CN109145458B - Simulation method for predicting reversible capacity of silicon-graphene composite material - Google Patents
Simulation method for predicting reversible capacity of silicon-graphene composite material Download PDFInfo
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Abstract
The invention discloses a simulation method for predicting reversible capacity of a silicon-graphene composite material, which comprises the following steps: (1) adding silicon and graphene into a simulation box to establish an initial model of the silicon-graphene composite material; (2) embedding lithium into the initial model of the silicon-graphene composite material; (3) calculating the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium intercalation; (4) and (4) calculating the reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation. The invention has good accuracy and practicability, and can effectively improve the development efficiency of the novel silicon-based electrode material of the lithium battery.
Description
Technical Field
The invention relates to a simulation method for a silicon-graphene composite material, in particular to a simulation method for predicting reversible capacity of the silicon-graphene composite material.
Background
Due to the shortage of traditional petrochemical energy and the increasingly serious environmental problems brought by the same, a great deal of research work is being carried out on the development and efficient utilization of new energy. Lithium ions have attracted general attention and attention from the international battery and scientific industries because of their advantages such as high energy, long service life, light weight, and small size. Particularly, with the strategic importance of energy safety and environmental protection in all countries in the world, electric vehicles are vigorously developed by all countries as strategic novel industries due to the characteristics of energy conservation and low emission.
The electrode material is a key factor for determining the comprehensive performance of the lithium battery, and the development of a new generation of high-performance electrode material is an important direction for the research of the lithium battery. Graphite is mainly used as a negative electrode material of a lithium battery in commercial use at present because of abundant resources and low cost. However, the energy density of graphite electrodes is low (capacity of 372 mAh/g) and deposition may cause safety problems, thereby limiting their further development. Silicon is a very potential negative electrode material for lithium batteries, which has the highest theoretical capacity (4200 mAh/g) among all negative electrode materials, but silicon undergoes a large volume change during cyclic charge and discharge, so that silicon particles are crushed and come out of contact with electrode sheets, rapidly causing capacity fade and shortening of cycle life. Graphene is a single carbon atom layer, and has attracted great attention due to its two-dimensional crystal structure of atomic thickness, unique electronic structure, high mechanical strength, high surface area, and excellent electronic conductivity. The graphene is combined with silicon, and the electrode material solution has a good application prospect. The graphene is superior to other carbon materials in the performance of contacting with silicon, and can effectively prevent the volume expansion, shrinkage and aggregation of silicon during the charge and discharge processes. The prepared material generally has gaps, and the space in the material can well relieve the influence of the volume change of silicon on the structure of the electrode. Efficient analysis and design of silicon-graphene composite materials are key to commercial application of the silicon-graphene composite materials, but methods for predicting electrochemical performance of the silicon-graphene composite materials are relatively lacking.
Because the traditional electrode material design method has the limitations of long period, high cost and the like, it is very difficult to search for the optimal design through a large amount of test research.
Disclosure of Invention
The invention aims to provide a simulation method for predicting reversible capacity of a silicon-graphene composite material. The invention has good accuracy and practicability, and can effectively improve the development efficiency of the novel silicon-based electrode material of the lithium battery.
The technical scheme of the invention is as follows: a simulation method for predicting reversible capacity of a silicon-graphene composite material is carried out according to the following steps:
(1) adding silicon and graphene into a simulation box to establish an initial model of the silicon-graphene composite material;
(2) embedding lithium into the initial model of the silicon-graphene composite material;
(3) calculating the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium intercalation;
(4) and (4) calculating the reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation.
In the simulation method for predicting the reversible capacity of the silicon-graphene composite material, the establishment method in the step (1) is,
a. selecting a gap layer between silicon and graphene to be 15 angstroms in thickness, adding a vacuum layer with the thickness of 50 angstroms above the simulation box, and adopting periodic boundary conditions in three directions;
b. the interactions between adsorbates and various surfaces were accurately described using generalized gradient approximation and the Perew-Burke-Ernzerhof functional; describing the interaction of electrons and ions by adopting an ultra-soft pseudopotential method; the plane wave cutoff energy was 380eV; the geometric optimization adopts a conjugate gradient method, and the convergence standard is set as the residual force
The brillouin zone employs a 3 x 3K-point network.
c. The simulated box dimensions are determined according to geometric optimization calculations.
In the simulation method for predicting the reversible capacity of the silicon-graphene composite material, the lithium intercalation method in the step (2) is to add lithium atoms into the initial model gap one by one.
According to the simulation method for predicting the reversible capacity of the silicon-graphene composite material, the lithium intercalation method is carried out according to the following steps:
1) For single crystal silicon, the minimum distance between the position of the inserted lithium atom and other atoms is defined
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating the total energy;
2) For the silicon to graphene gap, the minimum distance between the position of the inserted lithium atom and other atoms is defined
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating the total energy;
3) And repeating the steps until the quantity of the inserted lithium meets the set requirement.
In the simulation method for predicting the reversible capacity of the silicon-graphene composite material, in the step (3), the calculation of the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium insertion is performed according to the following steps:
1) Calculating the formation energy of a silicon-graphene composite material system when the lithium content is x according to the following formula:
Δ f E＝E total (Li x Si/Gra)-(x E total (Li)+E total (Si/Gra))
wherein E total (Li x Si/Gra) value is the energy value of the silicon-graphene structure with the Li content of x after geometric optimization divided by the number of silicon atoms in the structure, E total The (Li) value is the energy of a single Li atom in the body-centered cubic, E total The (Si/Gra) value is the energy value of the silicon-graphene structure after geometric optimization divided by the number of silicon atoms in the structure;
2) And calculating the formation energy of the silicon-graphene composite system under different lithium contents by taking 0.1 as the step length of the lithium content x.
With the increasing of x, when the difference between the formation energies of adjacent x is less than 5%, that is, it is determined that the number of intercalated lithium atoms in the initial model reaches saturation, the number of intercalated lithium atoms corresponds to the maximum theoretical capacity of the silicon-graphene composite material system.
In the above simulation method for predicting reversible capacity of a silicon-graphene composite material, in the step (4), the calculation of reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation is performed according to the following steps:
1) Finding out Si-C bonds formed by all silicon-graphene interfaces;
2) Finding out all Li-Si bond lengths around the Si-C bond
And Li-C bond length->
The lithium atom in these chemical bonds is defined as an irreversible lithium atom;
3) And subtracting the irreversible lithium atoms, and calculating to obtain the reversible lithium capacity of the silicon-graphene composite system by using the residual lithium atoms.
Compared with the prior art, the method has the advantages that the initial model of the silicon-graphene composite material is established, lithium embedding is carried out on the initial model of the silicon-graphene composite material, the maximum theoretical lithium capacity and the reversible lithium capacity are calculated on the initial model of the silicon-graphene composite material after lithium embedding, and the accuracy and the efficiency of the method are verified through comparison of experimental results. The electrochemical performance of the silicon-graphene composite material is calculated and predicted by the method, so that the problems of long experimental period, high research and development cost, difficulty in operation and the like in the traditional electrode material design method can be effectively solved, the method can be applied to a novel lithium battery silicon-based material with a similar nano structure to the silicon-graphene composite material, the experimental and development efficiency of the novel lithium battery silicon-based electrode material can be effectively improved, and the commercialized application of the novel lithium battery silicon-based electrode material is accelerated.
Drawings
Fig. 1 is a silicon-graphene composite initial model;
fig. 2 is a schematic diagram of irreversible lithium atoms at the silicon-graphene interface.
Detailed Description
The invention is further illustrated by the following figures and examples, which are not to be construed as limiting the invention.
Examples are given. A simulation method for predicting reversible capacity of a silicon-graphene composite material is carried out according to the following steps:
(1) adding silicon and graphene into a simulation box to establish an initial model of the silicon-graphene composite material;
(2) embedding lithium into the initial model of the silicon-graphene composite material;
(3) calculating the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium intercalation;
(4) and (4) calculating the reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation.
The establishing method in the step (1) is that,
a. the thickness of a gap layer between silicon and graphene is 15 angstroms, a vacuum layer with the thickness of 50 angstroms is added above the simulation box, and periodic boundary conditions are adopted in three directions;
b. generalized gradient approximation and Perew-Burke-Ernzerhof functional are used to accurately describe the phase between adsorbates and various surfaces(ii) an interaction; describing the interaction of electrons and ions by adopting an ultra-soft pseudopotential method; the plane wave cutoff energy was 380eV; the geometric optimization adopts a conjugate gradient method, and the convergence standard is set as the residual force
The Brillouin zone adopts a 3 multiplied by 3K point network;
c. the simulated box dimensions are determined according to geometric optimization calculations.
The lithium intercalation method in the step (2) is to add lithium atoms into the initial model gap one by one.
The lithium intercalation method comprises the following steps:
1) For single crystal silicon, the minimum distance between the position of the inserted lithium atom and other atoms is defined
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating the total energy;
2) For the silicon to graphene gap, the minimum distance between the inserted lithium atom position and other atoms is defined
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating the total energy;
3) And repeating the steps until the quantity of the embedded lithium meets the set requirement.
In the step (3), the calculation of the maximum theoretical lithium capacity of the lithium-intercalated silicon-graphene composite material system is carried out according to the following steps:
1) Calculating the formation energy of the silicon-graphene composite material system when the lithium content is x according to the following formula:
Δ f E＝E total (Li x Si/Gra)-(x E total (Li)+E total (Si/Gra))
wherein E total (Li x Si/Gra) value is the energy value of a silicon-graphene structure with Li content of x after geometric optimization divided by the number of silicon atoms in the structure, E total The (Li) value is the energy of a single Li atom in the body-centered cubic, E total The (Si/Gra) value is the energy value of the silicon-graphene structure after geometric optimization divided by the number of silicon atoms in the structure;
2) Calculating the formation energy of the silicon-graphene composite material system under different lithium contents by taking 0.1 as the step length of the lithium content x;
with the increasing of x, when the difference between the formation energies of adjacent x is less than 5%, that is, it is determined that the quantity of intercalated lithium in the initial model reaches saturation, the quantity of intercalated lithium atoms corresponds to the maximum theoretical capacity of the silicon-graphene composite material system.
In the process of lithium intercalation, with the increasing of lithium content, silicon generates huge volume expansion and finally forms Si-C bonds with graphene at the interface, and the formed Si-C bonds are shorter than the bonds of nearby Li-Si bonds and Li-C bonds, which indicates that very stable chemical bonds are formed and irreversible lithium capacity loss is caused.
In the step (4), the calculation of the reversible lithium capacity of the lithium-intercalated silicon-graphene composite material system is carried out according to the following steps:
1) Finding out Si-C bonds formed by all silicon-graphene interfaces, namely the Si-C distance is less than 1.9 angstroms;
2) Finding out all Li-Si bond lengths around the Si-C bond
And Li-C bond length->
The lithium atom in these chemical bonds is defined as an irreversible lithium atom;
3) And subtracting the irreversible lithium atoms, and calculating by using the residual lithium atoms to obtain the reversible lithium capacity of the silicon-graphene composite material system.
To verify the accuracy according to the present invention, 2 × 2Si (111) unit cell and 3 × 3 graphene unit cell were selected according to the above model construction method, resulting in an initial model of silicon-graphene composite as shown in fig. 1. Lithium atoms are inserted into the model one by one, the forming energy of the silicon-graphene composite material system under different lithium contents is calculated, and the maximum theoretical lithium capacity of the obtained silicon-graphene composite material is 2896mAh/g and is more consistent with an experimental value 2634 mAh/g. Based on the above definition of irreversible lithium capacity, the lithium ion in fig. 2 is determined as an irreversible lithium atom. The lithium atoms are removed, and the reversible lithium capacity of the silicon-graphene composite material calculated by using the remaining lithium atoms is 2383mAh/g and is also consistent with the experimental value of 2497 mAh/g. The above results show that the method can accurately predict the maximum theoretical lithium capacity and reversible lithium capacity of the silicon-graphene composite material, and provides an effective method for designing and analyzing a novel silicon-based lithium electrode.
Claims (4)
1. A simulation method for predicting reversible capacity of a silicon-graphene composite material is characterized by comprising the following steps:
(1) adding silicon and graphene into a simulation box to establish an initial model of the silicon-graphene composite material;
(2) embedding lithium into the initial model of the silicon-graphene composite material;
(3) calculating the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium intercalation;
(4) calculating reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation;
in the step (3), the calculation of the maximum theoretical lithium capacity of the silicon-graphene composite material system after lithium intercalation is carried out according to the following steps:
1) Calculating the formation energy of the silicon-graphene composite material system when the lithium content is x according to the following formula:
Δ f E＝E total (Li x Si/Gra)-(x E total (Li)+E total (Si/Gra))
wherein E total (Li x Si/Gra) value is the energy value of a silicon-graphene structure with Li content of x after geometric optimization divided by the number of silicon atoms in the structure, E total The (Li) value is the energy of a single Li atom in the body-centered cubic, E total The (Si/Gra) value is the energy value of the geometrically optimized silicon-graphene structure divided by the number of silicon atoms in the structure;
2) Calculating the formation energy of the silicon-graphene composite material system under different lithium contents by taking 0.1 as the step length of the lithium content x:
with the increasing of x, when the difference of the formation energy of adjacent x is less than 5%, determining that the quantity of lithium intercalation in the initial model reaches saturation, and the quantity of lithium atoms intercalated at the moment corresponds to the maximum theoretical capacity of the silicon-graphene composite material system;
in the step (4), the calculation of the reversible lithium capacity of the silicon-graphene composite material system after lithium intercalation is carried out according to the following steps:
1) Finding out Si-C bonds formed by all silicon-graphene interfaces;
2) Find out all Li-Si around the above Si-C bond
And Li-C->
The chemical bonds of (1), the lithium atoms in these chemical bonds are defined as irreversible lithium atoms;
3) And subtracting the irreversible lithium atoms, and calculating by using the residual lithium atoms to obtain the reversible lithium capacity of the silicon-graphene composite material system.
2. The simulation method for predicting reversible capacity of silicon-graphene composite material according to claim 1, wherein the step (1) is established by,
a. the thickness of a gap layer between silicon and graphene is 15 angstroms, a vacuum layer with the thickness of 50 angstroms is added above the simulation box, and periodic boundary conditions are adopted in three directions;
b. the interactions between adsorbates and various surfaces are described by generalized gradient approximation and the Perew-Burke-Ernzerhof functional; describing the interaction of electrons and ions by adopting an ultra-soft pseudo potential method; the cutting energy of the plane wave is 380eV; the geometric optimization adopts a conjugate gradient method and a convergence standardIs quasi arranged as
The Brillouin zone adopts a 3 multiplied by 3K point network;
c. the simulated box dimensions are determined according to geometric optimization calculations.
3. The simulation method for predicting the reversible capacity of the silicon-graphene composite material according to claim 1, wherein: the lithium intercalation method in the step (2) is to add lithium atoms into the initial model gap one by one.
4. The simulation method for predicting the reversible capacity of the silicon-graphene composite material according to claim 3, wherein the lithium intercalation method is performed according to the following steps:
1) For single crystal silicon, the position of the inserted lithium atom is defined with respect to other atoms
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating the total energy;
2) For the silicon-graphene interstitials, the position of the inserted lithium atom and the other atoms are defined
Optimizing the structure after lithium intercalation according to geometric optimization conditions, and calculating total energy;
3) And repeating the steps until the quantity of the inserted lithium meets the set requirement.
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| CN105762360A (en) * | 2016-04-01 | 2016-07-13 | 上海交通大学 | Graphene-silicon-coated composite negative electrode material and preparing method and application thereof |
| CN107145628A (en) * | 2017-03-31 | 2017-09-08 | 中南大学 | The method of prediction lithium battery cycle life based on electrochemical heat coupling model |
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