Accepted Manuscript Phase transitions and related electrochemical performances of Li-Rich layered cathode materials for high-energy lithium ion batteries Jianqing Zhao, Xiaoxiao Kuai, Xinyu Dong, Haibo Wang, Wei Zhao, Lijun Gao, Ying Wang, Ruiming Huang PII: S0925-8388(17)33615-0 DOI: 10.1016/j.jallcom.2017.10.179 Reference: JALCOM 43576 To appear in: Journal of Alloys and Compounds Received Date: 7 July 2017 Revised Date: 17 October 2017 Accepted Date: 22 October 2017 Please cite this article as: J. Zhao, X. Kuai, X. Dong, H. Wang, W. Zhao, L. Gao, Y. Wang, R. Huang, Phase transitions and related electrochemical performances of Li-Rich layered cathode materials for high-energy lithium ion batteries, Journal of Alloys and Compounds (2017), doi: 10.1016/ j.jallcom.2017.10.179. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. 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AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT Phase Transitions and Related Electrochemical Performances of Li-Rich Layered Cathode Materials for High-Energy Lithium Ion Batteries Jianqing Zhao a, b, d, Xiaoxiao Kuai a, b, Xinyu Dong a, b, Haibo Wang a, b, e, Wei Zhao f, a RI PT Lijun Gao a, b, *, Ying Wang d,**, Ruiming Huang c, *** Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Jiangsu Provincial Key Laboratory for Advanced Carbon Materials and Wearable Energy Technologies, Suzhou 215006, China c Department of Chemistry, Rutgers-Newark, The State University of New Jersey, Newark, New Jersey 07103, United States d M AN U b SC Soochow University, Suzhou 215006, China Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, TE D Louisiana 70803, United States Institute of Chemical Power Sources, Soochow University, Zhangjiagang 215600, China f Shanghai Haiying Machinery Plant, Shanghai 200436, China EP e *Corresponding Authors: AC C Prof. Lijun Gao, Tel: +86-512-65229905; Fax: +86-512-65229905; E-mail: email@example.com Prof. Ying Wang, Tel: +1-225-578-8577; Fax: +1-225-578-9162; E-mail: firstname.lastname@example.org Dr. Ruiming Huang, Tel: +1-973-353-1254; Fax: +1-1-973-353-1264; E-mail: email@example.com 1 ACCEPTED MANUSCRIPT Abstract: The present work systematically probes and tracks the phase transition of Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (marked as LMNCO) by using an ex-situ chemical activation that is realized through ion-exchange and post-annealing processes, in order to lithium-ion batteries. Ion exchanges H+-Li+ of and RI PT understand related electrochemical performances of Li-rich cathode materials for advanced subsequent TBA+-H+ (TBA: tetrabutylammonium) in LMNCO are carried out, resulting in its layered-to-spinel phase SC transition after optimal heat treatments. The resultant compound shows a Li4Mn5O12-type spinel structure. This converted spinel cathode material can deliver discharge capacities higher than 300 M AN U mAh/g at 0.1 C and 200 mAh/g at 1 C (1 C=250 mA/g), respectively, and also exhibits better cycling stability and rate capability in comparison with pristine layered LMNCO and other derivatives. This work offers a feasible route to study all changes of morphologies, crystal structures, chemical compositions, surface areas and related electrochemical lithium storage TE D behaviors during phase transitions of Li-rich layered cathode materials, and thus provides batteries. EP insights on optimizing electrochemical performances for high-energy and high-power lithium ion Keywords: Chemical activation, ion exchange, phase transition, Li-rich layered cathode material, AC C lithium ion battery 2 ACCEPTED MANUSCRIPT 1. Introduction The rechargeable lithium ion batteries have been demonstrated as highly effective power supplies for electric transportation system and portable electronic devices. Performances of RI PT lithium-ion batteries crucially rely on energy and power densities of electrode materials . Recently, tremendous research efforts focus on developing advanced cathode materials, which are expected to offer high specific capacity and operating voltage together with outstanding SC cycling stability and rate capability ,,. The Mn-based Li-rich layered oxides have attracted tremendous research efforts owing to the high lithium storage capacity and working M AN U potential. These cathode materials marked as Li[LixMnyMz]O2 (M=Co and Ni; x+y+z=1 and y>0.5) can be cycled over a broad voltage range of 2.0 - 4.8 V vs. Li+/Li and deliver specific capacities higher than 250 mAh/g, along with other merits including low cost, environmental friendliness and safety [5-18]. TE D Li[Li0.2Mn0.54Ni0.13Co0.13]O2 (marked as LMNCO) belongs to aforementioned Li-rich and Mnrich category, which has the desirable theoretical capacity (>300 mAh/g) and high working voltage (~ 4.0 V vs. Li+/Li) . As reported in literatures [6-8], Li-rich layered LMNCO is the EP product with a structural intergrowth of layered lithium-inactive Li2MnO3 (space group C2/m) and layered lithium-active LiMn1/3Ni1/3Co1/3O2 (space group R-3m) at a molar ratio of 1:1 AC C (0.5Li2MnO3·0.5LiMn1/3Ni1/3Co1/3O2). The high capacity of LMNCO cathode material can be achieved through the electrochemical activation of Li2MnO3 component in the first charge reaction above 4.5 V vs. Li+/Li. However, such a reaction leads to significantly irreversible capacity loss and low Coulombic efficiency in the first charge/discharge cycle, and further can trigger a detrimental layered-to-spinel phase transition during next electrochemical cycling of activated layered LMNCO. The structural similarity of cubic close-packed oxygen arrays in 3 ACCEPTED MANUSCRIPT layered and spinel configurations facilitates the layered-to-spinel phase transition in principle ,,,,. As a result, transition metal ions migrate to lithium layers in LMNCO and reside on vacant lithium ion sites permanently. This irreversible phase transformation is RI PT continued until a hybrid layered-spinel composite material is formed. Accordingly, the voltage plateau of working cathode is reduced from ~4.0 V (layered) to ~3.0 V (spinel) vs. Li+/Li ,. Overall, electrochemical activation of Li2MnO3 component causes structural instability SC and unfavorable phase transition, which accounts for the voltage fading and decreased energy density of Li-rich layered cathode materials . M AN U On the other hand, it has been reported that the layered-to-spinel phase transformation in Lirich layered cathode materials demonstrates the unexpectedly high-rate capability of hybrid layered and spinel cathode materials ,,, because the spinel phase is lithium-active with enhanced electronic conductivity and lithium ion diffusivity . The formation of the spinel TE D phase in Li-rich layered cathode materials has been demonstrated by high resolution STEM observations , high resolution TEM images with selected area electron diffraction (SAED) patterns , in-situ X-ray diffraction patterns, X-ray absorption spectroscopic and Raman EP studies ,, and is also reflected on charge/discharge curves , differential capacity plots , and cyclic voltammetric (CV) profiles ,. The intergrowth of spinel-layered phases AC C tends to alleviate the electrochemical inferiority of Li-rich layered cathode materials. However, structural details of these phases in the cycled electrodes have not been comprehensively understood yet. The crystal phase of the spinel formed in Li-rich layered oxides is always reported as either “spinel” or “spinel-like” phase. The effects of such a phase transformation (whether to improve or deteriorate performances of spinel-layered composite cathodes) are still 4 ACCEPTED MANUSCRIPT under debate. It is important to explore these effects, in order to understand the fundamental electrochemical behavior of high-capacity Li-rich layered cathode materials. As reported in literatures [16, 25], Li-rich layered cathode materials can be chemically RI PT activated via the protonation (H+-Li+ exchange) in an acidic environment, followed by removing H+ ions in a post-annealing treatment in air. As a result, the activated cathode material show considerably increased capacity and associated Coulombic efficiency in the initial cycle. SC Although H+-Li+ ion exchange can induce the formation of the spinel phase, the formation of spinel domains in the structure of Li-rich layered cathode material is very limited. Consequently, M AN U the phase transition successively takes place within the hybrid spinel-layered composite cathode during next electrochemical cycles, leading to structural instability and poor cycling stability . As referred to other ion-exchange reports ,,,, alkylammonium hydroxides, such as tetrabutylammonium hydroxide (TBA+·OH-) and tetramethylammonium hydroxide TE D (TMA+·OH-), have been widely employed to exfoliate protonated layered materials into twodimensional (2D) nanosheets. Due to the organic characteristics and larger molecular size of alkylammonium cations in comparison with protons and lithium ions, TBA+·OH- can be utilized EP for the second ion exchange (TBA+-H+) of protonated Li-rich layered cathode materials, in order to realize a complete layered-to-spinel phase conversion. AC C In our previous work , we realized the layered-to-spinel phase transformation of Li-rich Li[Li0.2Mn0.54Ni0.13Co0.13]O2 by employing ex-situ ion-exchange and post-annealing processes, and found that the completely-converted material shows a Li4Mn5O12-type spinel structure rather than commonly-reported LiMn2O4-type spinel. The approach we developed not only allows the comprehensive study of electrochemical effects resulting from the growth of a spinel phase within Li-rich layered cathode materials, but also offers a feasible route to precisely identify 5 ACCEPTED MANUSCRIPT different crystal structures during the formation of newly-formed spinel phase. Herein, we report a more comprehensive study during the phase transition of Li-rich layered materials with aspects to all changes of morphologies, crystal structures, chemical compositions, surface areas and RI PT related electrochemical lithium storage behaviors in details. Electrochemical performances indicate that introduction of a spinel phase significantly increases the specific capacity of ~100 mAh/g and results in much better high-rate performances as compared with original Li-rich SC layered cathode materials, but reduces the working voltage from 4.0 V to 3.0 V due to the activation of Mn3+/Mn4+ redox pair. It is also interesting to find that Li+-TBA+ can be carried out from the converted spinel phase. 2. Experimental M AN U in the ion-exchanged intermediate material for the possible recovery of the layered structure 2.1 Synthesis of Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles TE D Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles were synthesized by using coprecipitation method. Three precursor solutions were simultaneously prepared. 0.08 mol transition metal precursor at a molar ratio of Mn(CH3COO)2·4H2O : Ni(CH3COO)2·4H2O : EP Co(CH3COO)2·4H2O = 0.54 : 0.13 : 0.13 was dissolved in 50 mL ethanol; the lithium precursor solution was composed of 0.12 mol LiOH dissolved in 20 mL distilled water; and the surfactant AC C solution was 5.4 mmol F127 (EO106PO70EO106) dissolved in 50 mL ethanol. The F127/ethanol solution and transition metal precursor solution was first mixed together at 40°C under continuous stirring, and then the lithium precursor solution was dropwise added to precipitate transition metal ions. The resulting suspension was heated at 80 °C to completely remove the solvent and then dried in air at 120 °C for 12 h. The dried powder was annealed in air at 300 °C for 3 h at a temperature ramp of 1 °C/min, followed by sintering at 900 °C for 12 h at a 6 ACCEPTED MANUSCRIPT temperature ramp of 5 °C/min. Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles were obtained after cooling to room temperature. 2.2 Chemical activation of Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 via ion exchanges and RI PT post-heat treatments The chemical activation of Li-rich layered oxide was carried as follows: first, 1 g Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles were dispersed in 150 mL 2 M HCl aqueous solution for SC the H+-Li+ ion exchange of Li[Li0.2Mn0.54Ni0.13Co0.13]O2 at ambient temperature. The HCl solution was replaced every 2 days for 5 times in order to achieve deep protonation. The M AN U protonated intermediates dispersed in 150 mL aqueous HCl was obtained in a brown suspension. Secondly, an aqueous tetrabutylammonium (TBA·OH) solution (Sigma Aldrich) with a mass rate of 20 wt.% was employed to perform TBA+-H+ exchange of protonated particles. The volumetric ratio of TBA·OH solution over the brown suspension was set to 5 : 1, and these two solution was TE D mixed in a vortex stirrer for 30 min. All resulting ion-exchanged particles were collected via centrifugation and washed with distilled water for several times. Thirdly, the Li+-TBA+ exchange was carried out in 1 M LiOH aqueous solution to study the reversibility of different ion EP exchanges. Finally, all ion-exchanged derivatives were annealed in air at 500 °C for 3 h at a temperature ramp of 1 °C/min. AC C 2.3 Characterizations Crystallographic structures and phases of Li-rich layered nanoparticles and all derivatives were analyzed by X-ray diffraction (XRD) on a Panalytical X’pert Diffractometer with Cu Kα radiation. Morphology and particle characteristics of different samples were examined using a field emission scanning electron microscopy (FESEM, Hitachi S4800). Detailed structures of different samples were observed on transmission electron microscopy (TEM, FEI Tecnai G2 7 ACCEPTED MANUSCRIPT FEG) at an acceleration voltage of 300 kV. Porous structure and specific surface area of powders was measured by nitrogen adsorption/desorption at 77 K on a Quantachrome AS-1 instrument using the Brunauer-Emmet-Teller (BET) method. Chemical compositions of specimens were RI PT determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) on a SPCTRO CIROS elemental analyzer. 2.4 Electrochemical measurements SC The cathodes were consisted of 80 wt.% Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles or its derivatives, 10 wt.% acetylene black as the conductive carbon (Alfa Aesar, 99.5%), and 10 wt.% M AN U polyvinylidene fluoride (PVDF) as the binder (Alfa Aesar). These electrodes were assembled into CR2032-type coin cells for electrochemical measurements, with the metallic lithium foil as the anode and Celgard 2320 membrane as the separator. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) at a TE D volumetric ratio of 1:1:1. Galvanostatic charge and discharge were performed at different current densities in a voltage range of 2.0 - 4.8 V vs. Li+/Li using an 8-channel battery analyzer (MTI Corporation). Theoretical capacities of different cathode materials are all set to 250 mAh/g, i.e., EP current density corresponding to 1 C is 250 mA/g. Cyclic voltammetric (CV) curves of cathodes were recorded at a scanning rate of 0.1 mV/s between 2.0 and 4.8 V vs. Li+/Li using an AC C electrochemical analyzer (CHI 605C). 3. Results and discussion As shown in Fig. 1, ion-exchange processes and calcinations result in dramatically morphological changes of different derivatives. The pristine Li-rich Li[Li0.2Mn0.54Ni0.13Co0.13]O2 particles exhibit a distinct aggregation with an even particle size around ~250 nm in Fig. 1a, while H+-Li+ ion exchange in acidic environment gives rise to distinct layered cake-shaped 8 ACCEPTED MANUSCRIPT blocks of LHMNCO. We speculate that the multilayered morphology and structure of LHMNCO (Fig. 1b) is probably attributed to the structural introduction of layered LMNCO. As reported in literatures [26, 29, 31], the protonation is required for the subsequent H+-TBA+ ion exchange. RI PT After shaking the intermediate LHMNCO and HCl mixture (i.e., protonated LHMNCO particles dispersed in the final HCl solution) via violent vortexes in the aqueous TBA·OH solution at a volumetric ratio of 1:5, the collected LHMNCO TBA shows the interesting morphology of SC nanoflowers (Fig. 1c). Each particle is composed of numerous ultrathin nano-petals. This phenomenal morphology change is mostly resulted from the cooperative effects of the TBA- M AN U assisted exfoliation and the turbulence-induced reaction environment. It is interesting that further Li+-TBA+ ion exchange in a basic solution extensively unfolds petals of LHMNCO TBA nanoflowers into nanosheet stacks of LHMNCO TBA Li as shown in Fig. 1d. Fig. 1e-g reveal morphologies of ion-exchanged derivatives after post-annealing treatments, which apparently TE D cause aggregations and coarse structures of LHMNCO HT, LHMNCO TBA HT and LHMNCO TBA Li HT, respectively, after the removal of H+ and TBA+ ions along with other byproducts, such as H3O+ and OH-. EP Accordingly, phase transitions and structural reconstructions accompanying with morphology changes from initial layered LMNCO to different converted derivatives have been studied from AC C XRD and TEM characterizations in Fig. 2 and Fig. 3, respectively. In agreement with reported XRD patterns of Li-rich layered materials [7, 8, 17, 18], pristine LMNCO in Fig. 2a shows typical XRD peaks that are indicative of the intergrowth of monoclinic Li2MnO3 (space group: C2/m) and rhombohedral LiMn1/3Ni1/3Co1/3O2 (space group: R-3m) in the layered structure . The main layered structure of LiMn1/3Ni1/3Co1/3O2 can be determined from distinct peak splits of two (006)L2-(012)L2 and (108)L2-(110)L2 doublets at 2θ = 36 - 38º and 2θ = 64 - 66º, respectively. 9 ACCEPTED MANUSCRIPT The weak (020)L1 reflection at 2θ = 20 - 23º is belong to the layered Li2MnO3 phase, which is the superslattice within the parent layered structure . The intergrowth of these two layered components can be further confirmed from the other five peaks at 2θ = 18.7º, 36.9º, 37.9º, 38.5º RI PT and 44.5º, resulting in diffraction patterns of (001)L1/(003)L2, (200)L1/(101)L2, (113)L1/(006)L2, (131)L1/(012)L2, and (202)L1/(104)L2. A spinel-like impurity is found in the XRD pattern of LHMNCO (marked with red asterisks in Fig. 2b), which is accordant with that reported in the SC literatures [7, 22]. As shown in Fig. 2b, the peak merger of (113)L1/(006)L2 and (131)L1/(012)L2 pairs reveals the distortion of layered structure at a certain degree, while the preserved peak splits M AN U of (108)L2-(110)L2 doublets indicate the retained layered structure in the protonated LHMNCO intermediate . Furthermore, XRD pattern of LHMNCO TBA powder manifests the growth of spinel-like phase during TBA+-H+ exchange, since the intensity of one representative XRD peak at 2θ = 19.3º in LHMNCO for the spinel phase increases. The cubic close packed oxygen arrays TE D both in layered and spinel structures is essential to realize the phase transition in Li-rich layered transition metal oxides by migrating transition metal ions into lithium layers when lithium ion vacancies exist during ion-exchange processes ,. LHMNCO TBA Li shows identical EP XRD pattern to that of LHMNCO TBA, indicating limited reversibility of phase transition by Li+-TBA+ ion exchange. However, the peak splitting of (108)L2-(110)L2 doublets in its XRD AC C pattern reveals the prominent layered structure of LHMNCO TBA Li derivative, despite the spinel phase has been detected. Post-annealing treatment has been demonstrated as an effective way to remove H+ and TBA+ substituents in air, resulting in the generation of corresponding Li+ vacancies in lithium layers . Such facile process can accelerate the diffusion of transition metal ions into lithium ion sites, and thus promote the layered-to-spinel phase transition. The enlarged selected 2θ portions in Fig. 2b at 2θ =16-20º, 34-40º and 62-68º illustrate the phase 10 ACCEPTED MANUSCRIPT transitions when different ion-exchanged samples are subjected to calcinations in air. The merge of two separate peaks around 2θ = 19º into one peak occurs for LHMNCO, LHMNCO TBA and LHMNCO TBA Li after heat treatments, respectively, indicating dramatic phase transitions due RI PT to the removal of foreign H+ and TBA+ cations. It is surprising to find that the coupled (108)L2(110)L2 pair of LHMNCO TBA has merged to one broad peak for LHMNCO TBA HT at a lower 2θ position, while LHMNCO HT and LHMNCO TBA Li HT still show distinguishing peak SC splits of (108)L2-(110)L2 between 2θ = 62º and 64º. As mentioned before, the peak split of (108)L2-(110)L2 doublets is characteristic of layered structure, which is distinguished from the M AN U spinel phase showing the (440)S reflection at the same position. As a result, XRD pattern of LHMNCO TBA HT can be indexed to the spinel Li4Mn5O12 phase with a Fd-3m space group, indicating the complete layered-to-spinel phase transition from original Li-rich layered LMNCO to Li4Mn5O12-type spinel compound after two-step ion exchanges, followed by a post-annealing TE D process. In contract, XRD patterns of LHMNCO HT and LHMNCO TBA Li HT both reveal the coexistence of layered and spinel phases (Fig. 2). It is suggested that a second TBA+-H+ ion exchange is crucial to realize a complete phase conversion. We speculate that due to the larger EP size of TBA+ cations than protons, TBA+ substituents can increase c-axis of ion-exchanged layered derivative in comparison with the effect from H+ ions, which will increase the structure AC C instability that significantly facilitate migrations of transition metal ions when TBA+ are burned away in air. On the contrary, Li+ can be partially restored in the lithium layers through the Li+TBA+ ion exchange, resulting in hybrid layered-spinel structure of LHMNCO TBA Li HT. Fig. 3 shows the structural evolution from the pristine layered LMNCO to the converted Li4Mn5O12-type LHMNCO TBA HT spinel in TEM and HRTEM observations. In consistence with SEM image in Fig. 1a, LMNCO nanoparticles have the solid structure with an average 11 ACCEPTED MANUSCRIPT particle size of ~250 nm. The lattice fringe as shown in Fig. 3b indicates the high crystallinity of pristine Li-rich layered materials, due to the high synthetic heating temperature at 900 ºC. The characteristics of overlapped sheets can be observed in Fig. 3c of LHMNCO after H+-Li+ ion RI PT exchange in an acidic HCl solution. The formation of spinel phase during this process at the surface of particle is identified in HRTEM image (Fig. 3d), which is consistent with XRD results in Fig. 2. The continuous TBA+-H+ ion exchange not only tailors LHMNCO TBA to the SC nanoflower-like shape, but also generates porous structure as shown in Fig. 3e. Furthermore, due to the violent exfoliation effect of TBA+ cations, HRTEM image in Fig. 3f shows disordered M AN U lattice fringes both at the surface and in the bulk of LHMNCO TBA; the other reason leading to such a disordered structure possibly results from the partial decomposition of organic TBA+ cations under the attack of high-energy electron beam during HRTEM observations. On the other hand, fringes with the smaller d-space (d=0.455 and 0.438 nm in Fig. 3f) at the surface of the TE D specimen may also be attributed to the partial decomposition of TBA+ substituents. As shown in Fig. 1c and 1f, monodispersive LHMNCO TBA nanoflowers convert to LHMNCO TBA HT particles with irregular shapes after post-annealing processes due to the fold of nanopetals. EP Accordingly, the removal of TBA+ cations also contributes to the porous structure of LHMNCO TBA HT as shown in TEM image (Fig. 3g). According to the XRD results as shown in Fig. 2, AC C HRTEM image in Fig. 3h shows the spinel crystal structure of LHMNCO TBA HT with a dspace of (111)S equal to 0.471 nm. In general, ex-situ ion-exchange and heat treatments result in the complete phase transition from the layered LMNCO to a Li4Mn5O12-type spinel material, along with intriguing morphological and structural evolutions. The nitrogen adsorption and desorption isotherms and pore size distributions of pristine Lirich layered LMNCO, ion-exchanged LHMNCO TBA and annealed LHMNCO TBA HT are 12 ACCEPTED MANUSCRIPT shown in Fig. 4a and 4b, respectively. The corresponding porous characteristics in terms of surface area, pore volume and relative pore size are summarized in Table 1 together with Li/Mn/Ni/Co ratios of three samples. It is clear that substitution of Li+ ions within LMNCO by RI PT H+ protons, followed by continuous replacement with TBA+ cations results in significantly increased surface area to 11.109 m2/g of LHMNCO TBA, almost four times higher than that of original LMNCO particles (2.327 m2/g), which can be attributed to the exfoliation effect from SC TBA+ cations as shown in SEM (Fig. 1c) and TEM (Fig. 3e) images [27,31]. Accordingly, LHMNCO TBA also has a larger pore volume of 8.880e-2 cm3/g in comparison with 1.133e-2 M AN U cm3/g from LMNCO nanoparticles. Both SEM and TEM observations indicate that the pore volume of LMNCO powder is from special gaps between numerous agglomerated LMNCO nanoparticles (Fig. 1a and Fig. 3a), while the higher pore volume of LHMNCO TBA mostly arises from the porous structure of individual LHMNCO TBA nanoflowers (Fig. 1c and Fig. 3e). TE D Therefore, agglomerated LMNCO nanoparticles give rise to a relatively higher pore size distribution of ~6 nm in Fig. 4b as compared with ~4 nm from LHMNCO TBA nanoflowers with monodisperse characteristic. As aforementioned in XRD characterizations in Fig. 2, the post- EP annealing treatment plays a crucial role in realizing a complete layered-to-spinel phase transition, and the morphological and structural changes have been observed in SEM (Fig. 1f) and TEM AC C (Fig. 3g) images, respectively. As a result, heating LHMNCO TBA in air at 500 °C contributes to further increased surface area to 13.725 m2/g of LHMNCO TBA HT and two pore size distributions of ~3 and ~12 nm in Fig. 4d. Its reduced pore volume is probably due to the folded nanopetals to form an internal porous structure (Fig. 3g) and the obvious aggregation (Fig. 1f) after post-heat treatment. The chemical compositions of these three samples are compared in Table 1 in the form of Li/Mn/Ni/Co molar ratios. In comparison with the theoretical ratio of 13 ACCEPTED MANUSCRIPT Li/Mn/Ni/Co=1.2/0.54/0.13/0.13, the as-prepared LMNCO shows slightly less quantity of the lithium component that is probably assigned to the lithium loss during heat treatment at high temperature at 900°C together with the long duration time for 12 h. Such the harsh heat treatment RI PT is required for the synthesis of Li-rich LMNCO material, in order to achieve its nice integrated structure and high crystallinity, but leads to unfavorable lithium loss as measured by ICP results. After two step ion exchanges via H+-Li+ and TBA+-H+, LHMNCO TBA preserves 52.7 % of the SC original lithium ions in LMNCO. As reported in the literature , layered transition metal oxides can be fully exchanged and teared into transition metal oxide nanosheets. In our case, the M AN U partial ion exchange possibly is attributed to the formation of spinel phase at the surface of layered derivatives as shown in XRD results (Fig. 2) and HRTEM image (Fig. 3d). The detectable Ni loss in LHMNCO TBA after the ion-exchange processes may result from the cationic Li+-Ni2+ disorder in LMNCO. Consequently, a few Ni2+ ions occupied in lithium sites in TE D the lithium layer are replaced with H+ and TBA+ cations. The post-annealing process has few effect on the chemical composition of LHMNCO TBA HT, resulting in Li/M (M=Mn+Ni+Co)=0.75. Such value is much close to the Li/Mn ratio of 0.8 in Li4Mn5O12 spinel EP rather than 0.5 of LiMn2O4 spinel, which is consistent with the XRD result being indexed to Li4Mn5O12-type spinel for LHMNCO TBA HT. In short summary, ex-situ ion-exchange AC C processes along with post-heat treatments offer a feasible approach not only to tailor morphology and structure of Li-rich layered transition metal oxides, but also to control the phase transformation between layered and spinel phases. We speculate that the nanoarchitectured LHMNCO TBA HT spinel material should be favorable to facilely accommodate the electrolyte, maximize electrochemical active sites and release reaction strain during lithiation and delithiation; hence, LHMNCO TBA HT cathode with a pure spinel phase and porous structure is 14 ACCEPTED MANUSCRIPT expected to show enhanced rate capability and cycleability as compared with the pristine Li-rich layered LMNCO as well as other two hybrid LHMNCO HT and LHMNCO TBA Li HT with an intergrowth of layered and spinel phases. RI PT The cyclic voltammetric (CV) measurements of LMNCO, LHMNCO HT, LHMNCO TBA HT and LHMNCO TBA Li HT cathode materials are carried out, in order to study electrochemical properties related to phase transitions from Li-rich layered (LMNCO) to either SC layered-spinel (LHMNCO HT and LHMNCO TBA Li) or Li4Mn5O12-type spinel (LHMNCO TBA HT) phase. Fig. 5 shows resulting CV records in the first three cycles of four different M AN U samples. LMNCO reveals the typical electrochemical characteristics of Li-rich layered cathode materials as shown in Fig. 5a. The first anodic peak at 4.17 V in the initial charge curve is associated with the oxidation of Ni2+ to Ni4+, followed by Co3+ to Co4+, whereas Mn still remains as tetravalent in LiMn1/3Ni1/3Co1/3O2 component . The second anodic peak at 4.66 V TE D corresponds to the electrochemical activation of inert Li2MnO3 component, i.e., the decomposition of Li2MnO3 to Li2O and lithium-active MnO2, along with the unavoidable decomposition of electrolyte and the formation of solid electrolyte interphase (SEI) at such a EP high potential >4.5 V . Although the electrochemical activation process leads to the low Coulombic efficiency in the first cycle, but significantly results in high capacity of Li-excess AC C layered cathode materials in the successive cycles. Correspondingly, the reduction of Co4+/Co3+ and Ni4+/Ni3+/Ni2+ redox occurs at 3.66 V in the initial discharge curve. In the second cycle, the anodic peak at 4.66 V disappears, while an additional cathodic peak at 3.26 V appears, which can be attributed to the reduction of Mn4+ to Mn3+ from the as-activated MnO2 component. The third cycle shows the similar profile to the second cycle in less polarization and higher current density, indicating improved electrochemical reversibility after the electrochemical activation of Li-rich 15 ACCEPTED MANUSCRIPT layered LMNCO. It is clear to see that CV curves of LHMNCO HT, LHMNCO TBA HT and LHMNCO TBA Li HT are very similar to each other, but apparently different from that of LMNCO. Those three materials all show the dominant redox pair around 3.0 V in CV curves, RI PT together with two minor redox couples located near 4.0 and 4.6 V. The CV performance is consistent with the typical electrochemical characteristics of reported Ni/Co-doped Li4Mn5O12type spinel in a wide voltage range . The CV responses of LHMNCO HT, LHMNCO TBA SC HT and LHMNCO TBA Li HT support XRD results in Fig. 2, which reveal the Li4Mn5O12-type spinel structure of newly-formed spinel phase within the original layered structure of LMNCO M AN U after ion-exchange and post-annealing processes. The appearance of an anodic peak at ~4.6 V in the initial CV charges of LHMNCO HT and LHMNCO TBA Li HT indicate the existence of preserved layered structure, in accordance with XRD characterizations (Fig. 2). In contract, LHMNCO TBA HT reveals much lower current density of such anodic peak in the first CV TE D charge. Furthermore, an more intensive anodic peak at 2.95 V is generated, which can be attributed to the better complete phase transition of LHMNCO TBA HT in comparison with LHMNCO HT and LHMNCO TBA HT. LHMNCO TBA HT shows identical CV curves of EP second and third cycles, indicating outstanding electrochemical reversibility of this spinel materials. The anodic peak at 3.07 V and cathodic peak at 2.63 V are attributed to corresponding AC C oxidation and reduction reactions of Mn3+/Mn4+ redox pair, which are associated with extracting and inserting lithium ions on 16c sites in the spinel structure . Two small redox couples at ~4.0 and ~4.6 V probably result from the Co3+/Co4+ and Ni2+/Ni3+/Ni4+redox, respectively. Fig. 6 exhibits charge and discharge curves of four cathodes in the first five cycles at 0.1 C in a voltage range of 2.0-4.8 V vs. Li+/Li, which are in well accordance with CV profiles in Fig. 5. It is noticeable that introduction of a spinel phase with Li-rich layered cathode materials can 16 ACCEPTED MANUSCRIPT significantly increase the specific capacity. As shown in Fig. 6a-6d, charge/discharge curves of the fourth and fifth cycle are almost identical for all cathodes, suggesting that the cathodes are mostly stable after five electrochemical cycles. The pristine Li-rich layered LMNCO delivers a RI PT specific discharge capacity of 211.3 mAh/g at the fifth cycle with a voltage plateau around 3.7 V, revealing typical electrochemical behavior of Li-rich layered cathode materials. The XRD result in Fig. 2a demonstrates the coexistence of layered and spinel phases of LHMNCO HT after ex- SC situ ion-exchange and post-heat treatments, in which the spinel phase is dominant; hence LHMNCO HT shows a higher discharge capacity of 285. 8 mAh/g but along with a predominant M AN U voltage plateau at 2.6 V as well as a minor one at 4.4 V. The former voltage stage is due to the active Mn3+/Mn4+ redox in the spinel component, and the latter probably results from the Co4+/Co3+ and Ni4+/Ni3+/Ni2+ redox reactions in the reversed layered component. Furthermore, employing TBA+ cations for the second ion exchange of LHMNCO have contributed to the TE D complete phase conversion of Li-rich layered LMNCO, resulting in the spinel LHMNCO TBA HT with the Li4Mn5O12-type spinel characteristics (Fig. 2) and a mesoporous structure (Fig. 3g and Fig. 4b). As a result, an unexpectedly high discharge capacity of 343.2 mAh/g is achieved in EP the fifth cycle of LHMNCO TBA HT. There are three voltage plateaus located at 4.6, 4.0 and 2.8 V, respectively, which is distinctly different from the profile of LHMNCO HT, again AC C revealing different structural characteristics between LHMNCO TBA HT and LHMNCO HT. Those three voltage stages can be attributed to reductions from Ni4+/Ni3+/Ni2+, Co4+/Co3+ and Mn4+/Mn3+ redox pairs in the spinel structure and are favorable to preserve high-voltage performances of LHMNCO TBA HT. Fig. 3g reveals a porous mesoporous structure of nanoarchitectured spinel cathode, which would be favorable to accommodate electrolyte and effectively release reaction strains during lithiation/delithiation. Such a structure may enable to 17 ACCEPTED MANUSCRIPT absorb lithium ions within the porous structure, resulting in the additional capacity contribution [35, 36]. On the other hand, the porous structure may partially collapse when adsorbed lithium ions are extracted during the charge processes, leading to reduced lithium storage capacity in RI PT corresponding discharge processes. Accordingly, the LHMNCO TBA HT cathode (Fig. 6c) shows relatively low but gradually increased Coulombic efficiencies in initial cycles as compared with the other three cathode materials. In contrast, LHMNCO TBA Li HT shows SC similar electrochemical performance to LHMNCO HT with the intergrowth of layered and spinel structures, delivering a reduced capacity of 259.9mAh/g. This might result from the partially M AN U recovered layered structure via Li+-TBA+ ion exchange of LHMNCO TBA in LiOH aqueous solution. Fig. 7a and 7b show cycling and high-rate performances of LHMNCO HT, LHMNCO TBA HT and LHMNCO TBA Li HT cathodes in comparison with the pristine Li-rich layered TE D LMNCO, respectively. The effects from the introduced spinel phase within the layered LMNCO cathode material on improving specific capacity, cycling stability and rate capability are more phenomenal when cycled at higher current densities. As shown in Fig. 7a, LHMNCO TBA HT EP can retain a very high discharge capacity of 197.5 mAh/g with a corresponding capacity retention of 89.1% after 100 electrochemical cycles at 1C, much better than 58.1 mAh/g and 65.9% AC C of pristine layered LMNCO, 116.1 mAh/g and 85.1% of LHMNCO HT, and 77.9 mAh/g and 80.0% of LHMNCO TBA Li HT. Moreover, LHMNCO TBA HT delivers initial capacities of 313.6, 267.2, 203.9, 180.7, 126.3, and 89.4 mAh/g at 0.1, 0.5, 1, 2, 5, and 10 C, respectively, as exhibited in Fig. 7b. Such the remarkable cyclability and high-rate capability of LHMNCO TBA HT can be attributed to reconstructed spinel phase and hierarchical mesoporous structure for facile accommodation and diffusion of lithium ions, and effectively releasing reaction strains in 18 ACCEPTED MANUSCRIPT the “buffer” structure in high porous characteristics. Overall, generation of a spinel phase within Li-rich layered cathode materials can considerably increase the specific capacity and rate capability, but has to sacrifice the working voltage. Doping transition metal cations, such as Ni2+, RI PT Co3+ and Fe3+ ions, can contribute to the high-voltage performance. Ion-exchange method offers a desirable way to obtain enhanced electrochemical performance of Li-rich layered cathode materials. SC 4. Conclusions This work sheds light on fundamental understanding of layered-to-spinel phase transition and M AN U relevant electrochemical performances of Li-rich layered cathode materials via ex-situ ionexchange processes, followed by post-annealing treatments. Employing TBA+ cations for the second ion exchange of pronated Li-rich layered oxides is critical to realize a complete phase transition, resulting in a Li4Mn5O12-type spinel-structured material converted from Li-rich TE D layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2. Ion-exchange procedure also tailor the morphology and structure of solid Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles into nanostructured spinel material with high surface area and mesoporous porosity. In comparison with the pristine Li-rich layered EP cathode material, the final converted spinel cathode material with hierarchical porous structure reveals significantly increased specific capacity, better cycling stability and rate capability. This AC C work opens up a route to maximize electrochemical performance of Li-excess layered cathode materials for high-power and high-energy lithium ion batteries. Acknowledgements This work was supported by US National Science Foundation, the Division of Chemical, Bioengineering, Environmental and Transport Systems (NSF CBET) [grant number 1438493]; the US Small Business Technology Transfer (STTR) [grant number 1346496]; the Research 19 ACCEPTED MANUSCRIPT Enhancement Award (REA) program, Louisiana Space Consortium (LaSPACE) funded via the NASA Space Grant College & Fellowship Program Grant 2011-15 Cycle [grant number NASA/LEQSF(2010-2015)-LaSPACE]; the National Natural Science Foundation of China RI PT [grant number U1401248]; the Natural Science Foundation of Jiangsu Province, China [grant number BK20151227]; the General Financial Grant from the China Postdoctoral Science Foundation [grant number 2016M601876]. 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Bruce, Low temperature lithium manganese cobalt oxide spinels, Li4-xMn5-2xCo3xO12 (0<x<1), for use as cathode materials in rechargeable lithium batteries, J. Power Sources 97-98 (2001) 332-335. EP  Q. Zhang, E. Uchaker, S.L. Candelaria, G. Cao, Nanomaterials for energy conversion and storage, Chem. Soc. Rev. 42 (2013) 3127-3171. AC C  A. Vu, Y. Qian, A. Stein, Porous electrode materials for lithium-ion batteries-how to prepare them and what makes them special, Adv. Energy Mater. 2 (2012) 1056-1085. 24 ACCEPTED MANUSCRIPT Figure Captions Fig. 1 SEM images of (a) LMNCO, (b) LHMNCO, (c) LHMNCO TBA, (d) LHMNCO TBA Li, (e) LHMNCO HT, (f) LHMNCO TBA HT and (g) LHMNCO TBA Li HT. RI PT Fig. 2 XRD patterns of pristine Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles and corresponding derivatives in (a) full 2θ range and (b) enlarged 2θ portions between 16-22º, 3440º and 62-68º, respectively. LHMNCO TBA, and (g and h) LHMNCO TBA HT. SC Fig. 3 TEM and HRTEM images of (a and b) LMNCO, (c and d) LHMNCO, (e and f) M AN U Fig. 4 (a) Nitrogen adsorption/desorption isotherms and (b) corresponding pore size distributions of LMNCO and LHMNCO TBA and LHMNCO TBA HT. Fig. 5 Cyclic voltammetric (CV) curves of (a) LMNCO, (b) LHMNCO HT, (c) LHMNCO TBA HT and (d) LHMNCO TBA Li HT in the first three cycles at a scanning rate of 0.1 mV/s in a TE D voltage range of 2.0-4.8 V vs. Li+/Li. Fig. 6 Charge and discharge curves of (a) LMNCO, (b) LHMNCO HT, (c) LHMNCO TBA HT and (d) LHMNCO TBA Li HT in the first five cycles at a current density of 0.1 C in a voltage EP range of 2.0-4.8 V vs. Li+/Li. Fig. 7 (a) Cycling performances at 1 C and (d) high-rate performances at different current AC C densities of LHMNCO HT, LHMNCO TBA HT, LHMNCO TBA Li HT in comparison with the pristine layered LMNCO in a voltage range of 2.0-4.8 V vs. Li+/Li. 25 TE D M AN U SC RI PT ACCEPTED MANUSCRIPT Fig. 1 SEM images of (a) LMNCO, (b) LHMNCO, (c) LHMNCO TBA, (d) LHMNCO TBA Li, AC C EP (e) LHMNCO HT, (f) LHMNCO TBA HT and (g) LHMNCO TBA Li HT. 26 SC RI PT ACCEPTED MANUSCRIPT M AN U Fig. 2 XRD patterns of pristine Li-rich layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2 nanoparticles and corresponding derivatives in (a) full 2θ range and (b) enlarged 2θ portions between 16-22º, 34- AC C EP TE D 40º and 62-68º, respectively. 27 AC C EP TE D M AN U SC RI PT ACCEPTED MANUSCRIPT Figure 3. TEM and HRTEM images of (a and b) LMNCO, (c and d) LHMNCO, (e and f) LHMNCO TBA, and (g and h) LHMNCO TBA HT. 28 SC RI PT ACCEPTED MANUSCRIPT Fig. 4 (a) Nitrogen adsorption/desorption isotherms and (b) corresponding pore size distributions AC C EP TE D M AN U of LMNCO and LHMNCO TBA and LHMNCO TBA HT. 29 M AN U SC RI PT ACCEPTED MANUSCRIPT TE D Fig. 5 Cyclic voltammetric (CV) curves of (a) LMNCO, (b) LHMNCO HT, (c) LHMNCO TBA HT and (d) LHMNCO TBA Li HT in the first three cycles at a scanning rate of 0.1 mV/s in a AC C EP voltage range of 2.0-4.8 V vs. Li+/Li. 30 M AN U SC RI PT ACCEPTED MANUSCRIPT TE D Fig. 6 Charge and discharge curves of (a) LMNCO, (b) LHMNCO HT, (c) LHMNCO TBA HT and (d) LHMNCO TBA Li HT in the first five cycles at a current density of 0.1 C in a voltage AC C EP range of 2.0-4.8 V vs. Li+/Li. 31 RI PT ACCEPTED MANUSCRIPT SC . Fig. 7 (a) Cycling performances at 1 C and (d) high-rate performances at different current M AN U densities of LHMNCO HT, LHMNCO TBA HT, LHMNCO TBA Li HT in comparison with the AC C EP TE D pristine layered LMNCO in a voltage range of 2.0-4.8 V vs. Li+/Li. 32 ACCEPTED MANUSCRIPT Table 1. Porous characteristics and elemental composition of LMNCO, LHMNCO TBA and LHMNCO TBA HT. ICP elemental compositions Samples Pore volume (cm3/g) Pore size (nm) Li 2.327 1.133e-02 ~6 1.111 11.109 8.880e-02 ~4 13.725 5.921e-02 ~3 & ~12 Mn Ni Co 0.540 0.129 0.128 SC Surface area (m2/g) 0.586 0.540 0.119 0.126 0.585 0.540 0.118 0.126 M AN U AC C EP TE D Pristine LMNCO Ion-exchanged LHMNCO TBA Annealed LHMNCO TBA HT RI PT Porous characteristics 33 ACCEPTED MANUSCRIPT Highlights Ion exchanges of H+-Li+ and TBA+-H+ are performed on Li-rich layered oxides. The converted spinel phase has a Li4Mn5O12-type spinel structure. RI PT Layered-to-spinel phase transition of Li-rich layered oxides has been realized. Electrochemical performances of Li-rich layered cathodes are manipulated. AC C EP TE D M AN U SC The resulting spinel cathode delivers a capacity higher than 300 mAh/g at 0.1 C.