Investigating the Structure of an Active MaterialâCarbon Interface in

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Investigating the Structure of Active Material-Carbon Interface in the Monoclinic LiV(PO)/C Composite Cathode 3

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Hua Huo, Zeyu Lin, Di Wu, Guiming Zhong, Jinyu Shao, Xing Xu, Bingxing Xie, Yulin Ma, Changsong Dai, Chunyu Du, Pengjian Zuo, Geping Yin, and Luming Peng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00410 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Applied Energy Materials

Investigating the Structure of Active Material-Carbon Interface in the Monoclinic Li3V2(PO4)3/C Composite Cathode

Hua Huo*a, Zeyu Lina, Di Wu b, c, Guiming Zhongd, Jinyu Shao a, Xing Xu a, Bingxing Xie a, Yulin Ma a, Changsong Dai a, Chunyu Du a, Pengjian Zuo a, Geping Yin a and Luming Peng*c a

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage,

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92 West Da Zhi Street, Harbin, 150001, China b

Collaborative Innovation Center for Modern Grain Circulation and Safety, Key Laboratory of Grains

and Oils Quality Control and Processing, College of Food Science and Engineering, Nanjing University of Finance and Economics, Nanjing 210023, China c

Key Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing

University , Nanjing 210093, China d Xiamen

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

361021, China

*Corresponding Authors: E-mail: [email protected] E-mail: [email protected]

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ABSTRACT A widely adopted strategy to enhance the electronic conductivity of lithium transition metal phosphates is to form phosphate/C composite by introducing reagents (carbon sources) that can transform to carbon during calcination. In this present work, a systematic study combining XRD, SEM, (HR) TEM, SSNMR and electrochemical measurements has been conducted to investigate how the electrostatic interaction between the functional groups (carboxyl, hydroxyl, etc.) of a carbon source and the building units of Li3V2(PO4)3 (Li+, VO2+, PO43-, etc.) in the original precursor affects the structure of Li3V2(PO4)3-carbon interface in the final composite. It has been demonstrated that the types and concentrations of electro-negative functional groups in a carbon source play an important role in controlling not only the morphology of the product, but also the composition, the crystallinity and the micro-structure of Li3V2(PO4)3-carbon interface; and in turn the electrochemical behavior of Li3V2(PO4)3/C composite. This study provides guidance on carbon-lithium transition metal phosphate interface design and control.

Key words: Lithium ion battery; Lithium vanadium phosphate; Interface design; Carbon coating; SSNMR

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1. Introduction Lithium-ion batteries (LIBs) are currently the most important energy storage devices for hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs)1,2. Among all cathode materials for LIBs, monoclinic lithium vanadium phosphate, Li3V2(PO4)3 (LVP), has gained intense attention because of high charge-discharge voltage, relatively high theoretical speciﬁc capacity and thermodynamically stable structure3,4,5. Similar to other lithium transition metal phosphates, the electrochemical performance of LVP has been limited by its poor electronic conductivity due to the framework formed by VO6 octahedra corner-sharing with PO4 tetrahedra6. Carbon coating is an economic and convenient method to enhance the electronic conductivity of electrode materials, which can also alleviate the aggregation of LVP particles during calcination7,8,9. It is usually achieved by introducing carbon sources, such as graphene10, graphene oxide (GO)7,11,12, carbon black13, carbon nanoflakes14, citric acid15,16,17, glucose18 and humic acid19 etc. as raw materials. In the past decade, enormous research efforts have been devoted to optimizing the structure (morphology, porosity, thickness) and the degree of graphitization of the coating carbon. In terms of structure design, hierarchical structures with active materials surrounded by porous carbon (Li+ pathway) and further connected with long-range conductive carbon (electron pathway) have been demonstrated robust in improving rate performance. For instance, Rui et al. reported a hierarchical structure with LVP nanocrystals embedded within nano-porous carbon matrix and then attached to conductive rGO sheets, in which the nano-porous carbon both acted as an electrolyte container allowing fast Li+ migration and maintained the structural integrity during repeated charge-discharge processes16. Similarly, Wei et al. reported a LVP/C composite synthesized using cetyltrimethylammonium bromide (CTAB) as both the carbon source and the surfactant to form mesoporous carbon scaffolds8. However, some other studies suggested that the porosity around the active material allowing direct contact with the electrolyte may not be really necessary. For example, Fang et al. replaced the porous carbon with 4 nm compact carbon layers by CVD method in the Na3V2(PO4)3/C composite and obtained surprisingly good rate performance and cycling lifespan20. In a review article on LiFePO4 cathode, Yuan et al. pointed out that the degree of graphitization (i.e., the sp2/sp3 ratio) of coating carbon is the most important factor determining the conductivity and rate behavior of LiFePO4/C composite cathode, because graphite carbon (sp2-type) is more effective than disordered carbon (sp3-type) in both electron conduction and Li+ diffusion21. All these results mentioned above have clearly demonstrated that phosphate-carbon interface has profound impact on the electrochemical behaviors of phosphate/C composites. Therefore, basic chemistry and general rules that can help tailor the carbon coating become 3



essential. Carbon coating generates an interface between the carbon and active material, which can greatly affect the electrochemical performance of the electrode. Recently, the concept of “wettability” was proposed by Goodenough22 and coworkers in their study of all solid state lithium ion battery to describe the interfacial compatibility of two solids in contact, which has been widely borrowed in many other areas23, 24. From this perspective, the “wettability” of the interface between the carbon coating and lithium transition metal phosphates is of great importance but yet lack of fundamental understandings. Zhou et al. proposed the existence of P-O-C connections at the interface of LiMn0.75Fe0.25PO4 and graphene by comparing the X-ray absorption spectra (XAS) of free standing LiMn0.75Fe0.25PO4 with the graphene bonded one at Li, P, Mn, Fe and O edges25, which suggested the local structure of carbon/lithium transition metal phosphate interface is more complex than simply physical contact. To address the questions raised above about the fundamental chemistry of building LVP-carbon interface and the interfacial structure, it is necessary to trace the evolution of carbon source from the very beginning of synthesis. In a sol-gel based route, the initial solution/gel usually contains the building units of LVP (Li+, VO2+ and PO43-), the chelating agent (oxalic acid or citric acid) and the carbon source. In most of the cases, both chelating agent and carbon source are negatively charged in water as a result of the ionization of carboxylic acid and (phenolic) hydroxyl groups16,26,27. Although it is widely accepted that these cations (Li+, VO2+) and anions (PO43-, COO-, CO- etc.) can self-assemble via electrostatic attraction/repulsion, inconsistent mechanisms have been proposed by different researchers in regards of whether the LVP building units are attached to the carbon sources directly23 or via the chelating agent,8,16 which can be the key in determining the composition and structure of the interface. We hereby propose a competition model in Figure 1, suggesting that the electronegative groups of both carbon source and chelating agent can interact with the LVP building units simultaneously. However, the bonding between carbon source and LVP building units are always weakened by electrostatic and/or steric effects. In other words, carbon sources with strong electronegative groups (-COOH) and/or little steric hindrance (small molecule) can effectively compete with the chelating agent (oxalic acid) and form co-absorption on the surface of LVP building blocks. This co-absorption of chelating agent and carbon source may play a key role in controlling the morphology of LVP particles and the composition of LVP-carbon interface.

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Figure 1. The schematic illustration of the competition model of the interaction among LVP building blocks, chelating agent and carbon source in aqueous solution (graphene oxide is adopted in the illustration as an example of carbon source).

On the basis of this competition model, six carbon sources are selected in this work and divided into three categories according to their strength of electrostatic interaction with LVP building units (Figure 2). Both oxalic acid and citric acid are listed in the “strong electrostatic interaction/coordination” category, which are widely adopted as chelating agents. Citric acid (CA) is both the chelating agent and the carbon source for the composite denoted as LVP/CA; while oxalic acid, which can completely decompose to CO or CO2 during calcination, is the chelating agent adopted for pure LVP and the composites using other carbon sources. The (phenolic) hydroxyl groups in glucose and GO make them less electronegative in water and they are listed as carbon sources with medium electrostatic interaction. Graphene and super P can be considered as layered or bulk carbon with negligible electronegative groups. The denotations LVP/Glu, LVP/GO, LVP/G and LVP/SP stand for the composites synthesized with glucose, graphene oxide (GO), graphene (G) and super P (SP) as the carbon source, respectively, while oxalic acid is used 5



as the chelating agent. Note that the denotations adopted here indicate the original forms of the carbon sources only, not the actual compositions of the final LVP/C composites. The morphology, interfacial structures/compositions and the electrochemical performance of these LVP/C composites are investigated in this present work, in order to obtain fundamental understandings of LVP-carbon interface.

Figure 2. Categorization of carbon source base on the strength of electrostatic force between the electronegative functional groups in the carbon source and the LVP building units in the precursor. (a) oxalic acid (b) citric acid (c) glucose (d) graphene oxide (e) graphene (f) super P

2. Experimental Preparation of LVP and LVP/C composites 5 mmol pure LVP and the LVP/C composites were prepared by a sol-gel based method. Both oxalic acid and citric acid can be used as chelating agent as well as reducing agent. First, oxalic acid (BASF, >99.5%), V2O5 (Bodi Chemical, >99.6%), NH4H2PO4 (Bodi Chemical, >99.0%) and Li2CO3 (Bodi Chemical, >97.0%) in a stoichiometric ratio (3.5:1:3:1.75) were dissolved in deionized water and magnetically stirred at room temperature until a clear blue solution formed. 5% excess Li2CO3 was added to prevent the lack of lithium during high temperature process. The water was evaporated at 80 °C under stirring. The remaining solid was ground manually to fine powder, preheated at 350 °C for 4 h, and sintered at 800 °C for 8 h under H2 (5% in N2) gas flow to obtain pure LVP. Similarly, the LVP/CA composite was synthesized by using citric acid (Jinfeng Chemical, >99.5%), V2O5, NH4H2PO4 and Li2CO3 in a stoichiometric ratio (2:1:3:1.75). Likewise, the LVP/Glu composite was synthesized using glucose (Jizhun Reagent, >99.0%), V2O5, NH4H2PO4 and Li2CO3 in a stoichiometric ratio (2:1:3:1.75). LVP/GO, LVP/G and LVP/SP composites were prepared using oxalic acid as chelating and reducing agent, and the quantity of elemental carbon source (GO, graphene, super P) was controlled at a mass ratio of 8wt% (0.16 g), for optimized electrochemical performance12. GO, graphene (Ningbo Moxi Technology 6


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Co., >99.9%), or super P (Lizhiyuan Co., >99.9%) was added during stirring at 80 °C. Graphene oxide (GO) was synthesized from graphite by Hummers method28,29.

Characterizations Phase analyses were carried out using X-ray powder diffraction (XRD) on a Bruker AXS D8 advance X-ray diffractometer in the 2θ range from 10° to 90°, at a scan rate of 0.1°/s with filtered Cu Kα radiation. Rietveld refinements were performed using the GSAS software30. Scale factor, zero point, background, cell parameters, phase fraction and the particle size of minor phase were iteratively refined. The morphology of the composite was determined using field-emission scanning electron microscopy (FESEM; HELIOS NanoLab 600i) and high-resolution transmission electron microscopy (HRTEM, Tecnai G2-F30) with an accelerating voltage of 300 kV. 7Li, 31P

and

13C

magic-angle spinning (MAS) NMR spectra were acquired on a Bruker Avance

III-400 spectrometer. 7Li chemical shifts are referenced to 1M LiCl (aq.) at 0.0 ppm. 7Li (I = 3/2) single pulse experiments were employed with a 90° pulse duration of 1.6 μs and a recycle delay of 0.05 s, at two spinning rates of 30 and 60 kHz. 7Li 2D exchange NMR spectrum was acquired using a mixing time of 1 ms, at a spinning rate of 35 kHz. 31P chemical shifts are referenced to 1 M H3PO4 solution at 0.0 ppm. 31P (I = 1/2) single pulse was employed with a 90° pulse duration of 0.8 μs and a recycle delay of 0.1 s, at a spinning rate of 60 kHz. For 7Li and 31P NMR, the samples were packed into conventional 1.3 mm zirconia rotors and tested with a 1.3 mm HX probe.

13C

chemical shifts are referenced to adamantane (high frequency signal at 38.4 ppm). Rotor-synchronized echo-MAS (90°-τ-180°-τ) pulse sequence with 1H decoupling was employed for 13C NMR with a 90° pulse duration of 5 μs and a recycle delay of 0.5 s, at a spinning rate of 20 kHz. Hpdec pulse sequence (single pulse with high power decoupling) was adopted for recycle delay optimization. For

13C

NMR, the samples were packed into conventional 3.2 mm zirconia

rotors and tested with an HXY probe. Signal deconvolution and integration are performed using DMFIT software31. Electrochemical measurements Electrochemical measurements were performed by coin-type cells (CR2032) assembled in an argon-filled glove-box, where both moisture and oxygen levels were below 1 ppm. The active material (80 wt.%), Super P (10 wt.%) and PVDF (10 wt.%) were mixed in N-methyl-2-pyrrolidone (NMP). The slurry was stirred for 24 h and then coated on aluminum foil. After vacuum drying for 12 h at 120 ℃, the coated aluminum foil was cut into wafers and pressed 7



at a pressure of 10 MPa. Lithium wafers were used as anodes in coin-cells. The electrolyte was 1 M LiPF6 in ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DEC) (EC: EMC: DEC=1:1:1, by volume). The cells were galvanostatically charged and discharged using a multichannel battery tester (NEWARE) at two voltage ranges of 3.0-4.3 V and 3.0-4.8 V at room temperature. Cyclic voltammetry (CV) at different scan rates (0.05 mV/s, 0.1 mV/s, 0.3 mV/s, 0.5 mV/s, 1 mV/s) and electrochemical impedance spectroscopy (EIS) at the frequency range from 1 MHz to 0.01 Hz were conducted using a CHI-650D electrochemical workstation.

3. Results and Discussions The XRD patterns of the LVP/C composites are shown in Figure S1. Diffraction patterns of all the samples can be well indexed to monoclinic structure of PDF#01-078-5405 with the space group of P21/n. Trace amount of Li3PO4 may exist as a minor phase in these samples, since Li3PO4 is widely reported as an impurity in LVP formed during synthesis32,33. LVP- Li3PO4 two phase Rietveld refinement has been performed to examine the existence of Li3PO4. The refinement results (Table S1) suggest that some samples may contain Li3PO4 with small particle size. However, the readers must bear in mind that Rietveld refinement is not sensitive to amorphous impurity less than 5 wt%34.

SEM and TEM: The SEM images of various LVP/C composites are provided in Figure 3a-f. As shown in Figure 3a, the pure LVP particles can be described as smooth and interconnected round sticks. Presumably, the chelating agent oxalic acid plays an important role in forming this unique morphology. In contrast, the LVP/CA composite (Figure 3b) presents a completely different morphology with cubic/square shape and sharp edges. Obviously, the carbonization of citric acid has a strong influence on the morphology of the LVP/CA composite. Similar morphologies are also observed in the LVP/Glu (Figure 3c) and LVP/GO (Figure 3d) composites, indicating that the electrostatic interaction between the LVP building units and these carbon sources are strong enough to compete with oxalic acid and affect the final morphology. It is also worth noticing that the particle sizes of LVP/Glu and LVP/GO composites are relatively smaller than other samples. When bulk carbon materials with negligible electronegative groups like graphene and super P are directly adopted as carbon source, the final composites inherit the smooth and round shape of pure LVP with graphene or super P attached on the surface, as shown in Figure 3e and 3f.

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Figure 3. SEM images of the as-prepared LVP/C composites: (a) pure LVP; (b) LVP/CA; (c) LVP/Glu; (d) LVP/GO; (e) LVP/G; (f) LVP/SP;

The TEM images of four LVP/C composites including LVP/CA (strong interaction), LVP glucose (medium interaction, small molecule), LVP/GO (medium interaction, large molecule) and LVP/G (weak interaction), are shown in Figure 4a, c, e and g, respectively. The diameters of the LVP primary particles in LVP/CA (Figure 4a) and LVP/G (Figure 4g) composites are around 100-200 nm, while the LVP primary particles in LVP/Glu (Figure 4c) and LVP/GO (Figure 4e) composites are obviously smaller with the diameters about 30 - 50 nm. Presumably, the electronegative groups in these carbon sources (glucose and GO) and the chelating agent (oxalic acid) can co-absorb on the LVP building units in solution, and therefore prevent the LVP particles from growing up. In the LVP/C composites prepared using small molecules (citric acid, glucose) as carbon sources, the LVP particles are surrounded by amorphous carbon. In the LVP/C composites prepared using GO and graphene as carbon sources, the LVP particles are wrapped by rGO/graphene sheets. The HRTEM images of LVP/CA, LVP glucose, LVP/GO and LVP/G, are shown in Figure 4b, d, f and h, respectively. The lattice spacings (d) measured from these HRTEM images are 0.69 nm (Figure 4b), 0.29 nm (Figure 4d), 0.69 nm (Figure 4f) and 0.86 nm (Figure 4h), corresponding to 9



the (110), (212), (110) and (001) planes of monoclinic (P21/n space group) LVP, respectively. The HRTEM images illustrate more details about the LVP-carbon interface. For instance, small crystalline region embedded in the amorphous carbon coating layer can be observed in the LVP/CA composite (Figure 4b). The fast Fourier transform (FFT) pattern of the selected area in carbon coating (inset of Figure 4b) can be indexed to the (111) plane of diamond with lattice spacing of 0.199 nm. It is widely accepted that carbon source can alleviate the recrystallization of LVP during calcination4. The presence of crystalline carbon at the LVP-carbon interface of LVP/CA composite suggests that the recrystallization effect can be mutual, when strong interaction exists between the LVP building units and the carbon source. It is also interesting to compare the HRTEM images of the LVP/GO (Figure 4f) and LVP/G (Figure 4h) composites, which in principle should be identical. The LVP particle is wrapped by graphene film directly in the LVP/G composite, while an amorphous carbon layer exists between LVP particle and rGO/graphene in the LVP/GO composite. This difference probably origins from the electrostatic interaction between GO and the LVP building units.

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Figure 4. TEM images of LVP/C composite: (a), (b) LVP/CA. Inset: FFT pattern of the HRTEM image in the selected area; (c), (d) LVP/Glu; (e), (f) LVP/GO; (g), (h) LVP/G 11



7Li, 31P

and 13C MAS NMR

The 7Li MAS NMR spectra of the as prepared LVP/C composites acquired with a spinning speed of 30 kHz are shown in Figure 5a. The three 7Li NMR signals at 103, 52 and 17 ppm have been assigned to the three lithium sites Li3, Li1 and Li2, respectively35. The resonance at 0 ppm was previously attributed to residual Li2CO3 from raw materials by Goward et al36. However, the relative intensity of this signal clearly varies upon changing carbon sources, which implies that this 0 ppm signal represents some Li containing species formed during synthesis and related with the nature of the carbon source, possibly Li3PO4. Details about the integrations of this 0 ppm 7Li signal can be found in Table S2. Moreover, a broad signal centered around 70 ppm can be observed in the 7Li NMR spectrum of LVP/CA composite. This high frequency peak is out of the range of diamagnetic 7Li shifts (10 ~ -10 ppm). It can be due to either a new type of Li-O-V environment located in the LVP framework or some LiCx species37. To further understand the nature of the 0 ppm and the 70 ppm signals, 7Li NMR at a higher spinning speed of 60 kHz and 2D exchange 7Li NMR were employed and the data are shown in Figure 5b and 5c. It is well known that fast magic angle spinning can generate frictional heat, which will increase the sample temperature6, 38. In the 7Li MAS NMR spectra acquired at 60 kHz, the three resonances of LVP coalesced and became broader due to faster Li+ exchange at higher temperatures. Taking the spectra of pure LVP as a reference, the broadening and coalescence of the LVP/G composite is similar to pure LVP while that of the LVP/CA, LVP/Glu and LVP/GO composites is much severe, suggesting higher mobility of Li+ in these samples. This phenomenon can be related to multiple factors, including the heat conductivity of carbon coating, possible Li+ pathway at the LVP-carbon interface and the size of the primary particles. As shown in the 7Li 2D exchange spectrum, the three Li+ sites of LVP can exchange with each other, while the signals at 0 and the 70 ppm signals are not involved, indicating they are not in the LVP phase. Thus, the only plausible assignment of the peak at 70 ppm is LixC intermetallic compound formed at the LVP-carbon interface. The

31P

MAS NMR spectra of different LVP/C composites at the spinning rate of 60 kHz are

shown in Figure 5d. The three isotropic 31P signals at 4170, 2510 and 1960 ppm can be assigned to the P3, P2, and P1 sites in the monoclinic Li3V2(PO4)3 lattice, respectively36. Similar to the 7Li NMR spectra, a 31P signal at 10 ppm presents in all spectra with the same trend of intensity varies with carbon source. Details about the integrations of this signal can be found in Table S2. According to the NMR integration results, the 7Li signal at 0 ppm and the 31P signal at about 10 ppm are clearly associated together, suggesting the existence of a diamagnetic species containing both Li and P. The 7Li and 31P chemical shifts of Li3PO4 are reported at approximately 0 ppm and 9 ppm, respectively39, consistent with our results. A zoom-in review of the diamagnetic region in 12


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31P

NMR can be found in Figure S2. It is worth noticing that the 10 ppm

31P

signals are

broadened, very similar to the short-range ordered Li3PO4 observed by Deng et al39. This further confirms the 0 ppm (7Li) and 10 ppm (31P) signals are due to amorphous Li3PO4. The 31P signals of other possible P containing species like P2O74-, P3O105- and P4O124- etc. are reported within the range of -40 to 2 ppm40, which are quite different from our result. Moreover, it is widely reported that Li3PO4 exists as impurity in LVP32,33. Hence, it is reasonable to correlate the 7Li signal at 0 ppm with the 31P signal at 10 ppm, and assign them to Li3PO4 formed during synthesis. The

13C

MAS NMR spectra of LVP/CA, LVP/Glu and LVP/GO at the spinning rate of 20

kHz are shown in Figure 5e. The retention delay of 13C NMR has been optimized and proved to be extremely short, see Figure S3. The fast relaxation of presence of paramagnetic V3+. Each

13C

13C

magnetization is associated with the

spectrum can be clearly divided into two regions: 8-80

ppm corresponding to sp3 carbon and 90-180 ppm corresponding to sp2 carbon. 13C NMR has been widely adopted to determine the sp3: sp2 ratio in elemental carbon41. As shown in Figure 5e, the 13C

spectra of LVP/GO composite is dominated by a broad signal centered at 125 ppm in the sp2

carbon region, indicating that GO has been successfully reduced to sp2 bonded graphene with low content of sp3 carbon. In order to identify possible background signal from the probehead,

13C

experiment of empty rotor with the same experimental setup has been acquired and shown in Figure S4. This

13C

NMR spectrum of empty rotor clearly shows that the probe background has

been fully suppressed by using Hahn-echo pulse sequence at such a short recycle delay. Compared to LVP/GO, LVP/Glu and LVP/CA composites contain significant amounts of sp3 carbon. It is also worth noticing that the sp3 carbon signal in LVP/Glu is broad, which is consistent with the chemical shift dispersion of amorphous/disordered carbon, while the

13C

NMR spectrum of

LVP/CA composite shows sharp and well defined signals at 59 ppm (C-C, C-O), 30 ppm (C-C) and 16 ppm (C-C, C-H)42,43, suggesting ordered sp3 carbon. This result is in agreement with the existence of highly crystalline (diamond-like) sp3 carbon in the HRTEM data.

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Figure 5. MAS NMR spectra of different LVP/C composites：(a) 7Li NMR at the spinning rate of 30 kHz; (b) 7Li NMR at the spinning rate of 60 kHz; (c) 7Li 2D exchange spectrum of the LVP/CA composite, mixing time = 1 ms; (d) 31P NMR at the spinning rate of 60 kHz, the spinning sidebands are labeled with “*”; (e) 13C NMR at the spinning rate of 20 kHz.

Electrochemistry The first charge-discharge profiles at 0.1 C rate within the potential range of 3.0-4.3 V are shown in Figure 6a. All samples present three plateaus corresponding to three two-phase reactions within this voltage range. Pure LVP can only provide a capacity of ~100 mAh/g during the first charge due to poor electronic conductivity. Among all the LVP/C composites tested in this work, LVP/G shows the highest capacity of 129.2 mAh/g. Forming LVP/C composites can no doubt help LVP to approach its theoretical capacity 133 mAh/g by improving electronic conductivity. Two dotted rectangles (I) and (II) are drawn on the voltage profile; and their zoom-in views are shown in Figure 6b and c, respectively, in order to call particular attention to the rise/drop near the 14


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edges of charge/discharge plateaus. Pure LVP, LVP/SP and LVP/G present abrupt rise/drop, while LVP/GO, LVP/Glu and LVP/CA present sloped voltage curve at the edge of plateaus. Although these sloping features are widely observed in the voltage profiles of many LVP/C composites17,44, there is no discussion about this phenomenon reported to our knowledge. The sloping charge-discharge curve can be considered as a sum of the ideal voltage profile of LVP and the capacitive behavior of porous/activated carbon in the same voltage window. To further elucidate the difference among the voltage profiles of these LVP/C composites, the differential capacity curves converted from the charge-discharge profiles (Figure 6a) are shown in Figure 6d, and Figure 6e shows the zoom-in view of the third pair of peaks (4.02 - 4.10 V). The voltage of Li+ re-insertion significantly varies with the carbon source and therefore affects the redox peak potential difference (ΔE) value. These LVP/C composites can be divided into three groups according to their ΔE values: smaller than 20 mV ( LVP/CA), 25-30 mV (LVP/Glu and LVP/GO), and larger than 35 mV (LVP/SP, LVP/G and pure LVP). As a sensitive indicator of electrochemical reversibility, lower ΔE value is more desirable. A lower ΔE value also stands for less polarization e.g. faster Li+ and electron transportation at the electrode/electrolyte interface45. This result is consistent with the previous assumption that the LVP/CA, LVP/Glu and LVP/GO may contain porous/activated carbon that possesses capacitance within ~3 - 4.5 V. During Li+ reinsertion, porous carbon on the surface of LVP can absorb Li+ and help to eliminate the underpotential at LVP-electrolyte interface. Nonetheless, extra precautions must be taken to distinguish real changes in electrochemical dynamics caused by the synergistic effect of Li+ insertion-adsorption from simply apparent voltage changes when interpreting the improved electrochemical reversibility of these composites. To further address this issue, the rate performance of LVP/C composites at various current densities within 3 - 4.3 V has been tested (Figure 6f). LVP/G shows the highest capacity below 1 C (124.5 mAh/g at 1 C) due to the excellent electronic conductivity of graphene. However, at high rates of 5 C and 10 C, the capacity of LVP/G drops dramatically. In contrast, the capacities of LVP/CA and LVP/GO are the highest (~102 mAh/g) at 10 C. Obviously, the capacitive carbon plays an important role in maintaining the electrochemical performance at high rates. The relationship between electrochemical behavior and the LVP-carbon interfacial structure/composition will be addressed later in the discussion. The cycling performance at 1 C rate within 3 - 4.3 V is shown in Figure 6g. After 200 cycles, different materials show different capacities. LVP/G remains the highest capacity of 124.2 mAh/g and LVP display the lowest at 50.5 mAh/g. LVP/CA, LVP/Glu, LVP/GO and LVP/G composites show high capacity retention rates of 96.4 %, 98.3 %, 97.5% and 96.1 %, respectively. However, only medium capacity retention is achieved for LVP/SP composite (78.3 %) while LVP is associated with a retention as low as 48.1 %. Clearly, the cycling performance of LVP can be 15



improved by carbon-coating which increases the electronic conductivity. The first charge-discharge profiles at 0.1 C rate and the cycling performance at 1 C rate within 3 - 4.8 V are shown in Figure 6h and 6i, respectively. When cycled within the voltage window of 4.3 - 4.8 V, the LVP crystal structure undergoes a phase transformation35 leading to irreversible lithium intercalation and deintercalation between Li1V2(PO4)3 and V2(PO4)3. Hence, all cathodes display apparent capacity decay during the first three cycles. Similar to 3 - 4.3 V, when LVP is combined with carbon, the capacity and the cycling performance of LVP can be significantly improved. After 200 cycles, the capacity of pure LVP dropped to ~60 mAh/g. In comparison, LVP/G displays the best performance of 130.3 mAh/g and excellent capacity retention (72.6 %) after 200 cycles. Meanwhile, LVP/CA and LVP/GO also display good performance of 119.1 mAh/g and 117.4 mAh/g, and the capacity retention of 76.5 % and 70.5 %, respectively.

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Figure 6. The electrochemical performance of the as-prepared LVP/C composites over the potential ranges of (a) (g) 3.0 - 4.3 V and (h) - (i) 3.0 - 4.8 V: (a), (h) the first charge-discharge profiles at 0.1 C rate ; (b), (c) the zoom-in views of region (I), (II) in Figure 6a; (d) the differential capacity curves of the first cycle and (e) the zoom-in view of the third pair of peaks (4.02 - 4.10 V) in figure 6d; (f) rate performance tested at various cycling rates; (g),(i) cycling performance at 1 C rate.

To further determine the Li+ diffusion rate of each LVP-carbon composite, cyclic voltammetry (CV) was measured at different scanning rates (0.05, 0.1, 0.2, 0.5 and 1 mV/s) over the potential range of 3.0 - 4.3 V, Figure 7a - e. With the increasing scanning rate, the peak current increases, meanwhile the anodic and cathodic peaks shift to lower and higher potentials, respectively. As shown in the CV curves, all of the LVP-C cathodes undergo three complete phase transitions of Li3V2(PO4)3 to Li2.5V2(PO4)3, Li2.5V2(PO4)3 to Li2V2(PO4)3 and Li2V2(PO4)3 to Li1V2(PO4)3 at around 3.6/3.5 V, 3.7/3.6 V and 4.1/4.0 V46, respectively. Since the two CV peaks at 3.6/3.5 V and 3.7/3.6 V are too close to each other, they tend to merge into one peak at high scanning rate. It is impossible to extract accurate peak intensity from these two peaks. Therefore, the 4.0/4.1V peak was selected as an example for the calculation of Li+ diffusion coefficient in this work. 17



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A linear relationship between the 4.1/4.0 V redox peak current and the square root of different scan rates are depicted in Figure 7f, indicative of a diffusion controlled Li+ extraction/interaction process. Based on the Randles Sevchik equation (Eq. 1) for semi-infinite diffusion of Li+ into LVP, the apparent diffusion coefficients were calculated8.

I p = (2.69 105 )n 3/2 AD1/2 C*Li ν1/2

(1)

where Ip is the peak current (mA), n the number of electrons transferred per molecule, A the active surface area of the electrode (1.54 cm2), D the Li+ diffusion coefficient (cm2 s−1), ν the scan rate (V s−1), and C*Li is the concentration of lithium ions (1×10−3 mol cm−3). The slope of equation (1) (Ip/ν1/2) is plotted in Figure 7f. The apparent diffusion coefficients D of LVP, LVP/Glu, LVP/CA, LVP/GO and LVP/G are 2.4×10-9, 1.9×10-8, 1.8×10-8, 2.7×10-8 and 1.5×10-8 cm2 s−1 (Table S3), respectively. The apparent diffusion coefficient D of LVP/CA and LVP/GO is the biggest, which is in accordance with the best high rate electrochemical performance of them. The larger Li-ion diffusion coefficient of LVP/C composites than that of LVP among all the LVP samples implies the quicker lithium-ion transport and the better rate performance in carbon-coating LVP cathodes.

Figure 7. The CV test curve of different LVP/C composites at different scanning rate：(a) LVP. (b) LVP/Glu. (c) LVP/CA. (d) LVP/GO. (e) LVP/G. (f) Linear relationship between the redox peak current at 4.1 V/4.0 redox reaction and the square root of scan rate of different LVP/C composites.

The Relationship of LVP-Carbon Interfacial Structure and the Electrochemical Performances Since the LVP-carbon interface in the composite is directly derived from the LVP building units co-absorbed by the chelating agent and the carbon source (electronegative groups), it is reasonable to speculate that the interfacial structure is related to the strength of electrostatic force in 18


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precursor. The results of SEM, TEM and SSNMR have provided solid proofs for this hypothesis. In the following discussion, we are going to briefly summarize the composition and structure of LVP-carbon interface, and try to establish some correlation between these structural features and the electrochemical performance. First, the composition of the LVP-carbon interface is complex, especially for those composites synthesized with strong/medium carbon sources. For instance, Li3PO4 (the 0 ppm signals in both 7Li

and 31P NMR, also detected by XRD) tends to form at the LVP-carbon interface. The relative

content of Li3PO4 is most significant in LVP/CA and LVP/Glu, suggesting small molecule carbon source, i.e. citric acid and glucose, can effectively attract Li+ and PO43- to the LVP-carbon interface. Li3PO4 is a good Li+ conductor and widely applied as a protective coating for cathode materials47. Moreover, when strong carbon source (highly electronegative, small molecule) like citric acid is adopted, it is possible to form LixC intermetallic species (~ 70 ppm broad signal in 1D and 2D 7Li NMR) at the interface. Formation of Li-C mixing species can probably improve the “wettability” (Li+ and electronic conductivity) of LVP-carbon interface. Second, the composition and structure of carbon coating highly rely on carbon sources. The quality of carbon coating, including the carbon content, the degree of graphitization, the morphology and the distribution of carbon4, is a term widely adopted in literature. However, the degree of graphitization (sp2/sp3 ratio) has been taken by more and more researchers as the sole factor in evaluating the quality of carbon, while the contribution of sp3 carbon and amorphous carbon has been obviously underestimated. Our results have demonstrated that sp3 carbon with proper morphology also plays an important role in maintaining the electrochemical performance. Carbon coating formed from small molecule carbon sources can contain significant amounts of sp3 carbon. The sp3 carbon formed from glucose is disordered and associated with broad NMR resonances, while the sp3 carbon generated from citric acid is partially crystalline (diamond like) and associated with sharp 13C NMR signals. Presumably, recrystallization effect can take place on both LVP side and carbon side to form ordered (diamond like) sp3 carbon within amorphous carbon coating, when carbon source with strong electrostatic interaction like citric acid is adopted. Cheng et al.48 have demonstrated that the surface of nano-diamond is highly Li+ conductive and the nano-diamond itself is protective for the electrode. In addition, the sloping features related to the capacitive behavior of porous/activated carbon have been observed in the voltage profiles of LVP/CA, LVP/Glu and LVP/GO, suggesting the existence of porous carbon. According to our competition model, the carbon sources of these composites can compete with chelating agents and co-absorb on the LVP building units via negatively charged functional groups. CO2 generated from the decomposition of the chelating agent are expected to make amorphous carbon at the interface (mainly sp3 carbon derived from the carbon close to those electronegative groups) highly 19



porous and capacitive. This capacitive carbon layer can adsorb/ contain Li+, which buffers sudden change of Li+ concentration and decreases over-potential at higher charge-discharge rate. The schematic model of LVP-carbon interface is shown in Figure 8. As discussed above, the LVP particle is coated with three layers/components including the protective/wetting layer (Li3PO4, LixC and crystalline sp3 carbon), the capacitive/buffering carbon layer and the conductive carbon layer. The readers should bear in mind that this “three-layer” model is a highly simplified description about the composition of LVP/C composites, which does not necessarily reflect the complexity of the coating structure in real composites. The outstanding high rate performance of LVP/CA can be attributed to a synergistic effect of all three layers/components. The carbon coatings in both LVP/GO and LVP/G are electronic conductive graphene. The only difference is that LVP/GO composite possesses an additional capacitive carbon layer/component due to the electrostatic interaction between GO and LVP building units, which helps to maintain good Li+ conductivity at higher rates.

20


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Figure 8. The schematic illustration of the possible LVP-carbon interfacial structure of LVP/C composites. Please note that the aim of this picture is to roughly show all the possible species detected in this work and their relative positions to LVP. The exact species, amount and structure in each LVP/C composite can vary depending on the nature of its carbon source, and the interfacial structure in a real composite can be far more complex.

4. Conclusions A systematic study employing XRD, SEM, TEM, SSNMR and electrochemical measurements has been conducted to understand the formation mechanism and the structure of carbon-Li3V2(PO4)3 interface, as well as its impact on electrochemical performances. It has been demonstrated that carbon coating via sol-gel route is a process far more complex than simply putting carbon and active material into contact. The electrostatic interaction between the electronegative groups in the carbon source molecules and the LVP building units in the precursor plays a key role controlling the morphology of the final product and the structure of LVP-carbon interface. A specific model of LVP-carbon interfacial structure has been proposed to describe the interfacial structure and correlate it with the electrochemical performance. In this model, the carbon coating layer contains three components listed below: 1) A protective/wetting layer formed by species attracted to the interface by the strong attraction of –COOH groups in carbon source. It mainly contains Li3PO4 in most cases; and possibly LixC intermetalics and crystalline carbon due to recrystallization effect on the carbon side when a strong carbon source like citric acid was adopted. All these species are Li+ conductive, which protects LVP from electrolyte and provides a Li+ pathway between LVP and carbon. 2) A capacitive/buffering layer of porous carbon in close contact with LVP. The CO2 generated from the decomposition of chelating agent can make the carbon coating highly porous. This porous layer can absorb Li+ and significantly reduce the under potential caused by fast Li+ reinsertion during discharge. 3) A conductive layer of sp2 carbon formed by graphitization of the carbon sources, which plays the role of the fast electron pathway. This model of LVP-carbon interface provides guidance on the design and control of carbon-lithium transition metal phosphate interface and is of great significance for improving the electrochemical performance of cathode materials in lithium ion battery.

Supporting Information X-ray diffraction (XRD) patterns of LVP and LVP/C composites; The signal intensity of 13C NMR v.s. recycle delay plot; The 13C NMR spectra of empty rotor; The 7Li and 31P NMR 21



integral area percentages of samples; The Ip-ν1/2 fitting line slope of CV. The two-phase Rietveld refinement of of LVP/CA and LVP/Glu composites.

Conflicts of interest There are no conflicts to declare.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 21673065, 21403045, 21573013 and 91745202)

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It has been demonstrated that the types and concentrations of electro-negative functional groups in a carbon source play an important role in controlling not only the morphology of the product, but also the composition, the crystallinity and the micro-structure of Li3V2(PO4)3-carbon interface; and in turn the electrochemical behavior of Li3V2(PO4)3 composite 271x206mm (150 x 150 DPI)


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Figure 1 228x235mm (150 x 150 DPI)



Figure 2. 221x114mm (150 x 150 DPI)


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Figure 3. 219x224mm (150 x 150 DPI)



Figure 4. 147x296mm (150 x 150 DPI)


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Figure 5. 249x339mm (150 x 150 DPI)



Figure 6. The electrochemical performance of the as-prepared LVP/C composites over the potential ranges of (a) - (g) 3.0 - 4.3 V and (h) - (i) 3.0 - 4.8 V: (a), (h) the first charge-discharge profiles at 0.1 C rate ; (b), (c) the zoom-in views of region (I), (II) in Figure 6a; (d) the differential capacity curves of the first cycle and (e) the zoom-in view of the third pair of peaks (4.02 - 4.10 V) in figure 6d; (f) rate performance tested at various cycling rates; (g),(i) cycling performance at 1 C rate. 124x137mm (220 x 220 DPI)


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Figure 7. 334x192mm (150 x 150 DPI)



Figure 8. 138x196mm (150 x 150 DPI)


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Figure S1. 293x232mm (300 x 300 DPI)



Figure S2. The zoom-in review of the diamagnetic region (0-20 ppm) in the 31P NMR spectra of LVP and LVP/carbon composites. 267x230mm (300 x 300 DPI)


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Figure S3. The signal intensity of 13C NMR v.s. recycle delay (d1) plot 274x240mm (300 x 300 DPI)



Figure S4. The 13C NMR spectra of empty rotor acquired using Hahn-echo pulse sequence with a recycle delay of 0.5 s. 241x233mm (300 x 300 DPI)


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