Positive electrode materials for high energy rechargeable batteries

Abstract

With the growth of environmental concerns, rechargeable batteries – lithium ion batteries (LIBs) and sodium ion batteries (SIBs) - have been employed in a large number of different applications. To meet the market needs in terms of their performance, positive electrode materials with high energy density are in high demand.

The aim of this thesis work is to provide strategies which enhance electrochemical performance of LiCoPO₄ as a high voltage positive electrode material for LIBs (chapter 3 and chapter 4) and an insight into the mechanism which triggers oxygen redox activity of P3-type Na₀.₆₇M₀.₂Mn₀.₈O₂ (M= Mg and Ni in chapter 5 and 6, respectively) as potential candidates for high capacity positive electrode materials in SIBs.

Studies on the improvement of cyclability of LiCoPO₄ were carried out using aqueous binders, of which, sodium carboxymethyl cellulose (CMC) permits more stable cycling performance and better rate capability with respect to the conventional organic solvent-soluble binders. In addition, substitution of magnesium for cobalt was investigated, which demonstrates that doping with magnesium can be one of the solutions to obtain stable capacity on extended cycling.

Both P3-type Na₀.₆₇Mg₀.₂Mn₀.₈O₂ and Na₀.₆Ni₀.₂Mn₀.₈O₂ were synthesised by a co-precipitation method and studied to understand the origin of abnormal capacity on the first charge. Careful electrochemical and structural characterisation combined with bulk and surface spectroscopic techniques (XAS, XPS) reveal the oxygen redox activity in Na0.67Mg0.2Mn0.8O2. As a consequence of vacancies in the transition metal layers of Na₀.₆₇Mg₀.₂Mn₀.₈O₂ prepared under oxygen, reversible oxygen redox is enhanced. Subsequently, substitution of nickel for manganese was carried out to increase capacity using the Ni²⁺/Ni⁴⁺ redox couple of nickel. The presence of oxygen redox activity in Na₀.₆Ni₀.₂Mn₀.₈O₂ is also demonstrated by using a range of spectroscopic techniques (XAS, SXAS, RIXS), which is stabilised by reduction of nickel through the reductive coupling mechanism.