Insights into the Electrochemical Reaction Mechanism of a Novel

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Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi2(PO4)2/C for Li-ion Batteries Wengao Zhao, Guiming Zhong, Jian Zheng, Jianming Zheng, Junhua Song, Zhengliang Gong, Zheng Chen, Guorui Zheng, Zheng Jiang, and Yong Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15086 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Materials & Interfaces

Insights into the Electrochemical Reaction Mechanism of a Novel Cathode Material CuNi2(PO4)2/C for Li-ion Batteries Wengao Zhao1, Guiming Zhong2, 5, Jian Zheng3, Jianming Zheng4,Junhua Song4,Zhengliang Gong1, Zheng Chen1, Guorui Zheng2,Zheng Jiang6, Yong Yang1, 2∗ 1.

School of Energy Research, Xiamen University, Xiamen, Fujian, 361005, China

2.

State Key Lab of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials and Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China

3.

Institute for Integrated Catalysis, and Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States

4.

Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard,Richland, WA 99354, USA

5.

Xiamen Institute of Rare Earth Materials, Haixi institutes, Chinese Academy of Sciences,

6.

Shanghai Institute of Space Power-Sources, No. 2965 Dongchuan Road, Shanghai 200245,

Xiamen, 361021, China China

ABSTRACT In this work, we firstly report the composite of CuNi2(PO4)2/C (CNP/C) can be employed as the high capacity conversion-typed cathode material for rechargeableLi ion batteries (LIBs), delivering areversible capacity as high as 306 mAhg-1at a current density of 20mAg-1. Furthermore, CNP/C also presents good rateperformance and reasonable cyclingstability based on a non-traditional conversion reaction mode. X-ray diffraction (XRD) andhigh-resolution transmission electron microscopy (HRTEM) characterizationsshow thatCNPis reduced to form Cu/Ni and Li3PO4 during discharging process, which is reversed in the following charging process, demonstrating

thata

occurs.X-rayabsorption

reversible

conversion

spectroscopy(XAS)

reaction

disclosesthat

mechanism

Ni2+/Ni0exhibits

a

betterreversibilitycompared toCu2+/Cuduring conversion reactionprocess, whileCu0 is more difficult to be re-oxidized during the recharge process, leading to capacity loss 1

ACS Paragon Plus Environment

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as a consequence.The fundamental understanding obtained in this work provides some important clues to explore the high capacity conversion-typed cathode materials for rechargeableLIBs

KEYWORDS: CuNi2(PO4)2/C, high capacity, conversion reaction, XAS, reversibility

1.INTRODUCTION Rechargeable LIBs are regarded as an attractive power device for a series of cell-driven vehiclesand large-scale energy storage applications, such as smart grids, renewable solar and wind energy[1-5]. Especially, to realize the vehicle electrification, there is an urgent need to further improve the energy density of LIBs. Recently, researchers from all over the world are devoting much effort on the developments of alternative anode materials, especially in the high-capacity silicon based anodeand the lithium metal anode [6,7]. However, the upgrownof alternative cathode materials with high capacity and high operating voltage is still an importantway to further enhancethe energy density of LIBs.Current cathode materials for LIBs are mainlybased

on

layered,

spinel,andpolyanion-type

compounds

[8-12].Althoughconventional cathode materials exhibit excellent long-term cycling stability in LIBs, their reversible capacities and energy densities are limited by the intrinsic one electron transfer during repeated charge and dischargeprocess. Electrochemical conversion reactions appear to be a promisingapproach toenhanceenergy density of cathode materialsowing to thefull utilization of all charge of a multivalent transition-metal compound[13-14].Recently, much efforts have been 2


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devoted to the researchon transition metal oxides and fluorides (MaXb, X = O, F), which carry outby forming metal (M) and Li2O or LiF compounds during discharging and reconverting to the pristineMaXb phase upon charging according to the conversion reaction shown as follows[15]: MaXb + nb Li ⇋ bLinX + aM Because ofthe multivalent oxidation state change of the metal ion, Li incorporation is associated withmore than one electron transfer, leading to higher capacities than conventional insertion compounds such as LiCoO2 and LiFePO4. To date, among the MxNy family of conversion materials (M = Fe, Co, Ni, Cu, etc; N = N, O, F, P, S, etc),only

the

MFx

family

and

Cu3(PO4)2could

affordsufficiently

high

operatingvoltages during conversion reactions, validatingthem appealingcandidates for constructing high-energy-densityLIBs [14,16-26].It has been reported earlier that Cu3(PO4)2/C compositecouldbe used as anovel cathode materials for LIBs, which exhibited ahigh rechargeable capacity up to 280mAhg-1 and displayedtwo discharge plateaus at 2.7V and 2.1V, respectively. However, the crystalline Cu3(PO4)2shows limitedcyclingperformance because of the aggregation of Cu particles and dissolution of Cu+[14]. In this work,for further improve the cycling capability,Ni2+ modifiedCuNi2(PO4)2 (CNP) compound was synthesized and employed as a reliable cathode for rechargeable LIBs for the first time. It is demonstrated that the CNP/C composite is able to deliver a high specificdischarge capacity of 400 mA h g−1 with a voltage plateau of 2.3 V, implying an energy density of 920 W h kg−1, which is one of the 3



highest value ever reported for cathode materials.Furthermore, the CNP/C also demonstrates a good rate performance, confirming the 329 mAh g-1 even under the current density of 200 mA g-1 during the 1st discharge process. In addition, the material also exhibits an acceptable cycling performance that200mAh g-1 can be retained after 30 cycles at a current density of 200mA g-1. A variety of characterization techniques, such as ex-situ XRD, HRTEM and ex-situXAS were employed to disclose the detailed reversible electrochemical reaction mechanism ofrechargeable CNP/C cathode and a profound understanding on the reaction and capacity fading mechanism was obtained.

4


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2. EXPERIMENTAL SECTION 2.1. Materials Preparation.Crystalline CNPmaterial was prepared with a solid-state reaction

method

byhomogeneously

mixing

the

stoichiometric

amounts of

(NH4)2HPO4, CuO and NiO, and then the mixture was calcined underdifferent temperatures (700~850 oC) for 6h. The composite ofCNP/C was prepared through high energy ball-milling ofthe CNP particlesand carbon black. Either 1 or 2 g of a mixture composed of90 wt% CNPand 10 wt% carbon black was loaded into a zirconia milling jar (with two zirconia balls) in an glove box filled with Ar gas. The jar was sealed up before transferring to the ball mill, which was programmed to runat 500 rpm for 60min and 90min.

5



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2.2 Materials characterization.SEM (HITACHI S-4800) equipped with energy dispersive X-ray spectroscopy (EDS) were employed to investigate the morphology and

confirm

the

electrodeswerewashed

elements

of

the

usingdimethyl

CuNi2(PO4)2

carbonate

particles.The

(DMC)

MylarmembraneandKaptontapeforpreventingH2O

andO2

and

cycled

packedwitha contamination

beforeexaminationby ex-situ XRD, HRTEM and XAS. The diffraction pattern of the samples were collected by a XRD equipment (PANalytical, Netherlands). TheMylarmembraneproducedastrongpeakat

24°-28°,

which

however

did

notaffectthesignalcollectionfromthe electrodes.HRTEM images were collected with a Tecnai

F20

STTEM

operating

at

200

wasoperatedatbeamlineBL141WattheShanghaiSynchrotron

kV.

Ex-situ

XAS

Radiation

Facility(SSRF).The XAS signal was extracted in a standard way using Athena software packages. The oscillations were weighted with k2 and Fourier transformed within the limit k = 1.8–14 Å−1.

6


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2.3 ElectrochemicalCharacterization.The electrodes were prepared using CNP/C, super C and PVDF by the mass ratio of 8:1:1, and the loading of cathode material was about 2.5mg cm-2. Coin cells of CNP/C || Li were assembled in glove box using a CNP/C composite electrode, fresh Li metal, separator (Celgard 2400) and as-prepared electrolyte ( 1M LiPF6& EC : DMC = 4:6 wt ). The electrochemical performance of cells were carried out on the Land-CT2001A battery tester under 30oC, and the voltage range has been controlled between 1.5V and 4.5V under 10-200mA g-1 current densities. CHI660D electrochemical station were employed to test the reduction/oxidation behavior from 1.5V to 4.5V under 0.05mV s-1.

3. RESULTS AND DISCUSSIONS The synthesis and characterization of CNP/C. XRD patterns of CNP synthesized at different temperatures (700-850oC) are depicted in Figure1. At700oC, some peaks of impuritiesare obviouslyobserved at 2θ=15o-40o.Withthe temperature being increased to 750oC, the impurity peaks disappear,and all theXRD reflections of the obtained CNP matches well with diffraction peaks of pure phase (P-1 space group, JCPDS 01-089-6558) [27], indicating the high purity of CNP. However, when the temperature is further increased, the impurity peaks appearagain under 800 oCat 2θ= 26.12 and 2θ =32.79, which become much stronger at 850 oC and an additional impurity peak appears at 2θ = 36.10.Based on the phase equilibrium of the reaction product (CuO-NiO-P4O10) during synthesis process, the impurity peaks may belong to the Cu2NiO(PO4)2 orCuxNi2-xP2O7 compounds[28-29]. The above result indicates that the optimal calcination temperature for preparation of pure CNP is 750oC. 7



Figure2A presents SEM image of the CNP material, the sizes of the particles range from 400 to 800nm.The carbon coating is achieved by exfoliation of graphite particles through high-energy ball milling with CuNi2(PO4)2 [30-31].Afterhigh energy ball milling with carbon (10 wt% C), forming thecarbon-coated (CNP/C) composite material, the sizes of particle decreases to 100-300nm, as displayed in Figure 2B. The corresponding EDS (Figure 2C) data strongly suggest the existence of Cu/Ni/P/O and C elements in CNP/C composite.Figure2Dfurther comparesthe XRD patterns of CNP, CNP/C and the standard PDF card of CNP. Before ball milling, the XRD pattern of CNP shows sharp peaks with high intensity, indicating its decentcrystallinity and large primary particle size. After ball milling with carbon, the diffraction peaks of CNP/C obviously become weaker and broader. The broadened diffraction peaks demonstrate a loss of long-range ordering during the mechanic-chemical synthesis, indicating a visibledecrease in the particle size and crystallinity of CNP material.The formation of smaller particles could largelyshorten the Li+ diffusion distanceand enhance the interfacial reaction kinetics, which is beneficial forimproving the material’s charge/dischargeproperty,especially therate performance [32-34]. Electrochemical Performance.Figure3A demonstrates the voltage profiles of the CNP/Cat different stage of cyclingunder specific current of 40 mA g−1. At the initial discharge from 3.0V to 1.5V, the discharge capacity of CNP/C electrode can reach up to 400 mAh g-1, which is very close to the theoretical capacity of CNP (423mAh g-1), indicating that CNP particles are almost completely electrochemically reduced to metallic Cu and Ni as the discharge proceeds to 1.5V [14, 34-36]. After the formation 8


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cycle, the electrode still demonstratesa reversible capacity of 306mAh g-1, associating with a 72.4% utilization of the theoretical capacity (423 mAh g-1). The loss of capacity may be addressed to the following two reasons: firstly, the CNP cannot be fully converted to metallic Cu and Ni even at cut-off voltage of 1.5V, ascribed to the deposition of Cu causing a large polarization and thus the incomplete conversion. Furthermore, the dissolution of Cu+ is also an important factor accounting for the decreased capacity during subsequent discharge/charge process [13-14]. The discharge plateau (2.3V) at 2nd cycle is higher than that of the 1st cycle (2.1 V), which can be attributed to the following two reasons: 1) the decreased particle sizes of the materials shorten the Li+ ion diffusion route ; 2) the incomplete reaction of metallic Cu an Ni particles increased the electronic conductivity of CNP/C composite.

9



In order to assess the performance of the cathode material for LIBs, the electrochemical performance of CNP/C were tested under different current densities at room temperature. Figure3B compares the capacities and coulombic efficiencies (CEs) of the CNP/C composite at various current densities ranging from 20 to 200 mA g−1. The capacity of CNP/C declined slightly from 400 mAh g−1 to 374 mAh g−1 and then 338 mAh g−1, while the current density increased successively from 20 mA g−1 to 40 mA g−1 and further to 100 mAg−1, respectively. Even at the current density of 200 mA g−1, the composite cathode still delivers the capacity of 329 mAh g−1, showing a good rate performance.At the 2nd cycle, the CNP/C based cell exhibited the discharge capacity of 232mAh g-1, and it still delivers 200 mAh g-1 after 30 cycles. Moreover, the CNP/C all shows reasonable first CE and high CE during cycling performance under different currents.The good rate performance, high Coulombic efficiencyand reasonable cycling capability were observed for the CNP/C composite, demonstrating the new cathode materialhas a potential to be a promising candidate for conversion reaction.In order to further confirm theelectrochemical capability of CNP/C based cell under high current density, its capacity retention and columbic efficiency have been investigated after 30 cycles at 200mAg-1. Figure3C shows the cycling performance of CNP/C vs Li+/Li under the current density of 200mAg-1. The initial capacities of discharge/charge can maintain329 mAh g-1and 232mAh g-1, respectively. The capacity retention is 80.92% after 30 cycles, and the CE maintains at 99.2% over 30 cycles. The result shows CNP has reasonable cycle performance and good capacity retention. 10


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The Cyclic Voltammogram(CV) curve was collected with a scan rate of 0.05mV s−1 in Figure3D. There are two well-defined lithiation peaks and three delithiation peaks, indicating the transformation among Cu2+ to Cu+and then to Cu, and Ni2+ to Ni. During the second discharge process, the discharge plateaus increased from 1.55 V and 1.72 V to 1.70 V and 2.08 V. These higher discharge plateaus are beneficial for the improvement of energy density. The CNPalmost completely transformed into smaller Cu nanoparticles, Ni nanoparticles and Li3PO4after the 1st discharge, so the improved contact between smaller particles may enhance the electronic conductivity as well as shorten the diffusion path of Li+ion, raising the discharge plateau[37-38].

Structural evolution during reversible charging-discharging process.XRD is employed for investigating crystalline phase changes during discharge/charge process. We mark the rechargeable process combining cycle number and capacity, such as 1st DC means the full discharge at 1st cycle, and 2nd C200 indicating the capacity holds 200mAhg-1 during the 2nd discharge process. Figure4A depicts the XRD patterns of CNP/C at different lithiation/delithiation states. In order to analyze the reversible conversion reaction mechanism, Figure 4B shows a clearer phase transformation of CNP system during the 1st discharge.Obviously, the intensities of the XRD peaks ofCNP gradually decrease with the increase of state of discharge (SOD), and a new and broad Bragg peak appears at 43~45° under 1st discharge (DC), 2nd DC200and 2nd DC, indicative of the decomposition of CNP and the formation of metallic Cu (111) and Ni (111) during discharge[39-41].Some weak diffraction peaks are also observed at 22−24°, further confirming the formation of Li3PO4discharge product.In 11



addition, there is some residual of CNP ((131) and (310)) identified in the XRD patterns, which means CNP doesn’t fully involve in conversion reaction. This could explain why the actual discharge capacity is lower than the theoretical value of CuNi2 (PO4)2 as presented Figure3A. During charging, the broad peak around 43.5° disappears, suggesting that the crystalline Cu and Ni were re-oxidized during the charging process. During the 2nd discharge process, the broadNi (111) and Cu (111) diffraction peaksreappear, confirming the reversibility of the charge/discharge processes.

In order to further analyze the rechargeable reaction mechanism, Figure4B shows a more accurate analysis of CNP system during the 1st discharge. It can be clearly observed XRD peaks at 2θ ≈ 10−60corresponds to CNP with the P-1 space group.The main structure of CNP keeps intact until fully discharge proceeds, which is destroyed during fully discharge, concurrently with large phase changes. Furthermore, there is no new Bragg peaks are visible except the broad peak at 43-45°(Cu(111) and Ni (111)) and some weak peaks at 22−24° which belongs to Li3PO4, indicating that Li3PO4and any other products formed duringdischarging are almost amorphous. Thusthe ex-situ XRD technology just confirms that CNPis converted into Ni and Cu metalafterthe 1st cycle discharge process, while metallic Ni and Cuare re-oxidized during the rechargeable process [14, 34]. HRTEM was employed to explorethe nanostructure evolutionof CNP/C during the cycling process. Figure5A showsthat the pristine CNP consistsof well-crystallized particles, and the CNP(-101) exhibitsinterplanar crystal spacingof 0.38 nm.Figure 5B 12


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demonstrates an HRTEM image of CNP after 1st discharge, in which the phases of CNP (011) and Li3PO4 (101) are clearly observed, indicating that CNP has not been fully transformed intoLi3PO4. The HRTEM result matches well with XRD data, and is also consistent with the electrochemical capabilitythatthe initial discharge capacity of CNP (400 mAh g−1)is subordinate toits theoretical capacity (423 mAh g−1)(Figure 3A).Figure5C shows the phase structure after the 2ndcharge, where the crystalline CNP (021) and CNP (200) can be clearly identified, demonstrating the conversionreaction of Cu with surrounding Li3PO4. After the second full discharge, areas of Cu/Ni (111), Cu/Ni (200) and Li3PO4(120) can be seen in Figure5D, indicating that CNP is again converted to Cu/Ni and Li3PO4 during the second discharge. Therefore, in good accordance with the ex-situ XRD results, the HRTEM results further confirm the reversible conversion mechanism of the CNP/C cathode material. Reaction mechanism of Niand Cuin CNP/Cduring reversible cycling. With the aim to further confirmthe evolution ofreduction/oxidationof CNP/C cathode, the valence state and local coordination of Cu and Ni on CNP/C electrodes was further tracked by X-ray adsorption spectra (XANES and EXAFS of Ni and Cu K-edge).In order to precisely analyze the transformation during rechargeable process, the XANES and EXAFS spectroscopy of Ni0, Ni2, Cu0, Cu1+and Cu2+ are marked in Figure 6 and Figure7. As we can see, Figure6A shows the process ofNi2+transformed into Ni0 during the 1st discharge. The visible small peak at 833 eV in Figure 6A-C, belongs to the pre-edge signal of Ni2+ [42]. It is foundthat the signal of Ni2+ gradually decreases with the increasing of depth ofdischargeuntil almost all of Ni2+ was reduced 13



to Ni0. Meanwhile, Figure6D shows a similar process that the bulk Cu2+ was transformed into Cu0. The result is also well consistent with ex-situ XRD and HRTEM conclusion. Figure 6B and Figure 6E shows the evolution of Cu and Ni during the 2ndcharge process. The large part of Ni0 was transformed into Ni2+ when the electrode was fully charged to 4.5V. However, there is only a small fraction of Cu0 which is transformed into Cu2+, owing to fact that Ni has much lower oxidation potential and Cu, and therefore Cu0 is not easily oxide to Cu2+ when it co-exists with Ni [43-44].Figure6C and Figure6F show similar phenomenon/result with Figure6A and Figure6D during 2nd discharge process that almost all of Ni2+ and Cu2+are convertedinto Ni0 and Cu0. It is concluded that Ni0/Ni2+ is more electrochemically active in the reversible conversion reactions, whileCu/Cu2+redox couple is less reversiblein this system which mayascribe to the competitive reaction between the oxidation of Ni0 and Cu0[44].The similarresults in normalized Cu and Ni K-edge XANES spectra (Figure S1-2)indicatethat almost all of Cu2+ and Ni2+ were transformed into metallic formsafter 1st discharge cycle, and the main capacity comes from the transformation of Ni during rechargeable process. So the main reason of capacity loss is due to the irreversible conversion of Cu0 to Cu2+. The finding is different from early works that the capacity loss of Cu3(PO4)2 mainly derives from the formation ofisolated Cu particles which cannot actively involves in the rechargeable process [13-14, 34]. Figure7 shows Fourier transformed, K2-weighted Cu K-edge and Ni K-edge EXAFS spectra over the range of 0-5Å. Figure 7A exhibits the evolution process 14


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between Ni-O and Ni-Ni. At pristine state, no Ni-Ni scattering is observed. With the increasingof depth of discharge from 1st DC100 to 1st DC, the intensity of Ni-O interaction decreases and theNi-Ni bond increases. Finally, the Ni-O bond almost fully disappearsand that of Ni-Ni bond reaches the maximum. Figure7D shows similar process that most Cu-O bond disappears, simultaneously formingCu-Cu bond, although there is some residual Cu-O featuresafter 1st full discharge process [45]. The EXAFS result is consistent well with HRTEM in Figure 5C-D. Figure7B and Figure7E shows transformation from the metallic Ni and Cu species to correspondingoxides. With the enhancing of depth of charge, the bulk of metallic Ni converts into Ni oxide. However, there is only a small amount of metallic Cuinvolves conversion reaction between Cu and Cu2+.The approximate amount of Ni and Cu species changed during the charge and discharge processes are determined via linear combination fitting and the detailed results are listed in Table S1-6 in the Support Information. Figure7C and Figure7F show similar reaction pathway with the 1stdischarge process that almost all of Ni and Cu oxides were reduced to metallic forms. The above results substantiate that the irreversiblecapacity loss mainly comes from the Cu particles formed at the1st discharge process, which are not fully oxidized to Cu2+ during subsequentcharge process. The scheme of reaction mechanisms of CNP/C during rechargeable process. As discussed above result, the electrochemical reaction mechanisms of CNP/C ||Li cellsare summarized in Figure 8. During the first discharge cycle, ideal carbon coated CNP (CNP/C) composite modelshows a high capacity of 400 mAh g-1, combining 15



with the XRD and XAS results confirm that almost all of CNP converted into LixPO4 and nanoscale of Ni/Cu. During the charge process, the CNP/C electrode demonstrates impressive rechargeable capacity ranging to 306 mAh g-1and the existence of incomplete phase of Cu0 and LixPO4, as shown in the scheme. The capacity loss as disclosed by XAS attributed to the poor reversibility of Cu/Cu2+ during the conversion process, which is not comparable to the excellent reversibility of Ni/Ni2+.

4. CONCLUSION CNP/Cnanocomposite has been successfully synthesized and adopted as a promising cathode for rechargeable LIBs.It is shown that CNP/C cathode delivers a high specific capacity (rechargeable capacity of 306 mAhg-1at 20 mAg-1),a decent cyclingperformance as well as relativelygood rate performance, validating it a potential cathode candidate for next generation rechargeable LIBs. The reaction mechanism study indicates that Ni2+ and Cu2+ were almost fully transformed into Ni0 and Cu0 during the 1st discharge process. Furthermore, the evolution between Ni0 and Ni2+ is reversible during the 2ndcharge and 2nd discharge process. However, the majority of Cu0 could not be reconverted to Cu2+ at 2nd charge process, which isconsidered to be the primary reason forcapacity degradation during the 1st cycles. The revealed reaction mechanism of CNP/C cathodewill be beneficial for future development of high performanceCu-based/Ni-based conversion typedcathode materials for LIBs.

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ASSOCIATED CONTENTS Supporting Information This supporting information is available free of charge via the Internet at http://pubs.acs.org. The Ni/Cu K-edge XANES spectra, the Tables of Ni/Cu content.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] Notes The authors declare no competing financial interests

ACKNOWLEDGEMENT The authors thanks the support of National Natural Science Foundation of China (Grant no. 21233004, 21473148, 21428303 and21303147), and National Key Research and Development Program of China (grant no. 2016YFB0901500)

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REFERENCES 1) Kang K., Abruna, Meng, S. Y.; Breger, J.; Grey, C. P.; Gerbrand C. Electrodes with High Power and High Capacity for Rechargeable Lithium Batteries. Science 2006, 311,977-980. (2) Armand, A.; Tarascon, J. M. Building Better Batteries. Nature2008, 451, 652-657. (3) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587-603. (4) Li, T.; Ai, X. P.; Yang, H. X., Reversible Electrochemical Conversion Reaction of Li2O/CuO Nanocomposites and Their Application as High-Capacity Cathode Materials for Li-Ion Batteries. J. Phy. Chem. C2011, 115, 6167-6174. (5) Scrosati, B.; Garche, J., Lithium Batteries: Status, Prospects and Future. J. Power Sources 2010, 195, 2419-2430. (6) Jeong, S.; Li, X.; Zheng, J.; Yan, P.; Cao, R.; Jung, H. J.; Wang, C.; Liu, J.; Zhang, J. -G., Hard Carbon Coated Nano-Si/Graphite Composite as a High Performance Anode for Li-Ion Batteries. J. Power Sources 2016, 329, 323-329. (7) Zheng, J.; Engelhard, M. H.; Mei, D.; Jiao, S.; Polzin, B. J.; Zhang, J.-G.; Xu, W. Electrolyte Additive Enabled Fast Charging and Stable Cycling Lithium Metal Batteries. Nature Energy 2017, 2, 17012. (8) Peng, C.; Xiao, F.; Yang, J.; Li, Z.; Lei, G.; Xiao, Q.; Ding, Y.; Hu, Z., Carbon-Encapsulated Mn-Doped V2O5 Nanorods with Long Span Life for High-Power Rechargeable Lithium Batteries. Electrochim. Acta 2016, 192, 216-226. (9) Zheng, J. M.; Wu, X. B.; Yang, Y. Comparison of Preparation Method on the Electrochemical Performance of Cathode Material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 for Lithium Ion Battery. Electrochim. Acta 2011, 56, 3071-3078. (10) Zheng, J. M.; Zhang, Z. R.; Wu, X. B.; Dong, Z. X.; Zhu, Z.; Yang, Y. The Effects of AlF3 Coating on the Performance of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 Positive Electrode Material for Lithium-Ion Battery. J. Electrochem. Soc. 2008, 155, A775. (11) Ye, Z.;, Zhao, X.; Li, S. D.; Wu, S. Q.; Wu, P.; Nguyen, M. C.; Guo, J. H.; Mi, J. X.; Gong, Z. L.; Zhu, Z. Z.; Yang, Y.; Wang, C. Z.; Ho, K.M. Robust Diamond-Like Fe-Si Network in the Zero-Strain Na X Fesio 4 Cathode. Electrochim. Acta 2016, 212, 934-940. 18


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(12) Zheng, J.; Xiao, J.; Nie, Z.; Zhang, J.-G., Lattice Mn3+ Behaviors in Li4ti5o12/LiNii0.5 Mn1.5O4 Full Cells. J. Electrochem. Soc. 2013, 160, A1264-A1268. (13) Hua, X.; Robert, R.; Du, L.-S.; Wiaderek, K. M.; Leskes, M.; Chapman, K. W.; Chupas, P. J.; Grey, C. P. Comprehensive Study of the Cuf2conversion Reaction Mechanism in a Lithium Ion Battery. J. Phys. Chemistry C 2014, 118, 15169-15184. (14) Zhong, G. M.; Bai, J. Y.; Duchesne, P. N.; McDonald, M. J.; Li, Q.; Hou, X.; Tang, J. A.; Wang, Y.; Zhao, W. G.; Gong, Z. l.; Zhang, P.; Fu, R. Q.; Yang, Y.; Copper Phosphate as a Cathode Material for Rechargeable Li Batteries and Its Electrochemical Reaction Mechanism. Chem. Mater. 2015, 27, 5736-5744. (15) Grugeon,S.; Lascaud,S.;

Laruelle,S.;

Vidal,E.;

Dupont,L.;

Chevallier,F.; Taberna,P. L.; Simon,P.;

Gireaud,L.;

Yrieix, B.; Tarascon, J. -M. Combining Electrochemistry and Metallurgy for

New Electrode Designs in Li-Ion Batteries. Chem. Mater. 2005, 17, 5041-5047. (16) Yamakawa, N.; Jiang, M.; Baris, K.; Grey C. P. Identifying the Local Structures Formed During Lithiation of the Conversion Material, Iron Fluoride, in a Li Ion Battery a Solid-State Nmr, X-Ray Diffraction, and Pair Distribution Function Analysis Study. J. Am. Chem. Soc. 2009, 131, 10525–10536. (17) Amatucci, G. G.; Pereira, N. Fluoride Based Electrode Materials for Advanced Energy Storage Devices. J. Fluor.Chem. 2007, 128, 243-262. (18) Badway, F.; Pereira, N.; Cosandey, F.; Amatucci, G. G. Carbon-Metal Fluoride Nanocomposites. J. Electrochem. Soc. 2003, 150, A1209. (19) Cabana, J.; Monconduit, L.; Larcher, D.; Palacin, M. R., Beyond Intercalation-Based Li-Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting through Conversion Reactions. Adv.Mater. 2010, 22, E170-192. (20) Badway, F.;Mansour, A. N.; N. Pereira, N.; Al-Sharab, J. F.;F. Cosandey, F.; Plitz, I.; Amatucci, G. G.; Structure and Electrochemistry of Copper Fluoride Nanocomposites Utilizing Mixed Conducting Matrices. Chem. Mater. 2005, 17, 5041-5047. (21) Kim, S. W.; Seo, D. H.; Gwon, H.; Kim, J.; Kang, K. Fabrication of Fef3 Nanoflowers on CNT Branches and Their Application to High Power Lithium Rechargeable Batteries. Adv. Mater. 2010, 22, 5260-4.

19



(22) Li, T.; Chen, Z. X.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Transition-Metal Chlorides as Conversion Cathode Materials for Li-Ion Batteries. Electrochim. Acta 2012, 68, 202-205. (23) Liu, J.-l.; Cui, W.-J.; Wang, C.-X.; Xia, Y.-Y. Electrochemical Reaction of Lithium with Cocl2 in Nonaqueous Electrolyte. Electrochem. Commun. 2011, 13, 269-271. (24) Ma, D. L.; Cao, Z. Y.; Wang, H.-G.; Huang, X.-L.; Wang, L.-M.; Zhang, X. B. Three-Dimensionally Ordered Macroporous FeF3 and Its in Situ Homogenous Polymerization Coating for High Energy and Power Density Lithium Ion Batteries. Energy Environ. Sci. 2012, 5, 8538. (25) Yamakawa, N.; Jiang, M.; Grey, C. P., Investigation of the Conversion Reaction Mechanisms for Binary Copper(Ii) Compounds by Solid-State Nmr Spectroscopy and X-Ray Diffraction. Chem. Mater. 2009, 21, 3162-3176. (26) Zhang, W.; Ma, L.; Yue, H.; Yang, Y. Synthesis and Characterization of in Situ Fe2O3-Coated FeF3 Cathode Materials for Rechargeable Lithium Batteries. J. Mater. Chem. 2012, 22, 24769. (27) Goñi, A.; Luis Lezama, L.; José Luis Pizarro, J. L.; Jaione Escobal, J.; Arriortua, M. I.; Rojo, T.Intercalation of Cu2+ in the HNiPO4·H2O Layered Phosphate Study of the Structure, Spectroscopic, and Magnetic Properties. Chem. Mater. 1999, 11, 1752-1759. (28) Bamberger, C. E.; Specht, E.; Anovitz, L. M. Crystalline Copper Phosphates: Synthesis and Thermal Stability. J. Am. Chem. Soc. 1997, 80, 3133-3138. (29) Weimann, I.; Feller, J.; Žák, Z.Phase Equilibria in the System CuO‐NiO‐P4O10 and Synthesis, Crystal Structure, and Characterization of the New Copper Nickel Oxide Phosphate Cu3NiO(PO4)2. Z.Anorg.Allg.Chem.2017,299–305. (30) Liu, X.; Liu, H.; Zhao, Y.; Dong, Y.; Fan, Q.; Kuang, Q., Synthesis of the Carbon-Coated Nanoparticle Co9S8 and Its Electrochemical Performance as an Anode Material for Sodium-Ion Batteries. Langmuir 2016, 32, 12593-12602. (31) Lyu, H.; Gao, B.; He, F.; Ding, C.; Tang, J.; Crittenden, J. C., Ball-Milled Carbon Nanomaterials for Energy and Environmental Applications. ACS Sustainable Chem. Eng.2017, 5, 9568–9585 (32) Wang, Y.; Wang, Y.; Hosono, E.; Wang, K.; Zhou, H. The Design of a LiFePO4/Carbon Nanocomposite with a Core-Shell Structure and Its Synthesis by an in Situ Polymerization Restriction Method. Angew Chem Int Ed.2008, 47, 7461-7465.

20


Page 20 of 32

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(33) Wu, X.; Zheng, J.; Gong, Z.; Yang, Y. Sol–Gel Synthesis and Electrochemical Properties of Fluorophosphates Na2Fe1−XMnxPO4F/C (X = 0, 0.1, 0.3, 0.7, 1) Composite as Cathode Materials for Lithium Ion Battery. J. Mater. Chem. 2011, 21, 18630. (34)Zhao, W. G.; Zhong, G. M.;McDonald, M. J.; Gong, Z. L.; Liu, R.; Bai J. Y.; Yang. C.; Li, S .G.; Zhao, W. M.; Wang, H. C.; Fu, R. Q.; Jiang, Z.; Yang, Y. Cu3(PO4)2/C Composite as a High-Capacity Cathode Material for Rechargeable Na-Ion Batteries. Nano Energy 2016, 27, 420-429. (35) Débart, A.; Dupont, L.; Poizot, P.; Leriche, J. B.; Tarascon, J. M. A Transmission Electron Microscopy Study of the Reactivity Mechanism of Tailor-Made Cuo Particles toward Lithium. J. Electrochem. Soc. 2001, 148, A1266. (36) Morales, J.; Sánchez, L.; Martín, F.; Ramos-Barrado, J. R.; Sánchez, M. Nanostructured Cuo Thin Film Electrodes Prepared by Spray Pyrolysis: A Simple Method for Enhancing the Electrochemical Performance of CuO in Lithium Cells. Electrochim. Acta 2004, 49, 4589-4597. (37) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587–603. (38) Han, X.; Chen, H.; Liu, J.; Liu, H.; Wang, P.; Huang, K.; Li, C.; Chen, S.; Yang, Y. A Peanut Shell Inspired Scalable Synthesis of Three-Dimensional Carbon Coated Porous Silicon Particles as an Anode for Lithium-Ion Batteries. Electrochim. Acta 2015, 156, 11-19. (39) Pierno, M.; Bruschi, L.; Mistura, G.; Paolicelli, G.; Dibona, A.; Valeri, S.; Guerra, R.; Vanossi, A.; Tosatti, E. Frictional Transition from Superlubric Islands to Pinned Monolayers. Nat. Nanotechnol. 2015, 10, 714-718. (40) Roberts, F. S.; Kuhl, K. P.; Nilsson, A. High Selectivity for Ethylene from Carbon Dioxide Reduction over Copper Nanocube Electrocatalysts. Angew Chem. Int. E. Engl. 2015, 54, 5179-82. (41) Stefano, G.; Kathrin M.; Luca, B.; Juan C. M.; Tuan A. P.; Oleksii I.; Alexei, B.; Jonas, Björk.; Petra R.; Stöhr, A. M. Comparing Graphene Growth on Cu(111) Versus Oxidized Cu(111). Nano Lett. 2015, 15, 917-922. (42) Platero-Prats, A. E.; League, A. B.; Bernales, V.; Ye, J. Y.; Li, Z. Y.; Zheng, J.; Joseph T. Hupp, J. P.; Browning, N. D.; Fulton, J. L.; Camaioni, D. M.; Lercher, J. A.; Truhlar, D. G.; Gagliardi, L.; Christopher J. Cramer, C. J.; Karena W. Chapman, K. W. Bridging Zirconia Nodes within a Metal-Organic Framework Via Catalytic Ni-Hydroxo Clusters to Form Heterobimetallic Nanowires. J. Am. Chem. Soc. 2017, 139, 10410-10418. 21



(43) Débart, A.; Dupont, L.; Patrice, R.; Tarascon, J. M. Reactivity of Transition Metal (Co, Ni, Cu) Sulphides Versus Lithium: The Intriguing Case of the Copper Sulphide. Solid-State Sci. 2006, 8, 640-651. (44) Meuleman, W. R. A.; Roy, S.; Péter, L.; Varga, I., 40--Effect of Current and Potential Waveforms on Sublayer Thickness of Electrodeposited Copper-Nickel Multilayers. Journal of The Electrochemical Society 2002, 149, C479. (45) Ikuno, K.; Zheng, J.;Vjunov, A.; Sanchez-Sanchez, M.; Ortuño, M. A.; Pahls, D. S.; Fulton, J. L.; Donald M. Camaioni, D. M.; Li, Z. Y.; Ray, D.; Mehdi, B. L.; Nigel D. Browning, N. D.; Farha, O. K.; Joseph T. Hupp, J. T.; Cramer, C. J.; Gagliardi, L.; Lercher, J. A. Methane Oxidation to Methanol Catalyzed by Cu-OxO Clusters Stabilized in Nu-1000 Metal-Organic Framework. J. Am. Chem. Soc. 2017, 139, 10294-10301.

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Images for manuscript

TOC image

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Figure 1. The XRD patterns of CNP materials that were calcined at different temperatures for 6 h.

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Figure 2. (A) SEM images of CNP; (B) SEM and (C) EDS data of CNP/C; (D) XRD patternscorresponding to CuNi2(PO4)2, CuNi2(PO4)2/C and a standard PDF card.

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Figure 3. The electrochemical performance of CNP/C. (A) Charge/discharge profile under the current density of 40 mA g-1; (B) Cycle performance under different current densities; (C) Cycle performance and coulombic efficiency data at a current density of 200 mA g-1; (D) CV curves of CNP/C at a scan rate of 0.05 mV s-1.The voltage range is between 1.5 and 4.5 V.

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Figure 4. Ex-situ XRD patterns of CNP/C electrode under different discharged/charged states.(A) discharged/charged states of one and a half cycle process; (B) enlarged view of different discharged states of the 1st cycle.

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Figure 5. HRTEM images of pristine CNP and different discharged/charged states of CNP/C. (A) Pristine;(B) the 1st cycle discharge; (C) the 2nd cycle charge; (D) the 2nd cycle discharge.

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Figure 6. First derivative curves of the XANES spectra from crystalline CNP during the rechargeable process. (A) Ni 1st discharge; (B) Ni 1st charge; (C) Ni 2nd discharge; (D) Cu 1st discharge; (E) Cu 1st charge; (F) Cu 2nd discharge.

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Figure 7. R range curves of the EXAFS spectra from crystalline CNP during the rechargeable process. (A) Ni 1st discharge; (B) Ni 1st charge; (C) Ni 2nd discharge; (D) Cu 1st discharge; (E) Cu 1st charge; (F) Cu 2nd discharge.

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Figure 8. The scheme of reaction mechanisms of CNP/C during rechargeable process.

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The scheme of reaction mechanisms of CNP/C during rechargeable process 177x89mm (300 x 300 DPI)


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