Non-aqueous electrolyte compositions

Abstract

The invention relates to a non-aqueous electrolyte composition for an electrochemical cell comprising a presodiation reagent which comprises one or more sacrificial sodium salts. The invention also relates to an electrochemical cell which has been presodiated using said non- aqueous electrolyte. Devices, methods, and uses including said non-aqueous electrolyte are also disclosed.

Description

2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES NON-AQUEOUS ELECTROLYTE COMPOSITIONS FIELD OF THE INVENTION The present invention relates to novel non-aqueous electrolyte compositions particularly for an electrochemical cell. An electrochemical cell comprising said non-aqueous electrolyte composition and methods and uses including said novel electrolyte compositions are also disclosed. The invention also relates to an electrochemical cell which has used said non- aqueous electrolyte composition. BACKGROUND OF THE INVENTION Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion) battery is charging, Na+(or Li+) ions de- intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction. Sodium-ion batteries have generated considerable interest and are considered as one of the most promising candidates for next-generation battery technology. However, one area that needs more attention is the development of ways to increase the capacity and / or cycling lifespan of sodium-ion batteries in order to compete with existing lithium-ion batteries. For instance, any capacity loss of a sodium-ion cell, particularly in the initial cycle, is clearly undesirable. As explained below, such capacity loss is generally caused by the loss of ‘recyclable’ Na+ions which are consumed during the first cycle of operation. In a typical sodium-ion full cell, the cathode, as the only sodium-ion source, provides not only ‘recyclable’ Na+ions (i.e., reversible Na+ions which move between the anode and cathode as explained above), but also ‘non-recyclable’ Na+ions (i.e., irreversible Na+ions which are generally consumed during the first cycle). In particular, Na+ions are typically consumed during the formation of the solid electrolyte interphase (SEI) on the surface of the anode and / or the cathode electrolyte interface (CEI) on the surface of the cathode, particularly during the2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES first cycle of the sodium-ion cell. That is, the very first cycle experienced by the sodium-ion cell, which typically occurs during the initial ‘formation’ cycle of a sodium-ion cell. It is known in the prior art that increasing the content of reversible Na+ions in a sodium-ion full cell can improve electrochemical performance by alleviating the loss of Na+ions which are consumed during the SEI and / or CEI formation. This strategy of providing a further sodium- ion source to an electrochemical cell system (that is, in addition to the cathode active material sodium ion source), is known the art as ‘presodiation’. As capacity loss is normally induced by the consumption of sodium-ions which are restricted by the sodium source in the cathode active material, by providing a further sodium-ion source, the electrochemical cell system is then supplied with more Na+ions, when can therefore compensate for the loss of ‘recyclable’ Na+ions and the subsequent loss in capacity. Prior art ‘presodiation’ techniques are briefly summarised below from Mu et al., Energy Mater 2022; 2:200043 and take the form of: 1) Self-sacrificing sodium-containing additives for cathodes These are sacrificial sodium-containing additives which are combined with an active cathode material in the cathode electrode. Released from the cathode electrode, they provide an additional Na+source during the first cycle of the sodium-ion cell. Often provided as a salt, the associated anion is typically oxidised to a gas, which is then released from the sodium-ion cell. Examples include Na2C4O4, CH3COONa, NaNO2, NaN3, and Na2O2 which can be found in the following prior art: Na2C4O4 salt: Ilarduya et al., Electrochim. Acta, 2019, 321, 134693; Shanmukaraj et al., ChemSusChem, 2018, 11, 3286. Ch3COONa salt: Zou et al., Angew. Chem. Int. Ed., 2021, 60, 17070. NaNO2: Jo et al., J. Mater. Chem. A, 2019, 7, 3903 NaN3: Ilarduya et al., J. Power Sources, 2017, 337, 197 Na2O2: Guo et al., ACS Appl. Mater. Interfaces, 2021, 13, 2772 Whilst certain examples are said to be low cost, environmentally friendly and leave no additional residues after Na+extraction, the Applicant has evidenced within this specification the following drawbacks:2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES a) Adding a sacrificial sodium-containing additive to the cathode electrode composition reduces the amount of cathode active material that can be present in that composition. Thus, even though a gain in mAh / gcathode active materialmay be observed, the actual mAh of the cell might not increase. b) Adding a sacrificial sodium-containing additive to the cathode electrode composition reduces the integrity of the cathode electrode after the associated anion is oxidised. This has a significant impact on the performance of the sodium-ion cell upon subsequent cycling. The reader is directed to the experimental section of this specification for a greater technical explanation. 2) Self-presodiation cathode materials. These are Na-rich cathode active materials. One example includes a cathode compound of O3-type Na0.9Cu0.31Ni0.11Fe0.3Mn0.48Ti0.1O2, which was prepared by a quenching treatment resulting a high sodium content (nearly 0.9). See: Ding et al., Additive-Free Self-Presodiation Strategy for High-Performance Na-Ion Batteries. Adv. Funct. Mater.2021, 31, 2101475. This example demonstrated an alternative approach of presodiation. However, the additional steps required to make these Na-rich materials means this approach is unlikely to be successful in a commercial environment. 3) Direct contact with sodium foil / powder Another presodiation approach is to use sodium foil or power in direct contact with an anode material. See: Liu et al., ACS Appl. Mater. Interfaces 2019, 11, 26, 23207–23212 Metallic Na metal is however extremely active and difficult to store safely in air, so pre- sodiation can only be performed in an oxygen and moisture-free environment (e.g., a glove box). Additionally, the manufacture of Na powder is laborious, and the risk of thermal runaway exists when an anode and metallic Na powder are pressure coated. Therefore, this approach is also unlikely to be successful in a commercial environment. 4) Chemical presodiation of the anode2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Another presodiation approach is to directly contact the anode with a sodium-containing solvent which has been prepared from combining a solvent with metallic Na. The sodium- containing solvent can then be sprayed onto a commercially available anode material. See: Liu et al., A Simple Electrode-Level Chemical Presodiation Route by Solution Spraying to Improve the Energy Density of Sodium-Ion Batteries. Adv. Funct. Mater.2019, 29, 1903795. Although this approach avoids the use of metallic Na to prepare a presodiated anode, it still requires the use of metallic Na to prepare the sodium-containing solvent. Therefore, this additional complexity in the manufacturing process means this approach is unlikely to be commercially viable. 5) Electrochemical presodiation. A final prior art sodiation technique is to use an electrochemical process to provide a presodiated anode (often termed as ‘pre-cycling’ in the prior art). See: Liu, et al., Enabling high-performance sodium metal anode via a presodiated alloy-induced interphase, Chemical Engineering Journal, Volume 417, 2021, 128997. This approach involves additional assembly and disassembly of a half cell, to provide a presodiated anode, which can then be used in a sodium-ion cell full cell (specifically: first making sodium-half cells, cycling them, disassembling the cells, washing the electrode materials with appropriate solvents to get rid of unwanted decomposition products, drying the electrodes and then using these electrodes to make new cells). Therefore, these additional manufacturing steps means this approach is unlikely to be commercially viable. US 20150064568 A1 relates to a general disclosure in which a metal-ion additive may be incorporated into (i) one or more of the electrodes, (ii) the separator, or (iii) the electrolyte. The metal-ion additive provides additional donor ions corresponding to the metal ions stored and released by anode and cathode active material particles. Paragraph

[0035] of US 20150064568 A1 suggests a number of sacrificial salts which may supply additional Na in a sodium-ion battery. However, US 20150064568 A1 does not test nor exemplify any of these sacrificial salts in a sodium-ion battery because the focus of US 20150064568 A1 is lithium- containing sacrificial salts for use in a lithium-ion battery (see paragraph

[0010] and

[0018] to

[0024] . Further still, US 20150064568 A1 does not mention which solvent system should be used in conjunction with the sodium-containing sacrificial salts disclosed at paragraph

[0035] of US2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES 20150064568 A1, nor does it mention a particular concentration of the sodium-containing sacrificial salts disclosed at paragraph

[0035] of US 20150064568 A1, let alone when used in a particular solvent system. In fact, the only concentrations disclosed at paragraph

[0071] of US 20150064568 A1 are lithium-containing materials which bear no resemblance with the sodium-containing sacrificial salts disclosed at paragraph

[0035] of US 20150064568 A1. The present invention therefore aims to mitigate or eliminate one or more of the aforesaid disadvantages of the known art. In particular, the aim of the present invention is therefore to provide a novel presodiation technique which can be used in an electrochemical cell, particularly in a sodium-ion cell. The approach of the present invention will reduce and / or eliminate capacity loss that is generally caused by the loss of ‘recyclable’ Na+ions which are consumed during the first cycle of an electrochemical cell. The presodiation approach of the present invention will also be commercially attractive by avoiding the direct use of metallic sodium and / or additional manufacturing complexities such as the assembly of further cells. The presodiation approach of the present invention may also offer performance benefits in addition to the reduction and / or elimination of capacity loss, such as improved cell cycle life of an electrochemical cell. The present invention achieves these aims by providing a novel non-aqueous electrolyte for an electrochemical cell comprising: (a) a solvent system which comprises one or more organo carbonate-based solvents; and (b) a presodiation reagent which comprises one or more sacrificial sodium salts, optionally in a concentration of less than 1.0 mol / kg. Presodiation is pre-doping of sodium-ions. Therefore, for the purposes of this disclosure, a presodiation reagent as defined herein means any substance or compound (irrespective of its state e.g., solid or liquid) which is capable of being chemically changed to provide a source of sodium ions when subject to an electrochemical cell environment. In particular, upon cycling of an electrochemical cell, the presodiation reagent is able to provide sodium-ions to the reversibly active sodium-ion content in the electrochemical cell. In a typical sodium-ion cell, the active sodium-ion content is stored within the positive electrode (cathode). During the first charge of the cell (formation cycle), a certain amount of active sodium is typically lost, mostly by SEI formation, at the negative electrode (anode) so that the amount of remaining active sodium is reduced (i.e., there is less of it) after the first2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES charge / discharge cycle. In the present invention, the strategy to overcome this issue is to store a certain amount of active sodium using one or more sacrificial sodium salts in the non- aqueous electrolyte composition by presodiation prior to further charge / discharge cycling. This compensates for the active sodium lost, and thus leads to an increased sodium-ion content after the first cycle, and thus enhanced remaining cell energy. Indeed, the present invention contrasts with prior art methods in which the strategy to overcome the above issue has been to store a certain amount of active sodium in the negative and / or positive electrode by presodiation prior to charge / discharge cycling – e.g., using one or more sacrificial sodium salts contained in the cathode. More particularly, by providing a novel non-aqueous electrolyte composition according to the present invention, the total amount of Na+available to cycle reversibly in an electrochemical cell after the first cycle of operation is surprisingly increased. This not only increases the delivered mAh / gcathode active material capacity, but it also increases the total capacity (mAh) of the electrochemical cell. As evidenced by the specific examples disclosed herein, this contrasts a prior art presodiation approach of storing a certain amount of active sodium by using one or more sacrificial sodium salts in the negative electrode by presodiation prior to charge / discharge cycling. Furthermore, although the design of suitable electrolyte compositions is typically given less attention than active materials (electrodes), their importance should not be overlooked as they are in large part the key to battery life and for determining the practical performance achievable by a cell, for example in terms of capacity, rate capability, safety etc. However, to be a suitable electrolyte composition, it must fulfil a long list of attributes, these include: ^ Chemical stability – there must be no reactions during the cell operation, or the reactions need to occur in such a way so as to enable acceptable cycling after this reaction has occurred (for example, during SEI formation on the anode), including within the electrolyte itself or with the separator, the electrodes, current collectors or the packaging materials used; ^ Electrochemical stability – there must be a wide electrochemical stability window i.e., a large separation between the high and low onset potentials for decomposition by oxidation or reduction;2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES ^ Thermal stability – the electrolyte composition must not decompose or chemically break down during normal cell operation and operational temperature; ^ Physical properties – the electrolyte composition needs to be liquid therefore its melting and boiling points must be well outside the internal operating temperatures of the cell; ^ High ionic and low electronic conductivities are necessary to maintain cell operation by Na+transport and to minimize self-discharge of the cell, respectively; ^ Low toxicity; ^ Based on sustainable chemistries, i.e., made using abundant elements and via low impact syntheses (energy, pollution etc.) and ^ Cost effective production. It is clearly desirable to find novel presodiation techniques to reduce and / or eliminate the drawbacks associated with prior art approaches as discussed in detail above. However, it is also desirable to provide an electrolyte composition which is optimal for promoting high electrochemical cell performance. Indeed, an electrochemical cell relies on how the electrolyte composition interacts with the components of the electrochemical cell (e.g., the separator, positive and / or negative electrodes etc). This interaction is based on a mosaic of physical properties of the electrolyte composition, which cannot be predicted. In particular, as set out above, it is necessary for the electrolyte composition to have high ionic and ideally no electronic conductivities to maintain cell operation by Na+transport. However, such high ionic and low electronic conductivities must also be balanced at the very least with the ability to maintain chemical and electrochemical stability so that the electrochemical cell can charged and discharged over many cycles, and in certain instances, different voltage profiles. In the case of the novel non-aqueous electrolyte compositions, the Applicant has surprisingly discovered that not only does the present invention provide an improved technique to presodiation, but it also provides an electrolyte composition which promotes and / or maintains high electrochemical performance in an electrochemical cell when compared to an electrolyte composition according to the prior art. For instance, rate performance and / or cycling stability has been found to be similar to, or improved than, prior art electrolyte compositions. This was not known from, nor could have been predicted from, the prior art at the priority date of the present application.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES The phase ‘sacrificial sodium salt’ as used herein means a sodium salt, or a portion thereof, which is designed to be used up or destroyed during a charging operation of an electrochemical cell, with the purpose of providing an electrochemical cell with additional capacities as discussed below. In a preferred embodiment, the one or more sacrificial sodium salts of the present invention irreversibly provide sodium-ions to the anode (and / or to the surface of the anode) during charging of the electrochemical cell. As the electrochemical cell includes an anode, this is an anode of the electrochemical cell. In particular, the Applicant has found that presodiation using one or more sacrificial sodium salts in a non-aqueous electrolyte composition is advantageous when the salts ideally irreversibly donate sodium-ions during charging of the electrochemical cells because this eliminates the need for additional manufacturing steps in contrast to the prior art techniques as discussed above. The term ‘irreversibly” in this context therefore means that the one or more sacrificial sodium salts provide Na+ions and their respective anions, at least some of which, are consumed during the first charging (formation cycle) of the electrochemical cell. That is, they are consumed in the sense that they do not return to their previous condition as Na+ions together with their respective counteranions within the non-aqueous electrolyte composition. The Applicant demonstrates in the examples as disclosed herein that the further Na+provided by the one or more sacrificial salts of the present invention results in preserving the Na+in the cathode, which can then be used to deliver the additional capacities by the electrochemical cell (such as delivered mAh / gcathode active material capacity and / or total capacity (mAh)). Of course, to the extent that such one or more sodium-containing salts irreversibly provide sodium-ions to the anode (and / or to the surface of the anode) during charging of the electrochemical cell, such one or more sodium-containing salts may additionally provide ‘recyclable’ Na+ions (i.e., reversible Na+ions which move between the anode and cathode as explained above). Preferably, the one or more sacrificial sodium salts decompose (e.g., one or more anions of the one or more sacrificial sodium salts are oxidised) to provide sodium-ions to the anode during charging (e.g., first cycle) of the electrochemical cell. Therefore, it is preferred that the one or more sacrificial sodium salts comprise one or more anions which oxidise during charging of the electrochemical cell, more preferably during a first charging (i.e., first cycle) of the electrochemical cell. That is, the very first cycle experienced by the sodium-ion cell, which2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES typically occurs during the initial ‘formation’ cycle of a sodium-ion cell. The one or more sacrificial sodium salts of the present invention therefore ideally substantially decompose (i.e., one or more anions of the one or more sacrificial sodium salts are substantially oxidised) in response to an activation potential (i.e., a voltage) applied between an anode electrode and a cathode electrode of the electrochemical cell, typically during charging of the electrochemical cell. This therefore causes release of sodium-ions to the anode electrode. For this, the decomposition potential (i.e., a potential at which one or more anions are oxidised) of the one or more sacrificial sodium salts should ideally be lower than the upper cut-off voltage of the electrochemical cell to provide sufficient Na+during charging, preferably first charging, of the electrochemical cell. An ideal upper cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 4.5 V or 4.3 V. A most preferable upper cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 4.2 V, optionally about 3.75 V. An ideal lower cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 1.0 V, or alternatively 2.5 V. An electrochemical cell (ideally a sodium-ion cell) is preferably cycled from 1.0 V to 4.5 V, and more preferably from 1.0 V to 4.2 V. In one embodiment, a decomposition potential of the one or more sacrificial sodium salts is from about 0 V to about 5 V. This is ideally determined using an electrochemical cell comprising a reference electrode containing sodium metal (i.e., vs Na / Na+). This electrochemical cell is typically a three-electrode electrochemical cell, in which one electrode comprises sodium metal (as a reference electrode). Alternatively, the reference electrode may comprise another electrode active material (different to sodium metal), such as Na2+xV2(PO4)3, where -0.5 ≤ x ≤ 0.5, which typically has a redox couple around 3.4 V vs Na / Na+. Optionally, a decomposition potential of the one or more sacrificial sodium salts is from about from about 0.5 V to about 5 V vs Na / Na+(that is, versus the Na / Na+redox couple: it will be understood by those skilled in the art that 0.5 V to 5 V vs Na / Na+is an absolute potential range, that can be converted from one set of potential values using one scale, such as the Li / Li+, to another scale, such as the standard hydrogen electrode’s redox couple). Preferably, the decomposition potential of the one or more sacrificial sodium salts is from about 2.5 V to about 4.3 V vs Na / Na+, and highly preferably it is from about 2.75 V to about 3.9 V vs Na / Na+.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Most ideally, the decomposition potential of the one or more sacrificial sodium salts is from about 2.5 V to about 3.75 V vs Na / Na+. Indeed, specific values of 3.26 V (vs Na / Na+) and 3.74 V (vs Na / Na+) are also ideal. In one embodiment, the one or more sacrificial sodium salts have a decomposition potential of no greater than about 3.5 V (vs Na / Na+), ideally no greater than about 3.2 V (vs Na / Na+). To enable the one or more sacrificial sodium salts of the present invention to decompose in response to an activation potential (i.e., a voltage) applied between an anode electrode and a cathode electrode of the electrochemical cell, the electrochemical cell is charged to a voltage which is greater than the decomposition potential of the one or more sacrificial sodium salts. As set out above, the decomposition potential of the one or more sacrificial sodium salts should ideally be lower than the upper cut-off voltage of the electrochemical cell. This means that the upper cut-off voltage of the electrochemical cell should ideally be greater than the decomposition potential of the one or more sacrificial sodium salts. The decomposition potential of the one or more sacrificial sodium salts should ideally be greater than the lower cut-off voltage of the electrochemical cell. Thus, a preferred electrochemical cell may be cycled from 1 V to 4.2 V, and the preferred decomposition potential of the one or more sacrificial sodium salts may be from 2.5 V to 3.75 V. That is, the preferred one or more sacrificial sodium salts of the present invention will substantially decompose at potential from 2.5 V to 3.75 V. When the non-aqueous electrolyte composition according to the present invention comprises one or more non-sacrificial sodium-containing salts, the preferred one or more non-sacrificial sodium-containing salts of the present invention will not substantially decompose at potential from 2.5 V to 3.75 V as discussed in greater detail below. In one embodiment, the one or more sacrificial sodium salts do substantially decompose during charging of an electrochemical cell at a voltage from 1 V to 4.3 V, and preferably from 1 V to 4.2 V. That is, substantial oxidation of one or more anions of the one or more sacrificial salts occurs when the electrochemical cell is operated in this voltage range. Typically, substantial oxidation of one or more anions of the one or more sacrificial salts occurs when the cell is charged above 3 V, and in some cases, above 4 V. Thus, the one or more sacrificial sodium salts may substantially decompose during charging of an electrochemical cell at a voltage from greater than 3 V to 4.3 V, and preferably from 3 V to 4.2 V. In some cases, during charging of an electrochemical cell at a voltage from 4 V to2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES 4.3 V, and preferably from 4 V to 4.2 V. That is, substantial oxidation of one or more anions of the one or more sacrificial salts occurs when the electrochemical cell is operated in this voltage range. Ideally, oxidation of one or more anions of the one or more sacrificial sodium salts during charging of an electrochemical cell is about 25% or more, preferably about 50% or more, very preferably about 75% or more, further preferably 95% or more, and most preferably about 100%. The one or more sacrificial sodium salts of the present invention may comprise one or more anions containing atoms selected from the group consisting of sulfur atoms, nitrogen atoms, boron atoms, oxygen atoms, and halogen atoms (such as fluorine atoms). Ideally, the one or more sacrificial sodium salts may comprise one or more anions containing atoms selected from oxygen atoms and / or sulfur atoms, and most ideally, sulfur atoms. The one or more sacrificial sodium salts of the present invention may comprise one or more anions containing at least one atom selected from the group consisting of sulfur atoms, nitrogen atoms, boron atoms, oxygen atoms, and halogen atoms (such as fluorine atoms). Ideally, the one or more sacrificial sodium salts may comprise one or more anions containing at least one atom selected from oxygen atoms and / or sulfur atoms, and most ideally, sulfur atoms. Preferably, the one or more sacrificial sodium salts are selected from one or more compounds of the group consisting of thiocyanate-containing compounds, cyanate-containing compounds, sulfonate-containing compounds, sulfite-containing compounds, periodate-containing compounds, fluoro silicate-containing compounds, fluoro ferrate-containing compounds, alkoxide-containing compounds, boron-containing compounds, compounds of the formula NaSbmXx, thiolate-containing compounds, thiosulfate-containing compounds, cyanamide- containing compounds (including dicyanamide-containing compounds), azide-containing compounds, carboxylate-containing compounds, ketone-containing compounds (including rhodizonate-containing compounds), fluoro titanate-containing compounds, and arene- containing compounds (including terephthalate-containing compounds). Examples of terephthalate-containing compounds may include a terephthalate-containing compound that includes an alkyl group which may be straight chained, branched or cyclic, such as dimethyl, diethyl, and diisopropyl terephthalates. In the preferred arene-containing2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES compounds (including terephthalate-containing compounds), the total number of carbon atoms may be no greater than 14, optionally no greater than 10, further optionally no greater than 9. Additional one or more sacrificial sodium salts may be selected from one or more compounds of the group consisting of triflate-containing compounds, difluorophosphate-containing compounds, and tetraphenylborate -containing compounds. In the preferred one or more sacrificial sodium salts of the formula NaSbmXx, Sb is antimony and X is a group that comprises or consists of one or more halogens, preferably selected from fluorine, chlorine, bromine and iodine, further preferably fluorine. The amount x of halogen X is preferably x=1 to 16, further preferably x= 6. The amount, m, of antimony, Sb, is preferably m=1 to 3, further preferably m=1 to 2, and particularly preferably m=1. The most preferred one or more sacrificial sodium salts of the formula NaSbmXxis sodium hexafluoroantimonate(V) (NaSbF6). For example, the one or more sacrificial sodium salts may be selected from one or more compounds of the group consisting of: sodium sulfite (Na2SO3), sodium trifluoromethanesulfonate (NaSO3CF3) (also known as sodium triflate), sodium thiocyanate (NaSCN), sodium cyanate (NaOCN), sodium periodate (NaIO4), sodium 1-butene-2-olate (C4H7NaO), Sodium heptafluorobutyrate (C4F7NaO2), sodium tetraphenylborate (C6H5)4BNa), sodium hexafluoroantimonate(V) (NaSbF6), sodium methanethiolate (CH3NaS), sodium thiosulfate (Na2S2O3), sodium methoxide (CH3NaO), sodium cyanoborohydride (NaBH3CN), sodium dicyanamide (NaN(CN)2), sodium azide (NaN3), sodium hexafluorotitanate (Na2TiF6), disodium terephthalate (C8H4Na2O4), sodium terephthalate (C8H5NaO4), disodium rhodizonate (Na2C6O6), sodium hexafluoroferrate(III) (Na3FeF6),2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES sodium hexafluorosilicate (Na2[SiF6]), and sodium difluorophosphate (NaPO2F2), Most preferably, the one or more sacrificial sodium salts comprise cyanate-containing compounds or thiocyanate-containing compounds. The one or more sacrificial sodium salts will ideally have the general formula NaXCN, where X is a group 16 element (from the periodic table), optionally selected from oxygen and sulfur, preferably selected from sulfur. In a most preferred embodiment, the one or more sacrificial sodium salts will comprise sodium thiocyanate (NaSCN). Alternatively, or additionally, the preferred one or more sacrificial sodium salts may comprise one or more of sodium triflate (NaSO3CF3), sodium difluorophosphate (NaPO2F2), and sodium tetraphenylborate (NaBPh4). It is convenient to express the amount of the one or more sacrificial sodium salts of the present invention) in terms of its molality of the components in the solvent system; that is, the total number of moles of the one or more sacrificial sodium salt (i.e. solute), per kilogram of the solvent system – i.e. mol / kg. The one or more sacrificial sodium salts may be present in the non-aqueous electrolyte composition in a concentration of less than 1.0 mol / kg. That is, ideally from a concentration of greater than 0 mol / kg to less than 1.0 mol / kg. In a preferred embodiment, the one or more sacrificial sodium salts may be present in the non- aqueous electrolyte composition in a concentration of about 0.75 mol / kg or less. That is, ideally from a concentration of greater than 0 mol / kg to about 0.75 mol / kg. Similarly, the one or more sacrificial sodium salts may be present in the non-aqueous electrolyte composition in a concentration of about 0.50 mol / kg or less. That is, ideally from a concentration of greater than 0 mol / kg to about 0.50 mol / kg. In a highly preferred embodiment, the one or more sacrificial sodium salts may be present in a concentration from about 0.015 mol / kg to about 0.5 mol / kg. Increased capacities and / or excellent long term cycling were observed at such concentrations. In particular, excellent long term cycling was achieved at ideal concentrations of about 0.25 mol / kg. This was particularly apparent when the electrochemical cell was cycled to include a voltage range from about 3.75V to about 2.5V. Increased capacities were also observed at a concentration from about 0.015 mol / kg to about 0.1 mol / kg.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES In another highly preferred embodiment, the one or more sacrificial sodium salts may be present in the non-aqueous electrolyte composition in a concentration from about 0.020 mol / kg to about 0.025 mol / kg. Similarly, concentrations of about 0.020 mol / kg and about 0.025 mol / kg led to electrochemical cells with extended life cycles. The non-aqueous electrolyte composition of the present invention comprises a solvent system which comprises one or more organo carbonate-based solvents. The one or more organo carbonate-based solvents present in the solvent system, may be cyclic or non-cyclic compounds that are characterised by the fact that they contain a carbonate ester group, i.e. a carbonyl group that is flanked by one or two alkoxygroups: R1O(C=O)OR2. It will be appreciated that, where one or more cyclic organo carbonate-based solvents are present in the solvent system, R1and R2may connect to form a cyclic carbonate ester compound. The R1and R2groups are preferably independently selected (i.e. they may be the same or different from each other) from either hydrogen; or a C1 to C20- cyclic or non-cyclic, branched or unbranched, substituted or unsubstituted alkyl group; or a C1 to C20- cyclic or non- cyclic, branched or unbranched, substituted or unsubstituted alkenyl group; or a C1 to C20- branched or unbranched, substituted or unsubstituted cycloalkyl-, phenyl- or heterocycle- containing group. Highly suitable electrolyte solvents include C3-C10 cycloalkyl organo carbonates, such as propylene carbonate (C4H6O3) (PC) and ethylene carbonate (C3H4O3) (EC). Propylene carbonate (C4H6O3) shows particularly favourable compatibility with electrode materials and the high solubility, wide liquidus range and a high boiling point of this material also makes this solvent advantageous for use in sodium-ion batteries. Other highly suitable organo carbonate- based compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC) and vinylene carbonate (VC). Advantageous electrolyte compositions of the present invention may include a solvent system which comprises propylene carbonate in an amount of 20% or more by weight of the solvent system, preferably 25% or more by weight of the solvent system. Preferably, the non-aqueous electrolyte compositions of the present invention comprise a mixture of solvents. In particular, the preferred solvent system may comprise a mixture of solvents comprising propylene carbonate (PC) and a further solvent selected from the group2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES consisting of ethylene carbonate (EC), diethyl carbonate (DEC), and a hydrofluoroalkyl ether such as 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). Therefore, in one preferred embodiment, the solvent system comprises a mixture of solvents comprising propylene carbonate and a further solvent selected from the group consisting of ethylene carbonate (EC) and diethyl carbonate (DEC). This therefore encompasses a mixture of two solvent components (e.g. propylene carbonate with ethylene carbonate (EC) or propylene carbonate with diethyl carbonate (DEC)) or a mixture of three solvent components (e.g., propylene carbonate, ethylene carbonate (EC) and diethyl carbonate (DEC)). Preferably, the solvent system includes a mixture of ethylene carbonate (EC), diethyl carbonate (DEC) and propylene carbonate (PC), which is ideally in the weight ratio range 1 to 4 : 1 to 10 : 1 to 10 wt / wt and further ideally in the weight ratio 1 to 2 : 1 to 5: 1 to 2 wt / wt, and most preferably in the weight ratio 1:2:1 wt / wt (EC:DEC:PC). In another embodiment, the solvent system comprises propylene carbonate (PC) and diethyl carbonate (DEC). Preferably in this embodiment, the solvent system comprises propylene carbonate (PC) in amount from >55% to <100% by weight of the solvent system, and diethyl carbonate (DEC) in amount from >0 to <45% by weight of the solvent system. Most preferably in this embodiment, propylene carbonate (PC) is present in amount of about 76% by weight of the solvent system. Ideally, diethyl carbonate (DEC) is present in amount of about 20% by weight of the solvent system. In a preferred alternative embodiment, the solvent system comprises a mixture of solvents comprising propylene carbonate (PC) and a hydrofluoroalkyl ether such as 1,1,2,2- Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). Preferably in this embodiment, the solvent system comprises propylene carbonate (PC) in amount from >55% to <100% by weight of the solvent system, and a hydrofluoroalkyl ether such as 1,1,2,2-Tetrafluoroethyl- 2,2,3,3-tetrafluoropropyl ether (HFE) in amount from >0 to <45% by weight of the solvent system. Most preferably in this embodiment, propylene carbonate (PC) is present in amount of about 77% by weight of the solvent system, and 1,1,2,2-Tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (HFE) is present in amount of about 20% by weight of the solvent system. In a further preferred alternative embodiment, the solvent system comprises fluoroethylene carbonate (FEC) and a phosphate-based solvent such as triethyl phosphate (TEP). Preferably in this embodiment, the solvent system comprises fluoroethylene carbonate (FEC) in amount from >1% to <60% by weight of the solvent system, and a hydrofluoroalkyl ether such as2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) in amount from >40 to <99% by weight of the solvent system. Most preferably in this embodiment, fluoroethylene carbonate (FEC) is present in amount of about 5% by weight of the solvent system. The solvent system which comprises fluoroethylene carbonate (FEC) and a phosphate-based solvent such as triethyl phosphate (TEP) may preferably also include a hydrofluoroalkyl ether such as 1,1,2,2- Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). Most preferably in this embodiment, 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE) is present in amount of about 20% by weight of the solvent system. Additionally, the solvent system of the present invention may also include one or more performance additives which are different from the one or more sacrificial sodium salts. In particular, as the present invention comprises one or more solvents as a first component of the solvent system, the one or more performance additives may be included as a second component to the solvent system of the present invention. The one or more performance additives (in addition to the one or more sacrificial salts of the present invention) may enable the formation of a stable cathode-electrolyte interphase (CEI) on the cathode and / or a stable solid-electrolyte interphase (SEI) on the anode, or result in other benefits such as improved wetting of the electrolyte on the separator and / or electrode materials by reducing surface tension of the solvent-system and / or surface-adsorption effects which can effectively form artificial SEI / CEI layers, which in turn leads to advantages such as enhanced coulombic efficiencies and enhanced cathode discharge capacity. The addition of one or more performance additives (in addition to the one or more sacrificial salts of the present invention) to the non-aqueous electrolyte composition of the present invention may further enhance the electrolyte performance due to an unexpected synergistic effect (e.g. with the one or more sacrificial sodium salts of the present invention). The one or more performance additives as a second component of the solvent system are preferably selected from sulfur-containing compounds, boron-containing compounds, and surfactants. Ideally, these compounds are not sodium salts. Therefore, the one or more performance additives ideally do not comprise a sodium salt. Further still, unlike the presodiation reagent of the present invention which comprises one or more sacrificial sodium salts that ideally irreversibly provide sodium-ions to the anode during charging of the electrochemical cell, these one or more performance additives are not intended to (i.e., do not) provide sodium-ions to the anode during charging of the electrochemical cell.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES The one or more performance additives may be present as a second component of the solvent system in an amount of >0.5 to ≤10% by weight of the solvent system. For the avoidance of any doubt, the phrase “weight of the solvent system” as used herein means the weight of the first component of the solvent system combined with the weight of the second component of the solvent system. The second component of the solvent system may comprise two or more performance additives. The addition of two or more performance additives (in addition to the one or more sacrificial salts of the present invention) to the non-aqueous electrolyte composition of the present invention may further enhance the electrolyte performance due to an unexpected synergistic effect (e.g. with the one or more sacrificial salts of the present invention). The two or more performance additives may be present as a second component of the solvent system in an amount of >0.5 to ≤10% by weight of the solvent system. For example, the two or more performance additives as a second component of the solvent system may comprise a mixture of boron-containing compounds and surfactants. That is, one or more boron-containing compounds may be mixed with one or more surfactants to provide a mixture of two or more performance additives. Alternatively, the two or more performance additives as a second component of the solvent system may comprise a mixture of sulfur- containing compounds and surfactants. That is, one or more sulfur-containing compounds may be mixed with one or more surfactants to provide a mixture of two or more performance additives. The two or more performance additives as a second component of the solvent system may comprise a mixture of sulfur-containing compounds in an amount of about 2% by weight of the solvent system, and surfactants in an amount of about 1% by weight of the solvent system. This combination of two or more performance additives as a second component of the solvent system is particularly preferred when used in conjunction with a solvent system which comprises propylene carbonate (PC) and diethyl carbonate (DEC). Preferably, the second component of the solvent system may comprise three or more performance additives. The addition of three or more performance additives (in addition to the one or more sacrificial salts of the present invention) to the non-aqueous electrolyte composition of the present invention may further enhance the electrolyte performance due to an unexpected synergistic effect (e.g. with the one or more sacrificial salts of the present invention). The three or more performance additives may be present as a second component of the solvent system in an amount of >0.5 to ≤10% by weight of the solvent system.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES For example, the three or more performance additives as a second component of the solvent system may comprise a mixture of sulfur-containing compounds, boron-containing compounds and surfactants. That is, one or more sulfur-containing compounds may be mixed with one or more sulfur-containing compounds, one or more boron-containing compounds, and one or more surfactants to provide a mixture of three or more performance additives. The three or more performance additives as a second component of the solvent system may comprise a mixture of sulfur-containing compounds in an amount of about 1% by weight of the solvent system, boron-containing compounds in an amount of about 1% by weight of the solvent system, and surfactants in an amount of about 1% by weight of the solvent system. The use of three or more performance additives as a second component of the solvent system is particularly preferred when used in conjunction with a solvent system which comprises a mixture of solvents comprising propylene carbonate (PC) and a hydrofluoroalkyl ether such as 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE). The three or more performance additives as a second component of the solvent system alternatively may comprise a mixture of sulfur-containing compounds in an amount of about 2% by weight of the solvent system, boron-containing compounds in an amount of about 1% by weight of the solvent system, and surfactants in an amount of about 1% by weight of the solvent system. This combination of three or more performance additives as a second component of the solvent system is particularly preferred when used in conjunction with a solvent system which comprises propylene carbonate (PC) and diethyl carbonate (DEC). The one or more surfactants used in a second component of the solvent system according to the present invention are performance additives and are preferably selected to enhance the ability of the electrolyte composition to wet the separator (particularly a polyolefin separator) and / or the electrode of the battery which, in turn, advantageously promotes a longer battery cycle life. Furthermore, it is known in the literature that surfactants, by virtue of their surface- adsorption on different substrates, can lead to non-dendritic metal plating / stripping (and also inhibit corrosion). Preferred one or more surfactant additives are selected from anionic surfactants, cationic surfactants, non-ionic (hydrophilic) surfactants and amphoteric (zwitterionic) surfactants, with anionic surfactants and non-ionic (hydrophilic) surfactants being especially preferred. Such surfactants additives are ideally one or more selected from: 1) anionic (negatively charged) surfactants. Suitable examples include: carboxylates (such as alkyl carboxylates (e.g. fatty acid salts)), carboxylate fluoro-surfactants; sulfates2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES (such as alkyl sulfates (e.g. sodium lauryl sulfate), alkyl ether sulfates (e.g. sodium laureth sulfate)); sulfonates (such as docusates (e.g. dioctyl sodium sulfosuccinate) and alkyl benzene sulfonates); and mono- or di-phosphate esters (such as alkyl aryl ether phosphates and alkyl ether phosphates ; 2) Zwitterionic (amphoteric) surfactants, which can be anionic, cationic or non-ionic depending on the pH of the solution they are in. Examples include: RN+H2CH2COO-, RN+(CH3)2CH2CH2SO3-, phospholipids (such as phosphatidylcholine (lecithin)); 3) Cationic surfactants which bear a positive charge for example RN+H3Cl-, RN+(CH3)3Cl-, diotadecyldimethylammonium chloride, cetyl pyridinium chloride, benzalkonium chloride, hexadecyl trimethyl ammonium hexafluorophosphate, hexadecyl trimethylammonium chloride (CTAC) and hexadecyl trimethylammonium bromide (CTAB); and 4) Non-ionic surfactants. These are uncharged, and examples include: tri-alkyl phosphates, polyol esters (e.g. glycol, glycerol esters, sorbitan and sorbitan derivatives such as fatty acid esters of sorbitan (Spans) and their ethoxylated derivatives (Tweens)), polyoxyethylene esters, and poloxamers. The poloxamers may comprise block copolymers, for example Poloxamer 84, Poloxamer 105, Poloxamer 123, Poloxamer 124, Poloxamer 188, Poloxamer 237, Poloxamer 338, Poloxamer 407 and Poloxamer F127. Most preferably, the one or more surfactants used in a second component of the solvent system according to the present invention contain one or more non-ionic block copolymer- containing surfactants. Highly suitable examples include poloxamers. A very specific example includes Poloxamer 123. As discussed above, a second component of the solvent system according to the present invention may include one or more sulfur-containing compounds. The one or more sulfur- containing compounds of the present invention may be defined as described herein. The one or more sulfur-containing compounds used in a second component of the solvent system according to the present invention are performance additives, and are preferably selected to enable the formation of a stable cathode-electrolyte interphase (CEI) on the cathode and / or a stable solid-electrolyte interphase (SEI) on the anode, which in turn leads to advantages such as enhanced 1st cycle coulombic efficiencies (FCE = first cycle coulombic efficiency) and cycle life enhancement; alternatively, the additive might decompose significantly preferentially in the first cycle, which might lead to a low first cycle efficiency, but then improves subsequent cycling stability.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Suitable sulfur-containing compounds may include cyclic and / or non-cyclic sulfur-containing compounds. Preferably, the sulfur-containing compound is a sulfone-containing compound, a sulfate-containing compound, or a sulfonate-containing compound. In other words, the sulfur-containing compound may have a sulfonyl functional group which is attached to: two carbon atoms (sulfone), two oxygen atoms (sulfate), or one carbon atom and one oxygen atom in a cyclic or non-cyclic structure (sulfonate). When the sulfonate is cyclic, the compound may be referred to as a sultone. As used herein the term “sulfone” means that the central hexavalent sulfur atom is doubly- bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms. As used herein the term “sulfate” means that the central hexavalent sulfur atom is doubly- bonded to two of the oxygen atoms and has a single bond to the other two oxygen atoms, which are each further singly bonded to carbon atoms. As used herein the term “sulfonate” means that the central hexavalent sulfur atom is doubly- bonded to two of the oxygen atoms; it has a single bond to one carbon atom; and a single bond to the other oxygen atom, which is further singly bonded to a carbon atom, which if cyclically connected to the other carbon atom bonded to the sulfur atom, would be a sultone. The general formula of such compounds is RY(S=O)2Y’R’, in which: ^ Y and Y’ is independently selected from C or O (i.e. they may be the same or different from each other); ^ when Y and Y’ are the same, R and R’ may be independently selected (i.e. they may be the same or different from each other) from any straight or branched, substituted or unsubstituted C1to C6-alkyl group; a straight or branched, substituted or unsubstituted C1to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1to C6- alkoxy group; or wherein R and R’ may independently or together form a substituted or unsubstituted C3-C6-cycloalkyl-, phenyl- or heterocycle-containing group; and ^ when Y and Y’ are different from each other, R’ may be independently selected from any straight or branched, substituted or unsubstituted C1 to C6-alkyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkoxy group; or wherein R and R’2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES may independently or together form a substituted or unsubstituted C3-C6-cycloalkyl-, cycloalkenyl-, phenyl- or heterocycle-containing group. In some embodiments, the general formula of such compounds is R-SO2-R’, in which R and R’ may be independently selected (i.e. they may be the same or different from each other) from any straight or branched, substituted or unsubstituted C1to C6-alkyl group; a straight or branched, substituted or unsubstituted C1to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1to C6-alkoxy group; or wherein R and R’ may independently or together form a substituted or unsubstituted C3-C6-cycloalkyl-, phenyl- or heterocycle- containing group. Preferably, the general formula of such compounds is RO(S=O)2OR’, in which R and R’ may be independently selected (i.e. they may be the same or different from each other) from any straight or branched, substituted or unsubstituted C1to C6-alkyl group; a straight or branched, substituted or unsubstituted C1to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkoxy group; or wherein R and R’ may independently or together form a substituted or unsubstituted C3-C6-cycloalkyl-, phenyl- or heterocycle-containing group. Highly preferably, the sulfur-containing compound is selected from a cyclic sulfate (such as 1,3-propanediolcyclic sulfate (PCS) also known as 1,3,2-Dioxathiane 2,2-dioxide (DTD or (CH2)3SO4))); 1,5,2,4-dioxadithiane 2,2,4,4-tetraoxide; or 1,3,2-Dioxathiolane 2,2-dioxide. Suitable examples of cyclic sulfones include: sulfolane ((CH2)4SO2), 3-methyl sulfolane ((CH3)CH(CH2)3SO2),), and trimethyl sulfone ((CH2)3SO2). Suitable examples of non-cyclic sulfones include: methyl phenyl sulfone ((CH3)(C6H5)SO2). Suitable examples of examples of sultones include: 1-propene 1,3-sultone ((CH)2CH2SO3), and 1,3-propane sultone (CH2)3SO3, Most preferably, the sulfur-containing compound is 1,3-propanediolcyclic sulfate (PCS). As discussed above, a second component of the solvent system according to the present invention may include one or more boron-containing compounds. The one or more boron- containing compounds of the present invention may be defined as described herein. The boron-containing compounds used in a second solvent component of solvent system according to the present invention are performance additives, and are preferably selected to2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES enable the formation of a stable cathode-electrolyte interphase (CEI) on the cathode and / or a stable solid-electrolyte interphase (SEI) on the anode, which in turn leads to advantages such as enhanced 1st cycle coulombic efficiencies and / or cycle life enhancement. Suitable boron-containing compounds may include cyclic and / or non-cyclic boron-containing compounds. Preferably, the one or more boron-containing compounds are a borate or a boroxine. In other words, the one or more boron-containing compounds may have a central boron atom which is attached to three oxygen atoms, which are each further singly bonded to carbon or silicon atoms (borate). Or the one or more boron-containing compounds may comprise a 6 membered heterocyclic structure which is composed of three alternating boron atoms and oxygen atoms joined together by a shared single bond. Each boron atom is further singly bonded to a carbon atom or oxygen external to the heterocyclic structure. As used herein the term “borate” means that the central trivalent boron atom is singly bonded to three oxygen atoms, which are each further singly bonded to carbon or silicon atoms. As used herein the term “boroxine” means a 6 membered heterocyclic compound that is composed of singly bonded alternating trivalent boron atoms and divalent oxygen atoms, each boron atom being further singly bonded to an oxygen or carbon atom external to the heterocyclic structure. The general formula of such borate compounds is B(OYR)3, in which Y and R may be the same or vary in each individual attachment case. More specifically this may be written as: B(OYR)(OY’R’)(OY’’R’’), in which ^ Y, Y’, and Y’’ is independently selected from C or Si (i.e. they may be the same or different from each other); and ^ R, R’, and R’’ may be independently selected from hydrogen; any straight or branched, substituted or unsubstituted C1to C6-alkyl group; a straight or branched, substituted or unsubstituted C1to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkoxy group; or a substituted or unsubstituted C3-C6-cycloalkyl, phenyl- or heterocycle-containing group. Further preferably, the boron-containing compound is selected from a non-cyclic borate.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Suitable examples include tris(trimethylsilyl) borate (TMSB); Tris(2,2,2-trifluoroethyl) borate; Trimethyl borate; and Triethyl borate The general formula of such boroxine compounds is B3(YR)3O3, in which Y and R may be the same or vary in each individual attachment case. More specifically, this may be written as: Ba(YR)b(O)c, in which ^ Y is independently selected from C or O (i.e. they may be the same or different from each other); and ^ R may be the same or vary in each individual attachment case and may be independently selected from hydrogen; any straight or branched, substituted or unsubstituted C1 to C6-alkyl group; a straight or branched, substituted or unsubstituted C1to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1to C6- alkoxy group; or a substituted or unsubstituted C3-C6-cycloalkyl-, phenyl- or heterocycle-containing group; and ^ a = b = c, wherein a, b, and c are each 3. Suitable examples include: trimethoxyboroxine; and trimethylboroxine Most preferably, the boron-containing compound is tris(trimethylsilyl) borate (TMSB). In a preferred embodiment, the non-aqueous electrolyte composition of the present invention further comprises one or more sodium-containing salts in addition to a presodiation reagent in accordance with the present invention. Such additional salts may be considered as a ‘primary salt’. More particularly, unlike the presodiation reagent of the present invention which comprises one or more sacrificial sodium salts that ideally irreversibly provide sodium-ions to the anode during charging of the electrochemical cell, the anions of these additional one or more sodium- containing salts ideally are not intended to (i.e., do not) irreversibly provide sodium-ions to the anode during charging of the electrochemical cell. In particular, the anions of these additional one or more sodium-containing salts ideally are not intended to (i.e., do not) decompose (e.g. oxidise) to provide sodium-ions to the anode during charging of the electrochemical cell.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Instead, these additional one or more sodium-containing salts are intended to ideally facilitate the transfer of sodium ions in the electrochemical cell (e.g., from the cathode to the anode). Further still, unlike the presodiation reagent of the present invention which comprises one or more sacrificial sodium salts that ideally comprise one or more anions which oxidise during charging of the electrochemical cell, the one or more sodium-containing salts (in addition to the one or more sacrificial sodium salts of the present invention) are ideally substantially absent of one or more anions which oxidise during charging of the electrochemical cell. However, to the extent that such one or more sodium-containing salts contain one or more anions which do oxidise during charging of the electrochemical cell, they oxidise in an amount that was previously assumed to not result in benefit to electrochemical cell capacity. Such additional one or more (primary) sodium-containing salts may therefore be described as one or more (primary) non-sacrificial sodium salts. In one embodiment, the one or more non-sacrificial sodium-containing salts do not substantially decompose during charging of an electrochemical cell at a voltage from 1 V to 4.3 V, and preferably from 1 V to 4.2 V. That is, substantial oxidation of one or more anions of the one or more non-sacrificial sodium-containing salts does not occur (ideally no oxidation occurs) when the electrochemical cell is operated in this voltage range. Typically, substantial oxidation of the non-sacrificial salt will not occur when the cell is charged above 3 V, and in some cases, above 4 V. Ideally, oxidation of one or more anions of the one or more non-sacrificial sodium-containing salts during charging of an electrochemical cell is limited to no greater than about 5%, preferably no greater than about 3%, very preferably no greater than about 1%, and most preferably 0% (no oxidation occurs). Thus, the anions of the one or more non-sacrificial sodium-containing salts ideally do not substantially decompose (ideally do not decompose at all) during charging of an electrochemical cell at a voltage from greater than 3 V to 4.3 V, and preferably from 3 V to 4.2 V. In some cases, during charging of an electrochemical cell at a voltage from 4 V to 4.3 V, and preferably from 4 V to 4.2 V. Such one or more non-sacrificial sodium-containing salts (in addition to the one or more sacrificial sodium salts of the present invention) may be selected from one or more compounds of the formula NaMmXx; one or more compounds of the formula NaXO4; sodium salts of one2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES or more fluoro sulfonyl-containing compounds; sodium salts of one or more fluoro sulfonate- containing compounds; sodium salts of one or more oxalato borate compounds; sodium salts of one or more difluoro-oxalato borate compounds; and one or more compounds that contain a tetrahedral anion such as tetrakis[3,5-bis(trifluoromethyl)phenylborate anion (B[3,5- (CF3)2C6H3]− 4), tris(pentafluorophenyl)borate anion (B(C6F5)− 4), and tetrakis carboxy (trifluoromethyl)aluminate anion (Al[OC(CF3)3]− 4). Preferably, such one or more non-sacrificial sodium-containing salts are selected from: one or more compounds of the formula NaMmXx; wherein M is one or more metals and / or non- metals, and X is a group that comprises or consists of one or more halogens; sodium salts of one or more fluoro sulfonyl-containing compounds; sodium salts of one or more fluoro sulfonate-containing compounds; sodium salts of one or more oxalato borate compounds; and sodium salts of one or more difluoro-oxalato borate compounds. In the preferred one or more non-sacrificial sodium-containing salts of the formula NaMmXx, M is one or more metals and / or non-metals and X is a group that comprises or consists of one or more halogens, preferably selected from fluorine, chlorine, bromine and iodine, further preferably fluorine. The amount x of halogen X is preferably x=1 to 16, further preferably x=4 or 6. The amount, m, of the one or more metals and / or non-metals, M, is preferably m=1 to 3, further preferably m=1 to 2, and particularly preferably m=1. Ideally, the one or more metals and / or non-metals, M, is preferably selected from aluminium, boron, gallium, indium, iridium, platinum, scandium, Yttrium, lanthanum, antimony, arsenic and phosphorus. Particularly preferably, M is selected from aluminium, boron, gallium, phosphorus and arsenic. The most preferred sodium-containing salts of the general formula NaMnXx are one of more selected from sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6). In the preferred one or more sodium compounds of the formula NaXO4, the element X is preferably one or more halogens selected from fluorine, chlorine, bromine and iodine. Chlorine is particularly preferred, and the most preferred sodium-containing salt of the formula NaXO4 is NaClO4. Preferred non-sacrificial sodium salts of one or more fluoro sulfonyl-containing compounds include sodium bis(fluorosulfonyl)imide also known as NaFSI (Na N(SO2F)2) and sodium Bis(trifluoromethanesulfonyl)imide also known as NaTFSI (C2F6NNaO4S2). Preferred sodium salts of one or more one or more fluoro sulfonate-containing compounds include sodium trifluoromethanesulfonate, also known as sodium triflate or NaOTf (CF3NaSO3) when used in a cell at voltages <4.0V. Preferred sodium salts of one or more oxalate borate compounds2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES include sodium bis(oxalate), also known as borate NaBOB or NaB(C2O4)2; and sodium- difluoro(oxalato)borate, also known as NaDFOB. Highly preferably, the one or more non-sacrificial sodium-containing salts are selected from sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), and sodium bis(trifluoromethanesulfonyl)imide (NaTFSI). Most preferably, one or more non-sacrificial sodium-containing salts include sodium hexafluorophosphate (NaPF6). Alternatively, or additionally, a highly preferred non-sacrificial sodium-containing salt may comprise sodium tetrafluoroborate (NaBF4). It will be appreciated that certain salts which are non-sacrificial when cycled up to a certain maximum voltage can become sacrificial if cycled to voltages in excess of that operating voltage. Indeed, one such example is sodium triflate or NaOTf (CF3NaSO3). For instance, substantial oxidation of the anion of NaOTf occurs when an electrochemical cell is charged above 4 V as demonstrated herein. As used herein, the terms “non-sacrificial sodium-containing salt” or “primary sodium- containing salt” refer to any sodium-containing salts which contain anions which do not substantially decompose (i.e., do not substantially oxidise) (including no decomposition at all) under cell potentials as defined herein. For this, the decomposition potential of the one or more non-sacrificial sodium salts should ideally be higher than the upper cut-off voltage of the electrochemical cell. As set out above, an ideal upper cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 4.5 V or 4.3 V. A most preferable upper cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 4.2 V, optionally about 3.75 V. Therefore, the anion of a non-sacrificial (primary) sodium-containing salt should not substantially decompose (e.g. be oxidised) when cycled within an electrochemical cell to an upper cut-off voltage of 3.75V, ideally up to about 4.2V. Optionally, the anion of a non- sacrificial (primary) sodium-containing salt should not decompose (e.g. be oxidised) when cycled within an electrochemical cell to an upper cut-off voltage of 4.3V or about 4.5V. It is convenient to express the amount of the one or more sodium-containing salts (in addition to the one or more sacrificial sodium salts of the present invention) in terms of its molality of the components in the solvent system; that is, the total number of moles of the one or more sodium-containing salts (i.e., solute), per kilogram of the solvent system – i.e. mol / kg.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES The one or more sodium-containing salts (in addition to the one or more sacrificial sodium salts of the present invention) may be present in a concentration from about 0.1 mol / kg to about 5 mol / kg, further preferably about 0.3 mol / kg to about 5 mol / kg, more preferably from about 0.1 mol / kg to about ≤ 2.5 mol / kg, and most preferably from about 0.3 mol / kg to about 2.5 mol / kg. Highly preferably, particularly when the component is NaPF6, the molality is about 0.1 mol / kg to about 2.5 mol / kg, ideally about 0.3 mol / kg to about 1.5 mol / kg, and most preferably about 0.5 mol / kg to about 1 mol / kg. The total molality of the one or more sacrificial sodium salts of the present invention together with the additional one or more sodium-containing salts as defined above is preferably from about 0.1 mol / kg to about 5 mol / kg, further preferably about 0.3 mol / kg to about 5 mol / kg, more preferably about 0.3 mol / kg to about 3 mol / kg, and most preferably about 0.4 mol / kg to about 1.5 mol / kg. Highly preferably, when the present invention comprises a mixture of NaSCN and NaPF6, the total molality is from about 0.3 mol / kg to about 2 mol / kg, ideally about 0.4 mol / kg to about 1.5 mol / kg, and most preferably about 0.5 mol / kg to <1.5 mol / kg. Highly preferably, when the present invention comprises a mixture of NaSCN and NaFSI, the total molality is from about 0.3 mol / kg to about 2 mol / kg, ideally about 0.4 mol / kg to about 1.5 mol / kg, and most preferably about 0.5 mol / kg to <1.5 mol / kg. The non-aqueous electrolyte compositions of the present invention are particularly useful in an electrochemical cell. Therefore, in a further aspect, the present invention provides an electrochemical cell comprising a non-aqueous electrolyte composition as defined herein. The electrolyte compositions of the present invention are especially useful in sodium-ion cells, most preferably sodium-ion full cells (not sodium-metal half cells). Thus, in a further aspect, the present invention provides a sodium-ion cell comprising a non-aqueous electrolyte composition as defined herein. Preferably, the sodium-ion cell will comprise a negative electrode, a positive electrode, and a non-aqueous electrolyte composition as defined herein. A sodium-ion cell disclosed herein may also include an anode (negative) electrode that includes an anode current collector and / or a cathode (positive) electrode that includes a cathode current collector. The cathode current collector, the anode current collector, or both the cathode current collector and the anode current collector can be formed from aluminium or aluminium alloy (e.g., an alloy of aluminium and one or more of Mg, Mn, Cr, Zn, Si, Fe, and2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Ni). Preferably, the anode current collector comprises an aluminium current collector. Highly preferably, both the cathode current collector and the anode current collector comprise an aluminium current collector. Alternatively, copper, magnesium, carbon paper / foil / substrate and tin might also be used as current collector materials. Preferably, the anode (negative) current collector also includes one or more carbon-containing layers formed on one or more surfaces of the anode current collector prior to an initial first charge cycle of the sodium-ion cell. Such layers can comprise amorphous carbon (e.g., a carbon black, such as TIMCAL Super C65), ideally having a thickness from about 10 Angstrom to about 1000 µm. Alternatively, the anode (negative) current collector disclosed herein does not include one or more carbon-containing layers on one or more surfaces of the anode current collector prior to an initial first charge cycle of the sodium-ion cell. Therefore, the anode current collector is “pristine” prior to an initial first charge cycle of the sodium-ion cell. As used herein with respect to current collectors, the phrase "pristine" means that the current collector is in its “as made” state prior to an initial first charge cycle of the sodium-ion cell. In other words, the anode current collector is essentially the pure material from which it is formed from, barring impurities (e.g., surface oxide layers etc). As such the anode current collector is not coated with one or more carbon-containing layers (as discussed above), conventional active materials, binders, or the like. A sodium-ion cell disclosed herein may also include a separator located between the cathode and the anode current collector. A polyolefin separator is preferable. The anode (negative) electrode of the sodium-ion cell according to the present invention preferably further includes a polymeric binder. Typically, the polymeric binder is selected from polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC). Preferably, the polymer binder comprises carboxymethylcellulose (CMC). Highly preferably, the polymer binder comprises a mixture of styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC). The anode (negative) electrode of the sodium-ion cell according to the present invention ideally comprises a negative active material (A), one or more polymeric binders (B), and one or more conductive additives (C). Such a mixture is typically mixed with an aqueous or non- aqueous solvent (such as water or N-methyl Pyrrolidone (NMP)) and then disposed as a layer2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES or film on one or more surfaces of an anode current collector via techniques such as doctor blade or slot die methods. Typically, A:B:C are present in a weight ratio of from (80 to 98) : (1 to 19) : (1 to 19), and ideally in a weight ratio of from (85 to 98) : (1 to 9) : (1 to 9). When the polymer binder comprises polyvinylidene fluoride (PVDF), A:B:C are preferably in present in a weight ratio of about 88:9:3. When the polymer binder preferably comprises carboxymethylcellulose (CMC), and highly preferably comprises a mixture of styrene-butadiene rubber (SBR) and carboxymethylcellulose (CMC), A:B:C are preferably in a weight ratio of about 95:3.5:1.5. The one or more conductive additives (C) may include one or more of carbon black, carbon nanotubes, graphene, acetylene black, and carbon nanofiber. Preferably, the one or more conductive additives includes carbon black, such as TIMCAL Super C65. The anode (negative) electrode of the sodium-ion cell according to the present invention preferably further includes a negative active material. For example, the negative electrode which comprises a negative active material may be one or more of graphite material, soft carbon, hard carbon, carbon fibers, mesocarbon microbeads, silicon-based material, tin- based material, P-based material, Sb-based material, SnSb-based material, other types of materials that store charge via a conversion and / or alloying-type reaction (such as simple / binary / ternary etc oxides of Fe, Cu, Ni, Mn etc) and materials that store charge via a classical intercalation-type reaction such as lithium titanium oxide, different types of sodium titanates, titania etc; along with any combination of one or more of these preceding materials such as carbon / Sb, carbon / P, carbon / Fe2O3, either as mixtures generated by physical blends (such as carbon and Sb blended together by physical or chemical mixing methods such as ball-milling or spray-drying) or via an in-situ reaction (via an appropriate synthesis route such as solvo / hydrothermal, sol-gel, solution-based, reflux, co-precipitation, or solid-state reaction with or without subsequent heating / pyrolysis steps), the latter of which might result also result in doped materials (such as doped carbon with Sb or P or Sn) and / or substituted materials (such as substitution of Fe2O3 with a bit of TiO2). The anode electrode of the sodium-ion cell according to the present invention preferably includes a negative active material that comprises hard carbon. The non-aqueous electrolyte compositions of the present invention will be especially useful in sodium-ion cells which employ an anode electrode which includes a negative active material that comprises a non-graphitic carbon-containing material such as a hard carbon, or an anode which comprises a sodium insertion material, or a conversion and / or alloying anode material.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES The cathode (positive) electrode of the sodium-ion cell according to the present invention preferably further includes a polymeric binder. Typically, the polymeric binder is selected from polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC). Preferably, the polymer binder comprises polyvinylidene fluoride (PVDF). The cathode (positive) electrode of the sodium-ion cell according to the present invention ideally comprises a positive active material (A), one or more polymeric binders (B), and one or more conductive additives (C). Such a mixture is typically mixed with an aqueous or non- aqueous solvent (such as water or N-methyl Pyrrolidone (NMP)) and then disposed as a layer or film on one or more surfaces of an anode current collector via techniques such as doctor blade or slot die methods. Typically, A:B:C are present in a weight ratio of from (80 to 98) : (1 to 19) : (1 to 19). When the polymer binder comprises polyvinylidene fluoride (PVDF), A:B:C are preferably in present in a weight ratio of about 89:5:6, or alternatively, 92:4:4. The one or more conductive additives (C) may include one or more of carbon black, carbon nanotubes, graphene, acetylene black, and carbon nanofiber. Preferably, the one or more conductive additives includes carbon black, such as TIMCAL Super C65. The cathode (positive) electrode of the sodium-ion cell according to the present invention preferably further includes a sodium-containing positive active material which is adapted to allow the insertion / removal of sodium ions during charge / discharge. Examples of these include sodium transition metal oxides, polyanionic compounds (including fluorinated polyanionic compounds), Prussian blue analogue (PBA) compounds (such as Prussian White or Berlin Green), materials storing sodium via a conversion reaction, sodium transition metal fluorides, oxyfluorides, phosphates, sulfates, and silicates (and their fluorinated versions). Preferred examples are sodium transition metal oxides. Preferred sodium transition metal oxides are of the general formula: A 1 2 3 4 5 1±δ M VM WM X M YM ZO2-c wherein A is one or more alkali metals selected from sodium, potassium and lithium; M1comprises one or more redox active metals in oxidation state +2, preferably selected from the group consisting of nickel, copper, cobalt and manganese; M2comprises a metal in oxidation state greater than 0 to less than or equal to +4;2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES M3comprises a metal in oxidation state +2; M4comprises a metal in oxidation state greater than 0 to less than or equal to +4; M5comprises a metal in oxidation state +3; wherein 0 ≤ δ ≤ 1; V is > 0; W is ≥ 0; X is ≥ 0; Y is ≥ 0; at least one of W and Y is > 0 Z is ≥ 0; C is in the range 0 ≤ c < 2 wherein V, W, X, Y, Z and C are chosen to maintain electrochemical neutrality. Ideally, metal M2comprises one or more transition metals, and is preferably selected from manganese, titanium and zirconium; M3is preferably one or more selected from magnesium, calcium, copper, tin, zinc and cobalt; M4comprises one or more transition metals, preferably selected from manganese, titanium and zirconium; and M5is preferably one or more selected from aluminium, iron, cobalt, tin, molybdenum, chromium, vanadium, scandium and yttrium. A particularly preferred positive electrode active material for use in a sodium-ion cell will be a nickelate-based material. A sodium-containing active material with any crystalline structure may be used, however, preferably the structure will be O3 or P2 or a derivative thereof, but, specifically, it is also possible that the cathode electrode active material will comprise a mixture of phases, i.e. it will have a non-uniform structure composed of several different crystalline forms. For example, the cathode active material will comprise a compound with the general formula detailed above in a mixture of O3 and P2 phases. The ratio of O3:P2 phases is preferably 1 to 99: 99 to 1. The present invention also provides in another aspect the use of a non-aqueous electrolyte composition as defined herein in an electrochemical cell. Ideally, the electrochemical cell is a sodium-ion cell, and most ideally a sodium-ion full cell. The present invention also provides the use of one or more sacrificial sodium salts in an electrochemical cell, in which the use of one or more sacrificial sodium salts are as defined herein. Ideally, the electrochemical cell is a sodium-ion cell, and most ideally a sodium-ion full cell.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES As discussed above, most preferably, the one or more sacrificial sodium salts comprise cyanate-containing compounds or thiocyanate-containing compounds. The one or more sacrificial sodium salts will ideally have the general formula NaXCN, where X is a group 16 element (from the periodic table), optionally selected from oxygen and sulfur, preferably selected from sulfur. In a most preferred embodiment, the one or more sacrificial sodium salts will comprise sodium thiocyanate (NaSCN). Alternatively, or additionally, the preferred one or more sacrificial sodium salts may comprise one or more of sodium triflate (NaSO3CF3), sodium difluorophosphate (NaPO2F2), and sodium tetraphenylborate (NaBPh4). The present invention also provides in another aspect the use of one or more sacrificial sodium salts as defined herein as presodiation reagent, preferably for use in an electrochemical cell. Ideally, the electrochemical cell is a sodium-ion cell, and most ideally a sodium-ion full cell. The present invention also provides in another aspect the use of a non-aqueous electrolyte composition as defined herein as presodiation reagent, preferably for use in an electrochemical cell. Ideally, the electrochemical cell is a sodium-ion cell, and most ideally a sodium-ion full cell. The electrochemical cells of the present invention (ideally sodium-ion cell of the present invention) may be used in an energy storage device, for example a battery, a rechargeable battery, an electrochemical device and an electrochromic device. The present invention also provides in another aspect an apparatus comprising an electrochemical cell (ideally a sodium-ion cell) as disclosed herein. A typical apparatus may include a device such as a battery pack that may have use in either a stationary or a mobile application. The present invention also provides in another aspect a method of manufacturing a non- aqueous electrolyte composition as defined herein. The present invention also provides in another aspect an electrochemical cell which has been presodiated using a non-aqueous electrolyte composition as defined herein. That is, an electrochemical cell which has been presodiated by charging with a non-aqueous electrolyte composition as defined herein. Indeed, the one or more sacrificial sodium salts which are2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES present in the non-aqueous electrolyte composition as defined herein ideally provide sodium- ions to the anode during charging of the electrochemical cell. Therefore, more particularly, the present invention ideally provides an electrochemical cell comprising: a cathode; an electrolyte and an anode; and in which the anode has been presodiated using a non-aqueous electrolyte composition as defined herein. It will be appreciated that ‘an electrolyte’ in this context may or may not be a non-aqueous electrolyte composition as defined herein. This is because the one or more sacrificial sodium salts which are present in the non-aqueous electrolyte composition as defined herein ideally comprise one or more anions which oxidise during charging of the electrochemical cell. Indeed, such one or more sacrificial sodium salts, or a portion thereof, are designed to be used up or destroyed during charging of the electrochemical cell. Therefore, it could also be envisaged that a different electrolyte (i.e., different to a non- aqueous electrolyte composition as defined herein) may be used in the electrochemical cell after the anode has been presodiated. The electrochemical cell is ideally a sodium-ion cell, preferably a sodium-ion full cell. An ideal upper cut-off voltage of the electrochemical cell (preferably a sodium-ion cell) is about 4.5 V or 4.3 V. A most preferable upper cut-off voltage of an electrochemical cell (preferably a sodium-ion cell) is about 4.2 V, optionally about 3.75 V. The anode and cathode maybe defined as set out above. As such, it is preferable that the cathode includes a positive active material which comprises one or more sodium transition metal oxides. Similarly, it is preferable that the anode includes a negative active material which comprises hard carbon. Thus, it is highly preferable that an anode that includes a negative active material which comprises hard carbon has been presodiated using a non- aqueous electrolyte composition as defined herein. The present invention also includes in another aspect a process for forming an electrochemical cell, in which the process comprises using a non-aqueous electrolyte composition as defined herein. Ideally, a process for forming an electrochemical cell comprises:2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES a) providing an electrochemical cell with a non-aqueous electrolyte composition as defined herein; and b) charging the electrochemical cell to a voltage, optionally of ≤ 4.5V. The electrochemical cell (preferably sodium-ion cell) provided in step a) has not normally been previously subject to a charge operation. That is, step a) will usually take place straight after the electrochemical cell has been assembled by the manufacturer. Step b) comprises charging the electrochemical cell from a first voltage to a second voltage, in which the second voltage is preferably ≤ 4.5V. The second voltage is higher than first voltage. The first voltage is normally from -0.5V or greater. Ideally, the second voltage is from about 2V to about 4.5V. Charging in step b) may be performed at a C-rate from about 10C to about C / 50. It could be at 5C (C / 0.2), 2C (C / 0.5), C / 1, C / 2, C / 5, or C / 50. Ideally, it is from about C / 1 to about C / 10. The process may also include step c) which comprises discharging the electrochemical cell from the second voltage to a third voltage. The third voltage is normally from about 0V to about 3V. Discharging in step c) may be performed at a C-rate from about C / 0.2 (5C) to about C / 50. It could be at C / 0.5 (2C), C / 1 (1C), C / 2, C / 5, or C / 50. Ideally, it is from about C / 1 to about C / 10. The process may optionally comprise repeating steps b) and c) one or more times, perhaps two or more times. Preferably, the electrochemical cell is charged to a voltage which is greater than a decomposition potential of one or more anions of the one or more sacrificial sodium salts. Thus, the one or more anions of the one or more sacrificial sodium-containing salts may substantially decompose during charging. The electrochemical cell may be charged to a voltage of greater than 2V. The electrochemical cell may be charged to a voltage of greater than 3V. The electrochemical cell may be charged to a voltage of greater than 3.5V. The electrochemical cell may be charged to a voltage of greater than 3.8V. The electrochemical cell may be charged to a voltage of greater than 4.0V. Ideally, the electrochemical cell is charged to a voltage of 4.2V.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Preferably, the electrochemical cell is charged to a voltage from greater than 3 V to 4.3 V, and preferably from greater than 3 V to 4.2 V. In some cases, charged to a voltage from 4 V to 4.3 V, and preferably from 4 V to 4.2 V. The present invention also includes in another aspect an electrochemical cell, ideally a sodium-ion cell, which has been prepared by a process as defined herein. The purpose of the above formation process is to form stable interfaces at the surfaces between the electrodes and the electrolyte. As such, the formation process may generate gaseous by-products, which are either vented away (in cylindrical or prismatic-format cell), or siphoned away via an appropriate cut made in the packaging (for pouch cells), in a step called ‘degassing’. The inventors of the present application seek to take advantage of this formation process. In particular, it has been found by the inventors and shown in the examples disclosed herein that the sacrificial salts according to the present invention irreversibly oxidize during the first cycle (which is also the first formation cycle) – thus, any gaseous side products (if formed), can be removed during the degassing step (which would have occurred anyway). Thus, no extra process steps are incurred meaning the present invention as defined herein shows great commercial utility. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described with reference to the following figures in which: FIGURE 1 shows a plot of cathode discharge capacity (mAh / g) vs cycle number for cells that use electrolyte compositions of the present invention (samples TEL 22, TEL 25b, TEL 25c, TEL 25d, TEL 25e, TEL 25g, and TEL 25h), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 0). FIGURE 2 shows a plot of specific dQ / dV (mAh / g / V) vs cathode potential (V vs Na / Na+) for cells that use electrolyte compositions of the present invention (samples 25b, TEL 25c, and TEL 25d), compared against a cell which uses a control electrolyte composition (sample TEL 0).2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES FIGURE 3 shows a plot of energy retention (%) vs cycle number for a cell that uses electrolyte compositions of the present invention (sample TEL 25b). FIGURE 4 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 25e), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 0). FIGURE 5 shows a plot of discharge capacity (mAh / g) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 25e), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 0). FIGURE 6 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 45c), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 42d). FIGURE 7 shows a plot of discharge capacity (mAh / g) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 45c), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 42d). FIGURE 8 shows a plot of cathode specific capacity (mAh / g) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 76c), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 67a). FIGURE 9 shows a plot of cathode specific capacity (mAh / g) vs cycle number for a cell that uses an electrolyte composition of the present invention (sample TEL 146a), compared against the cycling stability of a cell which uses a control electrolyte composition (sample TEL 146). FIGURE 10 shows a plot of specific dQ / dV (mAh / g) vs voltage (V) for a cell that uses an electrolyte composition of the present invention (TEL 62a), compared against a cell which uses a control electrolyte composition (sample TEL 0). FIGURE 11 shows the plot of FIGURE 10 at cathode potential from 3.5 to 4.2V; FIGURE 12 shows a plot of specific dQ / dV (mAh / g) vs voltage (V) for a cell that uses an electrolyte composition of the present invention (TEL 67Ra), compared against a cell which uses a control electrolyte composition (sample TEL 67).2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES FIGURE 13 shows the plot of FIGURE 12 at cathode potential from 3.5 to 4.2V; and FIGURE 14 shows a plot of specific dQ / dV (mAh / g) vs voltage (V) for a cell that uses an electrolyte composition of the present invention (TEL 79c), compared against a cell which uses a control electrolyte composition (sample TEL 67a). EXAMPLES The electrolyte compositions under investigation were prepared using the following general procedure: Appropriate amounts of solvents for a desired solvent system were mixed together in a glass vial in the desired weight ratio. Where appropriate, one or more performance additives (e.g. one or more surfactants, one or more boron-containing compounds, one or more sulfur- containing compounds, and combinations thereof) were also added to prepare the solvent system. To dry the formed solvent system, activated 4A molecular sieves were added and the solvent system was allowed to dry at last for 18 h. In a separate glass bottle fitted with a magnetic pellet, required weights of NaPF6 salt and a presodiation reagent comprising one or more sacrificial sodium salts (e.g., NaSCN) sacrificial salt were added, followed by the required quantity of desired solvent system as prepared above. The electrolyte was then stirred via a magnetic pellet for a duration of 18 hrs or more, upon which, the electrolyte then was ready for use. The precise composition of each of the electrolyte compositions investigated, is detailed in Table 1 below: TABLE 1 Electrolyte Composition Composition Sample Ref. *TEL 0 (Control) 0.5 m NaPF6in (EC:DEC:PC = 1:2:1 wt / wt) TEL 25 TEL 0 with 0.5 m NaSCN2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES *TEL 25a TEL 0 with 1 m NaSCN TEL 25b TEL 0 with 0.25 m NaSCN TEL 25c TEL 0 with 0.1 m NaSCN TEL 25d TEL 0 with 0.05 m NaSCN TEL 25e TEL 0 with 0.025 m NaSCN TEL 25h TEL 0 with 0.02 m NaSCN TEL 25g TEL 0 with 0.015 m NaSCN *TEL 42d (Control) 1 m NaPF6in (PC with 20 wt% HFE-458, 1 wt% PCS, 1 wt% P123 and 1 wt% TMSB additives) TEL 45c TEL 42d with 0.05 m NaSCN *TEL 67a 1 m NaPF6 in (PC with 20 wt% DEC, 2 wt% PCS, 1 wt% P123 and 1 wt% TMSB) TEL 79c TEL 67a + 0.05m NaSO3CF3 *TEL 67 1 m NaPF6in (PC with 20 wt% DEC, 2 wt% PCS and 1 wt% P123) TEL 67Ra TEL 67 + 0.047 m NaPO2F2 *TEL 0c 1 m NaPF6 in (EC:DEC:PC = 1:2:1 wt / wt) TEL 62a TEL 0c + 0.05 m NaBPh4*TEL 114 1 m NaFSI in (EC:DEC:PC = 1:2:1 wt / wt) TEL 113a TEL 114 + 0.025 m NaSCN *TEL 147 1 m NaPF6 in (TEP with 20 wt% HFE and 5 wt% FEC) TEL 147a TEL 147 + 0.05 m NaSCN *TEL 146 1.2 m NaFSI in (TEP with 20 wt% HFE and 5 wt% FEC) TEL 146a TEL 146a + 0.05 m NaSCN * = not according to the present invention as defined by claim 1. For the avoidance of doubt, concentrations expressed by “m” (molality) above relate to the total number of moles of the non-sacrificial or sacrificial sodium salts (e.g. NaPF6 or NaSCN) per 1 kg or 1000g of the solvent system. Additionally, the % of the various solvent components is with respect to the total solvent weight and does not include the weight of the non-sacrificial or sacrificial sodium salts. Abbreviations used:2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES EC = Ethylene carbonate, DEC = Diethyl carbonate, PC = Propylene carbonate, PCS = 1,3- propanediolcyclic sulfate, P123 = Poloxamer (Pluronic) P123, TMSB = Tris(trimethylsilyl) borate, HFE = 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, NaSCN = Sodium thiocyanate, and NaPF6= Sodium hexafluorophosphate; NaPO2F2= Sodium Difluorophosphate; NaBPh4= Sodium tetraphenylborate; and NaFSI = Sodium bis(fluorosulfonyl)imide; NaSO3CF3= Sodium Triflate; TEP = Triethyl Phosphate; and FEC = Fluoroethylene carbonate; Please note that in the above Table 1, the wt% of the various solvents and / or performance additives are mentioned with respect to the total solvent system weight (they do not account for the weight(s) of the one or more non-sacrificial or sacrificial sodium salts). General Procedure to Make a Hard Carbon Na-ion Cell All cells were tested as Na-ion pouch cells. The electrolyte was prepared as follows. First, the co-solvents were mixed together in a glass vial in the desired weight ratio. To dry the solvent-mix, activated 4A molecular sieves were added and the solvent mix was allowed to dry at last for 18 h. In a separate glass bottle with a magnetic pellet, required weights of non-sacrificial salt (e.g. NaPF6) and sacrificial salt (e.g. NaSCN) were added, followed by the required quantity of the required solvent mix. The salts and solvent mix were magnetically stirred at least for 18 h, whereupon, the resultant electrolyte was ready for use. The cathode comprised O3 / P2 Nickelate oxide material as cathode active material, PVDF as binder and C65 as conductive additives, in 89:06:05 wt ratio, coated on C-coated current collectors. This formulation was used for all cells shown in Experiment 1 and Experiment 2. In alternative embodiments, the cathode formulation was active material:PVDF binder:C65 = 92:04:04 wt / wt. This formulation was used for all cells shown in Experiment 0, Experiment 3, Experiment 4 and Experiment 5. The anode comprised Type 1 hard carbon as active material, PVDF as binder and C65 as conductive additives, in a weight ratio of 88:09:03. This formulation was used for all cells shown in Experiment 1, and Experiment 2. In alternative embodiments, the anode used water-based CMC-SBR binder and in this embodiment, the weight ratio of Type 1 hard carbon:(CMC-SBR):C65 was 95:3.5:1.5. This formulation was used for all cells shown in Experiment 0, Experiment 3 and Experiment 4 and Experiment 5.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Aluminium tabs were connected to each of the electrodes and the cell was encased in a polymer-coated aluminium pouch. After coating the cathode and anode, they were stamped at the desired dimensions. The coating weights of the cathode and anode have been mentioned as a GSM value (grams per square metres). ‘C / A’ refers to the mass ratio of the cathode active to the anode active materials in the respective coatings. The separator used was a typical polyolefinic separator used commonly for any type of rechargeable lithium-ion or sodium-ion battery, such as Celgard 2500. The cathode / separator / anode assembly was placed within a pouch with two Al-based connecting tabs serving as the terminals, inside the glove box, and filled with the stated liquid electrolytes in quantities typical of such types of polyolefinic separator-containing lithium-ion or sodium- ion cells. These pouch cells were then sealed in the glove box and brought out for cell testing. In some examples (cells A3PC76, A3PC253, A3PC258 and A3PC272 in Experiment 1), three- electrode (3E) full cells were fabricated. These 3E cells are essentially identical to the 2E full cells in that they include a cathode and anode, but also now include a third reference electrode, to which a piece of Na metal has been attached – this Na metal can track the individual potentials of the cathode and anode (vs the Na / Na+redox couple). Two separator layers were used to fabricate this 3E cell, with the Na metal inserted between the two separators, outside of the footprint of the cathode / / anode stack. Details on the 3E cell design have been mentioned in the following prior art: Rudola et al., J. Electrochem. Soc., 2021, 168 (11), 110534. Cell testing The cells were tested using Constant Current (Galvanostatic) Cycling techniques. Generally speaking, the cells were first charged via the constant current (CC) mode to a pre-defined maximum voltage limit. Afterwards, the cell was made to undergo a constant voltage (CV) step at that maximum voltage limit, to either a pre-defined time or this CV step was made to last until the current dropped to a pre-defined value, as indicated in the examples. In some examples, the cells were made to undergo a series of different CC-modes to different voltage values, before undergoing the CV step only at the maximum voltage value. The discharge process was either conducted at CC-mode to the lower cut-off voltage, or in constant power (CP) mode, as indicated. A commercial battery cycler from MTI Inc. (Richmond, CA, USA) or Maccor (Tulsa, OK, USA) was used. On charge, alkali ions are inserted into the carbon-containing anode material.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES During discharge, alkali ions are extracted from the and re-inserted into the cathode active material. All cells were subject to cycling experiments at charge rates such as ±1C (this can also be stated as ±C / 1), ±C / 5 or ±C / 10 (or other rates, as mentioned for each experiment). Rate performance tests saw discharge rates of C / 5, C / 10, 1C and 2C also. All cells were rested for at least 4 - 24h prior to cycling. EXPERIMENT 0 (control) – The cycling performance of using a sacrificial salt in the cathode The aim of this experiment is to demonstrate the disadvantages of using one or more sacrificial sodium salts contained in the cathode in accordance with a prior art presodiation technique. In particular, the presence of one or more sacrificial sodium salts contained in the cathode is demonstrated to reduce the amount of cathode active material which can actually store sodium. Thus, even though the mAh / gcathode active material of a cell might increase, the actual mAh of the cell might not increase, thus defeating the purpose of this approach. This can be illustrated via Experiment 0 performed by the inventors of this application: 100 mAh Na-ion pouch cells were fabricated, with details shown in Table 0 below. Here, the following samples were prepared: 1. (FPC211002,03) used 92 wt% O3 / P2 oxide material as the cathode active material in the absence of one or more sacrificial sodium salts; 2. FPC210924 was a Na-ion pouch cell in which the cathode contained 10 wt% of CH3COONa sacrificial salt and 82 wt% O3 / P2 oxide cathode; and 3. FPC210928 was a Na-ion pouch cell in which the cathode contained 10 wt% of NaNO2 sacrificial salt and 82 wt% O3 / P2 oxide cathode. The cells were made to cycle as follows: Cycles 1 – 2: 4.1 – 1 V at ±C / 10 (to simulate ‘formation’ cycling). Cycles 3 – 10: 4.05 – 1.8 V at +C / 5 charge; discharge for two cycles each at C / 5, C / 2, 1C and 2C. Cycles 11 onwards: 4.05 – 1.8 V at ±1C.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Table 0 lists the various metrics. Here, the ‘g’ in mAh / g is based only on the weight of the O3 / P2 oxide ‘active material’, as the sacrificial salts are expected to oxidize completely after the first charge (to furnish the extra Na+, with the remainder emanating as gas, which can be removed during the de-gassing step): thus, after the first charge cycle, it is expected that the cathode would be 10 wt% lighter (as 10 wt% was the weights of the respective sacrificial salts added). In Table 0: D1 refers to the first discharge capacity of the full cell at ±C / 10 in units of mAh / g, where the weight (in grams, g) is the weight of the cathode active material, ‘FCE’ refers to the coulombic efficiency of cycle 1 (that is, the ratio of cycle 1 discharge capacity to cycle 1 charge capacity, expressed as a % value). D3 refers to the discharge capacity of the third cycle at 4.05 – 1.8 V at ±C / 5. D9 refers to the discharge capacity of the ninth cycle at 4.05 – 1.8 V at C / 5 charge, 2C discharge. Referring now to Table 0. It can be seen from Table 0 that, whilst the D1 capacities in mAh / gcathode active material were comparable for the two sacrificial salt cathode-based cells FPC210924 and FPC210928 relative to cells FPC211002 and FPC211003 at slow C / 10 and C / 5 rates, the actual cell capacity (in mAh) of cells FPC210924 and FPC210928 were actually 9 - 15 mAh lower than cells FPC211002 and FPC211003. TABLE 0 Cell Sac. C / A ±C / 10 D1 FCE C / 5 D3 2C D9 D1 Total Salt / wt% (mAh / g) (%) (mAh / g) (mAh / g) mAh FPC211002 - 2.58 114.8 89.9 99.2 88.8 98.6 FPC211003 - 2.62 114.3 91.1 97 87.9 99.7 FPC210924 CH3COON 2.55 111.9 71.8 100.1 77.6 83.9 a / 10 wt% FPC210928 NaNO2 / 10 2.57 100 64.8 96 77 90 wt% Another problem with this approach, which the inventors of the present application have found, is that the integrity of the cathode is significantly compromised after the sacrificial salt oxidises2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES in the first charge. This has an impact in that the rate performance of the cell is compromised significantly due to the higher internal resistance induced in the cell owing to the electrode’s microstructure getting affected due to the gas generation and void formation (these voids were previously occupied by the sacrificial salt). Proof of this is shown in Table 0 above: it can be seen that the 2C capacities of cells FPC210924 and FPC210928 were significantly lower (around 10 mAh / g lower) than the 2C capacities of cells FPC211002 FPC211003 and (even though the former’s slow rate capacities were similar / higher than the latter’s). As demonstrated below, the present invention aims to mitigate or eliminate one of more of these disadvantages of the known prior art. EXPERIMENT 1 - The cycling performance of cells using an electrolyte composition of the present invention (i.e., TEL 25, TEL 25b, TEL 25c, TEL 25d, TEL 25e, TEL 25h, and TEL 25g) Experiment 1 compares the cycling performance of Na-ion ~ 7 mAh (nominal capacity) pouch cells that were cycled with an electrolyte composition of the present invention (i.e., TEL 25, TEL 25b, TEL 25c, TEL 25d, TEL 25e, TEL 25h, and TEL 25g), compared to a Na-ion ~ 7 mAh (nominal capacity) pouch cell that was cycled with a comparative electrolyte composition (i.e., TEL 0). In particular, each Na-ion cell was cycled for four cycles at 4.2 – 1 V at ±C / 10 (to simulate formation cycling), followed by ±C / 2 cycling from cycle five onwards, also at 4.2 – 1 V. The performance metrics of these cells are shown below in Table 2. Figure 1 also shows the capacity vs cycle number graphs. TABLE 2 Cell Electrolyte C / A D1 FCE D5 Capacity Retention (mAh / g) (%) (mAh / g) (vs D5) at Cycle # APFC101 TEL 0 1.43 118.6 77.2 110.2 91.6% at Cycle 170 APFC132 TEL 25 1.41 128.5 60.1 110.5 78.5% at Cycle 23 APFC136 TEL 25b 1.39 125.6 60.3 115.9 75.8% at Cycle 61 APFC141 TEL 25c 1.42 123.5 64.8 115.8 75.7% at Cycle 70 APFC150 TEL 25d 1.39 118.2 66.5 114.7 79.2% at Cycle 1462024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES APFC157 TEL 25e 1.4 122.7 76.4 115.1 91.9% at Cycle 170 APFC209 TEL 25h 1.42 122.0 77.3 113.6 92% at Cycle 170 APFC202 TEL 25g 1.41 120.2 77.1 111.9 72.9% at Cycle 170 As shown above in Table 2: D1 refers to the first discharge capacity of the full cell at ±C / 10 in units of mAh / g, where the weight (in grams, g) is the weight of the cathode active material, ‘FCE’ refers to the coulombic efficiency of cycle 1 (that is, the ratio of cycle 1 discharge capacity to cycle 1 charge capacity, expressed as a % value). D5 refers to the first discharge capacity of the first cycle at C / 2 (fifth discharge overall). Furthermore, ‘Cycle Life’ refers to the cycle number to 20 % capacity fade (relative to D5). Analysis Capacity: As shown in Table 2, each Na-ion cell that was cycled with a non-aqueous electrolyte composition according to the present invention (i.e., TEL 25, TEL 25b, TEL 25c, TEL 25d, TEL 25e, TEL 25h, and TEL 25g), provided greater D1 and D5 discharge capacities when compared with a Na-ion cell that was cycled with a control sample (TEL 0; APFC101). For instance, the D1 of a cell (APFC132) cycled with TEL 25 was 128.5 mAh / g when compared to 118.6 mAh / g, which was the D1 of a cell (APFC101) cycled with control sample TEL 0. Similarly, the D5 of a cell (APFC136) cycled with TEL 25b was 115.9 mAh / g when compared to 110.2 mAh / g, which was the D5 of a cell (APFC101) cycled with control sample TEL 0. Table 2 shows that the D1 of a cell (APFC150) using TEL 25d was similar to a cell (APFC101) cycled with control sample TEL 0. However, Figure 1 shows that the second, third and fourth discharge capacities of APFC150 were roughly ~5 mAh / g higher than the corresponding capacities of APFC101 (this ~5 mAh / g capacity boost translates into D5, as shown in Table 1). Concentration of sacrificial salt:2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES As set out in Table 1 above, the concentration of sacrificial salt (NaSCN) was decreased from 0.5 m in TEL 25, to 0.015 m in TEL 25g. Indeed, it was not possible to cycle a sodium-ion cell with sample TEL 25a because NaSCN would not dissolve in the solvent mixture at a concentration of 1 m. Table 2 shows that as the concentration of sacrificial salt decreased, the FCE of the respective Na-ion cell increased progressively, before stabilizing around the FCE values shown for a cell cycled with control sample TEL 0. This is expected because the FCE is correlated with the irreversible oxidation of the anion of the sacrificial salt. As the concentration of sacrificial salt increases, the amount of charge used to irreversibly oxidise the sacrificial salt also increases, thus decreasing the FCE. It is also possible that at low concentrations of the sacrificial salt in the electrolyte, the irreversibility induced due to sacrificial salt oxidation is partly / fully offset by the extra Na+furnished, resulting in similar FCE values to the control sample. Therefore, the skilled person understands that a high FCE value is not the relevant metric for assessing the performance of the present invention because the sacrificial salt is expected to irreversibly oxidise as the cell is charged. Instead, it is whether the present invention can provide an additional source of sodium-ions which can then be accommodated in the anode, to boost the capacity of a sodium-ion cell. However, such additional capacity must also be balanced with the ability to maintain chemical and electrochemical stability so that the sodium-ion cell can be charged and discharged over many cycles. As set out above, when compared with a comparative electrolyte composition, the discharge capacities of sodium-ion cells were increased using an electrolyte composition of the present invention having a presodiation reagent which comprises one or more sacrificial sodium salts in a concentration of 0.5 mol / kg or less. Furthermore, higher discharge capacities and improved cycling stabilities were obtained using an electrolyte composition of the present invention having a presodiation reagent which comprises one or more sacrificial sodium salts in a concentration of 0.025 mol / kg or less, and particularly in a concentration of from about 0.025 mol / kg to about 0.020 mol / kg. For instance, the capacity retention after 170 cycles of a cell (APFC157) cycled with TEL 25e was 91.9%: this value was 91.6% after 170 cycles for a cell (APFC101) cycled with control sample TEL 0. Similarly, the capacity retention after 170 cycles of a cell (APFC209) cycled with TEL 25h was 92% when compared to 91.6% capacity retention after 170 cycles, for a cell (APFC101) cycled with control sample TEL 0.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Evidence of irreversible oxidation: TABLE 3: Cell Electrolyte C / A D1 FCE (mAh / g) (%) A3PC76 TEL 0 1.44 117 76.9 A3PC253 TEL 25b 1.35 126.3 65.1 A3PC258 TEL 25c 1.35 123.6 66 A3PC272 TEL 25d 1.39 123.2 70.6 Table 3 and Figure 2 clarify the differences in dQ / dV plots of separate 3E full cells filled with a non-aqueous electrolyte composition according to the present invention (i.e., TEL 25b, TEL 25c, and 25d), and a control sample (TEL 0). Figure 2 shows cathode’s differential capacity curve (dQ / dV), where the x-axis is the cathode potential vs the reference Na (hence, V vs Na / Na+). It can be seen that the sacrificial salt- containing electrolytes (TEL 25b, TEL 25c, TEL 25d) oxidizes around 3.26 V vs Na / Na+and 3.74 V vs Na / Na+, as gauged by the inflection points in the sacrificial salt containing electrolytes based 3E cells’ dQ / dV plots. It can also be seen that the discharge process (reduction) of all four cells were identical: this proves that the oxidation of the sacrificial salt is irreversible, suiting the intended usage. Conclusions Adding a sacrificial salt to the electrolyte in accordance with the present invention, unexpectedly provides a capacity increase in a sodium-ion cell. At certain concentrations, a sacrificial salt also provides a sodium-ion cell with increased cell cycle life as shown in the specific examples described above. EXPERIMENT 2: Alternate Voltage Window and Long-term Cycling Figure 2 of the present invention illustrated that oxidation of a sacrificial salt (i.e., NaSCN) was complete by around 3.74 V vs Na / Na+. Thus, a 60 mAh Na-ion pouch cell was fabricated using TEL 25b, with a C / A = 2.87, and made to cycle at 3.75 – 2.5 V, at ±C / 5 for 3500 cycles, followed by ±C / 2 cycling.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES It can be seen from Figure 3 that this cell, FPC190415, retained above 87 % of its initial energy, after 6900 cycles, proving that the sacrificial salt in electrolyte concept could result in cycle life in the thousands. Conclusion: Experiment 2 shows that the sacrificial salt in electrolyte concept can work well in different voltage windows, is scalable (data shown at 60 mAh scale, an order of magnitude higher than the cells shown in Experiment 1) and can result in Na-ion cells demonstrating thousands of cycles, which is very important from an industrial applicability viewpoint. EXPERIMENT 3: Rate Performance and Total Cell Capacity Boost Table 4, Figure 4 and Figure 5 show data on ~6 mAh Na-ion pouch cells where the cathode’s formulation was 92:04:04 and the anode’s formulation was 92:3.5:1.5 wt / wt. These cells were cycled as follows: Cycles 1 – 2: 4.2 – 1 V at ±C / 10 Cycles 3 – 10: 4.1 – 1.8 V at +C / 5 charge; discharge for two cycles each at C / 5, C / 2, 1C and 2C. Cycles 11 onwards: 4.1 – 1.8 V at ±1C. The relevant metrics for this experiment are shown in Table 4. Here, ‘2C:C / 5 Ratio’ means the capacity ratio delivered during discharge at 2C rate (cycle 9-10), vs C / 5 rate (cycles 3-4), expressed as a %. D11 means the first discharge capacity at ±1C (11th discharge in total). For D1 and D11, figures have been given for mAh / g (weight based on cathode active material), as well as mAh (the total charge stored by the cell in that cycle). TABLE 4 Cell Electrolyte C / A D1 mAh / g; FCE 2C:C / 5 D11 mAh / g; D1 mAh (%) Ratio (%) D11 mAh UM2C52 TEL 0 2.46 126.5; 6 85.5 88.6 93.8; 4.5 UM2C63 TEL 25e 2.46 129.1; 6.3 84.4 88.9 98.2; 4.8 From Table 4, it can be seen that UM2C63, using sacrificial salt-based TEL 25e, delivered not only higher D1 in mAh / gcathode active (+2.6 mAh / g), but also the total cell capacity in mAh (+0.32024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES mAh or +5 % of capacity boost), relative to TEL 0-based UM2C52. This experiment firstly reiterates that the sacrificial salt in electrolyte approach can be compatible with different electrode formulations and binders. More importantly, this experiment also shows that this approach not only boosts mAh / g, but also actually boosts the total cell capacity in mAh, a stark contrast to the conventional approach of adding sacrificial salts to the cathode as exemplified by experiment 0 disclosed herein. Furthermore, it can be seen that the rate performance of UM2C63 is actually marginally better than the control UM2C52 cell: the 2C:C / 5 capacity ratio for the former was 88.9 %, whilst that for the latter was 88.6 %. This enhanced rate performance is a huge benefit over the conventional approach of adding sacrificial salts to cathode as exemplified by experiment 0 as disclosed herein. This is also apparent in the faster rate ±1C cycling: the D11 capacity enhancement is +4.4 mAh / g (or +4.7 %) and +0.3 mAh (or +6.7 %) for the TEL 25e-based UM2C63, relative to the TEL 0-based UM2C52. As shown by Figure 4 and Figure 5, both cells demonstrated similar cycling stability over 450 cycles: in fact, after 450 cycles, not only were the cycling stabilities identical, but favourably, UM2C63 could deliver higher specific capacity (82.9 mAh / g vs 81.2 mAh / g) and higher cell capacity (4.05 mAh vs 3.86 mAh) relative to UM2C52. Conclusion: Experiment 3 shows that cycling performance using one or more sacrificial salts in a non- aqueous electrolyte is improved when compared to the prior art approach of using one or more sacrificial salts contained in the cathode. The former can not only lead to a capacity boost in terms of mAh / g, but also crucially, in terms of the total cell capacity (in mAh) – the latter cannot show capacity boost in terms of total cell capacity, on account of the lower content of the cathode active material in the cathode. Furthermore, the former approach actually leads to similar rate performance and long-term cycling stability: the latter approach significantly underperformed in both metrics. EXPERIMENT 4: Demonstration in Alternate Solvent Mix The use of one or more sacrificial salts in a non-aqueous electrolyte was investigated further in a different electrolyte composition. In particular, this was comprised of a carbonate-ester solvent (PC) and a non-carbonate-ester solvent (the hydrofluoroether 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, also known as ‘HFE-458’ or sometimes as ‘TTE’ in the prior2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES art). Table 5 (as well as Table 1) lists the control electrolyte, and the sacrificial salt-based electrolyte according to the present invention: TABLE 5 Electrolyte Composition TEL 42d (Control) 1 m NaPF6 in (PC with 20 wt% HFE-458, 1 wt% PCS, 1 wt% P123 and 1 wt% TMSB additives) TEL 45c TEL 42d with 0.05 m NaSCN The above two electrolytes were used in ~6 mAh Na-ion pouch cells where the cathode’s formulation was 92:04:04 and the anode’s formulation was 92:3.5:1.5 wt / wt. These cells were cycled as follows: Cycles 1 – 2: 4.2 – 1 V at ±C / 10 Cycles 3 – 10: 4.1 – 1.8 V at +C / 5 charge; discharge for two cycles each at C / 5, C / 2, 1C and 2C. Cycles 11 onwards: 4.1 – 1.8 V at ±1C. The relevant metrics for this experiments are shown in Table 6. TABLE 6 Cell Electrolyte C / A D1 FCE 2C:C / 5 D11 mAh / g; mAh / g; (%) Ratio (%) D11 mAh D1 mAh UM2C67 TEL 42d 2.42 126.9; 6.1 87.8 88.7 93.8; 4.54 UM2C88 TEL 45c 2.47 129.4; 6.4 85.8 89.3 98.2; 4.81 From Table 6, it can be seen that UM2C88, using sacrificial salt-based TEL 45c in accordance with the present invention, delivered not only higher D1 in mAh / gcathode active(+2.5 mAh / g), but also the total cell capacity in mAh (+0.3 mAh or +4.9 % of capacity boost), relative to control TEL 42d-based UM2C67. These conclusions are in line with those of Experiment 3 which demonstrates the use of compositions according to the present invention which do not contain HFE-458.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Furthermore, it can also be seen that the rate performance of UM2C88 is actually marginally better than the control UM2C67 cell: the 2C:C / 5 capacity ratio for the former was 89.3 %, whilst that for the latter was 88.7 %. This better rate performance is also apparent in the faster ±1C cycling: the D11 capacity enhancement is +4.4 mAh / g (or +4.7 %) and +0.27 mAh (or +5.9 %) for the TEL 45c-based UM2C88, relative to the TEL 42d-based UM2C67. As shown by Figure 6 and Figure 7, both cells demonstrated similar cycling stability over 150 cycles: in fact, after 150 cycles, not only were the cycling stabilities identical, but favourably, UM2C88 could deliver higher specific capacity (93 mAh / g vs 90.3 mAh / g) and higher cell capacity (4.57 mAh vs 4.35 mAh) relative to UM2C67. Conclusion: Experiment 4 shows that this invention can be compatible with different salt-solvent combinations. Importantly, Experiment 4 shows that the sacrificial salt in electrolyte based approach can show good performance even with non-carbonate-ester-based electrolytes such as a hydrofluoroether (e.g. HFE-458). FURTHER ANALYSIS OF RESULTS FROM EXPERIMENT 1 Table 7 below shows similar data as Table 2 of experiment 1. However, it further includes values for the expected boost in capacity based on the actual amount of sacrificial salt (NaSCN) content in the electrolyte, which was actually added to the cell. In Table 7 below: The ‘Electrolyte Added’ is the actual value of electrolyte which was added to the cell. Whilst this value should have been almost the same for each experiment (as the same electrolyte loading was used here), the slight discrepancies were within what would be expected from human error. Besides, this value (which was recorded) has been used for the actual calculations, thus ensuring accuracy of results. ‘D1 Boost from NaSCN (mAh / g)’, ‘D1 Boost vs TEL 0 (mAh / g)’ and ‘Unexpected mAh / g Boost’: the weight in grams accounts for the weight of the cathode active material. Furthermore, in these calculations, the FCE of that cell is also accounted for (for example, ‘D1 Boost from NaSCN (mAh / g)’ = FCE*’mAh Expected from NaSCN’ / (cathode active material wt in g)). TABLE 72024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Electrol mAh D1 Boost Unexpe NaSC D1 Boost yte expected from cted Cell Electrolyte C / A N wt in vs TEL 0 Added from NaSCN mAh / g cell (g) (mAh / g) (g) NaSCN (mAh / g) Boost APF *TEL 0 1.43 0.086 0 0.0000 0.000 0 C101 APF TEL 25 1.41 0.074 2.67 0.8819 9.64 9.9 0.26 C132 APF TEL 25b 1.39 0.091 1.67 0.5522 6.07 7 0.93 C136 APF TEL 25c 1.42 0.079 0.586 0.1939 2.27 4.9 2.63 C141 APF TEL 25d 1.39 0.08 0.298 0.0985 1.20 -0.4 4.9^ C150 APF TEL 25e 1.4 0.082 0.153 0.0506 0.71 4.1 3.40 C157 APF TEL 25h 1.42 0.076 0.114 0.0375 0.53 3.4 2.87 C209 APF TEL 25g 1.41 0.085 0.095 0.0315 0.44 1.6 1.16 C202 * = not according to the present invention. ^ = Based on D2 value (for this cell, D2 – D4 capacities were much higher than D1, thus, D2 – D4 capacities taken as representative. It can be seen from Table 7 that ALL cells which contain a non-aqueous electrolyte according to the invention (e.g. TEL 25, TEL 25b, TEL 25c, TEL 25d, TEL 25e, TEL 25h, TEL 25g), led to a higher capacity boost, than what would be predicted by just considering the amount of sacrificial salt added in the electrolyte of the cell. That is, when the one or more sacrificial sodium salts are present in a concentration from about 0.015 mol / kg to about 0.5 mol / kg. Furthermore, this capacity boost becomes disproportionately high at slower concentrations of the sacrificial salt (from 0.015 m to 0.1 m). Here, it should be stated that for APFC150 using TEL 25d, the capacity boost was seen in cycles 2 – 4 (D2 to D4) and also in the ±C / 2 cycling from cycle 5 onwards, proving that the unexpected capacity boost did in fact occur. The prior art does not teach nor suggest the use of one or more sacrificial salts in a non- aqueous electrolyte, let alone in the optimal concentrations as disclosed herein. The prior art2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES also does not mention the advantages in terms of a non-aqueous electrolyte according to the present invention providing electrochemical cells, particularly sodium-ion cells, with improved long term cycling and / or increased capacities. Therefore, the skilled would have no reason to consider the present invention without the benefit of hindsight or inventive capacity. EXPERIMENT 5: The cycling performance of further cells using an electrolyte composition of the present invention (i.e., TEL 79c, TEL 67Ra, TEL 62a, TEL 113a, TEL 147a, TEL 146a) Table 7 below sets out the electrochemical performance for sodium-ion cells using a layered oxide cathode / / hard carbon anode configuration and having a rated capacity of 6 – 10 mAh. These cells were fabricated in line with the general procedure to make a sodium-ion cell as discussed above. Table 7 shows the relevant electrochemical performance (the first discharge capacity of the cell) together with specific details of the testing conditions. TABLE 7 Example # Electrolyte Cell C / A; Voltage Cell Type D1 Capacity Sample Number Window; Cycling (mAh / g) Boost Rate (mAh / g) Comparative TEL 67a UM2C108 2.51; ±C / 10; 4.2 – 1 Na-ion 126.8 Example 5a V Cell Example 5a TEL 79c UM2C121 2.51; ±C / 10; 4.2 – 1 Na-ion 127.7 +0.9 V Cell Comparative TEL 67 UM2C95 2.42; ±C / 10; 4.2 – 1 Na-ion 125.2 Example 5b V Cell Example 5b TEL 67Ra UM2C72 2.42; ±C / 10; 4.2 – 1 Na-ion 128.6 +3.4 V Cell Comparative TEL 0c UM2C104 2.52; ±C / 10; 4.2 – 1 Na-ion 126.8 Example 5c V Cell Example 5c TEL 62a UM2C126 2.46; ±C / 10; 4.2 – 1 Na-ion 128.1 +1.3 V Cell Comparative TEL 114 UM2C124 2.46; ±C / 10; 4.2 – 1 Na-ion 127.6 Example 5d V Cell Example 5d TEL 113a UM2C122 2.47; ±C / 10; 4.2 – 1 Na-ion 133.4 +5.8 V Cell2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES Comparative TEL 147 F2400217 2.53; ±C / 10; 4.1 – 1 Na-ion 105.7 Example 5e V Cell Example 5e TEL 147a F2400215 2.53; ±C / 10; 4.1 – 1 Na-ion 108.1 +2.4 V Cell Comparative TEL 146 F2400216 2.54; ±C / 10; 4.1 – 1 Na-ion 108.1 Example 5f V Cell Example 5f TEL 146a F2400214 2.54; ±C / 10; 4.1 – 1 Na-ion 114.2 +6.1 V Cell It can be seen from Table 7 that in all Examples 5a to 5f, the use of a sacrificial salt-based electrolyte (TEL 79c, TEL 67Ra, TEL 62a, TEL 113a, TEL 147a, and TEL 146a) in a sodium- ion cell led to an enhancement in capacity compared to the use of a comparative composition (TEL 67a, TEL 67Ra, TEL 0c, TEL 114, TEL 147, and 146) in a sodium-ion cell, which was identical in every way other than the fact that the control electrolyte did not contain a sacrificial salt. In particular: Example 5a: shows a boost in cell capacity using the following: NaPF6 as the main non- sacrificial salt, NaSO3CF3 as the sacrificial salt and a PC-dominant electrolyte system with DEC as the other carbonate-based co-solvent. Figure 14 also shows dQ / dV plots of cells filled with a non-aqueous electrolyte composition according to the present invention (i.e., TEL 79c), and a control sample (TEL 67a). In particular, Figure 14 also shows the cathode’s differential capacity curve (dQ / dV). It can be seen that the sacrificial salt-containing electrolyte TEL 79c oxidizes around 4.05 V as gauged by the inflection points in the sacrificial salt containing electrolytes based 2E cells’ dQ / dV plots. Example 5b: shows a boost in cell capacity using the following: NaPF6as the main non- sacrificial salt, NaPO2F2as the sacrificial salt and a PC-dominant electrolyte system with DEC as the other carbonate-based co-solvent. Figures 12 and 13 also show dQ / dV plots of cells filled with a non-aqueous electrolyte composition according to the present invention (i.e., TEL 67Ra), and a control sample (TEL 67). In particular, Figures 12 and 13 also show the cathode’s differential capacity curve (dQ / dV), .2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES It can be seen that the sacrificial salt-containing electrolyte TEL 67Ra oxidizes around 4.05 V as gauged by the inflection points in the sacrificial salt containing electrolytes based 2E cells’ dQ / dV plots. Example 5c: shows a boost in cell capacity using the following: NaPF6as the main non- sacrificial salt, NaBPh4as the sacrificial salt and an (EC:DEC:PC = 1:2:1 wt / wt) solvent system as the electrolyte. Figures 10 and 11 also show dQ / dV plots of cells filled with a non-aqueous electrolyte composition according to the present invention (i.e., TEL 62a), and a control sample (TEL 0c). In particular, Figures 10 and 11 also show the cathode’s differential capacity curve (dQ / dV), It can be seen that the sacrificial salt-containing electrolyte TEL 62a oxidizes around 4.05 V to 4.2V as gauged by the inflection points in the sacrificial salt containing electrolytes based 2E cells’ dQ / dV plots. Example 5d: shows a boost in cell capacity using the following: NaFSI as the main non- sacrificial salt, NaSCN as the sacrificial salt and an (EC:DEC:PC = 1:2:1 wt / wt) solvent system as the electrolyte. Example 5e: shows a boost in cell capacity using the following: NaPF6 as the main salt, NaSCN as the sacrificial salt and a TEP+HFE+FEC-based solvent system as the electrolyte. Example 5f: shows that the sacrificial salt in electrolyte concept can show a boost in cell capacity using the following: NaFSI as the main salt, NaSCN as the sacrificial salt and a TEP+HFE+FEC-based solvent system as the electrolyte. EXPERIMENT 6: Further long-term cycling performance. This experiment provides long-term cycling data for Example 5a / Comparative Example 5a as discussed above in Experiment 5. The data is shown in Figure 8. In Figure 8, both cells UM2C108 (TEL 67a) and UM2C121 (TEL 79c) were cycled between 4.1 – 1 V at ±1C, after two cycles at 4.2 – 1 V at ±C / 10. Data over 1200 cycles has been shown. This experiment also provides long-term cycling data for Example 5f / Comparative Example 5f as discussed above in Experiment 5. The data is shown in Figure 9. In Figure 9, both cells2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES F2400216 (TEL 146) and F2400214 (TEL 146a) were cycled between 4.1 – 1 V at ±1C, after two cycles at 4.1 – 1 V at ±C / 10. Data over the first 100 cycles has been shown. Analysis With reference to Figure 8, it can be seen that cell UM2C121 utilising an electrolyte of the present invention (TEL 79c) demonstrated a cycle life of 1200 cycles to 20% capacity fade. In comparison, the comparative example cell UM2C108 utilising TEL 67a, showed a cycle life of just 885 cycles to 20% capacity fade. Thus, an electrolyte of the present invention (TEL 79c) resulted in a cycle life enhancement of 35.6% when used in Na-ion pouch cells, over another Na-ion pouch cell which did not use an electrolyte of the present invention (TEL 67a). With reference to Figure 9, it can be seen that cell F2400214 utilising the electrolyte of the present invention (TEL 146a) demonstrated a capacity retention of 93% in 100 cycles. In comparison, the comparative example cell F2400216 utilising TEL 146, showed a capacity retention of just 51.6% in 100 cycles . Thus, an electrolyte of the present invention (TEL 146a) resulted in a capacity retention enhancement of 80%, when used in Na-ion pouch cells, over another Na-ion pouch cell which did not use an electrolyte of the present invention (TEL 146). SUMMARY The experiments disclosed herein demonstrate that non-aqueous electrolyte compositions according to the present invention provide electrochemical cells, particularly sodium-ion cells, with a capacity boost. In some instances, this is not only in terms of mAh / g but also in terms of total cell capacity (mAh), with crucially, similar / better rate performance and also cycle lives. The experiments disclosed herein also demonstrate that non-aqueous electrolyte compositions according to the present invention result in better performance metrics over the conventional approach of using sacrificial salt in cathodes, which is the approach used in the prior art.

Claims

2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES CLAIMS 1. A non-aqueous electrolyte composition for an electrochemical cell comprising: a) a solvent system which comprises one or more organo carbonate-based solvents; b) one or more non-sacrificial sodium-containing salts; c) a presodiation reagent which comprises one or more sacrificial sodium salts in a concentration of less than 1.0 mol / kg; and in which the one or more sacrificial sodium salts irreversibly provide sodium-ions to the anode during charging of the electrochemical cell.

2. The non-aqueous electrolyte composition according to claim 1, in which the one or more sacrificial sodium salts comprise one or more anions which oxidise during charging of the electrochemical cell.

3. The non-aqueous electrolyte composition according to any one of claims 1 to 2, in which the one or more sacrificial sodium salts comprise one or more anions which contain at least one atom selected from the group consisting of sulfur atoms, nitrogen atoms, boron atoms, oxygen atoms, and fluorine atoms.

4. The non-aqueous electrolyte composition according to any one of claims 1 to 3, in which the one or more sacrificial sodium salts are selected from one or more compounds of the group consisting of thiocyanate-containing compounds, cyanate- containing compounds, sulfonate-containing compounds, sulfite-containing compounds, periodate-containing compounds, fluoro silicate-containing compounds, fluoro ferrate-containing compounds, alkoxide-containing compounds, boron- containing compounds, compounds of the formula NaSbmXx, thiolate-containing compounds, thiosulfate-containing compounds, cyanamide-containing compounds (including dicyanamide-containing compounds), azide-containing compounds, carboxylate-containing compounds, ketone-containing compounds (including rhodizonate-containing compounds), fluoro titanate-containing compounds, arene- containing compounds (including terephthalate-containing compounds), triflate- containing compounds, difluorophosphate-containing compounds and tetraphenylborate-containing compounds.2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES 5. The non-aqueous electrolyte composition according any one of claims 1 to 4, in which the one or more sacrificial sodium salts have the general formula NaXCN, where X is a group 16 element, preferably selected from oxygen and sulfur.

6. The non-aqueous electrolyte composition according to claim 5, in which the one or more sacrificial sodium salts comprise sodium thiocyanate (NaSCN).

7. The non-aqueous electrolyte composition according to any one of claims 1 to 6, in which a decomposition potential of the one or more sacrificial sodium salts is from about 2.75 V to about 4.3 V.

8. The non-aqueous electrolyte composition according to claim 4, in which the one or more sacrificial sodium salts comprise one or more of sodium triflate (NaSO3CF3), sodium difluorophosphate (NaPO2F2), and sodium tetraphenylborate (NaBPh4).

9. The non-aqueous electrolyte composition according to any one of claims 1 to 8, in which the one or more sacrificial sodium salts are present in a concentration of about 0.75 mol / kg or less.

10. The non-aqueous electrolyte composition according to claim 9, in which the one or more sacrificial sodium salts are present in a concentration of from about 0.015 mol / kg to about 0.5 mol / kg.

11. The non-aqueous electrolyte composition according to any one of claims 1 to 10, in which the solvent system comprises propylene carbonate (PC) and a further solvent selected from the group consisting of ethylene carbonate (EC), diethyl carbonate (DEC), and a hydrofluoroalkyl ether such as 1,1,2,2-Tetrafluoroethyl-2,2,3,3- tetrafluoropropyl ether (HFE).

12. The non-aqueous electrolyte composition according to claim any one of claims 1 to 11, in which the one or more non-sacrificial sodium-containing salts are selected from sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI).2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES 13. The non-aqueous electrolyte composition according to claim 12, in which the one or more non-sacrificial sodium-containing salts comprise sodium hexafluorophosphate (NaPF6) or sodium bis(fluorosulfonyl)imide (NaFSI).

14. The non-aqueous electrolyte composition according to any one of claims 1 to 13, in which the one or more non-sacrificial sodium-containing salts do not substantially decompose during charging of the electrochemical cell at a voltage from 1 V to 4.3 V.

15. The non-aqueous electrolyte composition according to any one of claims 1 to 13, in which the one or more sacrificial sodium salts do substantially decompose during charging of the electrochemical cell at a voltage from 1 V to 4.3 V.

16. The non-aqueous electrolyte composition according to any one of claims 1 to 15, in which the solvent system further includes one or more performance additives selected from one or more surfactants, one or more boron-containing compounds, and one or more sulfur-containing compounds.

17. An electrochemical cell comprising a non-aqueous electrolyte composition according to any one of claims 1 to 16.

18. An electrochemical cell which has been presodiated using a non-aqueous electrolyte composition according to any one of claims 1 to 16.

19. The electrochemical cell according to claim 17 or claim 18, in which the electrochemical cell is a sodium-ion cell.

20. An apparatus comprising an electrochemical cell according to any one of claims 17 to 19.

21. The use of a non-aqueous electrolyte composition according to any one of claims 1 to 16, in an electrochemical cell.

22. The use of a non-aqueous electrolyte composition according to any one of claims 1 to 16, to presodiate an electrochemical cell.

23. A process for forming an electrochemical cell comprising:2024.10.03 SPECIFICATION AS FILED 1121-10055WO ELECTROLYTES (a) providing an electrochemical cell with a non-aqueous electrolyte composition according any one of claims 1 to 16; and (b) charging the electrochemical cell to a voltage of ≤ 4.5V.

24. The process according to claim 23, in which the electrochemical is charged to a voltage of greater than 3V.

25. The process according to claim 23, in which the electrochemical is charged to a voltage of greater than 3.5 V.

26. The process according to claim 23, in which the electrochemical is charged to a voltage of greater than 3.8 V.

27. The process according to claim 23, in which the electrochemical is charged to a voltage of greater than 4 V.

28. An electrochemical cell prepared by the process according to any one of claim 23 to 27.

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