Qiyuan Lia,
Zheng Wena,
Changling Fan*ab,
Taotao Zenga and
Shaochang Hana
aCollege of Materials Science and Engineering, Hunan University, Changsha 410082, China. E-mail: fancl@hnu.edu.cn
bHunan Province Key Laboratory for Advanced Carbon Materials and Applied Technology, Hunan University, Changsha, Hunan 410082, China
First published on 13th February 2018
A new cathode LiVPO4F/C with a high working voltage of around 4.2 V was synthesized by a novel one-step method. The color of the solution turns green, which implies that V2O5 is successfully reduced to V3+. The reaction thermodynamics indicates that LiVPO4F/C is formed when the sintering temperature is higher than 650 °C, while the accompanying impurity phase Li3V2(PO4)3/C is also generated. The reaction kinetics proves that the reaction is third order and the activated energy is 208.9 kJ mol−1. X-ray photoelectron spectra imply that the components of LiVPO4F/C prepared at 800 °C (LVPF800) are in their appropriate valence. LVPF800 is composed of micron secondary particles aggregating from nano subglobose. The structural transformation shows that the V:P:F ratio in LVPF800 is close to 1:1:1. The reason behind generation of impurity Li3V2(PO4)3 at a high temperature of 850 °C is demonstrated directly, which is mainly due to the volatilization of VF3. The electrochemical performances of the cathode are related to the crystallite content of LiVPO4F/C and Li3V2(PO4)3/C. The specific capacities at 0.2 and 5C of LVPF800 are as high as 139.3 and 116.5 mA h g−1. Electrochemical analysis reveals that LVPF800 possesses an excellent reversibility in the extraction and insertion process and minimum charge transfer resistance.
Several electroactive materials with a particular crystallite structure, such as spinel LiMn2O4, layered LiNi1−x−yCoxMnyO2 and olivine LiFePO4, are commonly used as cathodes for lithium ion batteries. However, they cannot entirely meet the demanding aspects of structural stability, power density and safety performance. In 2002, Barker et al.1 synthesized a new cathode LiVPO4F with the same structure as natural minerals KAlPO4F and LiAlPO4F. The LiVPO4F phase comprises a three dimensional framework building up from a PO4 tetrahedron and a VO4F2 octahedron. Its crystal structure is a triclinic system and the lattice parameters are as follows: a = 5.1687(2) Å, b = 5.3062(2) Å, c = 7.5031(3) Å, α = 66.856(2)°, β = 67.004(2)°, γ = 81.583(2)°. In addition, the cell volume is 174.21(1) Å3.2 The V3+/V4+ redox couple potential is 4.2 V, which is 0.3 V and 0.8 V higher than those of LiCoO2 and LiFePO4.3 It results from the element fluorine with the highest electronegativity of 3.98, which makes the PO4 tetrahedron and VO4F2 octahedron more stable.4,5 The heat flow,6 self-heating rate7 and energy releasing reaction with electrolyte8 show that the thermal stability of LiVPO4F is superior to LiMn2O4, LiNi1−x−yCoxMnyO2 and LiFePO4. Hence, triclinic LiVPO4F delivers the advantages of high working voltage, stable crystallite structure and excellent thermal stability and is a possible candidate to replace the above cathode.
However, the electronic conductivity of LiVPO4F is very poor. Currently, studies have been primarily focused on the preparation of LiVPO4F with excellent performances by developing new methods, such as carbon coating and metallic ions-doping.
The studies on this new LiVPO4F cathode are few. LiVPO4F/C has often been prepared by two-step carbothermal reduction, which comprises of two sintering processes at high temperature.9–11 The intermediate VPO4 is synthesized by heating precursors at 700–800 °C for several hours and the energy consumption is large. In order to synthesize LiVPO4F/C in an energy-saving manner, a one-step method was developed, in which the synthesis of VPO4 was omitted. One typical treatment is the so-called hydrogel method, which is performed by adding H2O2 solution to V2O5 (ref. 12) or melting V2O5 in a crucible at 700 °C.13,14 The other method is mixing the raw materials using an agate mortar, followed by magnetic stirring and ball milling.15–18
Although carbon black can improve the performances of LiVPO4F/C by enhancing conductivity, the excessive carbon content (20–50%) used to reduce V2O5 is the other drawback of the two-step method. The high content of residual carbon will decrease the tap density and volumetric energy density. To the best of our knowledge, in enhancing the conductivity of the LiVPO4F/C cathode, carbon black is inferior to organic carbon sources.19 The abovementioned one-step method decreases the carbon content by introducing oxalic acid. Carbon sources such as sucrose,13 multi-walled carbon nanotubes20 and graphene21–24 have been used to modify LiVPO4F/C through a form conducting network and enhance the rate and cyclic performances.
Compared to carbon coating, doping of metallic ions is also an effective method to improve the intrinsic conductivity and the rate performances of crystallite LiFePO4.25 The substitution of V3+ with Al3+,26–28 Ti4+,4,29 Mn4+ (ref. 30 and 31) and Y3+ (ref. 32) enhances the performance of LiVPO4F; however, most of these doped materials are prepared by the traditional two-step carbothermal method.
Although the one-step method is superior to the traditional two-step method, most approaches take several hours to complete the reduction of V5+. In this paper, we propose a novel one-step method, in which the synthesis of VPO4 is omitted. The reduction of V5+ in NH4VO3 at an elevated temperature in a short time was systematically investigated. The chemical reaction characteristics including thermodynamics and kinetics were studied. The transformation law of the triclinic LiVPO4F/C crystallite with the sintering temperature was researched and the electrochemical performances were analyzed.
The LiVPO4F/C cathode was charged and discharged at different rates by a BT2000 Battery test system in the potential range of 3.000–4.500 V (vs. Li+/Li). The electrochemical impedance spectra and cyclic voltammogram of the cathode were obtained using a CHI 660C electrochemical workstation. The amplitude of potential was 5 mV and the frequency range was from 100 kHz to 0.01 Hz. The scanning rate of cyclic voltammogram was 0.1 mV s−1.
First, after the addition of the reducing agent H2C2O4, the color of the NH4VO3 solution turns gradually from white to yellow (a), blend color (b), blue (c) and green (d). The mixed color in (b) is a blend of yellow, blue and green. The ion constituent and valence of vanadium are VO2+ (V5+), VO2+ (V5+) (coexisting with VO2+ (V4+)), VO2+ (V4+) and V3+. Then, the color of the solution is still similar to Fig. 1(d) after the addition of LiF, NH4H2PO4, PVDF and CTAB. The color of the dry precursor (e) is green. It is well known that V3+ ion is typically green in color. The binding energy peak in the XPS spectrum of the dry precursor is around 571.2 eV and 524.6 eV. This indicates that the valency of vanadium is 3+ according to literature.9 These observations indicate that raw NH4VO3 is reduced completely to V3+ by H2C2O4 in 10 minutes at 60 °C. Therefore, the general reaction takes place according to eqn (1).
NH4VO3 + H2C2O4 + 3H+ → V3+ + 3H2O + NH3 + 2CO2 | (1) |
The reduction time of NH4VO3 is shorter compared to the long-time ball milling in other one-step methods.24
It can be observed that several visible stages of weight loss and thermal exchange appear in the TG–DSC curve of LiVPO4F/C. The first endothermic peak at 104.4 °C represents the release of absorbed water in the raw materials;33 the mass loss is 5.37%. The other stage of weight loss (35.61%) takes place between 120 °C and 420 °C. The endothermic peaks at around 231.2 °C and 306.7 °C represent the decomposition of H2C2O4.17 The exothermic peak at 493.2 °C is attributed to the decomposition of NH4H2PO4.16 The exothermic peak at around 656.4 °C shows that LiVPO4F crystallite is formed initially. The broad peak at 734.2 °C (700–800 °C) represents the crystallization process of LiVPO4F. It is concluded that the triclinic system LiVPO4F is formed when the sintering temperature is higher than 650 °C.
It is well known that monoclinic Li3V2(PO4)3 can be synthesized by a carbothermal reduction method17 using raw materials Li2CO3, V2O5, NH4H2PO4 and carbon sources (such as epoxy resin). These are very similar to LiF, NH4VO3, NH4H2PO4 and H2C2O4 used in this study. High valence V2O5 or NH4VO3 are reduced to +3 either by pyrolysis carbon from epoxy resin19 or H2C2O4 in this study. The most evident difference is the existence of F− in LiF and the reducing agent H2C2O4 in this study. The TG–DSC analysis of the dry precursor for Li3V2(PO4)3/C was performed, which was made by grinding the abovementioned mixture in a planetary mill. The endothermic peak at 269.1 °C arises from the decomposition of NH4H2PO4. The exothermic peak at around 572.9 °C represents the formation of Li3V2(PO4)3 according to eqn (2).
3Li2CO3 + 2V2O5 + 6NH4H2PO4 + 4C → 2Li3V2(PO4)3 + 9H2O + 6NH3 + 3CO2 + 4CO | (2) |
From the above analysis, it can be found that LiVPO4F will be synthesized successfully by the one-step method proposed in this paper. However, Li3V2(PO4)3 impurity is also generated in this process. Therefore, the growth of Li3V2(PO4)3 should be restrained in the synthesis of the high-purity LiVPO4F/C cathode.
After comparing with the standard pattern (shown in the bottom in Fig. 3), we found that the sample LVPF650 is made up of target LiVPO4F and impurity Li3V2(PO4)3. This proves that LiVPO4F is successfully prepared in this condition. However, the diffraction peaks are not strong enough and the crystallinity is low. In addition, this sintering temperature, used to prepare the sample LVPF650 is in the range of exothermic peaks at around 656.4 °C. Moreover, the strongest peak of impurity Li3V2(PO4)3 at 20.551° is visible. This is in accordance with the prediction based on reaction thermodynamics. It is concluded that the thermodynamics investigation is credible.
Upon increasing the sintering temperature, the intensity of the strongest peak of LiVPO4F at 26.978° increases gradually. On the contrary, the peak intensity of Li3V2(PO4)3 at 20.551° decreases. This infers that the crystallite LiVPO4F becomes increasingly perfect and its content also increases, accompanied with the drop in the content of the impurity. From the thermodynamics analysis, we know that the exothermic temperature of Li3V2(PO4)3 (572.9 °C) is lower than that of LiVPO4F, which implies that Li3V2(PO4)3 is easier to be prepared. Therefore, it is understandable that the formation of impurity Li3V2(PO4)3 accompanies the synthesis of LiVPO4F. When the sintering temperature reaches 800 °C, there is nearly no impurity Li3V2(PO4)3 in the LiVPO4F phase. Nonetheless, the impurity peak of Li3V2(PO4)3 appears again when the temperature rises to 850 °C.
It can be observed that the diffraction angle and peak intensity of the sample LVPF800 are very close to the standard pattern of LiVPO4F in the bottom. Also, its crystallite structure is a typical triclinic system. It is a three-dimensional network, which is built from PO4 tetrahedron and VO4F2 octahedron. Its thermal stability is excellent due to the strong covalence between phosphate and oxygen. The main peak (0–11) is sharp and symmetric, which shows that the crystallite structure is perfectly developed. The lattice parameters a, b, c and cell volume are 5.1672 Å, 5.3060 Å, 7.2893 Å and 173.97 Å3, respectively, which are similar to those obtained in previous studies.27,30
According to Scherer's equation, the crystallite sizes of LiVPO4F in the face of (0–11) at around 26.978° prepared at 650, 700, 750, 800 and 850 °C are 19, 21, 23, 31 and 40 nm, respectively. The crystallite size increases significantly. The crystallite size of LiVPO4F prepared at 800 °C is smaller than those reported in other literatures.12,34 It is concluded that this sample possesses a well-developed crystallite structure and moderate size.
In order to determine the activation energy, the degree of difficulty and reaction order of the preparation of the LiVPO4F/C cathode, the reaction kinetics was investigated from the viewpoint of the classical Arrhenius equation. The reactant concentration and chemical reaction rate constant of the synthesis reaction of LiVPO4F/C were calculated on the basis of the mole fractions of the resultant LiVPO4F/C, which were determined indirectly using Jade 6.0 software according to the XRD patterns shown in Fig. 3. After the calculation, we find that the mole fractions of the samples LVPF650, LVPF700, LVPF750, LVPF800 and LVPF850 are 0.7475, 0.8382, 0.9285, 0.9605 and 0.9133, respectively.
It can be noted that the mole fractions of LiVPO4F/C are very low when the sintering temperature is lower than 750 °C, which are 0.7475 (650 °C) and 0.8382 (700 °C). This indicates that there is a large amount of impurity Li3V2(PO4)3/C in the resultant, that is to say, the competition reaction of the formation of Li3V2(PO4)3/C is obvious. Therefore, low sintering temperature is not beneficial to the preparation of LiVPO4F/C. The mole fraction of LiVPO4F/C increases to 0.9285 and 0.9605 when the temperature rises to 750 and 800 °C. The mole fraction of LiVPO4F/C does not increase, but decreases to 0.9133 when the temperature increases to 850 °C. In other words, the content of impurity Li3V2(PO4)3/C increases again.
The Arrhenius equation, which is often used to discuss the kinetics of chemical reactions, was applied herein to investigate the kinetics of LiVPO4F/C prepared by the one-step method. The condition of 850 °C was eliminated in the subsequent analysis due to its inverse changing of mole fraction.
The initial reactant concentration (CA,0) was 0.2 mol L−1, in which 0.02 mol of reactant NH4VO3 was dissolved in the 100 mL reaction system of. Assuming that if x percent reactant was consumed at a given temperature, the reactant concentration (CA) would be 0.2(1 − x) mol L−1. The rate constants (kA) of the preparation reaction of LiVPO4F/C were calculated according to eqn (3)–(6) when the reaction order was supposed to be zero, first, second and third. In these equations, t is the reaction time and its unit is second (s).
Zero order,
CA,0 − CA = kAt | (3) |
First order,
ln(CA,0/CA) = kAt | (4) |
Second order,
1/CA − 1/CA,0 = kAt | (5) |
Third order,
(1/CA2 − 1/CA,02)/2 = kAt | (6) |
The Arrhenius eqn (7) was used to investigate the relationship between the rate constant (kA) and the absolute reaction temperature (T). Ea is the activation energy of the reaction and R is the universal gas constant. C is the pre-exponential factor. The Arrhenius equation can be expressed in the format of eqn (8). The plot of lnkA vs. 1/T was plotted at different supposed reaction orders. 1/T is transformed to 1000/T in order to make the scale of abscissa more appropriate. The results are shown in Fig. 4. As expected, the obtained plots linear and the slope can be calculated from linear fitting. Therefore, we can obtain the activation energy from eqn (9)
lnkA = −Ea/RT + C | (7) |
Y = A × X + B | (8) |
Ea = −A × R | (9) |
Fig. 4 Dependence of lnkA on 1000/T at supposed reaction orders, zero (a), first (b), second (c), third (d). |
It can be noted that lnkA fits well with 1000/T in different orders of reaction. Their coefficients are greater than 0.9. The estimated activated energies at the zero, first, second and third order reactions are 10.1, 46.0, 116.8 and 208.9 kJ mol−1, respectively. The coefficient of the third order reaction is as high as 0.9743, which is larger than that of the other orders. This indicates that the preparation process of the LiVPO4F/C cathode is very close to the third order reaction, which is a complex procedure.
From the viewpoint of chemical reaction, the activation energy is in the range of 60–250 kJ mol−1.35 The reaction activation energy of LiVPO4F/C is near to the upper limit. According to the classical activation theory, the consumed energy is very large and the synthesis reaction will be difficult. Therefore, it can be predicted that the reaction mechanism of LiVPO4F/C in this study is complex. It contains of not only the conversion of the initial reactant to the resultant LiVPO4F/C, but also the crystallinity transformation from low degree to high degrees.
When the sample is sintered at 800 °C, a large amount of subglobose nanoparticles are generated, which accumulate to form secondary larger particles with micron size. This conforms to the opinion that the primary particles of the cathode may be nanosized and the secondary particles must be micron sized. The nanoparticles enable the rapid extraction and insertion of lithium ions. We can realize its excellent rate performances when it is used as a cathode for lithium ion batteries. However, nanoparticles cannot be packed tightly and this will lower the tap density and volumetric energy density of the cathode. This is a typical feature of nanomaterials, where the tap density of acetylene black (d50 = 50 nm) is about 0.2 g cm−3. The aggregation of nanoparticles and the formation of micron particles in the sample LiVPO4F/C can improve the tap density and the volumetric energy density. This is the advantage of the particles in the sample LVPF800 obtained by the one-step method in this study.
It can be clearly observed that numerous thin layers of pyrolytic carbon are coated on the particles' surface of LVPF800 (Fig. 5(e)). The residual carbon content in its superficial layer is 51.09 wt%, which is much greater than the bulk content of 11.76 wt%. Therefore, its electronic conductivity is as high as 0.21 S cm−1. A visible peak belonging to fluorine can also be found in the EDS pattern of the sample LVPF800. This indicates that there is a certain content of elemental fluorine. The atomic percentages of V, P and F are 3.08%, 3.05% and 3.12%, which shows that the mole ratio of V:P:F is very close to the stoichiometric value of 1:1:1.
The atomic content and ratio of F:V in the above samples are illustrated in Table 1. It is known that the content of fluorine in the sample represents the content of LiVPO4F crystallite. The content of fluorine and the ratio of F:V are very low when the sample is prepared at 650 °C. This implies that the content of the LiVPO4F crystallite is low at this condition. On increasing the sintering temperature, the content of LiVPO4F crystallite increases due to the gradual increase in the content of fluorine and F:V. It should be noted that the content of phosphate is much higher than that of vanadium. This is because the ratio of P:V in Li3V2(PO4)3 is 1.5:1. The content of LiVPO4F crystallite in sample LVPF800 attains the largest value owing to the highest value of F:V (1.013) when the sintering temperature reaches 800 °C. These analyses are in accordance with the XRD results. It is noted that there is a difference between the ratio of F:V and the real content of LiVPO4F crystallite because EDS is only determined from the superficial layer and not from the bulk of the samples.
Samples | V (%) | P (%) | F (%) | F:V |
---|---|---|---|---|
LVPF650 | 4.23 | 5.76 | 0.95 | 0.225 |
LVPF700 | 3.86 | 5.36 | 1.06 | 0.275 |
LVPF750 | 3.76 | 4.56 | 1.31 | 0.348 |
LVPF800 | 3.08 | 3.05 | 3.12 | 1.013 |
LVPF850-solid | 7.39 | 7.23 | 4.95 | 0.670 |
LVPF850-dot | 8.32 | 12.29 | 0.36 | 0.043 |
When the sintering temperature increases to 850 °C (Fig. 5(g) and (h)), great changes take place in the morphology of the particles. A great deal of particles with large size and smooth surface appear and simultaneously, a large number of primary nanoparticles disappear. From the XRD patterns, we found that the crystallite size increases clearly from 31 nm (LVPF800) to 40 nm (LVPF850). The XRD analysis proves the formation of impurity Li3V2(PO4)3. For an in-depth analysis, the EDS analysis is conducted at two typical zones in the SEM images of sample LVPF850 and the results are presented in Fig. 5(i) and (j) and Table 1.
It can be noted that the peak attributed to fluorine in primary nanoparticles in the solid zone is visible, indicating the presence of LiVPO4F/C. However, the atomic ratio of F:V is 0.670, which is much lower than that of sample LVPF800. To our surprise, there is nearly no fluorine peak in the dot sample. The atomic content of fluorine in the dot sample is 0.36% and the ratio of F:V is only 0.043. This implies that there is nearly no LiVPO4F crystallites on the surface of large particles.
It is concluded that the sintering temperature plays a vital role in the preparation of sample LiVPO4F/C. The temperature of 800 °C is proved to be an ideal condition to synthesize LiVPO4F/C with well-developed crystallites.
3LiVPO4F → Li3V2(PO4)3 + VF3 | (10) |
The volatilization phenomenon exists in the solid zone, as shown in the Fig. 6(b), and drops the ratio of F:V in the nanoparticles, which results from the generation of Li3V2(PO4)3. These are similar to the XRD analysis, where impurity Li3V2(PO4)3 is formed again in sample LVPF850. However, the formation of impurity at 850 °C is rather different from the sample when the sintering is lower than 800 °C. These can be illustrated by the Scheme 1. When the temperature is between 650 and 800 °C, it is more beneficial to the crystallization of the LiVPO4F. Li3V2(PO4)3 is the accompanying phase because its formation temperature is much lower than LiVPO4F according to the thermodynamics analysis. However, when the temperature rises to 850 °C, the decomposition of the LiVPO4F becomes the dominant reaction compared to the formation of LiVPO4F; hence, more Li3V2(PO4)3 impurity is formed. This also proves that fluoride is not stable at high temperature.
Therefore, it is concluded that the formation mechanisms of impurity Li3V2(PO4)3 at different sintering temperature ranges are quite different. Moreover, there are significant differences in their crystallite structure. This can also be explained by the charge and discharge performance analysis, which were conducted subsequently. The partial curves belonging to impurity is smooth when the sintering temperature is lower than 800 °C. However, the corresponding curve in sample LVPF850 is not smooth but slant.
To further observe the morphology and micro structure of the samples, the typical TEM and HRTEM images of the sample LVPF800 and LVPF850 were recorded (Fig. 6). It can be observed that the primary particle size of sample LVPF800 and LVPF850 is about 200 nm and 500 nm. Moreover, we can clearly observe that each particle is coated by a non-uniform pyrolytic carbon layer and a core/shell structure is formed. The thickness of the carbon layer around the crystallite is about 3 nm. There is regular diffraction lattice in sample LVPF800. The layer distance is 0.447 nm. Its SAED pattern is a typical parallelogram, which is the characteristic of triclinic LiVPO4F/C. In case of sample LVPF850, there are two sets of diffraction lattice with different layer distances of 0.481 nm and 0.343 nm. Their SAED images are also rather different. Excluding the similar parallelogram pattern belonging to LiVPO4F/C, the other SAED pattern is attributed to the monoclinic Li3V2(PO4)3 impurity. This indicates that there are not only LiVPO4F crystallites, but also Li3V2(PO4)3 crystallites in the sample LVPF850.
It can be found that the binding energy peaks at around 517.2 eV and 524.6 eV are attributed to V 2p3/2 and V 2p1/2, respectively. This indicates that the valence of vanadium ion is +3.9,37 It is inferred that high valence vanadium was reduced effectively to low valence in the reaction. It is clear that the one-step method uses less carbon than the two-step method. This helps to inhibit the harmful effect of excessive carbon black in the cathode material, which is usually used to reduce V5+ at high temperature in the two-step method.
The binding energy peak at 133.8 eV is nearly the same as the P 2p3/2 of PO43−.38 However, it is different from the binding energy of VP (129.1 eV).39 This indicates that there is only one valence state of phosphorus. In addition, it shows that the reduction strength of H2C2O4 at 60 °C is moderate and it does not reduce P5+ in PO43− to P3+. Therefore, the PO43− anion can maintain its stability and the further reduction to VP does not occur. However, a similar substance Fe3P is formed sometimes in the preparation of LiFePO4.40 The binding energy peak of element fluorine at 1s is 685.7 eV, which is very near to 685.9 eV of LiF,41 which is rather different from that of the C–F bond (689.4 eV) in PVDF.42 This implies that F− exists in sample LVPF800 and C–F bonds in PVDF are entirely ruptured.
Furthermore, the carbon binding energy peak is closely attributed the highly oriented pyrolytic graphite (284.8 eV),43 which is a type of pyrolytic carbon. It is also far away from the carbon in H2C2O4 (289.9 eV) and PVDF (689.4 eV).42,44 This indicates that the carbonaceous materials are completely pyrolyzed and pyrolytic carbon with high conductivity is formed. This is beneficial to the formation of an excellent conductive network in the LiVPO4F/C cathode.
It can be concluded from the above analysis that the elemental components in LiVPO4F/C are in their appropriate valence. Therefore, triclinic LiVPO4F/C is successfully synthesized.
Fig. 8 Initial discharge curves of samples at 0.2C (a), 1C (b) and the curve of sample LVPF800 at 5C (dash line in (b)). |
Moreover, the other three plateaus are attributed to the formation of impurity Li3V2(PO4)3/C.46 It should be noted that these plateaus are short and the specific capacity of LVPF650 is only 92.1 mA h g−1, which is much lower than the theoretical capacity of LiVPO4F (156 mA h g−1) and Li3V2(PO4)3 (132 mA h g−1).2,19 This implies that the above two crystallites are far away from the well-developed structure.
The discharge plateau can remain stable when the temperature reaches 700 °C. Furthermore, the section of plateau belonging to LiVPO4F/C is elongating and the specific capacity increases to 102.1 mA h g−1. This indicates that LiVPO4F crystallite becomes increasingly regular and the crystallinity degree is also enhanced. When the sintering temperature reaches 750 °C, the plateau of LiVPO4F/C at around 4.2 V is lengthened and the corresponding capacity increases to 74.6 mA h g−1 compared to the samples LVPF650 (43.8 mA h g−1) and LVPF700 (59.9 mA h g−1). The plateaus (I, II and III) belonging to impurity decrease evidently. This is directly related to the dropping of the mole fraction of Li3V2(PO4)3/C in LiVPO4F/C. From the viewpoint of electrochemical reaction, it can also be concluded that Li3V2(PO4)3/C is the accompanying phase in the preparation of LiVPO4F/C, which is already proved in the former analysis.
There is only one discharge plateau at 4.2 V in sample LVPF800 and the plateaus of Li3V2(PO4)3 vanish entirely. The discharge plateau is very stable and the discharge capacity (139.3 mA h g−1) is close to the theoretical capacity of LiVPO4F. It should be noted that there is 11.76 wt% pyrolytic carbon, which is electrochemically inactive and will not contribute to the discharge capacity of LiVPO4F/C. If the content of residual carbon is eliminated, the discharge capacity of LiVPO4F crystallite in sample LVPF800 is about 157.9 mA h g−1 and is even higher than the theoretical capacity. This results from the nonuniform distribution of carbon in the sample. However, the inert pyrolytic carbon is indispensible to the LiVPO4F/C cathode. It should be noted that pure LiVPO4F is nearly an insulator owing to its poor conductivity (10−11 S cm−1).47 The charge and discharge experiments of pure LiVPO4F without carbon cannot be conducted and the specific capacity is very low. However, the conductivity of LVPF800 rises to 10−3 S cm−1 as it changes from insulator to semiconductor. Only in this condition, it delivers a satisfactory electrochemical performance.
The reversible electrochemical redox reaction in the charge and discharge processes of the LiVPO4F/C cathode take place according to the following eqn (11).
LiV3+PO4F ↔ Li1−xV4+PO4F + xLi+ + e− | (11) |
The reaction is related to the redox couple of V3+/V4+, i.e., the extraction and insertion of lithium ions from or into triclinic LiVPO4F crystallite. In the full discharged state, i.e., when the extraction degree of lithium ions is 100%, the lattice of VPO4F formed is near to that of LiVPO4F and the change in volume is only 3%. This implies that the lattice structure is rather stable. The covalent bond between stable PO43− and the strongest electronegativity element F is much greater than that of other cathodes. This improves the insertion potential of lithium ions to 4.2 V, which is superior to LiMn2O4, LiNi1−x−yCoxMnyO2 and LiFePO4.2,6
However, the plateau of sample LVPF850 at around 4.2 V becomes short and the plateau of impurity Li3V2(PO4)3/C reappears. This is due to the evaporation of fluorine according to eqn (10) and it is different from the samples prepared at 650, 700 and 750 °C. The discharge curve of impurity becomes slant because the crystallite is imperfect and the degree of crystallinity is low.
The rate performances of the LiVPO4F/C samples enhanced with an increase in the sintering temperature and most excellent performances are observed at 800 °C. Then, it drops significantly at 850 °C and it is even worse than that at 650 °C. The discharge capacity of sample LVPF800 at 1C is 130.3 mA h g−1, which is much larger than other samples. The capacity at 5C is 116.5 mA h g−1, which proves to be an excellent value compared with the literature.31
It is noted that the residual carbon in LVPF800 is 11.76 wt%. It is much lower than the traditional two-step reduction, in which the addition of carbon is about 20–50 wt%.4,12,31 Assuming that if the structure, real density and tap density of pyrolytic carbon and LiVPO4F in this study are similar to those reported in literature and the tap density of LiVPO4F/C is primarily determined by the content of residual carbon with low density, it is understandable that the tap density of LVPF800 will be evidently higher than the samples prepared by the two-step method.
The volumetric energy density (VED), which is an important index for the cathode in lithium ion batteries, can be calculated from eqn (12) and (13). In the equations, the symbols W, V, I, T, Q, U, D, m, c, ρ and A correspond to work, volume, current intensity, time, capacity, voltage, specific discharge capacity, mass, carbon content percentage, density and coefficient, respectively.
VED = W/V = UIT/V = UQ/V = UDm(1 − c)/(m/ρ) = ρUDm(1 − c)/m = Aρ(1 − c) | (12) |
VED = Aρ(1 − c) | (13) |
For the sake of simplicity, the volume of the cathode active material is applied to substitute the volume of batteries. Assuming that the voltage and discharge capacities of the samples prepared by the one-step or two-step method are nearly the same, it can be found that the volumetric energy density of LVPF800 will be significantly larger than that prepared by the traditional two-step method. This is also the advantage of the one-step method used in the study.
The electrochemical impedance spectra of the LiVPO4F/C samples are illustrated in Fig. 9(a and b). They have been fitted according to the given equivalent circuit in the figure and the results are listed in Table 2.
Samples | Rct (Ω) | i0 (mA) |
---|---|---|
LVPF650 | 234.6 | 0.071 |
LVPF700 | 183.9 | 0.091 |
LVPF750 | 122.3 | 0.137 |
LVPF800 | 96.8 | 0.172 |
LVPF850 | 378.1 | 0.044 |
It can be noted that there are only depressed semi-circles in the high frequency region and an inclined line in the low frequency region. The charge transfer resistance, Rct, in sample LVPF650, which represents the transfer process of charge, is very large (234.6 Ω). With an increase in sintering temperature, the resistance of the charge transfer process decreases gradually and reaches a minimum value of 96.8 Ω in sample LVPF800. This implies that it is charge-transfer occurs easily in sample LVPF800.
However, the transfer charge resistance of sample LVPF850 rises to 378.1 Ω, which is nearly three times larger than that of sample LVPF800. From the SEM images, we can observe that there are impurity Li3V2(PO4)3/C particles with large size. It is well known that the larger the particle size of the cathode, the slower will be the diffusing speed of charge. Therefore, it is understandable that the charge-transfer resistance of sample LVPF850 is much higher than that of sample LVPF800.
It can be noted that the variation in exchange current i0 is similar to that in rate performances, which is inverse to the charge transfer resistance according to eqn (14).
i0 = RT/nFRctS | (14) |
It is known that the larger the exchange current, the better will be the rate performance. The exchange current of sample LVPF800 is 0.172 mA, which is larger than other samples. Thus, we can understand that the rate performances of sample LVPF800 are excellent. The electrode kinetics parameters make an appropriate interpretation of the rate performances of cathode LiVPO4F/C.
The cyclic voltammogram of sample LVPF800 at the scanning speed of 0.1 mV s−1 is presented in Fig. 9(c). It can be observed that there is one pair of oxidation (charge) and reduction reaction (discharge) peaks in the figure. This can be interpreted by the reaction presented in eqn (11). The peak shape is symmetric and the intensity is high. The peak potentials are 4.377 V, and 4.145 V and the potential gap is as low as 0.232 V. This indicates that the reversibility in the electrochemical reaction in sample LVPF800 is very high. Thus, the charge and discharge reactions will be completed in a short time and rate performances will be satisfactory.
The cyclic performances of sample LVPF800 at 0.5C and 5C are illustrated in Fig. 9(c). From the insertion discharge curves, we can find that the discharge profiles of sample LVPF800 do not transform evidently after the 50 charge and discharge cycles at each rate of 0.5C or 5C. The profiles can also remain stable and the potential plateaus are very smooth. The only difference is that the capacity decreases slightly. It can be found that the discharge capacity at 0.5C is 135.3 mA h g−1, which decreases to 129.2 mA h g−1 in the 50th cycle. The specific capacity at 5C drops slowly from 116.5 mA h g−1 (51th cycle) to 114.0 mA h g−1 (100th cycle). The capacity retentions at 0.5C and 5C are 95.49% and 97.85%. Simultaneously, the charge and discharge efficiencies at 0.5C and 5C are clearly greater than 98%. Sample LVPF800 delivers an excellent reversibility of the crystallite structure in the extraction and insertion of lithium ions. Therefore, it is concluded that the sample LVPF800 synthesized by the novel one-step method in this study possesses excellent cyclic performances.
The one-step method is more energy saving than the traditional two-step carbothermal reduction. This is because it contains only one section of high temperature treatment. Moreover, the content of the residual carbon is low because the high valence vanadium was already reduced to low valence by chemical reduction. It is reduced in a PTFE beaker by magnetic stirring and this does not bring metal impurity from stainless steel media in the ball milling procedure. This is beneficial to the electrochemical performances of the cathode.
The reaction thermodynamics and kinetics of this method were investigated. The activation energy is 208.9 kJ mol−1. The transformation behavior of the crystallite and structure were investigated. The elements in LVPF800 are in their appropriate valence. The impurity Li3V2(PO4)3/C is the accompanying phase and can only be eliminated by maintaining the sintering temperature at 800 °C. The as-obtained sample LVPF800 exhibits excellent rate and cyclic performances. This method possesses an impressive future in the synthesis of LiVPO4F/C.
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