Can Doping of Transition Metal Oxide Cathode Materials Increase Achievable Voltages with Multivalent Metals?

We investigate from first principles the use of substitutional p-doping as a means to enhance the insertion energies of multivalent metals in transition metal oxides, and therefore the resulting voltages in an electrochemical cell, due to bandstructure modulation. Multivalent and earth-abundant metals such as magnesium or aluminium are attractive candidates to replace lithium in future high-performance secondary batteries with intercalation-type electrodes. Unfortunately, the achievable voltages obtained with this kind of elements still remain uncompetitively low. We study and compare the changes in insertion energetics (voltages) of single- and multivalent metals in semiconducting and insulating transition metal oxides upon substitutional p-doping with different metals, introducing different numbers of hole states. We use a single vanadium pentoxide monolayer as model system to study the effect of p-doping on achievable voltages and deduce general trends for transition metal oxides. Our investigations reveal the formation of n-hole polarons (with n>1) in form of oxygen dimers in p-doped vanadia caused by localized p holes on oxide ions in agreement with previous findings. We find that the oxygen dimer formation has an adverse effect on adsorption energetics compared to the single-hole case without dimerization. We find an analogous oxygen dimerization in other TMOs with oxygen-dominated valence bands like molybdenum trioxide and titanium dioxide, while strained systems like trigonal nickel- or titanium dioxide, or Mott-type systems like monoclinic vanadium dioxide with qualitatively different valence band composition do not exhibit oxygen dimerization with multi-hole doping. Our results demonstrate the advantages and limitations of TMO electrode p-doping and show a path to possible strategies to overcome detrimental effects.