Electrochemistry Communications 10 (2008) 1897–1900

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Electrochemistry Communications

journal homepage: www.elsevier .com/locate /e lecom

Unique control of bulk reactivity by surface phenomena in a positive electrodeof lithium battery

Nicolas Dupré a,*, Jean-Frédéric Martin a,b, Julie Oliveri a, Dominique Guyomard a,Atsuo Yamada b, Ryoji Kanno b

a Institut des Matériaux Jean Rouxel, 2 rue de la Houssinière, BP 32229, F-44322 Nantes Cedex 3, Franceb Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,4259 Nagatsuta, Midori, Yokohama 226-8502, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 July 2008Received in revised form 16 September2008Accepted 3 October 2008Available online 10 October 2008

Keywords:Lithium batteriesNMRInterfacesPositive electrodeImpedance spectroscopy

1388-2481/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.elecom.2008.10.004

* Corresponding author. Tel.: +33 2 40 37 39 33; faE-mail address: nicolas.dupre@cnrs-imn.fr (N. Dup

Layered LiNi1/2Mn1/2O2 compound prepared by the classical coprecipitation method delivers a capacityloss upon cycling in a lithium battery, which increases upon the 5 first cycles, and then becomes less vis-ible over the next 40 cycles. The charge–discharge polarization follows the same trend. The formation andthe evolution of lithium-containing species on the grain surface of layered LiNi1/2Mn1/2O2 and the inter-facial charge transfer resistance have been carefully investigated upon cycling, by coupling ex situ 7LiMAS NMR and in situ electrochemical impedance spectroscopy. An important increase in the amountof lithiated surface species is observed during the first electrochemical cycles along with an increase ofthe charge transfer resistance. After reaching a maximum, both integrated intensity of the NMR signaland charge transfer resistance decrease, indicating a strong correlation between these two different sur-face characteristics, obtained from ex situ and in situ experiments, respectively. The evolution of surfacespecies, probed by NMR and impedance spectroscopy, follow the same kind of variation as electrochem-ical parameters, demonstrating a unique control by surface phenomena of the overall electrochemicalbehavior of an electrode material of lithium battery.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

In a similar way to the solid electrolyte interphase (SEI) ob-served between the negative electrode and the electrolyte of aLi-ion battery, the rich chemistry taking place on the positiveelectrode surface is known to monitor the overall battery perfor-mance in terms of irreversible capacity loss, charge transfer kinet-ics and storage properties. Indeed, interfacial reactions and thegrowth of a passivation layer at the cathode surface upon cyclinghave been noticed and studied for different positive electrodematerials [1–3]. Techniques such as XPS and EIS have indicatedthe formation of surface films or species from the decompositionof the electrolyte components for various positive electrode mate-rial and its possible influence on the electrode conductivity [4–7].Various authors have found correlations between evolutions ofsurface of positive electrodes and long term cycling stability,demonstrating the need to control electrodes surface. Neverthe-less, no direct correlation between the evolution of electrode/elec-trolyte interphase and bulk electrochemical properties has beenobserved so far.

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x: +33 2 40 37 39 95.ré).

MAS NMR is a bulk technique, very rarely used to study surfacelayers [8,9]. We have demonstrated in a previous work [10] that 7LiNMR can be of high interest to detect and characterize the surfacelayer on a material, arising after contact with atmosphere or withthe electrolyte of a lithium battery.

Combining 7Li MAS NMR and electrochemical impedance spec-troscopy (EIS), we demonstrate here for the first time that surfacephenomena such as the formation of lithium-based interphase andcharge transfer resistance control the overall electrochemicalbehavior of LiNi1/2Mn1/2O2 positive electrode material.

2. Experimental

The LiNi1/2Mn1/2O2 samples used for this study have been ob-tained through the classical coprecipitation method described else-where [11].

7Li NMR measurements were carried out at room temperatureon a Bruker Avance-500 spectrometer (B0 = 11.8 T, Larmor fre-quency m0 = 194.369 MHz in 7Li resonance). Single-pulse MAS spec-tra were obtained by using a Bruker MAS probe with a cylindrical4-mm zirconia rotor. Spinning frequencies of 14 kHz were utilized.A short single pulse length of 1 ls corresponding to a p/2 pulse. Re-cycle time was in the 0.5–60 s range and a spectrometer dead time

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Potential vs. Li+/Li0 (V)

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ge c

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Fig. 2. Incremental capacity curves displaying the 1st, 5th and 20th cycle forLiNi1/2Mn1/2O2 obtained by coprecipitation method stored in ambient atmosphere.The specific capacity variation upon cycling curves for materials stored in ambientatmosphere (black dots) and argon (white dots) are shown in the inset.

1898 N. Dupré et al. / Electrochemistry Communications 10 (2008) 1897–1900

of 10 s was used before each acquisition. As shown by Ménétrieret al. [8], using a single pulse with a long preacquisition delay sup-presses the broad signal from lithium within the intercalationcompound which makes the signal assigned to the surface lithiummuch more pronounced and easier to analyze. The isotropic shiftsare relative to an external powder sample of LiCl set at 0 ppm. Thespectra displayed in this work were normalized to the number ofscans and the mass of the sample.

Galvanostatic cycling have been performed at C/20 rate, be-tween 2.0 and 4.5 V, using LiPF6 EC:DMC 1 M electrolyte, usingswagelock cells.

Details of EIS experiments are given elsewhere [12].

3. Results and discussion

X-ray diagrams for materials stored in air and stored in argonwere indexed and refined in the R-3m space group with a singlephase. The lattice parameters a and c were 2.891(1) Å and14.32(1) Å, which are close to the reported data (2.89 and14.30 Å, respectively [11]). The Rietveld refinements of the dif-fracted intensities were also consistent with the already reported10% exchange of Li and Ni between lithium layers and metal tran-sition layers [11] indicating that materials bulks were identical.

7Li MAS NMR performed on coprecipitation-synthesizedLiNi1/2Mn1/2O2 samples (Fig. 1) indicates the presence of Li2CO3

for the material stored in air, consistent with previous study [10]and confirmed by XPS.

The incremental capacity curves, derived from galvanostatic cy-cling experiments, for the sample obtained by the coprecipitationmethod, stored in ambient atmosphere, at the 1st, 5th and 20th cy-cle are displayed in Fig. 2.

Firstly, no evidence of an additional 4.3 V process [13], relatedto the decrease in the interlayer mixing of lithium and nickel,has been seen on our curves. Then, the apparent potential shift ofthe deintercalation process observed for the 5th cycle is assignedto a high polarization and to a low deintercalation kinetics. In addi-

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Pristine stored in air

Pristine stored in argon

reduction - 2.5 V

reduction - 2.5 V

reduction - 2.5 V

Fig. 1. Normalized 7Li MAS NMR spectra for pristine LiNi1/2Mn1/2O2, stored in argonand in ambient atmosphere, and LiNi1/2Mn1/2O2 initially stored in air, at the end ofdischarge after 1, 5 and 20 cycles.

tion, an overall decrease in intensity and the general broadening ofthe incremental capacity peaks along cycling can be observed. Thespecific capacity curves in function of the cycle number, obtainedat C/20 rate exhibits a fast and important capacity loss along the5 first cycles before reaching stabilization, with only a slight con-tinuous increase of the specific capacity for the following cycles.This behavior is consistent with performances shown in otherstudies [14]. The electrochemical behavior is clearly different com-pared to the same material stored in argon, showing the influenceof storage atmosphere and surface phenomena on the electro-chemical performances.

Recent work [15] showed that Li2CO3 additive can play animportant role in the electrochemical behavior of the active mate-rial. Therefore, it becomes crucial to follow the evolution of thematerial/electrolyte interphase upon the electrochemical cyclingbecause of the presence of initial Li2CO3 on the surface of the elec-trode active material. 7Li MAS NMR spectra of electrodes surface,made with a material that have been exposed to air and stabilizedat the end of discharge, at 2.5 V are displayed in Fig. 1. The NMRintegrated intensity goes through a clear maximum for the 5th cy-cle and then decreases although there is an overall increase alongthe electrochemical cycling from the 1st to the 20th cycle, indicat-ing an inherent accumulation of lithium-containing surface spe-cies. It is interesting to note that from the 5th cycle to the 40thcycle, the specific capacity is quite stable with a slight increase(Fig. 2), possibly due to the partial removal of resistive surface spe-cies making the intercalation process easier.

As NMR is an ex situ technique and may not be representative ofthe real evolution of the system, in situ electrochemical impedancespectroscopy (EIS) measurements with three electrodes cells havebeen performed in order to confirm the trend observed by NMR.For each tested electrode, the Nyquist diagram (Fig. 3) exhibitsone semi-circle at high frequency from which a capacitance inthe range 10–20 lF can be calculated, depending on the sample.This capacitance could be ascribed to a double layer capacitance(Cdl) according to the electrochemical area of the electrode(roughly 1 cm2) and the usual values of Cdl for solid electrodes inliquid electrolytes (10–50 lF cm�2). The decrease of the resistanceobserved between the 5th and 20th cycle demonstrates that the

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-Im(Z

) (O

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Fig. 3. Nyquist diagrams for LiNi1/2Mn1/2O2 obtained by the coprecipitation methodstored in ambient atmosphere for samples stabilized at the end of discharge after 1,5 and 20 cycles.

N. Dupré et al. / Electrochemistry Communications 10 (2008) 1897–1900 1899

dominating process is not the degradation of contact between par-ticles. In addition, in order to rule out the hypothesis of a solidresistive layer (SEI) covering completely the electrode surface,experiments performed at higher temperature (60 �C) showed adecrease in the resistance of the semi-circle and a stable capaci-tance, incompatible with a SEI. As a consequence, the resistive partis ascribed to a charge transfer process as the simplest model. Wewill not use in this paper more complicated models such as for in-stance a distributed resistance model of composite cathodes [16].

The resistance measured at the end of discharge passes througha clear maximum after 5 cycles before decreasing during furthercycling, following a similar evolution compared to the NMR signalacquired for samples stabilized at 2.5 V and respecting the overallincrease from the 1st to the 20th cycle (Fig. 4a).

Therefore, the similarity in the evolution of the intensity ofNMR signal and the charge transfer resistance indicates a correla-tion between the evolution of the amount of surface species and

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Fig. 4. Integrated 7Li NMR intensity (black dots) and charge transfer resistance (white dsquares) and the discharge capacity loss (black squares) as a function of the number of

the transfer of lithium between liquid electrolyte and solid crystal-line bulk during the intercalation/deintercalation process. Indeed,the partial covering of the surface of active material by an impor-tant amount of surface species could restrain the surface availablefor the intercalation/deintercalation process and could lead to anincrease in the charge transfer resistance. In addition, this resultconfirms that NMR is an appropriate technique, giving an accuratesight of the evolution of the interphase.

It is also possible to correlate the evolution of these surfaceproperties with the bulk electrochemical cycling behavior aroundthe 5th cycle (Fig. 4). As a matter of fact, the discharge capacity lossgoes through a maximum after a significant increase up to the 5thcycle before a slower decrease tending toward stabilization. Thistrend is consistent with the evolution of the peak-to-peak polariza-tion reported on the same figure. Not only surface phenomena(NMR integrated intensity, charge transfer resistance) and bulkphenomena (capacity loss and peak-to-peak polarization) have amaximum after approx. 5 cycles but their variations are moreimportant during the 5 first cycles and slower after the 5th cycle.As the materials are identical, this unexpected correlation indicatesthat the surface phenomena taking place at the interphase be-tween the positive electrode and the liquid electrolyte clearlyinfluence the overall electrochemical behavior of the cathodes. Itshows a very unique example of electrode bulk properties of inter-calation/deintercalation governed by surface reactions.

4. Conclusions

7Li MAS NMR permitted to follow the evolution of the inter-phase along the electrochemical cycling. The semi-circle observedby EIS has been attributed to the charge transfer resistance. Chargetransfer resistance and NMR signal corresponding to surface spe-cies follow a similar evolution, increasing during the first cyclesup to the 5th cycle and then slowly decreasing. This variation fol-lows as well that of the specific capacity loss and polarization,known as phenomena related to the active material bulk.

The good correlation between the evolution of surface phenom-ena and bulk properties shows in particular the influence of theformation of surface species on the electrochemical behavior.The observed correlation between the evolution of surface phe-nomena and that of bulk electrochemical properties for air-storedLiNi1/2Mn1/2O2 depicts a very exceptional situation where the over-all electrochemical behavior, which depends normally on bulk

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ots), at the end of discharge (a). Evolution of the peak-to-peak polarization (whitecycles for LiNi1/2Mn1/2O2 (b).

1900 N. Dupré et al. / Electrochemistry Communications 10 (2008) 1897–1900

properties, is strongly driven by the evolution of surfacephenomena.

References

[1] Y. Sundarayya, S.K.C. Kumara, C.S. Sunandana, Mater. Res. Bull. 42 (11) (2007)1942.

[2] D. Aurbach, M.D. Levi, E. Levi, H. Teller, B. Markovsky, G. Salitra, U. Heider, L.Heider, J. Electrochem. Soc. 145 (9) (1998) 3024.

[3] E. Ericksson, Ph.D. Thesis, Uppsala University, 2001.[4] D. Aurbach, B. Markovsky, M.D. Levi, E. Levi, A. Schechter, M. Moshkovich, Y.

Cohen, J. Power Sour. 81–82 (1999) 95.[5] S.S. Zhang, K. Xu, T.R. Jow, Electrochem. Solid State Lett. 5 (5) (2002) A92.[6] D. Aurbach, B. Markovsky, G. Salitra, E. Markevitch, Y. Talyossef, M. Koltypin, L.

Nazar, B. Ellis, D. Kovacheva, J. Power Sour. 165 (2) (2007) 491.[7] K. Edström, T. Gustafsson, J.O. Thomas, Electrochim. Acta 50 (2–3) (2004) 397.

[8] M. Ménétrier, C. Vaysse, L. Croguennec, C. Delmas, C. Jordy, F. Bonhomme, P.Biensan, Electrochem. Solid State Lett. 7 (6) (2004) A140.

[9] B. Meyer, N. Leifer, S. Sakamoto, S. Greenbaum, C.P. Grey, Electrochem. SolidState Lett. 8 (3) (2005) A145.

[10] N. Dupré, J.-F. Martin, D. Guyomard, A. Yamada, R. Kanno, J. Mater. Chem.(2008), doi:10.1039/b807778a.

[11] Z. Lu, L.Y. Beaulieu, R.A. Donaberger, C.L. Thomas, J.R. Dahn, J. Electrochem. Soc.149 (6) (2002) A778.

[12] N. Dupré, J.-F. Martin, J. Oliveri, P. Soudan, D. Guyomard, A. Yamada, R. Kanno,J. Electrochem. Soc., submitted for publication.

[13] N. Yabuuchi, S. Kumar, H.-H. Li, Y.-T. Kim, S.-H. Yang, J. Electrochem. Soc. 154(6) (2007) A566.

[14] J. Breger, N. Dupré, P.J. Chupas, P.L. Lee, T. Proffen, J.B. Parise, C.P. Grey, J. Am.Chem. Soc. 127 (2005) 7529.

[15] M. Koltypin, D. Aurbach, L. Nazar, B. Ellis, Electrochem. Solid State Lett. 10 (2)(2007) A40.

[16] M. Kerlau, M. Marcinek, V. Srinivasan, R.M. Kostecki, Electrochim. Acta 53 (3)(2007) 1385.