Towards rechargeable calcium-ion batteries : insertion chemistry and electrode stability

Abstract

Calcium-ion batteries (CIBs) have been considered promising multivalent ion battery systems due to the natural abundance and low redox potential of calcium (Ca). However, their practical realization is largely hampered by the lack of reliable electrode materials. The large radius and high charge density of Ca2+ commonly lead to sluggish diffusion kinetics, resulting in an inferior capacity and rate capability. This thesis aims to design high-performance electrodes and probe their reaction mechanisms for rechargeable CIBs. We first explore selenium as a new conversion-type electrode for both non-aqueous and aqueous CIBs. It provides a high specific capacity of 476 mAh g-1 with an average voltage of 2.2 V vs. Ca/Ca2+, offering a higher energy density than other reported cathode materials. The spectroscopy analysis and density functional theory calculations suggest multi-step conversion processes involving CaSe4 and Ca2Se5 polyselenides intermediates before reaching the final CaSe phase, exhibiting a distinct reaction pathway from those in other metal-Se batteries. Besides the inorganic Se electrode, the organic compound is explored due to its advantages of structure flexibility and unique ion coordination mechanism. Specifically, polytriphenylamine (PTPAn) material is developed as an ultra-stable and high-rate cathode, residing in the reversible combination/release of anions with a C-N bond. As a result, the graphite|PTPAn full cell exhibits superior stability of over 2000 cycles with extremely fast kinetics up to 50 C rate in non-aqueous electrolyte. Interestingly, PTPAn is also highly compatible with aqueous electrolytes, allowing the construction of an all-organic aqueous calcium-based dual ion battery by coupling with a 3,4,9,10-perylene-tetracarboxylic-diimide (PTCDI) anode. We further investigate organic PTCDI anode’s reaction mechanisms since PTCDI as a counter electrode shows a distinct electrochemical behavior in non-aqueous and aqueous electrolytes. It delivers an attractive specific capacity of 113 mAh g-1 at 0.2 A g-1 and capacity retention of 92% at 2 A g-1 in aqueous electrolyte, compared to 68 mAh g-1 and 74% in non-aqueous counterpart. The combined spectroscopic analysis and theoretical simulations reveal that such difference originates from the synergistic proton and Ca2+ insertion in the former. The co-storage strategy is also observed in the Mg-ion system, thus allowing the stable cycling of 1.1 V-class aqueous Mg-/Ca-ion full cells for over 2000 cycles.