Research On Mesoporous Metal Oxides As Anode Materials For High-rate Lithium Ion Batteries

Nowadays,lithium-ion batteries?LIBs?have become increasingly important in the development of social technology,and the pursuit for LIBs with both high energy density and fast-charge skill is becoming more and more urgent.Although graphite material remains the mainstream anode in the commercial LIBs market,it is difficult to meet future application needs due to its low theoretical specific capacity(372 m Ah g^-1).It is noted that transition metal oxides?TMOs?have obvious advantages,including high theoretical specific capacity(700–1000m Ah g^-1),easy preparation,and environmental friendliness,thus are considered to be a class of potential alternative anode materials.However,the serious volume expansion issue of TMOs as anodes during lithiation process can destroy the electrode structure,and their bulk structure has insufficient electrochemical performance at high rates.There are two universal strategies to solve stability problem:one is to alleviate volume expansion by designing reasonable nanostructures;the other is to maintain the whole structural stability through introducing coating material to form composites.Furthermore,it is highly advantageous and valuable to build mesopore structure for improving rate performances.Herein,this paper mainly aims to achieve the goal of long-term cycle stability at high rate by rational structure design for TMOs?Co₃O₄ and Fe₃O₄?.The specific works are as follows:?1?We successfully synthesized a three-dimensional?3D?peony-like Co₃O₄ architecture assembled with two-dimensional?2D?ultra-thin holey nanosheets by a solvothermal method followed annealing treatment.This peony-like Co₃O₄ anode showed excellent lithium storage ability under a wide temperature window:the specific capacity was as high as 1880 m Ah g^-1when accomplished 800 cycles at a current density of 500 m A g^-1 under room temperature?25°C?;this anode also retained remarkable rate capability,i.e.,an excellent capacity of 1141m Ah g^-1 at a large current density of 10 A g^-1.Even at low temperature?-25°C?,the electrode still afforded a reversible specific capacity of 642 m Ah g^-1 after running 50 cycles(current density:200 m A g^-1).?2?We successfully prepared Fe₃O₄@nitrogen-doped carbon nanocapsule with self-formed channel?denoted as Fe₃O₄@NC?based on a simple hydrothermal-coating-annealing route using low-cost Fe Cl₃ as raw material and dopamine hydrochloride as carbon source.When used as anode material of lithium ion battery,Fe₃O₄@NC nanocapsule manifested the admirable electrochemical behavior:there was no obvious capacity fading at an ultra-high rate(20 A g^-1),and it maintained a reversible specific capacity of 480 m Ah g^-1 after 1000 cycles,with average Coulombic efficiency of 99.97%.Furthermore,we also explored the common reason of achieving fast lithium storage for these two structures.The research found that abundant mesopores/channels can not only shorten the lithium ion diffusion and electron transfer paths,but also increase the pseudocapacitance behavior on the material surface.This synergistic effect provided the possibility for fast charge and discharge process.