ISRN Electrochemistry

Volume 2013 (2013), Article ID 851921, 5 pages

http://dx.doi.org/10.1155/2013/851921

Research Article

Improved Performance of Hydrothermally Synthesized by Ball Milling as a Positive Electrode for Li Ion Battery

Masashi Kotobuki

Department of Material and Environmental Engineering, Hakodate National College of Technology, 14-1 Tokura-cho, Hokkaido, Hakodate 042-8501, Japan

Received 4 December 2012; Accepted 23 December 2012

Academic Editors: A. I. Bhatt, X. He, A. A. Moya, C. M. Muller, and W. Xing

Copyright © 2013 Masashi Kotobuki. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

LiMnPO₄ is anticipated to be a promising cathode material for next generation lithium battery. A reduction of particle size is recognized as a good strategy to improve its performance and it can be achieved by ball milling. However, the ball milling including carbon addition forms small LiMnPO₄ particles with large carbon content, which leads to low volumetric energy density of electrode. In this study, carbon-coated LiMnPO₄ prepared by hydrothermal route was applied to the ball milling without carbon addition. The reduction of particle size of carbon-coated LiMnPO₄ was achieved by the ball milling without destroying the surface carbon layer. The ball-milled LiMnPO₄ particle revealed better cathodic performance than non-milled sample. This was attributed to shortening Li ion diffusion path, improvement of structural flexibility, and large surface area of electrode due to reduction of particle size. The ball milling is attested to be a promising method to improve cathodic performance of carbon-coated LiMnPO₄.

1. Introduction

LiMPO₄ compounds (M = Fe, Mn, Co, and Ni) with olivine structure have been attracted as alternative cathode materials for lithium ion batteries owing to their low cost, low toxicity, chemical and thermal stabilities compared with currently used LiCoO₂ [1]. Among these compounds, LiMnPO₄ has been recognized the most attractive compound due to its high operation voltage determined by Mn3+/4+ redox couple at 4.1 V versus Li/Li⁺ and is compatible with the system presently used in lithium ion battery [2].

However, the most serious problem of this cathode material is low intrinsic electronic and Li ion conductivities [3, 4]. Various experimental reports have indicated that carbon-coating can provide high electronic conductivity [5–7]. In fact, a large charge-discharge capacity of carbon-coated LiFePO₄ synthesized by a hydrothermal synthesis method has been reported [8].

As for improvement of Li ion conductivity, much effort has been paid for particle size reduction to shorten Li ion conduction path [9]. Drezen et al. reported reversible capacities for 140 and 270 nm diameter LiMnPO₄ particles prepared by sol-gel method were 81 and 7 mA h g⁻¹, respectively [10]. Some groups reported a good performance of ball-milled small LiMnPO₄ [11] and LiCoPO₄ particles [12]. However, their processes included conductive carbon addition (≤20 wt.%) before the ball milling to obtain a carbon composite. This large amount of carbon causes surely improvement of the electronic conductivity, however, such heavy carbon coating decreases volumetric capacity. Additionally, commercial battery foils typically only contain 2.5 weight percent (or lower) carbon blacks with close to 95 weight percent active materiel. Therefore, it may not appeal for practical application.

The carbon coated olivine type cathode materials prepared by hydrothermal route have been reported [13, 14]. The method allows us to prepare carbon-coated particles readily by only addition of a carbon source to start materials under appropriate condition. Consequently, it is guessed that if carbon-coated LiMnPO₄ (LiMnPO₄/C) prepared by hydrothermal condition was supplied to the ball milling, carbon amount could be suppressed by omission of further carbon addition. On the other hand, harmful influence by the ball-milling of LiMnPO₄/C is also surmised, such as destroying the carbon coating layer. Hence, research on effect of the ball-milling to LiMnPO₄/C on electrochemical properties is a worthwhile work.

Herein, we applied to the ball-milling technique to hydrothermally synthesized LiMnPO₄/C and its electrochemical property was compared with non-milled LiMnPO₄/C to clarify influence of the ball-milling technique on electrochemical property of LiMnPO₄/C.

2. Experimental

LiMnPO₄ was synthesized by a modification of hydrothermal process in previous reports for LiFePO₄ [15–17]. MnSO₄·5H₂O (43.84 g) and Li₃PO₄ (20.84 g) were dissolved into purified water (44 mL) under N₂ atmosphere. A molar ratio of Li : Mn : P in a precursor solution was 3 : 1 : 1. In order to prepare carbon-coated LiMnPO₄, 6.00 g of carboxy methyl cellulose (CMC) was added into the precursor solution [1]. The precursor solution was put into a glass-lined Parr reactor with N₂ gas sealed in a stainless steel autoclave, and then heated at 200°C for 3 h with stirring at 680 rpm. A precipitation was produced in the reactor under hydrothermal conditions. The precipitation was separated centrifugally at first, and then by a mean of freeze-drying at −50°C for 12 h. Yield was always higher than 95%. The obtained sample was dispersed into ethanol and ball milled at 400 rpm for 12 h with ZrO₂ ball using a planetary ball mill equipment (Pulverissette P-6, Fritsch GmbH). The milled and non-milled samples were treated at 700°C under 3% H₂/Ar flow for 1 h to obtain electroconductive graphite carbon [1, 14].

The crystalline phases of the synthesized sample were identified with X-ray diffraction (XRD, RINT-Ultima, Rigaku) with Cu Kα radiation. Raman measurement (NRS-1000, JASCO) was carried out at room temperature by a laser radiation of 532 nm. The morphology of the synthesized particle was observed by scanning electron microscope (SEM, JEOL). Coated carbon amount of the samples was estimated by Thermogravimetry (TG) analysis (DTG-60, Shimazu) and BET surface areas of milled and non-milled particles were measured using BELSORP-mini (BEL JAPAN Inc.).

Performances of milled and non-milled LiMnPO₄/C as a positive electrode were tested by a galvanostatic charge/discharge test. A composite LiMnPO₄/C electrode was prepared by mixing LiMnPO₄/C, Ketjen black, and Polyvinylidene difluoride (PVdF) in a weight ratio of 90 : 5 : 5 in 1.2 mL of NMP (N-methyl pyrrolidone). The LiMnPO₄/C electrode was painted onto a thin Al sheet and dried overnight at 85°C under reduced pressure for 12 h. The LiMnPO₄ electrode (14 mm diameter) was set in a coin cell 2032 with a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) (volume ratio = 1 : 1) containing 1 mol·dm⁻³ LiPF₆ as an electrolyte and with lithium metal as a negative and a reference electrodes. The galvanostatic charge/discharge tests of coin cell with LiMnPO₄ positive electrode were performed by using HJ1001SM8A (HOKUTODENKO). In the test, charging process was done under CC-CV (constant current-constant voltage) mode, that is, charge in a constant current condition of 0.1 C until 4.5 V and then constant voltage charging was performed at 4.5 V until current dropped to 0.01 C. The discharge process was carried out at constant current condition of 0.1 C. Cutoff voltages were 2.0 V and 4.5 V for discharge and charge processes, respectively.

3. Results

Figure 1 displays SEM images of non-milled and milled samples. In both cases, particles aggregated each other and formed secondary particles. The sizes of primary particles of non-milled and milled samples were estimated to be about 500 and 50 nm, respectively. The particle shapes of both samples were irregular. It is concluded that much smaller particles can be obtained by ball milling. BET surface area of non-milled and milled samples were 8.9 and 49.0 m2 g⁻¹, respectively. This result is well consistent with SEM observation.

(a) Non-milled LiMnPO₄

(b) milled LiMnPO₄

Figure 1: SEM images of the hydrothermally synthesized LiMnPO₄/C. (a) Non-milled and (b) milled samples.

XRD patterns of non-milled and milled samples are depicted in Figure 2. In non-milled sample, all diffraction peaks were attributed to LiMnPO₄ with olivine structure and no impurity phase was observed. In milled sample, the diffraction peaks were much broader due to small particle and low crystallinity given by the ball milling. An appearance of new phase was not confirmed. Therefore, it is inferred that the ball milling can crash the LiMnPO₄ particles, but did not produce any impurity phase.

Figure 2: XRD patterns of the hydrothermally synthesized LiMnPO₄/C. (a) Non-milled and (b) milled samples.

The Raman spectroscopy is sensitive to surface of the materials comparing with XRD. Dokko et al. have succeeded in detection of even small amount of impurity on the LiFePO₄ synthesized by the hydrothermal process [17] and carbon coating layer on the surface of the LiFePO₄/C [8], LiMnPO₄/C [1], and LiCoPO₄/C [14]. Thus, the Raman measurement was carried out to detect carbon layer on the samples that could not be detected by the XRD measurement. Figure 3 reveals the Raman spectra of non-milled and milled LiMnPO₄/C. In non-milled sample, clear peaks appeared at 948, 1361, and 1610 cm⁻¹. The peak at 948 cm⁻¹ is assigned to the symmetric vibration of the PO₄ group [18]. As mentioned above, clear diffraction peaks of the LiMnPO₄ were observed in the XRD patterns. The peak at 948 cm⁻¹ was consistent with the result of XRD and it was clear that the LiMnPO₄ has been synthesized successfully. Peaks at 1610 cm⁻¹ and 1361 cm⁻¹ were attributed to graphite and disorder carbon (G and D bands), respectively [19], indicating that conductive carbon layer was formed on the surface. In milled sample, the peak at 948 cm⁻¹ was not observed because of small particles and low crystallinity of the milled-sample. A high noise level of the spectrum is also possible reason. As for the G and D bands, they clearly appeared in the spectrum although their intensities became weaker, indicating that surface carbon layer still existed even after crashing the particles by the ball milling. The ratio of G band to D band, that is, quality of the carbon layer, was 1.4 and 1.3 for non-milled and milled samples, respectively. Therefore, the qualities of surface carbon layer of both samples are considered to be same. TG analysis was performed to estimate surface carbon amount. Estimated carbon amounts of non-milled and milled samples were 3.4 and 2.2 wt%, respectively. It seems that some carbon coatings were destroyed and peeled off during the ball milling.

Figure 3: Raman spectra of the hydrothermally synthesized LiMnPO₄/C. (a) Non-milled and (b) milled samples.

The charge/discharge test of non-milled and milled LiMnPO₄/C was carried out (Figure 4). In both cases, charge and discharge plateaus at around 4.1 V versus Li/Li⁺, which are attributed to intrinsic redox of LiMnPO₄, were observed. In non-milled LiMnPO₄, the plateau was much shorter than milled sample and the discharge capacity was 109 mA h g⁻¹. This was 64% of theoretical one (171 mA h g⁻¹) [20]. In milled LiMnPO₄/C, a potential difference of the plateau between charge and discharge curves was small owing to small internal resistance of the milled LiMnPO₄/C electrode. The discharge capacity improved, 145 mA h g⁻¹, 85% of theoretical capacity. A rate capability test also revealed favorable performance of milled sample (Figure 5). In this experiment, the charge process was performed at constant current condition of 0.1 C. The discharge capacities of milled and non-milled samples at 0.1 C were 127 and 98 mA h g⁻¹, respectively. Then, the capacity of non-milled sample decreased more quickly, only 6 mA h g⁻¹ at 5.0 C. On the other hand, the milled sample still maintained high capacity, 64 mA h g⁻¹ at same C rate. This corresponded to 50% of capacity retention.

Figure 4: Charge and discharge curves of the hydrothermally synthesized LiMnPO₄/C. (a) Non-milled and (b) milled samples.

Figure 5: Rate capabilities of the hydrothermally synthesized LiMnPO₄/C. (a) Milled and (b) Non-milled samples.

4. Discussion

In this study, we applied the ball milling to hydrothermally synthesized LiMnPO₄/C and its electrochemical property was compared with non-milled LiMnPO₄/C. By the ball milling, small particle of LiMnPO₄/C was obtained, although a little loss of surface carbon layer was confirmed. The ball-milled LiMnPO₄/C demonstrated larger discharge capacity than non-milled LiMnPO₄/C. Fey et al. studied on a dependency of discharge capacity on surface carbon amount of LiFePO₄ [21]. They reported the discharge capacities of LiFePO₄/C with coated carbon from 1.0 to 6.0 wt% were almost identical. In this study, carbon amounts of non-milled and milled samples were 3.4 and 2.2 wt.%, respectively. Consequently, it can be said that the carbon layer on the LiMnPO₄ particle surface was destroyed a little by the ball milling; however, the deconstruction would hardly influence on electrochemical property. The difference of cathodic performance between non-milled and milled samples seems to be attributed to difference in length of Li ion transport paths by diverse particle size. Also, it is fairly well known that the charged PO₄ phase undergoes a severe lattice deformation due to the asymmetric electronic configuration of Mn³⁺ ions (3d⁴()) [22, 23]. The Jahn-Teller deformation as well as the large lattice misfit between LiMnPO₄ and MnPO₄ phases destroys the integrity of lattice, thus leading to a low electrochemical activity. The small size of LiMnPO₄ enhances structural flexibility against lattice deformation [24]. Thus, improvement of integrity of the lattice by structural flexibility of small particles would be also another reason for superior performance of the milled sample. Moreover, the surface area of electrode increased with reduction of LiMnPO₄ particle size as indicated by BET surface area measurement, leading to large contact area between electrolyte and electrode. This enlargement of electrode area would also help improved performance of milled sample.

The reduction of LiMnPO₄/C particle size by the ball-milling was achieved and improved performance of ball-milled LiMnPO₄/C was confirmed. This is attributed to short Li ion diffusion paths, improvement of structure flexibility, and large surface area of electrode by reduction of LiMnPO₄/C particle size. The ball milling is attested to be a promising method to improve cathodic performance of carbon-coated LiMnPO₄.

5. Conclusion

The ball milling was applied to hydrothermally synthesized LiMnPO₄/C. The LiMnPO₄/C particle size was successfully reduced by the ball milling. Although a little loss of surface carbon layer by the ball milling was observed, this loss did not influence on electrochemical properties. The ball-milled LiMnPO₄/C demonstrated higher cathodic performance than non-milled sample. This would be attributed to short Li ion diffusion paths, improvement of structure flexibility, and large surface area provided by reduction of LiMnPO₄/C particle size. The ball milling is proven to be a promising method to improve cathodic performance of carbon-coated LiMnPO₄.

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