Long-cycle-life A2 B7 type superlattice hydrogen storage alloy and screening method thereof

Abstract

The invention relates to a screening method of a long-cycle-life A2B7 type superlattice hydrogen storage alloy. The screening method comprises the following steps: 1, establishing a stable-state unit cell structure; 2 and 3, establishing and screening a stable-state Y-substituted unit cell structure; 4, establishing and screening all stable-state unit cell structures; 5, subunit volume calculation and screening in the most stable state; 6 and 7, establishing a stable-state Al substituted unit cell structure; 8, establishing a stable-state unit cell structure; and 9, calculating and screening the volume of the steady-state subunit. In the requirements of screening conditions, the subunit volume difference value has obvious influence on the screening result. The screened steady-state alloy is La < 0.25 > Y < 1.75 > Ni < 6.75 > Al < 0.25 >, the maximum hydrogen absorption amount is 1.53 wt.%, and the cycle capacity retention ratio is greater than 96%; the change rate of tc is 12%, and tc is kept unchanged when hydrogen is absorbed for the tenth time and the 200th time.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen storage materials, and in particular to a long cycle life A2B7 type superlattice hydrogen storage alloy and a screening method thereof. Background Art

[0002] The traditional AB5 type hydrogen storage alloy has a low maximum discharge capacity due to the limitation of the single cell structure, which cannot meet the high capacity requirements of nickel-hydrogen batteries. Superlattice hydrogen storage alloys have the advantages of high capacity, easy activation, and good large current discharge capability. They are a new generation of hydrogen storage materials that replace traditional AB5 type alloys. At present, the main superlattice hydrogen storage alloy systems developed are La-Mg-Ni and La-Y-Ni. Since the La-Mg-Ni superlattice hydrogen storage alloy contains the volatile Mg element, it directly leads to the technical problems of difficult composition control and preparation of the La-Mg-Ni system; on the contrary, the La-Y-Ni superlattice hydrogen storage alloy does not contain the Mg element, and the La-Y-Ni superlattice hydrogen storage alloy has become the focus of research.

[0003] La-Y-Ni superlattice hydrogen storage alloys are formed by stacking [AB5] and [A2B4] subunits along the c-axis in a ratio of 1:1, 2:1, and 3:1, and can be divided into AB3 type, A2B7 type, and A5B 19 At the same time, since [A2B4] sub-metal has two structures, MgZn2 and MgCu2, each superlattice hydrogen storage structure can be divided into 2H and 3R types. For example, the La-Y-Ni Ni-rich corner phase diagram disclosed in the existing document 1 (Experimental investigation and thermodynamic assessment of La-Y-Ni ternary system in Ni-rich corner [J]. Rare Metals, 2022, 42 (4): 1316-1331) shows that the structure of the La-Y-Ni superlattice hydrogen storage alloy will change from 2H type to 3R type with the increase of Y content in the chemical composition, showing a 2H structure on the La-rich side and a 3R structure on the Y-rich side.

[0004] Each superlattice alloy with different structures has different hydrogen storage properties due to the different ratios of [AB5] and [A2B4] subunits. As the number of [AB5] subunits increases, the hydrogen storage capacity of the alloy decreases, while the cycle stability increases. Among them, the A2B7 type alloy has better cycle stability than the AB3 type alloy, and better cycle stability than the A5B 19 Type alloys have higher hydrogen storage capacity; however, due to the complex stacking structure of A2B7 type superlattice alloys, the alloy cycling stability is poor.

[0005] In order to solve the above problems, the inventors of the present invention have made preliminary research results. The existing document 2 (CN 17542441A A method for screening the composition of AB3 type La-Y-Ni superlattice hydrogen storage alloy) screens the alloy composition through first-principles calculation to solve the problem of improving the cyclic stability of the alloy, that is, the alloy composition is screened through first-principles calculation, which effectively improves the cyclic stability of the alloy. Specifically, a method for screening the composition of AB3 type La-Y-Ni superlattice hydrogen storage alloy is disclosed. Through first-principles calculation, La-Y-Ni AB3 type alloy composition with good structural stability in the long-term hydrogen absorption and dehydrogenation cycle is successfully screened out. After 50 cycles of hydrogen absorption and dehydrogenation, the capacity retention rate of the screened alloy is higher than 95%.

[0006] The inventor of the present invention found in subsequent research that the technical solution still has the following unresolved technical problems, which can be summarized into the following three problems:

[0007] 1. The screening condition 3 in the technical solution is that the difference in subunit volume between [A2B4] and [AB5] is less than It cannot be directly used as screening condition 3 for the composition of A2B7 type La-Y-Ni superlattice hydrogen storage alloy. The reason is that the ratios of [AB5] and [A2B4] subunits in AB3 type and A2B7 type superlattice hydrogen storage alloys are different, and the microstrain generated at the interface of the two subunits is different. Therefore, the screening condition 3 in the technical solution has a different screening condition range for the volume difference between [A2B4] and [AB5] subunits.

[0008] 2. It is necessary to further supplement the screening conditions for the composition of A2B7 type La-Y-Ni superlattice hydrogen storage alloy. The reason is that La-Y-Ni series A2B7 type alloy exists stably on the La-Ni side and the Y-Ni side. 2-x Y x Ni7 (x = 0.4-1.75) cannot exist stably within the composition range, so the calculated screening results need to be further screened within the stable existence range of the La-Y-Ni Ni-rich angular phase diagram;

[0009] 3. It is necessary to calculate the unit cell structures of 2H-La2Ni7 and 3R-La2Ni7 at the same time. The reason is that the La-Y-Ni series AB3 type alloy only exists stably on the Y-Ni side and presents a 3R structure. Therefore, only the 3R type structure AB3 alloy needs to be calculated during calculation. The La-Y-Ni series A2B7 type alloy presents a 2H type structure on the La-Ni side and a 3R structure on the Y-Ni side. Both types of structures need to be considered during calculation.

[0010] The above technical solution is to reduce the volume of the [AB5] and [A2B4] subunits and the volume difference between the two subunits by replacing the La element with a larger atomic radius with the Y element with a smaller atomic radius.

[0011] Since the Y element can enter both the [A2B4] subunit and the [AB5] subunit, an appropriate amount of the Y element preferentially enters the [A2B4] subunit, which can effectively reduce the volume of the [A2B4] subunit and reduce the volume difference between the [AB5] and [A2B4] subunits, thereby inhibiting the amorphization of the alloy during the hydrogen absorption and desorption process; however, an excessive amount of the Y element will also enter the [AB5] subunit, increasing the volume difference between the [AB5] and [A2B4] subunits, which is not conducive to the cycle performance of the alloy. Therefore, computational screening of the Y element content can improve the cycle performance of superlattice hydrogen storage alloys.

[0012] The Y element can only reduce the volume difference between the [AB5] and [A2B4] subunits by replacing the La element on the A side, but cannot reduce the volume difference between the [AB5] and [A2B4] subunits by replacing the Ni element on the B side. Compared with Ni, Al has a larger atomic radius and only occupies the [AB5] subunit, which can reduce the volume difference between the [AB5] and [A2B4] subunits. For example, the existing literature 3 (Effect of Al content on the structural and electrochemical properties of A2B7type La-Y-Ni based hydrogen storage alloy [J]. International Journal of Hydrogen Energy, 2020, 45 (33): 16677-16689) experimentally prepared a series of LaY 1.9 Ni 10.2-x Al x Mn 0.5 The effect of Al content on the A2B7 type LaY 1.9 Ni 10.2-x Al x Mn 0.5Influence of alloy structure and performance. The technical effect achieved is that when the Al content increases from 0 to 0.4, the capacity retention rate after 300 charge and discharge cycles increases from 59.4% to 62.7%. The reason for the improvement of the capacity retention rate of this technical solution is that after the Al element is added to the alloy, Al mainly replaces Ni at the 6h position between the two [AB5] subunits. After the substitution, the volume of the [AB5] subunit increases, while the volume of the [A2B4] subunit does not change much, which reduces the volume difference between the [AB5] and [A2B4] subunits. However, the inventors of the present invention have found that the existing document 3 does not constrain the volumes of the [AB5] and [A2B4] subunits and their differences when preparing the alloy, which directly leads to the inability to find the optimal amount of Al substitution through this technical solution, resulting in a small increase in the capacity retention rate.

[0013] Based on the above reasons, in order to use first-principles calculations to screen out La-Y-Ni A2B7 alloy compositions with good structural stability during long-term hydrogen absorption and dehydrogenation cycles, the following problems need to be solved:

[0014] 1. With the increase of Y content in the chemical composition, the structure of A2B7 superlattice alloy changes from 2H type to 3R type. Both types of structures need to be considered simultaneously in the calculation. However, due to the large unit cell structure of 3R-A2B7, the calculation is complex and the calculation does not converge. Therefore, the 2H type structure with a smaller unit cell structure is selected to screen the most stable structure of A2B7 type La-Y-Ni superlattice alloy;

[0015] 2. When the first-principles calculation is used to screen alloy components, the screening conditions need to be adjusted according to different structures;

[0016] 3. When introducing Al element into La-Y-Ni ternary alloy, appropriate calculation parameters need to be adjusted during first-principles calculation. Summary of the invention

[0017] In order to solve the problems of the prior art, the purpose of the present invention is to provide an A2B7 superlattice hydrogen storage alloy with a long cycle life and a screening method thereof. The cycle stability of the A2B7 La-Y-Ni alloy is preliminarily improved by adjusting the Y element content; and then by adding an appropriate amount of Al element, the matching degree of the [AB5] and [A2B4] subunits is adjusted to obtain a superlattice alloy with a long cycle life. Based on the A2B7 La-Y-Ni superlattice alloy with the most stable structure, the most stable structure of the A2B7 superlattice alloy at different Al contents is screened by first-principles calculation.

[0018] To achieve the above object, the present invention adopts the following technical solutions:

[0019] A method for screening an A2B7 superlattice hydrogen storage alloy with a long cycle life comprises the following steps:

[0020] Step 1, establishing the unit cell structure of stable La2Ni7. First, derive the unit cell structures of 2H-type La2Ni7 and 3R-type La2Ni7 from the existing public inorganic crystal structure database. The chemical composition of 2H-type La2Ni7 and 3R-type La2Ni7 is La2Ni7, which are collectively referred to as 2H-La / Y-Ni-0 and 3R-La / Y-Ni-0. Then, use the first principle calculation to optimize the unit cell structure model of 2H-type La2Ni7 and 3R-type La2Ni7, so that the unit cell structure is in a stable state. Among them, the unit cell structure optimization result of 3R-type La2Ni7 has not converged, and it will not be calculated further. The unit cell structure optimization result of 2H-type La2Ni7 has converged, and the unit cell structure of stable 2H-La / Y-Ni-0 is obtained, which is referred to as stable 2H-La / Y-Ni-0. At the same time, the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained.

[0021] In step 1, the specific method of structural optimization is that the method used for atomic position relaxation is the conjugate gradient method, that is, IBRION is 2; during structural optimization, the atomic position, unit cell shape, and unit cell volume are optimized simultaneously, that is, ISIF is 3; and the energy convergence standard EDIFF is less than or equal to 10 -6 eV, the convergence standard EDIFFG for single-atom forces is less than or equal to The cutoff energy ENCUT is 400eV, and the K-point grid k-point is 8×8×2; there are two cases of structural optimization calculation results. When the structural optimization calculation does not reach convergence, the unit cell structure is in an unstable state, referred to as an unstable state; when the structural optimization calculation reaches convergence, the unit cell structure is in a stable state, referred to as a stable state;

[0022] Step 2, the establishment and screening of the stable 2H-La / Y-Ni-1 unit cell structure, first, the first Y atom is substituted, that is, the La atom in the stable 2H-La / Y-Ni-0 in step 1 is replaced by one Y atom, and two kinds of La 1.75 Y 0.25 Ni7 unit cell structure, its chemical composition is La 1.75 Y 0.25Ni7, collectively referred to as 2H-La / Y-Ni-1; then, all 2H-La / Y-Ni-1 are structurally optimized, and the stable 2H-La / Y-Ni-1 unit cell structure is obtained, referred to as stable 2H-La / Y-Ni-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained; finally, the unit cell formation enthalpy of all stable 2H-La / Y-Ni-1 is compared, and the stable 2H-La / Y-Ni-1 with the smallest formation enthalpy can be obtained, referred to as the most stable 2H-La / Y-Ni-1;

[0023] Step 3: Establishment and screening of the stable 2H-La / Y-Ni-2 unit cell structure. First, the second Y atom is substituted to obtain 7 kinds of La 1.5 Y 0.5 Ni7 unit cell structure, its chemical composition is La 1.5 Y 0.5 Ni7, collectively referred to as 2H-La / Y-Ni-2; then, the structures of the seven types of 2H-La / Y-Ni-2 were optimized respectively, and the stable 2H-La / Y-Ni-2 unit cell structures were obtained respectively, referred to as stable 2H-La / Y-Ni-2, and the corresponding unit cell parameters, unit cell volumes and unit cell formation enthalpy were obtained at the same time; finally, the unit cell formation enthalpy of all the stable 2H-La / Y-Ni-2 were compared, and the stable 2H-La / Y-Ni-2 with the smallest formation enthalpy was obtained, referred to as the most stable 2H-La / Y-Ni-2;

[0024] Step 4, establishing and screening all stable 2H-La / Y-Ni-x unit cell structures, repeating the Y atom substitution, structure optimization, and formation enthalpy comparison of steps 2 and 3, and obtaining the stable 2H-La / Y-Ni-x and the most stable 2H-La / Y-Ni-x, respectively, and obtaining the corresponding unit cell parameters, unit cell volume, and unit cell formation enthalpy;

[0025] Step 5, calculation and screening of the volume of the [A2B4] subunit and [AB5] subunit in the most stable 2H-La / Y-Ni-x, by substituting the unit cell parameters and unit cell volume obtained in the previous steps into the following formula, V [A2B4] =a 2 ×h1×sin60° and V [AB5] =a 2 ×h2×sin60°, where a is the unit cell parameter, h1 is the height of the [A2B4] subunit, and h2 is the height of the [AB5] subunit. The volumes of the [A2B4] subunit and the [AB5] subunit in each most stable 2H-La / Y-Ni-x and the volume difference between the [A2B4] and [AB5] subunits can be calculated. Then, the most stable 2H-La / Y-Ni-x can be preliminarily selected under certain screening conditions.

[0026] In step 5, the screening condition is to meet the following four requirements at the same time:

[0027] (1) The subunit volume of [A2B4] is lower than

[0028] (2) The subunit volume of [AB5] is lower than

[0029] (3) having the smallest difference in subunit volume between [A2B4] and [AB5];

[0030] (4) A2B7 type superlattice alloy that exists stably in the La-Y-Ni phase diagram;

[0031] The most stable 2H-La / Y-Ni-x alloy preliminarily screened out is La 0.25 Y 1.75 The most stable state of Ni7 is 2H-La / Y-Ni-7;

[0032] Step 6, the establishment of the stable 2H-La / Y-Ni / Al-1 unit cell structure. First, based on the most stable La / Y-Ni-7 unit cell structure preliminarily screened in step 5, an Al atom is substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.875 Al 0.125 The unit cell structure is La 0.25 Y 1.75 Ni 6.875 Al 0.125 , collectively referred to as 2H-La / Y-Ni / Al-1, then, the structure of 2H-La / Y-Ni / Al-1 is optimized to make the unit cell structure stable, and the unit cell structure of the stable state 2H-La / Y-Ni / Al-1 is obtained, referred to as the stable state 2H-La / Y-Ni / Al-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained;

[0033] In step 6, since the original most stable 2H-La / Y-Ni-7 unit cell structure is a single cell structure, after one Al atom is substituted, La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure has a doping ratio that is too large, so the most stable 2H-La / Y-Ni-7 unit cell structure is expanded by 2×1×1. Due to the expansion of the unit cell structure, the specific method for structural optimization in step 6 is the same as step 1 for the contents not otherwise specified, except that the K-point grid k-point is 8×8×1.

[0034] Step 7: Establishment of the stable 2H-La / Y-Ni / Al-2 unit cell structure. First, two Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure is La 0.25 Y 1.75 Ni 6.75 Al 0.25 , collectively referred to as 2H-La / Y-Ni / Al-2, then, the structure of 2H-La / Y-Ni / Al-2 is optimized to make the unit cell structure stable, and the unit cell structure of the stable La / Y-Ni / Al-2 is obtained, referred to as stable 2H-La / Y-Ni / Al-2, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained;

[0035] Step 8: Establishment of the stable 2H-La / Y-Ni / Al-4 unit cell structure. First, four Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.5 Al 0.5 The unit cell structure is La 0.25 Y 1.75 Ni 6.5 Al 0.5 , collectively referred to as 2H-La / Y-Ni / Al-4, then, the structure of 2H-La / Y-Ni / Al-4 is optimized to make the unit cell structure stable, and the unit cell structure of the stable state 2H-La / Y-Ni / Al-4 is obtained, referred to as the stable state La / Y-Ni / Al-, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained;

[0036] Step 9, calculation and screening of the volumes of the [A2B4] subunit and the [AB5] subunit in the steady-state 2H-La / Y-Ni / Al-z, repeating the calculation of the volumes of the [A2B4] subunit and the [AB5] subunit in step 5, and obtaining the volumes of the [A2B4] subunit and the [AB5] subunit and the volume difference between the [A2B4] and [AB5] subunits in each steady-state 2H-La / Y-Ni / Al-z respectively; then, the steady-state 2H-La / Y-Ni / Al-z is finally screened out under certain screening conditions;

[0037] In step 9, the screening condition is to meet the following three requirements at the same time:

[0038] (1) The subunit volume of [A2B4] is lower than

[0039] (2) The subunit volume of [AB5] is lower than

[0040] (3) The difference in subunit volume between [A2B4] and [AB5] is less than

[0041] The final screened stable 2H-La / Y-Ni / Al-z alloy has a chemical composition of La 0.25 Y 1.75 Ni 6.75 Al 0.25 The steady state 2H-La / Y-Ni / Al-2;

[0042] The alloy finally selected was fully activated after one hydrogen absorption / dehydrogenation cycle. The hydrogen absorption amount measured by the second hydrogen absorption was the maximum hydrogen absorption amount of the alloy after activation, which was 1.53wt.%. The capacity retention rate after 200 hydrogen absorption / dehydrogenation cycles was 96.7%.

[0043] The alloy finally selected has a good t c The change rate is 12%. At the 10th and 200th hydrogen absorption, t c Remains almost unchanged.

[0044] XRD, hydrogen absorption / dehydrogenation cycle performance test and hydrogen absorption kinetic curve fitting show that the present invention has the following properties and characteristics:

[0045] The results of XRD test and XRD refinement show that the alloy is composed of 3R type superlattice phase (Y,La)2Ni7, 3R type superlattice phase (Y,La)Ni3 and superlattice phase (Y,La)Ni5, among which the mass content of 3R type superlattice phase (Y,La)2Ni7 is 48.5, the mass content of 3R type superlattice phase (Y,La)Ni3 is 44.6%, and the mass content of superlattice phase (Y,La)Ni5 is 6.9%.

[0046] The results of the hydrogen absorption / dehydrogenation cycle performance test show that under the hydrogen absorption conditions of 25°C and 4MPa, after one hydrogen absorption / dehydrogenation cycle, the alloy is fully activated. The hydrogen absorption amount measured in the second hydrogen absorption is the maximum hydrogen absorption amount of the alloy after activation, which is 1.53wt.%. The capacity retention rate after 200 hydrogen absorption / dehydrogenation cycles is 96.7%, which proves that the alloy screened by the present invention has a long cycle life.

[0047] The results of hydrogen absorption kinetic curve fitting show that the hydrogen absorption rate of the alloy gradually increases with the increase of hydrogen absorption times. During 200 hydrogen absorption / dehydrogenation cycles, t c The change rate is 12%. At the 10th and 200th hydrogen absorption, t cIt remains almost unchanged, proving that the alloy screened by the present invention has good hydrogen absorption rate stability, and proving that the alloy has good structural stability during long-term hydrogen absorption and dehydrogenation cycles, and also has good hydrogen absorption rate stability during the cycle.

[0048] Compared with the prior art, the present invention has the following advantages:

[0049] 1. When the alloy composition is screened by first-principles calculation, the alloy system is expanded from ternary alloy to quaternary alloy, which proves that the applicability of the screening method of the present invention is higher than that of the existing literature;

[0050] 2. After 200 cycles of hydrogen absorption and dehydrogenation, the capacity retention rate of the screened alloy is higher than 95%. During the 10th and 200th hydrogen absorptions, the hydrogen absorption rate remains unchanged, proving that the performance of the hydrogen storage alloy screened by the screening method of the present invention is higher than that of the existing literature. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 is the unit cell structure of the steady-state 2H-La / Y-Ni-0 calculated by first principles in Example 1, the unit cell parameters, unit cell volume and unit cell formation enthalpy of 2H-La / Y-Ni-x calculated by first principles in Example 1, wherein,

[0052] Figure 1 a is the unit cell structure of the stable 2H-La / Y-Ni-0.

[0053] Figure 1 b is the unit cell parameter a of 2H-La / Y-Ni-x,

[0054] Figure 1 c is the unit cell parameter c of 2H-La / Y-Ni-x,

[0055] Figure 1 d is the unit cell volume V of 2H-La / Y-Ni-x,

[0056] Figure 1 e is the unit cell formation enthalpy ΔH of 2H-La / Y-Ni-x;

[0057] Figure 2 Schematic diagram of the volume size of the [A2B4] subunit and [AB5] subunit of the most stable 2H-La / Y-Ni-x calculated by first principles in Example 1;

[0058] Figure 3 Schematic diagram of the volume difference between the [A2B4] subunit and the [AB5] subunit of the most stable 2H-La / Y-Ni-x calculated by first principles in Example 1;

[0059] Figure 4 It is the Ni-rich angular phase diagram of La-Y-Ni;

[0060] Figure 5 is the unit cell formation enthalpy ΔH of 2H-La / Y-Ni / Al-z calculated by first principles in Example 1;

[0061] Figure 6 is the unit cell structure of 2H-La / Y-Ni / Al-z calculated by first principles in Example 1, wherein:

[0062] Figure 6 a is the unit cell structure of the stable 2H-La / Y-Ni / Al-1.

[0063] Figure 6 b is the unit cell structure of the stable 2H-La / Y-Ni / Al-2.

[0064] Figure 6 c is the unit cell structure of the stable 2H-La / Y-Ni / Al-4;

[0065] Figure 7 Schematic diagram of the volume size of the [A2B4] subunit and [AB5] subunit of the steady-state 2H-La / Y-Ni / Al-z calculated by first principles in Example 1;

[0066] Figure 8 Schematic diagram of the volume difference between the [A2B4] subunit and the [AB5] subunit of the steady-state 2H-La / Y-Ni / Al-z calculated by first principles in Example 1;

[0067] Fig. 9 The XRD diagram and refinement result diagram of the 2H-La / Y-Ni / Al-2 alloy in Example 1;

[0068] Fig.10 The XRD pattern and refinement result of the 2H-La / Y-Ni-7 alloy in Example 1;

[0069] Fig.11 The first and second hydrogen absorption kinetic curves of the 2H-La / Y-Ni / Al-2 alloy and the 2H-La / Y-Ni-7 alloy in Example 1;

[0070] Fig.12 Capacity retention rate curves of 2H-La / Y-Ni / Al-2 alloy and 2H-La / Y-Ni-7 alloy in Example 1;

[0071] Fig.13 is the hydrogen absorption kinetics fitting curve of 2H-La / Y-Ni / Al-2 alloy and 2H-La / Y-Ni-7 alloy in Example 1, wherein:

[0072] Fig.13a is the fitting curve of the second hydrogen absorption kinetics of the two alloys.

[0073] Fig.13 b is the 10th hydrogen absorption kinetic fitting curve of the two alloys.

[0074] Fig.13 c is the 50th hydrogen absorption kinetic fitting curve of the two alloys.

[0075] Fig.13 d is the 200th hydrogen absorption kinetic fitting curve of the two alloys. DETAILED DESCRIPTION

[0076] The present invention is further described in detail through embodiments and in conjunction with the accompanying drawings, but the present invention is not limited thereto.

[0077] A method for screening an A2B7 superlattice hydrogen storage alloy with a long cycle life comprises the following steps:

[0078] Step 1, establishing the unit cell structure of stable La2Ni7. First, the unit cell structures of 2H-type La2Ni7 and 3R-type La2Ni7 are derived from the existing public inorganic crystal structure database. The chemical composition of 2H-type La2Ni7 and 3R-type La2Ni7 is La2Ni7, which are collectively referred to as 2H-La / Y-Ni-0 and 3R-La / Y-Ni-0. Then, the unit cell structure models of 2H-type La2Ni7 and 3R-type La2Ni7 are structurally optimized using first-principles calculations to make the unit cell structure stable. Among them, the unit cell structure optimization result of 3R-type La2Ni7 has not converged, and no further calculation is performed on it. The unit cell structure optimization result of 2H-type La2Ni7 has converged to obtain the unit cell structure of stable 2H-La / Y-Ni-0, referred to as stable 2H-La / Y-Ni-0. At the same time, the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained. The specific data are shown in Table 1.

[0079] Note: The naming rule of 2H-La / Y-Ni-0 is collectively referred to as 2H-La / Y-Ni-x, where x represents the number of Y atoms replaced in the stable 2H-La / Y-Ni unit cell, and the value range of x is 0≤x≤8. Specifically, in step 1, the number of Y atoms is 0, so it is named 2H-La / Y-Ni-0;

[0080] Table 1 Unit cell parameters, unit cell volume and unit cell formation enthalpy of 2H-La / Y-Ni-x (0≤x≤8)

[0081]

[0082]

[0083] The unit cell structure of the stable 2H-La / Y-Ni-0 in step 1 is as follows: Figure 1 As shown, it is composed of 8 La atoms and 28 Ni atoms. For the convenience of description, the above 8 La atoms are numbered as La1, La2, La3, La4, La5, La6, La7 and La8 respectively; due to the symmetry of the unit cell structure of the steady-state 2H-La / Y-Ni-0, there are only two different positions of La atoms, which are specifically divided into the La atomic position in the [A2B4] subunit and the La atomic position in the [AB5] subunit, among which La1, La2, La3 and La4 belong to the [AB5] subunit, and La5, La6, La7 and La8 belong to the [A2B4] subunit;

[0084] The specific method of structure optimization described in the present invention is that the method used for atomic position relaxation is the conjugate gradient method, that is, IBRION is 2; during structure optimization, the atomic position, unit cell shape, and unit cell volume are optimized simultaneously, that is, ISIF is 3; the energy convergence standard EDIFF is less than or equal to 10 -6 eV, the convergence standard EDIFFG for single-atom forces is less than or equal to The cutoff energy ENCUT is 400eV, and the K-point grid k-point is 8×8×2; there are two situations for the structural optimization calculation results. When the structural optimization calculation has not reached convergence, the unit cell structure is in an unstable state, referred to as an unstable state, represented by ○; when the structural optimization calculation reaches convergence, the unit cell structure is in a stable state, referred to as a steady state, represented by ■. If there are multiple steady states, please refer to the instructions in step 2.

[0085] Step 2, the establishment and screening of the stable 2H-La / Y-Ni-1 unit cell structure, first, the first Y atom is substituted, that is, the La atom in the stable 2H-La / Y-Ni-0 in step 1 is replaced by one Y atom to obtain La 1.75 Y 0.25 Ni7 unit cell structure, its chemical composition is La 1.75 Y 0.25Ni7, collectively referred to and abbreviated as 2H-La / Y-Ni-1, the individual naming rules can refer to step 1; since there are two La atoms in different positions in the stable state 2H-La / Y-Ni-0, two 2H-La / Y-Ni-1 unit cell structures can be obtained, which are named 2H-La / Y-Ni-1.1 and 2H-La / Y-Ni-1.2 respectively; then, the two 2H-La / Y-Ni-1 are structurally optimized respectively, and the stable state 2H-La / Y-Ni-1 unit cell structure is obtained respectively, which is referred to as the stable state 2H-La / Y-Ni-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained at the same time, as shown in Table 1; finally, the unit cell formation enthalpy of all stable state 2H-La / Y-Ni-1 is compared, and the stable state 2H-La / Y-Ni-1 with the smallest formation enthalpy can be obtained, which is referred to as the most stable state 2H-La / Y-Ni-1;

[0086] The specific method of Y atom substitution of the present invention takes step 2 as an example. As mentioned above, there are two kinds of La atoms in different positions in the stable 2H-La / Y-Ni-0, namely La1, La2, La3 and La4 in the [AB5] subunit and La5, La6, La7 and La8 in the [A2B4] subunit. Therefore, when performing Y atom substitution, one Y atom is used to replace the La8 atom in the [A2B4] subunit to obtain 2H-La / Y-Ni-1.1 and to replace the La3 atom in the [AB5] subunit to obtain 2H- La / Y-Ni-1.2, two unit cell structures of 2H-La / Y-Ni-1 can be obtained, and subsequent structural optimization can obtain steady-state 2H-La / Y-Ni-1.1 and steady-state 2H-La / Y-Ni-1.2. The unit cell formation enthalpies of steady-state 2H-La / Y-Ni-1.1 and steady-state 2H-La / Y-Ni-1.2 are compared, and the steady-state 2H-La / Y-Ni-1.1 with the smallest formation enthalpy is selected as the most stable 2H-La / Y-Ni-1, where the most stable state is represented by √.

[0087] Note: The naming convention of 2H-La / Y-Ni-1.1 and 2H-La / Y-Ni-1.2 is collectively referred to as 2H-La / Y-Ni-xy, wherein y represents the different La atomic positions in the stable 2H-La / Y-Ni unit cell when the number of Y atoms substituted is x. Specifically, there are two different La atomic positions in the stable 2H-La / Y-Ni-1 unit cell, so they are named 2H-La / Y-Ni-1.1 and 2H-La / Y-Ni-1.2.

[0088] Note that in Example 1, La8 in the [A2B4] subunit and La3 in the [AB5] subunit are randomly selected and only need to be atoms in the same position, which has no substantial effect on the calculation results.

[0089] Step 3, the establishment and screening of the stable 2H-La / Y-Ni-2 unit cell structure. First, the second Y atom is substituted. Since the first Y atom is substituted in step 2, the symmetry of the most stable 2H-La / Y-Ni-1 unit cell structure is destroyed, the most stable 2H-La / Y-Ni-1 has 7 La atoms in different positions, that is, the remaining 7 La atoms are all different La atoms. Therefore, 7 kinds of La can be obtained. 1.5 Y 0.5 Ni7 unit cell structure, its chemical composition is La 1.5 Y 0.5 Ni7, collectively referred to and abbreviated as 2H-La / Y-Ni-2, the individual naming rules can refer to step 1; then, the structures of the seven types of 2H-La / Y-Ni-2 are optimized respectively, and the stable 2H-La / Y-Ni-2 unit cell structures are obtained respectively, which are referred to as stable 2H-La / Y-Ni-2, and the corresponding unit cell parameters, unit cell volumes and unit cell formation enthalpy are obtained at the same time, as shown in Table 1; finally, the unit cell formation enthalpy of all stable 2H-La / Y-Ni-2 is compared, and the stable 2H-La / Y-Ni-2 with the smallest formation enthalpy can be obtained, which is referred to as the most stable 2H-La / Y-Ni-2;

[0090] Step 4, the establishment and screening of all stable 2H-La / Y-Ni-x unit cell structures, repeating the Y atom substitution, structure optimization, and formation enthalpy comparison of steps 2 and 3, respectively, to obtain the stable 2H-La / Y-Ni-x and the most stable 2H-La / Y-Ni-x, and at the same time obtain the corresponding unit cell parameters, unit cell volume, and unit cell formation enthalpy, as shown in Tables 1 and Figure 1 shown.

[0091] The unit cell screening results of all stable 2H-La / Y-Ni-x show that the unit cell formation enthalpy of the most stable 2H-La / Y-Ni-x decreases with the increase of x, indicating that the structural stability of the most stable 2H-La / Y-Ni-x is increasing.

[0092] Step 5, calculation and screening of the volume of the [A2B4] subunit and [AB5] subunit in the most stable 2H-La / Y-Ni-x, by substituting the unit cell parameters and unit cell volume obtained in the previous steps into the following formula, V [A2B4] =a 2 ×h1×sin60° and V [AB5] =a 2 ×h2×sin60°, where a is the unit cell parameter, h1 is the height of the [A2B4] subunit, and h2 is the height of the [AB5] subunit. The volumes of the [A2B4] subunit and the [AB5] subunit in each most stable 2H-La / Y-Ni-x and the volume difference between the [A2B4] and [AB5] subunits can be calculated, as shown in Table 2. Figure 2 and Figure 3 As shown; then, the most stable 2H-La / Y-Ni-x is preliminarily screened out under certain screening conditions; in specific embodiment 1, the most stable 2H-La / Y-Ni-7 meets the requirements.

[0093] Table 2 Volume and difference of [A2B4] and [AB5] subunits in the most stable 2H-La / Y-Ni-x (0≤x≤8)

[0094]

[0095]

[0096] According to the applicant's research, in the La-Y-Ni ternary alloy, in order to achieve the structural stability of the rare earth hydrogen storage alloy in the long-term hydrogen absorption and dehydrogenation cycle process, the screening condition is to simultaneously meet the following four requirements:

[0097] 1. [A2B4] subunit volume is lower than

[0098] 2. [AB5] subunit volume is lower than

[0099] 3. It has the smallest subunit volume difference between [A2B4] and [AB5];

[0100] 4. A2B7 type superlattice alloy that exists stably in La-Y-Ni phase diagram, La-Y-Ni Ni-rich angular phase diagram is as follows Figure 4 shown.

[0101] The most stable 2H-La / Y-Ni-x alloy preliminarily screened out is La 0.25 Y 1.75 Steady-state 2H-La / Y-Ni-7 of Ni8;

[0102] Step 6, the establishment of the stable 2H-La / Y-Ni / Al-1 unit cell structure. First, based on the most stable La / Y-Ni-7 unit cell structure preliminarily screened in step 5, an Al atom is substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.875 Al 0.125 The unit cell structure is La 0.25 Y 1.75 Ni 6.875 Al 0.125, collectively referred to as La / Y-Ni / Al-1, then the structure of 2H-La / Y-Ni / Al-1 is optimized to make the unit cell structure stable, and the unit cell structure of the stable state 2H-La / Y-Ni / Al-1 is obtained, referred to as steady-state 2H-La / Y-Ni / Al-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained, as shown in Table 3 and Figure 5 As shown;

[0103] Note: The naming rule of 2H-La / Y-Ni / Al-1 is collectively referred to as 2H-La / Y-Ni / Al-z, where z represents the number of replaced Al atoms in the stable 2H-La / Y-Ni / Al unit cell, and the value range of z is 1≤x≤4. Specifically, in step 6, the number of Al atoms is 1, so it is named 2H-La / Y-Ni / Al-1;

[0104] Table 3 Unit cell parameters, unit cell volume and unit cell formation enthalpy of 2H-La / Y-Ni / Al-z (1≤x≤4)

[0105]

[0106] In step 6, since the original most stable 2H-La / Y-Ni-7 unit cell structure is a single cell structure, after one Al atom is substituted, La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure has a doping ratio that is too large, so the most stable 2H-La / Y-Ni-7 unit cell structure is expanded by 2×1×1. Due to the expansion of the unit cell structure, the specific method for structural optimization in step 6 is the same as step 1 for the contents not otherwise specified, except that the K-point grid k-point is 8×8×1.

[0107] The unit cell structure of the stable 2H-La / Y-Ni / Al-1 in step 6 is as follows: Figure 6 As shown in a, it is composed of 14 La atoms, 2 Y atoms, 55 Ni atoms and 1 Al atom;

[0108] Step 7: Establishment of the stable 2H-La / Y-Ni / Al-2 unit cell structure. First, two Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure is La 0.25 Y 1.75 Ni 6.75 Al 0.25, collectively referred to as 2H-La / Y-Ni / Al-2, then the structure of 2H-La / Y-Ni / Al-2 is optimized to make the unit cell structure stable, and the unit cell structure of stable 2H-La / Y-Ni / Al-2 is obtained, referred to as stable 2H-La / Y-Ni / Al-2, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained, as shown in Table 3 and Figure 5 As shown;

[0109] The unit cell structure of the stable 2H-La / Y-Ni / Al-2 in step 7 is as follows: Figure 6 As shown in b, it is composed of 14 La atoms, 2 Y atoms, 54 Ni atoms and 2 Al atoms;

[0110] Step 8: Establishment of the stable 2H-La / Y-Ni / Al-4 unit cell structure. First, four Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.5 Al 0.5 The unit cell structure is La 0.25 Y 1.75 Ni 6.5 Al 0.5 , collectively referred to as 2H-La / Y-Ni / Al-4, then the structure of 2H-La / Y-Ni / Al-4 was optimized to make the unit cell structure stable, and the unit cell structure of stable 2H-La / Y-Ni / Al-4 was obtained, referred to as stable 2H-La / Y-Ni / Al-4, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy were obtained, as shown in Table 3 and Figure 5 As shown;

[0111] The unit cell structure of the stable 2H-La / Y-Ni / Al-4 in step 8 is as follows: Figure 6 As shown in c, it is composed of 14 La atoms, 2 Y atoms, 52 Ni atoms and 4 Al atoms;

[0112] Step 9, calculation and screening of the volume of the [A2B4] subunit and the [AB5] subunit in the steady-state 2H-La / Y-Ni / Al-z, repeat the calculation of the volume of the [A2B4] subunit and the [AB5] subunit in step 5, and obtain the volume of the [A2B4] subunit, the [AB5] subunit and the volume difference between the [A2B4] and [AB5] subunits in each steady-state 2H-La / Y-Ni / Al-z respectively; as shown in Table 4, Figure 7 and Figure 8 As shown; then, under certain screening conditions, the steady-state 2H-La / Y-Ni / Al-z is finally screened out; in specific embodiment 1, 2H-La / Y-Ni / Al-2 meets the requirements.

[0113] Table 4 Volumes and differences of [A2B4] and [AB5] subunits in steady-state 2H-La / Y-Ni / Al-z (1≤x≤4)

[0114]

[0115] According to the applicant's research, in the La-Y-Ni-Al quaternary alloy, in order to achieve the structural stability of the rare earth hydrogen storage alloy in the long-term hydrogen absorption and dehydrogenation cycle process, the screening condition is to meet the following three requirements at the same time:

[0116] In step 9, the screening condition is to meet the following three requirements at the same time:

[0117] 1. [A2B4] subunit volume is lower than

[0118] 2. [AB5] subunit volume is lower than

[0119] 3. The difference in subunit volume between [A2B4] and [AB5] is less than

[0120] The final screened stable 2H-La / Y-Ni / Al-z alloy has a chemical composition of La 0.25 Y 1.75 Ni 6.75 Al 0.25 The steady state 2H-La / Y-Ni / Al-2;

[0121] Taking Example 1 as an example, the specific screening process and reasons are as follows:

[0122] Since the atomic radius of La is 0.187nm and the atomic radius of Y is 0.180nm, when Y atoms replace La atoms, the following will occur:

[0123] 1. As x increases, the volume of the [A2B4] subunit in the most stable 2H-La / Y-Ni-x decreases. Therefore, when x = 5, 6, 7 and 8, the volume of the [A2B4] subunit in the most stable 2H-La / Y-Ni-x is less than

[0124] 2. As x increases, the volume of the [AB5] subunit in the most stable 2H-La / Y-Ni-x decreases. Therefore, when x = 0, 1, 2, 3, 4, 5, 6, 7 and 8, the volume of the [AB5] subunit in the most stable 2H-La / Y-Ni-x is less than

[0125] 3. The volume difference between [A2B4] and [AB5] subunits shows a trend of decreasing, increasing, decreasing again, and increasing again. Therefore, when x=5, the volume difference between [A2B4] and [AB5] subunits in the most stable 2H-La / Y-Ni-x is the smallest.

[0126] 4. The composition range of the A2B7 superlattice alloy that exists stably in the La-Y-Ni phase diagram is x=0-0.4 and 1.75-2. Therefore, specifically when x=0, 1, 7 and 8, the most stable 2H-La / Y-Ni-x exists stably in the La-Y-Ni phase diagram.

[0127] Therefore, the most stable 2H-La / Y-Ni-7 is the one that satisfies the above four screening requirements at the same time.

[0128] Since the atomic radius of Al is 0.143nm and the atomic radius of Ni is 0.125nm, when Al atoms replace Ni atoms, the following will occur:

[0129] 1. As z increases, the volume of the [A2B4] subunit in the steady-state 2H-La / Y-Ni / Al-z increases. Therefore, when z = 1 and 2, the volume of the [A2B4] subunit in the most stable 2H-La / Y-Ni / Al-z is lower than

[0130] 2. As z increases, the volume of the [AB5] subunit in the steady-state 2H-La / Y-Ni / Al-z increases. Therefore, when z = 1, 2, and 4, the volume of the [AB5] subunit in the steady-state 2H-La / Y-Ni / Al-z is lower than

[0131] 3. The volume difference between [A2B4] and [AB5] subunits shows a trend of decreasing first and then increasing. Therefore, when z = 2 and 4, the volume difference between [A2B4] and [AB5] subunits in the steady-state 2H-La / Y-Ni / Al-z is lower than

[0132] Therefore, the steady-state 2H-La / Y-Ni / Al-2 meets the above three screening requirements at the same time.

[0133] In order to verify the effectiveness, accuracy and screening effect of the screening method, two kinds of hydrogen storage alloys were prepared for testing.

[0134] 1. Rare earth hydrogen storage alloy 2H-La / Y-Ni / Al-2 that meets the three screening requirements in step 9, with the specific alloy composition being La 0.2 Y 1.8 Ni 6.8 Al 0.2 ;

[0135] 2. In order to compare the screening methods, 2H-La / Y-Ni-7 that does not meet the third screening condition is provided, that is, the difference in subunit volume between [A2B4] and [AB5] is higher than The specific alloy composition is La 0.2 Y 1.8 Ni7.

[0136] The alloy is prepared by a conventional preparation method, specifically, first, the target alloy composition is prepared, wherein La is added at 2wt.% on the basis of the target composition as a burn-in compensation, and Y is added at 1wt.% on the basis of the target composition as a burn-in compensation; then, the prepared raw materials are arc-melted under argon conditions, and then the alloy ingot is wrapped with tantalum foil and sealed in a vacuum quartz tube after the oxide layer is removed, and annealing is performed at an annealing temperature of 875°C and an annealing time of 48h, and water quenching is performed after the annealing is completed; finally, under air conditions, the surface oxide layer of the annealed alloy ingot is removed and mechanically crushed, and then sieved at 200 mesh to obtain rare earth hydrogen storage alloy powder.

[0137] In order to verify the phase composition of the rare earth hydrogen storage alloy, that is, the effectiveness of the screening method, XRD test was carried out, and in order to further confirm the phase content and obtain the unit cell parameters and unit cell volume of the rare earth hydrogen storage alloy, refinement was carried out.

[0138] The XRD test results of 2H-La / Y-Ni / Al-2 are as follows Fig. 9 As shown, it is composed of 3R type super lattice phase (Y, La) 2Ni7, 3R type super lattice phase (Y, La) Ni3 and super lattice phase (Y, La) Ni5. The mass content of 3R type super lattice phase (Y, La) 2Ni7 is 48.5, the mass content of 3R type super lattice phase (Y, La) Ni3 is 44.6%, and the mass content of super lattice phase (Y, La) Ni5 is 6.9%.

[0139] The XRD test results of 2H-La / Y-Ni-7 are as follows: Fig.10 As shown, it is composed of 3R type super lattice phase (Y, La) 2 Ni 7 and 3R type super lattice phase (Y, La) Ni 3. The mass content of 3R type super lattice phase (Y, La) 2 Ni 7 is 72.8, and the mass content of 3R type super lattice phase (Y, La) Ni 3 is 27.2%.

[0140] It can be seen from the XRD test results that the above two hydrogen storage alloys both contain 3R type superlattice phase (Y, La) 2 Ni 7, which proves that the screening method of the present invention is effective.

[0141] In order to evaluate the accuracy of the screening method of the present invention, the average deviation value is calculated by the following formula. The average deviation value formula is: average deviation value = | experimental value - calculated value | / experimental value × 100%, and the average deviation values ​​of the unit cell parameters a and c and the unit cell volume V are calculated respectively, as shown in Table 5, wherein the calculated value is the calculated value of the 2H-A2B7 unit cell structure, and the experimental value is the experimental value of the 3R-A2B7 unit cell structure. The difference between the 2H-A2B7 and 3R-A2B7 unit cell structures is that there are 3 groups of 2 [AB5] and 1 [A2B4] subunits in the 3R-A2B7 unit cell, and there are 2 groups of 2 [AB5] and 1 [A2B4] subunits in the 2H-A2B7 unit cell. For the convenience of comparison, the experimental value is reduced by 0.67 times and brought into the average deviation value formula for calculation.

[0142] The unit cell parameters and unit cell volume of 3R-(Y,La)2Ni7 phase in 2H-La / Y-Ni / Al-2 are shown in Table 5. The average deviations of the unit cell parameters a and c and the unit cell volume V are 0.20%, 0.33%, and 0.36%, respectively;

[0143] The unit cell parameters and unit cell volume of 3R-(Y,La)2Ni7 phase in 2H-La / Y-Ni-7 are shown in Table 5. The average deviations of the unit cell parameters a and c and the unit cell volume V are 0%, 0.37% and 0.16%, respectively.

[0144] The average deviation analysis results show that the average deviation values ​​of the unit cell parameters a and c and the unit cell volume V of the 3R-(Y,La)2Ni7 phase in the two alloys in Example 1 are less than 0.4%, which proves that the screening method of the present invention is accurate.

[0145] Table 5 Unit cell parameters and unit cell volume of La-Y-Ni / Al alloy 3R-(Y,La)2Ni7 phase

[0146]

[0147] In order to verify the screening effect of the screening method of the present invention, a hydrogen absorption / dehydrogenation cycle performance test was carried out, and the hydrogen absorption amount obtained by the test was substituted into the following formula, R 200 =(C 200 / C max )×100%, where C 200 is the hydrogen absorption amount of the 200th hydrogen absorption / dehydrogenation cycle of the activated alloy, C max The maximum hydrogen absorption amount of the alloy after activation can be calculated to obtain the capacity retention rate of the two hydrogen storage alloys after 200 hydrogen absorption / dehydrogenation cycles, and then the structural stability of the two hydrogen storage alloys in Example 1 during long-term hydrogen absorption and dehydrogenation cycles can be verified.

[0148] The specific method of a single hydrogen absorption / dehydrogenation cycle is:

[0149] The hydrogen absorption process is carried out under the conditions of a hydrogen pressure of 4 MPa, a hydrogen absorption temperature of 25°C, and a hydrogen absorption time of 30 min, and the amount of hydrogen absorption is measured;

[0150] The dehydrogenation process is to -3 MPa, dehydrogenation temperature of 400℃ and dehydrogenation time of 60min to completely dehydrogenate the alloy.

[0151] The first and second hydrogen absorption kinetic curves of the two hydrogen storage alloys are shown in Fig.11 As shown in the figure, the performance test results of 200 hydrogen absorption / dehydrogenation cycles are as follows Fig.12 shown.

[0152] The first hydrogen absorption kinetic curve of 2H-La / Y-Ni / Al-2 alloy showed an obvious latent period. After one hydrogen absorption / dehydrogenation cycle, the alloy was fully activated. The hydrogen absorption amount measured in the second hydrogen absorption was the maximum hydrogen absorption amount of the alloy after activation, which was 1.53wt.%. The capacity retention rate after 200 hydrogen absorption / dehydrogenation cycles was 96.7%.

[0153] The first hydrogen absorption kinetic curve of 2H-La / Y-Ni-7 alloy showed an obvious latent period. After one hydrogen absorption / dehydrogenation cycle, the alloy was fully activated. The hydrogen absorption amount measured in the second hydrogen absorption was the maximum hydrogen absorption amount of the alloy after activation, which was 1.65wt.%. The capacity retention rate after 200 hydrogen absorption / dehydrogenation cycles was 87.4%.

[0154] From the hydrogen absorption / dehydrogenation cycle performance test results, it can be seen that the 2H-La / Y-Ni / Al-2 alloy that meets the above three screening requirements at the same time has the highest capacity retention rate, that is, it has the best structural stability; while the capacity retention rate and structural stability of the 2H-La / Y-Ni-7 alloy that only meets two screening conditions cannot meet the application requirements.

[0155] In order to verify the structural stability of rare earth hydrogen storage alloys, the method of reflecting the structural stability through the stability of hydrogen absorption rate of hydrogen storage alloys was adopted for verification. Therefore, the hydrogen absorption kinetic curve fitting was performed. The hydrogen absorption kinetic fitting curves of the two hydrogen storage alloys for the 2nd, 10th, 50th and 200th times are shown in Figure 2. Fig.13 As shown, the characteristic time t c The speed of hydrogen absorption of the reaction hydrogen storage alloy, t c The larger the value, the slower the hydrogen absorption rate. The specific fitting results are as follows:

[0156] The hydrogen absorption rate of 2H-La / Y-Ni / Al-2 alloy gradually increases with the increase of hydrogen absorption times. During 200 hydrogen absorption / dehydrogenation cycles, t cThe change rate is 12%. At the 10th and 200th hydrogen absorption, t c It remains almost unchanged, indicating that the 2H-La / Y-Ni / Al-2 alloy has good structural stability during long-term hydrogen absorption and dehydrogenation cycles;

[0157] The hydrogen absorption rate of 2H-La / Y-Ni-7 alloy gradually increases with the increase of hydrogen absorption times. During 200 hydrogen absorption / dehydrogenation cycles, t c The change rate is 41%, indicating that the structural stability of 2H-La / Y-Ni-7 alloy in the long-term hydrogen absorption and dehydrogenation cycle is lower than that of 2H-La / Y-Ni / Al-2;

[0158] The fitting results also support the conclusions drawn from the hydrogen absorption / dehydrogenation cycle performance test; at the same time, it can also be concluded that the stability of the hydrogen absorption rate of hydrogen storage alloys is positively correlated with the structural stability, that is, the 2H-La / Y-Ni / Al-2 alloy has the best structural stability during long-term hydrogen absorption and dehydrogenation cycles, and also has the best hydrogen absorption rate stability during the cycle.

[0159] In summary, the above verification experiments show that the screening method of the present invention is effective and accurate, and the screening effect is demonstrated by hydrogen storage performance, capacity retention rate and structural stability; at the same time, by providing the test results of two hydrogen storage alloys, it is also proved that only when the above three screening requirements are met at the same time can the corresponding technical effect be obtained.

Claims

1. A method for screening A2B7 superlattice hydrogen storage alloys with long cycle life, characterized in that The following steps are involved: Step 1, establishing the unit cell structure of stable La2Ni7. First, derive the unit cell structures of 2H-type La2Ni7 and 3R-type La2Ni7 from the existing public inorganic crystal structure database. The chemical composition of 2H-type La2Ni7 and 3R-type La2Ni7 is La2Ni7, which are collectively referred to as 2H-La / Y-Ni-0 and 3R-La / Y-Ni-0. Then, use the first principle calculation to optimize the unit cell structure model of 2H-type La2Ni7 and 3R-type La2Ni7, so that the unit cell structure is in a stable state. Among them, the unit cell structure optimization result of 3R-type La2Ni7 has not converged, and it will not be calculated further. The unit cell structure optimization result of 2H-type La2Ni7 has converged, and the unit cell structure of stable 2H-La / Y-Ni-0 is obtained, which is referred to as stable 2H-La / Y-Ni-0. At the same time, the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained. Step 2, the establishment and screening of the stable 2H-La / Y-Ni-1 unit cell structure, first, the first Y atom is substituted, that is, the La atom in the stable 2H-La / Y-Ni-0 in step 1 is replaced by one Y atom, and two kinds of La 1.75 Y 0.25 Ni7 unit cell structure, its chemical composition is La 1.75 Y 0.25 Ni7, collectively referred to as 2H-La / Y-Ni-1; then, all 2H-La / Y-Ni-1 are structurally optimized, and the stable 2H-La / Y-Ni-1 unit cell structure is obtained, referred to as stable 2H-La / Y-Ni-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained; finally, the unit cell formation enthalpy of all stable 2H-La / Y-Ni-1 are compared, and the stable 2H-La / Y-Ni-1 with the smallest formation enthalpy can be obtained, referred to as the most stable La / Y-Ni-1; Step 3: Establishment and screening of the stable 2H-La / Y-Ni-2 unit cell structure. First, the second Y atom is substituted to obtain 7 kinds of La 1.5 Y 0.5 Ni7 unit cell structure, its chemical composition is La 1.5 Y 0.5 Ni7, collectively referred to as 2H-La / Y-Ni-2; then, the structures of the seven types of 2H-La / Y-Ni-2 were optimized respectively, and the stable 2H-La / Y-Ni-2 unit cell structures were obtained respectively, referred to as stable 2H-La / Y-Ni-2, and the corresponding unit cell parameters, unit cell volumes and unit cell formation enthalpy were obtained at the same time; finally, the unit cell formation enthalpy of all the stable 2H-La / Y-Ni-2 were compared, and the stable 2H-La / Y-Ni-2 with the smallest formation enthalpy was obtained, referred to as the most stable 2H-La / Y-Ni-2; Step 4, establishing and screening all stable 2H-La / Y-Ni-x unit cell structures, repeating the Y atom substitution, structure optimization, and formation enthalpy comparison of steps 2 and 3, and obtaining the stable 2H-La / Y-Ni-x and the most stable 2H-La / Y-Ni-x, respectively, and obtaining the corresponding unit cell parameters, unit cell volume, and unit cell formation enthalpy; Step 5, calculation and screening of the volume of the [A2B4] subunit and [AB5] subunit in the most stable 2H-La / Y-Ni-x, by substituting the unit cell parameters and unit cell volume obtained in the previous steps into the following formula, V [A2B4] =a 2 ×h1×sin60° and V [AB5] =a 2 ×h2×sin60°, where a is the unit cell parameter, h1 is the height of the [A2B4] subunit, and h2 is the height of the [AB5] subunit. The volumes of the [A2B4] subunit and the [AB5] subunit in each most stable 2H-La / Y-Ni-x and the volume difference between the [A2B4] and [AB5] subunits can be calculated. Then, the most stable 2H-La / Y-Ni-x can be preliminarily selected under certain screening conditions. Step 6, the establishment of the stable 2H-La / Y-Ni / Al-1 unit cell structure. First, based on the most stable La / Y-Ni-7 unit cell structure preliminarily screened in step 5, an Al atom is substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.875 Al 0.125 The unit cell structure is La 0.25 Y 1.75 Ni 6.875 Al 0.125 , collectively referred to as 2H-La / Y-Ni / Al-1, then, the structure of 2H-La / Y-Ni / Al-1 is optimized to make the unit cell structure stable, and the unit cell structure of the stable state 2H-La / Y-Ni / Al-1 is obtained, referred to as the stable state 2H-La / Y-Ni / Al-1, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained; Step 7: Establishment of the stable 2H-La / Y-Ni / Al-2 unit cell structure. First, two Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure is La 0.25 Y 1.75 Ni 6.75 Al 0.25 , collectively referred to as 2H-La / Y-Ni / Al-2, then, the structure of 2H-La / Y-Ni / Al-2 is optimized to make the unit cell structure stable, and the unit cell structure of the stable La / Y-Ni / Al-2 is obtained, referred to as stable 2H-La / Y-Ni / Al-2, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained; Step 8: Establishment of the stable 2H-La / Y-Ni / Al-4 unit cell structure. First, four Al atoms are substituted at the 6h position between the two [AB5] subunits to obtain La 0.25 Y 1.75 Ni 6.5 Al 0.5 The unit cell structure is La 0.25 Y 1.75 Ni 6.5 Al 0.5 , collectively referred to as 2H-La / Y-Ni / Al-4, then, the structure of 2H-La / Y-Ni / Al-4 is optimized to make the unit cell structure stable, and the unit cell structure of the stable state 2H-La / Y-Ni / Al-4 is obtained, referred to as the stable state La / Y-Ni / Al-, and the corresponding unit cell parameters, unit cell volume and unit cell formation enthalpy are obtained; Step 9, calculation and screening of the volumes of the [A2B4] subunit and the [AB5] subunit in the steady-state 2H-La / Y-Ni / Al-z, repeat the calculation of the volumes of the [A2B4] subunit and the [AB5] subunit in step 5, and obtain the volumes of the [A2B4] subunit and the [AB5] subunit and the volume difference between the [A2B4] and [AB5] subunits in each steady-state 2H-La / Y-Ni / Al-z respectively; then, under certain screening conditions, finally screen out the steady-state 2H-La / Y-Ni / Al-z.

2. The screening method according to claim 1, characterized in that: In step 1, the specific method of structural optimization is that the method used for atomic position relaxation is the conjugate gradient method, that is, IBRION is 2; during structural optimization, the atomic position, unit cell shape, and unit cell volume are optimized simultaneously, that is, ISIF is 3; and the energy convergence standard EDIFF is less than or equal to 10 -6 eV, the convergence standard EDIFFG for single-atom forces is less than or equal to The cutoff energy ENCUT is 400eV, and the K-point grid k-point is 8×8×2; there are two situations for the structural optimization calculation results. When the structural optimization calculation has not reached convergence, the unit cell structure is in an unstable state, referred to as an unstable state; when the structural optimization calculation reaches convergence, the unit cell structure is in a stable state, referred to as a steady state.

3. The screening method according to claim 1, characterized in that: In step 5, the screening condition is to meet the following four requirements at the same time: (1) The subunit volume of [A2B4] is lower than (2) The subunit volume of [AB5] is lower than (3) having the smallest difference in subunit volume between [A2B4] and [AB5]; (4) A2B7 type superlattice alloy that exists stably in the La-Y-Ni phase diagram.

4. The screening method according to claim 1, characterized in that: In step 6, since the original most stable 2H-La / Y-Ni-7 unit cell structure is a single cell structure, after one Al atom is substituted, La 0.25 Y 1.75 Ni 6.75 Al 0.25 The unit cell structure has a doping ratio that is too large, so the most stable 2H-La / Y-Ni-7 unit cell structure is expanded by 2×1×1. Due to the expansion of the unit cell structure, the specific method for structural optimization in step 6 is the same as step 1 for the contents not otherwise specified, except that the K-point grid k-point is 8×8×1.

5. The screening method according to claim 1, characterized in that: In step 9, the screening condition is to meet the following three requirements at the same time: (1) The subunit volume of [A2B4] is lower than (2) The subunit volume of [AB5] is lower than (3) The difference in subunit volume between [A2B4] and [AB5] is less than 6. The screening method according to claim 1, characterized in that: The most stable 2H-La / Y-Ni-x alloy preliminarily screened out is La 0.25 Y 1.75 The most stable state of Ni7 is 2H-La / Y-Ni-7; the final stable state 2H-La / Y-Ni / Al-z alloy screened out is, with a chemical composition of La 0.25 Y 1.75 Ni 6.75 Al 0.25 Steady-state 2H-La / Y-Ni / Al-2.

7. The screening method according to claim 1, characterized in that: Compared with experimental facts, the average deviation values ​​of the unit cell parameters a and c and the unit cell volume V of the finally screened alloys are a≤0.20%, c≤0.33% and V≤0.36%, respectively.

8. The screening method according to claim 1, characterized in that: The alloy finally screened out was fully activated after one hydrogen absorption / dehydrogenation cycle. The hydrogen absorption amount measured in the second hydrogen absorption was the maximum hydrogen absorption amount of the alloy after activation, which was 1.53wt.%. The capacity retention rate after 200 hydrogen absorption / dehydrogenation cycles was greater than 96%.

9. The screening method according to claim 1, characterized in that: The alloy finally selected has a good t c The change rate is 12%. At the 10th and 200th hydrogen absorption, t c Remain unchanged.

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