PARTICUOLOGY Picks | Exploring the Ultimate Capacity of Hard Carbon Below 0 V

Emerging Investigators

Utilizing the capacity below 0 V to maximize lithium storage of hard carbon anodes (Open Access)

Liu Ganlei, Dai Liqin *, Xie Lijing, Yi Zonglin, Song Mingxin, Fan Yafeng, Sun Guohua, Su Fangyuan, Chen Chengmeng *

Keywords: Low voltage; Li intercalation; Reversible capacity; Li dendrites; Hard carbon.

DOI: 10.1016/j.partic.2023.03.001

Hard carbon has become one of the candidate anode materials for lithium ion batteries (LIBs) due to its low redox potential and high reversible capacity. As described by the "House of Cards" model, hard carbon consists of a large number of randomly oriented graphite nanosheets. The nanostructure gives hard carbon large layer spacing (0.37-0.4 nm) and rich amorphous structure (defects, short-range crystallites and rich pore structure), which greatly improves the capacity of hard carbon low-pressure platform area (below 0V). Utilizing the plateau capacity of the hard carbon actually increases the operating voltage and energy density of the full cell when combined with the positive electrode. Compared with traditional graphite negative electrodes, due to the slow growth of dendrites on the surface of hard carbon below 0V, this makes this part of the capacity have the potential to be developed. However, limited by the experience gained from graphite anodes, a large number of reports on hard carbon lithium storage are mainly concentrated above 0V. In view of this, Chen Chengmeng's research team from Shanxi Institute of Coal and Technology, Chinese Academy of Sciences, was invited to publish a research paper at the PARTICUOLOGY. By regulating the lithiation range of hard carbon, the lithium storage performance of hard carbon was nearly doubled compared with discharge to 0V.

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Graphic Guide

In order to explore the limiting capacity of hard carbon below 0V, the battery was discharged to a series of degrees (V-0, C- 400, C- 500 and C- 600). The results are shown in Figure 1. V-0 represents a discharge to 0 V and C- 400 represents a discharge capacity of 400 mAh g-1, similar in C- 500 to C- 600. The V-0 exhibited a capacity of g-1 209 mAh, C- 400 the cut-off voltages of -0.028, -0.032 and -0.038V for C- 500 and C- 600, respectively. As shown in Figure 1a, C- 400,C-500 and C- 600 have a platform area below 0V. At the same time, the reaction of hard carbon with different degrees of lithiation was analyzed by dQ/dV (Figure 1c).

Figure 1. Time-voltage curve and dQ/dV curve from V-0 to C- 600;(a-b) time-voltage curve,(a) lithiation, (B) delithiation,(c)dQ/dV curve

Most researchers believe that the voltage below 0 V will form lithium dendrites, lithium dendrites can pierce the SEI layer, induce side effects with the electrolyte, thereby reducing the battery's coulombic efficiency (CE) during the cycle. Therefore, by observing the phenomenon of lithium plating on the surface of hard carbon, the suitable range of lithiation was initially found. SEM is often used to observe the lithium plating situation, and SEM is used here to observe the changes in the morphology of electrodes with different degrees of lithiation (original pole piece, V-0, C- 400, C- 500 and C- 600). These results imply that the resistance to lithiation in hard carbon should be less than the 500 mAh g-1,C-400 the potential for long-term cycling.

Figure 2. SEM images of initial electrode, V-0,C-400,C-500 and C- 600 at 50 mA g-1

In order to explore the availability of different degrees of lithiation, the CR2032 half-cell system evaluated its cycling performance at different current densities (Figure 3a). These results indicate that neither C- 500 nor C- 600 have the potential for long-term cycling at the three current densities. The V-0 can be cycled for 100 cycles at three current densities. C- 400 can be cycled for 100 cycles at current densities of 25 and 50 mA g-1. A current density of 100 mA g-1 will affect the battery cycle performance. Therefore, we believe that the C- 400 has the potential to be utilized at current densities below 50 mA g-1. At the same time, in order to analyze the reversibility of the capacity below 0V, it is necessary to quantify the CE of the capacity below 0 V (Figure 3b). Compared with C- 500 and C- 600, the coulombic efficiency of C- 400 lithium plating area can still reach 99.5% after 100 cycles, which means its ultra-high reversibility. Furthermore, Figure 3c shows the long-term cycling performance (capacity retention above 80%) of V-0, C- 400, C- 500 and C- 600 at a current density of 50 mA g "1. Compared with the V-0, the C- 400 has nearly twice the capacity and nearly the same cycle performance.

Figure 3. Long cycle and rate performance of hard carbon from V-0 to C- 600;(a) Three-dimensional plot including degree of lithiation, current density (g-1 25,50 100 mA) and number of cycles (CE above 98%), (B) Coulombic efficiency below 0 V at 50 mA g-1,(c) Long cycle performance at 50 mA g-1

In order to understand the effect of high lithiation degree on the cycle performance degradation of the battery, the voltage curve and dQ/dV curve during the cycle process were further analyzed. Here a comparison of C- 400 and C- 600 after 30, 50 and 80 cycles is taken (FIGS. 4a-c and d-f). Combined with C- 600 low CE during cycling, these results indicate that the peak "②" is detrimental to long-term cycling of hard carbon electrodes. Based on the previous SEM observations and the C- 600 capacity fading behavior, lithium dendrites are considered to be associated with the peak "②. The reason for the increase in polarization and capacity loss of the battery is the accumulation of "dead lithium" due to the formation of lithium dendrites and the continuous rupture and reconstruction of the unstable SEI during cycling.

Figure 4. Time voltage and dQ/dV curves of C- 400 and C- 600 at 50 mA g-1 after 30\50\80 cycles; Lithiation, delithiation and dQ/dV curves of (a-c)C-400 and (d-f)C-600

In order to analyze the lithium storage mechanism of hard carbon below 0V, a series of characterization analyses were carried out on the electrodes with different degrees of lithiation. Through the analysis of GITT, the lithium ion diffusion coefficient starts to rebound from close to 0V, and the 400 mAh g-1 reaches the minimum value. Through the analysis of XRD, it is found that the layer spacing of the hard carbon V-0 to the C- 400 increases. Following the XPS shows the surface composition of the electrode with different degrees of lithiation, combined with the deep sputtering characterization of C- 400 and C- 600 by AES, we believe that the reversible capacity of the C- 400 is derived from the lithium film deposited on the surface (about 140 nm) and the intercalation capacity of lithium in the hard carbon pseudo graphite domain.

Figure 5. Analysis of lithium storage behavior in hard carbon;(a) GITT curve of lithiation process, (B) lithium ion diffusion coefficient,(c)XRD pattern,(d)XPS pattern,(e-f) AES deep sputtering pattern of hard carbon electrode,(e)C-400,(f)C-600

Finally, FEM provides a deeper understanding of the mechanism of lithium plating on hard carbon. As shown in Figure 6, the state of charge (SOC) of the hard carbon electrode is defined as C1,surf/C1,max. The arrow in the figure represents the direction of movement of lithium ions, and the size and color represent the concentration of lithium ions. These results show that the SOC of the V-0 is 0.51, which means that there are still voids inside the hard carbon and the lithiation is incomplete. The SOC of C- 400 is 0.97 at the hc/sep interface, indicating that the lithium content in the hard carbon electrode is close to the maximum value. The SOC of C- 500 and C- 600 reached 1, indicating that the internal lithium intercalation has been completed. At this time, the hard carbon surface only has lithium plating reaction, and continued lithiation is called excessive lithium intercalation. Excessive lithiation can lead to the appearance of lithium dendrites on the electrode.

Figure 6. The mechanism of lithium plating is revealed by the finite element model;(a) Figure and arrow represent the distribution of lithium in hard carbon and the concentration gradient of lithium ions at the current collector-hard carbon interface and hard carbon-diaphragm interface, respectively, (B) Figure shows the overpotential of lithium plating

The above mechanistic analysis provides a comprehensive understanding of the hard carbon lithiation process. Finally, according to the understanding of lithium storage mechanism, we show the corresponding mechanism diagram. 7, the voltage profile of the hard carbon is divided into six stages. In the voltage ramp region (phase I) is the behavior of ion adsorption. In the second stage, the voltage drops to near 0V, and a plateau region appears. At this time, the plateau indicates the simultaneous action of lithium intercalation and lithium plating (lithium film) (stage II). Then there is an inflection point in the voltage curve, which means that the lithium intercalation in the hard carbon has been completed, and a single lithium plating reaction leads to the rapid growth of lithium dendrites (stage III). The other three stages are lithium exfoliation, lithium exfoliation-deintercalation and surface exfoliation of lithium, respectively.

Figure 7. Illustration of the lithium-ion storage mechanism;(a) the complete charge-discharge curve is divided into six stages, (B) the hard carbon structure of the different stages

research conclusion

This paper systematically discusses the availability of hard carbon capacity below 0V. The results show that the platform capacity below 0 V of hard carbon can be used when the capacity is controlled g-1 400 mAh and the current density is lower than 50 mA g-1. However, at higher current densities or higher lithiation depths, the resulting lithium dendrites can impair cycling performance. Benefiting from the effective adjustment of the lithiation boundary, the C- 400 exhibited an average CE higher than 99.5% at 50 mA g-1 and was run 172 times with a capacity retention of 80%. This boundary provides nearly twice the reversible capacity compared to discharging to 0V. Based on the results of GITT, XRD, XPS and AES, we found a new mechanism of lithium storage. Since the reaction voltages of lithium intercalation and lithium plating are close, the C- 400 platform capacity is attributed to the dual effect of lithium intercalation and reversible lithium film. Adsorption-intercalation and lithium plating (film)-lithium plating (lithium dendrite) are considered to be reasonable lithium storage behaviors of hard carbon. Compared with regular operations aimed at suppressing discharge below 0V, this work provides another effective strategy for developing hard carbon properties.

Introduction to the Author

Li Gan Lei

Liu Ganlei, a master's student at Shanxi Institute of Coal and Technology, Chinese Academy of Sciences, focuses on capacity development and storage mechanism of hard carbon negative electrodes for lithium/sodium batteries.

Chen Chengmeng

Chen Chengmeng, researcher of Shanxi Institute of Coal and Chemistry, Chinese Academy of Sciences, PhD supervisor, project leader, deputy director of the Key Laboratory of Carbon Materials of the Chinese Academy of Sciences, deputy director of the Graphene Engineering Laboratory of the Chinese Academy of Sciences, was funded by the National Natural Science Outstanding Youth Fund and was selected as the "MIT Science and Technology Review" China "35 people under the age of 35", "Qixing" outstanding talents of the Beijing Branch of the Chinese Academy of the Chinese Academy of the Chinese Academy of the Chinese Academy of the academic and the academic technology. He has presided over more than 20 national, local and enterprise projects, published 181 papers, cited 8765 times, published 1 English monograph, presided over the formulation of 4 international standards, 2 national standards and 38 authorized patents. He won the first prize of natural science in Shanxi Province, the first prize of innovation achievements of industry university research cooperation in China, and the second prize of technological invention of China Chemical Industry Association. Based on industry-university-research cooperation, we are committed to the research of carbon materials such as graphene, capacitive carbon, graphite and hard carbon, and have developed high-performance graphene thermal film and supercapacitors to form application demonstrations in the smart grid and aerospace fields.