The development of Na-ion battery (SIB) materials has increased rapidly in recent years thanks to the knowledge acquired from lithium-ion batteries, as they often show similar properties.

In fact, the three most studied families of cathode materials for Na-ion batteries (in essence, composed of earth-abundant elements such as Fe, Mn, P, C, N and O) are transition metal layered oxides, polyanionic compounds (both of which have analogous, though subtly different, materias in Li-ion technology), and Prussian blue Analogues.

With respect to the anode, as previously discussed in the first article of this series on Na-ion batteries, the fact that sodium is not intercalated in the graphite has led to disordered carbons becoming the preferred anode material for these systems. Disordered carbons are generally obtained from abundant and cheap precursors, which mainly come from biomass wastes, and can be classified into hard carbons and soft carbons (depending on their mechanical resistance), with hard carbon being the most commonly used.

In general, hard carbons offer a capacity of 300 - 350 mAh/g, an average voltage  of 0.3 - 0.4 V vs. Na⁺/Na, and a Coulombic efficiency above 80% - although some safety issues may result from its use due to the plateau below 100 mV (slightly above the sodium plating, which leads to possible dendrite formation and battery instability).

By contrast, soft carbons provide a capacity of 200 - 250 mAh/g, an  average voltage of 0.5 - 0.6 V vs. Na⁺/Na, and Coulombic efficiency between 70 - 80%. Although it has been shown that an optimization of soft carbons provides better electrochemical performance at high current intensities, hard carbons (due to their higher energy density) are considered the preferred anode material for Na-ion batteries (and consequently both safety and Coulombic efficiency remain key parameters to be optimized).

Like Li-ion batteries, each family of cathode materials has its advantages and disadvantages (such as in terms of electrochemical performance, sustainability, cost, etc.).

Considering this scenario, is there a winning chemistry? Or, is it more likely that several Na cathodes would coexist in order to fulfil the broad requirements resulting from the wide market for different applications?

Transition metal (T_M) layered oxides, or simply layered oxides (NaxT_MO₂), have two-dimensional layered structures (generally P2 and O3 structures) that allow reversible sodium insertion. The great advantage of these compounds lies in their low molecular weight, which results in a high theoretical specific capacity.

However, in addition to having a moderate redox potential, they undergo multiple phase transitions during the deinsertion/insertion of sodium ions - resulting in structural instability during charge/discharge, especially at high voltages, leading to low cyclability. This structural instability can be mitigated by reducing the voltage window, but at the expense of penalizing the capacity (which, for example, is typically reduced from 240 mAh/g to 120 - 150 mAh/g).

The key to maximizing electrochemical performance lies in carefully selecting the right combination of transition metals - allowing us to achieve energy density and cyclability values suitable for future practical applications.

Some spin-off, such as Faradion Limited (UK) and HiNa Battery Technology Co (China), have performed tests on prototype cells in pouch/cylindrical format using a layered oxide  with cathode materials possessing stoichiometries of Na_xNi_1-x-y-zMg_xMn_yTi_zO₂ and Na_0.9Cu_0.22Fe_0.3Mn_0.48O₂, respectively.

In 2017, Faradion´s research culminated in the announcement of a 400 Wh SIB, which provided an energy density of 80 Wh/kg - making it possible for an e-bike to travel around 35 km. Meanwhile, in 2019, HiNa Battery announced the installation of a 100 kWh/30 kW SIB for grid energy storage to provide electricity to Liyang City during peak hours. Currently, likely due to the commercial nature of these enterprises, no further data is known - especially about  the electrochemical performance (cyclability, rate capability, etc.).

Polyanionic compounds they also offer great versatility with respect to the development of new SIB cathode materials for SIBs,  as they are built from the combination of XO₄ tetrahedra (X = S, P, etc.) and MO₆ octahedra (M = Me, Fe). This enables a great deal of tailoring for optimization and maximizing material properties (energy density, stability over charge and discharge cycles, price, safety, sustainability, etc.), simply by adjusting the composition and stoichiometry of the materials.

Although these materials generally have lower capacitity than the layered oxides (typically due to their higher malecular weight) and low electronic conductivity, they are considered very attractive materials due to their higher voltage and structural stability (thanks, partly, to a low volumetric expansion and to the covalent X-O bonds of the polyanion group).

Among the most interesting materials (due to their low cost, sustainability, and abundant constituents),  NaFePO₄, Na₂FeP₂O₇, and Na₄(Fe,Mn)(PO₄)₂(P₂O₇), are the most studied polyanionic materials, - although they present moderate theoretical capacity (~ 100 - 150 mAh/g) and voltages (< 3 V vs. Na+/Na).

In contrast, from an energy density performance perspective, the most interesting materials are Na₃V₂(PO₄)₃ and Na₃V₂(PO₄)₂F₃, which provide a capacity between 100 - 120 mAh/g with an average voltage of 3.4 V vs. Na⁺/Na - although they also incorporate vanadium in their structures, which is considered  to be a  non-abundant and highly toxic element (especially in V⁵⁺ oxidation state).

At the proof-of-concept level , is worth highlighting the system developed by the French R2SE network, which subsequently founded the spin-off Tiamat (France). They announced the first cylindrical cell "186502" using Na₃V₂(PO₄)₂F₃ as cathode material, providing a specific energy density between 90 - 120 Wh/kg. In addition, it was reported to offer excellent cycling stability for a 75 Wh/kg cell (about 4,000 cycles with a capacity retention of more than 80%). In this particular case, the challenge would be to replace vanadium with abundant elements such as Mn and Fe - a strategy that has already been attempted, but so far without success.

The third family of interest are Prussian Blue Analogues (PBAs), with the stoichiometry Na_xMT_M[T_M´(CN)₆]_1-y-zH₂O (T_M = transition metal). They have a 3D structure with large channels through which sodium ions can diffuse rapidly. In addition, they are formed by abundant, cheap, and non-toxic elements.

Their main disadvantages would be their low density, which leads to a low volumetric energy density compared to layered oxides; the presence of water molecules within the crystal structure and the lack of knowledge of  its effect on electrochemistry; and the presence of CN groups in the structure, being able to release HCN (although these compounds have been shown to be thermally stable up to 200°C).

The Na_2-δMnFe(CN)₆-yH₂O composition, developed by Sharp Laboratories of America, Inc. provides a capacity of 140 mAh/g at an average potential of 3.4 V vs. Na⁺/Na, resulting in an energy density of 542 Wh/kg (at the material level) - values close to those of LiFePO₄ in commercial Li-ion batteries.

Subsequently, the spin-off Novasis Energies (USA) developed a proof-of-concept providing 100 - 130 Wh/kg (150 - 210 Wh/L). Thanks to the versatility of these compounds, which can be used as either cathodes or anodes (depending on the chemical composition), the American spin-off Natron Energy (USA) was able to develop a SIB using an with an aqueous electrolyte and a Prussian blue analog for both electrodes . This configuration provides lower energy densities than those obtained with an organic electrolyte, but, on the other hand, allows power density values of 775 W/kg (or 1550 W/L) to be achieved at a current intensity of 12C reaching 25,000 cycles. Moreover, it has been recently reported that Natron has developed a commercial battery – and the BlueTray™ 4000 is available for data centers or telecommunications.

Overview of the Na-ion battery spin-offs launched

Since the renaissance of Na-ion technology in 2010, there have been five sodium battery Spin-Offs founded - although only four of them have been consolidated up to date.

When comparing the different proofs-of-concept independently developed, in addition to assessing the different proposed KPIs, it is also necessary to analyze their advantages and disadvantages (as well as whether a commercial product has already been generated).

It is important to note that in order to make fair and reliable comparisons between the different KPIs it is important to differentiate between those values that have been verified and published (and can therefore be contrasted), and those that have only been announced or are predictions. All these aspects are reflected in the following comparative table:

To summarise, this in-depth analysis of the different Spin-Offs activities reveals that Na-ion is a very promising technology (taking into account its maturity).

In recent years, great progress has been made in the development of different proof-of-concept systems, with even a commercial product  having been generated (though it should be noted that, due to its energy density, its application field is limited). In spite of these great advances, SIB technology still requires research in the development of advanced materials to reach the Li-ion technology, and to be able to generate a competitive technology in the current framework. It is research towards that goal which CIC energiGUNE is currently undertaking.

Sodium-ion battery research at CIC energiGUNE

The aim of the Na-ion battery research line at CIC energiGUNE is to develop SIB materials (cathodes, anodes, and electrolytes), and to optimize  those with the best performance (in terms of physico-chemical, electrochemical, processing, and prototyping properties) in order to develop SIBs which are competitive with their Li-ion counterparts in terms of cost and safety, but with an even lower environmental impact.

To that end, an analysis of the different KPIs is also being carried out to assess the feasibility of Na-ion batteries for use in stationary applications, especially in grid applications, in order to identify those in which Na-ion technology could be applied.

In general, for each possible application, its use will be conditioned by different KPIs, and while cost may be one of the most important, durability, reliability, sustainability, etc. must also be balanced. 

Future research should be focused on developing advanced materials (anode and cathode) that allow us to develop SIBs with energy densities close to 250 Wh/kg, as established in the SET-PLAN Action 7 for 2030, in order to compete directly with LIBs. In addition, development of new advanced electrolytes that allow us to increase the performance (i.e. extend life cycle) at a wide range of temperatures will be a key factor – and it is reasonable to conclude that research targeting these goals will enable SIB implementation in large-scale stationary applications.