ALD/MLD process for coating battery grade graphite

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Kemian tekniikan korkeakoulu | Master's thesis
Funtional Materials for Global Challenges
Degree programme
Master's Programme in Advanced Materials for Innovation and Sustainability
Lithium-ion batteries (LIBs) are used in a broad variety of applications. Most common examples are portable electronics, electric vehicles, and energy storage systems, presenting a constantly increasing demand for LIBs and a need for improvements of performance. The employed positive and negative electrode materials play a crucial role in determining the battery properties. Graphite is the most commonly used negative electrode material due to its high electronic conductivity, layered structure, abundance and low costs. However, during cycling graphite undergoes a 10-30% volume expansion and a solid electrolyte interface (SEI) is formed due to electrolyte decomposition, causing an irreversible capacity loss of 10-20% within the first cycle. An improperly formed SEI layer leads to further undesired side reactions, which deteriorate the battery performance and thus, reduce its lifetime. This work presents an approach to overcome those disadvantages by the application of a nanometre thin aluminium oxide (AlO) and hybrid alucone (AlGL) coating on the graphite surface by ALD and ALD-MLD, respectively. In order to enhance the bonding between the graphite surface and the coating by oxygen functional groups, the graphite is pre-functionalized by an ozone treatment, for which the duration is varied from 40 min to 100 min. The effect of the ozone treatment duration as well as the thickness of the coatings are characterized structurally and electrochemically. The 40 min ozone-treated graphite exhibited the optimum electrochemical properties of the ozonated samples and thus, is employed for the subsequent coating. An increased long-term cycling stability is shown for the graphite powder coated with 100 cycles of AlO and the compound electrode coated with 35 cycles of AlGL, while the former also improved the rate capability capacity retention at 0.2C to 95% from 77% for pristine commercial graphite. The highest initial capacity at 0.1C with 375 mAh/g is obtained for the compound electrode coated with 50 cycles of AlGL.
Kallio, Tanja
Thesis advisor
Ahaliabadeh, Zahra
Miikkulainen, Ville
graphite, ozone treatment, ALD-MLD, lithium-ion batteries
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