Study On Structure And Properties Of Layered Oxide Cathodes For Sodium-Ion Batteries

High theoretical specific capacity O3-type transition metal oxides show great potential as cathode materials for sodium ion batteries,but their large-scale application is constrained by the following three factors.Firstly,when O3-type materials are exposed to air,lattice Na⁺will spontaneously escape from the lattice to surface accompanied by the formation of Na-deficient phase.The segregated Na will rapidly react with H₂O/CO₂ in ambient air to form residual alkali species,thus deteriorating the electrochemical performances.Secondly,the narrow layer spacing and tetrahedral sites with high diffusion barriers lead to the sluggish Na⁺diffusion between layers.Furthermore,the repeated extraction/insertion of Na⁺during the electrochemical reaction will destroy the crystal structure,leading to a limited cycling number.In response to these problems,we synthesizes a series of O3-type cathode materials with excellent performances and explains the intrinsic mechanism by combining physical characterization and theoretical calculations.This paper mainly contains the following two parts of work:（1）Study on improving the air stability and rate performance of O3 cathodes by introducing sodium vacancyBecause of the contradiction between Na containment suggested in air stability mechanism and enhanced Na diffusion mentioned in kinetics strategy,the improvement on the two handicaps is hard to simultaneously realize in O3-type layered oxides,thus searching for a simple and universal strategy to realize a simultaneous improvement on both air sensitivity and kinetics is critical.In this work,we suggested the introduction of proper Na vacancies into the framework to realize the simultaneous improvement.First,Na deficiencies can increase the valance state of transition metal ions,thus enhancing the oxidation resistance of material and suppressing the unfavorable spontaneous reaction between material and air.Secondly,the introduction of Na vacancies enhances the electrostatic repulsion between the upper and lower oxygen layers,thus widening the layer spacing.Furthermore,Na vacancies in the lattice can facilitate the Na⁺transformation from hard oxygen dumbbell hop（ODH）to facile tetrahedral site hop（TSH）.designed Na_0.93Li_0.12Ni_0.25Fe_0.15Mn_0.48O₂ have a decreased Na⁺diffusion barrier from≈1000 to 300me V and a high rate capability of 70.8%retention at 2000 m A g^-1.Remarkably,such a strategy can be easily realized by either pre-or post-treating,which exhibits excellent universality for various O3 materials,providing a new insight for the development of next generation high-performance Na-based cathode materials.（2）Study on the cyclic stability of integrated Layered Oxide cathode O’3/O3-Na0.93Li1/8Ni1/4Mn1/2Ti1/8O2The O3-type layered oxides have the advantage of high specific capacity due to its high initial Na content.However,the huge change in Na concentration upon electrochemical process will resut in a significant volume evolution.In addition,the strong interaction of TM layers leads to the structure collapse and eventually results in the deterioration of the electrochemical properties.Herein,we designed a unique O’3/O3symbiotic Na_0.93Li_1/8Ni_1/4Mn_1/2Ti_1/8O₂ layered oxide cathode to mitigate the strain upon Na extraction/insertion.O’3-structure,as an intermediate phase from O3 to P3,can shorten the phase transition process thus reducing the volume expansion.Additionally,Li⁺will migrate to the alkali metal layer in Na-deficient state,which can effectively maintain the structural stability at the high voltage stage.The O’3/O3 symbiotic cathode combines excellent long-cycle performance and considerable specific capacity,which shows a reversible specific capacity of 106.0 m Ah g^-1 at 0.5 C in the voltage range of 2.0-4.1V,and exhibits no capacity decay after 100 cycles.After 1000 cycles at 5 C,the electrode shows a superior capacity retention of 86.1%,and 2000 cycles of 79.7%.Especially in the second 1000 cycles,the average decay rate is just 0.0064%per cycle.The ingenious composite phase structure opens up a new approach for the development and design of long-cycle layered cathode for sodium-ion batteries.