Investigation on gas generation of Li4Ti5O12/LiNi1/3Co1/3Mn1/3O2 cells at elevated temperature
Introduction
Recently, lithium ion batteries have drawn tremendous attention due to the potential application in electric vehicles and smart grid. Lithium titanate (Li4Ti5O12, LTO), which was firstly proposed by J.R. Dahn et al. [1], [2], is regarded as one of the most attractive anode materials for an ultra-long life lithium ion battery owing to its zero volumetric variation [3], [4] and absence of SEI reformation. In addition, this material presented better rate capability in low temperature than graphite as a result of the lower de-solvation energy of Li+-solvents complex on its surface [5]. Therefore, lithium ion batteries with LTO anode have been proposed as one of the most attractive power batteries used in HEV [6], [7], [8].
However, batteries with LTO anode are still not widely used due to the unexpected swelling at elevated temperature, which seriously deteriorated its power density and cyclic stability [9], [10]. In our previous work, it was recognized that gases were produced from LTO side and H2 gas was the extra-dominant species [9]. Recently, He Y.B. et al. studied gassing behavior of LTO battery at room temperature and showed that carbon coated LTO did not cause considerable cell swelling after stored for 3 months or cycled 400 times at room temperature [11]. However, our previous work brought forward that gas generation of LTO battery at elevated temperature must be eliminated and storage at 80 °C for 120 h could be a good accelerating measurement for LTO battery based on our accelerated storage model in view of that its potential HEV and energy storage applications require more than 10 years of service life [9]. To the best of our knowledge, gas generation behavior of LTO battery at elevated temperature has not been fully understood and associated decomposition mechanism of solvents is not very clear yet. In this paper, the inherent reason of gas generation at elevated temperature and the key influencing factors are intensively investigated. The possible mechanisms of carbonates' decomposition are revealed based on cell gases and remnants on LTO electrodes.
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Experiments
Spinel Li4Ti5O12 was synthesized by a solid-state route. Stoichiometric amounts of Li2CO3 (Chinese Lithium Com., 99.9%) and TiO2 (99%, Pule Chem.) with appropriate amount of sugar were ball-milled for 4 h and sintered at 800 °C for 24 h under N2. After cooled down to room temperature, the powder was pulverized and vacuumed in Al-plastic laminate foil for storage. The home-made Li4Ti5O12 coated with 1.6 wt% carbon was used as anode active material. LiNi1/3Co1/3Mn1/3O2 (NMC) and the electrolyte
Influence of water content
The moisture in graphite electrode could be eliminated during formation since the reduction of water was peaked at 1.2 V vs. Li/Li+ [12]. However, large part of the absorbed moisture in LTO electrode remains intact after formation because its working potential is above 1.3 V vs. Li/Li+. The remained moisture will chemically react with PF6− to form POF3, which can catalyze the carbonate solvents decomposing to various species, including CO2 gas [13].
To investigate the influence of water content,
Summary
The mechanism of gas swelling in LTO cells has been investigated in this work. The inherent reason should be the electrochemical potential, instead of electrode moisture or others. As the reactants, electrolyte solvents underwent different decomposing pathways according to their structures. Linear carbonates mainly brought about H2 and soluble species, and cyclic carbonates tended to produce alkylene gas and insoluble salts. Among the investigated systems, PC + DMC (1:1) electrolyte generated
Acknowledgment
The authors appreciate Amperex Technology Limited Com. (ATL) for the financial support. Dr. Deyu Wang and Mr. Chenyun Wang are obliged to Ningbo Key Innovation Team (grant no 2011B82005), 973 project (grant no 2012CB722704), and 100 Talents Program, Chinese Academy of Science.
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