Slow-release H3 irreversible phase structure transformation method based on electrochemistry

Abstract

The invention discloses a slow-release H3 irreversible phase structure transformation method based on electrochemistry, and belongs to the technical field of lithium batteries. Constant voltage is applied for a certain period of time at the initial stage of charging, the constant voltage is 3.5-4.0 V, and the constant voltage time is 0.1-4 hours; and the phase structure transformation of the electrode material in the electrochemical cycle process is regulated and controlled, and the irreversible phase structure transformation is inhibited, so that the structure of the positive electrode material is stabilized, and the electrochemical cycle performance of the lithium battery is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a method for electrochemically slow-releasing irreversible phase structure transformation of H3. Background Art

[0002] The new energy industry has developed rapidly. As an important carrier for energy storage and conversion, lithium-ion batteries have faced more challenges. High capacity and long life have become the focus of lithium battery research. The high-nickel layered oxide cathode material LiNi x Co y Mn z O2 has attracted much attention from academia and industry due to its high energy density. Cathode materials with Ni element ratios of 0.5 and 0.6 have been widely used in commercial electric vehicles (EVs). However, these currently commercially dominant cathode materials cannot meet the continuous pursuit of electric vehicles with long driving ranges.

[0003] For high-nickel layered oxide cathode materials, the energy density of the cathode material is mainly improved by increasing the nickel content and working voltage. However, with the increase in nickel content, the irreversible H2-H3 phase structure transformation of high-nickel layered oxide cathode materials during the electrochemical reaction process intensifies, resulting in structural damage during the electrochemical cycling process of the cathode material, and causing poor thermal stability and cycling stability of the electrode material. Therefore, there is an urgent need to develop a method for stabilizing the structural stability of electrode materials during the electrochemical cycling process that is simple, low-cost, and does not require additional cumbersome operation processes. The present invention provides a method for electrochemically slow-releasing irreversible phase structure transformation of the cathode material H3. By applying a constant voltage for a certain period of time at the initial stage of charging, rapid de-lithiation of the electrode material occurs, regulating the phase structure transformation of the electrode material during the electrochemical cycling process, inhibiting the irreversible phase structure transformation, thereby stabilizing the structure of the cathode material and improving the electrochemical cycling performance of the lithium battery. Summary of the Invention

[0004] During the electrochemical charging stage of high-nickel cathode materials, the phase structure transformation of H1-M-H2-H3 will occur successively. Among them, H1, H2, and H3 all belong to the hexagonal structure, but due to different de-lithiation states, their lattice parameters are different, and are distinguished by H. The M phase is a transition phase between the H1 and H2 phases. During this process, due to the synergistic effect of the Jahn-Teller effect and the ordered arrangement of lithium / vacancies, the symmetry of the material is reduced, forming a monoclinic structure transition phase. The H2-H3 phase transformation process is due to the excessive extraction of lithium ions, resulting in a sharp contraction of the C axis and irreversible damage to the material structure, which is an important reason for the rapid loss of capacity of high-nickel cathode materials.

[0005] In view of the above problems, the present invention provides a method for irreversible phase structure transformation of an electrochemically slow-release positive electrode material H3. By applying a constant voltage for a certain period of time at the initial stage of charging, rapid de-lithiation of the electrode material occurs, regulating the phase structure transformation of the electrode material during the electrochemical cycle, inhibiting the irreversible phase structure transformation, thereby stabilizing the structure of the positive electrode material and improving the electrochemical cycle performance of the lithium battery.

[0006] At a low charging voltage, rapid de-lithiation of the positive electrode material occurs. In the positive electrode, the regions prone to reaction rapidly de-lithiate, causing a part of the H1 phase to rapidly transform into the H2 phase, and keeping the material phase structure in a state of coexistence of the H1 and H2 phases for a relatively long time. Continuing to charge, as the voltage increases, in the regions where the H2 phase structure appears first, due to more de-lithiation and larger lattice parameters, they are more prone to further reaction, and these regions first undergo a phase structure transformation into the H3 phase. Because the appearance of the H3 phase causes a sharp contraction of the unit cell parameters, a self-inhibiting reaction occurs, inhibiting the extraction of lithium ions. The un-extracted lithium ions act as pillar ions to stabilize the crystal structure of the material. This also causes the material to exhibit a phenomenon of coexistence of the H2 and H3 phases at a higher charging voltage, and this phase separation at high voltage inhibits the irreversible H3 phase structure transformation, avoiding the destruction of the material structure caused by the irreversible phase structure transformation.

[0007] To achieve the above object, the present invention particularly adopts a constant voltage for a period of time during the first-cycle charging process to control the lithium ion extraction rate during the charging process, and adopts the following technical solutions:

[0008] 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 coin-type lithium battery, and place the lithium battery in a constant-temperature environment with a constant temperature of 25°C - 45°C.

[0009] Step 2: Let the battery in Step 1 stand for a period of time, and then apply a short constant voltage to the lithium ion battery at the initial stage of the first-cycle charge and discharge. The constant voltage is 3.5 - 4.0V, and the constant voltage time is 0.1 - 4h;

[0010] Step 3: Continue to boost the voltage of the battery in Step 2 for charging, and then discharge to complete the first-cycle charge and discharge;

[0011] Step 4: Continue the charge and discharge electrochemical cycle process.

[0012] The charge and discharge rate in Step 3 and Step 4 is 0.01C - 0.5C, the charging cut-off voltage is 4.0 - 4.6V, and the discharging cut-off voltage is 2.5 - 3.0V.

[0013] The lithium-ion positive electrode material described above includes LiNi x A y B zO2, where 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.3, and x + y + z = 1. A is at least one element selected from Co, Mn, and Al; B is at least one element selected from Zr, Ti, Mg, Fe, and Ta.

[0014] The constant voltage mentioned above is the constant voltage applied at the initial stage of the first charge cycle, and the constant voltage value is 3.5 - 4.0V.

[0015] The constant voltage mentioned above is the constant voltage applied at the initial stage of the first charge cycle, and the constant voltage time is 0.1 - 4h.

[0016] Compared with the prior art, the present invention proposes an electrochemical method that is simple, without additional cumbersome operations and additional costs, to regulate the phase structure transformation of electrode materials during the electrochemical cycling process. That is, a short constant voltage treatment is provided in the first cycle, so that the material shows a state where H1 and H2 coexist for a long time in the low voltage region, and H3 phase separation occurs at a higher charging voltage, thereby inhibiting the irreversible H3 phase structure transformation of the material during charge and discharge, and thus achieving the purpose of stabilizing the bulk phase structure of the material and improving the comprehensive performance of the material. Description of the Drawings

[0017] Figure 1 It is the first charge-discharge curves of Example 1, Example 5, and Example 6.

[0018] Figure 2 It is the electrochemical cycling performance of Example 1, Example 5, and Example 6.

[0019] Figure 3 It is the dQ / dV curves of Example 1, Example 5, and Example 6.

[0020] Figure 4 It is the charge-discharge curves and in-situ XRD curves of Example 1 and Example 6. 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 stabilizing the structure of the LiNi 0.8 Co 0.1 Mn 0.1 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 the lithium battery in a constant temperature environment with a constant temperature of 25°C.

[0025] Step 2: Apply a constant voltage of 3.75 V to the lithium-ion battery in Step 1 for 1 h.

[0026] Step 3: Continue to perform rate charge and discharge on the battery in Step 2. Activate it at 0.1 C for two cycles, and then cycle at 0.5 C. The cut-off voltage is 2.8 - 4.4 V.

[0027] Example 2

[0028] This example provides a method for stabilizing the structure of the LiNi 0.7 Mn 0.2 Al 0.05 Ti 0.05 O2 cathode material, including the following steps:

[0029] 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 the lithium battery in a constant-temperature environment with a constant temperature of 45 °C.

[0030] Step 2: Apply a constant voltage of 3.5 V to the lithium-ion battery in Step 1 for 4 h.

[0031] Step 3: Perform rate charge and discharge on the battery in Step 2. Activate it at 0.1 C for two cycles, and then cycle at 0.5 C. The cut-off voltage is 3.0 - 4.6 V.

[0032] Example 3

[0033] This example provides a method for stabilizing the structure of the LiNi 0.8 Mn 0.1 Mg 0.08 Ta 0.02 O2 cathode material, including the following steps

[0034] 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 the lithium battery in a constant-temperature environment with a constant temperature of 35 °C.

[0035] Step 2: Apply a constant voltage of 4.0 V to the lithium-ion battery in Step 1 for 0.1 h.

[0036] Step 3: Perform rate charge and discharge on the battery in Step 2. Activate it at 0.1 C for two cycles, and then cycle at 0.5 C. The cut-off voltage is 2.8 - 4.3 V.

[0037] Example 4

[0038] This example provides a method for stabilizing the structure of the LiNi 0.92 Co 0.04 Mg0.02 Ti 0.02 Structuring method for Ti O2 cathode material, comprising the following steps:

[0039] 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 the lithium battery in a constant temperature environment with a constant temperature of 30°C.

[0040] Step 2: Apply a constant voltage of 3.75 V to the lithium-ion battery in Step 1 for 2 h.

[0041] Step 3: Perform rate charge and discharge on the battery in Step 2, activate it at 0.1C for two cycles, and then cycle at 0.5C with a cut-off voltage of 2.5 - 4.0 V.

[0042] Example 5

[0043] This example provides a method for stabilizing the structure of LiNi 0.8 Co 0.1 Mn 0.1 Structuring method for LiNiCoMnO2 cathode material, comprising the following steps:

[0044] 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 the lithium battery in a constant temperature environment with a constant temperature of 25°C.

[0045] Step 2: Apply a constant voltage of 3.75 V to the lithium-ion battery in Step 1 for 1.5 h.

[0046] Step 3: Perform rate charge and discharge on the battery in Step 2, activate it at 0.1C for two cycles, and then cycle at 0.5C with a cut-off voltage of 2.8 - 4.4 V.

[0047] Example 6 (comparative example)

[0048] 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 the lithium battery in a constant temperature environment with a constant temperature of 25°C.

[0049] Step 2: Perform rate charge and discharge on the battery in Step 2, activate it at 0.1C for two cycles, and then cycle at 0.5C with a cut-off voltage of 2.8 - 4.4 V.

[0050] From Figure 1 It can be clearly seen that the charge-discharge curve of Example 1 has an obvious constant voltage in the initial stage, and the phase transition around 4.2 V is effectively suppressed. This can also be seen from Figure 3It can be more intuitively reflected in the dQ / dV curve. In addition, compared with Example 6 where the capacity retention rate was only 71.2% after 50 cycles without any treatment, the capacity retention rate of Example 1 was significantly increased to 85.6%, and the capacity retention rate of Example 5 was significantly increased to 81.4%. From the in-situ XRD curve, it can be seen that a reversible H2-H3 phase structure transformation occurred during the electrochemical cycling process of Example 1, while an irreversible H2-H3 phase structure transformation occurred in Example 6.

Claims

1. An electrochemical-based method for the slow-release irreversible phase structure transformation of H3, characterized in that, During the initial charging process of the first cycle, a constant voltage is initially applied for a period of time to control the lithium ion extraction rate during the charging process. The following technical solutions are adopted: 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 button lithium battery, and place the lithium battery in a constant temperature environment with a constant temperature of 25°C - 45°C; Step 2: Let the battery in Step 1 stand for a period of time, and then apply a short constant voltage to the lithium ion battery at the initial stage of the first cycle charge and discharge. The constant voltage is 3.5 - 4.0V, and the constant voltage time is 0.1 - 4h; Step 3: Continue to boost the voltage of the battery in Step 2 for charging, and then discharge to complete the first cycle charge and discharge; Step 4: Continue the charge and discharge electrochemical cycling process.

2. A method for electrochemically-based sustained-release irreversible phase structure transformation of H3, characterized in that The charge and discharge rate in Step 3 and Step 4 is 0.01C - 0.5C, the charging cut-off voltage is 4.0 - 4.6V, and the discharging cut-off voltage is 2.5 - 3.0V.

3. A method for electrochemically based slow-release irreversible phase structure transformation of H3, characterized in that, The described lithium-ion cathode material includes LiNi x A y B z O2, where 0.5 ≤ x ≤ 1, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.3, and x + y + z = 1. The said A is at least one element among Co, Mn, and Al; the said B is at least one element among Zr, Ti, Mg, Fe, and Ta.

4. A method for electrochemically-based slow-release irreversible phase structure transformation of H3, characterized in that The constant voltage mentioned is the constant voltage applied at the initial stage of the first cycle charging.

Images