Positive electrode active material for secondary battery, and positive electrode comprising the same and secondary battery comprising the same

Abstract

A positive electrode active material for a secondary battery according to one embodiment of the present disclosure may include a compound represented by the following chemical formula 1. [Chemical Formula 1] LixMnFeP3O12(0≤x≤4)

Description

Technology Field

[0001] The present disclosure relates to a positive electrode active material for a secondary battery, a positive electrode comprising the same, and a secondary battery comprising the same. More specifically, the present disclosure relates to a positive electrode active material for a secondary battery, a positive electrode comprising the same, and a secondary battery comprising the same. Background Technology

[0002] As the utility of smart devices gradually increases, the demand for high-capacity rechargeable batteries is growing. Current rechargeable battery technology is predominantly based on lithium-ion batteries, and battery capacity is primarily determined by the cathode active material. Cathode active materials mainly used in commercially available lithium-ion batteries include LiCoO2, LiFePO4, LiMn2O4, and active materials with improved compositions of these materials.

[0003] However, Co, a key element of cathode active materials, is concentrated in specific countries and is exposed to the risk of geopolitical supply instability, while Li-Fe-P series cathode active materials have the disadvantage of lower energy density and capacity compared to Li-Co-O cathode active materials.

[0004] To resolve these issues and strengthen the competitiveness of the secondary battery materials market, the development of new high-performance cathode active materials for secondary batteries is urgent. The problem to be solved

[0005] According to one embodiment of the present disclosure, a positive electrode active material for a secondary battery having high energy density and average voltage, a positive electrode including the same, and a secondary battery including the same can be provided by using a quantum mechanics-based computational materials science methodology and a neural network-based machine learning.

[0006] According to one embodiment of the present disclosure, a cathode active material for a secondary battery capable of securing superior cathode material performance compared to commercial cathode active material LiFePO4, and a cathode including the same and a secondary battery including the same can be provided. means of solving the problem

[0007] A positive electrode active material for a secondary battery according to one embodiment of the present disclosure may include a compound represented by the following chemical formula 1.

[0008] [Chemical Formula 1]

[0009] Li x MnFeP3O 12 (0≤x≤4)

[0010] In a positive electrode active material for a secondary battery according to one embodiment of the present disclosure, the compound may be represented by the following chemical formula 2.

[0011] [Chemical Formula 2]

[0012] Li4MnFeP3O 12

[0013] A positive electrode active material for a secondary battery according to one embodiment of the present disclosure may include a compound represented by the following chemical formula 3.

[0014] [Chemical Formula 3]

[0015] Li4Mn y Fe 2-y P3O 12 (0 <y<2)

[0016] A positive electrode according to one embodiment of the present disclosure may include the positive electrode active material described above.

[0017] A lithium-ion secondary battery according to one embodiment of the present disclosure may include the positive electrode described above. Effects of the invention

[0018] The novel lithium iron phosphate cathode material according to the present disclosure has high energy density and average voltage.

[0019] The average discharge voltage of the novel cathode active material for secondary batteries according to the present disclosure is significantly higher than the average discharge voltage of existing commercially available LiFePO4. Therefore, the cathode active material according to the present disclosure has the potential to be applied to high-voltage operations.

[0020] The energy density per unit weight and energy density per unit volume of the novel cathode active material for secondary batteries according to the present disclosure are significantly higher than those of LiFePO4.

[0021] The effects of the present disclosure are not limited to those mentioned above, and other unmentioned effects will be clearly understood by those skilled in the art from the description below. Brief explanation of the drawing

[0022] Figure 1 is a schematic diagram of a model used to search for candidate materials for cathode active materials for secondary batteries and to evaluate their physical properties. Figure 2(a) shows the predicted formation energy according to the concentration of Li ions, and (b) shows the Li4MnFeP3O with respect to Li ion insertion. 12 Shows the voltage change of. Figure 3 shows the result of calculating the volume change rate of the positive active material. FIG. 4 shows Li4MnFeP3O, a novel cathode active material derived from the present disclosure. 12 It shows the crystal structure of. Specific details for implementing the invention

[0023] The embodiments described in this specification may be modified in various different forms, and the technology according to one embodiment is not limited to the embodiments described below. Furthermore, the embodiment of one example is provided to more fully explain the present disclosure to those skilled in the art.

[0024] Additionally, the singular form used in the specification and the appended claims may be intended to include the plural form unless specifically indicated otherwise in the context.

[0025] Additionally, numerical ranges used herein include lower and upper limits and all values ​​within the range, increments logically derived from the form and width of the defined range, all of which are limited values, and all possible combinations of upper and lower limits of numerical ranges defined in different forms. Unless otherwise specifically defined in this specification, values ​​outside the numerical range that may occur due to experimental error or rounding are also included in the defined numerical range.

[0026] Furthermore, throughout the specification, the term "comprising" a component means that, unless specifically stated otherwise, it does not exclude other components but rather may include additional components.

[0027] The terms "first," "second," etc., as used in this specification may be used to describe various components, but the components should not be limited by these terms. The terms are used solely for the purpose of distinguishing one component from another.

[0029] Recently, various studies utilizing density functional theory (DFT) and the like have been conducted to develop high-capacity cathode active materials. However, calculating the performance of electrodes for secondary batteries using these methods has limitations due to high computing costs stemming from quantum mechanical properties and insufficient results resulting from intuitive input. Recently, there have been attempts to overcome these limitations through machine learning (ML) based on artificial neural networks (ANN). For example, alternative cathode active materials with high energy density and minimal volume change can be extracted using a CGCNN (Crystal Graph Convolutional Neural Network) module.

[0030] Physical properties of the cathode of a secondary battery, such as energy density and voltage curves, are determined by electrochemical reactions. To accurately predict these electrochemical reactions, calculations must simultaneously consider the difference in chemical potential between the intercalated and deintercalated states. However, since the aforementioned CGCNN module processes data without considering these two states, it still has limitations in deriving alternative cathode active materials.

[0031] Meanwhile, Co, a key element of cathode active materials, is concentrated in specific countries and is exposed to the risk of geopolitical supply instability, and Li-Fe-P series cathode active materials have the disadvantage of lower energy density and capacity compared to Li-Co-O series cathode active materials. For example, the theoretical gravimetric capacity of the existing LiFePO4 cathode material used in secondary batteries is 169.9 mAh / g, which is lower than that of the NCM series cathode material. In addition, the theoretical average voltage of LiFePO4 is 3.52 V, the theoretical gravimetric energy density is 594.5 Wh / kg, and the theoretical volumetric energy density is 2063.4 Wh / l.

[0032] Therefore, it is necessary to secure new iron phosphate-based cathode active materials to strengthen the competitiveness of next-generation secondary battery cathode materials.

[0033] Figure 1 is a schematic diagram of a model used to search for candidate materials for cathode active materials for secondary batteries and to evaluate their physical properties.

[0034] In the present disclosure, as shown in FIG. 1, a newly proposed MCGCNN (Modular Crystal Graph Convolutional Neural Network) module composed of two independent CGCNN modules and a post-processing unit, and a cathode material database of the Materials Project are utilized to train an MCGCNN model, thereby searching for candidate cathode active materials for secondary batteries and evaluating their physical properties.

[0035] In this disclosure, a positive electrode active material having high energy density and average voltage was derived using a quantum mechanics-based computational materials science methodology and neural network-based machine learning.

[0036] Specifically, the positive electrode active material for a secondary battery according to the present disclosure may include a compound represented by the following chemical formula 1.

[0037] [Chemical Formula 1]

[0038] Li x MnFeP3O 12 (0≤x≤4)

[0039] The positive electrode active material for a secondary battery according to the present disclosure may include a compound represented by the following chemical formula 2.

[0040] [Chemical Formula 2]

[0041] Li4MnFeP3O 12

[0042] The positive active material for a secondary battery according to the present disclosure may include a compound represented by the following chemical formula 3.

[0043] [Chemical Formula 3]

[0044] Li4Mn y Fe 2-y P3O 12 (0 <y<2)

[0046] The novel lithium iron phosphate cathode material according to the present disclosure has high energy density and average voltage.

[0047] The average discharge voltage of the novel cathode active material for secondary batteries according to the present disclosure is significantly higher than the average discharge voltage of existing commercially available LiFePO4. Therefore, the cathode active material according to the present disclosure has the potential to be applied to high-voltage operations.

[0048] The energy density per unit weight and energy density per unit volume of the novel cathode active material for secondary batteries according to the present disclosure are significantly higher than those of LiFePO4.

[0050] The present invention will be described in more detail below.

[0052] 1. Derivation of candidate cathode active materials

[0053] An artificial neural network-based MCGCNN (Modular Crystal Graph Convolutional Neural Network) module was fabricated to predict material properties.

[0054] MCGCNN model training was performed using the Materials Project's cathode material database containing 4,904 battery data points.

[0055] Among the 154,718 inorganic materials retrieved from the Material Project, 780 materials containing all Li-Fe-PO were selected, and the trained MCGCNN was applied to predict battery performance based on average voltage.

[0056] At this time, considering the structural stability of the battery material, the prediction range of the average voltage of MCGCNN was set to 4V or higher and 4.5V or lower.

[0057] Considering the theoretical energy density of LiFePO4 of 595 Wh / kg, the target energy density was set to 700 Wh / kg, and the following new LFP cathode material, Li4MnFeP3O, was selected after excluding previously reported materials. 12 derived.

[0058] Meanwhile, the active material-related data mentioned in this disclosure was obtained by accessing the Materials Project database via the Materials Project REST API to search for candidate materials for cathode active materials, and by receiving and searching for intercalation electrode data having calculated battery characteristics, such as gravitational force and energy density per unit volume, from the said Materials Project.

[0059] In addition, in this disclosure, to support the prediction by the MCGCNN module, the physical properties of the cathode active material are recalculated using a first-principles calculation program called VASP (Vienna Ab initio Simulation Package), and the same calculation standards applied to the material project were applied.

[0061] 2. Theoretical Verification

[0062] (1) Perform first-principles calculations

[0063] First-principles calculations based on density functional theory (DFT) were performed to evaluate the performance of Li-Fe-P series battery candidate materials. The first-principles calculations were conducted using VASP (Vienna Ab-initio Simulation Package) software, which implements the projector-augmented-wave (PAW) method, and the results are summarized in [Table 1] below. In [Table 1], Voltage is the average voltage, and C gra is the energy density per unit weight, C vol Energy density per unit volume, V max represents the rate of change in volume.

[0065] Chemical Formula Voltage (V) C gra (mAh / g) C vol (Ah / L) △Vmax(%) Li 4 MnFeP 3 O 12 4.4 252 728 8.3

[0066] Experiments related to more specific first-principles calculations regarding the physical properties of the Li-Fe-P series battery candidate materials specified in Table 1 above will be explained in the <Experimental Examples> and <Experimental Results> below. In the experimental examples according to the present disclosure, the electron-to-electron exchange correlation energy function accompanying the first-principles calculation was represented by the Generalized Gradient Approximation (GGA) proposed by Perdew, Burke, Enzerhof, et al., and the kinetic energy cutoff for the Kohn-Sham wave function expression was set to 520 eV.

[0068] (2) Calculation of Formation Energy and Voltage

[0069] To calculate the voltage distribution due to Li insertion, a CGCNN model was applied to predict the formation energy according to the concentration of Li ions. The result is shown in Figure 2 (a).

[0070] After selecting the structure that exhibits the minimum formation energy at each Li ion concentration, the voltage was calculated using the following mathematical formula 1.

[0071] [Mathematical Formula 1]

[0072]

[0073] Here, E[LixMnFeP3O 12 ] and E[Li] represent the ground state energy of the positive active material with a Li ion concentration of x obtained through quantum mechanics calculations and the metallic phase Li, respectively, and F is the Faraday constant.

[0074] Figure 2(b) shows Li4MnFeP3O with Li ion insertion 12 Shows the voltage change of Li4MnFeP3O 12It was confirmed that the voltage was 5.3V under maximum charging conditions, and the voltage gradually decreased during the discharge process, reaching 3V under maximum discharge conditions, with an overall average voltage of 4.4V.

[0076] <Experimental Example>

[0077] The candidate substance model used in the experimental example is Li4MnFeP3O 12 As such, four formula units form one unit cell, and 16 Li atoms exist in the unit cell.

[0078] Li4MnFeP3O 12 To investigate the battery performance of LixMnFeP3O 12 For this, since first-principles calculations must be performed for the complete de-insertion of Li ions and all structures (0≤x≤4) occurring during the intermediate process, 3x3x2 gamma-centered k-point sampling generated by the Monkhorst-Pack method in the first Brillouin zone was applied.

[0079] To perform first-principles calculations on the structures occurring during the Li-ion deinsertion and insertion process, it is necessary to secure the energetically most stable structure for each step. To this end, possible arrangements of Li-ions were listed through electrostatic energy filtering, and the ground state structure exhibiting the lowest energy was selected to calculate battery characteristics.

[0080] <Experimental Results>

[0081] Li4MnFeP3O according to charge-discharge 12To investigate the structural stability of the cathode active material, the range of Li ion de-insertion [LixMnFeP3O 12 The Li composition ratio (x) of [0≤x≤4] was divided into 17 sections, and the volume of the cathode active material was calculated for each composition ratio.

[0082] The volume change rate (Vmax) of the positive electrode active material is the volume of the structure (V) generated during the process of working ion de-insertion and the volume of the positive electrode active material in the completely de-inserted state (V 탈리 It can be defined as [Mathematical Formula 2] below using ), and the result calculated accordingly is as shown in Fig. 3.

[0084] [Mathematical Formula 2]

[0085]

[0087] In addition, to verify the discharge voltage (V) of the positive active material, which is one of the important characteristics of the battery, [Lix1MnFeP3O 12 , Lix2MnFe P3O 12 The discharge voltage was calculated according to the preceding [Equation 1] using the first-principles calculation results.

[0088] Cathode active material for Li-Fe-P secondary batteries according to the present disclosure Li4MnFeP3O 12 The average discharge voltage is 4.4V, which is about 26% higher than the average discharge voltage of 3.5V of existing commercially available LiFePO4. Therefore, it has been confirmed that the cathode active material according to the present disclosure has the potential to be applied to high-voltage operations.

[0089] Li4MnFeP3O 12 The theoretical specific capacity and volumetric capacity of the cathode active material were calculated to be 252 mAh / g and 728 Ah / L, respectively, using Faraday's law. Specifically, the theoretical specific capacity is calculated using the following Equation 3.

[0090] [Mathematical Formula 3]

[0091] (Number of Li ions × Faraday constant) / (3600 × molecular weight of compound)

[0092] In addition, the volumetric capacity is calculated by multiplying the theoretical specific capacity calculated according to the above mathematical formula 3 by the molecular weight of the compound and then dividing by the volume.

[0093] Multiplying the above values ​​by the average discharge voltage of 4.4V yields an energy density of 1110Wh / kg per unit weight and 3205Wh / L per unit volume, respectively. This is significantly higher than the energy density of 594Wh / kg per unit weight and 2063Wh / L per unit volume of conventional LiFePO4.

[0095] FIG. 4 shows Li4MnFeP3O, a novel cathode active material derived from the present disclosure. 12 It shows the crystal structure of.

[0096] Li4MnFeP3O 12 It has a Pbcn space group, and Fe and Mn atoms bond with 6 oxygen atoms to form an octahedron.

[0097] As previously confirmed, Li4MnFeP3O 12 The average voltage of the Li ion insertion process was 4.4V, and the energy density per weight and energy density per volume were confirmed to be 1110.6Wh / kg and 3205.1Wh / L, respectively.

[0098] It can be seen that these properties are excellent when compared to LiFePO4, which has an average voltage of 3.52 V, an energy density per weight of 594.5 Wh / kg, and an energy density per volume of 2063.4 Wh / l.

[0100] Designing new cathode active materials requires the optimization of various parameters, such as minimal volume change, a flat discharge curve, and high energy density; therefore, the cathode active materials for secondary batteries derived according to the present disclosure are of great technical significance in that they satisfy the aforementioned parameters.

[0102] Embodiments of the present disclosure have been described above together with the drawings. This is illustrative and the present disclosure is not limited to the aforementioned embodiments and the contents of the drawings.

[0103] It is obvious to those skilled in the art that the present disclosure may be modified within the scope of the technical concept disclosed. The described embodiments should be regarded as part of the present disclosure, and the scope of the present disclosure should not be determined solely by the described embodiments.

[0104] The scope of the present disclosure should be determined by the technical ideas described in the claims. Furthermore, even if the operation or effect according to the configuration is not explicitly described while describing the embodiments of the present disclosure, it is obvious that the operation or effect predictable by said configuration should also be recognized as part of the present disclosure.

Claims

Claim 1 A positive electrode active material for a secondary battery comprising a compound represented by the following chemical formula 1. [Chemical Formula 1] Li x MnFeP3O 12 (0≤x≤4) Claim 2 A positive electrode active material for a secondary battery according to claim 1, wherein the compound is a compound represented by the following chemical formula 2. [Chemical Formula 2] Li4MnFeP3O 12 Claim 3 A positive electrode active material for a secondary battery comprising a compound represented by the following chemical formula 3. [Chemical Formula 3]Li4Mn y Fe 2-y P3O 12 (0 <y<2) Claim 4 A positive electrode comprising a positive electrode active material according to any one of paragraphs 1 to 3. Claim 5 A lithium-ion secondary battery comprising the positive electrode of paragraph 4.

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