Capacity Fading Research on Si Anode in Lithium Ion Batteries

Lithium ion batteries are considered to be the best choice for future hybrid electric vehicles (HEV) and full electric vehicles (FEV).  HEV and FEC require batteries with higher specific energy, energy density, and longer cycle life than those used in consumer electronics. The available lithium ion batteries materials cannot meet the requirements of batteries to be used in FEV and HEV.   The commonly used anode material is graphite. The theoretical specific capacity of graphite is 372 mAh g^-1 and the performance of commercial graphite materials almost reaches their limit.  Silicon anode material has attracted more attention of late due to its high capacity for lithium. Si can form an alloy with lithium (Li_4.4Si) with a theoretical capacity of 4200 mAh g^-1. In the past several decades, although silicon has been considered for use in lithium ion batteries, capacity decay with cycling is still a barrier to its application. It is believed that the large volume changes of Si (～300%) during lithiation and delithiation leads to rapid decay of capacity. Repeated cycling results in pulverization of silicon, which causes structural change of the Si electrode, and newly exposed surface area, which accommodates side reactions [1]. In this presentation, we researched the side reaction rate and investigated the capacity fading mechanism of Si anodes coupled with a lithium iron phosphate (LFP) counter electrode.

A composite electrode of Si (obtained from Umicore) was prepared by our previously reported method [2]. 2325 coin cells were assembled for electrochemical measurements.  LiPF₆-EC/DEC (1.2M, 3/7 wt)+30 wt% FEC was used as electrolyte.

Cycle performance of Si/LFP full cell is shown in Fig.1 a. Capacity fading happened continuously with cycling.  Although the average voltage on charge and discharge drops, the difference remains relatively constant, which means capacity fading is not due to resistance rise. From 3-electrode cell data (in Fig. 1 b), the charge process was limited by anode side, initially, and then switched to the cathode side with cycling. Lithium in the cathode was consumed gradually with cycling. We concluded that the lithium imbalance is one of the factors that lead to the capacity fading. For Si anode, consumed lithium may take part in SEI formation or be trapped in Si. Electron energy loss spectroscopy (EELS) results show that lithium accumulated in Si particles with cycling. Since there is no impedance rise with cycling, lithium accumulation is mainly from lithium trapping inside Si particles.

Particle isolation is another factor in Si anodes to cause capacity fading.  From charge and discharge end point data, we calculated capacity fading rate caused by particle isolation and side reactions. The rate of side reactions is higher than that of particle isolation. C-rate transition from C/10 to C/5 charge and C/3 discharge results in catastrophic particle isolation followed by high rates of side reactions. Eventually, the rate of side reactions drops to their initial rate and particle isolation drops to a low level but this is much past end of life.

Reference:

[1] C. Chan, H. Peng, Y. Cui et al, Nature Nanotechnology 2008, 3, 31-35.

[2] N. Yuca, H. Zhao, X. Song, M. Dogdu, W. Yuan, Y. Fu, V. S. Battaglia, X. Xiao, G. Liu, ACS Applied Materials & Interfaces. 2014, 6, 17111-17118.

   Fig.1. Electrochemical performance of LFP/Si (a. cycle performance, b. voltage profile).