Ⅲ.Material Issues in Lithium-Ion Secondary Batteries
Human beings currently have paid much attention to the energy issue, and the studies of lithium ion batteries have thus become an essential subject. Additionally, the portable electronic devices, such as cellular phone, personal computer (PC), MP3 players, and iPod, are now widely used. These applications require a high energy density of LIBs and consumers desire smaller, lighter, longer-running products. As compared to other secondary batteries, such as lead acid batteries, secondary alkaline batteries, nickel cadmium batteries, and nickel hydride batteries, lithium ion batteries (LIBs) exhibit attractively outstanding properties of (1) high working voltage, (2) good cycling life, (3) high gravimetric density and volumetric energy density, (4) quick charge acceptance, and (5) no memory effect. Therefore, LIBs are extensively used for 3C electronic devices, electric vehicles (EVs) and hybrid-electric vehicles (HEVs).
The performance of a Li-ion battery depends on many factors. The critical issues are the electrode materials and their evolution during cycling. During the cycling process, Li ions are migrated between the cathode and anode, while the intercalation compounds undergo the structural and electronic changes. Therefore, with respect to the application, the identification of the material evolution inside the battery is very important.
The most common commercialized LIB consists of a graphite anode, a LiCoO2 cathode and a separator soaked with a liquid solution of a lithium salt, LiPF6, in an organic solvent mixture. However, due to the high cost and toxicity of LiCoO2, other cathode materials have been investigated to replace it. Besides, in order to improve the structure stability of LiCoO2, surface modification is also executed. Moreover, reducing the particle size of powder was an efficient way to enhance the cycling performance in high C-rate. On the other hand, several anode materials have been synthesized to substitute graphite. Sn-based anode material is a promising material, owing to the high theoretical capacity. Nevertheless, overcoming the large volume change of anodes during cycling is a critical issue to enhance the electrochemical property of Li-ion batteries.
Many cathode materials in lithium ion battery have been developed in out lab since 2000, including 1D channel LiFePO4, 2D channel LiCO2, LiNi0.6Co0.4-xMnxO2 and 3D channel LiMn2O4, LiNi0.5Mn1.5O4. Since the capacity of these LIB materials fades dramatically due to instable structure during charge/discharge test, improving the stable structure of LIB is major issue. Besides, developing Sn-based anode materials is another task. The particle size of anodes is also reduced to further improve the cycle life of related-materials in high C-rate.
1. Present Study
1.1 The development of cathode materials in Li-ion batteries
1.1.1 Synthesizing nano-sized LiNi0.5Mn1.5O4 cathode materials by modified solid-state method
Lithium ion battery will possibly be applied in hybrid electric vehicles in the future, so high-energy, high-power and long-life energy sources are required. 5V cathode materials LiNi0.5Mn1.5O4 are developed to promote its capacity, cycling life at high temperature and the most important security issues now. LiNi0.5Mn1.5O4 can exhibit a promising electrochemical performance among these 5 V cathode materials, in which both Jahn-Teller distortion and dissolution of Mn are greatly alleviated. In addition, it could deliver the highest capacity above 4.5 V. Today, in pursuing a high power and energy LIB, the major goal is to synthesize impurity-free LiNi0.5Mn1.5O4 powder with high crystallinity, high initial capacity and favorable rate capability. This study has developed a modified solid state method at reduced fabricated temperature with uniform mixing. The electrochemical properties of LiNi0.5Mn1.5O4 at RT and HT (55 ℃) will be discussed. In order to achieve the demand of high power output in LIBs, rate capability of electrodes should be essentially enhanced. As the particle size of Li insertion materials is smaller, the diffusion length is shorter, and hence the rate capability of cathode materials is expected to be improved, due to the faster kinetics. In this study, the diffusion length is reduced through controlling particle size by adding different amount of H2C2O4, and the electrochemical properties of nano-sized LiNi0.5Mn1.5O4 at high cycling rate will be discussed.
1.1.2 Phase transition and oxidation state by in situ X-ray technique and electrochemical characterization of LiNi1-x-yCoyMnxO2 cathode material for Li-ion batteries
The most common cathode material in commercialized LIB is LiCoO2. However, due to the high cost and toxicity of LiCoO2, other cathode materials have been investigated to replace it. LiNiO2 with the iso-structure of LiCoO2 is one of the most attractive materials with the lower cost and higher discharge capacity. Nevertheless, LiNiO2 are found to possess unstable cycling performance, especially cycled at high C-rate. To overcome the instability, one way is the substitution Ni with other transition metals. A one-to-one solid solution of LiNiO2, LiCoO2, and LiMnO2, i.e. LiNi1/3Co1/3Mn1/3O2, with an α-NaFeO2 structure was investigated. With the change of Co and Mn substitution concentrations, new type layer-structured compounds Li(Ni1-x-yCoyMnx)O2 have been developed. In our lab, the Ni1-x-yCoyMnx(\(\(\(OH)2 precursor was obtained by the chemical co-precipitation method. Ni1-x-yCoyMnx(\(\(\(OH)2 and Li2CO3 were thoroughly mixed and then calcined at elevated temperature in the flowing oxygen atmosphere to obtain the pure LiNi1-x-yCoyMnxO2 cathode material. The initial discharge capacity of LiNi0.6Co0.25Mn0.15O2 was 178mAh/g and the capacity retention was 98.1% after 20 cycling tests. It is demonstrated that LiNi1-x-y CoyMnxO2 electrode should exhibit great potential for the future application in lithium-ion battery cathode.
For further improving the electrochemical performance and thermal stability, magnesium was chosen as the dopant in Li[Ni0.6-yMgyCo0.25Mn0.15]O2 cathode materials. LiNi0.6-yMgyCo0.25Mn0.15O2 (y=0~0.08) were successfully synthesized via the mixing hydroxide method. These materials exhibited α-NaFeO2 structure as indicated by the XRD patterns. The intensity ratio of (003) to (104) showed that Mg substitution could reduce the cation mixing. The electrochemical performance was also improved in both room temperature and 550C.
1.1.3 Synthesis of the LiFePO4/C cathode material by the solid-state method and associated electrochemical properties
LiFePO4 currently become the most promising material among all the cathodes, owing to the great thermal stability, good cycling performance, cheapness, and environmental friendliness. Besides, the theoretical capacity is around 170mAhg-1. Therefore, LiFePO4 can be applied to EV/HEV market. However, the poor conductivity would limit the commercialization of LiFePO4. In order to improve the conductivity of LiFePO4, the LiFePO4 powder was synthesized at 750 oC by the solid-state method. LiFePO4/C composites with glucose as the carbon source were also synthesized from 650 oC to 800 oC. The carbon coating method could improve the conductivity of LiFePO4cathode materials, and also enhance the retention and capacity of LiFePO4. The Fe2P impurity could improve the conductivity of LiFePO4/C composites. The polarization of LiFePO4/C with appropriate amount of Fe2P is reduced at a higher current rate. However, large amount of Fe2P impurities could prohibit the pathway of lithium ions of LiFePO4/C, causing the decrease in the capacity of LiFePO4/C at a higher current rate. Therefore, the content of Fe2P plays an important role in cycling performance. The amount of Fe2P was evaluated through Rietveld refinement method.
1.1.4 Synthesis of high voltage cathode material LiNi0.5Mn1.5O4 and improvement of its electrochemical performance
It is observed in literature that only LiNi0.5Mn1.5O4 can exhibit a promising electrochemical performance among these 5V cathode materials, since Jahn-Teller distortion and dissolution of Mn are greatly alleviated. In addition, it could deliver the highest capacity above 4.5V with an inappreciable voltage plateau at 4V among all cathode materials. In recent studies, it has been demonstrated that LiNi0.5Mn1.5O4 cathode material exhibited high capacity about 120-140mAhg-1, which is quite close to the theoretical value at 146mAhg-1. For this reason, LiNi0.5Mn1.5O4 is nowadays one of the most attention-focused materials. Furthermore, to increase the power capability and to meet the requirement of new generation LIBs for EV/HEV, LiNi0.5Mn1.5O4 is expected to be capable of fast charge/discharge. In this study, single phase LiNi0.5Mn1.5O4 powders with high current rate capability were successfully produced via a novel synthesizing procedure combining sol-gel route with combustion method. Through this approach, well-crystallized LiNi0.5Mn1.5O4 powders with nano-structure were obtained at a low temperature of 600oC. Nano-structured LiNi0.5Mn1.5O4 exhibited superior electrochemical performances. At a slower discharge rate, a capacity of 133mAhg-1 could be delivered. Moreover, the nano-structured LiNi0.5Mn1.5O4delivered a capacity of 122mAhg-1 at a discharge rate as high as 10C.
1.2 The development of anode material in Li-ion batteries
1.2.1 Synthesizing nano-sized Sn/C core-shell composite anode materials
In commercial lithium-ion batteries, the carbonaceous materials are generally used as anode materials, which only provide a theoretical capacity of 372mAhg-1. The metallic tin has been used as the anode material, since both the theoretical charge densities and specific charges of Li-Sn alloys are higher than those of the commercial lithiated graphite. The most critical problem of Sn-based anode materials is the severe volume expansion and contraction during the charge and discharge processes. Because of the huge volume change of LiXSn alloys, Sn metal particles are pulverized to lose its Li+ storage ability eventually. To solve this problem, further researches were devoted to prepare the Sn particles in nano-sized to reduce the absolute volume change during cycling and thus to improve the electrochemical property. Moreover, Sn/C composite anode material was investigated to enhance the retention, for ductile carbon could tolerate the stress induced by volume change during cycling. In this study, the nano-sized Sn/C core-shell composite anode materials were fabricated through sol-gel method, carbon-coating, and carbothermal reduction.
1.2.2 Preparation of tin/tin oxide/carbon anode by carbothermal reduction
In order to reduce the stress caused by large volume change during cycling, tin/tin oxide/carbon composite was selected as anode material. Tin oxide would be reduced to pure Sn and generated Li2O as buffer matrix. Therefore, appropriate ratio of tin to tin oxide could increase the capacity and enhance the cycling stability. In this study, tin/tin oxide/carbon composite anodes were synthesized through carbothermal reduction. The ratio of tin/tin oxide was adjusted by changing reducing temperature.
1.3 Fuel Cell – related issues
1.3.1 Preparation of non-precious metal oxygen reduction catalyst for PEM fuel cells by carbothermal reduction
Replacing platinum as a catalyst in hydrogen–air fuel cells, particularly in proton exchange membrane (PEM) fuel cells (FC), has long been an industry goal. Well-known drawbacks of using platinum as a catalyst are its price and scarcity. An additional, fundamental limitation is that the use of platinum makes the fuel cell operation energy inefficient. With this consideration, developing a suitable replacement for platinum as a catalyst for the oxygen reduction reaction (ORR) could contribute significantly to fuel cells becoming commercially viable. This study will focus on the performance of a non-precious metal catalyst (NPMC) based on nitroaniline precursors.
1.3.2 Synthesizing high nitrogen stainless steel for bipolar plates of proton exchange membrane fuel cells
Ideal bipolar plates for proton exchange membrane fuel cells need some specified properties, such as high corrosion resistance, high electrical conductivity, chemical stability and heat stability. Carbonaceous bipolar plates would be one of the best materials, yet problems, including brittleness and gas permeability, need to be further improved. From the point of view, stainless steels are very attractive, even though higher interfacial contact resistance (ICR) between stainless steel and gas diffusion layer (GDL) as compared with graphite remains to be solved. Furthermore, it has been well known that bearing of N element in stainless steels has a positive effect to enhance corrosion resistance, for that nitrogen enhances pitting resistance of stainless steels. In this study, nitrogen would be introduced by heat treatment in N2 atmosphere to enhance the corrosion resistance.
2. Previous Research Issues
2.1 The development of cathode materials
2.1.1 Microstructure and electrochemical properties of LBO-coated LiMn2O4 cathode material at elevated temperature for Li-ion battery
As the cathode electrode contacted with the Li-based electrolyte directly in Li-ion batteries, Mn dissolution was induced by the generation of acids like HF, due to the reactions of fluorinated anions with the manufactures of instable Li-based salt and solvent oxidation. Particularly, the phenomenon was obvious at higher temperature as 55℃. In order to solve this problem, surface modification of the cathode electrode is an effective way to reduce the side reactions. The surface treatment could decrease the surface area to retard the side reactions between the electrode and electrolyte, and to further diminish the Mn dissolution during cycling test. In this study, the microstructure and electrochemical property in the surface-modified LiMn2O4 were examined and probed. The Li2O-2B2O3 (LBO)-coated Li1+xMn2O4 was synthesized by either solid-state method or chemical solution method. The capacity fading of 0.3 wt％ LBO-coated Li1+xMn2O4 cathode material was 7 ％ after 20 cycles, showing much better cycleability than the un-coated one.
2.1.2 Investigation of copper-manganese oxide spinel cathode material at elevated current rate by in situ synchrotron X-ray for Li-ion batteries
This study focused on the in situ X-ray diffraction (XRD) techniques to investigate Cu substitutions in spinel lithium manganese oxide cathode with LiCuxMn2−xO4-coated LiMn2O4 composite. The objective was to make sure that surface modification of the cathode electrode is an effective way to reduce the side reactions. The phase transformation of both base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 during charging at 0.1 C, 0.5 C and 1C rate from 3 V to 4.5 V was confirmed by the in situ synchrotron X-ray diffractometer (in situ XRD). The plateau potential difference between the base LiMn2O4 and LiCuxMn2-xO4-coated LiMn2O4 composite was 50 mV. The decrease of the plateau can be related to the fact that the kinetics of the LiCuxMn2-xO4-coated LiMn2O4 composite cathode material was faster than that of the uncoated material. The XANES of Cu and Mn K-edge spectrum for LiCuxMn2-xO4-coated LiMn2O4 showed that the valence of Cu and Mn was close to Cu2+ and Mn4+, respectively. On the basis of the in situ XAS data, it was evidenced that Mn transferred toward Mn4+ to minimize the Jahn-Teller distortion by the technique of surface modification, and thus the better electrochemical property was achieved.
2.1.3 New surface modified material for LiMn2O4 cathode material in Li-ion battery
The dissolution of Mn into electrolyte was due to the cathode electrode contacted with the non-aqueous electrolyte directly in Li-ion batteries. Since the side reactions caused Mn2+ dissolution at the surface of the cathode powder during the cycling, surface modification of the cathode electrode is an efficient way to reduce the side reactions. The LiMn2O4 cathode powders derived from co-precipitation method was calcined with the surface modified material to form fine powder of single spinel phase with different particle size, size distribution and morphology. The LiCuxMn2−xO4-coated LiMn2O4 cathode powder with the fading rate of 11.98% at 0.5 C and 11.93% at 0.2 C showed better cyclability than the base one. It is demonstrated that the employment of LiCuxMn2−xO4-coated LiMn2O4 cathode material reduced the fading rate after cycling, especially at high C rate. The better performance was attributed to the significant reduction of the side reaction and Mn dissolution between the interface of the cathode electrode and electrolyte.
2.1.4 Valence change by in situ XAS in surface modified LiMn2O4 for Li-ion battery
The surface-modified cathode material in Li-ion battery was synthesized to decrease the side reactions at the interface between the cathode electrode and electrolyte. It is aimed to reduce the fading rate and to enhance the electrochemical performance, particularly at high C rate. The study is to investigate the variation of the oxidation state, electronic configuration, and site symmetry by the in situ X-ray absorption near edge structure (XANES) and inter atomic distance by the in situ extended X-ray absorption fine structure (EXAFS) during charge and discharge process. In this study, microstructure, valence change and variation of bonding state in the surface-modified LiMn2O4 were examined and probed. From the in situ EXAFS data, the bonding length of both Mn–O and Mn-M M (M=Mn or Cu) was in agreement with the oxidation state of Mn, which was decreased with Li deintercalation while increased with Li intercalation during cycling. On the basis of the in situ XAS data, it was evidenced that Mn transferred toward Mn4+to minimize the Jahn–Teller distortion, and thus the electrochemical property was improved.
2.1.5 LiCoO2 cathode material coated with nano-crystallized ZnO for Li-ion batteries
LiCoO2 is the most widely used cathode material for Li-ion batteries. However, the structural changes and cobalt dissolution into the electrolyte during cycling will lead to rapid capacity fading. Recently, surface modification of the cathode material with metal oxides is found to be a potential approach to improve the electrochemical properties. In this study, nano-crystallized ZnO was coated on the surface of LiCoO2 powders via a wet chemical method. The influence of the amount of coated ZnO on cycling behavior of surface-treated LiCoO2 powders was discussed. Moreover, the effect of the pristine-modified powders on the structural properties was also evaluated.
2.1.6 Improving the electrochemical performance of LiCoO2 cathode by nanocrystalline ZnO coating
The cycle life of LiCoO2 cathode cycled at high cutoff voltage can be largely improved by surface modification with ZnO coating. Moreover, the rate capability at high current density can also be notably improved. The role of ZnO coating played during cycling is discussed from the viewpoint of surface chemistry with the aid of SEM, TEM, EDS, and AC impedance. Furthermore, the correlation between cycling performance and surface properties of ZnO-coated LiCoO2 is explored.
2.1.7 Effect of calcination temperature on the electrochemical behavior of ZnO-coated LiCoO2 cathode
Surface modification of LiCoO2 by metal oxides was verified to be an effective way to retard the abrupt capacity fading of LiCoO2 cycled at cut-off voltage higher than 4.2 V. In this study, LiCoO2 powders were coated with ZnO by the wet chemical method with a calcination process under various temperatures. Morphological and structural characterization of the bare and ZnO-coated LiCoO2 before and after cycling was carried out. The electrochemical test results confirmed that ZnO coating was indeed a feasible approach to enhance the electrochemical performance of LiCoO2 at high voltage.
2.2 The development of anode materials
Because the capacity of graphite was low, raising the specific capacity of anodes by introducing tin-based compounds was the task in this work. To overcome the rapid capacity fading of tin-based anodes, carbonaceous materials were used as the matrix to accommodate the stress induced by the large volume changes of Li-Sn alloys during cycling. A modified electroless plating technique was adopted to prepare the composite electrodes of Sn compounds/carbonaceous material. Multi-phases of Sn compounds were deposited on mesophase graphite powders (MGP) by this process. The multiphase compounds containing metallic Sn, SnP3 and SnP2O7 were expected to provide a higher spectator to Sn ratio for improved cycleability.
2.2.2 Electrochemical properties of nano-sized Ni-Sn-P coated on MCMB anode for lithium secondary batteries
Sn/C composite was the most attractive and potential anode material in Li-ion batteries, due to its high capacity and cycling stability. In order to further enhance the retention, Ni was added into Sn/C composite and treated as buffer matrix during cycling. A novel Ni-Sn-P/mesophase carbon micro beads (MCMBs) composite material for lithium-ion batteries was prepared by an electroless plating method. On the basis of X-ray color mapping analysis by electron probe microanalyzer, the nano-sized Ni-Sn-P was precipitated not only on the surface but also in the interior of MCMB powders. In addition, the Ni-Sn-P/MCMB composite anode showed a significant improvement in electrochemical performance.
2.2.3 Preparation and characterization of Sn/C composite anode materials for Li-ion batteries by electroless plating method
To develop anode materials with higher specific capacity, tin-based compounds have been extensively investigated in the past years. Even metallic tin exhibits capacity as high as 991mAh/g, it suffers from rapid capacity fading. The poor cycleability is mainly caused by the pulverization of the anode materials which is associated with the large volume change during formation of Li-Sn alloys. To solve the problem, several types of tin compounds have received a great interest, such as Sn oxides, Sn-based alloys, Sn phosphides and Sn phosphates. These novel anodes exhibit superior initial capacity to carbonaceous materials and better capacity retention than metallic tin. However, these tin compounds still encounter the agglomeration of Sn during cycling, leading to the pulverization of these compounds after prolonged cycling. Many researchers are devoted to solve the problem. Recently, it was revealed that reducing the anode particle size and deriving a multiphase composition could retard the pulverization problem. Moreover, if metallic tin particles disperse well in the inactive matrix, the aggregation of Sn will be slowed down and hence the capacity retention could be improved. More recently, it is discovered that combining ductile carbonaceous materials with the metallic Sn, Sn oxides and Sn alloys could effectively improve the cycling stability. Electrochemical performance of the composite anodes is influenced by the material system, particle size and size distribution of the Sn compounds and the amount of deposited Sn. A favorable composite anode exhibits nano-sized and uniformly distributed tin compounds. In this study, the electroless plating method which has potential for commercialization will be adopted to prepare the Sn/C composite anode materials. By this systematical study, the Sn/C composite anode with the better electrochemical performance will be prepared under different pH value and precursor concnetration. In addition, the correlations among the physical property, microstructure and electrochemical properties of the materials are also probed and discussed.
2.2.4 Tin compounds coated on carbonaceous mixture by electroless plating method as anode material for Li-ion battery
Carbonaceous compounds are widely used in commercial Li-ion batteries. However, carbon only exhibits a theoretical capacity of 372 mAhg-1 after being lithiated to form LiC6. For the further improvement of lithium-ion batteries, the development of alternative anode materials, which possess larger capacity is required. Therefore, Li-Sn systems have been investigated for the development of new anode materials. Nevertheless, these materials have a serious drawback, which is the pulverization of tin caused by its volume expansion/shrinkage during cycling. In this study, it was aimed that the particle size distribution and Sn compounds particle size of the as-prepared electrode could be improve by the modified electroless plating procedure. It was hoped that as-obtained Sn compounds / C-C composite materials would be fabricated, showing a remarkable cycling performance and a higher reversible capacity.