In search of an optimized electrolyte for Na-ion batteries

Abstract

Electrolytes are essential for the proper functioning of any battery technology and the emerging Na-ion technology is no exception. Hence, a major focus on battery research is to identify the most appropriate formulation so as to minimize interface reactions and enhance both cell performances and safety aspects. In order to identify suitable electrolyte formulations for Na-ion chemistry we benchmarked various electrolytes containing diverse solvent mixtures (cyclic, acyclic carbonates, glymes) and Nabased salts having either F-based or perchlorate anions and measured viscosity, ionic conductivity, and thermal and electrochemical stability. The binary EC:PC solvent mixture has emerged as the best solvent formulation and has been used to test the performance of Na/hard carbon cells with both NaClO 4 and NaPF 6 as dissolved salts. Hard carbon electrodes having reversible capacities of 200 mA h g À1 with decent rate capability and excellent capacity retention (>180 cycles) were demonstrated. Moreover, DSC heating curves demonstrated that fully sodiated hard carbon cycled in NaPF 6 -EC:PC exhibits the highest exothermic onset temperature and nearly the lowest enthalpy of reaction, thus making this electrolyte most attractive for the development of Na-ion batteries.

Is single layer graphene a promising anode for sodium-ion batteries?

“…This capacity fading may be explained on the basis of the evolution of the charging and discharging cell overpotential during cycling since it was found to increase continuously from cycle 1 to cycle 20 (see plot of the normalized voltage vs capacity profile in Fig. S2 of the Electronic Supporting Information) which may be associated with the increase of the charge transfer resistance of the SEI layer [5]. Therefore, the formation of continuously growing SEI and electrolyte decomposition/consumption may be predicted.…”

“…All these experiments were performed using 1M NaPF 6 (EC:DEC, 1:1, w:w) as the electrolyte solution. In general terms, the intense cathodic and anodic peaks due to the insertion and de-insertion processes of Na + ions in carbon materials [5,7,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] do not appear in the CVs of SLG/Cu electrodes. More specifically, the CV of cycle 1 displays a significant cathodic current that has been previously observed in the study of the interaction of Li + ions with SLG [34] and attributed to the formation of a solid electrolyte interphase (SEI) layer as a consequence of the electrolyte irreversible reduction.…”

“…3, the cells start to show good electrochemical performance in terms of cycling stability as well as coulombic efficiency (≥ 90 %, ≥ 85 % for NaClO 4 in PC electrolyte) after 20 discharge-charge cycles. However, during the initial cycles (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20), these electrochemical parameters, particularly the capacity retention, were much lower.…”

“…To this end, a variety of electrode materials [1][2][3][4], cathode and anode, and electrolytes [1,2,5,6] for SIBs have been investigated, mainly, in the last two years. The results of the research in the field of cathode materials, several of them obtained by the simple replacement of lithium by sodium in the analogue compound, appear promising with a number of layered transition metal oxides, phosphates and fluorophosphates showing acceptable reversible capacity, stability and relatively high operating potentials [1,4].…”

“…Therefore, efforts have been focused on the identification of other host carbon materials to achieve a successful reversible intercalation/insertion of the larger Na + anions. Thus, a variety of carbons having a certain degree of porosity, a low-ordered structure consisting of few-layers graphite nanocrystallites (graphite domains) and different morphologies, including hard and soft carbons [5,7,[11][12][13][14][15], carbon fibers and nanofibers [16][17][18][19][20], spherical carbons [21], hollow carbon nanowires [22] and nanospheres [23], reduced graphene oxide, [24] pyrolytic carbons from graphite oxide [25], as well as highly disordered carbon composites [26,27] have been investigated as anodes for SIBs. Good results as regards capacity and cycling stability were achieved with some of these carbons, the rate capability and particularly, the low coulombic efficiency during the first charge/discharge cycle due to the relatively large surface area (as compared with graphitic carbons) being the main drawbacks to overcome.…”