Sol-gel derived V₂O₅ (aerogel) is an excellent host for intercalation, showing high capacity for guest species.  It was first reported as a host for Li⁺ ions [1], with a capacity of 4 Li/mole of host [2].  The capacity was shown to be reversible by GITT (galvanostatic intermittent titration technique [3]) for the entire range [1,2,4].  Polyvalent cations were also found to have an intercalation capacity of 4 equivalents/mole of V₂O₅ aerogel host (i.e. Mg⁺², Al⁺³, and Zn⁺²) [3,4].  Experimentally, the polyvalent cations were intercalated by chemical insertion from organometallic reagents, in parallel to the chemical insertion technique introduced for Li⁺ ions [5,6].

Thermodynamics of guest-host systems are of importance to the study of batteries that employ intercalation electrodes.  Even though thermodynamics does not specify the capacity of the host materials, the energy state of batteries is dictated for which charge can be extracted and stored.  Interaction between the guest species and with the host may diminish the system energy substantially as compared to an ideal guest-host process.  The diminished energy leads to a lower usable fraction of the available capacity, as reflected in low voltage or as irreversibility in some materials.

Our experience with amorphous V₂O₅, V₂O₅ xerogels and crystalline V₂O₅ as lithium hosts demonstrated that disordered structures are generally preferred for high intercalation capacity hosts.  The full usable capacity of four Li (per V₂O₅ unit) in the V₂O₅ spin-coated xerogel is not matched with amorphous V₂O₅ in our studies.  This illustrated that with proper processing modified structures can provide a significant enhancement for usable capacity.  The structural modification achieved with the V₂O₅ aerogel revealed that the inherent interaction in the M-V₂O₅ system is eliminated almost entirely.

The discussion here utilizes a model for the interactions as the most significant factor in providing a basis for the high capacity for the multivalent guests in the aerogel material [7,8].  Our analysis of the M-V₂O₅ systems predicts that the effect of interactions is reduced, to yield model predictions that agree extremely well with the experimental results.

References:

[1] Le, D.B., S Passerini, J. Guo, J. Ressler, B.B. Owens, and W.H. Smyrl, “High Surface Area
      V₂O₅ Aerogel Intercalation Electrodes”, J. Electrochem. Soc. 142(7), 2099-2104(1996)

[2] Le, D.B., S. Passerini, F. Coustier, J. Guo, T. Soderstrom, B.B. Owens, W.H. Smyrl,
      “Intercalation of Polyvalent Cations into V₂O₅ Aerogels”, Chem. Mater. 10, 682-684(1998)

[3] Weppner, W., and R.A. Huggins, “Electrochemical Methods for Determining Kinetic
      Properties of Solids”, Ann. Rev. Mater. Sci. 8, 269-311(1978)

[4] Le, D.B. PhD Thesis, University of Minnesota (2004)

[5] Murphy, D.W., F.J. DiSalvo, G.W. Hull, Jr., J.V. Waszczak, “Convenient Preparation and
      Physical Properties of Lithium Intercalation Compounds of Group 4B and 5B Layered
      Transition Metal Dichalcogenides”, Inorg. Chem. 15, 17-21(1976)

[6] Whittingham, M.S., and M.B. Dines, “n-Butyllithium-An Effective, General Cathode

      Screening Agent”, J.Electrochem.Soc. 124, 1387-1388(1977)

[7] McKinnon, W.R. and R.R. Haering, Modern Aspects of Electrochemistry No. 15, ed. R.E.
      White, J.O’M. Bockris, and B.E. Conway, Plenum Press, New York, 1983, pp 235-301

[8] Hill, T.L. An Introduction to Statistical Thermodynamics, Addison-Wesley, USA, 2^nd ed,
      1962