A positive electrode active material with low coulomb efficiency and a secondary battery

Abstract

The application discloses a kind of low coulomb efficiency positive electrode active material and secondary battery, belong to battery technical field.The low coulomb efficiency positive electrode active material provided by the application is Li x Ni y M 1‑y O2 Compound, compared with the same nickel, cobalt, manganese ratio in the prior art contrast commercial positive electrode material, the content of divalent nickel ion of the compound is increased, the content of trivalent nickel ion is reduced, so that more irreversible divalent nickel ion electrochemical oxidation occurs in formation and cycle process, an additional part of power consumption is consumed, and lithium ions in the compound are released.Therefore, the active lithium consumed by the SEI generated on the negative electrode can be supplemented, so that the positive and negative electrodes have mutually matched initial and / or cumulative coulomb efficiency.The specific capacity of the positive and negative electrodes is fully utilized without the need for additional addition of lithium supplementing additive, the manufacturing cost of the battery is reduced, the impedance in the cycle process of the battery is reduced, and the cycle life of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and particularly relates to a positive electrode active material with low coulombic efficiency and a secondary battery. Background Technology

[0002] The rapid development of technologies such as electric vehicles and electronic devices has led to a continuous increase in demand for high-energy-density lithium-ion batteries. As a crucial measure to improve battery energy density, the application of high-specific-capacity anode materials has also attracted increasing attention. However, because interfacial side reactions during the initial charge-discharge cycles consume a significant amount of active lithium, actual batteries using these anodes cannot fully utilize the specific capacity of the cathode.

[0003] Traditional techniques involve strictly controlling humidity during the positive electrode slurry preparation process and adding small amounts of lithium-supplementing additives, such as Li₂NiO₂ and Li₄Fe₅O₄, to enhance lithium content during charging. + The Li will be extracted from the lithium replenishment additive and re-intercalated into the lithium replenishment additive. + Very little, thus compensating for the irreversible capacity loss of the negative electrode during the first charge and discharge of the battery. For example, Chinese patent CN108232343A discloses a lithium-ion battery positive electrode supplementary additive, which is Li2AO2, Li2BO3, Li2CO4, Li3DO4, L i5 EO4, Li6FO4, Li x G y One or more of O2, wherein 1 < x < 2, 0 < y < 1; the average valence state of metals A, B, C, D, E, F, and G is lower than their highest oxidation state. The lithium-supplementing additive provides an excess lithium source to compensate for the lithium consumption caused by the formation of the SEI film at the positive and negative electrodes, thereby improving the battery's first-cycle coulombic efficiency and cycle performance. Alternatively, lithium powder or lithium foil can be added to the negative electrode. During the first discharge, the lithium powder or lithium foil on the negative electrode undergoes electrochemical oxidation, which can also compensate for the irreversible capacity loss of the negative electrode during the first charge and discharge of the battery. Existing patent CN116259712A discloses a positive electrode sheet and a battery. The positive electrode material layer includes a positive electrode active material, which includes lithium transition metal oxides and lithium-supplementing additives. The lithium transition metal oxides include LiCoO2, LiNiO2, LiFePO4, and Li... x Ni y M 1-y One or more of O2; M includes one or more of Co, Mn, Al, Mg, Ti, Fe, Cr, Mo, and Ca. For example, Chinese patent CN111446445A discloses a plasma treatment method for nickel-based lithium-ion cathode material precursors, which uses oxygen-based plasma to treat the nickel-based lithium-ion cathode material precursor, thereby reducing Ni... 2+ Oxidized to Ni 3+Inhibit Li + and Ni 2+ Mixing and matching can improve the initial coulombic efficiency. However, these methods all increase the manufacturing cost of the battery, and the products of delithiation of the positive electrode lithium replenishment additives significantly increase the battery impedance during cycling, thus reducing the battery cycle life. Summary of the Invention

[0004] Therefore, it is necessary to provide a positive electrode active material with low coulombic efficiency and a secondary battery to reduce the manufacturing cost of the battery, fully utilize the specific capacity of the positive and negative electrodes, reduce the impedance during battery cycling, and improve the cycle life of the battery.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a low coulombic efficiency positive electrode active material, wherein the positive electrode active material is a compound having the following general formula: Li x Ni y M 1-y O2; wherein 0.95≤x≤1.05, 0.6≤y<1; the compound contains Ni. 2+ and Ni 3+ , and Ni 2+ and Ni 3+ The content of Ni satisfies: 0.027 ≤ Ni 2+ / Ni 3+ ≤0.6.

[0006] Furthermore, M is selected from one or more of Co, Mn, Al, Mg, Ti, Fe, Sb, Nb, Cr and Mo.

[0007] This invention provides a method for preparing a low coulombic efficiency positive electrode active material as described in the above technical solution, comprising the following steps: S1. Ni hydroxide precursor y M 1-y (OH)2 is mixed with a lithium source and then heated and cooled to obtain a primary compound. S2. After washing the primary compound with water and / or treating it with boric acid, add M oxide and then perform a second heat treatment to obtain the low coulombic efficiency positive electrode active material.

[0008] The present invention also provides a secondary battery, the secondary battery comprising a positive electrode and a negative electrode, the positive electrode comprising a low coulombic efficiency positive electrode active material as described in the above technical solution, and the negative electrode comprising a negative electrode active material.

[0009] Furthermore, the difference between the initial coulombic efficiency a% of the positive electrode active material and the initial coulombic efficiency b% of the commercially available nickel, cobalt, and manganese active material of the same proportion satisfies: b%-a%≤7.3%.

[0010] Furthermore, the initial coulombic efficiency (a%) of the positive electrode active material is 80.7%~86.1%.

[0011] The initial coulombic efficiency (b%) of the commercially available nickel, cobalt, and manganese active materials in equal proportions is 88.0% to 90.0%.

[0012] Furthermore, the charging and discharging voltage of the secondary battery is 2.0V~4.5V.

[0013] The present invention also provides an energy storage device, including the secondary battery described in the above technical solution.

[0014] Compared with the prior art, the present invention has the following advantages and technical effects: The low coulombic efficiency positive electrode active material provided by this invention is Li x Ni y M 1-y O2 compounds, by increasing the Ni content in the compound 2+ with Ni 3+ Compared to existing commercially available nickel, cobalt, and manganese cathode materials with the same proportions, this compound has a higher content of divalent nickel ions and a lower content of trivalent nickel ions. This allows for irreversible oxidation of divalent nickel ions during formation and cycling, consuming additional energy and releasing lithium ions. The oxidized trivalent and tetravalent nickel ions still possess reversibility and electrochemical capacity, replenishing the active lithium ions and electrons consumed in SEI formation at the anode. This results in a matched first / cumulative coulombic efficiency between the positive and negative electrodes. Without the need for additional lithium replenishment additives, it fully utilizes the specific capacity of both electrodes, reducing battery manufacturing costs and minimizing impedance increases during cycling, thus improving cycle life. Attached Figure Description

[0015] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic diagram illustrating the principle of improving battery cycle life using a low coulombic efficiency positive electrode active material provided by this invention; Figure 2 XPS image of the positive electrode active material in Example 1; Figure 3 The image shows the XPS plot of the positive electrode active material in Comparative Example 1. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0018] This invention provides a low coulombic efficiency positive electrode active material, wherein the positive electrode active material is a compound having the following general formula: Li x Ni y M 1-y O2; wherein 0.95≤x≤1.05, 0.6≤y<1; the compound contains Ni. 2+ and Ni 3+ , and Ni 2+ and Ni 3+ The content of Ni satisfies: 0.027 ≤ Ni 2+ / Ni 3+ ≤0.6. The low coulombic efficiency cathode active material provided by this invention, compared to existing commercially available nickel, cobalt, and manganese cathode materials in the same proportion, has a lower efficiency than Ni before formation. 2+ / Ni 3+ Lower than commercially available cathode materials.

[0019] The schematic diagram illustrating the principle of improving battery cycle life using a low coulombic efficiency positive electrode active material provided by this invention is shown below. Figure 1 As can be seen, this invention controls the Ni content in the compound. 2+ with Ni 3+ The ratio of content can lead to Ni formation and cycling. 2+ Irreversible oxidation, Ni 2+ Converted to Ni 3+ In this irreversible oxidation reaction, Ni 2+ It loses an electron and is oxidized to Ni. 3+ At the same time, take off a Li + This causes the positive electrode active material to consume additional electricity, thereby reducing its initial coulombic efficiency, and thus requiring the positive and negative electrodes to have mutually matched initial / cumulative coulombic efficiencies. The removed Li + It can be used to replenish the active lithium consumed by the negative electrode, thus providing a certain degree of lithium replenishment. It should be noted that during this oxidation reaction, an equivalent amount of Li will not be produced during battery discharge. + Re-intercalation. And the oxidation product Ni... 3+ Having the same properties as Ni4+ Reversible electrochemical activity, meaning that reversible electrochemical reactions can occur during charge-discharge cycles, i.e., Ni 3+ Oxidized to Ni 4+ Ni 4+ Reduced to Ni 3+ Accompanied by Li + The extraction and re-intercalation. This reversible electrochemical reaction occurs dynamically and continuously, possessing electrochemical capacity; therefore, the Li in this invention... x Ni y M 1-y O2 compounds can achieve initial coulombic efficiency matching between the positive and negative electrodes, and also ensure that the specific capacity of the positive electrode material does not decrease significantly.

[0020] In this invention, "Ni" 2+ / Ni 3+ "Detection was performed using X-ray photoelectron spectroscopy. The specific detection process was as follows: first, the bulk phase of the positive electrode active material was exposed by argon ion etching, and then powder XPS was performed using AlKα rays to detect Ni2p binding energy appearing in the range of 850 eV to 870 eV." 3 / 2 Peak segmentation and fitting are performed, and the standard deviation of the fitting is ∑x. 2 <10%, then according to Ni 2+ and Ni 3+ The ratio of the corresponding peak areas To determine Ni 2+ and Ni 3+ relative content of Ni 2+ / Ni 3+ The argon ion etching process involved etching the material surface in a vacuum chamber using argon gas at an energy of 3 keV and a rate of 3.33 nm / s. The powder XPS measurement was performed on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) equipped with a 30 eV channel energy and a power of 100 W (10 kV and 10 mA), using a monochromatic AlKα X-ray source (hν = 1486.65 eV). All samples were within a range of less than 1.0 × 10⁻⁶ eV. -9 The analysis was performed under Pa pressure; spectra were acquired using Avantage software (version 5.979) in steps of 0.05 eV.

[0021] In a preferred embodiment, the y value satisfies: 0.6 ≤ y < 1, more preferably 0.6 ≤ y ≤ 0.98; exemplaryly, the y value can be, but is not limited to, 0.6, 0.7, 0.8, or 0.98. The smaller the y value, the larger the 1-y value. Since the average valence state of Ni and M is +3, in order to maintain the valence state balance of the compound, Ni... 2+The content of will increase, so the value of y should be controlled within the above range.

[0022] In a preferred embodiment, M is selected from one or more of Co, Mn, Al, Mg, Ti, Fe, Sb, Nb, Cr, and Mo, and more preferably from one or more of Mn, Co, Sb, and Ti. This invention adjusts the Ni content by reducing the oxygen content during sintering heat treatment or by adding high-valence dopant ions during preparation, thereby controlling the doping amount of high-valence transition metal ions. 2+ with Ni 3+ The ratio of the content; on the other hand, the doping of high-valence transition metal ions improves the diffusion kinetics of lithium ions, thereby improving the electrochemical performance of the battery.

[0023] In a preferred embodiment, the positive electrode active material is LiNi. 0.76 Co 0.095 Mn 0.095 Sb 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2 and LiNi 0.9 Co 0.05 Mn 0.05 O2.

[0024] In a preferred embodiment, the method for preparing the positive electrode active material includes the following steps: S1. Ni hydroxide precursor y M 1-y (OH)2 is mixed with a lithium source and then heated and cooled to obtain a primary compound.

[0025] S2. After washing the primary compound with water and / or treating it with boric acid, add M oxide and then perform a second heat treatment to obtain the low coulombic efficiency positive electrode active material.

[0026] In a preferred embodiment, in step S1, the hydroxide precursor Ni y M 1-y (OH)2 is selected from Ni 0.8 Co 0.1 Mn 0.1 (OH)₂. The hydroxide precursor is a dehydrated hydroxide precursor or a non-dehydrated hydroxide precursor. The dehydrated hydroxide precursor is prepared by heating the hydroxide precursor. The heating temperature is 300~600℃, and the heating time is 2~6h.

[0027] In a preferred embodiment, in step S1, the lithium source is selected from LiOH·H2O or Li2CO3; the amount of lithium source used is such that the molar ratio of Li / (Ni+M) is 1.02~1.08.

[0028] In a preferred embodiment, in step S1, the temperature of the heat treatment is 700~900℃, and the time of the heat treatment is 10~30h.

[0029] In a preferred embodiment, in step S1, the heat treatment is carried out in a flowing atmosphere, which includes a reducing gas, oxygen, and nitrogen. The reducing gas is selected from one or more of H2, CO, CH4, and NH3.

[0030] In a preferred embodiment, the volume percentage of reducing gas in the flowing atmosphere is 0.2% to 3%, and the volume percentage of oxygen is 16% to 90%. This invention regulates the Li content by adjusting the oxygen content and the amount of reducing gas in the flowing atmosphere. x Ni y M 1- y Ni in O2 compounds 2+ with Ni 3+ The ratio of content. The higher the proportion of reducing gas, the higher the proportion of divalent nickel. If the volume proportion of reducing gas exceeds 3%, it will lead to an excessively thick surface inert layer, high charge transfer and lithium-ion diffusion resistance, resulting in a low reversible specific capacity of the cathode active material with low coulombic efficiency. If the volume proportion of reducing gas is less than 0.2%, the reducing gas has almost no effect and cannot significantly increase the content of divalent nickel.

[0031] The present invention provides a secondary battery, the secondary battery comprising a positive electrode and a negative electrode, the positive electrode comprising the positive electrode active material described in the above technical solution, and the negative electrode comprising a negative electrode active material.

[0032] In a preferred embodiment, the difference between the initial coulombic efficiency a% of the positive electrode active material and the initial coulombic efficiency b% of the commercially available nickel, cobalt, and manganese active material of the same proportion satisfies: b%-a%≤7.3%.

[0033] In a preferred embodiment, the initial coulombic efficiency a% of the positive electrode active material is 80.7% to 86.1%, for example, a% is 80.7%, 81.4%, 85.4% or 86.1%.

[0034] In a preferred embodiment, the initial coulombic efficiency b% of the commercially available nickel, cobalt, and manganese active material in equal proportions is 88.0% to 90.0%.

[0035] This invention adjusts the initial and cumulative coulombic efficiencies of the positive electrode active material by regulating the content of divalent nickel ions, and adjusts the coulombic efficiency of the positive electrode active material according to the initial and cumulative coulombic efficiencies of the negative electrode active material. This achieves the matching of the initial and cumulative coulombic efficiencies of the positive and negative electrodes, allowing the positive electrode to continuously replenish additional lithium ions to the negative electrode during the first cycle and subsequent cycles, fully utilizing the specific capacity of both electrodes, and thus achieving a long cycle life for the entire battery.

[0036] In a preferred embodiment, the negative electrode active material is selected from one or more of silicon-containing materials, graphite materials, soft carbon materials, and hard carbon materials; the silicon-containing material is selected from one or more of silicon-oxygen composite materials, silicon-carbon composite materials, nano-silicon, micron-silicon, and novel silicon-based alloys.

[0037] In a preferred embodiment, the charge / discharge voltage of the secondary battery is 2.0V~4.5V. This invention, by regulating the charge / discharge voltage of the secondary battery, enables the negative electrode active material to release lithium ions as much as possible within the secondary battery, thereby improving the initial efficiency and discharge energy of the secondary battery.

[0038] This invention also provides an energy device, including the secondary battery described in the above technical solution. The secondary battery provided by this invention can be used as a power source for an electrical device or as an energy storage unit of an electrical device. The energy device may include mobile devices (e.g., mobile phones, tablets, e-readers, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric buses, electric trucks, etc.), ships and satellites, energy storage systems, etc.

[0039] In this embodiment of the invention, room temperature refers to "25±2℃".

[0040] Unless otherwise specified, all raw materials used in the embodiments of this invention were purchased through commercial channels.

[0041] Example 1 A positive electrode active material, namely LiNi 0.8 Co 0.1 Mn 0.1 O2 compound (NCM811), Ni 2+ / Ni 3+ =0.539.

[0042] The preparation method of the above positive electrode active material is as follows: Ni 0.8 Co 0.1 Mn 0.1 (OH)₂ was heat-treated at 400℃ for 4 hours to obtain Ni. 0.8 Co 0.1 Mn 0.1 O; will Ni 0.8 Co0.1 Mn 0.1 O and LiOH·H2O were mixed at a molar ratio of Li / (Ni+Co+Mn) of 1.02; the mixture was sintered at 810℃ for 12 h under a flowing atmosphere, and then cooled to room temperature to obtain LiNi. 0.8 Co 0.1 Mn 0.1 O2 compound (NCM811), wherein the volume percentage of reducing gas in the flowing atmosphere is 3% and the volume percentage of oxygen is 81%.

[0043] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer is composed of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer is composed of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0044] Figure 2 This is the XPS image of the positive electrode active material in Example 1. From... Figure 2 It can be seen that the positive electrode active material Ni in Example 1 2+ / Ni 3+ The value is 0.539.

[0045] Example 2 A positive electrode active material, namely LiNi 0.8 Co 0.1 Mn 0.1 O2 compound (NCM811), Ni 2+ / Ni 3+ =0.505.

[0046] The only difference between the above-mentioned method for preparing the positive electrode active material and Example 1 is that the volume ratio of the reducing gas in the flowing atmosphere is 2.2%.

[0047] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0048] Example 3 A positive electrode active material, namely LiNi 0.9 Co 0.05 Mn 0.05 O2 compound (NCM90505), Ni 2+ / Ni 3+ =0.471.

[0049] The difference between the above-mentioned method for preparing the positive electrode active material and Example 1 is that Ni is used. 0.9 Co 0.05 Mn 0.05 (OH)2 as a precursor (i.e., Ni) 0.9 Co 0.05 Mn 0.05 (OH)2 was heat-treated at 400℃ for 4 hours, and the proportion of hydrogen reducing gas in the flowing atmosphere was 0.2%.

[0050] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0051] Example 4 A positive electrode active material, namely LiNi 0.76 Co 0.095 Mn 0.095 Sb 0.05 O2, Ni 2+ / Ni 3+ =0.586.

[0052] The difference between the above-mentioned positive electrode active material preparation method and Example 1 is that 0.05 mol% of Sb2O3 was added in the precursor synthesis stage.

[0053] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0054] Comparative Example 1 A positive electrode active material, namely LiNi 0.8 Co 0.1 Mn 0.1 O2 compound (NCM811, purchased from Beijing Easpring Material Technology Co., Ltd.), Ni 2+ / Ni 3+ =0.466.

[0055] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0056] Figure 3 The image shows the XPS plot of the positive electrode active material in Comparative Example 1. Figure 3 It can be seen that in Comparative Example 1, the positive electrode active material Ni 2+ / Ni 3+ The value is 0.466.

[0057] Comparative Example 2 A positive electrode active material, namely LiNi 0.9 Co 0.05 Mn 0.05 O2 compound (NCM90505, purchased from Beijing Easpring Material Technology Co., Ltd.), Ni 2+ / Ni 3+ =0.459.

[0058] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0059] Comparative Example 3 A positive electrode active material, namely LiNi 0.8 Co0.1 Mn 0.1 O2 compound (NCM811), Ni 2+ / Ni 3+ =0.697.

[0060] The only difference between the above-mentioned method for preparing the positive electrode active material and Example 1 is that the volume ratio of the reducing gas in the flowing atmosphere is 6%.

[0061] A secondary battery includes a positive electrode and a negative electrode. The positive electrode includes an aluminum foil current collector and a positive electrode active material layer coated on the current collector. The positive electrode active material layer consists of 97.2 wt.% of the aforementioned positive electrode active material, 1.1 wt.% of SuperP, 0.6 wt.% of CNTs, and 1.1 wt.% of PVDF. The negative electrode includes a copper foil current collector and a negative electrode active material layer coated on the current collector. The negative electrode active material layer consists of 65 wt.% of graphite, 27.8 wt.% of a silicon-oxygen composite material (model S01, purchased from Lanxi Zhide New Energy Materials Co., Ltd.), 1.25 wt.% of SuperP, 4.5 wt.% of PAA, 1.2 wt.% of SBR, and 0.25 wt.% of SWCNTs (single-walled carbon nanotubes). The positive and negative electrodes are assembled with a polyethylene separator in a glove box to obtain the secondary battery.

[0062] The parameters of Examples 1-4 and Comparative Examples 1-3 are shown in Table 1.

[0063] Table 1 (1) Initial coulombic efficiency and lithium insertion / extraction capacity of the cathode were tested. At 25°C, the positive electrodes of Examples 1-4 and Comparative Examples 2-3 were made into coin cells and left to stand for 8 hours. They were then charged at a constant current of 0.05C to a voltage of 4.3V, and then discharged at a constant current of 0.05C to a voltage of 2.8V. Capacity data were collected.

[0064] At 25°C, the positive electrode of Comparative Example 1 was made into a coin cell and left to stand for 8 hours. It was then charged at a constant current of 0.05C to a voltage of 3.75V, and then discharged at a constant current of 0.05C to a voltage of 2.5V. Capacity data were collected.

[0065] Dividing the initial discharge capacity by the initial charge capacity yields the initial coulombic efficiency (a%) of the positive electrode active material. Dividing the initial discharge capacity by the effective mass of the positive electrode sheet yields the initial lithium insertion capacity of the positive electrode active material. Dividing the initial charge capacity by the effective mass of the positive electrode sheet yields the initial lithium extraction capacity of the positive electrode active material.

[0066] (2) Test of the first coulomb efficiency of the negative electrode At 25°C, the negative electrodes of Examples 1-4 and Comparative Examples 1-3 were fabricated into coin cells and left to stand for 8 hours. They were then discharged at a constant current of 0.05C to a voltage of 5.0 mV, and then charged at a constant current of 0.05C to a voltage of 1.5 V. Capacity data were collected. The initial charge capacity was divided by the initial discharge capacity to obtain the initial coulombic efficiency (e%) of the negative electrode active material.

[0067] (3) Cyclic stability test of secondary batteries The secondary batteries prepared in Examples 1-4 and Comparative Examples 1-3 were tested at room temperature (25°C ± 2°C). Step 1: Charge at a constant current rate of 1C to 4.25V, then charge at a constant voltage rate to 0.05C.

[0068] Step 2: Let stand for 5 minutes.

[0069] Step 3: Discharge at a constant current rate of 1C to 2.0V.

[0070] Step 4: Let stand for 5 minutes.

[0071] Step 5: Repeat steps 1 to 4 until the discharge capacity decays to 80% of the initial discharge capacity.

[0072] The performance test results of Examples 1-4 and Comparative Examples 1-3 are shown in Table 2.

[0073] Table 2 Comparing Examples 1-4 and Comparative Examples 1-3, it can be seen that the initial coulombic efficiency of the positive electrode activity provided by the present invention is highly matched with the negative electrode active material, resulting in a high cycle life of the secondary battery.

[0074] Comparing Examples 1 and 3, it can be seen that the positive electrode active material in Example 3 has a higher initial coulombic efficiency, which increases the cost and also generates excess lithium insertion / extraction vacancies due to the slightly poor matching of the initial coulombic efficiency with the negative electrode active material. These vacancies react with the electrolyte and affect the cycle life of the secondary battery to some extent. However, the performance is still significantly better than that of Comparative Examples 1 to 3.

[0075] Comparing Examples 1 and 4, it can be seen that after Sb doping, the Ni of the positive electrode active material in Example 4... 2+ / Ni 3+ The increased value makes its initial coulombic efficiency more compatible with the negative electrode active material compared to Example 1, resulting in a higher cycle life for its secondary battery.

[0076] Comparing Example 1 and Comparative Example 1, it can be seen that the initial coulombic efficiency of the cathode material used in Comparative Example 1 is too high, which leads to a mismatch between the decay rates of the cathode and the anode during the cycling process. The anode is in a state of high lithium insertion and high lithium extraction, which makes the lithium insertion and extraction depth of the silicon oxide material too large, resulting in rapid cycle decay of the anode.

[0077] Comparing Example 1 and Comparative Example 2, it can be seen that the initial coulombic efficiency of the positive electrode active material used in Comparative Example 2 is too high, and the cost is too high. This causes the excess lithium vacancies that can be inserted or removed from the positive electrode to undergo side reactions with the electrolyte during the cycling process, affecting the cycle life.

[0078] Comparing Example 1 and Comparative Example 3, it can be seen that when the volume ratio of reducing gas is too high, the surface inert layer is too thick, the charge transfer impedance is high, the reversible specific capacity is too low, and the cycle life is reduced.

[0079] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A cathode active material with low coulombic efficiency, characterized in that, The positive electrode active material is a compound having the following general formula: Li x Ni y M 1-y O2; where 0.95≤x≤1.05, 0.6≤y<1; The compound contains Ni. 2+ and Ni 3+ , and Ni 2+ and Ni 3+ The content of Ni satisfies: 0.027 ≤ Ni 2+ / Ni 3+ ≤0.

6.

2. The low coulombic efficiency positive electrode active material according to claim 1, characterized in that, The M is selected from one or more of Co, Mn, Al, Mg, Ti, Fe, Sb, Nb, Cr and Mo.

3. A method for preparing a low coulombic efficiency positive electrode active material as described in any one of claims 1 to 2, characterized in that, Includes the following steps: S1. Ni hydroxide precursor y M 1-y (OH)2 is mixed with a lithium source and then heated and cooled to obtain a primary compound. S2. After washing the primary compound with water and / or treating it with boric acid, add M oxide and then perform a second heat treatment to obtain the low coulombic efficiency positive electrode active material.

4. A secondary battery, characterized in that, The secondary battery includes a positive electrode and a negative electrode, wherein the positive electrode comprises a positive electrode active material with low coulombic efficiency as described in any one of claims 1 to 2, and the negative electrode comprises a negative electrode active material.

5. The secondary battery according to claim 4, characterized in that, The difference between the initial coulombic efficiency a% of the positive electrode active material and the initial coulombic efficiency b% of the commercially available nickel, cobalt, and manganese active materials of the same proportion satisfies: b%-a%≤7.3%.

6. The secondary battery according to claim 5, characterized in that, The initial coulombic efficiency (a%) of the positive electrode active material is 80.7%~86.1%. The initial coulombic efficiency (b%) of the commercially available nickel, cobalt, and manganese active materials in equal proportions is 88.0% to 90.0%.

7. The secondary battery according to any one of claims 4 to 6, characterized in that, The charging and discharging voltage of the secondary battery is 2.0V~4.5V.

8. An energy storage device, characterized in that, Includes the secondary battery as described in any one of claims 4 to 7.

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