As for the anode materials of next generation of Lithium Ion batteries, the silicon will be attractive one because of its large energy density, 3750-4200mAh/g. However the low first cycle efficiency and large volume change are major challenges to realize industrial application (1). The objectives of this study are to improve the first cycle efficiency and to achieve less volume change. We take an approach to find out a solution by adopting Nano-silicon particles modified by multi-wall CNT, agglomerated like a moss ball, and doing the first lithiation /delithiation to/from these particles directly in particulate form by electrolysis. The characteristics of these particles will be reported.
Experimental
The moss ball like Nano silicon, modified by multi-wall CNT, agglomerated particles are prepared as followings; the Nano-silicon particles are prepared by grinding from size of micron to Nano order. Nano silicon particles are mixed with nickel acetate and dried by spray dryer and calcined to make precursor of modified with multi-wall CNT. The precursor are fed with methane gas to the reactor (Direct Methane Reforming method) in order to grow multi-wall CNT on/in these particles at 600-700℃ (2)(3). The disk type electrode is made by pressing the moss ball like Nano silicon, modified by multi-wall CNT, agglomerated particles without binder in die, and the electrode is set in coin cell that provide Li metal counter electrode, separator and electrolyte, 1M Li-TFSI/EC:DEC(1:2). The cell is discharged (0.01C)/charged (0.005C) by one cycle. After discharge/ charge, the cell is dismounted to remove the disk electrode and wash it by DMC then the first cycle finished particles are obtained.
The similar process is applied to moss ball like Nano silicon, modified by multi-wall CNT, agglomerated particles by electrolysis treatment in a special designed equipment.
Results and Discussion
The discharge/ charge characteristics of disk type electrode cell and a coated type electrode cell, that use the same particles (76 wt. %) coated on Cu foil with binder, PAI (26 wt. %), show in Fig.1 and Fig.2. Comparing with these graph, discharge/charge performance are shown a similar tendency. After first cycle finished particles are analyzed by XRD and Raman spectrum. The results of analysis show Si (111) peak is reduce comparing with pristine one by XRD and the amorphous phase of Si is detected by Raman spectrum (4) (5). Based on these results, the large volume change in electrode will be expected to reduce, and first cycle efficiency will be also expected to improve by applying of this material for Li-ion batteries.
Acknowledgements
This work is supported by Nano carbon materials project in NEDO 2014.
Reference
1) NEDO “ The technology road map of rechargeable batteries in 2013” Aug. 2013
2) K.Ogisu, Y.Kouno,T.Ueno,T.Toda and N.Okazaki IMLB 2012 Poster #3P-10
3) A.Tada and N.Okazaki J. Fine Chemicals (in Japanese) 2015 44(8) 33-38
4) M.N.Obrovac and Lief Christensen Electrochemical and Solid State Letter 2004 7(5)A93
5) K.Ogata, E.Salager et al NATURE COMMUNICATIONS DOE|:10.1038/ncomms 4217