Patent Application
20170301918
The invention relates to a material made from lithium-borate having formula LiFe1-xCoxBO3, as well as to the use thereof as an electrode material and to its preparation method.
The field of use of the present invention particularly relates to energy storage, and more particularly to lithium-ion batteries.
During the last decades, different types of batteries have been developed to respond to the size, weight, and capacity requirements depending on the nature of the electronic devices. For example, lithium-ion batteries are particularly well adapted to portable electronic equipment in terms of energy density and of time stability (charge/discharge cycles).
Generally, a lithium-ion battery is an assembly of a positive electrode (cathode), comprising a material made from lithium, and of a negative electrode (anode) generally made from carbon (graphite, for example). Its operation is ensured by the reversible exchange of Li+ ions between the cathode and the anode, the electrodes being separated by an electrolyte made from lithium salt.
In the development of lithium-ion batteries, many positive electrode materials have been tested, and particularly LiMPO4 phosphates (M=Mn, Fe, or Co). Such materials are advantageous and arouse much interest due to the security that they provide and to their low cost. However, their theoretical specific capacity remains limited to 170 mAh/g for LiFePO4.
To obtain batteries having higher specific capacities, other materials have been envisaged, particularly LiMBO3 borates (M=Mn, Fe, or Co). These materials have the advantage of having a maximum theoretical capacity (220 mAh/g) greater than that of LiMPO4 phosphates, while being as attractive in terms of security.
However, the redox potentials of couples Fe2+/Fe3+ and Mn2+/Mn3+ are relatively low, which results in limiting the energy density of the LiFeBO3 and LiMnBO3 compounds.
The LiCoBO3 compound enables to improve the energy density, given that the Co2+/Co3+ redox couple of cobalt has a higher potential than couples Fe2+/Fe3+ and Mn2+/Mn3+. However, the disadvantage of the LiCoBO3 compound with respect to LiFeBO3 and LiMnBO3 compounds is its rather low experimental reversible capacity.
There thus is a need to improve the properties of lithium-borate materials, by developing a material having the following properties:
The present invention relates to lithium-borate compounds, having properties enabling to solve this problem. They comprise at the same time the two transition metals, iron and cobalt.
Further, the Applicant has observed that prior art methods do not enable to prepare such compounds comprising, at the same time, lithium iron and cobalt borates. Such methods do not result in the forming of a single phase containing the iron and the cobalt.
To overcome this technical issue, the Applicant has developed a novel method, in a plurality of steps, which takes into account possible incompatibilities relative to the respective reactivities of the cobalt and iron compounds.
The present invention aims at a lithium-borate material containing both iron and cobalt. The partial substitution of iron with cobalt in the LiFeBO3 material enables to improve the energy density by increase of the average reaction potential.
This material may be used in various fields of application, particularly in the field of energy storage. It appears to be particularly attractive to form the electronically-active material of the cathode of a lithium-ion battery.
More specifically, the present invention relates to an electrode material of formula LiFe1-xCoxBO3, where 0<x<1.
Advantageously, x is greater than 0 and smaller than or equal to 0.3 (0<x≦0.3), and, more advantageously still, x=0.3.
This material appears in crystal form and crystallizes in a monoclinic cell. It has a theoretical specific mass capacity in the range from 215 to 220 mAh/g, which makes it particularly adapted for a use as an active electrode material, advantageously for the cathode of a lithium-ion battery.
Another object of the invention relates to the method of solid-state preparation of such an electrode material of formula LiFe1-xCoxBO3, with 0<x<1. The method comprises the steps of:
This method requires separately synthesizing the iron and cobalt borates. The conditions adapted to each of these borates thus ease the stabilizing of the cobalt in the 2+ oxidation state during the forming of the mixed LiFe1-xCoxBO3 compound.
However, the mixed LiFe1-xCoxBO3 compound is not obtained when the iron, cobalt, and boron compounds are treated simultaneously in a single step. In this case, the cobalt 2+ can be easily reduced into metal cobalt during a thermal treatment under an inert atmosphere, while the iron 2+ can be easily oxidized into iron 3+ during a thermal treatment in air.
Generally, the iron compound used at step a1) is advantageously selected from the group comprising iron oxalate (FeC2O4, which may in particular be in hydrated or non-hydrated form); iron carbonate (FeCO3); and iron oxide (II) (FeO). A mixture of these compounds may also be used.
Generally, the cobalt compound used at step b1) is advantageously selected from the group comprising: cobalt oxalate (CoC2O4, which may in particular be in hydrated or non-hydrated form); cobalt carbonate (hydrated or non-hydrated CoCO3); and cobalt oxide (II) (CoO). A mixture of these compounds may also be used.
The boron compound used at steps a1) and b1) is advantageously boron oxide (B2O3) or boric acid (H3BO3). A mixture of these compounds may also be used.
The boron compound used at step a1) may be different from that used at step b1).
At step a1), the molar ratio between the iron compound and the boron compound is generally in the range from 0.6 to 1.2, more advantageously from 0.8 to 1.
At step b1), the molar ratio between the cobalt compound and the boron compound is generally in the range from 0.6 to 1.2, more advantageously from 0.8 to 1.
Molar ratio means the molar ratio of the molar quantity of iron, or of cobalt, to the molar quantity of boron.
The milling implemented at steps a1) and b1) may be performed by any adapted means known by those skilled in the art. It enables to reduce the solid iron, cobalt, and boron compounds into a fine powder.
The milling enables not only to obtain a fine homogeneous powder, but also to increase the specific surface area of these compounds and thus to improve their reactivity.
Advantageously, the milling of steps a1) and/or b1) may be carried out in a conventional ball mill.
According to a specific embodiment, when the mill is a ball mill, the rotation speed of the mill is advantageously in the range from 100 to 900 revolutions/minute, more advantageously from 250 to 750 revolutions/minute. It may also be equal to 500 revolutions/minute.
Advantageously, the duration of the milling of steps a1) and/or b1) is in the range from 0.5 to 24 hours, more advantageously from 1 to 12 hours. It may in particular be equal to 5 hours.
Preferably, the milled product resulting from step a1) or b1) appears in the form of a powder having an average grain diameter advantageously in the range from 0.1 to 30 micrometers, and more advantageously from 0.5 to 10 micrometers.
The milling conditions of steps a1) and b1) are independent from one another. In other words, the milling conditions for iron are not necessarily the same as for cobalt. Further, the average grain diameter of the mixture resulting from step a1) (iron) is not necessarily the same as that of the mixture resulting from step b1) (cobalt).
The thermal treatment (calcination) of the step a2) or b2), which follows mixing and milling step a1) or b1), enables the iron or cobalt compound to react with the boron compound to produce an iron borate or a cobalt borate.
As previously indicated, the thermal treatment of steps a2) and b2) is carried out under a different atmosphere for each material:
For these reasons, the iron and cobalt borates are prepared separately, prior to the synthesis of the LiFe1-xCoxBO3 compound by subsequent thermal treatment under an inert atmosphere at step d) and this, without reducing the cobalt 2+ into metal cobalt.
Advantageously, the thermal treatment (a2 and/or b2) may be a step of thermal quenching, that is, a treatment which comprise no progressive temperature rise.
According to another embodiment, the temperature of the thermal treatment (a2 and/or b2) is reached by applying a heating speed, advantageously in the range from 1 to 20° C./minute, more advantageously from 2 to 10° C./minute.
Preferably, the thermal treatment according to step a2) comprises heating the mixture resulting from step a1) up to a temperature advantageously in the range from 300 to 1,000° C., more advantageously from 550 to 850° C. It may also be in the range from 650 to 850° C., particularly when the thermal treatment is a thermal quenching step.
Preferably, the thermal treatment according to step b2) comprises heating the mixture resulting from step b1) up to a temperature advantageously in the range from 300 to 1,000° C., more advantageously from 550 to 850° C. It may also be in the range from 700 to 850° C., particularly when the thermal treatment is a thermal quenching step.
The duration of the thermal treatment (a2 and/or b2) is advantageously in the range from 5 to 1,200 minutes, more advantageously from 5 to 30 minutes. This duration may be in the range from 5 to 20 minutes, particularly when the thermal treatment is a thermal quenching step.
At the end of step a2) or b2), the iron or cobalt borate is cooled. The cooling speed is advantageously in the range from 2 to 20° C./minute, more advantageously from 5 to 10° C./minute, until the room temperature is reached.
Advantageously, the cooling is an air quenching step, that is, a treatment comprising no progressive temperature decrease. It is thus instantaneously passed from the thermal treatment temperature to the room temperature. For iron borate, it is an air quenching of the vessel (tube, for example) containing the sample (the closed vessel containing the sample is taken out in air but remains under an inert atmosphere).
The duration of the thermal treatment (a2 and/or b2) does not include the heating or cooling time.
The conditions of the thermal treatment of steps a2) and b2) are independent. In other words, the heating speed, the duration, and the temperature of the thermal treatment, and the cooling speed relative to iron borate are not necessarily the same as those relative to cobalt borate.
Further, the thermal treatment conditions, relative to the inert or oxidizing atmosphere, are adapted to the reactivity of the iron and cobalt compounds. Such a condition difference at steps a2) and b2) enables to subsequently obtain the monoclinic crystal compound of formula LiFe1-xCoxBO3 (with 0<x<1).
Once steps a) and b) have been carried out, the iron borate and the cobalt borate are mixed (and advantageously milled) in the presence of a precursor of lithium and of boric acid (step c)).
It will be within the abilities of those skilled in the art to adjust the respective quantities of these compounds to obtain the material of formula LiFe1-xCoxBO3 (with 0<x<1).
According to a specific embodiment, the mixture of step c) may comprise, for one mole of lithium:
The lithium precursor used at step c) is advantageously lithium carbonate (Li2CO3) or lithium hydroxide. Lithium hydroxide may appear in its hydrated or non-hydrated form (LiOH or LiOH.xH2O). It may also be a mixture of these compounds.
According to a specific embodiment, step c) may be followed by a milling step, prior to the thermal treatment of step d). This optional milling is advantageously performed in a ball mill. In this case, the mill rotation speed is advantageously in the range from 100 to 900 revolutions/minute, more advantageously from 250 to 750 revolutions/minute.
The duration of the optional milling preceding step d) is advantageously in the range from 0.5 to 24 hours, more advantageously from 1 to 12 hours.
The optional milling preceding step d) enables to obtain a homogeneous powder having an average diameter advantageously in the range from 0.1 to 30 micrometers, more advantageously from 0.5 to 10 micrometers.
The mixture resulting from step c) (possibly milled) is then thermally treated at step d).
The temperature of the thermal treatment of step d) is advantageously in the range from 300 to 900° C., more advantageously from 400 to 700° C., and more advantageously still from 400 to 600° C.
The heating speed is advantageously in the range from 1 to 20° C./minute, more advantageously from 2 to 10° C./minute.
As already indicated, the thermal treatment of step d) is performed under an inert atmosphere, for example, under argon, or under nitrogen. Preferably, it is performed under argon.
The duration of the thermal treatment of step d) is advantageously in the range from 15 to 1,200 minutes, more advantageously from 30 to 1,200 minutes, and more advantageously still from 45 to 180 minutes. It may in particular be equal to 120 minutes. It is advantageously greater than or equal to 75 or 90 minutes, particularly at 500° C.
Advantageously, the thermal treatment may be a step of thermal quenching, that is, a treatment which comprises no progressive temperature rise.
At the end of step d), the material is cooled. The cooling speed is advantageously in the range from 2 to 20° C./minute, more advantageously from 5 to 10° C./minute, until the room temperature is reached.
Advantageously, the cooling is an air quenching step, that is, a treatment comprising no progressive temperature decrease. It is thus instantaneously passed from the thermal treatment temperature to the room temperature.
The duration of the thermal treatment of step d) does not include the heating or cooling time.
According to a specific embodiment, the thermal treatment of step d) is advantageously a thermal quenching performed at a temperature in the range from 450 to 600° C., advantageously from 400 to 550° C., for a duration in the range from 15 to 120 minutes.
According to a preferred embodiment, the method comprises the steps of:
The final product obtained at step e) is a material of formula LiFe1-xCoxBO3 where x is greater than 0 and smaller than 1. In other words, the material necessarily comprises iron or cobalt.
It is a monoclinic crystal material. It generally appears in the form of particle agglomerates.
The average diameter of the agglomerates obtained after the thermal treatment of step d) is advantageously in the range from 0.5 to 10 micrometers, more advantageously from 0.5 to 5 micrometers. It depends, in particular, on the nature of the thermal treatment. It is advantageously in the range from 1 to 5 micrometers in the case of thermal quenching, while it is advantageously in the range from 4 to 10 micrometers when the thermal treatment does not correspond to a quenching step.
The average diameter of the primary particles forming the agglomerates and obtained after the thermal treatment of step d) is advantageously in the range from 0.1 to 1 micrometer, more advantageously from 0.1 to 0.5 micrometer. It depends, in particular, on the nature of the thermal treatment. It is advantageously in the range from 0.1 to 0.4 micrometer in the case of thermal quenching, while it is advantageously in the range from 0.5 to 1 micrometer when the thermal treatment does not correspond to a quenching step.
The present invention also relates to the lithium-ion battery comprising a cathode where the electronically-active material is the material of formula LiFe1-xCoxBO3, with 0<x<1.
It will be within the abilities of those skilled in the art to implement conventional techniques to prepare this battery, particularly by preparing a cathode by deposition of the LiFe1-xCoxBO3 material on a current collector.
As an example, the deposition may be that of an ink containing the active electrode material previously milled in the presence of an electron conductor Typically, the electron conductors used are vapor grown carbon fibers (VGCF), or more advantageously carbon black such as Ketjenblack®.
The invention and the resulting advantages will better appear from the following non-limiting drawings and examples, provided as an illustration of the invention.
Compounds of formula LiFe1-xCoxBO3 (x=0; 0.5; and 1) have been prepared according to an embodiment of the method forming of object of the present invention.
1/ Preparation of LiFe1-xCoxBO3 (x=0; 0.5; and 1)
The LiFe1-xCoxBO3 compound has been prepared according to the steps of:
In this method, the iron borate, Fe2B2O5, and the cobalt borate, Co3B2O6, are synthe-sized separately.
Step a): Iron Borate Synthesis
The boron and iron compounds are dispersed in cyclohexane and mixed for five hours at 500 revolutions per minute in a 50-ml bowl containing 10 stainless steel balls by means of a planetary mill (Retsch). The cyclohexane is then evaporated in air.
This mixture is then thermally treated in an alumina crucible under an inert atmosphere (argon) at 800° C. for 30 minutes with a temperature ramp of 5° C. per minute in rising mode and of 10° C. per minute in falling mode.
Step b): Cobalt Borate Synthesis
The boron and cobalt compounds are mixed in the same way as for the iron borate.
The mixture is then thermally treated in an alumina crucible in air at 800° C. for six hours with a temperature ramp of 5° C. per minute in rising mode and of 10° C. per minute in falling mode.
Step c): Preparation and Milling of a Mixture of Fe, Co, Li Compounds
The iron and cobalt borates are then mixed with the lithium salt and the boric acid. For this purpose, the compounds, in powder form, are dispersed in cyclohexane and mixed for five hours at 500 revolutions per minute in a 50-ml bowl containing 10 stainless steel balls by means of a planetary mill (Retsch).
The cyclohexane is then evaporated in air.
Steps d) and e): Thermal Treatment and Obtaining of LiFe1-xCoxBO3
The mixture is then thermally treated in an alumina crucible under an inert atmosphere (argon) at 500° C. for 90 minutes with a temperature ramp of 5° C. per minute in rising mode and of 20° C. per minute in falling mode.
The compounds used and the respective quantities are mentioned in tables 1 and 2. The lithium and the boron are introduced in slight excess relative to the iron and/or to the cobalt.
2/ Electrochemical Tests:
a) Preparation of the Positive Electrode
The active LiFe0,5Co0,5BO3 material is mixed by 85 wt. % with carbon of large specific surface area (Ketjenblack JD600) (15 wt. %) for 4 hours at 500 revolutions per minute in a 50-ml bowl containing 10 stainless steel balls by means of a planetary mill (Retsch).
Then, the obtained product is mixed by 90 wt. % with polyvinylidene fluoride (10 wt. %) dissolved in N-methyl-2-pyrrolidone.
Finally, the mixture is spread on an aluminum foil (100-μm) and then dried at 60° C.
The electrode is then made of 76.5 wt. % of active material; 13.5 wt. % of carbon, and 10 wt. % of polyvinylidene fluoride (PVDF).
b) Mounting of the Accumulator
The formed electrode is introduced into a cell of “button cell” type at format 2032.
The negative electrode is made of metal lithium.
Two types of separators have been used:
The electrolyte used is made of ethylene carbonate, of propylene carbonate, of dimethyl carbonate, and of lithium hexafluorophosphate (LiPF6) (Powerlyte's Electrolyte LP100).
c) Galvanostatic Cycling
At room temperature, a current is imposed to the system to obtain a C/20 rate, that is, the extraction/insertion of a lithium ion within 20 hours.
Further,
3/ Characterization of the LiFe1-xCoxBO3 Compound:
| Number | Date | Country | Kind |
|---|---|---|---|
| 1460004 | Oct 2014 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2015/052253 | 8/24/2015 | WO | 00 |
