Effective Electrolyte Additives for Dual-Ion Batteries

Abstract

This work identifies and evaluates effective electrolyte compositions for dual-ion batteries with carbon-based cathodes, supporting the simultaneous intercalation and deintercalation of Li+ and PF6-. The electrolytes were investigated in a variety of dual-ion configurations; aluminium dual-ion, dual-graphite, and lithium half-cells, using electrochemical characterisation techniques such as galvanostatic charge/discharge and cyclic voltammetry to determine their effectiveness during cycling. Dual graphite cells display the most reliable results, enabling prolonged cycling performance over 350 cycles with a 100 % capacity retention when the electrolyte is ethyl methyl carbonate (EMC) 2 M LiPF6 + 3 % vinylene carbonate (VC). Both VC and fluoroethylene carbonate (FEC) additives (3 %) were trialled in an EMC 2 M LiPF6 electrolyte displaying notable performance improvements in Li-half cells, with a ~ 20 % increase in the discharge capacities observed in comparison to the control electrolyte without the presence of the additives. The best Li-half cell performance is achieved when the cathode material is low-defect content graphite (SP1 graphite), resulting in a coulombic efficiency > 95 %. The improved performance of the VC and FEC containing Li-half cells indicates that these additives sufficiently stabilise the cathode/electrolyte interface. Enabling sustained anion intercalation into graphite whilst minimising the degree of parasitic reactions at the cathode due to electrolyte degradation at highly oxidising potentials. However, the transfer of the FEC containing electrolyte into a dual-graphite cell does not significantly improve the cell performance, indicating that the increase in cathode stability as a result of FEC addition is detrimental to the anode stability when a second graphite electrode is introduced. Alternatively, the addition of VC improved the dual-graphite cell performance with a discharge capacity of 61 mAh g-1 at 0.1 A g-1 reported after 350 cycles when the cathode is SP1 graphite. Finally, the importance of high-quality carbon-based cathode materials, that facilitate reversible anion storage, is highlighted throughout this work regardless of the dual-ion cell configuration.

Date of Award	31 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Eric Mcinnes (Supervisor)