Electrochemical cell

THIS INVENTION relates to an electrochemical cell.

According to the invention, there is provided an electrochemical cell, which comprises as at least part of an anode, a lithium transition metal 5 oxide or sulphide compound which has a [B₂]X₄ⁿ’ spinel-type framework structure of an A[B₂]X₄ spinel wherein A and B are metal cations selected from Li, Ti, V, Mn, Fe and Co, X is oxygen (0) or sulphur (S) , and n- refers to the overall charge of the structural unit [B₂] X₄ of the framework structure, and the 10 transition metal cation of which in its fully discharged state has a mean oxidation state greater than +3 for Ti, +3 for V, +3,5 for Mn, +2 for Fe and +2 for Co;

as at least part of a cathode, a lithium metal oxide or sulphide compound; and an electrically insulative lithium containing liquid or polymeric electronically conductive electrolyte between the anode and the cathode, such that, on discharging the cell, lithium ions are extracted from the spinel-type framework structure of the anode, with the oxidation state of the metal ions of the anode thereby increasing, while a concomitant insertion of lithium ions into the compound of the cathode takes place, with the oxidation state of the metal ions of the cathode decreasing correspondingly.

The compounds of the anode and cathode may, in particular, be lithium metal oxide compounds.

While the cell can be a primary cell, it is envisaged that it may, in particular, be a rechargeable or secondary cell in which the reverse reactions to those set out above, take place during charging of the cell.

Thus, spinel compounds have structures that can be represented by the general formula A[B₂]X₄ given hereinbefore, and in which the X atoms are ideally arranged in a cubic-close-packed fashion to form a negatively charged anion array comprised of facesharing and edge-sharing X tetrahedra and octahedra. In the formula A[B₂]X₄,.the A cations and B cations occupy tetrahedral and octahedral sites respectively. In the ideal spinel structure, with the origin of the unit cell at the centre (3m), the close-packed anions are located at the 32e positions of the space group Fd3m. Each unit cell contains 64 tetrahedral interstices situated at three crystallographically non-equivalent positions 8a, 8b and 48f, and 32 octahedral interstices situated at the crystallographically non-equivalent positions 16c and 16d.

In the A[B₂]X₄ spinel, the A cations reside in the 8a tetrahedral interstices and the B cations in the 16d octahedral interstices.

There are thus 56 empty tetrahedral and 16 empty octahedral sites per cubic unit cell.

The framework structure of the lithium metal oxide compound of the anode thus has, as its basic structural unit, a unit of the formula [B₂]X₄ⁿ־ as hereinbefore described.

In the anode of the cell of the present invention, therefore, the B cations of the [B₂] X₄ⁿ־ host framework structure may be regarded as being located at the 16d octahedral positions, and the X anions as being located at the 32e positions of the spinel structure. The tetrahedra defined by the 8a, 8b and 48f positions and the octahedra defined by the 16c positions of. the spinel structure thus form the interstitial space of the [B₂]X₄ⁿ־ framework structure for the diffusion of mobile Li⁺ cations.

The B cations of the framework structure may consist of one cationic type, or more than one cationic type of identical or mixed valence to provide various [B₂]X₄־״ framework structures, the overall charge of which can vary over a wide range.

Spinel compounds having the [B₂] X₄־ framework structure may also be characterized by crystallographic space groups other than the prototypic cubic space group Fd3m, and may therefore not have the ideal cubic-close-packed structures hereinbefore described. For example, in Li_1+X [Mn₂] O₄ compounds with 0<x<l, ie compounds in which A is Li, and B is Mn; the spinel structure is distorted, as a result of the Jahn-Teller Mn³*־ octahedral site ions, to tetragonal symmetry, and the compound is characterized by the tetragonal space groups F4!/ddm, or, alternatively, I4!/amd in which the tetrahedral and octahedral site nomenclature differs from that as defined by the space group Fd3m.

Furthermore, the anode need not necessarily be a stoichiometric spinel compound, but can instead be a defect spinel. Defect spinels are well known in the large family of spinel compounds and can have vacancies on the A sites, or on the B sites, or on both the A sites and B sites. For example, compounds can be synthesized in which defects are created by varying the quantity of B cations in the framework structure such that additional Li⁺cations can enter and. leave the framework. In these instances additional Li⁺ cations can partially occupy the 16d octahedral sites normally occupied by the B-type cations. Under such circumstances these partially occupied octahedra can be considered to form part of the interstitial space. Conversely, compounds can , also be synthesized, in which part of the interstitial space defined by the 8a, 8b and 48f tetrahedral and 160 octahedral interstices of the spinel structure can be occupied by B-type cations, thereby rendering these particular sites at least partially inaccessible to the mobile Li cations. The [B₂] X₄ⁿ־ framework structure can contain in certain instances a minor proportion, typically less than 10 atomic percent, of cations other than the mobile Li-type cations, or the A and B-type cations, within the framework structure or within the interstitial spaces of the framework structure, and that could serve to stabilize the structure. For example, doped spinels of stoichiometry Ώί_1+δΜη₂._δΟ₄ where 0<δ20,1, for example, Li₁₀₃Mn₁₉₇O₄ in which 5=0,03, and LiM^Mn^A where M=Mg or Zn and 0<5s0,05, for example, LiMg_{0 025}Mn₁₉₅O₄, are more stable to cycling than the stoichiometric spinel LiMn₂O₄.

The compound of the anode may be a stoichiometric spinel selected from the group comprising Li₄Mn₅O_12׳ which can be written as (Li) _8a [1ιί_0>33Μη₁₁₆₇] n5_dO₄ in ideal spinel notation; Li₄Ti₅O₁₂, which can be written as (Li) _8a [Li_{0 33}Ti₁₁₆₇] _16dO₄ in ideal spinel notation; LiTi₂O₄which can be written as (Li) _8a [Ti₂] _i6dO₄ in ideal spinel notation; LiV₂O₄, which can be written as (Li)_8a[V₂] _16dO₄ in ideal spinel notation; and LiFe₅O₈, which can be written as (Fe) _8a[Fe₁₅Li₀₁₅] !_6dO₄in ideal spinel notation.

Instead, the compound of the anode may be a defect spinel selected from the group comprising Li₂Mn₄O₉, which can be written as (LioaP,.8(״a[Μηι,Λ,221^04 in spinel notation; and Li₂Ti₃O₇, which can be written as (Li₀₈P]₁₁₅)_8a[Ti₁₁₇jLi₀₂₉] _16dO₄ in spinel notation. In defect spinels, the distribution of Li⁺ on the A and B sites can vary from compound to compound.

Instead, the compound of the anode may have a spinel-type structure, which can be a stoichiometric or defect spinel, with a mixture of transition metal cations such as a lithium-iron-titanium oxide in which the lithium and iron cations are located on the A-sites, and lithium, iron and titanium cations on the B-sites.

In a preferred embodiment of the invention, the transition metal cations, Ti, V, Mn, Fe and Co, reside predominantly or completely on the B-sites of the spinel structure, while the Li cations reside predominantly or completely on the A-sites of the structure.

The lithium metal oxide compound of the cathode may also have a spinel-type framework structure. Thus, the framework structure of the lithium metal oxide compound of the cathode may then also have, as its basic structural unit, a unit of the formula [B₂] X₄ⁿ־ of an A[B₂]X₄ spinel, as hereinbefore described, with the transition metal cations of the anode being more electropositive than those of the cathode.

In the compound of the cathode, A and B may be a metal cation of one type, or a mixture of different metal cations. The compound of the cathode may be a stoichiometric or defect spinel compound, as hereinbefore described.

When the compound of the cathode has a spinel-type structure, it may be selected from the group having as its B-type cations Li, Mn, Co or Ni, or mixtures thereof, such as Li_xMn₂O₄ where 0<xsl and Li_xCo₂0₄. where 0<xs2, optionally doped with additional metal cations to stabilize the structure as hereinbefore described.

Instead, the compound of the cathode may have another structure type, for example a layered type structure such as that found within a system defined by a formula Li_xCo₁._yNi_y0₂ where Osysl and

0<xsl.

In general, the anode compound will be selected from those spinel compounds that offer a relatively low voltage vs pure lithium, typically those that offer 3V or less, while the cathode compound will be selected from those spinel compounds that offer a relatively high voltage vs pure lithium, typically those that offer between 4,5V and 3V. For example, a Li/Li_4+xTi₅O₁₂ cell delivers on discharge at ΙΟΟμΑ/cm² (for 0<x<l) an average voltage of approximately 1,5V, while a Li/LixMn2O4 cell delivers on discharge at ΙΟΟμΑ/cm² (for 0<x<l) an average voltage of approximately 4V. Therefore, a cell in accordance with the invention can have Li4+xTi₅O₁₂ as an anode and Li_xMn₂O₄ as a cathode, and will deliver approximately 2,5V on discharge and which is approximately twice the voltage of a nickel-cadmium cell. In another example, a Li/Li₂Mn₄O₉ cell delivers a voltage of approximately 2,8V over most of the discharge. Thus, a cell in accordance with the invention can have a Li_2+xMn₄O₉ anode and Li_xMn₂O₄ as cathode, and delivers approximately 1,2V on discharge, which is the typical voltage of a nickel-cadmium cell. It is convenient to load such cells in a discharged state, ie with the following configurations:

Li₄Ti₅0₁₂/Electrolyte/LiMn₂0₄ ......(1)

Li₂Mn₄0₉/Electrolyte/LiMn₂0₄ ......(2)

Although it is convenient to load such cells in a discharged state, the cells may also be loaded in the charged state, if so desired. In this respect, the anodes of the invention have lithiated spinel structures and delithiated spinel structures that have the [B₂]X₄ spinel framework as defined hereinbefore.

In (1), Li⁺ ions are extracted from Li [Mn₂] 0₄ during charge with a concomitant oxidation of the manganese ions from an average valence of 3,5 to higher value's, and inserted into the Li₄Ti₅O₁₂electrode structure with a concomitant reduction of the titanium cations from the average valence state of +4 to lower values. During this process Li⁺ ions are shuttled between the oxide structures without the formation of any metallic lithium, the cell voltage being derived from changes in the oxidation state of the transition metal cations in the .anode and cathode structures.

The electrolyte may be a room temperature electrolyte such as LiC10₄, LiBF₄, or LiPF₆ dissolved in an appropriate organic salt such as propylene carbonate, ethylene carbonate, dimethyl carbonate, dimethoxyethane, or appropriate mixtures thereof. Instead, however, it may be any appropriate polymeric electrolyte such as polyethylene oxide (PEG) - LiC10₄, PEO - LiSO₃CF₃ and PEG - LiN(CF₃SO₂) ₂, that operates at room temperature or at elevated temperature, eg at about 12 0°C.

The invention will now be described by way of non-limiting examples, and with reference to the accompanying drawings in which:

FIGURE 1 shows powder X-ray diffraction patterns of compounds suitable for use as anode materials in rechargeable electrochemical cells according to the invention;

FIGURE 2 shows powder X-ray diffraction patterns of compounds suitable for use as cathode materials in rechargeable electrochemical cells according to the invention;

FIGURE 3 shows a plot of voltage vs capacity for a known Li/Li₂Mn₄O₉ cell;

FIGURE 4 shows a plot of Li/Li₄Mn₅O₁₂ cell;

FIGURE 5 shows a plot of Li/Li₄Ti₅O₁₂ cell;

FIGURE 6 shows a plot of Li/LiFe₅O_g cell;

FIGURE 7 shows a plot Li/Li-Fe-Ti oxide cell;

FIGURE 8 shows a plot of Li/LiMn₂O₄ cell;

FIGURE 9 shows a plot of L1 / L1!₀₃Mn 190₄ף cell;

voltage vs capacity for a known voltage vs capacity for a known voltage vs capacity for a known of voltage vs capacity for a voltage vs capacity for a known voltage vs capacity for a known

FIGURE 10 shows a plot of voltage vs capacity for a known Li/LiC00₂ cell;

FIGURE 11 shows a plot of voltage vs capacity for the cell of Example 1 and which is in accordance with the invention;

FIGURE 12 shows a plot of voltage vs capacity for the cell of Example 2 and which is in accordance with the invention;

FIGURE 13 shows a plot of voltage vs capacity for the cell of Example 3 and which is in accordance with the invention;

FIGURE 14 shows a plot of voltage vs capacity for the cell of Example 4 and which is in accordance with the invention;

FIGURE 15 shows plots of voltage vs capacity for the cells of Examples 5 and 6 and which are in accordance with the invention; and

FIGURE 16 shows a cyclic voltammogram of the Li/Li-Fe-Ti oxide spinel cell of Example 7.

The following stoichiometric spinel and defect spinel compounds were selected for use as anode materials in the examples following hereinafter:

a) Li₂Mn₄O_y

b) Li₄Mn₅O₁₂

c) Li₄Ti₅O₁₂

d) LiFe₅O₈

e) Li-Fe-Ti oxide spinel in which Li :Fe:Ti=2:2:1

Powder X-ray diffraction patterns of these compounds are given in Figure la-e respectively.

The following spinel and non-spinel compounds were selected for use as cathode materials in the examples following hereinafter:

a) LiMn₂O₄ (spinel-type structure)

b) Li₁₀₃Mn₁₉₇O₄ (spinel-type structure)

c) LiCo0₂ (layered-type structure)

Powder X-ray diffraction patterns of these compounds are given in Figure 2a-c respectively.

EXAMPLE 1

In view thereof that a Li/Li₂Mn₄O₉ cell delivers on discharge 150mAh/g at an average voltage of approximately 2,8V, as indicated in Figure 3, and a Li/LiMn₂O₄ cell delivers on discharge 120mAh/g at an average voltage of approximately 3,8V, as indicated in Figure 8, a cell in accordance with the invention and having the configuration Li₂Mn₄O₉(anode)/Electrolyte/LiMn₂0₄ (cathode) (2) was constructed.

The LiMn₂O₄ spinel compound of the cathode was synthesized by reaction of LiOH and γ-ΜηΟ₂ (chemically-prepared manganese dioxide, CMD) firstly at 450°C for 48 hours and thereafter at 750°C for 48 hours. The powder X-ray diffraction pattern of this compound is shown in Figure 2a.

Li₂Mn₄O₉ was synthesized by reaction of LiOH and MnCO₃ at 345°C for 32 hours. The powder X-ray diffraction pattern of this compound is shown in Figure la. The pattern is predominantly characteristic of the Li₂Mn₄O₉ defect spinel phase, but contains in addition a few very weak peaks, for example at 42^Ο2Θ and 53^Ο2Θ, that are indicative of a very minor proportion of lithiated γ-ΜηΟ₂ phase.

A cell of the format Li₂Mn₄0₉/Electrolyte/LiMn₂0₄ (2) was then constructed. The electrolyte used was IM LiC10₄ in propylene carbonate. The first 9 charge and 8 discharge cycles of the cell are shown in Figure 11. A current of 0,1mA was employed for both charge and discharge. The cell was cycled between upper and lower voltage limits of 1,5V and 0,45V respectively.

EXAMPLE 2

In view thereof that a Li/Li₄Mn₅O₁₂ cell delivers on discharge 150mAh/g at an average voltage of approximately 2,7V, as indicated in Figure 4, and a Li/Li₁₀₃Mn₁₁₉₇O₄ cell delivers on discharge lOOmAh/g at an average voltage of approximately 3,9V, as indicated in Figure 9, a cell in accordance with the invention and having the configuration Li₄Mn₅0₁₂/Electrolyte/ Li₁₀₃Mn₁₉₇O₄ (3) was constructed.

The Li!₀₃Mnj ₉₇O₄ spinel compound of the cathode was synthesized by the reaction of LiOH and γ-ΜηΟ₂ (chemically-prepared manganese dioxide, CMD) firstly at 450°C for 48 hours and thereafter at 650°C for 48 hours. The powder X-ray diffraction pattern of this compound is shown in Figure 2b.

Li₄Mn₅O₁₂ was synthesized by the reaction of Li₂CO₃ and MnCO₃ at 400°C for 10 hours. The powder X-ray diffraction pattern of this compound is shown in Figure lb. The pattern is predominantly characteristic of the Li₄Mn₅O₁₂ spinel phase.

A cell of the format Li₄Mn₅0₁₂/Electrolyte/Li_lo3Mn₁₉₇0₄ (3) was then constructed. The electrolyte used was IM LiClO₄ in propylene carbonate. The first 5 charge/discharge cycles of the cell are shown in Figure 12. A current of 0,1mA was employed for both charge and discharge. The cell was cycled between upper and lower voltage limits of 1,6V and 0,5V respectively.

EXAMPLE 3

In view thereof that a Li/Li₄Ti₅O₁₂ cell delivers on discharge 120mAh/g at an average voltage of approximately 1,5V, as indicated in Figure 5, and a Li/Li₁₀₃Mn₁₉₇O₄ cell delivers on discharge lOOmAh/g at an average voltage of approximately 3,9V, as indicated in Figure 9, a cell in accordance with the invention and having the configuration Li₄Ti₅0₁₂/Electrolyte/ Li₁₀₃Mn₁₉₇O₄ (4) was constructed.

The Li₁₀₃Mn₁₉₇O₄ spinel compound of the cathode was synthesized a.s in Example 2.

Li₄Ti₅O₁₂ was synthesized by the reaction of Li₂CO₃ and TiO₂, using a Li/Ti atomic ratio of 0,87, at 500°C for 12 hours and at 1000°C for 24 hours. A slight excess of lithium was used because of the volatility of Li₂0 at that temperature. The powder X-ray diffraction pattern of this compound is shown in Figure 1c. The pattern is predominantly characteristic of the Li₄Ti₅O₁₂ spinel phase.

A cell of the format Li₄Ti₅O₁₂/Electrolyte/Li₁₁₀₃Mn₁₉₇O₄ (4) was then constructed. The electrolyte used was IM LiClO₄ in propylene carbonate. The first 7 charge/discharge cycles of the cell are shown in Figure 13. A current of 0,1mA was employed for both charge and discharge.

The cell was cycled between upper and lower voltage limits of

2,8V and 1,9V respectively.

EXAMPLE 4

In view thereof that a Li/Li₄Ti₅O₁₂ cell delivers on discharge 120mA.Hrs/g at an average voltage of approximately 1,5V, as indicated in Figure 5, and a Li/LiCo0₂ cell delivers on discharge 140mA.Hrs/g at an average voltage of approximately 3,9V, as indicated in Figure 10, a cell in accordance with the invention and having the configuration Li₄Ti₅0₁₂/Electrolyte/LiCo0₂ (5) was constructed.

The LiCo0₂ spinel compound of the cathode was synthesized by the reaction of CoC0₃ and Li₂CO₃ firstly at 400°C for 48 hours and thereafter at 900°C for 48 hours. The powder X-ray'diffraction pattern of this compound is shown in Figure 2c.

Li₄Ti₅O₁₂ synthesized as in Example 3, was used for the anode in this example.

A cell of the format Li₄Ti₅0₁₂/Electrolyte/LiCo0₂ (5) was then constructed. The electrolyte used was IM LiCo0₄ in propylene carbonate. The first 3 charge/discharge cycles of the cell are shown in Figure 14. A current of 0,1mA was employed for both charge and discharge. The cell was cycled between upper and lower voltage limits of 2,8V and 1,9V respectively.

EXAMPLE 5

In view thereof that a Li/LiFe₅O₈ cell delivers on discharge lOOmAh/g at an average voltage of approximately 1,0V, as indicated in Figure 6, and a Li/Li₁₀5Mn_11?7O₄ cell delivers on discharge lOOmAh/g at an average voltage of approximately 3,9V, as indicated in Figure 9, a cell in accordance with the invention and having the configuration LiFe₅O₈/Electrolyte/Li₁₀₃Mn_1>970₄ (6) was constructed.

The Li₁₁₀₃Mn₁₉₇O₄ spinel compound of the cathode was synthesized as in Example 2.

LiFe₅O₈ was synthesized by reacting of Li₂CO₃ and a-Fe₂O₃ in a 1:5 molar ratio at 900°C for 24 hours. The powder X-ray diffraction pattern of this compound is shown in Figure Id.

A cell of the format LiFe₅O₈/Electrolyte/Li₁₀₃Mn_b97O₄ (6) was then constructed. The electrolyte used was IM LiC10₄ in propylene carbonate. The first charge cycle of the cell is shown in Figure 15a. A current of 0,1mA was employed for both charge and discharge. The cell had an upper voltage limit of 4,1V.

EXAMPLE 6

In view thereof that a Li/Li-Fe-Ti oxide spinel cell delivers on discharge 80mAh/g at an average voltage of approximately 0,6V, as indicated in Figure 7, and a Li/Li₁₀₃Mn_1>97O₄ cell delivers on discharge lOOmAh/g at an average voltage of approximately 3,9V, as indicated in Figure 9, a cell in accordance with the invention and having the configuration Li-Fe-Ti oxide spinel/Electrolyte/

Li₁₀₃Mn₁₉₇O₄ (7) was constructed.

The Li₁₀₃Mn₁₉₇O₄ spinel compound of the cathode was synthesized as in Example 2.

A Li-Fe-Ti oxide spinel was synthesized by the reaction of Li₂CO₃and Fe₂TiO₅, using a Li:Fe:Ti atomic ratio of 2:2:1, at 500°C for 6 hours and at 900°C for 16 hours. The powder X-ray diffraction pattern of this compound is shown Figure le. The pattern is predominantly characteristic of a spinel-type phase.

A cell of the format Li-Fe-Ti oxide spinel/Electrolyte/Li₁₀₃Mn₁₉₇O₄ (7) was then constructed. The electrolyte used was IM LiC10₄ in propylene carbonate. The first charge cycle of the cell is shown in Figure 15b. A current of 0,1mA was employed for both charge and discharge. The cell had an upper voltage limit of 4,4V.

EXAMPLE 7

A Li-Fe-Ti oxide spinel was synthesized by the reaction Li₂CO₃ and Fe₂TiO₃ using a Li:Fe:Ti atomic ratio of 1:2:1 at 500°C for 6 hours, and thereafter at 900 °C for 16 hours. A cyclic voltammogram of a Li/Li-Fe-Ti oxide spinel cell with an electrolyte of IM LiCO₄ in propylene carbonate is shown in Figure

16. It shows the rechargeable characteristics of the Li-Fe-Ti oxide spinel electrode, and in particular, the rechargeability of the Li insertion/extraction reaction that occurs at approximately 1,5V versus lithium.

Examples 5, 6 and 7 show, in particular, the potential of using spinel-type oxides containing iron as anodes because they provide a low voltage against lithium. Furthermore, the experimental data provided in the examples demonstrate the ability of transition metal oxides to provide an electrochemical couple for ׳rocking chair׳ rechargeable lithium cells in which lithium ions are transported between the two transition metal oxide electrodes, the anode of which has a spinel-type structure, and which uses a liquid or polymeric electrolyte containing Li⁺ ions. The electrochemical cells of the invention thus contain no metallic lithium anode, and are therefore inherently safer than lithium cells containing metallic lithium anodes and, indeed, lithium-carbon anodes. In particular, such cells have an added advantage of providing a more constant operating voltage than cells with carbon anodes. Although the cells of the invention are designed primarily for the use as rechargeable cells, they can also, as indicated hereinbefore, be utilized as primary cells, if desired.

Although the principles of this invention have been demonstrated by use of lithium-metal oxide compounds, the compounds of the electrodes, instead of being oxides, can be sulphides.