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Towards High Power Batteries: Pre-lithiated Carbon Nanospheres as High Rate Anode Material for Lithium Ion Batteries Florian Holtstiege, Tuncay Koç, Tobias Hundehege, Vassilios Siozios, Martin Winter, and Tobias Placke ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00945 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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ACS Applied Energy Materials

Towards High Power Batteries: Pre-lithiated Carbon Nanospheres as High Rate Anode Material for Lithium Ion Batteries Florian Holtstiegea, Tuncay Koça, Tobias Hundehegea, Vassilios Sioziosa, Martin Wintera,b, *, Tobias Plackea, * a

University of Münster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149 Münster, Germany b

Helmholtz Institute Münster, IEK-12, Forschungszentrum Jülich, Corrensstraße 46, 48149 Münster, Germany

Abstract In this work, carbon nanospheres (CS) are prepared by hydrothermal synthesis using glucose as precursor, followed by a subsequent carbonization step. By variation of the synthesis parameters, CS particles with different particle sizes are obtained. With particular focus on the fast charging capability, the electrochemical performance of CS as anode material in lithium ion batteries (LIBs) is investigated, including the influence of particle size and carbonization temperature. It is shown that CS possess an extraordinary good long-term cycling stability and a very good rate capability (up to 20C charge/discharge rate) at operating temperatures of 20 °C and 0 °C compared to graphitic carbon and Li4Ti5O12 (LTO)-based anodes. One major disadvantage of CS is the very low 1st cycle Coulombic efficiency (Ceff) and the related high active lithium loss, which prevents a usage of CS within LIB full cells. Nevertheless, in order to overcome this problem, we performed electrochemical pre-lithiation, which significantly improves the 1st cycle Ceff and enables the usage of CS within LIB full cells (vs. NMC-111), which is shown here for the first time. The improved rate capability of CS is also verified in electrochemically pre-lithiated NMC-based LIB full cells, in comparison to graphite and LTO anodes. Further, CS also display an improved specific energy (at ≥5C), energy efficiency (at ≥2C) and energy retention (at ≥2C) compared to graphite and LTO-based LIB full cells.

Keywords Pre-lithiation; carbon nanospheres; active lithium loss; rate capability; fast charging; anode; lithium ion battery

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Corresponding authors: *Dr. Tobias Placke

*Prof. Dr. Martin Winter

[email protected]

[email protected] [email protected]

Tel.: +49 251 83-36826

Tel.: +49 251 83-36031

Fax: +49 251 83-36032

Fax: +49 251 83-36032


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1. Introduction These days, in addition to high energy density,[1, 2] also high power capabilities, in particular the fast charging ability of lithium ion batteries (LIBs) finds increasing attention, e.g. in automotive applications.[3, 4] For this reason, it is necessary that a large amount of charge, with respect to the overall available capacity, can be accommodated or released within a short time period by the active materials, i.e. that they possess a high rate capability and that this results not into too high overpotentials. Low overpotentials and high rate capability can be subsumed in high power capability. Very often, only rate capability and not power capability is regarded in the battery materials science literature. One major issue, which prevents the rapid charge and discharge of LIBs, is the utilization of graphite as anode active material because it has only a limited rate capability.[5, 6] [7] The limited rate capability of graphite anodes, which is even enhanced at low operating temperatures, results also in increased safety issues, related to the high tendency for lithium metal plating on top of the graphite anode instead of lithium intercalation.[8-10] Established strategies to avoid Li metal plating are to tailor the anode/cathode capacity balancing [11, 12] or to optimize the anode material, e.g. by carbon coating.[13] In general, important factors influencing the rate capability of a material are the lithium diffusion lengths inside the particles and the contact area (=specific surface area) between active material and electrolyte, because a greater contact area/smaller particles support a fast lithiation and de-lithiation process, due to a smaller charge transfer resistance and reduced diffusion paths. electrolyte.

[14-17]

For graphite, the type of surface is determining the reactivity with the

[18-21]

One possibility to overcome these issues is to use nanomaterials, such as carbon nanospheres (CS), as active material for the anode. On basis of their small particle size and their high surface area, the rate capability can be remarkably increased.[22-24] Carbon spheres can be synthesized by using a variety of different synthesis techniques, e.g. chemical vapour deposition,[25, 26] arc-discharge,[27, 28] laser ablation,[27, 28] or hydrothermal heat treatment.[29-34] Furthermore, a wide variety of precursors can be used, like sucrose,[35] glucose,[15, agarose,[36] resin,[16,

37, 38]

29]

natural gas,[23] cyclohexachlorobenzene,[22] chloroform,[39] or

polymers like polypyrrole,[40] etc. Thereby, the choice of the precursor, the synthesis route and the synthesis conditions affect the electrochemical performance of the CS, like e.g. the reversible capacity.[22,

41-44]

By using a hydrothermal synthesis route in combination with

glucose as precursor, obtained from a renewable biomass (e.g. corn or potatoes), the production of CS is a low-cost, sustainable and non-toxic process. However, the major 3 ACS Paragon Plus Environment


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disadvantage of carbon nanospheres is the relatively low 1st cycle Coulombic efficiency (Ceff) of only 30-68% during lithiation/de-lithiation.[15,

23, 39]

This is mainly caused by the high

specific surface area and in particular by the high amount of “non-basal plane” surface area of CS, leading to an increased solid electrolyte interphase (SEI) formation, which is in turn linked to a higher active lithium loss.[21, 28, 45-47] For this reason, to the best of our knowledge, there are no publications in the literature so far showing the performance of CS within LIB full cells, i.e. the combination of a CS-based anode and lithium metal oxide cathode. The only data available so far show the great potential of CS as anode material in CS/Li metal cells, socalled “half-cells”.[6, 23, 40-43, 48] However, such a cell set-up -providing an excess amount of active lithium- seems to be unrealistic for any kind of application and shows different electrolyte decomposition and SEI formation mechanisms[14, 16]. In a real LIB full cell set-up, in which all the active lithium is stored in the cathode material prior to the first charge cycle, the low 1st cycle Ceff of CS would drastically decrease the lifetime of the cell. Pre-lithiation is known to be one of the most promising options to increase the 1st cycle Ceff. Thereby, the anode is pre-lithiated prior battery operation. The extra amount of active lithium, which is added in this step, is able to compensate the active lithium losses in the first cycle, causing a higher 1st cycle Ceff. There are different techniques in order to pre-lithiate the anode, namely chemical and electrochemical pre-lithiation as well as the pre-lithiation in direct contact to lithium metal or the pre-lithiation by use of lithiated active materials as anode additives. Each technique possesses different advantages and disadvantages with respect to practical application. Nevertheless, with all of these techniques it is possible to significantly increase the 1st cycle Ceff.[49-54] In this work, we report the preparation of carbon nanospheres (CS) and the influence of the carbonization temperature and the particle size on the electrochemical performance as anode material for LIBs. Furthermore, we show -for the first time- an outstanding electrochemical performance of CS within LIB full cells, which is realized by electrochemical pre-lithiation of the CS-based anodes prior to cell operation.


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2. Experimental Section 2.1 Synthesis of carbon nanospheres Carbon nanospheres (CS) were synthesized using a hydrothermal synthesis route. The resulting products were analysed by SEM imaging, thermogravimetric analysis and Raman spectroscopy in order to gain information about particle size, morphology, state of agglomeration, bulk structure and thermal behaviour during carbonization.

2.1.1 Carbon nanospheres with particle sizes of 200-350 nm at carbonization temperatures of 900 °C, 1200 °C and 1500 °C (CS900/CSsmall, CS1200, CS1500) A solution (67.5 g in 500 mL, 0.75M) of D-glucose (Fischer Chemical, laboratory reagent grade) dissolved in distilled water was used. The solution was transferred into a stainless steel autoclave (reactor volume 1 L) and stirred at 200 rpm. The solution was heated up to 180 °C within 2.5 hours and held for 3.5 hours. Afterwards, the autoclave was cooled down to room temperature. The obtained dark brownish suspension was filtered and the residue was washed with water, ethanol and acetone. The obtained brownish powder was dried at 60 °C in an atmospheric oven for 12 hours. Subsequently, the CS were carbonized under an argon flow (500 L h-1) inside a high temperature chamber furnace (HTK 25, Gero) at three different temperatures (900 °C, 1200 °C, 1500 °C) for 6 hours. For all carbonization temperatures, a heating ramp of 300 °C h-1 was chosen to obtain the products CS900, CS1200 and CS1500 as black solid. The product CS900 is also named CSsmall in the following. 2.1.2 Carbon nanospheres with particle sizes of 400-500 nm (CSmedium) at a carbonization temperature of 900 °C In order to obtain larger sized CS, a solution (47.29 g in 350 mL, 0.75M) of glucose in dist. water was transferred into an autoclave with smaller reactor volume (400 mL). A higher reaction temperature of 195 °C (within 1 hour) for 4 hours was used. After following the above described preparation and drying procedure (see 2.1.1), the brownish powder was also carbonized at 900 °C for 6 hours (heating ramp of 300 °C h-1) and the product (CSmedium) could be obtained as black solid.

2.1.3 Carbon spheres with particle sizes of 3-6 µm (CSlarge) at a carbonization temperature of 900 °C Finally, to further increase the size of the CS, the hydrothermal synthesis of CSmedium (see 2.1.2) was repeated. Thereby, 5 g of not carbonized CSmedium was added to a solution of 5 ACS Paragon Plus Environment


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glucose in dist. water (47.29 g in 350 mL, 0.75M). After following the above described preparation and drying procedure (see 2.1.1), with subsequent carbonization at 900 °C for 6 hours (heating ramp of 300 °C h-1), the product (CSlarge) could be obtained as black solid. 2.2 Analytical techniques In order to obtain insights into surface morphology and particle size, scanning electron microscopy (SEM) was used (Carl Zeiss AURIGA, Carl Zeiss Microscopy GmbH). Raman spectroscopy measurements were performed using a Raman dispersive microscope (Bruker SENTERRA, Bruker Optics Inc.). A green semiconductor laser operating at a wavelength of 532 nm and a laser power of 10 mW was utilized as laser source. The laser and the spectrometer were calibrated with a neon lamp. As detector a CCD (charge coupled device) detector with 1024 x 256 pixels, which was thermoelectrically cooled to -65 °C, and for the microscope a 20x objective was used. To collect the spectra, 10 integrations were carried out with an integration time of 60s. Finally, the measurements of three different sample positions were averaged. The content of oxygen containing functional groups of the CS in dependence of the carbonization temperature was determined by TGA-MS investigations. CS (8.5 mg) were placed in an alumina crucible and heated up with 10 K min-1 up to 1000 °C by a thermogravimetric analyzer (TGA Q500-IR, TA Instruments Inc., USA). Helium 5.0 was used as carrier gas (35 mL min-1). The identification of the decomposition products and the qualitative estimation of their ratios were carried out online by a coupled mass spectrometer (ThermoStar GSD 301 T3, Pfeiffer Vacuum GmbH). Furthermore, CS with different particle sizes were studied by nitrogen adsorption measurements to calculate the specific surface area with regard to the Brunauer-EmmettTeller (BET) theory. The measurements were performed and monitored at the temperature of liquid nitrogen (-196 °C) using a 3Flex Surface Characterization Analyzer (Micromeritics GmbH).

2.3 Electrode preparation The electrode dispersion for the anode was prepared using a composition of 90 wt.% active material [either graphite (SMG-A5, Hitachi, d50: 19.8 µm), CS or lithium titanate (Li4Ti5O12, Hombitec LTO5; Sachtleben, average particle size: 200-250 nm)], 5 wt.% of conductive carbon black agent C-nergy Super C65 (Imerys Graphite & Carbon) and 5 wt.% of sodium carboxymethyl cellulose (CMC; Walocel CRT 2000 PA; Dow Wolff Cellulosics) as binder. 6 ACS Paragon Plus Environment

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First, the CMC was dissolved in dist. water. Next, Super C65 was added and dispersed. After that, the active material was introduced into the solution. In order to eliminate agglomerates, the paste was dispersed at a speed of 10,000 rpm (Dissolver Dispermat LC30, VMAGetzmann GmbH) for 60 minutes. After dispersing, the electrode paste was cast onto dendritic copper foil (Carl Schlenk AG) using an automatic film applicator at a speed of 50 mm s-1. Afterwards, the electrode sheets were dried in an atmospheric oven at 80 °C for 60 minutes. Disks with a diameter of 12 mm were cut out and dried at 120 °C for 24 hours under reduced pressure of 10-2 bar. Electrodes with an areal capacity of ≈0.55 mAh cm-2 were obtained for the three different anode materials and used for all electrochemical measurements. The LiNi1/3Mn1/3Co1/3O2 (NMC-111; MX10, Cellcore, d50: 9.8 µm) composite cathode was prepared as described above, by the same procedure as for the anodes. However, as solvent N-methyl-2-pyrrolidone (NMP) and as binder polyvinylidene difluoride (PVDF, Solef 5130, Solvay) were used. Electrodes with areal capacity of ≈0.5 mAh cm-2 were obtained and used for all full cell measurements. The N/P-ratio of all full cells was 1.1/1.

2.4 Electrochemical measurements and pre-lithiation For all electrochemical measurements, Swagelok type T-cells with a three electrode set-up were used. The cell assembly was carried out in an argon-filled glove box with water and oxygen contents smaller than 1 ppm. CS were either studied in lithium metal cells using highpurity lithium metal (Albemarle Corporation) as counter (CE) and reference electrodes (RE) or in LIB full cells using the LiNi1/3Mn1/3Co1/3O2 (NMC-111) composite electrode as CE and lithium metal as RE. The electrolyte was made of 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with a mass ratio of 3:7 (BASF). For the studied system, a six-layer Freudenberg 2190 separator (Ø=13 mm) between working electrode (WE) and CE, and a three-layer Freudenberg 2190 separator (Ø=8 mm) for the RE, was chosen. The separators were wetted with 140 µL and 80 µL electrolyte, respectively. Galvanostatic charge/discharge experiments were carried out with current rates in a range of C/10 to 20C for both the charge and discharge step (1C: Graphite: 372 mA g-1, CS: 200 mA g-1, LTO: 175 mA g-1). The cell formation was performed using three formation cycles choosing a charge/discharge rate of C/10. The lithium metal cells with CS and graphite as WEs were cycled in a potential window of 0.01 V vs. Li/Li+ to 1.5 V vs. Li/Li+. In contrast, LTO was cycled with 1.0 V vs. Li/Li+ as lower cut off potential and 2.5 V vs. Li/Li+ as upper cut off potential. The LIB full cells for the C-rate capability investigation based on LTO were 7 ACS Paragon Plus Environment


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cycled between 2.7 V and 1.5 V, whereas the LIB full cells based on CS and graphite were cycled between 4.2 V and 1.5 V. The long-term cycling study of CS in pre-lithiated LIB full cells was carried out in a voltage window between 4.3 V and 2.5 V. In comparison, for the long-term cycling experiments of lithium metal/NMC-111 cells, 4.3 V vs. Li/Li+ and 2.0 V vs. Li/Li+ were chosen as upper and lower cut off potential (supporting information: Figure S3). For all LIB full cells an N/P-ratio of 1.1/1 was chosen. All experiments were performed using a Maccor Series 4000 automated test system (Maccor, Inc.,). Electrochemical pre-lithiation was performed, using constant current charging with a C-rate of C/10 to a cut-potential of 250 mV vs. Li/Li+ within a “half cell” with lithium metal as CE and RE. After pre-lithiation, the half cell was disassembled inside an argon filled glove box and a new full cell (CS/NMC) with an N/P-ratio of 1.1/1 was assembled.


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3. Results and discussion 3.1 Morphology, and particle size of carbon nanospheres Carbon spheres with different particle sizes (CSsmall, CSmedium and CSlarge) were synthesised via hydrothermal synthesis and analyzed by SEM imaging. Figure 1 shows SEM images of CSsmall, CSmedium and CSlarge at two different magnifications.

Figure 1: SEM images of CSsmall (a, b) CSmedium (c, d) and CSlarge (e, f) with two different magnifications (left column (a, c, e): 5000x; right column (b, d, f): 25000x).

The surfaces of all samples are very smooth. The three samples differ with regard to particle shape and particle size and all samples consist of spherical particles. However, in case of 9 ACS Paragon Plus Environment


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CSsmall, some of the particles are agglomerated to coils. This coil formation seems to be reduced when the particle size is increased, consequently there is no coil formation for CSlarge. The SEM images demonstrate that CSsmall, CSmedium and CSlarge possess particle sizes of 200-350 nm, 400-500 nm and 3-6 µm, respectively. However, also smaller particles with a particles size of ≈500 nm can be observed inside the CSlarge sample. Very likely, these particles are newly formed in the second hydrothermal synthesis step. However, the amount of these small particles in comparison to the rest of the sample is relatively low. Further material characterization of CS is reported in the supporting information, which particularly includes the specific surface area and DFT pore volume (Table S1), the structural characterization by Raman spectroscopy (Figure S1) and the analysis of oxygen-containing functional groups by thermogravimetric analysis (Figure S2). The CS show a tap density of ≈0.3 g/cm3, which is typical for carbon-based nanomaterials.

3.2 Carbon nanospheres as anode material for lithium ion batteries – Electrochemical studies vs. Li metal CS are intended to be studied as anode material in order to improve the fast charging capability of LIBs, based on their small particle size and large specific surface area. Therefore, the electrochemical performance of CS was first investigated in a “half cell” set-up using lithium metal as counter and reference electrodes. In the following, charge and discharge are referred to their designations in a LIB full cell set-up, i.e. charge refers to lithiation and discharge refers to de-lithiation of graphite, LTO or CS.


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Figure 2: Rate capability of graphite, lithium titanate (LTO) and CS. (a) Specific discharge capacity vs. cycle number using charge/discharge rates of 1C, 2C, 3C, 5C, 10C and 20C at 20 °C. (b) Specific discharge capacities vs. specific charge/discharge current at 20 °C. (c) Specific discharge capacity vs. cycle number using charge/discharge rates of 1C, 2C, 3C, 5C, 10C and 20C at 0 °C. (d) Specific discharge capacities vs. specific charge/discharge current at 0 °C. RE and CE: lithium metal. Error bars: Standard deviation of three cells.

Figure 2a/b compares the rate capability of these three different anode materials, i.e. CS, LTO and graphite. In addition, the potential profiles of all anode materials for C10, 1C, 3C, and 20C are shown in the supporting information (Figure S4). Graphite possesses the highest (372 mAh g-1) and LTO the lowest (161 mAh g-1) specific discharge capacity at a low charge and discharge rate of C/10 (Figure 2b). These are comparable to values shown in the literature for LTO[55,

56]

and graphite[5], and beside that, the specific capacity of graphite matches its

theoretical capacity of 372 mAh g-1. However, at a C-rate of 3C, the capacity of graphite significantly drops down (137 mAh g-1; Figure 2a), so that graphite only can achieve a 11 ACS Paragon Plus Environment


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specific discharge capacity comparable to LTO (115 mAh g-1). In contrast to that, CS shows a specific discharge capacity of 244 mAh g-1 at C/10, which only drops down to 167 mAh g-1 by using a charge/discharge rate of 3C. Hence, unlike graphite, which loses much of its specific capacity with increasing charge rate, the capacity reduction of CS and LTO is considerably smaller. The strong decrease of the specific capacity of graphite by applying higher C-rates is comparable to literature values published by Buqa et al., which showed a capacity reduction of ≈10-40% depending on the particle size by increasing the C-rate from 0.25C to 1C.[5] Cheng et al. showed an even worse rate capability of graphite even for already modified graphite. They could obtain specific capacities of 125 mAh g-1, 50 mAh g-1, 20 mAh g-1 for C-rates of 1C, 2C and 3C, respectively.[57] However, they also used slightly higher mass loading/electrode thickness than the electrodes that were used for this study. Nevertheless, this shows that the graphite electrodes, which were used in this work, already possess a good rate capability, in comparison to the literature. It should be kept in mind that the charge (lithiation) rate is always slower than the discharge (de-lithiation) rate of graphite.[5, 57] For this reason, sometimes the high rate capability of graphite is praised in the literature. In these cases, the charge process is normally performed by applying a much lower current rate in comparison to the discharge process. However, with respect to application often the charge rate is more important than the discharge rate. As can be seen from Figure 2a/b, the capacity retention at high charge rates is not only the best for CS with ≈101 mAh g-1 at a charge current of 4000 mA g-1 (20C), but also the specific discharge capacity is always higher than the one of LTO (38 mAh g-1 at 20C), which also possesses a relatively good rate capability. It has to be mentioned that for LTO an even higher rate capability was shown in the literature, for example for hollow microspheres with a shell of LTO nanosheets (131 mAh g-1 at 50C).[58] However, a higher rate capability often requires a complex nanostructuring of the used material and the capacity of LTO is always limited to its theoretical specific capacity of 175 mAh g-1. Furthermore, it also has to be kept in mind that LTO operates at a potential of 1.55 V vs. Li/Li+, which is not suitable for high energy battery applications due to the lowered cell voltage. In contrast, CS operate at potentials of

≈0.5 V vs. Li/Li+, similar to amorphous carbons.[59] Another disadvantage of graphite is the very poor rate capability at low operating temperatures, e.g. at 0 °C.[60,

61]

Due to the low lithiation potential small over-potentials

drastically decrease the specific capacity, because otherwise lithium metal plating is greatly accelerated, which leads to severe safety issues. The rate capability of CS, graphite and LTO at 0 °C is demonstrated in Figure 2c/d. Even at a charge/discharge rate of 1C, graphite 12 ACS Paragon Plus Environment

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possesses a specific capacity of only 35 mAh g-1. This is only 12% of the capacity which graphite possesses at 1C at 20 °C. In contrast to that, the capacity reduction at 0 °C is not that severe for LTO and CS, CS can still deliver a capacity of 178 mAh g-1 at 1C which is synonymous with a capacity retention of 92% in comparison to a cycling at 20 °C. Furthermore, in comparison to cycling at 20 °C, also at 0 °C CS possess always a higher specific capacity than LTO. According to the shown results, it can be summarized that the hypothesis, that CS should show a very good rate capability because of the small particle size and the large surface area is verified by the experimental data. In addition to the good rate capability of CS, they also show an extraordinary good long-term cycling stability as can be seen from Figure 3a. After 8,000 cycles at a charge/discharge rate of 10C (in a “half cell” set-up vs. lithium metal), CS display an extraordinary good capacity retention of 89% related to the 4th cycle (first cycle after formation). The existence of an amorphous carbon structure, which is demonstrated in the Raman spectrum in the supporting information (Figure S1), is confirmed by the sloping potential profiles of the lithium insertion/de-insertion in Figure 3b. After formation, the shape of the potential profile nearly does not change during cycling. However, there is a small capacity fade up to cycle 5,000 and, thereafter, a part of the capacity is recovered. Therefore, we believe that the capacity fading is not induced by the CS electrode but rather by the lithium metal electrode, which undergoes significant morphology changes (high surface area lithium (HSAL) formation) during such a long-term cycling. In the end, the potential profile of the 1,000th cycle and the 8,000th cycle are very comparable regarding shape and capacity. Another advantage of CS is that the Coulombic efficiency (Ceff) almost reaches 100% after the 6th cycle, which demonstrates the high reversibility of the lithiation and de-lithiation process and the formation of an effective SEI layer.



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Figure 3: (a) Long-term cycling of CSsmall at a charge/discharge rate of 10C. (b) Potential profiles of CSsmall at different cycle numbers. RE and CE: lithium metal. Cut-off potentials: 0.01-1.5 V vs. Li/Li+.

The influence of particle size and carbonization temperature on the electrochemical performance is demonstrated in Figure 4. Panel a and d show the specific discharge capacities of CS900, CS1200, CS1500, CSsmall, CSmedium and CSlarge for different charge currents and point out that the specific capacity decreases with increasing particle size and increasing carbonization temperature. However, the reduction of capacity is triggered by different reasons. In Figure 4e, it can be seen that the capacity retention at higher charge current drastically declines with increasing CS particle size. Therefore, the rate capability is reduced at the same time. For this reason, the specific capacity is very comparable at smaller C-rates for the different particle sizes, e.g. for 1C as illustrated for the charge/discharge cycling in Figure 4f. The reduced rate capability for larger CS particles is triggered through the smaller BET specific surface area, as demonstrated by the nitrogen adsorption measurements (supporting information, Table S1), and the longer diffusion path lengths of lithium ions into larger particles as well as the decreased contact area between CS particles and the electrolyte. In contrast to that, a higher carbonization temperature does not decrease the rate capability, even though the overall specific capacity is decreased, as shown in Figure 4b. Here, CS1500 particles show the best capacity retention with increasing charge rate. This is also clearly demonstrated by the larger capacity differences for different carbonization temperatures during charge/discharge cycling, as displayed in Figure 4c, in comparison to Figure 4f. Hence, the capacity reduction is not caused by a decreased rate capability, but rather the CS partially lose a part of their storage capability.


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Figure 4: Electrochemical performance of CS with different sizes and different carbonization temperatures. RE and CE: lithium metal. Cut-off potentials: 0.01-1.5 V vs. Li/Li+. Error bars (a,c,d,f): Standard deviation of three cells. Error bars (b,e): Standard deviation of five cycles. Cycling in panel c and f was performed with a C-rate of 1C. CSsmall, CSmedium and CSlarge were all carbonized at 900 °C.

This effect could also be observed in case of soft carbons and biomass-derived carbons.[62, 63] It is assumed, that carbons, which are treated at lower temperatures can offer higher capacities, because there is additional storage of Li ions in the vicinity of H-terminated edge carbon atoms possible.[62, 63] Finally, this leads to the reduction of the specific capacity for CS, 15 ACS Paragon Plus Environment


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which were synthesized using a higher carbonization temperature. To sum up, CS with a small particle size and a carbonization temperature of 900 °C shows the best electrochemical performance regarding reversible specific capacity and rate capability. However, CS (e.g. CSsmall) have one major disadvantage, namely the relatively low 1st cycle Ceff of ≈38% (±2). This is also visible from the potential profile of the 1st charge/discharge cycle of a CS electrode (Figure 5). The very high lithiation capacity (≈699 mAh g-1) is related to enhanced electrolyte decomposition and SEI formation, leading to a low Ceff.

Figure 5: Charge/discharge potential profile of the 1st cycle of a CS anode. RE and CE: lithium metal. Cut-off potentials: 0.01-1.5 V vs. Li/Li+. C-rate: C/10.

Table 1: 1st cycle irreversible capacity and 1st Coulombic efficiency of carbon spheres with different particle sizes and carbonization temperatures, including standard error of three different cells.

Coulombic efficiency (1st

CSsmall/

CS1200

CS1500

57 (±1)

57 (±1)

179 (±22) 277 (±25) 443 (±47)

147 (±2)

122 (±3)

203 (±24) 314 (±25) 521 (±62)

167 (±2)

142 (±4)

CSlarge

CSmedium

57 (±3)

47 (±2)

CS900 38 (±2)

cycle) / % Irreversible capacity (1st cycle) / mAh g-1 Accumulated irreversible capacity (first three cycles) / mAh g-1 The low 1st cycle Ceff is caused by the large specific surface area and the presence of residual oxygen-containing functional groups (supporting information, Figure S2), which both lead to 16 ACS Paragon Plus Environment

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a high degree of active lithium loss. Inversely, the 1st cycle Ceff increases with increasing particle size, and also when the carbonization temperature is raised from 900 °C to 1200 °C. The latter is likely, because the amount of oxygen-containing functional groups, which can irreversibly react with lithium, is reduced. Interestingly, a change of the carbonization temperature from 1200 °C to 1500 °C shows no further influence on the 1st cycle Ceff, which is related to the fact that CS1500 also possesses a smaller specific capacity. Taking into account the specific irreversible capacities, less active lithium might be consumed when 1500 °C is chosen as carbonization temperature (Table 1). This illustrates the disadvantage of solely focusing on the Ceff. Nevertheless, the difference between CS1200 and CS1500 is relatively small. Overall, as can be seen from Table 1, the highest achievable 1st cycle Ceff is still only 57%, which is not sufficient high to use CS in a LIB full cell set-up. This is due to the fact that within a LIB full cell the amount of active lithium is limited by the cathode active material, in contrast to the unlimited amount of active lithium inside a CS/Li metal cell, attributed to the lithium metal electrode. This means, that based on a anode/cathode capacity balancing of 1:1 -related to reversible capacities- a 1st cycle Ceff of 50% for the anode, would lead to a complete consumption of the whole active lithium content within the LIB full cell set-up.

3.3 Carbon nanospheres as anode material for lithium ion batteries – Electrochemical studies vs. NMC-111 cathodes

Figure 6: Long-term cycling performance of CS/NMC-111 LIB full cells, (a) without and (b) with pre-lithiation. Specific capacities are based on the mass of the cathode active material. N/P-ratio: 1.1/1. Cut-off voltages: 2.5-4.3 V. Rate in the first three cycles: C/10; later cycles: 1C.

Figure 6a shows exactly this behavior for a CS/NMC-111 LIB full cell. During the first charge, nearly the whole amount of active lithium is consumed for SEI formation and, 17 ACS Paragon Plus Environment


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therefore, only a small amount of lithium ions can be de-lithiated after charging, which leads to a very small reversible capacity of ≈38 mAh g-1 at 1C. The potential profiles for the anode and the cathode are demonstrated in the supporting information (Figure S5). It is shown that the NMC cathode potential rises up to 4.66 V vs. Li/Li+ in the first cycle, which is the only reason why there is still reversible capacity detectable. However, this causes very high delithiation degrees of NMC-111, leading to a stronger material and cell degradation. For this reason, CS electrodes cannot be used in a LIB without further pre-treatment. However, as already mentioned, with help of pre-lithiation it is possible to remarkably increase the 1st cycle Ceff. For this purpose, CS electrodes (CSsmall) were pre-lithiated using electrochemical pre-lithiation in combination with a cut-off potential of 250 mV vs. Li/Li+. After assembling of the LIB full cells (CS/NMC-111) the CS electrodes show a potential of 466 (±5) mV vs. Li/Li+. The electrochemical performance of these full cells is shown in Figure 6b. The 1st cycle Ceff is improved to 86.5% (±0.6) after pre-lithiation. This enables the reversible charge/discharge cycling of the CS/NMC-111 LIB full cells. Hence, we are able to show a working CS full cell for the first time. The pre-lithiated full cells show a reversible capacity of 134 mAh g-1 at 1C. After 362 cycles, a capacity retention of 80% could be obtained related to the 4th cycle (first cycle after formation). Furthermore, after 1000 cycles the capacity still account for 66% of the capacity of the 4th cycle, which might be further enhanced by an even optimized pre-lithiation or suitable electrolyte additives. Thereby, the potential profile of the anode always stays above 0 V vs. Li/Li+ (supporting information: Figure S6), which indicates that no lithium metal plating occurs during cycling. The results clearly demonstrate the extreme improvement, which can be obtained by pre-lithiation, when materials with a low 1st cycle Ceff are used. Finally, Figure 7 shows the rate capability with regard to specific discharge capacity (a) and specific energy (b) of CS/NMC-111, graphite/NMC-111 and LTO/NMC-111 LIB full cells, as well as the energy efficiency (c) and the energy retention (d) of these cells.


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Figure 7: Electrochemical performance of LIB full cells (CS/NMC-111, graphite/NMC-111, LTO/NMC-111) at different C-rates. (a) Specific capacity (based on the mass of the cathode active material), (b) specific energy (based on the masses of cathode and anode active material), (c) energy efficiency, (d) energy retention. N/P-ratio: 1.1/1. Cut-off voltages (CS- and graphite-based cells): 1.5-4.2 V. Cut-off voltages (LTO): 1.5-2.7 V. Error bars (a,b): Standard deviation of three cells. Error bars (c, d): Standard deviation of the cycles with the same C-rate.

The specific capacities (based on the mass of the positive electrode active material) for CS/NMC-111 and graphite/NMC LIB full cells are very comparable up to a charge/discharge rate of 1C (Figure 7a). For higher C-rates the capacity of the graphite/NMC cells drops down significantly in comparison to the specific capacity of CS/NMC-111 full cells (capacity retention at 10C: CS: 43%; graphite: 9%). This is caused by the better rate capability of CS in comparison to graphite, as shown before. Nevertheless, graphite has an advantage over CS with respect to specific energy, especially at smaller C-rates (Figure 7b). At C/10 the graphite/NMC cells possess a specific energy of 378 Wh g-1 (based on the mass of the active materials of anode and cathode) in comparison to CS which only possess 270 Wh g-1. This is 19 ACS Paragon Plus Environment


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mainly caused by the higher specific capacity of graphite at low C-rates and by the lower operating potential of graphite compared to CS (≈0.2 V compared to 0.5 V vs. Li/Li+). Furthermore, one striking point is the low specific energy of LTO even at low C-rates (154 Wh g-1). This is caused by the fact that LTO possesses the lowest practical and theoretical capacity as well as that LTO/NMC-111 cells show the lowest mean discharge voltage triggered by the high operation potential of LTO. At charge and discharge rates of 5C, the specific energy of CS/NMC-111 cells become larger (146 Wh g-1) than the one of graphite/NMC-111 cells (131 Wh g-1), caused by the good rate capability of CS. In addition, Figure 7d highlights the high energy retention of CS especially at higher C-rates (the specific energies at 0.1C are defined as 100% value). One further advantage of CS compared to graphite- and LTO-based LIB full cells is the high energy efficiency at C-rates ≥1C shown in Figure 7c. As the energy efficiency is the product of voltage efficiency and Coulombic efficiency,[59] this behavior, in dependence of the charge/discharge current, is mainly influenced by the high voltage efficiency of CS, as all LIB full cells achieve a high Ceff (≥99.9%) after formation cycles. In turn, the CS-based anode shows a low voltage hysteresis (=high voltage efficiency) even at high rates, compared to graphite and LTO, as can also been seen in the corresponding potential profiles (Figure S4, supporting information). All these results show that pre-lithiated CS can lead to a significant improvement of the rate capability of LIB full cells. With regard to energy density, it will be very interesting to investigate the specific energy, which can be obtained with CS possessing much higher specific capacities (370-821 mAh g-1)

[6, 23, 40, 43]

than graphite. With help of pre-lithiation, it

should be possible to use also these kind of CS within a LIB full cell set-up.


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4. Conclusion Carbon nanospheres (CS) were successfully synthesized by a hydrothermal synthesis route and subsequent carbonization. The production of CS is a low-cost, sustainable and non-toxic process, if glucose is used as precursor material. The morphology, particle size and surface area of CS particles were investigated and the corresponding electrochemical performance as anode material for LIBs was studied in detail, with particular focus on the fast charging capability. It could be shown that CS possess very good rate capabilities at operating temperatures of 20 °C and 0 °C, in comparison to graphitic carbon- (µm size range) or LTObased anodes. In particular the low temperature performance (0 °C) could be remarkably improved. Furthermore, CS showed an extraordinary good long-term cycling stability with a capacity retention of 89% after 8000 cycles at a charge/discharge rate of 10C. Furthermore, the influence of carbonization temperature and particle size was investigated. The specific capacity decreased with higher carbonization temperature and increasing particle size. However, the reduction of the specific capacity had two different origins. In the former case, the specific capacity decreased because the amount of lithium ions that could be stored inside the carbon particles is reduced with higher temperatures (within the chosen temperature window) and in the latter case, the specific capacity was decreased because larger particles possess a lower rate capability. Therefore, the smallest CS particles carbonized at 900 °C, showed the best electrochemical performance. One major disadvantage of CS, namely the very low 1st cycle Coulombic efficiency (38-57%), was successfully overcome with the help of pre-lithiation to effectively compensate active lithium losses during SEI formation. Electrochemical pre-lithiation made it possible to increase the 1st cycle Coulombic efficiency drastically, enabling the usage of CS within LIB full cells using NMC-111-based cathodes, with all related benefits, for the first time. The improved rate capability of CS was further verified in pre-lithiated NMC-based LIB full cells and showed major improvements compared to cells based on either graphite or LTO anodes. Finally, we could also show that CS even display an improved specific energy at rates ≥5C, enhanced energy efficiency at rates ≥2C and increased energy retention at rates ≥2C compared to graphite and LTO-based cells.

Acknowledgements The authors wish to thank the German Ministry of Education and Research (BMBF) for funding this work in the project “BenchBatt” (03XP0047A) and the German Federal Ministry for Economic Affairs and Energy (BMWi) for funding this work in the project “Go3” (03ETE002D). 21 ACS Paragon Plus Environment


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Supporting information Characterization of carbon spheres by nitrogen adsorption measurements, Raman spectroscopy and thermogravimetric analysis. Long-term cycling graph of Li metal/NMC-111 cells; potential profiles of CS/NMC-111 full cells with and without pre-lithiation and potential profiles of CS/Li, Graphite/Li, and LTO/Li cells for different C-rates.


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