First Principles Study Of The Interface Between Lithium Metal Anode And Electrolyte

Due to its high theoretical capacity(3861 mAh/g)and the lowest reduction potential(-3.04 V compared to standard hydrogen electrode),lithium metal is expected to be the next generation ideal anode material for energy density batteries.However,lithium metal anode with relatively active characteristics can easily react with electrolytes to form an unstable interface between anode and electrolytes,resulting in uncontrollable dendrite growth and then affecting the performance of batteries.LiF,as one of the main inorganic components of the solid electrolyte interphase(SEI)film,has been experimentally confirmed that the addition of LiF during the formation of SEI can regulate the morphology of lithium deposition and improve the electrochemical performance of batteries,while the specific mechanism has not yet been clarified.In this thesis,the interface between Li metal and LiF were constructed with computation to study the specific interaction mechanism at the interface.Based on the results of first principles calculations,main innovative results of this work are summarized below:(1)Surface models of Li and LiF were constructed and both the electronic properties and ion transport mechamisms of the related models were calculated.When the number of layers in the slab Li(001)model was five,the corresponding value of surface energy was 0.48 J/m2.As the number of layers continued to increase,the surface energy tended to converge.Combined the literature reference value with the calculation duration,the number of layers in slab LiF(001)model were determined to be three.Compared with bulk LiF,the band gap of slab LiF(001)reduced from 8.87 eV to 5.81 eV owing to the size effect.The migration of Li atom at two adjacent stable Hollow sites on the surface of lithium metal needed to overcome an adsorption energy barrier with 0.03 eV.The migration energy barriers were 0.64 eV and 0.74 eV respectively when Li+ migrated in the bulk LiF structure by the vacancy mechanism and the interstitial mechanism.(2)An interface of Li and LiF was built,and both electronic features and ion diffusion properties of the interface were simulated.The mechanism of LiF suppressing the lithium dendrite growth could be obtained simultaneously.The Li/LiF model constructed here had a lattice mismatch of 2.17%,which satisfied the requirement of less than 5%.The interfacial interaction energy is-3.81 meV/m2.The energy value was negative,indicating that the constructed interface model was thermodynamically feasible.The band gap of the interface near the Fermi level was zero,revealing that the interface system of Li/LiF had high conductivity and was beneficial to ion migration.At the interface of Li/LiF,the density of state about the Li-2s orbital in the Li layer and the F-2p orbital in the LiF layer near the interface overlapped each other,verifying that there was interaction between the Li layer and F layer,which had a certain influence on the distribution of electrons.The charge rearrangement was mainly concentrated on the interfacial layer,while the layers which were far from the interface had no obvious electrons gained and lost.Therefore,the charge concertration distribution of the entire interface system had a potential gradient,inducing Li+ in the electrolyte to aggregate at the interface.The migration energy barrier of Li+ at the Li/LiF interface(0.58 eV)was much smaller than the sum of that on lithium metal(0.03 eV)and in bulk LiF(0.64 eV),which indicated a low impedance at the interface,thus it facilitated the uniform deposition of ions aggregated at the interface and induced the formation of a controllable electrode/electrolyte interface.The redistribution of interfacial charge and the lower energy migration barrier at the interface worked together to make LiF stabilize the interface of lithium metal anode/electrolyte and protect the lithium metal anode.The calculation results from the atomic level proposed a new insight into the interaction mechanism of Li/LiF interface,and provided a new idea for exploring new interface protection materials or electrolyte additives in lithium metal batteries.The in-depth understanding of the interface between Li metal anode and LiF would be beneficial to the application and further development of lithium metal batteries,and would had certain guiding significance for the design of a stable lithium metal anode in the future.