Research On The Characteristics Of Energy Storage Lithium-ion Battery With Constant Current And Constant Power

With the development of new energy,smart grids and electric vehicles,as well as the proposal of energy internet,energy storage technology has been highly valued and developed rapidly.Now a number of large-scale energy storage stations such as the Zhangbei Wind-storage Demonstration Project and Shenzhen Baoqing Energy Storage Power Station have been built.Lithium-ion battery is one of the most commercialized technologies among chemical energy storage currently,whose installed capacity accounts for more than 66%in the field of chemical energy storage in China.In practical application,the dispatch order pass with power charge and discharge.Therefore,energy storage device needs to work at a constant power charge and discharge mode with the performance parameters measured by kW,MW or kWh and MWh.In general,capacity with a unit of ampere-hours is the key characteristic to estimate lithium-ion batteries among designing,producing and testing.However,the obtained electrochemical performance on this mode does not reflect the actual operating conditions of the batteries at constant power charge and discharge.Therefore,it is necessary to establish a testing method and standard based on constant power mode for energy storage batteries.In this thesis,the influence on electrochemical performance of charge and discharge modes is studied and the correlation between charge and discharge modes with the cycle performance of lithium ion batteries is demonstrated.Finally,the performance of TiO₂coated LiNi_0.5Mn_1.5O₄ is synthesized and investigated.The main results are as follows:Firstly,the 66 Ah LiFePO₄ energy storage batteries are tested under constant current and constant voltage charge-constant current discharge mode and constant power charge and discharge mode at different rates.The charge and discharge curves,capacity and energy of the battery under both modes are obtained.The rate performance is better in the constant current and constant voltage charge-constant current discharge mode,which displays a capacity retention of 98.97%at 1 hours discharge compared to the capacity at10 hours discharge.This value decrease to 91.38%in constant power charge and discharge mode due to the absent of constant voltage charge step.The energy of the battery are determined by combined the polarization voltage and discharge capacity.Besides,the supercapacitors and Lead-acid storage batteries are also tested.The supercapacitors with superior rate performance show consistent electrochemical performance,which the discharge energy and energy efficiency up to the peak value at the certain rate both under constant current and constant power.Lead-acid storage battery shows poor rate performance under both test modes as well as the low energy efficiency.When the rate exceeds 6 hours,the discharge capacity declines rapidly.Secondly,full cells are constructed using LiFePO₄,LiCoO₂ and LiMn₂O₄ as the cathode,respectively,and graphite as the anode in CR2032 coin cells.The cells are charged and discharged under constant current and constant voltage charge-constant current discharge mode and constant power charge and discharge mode at 3h rate,in order to explore the correlation between cycle performance and charge-discharge modes.After100 cycles,LiFePO₄ full cell displays a capacity and energy retention of above 99%,and energy efficiency of 95%.For LiCoO₂ full cell,the constant power mode significantly impair its capacity and energy.The energy loss exceeds 40%after 100 cycles.The capacity and energy performance of LiMn₂O₄ full cell are coincident under both operating modes.The capacity only remains 81.7%after 100 cycles.Lastly,the high-voltage cathode material LiNi_0.5Mn_1.5O₄ is synthesized via the thermopolymerization of acrylic acid method,following coated with TiO₂ using nano-TiN as the Ti source by milling and sintering.The effect on the morphology,structure and electrochemical performance of TiO₂ coating is investigated.TiO₂ coating can significantly improve the cycling performance of LiNi_0.5Mn_1.5O₄ at room temperature.The pristine LiNi_0.5Mn_1.5O₄ material exhibits an initial discharge specific capacity of 117mAh g^-1 at 0.1C rate,then the capacity decreases to 97 mAh g^-1 after 100 cycles at 1C rate,and a capacity retention of 82.9%;the 1%TiO₂ coated LiNi_0.5Mn_1.5O₄ material exhibits an initial discharge specific capacity of 120 mAh g^-1 at 0.1C rate,115 mAh g^-1after 100 cycles.The capacity retention is 95.8%.