(a) Which ion is smaller, Co3+ or Co+? (b) In a lithium-ion battery that is discharging to power a device, for every Li+ that inserts into the lithium cobalt oxide electrode, a Co4+ ion must be reduced to a Co3+ ion to balance charge. Using the CRCHandbook of Chemistry and Physics or other standard reference, find the ionic radii of Li+, Co3+, and Co4+. Order these ions from smallest to largest, (c) Will the lithium cobalt electrode expand or contract as lithium ions are inserted? (d) Lithium is not nearly as abundant as sodium. If sodium ion batteries were developed that function as lithium ion ones, do you think “sodium cobalt oxide" would still work as the electrode material? Explain, (e) If you don't think cobalt would work as the redox-active partner ion in the sodium version of the electrode, suggest an alternative metal ion and explain your reasoning.
Solution Step 1 3+ + Co will be smaller than Co .An ion with a greater positive charge will be smaller than an ion with lesser positive charge. Co has 27 protons and 24 electrons + Co has 27 protons and 26 electrons So the effective nuclear pull experienced by Co will be more and the ion will have a smaller size. Step 2 The ionic raii of Li =0.59A ° The ionic raii of Co =0.55A ° 4+ ° The ionic raii of Co =0.41A The decreasing order of size is as given + 3+ 4+ Li >Co >Co Step 3 + 4+ For every Li ion added to Lithium cobalt oxide electrode, we know that a Co gets reduced to Co .Thus the reduction results in an increase in size of the electrode since Co is larger in size 3+ 4+ than Co .Thus the Lithium cobalt electrode expands when Lithium ions are inserted