Journal of Power Sources 153(2006)
380–384
Short communication
A new look at the solid electrolyte interpha on
graphite anodes in Li-ion batteries
Kristina Edstr¨o m a ,∗,Marie Herstedt a ,Daniel P.Abraham b
a
˚Angstr¨o m Advanced Battery Centre,Department of Materials Chemistry,˚Angstr¨o m Laboratory,Uppsala University,
Box 538,SE-75121Uppsala,Sweden
b Chemical Technology Division,Argonne National Laboratory,9700South Cass Ave.,Argonne,IL 60439,USA
Available online 19July 2005
Abstract
The solid electrolyte interpha (SEI)of graphite electrodes has been extensively studied using surface nsitive techniques such as photoelectron spectroscopy (PES)and soft X-ray spectroscopy.By combining measurements of reference compounds with graphite electrodes cycled in different electrolytes and under different conditions,knowledge of the solid electrolyte interpha (SEI)chemistry can be obtained.In this article,conclusive results concerning the chemical composition of the inorganic part of the SEI is described.The results show that Li 2O often reported to be prent in the SEI could be an artifact from abusive Ar +sputtering.The prence of Li 2CO 3is a matter of debate;the compound is not obrved in anodes extracted from hermetically aled cells that are never expod to air.The results show that cell-design and sample handling are crucial to the obrved chemical composition of the SEI.©2005Elvier B.V .All rights rerved.
Keywords:Solid electrolyte interpha (SEI);Li-ion battery;XPS
1.Introduction
The Solid Electrolyte Interpha (SEI)on graphite elec-trode for Li-ion batteries has been studied extensively becau of its duality in both protecting the graphite from co-intercalation of electrolyte solvent molecules and of con-suming Li-ions in the formation of the layer leading to an irreversible lo
ss of battery capacity [1].However,there is still considerable debate as to the nature of the chemical species formed in the SEI during battery cycling.It is clear,how-ever,that at potentials below 0.8V versus Li/Li +electrolyte-solvent and -salt reduction products do form in the SEI.The general structural and morphological picture of the SEI that has emerged in recent years is one of a den inor-ganic matrix consisting mainly of LiF and Li 2CO 3clo to the electrode surface and a porous organic or polymeric layer extending further out from the electrode surface;typi-cal SEI thickness ranges from ∼20˚A
to veral hundreds ∗
家翻译Corresponding author.Tel.:+46184713713;fax:+4618513548.
cafe是什么意思
E-mail address:kristina.edstrom@mkem.uu. (K.Edstr¨o m).
of ˚Angstr¨o ms.Large crystals of LiF are also found in this
matrix (e schematic picture in Fig.1)[2].
It is also well established that the type of lithium-ion salt ud in the electrolyte will influence the ratio between the inorganic and organic components of the SEI-layer.Most salts (for instance,LiBF 4,LiPF
6,LiAsF 6,etc.)give ri to a layer of LiF clo to the electrode surface.The amount of LiF and the size of the LiF crystals in this layer depend on temperature,trace impurities and reactions with the organic solvents ud in many liquid electrolytes [2,3].One reason for today’s interest in the relatively new LiBOB salt (lithium bis(oxalato)borate)is that it does not form LiF during cell cycling [3].
The thermal stability of the SEI on a graphite anode also correlates with the amount of LiF in the layer.Greater LiF contents are associated with lower ont temperatures for thermally activated reactions in the SEI [4–6].
Due to the small SEI thickness (∼20˚A),
investigation of its chemical composition is limited to just a few experimental techniques,each with its limitations and advantages.Some of the more important techniques are:Fourier transform infrared
0378-7753/$–e front matter ©2005Elvier B.V .All rights rerved.doi:10.1016/j.jpowsour.2005.05.062
K.Edstr¨o m et al./Journal of Power Sources153(2006)380–384
381
阿瑟米勒Fig.1.A schematic picture of the SEI on a graphite particle. spectroscopy(FTIR)[7],photoelectron spectroscopy(PES, including XPS)[2,4–7,8–10]and soft X-ray absorption and emission spectroscopy
(XAS and XES)[11].Several groups have conducted in situ electrochemical cells FTIR and con-focal Raman spectroscopic studies of SEI species.However, the prence of different molecules having similar FTIR sig-natures and the low depth resolution(∼1m)of the confocal Raman beam,leads to a need for complementary studies using other surface nsitive techniques.In addition,some species such as inorganic salt reduction LiF) are not IR active and are hence not detected by the technique.attack
In this article,some of the different species reported to be prent in the SEI will be discusd in the light of ex situ PES results bad on both conventional in-hou XPS and synchrotron-bad PES.The SEI species investigated are the inorganic compounds,such as Li2O,Li2CO3,LiF,and salt reaction products.The results will be discusd in reference to earlier reported ex situ XPS results[9,10],and recent XAS and XES studies of SEI on graphite[11].
2.Experimental
Cell preparation and electrochemical characterisation.All cell preparation of graphite electrodes and cell cycling ud for the experiments has been carried out in the same way as described in Ref.[2,4–6,8–10].A short summery is prented here:
All electrodes were made by mixing80wt.%of active material with10wt.%carbon black and10wt.%binder.The electrodes were dried at120◦C over night in a vacuum fur-nace in an argonfilled glove box(O2,H2O<2ppm).The electrolyte ud was LiPF6in EC/DEC.The salt was dried at 80◦C before mixing the electrolyte and Karl Fischer titration showed the water content to be below10ppm.The cells were asmbled with lithium as the counter electrode,graphite as working electrode and a mechanical parator soaked in the electrolyte.The cell was aled in a pouch cell of polymer-laminated aluminium.
Surface analys of the electrodes by PES using Al K␣and synchrotron radiation.The electrochemical cells were dismantled in a glove-box and small pieces of the electrodes were cut out and mounted on a sample holder and transported to the PES equipments in an Ar(g)atmosphere using a spe-cially designed transport chamber to avoid contamination by air and moisture.Surface analysis by monochromatid Al K␣,at1486.6eV,was performed on a PHI5500system.The electrodes were dried by turbo pumping(∼1×10−6Torr) in the load-lock leading to the analysis chamber for>12h to remove electrolyte solvents from the electrode surface prior to PES characterisation.Depth analysis in the conventional monochromatic PES was performed by Ar+ion beam sputtering(4eV).The graphite peak was not discernible in all C1s spectra and the Al K␣spectra were therefore energy calibrated ag
ainst the main C1s peak t to286.0eV (the binding energy obtained for this peak in the spectra where the graphite C1s could be obrved at284.3eV).To allow asssment of relative peak intensities,the C1s and O1s spectra were intensity normalid against the main C1s peak(at286.0eV)and the F1s,P2p and Li1s spectra were intensity normalid against the F1s peak from Li x PF y compounds.
The synchrotron radiation bad PES measurements were performed at the beam line I411at the Swedish National Synchrotron Radiation Laboratory MAX[12].The excitation photon energy(hν)for a given core level was varied between454and1360eV to obtain a depth profile of the species within the surface layer.In the synchrotron radiation bad PES measurements,linear background correction and energy calibration versus the main C1s peak (286.0eV)have been applied in the data analysis.To allow an asssment of the relative amount of surface species, intensity normalisation was performed according to:C1s spectra were normalid vs.the main C1s peak(286.0eV) and F1s spectra were normalid versus the LiPF6peak (∼688eV).
Non-conducting salt species on the analyd surfaces caud some charging effects.Therefore,peak assignments were made after calibration with reference compounds,as previously published by us[2,4–6,8–10],rather than using tabulated binding energies.Furthermore,some of the refer-ence co
mpounds were spin coated or simply added with a pipette on graphite anodes and then measured,to determine the binding energies on our graphite electrodes.
3.Results and discussion
There is a whole-range of different chemical substances that have been ascribed to be in the inorganic part of the graphite SEI.Here,we concentrate on the most commonly reported inorganic compounds for a clor look at their PES spectra.The aim is to obtain a clearer picture of their prence in the graphite SEI.In this article,we prent results on Li2O, Li2CO3,LiF,and some other salt reduction products.The data
382K.Edstr¨o m et al./Journal of Power Sources 153(2006)
380–384language
Fig.2.The O1s depth profile of a graphite electrode cycled in 1M LiPF 6and EC/DEC using synchrotron bad PES.
网络辅导is bad on graphite that has an irreversible capacity of ∼12%during the first cycle.
3.1.Li 2O and its prence in the graphite SEI
Li 2O has been reported to be one of the compounds found clo to the graphite surface.It can be formed from trace amounts of water prent in the electrolyte or the compos-ite electrodes.In our studies,by synchrotron-bad XPS,we have not detected any Li 2O (e Fig.2),which should be obrvable at 528.3eV in the O1s spectrum and at 53.7eV in the Li1s spectrum.However,depth profiles obtained using Ar +sputtering always show Li 2O.We have shown earlier,using Li 2CO 3as a model compound,that Ar +sputtering gen-erates Li 2O (Li 2CO 3→Li 2O +CO 2)after some minutes of sputtering (Fig.3).The previously reported Li 2O could there-fore be an artifact of the sputtering process rather than a true SEI
component.
Fig.3.O1s spectrum (a)and Li1s spectrum (b)of Li 2CO 3,non-sputtered and Ar +etched 30min.
3.2.Li 2CO 3and its prence in the graphite SEI
Li 2CO 3has been reported to be one of the important如何构建和谐校园
products of the SEI.This has resulted in successful tests to pre-treat a graphite surface with Li 2CO 3to reduce the irre-versible capacity [13].There are also reports of more complex organic species prent in the SEI converting to Li 2CO 3at elevated temperature [14].However,there are also a num-ber of situations where Li 2CO 3has been reported not to be prent or not to be the main compound dominating the room temperature SEI [7,8].The factors that can explain Li 2CO 3abnce may include the following:(a)Li 2CO 3is not formed in the cell at any stage;(b)Li 2CO 3forms but reacts with trace amounts of HF from the electrolyte to form LiF,water and CO 2;(c)Li 2CO 3forms but reacts with PF 5to form CO 2,P-O type compounds,and LiF.The prence of Li 2CO 3appears to depend on the moisture content of the electrolyte and the type of cell ud.Anodes extracted from better-aled cells,such as the coffee-bag (pouch)cells,are less likely to show Li 2CO 3than commonly ud cells.
To test the influence of air and moisture,cycled graphite electrode surface was expod to air for 3h and then the PES spectrum was taken.A clear difference is en in both the ratio between the different oxygen-species prent imply-ing a larger amount of Li 2CO 3in the air-expod sample (Fig.4a),and in the broadening of the peak to lower photon energies.Not even Ar +etching the electro
de reveal any larger amounts of Li 2CO 3(e Fig.4b).This can be compared to our synchrotron bad PES results where the depth profile also shows very little prent Li 2CO 3clo to the electrode surface and no Li 2O (Fig.2).The effect of air on the SEI is also clearly en in the C1s spectrum (Fig.5)where the most of the organic species containing functional groups such as C O (at 286eV)have reacted and left the surface form-ing most likely forming CO 2so that the C H species
(at
Fig.4.O1s XPS spectra of a graphite electrode cycled one time non-expod and expod to air for 3h (a)compared to an O1s spectra of sputtered and non-sputtered cycled graphite electrode (b).All electrodes have been cycled in LiPF 6EC/EMC.
K.Edstr¨o m et al./Journal of Power Sources153(2006)380–384
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Fig.5.Influence of air on the C1s spectrum of a graphite electrode cycled in LiPF6EC/EMC.
285eV)become dominant.The data indicate that careful sample transfer into the spectrometer is very important for reliable results.
Results bad on soft X-ray spectroscopy,and in particular lective excitation resonant X-ray emission spectra,are also in agreement with our PES results.Comparing the graphite SEI spectra with tho from reference compounds showed that Li2CO3was not prent in the SEI[11].
3.3.LiF,salt-reduction products,and their prence in the graphite SEI
LiF may form as a by-product of Ar+etching.Our model sputtering studies of the pure salts,LiBF4and LiPF6,show that LiF are formed.LiF is easily distinguished in the F1s XPS spectra at686eV that is significantly lower binding energies than otherfluorine containing species[2,4,5–10].During electrochemical cycling of the cells there are,however, many reactions leading to LiF formation[14].In particular, there is an incread amount of LiF formed as a function of temperature[2,3,8].For cells stored at elevated temperatures and displaying different amounts of power fade there is also a correl
ation with incread amounts of LiF[5].The LiF formed is distributed as large crystals in the SEI-matrix.The size of the crystals is dependent on salt ud in the elec-trolyte,cycling temperature,and storage conditions.Rinsing cycled electrodes will mechanically remove LiF crystals, which also influences the amount of material available for analysis[15].
Salt reduction products consisting of Li x PF z are com-monly obrved in the SEI[1,3,5,6,8–11].In some cas also Li x PF y O z is detected.A part of the detected increa of LiF is a result of sputter-induced decomposition of the compounds[8].Ar+sputtering ems also to induce new P-bonds,probably P O or P P,spectra as en on sputtering of model compounds such as LiPF6and LiPF6in EC/DEC on a graphite surface[10].Rinsing the electrodes results mainly in a decrea in the amount of Li x PF z and an increa in Li x PF y O z due to uncovering of the compound clo to the electrode surface.During exposure to air Li x PF y is obrved to transform to Li x PF y O z[8].
Becau of the influence of sample preparation,handling procedures and sputtering conditions,quantitative analysis of the content of LiF and the salt reduction components Li x PF y and Li x PF y O z should be undertaken with caution.
4.Conclusions
The following conclusions are bad on data from our various PES analys:
•Li2O detected in the SEI by depth profiling can be an arti-fact of Ar+sputtering
•Li2CO3is not always obrved in the SEI.The graphite-type,electrolyte quality and hermetic-als on the lithium-ion cells may influence the prence and quantity of Li2CO3.Additional work is needed to conclusively iden-tify the effect of the parameters.
•LiF is formed during cell cycling;the amount measured in the SEI is influenced by veral factors that include storage temperature,cycling duration,and the type of lithium salt in the electrolyte.LiF can also be generated by Ar+etching.•Careful sample preparation and handling are important to determine the true constituents of the graphite SEI.Reac-tions with air and rin solvents and air can introduce artifacts,and alter the SEI composition.
This is part of an on-going study of SEI characterisa-tion using surface nsitive chemical analysis techniques. The work is now focud on organic compounds in the SEI.
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