Periodic Reporting for period 1 - ISOBEL (In Situ Observation of Batteries for Extending Lifetime)
Periodo di rendicontazione: 2021-08-01 al 2023-07-31
For the transition of society from being dependent on fossil fuels towards increased use of renewable energy sources, lithium-ion batteries are a key enabling technology. Despite being already widely used in portable electronics and the quickly growing market of plugin-hybrid and full-electric vehicles, further spreading of the technology requires batteries with longer life-time, lower prices, as well as higher energy densities. A considerable portion of the chemical reactions leading to degradation and eventual failure of the cells take place at the interfaces between the electrolyte and the electrodes. It has been found that especially nickel-rich cathode materials are more severely affected by the unwanted reactions with the electrolyte. However detecting these species are difficult due to the very reactive nature of these interfaces, where the surface can react with impurities even in Ar-filled gloveboxes. This project therefore targets developing x-ray based spectroscopic techniques to measure these interfaces without disassembling the cell exposing it to contaminants. Due to the presence of rare elements inside the batteries as well as the energetic processes required to make batteries from raw materials it is especially important to find new ways to recycle batteries where the active material can be recovered and restored to its pristine state before being used in new batteries.
The overall objectives of this project are to:
-Find out why the unwanted side reactions become more severe for higher nickel content electrodes.
-Develop in situ/operando techniques using x-rays to identify the species formed on the battery electrodes.
-Improve the recycling of the nickel-rich cathode material.
The overall objectives of this project are to:
-Find out why the unwanted side reactions become more severe for higher nickel content electrodes.
-Develop in situ/operando techniques using x-rays to identify the species formed on the battery electrodes.
-Improve the recycling of the nickel-rich cathode material.
The work performed for this project was carried out at the Department of Materials at the University of Oxford (UK). The focus has been on investigating how interfaces in battery cells behave during electrochemical cycling. X-ray spectroscopy together with modelling of the spectra as well as modelling of the electrochemistry have been used to study the side reactions taking place at the electrodes. This has included a study where the changes in degradation throughout the lifetime of the cells were investigated. It was found that the main source of capacity loss during the first 200 cycles in NMC811-graphite cells came from side reactions at the graphite electrode. However the side reactions cause a slippage of the potential profiles i.e. the graphite potential profile is shifted relative to the NMC’s so that the potential of both the NMC and the graphite becomes larger at end of charge. The increase in NMC electrode potential contribute to further degradation due to active material loss, transition metal dissolution as well as the reduction of transition metals at the NMC surface. The transition metal reduction is likely connected to the increase in impedance. Also the incorporation of transition metals into the SEI increases with cycle number and the ratio of transition metals does not follow the NMC stoichiometry, which has previously been assumed. Instead the Mn is much more prevalent during the initial cycles, but with the ratio of Ni increasing with cycle number. As the transition metals are incorporated into the SEI, the chemical surrounding around Ni changes.
Operando soft x-ray XAS was used to detect the SEI formation process on amorphous Si electrodes as a function of electrode potential using both a standard LP30 electrolyte and an LP30 electrolyte also containing the fluoroethylene carbonate FEC additive. The cells used in the study contained a silicon nitride membrane (100 nm thick) coated by a 20 nm Ni coating which in turn was coated with amorphous Si, which was thin enough to allow x-ray transparency. By using a modulated x-ray beam together with a lock in amplifier the total electron yield (TEY) could be extracted while the cell was held at an applied potential. It was possible to detect the formation of LiF and organic species as the electrode was lithiated.
NMC cathodes containing different concentrations of Ni were investigated how they reacted with common electrolyte solvents. It was found that the amount of lattice oxygen loss from the NMC depended on both the Ni concentration and the electrolyte solvent. Ni-rich NMCs were more prone to lattice oxygen release, especially in contact with ethylene carbonate (EC). The increased lattice oxygen release was found to be connected to higher cathode impedance, electrolyte decomposition and transition metal dissolution.
Direct recycling of NMC cathodes have been attempted through cell disassembly, dissolution of the cathode material in acetone followed by filtering and drying befor annealing the material together with lithium carbonate in a tube furnace under flowing oxygen atmosphere. After the recycling process the surface of the NMC particles remained reduced, indicating further steps are required for efficient recycling.
The reaction of NMC with various gasses was investigated to find out how the electrodes are affected by air exposure. The electrode material was sealed in pouches into which CO2 and or H2O was injected to allow differentiation of these with low level of contaminations. It was found that the NMC surface had little reactions with H2O, although the Ni became reduced. CO2 on the other hand caused more severe reactions leading to the formation of carbonates and the combination of H2O and CO2 lead to even more reactions. Photoemission electron microscopy was used to correlate the formation of carbonates with Ni reduction.
Operando soft x-ray XAS was used to detect the SEI formation process on amorphous Si electrodes as a function of electrode potential using both a standard LP30 electrolyte and an LP30 electrolyte also containing the fluoroethylene carbonate FEC additive. The cells used in the study contained a silicon nitride membrane (100 nm thick) coated by a 20 nm Ni coating which in turn was coated with amorphous Si, which was thin enough to allow x-ray transparency. By using a modulated x-ray beam together with a lock in amplifier the total electron yield (TEY) could be extracted while the cell was held at an applied potential. It was possible to detect the formation of LiF and organic species as the electrode was lithiated.
NMC cathodes containing different concentrations of Ni were investigated how they reacted with common electrolyte solvents. It was found that the amount of lattice oxygen loss from the NMC depended on both the Ni concentration and the electrolyte solvent. Ni-rich NMCs were more prone to lattice oxygen release, especially in contact with ethylene carbonate (EC). The increased lattice oxygen release was found to be connected to higher cathode impedance, electrolyte decomposition and transition metal dissolution.
Direct recycling of NMC cathodes have been attempted through cell disassembly, dissolution of the cathode material in acetone followed by filtering and drying befor annealing the material together with lithium carbonate in a tube furnace under flowing oxygen atmosphere. After the recycling process the surface of the NMC particles remained reduced, indicating further steps are required for efficient recycling.
The reaction of NMC with various gasses was investigated to find out how the electrodes are affected by air exposure. The electrode material was sealed in pouches into which CO2 and or H2O was injected to allow differentiation of these with low level of contaminations. It was found that the NMC surface had little reactions with H2O, although the Ni became reduced. CO2 on the other hand caused more severe reactions leading to the formation of carbonates and the combination of H2O and CO2 lead to even more reactions. Photoemission electron microscopy was used to correlate the formation of carbonates with Ni reduction.
The studies of x-ray photoelectron spectroscopy of electrode interfaces provide new understanding of how the cross talk reactions in lithium ion batteries work. Where it was shown how the formation of degradation products at the graphite lead to increased potential of the NMC electrodes, which in turn lead to more transition metal dissolution and reduction of the NMC surface increasing the cell impedance. They thus provide new insights into the processes of the mechanisms causing degradation in electrode materials currently used in commercial lithium-ion batteries, informing efforts to achieve a longer cycle lifetime in Ni-rich NMCs
To the author’s knowledge the first operando XAS total electron yield measurements of amorphous Si electrodes during cycling were performed. It has been demonstrated that this technique can detect the SEI formation process using a liquid electrolyte, both with and without additives. This makes it possible to measure the surface evolution on electrodes without interference form contaminations taking place during cell disassembly. The enhanced understanding of electrode interfaces obtained using this technique will benefit industries important to developing a sustainable society such as electrochemical energy storage.
The electrode interfaces of NMC electrodes using different Ni content in contact with common electrolyte solvents were measured. From this study it was found how ethylene carbonate (EC) reacts more with the electrodes than linear carbonates and that the problem is especially severe at Ni-rich electrodes. Since the recent development in commercial electrodes is to increase the Ni content, this highlight the importance of protecting the NMC surface e.g. through doping or surface coatings to achieve long cycle life.
To the author’s knowledge the first operando XAS total electron yield measurements of amorphous Si electrodes during cycling were performed. It has been demonstrated that this technique can detect the SEI formation process using a liquid electrolyte, both with and without additives. This makes it possible to measure the surface evolution on electrodes without interference form contaminations taking place during cell disassembly. The enhanced understanding of electrode interfaces obtained using this technique will benefit industries important to developing a sustainable society such as electrochemical energy storage.
The electrode interfaces of NMC electrodes using different Ni content in contact with common electrolyte solvents were measured. From this study it was found how ethylene carbonate (EC) reacts more with the electrodes than linear carbonates and that the problem is especially severe at Ni-rich electrodes. Since the recent development in commercial electrodes is to increase the Ni content, this highlight the importance of protecting the NMC surface e.g. through doping or surface coatings to achieve long cycle life.