An Electric Vehicle (EV) is an essential, green, environmentally friendly mode of transportation. EVs rely on Li-ion batteries, and although the lifetime of an EV battery (EVB) is quite long, eventually its performance declines, and it needs to be discarded. What happens with the aging of a battery, is a layer of Solid Electrolyte Interface (SEI) gets deposited on the electrode (Reference 1). This impedes the movement of Lithium ions at the Cathode surface, and, as a result, the battery's performance deteriorates. According to the United States Advanced Battery Consortium (USABC), an EVB needs to be discarded when its performance reaches 80% of its rated capacity. This is referred to as the End of Life (EOL) for the battery.
There are several options when an EVB reaches its EOL. These options include recycling, incineration, landfills, and reuse. Recycling will involve the use of chemicals, which will affect the cathode material and reduce the recycled battery's performance. Incineration will release certain toxic gases and heavy metals into the environment. Landfills are also vulnerable to battery toxicity, and there is a potential for heavy metals to be mixed into water supplies.
A valuable approach to handling LIB at EOL is to remove the deposited SEI, enabling the battery to be reused. This can be done using solvents, but they may alter the electrode’s chemistry and degrade performance. Another way to remove the SEL is to use the laser cleaning method, a non-contact method that does not use a secondary medium and does not pollute the environment, as a vacuum pump collects the ablated debris. This is a very cost-effective method and does not damage the electrodes if the laser operating parameters are appropriately chosen. In one study, a second-harmonic Nd:YAG laser at 532 nm was used to clean and activate the surfaces of platinum and glassy carbon electrodes (Reference 2). In another study, a near-IR Er:YAG laser was used to remove varnish from paintings (Reference 3).
In the study that is the focus of this summary note, an Nd:YAG laser at 1064 nm was used to clean the SEL from a Li-ion battery in pouch-cell form (Reference 1). The Lithium-ion rechargeable cells were made with a LiFePO4 cathode and carbon anodes, using dimethyl carbonate as the electrolyte. The laser parameters were carefully chosen to provide sufficient fluence to remove the SEI layer. A 20 Hz pulse repetition rate, 0.5 msec pulse width, and 0.4-0.28 pulse energy were used as the laser parameters. The fluence of the laser (energy/unit area) had a significant effect on cleaning performance, and this was adjusted by varying the pulse energy or the distance from the cathode surface. Laser pulse energies ranging from 0.4 to 4 joules were used at distances of 14 to 34 cm. This resulted in fluences ranging from 0.035 J/mm2 to 0.169 J/mm2. After the removal of the SEI layer, as shown in Figure 1, a scanning Electron Microscope (SEM) and an FTIR were used to assess the quality and the transmission of the electrode surface.
The SCM images before and after cleaning are shown in Figure 2. The laser-cleaned surface shows substantially reduced surface roughness, indicating the removal of SEL.
The results of SEM show that at a low fluence (0.035 J/mm2) the SEL is not entirely removed and at a very high fluence (0.169 J/mm2) some melting occurs and the cathode is damaged. The best fluence to remove all the SEL without melting the cathode was 0.142 J/mm2 (4 joules, 34 cm distance). FTIR measurements also showed that cathode transmission increased with surface laser cleaning and SEL removal.
In summary, it was demonstrated that the laser cleaning of SEL from the cathode surface for LIB at EOL is a safe, cost-effective, environmentally friendly, and damage-free method.
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References:
1 - Remanufacturing cathode from end-of-life of lithium-ion secondary batteries by Nd:YAG laser radiation, W. Liu et al, Clean Technologies and Environmental Policy, July 2015.
2 - Laser activation of carbon electrodes. Relationship between laser-induced surface effects and electron transfer activation. Poon et al., Anal Chem 60: 1725-1730 (1988)
3 - Surface cleaning of artwork by UV, VIS, and IR pulse laser radiation, J. Marczak, Salimbeni R (ed), Laser techniques and systems in art conservation. SPIE, Bellingham (2001).