First principles study of electrocatalytic behavior of olivine phosphates with mixed alkali and mixed transition metal atoms

Lithium transition metal olivine phosphates are well known Li-ion battery cathode materials, but these materials can also be used as electrocatalyst. Recent experimental studies showed that olivine phosphates with mixed alkali metals (Li and Na) and mixed transition metals (Ni and Fe) provide better electrocatalytic activity compared to single alkali and transition metal alternatives. In the current work, we analyzed the role of alkali metals, transition metals and vacancies on the reactivity of a series of olivine phosphates with different stoichiometries using first principles calculations. To this end, we investigated the adsorption of water at the surface of these materials. We found that water binds preferably at Ni surface sites for materials devoid of alkali ion vacancies. We further found correlation between the calculated adsorption energy with experimentally measured overpotentials for a series of olivine phosphates. Additionally, we found correlation between the adsorption energy of the systems with the total charge polarization of surface and adsorbate. To explain the computed trends, we analyzed the occupancies of the partial density of states of the Ni and Fe 3d states and Bader atomic charges.


Introduction
Lithium transition-metal olivine phosphates with the general formula LiMPO 4 (M ¼ Fe, Co, Ni, Mn) are promising and environmentally benign energy storage materials applied as positive electrodes in Li-ion batteries. These materials are cost effective and provide good thermal stability during lithiation and delithiation. [1][2][3][4][5] However, olivine phosphates with single transition-metal (TM) composition, such as LiFePO 4 , suffer from low electronic conductivity and poor redox kinetics. 6 There are many recent studies on mixed transition metal olivine systems (LiM 1Àx M 0 x PO 4 ; M and M 0 ¼ Fe, Ni, Co, Mn) by various group which have shown ability to overcome these issues. [7][8][9][10][11][12][13] Beyond this, the olivine phosphates are also known as good electrocatalyst for oxygen evolution reactions (OER). [14][15][16][17] Excellent OER electrocatalysts, such as ruthenium, iridium and their oxides exist, but these are costly, [18][19][20][21][22][23] and there is an ongoing search for non-noble metal catalysts for OER. [24][25][26][27][28][29] Olivine phosphate materials can provide cost-effective alternatives to noble metal catalysts like Pt and Ir, and Fe doped Ni-and Cobased olivine phosphate were found to be promising OER active materials. 30,31 Recently Gershinsky et al. synthesized mixed alkali (Li/Na) and mixed transition metal olivine phosphates and tested their OER activity. 32,33 They showed that a particular combination of alkali metals (Li and Na) and transition metals with specic (1 M to 6 M) KOH concentration can be an excellent OER catalyst. Moreover, this study also indicated that defects in the olivine phosphate improved the OER catalytic activity.
Olivine phosphates belong to the olivine family which has an orthorhombic structure within the Pnma space group. In olivine structures, Li ions are arranged in edge sharing octahedra and transition metals (TMs) are in a corner sharing octahedral environment with oxygen atoms, whereas phosphorus ions are in a tetrahedral environment. The strong P-O covalent bonds provide the material with good thermodynamic stability, by reducing the release of oxygen. 6 38 In the present work, we perform rst principles-based computations to understand the effect of mixed alkali and transition metals and defects on the reactivity of olivine phosphates by looking at the adsorption of water on the surface of these materials. OER involves different intermediate steps where binding of H 2 O, OH, O, OOH occurs at the metal site of the surface of the electro-catalyst. 16,39 It has been shown in earlier studies that these different steps are linearly correlated. 15,16,[40][41][42][43][44][45] Thus, the metal-oxygen interaction along the OER cascade is likely to play an important role in the rate limiting step. 40,45,46 Although water binding is presumably not the rate limiting step for OER, 16,46 water binding is expected to reect on the ability of the olivines to bind oxygen. Here, we systematically explore the binding of a water molecule at the surfaces of mixed alkali and mixed transition metal olivine phosphate using density functional theory (DFT) calculations. Our studies mainly focus on Li 0.8 Na 0.2 Ni 0.7 Fe 0.3 PO 4 without and with Livacancies. We further chose four different compounds LiNiPO 4 , LiNi 0.7 Fe 0.3 PO 4 , LiNi 0.8 Co 0.2 PO 4 , and LiNi 0.9 Fe 0.1 PO 4 , respectively for a comparative study following the work of Gershinsky et al. 33 Based on these studies we qualitatively correlate between the observed electrochemical activity for OER and the probability of binding water on the surface of these materials with and without Li-vacancies. To the best of our knowledge, this is the rst computational study focusing on the surface reactivity of olivine phosphate-based materials with application to OER.

Computational details
All the calculations were performed using plane wave-based DFT, as implemented in the Vienna Ab initio Simulation Package (VASP). 47, 48 We employ projector augmented wave (PAW) potentials, 49 in conjunction with the Perdew-Burke-Ernzerhof (PBE) functional 50 with HubbardÀU correction 51 (Dudarev's method 52 ). The applied effective value of U for Ni, Co, and Fe are 5.96, 5.7, and 4.3 eV, respectively. 7 The cut-off energy value for the plane wave basis was set to 520 eV. The convergence limit for the energy in a self-consistent run was set to 10 À5 eV, whereas 0.01 eV A À1 was used for the force convergence per atoms during geometry optimization. We chose a Gcentered k-mesh of 8 Â 4 Â 1 to sample the irreducible part of the Brillouin zone.

Structural model
We considered four formula units to model a bulk orthorhombic structure of LiNiPO 4 within the Pnma space group. To nd the adsorption energy of water at the surface of the olivine phosphates, we generated a (010) plane surface slab, as this plane is the most stable. [35][36][37] The slab model was created from an optimized bulk unit cell, and the slab model was reoptimized aer generation. The lattice parameters for the surface slab model are ca. 4.8 A and 10.2 A. The thickness of the slab is approximately 13.7 A, as this thickness produces converged values for calculated surface energies, as reported by Wang et al. 38 A vacuum of 20 A in the [010]-direction is employed to nullify the periodic image effect normal to the (010) plane. Our surface slab constitutes 10 formula units of LiMPO 4 . In this stoichiometric surface slab, one end is terminated with Li ions and other end is terminated with TMs. One water molecule is attached to a TM site as shown schematically in Fig. 1 and S1 in ESI. †

Results and discussion
In the following paragraphs we will rst discuss bulk material properties, followed by a discussion of the electronic structure of the (010) surface and the adsorption of a single water molecule on this surface for different combinations of alkali and transition metals with and without defects.
Initially, we consider bulk orthorhombic LiNiPO 4 within the Pnma space group. In this system, we nd that the antiferromagnetic conguration is more stable than the ferromagnetic conguration by 28 meV (see Table S1 in ESI †). From electronic structure calculations, we observed that Ni is in a 2+ oxidation state and in a high spin state (calculated moments of Ni ions are ca. 1.8 m B ). In these pure systems we introduce one Na atom within the bulk unit cell consisting of four formula units. The formation energy (DE f ) is calculated using the following formula for Na doping at a Li site: where E(Li 0.75 Na 0.25 MPO 4 ) and E(LiMPO 4 ) are the ground state energies of Li 0.75 Na 0.25 MPO 4 and LiMPO 4 respectively. M is a TM, like Ni. Based on these calculations, we nd that Na preferably replaces Li in LiNiPO 4 system (see Table S2 in ESI †). Further we model a bulk unit cell in the presence of Li-vacancies (25%) and nd that Na replaces Li in Ni-Fe mixed TM material. Replacement of Li by Na is expected since both Li and Na ions are in a +1 oxidation state.
We now compare the density of states (DOS) of the bare surfaces of LiNi 0.7 Fe 0.3 PO 4 (Fig. 2a) and Li 0.8 Na 0.2 Ni 0.7 Fe 0.3 PO 4 (Fig. 2b). We observe that TM-3d states appearing near the Hence, we expect that Ni ions will be reduced rst during water adsorption rather than Fe, due to the presence of antibonding Ni-3d states. In presence of Li-vacancies (e.g. Li 0.6 Na 0.2 Ni 0.7 -Fe 0.3 PO 4 ), the scenario is different as both the occupied and unoccupied levels near the Fermi level are composed of Fe-3d states (Fig. 2c). Hence, water is expected to adsorb at Fe-sites in the case of Li-vacancies.
Further, we calculated the adsorption energy of water to understand the difference in reactivity of these surfaces. The formula for the adsorption energy (DE ads ) is  Table 1.
From the calculation of adsorption energy of a water molecule at different TM sites, we observe that the Ni site is preferable in the absence of Li vacancies (Table 1), in agreement with the expectation based on the computed DOS (vide supra).
Moreover, both Ni and Fe are in a 2+ oxidation state, and the reduction potential of Ni 2+ is higher than Fe 2+ , hence also supporting water adsorption at Ni sites. Unlike the above systems which are devoid of vacancies, the introduction of vacancies was found to change the favorable adsorption site from Ni to Fe as indicated by the adsorption energy calculations for Li 0.8 Ni 0.7 Fe 0.3 PO 4 ( Table 1). The preference for Fe sites can be ascribed to the presence of unoccupied states which consist of Fe 3d states closer to the Fermi level than the Ni 3d states, as discussed above.
In the absence of alkali ion vacancy, doping LiNi 0.7 Fe 0.3 PO 4 with Na does not affect water adsorption to this surface significantly, as indicated by the comparable adsorption energy to that of Li 0.8 Na 0.2 Ni 0.7 Fe 0.3 PO 4 (Table 1). However, the presence of alkali ion vacancy was found to increase the reactivity of the system signicantly, as indicated by the reduction in adsorption energy of Li 0. 8  We further calculated the difference in charge density aer calculating charge density along the grid points for the bare  surface, surfaces with water, and for the water molecule at the same position (in absence of slab) using the following formula: 53 Dr ¼ r(surface with water) À r(bare surface) À r(water) Here r(surface with water), r(bare surface), and r(water) are the charge densities for the respective systems. Then we estimated the charge polarization between systems by multiplying Dr with the volume of the unit cell and with electronic charge, as shown in Table 2.
We observed that the adsorption energy increases (absolute values) with increasing charge polarization upon water binding to the surface slab. The charge polarization is more prominent in the presence of Li-vacancies than the doping of Na. Hence, these results also reveal that the reactivity is increased due to the presence of vacancies rather than Na-doping and this is accompanied by greater charge polarization.
We further calculated Bader charges at the atom sites (see Tables S3 and S4 Table S3  We observed that for different combinations of alkali Li/Na and transition metals Ni/Fe or Ni/Co the calculated adsorption energy of a single water molecule at the (010) surface of the olivine phosphate follows a trend similar to that observed by Gershinsky et al. 33 for the electrochemical activity of these systems for OER reactions based on overpotential measurements. We found that decreasing adsorption energy follows a trend of decreasing overpotential for the series of olivine phosphate, as shown in Fig. 3 and Table S5 (in ESI). †

Conclusions
We studied the binding of water at the surface of olivine phosphates with mixed alkali metals (Li and Na) and mixed TM atoms (Ni and Fe) using DFT. We considered the (010) plane of the olivine phosphate in our surface slab model, where one end is terminated with Li metal atoms and other end is terminated with TM atoms. The computed adsorption energy revealed that water preferably attaches to Ni sites in the absence of alkali ion vacancies. We explained this preference from the DOS of the bare surface where Fe-3d states compose the valence band whereas unoccupied Ni-3d states make up the conduction band, facilitating water binding via Ni sites. This trend of water adsorption changes to Fe-sites when Li-vacancies are present in the system. This is consistent with the DOS of the bare surface with Li-vacancies, which showed that Fe-3d states form the conduction band. This is also in line with the enhancement in calculated Bader charge at Fe sites in the presence of vacancies. Further we showed that there is qualitative correlation between the calculated adsorption energy of water at the surface and experimentally measured overpotential for the different combinations of mixed alkali and TMs olivine phosphates. Among the various experimentally reported systems considered in this study, Li 0.6 Na 0.2 Ni 0.7 Fe 0.3 PO 4 with mixed alkali Li and Na in presence of Li-vacancies shows best electrocatalytic behavior experimentally and also is the most potent water binder. In short, this study provides new insights into the role of transition metals, Na-doping, and the effect of vacancy on the surface reactivity of a series of olivine phosphates with different stoichiometry. Importantly, the current calculations provide insights that may guide future experimental efforts.

Conflicts of interest
There are no conicts of interest to declare.