Surface corrosion of electrolyte on the current collector of positive and negative electrodes

Since the surface of aluminum is usually covered with a protective oxide film, aluminum foil is very stable in air and electrolyte and will not be corroded. Therefore, aluminum is used as a positive electrode current collector material for commercial lithium-ion batteries. However, during the charging process, the positive electrode potential of a lithium-ion battery will exceed 4V. Under such a high potential (ie, strong oxidizing conditions), most metals will corrode to varying degrees. The current collector materials of lithium-ion batteries currently in use are all susceptible to environmental influences, resulting in performance degradation. Aluminum will have corrosion pits and copper will experience environmentally-caused fractures. Any corrosion to the current collector will shorten the life of the battery electrodes and cause battery safety issues. Preliminary research results show that if the battery is not activated (charged) after being assembled into a battery, the anode of a lithium-ion battery is prone to copper corrosion, resulting in a decrease in battery performance. Certain pollutants will oxidize the copper, and the copper foil substrate is not completely inert in the electrolyte of the lithium ion battery. Some impurities such as HF will oxidize the copper box substrate.

There has been a lot of research work on the corrosion mechanism of aluminum in lithium batteries, most of which have focused on the stability of aluminum in various electrolytes at high potentials (usually above 4V). Studies have shown that the corrosion process of aluminum strongly depends on the properties of the salts and solvents that make up the electrolyte. Among the various electrolyte salts that have been studied, organic electrolytes using Li(CF₃SO₂)(C₄F₉SO₂)N, LPF₆, and LiBF₄ have the least corrosiveness to aluminum. After the initial anodic reaction, they can decompose on the aluminum electrode to form a stable passivation layer to prevent further corrosion of aluminum. LiN(CF₃SO₂)₂ and LiCF₃SO₃ have many superior properties, such as being insensitive to a small amount of water in the electrolyte, and having relatively high thermal and electrochemical stability. However, because of their strong activity, they are also the most corrosive to aluminum. In the 1mol/L LiN(CF₃SO₂)₂/EC+DMC electrolyte, it only needs to go through a few cycles at 2~5V, and the corrosion depth of the electrolyte to aluminum can reach 1μm, and there are many corrosion pits on the surface of the aluminum electrode. Corrosion products are mainly Al[N(CF₃SO₂)₂]_3-y(OH)_y. If the aluminum current collector continues to be exposed to the electrolyte under the condition of the anode potential, this corrosion will continue.

The corrosiveness of LiN(C₂F₅SO₂)(LiBETI) solution to aluminum is between that of LiPF₆ and LiTFSI solutions. If a small amount of LiPF₆ or LiBF₄ is added to the organic solution of LiN(CF₃SO₂)₂ and LiCF₃SO₃, this corrosion effect can be significantly inhibited. The anodic stability of the aluminum electrode in the salt solution containing LiPF₆ is related to the AlF₃-rich film on the surface. The addition of LiBF₄ helps to form a stable passivation layer on the surface of aluminum, inhibits the reaction of aluminum with the electrolyte and the decomposition of the electrolyte solvent at high potentials. The passivation layer contains some organic precipitates produced by solvolysis, such as RCO₂M (M stands for Al and/or Li), lithium oxalate, LiOH, and soluble B-F salts such as Al(BF₄)₃. When adding fluoride to an organic solvent containing LiCF₃SO₃, comparing several fluoride lithium salt additives to inhibit the corrosion of aluminum in LiN(CF₃SO₂)₂ organic solutions, it is found that LiBF₄ can inhibit the corrosion of aluminum most effectively among LiBF₄, LiPF₆, LiAsF₆, LiSbF₆ and LiClO₄. This is because its oxidation potential is closest to that of CF₃SO₃^-anion. Among these fluoride additives, the corrosion current of aluminum decreases in the order of LiSbF₆> LiAsF₆>LiClO₄>LiPF₆>LiBF₄. The oxidation potential of ClO₄^- is almost the same as that of CF₃SO₃^-, but the corrosion current is smaller or slightly larger than the corrosion current in the solvent containing LiPF₆. The SEM study of the electrochemically oxidized aluminum sample shows that the degree of corrosion is in good agreement with the magnitude of the corrosion current.

The anodic behavior of aluminum in organic electrolytes includes two processes: surface film formation and dissolution. Studies have found that during the charging process of lithium-ion batteries, the surface protective film formed by aluminum in these electrolytes is damaged when the potential is above 3.5V, leading to the dissolution of the aluminum substrate and the premature failure of the battery. Using lithium tetrafluoroborate as an additive can avoid the damage of the protective film on the aluminum substrate and the corrosion of aluminum when the voltage is above 3.5V. On the contrary, for the imide lithium solution, it was found that the aluminum substrate was very stable in the methyl lithium electrolyte, and no obvious corrosion occurred until about 4.25V.

Study the dissolution of bare copper foil (Cu) and copper foil covered with graphite and adhesive (Cu-C) immersed in fresh and aged electrolyte, and found that, the electrolyte used is 1mol/L LiPF6 in three-component solvents (Ⅰ) PCEC-DMC and (Ⅱ) EC-DMC-MEC (methyl ethyl carbonate), the water content of the two electrolytes is less than 20μg/g . After the copper foil is put into the electrolyte without any agitation, the stability of the copper foil in the electrolyte is quantitatively analyzed by atomic absorption spectroscopy and it is found that the dissolution of the exposed copper foil in the fresh electrolyte is very slight, and the dissolution of copper was only 50μg/g after 20 weeks. The slight dissolution of copper is due to the oxidation of copper by a small amount of impurities (such as HF) in the electrolyte. Do the same experiment after exposing the fresh electrolyte to the air for 30 minutes, and it is found that a large amount of copper will dissolve. It shows that the dissolution mechanism of copper foil in the electrolyte under different conditions is different. A small amount of products (such as PF₅) that are decomposed during storage of the electrolyte may have a vital impact on the stability of copper. The H₂O and HF impurities in the electrolyte will accelerate the dissolution of copper, but their effect is far less than that caused by the aging of the electrolyte (which may generate PF₅). In terms of the solubility properties of copper foil, the results of adding H₂O and HF into the aged electrolyte are very similar to those of adding these impurities into the fresh electrolyte. In such electrolytes, the dissolution of copper is not as obvious as in dry and aged electrolytes (without doping impurities), and the degree of corrosion is similar to that in fresh electrolytes containing H₂O and HF. In order to further understand the role of LiPF₆ in the corrosion of copper, the copper foil was immersed in DMC without LiPF₆ for a comparative study. After 14 weeks, the dissolved amount of copper foil in DMC was less than 5μg/g, while the dissolved amount in the electrolyte reached 50μg/g. It can be seen that the presence of LiPF₆ promotes the dissolution of copper in the electrolyte.

Use cyclic voltammetry to study copper in three electrolytes [respectively LiPF₆ dissolved in PC:EC:DMC (1:1:3 volume ratio); soluble in EC:DMC:DEC (volume ratio 2:2:1); Soluble in EC:DMC:MEC (volume ratio 1:1:1)] in the electrochemical stability and redox behavior, it is found that the stability of copper is closely related to the impurities contained in the electrolyte. Impurities in the electrolyte significantly increase the oxidation tendency of copper. During the first charging of the battery, the electrolyte is reduced to form a passivation film covering the copper electrode, which can reduce the oxidative corrosion of copper by the electrolyte to a certain extent. However, this passivation film may be unstable, and any agitation to the electrolyte may cause the passivation layer to dissolve in the electrolyte and lose its protective effect on the copper electrode.

The dissolution of copper foil covered with graphite in fresh electrolyte is very slight. After 14 weeks, the dissolved amount in the same electrolyte was less than 1μg/g, which was even lower than that of bare copper foil. This is because the graphite covered on the copper foil adsorbs HF impurities in the electrolyte and slows the corrosion of copper. After exposing the electrolyte to the air for 30 minutes, the dissolved amount of copper was found to increase greatly, reaching 800 μg/g after 12 weeks. Therefore, a brief exposure to the air significantly increases the corrosiveness of the electrolyte to copper. Then the impurities H₂O (500μg/g) and HF (1000μg/g) were introduced into the fresh electrolyte and the aged electrolyte. After 8 weeks, it was found that the solubility of copper in the two electrolytes was similar. In fresh and aged electrolytes, the corrosion effects of H₂O and HF on copper are similar, and a small amount of impurities can significantly increase the dissolution tendency of copper in the two electrolytes.

Elemental analysis and thermogravimetric analysis were used to study the anodic polarization behavior of copper in LiClO₄/PC electrolyte. The results showed that the anodic polarization of copper caused the oxidation and dissolution of the metal substrate and the decomposition of the electrolyte on the metal surface. The scanning electron microscope photo shows as shown in Figure 1. The copper surface corroded by the electrolyte is full of corrosion pits and copper grain boundaries, and no solid phase corrosion products are observed on the copper surface. It shows that the corrosion of copper and the dissolution of copper ions caused by electrochemical oxidation proceed simultaneously, and the two compete with each other. The shape of the corrosion pits on the copper surface depends on the crystal phase orientation of the copper particles. The mechanism of oxidative dissolution of copper is proposed in terms of charge transfer and geometry. In actual use, since the cut-off potential of the negative electrode discharge is only 1.5V, no copper corrosion is observed.

Figure 1 - SEM images of the copper surface polarized in LiClO₄/PC solution [1000 times (a) and 10000 times (b)] The current density is 1.0mA/cm²

In addition to corroding the current collectors of the positive and negative electrodes, the electrolyte may also corrode the positive and negative materials with the same potential as the positive and negative current collectors. Both Jahn-Teller distortion and manganese dissolution during charging will cause capacity loss. The partial replacement of manganese with Cu²⁺ and Cr³⁺ reduces the capacity degradation caused by manganese leaching. This is because the higher trivalent chromium octahedron can stabilize the spinel structure of LiMn₂O₄, and copper and chromium doping can reduce the capacity attenuation of the spinel in the 4V region. Doping with lithium can inhibit the dissolution of manganese in the spinel, and increasing the doping amount of lithium in the spinel can reduce the dissolution of manganese and improve its cycle performance.