WO2023201393A1

WO2023201393A1 - Improved electrolyte for batteries

Info

Publication number: WO2023201393A1
Application number: PCT/AU2023/050321
Authority: WO
Inventors: Yanyan Wang; Zaiping Guo; Zhijie Wang
Original assignee: The University Of Adelaide
Priority date: 2022-04-21
Filing date: 2023-04-20
Publication date: 2023-10-26

Abstract

The present invention describes an improved non-flammable electrolyte for a rechargeable lithium metal battery that includes: a lithium metal salt, a carbonate, nitrate anions and tri- methyl phosphate, wherein the carbonate/ tri-methyl phosphate is in the ratio of approximately 7:3~1:1 v/v. The improved electrolyte has excellent non-flammable properties, improved Coulombic efficiency, low viscosity, high ionic conductivity and is economical.

Description

IMPROVED ELECTROLYTE FOR BATTERIES

FIELD OF THE INVENTION

The present invention relates to a non-flammable electrolyte for use in a lithium metal battery, the non-flammable electrolyte including a lithium metal salt, a carbonate and trimethyl phosphate.

BACKGROUND

Li metal -based batteries (LMBs) are of significant research interest as they have a high energy density and so as a source of power are desirable in applications with high energy demands such as in electric vehicles

Conventional LMB’s have a drawback of being relatively prone to combustion and can present as a fire hazard in certain applications. Design for safety in LMB’s has become most important, even at time outweighing the pursuit of high energy density. Using solid-state electrolytes has been regarded as a strategy to obviate threats to fire safety, but a practical difficulty has been to solve the high solid-solid interfacial resistance between electrolyte and electrodes.

Researchers have looked at salt-concentrated, high-salt-to-solvent ratio, all-fluorinated and ionic liquid electrolytes to maintain safety whilst boosting cycle life of LMBs. However, these electrolytes have some inherent disadvantages including, high viscosity, low ionic conductivity and present high cost limit practical applications.

SUMMARY

In accordance with a first embodiment of the invention there is a non-flammable electrolyte for a rechargeable lithium metal battery including: a lithium metal salt, a carbonate, nitrate anions and tri-methyl phosphate.

In certain embodiments the carbonate includes at least one carbonate selected from the group of: ethylene carbonate, diethyl carbonate (DEC), fluoroethylene carbonate (FEC). In certain embodiments the carbonate includes a mixture of carbonates selected from the group of: ethylene carbonate, diethyl carbonate (DEC), fluoroethylene carbonate (FEC).

In certain embodiments the carbonates ethylene carb onate: di ethyl carbonate (DEC):fluoroethylene carbonate (FEC) are in the ratio of approximately 4:4:2 N/N/N.

In certain embodiments the lithium metal salt is lithium hexafluorophosphate.

In certain embodiments the nitrate ions are from LiNOs.

In certain embodiments the concentration of lithium hexafluorophosphate in the electrolyte is between 0.01 M and 1.0 M.

In certain embodiments the concentration of lithium hexafluorophosphate in the electrolyte is 0.6 M.

In certain embodiments the concentration of LiNOs in the electrolyte is between 1.0 M and 1.3 M.

In certain embodiments the concentration of LiNOs in the electrolyte is 1.2 M.

In further embodiments there is a rechargeable battery having a non-flammable electrolyte for a rechargeable lithium metal battery including: a carbonate electrolyte, nitrate anions and the non-aqueous solvent tri-methyl phosphate.

In further embodiments there is a rechargeable battery having a non-flammable electrolyte including: a lithium metal salt, a carbonate, nitrate anions and tri-methyl phosphate, wherein the carbonate/ tri-methyl phosphate is in the ratio of approximately 7:3 to 1 : 1 v/v.

In further embodiments, the carbonate/ tri-methyl phosphate is in the ratio of approximately 3:2 v/v. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows the ionic conductivity of an embodiment of the CTLL of the present invention and comparison with state-of-the-art non-flammable electrolytes for LMBs. The embodiment of the CTLL electrolyte of the present invention has higher ionic conductivity compared with most of the reported non-flammable electrolyte;

Figure 2 shows a comparison of viscosity at room temperature of an embodiment of the CTLL of the present invention with other concentrated, or high-salt-to-solvent ratio, nonflammable electrolytes;

Figure 3 shows the coulombic efficiency (CE) for Li-Cu cells using of an embodiment of the CTLL of the present invention and CL electrolytes with fixed capacity of 1 mAh cm^-2 under current density 0.25 mA cm^-2;

Figure 4 shows Coulombic efficiency (CE) for Li-Cu cells with CL electrolyte and CTLL electrolyte;

Figure 5 shows Coulombic efficiency (CE) for Li-Cu cells containing different electrolyte;

Figure 6 shows Coulombic efficiency (CE) for Li-Cu cells containing two reference electrolytes: CT-1.8L (1.8 M LiPF 6 in EC/DEC/FEC/TMP) and CT-1.8LS (1.8 M LiFSI in EC/DEC/FEC/TMP) with current density of 0.25 mA cm '² and capacity of 1 mAh cm'²;

Figure 7 is the same as figure S14A but with current density of 1 mA cm'² and capacity of 1 mAh cm'²;

Figure 8 shows Coulombic efficiency (CE) comparison for Li-Cu cells containing CTLL electrolyte and C-1.8L electrolyte (1.8 M LiPF 6 in EC/DEC/FEC (4:4:2 by vol));

Figure 9 shows Coulombic efficiency (CE) comparison for Li-Cu cells with electrolyte containing different concentration of LiNOy

Figure 10 shows the cycling performance of LFP||Li foil cell at 0.5 C (1.07 mA cm'²); Figure 11 shows the cycling performance of NCM81 l||Li foil cell at 0.2 C (0.78 mA cm'²);

Figure 12 shows electrochemical performance of Li metal full cells with CTLL electrolyte of the present invention and CL electrolyte under practical conditions (high cathode loading, lean electrolyte, and low n/p ratios). LFP||Li full cell tests under high cathode loading and lean electrolyte conditions with n/p ratios of 5 (a) and 2 (b) at 0.2 C (-0.48 mA cm^-2);

Figure 13 shows voltage profiles for LFP||Li full cells with an n/p ratio of 5 in CTLL electrolyte (a) and CL electrolyte (b);

Figure 14 shows voltage profiles for LFP||Li full cells with an n/p ratio of 2 in CTLL electrolyte (a) and CL electrolyte (b);

Figure 15 shows the electrochemical performance of NCM||Li cell test under high cathode loading and lean electrolyte with n/p ratio of 4.5, in which the batteries are activated at 0.05 C and then cycled at 0.1 C (-0.47 mA cm^-2);

Figure 16 shows voltage profiles for NCM||Li full cells with an n/p ratio of 4.5 in CTLL electrolyte (a) and CL electrolyte (b);

Figure 17 shows a summary comparison of Coulombic efficiency (CE), cost and flammability of state-of-the-art electrolytes;

Figure 18 shows the Coulombic efficiency (CE) of battery using electrolytes with different percentages of TMP solution;

Figure 19 shows the average CE of battery using electrolytes with different percentages of TMP solution;

Figure 20 shows a comparison of properties and performances of embodiments of the CTLL of the present invention and CL, axes in the hexagons represent relative comparisons of properties and performances. DESCRIPTION OF THE INVENTION

Methods

Preparation of electrolytes. Lithium hexafluorophosphate (LiPFe, > 99.9 %) was used without further treatment’. Lithium nitrate (LiNO , 99.99%) was vacuum dried at 60 °C for 6 h before use. Ethylene carbonate (EC, anhydrous, 99 %), diethyl carbonate (DEC, anhydrous, > 99%), fluoroethylene carbonate (FEC, 99 %) and trimethyl phosphate (TMP >99%) were dried with molecular sieves (4 A) for two (2) days prior to use. All electrolytes were stored and used in an argon-filled glove-box with oxygen and moisture levels <0.1 ppm.

CTLL (Carbonate, Tri-methyl phosphate, Lithium hexafluorophosphate, Lithium nitrate) electrolyte was formulated via mixing 1 M LiPFe/ carb onate solution and 3 M LiNO /TMP solution with a volume ratio of 6:4, ie 3:2 v/v. The carbonate was composed of 40 % ethylene carbonate (EC), 40 % diethyl carbonate (DEC) and 20 % fluoroethylene carbonate (FEC) (by volume). CL electrolyte was formulated by addition of 0.6 M LiPFe into the same carbonate mixture (EC:DEC:FEC = 4:4:2 by volume).

Electrochemical tests. To determine Coulombic efficiency for Li plating/stripping, CR2032 type coin-cells with Cu-foil as cathode and Li-foil as anode were assembled. One (1) layer of Celgard 2400 polymer film was used as separator. The Cu-foils were pre-washed with boiling water, ethanol and diluted hydrochloric acid, respectively, to remove surface contamination. Fifty (50) pL of electrolyte was added to these Li-Cu cells.

To prepare high-mass-loaded lithium iron phosphate (LFP) electrodes , LFP powder, Super P and polyvinylidene fluoride (PVDF) were mixed in N-methyl-2-pyrrolidine in a mass ratio of 8: 1 : 1. The mixture was coated on aluminium-foil via a tape-casting method, following which the electrode was vacuum-dried at 120 °C for 12 h. The as-obtained LFP electrode was punched into discs with a diameter of 10 mm for later use. The NCM electrode was prepared by the same method with the mass ratio NCM (LiNio.8Coo.1Mno.1O2, NCM811), Super P and PVDF of 90:5:5. The active material mass loadings for the LFP and NCM electrodes were ~ 14.3 and -16.7 mg cm^-2, respectively. CR2032 type coin-cells were assembled to test the LFP||Li and NCM||Li batteries. The electrolyte was controlled at -10 pL mAh^-1. The Li metal electrodes for the LFP||Li and NCM||Li cells were prepared via pre-plating a controlled amount of Li on a Cu substrate at low current of 0.05 mA. Characterization. The Li-Cu cells were assessed in a NEW ARE battery test system, and the LFP||Li batteries tested with a LAND CT2001 A battery-tester. A JEOL JSM-7500FA SEM was used to observe the surface morphology of the Li metal electrodes. The XPS depth profiling was performed in a ThermoScientific NEXSA XPS system using a 20 eV Ar⁺ ion beam with a scanning area of 200 x 200 pm², for which the sputtering rate was 2 kV and the sputtering area 2 x 2 mm². A TOF.SIMS 5 instrument (ION-TOF, Munster, Germany) was used for TOF-SIMS depth profiling. The sputtering beam was Cs⁺ working at 1 keV.

The Li electrodes for SEM, XPS, and TOF-SIMS tests were cycled in Li-Cu cells for 50 cycles with a capacity of 1 mAh cm^-2. Prior to testing the cells were disassembled and the Li electrodes washed with DEC and TMP before drying in a glove-box. The cryo-TEM characterization was performed on a FEI Titan Krios microscope with an accelerating voltage of 300 kV. The high-resolution TEM images were obtained at a dose rate of ~18 e“ px^-1 s^-1 with a dosage of ~ 180 e A^-2. The TEM data were processed with DigitalMicrograph (Gatan) software. The Li for cryo-TEM characterization was deposited on bare Cu-grids without a supporting layer in coin-cells under a current density of 0.5 mA cm^-2 for 15 min. The Cu- grids were washed with DEC, dried in an Ar-filled glove-box and quickly transferred into a liquid N2 bath and loaded into the cassette without exposure to air.

To determine the flammability of the electrolytes, a layer of glass fibre (GF) was soaked in 200 pL liquid electrolyte and the GF electrolyte burned with a lighter. The ionic conductivity of the electrolyte was determined with a portable ionic meter (DDB-303 A, Shanghai Leici Instrument Factory). The viscosity of the electrolyte was measured with a rheometer (Anton Parr Physica MCR 301 Anton).

The electrolyte of the present invention is formulated by mixing 60 vol.% carbonate electrolyte, 1 M LiPFe in ethylene carbonate (EC)/ diethyl carbonate (DEC)/fluoroethylene carbonate (FEC) in a volume ratio of 4:4:2, with 40 vol.% 3 M LiNCh/TMP. The concentration of LiPFe and LiNOs in the as-formulated electrolyte of an embodiment of the present invention (denoted as CTLL: Carbonate, Tri-methyl phosphate, Lithium hexafluorophosphate, Lithium nitrate) is 0.6 M and 1.2 M, respectively. The electrolyte without TMP and LiNOs (0.6 M LiFPe in EC/DEC/FEC = 4:4:2) (denoted as CL: Carbonate, Lithium hexafluorophosphate,), and other electrolytes with varying concentration of TMP and LiNOs were used as references. The CTLL of the present invention was clear and transparent without apparent sediment or impurity. The CTLL electrolyte of the present invention could not be ignited by an ignition source because it exhibits a self-extingui shing time of 0 s g^-1. In contrast, the CL and other reference carbonate electrolytes without TMP are in highly flammable with a selfextinguishing time of > 60 s g^-1 as was confirmed via flame/ignition test. The ionic conductivity of the CTLL electrolyte was determined to be 5.42 mS cm^-1 (Figure 1) which is a greater value than for many non-flammable electrolytes. The viscosity of CTLL electrolyte is 4.04 mPa s^-1, a value significantly less than that for concentrated, high salt-to-solvent, and ionic liquid non-flammable electrolytes (Figure 2 and Table A). The viscosity value for the first three electrolytes is cited from the literature; the viscosity for CTLL and CTL is that at room temperature. It can be directly inferred from the table that the addition of LiNOs to the electrolyte did not significantly increase viscosity of the electrolyte. The advantage of ionic conductivity and viscosity of CTLL are attributed directly to the low salt concentration together with the size of NOV anion as it is significantly smaller than that of widely used bis(fluorosulfonyl)imide (FSE) and bis(trifluoromethanesulfonyl)imide (TFSE).

Table A

In addition, ignition tests, shown in Table B, were carried out on trimethyl phosphate (TMP) and carbonate mixtures (solid form), in which the percentage of TMP was 0%, 10%, 20%, 30%, 40%, and 50%, respectively. The carbonate solvent is composited with ethylene carbonate (EC), diethyl carbonate (DEC), and fluoroethylene carbonate (FEC) with a ratio of 4:4:2 by volume.

Table B

As shown in Figure 3, the Li-Cu cell using CTLL electrolyte exhibits a high average columbic efficiency (CE) of 99.49 % at 0.25 mA cm^-2, which was surprising with respect to non-flammable electrolytes. In comparison, the CE values for Li-Cu cells using CL electrolyte and other typical carbonate-based electrolytes are less. As shown in Figure 3, after activation, the average CE of Li-Cu cells in CTLL electrolyte is 99.49%, one of the highest values reported in non-flammable reported so far.

At high current density of 3 mA cm^-2 and a high capacity of 3 mAh cm^-2, the Li-Cu cell using CTLL electrolyte delivers a CE of -98%, also much higher than that using CL electrolyte (Figure 4). This confirms the poor stability of these electrolytes against Li metal anode. In the non-flammable electrolyte without LiNCh, (0.6 M LiPFe in EC/DEC/FEC/TMP, denoted as CTL), the CE falls rapidly with cycling, see figures S12, , current density at 1 mA cm ^-2and capacity 1 mAh cm^-2 and SI 3, current density of 3 mA cm^-2 and a capacity 3 mAh cm^-2. These results underscore that the LiNOs in CTLL both eliminates the negative effects caused by TMP and boosts stability of the basic carbonate electrolyte.

When increasing the concentration of traditional Li salts to 1.8 M in the electrolytes (carbonate-TMP mixture, without LiNCh), the CE for Li-Cu cells remains poor (Figure 7). In addition, the Li-Cu cell with 1.8 M LiPFe-carbonate electrolyte also remains poor CE (Figure 8), Current density 1 mA cm’² and capacity 1 mAh cm’². These results confirmed that LiNOs is useful to boost Li compatibility of the electrolyte.

In addition, in certain examples, we observed that when decreasing the concentration of LiNOs in the electrolyte, the CE of Li-Cu cells decline as well (Figure 9). CTLL-1 refers to 0.6 M LiPF₆ + 0.8 M LiNO₃ in EC/DEC/FEC/TMP, and CTLL-2 refers to 0.6 M LiPF₆ + 0.4 M LiNO₃ in EC/DEC/FEC/TMP, indicating high concentration of NO₃‘ is necessary to improve the stability of electrolyte.

Li metal full cells with LFP or NCM811 (LiNio.8Coo.1Mno.1O2,), as cathodes were tested to assess performance of CTLL electrolyte. Firstly, cycling performance of the LFP||Li foil and NCM81 1 ||Li foil cells with flooded electrolyte (60 pL) and high cathode mass loading were evaluated. As is shown in Figure 10, with a mass loading of LFP cathode is 12.68 mg cm -2 (2.15 mAh cm’² ), and the electrolyte amount is 60 pL, the LFP||Li foil cell using CTLL electrolyte remains a capacity of 80% even after 800 cycles, while that using CL electrolyte suffers an obvious capacity decay after 500 cycles. Similarly, the lifespan ofNCM811||Li foil cells with CTLL electrolyte (more than 300 cycles) also longer than that with CL electrolyte (-200 cycles) (Figure 11), with mass loading of NCM811 cathode is 13.7 mg cm’² (3.9 mAh cm’²), and the electrolyte amount is 60 pL. To achieve high energy density, practical LMBs must operate with lean electrolyte, high cathode capacity together with a low n/p ratio (capacity ratio of the negative electrode to positive electrode). Therefore, high mass-loading LFP (-14.3 mg cm^-2) (2.43 mAh cm^-2) and NCM (-16.7 mg cm^-2) (4.75 mAh cm^-2) electrodes and a limited capacity of Li were further used in the Li metal full cells, and the electrolyte was fixed at -10 pL mAh^-1. As is shown in Figure 12 (and Figure 13) when n/p ratio is controlled at 5, the LFP||Li battery using CTLL electrolyte exhibited a high initial CE (ICE) of 98.9% at 0.2 C (~ 0.48 mA cm^-2), greater than that for LFP||Li battery using CL electrolyte (97.8%).

Both the CE and discharge capacity of LFP||Li battery using CL electrolyte highly significantly fell after 60 cycles. Importantly however those for LFP||Li battery using CTLL electrolyte remained stable. Even after 150 cycles the LFP||Li battery using CTLL exhibited a high discharging capacity of 121.3 mAh g^-1 with capacity retention of 81.6%. When the n/p ratio was reduced to 2, as is shown in Figure 12-B and Figure 14, the ICE for LFP||Li battery using CTLL (99.03 %) was greater than that using CL (96.69 %). The CE and capacity of the LFP||Li battery using CL electrolyte reduced rapidly within 20 cycles. Significantly however the LFP||Li battery using CTLL continued for > 100 cycles with high capacity retention of 82.3% at 0.2 C (-0.45 mA cm^-2). When the CTLL electrolyte was applied in an NCM811 ||Li battery with an n/p ratio of 4.5 the battery retained more than 80% of its capacity after 100 cycles at 0.1 C (-0.47 mA cm^-2), whilst the capacity of the NCM81 1 ||Li battery with CL rapidly decayed to < 80% within 20 cycles (Figure 15 and Figure 16).

In harsh conditions the capacity decay of the full cell can be explained as follows, there is: 1) insufficient Li⁺ transport ability of the electrolyte to transfer all the necessary Li⁺ into and out from the relatively thick LFP/NCM electrodes, and; 2) depletion of electrolyte, or active Li, because of side reactions. CTLL electrolyte has a low viscosity and high ionic conductivity and therefore the capacity for high mass loading cathode to be delivered. Its boosted compatibility with Li ensures that the consumption of both active Li and electrolyte is minimized. We conclude therefore the lifespan of full cells using CTLL electrolyte will be extended.

As shown in Figure 17, the Columbic efficiency (CE) of Li-Cu cells was used to assess performance of the electrolytes. For traditional electrolytes it is practically difficult to simultaneously improve electrochemical performance and safety, whilst reducing cost. For example, electrolytes with functional additives, including adiponitrile, fluoroethylene carbonate and LiNOs, exhibit boosted CE but remain flammable. Carbonate electrolytes, with phosphates as flame retardants, boost safety but are accompanied by compromised battery performance. Although state-of-the-art electrolytes (e.g. salt-concentrated and all-fluorinated electrolytes) exhibit satisfactory CE, high cost limits commercialization. Additionally, a flame-retardant electrolyte can still be ignited, but it is just not as flammable as a conventional electrolyte. A non-flammable electrolyte however cannot be ignited. Significantly embodiments of the CTLL electrolyte of the present invention in this work exhibits excellent CE and is non-flammable, and has significantly lower cost than high-CE electrolytes, making it practical for application in LMBs.

Figure 18 shows the Coulombic efficiency (CE) of batteries using electrolytes with different percentages of TMP solution. The basic carbonate electrolyte is 1 M LiPFe in EC/DEC/FEC (4:4:2 by volume), and the TMP solution is 3 M LiNOs dissolved in TMP.

The ignition tests (Table B) confirm that 30% of TMP at least is required to eliminate the flammability of carbonate solvent. The Coulombic efficiency test reveals that a suitable amount of TMP solution boosts the electrochemical performance of non-flammable electrolytes, but excessive use of TMP has an opposite effect. A percentage of TMP solution in the non-flammable electrolyte is around 30%~50%.

The cost of the CTLL electrolyte of the present invention is significantly less than that for those of reported state-of-the-art high-performance electrolytes, including concentrated, high salt-to-solvent ratio and all-fluorinated non-flammable electrolytes. The boosted reduction stability, Li anode reversibility, full cell lifespan, and safety, as well as its appropriate physical properties and low cost (Figure 20), underscores that CTLL electrolyte is comprehensive and has practical potential for commercialization

Accordingly, the electrolyte of the present invention shows that the stability of electrolyte against Li metal can be boosted via the introduction of nitrate anions (NOL) into the solvation sheath. The electrolyte formulated by adding lithium nitrate / trimethyl phosphate (LiNCL/TMP) solution into carbonate electrolyte. Because NO3- has a DN of 22.2 kcal mol^-1, which is close to that of TMP (23.0 kcal mol^-1), the existence of TMP weakens the electrostatic attraction between Li⁺ and NO3-, but it is insufficient to “free” NO3-.

Consequently, the interaction amongst TMP, NO3- and Li⁺ reaches a balance and NO3- is introduced into the solvation structure. In this designed electrolyte, because of the involvement of NO3-, the electrophilicity of both coordinated carbonate and TMP molecules in the solvation structure is reduced, suppressing solvent decomposition on Li. This electrolyte is non-flammable with a zero self-extingui shing time. It also exhibits low viscosity (4.04 mPa s^-1) and high ionic conductivity (5.42 mS cm^-1). The reversibility of the Li plating/stripping in Li-Cu cells is boosted to 99.49%, one of the highest reports for nonflammable electrolytes. The lifespan of LMBs with lean electrolyte is significantly extended despite harsh conditions with mass loadings of, respectively, 14.3 mg cm^-2 and 16.7 mg cm^-2 for lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide cathodes (LiNio.8Coo.1Mno.1O2, NCM811) and n/p ratio <5.

The stability of an electrolyte against Li metal is significantly dependent on its solvation structure. Solvent molecules exhibit a reduced lowest unoccupied molecular orbital (LUMO) when complexed with Li⁺. In the solvation sheaths, polarized solvent molecules induced by the electrostatic field of Li⁺ are more susceptible to accepting electrons and decompose on Li metal surface than free solvents, because of increased regional electrophilicity. The polarization degree of solvent molecules can be reduced via introduction of anions into the solvation sheath, leading to lower possibility of accepting electrons. It appears that where the electrostatic attraction between anions and solvents molecules to Li⁺ reaches a delicate balance, the existence of anion in solvation sheath can be practically realized. Gutmann donor number (DN) is a widely accepted parameter to measure ability of compounds to act as electron donors. It can be used as a scale to assess the attraction between Li⁺ and other electrolyte components. We hypothesize that when anions and solvents with comparable DN are selected, anions can readily get into solvation sheaths which helps to improve reduction stability of the electrolyte. Nitrate anion (DN = 22.2 kcal mol^-1) and TMP (DN = 23.0 kcal mol^-1) are such a pair, and it's suggested that the non-flammable electrolyte of the present invention is based on this mechanism.

As can now be seen, the present invention provides a non-flammable electrolyte, has low viscosity, high ionic conductivity and is low cost and extended the lifespan of a practical lithium metal full cell by > 100%.

A. A non-flammable electrolyte for a rechargeable lithium metal battery including: a lithium metal salt, a carbonate, nitrate anions and tri-methyl phosphate, wherein the carbonate/ tri-methyl phosphate is in the ratio of 7:3-1 : 1 v/v.

B. The non-flammable electrolyte of A, wherein the carbonate includes a mixture of carbonates selected from the group of: ethylene carbonate, diethyl carbonate (DEC), fluoroethylene carbonate (FEC).

C. The non-flammable electrolyte of B, wherein the carbonates ethylene carb onate: di ethyl carbonate (DEC): fluoroethylene carbonate (FEC) are in the ratio of approximately 4:4:2 N/N/N.

D. The non-flammable electrolyte of any one of A-C, wherein the lithium metal salt is lithium hexafluorophosphate.

E. The non-flammable electrolyte of any one of A-D, wherein the nitrate ions are from LiNO₃. F. The non-flammable electrolyte of any one of the claims D or E, wherein the concentration of lithium hexafluorophosphate in the electrolyte is between 0.01 M and 1.0 M.

G. The non-flammable electrolyte of F, wherein the concentration of lithium hexafluorophosphate in the electrolyte is 0.6 M.

H. The non-flammable electrolyte of any one of claims E-G, wherein the concentration of LiNOs in the electrolyte is between 1.0 M and 1.3 M.

I. The non-flammable electrolyte of H, wherein the concentration of LiNOs in the electrolyte is 1.2 M.

J. A rechargeable battery having a non-flammable electrolyte of any one of A-I.

K. The non-flammable electrolyte of any one of A-H wherein the carbonate/ tri-methyl phosphate is in the ratio of 3 :2 v/v.

Claims

1. A non-flammable electrolyte for a rechargeable lithium metal battery including: a lithium metal salt, a carbonate, nitrate anions and tri-methyl phosphate, wherein the carbonate/ tri-methyl phosphate is in the ratio of approximately 7:3—1 : 1 v/v.

2. The non-flammable electrolyte of claim 1, wherein the carbonate includes a mixture of carbonates selected from the group of: ethylene carbonate, diethyl carbonate (DEC), fluoroethylene carbonate (FEC).

3. The non-flammable electrolyte of claim 2, wherein the carbonates ethylene carb onate: di ethyl carbonate (DEC): fluoroethylene carbonate (FEC) are in the ratio of approximately 4:4:2 N/N/N.

4. The non-flammable electrolyte of any one of claims 1-3, wherein the lithium metal salt is lithium hexafluorophosphate.

5. The non-flammable electrolyte of any one of claims 1-4, wherein the nitrate ions are from LiNOs.

6. The non-flammable electrolyte of any one of claims 4 or 5, wherein the concentration of lithium hexafluorophosphate in the electrolyte is between 0.01 M and 1.0 M.

7. The non-flammable electrolyte of claim 6, wherein the concentration of lithium hexafluorophosphate in the electrolyte is 0.6 M.

8. The non-flammable electrolyte of any one of claims 5-7, wherein the concentration of LiNOs in the electrolyte is between 1.0 M and 1.3 M.

9. The non-flammable electrolyte of claim 8, wherein the concentration of LiNOs in the electrolyte is 1.2 M.

10. The non-flammable electrolyte of any one of claims 1-9 wherein the carbonate/ trimethyl phosphate is in the ratio of 3 :2 v/v

11. A rechargeable battery having a non-flammable electrolyte of any one of claims 1-10.