| The development of electric vehicles(EVs)to replace conventional fossil-fueled cars has great significance in reducing environmental pollution and improving strategic energy security,and has been of global concem and promoted worldwide.However,a very important and serious challenge is emerging,namely,how to appropriately dispose of retired power batteries from numerous EVs.At the same time,renewable energy storage technologies have been rapidly developed in recent years for efficient utilization and storage of surplus solar,wind and electrical energy resources.Especially,the increase in installed capacity is the highest in lithium-ion battery(LIBs)energy storage of the electrochemical energy storage technologies,but the battery cost problem has not been completely solved.Although retired power batteries cannot meet the power requirements of EVs,they can still meet most of the basic requirements for energy storage,to realize the gradient utilization of the batteries.To achieve this goal,some key common technical problems must be solved regarding gradient utilization for retired power batteriesIn this dissertation,systematic and in-depth research on the thermo-electrochemical characteristics and thermal safety of retired LiFePO4(LFP)power batteries is reported,and important theoretical and technical support for their application in the field of energy storage provided.Electrochemical testing,micro/macro characterization,experimental testing,and computer simulation technologies were applied to retired LFP batteries to evaluate their thermo-electrochemical characteristics,and the basis and guiding principles were developed for screening battery state of health(SOH)and performance.Thus,the thermoelectric characteristics and critical working conditions were obtained.Based on this,the heat generation rate and hot stack rapidly increased after package assembly.Thus,various thermal management strategies were developed under different applications,and thermal reliability was systematically evaluated and improved.In addition,the electrochemical performance of LFP batteries will accelerate retrogradation at low ambient temperature.Hence,a comprehensive thermal management system that integrates low-temperature heating and heat dissipation technology was constructed to improve the high-low-temperature electrochemical performance of battery modules.Finally,the temperature rise,voltage-curve characteristics,and thermal runaway spread trend were also investigated under the overcharge and short circuit faults for retired LFP batteries,and a database of characteristic parameters established for early warning and diagnoses of overcharge and short circuit faults.The four conclusions of this study are the following.1.Thermo-electrochemical characteristics of retired LFP power batteries:Based on tests of remaining capacity,internal resistance,and voltage characteristic curve,the remaining capacity and internal resistance have a strong negative correlation,and the test process of residual energy detection significantly affects the test results.Meanwhile,the battery consistency of the voltage characteristic curve is poor,even when the internal resistance and capacity are slightly different.In addition,the retired LFP batteries were evaluated by conducting a thermoelectric reliability performance test at range of-15 to 50?,0.5 to 2.0 C and 500 charge-discharge cycles.An 80%SOH battery shows the greatest performance,and the capacity retention is over 90%after 500 cycles.The heat generation power increased with decreasing in in ambient temperature for a retired LFP battery,and the average power of the charge process is more than that during the discharge process at the same C-rate(0.3 W vs.0.15 W).Simultaneously,microstructure characterization also shows that the anode,cathode and separator materials exhibited significant collapse,particle agglomeration,hole annihilation and tearing under a lower SOH,and severely affected the electrochemical performance of retired LFP batteries.2.Heat generation characteristics of LFP power batteries:LIBs in the application process,as well as the behavior of heat-generation rate and hot stack could be rapidly increased after assembly,significantly affecting the temperature uniformity and thermal safety of battery module.Therefore,the advantage and disadvantage of various thermal management techniques(BMTs)were compared in terms of heat dissipation and temperature uniformity performance,and the relationship between BMTs and the electrochemical performance improvement mechanism was also investigated.Moreover,to improve the temperature uniformity index of BTMs,and give full play to the advantages of composite phase change materials(CPCMs)of BTMs,the energy density of the system was an important consideration.Therefore,a novel structure design of an S-shaped CPCM(S-CPCM-L)was successfully developed for the BMTs.The energy density increased by nearly 12.8%compared to the traditionally structured CPCM,reaching 121.6 Wh kg-1,and the S-CPCM-L also shows excellent thermal conductivity,mechanical property and shape-stability,respectively.Furthermore,the experimental and simulation results show that the convection flow field created by coupling with secondary heat dissipation has a significant strengthening effect on improving the temperature rise and heat release rate of S-CPCM-L.Even at a high ambient temperature of 40?,the maximum temperature rise and temperature difference within the battery module are less than 15? and 1? under 2 C fast-charging conditions,respectively.3.Low-temperature performance improvement of LFP power batteries:The CPCM BMTs demonstrated good heat dissipation performance,but cannot slow down the degeneration rate of electrochemical performance for a LFP battery under a low ambient temperature.Therefore,for a LFP battery module with a CPCM heat dissipation system,two low-temperature heating strategies with built-in and external heating plate were designed.The research results show that:(1)the charge-discharge performance and cycle capacity retention of a LFP battery when heating to 10? from-20? is better than 5?.(2)the inside and outside temperature consistency is poor when only external heating is used at a low temperature,so pulse heat preservation combined with internal heat source pre-heating must be adopted to improve the temperature gradient in the battery;and(3)in the low-temperature heating strategy performance evaluation model,the comprehensive evaluation function model constructed from the heating efficiency factor y,temperature deviation index ? and required heating time index ? shows that it has a better comprehensive performance evaluation function.4.Thermal runaway induced by overcharge and short-circuit faults for LFP power batteries:For the multi-dimensional passive prevention and control of thermal runaway,a combination of experimental testing and simulation methods was employed.The thermophysical properties of multilevel CPCM were modulated according to the thermal runaway behavioral characteristic;hence,the prepared multilevel CPCM has an anti-shape change capability under high temperature.Meanwhile,the key and typical characteristic parameters of temperature rise and voltage curves of a LFP battery were acquired,and different SOHs under overcharge and external short-circuit faults were experimentally tested and used for the early warning and diagnosis of overcharge and external short-circuit faults to provide characteristic parameters.In addition,the simulation result shows that the thermal runaway temperature field distribution was influenced by different assembly methods.At the same time,the main way that a thermal runaway spreads is not the convection heat transfer within the battery module,and the radiant heat transfer of high temperature and flame during the thermal runaway spread process could be a decisive effect. |
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