Journal of Alloys and Compounds 555 (2013) 10–15

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Journal of Alloys and Compounds

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Reversible sodiation in maricite NaMn1/3Co1/3Ni1/3PO4 for renewable energy storage

Manickam Minakshi ⇑, Danielle MeyrickFaculty of Science and Engineering, Murdoch University, Murdoch, WA 6150, Australia

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

Article history:Received 12 September 2012Received in revised form 26 November 2012Accepted 30 November 2012Available online 22 December 2012

Keywords:MariciteSodiumAqueousBatteryRechargeablePhosphate

0925-8388/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jallcom.2012.11.203

⇑ Corresponding author.E-mail addresses: minakshi@murdoch.edu.au,

(M. Minakshi).

Battery technology is now at a point where water based sodium batteries may take a place in large-scaleapplications such as electrical grid stabilization. The starting point for new aqueous based sodium batterytechnology is to understand the fundamental differences between lithium versus sodium in similar hostmaterials synthesized via non-ceramic synthetic approaches. The new sodium phosphate host com-pound, NaMn1/3Co1/3Ni1/3PO4, has been synthesized using sol–gel and combustion routes. The frameworkof sodium phosphate with a maricite structure is analogous to olivine based lithium phosphate com-pound, except the M(1) and M(2) sites have reverse occupancies. Physical and electrochemical character-ization indicate that the combustion synthesized sodium analogue is fully reversible (electrochemicallyintercalating and de-intercalating sodium ions) in water based NaOH electrolyte for multiple cycles.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Lithium ion secondary batteries play a vital role as the promi-nent power sources for electronic devices such as mobile phones,laptop computers, digital cameras and similar portable appliances.In recent years, plug-in hybrid electric vehicles (PHEVs) and hybridelectric vehicles (HEVs) have been developed that employ lithiumbatteries [1], an indication of the growing importance of renewableenergy production and storage to day-to-day life. The present chal-lenge for lithium battery technology is that much of the global un-tapped lithium is located in remote areas that may not be readilyaccessible in the future [2]. Another issue is the increasing costof lithium raw materials; these have roughly doubled from the firstapplication in 1991 to the present, and this cost is likely to con-tinue to rise in line with increasing demand for vehicles and otherapplications employing lithium batteries [3]. For sustainability oflow cost clean energy, the search for alternatives to lithium ionbattery technologies is necessary. Sodium-ion batteries may bethe key because of the wide availability of sodium, its low priceand the similarity of Li and Na insertion chemistries [4].

Research into sodium ion (Na-ion) batteries has been infrequentand the number of published reports on sodium batteries [5–10] issmall relative to those reporting Li-ion battery chemistry and asso-ciated successes. Nevertheless, sodium batteries are considered anattractive alternative to lithium counterparts as they could poten-tially be far less expensive, safer and environmentally benign

ll rights reserved.

lithiumbattery@hotmail.com

[9,10]. The mechanism of operation is similar to that observed inLi-ion cells; in Na-ion batteries, Na+ ions are shuttled betweenthe two electrodes (cathode and anode) during the electrochemicalprocesses with an electrolyte acting as the transportation medium[9,10]. The electrolyte in this case may be aqueous, and there isgreat potential for the development of water-based sodium batter-ies for large-scale applications such as electrical grid storage. Whilethe bulkiness of the sodium ion relative to the lithium ion maypresent some challenges in terms battery weight and footprint, thisis not important for stationary, electrical grid storage applications.

Despite the merits of using sodium compounds in battery tech-nology, there are few successful reports in the field of sodium oxideand phosphate based cathodes [7–10]. Among the phosphates, theolivines, LiMPO4 (M = Mn, Fe, Ni or Co), have emerged as a promis-ing class of cathode materials for Li-ion batteries [11]. In particular,lithium iron phosphate, LiFePO4 has found widespread industrialapplication. Although numerous publications exist on olivineLiFePO4 with regard to its significant electrochemical storage (dis-charge capacity) properties [12], the sodium equivalent (NaFePO4)has not been well characterised or documented in sodium cellapplications [13,14]. Most of the phosphates with the general for-mula ABPO4 (A: alkali cation; B: transition metal cation) crystallisewith either an olivine-, maricite- or zeolite-type structure. Thestructures of olivine and maricite are analogous, while that of zeo-lite shows some differences. When the size of the A alkali cation in-creases from Li, Na, to Cs the phosphates crystallise with thestructures of olivine, maricite and zeolite respectively [15]. Thetypical NaFePO4 maricite phase is reported [16] to be the thermo-dynamically favoured phase because it is obtained under high tem-perature synthetic conditions. The maricite structure is similar to

M. Minakshi, D. Meyrick / Journal of Alloys and Compounds 555 (2013) 10–15 11

the olivine LiFePO4 with the difference being the occupation of theM(2) site by the alkali cation (Na), and occupation of the M(1) siteby the transition metal [17]. A small number of early works [17,18]reported that NaFePO4 is not viable as a cathode material, suggest-ing that the closed (no cationic channels), one dimensional mari-cite framework results in entrapment of Na+, thus preventingreversible electrochemical processes. On the other hand, Moreauet al. [19] reported the synthesis of NaFePO4 by complete electro-chemical sodiation in a non-aqueous electrolyte. To the best of ourknowledge, no work has been reported on the maricite structurewith mixed transition metal cations (NaMn1/3Co1/3Ni1/3PO4) testedin aqueous solutions. The water based sodium battery used in thisstudy comprises benign electrodes and electrolyte salts. The cellscan be assembled in an open-air environment using simple equip-ment. The substitution of mixed transition metal cations and novelmaterials processing [20] have been employed in this work toovercome the poor electronic conductivity of iron-containing oliv-ine/maricite frameworks [12].

In this report, the mixed transition metal maricite (NaMn1/3

Co1/3Ni1/3PO4) samples of cathode material synthesized by eitherthe sol–gel (using citric acid and polyvinylpyrrolidone as chelatingagents) or combustion (using urea as fuel) technique are comparedand their electrochemical properties discussed. The chosen syn-thetic techniques led to a carbon coating on the material that notonly assisted in overcoming conductivity limitations but also en-hanced the percolation of ions from the electrolyte into the hostmaricite. The physical and electrochemical properties of the syn-thesized maricite cathode are demonstrated with a Zn anode inNaOH electrolyte in an aqueous rechargeable battery. The funda-mental differences between lithium versus sodium in similar hostmaterials (olivine and maricite) are discussed.

2. Experimental

2.1. Material synthesis

Sol–gel synthesis of NaMn1/3Co1/3Ni1/3PO4 was performed through mixing stoi-chiometric quantities of precursor’s sodium acetate, manganese acetate, cobalt ace-tate, nickel acetate, ammonium dihydrogen phosphate and citric acid (CA) orpolyvinylpyrrolidone (PVP) as chelating agent. All precursors were purchased from

Fig. 1. Schematic representation of the preparation of maricite NaMn1/3Co1/3N

Sigma Aldrich. The metal and phosphate reactants were dissolved in water at 80 �Cand stirred to obtain a homogeneous solution. The CA or PVP was added in 1:1weight ratio to the metal ions, and the pH of the solution was adjusted to 3.5 bythe addition of 70 v/v% nitric acid. Stirring and heating was continued until a thicktransparent gel was obtained. The gel was dried at 110 �C in air for 12 h, thenheated in a furnace at 300 �C for 8 h and at 600 �C for 6 h in air with intermittentgrinding. The maricite phase (NaMn1/3Co1/3Ni1/3PO4) powder obtained was groundfor further analysis. In the case of the combustion reaction, urea was used as a fuelwhile keeping the other reactants and conditions as for the sol–gel synthetic route.A schematic representation of the synthetic procedure is given in Fig. 1.

2.2. Instrumental methods

The structure of the synthesized NaMn1/3Co1/3Ni1/3PO4 was determined by Sie-mens D500 X-ray diffractometer 5635 using Cu-Ka radiation and a scan speed of 1�/min. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) forthe transparent gels were conducted using a TA instruments (SDT 2960). Surfaceanalysis of the materials was performed using a scanning electron microscope (Phi-lips Analytical XL series 20). Microtrac S3500 series particle size analyser with laserdiffraction technology was used for determining size distributions of NaMn1/3Co1/

3Ni1/3PO4 particles.

2.3. Electrochemical characterization

Zn foil (99.9%) from Advent Research Materials and sodium hydroxide from Sig-ma Aldrich were used in this study. The experimental procedures for cyclic voltam-metric studies were similar to those reported earlier [21]. The potentials weremeasured and reported against an Hg/HgO reference electrode. The electrolytewas a �7 M solution of sodium hydroxide, while the working electrode was mari-cite phase (NaCo1/3Mn1/3Ni1/3PO4) prepared by either the sol–gel or combustiontechnique. The counter electrode was a Zn strip. An EG&G Princeton Applied Re-search Versa Stat (3–200) model was used to scan the potential at a scan rate of25 lV/s in all cyclic voltammetric (CV) experiments.

3. Results and discussion

3.1. Synthesis of maricite NaMn1/3Co1/3Ni1/3PO4 powders – XRDanalysis

Polycrystalline powders of maricite NaFePO4 can be producedvia hydrothermal or Pechini methods as described in the literature[9,22]. Here, we describe a new wet chemical (non-ceramic) syn-thesis (i.e., polymer assisted sol–gel) technique and compare thiswith a combustion method to produce mixed transition metal

i1/3PO4 cathode synthesized by (a) sol–gel and (b) combustion methods.

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•• •(c)

(b)

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••

••

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nsity

/ a.

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(a)

Fig. 2. X-ray diffraction patterns of maricite NaMn1/3Co1/3Ni1/3PO4 synthesized bysol–gel method using (a) citric acid, CA and (b) polyvinylpyrrolidine, PVP aschelating agent; and (c) combustion method using urea as a fuel.

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•

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•

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•

•

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(b)

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nsity

/ a.

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(a)

Combustion

Fig. 3. X-ray diffraction patterns of analogous olivine LiMn1/3Co1/3Ni1/3PO4 synthe-sized by (a) sol–gel and (b) combustion method.

1 10 100size / μμm

1 10 100size / μm

a

b

c

12 M. Minakshi, D. Meyrick / Journal of Alloys and Compounds 555 (2013) 10–15

phosphate (NaMn1/3Co1/3Ni1/3PO4) of the maricite type. A variety oforganic solvents and polymers are available as chelating agents butfor the current study we have chosen a weak organic acid (citricacid – CA) and non-ionic surfactant (polyvinylpyrrolidone – PVP)for sol–gel synthesis and urea as a fuel for combustion.

In the wet chemical reaction, the metal acetate precursors aremixed in aqueous solution and heated at an elevated temperature(600 �C) under ambient atmosphere, a synthesis procedure similarto that for the counterpart Li (LiMn1/3Co1/3Ni1/3PO4) [23]. The X-raydiffraction (XRD) for maricite phase NaMn1/3Co1/3Ni1/3PO4 isshown in Fig. 2. Irrespective of the synthetic technique, theNaMn1/3Co1/3Ni1/3PO4 powders are crystallised in the orthorhom-bic structure of the maricite mineral [24]. The diffraction peaks as-signed for the NaMn1/3Co1/3Ni1/3PO4 are based on the naturalmaricite mineral (pattern No. 071–5040) and parent compoundNaFePO4 [9,19]. No impurity peaks were detected in the XRD pat-terns in Fig. 2, and the synthesis of maricite NaMn1/3Co1/3Ni1/3PO4

by a non-ceramic technique is reported here for the first time. It isclearly evidenced from the identical diffraction peaks (Fig. 2a–c)that no structural change is observed when different chelatingagents are employed, and both the sol–gel and combustion routesproduce products yielding the same XRD pattern. It is been re-ported earlier that maricite is the thermodynamically stable formof NaFePO4 [16] but interestingly in the present study, the mixedtransition metal maricite is also found to be stable, as indicatedby the observed diffraction pattern.

In order to understand the fundamental differences betweenthe alkali cations lithium and sodium in similar host materials(olivine and maricite), insight into structural differences throughXRD is essential. Figs. 2 and 3 compare the XRD patterns of olivineand maricite. Maricite (Na) is similar to the well known olivine (Li)in terms of its phosphate PO4 framework except that the M(1) andM(2) sites are occupied by transition metal cations (Mn, Co and Ni)and Na respectively, while metal ion arrangement is the reverse inolivine. The difference in cation occupancy (4a and 4c sites) be-tween these two structures is reflected in the change in diffractionpattern observed in Figs. 2 and 3. Maricite and olivine are identifiedin the orthorhombic Pmna space group but the observed peak posi-tions are different due to the change in cation positions.

1 10 100size / μm

Fig. 4. Particle size distribution of maricite NaMn1/3Co1/3Ni1/3PO4 synthesized bysol–gel method using (a) citric acid, CA and (b) polyvinyl pyrrolidine, PVP aschelating agent; and (c) combustion method using urea as a fuel.

3.2. Particle size and surface analysis

For sol–gel wet chemical synthesis, citric acid (CA) and polyvi-nylpyrrolidone (PVP) are used as chelating agents. CA, containinga carboxylic acid group, is widely used as a chelating agent and of-

ten gives rise to a homogenous precursor gel with fine particle size.The particle size distribution of the CA-synthesized sample in thisstudy (Fig. 4a) is wide, with particle size ranging from 10–100 lm.This variation may be due to the degradation of citric acid duringsynthesis, leaving particles heterogeneous. For particles synthes-ised with PVP as the chelating agent, a more uniform sample interms of particle size is produced (Fig. 4b). PVP decomposes duringsynthesis, forming an organic layer on maricite particles, providingstrong particle-to-particle contact and improving conductivity

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M. Minakshi, D. Meyrick / Journal of Alloys and Compounds 555 (2013) 10–15 13

between particles [25,26]. In the case of combustion, urea acts as afuel in producing internal heat and allows a combustion reaction[27]. The resultant heat accelerates the decomposition of theremaining constituents remarkably, leading to a narrow size distri-bution (Fig. 4c) and acceptable particle size of the product ob-tained. This diffusion barrier for urea synthesized NaMn1/3

Co1/3Ni1/3PO4 is shorter, allowing sodium ions to de-intercalate/intercalate from/into the host maricite, owing to the smaller parti-cle size and narrow size distribution.

The nature of the host structure has a significant impact on therelative difference between the sodium (maricite) and lithium(olivine) migration barriers (see Section (3.4)). The XRD patternsobtained for our sol–gel and combustion synthesized powders inthis study (Fig. 2) suggest the same product is formed in each case,while synthesis technique can have an important effect on particlesize range (Fig. 4). PVP as chelating agent and urea as fuel produceparticles with suitable sizes and size ranges. Samples synthesizedusing these reagents have been chosen for further studies.

Scanning electron microscopic images of the synthesized mari-cite NaMn1/3Co1/3Ni1/3PO4 powders are shown in Fig. 5. The parti-cles are well defined and interconnected, a feature that may aid thepercolation of sodium ions from the electrolyte for the combustionsynthesized product shown in Fig. 5b.

3.3. Thermogravimetric – differential thermal analysis (TG-DTA)studies

Fig. 6 shows the TG/DTA curves of the maricite (NaMn1/3

Co1/3Ni1/3PO4) precursors (transparent gel) for sol–gel and com-bustion routes. The TG curve for sol–gel using PVP as the chelating

Fig. 5. Scanning electron microscopy (SEM) images of maricite NaMn1/3Co1/3

Ni1/3PO4 (a) sol–gel (PVP) and (b) combustion (urea) method.

0 100 200 300 400 500 600 700 800 900Temperature \ oC

Fig. 6. Thermogravimetric (TG) and differential thermal analysis (DTA) curves oftransparent gel containing metal acetate with (a) sol–gel (PVP) as chelating agentand (b) combustion method (urea) as fuel.

agent in Fig. 6a shows three main steps of mass loss, whereas forthe combustion route in Fig. 6b, two short steps of mass loss areobserved. The small mass loss observed in between 50 �C and190 �C for sol gel sample (in Fig. 6a) is attributed to the releaseof ammonia and physically adsorbed water from the gel mixture.The DTA curve indicates that these processes are endothermicand the corresponding peak is observed at 150 �C. Finally, TG curveshows a steep weight loss, losing half of its initial mass, up to600 �C and then no further change is observed. In the DTA curve,several exothermic reactions are identified corresponding to peaksat 195, 340, 425 and 555 �C. Sharp exothermic peaks at 195, 340,425 �C were attributed to decomposition of acetate precursorsand pyrrolidine ring in PVP with nitrogen atoms. This decomposi-tion extends up to 555 �C but with further heating no significantweight loss is observed. The exothermic peak at 555 �C may bedue to the transformation of olivine to the maricite phase. TheTG and DTA curves observed for the product obtained by combus-tion (Fig. 6b) are significantly different and show negligible weightloss. The DTA curve indicates the process is endothermic at lowertemperature, while an exothermic peak is observed at a highertemperature (775 �C), which may be attributed to the reorganisa-tion of the crystal lattice [20]. The TG and DTA curves obtainedfor the olivine (LiMn1/3Co1/3Ni1/3PO4) [23] with PVP as chelatingagent are similar to those seen for sol–gel prepared maricite in thisstudy. Thermogravimetric analyses in this work support the ideathat the mechanisms involved in the sol–gel and combustion syn-theses are different, and these differences are reflected in the mor-phology and particle size distribution of the product.

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nsity

/ a.

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(a)

Fig. 8. X-ray diffraction patterns of maricite NaMn1/3Co1/3Ni1/3PO4 synthesized bycombustion method (a) before and (b) after oxidation (de-sodiation). Maricitestructure is still preserved after de-intercalation and sodium.

14 M. Minakshi, D. Meyrick / Journal of Alloys and Compounds 555 (2013) 10–15

3.4. Electrochemical characteristics of maricite NaMn1/3Co1/3Ni1/3PO4 –cyclic voltammetry

To investigate the redox activity of the synthesized cathodes,slow scan cyclic voltammetry was performed. Fig. 7 shows a typi-cal cyclic voltammogram of maricite NaMn1/3Co1/3Ni1/3PO4 synthe-sized via sol–gel and combustion routes. The scan was initiated at�0.6 V moving in the anodic direction to 0.2 V and then reversing itto the starting potential. While two anodic peaks A1 (0.008 V) andA2 (0.4 V) are observed during the anodic portion of the CV, onlyone reduction peak C1 (at �0.4 V) occurs during the reverse scan.This suggests that (a) irrespective of the synthetic routes, the mari-cite material undergoes only one reduction process at �0.4 V cor-responding to intercalation of sodium from the aqueous NaOHelectrolyte and (b) the product formed during the reduction pro-cess undergoes two separate oxidation processes during the re-verse anodic scan: A1 corresponds to de-intercalation of sodiumwhile A2 corresponds to oxidation of the MnOOH product formedduring reduction. As this is an aqueous electrolyte, formation ofoxy hydroxides cannot be ruled out. The electrochemical redoxprocess is, however, fully reversible. For the sol–gel prepared sam-ple (Fig. 7a), during the subsequent cycles the cathodic peak hasshifted to 0.1 V (less negative), suggesting the structure is versatilefor cyclability. The efficiency was found to be only 55% after the20th cycle. For the combustion product (Fig. 7b) both the cyclicefficiency and the cycling stability are excellent, with this cathode

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Fig. 7. Cyclic voltammogram (CV) of maricite NaMn1/3Co1/3Ni1/3PO4 synthesizedcathodes by (a) sol–gel and (b) combustion method, in aqueous LiOH electrolyte.

losing only 13% after 100 cycles. The controlled particle growth andparticle-to-particle uniformity (as observed through SEM inFig. 5b) arising from the combusted maricite product led toimproved electrochemical properties. This indicates the diffusionof sodium ions into the host structure may be influenced by thesynthetic technique and the associated physical properties.

Giving consideration to our earlier studies on olivine(LiMn1/3Co1/3Ni1/3PO4) [23], the current work suggests maricite be-haves favourably in terms of de-intercalation and intercalation ofsodium ions into the host. Importantly, the sol–gel synthesizedlithium counterpart [23] behaved comparatively poorly in termsof insertion and extraction of lithium. Fig. 8 shows the XRD pat-terns of maricite NaMn1/3Co1/3Ni1/3PO4 before and after de-sodia-tion. Ex-situ patterns were recorded for the oxidised electrode,showing the structural integrity is conserved after extracting so-dium (Fig. 8b). The intensity of the peaks is, however, reduced,illustrating that the number of M(2) sites occupied by sodium isdecreased as the battery is oxidised.

4. Conclusions

The combustion process produced maricite NaMn1/3Co1/3

Ni1/3PO4 powder with improved electrochemical behaviour andsuitable morphology. The maricite phase containing mixed transi-tion metal cations was reported for the first time. Irrespective ofthe synthetic route (sol–gel or combustion), we have obtained asingle-phase maricite without any detectable impurities. The nat-ure of the non-ceramic synthetic route apparently influences sam-ple homogeneity, particle size and size distribution. XRD analysissuggests that the insertion site in maricite (Na) is reversed with re-spect to that of olivine (Li) in the Mn1/3Co1/3Ni1/3PO4 matrix. Elec-trode materials synthesized via sol–gel or combustion appear to bepromising as a host material, with the combustion-produced elec-trode showed 87% efficiency after the 100th cycle.

Due to its long-term stability, low cost and environmentalfriendly materials, the water based sodium battery investigatedin this work is of interest for large-scale applications such as elec-trical grid stabilization.

Acknowledgements

The author (M.M.) wishes to acknowledge the Australian Re-search Council (ARC). This research was supported under Austra-lian Research Council (ARC) Discovery Project funding scheme

M. Minakshi, D. Meyrick / Journal of Alloys and Compounds 555 (2013) 10–15 15

(DP1092543). The views expressed herein are those of the authorsand are not necessarily those of the ARC.

References

[1] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J.C. Gonzalez, T. Rojo, EnergyEnviron. Sci. 5 (2012) 5884.

[2] F. Risacher, B. Fritz, Aquat. Geochem. 15 (2009) 123.[3] J.M. Tarascon, M. Armand, Nature 414 (2001) 359.[4] X. Ma, H. Chen, G. Ceder, J. Electrochem. Soc. 158 (2011) A1307.[5] R. Okuyama, E. Nomura, J. Power Sources 77 (1999) 164.[6] J.-S. Kim, D.-Y. Kim, G.-B. Cho, T.-H. Nam, K.-W. Kim, H.-S. Ryu, J.-H. Ahn, H.-J.

Ahn, J. Power Sources 189 (2009) 864.[7] I.D. Gocheva, M. Nishijima, T. Doi, S. Okada, J. Yamaki, T. Nishida, J. Power

Sources 187 (2009) 247.[8] T.B. Kim, J.W. Choi, H.S. Ryu, G.B. Cho, K.W. Kim, J.H. Ahn, K.K. Cho, H.J. Ahn, J.

Power Sources 174 (2007) 1275.[9] K. Zaghib, J. Trottier, P. Hovington, F. Brochu, A. Guerfi, A. Mauger, C.M. Julien, J.

Power Sources 196 (2011) 9612.[10] S.-W. Kim, D.-H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2 (2012) 710.[11] A. Yamada, S. Chung, K. Hinokuma, J. Electrochem. Soc. 148 (2001) A224.[12] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144

(1997) 1188.

[13] B. Ellis, W.R.M. Makahnouk, Y. Makimura, K. Toghill, L.F. Nazar, Nat. Mater. 6(2007) 749.

[14] A. Sun, A. Manivannan, ECS Trans. 35 (2011) 3.[15] M.C. Cabanas, V.V. Roddatis, D. Saurel, P. Kubiak, J.C. Gonzalez, V. Palomares, P.

Serras, T. Rojo, J. Mater. Chem. 22 (2012) 17421.[16] J.N. Bridson, S.E. Quinlan, P.R. Tremaine, Chem. Mater. 10 (1998) 763;

P. Tremaine, J. Chem. Thermodyn. 31 (1999) 1307.[17] S.P. Ong, V.L. Chevrier, G. Hautier, A. Jain, C. Moore, S. Kim, X. Ma, G. Ceder,

Energy Environ. Sci. 4 (2011) 3680.[18] B.L. Ellis, W.R.M. Makahnouk, W.N.R. Weetaluktuk, D.H. Ryan, L.F. Nazar,

Chem. Mater. 22 (2010) 1059.[19] P. Moreau, D. Guyomard, J. Gaubicher, F. Boucher, Chem. Mater. 22 (2010)

4126.[20] A.F. Liu, Z.H. Hu, Z.B. Wen, L. Lei, J. An, Ionics 16 (2010) 311.[21] M. Minakshi, Electrochim. Acta 55 (2010) 9174.[22] M.P. Paranthaman, C. Bridges, A. Huq, A. Sun, A. Manivannan, ECS Meet. Abstr.

1 (2011) 169.[23] M. Minakshi, S. Kandhasamy, D. Meyrick, J. Alloys Comp. 544 (2012) 62.[24] B.D. Sturman, J.A. Mandarino, M.I. Corlett, Can. Mineral. 15 (1977) 396.[25] M.P. Zheng, M.Y. Gu, Y.P. Jin, H.H. Wang, P.F. Zu, P. Tao, J.B. He, Mat. Sci. Eng. B

87 (2001) 197.[26] Y.K. Du, P. Yang, Z.G. Mou, N.P. Hua, L. Jiang, J. Appl. Polym. Sci. 99 (2006).[27] J.M. Amarilla, R.M. Rojas, J.M. Rojo, J. Power Sources 196 (2011) 5951.