Morphology and electrochemical properties of LiMn2O4 powders derived from the sol–gel route

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Materials Science and Engineering B79 (2001) 247 – 250

Letter

Morphology and electrochemical properties of LiMn

₂

O

₄

powders

derived from the sol – gel route

Chung-Hsin Lu *, Susanta Kumar Saha

Department of Chemical Engineering, National Taiwan Uni6ersity, Taipei,106Taiwan, ROC

Received 4 September 2000

Abstract

Spinel-lithium manganese oxide (LiMn2O4) powders are prepared by the sol – gel method using polyvinyl alcohol (PVA) in this

study, and evaluated as a positive electrode material for Li-ion batteries. PVA helps to maintain the homogeneity of the system in the precursors and thereby facilitates the formation of monophasic powders. The average primary particle size of the calcined LiMn2O4powders decreases with an increase in the amount of PVA used in solution. Increasing the amount of PVA also results

in a rise in the discharge capacity at 25°C because of the large interface area of small particles. On the other hand, the discharge capacity of the small LiMn2O4particles suffers markedly capacity fading when the batteries are operated at elevated temperatures.

Keywords:Homogeneity; Sol – gel method; Polyvinyl alcohol; Li Mn2O4

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1. Introduction

Nowadays, the demand for rechargeable batteries with high energy density and high voltage has increased rapidly due to the advancement and popularity of portable electronic devices. Lithium manganese oxide (LiMn2O4) is a promising cathode material for lithium

ion batteries because of its low cost and lower toxicity compared with the layered oxides LiCoO₂and LiNiO₂. For the consideration of practical application, it is important to produce LiMn2O4 powders with excellent

capacity and cyclability. The electrochemical properties of LiMn₂O₄ powders strongly depend on its synthesis process. The LiMn2O4powders are usually prepared by

the solid-state reaction [1 – 8]. This process requires extensive mechanical mixing, extended grinding, and prolonged heating time. These synthesis conditions in-evitably result in the unexpected grain growth of the spinel particles. To achieve good efficiency of Li

utiliza-tion at high current rates and improve the reliability of lithium secondary batteries, several chemical methods have been introduced to synthesize cathode materials with good homogeneity, uniform morphology and a narrow size distribution. The chemical methods such as co-precipitation [9], emulsion drying [10,11], sol – gel [12] and the Pechini process [13] have been developed to obtain the homogeneous LiMn2O4 powders with small

average particle size.

For simplifying the synthesis process, we have devel-oped a new sol – gel method for preparing the spinel LiMn2O4 powder with high capacity and good cycling

behavior. In this process, appropriate ratios of cations are dissolved in water, and then the aqueous solution is mixed with polyvinyl alcohol (PVA) solution. The solu-tion is evaporated from gel and subsequently dried to become precursor powder. The purpose of the study is to prepare homogeneous and monophasic LiMn2O4

powder. The charge – discharge characteristics of the cathode are also evaluated to examine the effects of particle size on the capacity and stability of LiMn₂O₄ powders.

* Corresponding author. Fax: + 886-2-3623040.

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2. Experimental

A solution containing a stoichiometric mixture of reagent grade lithium nitrate and manganese nitrate hexahydrate was firstly prepared. The concentration of the solution was maintained at 1 M with respect to metal ions. An aqueous solution of PVA was prepared by dissolving it in deionized water. The PVA solution was then added into the mixed metal nitrate solution with thorough stirring. The molar ratio of the metal ions to vinyl alcohol monomer units of PVA in the starting solution was maintained at 1:2 (sample A) and 2:1 (sample B), respectively. The resulting viscous solu-tion of metal nitrate and PVA mixture was evaporated with continuous stirring. The evaporated mass was then heated at various temperatures ranging from 400°C to 800°C.

TG/DTA studies of the evaporated powders were performed at a heating rate of 10°C min− 1_{in air. The}

FTIR spectra of the evaporated and heated powders were recorded from 400 to 2000 cm− 1_{. An X-ray}

diffractometer with Ni-filtered CuK_aradiation was used for the identification of the crystalline phases of the heat-treated powders. Transmission electron micro-scopic studies of the heat-treated powders were carried out using an electron microscope. The electrochemical behavior of the obtained LiMn2O4 powders was

exam-ined in a test cell. Lithium foil was used as the anode. 1 M LiPF6 in ethylene carbonate/dimethyl carbonate

(volume ratio = 1:1) was used as the electrolyte solu-tion. Cells were charged and discharged at 0.2 mA cm− 2_{within the potential range of 3.0 and 4.3 V.}

3. Results and discussion

Results of TG/DTA analysis indicate the formation of lithium manganese oxide at 400°C, at which temper-ature the precursor has attained a stable weight on TGA curve. This observation is also supported by the FTIR results. Only two characteristic vibrational bands at 495 and 610 cm− l_{, which correspond to metal}

oxy-gen bonds, are observed in the spectra of the powder heat-treated at 400°C or higher temperature. Fig. 1 shows the XRD patterns of LiMn2O4 powders of

sam-ples A and B calcined at 700°C and 800°C in air. It is confirmed by the XRD patterns that only spinel LiMn2O4 crystalline phase is formed in sample A at

both temperatures. All diffraction peaks for these cal-cined powders are assigned to the diffraction indices of LiMn₂O₄ spinel phase. However, the 700°C calcined powder of sample B exhibits both spinel LiMn₂O₄ phase and a trace of Mn₂O₃phase. This impurity phase completely disappears after calcining the sample at 800°C. Precursor powder of sample B is assumed to be less homogeneous which may be the reason for the appearance of Mn2O3 impurity phase at low

tempera-tures. Sufficient amount of PVA is required for the formation of polymer matrix with a whole range of homogeneously trapped metal ions in gel [14,15]. When the amount of PVA is insufficient, the homogeneity of the cation ions in the gel is poor (such as in sample B); therefore, certain regions lack of lithium ions will pro-duce Mn2O3 phase. Furthermore, homogeneous

distri-bution of the metal ions in the precursor powder reduces the diffusion distance among cation ions during heat treatment. This also accelerates the formation of monophasic spinel LiMn₂O₄

Fig. 2 shows the transmission electron micrographs of 800°C-calcined powders of samples A and B. Cal-cined powders of both systems are loosely bound ag-glomerates of nanosized particles with a spherical morphology. 800°C-calcined powder of sample A is comparatively smaller than that of sample B. The aver-age primary particle sizes of calcined samples A and B are 50 and 120 nm, respectively. The above observa-tions reveal that the powder prepared by this method has much smaller particle size and narrower particle size distribution than that obtained by solid state [8] and other solution method [10]. During the heating of the precursor powder, CO, CO₂, and H₂O are generated by the decomposition of organic species, which facili-tate the formation of the dispersed and nanosized parti-cles [16,17]. With a decrease in the amount of PVA used in sample B, the amount of generated gases is decreased, thereby resulting in an increase in the parti-cle size.

Fig. 3 shows the charge – discharge curves of 800°C-calcined powders of sample A as the cathode material operated at 25°C and 55°C. All the samples exhibit two Fig. 1. XRD patterns for the (a) 700°C- and (b) 800°C-calcined

powders obtained from precursor solutions having the molar ratio of the metal ions to the monomer units of PVA equal to 1:2 (sample A) and 2:1 (sample B).

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Fig. 2. Transmission electron micrographs of 800°C-calcined LiMn2O4powders from solutions having the molar ratio of the metal

ions to the monomer units of PVA equal to (a) 1:2 (sample A) and (b) 2:1 (sample B).

LiMn2O4 powders. When the particle size is reduced,

the overall surface area is increased. Thus, the cathode composed of small particles with large interface area can provide more lithium ions for diffusion, and there-fore the specific capacity is increased.

Variation of the specific discharge capacity with the cycle number is elucidated in Fig. 5. As shown in Fig. 5(a), the capacity losses over 10 cycles for the 800°C-calcined LiMn₂O₄ powders obtained from samples A and B at 25°C are 10% and 4% of the initial discharge capacity, respectively. When the operation temperature is raised to 55°C, the capacity loss for sample B over 10 cycles is around 4%; however, the capacity loss for sample A is markedly increased to 20%. Apparently, sample A suffers serious capacity fading at elevated

Fig. 3. Cycling charge/discharge curves over the potential range of 3.0 to 4.3 V at a current density of 0.2 mA cm− 2 _{using LiMn}

2O4.

800°C-calcined powders of sample A, operated at (a) 25°C and (b) 55°C.

Fig. 4. Cycling charge/discharge curves over the potential range of 3.0 to 4.3 V at a current density of 0.2 mA cm− 2 _{using LiMn}

2O4.

800°C-calcined powders of sample B, operated at (a) 25°C and (b) 55°C.

plateaus in the discharge curves. This is due to two-step reduction and oxidation process, which is a characteris-tic of lithium manganese oxide spinel [18,19]. The charge – discharge profile (Fig. 3(a)) shows that the elec-trochemical behavior of the powder exhibits an initial discharge capacity of 124 mA h g− 1 _{at 25°C. An}

elevated temperature operation at 55°C reduces the initial discharge capacity to 111 mA h g− 1_{. The}

charge – discharge electrochemical behavior of 800°C-calcined powders of sample B is illustrated in Fig. 4. The initial discharge capacity is relatively lower with respect to sample A at both operation temperatures of 25°C and 55°C. The initial discharge capacity of sample B at 25°C is 109 mA h g− 1_{, and is reduced to 104 mA}

h g− 1 _{at 55°C. The variation of initial charge –}

dis-charge capacity for different samples is considered to result from the difference in the particle size of

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Fig. 5. Variation of the specific discharge capacity with the cycle number for 800°C-calcined powders of samples A and B at (a) 25°C and (b) 55°C.

The average particle size of LiMn2O4 powder decreases

with an increase in the PVA amount used in the precursor solution. The obtained powders exhibit good electrochemical properties as cathode materials for the application to Li-ion batteries. LiMn₂O₄ particles with small particle size have large initial discharge capacity at room temperature, but fast fading rate of capacity at elevated temperature of operation. Controlling the par-ticle size of LiMn2O4 during synthesis processes is

important for raising the discharge capacity and im-proving the cyclability at elevated temperatures.

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[19] Y.M. Choi, S.I. Pyun, Solid St. Ionics 99 (1997) 173. temperatures. The faster capacity fading for sample A

is possibly related to smaller particle size of LiMn2O4

powders. Small particles provide more surface area for lithium ions to diffuse in and out of the host structure; however, the large surface area also enhances the reac-tivity between LiMn2O4 particles and electrolyte, which

reduces the structure stability of the spinel phase during charge – discharge cycling. Consequently, optimization of the particle size of LiMn2O4 powders by different

synthesis processes is crucial for increasing the dis-charge capacity and tailoring the cyclability at elevated temperatures.

4. Conclusions

Spinel LiMn2O4 powders have been synthesized by a

sol – gel method using an aqueous solution of metal nitrates containing PVA as a gelling agent. The LiMn2O4 powders obtained by this process have an

uniform morphology with a narrow size distribution.