Graphene-modified LiFePO4 cathode for lithium ion battery beyond theoretical capacity
ARTICLE
Received26Sep2012|Accepted5Mar2013|Published9Apr2013
DOI:10.1038/ncomms2705 Graphene-modi?ed LiFePO4cathode for lithium ion battery beyond theoretical capacity
By Lung-Hao Hu1,*,Feng-Yu Wu1,*,Cheng-T e Lin1,Andrei N.Khlobystov2&Lain-Jong Li1
The speci?c capacity of commercially available cathode carbon-coated lithium iron phosphate
is typically120–160mAh gà1,which is lower than the theoretical value170mAh gà1.Here
we report that the carbon-coated lithium iron phosphate,surface-modi?ed with2wt%of the
electrochemically exfoliated graphene layers,is able to reach208mAh gà1in speci?c
capacity.The excess capacity is attributed to the reversible reduction–oxidation reaction
between the lithium ions of the electrolyte and the exfoliated graphene?akes,where the
graphene?akes exhibit a capacity higher than2,000mAh gà1.The highly conductive
graphene?akes wrapping around carbon-coated lithium iron phosphate also assist the
electron migration during the charge/discharge processes,diminishing the irreversible
capacity at the?rst cycle and leading to B100%coulombic ef?ciency without fading at
various C-rates.Such a simple and scalable approach may also be applied to other cathode
systems,boosting up the capacity for various Li batteries.
1Institute of Atomic and Molecular Sciences,Academia Sinica,T aipei10617,T aiwan.2School of Chemistry,University of Nottingham,Nottingham NG72RD,UK.*These authors contributed equally to this work.Correspondence and requests for materials should be addressed to L.J.L. (email:lanceli@https://www.wendangku.net/doc/a1927229.html,.tw).
T
he serious climate change concern coupled with high fuel prices are driving the research and investment in the development of sustainable energies,such as solar energy,wind energy,hydroelectricity,wave power and geothermal energy.The sustainable energy,after its generation,needs to be ef?ciently stored for future use in devices as well as vehicles.The development in energy-storage devices with high capacity and power has progressed at an unprecedented high speed over last decade.Li ion battery is one of the most promising solutions for these storage systems because of its outstanding electro-chemical performance and high capacity.To meet such a strong demand in higher-capacity storage,the rate capability and energy density of Li tion battery must be improved,which urges the discovery of a novel composite cathode or anode.Lithium iron phosphate (LiFePO 4or LFP),one of the very popular commercial cathode materials for Li battery,exhibits several advantageous features for the energy storage such as low cost,environmental capability,relatively large capacity and intrinsic stability.Not unique to the LFP,all cathode materials exhibit their theoretical value in charge capacity,which is limited by intrinsic property of the material.Structural defects or other factors that may retard the Li tdiffusion further lower the actual capacity of the cathode.Current strategies to enhance the electrochemical performance of LFP include carbon coating on LFP (cLFP)1–6,metal doping 7–9,and LFP particle size reduction 10–15.The carbon coating process has been massively used in industry because the conductive carbon layer increases the electron migration rate during the charge/discharge processes.Metal doping is able to expand the Li tdiffusion channel and increase the output voltage of LFP-based Li ion battery.The reduction of LFP particle size results in the shortening of the Li tdiffusion path;however,the disadvantage is that the high surface area requires higher loading amount of binders to glue these small LFP particles together,where the inclusion of binders further decreases the capacity.Hence,the nano-sized materials are still not applicable to practical Li ion batteries.
Recently,graphene has become the spotlight in lithium ion battery research because it owns several desirable features,including high surface area and excellent electronic conductivity,
for improving the electrochemical performance of LFP.The most popular approach to synthesize graphene is to reduce the chemically exfoliated graphene oxide (GO)obtained by Hum-mer’s method,owing to that the method is potentially scalable.Several reports have demonstrated some enhancement in rate capacity using the reduced GO (rGO)/LFP composite material as the cathode,where the cathode was prepared by pyrolyzing the GO or rGO together with either LFP precursors or LFP particles 16–19.The capacity increase was attributed to that the rGO sheets helped to enhance the electron transport rate in cathodes.The speci?c capacity of the rGO-modi?ed LFP cathodes in these reports ranged from 146to 165mAh g à1.
Here we report that the incorporation of few-layer graphene obtained by another scalable method electrochemical exfolia-tion 20is able to deliver a capacity of 208mAh g à1,which is beyond the theoretical capacity of LFP 170mAh g à1.The energy density is up to 686Wh kg à1,much higher than the typical 500Wh kg à1of LFP.The excess capacity is attributed to the ultrahigh capacity (42,000mAh g à1)of the electrochemically exfoliated graphene (EG)?akes.The cyclic voltammetric measurement for the half cells with EG ?akes as the only active material reveals the reversible redox reaction that contributes to the Li tion storage capacity.The capacity enhancement effect is obvious such that only a very low weight percentage of graphene (o 2wt%)is required,and the low loading percentage avoids the unwanted voltage polarization effect that can been seen after introducing higher graphene percentage in cathodes.Meanwhile,the superior quality of the electrochemically EG,with a relatively higher carrier mobility compared with available rGO,greatly enhances the electron transfer during the charging and discharging processes.It is noteworthy that only B 0.8and 2wt%of graphene is needed to achieve the capacity 187and 208mAh g à1,respectively,without causing unfavourable voltage polarization.
Results
Electrochemically exfoliated graphene .Large quantity of EG thin ?akes were prepared using an electrochemical
exfoliation
6 μm
–2.8 nm 2.8 nm
C o u n t s Thickness (nm)
R a m a n i n t e n s i t y (a .u .)
Raman shift (cm –1)
296294292290288286284282280
C o u n t s (a .u .)
Binding energy (eV)
284.6 eV C=C 71.4%286.3 eV C-O 13.4%287.8 eV C=O 5.6%288.5 eV COOH 9.6%
Figure 1|Characterization of electrochemically exfoliated graphene ?akes.(a )Optical micrograph for the graphene ensemble.Scale bar,15m m.(b )AFM image of a selected graphene ?ake.(c )Statistical measurement results of AFM thickness for randomly selected 76?akes,where the thickness ranges from 1.5to 4.2nm (d )Typical Raman spectrum (excited by a 473nm laser)and (e )C1s binding energy pro?le measured by X-ray photoemission spectroscopy for the obtained few-layer graphene ensemble.
method previously reported by us20.In brief,a graphite rod was used as the source that can be exfoliated to produce graphene layers during the electro-oxidation process.A platinum wire was used as a grounded electrode,and the electrolyte solution has been optimized(SO42àat pH B12as described in Methods)to serve ef?cient exfoliation but without severely degrading the graphene layers.Once a suf?ciently high bias(10V)was applied to the graphite rod,it was quickly dissociated into small?akes spreading on the solution surfaces.These EG?akes were collected by?ltration and then re-dispersed in dimethylformamide(DMF). The as-dispersed graphene solution in DMF was further centrifuged to remove the unwanted side products,thick graphite particles.We have optimized the centrifugation condition to obtain the graphene layers with a narrowed distribution in thickness.Figure1a displays the typical optical micrograph,and Supplementary Fig.S1shows the scanning electron microscope(SEM)image for the graphene?ake ensemble we used in this study,where the lateral size of the ?akes ranges from a few to several tens of m m.Figure1b displays the atomic force microscope(AFM)image of a selected graphene ?ake.Figure1c shows the statistical measurement results of AFM thickness for randomly selected76?akes from the ensemble, where the thickness ranges from1.5to4.2nm(average2.5nm). Figure1d,e shows the typical Raman spectrum and the binding energy pro?le measured by X-ray photoemission spectroscopy, respectively.The observation of Raman D and D0bands,as well as the X-ray photoemission spectroscopy C1s binding energies at 286.4eV(C-OH),287.8eV(C?O)and288.9eV(O?C-OH), suggests that some oxygen-containing functional groups are present,where the atomic percentages of various carbons derived from the?tted peak areas are indicated in Fig.1e.However,the
existence of a sharp two-dimensional band(B2,700cmà1) indicates that the few-layer EG?akes are still with reasonably good graphitic structures,where their conductivity is superior to commonly used GO or rGO sheets20.
Morphology of the graphene-coated LiFePO4.In the study,we used commercially available LiFePO4powders(particle diameter B300nm),and these as-received particles were coated with a thin amorphous carbon layer(cLFP).To achieve homogenous coating of few-layer graphene?akes on the surface of cLFP,the DMF solution of EG?akes(concentration B250p.p.m.)was dropwise added to cLFP powders with gentle stirring at180°C. The EG?akes spread in the mixture with the aid of DMF solvent and wrapped the surfaces of cLFP particles through van der Waals interaction.Meanwhile,the DMF solvent gradually evaporated at the mixing temperature.The SEM image in Fig.2a illustrates that the EG?akes nicely cover the cLFP particles after added to cLFP.Figure2b shows the magni?ed SEM image, where the wrinkles of the graphene layers are clearly observed. Coin-sized half-cells with various cathode compositions,for example,cLFP materials with and without the addition of few-layer EG,were assembled using a lithium foil as the anode.
Speci?c capacity of the graphene-coated LiFePO4.The char-ging/discharging voltage pro?le for the cell using the cathode loaded with a very low EG percentage of0.13wt%does not show much difference compared with that using the cathode loaded with only cLFP(Supplementary Fig.S2).Figure2c shows that the voltage pro?les of the?rst cycle with0.1C as the charging and discharging rate for the cathodes with higher EG loading percentages become drastically different from that without EG. The EG-wrapped cLFP cathodes provide the capacity from187to 208mAh gà1,depending on the weight percentage of EG(from 0.8to2wt%).These values are higher than the reported values of 120–160mAh gà1for commercially available or synthetic LFP materials in research laboratories16,17,21,22.Surprisingly,these values are even in excess of the theoretical value of170mAh gà1 for LFP.Supplementary Fig.S3plots the speci?c discharge capacity value as a function of the percentage of EG based on the results of Fig.2c,where the discharge capacity of the EG,layers in the2wt%sample is estimated to be as high as2,720mAh gà1.
Discussion
It has been well established that the major LFP capacity is attributed to the Litinsertion to and extraction from the LFP particles(chemical equation:LiFePO4?FePO4tLitteà). Two major limiting processes have been reported to explain why some lithium ions cannot be fully extracted from the ordered-olivine structure which in turn causes some capacity loss.
(i)Limited lithium ion phase-boundary diffusion:the one-dimensional channels in LFP impose structural constraint,where the Litdiffusion can be interrupted by ionic disorder,foreign phases or stacking faults.Interruption of Litdiffusion impedes the motion of a LiFePO4/FePO4phase boundary that removes portions of the cathode from accessing to a reversible Litintercalation23.(ii)Low electron conductivity:during the charging and discharging processes,the charges must be kept in balance with the insertion/extraction of Litvia electron transfer. If electrons are not able to transfer rapidly,the electron mobility will limit the Litinsertion/extraction and cause a deteriorated electrochemical property.
To resolve these issues,many attempts have been made to expand the one-dimensional channels of the olivine structure for smooth Litdiffusion,such as the doping with impurities,metals or metal oxides24–26.Alternatively,signi?cant effort has been devoted to coating the LFP surfaces with electrically conductive materials,such as amorphous carbon and conducting polymers
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Figure2|Speci?c capacity of the graphene-coated LiFePO4.(a,b) SEM images of the carbon-coated LiFePO4particles wrapped by electrochemically EG sheets.Scale bar,100nm and10nm(in a and b, respectively).(c)The voltage pro?les for the?rst cycle charging and discharging(with a rate of0.1C)of the carbon-coated LiFePO4cathodes with various graphene-loading percentages(0,0.8,1.2and2wt%).
enhance the electrical conductivity of LFP surfaces27–32.Recent studies have adopted rGO or GO to cover the LFP surfaces16–19. However,studies so far have only shown certain improvements in the rate capability but no report has exceeded the theoretical limit of the capacity.Therefore,enhanced capacity over the theoretical value by adding EG layers must be associated with the unique features of EG layers.Figure3a shows the high-resolution transmission electron microscopy(HRTEM)image for the cLFP cathode materials with0.8wt%EG after charging(Litextraction from the cathode),where the cathode material was obtained by cleaving the Li-battery half-cell immediately after the charging procedure.HRTEM reveals that the initial amorphous carbon coating on cLFP particles is around1-nm thick,and the thickness of EG on top of the amorphous carbon is1.5–2nm(only few layers)for the selected site.Most of the graphene layers seem to stack around the surface of the cLFP particle.Figure3b displays the HRTEM images after the cathode material is discharged(Litinsertion to the cathode).It is observed that graphene layers become very disordered,with many layers oriented randomly with respect to the surface of the cLFP particle.Furthermore, statistical measurements of distances between graphene layers show an increase for the average interlayer distance from0.32nm for the charged cathode to0.38nm for the discharged one (Supplementary Fig.S4shows the TEM analysis of graphene layer-to-layer distances for over12sites).These observations indicate that Litions come into the few-layer EG after discharging,resulting in the expansion of interlayer distance. Note that it is not easy to visualize individual Litions in HRTEM;however,the occasionally found in-plane view of the graphitic layers at the edge of the cLFP particle as shown in Fig.3c shows a hexagonal periodicity that is typical for the two-dimensional lattice of Litions intercalated in graphite,providing an alternative proof for Litions insertion to EG.
In fact,the intercalation of Litions into graphite is typically seen in the electrochemical process occurring in a graphite anode;nevertheless,the theoretical capacity is B372mAh gà1and the reversible intercalation of lithium in graphite occurs at only B0.1V that is far to the3.4V.Highly disordered graphene nanosheets have been shown to exhibit a higher reversible Lition storage capacity up to1,100mAh gà1.However,the charge/ discharge pro?les should be with a large voltage hysteresis and without distinct potential plateaus33.These indicate that there should exist another Lition storage mechanism for the observed high capacity from EG.To reveal the possible origin of the excess capacity,we deposit EG?akes on inactive silica particles (diameter B5–10m m)as cathode materials and make coin-sized cells following the same method used for EG/cLFP. This allows us to directly measure the capacity of EG?akes. The second cycle voltage pro?les of the cathodes made by0and 1.8wt%EG on silica particles(EG/silica)are shown in Supplementary Fig.S5.It is observed that silica particles exhibit a small reversible capacity,whereas EG?akes demonstrate a signi?cantly large reversible capacity.Figure4a compares their cycling properties at a?xed charge/discharge current density 23mA gà1,corresponding to0.5and7.5C of rate capabilities for the cathodes with1.8and0wt%EG,respectively.The rate capability of1.8wt%EG/silica remains after six cycles;however, the rate capability of pure silica fades after a few cycles.This result indicates that EG?akes incorporated with the cathode
materials LiFePO4
core
Expected Li–Li
distance ≈ 3 ?
Figure3|TEM images.High-resolution TEM images of the carbon-coated
LiFePO4cathode materials with0.8wt%of graphene(a)after charging
(Litextraction),where the yellow square indicates the amorphous carbon
coating and the white square is the added graphene layers.Scale bar,1nm.
(b)After discharging(Litinsertion),the graphene layer-to-layer distance
expands and becomes disordered in stacking.Scale bar,1nm.(c)The in-
plane view of the graphitic layers at the edge of the carbon-coated LiFePO4
particle,which shows a possible indication of hexagonal lattice that is
typically formed by Lition intercalated graphite.Scale bar,0.7nm.
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Figure4|The electrochemical characteristics.(a)The cycling properties
for the coin cells with the cathodes made by0and1.8wt%graphene?akes
on silica particles.Charge/discharge rate:23mA gà1.(b)The cyclic
voltammetry characteristics for these two cells.The curve for silica was
collected at a rate of0.02Vsà1.
is able to enhance the capability of Li tion storage from EG,and this storage mechanism is reversible.Based on Fig.4a,the charge (discharge)capacity for the 1.8wt%EG/silica at the sixth cycle is 53.5(38.3)mAh g à1higher than that for pure silica,where the speci?c charge (discharge)capacity of EG is estimated to be B 2970(2,120)mAh g à1,consistent with the high capacity obtained in Supplementary Fig.S3.As the preparation of EG involves electrochemical reactions in a solution,it is very likely some redox active sites or defects are activated and remained on EG,where the sharp Raman D-band in Fig.1is perhaps the good indication for the presence of defects.Figure 4b shows the cyclic voltammetry characteristics for these two coil cells at various scan rates.No redox peak for silica particles is observed because the potential of the Li tion intercalation and extraction of silica is between 0.2and 0.5V.The slow scan rate 0.02V s à1reveals an oxidation peak at 3.5–4.5V and a reduction peak at 2.5–3.5V for the sample with EG,clearly demonstrating the presence of a reversible redox reaction that can contribute to Li tion storage capacity.Some Li tions may be stored reversibly between graphene planes,but the redox-based reversible storage should predominate,where the defects at edge sites or basal planes (vacancies and so on)of the EG may be involved.However,it is noteworthy that special care should to taken to avoid the re-stacking when applying EG ?akes to electrode materials because they tend to easily agglomerate to increase the polarization and decrease their surface areas.We have
deposited EG ?akes onto a ?at surface to form a thick ?lm;however,the measured speci?c capacity for the stacked EG ?akes is only B 350mAh g à1,much lower than the value obtained from EG/silica cathodes.
Another signi?cant change after EG addition to cLFP is the reduction in the ?rst cycle loss of capacity.Figure 2c shows that the irreversible capacity of EG/cLFP-based lithium battery is vanished for the cathodes loaded with 0.8and 1.2wt%of EG,respectively.The outstanding cycling performance is attributed to the highly conductive few-layers graphene homogenously distributed around cLFP particles,which serve as a fast path for electron migration during the charge/discharge processes 18.It is known that electron transfer in sp 2carbon is more effective than in sp 3amorphous carbon 34,35.The electrons are capable of spreading to the entire surface of the EG/cLPF particles through the EG sheets during charge/discharge,improving the kinetics and reversibility of the lithium insertion/extraction cycles.However,the irreversible capacity increases to 5%when EG percentage is increased to 2wt%.The irreversible capacity is also B 5%for the cell based on cLFP cathodes without EG.Yang et al.36have shown that the ?rst cycle loss percentage increases with the carbon content because lengthened Li tdiffusion path caused by the overall increase in carbon content increase may hinder the diffusion.It is noteworthy that the voltage plateau for the EG/cLFP cathode-based coin cell still remains at 3.4V,which indicates that the homogenous coating of EG does not cause the unwanted polarization effect,which is advantageous for the Li tbattery.However,if the EG coating around cLFP particles is not uniform,that is,EG layers stack together,it would be re?ected in additional voltage polarization.If the graphene ?akes are too thick,they would behave like graphite particles that would cause the unwanted voltage polarization 37.Such an electrochemical behaviour is typically known as for graphite particles.Supplementary Fig.S6shows the voltage pro?le for the cell with the cLFP cathode loaded with 2wt%EG but prepared with rough mixing (EG solution was not added in a dropwise manner),where the obvious voltage polarization is observed.As graphene layers tend to stack to form thicker aggregates,the structure is closer to graphite,due to the strong p àp stacking interaction.The cycle life is also increased because of the few-layer graphene coating,where the electrons can be fast transferred on the surface of cLFP to withstand the high discharging rate.For 0.8wt%EG-wrapped cLFP,the cycle life is substantially improved at higher discharging rate, 2.5C,comparing with commercial cLFP and lower weight percent of EG-wrapped cLFP as shown in Fig.5a.Figure 5b shows the discharge rate capability and coulombic ef?ciency at different charge/discharge rates for the lithium ion battery cells based on the cathode added with 1.2wt%of EG.The discharge capacity performance for the reference cell based on cLFP without EG is also shown for comparison.As expected,when the discharge rate is increased,both cathodes cannot sustain a high discharging current (fast Li tintercalation)and thus the capacity fades very quickly.However,the cell loaded with EG can deliver 423%higher capacity at lower rates 0.1–1.3C and 426%higher capacity at higher C-rates 2.5–28C,consistently demonstrating the advantages of adding EG.In particular,the composite cathode material,EG/cLFP,can deliver a capacity B 125mAh g à1at a high discharging rate 10C,which is around 26%better than the 98mAh g à1obtained from cLFP only.The Coulombic ef?ciency of EG/cLFP still maintains at 98–100%for various C-rates.
In summary,the electrochemically exfoliated few-layer graph-ene sheets have been applied to wrap the commercially available cathode material cLFP.The incorporation of low weight percent (2wt%)of graphene in cLFP is able to deliver a capacity of 208mAh g à1,which is beyond the theoretical 170mAh g à1
S p e c i f i c c a p a c i t y (m A h g –1)
Cycles
60
80100120140160180200220
S p e c i f i c c a p a c i t y (m A h g –1)
Numer of cycles
C o u l o m b i c e f f i c i e n c y (%)
Figure 5|The cycle life test for the battery.(a)The cycle life test for the Li tion battery cells based on the carbon-coated LiFePO 4cathodes added with 0and 0.8wt%of graphene.(b )The discharge rate capability and Coulombic ef?ciency at different charge/discharge rates for the Li tion battery cells based on the cathodes added with 0and 1.2wt%of graphene.
without causing obvious voltage polarization.The energy density is up to686Wh kgà1,higher than the typical value500Wh kgà1 for cLFP.The extra capacity is mainly attributed to the reversible redox reaction between the lithium ions of the electrolyte and the EG?akes.The addition of high-capacity materials to improve the capacity of an existing cathode material,like two capacitors connected in parallel,has been proved to be successful.The wide range of the redox potential of the EG?akes is also likely applicable to other cathode materials for Li battery,and this study may stimulate extensive research into fundamentals of this process and advance industrial applications.
Methods
Synthesis of electrochemically EG.The preparation of electrochemically exfo-liated few-layer graphene was based on the method developed in our previous report20as brie?y described below.Natural graphite?ake or graphite powders compressed as a rod was used as the graphene source as well as the working electrode.A Pt wire was chosen as a grounded electrode.An aqueous solution containing SO42à(2.4g of98%H2SO4in100ml of deionized water,and added with11ml of30%KOH solution to make its pH value around12)was used as an electrolyte.The static bias oft2.5V was?rst applied to the working electrode for 5–10min,followed by ramping the bias tot10V for graphene exfoliation.The initial low bias helps to wet the sample and likely causes gentle intercalation of SO42àions to the grain boundaries of graphite38,39.Before applying a high bias of t10V,the graphite working electrode still remained as a single piece.Once the high bias was applied,the graphite was quickly dissociated into thin sheets and spread in the solution surface.These EG sheets and related products were collected by?ltration and then re-dispersed in DMF.We note that the electrochemical exfoliation of graphene is very ef?cient as the whole exfoliation process can be ?nished in a few minutes but it generally produces thin sheets with large amounts of defects because of the fact that the H2SO4itself also results in strong oxidation of graphite.Switching the applied voltage betweent10andà10V during the exfoliation helps to reduce the oxidation of EG layers.The large-scale synthesis and puri?cations of the electrochemically EG?akes used in this study were assisted and performed in Nitronix Nanotechnology Corp.(Taiwan).
Synthesis of EG-wrapped LFP.The electrochemically EG sheets dispersed in a DMF solution(250p.p.m.)were dropwise added to the commercially available LiFePO4powders(diameter B300nm;from Tatung?ne chemicals Co.)with gentle stirring at180°C.It is noted that the commercially available LFP particles were normally coated with an amorphous carbon layer;hence,we name it as cLFP. The van der Waals forces between graphene sheets and cLFP particles drive the formation of cLFP particles wrapped with graphene layers on their surfaces,as shown in Fig.2.The good wrappability provides good protection for cLFP particles against the volume expansion or agglomeration.The EG-wrapped cLFP was used as the active material for the electrochemical test.
Electrochemical test.For the preparation of the lithium ion battery cathodes, 10wt%Super P was?rst mixed with10wt%polyvinylidene di?uoride in N-methyl-2-pyrrolidone followed by the addition of80wt%of the active material EG/cLFP, and all were mixed with stainless steel balls for ball-milling at400r.p.m.The resultant slurry,pasted on an Al foil,was dried at110°C for4h.The coin cells (2032)then were assembled in an argon-?lled glove box using a lithium foil as the anode,Celgard2600as the separator and1M LiPF6dissolved in ethyl methyl carbonate,dimethyl carbonate and vinylene carbonate with a volume ratio of1:1:1 as the electrolyte.Cells were tested at ambient temperature.The testing voltage in the constant current mode was in a range of2.0–3.8V,and the cells were charged in the constant voltage mode at3.8V until the current reached0.05C.
Characterizations.The AFM images were performed in a Veeco Dimension-Icon system.Raman spectra were collected in a confocal Raman system(from NT-MDT company).The wavelength of laser is473nm(2.63eV),and the spot size of the laser beam is B0.5?`m and the spectral resolution is3cmà1(obtained with a600 grooves per mm grating).The Si peak at520cmà1was used as a reference for wave number calibration.For TEM,the cathode material was rinsed by ethanol and dried in the glove box.The EG/cLFP was scratched off from the Al foil and then sonicated in ethanol.The EG/cLFP in ethanol solution was dropped on a copper grid for TEM observation.HRTEM imaging was performed on JEOL2100F FEG-TEM operated at100kV.
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This research was supported by Academia Sinica(IAMS and Nano program).A.N.K. acknowledges the European Research Council(ERC)for funding and the Nottingham Nanoscience and Nanotechnology Centre(NNNC)for access to TEM.
Author contributions
L.-H.H.and F.-Y.W.performed the major part of the experiments.A.N.K.performed TEM experiments.All authors discussed the results.L.-J.L.and L.-H.H.conceived the study and wrote the manuscript.
Additional information
Supplementary Information accompanies this paper on https://www.wendangku.net/doc/a1927229.html,/ naturecommunications
Competing?nancial interests:The authors declare no competing?nancial interests. Reprints and permission information is available online at https://www.wendangku.net/doc/a1927229.html,/ reprintsandpermissions/
How to cite this article:Lung-Hao Hu.et al.Graphene-modi?ed LiFePO4cathode for lithium ion battery beyond theoretical https://www.wendangku.net/doc/a1927229.html,mun.4:1687doi:10.1038/ ncomms2705(2013).