The Effect of Boron Doping on Structure and Electrochemical

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The Effect of Boron Doping on Structure and Electrochemical Performance of Lithium-rich Layered Oxide Materials Jiatu Liu, Shuangbao Wang, Zhengping Ding, Ruiqi Zhou, Qingbing Xia, Jinfang Zhang, Libao Chen, Weifeng Wei, and Peng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b03056 • Publication Date (Web): 23 Jun 2016 Downloaded from http://pubs.acs.org on June 24, 2016

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ACS Applied Materials & Interfaces

The Effect of Boron Doping on Structure and Electrochemical Performance of Lithium-rich Layered Oxide Materials Jiatu Liu,†,¶ Shuangbao Wang,‡,¶ Zhengping Ding,† Ruiqi Zhou,† Qingbing Xia,† Jinfang Zhang,† Libao Chen,† Weifeng Wei,∗,† and Peng Wang∗,‡ †State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P. R. China ‡National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences and Collaborative Innovation Center of Advanced Microctructures, Nanjing University, Nanjing 210023, P. R. China ¶These authors contributed equally E-mail: [email protected]; [email protected]

Abstract Polyanion doping shows great potentials to improve electrochemical performance of Li-rich layered oxide (LLO) materials. Here, by optimizing doping content and annealing temperature, boron doped LLO materials Li1.2 M n0.54 N i0.13 Co0.13 Bx O2 (x=0.04 and 0.06) with comprehensively improved performance (94% capacity retention after 100 cycles at 60mA/g current density and much elevated rate capability compared to pristine sample) were obtained at annealing temperatures of 750◦ C and 650◦ C separately, which are much lower than the traditional annealing temperature of similar material system without boron. A scenario of complex crystallization process was

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captured using Cs-corrected HAADF-STEM imaging technique. The existence of layered, NiO-type and spinel-like structures in a single particle induced by boron doping and optimization of annealing temperature is believed to contribute to the remarkable improvement of cycling stability and rate capability.

Keywords: lithium-rich layered oxide; cathode materials; boron doping; annealing temperature; HAADF-STEM; complex crystallization

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Introduction

Lithium-ion batteries (LIBs) are considered to be a promising energy-storage technology and are widely used in portable devices, electric vehicles and hybrid electric vehicles. The design of a cathode composed of environmentally benign, low-cost materials with high energy density has always been challenging. 1 Research interests and industrial efforts in past decades have been focusing on lithium transition metal oxides with a layered crystal structure denoted as LiM O2 (M=Mn, Co, Ni), showing specific capacity of around 160 mAh/g. 2 Lithium rich layered oxide materials, or LLO for short (Li1+x M1−x O2 , M=Mn, Ni, Co) have become a research focus owing to its high specific capacity (over 250mAh/g) since their possibility of outperforming traditional layered materials was illustrated. 3 Nevertheless, three fundamental problems need to be addressed before application of LLO materials to practical battery technology, including voltage instability 4 and capacity fading 5 during long-term cycling, and low capacity when charge/discharge at high rates. 6 Surface coating 7–12 and elemental substitution/doping are the two main ways to improve the performance of LLO materials. There are three main categories of doping according to the matching principle of the ionic radius and the interstitial size of cubic-close-packed (ccp) oxygen sub-lattice in the structure. Large ions would be introduced into octahedral interstitials occupied by lithium ions, expanding Li slabs of layered material crystal structure and improving lithium diffusion kinetics in consequence, as the cases with Na 13 and Mg. 14

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Medium sized ions, or metal atoms in coordination with oxygen, would replace transition metal atoms in octahedral interstitials, enhancing thermal and structural stability, in some case decreasing electronic and/or ionic resistance, as the cases with Al, 15 Ti, 16 Cr, 17,18 Fe, 19 Zn, 20 Mo, 21 Ru. 22 Even smaller ions, or non-metal atoms are supposed to position at tetrahedral interstitials of oxygen sub-lattice, changing electronic structure of materials profoundly and improving electrochemical stability consequently, as the cases with boron, 23 silicon and phosphorus. 24,25 The latter case are also called polyanion doping strategy because the doping unit could also be regarded as BO4 ,SiO4 and PO4 in terms of current understanding. Boron doping is equal to boracic doping in this study. Alc´ antara et al firstly showed improved reversibility of lithium (de)intercalation in LiCo0.95 B0.05 O2 and proposed a tetrahedral coordination for boron. 26 Li et al 23 further proved that incorporation of boracic polyanions into Lithium-rich layered cathode materials with formula Li1.2 M n0.54 N i0.13 Co0.13 B0.02 O2 leads to improved cycling stability, high thermal stability and enhanced redox potentials, all of which are attributed to enhanced oxygen stability brought by introduction of boron into vacancies of oxygen lattice. In both works, a phenomenon of growing particle size with increasing boron content at a fixed annealing temperature is observed. Inspired by this phenomenon, we research comprehensively on the effect of annealing temperature and boron doping content on LLO materials. Composition Li1.2 M n0.54 N i0.13 Co0.13 Bx O2 is chosen as the target material to be modified. Simple gel combustion method is adopted to obtain the precursors. By changing annealing temperature, we found that for a certain boron content there is an optimized annealing temperature, higher or lower of which would depreciate electrochemical performance. The reason for improvement in electrochemical performance is evidenced in XRD and SEM examinations. One step further, with the aid of Cs-corrected HADDF-STEM technology, we managed to propose some new insights into the electrochemical improvement resulted from boron doping.

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2 2.1

Experimental Section Sample synthesis

Li1.2 M n0.54 N i0.13 Co0.13 Bx O2 (x=0, 0.02, 0.04 and 0.06) samples were synthesized by a simple combustion method as reported in Ref. 27 Stoichiometric amounts of Li(CH3 COO) · 2H2 O, LiN O3 , N i(N O3 )2 ·6H2 O, Co(CH3 COO)2 ·4H2 O, Co(N O3 )2 ·6H2 O, M n(CH3 COO)2 ·4H2 O and H3 BO3 (All chemicals were purchased from Aladdin Industrial Co.) were dissolved in a minimum amount of distilled water and continuously stirred at 90◦ C in a beaker. An extra 8% of lithium salt was added to compensate for evaporation in annealing process. Flowing inert gas was employed to protect M n2+ from being oxidized and precipitation (causing inhomogeneity). The molar ratio of acetate to nitrate was adjusted to 3:1 to keep the combustion condition constant. As the water evaporated, the mixed solution turned into a viscous gel. This gel was then placed onto a corundum plate and was heated to 400◦ C before a nichrome wire was galvanized to ignite the already brownish gel. Upon ignition, selfpropagated combustion reaction took place, forming a porous and loose powder aggregate. A probing Differential Scanning Calorimetry test was conducted to give a hint on annealing temperature range. The decomposed powders were grounded, then heated at 500◦ C for 4h, followed by at 600◦ C∼950◦ C for 15 h in air and furnace cooled. The heating rate of 5◦ C/min was applied for all the temperature settings.

2.2

Sample characterization

Elemental content analysis was carried out on an IRIS plasma spectrometer (IRIS Advantage1000). Differential scanning calorimetry and thermal gravimetry (DSC-TG) analysis were taken on Netsch STA 449C Jupiter at a heating rate of 10◦ C/min from room temperature to 900◦ C. X-ray diffraction (XRD) were taken on a Rigaku D/Max-2500 Diffractometer (Cu Kα radiation) in the range of 10 ∼ 80◦ at a rate of 8◦ /min. Peak fitting was done by MDI Jade 6.0. Rietveld refinements are performed on MAUD 2.0 28 using XRD spectrum 4


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scanned at a rate of 0.02◦ /s. The morphology was evaluated using a Nova Nano SEM230 field emission scanning electron microscope (SEM). An ESCALAB 250Xi X-ray photoelectron spectrometer is employed to determine elemental depth profiles of metals and boron. A FEI Titan G2 cubed 60-300 double Cs corrected TEM, operating at 300kV, was employed in the present study. In order to interpret in terms of structures easily, HAADF-STEM imaging mode that truly reflect the real atomic structure are performed. The imaging parameters are as follows: operating power 300 keV, semi-angle of 22.4 mrad for probe convergence and a collection inner semi-angle of 36 mrad.

2.3

Electrochemical measurements

The annealed powder were mixed with acetylene black and polyvinylideneuoride in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone to form a slurry. Subsequently, a layer of 100µm slurry was coated onto Al foil and then dried at 110◦ C for 12 h in a vacuum oven to obtain as-prepared cathode, which is then punched into φ12mm wafers. The electrochemical testing was conducted with CR2016 coin-type half-cells assembled in an Ar-filled glove box. The half-cells consisted of a piece of cathode wafer, a Li metal anode, a Celgard 2500 separator and 1 M LiP F 6 in EC-DMC (weight ratio 1:1) electrolyte solution. The cells were cycled on a battery testing system (LANHE CT2001A, Wuhan LAND electronics Co., P. R. China) between 2.0 and 4.8 V at 30mA/g for the 1st cycle to activate Li2 MnO3 part of the cathode material, then at 60mA/g for cycling test and at 0.2C∼10C(1C=250mA/g) for rate capability test.

3 3.1

Results and Discussion Thermal analysis and XRD characterization

Precursors obtained through gel combustion process have almost the same phases(poorly crystallized lithium transition metal oxide and Li2 CO3 ) and chemical composition except for 5



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boron content(shown in Figure S1). To estimate annealing temperature, precursors of x=0, x=0.06 and x=0.12 were subjected to DSC-TG test. Figure 1 reveals that incorporation of borate significantly change phase formation process during heat treatment. One exothermic peak (1), corresponding to manganese oxidation, and two endothermic peaks (2) and (3), related to metal acetates decomposition and sinter reaction to form Li-rich layered phase, all shift to lower temperatures. It is clear that aforementioned three peaks are intensified as boron content increases. Precursor with composition x=0.12 was used to amplify the effect of boron doping. However, as the boron content reaches x=0.06 or higher, an impurity phase relevant to lithium borate is found after annealing (Figure S2). This is why we confined doping content within x=0.06. Precursors of pristine and boron doped materials were then annealed at 600◦ C∼950◦ C for 15 h in air and furnace cooled to obtain final products. Combustion method has led to uniform transition metal distribution within a particle, as revealed in EDS elemental mapping of typical materials in Figure S8. Although 900◦ C and 15h of annealing is believed to be optimized for pristine material according to Ref., 29 pristine materials were annealed at 850-950◦ C to evaluate the effect of annealing temperature. Two phenomena were instantly observed by ICP and SEM: (1)loss of lithium and boron content during annealing and (2)larger particle size when annealed at higher temperature, as shown in Figure 2. The structural information of materials with different boron contents and annealed at various temperature is displayed in Figure 3. The diffraction patterns of all materials studied could be primarily indexed to a R ¯3m layered structure with superlattice diffraction peaks at 2θ = 20 ∼ 30◦ characteristic of a C 2/m structure. The enlargement of diffraction patterns (insets of Figure 3 (b)-(d)) indicates that there is an upper annealing temperature limit for each doping content, i.e. 850◦ C for x=0.02, 800◦ C for x=0.04 and 750◦ C for x=0.06. Above these upper limits, extra diffraction peaks, indexed to an Fd¯3m spinel phase could be detected. A typical peak, indexed to (311) plane, is pointed out by black arrows in enlargement of 2θ = 35 ∼ 40◦ for sample 0.02@900, 0.04@850 and 0.06@800 in Figure 3

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oclinic phase characterized by parameter ”crystal size” is also refined and listed in column Refined Mono. size. The same trends are observed for pristine materials annealed at different temperatures. Apparently boron doping lowers the temperature needed for crystallization of LLO phase. Table 1: Crystallographic information based on XRD peak profiles and Rietveld refinements of the pristine and boron-doped samples materials x=0 @ 950◦ C x=0 @ 900◦ C x=0 @ 850◦ C x=0.02 @ 850◦ C x=0.02 @ 800◦ C x=0.02 @ 750◦ C x=0.04 @ 800◦ C x=0.04 @ 750◦ C x=0.04 @ 700◦ C x=0.06 @ 750◦ C x=0.06 @ 700◦ C x=0.06 @ 650◦ C x=0.06 @ 600◦ C

3.2

I(003)R I(104)R 1.37 1.36 1.18 1.30 1.02 0.93 1.62 1.45 1.32 1.62 1.31 1.25 1.15

FWHM of (003)R 0.178 0.233 0.257 0.255 0.334 0.417 0.167 0.189 0.194 0.174 0.197 0.243 0.329

FWHM of (020)C 0.197 1.311 1.428 0.699 1.255 1.359 0.188 0.263 0.357 0.197 0.345 1.175 1.276

Refined ionmixing (%) 1.27 2.98 16.35 6.00 13.91 26.75 1.37 5.62 7.71 2.10 5.21 10.53 29.98

Refined Mono. size (nm) 95 73 69 94 78 62 340 121 118 167 126 54 31

Electrochemical Performance

Figure 4(a)-(d) and Table S1 compare cycling performance of LLO materials with and without boron doping. For pristine materials, 0@900 shows the best cycling performance. Higher annealing temperature leads to faster deterioration of specific capacity due to better crystallization of Li2 MnO3 . While lower annealing temperature induces lower specific capacity because of worse crystallization of layered phase. The structural changes can be clearly told by Figure 3(a) and Table 1. Material 0@900 is thus chosen to act as a comparison for other samples. Figure 4(b)(c)(d) show that almost all boron doped materials show better electrochemical performance than pristine material 0@900. Also it is apparent that, for a certain composition, there is an optimal annealing temperature at which the highest spe10


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could be found in differential capacity profiles of 2nd and 102nd cycle in Figure 4(e)(f), which shows: 1) increased voltage position of main cathodic peak, related to electrochemical reaction M n4+ /M n3+ in layered structure 33 and in spinel-like structure developed upon cycling; 34 2) increased retention of the main cathodic peak area; 3)improved electrochemical reaction reversibility, marked by shrinkage of voltage difference between the main redox peak pair (marked as O and R). The result of improved electrochemical performance of material 0.02@850 coincides with the fact that boron doping could enhance electrochemical performance (revealed by Li et al 23 ). Combining the result of XRD analysis in Table 1 and SEM images in Figure 2, it is understandable that less interlayer ion mixing and smaller particle size give rise to improved performance for sample 0.04@750 (I(003)R /I(104)R =1.45) compared to pristine material(I(003)R /I(104)R =1.36). However, why does sample 0.06@650, with more interlayer ion mixing(I(003)R /I(104)R =1.25), outperforms pristine material markedly? And why is there an optimized annealing temperature for a certain composition?

3.3

Nanostructure Characterization

To get insight into the origin of improved performance for boron doped materials annealed at low temperatures, three samples, 0.06@700, 0.06@650 and 0.06@600 were observed by TEM and atomic resolution STEM. Figure 5(left) shows selected area electron diffraction(SAED) of a typical particle of material 0.06@700, which could be primarily indexed as spinel-like Fd¯3m (PDF#88-0459) in [0¯11] zone axis. The intensities of diffraction points, however, vary quite anomalously. Two sets of layered C2/m phases in [010] (PDF#84-1634) zone axis with different directions can also be indexed in the diffraction pattern. A closer observation using HAADF-STEM reveals that the border of the two parts of layered phase consists of spinel-like atomic configuration, as evidenced in FFT of (a), (b) and (c) in Figure 5(right). The orientation relationship can be written as: 12


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3.4

Discussion

For LLO materials, well crystallized layered structure with very small amount of ion mixing between TM layer and lithium layer can be acquired at quite high annealing temperatures (850 ∼ 1000 ◦ C ). The crystallization of Li2 MnO3 -typed structure with ordered Li-TM sites in TM layer requires higher temperature and longer annealing process especially. 35 While at lower temperature, NiO-typed structure with almost indistinguishable TM layer and lithium layer and tetragonal spinel with Li2 Mn2 O4 structure are more favored. 35 At even higher temperature (> 1000◦ C), with volatilization of lithium, cubic spinel with LiMn2 O4 -typed structure emerges. 30 Boron doping facilitates the formation of layered structure by lowering the crystallization temperature of layered structure, as can be also seen in DSC and XRD analysis. At annealing temperature as low as 650◦ C, layered structure with quite amount of ion-mixing shows the best electrochemical performance. Apart from structural stabilization brought by doping effect 23 and fast mass transportation as a result of small particle size, we believe that this unique nanostructure with moderate ion mixing contribute to the outstanding performance. The role of ion-mixing structure, i.e. NiO-type structure and spinel-like structure, has been controversial. Contrary to the traditional idea of thinking NiO-type and spinel-like structure are results of deterioration of active cathode materials and are inactive electrochemically, research in recent years have shown their special roles in stabilizing layered structure and facilitating mass transportation experimentally 36–38 and theoretically. 39 650◦ C and 15h of annealing gives the optimal combination of layered structure at outer part and NiO-type structure at the core of a particle. And a particle is mainly composed of subgrains with layered structure. Actually, existence of subgrains lower the particle size one step further, resulted in even shorter mass transport distance and offering much more possibilities to accommodate strain induced by electrochemical reaction. 40 The existence of spinel-like structure at the intersection parts of differently directional subgrains offers fast Li ion transport channel without putting these subgrains in contact with electrolyte, which 18


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can induce more side reaction otherwise. Lower temperature, e.g. 0.06@600, is not enough to form well crystallized layered structure. Electrochemically active at first though, low temperature synthesized metastable structure degrades into inactive structure during repeated cycling. Also, too small particle size could counteract the stabilization effect brought by boron doping because of larger contact area with electrolyte. Higher temperature, e.g. 0.06@700, brings about larger particle size and well developed layered structure without subgrains. Larger particle formed at increased annealing temperature would lead to lower specific capacity accordingly due to the fact that electrochemical reaction is hard to take place at core part of large particles. This is why we still can acquire high specific capacity for materials with quite amount of NiO-type structure at core (the core part offer very limited capacity).

4

Conclusion

Through adjusting doping content and annealing temperature, a variety of boron-doped Lirich layered oxide (LLO) materials were successfully prepared via a combustion method. The corresponding electrochemical performance of these materials reveals that the optimal annealing temperature decreases with increase of boron content. The optimized temperature is found to be 850◦ C, 750◦ C and as low as 650◦ C for materials x=0.02, 0.04 and 0.06 in formula Li1.2 M n0.54 N i0.13 Co0.13 Bx O2 . With the assistance of HAADF-STEM technique, detailed atomic configuration of the LLO materials with various local structures is observed and a complex crystallization theory is proposed to explain the effect of boron doping and process-performance relationship. The idea of crystallizing LLO materials with enhanced electrochemical performance at low temperatures and formation of a complex nanostructure within a single particle would be of importance in both engineering and scientific points of view.

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Supporting Information Available XRD and ICP of precursors (Figure S1); XRD of material 0.06@700 showing extra phase related to lithium borate (Figure S2); Rietveld refinement graphical results (Figure S3-S6); rate capabilities of top samples (Figure S7); XPS depth profiles and transition metal EDS elemental mappings of materials x=0.06 (Figure S8); detailed numerical analysis of electrochemical performance (Table S1); algebraic proof of the statement mentioned in section nanostructure characterization. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement This work was supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (51304248), the Program for New Century Excellent Talents in University (NCET-11-0525), the Doctoral Fund of Ministry of Education of China (20130162110002), the Program for Shenghua Overseas Talents from Central South University and the State Key Laboratory of Powder Metallurgy at Central South University.

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The Effect of Boron Doping on Structure and Electrochemical

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