Polynanocrystalline Graphite: A New Carbon Anode with Superior

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Polynanocrystalline Graphite: A New Carbon Anode with Superior Cycling Performance for K-ion Batteries Zhenyu Xing, Yitong Qi, Zelang Jian, and Xiulei Ji ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06767 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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ACS Applied Materials & Interfaces

Polynanocrystalline Graphite: A New Carbon Anode with Superior Cycling Performance for K-ion Batteries Zhenyu Xing, Yitong Qi, Zelang Jian and Xiulei Ji* Department of Chemistry, Oregon State University, Corvallis, OR, 97331, USA

Abstract We synthesized a new type of carbon—polynanocrystalline graphite—by chemical vapor deposition on a nanoporous graphenic carbon as an epitaxial template.

This

carbon is composed of nanodomains being highly graphitic along c-axis and very graphenic along ab plane directions, where the nanodomains are randomly packed to form micron-sized particles, thus forming a polynanocrystalline structure. polynanocrystalline

graphite

is

very

unique,

structurally

different

The from

low-dimensional nanocrystalline carbon materials, e.g. fullerenes, carbon nanotubes and graphene, nanoporous carbon, amorphous carbon and graphite, where it has a relatively low specific surface area of 91 m2/g as well as a low Archimedes density of 0.92 g/cc.

The structure is essentially hollow to a certain extent with randomly

arranged nanosized graphite building blocks.

This novel structure with disorder at

nanometric scales but strict order at atomic scales enables substantially superior long-term cycling life for K-ion storage as an anode, where it exhibits 50% capacity

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retention over 240 cycles, whereas, for graphite, it is only 6% retention over 140 cycles.

Keywords Polynanocrystalline graphite; Disorder, K-ion batteries; Anode; Cycling life

Introduction Li-ion batteries (LIBs) have gained great successes as the power source in portable electronics and electric vehicles due to their high energy density.1-8

However, the

scarcity and uneven global distribution of lithium reserve raise the concern of long-term sustainability for LIBs.9,10

Furthermore, there exists tremendous demand

for inexpensive alternative energy storage technologies that can enable widespread deployment of intermittent renewable energy sources, such as solar and wind. Hence, it is critical that battery systems on the basis of Earth-abundant elements be explored and investigated.

To date, several battery technologies based on

Earth-abundant elements have been under intense investigation, such as Na-ion batteries (NIBs),11-35 Mg-ion batteries (MIBs),36-38 Al-ion batteries (AIBs),39,40 and K-ion batteries (KIBs).41-50

Potassium is the 7th most abundant element in Earth’s

crust, nearly 900 times richer than lithium.

The abundance potentially renders the

production cost of such batteries much lower compared to Li-batteries. Recently, our group and others reported that graphite can reversibly store K-ions electrochemically, which opens up the possibility of equipping KIBs with insertion-type carbon anodes.51-53 Additionally, our group further reported K-ion storage properties of 2


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non-graphitic carbons, including soft carbon and hard carbon.54

Among these bulk

carbon materials, at the same specific conditions, graphite suffers the fastest capacity fading, where it loses 50% of its capacity after 50 cycles at C/2 (1C-rate is defined as 279 mA/g), while the capacity retentions for soft carbon and hard carbon are 81% after 50 cycles at 2C and 83% after 100 cycles at C/10, respectively.

Here, we hypothesize that the superior cycling stability of hard carbon and soft carbon to graphite in KIBs is due to the disordered arrangement of turbostratic nanodomains in these carbons.

We postulate that it is the volume expansion (61%) of the compact

graphite structure with ABAB registry that is responsible for the fast capacity fading. To test such a hypothesis, herein, we design a new type of carbon material for K-ion storage, which resembles the non-graphitic carbon structures at long-range scales but maintains a highly ordered graphitic registry locally for a few nanometers along the c-axis.

Namely, ordered graphitic nanodomains instead of turbostratic nanodomains

are randomly oriented in bulk-sized carbon particles.

Such a carbon would be,

indeed, short-range ordered but long-range disordered polynanocrystalline graphite. The closest analogue to our proposed carbon structure that exists would be some special hard carbons if they are annealed up to a very high temperature, i.e., > 2000 ºC. However, such hard carbon, or often referred to as “glassy carbon”, is not locally graphitic as it still does not have the short-range order along the c-axis 55.

In this contribution, we report a facile synthesis of polynanocrystalline graphite by

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chemical vapor deposition (CVD) using ethanol vapor as a precursor onto the surface of nanoporous graphenic carbon (NG) as an epitaxial template.

NG was synthesized

by our previously reported method, where a spontaneous metallothermic reduction of gaseous CO2 takes place on an Mg/Zn mixture as the reductant at high temperatures.56 We select NG as the template because CVD carbon deposition may occur toward different directions at nanometric scales on NG’s highly curved graphenic surface in the nanopores, thus generating a polynanocrystalline graphite.

The resulting

polynanocrystalline graphite we prepared, hereafter referred to as PG, shows very stable cycling life as an anode in KIBs with 50% of capacity retention over 240 cycles, in contrast to 6% over only 140 cycles by the commercial graphite.

The improved

cycling stability of PG is attributed to the random arrangements of graphitic nanodomains, which better accommodates the strain caused by K-ion insertion during cycling.

Experiment section Preparation of PG and NG NG is prepared following the method we reported before.1

To prepare PG by CVD

of carbon on NG, we place NG powder in an Al2O3 boat that is heated in a tube furnace at 1100 ºC for 20 h under ethanol vapor carried by Argon gas at a flow rate of 90 CCM. Characterization Methods: X-ray diffraction (XRD) patterns were recorded using a Rigaku Ultima IV 4


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Diffractometer with Cu Kα irradiation (λ= 1.5406 Å).

WITec confocal Raman

spectrometer with a 514 nm laser source was used to record the Raman spectra.

The

morphology was studied by field emission scanning electron microscopy (FESEM) using a Hitachi S5500 at 0.5 kV.

Transmission electron microscopy (TEM) images

were collected by FEI Titan 80-200.

Nitrogen sorption measurements were

performed on Micromeritics TriStar II 3020 analyzer.

Electrochemical Measurements: All the electrodes consist of 80 wt% carbon active mass, 10 wt% carbon black (C45) and 10 wt% sodium carboxymethyl cellulose (CMC) as the binder. The slurry was cast onto Cu foil and dried at 80 °C under vacuum for 12 h.

Coin cells were

assembled in an argon-filled glove box, with the as-prepared carbon electrode as the working electrode, potassium metal as the counter/reference electrode and glass fiber membrane as a separator.

The electrolyte was prepared by dissolving 0.8 M KPF6

into a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio: 1/1).

The active mass loading was around 2 mg/cm2.

The electrochemical

measurements were performed on an Arbin BT2000 system at room temperature, where the voltage range was from 0.01 to 2 V versus K+/K.

Results and discussion A polynanocrystalline material should exhibit a relatively low surface area and large particle sizes, which differentiates such a material from nanoporous materials or nanocrystalline materials.

Despite the low surface area and large particle sizes, its

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density should be much lower than the bulk single crystalline or polycrystalline analogs.

After the CVD treatment of NG to form PG, the Brunauer-Emmett-Teller

(BET) surface area dramatically decreases from 1900 to 91 m2/g.

A comparison of

the isotherms of NG (Type II) and PG (Type III) in Figure S1a and c shows that micropores and mesopores of NG observed at low and intermediate relative pressures, respectively, are nearly filled up after CVD.

By considering the scales of absorbed

volumes (y-axis), after CVD PG only maintains a small portion of macropores observed at high relative pressures (Figure S1b,d).

SEM images illustrate the macroscopic morphology change from NG to PG, where NG exhibits an irregular hollow semi-spherical morphology with very smooth surface (Figure 1a,b), whereas PG particles are of very solid texture with much rougher surface after CVD (Figure 1c,d).

Furthermore, we measured the Archimedes

density of NG and the seemingly “non-porous” PG, where the density of PG is only 0.95 g/cm3, compared with 0.54 g/cm3 for NG.

Considering that graphite’s density

is above 2 g/cm3, one can gain from the above results two important insights: (1) the CVD carbon deposition seals the nanoporosity of NG effectively, resulting in a low surface area of PG; (2) PG contains a hollow structure, but the internal structure is clearly not accessible to N2 molecules during sorption measurements.

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Figure 1. SEM images of NG (a) and PG (c). (b) and (d) are the enlarged sections marked in the yellow boxes in (a) and (c).

To characterize the structures of NG and PG, we conducted x-ray diffraction (XRD), transmission electron microscopy (TEM) as well as Raman spectroscopy measurements.

The goal is to determine whether PG exhibits a high degree of local

order but much disorder in nanometric or longer scales.

As shown in the XRD

patterns (Figure 2a), PG exhibits a much sharper (002) peak with a smaller value of full width at half maximum (FWHM) and a more pronounced (100) peak, compared to NG. PG.

The broad bump to the left of (002) peak of NG is completely vanished in

The d(002) of PG is calculated to be 0.345 nm, which is fairly close to that of

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graphite—0.335 nm.

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Based on the Scherrer equation, the coherence lengths: Lc

along c axis and La along ab planes are calculated to be 1.70 nm and 6.86 nm, respectively, for NG, and 3.71 nm and 8.42 nm, respectively, for PG.

The XRD

results showing a longer range of order along c-axis and ab planes indicate that PG comprises more graphitic and more graphenic domains than NG.

However, the

coherence range is still less than 10 nm, where the graphitic/graphenic domains in PG are, in fact, nanocrystalline; when considering the bulk particle size and low surface area of PG, PG is essentially polynanocrystalline.

Figure 2. (a,b) XRD patterns and Raman spectra of NG, PG and graphite (G).

Fitted

Raman data and their deconvolution into TPA (red), D (blue), A (green) and G (pink) bands for NG (c) and PG (d).

As shown in Figure 2b, graphite, PG and NG all give rise to both G band and D band, 8


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where the G-band corresponds to the sp2 carbon with E2g vibration, and the D-band is attributed to the A1g vibration of finite-sized graphenic domains.

To further analyze

the Raman results, we fit the experimental spectra of PG and NG by using TPA (transpolyacetylene) band, D band, A band, and G band, shown in Figure 2c and d.57 The TPA band can be observed as the shoulder to the left of the D band, and the A band contributes to the intensity between G band and D band.58-61

The peak

positions of G band for NG and PG are 1596 cm-1 and 1590 cm-1, respectively.

Here,

the redshift of G band is attributed to the increased graphenic degree, as revealed in the XRD results, which agrees with the trend reported before.62,63

The presence of A

band is commonly observed from disordered or non-graphitic carbon, where this band is attributed to point defects and/or graphene curvature.64,65

Interestingly, the fairly

pronounced A band in NG completely disappears in PG, clearly demonstrating the transition from a locally defective carbon to a much more graphenic carbon.

The

TPA band that is often attributed to transpolyacetylene-like structure also diminishes from NG to PG, also showing a better degree of order.66

The 2D band, the overtone

of D band, whose existence requires vibration from defect-free graphene domains, become much more pronounced from NG to PG, where the 2D band of PG shares the same wavenumber as graphite, further confirming the good extent of order along ab planes

67

.

The ratio between the integrated intensities of the G and D Raman bands

(IG/ID) along with the excitation laser wavelength are used to calculate the graphenic domain size, La, namely the coherence size along ab planes. The general equation correlating these parameters was introduced by Cancado et al.,68 as shown below:

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2.4 ⋅ 10 ⋅ where the excitation laser wavelength (λnm) is 514 nm. The IG/ID value is 0.25 for NG and 0.81 for PG, corresponding to La values of 4.18 nm and 13.56 nm for NG and PG, respectively.

The calculated La values from Raman

spectra further confirm the enhanced local order of PG compared to NG.

Figure 3.

HRTEM images of NG (a) and PG (b).

Inset of (b) is the SAED pattern

of PG. 10


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High-resolution

TEM

studies

provide

unequivocal

evidence

for

the

polynanocrystalline structure of PG, which is short-range ordered but long-range disordered.

As shown in Figure 3b, PG possesses vividly displayed lattice fringes

of graphitic nanodomains, where these domains are packed along random orientations, vastly different from the long-range ordered structure of graphite and completely disordered structure of amorphous carbon, i.e., hard carbon, indicating a unique polynanocrytalline structure.

Between these nanodomains, there are tiny voids,

which explain the low density of this material.

To provide a more complete

landscape for the unique structure of this polynanocrystalline graphite, additional TEM images are shown in Figure S2.

The formation of PG’s structure is attributed to the porosity and local atomic arrangement of NG as a CVD template, which exhibits a highly tortuous porous structure with graphenic walls (Figure 3a) and successfully serves as an epitaxial substrate for the growth of PG’s structure.

The polynanocrystalline structure of PG

is further confirmed by selected area electron diffraction (SAED), where the resolved diffraction rings indicate the isotropic nature in nanometric scales, very different from the typical SAED patterns of graphite with preferential crystallite orientation (Figure S3) and amorphous carbon that exhibits poorly resolved diffraction rings.

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Figure 4. (a) The first galvanostatic potassiation/depotassiation potential profiles of PG and Graphite in the potential range of 0.1–2 V vs. K+/K at a current rate of 20 mA/g.

(b) Rate performance for PG and Graphite at current densities from 20, 50,

100, 200, 500 to 1000 mA/g.

CV curves of the first three cycles for PG (c) and

Graphite (d).

For the electrodes with a potentially large volume change during cycling, such as silicon anode in LIBs, suitable binders can provide a structural buffer or a high adhesion force to retain the electrode’s integrity during cycling.

Sodium

carboxymethyl cellulose (CMC), also known as CMC, is a linear cellulose derivative, which is widely used in LIBs for silicon anodes.

In this study, we use CMC as the

binder for all carbon anodes, where their K-ion storage performance is studied.

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As

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shown in Figure 4a, PG exhibits more sloping potassiation/depotassiation potential profiles compared to those of graphite. clearly higher than that of graphite.

The average potassiation potential of PG is

The difference that PG presents a sloping profile

while graphite shows a plateau shape is more evident during depotassiation.

The

plateau profiles of graphite are attributed to the evolving stages of K-graphite intercalation compounds (K-GICs), whereas potassiation and depotassiation of PG occur in a manner more like a solid-solution process due to the polynanocrystalline structure.

If the stage-one K-graphite intercalation compound (GIC), KC8, is formed, the theoretical capacity of graphite and polynanocrystalline graphite is 279 mAh/g.

In

the first cycle, PG delivers a potassiation capacity of 414 mAh/g and a depotassiation capacity of 224 mAh/g with a coulombic efficiency of 54.1 % lower than 78.0 % of graphite, where PG exhibits a high level of reversibility in the following cycles (Figure S4a).

It is worth pointing out that the higher surface area of PG, 91 m2/g,

may play a role in the disparity of coulombic efficiency between PG and graphite (BET surface area: 1.2 m2/g).

We also compared the rate capability between PG and

graphite, as shown in Figure 4b, where PG delivers a reversible capacity of 221, 189, 142, 99, 43.2 and 13.6 mAh/g at current rates of 20, 50, 100, 200, 500 and 1000 mA/g, respectively, slightly lower than graphite at the same rates.

We hypothesize that the

tortuosity inside PG may slow down the ionic transport, which in turn lowers the rate performance.

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It should be pointed out that the capacities of PG and graphite mentioned above have excluded the contribution from carbon black (C45). We tested C45/K half-cells separately by having C45 as the sole active mass with CMC as the binder (mass ratio: 90/10).

The first two galvanostatic potassiation/depotassiation potential profiles of

C45 in the potential range of 0.1–2 V vs. K+/K at a current rate of 20 mA/g are shown in Figure S5.

The capacity values of potassiation and depotassiation in the first

cycle are 1707 mAh/g and 276 mAh/g, respectively.

The coulombic efficiency of

the first cycle is only 16.2%, which may partially explain the low coulombic efficiency of the graphite (78.0%) and PG (54.1%) electrodes we tested because C45 was added.

C45’s more stabilized capacity values of potassiation and depotassiation

in the second cycle are 387 and 227 mAh/g, respectively.

Here, we have factored the

capacity value of C45 in the 2nd depotassiation sweep out of the observed capacity values of the mixtures of PG/C45 and Graphite/C45 to calculate our results.

The

calculated depotassiation capacity of PG is 224 mAh/g, which is lower than the observed 252 mAh/g when not considering that C45 contributes to the total capacity.

To further understand the electrochemical behavior of PG, we also performed cyclic voltammetry (CV) at a scan rate of 0.1 mV/s in the potential range of 0.01-2 V. Surprisingly, there is no cathodic peak until 0.1 V vs. K+/K (Figure 4c). During the anodic scan for depotassiation, a single broad current peak centers at 0.5 V, which is in contrast to the multiple anodic peaks for graphite accounting for sequential

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transitions between staged K-GIC phases (Figure 4d).

The lack of phase

transformation for PG electrode may lead to minimal volume change during charge/discharge.

We notice that there is a large irreversible capacity during the first galvanostatic cycle, whereas there is a lack of massive cathodic current corresponding to the SEI formation in the first CV curves, which seems contradictory. If SEI were formed, there should be an obvious slope/plateau at relatively high potentials, i.e., > 0.2 V vs K+/K in the potassiation/depotassiation profiles. However, such a behavior was not observed, which is further confirmed by the CV results. It is thus worth emphasizing that the lack of SEI formation on carbon anodes in KIBs could be a significant advantage for potential KIBs.

Then, the question is whether the irreversible capacity in

galvanostatic cycling is reflected by CV curves. If we integrate the areas for the cathodic and anodic current over time in the CV curves for the PG electrode and graphite electrode, we find the ratios are 42.8% and 54.0% for PG and graphite, respectively.

These values, representing the degrees of the irreversible cathodic

reactions, are very close to the coulombic efficiency values obtained from the 1st galvanostatic potassiation/depotassiation potential profiles.

That being said, the

exact cause that leads to the irreversibility is currently under investigation. It is most likely due to the trapping mechanism, where some trapped K-ions cannot be extracted during depotassiation.

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It is also worth pointing out that high surface area of electrode materials typically induces more pronounced formation of solid electrolyte interphase (SEI).

NG with a

surface area of 1900 m2/g exhibits the first-cycle coulombic efficiency of only 10.9% (Figure S6a), which is confirmed by the massive irreversible cathodic current of the first CV curve (Figure S6c).

Also, its coulombic efficiency remains below 90% in

the first 20 cycles (Figure S6b, d).

Furthermore, NG presents linear galvanostatic

charge/discharge potential profiles and rectangular CV curves, which is typical capacitive charge storage behavior (Figure S6a, c).

Since the faster capacitive

mechanism plays a more dominant role than the intercalation mechanism, NG exhibits relatively good rate performance (Figure S6b), and fairly stable cycling performance especially after the SEI has been formed (Figure S6d).

The comparison of physical

and electrochemical properties of NG and PG is summarized in Table S1 in the supporting information.

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Figure 5. (a,b) Ex situ XRD patterns and ex situ Raman spectra of graphite and PG collected after 100 cycles. (c) Long-term cycling performance of graphite and PG at a current density of 100 mA/g.

PG exhibits much improved long-term cycling performance than graphite, where the cycling results from three PG/K cells, PG-1, PG-2, and PG-3 are shown in Figure 5c. Graphite retains ca. 6% of its original capacity after 140 cycles (from two cells, G-1 and G-2), whereas PG retained ca. 50% of its original capacity after 240 cycles.

The

capacity fading for PG is a linear curve, while graphite’s capacity remained stable for the first 50 cycles or so, and then went through an abrupt degradation afterwards. Figure

S7

shows

the

1st,

100th,

200th

and

17


300th

galvanostatic


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potassiation/depotassiation potential profiles of PG.

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Besides the capacity fading, the

polarization increases upon cycling, which may be related to the varying surface properties of the carbon electrode and/or the possible degradation of the electrolyte, considering that potassium counter electrode can be very reactive toward organic electrolyte.

To understand the fading mechanism, we conducted ex situ XRD to investigate the structural change of PG and graphite after 100 cycles, as shown in Figure 5a.

The

sharp (002) peak for pristine graphite evolves into a compounded peak, which is composed of the pure graphite peak in the middle, a disordered carbon shoulder on the left and the other shoulder on the right that may be attributed to a high-stage GIC phase.69

In fact, the shoulder on the right shows up even after the first cycle.52

The

long-term capacity fading of graphite may be attributed to the newly generated amorphous substructures that shut down the access of K-ions into the internal galleries of the large graphite crystals with rigid structures.

In contrast, interestingly,

the originally ordered local structure of PG along c-axis becomes completely amorphous after 100 cycles based on the XRD results (Figure 5a).

The (002) peak

of PG shifted from 25.8º to 17.4º after 100 cycles, where the d-spacing increases from 0.345 to 0.510 nm, and the much broadened peak with a large value of FWHM suggests effective exfoliation of graphitic nanodomains possibly into single or double layers of graphenes with no coherence along c-axis.

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Considering the disparity of cycling performance of graphite and PG as well as the ex situ post-cycling XRD results, it appears that the cycled graphite containing amorphous and crystalline portions does not provide efficient accessibility for K-ions any longer; however, the exfoliation of graphitic nanodomains taking place inside PG, albeit turning PG completely non-graphitic, enables much better long-term cyclability. This confirms our hypothesis that the disordered arrangement of graphene layers inside non-graphitic carbons leads to superior cycling performance, which has been observed in hard carbon and soft carbon in our prior reports.

Interestingly, there is

barely any change of the Raman spectra for both G and PG before and after cycling in Figure 5b, indicating that the ordered atomic structure along the ab planes remain unaffected during the long cycling in KIBs.

Conclusion In summary, we synthesized polynanocrystalline graphite by chemical vapor deposition on a nanoporous graphenic carbon.

This new type of carbon is composed

of randomly oriented graphitic nanodomains, facilitating a unique structure with a low density of 0.92 g/cc and a relatively low surface area of 91 m2/g.

The

polynanocrystalline graphite is structurally unique, compared to low-dimensional nanocrystalline carbon materials, nanoporous carbon, amorphous carbon and graphite. This polynanocrystalline graphite shows much improved long-term cycling life in KIBs, where it exhibits 50% capacity retention over 240 cycles in contrast to 6% over 140 cycles for graphite.

The improved cycling performance is attributed to the

polynanocrystalline nature of PG with disorder at nanometric scales, which allows for 19



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exfoliation of graphitic nanodomains inside the bulk particle of PG, thus still retaining the electrode structural integrity.

Our results provide new insights on carbon design

by differentiating structures at nanometric scales and atomic scales, while maintaining relatively low surface area.

This new type of carbon may well find applications

beyond KIBs.

ASSOCIATED CONTENT

Supporting Information.

BET results and supporting data are in Figure S1-S4.

This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-Mail: [email protected] Author Contributions The manuscript was written through contributions of all authors.

All authors have

given approval to the final version of the manuscript. Acknowledgments X. J. are thankful for the financial supports from National Science Foundation Award No.1551693.

We would like to thank Dr. Peter Eschbach and Ms. Teresa Sawyer for

the SEM measurements at the OSU Electron Microscopy Facility.

Additional

acknowledgements extend out to Mr. Joshua Razink for the TEM measurements at the Center for Advanced Materials Characterization at Oregon (CAMCOR) at the University of Oregon. We are thankful to Professor Chih-Hung Chang and Mr. 20


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Changqing Pan for Raman analysis.

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Polynanocrystalline Graphite: A New Carbon Anode with Superior

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