Sodium-ion battery

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 almost identical with those of the commercially widespread lithium-ion battery types, but sodium compounds are used instead of lithium compounds.
Sodium-ion batteries have received much academic and commercial interest in the 2010s and 2020s as a possible complementary technology to lithium-ion batteries, largely due to the uneven geographic distribution, high environmental impact and high cost of many of the elements required for lithium-ion batteries. Chief among these are lithium, cobalt, copper and nickel, which are not strictly required for many types of sodium-ion batteries. The largest advantage of sodium-ion batteries is the high natural abundance of sodium. This could make commercial production of sodium-ion batteries less expensive than lithium-ion batteries. 
As of 2020, sodium ion batteries have very little share of the battery market. The technology is unmentioned in a United States Energy Information Administration report on battery storage technologies. No electric vehicles use sodium ion batteries. Challenges to adoption include low energy density and a limited number of charge-discharge cycles. 
History.
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, by the 1990s, it had become clear that lithium-ion batteries had more commercial promise, causing interest in sodium-ion batteries to decline.[ In the early 2010s, sodium-ion batteries experienced a resurgence in interest, driven largely by the increasing demand for and cost of lithium-ion battery raw materials. The major advancements made in the field have been outlined below.
Operating principle.
Sodium-ion battery cells consist 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+ ions 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. In a practical rechargable battery, the anode and cathode materials must be able to withstand repeated cycles of sodium storage without degradation.
Materials.
Since the physical and electrochemical properties of sodium differ from those of lithium, the materials generally used for lithium-ion batteries, or even their sodium-containing analogues, are not always suitable for sodium-ion batteries.
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 in 2000. 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 demonstrated in 2003 which showed a high 3.7 V average voltage during discharge. 
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. 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+. Some sodium titanate phases such as Na2Ti3O7, or NaTiO2, 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, 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.
Researchers from the Tokyo University of Science achieved 478 mAh/g with nano‐sized magnesium particles as announced in December 2020.
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. 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. 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. 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+, 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. Such performance in full cell configuration is better or on par with commercial lithium-ion systems currently.
Apart from oxide cathodes, there has been 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 and fluorophosphate 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+). 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 and rhombohedral Prussian white Na1.88(5)Fe[Fe(CN)6]·0.18(9)H2O displaying initial capacity of 158 mAh/g and retaining 90% capacity after 50 cycles. 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. The limited electrochemical stability window of water, results in sodium-ion batteries of lower voltages and limited energy densities when aqueous electrolytes are used. 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 uses sodium hexafluorophosphate as the salt dissolved in a mixture of these solvents. Additionally, electrolyte additives can be used which can improve a host of performance metrics of the battery. Sodium has also been considered as a cathode material for semi-solid flow batteries.
Advantages and disadvantages over other battery technologies.
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 technologies in the market currently: the lithium-ion battery and the rechargeable lead-acid battery.
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 (140 – 150 Wh/kg at cell-level) with good rate performance till 3C and cycle lives of 300 (100% depth of discharge) to over 1,000 cycles (80% depth of discharge). 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 (at 0 V), 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, Li3M2(PO4)3, and Na3M2(PO4)2F3 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.  
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.
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.
Applications for batteries depend largely on the factors of lifetime cost per kilowatt-hour, specific energy density (energy per gram weight), safety, and power delivery characteristics. Compared to lithium-ion batteries, current sodium-ion batteries have somewhat higher cost, slightly lower energy density, better safety characteristics, and similar power delivery characteristics. If the cost of sodium-ion batteries is further reduced, they will be favored for grid-storage and home storage, where battery weight is not important. If, in addition to cost improvements, the energy density is increased, the batteries could be used for electric vehicles and power tools, and essentially any other application where lithium-ion batteries currently serve.