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Sodium-ion battery

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The sodium-ion battery (NIB) is a type of rechargeable battery analogous to the lithium-ion battery but using sodium ions (Na+) as the charge carriers. Its working principle and cell construction are identical with that of the commercially widespread lithium-ion battery with the only difference being that the lithium compounds are swapped with sodium compounds: in essence, it consists of a cathode based on a sodium containing material, an anode (not necessarily a sodium-based material) and a liquid electrolyte containing dissociated sodium salts in polar protic or aprotic solvents. During charging, Na+ are extracted from the cathode and inserted into the anode while the electrons travel through the external circuit; during discharging, the reverse process occurs where the Na+ are extracted from the anode and re-inserted in the cathode with the electrons travelling through the external circuit doing useful work. Ideally, the anode and cathode materials should be able to withstand repeated cycles of sodium storage without degradation.

Research progress

Development of the sodium-ion battery took place side-by-side with that of the lithium-ion battery in the 1970s and early 1980s, however, its development was superseded by that of the lithium-ion battery in 1990s and 2000s.[1][[2] From 2011, research interest in sodium-ion batteries has been revived. The major advancements made in the field have been outlined below.

Anodes: The dominant anode used in commercial lithium-ion batteries, graphite, cannot be used in sodium-ion batteries as it cannot store the larger sodium ion in appreciable quantities. Instead, a disordered carbon material consisting of a non-graphitizable, non-crystalline and amorphous carbon structure (called ‘hard carbon’) is the current preferred sodium-ion anode of choice. Hard carbon's sodium storage was discovered by Stevens and Dahn in 2000.[3] This anode was shown to deliver 300 mAh/g with a sloping potential profile above ⁓0.15 V vs Na/Na+ roughly accounting for half of the capacity and a flat potential profile (a potential plateau) below ⁓0.15 V vs Na/Na+. Such a storage performance is similar to that seen for lithium storage in graphite anode for lithium-ion batteries where capacities of 300 – 360 mAh/g are typical. The first sodium-ion cell using hard carbon was hence demonstrated in 2003 which showed a high 3.7 V average voltage during discharge.[4] There are now several companies offering hard carbon commercially for sodium-ion applications.

While hard carbon is clearly the most preferred anode due to its excellent combination of high capacity, lower working potentials and good cycling stability, there have been a few other notable developments in lower-performing anodes. Incidentally, it was discovered that graphite could store sodium through solvent co-intercalation in ether-based electrolytes in 2015: low capacities around 100 mAh/g were obtained with the working potentials being relatively high between 0 – 1.2 V vs Na/Na+.[5] Some sodium titanate phases such as Na2Ti3O7,[6][7][8] or NaTiO2,[9] can deliver capacities around 90 - 180 mAh/g at low working potentials (< 1 V vs Na/Na+), though cycling stability is currently limited to a few hundred cycles. There have been numerous reports of anode materials storing sodium via an alloy reaction mechanism and/or conversion reaction mechanism,[1] however, the severe stress-strain experienced on the material in the course of repeated storage cycles severely limits their cycling stability, especially in large-format cells, and is a major technical challenge that needs to be overcome by a cost-effective approach. 

Cathodes: Significant progress has been achieved in devising high energy density sodium-ion cathodes since 2011. Similar to all lithium-ion cathodes, sodium-ion cathodes also store sodium via intercalation reaction mechanism. Owing to their high tap density, high operating potentials and high capacities, cathodes based on sodium transition metal oxides have received the greatest attention. Furthermore, from a desire to keep costs low, significant research has been geared towards avoiding or reducing costly elements such as Co, Cr, Ni or V in the oxides. A P2-type Na2/3Fe1/2Mn1/2O2 oxide from earth-abundant Fe and Mn resources was demonstrated to reversibly store 190 mAh/g at average discharge voltage of 2.75 V vs Na/Na+ utilising the Fe3+/4+ redox couple in 2012 – such energy density was on par or better than commercial lithium-ion cathodes such as LiFePO4 or LiMn2O4.[10] However, its sodium deficient nature meant sacrifices in energy density in practical full cells. To overcome sodium deficiency inherent in P2 oxides, significant efforts were expended in developing Na richer oxides. A mixed P3/P2/O3-type Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2 was demonstrated to deliver 140 mAh/g at average discharge voltage of 3.2 V vs Na/Na+ in 2015.[11] Faradion Limited, a sodium-ion company based in the UK, has patented the highest energy density oxide-based cathodes currently known for sodium-ion applications. In particular, the O3-type NaNi1/4Na1/6Mn2/12Ti4/12Sn1/12O2 oxide can deliver 160 mAh/g at average voltage of 3.22 V vs Na/Na+,[12] while a series of doped Ni-based oxides of the stoichiometry NaaNi(1−x−y−z)MnxMgyTizO2 can deliver 157 mAh/g in a sodium-ion “full cell” with the anode being hard carbon (contrast with the “half-cell” terminology used when the anode is sodium metal) at average discharge voltage of 3.2 V utilising the Ni2+/4+ redox couple.[13] Such performance in full cell configuration is better or on par with commercial lithium-ion systems currently.

Apart from oxide cathodes, there has been tremendous research interest in developing cathodes based on polyanions. While these cathodes would be expected to have lower tap density than oxide-based cathodes (which would negatively impact energy density of the resulting sodium-ion battery) on account of the bulky anion, for many of such cathodes, the stronger covalent bonding of the polyanion translates to a more robust cathode which positively impacts cycle life and safety. Among such polyanion-based cathodes, sodium vanadium phosphate[14] and fluorophosphate[15] have demonstrated excellent cycling stability and in the case of the latter, an acceptably high capacity (⁓120 mAh/g) at high average discharge voltages (⁓3.6 V vs Na/Na+).[16] There have also been several promising reports on the use of various Prussian Blue Analogues (PBAs) as sodium-ion cathodes, with the patented rhombohedral Na2MnFe(CN)6 particularly attractive displaying 150 –160 mAh/g in capacity and a 3.4 V average discharge voltage.[17][18][19] Novasis Energies Inc. are currently working to commercialise sodium-ion batteries based on this material and hard carbon anode.

Electrolytes: Sodium-ion batteries can use aqueous as well as non-aqueous electrolytes. Aqueous electrolytes, owing to the limited electrochemical stability window of water, result in sodium-ion batteries of lower voltages and hence, limited energy densities. To extend the voltage range of sodium-ion batteries, the same non-aqueous carbonate ester polar aprotic solvents used in lithium-ion electrolytes, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, propylene carbonate etc. can be used. The current most widely used non-aqueous electrolyte utilises sodium hexafluorophosphate as the salt dissolved in a mixture of the aforementioned solvents. Additionally, electrolyte additives can be used which can beneficially affect a host of performance metrics of the battery.

Advantages

Sodium-ion batteries have several advantages over competing battery technologies. The table below compares how NIBs in general fare against the two established rechargeable battery technology in the market currently: the lithium-ion battery and the rechargeable lead-acid battery.[13][20]

Lead-acid battery Lithium-ion battery Sodium-ion battery
Cost Low High Low
Energy Density Low High Moderate/High
Safety Moderate Low High
Materials Toxic Scarce Earth-abundant
Cycling Stability Moderate (high self-discharge) High (negligible self-discharge) High (negligible self-discharge)
Efficiency Low (< 75%) High (> 90%) High (> 90%)
Temperature Range -40 °C to 60 °C -25 °C to 40 °C -40 °C to 60 °C
Remarks Mature technology; fast charging not possible Transportation restrictions at discharged state Less mature technology; easy transportation

Cost: As stated earlier, since 2011, there has been a revival of research interest in sodium-ion batteries. This is because of growing concerns about the availability of lithium resources and hence, about their future costs. Apart from being the sixth most abundant element in the Earth's crust, sodium can be extracted from seawater indicating that its resources are effectively infinite. Due to these facts, the consensus is that sodium-ion batteries’ costs would perpetually be low if the cathode and anode are also based on earth-abundant elements. Furthermore, sodium-ion batteries allow for usage of aluminium current collectors for the cathode as well as anode. In lithium-ion batteries, the anode current collector has to be the heavier and more costly copper as Al alloys with lithium at low potentials (sodium does not form an alloy with Al).

Another advantage is that sodium-ion batteries utilise the same manufacturing protocols and methodology as that required for commercial lithium-ion batteries owing to their similar working principles. Hence, sodium-ion batteries can be a drop-in replacement for lithium-ion batteries not only in terms of application but also during the production process. This fact indicates no additional capital costs are required for existing lithium-ion battery manufacturers to switch to sodium-ion technology.

Energy Density: It was assumed traditionally that NIBs would never display the same levels of energy densities as those delivered by LIBs. This rationale was assumed by taking into account the higher molecular weight of sodium vs lithium (23 vs 6.9 g/mol) and a higher standard electrode reduction potential of the Na/Na+ redox couple relative to the Li/Li+ redox couple (-2.71 V vs S.H.E. and -3.02 V vs S.H.E. respectively). Such a rationale is applicable only to metal batteries where the anode would be the concerned metal (sodium or lithium metal). In metal-ion batteries, the anode is any suitable host material other than the metal itself. Hence, strictly speaking, the energy density of metal-ion batteries is dictated by the individual capacities of the cathode and anode host materials as well as the difference in their working potentials (the higher the difference in working potentials, the higher the output voltage of the metal-ion battery). Considering this, there is no reason to assume that NIBs would be inferior to LIBs in terms of energy densities – recent research developments have already indicated several potential cathodes and anodes with performance similar or better than lithium-ion cathodes or anodes. Furthermore, the use of lighter Al current collector for anode helps enhancing the energy density of sodium-ion batteries.

With reference to rechargeable lead-acid batteries, the energy density of NIBs can be anywhere from 1 – 5 times the value, depending on the chemistry used for the sodium-ion battery.

Safety: Lead-acid batteries themselves are quite safe in operation, but the use of corrosive acid-based electrolytes hampers their safety. Lithium-ion batteries are quite stable if cycled with care but are susceptible to catching fire and exploding if overcharged thus necessitating strict controls on battery management systems. Another safety issue with lithium-ion batteries is that transportation cannot occur at fully discharged state – such batteries are required to be transported at least at 30% state of charge. In general, metal-ion batteries tend to be at their most unsafe state at the fully charged state, hence, the requirement for lithium-ion batteries to be transported at a partially charged state is not only cumbersome and more unsafe but also imposes additional costs. Such requirement for lithium-ion battery transport is on account of the dissolution concerns of Cu current collector if the lithium-ion battery's voltage drops too low.[13] Sodium-ion batteries, using Al current collector on the anode, suffers no such issue upon being fully discharged to 0 V – in fact, it has been demonstrated that keeping sodium-ion batteries at a shorted state (0 V) for prolonged periods does not hamper its cycle life at all.[13][21] While sodium-ion batteries can use many of the same solvents in the electrolyte as used by lithium-ion battery electrolytes, the compatibility of hard carbon with the more thermally stable propylene carbonate is a distinct advantage that sodium-ion batteries have over lithium-ion batteries. Hence, electrolytes with a higher percentage of propylene carbonate can be formulated for sodium-ion batteries as opposed to highly flammable diethyl carbonate or dimethyl carbonate (preferred for lithium-ion electrolytes) which would result in significantly enhanced safety for NIBs.

Commercialisation

At present, there are a few companies around the world developing commercial sodium-ion batteries for various different applications. The major companies are listed below.

Faradion Limited: Founded in 2011 in the United Kingdom, Faradion has developed an extensive IP portfolio (as of 2019, it has filed 26 patent families) covering a range of materials and methodologies for manufacture and system designs for sodium-ion batteries. Its chief cell design uses any of one of its patented high energy density oxide cathodes with hard carbon anode and with its patented liquid electrolyte. Primarily focusing on energy applications, Faradion pouch cells have demonstrated energy densities comparable to commercial Li-ion batteries (140 – 150 Wh/kg at cell-level) with good rate performance till 3C and excellent cycle lives of 300 (100% depth of discharge) to over 1,000 cycles (80% depth of discharge).[13] Faradion has been able to successfully demonstrate viability of its scaled-up battery packs for e-bike and e-scooter applications.[13] Faradion has also patented the concept of transporting sodium-ion cells in the shorted state (at 0 V), effectively eliminating any risks from commercial transport of such cells.[21] The company's CTO is Dr. Jerry Barker, co-inventor of several popularly used lithium-ion and sodium-ion electrode materials such as LiM1M2PO4,[22] Li3M2(PO4)3,[23] and Na3M2(PO4)2F3[24] and the carbothermal reduction[25] method of synthesis for battery electrode materials.

Tiamat: Founded in 2017 in France, TIAMAT has spun off from the CNRS/CEA following researches carried out by a task force around the Na-ion technology funded within the RS2E network and a H2020 EU-project called NAIADES.[26] With an exclusive licence for 6 patents from the CNRS and CEA, the solution developed by TIAMAT focuses on the development of 18650 cylindrical full cells based on polyanionic materials. With an energy density between 100 Wh/kg to 120 Wh/kg for this format, the technology targets applications in the fast charge and discharge markets. More than 4000 cycles have been recorded in terms of cycle life and rate capabilities exceed the 80% retention for a 6 min charge.[27][28][29] With a nominal operating voltage at 3.7 V, Na-ion cells are well-placed in the developing power market. The start-up has demonstrated several operational prototypes: e-bikes, e-scooters, start & stop 12V batteries, 48V batteries.  

Aquion Energy developed aqueous sodium-ion batteries and in 2014 offered a commercially available sodium-ion battery with cost/kWh similar to a lead-acid battery for use as a backup power source for electricity micro-grids.[30] According to the company, it was 85 percent efficient. Aquion Energy filed for Chapter 11 Bankruptcy in March 2017.

Novasis Energies, Inc.: Novasis originated from battery pioneer Prof. John B. Goodenough's group at the University of Texas at Austin in 2010 and further developed at the Sharp Laboratories of America. Reliant on PBAs as the cathode and hard carbon as the anode, Novasis’ sodium-ion batteries can deliver 100 – 130 Wh/kg with good cycling stability over 500 cycles and excellent rate capability till 10C.[13]

HiNa Battery Technology Co., Ltd: A spin-off from the Chinese Academy of Sciences (CAS), HiNa Battery was established in 2017 building off of the research conducted by Prof. Hu Yong-sheng's group at the Institute of Physics at CAS. HiNa's sodium-ion batteries are based on Na-Fe-Mn-Cu based oxide cathodes and anthracite-based carbon anode and can deliver 120 Wh/kg energy density. In 2019, it was reported that HiNa installed a 100 kWh sodium-ion battery power bank in East China.[31]

Natron Energy: A spin-off from Stanford University, Natron Energy uses PBAs for both cathode and anode while utilising an aqueous electrolyte.

Altris AB: In 2017 three researchers from Uppsala University, Sweden collaborated with EIT InnoEnergy to bring their invention in the field of rechargeable sodium batteries to commercialisation, leading to formation of Altris AB. Altris AB is a spin-off company coming from the Ångström Advanced Battery Centre lead by Prof. Kristina Edström at Uppsala University. EIT InnoEnergy has invested in the company from its inception. The company is selling Fennac®, an iron based Prussian blue analogue for the positive electrode in non-aqueous sodium ion batteries. Batteries containing Fennac® utilise hard carbon as the anode.

Applications

While the sodium-ion battery technology is very versatile and can essentially be tailored to suit any application, it is widely believed that the first usage of sodium-ion batteries would be for all applications which are currently being served by lead-acid batteries. For such lower energy density applications, sodium-ion batteries would essentially be delivering much higher energy densities than current lead-acid batteries (1 – 5 times higher) at similar costs with enhanced performance (efficiency, safety, faster charging/discharging capabilities and cycling stabilities). These applications could be for smart grids, grid-storage for renewable power plants, the car SLI battery, UPS, telecoms, home storage and for any other stationary energy storage applications.

The higher energy density sodium-ion batteries (typically those using non-aqueous electrolytes) would be well suited for those applications currently dominated by lithium-ion batteries. Among the lower energy density spectrum of such high energy density batteries, applications such as power tools, drones, low speed electric vehicles, e-bikes, e-scooters and e-buses would benefit from the lower costs of sodium-ion batteries with respect to those of lithium-ion batteries at similar performance levels (safety being in favour of sodium-ion batteries). 

It is expected that with the current rate of rapid progress in the field of sodium-ion batteries, such batteries would be eventually used in applications requiring very high energy density batteries (such as long-range electric vehicles and consumer electronics such as mobile phones and laptops) which are currently served by high cost and high energy density lithium-ion batteries.

See also

References

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