Prussian Blue Analogue Materials for Sodium-Ion Batteries

Abstract

Battery-based electrochemical storage is particularly attractive because of its high energy efficiency and ease of deployment, and lithium-ion batteries (LIBs) are one of the most well developed of these options. Sodium-ion batteries (SIBs), which replace lithium with abundant and inexpensive sodium, have received a great deal of attention recently. Similarities in manufacturing techniques between SIBs and LIBs may significantly accelerate their technological advance. Nevertheless, several scientific challenges still need to be resolved before the performance of SIBs becomes competitive with that of LIBs. In particular, the higher negative redox potential of Na compared to that of Li results in lower cell voltages and consequently lower energy densities. Moreover, the larger size of Na+ relative to Li+ causes slower solid-state diffusion in the active materials and leads to lower energy efficiencies when the batteries are rapidly charged or discharged. High capacity electrode materials with fast solid-state kinetics are therefore required in order to compensate for these intrinsic limitations. In this presentation, I will introduce low-vacancy, sodium manganese hexacyanomanganate (MnHCMn) as a viable cathode material for SIBs. The as-synthesized MnHCMn shows a monoclinic crystal structure composed of nonlinear Mn–N≡C–Mn bonds and containing eight large interstitial sites occupied by Na+ ions. Our experiments demonstrate a high specific capacity of 210 mAh g-1 and excellent capacity retention at high rates in a propylene carbonate electrolyte. We discovered a novel mechanism wherein small lattice distortions allow for the unprecedented storage of 50% more sodium cations than in the undistorted case. These results represent a step forward in the development of sodium-ion batteries.