Dissolution Mechanism Of Phosphorus Anode For Lithium/Potassium-ion Batteries And The Corresponding Strategies

Alkali-metal ion batteries have been attracting extensive attentions due to their high voltage,high energy density and environmental friendliness.Among them,lithium-ion batteries have become attractive energy storage for portable electronic devices and electric vehicles.However,with the rapid development of large-scale energy storage system and electric vehicles,lithium-ion batteries are severely hindered by the limited lithium shortage.Potassium-ion battery is one of the most promising alternatives/supplements to lithium-ion batteries for large-scale energy storage due to the abundant resource and wide distribution of potassium element.Anode materials are the key components of lithium/potassium-ion batteries,determining their performance.The development of high-performance anodes for lithium/potassium-ion batteries needs to be investigated urgently.Phosphorus is a promising anode material for fast-charging alkali-metal batteries because of the combined advantages of high mass and volume specific capacity and relatively low,yet safe lithiation potential.Even though lots of works were focused on the issues of volume expansion and low electronic conductivity,the poor cyclic stability and low coulombic efficiency hinder its practical application,and the development of phosphorus anode is severely restricted by the unclear reaction mechanism.In this thesis,we unlocked the new mechanism that soluble polyphosphides were generated when phosphorus anode was performed electrochemical lithiation process.These soluble lithium polyphosphides were able to pass through the separator and then corroded the counter electrode,leading to the loss of active components,which is one of the main issues for the capacity decay.To address this problem,three strategies were provided to modify P/CNT anode.（1）The functional LiF immobilizer that possesses the strong chemical adsorption effect on phosphorus and its derivatives was introduced into the P/CNT composite.It significantly enhanced the cyclic stability due to the inhibition of polyphosphides shuttle.（2）The black phosphorus/CNT anode was modified by WS₂,which can transform into Li₂S and W metal at a high potential.Li₂S can anchor lithium polyphosphides and restrict their free diffusion,meanwhile,W metal improves the electronic conductivity of electrode material,thus speeding up the reaction kinetics.（3）The SnO₂-modified P/CNT composites were prepared due to the strong electrostatic attraction between SnO₂ and P/CNT,in which SnO₂ can prevent phosphorus from the direct contact with electrolyte,thus inhibiting the dissolution of lithium polyphosphides.Furthermore,Li₂O formed after the first discharge that won’t participate in the subsequent electrochemical reaction and possesses strong chemical interactions with lithium polyphosphides.Thus,it is a good way to reduce the loss of active phosphorus that was originated from the side reactions between the soluble polyphosphides and the counter electrode.Besides,Sn and Li-Sn alloys that formed during cycling can improve the electronic conductivity.Therefore,the SnO₂-modified P/CNT electrode demonstrated an excellent cycling stability with the specific capacity of 1178.3 m Ah g^-1 at 200 m A g^-1 after 120 cycles.The potassium storage mechanism of phosphorus anode was also studied in this thesis.Soluble polyphosphides were also generated during potassiation process for phosphorus anode,and it was able to diffuse to the side of counter electrode and react with potassium metal,leading to the active phosphorus loss.The formation of stable and uniform solid electrolyte interface（SEI）by regulating the composition of the electrolyte can effectively inhibit the continuous dissolution of polyphosphides.Besides,LiF was introduced to provide the strong chemical adsorption of polyphosphides before the formation of SEI,contributing to the improved initial coulomb efficiency.Thus,it demonstrated an enhanced reversible specific capacity,excellent cycling stability and high-rate capability.Furthermore,the functional reagent containing phosphorus was prepared to construct a protective layer on Li metal anodes,inducing the uniform deposition of lithium metal and effectively reducing the safety risks caused by lithium-metal dendrites.Therefore,the symmetrical cell using the modified lithium metal showed a smaller overpotential and outstanding plating/stripping cycles.The above research provides critical insights for further study on the phosphorus anode and its application in various alkaline metal ion batteries.