Li Amalgam Alloy To Regulate Li-ion Deposition For Stable Lithium Metal Anode

Lithium metal is the ideal anode material for its super-high theoretical capacity of3860 m Ah g^-1 and the low potential of-3.04 V（vs.H⁺/H₂）in lithium-ion batteries（LIBs）.However,lithium metal leads to uneven deposition of Li⁺,resulting in the formation of lithium dendrites,which impale the diaphragm and cause fire and explosion and other safety problems,seriously limiting the application of lithium metal anode.Lithium alloy（Li_xM,M=Metal）as anode can solve the dendrite problem effectively and possess high theoretical capacity.However,the deep lithiation and very low voltage window are strictly required to achieve the highest specific discharge capacity of these alloys.Importantly,the stress changes of the electrode lead to the porous structure during insertion/removal of lithium ion.It can only be solved by forming nanostructures and compositing with carbon materials.On the other hand,Li_xM can serve as an artificial film to protect lithium metal,sustaining the stability of electrolyte/electrode interface and inhibiting the growth of dendrites on the surface of electrode.Yet the surface of the alloy is not sustained by gradually covered lithium in the repeated cycles and losing the effect of inhibiting dendrites.Based on this,the research mainly focuses on the following aspects:（1）We develop an ultra-thin and self-supporting LiHg film as anode to inhibit the lithium dendrites and promote the electrochemical performance of Limetal.The morphology and structure changes of LiHg film were studied systematically with different deposited capacity.After characterization,it was found that LiHg film was only～7μm thickness with 7 mg cm^-2 mass loading and Li_0.46Hg_0.54 molecular formula.LiHg₃,Li₃Hg were main phases and Liwas trace phase in the pristine LiHg film.The LiHg film was used as an electrode to deposit different lithium capacity(0.0,2.3,3.1,5.0,9.0,55.0 m Ah cm^-2)at the current density of 2 m A cm^-2.The surface morphology was changed from spherical to cylindrical structure and finally became a dense,smooth and bulk Li-rich solid solution interface（Li（Hg）,1 at.%Hg）.In half cell,LiHg film can cycle stably for 400 hours（hours,hrs）under 2 m A cm^-2(2 m Ah cm^-2)with completely100%Coulombic efficiency（CE）.With high loading cathode of LiFe PO₄(LFP,12 mg cm^-2),LiHg film anode can maintain 100 stable cycles at 2C rate.The theoretical calculation results reveals that the self-diffusion coefficient of Hg atom in the Li-rich region is as high as 1.55*10^-3 mm² s^-1 and the self-diffusion speed is 39μm s^-1,which is farther than the deposition speed of Liatom（9.7μm per hour）.Therefore,the Hg atoms in the LiHg film substrate can always diffuse to the surface of anode with high self-diffusion rate and always maintain the lithium-rich solid solution interface.The LiHg alloy film can effectively guide the Liatoms deposition resulting in a smooth and dense dendrite-free surface.（2）By controlling the amount of Hg atoms,we can control the thickness of LiHg film reasonably with a thickness of 100μm,which possesses a double-layer of LiHg₃+Li₃Hg composite structure(Li-Hg,Li_0.67Hg_0.33).The repeated stripping/plating were carried out at ultrahigh current densities of 50 m A cm^-2 in the symmetrical cell（the deposition/stripping time in each cycle is 1 h,respectively）.Li-Hg electrode shows much stable cycle performance over 95 hrs with small overpotential.Moreover,when coupled with high mass loading Li₄Ti₅O₁₂(LTO,10 mg cm^-2)electrode,the Li-Hg|LTO full battery can achieve 350 stable cycles at 5C rate with no-fading.By test analysis,the spherical Hg-rich LiHg₃ phase is preferentially alloyed to Li-rich Li₃Hg on the surface of Li-Hg electrode and then forming the Li-Li₃Hg core-shell structure with dendrite-free morphology as Lideposition.During stripping process of Liatoms,the size of the spherical particles returns to the original state while maintains the Li₃Hg alloy surface.This maybe related to the more higher binding energy of Li₃Hg than Li,which results to the surficial distribution of Li₃Hg in electrode structure to inhibit Lidendrites.This property makes Li-Hg electrode have excellent fast-charging electrochemical properties.（3）The ultra-thin LiHg alloy film（～7μm）was synthesized in-situ on the surface of pure lithium metal to form a composite alloy anode（LiHg-Li）in this paper.The results show that LiHg-Licomposite electrode can achieve long-term stability under large current density and capacity.In symmetric cells,LiHg-Lielectrode can cycle stably for more than 750 hours at 8 m A cm^-2(8 m Ah cm^-2).When coupled with high mass loading cathodes of LFP(12 mg cm^-2)and LiNi_0.6Co_0.2Mn_0.2O₂(NCM,12 mg cm^-2),the composite anode protected by LiHg alloy film can stably 100 cycles without attenuation,which significantly improves the electrochemical stability of the full battery,indicating its reliability for practical development of Limetal batteries.By characterization analysis,LiHg film forms the super stable Li₃Hg alloy after initial lithiation,and then deposited under the Li₃Hg alloy film next to the lithium metal upon followed Lideposition.Upon stripping process,Liwas extracted from Liunder the Li₃Hg film rather than from Liin Li₃Hg due to the higher energy consumption of in Li₃Hg alloy.Theoretical calculation results show that Hg atoms was around by the strong electron density and much higher electronic resistance in the stable Li₃Hg structure,which is more favorable to adsorb Li⁺than Liatoms on the electrode surface,and then Li⁺migrates to the lithium metal surface.Consequently,The unique alloy-deposition effect of LiHg-Lielectrode is an effective strategy to suppress dendrites and protect lithium metal anode,and makes LiHg-Lielectrode long-term stable cycle performance under large current density and capacity.（4）Soft metal indium（In）with low melting point and good ductility was added into the LiHg film to reduce Hg content considering the micro-toxicity of Hg.By adjusting the In/Hg atomic ratio（0.24:0.76,0.43:0.57,0.64:0.36）,a series of Li-Li_xIn_yHg_zcomposite alloy were synthesized with in-situ Li_xIn_yHg_z alloy film（2～5μm）to protect lithium metal anode.The electrochemical cycling performance were tested with different conditions(0.5 m A cm^-2(0.5 m Ah cm^-2),1 m A cm^-2(1 m Ah cm^-2)and 2 mA cm^-2(2 mAh cm^-2)).The composite electrode with In/Hg atom ratio of 0.64:0.36 could operate with long cycles and small overpotentials(3000 hrs（80 m V overpotential）.Coupled with the high-loading cathode LFP(～12 mg cm^-2)to full battery,composite anode could work with 200 cycles at 1C rate without attenuation.After characterization,the test results show that Li_xIn_yHg_z alloy film can guide the uniform deposition of Li⁺and inhibit the formation of dendrites during the cycle.The Li₁₃In₃ and Li₃Hg alloy on the composite electrode surface after many cycles,and the Li₁₃In₃ alloy phase is beneficial to reduce the overpotential.In general,due to the alloy reactivity between liquid mercury and most metals easily,Liamalgam can be synthesized by simple method at room temperature.The Liamalgam can be used as individual electrode or protective layer of lithium metal,which greatly improves the electrochemical performance of batteries and enables them to operate with long stable cycles under large current and capacity.By adding similar indium atom with low melting point into the LiHg film,the ternary alloy film can protect lithium metal and obtain stable cycling performance.