Abstract
Most rechargeable ion batteries employ transition metal oxides or phosphates as the positive electrode. To facilitate facile migration of the active ions (e.g. Li- or Na-ions), which to some extent governs the battery functionality, the electrodes are typically composed of crystalline materials, wherein the ions are intercalated via well-defined migration pathways. However, the electrode materials are rarely perfectly crystalline and will inherently contain some disorder, which may originate from the material preparation process or be induced by the ion-intercalation process. In some electrode materials the electrochemical performance is damaged by disorder, whereas in other cases good performance is retained even after severe order–disorder transitions. This agrees with the emergence of several ab origine disordered or amorphous oxide-based electrodes with promising electrochemical performance. The term disorder is spanning a wide variety of deviations from an ideal crystal periodicity, from classical defects such as point defects, vacancies, stacking faults etc., to the amorphous state. Disorder, beyond classical defects, in battery electrodes has previously been largely overlooked, and we know little about the nature of the disorder and how it affects the battery performance. Developments in methods for characterisation of local atomic structures now allow us to gain detailed structural knowledge on the disordered part of the electrodes and studies within this field are emerging. This perspective provides a summary of the state-of-the-art within this field and the tendencies we are beginning to see outlined. These will be illustrated through selected examples. Finally, we discuss the key research questions within the field of disorder in electrode materials and the perspectives of answering these.
Highlights
The intercalation-type rechargeable ion-battery is dependent on two types of functional materials: the electrodes and the electrolyte
Any further distribution governs the battery functionality, the electrodes are typically composed of crystalline materials, of this work must maintain attribution to wherein the ions are intercalated via well-defined migration pathways
Three types of materials have been utilised commercially: phospho-olivines (e.g. LiFePO4) [19,20,21,22], spinel-like metal oxides (e.g. LiMnO4, LiNi0.5Mn1.5O4) [23, 24] and layered metal oxides with α-NaFeO2 structure (e.g. LiCoO2, LiNixMnyCo1−x−yO2) [3, 25,26,27]. The latter type constitutes >70% of the Li-ion battery market, while the other two more or less fill the remaining share [28]. These cathode materials are all based on cubic close packed oxygen lattices with the transition metals sitting in the octahedral sites and lithium ions in the tetrahedral or octahedral sites
Summary
Traditional electrode materials for intercalation-type batteries are highly crystalline compounds with well-defined pathways for ion diffusion [4, 17]. Three types of materials have been utilised commercially: phospho-olivines (e.g. LiFePO4) [19,20,21,22], spinel-like metal oxides (e.g. LiMnO4, LiNi0.5Mn1.5O4) [23, 24] and layered metal oxides with α-NaFeO2 structure (e.g. LiCoO2, LiNixMnyCo1−x−yO2) [3, 25,26,27]. The latter type constitutes >70% of the Li-ion battery market, while the other two more or less fill the remaining share [28]. From these realisations a highly relevant question springs to mind: what if the electrodes are not perfectly crystalline?
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