Prussian Blue analogue materials for energy harvesting and storage

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. Sodium-ion batteries (SIBs), which replace lithium with abundant and inexpensive sodium, have received a great deal of attention recently. 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 talk, 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. Due to the minimal hysteresis in the galvanostatic charge/discharge curves of the electrochemical cell using Prussian Blue open-framework structures, a different approach is to explore thermodynamic cycles as is common in thermomechanical engines. The thermogalvanic effect, the dependence of an electrode’s electrochemical potential on temperature, can be used for such cycles. In the second part of the talk, the electrochemical thermodynamic cycle for thermal energy harvesting will be introduced. By utilizing novel electrode materials, this system can achieve very high efficiencies at low temperature ranges.