The discharge capacities of spinel-type Li 1.1 Mn 1.9 O 4 /graphite cells charged in electrolytes with solid electrolyte interphase ͑SEI͒-forming additives are investigated after being stored at 60°C. The presence of Mn deposits on the anode surface, which is responsible for the capacity fading of cells, is clearly shown by means of open-circuit voltage, ex situ X-ray diffraction, and energy-dispersive spectrometry measurements. Unlike fluoroethylene carbonate, using vinylene carbonate as an SEI former leads to a noticeable improvement in the discharge capacity retention of cells that were stored for 20 days at 60°C.Lithium-ion ͑Li-ion͒ batteries have been used as reliable power sources for portable electronic devices. 1 Furthermore, Li-ion batteries are very attractive for applications in electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles owing to their high energy density and long lifetime. A high energy density in batteries can be achieved by increasing the discharge capacity of the cathode or by augmenting the working potential of the cathode materials. 2 Lithium manganese oxides ͑LiMn 2 O 4 ͒ with a spinel structure have been extensively studied for high energy and high power batteries to replace lithium cobalt oxides ͑LiCoO 2 ͒ as cathode materials. LiMn 2 O 4 has the potential to create batteries that are low cost, eco-friendly, have a prolonged lifecycle, and are safe to operate. 3,4 The capacity fading mechanisms of LiMn 2 O 4 are complex, especially at elevated temperatures, and the capacity fading of batteries with spinel LiMn 2 O 4 cannot be explained solely via the loss of cathode materials. Dissolved Mn ions undergo reduction on the anode and thereby lead to self-discharge of the lithiated graphite anode. Even very small amounts of manganese in an electrolyte can affect the calendar life of Li-ion batteries for large-scale power sources. Therefore, it is necessary to restrain the manganese dissolution to attain highly reliable batteries. Recently, it has been established that the partial substitution of manganese ions with transitionmetal ions, such as Co, Cr, or Ni, enhances the structural stability and electrochemical performance of lithium manganese oxides with a spinel structure. 5 Surface coatings with lithium cobalt oxide and other oxides seem to improve the performance of spinel lithium manganese oxides at elevated temperatures. 6,7 It was reported that a considerable degree of Mn dissolution into the electrolyte occurs in the presence of HF that is formed by the hydrolysis of LiPF 6 salts and deposits on the anode surface. [8][9][10] In the present study, we aim to understand the influence of Mn dissolution from a delithiated lithium manganese oxide cathode on the capacity retention behavior of fully charged cells that have been stored at 60°C. We also report intriguing data on the roles of SEIforming additives that could mitigate severe self-discharge in a lithiated graphite anode caused by Mn deposits. ExperimentalAn electrolyte solution composed of a commerc...
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