WO1999052825A1

WO1999052825A1 - Lithium battery operating between 1.8 volts and 4.3 volts

Info

Publication number: WO1999052825A1
Application number: PCT/FR1999/000871
Authority: WO
Inventors: Didier Bloch; Carole Bourbon; Frédéric Le Cras; Franck Rouppert
Original assignee: Commissariat A L'energie Atomique
Priority date: 1998-04-14
Filing date: 1999-04-14
Publication date: 1999-10-21
Also published as: EP0991592A1; JP2002505798A; FR2777387A1; FR2777387B1; CA2293841A1; IL133498A0

Abstract

The invention concerns a lithium battery operating between 1.8 volts and 4.3 volts and comprising a negative electrode, an electrolyte and a positive electrode characterised in that said positive electrode comprises as active material, and before it has been subjected to a particular charging or discharging operation, a manganese oxide containing lithium or extra lithium having a spine structure obtainable by a preparation method comprising the following three steps performed successively or simultaneously: preparing a lithium alkoxide solution by dissolving metal lithium in the corresponding alcohol, said alcohol being selected among the alcohols derived from linear or branched alkanes containing at least 3 carbon atoms, the alcohols derived from unsaturated aliphatic hydrocarbons, and their mixtures; adding a manganese oxide powder to said lithium alkoxide solution to obtain a dispersion; controlled reduction of said manganese oxide by said alkoxide until a manganese oxide containing lithium or extra lithium having a stoichiometry Li: Metal is obtained. Said method further comprises the following steps: evaporating the residual alcohol; washing the resulting powder; drying the powder; optionally, an additional step of heat treatment after drying.

Description

Lithium battery operating between 1.8 Volts and 4.3 Volts

DESCRIPTION

The present invention relates to a lithium accumulator operating between 1.8 volts and 4.3 volts.

More specifically, the invention relates to a rechargeable lithium battery or electrochemical generator operating between 1.8 volts and 4.3 volts and comprising a negative electrode in particular a composite negative electrode, an electrolyte, and a positive electrode comprising as active material a manganese oxide of lithiated or superlithiated spinel structure.

The technical field of the invention can generally be considered as that of rechargeable lithium accumulators (in English "Secondary Lithium Cell" or "Secondary Lithium Battery")

A historical overview of the development of rechargeable lithium batteries is given in the document by K. BRANDT "Historical Development of Secondary Lithium Batteries", Solid State Ionics 69

(1994), 173-183. The technological sectors of lithium accumulators marketed or under development and some of their characteristics are detailed in Table I below:

TABLE I

SECTORS

"4 Volts" Li-metal / liquid electrolyte Li-metal / electrolyte

Li-ION "3 Volts - TADIRAN" solid polymer "3 Volts"

Carbon electrode (th: 120 μm) Metallic lithium (th: 120 μm) Lithium metal (th: 120 μm)

Negative

ELECTROLYT organic liquid or specific liquid electrolyte solid polymer without

E gel electrolyte solvent

Active material Cobalt, manganese oxide, vanadium oxide, nickel electrode or positive manganese oxide

Voltage & • 4 Volts. 3 Volts • 3 Volts

• Ambient temperature - 45-50 ° C • ambient temperature - 55 ° C. 50-80 ° C operation

. cyclability (600-1000 • DOUΓ V.E safety • high temperature of unit cell cycles) • high energy density: 130- operation:

Strengths • high density of 150 Wh / kg (200-240 km for interesting for electric vehicle V.E)

• Mn oxide reaches. safety for V.E cost and toxicity targets • high energy density: set for V.E 130-150Wh / kg (200-240

• high power density in km for V.E) discharge

. insufficient security • relatively cyclable. incoπroatibility between for V.E (Limited vehicle (500 cycles for the manganese and electric oxide) unit cells) the electrolyte t> olvmère

Weak points • cost of oxides of • relatively fast charge rate • relatively low cobalt and low nickel (C / 10) power density

• relatively low energy density: 110-

130 Wh / kg (170-200 km

for VE)

Among the three sectors described above, only the first and the second are currently marketed. The third ("lithium-polymer") is

lL800 / 66HdΛLDd SZ8ZS / 66 O still the subject of research and development work.

TADIRAN “3 Volts” supply chain with metallic lithium anode:

This die has, due to the presence of a negative lithium metal electrode, a limitation of its lifespan (500 cycles at present). In addition, the upper limit of the working voltage is limited to 3.4 Volts (upper operating terminal of the electrolyte). This limitation forces, if one wishes to have the greatest possible exchangeable capacity, to use only the active materials of positive electrode releasing their reversible capacity below 3.4 volts, and does not allow the use of positive electrode active materials based on manganese oxides "releasing" a significant part of their capacity above this value.

“4 Volts” Lithium-Ion system with carbon-based anode (SONY, SANYO, PANASONIC, SAFT, ...)

These channels have several limitations: = they are relatively limited in energy density (best performance: approx. 120 Wh / kg) real use for the time being as active materials of positive electrode of expensive oxides: oxides of cobalt or nickel . The operating principle of all these lithium accumulator systems is the same: at each charge or discharge of the accumulator, lithium in ionic form (Li ⁺ ) is exchanged between the positive and negative electrodes. The quantity of energy exchanged (supplied by the accumulator in discharge or supplied to the accumulator in charge) at each charge or discharge is exactly proportional to the quantity of lithium which it is possible to exchange during the electrochemical reaction . This "exchangeable" lithium must be supplied by a "source" of lithium. This source is the negative electrode in the case of the dies using a negative lithium metal electrode. For the dies using a negative carbon-based electrode, which in principle does not contain lithium by construction, the source of lithium must therefore be contained in the positive electrode. It is in this case the active material of positive electrode which plays this role of lithium source. We see that it is necessary to incorporate into the structure of this active material of positive electrode during its synthesis the largest amount of lithium possible in order to have a sufficient reserve of lithium to have interesting electrochemical performances.

An accumulator is characterized by its operating voltage, which is determined by the potential difference between the negative electrode and the positive electrode. The absolute potential (not measurable) of the negative electrode in metallic lithium is constant, because it is a pure metal.

The voltage of an accumulator with a negative lithium metal electrode is therefore entirely determined by the potential of the positive electrode, which depends on the crystallographic structure of the active material of positive electrode, and which changes as a function of the quantity of lithium contained. In this one. During a discharge of the accumulator, the lithium is inserted into the crystal structure of this active material whose potential drops regularly. The battery voltage drops. It is the opposite during a charge. The active materials all show a different evolution of their potential (vs Li / Li +) according to the quantity of lithium which they contain: each active material thus has a characteristic “electrochemical signature”. Some people insert lithium between 3.5 and 4.5 Volts (This is the case, for example, of cobalt oxides whose potential (vs Li / Li +) varies between 3.5 V (for LiCo0 ₂ ) and 4.5 Volts (for Liι_ _x Co0 ₂ , with x≈0.7 after charging the battery).

As another example, the potential (vs Li / Li ⁺ ) of the manganese oxides of composition similar to Lio ₃ no ₂ used by TADIRAN Batteries Ltd for accumulators produced according to the technology described in patent US-A-5,506,068 include the lithium between 3.4 Volts (the composition of the active material of positive electrode is then close to Li _0, _3, MnO ₂₎ and 2 volts (the composition of the active material of positive electrode is then close to LiMn0 ₂ ). This is the “Lithium-metal, 3 Volts, liquid electrolyte” process

Other materials based on manganese oxides are more versatile: this is how manganese oxides with a spinel structure usually have two "plateaus" of operating potential. For example, for the compound with a spinel structure of formula LiMn ₂ 0 ₄ , most of the lithium is extracted from this structure between approximately 3.2 volts and 4.4 volts (vs Li / Li ⁺ ) (the composition of the active material d positive electrode at the end of the charge up to 4.4 Volts is then close to Mn ₂ 0 ₄ ), while it is possible to insert lithium between approximately 3.2 Volts and 1.8 Volts in the structure LiMn ₂ 0 <ι (the composition of the active material of positive electrode at the end of the discharge up to 1.8 Volts of the accumulator is then close to Li ₂ Mn ₂ 0 ₄ ).

It is therefore seen that it is possible and even necessary to choose the active compound of the positive electrode in order to optimize the overall performance of the system.

“3 volts” metallic lithium system with liquid electrolyte:

Chronologically, the first lithium accumulators developed twenty years ago used a negative electrode made of metallic lithium. Although offering energy densities high thanks to the large reserve of lithium contained in the negative electrode, this sector was abandoned by most battery manufacturers due to the poor reconstruction of the metal surface at the negative electrode / electrolyte interface during cycles charging and discharging, leading to too short lifetimes (~ 200 cycles). Experience has shown that during successive charge / discharge cycles, dendritic growth phenomena (in the form of needles) appeared during the reconstitution of lithium metal. These needles would finish after about 200 cycles by filling the space between the negative electrode and the positive electrode, which caused short internal circuits. However, some battery manufacturers have managed to limit this phenomenon. For example, the document [11] by E. MENGERITSKY, P. DAN, I. WEISSMAN, A. ZABAN; D. AURBACH, “Safety and Performances of TADIRAN TLR-7103 Rechargeable Batteries”, J. Electrochem. Soc, vol. 143, n ° 7, July 1996 describes an accumulator operating between 2 and 3.4 volts with negative electrode of metallic lithium and with liquid electrolyte having an interesting lifetime (nevertheless limited to approximately 500 charge / discharge cycles) thanks to a new electrolyte formulation. This electrolyte, in addition to the fact that it significantly improves the electrochemical behavior of the accumulator, has another characteristic: In the event of an abnormal rise (greater than 3.5 Volts) in the voltage or the temperature of the accumulator, the electrolyte polymerizes and ceases to be ionic conductor.

This characteristic allows the accumulator to offer great safety in use. However, it can be seen that the upper terminal of the operating voltage of the accumulator is close to 3.5 volts. In a battery of this type, the negative metallic lithium electrode is alternately consumed (discharged) then reconstituted (charged) during a discharge / charge cycle. The thickness of lithium consumed during each discharge is usually between 10 and 20 micrometers of lithium (The exchange of a capacity of 1 mAh per unit of negative electrode surface area corresponds to the (theoretical) consumption of one "slice" of the negative metallic lithium electrode with a thickness equal to 4.88 micrometers. Usually, the capacity exchanged per unit of negative electrode area is between 2 and 4 mAh / cm ² ). In theory, therefore, it would only be necessary to use a negative metallic lithium electrode with a thickness close to 20 micrometers.

However, a certain amount of lithium is consumed and lost during the successive electrochemical exchanges.

In order to extend the cycle life of the accumulators, a negative electrode with a thickness greater than 20 micrometers is in fact used.

In general, at least 5 times more lithium is used than the theoretical amount required during the life of the battery: the negative electrode used by TADIRAN Batteries Ltd for example is a metal sheet of about 120 micrometers.

It is important to mention that the greater the thickness of lithium, the longer the service life.

This is how we obtain around 500 cycles with 120 microns but we can reach 600 cycles with 140 microns of lithium.

However, any increase in thickness of the lithium electrode consumes volume, and therefore takes place at the expense of the energy density of the accumulator. We therefore see that there is a compromise to be found between the cycle life and the energy density of the accumulator.

4-volt lithium-ion system

A different path was proposed in the early 1980s to overcome the difficulty of dendritic growth. It consists of replacing the negative lithium metal electrode with a carbon-based lithium insertion compound. In this case, the negative metallic lithium electrode is replaced by an electrode containing a carbon-based lithium insertion compound in which the lithium reversibly intercalates during successive cycles, exactly as it does in the compound. insertion of the positive electrode. This is the “Lithium-ION, 4 Volts” sector

However, because of this choice, 10

• the negative electrode is no longer able to act as a lithium reservoir necessary for the electrochemical reaction, which makes the use of a positive electrode compound containing by construction of lithium compulsory.

• part of the lithium from the positive electrode is irreversibly consumed by the carbon negative electrode when the battery is charged for the first time (corresponding to the first insertion of lithium into the carbonaceous negative electrode), which results in a equivalent loss of battery capacity.

These limitations imply that it is useful to be able to synthesize an active positive electrode material containing as much lithium as possible.

By way of example, the compounds based on manganese oxide which have so far exhibited the best electrochemical characteristics in the context of the “Lithium-ION, 4 Volts” sector described above are products of spinel structure. of composition close to LiMn ₂ 0 ₄ . They allow electrochemical cycling between 3.2 and 4.4 Volts (vs Li / Li ⁺ ). We see that in this case, part of the lithium contained in LiMn ₂ 0 "ι is irreversibly consumed during the first charge by the negative electrode.

The compounds selected by manufacturers for the development of this “Li-ION, 4 Volts” sector 11

were essentially mixed oxides of lithium and cobalt (LiCo0 ₂ ) or nickel (LiNi0 ₂ ). These compounds have in fact both the advantage of being able to be synthesized simply by heat treatment of suitable precursors and of maintaining an acceptable energy density, greater than 100 Wh / kg. Indeed, in this case the relatively small amount of lithium stored in the positive electrode (proportional to the capacity It) is compensated by the high operating voltage U close to 4 Volts (Energy = UIt).

Since the end of the 1980s, most manufacturers of accumulators have been developing this “Li-ION, 4 Volts” (Volts Lithium Ion Cell or 4-Volts Li-ION oven) which combines a positive electrode compound of Type LiCoOΣ cobalt oxide or nickel LiNi0 ₂ operating between 3.5 and 4.5 V (vs Li / Li ⁺⁾ and specific carbon negative electrode materials limiting the loss of capacity to the ^Er cycle [7]. These systems now reach energy densities of around 110 to 120 Wh / kg, corresponding to a range of 170 to 200 km for an electric vehicle, and have a lifespan of around 800 cycles. The disadvantages of this type of accumulator are the high cost of the oxides of cobalt and nickel, and their energy density which nevertheless remains relatively low (compared to the dies with negative electrode of metallic lithium).

The object of the present invention is to provide a lithium accumulator (rechargeable electrochemical generator) operating between 1.8 and 4.3 volts which does not 12

does not have the drawbacks, limitations, defects and disadvantages of accumulators of the prior art.

In particular, the object of the present invention is to provide a rechargeable electrochemical accumulator or generator operating between 1.8 and 4.3 volts, and in particular having improved performance expressed in terms of cycle life and in terms of density. energy, compared to those obtained so far with the available lithium batteries.

This object and others are achieved in accordance with the invention by providing a lithium battery operating between 1.8 and 4.3 volts and comprising a negative electrode, an electrolyte and a positive electrode characterized in that said positive electrode comprises as an active material and before it has undergone any charging or discharging operation a lithiated or superlithiated manganese oxide of spinel structure capable of being obtained by a preparation process comprising the following three steps, carried out successively or simultaneously: preparation of a solution of lithium alkoxide (alcoholate) by dissolving metallic lithium in the corresponding alcohol, said alcohol being chosen from alcohols derived from linear or branched alkanes comprising at least 3 carbon atoms, alcohols from unsaturated aliphatic hydrocarbons, and mixtures thereof; 13

adding a manganese oxide powder to said lithium alkoxide solution to obtain a dispersion; controlled reduction of said manganese oxide by said alkoxide until a “lithiated or over-lithium” manganese oxide is obtained having the defined (desired) stoichiometry Li: Metal, this stoichiometry is dependent on the composition and structure of the oxide initial manganese; said method then further comprising the following steps:

- evaporation of the residual alcohol;

- rinsing of the powder thus obtained;

- drying of the powder; - optionally also, additional heat treatment step carried out after drying.

By manganese oxide of spinel structure is also meant all the compounds formed of a mixture of spinel structure with another manganese oxide compound in all proportions.

By "superlithiation" is meant the insertion of lithium into the structure of a commercial transition metal oxide, prior to its incorporation into the positive electrode; compound ^v superlithium 'thus formed having similar characteristics (chemical, crystallographic, electrochemical) to those of the product obtained by the insertion of lithium in the initial commercial oxide as it is generated during a discharge infinitely 14

slowly from the accumulator to a voltage between the dropout voltage of this initial oxide (vs Li / Li ⁺ ) and 1.0 volt (vs Li / Li ⁺ ).

By lithiation, or lithiated product, is meant any intermediate step between the initial product and the overlithiation corresponding to the overlithiated product defined above.

An accumulator comprising as active material of positive electrode and prior to any charge or discharge, the lithiated or superlithiated manganese oxide specific above has never been mentioned or suggested in the prior art.

Such an accumulator provided with such a specific positive electrode active material makes it possible to achieve surprisingly improved performance in terms of energy density and cycle life.

The process used in the invention for preparing lithiated or superlithiated manganese oxide differs fundamentally from the processes of the prior art for the preparation of “lithiated or over-lithium” transition metal oxides and in particular oxides of Mn, in that it uses as reaction intermediate lithium alkoxides which are, according to an essential characteristic of this process, obtained by dissolving metallic lithium in the corresponding alcohol, the latter being an alcohol derived from a linear alkane or branched having at least 3 carbon atoms or an alcohol derived from an unsaturated aliphatic hydrocarbon. 15

It is understood that one can use any mixture of these alcohols in all proportions.

The use of lithium alkoxides in the preparation of positive electrode compounds for lithium accumulators has certainly already been mentioned, in particular for the preparation of lithiated vanadium oxides. (US-A-5,549,880). However, the process used in this document uses lithine LiOH and an alcohol as precursors for the preparation of the alkoxide, so that due to the chemical equilibrium constants between LiOH and the alcohols, the solution obtained cannot contain appreciable amounts of alkoxide only if the alcohol retained is a light alcohol (essentially methanol or ethanol).

This document discourages the use of heavier alcohols and expressly advises using methanol or ethanol as is the case in the unique example of this document.

The use of a heavier alcohol such as pentanol-1 in particular, under the conditions of the process of this document where one starts from lithine, only leads to the formation of alkoxide in very low concentration in the solution , and therefore at very low reaction kinetics.

However, the process used in the present application highlights the need for an alkoxide originating from the reaction between an alcohol derived from a linear or branched alkane, containing at least 3 16

carbon atoms or an alcohol from an unsaturated aliphatic hydrocarbon.

In general, it has been demonstrated that the oxides of lithiated or superlithiated transition metals and in particular the oxides of Mn lithiated or superlithiated delithelate in ethanol, thus, by way of example, we have been able to show that 'a LiMn204 spinel lithiated by the process used in the present application in pentanol-1 decays quickly during its stay in absolute ethanol.

This confirms the predominance of the stability of lithium ethoxide with respect to the manganese oxides lithiated or superlithiated by the present process, and therefore by the reciprocal impossibility of obtaining these same lithiated or superlithiated compounds in dispersion in a solution of 'light alcohol such as ethanol or methanol.

The preparation of the alkoxide from metallic lithium according to the first step of the process for the preparation of lithiated or superlithiated manganese oxide of the present application makes it possible, surprisingly, to obtain concentrated solutions of lithium alkoxide even with alcohols comprising 3 carbon atoms and more on the contrary of document US-A-5,549,880.

As a result, the kinetics of the lithiation of manganese oxide is faster, or even instantaneous, and the yield is close to 1 thanks to the more reducing nature of the heavy alkoxides (derived from alcohols derived from linear or branched alkanes having at least three carbon atoms, or alcohols 17

derived from unsaturated aliphatic hydrocarbons) compared to light alkoxides.

The preparation process used is in a different field which is that of so-called “soft” chemistry carried out in solution and at low temperature.

The preparation process used in the present application provides a solution to the problems posed by the processes of the prior art. Indeed, it only uses common reagents, inexpensive, readily available and which do not present any particular risks.

The preparation process used, unlike the processes of the prior art also has rapid kinetics and comprises only a few intermediate steps since the essential step of the process is the controlled reduction of manganese oxide.

The preparation process used is sufficiently "gentle" and precise not to deviate from the desired stoichiometry.

The reaction indeed stops at a Li: manganese stoichiometry perfectly defined by the choice of alcohol, even in the presence of a large excess of alkoxide, and which is a function of the composition and of the crystallographic structure of the initial manganese oxide.

By perfectly defined stoichiometry, we generally mean a Li: Manganese ratio greater than or equal to 0.5, preferably from 0.5 to 2. The preparation process used also has the advantage of being significantly less expensive than the other two first processes of the prior art described above. It could therefore be estimated, based on the costs evaluated by comparing the prices of one mole of lithiation reagent (mole of lithium), that the preparation process used had a cost divided by about 18 compared to the process of preparation using lithium iodide and a reaction time divided by 20, and a cost divided by 3 compared to the process using n-butyl lithium and a reaction time divided by at least 100.

The preparation process used has the advantage of being able to be carried out at low temperature; by low or low temperature, it is generally understood that the various stages are either carried out at ambient temperature - generally close to 20 ° C. - as is the case with the preparation of the lithium alkoxide solution, the addition of manganese oxide powder with lithium alkoxide solution or alternatively rinsing the powder obtained, either at a low or low temperature, as is the case with controlled reduction, which is generally carried out at a temperature of 50 or 70 to 260 ° C which corresponds substantially to the boiling temperature of the alcohol used (brought to reflux), or drying generally carried out at a temperature of 80 to 150 ° C. Of course, these temperature ranges can be lowered to room temperature or more 19

low, if instead of working at atmospheric pressure one operates under reduced pressure.

The use of low or low temperatures favors obtaining an electrochemically reversible compound and of Li: Metal stoichiometry perfectly defined.

The fact of using, in the accumulator according to the invention, a specific active positive electrode material based on manganese oxide of lithiated or superlithiated spinel structure capable of being obtained by the process described above, makes it possible, compared to the performances observed on the cells manufactured by the current manufacturers of lithium accumulators but not using this material, to obtain the following performances in terms of:

• energy density: 130 Wh / kg

• lifespan: 800 to 1000 cycles

• cost: reduced by at least 15%

It can therefore be seen that the performance is at least equivalent to that of the batteries currently on the market, at a cost reduced by at least 15%.

Indeed, the use of this type of material based on manganese oxide of lithiated or superlithiated spinel structure according to the preparation process described above in place of the compounds usually used makes it possible, with respect to the performances 20

presented in relation to the sectors marketed today:

• to conserve or even improve the cycle life thanks to the improvement of the electrochemical behavior of the active material of positive electrode based on specific lithiated or superlithiated manganese oxide: it is indeed "formed" during the prelithiation process in its "discharged" configuration in which the crystal is the most constrained (and in which it contains at least as much lithium as what it contains after a complete discharge of the accumulator) and introduced as is into the positive electrode of the accumulator during the development of the latter. When a non-lithiated or non-overlithiated manganese oxide material is integrated into a positive electrode, it undergoes during the first discharge (first insertion of lithium) strong stresses which can go as far as fracturing the grains of active material and thus harm (by loss of contact between grains of material) the good quality of the electronic and ionic contact within the positive electrode layer which is essential for correct electrochemical functioning.

• to conserve or even improve the energy density (Wh / kg and Wh / 1) of the accumulator, thanks to:

1) the choice of particular crystallographic structures of manganese oxides, which, 21

having undergone the lithiation or prelithiation process described above, release a reversible capacity greater than 180 mAh per gram of active material in an operating voltage range between 1.8 and 4.3 Volts: despite irreversible consumption, during of the first charge, of a part of the lithium coming from the active material of positive electrode in the carbonaceous material of negative electrode, the large quantity of lithium stored by construction in the active material of positive electrode compensates for this loss of capacity and the complete accumulator has an energy density close to 130 Wh / kg.

• significantly reduce the cost of accumulators (at least 15% of the cost)

The part of the active compound of positive electrode based on cobalt or nickel oxide in the total cost of the accumulator can be evaluated at 20%.

The cost of materials based on manganese oxides (including the lithiation process) being approximately three times lower than that of the abovementioned oxides, it can be estimated that their use as active material of positive electrode will allow a gain of the around 15% of the cost of accumulators, with equivalent electrochemical performance. 22

According to the invention, the lithiated or superlithiated manganese oxide of spinel structure capable of being obtained by the process for the preparation of a lithiated or superlithiated manganese oxide of spinel structure as described above is preferably obtained of an initial compound chosen from manganese oxides of spinel structure and located in the field of the phase diagram Li, Mn, 0 delimited by the compositions Mn0 ₂ , Li ₂ Mn ₂ 0 ₄ , Li ₄ Mn ₅ 0ι ₂ a treatment thermal is then not necessary.

This initial compound can also be β- Mn0 ₂ , one then obtains at the end of the lithiation treatment or over-lithiumization and without it being necessary to implement an additional heat treatment a spinel Li ₂ Mn ₂ 0 ₄ which is manganese oxide lithiated or superlithiated playing the role of active material of positive electrode.

The initial compound can be a compound of formula Li ₀ , 3 nO ₂ ("TADIRAN") or a compound of neighboring formula (for example Lio, ₃₃ Mn0 ₂ , ₀₃ ) at the end of the lithiation a compound of formula is obtained close to LiιMn0 ₂ ("TADIRAN Lithié") which does not have the spinel structure, this compound is stable and when it is heated at 250 ° C. for 1 hour does not exhibit any change in structure.

On the other hand, if this compound is subjected to a heat treatment at a temperature of 300 ° C. to 800 ° C. for a period of 30 minutes to 3 hours, preferably 1 hour, for example, if this compound is heated for 1 hour at 300 ° C., a compound formed from a mixture of phases is obtained 23

xLiιMn0 ₂ + (1-x) Li ₂ Mn ₂ 0 <j (spinel structure) with 0 <x <1

Indeed, the compound forming the active electrode material must generally always include even an infinitesimal quantity of spinel compound.

By varying the temperature of the heat treatment, for example from 300 ° C. to 800 ° C., preferably from 300 to 500 or 600 ° C., and its duration, for example, from 30 minutes to 3 hours, preferably from 1 to 3 hours, it is possible to varies x and we obtain all possible mixtures.

Finally, the starting compound can be gamma Mn0 ₂ or ε Mn0 ₂ . A heat treatment is then used at the end of the lithiation in order to obtain a particular structure described below.

The invention will be better understood on reading the description which follows given by way of illustration and not limitation with reference to the attached drawings in which:

- Figure 1 is an X-ray diffraction diagram (ordinate: Intensity in arbitrary unit, abscissa: 2 Theta in degrees, λ = 1.5406 Â) showing the evolution of the crystallographic structure of an initial compound of spinel type of formula Liι ₂ 9Mnι, ₇ ι0 ₄ with Li: Mn = 0.75 (bottom diagram), after lithiation by the process according to the invention (top diagram) and insertion of 0.56 mole of Li, as is described in Example 1. 24

- Figure 2A is a figure giving U (V) as a function of Dx (Li / Mn) for an electrode obtained from compound Li _0, ₃ 3Mn0 _{2 03} ( "TADIRAN") lithiated then heat-treated at 300 ° C for 1 hour; - Figure 2B is the derivative of Figure 2B and carries dx / dU as a function of U (V);

- Figures 3 and 4 respectively represent a 1st charge-discharge cycle and the first 7 charge-discharge cycles for the accumulator of Example 1, on the abscissa is indicated the capacity C in mAh / g and on the ordinate the voltage (U) in volts;

FIG. 5 is a figure giving U (V) as a function of Δx (Li / Mn) for the accumulator of Example 3.

More specifically, the lithiated or superlithiated manganese oxide can be obtained by a preparation process which therefore firstly comprises the preparation of a solution of lithium alkoxide in the corresponding alcohol.

The alcohol is according to the invention chosen from alcohols derived from linear or branched alkanes comprising at least 3 carbon atoms, preferably from 3 to 10 carbon atoms, alcohols derived from unsaturated aliphatic hydrocarbons, and their mixtures.

Pentanol-1 and isopropanol are preferred. Alcohol is usually found in excess compared to lithium. 25

The lithium alkoxide solution is generally prepared by adding lithium metal in alcohol at room temperature (generally 20 ° C) and under an atmosphere of inert gas such as argon or dry air.

The lithium alkoxide solution thus obtained generally has a concentration greater than or equal to 0.01 mole per liter, preferably a concentration of 0.1 to 10 moles per liter of lithium alkoxide. A transition manganese oxide powder is then added to the solution of lithium alkoxide in the corresponding alcohol prepared during the first step, in order to obtain a dispersion.

The lithium alkoxide is generally in slight excess relative to the corresponding alcohol.

Said manganese oxide is chosen from manganese oxides, and mixed manganese oxides with another metal chosen, for example, from alkali and alkaline earth metals. By oxide is also meant all the crystalline forms which the manganese oxides mentioned above can take.

The reduction step provided is generally carried out by heating the dispersion obtained in the previous step to reflux, then maintaining it at reflux at atmospheric pressure and at the reaction temperature, which therefore corresponds substantially to the boiling temperature. alcohol used. This boiling temperature can of course vary with the working pressure. 2 6

The reaction is preferably carried out with stirring and under an inert atmosphere, preferably under an argon sweep.

However, it can also be carried out under reduced pressure relative to atmospheric pressure, which makes it possible to reduce the boiling temperature of the alcohol used, and therefore to carry out the reaction at a lower temperature.

In the case where pentanol-1 is used as alcohol, the reflux can thus be carried out at atmospheric pressure at a temperature of 139 ° C. substantially equal to the boiling temperature of pentanol-1 at atmospheric pressure .

In this same case of use of pentanol-1 where one chooses to work under pressure lower than atmospheric pressure, the reflux is then carried out at a temperature below 139 ° C, and typically at a temperature between room temperature and 139 ° C. This reduction in the working temperature allows, at the industrial stage:

- reduce the production costs related to heating the solution;

- to carry out the lithiation treatment at a lower temperature.

During this stage, the alkoxide operates a controlled reduction of the manganese oxide until obtaining a Li: Metal stoichiometry defined thus in the case where one starts from Lio, 3Mn0 ₂ or a compound of formula neighbor we get as a lithiated product 27

or superlithium of LiιMn0 or a compound of similar formula.

As already indicated above, the three steps described above can be carried out successively or simultaneously.

The next step consists in evaporating the solvent, that is to say the residual alcohol; the temperature at which this step is carried out depends on the alcohol used and is generally from 50 or 70 to 260 ° C. under atmospheric pressure.

The powder obtained is then rinsed in an adequate liquid, for example in the alcohol previously used as solvent, hexane, tetrahydrofuran, in order to remove any excess alkoxide, then the powder is dried, generally under partial vacuum, and at a temperature for example of 80 to 150 ° C, preferably close to 150 ° C for an adequate period. The drying temperature can be higher, namely up to 250 ° C. or even 400 ° C., but on condition that this does not cause any modification of the structure of the lithiated or over-lithiated product.

The purpose of drying is to remove impurities and other organic elements which may be present on the surface of the product.

The method according to the invention may also comprise an additional step of heat treatment carried out after drying. The heat treatment is carried out at a temperature generally of 150 ° C to 800 ° C preferably 28

from 250 to 600 ° C, more preferably from 300 to 500 ° C and for a period of 30 minutes to 3 hours, preferably from 1 to 2 hours, generally in a neutral atmosphere. The main purpose of this heat treatment, which should not be confused with drying, is to possibly modify the structure and / or the electrochemical behavior of the lithiated or superlithiated material. Thus, the lithiation by the process according to the invention a crude product of composition Li _0, 33Mn0 ₂ o3 as used by TADIRAN Batteries leads to the synthesis of a composition similar product LiMn0 _2. If this product is subjected to a heat treatment under a neutral atmosphere at 250 ° C for 1 hour, its structure is not changed, which corresponds to that of the initial TADIRAN product having undergone an electrochemical discharge in an accumulator. If, on the other hand, the same product is subjected to a heat treatment at a higher temperature, for example at 300 ° C. for 1 hour, the partial transformation of the initial structure is observed into a quadratic spinel structure of Li ₂ Mn ₂ 0 _{4 type} . This product, consisting of a mixture of very disordered crystallographically phases, has the particularity of exhibiting an interesting electrochemical behavior between 1.8 and 4.3 volts.

Indeed, this behavior is intermediate between the electrochemical behavior of TADIRAN products and that of spinel structures of LiMn ₂ 0 _{4 type} . 29

It is therefore possible to modulate at will the relative importance of the electrochemical stages due to each of these phases, and thus to have a product whose discharge voltage changes more regularly as a function of the insertion of lithium than that of each. products taken separately.

This behavior is illustrated in FIGS. 2A and 2B.

We no longer observe the double plateau characteristic of the spinel structures illustrated in FIGS. 3 and 4 but a continuous and regular decrease in the discharge voltage as a function of the lithium insertion rate.

It is thus possible to synthesize a whole range of new products (constituted by mixtures of the two lithiated structures) having interesting electrochemical characteristics.

It is also possible to use a manganese oxide of type γ-Mn0 ₂ or ε-Mn0 ₂ , and to subject it to:

1) the lithiation treatment according to the process defined above, resulting for example in the composition LiMn0 ₂ .

2) an additional heat treatment for example between 250 ° C for 1 hour and 500 ° C for 1 hour which gives it a particular crystallographic structure (intermediate between a structure of type γ-Mn0 ₂ or ε-Mn0 ₂ and a very spinel structure disordered formulation Li ₂ Mn ₂ 0). Heat treatment 30

here relates to the production of a product whose electrochemical characteristics

(discharge and charge curves) are adapted as required. By high Li: Metal stoichiometry, we generally mean a Li: Metal ratio greater than or equal to

0.5, preferably 0.5 to 2.

The stability of the active positive electrode compound used in the accumulator according to the invention corresponds to a stable formulation thereof which is little affected, in particular by a prolonged stay in dry or ambient air, - by stay extended is generally understood to mean a stay lasting 24 to 240 hours - or by immersion in water. This stability is defined by the observed stability of the X-ray diffraction spectra and by the invariant nature of the degree of oxidation of manganese.

This stability of the product according to the invention makes it possible to package, store and handle it more easily.

In the accumulator according to the invention, the negative electrode is conventionally preferably a composite negative electrode, for example based on carbon or tin-based or also in SnO ₂ , or in lithium alloy or any other suitable negative electrode.

Similarly, the electrolyte used is a conventional electrolyte for this type of accumulator, namely a mixture of non-aqueous electrolyte known to those skilled in the art. 31

Generally speaking, the negative electrode,

1 electrolyte and the structure of the battery according to the invention are known to those skilled in the art with the exception of course active material of positive electrode specific to the invention.

The invention will now be described with reference to the following examples given by way of illustration and limitation.

EXAMPLES:

The following examples show the preparation of a manganese oxide lithiated by the process of the invention and its use in an accumulator as active material of positive electrode.

Example 1: Lithiation of an initial compound with cubic spinel structure.

This initial product has a formulation close to Liι ₂₉ Mnι, 7i0 with a Li: Mn ratio of 0.75 for an average oxidation degree of manganese determined by redox dosage of +3.90, and has a cubic mesh parameter close 8.15 angstroms. (the X-ray diffraction diagram for this compound is reproduced in FIG. 1 - diagram at the bottom)

This product was lithiated by the following process: 600 ml of pentanol-1 (purity> 99%) are introduced into a flask with three necks containing 1000 ml. In this balloon equipped with a condenser and whose interior is isolated from the ambient atmosphere, it is bubbled 32

for one hour and at a temperature in the region of 20 ° C of argon in alcohol subjected to slight stirring. 3.36 g (or 0.48 moles) of metallic lithium in the form of chips are introduced into the alcohol. The complete dissolution of lithium in pentanol-1 and therefore the formation of the corresponding alkoxide is carried out after two hours at room temperature. 34.8 g of manganese oxide of the spinel type (ie approximately 0.4 mole of manganese), previously dried under primary vacuum at 120 ° C. for one hour, are then introduced into the lithium alkoxide solution at room temperature .

The dispersion thus obtained is heated at atmospheric pressure to reflux at 139 ° C., ie the boiling point of pentanol, and is kept at this temperature for six hours.

• This dispersion can similarly be heated at reflux to a lower temperature, between room temperature and 139 ° C, if working at pressure below atmospheric pressure.

The dispersion, once cooled to a temperature close to ambient, is filtered with ambient air on sintered glass. The lithiated oxide is then rinsed with hexane, then dried and treated under primary vacuum at 300 ° C for one hour.

The final lithiated product then contains manganese with an average oxidation state close to +3, reflecting the insertion of approximately 0.6 mole of lithium per mole of manganese with a Li: Mn ratio of 1.08. 33

The crystallographic structure of the lithiated compound determined by X-ray diffraction is that of a deformed spinel with quadratic mesh parameters a = b = 5.691 Â and c = 8.768 Â (c / (V2-a) = 1.09) (figure 1 - top diagram). This is similar to that observed on compounds of the initial oxide type which have undergone electrochemical intercalation of lithium in an accumulator.

A positive electrode with a diameter of 8 mm and a thickness close to 300 microns integrating the lithiated compound prepared above is obtained by pressing at 10 tonnes an intimate mixture composed of 55% by mass of lithiated oxide, 30% by mass of black carbon and 15% by mass of graphite. An accumulator or electrochemical cell with a metallic lithium electrode is produced in a conventional Swagelock® type assembly. The electrolyte used is a molar solution of lithium perchlorate in an equimolar mixture of ethylene carbonate and dimethyl carbonate.

The separator used between the negative electrode and the positive electrode is a microporous polypropylene film of CELGARD® type.

The behavior of such an accumulator containing 10.8 milligrams of lithiated product in the positive electrodes, charged and discharged at 0.150 mA is shown in FIGS. 3 and. (1st charge-discharge cycle: figure 3; first 7 cycles indicated in figure 4). 34

Example 2: Lithiation of the initial compound of formulation close to LiMn ₂ 0 ₄ and of spinel structure for a degree of oxidation close to +3.58, determined by assay and which is a material suitable for 4V use in Li-ion accumulator .

The product was lithiated by the process of the invention in the following way: 745 ml of pentanol-1 are introduced into a flask with 3 necks containing 2000 ml. In this flask equipped with a condenser and the interior of which is isolated from the ambient atmosphere, argon is bubbled for 30 minutes and at a temperature close to 20 ° C. in alcohol subjected to slight agitation.

0.52 g (or 0.075 moles) of metallic lithium in the form of a shot and 13.38 g of LiMn ₂ 0 ₄ (or 0.145 mole) previously dried are introduced together into the alcohol. The contents of the reactor are then heated with stirring using a balloon heater. At a temperature in the region of 110 ° C, the lithium dissolves quickly and completely, which has the effect of bringing all the alcohol contained in the flask to the boil, ie a temperature in the region of 139 ° C. Thereafter, the boiling is maintained by heating for 16 hours. Once cooled, the dispersion is centrifuged under a dry atmosphere so as to separate the powder from the liquid phase. The oxide powder is rinsed twice in hexane, then dried at room temperature. The final lithiated product then contains manganese at an average degree of oxidation close to +3.08 35

reflecting the insertion of 0.50 mole of lithium per mole of manganese with a Li: Mn ratio close to 1.04.

A positive electrode with a diameter of 12 mm and a thickness close to 250 microns integrating the lithiated compound prepared above is obtained by using an intimate mixture of 80% by mass of lithiated oxide, 7.5% by mass of graphite , 7.5% by mass of carbon black and 5% by mass of PTFE binder on an aluminum grid. This electrode is then dried at 150 ° C under primary vacuum overnight.

An accumulator or an electrochemical cell with a metallic lithium electrode is produced in a CR2032 button cell. The electrolyte is a molar solution of lithium hexafluorophosphate (LiPF ₆ ) in a mixture made of 33% by weight of ethylene carbonate

(EC) and 67% by weight of di ethoxyethane (DME) (for example electrolyte LP32 from Merck). A Celgard type microporous polypropylene separator (Hoechst) is used between the two electrodes. The behavior of such an accumulator containing 62.25 mg of prelithiated product in the positive electrode, charged at 0.4175 mA and discharged at 0.835 mA is shown in FIG. 5.

Claims

3 6 CLAIMS

1. Lithium accumulator operating between 1.8 and 4.3 volts and comprising a negative electrode, an electrolyte and a positive electrode characterized in that said positive electrode comprises as active material, and before it has undergone any charge or discharge operation, a lithiated or superlithiated manganese oxide of spinel structure capable of being obtained by a preparation process comprising the following three steps, carried out successively or simultaneously:

- Preparation of a lithium alkoxide solution by dissolving metallic lithium in the corresponding alcohol, said alcohol being chosen from alcohols derived from linear or branched alkanes comprising at least 3 carbon atoms, alcohols derived from unsaturated aliphatic hydrocarbons , and mixtures thereof; - Adding a manganese oxide powder to said lithium alkoxide solution to obtain a dispersion; controlled reduction of said manganese oxide by said alkoxide until a lithiated or superlithiated manganese oxide having the defined stoichiometry Li: Metal is obtained; said process then further comprising the following steps:

- evaporation of the residual alcohol; - rinsing of the powder thus obtained;

- drying of the powder; 37

- possibly, additional heat treatment step carried out after drying.

2. Accumulator according to claim 1 wherein said heat treatment is carried out at a temperature of 150 ° C to 800 ° C for a period of 30 minutes to 3 hours.

3. Accumulator according to claim 1 or claim 2 in which the initial manganese oxide is chosen from manganese oxides of spinel structure located in the domain of the phase diagram LiMn, 0 delimited by the compositions Mn0 ₂ , Lι ₂ Mn ₂ 0 ₄ , Lι ₄ Mn ₅ 0i2, beta-Mn0 ₂ , gamma-Mn0 ₂ , epsιlon-Mn0 ₂ , the compounds of formula Lι ₀ , 3MnO ₂ or of similar formula.

4. Accumulator according to claim 3 wherein the initial manganese oxide is β-Mn0 ₂ or a manganese oxide of spinel structure located in the cited field and in which no additional heat treatment is carried out.

5. Accumulator according to claim 3 in which the initial manganese oxide is a compound of formula Lι ₀ 3MnO ₂ or of neighboring formula and in which it is subjected to a heat treatment at a temperature of 300 ° C to 800 ° C for a duration from 30 minutes to 3 hours.

6. Accumulator according to claims 1 and 5 wherein said compound forming the active material of positive electrode is formed from a mixture of phases xLiιMn0 ₂ + (1-x) Lι ₂ Mn ₂ 0 ₄ with 0 <x <1.