Sodium Ion Series
Part 1: Materials and landscape for the developing sodium ion market.
We are back with another deep dive series - this time it is sodium. This is the first edition of a multi-part series where we will jump into relevant literature and research related to sodium-ion batteries.
Part 1: Materials and Fundamentals of Sodium Ion
Sodium as a low-cost element is posing an interesting proposition for the battery market. A lot of buzz was generated when K.M. Abraham released an article in October 2020 comparing sodium ion to lithium-ion counterparts. Sodium-ion research has actually progressed alongside Li-ion since the 1980s, but lithium prevailed commercially by meeting the needs of electronic devices in the 1990s and 2000s. Abraham highlights three main papers by Yabuuchi (Japan), Mariyappan (France), and Zhang (Saudi Arabia) which were published well in advance of today’s battery frenzy.
In July of 2021, CATL announced visions of launching sodium-based rechargeable batteries that are competitive with LFP (lithium iron phosphate) counterparts. The Company could start production by the end of 2023.
The supply chain dilemma around the lithium-ion battery industry is well known with nearly every major research firm predicting a further squeeze on the price of battery metals. Battery-grade lithium and related materials are in heavy demand, and although more lithium mining capacity is expected to come online throughout the year, the announcement from CATL suggests forward-thinking on the sustainability of the battery industry. Sodium-ion battery growth estimates vary and Market Research Engine projects a >20% compound annual growth rate by 2025.
Diving into Sodium
An open-access paper by Tapia-Ruiz including various cited institutions provides a consolidated breakdown of the various materials that have been experimented with to construct Na-ion batteries. The readily abundant sodium (ahem, table salt) coupled with its ability to be used in existing manufacturing processes brings excitement to a rapidly evolving rechargeable battery industry.
The sodium concentration in Earth’s crust is about 2.6% or roughly 500 times that of lithium, it is also much easier to extract from seawater. The alkali metal is located directly beneath Li on the periodic table and contains an extra electron orbital which makes it unique when considering its application in a battery.
The redox chemistry is more complicated with sodium because various sodium active materials generating 2.5-3.5V can be considered. In each case during discharge, Na+ and an electron migrate to the cathode initiating the reduction and oxidation reactions.
The sodium ion toolbox of materials has been simplified into the below diagram with highlights and references from the sources referenced.
Cathode Active Materials
1.1 Layered transition-metal oxides P2/O3 Materials
The most promising Na-ion battery cathode materials have a 2D layered crystallographic structure like that of LiCoO2 (LCO). Sodium’s larger ionic radius due to an extra electron orbital enables the material to crystallize into a “trigonal prismatic configuration”, which enables added structural variation for unique research applications. The figure below shows the range of layered transition metal oxides evaluated in half cells.
While LCO carries a theoretical specific capacity of about 274 mAh/g, the corresponding Na alternative, NaCoO2, holds a specific capacity of about 235 mAh/g with an average voltage of about 3.0V. This relevant comparison is listed in the “How Comparable are Sodium-Ion Batteries paper by K.M Abraham.
LCO is a tried and true cathode active material, and the respective sodium alternative shows a more complicated charge/discharge profile (above) again highlighting various research opportunities.
1.2 Anion Redox Layered Transition-metal oxides
These materials are characterized by a distinctive overlap of bonding and antibonding orbitals shown in the figure below (b). The overlap results in a ‘continuum’ band which enables the removal of electrons from oxygen atoms. P2-type materials are highlighted as dominating the majority of the reported research. The potential for increased energy density is limited because these materials typically experience irreversible migration of certain metals towards sodium layers within metal oxide planes. Mortemard, et al reference Na2Mn3O7 as an interesting material for mitigating voltage hysteresis.
1.3 Polyatomic anion-based materials
Xiao, Hua et al use polyatomic anion insertion compounds which enable improved safety and the ability to extend the operating voltage of Na-ion cell to above 3.5V. Examples such as vanadium/fluoride-containing phosphates are some of the most promising. NASICON-type (Na3V2(PO4)3 can reversibly intercalate Na ions at 3.4V, while Na3V2(PO4)2F3 displayed an average operating voltage of 3.9V. The move to these materials has led to improved energy density and Bianchini, M demonstrated that a material Na3V2(PO4)2FO2 can accommodate an additional fourth Na-ion at 1.6V, which could increase specific energy further.
Theoretical specific capacities of some of the heavier polyatomic anion materials are lower than layered oxides, but the recent efforts of accommodating additional Na-ions when used as additives is a unique approach.
1.4 Prussian blue and analogues
Brant, William, and Mogensen are referenced for their work on these materials with the characteristic formula NaxM[M'(CN)6]1−y **where M and M’ are transition metals. Prussian blue analogues (PBAs) are some of the most cost-effective materials and provide the best price-to-performance ratio for cathodes.
A goal of capacities above 160 mAh/g and stable up to thousands of cycles is ideal to be competitive versus LFP. 8000 cycles have been demonstrated in aqueous electrolytes when capacity was limited to 60-80 mAh/g (image below).
Challenges for PBAs include the electrolyte system which is highlighted by moisture sensitivity and limited reversibility when cycling more than one atom of sodium per formula unit.
1.5 Organic Materials
These more sustainable materials address the potential problem of lithium, but even inorganic sodium-based active materials could be a strain on the environment. Organics are composed of carbon, hydrogen, oxygen, nitrogen, and sulfur. They function by their flexible crystal structure which enables them to bind to each other by Van der Waals forces which makes accommodation of cations such as Na+ or anions possible.
N-type cathode materials are required to make NIBs. The best-reported organics are polymerized perylene diimides, however, they have low operating potentials that do not exceed 2.5V.
Various materials are referenced in the image below. More research is needed to increase the operating voltage window of organics.
Anode Materials
2.1 Hard carbon
Hard carbons are the most popular choice for NIB anodes. They are different from graphite in that they have a higher interlayer spacing (>.34 nm) and are connected by disordered carbon regions. The larger spacing enables more efficient Na diffusion and intercalation sites at the anode. Research is cited to further understand the Na insertion mechanism and how various functional groups play a role.
One of the biggest challenges is to improve storage capacity over traditional graphite in LIBs (>372 mAh/g). The measured capacities for hard carbons vs. Na metal in half cells are around 300 mAh/g.
2.2 Titanium-based oxides
Titanium-based oxides are another of the most promising and versatile Na anode materials due to their low cost. They are safer than carbon-based anodes given their higher operating voltage preventing sodium plating.
One of the biggest hurdles is TiO2’s low electronic conductivity and slow Na+ ion (de) insertion kinetics. Ti4+ ions make the material an electrical insulator which hinders Na+ ion diffusion. Bulk and surface structure modifications have been done to try to improve these characteristics. Elemental doping at <5% concentration from the lanthanide series (Nb) has been shown to decrease the bandgap energy, improving performance.
Further enhancements in electronic conductivity and understanding of material/electrolyte interface layers are cited as keys to improving these materials.
2.3 Alloy and conversion materials
Alloys with Na are made with p-block elements and some transition metals enabling very high capacities. However, the formation of sodiated alloys causes volume expansion that is worse than LIBs. The large volume expansion is worsened by continuous electrolyte reduction and rapid cycling decay. Some of the most interesting cited materials are Na3P (2596 mAh/g) and Na3Sb (660 mAh/g) which have high capacities. Antimony is cited as one of the most promising materials since it possesses a large gravimetric charge storage capacity and several studies have shown long cyclability. Antimony composites coupled with hard carbon are suggested as a potential way to improve these materials.
2.4 2D transition-metal dichalcogenides
The stacking of 2D nanosheets of materials with high conductivity is considered a way to improve Na+ diffusion. Examples include 2D transition-metal dichalcogenides such as molybdenum disulphide which provides large channels and has a theoretical specific capacity of 670 mAh/g. There is not a lot of knowledge on the charge-storage mechanism, and characterization techniques are stated as ongoing efforts in this realm.
2.5 Organic materials
Sustainable anodes using organic molecules containing redox-active groups (carboxylate, azo, and imine) have shown some promise, but more efforts are needed to improve cycling stability and conductivities.
Electrolytes and SEI Engineering
Electrolyte use in LIBs is understood with 1M LiPF6 in ethylene carbonate/dimethlycarbonate as a standard. Na-ion batteries are in their infancy and authors in various publications hint at the need for further understanding sodium’s role in inter and deintercalating electrodes. Sodium salts in traditional ether and carbonate solvents are by default options given their use in lithium ion batteries.
The SEI growth over time during a sodium ion battery’s lifetime is dependent on a variety of factors and various authors cite this as an area of focus.
A Recap
In the Battery 2030+ sodium ion workshop last year, the lead scientist speaking about the materials highlighted a few. It appears that on the cathode there is similar thinking with layered oxides, Prussian blue analogues, and NVPF (Na3V2(PO4)2F3) as promising candidates.
Hard carbon appears to be a popular choice on the anode side, while the electrolyte system is unclear.
Driving the Future?
The implications of sodium ion suggest that it could be a potential alternative to traditional lithium ion batteries given that specific energies and cost are comparable. Sodium ion batteries also have the unique ability to be stored and transported at 0V. The lower cost reports are very appealing and given the level of innovation in structural design from recent companies (cell-to-pack), some could surmise further optimization. Another interesting feature of sodium ion is that it is not entirely novel like other technologies. Plenty of research is cited from the 2010-2015 timeframe, so the question is whether any of it is ready for primetime.
Thank you for reading part 1 of the sodium ion series. Stay tuned for more to come!
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Nice piece, I love sodium ion batteries. Looking forward to the rest of the series as well!