Effects of Reduced Dimensional Scales on the Electrochemical Reversible Li-Storage and Associated Integrity of Well-Ordered Graphitic/Graphenic Carbon
Authors: Sonia Farjana Jaishmin , Jangid Manoj Kumar , Ananthoju Balakrishna , Kali Ravi , Aslam M , Johari Priya , Mukhopadhyay Amartya 
Published: February 2020

Even though graphitic carbon is the most commonly used anode material for Li-ion battery, it possesses low gravimetric capacity and rate capability; which has stimulated research towards alternate anode materials. Recent investigations have indicated that graphene, the two dimensional building block of graphitic carbon, may possess enhanced Li-storage capacity, along with improved rate capability. However, the mechanism of reversible Li-storage and mechanical/structural integrity of graphene upon repeated Li-insertion/removal are yet to be understood; with too many contradictory reports are available in the literature concerning the Li-storage reversible capacity/behavior and cyclic stability of graphenic carbon [1, 2].
Therefore, using fairly pristine and well-ordered CVD-grown few layers graphene (FLG) film as model material and performing extensive electrochemical/structural investigations, also involving in-situ studies, we have been able to develop better insights into the inter-related phenomena concerning lithiation/delithiation mechanisms leading to higher Li-capacity, stress developments at different states of charges and mechanical integrity upon lithiation/delithiation of such nanoscaled ‘graphitic’ carbon. Presence of distinct features corresponding to formation of Li-intercalated graphite compounds below 0.3 V against Li/Li+ in chronopotentiograms (viz., potential plateaus) and cyclic voltammograms (viz., current peaks) obtained with FLG (~7 graphene layers), similar to thicker (~450 nm) bulk graphite (TBG) film, indicates that Li-intercalation/de-intercalation in the interlayer spaces does take place even for such reduced (nano)dimensional form of ‘graphitic’ carbon. Furthermore, the capacity obtained for FLG within just the same lower potential window (i.e., below 0.3 V) and even after subtracting entire possible contribution from Cu substrate underneath, is ~3-4 times greater than the overall specific capacity obtained with TBG (i.e., ~380 mAh/g) [3]. Analysis based on the electrochemical response indicated that this excess capacity in the case of FLG (or Li-storage in FLG below 0.25 V) has contributions from both diffusion controlled ‘classical’ bulk processes of Li-storage, as well as surface phenomena. In this context, simulation based on density functional theory indicated that the excess Li-capacity of graphene (beyond the LiC6 composition) is associated with additional stable Li-storage on the surface of the outer or topmost graphene layer in the form of two Li-layers (but different from Li-plating) and segregation of Li close to the ‘stepped’ exposed edges of the inner graphene layers; the relative contributions from which would go down with increase in dimensional scale [3].
Even though the nanoscaled FLG possess higher specific capacity, as well as improved rate capability, with respect to the thicker TBG, cyclic stability of FLG over the full range of lithiation/delithiation (i.e., between 2 and 0.001 V against Li/Li+) was observed to be inferior w.r.t. TBG. Interestingly, the cyclic stability, when compared with respect to the capacities obtained just within the lower potential window (i.e., below 0.25 V), was not inferior to that for TBG. In order to gain further insights into this, in-plane stress developments in the FLG was monitored in-situ during galvanostatic cycling. The net in-plane stress development upon full lithiation in FLG agrees reasonably well with the stress magnitude expected based on the net dilation along graphite a-axis upon ‘classical’ bulk Li-intercalation up to LiC6 composition. This tends to additionally indicate that the mechanisms of excess Li-storage (i.e., other than the ‘classical’ bulk Li-intercalation) leads to much lesser in-plane dimensional change (if any). Another interesting observation was that of ‘stress release’ in the stress-time profile of FLG, just within the potential window of 0.5-0.25 V (against Li/Li+) [4] where pristine graphene/graphite, dilute stages I and IV co-exist. Based on the evidences obtained from structural characterizations (including changes in Raman spectra after cycling within different potential windows) and geometrical modelling, such stress release is believed to be associated with mechanical/structural degradation taking place primarily during the initial stages of lithiation and later stages of delithiation as a result of stretching of individual graphene layers beyond fracture strain around intercalated Li at interfaces between the concerned co-existing stages [4]. Accordingly, the observed ‘stress release’ within the concerned potential window above 0.25 V and the as-believed mechanism for mechanical/structural degradation in the case of FLG tends to agree well with the fading of the overall capacity.
References
1. R. Raccichini, A. Varzi, S. Passerini and B. Scrosati, Nat. Mater. 2015, 14, 271.
2. D. Datta, J. Li, N. Koratkar and V. B. Shenoy, Carbon 2014, 80, 305.
3. F. J. Sonia, Manoj K. Jangid, Balakrishna Ananthoju, M. Aslam, Priya Johari, Amartya Mukhopadhyay, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA01978E.
4. F. J. Sonia, Balakrishna Ananthoju, Manoj K. Jangid, Ravi Kali, M. Aslam and Amartya Mukhopadhyay; Carbon 2015, 88, 206.