Electrochemical Performance of Polyoxometalates for Cathode Materials of Lithium Battery

Polyoxometalates (POM) are very famous cluster ions that can be synthesized self-assembly formation in the solution.  These cluster ions behave as independent molecules and show reversible redox activity in the solution.  These molecular cluster ions are expected to show different property from the usual cathode materials for lithium ion battery those are the bulk continuum ionic crystals.[1-3]      In this paper, we have synthesized Anderson-type POM Na₃[AlMo₆O₂₄H₆] (Fig.1 ) and examined its electrochemical property as cathode materials for lithium battery.  In Anderson-type POM, all [MoO₆] share the edge with hetero ion [AlO₆].  This covalent bond of [MoO₆] with unreactive hetero ion would stabilize the structure of the reduced POM.

     Anderson-type POMs were synthesized according to the reported method. [4] Obtained samples were mixed with acetylene black with the weight ratio of 1:2 using ball mill. The charge-discharge test and AC-impedance spectra measurements are performed using the coin cells at 25ºC. Lithium metal was used as anode and 1M solution of LiPF₆in EC:DEC (3:7 v/v%) was used as the electrolyte. After the charge-discharge measurements, ex-situ XRD and Raman measurements were carried out under Ar atmosphere again. 

     NMR, FT-IR and Raman spectra of obtained samples agreed with those for reported Anderson-type POM.  In Fig. 1, the discharge-charge curves for Na₃AlMo₆O₂₄H₆  were shown. Na₃AlMo₆O₂₄H₆ mixed by ball mill show large capacity over 420 mAh/g.  In Fig.2, cycle stability of Na₃AlMo₆O₂₄H₆  was shown. The cycle stability of Na₃AlMo₆O₂₄H₆ was higher than the most representative POM  K₃PMo₁₂O₄₀ with Keggin type structure. 90% of the initial discharge capacity for Anderson-type POM was retained after 50 cycles, whereas that for KPM decreased steeply in the first 10 cycles.  This result suggests Anderson type POM has higher stability than Keggin type POM.

 [1]  N. Sonoyama et al, J. Power Sources, 196 (2011) 6822.

[2] S. Uematsu et al, , J. Power Sources, J. Power Sources, 217, (2012)13.

[3] E. Ni et al. J Nanopart Res, 15(2013) 1732.

[4] Kenji Nomiya et al. Polyhedron, (1987) 213.