Exploration of Electrolytes for High Voltage Substituted Lithium Cobalt Phosphate Cathode Materia

Abstract

Next generation lithium ion batteries need higher energy density to displace the current technology, which operate around 4 V. Higher energy density can be achieved by increasing the specific capacity and/or the working voltage of the cathode active material. One such possibility is LiCoPO4 (LCP) which theoretically delivers 167 mAh g-1 at 4.8 V. Unfortunately, LCP undergoes rapid capacity fading in its pure form. The introduction of small quantities of dopants to create a substituted-LCP has greatly increased the cycle life over pure LCP [1]. With the development of new high voltage cathode materials such as LCP comes the need for electrolytes that will enable their implementation. Current state of the art (SOA) LiPF6/carbonate based electrolytes are only stable up to ~4.5 V. Two strategies used to increase the electrochemical window of electrolytes include using different solvents such as fluorinated solvents that are stable to higher voltages or using sacrificial additives that undergo reduction/oxidation reactions before the bulk electrolyte does. The latter approach was explored in a recent study using 4.7 V LiNi0.5Mn1.5O4 (LNMO) spinel [2]. Vinylene carbonate (VC), a common additive for SEI formation, did not work with LNMO/graphite cells due to the fact that VC oxidizes at a much lower potential than the bulk electrolyte but at a higher potential than SOA cathode materials (>4.2 V) as shown by cyclic voltammetry on glassy carbon electrodes. The oxidation of VC at the cathode prevents its incorporation into the anode SEI layer. Another additive, tris(trimethylsilyl) phosphate showed a similar reduction behavior to VC but was more oxidatively stable. This showed the importance of not only the oxidation stability but also the reduction stability that are both necessary for future electrolytes. The oxidation and reduction stabilities of electrolytes with different additives and combinations of additives using LCP cathode material will be reported.