Synthesis and Characterization of Co-free Ni-rich Single Crystalline Positive Electrode Materials for Li-ion Batteries
To lower the cost and increase the energy density of Li-ion batteries, a common approach is to use positive active electrode materials that are higher in Ni and lower in Co than currently used. However, a set of challenges face the use of Ni-rich materials, including poor cycling lifetime, low thermal stability and sensitivity to ambient atmosphere. Some of these challenges, such as poor cycling lifetime, are believed to stem from volume changes that exert anisotropic stress within a polycrystalline particle. Therefore, this thesis focuses on the development of Co-free Ni-rich single crystalline positive electrode materials. The thesis first systematically studies the impact of Mg substitution in various Ni-rich compositions. The study demonstrates that Co-free materials can have comparable performance to materials with Co, but the compositional study supports that capacity retention is correlated with capacity and the resulting volume changes. The thesis then focuses on the development of Co-free Ni-rich single crystalline materials containing either Al or Mg. The synthesis of single crystalline materials is investigated using either the one-step or two-step lithiation method. These studies help achieve understanding of several factors that impact grain growth and the trade-offs of parameters such as heating temperature. This thesis shows that the synthesis of Co-free Ni-rich SC materials that meet physical specification targets is achievable via various synthesis routes. However, the electrochemical performance of the synthesized SC materials is subpar, and this has been shown to mainly be a Li diffusion issue. It is expected that the rate capability and capacity of Co-free Ni-rich SC materials will be inherently limited by the large primary particles. Advances overcoming these limitations will be needed before Co-free Ni-rich SC materials become viable.