Journal of Solid State Electrochemistry https://doi.org/10.1007/s10008-017-3837-3
ORIGINAL PAPER
Co-precipitation synthesis of precursor with lactic acid acting as chelating agent and the electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium-ion battery Fei Zhou 1,2
&
Lipeng Xu 1,2 & Jizhou Kong 1,2
Received: 20 July 2017 / Revised: 1 November 2017 / Accepted: 15 November 2017 # Springer-Verlag GmbH Germany, part of Springer Nature 2017
Abstract Hydroxide precursor Ni0.5Co0.2Mn0.3(OH)2 was successfully prepared by co-precipitation using lactic acid as the environmentfriendly chelating agent. And the thermodynamics model of hydroxide co-precipitation was proposed. The influence of chelating agent ion concentration on the structure and morphology of the precursors was discussed. The LiNi0.5Co0.2Mn0.3O2 cathode materials were obtained by sintering the mixture of as-prepared Ni0.5Co0.2Mn0.3(OH)2 precursor and Li2CO3. The structural, morphological, and electrochemical performances of LiNi0.5Co0.2Mn0.3O2 cathode materials were investigated by using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and Land battery tester. The results showed that the quasi-spherical LiNi0.5Co0.2Mn0.3O2 with the size of about 5 μm exhibited the excellent electrochemical performance when its Ni0.5Co0.2Mn0.3(OH)2 precursor was synthesized at the molar ratio of 1:1 between lactate ion and transition metal ion. The initial discharge capacity was 194.2 mAh g−1 at 0.1 C, and the discharge capacities of 108.6 and 95.7 mAh g−1 were obtained at 3 and 5 C, respectively. In addition, the capacity retention rate was 93.3% after 100 cycles at 0.2 C. Keywords Lithium-ion battery . Co-precipitation . LiNi0.5Co0.2Mn0.3O2 cathode material . Lactic acid . Electrochemical performance
Introduction Rechargeable lithium-ion batteries have become the predominant power sources for many kinds of electronic devices such as smart mobile phones, laptop computers, electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs) [1–3]. The cathode materials have played an important role in improving the electrochemical performance of lithium-ion batteries in comparison to carbon anode with the capacity above 350 mAh g−1 [4]. At present, lithium cobalt oxide (LiCoO2) has been one of the most
* Fei Zhou
[email protected] 1
State Key Laboratory of Mechanics and Control of Mechanical Structure, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
2
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics and Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology, Nanjing 210016, China
commonly commercial cathode materials. However, some disadvantages of LiCoO2 such as high cost, limited capacity, toxicity of cobalt, instability at high potential (above 4.2 V) inhibit its further application in high-power and pricesensitive fields [5, 6]. LiNi1/3Co1/3Mn1/3O2 transition metal oxide reported by Ohzuku and Makimura [7] in 2001 delivered a large capacity of ~ 200 mAh g−1 at the first discharge with a current rate of 0.1 C over a voltage range of 2.5–4.6 V [8]. Due to its high capacity, excellent thermal stability, inexpensive, and low toxicity [9, 10], the LiNixCoyMn1-x-yO2 would be expected to replace the traditional LiCoO2 cathode material in the next generation of lithium-ion battery. Currently, the hydroxide precursor Ni x Co y Mn 1-x(OH) 2 for the LiNixCoyMn1-x-yO2 cathode materials are y mainly synthesized via co-precipitation using NH3·H2O as chelating agent [11]. As is known, ammonia (NH3) is a toxic, corrosive, and reactive inorganic gas, which is considered a highly hazardous material with an irritating odor to humans and aquatic animals even at a low concentration (> 300 ppm). Furthermore, the release of NH3 could pollute the environment and damage our health; thus, it is
J Solid State Electrochem
imperative to adopt environment-friendly organic chelating agents to synthesize hydroxide precursor with coprecipitation method. He et al. [12] have reported that the hydroxide precursor Mn(OH)2 was synthesized using citric acid and oxalic acid as the chelating agent to control the activity of Mn2+ in the solution, and then the spherical spinel LiMn2O4 cathode materials with tap density as high as 1.9 g cm−3 and the initial discharge capacity reaching 116 mAh g−1 were obtained. Zhang et al. [13] and Zhao et al. [14] have reported that the sol-gel synthesis of LiNi1/3Mn1/3Co1/3O2 and Li[Li0.2Co0.13Ni0.13Mn0.54]O2 was performed using oxalic acid, tartaric acid (TA), and succinic acid (SA) as chelating agents, and then indicated that the tartaric acid-derived cathode materials possess excellent coulombic efficiency. However, tartaric acid is a muscle toxin, which inhibits the production of malic acid. In addition, high doses of tartaric acid could cause human paralysis and death and the corrosion of steel tank reactor. Recently, Zhou’s group [15–17] reported that the spherical cathode materials with excellent electrochemical properties were synthesized using oxalic acid as chelating agent. Unfortunately, oxalic acid has toxic effects through oral consumption and/or skin contact, and cause kidney failure and congenital malformation in the fetus. Actually, the synthesis parameters are the effective and critical factors to improve the electrochemical properties of cathode materials. To obtain hydroxide precursor NixCoyMn1-x-y(OH)2 with the uniform element concentration, the activity of metal ions in the solution should be controlled by using chelating agents. As seen in Table 1, it is obvious that the stability constants of the coordination compounds generated from different chelating agents and transition metal ions (Ni2+, Co2+, Mn2+) are different from each other, which could cause the heterogeneous coprecipitation and poor electrochemical properties. But for lactic acid, its chelating reaction with transition metal ions generates single form of coordination compound, and the stability constants for these coordination compounds are Table 1 Stability constant of coordination compounds generated from different chelating agents and transition metal ion (Ni2+, Co2+, Mn2+) [19] Ligand
Metal ions
The number of ligand /n
log βn
Oxalic acid
Co2+
1,2,3
4.79, 6.7, 9.7
Mn2+
1,2 1,2,3 1,2 1 1,2 1 1 1
3.97, 5.80 5.3, 7.64, ~ 8.5 4.8, 12.5 3.67 5.11, 14.3 1.90 1.43 2.22
2+
Citric acid
Lactic acid
Ni Co2+ Mn2+ Ni2+ Co2+ Mn2+ Ni2+
consisted with the solubility product constants of hydroxide precipitations (Ni(OH)2, Co(OH)2, Mn(OH)2) [18]. This indicates that the transition metal ions could be coprecipitated homogeneously. As is known, lactic acid, also known as 2-hydroxy propionic acid, has a hydroxyl group adjacent to carboxyl group, making it an alphahydroxy acid (AHA). In medicine, lactate is one of the main components of lactated Ringer’s solution and Hartmann’s solution, which can be injected directly into human blood using intravenous fluid infusion. The quasispherical hydroxide precursor (Ni0.5Co0.2Mn0.3)(OH)2 has been synthesized using sodium lactate as chelating agent via co-precipitation method [19], and the corresponding cathode materials exhibit good rate capability and stable cyclability. However, the influence of lactic acid concentration on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode material has not yet been studied until now. In here, lactic acid was used to synthesize the spherical LiNi0.5Co0.2Mn0.3O2 cathode materials. The influences of lactic acid concentration on the structures, morphologies, and electrochemical performances of cathode material were investigated systematically, and the optimum chelating concentration for the synthesis of cathode materials with the excellent electrochemical properties was outlined.
Experimental procedures Synthesis of Ni0.5Co0.2Mn0.3(OH)2 precursor and LiNi0.5Co0.2Mn0.3O2 cathode material LiNi0.5Co0.2Mn0.3O2 was obtained via sintering the mixture of Li2CO3 and co-precipitated manganese-nickel-cobalt hydroxide precursor. The synthesis procedures of Ni 0.5 Co 0.2 Mn0.3(OH)2 precursor were shown in here: (1) the continuous stirred tank reactor was filled with argon gas to avoid the oxidation of Ni0.5Co0.2Mn0.3(OH)2 precursor; (2) the mixture of aqueous solution of NiSO4·7H2O, CoSO4·6H2O, and MnSO4·H2O (cationic ratio of Ni:Co:Mn = 5:2:3) with a concentration of 2 mol L−1 and lactic acid solution (aq.) as a chelating agent were pumped into the continuous stirred tank reactor by using the peristaltic pump at the adding speed of 5 ml/min, respectively. Synchronously, to keep the reaction solution with the pH value of 11.5, the NaOH solution (aq.) of 2 mol L−1 was poured into the same reactor using the peristaltic pump at the adding speed of 15 ml/min. The molar ratio between lactic acid and total transition-metal cations varied in the range of 0.1 ~ 1.2. The reactive temperature and stirring speed were 70 °C and 600 rpm, respectively; (3) after 12 h, the hydroxide precipitates were washed with distilled water repeatedly to remove the residual impurity of Na+,
J Solid State Electrochem
SO42−, etc.; and (4) the Ni0.5Co0.2Mn0.3(OH)2 precursor had been dried at 80 °C for 24 h in air. After that, the mixture of Ni0.5Co0.2Mn0.3(OH)2 precursor and Li2CO3 with the mole ratio of 1:1.1 was preheated at 500 °C for 5 h, and then calcined at 850 °C for 12 h in air. When the above mixture was cooled to room temperature and taken out from tube furnace, the LiNi0.5Co0.2Mn0.3O2 cathode material could be obtained.
Characterization of Ni0.5Co0.2Mn0.3(OH)2 precursor and LiNi0.5Co0.2Mn0.3O2 cathode material The crystalline phases of Ni0.5Co0.2Mn0.3(OH)2 precursor and LiNi0.5Co0.2Mn0.3O2 cathode material were characterized using D8-Advance X-ray diffraction (XRD) (Bruker, Germany) with Cu Kα radiation source (λ = 0.15404 nm). A continuous scan mode was used to collect 2θ data from 10 to 70° at the sampling pitch of 0.02 and the scan rate of 2°/min. The X-ray tube voltage and current were set at 40 kV and 40 mA, respectively, and the incident angle of X-ray beam was 0.25°.The morphologies of Ni0.5Co0.2Mn0.3(OH)2 precursor and LiNi0.5Co0.2Mn0.3O2 cathode material were observed using field-emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL). To measure the tap densities of LiNi0.5Co0.2Mn0.3O2 cathode powder, the cathode powder with the weight of 5 g was first put into the measuring tube with the volume of 10 ml. Then, the measuring tube was vibrated for 1500 times to know the volume of the powders. After that, the tap densities of all powders could be calculated. The electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode material were measured using galvanostatic cycling with two-electrode coin-cells (type CR2025). The procedures to prepare the positive electrode are showed as: (1) 80 wt% LiNi0.5Co0.2Mn0.3O2 powder, 10 wt% acetylene black, and 10 wt% polyvinylidene fluoride (PVDF) were dissolved in Nmethyl-2-pyrrolidone (NMP); (2) the above mixture slurry was coated uniformly on circular aluminum current-collector, and then dried at 110 °C in vacuum oven for 12 h; and (3) the electrodes with the diameter of 12 mm were fabricated via punching the above coated aluminum foil. The CR2025 coin cells were assembled in an argon-filled glove box (Mikrouna Super1220/750/900, Shanghai, China), using the prepared electrodes, metallic lithium foil as the counter electrode, 1 M LiPF6 dissolved in ethyl carbonate and dimethyl carbonate (1:1 in weight) as electrolyte, and a Cellgard 2700 membrane as the separator. Charge and discharge experiments were all performed at different current densities (1 C = 200 mA g−1) between 2.5 and 4.3 V at room temperature using Land battery testers (Land CT2001A, Wuhan, China). The assembled cells were measured for 100 cycles at current densities of 0.2 C (40 mAg−1) to analyze the cycling performances of the materials. The rate capability of LiNi0.5Co0.2Mn0.3O2 cathode material was measured at various coulombic rates (0.1, 0.2, 1.0, 3.0, and 5.0 C).
Results and discussion Influence of lactate ion concentration on the co-precipitation thermodynamics model of Ni0.5Co0.2Mn0.3(OH)2 In here, lactic acid was used as chelating agent to prepare the hydroxide precursor. Actually, when NaOH solution and lactic acid solution were simultaneously added into the reactor, lactic acid firstly reacted with hydroxyl ion to form the lactate ion [L−] (CH3CH(OH)COO−): CH3 CHðOHÞCOOH þ OH− →CH3 CHðOHÞCOO− þ H2 O
ð1Þ
The thermodynamic calculation of chemical equilibrium was determined based on the synthetic process conditions such as the pH value of reaction solution, the concentration of precipitant, and free metal ion. According to Lange’s Handbook of Chemistry [20], the chemical equations of lactate ions combining with the metallic ions and the equilibrium constant (KB) of co-precipitation reaction were summarized in Table 2. For the hydroxide precipitate M(OH)2(s) = M2++ 2OH− (M = transition metal), the concentration of residual metal ions could be calculated from the element mass balance equation expressed as: 2þ M ð2Þ ¼ K sp = K 1 2 102pH ¼ K sp =102pH−28 The concentration of DL-lactate[L], Nickel[Ni], Cobalt[Co], and Manganese[Mn] in the solution could be calculated as: − þ þ þ ½L ¼ ½HL þ ½L þ½NiL þ ½CoL þ ½MnL ð3Þ ¼ ½L− 10−pH =K 2 þ 1 þ K 3 Ni2þ þ K 8 Co2þ þ K 14 Mn2þ
½Ni ¼ Ni2þ þ NiðOHÞþ þ NiðOHÞ2 þ NiðOHÞ3 − þ ½NiLþ 2þ ¼ Ni 1 þ K 5 10pH−14 þ K 6 102pH−28 þ K 7 103pH−42 þ K 3 ½L− ;
ð4Þ
i h ½Co ¼ Co2þ þ CoðOHÞþ þ CoðOHÞ2 þ CoðOHÞ3 − þ CoðOHÞ4 2− þ ½CoLþ 2þ 1 þ K 10 10pH−14 þ K 11 102pH−28 þ K 12 103pH−42 þ K 13 104pH−56 þ K 8 ½L− ; ¼ Co
ð5Þ ½Mn ¼ Mn2þ þ MnðOHÞþ þ MnðOHÞ3 − þ ½MnLþ 2þ ¼ Mn 1 þ K 16 10pH−14 þ K 17 103pH−42 þ K 14 ½L− :
ð6Þ
Then, the above formula were solved using NewtonRaphson method. The variation of the residual metal ion concentrations with pH value at different lactate ion concentrations is shown in Fig. 2. Finally, the chemical co-precipitation
J Solid State Electrochem Table 2
Chemical equations and equilibrium constant (KB) of co-precipitation reaction
Reaction
logKB
Reaction
logKB
H2O = H+ + OH− HL = L− + H+ Ni2+ + L− = NiL+ Ni(OH)2(s) = Ni2+ + 2OH− Ni2+ + OH− = Ni(OH)+ Ni2+ + 2OH− = Ni(OH)2 Ni2+ + 3OH− = Ni(OH)3− Co2+ + L− = CoL+ Co(OH)2(s) = Co2+ + 2OH−
logK1 = −14.00 logK2 = −3.858 logK3 = 2.22 logK4 = −15.26 logK5 = 4.97 logK6 = 8.55 logK7 = 11.33 logK8 = 1.90 logK9 = −14.23
Co2+ + OH− = Co(OH)+ Co2+ + 2OH− = Co(OH)2 Co2+ + 3OH− = Co(OH)3− Co2+ + 4OH− = Co(OH)42− Mn2+ + L− = MnL+ Mn(OH)2(s) = Mn2+ + 2OH Mn2+ + OH− = Mn(OH)+ Mn2+ + 3OH− = Mn(OH)3−
logK10 = 4.3 logK11 = 8.4 logK12 = 9.7 logK13 = 10.2 logK14 = 1.43 logK15 = −12.72 logK16 = 3.9 logK17 = 8.3
occurred between the weak complex and hydroxy ion to form the hydroxide precursor Ni0.5Co0.2Mn0.3(OH)2. The curves of three metal ions are shown in Fig. 1. As seen in Fig. 1, when the pH value was lower than 11.0, all the metal ion contents decreased as the pH value increased, and a similar variation trend was observed. The highest precipitation efficiency of metal ions was shown at different pH values (pH(Ni2+) = 11.0, pH(Co2+) = 10.5, pH(Mn2+) = 12.5). Although the amount of Ni2+ and Mn2+ ions were different, their residual concentration distributions were basically same at different lactate ion concentrations. Until the molar ratio between lactate ion and metal cations reach 1:10, the residual concentration distributions of Ni2+ and Mn2+ were obviously different (Fig. 1d), which resulted from the different stability constant of coordination of lactate ion. 1
1
(a) [L]:[M]=1.2:1
0
Ni Co Mn
(b) [L]:[M]=1:1
0
Ni Co Mn
-1 log{ [ M ] / ( mol · L -1 ) }
log{ [M ]/(mol · L -1)}
-1 -2 -3 -4 -5 -6
-2 -3 -4 -5 -6 -7
-7
-8
-8 7
1
8
9
10 11 pH value
12
13
7
14
Ni Co Mn
9
10
11
12
13
(d) [L]:[M]=1:10
0
-2 -3 -4 -5
-2 -3 -4 -5
-6
-6
-7
-7
-8
14
Ni Co Mn
-1 log{ [M ]/(mol · L -1)}
-1
8
pH value 1
(c) [L]:[M]=0.8:1
0
log{ [M ]/(mol · L -1)}
Fig. 1 Plots of log[M]-pH at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
The relationship between the concentration of residual metal cations M2+ (M = Ni, Co, Mn) and pH value at different lactate ion concentrations was shown in Fig. 2. It was clear that the residual concentration of Ni, Co, and Mn decreased initially, and then increased as the pH value increased from 7 to 14. The solubility varies with the kinds of metal ions. Before reaching the minimum value of the curve (the highest precipitation efficiency), the concentration of residual metal ion M2+ increased with increasing the lactate ion concentration. Figure 2a shows that the trend of the concentration of Ni2+ was affected by the pH value at different lactate ion concentrations. When the pH value was higher than 11.0, the residual Ni2+ tended to be uniform and increased as the pH value increased. For Co ions, it was obvious from Fig. 2b that the residual Co2+ tended to be uniform at a pH value lower than 10.5. But after reaching the minimum value, Co2+ had a
-8 7
8
9
10 11 pH value
12
13
14
7
8
9
10 11 pH value
12
13
14
J Solid State Electrochem 1
-1
(a) M=Ni
[L] :[M]= 1:10 [L] :[M]= 0.8:1 [L] :[M]= 1:1 [L] :[M]= 1.2:1
-1 -2 -3 -4 -5
[L] :[M]= 1:10 [L] :[M]= 0.8:1 [L] :[M]= 1:1 [L] :[M]= 1.2:1
-3 -4 -5 -6 -7
-6 -7
(b) M=Co
-2 log{ [Co ]/(mol · L -1)}
0 log{ [Ni ]/(mol · L -1)}
Fig. 2 The log[M]-pH of the different metal cations at different lactate ion concentrations
7
8
9
10 11 pH value
12
13
14
-8
7
8
9
10 11 pH value
12
13
14
1
(c) M=Mn
0
[L] :[M]= 1:10 [L] :[M]= 0.8:1 [L] :[M]= 1:1 [L] :[M]= 1.2:1
log{ [Mn ]/(mol · L -1)}
-1 -2 -3 -4 -5 -6 -7 -8 -9
plateau when the pH value varied from 10.5 to 12.0. As seen in Fig. 2c, a similar variation trend of Mn ions with pH value was observed, just the residual Mn2+ ion tended to be uniform at the pH value higher than 11.5. The residual metal cations M2+ increased as the lactate ion concentration increased, so C3H6O3 as the chelating agent could slow down the reaction speed.
Influence of lactate ion concentration on the structural characteristics of Ni0.5Co0.2Mn0.3(OH)
7
8
9
10 11 pH value
12
13
14
among all patterns was not obvious at the different lactate ion concentrations. Figure 4 shows the morphologies of Ni0.5Co0.2Mn0.3(OH)2 precursor synthesized at different lactate ion concentrations. It was obvious that all the precursor particles had a spherical morphology with an average diameter of 5 μm. Actually, the lower stability constant of complex compounds was, the higher concentration of metal ions in the solution was. This meant that the precipitation reaction between metal ions and OH− occurred rapidly. Thus, the heterogeneous particle size distribution was observed at the lower chelating agent
2
(d)1:10
Intensity(a.u.)
Figure 3 shows the XRD patterns of Ni0.5Co0.2Mn0.3(OH)2 precursor synthesized at different lactate ion concentrations. It was obvious that the XRD patterns of Ni0.5Co0.2Mn0.3(OH)2 precursor in here were extremely similar with those using NH3·H2O as chelating agent [21–23]. All patterns were similar, and had low-diffraction peak intensity and broad peaks. Because the individual metal hydroxides such as nickel hydroxide, cobalt hydroxide, and manganese hydroxide could not be detected in Fig.3, so the hydroxide precursor was not the mixture of individual metal hydroxide (polyphase system). According to the XRD analysis in Fig.3, the hydroxide precursor was layered with ternary hydroxide with low crystallinity, and Ni, Co, and Mn elements were distributed homogeneously within Ni 0 . 5 Co 0 . 2 Mn 0 . 3 (OH) 2 particle. Furthermore, due to the homogeneous mixed small size particles, the diffraction peaks were quite broad. The difference
(c)0.8:1 (b)1:1
(a)1.2:1
10
20
30
40 o 2θ ( )
50
60
70
Fig. 3 XRD patterns of Ni0.5Co0.2Mn0.3(OH)2 synthesized at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
J Solid State Electrochem Fig. 4 SEM images of Ni0.5Co0.2Mn0.3(OH)2 synthesized at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
(110)/(108)
(113)
(107)
(104) (105)
(d)1:10
(101) (006)/(102)
Figure 5 shows the XRD patterns of LiNi0.5Co0.2Mn0.3O2 cathode materials corresponding to its precursors synthesized at different lactate ion concentrations. It was clear that all patterns could be indexed to a single phase of αNaFeO2 type with space group R-3m. This was attributed to the Miller indices (006, 102) and (108, 110) specific characteristics of layered structure [26, 27]. Table 3 lists the lattice parameters of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursors synthesized at different lactate ion concentrations. Generally speaking, the higher value of I(003)/I(104) ratio meant lower cation mixing degree [16, 28]. When the value of I(003)/I(104) ratio was below 1.2, this indicated undesirable cation ordering leading to poor electrochemical performance [29]. Furthermore, the c/a ratio revealed the well-defined layered structure. When the c/a ratio was higher than 4.9, the cathode material possessed good layered characteristics [30, 31]. As seen in Table 3, the values of I(003)/I(104) and c/a ratios for all samples were
(003)
Microstructure of LiNi0.5Co0.2Mn0.3O2 cathode corresponding to its precursor synthesized at different lactate ion concentrations
higher than 1.2 and 4.9. This indicated that all samples displayed better layered structure. The maximum values of I(003)/I(104) (1.82) and c/a (4.96) ratios for LiNi0.5Co0.2Mn0.3O2 cathode material were obtained simultaneously when the Ni0.5Co0.2Mn0.3(OH)2 precursor was synthesized at the molar ratio of 1:1 between lactate ion and total metal cations. This indicated that the LiNi0.5Co0.2Mn0.3O2 cathode material possessed the optimal layered structure and excellent electrochemical performance. Table 3 lists the tap density of LiNi0.5Co0.2Mn0.3O2
Intensity(a.u.)
concentration (the molar ratio of lactate ion and transition metal ion = 1:10). When the lactate ion concentration increased, the uniform and spherical particles with narrower particle size distribution were formed owing to the complex compounds with relatively high stability and the slow release of metal ions [24, 25].
(c)0.8:1 (b)1:1 (a)1.2:1
10
20
30
40
50
60
70
o
2θ ( ) Fig. 5 XRD patterns of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
J Solid State Electrochem Table 3 Lattice parameters and tap density of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations
n(L−):n(Ni2+ + Co2+ + Mn2+)
a(Å)
c(Å)
c/a
I(003)/I(104)
Tap density (g/cm3)
1.2:1
2.863
14.154
4.94
1.78
1.80
1:1 0.8:1
2.867 2.862
14.223 14.141
4.96 4.94
1.82 1.65
1.95 1.96
1:10
2.876
14.196
4.95
1.49
2.15
cathode materials. It was clear that the tap density of LiNi0.5Co0.2Mn0.3O2 cathode materials increased with an increase in the lactate ion concentration. As seen in Fig. 6, the LiNi0.5Co0.2Mn0.3O2 particles retained spherical shape with different size, the first particles were aggregated into second particles even if they were calcinated at high temperature. This showed the more homogeneous composition of the prepared samples [32]. When the lactate ion concentration increased, the Ni0.5Co0.2Mn 0.3 (OH) 2 precursors displayed uniform and spherical particles with narrow size distribution, but the average size for the LiNi 0.5 Co 0.2 Mn0.3O2 particles decreased from 5 to 3 μm, and the sphericity of particles became more complete. Because the low lactate ion concentration led to the low complexation ability in solution, the synthesis dynamic of precursor was biased toward the nucleation of new grains, and the growth rate of grain became low; thus, the precursor particle size became small, and then its corresponding cathode material particle size also became small, which caused the high tap density showed in Table 3.
Fig. 6 SEM images of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
Electrochemical properties of LiNi0.5Co0.2Mn0.3O2 cathode corresponding to its precursor synthesized at different lactate ion concentrations Figure 7 shows the initial charge/discharge curves of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations. It is clear that all samples showed good charge-discharge performance at the current density of 0.1 C in the voltage range of 2.5 ~ 4.3 V. A steady plateau was observed within charge-discharge period. Moreover, the mid-value voltage was 3.75 V, which was corresponded to the Ni2+/Ni4+ redox reaction occurred in this region [33]. When the lactate ion concentration for synthesizing Ni0.5Co0.2Mn0.3(OH)2 precursor increased, the initial charge/discharge capacities of its LiNi0.5Co0.2Mn0.3O2 cathode also increased. When the Ni0.5Co0.2Mn0.3(OH)2 precursor was synthesized at the molar ratio of 1:1, its corresponding LiNi0.5Co0.2Mn0.3O2 cathode material exhibited the highest charge/discharge capacities of 229.4/194.2 mAh g−1, but when the Ni0.5Co0.2Mn0.3(OH)2 precursor was synthesized
J Solid State Electrochem 4.8
3.2 1.2:1 1:1 0.8:1 1:10
1.6
0
50
100
150
200
250
-1
Capacity (mAh·g )
Fig. 7 Initial charge/discharge curves of LiNi 0.5 Co 0.2 Mn 0.3 O 2 corresponding to its precursor synthesized at different lactate ion concentrations
at the molar ratio of 1.2:1, the initial charge/discharge capacities for its corresponding LiNi0.5Co0.2Mn0.3O2 cathode material decrease to 213.0/184.6 mAh g−1. This indicated that the homogeneous precipitation of transition-metal cations might be inhibited owing to the complex compounds with higher stability [24] when the molar ratio between lactate ion and total metal cations was higher than one. Figure 8 shows the rate capability of LiNi0.5Co0.2Mn0.3O2 cells operated at different rates (0.1, 0.2, 1, 3, 5 C). It was obvious that the discharge plateau of all samples disappeared gradually when the current density increased due to the polarization of electrodes at high current [34]. Among them, the
4.8
4.8
0.1 C 0.2 C 1.0 C 3.0 C 5.0 C
Voltage(V)
(a)1.2:1 4.0
0
50 100 150 -1 Capacity (mAh· g )
200
4.0
3.2
2.4
0
50
100
-1
150
Capacity (mAh· g )
4.8
4.8 0.1 C 0.2 C 1.0 C 3.0 C 5.0 C
(c)0.8:1 4.0
3.2
2.4 0
0.1 C 0.2 C 1.0 C 3.0 C 5.0 C
(b)1:1
3.2
2.4
Voltage(V)
Fig. 8 Rate capabilities of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations. The molar ratio of lactate ion to metal cations: (a) 1.2:1, (b) 1:1, (c) 0.8:1, and (d) 1:10
Voltage(V)
2.4
50
100
-1
Capacity (mAh· g )
150
200
200
0.1 C 0.2 C 1.0 C 3.0 C 5.0 C
(d)1:10
Voltage(V)
V o lta g e (V )
4.0
LiNi0.5Co0.2Mn0.3O2 cathode corresponding to its precursor synthesized at the molar ratio of 1:1 exhibited the excellent rate capabilities, and the corresponding discharge capacities were 194.2,160.7,132.8,108.6, and 95.7 mAh g−1, respectively. It revealed that the rate capability of LiNi0.5Co0.2Mn0.3O2 samples could be improved when their Ni0.5Co0.2Mn0.3(OH)2 precursors were synthesized at the optimum lactate ion concentration. Figure 9 shows the cycle performance of LiNi0.5Co0.2 Mn0.3O2 cathode corresponding to its precursor synthesized at different lactate ion concentrations at 0.2 C rate. It was clear that the LiNi0.5Co0.2Mn0.3O2 cathode showed the highest capacity retention of 93.3%, with a discharge capacity of 145.2 mAh g −1 after 100 cycles when its Ni 0.5 Co 0.2 Mn0.3(OH)2 precursor was synthesized at the molar ratio of 1:1. The fluctuation in cycle performance curves was mainly influenced by the change of external environment temperature in the charge and discharge process. As is known, the suitable chelating agent concentration was effective to form the Ni0.5Co0.2Mn0.3(OH)2 precursor with the uniform and spherical particles with narrower particle size distribution, and then its LiNi0.5Co0.2Mn0.3O2 cathode displayed stable layered structure and low cation mixing [24, 25]. Because the maximum values of I(003)/I(104) (1.82) and c/a (4.96) ratios of LiNi0.5Co0.2Mn0.3O2 sample were obtained simultaneously when its Ni0.5Co0.2Mn0.3(OH)2 precursor was prepared at the molar ratio of 1.0; thus, it exhibited the excellent electrochemical performance.
4.0
3.2
2.4 0
50
100
-1
Capacity (mAh· g )
150
200
J Solid State Electrochem 200
-1
C a p a c ity (m A h · g )
0.2 C 150
3.
4.
100
5. 1.2:1.0 1.0:1.0 0.8:1.0 1.0:10
50
0
0
20
40
60
80
6.
100
Cycle number N Fig. 9 Cycle performance of LiNi0.5Co0.2Mn0.3O2 corresponding to its precursor synthesized at different lactate ion concentrations
7. 8.
9.
Conclusions Ni0.5Co0.2Mn0.3(OH)2 precursors were successfully synthesized by co-precipitation method using lactate ion as the chelating agent. At first, the thermodynamic model of hydroxide co-precipitation was proposed, then the LiNi0.5Co0.2Mn0.3O2 cathode materials were obtained by sintering the mixture of as-prepared Ni0.5Co0.2Mn0.3(OH)2 precursor and Li2CO3. When the lactate ion concentration increased, the Ni0.5Co0.2Mn0.3(OH)2 precursors displayed uniform and spherical particles with narrow size distribution, and then the sphericity of LiNi0.5Co0.2Mn0.3O2 particles became more complete with the average particles size increasing from 3 to 5 μm, the capacity of LiNi0.5Co0.2Mn0.3O2 samples first increases, and then decreases. When the Ni0.5Co0.2Mn0.3(OH)2 precursor was synthesized at the molar ratio of 1:1 between lactate ion and transition metal ion, its LiNi0.5Co0.2Mn0.3O2 cathode showed the highest discharge capacity (194.2 mAh g−1) and retention rate (93.3%) after 100 cycles.
10.
11.
12.
13.
14.
Funding information This work is financially supported by the Special Fund of the Scientific and Technological Achievements Transformation Project in Jiangsu Province (No.BA2013142), Jiangsu Province Natural Science Fund Project (No. BK20130800), Fundamental Research Funds for the Central Universities (No. NS2014054), Funding of Shanghai Academy of Spaceflight Technology (No. SAST201371), Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
15.
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