While the increase of the Ni ratio in NCM contributes to an enhanced specific discharge capacity, it also results in severe capacity degradation caused by cation mixing, surface side reactions, and crack propagation with structural instability1. To better understand these challenges, Jung et al. investigated the degradation mechanism of the phase transformation induced by cation mixing from the surface to bulk using ex situ structural analysis7. Similarly, Lin et al. described the surface reconstruction and chemical evolution of the rhombohedral layered structure to a cubic spinel structure using high-throughput X-ray absorption spectroscopy8. As theoretical approaches, electronic correlations for the redox reactions between the multivalent transition metals in the Ni-rich NCM9 and stability analysis with respect to the various ratios of the Ni, Co, and Mn components in the NCM10 have been performed through first-principles calculations. On the bases of these fundamental data, many researchers have suggested solutions to resolve the cyclic degradation problem. Along with diverse approaches such as morphology control11, elemental doping12,13,14,15 and surface coating16,17,18,19,20, Sun et al. have suggested various effective ways to reduce cyclic degradation and improve electrochemical performance through the design of core-shell21, gradient core-shell3,22, and full concentration gradient structures2,23,24 for Ni-rich NCM cathodes.
Recently, Meng et al. reported that the severe crack generation in NCM811 particles induces a significant performance degradation29. According to the scanning electron microscopy (SEM) observation, particle fractures and fragmentation of NCM811 particles were evident after cycles. Due to this crack generation, the discharge capacity of NCM811 was remarkably decreased with increasing overpotentials during cycles. From our fundamental understanding, we suggest that the origin of crack generation is the contraction of primary particles with a mechanical instability caused by heterogeneous phase transformation and anisotropic strain changes. In addition, the lower Gc at delithiated states contributes to a severe crack propagation. Finally, it is expected that these could be resolved by reducing the inhomogeneity and anisotropy of structural changes and increasing Gc.
Active reports 7 crack
Flexible electrodes and strain sensors have seen tremendous growth in their demand due to their ease of integration with non-conventional interfaces. In this regard, mainly two strategies have been adopted. One strategy relies on layered thin films cast on a flexible substrate, while an alternate strategy utilizes flexible composite films. While both have been studied extensively, each has its niche area of applications. For optoelectronic and device applications like solar cells1, transistors2, organic LEDs3, photodetector4, that necessitate energy band alignment, layered films have found favor due to the ease of interfacing. For applications in strain sensors, composites have shown more robust performance under mechanical deformation5,6 while layered films typically have relatively higher gauge factor. However, the success of these strategies highly depends upon their ability to withstand various forms of mechanically induced deformations like cracks and wrinkles. These deformations in layered morphology constitute as important attributes in flexible substrates and devices, and their effects have been studied extensively. Bowden et al.7 and others suggest methods to control the wrinkling/buckling of PDMS under oxygen plasma treatment, and thermal cycling on metal deposited substrates8,9. Conventional inorganic materials that have been employed as an electrode in such devices, have not been able to withstand strain more than 1.75%10,11, wherein the crack formation led to material failure, thus degrading the performance of the devices12,13. However, to develop commercially viable flexible devices, effect of cracks on electrode performance in such devices needs to be studied.
Graphene, an atomically thin 2-D membrane, due to its robust mechanical properties14 has been successfully shown to replace the existing conventional electrode materials in flexible devices15,16,17. Interestingly, when compared to pristine graphene, defected graphene was found to be more resistant to crack propagation, since the defects make the cracking process energetically less favorable18. Graphene oxide (GO) a chemically modified form of graphene, contains structural defects in the form of oxygen functional groups especially hydroxyls and epoxides which determine the crack formation and propagation19 and also the relative concentration of these groups controls mechanical failure of GO20. Thus one can tune the cracking of GO films by modifying the chemical composition which can be also achieved by reduction of GO to reduced graphene oxide (rGO), by chemical21 or thermal methods22. For commercial viability of rGO as an electrode material in flexible devices3,23,24 it is important to have an understanding of crack formation under strain and the limitations it imposes on electrical properties. Thomas et al. illustrated that buckling and cracking of GO films can be controlled by pre-straining the substrate25. Graphene layers have recently been used as a meta-interface to limit the cracking of Indium Tin Oxide (ITO) on polyethylene terephthalate (PET) substrates26. However, the interplay of strain-induced cracks, film thickness and the resultant strain-dependent electrical response of rGO on flexible substrates needs detailed investigation.
Here we report crack propagation and tunability in electrical response of conducting rGO films coated on PDMS under uniaxial strain. We show that by optimizing the thickness, rGO films can largely be made to retain their conductivity under strain up to 5%, a value which is well above the limit for conventional flexible electrode materials. Our study also reveals a facile way to achieve periodic cracking and control the crack density and crack width by varying the thickness. This is then utilized for strain sensing and also for strain resistant flexible photoconductor applications.
SEM images for (a) 1-coat (arrows indicate the position of cracks) (b) 3-coat (c) 6-coat rGO films on PDMS substrate at 5% applied strain. (d) Schematic diagram to illustrate the thickness dependent crack formation in rGO films under uniaxial strain.
To understand the cracking process in detail, we studied the progressive formation of cracks as a function of applied strain for different thickness. Figure S3 show the optical images of a representative 6-coat sample under the application of strain from 0 to 5%. The first notable feature about cracking is that there is a critical strain value (3.6%), after which the cracks begin to appear. Two modes of cracking have been described in the literature: (i) sequential cracking and (ii) simultaneous cracking39. The former is observed in our samples, since the crack density monotonically increasing with strain beyond the critical strain value. The optical images also reveal new cracks forming between existing cracks, confirming the sequential cracking.
So far we have discussed the morphology of crack formation at different values of uniaxial strain, and also the dependence of this phenomenon on film thickness. These morphological changes can also be expected to strongly influence the electrical response under strain. With this motivation, we prepared six kinds of samples ranging from 1-coat to 6-coat to evaluate the strain response as a function of thickness of rGO on PDMS substrate. The uniaxial strain was applied using a linear micro-manipulator stage. Figure 4a shows the variation of normalized resistance for 1-coat, 3-coat and 6-coat samples as a function of strain. For 1-coat sample, the resistance takes-off at an applied strain value of 3.5% and shows an eight fold increase in resistance for maximum strain value of 5%.
To get more insight into this change in resistance upon straining, a detailed study was conducted which is discussed as follows. Figure 5a shows the cross-section SEM of 3-coat sample under 5% strain. Arrows point towards the crack formation in both buckled and planar region of the sample. To further investigate the effect of cracks on fractional resistance change as the strain is varied for 3-coat sample, variation in crack density (n) was studied as a function of applied strain shown in Fig. 5b. Both the fractional resistance and crack density vary exponentially after a critical strain value (3.6%) is breached. The similar response of both parameters arises since crack density determines the fractional resistance change. The variation in crack density as a function of strain is shown in the representative optical images in Fig. 5c,d and e at 0, 3.6 and 5% strain, respectively.
So far we have demonstrated that very thin (1-coat) or thick (6-coat) films can be adapted for strain sensing. The intermediate thickness (3-coat) shows the least response to applied strain and is therefore suited in applications like flexible optoelectronics, where strain-related effects are not a desirable feature. To discuss the role of thickness-dependent crack propagation in a strained optoelectronic device, we prepared a hybrid of a standard UV active material - TiO2 nanoparticles with rGO on flexible PDMS substrate.
Figure 7a illustrates the UV response of TiO2-rGO system with 1-coat rGO as electrode. As can be seen from figure, the resistance under dark and light conditions continues to increases as a function of applied strain and does not show a stable response. For example, the strain dependence of dark current implies that the change in resistance of a flexible photodetector can be attributed either to the strain and its associated crack propagation or to the generation of photocarriers. The device, when used in the flexible configuration, is not capable of distinguishing these two factors since both influence the resistance. Figure 7b shows the response of TiO2-rGO system with 3-coat rGO as electrode. Both the dark resistance and resistance under illumination have a much weaker dependence on strain. For this case, a change of resistance of the device can directly be attributed to photodetection. Thus optimization of rGO thickness is crucial for applications which require resistance to strain. 2ff7e9595c
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