| With the wide and ever-increasing application of electric vehicles(EVs),lithium-ion batteries,as the only or the most important power source for the EV,attract more and more concerns.As the increasing of the specific energy of the battery,the range of EVs have been extended dramatically.However,comparing with the traditional internal combust engining vehicles,the charing time is still too long.Meanwhile,the safety,especially thermal stability,has been decreasing.Thus,the safety and charging time have become the bottleneck of EVs' marketability.(1)Reducing the charging time for EV is considered as a crucial factor to promote consumer interest and increase EVs' marketability.A novel self-adaptive fast charging algorithm is proposed,which is suitable for the high specific energy lithium-ion batter-ies,and it could prolong the cycle life of the battery extensively while implementing fast charge.Based on the combination of constant incremental capacity(dQ/dV)and constant current algorithm,the proposed charging algorithm could adaptively adjust the charging current through the entire-life according to the intrinsic properties of battery without heavy computational load,so as to achieve the goals of reducing charging time and prolonged cycle life at the same time.In this work,the NCM811 batteries has been tested,firstly with digital image correlation(DIC)to measure the volume expansion during normal charge,Li plating and overcharge;secondly,the cycle test results show that the battery using the proposed fast charging algorithm maintains a capacity of about 90%after 1150 cycles,which is far better than the constant current that the capacity re-mained 80%after 600-800 cycles,indicating that the proposed charging algorithm can improve the cycle life of the battery while decreasing the charging time.The fatigue analysis,anode potential evaluation and post-mortem results of cycled battery indicate that the capacity fading mechanism of proposed charging algorithm is mainly the loss of anode material,and there is no lithium deposition on anode,thus both the cycle life and safety have been improved significantly.(2)Since there is a huge amount of energy stored in the battery system,the thermal runaway(TR)propagation could induce fire or explosion,and would pose severe threat to the vehicle and passengers,thus it is necessary to eliminate or inhibit the TR propa-gation.In this work,the cylindrical 21700 batteries are conducted the TR experiment in cell and module levels with large number of samples,during which we discovered a non-standard failure mode,named as casing rupture,and verified its effect on the TR propagation.The cells are externally heated to trigger TR,and the casing rupture is found to be the key factors to cause an immediate cell-to-cell TR propagation.The appearance and cross-section microstructure show that the casing rupture could be cate-gorized as melting hole and tearing crack,and the mechanism of its formation has been studied.Furthermore,the simulative experiment has bee carried out to reproduce the casing rupture to verify the mechanism.It is found that the melting hole is caused by large current short circuit and the tearing crack is due to decreased mechanical strength with high temperature.Simulative experiments are conducted to further demonstrate the casing rupture mechanism.In addition,new designs with the enhanced casing thick-ness are implemented to inhibit the occurrence of casing rupture,and the effectiveness is further validated by numerous TR experiments.Our improvement has been applied in the commercial battery pack for more than 6 million batteries and over 150 million kilometers with no TR propagation in the field.(3)We design and fabricate a novel lithium ion battery system based on direct con-tact liquid cooling(DCLC).It is the first time that no gap battery arrangement has been implemented,and the contact area between battery and cooling liquid has been maxi-mized,thereby the battery system achieves maximum volume efficiency.The mass and volume integration ratio of the battery system are 91%and 72%,respectively,which are 1.1 and 1.5 times that of tube-based indirect contact liquid contact cooling(ICLC)sys-tem,respectively.The thermal management system has excellent performance,thanks to the significantly improved heat exchange efficiency.The simulation indicates that the max temperature rise and max temperature difference of the DCLC is only 20%-30%of that of DCLC.Using the NCM811 battery for the experiment,the temperature increment of the cells during 1C discharge does not exceed 13?,and the dynamic temperature difference is less than 8.8?.More importantly,the system can effectively prevent the thermal runaway propagation without any additional measures,thanks to the high heat dissipation rate and oxygen isolation.The research has successfully realized an oil-immersed battery system with high integration ratio and excellent safety,which provides a feasible solution for the demand of high safety and high specific energy of electric vehicle battery system. |
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