Regulation and control method for stabilizing crystal structure of lithium battery positive electrode material

Abstract

The invention discloses a regulation and control method for stabilizing a lithium battery positive electrode material crystal structure, and belongs to the technical field of lithium batteries. The lithium ion de-intercalation speed in different charging voltage stages is regulated and controlled by adopting different multiplying power for charging in the first circle, so that the strategy of stabilizing the crystal structure of the positive electrode material is realized. The first-circle higher charging rate accelerates the lithium ion removal of the material, and when the lithium removal rate is greater than the lithium ion balance rate in the material, the lithium removal amounts on the surface of the material and in the material are different, resulting in inconsistent phase change on the surface of the material and in the material. A slow phase change area in the material stabilizes the crystal structure of the material in the subsequent charging process, and crystal structure collapse caused by excessive lithium removal is prevented.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a method for regulating the crystal structure of a stable lithium battery cathode material. Background Art

[0002] With the progress of technology and the rapid development of society, fossil energy can be replaced by conventional clean energies (such as solar energy, wind energy, and tidal energy). However, due to their geographical and environmental limitations, these energies are intermittent and uncertain, and cannot be directly connected to the power grid for use. Therefore, energy storage has become crucial. Since rechargeable lithium-ion batteries were commercialized by Sony in 1991, they have received extensive attention from the academic and industrial communities. They can store electrical energy in the form of chemical energy and directly convert the energy released by chemical reactions of substances into electrical energy. Moreover, they have high energy density and power density, and are considered an important approach to solving the current energy storage problem.

[0003] Lithium batteries currently dominate the energy storage field, and their application scope has expanded from portable electrical and electronic devices to transportation. In recent years, due to the demand for higher-specification energy storage devices in the new energy electric vehicle field, the energy density of a single power battery cell has reached 500 Wh / kg. Therefore, there is an urgent need to develop lithium-ion batteries with high energy density, high safety, long cycle life, and low cost. As an important component of lithium-ion batteries, the cathode material directly determines the main characteristics of lithium-ion batteries. Therefore, developing cathode materials with high specific capacity and low cost is the key to promoting the application of high-energy-density and low-cost lithium-ion batteries. The high-nickel layered oxide material (Li[Ni x Co y Mn 1-x-y O2) has a high specific capacity (>220 mAh / g), and its energy density reaches 800 Wh / kg, making it one of the best choices for achieving high-energy-density power batteries.

[0004] The cation mixing of the high-nickel layered oxide cathode material intensifies with the increase in nickel content. During the charging process, a large amount of Li + is removed to form lithium vacancies, and Ni 2+ continually migrates to the lithium layer to occupy the lithium vacancies, forming lithium-nickel mixing. At the same time, some transition metal elements also migrate to the lithium layer to form cation mixing, causing the structure of the high-nickel layered oxide material to gradually change from a layered structure to a spinel structure or a cubic rock salt phase structure, resulting in poor structural stability of the material. During the cycling process, a large amount of high-valence Ni 4+ is easily reacted with the organic electrolyte, leading to the decomposition of the electrolyte. At the same time, the generated Ni 2+It is easy to dissolve, but the interfacial stability is poor. Surface treatment is considered the most common and effective strategy to improve the surface and interface stability of materials. However, the surface treatment method increases the process and raises the cost of materials. By regulating the charge-discharge regime of lithium batteries, controlling the speed of lithium-ion insertion and extraction during the charge-discharge process, and suppressing the irreversible phase structure transformation during the electrochemical cycling process of materials, realizing the improvement of the structural stability of electrode materials is a simple, low-cost and effective method, which has not been reported yet. Summary of the Invention

[0005] The object of the present invention is to provide a simple and easy-to-operate method for the structural damage of high-nickel cathode materials during the electrochemical reaction process, which leads to the reduction of thermal stability and cycling stability. By regulating the charge-discharge regime of lithium batteries in the first cycle, controlling the speed of lithium-ion insertion and extraction during the charge-discharge process, and suppressing the irreversible phase structure transformation during the electrochemical cycling process of materials, the cycling stability performance of lithium batteries is improved.

[0006] A relatively fast charging rate at the initial stage of charging causes the rapid de-lithiation of the cathode material. When the de-lithiation rate is greater than the lithium-ion equilibrium rate inside the material, the de-lithiation amounts on the surface and inside of the material are different, resulting in inconsistent phase transformations on the surface and inside of the material. The material is in a state where H1 and H2 coexist for a long time. In the region where the H2 phase transformation occurs first, due to the increase in the unit cell parameter, it is conducive to the extraction of lithium ions during further charging, and a self-catalytic reaction occurs, and then the H3 phase structure transformation occurs. After the H3 phase transformation occurs, the unit cell parameter shrinks sharply, which is not conducive to the further de-lithiation of the material, and a self-inhibition reaction occurs, suppressing the excessive H3 phase transformation. A part of the lithium ions that have not been extracted act as pillar ions in the material to stabilize the crystal structure of the material, and a small amount of lithium ions are extracted to avoid the collapse of the cathode material structure, thereby realizing the stability of the crystal structure of the cathode material.

[0007] To achieve the above object, the present invention especially adopts the method of regulating the charge-discharge regime of lithium batteries in the first cycle to control the speed of lithium-ion insertion and extraction during the charge-discharge process, and specifically adopts the following technical solutions:

[0008] Step 1: Mix the cathode material with a conductive agent and a binder to prepare a slurry, coat it on an Al foil, assemble it into a coin-type lithium battery, and place it in a constant-temperature oven for static placement, and the constant temperature is 25-45°C;

[0009] Step 2: Charge at a constant rate C1 to a specific capacity, the specific capacity is a, a is 5%-80% of the theoretical capacity, preferably 30%-50%, and the corresponding voltage is lower than 4.2V, and the charging rate C1 is 0.01C-0.5C, preferably 0.3-0.5C;

[0010] Step 3: Charge from a specific capacity of a to the cut-off voltage V1 at a constant rate C2. The cut-off voltage is 4.2 - 4.6V, and the charge rate C2 is 0.01C - 0.5C;

[0011] The charge rate C1 is greater than the charge rate C2;

[0012] Step 4: Discharge to the cut-off voltage V2 at a constant rate C3. The discharge cut-off voltage is 2.5 - 3.0V, and the discharge rate C3 is 0.01C - 0.5C;

[0013] The discharge rate C3 is less than or equal to the charge rate C2 in Step 3;

[0014] Steps 1 to 4 complete the first charge-discharge cycle;

[0015] Step 5: Cycle the battery in Step 4 at a constant rate C4. The constant rate is 0.1C - 2C, and the voltage range is V1 - V2.

[0016] The positive electrode material in Step 1 includes LiNi x A y B z O2, where x + y + z = 1, 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.3. The A is at least one element of Co, Mn, and Al; the B is one or more elements of Zr, Ti, Mg, Fe, Ta, etc.

[0017] Advantages of the present invention: The relatively fast charging rate in the first cycle of the present invention accelerates the extraction of lithium ions from the material. When the lithium extraction rate is greater than the lithium ion equilibrium rate inside the material, the amount of lithium extraction on the surface and inside of the material is different, resulting in inconsistent phase transitions on the surface and inside of the material. The slow phase transition region inside the material stabilizes the crystal structure of the material during subsequent charging processes and prevents the crystal structure from collapsing due to excessive lithium extraction. Therefore, the present invention regulates the crystal structure stability of the positive electrode material by adjusting the different rates in the charging stage. Description of the Drawings

[0018] Figure 1 are the first charge-discharge curves of charging to the corresponding cut-off capacity at different rates.

[0019] Figure 2 are the first dQ / dV curves of charging to the corresponding cut-off capacity at different rates.

[0020] Figure 3 are the electrochemical cycling curves of charging to the corresponding cut-off capacity at different rates. Detailed Embodiments

[0021] The following specific examples are used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto:

[0022] Example 1

[0023] This example provides a method for the structure of a stable LiNi 0.95 Mn 0.03 Al 0.02 O2 cathode material, including the following steps:

[0024] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in a thermostat for static placement. The constant temperature is 25 °C

[0025] Step 2: Charge the battery after the above static placement at a rate of 0.5C, and the cut-off capacity is 10 mAhg -1 .

[0026] Step 3: Charge the battery charged to 10 mAhg in Step 2 -1 continuously at a rate of 0.1C until 4.6V.

[0027] Step 4: Discharge the battery in Step 3 at a rate of 0.1C until 2.7V.

[0028] Step 5: Cycle the battery in Step 4 at a rate of 0.5C, and the cut-off voltage is 2.7 - 4.6V.

[0029] Example 2

[0030] This example provides a method for the structure of a stable LiNi 0.7 Mn 0.2 Mg 0.05 Ti 0.05 O2 cathode material, including the following steps:

[0031] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in a thermostat for static placement. The constant temperature is 30 °C;

[0032] Step 2: Charge the above static battery at a rate of 0.5C, and the cut-off capacity is 150 mAhg -1 .

[0033] Step 3: Charge the battery charged to 150 mAhg in Step 2 -1 at a rate of 0.05C until 4.2V, and then discharge it at a rate of 0.05C until 3.0V.

[0034] Step 4: Cycle the battery in Step 3 at a rate of 0.3C, and the charge and discharge cut-off voltage is 3.0 - 4.2V.

[0035] Example 3

[0036] This example provides a method for the structure of a stable LiNi 0.83 Co 0.11 Mn 0.06 O2 cathode material, including the following steps:

[0037] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in a constant-temperature oven for static settlement. The constant temperature is 25°C;

[0038] Step 2: Charge the static-settled battery at a rate of 0.5C, and the charging cut-off capacity is 50 mAhg -1 .

[0039] Step 3: Charge the battery in Step 2 to 50 mAhg -1 at a rate of 0.1C to 4.4V, and then discharge it at a rate of 0.1C to 2.7V.

[0040] Step 4: Cycle the battery in Step 3 at a rate of 0.5C, and the charge-discharge cut-off voltage is 2.7 - 4.4V.

[0041] Example 4

[0042] This example provides a method for the structure of a stable LiNi 0.83 Co 0.11 Mn 0.06 O2 cathode material, including the following steps:

[0043] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in a constant-temperature oven for static settlement. The constant temperature is 25°C;

[0044] Step 2: Charge the static-settled battery at a rate of 0.3C, and the cut-off capacity is 100 mAhg -1 .

[0045] Step 3: Charge the battery charged to 100 mAhg in Step 2 -1 at a rate of 0.1C to 4.4V and discharge it at 0.1C to 2.7V.

[0046] Step 4: Cycle the battery in Step 3 at a rate of 0.5C, and the cut-off voltage is 2.7 - 4.4V.

[0047] Example 5

[0048] This example provides a LiNi 0.9 Mn0.03 Mg 0.02 Ta 0.05 Method for stabilizing the structure of the O2 cathode material, comprising the following steps:

[0049] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in an incubator for static placement. The constant temperature is 45 °C

[0050] Step 2: Charge the static battery at a rate of 0.5C, and the cut-off capacity is 150 mAhg -1 。

[0051] Step 3: Charge the battery in Step 2 to 4.6V at a rate of 0.1C and discharge it to 2.7V at a rate of 0.1C.

[0052] Perform charge and discharge at a rate of 0.2C for two cycles, and then cycle at a rate of 0.5C. The cut-off voltage is 2.8 - 4.6V.

[0053] Example 6 (comparative example)

[0054] This example provides a method for stabilizing the structure of the LiNi 0.83 Co 0.11 Mn 0.06 O2 cathode material, comprising the following steps:

[0055] Step 1: Mix the cathode material, acetylene black, and PVDF in a ratio of 80:10:10, coat it on an Al foil, assemble it into a 2032 coin-type lithium battery, and place it in an incubator for static placement. The constant temperature is 30 °C;

[0056] Step 2: Perform charge and discharge on the static battery at a rate of 0.1C, activate it at a rate of 0.1C for two cycles, and then cycle at a rate of 0.5C. The cut-off voltage is 2.8 - 4.4V.

[0057] From Figure 1 It can be clearly seen that there is an obvious polarization voltage in the initial stage of charging in the charge and discharge curves of Example 3 and Example 4, and the H2-H3 phase structure transformation around 4.2V is effectively suppressed. From Figure 2 It is more intuitively shown in the dQ / dV curve. In addition, compared with Example 6 where the battery is not treated and tested, the electrochemical cycle stability is significantly improved. The capacity retention rates of Example 3 and Example 4 are 77.4% and 88.4% respectively, while the electrochemical cycle capacity retention rate of Example 6 is only 70.1%.

Claims

1. A method for regulating the crystal structure of a cathode material for a stable lithium battery, characterized in that, Adopt a charge-discharge regime for regulating the first cycle of lithium batteries to control the speed of lithium-ion insertion and extraction during the charge-discharge process. The following technical solutions are specifically adopted: Step 1: Mix the positive electrode material with a conductive agent and a binder to prepare a slurry, coat it on an Al foil, assemble it into a button lithium battery, and place it in a constant-temperature oven for static placement. The constant temperature is 25 - 45°C; Step 2: Charge at a constant rate C1 to a specific capacity, where the specific capacity is a, a is 5% - 80% of the theoretical capacity, and the corresponding voltage is lower than 4.2V. The charge rate C1 is 0.01C - 0.5C; Step 3: Charge from the specific capacity a to the cut-off voltage V1 at a constant rate C2. The cut-off voltage is 4.2 - 4.6V, and the charge rate C2 is 0.01C - 0.5C; The charge rate C1 is greater than the charge rate C2; Step 4: Discharge to the cut-off voltage V2 at a constant rate C3. The discharge cut-off voltage is 2.5 - 3.0V, and the discharge rate C3 is 0.01C - 0.5C; The discharge rate C3 is less than or equal to the charge rate C2 in Step 3; Steps 1 to 4 complete the first-cycle charge and discharge; Step 5: Cycle the battery in Step 4 at a constant rate C4. The constant rate is 0.1C - 2C, and the voltage range is V1 - V2.

2. A method for regulating the crystal structure of a stable lithium battery cathode material according to claim 1, characterized in that, The positive electrode material in Step 1 includes LiNi x A y B z O2, where x + y + z = 1, 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.3, A is at least one element among Co, Mn, and Al; B is one or more elements among Zr, Ti, Mg, Fe, Ta, etc.

3. A method for regulating the crystal structure of a stable lithium battery cathode material according to claim 1, characterized in that, In Step 2, a is 30% - 50% of the theoretical capacity, and the charge rate C1 is 0.3 - 0.5C.

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