Research On High-energy-density Composite Lithium Metal Batteries

Throughout the battery development history since 1800,energy density,cycle life and safety are the three core technical requirements for human to develop batteries.Since SONY commercialized lithium-ion batteries in 1991,the energy density of lithium-ion batteries has gradually increased from 90 Wh kg^-1 to up-to-date 300 Wh kg^-1.However,limited by the theoretical energy density of the commonly used intercalation anode and cathode materials,it is difficult for China to achieve the development goal of 500 Wh kg^-1 by 2030.In order to achieve this goal,we urgently need to develop new anode and cathode materials with high theoretical energy density.Lithium metal anode has a high theoretical specific capacity(3862 m Ah g^-1)and low electrode potential（-3.04 V vs.standard hydrogen electrode）,but high chemical reactivity,huge volume change and dendrite growth bring serious challenges for the practical application,and cycle life and safety problems become the biggest obstacles for the application of lithium metal anode.To solve these challenges,one effective strategy is to introduce solid-state electrolyte with good chemical stability and high mechanical strength to protect the interface of lithium metal anode and the electrolyte.To solve problems mentioned above,this paper focuses on the research on high-energy-density composite lithium metal batteries,and the work mainly includes three parts:（1）We screened for various high-energy-density,high-thermodynamic-equilibrium-voltage,low-cost,high-safety batteries by thermodynamical method,and estimated their practical energy density;（2）an organic-inorganic composite artificial SEI was constructed on the surface of lithium metal anode by in-situ polymerization to inhibit side reaction with commercial carbonate electrolyte and improve the cycle performance of lithium metal battery;（3）an integrated half-solid-state composite lithium metal anode was constructed by in-situ polymerization to further stabilize the lithium metal anode/electrolyte interface and improve the cycling performance and safety of lithium metal batteries.In the first part of this paper,we calculated the theoretical energy densities and electromotive forces of 1683 batteries based on conversion reaction by thermodynamical method,and selected 51 batteries with theoretical energy density over1000 Wh kg^-1,electromotive force over 1.50 V,low cost and low toxicity,for example,O₂/Li,O₂/Al,O₂/Mg,H₂O/Li,CO₂/Li,S/Li,CO₂/Al,H₂O/Al batteries.By comparing the theoretical energy densities of different types of batteries matched with the same cathode,it was found that the theoretical gravimetric energy densities of Li batteries and the theoretical volumetric energy densities of Al and Mg batteries were the most advantageous among Li,Na,K,Mg,Al and Zn batteries.Moreover,we also constructed a PEO/Li TFSI polymer pouch cell model to estimate the practical energy density of the selected batteries.The results showed that the ratio of the estimated gravimetric energy density and estimated volumetric energy density to the theoretical value were 0.48-0.67and 0.50–0.53,respectively.Finally,we summarized and forecasted the development of high-gravimetric-energy-density and high-volumetric-energy-density battery.In the short term,developing high nickel NCM cathode,composite silicon carbon anode and lithium carbon anode is still the primary strategy to improve the energy density of batteries.However,to develop batteries with energy density of more than 600 Wh kg^-1and 1300 Wh L^-1,it is essential to develop the 51 new batteries based on conversion reaction selected by this paper.In the second part of this paper,an organic-inorganic composite artificial SEI about5μm thick was grown on the surface of lithium metal by in-situ polymerization.It was mainly composed of organic and inorganic products of PEGDA and Li DFOB reacted with lithium metal.The surface of the SEI was more uniform than that of pure lithium without any treatment.After the artificial SEI contacting with the Li metal anode,more Li F was generated through the chemical reaction during cycling,improving the chemical and electrochemical stability of the artificial SEI,while the formation of lithiated polymer favored intimate contact with the lithium anode and improved the conductivity of lithium ions.The thermal stability of the artificial SEI was excellent and the initial thermal decomposition temperature was about 250℃.In addition,the artificial SEI had a good flexibility and a Young’s modulus of 14.3 MPa,which was about ten times higher than PEO.Thus,the artificial SEI can withstand the huge volume change and maintain the structural integrity during cycling,inhibiting the side reaction with the electrolyte and suppressing the formation of lithium dendrite,assuring long cycling performance of lithium metal battery.With the above advantages,artificial SEI modified Li||NCM 811 batteries showed a high capacity retention of 74.1%after 200cycles.Artificial SEI modified Li||Li battery achieved a stable cycle for 700 h(0.5 m A cm^-2,1 m Ah cm^-2)with a small overpotential of less than 60 m V.This work innovatively proposed a strategy of constructing artificial SEI on the surface of lithium metal by in-situ polymerization,which can greatly enhance the interaction between artificial SEI and lithium metal,and inhibit the side reaction between lithium metal and electrolyte,providing a new solution for improving the cycle performance of lithium metal battery.In the third part of this paper,in-situ polymerization was applied to bond lithium metal and separators with ceramic coating to a composite integrated half-solid-state lithium metal anode,which favored more uniform and denser lithium deposition morphology,improving the electrochemical stability of lithium metal anode/electrolyte interface,and reducing the polarization during cycling.The decomposition of SEI and the side reaction between anode and electrolyte are usually the initial sources of heat to induce the chain reaction of battery thermal runaway.Therefore,the structural design of half-solid-state lithium anode took advantage of the good thermal stability of solid-state electrolyte and greatly improved the safety of lithium metal battery.We also found that compared to Al₂O₃ coated separators,half-solid-state Li||NCM 811 batteries with LATP coated separators had a better cycle performance with lean electrolyte and at high temperature.With electrolyte of 30μL m Ah^-1,the capacity retention of NCM 811 was as high as 73.6%after 200 cycles.At 45℃,the average coulombic efficiency was up to 99.56%and the capacity retention was about 66.5%after 200 cycles,which was much higher than that paired with lithium anode with no treatment（12.2%）and lithium anode with artificial SEI modified（30.0%）.This may be because PEGMEA/Li DFOB precursor infiltrated into the gaps between LATP particles and formed organic-inorganic solid-state electrolyte with a complete ion transport network after in-situ polymerization induced by heat treatment,which can improve ionic conductivity,reduce electrode polarization,improve the practical specific capacity and inhibit the decomposition reaction of Li PF₆ in carbonate electrolyte on the surface of solid-state electrolyte.This work proposed a creative design of composite half-solid-state lithium anode to further improve the electrochemical stability of lithium/electrolyte interface based on the second part of this paper,providing a practical strategy to build safer lithium metal batteries for transition from liquid to all-solid-state batteries.In conclusion,this paper mainly focuses on the research on the energy density,cycle life and safety of high-energy-density composite lithium metal battery,which provides a suitable reference for the development of new high-energy-density battery and interface protection of lithium metal anode by in-situ polymerization.