Secondary battery and electric apparatus

Abstract

The present application relates to the technical field of electrochemistry, and specifically discloses a secondary battery and an electric apparatus. In the present application, a positive active material in a positive electrode sheet contains a lithium manganese iron phosphate material. A secondary battery using the positive electrode sheet is subjected to a charge / discharge test at 45°C under a 1C / 1C condition, and a dQ / dV-V curve is plotted. Within a range of 3.0 V to 3.85 V on the dQ / dV-V curve, a main reduction peak at a lower potential is denoted as peak A, and a main reduction peak at a higher potential is denoted as peak B. The peak potential difference ΔV between peak A and peak B and the half-peak width W of peak A satisfy the following relationship: 0.011≤△V 0.2×W≤0.145. In the present application, by means of controlling the oxidation-reduction reaction characteristics of a positive active material, a secondary battery containing the positive active material has a good high-temperature cycling characteristic and a good high-temperature storage performance.

Description

Secondary batteries and electrical appliances

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent Application No. 202411641270.0, filed on November 18, 2024, entitled "Secondary Battery and Electric Device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of electrochemical technology, specifically to secondary batteries and electrical devices. Background Technology

[0004] Lithium iron phosphate (LiFePO4, LFP) is a widely used battery cathode material in electric vehicles and energy storage devices, but its energy density is relatively low. Lithium manganese iron phosphate (LFP), as an upgraded product of LFP, has improved energy density. The theoretical voltage plateau of LFP is approximately 3.5V. With the introduction of manganese, the charge and discharge voltages of manganese and iron differ, resulting in two voltage plateaus for LFP, corresponding to the redox reactions of manganese and iron. The plateau near 3.5V corresponds to Fe. 2+ Converted to Fe 3+ Mn corresponds to a voltage around 4.1V. 2+ Transform into Mn 3+ In other words, the introduction of manganese allows the voltage platform of lithium manganese iron phosphate to reach 4.1V, and the theoretical energy density is 10-20% higher than that of lithium iron phosphate, which helps to improve the driving range of new energy vehicles.

[0005] For batteries using lithium manganese iron phosphate (LMFP) as the positive electrode active material, the battery's operating voltage changes due to failure during cycling or storage, and the consumption of active lithium. The negative electrode's cutoff potential rises sharply, causing the battery to reach its discharge cutoff voltage earlier than expected during discharge, even when the positive electrode potential is high. This prevents the iron plateau capacity from being fully utilized, resulting in a significant reduction in the battery's actual usable capacity during discharge, and rapid capacity decay in LMFP cells.

[0006] Under high temperature conditions, the rates of various side reactions inside the battery increase, the active lithium is consumed at a faster rate, and the high temperature exacerbates the structural degradation of electrode materials, resulting in a faster capacity decay rate for LMFP batteries and relatively poor high-temperature cycle performance.

[0007] Existing technologies have reported that doping LMFP with a certain amount of lithium iron phosphate (LFP) can improve the active lithium consumption rate, but the doping of LFP leads to a decrease in the battery's high-temperature storage performance and also affects its kinetic performance.

[0008] Therefore, it is necessary to develop a secondary battery that has both good high-temperature cycling performance and high-temperature storage performance. Summary of the Invention

[0009] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery and an electrical device. By controlling the redox reaction characteristics of the positive electrode active material in the secondary battery, the secondary battery of this application has excellent high-temperature cycle characteristics and high-temperature storage performance.

[0010] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a positive active material, which includes a lithium manganese iron phosphate material.

[0011] The secondary battery was charged and discharged at 45°C under 1C / 1C conditions. The differential of the charge and discharge curves was calculated, and a dQ / dV-V graph was plotted with voltage as the abscissa and capacity-voltage differential as the ordinate. The dQ / dV-V graph showed two main reduction peaks at different potentials in the range of 3.0V to 3.85V, with the lower potential main reduction peak being peak A. The peak position of peak A was V. A The main reduction peak at a higher potential is peak B, and the peak position value of peak B is V. B ;

[0012] The half-width at half-peak of peak A is denoted as W;

[0013] The difference in peak position between peak A and peak B is denoted as ΔV, where ΔV = V B -V A ;

[0014] △V and W satisfy the following relationship:

[0015] 0.011≤△V 0.2 ×W≤0.145;

[0016] The V A V B The units for ΔV and W are all V.

[0017] As an optional implementation of this application, ΔV and W satisfy the following relationship: 0.015 ≤ ΔV 0.2 ×W≤0.11.

[0018] As an optional implementation of this application, the range of ΔV is 0.05 to 0.3V.

[0019] As an optional implementation of this application, the range of ΔV is 0.08 to 0.27V.

[0020] As an optional implementation of this application, the range of W is 0.016 to 0.20 V.

[0021] As an optional implementation of this application, the range of W is 0.022 to 0.15V.

[0022] As an optional embodiment of this application, the positive electrode active material includes a first lithium manganese iron phosphate and a second lithium manganese iron phosphate, wherein the molar content of manganese in the first lithium manganese iron phosphate is greater than the molar content of manganese in the second lithium manganese iron phosphate.

[0023] As an optional embodiment of this application, the molar content of manganese in the first lithium manganese iron phosphate is 60-90%.

[0024] As an optional embodiment of this application, the molar content of manganese in the second lithium manganese iron phosphate is 0.05-25%.

[0025] As an optional implementation of this application, the mass ratio of the first lithium manganese iron phosphate and the second lithium manganese iron phosphate is (2.3~9)∶1.

[0026] In a second aspect, this application provides an electrical device comprising the aforementioned secondary battery.

[0027] The beneficial effects of this application are as follows:

[0028] This application provides a secondary battery and an electrical device. By controlling the redox reaction characteristics of the positive electrode active material in the secondary battery, this application enables the secondary battery to have excellent high-temperature cycle characteristics and high-temperature storage performance. Attached Figure Description

[0029] Figure 1 shows the dQ / dV-V diagram of the lithium-ion battery in Example 2. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0031] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0032] In this application, numerical ranges are involved. Unless otherwise specified, the numerical ranges mentioned above are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Optionally, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Furthermore, when multiple ranges are provided to describe features or characteristics, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0033] In this application, there are no particular restrictions on the specific dispersion and mixing methods.

[0034] Unless otherwise specified, all reagents or instruments used in this application are commercially available products.

[0035] One embodiment of this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer contains a positive active material, which includes a lithium manganese iron phosphate material.

[0036] The secondary battery was charged and discharged at 45°C under 1C / 1C conditions. The differential of the charge and discharge curves was calculated, and a dQ / dV-V graph was plotted with voltage as the abscissa and capacity-voltage differential as the ordinate. The dQ / dV-V graph showed two main reduction peaks at different potentials in the range of 3.0V to 3.85V, with the lower potential main reduction peak being peak A. The peak position of peak A was V. A The main reduction peak at a higher potential is peak B, and the peak position value of peak B is V. B ;

[0037] The half-width at half-peak of peak A is denoted as W;

[0038] The difference in peak position between peak A and peak B is denoted as ΔV, where ΔV = V B -V A ;

[0039] △V and W satisfy the following relationship:

[0040] 0.011≤△V 0.2 ×W≤0.145;

[0041] The V A V B The units for ΔV and W are all volts (V).

[0042] In this application, by controlling the redox reaction characteristics of the positive electrode active material, the secondary battery exhibits excellent high-temperature cycle characteristics and high-temperature storage performance.

[0043] In the dQ / dV-V diagram, the voltage peaks represent significant redox reactions at specific voltages. The presence of two main reduction peaks in the 3.0V to 3.85V range indicates that the cathode active material of this application exhibits two main electrochemical reactions within this voltage range, possessing at least two Fe reduction reaction plateaus with different potentials. In this application, ΔV represents the peak position difference between the two lowest potential main reduction peaks in the dQ / dV-V diagram, arranged in ascending order of potential, reflecting the Fe reduction in the cathode active material. 3+ The potential difference that exists when reduced; W represents the half-width at half-maximum (WHM) of peak A, which is the half-width at half-maximum of the main reduction peak with the lowest potential. It reflects the slope of the low-potential Fe reaction plateau and embodies the kinetic performance of the battery.

[0044] This study found that ΔV and W should be controlled comprehensively to ensure that |ΔV| 0.2 The range of ×W is 0.011 to 0.145. By comprehensively adjusting ΔV and W, the thermodynamic barrier and reaction kinetics of the redox platform are comprehensively controlled, a low-potential Fe reaction platform is introduced, and the platform potential and slope of the low-potential Fe reaction are controlled so that the intersection point of the low-potential Fe platform and the negative electrode discharge cutoff potential is later, the positive electrode capacity can be fully released, the high-temperature cycle performance of the battery is effectively improved, and the high-temperature storage performance is maintained.

[0045] When △V 0.2 When the value of ×W is below 0.011, it may indicate that the manganese content in the positive electrode active material is too high, or that the kinetic performance is excellent. During high-temperature cycling, the high manganese content may lead to excessive manganese dissolution from the positive electrode, and the high kinetic performance may result in highly active interfacial side reactions, leading to active lithium loss and impedance increase, thus causing poor high-temperature cycling performance of the battery. 0.2 When the value of ×W is higher than 0.145, the intersection of the low-potential Fe plateau and the negative electrode discharge cutoff potential occurs earlier, causing the overall battery voltage to reach its cutoff voltage prematurely. This narrows the voltage window of the positive electrode, resulting in insufficient utilization of the positive electrode active material's capacity and a reduction in actual capacity. Under high-temperature conditions, active lithium is consumed more rapidly, leading to rapid capacity decay and a significant decrease in high-temperature cycle performance. Furthermore, when ΔV... 0.2 When the value of ×W is too high, it can also lead to severe degradation of the battery's high-temperature storage performance. A certain degree of manganese leaching can help the negative electrode form a more stable SEI film; while when ΔV 0.2 When the W value is too high, the initial interaction between the positive electrode and the negative electrode is relatively small, resulting in poor film stability of the negative electrode. During high-temperature storage, the SEI film continuously cracks and re-repairs, which consumes a large amount of active lithium and electrolyte, leading to a deterioration in the high-temperature storage performance of the battery.

[0046] The electrochemical reactions in a battery are nonlinear, and the electrochemical reactions at different plateaus influence each other. By introducing a power of 0.2 for ΔV, the interference between different reduction reactions is balanced, thus ensuring that ΔV... 0.2 The ×W relationship can more accurately reflect the actual dynamic characteristics, thereby more accurately describing the interaction between different voltage platforms, so as to achieve the high-temperature cycling characteristics and high-temperature storage performance of the battery.

[0047] For example, in this application, △V 0.2 The value of ×W can be 0.011, 0.020, 0.025, 0.030, 0.040, 0.045, 0.050, 0.060, 0.070, 0.080, 0.085, 0.090, 0.095, or 0.145, or it can be any interval formed by any two of the above values.

[0048] In one alternative implementation, ΔV and W satisfy the following relationship: 0.015 ≤ ΔV 0.2 ×W≤0.11.

[0049] △V 0.2 When the W value falls within the above selectable range, the battery exhibits superior high-temperature cycle performance and high-temperature storage performance.

[0050] In one optional embodiment, the range of ΔV is 0.05 to 0.3V. For example, ΔV can be 0.05V, 0.10V, 0.15V, 0.20V, 0.25V, 0.28V, or 0.30V.

[0051] In one alternative implementation, the range of ΔV is 0.08 to 0.27V.

[0052] The value of ΔV should not be too high or too low. When ΔV is between 0.05 and 0.3V, and preferably between 0.08 and 0.27V, the overall electrochemical performance of the battery is optimized, resulting in better high-temperature cycle capacity retention and higher high-temperature storage performance. This study found that when the value of ΔV is small, the introduced low-potential Fe reaction plateau potential is higher, making the reaction energies of the two Fe plateaus in the positive electrode active material closer. This facilitates a later intersection point between the low-potential Fe plateau and the negative electrode discharge cutoff potential, allowing for full release of the positive electrode capacity and improving the battery's high-temperature cycle capacity retention. Simultaneously, the value of ΔV should not be too small. If the ΔV value is too small, manganese dissolution from the positive electrode active material increases, the high-temperature cycle failure reaction becomes more severe, active lithium loss is greater, and high-temperature cycle performance declines.

[0053] In one alternative embodiment, the range of W is 0.016 to 0.2V, for example, W can be 0.016V, 0.02V, 0.05V, 0.08V, 0.10V, 0.12V, or 0.2V.

[0054] In one alternative embodiment, the range of W is 0.022 to 0.15 V.

[0055] When W is within the above-mentioned selectable range, it indicates that the half-width at half-maximum (WHM) of peak A is within a suitable range. Within this reduction reaction range, the voltage change rate is appropriate, the battery kinetic performance is good, and it does not cause too many interfacial side reactions. The battery's high-temperature cycling and high-temperature storage performance are also excellent. When the value of W is too large, the WHM of peak A is larger, indicating that the voltage changes faster within this reduction reaction range, and the slope of the reduction reaction plateau is also larger. This may cause the battery to reach a low voltage before fully utilizing its capacity, reaching the discharge cutoff voltage prematurely. It also means that the voltage range in which the reduction reaction occurs is wider, indicating a slower reaction rate and poorer reduction reaction kinetic performance. When the value of W is too small, it indicates that there are relatively more interfacial active sites in the positive electrode active material, which may lead to severe positive electrode interfacial side reactions. It is also not conducive to assisting the formation of a more stable SEI film in the negative electrode through interaction, thus resulting in relatively poor high-temperature cycling and high-temperature storage performance of the battery.

[0056] The values ​​of ΔV and W are affected by a variety of factors of the positive electrode active material, such as the manganese content in lithium manganese iron phosphate materials, the particle size of the positive electrode active material, the carbon coating, and the processing technology.

[0057] This application does not limit the detection methods for ΔV and W. Those skilled in the art can use conventional technical means to perform charge and discharge tests on the secondary battery at 45°C under 1C / 1C conditions, plot the dQ / dV-V curve, and obtain ΔV and W.

[0058] For example, the detection methods for △V and W can be performed as follows:

[0059] (1) High-temperature constant-capacity setting: The secondary battery using the positive electrode sheet was left to stand at 45°C for 120 min, and then discharged at 1C constant current to 2.5V; after standing for 20 min, it was charged at 1C constant current and constant voltage until the cutoff voltage was 4.25V and the cutoff current was 0.33C; after standing for 20 min, it was discharged at 1C constant current to 2.5V; the constant-capacity setting was completed, and the charge-discharge curve was obtained;

[0060] (2) Plotting the dQ / dV-V curve: Based on the above charge and discharge curve, subtract the voltage and energy data of the nth data point from the voltage and energy data of the (n+1)th data point to obtain dV and dQ data; process all data in sequence to obtain a series of dV and dQ data; then divide dQ by dV to obtain dQ / dV; plot the dQ / dV-V curve with the capacity-voltage differential value (dQ / dV) as the vertical axis and the voltage value as the horizontal axis.

[0061] (3) In the order of potential from low to high, within the range of 3.0V to 3.85V in the dQ / dV-V diagram, the main reduction peak at the lower potential is peak A, and the peak position value of peak A is V. A The main reduction peak at a higher potential is peak B, and the peak position value of peak B is V. B The difference in peak position between peak A and peak B is denoted as ΔV, where ΔV = V B -V A ;

[0062] The half-width at half maximum (W) of peak A is given by the value of W.

[0063] V A V B The units for ΔV and W are all volts (V).

[0064] The peaks in the dQ / dV-V diagram correspond to Fe reduction reaction plateaus at different potentials. When the capacity utilization range corresponding to the Fe reduction reaction plateau is located in a different phase transition region of the negative electrode material, it may cause the reduction peak in the dQ / dV-V diagram to split (a peak splits into two or more peaks), resulting in multiple split peaks. When multiple split peaks appear in the dQ / dV-V diagram, the peak with the highest intensity in that group is the dominant reduction peak.

[0065] In one alternative embodiment, the positive electrode active material includes a first lithium manganese iron phosphate and a second lithium manganese iron phosphate, wherein the molar content of manganese in the first lithium manganese iron phosphate is greater than the molar content of manganese in the second lithium manganese iron phosphate.

[0066] The molar content of manganese in lithium iron phosphate (LFP) refers to the molar content of manganese relative to the other metal elements besides lithium. Similarly, the molar content of manganese in lithium iron phosphate (LFP) refers to the molar content of manganese relative to the other metal elements besides lithium.

[0067] Lithium iron phosphate materials with high and low manganese content were used. The Fe content of these two different manganese-containing lithium iron phosphate materials was analyzed. 3+ -Fe 2+ The different reaction potentials allow for the effective introduction of a low-potential Fe plateau and control of ΔV and W to satisfy the aforementioned relationship range.

[0068] In one alternative embodiment, the molar content of manganese in the first lithium manganese iron phosphate is 60-90%.

[0069] In one alternative embodiment, the molar content of manganese in the first lithium manganese iron phosphate is 75-85%.

[0070] In one alternative embodiment, the molar content of manganese in the second lithium manganese iron phosphate is 0.05-25%.

[0071] In one alternative embodiment, the molar content of manganese in the second lithium manganese iron phosphate is 0.1% to 10%.

[0072] The first type of lithium manganese iron phosphate (LFP) is a high-manganese-content LFP material, while the second type is a low-manganese-content LFP material. The high-manganese-content LFP material allows for appropriate manganese dissolution in the positive electrode active material, contributing to a more stable SEI film in the negative electrode and better high-temperature storage performance. The low-manganese-content LFP material primarily functions as a low-potential Fe plateau, providing capacity contribution at a lower voltage plateau. The appropriate introduction of low-manganese-content LFP material can effectively balance the potential difference between high and low potential plateaus, keeping the ΔV value within an appropriate range.

[0073] When the manganese content of the first and second lithium manganese iron phosphate exceeds the above-mentioned selectable range, it may be insufficient to improve lithium-ion loss, resulting in slightly poor high-temperature cycle performance of the battery; or it may be difficult to promote the stability of the negative electrode SEI film, resulting in poor high-temperature storage performance of the battery.

[0074] This application does not limit the method for detecting manganese content. Those skilled in the art can detect the manganese content of lithium manganese phosphate materials using conventional technical means.

[0075] For example, the molar content of manganese in the lithium manganese iron phosphate can be detected by the following method:

[0076] The battery was disassembled to obtain the positive electrode sheet, which was then processed to obtain the positive electrode active material powder. The molar content of manganese in lithium iron phosphate and lithium iron phosphate was obtained by electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

[0077] In one alternative embodiment, the mass ratio of the first lithium manganese iron phosphate and the second lithium manganese iron phosphate is (2.3 to 9): 1.

[0078] In one alternative embodiment, the mass ratio of the first lithium manganese iron phosphate and the second lithium manganese iron phosphate is (4-7):1.

[0079] When the mass ratio of lithium manganese iron phosphate (LFP) to lithium manganese iron phosphate (LFP) is within a suitable range, the battery achieves a better balance between high-temperature storage performance and high-temperature cycling performance, resulting in excellent overall electrochemical performance. When the mass ratio of LFP to LFP exceeds the above-mentioned selectable range, the electrode material may deteriorate more severely at high temperatures due to poor compatibility of high and low manganese ratios, leading to a slight decrease in the battery's high-temperature performance.

[0080] In particular, when the mass ratio of lithium iron phosphate (LFP) is too high, it may lead to insufficient charging capacity at high state of charge (SOC), increasing internal stress and side reactions, accelerating electrode material degradation at high temperatures, and shortening the battery's high-temperature cycle performance. Furthermore, under high-temperature storage conditions, the electrochemical reaction rate inside the battery accelerates. Self-discharge reactions or electrolyte decomposition may consume active lithium, causing the negative electrode potential to rise and the positive electrode potential to fall. This change in electrode potential alters the battery's cutoff potential. When the mass ratio of LFP is too high, a significant amount of LFP may not have participated in the electrochemical reaction by the time the battery reaches the cutoff potential, failing to contribute to the battery's actual capacity and resulting in poor high-temperature storage performance.

[0081] This application does not limit the method for detecting the mass ratio of lithium manganese iron phosphate and lithium manganese iron phosphate in the positive electrode active material of the positive electrode sheet.

[0082] For example, the mass ratio of lithium manganese iron phosphate (LFP) to lithium manganese iron phosphate (LFP) in the positive electrode active material of the positive electrode sheet can be detected by the following method:

[0083] The secondary battery containing the positive electrode sheet was discharged to 2V at 0.33C at room temperature, then charged to 4.25V at a constant current of 0.33C, charged to 0.05C at a constant voltage, and discharged to 2.5V at a constant current of 0.33C. This cycle was repeated for 2 weeks. The discharge data from the last week was used to plot a voltage versus capacity curve. The mass ratio of the first lithium manganese iron phosphate and the second lithium manganese iron phosphate with different manganese contents was determined based on the ratio of the manganese plateau to the iron plateau.

[0084] In one alternative embodiment, the surface of the second lithium manganese iron phosphate is coated with carbon material, and the mass ratio of carbon material to the second lithium manganese iron phosphate (denoted as carbon coating amount) is 1.0 to 3.0 wt.%.

[0085] This application does not limit the method for detecting carbon coating amount. Those skilled in the art can detect the carbon coating amount of positive electrode active material using conventional technical means.

[0086] For example, the carbon coating amount can be tested using the following method:

[0087] After being weighed by an electronic balance, the sample enters the combustion reaction cell. Under sufficient oxygen conditions, it is heated at high temperature in a high-frequency furnace, oxidizing carbon into carbon dioxide. This gas is then filtered and dried before entering the corresponding absorption cell, where it absorbs the corresponding infrared radiation spectrum (carbon dioxide 4200nm). The absorption is then converted into a corresponding electrical signal by a detector. This signal is acquired by a computer, linearly corrected, and converted into a value proportional to carbon dioxide. The values ​​from the entire analysis process are then accumulated. After the analysis is completed, this accumulated value is divided by the weighed value and multiplied by a correction factor in the computer to obtain the mass fraction of carbon in the sample.

[0088] This application does not limit the preparation method of lithium manganese iron phosphate, and those skilled in the art can prepare it using conventional technical means.

[0089] For example, a method for preparing lithium manganese iron phosphate may include the following steps:

[0090] Manganese, iron, phosphorus, lithium and carbon sources (if any) are mixed in a certain molar ratio and then ball-milled and spray-dried.

[0091] The spray-dried product was sintered in an atmosphere with an oxygen concentration of less than 150 ppm.

[0092] The sintered material is crushed and then screened to obtain LMFP.

[0093] The lithium source may include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, or lithium acetate.

[0094] The phosphorus source may include at least one of diammonium hydrogen phosphate, lithium dihydrogen phosphate, ammonium phosphate, or lithium phosphate.

[0095] The iron source may include at least one of ferrous oxalate, ferric hydroxide, ferrous hydroxide, ferric phosphate, ferrous phosphate, ferric acetate, ferrous acetate, ferric carbonate, ferrous carbonate, ferric oxide, ferric oxide, or ferric oxalate.

[0096] The manganese source may include at least one of manganese carbonate, manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, or manganese acetate.

[0097] The carbon source may include glucose.

[0098] The LMFP of this application may contain a certain amount of doping elements. Optionally, the doping element may be selected from at least one of the elements Mg, Ca, Sr, Ti, V, Cr, Ni, Co, Rb, Cu, Zn, La, Y, Mo, Nb, Al, and W.

[0099] When LMFP contains doping elements, a certain amount of doping element source can be mixed with manganese source, iron source, phosphorus source, lithium source, etc., as needed during the preparation of LMFP. Doping element sources include vanadium source (vanadium pentoxide), tungsten source (ammonium metatungstate), titanium source (titanium oxide), magnesium source (magnesium carbonate), etc., to obtain LMFP containing a certain amount of doping elements.

[0100] The chemical formula of lithium manganese iron phosphate is: LiMn x Fe y M z n PO4, where x is greater than 0 and less than 1; y is greater than 0 and less than 1; z is greater than or equal to 0 and less than 1; M refers to the dopant element, and n refers to the valence of the dopant element; satisfying: 2(x+y)+n×z=2.

[0101] Ferric manganese phosphate can be used as a source of manganese, iron and phosphorus simultaneously; iron phosphate can be used as a source of manganese, iron and phosphorus simultaneously; the lithium source can include at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate or lithium acetate.

[0102] The ball milling and spray drying processes can be repeated multiple times. For example, the step of mixing manganese, iron, phosphorus, lithium, and carbon sources (if any) in a certain molar ratio and then performing ball milling and spray drying may include the following:

[0103] Manganese, iron, phosphorus and lithium sources are mixed in a certain molar ratio and subjected to a first ball milling, followed by a first spray drying.

[0104] The product after the first spray drying is mixed with a carbon source (if any), and then subjected to a second ball milling, followed by a second spray drying.

[0105] The first ball mill and the second ball mill, as well as the first spray dryer and the second spray dryer, can all use different process conditions.

[0106] Optionally, the sintering can be performed under gradient sintering conditions.

[0107] The proportion of manganese content in LMFP can be controlled by adjusting the amount of manganese source, iron source, and phosphoric acid added.

[0108] The carbon coating ratio can be controlled by adjusting the amount of carbon source added.

[0109] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may also contain conductive agents and binders.

[0110] The conductive agent only needs to have suitable electronic conductivity and not cause adverse chemical changes in the battery; this application does not impose any particular limitation on the type of conductive agent. Specifically, the conductive agent can be at least one of carbon nanotubes, carbon black, or graphene.

[0111] The binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. In this application, the binder can be a conventional choice in the battery field. Specifically, the conductive agent can be at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, sodium carboxymethyl cellulose (CMC), or sodium alginate.

[0112] This application does not impose any particular restrictions on the positive electrode current collector, as long as it is conductive and will not cause adverse chemical changes in the battery, and can be made of, for example: stainless steel, aluminum, nickel, titanium, sintered carbon; or aluminum or stainless steel that has been surface treated with one of carbon, nickel, titanium, silver, etc.

[0113] In this application, the positive electrode sheet can be prepared according to conventional methods in the art. For example, the positive active material, conductive agent and binder are dispersed in a solvent to form a uniform positive electrode slurry, the positive electrode slurry is coated on the positive current collector, and after drying, rolling and other processes, the positive electrode sheet is obtained.

[0114] One embodiment of this application provides a secondary battery comprising the positive electrode sheet described above.

[0115] In addition to the positive electrode, the secondary battery also includes a negative electrode, a separator, and an electrolyte.

[0116] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, wherein the negative active material layer includes a negative active material. Regarding the negative active material, this application embodiment does not specifically limit the type of negative active material, and it can be selected according to actual needs. As an example, the negative active material can be natural graphite, artificial graphite, mesophase microcarbon spheres (MCMB), hard carbon, soft carbon, silicon, or silicon-carbon composites.

[0117] The separator is located between the positive and negative electrode plates, serving to separate them and prevent short circuits caused by contact. The separator can be any material suitable for separators in electrochemical energy storage devices. Specifically, the separator includes at least one of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers.

[0118] The electrolyte in this application can be any electrolyte suitable for electrochemical energy storage devices in the art. The electrolyte includes an electrolyte and a solvent, and the electrolyte typically includes a lithium salt.

[0119] Specifically, the lithium salt includes at least one selected from lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP). The concentration of the electrolyte in the electrolyte solution can be 0.5–5 mol / L.

[0120] Specifically, the solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butyl carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0121] One embodiment of this application provides an electrical device comprising the secondary battery described above.

[0122] The electrical device serves as the power source for the electrical device.

[0123] The term "electrical device" refers to any device that can utilize electrical energy and convert it into mechanical energy, thermal energy, light energy, or one or more other energy forms, such as electric motors, electric heaters, and electric light sources. Specifically, it can include, but is not limited to, mobile devices, electric vehicles, electric trains, ships and satellites, and energy storage systems. Mobile devices can include mobile phones, laptops, drones, robot vacuum cleaners, and e-cigarettes; electric vehicles can include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, and electric trucks.

[0124] The present application is further illustrated below with specific embodiments.

[0125] Example 1

[0126] Example 1 provides a lithium-ion battery, the preparation method of which is as follows:

[0127] (1) Preparation of positive electrode sheet

[0128] (1.1) Preparation of positive electrode active material

[0129] Preparation of lithium manganese iron phosphate:

[0130] According to the target chemical formula LiMn x1 Fe 1-x1 The molar proportions of Li, Mn, Fe and P in PO4 (where x1 is shown in Table 1) were weighed out and mixed with lithium carbonate, manganese carbonate, ferrous oxalate and diammonium hydrogen phosphate, and then ball-milled (at 120 rpm for 5 h).

[0131] The ball-milled slurry was then spray-dried (spray temperature 220℃, time 1h);

[0132] The spray-dried product was placed in a high-temperature tube furnace, high-purity nitrogen was introduced into the high-temperature tube furnace, vacuum was drawn, and it was calcined at 700℃ for 5 hours and then cooled to room temperature to obtain the first lithium manganese iron phosphate precursor.

[0133] The carbon source (glucose) and the above-mentioned first lithium manganese iron phosphate precursor were mixed, with the amount of carbon source added being 7% of the weight of the first lithium manganese iron phosphate precursor; then ball milling was performed again (at a speed of 120 rpm for 5 hours).

[0134] The ball-milled slurry was then spray-dried.

[0135] The spray-dried product was placed in a high-temperature tube furnace, high-purity nitrogen was introduced into the furnace, a vacuum was drawn, and the product was calcined at 650°C for 6 hours, then cooled to room temperature to obtain lithium manganese iron phosphate.

[0136] Preparation of lithium manganese iron phosphate:

[0137] S1. According to the target chemical formula LiMn x2 Fe 1-x2 The molar ratios of Li, Mn, Fe, and P in PO4 (where x2 is shown in Table 1) were determined by weighing FePO4, LiOH, NH4H2PO4, and MnSO4, with the molar ratio of FePO4 to LiOH being 1:1. A certain amount of glucose was also weighed.

[0138] S2. FePO4 and LiOH were mixed and ball-milled for the first time at 130 rpm for t1 min. The slurry after ball milling was then spray-dried for the first time (spray temperature 220℃, time 1 h). NH4H2PO4 was then added and ball-milled for the second time at 130 rpm for t2 min. The slurry after ball milling was then spray-dried for the second time (spray temperature 200℃, time 1.5 h). MnSO4 and glucose were then added and ball-milled for the third time at 130 rpm for t3 min. The slurry after ball milling was then spray-dried for the third time (spray temperature 220℃, time 1.5 h). The amount of glucose added was adjusted so that the mass ratio of carbon material coated on the surface of lithium manganese iron phosphate (carbon coating ratio) met the requirements shown in Table 1. t1, t2, and t3 are also shown in Table 1.

[0139] S3. The product after the last spray drying in step S2 is placed in a high-temperature tube furnace. High-purity nitrogen is introduced into the high-temperature tube furnace, and a vacuum is drawn for sintering. The sintering conditions are as follows: the temperature is increased from 25℃ at a rate of 5℃ / min, and the temperature is held for 10min when the temperature increases by 100℃. After the temperature reaches T℃, it is held for 6h. T is shown in Table 1.

[0140] S4. The sintered product is cooled to room temperature to obtain lithium manganese iron phosphate.

[0141] (1.2) The first lithium manganese iron phosphate and the second lithium manganese iron phosphate are mixed evenly according to the mass ratio shown in Table 1 to obtain the positive electrode active material;

[0142] The positive electrode active material is mixed with PVDF, SP and CNT in a mass ratio of 97:1.5:1:0.5 and dispersed in NMP to obtain a positive electrode slurry. Then, the mixed positive electrode slurry is uniformly coated on aluminum foil, dried in a vacuum furnace at 100°C, rolled, cut and baked to obtain a positive electrode sheet.

[0143] (2) Preparation of negative electrode sheet

[0144] The negative electrode active material (artificial graphite), conductive agent (SP), and binder (carboxymethyl cellulose, CMC) are mixed in a mass ratio of 96.4:1:2.6 and dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated onto the negative electrode current collector (copper foil). The negative electrode current collector coated with the negative electrode slurry is transferred to a vacuum environment in an oven and dried at 100°C. It is then rolled, cut, and baked again to obtain the negative electrode sheet.

[0145] (3) Preparation of electrolyte

[0146] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0147] (4) Preparation of the diaphragm

[0148] A polyethylene (PE) diaphragm is used.

[0149] (5) Battery manufacturing

[0150] The prepared positive electrode, separator, and negative electrode are wound to obtain a bare cell without electrolyte injection; the bare cell is placed in an outer packaging foil, and the prepared electrolyte is injected into the dried bare cell. After vacuum sealing, standing, formation, shaping, and sorting, a lithium-ion battery is obtained.

[0151] Examples 2-16 and Comparative Examples 1, 3, and 4

[0152] Examples 2-16 and Comparative Examples 1, 3, and 4 each provide a lithium-ion battery, the preparation method of which is basically the same as that of Example 1, the difference being in the preparation of the positive electrode active material:

[0153] The target chemical formulas of lithium manganese iron phosphate and lithium manganese iron phosphate are shown in Table 1.

[0154] In S2, the ball milling times t1, t2, and t3 of the second lithium manganese iron phosphate in Examples 2-6, Examples 8-15, and Comparative Examples 1, 3, and 4 are shown in Table 1. The mass ratio (carbon coating ratio) of the carbon material coated on the surface of the second lithium manganese iron phosphate is shown in Table 1. The remaining steps are the same as in Example 1.

[0155] In Example 7, step S2 in the preparation of lithium manganese iron phosphate is as follows:

[0156] FePO4, LiOH, NH4H2PO4, and MnSO4 were mixed and ball-milled for the first time at 130 rpm for t1 min. The ball-milled slurry was then spray-dried for the first time (spray temperature 220℃, time 1 h). Glucose was then added and ball-milled for the second time at 130 rpm for t2 min. The ball-milled slurry was then spray-dried for the second time (spray temperature 220℃, time 1.5 h). The amount of glucose added was adjusted so that the mass ratio of carbon material coated on the surface of lithium manganese iron phosphate (carbon coating ratio) met the requirements shown in Table 1. The t1 and t2 values ​​for Example 7 are shown in Table 1.

[0157] In Example 16, step S2 in the preparation of lithium manganese iron phosphate is as follows:

[0158] FePO4, LiOH, and NH4H2PO4 were mixed and ball-milled for the first time at 130 rpm for t1 min. The ball-milled slurry was then spray-dried for the first time (spray temperature 220℃, time 1 h). Then, MnSO4 and glucose were added and ball-milled for the second time at 130 rpm for t2 min. The ball-milled slurry was then spray-dried for the second time (spray temperature 220℃, time 1.5 h). The amount of glucose added was adjusted so that the mass ratio of carbon material coated on the surface of lithium manganese iron phosphate (carbon coating ratio) met the requirements shown in Table 1. The t1 and t2 values ​​for Example 16 are shown in Table 1.

[0159] In S3, the values ​​of T during the sintering of lithium manganese iron phosphate in Examples 2-9, Examples 11-16, and Comparative Examples 1, 3, and 4 are shown in Table 1. The remaining steps are the same as in Example 1.

[0160] In Example 10, step S3 in the preparation of lithium manganese iron phosphate is as follows:

[0161] The product after the last spray drying in step S2 is placed in a high-temperature tube furnace. High-purity nitrogen is introduced into the high-temperature tube furnace, and a vacuum is drawn for sintering. The sintering conditions are: the temperature is increased from 25℃ at a rate of 5℃ / min, and the temperature is continuously increased to T℃, and then held for 9 hours. T is shown in Table 1.

[0162] In step (1.2), the mass ratios of the first and second lithium manganese iron phosphate in Examples 2-16 and Comparative Examples 1, 3, and 4 are shown in Table 1. The remaining steps are the same as in Example 1. Comparative Example 2

[0163] Comparative Example 2 provides a lithium-ion battery, the preparation method of which is basically the same as that of Example 1, except that:

[0164] The second type of lithium manganese iron phosphate was replaced with lithium iron phosphate (LiFePO4). The preparation method of lithium iron phosphate was basically the same as that of the second type of lithium manganese iron phosphate in Example 1, except that:

[0165] The target chemical formula for lithium iron phosphate is LiFePO4, and no MnSO4 is added during its preparation.

[0166] The ball milling times t1, t2, and t3, the carbon coating ratio, and the value of T during sintering during the preparation of lithium iron phosphate are shown in Table 1.

[0167] Then, the first lithium manganese iron phosphate, which is the same as that in Example 1, and the lithium iron phosphate prepared in Comparative Example 2 are mixed evenly at a mass ratio of 8:2 to obtain the positive electrode active material.

[0168] Table 1

[0169] The ΔV and W of the lithium-ion batteries prepared in each embodiment and comparative example were measured, as shown in Table 2; and the high-temperature cycle performance and high-temperature storage performance of the lithium-ion batteries were tested, with the results shown in Table 2; the testing methods for the above items are as follows:

[0170] Test methods for △V and W:

[0171] (1) High-temperature constant-capacity setting: The lithium-ion battery was left to stand at 45°C for 120 min, and then discharged at 1C constant current to 2.5V; after standing for 20 min, it was charged at 1C constant current and constant voltage until the cutoff voltage was 4.25V and the cutoff current was 0.33C; after standing for 20 min, it was discharged at 1C constant current to 2.5V; the constant-capacity setting was completed, and the charge-discharge curve was obtained.

[0172] (2) Plotting the dQ / dV-V curve: Based on the above charge and discharge curve, subtract the voltage and energy data of the nth data point from the voltage and energy data of the (n+1)th data point to obtain dV and dQ data; process all data in sequence to obtain a series of dV and dQ data; then divide dQ by dV to obtain dQ / dV; plot the dQ / dV-V curve with the capacity-voltage differential value dQ / dV as the vertical axis and voltage as the horizontal axis.

[0173] (3) In the order of potential from low to high, in the range of 3.0V to 3.85V in the dQ / dV-V diagram, the main reduction peak at the lower potential is peak A, and the main reduction peak at the higher potential is peak B; the difference between the peak positions of peak A and peak B is ΔV, in V; the half-width of peak A is W, in V.

[0174] Test method for high-temperature cycling capacity retention:

[0175] The lithium-ion battery was left to stand at 45°C for 120 minutes, then fully charged at 1C constant current and constant voltage with a cutoff voltage of 4.25V and a cutoff current of 0.33C. After standing for 20 minutes, it was discharged at 1C constant current to 2.5V. This constitutes one cycle. The above steps were repeated for a total of 100 cycles (100 rounds) of charge and discharge. The discharge capacity of the second round (Q1) and the discharge capacity of the 100th round (Q2) were recorded. The high-temperature cycle capacity retention rate = Q2 / Q1 × 100%.

[0176] Test method for high-temperature storage capacity recovery rate:

[0177] The lithium-ion battery is discharged at a constant current of 0.33C to 2.5V, then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage to 0.05C (charging cutoff). It is then discharged at a constant current of 0.33C to 2.5V. This process is repeated three times to obtain the third discharge capacity Q3. The battery is then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage to 0.05C (cutoff). At this point, the battery is at 100% SOC, and it continues to be stored in this state.

[0178] After storing the battery at 60℃ and 100% SOC for 30 days, it was discharged at a constant current of 0.33C to 2.5V, then charged at a constant current of 0.33C to 4.25V, charged at a constant voltage to 0.05C to stop charging, and discharged at a constant current of 0.33C to 2.5V. The above steps were repeated for a total of 3 charge and discharge cycles to obtain the third discharge capacity Q4.

[0179] High-temperature storage capacity recovery rate = Q4 / Q3 × 100%.

[0180] The test results of each embodiment and comparative example are shown in Table 2.

[0181] The dQ / dV-V curves for Example 2 are shown in Figure 1. It can be seen that there are two main reduction peaks in the range of 3.0V to 3.85V. The main reduction peak at the lower potential on the left is peak A, and the main reduction peak at the relatively higher potential on the right is peak B. It should be noted that peak A exhibits a split peak; the peak with the highest intensity in this group is the main reduction peak (as indicated by the arrow).

[0182] Table 2

[0183] Based on the test results in Table 2, it can be seen that for a secondary battery when ΔV 0.2 When ×W meets the scope of this application (0.011 to 0.145), as shown in the various embodiments, the battery has excellent high-temperature cycling performance and high-temperature storage performance, with a capacity retention rate of ≥89.5% after 100 cycles at 45°C and a capacity recovery rate of ≥88.5% after 30 days of storage at 60°C.

[0184] According to Examples 12-13, it can be seen that when △V 0.2 When the value of ×W is optionally within the range of 0.015 to 0.11, the battery exhibits relatively better high-temperature cycle performance and high-temperature storage performance.

[0185] As can be seen from Examples 1-7 and Examples 8-11, when the value of ΔV is in the range of 0.08-0.27, or the value of W is in the range of 0.022-0.15, the lithium-ion battery exhibits better overall high-temperature cycle performance and high-temperature storage performance.

[0186] According to comparative examples 1 to 4, when ΔV 0.2 When the temperature coefficient of thermal conductivity (×W) is too low or too high, the high-temperature cycle performance and high-temperature storage performance of lithium-ion batteries are poor, the high-temperature cycle capacity retention rate is no higher than 83%, and the high-temperature storage capacity recovery rate is no higher than 83.5%.

[0187] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, the positive electrode active material layer containing a positive electrode active material, characterized in that, The positive electrode active material includes lithium manganese iron phosphate materials; The secondary battery was charged and discharged at 45°C under 1C / 1C conditions. The differential of the charge and discharge curves was calculated, and a dQ / dV-V graph was plotted with voltage as the abscissa and capacity-voltage differential as the ordinate. The dQ / dV-V graph showed two main reduction peaks at different potentials in the range of 3.0V to 3.85V, with the lower potential main reduction peak being peak A. The peak position of peak A was V. A The main reduction peak at a higher potential is peak B, and the peak position value of peak B is V. B ; The half-width at half-peak of peak A is denoted as W; The peak position difference value of peak A and peak B is recorded as ΔV, ΔV = V B - V A ; △V and W satisfy the following relationship: 0.011 < ΔV 0.2 x W < 0.145; The V A , V B , ΔV and W are all in V.

2. The secondary battery according to claim 1, characterized by The ΔV and W satisfy the following relationship: 0.015 ≤ ΔV 0.2 × W ≤ 0.

11.

3. The secondary battery according to claim 1 or 2, characterized by The range of ΔV is 0.05 to 0.3V.

4. The secondary battery according to claim 3, wherein The range of ΔV is 0.08 to 0.27V.

5. The secondary battery according to claim 1 or 2, wherein The range of W is 0.016 to 0.20 V.

6. The secondary battery according to claim 5, wherein The range of W is 0.022 to 0.15 V.

7. The secondary battery according to claim 1, wherein The positive electrode active material includes lithium manganese iron phosphate (LMFP) and lithium manganese iron phosphate (LMFP), wherein the molar content of manganese in the LFP is greater than that in the LFP.

8. The secondary battery according to claim 7, wherein The molar content of manganese in the first lithium manganese iron phosphate is 60-90%.

9. The secondary battery according to claim 7, wherein The molar content of manganese in the second lithium manganese iron phosphate is 0.05-25%.

10. The secondary battery according to claim 7, wherein The mass ratio of the first lithium manganese iron phosphate and the second lithium manganese iron phosphate is (2.3~9):

1.

11. An electrical device, characterized by The secondary battery comprising any one of claims 1 to 10.

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