A new generation of lithium-ion battery negative electrode material - silicon dioxide!

Due to the rapid development of lithium-ion batteries, people's daily life and production methods have undergone tremendous changes. At present, lithium-ion batteries are needed in everything from laptops, tablets, cameras, mobile phones to new energy vehicles, and lithium-ion battery products have spread to every corner of people's lives.

Lithium- ion battery composition

Lithium-ion batteries refer to secondary batteries that use two different compounds that can reversibly embed and extract lithium ions as positive and negative electrodes, respectively. Common lithium-ion batteries on the market are mainly cylindrical, square, button and soft-pack batteries, as shown in Figure 1. Although the appearance of lithium-ion batteries varies, they are basically composed of a shell, a positive electrode material, a separator, a negative electrode material and an electrolyte. The positive and negative electrode materials are used for the reversible embedding and extraction reaction of lithium ions; the separator is used to prevent the direct contact between the positive and negative electrode materials to avoid battery short circuit; the electrolyte is in direct contact with the positive and negative electrode materials as a medium for transmitting ions.

Figure 1. Structure of different types of lithium-ion batteries

How Lithium-ion Batteries Work

Lithium-ion battery is actually a lithium-ion concentration difference battery. The charging and discharging process corresponds to the embedding and de-embedding process of lithium ions. Its working principle is shown in Figure 2. When charging, under the action of external voltage, lithium ions gathered in the positive electrode are released and embedded into the negative electrode material through the electrolyte and the diaphragm. At the same time, electrons enter the negative electrode material through the external circuit to achieve charge balance between the positive and negative electrodes. When discharging, it is just the opposite.

 

Figure 2. Working principle of lithium-ion battery

Lithium-ion battery negative electrode materials

Lithium-ion battery negative electrode materials are key materials that determine the energy storage performance of lithium-ion batteries, and are also an important research topic for lithium-ion batteries. At present, commercial lithium-ion battery negative electrode materials are mainly carbon materials, which have the advantages of good cycle performance (greater than 1000 cycles), low electrode potential (less than 1.0 Vvs.LitLi), and low price. Carbon negative electrode materials are mainly divided into two categories, graphite and amorphous carbon. Among them, graphite is widely used. With the development of technology, the actual specific capacity of commercial graphite negative electrodes has been close to the theoretical specific capacity (372 mAh/g). Therefore, it is urgent to develop new lithium-ion battery negative electrode materials to further increase battery capacity. Silicon-based materials have many advantages and are considered to be the best choice to replace carbon negative electrode materials.

 

Silicon anode material

Advantages of silicon-based materials: ① Low operating voltage. The operating voltage is < 0.5V versus Li ⁺ /Li, which can provide a sufficiently high operating voltage to increase energy density. ② High theoretical specific capacity. When forming Li ₂₂ Si ₅ , the theoretical specific capacity reaches 4200 mAh/g, which is 10 times that of graphite. ③ No pollution. Environmentally friendly. ④ Low cost. Silicon is the second most abundant element in the earth's crust. Abundant reserves mean that raw materials are easy to obtain and costs are easy to control.

 

However, during the lithium intercalation process, silicon-based materials can experience volume expansion of over three times, as shown in Figure 3. This leads to repeated cracking and regeneration of the SEI (Solid Electrolyte Interphase) film formed on the surface of the silicon material, resulting in a significant amount of lithium ions being rendered ineffective, which lowers the initial coulombic efficiency. Furthermore, the repeated volume contraction and expansion during the charge and discharge processes can cause the silicon anode material to pulverize and significantly reduce the contact area between silicon and silicon, as well as between silicon and conductive agents, leading to a substantial decline in the cycling performance of the battery. This greatly limits the commercialization of silicon. To address the poor cycling performance of silicon, the development of silicon oxide materials with better cycling stability has become a research hotspot.

Figure 3. Schematic diagram of silicon volume expansion during charging and discharging

 

Silicon oxide negative electrode material

Silicon dioxide (SiOx, 0<x<2) is an inorganic compound and an incomplete oxide of Si. SiOx is dark brown or brownish yellow at room temperature and pressure, insoluble in water, and oxidized to silicon dioxide in the air. In a high temperature environment, a disproportionate reaction will occur to generate silicon and silicon dioxide. Silicon nanoparticles are evenly dispersed in the SiO2 _matrix . When actually used in batteries, silicon dioxide can buffer the volume expansion of nano-silicon, so it has a lower volume deformation and is relatively stable. In addition to being used as anode materials for lithium-ion batteries, SiOx materials can also be used as raw materials for the synthesis of fine ceramic materials, or as a semiconductor material, protective film , etc.

 

The structure of silicon oxide

_{SiOx materials are usually produced by reacting Si and SiO2} by vapor deposition under high temperature vacuum or inert atmosphere . However, the atomic structure of SiOx has always been a controversial topic. Early researchers proposed two types of SiOx microstructures: the first is the random bonding model (RB model), which believes that SiOx is a single-phase structure with randomly distributed Si-Si and Si-O bonds; the second is the random mixture model (RM model), which believes that SiOx is a multiphase structure composed of amorphous Si and amorphous SiO2 _phases . Based on the above two models and the test results of high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS), a third structural model, the Interface Clusters Mixture Model (ICM model), was proposed. This model believes that SiOx is composed of nano-Si, SiO2 _clusters , and the interface SiOx surrounding the two.

Figure 4. Schematic diagrams of (a) RB model, (b) RM model and (c) ICM model

 

Lithium storage mechanism of silicon dioxide

At present, the general view on the lithium storage mechanism of silicon oxide is that SiOx reacts with lithium first to generate elemental silicon, Li ₂ O and lithium silicate (Li ₄ SiO ₄ , Li ₂ SiO ₃ and Li ₂ SiO _5, etc.), and elemental silicon further reacts with Li to produce reversible capacity, while the generated Li ₂ O and lithium silicate no longer participate in the reaction in the subsequent electrochemical cycle, resulting in a very low initial coulombic efficiency of the material, but it can play a role in buffering volume expansion and protecting active materials. According to the research of H.Yamamura et al., the reaction of SiOx with lithium is shown in the formula: 4SiOx + 17.2Li → 3Li _4.4 Si + Li ₄ SiO ₄ .

The development and prospect of silicon oxide

As the application market of lithium-ion batteries continues to expand, people have higher and higher requirements for their energy and power density. The selection of high specific capacity negative electrode materials is an important strategy to achieve this goal. The theoretical specific capacity of silicon dioxide negative electrode ( 2600  mAh/g ) is high and the cycle stability is good. All major negative electrode material manufacturers have laid out silicon dioxide negative electrode, which is considered to be the most promising next-generation negative electrode material.

 

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