In-situ polymerized lithium polyacrylate (PAALi) as dual-functional lithium source for high-performance layered oxide cathodes

Accepted Manuscript Title: In-situ polymerized lithium polyacrylate (PAALi) as dual-functional lithium source for high-performance layered oxide cathodes Authors: Yuxiu Liu, Kun Qian, Jianfu He, Xiaodong Chu, Yan-Bing He, Mengyao Wu, Baohua Li, Feiyu Kang PII: DOI: Reference:

S0013-4686(17)31611-0 http://dx.doi.org/doi:10.1016/j.electacta.2017.07.170 EA 29987

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

13-3-2017 27-7-2017 27-7-2017

Please cite this article as: Yuxiu Liu, Kun Qian, Jianfu He, Xiaodong Chu, YanBing He, Mengyao Wu, Baohua Li, Feiyu Kang, In-situ polymerized lithium polyacrylate (PAALi) as dual-functional lithium source for high-performance layered oxide cathodes, Electrochimica Actahttp://dx.doi.org/10.1016/j.electacta.2017.07.170 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. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In-situ polymerized lithium polyacrylate (PAALi) as dual-functional lithium source for high-performance layered oxide cathodes

Yuxiu Liua,c,‡, Kun Qiana,b,c,‡, Jianfu Hed, Xiaodong Chua, Yan-Bing Hea,c, Mengyao Wud, Baohua Lia,c,*, Feiyu Kanga,b,c

a

Engineering Laboratory for Next Generation Power and Energy Storage Batteries, Graduate School at

Shenzhen, Tsinghua University, Shenzhen 518055, China b

Nano Energy Materials Laboratory (NEM), Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua

University, Shenzhen 518055, China c

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

d

Contemporary Amperex Technology Co., Limited, Ningde, Fujian 352100, China

‡ These authors contributed equally to this work.

* Corresponding Author: Tel.: +86 755 26036419; fax: +86755 26036419. E-mail address: [email protected] (B. Li).

Highlights



Lithium polyacrylate (PAALi) was using as lithium source to fabricate LiNi1/3Co1/3Mn1/3O2, and it shows much better cycle stability and rate capability than the sample prepared by the traditional method.



The in-situ polymerized PAALi will deliver lithium ion more efficiently, resulting in lower degree of cation mixing.



The PAALi can significantly limit the growth of the particle size under heat treatment.

Abstract 1 / 24

In this paper, a novel route using lithium polyacrylate (PAALi) as dual-functional lithium source was developed to produce layered oxide cathodes. The PAALi was insitu polymerized on the surface of Ni1/3Co1/3Mn1/3(OH)2 precursor. By means of spray drying, spherical powders composed of PAALi wrapped around the precursor were prepared. The organic compound PAALi can function as a particle size control agent as well as the highly dispersed lithium source in the proposed route. It can transform to carbon coating layers during pre-heat treatment at 450 °C under inert atmosphere. Those carbon coating layers not only significantly reduce agglomeration of particles and limit growth of the particle size, but also provide enough inner voids and paths after the subsequent sintering at 900 °C. The produced layered cathodes can deliver capacity retention of 84.4% after 200 cycles at 1 C and a capacity of 110.5 mA h g-1 at 10 C. In comparison, the sample prepared with traditional lithium source only delivers capacity retention of 63.0% after 200 cycles at 1 C and 70.9 mA h g-1 at 10 C. The proposed route using PAALi as lithium source is potentially generalized to a broad application in the preparation of high-performance electrode materials. Key words: Lithium polyacrylate; Layered oxide cathode; Lithium source; Spray drying; Lithium-ion battery 1. Introduction With the ever-increasing demand for high power and energy density lithium-ion batteries (LIBs), the development of prominent performance and low cost cathode materials has attracted tremendous attention[1-4]. Among various candidate cathode materials, LiNi1/3Co1/3Mn1/3O2 (NCM) shows very promising prospect owning to its high specific capacity (∼160 mA h g-1), good structural stability and thermal stability[57]

̅m space . NCM crystal possesses a hexagonal layered α-NaFeO2 structure with the R𝟑

group like LiNiO2, where the Ni sites were partially substituted by Co and Mn[8,9]. The main origin of capacity is the valence change of Ni between +2 and +4. With a certain content of Co ions, the conductivity of NCM can be improved which obviously increase the power density of the full cells and the addition of Co also provides some capacity when charging to a high voltage. Mn ions always maintain a stable valence of +4 during the whole charge and discharge process, which could greatly stabilize crystal structure 2 / 24

and remedy the lattice distortion caused by the chemical valence change of Ni [8,10-12]. In addition, the cost of Mn and Ni is relatively cheaper than Co, showing a competitive price in the market of cathodes. Hence, the NCM cathodes exhibit integrated higher capacity, better cycle stability and rate performance with lower toxicity and cost. For the synthesis of NCM, the lithium source, the transition metal source and the heat treatment are basically required. In 2001, Ohzuku[5] group firstly synthesized LiNi1/3Co1/3Mn1/3O2 by simple sintering method, mixing LiOH·H2O, CoCO3 and Ni1/2Mn1/2(OH)2, and then heating at 1000 °C for 15 h in air. After that, various synthesis techniques, such as co-precipitation method[6,13,14], sol-gel method[15,16], hydrothermal method[17,18], molten salt method[19,20], combustion method[21,22], inverse microemulsion[23], etc., have been developed to control the morphology, particle size and improve the property. Sinha et al.

[23]

in situ synthesized carbon-coated

submicrometer-sized NCM particles in an inverse microemulsion medium in the presence of glucose. The carbon coating imparts porosity as well as higher surface area, which can suppress the capacity fade and resistance increase. Kim[15] group used modified sol-gel process followed by an optimized stepwise heating and cooling crystallization process to get uniform composition and a porous NCM. The porous structure with a large specific surface area facilitated the electrochemical activity resulting in high capacity, high rate capability and excellent cycle stability. Fu et al. [6] synthesized NCM nanobricks with high degree of exposure of {010} active facets by co-precipitation method with the assistant of PVP and further calcination with LiOH·H2O, and it shows high rate capability because of more exposed active facets. Peng et al.[17] synthesized self-assembled nano-sheets NCM cathodes via a facile hydrothermal method. Different surface structures with various packing degrees were achieved by adjusting the concentration of raw materials. The sample with more exposed active facets and a loose packing possessed superior rate capability than that of dense packing one. Compared with all the methods mentioned above, the mixing of atoms and heat treatment are of great importance. The products obtained from the liquid phase like coprecipitation[6], hydrothermal process[17] or inverse microemulsion method[23], could 3 / 24

achieve better performance, since the Ni, Co, and Mn atoms were well mixed in liquid and then homogeneous distribution in precursors. This will short the diffusion paths of transition metal and lithium sources during the further heat treatment. Theoretically, the fully mix of lithium sources and transition elements could benefit more for the quality of NCM. However the lithium source generally used in the above methods, most are inorganic lithium salts, such as LiOH·H2O and Li2CO3. During the sintering, the anionic ion like -OH and -CO3 need to decompose and volatilize thoroughly as far as possible, otherwise it will do harm to the storage property and electrochemical performance of the products[24-27]. Moreover, the lithium salts usually not easy to get well-mixed with precursor contained transition metal in particle to particle contact, leading to lower crystalline quality, such as higher degree of Li/Ni disorder which will decrease the cycle and rate performance of the cathode electrodes greatly[28,29]. In addition, the particle size also will increase greatly during later high-temperature treatment, which are inferior to cycle stability and rate performance[15,30]. Although some methods like sol-gel method could achieve a uniform distribution of Li and transition elements in atomic level and novel architectures, it remains challenging to environmentally friendly prepare such cathode material on a large scale[31]. Herein, a combined synthesis route employing organic lithium sources is developed to prepare NCM particles with long cycle life and high rate capability. Specifically, we introduced in-situ polymerized PAALi as lithium source to facilitate the crystal process and constrain the particle growth. The PAALi was in-situ polymerized on the surface of Ni1/3Co1/3Mn1/3(OH)2 precursor prepared by co-precipitation and wrapped into spherical powders after spray drying. For one thing, the uniform coated PAALi as highly dispersed lithium ion supplier will deliver lithium ion more efficiently than the particle to particle contact using inorganic lithium salts like LiOH·H2O and Li2CO3 mixing in solid state. For another, the PAALi can transform to carbon coating layers during pre-heat treatment at 450 °C under N2 gas. Those carbon coating layers not only significantly reduce agglomeration of particles and limit growth of the particle size, but also leave enough inner voids and paths after the subsequent sintering at 900 °C in the air. The electrochemical characterization results demonstrate that the layered cathode 4 / 24

produced by the new route shows much better cycle life and rate performance than traditional route using LiOH·H2O as lithium source mixed in solid state. This new synthetic route is viable to expand on the preparation of other electrode materials. 2. Experimental 2.1. Preparation of Ni1/3Co1/3Mn1/3(OH)2 precursor The Ni1/3Co1/3Mn1/3(OH)2 precursor was prepared by co-precipitation method. Specifically, 23.66 g NiSO4·6H2O, 25.30 g CoSO4·7H2O, and 15.21 g MnSO4·H2O were added to 300 mL deionized water to get 0.9 mol L-1 transition metal salt solution with an equal stoichiometric ratio for Ni2+, Co2+, and Mn2+ (solution A). A 2 mol L-1 NaOH solution mixed with a proper amount of ammonia water (NH3·H2O) was used as the precipitator (solution B). The ammonia water utilized in the precipitator is aimed to control the speed of sedimentation and the uniformity of element distribution in the sediment. Then, solution A and solution B were simultaneously transferred into the reaction vessel with a constant velocity, which was precisely controlled by a peristaltic pump, to obtain Ni1/3Co1/3Mn1/3(OH)2 slurry i.e. the precursor for LiNi1/3Co1/3Mn1/3O2. The entire co-precipitation reaction process was protected with nitrogen gas (N2) to avoid the possible valence change of transition metal ions, which can be oxidized by the oxygen in the air and then formed the impurity phase. The slurry was filtrated, and the solid was washed with deionized water and anhydrous ethanol repeatedly. In the end, the precursor was dried at 110 °C in a vacuum oven. 2.2. Synthesis of LiNi1/3Co1/3Mn1/3O2 with different lithium sources In the synthesis of LiNi1/3Co1/3Mn1/3O2, two kinds of lithium sources were employed. A brief schematic diagram was adopted as shown in Fig.1 to illustrate the synthetic process. In the route A, 0.15 mol acrylic acid (CH2=CHCOOH) and 0.15 mol lithium hydroxide (LiOH·H2O) were mixed up in deionized water with vigorous stirring. During this process, the monomer of C3H3OLi was formed spontaneously by the neutralization. Then, stoichiometric Ni1/3Co1/3Mn1/3(OH)2 precursor was added to the solution and keep stirring for 2h in a water bath at 60 °C. After that, the initiator, i.e. 20 mL ammonium persulfate water solution ((NH4)2S2O8, 0.1 mol L-1), was dropwise added by 5ml at each time with an interval of 0.5h. Thus, the lithium polyacrylate 5 / 24

(PAALi) can be in situ polymerized on the surface of Ni1/3Co1/3Mn1/3(OH)2 precursor as the lithium source. This prepared slurry was then used in spray drying to produce spherical LiNi1/3Co1/3Mn1/3O2 powder. Finally, the LiNi1/3Co1/3Mn1/3O2 powder was firstly pre-heated at 450 °C for 9 h with nitrogen gas (named as P-A-NCM) and then sintered at 900 °C for 12 h (named as A-NCM) in the muffle furnace to improve the crystallinity. In comparison, the route B adopts LiOH·H2O as the lithium source. After intensively grinding Ni1/3Co1/3Mn1/3(OH)2 with LiOH·H2O, the mixture was handled by the same two-step heat treatment to obtain LiNi1/3Co1/3Mn1/3O2 materials (named as P-B-NCM and B-NCM, respectively). 2.3. Material characterization The crystal structure information was obtained from powder X-ray diffraction (XRD, Rigaku Dmax2500, Cu-K). The chemical composition of the as-prepared samples was tested by inductively coupled plasma optical emission spectroscopy (ICP-OES, ARCOSⅡ MV, Spectro Analytical Instruments). Fourier transformation infrared spectroscopy (FT-IR, Bruker vertex-70 spectrophotometer) was used to testify individual chemical bonds and confirmed whether PAALi succeeds coating on Ni1/3Co1/3Mn1/3(OH)2 precursor. Raman spectroscopy (HORIBA LabRAM HR800) was conducted on a micro-confocal Raman spectrometer to probe the surface composition of LiNi1/3Co1/3Mn1/3O2. The surface morphology of NCM particles was examined by scanning electron microscopy (SEM, Hitachi S-4800). Nitrogen adsorption-desorption experiments were conducted on a Micromeritics ASAP 2020 analyzer at 77 K. The specific surface area of A-NCM and B-NCM were calculated based on Brunauer–Emmett–Teller (BET) analyses of the adsorption isotherms. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted on a Mettler Toledo TGA thermo analyzer under air flow at a rate of 5 °C/min from room temperature to 900 °C.High-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2) was employed in viewing the microstructure and coating film of NCM. Moreover, an attached energy dispersive spectroscopy (EDS) was capable of checking the element distribution. The electrochemical measurements were carried out using coin-type cells (CR2032) assembled in the argon gas filled 6 / 24

glove-box. The cathode electrode was fabricated from a mixture of active material: acetylene black: Poly(vinylidene fluoride) (PVDF) 80:10:10 (mass%) pressed on an aluminum foil current collector. The loading amount of active material was fixed at about 3.2 mg cm-2. A Celgard film and a lithium metal foil were taken as separator and anode, respectively. The composition of the electrolyte is 1 M LiPF6 in EC/DMC/EMC (1:1:1vol %). Charge-discharge profile and rate capacity was measured on battery test system (LAND-CT 2001) at room temperature. The charge/discharge rates varied from 0.2 C to 10 C (1 C is 170 mA g-1) within the voltage window between 2.5 V and 4.5 V. Cyclic voltammetry (CV) test was conducted on the VMP3 system (Bio Logic) with a scan rate of 0.1 mV·s−1 over the voltage range 2.5-4.5 V. Electrochemical impedance spectra (EIS) of cells at charge state of 4.0 V were tested by the Solartron workstation with an amplitude of 5 mV over the frequency range from 100 kHz to 0.01 Hz.

3.

Results and discussion The selection of lithium source is vital to the synthesis of the high-performance

layered cathodes. Currently, the mass-produced cathode materials employ inorganic compounds, such as LiOH·H2O or Li2CO3, as their lithium source. After hightemperature treatment (> 700 °C), well-crystallized particle can be formed by the solidphase reaction[5,6,11,18]. Herein, the crystal structures of NCM particles produced from organic and inorganic lithium sources are compared. As shown in Fig.S1, the diffraction pattern of sample B-NCM, which uses LiOH·H2O as the lithium source, displays a group of peaks with a sharp appearance. This pattern well conforms to the reported powder diffraction pattern of LiNi1/3Co1/3Mn1/3O2, and no impurity phase can be found. When the lithium source changed to the organic compound of PAALi, it can also be seen that the sample (A-NCM) is well-crystallized and the pattern fits in with the hexagonal layered α-NaFeO2 structure of LiNi1/3Co1/3Mn1/3O2. The split of peaks of (006)/(102) and peaks of (108)/(110) in the diffraction pattern suggests that the synthetic cathodes possess an excellent layer structure[7-9]. Particularly, the calculation of the peak intensity ratio I(003)/I(004) (based on (003) and (104) peaks) is an experienced method to evaluate the degree of cation mixing of Li+ and Ni2+. The higher the value of 7 / 24

I(003)/I(004), the lower the cation mixing, and a faster lithium-ion diffusion rate could be achieved[27,29,32]. The I(003)/I(004) value of A-NCM (1.48) is higher than that of B-NCM (1.36), indicating that the substitution to PAALi as lithium source is prone to form a well-ordered crystal structure. The electrochemical properties of A-NCM and B-NCM were measured by testing the samples in half-cells. Galvanostatic charge-discharge curves at 1 C current are shown in Fig.2. There are no apparent irregular fluctuations at charge-discharge profiles, indicating that the cathodes can keep a stable structure cycling between 2.5 and 4.5 V (Fig.2a and b). However, from the voltage profiles of the 1st, 50th, 100th and 200th cycles, it can be seen that the B-NCM degrades much faster than A-NCM regarding both of capacity and voltage. This result apparently indicates that the A-NCM exhibits a much better cycling stability. A more detailed comparison with regard to cycling performance is shown in Fig.2c. The samples of A-NCM and B-NCM deliver an initial capacity of 165.2 mA h g-1 and 154.7 mA h g-1, respectively. After 100 cycles, the reversible capacity reduced to 153.9 mA h g-1 and 125.6 mA h g-1 with the capacity retention of 93.2% and 81.2%, respectively. With the cycling proceeding, the variance of capacity retention progressively grows up. At the 200th cycle, the A-NCM delivers a specific capacity of 139.4 mA h g-1, corresponding to 84.4 % of the initial capacity, while the B-NCM has an inferior performance with only 63.0% on capacity retention. In order to examine the rate capability of the NCM cathodes, half cells were charged and discharged at a wide current rate range from 0.2 C, 1 C, 2 C, 5 C to 10 C as shown in Fig.2d. At 0.2 C, A-NCM and B-NCM deliver a similar specific capacity ~180 mA h g-1, which is due to the complete electrochemical reaction at low current density. At 2 C and 5 C, specific charge capacities of the A-NCM are 152.8 mA h g-1 and 130.6 mA h g-1, respectively, while that of B-NCM is 142.1 mA h g-1 and 114.1 mA h g-1. It can be seen that the B-NCM suffers a greater loss when the current rate rises. When the current increases to as large as 10 C, the B-NCM can only deliver 70.9 mA h g-1, about 38.3% capacity of 0.2 C. Whereas, the A-NCM shows a much better rate capability with 110.5 mA h g-1 at 10 C, about 59.3% of the 0.2 C capacity. The rate capability (10 C) and cycling performance of A-NCM is also better than other samples reported on 8 / 24

references listed in the supporting information (Table S1, SI). The unique morphology and structure of A-NCM are responsible for the great improvement in cycling life and rate capability. From the SEM observation (Fig.3), it can be seen that the A-NCM particles are mainly spherical secondary particles composed of uniform primary particles within 200 nm. The diameters of the secondary particles are less than 5 μm. In comparison, B-NCM is just composed of irregular particles which have a larger size around 1 μm. For the sample of A-NCM, the small size of primary particle can provide a short distance for lithium ion transport, which benefit for the capacity delivery especially at high rate[33-35]. What’s more, the secondary particle can avoid the agglomeration of large amount of NCM crystal as BNCM did. Thus, more abundant space and diffusion path for electrolyte and lithium ion can be obtained. Also, the abundant void and space can ease the stress of the volume expansion during charge and discharge process, which benefits for the cycle life of the cathodes[36]. The results from nitrogen desorption isotherm test also support the observation from SEM. As shown in Fig.4. The specific surface area value of A-NCM is more than twice that of B-NCM. Generally, the smaller size of particles offers more surface area. Based on the experimental data from SEM and BET, it can be concluded that A-NCM have a smaller primary particle size and a higher specific surface area, which contributes to a shorter lithium-ion transport distance and a fast intercalation and de-intercalation of lithium-ion. On the control of the particle size, morphology and crystal structure during route A, the organic lithium source contribute a lot. The advantages of using PAALi as lithium source can be attributed to two aspects as the schematic diagram illustrated (Fig.5). Firstly, the in-situ polymerization can form a very homogeneous coating layer on the surface of NCM precursor, which is very different from the particle to particle contact in the cases of using inorganic lithium sources. The uniform coating will supply lithium ion from the outer layer of the precursors during the heat-treatment. It may benefit for obtaining low cation mixing cathodes since the lithium diffusion is uniform and wellordered during the crystal formation process. Secondly, the organic lithium source coating layer can transform to the carbon coating layer in the pre-heat period. This 9 / 24

carbon layer can effectively restrict the grains growth and hinder the aggregation and fusion of the small crystals, and then the final particle size is under control[32]. After sintering at 900 °C, those carbon layers are oxidized, leaving enough inner voids and paths for the permeation of electrolyte. Fig.6 displays the SEM images of the precursor and the spray dried particles. The co-precipitation formed precursor composed of small lamellas, which are highly aggregated (Fig.6a and b). After in-situ polymerization and spray drying, the precursors were granulated and the surface is covered by a thick and uniform coating layer (Fig.6c and d). The ICP-OES results (Table S2) demonstrate the stoichiometry of the transition metals (Ni:Co:Mn) for precursor and S-A-NCM are very close, approximate to nominal chemical composition 0.33:0.33:0.33, and the organic lithium source was successfully introduced into the particle by in-situ polymerization and spray-drying. The FT-IR spectra verified that the surface layer is mainly PAALi, supporting the success of insitu polymerization process. As can be seen in Fig.7a, there is no obvious IR absorption peaks in the sample of the Ni1/3Co1/3Mn1/3(OH)2 precursor. For the sample S-A-NCM, a series of IR absorption peaks were observed. For the -COOH, there is a characteristic IR peak at ~1700

cm-1, which is due to the stretching vibration of C=O in the carboxyl.

When the Li+ substitute the H+ (-COOLi), the peak at ~1700 cm-1 will disappear since the homogenization of C=O and C-O in the carboxyl and a new peak will emerge at ~1560 cm-1 due to the dipole moment change with the salt formation[37,38]. In our sample, the observed peak at 1563 cm-1 and the unobserved peak at ~1700 cm-1 indicates that the Li+ has completely substituted the H+. The peak at ~ 842 cm−1 is ascribed to carboncarbon chain absorption in polymeric compounds, and the peak at 1438 cm-1 are due to the bending vibration of -CH2 alkyl group[39]. The two peaks demonstrate that the C=C has been open and then the -C-C- chain and the alkyl group formed, which means the PAALi molecule have been successfully introduced. In the Raman spectra of the preheated samples (Fig.7b), the peaks located at 1365 cm-1 and 1590 cm-1 belongs to the D band and G band of the carbon coating layer, indicating the formation of carbon coating[40,41]. Meanwhile, the abroad peak at 500~600 cm-1 is attributed to a superposition of the Eg and A1g bands from the three constituent transition metals. 10 / 24

Specifically, the Eg band is assigned to δ(O-M-O) vibrations (around 500 cm−1) while the A1g band is from ν(MO6) vibrations (around 600 cm−1), where M = Ni, Co and Mn[42]. After 900 °C heat treatment, the carbon coating layers are oxidized and no carbon signals can be detect in the Raman spectra of A-NCM samples. Fig.7c and d show the thermogravimetry analysis of the precursor and the precursor coating with PAALi (S-A-NCM). It can be seen that the mass loss of Ni1/3Co1/3Mn1/3(OH)2 precursor mainly comes from two parts, the first one is the water loss (12.5%, Fig.7c) corresponding to the heat absorption peak at 72 °C (Fig.7d), the second one is the oxidation of the precursor (14.2%). While for the S-A-NCM, the mass loss is much larger than the precursor (42.7%). The loss mainly comes from the oxidation of the organic coating layer at 200~400 °C, and four heat emission peak of the PAALi were observed in the corresponding temperature range. Fig.8 displays the TEM and EDS mapping images of the P-A-NCM sample. As can be seen in Fig.8a and b, a uniform coating layer appears on the particle surface after pre-heat treatment. EDS mapping shows the elementary composition and distribution on P-A-NCM particle (Fig.8c-h). Transition-metallic element (Ni, Co, Mn) and O evenly distribute in the NCM particle, while the C have a broad distribution agreeing well with its property of surface coating layer. As can be seen in Table S3, the stoichiometry of the transition metals (Ni:Co:Mn) for all samples are identical to the precursor. Fig.9 illustrates the HR-TEM images of A-NCM and B-NCM. After sintering at 900 °C in the muffle furnace, the clear lattice spacing lines are observed in both A-NCM and B-NCM (Fig.9 b and e) and the value is measured to be approximately 0.475 nm, which well coincides with the (003) crystalline planes[6,13]. For A-NCM, although the carbon coating layer has been oxidized at high-temperature, some amorphous residue (~3nm) still can be found on the crystal surface (Fig.9b and c). This amorphous layer may function as a protective film to avoid parasitic reaction on the electrode-electrolyte interface, which benefit for the cycle life. In the designed route, the PAALi is functioned as a high-efficiency lithium ion supplier as well as a particle size control agent, which shows a great application prospect for high-performance cathode material preparation. In order to analysis the electrochemical properties in a further step, the initial cyclic 11 / 24

voltammetry curves of A-NCM and B-NCM were recorded as shown in Fig.10a. Both of the two samples obviously show a pair of redox peaks, which corresponding to the delithiation and lithiation process. Obviously, the A-NCM has lower cathodic peak (3.93 V) and higher anodic peak (3.65 V) than that of B-NCM (3.97 V and 3.63 V, respectively). The potential interval between the anodic and cathodic peaks represent the electrochemical process involving lithium diffusion in a solid phase and electron transfer process[43]. A smaller potential interval means less polarization and better electrochemical reversibility. The A-NCM with small primary crystals and plentiful diffusion channels displays a small potential interval, which indicates that the transferring of Li ions and electrons can be fast completed during charge and discharge. In addition, the peak current of A-NCM is also larger than B-NCM with a same loading of active material. This is also attributed to the small particle size and porous structure of A-NCM, which allow more active channels and sites participating in the lithiation and delithiation process. Thus, more crystals in A-NCM can be transformed in a short time compared with B-NCM, exhibiting a higher capacity and rate capability, which is in accord with the cycle and rate test before. The AC impedance of A-NCM and BNCM also conducted as shown in Fig. 10b. The inset equivalent circuit was used to fit the Nyquist plots. Specifically, Rb represents the bulk resistance, Rsei represents the resistance of the solid electrolyte interface, and Rct represents the charge transfer resistance. The fitting results, as can be seen in Table S3, show that the A-NCM has lower Rsei and Rct (24.64 and 9.99 Ω) than that of B-NCM (30.16and 11.67 Ω), which indicates that A-NCM possesses better electrochemical dynamics. Besides, the calculated values of lithium diffusion coefficient (DLi) are shown in Table S3. The DLi of A-NCM is 4.81×10-12 cm2 s−1 which is higher than that of B-NCM (4.2×10-12 cm2 s−1), indicating that the diffusion process in A-NCM is faster than traditional NCM samples. So taking above analyses into consideration, the sample A-NCM prepared with PAALi as lithium source has lower resistance and faster lithium ion diffusion rate, and those are the reasons for the better electrochemical performance.

4. Conclusions 12 / 24

Through in-situ polymerization reaction and pray drying, organic compound PAALi is successfully coated on Ni1/3Co1/3Mn1/3(OH)2 precursor. The PAALi is designed as a dual-functional lithium source, specially a highly dispersed lithium ion supplier as well as a particle size control agent. The uniform coated PAALi will deliver lithium ion more efficiently than the particle to particle contact using inorganic lithium sources. Moreover, this organic layer can transform to carbon coating layer during pre-heatment, which will limit growth of the particle size in the following sintering stage. By this novel route, the spherical LiNi1/3Co1/3Mn1/3O2 cathode with high rate capability and long cycle life was prepared. The superior cycle performance is attributed to the low cation mixing crystal structure and the stable primary-secondary particle structure, which possess good mechanical strength and can ease the stress of the volume expansion during charge and discharge process. The excellent rate capacity is due to the small particle size which can provide a short distance for lithium diffusion especially at high rate and the voids and paths generated after sintering.

Acknowledgements This work was supported by the National Key Basic Research Program of China (No. 2014CB932400); the National Nature Science Foundation of China (Nos. 51672156, U1401243 and 51232005); the NSAF (No. U1330123); and the Shenzhen Technical Plan Project (No. JCYJ20140902110354239).

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Figures

Fig.1 Schematic diagram of different synthetic routes. In the route A, the in-situ polymerized PAALi was used as lithium and carbon source. In the route B, LiOH·H2O was used as lithium source and mixed with precursor in solid state.

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Fig.2 Galvanostatic charge-discharge curves at 1 C of A-NCM (a) and B-NCM (b) electrodes; Cycling performance at 1 C (c) and rate capability (d) of A-NCM and B-NCM electrodes.

Fig.3 SEM images of the A-NCM (a, b) and B-NCM (c, d).

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Fig.4 Nitrogen adsorption-desorption isotherms for the A-NCM and B-NCM.

Fig.5 Schematic illustration of the advantages of using in-situ polymerization of PAALi as lithium source (a) to traditional mixing inorganic lithium salts with precursor in solid state (b).

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Fig.6 SEM images of the precursor (a, b) and spray dried particles S-A-NCM (c, d).

Fig.7 (a) FTIR spectra of precursor and S-A-NCM; (b) Raman spectra of P-A-NCM and A-NCM; (c) TGA curves of precursor and S-A-NCM; (d) DSC curves of precursor and S-A-NCM.

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Fig.8 TEM images (a,b) and EDS mapping images of C, O, Mn, Ni, Co (c-h) of P-A-NCM.

Fig.9 HR-TEM images of A-NCM (a-c) and B-NCM (d, e).

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Fig. 10 CV curves (a) and EIS (b) of A-NCM and B-NCM electrodes.

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Figure captions Fig.1 Schematic diagram of different synthetic routes. In the route A, the in-situ polymerized PAALi was used as lithium and carbon source. In the route B, LiOH·H2O was used as lithium source and mixed with precursor in solid state. Fig.2 Galvanostatic charge-discharge curves at 1 C of A-NCM (a) and B-NCM (b) electrodes; Cycling performance at 1 C (c) and rate capability (d) of A-NCM and B-NCM electrodes. Fig.3 SEM images of the A-NCM (a, b) and B-NCM (c, d). Fig.4 Nitrogen adsorption-desorption isotherms for the A-NCM and B-NCM. Fig.5 Schematic illustration of the advantages of using in-situ polymerization of PAALi as lithium source (a) to traditional mixing inorganic lithium salts with precursor in solid state (b). Fig.6 SEM images of the precursor (a, b) and spray dried particles S-A-NCM (c, d). Fig.7 (a) FTIR spectra of precursor and S-A-NCM; (b) Raman spectra of P-A-NCM and A-NCM; (c) TGA curves of precursor and S-A-NCM; (d) DSC curves of precursor and S-A-NCM. Fig.8 TEM images (a,b) and EDS mapping images of C, O, Mn, Ni, Co (c-h) of P-A-NCM. Fig.9 HR-TEM images of A-NCM (a-c) and B-NCM (d, e). Fig. 10 CV curves (a) and EIS (b) of A-NCM and B-NCM electrodes.

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