“Layered”/“cation-ordered”/O3-type
Li-TM-oxides (TM: transition metal) suffer from
structural instability due to “TM migration”
from the TM layer to the Li layer upon Li removal (viz., “cation disordering”). This phenomenon
gets exacerbated upon excessive Li removal, with Ni ions being particularly
prone to migration. When used as cathode material in Li-ion batteries,
the “TM migration” and associated structural
changes cause rapid impedance buildup and capacity fade, thus limiting
the cell voltages to ≤4.3 V for stable operation and lowering
the usable Li-storage capacity (concomitantly, energy density). Looking
closely at the “TM migration” pathway, one
realizes that the tetrahedral site (t-site) of the
Li layer forms an intermediate site. Accordingly, the present work
explores a new idea concerning suppression of “Ni migration”
by “blocking” the intermediate crystallographic site
(viz., the t-site) with a dopant,
which is the most stable at that site. In this regard, density functional
theory (DFT)-based simulations indicate that the concerned t-site is energetically the most preferred and stable site
for d
10 Zn2+. Detailed analysis
of crystallographic data (including bond valence sum) obtained with
the as-prepared Zn-doped Li-NMC supports the same. Furthermore, the
simulations also predict that Zn doping is likely to suppress “Ni
migration” upon Li removal. Supporting these predictions, galvanostatic
delithiation/lithiation studies with Zn-doped and undoped Li-NMCs
demonstrate significantly improved cyclic stability, near-complete
suppression of “cation mixing”, and negligible buildup
of impedance (as well as potential hysteresis) for the former, even
upon being subjected to long-term cycling using a high upper cut-off
potential of 4.7 V (vs Li/Li+). Accordingly, such subtle
tuning of the composition and structure, in the light of electronic
configuration of the dopant and specific crystallographic site occupancy,
is likely to pave the way toward the development of Ni-containing
stable high voltage O3-type Li-TM-oxide cathodes for the
next-generation Li-ion batteries.
NaxTMO2 type [TM: transition metal(s)] ‘layered’ oxides, having starting Na-content (x) ~1 (i.e., ‘O3’), are important as potential cathode materials for the upcoming Na-ion battery system. However, among other problems...
The present work proposes and establishes a universal
strategy
toward facilitating the development of desired structural types (viz., P-type vs O-type) of “layered” Na- transition
metal (TM) oxides, with the desired Na-content and properties.
In this regard, the structure type, allowable Na-content, and Na-layer/“inter-slab”
spacing have been found to depend on the “charge:size”
ratio of the TM-ions, concomitant electronegativity and
covalency of TM–O bonds, and the charge neutrality
aspect. Overall, increases in the average “charge:size”
ratio of the cation combination in the TM-layer and concomitant
TM–O bond covalency result in a lower effective
negative charge on the O-ions. This renders the prismatic coordination
of O-ions around the Na-ions more favorable even at a higher Na-content,
but with the latter needing some compromise over the charge neutrality
aspect. Accordingly, by careful selection of the combination of non-TM/TM-ions in the TM-layer, a high Na-containing
(viz., ∼0.84 per formula unit) P2-type Na0.84([]0.06Li0.04Mg0.02Ni0.22Mn0.66)O2 has been successfully developed
here, which, as a cathode material for Na-ion batteries, exhibits
a high desodiation capacity of ∼178 mAh/g (@ C/5; within 2–4
V vs Na/Na+), exceptional cyclic stability pertaining to
a ∼98% capacity retention after 500 galvanostatic desodiation/sodiation
cycles at a high current density (2.5C), and also stability upon exposure
to air/water. The suitable combination of a high Na-content and “charge:size”
ratio in the TM-layer of the as-developed P2-type Na-TM-oxide is again the factor responsible for the above properties/performances.
Furthermore, going with the proposed scientific basis, mere replacement
of Mn4+, having a higher “charge:size” ratio
(∼7.5 Å–1), with Ti4+, having
a lower “charge:size” ratio (∼6.5 Å–1), keeping everything else the same, has been found
to yield the O3-type structure.
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