Study On The Property Of All-Solid-State Lithium Ion Batteries Based On Thermal-Mechanical-Electrochemical Coupling

With the rapid development of new energy batteries and their large-scale applications in electronics,automotive,aviation and other fields,the shortcomings of traditional liquid lithium ion batteries,such as low energy density and low safety,have begun to emerge,which has been unable to meet the comprehensive needs of new generation energy consuming equipment,such as high safety,high energy density,flexible structure and wide operating temperature range.All-solid-state lithium ion batteries are widely concerned because of their high safety and high energy density,and are considered as the next generation battery technology to break through the bottleneck of lithium ion batteries.However,the bottleneck problems such as low yield and poor cycle stability of all-solid-state lithium ion batteries have hindered their commercialization and are in urgent need of technological breakthrough.At present,the design of all-solid-state lithium ion batteries,which are characterized by long development cycles,a wide range of systems and complex operating environments,urgently needs a mathematical model with universal model,efficient calculation and accurate results as a guide.Computer simulation technology is an effective means to accelerate the development and industrialization of all-solid-state lithium ion battery technology.Building an accurate,reliable and effective mathematical model is the key and prerequisite to obtain high-reliability simulation results.However,most of the existing battery mathematical models focus on a single physical field,without considering the complex coupling effect under the coexistence of multiple physical fields,forming a physical“island”phenomenon.Therefore,this paper focuses on the coupling modeling method and interaction mechanism of multiple physical fields in the working process of all-solid-state lithium ion batteries.The main works are as follows:（1）A modeling method of multi physical field coupling for all-solid-state lithium ion battery is proposed.This paper aims to solve the problems of single model,low accuracy and“island effect”in the simulation of all-solid-state lithium ion batteries.Based on the basic theories of thermodynamics and dynamics,the physical and chemical processes involved in the working process of all-solid-state lithium ion batteries are deeply analyzed.The basic theoretical equations such as Nernst equation,Fick’s diffusion law,Nernst-Planck mass transfer equation,generalized Hooke’s law and heat conduction equation are extended,and the multi-physical field coupling terms are added.Finally,the complete numerical models for all-solid-state lithium-ion batteries including electrochemistry,mass transfer,mechanics and heat transfer are established.（2）Based on Persson’s contact mechanics theory,a one-dimensional electrochemical-mechanical coupling model for all-solid-state lithium-ion batteries considering the interface contact area loss is proposed.The effects of external pressure load on the interface contact area loss and battery performance are systematically studied.A graphite|Li TFSI|Li Co O₂ thin film batteries is prepared,and the charge-discharge test device is assembled.The accuracy of the simulation model is verified by comparing with the experimental data.The influences of the contact area loss and pressure on the electrical and mechanical properties of the battery are studied with the help of the simulation model.The results show that the loss of contact area aggravates the deterioration of battery performance.The contact area loss at the cathode/solid state electrolyte interface leads to the lithium ion concentration falling to the minimum threshold in advance,resulting in the voltage and capacity reduction.The contact area loss at the anode/solid state electrolyte interface leads to the battery reaching the charging cutoff voltage in advance,resulting in the capacity reduction.Larger（>1MPa）and lower（0.05～0.4MPa）pressure loads cause the battery performance to decrease,while the best performance can be obtained at moderate（0.4～1MPa）pressure,where the interfacial stress difference is also minimal.（3）A one-dimensional electrochemical-mechanical coupling model suitable for all-solid-state lithium ion batteries with rigid constraints is established,and a novel method is proposed to improve the interfacial contact by using the volume expansion difference of electrodes.By comparing the simulation results with the existing experimental data,the effectiveness of the proposed model is verified.Considering the volume change during charging and discharging,the influences of the current and partial molar volume on interfacial contact and cycling performance are further investigated,and the characteristic equation of the battery under zero-stress condition is given.The results show that the change threshold of internal stress is related to current.Because the lithium embedded in the anode cannot all return to the cathode in discharging,a“residual stress”is created,which helps to improve interfacial contact.The interfacial contact loss leads to an increase in local current density.When the current density is 0.2m A/cm²,the local current density at the interface is as high as 0.66m A/cm²,which promotes the formation of lithium dendrites and eventually leads to a short circuit in the battery.The higher the partial molar volume of the cathode or anode,the lower the capacity retention rate of the battery.In addition,increasing the partial molar volume of cathode or decreasing that of anode leads to the reduction of stress,but this is not conducive to improving the interface contact.Therefore,it is recommended to use the cathode materials with low partial molar volume to improve interface contact and prevent deterioration of electrical properties,and for the negative electrode,a moderate partial molar volume（3.7～4.6cm³/mol）is selected to equalize the capacity and interfacial contact.（4）Aiming at the problem that the one-dimensional model is difficult to visually reflect the difference of particle properties at different locations,a heterogeneous model of all-solid-state lithium ion battery with spherical particle structure is established,and the effects of charging C-rate,active particle spacing and particle size on battery performance are analyzed.The proposed model is verified by the measured data.The results show that the high C-rate leads to the serious polarization of the battery in charging,which leads to the decrease of the capacity retention rate.Compared with the particle contact surface and the particle center,the free surface of the particle exhibits greater stress and smaller volumetric strain.The SOC,bulk energy density and stress of the battery are significantly affected by particle spacing.The SOC is positively correlated with the spacing of cathode,and the bulk energy density reaches the maximum when the spacing coefficient is 0.88.The SOC and volumetric energy density of the battery increased with the increase of the distance between the cathode particles.The higher the charging C-rate,the higher the requirements for particle size of the battery.（5）Aiming at the problem of poor performance prediction accuracy of existing battery models in low temperature environment,a thermal-mechanical-electrochemical fully coupled model for large soft-packing battery is constructed,and a method combining active heating and passive insulation is proposed to improve the low temperature performances of batteries.The simulation results at different temperatures are compared with the measured data to verify the effectiveness of the proposed model.The results show that the battery capacity decays with the decrease of ambient temperature and the increase of C-rate,and the lower the temperature and the higher the C-rate,the more serious the capacity decays.Using active heating device to preheat the battery is effective at high C-rate discharge,but the energy utilization rate is low.The combination of active heating and passive insulation can maximize battery performance.