Study On Interface Dynamics And State Estimation Of Lithium Ion Batteries

Since the 21 st century,the limitation and constant consumption of fossil fuels have led to soaring oil prices,while environmental damage has also increased people's pursuit of renewable energy.The discontinuity of renewable clean energy production,the difference between generation and consumption of space and time,in order to achieve its efficient use,the of energy storage devices are needed.At the same time,the electric vehicles(EV),hybrid electric vehicles(HEV)and mobile device have put forward higher requirements for lithiumion battery with high energy density,high power density and high safety.Because of its high energy density and power density,lithium ion battery has become a research hotspot.In order to improve the performance and estimate the state of lithium ion battery.From the perspective of electrochemistry,by means of simulation and measurement,we carried out the following research: Firstly,the multi-physical field simulation model of lithium iron phosphate batteries was established,and the mechanism of depolarization to improve the rate performance and capacity of lithium iron phosphate batteries was studied.Secondly,the largescale constant-current pulse method and the model were established,the lithium iron phosphate battery with different health levels,and the cathode and anode were studied.Thirdly,the reasons for the performance degradation of lithium iron phosphate battery was studied by Non-in-situ measurements.Finally,the effect of anode and cathode on battery performance was studied,and the following research results are obtained:(1)Lithium iron phosphate was coated with different nano-conductive carbon.The CV and charge-discharge curves shown that the surface contact has better performance.To analyze the internal mechanism,the multi-physical simulation model of lithium iron phosphate electrode was established.The charge-discharge curves,the diffusion flux,concentration distribution,current distribution and potential distribution of lithium ions in the electrode were obtained through multi-physical simulation,which proved that the effectiveness of surface contact in improving battery performance,and provide a theoretical basis for the design of the electrode.(2)A large galvanostatic pulse method(LGPM)was designed.This method can produce a transient change at the electrode/electrolyte interface,get the information of electrode interface.The first order RC circuit was used as an electrochemical model.(3)The lithium iron phosphate and graphite electrodes were tested by LGPM,and the first order RC circuit model was used for analysis.The parameters of lithium iron phosphate and graphite electrodes during charge and discharge were obtained.According to the changes of shunt capacitance and shunt resistance,the following conclusions were drawn: there is mutual charge between lithium iron phosphate particles in lithium iron phosphate battery electrodes,and the performance of graphite electrode determine the performance of lithium iron phosphate battery.The insufficient surface area of graphite electrode is made up by the rapid diffusion rate of lithium ion.(5)Combined with the LGMP,the lithium iron phosphate batteries with different health levels were analyzed.It can be inferred that lithium iron phosphate was not the main factor of the performance degradation of the battery.And that graphite negative electrode was the main factor for the performance degradation of lithium iron phosphate battery.The graphite electrode is covered by SEI film,it can store lithium ions and reduce the amount of lithium ions in graphite,which leads to the reduction of lithium ion battery capacity.And the shunt capacitance gets higher.(6)The parallel capacitance and resistance of graphite electrode and lithium iron phosphate electrode were extracted.The anode and cathode's model and parameters were used to construct the second-order RC circuit.The response voltage of the model was obtained under the LGMP methode.Using first-order RC circuit model and voltage response,the model value of first-order RC circuit was obtained by means of parameter estimation.The obtained circuit parameters were compared with those of the anode and cathode,and the anode and cathode' effects on battery performance were studied.