Properties of LiCoPO4-non-graphitic carbon foam composites

Available online at www.sciencedirect.com

ScienceDirect Journal of the European Ceramic Society 34 (2014) 933–941

Feature Article Properties of LiCoPO4-non-graphitic carbon foam composites Lucangelo Dimesso ∗ , Christina Spanheimer, Dirk Becker, Wolfram Jaegermann Technische Universitaet Darmstadt, Materials Science Department, Petersenstrasse 23, D-64287 Darmstadt, Germany Received 23 August 2013; received in revised form 11 October 2013; accepted 15 October 2013 Available online 26 November 2013

Abstract The properties of LiCoPO4 -non-graphitic carbon foams (LCP-NGCF) composites are reported. The composites are treated at 300 ◦ C for different times (t, from 0 to 12 h) in air, then at 730 ◦ C for 12 h in nitrogen. The diffraction analysis revealed LiCoPO4 as major crystalline phase, Li4 P2 O7 and Co2 P (t = 0 h), Co2 P (t > 0 h) as secondary phases. The morphology consists of crystalline “islands” with spongy-like features on the surface (for t = 0 h) and with acicular crystallites of different dimensions (2–20 ␮m) for t ≥ 0.1 h. The voltammetric curves show reduction potential values between 4.40 V and 4.60 V. The LCP-NGCF composites deliver a discharge specific capacity of 100 mAh g−1 (t = 0 h, discharge rate of C/25 and RT) and of 65 mAh g−1 (t > 0 h). The ac-impedance analysis reveal the formation of SEI-layer after high annealing times, which disfavors the kinetics of the Li-(de)intercalation processes. © 2013 Elsevier Ltd. All rights reserved. Keywords: Sol–gel; Cathode materials; Lithium cobalt phosphate; Composites; Foams

1. Introduction Li-ion re-chargeable battery is key component of today’s portable entertainment, computing and telecommunication gadgets as it provides a renewable and clean storage of energy. A battery is a transducer that converts chemical energy into electrical energy and vice versa. It contains an anode, a cathode, and an elecrtrolyte. In a lithium-ion battery, anode and cathode materials are mixed electronic/ionic conductors, which accomodate electrons and lithium-ions upon charge–discharge processes. The electrolyte enables the transport of lithium ions between electrodes and in a perfect battery the lithium ion transport number will be unity in the electrolyte. Many electrochemically active materials are not good electronic conductors, so it is necessary to add an electronically conductive material such as carbon black. To physically hold the electrode together, a binder is also added. In these cases the electrochemical reaction can only occur at those points where the active material, the conductive diluent, and electrolyte meet. Thus most electrodes are complex porous composites, which are designed to ensure fast

∗ Corresponding author at: Darmstadt University of Technology, Materials Science Department, Petersenstrasse 23, D-64287 Darmstadt, Germany. Tel.: +49 06151 16 69667; fax: +49 06151 16 6308. E-mail address: [email protected] (L. Dimesso).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.10.030

charge transport and, therefore, high power capability. The most common anode used in lithium-ion batteries is a graphitic carbon. Among carbon structures (fibers, tubes, beads), carbon and graphite foams offer properties such as chemical inertness, ultrahigh service temperatures, low coefficient of thermal expansion, and tailorable electrical/thermal conductivity.1 Carbon foams (CF) generally fall into two categories – graphitic and nongraphitic. The graphitic carbon foams (GCF) offer high thermal and electrical conductivity, but considerably lower mechanical strength than the non-graphitic foams. Non-graphitic carbon foams (NGCF) are generally stronger, act as thermal insulators and cost far less to manufacture. Carbon and graphite foams can be usually made from coal as feed stock and with uniform small or large pores, or a combination of large and small pores. In addition functionally graded carbon foams can be produced with a surface of one pore size and an interior of another.2 Lowcost NGCFs have also been identified as possible candidates in applications such as space-borne mirror substrates and thermal protection; fuel cell anode/cathode gas diffusion layers and catalysts support; and Li-ion batteries. Our group has investigated the properties of lithium iron phosphate (LiFePO4 ) – NGCF composites and their use as cathodes for Li-ion batteries.2,3 The promising results with this system has encouraged us to continue the investigation with high voltage (5 V) Li-ion materials that attracted high attention as potential power sources for electric vehicles due to the very

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high energy density of all the commercialized rechargeable batteries. Among the high-voltage materials in the lithium metals phosphates (LiMPO4 , M = Fe, Co, Mn, Ni) family, LiCoPO4 with an olivine structure possesses a high operating voltage (redox potential of 4.8 V vs. Li/Li+ ) and a high theoretical capacity of about 170 mAh g−1 .4 However, LiCoPO4 has shown a fast fading of discharge capacity upon charge–discharge cycling5–8 and our previous investigations on LiCoPO4 /carbon foams (LCP-CF) composites showed a dramatic dependence of the electrochemical performance on the annealing atmosphere.9–11 LiCoPO4 can be prepared by using solid state reactions12 and soft chemistry routes.9,13–17 Recently our group investigated the influence the annealing conditions during on the structural, microstructural and electrochemical properties of the LiCoPO4 system.9,15 Our results pointed out that the annealing in inert atmosphere (mostly nitrogen) is a necessary step to enhance the electrochemical performance of the LiCoPO4 phase. To optimize the properties of the LCP-CF composites, we have investigated a method of preparation called “2-steps annealing process”. The process consists of post-annealing at low temperature in flowing air (typycally T = 300 ◦ C) to decompose the organic precursors, followedby the anneal at higher temperature (typically T = 730 ◦ C) to favor the formation of the olivine structured phase. Improvements in the battery performance can be achieved by reconfiguring the electrode materials currently employed in 2-D batteries into 3-D architectures.3,10,18 The use of commercially available carbon foams as framework for cathode materials offers several advantages such as good interparticle conductivity, an efficient transport route for the solvated ions and hence possibly superior battery performances. In this work (Part I), we investigated the structural, micro-structural properties and the electrochemical behavior of the LCP-CF composites that have non-graphitic carbon foams as substrate. 2. Experimental The experimental procedures for the preparation and characterizations of the samples are similar to those reported earlier.7 The samples are prepared by dissolving in water Li(CH3 COO)·2H2 O (lithium acetate), Co(CH3 COO)2 ·4H2 O (cobalt(II) acetate) as precursors (molar ratio 1:1) with citric acid (2 × mol [Co]), then phosphoric acid in equimolar ratio with Li and Co ions was added. The composites were prepared by soaking the commercial non-graphitic foams (GRAFOAM® non-graphitic foam is a registered trade name from GRAFTECH International Ltd. [10 and references therein]) in the starting aqueous solution at 80 ◦ C for 2 h. After rinsing with distillate water the composites have been annealed in air at T = 300 ◦ C for t = 0, 0.1, 5 and 12 h respectively, then under nitrogen for t = 12 h at T = 730 ◦ C. The structural analysis, SEM imaging and electrochemical characterization of the samples were similar to those reported previously.3,19 For the electrochemical measurements, Swagelok-type cells were assembled in an argon-filled dry box with water and oxygen less than 5 ppm. To measure the composites, the foam disks were direct assembled into the cell and a few drops of the electrolyte were added. In the cell,

Li metal was used as anode, SelectiLyte LF30 (1 M Li-FAP in ethylene-carbonate:di-methyl-carbonate 1:1 (wt/wt), Merck KGaA, Germany) as electrolyte, Celgard® 2500 as separator. The electrochemical impedance spectroscopy (EIS) analysis was carried out by applying an ac voltage of 10 mV over the frequency range from 0.1 Hz to 300 kHz (MultEchemTM Potentiostates, Gamry Instruments, Inc.). All electrical measurements have been performed at room temperature. 3. Results 3.1. Structural analysis The XRD-diffractogramms of LiCoPO4 powders, prepared and treated under very similar conditions of the composites (in air at T = 300 ◦ C for t = 0, 0.1, 5 and 12 h then in nitrogen at T = 730 ◦ C for t = 12 h), are shown in Fig. 1. All the diffraction peaks have been indexed with the olivine-type symmetry which confirms that the structure of the prepared powders corresponds to the orthorhombic LiCoPO4 . Further, for t = 0 h, the presence of crystalline reflections attributed to Li4 P2 O7 and to Co2 P as secondary phases (indicated with 1 and with 2 respectively in Fig. 1) has been observed.6,20 After annealing in air the presence of Co2 P2 O7 (indicated with 3 in Fig. 1) and Co2 P at shorter times (t = 0.1 h) and Co2 P only for annealing times t ≥ 5 h have been detected. The formation of Co2 P occurs due to the reduction at the grain boundaries of the LiCoPO4 (and Co2 P2 O7 ) crystalline phase by heat treatment at high temperatures (≥700 ◦ C) under inert atmosphere and is favored by the presence of carbon as byproduct of the sol–gel process.20 The presence of Li4 P2 O7 for t = 0 h only can be explained by the oxidation of the pyrophosphate anions into phosphates in presence of the oxygen in air, according to the reaction P2 O7 4− + ½O2 ↔ 2PO4 3− . Whether the presence of these secondary phases leads to an improvement of the electrochemical performance of LiCoPO4 is still under debate. 3.2. Microstructural characterization The importance of a porous nano-architecture has been previously reported.21 The authors emphasize that in this “sponge” approach, the electrolyte layer is formed around a random 3-D network of electrode material. Short transport-path characteristics between the insertion electrodes are preserved with this arrangement. The micrograph of the LCP-NGCF composites, prepared for t = 0 h, shows the deposition of a crystalline spongylike material on the surface of the foam (Fig. 2a). On the other side, after annealing in air for longer times, the morphology consists of acicular crystallites with different dimensions (2–20 ␮m) as shown in Fig. 2b (t = 0.1 h) and Fig. 2c (t = 5 h). By soaking, the foam surface is supposed to be covered by a continuous layer of liquid in which the Li+ , Co2+ and (PO4 )3− ions are uniformly distributed. The slow evaporation of the solvent should lead to a “uniform” layer on the foam surface. The high viscosity of the Co-containing gel may be a hindrance to the homogeneous “penetration” of the solution throughout the porous structure of the foams. The consequence is the formation of “crystalline

20

2

2

2

2

40 30 50 2Θ (degree)

412 331 430

022 131 222 610 321

121 411 401

2 112

301

2

220

1 1 1

t=0h

2 2 2

20

60

30

2 2

50 40 2Θ (degree)

60

t = 12 h

Intensity (a.u.)

Intensity (a.u.)

t = 0.1 h

3

935 t=5h

Intensity (a.u.)

020

101 210 011

200

Intensity (a.u.)

111 201

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2 2

2 2 2

2 2

20

30

50 40 2Θ (degree)

60

20

50 30 40 2Θ (degree)

60

Fig. 1. X-Ray diffraction patterns of the prepared LiCoPO4 powders annealed in air at T = 300 ◦ C for different times (t), then in nitrogen at T = 730 ◦ C for t = 12 h. Secondary phases are indicated as (1) Li4 P2 O7 , (2) Co2 P and (3) Co2 P2 O7 respectively.

islands” instead of the formation of a homogeneous layer. A very similar behavior has been recently reported by our group during the preparation of composites consisting of LiCoPO4 deposited on carbon nano-fibers mats prepared by electrospinning and carbonization of organic solutions.22 This fact indicates and confirms the influence of the annealing atmosphere on the formation and growth mechanisms of crystalline phases from the gel. 3.3. Electrochemical measurements Cyclic voltammogramms (CVs) recorded for LCP-NGCF composites, annealed at T = 300 ◦ C in air for t = 0 h (in the range 3.0–5.0 V), t = 0.1 h and t = 12 h (in the range 3.0–5.3 V), then at T = 730 ◦ C for t = 12 h in nitrogen, using Li metal as counter and reference electrode are shown in Fig. 3a–c respectively. The deintercalation/intercalation potentials corresponding to the mean peak maxima for the all the samples are shown in Table 1. From the CV profiles and the data in Table 1 following important points are noted. For t = 0, firstly a shoulder at ∼4.67 V in the anodic sweep has been detected; this shoulder disappears by further cycling. Secondly, the surface of the CV-cycles decreases after each oxidation/reduction sweep. Thirdly, the mean peak maxima potentials in the reduction region are shifted toward higher values of potential (from 4.60 V to 4.65 V). For t = 0.1 h, a shoulder at ∼4.92 V in anodic sweep can be observed but no oxidation mean peak maxima are detected. During the reverse sweep a cathodic peak at ∼4.40 V (Fig. 3b) and the increase of the surface of the CV-profiles by further cycling (Fig. 3b) were seen. This fact may be explained by the oxidation of the

pyrophosphate anions into phosphates in presence of the oxygen in air which is supported by the XRD analysis of the powders annealed under very similar conditions. Finally, for t = 12 h a wider shoulder at ∼4.88 V in the anodic sweeps, a slight decrease of the surface in the CV cycles have been observed whereas the mean peak maxima potentials in the reduction region remain constant at a potential value of ∼4.67 V. Furthermore, mean peak maxima in the oxidation region were detected at 5.20 V for the first cycle, and between 5.10 V and 5.20 V from the second cycle on. The results are in good agreement with previous cyclic voltammetry investigations on LiCoPO4 system.2,7,8,23 The shoulders at ∼4.67 V (Fig. 3a), ∼4.92 V (Fig. 3b) and ∼4.88 V (Fig. 3c) in the oxidation regions of the CV-profiles could indicate a two-steps mechanism of lithium deintercalation Table 1 Values of the deintercalation/intercalation potentials (mean peak maxima in Fig. 3a–c) for the LCP-NGCF composites after annealing in air at 300 ◦ C for different times (t), then in nitrogen at 730 ◦ C for t = 12 h. t (h)

Cycle (nr)

Deintercalation potential (V)

Intercalation potential (V)

0

1 2 3 4–6 1 2 3 1 2 3

– – – – – – – 5.16 5.10 5.10

4.61 4.62 4.64 4.65 4.40 4.40 4.41 4.57 4.57 4.57

0.1

12

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0.2

I (mA)

0.1 0.0 -0.1 -0.2 0.6

I (mA)

0.4 0.2 0.0 -0.2

I (mA)

0.2

3.0

3.5

4.0

4.5

5.0

5.5

Voltage (V) Fig. 2. HREM pictures of carbon foam – LiCoPO4 composites after annealing in air at T = 300 ◦ C for (a) t = 0 h, (b) t = 0.1 h and (c) t = 5 h, then in nitrogen at T = 730 ◦ C for t = 12 h.

Fig. 3. Cyclic voltammograms recorded for the carbon foams/LiCoPO4 composites after annealing in air at T = 300 ◦ C for (a) t = 0 h (in the potential range 3.0–5.0 V vs. Li+ /Li), (b) t = 0.1 h and (c) t = 12 h (in the potential range 3.0–5.3 V vs. Li+ /Li), then in nitrogen at T = 730 ◦ C for t = 12 h (scan rate 0.05 mV s−1 ).

as previously reported.24 Extensive investigations on the electrochemical properties of the Lix CoPO4 system prepared by the sol–gel process report the presence of two plateaus in the charge curve (corresponding to 0.7 ≤ x ≤ 1.0 and 0 ≤ x ≤ 0.7 respectively in the stoichiometric compound) independently of the electrolyte used during the measurements.4 The results support the idea that two-step delithiation is an intrinsic property of the system as also confirmed by using in situ and ex situ XRD to explain the Li extraction/insertion mechanism in LiCoPO4 . Furthermore, a structural rearrangement in the LiCoPO4 phase takes place during the charge process. This rearrangement could be an intrinsic property of the LCP phase and independent of the preparation method as confirmed by similar voltammetric curves reported by Kishore and Varadaraju25 who prepared the LCP phase by a solid state reaction. The voltammetric profiles

indicate different (de)intercalation mechanisms which are strongly dependent upon the annealing atmospheres that means by the nature and amount of the secondary phases as well. The LCP-NGCF composites were cycled at different C-rates from C/25 to C/2.5 at room temperature (RT). The higher loading is 35–40 wt% LiCoPO4 on carbon, a value which is in agreement with the data previously reported.7,26 The values of discharge specific capacity at different C-rates for the first discharge cycle at RT have been summarized in Table 2. The discharge specific capacity (measured at C/25) as a function of the cycles number as well as the specific capacity-potential profiles of the first cycle for LCP-NGCF composites are shown in Fig. 4a and b respectively. For t = 0 h, the specific discharge capacity is 100 mAh g−1 7 whereas for t = 0.1 h and t = 12 h the discharge specific capacity were 62 mAh g−1 and 65 mAh g−1 respectively

L. Dimesso et al. / Journal of the European Ceramic Society 34 (2014) 933–941 Table 2 Discharge specific capacity (mAh g−1 ) measured at different discharge rates (1st cycle, RT) for LCP-NGCF composites, annealed at 300 ◦ C in air for different times, then at 730 ◦ C for t = 12 h in nitrogen. t (h)

C/25 C/10 C/5 C/2.5 C/10-ret

0.0

0.1

12

100 10 – – 8

62 16 9 5 12

65 19 9 <2 11

800 600 - Z'' (Ω)

Discharge rate(C)

400 200 0 0

200

(Fig. 4b). The increase of the capacity retention (measured after the 10th cycle) from 37% (t = 0 h) to 46% (t = 12 h) has been observed as well.

600

800

R1

R2

CPE1

CPE2

Rs

Fig. 5. (a) Nyquist plots of the non-graphitic carbon foam as delivered (blue triangles) and of the LCP-NGCF composites (red circles) annealed at 300 ◦ C for t = 5 h in air, then at 730 ◦ C for t = 12 h in nitrogen; (b) equivalent circuit model of the studied systems for the high and medium frequency ranges only. (For interpretation of the references to color in figure legend, the reader is referred to the web version of the article.)

80 60 40 20

Voltage (V)

-1

Specific capacity (mAhg )

100

400 Z' (Ω)

B

3.4. EIS For the purpose of investigation of the above electrochemical properties in the LCP-NGCF composites, ac-impedance (EIS) experiments have been performed. Fig. 5a depicts the Nyquist plots of the non-graphitic carbon foams (blue triangles, open circuit voltage (OCV) of ∼0.10 V) and of the LCP-NGF composites, annealed at 300 ◦ C for 5 h in air, then at 730 ◦ C for 12 h in nitrogen (red circles, (OCV of ∼1.38 V) before cycling. The

937

20

60 40 80 100 -1 Specific capacity (mAhg )

120

Fig. 4. (a) Discharge capacity as a function of cycling (discharge rate of C/25, RT) and (b) charge and discharge specific capacity/voltage profiles of the 1st cycle (discharge rate of C/25, RT) for carbon foam/LiCoPO4 composites annealed at different times in air, then in nitrogen at T = 730 ◦ C for t = 12 h.

Nyquist plot of the LCP phase consists of a semicircle alike in appearance of a half ellipse (in the high and intermediate frequency ranges) and a barely visible straight line with changing slope to the real axes (in the lower frequency range, Fig. 5ainset). Alike, the Nyquist plot of the LCP-NGFC composites consists of out-of-shaped semicircles (in the high and intermediate frequency ranges) and a straight line inclined with constant slope to the real axes. Further, a shoulder in the intermediate-low frequency range can be observed. The study of the ac-impedance results has been performed by using the approach outlined in Ref. [22 and references therein] and Ref. 27 by using a equivalent circuit model shown in Fig. 5b. The Rs , R1 and CPE1 values of the prepared cathode materials derived from the equivalent circuit fitting are presented in Table 3. Here Rs is the resistance associated with the impedance of the electrolyte (and separator) and consistent with the high frequency intercept in the ac impedance spectra; R1 and R2 components of the ac impedance semicircle in the high-to-medium frequency range related to the charge transfer resistance and constant phase elements CPE1 and CPE2 . As shown by the CVcurves and charge/discharge measurements multiple reactions, which overlap and affect the Li-ions diffusion, occur during the charge/discharge processes in the LCP-NGFC composites. In order to avoid misunderstandings and misleading conclusions the Warburg element was not included in the fitting of the ac impedance data.

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Table 3 Impedance parameters derived from the equivalent circuit for the NGCF-as delivered and LCP-NGCF composites (annealed at 300 ◦ C for t = 5 h in air, then at 730 ◦ C for t = 12 h in nitrogen, before cycling) respectively. Sample

OCV (V)

Rs ()

R1 ()

CPE1 -T (×10−6 ) (F)

CPE1 -P (×10−6 )

NGCF NGCF-LCP

0.10 1.37

27.80 13.90

114.60 576.50 (R1 + R2 )

35.08 0.338

0.741 0.753

The constant phase element involves two parameters, CPE1 -T (+CPE2 -T) which represent the capacity of the whole measured system (anode + cathode) and CPE1 -P (and CPE2 -P) which represents a deviation from the capacity.27 From Fig. 5a and the data reported in Table 3 some important observations can be drawn. The decrease of Rs and the increase of R1 (+R2 ) values in the LCP-NGCF phase would indicate an electrical behavior resembling that of a “insulator”. The increase in the charge transfer resistance (R1 + R2 ) may be due to the formation/change of phase and/or films during the cell operation and could be responsible for the electrochemical performance during cycling. The EIS measurements confirm the presence of “different phases” which show different conducting behavior and support the morphological analysis and the electrochemical data. A further confirmation comes from the Bode diagrams of the as-prepared LCP-NGFC composite before cycling shown in Fig. 6. The plot shows the presence of two slopes (indicated with the red arrows) in the high and intermediate frequency ranges. From the slopes of the plots, after cycling at different charge and discharge rates the presence of only one step can be observed which may indicate the formation of SEI-layers which slow down (or hinder) the kinetics of the (de)intercalation processes. Finally, the straight successions of triangles (NGCF) and circles (LCP-NGCF composites) in the low frequency range are attributed to the diffusion of the lithium ions into the bulk of the electrode material or so called Warburg diffusion. In the LCP-NGFC composites, the straight succession of circles (Fig. 5a-inset) is almost parallel to the first part of the semicircle (indicated with black arrows). This confirms that the whole 3

IZI (ohm)

10

2

10

uncycled C/25 C/10 C/5 C/2.5 C/10-ret

1

10 -2 -1 10 10

10

0

10

1

10

2

10

3

10

4

10

5

f (Hz) Fig. 6. Bode-diagram for the LCP-NGCF composites annealed at 300 ◦ C for t = 5 h in air, then at 730 ◦ C for t = 12 h in nitrogen, after cycling at different charge/discharge rates.

system shows an insulating behavior which would explain the poor electrochemical performance of the composites annealed in air for longer times. 4. Discussion The electrochemical activity of the LiCoPO4 system is still a controversial debate and has not been fully understood. Extensive investigations on the electrochemical properties of the Lix CoPO4 system, prepared by sol–gel process and annealed in inert atmosphere, claim that a two-step delithiation process occurs and that it is an intrinsic property of the system.28 The lithium extraction-insertion in LiCoPO4 was demonstrated to proceed in a reversible manner. Amine et al.29 reported that the diffraction pattern of the material after one complete cycle is very similar to the one of the pristine compound. This result would indicate that the capacity loss in the first cycle cannot be attributed to the irreversible phase transitions occurring upon cycling. This instability in cycling results from various factors such as limited electronic/ionic transport, solvent oxidation at high voltage, oxygen release or amorphization induced by HF from electrolyte. As example, Markevich et al.30 showed that the origin of the poor performance of LiCoPO4 cathodes in LiPF6 containing electrolyte solutions was a nucleophilic attack of F− anions in solution on the P atoms of the olivine compound in the delithiated (charged) state, resulting in the breaking of the P/O bonds of the phosphate anions and the formation of soluble LiPO2 F2 moieties. A further reason for the low chemical stability of the delithiated CoPO4 phase may be the fact that, unlike the other phosphates with Co3+ in octahedral coordination, in CoPO4 Co3+ exists in a high-spin configuration.31 The annealing treatment is an important factor to improve the electrochemical properties of the composites. The influence of the annealing in air on the electrochemical performance of the LCP-NGCF composites is shown in Fig. 4b and inset. At the end of the first discharge cycle, the specific discharge capacity is 65 mAh g−1 while this value decreases to 30 mAh g−1 after the 10th cycle. Furthermore, a discharge plateau appears at around 4.6 V for the LFC-NGCF composites. This value is consistent with the foregoing cyclic voltammetric data which show reduction maxima peaks at around 4.6 V. Although the lower capacity fade the CF-LCP composites pre-annealed in air delivered a lower specific capacity than those annealed under nitrogen only. This behavior can be partially explained by the different charge mechanism as confirmed by the charge profiles as a function of annealing time in Fig. 4b-inset. Indeed, the decomposition of the electrolyte could occur above 5.0 V as confirmed by the profile of the charge capacity in the first cycle where the sudden change of

L. Dimesso et al. / Journal of the European Ceramic Society 34 (2014) 933–941

the slopes in the charge curves as a function of (t) can be clearly observed. This change is related to the irreversible degradation of the electrolyte with the formation of secondary products which inhibit the kinetic of the deintercalation/intercalation processes of the Li-ions. Another important factor to be taken into account is the nature of the carbon substrate. As a matter of fact, all carbon materials can be lithiated to a certain extent. However, the amount of lithium reversibly incorporated in the carbon lattice (the reversible capacity), the faradaic losses during the first charge–discharge cycle (the irreversible capacity), the profile of the voltage curves during charging and discharging, all depend on the structure, the texture and heteroatom content of the carbon foam material. In the NGC-foams, the low crystallinity, the internal porosity, the presence of heteroatoms and of functional groups must have a strong influence on the reversible and irreversible capacities obtained by lithium insertion. Mild oxidation of carbon materials in air, oxygen, or other oxidizing substances has two main effects: opening of pores and formation of surfaces complexes. Obviously, the extent of these reactions, as concerns the pore shape and dimension, and the amount and nature of functional groups, depends on both the oxidation conditions and the surface characteristics of the starting carbon material. In fact, carbons made pyrolyzing epoxy-novolak resins at 1000 ◦ C were shown to poor electrochemical performance after oxidation in air at temperatures from 300 to 600 ◦ C.32 Initially, these materials had a significant nanoporosity but, due to the smallness of pore openings, the electrolyte could not penetrate the pores, so excellent behavior was observed. As the samples were oxidized, the pores did not grow significantly in volume, but the size of their openings apparently did, to the point where the electrolyte could penetrate the pores, leading to irreversible electrolyte decomposition reactions during the first electrochemical reaction of lithium with carbon and hence large irreversible capacity, even for burn-offs as small as 5%. In addition, surface oxides, resulting from oxygen chemisorptions, contribute to increase of the irreversible capacity. The origin of the high irreversible capacities observed during the first cycle has been studied by several groups[33 and references therein]. Part of the irreversible losses is due to the formation of a passivating layer on the carbon surface. The losses will increase if the carbon particles have micropores which can be penetrated by the passivating layer. Using various analytical techniques such as Fourier transform infrared attenuated total reflectance (FTIR-ATR), secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy, Matsumura et al.34 showed that a large portion of the irreversible capacity could come from the reaction of lithium with active sites in the bulk of the carbon electrode. These active sites can be hydroxylic groups or carbon radicals. It is well known that disordered carbons have many active sites associated with high concentrations of unpaired electron spin centers which give rise to surface complexes, mainly with oxygen but also with other elements. Kikuchi et al.35 showed that the irreversible current peak observed in the cyclic voltammograms disappeared after their pitch-based carbon fiber electrodes were heated at 980 ◦ C under vacuum.

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The effect of different gas exposures on the irreversible capacities of sugar carbons was recently studied by Xing and Dahn.36 They showed that carbon samples exposed only to Ar or N2 exhibit strongly reduced irreversible capacities compared to those observed after air exposure for several days. In our case, the irreversible capacity can also be explained by the acidic nature of the solution (pH 4–5) in which the C-foams are soaked. During the treatment into the solution the surface of the foam can be activated with the formation of C O containing layer(s) which are electronically insulating (passivating) but allow exchange of Li-ions. From our experimental results, the “layers” growth takes place for t = 0.1 h, longer annealing times in air (t = 12 h) do not affect the nature of the layer(s) but they seem to “stabilize” the (de)intercalation processes till 5.0 V (Fig. 4b). This layer is electronically insulating, permeable to Li-ions but impermeable to other electrolyte components and is manly composed of lithium carbonate and lithium alkylcarbonate.37,38

5. Conclusions In this work, we report the results on the characterization of LiCoPO4 /non-graphitic carbon foams composites, prepared by Pechini-assisted sol–gel method, pre-annealed in air for different times and then in nitrogen. The XRD analysis, performed on powders prepared and annealed under very similar conditions as the composites, revealed LiCoPO4 as major crystalline phase, Li4 P2 O7 and to Co2 P as secondary phases (for t = 0 h), Co2 P2 O7 and Co2 P (t = 0.1 h) and Co2 P only for t ≥ 5 h. The SEM-analysis shows the deposition of a crystalline spongy-like material on the surface of the foam for t = 0 h. For t ≥ 0.1 h, the morphology of the composites consists of acicular crystallites with various dimensions (2–20 ␮m). The CV curves show, for t = 0, a shoulder at ∼4.67 V in the anodic sweep, a decrease of the surface of CV-cycles and a shift of the mean peak maxima potentials in the reduction region toward higher values of potential (from 4.60 V to 4.65 V). For t = 0.1 h, the CV profiles show a small shoulder at ∼4.92 V in anodic sweep and no oxidation mean peak maxima; during the reverse sweep a cathodic peak at ∼4.40 V and an increase of the surface of CV-profiles were observed. Finally, for t = 12 h a wider shoulder at ∼4.88 V in the anodic sweeps, were detected whereas mean peak maxima potentials in the reduction region remain constant (∼4.67 V). The specific capacity of the LiCoPO4 /non-graphitic carbon foams composites, measured at a discharge rate of C/25, RT, was 100 mAh g−1 (for t = 0 h). For t = 0.1 h, the discharge specific capacity decreases to 62 mAh g−1 , no further dramatic changes in the specific capacity have been detected at longer annealing times (65 mAh g−1 for t = 12 h) and an increase of the capacity retention (measured after the 10th cycle) from 37% (t = 0 h) to 46% (t = 12 h) has been reported. The ac-impedance measurements confirm the insulating behavior of the LiCoPO4 -non graphitic composites. The Bode diagrams of the LCP-NGFC composite before and after cycling,

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shown in Fig. 6, reveal the formation of SEI-layers which slow down (or hinder) the kinetics of the (de)intercalation processes. Funding source The authors thank the Deutsche Forschungsgemeinschaft (DFG) (Project: Sonderforschungsinitiative Projekt PAK-177) for the financial support during this work. The DFG promote and support financially the publication of the scientific results obtained during the projects. The DFG had no involvement in study design; in the collection, analysis and interpretation of data; in the writing of the report, and in the decision to submit the paper for publication. Acknowledgement Many thanks are owed to Mr. J.-C. Jaud for the technical assistance in the XRD analysis. References 1. Spradling DM, Guth RA. Carbon foams. Adv Mater Process 2003;161(11):29–31. 2. Dimesso L, Spanheimer C, Jacke S, Jaegermann W. Synthesis and characterization of three dimensional carbon foams-LiFePO4 composites. J Power Sources 2011;196:6729–34. 3. Dimesso L, Jacke S, Spanheimer C, Jaegermann W. Investigation on 3dimensional carbon foams/LiFePO4 composites as function of the annealing time under inert atmosphere. J Alloys Compd 2011;509:3777–82. 4. Padhi AK, Nanjundaswami KS, Goodenough JB. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. J Electrochem Soc 1997;144:1188–94. 5. Wolfenstine J, Lee U, Poese B, Allen JL. Effect of oxygen partial pressure on the discharge capacity of LiCoPO4 . J Power Sources 2005;144: 226–30. 6. Jin B, Gu HB, Kim KW. Effect of different conductive additives on charge/discharge properties of LiCoPO4 /Li batteries. J Solid-State Electrochem 2008;12:105–11. 7. Tan L, Luo Z, Liu H, Yu Y. Synthesis of novel high-voltage cathode material LiCoPO4 via rheological phase method. J Alloys Compd 2010;502:407–10. 8. Bramnik NN, Bramnik KG, Buhrmester T, Bachtz C, Ehrenberg E, Fuess H. Electrochemical and structural study of LiCoPO4 -based electrodes. J Solid State Electrochem 2004;8:558–64. 9. Dimesso L, Jacke S, Spanheimer C, Jaegermann W. Investigation on LiCoPO4 powders as cathode materials annealed under different atmospheres. J Solid State Electrochem 2012;16:911–9. 10. Dimesso L, Cherkashinin G, Spanheimer C, Jaegermann W. Preparation and characterization of carbon foams-LiCoPO4 composites. J Alloys Compd 2012;516:119–25. 11. Schneider JJ, Khanderi J, Popp A, Engstler J, Tempel H, Sarapulova A, et al. Hybrid architectures from 3D aligned arrays of multiwall carbon nanotubes and nanoparticulate licopo4 : synthesis properties and evaluation of their electrochemical performance as cathode materials in lithium ion batteries. Eur J Inorg Chem 2011;2011(8):4349–59. 12. Lloris JM, P˙erez Vicente C, Tirado JL. Improvement of the electrochemical performance of LiCoPO4 5 V material using a novel synthesis procedure. Electrochem Solid-State Lett 2002;5:A234–7. 13. Bramnik NN, Nikolowski K, Trots DM, Ehrenberg H. Thermal stability of LiCoPO4 cathodes. Electrochem Solid-State Lett 2008;11(6): A89–93. 14. Gangulibabu, Bhuvaneswari D, Kalaiselvi N, Jayaprakash N, Periasamy P. CAM sol–gel synthesized LiMPO4 (M = Co Ni) cathodes for rechargeable lithium batteries. J Sol–Gel Sci Technol 2009;49: 137–44.

15. Bhuwaneswari MS, Dimesso L, Jaegermann W. Preparation of LiCoPO4 powders and films via sol–gel. J Sol–Gel Sci Technol 2010;56: 320–6. 16. Vasanthi RR, Kalpana D, Renganathan NG. Olivine-type nanoparticle for hybrid supercapacitors. J Solid-State Electrochem 2008;12:961–9. 17. Julien C. 4-V cathode materials for rechrgeable lithium batteries wet-chemistry synthesis structure and electrochemistry. Ionics 2000;6: 30–46. 18. Dimesso L, Spanheimer C, Jaegermann W. Influence of isovalent ions (Ca and Mg) on the properties of LiCoPO4 powders. J Power Sources 2013;243:668–75. 19. Ellis B, Herle Subramanya P, Rho YH, Nazar LF, Dunlap R, Perry LK, et al. Nanostructured materials for lithium-ion batteries: surface conductivity vs. bulk ion/electron transport. Faraday Discuss 2007;134:119–41. 20. Wolfenstine J, Read J, Allen JL. Effect of carbon on the electronic conductivity and discharge capacity of LiCoPO4 . J Power Sources 2007;163: 1070–3. 21. Long JW, Dunn B, Rolison DR, White HS. Three-dimensional battery architecture. Chem Rev 2004;104:4463–92. 22. Dimesso L, Spanheimer C, Jaegermann W, Zhang Y, Yarin AL. LiCoPO4 -3D carbon nanofiber composites as possible cathode materials for high voltage applications. J Appl Phys 2012;111:064307. 23. Wolfenstine J, Allen J. LiNiPO4 –LiCoPO4 solid solutions as cathodes. J Power Sources 2004;136:150–2. 24. Bramnik NN, Nikolowski K, Baehtz C, Bramnik KG, Ehrenberg H. Phase transition occurring upon lithium insertion–extraction of LiCoPO4 . Chem Mater 2007;19:908–15. 25. Satya Kishore MVVM, Varadaraju UV. Influence of isovalent ion substitution on the electrochemical performance of LiCoPO4 . Mater Resin 2005;40:1705–12. 26. Dudney NJ, Tiegs TN, Kiggans JO, Jang YI, Klett JW. Graphite foams for lithium-ion battery current collectors. ECS Trans 2007;3(27):23–8. 27. Dimesso L, Becker D, Spanheimer C, Jaegermann W. Investigation of graphitic carbon foams/LiNiPO4 composites. J Solid-State Electrochem 2012;16:3791–8. 28. Xie J, Imanishi N, Zhang T, Hirano A, Takeda Y, Yamamoto O. Li-ion diffusion kinetics in LiCoPO4 thin films deposited on NASICON-type glass ceramic electrolytes by magnetron sputtering. J Power Sources 2009;192:689–92. 29. Amine K, Yasuda H, Yamachi M. Olivine LiCoPO4 as 4.8 V electrode material for lithium batteries. Electrochem Solid-State Lett 2000;3: 178–9. 30. Markevich E, Sharabi R, Gottlieb H, Borgel V, Fridman K, Salitra G, et al. Reasons for capacity fading of LiCoPO4 cathodes in LiPF6 containing electrolyte solutions. Electrochem Commun 2012;15:22–5. 31. Ehrenberg H, Bramnik NN, Senyshyn A, Fuess H. Crystal and magnetic structures of electrochemically delithiathed Li1−x CoPO4 phases. Solid State Sci 2009;11:18–23. 32. Xue JS, Dahn JR. Dramatic effect of oxidation on lithium insertion in carbons made from epoxy resins. J Electrochem Soc 1995;142:3668–77. 33. Flandrois S, Simon B. Carbon materials for lithium-ion rechargeable batteries. Carbon 1999;37:165–80. 34. Matsumura Y, Wang S, Mondori JJ. Mechanism leading to irreversible capacity loss in Li-ion rechargeable batteries. J Electrochem Soc 1995;142:2914–8. 35. Kikuchi M, Ikezawa Y, Takamura T. Surface modification of pitch-based carbon fiber for the improvement of electrochemical lithium intercalation. J Electroanal Chem 1995;396:451–5. 36. Xing W, Dahn JR. Study of irreversible capacities for Li insertion in hard and graphitic carbons. J Electrochem Soc 1997;144:1195–201. 37. Naji A, Ghanbaja J, Humbert B, Willmann P, Billaud D. Electroreduction of graphite in LiClO4 –ethylene carbonate electrolyte. Characterization of the passivating layer by transmission electron microscopy and Fourier-transform infrared spectroscopy. J Power Sources 1996;63: 33–9. 38. Naji A, Ghanbaja J, Humbert B, Willmann P, Billaud D. Electrochemical reduction of graphite in LiClO4 –propylene carbonate electrolyte: influence of the nature of the surface protective layer. Carbon 1997;35:845–52.

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Dr. Lucangelo Dimesso, born in 1963, is currently a Solid State Chemist/Electrochemist at Department of Geo and Materials Sciences, Technische Universitaet Darmstadt, Darmstadt, Germany. Dimesso has a Ph.D. degree in Industrial Chemistry, Institute of Structural Chemistry, University of Parma, Italy in 1988, and specialized in Materials Science and Technology, University of Parma, Italy in 1991. In 1990, Dimesso received Prize Award “Cavaliere del Lavoro Rocco Bormioli” for the best Ph.D. thesis on Glass Chemistry in year 1989, University of Parma. He did his/her Master’s degree in REFA-Industrial Engineering, Darmstadt in 1999. He was a Post-doc fellow at Research Institute CNR-ITM, Milano from 1991 to 1993 and at Osaka Industrial Research Institute and ISTEC – Nagoya, Japan during 1993–1995. He received EU-fellowship at Technische Universitaet Darmstadt University of Technology, Materials Science Department, Thin Films Division during 1996–1998. He was a Senior Researcher at SusTech Darmstadt GmbH & Co. KG, Darmstadt during 2000–2008 and at Technische Universitaet Darmstadt, Materials Science Department, Surface Investigation Division during 2009–September 2012. His current interests include Investigation of lithium metal phosphates (LiMPO4 , M = Fe, Co, Ni, Mn) supported on bi- and three-dimensional carbon structures (nowoven carbon nanofibers, foams) as cathode materials for high-voltage Li-ions batteries prepared by a Pechini-assisted sol–gel process. Christina Spanheimer, born in 1971, has been working as Chemical lab assistant at Department of Geo and Materials Sciences, Technische Universitaet Darmstadt, Darmstadt, Germany from 2003 till now. She was a Chemical lab assistant at Merck Darmstadt, Germany in 1993, and at Institute of Pharmaceutical Chemistry, Goethe Universitaet Frankfurt during 1993–2003. Dipl.-Ing. Dirk Becker, born in 1964, is currently a Chemist/Electrochemist at Department of Geo and Materials Sciences, Technische Universität Darmstadt, Darmstadt, Germany. Becker completed a degree in Chemistry in 1990 at Technische Hochschule Darmstadt, Germany. Becker held the following positions: (1) research associate at Technische Universitaet Darmstadt, Institute for Chemical Engineering, Darmstadt, Germany during 1990–1993; (2) teaching job during 1994–1996 which included performance of independent professional services; (3) research associate at Technische Universitaet Darmstadt, Institute for Civil Engineering from 1996 to 2001; and (4) research associate at DECHEMA e.V., Karl-Winnacker Institute, Frankfurt am Main, Germany from 2001 to 2009. Becker was a Ph.D. student at Technische Universität Darmstadt, Department of Geo and Materials Sciences, Surface Investigation Division during 2009–2013. Becker’s current research interests are solid electrolytes for Li-ion batteries; solid electrolyte interfaces; electrode materials for Li-ion batteries: Li metal phosphates/carbon composites as cathode materials. Dr. Wolfram Jaegermann, born in 1954, is currently a professor at Department of Geo and Materials Sciences, Technische Universitaet Darmstadt, Darmstadt, Germany. His scientific education includes: (1) M.S. in Chemistry in 1977 at University of Dortmund, Germany; (2) Ph.D. in Inorganic Chemistry in 1981 at University of Bielefeld, Germany; and (3) Habilitation for Physical Chemistry in 1993 at FU Berlin, Germany. He worked in the following positions during his career: (1) “Visiting Research Scientist”, DuPont Experimental Station, CR&D, E.I. duPont de Nemours & Co, Willmington, USA during 1986–1987; (2) Head of Surface Particle Group, Vice-Director of the Solar Energy Department, Director: Prof. Dr. H. Tributsch, Hahn-Meitner-Institute, Berlin, Germany during 1987–1994; (3) Head of Surface Particle Department, Hahn-Meitner-Institute, Berlin, Germany during 1994–1997; and (4) Professor (C4) at Department of Geo and Materials Sciences, Technische Universitaet Darmstadt from 1997 till now. Jaegermann’s current research interests include electronic properties of nano materials, nanoparticles and nanostructures, transparent conducting oxides (TCOs), Cd–Te thin films solar cells, organic semiconductors, electrode materials for Li-ion batteries; Li metal phosphates/carbon composites as cathode materials, solid electrolyte interfaces, energy.