ELECTROLYTE COMPOSITION WITH FLUORINATED ACYCLIC ESTER AND FLUORINATED CYCLIC CARBONATE CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to European patent application No. 19213035.9 filed on December 3, 2019, the whole content of this application being incorporated herein by reference for all purposes. TECHNICAL FIELD The present invention relates to an electrolyte composition comprising a combination of a fluorinated acyclic ester compound, and a fluorinated cyclic carbonate compound. This electrolyte composition is useful in electrochemical cells, such as lithium ion batteries, especially those containing silicon and its derivatives as an anode material. BACKGROUND ART Lithium-ion batteries are a leading battery technology, since they offer efficient and high energy storage as well as high power density, and thus, they dominate the market for batteries used in portable electronic devices. However, large-scale applications like stationary energy storage and electric vehicles still require further improvement of the existing technology in terms of energy density, supplied power and cycle life. The use of silicon as an anode material for lithium ion batteries has attracted tremendous attention because of its high theoretical specific capacity (3580 mAh/g, nearly ten times higher than the typical graphite anode material, 372 mAh/g), appropriate lithium intercalation voltage and cost competency. One drawback of silicon as anode material is its large volume expansion during cycling (more than 300%), which causes cracking and pulverization of silicon particles resulting in sluggish kinetics and a poor cycle life because of the loss of active material and poor electrical contact. The use of silicon in a composite with other elements leads to a composite material showing lower anode capacity values than using pure silicon alone, but it shows better capacity retention with good cycle life. Representative silicon composite materials are silicon-carbon (Si/C) and silicon oxide-carbon (SiOa/C, wherein 0