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Using Nanoscale Domain Size to Control Charge Storage Kinetics in Pseudocapacitive Nanoporous LiMn2O4 Powders Benjamin K Lesel, John Berkeley Cook, Yan Yan, Terri Chai Lin, and Sarah H. Tolbert ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00634 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 7, 2017

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ACS Energy Letters

Using Nanoscale Domain Size to Control Charge Storage Kinetics in Pseudocapacitive Nanoporous LiMn2O4 Powders Benjamin K. Lesel,1 John B. Cook,1 Yan Yan,1 Terri Lin,1 and Sarah H. Tolbert1,2,3,* 1

Department of Chemistry and Biochemistry, UCLA, Los Angeles CA, 90095-1569

2

Department of Materials Science and Engineering, UCLA, Los Angeles, CA 90095 3

The California NanoSystems Institute, UCLA, Los Angeles, CA 90095 *

Address correspondence to: [email protected]

Abstract Pseudocapacitive materials can produce charge storage devices that have both high energy and power density. Although many pseudocapacitive anode materials have been identified, there is a lack of equivalently fast charging cathode materials necessary to create full-cell devices. Recently, thin-film studies from our group have identified nanoporous LiMn2O4 with ~15 nm domains as a pseudocapacitive cathode material. In this work, we use this insight to create nanoporous LiMn2O4 powders that can be used in practical, slurry-type thick electrode systems. Using these materials, we specifically examine the role of crystalline domain size in controlling charge storage kinetics. Four nanoporous LiMn2O4 powders were synthesized with crystallite sizes of 10, 20, 40 and 70 nm and their charge/discharge kinetics were studied. Smaller crystallite sizes showed lower capacity, but faster charge/discharge speeds, longer cycle life, and higher capacitive contribution based on kinetic analysis, whereas larger crystallite sizes showed the opposite trend. Importantly, there appeared to be a critical size, above which the charge/discharge kinetics and cycle life deteriorated significantly; materials with domain sizes just below this critical size showed the best combination of electrochemical performance characteristics. Key words: LiMn2O4, pseudocapacitor, lithium ion battery, high rate, nanoporous, cathode, kinetic analysis

The standard Li-ion battery found in mobile devices today has reasonable energy density, but requires charging on the hour time-scale. Capacitors are capable of charging on the seconds time-scale, but have low energy

1 ACS Paragon Plus Environment

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density. To bridge this gap, one can either make capacitors store more energy by significantly increasing their surface area, such as graphene supercapacitors, 1- 6 or as we will discuss here, make batteries charge faster by reducing the solid state diffusion distance for lithium ions within the electrodes. 7 -

15

Indeed, much the kinetic

limitation in lithium ion batteries that causes slow charging comes from the slow solid-state diffusion of Li-ions through the micron-length scale powders making up the electrodes. However, if the electrodes were made out of nano- rather than micron-scale powders, diffusion path lengths would be shorter and charging times could be faster. Care must be taken not to increase electrical resistance when using nanoscale materials, however. Li intercalation and extraction are an essential part to the charging and discharging of Li-ion batteries, since redox sites are located within the bulk of the electrode materials. When these diffusion processes are fast due to sufficiently small diffusion lengths with respect to the diffusion coefficient of the material (usually between 10-20 nm), the term intercalation pseudocapacitance is used to describe the fast intercalation/extraction redox processes.7-15 To date, many intercalation pseudocapacitive anode systems have been investigated including αMoO3, MoS2, T-Nb2O5 and others.8-14 Pseudocapacitive cathode materials, on the other hand, which typically display higher oxidation/reduction potentials (>3 V vs Li/Li+), are far more scarce, but are necessary for full cell fast charging Li-ion batteries. Despite this scarcity, there are several promising fast charging cathode systems which could potentially be pseudocapacitive, this includes nanosized LiCoO2, LiVPO4F, and Li(Ni0.8Co0.15Al0.05)O2, VOPO4 nanosheets, and nanoporous or nanostructured LiMn2O4.15- 20 One reason for the lack of pseudocapacitive cathode materials is the high crystallization temperature required to synthesize most cathode materials. This high crystallization temperature makes it difficult to form fine nano-structured materials and limits the applicability of solgel based polymer templating methods, which is one of our primary methods for making electrically interconnected yet still nanosized electrode materials. 21 Of these materials, LiMn2O4 is notable for its low crystallization temperature and ease of solution synthesis, and so we have chosen to focus on this system. In general, improving nano-scale architectures for intercalation pseudocapacitance is complex and requires optimization of many parameters. Simply reducing the grain size of a powder from the micron-scale to the nanoscale is not enough to improve kinetics. A typical slurry electrode consists of active material powder, conductive additives such as carbon black, and a binder such as PVDF. Active material consisting of individual nano-sized grains incorporated into the slurry can produce electrodes that are kinetically slow due to poor electrical conductivity between nanosized grains and reduced electrolyte penetration into regions of agglomerated nanoparticles, even though Li-ion diffusion lengths are short. One way to reduce the grain size and still maintain good conductivity and electrolyte penetration, is to use micron sized powders of porous materials with nano-scale pores and walls. These arrangements are commonly fabricated via soft or hard templating of nanocrystals or sol-gel precursors, which produces a porous structure when the template is removed. Unfortunately, with many nanostructured systems, new issues can arise due to the increased surface area. Specifically, either reduced or oxidized electrochemically inactive surfaces can form which can reduce overall capacity in systems with high surface areas. Anode materials can have oxidized surfaces, such as MoO2 with an inactive MoO3 surface or Si with an inactive SiO2 surface.12,22 By contrast, cathode materials can have reduced surfaces, such as LiCoO2 and LiMn2O4. In these materials Co2+ and Mn2+, respectively, can sit is surface Li+ sites,


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ACS Energy Letters

while the bulk is composed entirely of 3+ and 4+ transition metal ions.15,16,23,24 LiNi0.8Co0.15Al0.05O2 also suffers from an inactive surface layer of improperly mixed cations. 25- 27 Moreover, its high temperature of crystallization (~800°C) makes it particularly difficult to nanostructure.25-27 Some pseudocapacitive anode materials, such as MoS2 and Nb2O5, don’t seem to suffer from inactive surfaces and can thus be fabricated as small as needed for intercalation pseudocapacitive applications.10,11 To achieve full cells, optimized nanostructured cathode materials requires the most attention, and spinel LiMn2O4 is one of the most promising candidates. Although the capacity of LiMn2O4 is lower than the layered cathode materials, it is the easiest to fabricate into nanostructures due to its low crystallization temperature of ~550°C.15,23 This is in stark contrast to the layered cathode materials like LiCoO2 and LiNi0.8Co0.15Al0.05O2, which require temperatures several hundreds of degrees higher to form.16,25-27 Because of the low crystallization temperature of LiMn2O4, nanostructures can easily be synthesized through solid state conversions of nanostructured manganese oxides such as MnO2 or Mn3O4 mixed with a lithium salt.

15,28- 31

When heated to

temperatures around 550°C, the nanostructure remains intact and the correct crystal structure is achieved. Additionally, LiMn2O4 contains no particularly expensive or toxic elements, making it both affordable and environmentally benign. The electrochemical window of LiMn2O4 occur between 3.3V and 4.5V, which is nearly ideal as it is high enough to give a good cell voltage, but not so high that it causes electrolyte decomposition.15,32 In our previous report, we show that LiMn2O4 is capable very fast charge and discharge when finely nanostructured on conductive thin films.15 The capacity was reduced, however, due to the electrochemically inactive surface discussed above. We expect capacity and charge/discharge rate to be inversely proportional, and so in this letter, we study nanoporous LiMn2O4 materials with various crystallite sizes. It is important to note that we will be focusing on the kinetics of both charge and discharge in nanostructured LiMn2O4, since much of the literature in nanostructured LiMn2O4 is primarily focused on improved discharging rates. To maximize discharging capacity, slow charging can be employed. 33-35 The data presented here thus helps shed light on the different capacity and kinetic behavior seen throughout nanostructured LiMn2O4 in the literature. We note that earlier studies have shown crystallite size dependent trends for LiMn2O4 cycled below the cubic-to-tetragonal transition.23 Cycling through this transition destabilizes the structure, however, so is not relevant for typical cathode operation. Many nanostructured LiMn2O4 systems have been explored in the literature with various crystallite sizes and electrochemical performances.34,36- 39 As mentioned above, we focus here on the LiMn2O4 literature that show fast kinetics for both extraction and intercalation of Li-ions. One such example is hollow LiMn2O4 nanofibers with crystallite sizes of ~60 nm which shows 120 mAh/g charge capacity at 1C rate and retains 75% capacity retention at 10C.36 Another example is nanoporous LiMn2O4 spheres with crystallite size around 20 nm which has 100 mAh/g charge capacity at 1C and retains 90% of this capacity at 10C.37 It should be noted that some systems are less crystalline due to lower heating and often show slower kinetics as a result. 40,41 For example, polymer templated nanostructured LiMn2O4 heated to 500°C with 15 nm crystallite size shows 92 mAh/g charging capacity at 1C with 85% capacity retention at 10C.38 In this letter, we show kinetic analysis of a series of samples made in the same way and then flash crystallized at high temperatures to increase the domain size. We find increasing capacity with


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increased crystallite size and decreasing charge/discharge kinetics, but the non-linear nature opens the potential for material optimization.

Nanoporous LiMn2O4 powders were fabricated by polymer templating of Mn3O4 nanocrystals, followed by a solid-state conversion reaction with LiOH to form the LiMn2O4 spinel phase at 550°C for 2 hours. Full synthetic details are provided in the SI. The thickness and crystallinity of the LiMn2O4 walls was then modified by a high temperature coarsening step at 700, 800 or 900°C for 1 minute. This processed four nanoporous LiMn2O4 powder samples with increasing crystallite sizes and wall thicknesses that were used for electrochemical kinetic analysis. Figure 1A shows the X-ray diffraction patterns of all four nanoporous powders along with their average crystallite size calculated from the Scherrer equation. All patterns match LiMn2O4 (JCPDS card 00-35-0782). As expected, samples heated to lower temperatures show broader peaks corresponding to smaller crystallite sizes. The sample heated at 550°C for 2 hours gave a crystallite size of 10 nm; when heated for an additionally 1 minute at 700°C, 800°C and 900°C, the crystal size grew to 20 nm, 40 nm and 70 nm, respectively. Figure 1B shows SEM images of the four LiMn2O4 samples with different crystallite sizes. The SEM images show that the samples sintered at the lowest temperatures show the smallest grain size, which corresponds well the Scherrer analysis. Nitrogen adsorption/desportion isotherms are shown in Figure 2. Pore sizes, which tend to mimic wall thicknesses, for the same three samples calculated from the desorption isotherms are in good agreement with the calculated Scherrersize. The 70 nm sample is not included because its surface area was too small for effective isotherms to be measured without significant scale-up. Slurry electrodes from the nanoporous LiMn2O4 powders were prepared using 75% active mass, 10% carbon black, 5% carbon fibers and 10% PVDF with a final LiMn2O4 mass loading of 1-2 mg/cm2. Swagelok cells using the various porous LiMn2O4 electrodes were prepared in a glovebox with Li metal as the counter electrode and 1 M LiPF6 in EC:DMC 1:1 as the electrolyte. Charging and discharging was always done at the same rate. The samples were cycled galvanostatically at rates of 5, 10 and 20C (Figure 3A) with 1C defined using the theoretical current density of 148mA/g. The first galvanostatic charge/discharge curves for each sample at 5, 10 and 20C can be seen in Figure S1. The capacity increases with increasing crystallite size as the amount of electrochemically inactive surface is reduced.15,16,23 To better separate the effects of the inactive surfaces from capacity loss at high rate, capacities were also normalized to their 5C rate and the capacity drop with increasing rate was examined (Figure 3B). Surprisingly, the kinetics are rather similar for the 10 nm, 20 nm and 40 nm samples, but drop significantly for the 70 nm sample. Figure 3C shows that in addition to dramatically better rate behavior, the 40 nm sample also retains ~75% capacity after 2000 cycles at 10C, whereas the 70 nm sample shows significant capacity fade of ~50% after just 500 cycles at 10C. To better understand the differences in these samples, we examined the charge storage kinetics using sweep rate dependent cyclic voltammetry (CV). CV curves for the four LiMn2O4 samples with sweep rates (ν) of 1, 0.5 and 0.2 mV/s are shown in figure 4. The redox peaks in the CV curves are associated with bulk redox processes in LiMn2O4 during Li+ intercalation and detercalation. 42 Greater polarization shifts can be seen with increasing sweep rate, particularly for larger crystallite sizes. Fitting of this data can be used to differentiate the diffusion controlled


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current (normal battery intercalation, which varies as ν½) from the capacitive current (which should vary linearly with the scan rate, ν) for each given voltage (V) using the equation 1.8-15 i[V] = k1·ν + k2·ν1/2.

(1)

The current at any given sweep rate can then be divided into a capacitive and a diffusion controlled or standard battery-intercalation based component. The 0.2 mV/s sweep rate for each crystallite size shown in figure 4 contain shaded regions which correspond to the capacitive contribution to the current responses at every voltage. The percentage of the current that is shaded is therefore the percent capacitive character for a given material at slow rate, when both capacitive and diffusion controlled processes can occur. Note that there is less capacitive character around the redox peaks for all samples which indicates that these bulk (i,e, non-surface) redox process are the slowest. The 10, 20 and 40 nm samples show significantly improved capacitive character around the peaks compared to the 70 nm sample, however. As such, the smaller crystallite sized samples, which showed better kinetics in galvanostatic cycling, also show increased capacitive character in this kinetic analysys. Capacitive character increases with decreasing crystallite size, with the exception of the 10 nm sample, which is slightly less capacitive than the 20 nm sample, likely due to reduced crystallinity in this lower temperature sample.40,41 Close inspection of the capacitive contributions again shows that the 10, 20 and 40 nm samples are very similar, and that the 70 nm sample shows a significantly reduced capacitive fraction. An alternative analysis maps a single exponent (b) onto the sweep rate dependent data that can vary between 0.5 and 1, with a b-value of 0.5 again representing diffusion controlled processes, and a b-value of 1 representing capacitive processes. Here we focus only on the peak current, so that ipeak varies as νb, given by equation 2. 43 log(ipeak) = b*log(ν) + C

(2)

Figure S2 in the supplementary information shows the b-value analysis plots for all samples for the anodic (charge) peaks, with both charge and discharge data summarized in Table S1; peaks labels are shown in Figure S3. For 10 nm and 20 nm samples, b-values around 0.9 were calculated, clearly suggesting highly capacitive redox processes. The 40 nm sample displayed b-values averaging around 0.76, implying more mixed behavior. The 70 nm sample’s average b-value of 0.67 indicates that Li+ intercalation and deintercalation in this sample is dominantly diffusion controlled. It is interesting to speculate about the origin of the significant difference in kinetic behavior that occur with increasing crystallite size. These changes may result from suppression of the phase transitions that normally occur in bulk LiMn2O4 upon lithiation and delithiation.42 Such suppression of phase transition is seen in other nanoscale systems and is correlated with faster kinetics.12,13,44,45 One would expect suppression of phase transition to also effect long term cyclability since phase changes are a large contributor to capacity fade over many cycles. The data presented here suggest that such phase change suppression would likely occurs between 40 nm and 70 nm crystallite size in LiMn2O4 with some blurring of the range occurring due to polydispersity on the crystallite sizes of each sample. Samples just below this critical size (apparently ~40 nm) should show the best combination of good capacity due to a limited amount of inactive surface, and good rate capabilities. This is further supported by previous reports on LiMn2O4 which have suggested that a critical size exists around ~40 nm, below which a solid solution


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mechanism takes place when this material is cycled at the cubic to tetragonal transition (occurring below 3 V vs Li/Li+).23 Our work confirms this value and additionally shows that this phenomenon occurs not only below the 3 V range, but additionally in the more practical 4 V range. Nanoporous powders of the cathode material, LiMn2O4 were prepared by solid state conversion of a nanoporous Mn3O4 powder with LiOH. The resulting powders were flash crystallized at different temperatures leading to four distinct average crystallite sizes: 10, 20, 40 and 70 nm. The powders were characterized using both potentiostatic and galvanostatic cycling to determine their charge storage properties. Reduced capacity was observed in smaller crystallite sized LiMn2O4 powders due to an increased fraction of inactive surface sites, but charge storage kinetics were fast due to shorter Li-ion diffusion lengths. Kinetic analysis of the nanoporous powders showed that in general, smaller crystallite size gave increased capacitive character. The capacitive character was similar for 10, 20 and 40 nm samples (70-85 % capacitive) and differed from the 70 nm sample (~50 % capacitive). The large drop in capacitive character at 70 nm could result from suppression of Li-intercalation induced phase changes below 70 nm domain sizes. This idea is supported by long term cycling data for the 70 nm sample, which drops to 50% capacity retention before 500 cycles, while the 40 nm sample retains ~75 % capacity after 2000 cycles. Because the 40 nm sample has similar kinetics to the smaller crystallite sizes but has higher capacity, it is perhaps the most ideal of the crystallite sizes studied here for practical applications as a fast charging pseudocapacitive cathode material. The prevalence of fast charging and discharging as well as stable long term cycleability of the smaller nanoporous LiMn2O4 powders is ascribed to the direct access of the electrolyte through the porous structure, in combination with the short diffusion path lengths and possibly phase change suppression. These results suggest that nanoporous LiMn2O4 around 40 nm crystallite size may be a suitable cathode material for fast charging lithium ion batteries.

Acknowledgements This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under award DE-SC0014213.

Associated Content Detailed experimental, galvanostatic discharge curves and b-value calculations, tables and figures.


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A) Scherrer Width: ~10 nm

550°C, 2 hour

Scherrer Width: ~20 nm

B)

700°C, 1 min


800°C, 1 min


890°C, 1 min

10 nm

1 µm

100 nm

20 nm

40 nm

70 nm

1 µm

1 µm

1 µm

100 nm

100 nm

100 nm

Figure 1: A) XRD of nanoporous LiMn2O4 powders with different crystallite sizes obtained by thermal coarsening of the smallest samples. Scherrer domain sizes and heating conditions are shown on the figure. B) Low (top) and high (bottom) resolution SEM images of nanoporous LiMn2O4 powders with different average crystallite sizes (shown on the image). The images show both the primary and secondary particle sizes. All larger domain size samples were made by thermal coarsening of the 10 nm material.


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A)

10 nm

C)

40 nm

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B)

20 nm

D) 10 nm 20 nm 40 nm

Figure 2: Nitrogen adsorption/desorption isotherms A), B) and C) for 10, 20 and 40 nm samples, respectively and D) Desorption pores sizes, which tend to mirror wall-thickness, for 10, 20 and 40 nm LMO samples.


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A)

B)

5C 5C

10C 20C

70 nm 40 nm 20 nm

20 nm 40 nm 10 nm

10 nm 70 nm

C) 40 nm 70 nm

10C

Figure 3: A) Galvanostatic capacity testing at 5, 10 and 20C for four sizes of nanoporous LiMn2O4 powders as slurry electrodes. Smaller samples have lower capacity due to an increased fraction of inactive surface atoms. B) Charge time versus normalized capacity for nanoporous LiMn2O4 powders with different crystallite sizes. Samples with domain sizes up to 40 nm show similar capacity loss at higher rate, but the kinetics slow down appreciably for the 70 nm sample. C) Comparison of long term galvanostatic rate testing at 10C for 40 nm and 70 nm LiMn2O4 samples. The 70 nm sample shows rapid capacity fade, while the 40 nm sample shows much more stable cycling.


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20 nm

84% capacitive

81% capacitive

40 nm

70 nm

71% capacitive

52% capacitive

Figure 4: CV curves sweep rates of 1 (green), 0.5 (red) and 0.2 (black) mV/s for all nanoporous LiMn2O4 powders with average crystallite sizes shown on the figure. Shaded fits to equation 1 are shown within the 0.2 mV/s curve. Total percent capacitive contribution for each sample is noted in the bottom left corner of each plot. All samples with average crystallites sizes less than 70 nm show dominantly capacitive behavior.


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