Effect Of Prestress On Mass Transfer In Positive Electrode Of Lithium Air Battery

The theoretical energy density of lithium air battery is 11140wh/kg,which is about ten times of that of lithium ion battery.Because of its good application prospect,it has become a new favorite of electric vehicle power system.In addition to the development of porous electrode materials for lithium batteries,the internal mass transfer performance is also need to improve.In this paper,the performance of lithium air battery is studied by macro simulation method combined with the experimental verification,and the performance of lithium air battery under different influencing factors is analyzed,which has certain guiding significance for the future development of lithium air battery.In the aspect of experiment,the special positive electrode of lithium air battery was prepared on the basis of multi walled carbon nanotubes and Keqin carbon.Under the prestress of 1 to 6 MPa,the effects of prestress on the discharge capacity,impedance and cycle number of lithium air battery were compared and analyzed through deep discharge test,cyclic discharge test and electrochemical impedance test.The results showed that with the increase of prestress,the performance of the battery decreased obviously.With the increase of the prestress,the porosity of the positive electrode of the battery decreased,which leaded to the decrease of the storage space of the product lithium peroxide,and forced the reaction to stop.However,the contact of all reaction interfaces was poor when the preload was too small,which was not conducive to the full progress of mass transfer and electrochemical reaction.Therefore,4MPa is the best prestress.In the aspect of simulation,a two-dimensional model of mass transfer for lithium air battery was constructed based on the principles of electrochemical mass transfer,solid mechanics and solid state deposition reaction,and the validity of the model was verified by experimental data.The performance of lithium air battery was analyzed by finite element method under the assumption of transient and normal temperature.The mechanism of the effect of prestress on mass transfer in the battery was elaborated by obtaining the distribution of components at different discharge time.Referring to the above experimental conditions,the characteristics of lithium peroxide distribution,lithium ion transfer rate,specific capacity and positive porosity distribution of lithium air battery at different discharge time were compared and analyzed with different prestress as variables.The simulation results showed that with the increase of preload,the porosity of positive electrode decreased,the specific capacity and potential decreased,the distribution of oxygen and other components became worse,and the overall performance of the battery decreased.The simulation results were consistent with the experimental results,which further showed that the effect of excessive prestress on the battery is negative.Combined with experiment and simulation analysis,it can be concluded that the porosity of cathode had great influence on the performance of lithium air battery.By changing the pore structure of the positive electrode,a new viewpoint of gradual porosity was put forward,and the influence of linear porosity and single porosity on the performance of lithium air battery was compared.The results showed that the specific capacity of the battery with linear porosity of 0.65-0.85 was 2.5 times of that with single porosity of 0.75.The linear porosity had influence on the porosity distribution,oxygen concentration,Li2O2 volume fraction and oxygen diffusion coefficient during discharge.It showed that the structure optimization of the positive electrode(also known as porous electrode)plays a very important role in the performance of lithium air battery.