Three-Dimensional and Mesopore-Oriented Graphene Conductive

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Three-dimensional and mesopore-oriented graphene conductive framework anchored with nano-Li4Ti5O12 particles as an ultrahigh rate anode for lithium-ion batteries Yu Xiang, Pengcheng Zhao, Zhaoqing Jin, Bo Chen, Hai Ming, Hao Zhang, Wenfeng Zhang, Gaoping Cao, and Xiayu Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14774 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 18, 2018

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ACS Applied Materials & Interfaces

Three-dimensional conductive

and

framework

mesopore-oriented anchored

with

graphene

nano-Li4Ti5O12

particles as an ultrahigh rate anode for lithium-ion batteries Yu Xianga,b, Pengcheng Zhaoa,b, Zhaoqing Jina.b, Bo Chena, Hai Minga.b, Hao Zhanga,b, Wenfeng Zhanga,b, Gaoping Caoa.b,*,Xiayu Zhua,b a Research Institute of Chemical Defense, Beijing 100191, China b Beijing Key Laboratory of Advanced Chemical Energy Storage Technology and Materials, Beijing 100191, China

Abstract: Due to the disadvantages of commercial graphite anode for high power lithium-ion batteries, a kind of spinel nano-lithium titanate (Li4Ti5O12) /graphene microsphere composite (denoted as LTO/rGO) is successfully synthesized. The as-prepared composite is made up of curled graphene sheets which are anchored with nano-Li4Ti5O12 particles. These nano-Li4Ti5O12 particles are uniformly decorated on the conductive graphene framework and its sizes range from just 15 nm to 20 nm. In the as-prepared composite, the curled graphene sheets form unique mesopore-oriented structure which provides plenty of three-dimension channels for ions transportation. These structure characters make the electron conductivity and Li+ diffusion ability both greatly improved. The ration of pseudocapacitive capacity dramatically increases in the obtained LTO/rGO composite and generates excellent ultrahigh rate performances. The as-prepared LTO/rGO composite delivers a reversible capacity of 70.3 mA h g-1 at 200C and a capacity retention of 84.7% after 1000 cycles at 50C. As the current density varies from 30C to 100C, the special capacity retains unchanged (about 112 mA h g-1). These

1

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results show that the graphene framework supported nano-Li4Ti5O12 composite has potential application in high power lithium-ion batteries. Keywords: Graphene, conductive framework, mesopore-oriented, Nano-Li4Ti5O12, high power capability, pseudocapacitive capacity



INTRODUCTION

In order to eliminate global climate crisis and mitigate excessive fossil fuel consuming, recently renewable energies such as solar, wind, wave and so on have been developing fast.1-2 However, due to its unstable supply and seasonal variation characters, renewable energies must be cooperated with a large energy storage station for their practical application.3 The primarily used large energy station is usually made up of lithium-ion batteries (LIBS).1 Therefore, it is required that the LIBS of large energy station must own excellent power capability and enough security to cooperate with renewable energies.4 In commercial LIBS, the graphite anode cannot supply high power density and is easy to generate security troubles due to the formation of lithium dendrite at about 0 V (versus Li+/Li).5 Hence, many researches are exploring suitable anode materials for high power LIBS. Spinel lithium titanate (Li4Ti5O12) is considered as one of the most potential anode candidates for LIBS due to its unique properties.6 Li4Ti5O12 owns a flat lithium insertion plateau at approximate 1.55 V (versus Li+/Li) which is higher than the formation potential of lithium dendrite, so the safe risks can be greatly weakened.7 In practical applications, the cut-off voltage of Li4Ti5O12 is 1 V (versus Li+/Li) which is much higher than the

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reduction potential of electrolyte solvents.8-9 The high cut-off voltage of Li4Ti5O12 avoids the formation of solid electrolyte interface (SEI), which is benefic to fast surface ions transportation and high rate performances.10 Furthermore, Li4Ti5O12 is known as a zero-strain insertion material and owns negligible volume expansion during cycles.11 Therefore, Li4Ti5O12 can exhibit a long cycle life, prospective power capability and low security risks. Unfortunately, Li4Ti5O12 has some disadvantages such as the poor lithium diffusion coefficient (ca. 10-9 to 10-13 cm2 s-1) and the low electronic conductivity ( 100C) of the other recent references are not reported. However, according to the decay trend it could be inferred that the superhigh rate capabilities of these references are likely to be not very good. The composites in these references have either nano-Li4Ti5O12 particles or porous structure. Hence, though the Li+ diffusion speeds of these composites are enhanced, yet it could be not very satisfied to the demands at the superhigh rate (>100C). Unlikely to these references, the as-prepared LTO/rGO composite in this work combines the nano-Li4Ti5O12 particles and mesopore-oriented

structure

besides

the

three-dimension

conductive

graphene

framework. The speeds of Li+ and electron transportation are both enhanced greatly, so the superhigh rate capability of the as-prepared LTO/rGO composite are much better than other references. Mass loading of active material has notable influence on rate performance. Table S1 (supplement information) shows the active mass loads of the references in Fig. 5. To further accurately verify the high rate performance of LTO/rGO, two higher active mass loads of 3 mg cm-2 and 5 mg cm-2 are tested. The results shown in Fig. 6 demonstrate that the rate performances keep almost unchanged from 0.2C to 5C at the mass loading range from 1 mg cm-2 to 5 mg cm-2. Even at 10C, the special capacity at 1 mg cm-2 and 3 mg cm-2 are nearly the same except that the special capacity of 5 mg cm-2 emerges a little decrease (about 8 mA h g-1). However, as the rate exceeds 20C, the distinctions of the three level mass loadings come out. Higher mass loading results in thicker electrode, which will remarkedly increase the ion and electron diffusion resistances at high rate, so the high rate performances of high active mass loading always are very poor. However, 19



with nano-Li4Ti5O12 particles and unique mesopore-oriented graphene framework, the as-prepared LTO/rGO displays considerable rate performances at high mass loading. It is calculated that as the rate is 20C, the special capacities of LTO/rGO at 3 mg cm-2 and 5 mg cm-2 are 92 mA h g-1 (retention 66%) and 76 mA h g-1 (retention 54%) respectively. It is notable that the current densities at 20C are separately 10.5 mA cm-2 and 17.5 mA cm-2 for 3 mg cm-2 and 5 mg cm-2, which is much satisfied with particle applications. Even when the rate reaches 40C that the current densities are separately 21 mA cm-2 (3 mg cm-2) and 35 mA cm-2 (5 mg cm-2), the special capacities are still 70 mA h g-1 and 42 mA h g-1, separately. In contrast, the mass loads of these recent references are usually below 2 mg cm-2 in Table S1 and the current density is less than 7 mA cm-2. Hence, it could be concluded that the rationally designed structure of the as-prepare LTO/rGO composite indeed promotes the ion and electron diffusion processes and enhances the rate performances obviously.

Fig. 6. Effects of mass loads on high rate performances of LTO/rGO Fig. 7a shows the EIS measurements of the coin cells. The Nyquist plots consist of a semicircle in the medium-frequency region and a slanted line in the low frequency range. 20


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The results of EIS are simulated through the equivalent circuit in Fig. 7a. In the equivalent circuit, R1 represents the resistances of electrolyte and electrode. R2 is the resistance of charge transfer across the electrode interface. Warburg impedance (W) refers to the solid-state diffusion resistance, which is attributed to the slope line in the EIS measurement. Constant phase element (CPE) is used to represent the double-layer capacitance. The stimulate results are listed in Table 1. It is obvious that LTO/rGO has a much smaller R2 than LTO, indicating a faster dynamic character. The calculated results in Table 1 reveal R2 of LTO is 84 Ω which is nearly 5 times higher than that of LTO/rGO (R2 is 18.9 Ω). The simulation result of R2 further confirms the much better dynamic behavior of LTO/rGO.

Fig. 7. (a) Nyquist plots for LTO and LTO/rGO after the high rate test. (b) Z’ plots against ω-1/2 at the low frequency region of EIS

Table 1. Simulation results of EIS measurment Sample

R1/Ω

R2/Ω

21


σ/Ω·s-1/2

DLi+/cm2·s-1


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LTO

5.9

84

981.4

5.28×10-14

LTO/rGO

2.1

18.9

68.6

1.08×10-11

The diffusion coefficient of lithium ion (DLi+) could be obtained due to Equations (1).21 In the two equations, molar gas constant (R), absolute temperature (T), Faraday constant (F) are all constant. For the deintercalation/intercalation of Li+, the number of electrons transferred n=1. In this experiment, the area of the electrode surface (A) is about 2.0 cm2 and the molar concentration of Li+ (cLi+) is 4.17×10-3 mol cm-3. The Warburg impedance coefficient (σ) can be calculated from the slope of the lines in Fig. 7b according to equation (2).19 The calculated results of DLi+ are also listed in Table 1. The DLi+ of LTO/rGO is 3 order magnitudes higher than that of LTO, proving the process of Li+ diffusion has been greatly improved on the condition of nano-Li4Ti5O12 particles and mesopore-oriented porosity structure.

Additionally, R1 can also reflect the electron transformation process. Table 1 shows LTO/rGO owns a much lower R1. This means the electron conductivity of LTO should be much higher than LTO. That is to say, the 3D graphene conductive network also enhances the electron transportation speed in the as-prepared LTO/rGO. To further explore the root of the excellent high rate capability of LTO/rGO, a various of CV measurements are done from 0.5 mV s-1 to 10 mV s-1 and the results are shown in Fig. 8. Fig. 8a shows that with the increasement of scan rate the polarization of cathodic 22


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peak in LTO enhances greatly so that the cathodic peaks just appear partly from 2 mV s-1. The similar performances are also observed in the anodic peaks of LTO. On the contrary, the cathodic and anodic peaks of LTO/rGO can both appear completely at all scan rates and are still much sharper than LTO (Fig. 8b). Fig. 8c shows the polarization potentials (the potential difference between the cathodic and anodic peaks) at various scan rate. For LTO/rGO, the polarization potential increases from 0.15 V to 0.51 V under the sweep rate range. However, a 0.54 V polarization potential of the LTO emerges at the initial scan rate of 0.5 mV s-1. When the sweep rate increases to 10 mV s-1, the polarization potentials even reaches 1.13 V. All results show that LTO has a much severer polarization than LTO/rGO and further confirmed that LTO/rGO achieves much better kinetics character.

23



Fig. 8. Cyclic voltammograms of LTO (a) and LTO/rGO (b) at different scan rates. (c) Voltage hysteresis comparison of LTO and LTO/rGO composite. (d) The linear fitting between i ν -1/2 vs ν 1/2 of LTO and LTO/rGO at each cathodic and anodic peaks. (e) Pseudocapacitive contribution of LTO/rGO at 10 mV s-1. (f) The ratio of pseudocapacitive contribution at different scan rates. It is well known that the relation between peak currents and scan rates indicates the

24


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different electrochemical reaction mechanisms, including solid phase diffusion-controlled or surface-confined charge-transfer processes (referred to as pseudocapacitance).33-34 The distinct varieties of CV curves suggest LTO/rGO may combine the above two reaction mechanisms. The ratio of pseudocapacitive contribution can be separated according to equation (3), where ν is the scan rate, k1ν is the surface capacitance and k2ν1/2 is diffusion-controlled insertion capacity.34 The k1 and k2 can be obtained from the linear fitting between i ν-1/2 and ν-1/2 and the percentage of pseudocapacitive effect could be calculated.

Fig. 8d shows that for as-prepared LTO/rGO the k1 of cathodic and anodic peaks are 0.32 and 0.33 separately. It is implied the pseudocapacitive Li+ storage takes a mount of the total capacity. For LTO, Fig. 8d reveals that the slopes of the linear fitting of cathodic and anodic peaks are almost 0, indicating there is nearly no any surface Li+ storage capacity. On basis of the k1 and k2 of LTO/rGO, the ration of pseudocapacitive contribution at 10 mV s-1 is also calculated as shown in Fig. 8e. The dark shaded region represents the pseudocapacitive storage and its area takes up 37.5% of the total area (grey shaded region). The pseudocapacitive contributions at different sweep rates are shown in Fig. 8f. The percentage of pseudocapacitive capacity increases along the increasement of scan rate, which may account for the shorten distance of Li+ diffusion at high rate. According to previous reports, faradic pseudocapacitive effect will emerge as particle size decreases to nanometers. On the other hand, the mesopore-oriented structure of 25



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LTO/rGO microsphere provides plenty of 3D channels to shorten Li+ transport lengths. Additionally, the conductive 3D graphene framework also greatly decreases the resistance of electron transportation at high current density. The three mentioned causes are conducive to active pseudocapacitive effect. Due to the pseudocapacitive effect, the power capability of LTO/rGO is also enhanced greatly.



CONCLUSIONS In this work, a mesopore-oriented LTO/rGO microsphere composite consisting of

nano-Li4Ti5O12 particles is successfully prepared through a facile method. Under the condition of graphene sheets, nano-Li4Ti5O12 particles whose sizes range from 15 nm to 20 nm are uniformly deposited on graphene sheets. The curling graphene sheets form a mesopore-oriented structure and provide plenty of channels for electrolyte penetration and fast Li+ ion diffusion. Moreover, the 3D graphene framework also provides coherent electron conductive routines. The as-prepared LTO/rGO exhibits reduced voltage hysteresis and improved Li+ diffusion coefficient. A mount of the capacity originated from the surface pseudocapacitive effect, leading to an outstanding high rate capability (85 mA h g-1 at 35 A g-1), low decay rate quality and excellent ultra-long cycling stability (84.7% after 1000 cycles at 8.75 A g-1). It is first reported that as the current rate varies from 30C to 100C, the capacity retention keeps unchanged. Such excellent results demonstrate that the synthesized LTO/rGO microsphere composite is a very promising anode material for ultrahigh rate performance LIBs or Li-ion capacitor. In summary, it is believed that the successful

manipulation

of

the

mesopore-oriented

26


microsphere

consisting

of

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nano-Li4Ti5O12 particles will provide a new direction to synthesis the high-power electrode and can also be further extended to other materials. 

ASSOCIATED CONTENT Supporting Information Details on synthesize process of the as-prepared LTO/rGO composite and relative

CV results, TGA curve, XRD, SEM, Raman, N2 adsorption-desorption isotherm. 

AUTHOR INFORMATION

Corresponding author *E-mail: [email protected] Notes The authors declare no competing financial interest. 

ACKNOWLEDGEMENTS

The work was supported by the National Nature Science Foundation of China (NSFC 21703285). 

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Three-Dimensional and Mesopore-Oriented Graphene Conductive

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