Surface
modification is usually an effective strategy to improve
the cycling stability and rate capability of the Li-rich layered oxide
cathode materials. Herein, the high-crystallinity LaPO4 with good ionic conductivity was homogeneously deposited on the
surface of Li-rich layered oxide by the slow formation of LaPO4 nanoparticles because of chelating effect between citric
acid and La3+ as well as the using of appropriate phosphorus
source. The surface structure and electrochemical properties of Li-rich
Mn-based materials were characterized by X-ray diffraction, scanning
electron microscopy, high-resolution transmission electron microscopy
(HRTEM), and galvanostatic charge/discharge tests. The results indicate
that LaPO4 nanoparticles are homogeneously coated on the
surface of Li-rich layered Mn-based oxide, and the modification of
LaPO4 with appropriate nanoscale thickness can obviously
promote the cycling stability and rate capability of cathode material.
Especially, the material modified by 2 wt % LaPO4 shows
an optimum cycling stability with capacity retention of 83.2% after
200 cycles at 1 C, the best structure stability, and delivers a discharge
capacity of 146.2 mAh g–1 even at a high current
density of 10 C.
The LiNi1–x–y
Co
x
Al
y
O2 (NCA)-layered
materials are regarded as a research
focus of power lithium-ion batteries (LIBs) because of their high
capacity. However, NCA materials are still up against the defects
of cation mixing and surface erosion of electrolytes. Herein, a novel
design strategy is proposed to obtain a heterostructured cathode material
with a high-capacity LiNi0.88Co0.09Al0.03O2 layer (R3̅m) core and a stable LiNi0.5Mn1.5O4-like spinel (Fd3̅m) shell,
which is prepared through spontaneous redox reaction of the precursor
with KMnO4 in an alkaline solution and subsequent calcination
procedure. The structure, morphology, element distribution, and electrochemical
performances of the as-prepared NCA are studied by scanning electron
microscopy, transmission electron microscopy, X-ray diffraction, X-ray
photoelectron spectroscopy, and electrochemical techniques. The results
show that the LiNi0.5Mn1.5O4-like
spinel (Fd3̅m) shell layer
with a robust cubic close-packed crystal structure is uniformly adhered
to the surface of the NCA and can availably suppress the side reactions
with the electrolyte and surface-phase transformation, which will
facilitate insertion/extraction of Li+ ions during cycling.
Benefiting from the enhanced structural stability and improved kinetics,
the heterostructured NCA delivers a better cycling performance. The
discharge specific capacity is as high as 153.7 mA h g–1 at 10 C, and even at high charge voltage of 4.5 V, the capacity
retention can still increase 11% at 1 C (200 mA g–1) after 100 cycles. Besides, the material exhibits a prominent thermal
stability of 248 °C at 4.3 V. Therefore, this novel structure
design strategy can contribute to the development and commercialization
of high-performance cathode materials for power LIBs.
Na 2 FeSiO 4 , as one of the promising cathode materials in sodium-ion batteries, has attracted great interests. However, studies on the kinetic behaviors of Na ions insertion/extraction in Na 2 FeSiO 4 composite electrode have been barely considered, until now. Importantly, the specific capacity and rate capability of Na 2 FeSiO 4 cathode materials are greatly correlated with the kinetics of Na + transfer in the host material. Herein, on the basis of the characterizations of microstructure and morphologies (i.e., Rietveld refinement, FESEM, HRTEM, etc.), the electrochemical kinetics of Na ions extraction in Na 2 FeSiO 4 /C electrode are first studied in detail via two electrochemical techniques (EIS and GITT), establishing the ratecontrolling steps of Na + transport in Na 2 FeSiO 4 /C, evaluating series of kinetic parameters, as well as calculating the Na + diffusion coefficient at various state-of-desodiation. Changes of impedance response of Na 2 FeSiO 4 /C electrode depending on the different levels of desodiation show that a serial features of electrode process for Na ions migration have tremendous discrepancies, indicating that the kinetics of Na + extraction from Na 2 FeSiO 4 /C electrode are largely influenced by different electrode reaction processes. These results provide useful insight into the inner properties of Na 2 FeSiO 4 /C electrode, and it is significant to optimize the electrochemical performance of Na 2 FeSiO 4 /C. Moreover, two models of equivalent circuits are also constructed to simulate the electrode processes and describe the behaviors of Na ions transfer in Na 2 FeSiO 4 /C.
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