The invention provides an electrochemical cell which includes a first electrode and a second electrode which is a counter electrode to said first electrode, and an electrolyte material interposed there between. The first electrode comprises an electrode active material represented by the general nominal formula A_a[M_m,MI_n,MII_o](XY₄)_dZ_e, wherein at least one of M, MI and MII is a redox active element, 0m,n,o4, and [V(MI)+V(MII)]=V(M), wherein V(M) is the valence state of M, V(MI) is the valence state of MI, and V(MII) is the valence state of MII.