Use of a salt mixture as an additive in a lithium-gel battery

Abstract

The invention relates to the simultaneous use of a first salt comprising a nitrate anion (NO.sub.3.sup.−) and a second salt comprising an anion other than nitrate, at least one of the first and second salts being a lithium salt, as ionic conductivity promoters in a rechargeable lithium-metal-gel battery. The invention also relates to a lithium-gel battery comprising a mixture of said first salt and said second salt, to a non-aqueous gel electrolyte comprising such mixture and to a lithium battery positive electrode comprising said mixture.

Claims

1. An ion conductivity promoter in a rechargeable lithium-metal-gel battery comprising: i) a first salt S1 of the formula M.sub.α(NO.sub.3).sub.β in a molar number n1, ii) a second salt S2 of the formula M′.sub.γA.sub.δ in a molar number n2, for which: M and M′ are organic or inorganic cations, it being understood that at least one of M and M′ is a lithium cation, and A is an anion, α, β, γand δ are such that the electroneutrality of compounds of the formulae M.sub.α(NO.sub.3).sub.β and M′.sub.γA.sub.δ is respected, wherein: the molar ratio of salts S1 and S2 (RM.sub.S1/S2), defined by the following equation (1) $\begin{array}{cc}\frac{\beta .Math.n_1}{\delta .Math.n_2},& \left(1\right)\end{array}$ is greater than 1.5, and wherein said rechargeable lithium-metal-gel battery has at least one positive electrode, at least one non-aqueous electrolyte and at least one negative electrode based on lithium metal or a lithium alloy, said positive electrode and said electrolyte being one or both gel and forming an {electrolyte+positive electrode} complex, and wherein said battery is free of polysulfide ions.

2. The ion conductivity promoter as claimed in claim 1, wherein: 1) the electrolyte contains at least one salt S1 and at least one salt S2 and the positive electrode contains neither salt S1 nor salt S2, or 2) the electrolyte contains neither salt S1 nor salt S2 and the positive electrode contains at least one salt S1 and at least one salt S2, or 3) the electrolyte contains only one salt S1 and the positive electrode contains only one salt S2, or 4) the electrolyte contains only one salt S2 and the positive electrode contains only one salt S1, or 5) the electrolyte and the positive electrode each contain at least one salt S1 and at least one salt S2, the molar ratios of the salts S1 and S2 within the electrolyte (RM.sub.S1/S2 Electrolyte) and within the positive electrode (RM.sub.S1/S2 Elect. Positive) which may be identical or different from one another provided that the molar ratio RM.sub.S1/S2 within the battery is greater than 1.5, or 6) the electrolyte contains only one of the salts S1 and S2 and the positive electrode contains at least one salt S1 and at least one salt S2, or 7) the electrolyte contains at least one salt S1 and at least one salt S2 and the positive electrode contains only one of the salts S1 and S2.

3. The ion conductivity promoter as claimed in claim 1, wherein the molar ratio RM.sub.S1/S2 is greater than or equal to 10.

4. The ion conductivity promoter as claimed in claim 1, wherein the total content of salts S1 and S2 varies from 0.5 to 30 mass %, based on the mass of said {electrolyte+positive electrode} complex.

5. The ion conductivity promoter as claimed in claim 1, wherein the cations M and M′ of the salts S1 and S2 are selected from alkali metals.

6. The ion conductivity promoter as claimed in claim 5, wherein the alkali metals are selected from lithium, sodium, potassium, rubidium, cesium and francium.

7. The ion conductivity promoter as claimed in claim 1, wherein M and M′ are both lithium cations.

8. The ion conductivity promoter as claimed in claim 1, wherein the salt S1 is lithium nitrate.

9. The ion conductivity promoter as claimed in claim 1, wherein the anion A is selected from triflate, perchlorate, perfluorate, bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, bis(pentafluoroethylsulfonyl)imide, tetrafluoroborate and bis(oxalato)borate.

10. The ion conductivity promoter as claimed in claim 1, wherein the salt S2 is selected from lithium bis(trifluoromethylsulfonyl)imide and lithium bis(fluorosulfonyl)imide.

11. The ion conductivity promoter as claimed in claim 1, wherein the mixture comprises lithium nitrate as salt S1 and lithium bis(trifluoromethylsulfonyl)imide or lithium bis(fluorosulfonyl)imide as salt S2.

12. A composite gel positive electrode for a lithium-gel battery, said composite electrode comprising: at least one positive electrode active material reversibly inserting lithium ions, at least one polymer binder, at least one solvent, at least one gelling polymer and at least one mixture of: i) a first salt S1 of the formula M.sub.α(NO.sub.3).sub.β in a molar concentration C1 and a molar number n1 and, and ii) a second salt S2 of the formula M′.sub.γA.sub.δ in a molar concentration C2 and a molar number n2 and, for which: M and M′ are organic or inorganic cations, it being understood that at least one of M and M′ is a lithium cation, and A is an anion, α, β, γ and δ are such that the electroneutrality of compounds of the formulae M.sub.α(NO.sub.3).sub.β and M′.sub.γA.sub.δ is respected, said mixture being such that: the total molar concentration [C1+C2] of salts S1 and S2 varies from 0.5 to 10 mol/L, and the molar ratio of salts S1 and S2 (RM.sub.S1/S2), defined by the following equation (1) $\begin{array}{cc}\frac{\beta .Math.n_1}{\delta .Math.n_2},& \left(1\right)\end{array}$ is greater than 1.5.

13. The electrode as claimed in claim 12, wherein the mixture of salts S1 and S2 is 0.5 to 10 mass %, based on the total weight of said positive electrode.

14. The electrode as claimed in claim 12, wherein the positive electrode active material is 55 to 90 mass % relative to the total mass of the positive electrode material.

15. The electrode as claimed in claim 12, wherein said electrode is deposited on a current collector.

16. The electrode as claimed in claim 12, wherein the positive electrode active material is selected from lithium iron phosphates.

17. A lithium-gel battery comprising: a positive electrode, a negative electrode based on lithium metal or a lithium alloy, an electrolyte disposed between said positive electrode and said negative electrode, wherein said battery is free of polysulfide ions, and wherein said battery also comprises: i) a first salt S1 of the formula M.sub.α(NO.sub.3).sub.β in a molar number n1, and ii) a second salt S2 of the formula M′.sub.γA.sub.δ in a molar number n2, for which: M and M′ are organic or inorganic cations, it being understood that at least one of M and M′ is a lithium cation, and A is an anion, α, β, γ and δ are such that the electroneutrality of compounds of the formulae M.sub.α(NO.sub.3).sub.β and M′.sub.γA.sub.δ is respected, the molar ratio of salts S1 and S2 (RM.sub.S1/S2), defined by the following equation (1) $\begin{array}{cc}\frac{\beta .Math.n_1}{\delta .Math.n_2},& \left(1\right)\end{array}$ is greater than 1.5, said salts S1 and S2, independently of each other, being present, before the first charge/discharge cycle of said battery, within the electrolyte or within the positive electrode or within both the electrolyte and the positive electrode, one and/or the other of said positive electrode and said electrolyte being gel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the results from example 1, with the change in voltage (in V) is a function of time in hours, in accordance with one embodiment;

(2) FIG. 2 shows the results from example 2, with the voltage profile (in V) as a function of the discharge capacity of the battery (in mA.Math.h/g), in accordance with one embodiment; and

(3) FIG. 3 shows the results from example 2, with the change in the battery discharge capacity (in mAh/g) and the coulombic efficiency (in %) as a function of the number of cycles is shown, in accordance with one embodiment.

DETAILED DESCRIPTION

(4) The present invention is illustrated by the following examples, to which, however, it is not limited.

EXAMPLES

(5) The advantage of using a mixture of the salts S1 and S2 as defined according to the present invention in the composition of the electrolyte and/or the positive electrode can be measured by characterizing the lithium electrodeposition in a symmetrical lithium/electrolyte/lithium cell and by monitoring the cycling of complete cells.

Example 1

Demonstration of the Effect of the Mixture of Salts S1 and S2 on the Quality of Lithium Electrodeposition

(6) The quality of the lithium electrodeposition could be evaluated by cycling in symmetrical lithium/electrolyte/lithium cells. These tests made it possible to characterize the stability of non-aqueous gel electrolytes in accordance with the invention compared with a non-aqueous gel electrolyte not forming part of the present invention.

(7) The evaluations were carried out with lithium metal; the electrolyte solution alone was evaluated by impregnation of a polyolefin separator sold under the trade name BPF Bolloré Porous Film by Bolloré.

(8) Electrolyte solutions were prepared using LiTFSI as salt S2 (sold by 3M), LiNO.sub.3 as salt S1 (sold by Alfa Aesar) and polyethylene glycol) dimethyl ether (PEGDME 250 g/mol sold by Sigma Aldrich). The electrolyte solutions were prepared by dissolving the lithium salts in PEGDME under magnetic stirring at room temperature.

(9) Three electrolyte solutions A, B and C with the composition shown in Table 1 below were evaluated:

(10) TABLE-US-00001 TABLE 1 Solutions A (*) B C PEGDME (mass %) 88.00 55.00 69.00 LiNO.sub.3 (mass %) 3.60 13.50 24.80 LiTFSI (mass %) 8.40 31.50 6.20 Total salt concentration (in mol/L) 0.9 4.2 4.8 Molar ratio [NO.sub.3.sup.−]/[TFSI.sup.−] 1.8 1.8 16.7 NO.sub.3.sup.− concentration (in mol/L) 0.6 2.7 4.5 (*) comparative electrolyte solutions, not being part of the invention.

(11) Only solutions B and C are in accordance with the present invention. In particular, the comparative solution A has the same molar ratio [NO.sub.3.sup.−]/[TFSI.sup.−] as the solution B in accordance with the invention, but a total salt content of less than 1 mol/L.

(12) Three complete cells were then prepared, each using one of the electrolyte solutions A, B or C as prepared above.

(13) The separator was dipped in the electrolyte solution, the excess solution being removed with absorbent paper and then sandwiched between two sheets of lithium metal, each 50 μm thick. Three cells were thus obtained, referred to as cells A, B and C respectively.

(14) The cells were tested in galvanostatic cycling (constant current) at 40° C., at 300 μA/cm.sup.2 for 4 hours, then reversed current direction for 4 hours.

(15) The results obtained are shown in the appended FIG. 1, where the change in voltage (in V) is a function of time in hours. In this figure, the black curve corresponds to cell A comprising electrolyte solution A (cell not in accordance with the invention), the dark grey curve corresponds to cell B comprising electrolyte solution B and the light grey curve corresponds to cell C comprising electrolyte solution C.

(16) These results show the good stability of the cycling polarization of cells B and C in accordance with the invention, whereas cell A not in accordance with the invention has a very poor stability. These results also show that the higher the molar concentration of NO3.sup.−, the more stable the polarization is cycle after cycle.

Example 2

Preparation of a Lithium-Gel Battery in Accordance with the Present Invention

(17) A complete cell was prepared with the following constitution:

(18) Gel Electrolyte (According to the Second Subject Matter of the Invention): 20 g or 40 mass % of a solution comprising 13.75 mass % LiNO.sub.3 (or 2.45 mol/L) (Alfa Aesar) and 13.75 mass % LiTFSI (or 0.59 mol/L) (3M) in poly(ethylene glycol) dimethyl ether (PEGDME 250 g/mol sold by Aldrich); 20 g or 40 mass % PVdF Solef 21510 (Solvay); 10 g or 20 mass % polyoxyethylene (PEO 1L sold by Sumitomo Seika).

(19) The various components of the gel electrolyte were mixed in a mixer sold under the trade name Plastograph® by Brabender® temperature of 110° C. The resulting mixture was then laminated at 110° C. to form a gel electrolyte film with a thickness of about 20 μm.

(20) The gel electrolyte so prepared had the characteristics summarized in Table 2 below:

(21) TABLE-US-00002 TABLE 2 Components PEGDME (mass %) 29.00 LiNO.sub.3 (mass %) 5.5 LiTFSI (mass %) 5.5 Total salt concentration (in mol/L) 3.04 Molar ratio [NO.sub.3.sup.−]/[TFSI.sup.−] 4.16 NO.sub.3.sup.− concentration (in mol/L) 2.45

(22) Gel Positive Electrode (A Gel Positive Electrode Comprising a Mixture of LiNO.sub.3 and LiTFSI According to the Third Subject Matter of the Invention): 74 mass % LiFePO.sub.4 sold under the trade name LFP P600A by Pulead; 16 mass % of an electrolyte solution comprising 13.75 mass % (or 2.45 mol/L) LiNO.sub.3 (Alfa Aesar) and 13.75 mass % (or 0.49 mol/L) LiTFSI (3M) in poly(ethylene glycol) dimethyl ether (PEGDME 250 g/mol sold by Aldrich); 8 mass % poly(ethylene oxide) (PEO 1L sold by Sumitomo); 2 mass % carbon black sold under the trade name Ketjenblack® EC600JD by Akzo Nobel.

(23) The various components of the positive electrode were mixed in a mixer sold under the trade name Plastograph® by Brabender® at a temperature of 110° C. The resulting mixture was then laminated at 80° C. to form a gel positive electrode film with a thickness of about 30 μm.

(24) Cell Assembly:

(25) A 50 μm thick lithium metal strip was used as the negative electrode.

(26) An aluminum current collector having a carbonaceous coating (Armor) was used as a current collector for the positive electrode. The individual lithium/gel electrolyte/gel positive electrode/collector layers were laminated under 5 bar pressure at a temperature of 80° C. to produce the battery. The lamination was carried out in a controlled atmosphere (dew point—40° C.).

(27) The cells thus prepared were then enclosed in a sealed, heat-sealable package to protect them from moisture.

(28) The battery thus prepared was tested in galvanostatic cycling (constant current) at 40° C. The first cycle was carried out at C/10 (charge in 10 hours) and D/10 (discharge in 10 hours) and the following cycles at C/4 (charge in 4 hours) and D/2 (discharge in 2 hours).

(29) The voltage profile (in V) as a function of the discharge capacity of the battery (in mA.Math.h/g) is shown in the appended FIG. 2. In this figure the solid curve corresponds to cycle 1 (C/10; D/10) and the dotted curve corresponds to cycle 2 (C/4; D/2).

(30) The change in the battery discharge capacity (in mAh/g) and the coulombic efficiency (in %) as a function of the number of cycles is shown in the appended FIG. 3. In this figure, the curve with the full circles and the solid line corresponds to the discharge capacity and the curve with the empty circles and the dashed line corresponds to the coulombic efficiency.

(31) What emerges from these results is that the voltage profile shows a low cycling polarization reflecting good kinetics within the battery. In addition, the capacity and efficiency are stable, reflecting a good reversibility of the electrochemical process.