Using active materials to fabricate the negative and positive electrodes of lithium batteries, aluminum (Al) and copper (Cu) metal foil current collectors must be covered by a mixture of a large number of active materials, binders, and carbon. When a lithium battery is charged, the metallic copper coated with the negative active material approaches a very low potential (closer to that of metallic lithium). Although chemically stable, repeated charging and discharging for a long time can lead to physical cracks. On the other hand, metallic aluminum undergoes chemical corrosion at high potentials. This becomes more severe during long-term cycling and leads to an increase in the internal resistance of the battery.
- Intrinsic layer of aluminum
Since metal aluminum has an oxidation potential of 1.39 V and is a thermodynamically unstable substance, its surface is usually covered with thermodynamically stable Al2O3, light-based oxides and hydroxides. Due to this intrinsic layer, the metallic aluminum becomes stable and inhibits corrosion. In 1.0 M LiClO4/EC/DME electrolyte, the upper limit of the oxidation stability potential of metallic aluminum is 4.2 V.
The main form of the active material is a porous substance, and although the surface of the metal aluminum is an Al2O3 layer, it has a porous character. When the positive electrode coated on metal aluminum is placed in the electrolyte, the electrolyte will pass through the pores between the positive electrode particles and come into direct contact with the metal, thereby forming a metal-electrolyte interface. When a lithium battery is charged, the negative electrode and current collector must be maintained at a high potential. In addition, prolonged repeated cycling (accumulated charging at high potentials) may lead to corrosion of metallic aluminum. The aluminum-electrolyte interfacial reaction is directly related to battery safety. For example, pitting can damage aluminum and weaken the contact between electrodes and current collectors. This will not only shorten the life of the negative electrode, but also lead to the occurrence of short circuits.
- Corrosion of aluminum
The corrosion of aluminum is affected by the composition of the electrolyte. PC/DEC solvents corrode less than EC/DMC mixed solvents, but lithium salts have a greater effect on corrosion. Compared with other lithium salts, lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2) has more excellent ionic conductivity, cycling characteristics, thermodynamic stability and hydrolysis stability. However, it accelerates the corrosion of aluminum foil and produces carbonate compounds such as LiF and Li2CO3 on the aluminum surface. The corrosion and oxidation stability of aluminum can be judged using linear voltammetry over a wide positive operating voltage window (2.5–4.5 V). Figure 1 shows the stability of aluminum in different imide-based electrolytes.
From Figure 1 above, it can be seen that the stability of aluminum against oxidation corrosion can be arranged in ascending order as follows:
Imide-based lithium salts such as LiCF3SO3 and LiN(SO2CF3)2 induce aluminum corrosion at voltages below 3.8 V, while Li(C4F9SO2)CF3SO2N forms a passivation film. As for the imide-based electrolyte, the N(SO2CF3)2- anion can remain stable and will not be oxidized when the voltage reaches 4.5 V, and at the same time, it will combine with the Al3+ detached from the aluminum metal to form Al[N(CF3SO2)2 ] compound. As shown in Figure 2, Al[N(CF3SO2)2] is absorbed by the aluminum surface, while part of the deposits will dissolve in the electrolyte and cause corrosion by creating pits.
- Formation of passivation layer on aluminum surface
LiPF6 and LiBF4 form a passivation film on the aluminum surface, while imide-based lithium salts induce corrosion. In both cases, the deposits were created on the surface of the aluminum. LiBF4, in particular, produces a more stable passivation film with cycling characteristics under unfavorable conditions of high temperature and several hundred ppm water content. LiBF4 creates a passivation film and LiN(CF3SO2)2 causes pitting corrosion, but these properties can be combined to form LiN(CF3SO2)2. With the formation of the passivation film, corrosion is suppressed and internal resistance is improved. -SO2-, -CF3 groups of TFSI and -OH groups were found on the corroded metal surface. The components of the passivation film are -CH2CH3-, -OH, pulp group, ester group and B-F group. Organics such as CH3CH2-CO2M, -COOR and lithium oxalate (Li2C2O4), LiOH and inorganics with B-F groups were also detected.
Since the composition of these surface compounds is similar to that of the positive electrode SEI film, the formation of the passivation layer also has a similar formation mechanism. LiBOB can also produce passivation films, but its composition is still unknown.
The above-mentioned changes in the interfacial reaction brought about by lithium salts can only be applied to aluminum foils that do not cover the positive electrode. For cathodes like LiMn2O4 and LiFePO4 coated, in LiPF6-containing electrolytes, the oxidation reaction occurs at high voltages of 5.0–6.5 V vs. Li potential, which eventually leads to corrosion.