Simulation On Discharge Performance And Lithium Dendrites Growth In Li-N₂ Battery

The invention of Li-N₂ battery has important engineering significance as a new generation metal-gas battery.Li-N₂ battery can be used not only as an energy storage device with ultra-high theoretical capacity and high specific capacity,but also as a reversible nitrogen fixation device.Some researchers have performed a series of modification studies on Li-N₂ battery cathode materials continuously up to now.There is a lack of theoretical description of the material changes at atomistic and microscopic level during the discharge process.And it still lacks the correlation between battery materials with voltage-capacity.Nor a detailed simulation for the different factors affecting the discharge performance of batteries and the growth process of lithium metal cathode dendrites has been performed at the device and macroscopic level.In view of this,this thesis adopted computational methods based on the first principles method to explore the possible material changes at the molecular level during the discharge process of Li-N₂ batteries.The factors affecting the discharge performance,the lithium ion deposition,and dendrite growth processes on the surface of the anode lithium metal electrode were studied with the finite element method.The main findings are as below.Firstly,we simulated the possible material changes during the discharge of Li-N₂ battery with the first principles method.A graphene substrate model was established as the anode.It was found that the N₂ molecule interacting with graphene has a tendency of N atoms separation,and this tendency affects the distribution of Li and N on the graphene during discharge.The subsequent discharge simulations were performed on a 4N-graphene anode substrate model.We simulated and analyzed the free molecules of Li N,Li₂N,Li₃N,and Li₃N₂ in terms of their atomic configration,electrostatic potential,total density of states（TDOS）,HOMO,and LUMO.We confirmed that the experimentally discovered molecule Li₃N is the final stable product of Li-N₂ cell due to its stable molecular orbitals and suitable bond length.6 Li atoms are adsorbed sequentially on the 4N-graphene cathode model to study the discharge products and processes.After adsorption,Li atom loses electrons to be positively charged while the substrate is negatively charged.We found that the adsorption energy is linear with the amount of transferred charge.Using the Gibbs free energy changes of the adsorption system,the battery’s voltage-capacity relation was calculated without considering the volume work and entropy change.The electronic properties of the adsorbed system,such as TDOS,suggest that the adsorption changes graphene sbustrate from a semiconductor to a metallic one.This means that it is advantageous for graphene to be a cathode material in Li-N₂ batteries.In addition,the differential charge also indicates that the Li-N substance has a strong adsorption interaction with the substrate,which can play a dual role of electricity storage and nitrogen fixation.Secondly,we analyzed the factors influencing the discharge performance of Li-N₂batteries on the basic study of discharge products using a finite elemental one-dimensional model.Li-N₂ battery has better performances when discharging at low current density and with increasing temperature,porosity and nitrogen solubility factor.The ambient temperature has an overall important effect on the discharge voltage.When the ambient temperature is 350 K and the discharge current density drops from 0.50 m A/cm² to 0.05 m A/cm²,the initial discharge voltage of the battery rises by 0.16 V.If discharged at a small discharge current density of 0.05m A/cm² at 350 K,the initial discharge voltage of the battery will increase by 0.33 V.In addition,the nitrogen concentration distribution after discharge is mainly influenced by the cathode porosity and less influenced by the nitrogen solubility factor.The deposition of the final discharge product Li₃N is the main cause for the porosity change.The lower porosity of the porous electrode at the completion of discharge means that the pores of the porous electrode are occupied by more discharge product Li₃N and also means higher battery capacity.The two voltage-capacity curves we obtained using the finite element method and the first principles calculation are in general agreement with the variation trends of two experimental results.Finally,we investigated the lithium ion deposition and dendrite growth processes on the surface of lithium metal electrodes with different appearance based on a finite elemental two-dimensional model.It was found that the smooth electrode surfaces are more favorable for ions’uniform deposition.At the same time,lithium ion deposition and dendrite growth on the surface of lithium metal electrodes are two processes in which lithium ions accumulate in a certain area under the combined effect of different factors such as diffusion coefficient,ion concentration and electrode surface morphology,thus forming the dendrite morphology.The dendritic morphology on the electrode surface as the deposited layer is under the control of diffusion coefficient and system lithium ion concentration,which is formed by lithium ions accumulation to certain favorable regions due to diffusion inhomogeneity.At the initial stage of deposition,the lithium ions are deposited on the electrode surface to obtain a relatively uniform morphology.However,the original uniform morphology is gradually replaced by the undulating bumps with the passage of time,and then mossy lithium dendrites appear and grow.Throughout the whole simulation process,the deposition intensity in the edge region of the electrode is always larger than that in the middle region and the largest dendrite always appears in the edge region.Our simulated results would deepen the understanding for Li-N₂ batteries and could provide theoretical help for further improvement of the battery performance.