Thermodynamics of Metal Cation Intercalation into V2O5(aerogel) Hosts

Abstract

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.