Mixed Metal Phosphorous Oxides: Prospects for Secondary Lithium Based Batteries

Abstract

Cathode materials containing phosphate have met with success in lithium based batteries, with a notable example being lithium iron phosphate, LiFePO4. A recent focus for our research has been the investigation of bimetallic phosphates (MM’PxOy, where Mn+ = Ag, Cu), motivated by the desire to obtain the stability observed in phosphate based cathode materials and the opportunity for multiple electron transfers per formula unit due to the bimetallic composition of the materials. Our initial electrochemical studies focused on Ag2VO2PO4 where the material provided a discharge capacity of 272 mAh/g and high current pulse capability. In-situ formation of silver nanoparticles upon initiation of reduction was accompanied by a 15,000 fold increase in cathode conductivity. Our paradigm of current enhancement via reduction displacement was in evidence. An alternate composition Ag0.48VOPO4 material with lower silver content per formula unit shows a high operating voltage and characteristic multi-plateau voltage profile. Counter to Ag2VO2PO4, the reduction of Ag+ to Ag0 occurs later in the discharge process for Ag0.48VOPO4. Based on the delayed reduction of silver, we hypothesized that the MVOPO4 material class may demonstrate improved reversibility relative to the AgVO2PO4 system. The MVOPO4 material is explored for secondary lithium based battery applications. The electrochemistry of the analogous silver and copper containing MVOPO4 materials will be compared and contrasted in this presentation. The inclusion of copper instead of silver in the structure is motivated by lower cost of copper versus silver and the accessibility of divalent Cu2+ which can provide 2 electron equivalents per metal center. However, the silver containing material offers the advantage of higher operating voltage. The electrochemistry will be summarized in the context of prior work other members of the MM’PxOy and MM’O material families.