Understanding the structural and electrochemical behavior of high energy density layered oxide cathodes for sodium and lithium ion batteries

Thesis

The world’s crucial need for energy storage has provoked intense interest in cheaper, more sustainable battery chemistries, such as Na-ion. One of the main bottlenecks toward the utilization of Na-ion batteries, however, is the lack of an optimal cathode material. In this thesis, the class of cathode compounds known as layered transition metal oxides are investigated because of their ability to reversibly intercalate (insert/extract) a relatively large quantity of Na and Li-ions (i.e. high capacity).

P2-Na_2/3[Ni_1/3Mn_2/3]O₂ is first investigated to probe its utility as a cathode but is demonstrated to possess certain drawbacks including Na deficiency in the as-synthesized state, unsatisfactory cyclability, and a strong driving force for Na-ion ordering. These obstacles are shown to be sufficiently overcome by means of partial metal substitution of Fe as well as a new pre-treatment process using Na-naphthalide to intercalate extra Na-ions into the pristine structure.

Attention is then turned to the mechanism by which partial Fe substitution improves cyclability. It is found that this enhancement largely stems from an altered structural transition at high voltages (i.e. "Z"-phase) resulting in less overall volume changes. The true structural identity of the "Z"-phase is then resolved for the first time as an evolving P2:O2 intergrowth.

Finally, the possibility of using oxygen (instead of heavier transition metals) as a redox center is examined for a collection of layered transition metal oxide materials. It is discovered that oxygen-redox can be activated in the presence of Mg in P2-Na_2/3[Mg_0.28Mn_0.72]O₂ and is reversible over hundreds of cycles in O3- Li[Li_1/5Ni_1/5Mn_3/5]O₂. Moreover, through investigation of P2-Na_3y[Li_yMn_1−y]O₂ (y = 1/5, 1/4), it is determined that the discharge voltage of oxygen-redox compounds strongly depends on cation ordering within the transition metal layer.

In total, the aim of this thesis is to communicate new fundamental insight into the nature of layered transition metal oxide cathodes in order to provide a platform for their further development.

Authors: Somerville, JW

• DOI: 10.5287/ora-dqnp9de05
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Sodium Ion battery; Na-ion battery; Layered transition metal oxide; Lithium Ion Battery; Operando XRD; Cathode; Li-ion battery; High voltage; Battery; Z-phase; Oxygen Redox; P2
• Subjects: Materials science