Developing high-energy cathodes for Li-ion batteries based on Ni-rich layered oxides
Abstract
Energy storage in rechargeable batteries is essential for implementing more renewable energy into the global energy mix. Batteries can, for instance, be connected to solar or wind energy systems to handle the intermittency between generation and consumption of electric energy. They are also critical in electric vehicles, providing an alternative to combustion engines. Li-ion batteries have the highest energy densities among the available rechargeable battery technologies, making them the batteries of choice.
Ni-rich layered oxides are a class of high-energy cathode active materials that can deliver high capacities due to their high Ni-contents. However, they have limited cycling and thermal stabilities. LiNixMnyCozO2 (NMC) and LNixCoyAlzO2 (NCA) are two of the most familiar Ni-rich layered oxides, and they dominate the electric vehicle market. The trend towards higher Nicontents is driven by the desire to increase the capacity and reduce the Co-content, but this substitution has negative impacts on the cycle life and thermal stability. Common strategies to mitigate this are to add dopants to the bulk structure or to apply protective coatings to the surface.
The research related to this thesis aimed to develop high-energy cathode materials for Li-ion batteries based on Ni-rich layered oxides with high capacity, cycling stability and thermal stability. The work focused on developing NMC layered oxides with ≥85 at% Ni through bulk material synthesis and surface modifications at a laboratory scale. A considerable emphasis was to establish a good baseline Ni-rich layered oxide material since the bulk material will dictate the overall performance. Surface modifications were implemented to stabilise reactive surfaces by creating a protective barrier. The developed materials, with and without surface modifications, were evaluated by various physicochemical techniques, and their electrochemical performances were assessed in batteries. Post-mortem characterisation was also used to understand possible material degradation mechanisms. The results were reported in three manuscripts. A single-pot oxalic acid co-precipitation synthesis route for Ni-rich LiNi0.88Mn0.06Co0.06O2 was developed in the first project. Al-doped chemical variations were explored, and synthesis parameters were optimised to produce a high-quality baseline material. Although phase pure and well-ordered materials were achieved for all chemical variations, 2 at% Al-doping did not improve the electrochemical properties. Tuning the heat treatment temperature and amount of added Li precursor yielded a well-ordered Ni-rich layered oxide material with good capacity and high cycling stability without additional surface modifications.
The developed LiNi0.88Mn0.06Co0.06O2 material was surface-modified with an octadecyl phosphonic acid coupling agent through a wet-chemical route as part of the second stage of the project. The octadecyl phosphonic acid coating did improve the cycling stability, although the initial capacity was slightly reduced. Post-mortem investigations suggest that less LiF, an electrolyte decomposition product, is formed on the coated surface. Further heat treatments of the coated material were evaluated and did modify the hydrocarbon part of the coupling agent, but the electrochemical performance was worsened.
In the third project, Ni-rich LiNi0.8Mn0.1Co0.1O2 was coated with a cathode active material with the dual purpose of protecting the surface and providing additional capacity as compared to an inactive coating. The Ni-rich layered oxide was dry-coated with 2 wt% and 4 wt% LiFePO4. LiFePO4 is an established cathode active material with high cycling and thermal stability compared to the Ni-rich layered oxides. Cross-sectional SEM and EDS confirmed the coating, and battery testing revealed that the coating was electrochemically active. However, the coated materials did not show improved cycling or thermal stability. It is discussed whether increasing the coating thickness or altering the electrode fabrication process would be favourable for stabilising the Nirich layered oxide material.
Has parts
Paper 1: Pollen, Harald Norrud; Tolchard, Julian R; Svensson, Ann Mari; Wagner, Nils Peter. A Single-Pot Co-Precipitation Synthesis Route for Ni-Rich Layered Oxide Materials with High Cycling Stability. ChemElectroChem 2022 ;Volum 9.(19) s. 1-12. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License CC BY-NC. Available at: http://dx.doi.org/10.1002/celc.202200859Paper 2: Pollen, Harald Norrud; Nylund, Inger-Emma; Dahl, Øystein; Svensson, Ann Mari; Brandell, Daniel; Younesi, Reza; Tolchard, Julian R; Wagner, Nils Peter. Interphase Engineering of LiNi0.88Mn0.06Co0.06O2 Cathodes Using Octadecyl Phosphonic Acid Coupling Agents. ACS Applied Energy Materials 2023 ;Volum 6.(23) s. 12032-12042. This publication is Open Access under the license CC BY. Available at: http://dx.doi.org/10.1021/acsaem.3c02275
Paper 3: Pollen, Harald Norrud; Tolchard, Julian R; Din, Mir Mehraj Du; Svensson, Ann Mari; Rettenwander, Daniel; Svensson, Ann Mari; Wagner, Nils Peter. Dry-coated LiNi0.8Mn0.1Co0.1O2-LiFePO4 materials. This paper is submitted for publication and is therefore not included.