Electrochemical Energy Reviews
toward cathode degradation. Alternatively, graphite anodes
can function collaboratively with NMC-based cathodes
to provide promising performances at cell levels. How-
ever, the aging of graphite anodes due to the formation
of dendrites as caused by the massive plating of Li+ onto
graphite surfaces remains the main reason for increased
impedance. Furthermore, side reactions involving elec-
trolyte reduction (SEI formation) on graphite anodes can
cause Li loss after initial charging, and the thickening and
decomposition of SEIs in subsequent cycles can cause
aging behaviors.
To gain a deeper understanding of degradation mecha-
nisms of Ni-rich NMC/graphite LIBs, emerging testing
protocols and characterization methods have emerged in
which currently, research on LIB degradation is focused on
the elucidation of the degree of capacity fading in certain
mechanisms so as to distinguish major factors and allow
for the design of specifc remedies. And as a result of this
research, mitigation strategies, functioning individually or
synergistically, have been proposed to resolve long-term
cyclability issues, most of which have proven to be promis-
ing. Among these, dopants, gradient layers, coatings (both
Li-free and Li-contained), carbon matrixes and advanced
synthesis procedures have become mainstream methods to
enhance the performance of Ni-rich NMC-based cathodes.
In addition, coatings, advanced protocols and the use of new
electrolyte components can be applied to ameliorate anodic
performances (SEIs and graphite). And with these mitiga-
tion strategies, the morphology, element composition and
reactivity of electrodes can be improved.
Fig. 14 Comparison of a Coulombic efciency versus cycle number
and b the total sum of reversibly cycled Li over 50 cycles obtained
from LiFePO4/Cu cells (Reprinted with permission from Ref. [224].
Copyright © 2018, The Royal Society of Chemistry)
Although differences in the performance of various
Ni-rich NMC-based cathodes have been discovered, these
remain unexplained and the future of degradation mecha-
nism investigations needs to rely on more advanced physi-
cal techniques such as X-ray tomography, operando neutron
difraction and online electrochemical mass spectroscopy,
all of which can provide more comprehensive information
in operando and be less destructive to cells. And for the
deeper understanding of mitigation strategies for Ni-rich
NMC/graphite LIBs, trade-ofs in modifcation techniques
and controversies need to be taken into considerations. Here,
several prospective research directions for Ni-rich NMC/
graphite LIBs are proposed:
considered [219]. In addition, ionic liquids as substituents
capacity retentions at 90 °C [226].
5 Summary and Prospective
LIBs using Ni-rich NMC-based cathodes such as NMC811
can produce high specifc capacities (200–220 mAh g−1)
and promising energy densities (~ 800 Wh kg−1) in com-
parison with conventional LiCoO2 (~ 570 Wh kg−1) and
LiMn2O4 spinel (~ 440 Wh kg−1) materials. Because of
this, these materials have received extensive attention from
researchers. However, due to low portions of manganese
as structural stabilizers, large amounts of Ni4+ on cath-
ode surface layers/regions can trigger side reactions and
Ni2+ can cause cation mixing. As a result, enhanced mass-
specifc capacity comes at the expense of rate capability
and structural stability in these Ni-rich cathode materials,
leading to severe capacity fading. In addition, other factors
such as active material dissolution, oxygen release and the
intergranular cracking of primary particles also contribute
1. Future experiments should focus more on the quantita-
tive interpretation of diferent aging behaviors in NMC-
based cathodes with or without mitigation methods as
well as the understanding of correlations between dif-
ferent degradation mechanisms.
2. Eforts should be made toward profling the performance
of LIBs under harsher conditions (stressors) that more
closely represent practical operational conditions.
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