It is difficult for carbon materials to form thin films. The most researched ones are the thin films of metal and alloy materials, as well as the oxide thin films. The three types of thin film materials are mainly discussed here.
Carbon thin film material
The pyrolytic Carbon thin film is prepared by pressure pulse chemical vapor infiltration technology. The substrate used is two different conductive porous woods, one is carbonized wood, and the other is wood with TiN coating. Studies have shown that the pyrolytic Carbon thin film has a three-dimensional current path and good discharge performance at a large rate. The former Carbon thin film has a relatively high degree of crystallinity, while the latter is disordered. The capacity of the former Carbon thin film is lower than that of the latter, but the coulombic efficiency is higher than that of the latter. The charging curves of the pyrolytic Carbon thin film of TiN-coated wood under different currents are shown in Figure 1.
Figure 1 - Charging curves of pyrolytic Carbon thin films of TiN-coated wood under different currents
The preparation methods of carbon nanotube film include chemical vapor deposition method, microwave plasma chemical vapor deposition method, catalytic pyrolysis method and so on.
Metal and alloy thin film materials
Among the thin film materials, metal and alloy thin films have been studied more. The main reason is that metals and alloys have large theoretical specific capacities and are easier to form thin films. At the same time, the film can greatly improve the electrical performance of the negative electrode of the material, such as increasing the reversible capacity of the material, overcoming the powdering and chipping caused by the volume change of the material during the charge and discharge process, and improving the cycle performance of the electrode.
Aluminum is a common metal, and it may form three alloys with lithium: AlLi, Al2Li3, Al4Li9. The maximum lithium storage capacity of the aluminum electrode is 2.25 lithium, and its mass specific capacity can reach 2234mA·h/g (to form an Al4Li9 alloy), which is more than twice that of metal tin (994mA·h/g). In metal film formation, the evaporation coating method is mostly used. In a vacuum chamber with a vacuum degree of less than 10-3Pa, the inert metal Cu is used as the substrate to thermally evaporate the granular Al onto the substrate. The resulting Al film has a thickness of 0.1~1μm and a specific capacity of about 1000mA·h/g. The charge-discharge curve of the aluminum film is shown in Figure 2.
Figure 2 - Charge and discharge curve of aluminum film
There are mainly three potential regions in the charge-discharge curve of aluminum film, corresponding to three different electrochemical reactions. The first region is at 0.26~2.6V, which corresponds to the reduction of the oxide layer on the surface of the film. The voltage plateau of 0.26V belongs to the formation of LiAl alloy. XRD shows that the obtained LiAl alloy is amorphous rather than crystalline compound. In the potential region under 10mV, no Li-rich alloy Al4Li9 is formed. At the same time, the thicker the film, the smaller the reversible and irreversible capacity of the electrode, and the lower the charge and discharge efficiency. When the film thickness is 0.1μm, 0.3μm, 1μm, the discharge capacity of the electrodes were 800mA·h/g, 610mA·h/g, 420mA·h/g (C/4), and the first charge and discharge efficiency was 58%, 56%, and 41%, respectively.
Silver film has several advantages as a negative electrode material: First, it has a very high specific capacity, which can finally form AgLi12 compound; Second, the alloy and alloying potential range is very low (0.250~0V); third, the film is easy to prepare, such as by thermal evaporation or radio frequency sputtering.
The high frequency sputtering method (RF method) can prepare Ag films on the stainless steel substrate. The films prepared by the RF method have better adhesion and the thickness is easy to determine. The charge and discharge curve of the silver film electrode is shown in Figure 3. There are four voltage platforms between 0.400 and 0V, two voltage platforms between 0.06 and 0.04V, corresponding to AgLi5.2, and one voltage platform between 0.10V and 0.30V. The silver film in a micro battery with Li1.2Mn1.5Ni0.5O4 as the positive electrode and 1mol/L LiPF6-EC-PC-DMC as the electrolyte, the capacity is close to 254A·h/cm2 after 1000 cycles, and the average working voltage is 4.65 V.
Japan's Sanyo Electric Company has prepared and studied a Sn-Cu thin film electrode. Through electroplating, a thin layer of tin is formed on the copper substrate. The thin film is annealed at 200°C for 24 hours to improve the bonding strength of the tin layer and the substrate. After 10 weeks of cycling, the reversible capacity is still 800mA·h/g. Structural analysis and electron micrographs showed that two different Sn-Cu intermetallic compounds were formed between the tin layer and the copper current collector layer due to annealing. The concentration gradient of copper in the tin layer increases the strength of the action between the active material and the current collector, and improves the electrode performance.
Electroplating and magnetron sputtering methods are used to obtain metal antimony films of different forms. The antimony materials of different forms have the same charging and discharging platform, and the lithium insertion platform and the lithium discharge platform are about 0.8V and 1.0V, respectively. The electrochemical performance of thin-film antimony is better than that of antimony powder material, and the film produced by magnetron sputtering has the best performance, the first deintercalation capacity can reach 423mA·h/g, and its reversible capacity remains above 400mA·h/g after 15 cycles.
Oxide thin film materials
The research on anode oxide film materials mainly include SnO2, NiO, Li4Ti5O20 and other oxides. There are many methods for preparing SnO2 thin films, such as magnetron sputtering, chemical vapor deposition (CVD), spray pyrolysis (SP), electrostatic spray deposition (ESD), sol-gel method, electron beam evaporation method, etc.
The SnO2 film prepared by the ESD method has an amorphous structure. The amorphous structure can avoid the influence of stress changes on the crystal lattice, so that it has better cycle performance. The reversible capacity of this SnO2 thin-film electrode exceeds 1300mA·h/g at a charge and discharge current of 0.2mA·cm-2 between 0.05V and 2.5V; when the potential interval is 0~1.0V, the reversible capacity of 600mA·h/g can be maintained after more than 100 cycles.
The SnO2 thin film prepared by radio frequency magnetron sputtering has good reversible capacity and cycle performance, and the reversible capacity still exceeds 400mA·h/g after 75 cycles. The radio frequency magnetron sputtering method allows the film to be deposited on a lower temperature substrate, and improves the density, crystallinity and adhesion of the deposited film. In addition, the SnO2 film obtained by vacuum thermal evaporation method and chemical vapor deposition method also has good electrochemical performance.
As mentioned earlier, Li4/3Ti5/3O4 is a lithium-containing negative electrode material with a spinel structure. It has a stable structure and hardly any changes in volume during charging and discharging, so it has excellent cycle performance. Li4/3Ti5/3O4 thin film preparation methods include ESD, spin-coating technique (SCT) and chemical spraying technique (chemical spraying technique). The Li4/3Ti5/3O4 film prepared by the ESD method can use Li(CH3COO)2·4H2O and Ti(OC4H9)4 as raw materials， using CH3(CH2)3OCH2-CH2O (20%) solvent, a platinum wafer with a diameter of 14mm and a thickness of 30um was used as the substrate, and then the resulting film was heat-treated at a high temperature of 700℃. Using Sol-gel method, Li4/3Ti5/3O4 thin films were prepared by SCT. The specific process is: adding PVP (polyvinylpyrrolidone) to the raw material to form a sol, and then depositing the resulting sol on the Au substrate by spin coating, and finally heat-treating it at a high temperature of 500 ~ 800 ℃ to obtain the final film .
In addition, there are some other oxide film anode materials, such as NiO film, CeO2 film, CoFe2O4 film and so on. The preparation of NiO film mainly adopts methods such as magnetron sputtering, chemical vapor deposition, electroplating and PLD. The CoFe2O4 film can be obtained by pulsed laser deposition (PLD), the substrate is made of stainless steel, evacuated to vacuum, filled with pure oxygen, and deposited at 600°C for 1.5h. For example, the CeO2 film prepared by PLD has a specific capacity of about 150mA·h/g and good cycle performance. The slow scanning cycle curve shows that there are reduction peaks and oxidation peaks at 0.6V and 1.6V.