Mixed Metal Phosphorous Oxides: Prospects for Secondary Lithium Based Batteries

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’P_xO_y, 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 Ag_0.48VOPO₄ 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 Ag_0.48VOPO₄.

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’P_xO_y and MM’O material families.