A metal dicyandiamide negative electrode material

Abstract

The present invention discloses a metal dicyandiamide negative electrode material, wherein the metal dicyandiamide has the following chemical formula: II [N(CN)2]2 or Cu I [N(CN)2]; wherein M is Co, Ni, Mn, Cu or Zn; I and II refer to the valence state of the metal respectively. The metal dicyandiamide defined in the present invention acts as a medium for energy storage and release in the negative electrode of the battery, and has excellent electrochemical cycle stability and high specific capacity. Compared with existing metal nitride negative electrode materials, the system of the present invention involves nitrogen-containing anions (N(CN)2 ‑ ) provides reversible capacity and has superior specific capacity performance. In addition, these materials (such as Ni, Co, Cu, etc.) have an embedded electrochemical reaction mechanism. Compared with traditional conversion materials, the electrochemical reaction process does not have obvious phase transitions or volume changes, which improves safety.

Description

Technical Field

[0001] The present invention relates to the technical field of battery negative electrode materials, in particular to a metal dicyandiamide negative electrode material. Background Art

[0002] With the continuous advancement of technology, various portable electronic devices, such as mobile phones, laptops, and tablets, have become an integral part of people's daily lives. However, the energy consumption of these devices has also raised concerns about traditional energy sources. Therefore, the development of clean, renewable energy sources has become an urgent need. Against this backdrop, lithium batteries emerged and quickly became the energy source of choice for electronic devices. Compared to traditional batteries, lithium batteries offer advantages such as higher energy density, lighter weight, longer lifespan, and reduced pollution.

[0003] Electrode materials are key components of lithium-ion batteries, their primary function being to store and release energy. Electrode materials must meet a series of stringent requirements. First, they must have a high energy density, meaning they can store and release more energy per unit weight or volume of electrode material. Second, they must have a long lifespan, meaning their performance does not significantly degrade after repeated charge and discharge cycles. Furthermore, electrode materials must possess excellent electrical conductivity and chemical stability. Currently, the main cathode materials on the market include various layered metal salts, such as cobaltates, nickelates, and manganates. These materials enable reversible lithium ion insertion and extraction during charge and discharge, resulting in high energy density and excellent cycling performance. Negative electrode materials primarily include carbon and non-carbon materials. Carbon materials include graphite (natural graphite, artificial graphite, composite graphite, and mesocarbon microbeads) and other carbon-based materials (hard carbon, soft carbon, and graphene). Graphite is the preferred negative electrode material due to its light weight and high capacity. It also possesses advantages such as electrical conductivity, thermal conductivity, high-temperature resistance, lubricity, chemical stability, plasticity, and thermal shock resistance. However, the energy density of graphite is low, which limits the overall energy density of the battery. Non-carbon materials can be further divided into titanium-based materials, silicon-based materials, tin-based materials and metallic lithium. Among them, lithium titanate is considered to be a negative electrode material that is safer and has a longer life than carbon. It has the advantages of fast charging and discharging, a large number of cycles and high safety. In addition, lithium titanate negative electrode lithium-ion batteries have a long life. Because the structure of the lithium titanate negative electrode material itself is stable and the electrode structure remains stable during the charging and discharging process, the cycle life of the lithium-ion battery is greatly improved. At the same time, lithium titanate-ion batteries have excellent high and low temperature performance and fast charging and discharging functions that traditional lithium-ion batteries do not have. However, the disadvantage of lithium titanate is that its voltage platform is low, resulting in low energy density. Therefore, the development of new negative electrode materials with high energy density, high stability and low cost is an important research direction.

[0004] In addition, metal nitrogen-based negative electrode materials, especially lithium transition metal nitrides, are a typical type of non-carbon negative electrode materials. Lithium transition metal nitrides have good ionic conductivity, electronic conductivity and chemical stability. When used as negative electrode materials for lithium-ion batteries, their discharge voltage is usually above 1.0V. The discharge capacity, cycle performance and smoothness of the charge and discharge curves of the electrode vary greatly depending on the type of material. For example, when Li3FeN2 is used as the negative electrode of a lithium-ion battery, the discharge capacity is 150mAh / g and the discharge potential is 1.3V (vs.Li / Li + ), the charge and discharge curves are very flat, with no discharge hysteresis, but the capacity decays significantly during cyclic charge and discharge, and the specific capacity is low, making it difficult to meet practical applications. 2.6 Co 0.4 The charge and discharge of N occurs in the potential range of 0 to 1.4 V and shows a reversible capacity of up to 800 mAh / g, but the charge and discharge curves are not very smooth, with obvious potential hysteresis and capacity decay. In addition, nitride materials are very sensitive to moisture, which makes such materials face limitations in many fields. In 2016, Richard Dronskowski's group and Lorenzo Stievano's group collaborated to discover a new type of nitride containing carbodiimide groups (NCN 2- ) is a new type of layered metal nitrogen compound lithium battery negative electrode material. For example, when the chromium carbodiimide compound Cr2(NCN)3 is used as the negative electrode material, the specific capacity is as high as 600mAh·g -1 However, this type of material is of conversion type, and there are phase changes and volume changes during the electrochemical reaction, which may pose safety risks in practical applications. Summary of the Invention

[0005] The present invention aims to overcome these technical limitations by regulating the ratio of carbon and nitrogen in traditional nitrogen-based materials, further controlling the size of the pores formed by the connection between metal ions and ligands, thereby achieving structural control of electrochemical performance. To this end, the present invention proposes a metal dicyandiamide anode material that addresses the low specific capacity and poor safety issues inherent in existing technologies.

[0006] In a first aspect, the present invention provides a metal dicyandiamide negative electrode material, wherein the metal dicyandiamide has the following chemical formula: II [N(CN)2]2 or Cu I [N(CN)2]; wherein, M includes but is not limited to Co, Ni, Mn, Cu or Zn; I and II refer to the valence state of the metal respectively.

[0007] In a second aspect, the present invention provides a metal dicyandiamide having the following chemical formula: β-Mn II [N(CN)2]2 or β-Co II [N(CN)2]2.

[0008] In a third aspect, the present invention provides a method for preparing metal dicyandiamide, comprising the following steps:

[0009] Dissolve soluble Mn(ClO4).xH2O in deionized water and stir thoroughly to obtain a clear solution.

[0010] N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II;

[0011] Solution 1 was slowly added dropwise to solution 2 and allowed to stand until the solvent was completely evaporated to obtain a light blue single crystal of Mn II [N(CN)2]2.xH2O and light yellow powder precipitation of β-Mn II [N(CN)2]2.

[0012] In a fourth aspect, the present invention provides a method for preparing metal dicyandiamide, comprising the following steps:

[0013] Co 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution.

[0014] N(CN)2 - The soluble salt is dissolved in a mixed solution of water and pyridine, and a clear solution 2 is obtained after sufficient stirring;

[0015] Slowly add solution 1 dropwise to solution 2, let it stand, and then filter to obtain a precipitate;

[0016] The precipitate is heated to obtain β-Co II [N(CN)2].

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] The present invention proposes a new type of metal nitrogen-based battery negative electrode material M II [N(CN)2]2(M=Ni, Co, Mn, Cu or Zn) or Cu I [N(CN)2], which acts as a medium for energy storage and release in the negative electrode of the battery, has excellent electrochemical cycle stability and high specific capacity. Compared with existing metal nitride negative electrode materials, the system of the present invention involves nitrogen-containing anions (N(CN)2 - ) provides reversible capacity and has better specific capacity performance. In addition, such materials (such as Ni, Co, Cu, etc.) are embedded electrochemical reaction mechanisms, which are different from traditional conversion materials (such as the above-mentioned metal carbodiimide NCN 2-There is no obvious phase change and volume change during the electrochemical reaction process, which improves safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is the powder X-ray diffraction pattern of the α-Ni[N(CN)2]2 negative electrode material in Example 1;

[0020] Figure 2 The α-Ni[N(CN)2]2 negative electrode material in Example 1 is 100mA g in the voltage range of 0.01-3.0V. -1 Charge and discharge curves at current density of ;

[0021] Figure 3 The α-Ni[N(CN)2]2 negative electrode material in Example 1 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Circular curve diagram of

[0022] Figure 4 The α-Ni[N(CN)2]2 negative electrode material in Example 1 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Cyclic voltammetry curve of

[0023] Figure 5 is the powder X-ray diffraction pattern of the α-Co[N(CN)2]2 negative electrode material in Example 2;

[0024] Figure 6 The α-Co[N(CN)2]2 negative electrode material in Example 2 is 100mA g in the voltage range of 0.01-3.0V. -1 Charge and discharge curves at current density of ;

[0025] Figure 7 The α-Co[N(CN)2]2 negative electrode material in Example 2 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Circular curve diagram of

[0026] Figure 8 The α-Co[N(CN)2]2 negative electrode material in Example 2 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Cyclic voltammetry curve of

[0027] Figure 9 is the powder X-ray diffraction pattern of the α-Mn[N(CN)2]2 negative electrode material in Example 3;

[0028] Figure 10The α-Mn[N(CN)2]2 negative electrode material in Example 3 is 100mA g in the voltage range of 0.01-3.0V. -1 Charge and discharge curves at current density of ;

[0029] Figure 11 The α-Mn[N(CN)2]2 negative electrode material in Example 3 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Circular curve diagram of

[0030] Figure 12 The α-Mn[N(CN)2]2 negative electrode material in Example 3 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Cyclic voltammetry curve of

[0031] Figure 13 is the powder X-ray diffraction pattern of the β-Zn[N(CN)2]2 negative electrode material in Example 4;

[0032] Figure 14 The β-Zn[N(CN)2]2 negative electrode material in Example 4 is 100mA g in the voltage range of 0.01-3.0V. -1 Charge and discharge curves at current density of ;

[0033] Figure 15 The β-Zn[N(CN)2]2 negative electrode material in Example 4 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Circular curve diagram of

[0034] Figure 16 The β-Zn[N(CN)2]2 negative electrode material in Example 4 is in the voltage range of 0.01-3.0V and the current density is 100mA g -1 Cyclic voltammetry curve of

[0035] Figure 17 is the α-Cu in Example 5 II Powder X-ray diffraction pattern of [N(CN)2]2 negative electrode material;

[0036] Figure 18 is the α-Cu in Example 5 II [N(CN)2]2 negative electrode material in the voltage range of 0.01-3.0 V 100mA g -1 Charge and discharge curves at current density of ;

[0037] Figure 19 is the α-Cu in Example 5 II[N(CN)2]2 negative electrode material has a voltage range of 0.01-3.0 V and a current density of 100 mA g -1 Circular curve diagram of

[0038] Figure 20 is the α-Cu in Example 5 II [N(CN)2]2 negative electrode material has a voltage range of 0.01-3.0 V and a current density of 100 mA g -1 Cyclic voltammetry curve of

[0039] Figure 21 is Cu in Example 6 I Powder X-ray diffraction pattern of [N(CN)2] anode material;

[0040] Figure 22 is Cu in Example 6 I [N(CN)2] anode material in the voltage range of 0.01-3.0 V at 100 mA g -1 Charge and discharge curves at current density of ;

[0041] Figure 23 is Cu in Example 6 I [N(CN)2] anode material has a voltage range of 0.01-3.0 V and a current density of 100 mA g -1 Circular curve diagram of

[0042] Figure 24 is Cu in Example 6 I [N(CN)2] anode material has a voltage range of 0.01-3.0 V and a current density of 100 mA g -1 Cyclic voltammetry curves. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0044] In a first aspect, the present invention provides a metal dicyandiamide negative electrode material, wherein the metal dicyandiamide has the following chemical formula: II [N(CN)2]2 or Cu I [N(CN)2]; wherein, M includes but is not limited to Co, Ni, Mn, Cu or Zn; I and II refer to the valence state of the metal respectively.

[0045] The present invention adjusts the ratio of C and N elements to further adjust the pore size formed by the connection between metal ions and ligands, slowing down the volume change of the compound during the lithium insertion and removal process, making the structure more stable and improving safety. At the same time, such materials (such as Ni, Co, Cu, etc.) exhibit an insertion reaction mechanism during the electrochemical reaction. Compared with conversion materials, there is no obvious phase change during the electrochemical reaction, which also improves safety. On the other hand, the new nitrogen material nitrogen anion (N(CN)2 - ) participates in providing reversible capacity, making it have better specific capacity performance compared with other types of nitrogen-based materials.

[0046] In this embodiment, M II [N(CN)2]2 includes two isomers (α-M II [N(CN)2]2 and β-M II [N(CN)2]2). In order to distinguish different isomers, the present invention combines metal ions with N(CN)2 - The six-coordinate connection is collectively referred to as the α configuration, and the four-coordinate connection is collectively referred to as the β configuration.

[0047] In this embodiment, the metal dicyandiamide negative electrode material is a negative electrode material for a lithium battery or a sodium battery.

[0048] In this embodiment, the metal dicyandiamide has the following chemical formula: II [N(CN)2]2, and M is Co, Ni, Mn or Cu; the preparation method of the above metal dicyandiamide comprises the following steps:

[0049] S101, M 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution.

[0050] S102, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II;

[0051] S103, slowly adding solution 1 dropwise to solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate;

[0052] S104, heating the precipitate to obtain α-M II [N(CN)2]2.

[0053] In step S103, the metal ions and [N(CN)2] - Full response.

[0054] In step S104, heating is performed to decompose the M[N(CN)2]2.xH2O containing crystal water that may exist to obtain α-M II [N(CN)2]2, and the obtained precipitate is dried at the same time.

[0055] Furthermore, in step S101, M 2+ Soluble salts include, but are not limited to, at least one of a nitrate or a halide salt.

[0056] Furthermore, in step S101, in solution 1, the metal ion M 2+ The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0057] Furthermore, in step S102, N(CN)2 - The soluble salt is Na[N(CN)2].

[0058] Further, in step S102, in solution 2, N(CN)2 - The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0059] Furthermore, in step S103, during the process of slowly adding solution 1 to solution 2, the metal ion M 2+ With N(CN)2 - The molar ratio is 1:(1.8-2.2), and further 1:2.

[0060] Furthermore, in step S103, the dropping process is performed at room temperature.

[0061] Furthermore, in step S103, the standing temperature is room temperature, and the standing time is 12-24 hours.

[0062] Furthermore, in step S104, the heating method is vacuum heating, the vacuum heating temperature is 100-140° C., and the vacuum heating time is 8-14 hours.

[0063] In this embodiment, the metal dicyandiamide has the following chemical formula: β-Zn II [N(CN)2]2; The preparation method of the above-mentioned metal dicyandiamide comprises the following steps:

[0064] S201, Zn 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution.

[0065] S202, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II;

[0066] S203, slowly adding solution 1 dropwise to solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate;

[0067] S204, heating the precipitate to obtain β-Zn II [N(CN)2]2.

[0068] In step S203, the Zn 2+ and [N(CN)2] - Full response.

[0069] In step S204, the precipitate is dried by heating.

[0070] Furthermore, in step S201, Zn 2+ Soluble salts include, but are not limited to, at least one of a nitrate or a halide salt.

[0071] Furthermore, in step S201, in solution 1, metal ion Zn 2+ The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0072] Furthermore, in step S202, N(CN)2 - The soluble salt is Na[N(CN)2].

[0073] Further, in step S202, in solution 2, N(CN)2 - The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0074] Furthermore, in step S203, during the process of slowly adding solution 1 to solution 2, the metal ion M 2+ With N(CN)2 - The molar ratio is 1:(1.8-2.2), and further 1:2.

[0075] Furthermore, in step S203, the dropping process is performed at room temperature.

[0076] Furthermore, in step S203, the standing temperature is room temperature, and the standing time is 12-24 hours.

[0077] Furthermore, in step S204, the heating method is vacuum heating, the vacuum heating temperature is 100-140° C., and the vacuum heating time is 8-14 hours.

[0078] In this embodiment, the metal dicyandiamide has the following chemical formula: Cu I [N(CN)2]2; The preparation method of the above-mentioned metal dicyandiamide comprises the following steps:

[0079] S301, Cu 2+ The soluble salt is dissolved in deionized water, excess reducing agent is added, and the mixture is stirred thoroughly to obtain a solution containing Cu + of clear solution 1;

[0080] S302, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II;

[0081] S303, slowly adding solution 1 dropwise to solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate;

[0082] S304, heating the precipitate to obtain Cu I [N(CN)2]2.

[0083] In step S303, the Cu + and [N(CN)2] - Full response.

[0084] In step S304, the precipitate is dried by heating.

[0085] Furthermore, in step S301, Cu 2+ The soluble salts include, but are not limited to, at least one of nitrates, halides, or acetates.

[0086] Furthermore, in step S301, the reducing agent is K2[S2O5] or Na2[S2O5].

[0087] Furthermore, in step S301, Cu 2+ The molar ratio of the reducing agent is 1:(1.1-2), further 1:1.5.

[0088] Furthermore, in step S301, in solution 1, the metal ion Cu + The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0089] Furthermore, in step S302, N(CN)2 -The soluble salt is Na[N(CN)2].

[0090] Further, in step S302, in solution 2, N(CN)2 - The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0091] Furthermore, in step S303, during the process of slowly adding solution 1 to solution 2, the metal ion Cu + With N(CN)2 - The molar ratio is 1:(0.8-1.2), further 1:1.

[0092] Furthermore, in step S303, the dropping process is performed at room temperature.

[0093] Furthermore, in step S303, the standing temperature is room temperature, and the standing time is 12-24 hours.

[0094] Furthermore, in step S304, the heating method is vacuum heating, the vacuum heating temperature is 100-140° C., and the vacuum heating time is 8-14 hours.

[0095] In a second aspect, the present invention provides a metal dicyandiamide having the following chemical formula: β-Mn II [N(CN)2]2 or β-Co II [N(CN)2]2.

[0096] In a third aspect, the present invention provides a method for preparing metal dicyandiamide, comprising the following steps:

[0097] S401, dissolving soluble Mn(ClO4).xH2O in deionized water, and stirring thoroughly to obtain a clear solution 1;

[0098] S402, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II;

[0099] S403, slowly add solution 1 to solution 2, and let it stand until the solvent is completely evaporated to obtain a large piece of light blue single crystal Mn II [N(CN)2]2.xH2O and light yellow powder precipitate β-Mn at the bottom II [N(CN)2]2.

[0100] In this embodiment, in step S401, in solution 1, the metal ion Mn 2+ The concentration is 0.01-0.5 mol L -1, further 0.1 mol L -1 .

[0101] In this embodiment, in step S402, N(CN)2 - The soluble salt is Na[N(CN)2].

[0102] In this embodiment, in step S402, in solution 2, N(CN)2 - The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0103] In this embodiment, in step S403, during the process of slowly adding solution 1 to solution 2, the metal ion Mn 2+ With N(CN)2 - The molar ratio is 1:(1.8-2.2), further 1:2.

[0104] In this embodiment, in step 403, the dropping process is performed at room temperature.

[0105] In this embodiment, in step 403, the standing temperature is room temperature.

[0106] Furthermore, the standing time is 5-10 days.

[0107] In this embodiment, the above steps further include:

[0108] S404, heating the light blue single crystal Mn[N(CN)2]2.xH2O to obtain α-Mn without crystal water II [N(CN)2]2.

[0109] Furthermore, in step S404, the heating method is vacuum heating, the vacuum heating temperature is 100-140° C., and the vacuum heating time is 8-14 hours.

[0110] Through the above synthesis method, the inventors obtained β-Mn with good crystallinity. II [N(CN)2]2, and successfully resolved β-Mn for the first time II The structure of [N(CN)2]2.

[0111] In a fourth aspect, the present invention provides a method for preparing metal dicyandiamide, comprising the following steps:

[0112] S501, Co 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution.

[0113] S502, N(CN)2 -The soluble salt is dissolved in a mixed solution of water and pyridine (py), and after sufficient stirring, a clear solution 2 is obtained;

[0114] S503, slowly adding solution 1 dropwise to solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate;

[0115] S504, heating the precipitate to obtain β-Co II [N(CN)2].

[0116] In step S504, the metal ions are allowed to react with [N(CN)2] - Full response.

[0117] In step S504, the Co[N(CN)2]2.xpy containing crystalline water is decomposed by heating to obtain β-Co II [N(CN)2]2, and the obtained precipitate can be dried by heating at the same time.

[0118] Through the above-mentioned synthesis method, the inventors obtained β-Co[N(CN)2]2 with good crystallinity and successfully analyzed the structure of β-Co[N(CN)2]2 for the first time.

[0119] In this embodiment, in step S501, Co 2+ The soluble salts include, but are not limited to, at least one of nitrates, halides, or acetates.

[0120] In this embodiment, in step S501, in solution 1, the metal ion Co 2+ The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0121] In this embodiment, in step S502, N(CN)2 - The soluble salt is Na[N(CN)2].

[0122] In this embodiment, in step S502, in solution 2, N(CN)2 - The concentration is 0.01-0.5 mol L -1 , further 0.1 mol L -1 .

[0123] In this embodiment, in the mixed solution of water and py, the volume ratio of water to pyridine is (10-100):1, further 50:1.

[0124] In this embodiment, in step S503, during the process of slowly adding solution 1 to solution 2, the metal ion M 2+With N(CN)2 - The molar ratio is 1:(1.8-2.2), further 1:2.

[0125] In this embodiment, in step S503 , the dropping process is performed at room temperature.

[0126] In this embodiment, in step S503, the standing temperature is room temperature, and the standing time is 12-24 hours.

[0127] In this embodiment, in step S504, the heating method is vacuum heating, the vacuum heating temperature is 140-160° C., and the vacuum heating time is 8-14 hours.

[0128] In the present invention, it should be noted that the synthesis methods are quite different for different metals M. II Although the corresponding isomers of [N(CN)2]2 compound are not obtained, we also attribute it to β configuration due to its four-coordinate connection mode. At the same time, due to the β configuration of Co II [N(CN)2]2 and Mn II [N(CN)2]2 was not obtained in pure phase, so its electrochemical performance has not been characterized yet. However, due to the isostructural β-Zn II [N(CN)2]2 has excellent electrochemical properties, and those skilled in the art can reasonably expect that the above-mentioned β-configuration Co II [N(CN)2]2 and Mn II [N(CN)2]2 can also be used as a negative electrode material for lithium-sodium batteries, and has excellent electrochemical stability and high specific capacity performance.

[0129] In the present invention, those skilled in the art should understand that the above-mentioned nitrates, halide salts or acetates include but are not limited to their anhydrous forms or their hydrate forms.

[0130] Example 1

[0131] α-Ni II Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0132] α-Ni II The preparation method of [N(CN)2]2 comprises the following steps:

[0133] (1) Dissolve Ni(NO3)2.6H2O in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0134] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L-1 of clear solution 2;

[0135] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow Ni 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0136] (4) The precipitate was heated in vacuum at 120 °C for 12 h to obtain α-Ni II [N(CN)2]2.

[0137] See also Figure 1 ,pass Figure 1 It can be seen that the PXRD results of Example 1 of the present invention are consistent with those reported in the literature, proving that pure phase α-Ni II [N(CN)2]2.

[0138] The α-Ni obtained in this example II [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1 The constant current charge and discharge test and stability test were carried out at a current density of 100 nm. II [N(CN)2]2 electrode test see Figure 2-4 .

[0139] See also Figure 2-4 , in the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II [N(CN)2]2 first cycle discharge capacity is as high as 1308mAh g -1 However, the reversible capacity of the first cycle of charging is only 367mAh g -1 , and the corresponding coulombic efficiency is 28.1%. The initial capacity loss may be caused by the SEI film. However, as the number of cycles increases, the reversible capacity gradually increases (<360 cycles), reaching a maximum specific capacity of 660 mAh g -1 Around (360-370 cycles), then decreased again (370-450 cycles), and finally gradually stabilized (450-500 cycles).

[0140] In the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II The highest cycle specific capacity of [N(CN)2]2 is up to 660 mAh g -1Around, and has a stable cycle specific capacity (maintaining 600mAh g -1 The reversible capacity is >150 cycles).

[0141] At 0.1mVs -1 At the scanning rate, the cyclic voltammetry curve shows that α-Ni II [N(CN)2]2 has an oxidation / reduction couple at around 1.8V and 1.2V, corresponding to the redox process of nitrogen anions.

[0142] Example 2

[0143] α-Co II Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0144] α-Co II The preparation method of [N(CN)2]2 comprises the following steps:

[0145] (1) Dissolve Co(NO3)2.6H2O in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0146] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0147] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow the Co 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0148] (4) The precipitate was heated in vacuum at 120 °C for 12 h to obtain α-Co II [N(CN)2]2.

[0149] See also Figure 5 ,pass Figure 5 It can be seen that the PXRD results of Example 2 of the present invention are consistent with those of the literature, proving that pure phase α-Co II [N(CN)2]2.

[0150] The α-Co obtained in this example II [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1The constant current charge and discharge test and stability test were carried out at the current density of α-Co II [N(CN)2]2 electrode test see Figure 6-8 .

[0151] See also Figure 6-8 , in the potential range of 0.01-3.0V, 100mAg -1 At a current density of II [N(CN)2]2 first cycle discharge capacity is as high as 1447mAh g -1 However, the reversible capacity of the first cycle of charging is only 416mAh g -1 , the corresponding Coulombic efficiency is about 29%. II The capacity loss of [N(CN)2]2 in the first cycle may be caused by the SEI film. As the number of cycles increases, the reversible capacity gradually increases and reaches a maximum specific capacity of 780 mAh g -1 Then it decreases and finally gradually stabilizes.

[0152] In the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II [N(CN)2]2 can reach 780mAh g -1 The highest specific capacity of about 700mAh g -1 The reversible capacity is >150 cycles).

[0153] At 0.1mVs -1 At the scanning rate, the cyclic voltammetry curve shows that α-Co II [N(CN)2]2 has an oxidation / reduction couple at around 1.7V and 1.1V, corresponding to the redox process of nitrogen anions.

[0154] Example 3

[0155] α-Mn II Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0156] α-Mn II The preparation method of [N(CN)2]2 comprises the following steps:

[0157] (1) Dissolve Mn(NO3)2.4H2O in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0158] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0159] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow Mn 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0160] (4) The precipitate was heated in vacuum at 100 °C for 12 h to obtain α-Mn II [N(CN)2]2.

[0161] See also Figure 9 ,pass Figure 9 It can be seen that the PXRD results of Example 3 of the present invention are consistent with those reported in the literature, proving that pure phase α-Mn II [N(CN)2]2.

[0162] The α-Mn obtained in this example II [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1 The constant current charge and discharge test and stability test were carried out at a current density of 100 nm. II [N(CN)2]2 electrode test see Figure 10-12 .

[0163] See also Figure 10-12 , in the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II The first cycle charge and discharge coulombic efficiency of [N(CN)2]2 is about 40%. Similarly, α-Mn II The capacity loss of [N(CN)2]2 in the first cycle may be caused by the SEI film. As the number of cycles increases, the reversible capacity gradually increases and reaches a maximum specific capacity of 700 mAh g -1 above, then decrease, and finally gradually stabilize.

[0164] In the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II [N(CN)2]2 maintains 500 mAh g within 100 cycles -1 reversible capacity.

[0165] The cyclic voltammetry curves show that α-MnII [N(CN)2]2 corresponds to Mn at around 0.5V 2+ The reduction peak at around 1.0 V corresponds to the reduction peak of nitrogen anions.

[0166] Example 4

[0167] β-Zn II Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0168] β-Zn II The preparation method of [N(CN)2]2 comprises the following steps:

[0169] (1) Dissolve ZnCl2 in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0170] (2) Dissolve Na[N(CN)2]2 in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0171] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow Zn 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0172] (4) The precipitate was heated in vacuum at 100 °C for 12 h to obtain β-Zn II [N(CN)2]2.

[0173] See also Figure 13 ,pass Figure 13 It can be seen that the PXRD results of Example 4 of the present invention are consistent with those reported in the literature, proving that pure phase β-Zn II [N(CN)2]2.

[0174] The β-Zn obtained in this example II [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1 The β-Zn II [N(CN)2]2 electrode test see Figure 14-16 .

[0175] See also Figure 14-16 , in the potential range of 0.01–3.0 V, 100 mA g-1 At a current density of II The first cycle charge and discharge coulombic efficiency of [N(CN)2]2 is about 39%. Similarly, β-Zn II The initial capacity loss of [N(CN)2]2 is likely due to the SEI layer. With increasing cycles, the reversible capacity gradually increases. This may be due to grain fragmentation, which exposes more sites, or activation.

[0176] In the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of II The reversible capacity of [N(CN)2]2 gradually increases with the number of cycles within 300 cycles, and the maximum specific capacity is up to 700 mAh g -1 .

[0177] The cyclic voltammetry curves show that β-Zn II [N(CN)2]2 corresponds to Mn at around 0.5V 2+ The reduction peak at around 1.0 V corresponds to the reduction peak of nitrogen anions.

[0178] Example 5

[0179] α-Cu II Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0180] α-Cu II The preparation method of [N(CN)2]2 comprises the following steps:

[0181] (1) Dissolve CuCl2 in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0182] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0183] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow Cu 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0184] (4) The precipitate was heated in vacuum at 100 °C for 12 h to obtain α-Cu II [N(CN)2]2.

[0185] See also Figure 17 ,pass Figure 17It can be seen that Example 5 of the present invention is consistent with the literature report, proving that pure phase α-Cu II [N(CN)2]2.

[0186] The α-Cu obtained in this example II [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1 The constant current charge and discharge test and stability test were carried out at a current density of 100 nm. II [N(CN)2]2 electrode test see Figure 18-20 .

[0187] See also Figure 18-20 , in the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of Cu II The first cycle charge and discharge coulombic efficiency of [N(CN)2]2 is about 22%. II The capacity loss of [N(CN)2]2 in the first cycle may be caused by the SEI film. As the number of cycles increases, the reversible specific capacity value remains at 400 mAh g -1 about.

[0188] The cyclic voltammetry curves show that α-Cu II The broad peak of [N(CN)2]2 at around 2.0 V corresponds to Cu 2+ The reduction peak at around 0.6 V corresponds to the reduction peak of nitrogen anions.

[0189] Example 6

[0190] Cu I Preparation of [N(CN)2]2 negative electrode materials and their electrochemical performance as negative electrode materials for lithium-ion batteries.

[0191] Cu I The preparation method of [N(CN)2]2 comprises the following steps:

[0192] (1) Dissolve Cu(NO3)2·2.5H2O in deionized water, add excess K2[S2O5] and stir thoroughly to obtain Cu + The concentration is about 0.1 mol L -1 of clear solution 1;

[0193] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0194] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow Cu + and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0195] (4) The precipitate was heated in vacuum at 100°C for 12 h to obtain Cu I [N(CN)2]2.

[0196] See also Figure 21 ,pass Figure 21 It can be seen that the PXRD results of Example 6 of the present invention are consistent with those reported in the literature, proving that pure phase Cu was successfully prepared. I [N(CN)2]2.

[0197] The Cu obtained in this example I [N(CN)2]2, conductive agent acetylene black, and binder PVDF were mixed in a mass ratio of 6:3:1 and then coated to form an electrode sheet. -1 The constant current charge and discharge test and stability test were carried out at a current density of 100 nm. I [N(CN)2]2 electrode test see Figure 22-24 .

[0198] See also Figure 22-24 , in the potential range of 0.01–3.0 V, 100 mA g -1 At a current density of Cu I The first cycle charge and discharge coulombic efficiency of [N(CN)2]2 is about 33%. I The capacity loss of [N(CN)2]2 in the first cycle may be caused by the SEI film. As the number of cycles increases, the reversible specific capacity value gradually increases, and the specific capacity after 250 cycles is as high as 1200 mAh g -1 This may be related to interface activation.

[0199] The cyclic voltammetry curves show that Cu I [N(CN)2]2 corresponds to Cu at around 2.3V + The reduction peak at around 0.8 V corresponds to the reduction peak of nitrogen anions.

[0200] Example 7

[0201] β-Mn II The preparation method of [N(CN)2]2 comprises the following steps:

[0202] (1) Dissolve soluble Mn(ClO4)2.xH2O in deionized water and stir thoroughly to obtain a concentration of 0.1 mol / L-1 of clear solution 1;

[0203] (2) Dissolve Na[N(CN)2] in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 2;

[0204] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand until the solvent was completely evaporated, yielding large pieces of light blue single crystal Mn[N(CN)2]2.xH2O and a light yellow powder precipitate at the bottom (i.e., the new structure β-Mn II [N(CN)2]2);

[0205] (4) The light blue single crystal Mn[N(CN)2]2.xH2O was heated in vacuum at 120℃ for 12h to obtain α-Mn II [N(CN)2]2.

[0206] Example 8

[0207] β-Co II The preparation method of [N(CN)2]2 comprises the following steps:

[0208] (1) Dissolve Co(NO3)2.6H2O in deionized water and stir thoroughly to obtain a concentration of 0.1 mol L -1 of clear solution 1;

[0209] (2) Dissolve Na[N(CN)2] in a 50:1 v / v H2O:py solution and stir thoroughly to obtain a concentration of approximately 0.1 mol L -1 of clear solution 2;

[0210] (3) At room temperature, 20 ml of solution 1 was slowly added dropwise to 40 ml of solution 2 and allowed to stand for 24 h to allow the Co 2+ and [N(CN)2] - After sufficient reaction, filter to obtain a precipitate;

[0211] (4) The precipitate was heated in vacuum at 150 °C for 12 h to obtain β-Co II [N(CN)2]2.

[0212] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0213] (1) The present invention synthesized β-Co for the first time II [N(CN)2]2 and β-Mn II New materials including [N(CN)2]2.

[0214] (2) The present invention is the first to II [N(CN)2]2 and Cu I Nitrogen-based compounds such as α-Co[N(CN)2]2 are used as negative electrode materials for lithium-sodium ion batteries, and unexpectedly they have been found to have excellent electrochemical stability and high specific capacity performance. For example, α-Co[N(CN)2]2 has a high specific capacity at 100 mA g -1 The specific capacity is maintained at 700 mAh g -1 (>150 cycles), which is much higher than the theoretical specific capacity of graphite (~372 mAh g -1 ), providing a reference for the design and synthesis of new lithium battery negative electrode materials.

[0215] The specific embodiments of the present invention described above do not limit the scope of protection of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A metal dicyandiamide negative electrode material, characterized in that: The metal dicyandiamide has the following chemical formula: II [N(CN)2]2 or Cu I [N(CN)2]; wherein M is Co, Ni, Mn, Cu or Zn; I and II refer to the valence state of the metal, respectively.

2. The metal dicyandiamide negative electrode material according to claim 1, characterized in that: The M II [N(CN)2]2 includes two isomers; among them, the metal ion and N(CN)2 − The six-coordinated linkages are collectively referred to as α-M II [N(CN)2]2, metal ions and N(CN)2 − The tetracoordinated linkages are collectively referred to as β-M II [N(CN)2]2.

3. The metal dicyandiamide negative electrode material according to claim 1, characterized in that: The metal dicyandiamide has the following chemical formula: α-M II [N(CN)2]2, and M is Co, Ni, Mn or Cu; the preparation method of the metal dicyandiamide comprises the following steps: S101, M 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution. S102, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II; S103, slowly adding the solution 1 dropwise to the solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate; S104, heating the precipitate to obtain α-M II [N(CN)2]2; in, In step S101, the M 2+ The soluble salt includes at least one of a nitrate or a halide salt; in the solution, the metal ion M 2+ The concentration is 0.01-0.5 mol / L; In step S102, the N(CN)2 - The soluble salt is Na[N(CN)2]; in the second solution, N(CN)2 - The concentration is 0.01-0.5 mol / L; In step S103, during the process of slowly adding the solution 1 to the solution 2, the metal ion M 2+ With N(CN)2 - The molar ratio is 1: (1.8-2.2); the dropwise addition process is carried out at room temperature; the standing temperature is room temperature, and the standing time is 12-24 hours; In step S104, the heating method is vacuum heating, the vacuum heating temperature is 100-140 ° C, and the vacuum heating time is 8-14 hours.

4. The metal dicyandiamide negative electrode material according to claim 1, characterized in that: The metal dicyandiamide has the following chemical formula: β-Zn II [N(CN)2]2; The preparation method of the metal dicyandiamide comprises the following steps: S201, Zn 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution. S202, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II; S203, slowly adding the solution 1 dropwise to the solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate; S204, heating the precipitate to obtain β-Zn II [N(CN)2]2; in, In step S201, the Zn 2+ The soluble salt includes at least one of nitrate or halide salt; in the solution, the metal ion Zn 2+ The concentration is 0.01-0.5 mol / L; In step S202, the N(CN)2 - The soluble salt is Na[N(CN)2]; in the second solution, N(CN)2 - The concentration is 0.01-0.5 mol / L; In step S203, the solution 1 is slowly added dropwise to the solution 2, and the metal ion M 2+ With N(CN)2 - The molar ratio is 1: (1.8-2.2); the dropwise addition process is carried out at room temperature; the standing temperature is room temperature, and the standing time is 12-24 hours; In step S204, the heating method is vacuum heating, the vacuum heating temperature is 100-140 ° C, and the vacuum heating time is 8-14 hours.

5. The metal dicyandiamide negative electrode material according to claim 1, characterized in that: The metal dicyandiamide has the following chemical formula: Cu I [N(CN)2]2; The preparation method of the metal dicyandiamide comprises the following steps: S301, Cu 2+ The soluble salt is dissolved in deionized water, excess reducing agent is added, and the mixture is stirred thoroughly to obtain a solution containing Cu + of clear solution 1; S302, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II; S303, slowly adding the solution 1 dropwise to the solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate; S304, heating the precipitate to obtain Cu I [N(CN)2]2; where In step S301, the Cu 2+ The soluble salt includes at least one of nitrate, halide or acetate; the reducing agent is K2[S2O5] or Na2[S2O5]; the Cu 2+ The molar ratio of the reducing agent is 1: (1.1-2); in the solution 1, the metal ion Cu + The concentration is 0.01-0.5 mol / L; In step S302, the N(CN)2 - The soluble salt is Na[N(CN)2]; in the second solution, N(CN)2 - The concentration is 0.01-0.5 mol / L; In step S303, during the process of slowly adding the solution 1 to the solution 2, the metal ion Cu + With N(CN)2 - The molar ratio is 1: (0.8-1.2); the dropwise addition process is carried out at room temperature; the standing temperature is room temperature, and the standing time is 12-24 hours; In step S304, the heating method is vacuum heating, the vacuum heating temperature is 100-140 ° C, and the vacuum heating time is 8-14 hours.

6. A metal dicyandiamide, characterized in that The metal dicyandiamide has the following chemical formula: β-Mn II [N(CN)2]2 or β-Co II [N(CN)2]2, II refers to the valence state of the metal.

7. A method for preparing metal dicyandiamide, characterized in that: The metal dicyandiamide has the following chemical formula: β-Mn II [N(CN)2]2, II refers to the valence state of the metal; the preparation method of the metal dicyandiamide comprises the following steps: S401, soluble Mn(ClO4). x H2O was dissolved in deionized water and stirred thoroughly to obtain a clear solution. S402, N(CN)2 - The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution II; S403, slowly add the solution 1 to the solution 2, and let it stand until the solvent is completely evaporated to obtain a light blue single crystal Mn II [N(CN)2]2. x β-Mn precipitated from H2O and light yellow powder II [N(CN)2]2.

8. The method for preparing metal dicyandiamide according to claim 7, wherein In step S401, in the solution 1, the metal ion Mn 2+ The concentration is 0.01-0.5 mol / L; In step S402, the N(CN)2 - The soluble salt is Na[N(CN)2]; in the second solution, N(CN)2 - The concentration is 0.01-0.5 mol / L; In step S403, during the process of slowly adding the solution 1 to the solution 2, the metal ion Mn 2+ With N(CN)2 - The molar ratio is 1:(1.8-2.2); the dropwise addition process is carried out at room temperature; and the standing temperature is room temperature.

9. A method for preparing metal dicyandiamide, characterized in that: The metal dicyandiamide has the following chemical formula: β-Co II [N(CN)2], II refers to the valence state of the metal; the preparation method of the metal dicyandiamide comprises the following steps: S501, Co 2+ The soluble salt is dissolved in deionized water and stirred thoroughly to obtain a clear solution. S502, N(CN)2 - The soluble salt is dissolved in a mixed solution of water and pyridine, and a clear solution 2 is obtained after sufficient stirring; S503, slowly adding the solution 1 dropwise to the solution 2, allowing the mixture to stand, and then filtering to obtain a precipitate; S504, heating the precipitate to obtain β-Co II [N(CN)2].

10. The method for preparing metal dicyandiamide according to claim 9, wherein In step S501, the Co 2+ The soluble salt includes at least one of nitrate, halide or acetate; in the solution, the metal ion Co 2+ The concentration is 0.01-0.5 mol / L; In step S502, the N(CN)2 - The soluble salt is Na[N(CN)2]; in the mixed solution of water and pyridine, the volume ratio of water to pyridine is (10-100):1; in the second solution, N(CN)2 - The concentration is 0.01-0.5 mol / L; In step S503, during the process of slowly adding the solution 1 to the solution 2, the metal ion M 2+ With N(CN)2 - The molar ratio is 1: (1.8-2.2); the dropwise addition process is carried out at room temperature; the standing temperature is room temperature, and the standing time is 12-24 hours; In step S504, the heating method is vacuum heating, the vacuum heating temperature is 140-160 ° C, and the vacuum heating time is 8-14 hours.

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