New Electrode Materials with Novel Working Mechanisms for Lithium/Sodium-Ion Batteries

Abstract

These large-scale applications will place enormous demands on both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), requiring the development of new electrodes with higher energy and power densities, improved safety, longer life, lower cost and reduced environmental impact. This thesis explores several new promising electrodes for these two types of batteries. Na4Ni7(PO4)6 is identified as a candidate positive electrode for SIBs. Its complex Na conduction properties are experimentally and computationally investigated. Nudged elastic band calculations suggest a “two-ion ring exchange” Na-conduction mechanism with an energy barrier of 0.05-0.26 eV. Its complex low-temperature magnetic behaviour is also investigated, revealing three successive long-range spin-ordering transitions. Polyanion-type transition metal metaphosphates are examined for use as electrodes in both LIBs and SIBs. Co(PO3)2, Fe(PO3)2 and Fe(PO3)3 can reversibly react with both Li and Na through conversion reactions at a voltage lower than 1.0 V, and are investigated as negative electrodes for both LIBs and SIBs. Mn(PO3)2 and Ni(PO3)2 only work in LIBs, the former as a negative electrode and the latter as a positive electrode. The conversion reactions of these metaphosphates with Li/Na are systematically studied using a combination of in situ x-ray powder diffraction, in/ex situ x-ray absorption spectroscopy, and high-resolution transmission electron microscopy. In the 1st discharge, the crystalline metaphosphate is electrochemically activated through a Li/Na-driven amorphisation process accompanied by the reduction of the transition metal ion to the metallic state. A fine microstructure consisting of metal nanodomains embedded into a Li-/NaPO3 glass matrix forms in the electrode after the 1st lithiation. In the following 1st charge, the electrode converts back to a metaphosphate in terms of its composition but does not recrystallise.