Lithium insertion into Cr2(So4)3 frameworks: variation of electrochemical behavior with cation substitution and nature of polyanion

Electrochemical insertion of lithium into the Cr2(SO4)3 framework was carried out in a “LimidLiClO4:EC–PC (3:1)midcathode” cylindrical cell (EC: ethylene carbonate, PC: propylene carbonate). Preliminary electrochemical data revealed that the Cr2(SO4)3 sample exhibit poor performance compared to the cation- and polyanion-substituted samples. Depending on the cation (Nb) substitution and the nature of the polyanion (XO4) (X=P and S): (a) a clear dependence of the shape of the discharge curve versus capacity; and (b) an enhancement of voltage and capacity rate have been obtained and their difference is discussed in terms of redox potential. The cathodes with Nb substitution and polyanion having (PO4)3− are more tolerant to repeated lithium insertion and extraction than a standard Cr2(SO4)3 framework electrode in spite of a small reduction in the open-circuit voltage. The improvement in cycling performance is attributed to the presence of Nb cations that enhance the stability of the octahedral sites in the structure.