Synthesis And Electrochemical Performance Of Na-Rich Prussian Blue Analogues Cathodes

Prussian blue analogues(PBAs)with a 3D open framework are believed to be one of the most potential cathode materials for sodium ion batteries(NIBs)due to their high theoretical capacity,low cost and easy synthesis.The traditional synthesis methods mainly include co-precipitation,hydrothermal and electrochemical deposition.However,the as-prepared products usually have considerable interstitial H2O and lattice defects,resulting in lower Na content,lower capacity,and poor cycle performance.Therefore,the commercial application of PBAs requires effective control of the amount of lattice defects,interstitial H2O and sodium content.This work developed a solvent-free mechanochemical protocol to synthesize stable sodium-rich structure with low interstitial H2O via regulating the precursors water content coupled with post-heat treatment.(1)Firstly,the synthetic conditions have been optimized by regulating the crystal water content in precursors,changing the kind of manganese salt,controlling the ball-milling time,ball-milling speed,and drying conditions.Based on the optimized condition,manganese acetate was used to prepare and investigate the influence of crystal water content on the composition,morphology,structure and electrochemical performance of the product.The results show that higher crystal water content contributes to obtain monoclinic structure with more regular morphology,lower water content,lower defects,higher sodium content and initial discharge capacity.However,too much interstitial water in products leads to poor cycling performance.Thus,we further reduced the interstitial water through increasing the drying temperature,obtaining a monoclinic sodium-rich Nai.94Mn[Fe0.99(CN)6]0.95·□0.05·1.08H2O with high capacity(168.8 mA h g-1 at 10 mA g-1),good cycling performance(85%retention rate at 100 cycles at 100 mA g-1)and excellent rate performance(127 mA h g-1 at a current density of 2000 mA g-1).Besides,doping Fe and Ni via ball milling was also studied to improve the electrochemical performance,which indicated that the Na content and discharge capacity decrease with the increasing doping ratio of Fe,Ni.(2)To evaluate the performance of the optimized material for practical application,we have assembled a MnHCF/NTP full battery(140 mA h g-1 at 20 mA g-1)with MnHCF as the cathode and NTP as the anode.And the MnHCF/HC full cell was fabricated(15 mA h g-1 at 20 mA g-1)with hard carbon as the anode.Then the reason for the low discharge capacity of the MnHCF/HC full cell during the first cycle was focused on.In addition,the rhombohedral structure was obtained by removing interstitial water through high temperature heat treatment in argon.It delievers a specific capacity of 165 mA h g-1 in half cell,and 129 mA h g-1 in full cell at a current density of 20 mA g-1.Furthermore,KB coating and Ni,Cu trace doping were investigated to improve the cycle performance of the full cells,which indicated that 5%KB was effective,while Ni,Cu trace doping was demonstrated useless.