Patent Application
20180040887
The present invention relates to an electrode material for a sodium-ion battery. In particular, the present invention relates to an electrode material suitable for use as an anode of a sodium-ion battery wherein the electrode material comprises sodium titanate.
With the growing concerns about global warming in the recent years, much emphasis has been placed towards shifting from fossil fuel based energy harvesting plants to those relying on renewable sources of energy such as solar and wind. The intermittency of these sources results in the unavoidable need for energy storage. In this new age of renewable sources, energy storage in the form of stationary grid batteries will be as critical as the actual solar/wind farms and their performance as an integrated system will determine their market success. To the latter point, cost of these grid batteries would be the most critical. Lithium-ion batteries (LIBs), which are the state of the art and easily outstrip other traditional battery technologies in terms of performance, are ill-suited for grid application owing to the limited availability of lithium reserves. Furthermore, concerns about lithium's future price, especially when demand increases in the future owing to the rapid escalation of demand of LIBs for applications such as electric vehicles, would require an alternate battery technology that does not rely on lithium, yet has comparable performance to LIBs.
In this regard, room temperature sodium-ion batteries (NIBS) are very well suited since they have the same working principle as LIBs and recent reports suggest that they can rival or even surpass LIBs in terms of performance. Switching to NIBs would make sense since sodium is widely abundant and a battery technology relying on sodium would be much cheaper than that relying on lithium.
NIBs, similar to LIBs, require a cathode material capable of inserting/de-inserting sodium ions at a high potential, and an anode material capable of doing the same at low potentials. Obviously, the higher the operating potential of the cathode, termed as the “redox potential” (reduction/oxidation), and the lower the redox potential of the anode, the higher would be the operating potential of the full cell. The energy density of a full cell (the amount of energy stored per weight with the units typically being Wh/Kg) is equal to the operating potential (V) times the capacity of its cathode or anode (the amount of charge stored per weight of the electrode material, mAh/g). For grid storage batteries, unlike those needed for electric vehicles and consumer electronics, energy density is not the most important performance factor since there are no restrictions on the weight and size of these batteries. The primary factors instead are low cost, ultra-long cycle life, excellent safety and high round-trip energy efficiency (RTEE) of at least 80%. The RTEE is a product of the coulombic efficiency (ratio of the discharge capacity to the charge capacity) and the voltage efficiency (hysteresis in the voltage between the charge and discharge cycles). With these points in mind, the energy density of a full cell can easily be increased by increasing the operating potential of the full cell, this means combining a high potential cathode with a low potential anode. Also, the RTEE can be increased by ensuring that there is negligible voltage hysteresis (also called “polarization”) between the charging and discharging curves of the full cell. Since a full cell's galvanostatic curves are nothing but those of the anode subtracted from that of the cathode, what this means is that the individual galvanostatic curves of the cathode and anode (measured typically versus sodium metal in a “half-cell” approach) should exhibit very little voltage polarization.
Energy storage for the grid will be a vital component towards ushering in the age of the renewable sources of energy such as solar and wind as it solves the most glaring drawback of them—their intermittency. Hence, a cheap battery meant for grid application will make the cost of electricity coming from these plants very cost competitive for the consumers. Since cost is a big factor, LIBs are automatically disqualified owing to the lack of abundance of lithium resources. NIBs, relying on the globally abundant and therefore cheap sodium, are the best candidates.
While there have been some promising high voltage cathodes already demonstrated for NIBs, a suitable combination of a very low voltage redox potential and an attractive capacity anode material has not been demonstrated. Carbon based anodes for NIBs may have a high capacity and very low redox potential (<0.1 V vs Na/Na+), but they have serious safety concerns owing to the thermal instability of the sodiated material at low temperatures of around 100-150° C. as well as the possibility of sodium plating on the anode which may cause as internal short-circuit due to voltages dangerously close to 0 V.
Hence, there is a need for discovering electrode materials capable of storing sodium at attractive voltages with decent capacity, having low synthesis costs, able to display an ultra-long cycle life, having a high RTEE and being thermally stable in the sodium deficient and rich phases.
The listing or discussion of an apparently 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.
Any document referred to herein is hereby incorporated by reference in its entirety.
In the present invention, we have discovered a new phase in the sodium titanates, that of Na3Ti3O7. This material may be used as an anode in sodium ion batteries because, as an anode, it can store sodium in the lowest voltage ever reported for any non-carbon based anode material used in sodium-ion batteries. Our current findings demonstrate its very suitable low voltage sodium storage activity along with its attractive capacity which results in a high energy density full sodium-ion cell with the highest voltage plateau every reported in sodium-ion batteries for non-carbon based materials. Other attractive capabilities for battery application such as extremely good high rate performance, long cycle life, high efficiency, use of low cost materials as well as great thermal stability are also presented. It is envisaged that this material may play a key role in not just lithium/sodium based batteries, but also in other fields such as electrochromic windows, water splitting and catalytic reactions.
The most promising non-carbon anode material is the insertion based sodium titanate Na2Ti3O7, which has been shown to intercalate two moles of sodium (to nominally form Na4Ti3O7) at an average potential of 0.3 V vs Na/Na+ with a high capacity of 178 mAh/g. While it sounds very promising, there is a big drawback in this material. It displays a huge polarization in its charge and discharging potentials, being >0.4 V and <0.2 V, respectively. This >0.2 V difference will lead to a full cell operating with low RTEE.
Here, we present another sodium storage route for Na2Ti3O7 that involves limiting its sodium uptake to just one mole, forming the previously unknown “Na3Ti3O7” phase. This Na2Ti3O7<=>Na3Ti3O7 sodium storage pathway results in the lowest redox voltage of 0.20 V ever reported for a non-carbon based anode material with an attractive theoretical capacity of 88.9 mAh/g. It will also be demonstrated that there is very little voltage polarization in the charging and discharging curves of this material. When combined with a high voltage cathode, this material results in a high energy density full cell showing a high RTEE as well as an extremely high voltage plateau between 4 and 3.7 V vs Na/Na+. This would be the highest plateau displayed thus far for any non-carbon based NIB. In addition, the extremely good rate performance, excellent thermal stability and long cycle life for this material will be displayed.
In an aspect of the present invention, there is provided an electrode material for a sodium ion battery, the material comprising a sodium titanate having the formula Na3+xTi3O7, wherein the value of x is defined as −0.5≦x≦0.3.
In an embodiment, the sodium titanate has a formula of Na3Ti3O7, i.e. Na3+xTi3O7 (where x≈0), or Na3−xTi3O7 (where x≈0). In this application, when the formula Na3−xTi3O7 is used, it is also meant to refer to the scenario where x≈0, giving rise to the formula Na3Ti3O7.
Preferably, the electrode is an anode, and the material is an intermediate phase between Na2Ti3O7 and Na3+xTi3O7 when cycled against a cathode material having a higher voltage in an electrolyte. Preferably, the anode material is an intermediate phase between Na2Ti3O7 and Na4Ti3O7 when cycled against a cathode material having a higher voltage in an electrolyte. Preferably, that intermediate phase is Na3Ti3O7. Preferably, the cathode material is a sodium metal. The starting electrode (anode) material Na2Ti3O7, which may be synthesized by any suitable means, transforms to Na3+xTi3O7 (−0.5≦x≦0.3) in the course of cycling such that the electrode goes from Na2Ti3O7 to Na3+xTi3O7 (−0.5≦x≦0.3) during discharge in a half cell (in a full cell, the process would be the charge cycle) and back from Na3+xTi3O7 (−0.5≦x≦0.3) to Na2Ti3O7 during the charge process in a half cell (in a full cell, the corresponding process would be the discharge).
Any suitable electrolyte may be used, particularly any electrolyte suitable for an aqueous sodium-ion battery or non-aqueous sodium-ion battery of the present invention, or even any other solid-state batteries which use this Na3+xTi3O7 (−0.5≦x≦0.3) as anode. As such, the present invention includes any such electrolyte that is able to allow sodium ions to be shuttled from the cathode to the anode and vice versa. Such electrolytes may be ether/ionic liquids/any other polar aprotic solvents based electrolyte. In an embodiment, the electrolyte is a carbonate-based electrolyte.
In an embodiment, the electrode material further comprises a carbon matrix.
Preferably, the electrode is an anode and the anode material may further comprise any other additives that may be suitable when forming the sodium ion battery. For example, an electrically conductive additives (e.g. carbon black, acetylene black, graphite, carbon nanopowder, graphene, carbon, nitrogen-doped carbon), and/or a binder may be present at any suitable proportions and amounts.
In another aspect of the present invention, there is provided a sodium-ion battery comprising (a) an anode; (b) a cathode; (c) a separator disposed between the anode and the cathode, the separator is configured to conduct sodium ions; and (d) an electrolyte, wherein the anode comprises a material, the material comprising a sodium titanate having the formula Na3+xTi3O7, wherein the value of x is defined as −0.5≦x≦0.3.
Preferably, the material is an intermediate phase between Na2Ti3O7 and Na4Ti3O7 when cycled against a cathode material having a higher voltage in an electrolyte. The electrolyte may be a carbonate-based electrolyte. Preferably, the cathode material is a sodium metal.
In yet another aspect of the present invention, there is provided a method of producing an electrode for a sodium ion battery, the method comprising: (a) providing a sodium source and a titanium source; and (b) reacting the sodium source and titanium source in a solvo-thermal reaction in the presence of a carbon source and a medium to form a sodium titanate electrode material, wherein the electrode is formed by loading the sodium titanate electrode material with a conductive additive and a binder.
Preferably, the sodium source is sodium hydroxide and the titanium source is titanium isopropoxide.
Preferably, the conductive addictive is a Super P carbon and the binder is a sodium salt of carboxymethyl cellulose (CMC).
Preferably, the electrode material is Na2Ti3O7. More preferably, the electrode is an anode, and the material forms an intermediate phase between Na2Ti3O7 and Na3+xTi3O7 when cycled against a cathode material having a higher voltage in an electrolyte, wherein the value of x is defined as −0.5≦x≦0.3. Preferably, the electrode is an anode, and the material forms an intermediate phase between Na2Ti3O7 and Na4Ti3O7. Still more preferably, the intermediate phase is Na3Ti3O7.
Preferably, the electrolyte is a carbonate-based electrolyte.
Preferably, the weight ratio of the electrode material, Super P carbon and CMC may be 80:10:10, or 70:20:10. Alternatively, any other suitable weight ratio may be used.
The electrode formed from the above method may be used to form a sodium ion battery. Such sodium ion batteries may be applied in stationary grids or micro-grids, or in other applications such as in electric vehicles or consumer electronics are straightaway relevant. Other relevant uses could be in lithium-ion batteries, electrochromic windows, water splitting or catalyst reactions.
The present invention includes any material having the formula Na3+xTi3O7 with ‘x’ denoting the fact that a range of stoichiometries may be possible (−0.5≦x≦0.3, hence from Na2.5Ti3O7 to Na3.3Ti3O7). It should be recognized that all electrochemical, chemical or mechanical means to synthesize this Na3+xTi3O7 are included. As an example, we will present the electrochemical conversion of Na2Ti3O7, prepared from a solvo-thermal reaction, to Na3Ti3O7 when cycled against sodium metal in a carbonate based electrolyte. In addition to its use as an anode in NIBs, its application to LIBs, electrochromic windows, water splitting and catalytic reactions will also be covered.
In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative examples only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative figures.
In the Figures:
A new sodium storage pathway is presented here for the anode Na2Ti3O7 which involves the newly discovered intermediate phase of Na3−xTi3O7. Details about this Na2Ti3O7⇄Na3−xTi3O7 sodium storage pathway and how it relates to the conventional Na2Ti3O7⇄Na4Ti3O7 pathway are described below. This Na2Ti3O7⇄Na3−xTi3O7 pathway has the lowest redox voltage of 0.2 V vs Na/Na+ ever reported for any non-carbon based sodium-ion battery anode along with moderately high capacity approaching 89 mAh/g, negligible polarization, excellent rate performance (up to 80 C, or 45 s response) and good cycle life till 1,500 cycles. These results indicate this pathway's potential as an anode for sodium-ion batteries meant for diverse applications.
Na3Ti3O7 can be conveniently prepared electrochemically from Na2Ti3O7.
Na2Ti3O7 prepared from any means, be it solid-state reaction, hydro/solvo-thermal reaction etc. and with/without any carbon as a surface coating or a composite matrix will exhibit the characteristics displayed above. As an example, we have taken a solvo-thermally synthesized Na2Ti3O7 embedded in a carbon matrix, labelled as “Na2Ti3O7/C”. Also, the material properties of Na3Ti3O7, such as its morphology or surface area, which was synthesized electrochemically from Na2Ti3O7, would depend on the material properties of Na2Ti3O7 which may be subject to changes due to the synthesis. Another point to note is that the stated lower cut-off voltage may be found to be variable as it depends on the applied C rate of discharge, electrode thickness, ratio of conductive additive to Na2Ti3O7 present in the electrode, quality of cell fabrication, morphology of the synthesized Na2Ti3O7 as well as on the electrolyte used. Depending on these variables, it is possible for the lower cut-off voltage to stretch to as low as 0.0 V vs Na/Na+ to witness the Na2Ti3O7<=>Na3Ti3O7 storage pathway.
Therefore, in an embodiment of the present invention, Na2Ti3O7/C was synthesized using a solvo-thermal approach. The sodium and titanium sources were sodium hydroxide (32 mg) and titanium isopropoxide (296 μl) respectively taken in stoichiometric molar ratio with 20% molar excess of the former. Gluconic acid lactone (285 mg) was used as in-situ carbon source and absolute ethanol (15.3 ml) served as the medium. The reaction was carried out in a Teflon® vessel (26 ml) sealed in a stainless steel autoclave at 180° C. for 6 h and allowed to cool naturally. The resulting solution with precipitates was centrifuged once with absolute ethanol and then dried in an oven kept at 70° C. for 4 h. The obtained orange-light brown powder was then calcined at 800° C. for 4 h under argon gas flow in a tube furnace to yield the final product. Composite electrodes were made with the as-synthesized powder:Super P carbon:sodium salt of carboxymethyl cellulose (CMC) in the weight ratio 80:10:10 with the loading of Na2Ti3O7 being about 1.5-2.0 mg/cm2. 1M NaClO4 in EC:PC (1:1 v/v) was used as the electrolyte. For ex-situ XRD studies, solid-state synthesized Na2Ti3O7 was used in order to obtain better reflection intensities owing to its micrometer-sized particles. These electrodes had the weight ratio 70:20:10 (Na2Ti3O7:Super P carbon:CMC) and were cycled to respective states of discharge at C/5. The electrodes were opened in a glove box, covered with Kapton tape and all XRD patterns reported were obtained within 3 min as the discharged electrodes were found to be very unstable in air. The Kapton tape cover delayed phase transformation to Na2Ti3O7 long enough to obtain reliable XRD patterns. Please note that the same ex-situ XRD experiments were performed on the Na2Ti3O7/C electrodes prepared from the solvothermal synthesis and the results were identical as that with solid-state synthesized Na2Ti3O7. However, owing to its sub-micrometric particle size, the signal-to-noise ratio was quite low and hence, this data have not been shown. All other relevant experimental information has been published previously.
As depicted in
The first galvanostatic cycle of Na2Ti3O7/C cycled in a three-electrode cell between 2.5 and 0.01 V (deep discharged) is depicted in
To confirm this observation, ex-situ XRD patterns were collected at different stages of discharge (
Indeed, if the lower cut-off voltage is limited to around 0.155 V (shallow discharged) at C/5 instead of 0.01 V such that the material is allowed to intercalate sodium until the end of the upper discharge plateau (till the voltage step), the resulting galvanostatic profile of the charge curve is completely different (
The fact that two discharge plateaus are witnessed upon deep discharge corresponding to the Na2Ti3O7→Na3−xTi3O7 and Na3−xTi3O7→Na4Ti3O7 two-phase reactions respectively, but only the 0.44 V charge plateau (corresponding to that for the established Na4Ti3O7→Na2Ti3O7 two-phase reaction) is witnessed during charge, implies an irreversible transition during deep discharge to 0.01 V which causes the Na2Ti3O7⇄Na3−xTi3O7 pathway to be lost in subsequent cycles. If this was not the case, the charge cycle of a deep discharged Na2Ti3O7 electrode should have also displayed two separate charge plateaus. The different phases (Na2Ti3O7, Na3−xTi3O7 and Na4Ti3O7) involved in the sodium storage in Na2Ti3O7 may give rise to different sodium migration routes which would likely affect the chemical diffusion of sodium in them. This may help explain the large difference seen in the polarization of the Na2Ti3O7⇄Na3−xTi3O7 and Na2Ti3O7⇄Na4Ti3O7 pathways. Atomistic simulation studies on the resolved crystal structure of Na3−xTi3O7 from the in-situ XRD experiments would help in resolving this point. In summary, during deep discharge, Na2Ti3O7 forms the distinct phases of Na3−xTi3O7 and Na4Ti3O7, but during charge, Na4Ti3O7 transforms to Na2Ti3O7 directly without formation of the Na3−xTi3O7 phase. Hence, to witness the Na2Ti3O7⇄Na3−xTi3O7 pathway over many cycles, it is imperative that the irreversible transformation of Na3−xTi3O7 to Na4Ti3O7 is avoided by appropriately limiting the extent of sodiation during discharge (by limiting the voltage window).
The charge plateau close to 0.2 V makes the Na2Ti3O7⇄Na3−xTi3O7 pathway the lowest redox voltage non-carbon based NIB anode ever reported, being 0.2 V lower than the previous lowest voltage non-carbon anode, viz. the Na2Ti3O7⇄Na4Ti3O7 pathway demonstrating its charge plateau above 0.4 V. The close proximity of the oxidation and reduction peaks for the Na2Ti3O7⇄Na3−xTi3O7 pathway as revealed by CV implies lower polarization and good reversibility. Indeed, as seen from
In addition to the above, the present Na2Ti3O7<=>Na3Ti3O7 sodium storage mechanism demonstrates extremely good rate performance with little polarization even at high rates and stable cycling at all rates, as demonstrated in
Due to its superior properties involving intercalation and redox based reactions (as demonstrated by sodium intercalation), it is expected that this newly discovered compound Na3Ti3O7 may display very favourable performance in other applications which require one or both of these properties, such as in electrochromic windows, water splitting or catalyst reactions.
By storing two moles of sodium per formula unit through a two-phase reaction mechanism between Na2Ti3O7 and Na4Ti3O7, Na2Ti3O7 has a high capacity (178 mAh/g) and low and flat voltage of 0.4 V vs Na/Na+ upon sodium extraction. The major drawback of this material is its high polarization greater than 0.2 V. In fact, it has been proposed that the polarization is thermodynamically controlled (independent of particle size) and may not be below 0.225 V. Furthermore, the fully sodiated phase, Na4Ti3O7, has been shown to be unstable due to self-relaxation which may be the reason for its poor cycle life. Advantageously, the present invention results in a new sodium storage mechanism for a previously known compound (Na2Ti3O7) which not only results in superior sodium storage properties, but also results in the discovery of a new compound (Na3Ti3O7). When used as an anode material for sodium-ion battery application, this invention takes part in the lowest ever reported redox voltage activity for sodium storage for a non-carbon based anode material demonstrating excellent high rate performance as well as good stability over many cycles. When used in a full NIB paired with a high voltage cathode such as Na3V2(PO4)2F3, the highest voltage full sodium-ion cell ever fabricated using non carbon-based electrodes results which has obvious implications for not only a high energy density NIB, but also for powering loads which may not work if the voltage of a battery is too low. This anode supports high rate operation of NIB. Although voltage can always be increased by connecting identical cells in parallel, one would certainly require less of them in parallel for supplying current for high voltage needs, such as for grid storage batteries. These facts, along with the high RTEE exhibited by this reaction mechanism in a full NIB along with its inherent safety, would make the findings of this invention as the most suitable choice for a commercial high voltage NIB meant for grid storage batteries.
An intermediate phase has been discovered and isolated through galvanostatic cycling studies of Na2Ti3O7 in a sodium battery. This new anode reaction for Na2Ti3O7 results in it storing sodium via a two-phase reaction mechanism between the phases Na2Ti3O7⇄Na3−xTi3O7 and demonstrates the lowest charge plateau ever reported for any non-carbon based NIB anode at 0.2 V vs Na/Na+. Ex-situ XRD analysis revealed that Na2Ti3O7 undergoes two separate two-phase reactions in its very first discharge which results in two separate discharge plateaus separated by a faint voltage step. The lower discharge plateau causes an irreversible transformation which leads to the loss of the Na2Ti3O7⇄Na3−xTi3O7 sodium storage pathway in subsequent cycles. If this is avoided by restricting the voltage window, then the Na2Ti3O7⇄Na3−xTi3O7 pathway is capable of storing almost one mole of sodium resulting in ≈89 mAh/g capacity along with minimal polarization, an ultra-fast 80 C (45 s) response and a cycle life of 1,500 cycles at 1 C. This pathway's combination of earth-abundant elements, low voltage, decent capacity, good cycling stability and excellent rate performance would make it a promising NIB anode which could be used for diverse applications.
Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2016/050094 | 2/25/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62120463 | Feb 2015 | US |
