Study On Structural Design And Interfacial Chemistry Of Lithium Metal Anode

As an energy storage and conversion device between electric energy and chemical energy,electrochemical power source is a key carrier to improve energy utilization.It has received the widespread attention after commercialization.However,the current commercial Li-ions batteries(LIBs)based on intercalation chemistry are hardly to satisfy the growing demand for high energy density,thus,it is extremely urgent to develop novel battery chemistry with high energy density and long cycle life.As the core of LIBs,the development of electrode materials is the key factor restricting the improvement of energy density.Compared with commercial graphite anode,Li metal anode has been considered as the“Holy Grail”among all anode materials because of its high theoretical specific capacity,low electrochemical potential and density.Meanwhile,the novel battery systems based on Li metal anode,such as Li-S battery,Li-O₂ battery et al,also exhibit a high energy density of>500 Wh kg^-1,which is considered as an ideal candidate for the current LIBs.However,there are two challenges that need to be addressed before practical applications.On the one hand,it is easy to form uneven Li deposition during the continuous Li plating/stripping process because of the difference of local Li⁺mass transfer,causing the formation of Li dendrites,which may eventually pierce the separator and cause a series of safety problems such as:short circuit and even fire.On the other hand,Li metal anode will undergo severe volume expansion and contraction during cycling,leading to the continuous reconstitution of electrode/electrolyte interphase,thereby lowing the coulombic efficiency and cycle life.Based on the above background,this dissertation focuses on the fabrication of composite Li metal anode and the regulation of surface/interface chemical behavior of electrode,which promotes the uniform Li deposition and mitigates volume expansion of Li metal anode,thereby greatly improving the coulombic efficiency and cycling stability.The whole content is divided into five chapters:1.Developing an electro-deposition method to prepare stable Li metal anode with protected interface.The existing methods for preparing composite Li metal anode mainly focus on molten Li infusion and electrodeposition Li in coin cells,failing to carry out industrial scale-up.Thus,this chapter demonstrated a composite Li metal anode with high capacity achieved by electro-deposition of Li metal in a commercial carbon fiber cloth matrix.The 3D carbon fiber cloth can reduce the local current density and relieve volume expansion during cycling process.Meanwhile,a stable protective film with high Li⁺conductivity can be formed on the electrode surface,which not only ensures the rapid Li⁺transport at the electrode interface,but also inhibits the growth of Li dendrites.2.Proposing the interface regulation strategy of Li⁺based on complex mechanism.Although the growth of Li dendrites can be effectively suppressed by the composite Li metal anode,it is necessary to start from the electrode interface to fundamentally improve the uneven Li deposition behavior caused by inhomogeneous field distribution.Herein,this chapter introduced polyethylene glycol into electrolyte as additive,and after optimizing its molecular weight,promoting the uniform Li deposition and improving the cycling stability of Li metal anode.Polyethylene glycol can adsorb on the Li surface and form a stable surface film by complexing Li⁺with its repeated glycol unit,ensuring the homogeneous Li⁺diffusion at the electrode/electrolyte interface,thereby promoting the uniform Li deposition.3.Proposing the interface regulation strategy of Li⁺based on super-filling mechanism.Although the complexing mechanism proposed in last chapter can promote the uniform Li deposition to a certain extent,it is not conducive to the Li deposition under high current density and Li plating/stripping capacity because of the formation of a complexing barrier layer on the electrode surface.Thus,this chapter developed a series of sulfide-conjugated molecules as additives.Typified by thiourea molecule,it can be adsorbed on the Li surface and achieve uniform Li deposition at high current density and Li plating/stripping capacity by super-filling mechanism.Specifically,this mechanism works by accelerating Li deposition on the concave surface rather than the convex surface.With the extension of plating time,a smooth deposition layer without Li dendrites would be formed.The experimental results were also consistent with the theoretical simulation and as-obtained Li metal batteries also showed excellent discharge capacity and cycling stability especially at high current density.4.Improving the Li deposition behavior at the anode/electrolyte interface in solid state electrolyte.Although the use of solid state electrolyte can improve the safety of Li metal battery to a certain extent,it still does not change the inherent uneven Li deposition behavior on the anode/electrolyte interface.Based on the super-filling mechanism proposed in the last chapter,this chapter designed an asymmetric solid electrolyte,in which the organic-inorganic composite solid electrolyte was towards the cathode materials to ensure high interfacial compatibility,and the gel electrolyte containing thiourea was towards the Li metal anode for regulating Li deposition behavior.The introduction of thiourea not only ensured uniform Li deposition at the interface,but also improved the electrochemical kinetics of Li metal batteries.5.Optimizing the Li deposition behavior at the anode/electrolyte interface by constructing organophosphorus hybrid protection layer,based on the requirement of Li-S batteries with high energy density for Li metal anode(stability under high current density and Li plating/stripping capacity).Different from the traditional organic-inorganic protective film based on Li₂S_x,which is likely to react with Li polysulfide,the intermediate product of discharging,losing its protective effect.In this chapter,phytic acid was used as a Li metal surface treatment chemicals,and an organophosphorus hybrid flexible solid electrolyte interphase layer was achieved on Li metal surface by one-step surface chelation reaction.In this protective layer,the polynuclear complex between the PA and Li⁺serves as“flexible cross-linked connecter”,which not only ensures the flexibility of this protective layer,but also makes Li_xPO₄ uniformly distribute on the electrode surface,thereby improving the Li⁺transport at the electrode interface and promoting the uniform Li deposition.More importantly,the protective layer has high compatibility with Li-S batteries,which effectively improves the cycling stability of Li-S batteries.