In-situ And Ex-situ Soft X-ray Spectroscopy Of LiCoO₂ Lithium-ion Battery Electrodes And Interfaces

Lithium ion battery is one of the most promising system for energy storage due to its advantages of high volumetric and gravimetric energy density and long service life.Since first commercialized by Sony in 1991,lithium ion battery has dominated the field of energy storage for portable electrics like mobile phone and laptop.However,the cost,safety,lifetime,energy and power density of today's battery technology can not meet the requirements of the emerging mid-to-large scale energy storage,especially for application of electric vehicles?EVs?and grid scale electricity storage.The challenge for the development of more advanced energy storage device comes from the complicated fundamental mechanism involved in the battery cycling process which is related to the intrinsic battery material as well as interface layer.Typically,a lithium ion battery consists of three important components,the cathode,anode and electrolyte.Among which,cathode material is a key factor to determine the performance of compact full battery including the cell working voltage and capacity decay.LiCoO₂ remains a most important cathode material in the market of lithium ion battery after its 20 years of development.In additional to the layered LiMO₂?M = Co and Ni?,spinel LiMn2O4 and olivine LiFePO4 have been found to be promising cathode candidates,however the performance of corresponding battery should be further improved to meet the demand of commercialization.Considering the importance of LiCoO₂ cathode material in battery field,the battery performance optimization related to LiCoO₂ cathode will provide valuable information for practical applications.The well known problems of capacity fade and structural instability in LiCoO₂ especially in overcharge state,is a serious hindrance for its potential applications and lead to a reversible electrochemical cycling capacity of around 140 mAh/g.The safety concern of LiCoO₂ at high voltage further restricted its application in electric vehicles.In a simplified energy band scheme,the intrinsic voltage limitation resulted from pinning of redox couple at the top of anion p bands translates into a practical reversible capacity of only 0.5 lithium ion per LiCoO₂ formula since introduction of holes into O 2p states at voltage higher than 4.2 V will lead to oxygen loss from the lattice.Even though the fundamental scheme has long been proposed and widely accepted,the direct experimental proof has been missing,and the verification of the classic model will be of vital importance to guideline future material development.The verification of the interplay between Co 3d and O 2p electronic states is an important research field to both fundamental research and practical application.Throughout the research for alternative advanced cathode material,the lithium rich cathode material have attracted much recent research attention due to the high reversible capacity of?250 mAh/g and low cost.Even though the fundamental understanding of the large electrochemical capacity is quite complex and still under debate,the increasing experimental results point to the fact that O 2p state induced anionic redox couple is a key factor.The O 2p state induced anionic redox couple is an cross-disciplinary research field,which is not only very important to lithium ion battery but also to electrocatalysts for water splitting.Note that the reversible anionic redox couple in lithium rich cathode materials and the intrinsic voltage limitation of LiCoO₂ share the same fundamental principle that O 2p states is directly involved in the electrochemical process.This lead us to rethink about the typical understanding of O 2p state in battery cycle in LiCoO₂,which is of vital importance not only to the further development of LiCoO₂ cathode but also to the rational design of future lithium rich materials.From the fundamental physics point of view,the problems of intrinsic voltage limitation in LiCoO₂ and the anionic redox couple in lithium rich material comes from the reversibility of O 2p electronic states participating in electrochemical cycle.In order to achieve fundamental understanding of the battery operation mechanism,advanced tools that is capable of directly probing the key electronic states relevant to battery performance will play an important role.Synchrotron based soft X ray spectroscopy is the tool of choice that can directly measure both the occupied and unoccupied states in the vicinity of Fermi level by XES and XAS respectively,including transitional metal 3d states and anion 2p states that fundamentally determine material performance.In addition to XAS and XES,the development of 3rd generation synchrotron facility and innovation in optics design made resonant inelastic soft X ray scattering RIXS experimentally accessible,which is a new and powerful characterization tool for both the occupied and unoccupied electronic states characterization and will play an important role in the field of battery research.Considering that O 2p state is strongly hybridized with TM 3d states in typical TM oxide based cathode material,O 2p dominated states can not be well distinguish by XAS.The weak signal intensity restricted the utilization of RIXS in energy storage materials.In this work,we design and build up a unique high-resolution RIXS facility,which make the RIXS map collection experimental accessible,which make this work one of the first projects on the cathode material research by RIXS.In this work,we combined the in-situ and ex-situ soft X ray spectroscopy including soft X ray absorption,emission as well as resonant inelastic soft X ray scattering techniques to achieved systematic and fundamental understanding on LiCoO₂ material and the electrode/electrolyte interface related problems.The detailed for each part is as follows:Firstly,we prepared crystallized LiCoO₂ thin film by pulsed laser deposition,and mapped out the occupied and unoccupied electronic states evolution near Fermi level by soft X ray absorption and emission spectroscopy at different state of charge in battery cycle.In this work,the key electronic states evolution is well clarified with the combination of theoretical calculation and experimental spectra,the Co^3+/4+ redox couple is found to be available even in the over half delithiated LiCoO₂ which is in contrast to traditional understanding.The utilization of thin film LiCoO₂ excludes the signal contribution from organic binder and carbon addictives which is typical components in laminated cathode,and provide fundamental pure LiCoO₂ material.While the in-situ XAS characterization of thin film LiCoO₂ further proved the intrinsic property of spectra evolution in LiCoO₂ thin film.In this work,it is found that Co^3+/4+ redox couple is accessible in overcharged state and Co will be further oxidized over 4.2 V,which is in contrast to the previous understanding that Co and O is active in low voltage and high voltage respectively.Co-L edge XAS demonstrates that Co L3 peak shift to higher absorption energy compared to the charged 4.2 V state.On the other hand,with XES spectra fitting and theoretical calculation,we verified the interchange of Co 3d dominated state and O 2p dominated state in energy scale at the top of valance band.For the pristine LiCoO₂ thin film,the electronic states at the top of valence band is dominant by the anti-bonding Co 3d related states;while for overcharged to 4.8 V LiCoO₂ thin film,the top of valence band is dominant by the bonding O 2p related states.This work provide direct experimental proof for the verification of the classical model of the interplay between Co 3d states and O 2p states.Secondly,we measured the RIXS map of LiCoO₂ at different lithium content and verified the existence of electrochemical reversible anionic redox couple in bulk LiCoO₂.Even though the anionic redox couple decrease with cycle number from the localized peak intensity,RIXS map clearly shows that the reversibility of the anionic redox couple in LiCoO₂,which is a total surprise to traditional understanding that anionic redox doesn't exist in LiCoO₂ material and O 2p states is only related to the irreversible reaction in high voltage.More importantly,the directly observation of anionic redox couple in LiCoO₂ also implies that it is necessary to make correspoinding changes to the previous model for the understanding of Li rich cathode material.RIXS is also demonstrated to be a unique and powerful characterization methode for battery research.On the other hand,the combination of XAS in surface sensitive TEY and bulk sensitive TFY as well as RIXS implies that the chemical reaction between electrode and electrolyte at the interface and possible surface reconstruction is the reason for LiCoO₂ based battery failure mode in high voltage.Thirdly,we characterized the behavior of Au/electrolyte interface in in-situ static cell and verified the different molecule orientation effect in electric double layer compared to the bulk,the Li salt and molecule structure is found have important effect on the formation electric double layer.With the development of new method for data collection,spectra procession and normalization,we achieved the total electron yield TEY signal of interface electrolyte quasi-quantitatively.The signal difference between TEY and TFY demonstrated different molecule orientation effect between interface and bulk.By comparing three kinds of typical electrolyte 1M LiPF6 EC:EMC:DEC?4:2:4?,1M LiPF6 EC:DEC?1:1?and EC:DEC?1:1?,we found that 1M LiPF6 EC:DEC?1:1?demonstrated the strongest polarization effect which is robust to the introduction of extra holes at the interface.While 1M LiPF6 EC:EMC:DEC?4:2:4?demonstrates relative weak polarization which is sensitive to the perturbation and increase a lot with the introduction of external holes.By the comparison between electrolyte with and with out Li salt,it is found that the carbonate group tend to flourish at the electrode/electrolyte interface with the introduction of LiPF6 salt into the solvent.From the experimental point of view,we provide direct experimental proof of the electric double layer by in-situ soft X ray spectroscopy.