Studies On The Thermal Effects And Safety Of Lithium-ion Battery

Safety is a big challenge for rechargeable lithium batteries to become the unique candidate of advanced power source for the solution of environmental pollution and energy crisis. The key issue lies in if the cell system is kept running at high environment temperature or high cycling rate, the exothermic effect will induce the heat accumulation inside the cell, leading to thermal runaway and even the cell burning and explosion. Therefore when lithium-ion batteries are developed from small size extensively used in the portable electronic devices to up-scaling system investigated for the potential applications of hybrid electrical vehicles (HEVs) and satellites, safety concerns have come to the public attention.In this thesis, thermal characteristics and safety performance of lithium-ion battery have been investigated step by step: firstly, thermal effects of the overall cell and single electrodes were respectively studied with focus on the normal operation; secondly, thermal effects on the combustion reactions of active materials were systematically studied with attention to the ruptured cell after abnormal operation; at last, in pursuit of the key to the safety problem, functional additives were attempted with the aim to improve the safety performance of lithium-ion battery.â… . Thermal study on lithium-ion batteries1．1．Thermal effects of overall lithium-ion batteryIn this work, we studied the thermal effects of lithium-ion batteries with two different methods-potentiometric and electrochemical-calorimetric methods.On one hand, the thermodynamic parameters:â–³G,â–³S andâ–³H of Li_xC₆/ 1M LiPF₆/ Li_1-xCoO₂, Li AM LiPF₆/Li_1-xCoO₂, and Li AM LiPF₆/Li_xC₆ battery reactions were respectively obtained by potentiometric method-with the precise measurements of equilibrium voltage E_eq and entropy coefficient dE_eq/dT.The results revealed that during the discharge process of a fully charged Li_xC₆ AM LiPF₆/Li_1-xCoO₂ battery, the reversible cell discharge process, Li_1-xCoO₂+Li_xC₆→LiCoO₂ + 6C, is an exothermic reaction with AS of-29．78 J K^-1 mol^-1 and q_r of 8．874KJ mol^-1 . Comparing with the irreversible heat, this reversible heat can't be neglected. On the other hand, the total thermal effects for the discharge process of the overall cell and half cell were obtained by means of dc calorimetric measurements. Their results show that the value of AH obtained from potentiometric method is almost identical to those from electrochemical-calorimetric method. So we concluded that both methods are feasible and precise.1．2．Thermal effects of single electrodes in lithium-ion batteryThe AS and reversible Peltier heat q_r of cathode and anode reactions in lithium-ion battery were calculated from theâ–³S of Li/1M LiPF₆/Li_1-xCoO₂ and Li/1M LiPF₆ /Li_xC₆ cell reactions, and theâ–³S of Li electrode reaction respectively. For Li electrode reaction, theâ–³S and q_r were detected by both potentiometric and electrochemical-calorimetric methods.This work has made it more clearly for the heat-generated mechanism inside the Li-ion cell:For a fully charged Li_xC₆/1M LiPF₆/Li_1-xCoO₂ battery during reversible discharge process, the overall reaction Li_1-xCoO₂+Li_xC₆→LiCoO₂+6C presents exothermal heat effect withâ–³S of-29．78 J K^-1 mol^-1 and q_r of 8．874 KJ mol^-1 . Furthermore, the cathode reaction xLi⁺+xe^-+Li^1-xCoO₂→LiCoO₂ shows larger exothermic effect withâ–³S of-121．8 J K^-1 mol^-1 and q_r of 36．30 KJ mol^-1 , and the anode reaction Li_xC₆→xLi⁺+xe^-+6C shows smaller endothermic effect withâ–³S of 92．08 J K^-1 mol^-1 and q_r of-27．46 KJ mol^-1 . The heat produced at the positive electrode reaction is about three times more than that of overall battery reaction. The results should be taken into consideration when optimizing thermal system design especially for a high rate discharge lithium-ion battery.â…¡. Thermal study on the combustion reactions of active materials in lithium-ion battery2．1．Thermal effects in the combustion reactions of organic electrolytes with fully charged cathodes of lithium-ion batteriesExothermic effect of organic electrolytes /organic solvents with fully charged cathodic materials during the burning process of lithium-ion batteries have been firstly investigated by using oxygen bomb calorimeter under the rupture case.The pronounced exothermic effects have been studied during the combustion reactions of three organic electrolytes and its three salt-free solvents with cathodic materials. The results reveal that the additions of fully charged cathodic materials can accelerate not only the heat release by improving the kinetics of combustion reactions but also increase the total heat value of combustion reactions. More heat was generated with the higher mass ratio of cathodic materials to electrolyte or organic solvents.Linear relationships between the heat value and the added mass of cathodic material are well-determined in all six testing systems: EC:DEC=1:1, EC:DMC:DEC=1:1:1, and EC:DMC=1:1 (all in volume ratio) with or without 1M LiPF₆．Moreover, the heat of combustion for electrolytes or solvents (q°_{ele or sol}) and the heat rise per gram of cathodic material added to electrolytes or solvents (â–³q_cat) were calculated and systematically analyzed. The cathodic materials and electrolytes /solvents show the interactions during the period of combustion reaction and they are not simple physical mixture. It is worthy to note that electrolyte systems exhibit the less exothermic effects in combustion reactions than the corresponding solvent systems.X-ray diffraction analysis revealed that Co₃O₄, CoO and LiCoO₂ were the main solid products of combustion reactions. And there are more CoO and less LiCoO₂ products for the higher mass ratio of cathodic materials to electrolyte/solvent system and more amount of heat generated. It means that the combustion reaction, which produced CoO, generated more amount of heat than LiCoO₂．This study indicates that much more heat was released under sufficient oxygen atmosphere in a ruptured Li-ion cell than the sealed cell system. The violent exothermic reaction will cause an explosion of battery under a rupture case with enough oxygen.2．2．Thermal behaviors in the combustion reactions of P(VdF-HFP)-based and PMMA-based gel polymer electrolytesThis research firstly disclosed the thermal behaviors in combustion reactions of P(VdF-HFP)-based and PMMA-based gel electrolytes for polymer Li-ion battery by using oxygen bomb calorimeter. The heat amount of combustion was close to the pure liquid system in the P(VdF-HFP)-based gel electrolytes. And the heat release rates were gradually decreased with the increase of P(VdF-HFP) concentration. But the heat amount and release rates of combustion reactions greatly increased in the PMMA-based gel electrolytes. The similar characteristics of thermal behaviors have been observed in the polymer-based electrolytes even in which different original solvents were employed. It suggests that the pronounced difference of thermal behaviors between P(VdF-HFP) and PMMA gel electrolytes are absolutely independent on the types of liquid electrolyte as the original component.FT-IR spectra of gel electrolytes revealed that the functional groups of each polymer had chemical interactions with organic solvents. The signature functional groups of each polymer presented the direct effects on the combustion reactions of electrolytes. The effects of polymer depended on the polymer properties and the interactions between the polymer functional groups and solvents. The C-F group of P(VdF-HFP) can effectively prevent the oxygen transfer to organic solvents and retard the burning reactions. Alkyl and carbonyl groups of PMMA themselves are unstable and very flammable. The functional groups of polymer made the combustion reactions of gel polymer electrolytes more dramatic than organic solvents themselves. The thermal behaviors of PMMA and P(VdF-HFP) systems suggest that P(VdF-HFP) is a much safer gel system than PMMA for polymer Li-ion battery.â…¢. Studies of functional additives to improve the safety performance of lithium-ion battery3．1．Ammonium Perfluorocaprylate (APC)Our study revealed that the performance of carbonaceous anode for lithium-ion battery has been greatly improved by a novel selected electrolyte additive: Ammonium Perfluorocaprylate (APC).With the low content APC (<1wt.%) in 1M LiPF₆/EC: DEC=1:1(V/V) electrolyte, a passivating solid interface has been formed by the APC decomposed products and possible APC adsorption on the anode surface. An extraordinarily uniform and compact solid interface film was observed on the surface of carbonaceous anode after charge-discharge cycles by SEM images. The formed solid interface effectively suppressed the decomposition of organic electrolyte because in CV testing the additive was decomposed in 1．9¹ï¼Ž0 V vs. Li/Li⁺ on the surface of carbonaceous anode before the decomposition of organic electrolyte in a lower potential range (⁰．6 V vs. Li/Li⁺). The decomposed peak of organic electrolyte greatly decreased or almost disappeared with the increase of additive concentration. And the major components of this solid interface have been well-defined by Raman and FTIR analysis, which mainly consisted of APC reduced products, the probable APC adsorption on the anode surface, and good passivating agent Li₂CO₃ (considered as the major product from the probable reduction of trace electrolyte).It is worthy to note that lower charge transfer resistances of carbonaceous anode were determined by EIS characterization in the organic electrolyte system with APC additive. APC additive greatly improves the stability of solid interface on the carbonaceous electrode and thus can potentially enhance the cell performance of lithium-ion batteries.Based on above studies, APC has been further investigated to improve the safety performance of lithium-ion battery.In a UL 94 flammability test, even with a rather low concentration (<1 wt.%) of APC added in 1M LiPF₆/EC:DMC=1:1(V/V) electrolyte, its flame propagation rate was notably decreased by about 33% comparing with blank electrolyte system. So here the introduction of fluorine by additive APC can enhance the flame-retardant capability of organic electrolyte, by which thermal safety might be improved in lithium-ion batteries. And in DSC analysis, thermal stability of carbonaceous anode in cycled cell was prominently improved by additive APC: a higher onset temperature and less heat release in the rupture of solid interface layer as well as the subsequent reactions of lithiated anode with electrolyte. In this case, the additive APC can improve the safety performance of lithium-ion battery.A lower charge-transfer resistance of carbonaceous anode and enhanced stability of SEI film after long-term aging were both determined by EIS in the APC-contained electrolyte system. Both the carbonaceous anode and the overall cell with electrolyte additive APC presents the pronounced capacity retention capability in a long-term testing.Thus both the safety performance and electrochemical performance of lithium-ion battery have been notably improved by the novel electrolyte additive APC.3．2．Potassium nonafluoro-1-butanesulfonate (PNB)In this work, one novel flame-retardant material Potassium nonafluoro-1-butanesulfonate (PNB) has been successfully attempted as the electrolyte additive in lithium-ion battery. When optimizing the concentration of PNB in the electrolyte, the addition of PNB would greatly improve both the safety performance and electrochemical performance of lithium-ion battery.The UL 94 flammability test revealed that the flame-retardant effect of additive PNB on the organic electrolyte had become quite prominent even with a rather low concentration (0．8 wt.%) of PNB in 1M LiPF₆ /EC:DMC= 1:1(V/V) electrolyte. So the additive PNB can improve the thermal stability and safety performance of lithium-ion battery.By CV testing, the different concentration of PNB in electrolyte shows the quite different effects on the electrochemical performance of carbonaceous electrode. The concentration 0．5wt.% is too low to make the influence; The concentration more than lwt.% had the negative effect; and the 0．8wt.% PNB-contained electrolyte system can effectively enhance the reversible cycling capability of carbonaceous electrode.Lower SEI film resistances and charge transfer resistances of carbonaceous anode were determined by EIS characterization in the organic electrolyte system with PNB additive. An extraordinarily uniform and compact solid interface film was observed on the surface of carbonaceous anode after charge-discharge cycles by SEM images. Both EIS and SEM results had confirmed the addition of PNB can effective improve the stability of SEI film on the surface of carbonaceous anode.At last, the cell cycling test indicated that the electrolyte additive PNB with the optimized concentration (0．8wt.%) can notably improve the cell performance (both discharge capacity and coulomb efficiency) of overall lithium-ion battery. Therefore, novel flame-retardant material Potassium nonafluoro-1-butanesulfonate (PNB) can be applied into the practical (commercial) lithium-ion battery.As a whole, the work in this thesis is important to further understand heat-generated mechanism inside the Li-ion cell and thus optimize the thermal design with large capacity system; meanwhile, this thesis is valuable for investigation on the effective solution to improve the safety performance as well as cell performance of lithium-ion battery, thus leading the practical application of up-scale cells to the more widely fields (EV, HEV) and solving the environmental pollution and the energy crisis eventually.