Na-ion Batteries. Laure Monconduit

Na-ion Batteries - Laure Monconduit


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also competitive in terms of cost, performance and safety and is already developed to be introduced rapidly into the market.

      I.2.1. Positive electrodes

      The first two chapters are dedicated to the positive and negative electrodes for a NIB. Among the materials intensively studied as prospective positive electrode for NIBs are polyanionics, layered oxides, Prussian blue analogues and organic compounds.

      Among the promising positive electrode materials for NIBs, although achieving lower specific capacity than the layered oxides, polyanionic compounds represent an important class of materials. Indeed, they are characterized by high operating voltage, excellent chemical stability and life span. A large panel of structures and compositions exist, with as for layered oxides the possibility to tailor significantly the operating voltage, and thus the energy density, by varying the nature of the involved redox couple. Today, the most attractive materials are vanadium-containing phosphates, characterized by two different structures, Na3V2(PO4)3 and Na3V2(PO4)2F3. Chapter 2 shows the versatility of these families of compounds, highlighting their differences, their interesting properties and the challenges for their development in the next generations of NIBs.

      I.2.2. Negative electrodes

      Carbon materials present great interest as negative electrodes because of their low cost and environmental benignity, as well as their excellent electrochemical stability. In LIBs, graphite is the standard negative electrode. It exhibits a low operating voltage and 370 mAh/g specific capacity based on reversible Li deintercalation and reintercalation from/in between the graphene layers. Huge efforts were devoted to using it in NIBs (Stevens and Dahn 2001). Nevertheless, due to the thermodynamic instability of stage 1 Na-GIC formation, graphite was rapidly abandoned as the negative electrode. More recently, it was shown that when used in linear ether solvents, co-intercalation of ether molecules was observed and beneficial for Na+ intercalation in between graphene layers even though it does not lead to acceptable cycles performance (Kim et al. 2015).

      As the utilization of graphite as the negative electrode for NIBs failed, non-graphitic carbon materials such as hard carbon and soft carbon have been extensively investigated as alternatives (Dahbi et al. 2014). Chapter 3 discusses the main synthesis routes used to prepare hard carbons and the impact of both the nature of the precursor and synthesis conditions on the hard carbon features. Relationships between these synthesis conditions and the electrochemical properties are discussed in details, with a specific focus on the sodium storage mechanism, still a debate in literature.

      I.2.3. Electrolytes and the solid electrolyte interphase

      The electrolytes are essential for the proper functioning of any battery technology, with an important focus to minimize interface electrolyte/electrode reactions and enhance both performance and safety. First studies surveyed electrolytes prepared using classical alkyl carbonates solvents and mixtures, in combination with different Na salts (Ponrouch et al. 2012). Their viscosity, ionic conductivity, thermal and electrochemical stability were evaluated to establish some intrinsic trends to identify the first electrolyte formulations with the widest range of applicability for NIBs.

      More recently, IL, defined as room temperature molten salts and composed mainly of organic cations and (in)organic anions and presenting a huge versatility of structural variations, were proposed as major alternatives for the development of optimized electrolytes. Indeed, IL offer unique physical and chemical properties associated with low volatility that make them extremely interesting for the development of electrolytes with higher electrochemical and thermal stability. Chapter 5 will give the major trends for the IL-based electrolytes developed for NIBs.

      Although LIBs are unquestionably the leader for electrochemical energy storage, their costs remain high and dependent on the demand that continuously and exponentially increases. In contrast, owing to the abundance of sodium, the price of NIBs could be preserved even with a large demand for energy storage. Furthermore, due to the proximity of their chemistry, the technology developed for LIBs should be promptly transferable to NIBs and easily upscaled. Widespread commercialization and mass production


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