High Rate and Long Cycle Life Anodes via Atomic Layer Deposition of Titanium Dioxide Coatings on Mesoporous Activated Carbon

Titanium dioxide (TiO₂), a candidate as anode active material for lithium ion batteries, has attracted much attention from researchers due to its safety characteristics arising from its high electrochemical potential (~1.5V) vs. Li/Li⁺, low cost, material availability and environmental benignity.  However, low electric conductivity, sluggish Li⁺ diffusivity and poor cycle performance of TiO₂ have limited its wide acceptance in practical rechargeable lithium ion batteries.  Various strategies such as TiO₂-C-nano metal composites [1], nanofibers [2], and TiO₂ flakes [3], have been investigated in literature work.  While these studies led to improved performance to various degrees, they were limited either in rate (<20C) or cycle life (<100 cycles) or both.  A recent work [4] involved atomic layer deposition (ALD) of TiO₂on graphene powders and demonstrated vastly improved rate and cycle capabilities of the resulting materials.    

Here we present an alternative and promising method of preparing extremely high rate and long cycle life anodes via ALD coatings of TiO₂on mesoporous activated carbon (AC) electrodes.            

Figure 1 shows charge and discharge voltages between 1.5V and 3.0V (vs. Li/Li⁺) of an ALD TiO₂-AC and a mesoporous AC baseline coin half cell versus specific capacity based on total active materials.  Conventional carbonate-based lithium ion electrolyte was used.  The ALD TiO₂-AC cell demonstrated nearly five-fold capacity increase against the baseline with voltage profiles typical of nano TiO₂.  The contribution of ALD TiO₂to the observed capacity is ~83%, which leads to ~73 mAh/g specific capacity.  The relatively low specific capacity is because of the 1.5V cutoff voltage used in this study. 

Figure 2 shows rate performance of ALD TiO₂-AC vs. AC baseline half cells.  The ALD TiO₂-AC demonstrated very high rate capability, e.g. delivering 70% of low rate capacity at 400C rate, surpassing that of AC baseline.       

Figure 3 shows excellent cycle stability of ALD TiO₂-AC half cell with 80% capacity retention after 800 cycles. 

The remarkable electrochemical performance demonstrated from ALD TiO₂-AC is attributed to ALD enabled TiO₂nano structure.  Such nano structure is responsible for facile lithium insertion/extraction leading to extremely high rate capability and long cycle life due to stress/stain free nano architecture.  The results in this study suggest a promising material processing methodology for high rate and long cycle life anodes for lithium ion batteries used in applications that require fast rate capability, when implemented with high throughput, high volume ALD processes.  

Figure 1.  Charge and discharge voltage profiles of ALD TiO₂-AC/Li vs. AC/Li baseline half cells.

Figure 2.  Rate capability tests of ALD TiO₂-AC/Li vs. AC baseline half cells.

Figure 3.  Cycle life test of an ALD TiO₂-AC/Li cell.

References

1. Song Gyun Lee, Honggui Deng, Jun Hu, Lihui Zhou, Honglai Liu, Int. J. Electrochem. Sci., 8, 2204 (2013).
2. Zunxian Yang, Guodong Du, Qing Meng, Zaiping Guo, Xuebin Yu, Zhixin Chen, Tailiang Guo, and Rong Zeng, J. Mater. Chem., 22, 5848 (2012).
3. Ming-Che Yang, Yang-Yao Lee, Bo Xu, Kevin Powers, Ying Shirley Meng, J. Power Sources, 207, 166 (2012).
4. Chunmei Ban, Ming Xie, Xiang Sun, Jonathan J Travis, Gongkai Wang, Hongtao Sun, Anne C Dillon, Jie Lian and Steven M George, Nanotechnology 24, 424002 (2013).