Research On Some Complex Parasitic Reactions Of Li-Air Batteries

With the intensification of environmental pollution and the depletion of traditional fossil fuel,the development and utilization of renewable energy is the general trend of the future.Hence,there has been an increasing demand for new battery systems with higher energy density.As the state-of-art battery technology,the current prevailing Li-ion batteries may not be able to meet the future requirement for specific energy in many scenarios such as electronic vehicles and smart grid,even if they can reach the theoretical limits of the electrode materials.Therefore,we should seek for new battery chemistry with higher energy density.Recent years,a new battery system known as Li-air batteries have attracted people's attention.Different from the intercalation chemistry in conventional Li-ion battery,the operation of the new Li-air battery is based on the reversible formation/decomposition of the discharge product---Li2O2 on the porous cathode material.This electrode reaction is able to deliver a large theoretical specific energy of 3582 Wh·kg-1,making Li-air battery a promising candidate of future energy storage system.Besides,the active electrode material of Li-air batteries---oxygen---can be obtained directly from air,hence the using of transition can be avoided,making Li-air batteries an environmental friendly energy storage system.Of course,as an emerging energy storage system,the development of Li-air battery is still inhibited by several problems.One of the most serious problems is the generation of byproduct Li2CO3 and LiOH.These two byproducts are formed through the reaction between the discharge product Li2O2 and the CO2 and H2O in air.The accumulation of Li2CO3 and LiOH will increase the charging overpotential,aggravate the side reaction and jeopardize the stability of the battery.To achieve the complete decomposition of Li2CO3 and LiOH during charging,it is important to understand their decomposition mechanism and lower their charging overpotential.So far,several researches have reported that the decomposition process of both the two byproducts requires high over-potentail(>4 V vs.Li+/Li)and CO2 is released simultaneously.However,the actual mechanism of the decomposition process is still unclear.In this paper,we explored the decomposition mechanism of the byproduct LiOH and Li2CO3.Based on our findings,we also did some work on the performance optimization of Li-CO2 batteries---a battery system based on the reversible formation&decomposition of Li2CO3.In this part of work,we employed Ru nanoparticles as the cathode catalyst material for Li-CO2 batteries.In addition,considering the massive use of metallic lithium as the anode material in Li-air and Li-CO2 batteries,we proposed the first solar-powered,electro-dialysis-based method for lithium extraction from seawater.The major innovations in this work can be summarized as follow:1.Exploration of the electrochemical decomposition mechanism of Li2CO3:Considering the components in a discharged Li-air or Li-CO2 cell and other influential factors,we summarized three reaction pathways that are thermodynamically possible to describe the electrochemical decomposition of Li2CO3:Path ?.self-decomposition of Li2CO3 in which both CO2 and O2 are released;Path?.Li2CO3 reacts with carbon to release CO2 only;Path ?.Li2CO3 decomposes into Li+,CO2,O2 and O2.-at first,and then O2 and O2·-react with electrolyte solvent.We used electrodes pre-filled with Li2CO3 and different conductive additives to simulate a discharged Li-air cell or Li-CO2 cell,and thus we were able to exclude the irrelevant factors.In situ gas chromatography-mass spectrometry(GC-MS)measurements and isotopic tracing method were employed to detect the composition of the gas generated during charging the pre-filled electrode.X-ray diffraction(XRD),Fourier transform infrared(FTIR)spectroscopy analysis were also performed to characterize the component and structure of the electrode and electrolyte.Our goal is to eliminate the hypothesis reaction pathways that are inconsistent with our experiment results and to understand the decomposition mechanism of Li2CO3.In our experiments,we investigated the electrochemical reaction mechanism of Li2CO3 oxidation.During the charging process of Li2CO3 electrode,CO2 was detected as the main charging product while at the same time no gaseous O2 was released.With the help of isotopic tracing and GC-MS method,we were able to eliminate the possibility of the direct self-decomposition reaction of Li2CO3 as well as the reaction between Li2CO3 and carbon.Further experiments illustrated that superoxide radicals and dissolved oxygen generate during the charging process of Li2CO3 and will lead to electrolyte solvent decomposition.Combining the above results,we suggest that the decomposition mechanism of Li2CO3 can be described as follows:Li2CO3 decomposes into CO2,superoxide radicals and O2(formed by part of the superoxide radicals).The O2 and superoxide radicals are consumed by the further reaction with tetraglyme electrolyte solvent and thus fail to be detected.2.Application of Ru catalyst material in Li-CO2 batteries:As reported in our previous work,when no catalyst material is employed,Li2CO3 will decompose to CO2 and superoxide radicals.The superoxide radicals will further oxidize electrolyte solvent and lead to electrolyte decomposition.Besides,this reaction pathway is not the reversible process of the discharge reaction and hence will lead to a loss of the energy efficiency of the battery.In this work,we prepared a Ru@Super P cathode catalyst for Li-CO2 batteries.Benefiting from the superior catalytic activity of Ru@Super P material,we managed to optimize the electrochemical performance of the Li-CO2 battery and the charging potential was controlled below 4.4 V during the operation of the cell.The Li-CO2 cell delivered a discharge capacity of 8229 mAh·g-1 and showed a columbic efficiency of 86.2%in the first cycle of the full discharge-recharge test.Li2CO3 and carbon was confirmed to be the main discharge product by XRD and Raman spectra.The cell was able to be discharged and charged for over 70 cycles with a cutoff capacity of 1000 mAh g"1 at the current density of 100,200 and 300 mA·9-1,indicating an excellent cycle stability of the Ru@Super P cathode.In situ SERS and GC-MS results showed that Ru possess the catalytic activity of promoting the reaction between Li2CO3 and carbon during charging and electrolyte decomposition can be largely avoided in the operating potential range of Li-CO2 batteries.Our work shows that the exploration of a high efficient catalyst for further reduction of the charge potential is the key to achieve a reversible Li-CO2 battery.3.Exploration of the electrochemical decomposition mechanism of LiOH:In this work,we proposed three most possible reaction pathways that describe the decomposition reaction of LiOH:Path ?.self-decomposition of LiOH into H2O and O2;Path ?.LiOH reacts with carbon and release CO2;Path ?.LiOH react with electrolyte solvent.In the charging process of a LiOH-carbon electrode,we found only C02 was released,making Path I impossible.We further fabricated a carbon free electrode with RuO2 as conductive additive.During the charging process of this material,we still only detected the release of CO2.This result indicates that carbon is not involved in the decomposition process of LiOH,and the CO2 we detected is sourced from the decomposition product of electrolyte.Combining with previous works,we proposed that hydrogen peroxide anion is released from LiOH during charging,and the hydrogen peroxide further react with electrolyte solvent and lead to the release of CO2.Further proof of the existence of hydrogen peroxide is still required.4.Study on the electro-dialysis based lithium extraction technique with seawater as lithium source:In this work,we proposed a lithium extraction method based on solar-powered electrolysis technique with LISICON solid-state electrolyte as selective membrane.We show that the electrolysis method makes the lithium extraction process faster(five times higher lithium extraction rate than that of the electro-dialysis method)and more controllable than the adsorption and electro-dialysis and is able to overcome the limit of the concentration difference in electro-dialysis method.Moreover,with aprotic electrolyte in the anode side,we were able to directly obtain metallic lithium during the lithium extraction process.This method is easily scalable and may be suitable for future lithium extraction.The above results provide profound understanding to the decomposition mechanism of the byproducts in Li-air batteries,develop a new direction and strategy performance optimization and electrode(both cathode and anode)material screening of Li-CO2 and Li-air batteries.Therefore,it represents an important progress in the development of high performance and practical Li-O2 batteries.