Research Progress of Silicon Oxygen Anode Materials

Research Progress of Silicon Oxygen Anode Materials

At present, the lithium ion battery system with mature commercial application is graphite anode and lithium metal oxide anode. The specific capacity of graphite anode is about 360 mAh/g, while the specific capacity of lithium metal oxide anode is lower, generally less than 200 mAh/g, so it is necessary to develop high capacity materials to replace the materials currently used.

The theoretical specific capacity of silicon (4200 mAh/g) is 10 times higher than that of graphite; At the same time, raw materials from a wide range of sources, suitable for large-scale production and application, is the next generation of lithium ion battery anode ideal material. Silicon has a huge volume change (about 400%) in the process of lithium embedding and delithium, and has low intrinsic conductivity, poor cycling and multiplier performance, so it is difficult to be used for large-scale commercial use at present. Compared with simple silicon materials, SiOx anode material has slightly lower specific capacity (1965-4200 mAh/g, and decreases with the increase of oxygen content), but has more advantages in cycling performance, and is regarded as a silicon based anode material that is expected to be fully commercialized, and has been applied in a small amount in Tesla electric vehicle power battery.

In this paper, the structure, electrochemical performance, related mechanism and research progress of composite modification of SiOx materials are reviewed. Methods to improve the performance of SiOx anode materials are systematically summarized and the direction of improvement is prospected.

1.Structure of SiOx material

SiOx is a material with dominant amorphous structure. Philipp and Brady proposed random bonding (RB) and random mixing (RM) models respectively to explain the structure. The RB model describes SiOx as a continuous single-phase structure, Si-(SiYo4-Y) and Si- O bonds in SiOx are randomly distributed, and the network structure Si-(SiYo4-Y), (0< y < 4); In the RM model, SiOx is considered to be a Si-sio2 biphase material, which is composed of a small region of amorphous Si phase and SiO2 phase. These two models have their own limitations and cannot fully explain the electrochemical behavior of SiOx.

Based on the above two models and some test results, high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS), A.Hoh et al. proposed a structure model of SiOx -- interfacial cluster mixing (ICM) model. In the ICM model, SiOx is described as composed of Si clusters and SiO2 clusters and the region surrounding the hypoxic interface between them.

A.Hirata et al. systematically studied the atomic structure of amorphous SiO by using beam electron diffraction (ABED) technology and molecular dynamics simulation, providing direct evidence for atom-scale heterogeneity in amorphous SiOx. Synchronous accelerated High Energy X-ray diffraction (HEXRD) results reveal the unique interface structure between amorphous Si and SiO2 clusters.

2.Lithium embedding mechanism and main products of SiOx

Compared with the lithium embedding mechanism of elemental silicon, the reaction mechanism of SiOx and Li becomes more complicated due to the existence of oxygen element in SiOx. At present, the general understanding of SiOx lithium embedding mechanism is as follows: SiOx reacts with lithium first to form simple silicon, Li2O and lithium silicates (Li4SiO4, Li2SiO3 and Li2SiO5, etc.). The simple silicon further reacts with Li to produce a reversible capacity, while the Li2O and lithium silicates generated do not participate in the reaction in the subsequent electrochemical cycle, resulting in a very low first Coulomb efficiency (ICE) of the material. However, it can play a role in buffer volume expansion and protection of active materials.

According to the research of H. yamamura, the reaction between SiOx and lithium is shown in Equation (1) :
4SiO+17.2Li→2Li4.4Si+Li4SiO4（1）

According to formula (1), the theoretical specific capacity of initial charge and discharge is 2615 mAh/g and 2007 mAh/g, and the first Coulomb efficiency is 76.7%, but the measured first coulomb efficiency of SiOx is lower. With the decrease of oxygen content in SiOx, the reversible capacity and the first coulomb efficiency of the material will increase, but the capacity retention rate will decrease after the cycle.

The mechanism of lithium embedding and the main reaction products of SiOx have been clarified, and the relevant model can match the actual results well, which has important guiding significance for improving the electrochemical performance of SiOx. Due to the complexity of SiOx structure, the specific reaction mechanism of each component in SiOx, the specific type and content of the product, and the specific impact of each product are still unclear, which requires further in-depth study.

3.Performance and improvement of SiOx - based anode materials

There are two main problems in SiOx materials: low first coulomb efficiency and attenuation of cyclic properties. There are two reasons for the low initial coulomb efficiency. On the one hand, the organic electrolyte will decompose at the working potential of the negative electrode, forming a solid electrolyte phase interface (SEI) film, consuming the lithium released from the electrolyte and the positive material. On the other hand, due to the presence of oxygen atoms, SiOx material will generate lithium silicate and lithium oxide during the initial lithium embedding process, consuming active lithium. The lithium/silicon alloying process of SiOx is also accompanied by a drastic volume change. Although the oxygen atoms form the buffer matrix in situ, the total volume change is still large, which leads to the powder of the active material and the separation from the contact with the collector. The low intrinsic conductivity of SiOx will affect the performance of the material. The above factors lead to the rapid decay of cyclic properties of SiOx material.

 3.1 Structural modification of SiOx anode

The electrochemical properties of SiOx anode materials can be improved by reducing particle size, disproportionating reaction and constructing porous structure.

T.Huang et al. controlled the particle size of SiOx material by controlling the ball milling time (0~12 h) and studied the influence on the material properties. After ball milling for 10 h, the performance of SiOx was the best. The specific capacities of charge and discharge were 2091mAh/g and 2684mAh/g, respectively, with the current of 0.1a /g circulating in the range of 0.01-2.00V. The particle size decreased from 5μm of the original SiOx to 500 nm, and the specific surface area was 20.1m2 /g. It is more than 22 times of the original SiOx material (0.9 m2/g). The reduction of particle size and the increase of specific surface area shorten the mass transfer distance, inhibit the particle crushing and rupture, improve the utilization efficiency of active substances, and improve the electrochemical performance of the material.

C.M.Park et al. studied the influence of disproportionation temperature on the performance of SiOx material. SiOx treated at 1000 ℃ had the best performance, but when the temperature rose to 1200 ℃, the electrochemical performance of SiOx decreased sharply, presumably because a large number of Si4+ oxides with no electrochemical activity were generated. Blocking the transmission of Li+. B.C.Yu et al etched the SiOx anode material by NaOH (4h) after heat treatment and ball milling, and obtained the SiOx material with porous structure. This self-template etching technique using silicon crystal as pore-forming agent and SiO2 matrix as template can improve the cycling performance of SiOx material. The specific capacity of 100 cycles at 0.2C at 0~2.0 V is 1240 mAh/g without carbon coating.

3.2 SiOx and carbon composites

It is a simple and effective method to modify silicon and SiOx by coating the surface of material particles with carbon layer or composite with carbon material. The advantages of carbon composite with SiOx are as follows: (1) The conductivity of carbon is very good, and SiOx composite can improve the conductivity; (2) The volume change of lithium embedded in carbon material is small (about 8% of graphite), and the overall expansion of the composite electrode can be reduced by the combination of carbon and SiOx. The carbon material can protect the active material, avoid direct contact with the electrolyte, and the volume expansion of the buffer material, improve the cycle stability

J.Y.Zhang et al. prepared core-shell SiOx/C composites by pyrolysis and wet grinding. The first reversible specific capacity of SiOx/C composites was 1279 mAh/g when charged and discharged at 0.01~1.20 V at 0.1c. The capacity retention rate of 170 cycles was 97.3%. F.Dou et al. optimized the method of carbon coating by introducing TiO2 to promote the formation of ordered carbon layer on the surface of SiO particles, so that SiO has better interface stability and higher conductivity, charging and discharging at 0.005~1.000V. The coulomb efficiency of 3% Tio2-SiO /C anode reaches 99% in the first 10 cycles of 0.14 A/g, 400 cycles at 0.70 A/g, and the reversible specific capacity is 400 mAh/g.

Z.l. Li et al. prepared SiOx/C nanoparticles with dandelion structure by microemulsion method, with high specific capacity and good cyclic stability. In 0.01~1.50 V charge and discharge, 0.1a /g cycle 200 times discharge capacity is 1115.8 mAh/g; The discharge capacity is still 635mAh/g after 1000 cycles at 2.0A /g.

Carbon nanofiber (CNF) and carbon nanotube (CNT) are one-dimensional carbon materials with excellent electrical conductivity and mechanical strength. Composite with SiOx material can use the one-dimensional properties to form a network to improve the comprehensive properties of the material. Y.n.li et al. prepared SiO/ graphite /CNT/CNF composites by combining high-energy wet ball milling, spray drying and chemical vapor deposition. CNT and CNF grow in the interface and internal gap of SiOx and graphite materials at high temperature, forming a conductive network and providing a buffer medium for the volume expansion of the active material. The specific capacity is kept at 672.3mah /g with the current of 0.1a /g circulating at 0.01-2.00V for 100 times.

Graphene can be used as an electrode material due to its excellent electrical conductivity and high theoretical capacity. L.R.Shi et al. achieved the growth of vertically oriented graphene on micron SiOx particles by chemical vapor deposition (CVD). The vertically oriented graphene can establish a stable electrical contact between the active material and the surrounding conductive material, and allow the smooth passage of Li+. The first reversible specific capacity of the composite material is 1600mAh/g when the current is 0.16 A/g and the initial reversible specific capacity of the composite material is 1600mAh/g when the current is 0.32 A/g and the capacity retention rate is 93%.

3.3 SiOx composite with elemental silicon

Because of the presence of oxygen element, the specific capacity of SiOx is lower than that of simple silicon, so the combination of SiOx and simple silicon can not only improve the capacity, but also protect the simple silicon and buffer the bulk expansion of the simple silicon by using the irreversible components generated by SiOx in the initial lithium embedding process. J.G.Guo et al. soaked nano-Si in aqueous solution and prepared amorphous SiOx layers of different thickness. On the one hand, amorphous SiOx layer is used as expansion buffer layer. On the other hand, the Si-OH group on the surface of Si/SiOx provides a stable connection between particles, binder and conductive agent, forming a conductive network, and further reducing the volume change in the process of lithium intercalation. The nano-silicon particles (SiNPs) soaked for 8 days have the best performance, the first reversible specific capacity is 3590mAh/g and the first coulomb efficiency is 82.3% at the temperature of 0.01~2.00V, 0.1 A/g. After 500 cycles of 1.0 A/g, the reversible specific capacity is still about 1200 mAh/g.

Y.f. Chen et al. obtained MWCNT@Si/SiOx@C coaxial nanocomposites by magnesium thermal reduction and carbon coating of multi-walled carbon nanotubes (MWCNT) @sio2 nanocomposites. Compared with silicon and SiOx materials, the reversible capacity of the material is higher and the cycle performance is better. The coulomb efficiency is stable over 97% after 10 cycles with A current of 0.4A /g in the range of 0.001~2.000 V, and the reversible specific capacity is still 503 mAh/g after 100 cycles.

K.j. Kong et al. studied the influence of the crystallinity of SiOx coating on the properties of the material. SiOx coating formed by low temperature drying and oxidation has uniform and low crystallinity. Theoretically, continuous, uniform and low crystallinity coating is more conducive to lithium intercalation and structural stability. The electrochemical performance of the samples obtained by oxidation at 550 ℃ is the best. The discharge capacity of the samples is 1108 mAh/g after 100 cycles at 0.01-1.50 V with a current of 0.5C.

3.4 SiOx is recombined with metal and oxide

H.W.Yang et al. prepared Ti-SiOx@C composites by selective alcoholysis of SiCl4, TiCl4, ethylene glycol and benzene. The first reversible specific capacity of Ti-SiOx@C anode is 1304 mAh/g when the current is 0.1 A/g and the charge and discharge range is 0.01~1.50 V. The specific capacity of the 100th cycle is maintained at 1271 mAh/g; At A higher current of 1.0 A/g, the specific capacity of the 600th cycle is 833mAh/g and the capacity retention rate is 89%. Ti doping and carbon coating can improve electron transport and Li+ diffusion rate, and the improved structure can prevent structural degradation and ensure the stability of charge and discharge process.

C.J.Tang et al. designed a hollow Ni/SiO2 nanocomposite prepared by ultrafine nickel nanoparticles and SiO2 nanoribbons. Nano - ni particles can improve the electrical conductivity of the material, and the buffer volume of hollow structure expands. The smaller the size of Ni particles is, the higher the capacity of Ni/SiO2 composites is, and the better the cycling stability is. The reversible specific capacity of Ni/SiO2 composites is 676 mAh/g when the Ni/SiO2 composites are charged and discharged at 0.01~3.00 V, 0.1 A/g. At A high current of 10.0 A/g, the specific capacity of the 1000th cycle is still 337 mAh/g.

F. Cheng et al. composited ZrO2 and carbon onto SiOx materials by high-energy ball milling and chemical vapor deposition. The composite was cycled 100 times at 0.01-1.50 V at 0.8 A/g, and the reversible specific capacity was still 721 mAh/g.

3.5 Prelithiation of SiOx Anode

Most negative electrode materials produce SEI films during the first charge and discharge, resulting in the loss of active lithium, permanently reducing the lithium content in the positive electrode material, and thus reducing the usable capacity of the entire battery. In addition to the formation of SEI film, a large number of irreversible phases (Li2O and lithium silicate) are also formed in the SiOx anode material during the first charge and discharge, which further aggravates the loss of active lithium. Therefore, measures need to be taken to compensate for this defect. The pre-lithiation technology can greatly improve the first coulombic efficiency of SiOx, thereby increasing the energy density of the entire battery. Commonly used prelithiation techniques include electrochemical prelithiation, chemical prelithiation, direct contact prelithiation with lithium metal, and direct addition of prelithiation reagents.

Electrochemical pre-lithiation is similar to the electrochemical reaction process of lithium-ion batteries. By simulating the first charge and discharge process of the negative electrode material in the battery, lithium metal is used as the counter electrode, and the circuit is short-circuited outside the negative electrode and the lithium metal electrode. Pre-lithiation is performed by current charging and discharging. H.J.Kim et al designed a roll-to-roll device for continuous pre-lithiation, which can improve the above problems to a certain extent, and precisely control the degree of pre-lithiation to prevent lithium plating, so that the first 3 times of the graphite/SiOx composite electrode The Coulombic efficiencies of the cycles reached 94.9%, 95.7% and 97.2%.

Chemical pre-lithiation is to chemically react active reactants (lithium metal, Li-aromatic hydrocarbon complexes, etc.) with SiOx materials, and insert lithium in advance for pre-lithiation. The operation is simple and the pre-lithiation efficiency is high. J.Y.Jang et al. achieved higher efficiency prelithiation by designing different aromatic hydrocarbon molecular structures. After 30min of Li-aromatic hydrocarbon-ethylene glycol dimethyl ether (DME) solution impregnation, the first Coulombic efficiency was greatly improved ( increased from 57% to 107%); after immersion in 4,4-dimethylbiphenyl, 2-methylbiphenyl and 3,3,4,4-dimethylbiphenyl solutions, the first coulombic efficiencies of the electrodes were respectively 107%, 124% and 118%. Further control of the time and temperature of immersion enables fine control of the pre-lithiation degree of the entire electrode, which is expected to realize large-scale pre-lithiation of SiOx electrodes.

Lithium metal direct contact prelithiation is to directly contact the negative electrode material with lithium metal in the presence of an electrolyte, and use the potential difference between the two to make electrons flow directionally, while Li+ is released from lithium metal into the electrolyte. In order to achieve charge balance, Li+ in the electrolyte will be intercalated into the anode active material to complete the pre-lithiation. The direct addition of pre-lithiation reagents is to add lithium powder, stabilized metal lithium powder and lithium-silicon alloy particles (LixSi-Li2O) in the process of making batteries. The source particles are not uniformly distributed inside the electrode, which has a great impact on the safety and stability of the battery. Most of the solvent used in the existing negative electrode preparation process is water, and the pre-lithiation reagent has high activity and will react with water, so an additional stable organic solvent is required for dispersion.