WO2017168330A1

WO2017168330A1 - Lithium-ion cell

Info

Publication number: WO2017168330A1
Application number: PCT/IB2017/051777
Authority: WO
Inventors: Bartosz HAMANKIEWICZ; Andrzej CZERWIŃSKI; Michał KRAJEWSKI; Monika MICHALSKA; Ludwika LIPIŃSKA; Janusz Kozakiewicz; Jarosław PRZYBYLSKI; Krystyna SYLWESTRZAK; Witold SARNA
Original assignee: Uniwersytet Warszawski; Instytut Technologii Materiałów Elektronicznych; Instytut Chemii Przemysłowej
Priority date: 2016-03-31
Filing date: 2017-03-28
Publication date: 2017-10-05
Also published as: PL240408B1; PL416704A1

Abstract

A lithium-ion cell comprising a negative electrode applied to an aluminium current collector, a positive electrode applied to an aluminium current collector, a polymer separator serving the function of a solid electrolyte, a seal, a housing, a contact fluid, an ion charge carrier and a charge collector stabiliser is characterised in that the negative electrode (1 ) applied to an aluminium current collector (5) is made of lithium titanium oxide (LTO), the positive electrode (2) applied to an aluminium current collector (5) is made of lithium manganese oxide (LMO), the polymer separator (3) serving the function of a solid electrolyte is made of silicone-urethane prepolymer, and the contact fluid (4) constitutes a solution in dimethyl carbonate. The contact fluid (4) comprises a charge carrier in the form of LiTFSI and a current collector stabiliser in the form of LiBOB. The contact fluid (4) content of the cell amounts to below 10 μl/cm² in relation to the surfaces of the electrodes. The lithium-ion cell according to the invention is characterised by a specific energy of approx. 120 Wh/kg calculated per electrode masses and a cyclic resistance of approx. 55% after 50 cycles of deep charging/discharging. The lithium-ion cell according to the invention exhibits an increased safety level due to the extremely small amount of fluidic substances and due to the use of stable and non-reactive substances serving the function of a charge carrier and a current collector stabiliser.

Description

Lithium-ion cell

The object of the invention is a lithium-ion cell with an increased safety level, comprising a polymer separator serving the function of a solid electrolyte.

There is a demand for accumulators exhibiting high capacity and high power manifesting itself by the high current densities obtained. High capacity calculated per a unit of mass is desired due to economical reasons, while high power is necessary to effectively power receivers in moments of temporary increased demand for current, for example in an electric car during start or acceleration.

Three main types of accumulators are currently present in the market: lithium-ion, nickel-metal hydride and lead-acid. Each of those types of accumulators has its advantages allowing a specific application. The use of nickel-cadmium accumulators was recently discontinued due to the high toxicity of cadmium. Lead-acid accumulators are characterised by an operating voltage of 2.0-2.2 V. Lead-acid accumulators are a reliable and still the cheapest source of the acquired energy. Due to the very heavy weight of lead components, lead-acid accumulators are used only in devices for which the total mass increase does not constitute a significant problem, e.g. starter accumulators in combustion vehicles. Lead-acid accumulators are being considered as power sources for hybrid and electric cars (e.g. Ford). The greatest limitation of lead-acid accumulators is their large mass resulting from the necessity to use lead - one of the heaviest elements.

Nickel-metal hydride accumulators (Ni-MH) have an electromotive force of 1 .2-1 .3 V. Ni-MH accumulators feature good power parameters and their weight does not prevent them from being used in mobile devices. Due to this, nickel-metal hydride accumulators are used in mobile devices requiring high current densities, like e.g. photographic cameras, flash units. Nickel-metal hydride accumulators are also used to power hybrid cars provided with electric motors (e.g. Toyota Prius). The greatest limitation of nickel-metal hydride accumulators is their relatively low resistance to consecutive charging and discharging cycles.

Lithium-ion accumulators are characterised by an operating voltage of 3.2-4.0 V. The lithium-ion cells reaching the highest powers have power capacities comparable with nickel-metal hydride accumulators. Lithium-ion accumulators, due to their very small mass, are used mainly to power mobile devices requiring low power, e.g. mobile telephones and laptops.

Lithium-ion accumulators are currently being comprehensively studied with respect to their huge development potential. They have the highest known capacity resulting from the use of lithium cations (the lightest metal) as a charge carrier.

However, there are known problems and risks associated with the use of lithium-ion accumulators to power heavy duty mobile devices, such as cars. During high current draw, overheating and unsealing of the system can take place, which may lead to a fire of the whole device. Those risks result from the necessity to use liquid electrolytes. Liquid electrolytes known from state of the art are volatile and flammable.

Various types of lithium-ion cells are known, which use various substances for the construction of a positive electrode and a negative electrode, various substances for the construction of separators and various substances constituting the electrolyte. Classic solutions comprise a negative electrode (anode when discharged) made of carbon material (e.g. lithium intercalated in graphite) and a positive electrode (cathode when discharged) made of transition metal oxides: lithium-cobalt LCO, lithium-nickel- manganese-cobalt NMC, lithium ferrophosphate LFP or lithium manganese oxide LMO with the formula LiMn₂0₄ („Lithium Batteries Science and Technology", Springer 2003). The low potential of intercalation and deintercalation of lithium ions in carbon matrices causes the risk of underpotential deposition of metallic lithium under the conditions of high current density flow. In addition, manganese present in the cathode material disproportionates and dissolves in a fluidic organic electrolyte. The Mn²⁺ ions then diffuse towards the carbon electrode and participate in undesired reactions on its surface. These processes lead to a partial degradation of the cell, resulting in an intense decrease in the capacity of the graph ite/LiMn₂0₄ battery in subsequent galvanostatic cycles. A battery comprising carbon material as the negative electrode and LMO as the positive one exhibits a capacity of approx. 100 mAh/g and in room temperature it loses approx. 20% of its original capacity after 200 work cycles. In an elevated temperature of 60°C this loss amounts to over 50% after 100 cycles of galvanostatic charging /discharging {Electrochim. Acta, 130 (2014) 778).

Lithium titanium oxide with a spinel structure Li₄Ti₅0i₂ (LTO) may be used in a lithium-ion cell as an anode with a specific capacity of approx. 1 70 mAh/g. The high potential of reversible intercalation of lithium in its structure (approx. 1 .5 V in relation to metallic lithium) causes the chemical activity of Li⁺ in the LTO matrix to be significantly lower than in carbon material. In addition, in contrast to the graphite anode, there is no decomposition of the components of the electrolyte on titanate grains during first charging. Mn²⁺ ions, always present in the electrolyte of the cell comprising manganese materials, do not undergo electrochemical reduction on the surface of LTO and do not affect the course of processes taking place in the battery in subsequent working cycles. The above-mentioned advantages cause the LTO/LMO cell to be characterised by much more stable work, a lower decrease in the capacity during subsequent cycles of charging/discharging, a more predictable stoichiometry of the reactions which take place and the possibility of longer storage when charged. As a result, the operational safety of this type of lithium-ion batteries increases.

An LTO/LMO type lithium-ion cell is known, comprising a negative electrode made of lithium titanium oxide LTO with a spinel structure and the formula Li₄Ti₅0i₂ and a positive electrode of LMO (J. Electrochem. Soc, 141 (1994) L147; J. Electrochem. Soc, 142 (1995) 2558). LTO is characterised by a crystallographic structure enabling reversible intercalation/deintercalation of three moles of lithium onto the formula of the compound translating into a theoretical specific capacity of 168 mAh/g. The specific capacity decreased almost twofold and a higher intercalation potential of LTO compared to carbon materials are compensated by excellent cyclic resistance of LTO caused by the lack of a significant change in the volume of the material during the insertion of Li⁺ ions. In such systems, liquid electrolyte is used in the form of 1 M of LiCI0₄ solution in propylene carbonate, being a volatile flammable material. An LTO/LMO type lithium-ion cell is known, which uses liquid electrolyte comprising LiPF₆ dissolved in a mixture of ethylene carbonate and dimethyl carbonate (LiPF₆@EC/DMC) (J. Power Sources, 83 (1999) 1 56). The LTO/LMO cell in a coin-shaped form (CR2032) is characterised by an initial capacity of approx. 1 12 mAh/g (per the mass of cathode material) and loses 14% of its value during 50 cycles of charging/discharging.

An LTO/LNMO type lithium-ion cell is known, comprising a negative electrode of LTO and a positive electrode made of lithium manganese oxide substituted with nickel ions LiNi_0!5Mn_{l !5}O₄ (LNMO) using a liquid electrolyte LiPF₆@EC/DMC {Chem. Lett, 12 (2001 ) 1270; J. Power Sources, 1 19 (2003) 959). This battery is characterised by its operating voltage higher by approx. 0.5V compared to LTO/LMO cells. This is caused by the higher intercalation potential of lithium in LNMO. The LTO/LNMO cell is characterised by a capacity of 47.6 mAh/g (per the total mass of cathode and anode material) with a current load of 1 C. The battery does not exhibit a significant decrease in capacity with a higher load.

An LTO/LAMO type lithium-ion cell is known, comprising a negative electrode of LTO and a positive electrode made of LMO substituted with aluminium ions Li_{1 i1}AI_0i1 Mn_{1 i8}O₄ (LAMO). In this system, it is possible to use numerous types of liquid electrolytes. When using the LiPF₆@EC/DMC electrolyte, the specific capacity amounts to approx. 102 mAh/g (per the mass of cathode material) with a current load of 1 C and exhibits high resistance to charging/discharging cycles (Electrochem. Solid State Lett., 9 (2006) A557). The use of an electrolyte comprising LiBF₄ dissolved in acetonitrile (LiBF₄@ACN) causes a decrease in the capacity of the cell compared to a battery with the standard electrolyte (Chem. Lett, 35 (2006) 848). The LTO/LAMO system has been suggested as a new lead-free accumulator, because the operating voltage of five cells connected in series amounts to approx. 12 V (J. Electrochem. Soc. 156 (2009) A780; Chem. Lett, 38 (2009) 1202).

An LTO/LMO type lithium-ion cell is known with bipolar electrodes and liquid electrolyte comprising LiBF₄ dissolved in acetonitrile (J. Power Sources, 186 (2009) 508). The layout of the arrangement of individual half-cells looks as follows:

(-) collector|LTO|separator|LMO|collector|LTO|separator|LMO|collector (+)

The construction of the cell forced the use of an LMO electrode with twice as high capacity compared to LTO. The authors used active materials with 3-D morphology and spherical grains less than 10 μηι in diameter. This procedure enabled the creation of compounds with a synergic effect of decreased dissolution of manganese in the electrolyte (due to its small specific surface) and fast diffusion of Li⁺ ions (due to the short path of the movement of lithium in the original grains). In order to improve the conductive and mechanical properties, the electrodes comprised large amounts of carbon and binding agents, due to which the amount of active compound in the electrode mass amounted to just 65%. In spite of this, the presented battery is characterised by high specific energy.

An LTO/LMO type lithium-ion cell is known with LiPF₆@EC/DMC liquid electrolyte using electrode materials with bipartite morphology (Adv. Mater., 22 (2010) 3052). The grains with diameters below 10 nm form agglomerates 0.5-2 μηι in size. The cells exhibit a nominal capacity of 2-5 Ah and considerable resistance to high discharge currents.

An LTO/LMO type hybrid lithium-ion cell is known with LiBF₄@EC/DMC/EMC liquid electrolyte in a cell- capacitor system (J. Power Sources, 187 (2009) 635). The positive electrode comprises approx. 30% of the mass of LiMn₂0₄ and 45% of acetylene black serving the function of a supercapacitor. The cell is characterised by a specific energy of 10 Wh/kg with a direct current discharge and exhibits a slight decrease in the specific capacity amounting to 0.001 6%/cycle.

An LTO/LMO type lithium-ion cell is known with a liquid PC/EMC electrolyte comprising lithium salt Li₂B₁₂F₎₍H₁2 {J. Power Sources, 195 (2010) 1479). The cell is characterised by high specific capacity of approx. 120 mAh/g (per the mass of cathode material), however, after 25 cycles it retains only 76% of the original capacity.

There are few papers dedicated to LTO/LMO type lithium-ion cells which do not comprise a liquid electrolyte. The lithium-ion cells with a solid electrolyte exhibit an increased safety level due to the lack of volatile and flammable chemical compounds.

An LTO/LMO type lithium-ion cell is known with a solid polymer electrolyte consisting of polyacrylonitrile (PAN) saturated with LiPF₆@EC/DMC, which constitutes 79 mol% of the electrolyte (J. Electrochem. Soc, 145 (1998) 2615). Although the separator has been classified by the authors as a solid electrolyte, the considerable amount of volatile components should be noted. The battery exhibits a nominal capacity of 4.73 mAh.

An LTO/LMO type lithium-ion cell is known with a solid electrolyte based on poly(ethylene oxide) PEO and polyethylene glycol (PEG) (Solid State Ionics, 144 (2001 ) 1 85). In order to improve the ionic conductivity, a considerable amount of PEG with LiTFSI salt was added to the electrode masses, which unfortunately decreases the active mass content down to 60%. The system is characterised by a capacity of approx. 100 mAh/g (per the mass of cathode material) in an elevated operating temperature (40°C), which unfortunately decreases almost by half in room temperature.

An LTO/LMO type lithium-ion cell is known with a solid gel electrolyte made of poly(vinylene fluoride) (PVDF) saturated with a standard liquid electrolyte LiPF₆@EC/DMC (J. Electrochem. Soc, 152 (2005) A1949). A disadvantage of a LTO/PVDF/LMO button cell constructed in such a manner is the earlier activation of the negative electrode in the system with metallic lithium. The system is characterised by high specific capacity of approx. 130 mAh/g (per the mass of active material), but low resistance to high density currents and a 0.2% capacity decrease in each cycle.

An aluminium current collector undergoes electrodissolution above a potential of 3.5 V in relation to the lithium electrode. In the presence of fluorinated inorganic anions (in electrolytes comprising such salts as, e.g. LiPF₆, LiBF₄ etc.) this collector undergoes passivation creating a stable layer of aluminium fluoride preventing a progressive degradation. In the case of using other lithium salts (such as e.g. lithium bis(trifluoromethylsulphonyl)imide, LiTFSI) the aluminium collector undergoes constant oxidation reactions, which causes a gradual degradation of the cell and a decrease in its cyclic resistance. In such a case, additives ensuring the passivation of aluminium should be used. One of such compounds is lithium bisoxalatoborate, LiB(C₂0₄)₂ (abbreviation : LiBOB) (J. Power Sources, 296 (2015) 197).

In state of the art, LiPF₆ is used as the charge carrier, which at the same time causes the passivation of the aluminium charge carrier, ensuring its stabilisation. A disadvantage of LiPF₆ is its extreme reactivity in relation to humidity. LiPF₆ in contact with humidity generates hydrogen fluoride (HF), which causes the dissolution of the individual components of the cell leading to a decrease in its cyclic resistance and current efficiency. Furthermore, hydrogen fluoride is extremely dangerous for organisms. The presence of volatile organic solvents in lithium-ion cells has always caused concerns regarding the safety of their use. Constructions with electrolyte contents above 1 ml/cm² in relation to the surface of the electrodes are used in state of the art. During a battery short circuit, the energy accumulated in the cell becomes immediately released, emitting a large amount of heat, which results in the ignition of the volatile components of the accumulator. In batteries used in portable devices (laptops, mobile telephones), the accumulated energy is so low (up to 50 Wh) that the risk associated with a possible fire is small. Accumulators powering hybrid vehicles must be characterised by an energy of approx. 2 kWh and the construction of a fully electric car requires a battery system with an energy of approx. 40 kWh (Energy and Environmental Science, 4 (2011 ) 3243). Uncontrolled processes occurring in a package of cells with such high energy may lead to really serious consequences. As a result, alternative solutions are being sought, allowing the reduction of the amount of volatile components in the cells by, e.g. replacing the liquid electrolyte with a solid polymer electrolyte.

None of the known lithium-ion cells fulfil the basic safety requirement involving the elimination of liquid electrolyte comprising volatile and flammable chemical compounds. Furthermore, solutions from state of the art comprising LiPF₆ constitute a potential risk due to the possibility of emitting hydrogen fluoride in contact with humidity in the case of an unsealed cell.

The solution according to the present invention solves problems and inconveniences known from state of the art - it allows the creation of a safely operated lithium-ion accumulator, comprising a modern polymer separator serving the function of a solid electrolyte and minimum amounts of a non-volatile contact fluid, exhibiting the inability to emit hydrogen fluoride. The solution according to the present invention is characterised by high power.

Disclosure of Invention

A lithium-ion cell comprising a negative electrode applied to an aluminium charge collector, a positive electrode applied to an aluminium charge collector, a polymer separator serving the function of a solid electrolyte, a steel spacer, a disc spring, a seal, a housing, a contact fluid, an ion charge carrier and a charge collector stabiliser is characterised in that the negative electrode applied to an aluminium current collector is made of lithium titanium oxide, the positive electrode applied to an aluminium current collector is made of lithium manganese oxide, the polymer separator serving the function of a solid electrolyte is made of silicone-urethane prepolymer, the contact fluid constitutes a solution in dimethyl carbonate. The lithium-ion cell according to the invention is characterised in that lithium titanium oxide, Li₄Ti₅0₁₂ with a spinel structure for the construction of the negative electrode constitutes the product of a mechanochemical reaction of lithium oxide and titanium(IV) oxide in a molar ratio of 4:5, which mechanochemical reaction was conducted in a planetary ball mill, in the environment of ethyl alcohol for a minimum of 12 hours, preferably for 24 hours, using zirconium oxide balls with a diameter of 10 mm, and in the further stage the grinding product is subjected to a heating process in a two-stage regime, i.e. for 1 -5 hours in a temperature of 400-600°C, preferably 3 hours in a temperature of 500°C and subsequently for 10-30 hours in a temperature of 650-900°C, preferably for 20 hours in a temperature of 800°C.

The lithium-ion cell according to the invention is characterised in that the packing density of the electrode mass of the negative electrode on the current collector amounts to 1-5 mg/cm², preferably 3 mg/cm². The lithium-ion cell according to the invention is characterised in that lithium manganese oxide, Li(Mn₂0₄) for the construction of the positive electrode constitutes a product of a sol-gel reaction between lithium acetate and manganese acetate in a molar ratio of 1 :2, with 2-hydroxypropane-1 ,2,3-tricarboxylic acid, ethanoic acid, 2-hydroxyethanoic acid, ethane-1 ,2-diol having been used as a complexing agent, and subsequently having subjected the product of the reaction to heating in a temperature of 450-700°C with a temperature increase rate of 5°C/min, after which isothermal heating took place in a temperature of 700°C.

The lithium-ion cell according to the invention is characterised in that the packing density of the electrode mass of the positive electrode on the current collector amounts to 1-5 mg/cm², preferably 3 mg/cm².

The lithium-ion cell according to the invention is characterised in that a polymer separator is created in the process of hardening the mixture of a silicone-urethane prepolymer comprising polysiloxane and poly(oxyethylene) segments, along with lithium salt (LiTFSI - LiN(CF₃S0₂)2) and an ionic liquid with N- butyl N-methylpyrrolidinium bis(trifluoromethylsulphonyl)amide (PYR₁₄TFSI).

The lithium-ion cell according to the invention is characterised in that the contact fluid content of the cell amounts to below 100 μΙ/cm² in relation to the surfaces of the electrodes, preferably below 10 μΙ/cm² in relation to the surfaces of the electrodes. The contact fluid comprises a charge carrier in the form of lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) in a concentration of 0.1 -1 .5 mol/l, preferably 0.8 mol/l. The contact fluid comprises a current collector stabiliser in the form of lithium oxalatoborate (LiBOB) in a concentration of 0.05-0.5 mol/l, preferably 0.2 mol/l.

The lithium-ion cell with a polymer separator according to the invention is described below in embodiments with reference to the attached drawing, in which:

fig. 1 presents a layout of the system and the mutual positioning of the negative electrode (1 ) applied to an aluminium current collector (5), the positive electrode (2) applied to an aluminium current collector (5), the polymer separator (3) serving the function of a solid electrolyte with the contact fluid (4) applied to the surface in the lithium-ion cell prior to its assembly;

fig. 2 presents the results of the test of the lithium-ion cell according to the invention in a button system from an embodiment: a) galvanostatic charging of the cell; b) galvanostatic discharging of the cell; c) current efficiency of the cell ;

fig. 3 presents powder diffractograms of lithium titanium oxide used in the construction of the negative electrode, generated for various durations of grinding : a) 12 hours, b) 18 hours, c) 24 hours, d) 30 hours; fig. 4 presents SEM photographs of lithium titanium oxide used in the construction of the negative electrode, generated for various durations of grinding: a) 12 hours, b) 1 8 hours, c) 24 hours, d) 30 hours; fig. 5 presents a powder diffractogram of lithium manganese oxide used in the construction of the positive electrode;

fig. 6 presents SEM photographs of lithium manganese oxide used in the construction of the positive electrode magnified: a) 5000 times, b) 10000 times, c) 20000 times, d) 50000 times; fig. 7 presents the structure of the silicone-urethane prepolymer used to generate polymer membranes, where U stands for urethane group, POE - poly(oxyethylene) chain, rectangle - diisocyanate skeleton, broken line - poly(siloxanodiol) chain, NCO - isocyanate group;

fig. 8 presents the results of a thermogravimetric measurement of the silicone-urethane prepolymer used to create polymer membranes, marking the glass transitions of the polymer in temperatures below - 125.59°C, -73.77°C and -39.58°C.

Detailed Description of the Invention.

The negative electrode was made of lithium titanium oxide (LTO), created using the method of mechanochemical synthesis in the solid phase. This method involves a high-energy process of grinding a mixture of substrates in a stoichiometric ratio of LiO:Ti0₂ = 4:5. The grinding was conducted in a planetary (ball) mill, in the environment of ethyl alcohol for 24 hours using balls made of zirconium oxide with a diameter of 10 mm. In the next stage, the product of grinding is subjected to the process of heating in properly selected temperature and time conditions and atmosphere, i.e. 500°C for 3 hours and 800°C for 20 hours {Powder Technology, 266 (2014) 372-377). The result was lithium titanium oxide (Li₄Ti₅0₁₂) with a spinel structure. For longer grinding durations, the material exhibits an increase in the capacity for various discharge currents and resistance to high-current tests in relation to other materials of this class. This is particularly significant for the operation of the whole system. The electrode material described herein synthesised in the above-mentioned manner has not been previously used for the construction of lithium-ion cells.

The positive electrode was made of lithium manganese oxide (LMO), created using a modified sol-gel method. The developed technology allowed obtaining single-phase materials (with no foreign phases like e.g. manganese oxides) in an amount of 100 g per one synthesis with an efficiency of approx. 96%. Lithium and manganese acetates were introduced into the synthesis, being very well soluble in an aqueous environment, undergoing decomposition in the process of calcination. For each conducted synthesis the Li:Mn ratio = 1 :2. 2-hydroxypropane-1 ,2,3-tricarboxylic acid was introduced into this synthesis as a complexing agent. Apart from this compound, other compounds were also introduced, like, e.g. ethanoic acid, 2-hydroxyethanoic acid, ethane-1 ,2-diol. Properly selected conditions of the heating process (450-700°C) advantageously affected the form of the resulting final powder product. Complexing agents used in the synthesis of LMO affect the sizes of crystallites, which in this case amount to approx. 22-24 nm. The smaller the crystallites, the higher the values of specific capacity, the cyclic resistance and the resistance to high-current tests. The electrode material described herein (LiMn₂0₄) synthesised in the above-mentioned manner has not been previously used for the construction of lithium-ion cells.

The active material (Li₄Ti₅0₁₂ in the case of the negative electrode and LiMn₂0₄ in the case of the positive electrode) in an amount of 100-300 mg is preliminarily mixed with a conductive additive (Vulcan XC72R carbon) in an amount of 12-40 mg using the grinding method for approximately 15 minutes. Adhesive is dripped into this mixture in an amount of 12-40 mg in the form of a 5% poly(vinylene fluoride) solution (PVDF) dissolved in 1 -methyl-2-pyrrolidone (NMP). 20-50 mg of NMP solvent are dripped into such a suspension in order to obtain proper viscosity of the paste. After four hours of mixing by means of a magnetic stirrer everything is applied to a current collector. The current collector is made of aluminium. The stability of aluminium in the operating potential of the lithium-ion cell is ensured by adding LiBOB to the contact fluid. The thickness of the current collector ranges between 10 and 50 pm. Properly prepared electrode material is applied to the surface of the current collector and is subsequently uniformly spread by means of an automatic applicator with an aperture size of 50-250 pm. The surplus of NMP solvent was removed from the layer prepared in such a manner by drying it in a temperature of 30-70°C in the air for 1-3 hours and subsequently in a temperature of 100-150°C under the vacuum for 10-15 hours. The packing density of the electrode layer amounts to 1-5 mg/cm². Subsequently, electrodes with sizes adjusted to their specific use are cut from the material prepared in such a manner. The electrodes are additionally dried in a temperature of 100- 150°C under the vacuum for 10-20 hours.

A new type of polymer separator described in patent application P.413615 is used for the construction of the cell. The lithium-ion cell according to the invention is the first described construction using this type of polymer separator. The polymer separator serves the function of a solid electrolyte. The synthesis of the polymer used as a solid electrolyte in the cell according to the invention involved hardening the mixture of a silicone-urethane prepolymer comprising polysiloxane and poly(oxyethylene) segments, along with lithium salt (LiTFSI - LiN(CF₃S0₂)2) and an ionic liquid with N-butyl N-methylpyrrolidinium bis(trifluoromethylsulphonyl)amide (PYR₁₄TFSI). The polymer separator has a number of advantages in relation to state of the art. It is characterised by very good mechanical strength preventing its breakage during installation in the cell. It is also characterised by very high ionic conductivity, enabling fast diffusion of lithium ions while charging and discharging the cell.

The specific conductance of a polymer separator approx. 30 pm in thickness amounts to 1 .26-10 ³ S/cm and it has been determined by means of the EIS method. Thermogravimetric studies have proved that the polymer exhibits three glass transition temperatures, the lowest (below -120°C) corresponding to glass transition in the silicone phase and the remaining ones (-74°C and -30°C) to glass transitions in the organic phase. The polymer separator is characterised by its conductivity as well as thermal and mechanical properties appropriate for use in a lithium-ion cell.

The contact fluid improving the ionic contact of electrodes with the polymer separator and ensuring conductivity inside the pores of the electrodes is also a component of the cell. According to the present invention, the contact fluid constitutes a solution of LiTSI (lithium bis(trifluoromethylsulphonyl)imide) and LiBOB (lithium oxalatoborate) with a concentration of 0.2 mol/l in dimethyl carbonate (DMC). The solution exhibits low vapour pressure and is stable under the working conditions of the cell. The contact fluid is introduced into the cell directly before its assembly and hermetic sealing. The introduction of the contact fluid takes place by dripping it over both surfaces of the polymer separator, which tightly adheres to the surfaces of the electrodes. The contact fluid is introduced while assembling the cell just before its closure. The addition of the contact fluid is used in an amount of no more than 10 μΙ/cm² in relation to the surfaces of the electrodes.

In the contact fluid, DMC serves the function of a dispersing phase, LiTFSI serves the function of a charge carrier and LiBOB serves the function of an agent stabilising the aluminium charge collector. The ionic conductivity of LiTFSI is comparable to LiPF₆, however, LiTFSI has been chosen due to its stability in the presence of trace humidity, which translates into the safety of operation for the whole cell. LiBOB - this compound was chosen due to its stability in the operating conditions of the cell (with an operating voltage of approx. 2.5 V) and resistance in relation to humidity. LTO/LMO lithium-ion cells using LiBOB have not been previously known.

The lithium-ion cell according to the invention is characterised by a specific energy of approx. 120 Wh/kg calculated per electrode masses and a cyclic resistance of approx. 55% after 50 cycles of deep charging/discharging.

The lithium-ion cell according to the invention exhibits an increased safety level due to the extremely low amount of fluidic components, which translates into the low amount of volatile components which can cause an increase in pressure inside the cell during its operation. The only fluidic substance in the cell according to the invention is the contact fluid in an amount of less than 10 μΙ/cm² in relation to surface electrodes.

The lithium-ion cell according to the invention exhibits an increased safety level also due to the use of stable and non-reactive substances serving the function of a charge carrier (LiTFSI) and a current collector stabiliser (LiBOB).

Best Mode for Carrying out the Invention.

The lithium-ion cell in a button system has been made using LTO and LMO electrodes as well as a solid polymer electrolyte.

The positive and negative electrodes were created using a paste method on an aluminium collector (doctor blade slurry coating method). The active material (Li₄Ti₅0₁₂ in the case of the negative electrode and LiMn₂0₄ in the case of the positive electrode) in an amount of 200 mg was preliminarily mixed in a manual agate mortar along with a conducting additive (Vulcan XC72R carbon) in an amount of 25 mg using the grinding method for approximately 15 minutes. Adhesive was dripped into this mixture - poly(vinylene fluoride) (PVDF) in an amount of 25 mg, dissolved in the solvent - 1 -methyl-2-pyrrolidone (NMP) with a concentration of 5 wt%. 30 mg of NMP solvent were dripped into such a suspension in order to obtain proper viscosity of the paste. After four hours of mixing by means of a magnetic stirrer, everything was poured onto an aluminium collector with a thickness of 20 μηι and spread by means of an automatic applicator with an aperture size of 200 μηι. The surplus of NMP solvent was removed from the layer prepared in such a manner by drying it in a temperature of 50°C in the air for 1 h and 120°C under the vacuum for 12 h. The packing density of the electrode layer amounted to 3 mg/cm² for the negative electrode and 4 mg/cm² for the positive electrode. Electrodes with a diameter of 12 mm and geometric surface of 1 .13 cm² were subsequently cut from the layer and then dried in a temperature of 120°C under the vacuum for 1 5 h. After this operation, the electrodes were transferred to a glovebox with argon atmosphere, with humidity and oxygen content below 5 ppm, where they were used to construct a coin- shaped lithium-ion battery (coin cell) CR2032.

The negative electrode with a mass of 1 1 mg made of lithium titanium oxide with a spinel structure Li₄Ti₅0₁₂ was placed on the lower housing of the coin-shaped battery. A separator 19 mm in diameter was cut from a layer of polyurethane separator and placed on the negative electrode. The contact fluid was applied to it in an amount of 30 μΙ of lithium salt solution (LiTFSI and LiBOB) with a concentration of 0.2 mol/dm³ (in relation to LiBOB) dissolved in dimethyl carbonate (DMC). Subsequently, a positive electrode with a mass of 13 mg was applied to this system, made of lithium manganese oxide with a spinel structure LiMn₂0₄. A steel spacer, a disc spring and an upper housing (positive pole) were placed on a system of two electrodes separated by a polyurethane membrane. The battery prepared in such a manner was compressed on a special press under a pressure of 150 bar for 20 s. The battery ready for measurements weighed approx. 3.2 g.

Claims

1. A lithium-ion cell comprising a negative electrode applied to an aluminium charge collector, a positive electrode applied to an aluminium charge collector, a polymer separator serving the function of a solid electrolyte, a steel spacer, a disc spring, a seal, a housing, a contact fluid, an ion charge carrier and a charge collector stabiliser characterised in that the negative electrode (1 ) applied to an aluminium current collector (5) contains lithium titanium oxide, the positive electrode (2) applied to an aluminium current collector (5) contains lithium manganese oxide, the polymer separator (3) serving the function of a solid electrolyte contains silicone-urethane prepolymer, the contact fluid (4) constitutes a solution in dimethyl carbonate.

2. The lithium-ion cell according to claim 1 , characterised in that lithium titanium oxide, Li₄Ti₅0₁₂ with a spinel structure for the construction of the negative electrode (1 ) constitutes the product of a mechanochemical reaction of lithium oxide and titanium(IV) oxide in a molar ratio of 4:5, wherein the mechanochemical reaction is conducted in a planetary ball mill, in the environment of ethyl alcohol for a minimum of 12 hours, preferably for 24 hours, using zirconium oxide balls with a diameter of 10 mm, and in the further stage the grinding product is subjected to a heating process in a two-stage regime, i.e. for 1 -5 hours in a temperature of 400-600°C, preferably 3 hours in a temperature of 500°C and subsequently for 10-30 hours in a temperature of 650-900°C, preferably for 20 hours in a temperature of 800°C.

3. The lithium-ion cell according to claim 1 , characterised in that the packing density of the electrode mass of the negative electrode (1 ) on the current collector (5) amounts to 1-5 mg/cm², preferably 3 mg/cm².

4. The lithium-ion cell according to claim 1 , characterised in that lithium manganese oxide, Li(Mn₂0₄) for the construction of the positive electrode (2) constitutes a product of a sol-gel reaction between lithium acetate and manganese acetate in a molar ratio of 1 :2, with 2-hydroxypropane-1 ,2,3-tricarboxylic acid, ethanoic acid, 2-hydroxyethanoic acid, ethane-1 ,2-diol used as a complexing agent, and subsequently the product of the reaction is subjected to heating in a temperature of 450-700°C with a temperature increase rate of 5°C/min, after which isothermal heating takes place in a temperature of 700°C.

5. The lithium-ion cell according to claim 1 , characterised in that the packing density of the electrode mass of the positive electrode (2) on the current collector (5) amounts to 1-5 mg/cm², preferably 4 mg/cm².

6. The lithium-ion cell according to claim 1 , characterised in that the polymer separator (3) is formed in the process of hardening the mixture of a silicone-urethane prepolymer comprising polysiloxane and poly(oxyethylene) segments, along with lithium salt (LiTFSI - LiN(CF₃S0₂)₂) and an ionic liquid with N- butyl N-methylpyrrolidinium bis(trifluoromethylsulphonyl)amide (PYR₁₄TFSI).

7. The lithium-ion cell according to claim 1 , characterised in that the contact fluid (4) content of the cell amounts to below 100 μΙ/cm² in relation to the surfaces of the electrodes, preferably below 10 μΙ/cm² in relation to the surfaces of the electrodes.

8. The lithium-ion cell according to claim 7, characterised in that the contact fluid (4) comprises a charge carrier in the form of lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) in a concentration of 0.1-1 .5 mol/l, preferably 0.8 mol/l.

9. The lithium-ion cell according to claim 7, characterised in that the contact fluid (4) comprises a current collector stabiliser in the form of lithium oxalatoborate (LiBOB) in a concentration of 0.05- 0.5 mol/l, preferably 0.2 mol/l.