Development of novel electrode materials for rechargeable batteries

Christian Henriksen

Research output: ThesisPh.D. thesis

Abstract

A small technological revolution was started when Sony launched their rechargeable battery in 1991. Since
then, the rechargeable battery has become a household item for almost everyone on the planet. This is because
the Li-ion technology vastly outperformed its rechargeable predecessors, i.e. the nickel cadmium and Ni-metalhydride,
in terms of voltage and specific energy density. Some of the most utilized battery chemistries for
commercial rechargeable Li-ion batteries have glaring issues. They have decreased safety due to low thermal
runaway temperatures and use costly low abundance elements such as Ni and Co whose price is not only
subject to supply-and-demand but also geopolitical issues. Battery chemistries of the future should be cheaper,
safer and more environmentally friendly while being tailored to the specific need of the application.
The aim of this project is to investigate how changing the intercalation ion (Li and Na), the electrochemically
active species (first row transition metals) and/or the framework component of crystalline phosphate-based
cathode materials affects the electrochemical performance and the structural electrochemically-driven phase
transformation during cycling.
It was also found that by varying the synthesis conditions, the morphology of Fe2–y

y(PO4)(OH)3–3y(H2O)3y–2
particles could be fine-tuned by controlling the pH, synthesis time and addition of surfactant in the
hydrothermal synthesis. This made it possible to tune the overpotential and improving the capacity by
reduction of the particle size. Through SR-PXRD, it was found that Fe2–y

y(PO4)(OH)3–3y(H2O)3y–2 undergoes
a complete solid solution phase transformation over the entire xLi and xNa compositional range.
Fe2–y

y(PO4)(OH)3–3y(H2O)3y–2 was found to perform well as a cathode material for both Li-ion and Na-ion
intercalation.
Chemically delithiated phospho-olivine MnyFe1–yPO4 (y = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8) was found to be a
very versatile cathode material as it exhibits great electrochemical performance during Na-ion intercalation at
low Mn substitution. 10 – 20 % Mn substitution of Fe in the phospho-olivine structure yields >15 % higher
capacities compared to Mn-free FePO4 against Na with Mn0.1Fe0.9PO4 exhibiting the highest capacity of 145
mAh g–1. Even with substitution of Fe by Mn, it was found that the Fe3+/Fe2+ redox couple can be split into
two steps with the formation of a NaxMnyFe1–yPO4 (x > y) intermediate as observed in Mn-free FePO4 vs. Na.
Operando SR-PXRD showed that low Mn substitution leads to lowering of the lattice misfit whereas higher
Mn substitution leads to higher lattice misfits revealing a capacity-misfit correlation for MnyFe1–yPO4 against
Na. Thus, the effect of substituting Mn into the phospho-olivine structure is stronger than that of introducing
the larger Na-ion.
Furthermore, mixed transitions metal phospho-olivines (LiMn0.5–0.5yFe0.5–0.5yMyPO4, M = first row transition
metal) was investigated as cathode materials, a combined PXRD-XRF-mapping study about aging in
commercial LiFePO4 was investigated and in another study a route for optimization of electrode performance
was devised.
Original languageEnglish
Awarding Institution
  • University of Southern Denmark
Supervisors/Advisors
  • Ravnsbæk, Dorthe Bomholdt, Principal supervisor
Place of PublicationOdense
Publisher
Publication statusPublished - 14. Oct 2019

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