Many of the battery components in both sodium-ion and lithium-ion batteries are similar due to the similarities of the two technologies. This post provides a high-level overview of sodium-ion battery materials.
Cathode materials
- Polyanion-type materials: Similar in structure to LFP offering structural stability, with good cycling performance with a desirable operational voltage.[1] However, they are limited by poor conductivity. Researchers are studying numerous strategies for improving the conductivity, including surface-conducting modifications, doping and optimisation of morphology.[2]
- Prussian blue analogues (PBAs): Highly tuneable family cyano-coordination polymers that adopt nano porous open frameworks, providing rapid ion conduction.[3] PBAs experience minimal changes in crystallographic geometry during the (de-)intercalation of sodium, yielding long cycle lives. However, PBAS offer relatively low energy densities.
- Sodium layered oxides: Similar structures to lithium layered oxides with the general formula of NaxMO2, where M can be considered a mix of metals.[4] The precise composition determines the structure of the material and its properties. Leading layered oxides offer high theoretical specific capacity and energy density.[5] However, significant structural changes are common during cycling, which lead to interfacial degradation and significant capacity fading.
Anode materials
- Hard carbon: High capacity and low cost. However, there are safety issues because the charge reaction potential is very close to the deposition potential of metallic sodium. R&D is underway to enable higher capacities.
- Soft carbon: Higher voltage than hard carbon, but has the disadvantage of lower capacity.[6]
- Prussian blue analogues: These materials can be used as both anode and cathode materials. They are characterized by high current and long cycle life. However, it has the lowest energy density of the various candidates.
Metallic anodes are also being researched for Na-ion batteries, although these are not yet being considered for commercial applications.
Electrolytes and Cell Components
Electrolytes of sodium ion batteries are typically made up of a metal salt dissolved in an organic solvent. Sodium salts such as NaClO4 and NaPF6 can be used. However, NaClO4 comes with the risk of explosion, while NaPF6 comes with the risk of reacting with water to generate toxic hydrogen fluoride. Organic solvents such as those used in Lithium-ion systems (e.g.: dimethyl carbonate) can be used.
Other cell constituents, separators, are analogous to those used in Li-ion systems. Note that aluminium can be used for both anode and cathode current collectors as sodium does not alloy with aluminium at lower voltages.
[1] Qiao Ni et al., Polyanion‐Type Electrode Materials for Sodium‐Ion Batteries, 2017, Advanced Science, https://doi.org/10.1002/advs.201600275
[2] Yongjin Fang et al., Hierarchical Carbon Framework Wrapped Na3V2(PO4)3 as a Superior High‐Rate and Extended Lifespan Cathode for Sodium‐Ion Batteries, 2015, Advanced Materials, https://doi.org/10.1002/adma.201502018
[3] Kevin Hurlbutt et al, Prussian Blue Analogs as Battery Materials, 2018, Joule, https://doi.org/10.1016/j.joule.2018.07.017
[4] Kei Kubota et al., Layered oxides as positive electrode materials for Na-ion batteries, 2014, Cambridge University Press, https://doi.org/10.1557/mrs.2014.85
[5] Caihong Shi et al., Challenges of layer-structured cathodes for sodium-ion batteries, 2022, Nanoscale Horizons, https://doi.org/10.1039/D1NH00585E
[6] Hongshuai Hou et al., Carbon Anode Materials for Advanced Sodium-Ion Batteries, 2017, Advanced Energy Materials, https://doi.org/10.1002/aenm.201602898