Design,Performance And Mechanism Of Electrolytes For Lithium-air Battery

As the commercial Li-ion batteries approach their achievable energy densities,it is full of difficulties to satisfy the growing demand of higher capacity for long-lasting electric devices.Developing an advanced rechargeable battery has the possibility to solve this dilemma.Among the various battery systems,nonaqueous Li-O₂ batteries with an ultrahigh specific energy density of 3460 Wh kg^-1,more than two times higher than that of current Li-ion batteries,stand out.However,the surface discharge mechanism induced cathode passivation is a critical challenge that blocks the full liberation of the ultrahigh theoretical energy density of aprotic Li-O₂ battery.And the accumulation of insulated by-products(Li₂CO₃ and Li OH)from the decomposition of electrolyte and electrode will lead to the early death of the battery.Besides,the conventional electrolytes with a typical Li⁺transfer number of around 0.2-0.5 can induce concentration gradients,then the superfluous anions accumulated on the electrode will cause severe potential drop for a large concentration polarization,leading to a discount of energy density and rate capability of the battery.Furthermore,the uncontrollable Li dendrite growth in the anode side of Li-O₂ batteries can induce performance degradation and short-circuit,restricting the practical applications of the battery.In light of the above issues,this thesis is devoted to promoting the solution discharge in Li-O₂ batteries,inhibiting the formation and removing the accumulation of by-products,and realizing high Li⁺transfer number of the electrolyte and dendrite-free anode.The research contents mainly include the following aspects:1.Li-O₂ batteries with ultrahigh theoretical energy densities usually suffer from low practical discharge capacities and inferior cycling stability owing to the cathode passivation caused by insulating discharge products and by-products.Herein,we firstly introduce a new kind of trifunctional ether-based redox mediator,2,5-di-tert-butyl-1,4-dimethoxybenzene(DBDMB),into the electrolyte to capture the reactive O₂^-and alleviate the rigorous oxidative environment during cycling of Li-O₂ batteries.Thanks to the strong solvation effect of DBDMB towards Li⁺and O₂^-,it can not only reduce the formation of by-products(a high Li₂O₂ yield of 96.6%),but also promote the solution growth of large-sized Li₂O₂particles,avoiding the passivation of cathode as well as enabling a large discharge capacity.More encouragingly,DBDMB makes the oxidization of Li₂O₂ and the decomposition of main by-products(Li₂CO₃ and Li OH)proceed in a highly effective manner,prolonging the stability of Li-O₂ batteries(243 cycles at 1000 m Ah g^-1 and 1000 m A g^-1).DFT calculations also unravel the intrinsic functional mechanism of DBDMB for facilitating the reversible O₂ involved redox reactions.Our strategy of using trifunctional electrolyte additive to capture reactive discharge intermediates with reduced formation of by-products,regulate solution growth of Li₂O₂,and co-oxidize Li₂O₂ and by-products,will open up a new avenue towards the high-performance Li-air batteries.2.The surface discharge mechanism induced cathode passivation is a critical challenge that blocks the full liberation of the ultrahigh theoretical energy density of aprotic Li-O₂ battery.Therefore,we propose a facile universal concept of hydrogen bond-assisted solvation to trigger the solution discharge process for averting the shortcomings associated with surface discharge.And 2,5-di-tert-butylhydroquinone(DBHQ),an antioxidant with hydroxyl groups,is introduced as an exemplary soluble catalyst to promote the solution discharge by hydrogen bond-assisted solvation of O₂^-and Li₂O₂(O-H···O).Thus,the Li-O₂ battery with DBHQ delivers an extraordinary discharge capacity of 18945 m Ah g^-1(i.e.9.47 m Ah cm^-2),even surpassing the capacity endowed by the state-of-the-art reduction mediator of 2,5-di-tert-butyl-1,4-benzoquinone(DBBQ).Besides,an ultrahigh Li₂O₂ yield of 97.1%has also been achieved due to the depressed reactivity of the reduced oxygen-containing species(O₂^-,Li O₂,and Li₂O₂)by the solvating and antioxidative abilities of DBHQ.Consequently,the Li-O₂battery with DBHQ exhibits excellent performance for cycling lifetime and rate capability.Furthermore,the generalizability of this approach of hydrogen bond-assisted solution discharge has been verified by other soluble catalysts which contain-OH and-NH groups,with implications that could bring Li-O₂ batteries one step closer to being a viable technology.3.The severe performance degradation of high-capacity Li-O₂ battery induced by the Li dendrite growth and the concentration polarization from the low Li⁺transfer number of conventional electrolytes hinder its practical applications.Herein,lithiated Nafion(LN)with the sulfonic group immobilized on perfluorinated backbone has been designed as a soluble lithium salt for preparing less flammable polyelectrolyte solution,which not only simultaneously achieves high Li⁺transfer number(0.84)and conductivity(2.5 m S cm^-1),but also the perfluorinated anion of LN produces a Li F-rich SEI for protecting the Li anode from dendrite growth.Thus,the Li-O₂ battery with LN based electrolyte realizes all-round performance improvement,like low charge overpotential(0.18 V),large discharge capacity(9508 m Ah g^-1),and excellent cycling performance(225 cycles).Besides,the fabricated pouch-type Li-air cells exhibit promising applications to power electronic equipment with satisfactory safety.