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Synthesis of spinel LiNi0.5Mn1.5O4 with secondary plate morphology as cathode material for lithium ion batteries
Journal of Power Sources 293 (2015) 137e142
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Synthesis of spinel LiNi0.5Mn1.5O4 with secondary plate morphology as cathode material for lithium ion batteries Tim Risthaus, Jun Wang*, Alex Friesen, Andrea Wilken, Debbie Berghus, Martin Winter, Jie Li* MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstrasse 46, 48149 Muenster, Germany
h i g h l i g h t s This work reports a new synthesis method of LiNi0.5Mn1.5O4 cathode material by spray drying with PVP as a template. Specific morphology LiNi0.5Mn1.5O4 material with plate shape secondary particle can be obtained. The plate exhibits faster kinetics for Liþ intercalation and consequently shows improved discharge rate performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2015 Received in revised form 2 May 2015 Accepted 15 May 2015 Available online xxx
Spinel LiNi0.5Mn1.5O4 material has been synthesized by a spray drying process and subsequent solid state reaction. Polyvinylpyrrolidone (PVP) is given as additive to the spray drying precursor solution and its effects on structural and electrochemical properties are evaluated. By using PVP in the synthesis process, the obtained sample displays a secondary plate morphology which is consisting of densely arranged primary octahedrally shaped particles. The new cathode material has a lesser degree of impurity phases, a higher discharge capacity, a superior rate capability, and a slightly better cycling performance than the sample synthesized without PVP. In more detail, by the use of PVP the ratio of Mn3þ to Mn4þ in the final product decreases from 20.8 to 9.2%. The initial discharge capacity at 0.1 C exhibits an increase of about 14%. The normalized capacity at 20 C is 84.1% instead of 67.0%. A slightly improved cycling performance with the capacity retention increase from 93.8 to 97.9% could be observed as well. © 2015 Elsevier B.V. All rights reserved.
Keywords: Cathode material Spinel Spray drying Polyvinylpyrrolidone
1. Introduction As one of the promising candidates for the next generation lithium ion batteries [1,2] and lithium ion capacitors [3], LiNi0.5Mn1.5O4 (LNMO) has attracted much attention since its discovery in 1997. With a theoretical capacity of 146.7 mAh g1 and a high average operational potential of 4.7 V vs. Li/Liþ arising from the Ni2þ/Ni4þ redox couple it offers a larger energy density compared to the common cathode materials like LiCoO2, LiFePO4 or LiMn2O4 [4]. LNMO provides three dimensional diffusion paths for Liþ intercalation/deintercalation, leading to a good rate capability and good safety. In addition, it is more economical and less toxic than the materials employing cobalt.
* Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (J. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.05.056 0378-7753/© 2015 Elsevier B.V. All rights reserved.
The spinel structure of LNMO is reported to exist in two polymorphs, depending on synthesis conditions: an ordered structure (P4332), where the Mn and the Ni ions occupy different lattice sites (12d and 4a, respectively) and a disordered structure (Fd-3m), where the transition metal ions are randomly distributed on the 16d sites [5,6]. The disordered phase is easily obtained during solid state synthesis at temperatures higher than 700 C [7,8]. However, this phase is reported to be oxygen deficient and that its synthesis is accompanied by the formation of a rock salt LixNi1xO impurity phase. To maintain charge neutrality, a part of the Mn ions is present as Mn3þ (and not as Mn4þ) to compensate for the oxygen loss. This oxygen deficiency is reported to improve Li diffusion inside the material as well as the electronic conductivity which leads to an overall better rate capability [8e10]. However, a high degree of oxygen vacancies corresponds to a large content of Mn3þ ions, which in turn leads to a higher capacity fading. It is also reported that the disordered structure as well as the rock salt phase can be transferred back into the ordered phase by a sufficient long
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annealing time at lower temperatures under air or oxygen [1,2,4]. When discussing the effect of the particle morphology, it must be distinguished between the primary particle morphology and secondary particle morphology. Different primary particle morphologies, namely octahedrons, truncated octahedrons and plates or cubes have been reported [11e14]. Modification of primary particle morphology can mainly be achieved by changing the calcination profile. At low temperatures only the energetically favourable {111}-lattice plane shows a considerable growth rate (leading to octahedral particles), while at higher temperatures the growth of less favourable lattice planes {110} and {100} also contribute to the morphology. For LiMn2O4, it is reported that {110}-surfaces both improve the Liþ ion diffusion and the Mn dissolution [15,16]. Furthermore, low calcination temperatures lead to smaller particles with higher surface energies, which have the tendency to agglomerate into larger secondary particles, whereas for high temperatures almost unagglomerated single crystals could be observed [16]. However, only a few reports were focused on the effects of the secondary particle morphology, reflected by the agglomerate. It is also believed that the secondary particle morphology, i.e., the morphology of an agglomerate of primary particles would affect the structure and electrochemical performance of the final product. The structures reported so far are mainly consisting of spherical or hollow particles [17,18]. In this paper, we apply a spray drying method with assistance of PVP to synthesize a spinel LNMO material, which has a secondary plate morphology. The effects of the PVP additive on the structure and the electrochemical performance of final product are investigated. The obtained LiNi0.5Mn1.5O4 material with a novel morphological character exhibits an enhanced electrochemical performance compared to the conventional material synthesized from the spray drying method.
range from 15 to 70 . The resulting diffraction patterns were refined using the Rietveld method and TOPAS Academic Ver. 4.1 software. To determine the chemical composition of the final products, inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted with a Spectro ARCOS ICP-OES. Particle morphologies were compared by employing scanning electron microscopy (Zeiss Auriga Crossbeam Workstation). TGA measurements were performed with the precursors under air atmosphere. The heating and the cooling rates, respectively, during the measurements were set to 5 K min1. 2.3. Electrochemical measurements
2. Experimental
In order to prepare the electrodes, the obtained material together with a conductive carbon (TIMCAL Super P) and a binder Polyvinylidene difluoride (PVdF, ARKEMA KYNAR®) (weight ratio 90:5:5) was mixed in 1-Methyl-2-pyrrolidinone (NMP, ACROS Organics) and casted onto Al foil. After being dried, electrode tapes were then punched into Ø12 mm discs, pressed by a hydraulic press, and dried at 120 C under vacuum overnight. 2032-type coin cells were assembled in a dry room using LP30, (1 M LiPF6 in 1:1 weight ratio of ethylene carbonate: dimethyl carbonate (EC:DMC), BASF) as electrolyte, and a glass fibre filter (Whatman GF/D) as separator. The mass loading of active material was approximately 4.9 mg cm2. The cells were galvanostatically charged and discharged on Maccor series 4000 battery testers (USA) between 3.5 and 4.9 V at 20 C. The voltages reported in this work refer to the Li/ Liþ couple. For the cycling tests, three formation cycles with a 0.1 C and further 50 cycles with a rate of 2.0 C were conducted (nominal current 1C ¼ 146.7 mA g1). In the C-rate test, the charging rate was kept constant at 0.2 C. In addition, cyclic voltammetry (CV) was conducted using Swagelok-type cells with a three electrode set up. The voltage range was set at 3.5e4.9 V and the sweep rate was 0.1 mV s1.
2.1. Synthesis
3. Results and discussion
The LiNi0.5Mn1.5O4 materials were synthesized by spray drying followed by high-temperature calcination. To obtain the precursor solution for the spray drying process, an aqueous solution of Li(Ac)$ 2H2O, Ni(Ac)2$4H2O, and Mn(Ac)2$4H2O was prepared with the molar ratio of Li:Ni:Mn ¼ 2.04:1:3. Compared to the stoichiometric LiNi0.5Mn1.5O4 a molar excess of 2% of lithium was added to compensate for the lithium evaporation during calcination. In a comparison experiment, polyvinylpyrrolidone (PVP) was added into the precursor solution, as well. For 1 mol of targeted product of LiNi0.5Mn1.5O4, 5 g PVP was added. The spray drying process was carried out under air atmosphere at a temperature of 110 C. The precursor was calcined at 800 C for 3 h with a heating rate of 5 K min1, using an open-air muffle oven. Afterwards the sample was cooled naturally to room temperature. Since long dwell times at high temperatures lead to a distinct densification of the precursor particles, here a relatively short calcination process was chosen, for the densification not to overcome the influence of PVP addition on the morphology development, although in literature often longer calcination times are reported. However, a too low temperature and/or a too short calcination time would lead to a poor degree of crystallinity and a higher content of impurities. Hereafter, the samples synthesized without and with PVP are labelled as LNMO and LNMO-PVP, respectively.
3.1. Materials synthesis and structural and morphological properties
2.2. Structure and morphology The crystal structure was determined by X-ray diffraction (XRD) with Cu Ka radiation on the Bruker D8 Advance (Germany) in the 2q
The process scheme of synthesis of the LNMO-PVP material with secondary plate morphology is illustrated by Fig. 1. The PVP additive plays a crucial role in the formation of this structure. The synthesis can be approximately separated into two stages: (1) spray-drying synthesis of spherical precursor, and (2) growth of crystalline primary particles, which form plate-like secondary particles by the help of PVP. For the LNMO material, the first step is similar. However, without the assistance of PVP, the crystalline particles would agglomerate randomly during the second growth step. Fig. 2 shows the weight change during heating of the precursor of LNMO as well as of LNMO-PVP. For the precursor of LNMO, dehydration can be observed as a first reaction in the temperature range of 60 Ce100 C. The decomposition of the acetates takes place during an exothermic reaction in a narrow temperature window from 285 to 350 C. Afterwards the weight increases slightly, probably due to the uptake of oxygen, i.e., the formation of the oxide, and then remains constant. A decrease in weight due to oxygen loss at higher temperatures as reported in literature could not be observed here [8]. For the LNMO-PVP, the heating process, which is shown by the dashed line in Fig. 2, shows a somewhat more complex thermal behaviour due to the addition of a relatively large amount of PVP into the precursor solution, which could be completely decomposed at temperatures higher than 600 C. Beyond this temperature, the chemical composition of the material should not be influenced by PVP anymore. It can be noted that the main
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Fig. 1. Schematic illustration of the formation of LNMO-PVP secondary plate particles by using a spray drying process.
Fig. 2. Thermogravimetric analysis profiles for the spray-dried precursor of LNMO (solid) and LNMO-PVP (dash), recorded at a heating rate of 4 C min1.
decomposition of PVP is at higher temperatures than that of acetates. It follows that the formation of the active material particle morphology can be strongly influenced by using PVP as an additive. The chemical compositions of the LNMO and LNMO-PVP materials differ only slightly from each other as shown by the ICP-OES measurements, listed in Table 1. Both samples show little nickel deficiency compared to the stoichiometric LiNi0.5Mn1.5O4. Fig. 3 shows the XRD patterns of the as-obtained LNMO and LNMO-PVP materials. Both samples have adopted the disordered spinel structure Fd-3m, where the transition metal ions of nickel and manganese are randomly distributed on 16a sites in the spinel lattice. However, both materials exhibit some rock salt impurity peaks at the 2 theta angles of around 38.0, 43.4 and 63.5 . These peaks are more distinctive for the LNMO sample, indicating a higher content of impurity phase. The optical appearance of the LNMO and LNMO-PVP samples is shown in Fig. S1. The presence of an orange impurity can be seen on the LNMO sample (Fig. S1a). This impurity is restricted only to the surface and can not be found in the bulk of the sample. However, this impurity phase is not found when PVP has been used in the synthesis process, as shown in Fig. S1b. EDX measurement reveals that these surface impurities are mainly
Table 1 The molar ratio results of Ni and Mn obtained by ICP-OES analyses. Sample
Ni
Mn
LNMO LNMO-PVP
0.482 0.486
1.5 1.5
Fig. 3. XRD patterns of (a) LNMO, and (b) LNMO-PVP; LixNi1-xO impurity phase marked by arrows.
composed of oxygen and manganese with a little trace of nickel. Although it is difficult to determine the composition of the impurity phases, it could be concluded form the EDX measurement that directly after calcination of the pure LNMO sample surface impurities are present, which contain mainly manganese and oxygen. This accumulation of manganese on the surface should lead to relatively lower manganese concentration in the bulk material in turn and thus to more nickel-rich impurity for the bulk LNMO sample, which is in accordance with the XRD pattern. The Rietveld refinement results (Fig. S2) for the LNMO and LNMO-PVP samples show good fittings of the disordered spinel phase with the experimental data. Additionally, the lattice parameters were deducted from the refined structure. As shown in Fig. 3, the LNMO sample exhibits a larger lattice parameter and therefore a larger cell volume than the LNMO-PVP sample, which indicates that the LNMO sample has a higher content of Mn3þ ions in the lattice. In other words the LNMO sample exhibits a more disordered structure than the LNMO-PVP sample, since the radius of the Mn3þ ion is larger than Mn4þ ion. The morphology of the materials was examined by scanning electron microscopy. Fig. 4 shows the particle agglomerates directly after the spray drying process and after the calcination process. After the spray drying process, both precursors show a spherical morphology with a wrinkled surface (Fig. 4a and b). The diameter of both spherical precursors is in the range of 2e10 mm. The use of PVP did not result in much shape change of the precursors up to this point. After calcination, the spherical shapes could not be maintained for both samples. The primary particles for both samples
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Fig. 4. SEM images of the obtained (a, c, e) LNMO, and (b, d, f) LNMO-PVP samples.
have an octahedral shape, which is the most common crystal shape for spinel LiNi0.5Mn1.5O4 materials. The primary particles of the LNMO-PVP sample seem exhibit better developed octahedral shape, whereas the LNMO sample exhibits areas of strongly conjoined primary particles. At contrast, the differences in the secondary particle morphology between these two materials are more obvious. The secondary particles of LNMO-PVP exhibit a flake or plate-like morphology being consisting of densely aligned primary octahedrally shaped particles. The obtained flake is big, but it appears fragile (Fig. 4d). For the LNMO sample, the primary particles agglomerate in a random way and less dense structures are formed after calcination. Based on above results, it is worth to mention that the use of PVP in the synthesis process does decrease the content of impurity phases and the degree of disordered phase
in the final product. Although PVP has no effect on the morphology of the precursor after spray drying, it plays an important role in the formation of the plate-like LNMO-PVP sample, which exhibits more homogeneous and dense morphology than the LNMO sample. Flake-like particles were also observed by Mao et al. [15], using a PVP assisted gel combustion method for precursor synthesis and low temperatures for calcination. This observation was attributed to the small particle size and their corresponding tendency to agglomerate because of their high surface energy. From our studies however, it can be concluded that even when the LNMO sample shows a strong tendency to agglomerate, the flake-like secondary particles are only formed due to the presence of PVP in the calcination process.
T. Risthaus et al. / Journal of Power Sources 293 (2015) 137e142
3.2. Electrochemical performance The first charge/discharge voltage profiles of the Li/LNMO and Li/LNMO-PVP cells are shown in Fig. 5a. The cells are examined under a current density of 0.1 C in the voltage range of 3.5e4.9 V. The LNMO-PVP sample delivers an initial discharge capacity of 122.4 mAh g1, but the LNMO sample could only exhibit a lower discharge capacity of 108.9 mAh g1. This is not unexpected, since the LNMO-PVP shows higher purity according to the XRD patterns, thus the capacity is expected to be higher as well. Furthermore, both profiles exhibit a first voltage plateau in the region of 4.1 V due to the Mn3þ/Mn4þ transition. However, this plateau is more distinctive for the LNMO sample, reflecting a higher Mn3þ amount
Fig. 5. (a) The first charge/discharge voltage profiles of the Li/LNMO and Li/LNMO-PVP cells at a current density of 0.1 C (14.6 mA g1). (b) Rate capabilities at various current rates. (c) Cycling behaviours excluding the formation cycles and the corresponding Coulombic efficiencies at 2 C for 50 cycles.
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in this material. The relative Mn3þ amount can be estimated from the voltage profile by taking the discharge capacity in the voltage region from 3.8 V to 4.25 V and dividing it through the total discharge capacity. Therefore, the relative Mn3þ contents are estimated to be 20.8% for the LNMO sample, and 9.2% for the LNMOPVP sample, respectively. This is in accordance with the XRD patterns and the higher extent of an (electrochemically inactive) impurity phase in the LNMO sample, which means that the lower discharge capacity of the LNMO sample is intrinsic to the material. The Coulombic efficiencies for the first charge/discharge processes are 81.2% and 82.7% for the LNMO and LNMO-PVP materials, respectively. The results are similar which means that the adding of PVP has no negative effect on the Coulombic efficiency. Fig. 5b shows the rate capabilities of the Li/LNMO and Li/LNMOPVP cells. Both cells are charged under a constant current density value of 0.2 C, and discharged at C-rates from 0.2 to 40 C for three cycles each. The use of the PVP additive leads to higher capacities at various current densities when compared to the LNMO material. The LNMO sample delivers discharge capacities of 110.0, 111.0, 110.4, 109.6, 106.7, 95.1, 73.7, and 24.2 mAh g1, whereas the LNMOPVP sample exhibits 122.1, 125.2, 124.7, 124.2, 122.5, 116.8, 102.7, and 56.0 mAh g1, respectively at discharge C-rates of 0.2, 0.5, 1, 2, 5, 10, 20, and 40 C. The normalized discharge capacity values of the LNMO sample are 100.0, 100.9, 100.4, 99.6, 97.0, 86.5, 67.0, and 22.0%, and for LNMO-PVP samples the values are 100.0, 103.4, 102.1, 101.7, 100.3, 95.7, 84.1, and 45.9%, respectively at the corresponding discharge C-rates. Both the LNMO and LNMO-PVP samples show good rate capabilities, there is no obvious capacity decay up to 5 C, which is attributed to the good intrinsic electronic conductivity of spinel LiNi0.5Mn1.5O4 materials. The loss of capacity becomes obviously when the discharge current density is higher than 5 C that the LNMO-PVP sample displays much better rate capability than the LNMO sample, due to the specific morphology and better purity. The cycling properties of the Li/LNMO and Li/LNMO-PVP cells charged/discharged at 2 C between 3.5 and 4.9 V for 50 cycles are shown in Fig. 5c. For the LNMO sample, the initial discharge capacity is 105.8 mAh g1. After 50 cycles, only 99.3 mAh g1 is left, with a capacity retention of 94%. The discharge capacity of the LNMO-PVP sample in the first cycle is 121.0 mAh g1. The corresponding capacity retention after 50 cycles amounts to 98%, which is higher than that of the LNMO sample as expected by the lower Mn3þ content. The rate capability of the LNMO-PVP is comparable to the reported LiNi0.5Mn1.5O4 materials [19,20]. The Coulombic efficiencies at the beginning of the cycling processes are 97% and 98% for the LNMO and LNMO-PVP materials, respectively, and reach to 99.5% after the first cycling processes. The use of PVP also leads to a better cycling performance (Fig. 5c). Cyclic voltammetry (CV) tests were conducted in order to further clarify the difference in the electrochemical behaviours between the LNMO and LNMO-PVP samples. The Li/LNMO and Li/ LNMO-PVP cells were scanned at 0.1 mV s1 for 5 cycles. The first CV curves at a scan rate of 0.1 mV s1 are shown in Fig. 6a, the following four cycles at 0.1 mV s1 are plotted in Fig. 6b. From both CV plots of the Li/LNMO and Li/LNMO-PVP cells, three redox peaks can be observed: The peaks at around 4.0 V vs. Li/Liþ are due to the Mn3þ/Mn4þ redox couple. For the Li/LNMO sample, however, it can be discussed whether there is a splitting into two minor peaks. Such behaviour was observed for the nickel-free, lithium manganese oxides (LiMn2O4) spinel and attributed to an ordering phenomenon of Liþ ions on the 8a sites [21,22]. The subsequent two peaks are related to the Ni2þ/Ni3þ and Ni3þ/Ni4þ redox couples. This distinct splitting is characteristic for the disordered spinel structure Fd-3m, whereas the ordered (P4332) structure exhibits only a small separation of the two peaks [5,6,23]. For the LNMO sample, the
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impurities on the surface during the calcination process which in turn leads to lower Ni-rich impurities in the bulk and a higher discharge capacity and better capacity retention. This general route could also be extended to synthesize other materials. And other synthesis additives such as citric acid, and urea or polymers like polyvinyl alcohol, etc. are supposed to have the abilities to improve the electrochemical performance of the resulting cathode materials, as well. Acknowledgements The authors kindly acknowledge the financial support of Federal Ministry of Education and Research, Federal Ministry of Economic and Technology and Federal Ministry for the Environment (BMWi), Nature Conservation and Nuclear Safety (BMU) of Germany within the project KaLiPat (03EK3008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.05.056. References
Fig. 6. (a) The initial CV curves of Li/LNMO and Li/LNMO-PVP cells at a scan potential rate of 0.1 mV s1. (b) The following four-cycle CV curves at the same scan potential rate.
anodic reactions in the 4.7 V vs. Li/Liþ region occur at higher and the corresponding cathodic reactions at lower potentials, compared to the LNMO-PVP sample. After repeated cycling (Fig. 6b) the anodic nickel redox couples slightly moved towards lower voltages for the LNMO-PVP sample and towards higher values for the LNMO sample. Since the separation of cathodic and anodic peaks refers to the reversibility of the redox reactions, LNMO-PVP exhibits faster intercalation/deintercalation kinetics for Liþ ions, which is consistent with the rate-capability tests [24]. 4. Conclusions LiNi0.5Mn1.5O4 spinel material with secondary plate morphology has been successfully synthesized by a spray drying method with assistance of PVP. These plates exhibit faster kinetics for Liþ intercalation and consequently show improved discharge rate performance. The addition of PVP also prevents the enrichment of Mn
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Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Synthesis of spinel LiNi0.5Mn1.5O4 with secondary plate morphology as cathode material for lithium ion batteries Tim Risthaus, Jun Wang*, Alex Friesen, Andrea Wilken, Debbie Berghus, Martin Winter, Jie Li* MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstrasse 46, 48149 Muenster, Germany
h i g h l i g h t s This work reports a new synthesis method of LiNi0.5Mn1.5O4 cathode material by spray drying with PVP as a template. Specific morphology LiNi0.5Mn1.5O4 material with plate shape secondary particle can be obtained. The plate exhibits faster kinetics for Liþ intercalation and consequently shows improved discharge rate performance.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 March 2015 Received in revised form 2 May 2015 Accepted 15 May 2015 Available online xxx
Spinel LiNi0.5Mn1.5O4 material has been synthesized by a spray drying process and subsequent solid state reaction. Polyvinylpyrrolidone (PVP) is given as additive to the spray drying precursor solution and its effects on structural and electrochemical properties are evaluated. By using PVP in the synthesis process, the obtained sample displays a secondary plate morphology which is consisting of densely arranged primary octahedrally shaped particles. The new cathode material has a lesser degree of impurity phases, a higher discharge capacity, a superior rate capability, and a slightly better cycling performance than the sample synthesized without PVP. In more detail, by the use of PVP the ratio of Mn3þ to Mn4þ in the final product decreases from 20.8 to 9.2%. The initial discharge capacity at 0.1 C exhibits an increase of about 14%. The normalized capacity at 20 C is 84.1% instead of 67.0%. A slightly improved cycling performance with the capacity retention increase from 93.8 to 97.9% could be observed as well. © 2015 Elsevier B.V. All rights reserved.
Keywords: Cathode material Spinel Spray drying Polyvinylpyrrolidone
1. Introduction As one of the promising candidates for the next generation lithium ion batteries [1,2] and lithium ion capacitors [3], LiNi0.5Mn1.5O4 (LNMO) has attracted much attention since its discovery in 1997. With a theoretical capacity of 146.7 mAh g1 and a high average operational potential of 4.7 V vs. Li/Liþ arising from the Ni2þ/Ni4þ redox couple it offers a larger energy density compared to the common cathode materials like LiCoO2, LiFePO4 or LiMn2O4 [4]. LNMO provides three dimensional diffusion paths for Liþ intercalation/deintercalation, leading to a good rate capability and good safety. In addition, it is more economical and less toxic than the materials employing cobalt.
* Corresponding authors. E-mail addresses: [email protected] (J. Wang), [email protected] (J. Li). http://dx.doi.org/10.1016/j.jpowsour.2015.05.056 0378-7753/© 2015 Elsevier B.V. All rights reserved.
The spinel structure of LNMO is reported to exist in two polymorphs, depending on synthesis conditions: an ordered structure (P4332), where the Mn and the Ni ions occupy different lattice sites (12d and 4a, respectively) and a disordered structure (Fd-3m), where the transition metal ions are randomly distributed on the 16d sites [5,6]. The disordered phase is easily obtained during solid state synthesis at temperatures higher than 700 C [7,8]. However, this phase is reported to be oxygen deficient and that its synthesis is accompanied by the formation of a rock salt LixNi1xO impurity phase. To maintain charge neutrality, a part of the Mn ions is present as Mn3þ (and not as Mn4þ) to compensate for the oxygen loss. This oxygen deficiency is reported to improve Li diffusion inside the material as well as the electronic conductivity which leads to an overall better rate capability [8e10]. However, a high degree of oxygen vacancies corresponds to a large content of Mn3þ ions, which in turn leads to a higher capacity fading. It is also reported that the disordered structure as well as the rock salt phase can be transferred back into the ordered phase by a sufficient long
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annealing time at lower temperatures under air or oxygen [1,2,4]. When discussing the effect of the particle morphology, it must be distinguished between the primary particle morphology and secondary particle morphology. Different primary particle morphologies, namely octahedrons, truncated octahedrons and plates or cubes have been reported [11e14]. Modification of primary particle morphology can mainly be achieved by changing the calcination profile. At low temperatures only the energetically favourable {111}-lattice plane shows a considerable growth rate (leading to octahedral particles), while at higher temperatures the growth of less favourable lattice planes {110} and {100} also contribute to the morphology. For LiMn2O4, it is reported that {110}-surfaces both improve the Liþ ion diffusion and the Mn dissolution [15,16]. Furthermore, low calcination temperatures lead to smaller particles with higher surface energies, which have the tendency to agglomerate into larger secondary particles, whereas for high temperatures almost unagglomerated single crystals could be observed [16]. However, only a few reports were focused on the effects of the secondary particle morphology, reflected by the agglomerate. It is also believed that the secondary particle morphology, i.e., the morphology of an agglomerate of primary particles would affect the structure and electrochemical performance of the final product. The structures reported so far are mainly consisting of spherical or hollow particles [17,18]. In this paper, we apply a spray drying method with assistance of PVP to synthesize a spinel LNMO material, which has a secondary plate morphology. The effects of the PVP additive on the structure and the electrochemical performance of final product are investigated. The obtained LiNi0.5Mn1.5O4 material with a novel morphological character exhibits an enhanced electrochemical performance compared to the conventional material synthesized from the spray drying method.
range from 15 to 70 . The resulting diffraction patterns were refined using the Rietveld method and TOPAS Academic Ver. 4.1 software. To determine the chemical composition of the final products, inductively coupled plasma optical emission spectrometry (ICP-OES) was conducted with a Spectro ARCOS ICP-OES. Particle morphologies were compared by employing scanning electron microscopy (Zeiss Auriga Crossbeam Workstation). TGA measurements were performed with the precursors under air atmosphere. The heating and the cooling rates, respectively, during the measurements were set to 5 K min1. 2.3. Electrochemical measurements
2. Experimental
In order to prepare the electrodes, the obtained material together with a conductive carbon (TIMCAL Super P) and a binder Polyvinylidene difluoride (PVdF, ARKEMA KYNAR®) (weight ratio 90:5:5) was mixed in 1-Methyl-2-pyrrolidinone (NMP, ACROS Organics) and casted onto Al foil. After being dried, electrode tapes were then punched into Ø12 mm discs, pressed by a hydraulic press, and dried at 120 C under vacuum overnight. 2032-type coin cells were assembled in a dry room using LP30, (1 M LiPF6 in 1:1 weight ratio of ethylene carbonate: dimethyl carbonate (EC:DMC), BASF) as electrolyte, and a glass fibre filter (Whatman GF/D) as separator. The mass loading of active material was approximately 4.9 mg cm2. The cells were galvanostatically charged and discharged on Maccor series 4000 battery testers (USA) between 3.5 and 4.9 V at 20 C. The voltages reported in this work refer to the Li/ Liþ couple. For the cycling tests, three formation cycles with a 0.1 C and further 50 cycles with a rate of 2.0 C were conducted (nominal current 1C ¼ 146.7 mA g1). In the C-rate test, the charging rate was kept constant at 0.2 C. In addition, cyclic voltammetry (CV) was conducted using Swagelok-type cells with a three electrode set up. The voltage range was set at 3.5e4.9 V and the sweep rate was 0.1 mV s1.
2.1. Synthesis
3. Results and discussion
The LiNi0.5Mn1.5O4 materials were synthesized by spray drying followed by high-temperature calcination. To obtain the precursor solution for the spray drying process, an aqueous solution of Li(Ac)$ 2H2O, Ni(Ac)2$4H2O, and Mn(Ac)2$4H2O was prepared with the molar ratio of Li:Ni:Mn ¼ 2.04:1:3. Compared to the stoichiometric LiNi0.5Mn1.5O4 a molar excess of 2% of lithium was added to compensate for the lithium evaporation during calcination. In a comparison experiment, polyvinylpyrrolidone (PVP) was added into the precursor solution, as well. For 1 mol of targeted product of LiNi0.5Mn1.5O4, 5 g PVP was added. The spray drying process was carried out under air atmosphere at a temperature of 110 C. The precursor was calcined at 800 C for 3 h with a heating rate of 5 K min1, using an open-air muffle oven. Afterwards the sample was cooled naturally to room temperature. Since long dwell times at high temperatures lead to a distinct densification of the precursor particles, here a relatively short calcination process was chosen, for the densification not to overcome the influence of PVP addition on the morphology development, although in literature often longer calcination times are reported. However, a too low temperature and/or a too short calcination time would lead to a poor degree of crystallinity and a higher content of impurities. Hereafter, the samples synthesized without and with PVP are labelled as LNMO and LNMO-PVP, respectively.
3.1. Materials synthesis and structural and morphological properties
2.2. Structure and morphology The crystal structure was determined by X-ray diffraction (XRD) with Cu Ka radiation on the Bruker D8 Advance (Germany) in the 2q
The process scheme of synthesis of the LNMO-PVP material with secondary plate morphology is illustrated by Fig. 1. The PVP additive plays a crucial role in the formation of this structure. The synthesis can be approximately separated into two stages: (1) spray-drying synthesis of spherical precursor, and (2) growth of crystalline primary particles, which form plate-like secondary particles by the help of PVP. For the LNMO material, the first step is similar. However, without the assistance of PVP, the crystalline particles would agglomerate randomly during the second growth step. Fig. 2 shows the weight change during heating of the precursor of LNMO as well as of LNMO-PVP. For the precursor of LNMO, dehydration can be observed as a first reaction in the temperature range of 60 Ce100 C. The decomposition of the acetates takes place during an exothermic reaction in a narrow temperature window from 285 to 350 C. Afterwards the weight increases slightly, probably due to the uptake of oxygen, i.e., the formation of the oxide, and then remains constant. A decrease in weight due to oxygen loss at higher temperatures as reported in literature could not be observed here [8]. For the LNMO-PVP, the heating process, which is shown by the dashed line in Fig. 2, shows a somewhat more complex thermal behaviour due to the addition of a relatively large amount of PVP into the precursor solution, which could be completely decomposed at temperatures higher than 600 C. Beyond this temperature, the chemical composition of the material should not be influenced by PVP anymore. It can be noted that the main
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Fig. 1. Schematic illustration of the formation of LNMO-PVP secondary plate particles by using a spray drying process.
Fig. 2. Thermogravimetric analysis profiles for the spray-dried precursor of LNMO (solid) and LNMO-PVP (dash), recorded at a heating rate of 4 C min1.
decomposition of PVP is at higher temperatures than that of acetates. It follows that the formation of the active material particle morphology can be strongly influenced by using PVP as an additive. The chemical compositions of the LNMO and LNMO-PVP materials differ only slightly from each other as shown by the ICP-OES measurements, listed in Table 1. Both samples show little nickel deficiency compared to the stoichiometric LiNi0.5Mn1.5O4. Fig. 3 shows the XRD patterns of the as-obtained LNMO and LNMO-PVP materials. Both samples have adopted the disordered spinel structure Fd-3m, where the transition metal ions of nickel and manganese are randomly distributed on 16a sites in the spinel lattice. However, both materials exhibit some rock salt impurity peaks at the 2 theta angles of around 38.0, 43.4 and 63.5 . These peaks are more distinctive for the LNMO sample, indicating a higher content of impurity phase. The optical appearance of the LNMO and LNMO-PVP samples is shown in Fig. S1. The presence of an orange impurity can be seen on the LNMO sample (Fig. S1a). This impurity is restricted only to the surface and can not be found in the bulk of the sample. However, this impurity phase is not found when PVP has been used in the synthesis process, as shown in Fig. S1b. EDX measurement reveals that these surface impurities are mainly
Table 1 The molar ratio results of Ni and Mn obtained by ICP-OES analyses. Sample
Ni
Mn
LNMO LNMO-PVP
0.482 0.486
1.5 1.5
Fig. 3. XRD patterns of (a) LNMO, and (b) LNMO-PVP; LixNi1-xO impurity phase marked by arrows.
composed of oxygen and manganese with a little trace of nickel. Although it is difficult to determine the composition of the impurity phases, it could be concluded form the EDX measurement that directly after calcination of the pure LNMO sample surface impurities are present, which contain mainly manganese and oxygen. This accumulation of manganese on the surface should lead to relatively lower manganese concentration in the bulk material in turn and thus to more nickel-rich impurity for the bulk LNMO sample, which is in accordance with the XRD pattern. The Rietveld refinement results (Fig. S2) for the LNMO and LNMO-PVP samples show good fittings of the disordered spinel phase with the experimental data. Additionally, the lattice parameters were deducted from the refined structure. As shown in Fig. 3, the LNMO sample exhibits a larger lattice parameter and therefore a larger cell volume than the LNMO-PVP sample, which indicates that the LNMO sample has a higher content of Mn3þ ions in the lattice. In other words the LNMO sample exhibits a more disordered structure than the LNMO-PVP sample, since the radius of the Mn3þ ion is larger than Mn4þ ion. The morphology of the materials was examined by scanning electron microscopy. Fig. 4 shows the particle agglomerates directly after the spray drying process and after the calcination process. After the spray drying process, both precursors show a spherical morphology with a wrinkled surface (Fig. 4a and b). The diameter of both spherical precursors is in the range of 2e10 mm. The use of PVP did not result in much shape change of the precursors up to this point. After calcination, the spherical shapes could not be maintained for both samples. The primary particles for both samples
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Fig. 4. SEM images of the obtained (a, c, e) LNMO, and (b, d, f) LNMO-PVP samples.
have an octahedral shape, which is the most common crystal shape for spinel LiNi0.5Mn1.5O4 materials. The primary particles of the LNMO-PVP sample seem exhibit better developed octahedral shape, whereas the LNMO sample exhibits areas of strongly conjoined primary particles. At contrast, the differences in the secondary particle morphology between these two materials are more obvious. The secondary particles of LNMO-PVP exhibit a flake or plate-like morphology being consisting of densely aligned primary octahedrally shaped particles. The obtained flake is big, but it appears fragile (Fig. 4d). For the LNMO sample, the primary particles agglomerate in a random way and less dense structures are formed after calcination. Based on above results, it is worth to mention that the use of PVP in the synthesis process does decrease the content of impurity phases and the degree of disordered phase
in the final product. Although PVP has no effect on the morphology of the precursor after spray drying, it plays an important role in the formation of the plate-like LNMO-PVP sample, which exhibits more homogeneous and dense morphology than the LNMO sample. Flake-like particles were also observed by Mao et al. [15], using a PVP assisted gel combustion method for precursor synthesis and low temperatures for calcination. This observation was attributed to the small particle size and their corresponding tendency to agglomerate because of their high surface energy. From our studies however, it can be concluded that even when the LNMO sample shows a strong tendency to agglomerate, the flake-like secondary particles are only formed due to the presence of PVP in the calcination process.
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3.2. Electrochemical performance The first charge/discharge voltage profiles of the Li/LNMO and Li/LNMO-PVP cells are shown in Fig. 5a. The cells are examined under a current density of 0.1 C in the voltage range of 3.5e4.9 V. The LNMO-PVP sample delivers an initial discharge capacity of 122.4 mAh g1, but the LNMO sample could only exhibit a lower discharge capacity of 108.9 mAh g1. This is not unexpected, since the LNMO-PVP shows higher purity according to the XRD patterns, thus the capacity is expected to be higher as well. Furthermore, both profiles exhibit a first voltage plateau in the region of 4.1 V due to the Mn3þ/Mn4þ transition. However, this plateau is more distinctive for the LNMO sample, reflecting a higher Mn3þ amount
Fig. 5. (a) The first charge/discharge voltage profiles of the Li/LNMO and Li/LNMO-PVP cells at a current density of 0.1 C (14.6 mA g1). (b) Rate capabilities at various current rates. (c) Cycling behaviours excluding the formation cycles and the corresponding Coulombic efficiencies at 2 C for 50 cycles.
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in this material. The relative Mn3þ amount can be estimated from the voltage profile by taking the discharge capacity in the voltage region from 3.8 V to 4.25 V and dividing it through the total discharge capacity. Therefore, the relative Mn3þ contents are estimated to be 20.8% for the LNMO sample, and 9.2% for the LNMOPVP sample, respectively. This is in accordance with the XRD patterns and the higher extent of an (electrochemically inactive) impurity phase in the LNMO sample, which means that the lower discharge capacity of the LNMO sample is intrinsic to the material. The Coulombic efficiencies for the first charge/discharge processes are 81.2% and 82.7% for the LNMO and LNMO-PVP materials, respectively. The results are similar which means that the adding of PVP has no negative effect on the Coulombic efficiency. Fig. 5b shows the rate capabilities of the Li/LNMO and Li/LNMOPVP cells. Both cells are charged under a constant current density value of 0.2 C, and discharged at C-rates from 0.2 to 40 C for three cycles each. The use of the PVP additive leads to higher capacities at various current densities when compared to the LNMO material. The LNMO sample delivers discharge capacities of 110.0, 111.0, 110.4, 109.6, 106.7, 95.1, 73.7, and 24.2 mAh g1, whereas the LNMOPVP sample exhibits 122.1, 125.2, 124.7, 124.2, 122.5, 116.8, 102.7, and 56.0 mAh g1, respectively at discharge C-rates of 0.2, 0.5, 1, 2, 5, 10, 20, and 40 C. The normalized discharge capacity values of the LNMO sample are 100.0, 100.9, 100.4, 99.6, 97.0, 86.5, 67.0, and 22.0%, and for LNMO-PVP samples the values are 100.0, 103.4, 102.1, 101.7, 100.3, 95.7, 84.1, and 45.9%, respectively at the corresponding discharge C-rates. Both the LNMO and LNMO-PVP samples show good rate capabilities, there is no obvious capacity decay up to 5 C, which is attributed to the good intrinsic electronic conductivity of spinel LiNi0.5Mn1.5O4 materials. The loss of capacity becomes obviously when the discharge current density is higher than 5 C that the LNMO-PVP sample displays much better rate capability than the LNMO sample, due to the specific morphology and better purity. The cycling properties of the Li/LNMO and Li/LNMO-PVP cells charged/discharged at 2 C between 3.5 and 4.9 V for 50 cycles are shown in Fig. 5c. For the LNMO sample, the initial discharge capacity is 105.8 mAh g1. After 50 cycles, only 99.3 mAh g1 is left, with a capacity retention of 94%. The discharge capacity of the LNMO-PVP sample in the first cycle is 121.0 mAh g1. The corresponding capacity retention after 50 cycles amounts to 98%, which is higher than that of the LNMO sample as expected by the lower Mn3þ content. The rate capability of the LNMO-PVP is comparable to the reported LiNi0.5Mn1.5O4 materials [19,20]. The Coulombic efficiencies at the beginning of the cycling processes are 97% and 98% for the LNMO and LNMO-PVP materials, respectively, and reach to 99.5% after the first cycling processes. The use of PVP also leads to a better cycling performance (Fig. 5c). Cyclic voltammetry (CV) tests were conducted in order to further clarify the difference in the electrochemical behaviours between the LNMO and LNMO-PVP samples. The Li/LNMO and Li/ LNMO-PVP cells were scanned at 0.1 mV s1 for 5 cycles. The first CV curves at a scan rate of 0.1 mV s1 are shown in Fig. 6a, the following four cycles at 0.1 mV s1 are plotted in Fig. 6b. From both CV plots of the Li/LNMO and Li/LNMO-PVP cells, three redox peaks can be observed: The peaks at around 4.0 V vs. Li/Liþ are due to the Mn3þ/Mn4þ redox couple. For the Li/LNMO sample, however, it can be discussed whether there is a splitting into two minor peaks. Such behaviour was observed for the nickel-free, lithium manganese oxides (LiMn2O4) spinel and attributed to an ordering phenomenon of Liþ ions on the 8a sites [21,22]. The subsequent two peaks are related to the Ni2þ/Ni3þ and Ni3þ/Ni4þ redox couples. This distinct splitting is characteristic for the disordered spinel structure Fd-3m, whereas the ordered (P4332) structure exhibits only a small separation of the two peaks [5,6,23]. For the LNMO sample, the
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impurities on the surface during the calcination process which in turn leads to lower Ni-rich impurities in the bulk and a higher discharge capacity and better capacity retention. This general route could also be extended to synthesize other materials. And other synthesis additives such as citric acid, and urea or polymers like polyvinyl alcohol, etc. are supposed to have the abilities to improve the electrochemical performance of the resulting cathode materials, as well. Acknowledgements The authors kindly acknowledge the financial support of Federal Ministry of Education and Research, Federal Ministry of Economic and Technology and Federal Ministry for the Environment (BMWi), Nature Conservation and Nuclear Safety (BMU) of Germany within the project KaLiPat (03EK3008). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2015.05.056. References
Fig. 6. (a) The initial CV curves of Li/LNMO and Li/LNMO-PVP cells at a scan potential rate of 0.1 mV s1. (b) The following four-cycle CV curves at the same scan potential rate.
anodic reactions in the 4.7 V vs. Li/Liþ region occur at higher and the corresponding cathodic reactions at lower potentials, compared to the LNMO-PVP sample. After repeated cycling (Fig. 6b) the anodic nickel redox couples slightly moved towards lower voltages for the LNMO-PVP sample and towards higher values for the LNMO sample. Since the separation of cathodic and anodic peaks refers to the reversibility of the redox reactions, LNMO-PVP exhibits faster intercalation/deintercalation kinetics for Liþ ions, which is consistent with the rate-capability tests [24]. 4. Conclusions LiNi0.5Mn1.5O4 spinel material with secondary plate morphology has been successfully synthesized by a spray drying method with assistance of PVP. These plates exhibit faster kinetics for Liþ intercalation and consequently show improved discharge rate performance. The addition of PVP also prevents the enrichment of Mn
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