The effect of Ti doping on electrochemical properties of Li1.167Ni0.4Mn0.383Co0.05O2 for lithium-ion batteries

Solid State Ionics 296 (2016) 154–157

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The effect of Ti doping on electrochemical properties of Li1.167Ni0.4Mn0.383Co0.05O2 for lithium-ion batteries Yanjuan Kou *, Enshan Han, LingZhi Zhu, Lili Liu, Zhong'ai Zhang School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China

a r t i c l e

i n f o

Article history: Received 10 September 2015 Received in revised form 15 September 2016 Accepted 15 September 2016 Available online xxxx Keywords: Li1.167Ni0.4Mn0.383Co0.05O2 Ti additive Li-rich Cathode material

a b s t r a c t Li1.167Ni0.4-xMn0.383Co0.05TixO2 (x = 0, 0.02, 0.04 and 0.08) cathode materials have been synthesized via a coprecipitation method followed by a solid state reaction. The effects of Ti additive on the crystal structure features and electrochemical properties of the powders are studied in detail using X-ray (XRD), Scanning Electron Microscopy (SEM), galvanostatic charge-discharge test and Cyclic voltammetry (CV). It is found that Ti doping don't destroy the layered structure of Li1.167Ni0.4Mn0.383Co0.05O2, but can stabilize the crystal structure. The results showed Ti additive played an important role in the good cycling performance of Li1.167Ni0.4-xMn0.383Co0.05TixO2. In contrast, 0.04 Ti doped cathode material shows better cycle performance than bare sample. The Li1.167Ni0.36Mn0.383Co0.05Ti0.04O2 delivers an initial discharge capacity of 186.6 mAh/g at 0.1 C in the cut-off voltage of 2.0–4.8 V and has a capacity retention of 99.4% after 10 cycles at 0.1 C. © 2016 Elsevier B.V. All rights reserved.

1. Introduction During the past decades, Lithium ion batteries are receiving considerable attention as power source for electric vehicles (EV), hybrid electric vehicles (HEVS) and large electric power tools [1]. Recently, the layered structure series material LiNi1-x-yCoxMnyO2 (NCM) has received increasing attention [2,3]. Among these, LiNi0.8Co0.1Mn0.1O2 is one of special interests due to its high discharge capacity of about 200 mAh/g (4.3 V cut-off voltage at 0.1 C) [4]. However, compared with other cathode materials, such as LiFePO4, the cycling performance of LiNi0.8Co0.1Mn0.1O2 needs to be improved, due to high Ni content on the particle surface and cation mixing [5,6]. Many research groups studied the solid solutions of layered transition metal oxides with composition of xLi2MnO3-(1-x)LiMO2 (M = Mn, Co, Ni), due to their high discharge capacity, low cost and good safety [7]. The Li2MnO3 component can supply extra Li+ ions at voltages higher than 4.5 V (leading to increased operational voltage) and increase the structural and thermal stabilities [8]. In this class of materials, Li2MnO3 type domains are initially electrochemically inactive. But when the voltage increases beyond 4.5 V , Li2O can be extracted from the Li2MnO3 and form the electrochemically active MnO2 host, which can accommodate more Li+ ions into the lattice and provide additional capacity. In the paper, in order to improve the rate capacity of Li1.167Ni0.4Mn0.383Co0.05O2, we reported a series of compounds with nominal formula of Li1.167Ni0.4-xMn0.383Co0.05TixO2 (0.02 ≤ x ≤ 0.08) ⁎ Corresponding author. E-mail addresses: [email protected] (Y. Kou), [email protected] (E. Han).

http://dx.doi.org/10.1016/j.ssi.2016.09.020 0167-2738/© 2016 Elsevier B.V. All rights reserved.

synthesized via a co-precipitation method followed by a solid state reaction at high temperature. The effects of Ti additive on morphology, crystal structure and electrochemical properties of the as-prepared powders were investigated in detail. 2. Experimental 2.1. Synthesis Ni0.4-xMn0.383Co0.05(OH)1.666-2 × precursors were prepared as follows: (1) Stoichiometric amounts of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O were dissolved in distilled water with a concentration of 1.0 mol/L. The solution was pumped into a continuous stirred tank reactor. (2) The solution was heated at 50 °C in water bath kettle, after NH3·H2O, NaOH was slowly added into the solution to control the pH = 11.5. (3) After reaction, the mixtures were filtered, washed with distilled water and dried at 90 °C. This process was carried out without Ar or N2. Thus, our modified co-precipitation method possesses potential for large scale synthesis of these materials. Later, the precursors were thoroughly mixed with stoichiometric amounts of TiO2 and 2% excess Li2CO3. The obtained mixtures were sintered at 480 °C for 5 h, and then sintered at 850 °C for 16 h under air atmosphere. Then Li1.167Ni0.4-xMn0.383Co0.05TixO2 was obtained. 2.2. Sample characterization The powder X-ray diffraction (XRD, Rint-2000, Rigaku) using Cu Kα radiation was employed to identify the crystalline phase of the

Y. Kou et al. / Solid State Ionics 296 (2016) 154–157

synthesized material. The morphology was analyzed with scanning electron microscope (SEM). 2.3. Electrochemical tests The charge-discharge characteristics of cathodes were examined in a CR2032 coin cell. Cells were composed of a cathode and a lithium metal anode separated by a porous polypropylene separator (Celgard). Composite cathode was prepared by thoroughly mixing the active material (80%) with acetylene black (10%) and polyvinylidene fluoride (10%) in N-methyl-pyrrolidinone. The slurry was then cast on aluminum foil and dried at 110 °C for 10 h in vacuum. The obtained electrode film was subsequently pressed and punched into a circular disc. LiPF 6 (1 M) in a 1:1:1(v/v/v) mixture of dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC ) and ethylene carbonate (EC) was used as electrolyte. The coin cells were assembled in an argon-filled glove box. The cells were cycled at different C-rates between 2.0 V and 4.8 V with a Land battery testing system. The current value for different C-rates was calculated from the theoretical capacity of Li1.167Ni0.4-xMn0.383Co0.05TixO2. Cyclic voltammetry (CV) was conducted by a CHI660a electrochemical analyzer, operated at 0.1 mV/s between 2.5 V and 4.9 V. 3. Results and discussion 3.1. Crystal structure and particle morphology Fig. 1 shows the XRD patterns of bare and Li1.167Ni0.4-x Mn0.383Co0.05TixO2. Except for the feeble diffraction peaks between 20° and 25° which originate from the superlattice ordering of Li and Mn in transition metal layers (3a sites), the other diffraction peaks can be indexed as a layered oxide structure based on a hexagonal α-NaFeO2 structure with a space group R3-m. The small diffraction peaks at 20°25° that are generally attributed to Li2MnO3 phase with a space group C2/m [9,10]. No impurity peaks are observed in XRD patterns of Ti doping samples. The transition metal ions (Ni, Co, Mn) at 3b and oxygen ions at 6c sites. Since the ionic radii of Li+ (0.76 Å) and Ni2+ (0.69 Å) ions are similar, a partial disordering among the 3a and 3b sites is inevitable and it is called as “cation-mixing” [11]. It has been confirmed that cation-mixing deteriorates the electrochemical capabilities of the layered oxide materials. The integrated intensity ratio of the (003) to (104) lines (R) in the XRD patterns is shown to be a measurement of the cation mixing and a value of R b 1.2 is an indication of undesirable

003

104 101 015

018 110 113 107

Ti-0.08

Intensity/(a.u)

Ti-0.06

Ti-0.04

155

cation mixing [12]. In XRD patterns, The slight split of (006)/(102) and (108)/(110) indicates that the products possessed typical layered characteristics. And the greater the value of c/a, the better the layered structure develops and the less cation mixing occurs [13]. As can be seen in Fig. 1, clear peak splits of 006/102 and 108/110 are observed, indicating highly ordered layered structure of all materials. In summary, the increased c/a value and reduced cation mixing can influence the rate capacity and cycling performance of materials. As shown in Table 1. The lattice volumes increase with the amount of Ti additive, indicating that Ti substitute Ni because the ion of Ti4 +(60.5 pm) is larger then the Ni3+(56 pm). The c/a and R values of samples increase firstly and then decrease with the doping amount from 0.02 to 0.08. All values reach maximum until 0.04 Ti doping sample, which indicates best hexagonal structure and desirable cation mixing, moreover excellent rate capacity and cycling performance , which is proved by followed research. In order to observe morphologies of Li1.167Ni0.4Mn0.383Co0.05O2 and Ti doping Li1.167Ni0.4Mn0.383Co0.05O2, SEM measurements were performed at different magnifications. Fig. 2 shows SEM photographs of the bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2. It can be seen that all compounds maintain the near-spherical morphology, and the particles are comprised of densely packed particles. But different amounts of Tidoped sample, the degree of dispersion are not the same, the reunion of the bare and 0.02 Ti doped samples are more serious. The morphology of the 0.06 and 0.08 samples particles is changed obviously during Ti doping process. Agglomeration or severe fragmentation is not conducive to the diffusion of lithium ions. In contrast, the morphology of 0.04 Ti doping cathode is more better than others.

3.2. Electrochemical characteristics The intial charge-discharge curves of the prepared samples are shown in Fig. 3 between 2.0 and 4.8 V at 0.1C. All the voltage profiles exhibit two charge plateaus at about 3.9 V and 4.5 V. The first charge plateau is identified as the extraction of Li from the structure of space group R-3 m accompanying with the redox of Ni-ions between Ni2+ and Ni4+ (first mainly from Ni2+ to Ni3+, then only part of Ni3+ is oxidized to Ni4+) and Co3+/Co4+ [14,15]. The second one maybe the activation of the Li2MnO3-like region at approximately 4.5 V, corresponding to the delithiating process accompanying with the loss of oxygen from the Li2MnO3-like region (the removal of Li2O) and the formation of the MnO2 with layered structure [16]. Then the electrochemical inactive Li2MnO3-like region becomes active after removing Li2O from the lattice, thus the materials can deliver high discharge capacity during the subsequent discharge process [17]. As can be seen from Fig. 3 , the capacity decreases with increasing x, and a maximal capacity of 186.6 mAh/g is achieved when x = 0.04 . The Ti substitution will lead to a decreased discharge capacity for the first cycle. The first cycle, 10th cycle, 19th cycle, 41th cycle and 50th cycle charge-discharge curves of the prepared samples are shown in Fig. 4 between 2.0 and 4.8 V at different C-rates. The second platform that the activation of the Li2MnO3-like region at approximately 4.5 V disappeares after the first cycle. As the cycle progresses, the voltage platform decreases and no new platform appeares. It is obviously that the additive of Ti can inhibit phase transformation of Li2MnO3 from layered to spinel.

Ti-0.02

bare

20

40

60

80

2θ/° Fig. 1. XRD patterns of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2.

Table 1 Lattice parameters and R of bare and Li1.167Ni0.4-x Mn0.383Co0.05TixO2. samples

V/10−3 nm3

a/nm

b/nm

c/nm

c/a

R/%

bare 0.02 0.04 0.06 0.08

100.26 100.59 100.89 100.91 101.49

2.8621 2.8668 2.8682 2.8686 2.8687

2.8621 2.8668 2.8682 2.8686 2.8687

14.1329 14.1408 14.1792 14.1782 14.1772

4.9379 4.9326 4.9436 4.9425 4.9520

1.7374 1.3324 1.4186 1.4152 1.3253

156

Y. Kou et al. / Solid State Ionics 296 (2016) 154–157

bare

Ti-0.02

Ti-0.04

Ti-0.06

Ti-0.08

Fig. 2. SEM images of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2.

Fig. 5 shows the cycle performance of at 0.1C, 0.2C, 0.5C and 1C in the voltage range of 2.0 V–4.8 V. During the extend cycling , Li1.167Ni0.4-x Mn0.383Co0.05TixO2(x = 0, 0.02, 0.04, 0.06, 0.08) cathodes deliver initial charge and discharge capacity of 298.4 mAh/g, 187.8 mAh/g; 267.8 mAh/g, 180.8 mAh/g; 284 mAh/g , 186.6 mAh/g; 260 mAh/g , 160.2 mAh/g; 266.51 mAh/g, 157.2 mAh/g, respectively, with the initial

coulombic efficiencies of 62.94%, 67.21%, 65.77%, 61.62% and 58.99%. The capacity loss during the first cycle is attributed to the remove of Li2O from the Li2MnO3 region, which is irreversible [18,19]. According to Fig. 5, with the increase of substitution amount of Ti from 0 to 0.08, the initial discharge capacity show a trend of decrease. It is obviously

5.0 5.0

4.5

Charge 4.5

4.0

bare Ti-0.02 Ti-0.04 Ti-0.06 Ti-0.08

3.5

Potential/V

Potential/V

4.0

1 10 19 41 50

3.5

3.0

3.0

2.5

Discharge 2.5

2.0 2.0

0 0

50

100

150

200

250

300

50

100

150

200

250

300

specific capacity/mAh/g

Specific Capacity/mAh/g Fig. 3. Initial charge/discharge curves of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2 at 0.1C.

Fig. 4. The first cycle, 10th cycle, 19th cycle, 41th cycle and 50th cycle charge/discharge curves of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2.

Y. Kou et al. / Solid State Ionics 296 (2016) 154–157

200

peak at 4.0 V corresponds to the oxidation of Ni2 +/Ni4+. The intense oxidation peak at 4.6 V ascertains the loss of lattice oxygen and concomitant extraction of lithium from the monoclinic lattice. The inactive Li[Li1/3Mn2/3]O2 becomes active [MnO2], and during the following discharge process, high discharge capacity is obtained [20,21]. From the theoretical research, The intense oxidation peak at 4.6 V may be associated with the occupation of tetrahedral sites by lithium, in agreement with the reports of Hayley et al. [22]. As shown in Fig. 6, the anodic peak at about 4.6 V is not much sharper , it may mean that the activation of Li2MnO3-like regions is not completed thoroughly.

0.1C 0.2C

Specific Capacity/mAh/g

160

0.5C 1C 120

bare Ti-0.02 Ti-0.04 Ti-0.06 Ti-0.08

80

40

4. Conclusions

0 0

10

20

30

40

50

Cycle Number Fig. 5. Discharge capacity of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2 at different C-rates.

bare Ti-0.02 Ti-0.04 Ti-0.06 Ti-0.08

0.0015

Current/A

0.0010

The layered Li1.167Ni0.4-xMn0.383Co0.05TixO2 (x = 0, 0.02, 0.04, 0.06 and 0.08) cathode materials have been synthesized via co-precipitation method followed by a solid state reaction. The effects of Ti additive on the structure, morphology and electrochemical properties of Li1.167Ni0.4Mn0.383Co0.05O2 are studied in detail. The results indicate that Ti additive has an effective effect on the improvement of cycling performance of Li1.167Ni0.4Mn0.383Co0.05O2. Within the amount of 0– 0.08, the 0.04 Ti additive shows the best rate capability and highest capacity retention. References [1] [2] [3] [4]

0.0005

[5] 0.0000

[6] [7] [8]

-0.0005

[9] [10]

-0.0010 3.0

157

3.5

4.0

4.5

5.0

Potential/V Fig. 6. CV curves of bare and Li1.167Ni0.4-xMn0.383Co0.05TixO2.

[11] [12] [13] [14] [15] [16]

that the additive of Ti improves the rate capacity. The Li1.167Ni0.36Mn0.383Co0.05Ti0.04O2 (x = 0.04) cathode exhibit the highest discharge capacity compared with other samples at different rates. Fig. 6 shows the CV curves of bare and Li1.167Ni0.4xMn0.383Co0.05TixO2 electrodes in the potential range of 2.5–4.9 V at a scanning rate of 0.1 mV/s. Each of the curves has two main anodic peaks, one at about 4.0 V and another at about 4.6 V (vs. Li/Li+). The

[17] [18] [19] [20] [21] [22]

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