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Title: Post-mortem analysis of lithium-ion cells after accelerated lifetime testing
Author: Somerville, Limhi
ISNI:       0000 0004 6496 1919
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2017
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Lithium-ion cells are the most commonly used method of energy storage for portable electronics. However, the capacity and power reduces with time and is dependent on usage conditions. This is a challenge for electric vehicle battery packs that are expected to last for up to eight years. Vehicle manufacturers need to understand the causes of this phenomena to accurately predict the vehicles achievable range over its entire lifetime. For automotive manufacturers to tackle this challenge they must know what operating conditions impact cell capacity and power. And, once this has been established, how these operating conditions impact cell capacity and power. Answers to these questions would provide critical information for manufactures; allowing them to mitigate or plan for their impact. In this work, electrical testing was performed across seven cell conditions for between six and thirty months to determine changes in capacity and resistance. Due to the different facility requirements at least one of the following cell chemistries were used for each test, Nickel manganese cobalt (NMC), Nickel Cobalt Aluminium (NCA) and lithium cobalt dioxide (LiCoO2) / all with graphite negative electrodes. State of charge, temperature, current rate during charge, the quantity of the state of charge window utilised and vibration all impacted electrical performance. Cell orientation and external pressure had no effect on cell lifetime. Cell capacity and resistance change over its lifetime is a function of the parasitic chemical reactions occurring within the cell. Understanding how these operating conditions impact cell performance requires a study of the fundamental materials that are at fault. Therefore, materials characterisation of the negative electrode surface film (identified as the primary source of changes to cell capacity and resistance) was performed. Consistency of analytical methods to study this surface film is dependent on the processes of preparation. Those used within literature to open cells, and process the internal cell electrodes led to erroneous results through modification of the surface films chemical properties. A new method is introduced of opening 18650-type cells that is simpler, costs less and stops surface film damage and contamination. In addition, washing electrode surfaces with solvents, which is routinely done within literature, was found to affect the surface film. This work shows that washing can remove surface film and selectively solvate parts of it. It is therefore recommended that washing is not performed. After cell opening, samples were then analysed to determine material changes. A method is introduced to determine the relative surface film thickness (which relates to cell resistance) with x-ray photoelectron spectroscopy that is an improvement on the current method within literature. A wet chemistry method is also shown to selectively remove LiPF6 salt. This makes it possible to use high performance liquid chromatography to study the polymeric species without it reacting with hydrofluoric acid. Using these methods, a relationship is identified between current rate during charge and surface film thickness at the negative electrode up to rates of 4-C. At rates of 6-C and greater the surface film altered chemically. Cell vibration was found to cause the selectively formed film to be replaced with electrolyte reduction products, increasing cell resistance. Subjecting cells to different temperatures and states of charge (SoC) caused different films to form at each temperature. Coupled with electrical performance data, this could be reduced to two. One at 10o C and one at 45o C. SoC was also found to accelerate film formation but not chemically alter it at these two temperatures. Problems with the USABC test for a percentage change in state of charge for lithium-ion cells was identified, but these problems stopped surface film analysis. This work identifies what conditions impact cell performance and their effect on the negative electrode surface film. Changes in the surface film have significant implications on the users of electric-vehicles, most especially the range of the vehicle battery and how that reduces over its lifetime. Such information may directly impact the vehicle warranty, battery size and type of accelerated testing performed to predict cell lifetime. All of these factors represent considerable costs to manufacturers of electric vehicles. Accuracy is therefore of critical importance.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (D.Eng.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TK Electrical engineering. Electronics Nuclear engineering