Prussian blue analogue electrolyte

Abstract

The present invention relates to a sodium ion cell comprising: - a positive electrode comprising a sodium Prussian blue analogue according to formula AaMb[M'c(CN)6]d, wherein A is sodium, 1 < a ≤ 2, M and M' are individually one or more transition metals, 0 < b < 2, 1 ≤ c < 2, and 1 ≤ d < 2; - a negative electrode comprising a carbon-based sodium inserting material; - a separator, and - an electrolyte comprising a sodium salt, an organic carbonate-based solvent comprising at most 50 wt.% of cyclic organic carbonate, a first additive, and a second additive comprising an organic sulfate, wherein said first additive is a cyclic carbonate according to formula (I) or formula (II) and different from said cyclic organic carbonate in said organic carbonate-based solvent.

Description

[0001] PRUSSIAN BLUE ANALOGUE ELECTROLYTE

[0002] Technical field

[0003] The present invention relates to a sodium ion cell having a positive electrode comprising a sodium Prussian blue analogue and an electrolyte comprising a first and a second additive. The present invention further relates to a system comprising a plurality of said sodium ion cells.

[0004] Background

[0005] A rechargeable battery is one of several types of rechargeable energy storage systems. One of the most commonly used rechargeable batteries is the lithium-ion battery. Lithium-ion batteries provide high levels of charge but tend to be rather costly. Cheaper alternative to the lithium-ion battery is the sodium-ion battery which compared to lithium-ion batteries has better safety characteristics.

[0006] Sodium-ion battery (SIB), also referred to as sodium ion cell, uses sodium ions as charge carriers. SIB cells consist of a cathode based on a sodium containing material, an anode and a liquid electrolyte containing dissociated sodium salts in polar protic or aprotic solvents. During charging, sodium ions move from the cathode to the anode while electrons travel through the external circuit. During discharge, the reverse process occurs.

[0007] A problem with rechargeable batteries is that if they are used repeatedly, they lose capacity as the number of charge cycles increases, until they are eventually considered to have reached the end of their useful life. The battery is then to be worn out. Especially, operating battery cells at temperatures outside conventional conditions has become an issue in practical application of SIBs, because sluggish electrochemical reaction kinetics and unstable interfacial reactions deteriorate the performance and even cause safety issues. To achieve SIBs that sustain operation over a wide temperature range, the composition and structure of electrolyte and electrode materials must be studied and improved. Prussian blue and its analogues (PB / PBAs) are promising candidates of cathode materials of sodium-ion cells due to their stable frame structures, tunable redox sites, and facile synthesis methods. However, a challenge with Prussian blue analogue electrodes is that they suffer from a fast capacity fade and short cycle life due to decomposition of the electrolyte and phase changes in the material.

[0008] Accordingly, there is a need to provide an improved sodium-ion cell with lower capacity fade and improved cycle life.

[0009] Summary of the invention

[0010] The present invention provides ways to provide improved cycle life and better performance of a sodium ion cell. The scope of the present invention is set by the appended claims.

[0011] According to a first aspect of the present invention, this and other objects are achieved by a sodium ion cell comprising:

[0012] - a positive electrode comprising a sodium Prussian blue analogue according to formula AaMb[M'c(CN)6]d, wherein A is sodium, 1 < a < 2, M and M' are individually one or more transition metals, 0 < b < 2, 1 < c < 2, and 1 < d < 2;

[0013] - a negative electrode comprising a carbon-based sodium inserting material;

[0014] - a separator, and

[0015] - an electrolyte comprising a sodium salt, an organic carbonate-based solvent comprising at most 50 wt.% of cyclic organic carbonate, a first additive, and a second additive comprising an organic sulfate, wherein the first additive is a cyclic carbonate according to formula (I) or formula (II) and different from said cyclic organic carbonate in said organic carbonate-based solvent,

[0016] (I) wherein A and A' are independently selected from a cycloalkyl group, a substituted cycloalkyl group, a halogen group, an alkoxy group, a cyclic carbonate group, a substituted cyclic carbonate group, a hydrogen group, an alkyl group, or a substituted alkyl group, and wherein A and A' are not simultaneously a hydrogen group, or A and A' are not simultaneously a hydrogen group and a methyl group, and wherein B and B' are independently selected from a cycloalkyl group, a substituted cycloalkyl group, a halogen group, an alkoxy group, a cyclic carbonate group, a substituted cyclic carbonate group, a hydrogen group, an alkyl group, or a substituted alkyl group.

[0017] The present invention is based on the understanding that by providing the electrolyte of a sodium ion cell containing a PBA positive electrode with a first and a second additive, as described above, more distinct and defined upper and lower voltage plateaus may be exhibited, indicating enhanced efficiency and stability in the electrochemical reactions occurring within the sodium ion cell. The presence of the first and second additives may also lead to a reduction in voltage hysteresis, thereby rendering the charge and discharge curves more similar to each other. This similarity results in a narrower operating voltage range, which is advantageous for the overall efficiency of the sodium ion cell.

[0018] Cyclic organic carbonates and organic sulfates act as SEI-forming additives, and their combined presence contributes to a robust and uniformly distributed SEI layer, leading to improved mechanical, electrochemical, and thermal properties. It is believed that the selected organic carbonates and organic sulfates enhance the ionic conductivity of the SEI layer and reduces the impedance of the sodium ion cell. By doing so, organic carbonates and organic sulfates contribute to improved overall cycling performance of the sodium ion cell. The stable SEI formed by the combined action of organic carbonates and organic sulfates can effectively suppress further decomposition of the electrolyte on the anode surface. This reduces side reactions, which can extend the sodium ion cell's lifespan and improve its safety profile.

[0019] The alkyl group and the substituted alkyl group may be partially unsaturated. The cyclic carbonate may be referred to as a cyclic carbonic acid ester. One example of a Prussian blue analogue is Nai.8Fe[Fe(CN)e]. Prussian blue analogue can, with its open framework structure, accommodate the insertion / extraction of sodium ions with minimal volume changes, which is highly beneficial for maintaining structural stability during cycling.

[0020] During discharge, the potential (voltage) of Prussian blue decreases, exhibiting characteristic voltage plateaus at 3.0 and 3.3 V (vs. Na+ / Na). The voltage plateau corresponds to the two-electron redox reaction of iron centres in the Prussian blue analogue lattice.

[0021] During charge, the voltage increases, returning to its initial value.

[0022] The organic sulfate may be selected from compounds (Illa) to (lllf), wherein D, G, G*, GH, GH*, DH, Q, and QH are independently selected from a hydrogen group, a halogen group, an alkyl group, or a substituted alkyl group, wherein each of the alkyl group and the substituted alkyl group preferably comprises 1-6 carbon atoms.

[0023]

[0024] The organic sulfate may be a cyclic organic sulfate, preferably ethylene sulfate (DTD), ethylene sulfate - vinylene carbonate - ethylene sulfate (DTD-VC-DTD), or vinylene carbonate - ethylene sulfate (VC-DTD). DTD-VC-DTD may be referred to as 4-[3-(l,l-dioxo-lX6-thiolan-3-yl)-l, 3, 2X6-dioxathiolan-2-yl]-lX6-thiolane-1, 1-dione.

[0025] The Smile description for DTD-VC-DTD is:

[0026] O=C1OC(C2CCS(=O)(=O)C2)C(C2COS(=O)(=O)O2)O1. VC-DTD may be referred to as 4- (2-oxo-l,3-dioxolan-4-yl)-l,3,2X6-dioxathiolan-2-one. The Smile description for VC- DTD is O=C1OCC(C2COS(=O)(=O)O2)O1.

[0027] The cyclic carbonate may comprise fluoroethylene carbonate (FEC), vinylene carbonate-derivative, di-vinylene carbonate (Di-VC), di-vinylene carbonatederivative, vinylene carbonate (VC), and / or ethylene carbonate-derivative. Herein, Di-VC may also be referred to as 4-(2-oxo-l,3-dioxolan-4-yl)-l,3-dioxolan-2-one. The Smile description is O=C1OCC(C2COC(=O)O2)O1

[0028] The cyclic carbonate may comprise vinylene carbonate (VC). It is believed that VC helps in forming a robust and uniform solid electrolyte interface (SEI) layer on the anode. It gets preferentially reduced on the anode surface during the initial cycles, leading to the formation of polymeric compounds that are part of the SEI layer. This results in a reduction of irreversible capacity loss during the first few cycles and better cycling stability.

[0029] Organic carbonates, functioning as electrolyte solvents, serve to augment the solubility of electrolyte salts, thereby ensuring high ionic conductivity. Moreover, these solvents are capable to operate across a wide voltage range, concomitant with adequate thermal stability. Cyclic carbonates may form a stable solid electrolyte interface in sodium-ion cells, thereby enhancing the cell performance and longevity.

[0030] The cyclic carbonate may be selected from a compound according to formulas (IVa) to (IVe) wherein A, A', B, B', G*, GH, and GH* are selected from a halogen group, an alkoxy group, a hydrogen group, an alkyl group, or a substituted alkyl group, and wherein A' is not a hydrogen group or a methyl group.

[0031] (IVa) (IVb) (ivc) (IVd)

[0032] The alkyl group or the substituted alkyl group may be partially unsaturated.

[0033] The electrolyte may comprise from 0.1 to 5 wt.%, preferably from 0.5 to

[0034] 1 wt.% of organic sulfate.

[0035] The electrolyte may comprise from 0.1 to 5 wt.%, preferably from 1 to

[0036] 2.5 wt.% of cyclic carbonate. The organic carbonate-based solvent may comprise at least 50 wt.%, preferably at least 75 wt.%, and more preferably at least 90 wt.% of organic carbonate.

[0037] The organic carbonate-based solvent may further comprise linear organic carbonate, preferably dimethyl carbonate (DMC), diethyl carbonate (DEC) and / or ethyl methyl carbonate (EMC).

[0038] The organic carbonate-based solvent may comprise from 50 to 100 wt.%, preferably from 60 to 80 wt.% of linear organic carbonate.

[0039] The cyclic organic carbonate may comprise ethylene carbonate (EC) and / or propylene carbonate (PC).

[0040] The organic carbonate-based solvent may comprise from 5 to 45 wt.%, preferably from 10 to 30 wt.% of cyclic organic carbonate.

[0041] The first additive may be vinylene carbonate (VC) and the second additive may be ethylene sulfate (DTD). Alternatively, the first additive may be di-vinylene carbonate (Di-VC) and the second additive may be ethylene sulfate - vinylene carbonate - ethylene sulfate (DTD-VC-DTD). Both VC and DTD are SEI-forming additives, and their combined presence may lead to a robust and uniformly distributed SEI layer, leading to improved mechanical, electrochemical, and thermal properties. Similar to VC, it is believed that DTD enhances the ionic conductivity of the SEI layer and reduces the impedance of the sodium ion cell. By doing so, DTD contributes to improved overall cycling performance of the sodium ion cell. The stable SEI formed by the combined action of VC and DTD can effectively suppress further decomposition of the electrolyte on the anode surface. This reduces side reactions, which can extend the sodium ion cell's lifespan and improve its safety profile.

[0042] The electrolyte may comprise from 0.2 wt.% to 0.8 wt.% ethylene sulfate (DTD) and from 1 wt.% to 2 wt.% vinylene carbonate (VC) by mass of the electrolyte.

[0043] The electrolyte may for example comprise 1.5 wt.% VC and 0.5 wt.% DTD. The transition metals may comprise iron and / or manganese.

[0044] The sodium salt may comprise sodium hexafluorophosphate (NaPFe).

[0045] The carbon-based sodium inserting material may comprise hard carbon.

[0046] Hard carbon can reversibly store sodium ions. During charge (sodium deintercalation from cathode; sodium intercalation into anode), hard carbon undergoes a sloping voltage profile. The voltage typically starts from a high value and gradually decreases to around 0.01 V from about 0.8 V (vs. Na+ / Na).

[0047] During discharge (sodium intercalation into cathode; sodium deintercalation from anode), the cell voltage decreases as the state of charge (SOC) reduces.

[0048] During charge, the process is reversed. Sodium ions are extracted from the Prussian blue analogue cathode and inserted back into the hard carbon. The voltage increases as the SOC rises.

[0049] The sodium Prussian blue analogue may comprise sodium Prussian white.

[0050] The electrolyte may comprise from 0.1 wt.% to 5 wt.% of the first and the second additive respectively, based on the total weight of the electrolyte. This amount may facilitate the phase change of the lower plateau due to reduced impedance as described further up in the text, and which then promotes cycle life of the sodium ion cell. Alternatively, the electrolyte may comprise from 0.1 to 2.5 wt.% of the first and the second additive respectively, based on the total weight of the electrolyte.

[0051] According to a second aspect of the present invention, there is provided a system comprising a plurality of sodium ion cells according to the first aspect, and at least one electrical connector configured to electrically interconnect the plurality of sodium ion cells.

[0052] Brief of the

[0053] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention.

[0054] Figs, la-lc illustrate graphs of charge / discharge voltage profiles of coin cells with and without VC + DTD additives in an electrolyte with different solvent formulations;

[0055] Fig. 2 illustrates plots of Coulombic efficiency of the cells with VC + DTD additives in Figs, la-lc;

[0056] Figs. 3 illustrates plots of specific discharge capacity of the cells with VC + DTD additives in Figs, la-lc; Fig. 4 illustrates graphs of charge / discharge voltage profiles of cells with VC additive and with VC+DTD additive, respectively, in the electrolyte;

[0057] Fig. 5 illustrates plots of Coulombic efficiency of the cells in Fig. 4;

[0058] Fig. 6 illustrates plots of specific discharge capacity of the cells in Fig. 4;

[0059] Fig. 7 illustrates graphs of charge / discharge voltage profiles of coin cells with different electrolyte formulations at cycle one and 50;

[0060] Fig. 8 illustrates plots of specific discharge capacity of the cells in Fig. 7;

[0061] Fig. 9 illustrates plots of Coulombic efficiency of the cells in Fig. 7.

[0062] Detailed description and Examples

[0063] Fig. la-lc illustrate graphs of charge / discharge voltage profiles of coin cells with and without additives in an electrolyte with different electrolyte formulations. The electrolyte formulations tested comprised cyclic:linear carbonate ratios of 100:0 wt.%, 25:75 wt.%, and 0:100 wt.%, respectively.

[0064] The coins cells are comprised of hard carbon as an anode and Prussian blue analogue as a cathode, wherein 70 pl of the different electrolyte formulations is injected to respective cells. The capacity of the coin cells is approximately 30 mAh. It is important to note that these cells have undergone an initial formation process. The coin cells underwent galvanostatic cycling, post-formation, between 1.8 V - 3.8 V at a C-rate of C / 3. The cycling data presented here (Figs, la-lc), thus, excludes the formation cycles, with cycle number one indicating the initial cycle immediately following the formation stage.

[0065] The electrolyte used was NaPFe EC:PC:DEC:EMC with 2 wt.% vinylene carbonate (VC) and 0.5 wt.% ethylene sulfate (DTD). As a reference, a corresponding cell with the same components were used but with no additive contained in the electrolyte.

[0066] As can be seen from Figs, lb-lc, the samples containing VC and DTD additives exhibit more distinct and defined upper and lower voltage plateaus. This observation indicates enhanced efficiency and stability in the electrochemical reactions occurring within the cell. The presence of these additives also leads to a reduction in voltage hysteresis, thereby rendering the charge and discharge curves more similar to each other. This similarity results in a narrower operating voltage range, which is advantageous for the overall efficiency of the battery.

[0067] Figs. 2 and 3a-3b show that the incorporation of certain additives enhance the charge and discharge capacities in the ratios of 25:75 wt.%, and 0:100 wt.% when examining the cyclic:linear carbonate distributions. The initial specific discharge capacities are reported to be in the range of 135 - 138 mAh / g. However, there is an expected decline in these capacities to the range of 133 - 136 mAh / g after 30 cycles. In stark contrast, the 100:0 wt.% formulation exhibits a significantly reduced initial discharge capacity of 127.6 mAh / g, which further diminishes to 102.46 mAh / g post 30 cycles.

[0068] As can be seen in Figs, la to 3 and in Table 1, the introduction of certain additives leads to improvement within various electrolyte formulations, which are differentiated based on the proportion of cyclic to linear carbonate solvents.

[0069] It is noteworthy that the advantageous impacts of the additives are prominent at the ratios of 25:75 wt.% and 0:100 wt.% cyclic:linear, where capacity retention over 30 cycles is improved. However, such improvements are absent in the 100:0 wt.% cyclic:linear carbonate composition. The lower plateau of the voltage profile during discharge is observed to be extended, which yields a higher capacity, in the ratios of 25:75 wt.%, and 0:100 wt.% when the additives are incorporated. This is accompanied with a reduced impedance, increasing the average voltage during discharging which thereby increases the energy density of the sodium ion cell. The specific discharge capacities after 30 cycles are thus improved for the ratios of 25:75 wt.%, and 0:100 wt.%, highlighting the additives impact in increasing lifetime. For the ratio of 0:100 wt.% for e.g., the cell without additive only shows a discharge capacity of 3.95 mAh / g after 30 cycles, whereas its counterpart cell with additive shows 136.07 mAh / g after the same number of cycles. Such an improvement is not observed for the 100:0 wt.% cyclic:linear carbonate composition, where the introduction of additives shows a negative effect in terms of capacity and average voltage.

[0070] From these observations, it can be inferred that the additives exert a substantial effect within the range of 0 - 50 wt.% for cyclic carbonates solvent combination. From these observation, it can be inferred that the additives exert a substantial effect within the range and 100 - 50 wt.% for linear carbonates solvent combination.

[0071] Table 1. Comparative analysis of electrolyte formulations with cyclic:linear carbonate ratios of 100:0 wt.%, 25:75 wt.%, and 0:100 wt.%.

[0072] Figs. 4-6 and Table 2 illustrate results from large format cells (1.2 Ah) containing a carbonate electrolyte with a cyclic:linear carbonate ratio of 20:80 wt.%. One cell comprises 2 wt.% VC and the other cell comprises 1.5 wt.% VC and 0.5 wt.% DTD. The large format cells of capacity greater that 1 Ah are comprised hard carbon as an anode, Prussian blue analogue as a cathode, and a carbonate-based electrolyte. These cells have completed the formation process. The cycling data shown here omits the formation cycles. In this context, cycle number one refers to the very first cycle that occurs right after the completion of the formation stage.

[0073] An improvement in cycle life can be observed from the addition of both 1.5 wt.% VC and 0.5 wt.% DTD, where discharge capacity retention is at 90.68 % after 300 cycles. The cell with only 2 wt.% VC shows a lower specific discharge capacity retention of 86.14 % after the same number of cycles. Table 2. Comparison of VC and a combined VC + DTD additive based electrolyte

[0074] Fig. 7 illustrates graphs of charge / discharge voltage profiles of coin cells with different electrolyte formulations. The capacity of the coin cells is approximately 30 mAh. It is crucial to emphasize that the graphs include the formation cycles. In this context, cycle number one represents the initial cycle of the formation process. This formation phase is conducted at a C-rate of C / 10 and consists of a total of three cycles. After the formation phase, the subsequent cycling is carried out at a C-rate of C / 3. The galvanostatic cycling protocol was, thus: three cycles at C / 10 and 47 cycles at C / 3. As a reference, an electrolyte formulation containing EC:PC:DEC:EMC (1:1:4:4 wt.%) and 1.2 M NaPF6 was used. There was also one sample tested containing the reference electrolyte formulation and 2 wt.% VC and one sample containing the reference electrolyte formulation and 2 wt.% additives; i.e. 1 wt.% Di-VC (Al) and 1 wt.% DTD-VC-DTD (Bl). As may be seen in Fig. 8 and Table 3, all formulations exhibited a nearly uniform initial specific charge capacity at C / 10, oscillating between 161.00 and 163.76 mAh / g.

[0075] Table 3. Comparison of Reference electrolyte formulation with additive based electrolytes

[0076] The first cycle's data, as presented in Table 3 and Fig. 9, shows that the reference additive showcased an initial columbic efficiency of 74.50%. However, with the incorporation of 2 wt.% VC, a significant enhancement was observed, with the efficiency rising to 83.38%. Yet, the standout performance emanated from the novel additives (Al + Bl) introduced, which registered an elevated initial columbic efficiency of 86.28%.

[0077] This distinction was further mirrored in the first specific discharge capacity, see Fig. 8. Here, the electrolytes with novel additives (Al + Bl) achieved 138.92 mAh / g; a notable increment when contrasted with the 121.99 mAh / g from the reference electrolyte formulation and 134.44 mAh / g when 2 wt.% VC additives were utilized.

[0078] The observations, extending to the 50thcycle, corroborated these initial findings. The electrolyte formulations with novel additives consistently surpassed both the reference formulation and the one with 2 wt.% VC in terms of specific charge and discharge capacity.

[0079] In summation, introduction of VC additive into reference formulation improves initial columbic efficiency and thereby, specific discharge capacity. Introduction of the additives Al + Bl show a further increase in initial columbic efficiency as well as discharge capacity. As concerns Capacity Retention Over 50 Cycles, the capacity retention follows the same trend where the reference formulation fades the fastest, introducing VC increases capacity retention. However, the additives Al + Bl show the highest capacity retention. Hereby, the data accentuates the potential of the Al + Bl additives used in optimizing electrochemical performance.

[0080] The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

Claims

CLAIMS1. A sodium ion cell comprising:- a positive electrode comprising a sodium Prussian blue analogue according to formula AaMb[M'c(CN)6]d, wherein A is sodium, 1 < a < 2, M and M' are individually one or more transition metals, 0 < b < 2, 1 < c < 2, and 1 < d < 2;- a negative electrode comprising a carbon-based sodium inserting material;- a separator, and- an electrolyte comprising a sodium salt, an organic carbonate-based solvent comprising at most 50 wt.% of cyclic organic carbonate, a first additive, and a second additive comprising an organic sulfate, wherein said first additive is a cyclic carbonate according to formula (I) or formula (II) and different from said cyclic organic carbonate in said organic carbonate-based solvent,(I)wherein A and A' are independently selected from a cycloalkyl group, a substituted cycloalkyl group, a halogen group, an alkoxy group, a cyclic carbonate group, a substituted cyclic carbonate group, a hydrogen group, an alkyl group, or a substituted alkyl group, and wherein A and A' are not simultaneously a hydrogen group, or A and A' are not simultaneously a hydrogen group and a methyl group, and wherein B and B' are independently selected from a cycloalkyl group, a substituted cycloalkyl group, a halogen group, an alkoxy group, a cyclic carbonate group, a substituted cyclic carbonate group, a hydrogen group, an alkyl group, or a substituted alkyl group.

2. A sodium ion cell according to claim 1, wherein said organic sulfate is selected from compounds (Illa) to (lllf), wherein D, G, G*, GH, GH*, DH, Q, and QH are independently selected from a hydrogen group, a halogen group, an alkyl group, or a substituted alkyl group, wherein each of said alkyl group and said substituted alkyl group preferably comprises 1-6 carbon atoms. III nid3. A sodium ion cell according to claim 1, wherein said organic sulfate is a cyclic organic sulfate, preferably ethylene sulfate (DTD) or ethylene sulfate - vinylene carbonate - ethylene sulfate (DTD-VC-DTD).

4. A sodium ion cell according to any one of the preceding claims, wherein said cyclic carbonate comprises fluoroethylene carbonate (FEC), vinylene carbonate-derivative, di-vinylene carbonate (Di-VC), di-vinylene carbonate-derivative, vinylene carbonate (VC), and / or ethylene carbonate-derivative.

5. A sodium ion cell according to any one of the preceding claims, wherein said cyclic carbonate is selected from a compound according to formulas (IVa) to (IVe) wherein A, A', B, B', G*, GH, and GH* are selected from a halogen group, an alkoxy group, a hydrogen group, an alkyl group, or a substituted alkyl group, and wherein A' is not a hydrogen group or a methyl group.(IVa) (IVb) (IVc) (IVd)6. A sodium ion cell according to any one of the preceding claims, wherein said electrolyte comprises from 0.1 to 5 wt.%, preferably from 0.5 to 1 wt.% of said organic sulfate.

7. A sodium ion cell according to any one of the preceding claims, wherein said electrolyte comprises from 0.1 to 5 wt.%, preferably from 1 to 2.5 wt.% of said cyclic carbonate.

8. A sodium ion cell according to any one of the preceding claims, wherein said organic carbonate-based solvent comprises at least 50 wt.%, preferably at least 75 wt.%, and more preferably at least 90 wt.% of organic carbonate.

9. A sodium ion cell according to any one of the preceding claims, wherein said organic carbonate-based solvent further comprises linear organic carbonate, preferably dimethyl carbonate (DMC), diethyl carbonate (DEC) and / or ethyl methyl carbonate (EMC).

10. A sodium ion cell according to claim 9, wherein said organic carbonate-based solvent comprises from 50 to 100 wt.%, preferably from 60 to 80 wt.% of linear organic carbonate.

11. A sodium ion cell according to any one of the preceding claims, wherein said cyclic organic carbonate comprises ethylene carbonate (EC) and / or propylene carbonate (PC).

12. A sodium ion cell according to any one of the preceding claims, wherein said organic carbonate-based solvent comprises from 5 to 45 wt.%, preferably from 10 to 30 wt.% of said cyclic organic carbonate.

13. A sodium ion cell according to any one of the preceding claims, wherein said first additive is vinylene carbonate (VC) and said second additive is ethylene sulfate (DTD), or wherein said first additive is di-vinylene carbonate (Di-VC) and said second additive is ethylene sulfate - vinylene carbonate - ethylene sulfate (DTD-VC-DTD).

14. A sodium ion cell according to any one of the preceding claims, wherein said transition metals comprise iron and / or manganese.

15. A system comprising a plurality of sodium ion cells according to any one of the preceding claims, and at least one electrical connector configured to electrically interconnect said plurality of sodium ion cells.

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