The role of Li-ion battery electrolyte reactivity in performance decline and self-discharge

The purpose of this paper is to report on the reactivity of PF₅ and EC/linear carbonates to understand the thermal and electrochemical decomposition reactions of LiPF₆ in carbonate solvents and how these reactions lead to the formation of products that impact the performance of lithium-ion batteries. The behavior of other salts such as LiBF₄ and LiTFSI are also examined. Solid LiPF₆ is in equilibrium with solid LiF and PF₅ gas. In the bulk electrolyte, the equilibrium can move toward products as PF₅ reacts with the solvents. The Lewis acid property of the PF₅ induces a ring-opening polymerization of the EC that is present in the electrolyte and can lead to PEO-like polymers. The polymerization is endothermic until 170 °C and is driven by CO₂ evolution. Above this temperature the polymerization becomes exothermic and leads to a violent decomposition. The PEO-like polymers also react with the PF₅ to yield further products that may be soluble in the electrolyte or participate in solid electrolyte interphase (SEI) formation in real cells. GPC analysis of the heated electrolytes indicates the presence of material with M_w up to 5000. More details on the polymerization reactions and further reactions with PF₅ are reported. Transesterification and polymer products are observed in the electrolytes of cycled and aged Li-ion cells. Formation of polymer materials which are further cross-linked by reaction with acidic species leads to degradation of the transport properties of the electrolyte in the composite electrodes with the accompanying loss of power and energy density. Generation of CO₂ in lithium-ion cells leads to saturation of the electrolyte and cessation of the polymerization reaction. However, CO₂ is easily reduced at the anode to oxalate, carbonate and CO. The carbonate contributes to the SEI layer while the oxalate is sufficiently soluble to reach the cathode to be re-oxidized to CO₂ thus resulting in a shuttle mechanism that explains reversible self-discharge. Irreversible reduction of CO₂ to carbonate and CO partially accounts for irreversible self-discharge.