Reversible Activation of V~(4+)/V~(5+) Redox Couples in NASICON Phosphate Cathodes

摘要

Na superionic conductor structured Na_3V_2(PO_4)_3 cathodes have attractedgreat interest due to their long cycling lifespan and high thermal stabilityrendered by the robust 3D framework. However, their practical applicationis still hindered by the high cost of raw materials and limited energy density.Herein, a doping strategy with low-cost Fe~(2+) is developed to activateV~(4+)/V~(5+) redox, in an attempt to increase the energy density of phosphatecathodes. It is also revealed that reversible activation of V4+/V5+ redox isrelated to the Na positions (Na1, 6b; Na2, 18e). Only the V-based compoundswith enough Na2 content can activate the V~(4+)/V~(5+) reversibly. Moreimportantly, without presodiation treatment and addition of any sodiationagent, Na_(3.4)V_(1.6)Fe_(0.4)(PO_4)_3 is delicately designed as both cathode and the Naself-compensation agent in full cells, allowing a promising energy density of≈260 Wh kg~(?1). This work sheds light on enhancing the energy density, anddesigning Na self-compensation for practical Na-ions batteries.