Whereas the negolyte exhibits a linear behavior consistent with a binary mixture absorption model, the posolyte is known to contain a third component: a highly absorbing mixed-valence compound, V 2 O 3+ 3 , formed between V IV and V V , denoted here by subscript i = 45 . In Eq. 3, k = 2.09 and (C 45 , ϵ 45 ) represent the concentration and molar absorptivity of this complex, respectively. C 45 depends on C and SoC according to