Size-Dependent Reaction Mechanism of λ-MnO2 Particles as Cathodes in Aqueous Zinc-Ion Batteries

Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, USA Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, USA Department of Chemistry, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, USA Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, USA Beckman Institute of Advanced Science and Technology, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, USA

λ-MnO 2 has cubic spinel structure, with Mn and O ions occupying the octahedral 16d and 32e sites, respectively. Ideally, metal ions such as Zn 2+ can insert into or extract from the tetragonal sites with the spinel structure maintained (Figure 1(a)). In our previous work, the effect of particle size on Mg-ion intercalation in λ-MnO 2 has been thoroughly studied, where we show that reducing particle size can enhance Mg-ion intercalation [16]. This effect can be attrib-uted to the solid-solution type phase transition pathway of λ-MnO 2 NPs from cubic to tetragonal phase during Mgion insertion, which minimizes lattice mismatch and energy penalty for phase transformation.   Energy Material Advances Herein, we further illustrate the relations among the particle size, Zn-ion intercalation, side reactions, and the resulting electrochemical performance in ZIBs, by investigating λ-MnO 2 NPs (~80 nm) and micron-sized particles (MPs) (~900 nm) as a model system. Through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX), we show that reducing particle size enhances Zn-ion insertion in spinel lattice, but at the same time, severe water-induced Mn dissolution leads to quick capacity degrading. In contrast, MPs undergo an activation process due to the formation of amorphous MnO x nanosheets in the open spacings of the electrode during cycling, which can reduce charge transfer resistance and contribute to the capacity. This paper highlights the complicated and nonmonotonic size effects of cathode materials on the reaction mechanism and electrochemical property of ZIBs, which offers guidelines for integrating nanoengineering and high-performance cathode materials design for aqueous ZIBs. In order to study the effect of particle size on the performance of λ-MnO 2 , λ-MnO 2 NPs and MPs are collected by several steps of centrifugation (Eppendorf 5804 centrifuge). First, 0.2 g of λ-MnO 2 particles is dispersed in 30 mL of DI water in a centrifuge tube by sonication.

Experimental
To collect NPs, the dispersion is centrifuged at 3000 rpm for 5 min. The supernatant is then collected for one more round of centrifugation at 8000 rpm for 5 min. Next, the sediment at the bottom of the tube is collected and redispersed in DI water by sonication and finally dried at room temperature to get NPs.
To collect MPs, the dispersion is centrifuged at 2000 rpm for 2 min. Then, the supernatant is removed, and the sediment is collected and redispersed in 30 mL DI water for two more rounds of centrifugation (2000 rpm, 2 min), each time with the supernatant removed and the sediment redispersed in 30 mL of DI water. Finally, the suspension is dried at room temperature to get MPs.

Preparation of Working
Electrodes. Working electrodes are prepared by mixing NPs (or MPs) with carbon black and PVDF at a fixed weight ratio of 8 : 1 : 1 in a desirable amount of NMP solvent in an agate mortar. The mixture is ground for 15 min in the glovebox to achieve uniform slurry. The slurry is then cast on the stainless steel shim with a film casting doctor blade (MTI) to control the thickness of the slurry film (~15 μm). The corresponding mass loading is 2 mg/cm 2 . The stainless steel shim cast with slurry film is then left in the vacuum oven to dry overnight at room temperature to obtain the working electrode. Finally, the working electrode is punched into circular pieces (12 mm in diameter) for electrochemical tests.

Electrochemical Measurements.
A custom-made Swagelok cell and a multichannel potentiostat (VMP3, Bio-Logic Science Instruments) are used for all the electrochemical measurements. The cell is assembled using MnO 2 electrode as the cathode, glass fiber as the separator, Zn foil as the anode, and 0.2 M ZnSO 4 aqueous solution as the electrolyte. The galvanostatic measurements are conducted within the voltage range of 0.8 V to 1.9 V (vs. Zn 2+ /Zn) under different current densities. Cyclic voltammetry (CV) curves are measured in the voltage range of 0.8 V to 1.9 V at various scan rates. The electrochemical impedance spectroscopy (EIS) is performed with an amplitude of 10 mV in the frequency range from 200 kHz down to 5 mHz.

2.5.
Characterization. X-ray diffraction (XRD, Bruker D8 Advance) with Cu K α radiation (λ = 1:5418 Å) is used to characterize the MnO 2 electrodes before and after electrochemical measurements. Scanning electron microscopy (SEM, Hitachi 4800) is used to monitor the size of λ-MnO 2 particles after centrifugation and the surface morphology of MnO 2 electrodes before and after electrochemical measurements. Transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) are performed on JEOL 2100 Cryo TEM with a LaB 6 emitter at 200 kV. Energy-dispersive X-ray spectroscopy (EDX) and highangle annular dark-field (HAADF) imaging are performed on Themis Z analytical scanning TEM (STEM, Thermo Fisher) as operated at 300 kV. X-ray photoelectron spectroscopy (XPS, Kratos Axis ULTRA) is used for the surface composition analysis of the electrodes after cycling.

Results and Discussion
The λ-MnO 2 particles are prepared from as-purchased LiMn 2 O 4 particles by simple acid leaching (0.1 M HCl) to extract Li + ions. The XRD patterns of LiMn 2 O 4 and λ-MnO 2 ( Figure S1) show that the crystal structure of LiMn 2 O 4 (cubic phase, Fd 3m group) is maintained after acid leaching, with the diffraction peaks shifting towards higher angles, indicating the decrease of lattice parameter from 8.219 Å to 8.042 Å. The composition of λ-MnO 2 is 3 Energy Material Advances estimated to be Li 0.12 Mn 2 O 4 according to the lattice parameter [17]. The morphology of the particles has no obvious change after delithiation, with particle size highly polydispersed, ranging from several micrometers to tens of nanometers ( Figure S2). To study the effect of particle size on the performance of λ-MnO 2 , we employ several steps of centrifugation (see Section 2.2) to obtain λ-MnO 2 NPs (80 ± 15 nm, Figures 1(b) and 1(d)) and MPs (900 ± 295 nm, Figures 1(c) and 1(e)). The HAADF image and corresponding SAED pattern present the single-crystal nature of NPs ( Figure S3).
A custom-made Swagelok cell is used for all the electrochemical measurements, with λ-MnO 2 electrode as the cathode, glass fiber as the separator, Zn foil as the anode, and 0.2 M ZnSO 4 aqueous solution as the electrolyte. CV curves of NPs (Figure 1(f)) and MPs (Figure 1(g)) are measured between 0.8 and 1.9 V (vs. Zn 2+ /Zn) at a scan rate of 0.5 mV/s to reveal the redox reactions in the battery. For NPs, there is one strong peak at around 1.2 V and one relatively weak peak at around 1.4 V within the first discharge. According to the previous studies [18,19], the peak at 1.2 V corresponds to the intercalation of Zn 2+ , the current intensity of which drops during cycling, with the peak position shifting towards lower potentials, indicating the increase of Zn-ion insertion resistance. In comparison, MPs have one reduction peak at around 1.1 V in the first cycle, while two reduction peaks can be observed at 1.15 V and 1.35 V in the following cycles. The current intensity of the reduction peaks for MPs is much smaller than that of NPs at the beginning, which means lower electrochemical activity. However, the gradual increase of current during cycling suggests an activation process for MPs. The rate and cycling performance of NPs and MPs are consistent with the CV results. Figure 1(h) shows the discharge capacities of NPs and MPs at different current rates. Each capacity is the average value of five successive cycles. The capacities of NPs at 20, 100, 200, 500, and 1000 mA/g are 184, 163, 144, 124, and 91 mAh/g, respectively. In comparison, MPs exhibit much lower discharge capacities at these current rates. However, although NPs can deliver higher initial capacity, the cycling stability is poor, with only 10% capacity retained after 50 cycles at 200 mA/g, while MPs undergo an activation process in the first 50 cycles and then become stable with the capacity maintained at around 65 mAh/g (Figure 1(i)).
To investigate why NPs show better rate performance and higher initial capacity than MPs but suffer quick capacity fading, NP/Zn batteries are first charged to 1.9 V to remove residual Li + and then discharged to 1.4 V, 1.3 V, and 0.8 V, respectively, at a current density of 20 mA/g (Figure 2(a)). It is notable that the discharge capacity is larger than charge capacity in the first cycle (Figure 2(a)), but this difference does not persist in the later cycles ( Figure S4). We attribute this irreversible capacity in the first battery cycle to the formation of solid electrolyte interphase (SEI) film on the electrode surface or the trapping of Zn ions inside the spinel lattice due to lattice distortion during Zn-ion insertion, which has been observed in Li-ion batteries [20][21][22][23]. Electrodes are then collected at these cutoff voltages for XRD characterization ( Figure 2(b)). When the battery is discharged to 0.8 V, the peaks of cubic spinel λ-MnO 2 disappear, while a few new peaks emerge at 18.2°, 33.0°, and 36.4°, which can correspond to tetragonal spinel ZnMn 2 O 4 . To closely monitor the phase transition process, a zoomed-in view of the XRD pattern in the vicinity of c (111) peak is also provided (Figure 2(c)). The c (111) peak gradually shifts towards lower angles during Zn-ion insertion and finally turns into t (101), indicating a solid-solution type phase transition from cubic spinel λ-MnO 2 to tetragonal spinel ZnMn 2 O 4 . EDX mapping of NPs collected from state "C" also proves successful Zn-ion intercalation (Figure 2(f)). The uniform distribution of Zn 2+ in the particle further confirms the solid-solution type phase transformation. When the battery is recharged to 1.9 V, the t (101) peak shifts to higher angles but does not return to the original position of c (111) (Figure 2(c)). This means only part of the inserted Zn ions can be extracted from the spinel lattice, resulting in the irreversible capacity loss for the first cycle. Aside from Zn-ion insertion, proton insertion is also widely discussed for different phases of MnO 2 , often resulting in the formation of MnOOH that is detectable by XRD [4,18,24]. However, as shown in Figures 2(b) and 3(b), in our system, no obvious MnOOH peaks are detected by XRD for the cathode particles collected at different cutoff voltages, indicating that proton insertion is not the main reaction mechanism for spinel λ-MnO 2 . It is also notable that the peaks labeled with red " * " match well with Zn 4 SO 4 (OH) 6 •4H 2 O (ZHS). The SEM image of the electrode collected from state "C" (Figure 2(d)) shows the formation of ZHS flakes on the electrode surface, consistent with the observations in previous studies [25][26][27]. Part of these flakes disappears after recharging to state "D" (Figure 2(e)). It has been reported that the formation of ZHS is due to the increase of pH in the electrolyte during discharge, which is caused by the water-induced Mn dissolution (MnO 2 + 2H 2 O + 2e − ⟶ Mn 2+ + 4OH − ) [25]. Therefore, the large amounts of ZHS detected after discharge represent severe Mn dissolution, which causes the loss of active cathode materials and leads to capacity fading.
As a comparison, MP/Zn batteries are also charged and discharged at 20 mA/g (Figure 3(a)) and then characterized in the same way as NPs. The initial discharge capacity of MPs is 50 mAh/g, which is much smaller than that of NPs (250 mAh/g). This observation can be explained by the XRD patterns of MPs at different cutoff voltages, which show negligible changes for the diffraction peaks of λ-MnO 2 during discharge (Figures 3(b) and 3(c)), indicating limited Zn-ion insertion within the first cycle. ZHS peaks are also detected, with the peak intensity relatively weaker than that of NPs. The SEM image of the electrode collected from state "C" is consistent with the XRD results, showing smaller ZHS flakes on the electrode surface (Figure 3(d)), which totally disappear (Figure 3(e)) after recharging to state "D." Less ZHS formation indicates weaker Mn dissolution reaction compared to NPs due to smaller surface area (51.6 m 2 /g for NPs and 15.1 m 2 /g for MPs [16]). Element mapping of MPs collected from state "C" shows that Zn ions accumulate on the surface of MPs, forming a core-shell 4 Energy Material Advances structure, due to the slow Zn-ion diffusivity (Figure 3(f)). Based on these results, the enhanced initial capacity and rate performance of NPs can be attributed to the solid-solution type phase transition pathway, where the minimized lattice mismatch and energy penalty for accommodating new phases facilitate Zn-ion intercalation [16]. Next, CV curves of NPs (Figure 4(a)) and MPs (Figure 4(b)) at varying scan rates are measured to study the mechanism of charge storage for λ-MnO 2 with different particle sizes. The area under the curve represents the total stored charge contributed by the diffusion-limited faradaic process (intercalation) and the non-diffusion-limited redox pseudocapacitance. The relation between current i and scan rate v at a fixed voltage V can be expressed by a combination of these two factors: where a 1 v and a 2 v 0:5 represent capacitance and intercala-tion, respectively [16,28]. The values of a 1 and a 2 can be obtained by linearly fitting iðVÞ/v 0:5 as a function of v 0:5 at different voltages based on the CV curves. Then, CV curves representing intercalation (red) and capacitance (blue) are replotted at 2 mV/s as an example according to a 1 and a 2 ( Figure S5a,b). The stored charge corresponding to each contribution can be calculated from the area under each curve. As Figure 4(c) shows, for fresh electrodes, the total stored charge for NPs is 105 mAh/g, with 16% contributed by capacitance and 84% contributed by intercalation. In comparison, MPs only exhibit a total charge storage of 22 mAh/g, with 10% and 90% corresponding to capacitance and intercalation, respectively. The higher capacitance contribution of NPs can be attributed to the larger surface area than that of MPs, while the greatly improved intercalation is due to the solid-solution type phase transition pathway [16,29]. Similarly, CV curves of NPs (Figure 4(d)) and MPs (Figure 4(e)) at varying scan rates are also measured after 50 cycles of galvanostatic discharge and charge at 200 mA/g. As shown in Figure 4(f), 5 Energy Material Advances the total stored charge for NPs is decreased to 35 mAh/g after cycling (17% by capacitance and 87% by intercalation). In contrast, the stored charge for MPs is increased to 50 mAh/g (8% by capacitance and 92% by intercalation), which means the activation process of MPs can facilitate Zn-ion intercalation. The enhanced Zn-ion insertion can be proved by the XRD patterns of MPs after different cycles of discharge (Figure 4(g)). From the detailed view of the features in the vicinity of c (111) peak (Figure 4(h)), a new t (101) peak can be observed after 50 cycles, demonstrating the formation of tetragonal ZnMn 2 O 4 phase due to Zn-ion insertion. The element mapping of MPs after 100 th discharge (Figure 4(i)) shows more uniform Zn-ion distribution compared to that of the first discharge, which further supports the improved Zn-ion intercalation after the activation process.
To better understand the degrading mechanism of NPs and the activation process of MPs, EIS measurements are conducted after different cycles of charge (Figures 5(a)  and 5(b)). According to the equivalent circuit model ( Figure 5(c)), the values of series resistance (R s ), interfacial resistance between the electrolyte and electrode (R i ), and charge transfer resistance (R ct ) can be obtained by EIS fitting. As Table S1 shows, the series resistances of NPs (~9 Ω) and MPs (~8 Ω) do not show significant changes during cycling. After 100 cycles, the interfacial resistances of NPs and MPs increase from 5 to 11 Ω and 4 to 13 Ω, respectively, which may indicate a SEI film formation on the particle surface [30]. However, the interfacial resistance is negligible compared to the charge transfer resistance. The charge transfer resistance of NPs increases rapidly from 83 to 285 Ω during the first 50 cycles, which can be attributed to the formation of a passivated layer on the particle surface due to the severe reactions with the electrolyte [31]. The XRD patterns of NPs after different cycles of charge and discharge show negligible shift in peak positions ( Figure S6a,b), indicating suppressed Zn-ion insertion/ extraction due to the passivated layer. In comparison, the charge transfer resistance of MPs increases from 350 to 710 Ω during the first 20 cycles, which is much larger than  Energy Material Advances that of NPs, resulting in lower electrochemical activity. Interestingly, in the following cycles, R ct drops dramatically to 123 Ω and then becomes stable after 50 cycles. The corresponding XRD patterns show that the t (101) peaks disappear after charge ( Figure S6c,d), suggesting good reversibility of Zn-ion insertion/extraction. Therefore, we hypothesize that such significant impedance decrease may indicate a new phase formation, which can enhance Zn-ion diffusion and thus improve the capacity of MPs.
To confirm this hypothesis, the surface morphology change of NPs and MPs after cycling is studied by SEM and TEM. The SEM images show that there is nanosheet formation covering the electrode surface during cycling for both NPs and MPs ( Figure 6). The composition of the surface layer can be confirmed by XPS measurements. For both NP electrode and MP electrode, a peak is detected at 642.9 eV, which corresponds to Mn 2p 3/2 ( Figure S7). According to previous studies, these nanosheets are MnO x generated from the oxidation of Mn 2+ in the electrolyte during charge, which can reduce charge transfer resistance and contribute to the capacity [32]. Therefore, the activation process of MPs can be attributed to the formation of MnO x nanosheets during cycling. However, this conclusion still cannot explain why the nanosheet formation does not improve the cycling performance of NPs. For further investigation, SEM images of the cross-  Figure S9c) also shows the formation of nanosheets covering the MP surface. The corresponding SAED pattern ( Figure S9d) exhibits the amorphous character of the nanosheets. In contrast, there is no obvious surface morphology change for most of NPs after cycling ( Figure S9a,b), which is consistent with the cross-sectional SEM results. The HAADF images of NPs and MPs before and after 100 cycles show that the structure of the particles is maintained during cycling ( Figure S10a,b), without severe corrosion or structural collapse. This observation can further prove that the continuous increase of capacity for MPs is due to the formation of amorphous MnO x nanosheets instead of possible increase in particle surface area due to particle collapsing. According to the discussions above, the main reactions on the λ-MnO 2 electrode in ZIBs can be concluded by the following equations: As the schematic shows (Figure 7(a)), water-induced Mn dissolution (Equation (3)) occurs on both NP and MP elec-trodes during discharge. However, this reaction is more drastic for NPs due to their large surface area, which causes the degrading of capacity. As a result, more ZHS flakes form (Equation (4)) on the NP electrode surface compared to MPs. During charge, the dissolved Mn 2+ in the electrolyte will be oxidized and deposited back to the electrode to form amorphous MnO x nanosheets. The MnO x nanosheets can only overlay the top of the electrode for NPs, while they can fill the open spacings between MPs. At the same time, the H + generated from the oxidation of Mn 2+ will cause the dissolution of ZHS flakes on the electrode surface. At the single-particle level (Figure 7(b)), Zn-ion insertion (Equation (2)) in NPs is enhanced due to large surface area and solid-solution type phase transition pathway, resulting in a homogeneous tetragonal ZnMn 2 O 4 phase formation. However, the inserted Zn ions are only partially extractable due to the strong energy barrier for Zn-ion extraction. In contrast, Zn-ion insertion is limited for fresh MPs, with Zn 2+ accumulating on the particle surface, forming a coreshell structure. During charge, amorphous MnO x nanosheets will form on the surface of MPs, which can reduce charge transfer resistance and also act as a host for Zn-ion insertion. The gradual growth of MnO x nanosheets during cycling can correspond to the activation process of MPs.
After figuring out Mn dissolution as the major degrading mechanism of λ-MnO 2 NPs in aqueous ZIB system, a common strategy is applied to improve the cycling stability by adding 0.1 M MnSO 4 to the electrolyte, which can effectively reduce Mn dissolution [4]. As a result, the  9 Energy Material Advances cyclingperformance of NPs is significantly improved, with a capacity of 115 mAh/g retained for over 500 cycles at a current rate of 1 A/g (Figure 8(a)). As a comparison, MPs are first activated at 200 mAh/g for 50 cycles, after which the current is increased to 1 A/g for measurement. After activation, the capacity of MPs is maintained at about 70 mAh/g (Figure 8(b)), which is lower than that of NPs. It is also notable that the coulombic efficiencies for both NPs and MPs are around 100%, indicating good reversibility of the battery. The electrochemical performance of λ-MnO 2 NPs with Mn additive in the electrolyte is superior to most of the cathode materials that have been reported (Table S2) [4,8,9,11,15,33,34], demonstrating that λ-MnO 2 can also be a potential cathode material for ZIBs by controlling the particle size and Mn dissolution.

Conclusions
In this work, we reveal the effects of particle size on the performance and reaction mechanisms of λ-MnO 2 in aqueous Zn-ion batteries. We show that λ-MnO 2 MPs have limited Zn-ion intercalation initially but undergo an activation process during cycling due to the formation of amorphous MnO x nanosheets in the open spacings of the electrode. By reducing particle size, a higher capacity of 250 mAh/g can be obtained at 20 mA/g for λ-MnO 2 NPs, which is on account of the enhanced Zn-ion insertion due to large surface area and solid-solution type phase transition pathway. Meanwhile, reducing particle size can also lead to severer water-induced Mn dissolution, which causes the quick capacity degrading of NPs. To improve the cycling stability of NPs, MnSO 4 is added to the electrolyte to suppress Mn dissolution. As a result, a capacity of 115 mAh/g can be maintained over 500 cycles at 1 A/g for NPs, exceeding the performance of most Mn oxide-based electrodes. This paper not only highlights the advantages of using NPs as the cathode material for ZIBs but also points out the issues of possibly enhanced side reactions and corresponding solutions, which can offer guidelines for the design of highperformance cathode materials for ZIBs.

Data Availability
All data presented in the paper and the supporting information are available from the corresponding author upon reasonable request.

Supplementary Materials
XRD patterns of LiMn2O4 and λ-MnO2; SEM images of LiMn2O4 and λ-MnO2; TEM image and corresponding SAED pattern of a single NP; CV fitting for NPs and MPs at initial state and after 50th charge at 200 mA/g; XRD patterns of NPs and MPs after different cycles of discharge and charge at 200 mA/g; Mn 2p 3/2 XPS spectra of the electrode surface after 100 th charge at 200 mA/g; SEM images of the cross-section of fresh electrodes and electrodes after 100 th charge; TEM images of NPs and MPs after 100 th charge; HAADF images of MPs before and after 100 cycles; specific values of series resistance, interface resistance, and