LETTERS TO NATURE
`
`--lllllllllllllllllllll llll li!I I IIIJ:II'
`
`I I I
`
`0.5
`
`1.5
`
`d- spacing (A)
`
`8
`
`6 -
`
`•
`
`FIG. 1 a, Observed (dots) and calculated (solid line) neutron-diffraction
`profiles for monoclinic LiMn02 • The lower plot shows the difference/esti(cid:173)
`mated standard deviation b, Structure of LiMn02 emphasising the layered
`nature of the material. Mn, large dark circles; Li, smaller dark circles;/ 0,
`pale circles.
`
`TABLE 1 Crystallographic parameters of LiMn02
`
`Synthesis of layered LiMn02 as
`an electrode for rechargeable
`lithium batteries
`
`A. Robert Armstrong & Peter G. Bruce
`
`School of Chemistry, University of St Andrews, North Haugh,
`St Andrews, Fife KY16 9ST, UK
`
`REcHARGEABLE lithium batteries can store more than twice as
`much energy per unit weight and volume as other rechargeable
`2
`batteries1
`• They contain lithium ions in an electrolyte, which
`•
`shuttle back and forth between, and are intercalated by, the
`electrode materials. The first commercially successful recharge(cid:173)
`able lithium battery\ introduced by the Sony Corporation in
`1990, consists of a carbon-based negative electrode, layered
`LiCo02 as the positive electrode, and a non-aqueous liquid
`electrolyte. The high cost and toxicity of cobalt compounds,
`however, has prompted a search for alternative materials that
`intercalate lithium ions. One such is LiMn20 4, which has been
`much studied as a positive electrode material4-7
`; the cost of
`manganese is less than 1% of that of cobalt, and it is less toxic.
`Here we report the synthesis and electrochemical performance of
`a new material, layered LiMn021 which Is structurally analogous
`to LiCo02• The charge capacity of LiMn02 (~270mAhg- 1 )
`compares well with that of both LiCo02 and LiMn20 4, and
`preliminary results Indicate good stability over repeated
`charge-discharge cycles.
`Many attempts have been made to prepare layered LiMn02
`mainly involving the use of aqueous solutions8
`, The resulting
`10
`-
`products, though interesting, have stoichiometries which differ
`from LiMn02, contain water or protons, are of poor crystallinity
`or do not maintain their structure during cycling. In contrast we
`have succeeded in preparing layered, anhydrous and stoichio(cid:173)
`metric LiMn02 which is analogous to LiCo02 and may be cycled in
`a rechargeable battery; it is obtained by ion exchange from
`NaMn02• The sodium compound was synthesized by solid-state
`reaction between stoichiometric quantities of Na2C03 and man(cid:173)
`ganese (m) oxide at 700-730 oc for 18-72 hours under flowing
`argon 11
`• LiMn02 was obtained by reftuxing NaMn02 with an
`excess of LiCl or LiBr in n-hexanol at 145-150 oc for 6-8
`hours. After cooling to room temperature the product was filtered
`under suction and washed, first with n-hexanol and then with
`ethanol, and dried 12
`• Phase purity was established by powder X-ray
`diffraction.
`The layered structure of LiMn02 was confirmed by powder
`neutron diffraction carried out on the POLARIS diffractometer
`at the ISIS pulsed source (Rutherford Appleton Laboratory) (Fig.
`1a). The structure was refined by the Rietveld method using the
`program TF12LS based on the Cambridge Crystallographic Sub(cid:173)
`routine Library13
`• Adoption of a layered model yielded a final
`agreement factor, Rweightedprofile• of 2.06% compared with an Rcxpcctcd
`of 0.60%. A common structure obtained when attempts are made
`to prepare compounds with the LiMn02 composition is that of
`tetragonally distorted spinel Li2Mn20 4 (ref. 4). This
`structure was tested in the refinement process; how-
`ever, the fit obtained using the layered model was far
`superior to the fit assuming a tetragonal spinel struc-
`ture for which Rwp = 4. 79%. The layered structure of
`LiMn02 is shown in Fig. lb. The oxide ions are
`arranged in close-packed layers which are stacked in
`an ABC repeat sequence, that is, cubic close packing.
`Manganese ions are located in each octahedral site
`between the first and second oxide layers (Fig. lb).
`Between the second and third layers the Li+ ions
`reside also in octahedral sites. Refinement was carried
`out permitting the u+ and MnJ+ ions to occupy their
`
`Atom
`
`Wyckoff
`symbol
`
`X
`
`z
`
`Site occupancy
`
`s ..
`y
`2.4(2)
`Li1/Mn1
`0.91/0.09(4)
`2d
`0.5 0.5
`0
`0.10/0.90(3)
`0.72(6)
`0
`0
`2a
`Li2/Mn2
`0
`01
`0.2723(3) 0
`0.7706(2) 0.68(4) 1
`4i
`Monoclinic, sp~ce group C2/m (po. 12). Unit-cell dimensions: a = 5.4387(7) A,
`b = 2.80857(4) A, c = 5.3878(6) A, p = 116.006(3)", x2 = 11.83. R"~' = 0.60%,
`Rp = 1.86%, R..., = 2.06%, R, = 3.98%.)
`
`NATURE · VOL 381 · 6 JUNE 1996
`
`499
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`SONY EXHIBIT 1016
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`Page 1 of 2
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`

`
`LETTERS TO NATURE
`
`~ 4.0
`
`_ ____. ... -·······-
`
`5.0
`
`~ 4.5
`~ 4.0
`~
`!:! 3.5
`"' ·~
`
`3.0
`
`£
`0 c..
`
`0·0 o,.._.~-t.so_.....-'--';"t*'oo::---~1 5~o~-?2'*o~o ~-::-25~o~'""'J~oo
`Specific capacity (rnA h g - •)
`
`2.5
`
`0
`
`5
`
`tO
`Time (h)
`
`15
`
`20
`
`FIG. 2 Variation of electrode potential with capacity on charging LiMn0 2 at a
`current density of 10 /.lA cm- 2
`•
`
`own sites and those of each other. The Li+ and Mn3+ ion
`occupancies were free to refine on both sites, the only constraint
`being that the total occupancy of each site was set to equal one.
`The total lithium and manganese contents were respectively
`1.02(7) and 0.98(7), consistent with the LiMn02 stoichiometry.
`Atomic absorption analysis was carried out (Unicam PU9400X)
`and provided further confirmation of the stoichiometry. The
`maximum occupancies of Li on the Mn sites (2a) and Mn on the
`Li (2d) sites were respectively 0.10(3) and 0.09(4), demonstrating
`that the lithium and manganese ions reside predominantly on
`their own sites. Due to the strong Jahn-Teller nature of high-spin
`Mn3+(3d4
`), the local site symmetry around the MnJ+ ions is
`distorted somewhat from a regular octahedron and the crystal
`structure is not rhombohedral but monoclinic (space group
`C2/m ). The crystallographic data are given in Table 1.
`Interest in layered LiMn02 as a positive electrode in recharge(cid:173)
`able lithium batteries stems from the fact that u+ ions and
`electrons may be removed and reinserted into this compound,
`that is, it is an intercalation host for lithium. The electrochemical
`performance of LiMn02 was investigated with a three-electrode
`cell composed of lithium metal counter and reference electrodes,
`the working electrode being fabricated by compressing powdered
`LiMn02 (80%), carbon black (13.3%) and PTFE (6.7%) on to a
`metal grid. The electrolyte consisted of a 1M solution of LiCl04
`dissolved in propylene carbonate. The salt was rigorously dried by
`heating under vacuum at 150 °C and the solvent was distilled as
`described elsewhere 14
`• The cell was subjected to charging at a
`current density of 10 11Acm- 2
`• The resulting voltage curve is
`shown in Fig. 2. These data confirm that up to 0.95 lithiums per
`formula unit may be extracted from LiMn02 on initial charging,
`• Preliminary mea(cid:173)
`corresponding to a capacity of 270 rnA h g- 1
`surements at the much higher current density of 0.5mAcm-2
`suggest that a capacity approaching 200 mA h g- 1 may be obtained
`up to a cut-off potential of 4.3 V.
`The earliest lithium batteries, introduced in the 1980s, used
`lithium metal as the negative electrode. Despite some problems
`with flammability, there is still interest in using such cells for
`electric-vehicle applications because of their high gravimetric
`energy density. For such cells cathodes with a potential of 3 V
`are sufficient. When combined with carbon-based anodes in
`'rocking-chair' cells (in which both electrodes form intercalation
`
`FIG. 3 Cycling of the UMn02 electrode at a current density of 0.5 mA cm- 2
`and between voltage limits of 3.4 to 4.3V vs u+;u.
`
`compounds), cathodes with potentials of more than 3 V are
`required; however the move from petroleum coke to graphite
`has resulted in an anode with a potential closer to that of lithium
`metal over a greater range of composition. A practical capacity of
`~ 150 mA h g- 1 is obtainable from the cathode used in the present
`generation of commercially available rechargeable lithium bat(cid:173)
`teries, that is, LiCo02. A slightly smaller practical capacity may be
`obtained at 3 and 4 volts from LiMn,04• The capacities of200 and
`270 mA h g- 1 obtained on intially charging LiMn02 and already
`mentioned above, demonstrate that this compound is both scien(cid:173)
`tifically interesting and potentially attractive as a cathode for
`rechargeable lithium batteries. However a full analysis of the
`utility of this cathode in comparisson with others will require
`extensive measurements of the capacity variation on cycling at
`different current densities and over different voltage ranges. Some
`preliminary cycling data are presented below.
`X-raydiffraction data reveal that for (1 - x) ~ 0.5 in Li1_,Mn02
`the structure is rhombohedral. On removal of lithium from
`LiMn02> the Jahn-Teller active Mn3+ ion is oxidized to Mn4+
`with the resulting loss of the monoclinic distortion . The layered
`compound also exhibits a voltage transition at a composition of
`~Li0jMn02• This is the same composition (LiMn 20 4) at which
`lithium manganese oxide with the spinel structure transforms
`from a 3 - V to 4- V cathode, suggesting that the voltage transition
`is not related specifically to the spinel structure. However, a full
`discussion of these more detailed aspects must await the avail(cid:173)
`ability of further structural data.
`Some preliminary cycling data are presented in Fig. 3. The cell
`was cycled at a constant current density of 0.5 mAcm- 2 between
`the potential limits 3.4 and 4.3 V. Previous studies using cyclic
`voltammetry indicated that the electrolyte was stable in contact
`with this electrode up to at least 4.4 V. X-ray diffraction carried
`out on the layered material at different stages of cycling indicate
`that the layered structure, is retained during lithium removal and
`reinsertion. Although the capacity declines on eyeing, it must be
`stressed that the voltage range has not been opitimized and
`includes both voltage plateaux. In the case of the LiMn20 4
`spinel, cycling over both plateaux results in a greater capacity
`fade than if confined to just one plateau. It should also be recalled
`that the early spinel materials showed very poor cyclability, and
`only later optimization yielded satisfactory results.
`D
`
`Received 14 February; accepted 2 May 1996.
`
`1. Oyama, N., Tatsuma, T., Sate, T. & Sotomura, T. Nature 373, 598-600 (1995).
`2. Scrosati, B. Nature 373,557-558 (1995).
`3. Nagaura, T. 3rd Int. Batte/)' Seminar (Deer1~eld Beach, FL. 1990).
`4. Thackeray, M. M., David, W. 1, F., Bruce, P. G. & Goodenough, J. B. Macer. Res. Bu//18, 461-
`472 (1983).
`5. Huang, H. & Bruce, P. G. J. Power Sources 14, 52-57 (1995).
`6. Tarascon, J.·M. & Guyomard, D. E/eclrochimica Acta 38 , 1221-1231 (1993).
`7. Pistoia, G. & Wang, G. Solid St. /on/cs 68, 135-142 (1993).
`
`8. Rossouw, M. H. , Ules, D. C. & Thackeray, M. M. J. So//d Sr. Chern. 104, 464- 466 (1993).
`9. Thackeray, M. M. J. electrochem. Soc. 142, 2558-2563 (1995).
`10. Leroux, F., Guyomard, D. & Piffard, Y. Solid St. /onlcs 80, 299-306 (1995).
`11. Fuchs, B. & Kemmler·Sack, S. Solid St. /on/cs 88, 279-295 (1994).
`12. Bruce, P. G. & Armstrong, A. R. UK Patent Application January 1996.
`13. Matthewman, J. C., Thompson, P. & Brown, P. J. J. app/. CI}'SCal/ogr. 1&, 167-173 (1982).
`14. Huang, H. & Bruce, P. G. J. e/ectrochem. Soc. 141, L106-L107 (1994).
`
`ACKNOWLEDGEMENTS. We thank H. Huang for her assistance w~h the electrochemical measure·
`ments. P.G. B. is grateful to the EPSRC for financial support.
`
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`NATURE · VOL 381 · 6 JUNE 1996
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