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N3.05 - Surface Modified Lithiated H2V3O8: A Stable Vanadate for Cathodic Application in Lithium-Ion Batteries Containing LiPF6 Electrolytes 
Date/Time:
April 22, 2014   2:45pm - 3:00pm
 
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H2V3O8 is a promising cathode material for Li-ion batteries, allowing intercalation of up to four lithium equivalents, ca. 400 Ah kg-1, between 4.2 V and 1.5 V vs. Li/Li+ with a mean potential close to 2.7 V, leading to a specific energy density above 1 kWh kg-1. However, poor stability under cycling still hinders its dissemination, especially when LiPF6 based electrolytes are used. Recently, Wu et al. [1] revealed that traces of water present in the battery electro active core can lead to HF formation, reacting with V2O5 cathodes through a self-catalyzed process, both at open circuit and under potential. The formation of vanadium oxyfluorides could be responsible for the severe capacity fade under charge/discharge cycles.Aiming to hinder side reactions between H2V3O8 surface and the electrolyte, a process was developed to deposit aluminum oxyhydroxide on the surface of lithiated H2V3O8 at low temperature. The deposition was performed by a one pot multistep reaction starting with phase pure H2V3O8, following by a chemical lithiation, in which the concentration of vanadium (IV) in the structure increases with the lithiation extent, and subsequent surface coating with an aluminum oxyhydroxide. Electro active materials with nominal aluminum concentrations ranging from 0.5 wt.% to 3 wt.% were prepared and characterized by several techniques (TEM, SEM/EDX, He-ion microscopy, XRD, XPS, TGA-DSC and electrochemically).Physicochemical analysis revealed that the crystal structure of LixH2-xV3O8 was unaffected by the presence of AlOx(OH)y and the later one most likely has an amorphous structure. A homogeneous surface coverage with 3 to 10 µm thickness was measured by TEM on the 3 wt.% Al sample, while for low Al contents no clear coverage could be detected by imaging. EDX and XPS results confirmed the presence of aluminum in all samples. Electrochemical characterization in half cells assembled with 1.5 wt.% Al coated sample showed higher stability for Li-ion intercalation after 200 cycles compared to bare LixH2-xV3O8. For instance, after 200 charge and discharge cycles between 4.05 and 2.2 V vs. Li/Li+ at a rate of 100 A kg-1, LixH2-xV3O8 coated with a nominal Al concentration of 1.5 wt.% shows a capacity retention of 89% compared to 67% of the uncoated LixH2-xV3O8, allowing for a specific energy higher that 0.5 Wh kg-1 for more than 200 cycles.[1] - J. Wu, N. Membreno, W.-Y. Yu, J. D. Wiggins-Camacho, D. W. Flaherty, C. B. Mullins and K. J. Stevenson, J. Phys. Chem. C 2012, 116, 21208
 


 
 
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