2 ). Since the bands in this region are caused by hydrogen phosphate ions, it was confirmed that hydrogen phosphate ions were adsorbed on the surface of the iron hydroxynitrate in the case of Preparation Examples 1 to 7.

[0189] In addition, the surface charge of the iron hydroxynitrate prepared in Preparation Example 1 and Comparative Preparation Example 1 was measured using ζ-potential (Malvern Panalytical company, Zetasizer Nano ZS90). For the measurement, an aqueous sample was used, and the pH of the aqueous solution was adjusted to 7 using an aqueous phosphoric acid solution or an aqueous lithium hydroxide solution. The ζ-potential was measured 3 times repeatedly, and the results are shown in Table 1 and Figure 3 .

[0190] Table 1:

[0191] Preparation Example 1 Comparative Preparation Example 1 First measurement -16.2 mV 3.14 mV Second measurement -18.1 mV 5.19 mV Third measurement -17.9 mV 4.35 mV Average value -17.4 mV 4.2 mV

[0192] Based on the above results, it was confirmed that Preparation Example 1 had a surface negative charge, while Comparative Preparation Example 1 had a surface positive charge.

[0193] That is, it was confirmed that in the case of Preparation Example 1, a coating made of hydrogen phosphate ions was formed on the surface of the iron hydroxynitrate, while in the case of Comparative Preparation Example 1, no such coating was formed. This was expected for Preparation Example 1 due to the fact that the hydrogen phosphate ions adsorbed on the surface changed the isoelectric point of the iron hydroxynitrate.

[0194] [Experimental Example 2] Observation of the Degree of Agglomeration with the Binder

[0195] After mixing 7.6 wt% of Li-AG having the formula 2 (Sumitomo Seika Chemicals Co., Ltd., Japan), a binder solution (2.63 g, 0.2 g of solid material), 0.2 g of an additive, and 5 g of water (DIW), the mixture was reacted for 10 minutes by 80 g of acoustic mixing to prepare a solution in which the binder and the additive were mixed, and the degree of agglomeration of the additive and the binder was observed using a particle size analyzer (Microtrac company, S3500).

[0196] [Formula 2]

[0197]

[0198] At this time, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on the surface prepared in Preparation Example 1, hydroxyl iron nitrate in Comparative Preparation Example 1, and hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (H2PO4 - ) on the surface in Comparative Preparation Example 3 were used to prepare various mixed solutions.

[0199] In Figure 4 's results, in the case of Comparative Preparation Example 3, two peaks were observed. This means that due to the low pH (4.7), hydrogen phosphate ions are adsorbed onto hydroxyl iron nitrate in the form of H2PO4 - , causing changes in hydroxyl iron nitrate, and it can be seen that the agglomeration phenomenon with the binder is enhanced due to the weak repulsive force with the carboxylate group.

[0200] On the other hand, similar peaks were observed in Preparation Example 1 and Comparative Preparation Example 1. These peaks were observed in more detail by the change in particle size over time.

[0201] Figures 5 to 7 As a result of measuring the change in particle size over time in Preparation Example 1 and Comparative Preparation Example 1, this shows that compared with hydroxyl iron nitrate without hydrogen phosphate ions on the surface in Comparative Preparation Example 1, even after a period of time, hydroxyl iron nitrate adsorbed with hydrogen phosphate ions on the surface in Preparation Example 1 shows a small particle size at all particle size indices (D 50 , D 90 , average value (MV)). Thus, it was confirmed that compared with Comparative Preparation Example 1, Preparation Example 1 significantly reduced the agglomeration phenomenon with the binder.

[0202] That is, it can be seen that when hydroxyl iron nitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) on the surface is used together with a binder containing a carboxylate group functional group, because hydrogen phosphate ions (HPO4 2- ) repel the carboxylate group, the agglomeration phenomenon with the binder can be solved.

[0203] [Experimental Example 3] CV Measurement of Hydroxyl Iron Nitrate

[0204] Cyclic voltammetry (CV) analysis was performed to confirm the electroactivity of hydroxyl iron nitrate in Preparation Example 1 and Comparative Preparation Example 1.

[0205] Specifically, a catalyst electrode containing ferric oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on the surface prepared in Preparation Example 1, a conductive material (carbon nanotube), and an adhesive (PAA) in a weight ratio of 6:2:2 was prepared. The loading amount of the catalyst of the electrode was about 1 mg / cm 2 . A coin cell having a structure including the catalyst electrode, an electrolyte solution, and a Li electrode was fabricated, and charge / discharge was performed for 5 cycles.

[0206] After completing the cycles in the charged state, the coin cell was disassembled to recover the catalyst electrode, and a cell having a symmetric structure including the catalyst electrode, an electrolyte solution, and the catalyst electrode was prepared using the recovered catalyst electrode. At this time, as the electrolyte solution used in the cell having the symmetric structure, an electrolyte solution containing 0.53 M of Li2S6 was used. After wetting for 1 day, CV was measured under the conditions of scan rate = 3 mV / S and voltage window = -1 V to +1 V.

[0207] In addition, CV was measured in the same manner as above by using ferric oxyhydroxide nitrate of Comparative Preparation Example 1 instead of ferric oxyhydroxide nitrate of Preparation Example 1.

[0208] The results are shown in Figure 8 , and it was confirmed that Preparation Example 1 showed a higher current density than Comparative Preparation Example 1. This means that Preparation Example 1 can react with polysulfide (Li2S6) contained in the electrolyte solution at a faster rate than Comparative Preparation Example 1, and showed a result that Preparation Example 1 has very good catalytic activity compared with Comparative Preparation Example 1.

[0209] <Manufacture of Positive Electrode for Lithium-Sulfur Battery>

[0210] [Example 1]

[0211] First, based on the total weight (100 parts by weight) of the base solid materials (active material, conductive material, and adhesive), 10 parts by weight of ferric oxyhydroxide nitrate containing a coating containing hydrogen phosphate ions on the surface prepared in Preparation Example 1 was added to water as a solvent and dissolved.

[0212] Thereafter, based on the obtained solution, 100 parts by weight of the base solid materials were added, namely 84 parts by weight of a sulfur-carbon composite material (S / C 75:25 parts by weight) as an active material, 5 parts by weight of GCNT as a conductive material, 2.5 parts by weight of lithiated carboxymethyl cellulose (LiCMC) as an adhesive, and 3.5 parts by weight of styrene-butadiene (SBR) and mixed to prepare a slurry composition for the positive electrode.

[0213] Subsequently, the prepared slurry composition was coated on a current collector (aluminum foil), dried at 50 °C for 12 hours, and pressed with a rolling device to fabricate a positive electrode for a lithium-sulfur battery. At this time, the loading amount was 5.5 mAh / cm 2 , and the porosity of the electrode was 70%.

[0214] [Example 2]

[0215] Except for using the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 2 instead of the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, the positive electrode for a lithium-sulfur battery of Example 2 was fabricated in the same manner as in Example 1.

[0216] [Example 3]

[0217] Except for using the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 3 instead of the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, the positive electrode for a lithium-sulfur battery of Example 3 was fabricated in the same manner as in Example 1.

[0218] [Example 4]

[0219] Except for using the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 4 instead of the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, the positive electrode for a lithium-sulfur battery of Example 4 was fabricated in the same manner as in Example 1.

[0220] [Example 5]

[0221] Except for using the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 5 instead of the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, the positive electrode for a lithium-sulfur battery of Example 5 was fabricated in the same manner as in Example 1.

[0222] [Example 6]

[0223] Except for using the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 6 instead of the iron hydroxyoxynitrate adsorbed with hydrogen phosphate ions (HPO42- ) In addition to iron hydroxyoxynitrate with adsorbed hydrogen phosphate ions, a positive electrode for a lithium-sulfur battery of Example 6 was produced in the same manner as in Example 1.

[0224] [Example 7]

[0225] Except for using iron hydroxyoxynitrate with adsorbed hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 7 instead of the iron hydroxyoxynitrate with adsorbed hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, a positive electrode for a lithium-sulfur battery of Example 7 was produced in the same manner as in Example 1.

[0226] [Example 8]

[0227] First, based on the total weight (100 parts by weight) of the base solid materials (active material, conductive material, and binder) of iron hydroxyoxynitrate with a coating containing hydrogen phosphate ions on the surface prepared in Preparation Example 6, 5 parts by weight of iron hydroxyoxynitrate with a coating containing hydrogen phosphate ions on the surface prepared in Preparation Example 6 was added to water as a solvent and dissolved.

[0228] Thereafter, based on the obtained solution, 100 parts by weight of the base solid materials were added, namely 91 parts by weight of a sulfur-carbon composite material (S / C 75:25 parts by weight) as an active material, 1.5 parts by weight of lithiated carboxymethyl cellulose (LiCMC) as a binder, and 2.5 parts by weight of styrene-butadiene (SBR) and mixed to prepare a slurry composition for the positive electrode.

[0229] Subsequently, the prepared slurry composition was coated on a current collector (aluminum foil), dried at 50 °C for 12 hours, and pressed with a rolling press to produce a positive electrode for a lithium-sulfur battery. At this time, the loading amount was 3.5 mAh / cm 2 , and the porosity of the electrode was 70%.

[0230] [Comparative Example 1]

[0231] Except for using iron hydroxyoxynitrate prepared in Comparative Preparation Example 1 instead of the iron hydroxyoxynitrate with adsorbed hydrogen phosphate ions (HPO4 2- ) prepared in Preparation Example 1 above, a positive electrode for a lithium-sulfur battery of Comparative Example 1 was produced in the same manner as in Example 1.

[0232] [Comparative Example 2]

[0233] Except for using iron oxide with adsorbed hydrogen phosphate ions (HPO4 2- ) prepared in Comparative Preparation Example 2 instead of the iron oxide with adsorbed hydrogen phosphate ions (HPO4 prepared in Preparation Example 6 above,2- ) In addition to iron hydroxynitrate with adsorbed hydrogen phosphate ions, a positive electrode for a lithium-sulfur battery of Comparative Example 2 was manufactured in the same manner as in Example 8.

[0234] [Experimental Example 4] Measurement of Adhesion of Positive Electrode for Lithium-Sulfur Battery

[0235] The adhesion of the positive electrodes for lithium-sulfur batteries prepared in Examples 1 to 7 and Comparative Example 1 was measured. The adhesion of the positive electrode was measured by using a peel test device to pull the electrode in the 90-degree direction and measuring the force when the positive electrode active material layer detached, and the results are shown in Table 2 below. The adhesion refers to the adhesion of the positive electrode active material layer to the positive electrode current collector.

[0236] Table 2:

[0237] Adhesion force (gf / cm) Example 1 24.3 Example 2 21 Example 3 21.7 Example 4 16.9 Example 5 17.1 Example 6 14.4 Example 7 11.5 Comparative Example 1 1.5

[0238] Examples 1 to 7 contain iron hydroxynitrate with adsorbed hydrogen phosphate ions (HPO4 2- ) as an additive, and Comparative Example 1 contains iron hydroxynitrate without adsorbed hydrogen phosphate ions on its surface.

[0239] In the results of Table 2, Examples 1 to 7 showed an adhesion force more than about 10 times that of Comparative Example 1, and the higher the concentration of hydrogen phosphate ions, the stronger the adhesion force.

[0240] From the above results, it was found that the hydrogen phosphate ions adsorbed on the surface repel the carboxylate group of the binder, weakening the interaction, thereby improving the adhesion function of the binder. On the other hand, it was found that when hydrogen phosphate ions are not adsorbed on the surface, the surface of iron hydroxynitrate interacts with the carboxylate group of the binder, thereby deteriorating the adhesion function of the binder.

[0241] [Experimental Example 5] Evaluation of Charge / Discharge Characteristics of Lithium-Sulfur Battery (1)

[0242] A polyethylene separator was punched to 19φ, lithium metal with a thickness of 45 μm as the negative electrode was punched to 16φ, the positive electrodes prepared in Examples 1 to 7 and Comparative Example 1 were respectively punched to 14φ, and various coin-type lithium-sulfur batteries of Examples 1 to 7 and Comparative Example 1 were manufactured by using an organic solvent dissolved with a lithium salt as the electrolyte.

[0243] The discharge capacity of the fabricated coin-type lithium-sulfur battery was measured from 1.8 V to 2.5 V using a charge / discharge measurement device (PESC05-0.01, PNE solution, Korea). After charging / discharging for 3 cycles at a current density of 0.1 C and then charging / discharging for 3 cycles at 0.3 C, the charge / discharge cycle was performed according to the experimental method of charging at 0.3 C and discharging at 0.5 C, and the results are shown in Table 3 below and Figure 9 in.

[0244] Table 3:

[0245]

[0246] Example 1 showed an initial discharge capacity of 1121 mAh / g at 0.1 C, Examples 2-7 showed an initial discharge capacity of 1152-1166 mAh / g at 0.1 C, and Comparative Example 1 showed an initial discharge capacity of 1170 mAh / g at 0.1 C, thus showing a higher discharge capacity for Comparative Example 1. However, the discharge capacity of the 3rd cycle at 0.1 C was 954 mAh / g for Comparative Example 1 and 981-1006 mAh / g for Examples 2-7, indicating a higher discharge capacity. In addition, even at the 0.5 C discharge capacity, Examples 2-7 also showed a higher discharge capacity than Comparative Example 1. Although an aqueous phosphoric acid solution with a higher concentration than that of Examples 2-7 was used, it was expected that Example 1 would have a lower discharge capacity than Examples 2-7.

[0247] From the above results, it was found that when an appropriate amount of hydrogen phosphate ions was adsorbed on the surface of iron oxyhydroxide nitrate, the capacity of the positive electrode active material was higher than that of iron oxyhydroxide nitrate on which hydrogen phosphate ions were not adsorbed on the surface.

[0248] Therefore, it was confirmed that iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions on the surface improved the initial discharge capacity of the battery while increasing the adhesion of the positive electrode active material layer to the current collector.

[0249] [Experimental Example 6] Evaluation of Charge / Discharge Characteristics of Lithium-Sulfur Battery (2)

[0250] A polyethylene separator was punched to 19φ, lithium metal with a thickness of 45 μm as the negative electrode was punched to 16φ, the positive electrodes prepared in Example 8 and Comparative Example 2 were respectively punched to 14φ, and various coin-type lithium-sulfur batteries of Example 8 and Comparative Example 2 were fabricated by using an organic solvent dissolved with a lithium salt as the electrolyte.

[0251] The discharge capacity of the fabricated coin-type lithium-sulfur battery from 1.8 V to 2.5 V was measured using a charge / discharge measurement device (PESC05-0.01, PNE solution, Korea). After charging / discharging 3 cycles at a current density of 0.1 C and then charging / discharging 3 cycles at 0.3 C, the charge / discharge cycles were performed according to the experimental method of charging at 0.3 C and discharging at 0.5 C, and the results are shown in Table 4 below and Figure 10 and 11 in

[0252] Table 4:

[0253]

[0254] As a result of the performance test of the coin-type batteries of Example 8 and Comparative Example 2, as Figure 10 and 11 shown, it was confirmed that iron oxyhydroxide nitrate adsorbed with hydrogen phosphate ions was more excellent than iron oxide adsorbed with hydrogen phosphate ions in terms of the first discharge capacity and the third discharge capacity at 0.1 C and the discharge capacity at 0.5 C.

Claims

1. An iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface, wherein the iron oxyhydroxide nitrate has the following formula 1: [Formula 1] FeO(NO3) x (OH) 1-x where 0 < x < 1, wherein the iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface is an additive for a positive electrode of a lithium secondary battery, the positive electrode includes a binder, and the binder contains a carboxylate group.

2. The iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface according to claim 1, wherein the hydrogen phosphate ions are adsorbed on the surface of the iron oxyhydroxide nitrate in a monolayer form.

3. The iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface according to claim 1, wherein the average particle size of the iron oxyhydroxide nitrate having formula 1 is 1 μm to 200 μm.

4. A method for preparing the iron oxyhydroxide nitrate with hydrogen phosphate ions adsorbed on its surface according to claim 1, the method comprising the following steps: (1) Prepare an Fe(NO3)3·9H2O solution by dissolving Fe(NO3)3·9H2O in a mixed solvent of an aqueous solvent and an organic solvent; (2) Dry the Fe(NO3)3·9H2O solution to obtain an iron oxyhydroxide nitrate having the following formula 1; (3) Prepare an aqueous phosphoric acid solution with a pH of 6 to 12; (4) Prepare a mixed solution by mixing the obtained iron oxyhydroxide nitrate having formula 1 with the aqueous phosphoric acid solution with a pH of 6 to 12; and (5) After the reaction of the mixed solution is completed, recover the solid material and dry it, [Formula 1] FeO(NO3) x (OH) 1-x where 0 < x < 1.

5. The method according to claim 4, wherein the concentration of the Fe(NO3)3·9H2O solution in step (1) is 0.5 M to 2.5 M.

6. The method according to claim 4, wherein the aqueous solvent and the organic solvent in step (1) are mixed at a weight ratio of 30:70 to 70:

30.

7. According to the method of claim 4, wherein the concentration of the aqueous phosphoric acid solution in step (3) is 1×10 -5 M to 1×10 -1 M.

8. The method according to claim 4, wherein the content of the iron oxyhydroxide nitrate having formula 1 is 10% by weight to 80% by weight relative to the total weight of the mixed solution in step (4).

9. The method according to claim 4, wherein after step (4), the method further comprises a step of readjusting the pH of the mixed solution to 6 to 12.

10. A positive electrode for a lithium secondary battery, the positive electrode for a lithium secondary battery comprising: a positive electrode current collector; and a positive electrode active material layer located on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer includes a positive electrode active material, a conductive material, a binder, and an additive, wherein the binder contains a carboxylate group, and wherein the additive includes the iron oxyhydroxide nitrate according to claim 1.

11. The positive electrode for a lithium secondary battery according to claim 10, wherein the content of the additive is 0.1 part by weight to 40 parts by weight relative to 100 parts by weight of the base solid material contained in the positive electrode active material layer.

12. The positive electrode for a lithium secondary battery according to claim 10, wherein the positive electrode active material includes at least one selected from the following: elemental sulfur S8, organic sulfur compounds, and sulfur-carbon composite materials.

13. A lithium secondary battery, comprising: a positive electrode; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, wherein the positive electrode is the positive electrode according to claim 10.

14. The lithium secondary battery according to claim 13, wherein the lithium secondary battery is a lithium-sulfur battery.

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