An aqueous rechargeable lithium ion battery with metallic cadmium as the negative electrode, LiCoO2 nanoparticles as the positive electrode, and a neutral aqueous electrolyte solution of 0.5 M Li2SO4 and 10 mM Cd(Ac)2 is reported. It has good electrochemical performance. The calculated energy density based on the practically available capacity of the two electrodes is 72 W h kg–1; this is comparable to that of Ni–Cd batteries. The positive mercury electrode of the Weston cell is replaced by a LiCoO2 electrode, the negative cadmium amalgam by a plain cadmium metal electrode. Mercury is completely avoided. Compared with Ni–Cd batteries an alkaline electrolyte solution is not needed making the system more environment-friendly.