Preparation and structure of Na 2 Ag 5 Fe 3 (P 2 O 7 ) 4 -Ag metal composite: Insights on electrochemistry
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Preparation and structure of Na2Ag5Fe3(P2O7)4 -Ag metal composite: Insights on electrochemistry Yiman Zhang,a Amy C. Marschilok,a,b,* Esther S. Takeuchi,a,b,c,*Kenneth J. Takeuchia,b,* a Department of Chemistry, Stony Brook University, NY 11794, USA b Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, NY 11794 c Energy Sciences Directorate, Brookhaven National Laboratory, Upton, NY 11973 * Corresponding authors: (ACM) [email protected], (EST) [email protected], (KJT) [email protected] ABSTRACT Ag7Fe3(P2O7)4 is a 3D structured material which has been recently studied as a possible cathode material for lithium batteries. Notably, Na7Fe3(P2O7)4 is reported to be a fast-ion conductor, yet poor electrical conductor. Here, partial replacement of Na+ for Ag+ yielded Na2Ag5Fe3(P2O7)4 pyrophosphate framework where the formation of Ag metal is proposed to increase the intrinsic low electrical conductivity of this polyanion electrode. Specifically, the Ag5Na2Fe3(P2O7)4 -Ag composite is synthesized via chemical reduction of Ag7Fe3(P2O7)4 using NaBH4. The occupancy of Ag+ and Na+ in each site was determined via Rietveld analysis of the diffraction pattern. Electrochemistry of the Ag5Na2Fe3(P2O7)4 -Ag metal composite was explored with voltammetry and galvanostatic charge/discharge cycling. The Ag5Na2Fe3(P2O7)4 -Ag metal composite electrodes displayed good rate capability assisted by the presence of Ag metal from the chemical reduction and in-situ electrochemical formation of a Ag conductive network. INTRODUCTION Currently, lithium ion batteries are regarded as the most advanced energy storage systems, with applications ranging from portable devices to electric and hybrid electric vehicles.1 While different functional demands are required in these diverse applications, environmental friendliness and safety are commonly desired characteristics. Phosphate or pyrophosphate polyanions have shown increased stability over oxides.2 Further, earth-abundant iron redox center materials are appealing. Thus, iron-based polyanion materials such as LiFePO4,3 Li2FeP2O7,4-7 LiFeP2O7,8, 9 Li3Fe2(PO4)3,10 and the mixed phosphate and pyrophosphate materials Li9Fe3(P2O7)3(PO4)211 have been studied. Among that group, olivine LiFePO4 has been the most widely studied due to stable discharge plateau at high voltage (3.5 V vs. Li) and long cycle life.3 Despite possible advantages, detrimental features of LiFePO4 cathode material are its low electronic conductivity (ca. 10-10 S cm-1) and slow Li+ diffusion kinetics (ca. 10-16 cm2 s-1).8, 12 Modifications to LiFePO4 electrodes included formation of a carbon / polypyrrole (PPy) composite to increase conductivity.13 Ag metal, well known for its high electrical conductivity, has also been proposed as a possible LiFePO4 coating. Research on LiFePO4-incorporated Ag nanoparticles established the LiFePO4 / Carbon (C) – Ag electrode delivered 6X higher capacity than LiFePO4/C due to more facile Li+ diffusion.14 A LiFePO4/C PVDF/Ag-based cathode
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