1//llljc/)11 /{!/'-- f ;J f7 v
`~-Cf'-/-!O(o3 --1/
`
`LITIDATEDOXIDESFORLnrnrnrnM~ONBATIERmS
`
`by
`
`M. M. Thackeray
`Argonne National Laboratory
`Electrochemical Technology Program
`Chemical Technology Division
`9700 South Cass Avenue
`Argonne, IL 60439
`
`The submitted manuscrapt has been authored
`by a contractor of the U. S. Government
`under
`contract No. W·31·109-ENG·38.
`Accordingly. the U. S. Government retains a
`nonexclusive. royalty·free license to publish
`or reproduce
`the published
`form of this
`contribution, or allow others to do so, for
`U. S. Government purposes.
`
`October 9-14, 1994
`
`To he presented at The I 86th Meeting, The Electrochemical Society
`Symposium, October 9-14, 1994, Miami Beach, Florida
`
`SONY EXHIBIT 1014
`
`Page 1 of 14
`
`

`
`DISCLAIMER
`
`Portions of this document may be illegible
`in electronic image products. Images are
`produced from the best available original
`document.
`
`Page 2 of 14
`
`

`
`LITHIA TED OXIDES FOR LITHIUM-ION BATTERIES
`by
`M. M. Thackeray
`Argonne National Laboratory
`Electrochemical Technology Program
`Chemical Technology Division
`9700 South Cass Avenue
`Argonne, IL 60439, USA
`
`ABSTRACT
`
`Rechargeable lithium batteries that can be assembled in the discharged state
`with lithiated metal oxide cathodes and carbon anodes are being developed
`to minimize the safety hazards associated with batteries that use pure metallic
`lithium anodes. This paper highlights the progress that has been made to
`develop lithiated transition metal oxide cathodes that have a layered
`structure, L~02, and those that have a spinel-type structure, L~204,
`(M = Co, Ni, Mn, V). Emphasis is placed on the structural properties of
`insertion electrodes that control their stability during electrochemical cycling.
`The use of lithiated spinel oxide instead of lithiated carbon (LiC6) as the
`anode is briefly discussed.
`
`INTRODUCTION
`
`Insertion compounds have been widely studied over the last two decades as
`electrodes for lithium batteries. Early work was carried out on cathode materials such as
`TiS2, W03, V 20 5, and Mn02, which were evaluated in cells with metallic lithium anodes [1].
`in
`Recently, the focus of lithium battery technology has shifted to rechargeable systems -
`particular, 11lithium-ion 11 systems in which lithium ions are shuttled between an anode host
`structure and a cathode host structure during charge and discharge. Much of today's research
`is spent on developing cells with lithium-carbon anodes (LixC6, O<x<1) [2] and high-voltage
`cathodes, such as Li1_xCo02 [3-5], Li1_~i02 [5-7], and Li,Mfl20 4 [8-12] (O<x<l.O). Lithium
`insertion into carbon takes place between 1 and 0 V with respect to a lithium reference
`electrode, whereas lithium extraction from LiCo02, LiNi02, and LiMn20 4 occurs at
`approximately 4 V. In practice, therefore, lithium-ion cells with carbon anodes and these
`high-voltage cathodes charge and discharge between 4 V and 3 V.
`
`The structural stability of a host electrode to the repeated insertion and extraction of
`lithium is undoubtedly one of the key properties for ensuring that a lithium-ion cell operates
`In transition-metal oxides, both stability of the
`with good electrochemical efficiency.
`
`QCUMENT IS UNLIMITED
`t~~Slf.R.
`DISTRIBUTION or THIS o
`(J(
`
`Page 3 of 14
`
`

`
`-, ... /.
`
`oxygen-ion array and minimum displacements of the transition metal cations in the host are
`required to ensure good reversibility. Structures with a cubic-close-packed ( ccp) oxygen
`array are more stable to lithium insertion/extraction than hexagonal-close-packed (hcp)
`structures. For example, the oxygen array in hcp structures such as a-Fe20 3 [13], ~-Mn02
`[14], and Mn02 (ramsdellite) [15] shears toward cubic close packing on lithium insertion
`in response to electrostatic interactions between the incoming lithium ions and transition
`metal cations in face-shared octahedral sites. The shear process is often accompanied by
`displacements of the transition metal cations within the oxygen lattice. These structural
`modifications tend to degrade the integrity of an insertion electrode, particularly when it is
`subjected to repeated charge and discharge.
`
`Structures of transition metal oxide insertion electrodes are, in general, not tolerant
`to overcharge or overdischarge. It is, therefore, critically important to carefully control
`fabrication conditions so that the composition of the electrode corresponds to that of the
`most stable host structure of the metal oxide system. Furthermore, cell operating conditions,
`such as voltage limits and current drain, must be carefully controlled to ensure that the
`structural integrity of the electrodes is not destroyed on cycling.
`
`The results in this paper illustrate the importance of limiting the structural changes
`within an insertion electrode during electrochemical cycling to the absolute minimum. The
`discussion is focused primarily on the advantages of the high-voltage cathodes of current
`interest, namely, LiCo02, LiNi02, and LiMn20 4. Examples of electrodes that, from a
`structural viewpoint, are not tolerant to lithium insertion and extraction are also provided for
`comparison. The use of the low-voltage spinel Li4 Ti50 12, which discharges at approximately
`1.5 V against lithium, as an anode in lithium-ion cells is briefly addressed.
`
`CATHODE MATERIALS
`
`(i)
`
`Layered Structures. LiM02 CM =Co. Ni, Mn. Y).
`
`LiCo02 and LiNi02. The ideal layered LiM02 structure (M =Co, Ni, V) has
`a)
`a close-packed oxygen array which is slightly distorted from ideal cubic close packing
`(Fig. 1). The structure has trigonal (RJm) symmetry; the unit cell parameters are usually
`defined in terms ofthe hexagonal setting. Both LiCo02 and LiNi02 are of particular interest
`for lithium battery applications [3-7]. They are synthesized by solid state reaction of the
`appropriate precursors (for example, CoC03, NiO, and LiOH) at temperatures that typically
`range from 700°C to 850°C. At present, LiCo02 is the material of choice, despite its high
`cost, because it is relatively easy to prepare a high-quality electrode with essentially the ideal
`layered structure. The ideal layered LiNi02 structure is more difficult to synthesize; in this
`case, the product often contains a small proportion of nickel within the lithium layers, which
`can seriously damage electrode performance. An optimized LiNi02 electrode is, however,
`
`-~-~--
`
`Page 4 of 14
`
`

`
`more forgiving than LiCo02 and is more stable to charge, particularly at low lithium
`loadings; rechargeable capacities of more than 150 mAb/g of cathode have been
`reported [5,6].
`
`Fig. 1. Schematic repesentation of layered LiM02 compounds (M = V, Co, Ni).
`M atoms are located within the layers of shaded octahedra;
`Li atoms are in alternate layers of unshaded octahedra.
`
`Both Li1_xCo02 and Li1_x~H02 undergo reversible phase transitions, which have been
`attributed to changes in crystal symmetry from trigonal to monoclinic [6,7]. The transitions
`are attributed to order-disorder phenomena associated with the lithium ions. An important
`feature of these transitions is that they occur with only relatively small changes of the unit
`cell parameters [7]. Because the oxygen array of the Co02 and Ni02 framework structures
`is not substantially distorted by the transitions, and the metal cations of the host do not enter
`the interstitial space of the M02 framework, these two electrode systems exhibit the good
`reversibility. Table 1 shows the change in the lattice parameters of a Li1_~i02 electrode on
`delithiation.
`
`Table 1. Changes in lattice parameters and crystal symmetry of Li1_xNi02
`(after Dahn et al r7l).
`a
`(A)
`
`p
`(0)
`
`Crystal
`Symmetry
`
`xin
`Li1_,.Ni0,
`
`b
`(A)
`
`c
`(A)
`
`0.01
`0.06
`0.11
`0.21
`
`0.21
`0.31
`0.41
`0.51
`
`0.51
`0.61
`0.71
`0.82
`
`0.82
`
`2.883
`2.878
`2.872
`2.869
`
`5.002
`4.996
`4.955
`4.944
`
`2.826
`2.826
`2.823
`2.825
`
`2.821
`
`2.883
`2.878
`2.872
`2.869
`
`2.841
`2.832
`2.831
`2.820
`
`2.826
`2.826
`2.823
`2.825
`
`2.821
`
`14.215
`14.217
`14.234
`14.241
`
`14.303
`14.340
`14.416
`14.469
`
`14.445
`14.397
`14.400
`14.427
`
`14.434
`
`90.45
`90.56
`90.45
`90.45
`
`Trigonal
`
`Monoclinic
`
`Trigonal
`
`Trigonal
`
`:·---~t''l::.
`
`--~J--- .
`
`-
`
`Page 5 of 14
`
`

`
`Note that the b and c parameters of the monoclinic unit cell relate directly to the a and c
`parameters of the trigonal unit cell, and that amonoclinic::::: 2atrigonal Sin 60; all parameters vary
`gradually as lithium is extracted from the structure. The unit cell volume of Li1_x~H02
`decreases marginally, by 2.7%, between x = 0 and x = 0.6, the range over which the
`electrode is usually cycled. The interlayer spacing increases slightly on delithiation, in
`response to a decrease in the binding energy within the lithium-depleted layers, which allows
`rapid lithium-ion diffusion. These layered structures are therefore able to cope well with
`lithium insertion/extraction reactions; structural integrity of the electrodes is maintained
`provided that Li/Li1_xCo02 and Li/Li1_~i02 cells operate between carefully controlled
`compositional (voltage) limits. At low lithium loadings (x<0.5), the high oxidation potential
`of these cathodes threatens their structural stability, and chemical decay (for example, by
`reaction with the organic electrolyte) becomes a factor that can limit the life of the lithium(cid:173)
`ion cells.
`
`LiV02. Unlike LiCo02 and LiNi02, the layered structure of LiV02 is
`b)
`destabilized by lithium extraction; at x:::::0.3 in Li1_x V02, approximately one-third of the
`vanadium ions migrate from the vanadium layer to the lithium-depleted layer [16]. This
`process destroys the two-dimensional space for lithium-ion diffusion. The product has a
`defect rock-salt structure that offers limited electrochemical activity [17]. Reinsertion of
`lithium does not regenerate the layered structure. The structural degradation of the layered
`V02 host is clearly evident from the X-ray diffraction patterns of the original material and
`the delithiated product (Fig. 2). In particular, the significant decrease in the intensity of the
`[003] peak at approximately 18 °26 (which can be used as a yardstick to measure the degree
`of layering within cubic-close-packed oxygen arrays) indicates the movement of vanadium
`ions into adjacent layers.
`
`"LiMnO/!_. A layered LiMn02 compound that is isostructural with LiCo02,
`c)
`LiNi02, and LiV02 has not yet been synthesized. However, a recent report has indicated
`that a layered compound with approximate composition LiMn02 can be prepared, by
`leaching Li20 from the layered rock-salt compound Li2Mn03 (Li20•Mn0~ with acid and,
`thereafter, relithiating the resulting manganese oxide with lithium [18]. This technique
`results in a layered lithium manganese oxide product, Li1.0~n0_91 02, with a structure that
`closely resembles that ofLiCo02 and LiNi02; it has a small amount (9%) of lithium in the
`manganese layers (Fig. 3a). However, unlike LiCo02 and LiNi02, Li1.09Mn0_910 2 is a 3 V,
`not a 4 V, electrode. The oxygen array of this layered structure is unstable to lithium
`extraction; delithiation causes a shearing of the oxygen planes, which keeps the manganese
`ions octahedrally coordinated but places the lithium ions in trigonal prismatic coordination
`(Fig. 3b ). The instability of the oxygen array to lithium insertion/extraction limits the use of
`this compound as a rechargeable electrode in lithium cells; further work is required to
`improve its cycling behavior.
`
`Page 6 of 14
`
`

`
`OOl
`
`....
`
`100:11
`
`...
`
`IOOol
`
`jl041
`.... 04
`
`.•.
`
`Lio.uV02
`... ...
`
`(a)
`
`(b)
`
`..
`
`zo
`
`,.
`40
`TWO-TIIETA
`
`...
`
`oor
`
`tO
`
`II!
`
`... I
`
`ro
`
`ff!oo'l
`
`- · 4
`
`..
`
`Fig. 3. The layered structures of
`a) Li1.09Mn0•91 0 2 and
`b) Lio.36Mno.9t 02
`(taken from reference [18]).
`
`Fig. 2. The X-ray diffraction patterns
`of a) LiV02, b) Li0•5V02, and
`c) Li0•2 V02 showing the large
`decrease in intensity of the [003]
`peak at approximately 18°26 in
`response to a migration of the
`V atoms into the lithium layers
`(taken from reference [21]).
`
`(ii)
`
`Spinel Materials Li[M2J.Q..
`
`The [MiJ04 framework of a Li[Mi!04 spinel is an attractive host structure for lithium
`insertion/extraction reactions because it provides a three-dimensional network of face(cid:173)
`sharing tetrahedra and octahedra for lithium-ion diffusion (Fig. 4). In the [M2]04 spinel
`framework which has cubic symmetry Fd3m, 75% of the metal cations occupy alternate
`layers between the cubic-close-packed oxygen planes; the remaining 25% of the metal
`cations are located in the adjacent layers. Therefore, sufficient M cations exist in every layer
`to provide, on delithiation, a sufficiently high binding energy to maintain an ideal cubic(cid:173)
`close-packed oxygen array. In most cases, spinel structures expand and contract isotropically
`
`Page 7 of 14
`
`

`
`during lithium insertion and extraction. Such cubic electrodes are more stable to
`electrochemical cycling than those in which there is a large anisotropic expansion or
`contraction of the lattice parameters. For example, the spinels Li4M50 12 (or alternatively,
`Li[M1.67Li0.33]04, M = Mn, Ti) are extremely tolerant to cycling because the cubic unit cell
`expands and contracts by less than 1% within controlled compositional limits [19,20]. By
`contrast,
`the orthorhombic unit cell of a Mn02 (ramsdellite) electrode expands
`anisotropically when lithiated to a composition Lio_gMn02; the unit cell parameters a, b and
`c change by+5.7%, +16.5%, and -1.4%, respectively, which represents a 21.5% increase in
`the unit cell volume. In this case, the strain that is imposed on the host structure is far too
`severe for the electrode to survive repeated cycling.
`
`Fig. 4. The [M2]04 framework of a Li[M2]04 spinel.
`
`(a)
`LiMn2~. Of the lithium spinels, the system Li,Mn20 4 is the most attractive
`for lithium-ion batteries; it offers the highest voltage ( 4 V) against lithium and provides a
`stable cubic [Mn:iJ04 framework over the whole 4 V compositional range O<x<1 [8,9]. But
`Lix[Mn2]04 has two main disadvantages. Firstly, it is difficult to extract, electrochemically,
`all the lithium from the structure at practical voltages . This limits the rechargeable capacity
`at 4 V to about 120 mAh/g. Secondly, at the end of discharge, when, under non(cid:173)
`equilibrium conditions, the overall composition of the electrode reaches LiMn20 4, the
`surface of some particles is more extensively lithiated than the bulk and can reach a
`Lil+0Mn20 4 composition in which the average Mn oxidation state falls below 3.5. For o>O,
`a Jahn-Teller distortion occurs at the particle surface and reduces the symmetry of the spinel
`from cubic to tetragonal (cia = 1.16). The large anisotropic expansion of the unit cell that
`is reflected by the 16% increase in the c parameter is too severe for the cubic and tetragonal
`phases to remain as one intergrown structure. The incompatibility between the oxygen arrays
`of the cubic and tetragonal phases causes the electrode to fracture at the particle surface.
`This effect destroys structural integrity and particle-to-particle contacts, which are essential
`for maintaining the electronic conductivity of the electrode. Gummow et al. [12] have
`proposed that the structural damage is a reason for the slow loss of capacity that occurs on
`cycling Li/Li1_xMn20 4 cells (Fig. 5a). This damaging effect can be reduced by suppressing
`the onset of the J ahn-Teller distortion by modifying the composition of the spinel electrode
`
`Page 8 of 14
`
`

`
`to keep the average Mn oxidation state slightly above 3.5 at the end of the 4 V plateau [12].
`This has been accomplished by using electrodes of general formula Li1+0Mn2_00 4 or
`Li1_0Mn2_200 4 with small values of o, for example, o = 0.05. In general, materials with
`Mnn+ slightly greater than 3.5 are found within the darkened triangle of the Li-Mn-0 phase
`diagram (Fig. 6). Although these modified spinel electrodes offer slightly inferior electrode
`capacities to Li[Mn~O 4, they do exhibit very stable cycling behavior and deliver
`rechargeable capacities in excess of 100 mAh/g (Fig. 5b ). In principle, other cations can be
`used to "stabilize" Li[Mn:z]04, for example, LiM012Mn2_60 4 (M = Mg or Zn).
`
`0
`
`120
`80
`40
`Capacity (mAb/g)
`
`0
`
`120
`80
`40
`Capacity (mAb/g)
`
`Fig. 5. Cycling behavior for the first 10 cycles of a) Li/LiMn20 4 and
`b) Li/Li1.03Mn1•970 4 cells, showing the improved capacity retention in b)
`(taken from reference [12]).
`
`UMn 20 4
`
`Fig. 6. An expanded view of the Li-Mn-0 phase diagram. The darkened area
`shows the composition of spinel electrodes of interest in which the
`oxidation state of the manganese ions is slightly greater than 3.5 (taken
`from reference [12]).
`
`LiV2~. Like LiV02, LiV20 4 is destabilized by lithium extraction, which
`b)
`limits its use as an insertion electrode [17,21]. Removal oflithium from the Li1_x V20 4 spinel
`structure is accompanied, at x:::0.33, by the displacement of approximately one-ninth of the
`vanadium ions in the vanadium-rich layer into adjacent layers. This destroys the [V~04
`spinel framework and the ordered three-dimensional interstitial space for lithium-ion
`
`Page 9 of 14
`
`

`
`diffusion. The product has a defect rock-salt structure, similar to that formed when lithium
`is extracted from layered LiV02 [16].
`
`LiCo2(4. When LiCo02 is prepared at 400oc (often referred to as
`c)
`L T -LiCo02), the structural and electrochemical properties of the material are significantly
`different from those ofLiCo02 prepared at 850°C (referred to as HT-LiCoO~ [22-24]. The
`LT-Li1_xCo02 has a cubic-close packed oxygen lattice that remains unaffected by lithium
`extraction, whereas HT -Li1_xCo02 has trigonal symmetry with a cia ratio that increases from
`4.99 at x = 1 to 5.12 at x = 0.5. The LT-Li1_xCo02 electrodes discharge at approximately
`0.5 V lower than HT-Li1_xCo02 but, in comparison to HT-Li1_xCo02, do not cycle well.
`Despite a structural anomaly that makes it impossible to distinguish, by powder X-ray or
`neutron diffraction techniques, a layered LiCo02 structure with an ideal cubic-close packed
`oxygen array from a cubic, lithiated spinel Li2[Co2]04 (both of which have cia= 4.899),
`high-resolution neutron diffraction data have shown recently that the oxygen array in
`LT-LiCo02 is not quite ideally cubic close packed (cia = 4.914 [25]). The model for
`LT-LiCo02 that best fits the neutron diffraction data is one that can be described as having
`a structure with a cation distribution which is intermediate between an ideal layered and an
`ideal lithiated spinel structure. This interpretation, along with the poor electrochemical
`performance ofLT-LiCo02, is consistent with the performance ofLiNi02 electrodes that
`contain a significant amount of nickel in the lithium layers. Note, however, that removing
`lithium from LT-LiCo02 in acid generates an almost ideal spinel Li[Co2]04 compound [25],
`according to the reaction:
`
`Acid-leached spinel samples derived from nickel-doped LT-LiCo1 _~ix02 compounds
`(O~x~0.2) show a significantly improved cycling ability over LT-LiCo02 products (Fig. 7)
`[26];
`the resulting spinel structure expands/contracts by only 0.2% on
`lithium
`insertion/extraction.
`
`[1]
`
`4.2 ~---"""7"--=-.,..-----""7"1
`4
`3.8
`
`20
`
`100 120 140 160
`80
`60
`40
`Capacity {mAhr /g)
`
`CJ)
`
`4.2 .-----------r,..,.....,,...,
`4
`3.8
`~3.6
`~ 3.4 1~~=====::=::::::::::--.
`~ 3.2
`~ 3
`2.8
`2.6
`2.4~-~~-~-~-~-~
`0
`20
`40
`60
`80 100 120
`Ccpccity (mAhr /g)
`
`Fig. 7. Cycling behavior of a) Li/LT-LiCo0•9Ni0•10 2 and b) Li/LT-Li0•4Co0•8Ni0•10 2
`(acid-leached) cells, showing the improved capacity retention in b)
`(taken from reference [26]).
`
`Page 10 of 14
`
`

`
`Although further work is necessary to fabricate the perfect [CoiJ04 framework, it can be
`concluded from the studies undertaken thus far that this framework should be stable to
`lithium insertion/extraction. Because LiCo02 and LiNi02 compounds, with a cation
`distribution that is intermediate between a layered and a spinel structure, are not tolerant to
`electrochemical cycling, fabrication conditions must be carefully controlled to ensure that
`deviations from the desired stoichiometry and structure of an electrode are minimal.
`
`ANODE MATERIALS
`
`Spinel Oxides
`
`Transition metal oxides that offer a low voltage against pure lithium, for example,
`W02 [26], Mo02 [27], and Li,te20 3 [28,29], have been investigated in the past as anode
`materials for lithium-ion cells. Those materials that have rutile or corundum-type structures
`with hexagonally-close-packed oxygen arrays are not sufficiently stable to lithium
`insertion/extraction to be practical in rechargeable lithium cells. However, because the
`[B21X4 framework of an A[Bl1X4 spinel is stable to lithium insertion and extraction, and the
`voltage of the electrode can be tailored by changing the metal cation on the B sites of the
`framework, this family of spinels is an attractive candidate for fmding a stable anode for
`in spinel notation
`is Li4Ti50 12,
`lithium-ion cells
`[30,31]. One such material
`Li[Ti0.67Li0.33]0 4; it offers a stable operating voltage of approximately 1.5 V vs. lithium. In
`principle, therefore, this material can be coupled with a 4 V electrode such as LiCo02,
`LiNi02, or LiMn20 4 to provide a cell with an operating voltage of approximately 2.5 V,
`which is twice that of a nickel-cadmium or nickel-metal hydride cell. Lithium reacts with
`Li4 Ti50 12 according to the reaction:
`
`[2]
`
`Because lithium insertion into the Li4 Ti50 12 spinel displaces tetrahedrally coordinated
`lithium ions into octahedral sites with the formation of a rock-salt-type Li7 Ti50 12 product,
`the electrode is two-phase and, therefore, provides a flat voltage response to lithium
`insertion. The theoretical capacity of Li4 Ti50 12 is 175 mAh/g. The exceptional stability of
`this electrode to electrochemical cycling has been demonstrated by Rossen et al. [24] and can
`be attributed to the stability of the [Ti1.67Li0_33]04 framework and to the minimal dilation
`(<I%) of the cubic unit cell that occurs on lithium insertion.
`
`The principle of using two spinel oxides as anode and cathode in a lithium-ion cell has
`recently been demonstrated by using Li4 Ti50 12 as anode and a stabilized spinel,
`Li1.03Mn1.970 4 or LiZn0_05Mn1.950 4, as cathode [31]. In a balanced cell, with a
`LiZn0_025Mn1.950 4 cathode, the reaction is:
`
`·
`
`· '"7" - -~ - -
`
`•• •
`
`..
`
`. ~ - ~~: : ;.• <" '.
`
`'- ...,!
`
`•
`
`..-.
`
`....... ~--·-~ -
`
`.
`
`Page 11 of 14
`
`

`
`These cells are assembled in the discharged state from spinel products, which are fairly easy
`to prepare by solid-state reaction techniques. During the first charge cycle, lithium is inserted
`into Li4 Ti50 12 with a concomitant reduction of the titanium cations, and it is extracted from
`LiZn0.025Mn1.950 4 with a concomitant oxidation of the manganese cations. The theoretical
`cell capacity and energy density (based on an average discharge voltage of 2.5 V and the
`masses of the electrode materials only) for the reaction above are 63.5 Ahlkg and
`159 Whlkg, respectively. The excellent electrochemical reversibility of such cells for the
`first five cycles is shown in Fig. 8. This figure illustrates the independent flat voltage
`response that is obtained at the anode and cathode, when monitored against a metallic
`lithium reference electrode during charge and discharge. Higher specific energies can be
`achieved if LiCo02 or LiNi02 electrodes are used. Because of their comparative specific
`energy, Li4+yTi50 121Li1_xCo02 and Li4+yTi50121Li1_~i02 cells may provide alternatives to
`nickel-cadmium cells. For x = 0.6 andy= 3, the theoretical energy densities of the former
`two systems are close to 200 Whlkg; the nickel-cadmium system provides 194 Whlk:g [31].
`
`,-------------.L...---;:-:....----'--r--,
`4.5.-
`
`1
`
`4 I~"'"
`3.5
`
`0.5
`
`o+--~-~--~-~-~--~~
`0
`20
`40
`60
`80
`100
`120
`140
`Capacity (mAh/g)
`
`Fig. 8. Charge/discharge characteristics ofLi4Ti50 12/LiZn0•025l\'In1.950 4 cell, showing
`the voltage response at both anode and cathode against a metallic lithium
`reference electrode [31]. The x-axis corresponds to the capacity delivered by the
`cathode.
`
`CONCLUSIONS
`
`The structural properties of lithiated transition metal oxide insertion electrodes play
`a key role in determining their rechargeability. Lithium insertion/extraction reactions should
`be accompanied by a minimum distortion of the oxygen-ion array and minimum
`
`-~ ---
`
`Page 12 of 14
`
`

`
`displacement of the transition metal cations within the host. Ideal layered and ideal spinel
`structures operate significantly better than structures that have "intermediate-type"
`In principle, optimum electrochemical rechargeability can be expected from
`structures.
`cubic structures that expand and contract isotropically with minimum change to the unit cell
`volume.
`
`REFERENCES
`
`1.
`2.
`
`3.
`
`4.
`5.
`
`6.
`7.
`8.
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`DISCLAIMER
`
`This report was prepared as an account of work sponsored by an agency of the United States
`Government. Neither the United States Government nor any agency thereof, nor any of their
`employees, makes any warranty, express or implied, or assumes any legal liability or responsi(cid:173)
`bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or
`process disclosed, or represents that its usc would not infringe privately owned rights. Refer(cid:173)
`ence herein to any specific commercial product, process, or service by trade name, trademark,
`manufacturer, or otherwise docs not necessarily constitute or imply its endorsement, recom(cid:173)
`mendation, or favoring by the United States Government or any agency thereof. The views
`and opinions of authors expressed herein do not necessarily state or reflect those of the
`United States Government or any agency thereof.
`
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