WO2014018546A1

WO2014018546A1 - Hybrid energy storage devices having sodium

Info

Publication number: WO2014018546A1
Application number: PCT/US2013/051711
Authority: WO
Inventors: Xiaochuan Lu; Jin Yong Kim; Guosheng Li; John P. Lemmon; Vincent L. Sprenkle
Original assignee: Battelle Memorial Institute Inc
Current assignee: Battelle Memorial Institute Inc
Priority date: 2012-07-23
Filing date: 2013-07-23
Publication date: 2014-01-30
Anticipated expiration: 2015-01-23
Also published as: US9252461B2; CN104488131B; CN104488131A; AU2013293157A1; US20140023903A1; KR20150036573A; AU2013293157B2; CA2878177A1

Description

Hybrid Eaergy Storage Devkes Havisg Sodium

Priority

fOeillJ This n ent on claims priority from U.S. Provisional Patent Application No. 61 674,609, entitled Hybrid Hxergy Storage Devices Having Sodium, ii ed July 23, 2(112, and from U.S. Patent Application No. 13/94SJ57, entitled Hybrid Energy Storage Devices Having Sodium, filed Inly 23, 2013.

Staterae&i Rega in Federally S ons r d Resea ch Or Development

flM 12| This invention was made ith Government support under Contract

DE-AC0S76RLOi 83O awarded by the U.S. Department of Energy. Th Ckjverrrment has certain rights in the invention.

Backg und

|1NH 3| Sodin -beia alumina batteries (NBBs), based on a molten Na anode and β"~ A Oi solid electrolyte (BASE), have recently gained increasing interests as an ekctrieal energy storage device for renewable integration and grid applications, along with commercial or fleet transportation. There are mainly two types of NBBs that have been widely studied, based on the particular cathode materials. One is a sodium-smiirr (Na-S) battery of which the cathode is molten snifnr, follo wing the cell reaction:

xS 4 2/Va Na_:gH ix^{■■■■■■} 5-3 E ^■ 2.08-1.78 V at 35iPC (1 } Na-S chemistry has a high theoretical energy density (-760 Wh/kgf high energy efficiency and acceptable cycle life. The materials of the sodium-sulfur battery (i.e., alumina, sulfur and sodium) are relatively nouaoxie, inexpensive and readily available. The combination of these features makes it extremely attractive compared to other technologies for grid storage such, as lithium-ion, Ni- -metal hydride or Pb-acid batteries. The traditional Na-S battery uses thick solid β^-Α^Ο^ membrane (>1 mm) as electrolyte to separate the sulfur cathode and sodium anode, and operate at high temperatures {3O0~350^CC)« ^'The high temperature is necessary for both the BASE and cathode constituents (i.e., sodium poiysal t alcs j to achieve satisfactory electrochemical activities. I !o vever. the drawbacks of Na-S battery can include: 1 ) intrinsic corrosive behavior of olysaffi e melts, which limits material selections for both, cathode current collector and battery casing: 2) high operating temperature and open circuit cell failure mode. If the BASE is broken during batter operation, molten sulfides come in direct contact with molten sodium and the reactions between them are inherently vigorous. This can potentially result in a. Ore and even explosion since the battery operation temperature is close to boiling point of sudor (440°C). Neighboring ceils can also be affected by such an event and result in severe power loss due to open circuit.

[OeeSj The second type of NBB is the ZEBRA battery, in which solid transition uretai hahdes. which can include NICE, Fe(¾ and ZnCh, are used as active materials in the cathode. The ZEBRA battery typically needs a molten secondary electrolyte (i.e., NaAICL;) in the cathode so as to ensure facile sodium ion transport between the BASE and solid cathode materials. The electrochemical reaction of -NiC^ cells is as follows:

N iC - 2/Va - - ^'INaCl a - 2.58 V at 300^CC (2) |0ΘΘ6| The ZEBRA battery exhibits a numbe of advantages over the Na~S battery, which include higher voltage, facile assembly in discharged state, less corrosive nature of cathode materials, lower operating temperature, safer ceil failure mode, and better tolerance against overcharging. One notable disadvantage of the current ZEBRA technologies is the lower energy density compared to Na-S battery. Accordingly, a need exists for sodium energy storage devices exhibiting at least some advantages of both ZEBRA and Na-S technologies.

Summary

!§00^'?! This document describes hybrid sodium energy storage devices employing aspects of both ZEBRA batteries and traditional Na-S batteries. The hybrid sodium energy storage devices include a catholyte comprising a molten sodium salt and a positive electrode that has active species comprising sulfur. Additional active species can further include NaCl and a transition metal source. The sulfur is present in amounts greater than is used when sulfur is employed as a mere additive, The resultant energy storage devices exhibit lower operating temperatures, higher energy density, better cycle life, and improved safety compared to Na-S and/or ZEBRA batteries. This can make them useful in a variety of applications including, but not limited to, renewable integration and grid applications as well as commercial and fleet transportation.

f 0808] In one embodiment, the energy storage device has a negative electrode comprising sodium, a positive electrode having an active species comprising .sulfur, and a catholyte comprising a molten sodium salt. A beta-alumina solid electrolyte (BASE) separates the positive and negative eieetrodes. As a product of the energy discharge process. r¾S_x forms in which x is less than three. In traditional Na-S batteries, solid Na^S* formation typically results in. baited discharge, in preferred embodiments, an energy discharge product comprises ^S. In some embodiments, a product of the energy charge process can comprise NaaS_y, wherein y is greater than or equal to 3.

| MHI9 According to various embodiments, the amount, of active species that comprises sulfur can differ. For example, substantially 100% of the active species can comprise suiiur. An electrically conducive material can be employed as a positive current collector, bet the conductive material docs not participate as an active species. Examples of suitable electrically conductive materials can include, but axe not limited to, carbon, graphite, graphene, transition metals, and combinations thereof Alternatively, the active species can comprise a plurality of materials. In such eases, at least 10% of the active species comprises sulfur. Alternatively, at lees! 30% of the active species comprises sulfur. Preferably, at least 50% of the active species comprises sulfur. In. one embodiment, the additional active species comprises a transition metal source and NaCL The transition, metal can include, but is not limited io, Ni. Cu, Fe, Zn, Ag, lv:ln, Co, Ti, and combinations thereof Examples of transition metal sources can include, but are not limited to, NiC , C«<¾, FeCl_¾

Z«(¾,Ag€l, nCb,€o(¾, TIC),*_* una combinations thereof.

j^'$01 ] As used herein, molten sodium salts refer to sodium containing salts that are molten at the operating temperature of the energy storage device, lire salt is not necessarily always molten. For example, if the energy storage device is cooled when not operating, the sodium salt can solidify, Examples of molten sodium salts can include, but are not limited to, sodium polysulildcs, sodium metal halides, and combinations thereof- In a preferred embodiment, the molten sodium salt comprises NaAlCU. 011] In one embodiment, the operating temperature of the energy storage devices described herein can be below 400 °C. Preferably, the operating temperature is below 300

|^' 012| The purpose of the summary provided here n is to enable the United States Patent and Trademark Office and the public generally, especially the scientists, engineers, and practitioners in the art who are not familiar with patent or egal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The summary is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.

.Des«rif)fe.o of Drawin s

1101.3] Embodiments of the invention are described below with reference to the following accompanying drawings.

|^" 14j Fig, 1 is a schematic diagram conceptually depicting a sodium energy storage device having an active species comprising sulfur in the positive electrode according to embodiments of the present invention,

fOOlSJ Fig, 2 i s a graph of the initial charge and discharge curves of a hybrid a»8 NiCb cell at 280 ^aC according to embodiments of the present invention.

|0θί ) Figs. 3 A ···· 3D include X-ray diixraetioo. (XIID) patterns of a hybrid Na-S/NiC¼ eels at (A) zero and ( ) one-hundred percent state of charge (SOC) as well as (€> zero and (D) 100% depth of discharge OD) for ^'Na-S p rtions of the cathode according to embodiments of the present invention.

[0 l?f Figs. 4A -- 4C include graphs of (A) ceil voltage profiles during l ^s\ 15* 30*, 5* and 60"^' cycles at 280 °C; (B s cell charge/discharge capacity versus cycle mariners at 280 °C; and (C) cell charge/discharge energy density and conlombic efficiency versus cycle numbers 280 C for a hybrid Na-S/NiC ceil according to em d men s of the present invention,

(«0181 log. S includes a graph of initial charge and discharge curves for a Na-S/hiaAlC call according to embodiments of the present invention,

(«0i J Figs. 6A - 6C include graphs of (A) cell voltage profiles during 1 ^st _s 10*, 20*, 30*, 40^th, 50*. and 60^th cycles at 280°C; (B) End-of~charge and end-of-discharge voltage during cycling as 280°C; and ((f) Cell charge/discharge capacity during even eg at 80^"-^'C ibr a Na-S/NaAICU cell according to embodiments of the present invention.

Description

(««201 Various advantages and novel fea ures of the present invention are described herein and will become farther readily apparent io those skilled in this art from the following detailed description. In the preceding and following descriptions, the various embodiments, including the preferred embodiments, have been shown and described. Included herein is a description of the best mode contemplated ibr carrying out the invention. As will he realized, the invention is capable of modification in various respects without departing from the invention. Accordingly, the drawings and description of the preferred embodiments set forth hereaiter are to be regarded as illustrative in nature, and not as restrictive. |^'0 21| Furthermore, the following description includes the preferred be t mode of one embodiment of the present invention. It will be clear from this description of the invention that the invention is not limited to these illustrated embodiments but that ihe invention also includes a variety of modifications and embodiments thereto. Therefore the present description should be seen as illustrative aid not limiting. While the invention is susceptible of various rnodifi cations and alternative constructi ns, it should be understood, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents fading within the spirit and scope of the invention as defined in the claims.

\iWZ2) As described elsewhere herein, two technologies (e.g., sodium -sul tin and sodium-metal halide) have traditionally been available in sodium beta batteries. Referring to Fig. 1₅ embodiments described herein employ a combination of these two chemistries with a hybrid cathode J 02 ana an anode 1111 comprising sodium. The anode and eathode are separated by BASE 105. In one example, the cathode 102 of a cell has an active species comprising Na- S and a cathoiyte comprising NaAlCL. in another example, the eathode 102 of a cell consisted of molten NaAICi* as a cathoiyte and a mixture of Ni, aCl and hh¾S as active species. Positive and negative current collectors are depicted as elements 1 )3 and 104, respectively. During discharge to a load J0&, sodium in the anode loses electrons 110, which can be collected by the negative current collector 104. The result nt sodium ions 10S cross the BASK to the cathode 102 where they react with the suiiur containing active species and any other active specks enabled by the cathoiyte. During charging (e.g., from a source .1.07), the active species is oxidized. The resultant electron 111 is eoilceted hy the positive current collector and the resultant sodium ion 109 crosses the BASE to the anode where it is reduced to form sodium. The stable reduction of sulfur in the cathodes described herein can lead to an increase in overall energy density.

[0823 j Examples below are illustrative of embodiments described herein. BASE discs were fabricated using a vapor phase process as described previously. Starting powders were high purity a--Abi¾ (Aimatie, >99J%) and yttria-stabilized zireonia (BYSZ, UCM

Advanced Ceramics). 70 vol% u-A Cu and 30 voi% YSZ were ball-nulled with a d spersant (Phospholan PS-236, Akzo Nobel), solvents (ME Etfaa&ol), a plasdcker (benzyl butyl phthaiate, Aldrioh) and a binder (Buivar® B-79) to make a slurry. Alter the slurry was cast into thin sheets (- 125 μηϊ), the sheets were laminated and laser-eat to circular discs. The djses were tired at 160(FC in air to achieve full density (> 99%). t he sintered a-AbO YSZ discs were then placed in a loose jF-A!d¾ powder and heat-treated at 1450^" C in air in order to convert α ·Αί ··>. ¾ into β^"-·Αΐ2(¾. The conversion occurred by a coupled transport of sodium and oxygen ions from the

powder to the samples. Tbe β"~ΑΜ>3 powder used for the conversion process was synthesized using boehmite, Na?CC and L½CO:_> via a solid- state reaction, ^'The thickness of the convened composite p^i!~AhO ^:YSZ discs was -600 urn.

|002 1 In a first example, a hybrid Na-S/ iCb cell employed a cathode comprising a mixture of Nh NaCl a.nd a^S as the active materials and NaAlCLj as the eathoiyte. This cell retains most of the advantages of the state-of-the-art Na~S and EBRA batteries while overcoming the deficits previously discussed. T he addition of NaAlCU eathoiyte allows for lower operating temperatures compared io traditional Na-S battery while retaining the benign failure mode inherent to the ZEBRA chemistry. Another feature is that the mixed chemistry exhibits higher energy density than traditional Na-S cnemisby. The improvement in capacity appears io be a result of increased oxidation of sulfur thai can form solid a¾S_fi with n < 3 ith the presence of NaA!CL* catholyte. It is unlike that of traditional Na-S battery, n which discharge can halt with the formation of solids such as ^S.?. Daring cycling, two reversible plateaus were observed in cell voltage profiles, which matched electrochemical reactions for Na-S and Na-NiCb redox couples. In some instances, an irreversible reaction between suitor species and Ni was identified during initial charge at 280%^', which led to a decrease In ceil capacity. Despite the initial drop in ceil capacity, the hybrid cathode demonstrated relatively stable cycling with more than 95% of capacit retained over 60 cycles under 1 OmA/cm .

[0025] A BASE disc with the diameter of 26 mm was glass-sealed to an a-AiA^'A ring and the cell active cell area was 3 end. The ceil assembly was then moved into a glove box with 1 g of cathode powders consisting of N ^ , NaCL Ni and small amonms of additives. The mole ratio between Na;S and NaCl was 1 :2. After the powders were dried at 200°C under vacuum to remove ail traces of moisture, NaA!CLi melt was infiltrated into the cathode. A loll and a spring made of Mo were placed on the top of the cathode as a current collector. A spring-loaded stainless steel shim, which served as a molten sodium reservoir, was inserted into the anode compartment. Anode and cathode end plates were then compression-sealed to both sides of a-AW¾ ring using gold o~rbvgs. Nickel leads, which served as current collectors, were welded to the electrode end plates.

§02< | The assembled cells were heated in air to 2S0°C. The ga!vanostatic

charge/di charge test was carried out with a BT-2000 Arbin Battery Testing system. The cells were initially charged up to 2,8V under a current of 10 mA. The cells were then discharged back to 80% of the charge capacity using the same current. After the initial charge/discharge, the cells were cycled under the current of 30 mA to tesi the performance stability. The voltage limits of 2,8 and 1.8 V were applied to avoid overcharging and overdiseharging, respectively. After cell testing, several eel! cathodes were analyzed using room temperature powder XRD. The cathode samples were crushed ami ground to obtain fine-grained powders ibr XRD a alysis. The measurement was carried out in the 20 range of 20-80° wfth Co radiation,

| 27| Fig. 2 shows the initial charge and discharge curves of the hybrid Na-S/Ni€¾ ceil at 28CCC. Two plateaus were observed in the curves below the cut-off voltage of 2,8 V. indicating two separate steps for ceil reactions. This is unlike traditional ZEBRA chemistry, in which there Is only one plateau due to reaction between Ni and aCf As seen in Fig. 2, the open-circuit voltages (OCVs) of the two separate plateaus were around 2.1.5 and 2.58 V vs Na, respectively. These values correspond to the OCVs of Na-N and Na-NiC¾ batteries ai 300°C indicating co-existence of the two chemistries.

|0 28] To verify the assumption, cathode materials before and after various

charged/discharged states were analyzed using XRD, as shown in Fig. 3. The diiif ction pattern before initial charge shows peaks from a^S, NaCi and Ni (Fig. 3 A). During the first step of charge, the peaks for NB JS disappeared while those for Na^Ss and Na^S* were observed, as seen in Fig. 3B_;t confirming that the first step of charge was due to the electrochemical oxidation ofN aS to sodium poiysulf es with lower- valence sulfur. From Fig. 3B, the reaction products during the charge state were a^¾ and Na_;:S_>. The data also suggest the absence of elemental sulfur. However, this does not account, or amorphous Fains of sulfur. Fig. 313 also shows the presence of NiS, which was likely due the chemical reaction between Ni and sulfur species. NiS appealed to be stable in the cathode at following discharge as the peaks of NiS were clearly visible a; the start (Fig. 30 ) and end {^'big. 3D) of sodium poiysidt!de reduction reactions. The formation, of NiS from this irreversible reaction can cause a. reduction of active materials involved into the electrochemical reactions, and is not preferred. During discharging, peaks for

disappeared while those ibr Na->$ were not observed, which indicated thai the polysufides reacted with sodium Ion to form. with x < 3. Accordingly, the electrochemical cell reactions were proposed as follows:

Na₂S_y + Na /Va,S_y (x < 3 < y)_; E ~ 2.1 S V at 280*0 (3)

|ΘΘ29| The reversibi lity of polysulfid.es is significantly different from the traditional a~ S chemistry, in which discharge capacity is typically limited to the formation ofN;¾¾. Further discharging leads to the formation, of high melting solid, species such as a?S2 arid consequently increased resistance in the cathode, hi the embodiments described herein, molten NaAlC¾ wa maintained at the solid electrolyte/electrode mterhrce for rapid ion and mass transport so that solid species such as NaaS_x with x < 3 were formed during discharge, according to Reaction G ).

fCHB j As mentioned elsewhere herein, capacity losses can be observed during the initial charge and discharge of the hybrid cathode ceil. These losses might be attributed to side reaction between sulfur and Ni, which eventually could impact performance and cycle life. However, unexpectedly; cycle stability is at least comparable with some state-of-the-art devices The hybrid Na-S/NiC!_.; cells were cycled at C/5 rate GOmA) wit!- a capacity of 15b mAh. Cell voltage profiles of foe Γ. 15* 30 . 45^Jii and 60* cycles are show; in Fig. 4Λ The cell voltage at the start of the two redox plateaus was stable during cycling for both charge d discharge, while changes in voltage were observed for the ond-of-charge nd discharge voltage for the Na-NiCh couple. Figure 4A shows that, the end-ob-charge and discharge voltage reached cut-off limits of 2 J and 1.8 V for the 15^m and 30* cycles, respectively, Once the voltage limits were reached, the cell was not able to cycle at 150 niAh and cell capacity fade occurred. As showed in Fig. 48, the foil charge and discharge capacit of 150 mAh was maintained during the initial 10—15 cycles and ceil performance began to degrade afterwards, Overall, this hybrid cathode cell showed acceptable stability over 60 cycles with a capacity lade rate of 5%. Stability exceeds other Na-S batteries owing to the ceramic membrane electrolyte that can fully block imewdiffuslon id side reactions between sulfur species and sodium I bte anode. Charge and discharge capacity were calculated from Fig, 4A and are listed, in fable 1 below.

Table I . Charge and discharge capacity for a- -Cb and Na- S portions of hybrid Ha-S/NiCb

n t: mAh jO J The charge capacity of Na-NiCh portion of the cell was much higher than that of Na-S counterpart daring the first cycle at C/5 rate, which was unlike that at low rates daring initial cycles (Fig, 2). However, the capacity of Na-S portion continuously increased from 60 to 74 mAh during 60 cycles, suggesting that the sodium poiysulfides became more accessible in the electrochemical reactions, A similar trend was observed during discharge. Fig. 4C shows the specific energy of the hybrid Na-S/NiC!;; ceil as a function of cycling. The initial charge and discharge energy was 248 arid 230 Wh kg (per cathode and anode), and 95 and 97. S% of the energy was retained after 60 cycles, respecti ely. The energy efficiency as a function of cycle is also shown in Fig. 4C and is greater than 90%, The energy density of the hybrid Ha--$ Ni<¾ ceil was higher than that of a pure a-NiCb battery (150- 200 Wh kg) under similar conditions. Optimization of this mixed chemistry can further improve the performance.

|O032j i another example, a ceil employed a cathode comprising Na?S as substantially 100% of the active materia! with Ni as die conducting material i was not necessary arid other conducting materials can be suitable, aAICu was selected as the catholyie. During cycling of the instant a~S/NaAlC ceil, only one reversible plateau was observed in cell voltage profiles, which was attributed to the electrochemical reactions for the Na-S redox couple. Similar to ie hybrid Na-S NiC cell, irreversible reactions between sulfur species and Ni can he observed during initial charge, which can lead io a decrease in cell capacity compared to the theoretical value. Accordingly, other conductive materials can he utilized.

| t 33] ^'The Na-S/NaAlCL; cell with pure Na?S cathode was fabricated, assembled and tested, using techniques similar to those of hybrid Na-S/NiC¾ cell. The difference being thai the cathode was fabricated with 0.?g of powder comprising ^S and Ni as the active species and the conductive material, respectively. There was no ai.^'l added into the cathode.

|0034| Fig. 5 shows the initial charge and discharge curves of the Na-S/NaAICL* cell at 280°C. Only one plateau was observed in the curves below the cut-off voUage of 2.55 V. The voltage profiles wore quite similar to the Ha-S couple n the hybrid system, indicating the plateau was due to the electrochemical redox reactions from sulfur species,

[0035] To verify the long-term stability of the Na-S/ aAlCu coll a ceil was cycled at C/3 rate (30mA) with a capacity of 90 mAh. Cell voltage profiles of the Γ, 10*, 2if 30*, 40* 50^:'\ aud 60* cycles are shown in. Fig. 6A< It cars be see that end -of-charge and discharge voltage gradually changed during cycling. The end-oheharge and end-of- discharge voltage was plotted in Fig. 6.8. The end -of charge voltage increased and ersd-of- discharge voltage decreased with time, which was an indication of ceil performance degradation, over time. Fig. 6C shows the cell charge/discharge capacity during cycling. There was no capacity fade observed during 60 cycles. The more obvious performance fade was observed for the hybrid Na-S/NiC¾ cell which was doe to the wider cycling window and larger cycling capacity in the former. Side reactions between Ni and active material of ^ were believed to result in the formation of nickel po!y uUklc species as discussed earlier (see Fig. 3), which can eventually cause cell performance degr da ion. Accordingly, in some embodiments, the conductive material uses loss Ni or does not comprise i at all.

T able 2, Energy densities of Na-ThCb and a- S batteries conrpared to that of the hybrid Ns-S/NiCb

T siihiknsid My ri Na-

Na-NiOj Na-S S/MC¾ battery

battery bak r batter

Theoretical energy 790 760" 970° ! 250*

density

Energy density <aT - ^"6 ^': --10?° -510^s

including cathol te

and current eollecior

unit: Wh/kg

⁸ Capacity ratio between a-NiCh and Na-S batteries ish

u Assuming the finai discharging product is molten a>S₅,

8 <¼summg the f\m\ discharging product is solid NJOS. ^β The energy is calculated based oa cathode active materials of N&C arid i (with Na_:;S in die hybrid cathode), NaAiCh catholyte, and excessive Ni as current collector,

* Assuming carbon felt as c wrest collector,

^! Assuming the final discharging product is solid Na_;;S,

⁸ The e ergy is calculated based on cathode active sBateria!s of bhoS, NaAICh catholyte, and Ni as current celleeioo

[0036 f The energy densities for the i d cathode chemistry of the hybrid Na-S/Ni€¾ said Na-S aAlC ceils are com ared ith those of traditional Na- iClj and Na-S batteries in T able 2. The theoretical energy densities of the hybrid and Na-S NaAICU cathode are higher than thai of a conventional Na-S battery. As long as the molten catholyte (e.g., NaAiCiU) is maintained in the cathode, the reactants are not limited to rno en sodium polysuiildes as is typical in conventional a~S batteries. On the contrary, solid species of Να_? χ with * < 3 can be utilized during discharge, which eventually leads to higher theoretical energy density than the traditional Na-S battery. As discussed earlier, the discharging products of embodiments described herein comprises Na₂S_x with x < 3. indicating that solid polysidiide species participated in the electrochemical, reactions.

|IHB7| As shown in 'Fable 2, the energy densities of the two embodiments described above (i.e., the Na-S NiC¾ a the Na-S/NaAlCi₄ ceils) are at least 400 W tg. This represents t. least a 50% increase in actual energy density over traditional Na-NiCb chemistry. The difference in actual and theoretical capacities can be a result of (he use of Ni in the cathode to maintain electrical conductivity and the use of NaAlC as catholyte (see Table 2). Accordingly, in some embodiments, various ratios between the cathode active materials (e.g., stdfur-eontaining material, NaCI and/or transition metal), or between active species, current collector and catholyte cars be utilized to improve the energy density of the energ storage device. Furthermore other electrically conductive materials, such as carbon, graphite, aph me, and/or transition rneials can be sed as a con nctirsg material and/or current collector,

|Θ038] Whil a. number of embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes aral modifications may be made without departing from the invention in its broader aspects. The appended claims, therefore, are intended to cover ail such changes and modifications as they fall within the true spirit and scope of the invention.

Claims

Claims We claim:

1. An energy storage device having a. negative electrode comprising sodkan, the device characterised by a positive electrode having an active species comprising sulfur, by a oatholy e comprising a molten sodium sail, by an energy discharge product comprising a^S* in which, x is less than three, and by a beta-alumina solid electrolyte (BASE) separating ihe positive and negative electrodes,

2. The energy storage device of Claim C wherein the energy discharge product

comprises Na->S.

3. Tire energy storage device of Claim 1 , further comprising an energy eharge product comprising a_? . in which, y is greater than or equal to 3,

4. The energy storage device of Claim 1 , wherein 100% of the acti ve species comprises sulfur.

5. The energy storage device oiClauu L wherein at least 50% of the acuve species comprises.

6. The energy storage device of Claim 1 , wherein at least 10% of the active species comprises.

?.. The energy storage device of Claim 1 , wherein the active species further comprises a transition metal source and NaCl

8. The energy storage device of Claim ?, wherein the transition metal source comprises an element selected from the group consisting of h Cm Fe„ Zn, Ag, Mm Co, IT and combinations thereof.

9. The energy storage device of Claim 7, wherein the transition me al source comprises a metal alide selected from the group consisting of Ni(¾₅ Cul¾ !½(¾_>

ZnCh_?AgCL MnCk, CoCfe, TiCk and combinations thereof.

10. T he energy storage device of Claim 1 , wherein the molten sodium salt comprises a compound selected from the group consisting of is selected from the group consisting of sodium poiysu!fides, sodium metal hahdes, and combinations thereof

1 1. The energy storage device of Claim 10, wherein the molten sodium salt comprises NaAiCk

12. The energy storage device of Claim ! , further having a positive current collector contacting at least a portion of the positive electrode and comprising an electrically conductive material.

1 . The energy storage device of Claim 12, wherein the electrical ly conductive material comprises a material selected from the group consisting of carbon, graphite, graphene, transition metals, and combinations thereof

14. The energy storage device of Claim L having an operating temperature below 400 ^' I

15. The energy storage device of Claim 1 , having an operating temperature below 300

16. An energy storage device having a negative electrode comprising sodium, the device characterized by an operating temperature below 300 °C, by a positive electrode having an active species comprising a sulfur source, by a catholyte comprising molten aAlCfr. by an energy discharge product comprising f¾?Sv in which x is less than three, and by a beta-alumina solid electrolyte (BASE) separating the positive avid negative electrodes, wherein at least 50 wi% of the active species comprises sulfur.

17, The energy storage devlee of Claim 16, wherein the energy discharge product

comprises Na?S,

I S, The energy storage device of Claim 16. further comprising an energy charge product comprising Na.-iv in which y is greater than or equal to 3.

1 . The energy storage device of Claim 16, wherein the active species comprises a

mixture having a NiiCh, the sulfur source, and NaCl

20. The energy storage device of Claim 16, wherein 100% of the active species

comprises sulfur.