Charge and discharge methods for electrochemical devices

Abstract

This invention provides a charging and discharging method for an electrochemical device. The positive electrode of the electrochemical device includes lithium manganese iron phosphate and a ternary cathode material. The charging and discharging method includes: charging the electrochemical device at a low rate in a first voltage range and charging it at a high rate in a second voltage range; and / or discharging the electrochemical device at a low rate in a third voltage range and discharging it at a high rate in a fourth voltage range. The first and second voltage ranges are respectively non-overlapping and overlapping charging voltage platforms of lithium manganese iron phosphate and the ternary cathode material; the third and fourth voltage ranges are respectively non-overlapping and overlapping discharging voltage platforms of lithium manganese iron phosphate and the ternary cathode material. This invention solves the problem of voltage platform mismatch and poor capacity utilization between lithium manganese iron phosphate and the ternary cathode material, and improves the cycle stability and service life of the electrochemical device.

Description

Technical Field

[0001] This invention belongs to the field of battery technology and relates to a charging and discharging method for an electrochemical device. Background Technology

[0002] Lithium manganese phosphate is an olivine-type positive electrode active material for lithium-ion batteries. It has a high voltage plateau, but its intrinsic conductivity is low. In the existing technology, lithium manganese iron phosphate is generally obtained by doping or replacing the manganese sites of lithium manganese phosphate with iron to solve the problem of low conductivity of lithium manganese phosphate.

[0003] To meet energy density requirements, lithium manganese iron phosphate (LFP) often needs to increase the manganese-to-iron ratio to 6:4 to 8:2. However, at this ratio, LFP exhibits low conductivity, hindering its electrochemical performance. To address this issue, existing technologies incorporate ternary cathode materials for blending, achieving higher energy density and better conductivity. However, LFP and ternary cathode materials have different charge-discharge plateaus. Therefore, within their respective voltage plateau ranges, current density increases significantly, leading to abnormal battery capacity and thus affecting the application of LFP / ternary cathode blends in electrochemical devices. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a charging and discharging method for an electrochemical device. This invention performs high-rate charging and / or discharging within the voltage range where the lithium manganese iron phosphate and the ternary cathode material are matched, and low-rate charging and / or discharging within the voltage range where they are mismatched. This solves the problems of voltage mismatch and poor capacity utilization between lithium manganese iron phosphate and the ternary cathode material, improves the cycle stability of the electrochemical device, and extends its service life.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a charging and discharging method for an electrochemical device, wherein the positive electrode of the electrochemical device comprises lithium manganese iron phosphate and a ternary positive electrode material, and the charging and discharging method includes:

[0007] The electrochemical device is charged at a low rate in a first voltage range and at a high rate in a second voltage range; and / or

[0008] The electrochemical device is discharged at a low rate in the third voltage range and at a high rate in the fourth voltage range.

[0009] The first voltage range is the voltage range in which the charging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material do not overlap; the second voltage range is the voltage range in which the charging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material overlap; the third voltage range is the voltage range in which the discharging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material do not overlap; and the fourth voltage range is the voltage range in which the discharging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material overlap.

[0010] The combined use of lithium manganese iron phosphate (LMP) and ternary cathode materials can meet the energy density requirements of electrochemical devices while addressing the issues of low conductivity and ineffective electrochemical performance when LMP is used alone, thus improving the conductivity and capacity of the electrochemical device. However, the charge-discharge platforms of LMP and ternary cathode materials differ significantly. LMP shows almost no capacity utilization in the voltage range >4.0V and 3.5V to 3.9V, while ternary cathode materials show almost no capacity utilization in the <3.4V range. This leads to a substantial increase in current density experienced by both materials within their respective voltage ranges, resulting in high-rate discharge. Calculations show that in the voltage mismatch region, a 1C rate will cause the ternary material to discharge at a rate of 3C to 4C for a battery with 20% ternary cathode material. This voltage mismatch will lead to abnormal battery capacity utilization, and long-term high-rate charge-discharge will also result in poor battery cycle performance.

[0011] This invention performs high-rate charging and / or discharging (i.e., fast charging / fast discharging) in the range where the voltages of lithium manganese iron phosphate and ternary cathode materials match, while performing low-rate charging and / or discharging in the range where their voltages do not match. This intelligent step-charging mode solves the problem of voltage mismatch and poor capacity utilization between lithium manganese iron phosphate and ternary cathode materials, improves the cycle stability of the electrochemical device, and extends the service life of the electrochemical device.

[0012] Preferably, the first voltage range is (2.8V to 3.4V), (3.6V to 3.9V) and (4.1V to xV), where x is (4.2V to 4.4V), for example, it can be 4.2V, 4.25V, 4.26V, 4.28V, 4.3V, 4.35V or 4.4V, etc.

[0013] It should be noted that in the first voltage range (4.1V to xV) of this invention, xV is the full charge voltage. This voltage varies depending on the chemical system of the electrolyte or positive electrode material. Generally, xV can reach 4.2V to 4.4V, that is, (4.1V to xV). For example, it can be (4.1V to 4.20V), (4.1V to 4.26V), (4.1V to 4.28V), (4.1V to 4.3V), (4.1V to 4.35V), or (4.1V to 4.4V), etc.

[0014] Preferably, the second voltage range is (3.4V to 3.6V) and (3.9V to 4.1V).

[0015] Preferably, the third voltage range is (3.4V to 3.7V) and (3.9V to yV), where y is (4.2V to 4.4V).

[0016] Preferably, the fourth voltage range is (2.8V to 3.4V) and (3.7V to 3.9V).

[0017] In this invention, by performing rate charging and discharging within the aforementioned voltage range, the synergistic effect of lithium manganese iron phosphate and ternary cathode materials can be further enhanced, solving the problems of voltage platform mismatch and poor capacity utilization after the two are mixed, and further improving the capacity, cycle stability and service life of the electrochemical device.

[0018] Preferably, the rate of the low-rate charging and the rate of the low-rate discharging are less than or equal to 0.5C, for example, 0.01C, 0.1C, 0.2C, 0.3C, 0.4C or 0.5C, etc.

[0019] Preferably, the rate of the low-rate charging and the rate of the low-rate discharging are 0.2C to 0.5C.

[0020] Preferably, the rate of the high-rate charging and the high-rate discharging are greater than 0.5C, for example, 0.55C, 0.6C, 0.7C, 0.8C, 0.9C, 1C, 1.1C, 1.2C, 1.5C, 2C, 3C or 5C, etc.

[0021] Preferably, the rate of high-rate charging and high-rate discharging is from 0.5C to 3C.

[0022] The charging and discharging method provided by this invention, by matching appropriate rates in different ranges during charging and discharging, fully utilizes the capacity and rate performance of lithium manganese iron phosphate and ternary cathode materials in their respective charging and discharging ranges, thereby improving the rate performance and cycle performance of the electrochemical device while meeting energy density requirements.

[0023] As a preferred method of the charging and discharging method of the present invention, the mass ratio of lithium manganese iron phosphate to ternary cathode material is (1:9) to (9:1), for example, it can be 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2 or 9:1, etc.

[0024] Preferably, the chemical formula of the lithium manganese iron phosphate is Li. m Mn n Fe 1-nPO4, where 0 < n < 1, for example, it can be 0, 0.1, 0.3, 0.5, 0.7, 0.9 or 1, etc., and is further preferably 0.5 < n < 0.8; 0.9 ≤ m ≤ 1.1, for example, it can be 0.9, 1 or 1.1, etc., and is further preferably 1 ≤ m ≤ 1.03.

[0025] Preferably, the morphology of the lithium iron manganese phosphate is a nano - morphology and / or a secondary particle morphology.

[0026] Preferably, for the lithium iron manganese phosphate in nano - morphology, the particle size Dmin is from 0.1 μm to 0.3 μm, for example, it can be 0.1 μm, 0.15 μm, 0.2 μm, 0.25 μm or 0.3 μm, etc.; D10 is from 0.3 μm to 0.6 μm, for example, it can be 0.3 μm, 0.35 μm, 0.4 μm, 0.45 μm, 0.5 μm, 0.55 μm or 0.6 μm, etc.; D50 is from 0.7 μm to 3 μm, for example, it can be 0.7 μm, 0.9 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2..4 μm, 2.6 μm, 2.8 μm or 3 μm, etc.; D90 is from 3.0 μm to 12 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm or 12 μm, etc.

[0027] Preferably, for the lithium iron manganese phosphate in secondary particle morphology, the particle size Dmin is from 0.2 μm to 0.4 μm, for example, it can be 0.2 μm, 0.25 μm, 0.3 μm, 0.35 μm or 0.4 μm, etc.; D10 is from 1 μm to 3 μm, for example, it can be 1 μm, 1.5 μm, 2 μm, 2.5 μm or 3 μm, etc.; D50 is from 7 μm to 11 μm, for example, it can be 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm, 10 μm, 10.5 μm or 11 μm, etc.; D90 is from 15 μm to 25 μm, for example, it can be 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm or 25 μm, etc.

[0028] Preferably, the chemical formula of the ternary cathode material is LiNi a Co b Mn 1-a-b O2, where 0.5 ≤ a ≤ 0.9, for example, it can be 0.5, 0.6, 0.7, 0.8 or 0.9, etc., 0 < b ≤ <0.2, for example, it can be 0.01, 0.05, <0.1, <0.05 or <0.2, etc.

[0029] Preferably, the morphology of the ternary cathode material is a single - crystal morphology and / or a secondary particle morphology.

[0030] Preferably, the particle size D50 of the single-crystal ternary cathode material is 3μm to 5μm, for example, it can be 3μm, 3.2μm, 3.4μm, 3.6μm, 3.8μm, 4μm, 4.2μm, 4.4μm, 4.6μm, 4.8μm or 5μm, etc.

[0031] Preferably, the particle size D50 of the secondary particle ternary cathode material is 10 μm to 20 μm, for example, it can be 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm, etc.

[0032] Preferably, the particle size D50 of the primary particles in the ternary cathode material with secondary particle morphology is 0.1 μm to 3 μm, for example, it can be 0.1 μm, 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, or 3 μm, etc.

[0033] Preferably, the particle size D50 of the primary particles in the ternary cathode material with secondary particle morphology is 0.1 μm to 1 μm.

[0034] This invention optimizes the morphology and primary and secondary particle sizes of lithium manganese iron phosphate and ternary cathode materials, selects suitable admixtures as cathode active materials for electrochemical devices, improves the electrochemical performance of the materials, further enhances the matching between lithium manganese iron phosphate and ternary cathode materials in the electrochemical device, and improves the capacity, cycle stability and service life of the electrochemical device by combining with specific charge and discharge modes.

[0035] Preferably, the surface of the lithium manganese iron phosphate is further coated with carbon.

[0036] As a preferred embodiment of the charging and discharging method of the present invention, the charging and discharging method includes:

[0037] The electrochemical device is charged at a low rate within the voltage ranges of (2.8V to 3.4V), (3.6V to 3.9V), and (4.1V to xV), where x is 4.2V to 4.4V, with a rate less than or equal to 0.5C; and charged at a high rate within the voltage ranges of (3.4V to 3.6V) and (3.9V to 4.1V), with a rate greater than 0.5C; and / or the electrochemical device is discharged at a low rate within the voltage ranges of (3.4V to 3.7V) and (3.9V to yV), where y is (4.2V to 4.4V), with a rate less than or equal to 0.5C; and discharged at a high rate within the voltage ranges of (2.8V to 3.4V) and (3.7V to 3.9V), with a rate greater than 0.5C, to complete the charge-discharge process.

[0038] It should be noted that the electrochemical device is not specifically limited in this invention. In one optional embodiment, the electrochemical device is a lithium-ion battery.

[0039] In one alternative embodiment, the positive electrode of the electrochemical device comprises a positive electrode active material, a conductive agent, and a binder.

[0040] Preferably, the conductive agent includes conductive carbon black (SP) and / or carbon nanotubes (CNTs).

[0041] Preferably, the adhesive comprises polyvinylidene fluoride (PVDF).

[0042] Preferably, the mass ratio of the positive electrode active material, SP, CNT and PVDF is (90 to 99):1:0.5:2, for example, it can be 90:1:0.5:2, 92:1:0.5:2, 94:1:0.5:2, 96:1:0.5:2 or 99:1:0.5:2, etc.

[0043] In an optional embodiment, the negative electrode of the electrochemical device comprises graphite, SP, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR), wherein the mass ratio of graphite, SP, CMC, and SBR is (90 to 99):1:1.5:2, for example, 90:1:1.5:2, 92:1:1.5:2, 94:1:1.5:2, 96:1:1.5:2, 98:1:1.5:2, or 99:1:1.5:2, etc.

[0044] In one alternative embodiment, the electrolyte of the electrochemical device comprises a lithium salt and a solvent.

[0045] In an alternative embodiment, the lithium salt comprises LiPF6.

[0046] In an optional embodiment, the lithium salt content is 4 wt% to 24 wt% based on 100 wt% of the electrolyte, for example, it can be 4 wt%, 8 wt%, 10 wt%, 15 wt%, 20 wt%, or 24 wt%.

[0047] In an optional embodiment, the solvent includes at least one or any combination of two of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and polycarbonate (PC), such as a combination of EC and EMC, a combination of DMC and PC, a combination of EC, EMC, and DMC, or a combination of EC, EMC, DMC, and PC, etc.

[0048] In an optional embodiment, the mass ratio of EC, EMC, DMC, and PC in the solvent is (2 to 4):(3 to 5):(2 to 4):(0 to 1). The range of EC (2 to 4) can be, for example, 2, 2.5, 3, 3.5, or 4. The range of EMC (3 to 5) can be, for example, 3, 3.5, 4, 4.5, or 5. The range of DMC (2 to 4) can be, for example, 2, 2.5, 3, 3.5, or 4. The range of PC (0 to 1) can be, for example, 0, 0.1, 0.2, 0.3, 0.5, 0.7, or 1. When PC is 0, it means that the solvent does not contain PC.

[0049] In this invention, the method of assembling the electrochemical device using the aforementioned positive electrode, negative electrode, and separator is prior art, and those skilled in the art can refer to the methods disclosed in the prior art for assembly. Taking a lithium-ion battery as an example, the positive electrode, separator, and negative electrode are wound or stacked in sequence to form a cell, which is then installed in a battery case, injected with electrolyte, formed, and packaged to obtain the electrochemical device.

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] This invention performs high-rate charging and / or discharging (i.e., fast charging / fast discharging) in the range where the voltages of lithium manganese iron phosphate and ternary cathode materials match, while performing low-rate charging and / or discharging in the range where their voltages do not match. This intelligent step-charging mode solves the problem of voltage mismatch and poor capacity utilization between lithium manganese iron phosphate and ternary cathode materials, improves the cycle stability of the electrochemical device, and extends the service life of the electrochemical device. Detailed Implementation

[0052] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0053] Example 1

[0054] This embodiment provides a charging and discharging method for an electrochemical device, the charging and discharging method comprising:

[0055] The electrochemical device was placed in the battery performance testing system (equipment model: BTS05 / 10C8D-HP) of Shenghong Electric Co., Ltd. At 25°C, it was charged to 3.4V at 0.33C, then charged to 3.6V at 1C, then charged to 3.9V at 0.33C, then charged to 4.1V at 1C, then fully charged to 4.3V at 0.33C, and then charged at a constant voltage to 0.05C to obtain the charging capacity; then discharged to 2.8V at 1C to obtain its discharge capacity; the above charge and discharge steps were repeated 200 times, and the discharge capacity of the 200th cycle was divided by the discharge capacity of the first cycle to obtain the capacity retention rate after 200 cycles.

[0056] The positive electrode of the aforementioned electrochemical device includes a positive electrode active material (lithium manganese iron phosphate LiMn). 0.6 Fe 0.4 PO4 and ternary cathode material LiNi 0.5 Co 0.2 Mn 0.3 The mass ratio of O2 is 7:3), SP and PVDF, the morphology of lithium manganese iron phosphate is nano-sized with a particle size D50 of 1μm, and the morphology of ternary cathode material is single crystal with a particle size D50 of 3.8μm.

[0057] This embodiment also provides a method for preparing the above-mentioned electrochemical device, including:

[0058] (1) Preparation of the positive electrode: The positive electrode active material (lithium manganese iron phosphate LiMn) is prepared. 0.6 Fe 0.4 PO4 and ternary cathode material LiNi 0.5 Co 0.2 Mn 0.3 O2 (mass ratio 7:3), SP, PVDF and N-methylpyrrolidone (NMP) were mixed in a mass ratio of 99:1.5:1:40 and stirred at high speed for 2 hours to obtain a positive electrode slurry. The positive electrode slurry was then evenly coated onto aluminum foil with a scraper and placed in a forced-air drying oven to dry at 120°C for 20 minutes. The dried electrode sheet was then rolled and cut to prepare the positive electrode.

[0059] (2) Preparation of negative electrode: Graphite, SP, CMC and SBR are mixed in a mass ratio of 95.5:1:1.5:2 to prepare a slurry and coated on copper foil, and then rolled to obtain the negative electrode;

[0060] (3) Preparation of lithium-ion battery: Using the above positive and negative electrodes, 1M LiPF6 electrolyte, the electrolyte solvent is ethylene carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) electrolyte with a mass ratio of 1:1:1, and PE base film, a 1Ah soft pack battery is assembled. The battery is formed and aged, and then charged and discharged once at a rate of 0.33C with a voltage window of 2.0V to 4.3V to define the actual capacity of the battery and obtain the electrochemical device.

[0061] Example 2

[0062] This embodiment provides a charging and discharging method for an electrochemical device, wherein the charging and discharging method is as follows:

[0063] The same electrochemical apparatus as in Example 1 was placed in the battery performance testing system (equipment model: BTS05 / 10C8D-HP) of Shenghong Electric Co., Ltd. At 25°C, it was fully charged to 4.3V at 0.5C, and then charged at a constant voltage to 0.05C to obtain the charging capacity. Then it was discharged to 3.9V at 0.33C, then discharged to 3.7V at 1C, then discharged to 3.4V at 0.33C, and then discharged to 2.8V at 1C to obtain the discharge capacity. The above charge and discharge steps were repeated 200 times. The discharge capacity of the 200th cycle was divided by the discharge capacity of the first cycle to obtain the capacity retention rate after 200 cycles.

[0064] Examples 3 to 9 are based on the steps of Example 1 with parameter changes. The specific changed parameters and test results are shown in Tables 1 and 2. The test results of Comparative Examples 1 to 2 are shown in Table 3.

[0065] Table 1

[0066]

[0067] By comparing Examples 1 with Examples 3 and 4, and Examples 2 with Examples 5 and 6 in Table 1, it can be seen that using appropriate high and low charging rates can further improve the performance of the electrochemical device. When the low charging rate is too small, the difference in electrical performance is not significant, but the charging time cost increases exponentially as the rate decreases. When the high charging rate is too large, both the specific capacity and cycle life deteriorate. Therefore, in Example 1, using an appropriate charging rate results in the best capacity utilization and cycle performance of the electrochemical device.

[0068] Table 2

[0069]

[0070] By comparing Example 1 with Examples 7 to 9 in Table 2, it can be seen that the use of lithium manganese iron phosphate with appropriate particle size and ternary cathode material in this invention can further improve the matching between the two. Combined with a specific charge and discharge mode, it can improve the capacity, cycle stability and service life of the electrochemical device.

[0071] Comparative Example 1

[0072] This comparative example provides a charging and discharging method for an electrochemical device, wherein the charging and discharging method is as follows:

[0073] The same electrochemical device as in Example 1 was placed in the battery performance testing system (equipment model: BTS05 / 10C8D-HP) of Shenghong Electric Co., Ltd. At 25°C, it was fully charged to 4.3V at 0.5C, and then charged at a constant voltage to 0.05C to obtain the charging capacity; then it was discharged to 2.8V at 1C to obtain the discharge capacity; the above charge and discharge steps were repeated 200 times, and the discharge capacity of the 200th cycle was divided by the discharge capacity of the first cycle to obtain the capacity retention rate after 200 cycles.

[0074] Comparative Example 2

[0075] This comparative example provides a charging and discharging method for an electrochemical device, wherein the charging and discharging method is as follows:

[0076] The same electrochemical device as in Example 1 was placed in the battery performance testing system (equipment model: BTS05 / 10C8D-HP) of Shenghong Electric Co., Ltd. At 25°C, it was fully charged to 4.3V at 0.5C, and then charged at a constant voltage to 0.05C to obtain the charging capacity; then discharged at 0.5C to 2.8V to obtain the discharge capacity; the above charge and discharge steps were repeated 200 times, and the discharge capacity of the 200th cycle was divided by the discharge capacity of the first cycle to obtain the capacity retention rate after 200 cycles.

[0077] Table 3

[0078] Charging capacity (mAh / g) Discharge capacity (mAh / g) Capacity retention rate after 200 cycles (%) Example 1 162.4 148.1 95.6 Comparative Example 1 158.4 138.1 92.1 Example 2 159.5 147.2 97.5 Comparative Example 2 156.3 140.4 93.7

[0079] By comparing Examples 1 to 2 with Comparative Examples 1 to 2 in Table 3, it can be seen that the present invention adopts a step-charge / step-discharge mode, using a small current in the region where the voltage ranges of lithium manganese iron phosphate and ternary cathode materials are mismatched, and using a large current in the region where the voltage ranges of the two are matched, thus avoiding high-rate charge and discharge of the materials. Therefore, the charge and discharge capacity of the electrochemical device in the examples and the capacity retention rate after 200 cycles are better than those of Comparative Examples 1 to 2.

[0080] In summary, this invention performs high-rate charging and / or discharging within the voltage range where the lithium manganese iron phosphate and ternary cathode materials match, and low-rate charging and / or discharging within the voltage range where they do not match. This solves the problem of voltage mismatch and poor capacity utilization between lithium manganese iron phosphate and ternary cathode materials, improves the cycle stability of the electrochemical device, and extends the service life of the electrochemical device.

[0081] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A charging and discharging method for an electrochemical device, characterized in that, The positive electrode of the electrochemical device includes lithium manganese iron phosphate and a ternary positive electrode material, and the charging and discharging method includes: The electrochemical device is charged at a low rate in a first voltage range and at a high rate in a second voltage range; and / or The electrochemical device is discharged at a low rate in the third voltage range and at a high rate in the fourth voltage range. The first voltage range is the voltage range in which the charging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material do not overlap; the second voltage range is the voltage range in which the charging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material overlap; the third voltage range is the voltage range in which the discharging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material do not overlap; and the fourth voltage range is the voltage range in which the discharging voltage platforms of the lithium manganese iron phosphate and the ternary cathode material overlap. The first voltage range, the second voltage range, the third voltage range, and the fourth voltage range satisfy any one of the following conditions (a) to (d): (a) The first voltage range is (2.8V to 3.4V), (3.6V to 3.9V) and (4.1V to xV), where x is (4.2V to 4.4V); (b) The second voltage range is (3.4V to 3.6V) and (3.9V to 4.1V); (c) The third voltage range is (3.4V to 3.7V) and (3.9V to yV), where y is (4.2V to 4.4V); (d) The fourth voltage range is (2.8V to 3.4V) and (3.7V to 3.9V).

2. The charging and discharging method according to claim 1, characterized in that, The low-rate charging, high-rate charging, low-rate discharging, and high-rate discharging satisfy any one of the following conditions (e) to (h): (e) The rate of the low-rate charging and the low-rate discharging are less than or equal to 0.5C; (f) The rate of the low-rate charging and the rate of the low-rate discharging are from 0.2C to 0.5C; (g) The rate of the high-rate charging and the high-rate discharging are greater than 0.5C; (h) The rate of high-rate charging and high-rate discharging is from 0.5C to 3C.

3. The charging and discharging method according to claim 1, characterized in that, The mass ratio of lithium manganese iron phosphate to ternary cathode material is (1:9) to (9:1).

4. The charging and discharging method according to claim 1, characterized in that, The chemical formula of the lithium manganese iron phosphate is Li m Mn n Fe 1-n PO4, where 0 <n<1,0.9≤m≤1.1。 5. The charging and discharging method according to claim 1, characterized in that, The lithium manganese iron phosphate is in the form of nano-sized particles and / or secondary particles, and the nano-sized lithium manganese iron phosphate and the secondary particle-shaped lithium manganese iron phosphate satisfy any one of the following conditions (i) to (j): (i) The nano-sized lithium manganese iron phosphate has a particle size Dmin of 0.1 μm to 0.3 μm, D10 of 0.3 μm to 0.6 μm, D50 of 0.7 μm to 3 μm, and D90 of 3.0 μm to 12 μm; (j) The secondary particulate lithium manganese iron phosphate has a particle size Dmin of 0.2 μm to 0.4 μm, D10 of 1 μm to 3 μm, D50 of 7 μm to 11 μm, and D90 of 15 μm to 25 μm.

6. The charging and discharging method according to claim 1, characterized in that, The chemical formula of the ternary cathode material is LiNi. a Co b Mn 1-a-b O2, where 0.5 ≤ a ≤ 0.9, 0 <b≤0.2。 7. The charging and discharging method according to claim 1, characterized in that, The ternary cathode material is in the form of a single crystal and / or a secondary particle form, and the single-crystal ternary cathode material and the secondary particle ternary cathode material satisfy any one of the following conditions (k) to (n): (k) The particle size D50 of the single-crystal ternary cathode material is 3 μm to 5 μm; (l) The particle size D50 of the secondary particle ternary cathode material is 10 μm to 20 μm; (m) The particle size D50 of the primary particles in the ternary cathode material with secondary particle morphology is 0.1 μm to 3 μm; (n) The particle size D50 of the primary particles in the ternary cathode material with secondary particle morphology is 0.1 μm to 1 μm.

8. The charging and discharging method according to claim 1, characterized in that, The surface of the lithium manganese iron phosphate is also coated with carbon.

9. The charging and discharging method according to claim 1, characterized in that, The charging and discharging method includes: The electrochemical device is charged at a low rate within the voltage ranges of (2.8V to 3.4V), (3.6V to 3.9V), and (4.1V to xV), where x is 4.2V to 4.4V, with a rate less than or equal to 0.5C; and charged at a high rate within the voltage ranges of (3.4V to 3.6V) and (3.9V to 4.1V), with a rate greater than 0.5C; and / or the electrochemical device is discharged at a low rate within the voltage ranges of (3.4V to 3.7V) and (3.9V to yV), where y is (4.2V to 4.4V), with a rate less than or equal to 0.5C; and discharged at a high rate within the voltage ranges of (2.8V to 3.4V) and (3.7V to 3.9V), with a rate greater than 0.5C, to complete the charge-discharge process.

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