Hybridizing δ-Type MnO2 With Lignin-Derived Porous Carbon as a Stable Cathode Material for Aqueous Zn–MnO2 Batteries

With the advantages of intrinsic safety, low price, high output potential, and acceptable energy density, aqueous rechargeable Zn–MnO2 batteries are gradually emerging as promising energy storage devices in recent years. Unfortunately, the structural instability and poor electrical conductivity of manganese dioxides still hinder their further applications. To tackle these issues, we demonstrate a high-performance cathode material for Zn–MnO2 batteries by hybridizing lignin-derived porous carbon with δ-MnO2 (denoted as LPC/δ-MnO2). Benefiting from the high electrical conductivity of porous carbon, the LPC/δ-MnO2 cathode material can offer high discharge capacity of 332.3 mAh g–1 at 0.2 A g–1 and excellent high-rate capability (196.1 mAh g–1 at 5 A g–1). Meanwhile, the assembled Zn–MnO2 battery displays good long-term cycling stability (82% capacity retention after 1000 cycles). Such superior performances are attributed to the synergetic effect of nanostructured δ-MnO2 and porous carbon scaffold, which is in favor of rapid Zn2+ ion diffusion and large contribution ratio of pseudocapacitive Zn2+ ion intercalation. The results of this study show great prospects of hybridizing biomass-derived carbon framework with electrochemically active materials toward advanced energy storage materials.


INTRODUCTION
Considering the growing demands for efficient and reliable energy storage devices in modern society, low-cost, good-safety, and long-lifespan rechargeable batteries are urgently needed. Despite the wide-range applications of lithium ion batteries (LIBs), LIBs still suffer from a series of problems because of the often-used organic electrolytes (Randau et al., 2020). Thus, it is extremely important to develop alternative energy storage devices. Recently, aqueous zinc-ion batteries (AZIBs) have attracted significant attention, mainly attributed to the advantages of aqueous electrolytes and zinc metal anodes (Tang et al., 2019;Li et al., 2020). In particular, the zinc metal shows high theoretical capacity, adequate redox potential for aqueous energy storage, and high abundance with low cost (Wan et al., 2018;Wang et al., 2018). Moreover, the ionic radius of Zn 2+ (0.74 Å) is smaller than that of Na + (1.02 Å) and close to that of Li + (0.69 Å) (Kundu et al., 2016;Zhang et al., 2017), making AZIBs a promising candidate for future energy storage devices. Actually, various suitable and advanced cathode materials such as MnO 2 polymorphs [e.g., α- , β- , γ- , and δ- (Nam et al., 2019)], vanadium-based oxides (Kundu et al., 2016;Soundharrajan et al., 2018), Prussian blue analogs (Zhang et al., 2015;Yang et al., 2019), and organic compounds (Guo et al., 2018;Tie et al., 2020) have been extensively explored.
As one of the most promising cathode materials, MnO 2 possesses the merits of low cost, high theoretical capacity (308 mAh g −1 , based on one electron reaction), high output voltage (∼1.4 V), and environmental friendliness (Jin et al., 2019;Zhang et al., 2019;Chen et al., 2020). Ever since Yamamoto et al. developed Zn//ZnSO 4 //MnO 2 cell chemistry in 1986 for the first time, lots of impressive works have been dedicated to this field (Yamamoto and Shoji, 1986;Guo et al., 2020;Xie et al., 2020). For instance, Pan et al. (2016) studied the electrochemical performances of aqueous Zn//α-MnO 2 battery in a ZnSO 4 aqueous electrolyte with MnSO 4 additive and realized excellent rate capability and great cyclability (92% capacity retention after 5000 cycles. Zhong et al. (2020) recently proposed an electrolyte-decoupling strategy to widen the working potential of Zn-MnO 2 battery and achieved a large open-circuit voltage of 2.83 V as well as good cyclability. Among various crystallographic polymorphs of MnO 2 , layered δ-type MnO 2 exhibits a relatively large interlayer spacing (about 0.7 nm in the (0 0 1) plane), enabling this type of material to be suitable for facile and reversible (de-)intercalation of Zn 2+ ions (Wang et al., 2019a). However, as a semiconductor, MnO 2 has intrinsically poor conductivity (around 10 −5 to 10 −6 S cm −1 ) that is not favorable for high-rate applications (Dong, 2019). To further optimize the electrical conductivity of MnO 2 , various strategies have been developed, mostly through introducing conductive additives [such as graphene (Wu et al., 2018), carbon nanotubes , and conductive polymers ]. For instance, Wu et al. (2018) proposed a graphene-coated α-MnO 2 as the cathode material for AZIBs. The graphene coating can not only reduce the dissolution of the MnO 2 during cycling, but also provide high connectivity for rapid ions/electrons transfer (Wu et al., 2018). However, it remains a great challenge to improve the electrical conductivity and stability of MnO 2 through a facile and cost-effective method.
Meaningfully, Lignin, as the second most abundant natural polymer, has been successfully demonstrated to be a viable precursor or feedstock for the energy storage devices, such as Li-ion batteries (Cao et al., 2019;Chinomso et al., 2019), Na-ion batteries (Lin et al., 2020), and supercapacitors (Wang et al., 2019d;Yu et al., 2019). Endowed with abundant aromatic polymer with a carbon content of about 60 wt.%, lignin is suitable for preparing porous carbon framework (Yu et al., 2019;Lin et al., 2020). Nevertheless, to the best of our knowledge, there have been no reports on lignin-derived carbon for AZIB storage systems (Yuan et al., 2019). Hence, it is highly meaningful to construct an integrated lignin-derived porous carbon (LPC) and MnO 2 nanocomposite with good electrical conductivity, superior structural stability, and satisfying electrochemical performances. In this work, we use a one-step hydrothermal method to combine δ-MnO 2 with LPC scaffold. The prepared LPC/δ-MnO 2 nanocomposite can provide considerably improved electrochemical performances compared to that of neat δ-MnO 2 sample. Impressively, the LPC/δ-MnO 2 cathode material shows a large reversible capacity of 332.3 mAh g −1 at 0.2 A g −1 , excellent rate capability of 196.1 mAh g −1 at a large current density of 5 A g −1 , and good cycling stability of 82% capacity retention over 1000 cycles, much better than that of δ-MnO 2 counterpart. Also, the pseudocapacitive Zn 2+ ion intercalation occupies a large proportion of the total capacity of LPC/δ-MnO 2 , e.g., 72.8% at 2.0 mV s −1 . Such great performances of LPC/δ-MnO 2 nanocomposite can be explained by the following reasons. Firstly, δ-MnO 2 active material with relatively large interlayer spacing (∼0.7 nm) can provide a fast Zn 2+ transfer channel during charge/discharge processes. Secondly, the porous carbon skeleton ensures fast transmission of electrons, which improves the whole electrical conductivity. Thirdly, the porous carbon scaffold can help accommodate the volume change caused by the electrochemical redox reactions of MnO 2 .

Preparation of Neat δ-MnO 2
In a typical synthesis, 100 mL of 0.1 M MnSO 4 ·H 2 O aqueous solution was dropwise added into 100 mL of 0.1 M K 2 S 2 O 8 aqueous solution under stirring. After that, the mixed solution was stirred for 2 h. Next, 60 mL of 1.2 M NaOH aqueous solution was added slowly into the above-mentioned solution with stirring for another 1 h, followed by aging for 2 h in the air. The obtained precipitate was washed with de-ionized (DI) water and absolute ethanol for 6 times, and subsequently freeze-dried to obtain δ-MnO 2 .

Preparation of Lignin-Derived Porous Carbon and LPC/δ-MnO 2 Nanocomposite
Lignin (de-alkaline) was purchased from Aladdin Chemicals, Shanghai, China. The LPC was prepared by ball milling and subsequent calcination process. In specific, 0.5 g lignin and 0.5 g K 2 CO 3 (as the activating agent) were added with 3.0 g zirconia grinding balls into a zirconia grinding bowl. The mixture was then milled for six cycles (30-min run and 5-min rest, forward and reverse running) for a total effective ball grinding time of 180 minutes. The mixed powder was then carbonized at 800 • C for 2 h under Ar atmosphere. Then, 2 M HCl aqueous solution and sufficient DI water were used to wash the obtained black powder thoroughly. After drying, the LPC was obtained. For the synthesis of LPC/δ-MnO 2 , 120.0 mg LPC and 252.9 mg KMnO 4 were added into 60 mL DI water. After that, the hydrothermal treatment was carried out at 140 • C for 8 h. After being washed with DI water and ethanol and dried at 80 • C for 12 h, the final product (LPC/δ-MnO 2 ) was obtained. For comparison, neat δ-MnO 2 without LPC was also prepared.

Characterization and Electrochemical Measurements
The morphology, structure, and chemical composition of samples were analyzed by field emission scanning electron microscope (FE-SEM, JEOL JSM-7600F), transmission electron microscope (TEM, JEOL JEM-2100F), powder X-ray diffractometer (XRD, Rigaku Ultima IV) with Cu Kα radiation source (λ = 1.5406 Å), X-ray photoelectron spectrometer (XPS, Kratos AXIS UltraDLD), elemental analyzer (Elementar Vario EL Cube), and N 2 adsorption/desorption analyzer (Quantachrome Autosorb-iQ2-MP). For the preparation of cathodes, 70% LPC/δ-MnO 2 , 20% carbon black, and 10% polyvinylidene fluoride dispersed in N-methylpyrrolidone were coated onto Ti foils and then dried at 80 • C for at least 6 h in an oven. After being pressed at 10 MPa, the Ti foils were sliced into rounds with 12 mm in diameter, serving as working electrodes. The mass loading of LPC/δ-MnO 2 is controlled at ∼1.25 mg cm −2 . The electrochemical performances of LPC/δ-MnO 2 were evaluated with CR2016-type coin cells, which were assembled using Zn foil as the counter/reference electrode, Whatman filter paper (GF/A) as the separator, and 2 M ZnSO 4 /0.2 M MnSO 4 aqueous solution as the electrolyte. The Galvanostatic charge/discharge (GCD) measurements were performed on a LAND CT2001A instrument. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out on a Bio-Logic VSP-300 electrochemical workstation.

RESULTS
The crystal structures of LPC, δ-MnO 2 , and LPC/δ-MnO 2 were characterized by XRD measurements, and the results are shown in Figure 1A. The XRD pattern of LPC shows the broad peaks at around 23 • and 43 • , which are attributed to the crystallographic planes of (0 0 2) and (1 0 0) of graphitic carbon, respectively, indicating the pseudographitic structure in the carbon framework. The peaks at 30 • , 43 • , and 60 • are indexed to the impurities of SiO 2 and Na 4 SiO 4 from the carbonized lignin biomass. Besides, the characteristic diffraction peaks of neat δ-MnO 2 match with the standard PDF card (JCPDS 42-1317). As for the LPC/δ-MnO 2 nanocomposite, four peaks at around 12 • , 24 • , 36 • , and 65 • can be assigned to the (0 0 1), (0 0 2), (1 1 -1), and (3 1 -2) planes of birnessite-type MnO 2 with a layered structure (Liu et al., 2016;. In addition to δ-MnO 2 , some other peaks in the LPC/δ-MnO 2 can be indexed to a hydrothermal by-product (K 6 Mn 2 O 6 , JCPDS 36-0873). As shown in the insert of Figure 1A, in the crystal structure of δ-MnO 2 , K + ions and H 2 O molecules reside in the interlayer space of MnO 6 layers. The pre-intercalated K + ions and H 2 O molecules can act as pillars to expand the interlayer distance and improve the structural stability (Wang et al., 2019b;Sun et al., 2020). The detailed valence states of Mn and C elements were investigated through XPS analysis. As shown in Figure 1B, the high-resolution spectrum of Mn 2p can be split into two strong peaks situated at 654.3 and 642.5 eV, which are attributed to Mn 2p 1/2 and 2p 3/2 , respectively. It should be noted that the spin-energy separation between 2p 1/2 and 2p 3/2 is 11.7 eV, revealing the presence of Mn 4+ in LPC/δ-MnO 2 nanocomposite (Wang et al., 2019a). The XPS spectrum of C 1s in Figure 1C provides three subpeaks peaks, corresponding to C-C/C=C, C-OH, and C=O/C-OOH, respectively. Such rich oxygen-containing functional groups can make sure of good electrolyte wettability for the active material (Gao Q. et al., 2020). The LPC/δ-MnO 2 was further characterized by SEM and TEM techniques. As can be seen from Figures 1D,E, the surface of δ-MnO 2 is covered by LPC with a uniform dispersion state, and the LPC adopts two different morphologies, i.e., nanoparticles and nanoplates. Such unique morphologies are beneficial to the penetration of Zn 2+ ions and the exposure of sufficient active sites . Meanwhile, the δ-MnO 2 with shape of homogenous flower-like morphology is composed of many curved nanosheets stacked in a cross way and the nanosheets have a thickness of about 50 nm. The HRTEM image in Figure 1F reveals the amorphous carbon region of LPC and two obvious lattice fringes of 0.608 and 0.241 nm, corresponding to the (0 0 1) and (1 1 -1) planes of δ-MnO 2 , respectively. To further evaluate the structure of the LPC/δ-MnO 2 , N 2 adsorption/desorption measurement was carried out. The Brunauer-Emmett-Teller (BET) specific surface area of LPC/δ-MnO 2 is calculated to be 39.89 m 2 g −1 , providing sufficient channels for the facile infiltration of electrolyte. Based on the elemental analysis, the mass ratio of LPC in LPC/δ-MnO 2 nanocomposite is determined as 6.2%. Although the mass ratio of LPC is relatively low, it acts a pivotal part in enhancing electrochemical performances of LPC/δ-MnO 2 . If the content of LPC is higher, the overall specific capacity of LPC/δ-MnO 2 would become lower.
As illustrated in the schematic diagram in Figure 2A, the electrochemical Zn 2+ ion storage performances were investigated by using LPC/δ-MnO 2 nanocomposite, 2 M ZnSO 4 /0.2 M MnSO 4 aqueous solution, and Zn foil as cathode material, electrolyte, and anode, respectively. High rate capability is critical for the practical applications of Zn-MnO 2 batteries. As can be seen from Figure 2B, at all the current densities, the LPC/δ-MnO 2 nanocomposite exhibits much higher capacities than that of the neat δ-MnO 2 . As the current density increases from 0.2 A g −1 gradually, the discharge capacity of LPC/δ-MnO 2 at the end of each current density declines from 332.3 to 297. 1, 276.2, 248.5, and 196.1 mAh g −1 at 0.5, 1, 2, and 5 A g −1 , respectively. In addition, when recovering to 0.2 A g −1 , the discharge capacity of LPC/δ-MnO 2 can return to a high value of 339.1 mAh g −1 , showing good reversibility of the LPC/δ-MnO 2 nanocomposite. However, in sharp contrast, the capacity of δ-MnO 2 is 196.8 mAh g −1 at 0.2 A g −1 , which falls rapidly to 149.9, 93.1, and 65.7 mAh g −1 at 0.5, 1, and 2 A g −1 , respectively. Worse still, merely 14.1 mAh g −1 is maintained at a relatively large current density of 5 A g −1 , manifesting the extremely poor rate performances of pure MnO 2 material . Figure 2C presents the GCD curves of LPC/δ-MnO 2 in initial three cycles at 0.2 A g −1 . It is seen that the LPC/δ-MnO 2 gives an initial discharge capacity of 284.7 mAh g −1 , which rises to 321.2 and 324.0mAh g −1 at the 2nd and 3rd cycles, respectively. The increasement in capacity is attributed to the gradual activation of manganese oxide materials and similar phenomenon has been shown in previous reports Zhang et al., 2019;Gao Q. et al., 2020). Figure 2D shows the corresponding GCD profiles of LPC/δ-MnO 2 at current densities ranging from 0.2 to 5 A g −1 . The curve profiles manifest nearly the same shape and small electrochemical polarization at different rates, implying fast charge transfer kinetics. Notably, the capacity fading along with the increased current density mainly comes from the shrinkage of the second discharge plateau. These results highlight the significant structural stability of the Zn-MnO 2 battery in delivering high capacities at various currents (Pan et al., 2016;Huang et al., 2018;. Cycling stability is a vital criterion for Zn-MnO 2 batteries. As shown in Figure 3A, at a low current density of 0.2 A g −1 , the LPC/δ-MnO 2 exhibits discharge capacities of 282.1 and 329.1 mAh g −1 for the initial two cycles. After 300 cycles, the capacity declines to 292.5 mAh g −1 gradually, corresponding to a high capacity retention of 89% (compared with the second cycle). Instead, when LPC is not employed, neat δ-MnO 2 only provides 215.5 and 253.3 mAh g −1 for the initial two cycles. After 300 cycles, the capacity of neat δ-MnO 2 drops Frontiers in Energy Research | www.frontiersin.org rapidly to 160.9 mAh g −1 , showing inferior cyclability (64% capacity retention). These results further indicate that the LPC framework plays an important role in stabilizing the electrochemical properties of δ-MnO 2 . At the same time, in order to obtain more information about the electrochemical kinetics of LPC/δ-MnO 2 , the EIS measurements were carried out using 5 mV amplitude with frequency ranging from 10 mHz to 100 kHz during the cycling process. The Nyquist plots of LPC/δ-MnO 2 and δ-MnO 2 are shown in Figure 3B. All the EIS curves show depressed semicircles in the high frequency region and sloped lines at low frequencies. Generally, the semicircle reflects charge transfer resistance (R ct ), while the sloped line is ascribed to ion diffusion in the bulk electrodes Hao et al., 2019;Xu et al., 2019). It is found that the R ct values of LPC/δ-MnO 2 are smaller than that of δ-MnO 2 at different states during the whole cycling test, indicating better electrochemical kinetics of LPC/δ-MnO 2 . In detail, at the initial state, the R ct value of LPC/δ-MnO 2 is 681.2 . After the 1st and 300th cycles, the R ct decreases to 229.2 and 39.8 , respectively. In contrast, the R ct of δ-MnO 2 at different states are much higher (947.3 at initial state, 610.9 after 1st cycle, and 99.4 after 300th cycle). The EIS results demonstrate that the charge transfer kinetics is improved and the electrochemical activity is enhanced when δ-MnO 2 is modified by LPC networks. The long-term cycling stability of LPC/δ-MnO 2 was further tested at 5 A g −1 (Figure 3C). Even after 1000 cycles, a high reversible capacity of 105.4 mAh g −1 is retained. Moreover, the coulombic efficiencies (CE) approach 100% after the 1st cycle, displaying good reversibility and excellent long-term cycling stability. These results further confirm that utilizing LPC as conductive framework and structural stabilizer for Mn-based cathode materials can substantially improve the performances of Zn-MnO 2 batteries in terms of rate capability and cyclability.
The electrochemical redox behaviors were further investigated by CV tests from 1.0 to 1.85 V. Figure 4A depicts the CV curves of LPC/δ-MnO 2 at initial six cycles at 0.2 mV s −1 . Obviously, there are two well separated reduction peaks at around 1.37 and 1.26 V (vs. Zn 2+ /Zn) on cathodic sweeping, corresponding to the insertion processes of H + and Zn 2+ , respectively  (Wang et al., 2019c;You et al., 2020). Besides, it is found that the average peak current has no obvious loss in the initial six cycles, indicative of excellent electrochemical reversibility. CV measurements at different scan rates from 0.2 to 2.0 mV s −1 were also carried out to analyze the Zn-MnO 2 battery with LPC/δ-MnO 2 nanocomposite cathode material. As shown in Figure 4B, as the scan rate increases, the anodic peaks shift to higher potential, while the cathodic peaks move to lower potential at the same time. When the scan rate increases to a larger degree, these CV peaks get broader gradually. This phenomenon is due to the increased electrochemical polarization at a higher scan rate (Wang et al., 2019c). Based on the empirical power-law relationship between the peak current (i) and scan rate (ν), the following equation is used (Augustyn et al., 2013): Here, a and b are variable parameters . According to the slop of log (i) vs. log (ν) in Figure 4C, the b values associated with four redox peaks are 0.61, 0.68, 0.79, and 0.81, respectively, suggesting that the capacitive intercalation behaviors dominate the corresponding redox reactions in Zn-MnO 2 batteries. In addition, i(V), the current response, can be quantitatively separated into capacitive (k 1 v) and diffusioncontrolled (k 2 v 1/2 ) parts based on the following equation (Ming et al., 2018;Han et al., 2020): By determining the k 1 parameter, the capacitive contribution can be calculated. As illustrated in Figure 4D, when the scan rate increases from 0.2 to 2.0 mV s −1 , the capacitive contribution ratio rises from 52.4 to 72.8% gradually. That is, considerable portion of pseudocapacitive intercalation takes part in the Zn 2+ ion storage process, especially at high rates. Hence, it is not surprising that the LPC/δ-MnO 2 shows great rate capabilities in Figure 2B.

CONCLUSION
Benefiting from large specific capacity and high output potential, MnO 2 has received great attention as the cathode material for AZIBs. However, the bottleneck of MnO 2 is severe capacity fading and structural instability during cycling. In this study, the LPC/δ-MnO 2 nanocomposite with enhanced structural stability and improved electrical conductivity is constructed through a facile hydrothermal method. At 0.2 A g −1 , the LPC/δ-MnO 2 delivers a high specific capacity of 332.3 mAh g −1 , 74.8 and 59.0% of which can be retained when the current density is as high as 2 and 5 A g −1 , respectively. Moreover, the capacity retention of LPC/δ-MnO 2 can reaches 82% over a long term of 1000 cycles. Such great performances are attributed to the large specific surface area, low charge transfer impedance, and large contribution ratio of pseudocapacitive Zn 2+ ion intercalation. In short, this study not only offers new insights into enhancing the performances of MnO 2 for AZIBs but also put forwards a novel strategy for exploring low-cost biomass-derived porous carbon for advanced energy storage devices.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.