Formation method of lithium iron phosphate battery blended and doped with lithium ferrite and lithium nickelate

Abstract

The invention discloses a formation method of a lithium iron phosphate battery blended and doped with lithium ferrite and lithium nickelate, and relates to the technical field of batteries, the method comprises the following steps: step 1, heating N-methyl pyrrolidone to 45 DEG C; and 2, uniformly mixing a conductive agent and lithium iron phosphate powder which are weighed in advance with PVDF, and meanwhile, adding 0-5wt% of LNO and LFO blended lithium supplement agent into the LFP dry mixed material. According to the lithium iron phosphate battery with the positive electrode mixed with the lithium ferrite and the lithium nickelate, a stepped formation process mode is adopted, and a blending lithium supplement agent is adopted, so that the LNO is subjected to lithium deintercalation and intercalation to a certain extent, the long-time capacity retention rate can be realized, the unstable factors are compensated by the LFO, a more stable SEI film is formed through small-current stepped charging, and the capacity retention rate is improved. Therefore, high energy density and safe long cycle performance are maintained to the greatest extent, and the risk of lithium precipitation of the negative pole piece is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of batteries, and in particular to a formation method of a lithium iron phosphate battery in which lithium ferrite and lithium nickelate are co-doped. Background Art

[0002] Lithium-ion batteries are usually composed of four key materials: positive electrode materials, negative electrode materials, electrolytes, and separators. Among them, ternary materials (NCM / NCA) and lithium cobalt oxide (LCO) materials are layered structures, which are prone to structural instability, excessive metal dissolution, and particle breakage during the cycle process, and often cannot meet the long service life requirements of energy storage power stations. Therefore, lithium iron phosphate (LFP) is usually used for the positive electrode, and graphite is usually used for the negative electrode. LFP has a strong PO bond and plays a good structural support role, which can better accept the negative impact of the continuous insertion and removal of Li+ on the structure during the cycle.

[0003] In recent years, with the continuous development of lithium iron phosphate batteries, energy density, cycle performance, etc. have been further improved, but irreversible capacity loss still restricts the further improvement of lithium-ion battery performance. Since lithium ions will generate a passivation film, namely SEI film, during the first charge and discharge process, the process of forming these lithium-containing SEI films is irreversible, so it will permanently consume a part of the Li+ from the positive electrode, reduce the first cycle Coulomb efficiency (ICE), and cause the energy and capacity density of lithium-ion batteries to be low.

[0004] Based on this, a formation method of a lithium iron phosphate battery in which lithium ferrite and lithium nickelate are co-doped is now provided, which can eliminate the disadvantages of the existing device. Summary of the invention

[0005] The object of the present invention is to provide a formation method of a lithium iron phosphate battery in which lithium ferrite and lithium nickelate are co-doped, so as to solve the shortcomings of the current product in the background technology.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A formation method for a lithium iron phosphate battery in which lithium ferrite and lithium nickelate are co-doped comprises the following steps:

[0008] The following steps are involved:

[0009] Step 1: Heat N-methylpyrrolidone to 45°C;

[0010] Step 2: Evenly mix the weighed PVDF, conductive agent and lithium iron phosphate powder to obtain a mixed material 1;

[0011] Take LNO and LFO mixed with lithium supplement, and add them into the mixed material 1 at 0wt%-5wt% to prepare the mixed material 2;

[0012] Step 3: adding the mixed material 2 in step 2 to the heated N-methylpyrrolidone, and mixing them thoroughly to prepare a positive electrode sheet, and assembling the positive electrode sheet with the negative electrode sheet, the isolation film, and the electrolyte into a hard shell battery cell;

[0013] Step 4: placing the hard shell battery cell in step 3 at 40-55° C. for 12-24 hours to obtain a shelf battery cell;

[0014] Step 5: Charge the battery in step 4 once, and then let it stand for 5-30 minutes to obtain a charged battery cell;

[0015] Step 6: recharge the primary rechargeable battery cell in step 5, and then let it stand for 5 min-30 min to obtain a secondary rechargeable battery cell;

[0016] Step 7: Charge the secondary rechargeable battery cell in step 6 three times, and then let it stand for 10-20 minutes to obtain a completed battery cell.

[0017] On the basis of the above technical solution, the present invention also provides the following optional technical solution:

[0018] In an optional solution: when the battery is left on hold for charging in step 5, a constant current of 0.1C-0.16C is introduced to charge the battery to 3.25V.

[0019] In an optional solution: when charging the primary rechargeable battery in step 6, a constant current of 0.03C-0.05C is passed through the primary rechargeable battery to charge the battery to 3.95V.

[0020] In an optional solution: when charging the secondary rechargeable battery cell in step seven, a 0.02C constant current is passed through the secondary rechargeable battery cell to charge the secondary rechargeable battery cell to 4.15V.

[0021] In an optional solution: the charging environment temperature of the shelf battery, the primary rechargeable battery cell, and the secondary rechargeable battery cell is 40°C-50°C.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. The present invention achieves a higher cycle retention rate by adding LNO and LFO mixed lithium supplement agents into the positive electrode material in a certain proportion and adopting different formation process voltages, which is beneficial to improving the low energy density of LFP battery cells during use and extending the service life of the battery cells. On the basis of the existing LFP safety and stability and the existing capacity performance, the long cycle performance requirements are further improved.

[0024] 2. The lithium iron phosphate battery with lithium iron oxide and lithium nickel oxide mixed in the positive electrode of the present invention adopts a step-by-step formation process and a co-mixed lithium supplement, so that LNO can deintercalate lithium to a certain extent, and a long-term capacity retention rate can be achieved. Its unstable factors are compensated by LFO, and a small current step charging forms a more stable SEI film, thereby achieving high energy density and safe long-cycle performance to the greatest extent, and reducing the risk of lithium plating in the negative electrode. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a SEM image of the surface of the electrode with added lithium supplement agent of the present invention.

[0026] Figure 2 For the present invention Figure 1 Partial enlarged image.

[0027] Figure 3 Graph showing the cyclic decay rate of various examples of the present invention.

[0028] Figure 4 The following are data charts of cycle capacity attenuation of various examples of the present invention. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0030] In one embodiment, Figure 1-Figure 4 As shown, a formation method of a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are mixed and doped comprises the following steps:

[0031] Step 1: Heat N-methylpyrrolidone to 45°C;

[0032] Step 2: Evenly mix the weighed PVDF, conductive agent and lithium iron phosphate powder to obtain a mixed material 1;

[0033] Take LNO and LFO mixed with lithium supplement, and add them into the mixed material 1 at 0wt%-5wt% to prepare the mixed material 2;

[0034] Step 3: Add the mixed material 2 to the heated N-methylpyrrolidone, mix thoroughly, and prepare a positive electrode sheet;

[0035] The positive electrode sheet, the negative electrode sheet, the separator and the electrolyte are assembled into a hard shell battery cell;

[0036] Step 4: Place the hard shell battery cell in a 40-55°C environment for 12-24 hours to obtain a shelf battery cell;

[0037] Step 5: Charge the battery in step 4 once, and then let it stand for 5-30 minutes to obtain a charged battery cell;

[0038] Step 6: recharge the primary rechargeable battery cell in step 5, and then let it stand for 5 min-30 min to obtain a secondary rechargeable battery cell;

[0039] Step 7: Charge the secondary rechargeable battery cell in step 6 three times, and then let it stand for 10-20 minutes to obtain a completed battery cell.

[0040] By adding LNO and LFO mixed lithium supplements into the positive electrode material in a certain proportion and using different formation process voltages, a higher cycle retention rate can be achieved, which is beneficial to improve the low energy density of LFP cells during use and extend the service life of the cells. On the basis of the existing LFP safety and stability and the performance of the existing capacity, the long cycle performance requirements are further improved;

[0041] The lithium iron phosphate battery with a positive electrode mixed with lithium iron oxide and lithium nickel oxide adopts a step-by-step formation process and a co-mixed lithium supplement, which allows LNO to deintercalate lithium to a certain extent, thereby achieving a long-term capacity retention rate. Its unstable factors are compensated by LFO, and small current step charging forms a more stable SEI film, thereby achieving high energy density and safe long-cycle performance to the greatest extent, reducing the risk of lithium plating on the negative electrode.

[0042] In one embodiment, when the battery is left on hold for charging in step 5, a constant current of 0.1C-0.16C is introduced to charge the battery to 3.25V.

[0043] In one embodiment, when charging the primary rechargeable cell in step six, a constant current of 0.03C-0.05C is passed through the primary rechargeable cell to charge it to 3.95V.

[0044] In one embodiment, when the secondary rechargeable battery cell is charged in step seven, a 0.02C constant current is passed through the secondary rechargeable battery cell to charge the secondary rechargeable battery cell to 4.15V.

[0045] In one embodiment, the charging environment temperature of the storage battery, the primary rechargeable battery cell, and the secondary rechargeable battery cell is 40° C.-50° C.

[0046] The above embodiment discloses a formation method of a lithium iron phosphate battery in which lithium ferrite and lithium nickelate are co-doped. The specific comparative experimental process and results are as follows:

[0047] Experiment 1:

[0048] The 93% LCO positive electrode sheet was assembled with the graphite electrode sheet, isolation membrane and electrolyte into a lithium-ion hard shell battery cell, and finally a 500-cycle experiment was performed.

[0049] Lithium iron phosphate slurry preparation process:

[0050] First, heat N-methylpyrrolidone to 45°C, then mix the pre-weighed PVDF, conductive agent and lithium iron phosphate powder in a ratio of 85:12.5:2.5, add the mixed dry materials into the preheated NMP, and fully mix them into a slurry with a certain viscosity.

[0051] Hard shell battery preparation process:

[0052] S1: Lithium iron phosphate slurry is coated on an aluminum foil sheet, then rolled using a roller press and divided using a slitting machine to form a positive electrode sheet of appropriate size;

[0053] S2: Mix graphite and PVDF and coat them on copper foil, then roll them using a roller press and split them using a slitting machine to make negative electrode sheets of appropriate size;

[0054] S3: According to the specifications of the prepared battery, the negative electrode, the separator, and the positive electrode are wound to form a battery cell and then incorporated into an aluminum shell;

[0055] S4: Place the battery cell in the liquid injection machine and inject an appropriate amount of electrolyte twice. After injection, seal the battery to ensure that the electrolyte will not leak. After welding the pole ears and wrapping the insulation layer, the lithium iron phosphate hard shell battery is completed.

[0056] Hard shell battery formation and capacity process:

[0057] S1: placing the hard shell battery cell in an environment of 40-55°C for 12-24 hours to obtain a shelf battery cell;

[0058] S2: At an ambient temperature of 40°C-50°C, the battery in S1 is charged once, with a constant current of 0.1C-0.16C, charged to 3.25V, and then left to stand for 5min-30min to obtain a charged cell;

[0059] S3: At an ambient temperature of 40°C-50°C, the primary rechargeable battery in S2 is recharged under the following charging conditions: 0.03C-0.05C constant current, charged to 3.95V, and then left to stand for 5min-30min to obtain a secondary rechargeable battery;

[0060] S4: At an ambient temperature of 40°C-50°C, the secondary rechargeable battery cell in S3 is charged three times. The charging conditions are: 0.02C constant current is passed, charged to 4.15V, and then left to stand for 10-20 minutes to obtain a completed battery cell.

[0061] The final lithium iron phosphate battery was placed in a temperature control device, the required temperature was set to 20℃-30℃, constant power charged to 3.65V at 1P power, allowed to stand for 5 minutes, constant power discharged to 2.5V, allowed to stand for 5 minutes, one charge and discharge was recorded as one cycle, and after 500 cycles, the capacity decay rate cycle curve and battery capacity change curve were recorded.

[0062] like Figure 3 As shown, the lithium iron phosphate hard shell battery without adding lithium supplement agent was finally obtained. The capacity decay rate was 96% at 500cls in the cycle test, and the capacity decay rate was expected to reach 80% after 3500cls+ cycle.

[0063] like Figure 4 As shown, as the number of cycles increases, the battery capacity decreases by about 1Ah from the maximum capacity;

[0064] The main reasons are the irreversible loss of Li+ when SEI is formed and the loss of Li+ when shuttling back and forth between the positive and negative electrodes during the cycle, which leads to the rapid depletion of the battery cell capacity and affects the cycle life of the lithium iron phosphate battery.

[0065] Experiment 2:

[0066] The 93% LCO positive electrode sheet was assembled with the graphite electrode sheet, isolation membrane and electrolyte into a lithium-ion hard shell battery cell, and finally a 500-cycle experiment was performed.

[0067] Lithium iron phosphate slurry preparation process:

[0068] First, heat N-methylpyrrolidone to 45°C, then mix the pre-weighed PVDF, conductive agent and lithium iron phosphate powder in a ratio of 85:12.5:2.5, add 3wt% of the blended lithium supplement agent, stir evenly again, pay attention to controlling the ambient humidity, and finally add the mixed dry materials to the preheated NMP and fully mix them into a slurry with a certain viscosity.

[0069] Hard shell battery preparation process:

[0070] Same as Experiment 1 above.

[0071] Hard shell battery formation and capacity process:

[0072] Same as Experiment 1 above.

[0073] like Figure 3 As shown, in Experiment 2, a lithium iron phosphate hard shell battery with 3wt% lithium supplement added was finally obtained. In the cycle test, the capacity decay rate was 101.05% at 500cls, and the capacity decay rate was expected to reach 80% after 6000cls+ cycle.

[0074] like Figure 4As shown, with the increase of cycle number, the battery capacity decreases by about 0.25Ah from the maximum capacity.

[0075] This is because when SEI is formed, the lithium supplement agent promptly replenishes the irreversible Li+ loss, and part of the lithium supplement agent participates in the cycle process, allowing Li+ to shuttle back and forth between the positive and negative electrodes, reducing capacity attenuation and maintaining a high energy density, thereby increasing the cycle life of the lithium iron phosphate battery.

[0076] Experiment 3:

[0077] The 93% LCO positive electrode sheet was assembled with the graphite electrode sheet, isolation membrane and electrolyte into a lithium-ion hard shell battery cell, and finally a 500-cycle experiment was performed.

[0078] Lithium iron phosphate slurry preparation process:

[0079] First, heat N-methylpyrrolidone to 45°C, then mix the pre-weighed PVDF, conductive agent and lithium iron phosphate powder in a ratio of 85:12.5:2.5, add 5wt% of the blended lithium supplement agent, stir evenly again, pay attention to controlling the ambient humidity, and finally add the mixed dry materials into the preheated NMP and fully mix them into a slurry with a certain viscosity.

[0080] Hard shell battery preparation process:

[0081] Same as Experiment 1 above.

[0082] Hard shell battery formation and capacity process:

[0083] Same as Experiment 1 above.

[0084] like Figure 3 As shown, in Experiment 3, a lithium iron phosphate hard shell battery with 5wt% lithium supplement was finally obtained. The capacity decay rate was 103.61% at 500cls in the cycle test, and the capacity decay rate was expected to reach 80% after 15000cls+ cycle.

[0085] like Figure 4 As shown, as the number of cycles increases, the battery capacity decreases from the highest capacity to 0Ah, but still maintains the highest capacity;

[0086] This is because the addition of 5wt% lithium supplement provides the battery with abundant Li+ reserves, which not only replenishes the irreversible Li+ loss in time when SEI is formed, but also has a large amount of lithium supplement capable of participating in the shuttle of Li+ between the positive and negative electrodes during the cycle, greatly reducing the capacity attenuation, and maintaining the energy density basically unchanged in a cycle, thereby greatly improving the cycle life of lithium iron phosphate batteries.

[0087] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A formation method for a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are co-doped, characterized in that: The following steps are involved: Step 1: Heat N-methylpyrrolidone to 45°C; Step 2: Evenly mix the weighed PVDF, conductive agent and lithium iron phosphate powder to obtain a mixed material 1; Take LNO and LFO mixed with lithium supplement, and add them into the mixed material 1 at 0wt%-5wt% to prepare the mixed material 2; Step 3: adding the mixed material 2 in step 2 into the heated N-methylpyrrolidone, and mixing them thoroughly to prepare a positive electrode sheet; The positive electrode sheet and the negative electrode sheet, the separator and the electrolyte are assembled into a hard shell battery cell; Step 4: placing the hard shell battery cell in step 3 at 40-55° C. for 12-24 hours to obtain a shelf battery cell; Step 5: Charge the battery in step 4 once, and then let it stand for 5-30 minutes to obtain a charged battery cell; Step 6: recharge the primary rechargeable battery cell in step 5, and then let it stand for 5 min-30 min to obtain a secondary rechargeable battery cell; Step 7: Charge the secondary rechargeable battery cell in step 6 three times, and then let it stand for 10-20 minutes to obtain a completed battery cell.

2. The formation method of a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are co-doped according to claim 1, characterized in that: When the battery is left on hold for charging in step 5, a constant current of 0.1C-0.16C is introduced to charge the battery to 3.25V.

3. The formation method of a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are co-doped according to claim 1, characterized in that: When charging the primary rechargeable battery in step 6, a constant current of 0.03C-0.05C is passed through the primary rechargeable battery to charge the battery to 3.95V.

4. The formation method of a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are co-doped according to claim 1, characterized in that: When the secondary rechargeable battery cell is charged in step 7, 0.02C constant current is passed through and charged to 4.15V.

5. The formation method of a lithium iron phosphate battery in which lithium iron oxide and lithium nickel oxide are co-doped according to claim 1, characterized in that: The charging environment temperature of the shelved battery, the primary charging cell and the secondary charging cell is 40°C-50°C.

Images