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Electrochemical properties of Mg-added lithium nickel cobalt oxide induced by structural characteristics depending on the synthetic process
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Electrochemical properties of Mg-added lithium nickel cobalt oxide induced by structural characteristics depending on the synthetic process ⁎
Gene Jaehyoung Yang, Yongseon Kim
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Powders: chemical preparation B. Defects C. Electrical properties D. Transition metal oxides E. Batteries
The doping of various foreign elements into Ni-based layer-structured cathode materials has been investigated to improve their electrochemical performance. However, the dependence of structural features and the resultant electrochemical performance on the processing methods remains unclear. In this study, the effects of varied Mg addition methods on the properties of lithium nickel cobalt oxide (LNCO) are investigated by experimental and first-principles simulation approaches. Coprecipitating Mg with the transition metals causes no differences in the morphological and initial charge/discharge characteristics compared to the method of mixing the Mg-source powder with the other raw materials, but performance relating to long-term stability, such as cyclic capacity retention and swelling by gas evolution, is greatly improved. The simulation results indicate that cases in which Mg is doped within the LNCO crystal and in which Mg forms a separate MgO phase are both thermodynamically allowable, and that increasing the Mg content in the LNCO crystal structure suppresses the formation of VO- and NiLi-type point defects. This is proposed as a new mechanism of structural stabilization by Mg doping that explains the improved stability with the introduction of Mg by coprecipitation.
1. Introduction Lithium ion batteries (LIBs) are widely used as power supplies for mobile electronic devices, with application extending to mid- and largesized systems such as electric vehicles (EVs) and energy storage systems [1–3]. Despite the rapid growth of the market, the technical performance of LIBs does not satisfy consumer expectations for the energy storage density, charge/discharge speed, safety, and price. Most of all, increases in energy capacity are constantly in high demand. Smartphone usage continuously lengthens with the integration of new functions, while vehicular mileage per charge is considered to determine the future of the EV industry [4,5]. Adopting high-capacity cathode materials is one way to address this demand, and new materials that can replace the conventionally used LiCoO2 (LCO) are actively researched [4,6–10]. Unfortunately, new materials still cannot fully satisfy every aspect of performance required for application in current LIB systems; therefore, the most pragmatic near-term solution for LIB cathodes may be the development of Nibased layer-structured materials with the same crystal structure as LCO but higher energy capacities. The commercialization of some materials in this category, represented by LiNi1−x−yCoxAlyO2, is actively underway [6,7,11,12]. It is well known that increased capacity under conventional
⁎
charging voltage conditions (generally 4.2–4.3 V) is obtainable as the amount of Ni replacing Co increases, but the thermal and structural stability of the cathode degrades simultaneously, decreasing the safety of the LIBs. To address this problem, the doping of various impurity elements has been tested to achieve higher stability in the crystal structure after delithiation [13–18]. Not only the selection of the doping element, but also the doping methodology has been examined by researchers: sol-gel, coprecipitation, and hydrothermal routes have been tested, as well as conventional solid-state reactions which mainly comprise raw-material powder mixing and high-temperature calcination processes [19–22]. In spite of extensive research on the topic, it remains necessary to systematically investigate the effects on the structural and electrochemical properties induced by different doping processes. Although many reports exist on synthetic methods for doping or the optimization of processing conditions, organized examinations of the interrelations among the doping and synthesis methods, the structural and defect characteristics, and the short- and long-term electrochemical performances of the synthesized materials are unavailable, to the best of our knowledge. To accurately interpret the mechanisms underlying changes in properties with the introduction of foreign elements, it is necessary to proactively examine whether the elements are doped in the crystal as intended or segregated on the surfaces or grain boundaries, as well as
Corresponding author. E-mail address: [email protected] (Y. Kim).
http://dx.doi.org/10.1016/j.ceramint.2017.10.175 Received 26 September 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Yang, G.J., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.10.175
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significantly decreased as the Mg-content exceeded 2.0%, thus indicating 1.0–1.5% to be the optimum doping amount of Mg. The powder mixture was placed in Al2O3 crucibles and heated at 800 °C for 10 h under flowing O2. The metal-hydroxide precursors were prepared by a coprecipitation method. NiSO4·6H2O and CoSO4·7H2O, with or without MgSO4·7H2O (all EP grade from Samchun Chemicals, Korea), were dissolved in distilled water. The solution of 1 M concentration was injected into a 2-L reactor at the rate of 50 mL/h. An ammonia solution (~ 28–30%, Samchun Chemicals, Korea) was added to the reactor, maintaining the ratio of NH3 to Ni + Co (+ Mg) at 1:1. The pH inside the reactor was controlled at 11.5–12.0 by changing the injection rate of an aqueous NaOH solution (EP, Duksan Pure Chemicals, Korea). After 12 h of injection while stirring the solution inside the reactor, the coprecipitated powder was washed several times with water and dried at 120 °C. 2032-type coin cells were fabricated using the synthesized samples as the cathode, and the electrochemical properties were measured. The active cathode material and acetylene black (AB) were mixed with a solution of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidone. The weight ratio of the active material, AB, and PVDF was set at 94:3:3 wt%. This slurry was coated onto an Al foil, dried, and rollpressed to produce the cathode. The loading level was fixed at ~ 15 mg/cm2. After vacuum-drying of the cathode, coin cells were fabricated. Li metal foil, 16-μm-thick polypropylene porous film (CS Tech, Korea), and 1.3-M LiPF6 solution (Panax Etec, Korea) in a 3:3:4 v/v mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were used as the counter electrode, separator, and electrolyte, respectively. The initial charge/discharge profile was obtained from galvanostatic measurement at rates of 0.2, 0.5, 1.0, and 0.2 C (1 C = 190 mA/ g), and then the cyclic performance was monitored with repeated charge/discharge at a 1-C rate between 3.0 and 4.3 V vs. Li+/Li. Gas evolution during a high-temperature storage test was measured from the volume change of pouches containing cathodes separated from the coin cells after the first charge cycle and fresh electrolyte solution. The pouch volume was measured using Archimedes’ principle; thus, the amount of gas generated inside the pouch was monitored with respect to the time for which the pouch was held at 60 °C.
whether the introduction of the dopants induces the formation of other point defects. Such secondary point defects may cause property changes rather than the dopant itself. With these considerations, this study investigated Mg-added lithium nickel cobalt oxide (Mg-added LNCO) through experimental and firstprinciples simulation methods. Two different doping processes of Mg coprecipitation with Ni and Co during the preparation of the metal hydroxide precursor and a general solid-state synthesis of mixing the Mg source with other raw-material powders were implemented. The properties of the resulting materials were comparatively investigated. The thermodynamically stable states and related point defects were theoretically analyzed by simulating phase diagrams including various types of defect-containing phases based on first-principles calculations. Combining the experimental and computational investigations, the synthesis process-dependent structural features, and the associated electrochemical properties were interpreted. The coprecipitation and solid-state reactions examined in this study are the most widely used methods for the synthesis of layer-structured cathode materials in the LIB industry, yet the timing and manner for adding doping elements during the synthesis remains controversial. The results of this study are expected to yield useful information on these matters, thereby guiding the design of optimized production processes for cathode materials.
2. Material and methods 2.1. Experimental Mg-added LNCO samples were prepared with different synthetic processes (Fig. 1). One sample was obtained by calcining a mixture of LiOH·H2O (98%, Duksan Pure Chemicals, Korea), (Ni, Co)(OH)2 precursor, and Mg(OH)2 (extra pure (EP), Samchun Chemicals, Korea) powders, whereas another sample was synthesized using (Ni, Co, Mg) (OH)2 and LiOH·H2O as the starting material, thus providing varied processing conditions of the timing and manner for adding Mg. The ratio of Li to Ni +Co + Mg was set at 1.05, considering the loss of Li during calcination. The raw materials of Ni, Co, and Mg were mixed with a stoichiometric ratio targeting the synthesis of Li (Ni0.835Co0.150Mg0.015)O2. The Mg-content was determined by referring to our previous experiments, which showed the discharge capacity
Fig. 1. Schematic of the experimental and first-principles simulation processes of this study.
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2.2. First-principles simulation
Table 1 Phases of [LNCMgO + defects] and [LNCO + defects + MgO] included in the simulation of phase diagrams. Mg is assumed to be doped in the LNCO crystal for the [LNCMgO + defect] combinations, while it forms a secondary MgO phase for [LNCO + defect + MgO] combinations.
The phase diagram of the Li–M–O (M = 20/24Ni + 3/24Co + 1/ 24Mg) system was simulated as a function of temperature, by comparing the normalized Gibbs free energy G̅ of the phases [23]:
LNCMgO + defects
G[Lia Mb Oc (CO2)d (H2 O)e] =
G[Lia Mb Oc (CO2)d (H2 O)e]−dGCO2 −eGH2O a+b+c
The equation was designed to reflect the energy changes related to the addition or removal of CO2 and H2O, which enabled the examination of hydroxide or carbonate phases as well as the metallic and oxide states. By assigning a 3-D coordinate point to each phase (x and y coordinates according to the chemical composition and z coordinate according to G ) and taking the 2-D projection of the convex hull [24] in the – z direction, ternary phase diagrams showing stable phases and their tie-lines could be obtained [25,26]. The energy of each solid phase was calculated by the density functional theory (DFT) method, based on the Perdew–Burke–Ernzerhof (PBE) generalized-gradient approximation with Kresse–Joubert projector augmented-wave (KJPAW)-type PBEsol pseudopotentials [27,28]. QUANTUM-ESPRESSO code [29] was used with a kinetic energy cutoff of 50 Ry for the wavefunctions and a convergence threshold of 10−3 Ry/Bohr on the forces for ionic minimization. The k-point spacing equivalent to 0.3 Å−1 was used, with full relaxation of atomic positions and lattice vectors allowed. The Gibbs free energy of each solid phase was calculated as a function of temperature using the isobaric heat capacity obtained from quasi-harmonic approximation (QHA) calculations provided in the Thermo-PW software package [30]. The standard-state chemical potentials of the gas phases were determined semi-empirically to minimize discrepancies between the experimentally reported formation energies of oxide, hydroxide, or carbonate materials and those obtained from the DFT calculations (see Refs. [26,31] for details). The effects of temperature and pressure on the gas-phase chemical potential were accommodated by referring to the JANAF Thermochemical Table [32]. The partial pressures of O2, CO2, and H2O were fixed at 0.02, 3.0 × 10−5, and 1.5 × 10−3 MPa, respectively, during the simulations assuming a typical ambient atmosphere. Various types of associated Mg and point defects were designed by changing the Mg doping site and combining the point defects: combinations of MgNi, MgLi, and Mgtet with VLi, VO, VNi, NiLi, and LiNi were investigated. Point defects other than these were excluded by prescreening because they had high formation energies. Thus, 22 types of [Mg-doping + point defect(s)] combinations are considered. Model crystals for DFT calculations of these phases were obtained by introducing each combination to a Li24M24O48 frame crystal in which three Co atoms substitute at Ni sites with positions optimized in advance. Differently sized frame crystals were used as necessary for cases including Mgtet or MgLi. For each case, the most stable arrangement of Mg and the associated defects was determined by changing their positions and comparing the energies. In addition to these Mg-doped models (LNCMgO), cases in which Mg was not doped in the crystal but instead formed a secondary MgO phase (LNCO + MgO) were considered. Thus, 39 total cases of [LNCMgO + defect(s)] or [LNCO + defect(s) + MgO] were produced (Table 1). The [LNCMgO + defects(s)] phases, in which Mg is doped in the LNCO crystal, are expressed as numbers in solid circles, such as ➊, ➋, ➌…, whereas the [LNCO + defect(s) + MgO] phases, in which Mg is not doped but forms a secondary MgO phase, are indicated as emptycircle numbers (①, ②, ③…) + MgO. These were included in the simulation of the phase diagrams as if each were an independent phase, in addition to the regular phases of the Li–M–O system as reported in the Inorganic Crystal Structure Database [33].
LNCO + defects + MgO
Description
Defects
Description
Defects
➊ ➋–1 ➋–2 ➌ ➍ ➎–1 ➎–2 ➏–1 ➏–2 ➐ ➑–1 ➑–2 ➒ ➓–1 ➓–2
MgNi MgNi + NiLiLiNi MgLi + LiNi Mgtet + VNi MgNi + VLi MgNi + NiLiVNi MgLi + VNi MgNi + VLiNiLiLiNi MgLi + VLiLiNi MgNi + VO MgNi + VONiLiLiNi MgLi + VOLiNi MgNi + VOVLi MgNi + VONiLiVNi MgLi + VOVNi MgNi + NiLi MgLi Mgtet + VLi MgNi + VONiLi MgLi + VO MgNi + VLiNiLi Mgtet
① + MgO ②–1 + MgO ②–2 + MgO ③ + MgO ➃ + MgO ➄ + MgO
No defect NiLiLiNi LiNi VNi VLi NiLiVNi
➅–1 + MgO ➅–2 + MgO ⑦ + MgO ➇–1 + MgO ➇–2 + MgO ➈ + MgO ➉–1 + MgO ➉–2 + MgO ⑪ + MgO
VLiNiLiLiNi VLiLiNi VO VONiLiLiNi VOLiNi VOVLi VONiLiVNi VOVNi NiLi
⑬ + MgO
VONiLi
⑭ + MgO
VLiNiLi
⓫–1 ⓫–2 ⓬ ⓭–1 ⓭–2 ⓮ ⓯
3. Results 3.1. Experimental results Scanning electron microscopy (SEM) images of the metal-hydroxide precursors and the active materials prepared using the precursors are presented in Fig. 2. Comparing the morphologies of the metal-hydroxide precursors (Fig. 2(a, d)), it appears that particles of the Mg-containing material (Fig. 2(a)) show dimply surface. However, the overall morphological characteristic of small, needle-shaped primary particles is shared by the hydroxide precursors both with and without Mg. As can be seen from Fig. 2(b) and (e), the active materials synthesized from the precursors also show similar particle shapes. Thus, it seems that the step at which Mg is added does not significantly affect the particle shape, indicating that influence of the morphology can be ruled out in comparing the electrochemical performances of the two materials. Fig. 2(c) and (f) show the energy-dispersive X-ray spectroscopy (EDS) maps of Mg for the (Ni, Co, Mg)(OH)2 precursor and the active materials synthesized using either (Ni, Co)(OH)2 or (Ni, Co, Mg)(OH)2 precursors. Mg is homogeneously distributed throughout both the surface and bulk regions, indicating that Mg is effectively mixed with Ni and Co by the coprecipitation method. Fig. 3(a) and (b) show the charge/discharge profiles of the two samples. All features of the profile, such as capacity, rate performance, length of constant voltage (CV)-mode charging section, and average operation voltage, are almost identical. This similarity in basic electrochemical performance at the initial stage, along with that in the morphological characteristics between the two samples as discussed above, could allow the hasty conclusion that the Mg-addition step during the synthesis does not affect the properties of Mg-added LNCO. However, examination of the performance parameters relating to the long-term stability shows notable differences: The active material prepared using the Ni–Co–Mg hydroxide coprecipitated precursor shows enhanced cycle stability, as presented in Fig. 3(c). This sample also shows better performance in the storage test: Fig. 3(d) shows the amount of gas evolved from the charged state (4.3 V vs. Li+/Li) of the cathodes as a function of storage time at 60 °C, which indicates that the
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Fig. 2. SEM images of the hydroxide precursors prepared by coprecipitation process, final products of Mg-added LNCO active materials, and EDS mapping of Mg: (a, d) (Ni, Co, Mg)(OH)2 and (Ni, Co)(OH)2 precursor, (b, e) active materials synthesized using precursors of (a) and (d), respectively, (c) EDS mapping of Mg on the cross-section of (d), and (f) EDS mapping of Mg on the surface of (b, left) and (e, right).
be affected by the method of Mg addition, the long-term performance appears to be greatly improved by introducing Mg via the coprecipitation process.
coprecipitation of Mg with Ni and Co greatly reduces gas generation during the high-temperature storage of the charged cathode material. Thus, although the initial charge/discharge properties do not seem to
Fig. 3. Electrochemical properties of Mg-added LNCO active materials: (a, b) charge/discharge profiles of the samples prepared by coprecipitation of Mg and by mixing of Mg-source powder (3.0–4.3 V vs. Li+/Li; 0.2, 0.5, 1.0, and 0.2 C rate consecutively (1 C = 190 mA/g)), (c) retention of discharge capacity with cycles (1 C rate), and (d) amount of gas evolution with respect to the storage time at 60 °C. (Mg-LNCO_MX and Mg-LNCO_CP in (c) and (d) denote the samples prepared by adding Mg by powder mixing and coprecipitation, respectively.).
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Fig. 4. Phase diagrams of Li–M–O (M = 20/24Ni + 3/24Co + 1/24Mg) system, simulated as a function of temperature, including defect phases of Table 1.
fluctuations, thus maintaining a stoichiometric composition at all points and times, only the defect-free phase ❶ is synthesized in theory. However, in an actual synthesis, the imperfect mixing of raw materials and/or local insufficiencies of O2 may cause defect formation. Therefore, the synthesized material is likely to be a mixture of perfect and defect-containing phases, mostly comprising the thermodynamically probable phases ❶, ⑦, and ⑪ with a separate MgO phase, as predicted in Fig. 4(c–f). Thus, the synthesized material likely contains oxygen vacancies (phase ⑦) and cation antisites (phase ⑪) as the main point-defect species and the secondary MgO phase. The Mg content in the layer-structured LNCMgO is determined by the proportion of the perfectly crystalline LNCMgO phase ❶.
3.2. Simulation results The simulation result of the phase diagrams of Li–M–O is presented in Fig. 4. In the metallic state, the formation of a Ni–Mg solid solution is stable, while Ni–Co alloying is thermodynamically unfavorable. For the binary system of M–O, only MO-type compounds are stable phases, and Ni, Co, and Mg form a stable solid solution rather than a mixture of separate phases. On the Li–O line, a stable phase appears at the compositional point of Li:O = 2:1, at which Li2CO3 is the stable phase below ~ 800 K. This transforms to Li2O by releasing CO2 at higher temperatures. As for ternary phases, spinel and layer-structured materials appear to be stable on the phase diagrams. The spinel LiM2O4 is simulated as stable under ~ 1200 K, wherein Ni and Co form a complete solid solution but Mg exists in a separate secondary phase of MgO. The layerstructured LiMO2, which is of primary interest as an LIB cathode material, is predicted to be unstable at room-temperature regimes of ~ 300 K (blue arrow in Fig. 4(a)). This may seem inconsistent with the common use of Ni-based layer-structured cathode materials at room temperatures. However, because the material is generally synthesized at temperatures exceeding 1000 K, and because the stable–unstable transition temperature is ~ 400–500 K—at which the temperature is low enough to prevent the atomic movement required for the decomposition reaction of LiMO2 + CO2 → Li2CO3 + (spinel + MgO) + MO, as predicted from the phase diagram of Fig. 4(a)—LiMO2 is expected to maintain a layered crystal structure at room temperature. At the surface region, where structural instability is higher than it is in the bulk, decomposition reactions may proceed with few environmental effects, resulting in the commonly observed degradation of Ni-based cathodes after long-term storage under ambient conditions. Above ~ 700 K, three phases of LiMO2, which are ❶, ⑦ + MgO, and ⑪ + MgO, appear stable on the phase diagrams. If Li, M, and O are assumed to mix perfectly throughout the sample with no compositional
4. Discussion Phase ❶ is the only stable LiMO2 phase in which Mg is doped in the crystal. The other stable phases, ⑦ and ⑪, do not contain Mg as a dopant, but a separate MgO phase coexists with them. Because ❶ contains no other point defects, the Mg-doped domains of the material would probably have perfect crystal structures. Considering this from the perspective of synthesis, it is proposed that the proportion of phase ❶ may be increased if more Mg is doped into the crystal structure by any method, for ❶ is the only thermodynamically stable Mg-containing phase. The concentrations of point defects such as VO and NiLi, which are included in phases ⑦ and ⑪, would be decreased by increasing the fraction of phase ❶. In summary, the simulation result indicates that doping as much Mg in the LiMO2 crystal as possible by optimizing the synthetic process may be effective in reducing defect generation and improving the crystallinity of the cathode material. As seen from the experimental results, introducing Mg during the coprecipitation process is effective in improving the cyclic longevity and reducing gas evolution in the cathode material. The coprecipitation method is effective in mixing Mg with Ni and Co because atomic-level 5
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mixing is possible with solution-based processes. Increased doping of Mg would increase the proportion of phase ❶ over those of ⑦ and ⑪, thereby decreasing the concentration of VO and NiLi. In a previous report, we suggested that the VO defect could act as a route for the diffusion of Ni to Li sites by a vacancy diffusion mechanism at the delithiated states of Ni-based layer-structured cathode materials [34]. Cation mixing between Ni and Li has been reported to be a main factor in the stability deterioration of Ni-based cathodes [6]. Thus, introducing Mg during coprecipitation is proposed as an effective method to improve the structural stability of Ni-based cathodes by reducing the defect concentration, which contributes to increased longevity and decreased swelling of LIBs that use the cathode material. Although the possibility may not be excluded that undoped Mg might form a secondary phase and provide a little surface coating effect, but it seems the effect of suppressed formation of point defects was more effective in improving the cathode performance. In conclusion, contrary to previous studies focused on the role of Mg itself in stabilizing the structure of Nibased cathode materials [13,14], we suggest that the suppressed formation of intrinsic point defects by Mg doping is a new stabilization mechanism worthy of further investigation. 5. Conclusions The effects of the stage of Mg introduction during the synthesis of Mg-added LNCO on the properties of the material when applied as a cathode in LIBs were examined. One cathode material was prepared by coprecipitating Mg with Ni and Co before calcining the (Ni, Co, Mg) (OH)2 precursor mixed with LiOH·H2O. A second cathode was prepared by introducing Mg as a powdered compound to mix with LiOH·H2O and a (Ni, Co)(OH)2 precursor before calcination. No significant differences were observed in either the particle morphology or the initial electrochemical properties, such as charge/discharge capacity and rate performance, between the two cases. However, the long-term stability-related characteristics showed great improvement with the coprecipitation of Mg with the transition metals. The capacity retention with repeated cycling was enhanced, and the gas evolution was remarkably reduced during high-temperature storage. Simulations based on first-principles calculations indicated that the concentration of point defects could be reduced by increasing the content of doped Mg in the LNCO crystal structure. Both cases of doping Mg in the crystal and forming a separate MgO phase were found to be thermodynamically stable states through the simulation. Therefore, optimizing the synthetic processing conditions to dope as much Mg as possible in the crystal is suggested to be the determining factor in suppressing the formation of point defects, thereby increasing the crystal quality and enhancing the structural stability of the cathode material. Introducing Mg during the coprecipitation process, which permits the atomic-level homogeneous mixture of elements, is confirmed to be effective for this purpose. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03933704). References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] L.A. Dominey, Lithium Batteries, Elsevier, Netherlands, 1994. [3] G.A. Nazri, G. Pistoia, Lithium Batteries: Science and Technology, Kluwer, Boston, 2004. [4] J.B. Goodenough, K. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [5] J. Chen, Recent progress in advanced materials for lithium ion batteries, Materials 6 (2013) 156–183.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Electrochemical properties of Mg-added lithium nickel cobalt oxide induced by structural characteristics depending on the synthetic process ⁎
Gene Jaehyoung Yang, Yongseon Kim
Department of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Powders: chemical preparation B. Defects C. Electrical properties D. Transition metal oxides E. Batteries
The doping of various foreign elements into Ni-based layer-structured cathode materials has been investigated to improve their electrochemical performance. However, the dependence of structural features and the resultant electrochemical performance on the processing methods remains unclear. In this study, the effects of varied Mg addition methods on the properties of lithium nickel cobalt oxide (LNCO) are investigated by experimental and first-principles simulation approaches. Coprecipitating Mg with the transition metals causes no differences in the morphological and initial charge/discharge characteristics compared to the method of mixing the Mg-source powder with the other raw materials, but performance relating to long-term stability, such as cyclic capacity retention and swelling by gas evolution, is greatly improved. The simulation results indicate that cases in which Mg is doped within the LNCO crystal and in which Mg forms a separate MgO phase are both thermodynamically allowable, and that increasing the Mg content in the LNCO crystal structure suppresses the formation of VO- and NiLi-type point defects. This is proposed as a new mechanism of structural stabilization by Mg doping that explains the improved stability with the introduction of Mg by coprecipitation.
1. Introduction Lithium ion batteries (LIBs) are widely used as power supplies for mobile electronic devices, with application extending to mid- and largesized systems such as electric vehicles (EVs) and energy storage systems [1–3]. Despite the rapid growth of the market, the technical performance of LIBs does not satisfy consumer expectations for the energy storage density, charge/discharge speed, safety, and price. Most of all, increases in energy capacity are constantly in high demand. Smartphone usage continuously lengthens with the integration of new functions, while vehicular mileage per charge is considered to determine the future of the EV industry [4,5]. Adopting high-capacity cathode materials is one way to address this demand, and new materials that can replace the conventionally used LiCoO2 (LCO) are actively researched [4,6–10]. Unfortunately, new materials still cannot fully satisfy every aspect of performance required for application in current LIB systems; therefore, the most pragmatic near-term solution for LIB cathodes may be the development of Nibased layer-structured materials with the same crystal structure as LCO but higher energy capacities. The commercialization of some materials in this category, represented by LiNi1−x−yCoxAlyO2, is actively underway [6,7,11,12]. It is well known that increased capacity under conventional
⁎
charging voltage conditions (generally 4.2–4.3 V) is obtainable as the amount of Ni replacing Co increases, but the thermal and structural stability of the cathode degrades simultaneously, decreasing the safety of the LIBs. To address this problem, the doping of various impurity elements has been tested to achieve higher stability in the crystal structure after delithiation [13–18]. Not only the selection of the doping element, but also the doping methodology has been examined by researchers: sol-gel, coprecipitation, and hydrothermal routes have been tested, as well as conventional solid-state reactions which mainly comprise raw-material powder mixing and high-temperature calcination processes [19–22]. In spite of extensive research on the topic, it remains necessary to systematically investigate the effects on the structural and electrochemical properties induced by different doping processes. Although many reports exist on synthetic methods for doping or the optimization of processing conditions, organized examinations of the interrelations among the doping and synthesis methods, the structural and defect characteristics, and the short- and long-term electrochemical performances of the synthesized materials are unavailable, to the best of our knowledge. To accurately interpret the mechanisms underlying changes in properties with the introduction of foreign elements, it is necessary to proactively examine whether the elements are doped in the crystal as intended or segregated on the surfaces or grain boundaries, as well as
Corresponding author. E-mail address: [email protected] (Y. Kim).
http://dx.doi.org/10.1016/j.ceramint.2017.10.175 Received 26 September 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 0272-8842/ © 2017 Published by Elsevier Ltd.
Please cite this article as: Yang, G.J., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.10.175
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significantly decreased as the Mg-content exceeded 2.0%, thus indicating 1.0–1.5% to be the optimum doping amount of Mg. The powder mixture was placed in Al2O3 crucibles and heated at 800 °C for 10 h under flowing O2. The metal-hydroxide precursors were prepared by a coprecipitation method. NiSO4·6H2O and CoSO4·7H2O, with or without MgSO4·7H2O (all EP grade from Samchun Chemicals, Korea), were dissolved in distilled water. The solution of 1 M concentration was injected into a 2-L reactor at the rate of 50 mL/h. An ammonia solution (~ 28–30%, Samchun Chemicals, Korea) was added to the reactor, maintaining the ratio of NH3 to Ni + Co (+ Mg) at 1:1. The pH inside the reactor was controlled at 11.5–12.0 by changing the injection rate of an aqueous NaOH solution (EP, Duksan Pure Chemicals, Korea). After 12 h of injection while stirring the solution inside the reactor, the coprecipitated powder was washed several times with water and dried at 120 °C. 2032-type coin cells were fabricated using the synthesized samples as the cathode, and the electrochemical properties were measured. The active cathode material and acetylene black (AB) were mixed with a solution of polyvinylidene fluoride (PVDF) dissolved in N-methyl-2pyrrolidone. The weight ratio of the active material, AB, and PVDF was set at 94:3:3 wt%. This slurry was coated onto an Al foil, dried, and rollpressed to produce the cathode. The loading level was fixed at ~ 15 mg/cm2. After vacuum-drying of the cathode, coin cells were fabricated. Li metal foil, 16-μm-thick polypropylene porous film (CS Tech, Korea), and 1.3-M LiPF6 solution (Panax Etec, Korea) in a 3:3:4 v/v mixed solvent of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate were used as the counter electrode, separator, and electrolyte, respectively. The initial charge/discharge profile was obtained from galvanostatic measurement at rates of 0.2, 0.5, 1.0, and 0.2 C (1 C = 190 mA/ g), and then the cyclic performance was monitored with repeated charge/discharge at a 1-C rate between 3.0 and 4.3 V vs. Li+/Li. Gas evolution during a high-temperature storage test was measured from the volume change of pouches containing cathodes separated from the coin cells after the first charge cycle and fresh electrolyte solution. The pouch volume was measured using Archimedes’ principle; thus, the amount of gas generated inside the pouch was monitored with respect to the time for which the pouch was held at 60 °C.
whether the introduction of the dopants induces the formation of other point defects. Such secondary point defects may cause property changes rather than the dopant itself. With these considerations, this study investigated Mg-added lithium nickel cobalt oxide (Mg-added LNCO) through experimental and firstprinciples simulation methods. Two different doping processes of Mg coprecipitation with Ni and Co during the preparation of the metal hydroxide precursor and a general solid-state synthesis of mixing the Mg source with other raw-material powders were implemented. The properties of the resulting materials were comparatively investigated. The thermodynamically stable states and related point defects were theoretically analyzed by simulating phase diagrams including various types of defect-containing phases based on first-principles calculations. Combining the experimental and computational investigations, the synthesis process-dependent structural features, and the associated electrochemical properties were interpreted. The coprecipitation and solid-state reactions examined in this study are the most widely used methods for the synthesis of layer-structured cathode materials in the LIB industry, yet the timing and manner for adding doping elements during the synthesis remains controversial. The results of this study are expected to yield useful information on these matters, thereby guiding the design of optimized production processes for cathode materials.
2. Material and methods 2.1. Experimental Mg-added LNCO samples were prepared with different synthetic processes (Fig. 1). One sample was obtained by calcining a mixture of LiOH·H2O (98%, Duksan Pure Chemicals, Korea), (Ni, Co)(OH)2 precursor, and Mg(OH)2 (extra pure (EP), Samchun Chemicals, Korea) powders, whereas another sample was synthesized using (Ni, Co, Mg) (OH)2 and LiOH·H2O as the starting material, thus providing varied processing conditions of the timing and manner for adding Mg. The ratio of Li to Ni +Co + Mg was set at 1.05, considering the loss of Li during calcination. The raw materials of Ni, Co, and Mg were mixed with a stoichiometric ratio targeting the synthesis of Li (Ni0.835Co0.150Mg0.015)O2. The Mg-content was determined by referring to our previous experiments, which showed the discharge capacity
Fig. 1. Schematic of the experimental and first-principles simulation processes of this study.
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2.2. First-principles simulation
Table 1 Phases of [LNCMgO + defects] and [LNCO + defects + MgO] included in the simulation of phase diagrams. Mg is assumed to be doped in the LNCO crystal for the [LNCMgO + defect] combinations, while it forms a secondary MgO phase for [LNCO + defect + MgO] combinations.
The phase diagram of the Li–M–O (M = 20/24Ni + 3/24Co + 1/ 24Mg) system was simulated as a function of temperature, by comparing the normalized Gibbs free energy G̅ of the phases [23]:
LNCMgO + defects
G[Lia Mb Oc (CO2)d (H2 O)e] =
G[Lia Mb Oc (CO2)d (H2 O)e]−dGCO2 −eGH2O a+b+c
The equation was designed to reflect the energy changes related to the addition or removal of CO2 and H2O, which enabled the examination of hydroxide or carbonate phases as well as the metallic and oxide states. By assigning a 3-D coordinate point to each phase (x and y coordinates according to the chemical composition and z coordinate according to G ) and taking the 2-D projection of the convex hull [24] in the – z direction, ternary phase diagrams showing stable phases and their tie-lines could be obtained [25,26]. The energy of each solid phase was calculated by the density functional theory (DFT) method, based on the Perdew–Burke–Ernzerhof (PBE) generalized-gradient approximation with Kresse–Joubert projector augmented-wave (KJPAW)-type PBEsol pseudopotentials [27,28]. QUANTUM-ESPRESSO code [29] was used with a kinetic energy cutoff of 50 Ry for the wavefunctions and a convergence threshold of 10−3 Ry/Bohr on the forces for ionic minimization. The k-point spacing equivalent to 0.3 Å−1 was used, with full relaxation of atomic positions and lattice vectors allowed. The Gibbs free energy of each solid phase was calculated as a function of temperature using the isobaric heat capacity obtained from quasi-harmonic approximation (QHA) calculations provided in the Thermo-PW software package [30]. The standard-state chemical potentials of the gas phases were determined semi-empirically to minimize discrepancies between the experimentally reported formation energies of oxide, hydroxide, or carbonate materials and those obtained from the DFT calculations (see Refs. [26,31] for details). The effects of temperature and pressure on the gas-phase chemical potential were accommodated by referring to the JANAF Thermochemical Table [32]. The partial pressures of O2, CO2, and H2O were fixed at 0.02, 3.0 × 10−5, and 1.5 × 10−3 MPa, respectively, during the simulations assuming a typical ambient atmosphere. Various types of associated Mg and point defects were designed by changing the Mg doping site and combining the point defects: combinations of MgNi, MgLi, and Mgtet with VLi, VO, VNi, NiLi, and LiNi were investigated. Point defects other than these were excluded by prescreening because they had high formation energies. Thus, 22 types of [Mg-doping + point defect(s)] combinations are considered. Model crystals for DFT calculations of these phases were obtained by introducing each combination to a Li24M24O48 frame crystal in which three Co atoms substitute at Ni sites with positions optimized in advance. Differently sized frame crystals were used as necessary for cases including Mgtet or MgLi. For each case, the most stable arrangement of Mg and the associated defects was determined by changing their positions and comparing the energies. In addition to these Mg-doped models (LNCMgO), cases in which Mg was not doped in the crystal but instead formed a secondary MgO phase (LNCO + MgO) were considered. Thus, 39 total cases of [LNCMgO + defect(s)] or [LNCO + defect(s) + MgO] were produced (Table 1). The [LNCMgO + defects(s)] phases, in which Mg is doped in the LNCO crystal, are expressed as numbers in solid circles, such as ➊, ➋, ➌…, whereas the [LNCO + defect(s) + MgO] phases, in which Mg is not doped but forms a secondary MgO phase, are indicated as emptycircle numbers (①, ②, ③…) + MgO. These were included in the simulation of the phase diagrams as if each were an independent phase, in addition to the regular phases of the Li–M–O system as reported in the Inorganic Crystal Structure Database [33].
LNCO + defects + MgO
Description
Defects
Description
Defects
➊ ➋–1 ➋–2 ➌ ➍ ➎–1 ➎–2 ➏–1 ➏–2 ➐ ➑–1 ➑–2 ➒ ➓–1 ➓–2
MgNi MgNi + NiLiLiNi MgLi + LiNi Mgtet + VNi MgNi + VLi MgNi + NiLiVNi MgLi + VNi MgNi + VLiNiLiLiNi MgLi + VLiLiNi MgNi + VO MgNi + VONiLiLiNi MgLi + VOLiNi MgNi + VOVLi MgNi + VONiLiVNi MgLi + VOVNi MgNi + NiLi MgLi Mgtet + VLi MgNi + VONiLi MgLi + VO MgNi + VLiNiLi Mgtet
① + MgO ②–1 + MgO ②–2 + MgO ③ + MgO ➃ + MgO ➄ + MgO
No defect NiLiLiNi LiNi VNi VLi NiLiVNi
➅–1 + MgO ➅–2 + MgO ⑦ + MgO ➇–1 + MgO ➇–2 + MgO ➈ + MgO ➉–1 + MgO ➉–2 + MgO ⑪ + MgO
VLiNiLiLiNi VLiLiNi VO VONiLiLiNi VOLiNi VOVLi VONiLiVNi VOVNi NiLi
⑬ + MgO
VONiLi
⑭ + MgO
VLiNiLi
⓫–1 ⓫–2 ⓬ ⓭–1 ⓭–2 ⓮ ⓯
3. Results 3.1. Experimental results Scanning electron microscopy (SEM) images of the metal-hydroxide precursors and the active materials prepared using the precursors are presented in Fig. 2. Comparing the morphologies of the metal-hydroxide precursors (Fig. 2(a, d)), it appears that particles of the Mg-containing material (Fig. 2(a)) show dimply surface. However, the overall morphological characteristic of small, needle-shaped primary particles is shared by the hydroxide precursors both with and without Mg. As can be seen from Fig. 2(b) and (e), the active materials synthesized from the precursors also show similar particle shapes. Thus, it seems that the step at which Mg is added does not significantly affect the particle shape, indicating that influence of the morphology can be ruled out in comparing the electrochemical performances of the two materials. Fig. 2(c) and (f) show the energy-dispersive X-ray spectroscopy (EDS) maps of Mg for the (Ni, Co, Mg)(OH)2 precursor and the active materials synthesized using either (Ni, Co)(OH)2 or (Ni, Co, Mg)(OH)2 precursors. Mg is homogeneously distributed throughout both the surface and bulk regions, indicating that Mg is effectively mixed with Ni and Co by the coprecipitation method. Fig. 3(a) and (b) show the charge/discharge profiles of the two samples. All features of the profile, such as capacity, rate performance, length of constant voltage (CV)-mode charging section, and average operation voltage, are almost identical. This similarity in basic electrochemical performance at the initial stage, along with that in the morphological characteristics between the two samples as discussed above, could allow the hasty conclusion that the Mg-addition step during the synthesis does not affect the properties of Mg-added LNCO. However, examination of the performance parameters relating to the long-term stability shows notable differences: The active material prepared using the Ni–Co–Mg hydroxide coprecipitated precursor shows enhanced cycle stability, as presented in Fig. 3(c). This sample also shows better performance in the storage test: Fig. 3(d) shows the amount of gas evolved from the charged state (4.3 V vs. Li+/Li) of the cathodes as a function of storage time at 60 °C, which indicates that the
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Fig. 2. SEM images of the hydroxide precursors prepared by coprecipitation process, final products of Mg-added LNCO active materials, and EDS mapping of Mg: (a, d) (Ni, Co, Mg)(OH)2 and (Ni, Co)(OH)2 precursor, (b, e) active materials synthesized using precursors of (a) and (d), respectively, (c) EDS mapping of Mg on the cross-section of (d), and (f) EDS mapping of Mg on the surface of (b, left) and (e, right).
be affected by the method of Mg addition, the long-term performance appears to be greatly improved by introducing Mg via the coprecipitation process.
coprecipitation of Mg with Ni and Co greatly reduces gas generation during the high-temperature storage of the charged cathode material. Thus, although the initial charge/discharge properties do not seem to
Fig. 3. Electrochemical properties of Mg-added LNCO active materials: (a, b) charge/discharge profiles of the samples prepared by coprecipitation of Mg and by mixing of Mg-source powder (3.0–4.3 V vs. Li+/Li; 0.2, 0.5, 1.0, and 0.2 C rate consecutively (1 C = 190 mA/g)), (c) retention of discharge capacity with cycles (1 C rate), and (d) amount of gas evolution with respect to the storage time at 60 °C. (Mg-LNCO_MX and Mg-LNCO_CP in (c) and (d) denote the samples prepared by adding Mg by powder mixing and coprecipitation, respectively.).
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Fig. 4. Phase diagrams of Li–M–O (M = 20/24Ni + 3/24Co + 1/24Mg) system, simulated as a function of temperature, including defect phases of Table 1.
fluctuations, thus maintaining a stoichiometric composition at all points and times, only the defect-free phase ❶ is synthesized in theory. However, in an actual synthesis, the imperfect mixing of raw materials and/or local insufficiencies of O2 may cause defect formation. Therefore, the synthesized material is likely to be a mixture of perfect and defect-containing phases, mostly comprising the thermodynamically probable phases ❶, ⑦, and ⑪ with a separate MgO phase, as predicted in Fig. 4(c–f). Thus, the synthesized material likely contains oxygen vacancies (phase ⑦) and cation antisites (phase ⑪) as the main point-defect species and the secondary MgO phase. The Mg content in the layer-structured LNCMgO is determined by the proportion of the perfectly crystalline LNCMgO phase ❶.
3.2. Simulation results The simulation result of the phase diagrams of Li–M–O is presented in Fig. 4. In the metallic state, the formation of a Ni–Mg solid solution is stable, while Ni–Co alloying is thermodynamically unfavorable. For the binary system of M–O, only MO-type compounds are stable phases, and Ni, Co, and Mg form a stable solid solution rather than a mixture of separate phases. On the Li–O line, a stable phase appears at the compositional point of Li:O = 2:1, at which Li2CO3 is the stable phase below ~ 800 K. This transforms to Li2O by releasing CO2 at higher temperatures. As for ternary phases, spinel and layer-structured materials appear to be stable on the phase diagrams. The spinel LiM2O4 is simulated as stable under ~ 1200 K, wherein Ni and Co form a complete solid solution but Mg exists in a separate secondary phase of MgO. The layerstructured LiMO2, which is of primary interest as an LIB cathode material, is predicted to be unstable at room-temperature regimes of ~ 300 K (blue arrow in Fig. 4(a)). This may seem inconsistent with the common use of Ni-based layer-structured cathode materials at room temperatures. However, because the material is generally synthesized at temperatures exceeding 1000 K, and because the stable–unstable transition temperature is ~ 400–500 K—at which the temperature is low enough to prevent the atomic movement required for the decomposition reaction of LiMO2 + CO2 → Li2CO3 + (spinel + MgO) + MO, as predicted from the phase diagram of Fig. 4(a)—LiMO2 is expected to maintain a layered crystal structure at room temperature. At the surface region, where structural instability is higher than it is in the bulk, decomposition reactions may proceed with few environmental effects, resulting in the commonly observed degradation of Ni-based cathodes after long-term storage under ambient conditions. Above ~ 700 K, three phases of LiMO2, which are ❶, ⑦ + MgO, and ⑪ + MgO, appear stable on the phase diagrams. If Li, M, and O are assumed to mix perfectly throughout the sample with no compositional
4. Discussion Phase ❶ is the only stable LiMO2 phase in which Mg is doped in the crystal. The other stable phases, ⑦ and ⑪, do not contain Mg as a dopant, but a separate MgO phase coexists with them. Because ❶ contains no other point defects, the Mg-doped domains of the material would probably have perfect crystal structures. Considering this from the perspective of synthesis, it is proposed that the proportion of phase ❶ may be increased if more Mg is doped into the crystal structure by any method, for ❶ is the only thermodynamically stable Mg-containing phase. The concentrations of point defects such as VO and NiLi, which are included in phases ⑦ and ⑪, would be decreased by increasing the fraction of phase ❶. In summary, the simulation result indicates that doping as much Mg in the LiMO2 crystal as possible by optimizing the synthetic process may be effective in reducing defect generation and improving the crystallinity of the cathode material. As seen from the experimental results, introducing Mg during the coprecipitation process is effective in improving the cyclic longevity and reducing gas evolution in the cathode material. The coprecipitation method is effective in mixing Mg with Ni and Co because atomic-level 5
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mixing is possible with solution-based processes. Increased doping of Mg would increase the proportion of phase ❶ over those of ⑦ and ⑪, thereby decreasing the concentration of VO and NiLi. In a previous report, we suggested that the VO defect could act as a route for the diffusion of Ni to Li sites by a vacancy diffusion mechanism at the delithiated states of Ni-based layer-structured cathode materials [34]. Cation mixing between Ni and Li has been reported to be a main factor in the stability deterioration of Ni-based cathodes [6]. Thus, introducing Mg during coprecipitation is proposed as an effective method to improve the structural stability of Ni-based cathodes by reducing the defect concentration, which contributes to increased longevity and decreased swelling of LIBs that use the cathode material. Although the possibility may not be excluded that undoped Mg might form a secondary phase and provide a little surface coating effect, but it seems the effect of suppressed formation of point defects was more effective in improving the cathode performance. In conclusion, contrary to previous studies focused on the role of Mg itself in stabilizing the structure of Nibased cathode materials [13,14], we suggest that the suppressed formation of intrinsic point defects by Mg doping is a new stabilization mechanism worthy of further investigation. 5. Conclusions The effects of the stage of Mg introduction during the synthesis of Mg-added LNCO on the properties of the material when applied as a cathode in LIBs were examined. One cathode material was prepared by coprecipitating Mg with Ni and Co before calcining the (Ni, Co, Mg) (OH)2 precursor mixed with LiOH·H2O. A second cathode was prepared by introducing Mg as a powdered compound to mix with LiOH·H2O and a (Ni, Co)(OH)2 precursor before calcination. No significant differences were observed in either the particle morphology or the initial electrochemical properties, such as charge/discharge capacity and rate performance, between the two cases. However, the long-term stability-related characteristics showed great improvement with the coprecipitation of Mg with the transition metals. The capacity retention with repeated cycling was enhanced, and the gas evolution was remarkably reduced during high-temperature storage. Simulations based on first-principles calculations indicated that the concentration of point defects could be reduced by increasing the content of doped Mg in the LNCO crystal structure. Both cases of doping Mg in the crystal and forming a separate MgO phase were found to be thermodynamically stable states through the simulation. Therefore, optimizing the synthetic processing conditions to dope as much Mg as possible in the crystal is suggested to be the determining factor in suppressing the formation of point defects, thereby increasing the crystal quality and enhancing the structural stability of the cathode material. Introducing Mg during the coprecipitation process, which permits the atomic-level homogeneous mixture of elements, is confirmed to be effective for this purpose. Acknowledgement This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03933704). References [1] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367. [2] L.A. Dominey, Lithium Batteries, Elsevier, Netherlands, 1994. [3] G.A. Nazri, G. Pistoia, Lithium Batteries: Science and Technology, Kluwer, Boston, 2004. [4] J.B. Goodenough, K. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [5] J. Chen, Recent progress in advanced materials for lithium ion batteries, Materials 6 (2013) 156–183.
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