P2-structured Na
0.67
Ni
0.33
Mn
0.67
O
2
(PNNMO)
is a promising Na-ion battery cathode material,
but its rapid capacity decay during cycling remains a hurdle. Li doping
in layered transition-metal oxide (TMO) cathode materials is known
to enhance their electrochemical properties. Nevertheless, the influence
of Li at different locations in the structure has not been investigated.
Here, the crystallographic role and electrochemical impact of lithium
on different sites in PNNMO is investigated in Li
x
Na
0.67–
y
Ni
0.33
Mn
0.67
O
2+δ
(0.00 ≤
x
≤ 0.2,
y
= 0, 0.1). Lithium occupancy on
prismatic Na sites is promoted in Na-deficient (Na < 0.67) PNNMO,
evidenced by
ex situ
and operando synchrotron X-ray
diffraction, X-ray absorption spectroscopy, and
7
Li solid-state
nuclear magnetic resonance. Partial substitution of Na with Li leads
to enhanced stability and slightly increased specific capacity compared
to PNNMO. In contrast, when lithium is located primarily on octahedral
TM sites, capacity is increased but at the cost of stability.
As a promising alternative
to the market-leading lithium-ion batteries,
low-cost sodium-ion batteries (SIBs) are attractive for applications
such as large-scale electrical energy storage systems. The energy
density, cycling life, and rate performance of SIBs are fundamentally
dependent on dynamic physiochemical reactions, structural change,
and morphological evolution. Therefore, it is essential to holistically
understand SIBs reaction processes, degradation mechanisms, and thermal/mechanical
behaviors in complex working environments. The recent developments
of advanced in situ and operando characterization
enable the establishment of the structure–processing–property–performance
relationship in SIBs under operating conditions. This Review summarizes
significant recent progress in SIBs exploiting in situ and operando techniques based on X-ray and electron analyses at
different time and length scales. Through the combination of spectroscopy,
imaging, and diffraction, local and global changes in SIBs can be
elucidated for improving materials design. The fundamental principles
and state-of-the-art capabilities of different techniques are presented,
followed by elaborative discussions of major challenges and perspectives.
Multiphase layered transition metal oxides (LTMOs) for sodium ion battery (SIB) positive electrodes with phase interfaces across multiple length scales are a promising avenue toward practical, high-performance SIBs. Combinations of phases can complement each other's strengths and mitigate their weaknesses if their interfaces are carefully controlled. Intra-and interparticle phase interactions from nanoscale to macroscale must be carefully tuned to generate distinct effects on properties and performance. An informed design strategy must be paired with relevant synthesis techniques and complemented by spatially resolved characterization tools to manipulate different length scales and interfaces. This review examines the design, synthesis, and characterization strategies that have been demonstrated for the preparation of heterogeneous, multiphasic LTMOs with phase interfaces across varied length scales.
Layered
NaNi
x
Fe
y
Mn
z
O2 cathode (NFM) is of great interest
in sodium ion batteries because of its high theoretical capacity and
utilization of abundant, low-cost, environmentally friendly raw materials.
Nevertheless, there remains insufficient understanding on the concurrent
local environment evolution in each transition metal (TM) that largely
influences the reversibility of the cathode materials upon cycling.
In this work, we investigate the reversibility of TM ions in layered
NFMs with varying Fe contents and potential windows. Utilizing ex situ synchrotron X-ray absorption near-edge spectroscopy
and extended X-ray absorption fine structure of precycled samples,
the valence and bonding evolution of the TMs are elucidated. It is
found that Mn is electrochemically inactive, as indicated by the insignificant
change of Mn valence and the Mn–O bonding distance. Fe is electrochemically
inactive after the first five cycles. The Ni redox couple contributes
most of the charge compensation for NFMs. Ni redox is quite reversible
in the cathodes with less Fe content. However, the Ni redox couple
shows significant irreversibility with a high Fe content of 0.8. The
electrochemical reversibility of the NFM cathode becomes increasingly
enhanced with the decrease of either Fe content or with lower upper
charge cutoff potential.
Sodium ion batteries (SIBs) are attractive alternative energy storage technology to lithium-ion batteries due to its low-cost. There has been growing attention in developing new electrode materials for sodium ion batteries. Compared to lithium ion batteries, SIBs suffer from more issues in long-term stability, resulted from the sluggish kinetics, large volume change due to the much larger Na+ ion (~ two times the size of Li+) as well as multiple phase transitions upon cycling. Several approaches to enhance the electrode performance have been explored such as doping, nanostructuring, and carbon coating. Here, we will discuss our recent work on developing advanced electrode materials for SIBs through interfacial engineering. We have developed a coaxial core-shell nanostructured negative composite electrode composed of carbon nanotube (CNT) as the core and TiO2@MoO2@C as shells. The 1D tubular nanostructure can effectively reduce ion diffusion path, increase electrical conductivity, accommodate the stress due to volume change upon cycling, and provide additional interfacial active sites for enhanced charge storage and transport properties. Significantly, a synergistic effect between TiO2 and MoO2 nanostructures is investigated through ex-situ solid state nuclear magnetic resonance. We also developed multi-phased transition metel oxide positive electrodes, which exhibits enhanced capacity and cycling stability.
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