Study On Fading Mechanismsfor Lithium-ion Batteries At Energy Storage Systems

The amount of the retired LNCMO batteries has increased year by year with the extensive development of electric vehicles.The retired LNCMO batteries is used in energy storage systems with lower performance requirements,such as electrical energy storage and home energy storage.To a certain extent,it can better play the residual value of the retired LNCMO batteries and reduce the application cost of the power battery in the field of electric vehicles.Studying the performance degradation mechanism of retired batteries under energy storage conditions is of great significance for the secondary utilization of retired batteries.This article mainly carries out long-term cyclic tests on LNCMO batteries under simulated peaking operations,frequency modulation conditions,and uninterruptible power system(UPS).The performance degradation of the LNCMO battery was analyzed from the aspects of active lithium-ion loss,positive/negative electrode material structural damage,and changes in the composition of graphite surface compounds.The performance degradation of the LNCMO battery is analyzed from the aspects of active lithium-ion loss,destruction of the positive/negative electrode material structure,and changes in graphite surface composition.The main research contents are as follows:The retired LNCMO battery is charged and discharged under simulated peaking operations.The cell was cycled at 1C rate between 3.0V and 4.15V at 25?.The electrochemical impedance spectroscopy is tested every 200 cycles(SOC=100%).The capacity of cell was tested every 400 cycles at 1/3C rate.The retired LNCMO cell was disassembled in the glove box at different cycle numbers,and analyze the aging state of the internal materials of the cell.The capacity of the retired cell was significantly decreased after 1200 cycles.The cycle life of the retired LNCMO cells under peaking operations is about 2000 times.The values of ohmic resistance,charge transfer impedance and diffusion impedance gradually increase with the increase of the cycle numbers through equivalent circuit fitting.It is found that the NCM crystal structure is damaged and the graphite layer distance is gradually expanded by XRD analysis.According to the analysis of XPS spectrum tests,the relative content of Li2O,LiF,P-O/P=O and C-Li compounds is increased gradually with the increase of the cycle numbers.The graphite inner layer compound is mainly composed of Li2CO3 and ROCO2Li by sputtering depth analysis.The electrochemical tests and ICP analysis shows that the capacity loss caused by active lithium ions accounted for about 55%of capacity loss of the full-cell and the destruction of NCM active materials accounts for about 30%of capacity loss of the full-cell.The retired LNCMO battery is charged and discharged under simulated frequency modulation conditions.The cell was cycled at 0.5C rate between SOC=45%and 55%at 25?.The electrochemical impedance spectroscopy is tested every 2000 cycles(SOC=100%).The capacity of cell was tested every 4000 cycles at 1/3C rate.The cycle life of the retired LNCMO cells under frequency modulation conditions is about 18200 times.It is found that the NCM material particles are broken and the graphite layer distance is gradually expanded by XRD and SEM tests,According to the analysis of XPS spectrum tests,the relative content of organic and inorganic compounds is increased gradually with the increase of the cycle numbers,and these changes cause an increase in the charge transfer resistance between the active material and the electrolyte.The electrochemical tests and ICP analysis shows that the capacity loss caused by active lithium ions accounted for about 49%of capacity loss of the full-cell and the destruction of NCM active materials accounts for about 45%of capacity loss of the full-cell.The impact of the destruction of the NCM material structure on the capacity loss gradually increases with the number of cycles increases.In the fully charged state,the LNCMO cells are long-term stored under simulated UPS.The LNCMO cells perform capacity calibration and impedance testing every 28 days.The capacity of the cell declines about 2.7%after storage 308 days.The life of the LNCMO cell is about 6 to 7 years under simulated UPS.During the long-term storage,the equivalent circuit fitting shows that the ohmic internal resistance,charge transfer impedance and diffusion impedance of the LNCMO cell increase significantly.The phenomenon of capacity "diving" appeared in the LNCMO cell during long-term cycling.The LNCMO cell is charged and discharged at a constant current of 1C at 45?.The capacity of the LNCMO cell declined rapidly after 850 cycles.According to XRD and SEM analysis,it is found that the mixed arrangement of Li and Ni is more serious in the NCM lattice structure,the pores of the diaphragm are seriously blocked and the graphite material peels off.XPS analysis showed that a large amount of reduction products of electrolyte were deposited on the graphite surface,which is dominated by Li2CO3,LiF and P-O/P=O compounds.From the charge and discharge capacity of the half-cell,it can be seen that NCM and graphite have a certain ability to remove/intercalate lithium.According to the ICP test results,the capacity loss caused by the lithium content deposited on the graphite surface is much larger than the capacity loss of the cell before "diving".The lithium precipitation of the graphite anode accelerates the loss of active lithium-ions during the "diving" of the capacity.