Preparation And Properties Of Cr-Doped β/δ-MnO₂ Cathode Materials For Zinc Ion Batteries

With the development of society,new electrochemical energy storage devices attract the attention of many researchers.Due to the use of flammable organic electrolyte,the widely used lithium-ion battery has hidden dangers such as spontaneous combustion and environmental pollution.Hence,aqueous battery become a breakthrough direction.Among many aqueous battery devices,with high theoretical capacity(819 m Ah·g^-1),stable chemical properties and environmental protection of zinc metal,aqueous zinc ion battery becomes the first choice.The key of aqueous zinc ion battery is cathode materials and manganese dioxide is preferred one.This project focus onβ-MnO₂（stable thermodynamic properties）andδ-MnO₂（a wider layer spacing）.Based on theoretical calculation of Cr-O binding bond(461 k J·mol^-1)and Mn-O binding bond(362 k J·mol^-1),Cr was added intoβ/δ-MnO₂ to strengthen their structural stability.Through the structural characterization and electrochemical tests,the performance improvement mechanism of Cr doping inβ/δ-MnO₂ as the cathode material of aqueous zinc ion battery was explored.The details are as belows:（1）Different proportions（1%,3%,5%）of Cr were introduced intoβ-MnO₂ during the hydrothermal method process,and the tunnel structure of Cr-dopedβ-MnO₂ was synthesized.A series of characterizations show that Cr atoms has successfully entered the lattice structure ofβ-MnO₂,partially replaced the site of Mn in the lattice structure ofβ-MnO₂.The electrochemical tests show that the specific discharge capacity ofβ-MnO₂ and 5%Cr-dopedβ-MnO₂are 123 m Ah·g^-1 and 180 m Ah·g^-1.Due to the introduction of Cr inβ-MnO₂,the crystal structure of inhibits the dissolution of Mn²⁺caused by Jahn-teller effect during charging and discharging.In addition,the introduction of Cr inβ-MnO₂enlarges its（110）crystal plane spacing,which increases the specific capacity and cycling stability ofβ-MnO₂.（2）Different proportions of（1%,3%,5%,7%and 9%）Cr-doped during the synthesis process,the doped layeredδ-MnO₂ materials are synthesized by hydrothermal method.Through a series of characterization tests,it is proved that theδ-MnO₂ transformed toα-MnO₂ while high ratio（7%and 9%）hydrothermal doping occurred.The Cr atoms has successfully entered theδ-MnO₂ lattice structure,which partially replacing the site of Mn inδ-MnO₂.The electrochemical tests show that the first specific discharge capacity ofδ-MnO₂ and（1%,3%and 5%）Cr doped inδ-MnO₂are 159 m Ah·g^-1,165 m Ah·g^-1,179 m Ah·g^-1 and 183 m Ah·g^-1 at 0.1 A·g^-1,with their capacity retention rate are 50.9%,67.9%,52%and 82.5%after 100 cycles.The introduction of Cr stabilizes the layered structure ofδ-MnO₂ and effectively inhibits the manganese dissolution caused by Jahn-Teller effect.The（001）crystal spacing ofδ-MnO₂ was enlarged and the lattice defects were introduced intoδ-MnO₂,which improved the specific capacity and long cycle performance ofδ-MnO₂.