V i ew A rticle Online I Journal Homepage I T able of Coments for this i ssue
`
`journal of
`Materials Chemistry
`
`www.rsc.org/materials
`
`Volume 17 I Number 30 I 14 August 2007 I Pages 3053-3272
`
`.~·
`~·
`.~·
`
`ISSN 09S9-9428
`
`Theme Issue: New Energy Materials
`
`RSC Publishing
`
`FEATURE ARTICLE
`Michael M. Thackeray eta/.
`Li2Mn03-stabilized LiM02 (M = Mn,
`Ni, Co) electrodes for lithium-ion
`batteries
`
`[ti 1 \§, 1!$ i Ga ,j 1 r•l@j
`
`In this issue ...
`
`II I II
`
`0959-9428(2007) 17:30;1-0
`
`SONY EXHIBIT 1013
`
`Page 1 of 15
`
`

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`FEATURE ARTICLE
`
`www.rsc.org/materials I Journal of Materials Chemistry
`
`Li2Mn03-stabilized LiM02 (M
`batteriest
`
`Mn, Ni, Co) electrodes for lithium-ion
`
`Michael M. Thackeray,*a Sun-Ho Kang,° Christopher S. Johnson,0 John T. Vaughey,0 Roy Benedek0 and
`S. A. Hackneyh
`
`Received 16th Febmary 2007, Acceptell 29th March 2007
`First pub/is/ted as all Adva11ce Article 0 11 the web 20th April 2007
`DOl: 10.1039/b702425h
`
`). Li2Mn03-stabilized LiM02 (M = Mn,
`A strategy used to design high capacity (>200 mAh g- 1
`Ni, Co) electrodes for lithium-ion batteries is discussed. The advantages of the Li2Mn03
`component and its influence on the structural stability and electrochemical properties of these
`layered xLi 2Mn03·(1 - x) LiM02 electrodes are highJigbted. Structural, chemical, electrochemical
`and thermal properties of x Li2Mn0y( l - x)LiM02 electrodes are considered in the context of
`other commercially exploited electrode system , such as LiCo02, Li
`i0 .8Co0 .15Al0.050 2,
`Li 1+.,.Mn2- .,·04 and LiFeP04.
`
`I. Introduction
`
`The introduction of non-aqueous rechargeable lithium-ion
`batteries in the 1990s to power portable electronic device
`created a revolution in battery technology and a marked
`swing away from
`the relatively low-voltage, water-based
`systems such as nickel-cadmium and nickel- metal hydride
`batteries and high-temperature systems.' The first commercia l
`lithium-ion battery, introduced into the market by Sony
`Corporation
`in 1991, was based on a LixC<fLi 1- .--Co02
`couple. During cell operation between 4.2- 3.0 V, lithium
`the
`two
`transported electrochemically between
`ions are
`electrodes. with concomitant oxidat ion and reduction of the
`
`"Battery Technology Department. Chemical Engineering Dil•ision.
`Argonne National Litboratory, Argonne. !1/inois 60439. USA
`hDeparrment of Metallurgical and Materials Engineering, Michigan
`Technological Unil•ersity, Houghton, Michigan 49931, USA
`t This paper is part of a Journal of Materials Chemistry theme issue on
`ew Energy Materials. Guest editor: M. Saiful Islam.
`
`3 and Li 1- xCo02 cathode,4 respectively, during
`Li."C6 anode2
`•
`discharge and vice versa during charge. The surface reactivity
`and instabi lity of delithiated Li 1_.,.Co02 limit the practical
`capacity of LiCo02 electrodes to approximately 140 mAh g- 1
`,
`which corresponds to x ~ 0.5
`i. e., ~50% of its theoretical
`value (273 mAh g- 1
`). These limitations and the relatively
`high cost of cobalt, have led to enom10us efforts since 1991
`to find alternative cathode materials to LiCo02 that provide
`lithium-ion cells with superior capacity, energy, safety and
`cycle life.
`Several alternative cathode materials to LiCo02 have been
`exploited by the lithium battery industry over the past decade;
`they include compositional variations of the layered LiCo02
`5 spinel electrodes
`structure, such as Li
`i0.80Co0. , 5Al0.050 2,
`6
`7 such as litllium-rich compounds in
`derived from LiMn 20 4,
`·
`the Li 1+xMn 2- .\'0 4 system,8 and LiFeP04, which has an
`olivine-type structure.9
`10 Although Li
`io.soCoo.15AJo.o50 2
`·
`provides
`a
`slightly higher
`practical
`capacity
`(160-
`180 mAh g - I) than LiCo02,
`its thermal instabi lity on
`
`Michael Thackeray receil•ed his
`B.S. {1970) , M.S. {1973) and
`Ph. D.
`( 1977 ) degr ees in
`Chemistry from the Unirersity
`of Cape Town. Solllh Africa
`and studied as a postdoctoral
`fellow with Professor John B.
`Good enough at O xfo rd
`University, UK. After spending
`twenty years at the Council for
`Scientific and Indust rial
`Research ( CSI R ) , Pretoria.
`South Africa {1973- 1994) on
`ballery-related research, he
`moved to the United Swtes
`where he is current ly an
`Argonne Dislinguished Fellow and Group Leader at Argonne
`National Laboratory ow ide Chicago. His primary research interest
`is determining the structure-electrochemical properties of solid
`electrolyte and electrode materials for electrochemical applications.
`
`Michael Thackeray
`
`Sun-Ho Kang received his
`B.S. {1992). M.S. {1994),
`and Ph. D.
`( 1 998 )
`in
`Materials Sci ence and
`Engineering from Seoul
`National University . South
`Korea. After swdying as a
`postdoctoral fellow with
`J o h 11
`B .
`P r of e s s o r
`Goodenough at the University
`of Texas at Austin ( 1999-
`2000) , he joined the Chemical
`Engineering Divis ion at
`Argonne National
`Laboratory. His primary
`research
`interesrs
`include
`synthesis, electrochemical and transport properties, and struc(cid:173)
`wre- property relationships of electrode materials for energy
`storage and conversion systems.
`
`Sun-Ho Kang
`
`31 1 2 I J. Mater. Chem., 2007, 17, 3112-3125
`
`This journal is © The Royal Society of Chemistry 2007
`
`Page 2 of 15
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`Page 2 of 15
`
`

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`delithiation compromises the safety of lithium-ion cells. 11
`On the other ha nd , spinel LiMn20 4 and olivine LiFeP04
`electrodes are significantly more stable to lithium extraction
`than the layered Co- and
`i-based electrodes, both structurally
`and
`thermally, but
`they deliver relatively
`low practical
`capacities above 3 V
`in a
`lithium cell,
`typically 100-
`120 mAh g- 1 a t moderate current rates. (The theoretical
`capacities of Li 1- xMn20 4 and Li 1- xFeP04 for the complete
`extraction of lithium (x = I) are 148 and 170 mAh g- 1
`,
`respectively.) It was clear by the end of the 1990s that new
`strategies would have to be developed to design alternative
`high potential cathode materials (>3 V vs. Li~ with superior
`capacities to sta ndard LiCoOr, LiMn 20 4- a nd LiFeP04-type
`electrodes without compromising the structural stability or
`rate capability of the electrodes or the cycle life of the cells. In
`this Feature Article, the strategy that was adopted at Argonne
`ational Laboratory to develop a fa mily of high capacity,
`Li 2Mn03-stabilized LiM02 (M = Mn,
`i, Co) electrodes for
`lithium-ion batteries is highlighted; progress made at Argonne
`and by others in advancing this class of e lectrode materials is
`brieOy reviewed.
`
`2. Strategy
`
`Insertion electrodes for lithium-ion electrochemical cells need
`to have stable structures o ver a wide compositional range so
`that as much lithium as possible ca n be inserted and extracted
`during repeated discharge and charge to maximize cell
`capacity, energy and cycle life. Furthermore, the host structure
`must have an interstitia l space tha t provides energetically
`favorable pathways for fast Lithium-ion transport, i.e. , a high
`power capability. A common procedure u ed to stabilize meta l
`oxide electrode structures is cation or anion substitution. For
`i0.80Co0.15A10.050 2, Al3+ ions are used to
`example, 1) in Li
`provide greater bindi11g energy to the oxygen sheets of the
`layered structure on de litbiation, 2) Li+ substitution for Mn3+
`in LiMn20 4 , i. e., Li 1+_.Mn2_.,0 4 , increases the relative Mn4 +(cid:173)
`ion concentration in the spinel structure which s uppresse the
`12
`manganese so lubility in lithium-ion battery electrolytes,8
`•
`14 and
`and 3) fluorine substitution for oxygen in spineJ13
`·
`layered 15
`16 electrodes, particularly at the pa rticle surface,
`·
`provides enhanced cycEng stability. An alternative approach
`to impart structura l stability a nd improve the elect rochemical
`
`Christopher S. Johnson holds a
`in Chemistry from the
`B.S.
`University of North Carolina
`at Chapel Hill ( 1987) and a
`Ph D. in Inorganic Chemistry
`from Northwestern University
`( 1992 ). He is currently a staff
`ch em is t
`in
`th e Chemica l
`Eng in e e rin g Divis ion at
`Argonne National LaboraLory.
`Dr J ohnson 's professional
`interests include the develop(cid:173)
`ment of new synthetic methods
`and the use of spectroscopy and
`electrochemistry to soh•e pro(cid:173)
`blems in materials science. He
`conducts applied research in lithium balleries, and basic research in
`studies of ionic transport and mechanisms, electron transfer
`processes, intercalation structures and phase diagrams ofmaterials.
`
`Christopher S. Johnson
`
`John Vaughey received his B.S.
`in Chemislly from Worcester
`Polyceclmic Ins/illite ( 1987)
`and his Ph. D. in Inorganic
`Chemistry from Northwestern
`University ( 1992) under the
`direct ion of Professor Ken
`Poeppelmeier. Since completing
`postdoctoral positions with
`Professor A //an Jacobson at
`the University of Houston and
`Professor John D. Corbell at
`Iowa State University/Ames
`Laboratory, he has worked in
`the Chemical Engineering
`Division of Argonne National
`Laboratory since 1997. His primary research interests include
`strucwre- property relationships in metal oxides, primary and
`secondary lithium-ion ballery materials.
`
`John Vaughey
`
`Roy Benedek received B.S.
`and M . Eng. de g r ees in
`Eng ineering Physics from
`Cornell University, and a
`in Theoretical Solid
`Ph. D.
`Srate Physics from Bro wn
`University. He was a post(cid:173)
`doctoral research associate at
`Cornell University before join(cid:173)
`ing
`the Materials Science
`Division at Argonne National
`Laboratory. He is presently a
`Special Term Appointee with
`the Battery Materials Group in
`the Chemica l Enginee ring
`Division at Argonne. His pri(cid:173)
`mary research interest is the simulation of structural and
`electronic properties of materials.
`
`Roy Benedek
`
`Stephen Hackney received his
`B.S. in Chemistry from James
`Madison University ( 1980), and
`M.S. ( 1982) and Ph.D. (1985)
`degrees in Materials Science
`from the Unil'ersity of Virginia.
`In 1985 6, he spent 18 mowhs
`as a postdoctoral feiiOiv wirh
`Professor John W. Calm at
`N IST. He spent the following
`20 years as a faculty member in
`the Department of Materials
`S cience and Engineering at
`Mi c higa n Technological
`UniversiTy. His primary research
`interests have been materials
`charac/eri=ation and crystallographic phase transformations, with
`a focus on electrode materials for electrochemical applications.
`
`S tephen Hackney
`
`This journal is © The Royal Socie ty of Chemistry 2007
`
`J. Mater. Chem., 2007, 17,3112- 3 125 I 3113
`
`Page 3 of 15
`
`Page 3 of 15
`
`

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`(a)
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`(c)
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`(d)
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`• A I
`•
`'a
`o o
`
`macti'-e
`r-AIP1
`sp/ntl
`block
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`inucth-e
`r-Aip 1
`t;pinel
`block
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`o I
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`• Ag
`
`[001] projection
`
`Fig. I Schema tic illustra tions of the structures of a) y-M n0 2 [a fter ref. 17); b) Li20-stabilized <X-Mn02 [after ref. 19]; c) ~" -alumina,
`a20 ·5.3Al20 3 after [ref. 20] a nd d) 44Agl·3(C 11 H22 3)13 [after ref. 22]. Figures reproduced from the respective references, Fig Ia. I b and lc with
`permission from Elsevier and Fig. ld from the International Union of Crystallography (http:l/journals.iucr.org!).
`
`p roperties o f solid electrode a nd solid electrolyte materials is to
`use structural units ra ther tha n cation o r a nion substituents.
`Fo r example, in the family of Mn02 electrodes, y-Mn0 2,
`which is used in 3 V lithium cells, has a composite structure
`containing unidime nsional (2 x
`I) ramsdellite-type tunnels
`(designa ted r-Mn02 in Fig. Ia) tha t ca n accommoda te lithium,
`a nd na rrow ( I x I) rutile-like tunnel (de igna ted P-Mn02 in
`Fig. I a) tha t a re essentially impervio us to lithium and impa rt
`sta bility to the o verall structure. 17 Ano ther exa mple of a 3 V
`electrode is ct-Mn02 with a holla ndite-type structure tha t
`provides enha nced electrochemica l performa nce whe n sta bi(cid:173)
`19
`l b). 18
`lized by Li 20 units in the (2 x 2) tun nels (Fig.
`·
`ln the class of solid electro lytes, the fast
`a + -ion cond uctor,
`::::::: 0.3 S cm- 1 a t 300 "C)
`13"-alumina (Na20 ·5.3Al20 3, u
`consists of y-AI20 3 spinel units tha t sta bilize the
`a+(cid:173)
`2 1 whereas
`lc)20
`ton cond ucting
`layers
`(Fig.
`(Na 20 )
`•
`A~ I53(C 11 H30 3h (44Agl ·3C11H 30 313) is an example of a
`Ag+-io n conductor (u::::::: l0- 2 S cm- 1 a t 25 "C) in which tri(cid:173)
`a mine cationic units. C11H 30 /+, serve
`to quench a nd
`sta bilize a highly conducting Agl component a t
`room
`tempera ture (F ig. ld).22.23
`Layered LiM02 a nd spinel LiM 20 4 compo unds, in which M
`is typically an electrochemically active first row transitio n
`ion, such as ma nganese, 11ickel a nd cobalt, a nd
`metal
`sta bilizing substituen t cations, such as lithium, magnesium
`a nd aluminium, have close-packed structures with a close(cid:173)
`packed interlayer distance of a pproximately 4.7 A. Layered
`Li 1_,M02 structures tend to become unsta ble a t high levels of
`delithia tion, typica lly when x exceed s 0.5. When M =Co o r
`i,
`
`the instability is attributed la rgely to the l1igh ly oxidizing
`na ture o f tetrava lent cobalt a nd nickel. which results in the loss
`of oxygen and the migration of the tra nsition me ta l ions to the
`layer. When M = Mn,
`lithium-depleted
`the delithiated
`structure transforms to a more sta ble spinel-type configura(cid:173)
`this process degrades its high-potential (4 V)
`tio n, b ut
`electrochemical performa nce. 24
`In principle, however, a
`layered Li 1- xMn02 structure if kept intact over the full range
`of x (0 < x < I) would yield an a ttracti ve 286 mAh g- 1
`between 4 V a nd 3 V.25
`( ote tha t
`the corresponding
`Li 1- xMn02 spinel system (i. e., LiyMn20 4; 0 < y < 2, y =
`2 - 2x) offers the same capacity, one half o f which is delivered
`at ~ 4 V a nd the o ther ha lf below 3 V .) Building on the co ncept
`of using a structura l uni t to sta bilize electrochemically active
`ma terials, as described a bove, a nd guid ed by our ea rlier work
`in stabilizi ng Mn02 compounds with Li20 , 18
`26 we ado pted a
`•
`simila r strategy to stabilize layered LiM02 and LiM20 4 spinel
`electrodes by integra ting a st ructu rally compatible component
`tha t was electrochemically inactive (with respect to capacity
`genera tion) between 4 a nd 3 V. Our intention was to increase
`the sta bility of layered a nd spinel electrodes over a wider
`compositio nal ra nge tha n was previously possible witho ut
`com promising the power or cycle life of the cells.
`Fo llowing our success in fabricating Li2Mn03-stabibzed
`layered Mn02 electrode structures by I) leaching Li20 from
`Li2Mn03 by acid treatment, 2) subseq uent relithiation, either
`electrochemica lly in a lithium cell27•28 or chemically with
`n-butylli thium,29 a nd 3) recognizing the structural compa ti(cid:173)
`bility between Li 2M '0 3 (e.g., Mn, Ti, Zr) a nd layered LiM02
`
`31 14 I J. Mater. Chem., 2007, 17, 3112- 3125
`
`This journal is © The Royal Society of Chemistry 2007
`
`Page 4 of 15
`
`Page 4 of 15
`
`

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`
`and spinel LiM 20 4 compounds,30 we initiated efforts to synthe(cid:173)
`' layered- layered' x Li2M'03·
`size
`structurally
`integrated
`31-33 and ' layered- spinel' xLi2M 10 y ( l - x)
`{I - x) LiM02
`LiM20 }3-35 compounds in which the Li2M '0 3 component
`would act to provide additiona l stability to a layered LiM02
`or a LiM 20 4 spinel electrode over a wide operating voltage
`wind ow of a lithium-ion cell, for example, 4.5 to 2.0 V. At
`present,
`'layered- layered' e lectrodes appear to show the
`greatest promise of the two systems, particula rly x Li2Mn03·
`{l - x)LiM02 (M = Mn,
`i, Co) electrodes; the discussions
`that follow therefore focus on the structural, electrochemical
`and thermal properties of xLi2Mn03·(1 - x)LiM02 electrodes
`which emphasize their ad vantages over conventional lithium(cid:173)
`ion battery e lectrodes.
`
`3. Structurally integrated xLhMn03·(1 - x)LiM02
`electrodes
`
`3.1 Structural considerations
`Schematic illustrations of idealized Li2Mn03 a nd LiM02 (M =
`Co,
`i, Mn) layered structures are shown in Fig. 2a and b,
`respectively. In LiM02 compounds, the average oxidation
`sta te of the M cation
`is three, as exemplilied by Li2Mn03
`(Li[L i113Mnu 3]02) in which tetravalent manganese and mono(cid:173)
`valent li thium comprise the M layer. Despite the variation in
`crysta llographic space group symmetry brought about by
`differences in atomic and electro nic configura tions (mo no(cid:173)
`37 trigonal, R'3m for
`clinic, C2/m for L i2MnOl 6 and LiMn02;
`39 the close-packed layers in each o f
`LiCoOl 8 a nd LiNi02)
`these com pounds ((OOI)monoclinic a nd (003)1rigonaJ) have an
`interlayer spacing close to 4.7 A. The compatibility of the
`close-packed layers allows the integra tion of a Li2Mn03 com(cid:173)
`ponent with a LiM02 component at the a tomic level , if one
`allows for some disorder between the Mn and M cations,40 in
`much
`the same way tha t
`the hexagona lly close-packed
`structures of P-Mn02 (rutile-type) a nd ramsdellite-Mn02 are
`integrated to form the composite structure of y-Mn02 (Fig. I a).
`41 and Iayered- spine!33 composite
`However, unlike y-Mn02
`structures in which the respective rutile/ramsdellite and layered/
`spinel components can be readily distinguished from one
`another by high-resolution tra nsmission electron microscopy
`(HRTEM) it is more diflicult to differentiate two structurally
`compatible
`layered components with
`this
`technique, as
`illust rated m
`the HRTEM
`image of a 0 .3Li2 Mn03 ·
`0.7LiMno.s
`io.50 2 composite structure in which the (001)
`and (003) la ttice fringes of tbe Li 2Mn03 a nd LiMn0.5N i0.50 2
`components, respectively, are coincident (Fig. 3).
`Both Li2Mn03 and LiM02 components of x Li2Mn03·
`{I - x)LiM02 electrodes have rocksalt structures in which a ll
`
`(b) LiM02
`
`M06
`octahedra
`
`0 Li
`
`Fig. 2 Layered structures of a) Li2Mn03• b) LiM02 (M = Co,
`i. Mn).
`
`-4.7 A lattice spacing
`
`Fig. 3 TEM image of a 0.3Li2Mn0y 0.7LiM n0.5Ni0.50 2 electrode.
`
`the octahedral sites of the (cubic) close-packed oxygen a rray
`a re occupied. Between -4.4 a nd 2.0 V vs. Li0
`, the LiM02
`component is electrochemically active and operates as a true
`insertion electrode. Duri ng charge, lithium extraction occurs
`with the concomitant oxidation of the M ions; the reverse
`process occurs during discharge. By contrast, the L i2M n03
`component is electrochemically inactive over this potential
`wi ndow, in the sense that it does not contribute to electro(cid:173)
`chemical capacity, because a ll
`the manga nese
`ions are
`tetravalent and cannot be oxidized fu rther. Lith ium insertion
`into Li2Mn03, with a concomitant reductio n of the manga nese
`ions, is also prohibited because there a re no energetically
`favorable interstitial sites for the guest ions. Under such
`conditions, the L i2Mn03 component acts as a stabilizing unit
`in
`the electrode structure.
`l f, however,
`the electron ically
`insulating Li2Mn03 regions are extremely small (nano(cid:173)
`dimensio nal) a nd if they are distributed ra ndomly throughout
`the composite structure, then these regions will fu nctio n as
`solid electrolyte constituents in facilitating Li+ -ion transport
`between
`regions of the
`the capacity-generating LiM02
`structure; in this respect, the Li+ -ion conductivity in a single(cid:173)
`phase Li2Mn03 specimen has been reported to be in the range
`10- 6 to 10- 3 S cm- 1 between 18 and 400 oc with an activation
`•42
`energy of 44.87 kJ mol - 1
`
`3.2 Nomenclature and the advantages of using a two-component
`xLhMn03·(1 - x)LiM02 notation
`
`Because Li2Mn03 can be reformulated as Li[Li 113Mn213]02, it
`is possible to normalize x Li 2Mn03·( I - x)LiM02 electrodes in
`standard layered notation as Li t+(.,.12+.<>M ' l+(.,·t(l+x))0 2 in whic h
`M, = Mn + M or, more simply, Li 1 ... yM' 1-y0 2 where y =
`(x/(2 + x)). Layered xL i2Mn03·(1 - x)LiM02 electrodes can,
`therefore, be considered to be lithium-rich com pounds with
`Mn4+ ions in the transition metal layers. Both nota tions have
`been used extensively in the literature a nd we use them
`intercha ngeably in this paper. For example, the lithium-rich
`Li[Lio.2Mno.4Cro.4]02 electrode43 can be represented in two(cid:173)
`component notation as 0.4Li2Mn03·0.4LiCr02 whereas the
`for 0 < x < 1,44 a nd
`systems Li[Li.,nC01 -xMn2x13]02
`45
`ixMnu3- xt3]02 for 0 < x < 0.5
`Li[L il/3- zx/3
`can be rewritten
`as (2x)Li2Mn03"3(1 - x)LiCo02 and { I -
`2x)Li2Mn03·
`i0.50 2, respectively, for the same ranges o f x.
`(3x)LiMno.s
`
`This journal is © The Royal Society of Chemistry 2007
`
`J. Mater. Chem., 2007, 17,3112- 3125 I 3115
`
`Page 5 of 15
`
`Page 5 of 15
`
`

`
`M02
`(x-c5}Li.JAn03 •c5Mn0 2-(1-x)M02 ---:::--...
`-Li20
`>4.4V
`
`(ideal CdCI2-type)
`
`:::: . .......... ....
`
`'
`
`••• ...--<::44V
`.
`.....
`
`~
`~ 3
`
`<4.4V
`
`'
`
`\
`\
`
`50
`
`150 200 250
`100
`Capacity I mAh g ·•
`
`30C
`
`x=0.3 / '
`/
`(M=Mn, Ni, Co)
`xLizMnOJ.f1-x)LiM02
`/
`(x-c5}Li.JAn03•0LiMn02-(1-x)LiM02
`
`Fig. 4 Compositional phase diagram showing the electrochemical reaction pathways for a xLi2Mn03·(1 - x)LiM02 electrode.
`
`two(cid:173)
`There are several distinct advantages of using a
`component notation to describe xLi2Mn0y (l - x)LiM02
`electrodes because the Li 2Mn03-like regions in these lithium(cid:173)
`rich compounds play a major role in dictating their structural
`stability and electrochemical properties during charge and
`discharge. Furthermore, with this notation, it is possible to
`follow the electrochemical reactions of the xLi 2Mn03·(1 - x)
`LiM02 electrodes on a Li2MnOr LiM02- M02 compositional
`pha e diagram as illustrated in Fig. 4, in which 0.3Li2Mn03 •
`i0.50 2 (i.e., M = Mn0.5
`is used as the
`i0 5 )
`0.7LiMn0.5
`composition of a parent reference electrode. T he correspond(cid:173)
`ing profile of the
`initial charge/discharge cycle of a
`Li/0.3Li2Mn03·0.7LiMn0.5Ni 05 0 2 cell is al o provided in
`Fig. 4. During the initia l charge of the cell to approximately
`i0.50 2 component
`4.4 V, lithium is extracted from the LiMn0.5
`with a concomitant oxidation of the divalent nickel ions to the
`tetravalent sta te while the manganese 'spectator' ions remain
`tetravalent until the Li2MnOr M02 (Mn0.5Ni0.50 2) tie-line is
`reached. During this reaction, depletion of lithium ions from
`the lithium layer is compensated by the d iffusion of lithium
`from octahedral sites in the manganese layer of the Li 2Mn03
`component to tetrahedral sites in the lithium-depleted layer
`thereby providing the additional binding energy necessary to
`maintain structural stability. T his phenomenon has been
`observed experimentally by MAS MR46 and confirmed by
`theoretical calculations.41 In
`this regard,
`therefore,
`the
`Li2Mn03 component act as a reservoir of surplus li thium
`that can be used to stabi.l ize the electrode structure at low
`lithium loadings. A schematic illustration of a lithium-depleted
`Li2Mn03-!ike unit is shown in Fig. 5. The co-existence of
`
`M06 (M=Mn, Li) octahedra
`~ vacant octahedra
`o = octahedral Li
`b. = tetrahedral Li
`
`Fig. 5 Schematic illustrat ion showing the 2-D spinel-like configura(cid:173)
`tion in a dclithiatcd Li2Mn03-like region of a x Li2Mn0r ( l - x)
`LiM02 electrode structure (M = Mn, Ni, Co).
`
`tetrahedral and octahedral lithium in the lithium-depleted
`layers gives the electrode structure two-dimensional, qua i(cid:173)
`spinel-like features, reminiscent of the three-dimensional
`tetrahedral (8a)- octahedral (16c) interstitial network of the
`Mn204 spinel framework (space group Fd3m),48 sugge ting
`that
`the
`lithium-depleted
`-
`x)
`layers of x Li 2Mn0 3·(1
`Li 1-yM02 electrode
`provide an energetically
`favorable
`interstitial space for lithium, thereby ensuring fast reaction
`kinetics. Indeed , there have been reports that a 10-15%
`excess lithium in Li 1 +., M ,_ ,.0 2 electrodes enhances their rate
`capability,49 consistent with our hypothesis. Furthermore,
`when fabricating layered Li l+xM 1_.,.02 electrodes at elevated
`temperatures, particularly those containing nickel, the surplus
`lithium is beneficial for minimizing the contamination of the
`lithium layers by transition metal ions. 50
`If the electrochemical potential of a Li/0.3Li2Mn03•
`0.7LiM02 cell is raised above 4.4 V during charge, then
`further lithium can be extracted from the electrode as indicated
`by the solid line in the compositional phase diagram and
`electrochemical plot of the Li/0.3Li2:'vtn03·0.7LiM02 cell in
`Fig. 4. [n this case, lithium is extracted from the Li 2Mn03
`5 1
`·52
`component with the simultaneous release of oxygen;32·45·
`the net loss from the xLi2Mn03·(1 - x)M02 is Li 20. This
`reaction drives the composition of the electrode irreversibly
`towards the M02 apex of the Li2Mn03- LiMOr M02 phase
`diagram. Complete extraction of Li 20 from the Li 2Mn03
`component
`(Li20· Mn02) yields electrochemically active
`Mn02. Reinsertion of lith ium into the Mn02 component is
`now possible with the concomitant red uction of manganese
`until the rocksalt stoichiometry LiMn02 is reached. Such an
`electrochemical activation process, therefore, provides an
`excellent method not only for ta iloring the Li 2Mn03 content
`and the amount of litJ1ium in the Li2Mn03 reservoir required
`to stabilize delithiated xLi2Mn03·(1 - x)LiM02 electrodes, it
`also controls the amount of electrochemically active manga(cid:173)
`nese in the electrodes after activation above 4.6 V. 52 The
`extraction of two Lt ions per Li 2Mn03 unit on the initial
`charge and the reintroduction of only one Li+ ion on the
`subsequent discharge into the resulting Mn02 unit necessari ly
`means that there must be an irreversible capacity loss on the
`
`31 16 I J. Mater. Chem., 2007, 17, 3112- 3125
`
`This journal is © The Royal Society of Chemistry 2007
`
`Page 6 of 15
`
`Page 6 of 15
`
`

`
`initial cycle, as clearly demonstrated in the electrochemical plo t
`of Fig. 4. This phenomenon can, however, be advantageous,
`because it allows the possibility of using the surplus lithium in
`the parent x Li2Mn0y{ l - x)LiM02 electrode to offset a ny
`first-cycle, irreversible capacity loss that may be encountered
`a t the negative electrode, notably the irreversible capacity
`losses that are typical of intermetallic a nodes? 3
`
`3.3 The structural complexity of xLi2M n03·(1 - x) LiM02
`electrodes
`
`The powder X-ray diffraction patterns of two x Li2Mn0y
`io.s (x = 0, 0.3,
`(I - x) LiM02 systems in which M = Mn0.5
`0.4, 0.5) and M = Mno.m
`i0.mCoo.33J (x = 0, 0.3. 0 .5 and 0.7)
`a re shown in Fig. 6a and b, respectively. Because of the
`remarkable structural compatibility of layered Li2Mn03,
`LiMn0.5
`i0.50 2 and LiMn0.333
`i0.333Co0.3330 2 compounds,
`all the strong diffraction peaks can be indexed to a pseudo(cid:173)
`trigonal unit cell with R3m symmetry.40 Several weak peaks
`between 21 and 2SO 20 (circled in 1::-ig. 6a a nd b), which cannot
`be indexed to R3m symmetry, are consistent with the LiMn6
`cation ordering that occurs in the transition metal layers of
`Li2Mn03; they can be indexed to the monoclinic unit cell,
`C2/m, that characterizes Li 2Mn03•36 The magnitude of these
`the value of x
`weak peaks
`increases with
`for both
`i0.50 2 and x Li2Mn03·(1 - x)
`x Li2Mn0y {l -
`x)LiMn 05
`LiMno.JJJ
`io.333Coo.3330 2 systems, consistent with the increas(cid:173)
`ing Li2 Mn0rlike character within the structures.
`For low values of x in xLi2M n0y (l - x)LiM02 com(cid:173)
`pounds, i. e. , when small amounts of excess lithium are used,
`the Li2Mn0r like features are sometimes difficult to discern
`in the X-ray diffraction patterns, parlicularly when cobalt is
`present, as in the xLi2MnOJ"( l - x)LiMno.333
`i0.333Coo.3330 2
`system. In these instances, other analytical techniques, such as
`convergent beam electron diffraction (CBED) 53 and MAS
`MR spectroscopy that probes local atomic environments54
`·55
`have to be used. Differences in the X-ray diffraction patterns
`
`i0.50 2 and xLi2Mn03"(1 - x )
`of x Li2Mn03"( I - x)LiMn0.5
`LiMno.333Ni0.333Coo.3330 2 systems were recently highlighted in
`i- 0 and
`a comparative study of layered, lithium-rich Li- Mn-
`Li- Mn-
`i- Co-0 electrodes, in which the alternative nota(cid:173)
`tions
`Lil +x(Mno.s
`io.s)l - x0 2
`ru1d
`Lil +x(Mno.333
`io.333-
`Co0.333)1 - x0 2 were used to define the amount of excess
`lithium
`in the electrode structures, fo r example, Li~.024-
`(0.05Li2M n03 ·0.95LiM n0.298-
`[M no.333
`io.mCoo.mlo.9760 2
`io.Js 1 Coo.Js l 0 2)
`and
`Li J.04s[Mno.333
`io.333Coo.JJJ]o.9s202
`40 even at these
`(O.IOLi2Mn03·0.90LiMno.2s6Nio.m Coo.m0 2) ;
`low levels of excess lithium, evidence of Li2Mn03-likc regions
`could be detected unequivocally by MAS NMR.
`The complexity of structurally integrated xLi2 Mn03·(1 - x )
`LiM02 compounds (M = Mn ,
`\ Co) manifests itself in
`pronounced differences in the peak intensities of the weak
`ordering
`reflections
`in
`the XRD patterns of Li1+x(cid:173)
`(Mno.sNio.s) 1- _,.0 2
`and Li 1 +x(Mno.333N io.333Coo.333) 1- .,.0 2
`compounds as shown in Fig. 7a and b, respectively. 40 I n
`io.5) 1-x0 2 the ordering peaks increase with x ,
`Li1 +AMno.s
`consistent with an increasing Li2Mn03 content, whereas for
`io 333Coo_n3)1 - •0 2 system
`the ordering
`the Li1 +,.(Mno 3 33
`peaks are significantly weaker and remain essentially unaltered
`as x increases. The structural complexity and differences in
`cation distributions between these compounds can be better
`understood by considering the schematic illustrations of
`idealized cation ordering arrangements in Li2Mn03 (Fig. 8a),
`LiMno.s
`io.s0 2 (Fig. 8b), LiMno.333
`io.m Coo.m0 2 (Fig. 8d),
`and proposed models for cation arrangements in the highly
`complex composite structures of lithium-rich Li 1 +.,.(Mn0.s-
`io.s)l- x0 2 (Fig. 8c) and Li l+x(Mno.J33Nio.JJ3Coo.33J)I - x0 2
`(Fig. 8e) compounds. Representative X-ray diffraction pat(cid:173)
`terns of each of the compounds are provided in F ig. 8(a)- (e),
`to aid in the discussion that follows.
`
`a) Li2Mn03• Fig. Sa shows a slice of the manganese-rich
`layer of the Li2Mn03 rock salt structure (C2/m symmetry)
`
`:J
`
`=-·c:
`~ co
`.!=
`:.0
`.....
`~
`
`>. -·u;
`c
`2 c
`
`x=0.7
`
`x=0.5
`
`-..... ·c:
`
`:J
`
`~ co
`.!=
`:.0
`.....
`~
`>.
`:!:::!
`(/) c
`2 c
`
`20
`
`40
`29
`
`60
`
`80
`
`20
`
`60
`
`80
`
`40
`29
`
`Fig. 6 X-Ray difTraction patterns of (a) xLi2Mn03·(1
`io.mCo0 .3330 2 for x = 0, 0.3. 0.5, 0.7.
`LiMno.m
`
`-
`
`x)LiMno.s i0.50 2 for x = 0, 0.3. 0.4, 0.5; a nd (b) x Li2Mn03·(1
`
`-
`
`x)
`
`This journal is © The Royal Society of Chemistry 2007
`
`J. Mater. Chem., 2007, 17,3112-3125 I 3117
`
`Page 7 of 15
`
`Page 7 of 15
`
`

`
`N
`
`J:
`
`"" 0
`>. on a N
`"""
`2 N
`., r-
`~ 0
`"'- Cll
`co 0;
`t:: "" 0
`0
`~~
`0 ·o e "0
`.B E!l
`j
`0
`t.i
`-;;
`t:: ~
`.~ ""
`;:;; or>
`:::>
`z Q.
`::::: a.
`~
`±:::
`0
`., t::
`§ ..r:.
`,..
`0
`:.:::; r-
`8 8
`~ :r:
`:::
`0..
`<(
`j
`0
`>,N
`~ t::
`0
`13
`..,
`"0
`"' ~
`.2 = :0
`:::
`:::>
`0 c..
`0
`
`N
`
`t::
`
`"0
`
`I
`-~ t
`I
`
`'4!f;
`
`~~~
`
`lJ
`
`11UI'GHI
`
`I
`!Nf\• 1
`I
`.. ~ l.
`20
`40
`
`30
`
`20
`
`30
`
`40
`
`50
`29
`
`60
`
`70
`
`80
`
`x=O
`
`l u
`x=0.024
`
`ll
`,
`x=0.048
`
`l
`
`), J!_l
`x=0.143
`
`u
`70
`
`60
`
`80
`
`50
`29
`
`io.s)J- ,0 2 for x = 0, 0.05. 0.10 and 0.15; and (b) Li 1+x(Mno.JJJ
`Fig. 7 X-Ray difTraction patterns of(a) Li1 +lMno.s
`0.024, 0.048 and 0. 143 [after ref. 40]. Figures reproduced from ref. 40 with permission from Elsevier.
`
`i,1.m Coo.m)J - x0 2 for x = 0.
`
`which contains 67% Mn and 33% Li; the manganese and
`lithium ions occupy all the octahedral sites of the layer; the
`oxygen ions at the apices of the octahedra are represented by
`small dots. The lithium layers are located immediately