Design And Electrochemical Performance Of Positive Electrode And Electrolyte Materials For Liquid Metal Battery

Liquid metal battery(LMB)is featured by all-liquid battery structure and earth-abundant electrode and electrolyte materials.These characteristics endow it with desirable cost-efficiency,long cycle lifetime,and flexible scalability,and make it an attractive candidate for grid-scale energy storage.Nevertheless,the development of the LMB still faces many challenges.The low Li diffusion rate in solid discharge products and excessive addition of inert alloy components seriously reduce the energy efficiency and energy density,and the high operating temperature is detrimental to the long cycle stability of the battery.Based on the thermodynamical and electrochemical theories,high-performance electrode and electrolyte materials are designed in this thesis.Rapid Li diffusion paths are elaborately constructed among the solid intermetallic compound layer to accelerate the electrode reaction kinetics.Advanced dual-active components alloy electrodes with a novel lithiation mechanism are prepared to improve the specific capacity and discharge voltage,and therefore enhance the energy density of LMB.A new-type low-melting-point electrolyte is developed with the help of the theoretical calculation based on the mass triangle model to decrease the battery working temperature,and so realize stable operation of high-performance LMB.The correlations between material composition and electrochemical properties of the designed electrode and electrolyte materials are systematically investigated.The main results of this thesis are listed below.(1)To solve the sluggish electrode kinetics issue,Bi-Ga alloy electrode materials are designed based on the physicochemical properties of materials and CALPHAD.During discharge,the fast generation of solid Li3Bi at positive electrode/electrolyte interface induces Ga-rich melts formation in the intermetallic compound layer,which provides rapid Li diffusion paths and so enables accelerated electrode reaction kinetics.The electrochemical tests indicate that the introduction of Ga significantly increases the Li diffusion coefficient of the positive electrode and decreases the voltage polarization.The Li‖Bi-Ga battery presents high discharge voltage and so increased energy efficiency,as well as outstanding rate performance.A high discharge voltage of 0.67 V is registered at 200 mA cm-2,corresponding to a low polarization of 4.3%compared to its electromotive force.At 800 mA cm-2,the energy efficiency increases from 44%(the unmodified Li‖Bi battery)to 61%of the Li‖Bi-Ga battery.(2)A Bi-Zn alloy positive electrode with high voltage charateristics is designed,and a unique lithiation mechanism is proposed in the LMB field.It is demonstrated that the LiZnBi ternary intermetallic compound is generated during discharge,which can effectively increase the discharge voltage of the battery.Moreover,the introduction of Zn also helps to improve the electronic conductivity of the electrode.The assembled Li‖Bi70Zn30 battery shows excellent electrochemical performance with specific capacity of 316.97 mAh g-1 and high discharge of 0.73 V at 100 mA cm-2,which is even higher than the electromotive force of the Li‖Bi battery.At 200 mA cm-2,the discharge voltage is 0.70 V.Moreover,a reversible specific capacity of 269.08 mAh g-1(85%capacity retention relating to that at 100 mA cm-2)is attained at 1200 mA cm-2,indicating an outstanding rate capability.In addition,the capacity fade rate is only 0.08%per cycle during 200 repeated cycles at 800 mA cm-2,suggesting an excellent long-term cycle performance.(3)The Zn introduction can significantly improve the voltage characteristic of the Sb-based materials.The formation of LiZnSb ternary intermetallic compound enables an attractive voltage platform of 1.1 V.Meanwhile,the dual-active feature of the Sb-Zn alloy increases the specific capacity of the positive electrode,and so improves the battery energy density.The conversion reaction of LiZnSb to Li3Sb during deep lithiation process is accompanied by the in-situ exsolution of Zn.The regenerated Zn melt disperses in the solid discharge product layer,building up fast Li and electron percolating networks,which dramatically improve the electrode reaction kinetics.The electrochemical characterizations show that the Sb30Zn70 electrode delivers high specific capacity of 423.76 mAh g-1,and Li‖Sb30Zn70 battery registers a high discharge voltage of 0.76 V and superior energy density of 290.6 Wh kg-1 at 100 mA cm-2.The capacity retention can attain 93%when the current density increases from 100 to 1000 mA cm-2,indicating an outstanding rate performance.(4)Based on the phase diagram analysis,a high-energy-density Sb-Cd alloy electrode is proposed.The liquid phase temperature of Sb-based materials can be decreased with relatively low Cd addition.The ternary intermetallic LiCdSb formation during discharge and the Li-Cd alloying reaction improve the discharge voltage and increase the specific capacity of the Sb-Cd material.As a result,a superior energy density is achieved in the Li‖Sb-Cd battery.Furthermore,the exsolved Cd melt exists in the solid intermetallic layer,which helps to enhance the conductivity of the Sb-Cd electrode and so accelerate the electrode reaction kinetics.The effect of Cd content on the electrochemical performance is studied.The Sb80Cd20 electrode presents the optimal electrochemical performance with a high specific capacity of 541.62 mAh g-1,enabling a preeminent energy density of 398.4 Wh kg-1 to the Li‖Sb80Cd20 battery at 100 mA cm-2,which far outperforms most reported LMB systems.Even at the high current density of 2400 mA cm-2,a specific capacity of 487.91 mAh g-1 can still be attained,corresponding to a capacity retention of 90%.(5)LiCl-LiBr-KBr electrolyte is elaborately prepared to reconcile the melting point and ionic conductivity by using mass triangle model calculation.The optimized LiCl-LiBr-KBr(33:29:38 mol%)registers low cost(21.23 $ kg-1),low melting point(327 ℃),and high ionic conductivity(1.573 S cm-1 at 420 ℃),which renders the assembled battery to operate stably with excellent electrochemical performance at 420℃,meaning at least 80℃ decrease in operating temperature compared to promising Li|LiF-LiCl-LiBr|Sb-Sn and Li|LiF-LiCl|Bi LMBs.The Li|LiCl-LiBr-KBr|Bi battery delivers a high discharge voltage of 0.69 V at 100 mA cm-2.When the current density increases to 400 mA cm-2,the discharge voltage decreases slightly to 0.54 V,corresponding to a small polarization voltage of 0.15 V.This cell can still operate stably after freeze-thaw cycle with negligible voltage and capacity variation relative to that before the test,showing superior ability against wide-range temperature fluctuation.