In 2012, Amartya Mukhopadhyay of the Brown University of Engineering and Technology in the United States adopted an optical stress sensing device, Real-time (in-situ) measurement of the stress in the film, used to study the development of irreversible stress in the graphite electrode during lithium-ion battery cycling. The author experimentally explored the electrochemical behavior of the graphite electrode in the initial cycle, and deeply analyzed the evolution of the irreversible stress of the electrode from the perspective of electrochemistry, cycle times, electrode thickness, voltage, etc.( Irreversible stress: the difference in stress change during the lithium insertion/delithiation cycle),. it provides a new idea for the study of the actual mechanical properties of lithium-ion batteries.
Focus a parallel laser beam on the back of the quartz substrate (graphite electrode carrier), the rigid substrate limits the expansion and contraction of the in-plane dimensions of the active film (graphitic carbon here) during the lithium insertion/delithiation period, causing the substrate/film system to bend, when the laser beam is reflected back from the substrate, the laser beam is deflected. Since the quartz substrate will undergo elastic deformation, the stress in the film is proportional to the induced change in the curvature of the wafer, by monitoring the change of the spot pitch of the deflected beam reflected from the back of the substrate, it is possible to measure the stress development in the graphite electrode in situ.
Sample information: Graphite carbon film (CVD C), the graphite carbon layer is prepared on a quartz wafer with a thickness of 250 nm and a diameter of 1 inch by the principle of chemical vapor deposition (CVD) and graphitization; And deposited a 0.5 nm thick Al2O3 layer on the graphitic carbon layer by atomic layer deposition (ALD) to block the formation of SEI and the intercalation of solvated ions on the anode material to study its electrochemical behavior and subsequent the effect of stress.
Test content: In customized electrochemical cells, assemble the CVD C film on the lithium metal model battery, and carry out the constant current discharge/charge cycle, by monitoring the curvature of the substrate (the bending of the substrate/film system), the electrochemical behavior of the CVD C film and the resulting stress changes are studied.
1. Electrochemical behavior in initial cycles
Figure 1. Changes in potential and stress of CVD C lithium insertion/delithiation cycle over time
In the first cycle, a huge Li capacity was observed during the half cycle of Li insertion, while the reversibility was relatively small during the half cycle of Li removal, after removing some obvious irreversible capacity loss in the first cycle, most of the capacity in other cycles is reversible and consistent for different cycle rates. At C/5, C/10 and C/20 electrochemical cycling rates, graphite film electrode (CVD C) is repeatedly charged with Li to close to the theoretical capacity, the similarity of the three rate capacities means that Li diffusion is not rate-limited in this material. In addition, after 50 cycles, there is no capacity decay or any obvious macro and micro structure damage, and the voltage data is stable.
Figure 2. (a) The change of Li capacity with the number of cycles during the half cycle of lithium insertion and delithiation; (b) The nominal stress measured during the half cycle of lithium insertion and delithiation; (c) Coulomb efficiency and irreversible stress; (d) The stress existing in the sample at the beginning of each cycle;
Irreversible capacity, that is, the difference in Li capacity during lithium insertion/delithiation process, is described by Coulomb efficiency (CE). The article mentioned that the cumulative irreversible stress in the initial cycle is 2 times higher than the stress caused by the reversible stress, and the surface effect of irreversible stress (film thickness effect) shows that the irreversible component is mainly determined by one or more phenomena occurring near the surface of the film. In the first cycle, during the lithium insertion half cycle, the high capacity and consequent high stress were recorded, the capacity and stress reversal during delithiation is relatively low, resulting in a large difference between the capacity and stress during the first two half cycles, The Coulomb efficiency (CE) value in the first cycle is very low. The correlation between the electrochemical behavior and the stress development in the first 20 cycles and the irreversible stress are only derived from surface phenomena, indicating that, a large number of irreversible stresses are directly related to the formation of the SEI layer.
The stress evolution in the initial cycle is also shown in more detail in Figures 2b-2d, the author designed a graphitic carbon film with a lattice coefficient parallel to the substrate so that the stress development is only related to the change of the film. During discharge (half cycle of lithium insertion), a net compressive stress will be generated, and then it will be reversed during charging (half cycle of lithium removal). In the first lithium insertion cycle, a large part of the compressive stress is not reversed (equivalent to the initial residual stress). The magnitude of the irreversible stress is significantly reduced in the second cycle, and this trend continues until the difference between the stresses of the two half cycles in the later period is negligible, the stress development during the delithiation/intercalation process is almost completely reversed. In this experiment, The maximum in-plane stress change caused by the insertion of lithium is about -0.25 GPa (reversible stress, Figure 2b), the irreversible net compressive stress formed in the initial cycle is much higher (about -0.5 GPa, see Figure 2d). The author believes that this irreversible stress is an important part of the characteristics of feedback materials.
2. Sources of irreversible pressure
2.1Surface ef0fect of irreversible stress
The author verified by hypothesis that the irreversible stress originates from the surface layer of the graphitic carbon layer and is independent of the thickness of the CVD C film. By changing the thickness study, if irreversible stress appears inside the film, the corresponding stress thickness is proportional to the film thickness; Conversely, if this contribution originates from the surface layer of the graphitic carbon layer, the stress thickness is independent of the film thickness.
Figure 3. (a) The actual stress thickness of irreversible stress under different thickness films
2.2Stress changes under different voltages
Figure 4. (a) The electric potential and nominal stress of the first cycle (equivalent to the residual stress of the deposited film) over time;
It is known that below ~0.25 V is mainly the actual Li intercalation. The rapid change in compressive stress can be seen from the electric potential. Once the electric potential reaches about 1.0 V, a protective SEI layer is formed, which almost completely suppresses the co-intercalation of solvated ions. Figure 4b-e summarizes the stress contributions measured in different voltage ranges during the lithium insertion and delithiation processes of cycle 1 and cycle 2. The author shows that most of the compressive stress in the half cycle of lithium insertion occurs below 0.25 V, and the occurrence of stress is the actual insertion of lithium ions,and in the first few cycles, this voltage range is accompanied by the continuous formation of the SEI layer, that is, the small amount of irreversible compressive stress observed after the second cycle is also due to the formation of the SEI layer; However, any stress of higher potential (>0.25V) in the first cycle is directly related to the formation of SEI or the possible co-intercalation of solvated ions.
Figure 5. (a) The first half-cycle stress change of lithium insertion; (b) The second half-cycle stress change of lithium insertion; (c) The first half-cycle stress change of lithium insertion (Al2O3 coated CVD C sheet)
Fig. 5a The stress change is explained as the intercalation of Li + the formation of SEI + the intercalation of solvated ions; Figure 5b shows the continuous and slow formation of Li intercalation + passivation SEI; Figure 5c shows the actual Li intercalation in the graphite (Al2O3 coating blocks the formation of SEI), which further confirms that the main source of irreversible stress is the formation of SEI.
Figure 6. (a) Schematic diagram of the SEI film formed on the thin-film graphite electrode and the accompanying stress; (b) the expected behavior in the graphite particles and the surrounding SEI layer
In this research, an optical stress sensing device is used to measure the stress in the film in real time (in situ). The first few electrochemical cycles of graphite electrodes against Li metal are reported for the first time. Discuss the evolution of the huge irreversible compressive stress generated in the thin-film graphite electrode. The relationship between the irreversible stress and the formation of SEI is determined from multiple angles, which provides important guidance for the study of the electrochemical behavior and actual mechanical properties of lithium-ion batteries.
SWE series in-situ expansion analysis system (IEST Yuanneng Technology):u
uUsing a highly stable and reliable automation platform, equipped with high-precision thickness measurement sensors, to achieve the measurement of the thickness change and rate of change during the entire charging and discharging process of the battery, and can achieve long-term stability and accuracy;
uSWE series equipment can simultaneously measure the expansion thickness and expansion force of the battery during the charging and discharging process, quantify the change of the expansion thickness and expansion force of the battery, and assist in the study of the expansion behavior of the battery.
Mukhopadhyay A, Tokranov A, Xiao X, et al. Stress development due to surface processes in graphite electrodes for Li-ion batteries: A first report[J]. Electrochimica Acta, 2012, 66(none):28-37