## 3 Results and Discussion
### Mechanical Properties  
As mentioned above, the good morphological characteristics and tunnel structure of the NTO NWs can be used as anode materials in SIBs. However, for the stability of the SIBs, the anode materials  
Figure 2: Microstructure of the NTO NW. (**a**) HRTEM images; (**b**) Fourier transform diffractions; (**c**) Magnified local HRTEM image; (**d**) Simulation HRTEM images of the Na\({}_{2}\)Ti\({}_{7}\)O\({}_{15}\) structure at different levels of defocus and thickness. should have good mechanical stability for the volume change effect in the charge-discharge process. Here, we present the experimental results of studying the in situ mechanical properties. First, a commercial STM-TEM probing device was used to test in situ the elasticity of nanowires under transmission electron microscopy. As shown in Figure 3a, the NTO NWs were able to stand independently and had good strength, and one of the appropriate small NTO NWs was selected and extracted. The diameter of the nanowire was about 100 nm, the length was more than 10 \(\upmu\)m, and the aspect ratio was more than 100. The STM probe was used to push the nanowire into contact with another fixed end and perform the reciprocating compression elasticity test. As shown in Figure 3b-i, the nanowire was gradually pressurized by the micro-movement of the needle tip. When the nanowire was bent into a certain curvature, the pressure on the needle tip was slowly released, and then the nanowire could be restored to the original state, thus completing one loading cycle. During the experiment, one cycle could last less than 5 s, and the nanowire could quickly restore to its original state. After ten cycles, the nanowire was still intact. This shows that the NTO NWs had good elastic deformation characteristics and mechanical structure stability. Using the maximum deformation position obtained from the in situ TEM images, the curvature radius \(r\) was about 3.81 \(\upmu\)m, obtained by fitting Figure 3i. The nanowire diameter \(d\) of the selected NTO NW was about 0.1 \(\upmu\)m. The elastic strain \(\varepsilon\) can be simply described by the bending deformation model:  
\[\varepsilon=\frac{d}{d+R}\times 100\% \tag{1}\]  
The calculated elastic strain was about 2.56%. However, with in situ tests, because one end of the nanowire is not fixed, it is not easy for the fixed end to slip on the contact surface and restore the linear shape when the external force is too large. As a result, the elastic strain limit of the NTO NWs was not reached. Due to the good recovery of the multi-cycle deformation test, it can be deduced that the actual elastic strain may be far greater than 2.56%, which will be beneficial for anode materials in high-stability SIBs [21]. Figure 3: In situ TEM elastic strain analysis of the NTO NW. (**a**) Selection and extraction in the NTO NWs for deformation test; (**b–i**) Different states during the deformation of the NTO NW. For the anode materials of high-performance SIBs, a flexible structure with a low modulus of elasticity along the stress direction is necessary [22]. The elastic constant of the NTO NW was then further studied using a SEM/SPM combined system. In Figure 4a and the inset schematic diagram, the SEM images of the mechanical test and experimental model are presented. The result of the relationship curve between the load and displacement of the NTO NW, as shown in Figure 4b, is a proportional relation. Therefore, the Young's modulus E can be computed using the following equation [23]:  
\[E=\frac{F\cdot L^{3}}{k\cdot\Delta d\cdot I} \tag{2}\]  
Here, \(k\) is a parameter related to the boundary conditions, where \(k\) = 192 when ends-clamped and \(k\) = 48 when ends-free. In this test, both ends of the nanowire are free. \(F\) is the force, \(\Delta d\) is the midpoint displacement of the nanowire, \(L\) is the hanging length of the nanowire, \(I\) is the moment of inertia (for cylindrical nanowires, \(I=\frac{\pi\cdot r^{4}}{4}\), \(r\) is the radius of the NWs), and \(F/\Delta d\) is the slope of the loading curve.