To
advance current Li rechargeable batteries further, tremendous
emphasis has been made on the development of anode materials with
higher capacities than the widely commercialized graphite. Some of
these anode materials exhibit capacities above the theoretical value
predicted based on conventional mechanisms of Li storage, namely insertion,
alloying, and conversion. In addition, in contrast to conventional
observations of loss upon cycling, the capacity has been found to
increase during repeated cycling in a significant number of cases.
As the internal environment in the battery is very complicated and
continuously changing, these abnormal charge storage behaviors are
caused by diverse reactions. In this review, we will introduce our
current understanding of reported reactions accounting for the extra
capacity. It includes formation/decomposition of electrolyte-derived
surface layer, the possibility of additional charge storage at sharp
interfaces between electronic and ionic sinks, redox reactions of
Li-containing species, unconventional activity of structural defects,
and metallic-cluster like Li storage. We will also discuss how the
changes in the anode can induce capacity increase upon cycling. With
this knowledge, new insights into possible strategies to effectively
and sustainably utilize these abnormal charge storage mechanisms to
produce vertical leaps in performance of anode materials will be laid
out.
Figure 4. a) Rietveld refined XRD patterns for disordered (top) and ordered (bottom) spinel. b) Raman and c) FTIR spectra of the disordered (Fd3m,black) and ordered spinel (P4 3 32, red) material. Reproducedf rom Refs. [35, 37] with
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