The experimental results show that the silicon-aluminum composite thin-film anode material prepared by magnetron sputtering can effectively alleviate the volume expansion of the electrode and has good cycle performance.
Lithium-ion batteries have become the mainstream of secondary batteries due to their high energy density and long cycle life, and have been widely used in portable electronic equipment, electric vehicles, and energy storage industries [1-2]. As the existing lithium-ion battery development and application technologies mature, people have put forward higher requirements for secondary power sources led by lithium-ion batteries. The development and application of graphite-based carbon anode materials, one of the key materials of lithium-ion batteries, has approached the limit level and cannot meet the needs of higher volume-specific capacity batteries [3].
In the current negative electrode system, silicon has the highest theoretical lithium insertion capacity (4 200 mAh/g, about 11 times that of graphite) and a relatively moderate lithium insertion and removal potential (~0.5V), which has attracted much attention. One of the potential anode materials for next-generation lithium-ion batteries. However, powdered silicon as an electrode active material limits the further application of the material due to its poor electrical conductivity, serious volume effect (volume expansion during lithium insertion process of about 300%), unstable material structure (structural damage during charge and discharge), and low initial efficiency. .
The current solutions include nano-silicon, silicon oxide and graphite composite methods, but the expansion level is still much higher than that of graphite anode materials, and there are still poor cycle performance, low first-time efficiency, and specific capacity advantages in practical applications. Unobvious law and other issues. At present, many researchers have made many attempts on traditional lithium-ion batteries to improve the volume effect of silicon-based electrode materials to improve electrochemical performance [4-7].
Compared with traditional silicon powder materials for lithium-ion batteries, the film structure can reduce the volume change in the direction perpendicular to the film and maintain the structural integrity of the electrode [8]. However, after many cycles, the silicon film is easily broken and separated from the substrate. The metal component in the silicon alloy can improve the electronic conductivity of the material, reduce the polarization of the silicon material, and improve the rate performance of the silicon material. The ductility of metal can restrain the volume effect of silicon material to a certain extent and improve the cycle performance.
In addition, the preparation of the active material and the forming of the electrode are completed at the same time without adding a conductive agent and an adhesive, which simplifies the electrode preparation process and reduces the process cost. In terms of thin-film silicon research, some teams have made positive progress [9-10]. Due to the inherent existence of internal stress due to volume changes, especially the research on deposition substrates/current collectors, it is still in the exploratory stage.
The porous copper structure facilitates the transportation of liquid electrolyte, provides a fast lithium ion transmission path, and the active material is not easy to fall off, so the electrode is stable and the cycle life and coulomb efficiency of the electrode are improved. In the porous structure, rough copper foil with a two-dimensional (2d) micro-nano porous structure (RCF for short) and porous copper with a three-dimensional (3d) network structure (PC for short) were used as current collectors, and the following research was conducted.
Experiment
1.1 Preparation of the electrode
Copper foil with rough surface (referred to as RCF, surface density 7.308mg/cm2, thickness 12μm, Ra=0.26, Rz=1.87) and porous copper foil (referred to as PC, pore size 130PPI, surface density 8.557 mg/cm2, thickness 100μm) respectively It is the substrate/current collector, cleaned and dried on the surface, placed in a magnetron sputtering system (TechnolJCP-350) cavity, using silicon and aluminum as targets, co-sputtering deposited on the surface of the substrate, and controlling the silicon through time and current- The thickness of the aluminum composite film is deposited to obtain a Si-Al composite electrode. The surface density of the active material is calculated by the difference between the mass of the substrate before and after the deposition.
1.2 Characterization of electrode morphology
The structure and morphology of the substrate and the deposited Si-Al film were observed by scanning electron microscope (SEM, FEIQuanta 200, USA). The chemical composition of the Si-Al film was tested by the energy spectrum X-ray analysis (EDX, AMETEKApollo XP, USA) supporting the SEM. The phase structure of the Si-Al film was analyzed by XRD (Bruker D8 Advance, Germany), and the Cu target Kα radiation, and the scanning range was 20° to 80°. The cross-sectional morphology, crystal structure and thickness of Si-Al film were characterized by transmission electron microscope (TEM, FEITECNAI G2 S-TWIN F20, USA).
1.3 Characterization of the electrochemical performance of the electrode
The 2430 half-cell was used to evaluate the electrochemical performance of the Si-Al composite film. The Si-Al film deposited on the above two substrates is made into a 2 cm diameter pole piece, and then the diaphragm, electrolyte, and lithium foil as the counter electrode are placed in a glove box (Vacuum) filled with argon (99.995%). Atmospheres, Mikrouna, Universal 1220/1000, <10×10-6 O2 and H2O) are assembled into button batteries. The organic electrolyte system used in the experiment is LIPF6/(EC+EMC) (volume ratio 3:7, containing 10% FEC additive). The button cell assembled from the glove box is taken out and pressed, and left to stand for 12 h Then, the constant current cycle test was carried out in the Arbin battery test system (Arbin, BT 2000-8A). The button cell is first discharged from the open circuit potential to 0.005 V relative to the Li+/Li electrode, and the cycle test is performed at 0.005 to 2.0 V after the first discharge. At room temperature, all charge/discharge experiments were run at a constant current of 0.1 C to 50 times.
2. Results and discussion
2.1 Electrode morphology characteristics
The SEM surface topography of silicon-aluminum composite films deposited on two different substrates. The two Si-Al films exhibit different surfaces due to the surface profile of the substrate. The active material has some cracks or voids, but no cracks. The silicon-aluminum composite film deposited on RCF is nearly spherical, with a diameter of about 1 to 2 μm, and relatively loose, while the silicon-aluminum composite film deposited on PC is island-shaped and relatively dense.
Therefore, it can be seen that the surface morphology of the substrate has a great influence on the bonding state of the film in the magnetron sputtering process. The bonding state of the silicon-aluminum composite film deposited on the RFC is in the form of point contact, which causes the electrical conductivity of the film to decrease and affects the full play of the active material. The bonding state of the Si-Al composite film deposited on the PC exhibits a surface contact form, thereby increasing the adhesion between the film and the substrate. The Si-Al composite film deposited on the RCF is in the form of a two-dimensional (2D) structure, but silicon-aluminum can be deposited into the internal pores of the PC three-dimensional structure.
Therefore, the volume expansion during charging and discharging can be well buffered by the macroporous structure in PC.
The composition of the silicon-aluminum composite film deposited on two different substrates detected by EDS is basically the same, and the ratio of Si content to Al content is about 10:1. The thin film material contains two active components (Si and Al), which can realize the characteristics of high-capacity electrodes. The theoretical specific capacity of aluminum is 2 234 mAh/g, which has high conductivity. Al and Si have different lithiation potentials. Effectively alleviate the volume expansion during charging and discharging, and improve the cycle stability of the negative electrode material.
XRD patterns of Si-Al composite films deposited on two different substrates. All the main diffraction peaks are mainly from the copper matrix, and the weak peak at 2θ of 28° corresponds to the crystal Si(111). No peaks corresponding to Al and Si-Al alloys are observed. These indicate that the prepared Si-Al composite film is similar to an amorphous structure.
TEM images of cross-sections of silicon-aluminum composite films deposited on two different substrates. In order to increase the energy density of the material and meet practical applications, the thickness of the silicon-aluminum composite film deposited on two different substrates is approximately the same, about 2.3 μm. It is observed that the two cross-sections are very different. There is a certain gap between the silicon-aluminum composite film particles deposited on the RCF [such as the dentate structure, Figure 5(a)], and the interface between the film and the RCF is relatively loose , And the interface between the film and PC is very good [Figure 5(c)]. It can be seen that there are no parallel lattice fringes (or lines) with different orientations on the surface, and the selected area electron diffraction (SAED) pattern is diffusely reflective and very fuzzy. This also proves that the structure of the prepared silicon-aluminum composite film is mainly amorphous.
The amorphous structure of the electrode prevents the expansion of the crystal lattice formation better than the crystal structure. Lee et al. reported that this amorphous structure is beneficial to the electrochemical performance of thin film anodes (such as enhancing cycle performance), because amorphous materials exhibit uniform volume expansion/shrinkage during charge and discharge. This amorphous structure can sufficiently prevent crystallization. The lattice expands and provides more diffusion paths for lithium ions.
2.2 Electrode electrical performance indicators
The 0.1C cycle performance of silicon-aluminum composite films deposited on two different substrates.
The charge-discharge curves of silicon-aluminum composite films deposited on two different substrates at a constant current of 0.05 and 2 V and 0.1 C. The initial specific capacity and first-time efficiency of the silicon-aluminum composite film deposited on RCF were 2379 mAh/g and 92.4%, respectively, while the initial specific capacity and first-time efficiency of the silicon-aluminum composite film deposited on the PC substrate were 2 634mAh/g, respectively And 95.5%. The voltage plateaus of the silicon-aluminum composite films deposited on two different substrates are both 0.2~0.6 V (charge) and 0.4~0.05 V (discharge). Moreover, the silicon-aluminum composite film has a smooth and inclined charge/discharge curve, which can be explained by the amorphous structure material without any phase change conversion.
However, after the increase in the number of cycles, the voltage of the silicon-aluminum composite film deposited on the RCF increases rapidly, and the charge/discharge capacity decreases rapidly. However, the charge-discharge curve of the silicon-aluminum composite film deposited on the PC is The voltage platforms are almost overlapping and have a higher capacity retention rate. After 50 cycles of the silicon-aluminum composite film deposited on PC, the specific capacity is still 2 497 mAh/g, and the capacity remains 94.8%; while the silicon-aluminum composite film deposited on RCF after 50 cycles, the remaining The specific capacity is only about 1 117 mAh/g, and the capacity remains reduced by 46.9%.
The results show that the difference in the morphology of the current collector has a considerable impact on the charge/discharge behavior, and the Si-Al composite film deposited on PC has better cycle performance. This is because the three-dimensional porous structure can increase the active material and therefore, from the perspective of cycle stability, the silicon-aluminum composite film deposited on PC has better electrochemical performance than the silicon-aluminum composite film deposited on RCF.