Understand the core issues in lithium-ion batteries?

With the acceleration of social development, people's demand for and dependence on batteries is increasing. Batteries are closely related to people's lives. They are widely used in various small portable electronic devices. With the shortage of energy and environmental pollution, they have become the core of large-scale equipment such as electric vehicles and clean energy storage devices. Among the many battery systems, as shown in Figure 1, the most attractive one is the lithium-ion battery. At present, the actual capacity of cathode materials in lithium-ion batteries is generally low, which has become the focus and difficulty of research. The understanding of the structure and working principle of the current common lithium-ion battery cathode materials can help us understand the core issues in lithium-ion batteries.

Lithium-ion batteries are devices that convert and deintercalate between the positive and negative materials through lithium ions to realize mutual conversion of chemical energy and electrical energy. They are also described as rocking chair batteries, which were first proposed by A.Armand in 1980. The structure and charging and discharging principle are shown in Figure 2. Lithium-ion battery cathode candidate materials can be mainly divided into the following three categories according to the structure: (1) layered LiMO2 (M=Co, Ni, Mn) cathode material; (2) spinel-structured LiMn2O4 cathode material; An olivine-structured LiFePO4 cathode material.

1. Layered LiMO2 (M=Co, Ni, Mn) cathode material

The layered LiMO2 (M=Co, Ni, Mn) cathode material was developed on the basis of the layered LiCoO2 material, and the structure was similar to that of the layered LiCoO2 by replacing part of Co with Ni and Mn metals. As shown in Figure 3, Li+ is located between the regular octahedral plates, showing a layered arrangement.

Therefore, during charging and discharging, lithium ions can move two-dimensionally from the plane in which they are located, and the insertion and deintercalation of lithium ions is faster. The electrochemical process is as follows:

LiMO2 Li1-xMO2 + xLi+ + xe-

Among the layered structures of LiMO2 (M=Co, Ni, Mn), different transition metal materials are slightly different in synthesis and electrochemical properties, summarized as follows: (1) Reversible embedding of lithium ions in layered LiCoO2 structures The amount of deintercalation is only 0.5 units, and when it is more than 0.5, the material undergoes an irreversible phase change, resulting in capacity decay. Therefore, the overcharge resistance of LiCoO2 is poor. The range of x in Li1-xCoO2 is 0 ≤ x ≤ 0.5, and the theoretical capacity is only 156 mAh/g. Further, lithium cobaltate Li1-xCoO2 (x>0) in a charged state is liable to undergo an oxygen evolution reaction at a high temperature to release oxygen.

Li0.5CoO2 0.5 LiCoO2 + 1/6 Co3O4 + 1/6 O2

The theoretical capacity of the layered LiNiO2 is 275 mAh/g, and the actual capacity is 190-200 mAh/g. However, since the ionic radius of nickel ions is smaller than that of lithium ions, nickel ions easily occupy the position of lithium ions during charging and discharging, and cation misalignment occurs, resulting in collapse of local interlayer structure of LiNiO2, resulting in a decrease in material capacity. In addition, LiNiO2 materials also have various problems such as poor thermal stability, large heat release, and poor overcharge resistance.

The layered LiMnO2 is slightly different from the layered LiCoO2 structure, and the oxygen atoms are arranged in a twisted tetragonal close-packed manner, which is a layered rock salt structure. The theoretical capacity is 285 mAh/g, but its cycle performance is poor. The material is unstable after delithiation and will slowly transform into the spinel-type LiMn2O4 structure. At this time, lithium ions will enter the manganese ion layer, causing capacity degradation. In addition, manganese ions are easily side-reacted with the electrolyte and dissolved in the electrolyte. At high temperatures, the material is also prone to reactions that produce heterogeneous phases.

3LiMnO2 + 1/2O2 LiMn2O4 + Li2Mn2O3

Among the ternary positive electrode materials, LiNi1-x-yCoxMnyO2 is the most typical LiNi1/3Co1/3Mn1/3O2 compound with a nickel-cobalt-manganese ratio of 1:1:1, and its theoretical capacity is 277 mAh/g. The LiNi1-x-yCoxMnyO2 material can adjust the properties of the material by adjusting the ratio of Ni, Co, and Mn, but the stability and safety of the material still exist, and the mixing of various elements also brings difficulties in the synthesis process.

2. Spinel structure LiMn2O4 cathode material

As early as 1983, M. Thackeray and J. Good enough discovered that manganese spinel (LiMn2O4) can be used as a cathode material for lithium ion batteries with a theoretical capacity of 148 mAh/g. In the structure of spinel LiMn2O4, oxygen is arranged in a cubic close-packed manner to form its unit cell skeleton, in which Li+ occupies 1/8 of the oxygen tetrahedron 8a position, and Mn atom occupies 1/2 oxygen octahedron 16d. There are two valence states of manganese in the structure, namely Mn3+ and Mn4+, each accounting for 50%. The material structure is shown in Figure 4.

In the LiMn2O4 structure, the empty oxygen tetrahedron and the oxygen octahedron are connected in a coplanar and coplanar manner. These vacancies constitute a three-dimensional lithium ion diffusion channel, so the lithium ion conductivity of the material is very good, and the lithium ion diffusion coefficient is 10-10. ~10-8cm2/s. The electrode reaction is as follows:

LiMn2O4 Li1-xMn2O4 + xLi+ + xe-

When lithium ions are intercalated and deintercalated, the manganese atoms in the structure can stabilize the oxygen of the cubic dense pile and support the entire structure, so the structure of the spinel LiMn2O4 is relatively stable. The main problem of spinel LiMn2O4 material is that its capacity decays too fast, and the main reasons for its capacity attenuation are: (1) LiMn2O4 is converted into tetragonal phase Li2Mn2O4 during deep discharge or high power charge and discharge, and Mn in the material It was restored to trivalent. This change in valence causes the Jahn-Teller effect to cause deformation of the material, causing a 6.5% increase in unit cell volume, destroying the crystal structure of the material, causing capacity decay. (2) During the reaction, Mn3+ will disproportionate to form Mn4+ and Mn2+, and the divalent manganese ions will dissolve into the electrolyte, causing the active material to be lost.

3. Olivine structure LiFePO4 cathode material

In 1997, John B. Good enough reported that olivine-structured lithium iron phosphate can also be used as a cathode material for lithium ion batteries. The theoretical capacity of LiFePO4 is 170 mAh/g. The olivine-structured LiFePO4 belongs to the orthorhombic system, and its structure is shown in Fig. 5.

The oxygen atoms form the basic skeleton of the unit cell in a slightly twisted hexagonal close-packed manner. The FeO6 octahedrons are connected by the apex sharing oxygen atoms, and the LiO6 octahedrons are connected by the co-edges to form a chain. Each PO4 tetrahedron is separated from a FeO6 octahedron, coexisting with two LiO6 octahedrons. All oxygen ions are combined with pentavalent phosphorus atoms through covalent bonds. Due to the strong P-O bond, P acts to stabilize the entire skeleton. The thermal stability of the material is very good and the overcharge resistance is strong. The electrode reaction is as follows:

LiFePO4 Li1-xFePO4 + xLi+ + xe–

However, in practical applications, the capacity and rate of LiFePO4 materials are much lower than the theoretical values, mainly due to the poor conductivity and lithium-lead properties of the materials. The calculation results show that in the olivine-type LiFePO4 structure, the diffusion barrier of lithium ions from the b axes is too high, and can only diffuse along the lower c-axis of the diffusion barrier. Therefore, in the LiFePO4 material, the diffusion channel of lithium ions is one-dimensional, and lithium ions can only diffuse in the c-axis direction (corresponding to the [010] direction of the crystal). In addition, since the FeO6 octahedrons are only connected by a common apex and have no co-edges, a continuous network structure is not formed, resulting in a low electronic conductivity of the material.

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