Research of the Electrode|Electrolyte Interface in Lithium-Oxygen Batteries

Author:Liu Zuo Jie

Supervisor:peng zhang quan


Degree Year:2019





Society’s ever-growing energy storage demands have generated a great deal of interest in exploring novel battery chemistries,with the hope of realizing practical energy storage devices having higher capacity and lower cost than today’s state-of-art Li-ion batteries.Among these novel battery systems,the aprotic Li-O2 battery has received wide attention due to its extremely high theoretical energy density.However,current Li-O2 battery is hindered by the unstability of solid electrolyte interphase(SEI)film,which caused by severe parasitic reactions and uncontrollable lithium dendritic growth on the lithium anode.Moreover,the aprotic Li-O2 battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery,leading to the "sudden death" of Li-O2 batteries.The key to solving this dilemma lays in tailoring SEI film on the lithium anode and building more stable cathode | electrolyte interface in Li-O2 batteries.In order to solve the problem of high charging overpotential of Li-O2 batteries,the research of biflinctional catalyst has attracted much attention,but its catalytic mechanism is not clear.Herein,the above key scientific problems are explored and analyzed.(1)There is a growing concern about the cyclability and safety in particular of the high-energy density lithium-metal batteries,due to the long-term unsolved parasitic side reactions between the lithium metal anode and the electrolyte and the uncontrollable lithium dendritic growth upon lithium plating and stripping.This concern is even greater for the topical Li-O2 batteries because a new vexing player,the O2 transpired from the cathode to the anode compartment,could exacerbate the side reactions and dendrite growth of the lithium metal anode.Here we report that a new electrolyte,formed from LiFSI as the salt and a mixture of tetraethylene glycol dimethyl ether(TEGDME)and polymeric ionic liquid(PIL)of P[C5O2NMA.11] FSI as the solvent,can produce stable electrode(both cathode and anode)| electrolyte interface in Li-O2 batteries.Specifically,this new electrolyte,when in contact with lithium metal anode,has the ability to produce a uniform SEI with high ionic conductivity for Li+transport and desired mechanical property for suppression of dendritic lithium growth.A symmetric Li | Li cell containing this new electrolyte demonstrates a long cycle life(>140 cycles),low polarization(<0.03 V)and high average coulombic efficiency(94.6%vs 86.0%of the prevailing LiFSI-TEGDME electrolyte)when operated at a current density of 0.2 mA cm-2.Moreover,this new electrolyte possesses a high oxidation tolerance(up to 4.53 V vs Li/Li+)that is very beneficial to the oxygen electrochemistry(i.e.,formation and decomposition of Li2O2)on the positive electrode of Li-O2 batteries.As a result,enhanced reversibility(O2 recover efficiency 94.4%)and cycle life(35 cycles)have been realized for the resultant Li-O2 batteries,while for Li-O2 batteries containing the benchmark LiFSI-TEGDME electrolyte,limited reversibility(O2 recover efficiency 63.5%)and cycle life(14 cycles)have been obtained.The results reported in this work indicates that tailor-design of functional electrolytes constitutes a promising solution to unlocking the energy capabilities of Li-O2 batteries by producing more stable electrode | electrolyte interfaces.(2)For years the aprotic Li-O2 battery suffered from a severe capacity-current trade-off that would be unacceptable for a beyond Li-ion battery.Recent fundamental study of Li-O2 electrochemistry revealed that this dilemma is caused by the growth of Li2O2 on the cathode surface and can be solved by discharging Li2O2 in the electrolyte solution.Among the strategies that can promote solution growth of Li2O2,redox mediators(i.e.,soluble catalysts)demonstrate prominent performance.However,soluble redox mediators may shuttle from the cathode to the lithium anode and decompose thereon causing severe deterioration of the lithium anode and degradation of the mediators functionality.Here,we report that immobilized redox mediators(e.g.,anthraquinone,AQ)in the form of a thin conductive polymer film(PAQ)on the cathode can effectively promote solution growth of LibO2 even in weakly solvating electrolyte solutions that would otherwise lead to surface film growth and early cell death.The PAQ-catalyzed Li-O2 battery can deliver a discharge capacity that is up to~50 times what its pristine counterpart does at the same current densities and is comparable to the capacity realized by soluble AQ-catalyzed Li-O2 batteries.Most importantly,the adverse "cross-talk" between the lithium anode and the redox mediators immobilized on the cathode has been completely eliminated.(3)As we all know,the efficient bifunctional catalyst on the cathode is essential to achieve high power density and round-trip efficiency for Li-O2 batteries.Albeit a lot of highly efficient bifunctionl catalysts have been reported,it still remain unknown that how the catalyst accelerate the kinetics of ORR and OER.In this dissertation,CoxOy thin film with highly dispersed oxygen defects were formed on the rough Au electrodes by traditional cyclic voltammetric methods.Further,we employed in situ surface enhanced Raman spectroscopy to understand the essential role of the transition metal oxides catalyst toward reducing the charging overpotential.It is found that the Raman signal of O2-on the electrode surface exists during the whole discharge porcess,indicating that the CoxOy film with highly dispersed oxygen defects can stabilize the O2-during discharge,and thus reducing the charge overpotential.