Synthesis and electrolyte design for sodium-ion batteries Darren Ould, Svetlana Menkin, Holly Smith, Christopher A. O’Keefe, Clare P. Grey, Dominic S. Wright Cambridge University, UK The global drive towards electrification means suitable energy storage solutions are needed. Lithium-ion batteries (LIBs) currently lead the way in battery technology, but due to the relative low abundance and rising cost of lithium, alternative battery technologies are urgently required. Sodium-ion batteries (SIBs) are a promising emerging technology, given the greater availability of sodium and lower cost. Moreover, the use of sodium has further sustainability implications, as it allows cobalt-free cathodes to be used and the copper current collectors at the anode to be replaced by aluminium.[1] Currently, NaPF6 dissolved in carbonate solvent is the benchmark electrolyte for SIBs.[2] However, NaPF6 is highly hygroscopic and reacts with water to form NaF, POF3 and HF.[3] The formation of NaF causes solubility problems and is often present in commercial supplies,[4] whereas POF3 and HF are toxic and present severe safety concerns. This poster firstly showcases the synthesis of battery-grade NaPF6 from the addition of sodium metal with NH4PF6, which allows for studies of higher concentration (beyond 1 M) due to the absence of NaF. Following this, the use of sodium borate complexes as alternative electrolyte salts is presented.[5] The borate salts can be prepared by either using sodium borohydride or a tricoordinate borate as starting materials; the chosen synthesis route is dependent on the acidity of the starting alcohol. Electrochemical battery testing on the sodium borate salts found that Na[B(hfip)4]·DME (hfip = hexafluoroisopropyloxy, OiPrF) and Na[B(pp)2] (pp = perfluorinated pinacolato, O2C2(CF3)4) have superior electrochemical performance. These two salts give greater cycling stability, a more stable electrode-electrolyte interface and comparable cycling capacity compared to NaPF6. Furthermore, DFT studies were undertaken to compare the ion-pair association and oxidative stability of the synthesised salts, which aided explanation to the superiority of Na[B(hfip)4]·DME and Na[B(pp)2]. References 1. P. K. Nayak, L. Yang, W. Brehm, P. Adelhelm, Angew. Chem. Int. Ed., 2018, 57, 102–120. 2. A. Ponrouch, E. Marchante, M. Courty, J.-M. Tarascon, M. R. Palacín, Energy Environ. Sci., 2012, 5, 8572–8583. 3. P. Barnes, K. Smith, R. Parrish, C. Jones, P. Skinner, E. Storch, Q. White, C. Deng, D. Karsann, M. L. Lau, J. J. Dumais, E. J. Dufek, H. Xiong, J. Power Sources, 2020, 447, 227363. 4. D. M. C. Ould, S. Menkin, C. A. O’Keefe, F. Coowar, J. Barker, C. P. Grey, D. S. Wright, Angew. Chem. Int. Ed. 2021, 60, 24882–24887. 5. D. M. C. Ould, S. Menkin, H. E. Smith, V. Riesgo-Gonzalez, E. Jónsson, C. A. O’Keefe, F. Coowar, J. Barker, A. D. Bond, C. P. Grey, D. S. Wright, Angew. Chem. Int. Ed. 2022, e202202133.
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