| Lithium-ion batteries?LIBs?have been attractive candidates as the energy storage devices for a wide variety of applications such as hybrid electric vehicles?HEV?,electric vehicles?EV?.The cycle life and safety issues facing lithium ion battery are still bottlenecks in powering HEV and EV.One of the major safety challenges in LIBs is dendritic lithium formation on anode.The formation of dendritic Li on the“standard anode”graphite in preference to intercalation at critical charging conditions accelerates capacity fade and poses safety hazards.In this thesis,unwanted lithium plating in LIBs was well studied,especially effects of operation conditions and electrolyte components on lithium plating.Based on these understandings,novel electrolyte additives was used to improve high rate performance without lithium plating.Lithium plating behavior during charging at low temperatures and high C-rates in the LCO/MCMB full cells was studied.The high voltage plateau in the discharge profile was used to study lithium plating at low temperatures?-20oC and-10oC?.The result showed that low temperature and high charge voltage were more prone to lithium plating at lower charge rate.When charged to 4.2V,Li plating occurred at 3C at 30oC while 0.2C at-10oC.Charge transfer resistance and Li solid diffusion were influence factors that induced Li plating at low temperatures.An electrochemical model was established and used to simulate the plating overpotential and lithium concertration which showed that lithium plating was more prone to occur on the graphite/electrolyte intephase.Moreover,optimization of material properties by simulation showed that charge-transfer kinetic of the insertion reaction on negative electrode instead of solid-state lithium diffusion in graphite particles was the main factor that influenced Li plating.The results of experiment and modeling on lithium plating provided support for the optimization of fast charging procedures and material properties to minimize lithium plating in lithium ion cells.High charge rates and low temperatures were applied to Li[Ni1/3Mn1/3Co1/3]O2/graphite?NMC111/graphite?pouch cells containing ethylene carbonate?EC?-free ethyl methyl carbonate?EMC?-based electrolytes with additives to study unwanted lithium plating.EMC electrolytes,such as 1 M LiPF6 in 98%EMC:2%vinylene carbonate?VC?,have been shown to passivate lithiated graphite effectively and allow Li[Ni0.42Mn0.42Co0.16]O2/graphite?NMC442/graphite?Li-ion cells to operate effectively for hundreds of charge discharge cycles.In cells having both EC/EMC and EMC electrolytes,lithium plating was the cause of rapid capacity loss at high charge rates.In EMC electrolytes,the plated lithium metal was not well passivated and reacted to create gas while in the same cells with 1M LiPF6 EC:EMC?3:7?electrolyte no gas was observed when Li plating occurred because EC passivated metallic Li well.The volume of the pouch cells with EC-free electrolytes increased sharply when Li plating occurred as measured using in-situ methods.Therefore,solvents that could not passivate lithium well could not be used alone,or will cause gas production once lithum plating occurred,which provided guidance for the rational choice of electrolyte solvents.A variety of electrolyte additives,which extend lifetime during low rate cycling,were used in Li[Ni1/3Mn1/3Co1/3]O2/graphite?NMC111/graphite?pouch cells subjected to high rate charging at 20oC.It was found that additives and electrolytes which increased the negative electrode area-specific resistance,Rnegative,decreased the onset current,ILi plating,for unwanted lithium plating.Cells with control or 2%VC+1%ES had small Rnegativeegative with?36.0?×cm2 thus high ILi plating,1.5C;while cells with PES211 and VC had lower ILi plating because the high impedance caused by the formation of SEI by these additives.This relation between ILi plating and Rnegative will help guide researchers for the choose of electrolyte additives that simultaneously increase lifetime and allow fast charging.The effect of the additive lithium difluorophosphate?LiPO2F2?on lithium plating at high C-rates in lithium-ion cells was studied in this work.Li[Ni1/3Mn1/3Co1/3]O2/graphite pouch cells were cycled at high charge rates where unwanted lithium plating was identified as the major aging mechanism.LiPO2F2 was chemically reduced on the graphite electrolde,reducing cell impedance.In addition,1%LiPO2F2 reduced the growth of cell impedance during cycling even at high charge rate?i.e.2C?at 20?°C.EIS measurements on symmetric cells showed that LiPO2F2 slowed the growth of both positive and negative electrode impedances during cycling.Although LiPO2F2 could reduce the charge transfer resistance of the graphite electrode,it cannot prevent lithium plating on graphite which was likely to be controlled by lithium diffusion of lithium into graphite due to the large size of artificial graphite particles of 15-30?m.Cells with smaller graphite particles(dMCMB=10-20?m)showed higher charge rate without lithium plating.Physical characterizations showed that LiPO2F2 participated in the SEI formation with inorganic components including LixPOyFz and LiF,which improved high rate performance and mitigate lithum plaing. |
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