Transition Metal Sulfides As Lithium Metal Battery Cathodes

Conversion-type cathodes,which have multiple advantages such as high specific capacity and high energy density,are considered to be promising for next-generation LIBs.Among those conversion-type cathodes,transition metal sulfides（MS）are selected as research objects because of their low price and high reactivity.Specifically,due to their better application prospect,Cu S,Fe S₂,and Fe S_x（1≤x≤1.14）are extensively studied with detailed macroscopic and microscopic phase changes.In a word,a comprehensive evaluation of materials preparation,performance investing,mechanism analyzing,materials utilization,and materials modification is conducted,which provides valuable information for further research on similar materials.In particular,the novelty and main points in this work are concluded as follows.First,Cu S is chosen to conduct a further investigation due to its appealing energy density among a series of transition metal monosulfides（Ti S,VS,Cr S,Mn S,Fe S,Co S,Ni S,Cu S）.As Cu S faces the problem of voltage decay,its reaction mechanism and interfacial phase transformation are carefully studied.Based on the acquired results,a solution to cure voltage degradation is proposed.Specifically,Two new Li-Cu-S complexα-Li Cu S（Pmna space group）and Li_2-xCu_xS（Fm（?）m space group）are identified for the first time.Besides,another intermediateβ-Li Cu S（Ibam space group）is observed in the electrochemical process for the first time.Based on these vital intermediates,multiple topotactic reactions are discovered during the electrochemical reaction of Cu S and Cu₂S.According to the in-situ XRD and ex-situ XRD results,new reaction mechanisms of Cu S and Cu₂S are concluded.Moreover,XRD results hint that the highly overlapped reflection pattern ofα-Li Cu S,β-Li Cu S,Li_2-xCu_xS,and Li₂S around 27°is the main reason that leads to a misjudgment of the reaction product.As for the asymmetric overpotential of Cu S,it is attributed to the“Cu⁺diffusion control mechanism”and“local self-discharge mechanism”.Furthermore,it is found that this Cu⁺diffusion is also the origin of Cu S voltage decay in a macroscopical view.In other words,the reaction between Cu S and Cu current collector causes the erosion of the current collector and the degradation of Cu S(Cu S→Cu_1.81S).Whereupon,a solution that involves Al current collector replacement is provided in this work and successfully suppressed the voltage fading of Cu S.Considering that the energy density is limited due to Cu S containing only one unit of active sulfur,another typical conversion compound Fe S₂ is investigated because of its higher energy density and lower price.Specifically,an overall understanding of the electrochemical properties of Fe S₂ is acquired via multiple factors adjustments,such as electrolyte,voltage window,and conductive carbon content.Regarding the reaction mechanism,the room-temperature（25℃）reaction and high-temperature（60℃）reaction is analyzed separately.We found that it is a one-step-reaction-dominated mechanism at room temperature while it is a two-step-reaction-dominated mechanism.Based on this result,a competitive reaction mechanism and a spontaneous reaction mechanism are concluded.As for the capacity decay of Fe S₂,it is attributed to the dual dissolution of polysulfide(S_n^2-)and Fe²⁺,combined with particle pulverization.To restrain the degradation of Fe S₂,three possible modification strategies are proposed.These strategies are PEO-based solid-state batteries,DOL in-situ gelation batteries,and repolymerization of PVDF coating Fe S₂ batteries.It is found the third way demonstrates the best electrochemical performance.Specifically,this strategy uses Li₂S₄ triggering repolymerization of PVDF to build a crossed P-PVDF matrix.The in-situ formed product on Fe S₂ surface can decrease the dissolution of polysulfides and prevent the crush of Fe S₂particles.Though the above method may improve the cycling performance of Fe S₂ to some extent,it still needs improvement to obtain a satisfactory cycling life.As the capacity fading of Fe S₂ is due to the polysulfides shutting and large volume variation,this work tries to sacrifice a part of the active sulfur to suppress both of these two shortages.We use Fe S₂ pyrolysis to prepare Fe S_x（1≤x≤1.14）complexes with different Fe/S ratios.The stable cycling of all these compounds proves that this sacrifice is viable.Typically,Fe S can keep a stable cycling for 700 cycles at 1C current with a capacity retention of 82%.Subsequently,the origin to have such stability of Fe S_x is investigated.With combined in-situ and ex-situ XRD measurements,a key intermediate N-Fe S is found for the first time.Then,a topotactic transformation mechanism is proposed to explain the formation of N-Fe S.Based on these findings,a new electrochemical mechanism is provided.In the end,Fe₇S₈ is chosen due to its relatively higher energy density to study the practicability.A pouch cell with a designed capacity of 9.4 Ah is fabricated.This battery realizes an energy density of 389 Wh·kg^-1.A further step,the battery failure is discussed in this work.It is attributed to the electrolyte consumption and the generation of dead Li on the lithium side,while there are no obvious changes on the Fe₇S₈ cathode side.