Enhancing Na+ Extraction Limit Through High Voltage Activation of

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Enhancing Na Extraction Limit Through High Voltage Activation of the NASICON-type NaMnV(PO) Cathode 4

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Maxim V. Zakharkin, Oleg A. Drozhzhin, Ivan Tereshchenko, Dmitry Chernyshov, Artem Abakumov, Evgeny Antipov, and Keith J. Stevenson ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01269 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 4, 2018

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ACS Applied Energy Materials

Enhancing Na+ Extraction Limit Through High Voltage Activation of the NASICON-type Na4MnV(PO4)3 Cathode Maxim V. Zakharkin1,2,*, Oleg A. Drozhzhin1,2, Ivan V. Tereshchenko1,2, Dmitry Chernyshov3,4, Artem M. Abakumov1, Evgeny V. Antipov1,2, Keith J. Stevenson1

1Skolkovo

Institute of Science and Technology, 3 Nobel St, Moscow, 143025, Russia

2Lomonosov

Moscow State University, 1/3 Leninskie gory, Moscow, 119991, Russia

3Swiss–Norwegian

Beamlines, European Synchrotron, 71 Rue des Martyrs, Grenoble, 38043, France

4Peter

the Great St. Petersburg Polytechnic University, 29 Polytekhnicheskaya St, SaintPetersburg, 195251, Russia

KEYWORDS. Sodium-ion battery; NASICON; Na4MnV(PO4)3; phase transitions; operando XRD 1 ACS Paragon Plus Environment

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ABSTRACT. Sodium (de)intercalation in the Na4MnV(PO4)3 NASICON-type cathode has been studied with operando synchrotron X-ray powder diffraction and galvanostatic cycling up to 3.8 V and 4.0 V cut-off voltages. Symmetry reduction from rhombohedral to monoclinic was observed immediately after the start of the electrochemical desodiation, with restoring of the rhombohedral phase at 3.8 V. Cycling within 2.5-3.8 V potential range proceeds through both solid solution and two-phase processes. An additional voltage plateau at ~3.9 V is observed, associated with “unlocking” of the Na1 site in the rhombohedral phase. Reverse insertion of Na+ cations proceeds via the entire solid solution region. The experimentally observed discharge capacity increases by ≈14% after raising cut-off voltage.

The prospects of developing cheaper energy storage technologies along with the applicability of knowledge gained from designing lithium insertion electrodes motivate studies on Na-ion batteries. Extensively investigated Na3V2(PO4)3 (NVP) positive electrode (cathode) material crystallizes in a rhombohedral NASICON-type structure1–4 consisting of corner-sharing VO6 octahedra and PO4 tetrahedra and providing a 3D-network of Na+ diffusion pathways. NVP delivers highly reversible capacity of more than 110 mAh/g,1 with a voltage plateau at ~3.4 V resulting in energy density up to 400 Wh/kg. Nevertheless, it is highly desirable to improve this value in order for sodium-ion batteries to become competitive with the already matured lithiumion technology.5 NVP’s theoretical capacity of 118 mAh/g corresponds to (de)intercalation of 2 Na ions per formula unit. Partial substitution of vanadium in NVP could increase sodium (de)intercalation voltage, mainly due to activation of V4+/V5+ redox pair.6,7,8 Unfortunately, so far substitution of vanadium by other metals in NASICON-type cathodes did not result in increasing 2 ACS Paragon Plus Environment

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amount of the reversibly cycled Na cations. Discharge capacities corresponding to 2 Na ions per formula unit were reported for different Na3+δV2-xMx(PO4)3 materials, with M = Al,7,9 Mg,8 Fe,2,10,11 Cr,6,12 Mn,2,13 etc. One of these representatives is recently reported Na4MnV(PO4)3. The replacement of one vanadium with Mn in NVP lowers the cost of the material and enhances the operation voltage compared to that in Na3V2(PO4)3. Additionally, larger Mn2+ expands interstitial spaces for fast sodium ion diffusion in Na4MnV(PO4)3 making it capable of rapid charge and discharge.2 In this work, we studied phase transformations in Na4MnV(PO4)3 cathode material upon charge and discharge within different voltage limits. By raising the potential above 3.8 V we demonstrate “unlocking” of the Na1 site and extraction of ≈14% more sodium accompanied by highly unusual phase transformation behavior. Na4MnV(PO4)3 was prepared by a co-precipitation technique followed by annealing at high temperature (see Supporting information for details). The obtained material crystallizes in the R

3 c NASICON-type structure with the unit cell parameters a = 8.9640(1) Å, c = 21.4772(3) Å (Fig. S1 of Supporting information). The sample consists of irregularly-shaped agglomerated platelets with the size of 0.1 – 0.5 µm (Fig. S2). In order to reveal the (de)intercalation mechanism, operando synchrotron X-ray powder diffraction (SXPD) experiments have been performed at the European Synchrotron Radiation Facilities (ESRF) using an electrochemical cell with sapphire windows.14,15 As it has already been reported,2 sodium extraction and insertion in Na4MnV(PO4)3 proceeds via two steps in the 2.5-3.8 voltage window, with one sloping and one flat plateau at appr. 3.4 and 3.6 V (Figure 1a). After the very beginning of the Na+ deintercalation, SXPD patterns reflect a symmetry reduction from rhombohedral to monoclinic, which is evident from appearance of additional reflections (Figure 1d). Therefore for the structure refinements we used both rhombohedral and a well-known monoclinic modification of 3 ACS Paragon Plus Environment


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the NASICON structure with C2/c symmetry (Figure 2).16 It must be pointed out that X-ray

Figure 1. Charge-discharge curves and transformations of the selected regions of SXPD patterns and unit cell parameters of Na4-xMnV(PO4)3 cathode material within 2.5 – 3.8 V (a, b, c, d) and 2.5 – 4.0 V (e, f, g, h) voltage range.

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powder diffraction patterns of rhombohedral and monoclinic Na4-xMnV(PO4)3 are very similar (Figure S4). The monoclinic phase manifests itself in the 110, -423 and 040 reflections (at ≈ 5.5o, 16.5o and 18.0o 2Ɵ for λ = 0.68987 Å) which are absent in the rhombohedral pattern. Their intensity is very sensitive to the Na distribution between available crystallographic positions (see examples in Figure S5). Evolution of the intensity of 110 monoclinic reflection during cycling is shown in Figure 1d. Unit cell parameters of the initial electrode in the monoclinic setting are a = 15.4679(7) Å, b = 8.9438(4) Å, c = 8.8048(3) Å, 𝛽 = 125.756(3)o. The first step of the charge process up to 3.5 V is a solid solution reaction, which is illustrated by a continuous shift of the reflection positions (Figure 1b) and variation of the unit cell parameters (Figure 1c, Table S1) associated with extraction of Na+ until the “Na3MnV(PO4)3” composition is reached. Such behavior is quite atypical for the NASICON-based cathode materials, where the initial extraction of Na or Li usually proceeds via a two-phase reaction17,18 because of cation-vacancy ordering. The maximum of the intensity of additional monoclinic reflections corresponds to the end of the solid solution region (Figure 1d) concurrent with strong distortions of the transition metal octahedra MO6 (compare the octahedral distortion parameter d at different state of charge in Table S2). Further charge follows a two-phase reaction at 3.65 V plateau, resulting in a formation of sodium-poor “Na2MnV(PO4)3” at 3.8 V. Since the typical monoclinic reflections are absent in the SXPD pattern of the charged electrode, we designated “Na2MnV(PO4)3” phase as rhombohedral. Transition metal octahedral distortions at this point are minimal. Dependence of the unit cell volumes of the monoclinic and rhombohedral phases on the state of charge is shown in Figures 1c and 1g.



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In case of the 4.0 V cut-off, a new voltage step is observed on charge at 3.9 V (Figure 1e). This step is associated with a solid-solution-type reaction illustrated by a smooth shift of reflections towards higher 2θ angles (Figure 1f). Extraction of Na+ up to the Na1.77MnV(PO4)3 composition is accompanied by 1.6% volume contraction. Experimental reversible capacity is above the theoretical value of 111 mAh/g for 2 Na+ ions extraction, in contrast with ≈100 mAh/g obtained after charge to 3.8 V.

Figure 2. Crystal structure of the (a) initial (Na3.66MnV(PO4)3, rhombohedral structure) and (b) charged to 3.5 V (Na2.96MnV(PO4)3, monoclinic structure) phases19. Details of the refinement are given in supplementary materials (Tables S3-S7, Fig. S6-S10).

Three Na positions exist in the monoclinic structure: 4d (Na1), 4e (Na2) and 8f (Na3), and only two - 6b (Na1) and 18e (Na2) – in the rhombohedral structure (Figure 2). Rhombohedral Na2 (contains 3 Na atoms per f.u.) site splits into Na2 (1 Na per f.u.) and Na3 (2 Na per f.u.) in the monoclinic cell. During desodiation Na+ cations are extracted mainly from the Na2 and Na3 positions while occupancy of Na1 remains almost steady. Inequality of Na2 and Na3 sites and 6 ACS Paragon Plus Environment

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their uneven occupation may result from distortions of transition metal octahedra with one short M-O distance of 1.82(3) Å (Table S2) possibly due to formation of a vanadyl bond. During the 3.4 V voltage step transition metal octahedral deformations increase by an order of magnitude from d(pristine) = 0.398∙10-3 to d(3.5V) = 3.85∙10-3. Nevertheless, at the beginning of the twophase process occupation of the Na2 and Na3 positions becomes almost equal. This is followed by consolidation of the Na2 and Na3 sites back into the single Na2 position of the rhombohedral cell. Overall Na content in the rhombohedral phase at 3.8 V was refined to 1.9, with the Na1 and Na2 occupancy factors of 1 and 0.300(5), respectively. Transition metal octahedra at this point are almost regular with the distortion value of d = 3∙10-6, which is atypical for Jahn-Teller active Mn3+ octahedra, shown to be present in “Na2MnV(PO4)3” upon charge to 3.8 V.2 Such low distortions may indicate either suppression of cooperative Jahn-Teller distortion because of the Mn3+/V4+ mixing or concurrent existence of both Mn3+ octahedra with long Mn-O bonds20 and V4+ octahedra with short V-O bonds21 at the local level. These effects can’t be resolved by X-ray powder diffraction. Further charge from 3.8 to 4.0 V is accompanied by extraction of Na+ mainly from the Na1 position; its occupancy decreases to 0.87(1). In contrast to Na3V2(PO4)3,4,22 Na4MnV(PO4)3 and desodiated NaV2(PO4)3,23 at this state of charge the average Na1-O distance is longer than the average Na2-O distance (Table S2), which can be related to the “unlocking” of the Na1 site. Total Na content in the electrode charged to 4.0 V, as determined from Rietveld refinement, is 1.77, which is in a good agreement with the electrochemical data. Short-range cycling of the electrodes within different voltage thresholds (with upper potential varying from 3.8 to 4.4 V, Figure 3) revealed 2.5 – 4.0 V interval as the most promising for practical application because of combining sufficient discharge capacity and cycling stability. 7 ACS Paragon Plus Environment


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Figure 3. (a) Discharge capacity retention and coulombic efficiency for Na4MnV(PO4)3 electrodes cycled at C/5 current rate within different potential intervals. At first cycle potential limit was set as 3.8V for all cells. (b) Charge-discharge curves corresponding to the 2nd cycle.

Thus, we conclude that raising the voltage above 3.8 V enhances the limit of Na+ extraction above 2 sodium atoms per formula unit through depletion of the Na1 position, which was not shown to take part in desodiation of the Na3V2(PO4)3.22 This activation drastically affects the nature of the reverse charge process. A sloping discharge curve was observed (Figure 1e), corresponding to continuous shift of the SXPD reflections and unit cell parameters without any 8 ACS Paragon Plus Environment

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signature of the first order phase transition. Reinsertion of Na+ into Na1.77MnV(PO4)3 on discharge down to 2.5 V proceeds via continuous solid solution in contrast with biphasic Na+ reinsertion into Na1.9MnV(PO4)3. The observed relation between Na+ extraction level and the (de)intercalation mechanism in Na4-xMnV(PO4)3 is in part consistent with the results of Chen et al.24 Moreover, such asymmetry was previously observed for the Li+ extraction from monoclinic LiV2(PO4)3 causing oxidation of V4+ to V5+ up to the V2(PO4)3 composition followed by a Li+ reinsertion via a solid-solution mechanism, which persisted until the Li2V2(PO4)3 composition.25 V4+/V3+ and Li+ ordering were found to govern (de)intercalation by a two-phase mechanism, while suppression of the charge ordering in the fully extracted V2(PO4)3 induced random population of the Li sites on reinsertion resulting in a solid-solution regime.26,27

ASSOCIATED CONTENT Supporting Information. Synthesis, SXPD and electrochemical measurements details. XRD pattern, SEM image and TG curve for the as-synthesized Na4MnV(PO4)3. SXPD patterns, fractional atomic coordinates, occupancy factors, atomic displacement parameters and selected interatomic distances at different states of charge and discharge. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions



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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources M.Z., O.D., I.T., A.A. and E.A. are grateful to the Russian Science Foundation (Grant No. 1773-30006). The work was carried out within the framework of the MSU-Skoltech Center for Electrochemical Energy Storage and the Moscow State University Program of Development. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors greatly acknowledge Dr. Dmitry Rupasov for TG measurements, Dr. Iurii Dovgaliuk and all SNBL staff for their support. ABBREVIATIONS XRD X-ray Diffraction; NASICON NA SuperIonic CONductor; NVP Na3V2(PO4)3; SXPD Synchrotron X-ray Powder Diffraction; ESRF European Synchrotron Radiation Facilities; SEM Scanning Electron Microscopy; TG thermogravimetry; REFERENCES (1)

Saravanan, K.; Mason, C. W.; Rudola, A.; Wong, K. H.; Balaya, P. The First Report on

Excellent Cycling Stability and Superior Rate Capability of Na3V2(PO4)3 for Sodium Ion Batteries. Adv. Energy Mater. 2013, 3, 444–450.


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Table of Contents



Charge-discharge curves and transformations of the selected regions of SXPD patterns and unit cell parameters of Na4-xMnV(PO4)3 cathode material within 2.5 – 3.8 V (a, b, c, d) and 2.5 – 4.0 V (e, f, g, h) voltage range.

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Crystal structure of the (a) initial (Na3.66MnV(PO4)3, rhombohedral structure) and (b) charged to 3.5 V (Na2.96MnV(PO4)3, monoclinic structure) phases. Details of the refinement are given in supplementary materials (Tables S3-S7, Fig. S6-S10). 164x72mm (220 x 220 DPI)



(a) Discharge capacity retention and coulombic efficiency for Na4MnV(PO4)3 electrodes cycled at C/5 current rate within different potential intervals. At first cycle potential limit was set as 3.8V for all cells. (b) Chargedischarge curves corresponding to the 2nd cycle.


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TOC 66x35mm (300 x 300 DPI)


Enhancing Na+ Extraction Limit Through High Voltage Activation of

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