The aqueous synthesis and electrochemical properties of nanocrystalline M x V 2 O 5 A y ' nH 2 O are described. It is easily and quickly prepared by precipitation from acidi5ed vanadate solutions. M x V 2 O 5 A y ' nH 2 O has been characterized by X-ray powder di4raction, electron microscopy, TGA, chemical analyses, and electrochemical studies. The atomic structure is related to that of xerogel-derived V 2 O 5 ' nH 2 O. In M x V 2 O 5 A y ' nH 2 O, M is a cation from the starting vanadate salt and A is an anion from the mineral acid. This material exhibits high, reversible Li capacity and may be considered for use in a cathode in primary and secondary batteries. The lithium capacity of an electrode composed of M x V 2 O 5 A y ' nH 2 O/EPDM/carbon (88/4/8) is &380 (mA h)/g (C/80 rate) and the energy density is &1000 (W h)/kg (120-m-thick cathode, 4+1.5 V, versus Li metal anode). Critical parameters identi5ed in the synthesis of M x V 2 O 5 A y ' nH 2 O, with respect to achieving high Li-ion insertion capacity, are acid/vanadium ratio, starting vanadate salt, and temperature. Inclusion of carbon black in the synthesis yields a composite that maintains the high Li capacity, lowers the electrochemical-cell polarization, and preserves the lithium capacity at higher discharge rates. Li-ion coin cells, using prelithiated graphite anodes, exhibit electrochemical performance comparable to that of Li-metal coin cells.