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Design of Nickel-rich Layered Oxides Using d Electronic Donor for Redox Reactions Duho Kim, Jin-Myoung Lim, Young-Geun Lim, Ji-Sang Yu, Min-Sik Park, Maenghyo Cho, and Kyeongjae Cho Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02697 • Publication Date (Web): 01 Sep 2015 Downloaded from http://pubs.acs.org on September 5, 2015
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Chemistry of Materials
Design of Nickel-rich Layered Oxides Using d Electronic Donor for Redox Reactions Duho Kim,† Jin-Myoung Lim,† Young-Geun Lim,‡ Ji-Sang Yu,‡ Min-Sik Park,*,‡ Maenghyo Cho,*,† Kyeongjae Cho*,†,|| †
Department of Mechanical and Aerospace Engineering, Seoul National University, 599 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea ‡ Advanced Batteries Research Center, Korea Electronics Technology Institute, 68 Yatap-dong, Bundang-gu, Seongnam, 463-816, Republic of Korea || Department of Materials Science and Engineering and Department of Physics, University of Texas at Dallas, Richardson, TX 75080, USA ABSTRACT: Through first-principles calculations and experimental observations, we firstly present the correlation between Ni and Mn ratio, and the redox behaviors of the layered NCM cathodes. The equilibrium potentials based on redox reactions of Ni2+/Ni3+ are highly dependent on the Mn ratio (NCM523 and NCM721: ~3.7 V and 3.5 V) because of a donor electron, in eg band, transferred from Mn to Ni owing to their crystal field splitting (CFS) with different electronegativities, leading to oxidation states of Ni2+-like and Mn4+. Considering the electronic donor (Mn) based on CFS with electronegativity of transition metals (TMs), we finally expect V as a promising doping source to provide donor electrons for Ni redox reactions in Ni-rich layered oxides, leading to be higher de-lithiation potentials (NCV523: 3.8 V). From our theoretical calculations in the NCV oxide, the oxidation state of Ni and V are stable Ni2+-like and V5+, respectively, and the fractional d-band fillings of Ni is the highest value as compared with NCM523 and LiNiO2 because of two donor electrons in t2g band. Based on the underlying understanding on the CFS with electronegativity of TMs, it would be possible to design new Ni-rich layered cathodes with higher energy for use in Li-ion batteries.
1. INTRODUCTION A growing demand for large-scale energy storage applications (e.g., electric vehicles and electric energy storage systems) has expedited the need for cathode materials with high Li+ storage capability; such materials will aid in the fabrication of advanced lithium ion batteries (LIBs) with high energy and high power.1,2 So far, among commercialized cathode materials, LiCoO2 has widely been used in LIBs because of its superior charge-discharge rate capability and its high working voltage (i.e., ~4.0 V).3-6 However, the use of LiCoO2 is gradually decreasing because of its small practical capacity, high production cost, and the toxicity of the Co it contains. Such shortcomings have encouraged many researchers to develop other layered cathode materials consisting of lower Co ratios.7-9 More recently, Ni-rich layered oxides have been extensively studied as an alternative cathode material because of their relatively higher reversible capacity (~180mAhg-1) and lower cost as compared with commercialized LiCoO2.10,11 In this regard, multi-component layered LiNi1-x-yCoxMnyO2 (NCM) cathodes have demonstrated significant success in commercial use. Because it is essential to increase the Ni content in order to further increase the reversible capacity, significant attention has been continuously devoted to increasing the Ni content in the layered cathodes.12,13 In fact, even small increases in the Ni
content can allow for a higher reversible capacity, although significant performance fading (as regards cyclability and rate capability) of the cathode results in structural instability that arises from the high Ni2+ content.14,15 To resolve the abovementioned problems, many research groups have focused on i) adjusting the composi tion of transition metals (TMs) and ii)16,17 substituting different TMs or non-TMs into the layered cathode structure in order to ensure high reversible capacity, stable cyclic performance, and the structural stability of the cathode.18-22 Recently, Sun et al. reported on a concentration-gradient layered cathode, which was designed to improve electrochemical performance and meet rigorous safety requirements for commercial use.23 In another study by Cho and co-workers, a pillar layer was introduced on the surface of the layered cathode in order to suppress interslab collapse and prevent particle pulverization.24 Nevertheless, even if such efforts represent significant improvements upon the performance of layered cathodes with high Ni content through the modification of the physicochemical properties (i.e., extrinsic characteristics) of materials, further improvement is still required in order to secure successful implementation on a commercial scale.
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Although studies of developing battery performance from the practical perspective have been intensively conducted, theoretically in-depth understanding of the multi-component layered oxides has not yet been clearly understood. Considering those underlying viewpoints, it is strongly important to control the intrinsic properties of materials, which directly affect the electrochemical redox behavior of TMs in the layered cathodes. In this regard, we introduce a material design concept of multicomponent layered cathodes using an electronic donor based on a crystal field splitting with the electronegativity (i.e., the tendency of an atom to attract electrons towards itself) of the TMs they contain. We then demonstrate its correlation with electrochemically redox behaviors. In other words, oxidation states of TMs in the multi-component system could be changed into different chemical states, because the redox electrons of TMs are affected by the difference in their electronegativities considered with a crystal field splitting.25 Thus, it is expected that the electrochemical performance of the multicomponent layered cathode could be directly affected by considering a crystal field splitting and adjusting the electronegativity of TMs in the structure. In this paper, we investigate the interactions between Ni and Mn in light of their electronic structures (e.g., crystal field splitting) and different electronegativities through a combined study of first-principles calculations and experimental observations. The electrochemical equilibrium potentials derived by redox reactions of Ni are varied according to the content of Mn in the structure. The theoretical calculations and experimental measurements suggest that Mn, functioning as an electronic donor, affects the electrochemical potential of Ni in the layered structure because of its lower electronegativity. Considering a crystal field splitting with different electronegativity of TMs, we suggest the potential use of V as a promising doping source to provide more useable redox electrons. Furthermore, our findings provide an important guideline for developing robust and high-performance cathode materials for LIB applications.
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electronic structures, atomic coordinates, and cell parameters were fully relaxed. 2.2 Experimental Study. Concerning the structural characterization of NCM523, NCM712, and NCM721,the three powder samples supplied from ECOPRO were characterized by an X-ray diffractometer (XRD) equipped with a three-dimensional (3D) pixel semiconductor detector; it used Cu-K α radiation (λ = 1.54056 Å) in the 2θ range from 10º to 80º. The particle morphology and size of the three samples were investigated by a field-emission scanning electron microscope (FESEM, JEOL JSM-7000F). The chemical composition of the compounds was confirmed by inductively coupled plasma mass spectroscopy (ICP-MS, Bruker Aurora M90) analysis. In order to measure the electrochemical performances of the three samples, the coin cells (CR2032) as a half type were fabricated as follows. A cathode slurry mixed with the active material (90 wt%), a conducting agent (Super-P, 5wt%), and a polyvinylidene fluoride (PVDF, 5 wt%) binder dissolved in N-methyl pyrrolidinone (NMP) solution was prepared for the electrode. The slurry was coated on an Al foil that was used as a current collector, and then dried at 80 ºC for 3h to evaporate the NMP. After the sufficiently dried electrode was uniformly pressed, it was punched and then additionally dried at 120 ºC overnight in a vacuum oven. The coin cells were assembled in a dry environment with Li metal functioning as the reference and counter electrodes. A porous polyethylene (PE) membrane was used as a separator. 1 M LiPF6 in ethylene carbonate (EC) /ethylmethyl carbonate (EMC) in a 1:2 volume ratio was used as the electrolyte (PANAX Etec Co. Ltd.). The loading amount of active materials and electrode density were fixed at 3.9 mgcm−2 and 1.0 gcm−3, respectively. To conduct GITT, the beaker-type cells (composed of a positive electrode, a negative electrode, and a Li electrode as the reference electrode) were carefully assembled in a dry room.
2. EXPERIMENTAL SECTION 2.1 Computational Study. In our computational calculations, the DFT method was performed using the Vienna Ab Initio Simulation Package (VASP). The spin-polarized generalized gradient approximation (GGA) to the exchange-correlation functional (according to Perdew-Wang 91) was employed in the calculation. To calculate more accurate electrochemical potentials and electronic properties, the GGA with a Hubbard-type U correction (GGA+U) was used, taking into account the strong correlations of the 3d orbitals of Ni, Co, Mn and V ions. The U values for Ni, Co, Mn and V in the layered structure (R 3 m) were 6.7, 4.91, 4.64 and 4.0 eV, respectively, and were determined from previously reported results.26-28 As the standard computational parameters, a kinetic energy cutoff of 400 eV and reciprocal-space k-point meshes of 4 × 4 × 1 for the Brillouin zone sampling were included in all calculations. In order to obtain optimized crystal structures, the thermodynamic quantities,
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Figure 1. (a) Galvanostatic first charge curves of NCM523 (black line), NCM712 (blue line), and NCM721 (red line) recorded with a constant specific current of 0.5 C rate in a voltage range between 2.5 and 4.3 V vs. Li/Li+. (b) A combined graph of galvanostatic intermittent titration technique (GITT) profiles during the initial charge to 4.0 V vs. Li/Li+ and calculated de-lithiation potentials for NCM523 and NCM721. (c) Atomic models of NCM523 and NCM721with octahedra of NiO6, MnO6, and CoO6. 3. RESULTS AND DISCUSSION The phase purities of commercial LiNi0.5Co0.2Mn0.3O2 (denoted as NCM523 hereafter), LiNi0.7Co0.1Mn0.2O2 (denoted as NCM712 hereafter), and LiNi0.7Co0.2Mn0.1O2 (denoted as NCM721 hereafter) were confirmed by powder X-ray diffraction (XRD, see Figure S1). All XRD patterns showed a typical layered structure (α-NaFeO2 structure), which belongs to the R
3 m space group. The clearly separated (006)/(012) and (018)/(110) peaks were evident in the patterns, indicating well-developed layered structures. Corresponding lattice parameters of the samples were calculated by Rietveld refinements (summarized in Table S1), which are in good agreement with the previous reports.29,30 The chemical composition of each sample was also confirmed by inductively coupled plasma mass spectroscopy (ICP-MS, see Table S2). Furthermore, field-emission scanning electron microscopy (FESEM) observations (Figure S2) indicated that all samples possessed a spherical shape with an average particle size of approximately 8 µm.
well as in our density functional theory (DFT) calculations (Figure 1b) based on representative structure modes of NCM523 and NCM721. To clarify the correlation between the TM ratio and the electrochemical behavior of the NCM cathode, it is necessary to understand the fundamental characteristics of TMs and their interactions in a given structure. Despite the fact that many researchers have paid much attention to this correlation in various ways, it has not been clearly understood yet. In this respect, detailed theoretical studies based on quantum mechanics are quite necessary to clearly understand this correlation, thereby allowing us to build better cathode materials for LIBs.
Figure 1a shows galvanostatic charge profiles of NCM523, NCM712, and NCM721 cathodes with different Mn ratios. During the first charge (Li+ extraction) to 4.3 V vs. Li/Li+ at room temperature, we found that electrochemical potentials of the cathodes were indeed affected by the Mn ratio of the structures. Compared with NCM523, both high Ni-content NCM samples (i.e., NCM712 and NCM721) showed relatively lower voltage profiles. Even though the high Ni-content samples have the same Ni ratios, NCM712 presents a higher voltage profile as compared with NCM721. Similar behaviors were also found in galvanostatic intermittent titration technique (GITT) profiles measured at a pseudo-equilibrium state, as
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Figure 2. (a) 3d-electron partial density of states(PDOS) of Ni and Mn ions in both NCM523, NCM721, and each pristine layered oxide: LiNiO2 and LiMnO2. (b) Occupied electrons in 3d orbitals of Ni3+ and Mn3+ positioned in an octahedral site and the corresponding schematic energy levels known as eg and t2g band. Fractional band fillings of (c) Ni ions and (d) Mn ions calculated from the above structures, respectively. For the theoretical investigations, two crystal structures (space group: R 3 m), which represented NCM523 and NCM721, were modeled as described in Figure 1c, in which Li, TMs, and O occupy the 3b, 3a, and 6c sites, respectively, with respect to the Wyckoff positions. Both NCM523 and NCM721 structures were relaxed with 12 formula units (f.u.) consisting of 48 atoms. The TM ratios involved in NCM523 and NCM721 were (6, 3, and 3) and (8, 2, and 2), respectively (Ni, Co, and Mn in sequence). With those structures, the delithiation potentials were calculated based on the thermody-
∆G
r ). As namic energy of formation (i.e., Gibbs free energy: with the experimental results, the NCM523 exhibited a higher de-lithiation potential than that of NCM721 (Figure 1b).31
Considering the fact that the de-lithiation potentials of NCM samples are directly correlated with the electronic structures of the TMs they contain, we calculated the partial density of states (PDOS) of each TM in the structure. Figure 2a shows PDOS profiles for the projected 3d orbital electrons of Ni and Mn in both NCM523 and NCM721 structures (shaded regions). The solid lines represent the PDOS of Ni and Mn in the pristine LiNiO2 and LiMnO2, respectively. In Figure 2a, the detailed electronic structures of Ni in NCM523 seem to be Ni2+like at low-spin states, which is different from that of Ni in the pristine LiNiO2 (represented as Ni3+). More specifically, from the crystal field theory, the spin-up electron of eg1 in the conduction band of the pristine LiNiO2 is positioned between the Fermi level and 1.0 eV. By way of contrast, the spin-up electron of eg1 in the acceptor band is almost positioned below the Fermi level in the NCM523 structure. From the perspective of ligand field theory, the O 2p orbitals forming σ antibonding orbitals with the Ni eg1 orbital are relatively smaller in NCM523 than those in the LiNiO2 (Figure S3).
In order to further investigate the acceptor band, we also conducted PDOS calculations for the Mn in the NCM523, as shown in Figure 2a. Although the absolute charge valence of Mn should beMn3+ in the NCM523 structure, the calculated PDOS of Mn indicates the presence of Mn4+. We suggest that the abnormal charge valence of Mn would be caused by the different electronegativities between Ni (1.91) and Mn (1.55). Considering the electronic configurations of Ni and Mn, it seems that the spin-up electron of eg1 (donor band) of Mn could be transferred to the acceptor band of Ni. To better understand, the schematic energy state divided into two distinct sets (e.g., eg and t2g) is illustrated in Figure 2b. Based on a crystal field splitting, an electron occupied (in dashed box) in eg band of Mn3+ could be a role of electronic donor, whereas, the rest electrons fully occupied in t2g band are strongly hard to donate. For the purpose of comparison, we examined the electronic structures of Ni and Mn in the NCM523 and NCM721 structure configured with a higher Ni ratio. According to the PDOS of Ni in NCM721 in Figure 2a, Ni exhibits a Ni2+-like electronic structure (rather than a Ni3+-like structure) as a result of electron transfer from the donor band of eg1 in Mn. It can be inferred from the PDOS (0 to 1 eV) of Ni in the NCM523 and NCM721 structures that acceptor electrons were transferred from Mn, indicating that the Ni in the NCM523 is more likely to be Ni2+-like, as compared with the Ni in NCM721. In a similar manner, these tendencies are directly reflected in the PDOS of O forming σ antibonding orbitals, as shown in Figure S3. From the comparison, it should be noted that the electronic structure of Ni in NCM523 is more stable than that of Ni in NCM721, leading to the higher equilibrium potential for Ni oxidation during the charge, as evidenced by the GITT potential profiles.
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Compounds
Area ratios Ni3+/Ni2+
Mn3+/Mn4+
NCM721
0.6944
0.4625
NCM712
0.5974
0.4565
NCM523
0.4580
0.4559
Table 1. Relative peak area ratios of NCM721, NCM712 and NCM523.
f
In the same way, the fractional d-band fillings of Mn ( Mn ) are presented in Figure 2d with a LiMnO2 reference. As the Mn ratio in the structure increases, the fractional band fillings in terms of Ni tend to increase, suggesting that the Ni in NCM523 is more likely to be Ni2+ as compared with NCM721. On the other hand, the fractional band fillings of Mn in the structures are maintained regardless of the Mn ratios because of the lower electronegativity as compared with Ni’s electronegativity. Importantly, the Mn exists in the form of Mn4+ by donating its electron to the Ni, which possesses a higher electronegativity. Furthermore, the average net charges of Ni and Figure 3. X-ray photoelectron spectroscopy (XPS) spectra of (a) Ni 2p and (b) Mn 2p in NCM523, NCM712, and NCM721. For precisely quantifying transferred electron by the electronegativity effects, calculations of fractional band filling were conducted with respect to Ni and Mn in both NCM523 and NCM721 structures. The fractional band filling (denoted as
f ) was calculated according to Equation 1. f =
Noccupiedstates
where
N states
Ef
−∞ ∞ −∞
f (Figure
In order to experimentally validate these theoretical findings, we conducted X-ray photoelectron spectroscopy (XPS), as shown in Figure 3. We focused on the variations in the valence of Ni and Mn in the layered oxides with different Mn ratios. The Ni 2p3/2 and Mn 2p3/2 spectra
g ( E )dE g ( E )dE
(1)
g ( E ) is the projected DOS of the atomic orbitals,
N occupiedstates bands,
∫ = ∫
Mn show identical behaviors with respect to both S4).
N states
refers to the total number of states in occupied refers to the total number of states in occupied
and unoccupied bands, and
Ef
refers to the Fermi level.
f
Figure 2c shows the fractional d-band fillings of Ni ( Ni ), which are calculated from the PDOS,in the structures LiNiO2, NCM721, and NCM523.
Figure 4. The 1stcharge (dashed lines) and 10th charge (solid lines) of the three samples with a constant specific current of 0.5 C rate and a voltage ranging from 2.5 V to 4.3 V vs. Li/Li+.
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Figure 5. (a) Occupied electrons in 3d orbitals of Ni3+ and V3+ positioned in an octahedral site and the corresponding schematic energy levels known as eg and t2g band.(b) Atomic models of NCV523 with octahedra of NiO6, CoO6, and VO6. (c) 3d-electron partial density of states (PDOS) of Ni and V ions in NCV523 (shaded regions), and each pristine layered oxide: LiNiO2 and LiVO2 (solid line). (d) Fractional band fillings of Ni ions in pristine layered oxide (LiNiO2), NCM523 and NCV523. (e) Calculated delithiation potentials for NCM721, NCM523 and NCV523. were carefully fitted to the C 1s spectra at 284.8 eV. From the Ni 2p spectra, a dominant peak with respect to Ni was found at 857.7 eV in NCM523, corresponding to Ni2+. As the Mn ratio was reduced, the Ni2+ component gradually decreased, whereas the growth of the Ni3+ component at 856 eV became evident. Moreover, predominant peaks corresponding to Mn4+ were mainly observed at 642.6 eV in Mn 2p spectra and there was no difference in all structures (i.e., NCM523, NCM712 and NCM721) as summarized in Table 1. Because of the fixed chemical state of Mn4+ in all structures, the Ni2+/Ni3+ redox reaction initially contributes to the electrochemical reaction (i.e., Li+ extraction), and the Ni3+/Ni4+ redox reaction is thereafter activated in the structures. Therefore, the chemical state of Ni2+ is more stable in the NCM523 structure than that of Ni2+ in the NCM721, resulting in higher de-lithiation potential during the charge. The different de-lithiation potentials induced by the Ni redox reaction were still found even after 10 cycles. Figure 4 compares charge profiles of all samples at the 1st charge (dotted line) and the 10th charge (solid line). As mentioned above, the NCM523 exhibited a higher de-lithiation potential than the other structures. Interestingly, this phenomenon still persisted after 10 cycles. These findings demonstrate that the delithiation potentials can be significantly affected by the electronegativity considered with a crystal field splitting of TMs in the layered system. Considering the correlation between the TM ratio (e.g., a crystal field splitting with electronegativity effects) and the electrochemical behaviors of the NCM cathode, we suggest that V would be a promising candidate for a doping element possessing more transferable electrons as compared with Mn. To better understand, the schematic energy state of Ni3+ and V3+ is illustrated in Figure 5a. Considering a crystal field splitting, t2g band with 3 degeneracies could maximally provide
two transferable electrons, whereas, eg band with 2 degeneracies could maximally provide one transferable electron. In this respect, we expect that V would be the most hopeful doping source among TMs configured with oxidation state of 3+. As shown in Figure 5a, the two electrons occupied (in dashed box) in t2g band of V3+ would be donated for Ni redox reactions, resulting in V5+ because of its lower electronegativity in comparison with Ni’s electronegativity. For theoretical expectations, we calculated electronic structures and de-lithiation potentials in NCV523 layered structure as NCM523. NCV523 was modeled as described in Figure 5b and fully relaxed with 12 f.u. consisting of 48 atoms, and the TM ratios involved in NCV523 were 6, 3, 3 (Ni, Co and V in sequence). In order to investigate roles as an electronic donor of V, we also conducted PDOS calculations for the Ni and V in the NCV523, as described in Figure 5c. The electronic structures of Ni in NCV523 almost seem to be Ni2+-like and those of V in NCV523 indicate the presence of V5+, although the absolute charge valence of V should be V3+. Taking into account the electronic configurations of Ni and V, it is expected that the spin-up electrons of t2g2 (donor band) of V could be transferred to the acceptor band of Ni. For the purpose of quantification and comparison of the electronegativity
f
effects, Ni were calculated from the PDOS, in the structures LiNiO2, NCM523, and NCV523 as shown in Figure 5d. As the Mn ratio in the structure is substituted with V, the fractional band fillings in terms of Ni tend to increase, indicating that the Ni in NCV523 is more likely to be Ni2+ as compared with NCM523. These electronic behaviors by a crystal field splitting with electronegativity effects lead to the higher delithiation potential for Ni oxidation in NCV523 as compared with the two structures (e.g., NCM721 and NCM523), as shown in Figure 5e. We also believe that V, functioning as an electronic donor with more transferable electrons than Mn, would provide useable electrons for Ni redox reactions.
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4. CONCLUSION In this work, we firstly investigated the correlation between the Mn ratio and the electrochemical behavior of the layered NCM cathode based on the theoretical calculations and experimental observations. Through GITT measurements, we found that the equilibrium potential of the NCM cathode was highly dependent on the Mn ratio in the structure; this can be explained by the fact that electrons can be transferred from Mn to Ni owing to their electronic structures based on a crystal field splitting and their different electronegativities. Taking into account a crystal field splitting of TMs with electronegativities, we finally suggest that V would be a promising candidate for a doping element, functioning as an electronic donor with more transferable electrons as compared with Mn. Our findings from the theoretical calculations and experimental observations are not only helpful to understanding the underlying relationship between a crystal field splitting with electronegativity and TMs interactions, but may also provide a conceptual design method based on the perspective of electronic structures. Considering our theoretical calculations in the NCV layered structure, elements possessing more transferable electrons with functioning as an electronic donor to redox TMs (e.g., Ni) would be a promising candidate to realize electrochemically better performance in the layered cathodes (i.e., high Ni-content layered oxides). We also believe that the design concept will be further applied to many other associated with materials in various fields, and our group will report designed cathode materials.
ASSOCIATED CONTENT Supporting Information. Additional figures and tables (MS Word). This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author * E-mail addresses:
[email protected] (K. Cho),
[email protected] (M. Cho),
[email protected] (M.-S. Park).
ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (2012R1A3A2048841). This work was also supported by the IT R&D program (10046306, Development of Li-rich Cathode (≥ 240 mAh/g) and Carbon-free Anode Materials (≥1,000 mAh/g) for High Capacity/High Rate Lithium Secondary Batteries), funded by the Ministry of Trade, Industry, & Energy (MOTIE) of the Republic of Korea.
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