Electrode materials for lithium-ion batteries (LIBs) typically show spherical particle shapes. For cathode materials, the spherical shape is obtained through the synthesis method. For graphite, the by far most popular anode material for LIBs, spherical particles are obtained through a spheroidization process. The yield of that process is quite low and limited to about 50%, leaving substantial amounts of by-products. Using such lower quality by-products would be quite attractive for developing low-cost energy stores like sodium-ion batteries (SIBs), for which the requirements for particle sizes and shapes might be less strict as compared to high performing LIBs. Here, we study three different graphite 'waste fractions' as anode material for SIBs that are obtained from the spheroidization process and how they compare to LIB battery grade material. Only negligible differences between the fractions are found when analyzing them with x-ray diffraction (XRD), Raman spectroscopy and elemental analysis (EA). More clear differences can be seen from N₂ physisorption, scanning electron microscopy (SEM) and particle size analysis. For example, the surface areas of the 'waste fractions' can become roughly up to twice as large as compared to the battery grade fraction and the d₅₀ values shift by up to 11.9 µm to lower numbers. Electrochemical measurements show that the 'waste fractions' can deliver the full electrode capacity and behave similar to the battery grade fraction up to 10 C. However, the higher surface areas lead to more irreversible losses in the first cycle. A surprising finding is that all graphite fractions show almost identical discharge voltages, while the charging voltages differ by as much as 200 mV. This asymmetric behavior only occurs in SIBs and not in LIBs, which indicates a more complex storage behavior in case of sodium.

We present a combination of experiments and theory to study the effect of sulfur doping in hard carbons anodes for sodium-ion batteries. Hard carbons are synthesised through a two step process: hydrothermal carbonisation followed by pyrolysis of a biomass-derived carbon precursor. Subsequent sulfur doping is introduced via chemical-vapour deposition. The resulting sulfur-doped hard carbon shows enhanced sodium storage capacity with respect to the pristine material, with significantly improved cycling reversibility. Atomistic first principles simulations give insight into this behaviour, revealing that sulfur chemisorbed onto the hard carbon increases the sodium adsorption energies and facilitates sodium desorption. This mechanism would increase reversible Na storage, confirming our experimental observations and opening a pathway towards more efficient Na-ion batteries.

Sodium layered oxides which display oxygen anion redox behaviour are considered promising positive electrodes for sodium-ion batteries because they offer increased specific capacities. However, they suffer from irreversible structural changes resulting in significant capacity loss and limited oxygen redox reversibility. Here the effect of Cu substitution on the electrochemical performance of P3-type sodium manganese oxide is examined by evaluating the structural and electronic structural evolution upon cycling, supported by density functional theory (DFT) calculations. Over the voltage range 1.8–3.8 V vs. Na/Na⁺, where the redox reactions of the transition metal ions contribute entirely towards the charge compensation mechanism, stable cycling performance is maintained, showing a capacity retention of 90% of the initial discharge capacity of 166 mA h g⁻¹ after 40 cycles at 10 mA g⁻¹. Over an extended voltage range of 1.8–4.3 V vs. Na/Na⁺, oxygen anion redox is invoked, with a voltage hysteresis of 110 mV and a greater initial discharge capacity of 195 mA h g⁻¹ at 10 mA g⁻¹ is reached. Ex-situ powder x-ray diffraction patterns reveal distortion of the P3 structure to P'3 after charge to 4.3 V, and then transformation to O'3 upon discharge to 1.8 V, which contributes towards the capacity fade observed between the voltage range 1.8–4.3 V. DFT with projected density of states calculations reveal a strong covalency between the copper and oxygen atoms which facilitate both the cationic and anionic redox reactions in P3-type Na_0.67Mn_0.9Cu_0.1O₂.

Prussian white (PW, ${\text{N}}{{\text{a}}_2}{\text{Fe}}\left[ {{\text{Fe}}{{\left( {{\text{CN}}} \right)}_6}} \right] \cdot {\text{z}}{{\text{H}}_2}{\text{O}}$) is a promising cathode material for use in sodium-ion batteries for large-scale energy storage applications, which demand long cycling life-times. However, for non-aqueous battery applications PW must not contain any water, and yet dehydration induces a large volume change destabilizing the structure and reducing the cycling life. The material undergoes multiple phase transitions as a function of both the sodium and water content, however, the mechanism behind is poorly understood. Here, we use neutron diffraction to explore the influence of water on the structure of PW. For the first time, two structures for a single composition of PW were observed near room temperature independent of the synthesis method. These structures differ in the FeN₆ and FeC₆ octahedral tilting configurations, which is connected to the ordering of water in the framework. The removal of water modulates the magnitude of pre-existing structural distortions, if it is itself disordered within the structure, rather than modifying the nature of the distortions. These results provide a robust fundamental understanding of the chemical driving force impacting the nature and magnitude of structural distortions in Prussian blue analogues. The insights provide guidance for designing tilt-engineering ultimately enabling new materials with enhanced long-term electrochemical performance in battery applications.

With the increasing energy demand for both electronic portable devices and energy storage for fluctuating renewable energy sources, there is a strong need for alternatives beyond lithium batteries. Sodium batteries have been attracting great attention recently due to the abundance and low supply cost of the raw materials. However, they require highly conductive, safe and electrochemically stable electrolytes in order to enable their practical realization. In this work we present the promising physicochemical properties of the electrolyte based on hexamethylguanidinium bis(fluorosulfonyl)imide [FSI] at a sodium concentration of 25 mol% NaFSI. The liquid-state electrolyte supports stable Na plating and stripping at 1 h polarization times at 0.5 mA cm⁻² current density in a Na symmetrical coin cell at 50 °C, maintaining a low polarization potential of ≈45 mV throughout 160 cycles. Moreover, this electrolyte is characterized by relatively high Na-ion transference number of 0.36 ± 0.03 at 50 °C. A long cycle life of 300 cycles with 285 mAh g⁻¹ is achieved in a half cell set up with hard carbon. The solid-electrolyte interphase layer on the anode, which contributes to this high capacity, is investigated by x-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance spectroscopy. The long-term cycling performance of Na|NaFePO₄ cell is also demonstrated with a high specific capacity of 106 mAh g⁻¹ and 80% capacity retention after 110 cycles.

Practical utilisation of renewable energy from intermittent sustainable sources such as solar and wind relies on safe, reliable, cost-effective, and high-capacity energy storage systems to be incorporated into the grid. Among the most promising technologies aimed towards this application are sodium-ion batteries(SIBs). Currently, hard carbon is the leading negative electrode material for SIBs given its relatively good electrochemical performance and low cost. Furthermore, hard carbon can be produced from a diverse range of readily available waste and renewable biomass sources making this an ideal material for the circular economy. In facilitating future developments on the use of hard carbon-based electrode materials for SIBs, this review curates several analytical techniques that have been useful in providing structure-property insight and stresses the need for overall assessment to be based on a combination of complementary techniques. It also emphasises several key challenges in the characterisation of hard carbons and how various in situ and operando techniques can help unravel those challenges by providing us with a better understanding of these systems during operation thereby allowing us to design high-performance hard carbon materials for next-generation batteries.