Electrochimica Acta 223 (2017) 109–114

Novel insights into higher capacity from the Li-ion battery cathodematerial Li2FeSiO4

Anti Liivata,*, Josh Thomasa, Jianghuai Guob, Yong Yangb

aÅngström Advanced Battery Centre (ÅABC), Department of Chemistry, Uppsala University, Box 538, SE-751 21 Uppsala, Swedenb State Key Lab for the Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, PR China

A R T I C L E I N F O

Article history:Received 20 April 2016Received in revised form 15 November 2016Accepted 2 December 2016Available online 5 December 2016

Keywords:Lithium-ion batteriespositive electrodeLi2FeSiO4

in situ Mössbauer spectroscopyFe4+

A B S T R A C T

A highly reactive composite cathode material incorporating nano-particles of the popular Li-ion batterycathode material Li2FeSiO4 (LFS) is here studied to probe the activation of the controversial Fe3+/Fe4+

redox couple in exploiting the second Li-ion in the formula unit – for use in rechargeable Li-ion batteries.A novel form of in situ Mössbauer spectroscopy is used to monitor the oxidation state of the Fe-ions insymmetric LFS || LFS cells. This is based on mapping the poorly resolvable Mössbauer spectra from theexpected Fe3+/Fe4+ redox couple in the working electrode onto the highly resolvable Fe2+/Fe3+ spectrafrom the counter electrode. Comparison of such data from half-delithiated Li(1)Fe3+SiO4 || Li(1)Fe3+SiO4

and almost lithium-free “Li(0)Fe4+SiO4 || Li(0)Fe4+SiO4” symmetric cells is demonstrated – to distinguishthe electrode reactions from the those involving the electrolyte. Lithium is shown to cycle reversibly inthe symmetric cells. However, a large proportion of the cycled lithium (�70%) does not derive from thebulk of the electrodes, but is rather a result of high-V electrolyte degradation, where charge balance ismaintained by leaching lithium from the electrolyte and inserting it into the electrodes.

© 2016 Elsevier Ltd. All rights reserved.

Contents lists available at ScienceDirect

Electrochimica Acta

journa l home page : www.e l sev ier .com/ loca te /e le cta cta

Introduction

Reversible cycling of two lithium ions (along with twoelectrons) per transition-metal (TM) ion in a Li-ion battery hasbeen seen as a most attractive way to increase the capacity ofcathode materials to better match the capacities of today’s anodematerials � typically, graphite. Such a process is more likely tooccur in polyanionic cathode materials, since simple TM-oxidestend to be less stable [1]. Low-cost Fe- or Mn-based silicates, suchas lithium iron silicate (Li2FeSiO4) [2], are ideal candidates in thiscontext, provided they do not suffer from slow reaction kinetics orstructural instability.

Highly reactive nano-composites of Li2FeSiO4 and carbon haveindeed been shown to cycle with capacities corresponding to theexchange of >1.5 electrons per formula unit (f.u.) (e.g. [3–6]). Whathas remained unclear, however, is whether this enhanced capacityderives solely from the active cathode material, or whether there issome contribution from high-V electrolyte degradation reactions.Indeed, such a possibility is not unreasonable since this enhancedcapacity is observed only when charging above ca. 4.5 V vs.Li/Li+ – where conventional electrolytes become unstable with

* Corresponding author.E-mail address: anti.liivat@kemi.uu.se (A. Liivat).

http://dx.doi.org/10.1016/j.electacta.2016.12.0080013-4686/© 2016 Elsevier Ltd. All rights reserved.

respect to oxidation, especially for high surface-area nano-structured electrodes (as is the case here) [7]. DFT calculationsalso suggest that the potential for the cathode reaction:

LiFe3+SiO4! Fe4+SiO4 + e� + Li+ (1)

lies above 4.7 V vs. Li/Li+ [8]. Experimental evidence for thisformation of Fe4+ relies typically on in situ X-ray absorption near-edge spectroscopy (XANES) and Mössbauer Spectroscopy (MS)studies. In lithium iron silicate, the continuous increase in theobserved centre position for the Fe K-edge in the XANES spectrumon charging to 4.2 V is interpreted as the oxidation of Fe2+ to Fe3+;the larger centre-shift above 4.2 V is attributed to the formation ofFe4+ [9].

In situ MS studies are ideally suited to monitor the Fe2+ to Fe3+

conversion, since these species are clearly distinguishable [10],while the Fe3+ to Fe4+ conversion is much more difficult to monitor,since the Fe3+ and Fe4+ spectra closely overlap [11,12]. Conflictingconclusions relating to the electrochemical formation of Fe4+

species are therefore not common in the literature, e.g., [13], vs[14].

Using a novel in situ MS approach, we have here addressed thechallenge of probing the occurrence of a > 1 el. reaction inLi2FeSiO4. This has been done by simultaneous in situ MSmonitoring the readily identifiable Fe2+/Fe3+ and the virtually

110 A. Liivat et al. / Electrochimica Acta 223 (2017) 109–114

indistinguishable Fe3+/Fe4+ redox states during electrochemicalcycling of symmetric Li(1)FeSiO4 || Li(1)FeSiO4 pouch cells. This is,to the best of our knowledge, the first documented use ofsymmetric electrochemical cells for in situ MS studies in general,and for studies of Li2FeSiO4 in particular.

Experimental

Electrode coatings were deposited onto Al-foil using an in-house lab-scale coater. The slurry used was composed of the “LFS”composite 0.8Li2FeSiO4�0.4Li2SiO3/C used in the initial study [6]),carbon black (Super-P) and a PVdF binder (Solef 5130) � mixed inthe proportions 85:5:10 using an NMP solvent. The mix wasball-milled using a Retsch low-energy ball-mill (at 200 rpm for1 h). 1 cm radius disk electrodes were punched from the resultingcoatings, dried for 12 h under vacuum at 120� C, and stored in anAr-filled glove-box ( < 7 ppm H2O, <1 ppm O2). The effectiveloading of Li2FeSiO4 active material was �2 mg/cm2. The mass ofactive material used (stoichiometry: �Li1.4FeSiO4) (Fig. S2) couldactually be up to 10% lower than estimated, since such aLi-deficient powder can typically contain extracted lithiumcontained in a Li2CO3 deposit [15]. However, this uncertaintyhas no qualitative influence on our results, since we focus here onrelative changes in the Fe2+:Fe3+ ratio.

Half-cells contained a Li-foil anode and a glass-fibre separatorimpregnated with fresh 1 M LiPF6 in EC:DEC (1:1) electrolyte.These cells were cycled galvanostatically at RT (�21 �C) using aC/20-rate between a lower cut-off at 1.5 V and an upper cut-off at4 V and 4.8 V. The 4.8 V cut-off value was selected to correspond tothe value used in earlier studies of this “LFS” material [6].

Symmetric whole-cells of half-delithiated Li(1)Fe3+SiO4 || Li(1)Fe3+SiO4 and deeply delithiated “Li(0)Fe4+SiO4 || Li(0)Fe4+SiO4”

electrodes of LFS were made by assembling identical pairs of “LFS”electrodes extracted from the pre-charged “LFS” half-cells (Fig. S3,see also [16]). Fresh electrolyte together with a fresh glass-fibreseparator were also used in preparing the cells. Some of these cellswere prepared with an additional Li reference electrode insertedbetween two pieces of separator at the edge of the main electrodes.This was done to monitor the potential of the individual electrodes.

Fig. 1. (a) The 10th galvanostatic cycling curve for a “LFS” half-cell using a 1.5V–4.0 V volfirst 10 cycles; (b) Mössbauer spectra recorded at points A, B and C on the galvanostat

Cell assembly and disassembly was performed in the glove-box.The electrodes were sealed in aluminized plastic “pouches”, givinga total cell thickness of <1 mm, and thereby facilitating non-invasive in situ MS studies in transmission mode; see [17].

57Fe 14.4 keV absorption was measured using in-house512-channel Mössbauer spectrometer working in a constantacceleration mode. The measurements was made at RT for bothhalf-cells and symmetric whole-cells in an OCV state beforecharging/cycling and after being charged/cycled to the desiredcutoff potentials. Note that the effect of polarizing the cell to thecut-off potential over the duration of the MS data collection wasalso tested, but found to be virtually identical to that done in OCVstate (Fig. S4).

A cut-off of �4 V vs. Li/Li+ was used when the state-of-chargewas limited to 1 el./f.u. A cut-off of 4.8 V vs. Li/Li+ was used whenstudying behaviour beyond 1el./f.u. Cut-offs at �2.8 V and �3.2 Vvs. Li(1)Fe3+SiO4/Li(2)Fe2+SiO4 (estimated from three-electrode cellstudies) were used when cycling symmetric cells lacking aLi-reference electrode. The MS spectra were fitted to the differentFe-ion species occurring in the electrodes using Lorentzian profiles(Fig. S2). Trace amounts of Fe present in the Al-foil current-collectors and in the pouch laminate material contributed �0.5% tothe total absorption area (Fig. S1). This was corrected for in theanalysis.

Results and discussion

A series of novel in situ MS studies were made using differentelectrochemical cell types:

Conventional half-cell studies

Half-cells containing the Li2FeSiO4/Li2CO3/C (“LFS”) compositematerial [6] cycle to �0.85el./f.u. within the voltage window1.5V-4.0 V, as Fe2+ oxidises to Fe3+ and reduces back to Fe2+

(Fig. 1(a)). Mössbauer absorption spectra recorded for differentstates-of-charge (SOC) (Fig. 1(b)) contain two readily distinguish-able sub-spectra. Each comprises a pair of equal-area Lorentzianfunctions: one pair with a larger separation, larger quadrupole-

tage window; the inset shows charge retention (in lithium equivalent: DLi) over theic curve, showing readily resolvable sub-spectra for Fe2+ (black) and Fe3+ (grey).

Fig. 3. “LFS” half-cell discharge curves (superposed for comparison) after 4.0 V(black) and 4.8 V (red) charging; 0.85 el./f.u. was reinserted after the cells werecharged to 4.0 V, whereas �1.1el./f.u. was reinserted after charging to 4.8 V.

A. Liivat et al. / Electrochimica Acta 223 (2017) 109–114 111

splitting (QS) and larger doublet-centre velocity � the centre-shift(CS) corresponds to high-spin Fe2+; the other sub-spectruminvolves smaller parameter values and corresponds to high-spinFe3+. The absorption area is proportional to the concentrations ofFe2+ and Fe3+ ions � shown as shaded black and grey areas,respectively, in Fig. 1(a). Their relative areas vary with the SOC ofthe cell, e.g., the amount of Fe2+ increases from A (charged cell)through B to C (discharged cell) in Fig. 1.

Further charging to 4.8 V would appear to result in theextraction of second electron/f.u. (Fig. 2(a)), but in only minimalchanges to the MS spectra compared to those at 4.0 V (Fig. 2(b)).The expected Fe4+ contribution (near zero Doppler velocity) ispresent, but not in amounts consistent with the electrochemicaldata. Interestingly, the spectra collected from cells polarised at4.8 V are identical to those collected subsequently from the relaxedcells (in OCV state) (see Fig. S4) suggesting that, once charged, theapparent Fe redox state in a highly charged electrode does notchange over many tens of hours.

The capacity recovered on subsequent discharge, �1.1 el./f.u., is25% higher than for cells charged to only 4.0 V (Fig. 3). This isconsiderably less than the corresponding value of �1.7el./f.u. oncharging to 4.8 V. It is problematical, however, to ascertain from MSdata how much of this extra capacity above 4.4 V actually derivesfrom a Fe3+ to Fe4+ reaction, and how much is parasitic capacityfrom electrolyte oxidation. It is also remarkable that charging to ahigher cut-off voltage does not result in a correspondingly higherdischarge potential. An alternative explanation to electrolytedegradation, such as the structural degradation of Li2FeSiO4 itselfon deep delithiation is unlikely considering the available evidencefrom recent XANES and EXAFS study [18]. Such structuraldegradation is indeed known to occur in Li2MnSiO4, leading to

Fig. 2. Charging curve for the “LFS” half-cell to 4.0 V (black) and 4.8 V (red) high-Vcut-offs showing: (a) the apparent difference between 1 el. and 2 el. extraction, and(b) that the corresponding Mössbauer spectra (at OCV) are predominately of Fe3+

character, and differ only slightly from one another; the region of the uncertain Fe4+

contribution is circled.

the blocking of ion-diffusion pathways and a rapid loss of capacitywithin a few cycles [19]. Further evidence for the stability ofLi2FeSiO4 derives from our observation that a cell charged to 4.8 Vshows a lithiation curve very similar to that for charging to only 4 V(Fig. 2).

Symmetric Li(1)FeSiO4 || Li(1)FeSiO4 cell studies

Our inability to extract the quantity of Fe4+ from the MSspectrum for a charged half-cell containing predominately Fe3+

provokes the use of a totally novel approach to monitor theFe-redox activity in the “ < 1 el./f.u.” regime. We have here used asymmetric Li(1)FeSiO4 || Li(1)FeSiO4 cell design, in which theobscure “Fe3+/Fe4+“ redox activity in the one working electrode isimplicitly coupled to the readily quantifiable Fe3+/Fe2+ redoxactivity in the other. This cell chemistry is compared in Fig. 4 to thetype of cell used earlier to study symmetric cell behaviour of theLiMn2O4 spinel cathode material [20]. The expected reaction hereis:

Li1Fe3+SiO4 || Li1Fe3+SiO4! “Li1-xFe4+xFe3+1-xSiO4” || Li1+xFe2+xFe3+1-xSiO4 (2)

Since in situ MS (in transmission mode) is sensitive to all Fe-ionoxidation states in the bulk of the two electrodes, any changesobserved in the spectra would reflect charge insertion (extraction)into(from) the electrodes, and rule out extraneous reactions suchas high-V redox shuttles. Step-wise charging curves and their

Fig. 4. The symmetric cell principle – illustrated through comparison with asymmetric LiMn2O4 vs. LiMn2O4 cell design; such a cell works at �1 V, exploitingMn2+/Mn3+ redox activity at 3 V (blue) in one electrode (lithium insertion) coupledto Mn3+/Mn4+ redox activity at 4 V (red) (lithium extraction) in the other. Analogoussymmetric cells have here been made from pairs of identical half-delithiatedelectrodes, each with stoichiometry Li(1)FeSiO4 (Fig. S3).

112 A. Liivat et al. / Electrochimica Acta 223 (2017) 109–114

corresponding in situ MS spectra are shown in Fig. 5 for thesesymmetric cells at electrode potentials 4.4 V, 4.6 V and 4.8 V. Thehigh-velocity non-overlapping branch of the Fe2+ doublet (ataround v�2 mm/s) is very sensitive to changes in the Fe oxidationstates in the symmetric cell (Fig. 5(b)), and can therefore serve asan ideal fingerprint of bulk electrode reactions.

Up to the 4.6 V cut-off, the charge passed is proportional to thefraction of Fe2+ registered in the in situ MS spectra (Fig. 5(b). Thiscorrespondence between the measured fraction of Fe2+ in thesymmetric whole-cell and its distribution between the twoelectrodes was also confirmed by dismantling the charged cellsand re-measuring the individual electrodes separately (Fig. S5).This means that, when one electrode is charged to 4.6 V, �0.4 el./f.u. is inserted into the counter electrode � resulting in thestoichiometry Li1.4Fe0.63+Fe0.42+SiO4. Comparison of the amount ofFe2+ deduced from the MS spectra and the charge x deduced fromthe galvanostatic data, reveals a clear discrepancy as the potentialapproaches 4.8 V. At the 4.8 V cut-off, the fraction of Fe2+ saturatesat �0.7el./f.u., while the charge capacity has already reachedalmost 0.8el./f.u. The high-V cut-off was thus set at 3.3 V insubsequent cycling of symmetric cells in the absence of a Li-reference electrode.

Symmetric “Li(0)FeSiO4” || “Li(0)FeSiO4“ cell studies

We have seen that in situ MS studies of symmetric Li(1)FeSiO4 ||Li(1)FeSiO4 cells has allowed the quantification of the amount of Lire-inserted into the counter electrode as a result of oxidation of theworking electrode. It still remains unclear, however, how much Li isactually extracted from the bulk of the working electrode, and howmuch results from charge compensation following electrolyte

Fig. 5. (a) Charging symmetric Li(1)FeSiO4 || Li-ref. || Li(1)FeSiO4 cells sequentiallyto cut-offs at 4.4 V, 4.6 V and 4.8 V vs. Li/Li+; (b) Mössbauer spectra (M0-M3)recorded (under OCV) at each cut-off point.

oxidation at high-V [21]. This question has here been address usingsymmetric cells comprising two deeply delithiated electrodes oftype “Li(0)FeSiO4 || Li-ref. || Li(0)FeSiO4”, where each electrode waspre-charged to 4.8 V vs. Li/Li+(Fig. S3(b)). The charging of such a cellup to 2.8 V is shown in Fig. 6; each electrode potential vs Li/Li+ ismonitored separately. In contrast to the case of Li(1)FeSiO4 || Li-ref.|| Li(1)FeSiO4 cells, almost negligible (�5%) SOC is measured for “Li(0)FeSiO4 || Li-ref. || Li(0)FeSiO4“ cells below 4.8 V vs Li/Li+. Pre-charging in a half-cell to 4.8 V has apparently passivated theelectrode interface to such an extent that side reactions arerelatively modest in subsequent cycling over the same voltagewindow. This is an important factor in assessing the origin of thecapacity in Li(1)FeSiO4 || Li(1)FeSiO4 symmetric cells; if theircapacity persists beyond the first cycle, then it does not originatefrom side reactions such as electrolyte degradation. Note that thepotential of the working electrodes in these symmetric “Li(0)FeSiO4 || Li-ref. || Li(0)FeSiO4“ cells increases beyond 4.8 V vs Li/Li+,the decomposition reactions increase rapidly (Fig. 6(a)), and thelithiation of the counter electrode is reflected by the emergence ofthe Fe2+ peak in the MS spectrum (Fig. 6(b)). There is again aqualitative correspondence between the galvanostatically mea-sured charge and the fraction of Fe2+ deduced from the MSspectrum.

Discharge capacity of Li(1)FeSiO4 || Li(1)FeSiO4 symmetric cells

Despite the fact that Li(1)FeSiO4 half-cells respond to high-Vcharging by giving a �25% increase in discharge capacity, thedischarge curve exhibits no high-V features (Fig. 3) � unlike, forexample, in the case of LiMn2O4 cells (Fig. 4). This extra capacity

Fig. 6. (a) Voltage curves of the two identical electrodes in the symmetric “Li(0)FeSiO4 || Li-ref. || Li(0)FeSiO4“ cell during charging up to 2.8 V (corresponding 5.2 Vvs Li/Li+); (b) in situ MS spectra before (black) and after (red) charging – show the Fereduction up to 70% Fe2+.

A. Liivat et al. / Electrochimica Acta 223 (2017) 109–114 113

(and voltage), resulting from charging “beyond the 1el. limit”, hashere been quantified by cycling symmetric Li(1)FeSiO4 || Li(1)FeSiO4 cells. This is shown in Fig. 7(a), where �0.9el./f.u. isextracted on charging to 2.8 V, but only �0.2el./f.u. is reinserted ondischarge. Furthermore, discharge occurs at a very low averagevoltage (0.35 V), especially when compared to the average chargingvoltage of 2 V. This also explains why the discharge voltage is notseen to increase noticeably in half-cells cycled with a high-Vcut-off.

But what happens to the remaining 0.9el. � 0.2el. = 0.7el? Thischarge is reinserted at the “normal” lithiation potential forLi2FeSiO4, i.e. at < 2.8 V vs. Li/Li+ or at <0 V in symmetric cells(Fig. 7(b)).

In subsequent cycles, Fe2+/Fe3+ redox activity dominates (theshaded green region in Fig. 7(b)), which is not surprising since thefirst charge to high-V induced 0.5-0.8 el./f.u. of Fe2+ into theelectrodes.

To summarize

It was not possible to identify a clearly distinguishable voltageassignable to Fe3+/Fe4+ redox activity in “LFS” � as was possible forthe case of the Mn3+/Mn4+ redox couple in LiMn2O4. This is partlybecause the oxidation potential for the Fe3+/Fe4+ couple is wellabove 4 V, and is therefore obscured by electrolyte decompositioneffects. However, this problem can also derive from morefundamental issues, such as the nature of the hybridisation ofFe4+— O2�— Si4+ electronic states, which can have a predominatelyoxygen character, i.e., “Fe3+— O� — Si4+”. In this context, it isrelevant to note the recently demonstrated complex redox reactionmechanisms occurring in layered transition-metal oxides, involv-ing reversible oxygen oxidation and reduction [22], in addition tothe more conventional redox behaviour in transition-metald-states. Such anion-dominated redox activity has rarely been

Fig. 7. (a) The first charge and discharge curves for a symmetric Li(1)FeSiO4 || Li(1)FeSiO4 cell (solid black line), showing the average voltages (Vavg); (b) the first chargeand discharge curve (black) is shown together with the subsequent sweep tonegative cutoff �2.8 V and back to +2.8 V (dotted) together with 10 such cycles(dashed) demonstrating the cyclability of the symmetric cell.

investigated in polyanion systems. Resilience of the silicatestructure to such wide V-range cycling would, indeed, be attractivefor low-cost energy storage applications � and provide newinsights into the research of polyanion-type electrode materials.For iron-containing systems in particular, the benefits of using thisnovel symmetric-cell in situ Mössbauer approach is shown to bevery fruitful.

Conclusions:

In situ MS studies of lithium deficient Li2-xFeSiO4 suggest that:

Charging above 4 V can be monitored more definitively usingsymmetric cells.

Charging above 4 V also increases the discharge capacity to above1Li/f.u., of which �20% constitutes “beyond 1el. reactivity”.

�Such charging results in a very modest increase in dischargevoltage above the conventional �2.8 V.

�Electrolyte decomposition limits the realisation of >1 Li/f.u.

The symmetric-cell approach also revealed that:

0.6-0.7 Li/f.u. can be extracted from Li(1)FeSiO4, but only<0.05Li/f.u. from “Li(0)FeSiO4“ electrodes, when charging to4.8 V vs. Li/Li+.

Electrolyte decomposition leads to re-lithiation of the Li(1)FeSiO4 electrodes in the symmetric cells.

Such re-lithiated cells can exhibit >10 cycles with a dominantcontribution from the Fe2+/Fe3+ redox couple.

Generally, the resolution of the in situ Mössbauer data can besubstantially improved for any system working beyond Fe2+/Fe3+

redox activity.

Acknowledgements

This work has been supported by the Swedish Energy Agency(STEM), Swedish Innovation Agency (VINNOVA) and the SwedishScience Research Council (VR).). Y. Yang acknowledges that silicatematerials synthesis is financially supported by the National NaturalScience Foundation of China (Grants No. 21233004, 21473148)

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.electacta.2016.12.008.

References

[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144(1997) 1188–1194.

[2] A. Nytén, A. Abouimrane, M. Armand, T. Gustafsson, J.O. Thomas, Electrochem.Commun. 7 (2005) 156–160.

[3] J. Yang, X. Kang, D. He, A. Zheng, M. Pan, S. Mu, J. Mater. Chem. A 3 (2015)16567–16573.

[4] H. Zhu, X. Wu, L. Zan, Y. Zhang, Electrochim. Acta 117 (2014) 34–40.[5] S. Singh, S. Mitra, Electrochim. Acta 123 (2014) 378–386.[6] J. Bai, Z. Gong, D. Lv, Y. Li, H. Zou, Y. Yang, J. Mater. Chem. 22 (2012)

12128–12132.[7] S.R. Li, C.H. Chen, J. Camardese, J.R. Dahn, J. Electrochem. Soc. 160 (2013)

A1517–A1523.[8] P. Zhang, Y. Zheng, S. Yu, S. Wu, Y. Wen, Z. Zhu, Y. Yang, Electrochim. Acta 111

(2013) 172–178.[9] D. Lv, J. Bai, P. Zhang, S. Wu, Y. Li, W. Wen, Z. Jiang, J. Mi, Z. Zhu, Y. Yang, Chem.

Mater. 25 (2013) 2014–2020.[10] A. Nytén, S. Kamali, L. Häggström, T. Gustafsson, J.O. Thomas, J. Mater. Chem.16

(2006) 2266.[11] C. Jeannot, B. Malaman, R. Gerardin, B. Oulladiaf, J. Solid State Chem.165 (2002)

266–277.

114 A. Liivat et al. / Electrochimica Acta 223 (2017) 109–114

[12] M. Tabuchi, A. Nakashima, H. Shigemura, K. Ado, H. Kobayashi, H. Sakaebe, K.Tatsumi, H. Kageyama, T. Nakamura, R. Kanno, J. Mater. Chem. 13 (2003)1747–1757.

[13] D. Lv, W. Wen, X. Huang, J. Bai, J. Mi, S. Wu, Y. Yang, J. Mater. Chem. 21 (2011)9506.

[14] A.S. Christiansen, R.E. Johnsen, P. Norby, C. Frandsen, S. Morup, S.H. Jensen, K.K.Hansen, P. Holtappels, J. Electrochem. Soc. 162 (2015) A531–A537.

[15] C. Deng, S. Zhang, Y. Gao, B. Wu, L. Ma, Y. Sun, B. Fu, Q. Wu, F. Liu, Electrochim.Acta 56 (2011) 7327–7333.

[16] J.C. Burns, L.J. Krause, D.-B. Le, L.D. Jensen, A.J. Smith, D. Xiong, J.R. Dahn, J.Electrochem. Soc. 158 (2011) A1417–A1422.

[17] T. Gustafsson, J.O. Thomas, R. Koksbang, G. Farrington, Electrochim. Acta 37(1992) 1639–1643.

[18] A.W. Brownrigg, G. Mountjoy, A.V. Chadwick, M. Alfredsson, W. Bras, J. Billaud,A.R. Armstrong, P.G. Bruce, R. Dominko, E.M. Kelder, J. Mater. Chem. A 3 (2015)7314–7322.

[19] A. Kokalj, R. Dominko, G. Mali, A. Meden, M. Gaberscek, J. Jamnik, Chem. Mater.19 (2007) 3633–3640.

[20] W. Li, J.R. Dahn, J. Electrochem. Soc. 142 (1995) 1742–1746.[21] A.J. Smith, J.C. Burns, D. Xiong, J.R. Dahn, J. Electrochem. Soc. 158 (2011)

A1136–A1142.[22] E. McCalla, A.S. Prakash, E. Berg, M. Saubanere, A.M. Abakumov, D. Foix, B.

Klobes, M.-T. Sougrati, G. Rousse, F. Lepoivre, S. Mariyappan, M.-L. Doublet, D.Gonbeau, P. Novak, G. Van Tendeloo, R.P. Hermann, J.-M. Tarascon, J.Electrochem. Soc. 162 (2015) A1341–A1351.