LiNi0.5Mn1.5O4 Spinel and Its Derivatives as Cathodes for Li-Ion Batteries

It is well known that lithium-ion batteries are common in consumer electronics. It is one of the most popular types of rechargeable battery for portable electronics, with the best energy densities, no memory effect, and a slow loss of charge when not in use [1, 2]. Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle, and aerospace applications. Its excellent properties originate from its materials including cathode, anode and electrolyte and so on. For cathode materials, there are mainly three kinds of materials which have been widely studied and applied commercially, including layered oxide LiCoO2, spinel LiMn2O4 and olivine LiFePO4. Among the cathode materials, LiCoO2 has been used since the invention of LIB [3], while LiMn2O4 and LiFPO4 are considered as promising ones due to less toxicity, low cost, more safety and good electrochemical properties [4, 5]. In term of redox energy level, these materials can be charged and discharged at around 4 V, which limits their energy density. The spinel LiNi0.5Mn1.5O4 is becoming a research focus recently. The most remarkable property of spinel LiNi0.5Mn1.5O4 is its discharge voltage plateau at around 4.7 V. In some cases, using LiNi0.5Mn1.5O4 will lead fewer cells at the battery pack level. For example, hundreds of ordinary lithium ion batteries are needed to meet the requirement of electric vehicle (EV) in the state of start-up, accelerate and climb-up [6] because more energy is needed in this case. If the high voltage cells are utilized, the amount of batteries used for EV can decrease greatly. This chapter gives a detailed introduction on LiNi0.5Mn1.5O4 spinel and the latest research advances in this area.


Introduction
It is well known that lithium-ion batteries are common in consumer electronics.It is one of the most popular types of rechargeable battery for portable electronics, with the best energy densities, no memory effect, and a slow loss of charge when not in use [1,2].Beyond consumer electronics, LIBs are also growing in popularity for military, electric vehicle, and aerospace applications.Its excellent properties originate from its materials including cathode, anode and electrolyte and so on.For cathode materials, there are mainly three kinds of materials which have been widely studied and applied commercially, including layered oxide LiCoO 2 , spinel LiMn 2 O 4 and olivine LiFePO 4 .Among the cathode materials, LiCoO 2 has been used since the invention of LIB [3], while LiMn 2 O 4 and LiFPO 4 are considered as promising ones due to less toxicity, low cost, more safety and good electrochemical properties [4,5].In term of redox energy level, these materials can be charged and discharged at around 4 V, which limits their energy density.The spinel LiNi 0.5 Mn 1.5 O 4 is becoming a research focus recently.The most remarkable property of spinel LiNi 0.5 Mn 1.5 O 4 is its discharge voltage plateau at around 4.7 V.In some cases, using LiNi 0.5 Mn 1.5 O 4 will lead fewer cells at the battery pack level.For example, hundreds of ordinary lithium ion batteries are needed to meet the requirement of electric vehicle (EV) in the state of start-up, accelerate and climb-up [6] because more energy is needed in this case.If the high voltage cells are utilized, the amount of batteries used for EV can decrease greatly.This chapter gives a detailed introduction on LiNi 0.5 Mn 1.5 O 4 spinel and the latest research advances in this area.

Structures of LiNi 0.5 Mn 1.5 O 4
There are two kinds of crystal structure for spinel LiNi 0.5 Mn 1.5 O 4 , i.e. face-centered spinel (Fd3m) and primitive simple cubic crystal (P4 3 32).For LiNi 0.5 Mn 1.5 O 4 with a face-centered structure (Fd3m), the lithium ions are located in the 8a sites of the structure, the manganese and nickel ions are randomly distributed in the 16d sites.The oxygen ions which are cubicclose-packed (ccp) occupy the 32e positions.For LiNi 0.5 Mn 1.5 O 4 (P4 3 32) with a primitive simple cubic structure, the manganese ions are distributed in 12d sites, and nickel ions in 4a sites.The oxygen ions occupy the 24e and 8c positions, while the lithium ions are located in the 8c sites.In this case, the Ni and Mn ions are ordered regularly [7][8][9].Whether LiNi 0.5 Mn 1.5 O 4 has a structure of face-centered spinel (Fd3m) or primitive simple cubic (P4 3 32) depends on its synthetic routes.In synthesizing LiNi 0.5 Mn 1.5 O 4 , annealing process at 700°C after calcination led to the ordering of Ni and Mn ions, making it transformed from face-centered spinel (Fd3m) to primitive cubic crystal (P4 3 32).Schematic drawing of the structures of LiNi 0.5 Mn 1.5 O 4 is shown in Fig. 1 [10].Fig. 1.Schematic drawing of the structures of LiNi 0.5 Mn 1.5 O 4 spinel lattice: a) face-centered spinel (Fd3m) b) primitive simple cubic (P4 3 32) [10] Infrared spectroscopy is an effective method to distinguish these two structures.Infrared spectra of ordered (P4 3 32) and disordered (Fd3m) LiNi 0.5 Mn 1.5 O 4 exhibit different patterns between 650 and 450 cm -1 .At this band range, there are apparent spectra at 588 and 430 cm -1 for ordered LiNi 0.5 Mn 1.5 O 4 .The intensity ratio of two bands at 619 and 588 cm -1 can be used qualitatively to assess percentage of ordering in spinel which contains both ordered and disordered LiNi 0.5 Mn 1.5 O 4 [11].
The diffusion path of Li in the spinel structure is a three-dimensional network.Lithium moves from one tetrahedral site to the next through a vacant octahedral site.The activation barriers of migration are greatly influenced by the electrostatic repulsion from the nearest transition metal.Because the distribution of Ni and Mn is different in ordered (P4 3 32) and disordered (Fd3m) LiNi 0.5 Mn 1.5 O 4 , the activation barriers for migration of Li will be different from each other.Although the previous studies showed that disordered LiNi 0.5 Mn 1.5 O 4 exhibited better cycling performance than ordered LiNi 0.5 Mn 1.5 O 4 at high rates [12][13], a recent study shows that the activation barriers for Li ion transportation in ordered (P4 3 32) LiNi 0.5 Mn 1.5 O 4 can be as low as around 300 meV according to first-principles calculation, so the ordered LiNi 0.5 Mn 1.5 O 4 can exhibit good cycle ability at high current rates [14].
In the synthesis of LiNi 0.5 Mn 1.5 O 4 , the high calcination temperature sometimes leads to the reduction of the Mn oxidation state from +4 to +3, which results in the formation of Fd3m structure.When annealed at 700 °C in air after a high-temperature calcination at 1000 °C, the resulting powders does not contain Mn 3+ [15].It was reported that LiNi 0.5 Mn 1.5 O 4 synthesized under O 2 atmosphere has the cubic spinel structure with a space group of P4 3 32 instead of Fd3m [16,17].

Mechanism of high voltage and insertion/deinsertion
Based on the results obtained with the systems LiMn 2−y Ni y O 4 and LiCr y Mn 2-y O 4 , Dahn and Sigala [18,19] previously pointed out that the high voltage originated from the oxidation of nickel and chromium ion.The 4.1 V plateau was related to the oxidation of Mn 3+ to Mn 4+ and the 4.7 V plateau to the oxidation of Ni 2+ to Ni 4+ .The oxidation of chromium ion could bring about a high voltage of 4.9 V. Yang [20] suggested that a significant amount of Mn 4+ ion in the spinel framework was essential for electrochemical reaction to occur at around 5 V. His view was supported by Kawai [21] who argued that the presence of manganese was necessary to keep the high voltage capacity because manganese-free spinel oxides, such as Li 2 NiGe 3 O 8 , did not show any capacity above 4.5 V.The influence of doping metals including M = Cu [22][23][24], Co [25], Cr [26][27][28][29], Fe [30][31][32], Al [33,34], and Zn [35] on the properties of LiM 0.5 Mn 1.5 O 4 have been investigated.Among these materials, Ni-doped compound LiM 0.5 Mn 1.5 O 4 displays higher capacity and better cycle ability.For spinel LiNi 0.5 Mn 1.5 O 4 , there is a capacity occurring at 4.6-4.7 V, which can be attributed to a two electron process, Ni 2+ /Ni 4+ .While in the 4 V region, the electrode sometimes shows some minor redox behavior, related to the Mn 3+ /Mn 4+ couple.When there are more Mn 4+ and Ni 2+ in LiNi 0.5 Mn 1.5 O 4 , then the corresponding capacity at 4 V will be less and that at 5 V will be large.[36,37].
Gao [38] put forward an explanation for the origin of high voltage.As an electron is removed from Mn 3+ , it is removed from Mn eg (↑) which has an electron binding energy at around 1.5-1.6 eV, and this accounts for the 4.1V plateau.When there are no more electrons left on Mn eg (↑) (all Mn are Mn 4+ ), electrons are removed from Ni eg (↑↓) which has an electron binding energy of about 2.1 eV, and the voltage plateau moves up to 4.7 V because of the increased energy needed to remove electrons.
Terada [39] studied the mechanism of the oxidation reaction during Li deintercalation by measuring the in situ XAFS spectra of Li 1-x (Mn,M) 2 O 4 (M=Cr, Co, Ni).It is found from the Ni K-edge XAFS analysis that Ni in Li 1-x Mn 1.69 Ni 0.31 O 4 experiences three distinct valence states during Li deintercalation, Ni 2+ , Ni 3+ and Ni 4+ .The X-ray absorption near-edge structures (XANES) of Mn and M shows that the high voltage (～5 V) in the cathode materials is due to the oxidation of M 3+ to M 4+ (M = Cr, Co), and M 2+ to M 4+ (M = Ni).The origin of the low voltage (3.9-4.3V) is ascribed to the oxidation of Mn 3+ to Mn 4+ .
As for the impurity phase of Li x Ni 1-x O in product, it is believed that they come from the loss of oxygen at high temperatures.The tetravalent manganese (Mn 4+ ) is unstable at high temperatures and can be converted to trivalent (Mn 3+ ) so that oxygen may partially evolve out of the lattice to form LiN i0.5 Mn 1.5 O 4−y .When x value becomes large, this phase becomes unstable and may decompose into two phases, i.e., LiNi 0.5−z Mn 1.5 O 4−y and Li x Ni 1−x O.The overall reaction process can be depicted as follows:  Figure 3 (a) shows the charge-discharge curves of LiNi 0.5 Mn 1.5 O 4 (1) which was synthesized without annealing process.Its discharge capacities were 121.5 mAh g -1 at 0.2 C and 117.6 mAh g -1 at 0.7 C, respectively.The cycle performance at 0.7 C was displayed in Fig. 3 (b).It can be found that there is only small capacity decay after 50 cycles.The theoretical capacity of LiNi 0.5 Mn 1.5 O 4 is about 148 mAh g -1 .There is a capacity of about 26 mAh g -1 that is not delivered by the sample LiNi 0.5 Mn 1.5 O 4 (1).Figure 4 (a) illustrates the charge-discharge curves of the sample LiNi 0.5 Mn 1.5 O 4 (2).This test was conducted at 0.5 C charge current and different discharge current rates.The discharge capacities were 119.5 mAh g -1 at 0.5 C and 116.3 mAh g -1 at 1 C, respectively.The specific capacity around 4 V was about 13.6 mAh g -1 .It is less than that of LiNi 0.5 Mn 1.5 O 4 (1).The specific capacity of sample LiNi 0.5 Mn 1.5 O 4 (1) around 4 V was about 17.0 mAh g -1 .This proves that there was less amount of Mn 3+ in sample LiNi 0.5 Mn 1.5 O 4 (2) than sample LiNi 0.5 Mn 1.5 O 4 (1).The reason is that there is less oxygen deficiency due to the annealing process.
The cycle performances of sample LiNi 0.5 Mn 1.5 O 4 (2) are shown in Fig. 4 (b).Its discharge capacities at 2 C and 4 C were 107.5 and 98.5 mAh g -1 , respectively.The capacity retention was good for every current rate.The above results demonstrate that the impurity Li x Ni 1-x O can reduce the specific capacity of LiNi 0.5 Mn 1.5 O 4 .However, there is no obvious evidence that the impurity Li x Ni 1-x O impairs the cycle performances of products.According to previous reports, the Li x Ni 1-x O phase can be used as an anode material for lithium ion batteries, exhibiting good electrochemical properties.At 100 mA g -1 , its discharge capacity of the first cycle was up to 1480 mAh g −1 below 1.5 V [96].

Nano-sized LiNi 0.5 Mn 1.5 O 4 spinels
Nanostructure materials have both advantages and disadvantages for lithium batteries.The advantages include short path lengths for Li + transport, short path lengths for electronic transport, higher electrode/electrolyte contact area leading to higher charge/discharge rates, while the disadvantage include an increase in undesirable electrode/electrolyte reactions due to high surface area, leading to self-discharge, poor cycling and calendar life [97,98].
Usually, nano-sized LiNi 0.5 Mn 1.5 O 4 can be obtained via wet chemical methods.In this process, the precursor compounds including Li, Ni and Mn salts are mixed homogenously at atomic scale.After further calcination, nano-sized LiNi 0.5 Mn 1.5 O 4 particles can be obtained at an low temperatures.When sintering temperature continues to go up, the particle size of LiNi 0.5 Mn 1.5 O 4 increase, and finally they will turn into micro-sized products.
Based on the research results that have been reported, the formation temperature of spinel phase is as low as 450 °C, whereas the growth of integrated LiNi 0.5 Mn 1.5 O 4 crystals takes place at relatively higher calcination temperature.The calcination temperature has significant effects on the structure and morphology of the materials so as to affect their electrochemical performance.The higher calcination temperature leads to higher crystallinity that helps to increase the electrode capacity while it may produce particles with relatively large size and long diffusion distances for lithium ions, which makes lithium ions insertion-extraction difficult.Therefore, with the combination of these two factors, the powders calcined at proper temperature will deliver the highest discharge capacity.Some researches relative to nano-sized LiNi 0.5 Mn 1.5 O 4 spinels have been reported.In general, the nanometer particles exhibit a good performance at high rates due to the shortened diffusion paths, whereas at low rates the reactivity towards the electrolyte increases and the cell performance is lowered.Micrometric particles, which are less reactive towards the electrolyte, are a better choice for making electrodes under these latter conditions.
Recently, some improvements have been achieved.Nanometer LiNi 0.5 Mn 1.5 O 4 with good electrochemical performance over a wide range of rate capabilities by modifying the experimental synthetic conditions has also been reported.For example, Lafont [103] synthesized a nano material LiMg 0.05 Ni 0.45 Mn 1.5 O 4 of about 50 nm in size with an ordered cubic spinel phase (P4 3 32) by auto-ignition method.It displayed good capacity retention of 131 mAh g -1 at C/10 and 90 mAh g -1 at 5C.By using a template method, Arrebola [104] synthesized LiNi 0.5 Mn 1.5 O 4 nanorods and nanoparticles using PEG 800 (PEG: polyethyleneglycol) as the template.Highly crystalline nanometric LiNi 0.5 Mn 1.5 O 4 of 70-80 nm was prepared at 800°C.Its electrochemical properties were measured at different charge/discharge rates of C/4, 2C, 4C, 8C and 15C, the capacity values were 121 mAh g -1 at 2C to 98 mAh g -1 at 15C, and faded slowly on cycling.
Hydrothermal synthesis includes various techniques of crystallizing substances from hightemperature aqueous solutions at high vapor pressures.The method is also particularly suitable for the growth of large good-quality crystals while maintaining good control over their composition.Now it is often used to synthesize nano scale materials including electrode materials for lithium ion batteries.Recently, it is reported that nano LiNi 0.5 Mn 1.5 O 4 was fabricated by this approach, and the products exhibited good performances.For example, LiOH•H 2 O, MnSO 4 •H 2 O, NiSO 4 •6H 2 O, (NH 4 ) 2 S 2 O 8 were used as reactants, and they were dissolved in deionized water in a Teflon-lined stainless steel autoclave.Then, the autoclave was sealed and heated at 180°C for some time.The nano scale products were finally obtained.It delivered 100, 91, 74, and 73 mAh g -1 at current densities of 28, 140, 1400, and 2800 mA g -1 , respectively.The rate capability of such a nanosized 5 V spinel is better than those of a submicron LiNi 0.5 Mn 1.5 O 4 [105].Fig. 5 (a) and (b) show the SEM photographs and charge-discharge curves, respectively.Besides particle sizes, particle morphology and crystallinity also play a role in properties of materials.Kunduraci [106] synthesized a three dimensional mesoporous network structure with nanosize particles and high crystallinity.This morphology allows easy electrolyte penetration into pores and continuous interconnectivity of particles, yielding high power densities at fast discharges.
At present the electrode materials have reached their intrinsic limit, nano materials provide a new chance to improve their properties.It is no doubt that nano-sized electrode materials including nano LiNi 0.5 Mn 1.5 O 4 will gradually be applied in future high-energy lithium ion batteries.To realize the commercial application of nano materials, some technical obstacles such as undesirable electrode/electrolyte reactions due to high surface area, self-discharge and poor calendar life, etc have to be solved.The structural and electrochemical properties of the LiNi 0.5 Mn 1.5 O 4 could also be affected by the substitution of other metal ions.Cation doping is considered to be an effective way to modify the intrinsic properties of electrode materials.Taking doping Cu as an example [107], the amount of Cu will affect the lattice parameters, the cation disorder in the spinel lattice, the particle morphology, as well as the electrochemical properties.In situ XAS experiment, the Cu K-edge XANES spectra of LiCu 0.25 Ni 0.25 Mn 1.5 O 4 shows that the Cu valence only changes between 4.2 and 4.7 V. Therefore Cu can participate in the charge process in this range may be due to the oxidation of Cu 2+ to Cu 3+ .Although the reversible discharge capacity decreases with increasing Cu amount, optimized composition such as LiCu 0.25 Ni 0.25 Mn 1.5 O 4 exhibits high capacities at high rates.In addition, the doping with appropriate amount in LiNi 0.5 Mn 1.5 O 4 can improve electrical conductivity, and help to improve electrochemical performances.For example, the electronic conductivity conductivities of the LiNi 0.5 Mn 1.5 O 4 , Li 1.1 Ni 0.35 Ru 0.05 Mn 1.5 O 4 , and LiNi 0.4 Ru 0.05 Mn 1.5 O 4 measured from EIS at room temperature are 1.18×10 −4 , 5.32×10 −4 , and 4.73×10 −4 S cm −1 , respectively.Although substitution of Ni 2+ ions with heavier Ru 4+ ions may reduce the theoretical capacity, the results show that a small doping content does not affect the accessible capacity at low current rates; on the contrary, larger accessible capacity can be obtained due to enhanced conductivity.
Cation doping like doping Ru and Fe has achieved some encouraging results, improving the rate capability to a certain extent.Cation doping can improve conductivity, enlarge lattice constants and form stronger M-O bond, etc., which are favorable for the migration of lithium ions and maintaining stable crystal structure.When choosing appropriate element and amount better electrochemical properties can be expected.Perhaps electronic structure of the crystal can provide another theoretical explanation to the role of cation doping.
Besides cation doping, there are some researches relative to the substitution of small amount of F -for O 2-anion [127][128][129].In this case it is assumed that O 2− and F − ions are located at the www.intechopen.com32e sites.The doped compounds like LiNi 0.5 Mn 1.5 O 4-x F x have smaller lattice parameter than LiNi 0.5 Mn 1.5 O 4 because fluorine substitution changes the oxidation state of transition metal components and more Mn 3+ ions with larger ionic radius (r = 0.645 Å) will replace Mn 4+ ions (r = 0.53 Å) for electro-neutrality.The content of fluorine has influence on electrochemical properties of the doped compounds.On one hand, strong Li-F bonding may hinder Li + extraction, leading to a lower reversible capacity.On the other hand, fluorine doping makes spinel structure more stable due to the strong M-F bonding, which is favorable for the cyclic stability.According to the previous research report [129], the compound LiNi 0.5 Mn 1.5 O 3.9 F 0.1 displayed good electrochemical properties of an initial capacity of 122 mAh g -1 and a capacity retention of 91% after 100 cycles.In addition, Oh [127] studied the effect of fluorine substitution on thermal stability.He reported that the LiNi 0.5 Mn 1.5 O 4 electrode had an abrupt exothermic peak at around 238.3 o C (1958 J g -1 ) when charged to 5.0V, while Li δ Ni 0.5 Mn 1.5 O 3.9 F 0.1 electrodes exhibited smaller exothermic peaks at higher temperatures, i.e., 246.3 o C (464.2 J g -1 ).So fluorine substitution is advantageous for the thermal stability of Li δ Ni 0.5 Mn 1.5 O 4−x F x spinel.

Surface modification
Although surface modifications applied to high voltage material LiNi 0.5 Mn 1.5 O 4 are much less than those applied to cathode materials with layer structure like LiCoO 2 , they are also effective ways to improve the properties of LiNi 0.5 Mn 1.5 O 4 .It is believed that the high surface reactivity of the LiNi 0.5 Mn 1.5 O 4 with the electrolyte at high operating voltage results in the formation of SEI film, which significantly hinders the insertion/extraction of Li + ion, the charge transfer and hence the kinetics of the electrochemical processes.In order to improve its electrochemical behavior, coating the electrode material LiNi 0.5 Mn 1.5 O 4 with chemically stable compounds has been applied.The coating layer can hinder the formation of SEI film, and protect cathode materials from being attacked by HF.So far, surface modification of 5 V spinels has been limited mainly to Bi 2 O 3 130, Al 2 O 3 131, 132, ZnO 133-135, Li 3 PO 4 136, SiO 2 137, Zn 138, Au 139, AlPO 4 140, ZrP 2 O 7 141, BiOF 142 which lead to better cycle performance and rate capability retention.However, the effect of coating the nanometric spinel LiNi 0.5 Mn 1.5 O 4 with Ag on its rate capability was negative 143.According to Liu 131, Al 2 O 3 -modified sample exhibited the best cyclability (99% capacity retention in 50 cycles) with a capacity of 120 mAh g -1 , while Bi 2 O 3 -coated sample exhibited the best rate capability.At a rate of 10C, the Bi 2 O 3 -coated sample could deliver a capacity of about 90 mAh g -1 after 50 cycles.Liu 132 thought that Al 2 O 3 reacted with the surface of LiMn 1.42 Ni 0.42 Co 0.16 O 4 during the annealing process and formed LiAlO 2 that exhibited good lithium-ion conductivity.Therefore, "Al 2 O 3 " modification layer acts as both a protection shell and as a fast lithium-ion diffusion channel, rendering both excellent cycling performance and good rate capability for the Al 2 O 3 -coated LiMn 1.42 Ni 0.42 Co 0.16 O 4 .Similarly, Bi 2 O 3 is reduced on the cathode surface during electrochemical cycling to metallic Bi, which is an electronic conductor, rendering both excellent rate capability and good cycling performance for the Bi 2 O 3 -coated LiMn 1.42 Ni 0.42 Co 0.16 O 4 .In addition, the microstructure of the surface modification layer plays an important role in determining the electrochemical performances of the active material.Some experimental results indicate that the surface modifications neither change the bulk structure nor cause any change in the cation disorder of the spinel sample.In addition, electrolyte is easy to decompose on the surface of the 5 V spinel cathodes because of the higher operating voltage, resulting in the formation of thick Besides the above mentioned coating layers, coating carbon should also be a good choice because it is a better conductor.Recently, it was reported that the carbon-coated material LiNi 0.5 Mn 1.5 O 4 was synthesized by a sol-gel method.The XRD patterns demonstrate that the spinel structure is not affected after coating the LiNi 0.5 Mn 1.5 O 4 powder with carbon.The lattice parameter was 0.8178 nm for pristine LiNi 0.5 Mn 1.5 O 4 , while lattice parameters of LiNi 0.5 Mn 1.5 O 4 coated with different amount of carbon varied from 0.8171 to 0.8177 nm.The carbon layer was consecutive, and the thickness range of carbon layer was from approximate 10 to 20 nm.The carbon coating made the powders coarser and more agglomerated.The conductive carbon layer not only avoided the direct contact between the active cathode material and the electrolyte, but also provided pathways for electron transfer.Accordingly, the electrochemical properties of LiNi 0.5 Mn 1.5 O 4 were also improved duo to carbon layer, for example, when the LiNi 0.5 Mn 1.5 O 4 was modified with optimal 1wt.% sucrose, its discharge capacity could reach 130 mAh g −1 at 1 C discharge rate with a high retention of 92% after 100 cycles and a high 114 mAh g −1 at 5 C discharge rate 144.
The graphite anode with brittle layer structure can suffer from exfoliation when lithium ion inserts into its structure in electrolyte, which deteriorates the properties of batteries.Also, it is believed that the operating potential plateau of the carbon anode is close to that of metal lithium so that "dendrite" could still be unavoidable.And the solid electrolyte interface (SEI) layer on the carbon electrode, which is usually formed at the potential below 0.8V versus Li + /Li and accompanied over time with active lithium loss, an increase in impedance and a decrease in specific capacity, limits the lifetime and rate capability of the lithium-ion batteries.Furthermore, there are some other drawbacks, such as thermal stability concerns, and the bad compatibility with propylene carbonate-based electrolytes and some functional electrolytes, i.e. the flame-retardant electrolytes containing phosphates or phosphonate.The spinel Li 4 Ti 5 O 12 (LTO) has been considered as a zero-strain insertion material duo to its excellent reversibility and structural stability in the charge-discharge process.So it is a promising alternative anode material.In addition, its structure remains nearly unchanged in PC-containing electrolyte, which makes batteries safer than those with graphite anode [155,156].In recent years, there have been some researches about full lithium-ion cells.Many 2V lithium-ion battery systems have been studied, such as LiCoO 2 /LTO, LiMn 2 O 4 /LTO, LiFePO 4 /LTO, LiNi 1/3 Co 1/3 Mn 1/3 O 2 /LTO, etc.Although these batteries exhibit good cycleability, rate capability and stability associated with safety, the low voltage indicates that the battery has low energy density.
Because the operating voltage of spinel LiNi 0.5 Mn 1.5 O 4 can reach 4.7V, the full batteries can output a voltage of over 3 V if using LiNi 0.5 Mn 1.5 O 4 as cathode and Li 4 Ti 5 O 12 as anode respectively.Fig. 8   In addition, Arrebola [157] have tried to combine LiNi 0.5 Mn 1.5 O 4 spinel and Si nanoparticles to fabricate new Li-ion batteries.Because Si composite could deliver capacities as high as 3850 (with super P) and 4300 (with MCMC) mAh g -1 , this LiNi 0.5 Mn 1.5 O 4 /Si cell was expected to have higher capacity.At present, the battery could deliver a capacity of around 1000 mAh g -1 after 30 cycles with good cycling properties.Xia [158] reported that the properties of LiNi 0.5 Mn 1.According to the previous studies, the spinel with Fd3m structure exhibits better cycling performance than spinel with P4 3 32 structure at high rates.However, recent research demonstrates that the activation barrier for Li ion transportation in ordered (P4 3 32) LiNi 0.5 Mn 1.5 O 4 is the lowest one, so the ordered LiNi 0.5 Mn 1.5 O 4 can exhibit the best cycle ability at high current rates.Further studies are needed to solve this disagreement.The microstructure and surface are the key factors affecting its electrochemical properties.Doping elements can improve electrochemical properties, such as doping Ru, Fe and so on.It is well known that the doped elements can make crystal structure more stable, which is in favor of Li insertion/deinsertion.However, the transportation of Li ion in structure is also affected by electronic structure of materials.So far there has not been satisfactory and scientific explanation for this aspect.Making surface modification on the surface of LiNi 0.5 Mn 1.5 O 4 , can not only protect electrode materials from attacking of HF which generates from electrolyte decomposing, but also suppress the development of the SEI layer.This also helps to improve the electrochemical properties of spinel LiNi 0.5 Mn 1.5 O 4 .The Li insertion/deinsertion is affected by particle morphology and size which depend on synthesis methods as well.Nano materials can lead to higher charge/discharge rates.New lithium-ion battery system can be put into practice when combining LiNi 0.5 Mn 1.5 O 4 and other anode materials such as Li 4 Ti 5 O 12 .This system has exhibited excellent electrochemical properties.
To get rid of the impurities, an annealing process after the high temperature treatment is usually necessary.It is acknowledged that impurity Li x Ni 1−x O can deteriorate the electrochemical properties of products.However, so far there have not been special researches about how the impurity Li x Ni 1-x O affects the electrochemical performances of products.In order to further investigate the effect of impurity Li x Ni 1-x O.The LiNi 0.5 Mn 1.5 O 4 compounds were synthesized by solid state reaction method.Fig.2(a) and (b) show the XRD patterns of two products.One LiNi 0.5 Mn 1.5 O 4 (1) was synthesized at 850 0 C for 12 h, and the other LiNi 0.5 Mn 1.5 O 4 (2) was synthesized at 850 0 C for 12 h and then annealed at 600 0 C for 12 h.The reference material Li 0.26 Ni 0.72 O is also illustrated in Fig.2 (a).It can be seen that there are small peaks at 37.5 0 , 43.6 0 and 63.3 0 in the patterns of two products, illustrating that there was a secondary phase Li x Ni 1-x O.The intensity of the impurity Li x Ni 1-x O peaks decreased due to the annealing process.

Fig. 6 .
Fig. 6. a) Scanning electron microscopy of the b) Capacity retention of PAS-LiNi0.5-2xRuxMn1.5O4PA-LiNi 0.5-2x Ru x Mn 1.5-x O 4 : (a) x = 0, (b) x = 0.01, (c) x = 0.03, (d) x = 0.05 [126] Generally speaking, doping elements can substitute for Ni or Mn in LiNi 0.5 Mn 1.5 O 4 .For instance, element Cr will substitute for Ni in LiNi 0.5 Mn 1.5 O 4 because the ionic radius of Cr 3+ is 0.615 Å which is close to that of Ni 2+ (0.69 Å).Partial replacement of Ni in LiNi 0.5 Mn 1.5 O 4 with Cr is an effective approach to improve the electrochemical properties of LiNi 0.5 Mn 1.5 O 4 because the bonding energy of Cr-O is stronger than that of Mn-O and Ni-O.The stronger Cr-O bond is in favor of maintaining the spinel structure during cycling.This prevents the structural disintegration of the material.Besides replacing Ni, Mn in LiNi 0.5 Mn 1.5 O 4 can also be substituted for.In the case of Al doping, the ionic radius of Al 3+ is 0.62 Å, which is nearly the same as that of Mn 4+ (0.54 Å), so Al can substitute for Mn in LiNi 0.5 Mn 1.5 O 4 .The strong Al-O bond is also beneficial to improving the electrochemical properties of LiNi 0.5 Mn 1.5 O 4 .Doping with Fe has also achieved good experimental results.The LiMn 1.5 Ni 0.42 Fe 0.08 O 4 delivered a capacity of 136 mAhg -1 at C/6 rate with capacity retention of 100% in 100 cycles and a remarkably high capacity of 106 mAhg -1 at 10 C rate.The material could deliver capacities of 143, 118 and 111 mAh g -1 at current densities of 1.0, 4.0 and 5.0 mA cm -2 with excellent capacity retention, respectively.

Fig. 8 .
Fig. 8. a) Charge-discharge curves; b) Cyclic voltammogramsThe properties of new battery LiNi 0.5 Mn 1.5 O 4 /Li 4 Ti 5 O 12 depended on both cathode and anode materials.Nano scale Li 4 Ti 5 O 12 displays good electrochemical properties, and will be applied in this battery.Because cathode LiNi 0.5 Mn 1.5 O 4 and anode Li 4 Ti 5 O 12 have different specific capacities, there are two ways to fabricate the full batteries.One is LNMO-limited cell, another is LTO-limited cell.The experiment results indicate that the LNMO/LTO cell system with the capacity limited by LTO has the better cycling performance than that limited by LNMO[145].