NO880921L - ANODE FOR BATTERIES. - Google Patents

Abstract

Anode material comprising a solid solution of 1 lithium metal 1 and 1 lithium nitride for use in lithium-based electrochemical cells. The anode material may comprise 1 lithium alloys, carbon and polymeric materials.

Description

The invention relates to the use of a mixture of lithium

in*

and nitrogen as anode material in rechargeable or non-rechargeable high energy density batteries.

The problems in connection with the use of pure 1 itium meta 11 anodes in electrochemical systems, and in particular in rechargeable cells, are well described in the scientific and technical literature (see, for example, U.S. Patents 4,550,064, 4,499,161, 4,118,550 , 4.071664, 4.086.403). The most important problem is the thermodynamic and kinetic instability of lithium towards cell materials "and the electrolyte, especially at the anode-electrolyte interface. In electrochemical cells, this results in corrosion of the lithium electrode. Thus, attempts to recycle lithium result in dendritic growth, formation of poorly adherent deposits with a large surface area of lithium metal during charging and slow chemical breakdown of the electrolyte. Finally, cell failure occurs due to cell short-circuiting, depletion and/or isolation of the anode, and anode passivation from e1 electrode degradation products. Short-term solutions to the above problems have been the use of excess lithium metal and/or conductive/protective films. Another approach has been the use of composite electrodes such as 1 itium-a 1uminium alloy. In such cases, promotes the immoaile - host material by an even distribution of lithium with reduced chemical activity.Li Tium-a1luminium alloys have been studied very extensively in non-aqueous electrolyte systems. However, the electrode undergoes large volume changes during deposition/stripping in connection with phase transformation. Such electrodes, with extensive recycling, exhibit severe instability. Other disadvantages associated with the use of alloys include large voltage and capacity losses compared to pure 1 i t i u-m anodes.

It has now been found that the above problems with anodes made from lithium iron or lithium-aluminum alloy can be reduced or avoided by using anodes made from -

of a mixture of lithium and nitrogen. The present invention is therefore primarily directed at lithium-based electrochemical cells with a new anode material comprising a mixture in the solid state of 1 itium-raeta 11

and nitrogen wherein the nitrogen is in the form of 1 lithium nitride id.

Anodes comprising the solid mixture or solution formed

from this mixture', (said mixture or solution is sometimes referred to in the present application as "Li/Li^N")

should be used to replace 1 the lithium anodes in lithium-

based electrochemical cells, The terms "mixture" and "solution" are used interchangeably in the present invention;

the characterization of the solid anode material as a: "mixture" or a "solution" is without meaning- The other elements in the 1 itium-based electrochemical cells, such as the cathode, the separator and the electrolyte, can be used without modifications in the electrochemical cells which are produced according to the present invention and which have anodes of Li/Li^N. Such elements are extensively described in the literature relating to lithium batteries.

Mixtures of lithium and lithium nitride have been identified in the scientific literature (e.g., U.S. Patent 4,447,379, Wagner) as intermediates in the preparation of a solid electrolyte, and lithium nitride has been used as a solid ion conductor in solid state cells. state, as explained by A. Rabenau ( Solid State Ionics) , 6 ( 1 982 ) 277 ). Surface films of li^N on lithium foil anodes have also been investigated by Thevenin et al. (Lawrence Berkeley Lab., CA (USA), Jan. 1986, Report No. LBL 20659). However, it has so far not been known to use Li/Li^N as an anode material.

The new anode material has reduced chemical/electrochemical activity towards the electrolyte, particularly at the anode/electrolyte interface, can recycle lithium very efficiently and exhibits no loss of ce1le voltage. Li/LiN significantly reduces the degree of interaction between elemental lithium and electrolyte during charge/discharge cycles and during storage. Therefore, an improved lithium-based anode material formed from lithium and lithium nitride, which can be used with a suitable electrolyte, separator and cathode for use in an electrochemical cell, and, in particular, a lithium rechargeable cell, is disclosed in the present invention.

Lithium nitride is known to have the highest Li+ ion conductivity of any inorganic lithium salt and has been extensively investigated as a solid electrolyte in solid cells. However, the relatively low breakdown potential of lithium nitride (Ed=0.44V) has limited its commercial use in solid cells. In contrast, lithium nitride is thermodynamically stable to lithium metal and forms,

in combination with lithium metal, a very stable anode material.

Fig. 1 is the polarization curve for Li/Li^N (11 mol?oN) in a 1M LiAsF 6/2-MeTHF electrolyte at room temperature. ■ Figs 2 and 3 show the activity of Li/Li^N (11 m o 1 ?o N ) and clean 1 iti urn 1 electrodes during storage in 1M LiAsF6/2-MeTHF - base rt electrolyte and 1M LiAsF^/THF- respectively electrolyte at room temperature. Fig. 4 shows the alternating current of Li/Li^N (11 m o 1 ?o N ) and pure lithium electrodes in LiAlCl 4/1 i on y 1 k 1 o r i d-e 1 e k t rt) 1 y 11 at room temperature. Fig. 5 shows the degree of passivation of pure lithium and Li/Li^<N> (11 mol?oN) during the anodic step in a 1M LiAsF^dioxolane-based electrolyte at -20°c. Fig. 6 shows the cyclic performance of lithium and the Li/Li^N electrodes (1 1 molSKN) r in a 1M LiAsF6,/2-MeTHF electrolyte at

••room temperature.

Anode material according to the present invention

can be prepared by melting 1 lithium meta 11 foil in a crucible in a dry container filled with argon, adding nitrogen in the form of lithium nitride powder, nitrogen gas or another suitable nitrogen-containing material to form a homogeneous material, and cooling the material to room temperature. The anode material thus formed comprises a solid solution

of lithium metal and lithium nitride. Whether -nitrogen is added to liquid lithium in the form of lithium nitride, nitrogen gas or any other nitrogen-containing substance that will release its nitrogen in molten lithium, it has been found that the nitrogen in the solution or mixture is in the form of lithium nitride. The exact composition of Li/Li^N-rates in the 1et depends on the process temperature and the cooling rate. The temperature dependence is particularly defined by means of

the material's phase diagram (see P.F. Adams et al. J. of the U- ss-Common Metals, 42 ( 1 975 ) .1 -1 1, 325-334). Even small amounts

avLi/Li-jN in the anode is considered an improvement over a pure lithium anode, and electrochemical cells using such anodes are within the scope of the present invention.

In the particular case of the 11mol«?oN Li/Li^N mixture used for the data represented herein, approximately 2g of Li^N powder was slowly added to 2g of molten lithium metal in

o

a nickel crucible at about 300 C. The material was stirred for several minutes until homogeneity and was then cooled to room temperature. The resulting anode material was pressed at about -4082; kg/cm 2 to the desired shape.

The anode material can be rolled or extruded into a foil,

which is considered a preferred form for use in electrochemical cells.

An anode material comprising a solid solution of 1.12 mol% nitrogen was prepared by slowly dissolving 6.34 g of Li-jN in 108.21 g of pure molten lithium at a temperature of about

o

445 C. The melt was then poured into a mold with a large heat loss to allow rapid cooling. " The material solidified almost instantly and the mold was transferred to an extruder for further processing.

In embodiments of the present invention, the ratio of Li^N in the mixture may be at any effective level. At levels in which the ratio of N in the mixture varies from approximately 0.06 to 20 mol%, Li<g>g Li^N forms a solid solution at elevated temperatures, and provides a satisfactory, homogeneous anode material. Within this large area, a smaller area of approximately 0.1 to 10 mo 1S5 N is considered preferred.

An electrochemical system using the Li/Li^N anode comprises a separator, an electrolyte and a cathode.

Suitable separator materials** include those commonly used in lithium cells such as porous polypropylene and materials of glass microfibres which are sufficiently porous and can be wetted using the appropriate electrolyte.

Suitable non-aqueous solution systems for the electrolyte include all solutions normally used in lithium cells with particular emphasis on aprotic ether or sulfur analogue systems, preferably dioxolanes, furans, glyme, methoxy-* methanes, glycol sulfites, sulfolanes, propylene carbonate and combinations hence. Suitable lithium salts' are chosen based on their high solubility (e.g.=0.5M) and conductivity in the chosen solution system, but will more particularly include anions such as perchlorates, sulphonates, acetates, borates, closoboranes, hexafluoro- or tetrafluoro- metals, halides, aluminates and derivatives thereof. One should note that normal lithium foil anode dependencies and/or bounds in the choice of electrolyte,

(eg solute/solvent combinations) do not. applies to systems using Li/Li^N anode material.

This means that far more electrolytes can be used. Some of these electrolytes show clearly improved performance characteristics compared to standard lithium anode material

Solid electrolytes such as lithium nitride, lithium iodide and lithium aluminum nitride, as well as polymeric ion conductors or combinations thereof, can also be used in the present invention.

The use of electrolytes based on inorganic solvents such as sulfur dioxide, thionyl chloride or sulfuryl chloride, with a suitable conductive lithium salt, is also contemplated.

A suitable cathode includes each of those described in technical and patent literature for lithium-based electrochemical systems, . E.g. cathodes comprising a halogen or halide, a metal oxide, sulfides, selenides, oxyhalides, sulfur dioxide, and carbon can be used.

Cathodes with an active material comprising a ciialcogen or. a cha 1kogenide compound 1se of a transition metal is also suitable. In the case of rechargeable cells, the use of deposit-type materials and organic metals (e.g. TiS or MoS) or oxides (e.g. V 0 ) is , preferred.

22yv V 6 13

Anodes produced in accordance with the present invention may alone or substantially alone comprise Li/Li^N, or they may comprise composite materials, in which Li/Li^N is a component. For example, anode compositions comprising plastic or elastomeric ion-conducting macromolecular material, lithium alloys, and carbon compounds are described in U.S. Pat. Patent 4,517,265 (Belanger et al.).''- Similar anode compositions can be prepared according to the present invention which will include Li/Li^N as a component.

It is also known to use alloys of lithium and aluminum as an anode material (e.g. U.S. Patent 4,002,492 (Rao). Anodes can be produced according to the present invention using mixtures of Li/Li N and lithium-aluminum alloys , or mixtures of lithium nitride and lithium-aluminum alloys.

The use of other lithium alloys as anode material is also known in the prior art, and the use of mixtures of lithium nitride and one or more lithium alloys such as Li-Si, Li-Sn, Li-Fe, Li-Sb, Li-Bi, Li-B, Li-Pb, Li-As, etc. and other combinations thereof as anode materials are contemplated in accordance with the present invention.

As shown in Fig. 1 of the drawings, polarization shows

the curve for Li/Li^N at thorn t empe r a t u.r that at the anodic stage the current increases to large values. Despite the relatively low breakdown potential of Li 3 N (0.40V),2 the cell using Li/Li^N can be discharged up to 7.0 mA/cm in most aprotic solvents commonly used in 1 lithium rechargeable cells . The polarization curve also indicates that Li/Li N anodes cycled at low current densities will only show smaller over-potentials In these cases, the over-potentials are smaller than the breakdown potential of Li^N.

Fig. 2 shows the relative activity of lithium metal and Li/Li^N during storage at room temperature. Generally, the formation of insoluble layers on the electrode during the storage periods causes an associated reduction of its active surface area. The development of the corresponding loss of activity has been followed

to measure the ratio i /i (i is the exchange current at

t o o

the initial time, i ter the exchange current after a later specified time) by means of AC impedance spectroscopy at the end of the initial treatment (io) and after an open-circuit dwell time (t) (i^)■

The curves in Fig. 2 show that the chemical stability of lithium compared to the electrolyte falls seriously with time,

while Li/LiN shows an excellent stability compared to the solvent under similar experimental conditions.

A similar experiment was carried out using a relatively more reactive solvent, such as

tetrahydrofuran ("THF"). THF is known to be far too reactive for use in secondary cells. Even very pure THF reacts rapidly with lithium to form a gel-like film. From Fig.3 you can clearly see that the surface activity of Li/Li^N decreases moderately during a storage time of 50 hours

at room temperature, while the surface of a pure lithium electrode became totally inactive during the same period.

These physical phenomena support the conclusion that Li/Li^N

gives a significantly longer shelf life, in which an immobile host material such as Li^N promotes a uniform lithium distribution of reduced activity. This is particularly the case for lithium electrodes in primary cells, where problems arising from the presence of passivating layers on the surface of the lithium metal, which limit the transport of the active materials to the interface, are constantly encountered. Moreover, a pronounced passivation of the pure lithium anode will entail a long-lasting delay in reaching an equilibrium voltage on initial discharge. All these problems can be mitigated considerably by using a Li/Li^N anode, where significantly reduced chemical activity is observed.

Also, cells using Li/Li^N anodes can be very important in today's primary cell technology. One of the most promising of these is the lithium thionyl chloride cell, in which LiAlCl^ is added to liquid thionyl chloride to increase conductivity and facilitate Li + -ion transport. The high stability of lithium in LiAlCl^/ SOCl^ solution is due to a protective surface film of LiCl

which is formed when the electrode comes into contact with the electrolyte.

While the film makes it possible to build a battery from a thermodynamically unstable combination such as Li in SOC^» an emerging voltage delay must be overcome to obtain a practical system.

In this context, Li/Li3N~electrodes have been investigated

in thionyl chloride and sulfur dioxide solutions. Fig. 4 shows the percentage of the exchange current in the cell using Li and L i/L i -jN (11 mol^N) in t i ony 1 kl ori d/L i Al Cl (1.6M) during storage at room temperature. The magnitude of the exchange current is indicative of the surface activity of the electrode which in turn is directly related to the thickness of the passive film formed on the electrode surface. From Fig. 4, it can be clearly seen that the exchange current for the inner electrode decreases with increasing storage time, while the cell using a Li/L i^N-e electrode exhibits a greater exchange current under similar conditions.

The LiCl film growing continuously on lithium causes an initial voltage delay, which is discussed earlier. It is further assumed that the initial voltage delay for Li/sOCl^~ce11en depends on the effective growth rate

of LiC1-fi 1mtykke1sen. Thus, the initial voltage delay for a cell using a Li/L i^N anode will be significantly less than the cell using a Li-meta 11 anode. This is particularly the case at longer storage times, in which the Li cell suffers significantly from the continuous growth of LiCl film on lithium with a corresponding gradual increase in the cell's initial voltage delay.

Fig. 5 shows the activity of lithium metal and Li/L^N during anodic stripping at 1.0 mA/cm at -20 C. The formation of insulating layers on the lithium metal surface can be clearly seen in Fig. 5. These layers have been attributed to , a solvent polymerization initiated by LiAsF degradation products. Impairment of the electrochemical properties in the metal-electrolyte interface due to the formation of a passivating layer can be attributed to a significant overpotential of resistance drop due to lower conductivity as well as higher

Consequently, this limits the amount of lithium metal in each

cycle. It can also be seen from Fig. 5 that the degree of passivation for Li/Li,N is not as severe as that for lithium electrodes at -20 oC during an anodic stripping at

2

1.0 mA/cm , despite the larger resistance drop in the cell using Li/Li^N.

Fig. 6 shows the discharge capacity of lithium and Li/Li^N as a function of the number of cycles at room temperature. The cathode was produced • from a deposit material• l, namely TiS 2•

The cell was completed with a glass fiber separator soaked with 1.0 M L iAsF 2 in 2-methylether ahydrofuran. The results from the cycle tests show the higher theoretical capacity for the Li/Li N electrode compared to pure lithium anodes after run-through. In further testing, the cell with a pure lithium anode completed only 5 cycles at 30% of its theoretical capacity, while the cell with the Li/L i^N anode completed 125 cycles at 40% of its theoretical capacity.

From the above data, it can be seen that the present invention provides a solution to a number of problems in connection with the use of pure 1 lithium meta 11 anodes in lithium galvanic cells and, in particular, lithium rechargeable cells. A proposed explanation for the superior properties of Li/Li^N as an anode material is related to the formation of micro-pores on the surface of the electrode during the discharge process. The porous Li/L i^N-e1 electrode formed by discharge can act as a three-dimensional electrode, thereby significantly reducing the local current density and eliminating the mass distribution of lithium during extended run-through. Furthermore, morphological investigations carried out on Li/Li-jN using a scanning electron microscope have shown that anodic polarization effectively cleans the anode surface. The microporous structure was clearly revealed after the electrode surface was anodically polarized at approximately +50 mV overpotential and 10 coulomb/. cm<2>charge density.

During the cathodic stage, lithium begins to grow on. a clean surface. It is proposed that the improvement in electrode properties is directly tied to the presence of micro-cavities formed on the electrode surface during the initial anode step. Moreover, the incorporation of lithium into the Li/Li^N anode during subsequent cathodic steps will take place through these micro-hu1 space. However, the size of the voids only ensures the passage of small molecules such as lithium, while bulk solvent molecules are unable to penetrate the voids. Based on this -model, the initial anodic polarization plays a decisive role when concerns the quality of the electrode properties, and the formation of cavities that allow the passage of lithium ions alone and not the solvent molecules or solute molecules.As part of this model, it is further expected that lithium nitride, in addition to acting as a host species, can also promote lithium ion conductivity inside the anode Additionally, initial lithium NMR studies indicate complete delocalization of 1 iti urne 1ektrons which equalize and reduce the total lithium activity. It is assumed that the combination of all these effects results in improved cycling and longer durability as observed in the investigations with the Li/Li^N-e1 electrode.

Claims (7)

1. An electrochemical cell, characterized in that it includes: (a) an anode comprising • a solid solution of lithium metal and lithium nitride; (b) a cathode; (c) a non-aqueous electrolyte comprising a solvent and a lithium salt dissolved therein; (d) a porous separator. 2. Electrochemical cell as stated in claim 1, characterized in that the proportion of nitrogen in said solution is in the range from approximately 0.06 to 20 mol%. 3. Electrochemical cell, characterized in that it includes: (a) an anode comprising a solid solution of lithium nitride and an alloy selected from the group consisting of Li-Al, L.t-Si, Li-Sn, Li-Fe, Li-5b, Li-Bi, Li-B and Li-Pb , wherein the proportion of nitrogen in said solution is in the range of approximately 0.06 to 20 mol%. (b) a cathode; (c) a non-aqueous electrolyte comprising a solvent and a lithium salt dissolved therein; and (d) a porous separator. 4. Electrochemical cell as set forth in claim 1, characterized in that said lithium salt has an anion selected from the group consisting of halide ions, hexafluorometalate ions, tetrafluorometalate ions, perchlorate ions, sulfonate ions, borate ions, fiocyanate ions, aluminate ions, closoborane anions and acetate ions and derivatives thereof . 5. Electrochemical cell as stated in claim 1, characterized in that said electrolyte is a solid electrolyte selected from the group consisting of lithium nitride, lithium iodide and lithium aluminum nitride. 6. An improved lithium-based electrochemical cell of the type with an anode, a cathode, an electrolyte and a -separator characterized by a-t the improvement comprises an anode comprising lithium and lithium nitride, said anode being formed by adding nitrogen to molten lithium and cooling and shaping the obtained material. 7 . An anode for use in a lithium-based electrochemical cell, characterized in that it comprises a solid solution of lithium metal and lithium nitride, where the proportion of nitrogen in said solution is in the range from 0.06 to 20 mol%, and where said anode is in the form of a foil.