A bi-functional strategy involving surface coating and subsurface gradient co-doping for enhanced cycle stability of LiCoO_(2) at 4.6 V

Layered LiCoO_(2)(LCO)acts as a dominant cathode material for lithium-ion batteries(LIBs)in 3C products because of its high compacted density and volumetric energy density.Although improving the high cutoff voltage is an effective strategy to increase its capacity,such behavior would trigger rapid capacity decay due to the surface or/and structure degradation.Herein,we propose a bi-functional surface strategy involving constructing a robust spinel-like phase coating layer with great integrity and compatibility to LiCoO_(2) and modulating crystal lattice by anion and cation gradient co-doping at the subsurface.As a result,the modified LiCoO_(2)(AFM-LCO)shows a capacity retention of 80.9%after 500 cycles between 3.0and 4.6 V.The Al,F,Mg enriched spinel-like phase coating layer serves as a robust physical barrier to effectively inhibit the undesired side reactions between the electrolyte and the cathode.Meanwhile,the Al,F,Mg gradient co-doping significantly enhances the surficial structure stability,suppresses Co dissolution and oxygen release,providing a stable path for Li-ions mobility all through the long-term cycles.Thus,the surface bi-functional strategy is an effective method to synergistically improve the electrochemical performances of LCO at a high cut-off voltage of 4.6 V.

Effect of Fluorine Substitution on the Electrochemical Property and Structural Stability of a Lithium-Excess Cation Disordered Rock-Salt Cathode

Lithium-excess cation disordered rock-salt materials have received much attention because of their high-capacity as a candidate for cathodes for lithium-ion batteries.The ultra-high specific capacity comes from the coordinated charge compensation of both transition metal and lattice oxygen.However,the oxygen redox at high voltage usually leads to irreversible oxygen release,thereby degrading the structure stability and electrochemical performance.Lithium-excess Li_(1.14)Ni_(0.57+0.5 x)Ti_(0.19-0.5 x)Mo_(0.10O2-x)F_(x)(x=0,0.05,0.10,0.15,and 0.20) with different amounts of fluorine substitution were synthesized.Among them,Li_(1.14)Ni_(0.620)Ti_(0.140)Mo_(0.10)O_(1.85)F_(0.15)exhibits a lower capacity decline,better rate performance,and lower structure damage.The effects of fluorine substitution on the electrochemical property and structural stability were systematic studied by x-ray photoelectron spectroscopy and in situ XRD etc.Results show that fluorine substitution reduces the average valence of the anion,allowing a larger proportion of low-valent redox active transition metals,increasing the transition metal redox capacity,inhibiting irreversible oxygen release and side reaction.Fluorine substitution further improves the structural stability and suppresses lattice deformation of the material.

Deciphering the Oxygen Absorption Pre-edge: A Caveat on its Application for Probing Oxygen Redox Reactions in Batteries

The pre-edges of oxygen-K X-ray absorption spectra have been ubiquitous in transition metal(TM)oxide studies in various fields,especially on the fervent topic of oxygen redox states in battery electrodes.However,critical debates remain on the use of the O-K pre-edge variations upon electrochemical cycling as evidences of oxygen redox reactions,which has been a popular practice in the battery field.This study presents an investigation of the O-K pre-edge of 55 oxides covering all 3d TMs with different elements,structures,and electrochemical states through combined experimental and theoretical analyses.It is shown unambiguously that the O-K pre-edge variation in battery cathodes is dominated by changing TM-d states.Furthermore,the pre-edge enables a unique opportunity to project the lowest unoccupied TM-d states onto one common energy window,leading to a summary map of the relative energy positions of the low-lying TM states,with higher TM oxidation states at lower energies,corresponding to higher electrochemical potentials.The results naturally clarify some unusual redox reactions,such as Cr^(3+/6+).This work provides a critical clarification on O-K pre-edge interpretation and more importantly a benchmark database of O-K pre-edge for characterizing redox reactions in batteries and other energy materials.

High-throughput characterization methods for lithium batteries

The development of high-performance lithium ion batteries requires the discovery of new materials and the optimization of key components.By contrast with traditional one-by-one method,high-throughput method can synthesize and characterize a large number of compositionally varying samples,which is able to accelerate the pace of discovery,development and optimization process of materials.Because of rapid progress in thin film and automatic control technologies,thousands of compounds with different compositions could be synthesized rapidly right now,even in a single experiment.However,the lack of rapid or combinatorial characterization technologies to match with high-throughput synthesis methods,limit the application of high-throughput technology.Here,we review a series of representative highthroughput characterization methods used in lithium batteries,including high-throughput structural and electrochemical characterization methods and rapid measuring technologies based on synchrotron light sources.