Triboelectric Nanogenerator Enabled Dendrite-Free Lithium Metal

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Energy, Environmental, and Catalysis Applications

Triboelectric Nanogenerator Enabled Dendrite-Free Lithium Metal Batteries Nian-Wu Li, Yingying Yin, Xinyu Du, Xiuling Zhang, Zuqing Yuan, Huidan Niu, Ran Cao, Wei Fan, Yang Zhang, Weihua Xu, and Congju Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17364 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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ACS Applied Materials & Interfaces

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Triboelectric Nanogenerator Enabled Dendrite-Free Lithium Metal Batteries Nian-Wu Li1,2⊥, Yingying Yin1,2⊥, Xinyu Du1,2⊥, Xiuling Zhang1,2, Zuqing Yuan 1,2, Huidan Niu 1,2,

Ran Cao 1,2, Wei Fan 1,2, Yang Zhang 1,2, Weihua Xu1,2,3, Congju Li1,2,3*

1Beijing

Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing

100083, P. R. China. 2School

of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing

100049, P. R. China. 3School

of Energy and Environmental Engineering, University of Science and Technology

Beijing, Beijing Key Laboratory of Resource-oriented Treatment of Industrial pollutants, Beijing 100083, China. E-mail: [email protected] KEYWORDS: triboelectric nanogenerator, lithium metal anode, lithium dendrites, pulse output, batteries

ACS Paragon Plus Environment

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ABSTRACT

Lithium metal batteries (LMBs) are prominent among next-generation energy-storage systems because of their high energy density. Unfortunately, the commercial application of LMBs is hindered by dendrites growth issue during charging process. Herein, we report that the triboelectric nanogenerator (TENG) based pulse output with novel waveform and frequency has restrained the formation of dendrites in LMBs. The waveform and operation frequency of TENG can be regulated by TENG designed and smart power management circuits. By regulating the waveform and frequency of TENG based pulse output, the pulse duration time is shorter than the lithium dendrites formation time at any current of pulse waveform, and lithium-ion can replenish in the entire electrode surface during rest periods eliminating concentration polarization. Therefore, the optimized TENG based charging strategy can improve the Coulombic efficiency of lithium plating/stripping and realize the homogenous lithium plating in LMBs. This TENG based charging technology provides an innovative strategy to address the Li dendrite growth issues in LMBs, and accelerates the application of TENG based energy collection systems.

INTRODUCTION Energy storage systems with high energy density are urgently needed to satisfy the everincreasing demand in electric vehicles, consumer electronics, and grid-scale storage. However, the energy density of the existing lithium (Li)-ion batteries remains insufficient


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for many of these applications.1-3_ENREF_1 Li metal is the ultimate anode for rechargeable batteries because it has the most negative reduction potential (-3.04 V versus standard hydrogen electrode) and the ultrahigh theoretical specific capacity (3860 mAh g-1).4-5 Furthermore, Li metal anode is indispensable for next-generation energy-storage systems, including Li-S, Li-air, and other Li batteries using intercalation compounds or conversion compounds as cathode materials.6-9 Unfortunately, the practical application of Li metal batteries (LMBs) is hindered by dendritic deposition during charging process, unstable solid electrolyte interphase (SEI), and almost infinite volume change during cycling. The dendrite growth also can accelerate the side reactions, leading to serious safety issue and low Coulombic efficiency.10-12 Many efforts have been made to address the dendritic deposition issues including 3D current collector13-15, interfacial protective layer16-21, and electrolyte optimization22-26. In addition, the formation mechanism of lithium dendrites has been studied through theoretical simulation and experiment and at various scales.27-28 Based on these research, the dendrite growth is driven by the cation diffusion and electromigration at the anode/SEI interface under electric driving force.29-31 Thus, the uncontrolled dendrite growth can be reduced by the pulse charging strategies, which have been successfully used for electroplating of metals (such as Cu and Ni).32-33 The concentration of Li-ion is replenished in the entire electrode by diffusion during the rest periods, leading to a more homogenous Li deposition.34-35 Notably, all these pulse charging strategies are based on square-wave and complicated compact power, limiting the development of pulse charging technology.


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Triboelectric nanogenerator (TENG) is a newly developed portable device for micro-scale energy harvesting.36 TENGs have shown great potential in scavenging various low-frequency mechanical energy such as human motion, wind, and water wave.37-39 Pulse output is the character of TENG, and the pulse waveforms can be regulated by structure designed, power management circuits, and outside mechanical frequency. This regulated pulse waveform is beyond the reach of conventional power source, thereby expanding the understanding of electric driven dendrite growth mechanism and providing a promising charging technology for LMBs. Herein, we report that the ultra-robust TENG (UR-TENG) with novel pulse waveform has restrained the formation of dendrites in LMBs (Figure 1). The waveform and operation frequency of UR-TENG can be regulated by TENG designed and smart power management circuits. At the same effective current density, the Coulombic efficiency of Li plating/stripping can be improved by regulating the pulse waveform and frequency of TENG output. Furthermore, the Li metal anode cannot reach the dendrite formation time during pulse duration time, and the Li-ion can replenish in the entire electrode surface during rest periods eliminating the concentration polarization. Therefore, the optimized TENG based pulse output can achieve the stable and homogenous Li plating during cycling in LFP|Li batteries. RESULTS AND DISCUSSION The self-powered system is mainly composed of UR-TENG, smart power management circuit, and LMB (Figure 1). The UR-TENG consists of a three-dimensional printed polylactic


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acid (PLA) circular cylinder stator and a PLA rotor, which has been improved on the basis of our previous works.40-41 The fluorinated ethylene propylene (FEP) film coated with a Cu electrode (back electrode) is one triboelectric layer, and the Cu electrode (top electrode) with kapton film coating is another triboelectric layer (Figure 1). Aiming to increase the output frequency of pulse wave, eight electrodes are assembled on the stator and the rotor has four rollers. With the rotor rotating, the top Cu electrode will contact and separate with the FEP film, thereby the electron will flow through the external circuit because of the electrostatic induction effect.40 This process will produce an alternating current (AC) output and the frequency of the AC output is consistent with the theoretical calculation using the following equation f = nv, where n is the number of flexible layers and v is the rotation rate (rps). A smart power management circuit constituted of various transformer and bridge rectifier is designed to regulate the pulse output of UR-TENG (Figure 1). The Li|Cu cells are used to evaluate the Coulombic efficiency of Li plating/stripping under TENG based pulse output with various waveform and frequency. By regulating the size of triboelectric layer, coil ratio of transformer, bridge rectifier, and rotational frequency, the UR-TENG based pulse output with various waveform and frequency can be evaluated at the same equivalent current. The UR-TENG pulse outputs with equivalent current of 0.405 mA and rotational frequency of 1.9, 2.5, 2.8, and 4 Hz are used to plate Li in Li|Cu cell (Figure 2 and S1), which are labeled as TENG1, TENG2, TENG3, and TENG4, respectively. The output currents in Figure S1 are stable within the acceptable range. The output currents show a little instability because of the little change of battery impedance during Li plating and the


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sampling rate we used in Figure S1 is 50 s-1. It is worth noting that the effective current value shows negligible difference under sampling rate of 50 and 1000 s-1 (Figure S2). The Li|Cu cell charging by TENG2 show the highest Coulombic efficiency of Li plating/stripping (Figure S3 and S4). The Toff/Ton (Ton is the time for pulse duration, Toff is the intermittent time for pulse) plays important role in restraining the Li dendrite growth, which is highlighted in the previous reports.29,31 For the TENG based pulse charging protocols (TPCPs), the Coulombic efficiency of Li plating/stripping does not increase with the increase of Toff/Ton (Figure 2e and S3). It is worth noting that TENG3 show the highest Toff/Ton, while the corresponding Coulombic efficiency of Li plating/stripping is less than the Li|Cu cell charging by TENG2 and TENG4. Therefore, the shape of pulse waveform may play a crucial role in the Coulombic efficiency of Li plating/stripping. The “Sand time” model is widely accepted to describe the dendrite formation time of Li metal anode.10,27 The “Sand time” is the time for ionic concentration at lithium anode surface dropping to zero and the dendritic lithium formation. Therefore, the “Sand time” is introduced to evaluate the waveform of TPCPs. The “Sand time” τ can be calculated by the following equations27, τ = 𝜋𝐷

𝑒𝐶0 2

( ) 2𝐽𝑡𝑎

𝜇𝑎

ta = 𝜇 𝑎 + 𝜇

𝐿𝑖 +

≈ 1 – 𝑡𝐿𝑖 +

(1) (2)

where D is the diffusion coefficient, e is the electronic charge, C0 is the initial cation (Li salt) concentration, J is current density, ta is the anionic transport number, 𝜇𝑎 and 𝜇𝐿𝑖 +


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represent the anionic and Li+ transference number, respectively. The Sand time model predicts that the dendrite formation time is proportional to J-1/2 and D. The Li-ion transference number is calculated by Bruce-Vincent method42, as shown in Figure S5. The diffusion coefficient of electrolyte (De) can be calculated by Nernst-Einstein equation43. 𝜆𝑘𝑇

De = 𝑛𝑒2

(3)

Where 𝜆 is the ionic conductivity (7.6ｘ10-3 S cm-1), k is the Boltzmann constant, T is the temperature (K), n is the ion concentration, e is the charge of the diffusion species. The calculated diffusion coefficient is 2.03ｘ10-6 cm2 s-1. Based on these results, the calculated τ is 2.5 ｘ 104 s at current density of 1 mA cm-2. However, the size of Li dendrite can reach to several micrometer within 120 s at current density of 1 mA cm-2 or less in the previous experimental results.16,44 This is because the Li+ diffusion coefficient in the SEI is much lower than Li+ diffusion coefficient in the liquid electrolyte, which is different to the solid polymer batteries. In the LMBs using liquid electrolyte, the SEI will reach to the “Sand time” quickly, at which time the ionic concentration will go to zero at the Li metal anode and the potential will eventually diverge at the interphase leading to the formation of Li dendrite. The diffusion coefficient of Li+ in the SEI can be obtained from the follow equation45.

D = 0.5(

𝑅𝑇 2 2

)2

𝐴𝑛 𝐹 𝜎𝜔𝐶

(4)

Where R is the gas constant, T is the temperature (K), A is the area of electrode surface, n is the number of electrons per molecule during reduction, F is Faraday’s constant, 𝜎𝑤 is the Warburg factor, respectively. 𝜎𝑤 is the slope for the plot of Z´ vs the reciprocal root square


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of the lower angular frequencies (𝜔-1/2), as shown in Figure S6. The Li+ diffusion coefficient is obtained (1.11ｘ10-12 cm2 s-1). Based on the calculated Sand time, the dendrite formation curve at corresponding current is shown in the typical peak of TENG2 (Figure 2f). The pulse duration time is shorter than the dendrite formation time at any current of pulse waveform for TENG2. The pulse duration time of TENG1 and TENG3 is longer than the dendrite formation time at certain current, while the pulse duration time of TENG2 and TENG4 is shorter than dendrite formation time (Figure 2g and S7). Therefore, Li dendrite can generate at pulse duration time using TENG1 and TENG3 based charging technologies, thereby reducing the Coulombic efficiency of Li plating/stripping (Figure 2h). By contrast, Li dendrite cannot generate at pulse duration time charging by TENG2 and TENG4, thereby improving the Coulombic efficiency of Li plating/stripping (Figure 2h and S3). Figure 2h shows there is a negative correlation between the ratio of Tpulse duration/Tdendrite formation and Coulombic efficiency of Li plating/stripping excepting TENG4. This is because that the TENG4 possesses a low Toff/Ton value, which is disadvantage for the Li-ion replenishing in the entire electrode surface by diffusion. To further optimization of TPCP, the UR-TENGs with different waveform at fixed rotational frequency (2.5 Hz) have been applied to the Li|Cu cells, which are labeled as TENG5 and TENG6 (Figure S8), respectively. The typical waveforms of TENG5 and TENG6 are showed in Figure 3a and 3b. By integral calculation, the effective current of TENG5, TENG3, and TENG6 are 0.33, 0.52, 0.58 mA cm-2, respectively. Notably, the Coulombic


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efficiency of Li plating/stripping is 95.3% using TENG5 based charging strategy, which is higher than the direct current (DC) charging strategy. The ratio of Tpulse duration/Tdendrite formation increases with the increasing of effective current (Figure 3c and S9). The ratio of Tpulse duration/Tdendrite formation

has a negative correlation to the Coulombic efficiency of Li

plating/stripping (Figure 3d and S10). Thus, the ratio of Tpulse

duration/Tdendrite formation

can be

used as an important evaluation criterion for TPCPs. TENG is a promising device to harvest low frequency (typically 0.1-3 Hz) mechanical energy (such as human motions, ocean waves).46 Therefore, the self-power systems constituted by TENG and Li metal batteries have a wide applications in low frequency mechanical energy collection. Furthermore, in order to restrain the dendrite growth issues at higher current (> 1 mA cm-2), the optimization of existing DC charging technology using the TPCP, and the integration of existing dendrite suppressing strategies with TPCP will be carried out in our lab for further investigations. The LMBs using lithium iron phosphate (LFP) as cathode are used to evaluate the TPCP. Figure 4a shows the typical waveform of optimized TPCP and the charging current for 4 h is showed in Figure S11. The pulse duration time of the optimized TPCP is lower than the dendrite formation time (Figure S12) and the value of Toff/Ton is 3.29. In order to investigate the morphology of Li plating using different charging technologies, the LFP|Li batteries are charged at the same effective current. Furthermore, these LFP|Li are disassembled at 1, 2, and 4 h for morphology detection (Figure 4b and 4c). The LFP|Li battery charging by TPCP demonstrates lower charging voltage than the LFP|Li charging by DC at initial stage (Figure 4b and 4c), indicating a lower charging resistance. By using DC charging technology, the Li


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metal anode shows porous dendrite structure at micrometer scale after 1 h, exhibits moss-like dendrite structure after 2 h, and ultimately demonstrates bush-like dendrite structure after 4 h (Figure 4d-f). These moss-like and bush-like structure possess sharp edge at nanoscale, thereby promoting the side reactions between Li metal and electrolytes. By using optimized TPCP, the Li metal anode shows stone-like structure at dozens of microns after 1 h, and exhibits smooth Li plating structure after 2 and 4 h (Figure 4g-i). These stone-like structures possess round-shaped edges at micrometer scale, restricting their ability to penetrate the porous separators. The interphase polarization and dendrite formation cannot occur during the pulse duration time because that the pulse duration time is much less than the dendrite formation time. In other words, the diffusion of Li-ion can match to the electromigration during the pulse duration time, thereby reducing the concentration polarization and the Li dendrite formation. Furthermore, the Li-ion is periodically replenished in the entire electrode surface at relaxation period, eliminating the concentration polarization at the sharp surface and result in a more homogenous Li plating. Consequently, by using the optimized TPCP, the formation of Li dendrite has been restrained. The typical charge/discharge curves of LFP|Li batteries using optimized TPCP and DC charging strategy at the same effective current are shown in Figure 5a. The polarization between the charge and discharge plateaus for the LFP|Li battery using optimized TPCP is lower than that using DC charging strategy, elucidating that the resistance of LFP|Li battery are reduced by using TPCP. To further investigate the kinetics and charge transfer behavior of LFP using different charging strategies, the electrochemical impedance spectroscopy (EIS)


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is applied to the LFP|Li batteries. Seen from the Nyquist plots (Figure 5b and S13), the LFP|Li battery charging by optimized TPCP shows lower charge transfer and interface impedance than the LFP|Li battery charging by DC, thereby enhancing the battery performance especially after 100 cycles (Figure S14). Furthermore, the LFP|Li battery shows high charging stability for more than 120 cycles (Figure S14). The morphology of Li metal anodes using different charging strategies for 20 cycles are showed in Figure 5c and 5d. The Li metal anode using DC charging strategy shows bush-like dendrite structure at micrometer scale, thereby increasing the side reactions between the Li metal and electrolytes. In addition, part of these bush-like dendrite structure detaches from the current collector forming electrochemically inactive “dead Li”, increasing the interface resistance and consuming a lot of Li metal. In contrast, the Li metal anode using TPCP shows stone-like Li plating structure with smooth surface at hundreds of micrometers. Besides, all these stone-like Li are connected together and constituted to one mainland, thereby restraining the formation of electrochemical “dead Li” and reducing the interface resistance. By using the TPCP, the LMBs can realize the stable Li plating. The interface composition and valence on the surface of Li metal anodes were detected by X-ray photoelectron spectroscopy (XPS). Seen from the C 1s and F 1s spectra (Figure 5e,f and S15), The reduction products of electrolyte including COR, C=O, Li2CO3, ROCO2Li, and LiF are founded on the surface of Li metal anodes, which are the main composition of SEI layers.7,19,24 Notably, the content of these side reaction products is clearly decreased on the surface of Li metal anode by using the TPCP (Figure 5f and S16). Combing with the SEM


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results, the bush-like Li dendrite with high surface area possesses a lot of reduction products using the DC charging strategy. While by using the TPCP, the stone-like Li with smooth surface can markedly reduce the sides reactions between the Li metal and electrolytes. Thus, the stable and homogenous Li deposition can be achieved during cycling. These results will exert far-reaching influence on the development of TENG and LMBs. TENG based output can be efficiently stored in LMBs by using optimization design and smart power management circuit, promoting the application of TENG in blue energy, wind energy, and wearable electronics. In addition, with the waveform and frequency regulation technology, the TENG based charging technology can expand the understanding of electric driven dendrite growth mechanism and provide an innovative strategy to address the Li dendrite growth issues in LMBs. CONCLUSION In summary, TENG based pulse current charging technology is demonstrated to be a promising strategy for achieving the stable and homogeneous Li plating in LMBs. The waveform and operation frequency of TENG output can be regulated by TENG design and smart power management circuit. By regulating the waveform and frequency, the pulse duration time is shorter than the formation time of Li dendrite at any current of pulse waveform, and the high Toff/Ton enables Li-ion to replenish in the entire electrode eliminating concentration polarization, thereby improving the Coulombic efficiency of Li plating/stripping. Besides, the homogenous Li plating have been realized in the LFP|Li batteries during cycling. This TENG based charging technology provides an innovative


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strategy to address the Li dendrite growth issues in LMBs, and accelerates the application of TENG based energy collection systems. METHODS Fabrication of UR-TENG The UR-TENG was prepared by 3D printing and the surface modification, which has been reported in our previously work.40 UR-TENGs with various size were prepared. The coil ratio of transformer used in the power management circuits are 36, 50, and 75. To obtain the same effective current in the Li|Cu cell. By regulating the size of rectangular triboelectric layer (from 5*2.5 cm to 10*2.5cm), coil ratio of transformer, and rational frequency, four URTENG based output at same effective current (0.405 mA) were obtained. These UR-TENG based output at rotational frequency of 1.9, 2.5, 2.8, and 4 Hz were labeled as TENG1, TENG2, TENG3, and TENG4, respectively. We choose the low rotational frequency because that TENG has a better performance than the electromagnetic generators (EMGs) at low frequency. The coil ratio of transformer, size of triboelectric layer, and rotational frequency of motor are positively related to the output of TENG. When the TENG triboelectric layer and coil ratio of transformer are selected, the frequency of TENG is also fixed for the same current output (0.405 mA). The frequency of TENG is regulated by the rotational speed of motor. Thus, regulation process is stability and reliability. Preparation of Batteries The Li|Cu cell was fabricated using the Cu foil, a separator, Li metal anode, and electrolyte. For LFP|Li battery, the cathode slurry was prepared by mixing 80 wt% LFP, 10 wt% Super P,


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and 10 wt% polyvinylidene difluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP). The cathodes were produced by coating the slurry onto aluminum foil and drying at 80 °C for 12 h. The 2032 coin cells were fabricated in an argon-filled glove box using Li metal as the counter electrode and celgard 2325 as the separator. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 by volume). Characterization For the Li|Cu cell, UR-TENGs with various waveform and frequency were used as the power source for Li plating, and the Li stripping processes were carried out by LAND testing system. For the LFP|Li batteries, the charge processses were conducted by optimized TPCP and the discharge processes were performed on LAND testing system. All the pulse currents were recorded by electrometer (Keithley 6514), and corresponding voltages were recorded by multimeter (UT61E) with help of computer. The coin-cells of LFP|Li batteries were disassembled in a glove box and thus the Li metal anodes were washed for three times using DMC to remove the residual electrolyte. After that the Li metal anodes were transferred to special designed devices for ex situ characterization without exposing the samples to air including scanning electron microscopy (SEM) and Xray photoelectron spectroscopy (XPS). The microstructure of the samples was performed on a Hitachi SU8020 field-emission scanning electron microscope (FE-SEM). XPS was carried out on a Thermo Scientific ESCALab 250Xi using 200 W monochromated Al-Kα radiation.


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Figure 1. Charging of LMBs using constant current power source (a) and UR-TENG (b).


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Figure 2. The waveform of TENG1 (a), TENG2 (b), TENG3 (c), and TENG4 (d) with the same equivalent current of 0.405 mA. The relationship between Coulombic efficiency of Li plating/stripping and Toff/Ton of TENG based output (e). The schematic of pulse duration time and dendrite formation time of TENG2 (f). The ratios of Tpulse duration/Tdendrite formation (g) and their relationship with Coulombic efficiency of Li plating/stripping (h).


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Figure 3. The typical waveform of TENG5 (a) and TENG6 (b). The ratios of Tpulse duration/Tdendrite

formation

(c) and their relationship with Coulombic efficiency of Li

plating/stripping (d).


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Figure 4. (a) The typical waveform of optimized TPCP for LFP|Li battery. The charging curves of LFP|Li batteries using DC (b) and optimized TPCP (c). SEM images of Li metal anode charging by DC for 1 (d), 2 (e), and 4 h (f), respectively. SEM images of Li metal anode charging by optimized TPCP for 1 (g), 2 (h), and 4 h (i), respectively.


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Figure 5. The charge/discharge curves (a) and Nyquist plots (b) of LFP|Li batteries. SEM images of Li metal anode charging by DC (c) and TPCP (d) after 20 cycles. XPS spectra of C 1s for Li metal anode charging by DC (e) and TPCP (f) after cycles.


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ASSOCIATED CONTENT Supporting Information. The current of Li-Cu cells charging by different TENG, the Li stripping curves of various Li-Cu cells, the calculation method of lithium-ion transference number and Warburg factor, the comparison of TENG pulse duration time and dendrite formation time for various TENG, the cycling performance of LFP|Li batteries, and the XPS results of Li metal anode are showed in the Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ⊥These

authors contributed equally to this work.

Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Postdoctoral Program for Innovative Talents and the General Financial Grant from the China Postdoctoral Science Foundation (Grant Nos. BX201700061 and 2017M620710), the National Natural Science Foundation of China (NSFC Nos. 21703010, 51503005 and 21274006), National Key R&D Project from Minister of Science and Technology (2016YFA0202703, 2016YFA0202702 and 2016YFA0202704), and the Programs for Beijing Science and Technology Leading Talent (Grant no. Z161100004916168), and the Fundamental Research Funds for the Central Universities (Grant no. 06501000), and the “Ten thousand plan”-National High-level personnel of special support program, and the “Thousands Talents” Program for Pioneer Researcher and His Innovation Team, China. REFERENCES 1.

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(

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