Lithium Iron …II …Phospho-olivines Prepared by a Novel Carbothermal Reduction Method
J.Barker,*,z M.Y.Saidi,*and J.L.Swoyer *
Valence Technology,Incorporated,Henderson,Nevada 89015,USA
The electroactive materials LiFePO 4and LiFe 0.9Mg 0.1PO 4have been synthesized by a novel carbothermal reduction ͑CTR ͒method.The transition metal reduction and lithium incorporation processes are each facilitated by the high temperature carboth-ermal reaction based on the C →CO transition.These CTR conditions favor stabilization of the iron as Fe 2ϩas well as offering some control of the product morphology and conductivity.Electrochemical evaluation of the CTR LiFe 0.9Mg 0.1PO 4reveals a lithium insertion plateau around 3.4V vs.Li together with a specific capacity of over 150mAh/g.Differential capacity data confirm the two-phase nature of the insertion reactions as well as the outstanding ionic reversibility.Few technical obstacles have been encountered in scaling the CTR process from a laboratory process to that required for pilot production.©2003The Electrochemical Society.͓DOI:10.1149/1.1544211͔All rights reserved.
Manuscript submitted September 6,2002;revised manuscript received November 25,2002.Available electronically January 14,2003.
Commercial lithium-ion batteries rely on application of one of the well-known lithium insertion ,LiMn 2O 4,LiCoO 2,or LiNiO 2.1Recently,novel materials based on lithiated transition metal polyanions have also been proposed,and phosphate com-pounds crystallizing in either the olivine or Nasicon structures ap-pear to hold particular promise.2,3For instance,it has been sug-gested that the lithium iron phospho-olivine,LiFePO 4,may offer the optimal combination of low cost,favorable electrochemical ac-tivity,and low environmental impact.2The LiFePO 4material dem-onstrates reversible lithium insertion at around 3.4V vs.Li together with a theoretical specific capacity of 170mAh/g.The strong induc-tive effect of the PO 43Ϫpolyanion moderates the Fe 2ϩ/Fe
3ϩ
redox couple to generate the relatively high operating potential found for this compound.4Preliminary trials indicated that the performance of the LiFePO 4was limited by the relatively poor electronic conduc-tivity of the material as well as by the characteristics of the two-phase insertion mechanism.
Recent investigations have led to im-provements in the material utilization by the addition of a conductive carbon layer 5and by use of preparative approaches that control the product morphology.6Partial substitution of Mn for Fe has also been used to increase the operating potential of the active material.2Solid-state preparative approaches used to date have re-lied on the use of Fe 2ϩprecursor compounds,typically iron ͑II ͒ox-alate,FeC 2O 4•2H 2O,2or iron ͑II ͒acetate,Fe(OOCH 3)2.4Apart from being expensive,these multistage preparative strategies are not considered commercially favorable.Low temperature approaches have involved hydrothermal methods based on either FeSO 4•7H 2O 6,7or Fe(NO 3)38as the iron source.In this study,we have applied a carbothermal reduction ͑CTR ͒technique 9as a con-venient and energy efficient method to synthesize Li
FePO 4and the Mg-substituted compound,LiFe 0.9Mg 0.1PO 4.The CTR approach has been described previously by this group as a method to prepare other electroactive materials such as ␥-LiV 2O 5and Li 3V 2(PO 4)3.9On a general point,we believe the carbothermal approach provides an extremely promising method for mass production of these lithi-ated transition metal compounds.
The underlying CTR process is used extensively in the extraction metallurgy industry 10,11to reduce metal oxides ͑and other com-pounds ͒to the pure metal state and relies on the application of the two carbon oxidation reactions
C ϩO 2→CO 2͓1͔2C ϩO 2→2CO
͓2͔
Using binary oxides as an example system,it is found that the ease of reduction for a particular metal oxide depends on the affinity of that metal for the oxide lattice,a property characterized by the standard free energy of formation for the oxide.If we include the oxidation of carbon in these considerations then it becomes clear that Reaction 1is favored at temperatures lower than about 650°C whereas Reaction 2is expected to dominate at higher temperatures.Stronger reducing conditions are produced at higher reaction tem-peratures,while lower temperatures favor less reductive conditions and slower kinetics.Im
portant consequences occur due to entropy ,since Reaction 1only incurs a small change in volume the entropy,change is negligible.Reaction 2,in contrast,involves an increase in volume and also,therefore,in entropy.11The conse-quence of this property means that provided that a high enough temperature is reached,carbon can theoretically reduce any oxide.Building on this basic concept,we have extended this CTR meth-odology by allowing selective and controlled reduction of appropri-ate metal precursors together with simultaneous incorporation of lithium.9,12,13In this work no attempt has been made to separate out the residual carbon from the reaction products since the composite materials prepared are to be used in electrode formulations requiring the use of diluent carbon.
A number of reductive reaction schemes based on the CTR ap-proach may be used to prepare single-phase LiFePO 4and LiFe 1Ϫx Mg x PO 4materials.In this study we have selected Fe 2O 3as a very inexpensive and readily available precursor source,although alternative reaction schemes based on,for example,FePO 4or Fe 3O 4,have been found to be equally successful.13Regardless of the Fe 3ϩprecursor,the CTR approach allows selective reduction and simultaneous lithium incorporation.The precise CTR reaction conditions were determined by a semiempirical approach based on the thermodynamics ͑free energy-temperature relation ͒requirements for the required Fe 3ϩto Fe 2ϩtransition.10Since the temperature requirements for this reduction ͑i.e.,Ͼ650°C ͒favor the C →CO carbothermal reaction mechanism,the CTR reactions involved may be summarized
1.0LiH 2PO 4ϩ0.50Fe 2O 3ϩ0.50C →LiFePO 4ϩ0.50CO
ϩ1.0H 2O
͓3͔
1.0LiH 2PO 4ϩ0.45Fe 2O 3ϩ0.10Mg ͑OH ͒2ϩ0.45C
→LiFe 0.9Mg 0.1PO 4ϩ0.45CO ϩ1.1H 2O
͓4͔
Typically a 25%mass excess of carbon was used over the sto-ichiometric conditions based on Reactions 3and 4above.In each case,the following experimental procedure was followed.The pre-cursors were intimately mixed and then pelletized using a hydraulic pellet press.The reaction mixture was then transferred to a temperature-controlled tube furnace ͑Carbolite Ltd,England ͒equipped with a flowing Ar atmosphere and heated to an ultimate
*Electrochemical Society Active Member.
z
E-mail:JerryBarker@compuserve
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0013-4651/2003/6͑3͒/A53/3/$7.00©The Electrochemical Society,Inc.
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reaction temperature of 750°C and held for 8h.Following slow cooling to room temperature,the materials were removed from the furnace for inspection and evaluation.
Figure 1shows the X-ray diffraction data collected for the LiFePO 4and LiFe 0.9Mg 0.1PO 4materials.The diffraction patterns were refined based on an ordered olivine ͑orthorhombic ͒structure using space group Pnma .This olivine structure comprises PO 4tet-rahedra and distorted FeO 6octahedra,which produce a two-dimensional pathway for lithium-ion diffusion.The refined cell pa-rameters for the CTR LiFePO 4sample were a ϭ10.288Å,b ϭ5.976Å,c ϭ4.672Å,and volume ϭ287.23Å3.For the LiFe 0.9Mg 0.1PO 4,a ϭ10.269Å,b ϭ5.971Å,c ϭ4.676Å,and volume ϭ286.72Å3.For comparative purposes,the related lithium magnesium phosphate,LiMgPO 4,14,15made by a similar solid-state procedure could also be indexed using the Pnma space group with a ϭ10.147Å,b ϭ5.909Å,c
ϭ4.692Å,and volume ϭ281.32Å3.The single-step CTR experimental conditions were adequate to produce reaction products with no unwanted impurity phases such as Fe 2O 3,Fe 3O 4or ‘‘FeO.’’Conventional solid-state methods,although carried out under carefully controlled conditions,have indicated the unwanted presence of the impurity phases Fe 2O 3and Li 3Fe 2(PO 4)3.4
The insertion properties of the test materials were evaluated in metallic lithium test cells using the electrochemical voltage spec-troscopy ͑EVS ͒technique.16EVS is a voltage step method,which provides a high-resolution approximation to the open circuit voltage curve for the electrochemical system under investigation.The ex-perimental conditions are roughly equivalent to a C/20rate for charge and discharge.The positive electrodes comprised 84wt %active materials,5wt %Super P ͑conductive carbon ͒,and 11wt %poly ͑vinylidene fluoride ͒-HFP copolymer ͑SOLEF,Solvay Chemi-cal ͒binder.The electrolyte was a 1M solution of LiPF 6in ethylene carbonate/dimethyl carbonate ͑2:1by weight ͒.
The 23°C EVS voltage profile and differential capacity data for a representative LiFe 0.9Mg 0.1PO 4sample are shown in Fig.2and 3,respectively.The lithium extraction/insertion behavior for this active material relies on the reversibility of the Fe 2ϩ/Fe 3ϩredox couple LiFe 0.9Mg 0.1PO 4↔Li 0.1Fe 0.9Mg 0.1PO 4ϩ0.9Li ϩϩ0.9e Ϫ
͓5͔
Note from Reaction 3that the material utilization of the LiFe 0.9Mg 0.1PO 4phase is determined by the Fe activity,such that the extraction process is limited to 0.9Li per formula unit.This process is equivalent to a theoretical specific capacity of 156mAh/g,a figure that compares favorably with commercially available cath-ode materials.An important consequence of this limitation is that at no stage during the charge/discharge process is a fully delithiated olivine phase produced.It has been suggested previously that the coexistence of Li 1Ϫx FePO 4and the fully delithiated FePO 4phase within individual olivine particles may lead to a significant diffu-sional limitation.2,17
The insertion properties in the iron phospho-olivines have been shown to be dependent on several factors including particle mor-phology,the impurity content,and the material conductivity.2,4,7,8,17For instance,Ravet et al.5obviated the limitation of the
material
reaction member
Figure 1.X-ray diffraction patterns using Cu K ␣radiation for CTR samples of LiFePO 4and LiFe 0.9Mg 0.1PO 4.The diffraction patterns were refined based on an ordered olivine ͑orthorhombic ͒structure using space group Pnma ͑see text for unit cell parameters ͒.The corresponding Miller indices are shown by annotation on the
figure.
Figure 2.EVS voltage profile for a typical Li//CTR LiFe 0.9Mg 0.1PO 4cell cycled between 2.80-3.80V .The data shown were collected at 23°C and are for the first cycle.The lower x axis indicates the lithium concentration,x ,in Li 1Ϫx Fe 0.9Mg 0.1PO 4,while the upper x axis shows the material specific ca-
pacity.
Figure    3.EVS differential capacity profile for a typical Li//CTR LiFe 0.9Mg 0.1PO 4cell cycled between 2.80-3.80V .The data shown were col-lected at 23°C and are for the first cycle.
Electrochemical and Solid-State Letters ,6͑3͒A53-A55͑2003͒
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conductivity by coating the active particles with a carbon-based sur-face layer.This conductive layer was created in a postdeposition treatment and allows improved material utilization.In contrast,the carbothermal approach offers a direct,single-step method for pro-ducing an olivine material with optimized electrochemical perfor-mance.No post-deposition enhancements are required to allow op-timized material utilization.Yamada and co-workers,4on the other hand,reported that the optimum performance for LiFePO4may only be achieved by minimizing grain growth and avoiding the presence of unwanted Fe3ϩphases in the reaction product.It was reported that an abrupt increase in particle growth at reaction temperatures above600°C resulted in a sharp decrease in material capacity.4As evidenced by scanning electron microscope͑SEM͒analysis,the relatively mild CTR reaction conditions combined with the presence in the reaction mixture of high surface-area carbon,minimized the possibili
ty of significant grain growth.Particle size analysis͑Coulter Counter,LS particle size analyzer͒and SEM examination of the powders indicated the presence of considerable material agglomera-tion.The agglomerates themselves averaged around100␮m in size, although these could be easily dispersed by either mechanical mix-ing or ultrasonic agitation.Indeed the measured particle size distri-bution was comparable to that for the Fe2O3precursor,suggesting that the CTR conditions did not promote significant sintering or grain growth.The SEM micrographs also detected the presence of residual carbon in the reaction products.The residual carbon was uniformly dispersed throughout the‘‘composite’’samples,most clearly accumulating at the agglomerate surface.
The voltage plateau behavior in Fig.2is indicative of the two-phase nature of the lithium extraction/insertion reactions.2This ob-servation is also supported by the EVS transient data,which indi-cates a non-Cottrell current-time relationship characteristic of phase nucleation behavior.18The reversible specific capacity corresponds to151mAh/g͑i.e.,xϭ0.87in Li1Ϫx Fe0.9Mg0.1PO4)which amounts to a97%utilization efficiency.In addition,the low rate EVS cycle incurs only a minor coulombic inefficiency evidenced by the near equivalence of the charge and discharge capacities.At the test temperature of23°C,these data clearly represent outstanding insertion reversibility and indicate the remarkable performance of the CTR prepared LiFe0.9Mg0.1PO4active material.Similar mea-surement
s carried out at60°C effectively removed all the kinetic limitations in the system,and demonstrated that,essentially,all the extractable lithium may be effectively removed from the Li1Ϫx Fe0.9Mg0.1PO4phase.Comparable23°C voltage data were de-termined for the CTR LiFePO4material,although somewhat sur-prisingly the material utilizationfigure was determined to be slightly inferior,even though this material possesses a higher theoretical specific capacity.The elevated temperature performance for this ma-terial showed utilization close to the theoretical value.
The EVS differential capacity data are shown in Fig.3and con-firm the outstanding reversibility of the lithium extraction/insertion reactions.It should be noted that sharp peaks in differential capacity plots are expected to indicate structural phase transitions,whereas minima correspond to single phase regions and/or ordering effects for the insertion ion.14The well-defined peaks and symmetrical form of these data thus confirm the two-phase behavior reactions while also indicate the outstanding insertion reversibility of the CTR-produced active material.Since the EVS data are collected at con-ditions approximating to thermodynamic equilibrium for the system,the differential capacity peaks may be used to accurately define the potentials for the extraction and insertion reactions.Under the pre-vailing EVS conditions,the main lithium extraction process is cen-tered at a potential of3.46V vs.Li,while the corresponding lithium insertion reaction is located at a voltage of3.43V vs.Li.The volt-age separation,⌬
V,amounts to just0.03V,a surprisingly small value considering the diffusional polarization described previously for the Li/LiFePO4system.2The quality of these EVS data appear superior to comparable results presented for the LiFePO4material made from convention solid-state methods.2,4,19
Long-term cycling of the Li1Ϫx Fe0.9Mg0.1PO4and LiFePO4com-pounds at various charge/discharge rates in both metallic lithium and lithium-ion configurations confirm the excellent reversibility of the insertion reactions.In these test systems,the CTR materials each demonstrate several hundred charge/discharge cycles without appre-ciable capacity fade.The LiFePO4and Li1Ϫx Fe0.9Mg0.1PO4both of-fer outstanding potential as electroactive materials for lithium-ion applications.
In summary,the CTR approach has been successfully applied to the preparation of these lithiated olivine phases.In related studies, we have utilized similar reductive procedures to produce a range of lithiated transition metal oxides and phosphates.The CTR condi-tions seem ideally suited to favoring stabilization of iron as Fe2ϩ,as well as offering a degree of control of the material morphology and conductivity.Few technical obstacles have been encountered in transferring the CTR process from laboratory scale to that required for pilot production of the olivine phases.We believe that the CTR approach offers a unique and scalable process for preparation of a great many electroactive phases.
Valence Technology,Inc.,assisted in meeting the publication costs of this article.
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Electrochemical and Solid-State Letters,6͑3͒A53-A55͑2003͒A55

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