Ni Disorder and Improving the Electrochemical

Jun 26, 2017 - Received: April 24, 2017 ... doping with Ca and a 6% Ca-doped sample exhibited the best .... hexagonal α-NaFeO2 structure (space group...
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Decreasing Li/Ni Disorder and Improving the Electrochemical Performances of Ni-Rich LiNi0.8Co0.1Mn0.1O2 by Ca Doping Minmin Chen,†,‡ Enyue Zhao,†,‡ Dongfeng Chen,§ Meimei Wu,§ Songbai Han,§ Qingzhen Huang,⊥ Limei Yang,† Xiaoling Xiao,*,† and Zhongbo Hu*,† †

College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Department of Nuclear Physics, China Institute of Atomic Energy, Beijing 102413, China ⊥ Center for Neutron Research, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899-6102, United States ABSTRACT: Decreasing Li/Ni disorder has been a challenging problem for layered oxide materials, where disorder seriously restricts their electrochemical performances for lithium-ion batteries (LIBs). Element doping is a great strategy that has been widely used to stabilize the structure of the cathode material of an LIB and improve its electrochemical performance. On the basis of the results of previous studies, we hypothesized that the element of Ca, which has a lower valence state and larger radius compared to Ni2+, would be an ideal doping element to decrease the Li/Ni disorder of LiMO2 materials and enhance their electrochemical performances. A Ni-rich LiNi0.8Mn0.1Co0.1O2 cathode material was selected as the bare material, which usually shows severe Li/Ni disorder and serious capacity attenuation at a high cutoff voltage. So, a series of Ca-doped LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) samples were synthesized by a traditional solid-state method. As hypothesized, neutron diffraction showed that Ca-doped LiNi0.8Co0.1Mn0.1O2 possessed a lower degree of Li/Ni disorder, and potentiostatic intermittent titration results showed a faster diffusion coefficient of Li+ compared with that of LiNi0.8Mn0.1Co0.1O2. The Ca-doped LiNi0.8Mn0.1Co0.1O2 samples exhibited higher discharge capacities and better cycle stabilities and rate capabilities, especially under a high cutoff voltage with 4.5 V. In addition, the problems of polarization and voltage reduction of LiNi0.8Mn0.1Co0.1O2 were also alleviated by doping with Ca. More importantly, we infer that it is crucial to choose an appropriate doping element and our findings will help in the research of other layered oxide materials.

1. INTRODUCTION Lithium-ion batteries (LIBs) have garnered a great deal of attention because of their high capacity, long applicable cycle lives, comparatively lower cost, and better safety.1−10 At present, the commonly used cathode material for commercial rechargeable LIBs is LiCoO2, which displays a high operation voltage and excellent cycle stability.11−14 Nonetheless, the high price of Co and the low theoretical capacity of LiCoO2 limit the performances of LIBs, which cannot fully satisfy the growing demand of portable electronics and electric vehicles.15−19 Layered oxide LiMO2 (M = Mn, Ni, and Co) materials have been studied widely as promising cathode materials to replace LiCoO2 because of their higher capacities, better structural stabilities, and attractive operation voltage ranges.15,20−27 However, many drawbacks restrict the further popularization and application of LiMO2 materials. Because of the similarity radii of Li+ (0.076 nm) and Ni2+ (0.069 nm), a certain degree of mixing of Li+ and Ni2+ between the transition-metal and Li layers is inevitable.12,28 Li/Ni disorder not only decreases the diffusion rate of Li+ but also reduces the amount of Li+ © XXXX American Chemical Society

participating in the charge−discharge reaction. Thus, Li/Ni disorder contributes to the limited rate capabilities and specific capacities of layered LiMO2 materials. Elemental doping with electrochemical inactive ions has been proven to be a valid method to reduce the degree of Li/Ni disorder and improve the performances in layered cathode materials.29−33 Myung et al. used Al and Ti as the adulterants to prepare LiNi0.475Al0.05Mn0.475O2 and LiNi0.5Mn0.45Ti0.05O2, and the cation disorder of LiNi0.5Mn0.5O2 was observably reduced by the substitution of Al and Ti. The pristine showed an initial capacity of about 155 mAh/g and gradually faded to 130 mAh/ g. In contrast, Al-doped samples LiNi0.475Al0.05Mn0.475O2 delivered discharge capacities of about 175 mAh/g with high reversibility, which showed the smallest cation mixing.33 It was reported that the cathode material LiNi0.6Mn0.2Co0.15Al0.025Fe0.025O2 doped with Al and Fe showed higher performance and better cycle stability than those of the Received: April 24, 2017

A

DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX

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voltage reduction of LiNi0.8Co0.1Mn0.1O2 were also mitigated. More importantly, we infer that selecting a doping element with a lower valence state and a larger radius compared to Ni2+ could improve the electrochemical performance and our findings represent a good strategy to decrease Li/Ni disorder of LMO2 materials and enhance their electrochemical performances for LIBs.

pristine material. A considerable decrease of the cationic disorder was exposed by Rietveld refinement analysis after metal substitution, which is closely connected to the improved electrochemical behavior. 10 Kang et al. prepared Li(Ni0.475Mn0.475)Co0.05O2, Li(Ni0.475Mn0.475)Al0.05O2, and Li(Ni0.475Mn0.475)Ti0.05O2 with Co, Al, and Ti as dopants. They found that the discharge performances were increased from 120 mAh/g to 140, 142, and 132 mAh/g, respectively. Meanwhile, there is almost no observation of capacity decay.34 Despite these successes, there is little in the existing literature that reports on the principles of the selection of substitute elements. Theoretical calculations have revealed that the substitution of transition-metal ions by low-valence ions is more favorable for the diffusion of Li+.35 Our previous study also showed that lowvalence Mg was more effective than high-valence Al when used as a doping element for Li-rich materials.28 In addition to the valence, the radius of the doping atoms is another significant property that affects the Li/Ni disorder and electrochemical performance of layered materials. Ceder et al. reported that the layered α-NaFeO2 structure is more stable with an increase of the size difference between cations through theoretical calculations.36 Because low-valence ions are more effective as doping elements, we mainly investigated divalent metal ions. The radii of divalent metal ions are shown in Table 1.37

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. All of the Ca-doped compositions and the pristine material were synthesized using the conventional solid-state process. Stoichiometric amounts of nickel acetate tetrahydrate, manganese acetate tetrahydrate, cobalt acetate tetrahydrate, lithium hydroxide monohydrate, and calcium nitrate terahydrate were thoroughly mixed in an agate mortar and ground. Zhou et al. reported that Ca in the material occupies the Li-ion sites. In this work, LiOH·H2O, Ni(OH)2, Co2O3, and Ca(OH)2 were fully mixed by grinding in a molar ratio of 1:0.8:0.1:0.02. The partial occupancy of Ca2+ ions in the Li lattice sites is due to the evaporation of Li because of the high steam pressure of lithium oxide above 700 °C.40 In our work, in order to ensure Ca in the LiNi0.8Co0.1Mn0.1O2 material occupied by the transition-metal sites, the atomic ratio of LiOH·H2O, NiC4H6O4·4H2O, MnC4H6O4·4H2O, CoC4H6O4·4H2O, Ca(NO3)2· 4H2O was 1.05:0.8(1 − x):0.1:0.1:0.8x. Excess Li was used to remedy the loss of Li evaporation during firing at higher temperature.41,42 The ground mixtures were heated with a heating rate of 5 °C/min at 500 °C for 5 h and then continuously heated with a heating rate of 5 °C/ min at 800 °C for 12 h. When the reaction was complete, the samples were cooled naturally in a furnace to room temperature. 2.2. Materials Characterization. To investigate crystal structures of the samples, X-ray diffraction (XRD) pattern measurements were carried out using an X-ray diffractometer (Persee XD2) equipped with Cu Ka radiation (α = 1.5418) in the 2θ range of 10−80°. The structural parameters were analyzed by the Rietveld refinement program Fullprof 2000, and powder neutron diffraction experiments were performed at room temperature with a high-resolution powder neutron diffractometer (BT-1) at the Center for Neutron Research, NIST. A Cu(311) monochromator was used to produce monochromatic neutron beams with a wavelength of 1.5403 Å. Collimators with horizontal divergences of 15, 20, and 7 ft. were used before and after the monochromator and after the sample, respectively. Inductively coupled plasma (ICP) measurements were carried out on an Agilent 7500ce. 2.3. Electrochemical Measurement. The cathode electrodes were prepared by pasting a mixture of 80 wt % active materials, 10 wt % carbon black, and 10 wt % poly(vinylidene fluoride) in Nmethylpyrrolidinone onto an Al foil current collector with a thickness of 150 μm, and then the electrodes were dried under vacuum at 120 °C for about 12 h. For electrochemical investigation, a coin-type cell (2016) was prepared and consisted of a cathode, Li foil as the counter electrode, a separator, and 1 mol/L LiPF6 in a 1:1 mixture of ethylene carbonate/dimethyl carbonate as the electrolyte. The assembly of the cells was carried out in an Ar-filled glovebox, after which they were aged for 12 h before being electrochemically cycled. Galvanostatic charge−discharge cycling was performed in the 3.0−4.3 and 2.5−4.5 V (vs Li/Li+) potential ranges using an automatic galvanostat (NEWARE) at different current densities of 16, 32, 80, 160, 320, and 800 mA/g at room temperature. PITTs were carried out using an PGSTAT 302N (Metrohm-Autolab) instrument.

Table 1. Radii (nm) of Common Divalent Metal Ions Be2+

Mn2+

Ni2+

Mg2+

Cu2+

Zn2+

0.045 Co2+

0.083 Fe2+

0.069 Pd2+

0.072 Cd2+

0.073 Ca2+

0.074 Ba2+

0.075

0.078

0.086

0.095

0.1

0.135

It can be concluded from Table 1 that, first, Cd2+and Pd2+ are not suitable as doping elements because of their high toxicities and high costs; second, Ba2+, other rare-earth elements, and Be2+ have difficulty forming solid solutions with the parent material, theoretically, because of oversized or undersized radii compared with Ni2+, respectively. Third, Ni2+, Co2+, and Mn2+ are no longer taken into account because they are already contained in the parent material. Excluding the elements mentioned above, the remaining elements are divided into two categories. One category is characterized by similar radii with Ni2+, including Mg2+, Fe2+, Cu2+, and Zn2+. The other category only includes Ca2+ because its radius is bigger than that of Ni2+. On the basis of the above analysis, we hypothesized that Ca, which possesses a lower valence state compared with that of Ni2+ and a larger radius compared with that of Ni2+ as well as that of Li+, is most likely to be an ideal doping element. Meanwhile, we selected the layered cathode material LiNi0.8Co0.1Mn0.1O2, which possesses a high proportion of Li/ Ni disorder because of a higher Ni content, as a representative of LiMO2.38 In order to verify the effectiveness of our hypothesis, a series of Ca-doped LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) samples were synthesized through a conventional solid-state reaction.39 As hypothesized, the electrochemical productivity of LiNi0.8Co0.1Mn0.1O2 was increased markedly by doping with Ca and a 6% Ca-doped sample exhibited the best performance. Neutron diffraction and potentiostatic intermittent titration technique (PITT) showed that Ca doping could effectively reduce Li/Ni disorder of LiNi0.8Co0.1Mn0.1O2 and improve the diffusion coefficient of Li+ (DLi), respectively. Therefore, Ca-doped samples showed higher capacities and better rate performances. In addition, the polarization and

3. RESULTS AND DISCUSSION XRD patterns of the Ca-doped LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) materials are shown in Figure 1. In all cases, no impurity phase was observed, and this indicates that singlephase-layered samples were obtained. All of the peaks are sharp and well-matched, which indicates high crystallinity. All of the diffraction lines of the prepared materials can be indexed as a B

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Figure 1. XRD patterns of LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) samples.

hexagonal α-NaFeO2 structure (space group: R3̅m), in which Li and transition-metal ions occupied the 3b and 3a octahedral sites, respectively. Furthermore, the clear splits of the (006)/ (102) and (108)/(110) couples manifest an ordered layered structure for all of the powders.32 The I(003)/I(104) ratio directly shows cation disorder in the Li layer between Li+ and Ni2+.43 The value of the I(003)/I(104) ratio decreases with an increase of the Ni content, and the value of LiNi0.8Co0.1Mn0.1O2 is about 1.19.38 This is also the reason why we selected LiNi0.8Co0.1Mn0.1O2 as the bare material. In our work, the I(003)/I(104) ratios of LiNi 0.8 Co 0.1 Mn 0.1 O 2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) were 1.186 and 1.198, respectively. A lower I(003)/I(104) ratio shows decreased cation disorder. Besides, the ICP results in Table 2 show that the actual contents of the elements and the theoretical contents in LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) are in good agreement. To study the impact of Ca doping on the electrochemical properties, doped samples and pristine LiNi0.8Co0.1Mn0.1O2 cells with Li anodes were tested in the voltage range of 3.0− 4.3 V at a current density of 0.2C (1C = 160 mAh/g). Figure 2 shows the charge−discharge capacity of LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) electrodes and the capacity curves of LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) at the 1st and 50th cycles, respectively. As shown in Figure 2, the discharge capacities of 2, 4, 6, and 8 mol % Ca-doped electrodes were about 117, 117, 122, and 96 mAh/g after 50 cycles, respectively. Evidently, all of the Ca-doped materials delivered a higher value than the pristine electrode, which only shows 91 mAh/g after 50 cycles. Especially, compared with the pristine electrode, the 6% Ca-doped sample showed the best discharge properties, which increased by 33.3% after 50 cycles. However, the 8% Ca-doped one performs worse than the 6% sample; the reason for this result may be that Ca is not an active substance,

Figure 2. (a) Discharge capacity curves o f the LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) pole in the voltage range of 3.0−4.3 V at 0.2C. Capacity versus potential curves at the (b) 1st and (c) 50th cycles for the LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) electrodes, respectively, in the range of 3.0−4.3 V at 0.2C.

and too much doping will affect the performance of the battery.28 There is a trivial charge curve for the 8% sample at the first cycle different from the others. Perhaps the external environment has a slight influence on the charge−discharge curve. In addition to the first cycle, all other curves, such as the curve at the 50th cycle, are normal. The capacity retention ratios (%) of LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) are shown in Table 3. The values of 2, 4, 6, and 8 mol % Ca-doped electrodes after 50 cycles were about 69%, 78%, 81%, and 67%, respectively. Also, the capacity retention of the 6% Ca-doped

Table 2. ICP Results of LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%)

LiNi0.8Mn0.1Co0.1O2 LiNi0.8(1−x)Mn0.1Co0.1Ca0.8xO2 (x = 6%)

theoretical content actual content theoretical content actual content C

Li

Ni

Co

Mn

Ca

1 1.014 1 1

0.8 0.796 0.752 0.749

0.1 0.1 0.1 0.105

0.1 0.103 0.1 0.101

0.048 0.043

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4.5 V. The pristine and 6% Ca-doped materials were tested between 2.5 and 4.5 V at a current density of 0.2C and activation at a current density of 0.1C for two cycles. Parts a and b of Figure 3 indicate that the Ca substitution materials have a good influence on the capacity retention and enhance the cycle performance. The initial discharge capacities of the pristine and Ca-doped electrodes were about 202 and 195 mAh/g, respectively. From the beginning of the second cycle, two kinds of electrodes exhibit good cycle stability up to 50 cycles. Surprisingly, after 50 cycles, the discharge capacity of the Ca-doped electrode decreased slightly, while the discharge capacity of the pristine electrode declined sharply. Also, at even higher cycle numbers, the Ca-doped material showed a better cycle performance. Specifically, the discharge capacity of the Ca-doped electrode decreased by 8.24% compared with that of LiNi0.8Co0.1Mn0.1O2 after 50 cycles. The discharge property of the Ca-doped sample approached 131 mAh/g after 100 cycles, 27% higher than that of LiNi0.8Co0.1Mn0.1O2, which only was about 103 mAh/g. The capacity retention versus cycle curves for two kinds of cells are plotted in Figure 3b. The capacity retention of the pristine sample reached 90% after 12 cycles at a

Table 3. Capacity Retention of LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) in the range of 3.0−4.3 V at 0.2C discharge capacity (mAh/g) LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 x x x x x

= = = = =

0% 2% 4% 6% 8%

1st cycle

50th cycle

capacity retention ratio (%)

132.95 149.52 151.19 150.20 141.91

91.37 116.72 117.23 121.81 95.6

68.72 78.06 77.74 81.10 67.37

material was also markedly higher than that of the other materials. According to other literatures, the capacity of Li[NixCoyMnz]O2 electrodes can be improved by increasing the relative content of Ni, but the capacity retention results from this study show a nearly linear decrease especially at higher charge voltages.44,45 Therefore, we explored the effect of Ca doping on the cycle performance in the voltage range of 2.5−

Figure 3. (a) Discharge capacity versus cycle number for the pristine and 6% Ca-doped electrodes in the voltage range of 2.5−4.5 V. (b) Capacity retention versus cycle number for the pristine and 6% Ca-doped samples. Voltage versus capacity for LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0 and 6%) electrodes at the (c) 1st and (d) 100th cycles. (e and f) Discharge capacity versus voltage curves at the 1st, 5th, 20th, 50th, and 80th cycles for LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0 and 6%). The inset histograms show variation of the discharge voltage after different cycle numbers. D

DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Rate performance of the pristine and Ca-doped electrodes. The capacity curves of (b) LiNi0.8Co0.1Mn0.1O2 and (c) LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) electrodes at various rates in the voltage range of 2.5−4.5 V.

Figure 5. Neutron diffraction patterns of the (a) pristine and (b) 6% Ca-doped samples by Rietveld refinement. The red dots, black line, blue line, and green vertical tick marks represent the observed, calculated, difference pattern, and Bragg peaks, respectively.

current density of 0.2C. As for the Ca-doped sample, it took 37 cycles to reach the same capacity retention, which is over 3 times that of the pristine sample. This indicates that Ca substitution can effectively inhibit the capacity fading and greatly enhance the cycle performances of these types of materials. It also can be seen from Figure 3c that the discharge property of the Ca-doped material is slightly lower than that of LiNi0.8Co0.1Mn0.1O2 at the first cycle. The reason may be that Ca is not an active substance, and Ca doping decreases the content of Ni, which leads to a decline in the capacity of the battery. The smaller gaps between the charge and discharge

plateau regions in Figure 3d indicate that the LiNi0.8Mn0.1Co0.1O2 electrode doped by Ca shows a lower polarization. Parts e and f of Figure 3 show that the gap of the discharge property curve of the Ca-doped electrode was much smaller than that of LiNi0.8Co0.1Mn0.1O2. The histogram shows that the discharge voltage plateau of the pristine sample decreased rapidly with increasing cycle. Despite this, the Cadoped electrode decreased slowly, indicating that Ca doping can effectively inhibit the voltage drop during charge−discharge cycling. It can be concluded from the result that Ca doping can evidently improve the cycle stability of LiNi0.8Mn0.1Co0.1O2, E

DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry lower polarization, and alleviate the voltage reduction during charge−discharge cycling. Figure 4a shows the rate performances of the pristine and 6% Ca-doped electrodes, which were tested for five cycles at current densities of 0.2, 0.5, 1, 2, and 5C, respectively. Parts b and c of Figure 4 exhibit the capacity curves of the LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) electrodes at different rates in the voltage range of 2.5−4.5 V, respectively. For consistency, we selected the third cycle for both to prepare the capacity curves of the LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1 Mn0.1Ca0.8xO2 (x = 6%) electrodes at various rates. The electrochemical data in Figure 4a at a current density of 0.2C are in agreement with those of Figure 3a, which shows the cycle performance in the voltage range of 2.5−4.5 V at a current density of 0.2C and activation at a current density of 0.1C for two cycles. Specifically, the discharge capacities of the pristine sample at 0.2, 0.5, 1, 2, and 5C are 181, 151, 127, 97, and 52 mAh/g, respectively. The corresponding values for the 6% Ca-doped electrodes are 183, 160, 139, 115, and 74 mAh/g, respectively. By a comparison of the discharge capacities of the two samples, it can be seen that the 6% Cadoped sample displayed a higher property than the pristine sample at different charge−discharge rates from 0.2 to 5C. Meanwhile, with increased charge−discharge current densities, the gap between the discharge capacities of the pristine and 6% Ca-doped samples became larger. For example, the discharge capacity of the 6% Ca-doped material increased by 9.45% compared with that of the pristine sample at 1C, while the value increased by 42.31% at a rate of 5C. From Figure 4, we can conclude that Ca-doped electrodes exhibit excellent rate performances. Especially in the case of high rates, the 6% Cadoped sample has a distinct advantage. From the above results, we find that the 6% Ca-doped electrode shows a higher discharge capacity, better rate capacity, and outstanding cycle stability at high voltages. As is commonly known, the structure of the material directly determines its performance. In order to further clarify the reason for the obtained observations, we employed the Rietveld method to refine the neutron diffraction patterns of LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) and then analyzed the correlation between the crystal structure and electrochemical performance. The results of the Rietveld refinement pattern and the resultant parameters are shown in Figure 5 and Table 4. From the data in Table 4, we can see that doping with Ca can inhibit Li from occupying the transition-metal layer and also can inhibit Ni from occupying the Li layer. In particular, there were 8.1% and 4.7% Ni in the Li interslabs for LiNi0.8Co0.1Mn0.1O2 and 6% Ca-doped samples, respectively. The proportion of Li/Ni disorder was lessened by about 42% for the 6% Ca-doped sample compared to the pristine materials. Different ratios of Li/Ni disorder indicate the following: (1) The lower ratio of Li/Ni disorder signifies less Ni2+ movement into the Li layer, resulting in more active Ni ions to participate in the discharge reaction, which achieves a higher discharge capacity.24 (2) Li/Ni disorder leads to decreases in the layer spacing, and the existence of Ni2+ in the Li layers can disrupt and hamper the Li+ mobility in the Li slab space, which reduces the diffusion rate of Li+12. Therefore, the 6% Ca-doped electrode with a lower ratio of Li/Ni disorder acquired a fast diffusion rate of Li+ and an excellent rate performance. (3) The lower ratio of Li/Ni disorder means that the structure is more perfect, which leads to prominent high voltage stability. Through refinement of the neutron diffraction

Table 4. Rietveld Fitting Parameters for the Pristine and 6% Ca-Doped Sample LiNi0.8Co0.1Mn0.1O2 atom

site

x

y

z

occupancy

Li Ni Ni Li Mn Co O

3b 3b 3a 3a 3a 3a 6c

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0.5 0.5 0 0 0 0 0.25899

0.91896 0.08100 0.71988 0.08100 0.10000 0.10000 1.00000 Rwp = 5.63%

LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) atom

site

x

y

z

occupancy

Li Ni Ni Li Ca Mn Co O

3b 3b 3a 3a 3a 3a 3a 6c

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0.5 0.5 0 0 0 0 0 0.25883

0.95304 0.04704 0.70500 0.04704 0.04800 0.10000 0.10000 1.00000 Rwp = 6.84%

patterns, we learned that the high discharge capacity, excellent rate performance, and prominent high voltage stability can be ascribed to the lessened proportion of Li/Ni disorder. For further identification of the effects of a decreased ratio of Li/Ni mixing, a PITT test was carried out in this work, and the details are described in Figure 6a,b. The test was executed after the cells were charged and discharged after two cycles at a current density of 0.1C and one cycle at the rate of 0.2C between 2.5 and 4.5 V. Assuming that Li transport in the electrode obeys Fick’s second law, DLi can be obtained by the following equation: DLi = −

d ln I 4L2 dt π 2

where L is the thickness of the electrode active material on the Al foil.46 Parts c and d of Figure 6 show examples of the PITT curves of pristine and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) for a potential step of 0.1 V on the discharge process (3.7−3.8 V vs Li/Li+) and its plot of ln(I) versus t. Figure 6e plots the calculate Li diffusion coefficient (DLi) versus discharge voltage. As can be seen in Figure 6e, LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) shows a higher DLi during discharge processes compared to the pristine electrode. For instance, the DLi values of the pristine and Ca-doped electrodes at 3.8 V upon discharge were calculated to be 3.823 × 10−10 and 4.896 × 10−10 cm2 s−1, respectively. The data are in agreement with the cycle and rate performances of the materials. The improved DLi values are closely related to the lower ratio of Li/Ni disorder and resulted in higher electrochemical performance.

4. CONCLUSIONS In this work, we selected Ca, which has a lower valence state and larger radius compared to Ni2+, as the doping element to modify a Ni-rich LiNi0.8Co0.1Mn0.1O2 cathode material. A series of Ca-doped LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 0−8%) F

DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. PITT tests of the (a) pristine and (b) Ca-doped materials. The DLi values of both the pristine and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) electrodes were extracted from a step size of 0.1 V. (c) PITT curves of the pristine LiNi0.8Co0.1Mn0.1O2 and LiNi0.8(1−x)Co0.1Mn0.1Ca0.8xO2 (x = 6%) electrodes as a function of time for a potential step of 3.7−3.8 V after three cycles and (d) their plots of ln(I) versus t. (e) Li diffusion coefficients (DLi) calculated from the PITT curves versus voltage during discharge processes.

the Youth Innovation Promotion Association CAS (Grant 2016152).

samples were successfully synthesized. As hypothesized, the electrochemical performances of LiNi0.8Co0.1Mn0.1O2 were markedly enhanced by doping elemental Ca and the 6% Cadoped material exhibited the most superior performance. The neutron diffraction and PITT results showed that Ca doping can effectively reduce Li/Ni disorder of LiNi0.8Co0.1Mn0.1O2 and improve the diffusion coefficient of Li+ (DLi); thus, the Cadoped samples showed higher capacity and better rate performances. More importantly, we infer that selecting doping elements with lower valence states and large radii compared with Ni2+ would improve the electrochemical performances and our findings demonstrate a good strategy to decrease Li/Ni disorder of LMO2 materials and enhance their electrochemical performances for LIBs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel. +86 10 8825 6655. *E-mail: [email protected]. Tel. +86 10 8825 6655. ORCID

Xiaoling Xiao: 0000-0002-9204-3715 Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Beijing Nova Program (Grant Z141103001814065), the State Key Project of Fundamental Research (Grants 2014CB931900 and 2012CB932504), and G

DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b01035 Inorg. Chem. XXXX, XXX, XXX−XXX