Rechargeable lithium-ion batteries (LIB) with high specific energy and high specific power, long life, low self-discharge, and wide work-temperature range, have been considered to be one of the most promising energy storage system. In daily use lithium-ion batteries are often exposed to great stress like high ambient temperature and high storage voltages, so calendar aging is permanently present, arising independently from other stress like cycling. This may influence electrochemical performance, kinetics and safety of the battery.

Overcharge performance is an important feature of a battery. Most of these studies are emphasizing the importance of the overcharge performances of the battery materials. In present study, the heat-release and gas evolution during overcharge process for calendar aged 10 Ah LTO pouch batteries  are investigated by several analytical methods, with the positive of a mixture of LiNi_1/3Mn_1/3Co_1/3O₂ and LiCoO₂ ( m : m = 7:3) and the electrolyte of 1.2 M LiPF₆ in 3:7 EC/EMC). Ten identical cells are stored at 55℃ and a voltage of 2.75 V (SOC100%). The batteries are stored at open circuit potential and the storage is over up to 5 months.

The trace gas is observed when the storage time is arrived to 3 months. Seriously, the amount of evolution gas is large when up to 5 month. The evolved gase consists of carbon dioxide (CO₂), carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), propylene (C₃H₆), and Propane (C₃H₈), as shown in Fig. 1. Especially, the amount of CO₂ gas is much higher than the other gas. The carbon dioxide (CO₂) is mainly from the decomposition of EC ^[1]. Meanwhile, the residual capacity of the batteries is gradually decreased to 86% after stored with 5 months, accompanied by the increasing of the internal resistance.

The LTO pouch batteries with the pristine, aging for 3 months, 4 months and 5months are test by overcharge under adiabatic condition. The batteries are charged with 1C from SOC 100% to a voltage of 4 V. All the LTO pouch batteries show no thermal runaway in the overcharge process. With the decrease of the residual capacity, the raising range of the temperature and average heat power of the battery in the overcharge process decreases, suggesting the overcharge property of LTO batteries aged for 5 months is improved, as shown in Fig.2. The internal gas of LTO pouch batteries with pristine and aged for 5 months after overcharged are collected by a micro-syringe, and analyzed by gas chromatography - mass spectroscopy, as shown in Fig. 1. The amount of methane (CH₄) and ethane (C₂H₆) is obviously increasing after overcharge, which may be from the decomposition of EMC ^[2].

This work was supported by National 863 Program (No. 2013AA050902) and Shanghai technological innovation (No.14JC1491800).

References

[1] I. Belharouak, G.M. Koenig, Jr., T. Tan, H. Yumoto, N. Ota, K. Amine,  J. Electrochem. Soc., 159 (8) A1165-A1170 (2012).

[2] Y. Takei, K. Takeno, H. Morimoto, S. Tobishima, J. Power Sources 228 (2013) 32-38.