New battery cathode materials:: Synthesis, characterization, and electrochemical performance of M1-xV3O8-yFz•nH2(M = NH4, K)

The aqueous synthesis and electrochemical properties of the new nanocrystalline material M1-xV3O8-yFz.nH(2)O (M = NH4, K; x approximate to 0.0-0.2; gamma approximate to 0.0-0.2; z approximate to 0.1-0.2; n approximate to 1) are described. This material is easily prepared by precipitation from aqueous HF-acidified ammonium or potassium vanadate solutions. M1-xV3O8-yFz.nH(2)O has been characterized by X-ray powder diffraction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The proposed atomic structure is related to that of (NH4)(2)V6O16, with V6O16 layers separated by layers of hydrated ammonium cations. The M = NH4 material exhibits a high Li capacity with good reversibility within the first few charge-discharge cycles. The lithium capacity of an electrode composed of (NH4)(0.9)V3O7.9F0.1.-nH(2)O/EPDM/carbon (88/4/8) is 409 (mA h)/g (C/80 rate), and the energy density is similar to1000 (W h)/kg (128-mum-thick cathode, 4-1.5 V versus Li metal anode). The HF concentration, starting vanadate salt, and temperature are important parameters in the synthesis of M1-xV3O8-yF2.nH(2)O,with respect to achieving the optimum Li-ion capacity and phase purity. Precipitating M1-xV3O8-yFz.nH(2)O in the presence of carbon black gives a composite that maintains the Li capacity and lowers the electrochemical-cell polarization.