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First-principles prediction of enhancement in the electrochemical potential of LiCoO with aluminum substitution has been realized through earlier experiments. For safer and less expensive Li-ion batteries, it is desirable to have a similar enhancement for alternative cathode materials, LiFePO and LiCoPO. Here, we present first-principles density functional theory based analysis of the effects of aluminum substitution on electrochemical potential of LiCoO, LiFePO and LiCoPO. While Al substitution for transition metal results in increase in electrochemical potential of LiCoO, it leads to reduction in LiFePO and LiCoPO. Through comparative topological analysis of charge density of these materials, we identify a ratio of Bader charges that correlates with electrochemical potential and determine the chemical origin of these contrasting effects: while electronic charge from lithium is transferred largely to oxygen in LiCoO, it gets shared by the oxygen and Co/Fe in olivine phosphates due to strong covalency between O and Co/Fe. Our work shows that covalency of transition metal-oxygen bond plays a key role in determining battery potential. [ABSTRACT FROM AUTHOR]