Arturo Gutierrez is a Ph.D. student working with Professor Arumugam Manthiram on the synthesis and characterization of cathode materials used for lithium ion batteries. Polyanion cathodes represent a special class of materials. The research on polyanion cathodes has continued to gain momentum since the first report on the electrochemical performance of LiFePO4. The interest in polyanion cathodes comes from added safety and higher voltage values in comparison with traditional oxide cathode materials. The strong covalent bonding in the polyanion structure has been credited with the added safety and increase in voltage. These inherent characteristics in polyanion cathodes have promoted the investigation of other polyanion chemistries for use in lithium ion batteries.
Among the new chemistries are the silicates Li2MSiO4, pyrophosphates Li2MP2O7, and borates LiMBO3. Each of these chemistries possesses additional beneficial characteristics as cathode materials. The borates contain the lightest of the polyanion units (BO3) and, therefore, have a higher theoretical capacity (~ 200 mAh g-1) than LiFePO4 (~170 mAh g-1). The pyrophosphates and silicates offer the appealing possibility of extracting/inserting two lithium ions for every transition metal ion in the material, which further increases the theoretical capacity (~ 220 and 330 mAh g-1, respectively). Additionally, because silicon is one of the most abundant elements on earth, the cost of mass production of the silicates may be reduced.
The voltage of a given cathode material can be seen as the location of the Mn+/(n+1)+ redox couple relative to the Fermi level of lithium (Li, anode used as reference in this work). The voltage delivered by a cathode material vs. Li has a direct consequence on the energy stored/delivered (Energy = Voltage * Capacity). Therefore, it is imperative to understand how the voltage of a material can be controlled through intelligent design.
The ability of the polyanion to lower the Mn+/(n+1)+ redox couple and increase the voltage, known as the inductive effect, was originally used to explain the voltage differences in isostructural compounds. It was shown that a more covalent polyanion structure lowers the Mn+/(n+1)+ redox couple further below the Fermi level in lithium and increases the voltage. Because the structure of the compounds investigated was the same, the strength of the polyanion covalency was based on the electronegativity of the countercation (X in XOn) in the material. The novel chemistries currently being investigated as polyanion cathode materials are not isostructural. For instance, the coordination number of the transition metal and counter cation (X in XOn) is not the same in all compounds. These structural differences have an affect on the redox energy.
A structural analysis was completed on the new generation of polyanion cathodes in order to provide a guide to how the structural differences may shift the M2+/3+ redox energy and increase/decrease the voltage they deliver vs. Li. Among the structural differences, special attention was paid to the transition metal coordination (Figure 1), polyhedron edge sharing (Figure 2), and polyanion covalency based on hybridization and resonant forms occurring (Figure 3). The results could be used for smarter design of cathode materials used for lithium ion batteries.