Integrated Solid/Nanoporous Copper/Oxide Hybrid Bulk Electrodes for High-performance Lithium-Ion Batteries

Nanoarchitectured electroactive materials can boost rates of Li insertion/extraction, showing genuine potential to increase power output of Li-ion batteries. However, electrodes assembled with low-dimensional nanostructured transition metal oxides by conventional approach suffer from dramatic reductions in energy capacities owing to sluggish ion and electron transport kinetics. Here we report that flexible bulk electrodes, made of three-dimensional bicontinuous nanoporous Cu/MnO2 hybrid and seamlessly integrated with Cu solid current collector, substantially optimizes Li storage behavior of the constituent MnO2. As a result of the unique integration of solid/nanoporous hybrid architecture that simultaneously enhances the electron transport of MnO2, facilitates fast ion diffusion and accommodates large volume changes on Li insertion/extraction of MnO2, the supported MnO2 exhibits a stable capacity of as high as ~1100 mA h g−1 for 1000 cycles, and ultrahigh charge/discharge rates. It makes the environmentally friendly and low-cost electrode as a promising anode for high-performance Li-ion battery applications.

transport and cycling performance, there have been initial explorations on developing composite electrodes by employing conductive agents, such as metal pillars 23 or substrates 35 , carbon nanotubes (CNTs) 8,10,38 and nanohorns 39 , graphene 9,40,41 , to serve as conductive pathways of metal oxides. Nevertheless, the electrodes, which are assembled by a traditional approach to mix these low-dimensional active nanocomposites using polymeric binders, exhibit undesirably low electrical conductivity and ion transport owing to exceptionally low electron conductivity in the nanomaterials as well as the high contact resistances within nanomaterials and between the current collector and electrodes, essentially impeding their wide use in practical high-power Li-ion batteries [7][8][9]19,23,42 . Moreover, large volume changes during Li insertion/extraction lead to low power capability, severe capacity fading, and even electrode failure as a result of pulverization, aggregation and loss of electrical contact 43 .
Here, we report seamlessly integrated solid/nanoporous (S/NP) Cu/MnO 2 hybrid bulk Li-ion battery electrodes, of which the three-dimensional (3D) NP Cu/MnO 2 layer with bicontinuous nanopores/ligaments and intimate Cu/MnO 2 interface enhances ion and electron transports while the charge storage of MnO 2 is facilitated by a dual mechanism of capacitive and Li insertion/extraction processes. As a result of concurrent realization of minimizing primary resistances, producing stable Cu/MnO 2 interface and accommodating large volume changes during charge/discharge, the constituent MnO 2 delivers high energy at ultrahigh rates with outstanding cyclability.

Results
The S/NP Cu/MnO 2 electrodes are fabricated by a facile procedure, which involves synthesis of seamlessly integrated S/NP Cu skeletons and electroless plating of MnO 2 into the nanoporous layer (Figs. 1a-e and Supplementary Fig. S1) 17 , for the use in coil cells (Figs. 1f, g). Following the deposition of Cu 30 Mn 70 alloy films onto Cu foils via magnetron sputtering ( Supplementary Fig. S2a), S/NP Cu skeletons are produced by chemical dealloying in the N 2 -bubbled HCl solution, during which less noble Mn is selectively dissolved while remained Cu forms nanoporous structure ( Supplementary Fig. S2b) [44][45][46] . Figure 2a shows typical top-view scan electron microscope (SEM) image of as-dealloyed S/NP copper, demonstrating that a 800-nmthick 3D bicontinuous nanoporous layer consisting of quasi-periodic Cu ligaments and nanopore channels with a pore size of ,50 nm ( Fig. 2a and Supplementary Fig. S3a) 47,48 is seamlessly jointed with Cu foil (Fig. 2b, Supplementary Fig. S4). Elemental mapping for Cu, Mn and O in the as-dealloyed NP Cu layer ( Supplementary Fig. S5) reveals the negligible Mn (,1.14 wt.%) and O (,0.25 wt.%) remains that cannot be identified in high-resolution transmission electron microscope (HRTEM) image of Cu ligaments ( Supplementary Fig.  S6) and X-ray diffraction pattern ( Supplementary Fig. S7). After electroless plating, MnO 2 nanoparticles with the amount of ,12 wt.% are uniformly incorporated into pore channels of the entire NP Cu layer without any defect (Fig. 2e Supplementary Fig. S3b). The loading of rough MnO 2 nanocrystals with diameter of ,5 nm gives rise to a larger real surface area of S/NP Cu/MnO 2 than that of bare S/NP Cu ( Supplementary Figs. S3c, S8). HRTEM micrograph shows that well-crystallized MnO 2 directly grows along the Cu ligaments with end-bonded contact (Fig. 2f) 49 , offering excellent electrical conductivity and stable interface between Cu and MnO 2 17,49 .
The chemical state of MnO 2 is verified by X-ray photoelectron spectroscopy (XPS) survey, wherein Mn-2p core level spectrum displays two peaks at the binding energies of 642.5 eV and 654.4 eV, corresponding to Mn-2p 3/2 and 2p 1/2 orbits of Mn 41 with a separation of 11.9 eV (Fig. 2g) 50 . Raman spectrum of MnO 2 with the characteristic peaks at 491, 568 and 635 cm 21 indicates the birnessite-type crystalline structure (Fig. 2h) 51 . The binder-free procedure facilely realizes the integration of nanoporous metal/oxide composites with current collectors without any additional contact resistance while the bicontinuous nanoporous channels and Cu skeleton facilitate ion and electron transport kinetics. These advantages enlist the hybrid electrodes to not only exhibit exceptional mechanical flexibility and stability (Fig. 1e, Supplementary Fig. S1e), but minimize the primary resistances in the entire hybrid electrodes for the enhanced charge storage.
The electrochemical properties of the S/NP Cu/MnO 2 hybrid bulk electrode are tested in a two-electrode configuration (Figs. 1f, g), in which a lithium foil and a porous polymer film are used as a counter electrode and a separator, respectively. Figure 3a shows cyclic voltammetry (CV) curves of the first three cycles of the S/NP Cu/MnO 2 hybrid electrode in the range of 0.01 and 3 V (vs. Li 1 /Li) at a scan rate The current of two cathodic peaks in the sequent cycles slightly reduces most likely due to formation of solid-electrolyte-interphase (SEI) layer on the electrode surface during the first discharge step 8,19,40 . As a consequence, the first discharge and charge steps deliver specific capacities of ,1324 and ,1179 mA h g 21 , respectively, with a Coulombic efficiency of ,89% (Fig. 3b). The most overlapping of the sequent CV and charge/discharge curves implies an outstanding reversibility of conversion reaction of MnO 2 during Li insertion/extraction (Figs. 3a, b) although it is probably accompanied by the oxidation of the intimate contact Cu layer 52 . The negligible influence of Cu oxidation on the charge/discharge behavior of the whole electrode is demonstrated by electrochemical impedance spectroscopy (EIS) measurements, which are performed on the cell with the S/NP Cu/ MnO 2 electrode before and after cycling test (Fig. 4). As shown in the Nyquist plot, both EIS spectra exhibit a characteristic semicircle in the high-and middle-frequency range, followed by a inclined line in the low-frequency range, with inconspicuous shape variations. This suggests the exceptional retention of charge transfer and Li 1 ion diffusion in the S/NP Cu/MnO 2 electrode during cycling as a result of the stable architecture (inset of Fig. 3c), of which 3D bicontinuous nanoporous channels with extremely large specific surface area of electrode/electrolyte interface offers the short ion diffusion and the Cu/MnO 2 network seamlessly integrated with Cu current collector facilitates electron transport. At the low frequency, the inclined lines of both fresh and tested S/NP Cu/MnO 2 electrodes exhibit the similar slopes, revealing the almost same solid-state diffusion of Li 1 ions in electrodes 53,54 . While the small difference of semicircles at the middle frequency is due to the formation of SEI layer after the discharge, giving rise to a slightly larger charge transfer resistance at the electrode/electrolyte interface 53,54 . These primary properties enable excellent cyclability of the S/NP Cu/MnO 2 electrode, which is further verified by the cycling performance in the voltage range of 0.01-3 V vs. Li 1 /Li (Fig. 3c, Supplementary Fig. S9). After the first cycle the Coulombic efficiency increases to more than 98%, and the discharge capacity gradually increases from ,1135 mA h g 21 to ,1320 mA h g 21 due to the improvement of lithium ion accessibility in the electrode during the initial 150 charge/discharge cycles at the current density of 4.2 A g 21 55 . Even the current density is increased to 8.4 A g 21 , a stably reversible capacity more than 1100 mA h g 21 can be  maintained for ,1000 cycles with the Coulombic efficiency of ,99% (Fig. 3c, Supplementary Fig. S9). The attractive capacity retention results from the solid Cu/MnO 2 interface (inset of Fig. 3c) and the stable 3D nanoporous architecture, which affords enough space to accommodate the volume changes during the charge/discharge processes [1][2][3][4]8,22,23 .
Whereas MnO 2 is known to suffer from poor conductivity 1,8,9,17,40 , its rate capability is substantially enhanced by the seamlessly integrated architecture of S/NP Cu/MnO 2 (Fig. 3d). The discharge capacity of the constituent MnO 2 in the hybrid bulk electrodes reaches ,1270 mA h g 21 at low rate such as 0.4 A g 21 , and retains ,78% (,996 mA h g 21 ) and ,51% (,652 mA h g 21 ) at the exceptionally high discharge rates of 18 A g 21 and 143 A g 21 (corresponding to full discharge in 14 and 116 C, Supplementary Fig. S10), respectively. Even at 377 C the hybrid electrode delivers the capacity of ,240 mA h g 21 (Supplementary Fig. S10), and the discharge capacity reverts to ,1270 mA h g 21 when the current density returns to 0.4 A g 21 (Supplementary Fig. S11). Moreover, the further increase of the MnO 2 loading by thickening nanoporous Cu/MnO 2 layer to 1.2 mm does not lead to remarkable capacity fading of the constituent MnO 2 at high current densities (Supplementary Fig. S12). The rate performance is in distinct contrast with that of MnO 2 nanoparticles (similar to those incorporated into S/NP Cu skeleton) supported by Cu foil (the middle plot of Fig. 3d, Supplementary Fig. S13) and directly grown onto Cu foil at the same electroless plating conditions ( Supplementary Fig. S14). For comparison, the rate capabilities of carbon nanotubes/MnO 2 8 , carbon nanohorns/MnO 2 39 , graphene/MnO 2 40,41 , graphene/Mn 3 O 4 9 and carbon nanotube/CuO 10 nanocomposites are also included in Supplementary Fig. S14. Although low-dimensional nanostructures can shorten solid-state   ion diffusion length 2,3,7,20 and conductive agents can ameliorate the electron transport 4,[8][9][10]13,40 , the entire electrodes assembled with these nanostructures using a conventional approach exhibit much lower capacity and remarkable fading for metal oxide-based composite nanostructures even at the current densities below 10 A g 21 (Fig. 3d, Supplementary Figs. S11 and S14) 8,9,40 . This further highlights the merits of S/NP Cu architecture in the high-performance lithium storage of S/NP Cu/MnO 2 hybrid electrodes. To assess the contribution of total capacity from oxides that are produced from the oxidation of NP Cu skeleton [29][30][31][32][33][34][35][36] and the remaining Mn in KMnO 4 solution 26 , the charge/discharge profiles of bare and KMnO 4 -treated NP Cu foils are measured at the same conditions (Supplementary Figs. S15 and S16). It demonstrates that at the low current density of 0.02 mA cm 22 , the areal capacity of the produced oxides is ,0.0074 mA h cm 22 , accounting for only ,6.7% of the total capacity of S/NP Cu/MnO 2 . While the current density increases to 2 mA cm 22 , this contribution further decreases to less than 1% (Supplementary Fig. S17).

Discussion
To analyze the charge storage of S/NP Cu/MnO 2 hybrid electrodes, the voltammetric behavior at various scan rates (v) is reexamined in the voltage range of 0.01-3 V (vs. Li 1 /Li) ( Supplementary Fig. S18a), wherein the current response (i) at a fixed voltage (V) is described as the combination of capacitive effect (k 1 v) and diffusion-controlled Li insertion/extraction (k 2 v 1/2 ) 56,57 , i.e., Their different scan-rate dependence of current response is employed to distinguish the fraction of current arising from capacitive and Li insertion processes by determining both k 1 and k 2 in the light of the methodology proposed in Refs. 56 and 57. For analytic purpose, Eq.
(1) can be rearranged to i(V)/v 1/2 5 k 1 (V)v 1/2 1 k 2 (V), according to which the values of k 1 (V) and k 2 (V) are obtained from the slope and the y-axis intercept point for i(V)/v 1/2 as a linear function of v 1/2 , respectively, at each fixed potential 56,57 . Figure 5a shows the typical voltage profile for the insertion/extraction current (shaded region) approximately estimated according to Eq. (1), in comparison with the total current of S/NP Cu/MnO 2 electrode at a scan rate of 5 mV s 21 , wherein the current of Li insertion/extraction is estimated in terms of the equation i insertion (V) 5 k 2 (V)v 1/2 . It illustrates that the total stored charge consists of both Li insertion and capacitive processes and their tradeoff depends on the scan rate ( Fig. 5b) 56,57 . For relatively low scan rates (,10 mV s 21 ), the insertion/extraction process delivers .60% of the total capacities; whereas at higher scan rates, the insertion capacity drops to ,32% and the capacitive charge storage primarily resulting from pseudocapacitive contribution becomes dominant (,68%) (Fig. 5b) 17 although the double-layer capacitance of bare S/ NP Cu decreases from ,13% to ,6% ( Supplementary Fig. S17). The unique feature combining both battery-and supercapacitor-like behaviors in this hybrid bulk electrode offers ultrahigh rate capability without remarkable fading of capacity in the 0.01-3 V range (Fig. 3d, Supplementary Figs. S10 and S11) 4,14 , and enlists the constituent MnO 2 to exhibit high gravimetric energy (200 W h kg 21 ) delivered at an exceptionally high power of 430 kW kg 21 in Li//S/NP Cu/ MnO 2 cells, much higher than the active materials in CNT/FePO 4 3 , functionalized LBL-CNTs 14 , graphene/V 2 O 5 58 , nanoporous carbon/ LiFePO 4 14,59 , and LiNi 0.5 Mn 0.5 O 2 14,60 , as well as supercapacitive electrodes 11,14 (Fig. 5c). Even the mass of nanoporous Cu layer is included, the power and energy densities of NP Cu/MnO 2 reach maxima of ,74 kW kg 21 and ,360 W h kg 21 (Supplementary Fig.  S19). Although MnO 2 has intrinsically low conductivity that limits its charge/discharge rate, the charge storage performance can be significantly enhanced by the dual mechanism of capacitive storage and Li insertion in the unique integration of S/NP Cu/MnO 2 architecture, wherein (i) the intimate Cu/MnO 2 interface accelerates the electron transport between the nanocrystalline MnO 2 and Cu ligaments and stabilizes the hybrid structure; (ii) the interconnected-pore and -ligament Cu network shortens the ion diffusion path length and improves the electrical conductivity, respectively; (iii) the seamless integration minimizes the contact resistances between NP Cu/MnO 2 and copper current collector; (iv) the good nanoporous structure provides a large specific surface area to facilitate the full use of the capacitive charge storage of MnO 2 while also accommodating large volume changes on Li insertion/extraction of MnO 2 for the improved cyclability. These four advantages make the high-rate feature comparable to that of LBL-CNT electrodes assembled with the functionalized CNTs by layer-by-layer technique, which stores charge by only surface-redox process 14 .
In summary, we have developed hybrid bulk electrodes with seamlessly integrated nanoarchitecture of S/NP Cu/MnO 2 by a procedure combining physical deposition with chemical dealloying and modified electroless plating. The hybrid bulk electrodes store/deliver high energy at ultrahigh rates with excellent stability by a dual mechanism of pseudocapacitive and Li insertion/extraction processes, making the NP copper/oxide hybrids promising use in miniaturized devices for high power-and energy-density applications. Such exceptional charge-storage performance results from the intimate contact of Cu/ MnO 2 and the unique S/NP integration architecture, simultaneously minimizing the primary resistances during charge and discharge and accommodating large volume change during Li insertion/extraction.

Methods
Fabrication of seamless S/NP Cu/MnO 2 hybrid bulk electrodes. Seamlessly integrated S/NP Cu foils were fabricated by a combination of physical deposition and chemical dealloying. Cu 30 Mn 70 (atomic ratio) alloy films with a thickness of ,800 nm were deposited on solid Cu foils with dimensions of ,3 cm 3 2 cm 3 10 mm by direct current magnetron sputtering with a power of 200 W for 20 min at room temperature. The solid Cu foils were cleaned thoroughly with acetone, 1 M HCl solution and deionized water (18.2 MV?cm) before the physical deposition. Nanoporous Cu layer on the solid Cu substrate was produced by chemically dealloying Cu 30 Mn 70 alloy film for 5 h at room temperature in 10 mM HCl solution that was firstly bubbled by N 2 gas for 30 min [44][45][46] . The residual acid in nanoporous copper was removed by N 2 bubbled water rinsing. MnO 2 nanocrystals were plated onto the clean S/NP copper foils by a modified electroless plating technique in the aqueous mixture of 5 mM KMnO 4 and 10 mM KOH for 30 minutes at room temperature under the gas reagent of hydrazine (N 2 H 4 ). The complete immersion of S/NP Cu foils in the aqueous solution allows the uniform growth of MnO 2 nanocrystals along the Cu ligaments. All specimens are dried in vacuum (,10 24 torr) after thorough water rinsing. The mass of loading MnO 2 is calculated according to the mass of nanoporous Cu skeleton and the weight ratio of nanoporous Cu/MnO 2 determined by EDS measurements (Fig. S2c).
Structural characterization. The microstructure and chemical composition of the specimens were investigated using a field-emission scanning electron microscope (JEOL 6700F) equipped with an X-ray energy-dispersive microscopy (EDS), and a transmission electron microscope (JEOL JEM-2100F, 200 keV). X-ray photoelectron spectroscopy (XPS, AxIS-ULTRA-DLD) with Al Ka (mono) anode at energy of 150 W in a vacuum of 10 27 Pa. X-ray diffraction measurement was carried out on a D/Max2500pc diffractometer using Cu Ka radiation. Raman spectrum was collected using a micro-Raman spectrometer (Renishaw) with a laser of 532 nm wavelength.
Construction of the lithium ion battery and electrochemical measurement. Cointype cells (2016) were assembled in an argon-filled dry glove box (both moisture and oxygen levels were kept below 1 ppm) using the S/NP Cu/MnO 2 as the positive electrode and the Li foil as the negative electrode. Both positive and negative electrodes were electronically separated by Celgard 2400 film in non-aqueous electrolyte (1 M LiPF 6 in 15151 volume ratio mixture of ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC)). CV was performed on an IVIUM electrochemical analyzer, and the charge/discharge measurements were carried out on a battery test system in the voltage range between 0.01 and 3.0 V (vs. Li 1 /Li) at room temperature. EIS measurements were carried out over the frequency range from 10 mHz to 10 kHz with an amplitude of 5 mV.