High-rate electrochemical energy storage through Li +intercalation pseudocapacitance
Veronica Augustyn 1,Jérémy Come 2,3,Michael A.Lowe 4,Jong Woung Kim 1,Pierre-Louis Taberna 2,3,Sarah H.T olbert 5,Héctor D.Abruña 4,Patrice Simon 2,3and Bruce Dunn 1*
Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions,as observed with RuO 2·x H 2O in an acidic electrolyte.However,we recently demonstrated that a pseudocapacitive mechanism occurs when lithium ions are inserted into mesoporous and nanocrystal films of orthorhombic Nb 2O 5(T -Nb 2O 5;refs 1,2).Here,we quantify the kinetics of charge storage in T -Nb 2O 5:currents that vary inversely with time,charge-storage capacity that is mostly independent of rate,and redox peaks that exhibit small voltage offsets even at high rates.We also define the structural characteristics necessary for this process,termed intercalation pseudocapacitance,which are a crystalline network that offers two-dimensional transport pathways and little structural change on intercalation.The principal benefit realized from intercalation pseudocapacitance is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-st
ate diffusion.Thick electrodes (up to 40µm thick)prepared with T -Nb 2O 5offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices.
Pseudocapacitance occurs whenever the charge (Q )depends on the change in potential (d E ),yielding a capacitance (d Q /d E )(ref.3).The capacity can be due to monolayer adsorption of ions at an electrode surface,as in the underpotential deposition of metals 4;surface redox reactions as in RuO 2;or ion intercalation that does not result in a phase change.Although these redox processes are Faradaic in nature,their phenomenological behaviour,and response to experimental variables such as sweep rate,are those typical of capacitors.All of these scenarios produce a relationship between the fractional extent of charge storage,X ,and the potential of the form 3,5:
E ∼
RT nF ln X (1−X )
(1)where R is the ideal gas constant (J mol −1K −1),T is the temperature (K),F is Faraday’s constant (96,485As mol −1)and n is the number of electrons involved in the reaction.In all of these cases,a constant-current experiment yields a potential E that changes with the extent of charge Q according to:
Q =C E
where Q is the charge passed (Coulombs), E is the potential change (V)and C is the pseudocapacitance (F).This behaviour
1Department
of Materials Science and Engineering,University of California,Los Angeles,California 90095,USA,2Department of Materials Science,
UniversitéPaul Sabatier,CIRIMAT UMR CNRS 5085,T oulouse 31062,France,3Réseau sur le Stockage Electrochimique de l’Energie (RS2E),FR CNRS 3459,France,4Department of Chemistry and Chemical Biology,Cornell University,Ithaca,New York 14853,USA,5Department of Chemistry and Biochemistry,University of California,Los Angeles,California 90095,USA.*e-mail:bdunn@ucla.edu .
is typical of capacitive charge/discharge,thus leading to the
term pseudocapacitance 3.
Of the three pseudocapacitive mechanisms mentioned above,underpotential deposition and surface redox reaction pseudoca-pacitance exhibit kinetics indicative of surface-controlled electro-chemical processes 6:
i =Cv
where i is the current (A)and v is the sweep rate (mV s −1)of a cyclic voltammetry experiment.However,in intercalation pseudocapacitance,as described herein,charge storage does not occur on the surface but in the bulk material.The kinetics are not diffusion-limited and instead are limited by surface processes so that the overall behaviour seems capacitive.Intercalation pseudocapacitance is rarely observed because in most intercalation materials charge storage (even in thin films,as in anatase TiO 2;refs 7,8)is limited by solid-state diffusion and therefore the peak currents scale with v 1/2.
Here we investigate the phenomenon of intercalation pseudo-capacitance and the high-rate behaviour of T -Nb 2O 5using two different electrode techniques that provide a wide variation in sweep rates.For timescales between ∼3h and 60s (sweep rates of 0.1–20mV s −1within a voltage window of 1.2V),we used a thin-film electrode.For shorter timescales where ohmic polariza-tion is significant (60–500mV s −1),we used a cavity microelectrode where the active material was mixed with a conductive carbon black to alleviate the loss of electrical transport (ohmic losses)9.To confirm the small ohmic drop of this electrode,we performed cyclic voltammetry from 100–500mV s −1in a bulky-ion electrolyte (Sup-plementary Fig.S1).In addition,we prepared thick films (∼40µm)of T -Nb 2O 5to determine whether the
high-rate capability was limited to thin films.
Charge storage from the intercalation of lithium ions into Nb 2O 5can be expressed as:
Nb 2O 5+x Li ++x e −↔Li x Nb 2O 5
where the maximum capacity is x =2(ref.10).Figure 1a shows cyclic voltammograms from 100to 500mV s −1in a cavity microelectrode where it is evident that both anodic and cathodic peaks are broad,about 600mV.There is also a noticeable peak shift (and increase in peak separation E p )as the sweep rate increases,but the capacity remains reversible.Figure 1b presents a plot of log(i )versus log(ν)from 0.1to 500mV s −1for both cathodic and anodic peaks.Assuming that the current obeys a power-law
reaction rate1 × 10a
C u r r e n t (m A )
N o r m a l i z e d c a p a c i t y
Sweep rate ¬1/2 (s 1/2 mV ¬1/2)
¬1 × 10log(sweep rate, mV s ¬1)
c
Figure 1|Kinetic analysis of the electrochemical behaviour of T -Nb 2O 5.a ,Cyclic voltammograms from 100to 500mV s −1demonstrate the high-rate capability of the material.b ,b -value determination of the peak anodic and cathodic currents shows that this value is approximately 1up to 50mV s −1.This indicates that even at the peak currents,charge storage is capacitive.c ,Capacity versus v −1/2allows for the separation of diffusion-controlled capacity from capacitive-controlled capacity;two distinct kinetic regions emerge when the sweep rate is varied from 1to 500mV s −1.The dashed diagonal line corresponds to the extrapolation of the infinite sweep rate capacitance using the capacity between 2and 20mV s −1.d ,The variation of the cathodic peak voltage with the sweep rate exhibits a region of small peak separation followed by increased separation at 20mV s −1,and represents another method of identifying systems with facile intercalation kinetics.
relationship with the sweep rate leads to 8:
i =av b
numerous sources including an increase of the ohmic contribution (active material resistance,solid–electrolyte interphase resistance)and/or diffusion constraints/limitations 11.In the limit of slow diffusion,b would approach a value of 0.5as described above.
The relationship between capacity and sweep rate can also establish the rate-limiting step of a charge-storage mechanism 12.In a plot of Q versus v −1/2,regions that are linear represent capacity limited by semi-infinite linear diffusion whereas capacitive contributions are independent of the sweep rate.At sw
eep rates below 20mV s −1,the extrapolated y -intercept yields the infinite sweep rate capacitance 13.Figure 1c shows the plot of normalized capacity versus v −1/2for T -Nb 2O 5from 1to 500mV s −1(the gravimetric capacity for the thin-film electrode is shown in Supplementary Fig.S2).Analogous to the behaviour of the peak current in Fig.1b,there are two distinct regions in Fig.1c.In region 1,at sweep rates <20mV s −1,the capacity is mostly independent of sweep rate.The magnitude of the capacity is ∼130mAh g −1or ∼65%of the theoretical value based on a two-electron redox reaction with Nb 2O 5.In this range,solid-state lithium-ion diffusion is not the rate-limiting step for charge storage.In region 2,from 50to 500mV s −1,the capacity decreases linearly with v −1/2.This indicates that charge storage is mainly diffusion-controlled at high sweep rates.That is,for charging times
a
b
C rate
P o t e n t i a l (V v e r s u s L i /L i +)
Capacity (mAh g ¬1)
x in Li x Nb 2O 5
1.21.51.8
2.12.42.7
3.0
Figure 2|Electrochemical cycling of a 40-µm-thick T-Nb 2O 5electrode.a ,Galvanostatic cycling of a thick Nb 2O 5electrode at a 10C rate.b ,Comparison
of the rate capability of T -Nb 2O 5with a high-rate lithium-ion anode,Li 4Ti 5O 12,at various C-rates (Li 4Ti 5O 12data reproduced from ref.16).
of <20s,diffusion is rate-limiting,similar to most traditional battery electrodes.However,for charging times of 1min (60C)or longer,
there is no indication of diffusion limitations and this intercalation-based system behaves in a fully capacitive manner.Another feature of T -Nb 2O 5at sweep rates <20mV s −1is that the peak voltage shifts with sweep rate are small (Fig.1d).The cathodic peak shift is <0.1V at sweep rates below 10mV s −1.As a result,the anodic and cathodic peaks overlap at 0.1mV s −1(Supplementary Fig.S3)and it is in this behaviour that the similarity to surface redox reactions is most apparent 14.In many lithium-ion intercalation materials,the peak separation is significant even in thin films and at slow sweep rates (for
example, E p =0.13V for LiCoO 2at 0.1mV s −1;ref.15).This type of behaviour is often associated with crystallographic phase changes during the Faradaic process,and contrasts with intercalation materials that form a solid solution,such as T -Nb 2O 5.Besides identifying facile intercalation,the peak voltage separation is related to the high-power capability of a material.As the charging time decreases,that is,at higher current densities,the peak separation in a battery material increases owing to polarization (reflecting the higher overpotentials necessary to deliver the higher currents),so that at higher rates the energy required to fully charge the material is significantly larger than the energy available on discharge.
The high-rate behaviour of T -Nb 2O 5is not limited to thin films or to experiments with small amounts of active material.The constant-current charge/discharge of a 40-µm-thick T -Nb 2O 5(1mg cm −2)electrode at a 10C rate is shown in Fig.2a.At this rate,the capacity is 130mAh g −1and E varies linearly with Q as expected for a pseudocapacitive process from equation (1).This represents capacities typical of battery materials but at rates closer to those of supercapacitors.The rate capability of T -Nb 2O 5from 1to 1,000C is shown in Fig.2b and compared with that of Li 4Ti 5O 12(of comparable electrode dimensions)16.Li 4Ti 5O 12is chosen as an example of a high-rate lithium-ion anode material.The rate capability for T -Nb 2O 5is significantly better than Li 4Ti 5O 12above 30C and
even at a 1,000C rate the capacity of the thick T -Nb 2O 5electrode is ∼40mAh g −1.The thick electrode results verify that the intercalation pseudocapacitance mechanism is not due to thin-film or surface effects,such as vacancies or contributions of the first few atoms from the surface.Indeed,even for the thick electrode,the b -values for the anodic and cathodic peak currents are 1from 1to 10mV s −1(Supplementary Fig.S4).The high-rate capability of T -Nb 2O 5implies that the crystal structure permits exceptionally rapid ionic transport.As shown in Fig.3a,the unit cell has sheets of edge-or corner-sharing distorted polyhedra lying parallel to the (001)plane,with each Nb 5+surrounded by either 6or 7O 2−.The polyhedra are exclusively corner-sharing along the [001]direction with 5%of the Nb 5+ions randomly located in 9-coordinate sites between the (001)poly-hedral planes 17.The mostly empty octahedral sites between (001)planes provide natural tunnels for lithium-ion transport through-out the a –b plane.Calculations indicate that the (001)plane allows degenerate pathways with low energy barriers for ion transport 18.A previous in situ X-ray diffraction study showed that the insertion of lithium into T -Nb 2O 5results in a solid solution with no apparent phase changes 19,and negligible changes to lattice constants 2and unit-cell volume 20up to ∼1.25Li +/Nb 2O 5.In situ X-ray absorption spectroscopy (XAS)demonstrated that lithiation reduces Nb 5+to Nb 4+(ref.19).The in situ XAS studies carried out here confirm that lithiation results in a continuous change in oxidation state (Fig.3b)and the Fourier-transform of the extended X-ray absorption fine structure (EXAFS)indicates that the insertion r
eaction proceeds through two stages (Fig.3c).From 2.5to 1.75V,the EXAFS signal from the various Nb–O bond lengths in Nb 2O 5(1.40–1.85Å)merges to a single peak at an intermediate bond length (1.75Å),indicating that lithiation increases the Nb-centred symmetry.The lithiation is probably faster at low Li +levels 20owing to greater availability of sites and less interaction between cations.At lower potentials,the new EXAFS peak shifts to longer bond distances (1.85Å)as a consequence of increased Li–O interactions at higher Li +content.These structural studies emphasize the value of an open,layered structure to enable rapid ion transport within the active material.
The results presented in this study establish that T -Nb 2O 5exhibits electrochemical features of a pseudocapacitive material despite charge storage occurring in the bulk.Such behaviour is consistent with intercalation pseudocapacitance.The electrochem-ical features indicative of this mechanism are currents that are linearly proportional to the sweep rate,capacity that does not vary significantly with charging time,and peak potentials that do not shift significantly with sweep rate.A key design rule for intercalation pseudocapacitance at the atomic scale is a structure that does not undergo phase transformations on intercalation.In addition,facile two-dimensional (2D)lithium-ion diffusion pathways are
18,980
19,00019,02019,040
1
2
3
4
D e r i v a t i v e , n o r m a l i z e d a b s o r p t i o n
Incident energy (eV)
a
k 2|(R )| (A n g ¬3)
χR (Å)
b c
2.50 V 1.80 V 1.65 V 1.50 V 1.35 V
0.0
0.5
1.01.5
Figure 3|Structural features of lithium intercalation in T -Nb 2O 5.a ,The structure of T -Nb 2O 5stacked along the c axis demonstrates the layered
arrangement of oxygen (red)and niobium (inside polyhedra)atoms along the a –b plane.b ,Derivative of Nb K-edge X-ray absorption near-edge spectra at selected cell voltages,showing a systematic shift to lower energies
as Nb 5+is reduced to Nb 4+.c ,k 2-weighted Fourier-transformed Nb K-edge EXAFS at selected cell voltages.
important.Charge storage that behaves as a quasi-2D process ex-hibits similar behaviour to 2D surface adsorption reactions 14.These features contrast with those of pseudocapacitive RuO 2·x H 2O where charge storage occurs mainly on the surface or near-surface 21–24as summarized in Table 1.
The results here are exciting because they demonstrate that for charging times as fast as 1min (60C rate),there are no diffusion limitations in T -Nb 2O 5.As the high-rate capability is
due to fast ion diffusion in the bulk,this mechanism may be very good for thick electrodes because surface exposure to the electrolyte is not critical.To achieve devices with high energy density,further engineering at the nanoscale and beyond will be necessary to preserve the atomic-scale behaviour observed in thin films and microelectrodes.In particular,maintaining proper electronic conduction pathways will be critical.
Methods
Synthesis.The synthesis of T -Nb 2O 5nanocrystals was reported previously 2.Briefly,2.56mmol of NbCl 5(Sigma-Aldrich)was dissolved in 2ml of ethanol (Fisher Scientific).In a separate vial,0.23ml of deionized water was mixed with 2ml of ethanol.Both vials were then chilled for 2h.The two solutions were then mixed together while 1ml of propylene oxide (Sigma-Aldrich)was slowly added,forming a tra
nsparent gel.This gel was aged for 1day and then soaked in acetone (Fisher Scientific)for 5days.After supercritical drying with CO 2,the gel was transformed into an amorphous Nb 2O 5aerogel.Crystallization to the T -phase occurred by heat treatment at 600◦C for 2h in air.
Characterization.Thin-film electrodes were fabricated by drop-casting a well-sonicated solution of T -Nb 2O 5in ethanol onto an oxygen plasma etched stainless-steel foil (Alfa Aesar).For thin-film measurements,lithium foils
(Sigma-Aldrich)served as the reference and counter electrodes,and the electrolyte was 1M LiClO 4(Sigma-Aldrich)in propylene carbonate (Sigma-Aldrich).The microelectrode preparation was described elsewhere 9.Briefly,T -Nb 2O 5active material and carbon black (Timcal Super C65)powders were mixed in a 1:1weight ratio to ensure good electrical contact.The mixture was packed into the microcavity (30µm depth,30µm width),and subsequently immersed in the electrolyte.A 1cm 2platinum foil (Alfa Aesar)was used as the counter electrode
and lithium foil served as the reference.Thick Nb2O5electrodes were prepared by mixing the active material,carbon black(Timcal Super C65)and PVdF(Kynar) binder in a80:10:10weight ratio.The slurry
was drop-cast onto12-mm-diameter aluminium-disc current collectors and dried at80◦C for12h.The electrode loading was between1and1.5mg cm−2with a thickness of40±5µm.Measurements of the thick Nb2O5electrodes were performed in three-electrode Swagelok cells with the Nb2O5electrode as the working electrode,activated carbon as the counter electrode,a glass fibre separator(Whatman,GF/A)saturated with1M LiClO4
in propylene carbonate,and a lithium foil reference.All measurements were carried out in argon-filled glove boxes between1.2and3V(versus Li/Li+)with oxygen and moisture levels of<1ppm and using PAR EG&G273and Bio-Logic VMP3potentiostats.
In situ X-ray absorption was performed using thin films of T-Nb2O5cast from a well-sonicated ethanol solution onto carbon-coated aluminium.2032-type coin cells were modified to include an X-ray window by drilling∼3-mm-diameter holes through the cathode casing and epoxying a125-µm-thick Kapton window on the outside of the casing.Both sides of the window were then coated with 100-nm-thick aluminium layers using a vacuum evaporator.Cells were assembled with a lithium anode,a1M LiClO4in propylene carbonate electrolyte,and a Whatman glassy fibre separator.
Received17August2012;accepted19February2013; published online14April2013
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Acknowledgements
This work was supported by the Molecularly Engineered Energy Materials and the Energy Materials Center at Cornell,Energy Frontiers Research Centers funded by the US DOE Office of Basic Energy Sciences(DE-SC001342and DE-SC0001086,respectively). XAS was performed at the Cornell High Energy Synchrotron Source,supported by
the NSF and NIH/NIGMS(DMR-0936384).M.A.L.acknowledges support from the US DOD National Defense Science and Engineering Fellowship.J.C.was supported
by Delegation Generale pour l’Armement(DGA).P.S.and P-L.T.acknowledge the support from the European Research Council(ERC,Advanced Grant,ERC-2011-AdG, Project291543—IONACES)and the Chair of Excellence‘Embedded multi-functional nanomaterials’from the EADS Foundation.
Author contributions
V.A.,J.C.,M.A.L.and J.W.K.:experimental work,data analysis.P-L.T.,S.H.T.,H.D.A., P.S.,B.D.:project planning,data analysis.
Additional information
Supplementary information is available in the online version of the paper.Reprints and permissions information is available online at www.nature/reprints.Correspondence and requests for materials should be addressed to B.D.
Competingfinancial interests
The authors declare no competing financial interests.
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