WO2018151674A1

WO2018151674A1 - In-situ sodium plated battery

Info

Publication number: WO2018151674A1
Application number: PCT/SG2018/050065
Authority: WO
Inventors: Balaya PALANI; Ashish RUDOLA
Original assignee: National University Of Singapore
Priority date: 2017-02-14
Filing date: 2018-02-13
Publication date: 2018-08-23

Abstract

Disclosed herein is an in-situ sodium plated battery where the anode is simply an anode current collector that is not coated with an active material and the electrolyte contains a glyme solvent.

Description

In-situ Sodium Plated Battery

Field of Invention This invention relates to sodium-based batteries and in particular batteries with current collector foil/sheet that acts as the anode.

Background The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

The most advanced room-temperature rechargeable battery technology, the lithium-ion battery (LIB), relies on the scarce elemental resources of Li. This may not be attractive from a future sustainability and economics point of view. From future supply-and-demand perspective, high energy density batteries based on globally abundant sodium resources, also present in unlimited quantities in seawater, would be attractive to potentially keep costs low. Unfortunately, the sodium analogue to LIBs, sodium-ion batteries (NIBs), currently does not have high energy densities, implying they might be attractive for large-scale grid storage applications and perhaps not for electric vehicles/consumer electronics.

A rechargeable NIB consists of a cathode as the positive (higher potential) electrode and an anode as the negative (lower potential) electrode, separated by a porous polymer membrane, which physically prevents contact between the cathode and anode. The whole arrangement is immersed in an electrolyte solution; the separator allows electrolyte flow and ionic diffusion between cathode and anode. During charging, typically, sodium ions shuttle from the cathode and are inserted into the anode, while during discharging the sodium ions are extracted from the anode and are inserted back into the cathode, with the electrolyte serving as a medium to allow the sodium ions to travel to and fro between the two electrodes. Simultaneously, the electrons move though the external circuit, doing useful work. To achieve high sodium storage capacities, the cathode should supply as much sodium as possible during charging and the anode should correspondingly be able to store this sodium. The overall gravimetric energy density of the sodium-ion battery is estimated from the sodium-ion insertion/extraction potential difference between cathode and anode, storage capacities of cathode and anode, and the overall weight of cathode and anode materials as well as the weight of separator, electrolyte, can and cap of the container. Typically, the cathode and anode consist of an "active material" (AM) that is capable of storing sodium. This AM may be mixed with electronic conductive additives and/or binder material and coated on a current collector foil (such as aluminium, copper or stainless steel) to finally form an electrode. Therefore, the cathode AM must store sodium at as high potential as possible, while the anode AM must store sodium at as low potential as possible, to provide a larger potential difference for the sodium storage in order to boost the energy density of a battery. Since there can be no potential lower than that of the Na/Na+ redox potential for a sodium based battery (0 V vs Na/Na+), the ideal anode for sodium storage would be Na metal itself, from a strictly voltage perspective. Furthermore, sodium metal possesses a high gravimetric storage capacity (1166 mAh/g), hence, use of sodium metal as the anode would certainly improve the gravimetric specific energy density of a NIB full cell (for the same cathode and electrolyte) as compared to an NIB which uses Hard Carbon anode, which is the state-of-the-art NIB anode of today as Hard Carbon's gravimetric specific capacity is typically limited to around or under 300 mAh/g at a higher average potential as compared with that of sodium metal. However, until very recently, reversible sodium storage in rechargeable sodium batteries using Na metal as the anode over many cycles (as would be required from a practical viewpoint) was not shown to be practical. This is because it is commonly known in the field that Na metal exhibits dendrite formation with repeated Na plating and stripping processes; these dendrites grow with each charge- discharge cycle and eventually pierce through the separator layer (porous polymer membrane separating the cathode and anode), thus causing an internal short circuit. This is extremely undesirable as it can lead to explosions. By switching from traditional non-aqueous electrolytes based on carbonate solvents to glyme-based solvents (Z. W. Seh et al., ACS Central Science, 2015, 1 , 449-455; L. Schafzhal et al., ChemSusChem, 2017, 10, 401-408; R. Cao et al., Nano Energy, 2016, 30, 825-830), or NaAICI₄.S0₂-based electrolytes (J. Song et al., ACS Applied Materials & Interfaces, 2015, 7, 27206-27214), it has been shown that reversible and non-dendritic Na plating/stripping processes over many charge-discharge cycles can be achieved in "sodium batteries", meaning batteries where sodium metal is employed as the reference as well as the counter electrode (with a suitable AM coated on Al foil as the working electrode). It is believed that these proposed solutions provide non-dendritic Na metal cycling by the formation of a more inorganic rich passivation layer on Na, also called the solid electrolyte interphase (SEI). As previously mentioned, a non-aqueous "sodium-ion battery" (NIB) consists of an AM layer coated on a current collector foil to form a cathode and a similar arrangement exists for a corresponding anode. The cathode and anode are physically separated by a separator which allows for a flow of ions from the non-aqueous liquid electrolyte, which is present uniformly within the cell and wets the entire cathode, anode and separator. Hence, during charging in NIBs, sodium ions shuttle from the cathode AM and are inserted into the anode AM (electrons flow through the external circuit) and the reverse process occurs during discharging (sodium ions are extracted from the anode AM and are inserted into the cathode AM with the electrons flowing through the external circuit, doing the useful work). In contrast, for "sodium batteries", the anode is necessarily composed of sodium metal (either as a standalone Na metal foil/sheet or as sodium metal foil/sheet laminated upon another current collector foil sheet) with the cathode, separator and electrolyte being the same as that used in the above NIB. Hence, in sodium batteries, during each charge/discharge cycle, Na plating/stripping occurs on the Na metal foil (the cathode behaves in the same manner as that in NIBs).

It is noted that the Seh, Schafzhal and Cao papers (ibid) use a half-cell arrangement to demonstrate the majority of their results, where sodium metal is provided as the counter and reference electrode to a copper foil working electrode (half-cell arrangement). Obviously, the Cu//Na cell demonstrated in these papers cannot be actually used for practical purposes as for both electrodes, Na plating/stripping occurs, which means that the effective voltage of the cell will be 0 V, which is useless for practical application. Similarly, the Song paper (ibid) also uses sodium half-cells or sodium symmetric cells (Na//Na cells, where again Na plating/stripping will occur at both electrodes with an effective average voltage of 0 V). While Song did not show any "full cell", Seh disclosed a Na-S battery by way of demonstrating full cell results, where Na metal was used as a counter and reference electrode and a sulphur AM layer coated on an aluminium current collector foil was used as the cathode. In addition, Schafzhal and Cao both demonstrated sodium batteries with Na metal as the counter and reference electrodes and a Na₃V₂(P0₄)3 AM layer coated on an aluminium current collector foil as the cathode. However, as will be appreciated, for practical sodium-based batteries, Na metal as the anode (i.e., a "sodium battery" as defined in the previous paragraphs) may not be the most ideal candidate, especially from a battery production point of view. Currently, it is a common practice to fabricate the electrodes of non-aqueous batteries, such as lithium ion batteries (LIBs) and NIBs, in dry room conditions with less relative humidity (<5-20%). Na is an extremely reactive metal, spontaneously forming sodium oxide in air. Also, Na metal violently reacts with water and such a reaction could lead to an explosion. Hence, the use of Na metal as an anode in a sodium battery would necessitate the use of an inert atmosphere during the electrode manufacturing process (for commercial NIBs/LIBs, these steps, as mentioned previously, can be done in dry room atmosphere because an inert atmosphere is not required, as the cathode and anode AMs used in LIBs/NIBs are not as air and moisture sensitive as Na metal). This would be expected to significantly raise battery manufacturing costs.

As mentioned above, NIBs cannot currently compete with existing commercial LIBs in terms of gravimetric energy density. However, sodium batteries (using Na metal as the anode) could have higher energy densities than existing commercial LIBs, but these would currently be expensive/more cumbersome to manufacture due to the reasons mentioned above. Thus, there remains a need to devise a room temperature rechargeable sodium based battery which can simultaneously achieve specific energy densities at par or better than existing commercial LIBs while also being inexpensive and easy to manufacture. Recently, Cohn et al. (A. P. Cohn, et al., Nano Lett., 2017, 17, 1296-1301) demonstrated an anode-free sodium battery (AFNB), also called an in-situ sodium plated battery (INPB), using carbon nucleation layer coated on Al current collector as anode, 1 M NaPF₆ in diglyme as electrolyte and pre-sodiated FeS₂ as cathode. They showed over 40 charge/discharge cycles with some of the cycles displaying a coloumbic efficiency of only around 75-80%. It is noted that pre-sodiation is an expensive and difficult process to conduct. This is because such pre-sodiation approaches would be commercially cumbersome to implement as they typically involve firstly fabricating a Na half-cell with the cathode, cycling such a half-cell to pre-sodiate the cathode, subsequent disassembly of that half-cell and then fabrication of a full cell (the desired INPB) with the resulting pre-sodiated cathode. Naturally, such extra steps, all requiring an inert atmosphere (except for the cycling step, which can be conducted in ambient air as the fabricated half-cell would have been hermetically sealed in the inert atmosphere; if the half-cell is not hermetically sealed in inert atmosphere, then this cycling step will also need to be conducted in inert atmosphere), would increase costs, undermining the purpose of using sodium based batteries in the first place. Furthermore, the use of a carbon nucleation AM layer coated on Al current collector is also an extra step and would incur costs associated with material synthesis/purchase and electrode fabrication of this AM layer on the anode current collector. Therefore, the commercial utility of this disclosure is doubtful.

As such, there remains a need for new in-situ sodium plated battery systems that display improved performance (high specific energy densities and high coulombic efficiencies), uses cheap materials and does not employ any pre-sodiation/pre-cycling steps. If such an INPB can be demonstrated, it could result in a low-cost and high energy density battery technology which could be used in various applications, all the while being based on abundant Na resources. Summary of Invention

The battery concept described herein does not suffer from any of the above limitations. By simply using a current collector as an anode without any active material and relying only on the cathode to supply sodium (all existing commercial LIBs and NIBs already rely on the cathode to supply lithium/sodium), the battery manufacturing process is expected to be greatly simplified. The manufacturing costs of the battery are also expected to be much lesser as there would be no costs associated with the synthesis of anode active material, binder, conductive carbon etc, as well as anode processing costs. Furthermore, the energy density of such an anode-free sodium battery (AFNB), also called an in-situ sodium plated battery (INPB), is expected to be higher with respect to that of an analogous sodium metal based battery or NIB (if paired with the same cathode), assuming 100% coulombic efficiency (ratio of discharge to charge capacity) for the batteries considered. When considering the more expensive manufacturing costs for the Na metal based battery and/or NIB, an analogous AFNB/INPB would lead to significantly lower costs of storing energy per weight or volume of the battery.

Thus, this invention details a new rechargeable sodium-based battery concept with an operating principle different from that of the promising rechargeable sodium-ion batteries (NIBs). Traditionally, NIBs have an "active material" that partakes in the actual sodium storage on the (high potential) cathode as well as on the (low potential) anode. For rechargeable NIBs to display long cycle life, both the cathode and anode active materials should reversibly accept and then release the sodium ions at each charge/discharge cycle with preferably 100% efficiency. In such NIBs, the sodium source in an overwhelming majority of cases comes from the cathode active material only while the anode serves as a sink for this sodium. In this invention, we disclose a Na based rechargeable battery with the active material present only at the cathode and without any active material at the anode; the anode simply consists of a current collector foil/sheet. We show that the mechanism of sodium storage in such an anode-free sodium battery (AFNB) also called an in-situ sodium plated battery (INPB) is of repeated sodium plating and stripping on the current collector foil/sheet during each charge and discharge cycle, respectively. This reliable and non- dendritic Na plating-stripping process is caused by the choice of electrolyte and current collector used. We shall reveal various such details pertaining to this invention and also demonstrate an example of an AFNB/INPB with a selected cathode, electrolyte and current collector foil/sheet serving as the anode leading to a high energy density and inexpensive AFNB/INPB with a high degree of safety. In fact, we demonstrate an AFNB/INPB whose gravimetric specific energy density is greater or on par with existing commercial LIBs of today, without any pre-sodiation/pre-cycling approaches. It is believed that this is a commercially feasible battery and it is expected to provide low-cost and high energy density batteries based on the abundant sodium resources.

Aspects and embodiments of the current invention are provided in the numbered clauses below.

1. An in-situ sodium plated battery, comprising:

a cathode comprising a sodium containing active material;

an anode current collector;

a separator located between the cathode and anode current collector; and

an electrolyte, wherein

the anode current collector is substantially unmodified and substantially uncoated and the electrolyte comprises a salt and a glyme solvent.

2. The battery according to Clause 1 , wherein the salt is selected from one or more of the group consisting of NaBF₄, NaPF₆, NaAICI₄.2S0₂, NaCN, NaCI0₄, NaAsF₆,

NaPF₆._x(C_nF₂n₊i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀CI₁₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate.

3. The battery according to Clause 2, wherein the salt is selected from one or more of the group consisting of NaBF₄, NaPF₆, NaCN, NaCI0₄, NaAsF₆, NaPF₆._x(CnF_2n+i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀Cli ₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n- C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate, optionally wherein the salt is selected from one or more of the group consisting of NaBF₄ and NaPF₆.

4. The battery according to Clause 3, wherein the salt is NaBF₄. 5. The battery according to any one of Clauses 2 to 4, wherein each of the one or more salts of any one of Clauses 2 to 4, when present, are provided in a concentration of greater than 0 to 2.5 M.

6. The battery according to Clause 4, wherein the NaBF₄ is provided in a concentration of about 1.0 M.

7. The battery according to any one of the preceding clauses, wherein the glyme solvent is selected from one or more of the group consisting of ethylene glycol dimetheyl ether (monoglyme), diglyme, triglyme, tetraglyme, methyl nonafluorobutyl ether (MFE) and analogues thereof.

8. The battery according to Clause 7, wherein the glyme solvent is tetraglyme.

9. The battery according to any one of the preceding clauses, wherein the glyme solvent further comprises one or more solvents selected from the group consisting of a cyclic carbonate, a linear carbonate, a cyclic ester, a linear ester, a cyclic or linear ether other than a glyme, a nitrile, dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof.

10. The battery according to Clause 9, wherein the glyme solvent further comprises one or more of the group selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, sulfolane, and acetonitrile.

1 1 . The battery according to any one of the preceding clauses, wherein the cathode comprises an active material selected from one or more of the group consisting of M- Na₂Fe₂(CN)₆.2H₂0, R-Na₂Fe₂(CN)₆, NVP, Na_a[Cu_bFe_cMn_dNi_eTi_fMg]0₂, Na₄Mn₃(P0₄)₂(P₂0₇), M-Na₂Fe₂(CN)₆.2H₂0, and Na₄Mn₃(P0₄)₂(P₂0₇), where: 0≤ a≤ 1 ; 0≤ b≤ 0.3; 0≤ c≤ 0.5; 0 ≤ d≤ 0.6; 0≤ e≤ 0.3; 0≤ f ≤ 0.2; and 0≤ g≤ 0.4, and M is selected from one or more of the group consisting of Mo, Zn, Mg, Cr, Co, Zr, Al, Ca, K, Sr, Li, H, Sn, Te, Sb, Nb, Sc, Rb, Cs, and Na. 12. The battery according to Clause 1 1 , wherein the cathode comprises the active material R-Na₂Fe₂(CN)₆. 13. The battery according to any one of the preceding clauses, wherein the electrolyte further comprises an additive selected from one or more of the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and adiponitrile.

14. The battery according to any one of the preceding clauses, wherein the anode is in the form of a foil and/or a sheet.

15. The battery according to any one of the preceding clauses, wherein the anode is selected from graphite, tin, antimony, tin/antimony composites in any weight ratio or more particularly, copper, stainless steel, aluminium, molybdenum, carbon cloth, nickel, zinc, magnesium, silver, gold, platinum, palladium and tungsten.

16. The battery according to any one of the preceding clauses, wherein the cathode is not pre-sodiated.

17. A battery according to any one of the preceding clauses, wherein the battery is provided after a formation cycle, wherein the formation cycle comprises performing from 1 to 200 charge discharge cycles, where:

(a) for each cycle, the charge provided is from 20 to 80% of the maximum upper cut-off voltage for the battery and the battery is then discharged fully to the normal full range lower cut-off voltage; and/or

(b) for each cycle, the charge rate is from C/20 to 4C and the discharge rate is from C/10 to 20C, provided that the charge rate is slower than the discharge rate.

18. A method of operating an in-situ sodium plated battery according to any one of the preceding clauses, wherein the charge rate for the battery is at a slower rate than the discharging rate. 19. The method of Clause 18, wherein the charge rate is from C/20 to 4C and the discharge rate is from C/10 to 20C.

Drawings Fig. 1 Depicts Cyclic Voltammetry curves of a Na half-cell with a Cu current collector as the working electrode in 1 M NaBF₄ in tetraglyme. Fig. 2 Depicts Na plating and stripping characteristics of a Na half-cell with a Cu current collector as the working electrode: (a) coulombic efficiency vs cycle number for 400 cycles for the half-cell in 1 M NaBF₄ in tetraglyme; (b) cycling profiles of the 1st, 2nd, 10th, 100th and 400th Na plating-stripping cycles on Cu in 1 M NaBF₄ in tetraglyme. Na was plated at 0.2 mA (0.1 mA/cm² current density) for 30 min; (c) Na plating and stripping on Cu in a carbonate-based electrolyte (1 M of NaCI0₄ in EC: PC, 1 : 1 v/v), at conditions similar to (a); and (d) effect of Na plating-stripping on Cu in 1 M NaBF₄ in tetraglyme, for higher areal capacities and higher current densities. Na was plated at 0.1 mA/cm² but for 5 h, resulting in a total plated sodium areal capacity of 0.5 mAh/cm². Na was stripped at a much higher current density of 0.5 mA/cm².

Fig. 3 Depicts Na plating and stripping characteristics of Na half-cells with either an aluminium foil or stainless steel (SS) foil as current collectors as the working electrode for 6 cycles in 1 M NaBF₄ in tetraglyme. Na was plated at 0.2 mA (0.1 mA/cm² current density) for 30 min and Na was stripped at 0.2 mA (0.1 mA/cm² current density).

Fig. 4 Depicts FESEM-EDX characterisation of the Na deposits on Cu foil in a Na half-cell using 1 M NaBF₄ in tetraglyme as electrolyte: (a) and (c) FESEM images of the Na deposits showing uniform and large micrometric-sized spherical-type deposits; and (b) and (d) EDX spectra of the Na deposits.

Fig. 5 Depicts FESEM-EDX characterisation of the Na deposit on Cu foil in a Na half-cell using 1 M NaCI0₄ in EC: PC, 1 : 1 v/v, as the electrolyte: (a) FESEM image of the Na deposits showing non-uniform deposits with a tendency for vertical growth; and (b) EDX spectrum of the Na deposits.

Fig. 6 Depicts the performance of a cathode (R-Na₂Fe₂(CN)₆) and anode (Cu current collector) combination in a full cell, using 1 M NaBF₄ in tetraglyme with cycling (charging and discharging) at C/5 rate from 3.9 - 2.0 V: (a) a representative galvanostatic cycle of a R- Na₂Fe₂(CN)₆//Cu INPB. Part of the lower charge plateau of R-Na₂Fe₂(CN)₆ was used for coulombic inefficiency compensation, hence, the lower charge-discharge plateau is correspondingly shorter. Despite this, a very high energy density of 285 Wh/kg could be obtained; (b) long term cycling of the INPB at 17mA/g current density demonstrating the relative stable cycling over 60 cycles.

Fig. 7 Depicts the performance of a cathode (R-Na₂Fe₂(CN)₆) and anode (Cu current collector) combination in a full cell, using 1 M NaBF₄ in tetraglyme with a cycling protocol of slower charge/faster discharge and limited cut-off voltages in the initial cycles: (a) initial cycling within 3.25 V - 2.0 V voltage window; (b) cycling profiles of 2^nd and 100^th cycles within 3.9 - 2.0 V voltage window, after initial 3.25 V - 2.0 V cycling. Specific capacity on x- axis is based on cathode AM weight; (c) comparison of the specific energy densities of the INPB with reported NIB (obtained at coin-cell level) and commercial LIB full cell configurations; and (d) areal capacity and coulombic efficiency over 100 cycles of the INPB shown in (b).

Fig. 8 Depicts FESEM-EDX characterisation of the Na deposits on Cu current collector during the charge cycle of a R-Na₂Fe₂(CN)₆//Cu INPB using 1 M NaBF₄ in tetraglyme as the electrolyte: (a) FESEM image of the Na deposits revealing large micrometric-sized Na deposits; and (b) EDX spectrum of an isolated Na particle indicating Na plating had occurred with an inorganic rich SEI similar to that observed in a Cu//Na half-cell, as shown in Fig. 4b and d.

Description

It has been surprisingly found that an insitu sodium plated battery may be formed without the need of expensive production steps and/or materials. Thus, there is provided an insitu sodium plated battery, comprising:

a cathode comprising a sodium containing active material;

an anode current collector;

a separator located between the cathode and anode current collector; and

an electrolyte, wherein

When used herein, an "insitu sodium plated battery" refers to a battery wherein the working principle of the battery involves Na extraction/insertion from the cathode active material (AM) and Na plating/stripping at the anode current collector (e.g. in the form of a foil or sheet without active material on the anode current collector). This is different from a "sodium ion battery" (Na extraction/insertion at cathode AM and Na insertion/extraction at anode AM) or a "sodium battery" (Na extraction/insertion at cathode AM and Na plating/stripping at Na metal anode), which have different working mechanisms.

When used herein, "substantially unmodified and substantially uncoated" in relation to the anode current collector means that in a pristine state, the current collector is essentially the pure material from which it is formed from, barring minor impurities, meaning that the current collector is not coated with any conventional active materials, binders or the like. Following charge and discharge cycles, the current collector in a fully Na-stripped state may be identical to the pristine state or it may incorporate a coating substantially comprising inorganic-rich materials. This coating may be across the entire surface of the current collector or it may be a partial coating (e.g. a coating of some regions).

In embodiments herein, the word "comprising" may be interpreted as requiring the features mentioned, but not limiting the presence of other features. Alternatively, the word "comprising" may also relate to the situation where only the components/features listed are intended to be present (e.g. the word "comprising" may be replaced by the phrases "consists of" or "consists essentially of"). It is explicitly contemplated that both the broader and narrower interpretations can be applied to all aspects and embodiments of the present invention. In other words, the word "comprising" and synonyms thereof may be replaced by the phrase "consisting of" or the phrase "consists essentially of" or synonyms thereof and vice versa.

It will be appreciated that the in-situ sodium plated battery may be provided in any suitable configuration. Examples of suitable configurations include, but are not limited to, cylindrical cells, prismatic cells, button/coin cells, pouch cells and the like.

Advantages associated with the disclosed batteries are discussed hereinbelow.

As noted above, the in-situ sodium plated battery disclosed herein does not include an active material on the anode. As such, there are significant weight and volume savings due to the fact that no active material and associated conductive additive and binder (if any) exist on the anode. This could lead to a very significant boost in both gravimetric and volumetric energy/power densities for an in-situ sodium plated battery with respect to that of an analogous NIB or a Na metal based battery, with all other factors remaining the same.

Since no anode active material is used, all the costs and manufacturing steps associated with the formation of a traditional anode are eliminated. This will also make the battery manufacturing process simpler and faster, further reducing costs.

The electrolytes disclosed herein for use in the in-situ sodium plated battery demonstrated herein are essentially non-flammable. This helps to alleviate concerns about the battery catching fire as traditionally, battery fires occur due to the flammable electrolytes used in them.

The cathode (and electrolyte) can be changed to yield in-situ sodium plated batteries of different energy/power/volumetric densities to suit any specific application. Examples of such changes are discussed in more detail below.

The batteries disclosed herein may have cycle lives of from 50 cycles to 50,000 charge/discharge cycles, such as from 100 cycles to 25,000 charge/discharge cycles, such as 300 cycles to 10,000 charge/discharge cycles. Additional suitable cycle lives may be from 50 to 5,000 charge/discharge cycles, such as from 100 cycles to 4,000 charge/discharge cycles, such as 300 cycles to 3,000 charge/discharge cycles. It will be appreciated that any of the low-end range numbers here (e.g. 50, 100, 300) may be combined with any of the higher range numbers (e.g. 3000, 4000, 5000, 10000, 25000, 50000) to provide additional preferred ranges. When used herein, "cycle life" refers to the cycle number whereby the cell can deliver 20 % of the capacities it could deliver in the initial cycles.

Based on the disclosures herein, it is believed that an in-situ sodium plated battery according to the current invention may be formed using any type of Na containing active material serving as the cathode, if such an active material has already been shown to function well in a sodium based battery or NIB. Hence, any such known Na containing cathode for application in in-situ sodium plated batteries is contemplated. For example, active materials that may be used in the cathode include, but are not limited to, Na_a[Cu Fe_cMn_dNi_eTi_fMg]02 (where: 0≤a≤1 ; 0≤b≤ 0.3; 0≤ c≤ 0.5; 0≤ d≤ 0.6; 0≤ e≤ 0.3; 0≤ f≤ 0.2; and 0≤ g≤ 0.4, and M is selected from one or more of the group consisting of Mo, Zn, Mg, Cr, Co, Zr, Al, Ca, K, Sr, Li, H, Sn, Te, Sb, Nb, Sc, Rb, Cs, and Na), or more particularly, M-Na₂Fe2(CN)₆.2H₂0; R-Na₂Fe₂(CN)₆, Na₃V₂(P0₄)3 (NVP), and Na₄Mn₃(P0₄)₂(P₂0₇). It will be appreciated that the above materials may be used individually. That is, a cathode may only contain one of the above active materials. However, it is also possible for a single cathode to contain more than one of the above materials in combination. Any suitable weight ratio may be used when the active materials above are used in combination. For example, the weight ratio for two active materials in a single cathode may range from 1 : 100 to 100: 1 , such as from 1 :50 to 50: 1 , for example 1 : 1.

For the R-Na₂Fe₂(CN)₆ cathode active material, both its 3.9 - 2.0 V cycling (two mole sodium storage per mole of R-Na₂Fe₂(CN)₆ resulting in 170.85 mAh/g theoretical capacity) and its 3.9 - 3.0 V cycling (i.e. cycling just within its upper charge-discharge plateaus) are intended to be covered herein by mention of this active material.

When the active material is chosen from the materials covered by the formula Na_a[Cu Fe_cMn_dNi_eTi_fMg]0₂, it will be appreciated that the values for a-g and M are chosen so as to maintain charge balance. Examples of active materials that fall within the formula Naa[Cu Fe_cMn_dNieTi_fMg]02 that may be mentioned herein include Na₀.9[Cuo.22Feo.3oMn₀.48]02 and Nao.9fCuo.12Nio.1₀Feo.₃₀Mno.4₃Tio.05 2. When Na₃V₂(P0₄)3 (NVP) is used as the positive active material, it may be used as undoped NVP or as doped NVP (maximum of 10 % of dopant(s)), where the dopant(s) may be selected from any suitable metal, such as one or more of the group including, but not limited to, Mg, Zn, Al, and the like. A particular NVP that may be mentioned herein is NVP doped with Zn. It is explicitly contemplated that the above-mentioned undoped NVP and doped NVPs may be used as the cathode active material wherever NVP is mentioned herein.

In particular embodiments of the invention that may be mentioned herein, the cathode is not pre-sodiated. The binder improves binding properties of the positive active material particles (e.g. M- Na₂Fe2(CN)6.2H₂0 or R-Na₂Fe₂(CN)₆) with one another and the current collector. The binder may be a non-aqueous binder, an aqueous binder, or a combination thereof. The binder is not particularly limited as long as it binds the positive active material and the conductive material on a current collector, and simultaneously (or concurrently) has oxidation resistance for high potential of a cathode and electrolyte stability.

Non-aqueous binders that may be mentioned herein include, but are not limited to, polyvinylchloride, carboxylated polyvinylchloride, polyvinylfluoride, an ethylene oxide- containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

Aqueous binders that may be mentioned herein include, but are not limited to, a rubber- based binder or a polymer resin binder. Rubber-based binders may be selected from a styrene-butadiene rubber, an acrylated styrene-butadiene rubber (SBR), an acrylonitrile- butadiene rubber, an acrylic rubber, a butyl rubber, a fluorine rubber, and a combination thereof. Polymer resin binders may be selected from ethylenepropylene copolymer, epichlorohydrin, polyphosphazene, polyacrylonitrile, polystyrene, ethylenepropylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, a polyester resin, an acrylic resin, a phenolic resin, an epoxy resin, polyvinyl alcohol and a combination thereof. A cellulose-based compound may be used as the binder (or in combination with other materials). Examples of suitable cellulose-based materials includes, but is not limited to, one or more of carboxyl methyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. Such a cellulose-based compound may be included in an amount of about 0.1 parts by weight to about 20 parts by weight based on 100 parts by weight of the active material. A particular cellulose-based binder that may be mentioned herein is the sodium salt of carboxyl methyl cellulose.

The conductive material improves conductivity of an electrode. Any electrically conductive material may be used as a conductive material, unless it causes a chemical change, and examples thereof may be natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber and/or like carbon-based material; copper, nickel, aluminum, silver, and/or like metal powder or metal fiber and/or like metal-based material; polyphenylene derivative and/or like conductive polymer; and/or a mixture thereof. Cathodes of the current invention may be manufactured using the following method. First, the active material(s), the conductive material, and the binder are mixed in a desirable ratio (e.g. active material(s):additive:binder ratio of from 70:20: 10 to 96:2:2, specific ratios that may be mentioned include, but are not limited to 85: 10:5 and 90:5:5) and dispersed in an aqueous solution and/or an organic solvent (such as N-methyl-2-pyrrolidone) to form a slurry. Additionally or alternatively, the amount of active substance in the cathodes may be from 70 to 96 wt%, the amount of additive (e.g. conductive carbon) may be from 2 to 20 wt% and the amount of binder may also be from 2 to 10 wt%. Subsequently, the slurry is coated on a current collector and then dried to form an active material layer. Herein, the coating method is not particularly limited, and may be, for example, a knife coating method (e.g. Doctor knife coating), a gravure coating method, and/or the like. Then, the active material layer is compressed utilizing a compressor (such as a roll press) to a desirable thickness to manufacture an electrode. A thickness of the active material layer is not particularly limited, and may be any suitable thickness that is applicable to a positive active material layer of a rechargeable lithium-ion or sodium-ion battery. The active material loading may be from 1 to 50 mg cm^"2, for example the active material loading may be from 5 to 40 mg cm^"2, such as from 8 to 30 mg cm^"2. As noted hereinbefore, the anode current collector is essentially provided in uncoated form, but may over many charge/discharge cycles form a substantially inorganic passivation layer across the whole or part of the anode current collector. The anode current collector may preferably be provided in the form of a foil and/or a sheet. Any suitable type of electronically conducting foil/sheet may serve as the 'anode' as long as it promotes stable and efficient Na plating-stripping. A suitable anode current collector material may be, but is not limited to, graphite, or more particularly, tin, antimony, tin/antimony composites in any weight ratio or more particularly, copper, stainless steel, aluminium, molybdenum, carbon cloth, nickel, zinc, magnesium, silver, gold, platinum, palladium and tungsten. In certain embodiments, metallic materials may be preferred for the anode current collector material.

The batteries disclosed herein also include a separator. The separator is not particularly limited, and may be any suitable separator utilized for a sodium-ion battery. For example, a porous layer or a nonwoven fabric showing excellent high rate discharge performance and/or the like may be utilized alone or as a mixture (e.g., in a laminated structure).

A substrate of the separator may include, for example, a polyolefin-based resin, a polyester- based resin, polyvinylidene difluoride (PVDF), a vinylidene difluoride-hexafluoropropylene copolymer, a vinylidene difluoride-perfluorovinylether copolymer, a vinylidene difluoride- tetrafluoroethylene copolymer, a vinylidene difluoride-trifluoroethylene copolymer, a vinylidene difluoride-fluoroethylene copolymer, a vinylidene difluoride-hexafluoroacetone copolymer, a vinylidene difluoride-ethylene copolymer, a vinylidene difluoride-propylene copolymer, a vinylidene difluoride-trifluoropropylene copolymer, a vinylidene difluoride- tetrafluoroethylene-hexafluoropropylene copolymer, a vinylidene difluoride-ethylene- tetrafluoroethylene copolymer, and/or the like. The polyolefin-based resin may be polyethylene, polypropylene, and/or the like; and the polyester-based resin may be polyethylene terephthalate, polybutylene terephthalate, and/or the like.

The porosity of the separator is not particularly limited, and may be any suitable porosity that a separator of a lithium-ion or sodium-ion battery may have.

The separator may include a coating layer including an inorganic filler may be formed on at least one side of the substrate. The inorganic filler may include Al₂0₃, Mg(OH)₂, Si0₂, and/or the like. The coating layer including the inorganic filler may inhibit direct contact between the positive electrode and the separator, inhibit oxidation and decomposition of an electrolyte on the surface of the positive electrode during storage at a high temperature, and suppress the generation of gas which is a decomposed product of the electrolyte. A suitable separator that may be mentioned herein is a glass fibre separator.

It will be appreciated that any of the above separators may be used in the aspects and embodiments of the current invention, provided that they are a technically sensible choice.

As noted herein, the in-situ sodium plated battery includes an electrolyte that comprises a salt and a glyme solvent. Any suitable salt may be used in electrolyte of the batteries disclosed herein. Suitable salts that may be mentioned herein include, but are not limited to one or more of NaBF₄, NaPF₆, NaAICI₄.2S0₂, NaCN, NaCI0₄, NaAsF₆, NaPF₆._x(C_nF₂n₊i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀CI₁₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate. For example, the salt(s) may be selected from one or more of the group consisting of NaBF₄, NaPF₆, NaCN, NaCI0₄, NaAsF₆, NaPF₆._x(C_nF_2n+i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀CI₁₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate. Particularly suitable salts that may be mentioned herein include, but are not limited to, NaBF₄ and NaPF₆. When any of these salts are included in the electrolyte composition, each salt may be provided in a suitable concentration. Suitable concentrations for each of these salts include, but are not limited to from greater than 0 to to 2.5 M, from 0.75 to 2.5 M, from 1.0 to 2.5 M, from 1.5 to 2.5 M, from 2.0 to 2.5 M, from 0.5 to 2.0 M, from 0.5 to 1.5 M, from 0.5 to 1.0 M, and from 0.5 to 0.75 M in the glyme solvent. For example, each salt (when present) may be present in a concentration of about 1.0 M in the glyme solvent. For example, a particular salt that may be present in the electrolyte is NaBF₄, which may be provided at a concentration in keeping with the values presented above (e.g. when NaBF₄ is present in the electrolyte, it may be provided at a concentration of about 1.0 M).

The glyme solvent may be selected from one or more of the group consisting of ethylene glycol dimetheyl ether (monoglyme), diglyme, triglyme, tetraglyme, methyl nonafluorobutyl ether (MFE) and analogues thereof. Analogues of tetraglyme (CH₃(0(CH2)2)₄0CH₃) that may be mentioned include, but are not limited to, compounds where one or both of its CH₃ end members may be modified to either -C₂H₅ or to -CH₂CH₂CI, or other similar substitutions. In certain embodiments of the invention that may be mentioned herein, the glyme solvent is tetraglyme.

While the glyme solvent may only contain a glyme-based solvent, it may in certain embodiments also contain an additional suitable solvent, that is a solvent compatible for use in a sodium-ion battery. Suitable solvents that may be mentioned herein include, but are not limited to one or more of a cyclic carbonate (such as propylene carbonate, ethylene carbonate, diethyl carbonate butylene carbonate, fluoroethylene carbonate, chloroethylene carbonate, vinylene carbonate, and/or the like), a linear carbonate (such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and the like), a cyclic ester (such as v- butyrolactone, γ-valerolactone, and the like), a linear ester (such as methyl formate, methyl acetate, methyl butyrate, and the like), a cyclic or linear ether other than a glyme (such as tetrahydrofuran (and derivatives thereof), 1 ,3-dioxane, 1 ,4-dioxane, 1 ,2-dimethoxy ethane, 1 ,4-dibutoxyethane, and the like), a nitrile (such as acetonitrile, benzonitrile, and/or the like), dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof. These solvents may be used in any suitable weight ratio with respect to the glyme solvent (e.g. tetraglyme). For example, the additional solvents may be selected from one or more of the group selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, sulfolane, and acetonitrile.

Particular solvent and salt combinations that may be mentioned herein include, but are not limited to X.NaBF₄ in tetraglyme, different concentrations (0 < X < 2.5 M, e.g. around 1.0 M), 1 M NaPF₆ in one or more of monoglyme, diglyme and tetraglyme or X. NaN(FS0₂)2 in monoglyme (0≤ X < 2.5 M).

The electrolyte may further include various suitable additives such as a negative electrode SEI (Solid Electrolyte Interface) forming agent, a surfactant, and/or the like. Such additives may be, for example, succinic anhydride, lithium bis(oxalato) borate, sodium bis(oxalato) borate, lithium tetrafluoroborate, a dinitrile compound, propane sultone, butane sultone, propene sultone, 3-sulfolene, a fluorinated allylether, a fluorinated acrylate, carbonates such as vinylene carbonate, vinyl ethylene carbonate and fluoroethylene carbonate and/or the like. The concentration of the additives may be any suitable one that is utilized in a sodium-ion battery. Particular additives that may be included in the electrolyte are those selected from one or more of the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and adiponitrile. The above additives may be present in any suitable weight ratio.

In an in-situ sodium plated battery, the separator may be disposed between the cathode and the anode current collector (i.e. anode) to manufacture an electrode structure, and the electrode structure is processed to have a desired shape, for example, a cylinder, a prismatic shape, a laminate shape, a button shape, and/or the like, and inserted into a container having the same shape. Then, the non-aqueous electrolyte is injected into the container, and the electrolyte is impregnated in the pores in the separator. Finally, the cell is hermetically sealed, thereby manufacturing an in-situ sodium plated battery.

As is normal, the batteries disclosed herein may require a formation cycle(s). That is, the batteries disclosed herein may need to undergo a number of initial charge/discharge cycles to prepare the battery for use to its full voltage potential. For the batteries disclosed herein, the battery may be cycled in such a way that, during the initial cycles, it is cycled within a narrower range than its full voltage range for a few cycles (such as from 1 to 200 cycles). In preferred embodiments, the battery may be cycled to a voltage value corresponding to anywhere within 20% - 80% of the value corresponding to its full range (maximum) upper cut-off voltage, during the charging cycle, and discharged fully to the normal full range lower cut-off voltage. Subsequently (after such an initial cycling step for 1 - 200 cycles), it may be cycled within its full voltage range. In an additional or alternative embodiment, the cycling procedure for both this initial cycling as well as the later subsequent (full range) cycling may entail the charging cycle to be conducted at a slower rate than the discharging cycle. In preferred embodiments, the charging rate may be anywhere between C/20 to 4C, while the discharging rate may be anywhere from C/10 to 20C.

Hereinafter, embodiments of the invention are illustrated in more detail with reference to the following examples. However, the present disclosure is not limited thereto. Furthermore, what is not described in this disclosure may be sufficiently understood by those who have knowledge in this field and will not be illustrated herein.

Examples

The INPB can be prepared as described below. The cathode consists of a sodium containing active material (AM) which may or may not be mixed with any type of conductive additive such as carbon black (to enhance the electronic conductivity of the AM phase) and/or a binder material (water or organic solvent soluble). This cathode AM mixture composition is then spread on an aluminium (or other metal) current collector to form the cathode, using the traditional electrode processing technique.

The anode is simply a thin metal foil such as a copper, stainless steel or any other metal foil (similar to the current collectors that are generally used in batteries currently) without any coating of AM. These cathode and anode (in this case metal foil) can then be fabricated into any type of battery configuration (cylindrical/button/coin/prismatic/pouch cells or of any other type of configuration) with a separator layer(s) in between with a non-aqueous electrolyte filled inside these sealed cells.

It can be seen that the methodology of fabricating such INPBs is exactly the same as current non-aqueous LIBs and NIBs with the added simplicity of not having any AM on the anode; the anode is simply a current collector which would have been used in a traditional LIB/NIB anode anyway. As such, an INPB will always be much lighter than an analogous NIB (for the same cathode and electrolyte used in both cases).

Synthesis of rhombohedral NapFepfCNOg R-NapFepfCNQfi)

R-Na₂Fe₂(CN)₆ was synthesised based on the conversion of monoclinic M- Na₂Fe₂(CN)₆.2H₂0 into R-Na₂Fe₂(CN)₆ by heating M-Na₂Fe₂(CN)₆.2H₂0 (in electrode form or otherwise) above 240 °C in Ar atmosphere, as described in PCT application No. PCT/SG2017/050203 and J. Electrochem. Soc, 2017, 164, A1098-A1 109.

Preparation of electrolytes

The solvents, ethylene carbonate (EC, Alfa Aesar), propylene carbonate (PC, Sigma Aldrich) and tetraethylene glycol dimethyl ether (tetraglyme,≥ 99%, Sigma Aldrich) were obtained from commercial sources and used without further purification. EC-PC (1 : 1 , v/v) mixture was prepared in-house accordingly. Similarly, the Na salts, NaCI0₄ (98+%, anhydrous, Alfa Aesar) and NaBF₄ (98%, Sigma Aldrich) were obtained from commercial sources and used without further purification.

The 1 M NaBF₄ in tetraglyme electrolyte was prepared by dissolving NaBF₄ salt in a required quantity of tetraglyme. The mixture was then stirred to fully dissolve the salt to obtain a clear transparent solution. Prior to the preparation of the electrolyte, the NaBF₄ salt was dried at 120 °C in vacuum for 18 h and the tetraglyme solvent was dried over molecular sieves for 24 h. Electrode preparation, cell assembly and electrochemical evaluation

2016-type (MTI Corporation) coin cells were used in all cycling experiments and all electrodes with cross-sectional area of 2 cm² were used. Copper (Cu) foil (10 μηι thick) was obtained from Eager Corporation (Japan) and used as-received without any modification or surface treatment. Whatman glass fiber separator (Grade GF/A) was used in all cycling experiments. For the Cu//Na half-cell cycling, the cells were initially cycled between 0.0-1.0 V vs Na/Na⁺ for 10 cycles at a very low 5 μΑ/cm² current density to remove any surface contaminants. The composition of the R-Na₂Fe₂(CN)₆ cathode consists of R-Na₂Fe₂(CN)₆ AM: Super P carbon black (Alfa Aesar): polyvinylidene fluoride (PVDF) binder (Kynar 2801) in a weight ratio 90:5:5. To obtain high areal capacities, the loading of R-Na₂Fe₂(CN)₆ in the electrode was 10.6 mg/cm² During the initial testing of the R-Na₂Fe₂(CN)₆//Cu full cell, the cell was cycled for 10 cycles at 5 μΑ/cm² between 1.0-2.0 V to simulate the surface contamination cleaning cycles that were applied for the half-cells. In Example 6, the cell was cycled at C/5 rate from 3.9 - 2.0 V. However, in Example 7, the cell was tested between 3.25 - 2.0 V for 20 cycles with charging at C/5 and discharging at 1C. Finally, the cell was placed for long term cycling between 3.9- 2.0 V.

Characterisation of the Na deposits on the Cu foil

For the Field Emission Scanning Microscopy-Energy Dispersive X-ray Analysis (FESEM- EDX) studies, the cell was opened in a glove box (H₂0 and 0₂ < 5.0 ppm) and the Cu current collector deposited with Na was washed thoroughly with either diglyme (if the electrolyte used was 1 M NaBF₄ in tetraglyme) or PC (if the electrolyte solution was 1 M NaCI0₄ in EC: PC), to remove residual electrolyte salt. The washed Cu current collector anode was then vacuum-dried and transported to the FESEM in Ar-filled containers.

Example 1

Cyclic voltammetry of Na half-cell with Cu current collector, using 1 M NaBF₄ in tetraglyme electrolyte To determine the reliability of the Na plating-stripping process on the Cu foil, Na half-cells with Cu current collector using the non-flammable 1 M NaBF₄ in tetraglyme electrolyte was fabricated. Cyclic voltammetry (CV) experiment revealed that Na plating and stripping had indeed occurred on Cu foil, as shown in the reversible reduction of Na⁺ to Na metal (Na plating) between 0.0 to -0.1 V vs Na/Na⁺ during the reduction cycle, and oxidation of Na metal to Na⁺ (Na stripping) around 0.10 V vs Na/Na⁺ during the oxidation cycle, respectively (Figure 1).

Example 2

Performance of a Na half-cell with a Cu current collector (as the working electrode) in different electrolytes

Figure 2a shows the Na plating-stripping process of such a half-cell over 400 cycles. It was observed that a stable coulombic efficiency close to 100 % was achieved throughout the cycling, which was indicative of very stable Na plating and stripping. In addition, the cycling curves of the Na plating-stripping process (as shown in Figure 2b) showed a minimal voltage efficiency or polarization (the difference between Na plating and stripping voltages) of about 20 mV. This suggests a potential for high Round Trip Energy Efficiency (RTEE) for the INPBs, provided that the RTEE of the cathode is also high (RTEE = coulombic efficiency x voltage efficiency).

In contrast, an identical cell arrangement using a carbonate-based electrolyte (1 M NaCI0₄ in EC: PC (1 : 1 v/v)) gave very poor Na plating-stripping characteristics for the same cycling protocol. Figure 2c shows that a maximum coulombic efficiency of 23% was achieved in the initial cycle and this dropped continuously to lower values in subsequent cycles. This was expected as the Na plating-stripping in typical carbonate-based electrolytes was known to be very unstable and highly inefficient, as observed by Seh et al. in ACS Cent. Sci., 2015, 1 , 449-455. It was observed that the current collector used as anode also played a significant role in achieving reliable and efficient Na plating/stripping process even with 1 M NaBF₄ in tetraglyme as the electrolyte (discussed further in Example 3).

On the other hand, the Na plating-stripping process in 1 M NaBF₄ in tetraglyme was very stable at higher current rates and for higher areal capacities. As shown in Figure 2d, Na stripping could be achieved with almost 100% coulombic efficiency at 1 mA or 0.5 mA/cm² (current rate 5 times higher than that shown in Fig. 2a and 2b) for over 50 cycles. Furthermore, the total areal capacity of 0.5 mAh/cm² indicates that INPBs using this electrolyte can also handle large amounts of Na deposition/stripping. The ability for the highly efficient and reversible Na plating/stripping process at high current densities and areal capacities suggests the possibility of high energy and power density INPBs, when paired with a suitable cathode.

Example 3

Performance of a Na half-cell with either a stainless steel or an aluminium current collector in 1 M NaBF₄ in tetraglyme

The cycling results mentioned in Example 2 were extended to the use of other metal current collectors as well in sodium half-cells, though the most efficient Na plating and stripping was achieved using Cu as the current collector. Na plating and stripping of Na half-cells with either an aluminium foil or stainless steel (SS) foil as current collectors (working electrode) were carried out for 6 cycles in 1 M NaBF₄ in tetraglyme. Na was plated at 0.2 mA (0.1 mA/cm² current density) for 30 min and Na was stripped at 0.2 mA (0.1 mA/cm² current density). The use of a stainless steel current collector resulted in a lower efficiency of about 97-98% (in comparison to the use of Cu as the current collector). In addition, the use of an aluminium current collector was also possible but this gave unstable cycling and coulombic efficiencies lower than 80%.

Example 4

FESEM-EDX characterisation of the Na deposits on the Cu current collector after cycling in different electrolytes

To investigate the nature of the Na deposits on the Cu current collector in 1 M NaBF₄ in tetraglyme electrolyte and in 1 M NaCI0₄ in EC: PC, the Na deposits on the Cu current collector used in each of the electrolytes were characterised by FESEM-EDX. Figures 4a and c show representative morphologies of the FESEM images of the Na deposits with 1 M NaBF₄ in tetraglyme electrolyte solution while Fig 5a shows that in 1 M NaCI0₄ in EC: PC solution, respectively. In the case of the former electrolyte, large micrometric-sized uniform spherical deposits were obtained (Figures 4a and c). In contrast, for the carbonate-based solution, highly non-uniform deposits with a preference for vertical growth (dendritic Na plating-stripping process) was observed (Figure 5a).

The non-uniform vertical growth of Na in the carbonate-based electrolyte may cause internal short-circuit as these pointed Na dendritic deposits may vertically grow with each cycle and could puncture the separator. However, for the 1 M NaBF₄ in tetraglyme, the highly uniform, smooth and spherical-type Na deposits should alleviate such internal short-circuit concerns as these large Na particles (≥ 1 μηι) would most probably not puncture the separator in actual INPBs, therefore suggesting prospects for safe and reliable cycling. This is supported by the result that this electrolyte solution can promote Na plating and stripping for over 400 cycles in a very efficient and stable manner (Figure 2a).

In addition, the representative EDX spectra indicated that these deposits were pure Na with low amount of organic material present in the solid electrolyte interface (SEI), as suggested by the low intensity of carbon (C) signal (Figure 4b and d). The F and O signals indicated that the SEI on Na may be dominated by Na₂0, NaOH and NaF, which was consistent with the inorganic rich SEI observed on Na metal counter electrodes in previous reports. In contrast, the EDX spectra on the Na deposits in the carbonate-based electrolyte (Figure 5b) revealed significant presence of C and CI, suggesting that the Na deposits were covered with organic-rich SEI due to substantial solvent decomposition.

Given the low coulombic efficiencies of around 16% obtained using such carbonate-based electrolyte (Figure 2c), it was deduced that significant electrolyte degradation occurred during each cycle, which led to the presence of CI in the Na deposits. The plated Na may be covered by unstable surface layers formed by electrolyte degradation.

Example 5

Quantification of Na on the Cu current collector by ICP-OES The amount of Na deposited on the Cu foil (in 1 M NaBF₄ in tetraglyme) was quantified by inductively coupled plasma optical emission spectroscopy (ICP-OES) and was found to be consistent with the calculated amount (Table 1). This further proved that Na metal deposition had occurred. Table 1. Measured Na values from ICP-OES for 0.5 mAh/cm² worth of plated Na on Cu current collector foil in a Na half-cell. Data from three different samples were obtained, with the expected quantity as indicated.

Measured Na (ppm)

Average Measured Na (ppm)

Expected (deviation from expected amount in

(deviation from expected amount Na (ppm) ppm)

in ppm)

Sample 1 Sample 2 Sample 3

85.78 84.67 82.98 80.19 82.61 (1 .1 1 ) (2.8) (5.59) (3.17)

Example 6

Performance of a cathode (R-Na₂Fe₂(CN)₆) and anode (Cu current collector) combination in a full cell, using 1 M NaBF₄ in tetraglyme electrolyte with cycling (charging and discharging) at C/5 rate from 3.9 - 2.0 V

To verify the performance of the INPB in a full cell using a Cu current collector as anode and 1 M NaBF₄ in tetraglyme as the electrolyte, the rhombohedral R3 phase of Na₂Fe₂(CN)₆ (abbreviated as R-Na₂Fe₂(CN)₆ henceforth) cathode was chosen as it delivers a high capacity (theoretical capacity of 170.85 mAh/g) with two charge-discharge plateaus centered at 3.1 and 3.3 V vs Na/Na⁺ (see International Patent Application No. PCT/SG2017/050203 and J. Electochem. Soc. 2017, 164, A1098-A1 109). As described in PCT/SG2017/050539, the lower charge plateau could be used for coulombic inefficiency compensation in NIBs in the initial cycles, with only the upper charge and discharge plateaus partaking in sodium storage in the subsequent cycles. This was applied in this case as well, since the initial coulombic efficiencies of Na plating stripping processes would typically increase from 87 % in the first cycle, to 94% in the second cycle before increasing above 99% within 10 cycles (as indicated by the half-cell cycling in Figure 2a).

After a few initial formation cycles, the representative galvanostatic (constant current) cycle of such an INPB was as shown in Figure 6a, with the charging and discharging cycle conducted at C/5 rate (17 mA/g) in accordance with one mole of sodium storage per mole of R-Na₂Fe₂(CN)₆ in 5 h (assuming theoretical capacity of 85.425 mAh/g is reached).

During the charge cycle of the INPB, Na was extracted from the R-Na₂Fe₂(CN)₆ cathode and plated onto the Cu current collector, while during the discharge cycle, Na was stripped from the Na deposits on the Cu current collector and inserted back into the cathode. It was noticed that part of the lower charge plateau was used up for the coulombic inefficiency compensation of the INPB in the initial cycles, resulting in the lower charge-discharge plateau (Figure 6a) to have a smaller capacity as compared to the theoretical capacity of 85.425 mAh/g). After the initial cycling, this R-Na₂Fe₂(CN)₆//Cu INPB was able to deliver a capacity of approximately 90 mAh/g at an average discharge voltage of 3.22 V. Due to the absence of any anode AM, the INPB shown in Figure 6a delivered a very high energy density of about 285 Wh/kg of cathode AM weight (the anode did not contain any AM, as made clear in the preceding sections). The cycling of such INPB was quite stable, displaying capacity retention of approximately 76% of its initial values after 60 cycles (Figure 6b).

Since the INPB is not specific to any particular cathode or current collector, further optimisation with careful selection of new cathode AM and/or current collectors and/or electrolyte will boost the energy density values. Further optimisation can also improve the cycling stability of a R-Na₂Fe₂(CN)₆//Cu INPB to the same level as that of R-Na₂Fe₂(CN)₆ in NIBs. The electrolyte of 1 M NaBF₄ in tetraglyme also has the advantage of being nonflammable, therefore making this INPB very attractive for many commercial applications.

Example 7

Performance of a cathode (R-Na₂Fe₂(CN)₆) and anode (Cu current collector) combination in a full cell, using 1 M NaBF₄ in tetraglyme electrolyte with a cycling protocol of slower charge/faster discharge and limited cut-off voltages in the initial cycles

In addition to Example 6, the charging cycle of the INPB was conducted at slow C/5 rate (0.18 mA/cm²), in accordance with one mole sodium storage per mole of cathode in 5 h assuming theoretical capacity of 85.4 mAh/g, with discharging cycle performed at 1 C (0.9 mA/cm²). The initial cycling of such a Na₂Fe₂(CN)₆//Cu INPB is shown in Figure 7a between 3.25 - 2V (within the lower charge-discharge plateaus of R-Na₂Fe₂(CN)₆). It can be seen that the lower coulombic efficiencies in the initial cycles led to a gradual decrease in capacity (of the lower charge-discharge plateaus). However, with each cycle in this initial cycling period, the coulombic efficiency improved. Finally, after these initial cycles and with the above- stated cycling protocol, the cycling curves of the R-Na₂Fe₂(CN)₆//Cu INPB obtained for the 2^nd and 100^th cycles are presented in Figure 7b. The R-Na₂Fe₂(CN)₆//Cu INPB could deliver an areal capacity of 1.14 mAh/cm² with specific energy density close to 336 Wh/kg based on the cathode AM weight (anode did not contain any AM) at an average voltage of 3.1 1 V.

In addition, the high specific energy density of this INPB full cell easily surpasses that of the best reported NIB full cells and is comparable with that of existing commercial LIB full cells (Figure 7c) (Yabuuchi, et al., Chem. Rev. 2014, 1 14, 1 1636; Cohn, et al., Nano Lett. 2017, 17, 1296; Wang, et al., J. Am. Chem. Soc. 2015, 137, Dugas, et al., J. Electrochem. Soc. 2016, 163, A867; Mu, et al., Adv. Mater. 2015, 27, 6928; Wang, et al., Adv. Mater. 2016, 28, 4126; David, et al., Handbook of Batteries, 2001). The capacity of both LiFeP0₄ and Li₄Ti₅0i2 is assumed as 170 mAh/g and the operating voltage is taken as 1.9 V. Hence, the estimated specific energy density is (170* 1.9)/2=161.5 Wh/kg. In compiling Figure 7c, full cells where cathodes or anodes were pre-sodiated/pre-cycled prior to full cell fabrication were omitted as this procedure unfairly boosts the specific energy density of full cells. Such pre-sodiation/cycling steps are also expensive and certainly cumbersome from commercial perspective.

Further, it was shown that the cycling of such INPB was quite stable, displaying capacity retention of 76 % of its initial values in 100 cycles with highly stable coulombic efficiency around 99-100 % after a few cycles (Figure 7d). The lower coulombic efficiencies in the initial 3.9 - 2.0 V cycles of the R-Na₂Fe₂(CN)₆//Cu INPB led to a gradual disappearance of the lower charge-discharge plateaus (see the profile of cycle 100 in Figure 7b) as sacrificial capacity loss. Despite this, the INPB could still deliver specific energy density of 238 Wh/kg even after 100 cycles. It is believed that the cycling protocol of slower charge/faster discharge and limited cut-off voltages in the initial cycles to compensate for lower initial coulombic efficiencies for the INPB concept may also work well with many other types of cathodes which display sloping potential profiles and/or profiles with two or more plateaus. Example 8

FESEM-EDX characterisation of the Na deposits on the Cu current collector of a R- Na₂Fe₂(CN)₆//Cu INPB using 1 M NaBF₄ in tetraglyme

Figure 8a shows a FESEM image of Na deposits on a Cu current collector during the charge cycle of a R-Na₂Fe₂(CN)₆//Cu INPB using 1 M NaBF₄ in tetraglyme after more than 100 cycles. 0.83 mAh/cm² worth of Na was plated at a current density of 0.2 mA/cm² corresponding to C/5 for one mole sodium storage per mole of cathode. The image shows large micrometric-sized Na deposits similar to that obtained in the Cu//Na half-cell in Figure 3a. The EDX spectrum of an isolated Na particle, as shown in Figure 8b, demonstrated that Na plating had occurred with an inorganic rich SEI, similar to that observed in the Cu//Na half-cell (Figure 4b and d). These results were obtained on a separate R-Na₂Fe₂(CN)₆//Cu INPB cell (different from that as shown in Figures 6 and 7). The cathode AM loading of this cell was 1 1.8 mg/cm².

Claims

1. An in-situ sodium plated battery, comprising:

a cathode comprising a sodium containing active material;

an anode current collector;

a separator located between the cathode and anode current collector; and

an electrolyte, wherein

2. The battery according to Claim 1 , wherein the salt is selected from one or more of the group consisting of NaBF₄, NaPF₆, NaAICI₄.2S0₂, NaCN, NaCI0₄, NaAsF₆, NaPF₆._x(C_nF₂n₊i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀CI₁₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n-C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N- phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate.

3. The battery according to Claim 2, wherein the salt is selected from one or more of the group consisting of NaBF₄, NaPF₆, NaCN, NaCI0₄, NaAsF₆, NaPF₆._x(CnF_2n+i)x (1 <x<6, n=1 or 2), NaSCN, NaBr, Nal, Na₂S0₄, Na₂B₁₀CI₁₀, NaCI, NaF, NaPF₄, NaOCN, Na(CF₃S0₃), NaN(CF₃S0₂)₂, NaN(FS0₂)₂, NaN(C₂F₅S0₂)₂, NaN(CF₃S0₂)(C₄F₉S0₂), NaC(CF₃S0₂)₃, NaC(C₂F₅S0₂)₃, (CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NCI0₄, (C₂H₅)₄NI, (C₃H₇)₄NBr, (n- C₄H₉)₄NCI0₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate, (C₂H₅)₄N-phtalate, sodium stearyl sulfonate, sodium octyl sulfonate, and sodium dodecylbenzene sulfonate, optionally wherein the salt is selected from one or more of the group consisting of NaBF₄ and NaPF₆.

4. The battery according to Claim 3, wherein the salt is NaBF₄.

5. The battery according to Claim 2, wherein each of the one or more salts of any one of Claims 2 to 4, when present, are provided in a concentration of greater than 0 to 2.5 M.

6. The battery according to Claim 4, wherein the NaBF₄ is provided in a concentration of about 1.0 M.

7. The battery according to Claim 1 , wherein the glyme solvent is selected from one or more of the group consisting of ethylene glycol dimetheyl ether (monoglyme), diglyme, triglyme, tetraglyme, methyl nonafluorobutyl ether (MFE) and analogues thereof.

8. The battery according to Claim 7, wherein the glyme solvent is tetraglyme.

9. The battery according to Claim 1 , wherein the glyme solvent further comprises one or more solvents selected from the group consisting of a cyclic carbonate, a linear carbonate, a cyclic ester, a linear ester, a cyclic or linear ether other than a glyme, a nitrile, dioxolane or a derivative thereof, ethylene sulfide, sulfolane, and sultone or a derivative thereof.

10. The battery according to Claim 9, wherein the glyme solvent further comprises one or more of the group selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, sulfolane, and acetonitrile.

11. The battery according to Claim 1 , wherein the cathode comprises an active material selected from one or more of the group consisting of M-Na₂Fe₂(CN)6.2H₂0, R-Na₂Fe₂(CN)₆, NVP, Na_a[Cu_bFe_cMn_dNi_eTi_fMg]0₂, Na₄Mn₃(P0₄)₂(P₂07), M-Na₂Fe₂(CN)₆.2H₂0, and Na₄Mn₃(P0₄)₂(P₂0₇), where: 0≤a≤ 1 ; 0≤b≤ 0.3; 0≤ c≤ 0.5; 0≤ d≤ 0.6; 0≤ e≤ 0.3; 0≤ f ≤ 0.2; and 0≤ g≤ 0.4, and M is selected from one or more of the group consisting of Mo, Zn, Mg, Cr, Co, Zr, Al, Ca, K, Sr, Li, H, Sn, Te, Sb, Nb, Sc, Rb, Cs, and Na.

12. The battery according to Claim 11 , wherein the cathode comprises the active material R-Na₂Fe₂(CN)₆.

13. The battery according to Claim 1 , wherein the electrolyte further comprises an additive selected from one or more of the group consisting of fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and adiponitrile.

14. The battery according to Claim 1 , wherein the anode is in the form of a foil and/or a sheet.

15. The battery according to Claim 1 , wherein the anode current collector material is selected from graphite, tin, antimony, tin/antimony composites in any weight ratio or more particularly, copper, stainless steel, aluminium, molybdenum, carbon cloth, nickel, zinc, magnesium, silver, gold, platinum, palladium and tungsten.

16. The battery according to Claim 1 , wherein the cathode is not pre-sodiated.

17. A battery according to Claim 1 , wherein the battery is provided after a formation cycle, wherein the formation cycle comprises performing from 1 to 200 charge discharge cycles, where:

18. A method of operating an in-situ sodium plated battery according to Claim 1 , wherein the charge rate for the battery is at a slower rate than the discharging rate.

19. The method of Claim 18, wherein the charge rate is from C/20 to 4C and the discharge rate is from C/10 to 20C.