Sodium-ion battery
The sodium-ion battery is a type of rechargeable battery analogous to the lithium-ion battery but using sodium ions 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 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took place side-by-side with that of the lithium-ion battery in the 1970s and early 1980s, however, its development was superseded by that of the lithium-ion battery in 1990s and 2000s., a sodium-ion company based in the UK, has patented the highest energy density oxide-based cathodes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+, while a series of doped Ni-based oxides of the stoichiometry NaaNiMnxMgyTizO2 can deliver 157 mAh/g in a sodium-ion “full cell” with the anode being hard carbon at average discharge voltage of 3.2 V utilising the Ni2+/4+ redox couple. 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 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 and fluorophosphate have demonstrated excellent cycling stability and in the case of the latter, an acceptably high capacity at high average discharge voltages. There have also been several promising reports on the use of various Prussian Blue Analogues as sodium-ion cathodes, with the patented rhombohedral Na2MnFe6 particularly attractive displaying 150 –160 mAh/g in capacity and a 3.4 V average discharge voltage. 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.| Sodium-ion battery | Lithium-ion battery | Lead-acid battery | |
| Cost | Low | High | Low |
| Energy Density | Moderate/High | High | Low |
| Safety | High | Low | Moderate |
| Materials | Earth-abundant | Scarce | Toxic |
| Cycling Stability | High | High | Moderate |
| Efficiency | High | High | Low |
| Temperature Range | |||
| Remarks | Less mature technology; easy transportation | Transportation restrictions at discharged state | Mature technology; fast charging not possible |
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.
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 and a higher standard electrode reduction potential of the Na/Na+ redox couple relative to the Li/Li+ redox couple. Such a rationale is applicable only to metal batteries where the anode would be the concerned 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. 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. 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 for prolonged periods does not hamper its cycle life at all. 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 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, their chief cell design uses oxide cathodes with hard carbon anode and a liquid electrolyte. Their pouch cells have energy densities comparable to commercial Li-ion batteries with good rate performance till 3C and cycle lives of 300 to over 1,000 cycles. The viability of its scaled-up battery packs for e-bike and e-scooter applications has been shown. They have also demonstrated transporting sodium-ion cells in the shorted state, effectively eliminating any risks from commercial transport of such cells. The company's CTO is Dr. Jerry Barker, co-inventor of several popularly used lithium-ion and sodium-ion electrode materials such as LiM1M2PO4, Li3M23, and Na3M22F3ref> and the carbothermal reduction 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. With an exclusive licence for 6 patents from the CNRS and CEA, the solution developed by TIAMAT focuses on the development of 18650-format 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. 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. According to the company, it was 85 percent efficient. Aquion Energy filed for Chapter 11 Bankruptcy in March 2017.
Novasis Energies, Inc.: 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 Prussian Blue analogues as the cathode and hard carbon as the anode, their sodium-ion batteries can deliver 100 – 130 Wh/kg with good cycling stability over 500 cycles and good rate capability till 10C.
HiNa Battery Technology Co., Ltd: A spin-off from the Chinese Academy of Sciences, 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.
Natron Energy: A spin-off from Stanford University, Natron Energy uses Prussian Blue analogues for both cathode and anode with 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 a proprietary iron based Prussian blue analogue for the positive electrode in non-aqueous sodium ion batteries that use 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 at similar costs with enhanced performance. 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 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.
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 which are currently served by high cost and high energy density lithium-ion batteries.