Investigation On Fabrication And Properties Of Lithium Ion Power Batteries

The development of Li-ion power batteries are reviewed in detail. The fabrication and properties of LiMn₂O₄ and LiFePO₄ batteries are studied in this paper.The electrochemical,storage and safety performance and mechanism are studied and improved through cathode,anode and electrolyte studying.The fabrication,performance and mechanism of LiFePO4 battery are studied.Several LiMn₂O₄ cathode,anode and electrolyte samples are compared for battery fabrication.Conformable LiMn₂O₄,artifical graphite and electrolyte sample are chosen for power batteries fabrication.The cell balances are designed as 1.33,1.19 and 1.08, respectively.The capacities of LiMn₂O₄ are 101,105 and 107mAh/g,and the capacity retention ratios are 87.3,85.4 and 84.1%after 170 cycles, respectively.2wt%Li₂CO₃,MgO and LiF is added in the cathode, respectively.The capacities of LiMn₂O₄ batteries are 106.1,107.2 and 107.5mAh/g,and the capacity retention ratio after 100 cycles are 90.8, 91.8 and 93%,respectively.The electrochemical performance is best when content of additive LiF is 2 wt%.It is found that the discharge capacity at 20C rate is equivalent with 94.1%at 1C rate when the area density is 2.5g/dm² and conductive is 3wt%.The capacity retention ratio is 92%with 1C charged and 5C discharged after 100 cycles.The content of electrolyte salt is EC:EMC:E=2:7:1,1.0mol/L LiPF₆.The results show that the discharge capacity at 0.2C and 1C at -40℃are and 81.1%and 63.4%of that at room temperature,respectively.The ratio of capacity recovery of LiMn₂O₄ battery after storage at room temperature for a month is 96.3%.The capacity retention ratio is 94.1%after 100 cycles.The XRD,SEM,TEM and XPS results show that the film covered on the cathode is composed by MnO₂,R-CO₃Li,Li₂CO₃, Li_xPF_y and LiF after storage.And oxygen deficiency in the LiMn₂O₄ electrode is detected.The migration resistance of LiMn₂O₄/electrolyte is increased from 20.28 to 53.31Ωafter storage.And the exchange current is increased from 0.69 to 1.01mA/cm² after storage.AC impedance for anode result shows that the SEI film is incrassated and compacted,and the impedance is increased from 183.1 to 310Ω.FTIR results show that LiPF₆ decomposes at a certain degree after storage.And the solvent in the electrolyte is oxided to small molecular weight substance.The polarization of electrode,Mn dissolution and Oxygen deficiency in the LiMn₂O₄,decomposition and oxygenation of electrolyte,Li⁺ consuming during the incrassated and compacted SEI film is the reason of capacity fading during storage.The capacity lost in the first day is maximal during the high temperature storage.The capacity lost in every day is decreased with the time extending.The improvement of cycling performance of battery after storage is increased with time extending.The capacity loss is slow because of thickness of film covered on the cathode increasing,shrinked and strengthened of LiMn₂O₄ structure,decreasing area of particle/electrolyte,weakened oxygen of electrode.The ratio of capacity recovery of LiMn₂O₄ battery at discharge state is highest,99.2%;and that is 93.5%at charged state.The improvement of cycling performance after charged storage is best.The difference of capacity fading of LiMn₂O₄ battery with different charge state is because of different Mn dissolution and polarization of electrode.Mn⁴⁺ may be deoxidized to Mn³⁺ first,and then reacts with H⁺ to create Mn²⁺ with charged increased.The cycling performance is improved because of the the film covered on the cathode,strengthened spinel structure and the improved SEI on the anode.The capacity recovery of LiMn₂O₄ battery with half charged is increased from 96.3 to 98%after storage with LiF added in the cathode. The results show that the LiF restrain the dissolution of Mn and the polarization of cathode after storage.The LiMn₂O₄ battery(347080-16Ah) is tested by heat concussion, puncture,short circuit and overcharge.The LiMn₂O₄ battery shows good safety performance and doesn't explode.But the battery blast after 3C/10V overcharge.The maximal temperature of battery surface arrives at 290℃after blast.The carbon,MnO,and Li₂CO₃ are observed in the exploded powders.Cracks in the cathode electrode particles are detected with the voltage increased to 5.0V.Cracks may be the reason of voltage and temperature increasing rapidly and blast.The battery is fabricated with Al₂O₃ coated LiMn₂O₄ and doesn't explode after 3C/10V overcharge.Three LiFePO₄ samples are compared for battery fabrication.Effect of carbon nanotube on the electrochemical performance of C-LiFePO₄/graphite battery is studied.The capacity ratio of 0.1C/1C is increased from 92.1 to 96.3%with CNT added,but the capacities discharge at 0.1C are 124mAh/g.Cyclic voltammograms and AC impedance results show that charge transfer resistance is decreased from 173.1 to 36.88Ωwith CNT added,and the polarization of electrode is decreased.And the capacity retention ratio is increased from 93.7 to 96.6%after 200 cycles.The first discharge capacity of LiFePO₄ battery with LA133 and PVDF-binder is 120 and 124 mAh/g discharged at 1/3C, respectively.The capacity retention ratios are 96.3 and 93.2%.347080 size LiFePO₄ power battery is fabricated with two LiFePO₄ sample mixed according to their characteristic.The capacity ratio of 1C/10C is 95.9%, and the capacity retention ratio is 96.9%after 40 cycles with 3C charged and 10 discharged.The battery doesn't explode under heat concussion, puncture,short circuit and overcharge.