CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to European patent application No. 19216479.6 filed on December 16, 2019, the whole content of this application being incorporated herein by reference for all purposes. TECHNICAL FIELD

The present invention relates to a process for manufacturing a composite material, comprising the steps of a) spraying an aqueous solution of a tetrakis(hydroxyorgano) phosphonium (THP) salt, an organic nitrogen compound and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms onto a i) porous substrate; b) drying the substrate; c) curing the substrate with ammonia; d) oxidizing the substrate; e) treating the substrate with a metalbi sulfite salt; and f) washing the substrate with excessive water and also to a composite material produced by the process, wherein said step a) is implemented at a speed of 3.0 ml/sec or less of the aqeuous solution, preferably 2.0 ml/sec or less of the aqeuous solution. The present invention also relates to a composite material comprising a i) porous substrate and a ii) reaction product of a THP salt, an organic nitrogen compound, and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms, wherein the ii) reaction product is impregnated into the pores of the i) porous substrate; to an electrochemical device comprising the composite material as a separator, and to the use of the composite material as a separator in an electrochemical device, in particular a Li-ion secondary battery, a lead-acid battery or a supercapacitor for improving safety performance.

TECHNICAL BACKGROUND For more than two decades, Li-ion batteries have retained a dominant position in the market of rechargeable energy storage devices due to their many benefits comprising light-weight, reasonable energy density, and good cycle life. Nevertheless, current Li-ion batteries still suffer from poor safety and relatively low energy density with respect to the required energy density for high power applications, such as electrical vehicles (EVs), hybrid electrical vehicles (HEVs), grid energy storage, etc. An electrolyte is a substance which produces an electrically conducting solution when it is dissolved in a polar solvent. The dissolved electrolyte splits into cations and anions, which disperse through the solvent in a uniform manner. Such a solution is electrically neutral, but if an electrical potential is applied, the cations in the solution move to the electrode having abundant electrons, whereas the anions move to the electrode having a deficit of electrons. That is, the movement of cations and anions in opposite directions results in an electrical current.

Basic requirements to be a suitable electrolyte for an electrochemical cell include high ionic conductivity, low melting, high boiling point, (electro)chemical stability, and safety. The conventional electrolyte, which is in liquid, has played an essential and dominant role in the field of electrochemical energy storage for several decades due to its high ionic conductivity and good interface with electrodes. However, such a liquid electrolyte has brought safety issues caused by its leakage and inherent explosive nature, e.g., combustion of the organic electrolyte.

Accordingly, safety has been a prerequisite for batteries. Several protective mechanisms have been considered as measures to ensure battery safety. External protection relies on electronic devices such as temperature sensors and pressure valves, which eventually increase the volume/weight of the battery and are unreliable under thermal/pressure abuse conditions. Internal protection schemes focus on using intrinsically safe materials for battery components and are hence considered to be the more appropriate solution for battery safety.

The presence of a separator in an electrochemical device, in particular, in secondary batteries, is necessary to keep the two electrodes apart to prevent electrical short circuits, while permitting the transport of ionic charge carriers which are needed to close the circuit during the passage of current in an electrochemical cell. Separators are critical components in batteries with liquid electrolyte because their structure and properties considerably affect the battery performance, including energy/power densities, cycle life, and safety. A separator generally consists of a polymeric membrane forming a microporous layer. The separator must be (electro)chemically stable with regard to the electrolyte and electrode materials, and also mechanically strong enough to withstand the high tension during the process of battery assembly.

Notably, for safety, the battery must be able to shut down when overheating occurs, so as to avoid thermal runaway, which may cause dimensional shrinkage or even melting of the separator, eventually resulting in the physical contact of the electrodes.

There have been strong needs in the field for a separator having good safety, without compromising its thermal and mechanical properties. For instance, inorganic composite membranes have been investigated for this purpose, and are widely used as separators for electrochemical devices including secondary batteries, in particular Li-ion batteries. Examples of porous inorganic composite membranes are disclosed in many prior art documents, including US 2002/197413A (Teijin Limited). However, inorganic composite separators are usually not mechanically strong enough to withstand stress applied during cell assembly, even though they offer excellent wettability of electrolytes and good thermal stability.

There is thus still the need in the field for a separator that has good safety as well as good mechanical properties.

In this regards, there is a well-known process for the treatment of fabrics, which consists of several consecutive steps comprising impregnation of the fabric substrate with an aqueous solution of a THP salt and an organic nitrogen compound, such as urea or thiourea, to impart excellent flame-retardant properties. There treatment also provides flexibility to the fabrics. These treated fabrics have been sold for decades under the registered trademark PROBAN® of Albright & Wilson Limited (acquired by Rhodia, now Solvay). The flame- retardant treatments of this type are also well known and have been described in several prior art documents, e.g., GB2040299B, EP0709518A1, EP0294234A2, etc.

Such a conventional process for the flame-retardant treatments, which have been used only for textiles and garments for several decades, usually requires padding and mangling steps for effective impregnation of the chemicals into the textiles, i.e., passing the target substrate through an aqueous bath containing a solution of the flame-retardant agent and any other additives, and subsequently through rollers. Moreover, existing methods to apply an aqueous solution of a THP salt and an organic nitrogen compound onto textiles are typically continuous processes, where the textiles need to be dipped into the aqueous solution and subsequently the excess water needs to be removed by either roller or mangle. These existing methods are hence inappropriate for a nonwoven material, because the methods will eventually result in tearing and destruction of the nonwoven material which, unlike woven textile, becomes fragile when an aqueous solution is applied thereon. In short, such methods inevitably apply a certain pressure/stress onto the substrate. However, a battery separator should be thin enough to facilitate the energy/power density of the battery, without compromising its mechanical strength.

Accordingly, said process is not appropriate to be used in manufacturing a battery separator with a purpose to impart better safety performance thereto without a sacrifice in other properties, and hence the state of the art has been inactive in the field of batteries over a long period prior to the present invention, even though during that time an urgent need for the improvement of safety in batteries has demonstrably and continuously existed.

The inventors found that by using the specific process as defined above, the reaction product from the aqueous solution of a THP salt, an organic nitrogen compound, and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms can be effectively impregnated into the pores of the i) porous substrate. In the present invention, the reaction product is a water- insoluble cross-linked polymer, which becomes mechanically fixed across the pores of the i) porous substrate, and the resulting composite material is suitable to be used as a separator in an electrochemical device having good safety performance and flexibility as well as good cycle performance.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a process for manufacturing a composite material comprising the steps of a) spraying an aqueous solution of a tetrakis(hydroxyorgano) phosphonium (THP) salt, an organic nitrogen compound and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms onto a i) porous substrate; b) drying the substrate; c) curing the substrate with ammonia; d) oxidizing the substrate; e) treating the substrate with a metalbi sulfite salt; and f) washing the substrate with excessive water, wherein said step a) is implemented at a speed of 3.0 ml/sec or less of the aqeuous solution, preferably 2.0 ml/sec or less of the aqeuous solution.

A second object of the present invention is to provide a composite material produced by the process according to the present invention. A third object of the present invention is to provide a composite material comprising a i) porous substrate and a ii) reaction product of a THP salt, an organic nitrogen compound, and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms, wherein the ii) reaction product is impregnated into the pores of the i) porous substrate.

A fourth object of the present invention is to provide an electrochemical device comprising the composite materials as described above as a separator.

Another object of the present invention is the use of the composite material as described above as a separator in an electrochemical device, in particular, a Li-ion secondary battery, a lead-acid battery, or a supercapacitor for improving safety performance.

It has been surprisingly found by the inventors that said flame-retardant treatments, which have been used only for textiles and garments for several decades, can be applied for manufacturing a composite material which may be used as a separator for batteries having improved safety performance, notably improved flame-retardant properties.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is SEM images of the surface of a separator according to the present invention (A) and of the surface of separator made of the original nonwoven cellulose (B).

Figure 2 is cross-section SEM images of the separator according to the present invention (C) and of the surface of separator made of the original nonwoven cellulose (D).

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Throughout this specification, unless the context requires otherwise, the word "comprise" or “include”, or variations such as "comprises", "comprising", “includes”, including” will be understood to imply the inclusion of a stated element or method step or group of elements or method steps, but not the exclusion of any other element or method step or group of elements or method steps. According to preferred embodiments, the word "comprise" and “include”, and their variations mean “consist exclusively of’.

As used in this specification, the singular forms "a", "an" and "the" include plural aspects unless the context clearly dictates otherwise. The term “and/or” includes the meanings “and”, “or” and also all the other possible combinations of the elements connected to this term. The term “between” should be understood as being inclusive of the limits.

As used herein, "alkyl" groups include saturated hydrocarbons having one or more carbon atoms, including straight-chain alkyl groups, such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, cyclic alkyl groups (or "cycloalkyl" or "alicyclic" or "carbocyclic" groups), such as cyclopropyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl, branched-chain alkyl groups, such as isopropyl, tert-butyl, sec-butyl, and isobutyl, and alkyl- substituted alkyl groups, such as alkyl-substituted cycloalkyl groups and cycloalkyl-substituted alkyl groups.

The term "aliphatic group" includes organic moieties characterized by straight or branched-chains, typically having between 1 and 18 carbon atoms. In complex structures, the chains may be branched, bridged, or cross-linked. Aliphatic groups include alkyl groups, alkenyl groups, and alkynyl groups.

Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a temperature range of about 120°C to about 150°C should be interpreted to include not only the explicitly recited limits of about 120°C to about 150°C, but also to include sub-ranges, such as 125°C to 145°C, 130°C to 150°C, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 122.2°C, 140.6°C, and 141.3°C, for example.

Unless otherwise specified, in the context of the present invention the amount of a component in a composition is indicated as the ratio between the weight of the component and the total weight of the composition multiplied by 100 (i.e., % by weight or wt%).

By the term "separator", it is hereby intended to denote a monolayer or multilayer polymeric or inorganic material, which electrically and physically separates the electrodes of opposite polarities in an electrochemical cell and is permeable to ions flowing between them.

By the term "porous substrate", it is hereby intended to denote a substrate containing pores of finite dimensions, which is electrically and chemically inert. The substrate has typically a porosity advantageously of at least 5%, preferably of at least 10%, more preferably of at least 20% or at least 40% and advantageously of at most 90%, preferably of at most 80%, e.g. measured via Gurley number as described in method ISO 5636-5.

By the term "electrochemical cell", it is hereby intended to denote an electrochemical device/assembly comprising a positive electrode, a negative electrode and a liquid electrolyte, wherein a monolayer or multilayer separator is adhered to at least one surface of one of the said electrodes. Non-limitative examples of suitable electrochemical devices include, notably, secondary batteries, especially, alkaline or an alkaline-earth secondary batteries such as lithium ion batteries, lead-acid batteries, and capacitors, especially lithium ion- based capacitors and electric double-layer capacitors (supercapacitors).

The constituents of the process for manufacturing a composite material according to the present invention are described hereinafter in the details. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. Accordingly, various changes and modifications described herein will be apparent to those skilled in the art. Moreover, descriptions of well-known functions and constructions may be omitted for clarity and conciseness.

The present invention provides a process for manufacturing a composite material comprising the steps of a) spraying an aqueous solution of a THP salt, an organic nitrogen compound and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms onto a i) porous substrate; b) drying the substrate; c) curing the substrate with ammonia; d) oxidizing the substrate; e) treating the substrate with a metalbi sulfite salt; and f) washing the substrate with excessive water, wherein said step a) is implemented at a speed of 3.0 ml/sec or less of the aqeuous solution, preferably 2.0 ml/sec or less of the aqeuous solution.

In an embodiment, the step a) of spraying is implemented onto the porous substrate suspended by a support frame in a horizontal direction. Preferably, the step a) is carried out with less tension as much as possible so that the substrate can withstand stress once being sprayed. In an embodiment, once the step a) is completed, the porous substrate remains soaked within the aqueous solution for at least 3 secons, preferably for at least 5 seconds before the step b) of drying.

The i) porous substrate can be made with any porous substrate commonly used for a separator in an electrochemical device, comprising at least one material selected from the group consisting of cellulose, polyester such as polyethylene terephthalate and polybutylene terephthalate, polyphenylene sulphide, polyacetal, polyamide, polycarbonate, polyimide, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, polyethylene oxide, polyacrylonitrile, polyolefin such as polyethylene and polypropylene, or mixtures thereof.

In an embodiment, the i) porous substrate is a nonwoven material, which comprises cellulosic nonwoven and optionally non-cellulosic nonwoven. In another embodiment, the non-cellulosic nonwoven is selected from the group consisting of polyester nonwoven, polyamide nonwoven, acrylic nonwoven, aramid nonwoven and polybenzimidazole nonwoven, preferably polyester nonwoven. In a specific embodiment, the nonwoven is a blend of cellulose and polyester. In a more specific embodiment, the nonwoven is a blend of 50 wt% of cellulose and 50 wt% of polyester.

In another embodiment, the porous substrate is a nonwoven material consisting of cellulose only.

In the present invention, the term “nonwoven” is intended to denote a planar structure obtainable by randomly interlocking or bonding mechanically, thermally or chemically one or more sets of polymer materials, leading to a structure with numerous pores.

By the term “cellulose”, it is hereby intended to denote a component of the cell walls of plants. Examples of celluloe include cotton, rayon, linen, hemp and cellulose acetate, while the most common example is cotton.

The THP salt according to the present invention is preferably a tetrakis(hydroxyalkyl) phosphonium salt, for example tetrakis(hydroxymethyl) phosphonium chloride (THPC) or tetrakis(hydroxymethyl) phosphonium sulphate (THPS).

The organic nitrogen compound according to the present invention is a compound containing nitrogen which may form a condensate with a THP salt. Without wishing to be bound by any particular theory, it is believed that the adjustment of the pH of THP salt to about 6.0 may render the salt more reactive towards the organic nitrogen compound.

In an embodiment, the organic nitrogen compound is an amide, for example urea or thiourea.

Examples of the aliphatic amine include, but not limited to, a primary amine, a secondary amine, a quaternary ammonium salt, an ethoxylated amine, an ethoxylated diamine, an amide oxide, an alkylamino-substituted carboxylic acid, an amide, an ethoxylated amide, an amido-imidazoline, a siolxane, and a silane derivative.

Where the aliphatic amine having at least one alkyl group having at least 12 carbon atoms is an amine, it may, for example, consist essentially of n- dodecylamine, n-octadecylamine, n-hexadecylamine, n-eicosylamine, or mixtures thereof.

After the step of a) spraying, the substrate is dried. In a specific embodiment, the step of b) drying is performed at a temperature of between 1°C and 200°C, preferably between 20°C and 100°C. Care should be taken not to dry the material too quickly as migration of the chemical may occur. In another specific embodiment, the substrate is dried to a moisture content of 20% or less, preferably between 1% and 15%, and more preferably between 5% and 10%.

The moisture content can be measured from the increase in weight of the substrate and the weight of chemicals applied thereto by spraying.

Following the steps of a) spraying and b) drying, the substrate is cured with ammonia, usually gaseous ammonia, so as to produce a water-insoluble cross- linked polymer, corresponding to the reaction product from the aqueous solution of a THP salt, an organic nitrogen compound and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms, which is mechanically fixed across the pores of the i) porous substrate. This is an irreversible process so that said cross-linked polymer becomes embedded in the substrate. In a specific embodiment, the step of c) curing is performed at a temperature of 100°C or less, preferably between 1°C and 90°C, and more preferably between 15°C and 50°C.

After curing, an oxidation corresponding to the step d) is carried out using an oxidant, for instance, a peroxy compound such as aqueous hydrogen peroxide solution and sodium perborate solution, so as to convert the trivalent phosphorus to its stable form of pentavalent phosphorus, followed by a treatment with a base, such as sodium carbonate for neutralization. Alternatively, the oxidation may be performed with a gas containing molecular oxygen, preferably air.

Subsequent to the step of d) oxidation, the substrate is treated with a metabi sulfite salt, preferably sodium metabi sulfite to reduce the amount of formaldehyde present in the final product, which is then washed with excessive water and dried. Softened water is chosen for the final wash of the process in an attempt to minimize the number of metal ions that may be left on the final product.

A second object of the present invention is to provide a composite material produced by the process according to the present invention.

A third object of the present invention is to provide a composite material comprising a i) porous substrate; and a ii) reaction product of a THP salt, an organic nitrogen compound, and optionally an aliphatic amine having at least one alkyl group having at least 12 carbon atoms, wherein the ii) reaction product is impregnated into the pores of the i) porous substrate.

In the present invention, the ii) reaction product is a water-insoluble cross- linked polymer, which becomes mechanically fixed across the pores of the i) porous substrate.

The composite material according to the present invention exhibits good thermal resistivity and flexibility, while maintaining good cycle performance, suitable to be used as a separator in an electrochemical cell.

A separator in an electrochemical cell must have sufficient pore density to hold the liquid electrolyte which enables ions to move between two electrodes. However, excessive porosity also can hinder the ability of the pores to close, which is vital to allow the separator to shut down an overheated battery.

In other words, the pore size of a separator is a key factor in an electrochemical cell. Undersize pore will decrease the transmission rate of a metal ion, e.g., Li ion. A relatively large pore size is necessary for low ionic resistance, which can result in good charge/discharge acceptance, particularly at high C-rate. On the other side, however, in order to suppress the internal short circuits of batteries, separators should have relatively small pore size and sufficiently narrow distribution of pore sizes. Accordingly, it’s very important to have optimal pore size, which is thin enough and at the same time is not detrimental in view of internal short circuits. The average pore size of polyolefin-based membrances is between 0.02 pm and 0.2 pm, preferably between 0.03 pm and 0.1 pm. In addition, the average pore size of nonwoven membranes is between 1 pm and 10 pm.

In an embodiment, the composite material according to the present invention has the average pore size of between 0.1 pm and 1 pm, preferably between 0.2 pm and 0.8 pm, and more preferably between 0.4 pm and 0.6 pm.

Porosity can be measured using liquid or gas absorption methods according to the ASTM D-2873. Typically, a separator for Li-ion battery provides a porosity of 40%.

In an embodiment, the composite material according to the present invention has porosity of between 10% and 90%, preferably between 30% and 80% and more preferably between 40% and 80%.

Ideally, the pores should be uniformly distributed while also having a tortuous structure, which enables uniform distribution of current throughout the separator, while suppressing the growth of Li dendrites on the anode. The distribution and structure of pores can be analyzed using a capillary flow porometer or a scanning electron microscope (SEM).

A separator in an electrochemical cell should be thin to facilitate the energy density of the battery. Its thickness should be defined in consideration of mechanical strength and safety performance. The thickness should be also uniform to support many charging cycles.

In a specific embodiment, the thickness of the composite material according to the present invention is from 3 to 100 pm, preferably from 5 to 50 pm, and more preferably from 8 to 40 pm.

Determination of the thickness can be performed by any suitable method. The thickness is preferably determined according to ISO 4593 standard procedure.

A fourth object of the present invention is an electrochemical device comprising the composite materials as described above as a separator.

Another object of the present invention is the use of the composite material as described above as a separator in an electrochemical device, in particular a Li- ion secondary battery, a lead-acid battery or a supercapacitor for improving safety performance.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The invention will be now explained in more detail with reference to the following examples, whose purpose is merely illustrative and not intended to limit the scope of the invention.

EXAMPLES

A/ Formulation of the Electrolyte Compositions

The electrolyte composition was prepared by mixing the different compounds using a magnetic stirrer. All required components were added to one bottle and mixed while stirring until a transparent solution was obtained. First, Li salt (LiPF ₆ (lithium hexafluorophosphate); 1 mol.L ¹ ) was dissolved in the solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (3:7 v/v), with 2 wt% of vinylene carbonate (VC) and 0.5 wt% of 1,3-propane sultone (PS) as additives.

B / Preparation of the Separators

1- Inventive Example 1 (El)

1.1 A substrate of original nonwoven cellulose (NW; thickness: 35 pm; TF4035 available from Nippon Kodoshi Corp.) was suspended from a frame (dimension : 600 mm x 150 mm) under slight tension. A 20% w/w solution of Perform™ LF (an aqueous condensate solution of a THPS salt and urea; available from Solvay Solutions UK) was prepared with distilled water. Perform™ LF solution was sprayed onto the horizontally-oriented NW until being saturated at a speed of about 2.0 ml/sec. Wet NW was left within Perform™ LF solution for 5 seconds and was found to have poor wet strength at this stage. Wet NW was then dried in an oven and left in ambient humidity after drying to re-gain moisture.

1.2 Dried NW was placed into an ammonia curing chamber, which was filled with anhydrous ammonia gas and then was left for 10 minutes. After 10 minutes, the chamber was purged of ammonia and the sample was removed.

1.3 Cured NW was immersed in a 10% aqueous solution of hydrogen peroxide and left for 5 minutes. Oxidized NW was then neutralized with sodium carbonate solution (pH about 11).

1.4 NW was placed in an aqueous solution of sodium metabisulfite at 45°C for 10 minutes. The pH of this solution was increased to pH 9 by addition of sodium carbonate. Sodium metabisulfite was used to reduce the amount of formaldehyde present on the final product. Neutralized NW was then soaked in water baths until pH remained stable (about pH 7). Processed NW was then left to air-dry to obtain the composite material of interest.

2. Comparative Examples (CE1 and CE2)

CE1 and CE2 were prepared in the same manner as El, respectively, except that different separator was used. That is, polyethylene (PE; thickness : 20 pm; porosity : about 40%; available from Toray Tonen Specialty Separator Godo Kaisha) was used for CE1 and original NW (thickness : 30 pm) without the treatment as El was used for CE2.

C/ Assembly of the Cells

A separator made of the composite material according to the present invention (El) and also comparative separators (CE1 and CE2) was placed between the cathode (NCM622) and the anode (natural graphite), respectively to produce the mono cells. All mono cells were designed to have 30 mAh of discharge capacity.

D / Injection of Electrolyte to Dry Cells

The liquid electrolyte as prepared was injected into the dry cells by pipette. The amount of liquid electrolyte composition injected into each cell was 100 pi.

E/ Aging for Wetting and Sealing

After injection, the dry cells were kept in a vacuum container for wetting of the liquid electrolyte to the electrode components. After releasing the vacuum, the cells were kept as such for 1 minute for better wetting. After wetting, the cells were sealed by vacuum sealing machine and the cells were kept in the laboratory additionally for 1 day at room temperature.

F/ Cell Activation and Measurement of Performance

1- Formation (=activation of lithium ion battery cell): Aged mono cells were charged to 30 % charge level (State of Charge (SOC): 30 %) and were kept at room temperature for 1 day.

2- Degassing: Generated gas during the formation was removed by cutting the extra part of the mono cell and the mono cell was re-sealed by vacuum..

3- Measurement of Performance: The mono cells were evaluated in terms of cycling performance (at room temperature) and thermal resistivity at various test conditions detailed as the following :

Cycling at room temperature

- Charge: 0.5C or 1.0C / 4.2V / 0.05C (Constant Current-Constant Voltage)

- Discharge: 0.5C or 1.0C / 3.0V (Constant Current)

Thermal resistivity - Test voltage: 4.2 V

- Temperature range: between 25 and 200 °C

- Heating rate: 5 °C/min

RESULTS A/ Morphology

The surface and cross-section morphologies of the composite separators were characterized by SEM images. The separator of CE2 with original nonwoven cellulose showed many open pores on the surface, whereas the separator of El according to the present invention showed smaller pores in comparison with CE2 (Figure 1). In addition, the cross section of CE2 was circular, while the cross section of El clearly showed the difference, i.e., the polymeric layer around the cellulose fiber.

B / Cycle Performance (Charge-Discharge) at room temperature

The initial discharge capacity of the test cells with CE1 and CE2 was respectively 30.7 mAh and 30.9 mAh, while the initial discharge capacity of the test cell with El was 25.9 mAh, about 13.7 % lower than the discharge capacity as designed (30 mAh). However, the discharge capacity was recovered during first 30 cycles with 0.5C and first 50 cycles with 1.0C.

C/ Safety Performance (Thermal Stability/Exposure Test) Understanding the behavior of Li-ion cells during thermal runaway is critical to evaluate the safety of the energy storage devices under outstanding conditions. Li-ion cells possess a high energy density and are used to store and supply energy to diverse applications. Incidents relating to the overheating or thermal runaway of the cells may cause severe damage, through fire or explosion. Accordingly, thermal studies of Li-ion cells are very important for ensuring the safety and reliability of cells. This study is to evaluate the thermal behavior of Li-ion cells under adiabatic conditions. The thermal runaway of a Li- ion cell is dominated by the exothermic reaction between the electrolyte and electrode materials. Thermal runaway occurs when the exothermic reactions go out of control so that the self-heating rate of the cell increases to the point that it begins to generate more heat than what can be dissipated.

The thermal exposure tests were implemented by using GL800 (a data logger, available from Graphtec Corporation) in an explosion proof chamber under the following conditions: - heating up to 200°C with a heating rate of 5°C/min; and

- holding for 60 min at 200°C. The onset temperatures of thermal runaway (a.k.a. thermal resistivity temperatures) of CE1, CE2 and El were 162 °C, 175 °C, and 187 °C, respectively. The thermal resistivity temperature of El was higher than those of CE1 and CE2, which means that El according to the present invention increases the safety performance in terms of thermal resistivity temperatures in comparison with CE1 and CE2.

In a nutshell, it was demonstrated that a separator made of the composite material according to the present invention exhibits comparable or superior thermal resistivity performance while maintaining good cycling performance.