A first review of hard carbon materials as negative electrodes for sodium ion batteries is presented, covering not only the electrochemical performance but also the synthetic methods and microstructures. The relation between the reversible and irreversible capacities achieved and microstructural features is described and illustrated with specific experiments while discussing also the effect of the electrolyte. A summary of the current knowledge is given while emphasizing the possibility of further performance improvements by thoroughly mastering structure-property relationships and also discussing the main current bottleneck to maximize energy density in real applications: the first cycle irreversible capacity. Finally, a short conclusion and perspectives session is provided highlighting necessary developments in the field to turn the present optimistic research prospects into tangible practical products. Intensive efforts aiming at the development of a sodium-ion battery (SIB) technology operating at room temperature and based on a concept analogy with the ubiquitous lithium-ion (LIB) have emerged in the last few years.1-6 Such technology would base on the use of organic solvent based electrolytes (commonly mixtures of alkylcarbonates with a dissolved sodium salt, typically NaPF 6 ) and two high and low potential operation electrodes which would exhibit reversible redox reactions involving sodium ions.In contrast to the large spectrum of suitable positive electrode materials identified, the choice is more restricted for the negative side, as is also the case for LIB. Indeed, sodium titanium oxides operating through intercalation reactions exhibit poor capacity retention 7,8 and alloy based electrodes 5 though promising at the laboratory scale, might suffer from practical bottlenecks derived from the large volumetric changes associated to their redox operation as is the case in LIB. 9,10 To date thus, only carbonaceous materials have practically proved viability.Most types of carbon react with lithium ions to a certain extent at low potential (∼0.1-1 V vs. Li + /Li) and are thus suitable for use as negative electrode materials. Hard carbons can deliver high capacity since the random alignment of small-dimensional graphene layers provides significant porosity able to accommodate lithium, 11 yet the rate capability (power performance) is usually limited and the irreversible capacity (mostly consumed in the formation of the Solid Electrolyte Interphase (SEI)) is higher than that of graphite. This fact coupled to its higher density which results in higher volumetric capacity has contributed to graphite being the most widely used commercial negative electrode material in LIB. Since graphite is not able to insert sodium ions 12,13 unless solvated to form ternary intercalation compounds, 14 non-graphitic carbons were already investigated a few years ago. 15,16These were found to exhibit first cycle reversible capacities in the range of 100-300 mAh/g with substantial fading upon cycling but still enabled the realization...

Sodium-ion batteries (SIBs) are currently being considered for large-scale energy storage. Optimisation of SIB electrolytes is, however, still largely lacking. Here we exhaustively evaluate NaPF 6 in diglyme as an electrolyte of choice-both physico-chemical properties and extensive electrochemical tests including half as well as full cells. Fundamentally, the ionic conductivity is found to be quite comparable to carbonate based electrolytes and obeying the fractional Walden rule with viscosity. We find Na metal to work well as a reference electrode and the electrochemical stability, evaluated potentiostatically for various electrodes and corroborated by DFT calculations, to be satisfactory in the entire voltage range of 0-4.4 V. Galvanostatic cycling at C/10 of half and full cells using Na 3 V 2 (PO 4) 3 (NVP) or Na 3 V 2 (PO 4) 2 F 3 (NVPF) as cathodes and hard carbon (HC) as anodes indicate rapid capacity fading in cells with HC anodes, possibly originating in a lack of a stable SEI or by trapping of sodium. Aiming to understand this capacity fade further, we conducted a GC/MS analysis to determine electrolyte reduction products and to propose reduction pathways, concluding that oligomer and/or alkoxide formation is possible. Overall, the promising results should warrant further investigations of diglyme based electrolytes for modern SIB development, albeit avoiding HC anodes.

A study of the Solid Electrolyte Interphase (SEI) on hard-carbon (HC) electrodes in sodium half-cells is presented. Electrochemical performances over > 100 cycles were compared with two different salts (NaPF6, NaTFSI) and two different electrolyte additives (FEC, DMCF) in a mixture of EC and DMC solvents. The best electrochemical performances were observed with NaPF6 salt in conjunction with 3% FEC. The DMCF additive had a detrimental effect in all electrolyte combinations. The chemical characterization of the SEI was carried out by X-ray Photoelectron Spectroscopy (XPS) and showed that the best electrochemical behavior was related to an SEI composition based on sodium ethylene dicarbonate and NaF, whereas poorer electrochemical performances were associated to either low NaF or high Na2CO3 content. The results reported herein provide an insight on the SEI chemistry on hard carbon electrodes in sodium cells after long-term cycling, as a complement to previous studies dealing with the first cycles.

Hard carbon materials were prepared from different precursors (phenolic resin and commercially available cellulose and lignin) under different pyrolysis and processing conditions using industrially adapted syntheses protocols. The study of their microstructural features enabled to assess that the nature of the precursor and the temperature of pyrolysis are the major factors determining the carbon yield and the surface area, the latter one having a major effect on the electrochemical capacity.Finally, the presence of surface groups and physisorbed water can also play a role both on the maximum reversible capacity achievable (by influencing the interaction of sodium ions with the hard carbon surface) and the irreversible capacity.Phenolic resin combining high carbon yield (~50%), tap density (0.7 gcm -3 ) and reversible capacity (249 mAh/g) was found to be the precursor producing the most suitable hard carbon for practical use in Na-ion batteries. Cellulose can be a good candidate as well, the lower carbon yield being counterbalanced by its lower price and higher capacity (280 mAh/g).