Solid State lonics 57 (1992) 311-318
`North-Holland
`
`SOLID
`STAn
`IOIICS
`
`Structure and electrochemistry ofLixMnyNi1_y02
`
`E. Rossen, C.D.W. Jones and J.R. Dahn
`Department of Physics, Simon Fraser University, Burnaby, B.C., Canada V5A JS6
`
`Received 16 March 1992; accepted for publication 6 April 1992
`
`We show that the solid-solution series Li_.Mn,.Ni 1_y02 can be prepared as a single phase for x near 1 and OsysO.S. Using
`powder X-ray diffraction and Rietveld proflle refinement, we show that these solid solutions are isostructural with LiNi02 but
`with Mn and Ni sharing the Ni sites. However, subtle changes in the diffraction profiles show that as y increases some mixing of
`Li atoms into Ni, Mn sites and Ni, Mn atoms into Li sites occurs. In addition, the ratio ofLi atoms toNi, Mn atoms in the material
`increases with y at fixed x, suggesting the formation of Mn4+ as y increases. The cation mixing and the presence of Mn4+ pro(cid:173)
`foundly affects the electrochemical deintercalation of Li from Li..Mn,.Ni1_y02• Electrochemical measurements using Li/
`LixMn,.Ni 1_y02 cells show that as y increases the amount ofLi which can be reversibly cycled in these materials decreases, thus
`limiting the practical usefulness of these materials.
`
`I. Introduction
`
`LiNi02 has been used as a cathode material in sec(cid:173)
`ondary Li batteries [ 1,2] and in rechargeable Li(cid:173)
`Ni02/ carbon cells [ 3]. Solid solutions of Li(cid:173)
`CoyNi1_y02 [ 4,5] have also been prepared and used
`as electrodes in such cells, showing advantages over
`the pure end members LiCo02 and LiNi02. For
`y=0.5 in LiCoyNi1_y02 the compound is easier to
`prepare than LiNi02 but has almost the same voltage
`profile [ 4]. The voltage of Li/LiNi02 cells is about
`0.25 V lower than Li/LiCo02 cells making the for(cid:173)
`mer less prone to electrolyte oxidation problems at
`high temperature ( 6]. In addition, Ni is less expen(cid:173)
`sive than cobalt and is more abundant, so Li(cid:173)
`NiyCo1_y02 may be more useful than LiCo02. Ex(cid:173)
`tending this economic logic suggests that the solid
`solutions LixMnyNi1_y02 with x near 1 should be in(cid:173)
`vestigated since the cost of manganese is roughly one
`tenth that of nickel. An extensive but poorly de(cid:173)
`scribed study of LixCo1_yMy02 materials (M=W,
`Mn, Ta, Ti, Nb) has been published in the patent
`literature [ 7 ] suggesting such approaches are indus(cid:173)
`trially relevant.
`LiMn02 apparently cannot be prepared with the
`LiNi02 structure shown in fig. 1. Instead, it forms an
`orthorhombic structure where the Li and Mn atoms
`
`• Li
`e Ni or Mn
`0 0
`
`Fig. l. A portion ofthe unit cell of LiMn,.Ni 1_y02 showing the
`predominantly Li-filled layers and the predominantly Ni, Mn(cid:173)
`filled layers.
`
`do not separate into clear layers as do the Li and Ni
`in LiNi02 [8,9]. This suggests that any attempt to
`prepare LixMnyNi1_y02 solid solutions with the
`LiNi02 structure will have difficulty when y is near
`1. Li2Mn03 bas a monoclinic structure [ 10] which
`is related to the LiNi02 structure. If the stoichio(cid:173)
`metry of Li2Mn03 is written as Li(Li, 13Mn213 )02
`then the composition of the cation layers in this
`compound is made clear. There are pure Li layers
`
`0167-2738/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
`
`SONY EXHIBIT 1011
`
`Page 1 of 8
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`

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`312
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`E. Rossen eta/. I Structure and electrochemistry of LixMnyNi 1_y02
`
`and layers of composition Li113Mn213• This shows
`that Mn atoms can be incorporated into a layered
`oxide with a structure similar to LiNi02. Finally, the
`ionic radii of Mn3+ and NP+ in low-spin states are
`within 0.02 A [ 11 ] . These facts convinced us that it
`should be possible to prepare LixMnyNi 1_ y02 solid
`solutions with the LiNi02 structure for x near 1 and
`at least for small y. Fig. 1 shows a portion of the unit
`cell of this solid solution. To our knowledge such solid
`solutions have not been previously reported.
`Each of the materials described above is based on
`a structure containing layers of close-packed oxygen
`atoms. In such materials, there is usually one metal
`atom per oxygen atom before Li is deintercalated
`electrochemically.
`The
`solid-solution
`series
`LizNi2-z02 has been extensively studied and main(cid:173)
`tains one oxygen per metal for 0 < z < I [ 12, 13 ] . As
`z decreases from I, Ni atoms occupy sites in the pre(cid:173)
`dominantly Li filled layers. Disorder of this nature
`is known to degrade battery performance [ 2]. There
`has only been one report ofLizNi2_z02 prepared with
`z> I [ 14] where Li atoms must move into the Ni
`layers although in other studies material with z sub(cid:173)
`stantially greater than 1 could not be prepared
`[ 12, 13]. Therefore, to prepare good LiNi02, it is
`common to react with an excess of Li salt since these
`are somewhat volatile; if the heating during reaction
`is carried out under oxygen some LhO then coexists
`with the LiNi02 product. Since Li2Mn03 has Li in
`the Mn-containing layer, one might expect Li in the
`Ni, Mn layer of LixMnyNi 1_y02 materials for y#O
`and single-phase materials with x> 1. We still be(cid:173)
`lieve that the number of cations will equal the num(cid:173)
`ber of oxygen atoms in these materials prepared at
`high
`temperature, so when we
`refer
`to Lix(cid:173)
`MnyNi1_y02 with x near 1 we actually believe the
`stoichiometry is LixMnyNi1_y01 +x• but use the for(cid:173)
`mer notation for convenience.
`
`2. Experimental
`
`In our first attempts to prepare LixMnyNi 1_y02,
`stoichiometric mixtures of LiOH · H20, NiO or
`Ni(OHh and Mn02 were reacted at temperatures
`between 500 and 900 o C in air and in oxygen. Re(cid:173)
`peated grindings and firings were made in attempts
`to prepare single-phase materials. These were un-
`
`successful and two-phase mixtures of Li2Mn03 and
`LizNi2_z02 resulted.
`In order to prepare single-phase materials it was
`necessary to use MnO as the source of manganese.
`To make single-phase products, stoichiometric mix(cid:173)
`tures ofLiOH· H20 (99.5%, Lithium Corporation of
`America), NiO (99.5%, Inco) and MnO (99.5%,
`Johnson-Matthey) were ground in a motorized ag(cid:173)
`ate mortar and pestle (Retsch RM-0) for 10 to 20
`min. The mixtures were then heated under extra dry
`air for 4 to 16 h in alumina boats. The final products
`were uniform in appearance, were black and hard and
`usually had air bubbles throughout them caused by
`the decomposition of the LiOH · H20. The Lix(cid:173)
`MnyNi 1_ y02 was then ground again until the powder
`could pass through an 80 mesh screen ( 180 f.l.m).
`Powder X-ray diffraction measurements were
`made using a Philips diffractometer equipped with
`a diffracted beam monochromator and Cu Ka ra(cid:173)
`diation. We used Hill and Howard's version of the
`Rietveld program [ 15 ] to analyze the powder dif(cid:173)
`fraction profiles. Lattice constants were determined
`using the Rietveld program and also by direct least(cid:173)
`squares refinement (using a program called HEX(cid:173)
`OFF) to the observed peak positions including are(cid:173)
`fined off-axis correction for sample position in the
`beam [ 16] . It is our experience that the latter method
`gives more reliable lattice constants since the Riet(cid:173)
`veld program does not correct for off-axis displace(cid:173)
`ments. Nevertheless, the two methods agree well for
`the samples measured here.
`Hermetically sealed coin cells similar to those de(cid:173)
`scribed in ref. [ 2 ] were used to test the electrochem(cid:173)
`ical properties of these materials. Electrodes from
`LixMnvNi1_y02 with 10% by weight of Super-S car(cid:173)
`bon black (Chemetals Inc., Baltimore, MD, USA)
`were made as described Qefore [ 2] . Typical elec(cid:173)
`trodes had a coverage of 20 mg/cm2 and a thickness
`of 120 IJ.m. The electrodes were 1.2 em X 1.2 em and
`had active masses near 28 mg. Coin cells were con(cid:173)
`structed using these electrodes, lithium foil (Lith(cid:173)
`ium Corporation of America), porous polypropyl(cid:173)
`ene separators ( Celgard 2502) wetted with I M
`LiC104 dissolved in equal volumes of propylene car(cid:173)
`bonate and ethylene carbonate. All cell assembly was
`carried out in an argon-filled dry box with moisture
`and oxygen levels near I ppm.
`Cells were charged and discharged using constant-
`
`Page 2 of 8
`
`

`
`E. Rossen eta/. I Structure and electrochemistry ofLixMnyNil-y02
`
`313
`
`current cyclers with ± 1% current stability. The cells
`were thermostated at 30 ± 1 o C. Data were logged
`whenever the cell voltage changed by more than 0.002
`V. The changes, Ax, in Liu-a.-Mn,Ni 1_y02 were
`calculated from the cathode mass, the constant cur(cid:173)
`rent and the duration of current flow.
`
`3. Structural results
`
`Fig. 2 shows the plane in the Li-Ni-Mn-0 qua(cid:173)
`ternary phase diagram where the number of cations
`equals the number of anions. This pseudotemary
`phase diagram shows the locations of LiNi02,
`LiMn02 and LizMn03 as well as the stoichiometries
`of the reactants we mixed to prepare predominantly
`single-phase samples. The line in fig. 2 running from
`near LiNi02 to near LiMn02 (labelled c) corre(cid:173)
`sponds
`to
`the samples LiuMn,Ni 1_y02 with
`O~y~0.6. (Recall that these samples should be plot(cid:173)
`ted on the diagram as LiuMn,Ni 1_y02. 1 and this is
`what we have done.) The samples on the line la(cid:173)
`belled a correspond to the series LixMno.6Nio.402 with
`0.9~x~ 1.4 and those on the line labelled b corre(cid:173)
`spond to the series LixMno.3Nio.702 with 0.9 ~x~ 1.2.
`Fig. 3. shows the results of Rietveld refinement for
`the sample LiuMno.3Ni0.702 based on the LiNi02
`structure. The refinement fits well (R8 = 3.2 where
`Rs= 100L (I lobs -/ca~cl /lobs)) and shows that there
`is little impurity in the sample. Miller indices for all
`
`b
`
`----
`
`---
`
`Fig. 2. The location on the Gibbs triangle of the LixMnyNi 1_y02
`samples synthesized based on the stoichiometry of the mixed
`reactants. The lines labeled a, b and c are described in the text.
`
`1103
`
`104
`:
`
`LiuMno.aNiD.?Oa
`Ra=3.23
`
`101
`
`018.
`
`·~ J
`=·
`r--.J'- L or A ~X
`
`10
`
`20
`
`...
`j
`60
`70
`60
`50
`40
`30
`SCATTERING ANGLE (DEGREES)
`
`90
`
`Fig. 3. Rietveld proflle refinement of powder X-ray data for
`LiuMDo.3Nio.102. The points are the data and the solid line is
`the calculated profile. The Bragg R-factor, R8 , is defined in the
`text. Miller indices of the Bragg peaks are indicated next to each
`one.
`
`reflections below 20= 70° are labelled in the figure.
`Refinement parameters included the lattice con(cid:173)
`stants, the scale factor, the scattering angle zero(cid:173)
`point, three halfwidth parameters, an overall iso(cid:173)
`tropic temperature factor, the oxygen z coordinate
`(the only positional parameter which can be freely
`varied in this space group) and four parameters
`which characterize the smoothly varying back(cid:173)
`ground. The Mn: Ni ratio in the sample was fixed to
`be that in the starting mixture and an "average" Ni,
`Mn atom was used to represent the heavy cations;
`for this sample the average atom was made up of 70%
`Ni and 30% Mn. The refinement was constrained so
`that there was always one metal atom per oxygen
`atom, but the occupancies of Li and Ni, Mn on each
`cation site were allowed to vary such that their sum
`equalled I. Although the scattering power of Li is very
`small compared to Ni and Mn, using the assumption
`that there is one cation per anion we remain sensi(cid:173)
`tive to the presence of Li mainly due to an absence
`ofNi and Mn. Using this method, we are able to de(cid:173)
`termine the extent of the cation mixing which occurs
`in these samples. With reference to fig. 1, we can de(cid:173)
`termine what fraction (on average) of the sites in
`the predominantly Li-filled layer are occupied by Ni,
`Mn atoms and what fraction of sites in the Ni, Mn
`layer are filled by Li atoms and hence estimate the
`sample stoichiometry.
`
`Page 3 of 8
`
`

`
`314
`
`E. Rossen et at. I Structure and electrochemistry of LixMnyNi 1_ y02
`
`Fig. 4 shows diffraction data measured for the
`LiuMn,Nit-y02 series and fig. 5 shows data for
`LixMI1o.JNio.102 series. These data show that pre(cid:173)
`dominantly single-phase samples are obtained at each
`of these compositions. Each of these data sets has
`been analyzed using Rietveld refinement as de(cid:173)
`scribed above yielding an average Bragg R-factor, for
`these eight samples, of 2.8. There are two weak im(cid:173)
`purity peaks which deserve mention: ( 1 ) the small
`peak near 15° for Y=0.4 and y=0.5 in fig. 4 is due
`to LiMn02, and (2) the shoulder on the low-angle
`side of the ( 104) peak, present for many of the sam(cid:173)
`ples in fig. 4, is due to LiMn20 4• The weak broad peak
`near 21 o in most of the samples with y~0.3 coin(cid:173)
`cides with the position of a strong peak from LhC03
`but other strong peaks from that compound are
`missing, suggesting it is absent. It is more likely that
`this peak arises from partial ordering of the Ni, Mn
`and Li atoms within predominantly Ni, Mn filled
`layers on a 31 12 X 3112 in-plane superlattice as is found
`in the Li113Mn213 layers of Li2Mn03• The peak po(cid:173)
`sition falls exactly where expected for such a super(cid:173)
`lattice; presumably the peak is broad because only
`short-ranged correlations are present. The samples
`for Y=0.6 in LixMn,Ni1_y02 (line a in fig. 2) showed
`
`Li1.1Mn,.Ni(1-y)Oa
`
`L l
`
`1 1
`
`1
`
`y=O.l
`
`.A.
`
`..
`
`y=0.2
`
`1 .A
`
`y=0.4
`
`A
`
`y=0.5
`
`~ 1
`10 20 30 40 50 60 70 80 90
`SCATTERING ANGLE (DEGREES)
`Fig. 4. PowderX-rayprofll.esfor Li~.~Mn,.Ni 1 _y02 for0.1 ~y~O.S.
`
`l_l
`
`Li,.Mno.aNio . .,Oa
`
`h ..A.
`
`x=0.9
`A ll
`
`x=l.O
`
`1 _j,_
`
`_A
`
`1 _A
`
`x=l.l
`A A
`
`.A
`
`x=1.2
`
`L _j,_
`_All
`10 20 30 40 50 60 70 80 90
`SCATTERING ANGLE (DEGREES)
`
`Fig. 5. Powder X-ray profi.J.es for LixM11o.3Nio.,02 for 0.9 ~x~ 1.2.
`
`than the
`more impurities (especially LhMn03 )
`samples described by figs. 4 and 5 so we are uncer(cid:173)
`tain whether they can be made single phase or not.
`As a result, these samples were not analyzed further.
`Figs. 6 and 7 show the lattice constants and the
`volume of the rhombohedral primitive cell ( 1 I 3 the
`volume of
`the
`hexagonal
`cell)
`for
`the
`LiuMn,Ni1_y02 series and the LixMn0.3Nio.702 se(cid:173)
`ries respectively. There are only minor changes with
`yin fig. 6, presumably because Ni3+ and Mn3+ have
`similar size. In fig. 7, the lattice is larger for small x,
`just as it is in LizNi2-z02 for small z [ 12,131 indi(cid:173)
`cating that there is some Ni, Mn in the predomi(cid:173)
`nantly Li-filled layer for the x=0.9 sample. The vol(cid:173)
`ume of the rhombohedral cell of LiNi02 is 33.9 A3
`[ 121 suggesting that Li is occupying sites in the Ni,
`Mn layer for x= 1.1 and 1.2 since they have smaller
`cell volumes.
`We have shown earlier that the ratio of the inten(cid:173)
`sity of the ( 0 12, 006) overlapping Bragg peak near
`20=38° to the intensity of the (101) peak near
`20=36.5° is a sensitive measure of the cation mix(cid:173)
`ing in LizNi2_z02 [2]. In those materials, little oc(cid:173)
`cupation ofthe Ni layer by Li occurs for z> 0.9, and
`the main effect is an occupation of the Li-filled layer
`
`Page 4 of 8
`
`

`
`E. Rossen eta/. I Structure and electrochemistry of LixMnyNi 1-y02
`
`315
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`~~ 0.05
`0.04
`iS
`0.16
`0.14
`;:3~ 0.12
`Zc 0.10
`0....:1 0.08
`~~ 0.06
`::s.l=l 0.04
`&::::11 0.02
`6
`0.000
`.0 0.1 0.2 0.3 0.4 0.5 0.
`y in Li1.1Mn,Ni(1-:v)Oa
`
`..
`
`0.51
`~0.50
`..........
`)0.49
`..
`+ 0.48
`,_$0.47 -0.46
`
`~
`:i~ 0.07
`~:s 0.06
`
`2.878
`2.876
`2.874
`
`a 2.872
`
`Cl 2.870
`2.868
`2.866
`14.22
`14.21
`
`CJ 14.19
`14.18
`14.17
`
`a 14.20
`-=< - 33.9
`~ 33.8
`g 33.7
`
`.o
`
`•
`•
`
`I
`
`•
`•
`
`• • •
`
`•
`•
`
`• •
`
`I
`
`•
`•
`
`•
`•
`
`I
`
`I
`
`•
`•
`
`I
`
`•
`•
`• HIXOW
`• RII'l'VILD
`o.5
`0.3
`0.2
`0. 6
`0.4
`0.1
`y in Liu:Yn,Ni(1..,.)0a
`
`Fig. 6. Lattice constants and rhombohedral unit-cell volume of
`LiuMn,.Ni,_y02 versus y. (a) a-axis, (b) c-axis, (c) volume.
`
`2.882
`
`2.878
`
`8:2.874
`
`Cl 2.870
`
`2.868
`14.23
`14.22
`.;< 1u1
`-14.20
`CJ 14.19
`14.18
`14.17
`
`-=< 34.1
`......... 34-.0
`£:'! 33.9
`s 33.8
`8 33.7
`> 33.8
`33.5
`0.8
`
`•
`•
`
`•
`•
`
`•
`•
`
`•
`
`•
`
`•
`
`•
`•
`
`•
`•
`
`I
`
`•
`•
`
`•
`•
`
`• •
`
`• HIXOW
`• RII'l'VILD
`1.2
`1.3
`1.0
`0.9
`1.1
`x in Li,.llno.aNio . .,Oa
`
`Fig. 7. Lattice constants and rhombohedral unit cell volume of
`LixMno.3Nio.,02 versusx. (a) a-axis, (b) c-axis, (c) volume.
`
`by Ni causing a strong dependence of the intensity
`ratio on z. Fig. Sa shows the variation of this inten(cid:173)
`ratio with y
`sity
`in Li~..MnyNi 1 _y02. In
`LiuMnyNi 1_y02, the intensity ratio does not vary
`
`Fig. 8. (a) (012+006)/(101) Bragg peak intensity ratio, (b)
`fraction of sites in the Li layers filled by Mn, Ni and (c) fraction
`of sites in the Mn, Ni layers filled by Li, all plotted versus y in
`Li~.,Mn,.Ni1 _y02• The standard deviations in the site occupa(cid:173)
`tions are ± 0.01 as determined by Rietveld analysis.
`
`strongly withy as expected if cation mixing does not
`occur but the results of the Rietveld refinements,
`shown in fig. 8b and 8c, show that cation mixing is
`nevertheless occuring. In fig. 9, we have replotted the
`data in figs. 8b and 8c to show how the amount of
`Ni, Mn in the Li layer is correlated to the amount of
`Li in the Ni, Mn layer. We have also included the(cid:173)
`oretical calculations of the cation mixing needed to
`give constant ( 012, 006) I (1 01 ) intensity ratios of
`0.45 and 0.5. These results show that if the cation
`mixing varies appropriately then the intensity ratio
`can be approximately constant.
`Fig. 1 Oa shows the ( 0 12, 006) I ( 1 01 ) intensity ra(cid:173)
`tio versus x in LixMn0.3Nio. 102 and figs. 1 Ob and 1 Oc
`show the fraction of sites in the Li layer occupied by
`Ni, Mn and the fraction of sites in the Ni, Mn layer
`occupied by Li respectively. When xis 0.9, consid(cid:173)
`erable Ni, Mn is found in the Li layers as expected
`based on our earlier experience with LizNi2_z02. As
`x increases toward 1.2, the amount of Ni, Mn in the
`Li layer is reduced, and excess Li appears in the Ni,
`Mn layer.
`
`Page 5 of 8
`
`

`
`316
`
`E. Rossen et at. I Structure and electrochemistry ofLixMnyNiJ_y02
`
`~0.10
`j
`~0.08
`z
`-0.06
`:i .....
`Zo.04
`z
`0 e:: uo.o2
`~
`
`POINTS - EXPERIMENT
`IJNE - THEORY
`
`•
`0.00 +----.--,---.----.---.--,...----,.--1
`0. 0
`0. 5
`0.10
`0.15
`0.20
`FRACTION Li IN Ni,Mn LAYER
`
`Fig. 9. Comparing the fraction of sites in the Li layer filled by Ni,
`Mn with the fraction of the sites in the Ni, Mn layer filled by Li
`for the series LiuMnyNi1_y02. The lines in the figure indicate
`trajectories of constant ( 012 + 006) I ( 101 ) Bragg peak intensity
`ratio, R. The squares are for LiNi02 from ref. [ 13).
`
`0.70
`§o.65
`'-..-..0.60
`jo.55
`t,o.5o
`
`..§o.45 -0.40
`
`~ 0.10
`~5 0.09
`:z:!i=; 0.08
`§:5 0.07
`::5;::!1 0.06
`l:!i 0.05
`0.04
`i!i
`0.18
`;::!15 0.16
`:z:5 0.14
`O,.;r 0.12
`~~ 0.10
`I::Z: 0.08
`0.06
`0.8
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`
`•
`1.3
`1.2
`1.1
`1.0
`0.9
`x in Li~:Mno.sNio.70a
`
`Fig. 10. (a) (012+006)/(101) Bragg peak intensity ratio, (b)
`fraction of sites in the Li layers filled by Mn, Ni and (c) fraction
`of sites in the Mn, Ni layers filled by Li, all plotted versus x in
`LixMDo.3Nio. ,02. The standard deviations in the site occupations
`are ± 0.01 as determined by Rietveld analysis.
`
`LiMn02
`
`/
`
`/
`
`/
`
`/
`
`/
`
`\
`111._0
`
`LiNiO
`
`Fig. 11. A portion of the Gibbs triangle showing the planned sto(cid:173)
`ichiometries of our samples (solid symbols) and the stoichiom(cid:173)
`etries determined by Rietveld refinement (open symbols).
`
`Based on our Rietveld refmements it is possible to
`measure the stoichiometries of all the prepared sam(cid:173)
`ples. These are shown in fig. 11, where the solid sym(cid:173)
`bols are the stoichiometries of our samples based on
`the mole ratios of the reactants and the correspond(cid:173)
`ing open symbols are the stoichiometries predicted
`by the Rietveld program. On the line joining LiNi02
`and LiMn02 the Mn in these materials is presum(cid:173)
`ably found as MnH, but to the left of this line, some
`Mn4 + (as in LhMn03 ) must be incorporated into
`the compound assuming equal numbers of oxygen
`and metal atoms in these structures. The results in
`this figure show that there is probably substantial
`Mn4 + in many of the samples, especially those with
`large x and/ or y.
`
`4. Electrochemical results
`
`Fig. 12 shows the cell voltage plotted versus !lx for
`the first recharge and subsequent discharge of Li/
`Liu-AxMnyNi 1_y02 cells for four values of y. The
`cells were cycled using currents which corresponded
`to a change llx= 1 in 100 h. Fig. 13 shows corre(cid:173)
`sponding results for Li/Lix_;uMno.JNio.,02 cells for
`the four samples studied. Although only the first
`cycles are shown in figs. 12 and 13, the cells cycled
`reversibly for many subsequent cycles with the same
`capacity as the discharge shown. The difference in
`capacity between the first charge and the subsequent
`
`Page 6 of 8
`
`

`
`E. Rossen eta/. I Structure and electrochemistry of LixMnyNi 1_ y02
`
`317
`
`cycles is usually observed for LiNi02, except when
`the powder is flrst rinsed in water [ 2]. The irrever(cid:173)
`sible capacity amounts to about Ax= 0.1 for the sam(cid:173)
`ples studied here which is typical of good LiNi02.
`Figs. 14 and 15 summarize the cell capacity be(cid:173)
`tween 2.0 V and 4.2 V as a function of y and x re(cid:173)
`spectively. The results for two cells of each compo(cid:173)
`sition are included. The data in flg. 14 extrapolate
`well to capacities obtained for good LiNi02. As man-
`
`0.70 : r - - - - - - - - - - - - - - - - ,
`
`•
`•
`•
`•
`
`•
`•
`
`I
`
`0.60
`
`]'
`
`.......... 0.50
`
`~
`~ 0.40
`ea u
`
`0.30
`
`•
`•
`•
`•
`
`•
`
`•
`
`•
`•
`
`I
`
`• FIRST CHARGE TO 4.2 V
`• SUBSEQUENT DISCHARGE TO 3.0V
`0.20 +-----,.------,.---.,.-----.---.-----l
`0.1
`0.2
`0.3
`0.4
`0.5
`0.6
`0.0
`y in LiuMn,Ni(l-:r)Oa
`
`for Li/
`Fig. 14. Capacity between 3.0 and 4.2 V
`Lit.t-AxMn,.Ni 1_y02 cells versus y. The circles were measured
`during the first charge and the squares during the subsequent
`discharge.
`
`4.2
`3.8
`3.4
`3.0
`4.2
`
`t
`
`y=0.1
`
`y=0.2
`
`-3.8
`.e::-3.4
`~
`3.0
`4.2
`~ ...:I
`3.8
`~
`3.4
`3.0 +--r-1-......-.--...-........ --...-................ ,.....,..-.--r-i
`4.2
`3.8
`3.4
`3.0 +--r--r"T-.--...-,..,.-,-T""T-.-..-r-r-r-i
`0.0 0.1 0.2 0.3 0.4 0. 0.6 0. 0.8
`ax in Li(1.1-u)Mn,Ni(1-:r)Oa
`
`y=0.4
`
`y=0.5
`
`Fig. 12. Voltage versus composition for Li/Lii.I-AxMn,.Ni1_y02
`cells for four values of y. The cells were charged and discharged
`using currents corresponding to a change dx= 1 in 100 h.
`
`3.0
`
`x=0.9
`
`2.0 hr-.-"r-..r-.-..,......,--.-......-. ....... ........,.--.--r---l
`4.0 1---~=------
`>3.0
`..........
`
`x=l.O
`
`~ 2.0 ~..-+....-........ -.-......... --.-........,. ....... .,............,....-l
`< 4.0 !---~---
`t1
`~ 3.0
`
`x=1.1
`
`2.0 't-o-'1-..,......,--.-..--. ....... ........,.--...-,....,. ........ .--~
`
`4.0 1--=-==----
`
`x=1.2
`
`3.0
`
`2.0 h""--.-,r-.-........... --.-.,.-,--.-.......... --.--r---l
`0.0 0.1 0.2 0.3 0.4 0.5 0.6 o. 0.8
`ax in Li(:r-.u)Mno.sNiuOa
`
`0.60
`
`0.55
`
`.,........
`>< 0.50
`<I
`
`-
`~
`0.45
`~
`ea
`0.40
`u
`
`0.35
`
`0.30
`0.8
`
`•
`•
`
`I
`
`•
`•
`
`•
`•
`
`•
`•
`
`•
`•
`•
`•
`
`•
`•
`• FIRST CHARGE TO 4.2 V
`• SUBSEQUENT DISCHARGE TO 2.0V
`0.9
`1.0
`1.1
`1.2
`1.3
`x in Li,.Mno.sNiuOa
`
`Fig. 13. Voltage versus composition for Li/Lix-~11o.3Nio.702
`cells for four values of x. The cells were charged and discharged
`using currents corresponding to a change dx= 1 in 100 h.
`
`Fig. 15. Capacity between 2.0 and 4.2 V for Li/Lix-~llo.~io.,02
`cells versus x. The circles were measured during the first charge
`and the squares during the subsequent discharge.
`
`Page 7 of 8
`
`

`
`318
`
`E. Rossen et a/. I Structure and electrochemistry of LixMnyNi I- y02
`
`ganese is substituted for nickel, the cell capacity de(cid:173)
`creases (fig. 14) linearly. The structural results in fig.
`8 do show that more Ni, Mn is incorporated in the
`Li layer as y increases, but even for LiuMn0.5Ni0.50 2
`the amount is less than for Lio.9Mno.3Nio.102 (fig. 10)
`and cells of the former have smaller capacity than
`cells of the latter. This suggests that there is another
`mechanism also responsible for the variation of the
`capacity of these materials with x andy. We note that
`fig. 11 shows that the samples with the most Mn4 +
`are those with the largest yin LiuMn,Ni 1_y02. Fig.
`15 shows a maximum in cell capacity near X= 1 in
`LixMn0.3Ni0.70 2; for x< 1 structural results (fig. 10)
`show substantial Ni, Mn in the Li layer, while for
`x> 1.1 substantial Mn4 + is created (fig. 11 ). We
`know that Li cannot be extracted from LhMn03 in
`electrochemical cells even up to 4.5 V versus Li metal.
`This is presumably not for structural reasons for there
`are layers containing only Li atoms in this material.
`More likely, is that the removal ofLi would force the
`oxidation of Mn4 + to Mn5+ which will occur at much
`larger voltages than the oxidation of Mn3+ to Mn4 +
`which takes place near 4 V in Li 1_xMn20 4 [17].
`Therefore, we believe that the capacity variations
`seen in our LixMn,Ni 1_y02 cells as a function of y
`can be explained by the coupled effects of cation
`mixing and the creation of Mn4+.
`
`S. Conclusions
`
`Through careful structural studies we have shown
`that solid solutions of LixMn,Ni1_y02 can be pre(cid:173)
`pared for x near 1 and for 0:Sy:S0.6. The capacity
`of Li/LixMn,Ni,_y02 cells decreases as y increases
`and when x differs substantially from 1. This will
`probably limit the usefulness of these materials as
`cathodes in practical cells. We have shown the im(cid:173)
`portance of careful structural analysis in the under(cid:173)
`standing of the capacity reduction in these materials
`and suggest that similar studies may be needed for
`
`other quaternary or pseudotemary materials pro(cid:173)
`posed as electrodes for secondary Li cells.
`
`Acknowledgement
`
`We thank Moli Energy ( 1990) Ltd. for use of fur(cid:173)
`nace facilities for some aspects of this work. Funding
`from the NSERC strategic grants program is grate(cid:173)
`fully acknowledged. We thank Ulrich von Sacken and
`Jan Reimers for useful discussions.
`
`References
`
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`
`Page 8 of 8